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Improvement of CH3NH3PbI3 formation for efficient and better reproducible mesoscopic perovskite solar cells Changyun Jiang, Siew Lay Lim, Wei Peng Goh, Fengxia Wei, and Jie Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07446 • Publication Date (Web): 22 Oct 2015 Downloaded from http://pubs.acs.org on October 23, 2015
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ACS Applied Materials & Interfaces
Improvement of CH3NH3PbI3 formation for efficient and better reproducible mesoscopic perovskite solar cells Changyun JIANG, Siew Lay LIM, Wei Peng GOH, Feng Xia WEI, Jie ZHANG* Institute of Materials Research and Engineering, A*STAR, 3 Research Link, Singapore 117602 Abstract High performance perovskite solar cells (PSCs) are obtained through optimization of the formation of CH3NH3PbI3 nanocrystals on mesoporous TiO2 film, using a two-step sequential deposition process by first spin-coating a PbI2 film and then submerging it into CH3NH3I solution for perovskite conversion (PbI2 + CH3NH3I -> CH3NH3PbI3). It is found that the PbI2 morphology from different film formation process (thermal drying, solvent extraction, and asdeposition) has a profound effect on the CH3NH3PbI3 active layer formation and its nanocrystalline composition. The residual PbI2 in the active layer contributes to substantial photocurrent losses, thus resulting in low and inconsistent PSC performances. The PbI2 film dried by solvent extraction shows enhanced CH3NH3PbI3 conversion as the loosely packed disk-like PbI2 crystals allow better CH3NH3I penetration and reaction in comparison to the multi-crystal aggregates that are commonly obtained in the thermally dried PbI2 film. The asdeposited PbI2 wet film, without any further drying, exhibits complete conversion to CH3NH3PbI3 in MAI solution. The resulting PSCs reveal high power conversion efficiency of 15.60 % with a batch-to-batch consistency of 14.60 ± 0.55 %, whereas a lower efficiency of 13.80 % with a poorer consistency of 11.20 ± 3.10 % are obtained from the PSCs using thermally dried PbI2 films. Keywords: Perovskite solar cells • Mesoscopic structure • Perovskite conversion • Two-step sequential deposition • Reproducibility
Introduction The search for effective usage of the abundant clean energy on earth has encouraged extensive research in solar photovoltaics. Recently, the organic-inorganic hybrid perovskite, methylammonium lead halide MAPbX3 (MA = CH3NH3; X = Cl, Br, I), has been studied extensively as photo-harvesting materials by the clean energy research community.1-5 MAPbX3 exhibits long electron/hole diffusion length, high absorption coefficient and excellent defect tolerance.6-9 A high power conversion efficiency (PCE) perovskite solar cell (PSC) of
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20.1 % has been certified recently,10,11 as compared to 3.8 % when MAPbX3 was first studied as a photo-harvester in 2009.12 The high performance, low material cost and solution processability make PSC an excellent candidate for industry adoption to eventually develop into a central role in solar energy harvesting. A variety of solution based fabrication processes, such as spin coating, drop casting, dipping, solvent/vapour extraction and so on, can be used to fabricate the PSCs with a planar or mesoscopic structure.13-18 CH3NH3PbI3 (MAPbI3) perovskite films are usually formed by either a one-step or a two-step sequential deposition method. In the one-step method, the PbI2 (or PbCl2) and CH3NH3I (MAI) are premixed in a solution before the deposition. The perovskite crystals are formed as the carrier solvent evaporates during the deposition.19-21 In the two-step sequential method, PbI2 is first spin coated, followed by dipping into MAI solution for the perovskite conversion (PbI2 + CH3NH3I -> CH3NH3PbI3).22,23 The two-step sequential method is an ideal approach to mesoscopic PSCs as it allows control of the perovskite crystallization in the mesoscopic pores and on the TiO2 film surface. However, the two-step process usually leads to incomplete perovskite conversion due to incomplete intercalation of MAI into the PbI2 film.2426
Therefore, it is more challenging to realize efficient perovskite conversion. Consequently,
lower and inconsistent PSC performances are obtained. The recent study by Wu et. al. suggested that retarding the crystallization of initially deposited PbI2 film by using dimethylsulfoxide (DMSO) as solvent could yield more complete perovskite conversion.27 In this study, our focus is to address the PSC fabrication consistency and performance by optimizing the perovskite film quality through controlling the PbI2 layer formation by different post-deposition processes. The selected post-deposition process conditions are thermal drying, solvent extraction by dichloromethane (DCM), and using the as-deposited wet PbI2 film directly. Improved perovskite conversion is obtained by DCM-extraction compared to thermal drying. Complete perovskite conversion can be achieved by using asdeposited wet PbI2 film to realize PSCs with higher power conversion efficiency and better reproducibility. Experimental A compact TiO2 film of 25 nm was deposited on fluorine-doped tin oxide (FTO) coated substrates (Solaronix, 15 Ω/square) by spray pyrolysis at 300 °C, followed by sintering at 500 °C for 60 min.28 This compact TiO2 layer is also called blocking layer (BL-TiO2). The subsequent mesoporous TiO2 (mp-TiO2) film was formed by spin coating on the BL-TiO2 layer using a TiO2 nanoparticle paste (Dyesol, 18NR-T) at 5000 rpm followed by sintering at 500 °C for 30 min. The TiO2 paste was diluted in ethanol at a ratio of 1:3.5 before use. The resulting mp-TiO2 film had a thickness of 250 nm. 2 ACS Paragon Plus Environment
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The two-step MAPbI3 formation comprised of 1) spin coating of PbI2 film, and 2) dipping the PbI2 film in MAI solution (1 % MAI in isopropyl alcohol (IPA)) for 10 min to convert PbI2 into MAPbI3. In the first step, the PbI2 solution (1 M in N,N-dimethylformamide (DMF), heated at 70 °C) was spin coated on the mp-TiO2 substrate at 4000 rpm for 5 s in a N2 purged glove box. The as-coated PbI2 films were treated by three different processes before dipping in the MAI solution: a) thermal drying at 100 °C for 5 min (thermal drying), b) drying by solvent extraction through soaking as-coated films in DCM for 30 s before blow drying the films using N2 (DCM-extraction), and c) directly using the as coated wet films (as-deposition). During the second step, the PbI2 coated substrate was dipped into MAI solution for 10 min, followed by drying at 70 °C for 10 min to form the perovskite layer. Spiro-OMeTAD was used as the hole transporting material (HTM). Spiro-OMeTAD solution (80 mg/mL in chlorobenzene) containing 35 mM Li[CF3SO2]2N and 200 mM tertbutylpyridine (tBP) was spin coated at 3000 rpm for 40 s on the perovskite layer. Finally, a 150 nm thick Ag layer as the anode was formed by thermal evaporation through a shadow mask to define the active cell area (0.2 cm2). PbI2 and perovskite films on FTO/mp-TiO2 substrates were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis spectroscopy. Separately, PbI2 films were prepared for transmission electron microscopy (TEM) characterization by spin coating the PbI2 solution on holely Cu grids, followed by thermal and DCM-extraction drying processes. Photovoltaic current-voltage (J-V) characteristics of solar cell devices were measured in ambient conditions with a Keithley 2400 source-meter unit under 100 mW/cm2 illumination (AM 1.5G) from a 300 W solar simulator (Newport 91160). The intensity of the light source was determined using a Si reference cell (calibrated by Fraunhofer Institute for Solar Energy, ISE) and corrected for spectral mismatch. The fabricated devices were stored in a dry box for an hour before the measurements, without additional conditioning. J-V measurements were performed in forward voltage scan mode (voltage swept from negative to positive), at a moderate scan rate of 50 mV/s. Reverse voltage scans (voltage swept from positive to negative) were also performed on some devices in order to study the hysteresis effects (see Supporting Information). Results and Discussion The mp-TiO2 based PSC was selected as the device structure in this study to take advantage of the large interfacial area between TiO2 and perovskite for efficient charge carrier separation.29-30 The optimized two-step sequential perovskite formation process yielded defect-free solar cells. A cross-sectional SEM image of a typical solar cell device is given in Figure 1. Through controlling the mp-TiO2 thickness and porosity, and the PbI2 film formation process, perovskite crystals filled the pores in the mp-TiO2 film and subsequently 3 ACS Paragon Plus Environment
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formed a capping layer on the surface of the mp-TiO2 layer. The overall photoactive layer (mp-TiO2 + MAPbI3) was ~530 nm, consisting of ~250 nm of mp-TiO2:MAPbI3 mesoscopic layer and ~280 nm of MAPbI3 planar capping layer. All pores in the mp-TiO2 film appeared to be filled by MAPbI3, which is important in realizing high performance mesoscopic perovskite solar cells.31 The solar cell with the combination of the mesoscopic and planar MAPbI3 layer has unique advantages. The mesoscopic layer provides efficient charge separation at the TiO2/MAPbI3 interface. The complete infiltration of perovskite in the mp-TiO2 layer can also prevent interfacial recombination by ensuring that spiro-OMeTAD (hole transporting) is isolated from TiO2 (electron transporting).32 The capping-perovskite layer with larger crystals facilitates efficient charge transport6-7 and provides higher perovskite loading compared to the mp-TiO2:MAPbI3 layer for efficient charge collection and maximum light absorption. Note that the BL-TiO2 between the FTO and the mp-TiO2 layer is not clearly visible from the SEM image. This is due to the thin thickness of 25 nm. The thin BL-TiO2 layer not only limits the resistance to electron transport, but also effectively blocks hole injection from the perovskite into the FTO cathode. This is a prerequisite for achieving high device performance.
Figure 1. Cross-sectional SEM image of a device with layers of FTO, BL-TiO2, mp-TiO2, MAPbI3, HTM (spiro-OMeTAD) and Ag. (Scale bar: 200 nm) Through the two-step MAPbI3 formation process, the morphology of PbI2 film, the intermediate product from the first deposition step, has profound influence on the final perovskite film quality and composition, thus affecting the PSC performances. To be able to systematically study this effect, three different post treatments on spin coated PbI2 precursor films were introduced: a) thermal drying (at 100 °C for 5 min), b) DCM-extraction by soaking the as-coated films in DCM for 30 s to extract the residual DMF followed by blow drying the films using N2, and c) using the as-coated wet PbI2 films directly for dipping (as-deposition). All the PbI2 precursor films were prepared using the same spin coating conditions for consistency in the PbI2 precursor loading and the film thickness on mp-TiO2 layer.
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The perovskite film morphology and crystal structures were studied to investigate the perovskite conversion on PbI2 films processed under different drying conditions. The results are shown in Figure 2. The perovskite crystals have similar cuboid shapes for all three perovskite films obtained from PbI2 films with different post processing conditions (Figure 2 a-c). The sizes of the crystals range from 100 to 250 nm, as determined from SEM images. Slightly larger crystal sizes are observed for the perovskite film obtained from the asdeposited wet PbI2 film (Figure 2c). From the XRD spectra (Figure 2d), a strong crystalline PbI2 peak at 12.6 ° is observed in the perovskite film processed from the thermally dried PbI2. This indicates a considerable amount of residual PbI2 remaining in the MAPbI3 film, even after dipping in MAI solution for 10 min. The intensity of the residual PbI2 peak decreases in the MAPbI3 film when the DCM-extraction is used to prepare the PbI2 film whereas the peak (at 12.6 °) is almost completely eliminated when the as-deposited wet PbI2 film is used for MAPbI3 conversion. After dipping in MAI for 10 min, the weight percentage of PbI2 in the PbI2+MAPbI3 composite in the perovskite films obtained from thermal drying, DCMextraction and as-deposition processes is 19.0%, 4.0% and
