Anatase TiO2 Single-Crystalline Nanoparticle Electron

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Compact TiO2/Anatase TiO2 Single-Crystalline Nanoparticle Electron Transport Bilayer for Efficient Planar Perovskite Solar Cells Md. Shahiduzzaman, Hiroto Ashikawa, Mizuki Kuniyoshi, Sem Visal, Seiya Sakakibara, Tetsuya Kaneko, Tetsuhiro Katsumata, Tetsuya Taima, Satoru Iwamori, Masao Isomura, and Koji Tomita ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02406 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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ACS Sustainable Chemistry & Engineering

Compact TiO2/Anatase TiO2 Single-Crystalline Nanoparticle Electron Transport Bilayer for Efficient Planar Perovskite Solar Cells Md. Shahiduzzaman,1,2* Hiroto Ashikawa,3 Mizuki Kuniyoshi,1 Sem Visal, 3 Seiya Sakakibara,3 Tetsuya Kaneko,3 Tetsuhiro Katsumata,1 Tetsuya Taima,2 Satoru Iwamori,3 Masao Isomura,3* and Koji Tomita1* 1

Department of Chemistry, School of Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka City, Kanagawa 259-1292, Japan 2

Institute for Frontier Science Initiative (InFiniti), Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan

3

Graduate School of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan

*E-mail: [email protected] (MS); [email protected] (MI); [email protected] (KT)

Abstract Electron transport layer (ETL)/perovskite interface modification plays a key task for producing

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efficient planar perovskite solar cells (PSCs). In this study, interfacial modification of compact TiO2 using a novel one-step hydrothermally synthesized single-crystalline anatase (AT) titania nanoparticles (TiO2 NPs) (average diameter = 6–10 nm) was applied as an ETL bilayer to enhance the efficient charge generation and extraction and eliminate the electron–hole recombination ratio. We report here an easy approach for enhancing the performance of planar PSCs by introducing a compact TiO2/AT TiO2 NPs bilayer through spray pyrolysis (SP) deposition and spin-coating (SC) techniques, respectively. The enhanced performance of the devices with an SP-TiO2/SC-AT TiO2 NPs bilayer facilitated more efficient electron transport, charge extraction, and low interfacial recombination. Ultimately, the best device had a 17.05% power conversion efficiency resulting from the significant decrease in J–V hysteresis, presenting almost a 12% performance improvement compared to the TiO2 only layer-based counterpart. Thus, the present study provides an important advance to the design of photovoltaic devices with respect to charge transport and electron–hole recombination. Keywords: Planar Perovskite Solar Cells, Compact SP-TiO2/SC-AT TiO2 Nanoparticle Bilayer. Introduction Perovskite solar cells (PSCs) have appeared as rising stars in third-generation thin-film photovoltaics, attracting broad interest in both academic and industrial communities. PSCs are low-cost, solution-processable materials, and have powerful absorption coefficients.1 Perovskites as a light-absorbing material was first reported with power conversion efficiency (PCE) of 3.8% by Miyasaka and his colleagues.2 The best performing PSC discovered to date has a notable PCE of 22.7%.3 At present, there are two leading device structures for PSCs such as mesoporous and planar heterojunction (PHJ) fabrications.4-5 In particular, PHJ PSCs have gained more research


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interest than mesoporous PSCs, in order to their easy device framework and efficient performance characteristics. The electron transport layer (ETL) act on a key function in transporting electrons and blocking holes, eliminating the electrical shunt between the transparent electrode/perovskite and transparent electrode/hole transport material (HTM) interfaces to obtain high-performance PHJ PSCs.6-7 Titanium oxide (TiO2) is a commonly used n-type ETL material in PSCs because it has an environmentally friendly nature, tunable electronic-property,

cost-effectiveness, and

corresponding energy levels with perovskite. In addition, TiO2 can be easily prepared as a thin film using various solution-processing methods.8-9 However, using TiO2 in PHJ PSCs leads to the following drawbacks. (i) Low point electron mobility and conductivity led unflattering for electron collection and transport.10-11 (ii) Introduction of TiO2 to UV lights additionally influences the configuration formation of oxygen vacancies in the surface and grain boundaries of TiO2. This acts as charge traps and results in severe recombination loss of the photo-generated carriers.8, 12 Thus, the TiO2/perovskite interface restrains the photo-reply of the resultant devices, which facilitates robust hysteresis responses subsequently tough to get precise efficiency measurements.11 To enhance the performances of PSCs, notable achievements have been reported to the modification of TiO2 ETL, such as e like element doping and interface engineering. Adhikari et al.13 reported that replacing PCBM with perylene diimide (PDI) glass resulted in a performance enhancement of 39%. We have developed an interface engineering approach toward bridging perovskite and amorphous compact TiOx to fabricate efficient PHJ PSCs.14-16 Wu et al.17 revealed that the hysteresis phenomenon influenced to the rich number of both oxygen vacancies and electron traps on the surface of TiO2. Several teams have circumvented the hysteresis phenomenon by passivating the TiO2 layer together with C60/PCBM18 or employing doped TiO2 ETL19-20 to


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match the energy level configuration to the perovskite conduction band; consequently, this improves electron collection and transport and reduces the hysteresis. Yang et al.21 reported yttrium-doped tin dioxide (Y-SnO2) nanosheets as ETL-based PSCs and improved the PCE to 17.29% along with reduction in hysteresis. Huang et al.22 revealed that the PSCs with reduced hysteresis and enhanced PCE of 15.45% can be obtained using a phosphotungstic acid (PW12)TiO2 composite layer. Li et al.23 demonstrated that titanium chelate titanium (diisopropoxide) bis(2,4-pentanedionate) ETL was cast-off to formulate an ohmic contact with the unfavorable electrode enhanced the performance by increasing the efficiency of charge extraction and suppressing charge recombination. Li et al.24 reported that PSC films with enhanced crystallinity can be obtained by incorporating methylammonium halide intermediates through intramolecular exchange processes. Guo et al.25 revealed that the smooth perovskite morphology and large crystallized achieved by examining the PbI2 and CH3NH3I films thickness: controlling the reaction time and distance. They also reported that PDI-based polymers, co-polymerized PDI with different conjugated elements could be introduced for ETL-based PSCs.26 Wu et al.27 demonstrated an additive-assisted strategy for p-type molecular doping of solution-bladed perovskite films for facilitating the extraction of photo-excited holes from perovskite to ITO electrons, resulting in a PCE improvement of over 20.0% with almost no J–V hysteresis. Wu et al.28 published a recent review of the n-type materials that could be used for ETL-based PSCs, demonstrating the effect of perovskite film morphology. Wu et al.29 revealed that the recent progress made in the layered stacking technique for creating TiO2 ﬁlms with double-layered, tri-layered, and even multilayered structures enabled efficient formulations of photoelectrodes for solar cells. Wu et al.30 also reported a gas-assisted high-quality PSCs by using a mixed solvent based perovskite precursor solution.


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Extensive efforts have been made on PSCs utilizing nanostructure-based ETL materials. Huang et al.31 reported efficient PSCs with a PCE of 15.08% by using TiO2 nanopillar layers prepared by employing sputtering technique. PSCs with an enhanced PCE of 11.4% were obtained by introducing SrTiO3 layer on the meso scaffold TiO2.32 Li et al.33 demonstrated that the PCEs was significantly improved up to 17.2% by adding a lanthanum-doped compact TiO2 layer. Wang et al.34 reported the enhanced photovoltaic performance of PSCs by adding a graphene nanoflakes and anatase-TiO2 nanoparticles (NPs) as an ETL. Wu et al.35 reported Zn-doped TiO2-based PSC with a PCE of 14.0%. PSCs with a 14.8% PCE were fabricated using a low-temperature solutionprocessed composite layer of SiO2–TiO2.36 Wu et al.37 reported TiO2 nanowire-based PSCs with a PCE of 19.5%. Gan et al.38 reported a PHJ PSCs with a PCEs of 3.7% by adjusting the TiO2 nanorod's scaffolds. PSCs with a PCE of 7.5% was obtained by TiO2 and NiO nanosheets as ETL and HTM, separately.39 Yang et al.40 revealed annealing-free TiO2-based PSCs that achieved a PCE of 18.29%. He et al.41 demonstrated the use of compact TiO2 single crystalline nanorod layers for the fabrication of PSCs, which enhanced the PCE to over 17%. Aeineh et al.42 reported enhanced PSC performance, achieving PCEs of 17.55%, using inorganic surface engineering. Wei et al.43 reported a novel embedded structure utilizing TiO2 NPs embedded into perovskite films with improved PCE (from 16% to 18%) compared to that of standard PHJ PSCs and a highest efficiency of up to 19.2% for the embedded PSC. In addition, Shao et al.44 further developed the TiO2 NP-embedded perovskite film system and enhanced the PCE to 17.42%. The ability of charge separation and transport can be higher by promoting the internal electric field of the heterojunction by using a bilayer. Xu et al.45 designed PSCs by employing the type-II energy band structure of a TiO2/ZnO as the ETL to suppress charge assemble and recombination at the interface of ETL/perovskite. In addition, Li et al.46 reported a novel TiO2 bilayer for the ETL


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by combining atomic layer deposition and spin-coating techniques to enhance PHJ PSCs, achieving a PCE of 16.5%. Wu et al.47 demonstrated that the PCE of PSCs could be significantly increased from 12.3% to 16.1% using solution-processed ZnO/AZO bilayer ETLs. Consequently, photovoltaic performance enhancement can be expected by utilizing a TiO2 bilayer as the ETL in PHJ PSCs, owing to promote charge separations and defeat the recombination ratio at the interface. These results indicate that ETL modification is crucial for obtaining highly efficient PHJ PSCs. Commonly, highly acidic solutions and organic solvents are used in the synthesis of TiO2; therefore, TiO2 coexists with toxic, flammable, and corrosive products. Thus, the synthesis of nontoxic, low-environmental impact, and high-potential anatase (AT) TiO2 NPs via the lowtemperature manufacturing technique is challenging for obtaining an efficient ETL bilayer in PHJ PSCs (achieving more charge transport, suppressing hysteresis, and improving the photovoltaic performance). This paper reports a novel, higher potential, non-toxic, and environmentally friendly approach for preparing single-crystalline AT TiO2 NPs using a simple, scalable one-step hydrothermal route48 starting from a water-soluble titanium complex as the titanium source. The resultant AT TiO2 NPs perform well as the ETL bilayer in PHJ PSCs. In addition, we developed a TiO2/AT TiO2 NPs bilayer using spray pyrolysis (SP) deposition and spin-coating (SC) process, and displayed as ETLs to enhance PHJ PSC performance. To investigate the significance of varying the cycle coating number of SC-TiO2 NPs layers, the resulting device performances were presented and charge transport through the interface between TiO2 and perovskite was investigated by photoluminescence (PL) spectroscopy.


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Experimental methods Materials Lead iodide (PbI2) and methylammonium iodide (CH3NH3I) were purchased from Tokyo Chemical Industry (Tokyo, Japan). 0.35 M titanium diisopropoxide bis-(acetylacetonate; 75 wt% in isopropanol, Sigma-Aldrich,) in a 2-propanol (purity 99.9%, Wako chemical) were supplied. N,N-dimethylformamide (DMF, purity 99.5%) and dimethyl sulfoxide (DMSO, purity 99.5%) were also provided by Wako Chemical (Tokyo, Japan). AT NPs were synthesized according to the literature procedures.48 Characterization Field emission scanning electron microscopy (FE-SEM; S-4800, Hitachi High-Tech, Tokyo, Japan) and transmission electron microscopy (TEM; HF-2200 TU, Hitachi Ltd., Tokyo, Japan) were used to examine the morphology and NP diameter sizes, respectively. Raman spectroscopy (STR150, AIRIX, Tokyo, Japan) using a 50 mW to 532 nm diode laser was used to measure and further confirm the selectively synthesized AT TiO2 NPs. Ultraviolet-visible near-infrared absorption spectroscopy was performed using a Jasco spectrophotometer (UV-Vis-NIR; V-670, Jasco Corporation, Tokyo, Japan). The specific surface area was measured using a Bellsorp system (BET; Bellsorp mini II, Microtrac BEL, Tokyo, Japan). The X-ray diffraction (XRD) patterns of TiO2 nanocrystals and perovskite thin-films were measured using a Bruker X-ray diffractometer (D8 Discover, Bruker AXS Co. Ltd, Tokyo, Japan) together with an X-ray tube. PL spectroscopy was performed using a Jasco spectrometer (FP-8600, Jasco Corporation, Tokyo, Japan) to analyze charge transfer through the perovskite and varying numbers of TiO2 layers. The current density versus voltage (J–V) characteristics were scanned at a speed of 0.05 V/s in the forward-scan (FS; from -0.1 V to 1.2 V). In the reverse-scan (RS; from 1.2 V to -0.1 V), the devices were analyzed


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according to the simulated solar conditions (100 mW/cm2, AM1.5, 1 sun intensity) using a Keithley 2401 digital source meter. The incident photon-to-electron conversion efficiency (IPCE) of the resultant devices was measured by a monochromatic xenon arc light scheme (Bunkoukeiki, SMI250JA). Whole devices were measured in air with humidity differing from 40% to 50% at a temperature of 20 °C. The active area of the fabricated devices was 0.09 cm2. Synthesis of AT TiO2 NPs: Nanocrystalline AT NPs were synthesized by hydrothermal conditions using a water-soluble titanium complex as a titanium source.48 Finely powdered titanium (5 mmol) was liquefied in a mixed solution of 30% H2O2 aq. and 28% NH3 aq., to which Malic acid (5 mmol) was added. After the solution was dry at 50 °C to detach remaining H2O2 and NH3, the 0.25 M Ti–malate complex solution was prepared by adding 20 mL distilled water. The Ti complex solution was heat-treated in an autoclave at 200 °C for 5 h. The formed TiO2 particles were separated twice by centrifugation with 12,000 rpm for 10 min, and then re-dispersed in 10 mL distilled water. Preparation of the SP-TiO2/SC-AT TiO2 NP Bilayer: The FTO glass substrates (sheet resistance 10 Ωsq-1) were cleaned sequentially with a soap solution, distilled water, acetone, ethyl alcohol, and again distilled water. Thereafter, they were subjected to UV ozone action for 15 min. Then, 70 nm of TiO2 (optimized) was placed on the FTO glass via SP deposition at 450 °C from a solution (0.35 M titanium diisopropoxide bis(acetylacetonate) in isopropanol) according to the process depicted by Wakamiya et al.49 After spraying, the substrates were dried at 450 °C for 30 min in a muffle furnace and allowed to cool slowly to room temperature. The single-phase AT TiO2 NPs films were synthesized under a novel, simple, one-step hydrothermal approach corresponding to the procedure described by Tomita et al.48 As synthesized nano-colloidal AT TiO2 solution was deposited on the compact SP-TiO2 layer


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using the SC method at 2000 rpm for 30 s and annealed at 100 °C for 5 min, followed by sintering at 450 °C for 30 min in a muffle furnace. The as-synthesized AT TiO2 NPs were crystalline in form, as shown in Figure 1a. Finally, a compact SP-TiO2/SC-AT TiO2 NP bilayer was obtained by combining the SP deposition and SC techniques, and used as the ETLs. Fabrication of the Solar Cells: To prepare the perovskite precursor solution, 1M PbI2 and 1M CH3NH3I were dispersed in a mixed solvent of DMF and DMSO (4:1), and stirred at 60 °C for 1 h. The perovskite film was fabricated by spin-coating the precursor solution at 6000 rpm for 60 s with dripping chlorobenzene (CB) (500 µl) 8 s after the spin-coating started. The precursor-coated substrates were then allowed to anneal at 100 °C on a hot plate for 1 h to crystallize the perovskite. The HTL was SC at 3000 rpm for 30 s as of a solution of 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)9,9ʹspirobifluorene (spiro-OMeTAD) in CB (0.058 M) with 4-tert-butylpyridine (0.19 M), lithium bis(trifluoromethylsulfonyl)imide

(0.031

M),

and

tris[2-(1H-pyrazol-1-y1)-4-tert-

butylpyridine]cobalt(III) tris[bis(trifluoromethysulfonyl)imide] (5.6 × 10−3 M).49-50 Thereafter, 60 nm of gold (Au) was placed on the hole-transport layer (HTL) layer to form the electrodes. The optimized thicknesses of SP-TiO2, SC-AT TiO2 (three cycle-coating), perovskite layer, HTL, and Au layers were measured to be

70,

50,

270,

250, and

60 nm, respectively.

Results and discussion Novel, higher potential, single-crystalline AT TiO2 NPs were synthesized using the one-step hydrothermal route described by Tomita et al.48 From an environmental viewpoint, using watersoluble titanium complexes to prepare titanium-containing functional materials has proven to be very promising in terms of lowering the carbon content used per titanium atom. The as-synthesized nanocrystalline AT TiO2 NPs can formulate stable (more than a month) nano-colloidal aqueous


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suspensions (inset of Fig. 1a; concentration = 10 mg/mL). The XRD pattern of AT-TiO2 is depicted in Fig. 1a. The AT-TiO2 diffraction peaks were observed at 2θ of 25.4° 38.28°, 47.46°, and 54.26°, and assigned to the (101), (004), (200), and (105) crystal planes, respectively. The peak positions were unchanging with the tetragonal phase AT-TiO2 (ICSD No. 9852). Thus, the as-synthesized TiO2 NPs were confirmed to contain a significant number of AT domains. The mean crystallite size of the AT-TiO2 crystal was 6.79 nm, as estimated by Scherrer’s formula.51 Furthermore, the AT-TiO2 NPs were selectively synthesized, as evidenced by the Raman spectra52 shown in Fig. S1a. These results also reveal that the TiO2 NPs contained a large ratio of AT nanocrystalline phases, suggesting that it had a minimized number of surface defect traps. This may lead to enhanced charge transport at TiO2/perovskite interface. The crystallographic structure of AT TiO2 can be seen in the inset of Fig. S1a. The cross-sectional SEM image of the AT TiO2 NP film on FTO was obtained from five cycles of coating and revealed the formation of uniform and denser scaffolds, which facilitated more efficient charge extraction (Fig. S1b).

Fig. 1. (a) XRD pattern of AT TiO2 NPs. Inset shows the AT TiO2 nano-colloidal suspension in water. (b) Transmittance spectra of an SC-AT TiO2 NP film. Inset showing a representative STEM image of SC-AT TiO2 NPs deposited on a FTO/glass substrate.


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BET analysis revealed that the AT TiO2 NPs were uniform, dense, and had a large specific surface area of 170.2 m2g-1 (Fig. S1d). The resultant AT TiO2 NPs film exhibited well-behaved transmittance superior than 80% over the whole perceptible area (Fig. 1b). In addition, the resultant AT TiO2 NPs showed a significant reduction in reflectance (Fig. S1c) over the entire wavelength region, improving the transmittance by more than 80%. To know further understanding into the nanoscale morphology and sizes of AT TiO2 NPs, STEM was performed (inset Figure 1b). The STEM image revealed that the AT TiO2 NPs were spherical, with the particle size ranging from 6 to 10 nm in diameter. The spherical AT TiO2 formed denser scaffolds, and thus, offered multiple advantages, including more efficient charge extraction, superior electron pathways, and probably, an exceptional hole-blocking effect because of this dense packing. Precursor solution of perovskite was placed on the substrates and processed with an antisolvent dipping method.53-54 During the SC process, CB was rapidly dropped on the substrate to prompt crystallization of the perovskite. The XRD of the perovskite thin-films fabricated with, and without, the bilayer are showed in Figure 2. Diffraction peaks were detected at 2θ = 14.01°, 28.40°, 31.91°, and 40.74° for both SP-TiO2/perovskite and SP-TiO2/SC-AT TiO2 NPs/perovskite films, and were assigned with the key phases of perovskite to the (110), (220), (310), and (224) planes, separately.55 The XRD peak at 14.15° grew slightly longer a the full width at half-maximum (FWHM) of the entire peak decreased from 0.11 on SP-TiO2/perovskite to 0.10 on SP-TiO2/SC-AT TiO2 NPs/perovskite. The single SP-TiO2 layer-based perovskite film yielded a polycrystalline morphology with an average grain size of 351 nm, while the SP-TiO2/SC-AT TiO2 NPs bilayerbased perovskite film revealed grain size of 765 nm. (estimated by FWHM from by Scherrer equation,51 Table S1). The calculated values were consistent with the top-view morphologies, as exhibited in the SEM images in Fig. 3c and 3d. In particular, the single SC-TiO2 NP layer-based


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perovskite film also exhibited a large grain size (average around 642 nm) when compared to the single SP-TiO2 layer-based perovskite film, as shown in Table S1 and Fig. S2b. Higher crystallinity was observed for SP-TiO2/SC-AT TiO2 NP/perovskite because the as-synthesized ATTiO2 material started with better crystallinity, as evident in Fig. 1a. Perovskite film quality and crystallinity are correlated ultimately to the efficient performance and properties.5, 53 Considering perovskite films, the morphology and crystallinity and their photovoltaic properties have been improved in terms of incorporating with a compact TiO2 in combination with AT TiO2 NPs bilayer.

Fig. 2. XRD spectra of perovskite films fabricated with single-layer SP-TiO2 and the SP-TiO2/SCAT TiO2 NPs bilayer. To investigate the morphological properties of SP-TiO2 and the SP-TiO2/SC-AT TiO2 NPs bilayer, their corresponding perovskite films were analyzed by SEM (Fig. 3a-d). The SEM image in Figure 3a reveals that the 70-nm-thick compact TiO2 single layer obtained from the SP deposition method was smoothly distributed. In addition, some cracks (specified by red circles) can be observed in the SP-TiO2 layer that correlates to the grain boundaries of FTO, resulting in yield connection between FTO and perovskite and leading to significant charge recombination. In


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contrast, interfacial modification of the compact SP-TiO2 by a novel 50-nm-thick single-crystalline SC-AT TiO2 NP layer was homogenously distributed, allowing for uniform and denser SPTiO2/SC-AT TiO2 NP bilayer scaffolds (Figure 3b) without visible cracks; thus, a more efficient charge separation and recombination rate suppression was enabled. Large crystals with smooth morphology were seen in the presence of an SP-TiO2/SC-AT TiO2 NP bilayer film in order to promote bonding and surface coverage of perovskite on the SC-AT TiO2 NP surface with higher homogeneity was possible using the SC-AT TiO2 NP layer instead of the single SP-TiO2 layer, as shown in Fig. 3c and 3d. The smoother surface morphology and larger grain size of the perovskite film possessed fewer traps and grain boundaries, thus reducing the charge carrier losses led to the grain boundaries through the trap states.56 Hence, it can be concluded that the improved morphology, together with the improved consistency of the interface of SP-TiO2/perovskite, owing to the presence of AT TiO2 NPs, may enhance charge carrier extraction in terms of significantly quenching trap states, inducing slightly higher absorption, and increasing the quantum efficiency, as per revealed in Figs. 4, S3, and 6c, individually. Additionally, the bigger crystal grains and dense morphology of the SP-TiO2/SC-AT TiO2 NPs bilayer-based perovskite film reduced the bad effect of oxygen and moisture, promoting the long-term stability of the resultant devices. A crosssection SEM image of the PSCs processed with a TiO2 NP-based bilayer is shown in Fig. 6a, and the resultant layers thickness was measured. In a cross-sectional SEM image, we assumed that the existence of voids (pin-holes) might be due to the addition of a small amount of TBP and LiTFSI in the resulting spiro-OMeTAD. 57 They also further confirmed that there is no role of perovskite in creating such defects (pin-holes) in the spiro-OMeTAD. PL spectroscopy was performed to know understanding on the charge transfer mechanism through the perovskite/compact TiO2 layer interface. The PL analysis provided the evidence on


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charge transport and separation capability of the ETLs.58 The PLs of SP-TiO2/perovskite, SC-AT TiO2 NP/perovskite, and SP-TiO2/SC-AT TiO2 NP/perovskite films are shown in Fig. 4. Perovskite PL was quenched significantly in the presence of the SP-TiO2/SC-AT TiO2 NP bilayer. The PL for the SP-TiO2/SC-AT TiO2 NP/perovskite film showed even stronger quenching, revealing efficient charge transfer at the interface between the photo-excited perovskite and the SP-TiO2/SC-AT TiO2 NP bilayer.

Fig. 3. SEM images of the (a) SP-TiO2 layer and (b) SP-TiO2/SC-AT TiO2 NP bilayers, and the perovskite films grown on the (c) SP-TiO2 layer and (d) SP-TiO2/SC-AT TiO2 NP bilayers.


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Fig. 4. PL spectra of perovskite films fabricated on substrates with or without the NP bilayer.

Fig. 5. Schematic figure of the nucleation and growth of perovskite grains.


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Herein, we introduce a persuasive mechanism to account for the insertion effect of the AT TiO2 NPs between perovskite and SP-TiO2 for enhancing the performance of the PHJ PSCs. For the perovskite film placed on the compact SP-TiO2 layer, mostly narrow nucleation sites for CH3NH3PbI3 were available on the flat SP TiO2 surface. Following growth of the perovskite crystals, a limited space framework was formed by interconnecting the resultant nuclei at each nucleation site, resulting in small grain-sized perovskite crystals, as shown in Figure 5a. On the other hand, crystallization of the perovskite film grown on single-crystalline compact AT TiO2 NPs involved two stages—nucleation and crystal growth—as shown in Figure 5b and 5c. The perovskite solution was spun cast on single-crystalline compact AT TiO2 NPs under the optimized conditions. Owing to the better interparticle adhesion across the interface between perovskite and single-crystalline AT TiO2 NPs, fewer nucleation sites are produced, which facilitates the perovskite nucleation because of the inferior nucleation energy boundary at the interface.59 Consequently, perovskite growth leads to a smooth interface containing less grain boundaries with larger crystal domains, a more continuous and uniform morphology, and enhanced crystallinity. Primary nucleation dominates by the resultant single-crystalline compact AT TiO2 NPs. Recently, Trilok and Miyasaka group revealed that the growth of perovskite layer is completely prompted by the quality of bottom substrates.60 Large anatase single crystals hold superior mobility created by thermal excitement of shallow donor circumstances described by Forro et al.61 Single-crystalline AT TiO2 NPs has more benefit than compact TiO2 layer due to large interface area for election injection and following transport that further balances the charge carriers. Therefore, it can be concluded that the large grains, with fewer grain boundaries, on the perovskite film were induced by the presence of single-crystalline AT TiO2 NPs between the perovskite and SP TiO2. This is one of the primary origins for high-performance PSCs.


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Fig. 6. (a) Cross-sectional SEM image of the PSC processed with a TiO2 NP-based bilayer. (b) RS J–V curves obtained for solar cells based on substrates with and without the NP bilayer. (c) IPCE spectra of representative devices containing SP-TiO2 single and SP-TiO2/SC-AT TiO2 NP bilayers as the ETLs. (d) FS and RS J–V characteristics of SP-TiO2 single and SP-TiO2/SC-AT TiO2 NP bilayer-based devices. The J–V characteristics of PSCs in the FS and RS directions for the substrates with and without bilayers were analyzed under simulated sunlight (Fig. 6). Whole devices were measured in air with humidity fluctuating from 30% to 40% at 20 °C without encapsulation. The corresponding device parameters are outlined in Table 1. Furthermore, the FS and RS are summarized in greater detail in Table S2. The SP-TiO2 single layer-based champion (RS) PSCs exhibited a considerably low short-circuit current density (Jsc) of 20.68 mAcm-2, open-circuit voltage (Voc) of 1.05 V, fill factor (FF) of 0.70, and PCE of 15.33%, as shown in Fig. 6b. Conversely, introducing of an SP-TiO2/SC-


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AT TiO2 NP bilayer as an ETL caused Jsc to increase to 21.04 mAcm-2, while it was 21.06 mAcm2

for the device with an SC-AT TiO2 NP single layer. The FF was improved from 0.70 to 0.75 and

0.71 by inserting SP-TiO2/SC-AT TiO2 NP bilayer and SC-AT TiO2 NP single layers, respectively (shown in Figs. 6b and S4a). We assumed that the performance enhancement from utilizing an SP-TiO2/SC-AT TiO2 NP bilayer ETL was caused by the highly nanocrystalline AT phase content within the compact layer and that the greatly homogeneous morphology favored electron extraction through minimizing the ratio of carrier recombination than that of SP-TiO2 and the SCTiO2 NP single layer.62-64 In addition, the device formed from the SP-TiO2/SC-AT TiO2 NP bilayer exhibited an average series resistance (Rs) of 52.77 ± 1.06 Ωcm2. This was significantly lower than that of the PSCs using an SP-TiO2 single layer (Rs = 62.87 ± 6.16 Ωcm2), suggesting that the incorporation of an SC-AT TiO2 NP layer on the SP-TiO2/FTO glass substrate lowered the Rs and improved carrier extraction (Fig. 4) from the perovskite to the FTO electrode, enhancing the Jsc, Voc, FF, and PCE. The J–V characteristics with FS and RS of the resultant device with the optimized SP-TiO2/SC-AT TiO2 NP bilayer (three-cycle coating) exhibited lower hysteresis than the single SP-TiO2 layer, as shown in Fig. 6d. In terms of bilayer-based PSCs, the low-hysteresis characteristic was exhibited that attributes to the bigger size grain of the perovskite, and the stronger electron extraction efficiency facilitated the reduction of charge carrier recombination.65 Besides, bilayer-based perovskite with fine grains created fewer disorganized lattices and could solve the problem of unbalanced ion aggregation close to the grains, ensuring slower electrode polarization.

66

Thus, the bilayer-based PSCs showed efficient device performance than the TiO2

single layer-based counterparts, indicating the presence of well-organized charge transfer at the TiO2/perovskite interface. The incorporation of AT TiO2 NPs on the SP-TiO2 layer as a bilayer in PSC fabrication clearly led to a significant impact on the device performance, as evident by the J–


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V characteristic analysis (Fig. 6b). To examine the effects of AT TiO2 layer thickness on the performance enhancement of the bilayer-based PHJ PSCs, a series of devices was prepared with one, three, and five coating cycles of AT TiO2 NPs (30, 50, and 70 nm, respectively) on the SPTiO2 substrate, as shown in Fig. S5.

Table 1. Outline of the photovoltaic device performance characteristic of FTO/SP-TiO2/SC-AT TiO2 and perovskite/spiro-OMeTAD/Au. Statistical analysis (average ± standard deviation) is based on the measurement of ten and eight individual devices for both SP-TiO2 and SP-TiO2/ SCAT TiO2 films, respectively. “Champion” states to the best device performances measurement data. Jsc

Compact Layer SP-TiO2 only

SP-TiO2/SC-AT TiO2 NP bilayer

(mA/cm2)

Voc (V)

FF

PCE (%)

Rs/ Ωcm2

Champion

20.68

1.05

0.70

15.33

48.40

Average

19.24

1.01

0.66

12.92 ± 1.84

62.87 ± 6.16

Champion

21.04

1.08

0.75

17.05

47.25

Average

20.42

1.06

0.74

16.11 ± 0.63

52.77 ± 1.06


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Fig. 7. Average values of (a) Jsc, (b) Voc, (C) FF, and (d) PCE obtained for the SC-TiO2 and SCTiO2/SC-AT TiO2 NP bilayer-based PSCs. Error bars indicate ± one standard deviation from the mean. Ten and eight individual devices based on SP-TiO2 and SP-TiO2/SC-AT TiO2 films, respectively, were used. Figure 7a–d presents the average Jsc, Voc, FF, and PCE values for the PSCs, respectively, for the SC-TiO2 and SC-TiO2/SC-AT TiO2 NP bilayer devices, respectively. Ten and eight individual devices were fabricated based on SP-TiO2 and SP-TiO2/ SC-AT TiO2 films, respectively. The SPTiO2/SC-AT TiO2 NP bilayer ETL-based devices yielded better reproducibility and superior PCEs of 16.11 ± 0.63% compared to the single SP-TiO2 and AT TiO2 layer devices (PCEs of 12.92 ± 1.84% and 15.30 ± 0.10%, respectively). The IPCE data of devices with and without the bilayer are displayed in Figs. 6c and S4b, respectively. The improved PCEs of the devices with an SP-TiO2/SC-AT TiO2 NP bilayer or single


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SP-TiO2 and SC-AT TiO2 NP layers were unchanged with the IPCE data than those of cells without the bilayer. The photo-currents insistent on the IPCE data were 19.09, 19.14, and 19.64 mA/cm2 for devices based on an SP-TiO2 single layer, SC-AT TiO2 NP single layer, and SP-TiO2/SC-AT TiO2 NP bilayer, respectively. The PHJ PSCs demonstrated a smooth absorption peak at 380–750 nm (80–85% intensity). The broader IPCE for the device with a bilayer compared to that of other devices indicated that the bilayer collected more efficient electrons at the perovskite/TiO2 edge because it effectively reduced the limit of interfacial energy barrier.

Fig. 8. Normalized PCE in terms of time (in days) for PSCs processed with and without the SCAT TiO2 NP-based ETL bilayer. In terms of investigating the stability of single and bilayer-based PSCs, a non-persistent measuring approach was applied every seven days to devices stored under a dark nitrogen atmosphere before and after the J–V measurements in air (without encapsulation). Figure 8 reveals that the bilayer-based PSCs degraded more slowly than the single layer-based PSCs over time. The PCE of the bilayer-based PSCs presented notable long-term stability, holding 53% of the initial PCE even after 28 days. This stability was promoted by introducing higher-potential AT


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TiO2 NPs. Previous studies67-68 have reported that PSCs with the bigger grain of perovskite and a dense perovskite film facilitate improved device stability because the bigger grains and the dense film can reduce the damaging impacts of oxygen and moisture. Additionally, the SP-TiO2/SC-AT TiO2 NP bilayer-based device exhibited improved PCE owing to the oxidation of spiroOMeTAD.69-70 Only 38% deterioration of its original PCE was observed after 47 days when kept in a dry dark N2 environment, as shown in Fig. S6. In accordance with these findings, our results assert that the insertion of an SC-AT TiO2 NP layer between SP-TiO2 and perovskite yielded a compact perovskite with the bigger grain sizes (as showed in SEM images in Fig. 3d). Thus, better device stability was obtained. This facile synthesis process and the significant performance enhancements revealed that the synthesized bilayer is a strong ETL candidate for highperformance PHJ PSCs. Conclusions We altered the surface morphology of SP-TiO2 films by adding SC-AT TiO2 NPs into the PHJ PSCs to enhance their PCEs. The SC-AT TiO2 NP-based PHJ PSCs exhibited outstanding behavior and prolonged stability compared to ordinary PHJ PSCs. The improved electron extraction and broadened perovskite grain size, aided by single-crystalline AT TiO2 NPs, were responsible for enhanced PSC performance. This SP-TiO2/SC-AT TiO2 NP bilayer ETL effectively eliminated the through contact between the perovskite and the electrode. The optimized number of SC-AT TiO2 NP coating layers on SP-TiO2 was three, resulting in a thickness of 50 nm. A maximum PCE of 17.05% was obtained for the resulting PSC. The notable performance of ETL bilayer-based device was referred to the enhanced photo-generated charge carrier site content and the reduced particle aggregation and hole trapping at the SP–TiO2 interface, as evident from the PL analysis. This


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modified ETL structure has been proven to be an encouraging and easy procedure to further enhance the performance of PHJ PSCs. Associated Content Supporting Information Description The Supporting Information is available free of charge on the ACS Publications website at DOI: Raman spectra of AT TiO2 NPs; Cross-sectional SEM image of SC-AT TiO2 NPs; Diffuse reflectance image of AT TiO2 NPs; BET analysis data of AT TiO2 NPs; Top view of the SEM images for (a) SC-AT TiO2 NP single layer (three-cycle coating on an FTO substrate) and (b) perovskite films grown on an SC-AT TiO2 NP layer; UV-vis characteristics of perovskite with and without the SC-AT TiO2 NP bilayer; (a) J–V characteristics and (b) IPCE spectra of the device fabricated using an SC-TiO2 NP single layer; J–V characteristics for varying the cycle spin-coating layers of SC-AT TiO2 NP-based bilayers of SP-TiO2 in the RS (1.2 V to -0.1 V); Normalized PCE in terms of time (in days) for PSCs processed with SP-TiO2/SC-AT TiO2 NPs based on an ETL bilayer. (PDF) Corresponding Author *E-mail: [email protected] (MS); [email protected] (MI); [email protected] (KT) Tel/Fax: +81-463-58-1211(ext.3742) Acknowledgements This study was supported in part by Research and Study Project of Tokai University General Research Organization. We thank Professor Dr. Tsutomu Miyasaka and Dr. Ajay Kumar Jena (Toin University of Yokohama) for valuable discussions.


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(63) Ma, J.; Yang, G.; Qin, M.; Zheng, X.; Lei, H.; Chen, C.; Chen, Z.; Guo, Y.; Han, H.; Zhao, X.; Fang, G. MgO nanoparticle modified anode for highly efficient SnO2-based planar perovskite solar cells. Adv. Sci. 2017, 4 (9), 1700031-n/a. (64) Liu, Z.; Chen, Q.; Hong, Z.; Zhou, H.; Xu, X.; De Marco, N.; Sun, P.; Zhao, Z.; Cheng, Y.B.; Yang, Y. Low-temperature TiOx compact layer for planar heterojunction perovskite solar cells. ACS Appl Mater Inter 2016, 8 (17), 11076-11083. (65) Almora, O.; Zarazua, I.; Mas-Marza, E.; Mora-Sero, I.; Bisquert, J.; Garcia-Belmonte, G. Capacitive dark currents, hysteresis, and electrode polarization in lead halide perovskite solar cells. J. Phys. Chem. Lett. 2015, 6 (9), 1645-1652. (66) Peng, W.; Wang, L.; Murali, B.; Ho, K.-T.; Bera, A.; Cho, N.; Kang, C.-F.; Burlakov, V. M.; Pan, J.; Sinatra, L.; Ma, C.; Xu, W.; Shi, D.; Alarousu, E.; Goriely, A.; He, J.-H.; Mohammed, O. F.; Wu, T.; Bakr, O. M. Solution-grown monocrystalline hybrid perovskite films for holetransporter-free solar cells. Adv. Mater. 2016, 28 (17), 3383-3390. (67) Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air. J. Am. Chem. Soc. 2015, 137 (4), 1530-1538. (68) Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A layered hybrid perovskite solar-cell absorber with enhanced moisture stability. Angew. Chem. 2014, 126 (42), 11414-11417. (69) Ginting, R. T.; Jeon, M.-K.; Lee, K.-J.; Jin, W.-Y.; Kim, T.-W.; Kang, J.-W. Degradation mechanism of planar-perovskite solar cells: correlating evolution of iodine distribution and photocurrent hysteresis. J Mater Chem A 2017, 5 (9), 4527-4534. (70) Cappel, U. B.; Daeneke, T.; Bach, U. Oxygen-induced doping of spiro-MeOTAD in solidstate dye-sensitized solar cells and its impact on device performance. Nano Lett. 2012, 12 (9), 4925-4931.


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TOC Graphic Synopsis: Incorporating SC-TiO2/SC-AT TiO2 NPs bilayer approach to achieve high-quality perovskite film in planar PSCs for green and sustainable energy conversion.


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Graphical abstract


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Anatase TiO2 Single-Crystalline Nanoparticle Electron

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