Strategies for Modifying TiO2 Based Electron Transport Layers to

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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Strategies for Modifying TiO2 Based Electron Transport Layers to Boost Perovskite Solar Cells Chao Zhen,†,‡ Tingting Wu,†,‡ Runze Chen,† Lianzhou Wang,*,§ Gang Liu,*,†,‡ and Hui-Ming Cheng†,∥

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Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua RD, Shenyang 110016, China ‡ School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang 110016, China § Nanomaterials Centre, School of Chemical Engineering and AIBN, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia ∥ Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, 1001 Xueyuan Road, Shenzhen 518055, China ABSTRACT: The research on solution processed metal halide perovskite solar cells (PSCs) as a new type of solar cells has experienced explosive growth since the first report in 2009. It is impressive that solar energy conversion efficiency has increased to over 23%. Outstanding optoelectronic properties including high absorption coefficient, high mobility, and long diffusion length of charge carriers have been revealed in the family of hybrid organic inorganic halide perovskite materials that are considered the heart of solar cells. A long-anticipated feature for solar cells that the diffusion lengths of charge carriers outstrip the active layer thickness of a device has been demonstrated in PSCs so that the efficiency of extracting photocarriers, particularly electrons at the interfaces becomes a key parameter controlling global device performance. The n-type semiconductor TiO2 with the merits of thermal and chemical stability, low cost, and suitable band edge positions has been regarded an ideal electron transporting layer (ETL) material in PSCs performing the function of selectively extracting photoelectrons and subsequently delivering them toward a current collector. Besides the highly concerning energy conversion efficiency of PSCs, the challenge of the current−voltage hysteresis phenomenon and instability of PSCs are also revealed to be closely related with TiO2 ETLs. In this review, the recent progress on strategies for modifying TiO2 ETLs by controlling morphology, surface modification, doping, and constructing composites to improve global performance of PSCs is reviewed. Moreover, the perspective on future development of TiO2 based ETLs for high performance PSCs is proposed on the basis of the comprehensive and deep understanding of TiO2 from the area of photocatalysis. It is anticipated that finely tailoring the features and properties of TiO2 ETLs will further release large room for exciting enhancements in the global performance of PSCs. KEYWORDS: Solar cells, Perovskite, Electron transport layer, TiO2

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single crystals).7,21−28 Metal halide perovskites generally have the chemical formula of ABX3 (Figure 1), where A, B, and X represent an organic or alkaline metal cation, metal cation, and halide anion, respectively. Through varying these crystal components, a wide range of tunable band gaps from the ultraviolet to infrared region can be obtained in perovskite materials.29−35 From 2006 to 2009, metal halide perovskite materials (CH3NH3PbI3 and CH3NH3PbBr3) were first used by Miyasaka and co-workers as dyes loaded on mesoporous TiO2 photoanodes in liquid DSSCs, achieving a maximum efficiency of around 3.8%.6,36 Park and co-workers used perovskite nanocrystals as quantum dot sensitizers in liquid solar cells and

INTRODUCTION Metal halide perovskite solar cells (PSCs) have attracted intensive attention in the past years due to their simple and low cost solution processing and high photovoltaic performance.1−4 The power conversion efficiency (PCE) has been continuously improved from 3.8% in 2009 to recently over 22%,5−20 approaching that of traditional photovoltaics such as Si, Cu(In, Ga)Se2, and CdTe and exceeding that of other new types of solar cells such as organic photovoltaics (OPVs), quantum dot solar cells (QDSCs), and dye sensitized solar cells (DSSCs). The extraordinary development originates from the super-opto-electronic properties of this family of halide perovskite materials, such as large light absorption coefficient (1.5 × 104 cm−1 at 550 nm), high photocarrier mobility (up to 150 cm2 V−1 s−1), long photocarrier lifetime (250 ns or longer), and long diffusion lengths (hundreds of nanometers to a few micrometers for polycrystalline films and tens to over 175 μm for © XXXX American Chemical Society

Received: December 15, 2018 Revised: January 27, 2019

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DOI: 10.1021/acssuschemeng.8b06580 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) Crystal structure of the perovskite absorber adopting the perovskite ABX3 form, where A is methylammonium, B is Pb, and X is I or Cl [Reprinted with permission from ref 12. Copyright 2013 Springer Nature]. (b) Optical images, absorption spectra, and XRD spectra of MAPb0.75Sn0.25(I1−yBry)3 perovskites with different Br content [Reprinted with permission from ref 35. Copyright 2016 American Chemical Society].

Figure 2. Structure evolution of perovskite solar cells.

conclusion of ambipolar carrier transport abilities of perovskite materials can be easily deduced from above-mentioned works. Consequently, Snaith and co-workers reported a planar structure PSC with a simplified architecture of a film of perovskite sandwiched between a TiO2 compact layer and an HTM layer, where the perovskite absorber additionally performs the tasks of charge-separation and ambipolar charge-transport of both carrier species.12 The evolutionary process of PSCs depicted above is schematically shown in Figure 2. Since 2013, the mesoporous PSCs with a perovskite capping layer and the planar PSCs have been extensively investigated as two typical models. The working mechanism of these solid-state PSCs is similar to that of inorganic thin film photovoltaic cells with p−i− n junction structure. The photoelectrons and holes in the perovskite absorber layer will be transferred into n-type and ptype semiconductor layers as electron transporting layers (ETLs) and hole transporting layer (HTLs), respectively. In planar PSCs, the n-type semiconductor (like TiO2) compact layer coated on a conductive substrate is regarded as the ETL, and the combination of top n-type semiconductor mesoporous film and bottom n-type semiconductor compact layer performs as the ETL in mesoporous PSCs. Photocarrier diffusion lengths and extraction efficiency at interfaces are key parameters affecting the performance of PSCs. It has been known that the carrier diffusion lengths in perovskite materials are much larger than the optimum thickness of the

obtained an improved efficiency of over 6% by optimizing parameter modifications.7 However, the dissolution of perovskite materials in liquid electrolyte limits the development of liquid based PSCs. A surge of recent advances in PSCs started from the emergence of the solid-state devices with efficiency reaching 10%,8 where a transparent p-type hole conductor 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) was used to replace redox mediator containing liquid electrolyte as the hole transporting material (HTM). Simultaneously, a so-called “meso-super structured solar cell” (MSSC) was proposed by Snaith’s group in 2012, where the mesoporous structure component was changed from an n-type semiconductor TiO2 to insulator Al2O3.9 The feasibility and superior performance of the MSSC demonstrated that the photoelectrons can be effectively transported through perovskite material itself to a TiO2 compact layer without the mesoporous TiO2 layer. The full coverage of the perovskite material on the mesoporous Al2O3 film is crucial in MSSCs in order to enable a smooth transport of photoelectrons to the TiO2 compact layer. In 2013, Grätzel’s group reported a mesoporous PSC with the perovskite capping layer at the top using a sequential deposition method, achieving a maximum efficiency of ∼15%.11 The existence of a perovskite capping layer between the HTM and perovskite fully filled mesoporous TiO2 film demonstrates that the photoholes can be also effectively transported through the perovskite material to the HTM layer. A B


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Figure 3. Basic principle of solid-state perovskite solar cells with p−i−n junction structure.

perovskite absorber film (few hundreds of nanometers) in devices, meaning that the photocarriers can effectively reach interfaces before recombination. Therefore, carrier extraction efficiency at interfaces could be the key parameter limiting device performance. The n-type semiconductor ETL, locating between the anode electrode and the perovskite absorber layer, performs multiple functions of selectively extracting photoelectrons (with blocking photoholes) and delivering them to the anode electrode. Both the electron extraction efficiency and the electron collection efficiency are sensitively influenced by ETLs. Besides device performance, the challenges of PSCs such as the hysteresis phenomenon during the current−voltage polar test and their instability also intimately correlate with ETLs. The ntype semiconductor TiO2, possessing the merits of thermal and chemical stabilities, low cost, suitable band alignments, and simple processing, has been regarded an ideal ETL material and extensively investigated in PSCs. Nearly all record efficiencies that PSCs experienced were achieved in the devices based on TiO2 ETLs. In this paper, the recent progress in improving device performance and solving challenges of the hysteresis phenomenon and instability of PSCs by modifying TiO2 ETLs is reviewed. The perspective of the development of TiO2 based ETLs for high performance PSCs has been proposed.

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transfer to the anode electrode (or cathode electrode) are three domain desirable dynamics for effective photovoltaic cells, which are drawn in thick lines and labeled as paths ①, ②, and ③, respectively. Photocarrier recombination (like ④ radiative recombination, ⑤ nonradiative recombination, and ⑥, ⑦, and ⑧ interface recombination) and trapping in defect states ⑨ at interfaces are undesirable dynamics, which deteriorate device performance and contribute to the emergence of hysteresis phenomenon during current−voltage polar tests. Meanwhile, ntype metal oxide based ETLs can also be excited under the irradiation of sunlight and produce highly oxidative photoholes in their valence bands. These photoholes could be either trapped at interfacial defect states (path I) or transferred to the perovskite absorbers (path II), the latter pathway may result in the destruction of the perovskite absorbers and therefore reduce device stability. In order to achieve highly efficient PSCs with hysteresis free (or less) and improved durability, the ETLs should be rationally designed to enable the desirable photocarrier dynamics to overcome the undesirable ones. The photocarrier dynamics correlate with the parasitic resistances in photovoltaic cells such as series resistance (Rs) and shunt resistance (Rsh), affecting device performance determined parameters of short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF). The photoelectron transportation through bulks (perovskite absorbing layer and TiO2 ETL) and transfer at interfaces (FTO/TiO2 and TiO2/perovskite) constitute the parasitic Rs, mainly influencing Jsc and FF of devices. Meanwhile the photocarrier recombination at interfaces is regarded as a domain component of parasitic Rsh, determining the device parameters of Voc and FF. Generally, an ideal photovoltaic device should possess minimum Rs for facilitating photocarrier collection and maximum Rsh for inhibiting photocarrier recombination, leading to maximum Jsc and Voc, respectively. Additionally, the device performance determined parameter FF is also closely related to the parasitic resistances and its relationships with parasitic resistances have

INTERFACIAL PHOTOCARRIER DYNAMICS IN PSCS

The photocarrier dynamics at interfaces such as injection, recombination (radiative and nonradiative), and trapping have much influence on the performance, hysteresis phenomenon, and stability of PSCs. Therefore, a full comprehension on carrier dynamics at interfaces is indispensable and helpful for the construction of high performance PSCs through rational design of ETLs. The general principle of the solid-state PSCs with a p− i−n junction structure has been mentioned before and schematically shown in Figure 3, where possible photocarrier relaxation pathways were also exhibited. Photoelectron (or photohole) injection from the perovskite absorber to the ETL (or HTL), their transportation through the ETL (or HTL), and C


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Figure 4. (a−c) Cross sections of SC-TiO2 with average thickness of 20, 40, and 80 nm, respectively, on FTO glass. The red circle shows a pinhole in the SC-TiO2 layer. The light red regions of the cross-sectional SEM images indicate the SC-TiO2 layer. (d) Schematic illustration of the predicted electron collection process in the SC-TiO2 for various thicknesses. (e) Photocurrent−photovoltage (J−V) properties of the planar perovskite solar cells prepared using SC-TiO2 ETLs with various thicknesses, measured under AM 1.5 solar illumination [Reprinted with permission from ref 50. Copyright 2016 American Chemical Society].

Rsh. A thin compact TiO2 film (10−100 nm) without cracks and pinholes is generally regard as an important component of high efficiency PSCs and varied film deposition methods have been used for this purpose such as spray pyrolysis, atomic layer deposition, spin coating, magnetron sputtering, and more.45−51 Solution processing methods (like spin coating, spray pyrolysis) with features of low cost and simple processing have been intensively adopted in PSCs. The resultant compact TiO2 films from the spin coating method (SC-TiO2) generally show cracks and pinholes due to the further crystallinity and thermal stress during the subsequent heat treatment.45,46,48−50 Meanwhile, the resultant compact layers show a relative smooth surface compared with the bared FTO substrate due to the filling of the FTO valleys with TiO2, meaning that the thickness of the SC-TiO 2 layer formed on FTO is heterogeneous and considerable gaps may exist at the bottom of valleys. Generally, the surface roughness of the commercial FTO is on the scale of tens of nanometers. Consequently, the optimum thickness of the SC-TiO2 ETL on the FTO is on the range of 40−60 nm depending on the roughness of the used FTO,45,50 which makes the SC-TiO2 just fully cover the FTO surface without exposing rough FTO peaks and effectively transport photoelectrons to the anode electrode (Figure 4). While, the compact TiO2 films produced from spray pyrolysis method (SP-TiO2) show similar surface roughness to that of the FTO and uniform thickness with much less cracks and pinholes.45,46 The SP-TiO2 ETL duplicated the morphology of the FTO due to the in situ crystallization of TiO2 on hot FTO substrates. The optimum thickness of SP-TiO2 ETL is ∼20 nm.45 It is surprising that PSCs equipped with these two different TiO2 compact layers show almost the same performance, meaning SC-TiO2 ETL can also effectively inhibit the back flow of photoelectrons to the perovskite absorber or HTM. A reasonable explanation may be that the pore size in SC-TiO2 ETL is small enough that the perovskite and HTM layers cannot be fully filled in, avoiding directly intimate contact between the FTO and perovskite (or HTM) layer. Formation of high quality TiO2 compact layers with much less pinholes and cracks is highly desired for high efficient PSCs, atomic layer deposition (ALD) is a powerful tool for this purpose and has been widely used to deposit high quality TiO2 dense films as the protective layers on the unstable photoelectrode surface in photo-electro-chemical (PEC) water splitting cells.52−54 The ALD deposited TiO2 compact layers (ALD-TiO2) performing as the ETL in PSCs was first reported by Wu and the resultant PSCs with the ALD-TiO2 ETL (50 nm

been well summarized in photovoltaic cells as shown in the following equations: FFs = FF0(1 − rs)

ij 1 yzz FFsh = FF0jjj1 − z j rsh zz{ k

(1)

(2)

Where, FF0 is the fill factor of an ideal device with zero Rs and infinity Rsh, FFs is the fill factor of a device with some Rs and infinity Rsh, FFsh is the fill factor of a device with zero Rs and limited Rsh, rs is the normalized series resistance (rs = Rs/RCH, where RCH is the characteristic resistance of a device and is defined as RCH ≈ Voc/Jsc), and rsh is the normalized shunt resistance (rsh = Rsh/RCH). It is obvious that the FF increases with reducing Rs and enlarging Rsh.

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TIO2-BASED ELECTRON TRANSPORTING LAYERS TiO2 as a typical n-type semiconductor with a wide band of 3.0− 3.2 eV, possessing merits of thermal and chemical stability, low cost, high photocatalytic activities, and easy fabrication, has been widely investigated in photocatalytic systems and sensitized photovoltaic solar cells.37−44 Consequently, its intrinsic properties and corresponding tailored strategies have been well understood and its morphologies and polymorphs can also be precisely controlled with numerous synthesis methods, which provide convenient information for researchers to rationally design and decorate TiO2 ETLs so as to obtain high performance PSCs. Morphology Control of the TiO2 ETL. Morphology of the TiO2 ETL directly correlates to the dynamics of photoelectron transfer across interfaces and its transportation toward the anode electrode and, also, can improve the light absorption efficiency through rational structure design.37 As mentioned above, a small value of Rs and an enlarged Rsh are desirable for high performance photovoltaic cells. In PSCs, an effective ETL should possess features of full contact with both the perovskite absorber film and the collector electrode, fast transportation of electrons, and dense coverage on carrier collector electrode (like FTO). The former two features facilitate interface transfer and bulk transportation of photoelectrons and, therefore, reduce Rs. The third one, avoiding the direct contact of the carrier collector electrode with the perovskite absorber layer (or HTM), inhibits interface recombination stemmed from the backflow of photoelectrons to perovskite (or HTM), and results in large D


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ACS Sustainable Chemistry & Engineering in thickness) showed enlarged Rsh and reduced Rs in comparison with devices with the SC-TiO2 or SP-TiO2ETLs, leading to an improvement of more than 50% on device efficiency.46 Recently, it was found that the thickness of ALD-TiO2 ETLs could be thinned to few nanometers in PSCs, showing similar photovoltaic performance to that of 50 nm SC-TiO2 ETLs.47 Magnetron sputtering is an alternative low cost and scalable method for deposition of high quality TiO2 films with features of thickness homogeneity, high degree of compaction, excellent transmittance, and strong interaction with substrates. The TiO2 compact layers prepared from magnetron sputtering (MS-TiO2) generally show higher conductivity, better hole blocking, and photoluminescence quenching abilities in comparison with both SP-TiO2 and SC-TiO2 (sequence in MS > SP > SC). Consequently, the optimum efficiency of PSCs with the MSTiO2 ETL can be improved by over 16% relative to devices with the SP-TiO2 or SC-TiO2ETLs.48,49 Novel methods combining magnetron sputtering deposition of Ti with following oxidization procedures have also been adopted for the construction of high quality TiO2 compact layers.50,51 Thermal oxidation of sputtered Ti film together with the top porous TiO2 layer can effectively improve their interface connection, reducing interface charge recombination probability, and interface charge transfer resistance. A maximum efficiency of 15.07% has been achieved by the PSC with optimum thickness (15 nm) of this thermal oxidized TiO2 compact film.51 Anodization of the sputtered Ti film on the FTO substrate in a F− ions contained electrolyte following by a thermal treatment can produce a well-defined nanostructured TiO2 ETL, which has an excellent transmittance, single-crystalline properties, a uniform film thickness, a large effective area, and defect-free physical contact with a rough substrate, providing outstanding electron extraction and transportation, and hole blocking in a planar PSC.50 The anodized TiO2 (A-TiO2) ETL with a thickness of 40 nm can increase device efficiency by 22% (from 12.5 to 15.2%) compared with the SC-TiO2.50 TiO2 compact layers prepared from different methods generally show different features on morphology (Figure 5), compactness, conductivity, and more. Consequently, their optimum thickness in PSCs, which were obtained by balancing hole blocking and electron transporting capacities, were varied from 10 to 100 nm.45−51 Besides TiO2 compact layers that are generally required for efficient PSCs, atop porous TiO2 films also play an important role in performance improvement for meso-structured devices. Carrier diffusion lengths in the two most common perovskite solar cells, i.e., MAPbI3−xClx and MAPbI3 solar cells, have been characterized by the direct measurement of electron beaminduced current (EBIC) profiles of the cross sections of the solar cells.52 Electron and hole diffusion lengths in the MAPbI3−xClx are comparable falling in the range of 2 μm. However, the effective diffusion length of electrons is shorter than that (∼1 μm) of holes in the MAPbI3-based cells. It explains well that a mesoscopic device configuration with the mesoporous TiO2 as an electron transporter is required to overcome the impediment for efficient pure iodide-based PSCs. The porous TiO2 film performs tasks of partially collecting photoelectrons from the perovskite absorber and delivering them through the porous frame to the TiO2 compact layer. The photoelectrons can be quickly injected from the conduction band (CB) of the perovskite absorber into that of the porous TiO2 due to their favorable band alignments and enlarged contact interface area. The injection process generally happens on the time scale of 260−307 ps (ps) that is much short than that of photocarrier

Figure 5. FESEM cross-sectional images of compact TiO2 with MS, SP, and SC deposition methods (left). FESEM top views of compact TiO2 with MS (a and b), SP (c and d), and SC (e and f) (right) [Reprinted with permission from ref 48. Copyright 2017 The Chemical Society of Japan].

lifetime (tens of microseconds).24,53 This rapid injection of photoelectrons into porous TiO2 can effectively realize space separation of photocarriers, largely reducing the recombination probability of photoelectrons with photoholes. While the electron mobility in TiO2 is only on the scale of 10−2−1 cm2 V−1 s−1, which is a few magnitudes smaller than that in perovskite materials,54 moreover, numerous boundaries exist in TiO2 porous films that consist of nanoparticles. Therefore, it will take a much longer time to deliver the photoelectrons through the porous TiO2 frame to the TiO2 compact layer in comparison with that transporting through the perovskite itself. Rapid photoelectron injection from the perovskite absorber to the porous TiO2 and slow transportation of the photoelectrons in the porous TiO2 layer result in a compromised porous TiO2 layer with the thickness of a few hundreds of nanometers for efficient meso-structured PSCs. Leijtens and co-workers investigated influence of the porous TiO2 thickness on the perovskite pore filling fraction and performance of the mesostructured PSCs. By decreasing the thickness of the porous TiO2 scaffold layer from 750 to 260 nm, the solar cell architecture was changed from perovskite-sensitized to completely perovskitefilled. The latter case leads to improvements in photovoltaic performance due to the following reasons: (1) higher electron densities can be sustained in the thin TiO2 scaffold layer, improving electron transport rates and photovoltage; (2) the primary recombination pathway between the TiO2 and the hole transporting material is blocked by the perovskite itself (Figure 6).55 Besides thickness, the size and morphology of the TiO2 nanoparticle components in the porous films also play important roles in the perovskite pore filling fraction, directly affecting the device performance. Yang and co-workers investigated the TiO2 nanoparticle size effects on the performance of printable mesoscopic PSCs with carbon as cathode electrodes. When gradually increasing the TiO2 nanoparticle size from 10 to 30 nm, the pore size in the porous TiO2 layer was gradually increased from 14.7 to 25.8 nm and the specific surface area was gradually reduced from 139.8 to 59.6 m2 g−1. The Jsc increased first and then decreased with further enlarging particle size due E


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Figure 6. Proposed recombination mechanisms for solar cells with incomplete (a) and complete (b) perovskite pore filling fractions. Step 1 (white arrow) stands for photo-excitation of the perovskite absorber, and step 2 (light blue arrow) is electron injection into TiO2 and hole transfer to the HTM, while step 3 (black arrows) represents the recombination pathways [Reprinted with permission from ref 55. Copyright 2014 American Chemical Society].

to a trade-off between the improved pore filling and lowed filling amount of perovskite, and the FF increased with enlarging particle size due to the improved photoelectron injection rate and transportation, resulting from better pore filling of perovskite and reduced electron transportation resistance, respectively. The recombination rate increased with enlarging particle size because the contact between the TiO2 and the carbon cathode electrode occurs with more probability for larger particles. Consequently, an optimum particle size of 25 nm was obtained by compromising between device parameters of Jsc, Voc, and FF, and an efficiency of 13.41% was achieved.56 A similar experiment on evaluating the TiO2 particle size effects on the performance of standard meso-structured PSCs was also carried out by Numata and co-workers. Anatase TiO2 particles with different sizes (5, 30, 60, and 90 nm) were used as the porous scaffold layer components, and the best efficiency of 11% was obtained in a device with the TiO2 particle size of 60 nm.57 The shape of TiO2 nanoparticles influences the pore structure in the porous TiO2 layer, as well as the connectivity among the nanoparticles. As compared with TiO2 nanoparticles with welldefined facets, TiO2 nanoparticles with spherical or round-edge shapes can greatly improve contacts and connectivity among the nanoparticles, as well as the good channel connectivity of pores with uniform sizes, which will be beneficial for the efficient infiltration of perovskite material into the porous TiO2 layer. Sung and co-workers fabricated spherical TiO2 nanocrystals with controllable sizes (30, 40, 50, and 65 nm) through a solvent modified hydrothermal method. The resultant porous TiO2 scaffold layers show different pore sizes (24, 34, 45, and 56 nm) and surface specific areas (57, 40, 34, and 28 m2 g−1). The mesostructured PSCs with 50 nm spherical TiO2 nanocrystals show the best photovoltaic performance with the maximum efficiency of 17.19%. The improvement on efficiency in the mesostructured PSC with 50 nm spherical TiO2 nanocrystals stemmed from the better pore filling of perovskite and reduced electron transportation resistance, leading to the increase of Voc and FF but not Jsh.58 The contradiction between the requirements of high specific surface area for efficient photoelectron injection and suitable large pore size for fully filling of perovskite needs to be carefully solved in porous TiO2 scaffold layers for building efficient meso-structured PSCs. A novel wet etching process was proposed by Kwon and co-workers to tailor the inner space of TiO2 porous scaffold layers without sacrificing specific surface area. It was found that the porous TiO2 layer treated with a HF solution exhibited remarkably enhanced power conversion efficiencies in comparison with those of pristine ones.59 Monodispersed porous TiO2 spherical aggregates with diameters of 100 nm has been successfully prepared

by Yang and co-workers. The resultant porous TiO2 scaffold layers with this novel spherical TiO2 aggregates possess both suitable pore sizes (50 and 90 nm) for infiltration of perovskite and large specific surface area (112 m2 g−1) for efficient photoelectron injection, which derive from the external pores formed by close stack of the spherical aggregates and small internal pores (12 nm) in the spherical aggregates, respectively. The meso-structured device with the spherical TiO2 aggregates showed much higher efficiency (17.06%) than that with 30 nm TiO2 nanoparticles (13.30%) and even higher than that with 50 nm TiO2 nanoparticles (16.53%). Especially after toluene dropping treatment which promotes further infiltration of perovskite into tiny internal pores, a highest efficiency of 18.41% has been achieved.60 As compared to the particle structure, one-dimensional (1D) structure may be better for the perovskite infiltration due to its open porous structures. Moreover, it has been reported to be better in carrier transportation and recombination behavior. Electro-spun TiO2 nanofiber films on FTO substrates have been used as porous scaffold layers in meso-structured PSCs with the hope of achieving fast electron transportation and slow recombination. But the extremely large pore size between nanofibers dramatically reduced the contact surface area between the TiO2 nanofibers and the perovskite absorber, limiting the photoelectron injection rate and therefore device efficiency (9.8%).61 In order to conquer this limitation in the one-dimensional structure, an efficient porous TiO2 scaffold layer has been designed by Islam through blending onedimensional structure with particle structure. In comparison with the pure TiO2 nanoparticle based device, the PCE was increased by about 27% when 10% of the TiO2 nanorods were incorporated due to the enhanced charge carrier collection and light harvesting efficiencies.62 One-dimensional (1D) aligned TiO2 nanostructure arrays have been regarded as ideal building blocks for the construction of efficient meso-structured PSCs because they provide a direct path for the rapid transportation of photoelectrons, open voids for efficient infiltration of perovskite or HTMs, and tunable specific surface area for fast photoelectron injection. Vertically aligned rutile TiO2 nanorod/ nanowire arrays fabricated by hydrothermal methods have been widely investigated in DSSCs as the porous photoanodes for loading dye sensitizers.37,63,64 The disadvantage of these aligned arrays as photoanodes in DSSCs is their lower loading amount of dyes due to their relatively smaller specific surface area as compared with the nanoparticle based porous films, limiting incident the light harvesting efficiency. Meanwhile, this disadvantage can be effectively reduced in meso-structured PSCs due to the high light absorption coefficient of perovskite F


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intensively investigated in DSSCs for remedying the shortage of small specific surface area of TiO2 nanowires/nanorods, increasing loading amount of dyes.73−75 While the aim of increasing specific surface area in PSCs is not at loading more perovskite but at increasing contact surface area between the TiO2 and perovskite absorber and enhancing light absorption. Mahmood and co-workers introduce a unique and scalable multistage electrospinning and hydrothermal route for the development of 3D hyperbranched anatase TiO2 nanorod− nanofiber arrays as the ETL. The hyperbranched ETL with optimal electron transport and carrier lifetime leads to highly efficient meso-structured PSCs with an average PCE of 15.03% and a maximum PCE of 15.50%.76 Wu and co-workers reported a facile one-step hydrothermal process for fabrication of highly branched anatase TiO2 nanowires (ATNWs) with varied orientation on the FTO. Solar cells constructed by only using the optimized ATNW thin films (220 nm in thickness) without the compact layer yield a PCE up to 14.2%.77 The 3D TiO2 dendritic nanowire arrays have been synthesized via a surfacereaction-limited pulsed chemical vapor deposition (SPCVD) technique. The highest efficiency of 9.0% was achieved with ∼600 nm (in length) 3D TiO2 NW structures, which is higher than those achieved with the one-dimensional TiO2, ZnO, and their core/shell composite arrays.78 The hierarchical nanorod (HNR) films, which consist of TiO2 nanorod trunks with optimal lengths of ∼540 nm and TiO2 nanobranches with lengths of ∼45 nm, have also been synthesized by a two-step low temperature (180 °C) hydrothermal method. PSCs based on the TiO2 HNR achieved higher PCEs than devices with the TiO2 nanorod arrays. This improved efficiency was mainly ascribed to the lower charge carrier recombination as determined by the impedance spectroscopy.79 The TiO2 HNR arrays in PSCs serve as not only the electron transport medium but also as a nanophotonic light trapping structure, enhancing both electron collection and light harvesting efficiency. Lin and co-workers found that the HNR in the MAPbI3 matrix exhibits superior light trapping performance compared to the nanorod (NR) via using a finite difference time domain (FDTD) simulation method, The enhanced electron collection in the TiO2 HNR based PSC was confirmed by its higher internal quantum efficiencies (IQE), especially at longer wavelengths. Compared to the TiO2 NR based solar cell, a 25% improvement in the average efficiency was attained in the TiO2 HNR based solar cells.80 Other onedimensional TiO2 nanostructures such as TiO2 nanotubes, nanocones, and nanohelices have also been used as ETLs in meso-structured PSCs.81−83,87−89 Freestanding TiO2 nanotube arrays (TNTs) fabricated by the electrochemical anodization were first used as ETLs by Gao and co-workers in liquid based perovskite-sensitized solar cells.83 The device demonstrates improved light absorption with more than 90% of light absorbed in the whole visible range and a reduced charge recombination rate, leading to a significant improvement on efficiency compared to that based on the TiO2 particle film. A flexible solid state PSCs based on TNTs was first reported by Wang and co-workers and a highest efficiency of 8.31% was achieved, in which thin Ti foils as the flexible substrates and transparent carbon nanotube films as the cathode electrodes.82 TNTs has also been synthesized by using ZnO nanorod arrays as the templates, the resultant TNTs shows open space between tubes, facilitating infiltration of the perovskite or HTM. A maximum efficiency of 11.30% has been achieved in this TNTs based PSCs.83 Nanocone arrays were reported to be beneficial for light harvesting. Moreover, it was reported that there is an electric

materials. At early stages, the perovskite materials were mainly used as sensitizers loaded on vertical aligned TiO2 nanorod/ nanowire arrays and noncontinuously distributed on their surface. The resultant perovskite-sensitized PSCs exhibited fast improvement on efficiency, whose efficiency increased from pioneer 4.87% to 9.4% just in two months.65,66 Tuning the nanorod thickness, length, and density can lead to a better infiltration of both the absorber and the HTM, an optimum nanorod length of 600 nm has been demonstrated in perovskitesensitized PSCs.66 Further improved efficiency of 11.7% was obtained by Jiang in the perovskite-filled meso-structured PSCs with well separated TiO2 nanorod arrays of 900 nm thickness.67 The hydrothermal conditions directly influence the morphology, intrinsic physical properties, and surface properties of resultant rutile TiO2 nanorod arrays. The resultant TiO2 nanorod arrays from water−HCl solution exhibited special orientation, high conductivity, improved morphology, good optical properties, fast charge transfer, and reduced charge recombination as compared with that derived from ethanol− HCl solution. The best efficiency of 11.8% has been achieved in the PSCs based on the TiO2 nanorod arrays with rod lengths of 500 nm derived from water−HCl solution.68 The strong acid conditions for the growth of TiO2 nanorod arrays restricts their applications, increasing the equipment cost and even causing environmental damage. A mild condition using an acid-free hydrothermal method for the growth of TiO2 nanorod arrays has been proposed by Cai and a comparable efficiency of 11.1% has been achieved in devices based on these TiO2 nanorod arrays with rod lengths of 700 nm.69 Besides the hydrothermal method, other methods such as physical vapor deposition in an oblique angle configuration (PVD-OAD) and block copolymer (BCP) template approach have also been used for the fabrication of ordered TiO2 nanorod arrays. The resultant TiO2 nanorod arrays from PVD-OAD showed paralleled columnar morphology with an unformed tilted angle to FTO substrates. The tilted microstructure was reported to be efficient for the photon collection and the best efficiency of 10.59% was achieved in the device with a thin porous layer of 200 nm.70 Seo and co-workers achieved an efficiency of over 15% in meso-structured PSCs with short (120 nm) straightforward aligned TiO2 nanorod arrays fabricated by using a nanoporous template of BCPs.71 In order to reveal the impact of the properties of TiO2 nanowire/nanorod array on device performance, a two-dimensional modeling of the TiO2 nanowire-based meso-structured PSCs has been performed by Wu and co-workers through combining the optical and electrical responses.72 Simulation results show that the device performance greatly correlates with the electron concentration of TiO2 nanowires, which decided the distribution of electron field inside cells and an optimum thickness of 600 nm is obtained for the TiO2 nanowires with low electron concentration. Another conclusion deduced from the simulation is the transportation of electrons is primarily carried out by the perovskite itself and the transporting ratio through TiO2 nanowires is less than 5%. Although one-dimensional TiO2 nanowire/nanorod based scaffold layers provide advantages of open pores for the infiltration of perovskite, direct pathways for fast electron transportation and less surface defects for slow recombination, the relatively small specific surface area is their main limiting factor for efficient meso-structured PSCs, which limits the photoelectron injection process due to the reduced contact surface area between the TiO2 nanostructures and perovskite absorber. Hierarchical TiO2 nanowires/nanorods as promising build blocks for porous scaffold layers have been G


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Figure 7. (a) TiO2 nanofiber ETL. (inset) Magnified view of nanofibers showing their highly porous nature [Reprinted with permission from ref 76. Copyright 2015 John Wiley and Sons]. (b) Cross-sectional FESEM images of rutile TiO2 nanorods grown on FTO substrate [Reprinted with permission from ref 68. Copyright 2013 American Chemical Society]. (c) Cross-sectional SEM image of TiO2 nanocones on FTO [Reprinted with permission from ref 88. Copyright 2014 Elsevier Ltd]. (d) Cross-sectional SEM image of TiO2 nanotubes [Reprinted with permission from ref 81. Copyright 2014 The Royal Society of Chemistry]. (e) Helical TiO2 deposited on a compact TiO2 under layer coated on FTO glass [Reprinted with permission from ref 89. Copyright 2015 The Royal Society of Chemistry]. (f) Hyperbranched nanofiber−nanorod array ETL [Reprinted with permission from ref 76. Copyright 2015 John Wiley and Sons]. (g) Planar view of a 3D TiO2 NW architecture. (inset) Enlarged SEM image showing the treelike branched NR structure [Reprinted with permission from ref 78. Copyright 2015 American Chemical Society]. (h) SEM image of the bare IOT-ETL film [Reprinted with permission from ref 95. Copyright 2015 John Wiley and Sons]. (i) Moth-eye patterned mp-TiO2 layer (moth-eye TiO2) [Reprinted with permission from ref 98. Copyright 2016 John Wiley and Sons].

field along the axial direction of an n-type semiconductor cone when it was embodied in a p-type semiconductor matrix,84−86 which would drive electron moving faster from perovskite to the nanocones. Consequently, the TiO2 nanocone array based meso-structured PSCs show superior performance to devices with the TiO2 nanorod arrays due to the fasten electron injection at the TiO2 nanocone/perovskite interface.87,88 Beside hierarchical structures, twisting long nanowires into helical nanosprings with an optimum length is an alternative effective way to increase the light absorption efficiency and contact area between the TiO2 and perovskite absorber without the loss of carrier collection efficiency. Lee and co-workers synthesized well aligned helical TiO2 arrays on the FTO by oblique-angle electron beam evaporation and achieved an average PCE of 12.03 ± 0.07%.89 Light harvesting efficiency of absorber materials directly determined the value of Jsc, influencing final conversion efficiency of solar cells. The almost 100% IQE has been obtained in planar solid-state PSCs with larger grain size. Simulated results indicate that a photocurrent density of 27.2 mA cm−2 can be obtained if the photons with wavelength in the 280−800 nm range could be totally used to generate electricity,90 while perovskite solar cells generally obtain a photocurrent density of 20% retained 90% (97% after dark recovery) of their initial performance after 500 h continuous room-temperature operation at their maximum power point under one-sun illumination (Figure 13g).167 Besides incorporation of a monolayer of functional organic molecules, decorating a thin layer of inorganic (or organic) materials on the TiO2 ETL surface is another popular strategy to model interface carrier dynamics for enhancing performance/ stability of PSCs. Generally, the aim of introducing a thin layer of materials at the TiO2/perovskite interface is to expedite electron injection or/and inhibiting interface recombination. If an appropriate intermediate layer could be introduced as a bridge to connect TiO2 and CH3NH3PbI3 for facilitating electrons transfer, the performance of PSCs would be greatly enhanced through increase of Jsc. Zhu et al. decorated the graphene quantum dots (GQDs) on mesoporous TiO2 scaffold before the deposition of a perovskite absorber layer. The GQDs in the device configuration served as a superfast bridge to facilitate the electron injection from the perovskite into TiO2, leading to a significantly enhanced photocurrent and efficiency of the corresponding solar cells. The electron extraction time was reduced from 280 to 90 ps and efficiency of PSCs was improved from 8.81% to 10.15% after inserting the ultrathin GQDs layer (Figure 14a).53 The lithium-neutralized graphene oxide (GOLi) layer has been already used as ETL in organic photovoltaic devices by Kakavelakis et al. The replacement of H atoms in the carboxyl groups of GO by Li atoms can effectively reduce the working function (WF) of GO from 4.9 to 4.3 ± 0.1 eV, matching with the lowest unoccupied molecular orbital (LUMO) level of the mesoporous TiO2.168 Inspired by this work, Agresti et al. proposed a new PSC structure by including GO-Li as the ETL on top of the mesoporous TiO2 substrate. The GO-Li ETL enhances the electron injection from the perovskite to the TiO2, as well as partially passivates the oxygen vacancies/defects of the TiO2 and the resulting devices exhibit an improved efficiency (from 10.3 to 11.8%), a reduced hysteresis, and an enhance stability.169 1-Butyl-3-methylimidazolium tetrafluoroborate ionic liquid (IL) has been reported to possess merits of large electrical conductivity, high charge mobility, and superior optical transparency;170 Yang et al. introduced this IL material to optimize surface structures of the

compact TiO2 ETL before the perovskite active layer deposition. They demonstrated that the anion group of the IL bonds to TiO2, leading to a higher electron mobility and a well-matched work function. Meanwhile, the cation group interfaces with adjacent perovskite grains to provide an effective channel for the electron transport and a suitable setting to grow a low trap-state density perovskite absorber layer. As a consequence, the efficiency of the planar PSC was promoted to as high as 19.62% (the certified efficiency is 19.42%), exceeding the previous highest efficiency recorded on planar CH3NH3PbI3 PSCs.171 SrTiO3 with the same perovskite structure has a good chemical affinity and a suitable band alignment with both TiO2 and CH3NH3PbI3. Hou et al. inserted a SrTiO3 layer between the mesoporous TiO2 and the perovskite absorber layer for building up a compatible interface. As a result, an average efficiency of 11.4% has been achieved due to the formation of high quality perovskite and the rapid carrier interface transfer, which was enhanced by ∼46% in comparison with that (7.8%) of the control devices.172 In order to suppress the interface recombination, the defects on TiO2 surface, which generally work as carrier trap centers for recombination, should be rationally passivated. Chandiran and co-workers deposited an ultrathin layer of TiO2 on the mesoporous TiO2 scaffold layer by use of the ALD method to passivate surface defects of the mesoporous TiO2 and in addition to further cover pinholes existing in the bottom compact TiO2 layer for the suppression of carrier interface recombination.173 A 2 nm TiO2 can block the electron back reaction effectively, both from the FTO and TiO2 surface, leading to an improved efficiency up to 11.5% from the original 7%. Coincidentally, Mali et al. deposited a thin layer of ALD-TiO2 on 1D rutile TiO2 nanorod arrays derived from hydrothermal growth for the surface passivation. It has been demonstrated that a fine-tuned 4.8 nm deposited ultrathin TiO2 layer can effectively reduce the recombination rate, leading to an improved efficiency up to 13.45% from the original 9.93%.174 The open pore structure in 1D nanorod arrays allows thicker ALD-TiO2 deposition without affecting the infiltration of perovskite materials. Depositing a thin layer of metal sulfides (e.g., ZnS and CdS), which generally show high carrier mobility and slow surface carrier recombination rate, is an alternative strategy to passivate surface defects of TiO2 ETLs for blocking interface recombination. Ito et al. modified the mesoporous TiO2 scaffold layer with a thin layer of Sb2S3 by use of a chemical bath deposition method. However, the improvement in overall device performance seemed to be a result of increased Jsc and FF rather than Voc. Moreover, the PSCs with Sb2S3 became stable against light exposure without encapsulation due to the passivation effect of the TiO2 photocatalysis, avoiding decomposition of perovskite absorber layer induced by the highly oxidative holes generated in TiO2 (Figure 14b).175 A CdS layer has also been inserted at interface between the TiO2 compact layer and perovskite in planar PSCs, the introduction of the CdS layer effectively suppresses the carrier recombination at the interface and enhances charge carrier transportation. At the optimum CdS thickness, a champion PCE of 14.26% was achieved, higher than that (10.31%) of the reference CdS-free PSCs. Similar to the introduction of Sb2S3, the CdS modified PSCs also demonstrated the enhanced air stability due to the suppression of highly oxidative holes injection from TiO2 to perovskite.176 An enhanced stability under the continuous illumination was also observed by Hwang and co-workers in the PSC using the TiO2/CdS core/shell ETL. But, this enhanced stability to light was attributed to the passivation of the surface R


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Figure 15. Schematic illustration of the control perovskite solar cell (a) before and (b) after UV irradiation and perovskite solar cell with CsBr interface modification (c) before and (d) after UV irradiation. (e) EQE result for the device with and without CsBr modification. (f) Normalized PCE decay of devices upon UV irradiation [Reprinted with permission from ref 183. Copyright 2016 The Royal Society of Chemistry].

efficiency was obtained with the optimized modifying process, enhanced by nearly 20% compared with the control devices.179 The improvement mainly comes from the retarded back recombination. MgO-coated mesoporous TiO2 ETLs were fabricated to prevent charge recombination at the perovskite/ TiO2 interface by Han and co-workers. The efficiency was increased from 11.4% to 12.7% due to the improved Voc and FF resulting from the triply prolonged carrier recombination times, demonstrating that MgO ultrathin layers can effectively retard charge recombination at the perovskite/TiO2 interface in PSCs (Figure 14d).180 Controlled interfacial electron injection was investigated by Kang and co-workers through surface modification of the mesoporous TiO2 film with an insulating thin ZrO2 layer in a meso-structured PSC. A PCE was improved from 11.7% to 13.6% by introducing a 1.1 nm-thick ZrO2 layer on the TiO2 surface.181 Marin-Beloqui and co-workers modified the mesoporous TiO2/perovskite interface with a nanoscopic layer of insulating Al2O3. Due to the prolonged carrier recombination times after the Al2O3 decoration, the resulting PSCs exhibited an improved average efficiency of 12% and an enhanced average Voc of 940 mV in contrast to standard PSCs, which have a standard average efficiency of 10.2% and an average Voc of 867 mV, respectively.182 Li and co-workers demonstrated that cesium bromide (CsBr), as an interfacial modifier between the compact TiO2 ETL and the perovskite absorber layer, can effectively not only enhance the stability of planar PSCs under UV light soaking, but also improve the device performance due to the alleviated defects at the perovskite/TiO2 heterojunction and enhanced the electron extraction.183 The mechanism for the improved UV stability was attributed to the dual effect of the reduced chemical reactivity of TiO2 after the CsBr modification and the reduced defect density at the compact TiO2/perovskite interface. Since the CsBr appeared to form clusters but not a continuous film, the authors inferred that it was unlikely that CsBr acts as a buffer layer in the traditional sense, inhibiting the contact between TiO2 and perovskite. Rather, it was likely that the Cs ions inhibit the photocatalytically active sites on the TiO2

traps at TiO2 (Figure 14c), which are active for the O2 adsorption in dark and set O2 free under the UV-light illumination, rather than the prevention of perovskite decomposition.177 To date, the dominant reason for the decreasing performance of perovskite solar cells under UV-light illumination remains unclear and debatable. It has been demonstrated that doping MAPbI3 perovskite by the substitution of Pb with Sn increases charge recombination across the perovskite/TiO2 interface, leasing to to loss of Voc.166 Achieving high Voc for tin-based perovskite solar cells is challenging. Ke and co-workers demonstrated that a ZnS interfacial layer can improve the Voc and photovoltaic performance of formamidinium tin iodide (FASnI3) based PSCs. The presence of a ZnS interlayer can effectively reduce the interfacial charge recombination and facilitate electron transfer. An enhanced efficiency of 5.27% and a higher Voc of 0.380 V, were achieved, which were improved by ∼43% and 31%, respectively, compared to the device based on the neat TiO2 ETL. Similar but smaller enhancements have also be observed when CdS replaced ZnS as the interfacial modifier.178 Introducing a thin layer of insulator materials at the TiO2/perovskite interface is a two-edged sword. The positive side is that the interface recombination would be largely prohibited due to the inhibition of back follow of electrons at presence of the insulator interlayer and/or the passivation of surface trap sits, leading to an enlarged Voc. The negative side is that the carrier extraction efficiency at the interface would be reduced due to the presence of interfacial transfer barrier originating from the insulator interlayer, resulting in a reduced Jsc. Fortunately, the barrier can be effectively tunneled through by electrons when the barrier width is thin enough. As long as the positive side prevails over the negative side, inserting a thin layer of insulator materials would enhance device performance. Insulators like Cs2CO3 Al2O3, ZrO2, MgO, and CsBr have been widely investigated as ultrathin interfacial modifiers for enhancing PSC performance.179−183 Dong et al. employed Cs2CO3 as a new surface modification material decorating the TiO2 ETL for efficient PSCs. A 14.2% S


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Figure 16. (a) Cross-sectional SEM micrographs with color-enhanced and annotated cross section showing a general scheme of the solar cell architecture. (b) Schematic illustration of energy levels of the materials used in this study. The graphene and TiO2 are blended into a single composite layer and not layered as depicted in the energy level diagram. (c) Current−voltage characteristics of different electron collection layers under simulated AM 1.5, 100 mW cm−2 solar irradiation (solid line), and in the dark (dotted line). (d) Best performing (η = 15.6%) solar cell based on a graphene− TiO2 nanocomposites under simulated AM 1.5, 106.5 mW cm−2 solar irradiation (solid line), and in the dark (dotted line), processed at temperatures not exceeding 150 °C. Solar cell performance parameters are given in the inset [Reprinted with permission from ref 185. Copyright 2014 American Chemical Society].

suppress the electron recombination with holes in perovskite or HTM, leading to not only a largely reduced Rs but also an enlarged Rsh. Therefore, the efficiency of PSCs with the incorporation of conductive networks into TiO2 ETLs would be enhanced by improving Jsc, Voc, and FF. Graphene as an atomthick two-dimensional (2D) material has remarkably high charge carrier mobility and electrical conductivity due to the zero mass of its charge carriers. Moreover, graphene has a WF between that of FTO and the TiO2 CB edge; it may reduce the formation of energy barriers at the material interfaces and therefore behave as a good electron collector when incorporated into TiO2 ETLs. Wang and co-workers employed nanocomposites of graphene and TiO2 nanoparticles as compact ETLs in MSSCs through a low-cost, solution-based deposition procedure. Upon careful sonication before spin-coating, a dense and well-distributed nanocomposite could be produced, with graphene acting as a continuous 2D conductive framework on which TiO2 nanoparticles were allowed to be anchored. The optimum graphene content of 0.6 wt % corresponds to just over 1 monolayer coverage of the graphene by TiO2 nanoparticles. The devices with graphene/TiO2 composite ETLs exhibited remarkable photovoltaic performance with an efficiency up to 15.6%, over 50% enhancement compared to the control devices with neat TiO2 ETLs (Figure 16).185 Han et al. introduced a reduced graphene oxide (rGO) into the mesoporous TiO2 scaffold layer to reduce the interfacial resistance for enhancing electron transportation property and, hence, improving charge collection efficiency. The meso-structured PSCs based on the rGO/mesoporous TiO2 nanocomposites with an optimal rGO

and assist in the formation of less defective perovskite at this interface. The devices incorporating the CsBr modification achieved an average efficiency of 15.3%, which is significantly improved compared to the control devices with an average efficiency of 11.5%. And, the normalized efficiency remained more than 70% of its initial value even after a 20 min UV irradiation in air for the modified device, which was nearly zero for the control device (Figure 15). Different from inorganic insulating modifiers, for the modification of a thin layer of insulating polar polymer, besides electron−hole recombination being retarded, the work function of the TiO2 ETL and the series resistance could also be reduced due to the presence of the interfacial dipole after modification. Dong et al. report the modification of the compact TiO2 ETL using a thin layer of polyoxyethylene (PEO). Compared with the control devices with TiO2 only, devices based on the modified ETL gave a nearly 15% enhancement to the efficiency, with improved both Voc and Jsc.184 The improved performance was mainly attributed to the better retardation of back recombination and the enhanced electron collection efficiency by means of the PEO thin layer modification. TiO2 Based Composites. Apart from morphology control, doping, and surface modification, rationally constructing composite TiO2 ETLs by introducing varied functional materials into the TiO2 ETL is another effective strategy to improve the photovoltaic performance of PSCs. Implanting high conductive networks into TiO2 ETLs is an efficient way to offset the intrinsic shortage of low carrier mobility in TiO2, which can expedite electron transportation through ETL and consequently T


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Figure 17. (a) Scheme diagram of the device structure (ITO/TiOx-Au-NPs/CH3 NH3PbI3−xClx/HTM/Ag). Au-NPs are sandwiched between two TiOx layers. HTL can be either P3HT or Sprio-OMeTAD. (b) Cross-section SEM image of the device; here, P3HT acts as an HTL. (c) Electron injection current measurement: J−V curves of conductivity test under illumination conditions. (d) Schematic electrical diagram of hot carrier injection process from Au-NPs to TiOx. (e) J−V curves and (f) EQE spectra for the devices based on TiOx and TiOx/Au-NPs composite films. Here, SpiroOMeTAD acts as an HTL [Reprinted with permission from ref 189. Copyright 2015 John Wiley and Sons].

content of 0.4 vol % showed 18% higher efficiency compared with the TiO2 nanoparticle based devices.186 Umeyama et al. incorporated rGO both into compact TiO2 layers and mesoporous TiO2 scaffold layer in meso-structured PSCs, The presence of rGO with 0.15 wt % in the compact TiO2 layer and 0.015 wt % in the mesoporous TiO2 layer led to a 40% enhancement in efficiency.187 Silver possesses similar electronic properties to that of graphene, like high conductivity and a proper work function of ∼4.4 eV. A network of silver nanowires (AgNWs) sandwiched between TiO2 films fabricated by a solution process was applied as the ETL in a MAPbIxCl3−x based planar PSC by Huang and co-workers. As a result, the efficiency was improved from 11.07% to 12.55% after the AgNW network incorporation.188 One thing that should be noted is the incorporated conductive networks in compact TiO2 layers should be fully covered by TiO2 in case of direct contact with the perovskite layer or the conductive networks would perform as the recombination centers and deteriorate performance of PSCs. Besides the conductive network incorporation, embedding

plasmonic gold nanoparticles (Au-NPs) into TiO2 layers is an alternative strategy for carrier transportation property improvement in TiO2 ETLs. Yuan and co-workers reported a structure of Au-NPs sandwiched between two layers of low temperature processed TiOx film performing as the ETL to improve the charge extraction efficiency in PSCs (Figure 17a and b). The plasmon-induced charge injection across the metal−semiconductor Schottky barrier from Au-NPs to TiOx would fill the trap state sites in TiOx matrix and result in improved conductivity as well as the increased surface potential of the TiOx composites (Figure 17c and d). Accordingly, the PSCs based on the TiOx-Au-NPs composites achieved a PCE of 16.2% due to the improved charge extraction capability, which is over 22% higher than that of the control devices (Figure 17e and f).189 The use of metallic nanostructures has been frequently proposed as an efficient means to enhance the performance of photovoltaic devices.190−192 Generally the enhanced photovoltaic performance has been attributed to the increased light absorption through plasmonic effects. Upon surface plasmon U


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ACS Sustainable Chemistry & Engineering excitation, the strong near-field electric field around metal NPs could enhance the effective light absorption cross-section of a solar cell. Meanwhile, the optical path length of the incident light would be increased by being scattered off the plasmonic metal NPs. Moreover, metal NPs could directly act as a sensitizer to harvest light and inject photoelectrons to an electron acceptor. It has been mentioned before that the photon capture in perovskite films is not as high in the range of near band edge (600 < λ < 780 nm) as it is for shorter wavelengths because the extinction coefficient rapidly decays for red frequencies. Therefore, it is desired to introduce plasmonic metal NPs with proper size, shape and composite into the perovskite absorber layer for further enhancing the absorption of light with long wavelengths. The initial study on the incorporation of plasmonic Au@SiO2 core/shell NPs into PSCs was implemented by Snaith’s group.193 Interestingly, the significantly enhanced photocurrent density after the inclusion of Au@SiO2 core/ shell NPs was attributed to the reduced exciton bonding energy from 100 to 35 meV in perovskite but not to the improvement of light harvesting that was not observed. The efficiency of PSCs with Au@SiO2 core/shell NPs was improved to 11.4% from 10.7% of the control devices. Further, Ag@TiO2 core/shell nanoparticles were also inserted into perovskite absorber layer by Snaith’s group and similar results of enhanced photocurrent density and invariable light harvesting efficiency were characterized.194 But the enhanced photocurrent density after the inclusion of Ag@TiO2 core/shell NPs was not seemingly attributed to the reduced exciton binding energy but to the enhanced recycling light absorption. Finally, an efficiency up to 16.3% was obtained for PSCs with the Ag@TiO2 core/shell NPs, which is an over 12% enhancement compared with the control devices. The metal oxide (SiO2 and TiO2) shells coated on plasmonic metal NPs perform tasks of improving stabilities of plasmonic metal NPs and preventing plasmonic metal NPs from acting as charge carrier recombination centers. The unobserved enhancement of light absorption in these cells may be attributed to the suppression of narrow localized surface plasmon (LSP) resonant peaks by the strong light absorption of perovskite. Lu and co-workers introduced the irregular Au−Ag alloy popcorn NPs into the mesoporous TiO2 scaffold layers of mesostructured PSCs.195 Different from the results of Snaith’s group, the optical absorption property of the perovskite layer was improved significantly due to these plasmonic popcorn NPs exhibited broadband LSP resonances from the ultraviolet to the near-infrared wavelength range. Moreover, the electron transfer property of the device was also improved after the inclusion of these plasmonic popcorn NPs. Finally, with the aid of plasmonic popcorn-shaped NPs, the device’s maximum efficiency increased by 15.7%, from 8.9% to 10.3%. The plasmonic metal Au NPs were also in situ embedded into the TiO2 nanofiber based porous scaffold layers through an electrospinning technique by Mali and co-workers. The carrier recombination rate at the perovskite/TiO2 interface was effectively reduced after the inclusion of Au metal NPs into TiO2 nanofibers, which may result from the filling of surface trap state sites of TiO2 by hot electrons derived from Au LSP resonance. Combining the enhancement of light absorption and the reduced recombination rate, an encouraging efficiency of 14.92% was demonstrated for PSCs based on the Au@TiO2 nanofibers, an over 30% enhancement compared to that of the control devices based on the pure TiO2 nanofibers.196 The optimum content of Au NPs of 0.3 wt % should correspond to just no exposure of Au NPs on the surface of TiO2 nanofibers, avoiding formation of recombination

centers. Yu and co-workers introduced silica coated Ag NPs into the anatase orchid-like TiO2 nanowire (OC-TiO2 NW) based porous scaffold layers for enhancing light harvesting and reducing exciton bindings by plasmonic effects.197 Consequently, a PSC based on this collaborative scaffold exhibited an average efficiency of 14.18%, which is an improvement of ∼10% over a PSC based on a neat OC-TiO2 NW scaffold layer, where the average PCE was 12.97%. The pure Ag NPs has also been directly blended into porous TiO2 scaffold layers for enhancing light absorption, although they would also perform as carrier recombination centers when sandwiching between the TiO2 ETL and the perovskite absorber layer. As long as the positive effect (light absorption enhancement) prevails over the negative one (carrier recombination enhancement), the final improved photovoltaic performance would be obtained. Yang et al. systematically investigated the effect of Ag NPs on the efficiency of PSCs through embedding different amounts of neat, spherical Ag NPs with loading levels of 0.0, 0.5, 1.0, and 2.0 wt % into the TiO2 porous scaffold layers.198 They found that the device with 0.5 wt % of Ag NPs showed a maximum efficiency of 11.96%, higher than that (10.96%) of the standard device without Ag NPs. Besides plasmonic metal NPs, inclusion of down-shifting materials into TiO2 ETLs is an alternative approach to increasing light utilization efficiency through splitting one high energy photon (like UV photons) into two or more photons with low energy, which can excite perovskite to generated more photocarriers. Moreover, since PSCs generally show instability under the illumination of UV light, incorporation of down-shift materials could not only increase the number of photocarriers but also improve the stability of PSCs under whole light illumination.199 Inspired by this motivation, Hou et al. incorporated nanoparticles of down-shifting material ZnGa2O4:Eu3+ into TiO2 mesoporous scaffold layers. The average efficiency of PSCs was increased to 13.80% and a maximum efficiency of 14.34% was achieved after the incorporation of suitable amount of ZnGa2O4:Eu3+, much higher than that (10.67%) of the devices without ZnGa2O4:Eu3+.200

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SUMMARY AND PERSPECTIVE Summary. The performance of a photovoltaic solar cell is mainly determined by the light absorber active material and the selective contact interfaces. The active material decides the number of generated photocarriers and the ratio of photocarriers that could reach interfaces, and the following photocarrier extraction efficiency lies on the structures of selective contact interfaces. A desired feature that carrier diffusion length outstrips the optimum thickness of metal halide perovskite active layer has been well demonstrated in PSCs, which means that almost 100% of generated photocarriers in PSCs could arrive at selective contact interfaces before recombination. Meanwhile, the injection times of electrons and holes at interfaces have been measured to be hundreds of picoseconds, which is still orders of magnitude longer than the hot carrier cooling (or thermalization) time (≈ 0.4 ps),27,53 meaning that a large amount of the converted photon energy is wasted in the thermalization and trapping processes. Consequently, the photocarrier extraction efficiency at interfaces becomes a crucial parameter, limiting the further improvement of photovoltaic performance of PSCs. In order to further improve the performance of TiO2 based PSCs, some effective strategies have been intensively adopted to modify TiO2 ETLs to conquer the intrinsic shortcomings (e.g., low carrier mobilities and V


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Figure 18. Schematic diagram of summarized effective strategies for modification of TiO2 ETLs for high photovoltaic performance PSCs.

inevitable defects) of TiO2. Meanwhile, the challenges of I−V hysteresis phenomenon and instability of PSCs have also been alleviated (or eliminated) after the proper modifications. The reported effective strategies on modification of TiO2 ETLs for high photovoltaic performance PSCs are schematically summarized in Figure 18, including morphology control, doping, surface modification, and compositing. From the perspective of morphology control, the compact TiO2 films should fully and tightly cover on the conductive substrate with uniform thickness, avoiding direct contact of the electrode with the perovskite absorber layer or HTM and thus inhibiting interface recombination stemming from the backflow of photoelectrons to perovskite or HTM. The mesoporous TiO2 scaffold layers should possess proper pore size for the effective filtration of perovskite absorber materials and high specific area for increasing contact interface area. Effective filtration of perovskites could inhibit the carrier recombination at the interface and the large contact interface area could facilitate the carrier interface transfer. Therefore, the size and shape of particle component in the mesoporous TiO2 scaffold layers should be finely tuned and carefully designed for efficient PSCs. 1D TiO2 components in mesoporous scaffold layers could provide not only open pore structures but also better behaviors in carrier transportation, especially for aligned 1D TiO2 nanostructure arrays that provide a direct path for the rapid transportation of photoelectrons. Moreover, the interface recombination behavior could also be suppressed due to less boundaries and surface defects in 1D TiO2 components. However, the relatively small specific surface area of the mesoporous scaffold layers consisting of 1D TiO2 nanostruc-

tures is the main limiting factor for efficient meso-structured PSCs, which limits the photoelectron injection process due to the reduced contact surface area between TiO2 and perovskite. 3D hyperbranched TiO2 nanostructures could effectively increase specific surface area compared to aligned 1D TiO2 arrays, enhancing the carrier interface transfer, and the light absorption efficiency of PSCs could be enhanced through the light trapping effect in 3D hyperbranched TiO2 arrays. The presence of macropores (or macroparticles) with subwavelength sizes can effectively enhance the light harvesting efficiency in the red part of solar irradiation by increasing the effective optical path length via scattering, which is particularly attractive to PSCs since the light absorption of CH3NH3PbI3 typically decreases precisely in the long wavelength from 500 to 800 nm.7,9 Therefore, TiO2 ETLs with periodical arrangement of macropores (e.g., inverse opal-like TiO2 ETL and moth-eye TiO2 ETL) have been widely investigated in PSCs for light harvesting efficiency enhancement. Different from the morphology control method, doping TiO2 with heteroatoms could modify its electronic structures and intrinsic properties. The improved electrical conductivity of the TiO2 ETL can be generally achieved by doping through either increasing carrier density or improving carrier mobility. Accompanied with electrical conductivity improvement, the band gap and alignment as well as interface trap state density and distribution can also be modified after doping. Doped TiO2 ETLs with higher positive valent dopants (e,g, Nb5+ and W6+), denoted as n-type doping, exhibited not only high conductivity but also an improved photoelectron interface injection process as results of extra electrons from dopants and positive shift of W


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trap defects in TiO2, and the excess-halide-associated defects at grain boundaries have been reported to be a dominant source of electronic traps in MAPbI3 based perovskites.144−146 The existence of these surface trap states may slow down the photoelectron injection/extraction efficiency and increase the recombination probability, leading to an enlarged Rs and a reduced Rsh. Furthermore, the accumulation of photocarriers may also occur due to the reduced injection and extraction efficiency, resulting in an obvious I−V hysteresis phenomenon. Therefore, inserting a monolayer of materials with the ability of passivating surface defects of both TiO2 and perovskite, reducing or removing the interfacial trap states, is regarded a promising approach to both improve the PSC efficiency and reduce (or remove) their I−V hysteresis phenomena. Fullerene derivatives have the ability of suppressing the formation of deep traps in perovskites,149 and functional groups containing oxygen species with vacant orbitals (e.g., −COOH) prefer to adsorbed on the TiO2 surface for the defective trap state passivation. Consequently, various fullerene derivatives substituted with different oxygen-containing functional groups have been intensively adopted as the surface monolayer modifiers of TiO2 ETLs for high photovoltaic performance PSCs, such as PCBM, fullerenol, PCBA, PCBB-2CN-2C8, and MCA. The fullerene derivative modified devices generally exhibited enhanced performance, as well as removed (or reduced) hysteresis phenomenon due to the effective passivation of the interfacial trap states. Similarly, small organic molecules with two functional groups at opposite ends have also been frequently introduced at the TiO2/perovskite interface for the trap state passivation. Oxygen-containing groups at one ending position prefer to adsorb on the surface of the TiO2 ETL for passivating its surface defect states, and the directional groups (e.g., −Cl, −Br, −NH2, −SH, −NH3I) at the other end generally induce the growth of less defective perovskite layers. Meanwhile, the benzene ring unite was intensively adopted as the bone of small organic molecules due to its high conductivity resulting from the delocalized conjugate π electrons. Besides organic molecule monolayers, introducing a thin layer of proper materials is an alternative approach to facilitate electron interfacial transfer and/or suppress interfacial recombination. Conductive materials with appropriate Fermi level positions, e.g. graphene based materials (e.i. GQDs and GO-Li) and IL (1-butyl-3-methylimidazolium tetrafluoroborate), have been inserted at the TiO2/ perovskite interface to facilitate the electron interfacial transfer. Semiconductive metal sulfides with high carrier mobility and slow surface carrier recombination rate have been frequently used to passivate the surface defects of TiO2 ETLs for blocking interface recombination. Moreover, the improved performance stability has also been obtained in devices with metal sulfide modification due to the passivation of TiO2 defect states or the reduction of oxidative activity of photoholes generated in TiO2. Ultrathin insulator layers have also been introduced at the TiO2/ perovskite interface to prohibit the interface recombination, leading to enlarged Voc values. Incorporating functional materials is an effective way to directionally complement shortcomings of TiO2 ETLs or perovskite materials. Implantation of high conductive networks into TiO2 ETLs can expedite the electron transportation through the ETL and consequently suppress the back electron recombination. Graphene has remarkably high charge mobility and electronic conductivity and a proper work function; it has been frequently introduced into TiO2 ETLs to construct the conductive framework on which TiO2 nanoparticles were

TiO2 CB, respectively. Moreover, the electrical conductivity of TiO2 can also be increased by improving the electron mobility in TiO2 via introducing metal dopants (e.g., Sn) whose oxide compounds possess high electron mobility. Introducing equal valent metal ion dopants to partially replace Ti4+ lattice generally induces changes of band gap and alignment, carrier mobility, and trap-state density but not carrier density due to having no extra electrons from dopants. For example, Zr doped TiO2 was speculated to have reduced interfacial trap states and little upward shift of CB and Fermi level.116 Introducing metal ions with a lower valency into Ti4+ lattice sites would induce p-type doping, generally leading to the loss of conductivity due to the reduced electron density. Strikingly, low-valent metal ion doped TiO2 ETLs with low concentration showed remarkable improvement on electrical conductivity, which was attributed to the suppression of charged defects. Because the deep donor defects arising from oxygen vacancies or Ti3+ ions can be effectively removed by doping TiO2 with Al3+ dopants, the sealed PSCs with Al3+ doped TiO2 ETLs show much improved performance stability under testing in inert conditions.118 A loss of conductivity generally happened after high level p-type doping TiO2 with low valent metal ions, leading to a reduction of Jsc, but the upshift of CB in TiO2 after p-type doping could enlarge Voc. Introducing lower valent metal ions with small radius into interstitial sites of TiO2 can also lead to n-type doping due to the contribution of extra electrons from dopants. The Li+ doped TiO2 ETL exhibited down-shift of conduction band edge, improved conductivity, and reduced trap density, leading to improvements on charge separation/injection at perovskite/TiO2 interface, charge transport in mesoscopic TiO2, and recombination resistance at interface. The carrier transporting capacity in Li+ doped TiO2 ETL is comparable to that in perovskite itself, which means that the contradiction between fast electron injection at the perovskite/TiO2 interface and slow electron transportation through TiO2 in TiO2 based mesostructured PSCs can be effectively solved by doping. Intrinsic defects in TiO2 play different roles in performance of PSCs depending on the position of their corresponding local energy levels. If the introduced native defects with localized states are located below CB edge, photoelectrons from the perovskite absorber would be easily trapped there, resulting in a loss of efficiency. However, if the localized defect states presented between the VB edge and middle gap, photoelectrons from the perovskite absorber would not be trapped but the photoholes generated in the TiO2 ETL under UV light irradiation would be easily captured. Meanwhile, photoelectrons were left in the TiO2 CB which contributes to the improved photoconductivity of the TiO2 ETL, leading to the improved efficiency of PSCs. Moreover, PSCs with defective TiO2 ETLs showed improved stability due to the existence of these deep level trap states that block transfer of highly oxidative photoholes from TiO2 into perovskites. The defective TiO2 ETLs fabricated by different methods and conditions generally possess different distributions and densities of localized trap states, and consequently, both negative and positive effects of native defects in TiO2 ETLs on PSC performance appear in published papers, depending on the distribution of localized trap levels in the band gap arising from introduced native defects. Facilitating the photoelectron transfer process and suppressing the photocarrier recombination at interfaces are the two main purposes of surface modification of TiO2 ETLs, leading to a reduced Rs and an enlarged Rsh, respectively. The oxygen vacancies and/or the Ti3+ interstitials are the domain electronic X


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ACS Sustainable Chemistry & Engineering Table 1. Abbreviations and Their Corresponding Full Names in This Paper full name perovskite solar cells electron transporting layer power conversion efficiency organic photovoltaics quantum dot solar cells dye sensitized solar cells hole transporting material 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)9,9′spirobifluorene “meso-super structured solar cell” hole transporting layer series resistance shunt resistance short circuit current density open circuit voltage fill factor fill factor of an ideal device with zero Rs and infinity Rsh fill factor of a device with some Rs and infinity Rsh fill factor of a device with zero Rs and limited Rsh characteristic resistance of a device (≈ Voc/Jsc) normalized series resistance (Rsh/RCH) normalized shunt resistance (Rs/RCH) compact TiO2 films from spin coating method compact TiO2 films produced from spray pyrolysis method atomic layer deposition ALD deposited TiO2 compact layers TiO2 compact layers prepared from magnetron sputtering TiO2 compact layers prepared from anodizing Ti films electron beam-induced current conduction band anatase TiO2 nanowires surface-reaction-limited pulsed chemical vapor deposition hierarchical nanorod

abbreviation

full name

PSCs ETL PCE OPVs QDSCs DSSCs HTM spiroOMeTAD MSSC HTL Rs Rsh Jsc Voc FF FF0 FFs FFsh RCH rs rsh SC-TiO2 SP-TiO2 ALD ALD-TiO2 MS-TiO2 A-TiO2 EBIC CB ATNWs SPCVD HNR

abbreviation

internal quantum efficiencies TiO2 nanotube arrays light harvesting efficiency polydimethylsiloxane moth-eye patterned mesoporous TiO2 mesoporous TiO2 flat-band potential [6,6]-phenyl-C61-butyric acid methyl ester density functional theory C60-substituted benzoic acid self-assembled monolayer carboxylicgroup functionalized water-soluble fullerene derivative [6,6]-phenyl-C61-butyric acid [6,6]-phenyl-C61-butyric acid-dioctyl-3,3′-(5-hydroxy-1,3phenylene)-bis(2-cyanoacrylate) ester ditert-butyl methano[60]fullerene-61,61-dicarboxylate methano[60]fullerene-61-carboxylic acid heterojunction solar cells graphene quantum dots lithium-neutralized graphene oxide working function lowest unoccupied molecular orbital ionic liquid polyoxyethylene two-dimensional one-dimensional reduced graphene oxide silver nanowires gold nanoparticles localized surface plasmon orchid-like TiO2 nanowire

IQE TNTs LHE PDMS moth-eye TiO2 mp-TiO2 Vfb PCBM DFT C60SAM C60-Ac10 PCBA PCBB2CN-2C8 DBMD MCA HSCs GQDs GO-Li WF LUMO IL PEO 2D 1D rGO AgNWs Au-NPs LSP OC-TiO2 NW

alternative approach to increase LHE through splitting one high energy photon (like UV photons) into two or more photons with low energy, which can excite perovskite to generate more photocarriers. Furthermore, the stability of PSCs under whole light illumination could be improved due to the removal of UV light. The strategies introduced in this review on the modification of the TiO2 ETL in PSCs could be further extendable to other semiconductor related applications including semiconductor photoelectrodes for solar driven water splitting,201−204 semiconductor based electrocatalysts,205,206 and energy storage devices.207−210 Perspective. To date, PSCs have shown impressive high performance, greatly improved stability, and weak hysteresis behavior after proper modification of TiO2 ETLs. However, the performance of PSCs is intimately correlated with the size of devices due to the structure heterogeneity as a consequence of the simple solution processes. Generally, a small device shows high efficiency. Microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy have been employed to study charge carrier dynamics in PSCs based on TiO2 ETLs. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers.211 A stripe pattern radiating in all directions from the center of the device was observed in PL images, which is similar to the pattern of mesoporous TiO2/compact TiO2 on a glass substrate fabricated

allowed to be anchored, improving charge carrier collection efficiency. Silver possesses similar electronic properties to that of graphene like high conductivity and a proper work function of ∼4.4 eV. A conductive network consisting of silver nanowires (AgNWs) has also been incorporated into TiO2 ETLs for improving charge carrier collection efficiency. Embedding plasmonic gold nanoparticles (Au-NPs) into TiO2 layers is an alternative strategy for the carrier transportation property improvement in TiO2 ETLs. The trap state sites in the TiOx matrix would be filled by the plasmon-induced charge carriers and result in improved conductivity, as well as increased surface potential of the TiOx composites. It has been mentioned that the LHE in the range of near band edge (600 < λ < 780 nm) for perovskite films is not as high as it is in the shorter wavelength range. The use of metallic nanostructures could increase light absorption through plasmonic effects. Upon surface plasmon excitation, the strong near-field electric field around metal NPs could enhance the effective light absorption cross section of a solar cell and the optical path length of the incident light would be increased by being scattered off the plasmonic metal NPs; the metal NPs could directly act as a sensitizer to harvest light and inject photoelectrons to an electron acceptor. Therefore, plasmonic metal NPs with proper size, shape, and composition have been frequently introduced into TiO2 ETLs for further enhancing light absorption of perovskite in the long wavelength range. Inclusion of down-shifting materials into TiO2 ETLs is an Y



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by the spin-coating method. Meanwhile, the PL image of the perovskite film on a quartz substrate exhibits no radial pattern. Therefore, the observed PL pattern is attributed to the inhomogeneous mesoporous TiO2 layer fabricated by spincoating. Due to the competition between photocarrier injection at perovskite/TiO2 interface and recombination within the perovskite layer, a negative correlation has been observed between PL and PC intensities. The device constructed on the positions where show low PL intensity and high PC intensities would display high performance. For the device with large active area, the performance reflects the average efficiency. A large heterogeneity of the carrier extraction efficiency at the perovskite grain−electrode interface has also been observed in PSCs with grain sizes larger than the film thickness.212 The long carrier diffusion length means that the photocarriers can be effectively transported to the grain/collecting electrode interfaces through fast intragrain diffusion. However, if there are defects (or barriers) at the perovskite grain/electrode interface, the photocarriers cannot find alternative collection pathways because of the slow lateral intergrain diffusion. Consequently, improving homogeneity at perovskite−electrode contacts is thus a promising direction for improving the performance of PSCs. Besides the heterogeneity at perovskite/ TiO2 interface, attention should be paid to the transparent oxide conductors (TOC)/TiO2 interfacial structure heterogeneity. The existence of gaps at the FTO/TiO2 interface has been demonstrated for TiO2 ETLs coated on FTO substrates, which strongly influences the photocarrier collection efficiency. However, the influence of TOC/TiO2 interface structures on the photoelectron collection efficiency has not been paid as much attention as that on the TiO2/perovskite interface and TiO2 structures themselves. Hetero-epitaxial growth of TiO2 single crystal ETLs on TOC is an ideal approach for construction of homogeneous TOC/TiO2 interface due to the coherent growth behavior. In addition, the photoelectron collection efficiency could be further improved in TiO2 single crystal ETLs epitaxially grown on TOCs by removing the strong scattering effects of grain boundaries. Coincidently, rutile TiO2 possesses the same crystal structure and similar crystal parameters to those of SnO2; the matrix material of FTO substrates. An FTO film is composed of F− doped SnO2 single crystals densely arrayed on glass substrate. It is therefore feasible to fabricate dense TiO2 single crystal arrays epitaxially grown on the FTO as an efficient ETL for high photovoltaic performance PSCs. It is also expected that the surface defects would be remarkably reduced due to the single crystal nature. A layer of mesoporous TiO2 single crystal arrays homoepitaxially grown on the dense TiO2 single crystal array film can improve the device performance. The increased interface would improve the photoelectron injection at the perovskite/TiO2 interface. Consequently, the heterogeneity at perovskite/TiO2 and TOC/TiO2 interfaces could be effectively alleviated by the mesoporous-compact integrated TiO2 films heteroepitaxially grown on FTO substrates as ETLs in PSCs. Moreover, the photoelectron collection efficiency could also be enhanced due to the coherent interfaces, single crystal nature and the enlarged perovskite/TiO2 contact interface. It is anticipated that high performance large size PSC could be achieved by use of this ideal TiO2 ETL model. The abbreviations appeared in this article are listed in Table 1.

Perspective

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Z.W.). *E-mail: [email protected] (G.L.). ORCID

Lianzhou Wang: 0000-0002-5947-306X Gang Liu: 0000-0002-6946-7552 Hui-Ming Cheng: 0000-0002-5387-4241 Notes

The authors declare no competing financial interest. Biographies

Chao Zhen received his Bachelor’s degree in Materials Physics from Jilin University in 2007. He obtained his Ph.D. degree in Materials Science from the Institute of Metal Research (IMR), Chinese Academy of Sciences, in 2013. He worked as an assistant professor of materials at IMR between 2013 and 2015, and now, he is an associate professor of materials science at IMR. His current research interest is developing efficient solar energy conversion devices including photo-electrochemical water splitting cells and new types of photovoltaic cells.

Tingting Wu received her Bachelor’s degree in Materials Science and Engineering from Shandong University of Science and Technology in 2012. She obtained her Master’s degree in Materials Engineering from the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), in 2015. She is a Ph.D. candidate at the Institute of Metal Research, University of Science and Technology of China. Her research interests focus on solar cells and photocatalysis under the supervision of Professor Gang Liu and Professor Hui-Ming Cheng. Z


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his Ph.D. degree in Materials Science from the Institute of Metal Research in 2009. His main research interests focus on solar-driven photocatalyitc materials for renewable energy.

Runze Chen received his Bachelor’s and Master’s degrees in Materials Science and Technology from the Northeastern University of China in 2012 and 2014, respectively. Now he is a Ph.D. candidate in a joint Ph.D. training program of the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), and Northeastern University of China. His current research is focused on photo-electro-chemical water splitting and perovskite solar cells under the supervision of Professor Gang Liu and Professor Hui-Ming Cheng.

Hui-Ming Cheng is a Professor and Director of the Advanced Carbon Research Division of Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, and the Low-Dimensional Material and Device Laboratory of the Tsinghua-Berkeley Shenzhen Institute, Tsinghua University. His research focuses on carbon nanotubes, graphene, two-dimensional materials, energy storage materials, photocatalytic semiconducting materials, and bulk carbon materials. He is a highly cited researcher in the fields of Materials Science and Chemistry. He is now the founding Editor-in-Chief of Energy Storage Materials and Associate Editor of Science China Materials. He was elected a member of CAS and a fellow of TWAS.

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ACKNOWLEDGMENTS The authors thank National Natural Science Foundation of China (Nos. 51825204, 51572266, 51629201) and the Key Research Program of Frontier Sciences CAS (QYZDB-SSWJSC039) for the financial support.

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Lianzhou Wang is a Professor at the School of Chemical Engineering and Director of Nanomaterials Centre, the University of Queensland (UQ), Australia. He received his Ph.D. degree from the Chinese Academy of Sciences in 1999. Before joining UQ in 2004, he worked at two national institutes (NIMS and AIST) of Japan for five years. Wang’s research interests include the design and application of semiconducting nanomaterials in renewable energy conversion/storage systems, including photocatalysts and photo-electro-chemical devices.

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