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Inorganic Hole Transporting Materials for Stable and High Efficiency Perovskite Solar Cells Jiangzhao Chen, and Nam-Gyu Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01177 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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The Journal of Physical Chemistry

Inorganic Hole Transporting Materials for Stable and High Efficiency Perovskite Solar Cells Jiangzhao Chen, Nam-Gyu Park* School of Chemical Engineering, Sungkyunkwan Univeristy (SKKU), Suwon 440-746, Korea *Corresponding author: Email: [email protected]

ABSTRACT Organic-inorganic hybrid perovskite solar cells (PSCs) have received considerable attentions due to their low cost, easy fabrication and high power conversion efficiency (PCE), which achieved a certified PCE of 22.7%. To date, most of high efficiency PSCs were fabricated based on organic hole transporting materials (HTMs) such as molecular spiro-MeOTAD or polymeric PTAA. However, poor stability of PSCs limits its large scale commercial application because of use of additives like tert-butylpyridine (t-BP) and lithium salt. Moreover, relatively low-temperature degradation of organic HTMs is responsible for poor thermal stability of PSCs. Consequently, HTM play a crucial role in realization of efficient and stable PSCs. In order to improve the stability of PCSs, various inorganic HTMs have been developed and applied into PSCs. Recently, the devices based on CuSCN and Cu:NiOx HTMs have demonstrated PCEs over 20%, which is comparable to PCEs of devices based on organic HTMs. Most importantly, stability of PCSs are much improved by the inorganic HTM, which indicates clearly that inorganic HTMs are promising alternative to organic HTMs. Herein, we review recent progress on application of inorganic HTMs in PSCs. We highlight the importance of systematic engineering for each layer and respective interface in the whole device for further improvement of PCE and stability. 1. INTRODUCTION Solar cells, which directly convert sunlight light into the electricity, can be fabricated with light absorbing materials such as inorganic semiconductors, such as silicon (Si), gallium arsenide (GaAs), and copper indium gallium selenide (CIGS). Those light absorbing semiconductors exhibit power conversion efficiency (PCE) exceeding 20%.1 However, the high 1 ACS Paragon Plus Environment

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fabrication cost hinders their large-scale extensive applications. To solve the cost issue, dyesensitized solar cell (DSSC) was developed by Grätzel et al.,2 and subsequently attract a great deal of attentions in academia and industrial community due to low cost.3-4 Nevertheless, lower PCE of DSSC than the inorganic semiconductor based solar cells has been bottleneck for further expansion. Development of light absorbers with high absorption coefficient is the key technology toward higher PCE of DSSC. To that end is finding inorganic materials instead of molecular sensitizers because of higher absorption coefficient expected. Organic-inorganic hybrid perovskite emerges as a kind of promising light-harvesting materials. Generally, three-dimensional organic-inorganic hybrid perovskite with ABX3 chemical formula consists of corner-sharing BX6 octahedra and AX12 cubo-octahedra.5 In 2009, Miyasaka group.6 reported a 3.8% efficient PSC based on MAPbI3 (MA = CH3NH3+) that used DSSC device configuration having liquid electrolyte. In 2011, a nearly doubled PCE of 6.5% was realized by Park group,7 using high concentration perovskite precursor solution. Although higher PCE was obtained, little attention has been paid to liquid junction PSC because of extremely poor stability due to dissolution of MAPbI3 in polar liquid electrolyte. In order to overcome the instability of liquid PSCs, solid-state hole transporting material (HTM) spiroMeOTAD

(2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene)

was

employed to replace liquid electrolyte, which demonstrated a PCE of 9.7% and 500 h stability without encapsulation.8 Since then, many efforts have been made to improve photovoltaic performance of PSCs from different perspectives, including fabrication methods for perovskite layer, device architecture design, interface engineering, and development of electron transporting materials (ETM) and hole transporting material (HTM). On the basis of the above optimization, a certified PCE of 22.7% was achieved.1 Although presently obtained PCE is appealing, the PSCs still suffer from poor stability (thermal, moisture and light stability), which may impede its commercialization. It is well-known that the structure of PSCs is mainly classified into two kinds: one is FTO/ETM/perovskite/HTM/electrode (Au or Ag) (denoted as normal (or regular) structure); the other is FTO or ITO/HTM/perovskite/ETM/electrode (denoted as inverted structure). Inspired by the demonstration of bipolar charge transfer of metal organic halide perovskite materials (like MAPbI3 and FAPbI3),9-10 hole-conductor-free PSC was developed to reduce device fabrication 2 ACS Paragon Plus Environment

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cost and simplify the device configuration.10 Although great progresses have been made, much lower PCE is found in hole-conductor-free PSC11-14 as compared to devices employing HTM.1519

It indicates that HTM is indispensable for effective hole extraction from perovskite. Moreover,

device stability also depends on the stability of HTM to a great extent. In inverted PSCs, the most commonly used HTM is poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS).20-21 However, its hygroscopic and acidic nature is detrimental to long-term stability. Most of high-efficiency PSCs are fabricated based on the normal structure with organic HTMs (like spiro-MeOTAD and PTAA).15-19, 22-23 However, the additives of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and t-BP (tert-butylpyridine) in the organic HTMs were found to have negative effect on stability.24-26 Furthermore, spiro-MeOTAD itself as an organic HTM suffers from poor long-term UV and thermal stability. In order to overcome the moisture instability of HTM with additives, various dopant-free organic HTMs25-31 have been proposed for PSCs but issues on thermal and UV stability remain due to their organic characteristics. Thus, it is highly desired to develop stable and efficient HTM toward stable PSCs. Since inorganic materials are expected to be more stable than organic ones in terms of high temperature and moisture, a variety of inorganic HTMs have been developed and applied into the PSCs to surmount the drawbacks of organic HTMs, such as carbon,32-33 vanadium(V) oxide (V2O5),34-35 MoOx and WOx,36 spinel CoOx,37 nickel oxide (NiOx),38-40 Kesterite Cu2ZnSnS4,41 CuCrO2,42 CuAlO2,43 CuGaO2,44 CuS,45 CuI,46 CuOx,47-48 and CuSCN.49-50 The inverted devices based on NiOx HTM have delivered an impressive PCE over 20%.39-40 CuOxbased inverted PSC employing MAPbI3-xClx demonstrated a PCE of as high as 19%.48 Recently, an exciting PCE of up to 20.4% was realized in regular PSCs through use of CuSCN as HTM and TiO2 as ETM, which is comparable to 20.9% of spiro-MeOTAD HTM.51 As we expected, PSCs with inorganic HTM showed stability under humid and high temperature condition.51-52 In this review, we present progress of inorganic HTM-based PSCs in terms of photovoltaic performance and stability.

2. DEVICE STRUCTURE AND WORKING PRINCIPLE

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As we mentioned above, mainly two kinds of device structures are adopted in PSCs (normal structure and inverted structure), as shown in Figure 1a and b. Obviously, both structures primarily consist of transparent conductive oxide (TCO) (like FTO or ITO), ETM layer, perovskite layer, HTM layer and back contact electrode (like Au and Ag). Figure 1c schematically illustrates the energy level diagram of normal configuration showing the transporting direction of electrons and holes during operation. When sunlight illuminates the perovskite active layer, excitons are generated and then separated into free carriers. Afterward, the generated electrons and holes can be transported to each interface and then injected to ETM and HTM, respectively. Finally, electrons and holes in ETM and HTM are collected by working and counter electrodes, respectively, transported to external circuit and produce current.53-54 According to transient absorption spectroscopic study, charge separation between MAPbI3 and HTM such as spiro-MeOTAD was clearly detected, while electron injection at open-circuit condition was not clearly detected.8 This indicates that HTM plays a crucial role in carrier separation and transport in PSCs.

Figure 1. Schematically illustrated device structures depending on electrode polarity and light absorption direction: (a) normal structure with n-type ETM facing incoming light and (b) 4 ACS Paragon Plus Environment

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inverted structure with p-type HTM facing incoming light. (c) Energy level diagram of normal configuration showing carrier transport.

3. PHOTOVOLTAIC PERFORMANCE Since the main function of HTM in PSCs is to collect and transport holes from perovskite layer, the following prerequisites are satisfied for ideal HTM. First, the valence band maximum (VBM) of HTM should be higher than that of perovskite semiconductor. Simultaneously, higher conduction band minimum (CBM) of HTM than perovskite should be guaranteed in order to effectively block the recombination between the electrons in perovskite and holes in HTM. For convenient discussion and description, the energy level diagram of representative inorganic HTMs (CuI, CuSCN, Cu2O, CuO, CuS, Cu2ZnSnS4, NiOx, MoO3, V2O5 and CuGaO2) is displayed in Figure 2.44, 55-56 Second, high hole mobility is required for ensuring that holes can be transported to back contact electrode. Table 1 shows hole mobility of some representative inorganic HTMs along with spiro-MeOTAD. Third, in case of deposition of inorganic HTM in solution process, solubility in organic solvents and good film-forming ability is important. In normal PSCs with inorganic HTM on the top of perovskite layer, the organic solvent used for dissolving the HTM must be inert for underlying perovskite film. Fourth, photo- and chemicalstability of HTM are equally required for long-term stability. Finally, high optical transmittance is essential especially in case of inverted structure in order not to lose incoming photon. Based on the above fundamental requirements, all kinds of inorganic HTMs have been developed for the purposes of high-performance and stable PSCs in the past several years.



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Figure 2. The energy level diagram of representative inorganic HTMs (CuI, CuSCN, Cu2O, CuO, CuS, Cu2ZnSnS4, NiOx, MoO3, V2O5 and CuGaO2), along with HOMO level of spiro-MeOTAD. The data were taken from references 44, 55-56.

Table 1. Hole mobility of some representative inorganic HTMs along with spiro-MeOTAD. HTM spiro-MeOTAD Cu2ZnSnS4 Cu2O CuO CuSCN CuAlO2 CuCrO2 CuGaO2

Hole mobility (cm2/Vs) 4 ×10-5 6-30 100 0.129 0.01-0.1 3.6 7.7 0.01–10

Ref 56 56 56 56 56 57 58 44

3.1. Copper based HTMs 3.1.1. Copper Iodide (CuI), Copper Sulphide (CuS) and Kesterite Cu2ZnSnS4 In late 2013, CuI was proposed as an inexpensive, stable and wide band gap inorganic HTM, which was prepared via drop casting method (Figure 3a) and introduced into normal 6 ACS Paragon Plus Environment

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PSCs with MAPbI3 as light absorber, achieving a short-circuit photocurrent (Jsc) of 17.80 mA/cm2, an open-circuit voltage (Voc) of 0.550 V, and a fill factor (FF) of 0.62, corresponding to a PCE of 6.0%. In the same condition, spiro-MeOTAD-based device shows a Jsc of 16.10 mA/cm2, a Voc of 0.790 V, and a FF of 0.61, corresponding to a PCE of 7.9%, as illustrated in Figure 3b.46 The impedance spectroscopic result indicates that much lower Voc in CuI-based device as compared to spiro-MeOTAD-based one is ascribed to more serious recombination, associated with CuI film thickness. In order to compare the photostability, the unencapsulated devices were exposed to constant 100 mW/cm2 AM 1.5G illumination at short-circuit condition for 2 h under ambient condition. It was found that CuI-based device maintained a constant current while spiro-MeOTAD-based device showed an about 10% decrease as presented in Figure 3c. Inspired by the above improved photostability of CuI-based device, Nazari et al.57 achieved a PCE of 9.24% (Table 2) in normal structure, where CuI layer was in-situ formed on the perovskite layer via the reaction between the excess MAI and Cu. Although much higher Jsc and Voc were obtained through the in-situ interface engineered CuI compared with previous solution-processed CuI, significantly low FF remains an intractable problem for realizing high PCE.



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Figure 3. (a) Automated drop-casting apparatus used for the solution deposition of CuI onto mesoporous TiO2/MAPbI3 films. (b) J−V curves of champion CuI and spiro-MeOTAD-based devices measured under AM 1.5G one sun (100 mW/cm2) illumination. (c) Jsc evolution of devices based on CuI and spiro-MeOTAD upon 2 h continuous one sun illumination without encapsulation. (d) J–V curves of the best performing CuI-based device measured at both forward and reverse scan modes with a scan rate of 300 mV/s. (e) PCE as a function of exposure time to ambient atmosphere for unencapsulated PSCs with CuI or PEDOT:PSS HTMs. (a-c) Reprinted with permission from ref. 46. Copyright 2013 American Chemical Society. (d-e) Reprinted with permission from ref. 59. 2016 Royal Society of Chemistry.

Table 2. Photovoltaic parameters of PSCs based on copper iodide (CuI), copper sulphide (CuS) and kesterite Cu2ZnSnS4 inorganic HTMs. HTM

Device structure

Perovskite

Jsc (mA/cm2)

Voc (V)


FF

PCE (%)

Year

Ref.

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CuI CuI CuI CuI CuI CuS Cu2ZnSnS4

Normal Normal Inverted Inverted Inverted Inverted Normal

MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3

17.80 22.99 21.06 22.80 20.90 22.30 20.54

0.550 0.850 1.040 1.010 1.040 1.020 1.060

0.62 0.47 0.62 0.73 0.68 0.71 0.59

6.0 9.24 13.58 16.80 14.70 16.20 12.75

2013 2017 2015 2016 2017 2016 2015

46 57 58 59 60 45 41

Except for the applications of CuI HTM in normal structure, it can be utilized in inverted structure since CuI possesses high hole mobility and good optical transmittance. In 2015, Chen et al.58 incorporated solution-processed CuI as HTM in inverted PSCs with a PCE of 13.58%, which is slightly higher than 13.28% of PEDOT:PSS-based device (Table 2). More importantly, improved air stability can be found in CuI-based device. In order to further enhance PCE, Wang et al.60 synthesized CuI film through exposing a thermally evaporated copper film to iodine vapor and applied it as HTM in inverted planar PSCs. A PCE of 14.7% was achieved along with long-term stability at ambient condition due to the hydrophobic nature of CuI layer. This work implies that development of novel preparation method of CuI could hold great potential in achieving highly efficient and stable PSCs. Additionally, Sun et al.59 reported room-temperature and solution-processed CuI film as HTM in inverted PSCs with an impressive PCE of 16.8% as shown in Figure 3d and Table 2. To the best of our knowledge, this PCE is the highest among all of CuI-based devices reported so far. It is worth noting that the unencapsulated CuI-based device exhibits much better ambient stability as compared to PEDOT:PSS counterpart, as displayed in Figure 3e. When comparing the photovoltaic performance of CuI-based PSCs in normal and inverted devices, we can see clearly that inverted devices produce remarkably higher PCE than normal devices (Table 2). It is easy to understand that compact and uniform CuI film is easy to be fabricated on FTO or ITO substrate by solution process but it is hard to obtain fully covered and homogeneous CuI film on perovskite film. One of important reasons is that most of solvents for deposition of CuI are likely to dissolve the perovskite layer in normal layout. Consequently, it may be a good direction to develop new deposition method of CuI film on perovskite films for further enhancement of CuI-based normal PSCs. For the CuS HTM, as shown in Figure 2, CBM and VBM are -2.9 eV and -5.1 eV, respectively, which are energetically well-matched with 3.9 eV for CBM and 5.4 eV for VBM of 9 ACS Paragon Plus Environment


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MAPbI3.8, 55 Rao et al.45 first used CuS nanoparticles as HTM in inverted planar heterojunction PSCs. It was found that improved hole extraction efficiency was achieved because CuS nanoparticles tuned the surface work function from 4.9 to 5.1 eV while maintaining good surface coverage and transmittance. As a result, the optimized device showed a maximum PCE of 16.2% with negligible hysteresis and good stability, which means that CuS nanoparticle is one of promising inorganic HTMs. Kesterite Cu2ZnSnS4, which possesses advantages of decent band gap, large absorption coefficient and high hole mobility up to 6-30 cm2 V-1s-1 (Table 1), has been extensively employed as light absorber in thin film solar cells.61-63 Wu et al.41 used Cu2ZnSnS4 as HTM in normal structured PSC employing MAPbI3 and obtained a PCE of 12.75% being comparable to 13.23% of spiro-MeOTAD control device. Although an impressive PCE has been obtained based on Cu2ZnSnS4 HTM, Cu2ZnSnS4 with MAPbI3 may not be ideal combination since both CBM (-4.08 eV) and VBM (-5.57 eV) of Cu2ZnSnS4 are very close to those of MAPbI3. In other words, it is necessary to find a suitable perovskite material with lower VBM than Cu2ZnSnS4 or to engineer interface for the case of MAPbI3 to ensure enough hole extraction driving force and inhibit back electron transfer.

3.1.2. Cuprous Oxide (Cu2O) and Copper Oxide (CuO) Table 3. Photovoltaic parameters of PSCs based on Cu2O and CuO inorganic HTMs. HTM Cu2O Cu2O CuO Cu2O CuxO CuxO Cu2O CuxO

Device structure Normal Inverted Inverted Inverted Inverted Inverted Inverted Inverted

Perovskite MAPbI3-xClx MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3-xClx MAPbI3 MAPbI3-xClx

Jsc (mA/cm2) 15.80 16.52 15.82 16.52 23.20 17.22 18.03 22.50

Voc (V) 0.960 1.070 1.060 0.890 0.990 0.700 0.880 1.110


FF 0.59 0.76 0.73 0.56 0.74 0.48 0.61 0.76

PCE (%) 8.93 13.35 12.16 8.23 17.10 5.83 9.64 19.00

Year

Ref.

2016 2015 2015 2016 2016 2017 2016 2016

64 47 47 65 66 67 68 48

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Hossain et al.69 calculated the key characteristics of PSCs with various HTM layers (spiro-MeOTAD, NiO, CuI, CuSCN, and Cu2O) by using wxAMPS and SCAPS software on the basis of assumption of defect free MAPbI3 and HTM layers. The calculation results showed that the PCE over 24% was expected for Cu2O-based device, which outperformed all other inorganic HTMs investigated. Consequently, this report provides a theoretically reasonable guidance for future research work on inorganic HTMs and stresses the great potential of application of Cu2O in PSCs. Cu2O is believed to be one of promising inorganic HTMs due to its hole mobility as high as 100 cm2/Vs (Table 1), wide band gap (Figure 2), and high optical transmittance. Its VBM (-5.4 eV) matches well with those of most perovskite compositions (like MAPbI3, FA0.85MA0.15PbI2.55Br0.45 and FA0.9Cs0.1PbI3).54 However, application of Cu2O in normal PSC still faces difficulty because of lack of appropriate solution deposition method. To overcome this issue, reactive magnetron sputtering method was used to prepare Cu2O film on perovskite MAPbI3-xClx and as-prepared Cu2O film was uniform, compact and crack-free. As a consequence, the optimized Cu2O-based normal PSC yielded a PCE of 8.93% (Table 3), which was comparable to 9.24% obtained from CuI-based normal PSC.57, 64 However, much lower FF was observed for Cu2O-based device relative to spiro-MeOTAD-based device, which was attributed to diffusion of Cu during magnetron sputtering. Zuo et al.47 introduced Cu2O and CuO as HTMs in inverted PSCs and demonstrated PCE of 13.35% and 12.16%, respectively, which was higher than 11.04% of PEDOT:PSS counterpart. Cu2O film was prepared by immersing spin-coated CuI film in NaOH aqueous solution during which CuI can react with NaOH. CuO film was prepared by oxidizing Cu2O. The characteristic X-ray diffraction peak intensity of MAPbI3 on different HTM layers was in the order of Cu2O＞CuO＞PEDOT:PSS. This is indicative of improved crystallinity for MAPbI3 films on Cu2O and CuO as compared to PEDOT:PSS, which was responsible for the enhanced Jsc for Cu2O or CuO-based devices compared with PEDOT:PSSbased device. This highlights the importance of substrate for perovskite crystallization. Additionally, higher transmittance for Cu2O and CuO films was also one of main reasons for the improved Jsc. Finally, higher Voc for Cu2O and CuO-based devices was associated with their lower VBMs than that of PEDOT:PSS. However, it could be difficult to guarantee complete reaction from CuI to Cu2O or from Cu2O to CuO. In another work, it was also reported that electrodeposited Cu2O particle on ITO substrate could promote the growth of perovskite MAPbI3 11 ACS Paragon Plus Environment


on it.68 They proposed that high quality of the perovskite films was due to the fact that Cu2O particles served as the nucleation site for perovskite crystals during annealing. However, a PCE of 9.64% was still lower as compared to conventional organic HTM. Apart from the magnetron sputtering and CuI reaction methods, successive ionic layer adsorption and reaction (SILAR) method was applied to prepare Cu2O film that was used for inverted PSC, which showed a PCE of 8.23%.65 It is interesting that type II band alignment was observed at perovskite/Cu2O interface, which is favorable for efficient hole extraction. Electrospray was used to prepare CuxO film in inverted structure, which however showed poor PCE of 5.83%.67 To date, several deposition methods for CuxO film as HTM have been developed and applied in PSCs as mentioned above. However, simple and inexpensive solution process is highly expected to finally commercialize the PSCs. Encouraged by this purpose, Sun et al.66 achieved an impressive PCE of 17.1% based on solution-processed CuxO HTM using pure MAPbI3 as shown in Figure 4a (see also Table 3). Impedance and photoluminence (PL) results indicated that high performance of CuxO-based devices was attributed to fast hole extraction and low contact resistance of CuxO layer. Besides, negligible hysteresis was found in CuxO-based devices as presented in Figure 4b. Regarding long-term stability, the CuxO-based device retained around 90% of its initial PCE after 200 h while PEDOT:PSS-based device degraded to 50% in the same aging condition as exhibited in Figure 4c. Actually, realization of high PCE for inorganic HTM-based PSCs depends on not only inorganic HTM itself but also optical and electrical properties of perovskite. Based on this point, MAPbI3-xClx perovskite as light absorber instead of pure MAPbI3 was incorporated by the same group48 into solution-processed CuxObased inverted PSCs with PCBM as ETM for further PCE improvement. As a result, the optimized CuxO-based device showed a very high PCE of 19.0% (Figure 4d), which was the highest PCE among inorganic HTM-based PSCs at that time. Photovoltaic performance of devices based on MAPbI3 and MAPbI3-xClx was compared, where the latter showed significantly improved photovoltaic parameters than the former due to reduced defect density and increased recombination resistance by improved morphology and increased grain size as confirmed by topview SEM in Figure 4e and f. In fact, it has been reported previously that incorporation of Cl- in pure MAPbI3 can improve the device performance as a consequence of improved carrier lifetimes and transport induced by improved morphology and crystallization.70-72 The above 12 ACS Paragon Plus Environment

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successful combination of solution-processed CuxO HTM with Cl doped perovskite MAPbI3-xClx strongly suggests that each layer and their interfacial compatibility in PSCs should be taken into consideration simultaneously. As shown in photovoltaic performance in Table 3, a broad spectrum of PCE ranging from 6% to 19% seems to depend significantly on preparation methods of inorganic CuxO HTM layer. Thus it is required to find a standard method for high efficiency CuxO-based PSCs.

Figure 4. (a) J–V curve of the CuOx-based PSC measured under AM 1.5 G one sun illumination. The device structure used in this work is illustrated in the inset. (b) J–V curves of one of devices based on CuOx HTM measured in reverse and forward scans with a scan rate of 300 mV/s. (c) PCE as a function of ambient aging time for the unencapsulated PSCs based on PEDOT:PSS or CuOx. (d) J–V curve of the champion CuOx-based PSC employing MAPbI3-xClx as light absorber measured in the forward and reverse scan direction. SEM images of (e) MAPbI3 and (f) MAPbI3xClx film deposited on CuxO-deposited ITO substrates. (a-c) Reprinted with permission from ref. 68. 2016 Royal Society of Chemistry. (d-f) Reprinted with permission from ref. 48. 2016 Elsevier.

3.1.3. CuAlO2, CuCrO2 and CuGaO2 CuAlO2, CuCrO2 and CuGaO2 known to be delafossite structure with general formula of ABO2, where a sheet of linearly coordinated A cations are stacked between edge-shared octahedral BO6 layers, have been investigated in DSSCs and organic photovoltaics as p-type 13 ACS Paragon Plus Environment


semiconductors because of their wide bandgap, high hole mobility and chemical stability.73-76 Recent two years, some attentions have been paid to these inorganic HTMs in PSCs. Igbari et al.43 developed room-temperature amorphous CuAlO2, applied it as HTM in inverted PSCs combined with MAPbI3-xClx light absorber and the corresponding device gave rise to a PCE of 10.14%. CuAlO2-based devices showed improved stability as compared to PEDOT:PSS counterpart. Dunlap-Shohl et al.42 used room-temperature-prepared CuCrO2 as HTM in inverted MAPbI3-based PSCs and achieved a stabilized PCE exceeding 14% along with a negligible hysteresis. Zhang et al.44 reported the employment of solution-processed inorganic nanoplates CuGaO2 in normal PSC structure employing MAPbI3-xClx (Figure 5a). From the energy level diagram in Figure 5b, we can see obviously that CBM (-1.71 eV) and VBM (-5.29 eV) of CuGaO2 matches well with CBM (-3.75 eV) and VBM (-5.30 eV) of MAPbI3-xClx. So efficient hole extraction and preventing back electron transfer are expected, which is confirmed by high Voc (Table 4) and PL results. Additionally, XRD and XPS confirmed the synthesis of CuGaO2. As-prepared CuGaO2 film was homogeneously and fully covered on the top of perovskite layer as revealed by top-view SEM result. As consequence, the optimized CuGaO2-based device shows higher average PCE than spiro-MeOTAD-based device (Figure 5c). They thought that slightly improved Voc could originate from better energy level alignment which minimizes the hole-extraction barrier and enlarges the built-in potential across the device. The best-performing CuGaO2-based device delivered a PCE of 18.51% (Table 4), together with Jsc of 21.66 mA/cm2, Voc of 1.110 V, and FF of 0.77. In order to check the device stability, the authors directly exposed the unencapsulated devices to ambient condition with a relative humidity ranging between 30% and 55% at 25 °C. Figure 5d shows that the CuGaO2-based PSC is much more stable than the spiro-MeOTAD-based one. Compared with the previously discussed HTMs of CuI and CuxO in normal device configuration, much higher PCE was testified from CuGaO2 in normal PSC, which strongly indicates that CuGaO2 is potential candidate as an effective HTM for efficient and stable PSCs in future.


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Figure 5. (a) The cross-sectional SEM image of the PSC with CuGaO2 as HTM. (b) The energy band diagrams of the corresponding device composition. (c) Statistic PCE diagram of 50 cells based on CuGaO2 and spiro-MeOTAD HTMs. (d) PCE evolution depending on the ambient exposure time in the humidity range of 30%–55% at 25 °C. Reprinted with permission from ref. 44. Copyright 2017 John Wiley and Sons.

Table 4. Comparison of photovoltaic parameters of PSCs based on CuAlO2 and CuGaO2 inorganic HTMs. Reprinted with permission from ref. 44. HTM CuAlO2 CuGaO2

Device structure Inverted Normal

Perovskite MAPbI3-xClx MAPbI3-xClx

Jsc (mA/cm2) 18.58 21.66

Voc (V) 0.880 1.110

FF 0.62 0.77

PCE (%) 10.14 18.51

Year

Ref.

2016 2017

43 44

3.1.4. Copper Thiocyanate (CuSCN) CuSCN has been used as HTM for both normal and inverted PSCs employing various perovskite compositions, which is summarized and listed in Table 5.



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Table 5. Photovoltaic parameters of PSCs based on CuSCN inorganic HTM. The solvent used for dissolving CuSCN was adipropyl sulfide (DPS), bdiethyl sulfide (DES), cvarious mixed solvents, and daqueous ammonia. Deposition method of CuSCN Spin-coatinga Electrodeposition Spin-coatingd Electrodeposition Doctor bladea Doctor blade Drop-castinga Doctor bladec Doctor bladea Spray depositiona

Device structure Inverted Inverted Inverted Inverted Normal Normal Normal Normal Normal Normal

Spin-coatingb

Normal

Spin-coatingb

Normal

Spin-coatingb

Normal

Perovskite MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3-xClx MAPbI3 MAPbI3 MAPbI3 (FAPbI3)0.85 (MAPbBr3)0.15 Cs0.05(MA0.17 FA0.83)0.95 Pb(I0.83Br0.17)3 (FAPbI3)0.88 (CsPbBr3)0.12

Jsc (mA/cm2) 15.7 21.9 22.7 19.79 19.7 14.5 14.4 19.42 19.3 23.10

Voc (V) 1.06 1.00 1.10 0.90 1.016 0.63 0.727 0.92 0.835 1.013

0.65 0.76 0.71 0.64 0.62 0.53 0.62 0.56 0.60 0.73

PCE (%) 10.8 16.6 17.5 11.40 12.4 4.85 6.4 10.07 9.6 17.10

2015 2015 2017 2017 2014 2014 2014 2017 2017 2017

77 78 79 80 50 81 49 82 83 84

23.1

1.04

0.75

18.0

2016

85

23.24

1.112 0.78

20.4

2017

51

21.93

1.068 0.72 16.75 2018

86

FF

Year Ref.

CuSCN has two types of crystal phases: orthorhombic (α phase) and hexagonal or rhombohedral (β phase).87 Although some reports suggest that two phases simultaneously exist in polymorphic

systems, the films commonly behave as β phase.88 The p-type conductivity of CuSCN is endowed by copper vacancies in the crystal lattice induced by the excess of SCN-.87 Moreover, such a copper vacancy defect can broaden optical bandgap and accordingly contribute to optical transparency. CuSCN is famous for following advantages, such as wide bandgap of 3.9 eV (CBM of -1.4 eV and VBM of -5.3 eV as shown in Figure 2), high hole mobility of 0.01-0.1 cm2/Vs (Table 1), good optical transmittance and solution processibility, and good thermal stability.87 The aforementioned merits of CuSCN HTM render its versatility like the applications in thin-film transistor,89-90 quantum-dot-sensitized solar cell,91 organic photovoltaics,92 DSSC93 and PSC.50 In 2015, Zhao et al.77 reported the application of solution-processed CuSCN in inverted PSCs with MAPbI3 as light absorber and achieved a PCE of 10.8% with negligible hysteresis, 16 ACS Paragon Plus Environment

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which is higher than 9.7% of PEDOT:PSS-based control device. CuSCN film was prepared by spin-coating the solution of CuSCN in dipropylsulfide (DPS) and the CuSCN layer was found to provide template for growth of perovskite crystals. It is well known that it is important to deposit uniform and compact CuSCN films on conductive substrate for highly efficient inverted PSCs. In order to further improve the morphology of CuSCN film, Wijeyasinghe et al.79 developed a novel deposition method of CuSCN films with aqueous ammonia as solvent instead of conventional n-alkyl sulfide solvents. It was found that CuSCN film from aqueous ammonia showed better and uniform coverage than sulfide solvent (Figure 6a-h), which was ascribed to the complexation between CuSCN and NH3 as revealed by solution color change in Figure 6i. This morphology change led to increase in hole mobility. The hole mobility of the NH3(aq)processed CuSCN devices was estimated to be ~0.05 cm2/Vs, which was higher than that (0.01 cm2/Vs) of the DES (diethyl sulfide)-processed CuSCN device. Consequently, a much higher PCE of 17.5% was obtained from NH3(aq)-processed CuSCN film than that (10.2%) of the DESprocessed one, and also higher than the PCE of 13.6% for PEDOT:PSS counterpart (Figure 6j). In order to explore alternative deposition approaches to spin-coating of CuSCN films for the scale-up purpose, electrodeposition method was proposed.78 The authors compared the photovoltaic performance of devices based on MAPbI3 prepared by one-step or two-step method. It was found that the device based on one-step method exhibited much higher PCE (16.6%) in comparison with two-step method (13.4%). They proposed that higher quality perovskite films prepared through one-step fast deposition-crystallization method should be responsible for the improved PCE compared with sequential deposition approach. This also indicates that templating role of the CuSCN layer needs to be considered carefully for further PCE optimization of inverted PSCs. Actually, many researchers have highlighted the critical function of template in the process of nucleation and crystal growth of high-quality perovskite grains.15,

94-95

Electrodeposition method was employed by another group to deposit different CuSCN nanostructures (hexagonal prism-like (3D), pyramid-like (2D) and nanowire structures (NWs)), which were applied as HTMs in inverted PSCs.80 They found that the device based on 3D hexagonal prism-like gave the best photovoltaic performance as compared to the other morphologies. They thought that 3D hexagonal prism-like regulated well the morphology and the crystal orientation of perovskite and finally contributed to the formation of high-quality perovskite film as confirmed by XRD results. However, rather low PCE of 11.4% was achieved, 17 ACS Paragon Plus Environment


which was much lower than the same electrodeposition method showing 16.6%78 due to different perovskite deposition method. This implies that harmony between perovskite and inorganic HTM is important rather than focusing only selective contact materials.

Figure 6. AFM images of (a) glass substrate, (b) CuSCN/NH3(aq) spin-coated on glass at 2000 rpm from a 15 mg/mL solution, (c) CuSCN/NH3(aq) spin-coated on glass at 800 rpm from a 10 mg/mL solution, (d) CuSCN/DES spin-coated on glass at 800 rpm from a 10 mg/mL solution, (e) Glass/ITO surface, (f) PEDOT:PSS spin-coated on glass/ITO at 7000 rpm and annealed at 140 °C, (g) CuSCN/NH3 spin-cast on glass/ITO at 2000 rpm from a 15 mg/mL solution, and (h) CuSCN/DES (diethyl sulfide) spin-coated on glass/ITO at 2000 rpm from a 15 mg/mL solution. (i) Photographs of the CuSCN solutions prepared by NH3(aq) and DES solvents, respectively. The vials were placed behind 2 × 2 cm2 glass substrates coated with thin layers of CuSCN. (j) J– V curves of devices based on CuSCN/NH3(aq) and PEDOT:PSS HTMs measured under simulated one sun illumination. Reprinted with permission from ref. 79. Copyright 2017 John Wiley and Sons.


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Except for applications of CuSCN HTM in inverted PSCs, it was used in normal architecture PSCs. To the best of our knowledge, Ito et al. first reported on the CuSCN HTM in normal PSCs and demonstrated a PCE of 4.85%,81 which was further improved to 12.4% (Table 5) with doctor-blading-deposited thick CuSCN layer as displayed in Figure 7a.50 From the crosssectional SEM image in Figure 7b, we can see that the thickness of CuSCN layer is exceeding 400 nm, which could be too thick to improve further PCE. As presented in J-V curves in Figure 7c, remarkably improved PCE can be observed in PSCs with CuSCN HTM compared with without CuSCN HTM. Doctor blade deposition method was utilized by other groups to prepare CuSCN films in normal PSCs,82-83 which however did not improve PCE. These results suggest that doctor blade deposition method could be not appropriate for preparing thinner compact CuSCN film. In order to enrich deposition method of CuSCN films, drop casting was developed for normal PSCs but a PCE as low as 6.4% was achieved.49 While searching appropriate deposition methods, spray deposition method was proposed to deposit CuSCN films using a home-made deposition apparatus as illustrated in Figure 8a.84 It can be seen from top-view SEM images that homogeneous full coverage was realized. Compared with doctor-blade method, spray-deposited CuSCN showed much higher performance (Figure 8b). Besides, significantly improved long-term moisture stability was monitored in spray-deposition-based device compared with a control device with spiro-MeOTAD (Figure 8c). This indicates that spray deposition could be superior to doctor blade approach. However, since the absolute PCE is still lower than high efficiency organic HTM, further improvement in CuSCN is needed.

Figure 7. (a) Schematically illustrated device structure with CUSCN as HTM. (b) Crosssectional SEM image of complete device. The scale bar is 200 nm. (c) J-V curves of PSCs with



Page 20 of 64

(red) and without CuSCN (black) measured in the dark and under one sun illumination. Reprinted with permission from ref. 50. Copyright 2014 Nature publication group.

Figure 8. (a) Schematic diagram of home-made spray deposition apparatus for depositing CuSCN layer. (b) J-V curves of PSCs depending on deposition method for CuSCN along with spiro-MeOTAD with and without additive. (c) PCE as a function of aging time at indoor ambient atmosphere with relative humidity of 30%. Reprinted with permission from ref. 84. Copyright 2017 Elsevier.

Analyzed from photovoltaic performance in Table 5, most of reports still used pure MAPbI3 as light absorber although good stability and PCEs over 21% have been proved in mixed cation/anion composition systems in PSCs based on organic HTMs.16-17,

19

In other words,

progress obtained in organic HTMs is not well integrated into CuSCN-based PSCs. Regarding coating methods, spin-coating could be good choice for depositing compact and uniform thin CuSCN films in normal PSCs since CuSCN can be dissolved in DPS and DES solvents. Although CuSCN in inverted structure showed PCE as high as 17.5%,79 highest PCE can be achieved from normal structure (see Table 5). In order to improve the thermal stability of PSCs, Jung et al.85 incorporated classical perovskite composition (FAPbI3)0.85(MAPbBr3)0.15 instead of 20 ACS Paragon Plus Environment

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pure MAPbI3 into solution-processed CuSCN-based normal PSCs with TiO2 as ETM and obtained a PCE as high as 18.0% as listed in Table 5. As revealed by J-V curves and IPCE spectra in Figure 9a and b, slightly enhanced Jsc is found from CuSCN-based device as compared to spiro-MeOTAD counterpart. As indicated by time-resolved photoluminescence (TRPL) results in Figure 9c, improved hole extraction was thought to be main origin of increased Jsc. More importantly, the unencapsulated CuSCN-based device maintains about 60% of its initial PCE after annealing for 2 h at 125 oC in air under 40% average relative humidity, whereas merely 25% for the control device with spiro-MeOTAD, as displayed in Figure 9d. As we mentioned in introduction, employment of inorganic HTM is of the essence for long-term thermal stability of PSCs. Most recently, Arora et al51 reported a record PCE of 20.4% in normal PSCs (as shown in the inset in Figure 10a) based on solution-processed CuSCN film as HTM and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 as light absorber. In case of CuSCN-based device, it should be stressed that reduced graphene oxide (rGO) as a spacer layer was inserted between CuSCN and Au electrode to improve device stability. The stable output PCE was 20.5% for spiro-MeOTAD-based device and comparably 20.2% for CuSCN-based device as shown in Figure 10a. TRPL in Figure 10b confirmed more hole extraction from CuSCN/perovskite film than spiro-MeOTAD/perovskite reference film. They measured the thermal stability of CuSCNbased device coated with a thin layer of poly(methyl methacrylate) polymer at 85 °C in ambient conditions in the dark. It was found that over 85% of initial PCE was retained for the CuSCNbased device after 1000 h aging. Figure 10c compares the photostability of CuSCN- and spiroMeOTAD-based devices measured at maximum power point under continuous one sun at 60 °C. The CuSCN-based device maintained over 95% of initial efficiency after aging for 1000 h, which is much better than that of spiro-MeOTAD-based control device. The long-term operational stability achieved paves way for large scale commercial application of PSCs.



Figure 9. (a) J-V curves and (b) external quantum efficiency (EQE) spectra of the bestperforming PSCs with CuSCN (red) and spiro-MeOTAD (blue) HTMs. The scale bar is 500 nm. (c) Time-resolved PL spectra of ITO/CuSCN/perovskite film, ITO/spiro-MeOTAD/perovskite film, and HTM-free perovskite/ITO film. The excitation wavelength of 470 nm with a maximum average power of 5 mW was used for the PL measurements. (d) PCE evolution depending on aging time at 125 oC in air under 40% average relative humidity in the dark. Reprinted with permission from ref. 85. Copyright 2016 John Wiley and Sons.


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Figure 10. (a) Stabilized PCE output at the maximum power point (MPP) for spiro-MeOTAD and CuSCN based devices. The inset displays the cross-sectional SEM image of the complete device. (b) Time-resolved PL spectra of perovskite, perovskite/TiO2, perovskite/spiro-MeOTAD and perovskite/CuSCN films. (c) Operational stability of un-encapsulated CuSCN based device and un-encapsulated CuSCN based device containing a thin layer of rGO (as a spacer layer between CuSCN and gold layers), examined at a maximum power point under continuous fullsun illumination at 60°C in nitrogen atmosphere. Reprinted with permission from ref. 51. Copyright 2017 American Association for the Advancement of Science.

It is widely recognized that CuSCN-based device faces interface degradation problem.51, 96 In order to overcome the interface degradation issue, Chen et al. incorporated in-situ formed 2D perovskite (5-AVA)2PbI4 at interface between (FAPbI3)0.88(CsPbBr3)0.12 and the hole transporting CuSCN prepared by solution process.86 As revealed by energy levels in Figure 11a, incorporation of ultra-thin 2D can form type II heterojunction with (FAPbI3)0.88(CsPbBr3)0.12 layer, which is expected to lead to better hole extraction and suppression of back electron transfer. A thin layer 2D formed on the surface of (FAPbI3)0.88(CsPbBr3)0.12 after 5-AVAI treatment was investigated using plane-view SEM (Figure 11b and c), which was also



confirmed by X-ray photoelectron spectroscopy (XPS). Devices A, B, C and D were prepared for comparative study, where A: perovskite prepared from stoichiometric (FAPbI3)0.88(CsPbBr3)0.12 precursor solution, B: perovskite prepared from non-stoichiometric (FAPbI3)0.88(CsPbBr3)0.12 precursor solution with excess 8 mol% PbI2, C: post-treatment of A with 5-AVAI, and D: posttreatment of B with 5-AVAI. It was found that J-V property of device D was superior to other devices, especially much better performance as compared to device B (Figure 11d), resulting in the highest PCE, and moreover device B could not outperform device A. This suggests that improved PCE of device D is not ascribed to excess PbI2 but to the in-situ formation of 2D perovskite layer. Additionally, introduction of 2D passivating layer reduced J-V hysteresis, which is attributed to reduced defects or trap states on the surface of perovskite active layer as confirmed by steady-state PL. As a consequence of effective passivation of the defect and trap states on surface of perovskite (FAPbI3)0.88(CsPbBr3)0.12 and effective suppression of back electron transfer, PCE was increased from 13.72% (device A) to 16.75% (device D). Stability against moisture was improved by 2D passivation layer (Figure 12a-i), which indicates that 2D layer plays important role in protecting moisture intrusion. We also exposed four kinds of devices to dry air with a relative humidity of about 10% in the dark at room temperature as presented in Figure 12j. The unencapsulated device D was found to maintain 98% of its initial PCE after aging for 63 days. The recently reported above two works demonstrate the importance of interface engineering in CuSCN-based PSCs for the purpose of high PCE and excellent longterm stability. Consequently, development of novel interface engineering is highly desirable for further optimization of CuSCN-based PSCs.


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Figure 11. (a) The energy level diagram of device composition. Film B and D represent (FAPbI3)0.88(CsPbBr3)0.12 prepared from non-stoichiometric precursor solution with excess 8 mol% PbI2 and post-treatment of film B with 5-AVAI, respectively. Plane-view SEM images of (b) films B and (c) film D. (d) J-V curves of the best performing devices A, B, C and D measured in reverse and forward scan directions with a scan rate of 150 mV/s under AM 1.5G one sun illumination. RS and FS stand for reverse scan and forward scan, respectively. Reprinted with permission from ref. 86. Copyright 2018 John Wiley and Sons.



Figure 12. (a) Home-made moisture test chamber equipped with a hygrometer and a bottle with deionized water. Relative humidity (RH) of 85±10% was generated in a closed vessel. Optical images of the perovskite films (b) A, (c) B, (d) C and (e) D as a function of humidity exposure time. UV-vis absorption spectra of the aged films (f) A, (g) B, (h) C and (i) D. (j) Timedependent normalized PCE of the unencapsulated devices A, B, C and D under low relative humidity of about 10% in the dark. Reprinted with permission from ref. 86. Copyright 2018 John Wiley and Sons.

3.2. Nickel oxide (NiOx) In Table 6 and 7, photovoltaic data are listed for PSCs employing NiOx and doped NiOx as HTMs. 26 ACS Paragon Plus Environment

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Table 6. Photovoltaic parameters of representative PSCs based on NiOx HTM. aThe device was fabricated based on carbon electrode. bp-type perovskite sensitized solar cell was fabricated based on I3-/I- liquid electrolyte. cPSC was fabricated based on mesoporous NiOx as HTM and PC61BM as ETM. dPCBM/Ti(Nb)Ox bilayer was used as ETM. eThe active area for J-V measurement was 1.02 cm2. fZnO was employed as ETM. gAdditive engineering (ionic additive of MAAc and a molecular additive of TSC) was adopted. Jsc (mA/c m2) 18.2 21.36 21.62

Voc (V)

FF

PCE (%)

Year

Ref.

0.890 0.917 0.915

0.71 0.76 0.76

11.4 14.9 15.03

2015 2015 2015

97 98 99

19.49

0.881

0.53

9.11

2017

100

22.38 9.47 13.24

0.903 0.205 1.040

0.76 0.36 0.69

15.38 0.71 9.51

2017 2014 2014

101 102 103

MAPbI3

19.8

0.96

0.61

11.6

2014

104

Inverted

MAPbI3

15.17

1.10

0.59

9.84

2014

105

Inverted

MAPbI3 MAPbI3xClx MAPbI3

15.4

1.05

0.48

7.6

2014

106

14.9

0.936

0.75

7.25

2014

107

17.99

1.036

0.72

13.49

2014

108

1.072

0.75

16.2

e

2015

52

21.0 19.01 22.23 20.2

1.01 1.11 1.05 1.06

0.76 0.73 0.76 0.81

16.1 15.40 17.74 17.3

2015 2014 2015 2015

109 110 111 112

21.4

1.03

0.78

17.2

2017

113

21.77

1.120

0.79

19.35

2017

114

22.68

1.12

0.77

19.58

2017

115

21.79

1.12

0.74

18.0

2017

116

HTM

Deposition method of NiOx

Device structure

mp-NiOx mp-NiOx mp-NiOx

Screen print Doctor blade Screen print

Normala Normala Normala

NiOx

Spin-coating

Normal

NiOx mp-NiOx mp-NiOx

Drop-casting Screen print Spin-coating Magnetron sputter Magnetron sputter Spin-coating Electrodepositi on Spray pyrolysis

Normal Invertedb Invertedc

MAPbI3 MAPbI3 MAPbI3 MAPbI3xClx MAPbI3 MAPbI3 MAPbI3

Inverted

Spray pyrolysis

d

MAPbI3

20.62

f

NiOx NiOx NiOx NiOx NiOx Li0.05Mg0.15 Ni0.8O NiOx Cu:NiOx Cu:NiOx NiOx

Inverted Inverted Inverted

Perovskite

Spin-coating Spin-coating Combustion Pulsed laser

Inverted Inverted Inverted Inverted

Inverted MAPbI3 MAPbI3 MAPbI3 Cs0.05(MA0.

Cs:NiOx

Spin-coating

Inverted

FA0.83)0.95P b(I0.83Br0.17)

Cs:NiOx Li0.05Mg0.15 Ni0.8O

Spin-coating

Inverted

Spray pyrolysis

Inverted

MAPbI3

Li:NiOx

Spin-coating

Inverted

MAPbI3xClx

17

3

MAPbI3 g

Table 7. Photovoltaic parameters of representative PSCs based on nickel oxide (NiOx) HTM. The active area for J-V measurement was 1.084 cm2.

a



HTM NiOx NiOx Cu:NiOx NiOx NiOx Cu:NiOx Li0.05Mg0. 15Ni0.8O NiOx NiOx

Deposition method of NiOx Spin-coating Vacuum deposition Spin-coating Spin-coating Electrodeposition Spin-coating

Device structure Inverted Inverted Inverted Inverted Inverted Inverted

Spray pyrolysis

Page 28 of 64

MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3-xClx

Jsc (mA/cm2) 22.17 18.6 20.76 21.22 22.6 -

Voc (V) 1.079 1.06 1.11 1.04 1.05 -

0.78 0.78 0.81 0.75 0.72 -

Inverted

FA1-xMAxPbI3

23.09

Spin-coating

Inverted

Spin-coating

Inverted

MAPbI3 Csy(FA1xMAx)1yPbBrzI3-z

Perovskite

FF

PCE (%) 18.69 15.4 18.66 16.55 17.1a 20.5

Year

Ref.

2017 2017 2017 2017 2017 2017

117 118 119 120 121 40

1.10

0.81 20.65 2017

39

21.25

1.09

0.79 18.21 2018

122

22.8

1.09

0.75

123

18.6

2017

NiOx was extensively applied in organic solar cells, DSSCs and other optoelectronic devices.124-130 because of high hole mobility, high optical transmittance and good thermal stability. Encouraged by p-type DSSCs, Wang et al.102 reported mesoporous NiOx-based p-type perovskite-sensitized solar cells with I3-/I- as liquid electrolyte and achieved a PCE of 0.71%. Another group103 also developed mesoporous NiOx as photoanode in p-type solid-state perovskite-sensitized solar cells using PC61BM ETM as alternative to I3-/I- electrolyte and a PCE of 9.51% was obtained under AM 1.5 G illumination. This indicates I3-/I- liquid electrolyte might not be good for electron collection due to mismatch of energy level. Liu et al100 demonstrated the application of as-synthesized NiOx nanoparticle as HTM in normal PSCs combined with MAPbI3-xClx, yielding a PCE of 9.11%. Yang et al.101 also reported the use of as-synthesized NiOx nanoparticle as HTM in MAPbI3-based normal PSCs with multi-walled carbon nanotube ETM and achieved a PCE of 15.38%. Compared with conventional normal PSCs with Au or Ag electrode, much higher PCEs have been obtained in fully printable mesoporous PSCs with carbon counter electrode through replacement of ZrO2 insulating layer with NiOx layer97 or inserting NiOx layer between ZrO2 or Al2O3 layer and carbon layer.98-99 Since spin-coating has been frequently adopted for depositing NiOx films, attentions have been paid to the optimization of NiOx-based inverted PSCs through spin-coating method.106,

117, 120, 123

Except for screen

printing and spin-coating, magnetron sputter was also utilized to deposit NiOx film.104-105 In addition, electrodeposition or vacuum-deposition was proposed as alternative method to spincoating.107, 118, 121 By learning from the deposition methods (electro-deposition, sol–gel process, 28 ACS Paragon Plus Environment

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sputtering and pulsed laser deposition) of NiOx film in OPV and DSSCs, Park et al.112 fabricated NiOx film using pulsed laser deposited technique in inverted PSCs, which showed a PCE of 17.3% along with a high FF up to 0.81 after optimization of oxygen partial pressure and NiOx film thickness. Authors explained that high PCE and FF were originated from the uniform and dense NiOx film with (111)-orientation and good optical transparency as revealed by SEM, XRD and transmittance spectra, leading to improvement of hole extraction and inhibition of recombination. Although sol-gel method has been widely adopted for depositing NiOx film in PSCs, it requires high temperature for crystallization and remove of organic impurities since sol-gel reaction is endothermic. In contrast, combustion deposition method can avoid high temperature process due to unique self-energy generation and exothermic reaction, which is extensively used for preparing transition metal oxide in thin-film transistors.131 Jung et al.111 developed lowtemperature solution-processable combustion deposition method for preparing high-quality Cu:NiOx film as an efficient HTM in inverted PSCs, which demonstrated a PCE of 17.74%, outperforming 15.52% of high-temperature-processed Cu:NiOx-based control device. Based on this low-temperature solution-processable combustion deposition method, the ITO-free PSCs showed a PCE of 13.42%. Besides, spray pyrolysis is well known method for preparing metal oxide films which has been intensively explored in deposition of TiO2 blocking layer in DSSCs.3 Chen et al.108 used spray pyrolysis to prepare NiOx film for inverted PSCs employing MAPbI3 and achieved a PCE of 13.49%, where mesoporous Al2O3 scaffold was incorporated as interfacial layer. Although various deposition methods have been developed for NiOx films, spincoating and spray pyrolysis methods showed relatively high PCEs over 19%, which was compared with other methods in Table 6 and 7. Crystal structure and chemical composition were found to have influence on optical and electrical properties of NiOx films. Furthermore, low Jsc and FF of devices based on pristine NiOx HTM were interpreted mainly due to its low conductivity. In this regard, doping to NiOx is expected to increase conductivity. Kim et al.110 significantly improved the conductivity of pristine NiOx film via Cu doping as confirmed by conductive atomic force microscopy (c-AFM) and J-V measurements (2.2 × 10−6 S/cm for pristine NiOx and 8.4 × 10−4 S/cm for Cu-doped NiOx) and employed it as HTM in inverted PSCs, delivering an substantially enhanced PCE of 15.4% as compared to 8.94% of pristine-NiOx-based control device. Similar with previously 29 ACS Paragon Plus Environment


discussed CuSCN-based devices, significantly improved ambient moisture stability was realized in Cu:NiOx-based device compared with PEDOT:PSS-based device. The latter often shows poor moisture stability because of acidic and hygroscopic properties of PEDOT:PSS. Subsequently, the same group111 reported the utilization of Cu doped NiOx HTM prepared by low temperature solution-processable combustion method in inverted PSCs and demonstrated a PCE of 17.74%, which is much higher than 15.52% of sol-gel-processed Cu:NiOx-based counterpart. He et al119 also incorporated Cu into NiOx in inverted PSCs and realized a PCE of 18.66% at the area of 0.10 cm2. Moreover, the flexible devices achieved relatively high PCE of 17.16% at small area of 0.10 cm2 and 15.42% at large area of 1.08 cm2. Yue et al.40 systematically engineered HTM layer, perovskite layer and interface between PC61BM and Al electrode (Figure 13a), where Cu doping strategy was adopted in the NiOx HTM layer based on the previous reports,110-111, 119 Clcontaining perovskite composition was employed to prolong the carrier lifetimes and diffusion length, and finally, zirconium acetylacetonate (ZrAcAc) was introduced to modify Al electrode for improving electron extraction and transport. After the optimization of ZrAcAc concentrations (Figure 13b) and doping concentrations of Cu in NiOx film (Figure 13c), device performance depending on different conductive substrates (FTO and ITO) was further compared and it was found that FTO-based device displayed a much higher PCE than ITO-based device. The impedance results show that FTO-based device showed a significantly reduced charge transport resistance in comparison with ITO-based one. It suggests importance of systematic optimization of the whole device (including each layer and corresponding interfaces).


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Figure 13. (a) The schematic illustration of PSC configuration based on Cu:NiOx HTM and interface engineering. (b) The J-V curves of devices based on different concentrations of ZrAcac solutions measured under AM 1.5G one sun illumination. (c) The J-V curves of devices based on NiOx film with different doping concentrations of Cu. Reprinted with permission from ref. 40. 2017 Royal Society of Chemistry.

Except for Cu element doping, other elements are encouraged to dope pristine NiOx film for conductivity enhancement. Alkali metal Li was used to dope pristine NiOx film in inverted PSCs and Voc of 1.12 V was obtained due to better energy band alignment at perovskite/NiOx interface.116 Additionally, Chen et al.114 demonstrated the application of Cs-doped NiOx as HTM in inverted PSCs (Figure 14a) showing a promising PCE of 19.35%, outperforming the 16.04% 31 ACS Paragon Plus Environment


of pristine NiOx-based control device as shown in Figure 14b. The improved PCE was attributed to enhanced hole extraction associated with band alignment as revealed by PL study. As illustrated in Figure 14c, Cs:NiOx-based device maintained 98% of the initial efficiency after aging for 70 days in inert environment, which was due to chemical stability of inorganic Cs:NiOx and stable cathode interfacial layer. Kim et al.113 also reported the use of Cs:NiOx as HTM in inverted PSCs, where state-of-the-art triple-cation perovskite was employed to deliver a PCE of 17.2%.

Figure 14. (a) The schematic device architecture of the inverted planar PSCs based on pristine NiOx or Cs:NiOx as HTMs. (b) The J–V characteristics of inverted PSCs with NiOx and Cs:NiOx measured in forward ( −0.2 to 1.2 V) and reverse scan (1.2 to −0.2 V) with a scan rate of 125 mV/s. (c) J–V curves of aged Cs:NiOx-based device for 70 days in inert environment. All devices were encapsulated with cover glass where the edge areas were sealed with epoxy. The J-V measurements were conducted outside glove box while the devices were stored in glove box before and after measurement. Reprinted with permission from ref. 114. Copyright 2017 John Wiley and Sons.

In the past several years, investigation on doping of pristine NiOx focused on not only one of metals such as Cu, Li or Cs but also mixed metals like Li and Mg. Chen et al.52 co-doped Li and Mg in NiOx (denoted as Li0.05Mg0.15Ni0.8O) as HTM that was applied to inverted PSC employing Ti(Nb)Ox/PCBM bilayer as ETM and MAPbI3 perovskite as illustrated in Figure 15a, which demonstrated a PCE of 16.2% at area of 1.02 cm2 (Figure 15b). Improved charge extraction was responsible for the high PCE. Most importantly, combination of inorganic HTM (Li0.05Mg0.15Ni0.8O) and ETM interfacial interlayer (Ti(Nb)Ox) enables the realization of longterm photostability (The device retained over 90% of its initial PCE after 1000 hours light 32 ACS Paragon Plus Environment

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soaking) as displayed in Figure 15c. This work indicates that simultaneous employment of inorganic HTM and ETM is important for solving the long-term stability while maintaining high PCE. Replacement of organic cation with inorganic one in perovskite layer can be also a way toward stable PSCs.132-133 Subsequently, the same group134 incorporated nanostructured carbon layer at PCBM/Ag interface to suppress the diffusion of ions/molecules in inverted Li0.05Mg0.15Ni0.8O-based PSCs and found that excellent photo and thermal stability can be attained for N-doped graphene and carbon quantum dots-incorporated device after aging test at 85 oC for over 500 h or light soaking for 1000 h. Additionally, additive engineering strategy was also developed to further improve thermal stability of Li0.05Mg0.15Ni0.8O-based inverted PSCs with MAPbI3, accompanied by a high PCE of 19.19% at an aperture area of 1.025 cm2.115 Over 80% of the initial PCE after 500 h of thermal aging at 85 °C can be maintained, which are among the best results of MAPbI3-based PSCs. Most recently, Xie et al.39 developed a perovskite grain growth method namely ‘‘vertical recrystallization” to prepare FA-based perovskite film (Figure 16a). As shown in Figure 16b, highly crystalline FA-based perovskite film was obtained through this vertical recrystallization approach. Furthermore, vertically oriented perovskite grain was observed from cross-sectional SEM image in Figure 16c, which minimizes grain boundary and trap site in the films. From the light intensity dependent voltage, ideality factor was estimated to be 1.26 that was smaller than 1.85 for the non-vertically grown FAPbI3. Since the ideal diode with ideality factor = 1 is expected not to have trap states, the smaller ideality factor approaching unit indicates that free carrier recombination (band-to-band recombination) dominates. Increase in FF and Voc after vertical growth is thus related to ideality factor and reduced trap-assisted recombination. The optimized device based on Li0.05Mg0.15Ni0.8O as HTM produced a PCE as high as 20.65% (Figure 16d), which is the highest value among NiOx-based inverted PSCs. Finally, it was found that vertically grown perovskite exhibited much better stability than pristine FAPbI3 (Figure 16e).



Figure 15. (a) Schematically illustrated inverted PSC structure highlighting the doped charge carrier extraction layers. The right insets shows the composition of Ti(Nb)Ox and the crystal structure of lithium doped NixMg1–xO, denoted as NiMg(Li)O. (b) The J–V curve of the best large cell endowed with anti–reflection film. (c) PCE evolution of encapsulated devices kept in the dark or under simulated solar light (AM 1.5, 100 mW/cm2, using a 420 nm UV light cut–off filter). Reprinted with permission from ref. 52. Copyright 2015 American Association for the Advancement of Science.


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Figure 16. (a) Schematically illustrated vertical recrystallization method for the formation of FA1-xMAxPbI3 film. (b) XRD pattern and (c) cross-sectional SEM image of FAPbI3/MACl recrystallization film, annealed at 100 oC for 10 min and then 140 oC for 20 min. (d) J–V curves of perovskite devices using FAPbI3 with 0, 1, 5 and 10 mg/ml MACl treatment. (e) Stability data of pristine FAPbI3-based PSC and FAPbI3/MACl (5 mg/ml)-based PSC, aged under continuous AM 1.5 light soaking (square dot lines, at maximum power point, temperature of ～25 oC), or aged at 85 oC (triangle dot lines, at dark). Reprinted with permission from ref. 39. 2017 Royal Society of Chemistry.

3.3. Carbon-based HTMs In Table 8, photovoltaic performance of PSCs based on carbon-based HTM is listed. Table 8. Photovoltaic parameters of PSCs based on carbon-based HTMs. Device structure FTO/bl-TiO2/mpTiO2/MAPbI3/nanographene/Au ITO/GO/MAPbI3/C60/BCP/Au

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Year

Ref.

20.56

0.95

0.66

12.81

2015

135

21.6

1.00

0.76

16.5

2017

136



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Since calcium carbonate or graphite can be classified into inorganic carbon, we like to describe carbon-based HTM as part of inorganic HTM in this section. Carbon-based materials, including porous carbon,137 graphene,138 graphdiyne,139 graphene oxide,140 single-walled carbon nanotube141 and multi-walled carbon nanotube142, are widely applied into optoelectronic devices due to high conductivity and hole mobility. Cao et al.135 demonstrated well-defined thiolated nanographene as HTM in normal PSCs, affording a PCE of 12.81% (Table 8). The device with thiolated nanographene showed improved moisture stability as well, which is due to hydrophobic nature of nanographene. By contrast, Yang et al.136 reported the use of graphene oxide as HTM in inverted PSCs with the device structure of ITO/GO/MAPbI3/C60/BCP/Au, which delivered a PCE of 16.5% (Table 8). These results suggest that carbon-based HTM materials can be alternatives to organic HTM.

3.4. Co3O4, VOx, WO3 and MoOx : Transition metal oxide HTMs Table 9 summarizes photovoltaic performance of PSCs with transition metal oxide HTMs, including cobalt, vanadium, tungsten and molybdenum transition metals.

Table 9. Photovoltaic parameters of PSCs based on Co3O4, V2O5, WOx and MoOx HTMs. HTM

Device structure

Co3O4

Regular

VOx WOx MoOx

Inverted Inverted Inverted

Perovskite (5-AVA)x MA1-xPbI3 MAPbI3 MAPbI3 MAPbI3

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Year Ref.

23.43

0.88

0.64

13.27

2017

37

19.42 16.6 18.8

0.96 0.93 0.99

0.75 0.64 0.71

14.04 9.8 13.1

2017 2016 2016

143 36 36

Transition metal oxides like Co3O4, V2O5, WOx and MoOx have been employed as HTM in optoelectronic devices because of their high hole mobility, high optical transmittance and chemical stability.144-145 Bashir et al.37 synthesized spinel Co3O4 nanomaterials for use in carbon36 ACS Paragon Plus Environment

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based printed PSCs and found that incorporation of Co3O4 as HTM between ZrO2 layer and carbon layer improved not only the PCE but also stability. The Co3O4 interlayer was found to play important role in suppressing charge recombination and extracting hole efficiently, while photovoltaic performance of carbon-based monolithic device without Co3O4 was restricted by the charge carrier transport and recombination processes at the carbon-perovskite interface. For the case of VOx, since VBM of VOx (Figure 2) is higher than that of MAPbI3, Voc is expected to be declined when assuming that Voc is determined by difference between EFn (Fermi level of ntype) of MAPbI3 and EFp (Fermi level of p type) of VOx. Yao et al.143 decreased this gap by lowering the VBM of VOx through modifying its surface with aminopropanoic acid (APPA). As a consequence, the device with VOx/APPA gave a higher PCE (14.04%) than that with only VOx (11.69%). Transient photovoltage measurement revealed that APPA treated VOx showed longer lifetime for charge recombination than VOx without treatment. WOx and MoOx, used for OPV, were applied to PSC. Thermally evaporated WOx and MoOx thin layers were used as HTMs in inverted structure, which led to PCE of 13.1% for MoOx and 9.8% for WOx. The higher PCE was related to better hole extraction capability of MoOx.36 When comparing a conventional PEDOT:PSS HTM, MoOx-based device exhibited better stability. Since the studied transition metal oxide HTMs were mostly prepared by vacuum deposition techniques, solution-process for oxide thin films will be one of challenging topic.

3.5. Hybrid HTMs Various hybrid HTMs have been studied for improving photovoltaic performance of PSCs, which is summarized in Table 10.

Table 10. Photovoltaic parameters of PSCs based on organic/inorganic or inorganic/inorganic hybrid HTMs. acomposite HTMs and bbilayer HTMs. SWCNTs stands for single-walled carbon nanotubes. MWCNT stands for multi-walled carbon nanotubes. GO stands for graphene oxide. NiPc represents nickel phthalocyanine. PhNa-1T and FBT-Th4 are organic HTMs. HTM SWCNTs/P3HTa

Device structure Normal

Perovskite MAPbI3-xClx

Jsc (mA/cm2) 22.71

Voc (V)

FF

1.02

0.66


PCE (%) 15.3

Year

Ref.

2014

146


CuSCN/spiroMeOTADa SWCNTs/spiroMeOTADa CuAlO2/PEDOT:P SSb SWCNTs/PEDOT: PSSb V2O5/NiPcb

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Normal

MAPbI3-xClx

22.01

1.06

0.77

18.02

2016

147

Normal

(FAPbI3)0.85 (MAPbBr3)0.15

-

-

-

15.5

2016

33

Inverted

MAPbI3-xClx

21.98

0.88

0.75

14.52

2016

43

Inverted

MAPbI3

17.98

0.99

0.70

12.4

2017

148

Normal

(FAPbI3)0.85 (MAPbBr3)0.15

23.1

1.080

0.73

18.3

2017

34

Inverted

MAPbI3-xClx

22.41

0.94

0.75

15.86

2017

35

Inverted Inverted

MAPbI3 MAPbI3-xClx (FAPbI3)1-x (MAPbBr3)x

20.3 -

1.09 -

0.78 -

17.2 16.89

2017 2018

149 150

22.68

1.08

0.70

17.2

2017

151

V2O5/ PEDOT:PSSb r-GO/PTAAb PTAA/MoS2b NiOx/spiroMeOTADb Cu:NiOx/PhNa1Tb FBT-Th4/CuxOb SWCNTs/GOb NiOx/GOb

Inverted

MAPbI3

21.4

1.03

0.77

17.0

2018

152

Normal Normal Inverted

21.77 17.7 18.6

1.11 0.97 0.97

0.73 0.60 0.62

17.74 10.4 11.2

2018 2016 2018

153 32 154

NiOx/CuSCNb

Normal

21.04

1.10

0.65

15.03

2017

151

bl-NiOx/mp-NiOxb NiOx/MWCNTsb

Inverted Normal

MAPbI3 MAPbI3 MAPbI3-xClx (FAPbI3)1-x (MAPbBr3)x MAPbI3 MAPbI3

21.66 22.38

1.11 0.903

0.82 0.76

19.79 15.38

2017 2017

155 101

Normal

The commonly used organic HTMs usually contain additives (t-BP and LiTFSI) to increase mobility. However, use of hygroscopic additives is at the cost of sacrificing stability. In order to improve stability of organic HTMs, design of organic and inorganic composite HTMs is one of methods to solve instability of organic HTM. Based on this concept, Habisreutinger et al.146 developed polymer-functionalized single-walled carbon nanotube (SWNTs)/P3HT composite HTM and applied it in normal PSCs. The optimized device delivered a PCE of 15.3% along with improved thermal and moisture stability as compared to organic-HTM-based devices (spiroMeOTAD, P3HT and PTAA). Besides, SWCNTs was mixed with spiro-MeOTAD and applied to (FAPbI3)0.85(MAPbBr3)0.15 based PSC as HTM, which yielded a PCE of 15.5%.33 SWCNT showed dual function of hole transport and collection. Except for SWCNTs, CuSCN and CuI were also utilized to form composite HTMs with spiro-MeOTAD (Figure 17a). Higher PCEs of 18.02% for CuSCN/spiro-MeOTAD and 16.67% for CuI/spiro-MeOTAD were achieved than the spiro-MeOTAD only (14.82%) (Figure 17b). The improvement of PCE was attributed to improved conductivity and mobility by introduction of inorganic HTM in spiro-MeOTAD.147 38 ACS Paragon Plus Environment

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Moreover, moisture stability was better in composite HTMs-based devices compared with spiroMeOTAD-based device as shown in Figure 17c. Therefore, organic/inorganic composite HTMs are a promising strategy for the achievement of efficient and stable PSCs. However, further optimization is needed for obtaining comparable PCE to that of organic HTM.

Figure 17. (a) Schematically illustrated device configuration of planar PSC along with structures of spiro-MeOTAD, CuSCN and CuI. (b) J–V curves of the PSCs using spiro-MeOTAD, spiroMeOTAD:CuSCN (33 mol%), and spiro-MeOTAD:CuI (32 mol%) measured under simulated AM 1.5 one sun light intensity. (c) PCE depending on aging time in ambient condition. Reprinted with permission from ref. 147. Copyright 2016 John Wiley and Sons.

In order to further improve the stability of PSCs, organic-inorganic or inorganic-inorganic bilayer HTMs was designed as alternative approach to composite HTMs because moisture intrusion into organic HTM might be protected by inorganic HTM layer and/or rational band alignment between two kinds of HTMs is beneficial for improving hole extraction and reducing recombination. As we mentioned previously, PEDOT:PSS-based device is unstable due to 39 ACS Paragon Plus Environment


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hygroscopic and acidic properties. So bilayer HTM methods have been adopted to address the above concern. Yoon et al.148 designed SWCNTs/PEDOT:PSS hybrid HTM in inverted PSCs and demonstrated a slightly improved PCE (12.4%) in comparison with PEDOT:PSS only device (12.0%), which is due to better energy level alignment. Igbari et al.43 demonstrated improved stability and performance from CuAlO2/PEDOT:PSS bilayer HTMs in inverted PSCs where CuAlO2 film was prepared by magnetron sputtering. Introduction of VOx/PEDOT:PSS bilayer HTMs in inverted PSCs showed a PCE of 15.86% that was higher than the PEDOT:PSS reference (13.56%).35 Cheng et al.34 demonstrated an organic-inorganic bilayer HTMs composed of the solution-processed nickel phthalocyanine (NiPc), abbreviated as NiPc-(OBu)8, and vanadium(V) oxide (V2O5) in normal mesoporous PSCs employing (FAPbI3)0.85(MAPbBr3)0.15 as light absorber. Figure 18a shows that type II band alignment is formed between NiPc-(OBu)8 and perovskite or between NiPc-(OBu)8 and V2O5, which is appropriate for hole extraction and suppressing back electron transfer. The PCE of dopant-free NiPc-(OBu)8/V2O5-based device (18.3%) was higher than that of doped NiPc-(OBu)8-based device (17.9%) whereas a little lower than that of doped spiro-MeOTAD-based device (19.4%) as revealed by J-V curves in Figure 18b and c. The stability of unencapsulated PSCs under ambient conditions in the dark was evaluated as presented in Figure 18d. 75% of initial PCE was retained for the device based on dopant-free NiPc-(OBu)8/V2O5 bilayer HTMs, whereas significant deterioration in PCE by more than 50% was observed for doped spiro-MeOTAD-based device. So far, HTM for the highest PCE in PSCs was polytriarylamine (PTAA) organic polymeric HTM.17 Nevertheless, the device with PTAA still suffers from poor moisture stability because of hygroscopic properties of additives in PTAA. Although the highest PCE of more than 22% was achieved using normal mesoscopic structure with PTAA, in recent two years, PTAA was intensively investigated in planar inverted PSCs except for in normal mesoporous architecture and impressive PCE has been demonstrated.156 Zhou et al.149 engineered a reduced graphene oxide (r-GO)/PTAA integrated bilayer

HTMs

and

demonstrated

a

PCE

of

17.2%

based

on

ITO/r-GO/PTAA

MAPbI3/PCBM/bathocuproine (BCP)/Ag. Furthermore, the PSC based the bilayer HTMs retained about 90% of its original PCE after continuous illumination for 500 h at one sun illumination. MoS2 was combined with PTAA to form bilayer HTMs in a device configuration of ITO/PTAA/MoS2/MAPbI3/PCBM/PFN/Al, which led to a PCE of 16.89% slightly higher than 16.25% of pristine PTAA-based control device and improved stability of keeping 80% of initial 40 ACS Paragon Plus Environment

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PCE after about 560 h.150 Cao et al.151 compared the photovoltaic performance of NiOx, NiOx/CuSCN, and NiOx/spiro-MeOTAD HTMs in normal device layout and found that the device based on NiOx/spiro-MeOTAD bilayer HTMs showed the highest PCE, while the best moisture and thermal stability was realized in NiOx/CuSCN HTM. A PCE of 17.0% was reported from Cu:NiOx/PhNa-1T bilayer HTMs in inverted structure, which was much higher than that (13.5%) of inorganic HTM only device.152 Thermally evaporated CuxO was combined with FBTTh4, which was applied to SnO2-based normal PSCs showing a PCE of 18.85% with negligible hysteresis.153 Moreover, the studied bilayer HTMs realized improved long-term stability of PSCs under high humidity of about 75%. Although relatively high PCE and good stability have been achieved with organic-inorganic bilayer HTMs, the achievement of competitive PCE with those of organic HTMs like PTAA and spiro-MeOTAD remains to be further optimized through systematically considering each layer and respective interfaces in the whole device.

Figure 18. (a) Energy levels in PSC with NiPc-(OBu)8/V2O5 hybrid HTM. J–V curves of the PSCs based on (b) spiro-MeOTAD doped with LiTFSI, t-BP, and FK209 and (c) dopant-free NiPc-(OBu)8/V2O5 HTM. (d) Comparison of stability of unencapsulated PSCs stored under dark 41 ACS Paragon Plus Environment


ambient conditions (humidity about 40%–45% and temperature of 20–25 °C). Reprinted with permission from ref. 34. Copyright 2017 John Wiley and Sons.

Regarding inorganic-inorganic bilayer HTM, higher PCE than single inorganic HTM is expected because of minimizing recombination resulting from better energy band alignment. On the basis of above considerations, considerable attentions have been focused on the inorganicinorganic bilayer HTMs engineering. Wang et al.32 reported SWCNT/GO/PMMA bilayer HTM that was included in normal mesoporous PSCs and PCE was measured to be 13.3%, where the bilayer HTM showed better stability as than spiro-MeOTAD. NiO/GO bilayer HTMs was developed for inverted PSCs and found to deliver a PCE of 11.2%.154 NiO was also tried to combine with MWCNTs to form bilayer HTM, which exhibited higher PCE of 15.38%101 A hysteresis-free inverted mesoporous PSC was fabricated based on integrated Cu:NiO blocking layer and Cu:NiO mesoporous layer (Figure 19a), so-called mesoscopic inverted PSC.155 Among the tested samples (cell A: bl-NiOx, cell B: bl-Cu:NiOx, cell C: bl-NiOx/mp-NiOx, cell D: bl-NiOx/mp-Cu:NiOx, and cell E: bl-Cu:NiOx/mp-Cu:NiOx), the bilayer HTM composed of blCu:NiOx/mp-Cu:NiOx yielded the highest PCE as shown in Figure 19b. The results indicate that both the Cu doping in NiOx and the bilayer integration of bl-Cu:NiOx with mp-Cu:NiOx play crucial role in enhancing photovoltaic performance due to better hole extraction as confirmed by time-resolved PL measurement in Figure 19c. In addition, moisture- and photo-stability was improved by the bilayer structure in cell E as compared to device without mp-NiOx (cell A) (Figure 19d), which is indicative of importance of mesoporous layer in terms of stability.


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Figure 19. (a) Device architecture of the inverted mesoporous device containing mp-NiOx. (b) JV curves for the devices fabricated from various HTM compositions. Cell A: bl-NiOx, Cell B: blCu:NiOx, Cell C: bl-NiOx/mp-NiOx, Cell D: bl-NiOx/mp-Cu:NiOx, and Cell E: bl-Cu:NiOx/mpCu:NiOx. (c) Time-resolved PL of perovskite films deposited on various HTM compositions. The solid lines were obtained by fitting the data using a bi-exponential equation. (d) The stability of the cells without sealing, based on planar and mesoporous structures in the dark or under constant one sun illumination without UV-filter. The cells were kept in a dry cabinet (< 30% humidity) in the dark and measured in ambient air. Reprinted with permission from ref. 155. Copyright 2017 Elsevier.

3.6. Perovskite/Inorganic HTM Composite Stability of PSCs is related with not only selective contact materials but also perovskite composition and morphology. Most of high-efficiency perovskite compositions are sensitive to moisture and degraded somewhat rapidly. To overcome such instability, inorganic HTM is proposed to be incorporated in bulk perovskite layer to improve device stability. Wang et al.157 reported on incorporation of NiO in perovskite MAPbI3−xClx (Figure 20a), where NiO HTM and TiO2 ETM was separated by Al2O3 in order to facilitate charge separation. The best PCE of 18.14% (Figure 20b and Table 11) was achieved as well as negligible hysteresis (Figure 20c). 43 ACS Paragon Plus Environment


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Moisture stability was improved as compared to pristine perovskite without mp-NiO (Figure 20d). Additionally, another interesting work was reported regarding incorporation of Cu(thiourea)I (Cu(Tu)I) into MAPbI3−xClx, which was demonstrated in inverted PSCs, delivering a PCE of 20.0% (Figure 21a) without almost no hysteresis.158 PL data showed that carrier lifetimes was enhanced upon introduction of Cu(Tu)I into perovskite layer as displayed in Figure 21b, which was consistent with increased recombination resistance (Figure 21c). As shown in possible mechanism for the trap state passivation in Figure 21d, the authors proposed that Cu(Tu)I can effectively passivate the trap states of perovskite via interacting with the undercoordinated metal cations and halide anions at the perovskite crystal surface.

Table 11. Photovoltaic parameters of PSCs based on inorganic HTM/perovskite composite. HTM/Perovskite NiOx-MAPbI3-xClx Cu(Tu)I-MAPbI3-xClx

Device structure Normal Inverted

Jsc (mA/cm2) 23.02 22.3

Voc (V)

FF

PCE (%)

Year

Ref.

1.01 1.12

0.78 0.80

18.14 20.0

2017 2017

157 158


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Figure 20. (a) Schematic of device configuration of FTO/c-TiO2/mpTiO2/Al2O3/NiO/MAPbI3−xClx-NiO/Spiro-MeOTAD/Au, along with transporting paths of charge carriers. (b) J-V curve of the champion device under one sun illumination. (c) Scan direction dependent J-V curves. (d) Stability comparison between MAPbI3−xClx (black) and MAPbI3−xClxNiO (blue). Data were obtained from the devices stored an ambient environment (45–55% humidity, T = 25 oC). Reprinted with permission from ref. 157. Copyright 2017 Elsevier.

Figure 21. (a) J−V curves of the device with Cu(Tu)I incorporated in perovskite layer. (b) PL decay spectra of the pristine MAPbI3−xClx film and the perovskite−Cu(Tu)I hybrid film deposited on glass substrates prepared from the mixed precursor solution with CuI, Cu(Tu)I, or Cu(Tu)Cl. (c) Resistance for recombination (Rrec) depending on Cu(Tu)I additives. (d) Schematic illustration of possible mechanism for the trap state passivation. Reprinted with permission from ref. 158. Reprinted with permission from ref. 158. Copyright 2017 American Chemical Society.

4. SUMMARY AND OUTLOOK In this review, we have investigated in detail the progress of inorganic HTM-based PSCs and discussed the effect of inorganic HTM on PCE and stability. So far, a variety of inorganic HTMs have been incorporated in both normal and inverted PSCs, such as CuI, CuS, Cu2ZnSnS4, Cu2O, CuO, CuAlO2, CuCrO2, CuGaO2, CuSCN, NiOx, Co3O4, carbon materials, VOx, WO3, MoOx, 45 ACS Paragon Plus Environment


hybrid HTMs and inorganic HTM-perovskite composite. Among them, CuSCN, Cu:NiOx and Li0.05Mg0.15Ni0.8O based devices have achieved PCEs exceeding 20%, which is comparable to those of organic HTM-based devices. More importantly, excellent stability especially moisture and thermal stability has been demonstrated in inorganic HTM-based PSCs. Except for single inorganic HTM, development of hybrid HTMs especially for inorganic-inorganic hybrid HTMs is of importance in future research direction. In order to improve further PCE and stability based on inorganic HTMs, the following several points need be considered. First, good energy level alignment should be adjusted. Second, novel solution-processable methods are to be developed for thin inorganic HTM layers. Finally, interface compatibility should be also taken into account. From the wet chemistry point of view, solution-processable inorganic HTMs like CuI, CuSCN and Cu2O could be highly desirable especially in normal PSCs. In view of band alignment, wide bandgap HTM is advantageous for forming type II heterojunction, which can accelerate hole extraction and suppress back electron transfer and thus reduce recombination. As for stability, more attentions should be focused on interface, including interface defect passivation, interface compatibility, charge transfer kinetics and interface templated oriented grain growth. Additionally, in-depth understanding of interface charge transfer and recombination is necessary for developing novel inorganic HTM. From the perspective of optimizing film quality of inorganic HTMs, it is of great importance to prepare high-quality homogeneous inorganic HTM films with less defects and desirable conductivity by optimizing or developing deposition techniques (vacuum deposition, spin-coating, electrodeposition, drop casting, doctor blade, pulsed laser, screen printing, spray pyrolysis and so on). As of Jan. 2018, the highest PCE was attained in inverted PSCs based on Li0.05Mg0.15Ni0.8O as HTM instead of pristine NiOx, which is ascribed to improved conductivity promoting hole extraction and transport. This result suggests that doping is one promising approach for improving the electrical properties of inorganic HTMs. In addition, incorporation of inorganic HTMs into perovskite layer has been demonstrated. Through this perovskite-inorganic HTM composite engineering, it is beneficial for hole extraction because of the formation of bulk heterojunction and enhancement of moisture stability of perovskite layer. In future, integration of several kinds of strategies (perovskite composition engineering, interface engineering, perovskite-inorganic HTM composite and hybrid HTMs) could be a meaningful and important research direction for simultaneously achieving efficient and stable PSCs required for final commercial application. 46 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts No. NRF2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System), and NRF-2015M1A2A2053004 (Climate Change Management Program). This was also supported in part by NRF-2016M3D1A1027663 and NRF-2016M3D1A1027664 (Future Materials Discovery Program).

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