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Enhanced photovoltaic performance and stability in mixed-cation perovskite solar cells via compositional modulation
Accepted Manuscript Title: Enhanced photovoltaic performance and stability in mixed-cation perovskite solar cells via compositional modulation Authors: Xin Li, Junyou Yang, Qinghui Jiang, Weijing Chu, Dan Zhang, Zhiwei Zhou, Yangyang Ren, Jiwu Xin PII: DOI: Reference:
S0013-4686(17)31457-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.07.040 EA 29857
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
15-2-2017 1-5-2017 6-7-2017
Please cite this article as: Xin Li, Junyou Yang, Qinghui Jiang, Weijing Chu, Dan Zhang, Zhiwei Zhou, Yangyang Ren, Jiwu Xin, Enhanced photovoltaic performance and stability in mixed-cation perovskite solar cells via compositional modulation, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced photovoltaic performance and stability in mixed-cation perovskite solar cells via compositional modulation
Xin Li1,2, Junyou Yang1,2*, Qinghui Jiang1,2, Weijing Chu1,2, Dan Zhang1,2, Zhiwei Zhou1,2, Yangyang Ren1,2, Jiwu Xin1,2
1. State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, P.R. China 2. Shenzhen Institute of Huazhong University of Science & Technology, Shenzhen 51800, P.R. China *To whom correspondence should be addressed, email: [email protected]
Graphical abstract
1
Highlights A pure phase of FA1-xMAxPbI3 perovskite FA1-xMAxPbI3 was prepared. Discussing effect of chemical composition and morphology on the photovoltaic performance in detail. The PCE of PKSC based on FA0.7MA0.3PbI3 reaches up to 16.0%. The long term stability and reproducibility is obtained.
Abstract Preparation of high-quality and pure phase mixed-organic-cation FA1-xMAxPbI3 perovskite film is a big challenge, especially on a porous oxide scaffold for mesoscopic perovskite solar cells. In this study, we fabricated a pure phase of mixed-organic-cation perovskite FA1-xMAxPbI3 simply using a conventional two-step method. Effect of chemical composition and morphology on the photovoltaic performance and stability of the mixed-organic-cation perovskite has been studied in details. Results show that the FA1-xMAxPbI3 perovskite thin films with a FA+:MA+ molar ratio of 0.7:0.3 demonstrate the best photovoltaic performance. Mesoscopic perovskite solar cells fabricated using these FA0.7MA0.3PbI3 thin films yield a power conversion efficiency (PCE) up to 16.0%. 2
Moreover, the devices present a superior stability and the unsealed devices exposed in ambient condition (25 ℃, 50%-60% relative humidity) can maintain their photoelectric performance without apparent PCE loss for up to 20 days.
Keywords: Compositional modulation; Mixed organic cation; Enhanced photoelectric performance; Long term stability; Perovskite solar cells.
1. Introduction Organolead halide perovskite solar cells (PKSCs) have received much attention over the last several years, predominantly driven by the inevitable and huge demands for photoelectric active materials with high absorption coefficient, broad absorption range, large charge carriers mobility, suitable direct band gap, long carrier diffusion length, facile solution fabrication process and outstanding power conversion efficiency (PCE) [1-5]. Nowadays, the highest certified power conversion efficiency of PKSCs has reached over 22.1% [6], and has been regarded as one of the best candidates for replacing the conventional Si-based solar cells. The chemical formula of perovskite is ABX3, where A is the organic cation (A=CH3NH2+ or HC(CH2)2+), B is the metal cation (B=Pb2+ or Sn2+) 3
and X is the halide anion (X=I-, Br- or Cl-) [1-5]. Up to now, most of the reported PKSCs are based on the CH3NH3PbI3 perovskite (MAPbI3). Although the MAPbI3 has excellent photovoltaic properties with a bandgap of ~1.55 eV, it is less stable and decomposes rapidly even at a mediate elevated temperature less than 100 ℃once exposed in the air [7-9]. Comparatively, the HC(NH2)2PbI3 perovskite (FAPbI3) is relatively more stable, but it is less studied yet. In general, the FAPbI3 has two possible crystal structures at ambient temperature [10-12]. The first one is α-FAPbI3 phase (space group P3m1), which is black with good photoelectric properties, and the second one is δ-FAPbI3 phase (space group P63mc), which is yellow with an indirect bandgap of 2.48 eV. In comparison with MAPbI3, α-FAPbI3 has a suitable direct bandgap of about 1.47 eV and superior properties [12-15], such as extended absorption range [10-13], higher phase conversion and photostability [12]. However, preparation technique of high-quality FAPbI3 perovskite films is still a big challenge, especially for those based on a mesoscopic scaffold of oxides, e.g. mesoporous TiO2 films. On one hand, the ion size of FA+ (2.79 Å) is larger than that of the MA+ (2.70 Å), and thus the intercalation FA+ cation into the PbX6 octahedral framework is kinetically more difficult, which may result in some unreacted PbI2 4
residues [16-17]. On the other hand, compared with the single phase homomorphic MAPbI3, it is hard to prepare a pure α-FAPbI3 phase because of the formation of some δ-FAPbI3 phases accompanying along the crystallization process of α-FAPbI3 in the thin film. In theory, the δ-FAPbI3 phase can transform to the photoactive α-FAPbI3 phase at an elevated temperature (>150 ℃) [10, 13, 15, 18]. However, it is not easy for δ-FAPbI3 to be fully transformed into α-FAPbI3 within the scaffold layer probably because of the stress from the substrate, the residual PbI2 and related δ-FAPbI3 may degrade the photoelectric performance and affect its successful application for solar cells [5]. Zhou et al. demonstrated that the microstructurally engineered PbI2 thin films with porous and low crystallized microstructures are the most favorable precursors for uniform-coverage, pure phase α-FAPbI3 perovskite thin films, by which a planar perovskite solar cell was fabricated and a PCE of 13.8% had been achieved [17]. However, it is difficult to apply the same method to fabricate a mesoporous TiO2 scaffold. Pellet et al. [14] reported an improved PCE 13.4% for the PKSCs using mixed cation lead iodide perovskites (FA1-xMAxPbI3), but the ratio of FA+ to MA+ in the FA1-xMAxPbI3 perovskite is not yet optimized. Moreover, the long term stability of the FA1-xMAxPbI3 perovskite solar cells has not been confirmed yet. 5
Herein, we fabricated a single phase of mixed-organic-cation perovskite FA1-xMAxPbI3 on a mesoporous TiO2 substrate simply using a conventional two-step method. Effects of chemical composition on the photovoltaic performance and long term stability of the mixed-organic-cation perovskite have been studied in details. Due to the synergistic effect of the mixed-organic-cation, a maximum PCE of 16.0% has been achieved in the FA0.7MA0.3PbI3 perovskite solar cells. Meanwhile, the devices showed a good stability, and could maintain the performance without apparent PCE loss when they were exposed in ambient condition (25 ℃, 50%-60% relative humidity, without sealing) for about 20 days.
2. Experimental 2.1. Materials Unless otherwise stated, all of materials were purchased from Alfa Aesar. The process procedures for the MAI and FAI were previously reported [5]. The FAI was firstly prepared by mixing 30 ml hydroiodic acid (57% in water, Aladdin) and 15 g formamidine acetate (Sigma-Aldrich) at 0 ℃ for 2 h with stirring. The precipitates were obtained by evaporating the solutions at 65 ℃ for 1 h. The white products were dissolved in ethanol, recrystallized using 6
diethyl ether, and finally dried at 60 ℃ in a vacuum oven for 24 h. Preparation of the MAI was followed the similar procedures. 2.2. Solar cell fabrication FTO glass substrate was etched using Zn powder and hydrochloric acid. The etched substrate was then cleaned with ethanol, acetone, and deionized water, followed by drying with dry air. Subsequently, a 30 nm-thick TiO2 compact layer (c-TiO2) was spray coated onto the hot FTO glass (450 ℃) using 0.2 M solution of Ti(IV) bis(ethyl acetoacetate)-diisopropoxide in 1-butanol and subsequently annealed at 450 ℃ for 1 h. A mesoporous TiO2 (m-TiO2) layer was deposited by spin coating at 3500 rpm for 60 s using a TiO2 paste (Dyesol 18NRT) diluted in ethanol (1:3 with ethanol by weight). The TiO2 films were then annealed at 500 ℃ for 30 min. PbI2 was dissolved in N,N-dimethylformamide at a concentration of 462 mg mL-1 (1 M), and then spin-coated on the TiO2 coated FTO at 3000 rpm for 60 s, followed by drying at 70 ℃ for 30 min. The films were dipped in a mixed solution of FAI and MAI with different molar ratios (x=0.1, 0.2, 0.3, 0.4, 0.5) in 2-propanol (10 mg mL-1) for 5 min, rinsed with 2-propanol, and dried at 160 ℃ for 15 min. A solution of hole-transporting material (HTM) was prepared by dissolving 72.3 mg spiro-MeOTAD in 1 ml chlorobenzene, to which 28.8 μL 4-tert-butylpyridine and 17.5 μL LiN(CF3SO2)2N (LITSFI) solution 7
(520 mg LITSFI in 1 mL acetonitrile) were added. The HTM layer was deposited onto the FTO glass by spin-coating (3000 rpm, 60 s). Finally, a 80 nm Au electrode was thermally-evaporated to complete the solar cells. 2.3. Material and device characterization Crystalline structures of the films were analyzed using X-ray diffraction (XRD) with a Philip X’Pert PRO X-ray diffractometer (Cu Kα irradiation (λ=1.5406 Å)). The cross-sectional and top-view morphologies of thin films were observed using a Nova NanoSEM 450 scanning electron microscope (SEM, FEI Company). The light absorption measurement was performed using a UV-vis spectrophotometer (Lambda 950, PerkinElmer). The photocurrent density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under one-sun AM 1.5G (100 mW cm-2) illumination with a solar light simulator (Oriel, Model 71675-71580). Photoluminescence (PL excitated at 532 nm) spectra were obtained with a LabRM HR800 spectrometer (Horiba Jobin Yvon Company). The incident photon conversion efficiency (IPCE) was measured with a 150 W xenon lamp (Oriel) fitted with a monochromator (Cornerstone 74004) using a monochromatic light source. The impedance measurements were performed using a potentiostat (IviumStat 10800, Ivium Technologies) in a dark environment. The 8
frequency range was from 1 MHz to 100 mHz.
3.Results and discussion Fig.1a and b depict XRD patterns of the FA1-xMAxPbI3 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) perovskites with different organic cation ratios. As shown in Fig.1a, it can be seen that α-phase is retained in all the samples irrespective of the increase of methylammonium cation concentration. Moreover, there are PbI2 residues in the FA1-xMAxPbI3 (x=0.1, 0.2, 0.4, 0.5) samples except in the FA0.7MA0.3PbI3 sample. The FAPbI3 sample presents the highest intensity of PbI2 peaks, and the relative intensity of PbI2 (001) peak decreases while the intensity ratio of the FA1-xMAxPbI3 (110) peak to PbI2 (001) peak increases gradually with the replacement of FA+ by MA+ to form the FA1-xMAxPbI3 (x=0.1, 0.2) solid solution. It is noted that a pure α-phase FA0.7MA0.3PbI3 perovskite has been obtained without any unreacted PbI2 and δ-FAPbI3 phases when the molar ratio reaches to x=0.3. However, further increasing the molar ratio to x=0.4 and 0.5 results in reappearance of the PbI2 residue, indicating that the stability of FA1-xMAxPbI3 perovskite decreases when x >0.3. As shown in Fig.1b, the (110) peak of the perovskites shifts towards the high angle side with the increase of x [19-21]. Due to the smaller size of MA+ than the FA+ cation, the replacement of FA+ with MA+ 9
results in the lattice shrinkage of perovskite and thus the peak shifts to the higher angle side according to the Bragg equation 2d sinθ = λ [22]. Fig.2a-f show the top-view FESEM images of the perovskite films with different ratios of organic cations. The concentration of MA+ in FA1-xMAxPbI3, which is used to enhance the stability of the α-FAPbI3, has a significant influence on the morphology of the final perovskite films. Due to the larger size of FA+ (2.79 Å), it is kinetically more difficult for the FA+ cations to intercalate into the PbX6 octahedral framework, which may result in some unreacted PbI2 residues. In addition, it is not easy for δ-FAPbI3 to fully transform to α-FAPbI3 within the mesoporous TiO2 scaffold layer due to the stress and space limitation. Therefore, there are some PbI2 residues in the XRD pattern and lots of pinholes in the SEM image of the nominal FAPbI3 sample. As is well known, the surface morphology of the FAPbI3 perovskite layer is primarily affected by the reaction kinetics during its formation [5, 23-24], the addition of MAI can infulence the reaction kinetics and thus greatly influence the final morphology of the perovskite films. Therefore, the morphology and coverage of FA1-xMAxPbI3 perovskite films formed on the mesoporous TiO2 (mp-TiO2) scaffolds are significantly changed compared with pure FAPbI3 when the concentration of MA+ 10
is increased from x=0.1 to x=0.5. For FA1-xMAxPbI3 solid solutions, due to the smaller size of MA+ (2.70 Å) than FA+ (2.79 Å), MA+ is easier to intercalate the PbI6 octahedral framework to form primary MAPbI3 perovskite during the crystallization process, and the primary MAPbI3 phase will then be the nuclei in the subsequent crystallization of FA1-xMAxPbI3 solid solution. Therefore, the residue of PbI2 decreases and the fraction of FA1-xMAxPbI3 solid solution increases with the increase of x from 0 to 0.3, and the surface quality of FA1-xMAxPbI3 solid solution also improves accordingly, and the FA0.7MA0.3PbI3 sample presents a single phase perovskite structure with a homogeneous particle size and without evident voids or pinholes. Further increasing the nominal MA+ content x to 0.4 or 0.5, large amount of primary MAPbI3 perovskite can be formed accordingly. Some of primary MAPbI3 decomposes to PbI2 before the formation of FA1-xMAxPbI3 solid solution because of its unstability at a high annealing temperature of 160oC. That is why PbI2 residue peak reappears in the corresponding XRD patterns, and there are many pin-holes in the SEM morphology of FA1-xMAxPbI3 samples. The low magnified images of all the samples are shown in the supporting information (Fig.S1). Therefore, to achieve a high quality FA1-xMAxPbI3 perovskite film, the ratio of MA+/FA+ in the mixed solution of FAI and MAI is crucial. The microstructure results 11
from SEM observation are consistent with the results of XRD, both of which confirm that FA1-xMAxPbI3 with x=0.3 is the appropriate composition to acquire a pure FA1-xMAxPbI3 perovskite structure with a good surface quality. Fig.3 shows the UV-vis absorption spectra of the FA1-xMAxPbI3 thin films. When the MA+ content is increased from x=0.1 to 0.3, the absorption intensity is enhanced in the shorter wavelength range between 500 nm and 750 nm. However, with further increase of the MA+ content to x=0.4 and 0.5, the absorption intensity decreases dramatically, especially for the x=0.5 sample. As is well known, the UV-vis absorbance ability is closely related to the phase and microstructures of the FA1-xMAxPbI3 perovskite thin films. As demonstrated by the above XRD and SEM results, the FA0.7MA0.3PbI3 sample has a single perovskite phase and good surface quality/coverage on the mesoporous TiO2, therefore, it shows the highest UV-vis absorbance among the samples in the range of 500-750 nm. Fig.4a shows the J-V curves of devices based on the FA1-xMAxPbI3 films with MA+ content ranging from x=0.1 to x=0.5. Fig.4b shows a cross-sectional SEM image of a typical mesoscopic FA0.7MA0.3PbI3 perovskite solar cell, which has a cross-structure of FTO/c-TiO2 (~30 nm)/mesoscopic TiO2 (~300 nm)/FA0.7MA0.3PbI3 12
perovskite/HTM/Au. The FA0.7MA0.3PbI3 perovskite infiltrates into the mesoscopic TiO2 and forms a perovskite overlayer about 200 nm in thickness on the top of mesoscopic TiO2 (Fig.4b). Therefore, the FA0.7MA0.3PbI3 based device has a good structural and chemical affinity between the mp-TiO2/FA0.7MA0.3PbI3 perovskite and HTM layer. The details of the photovoltaic parameters were calculated and are listed in Table S1. The maximum power conversion efficiency of 16.0% is achieved in the device based on the FA0.7MA0.3PbI3 with a short-circuit of 22.6 mA cm-2, an open-circuit voltage of 1.03 V, and a fill factor of 0.68. These good photovoltaic parameters are attributed to the formation of a pure α-phase, a good surface quality/coverage of the FA0.7MA0.3PbI3 sample, as demonstrated by the XRD and SEM results shown above. However, the device’s efficiency decreases with further increases of x from 0.3 to 0.5 or decreasing the MA+ content from x=0.3 to 0. The decreases in the Voc, Jsc and FF values are mainly attributed to the increased recombination of the charge carriers in FA1-xMAxPbI3 (x=0.1, 0.2, 0.4, 0.5) due to the increase of PbI2 phase and the decrease of surface quality and coverage, as verified from results of XRD and SEM. The partial coverage of the perovskite thin film may lead to a direct contact between the hole transporting layer (HTM) and TiO2 contact layer, forming an internal shunt thus resulting in a lower Voc 13
value. The changing trends of Jsc values in all devices are also in good agreements with the UV-vis absorption spectra shown in Fig.3. The device based on the pure phase perovskite yields higher values of Jsc and Voc, indicating efficient charge generation/transport and a better perovskite/ETL junction for the completely converted FA0.7MA0.3PbI3 α-phase. The J-V curves of devices using pure α-phase FA0.7MA0.3PbI3 thin film under the forward and reverse scans at a rate of 10 mV per step are shown in Fig.S2. The maximum PCE of 16.0% is achieved in the reverse scan. Under the forward scan, the value of Voc is stable and the values of Jsc and FF are slightly changed. It indicates that the device has a less hysteresis. As shown in Fig.5b, a stable photocurrent of 20.39 mA cm-2 is obtained, and the steady-state PCE of the device is ~15.5%, which lies between the values extracted from J-V curves measured in the reverse and forward scan directions. The integrated short-circuit photocurrent density (Jsc) values obtained from IPCE spectra is 21.98 mA cm-2 as shown in Fig.5c, which is in good consistence with the Jsc of 22.6 mA cm-2 from the J-V measurements. The PL spectra are effective to be used to explore the recombination properties of light-excited electrons and holes in semiconductors and are related to the morphology of the perovskite 14
and the electronic coupling with the charge carrier quenching layer [21, 25-27]. Fig.6 shows the PL spectra of FA1-xMAxPbI3 layers deposited on mesoporous TiO2 samples. It can be seen that the emission peaks of all samples are located at around 800 nm (corresponding to an excitation wavelength of 532 nm). However, the PL intensity of the samples vary a lot and decrease from FAPbI3 to FA0.7MA0.3PbI3 (from x=0 to x=0.3) and then increases from FA0.7MA0.3PbI3 to FA0.5MA0.5PbI3 (from x=0.3 to x=0.5). The FA0.7MA0.3PbI3 sample presents the lowest peak intensity, indicating a minimum recombination and linking with the best photovoltaic performance. In contrast, the FA0.5MA0.5PbI3 sample exhibits the highest PL signal and thus a higher carrier recombination rate than the other samples. The results of the PL spectra are also consistent with the J-V curves as shown in the Fig.4a. The Nyquist plots of the device in the dark over different forward biases with the FAPbI3 and FA0.7MA0.3PbI3 as the light absorber were obtained and are shown in Fig.7a and 7b, which reveal the interfacial charge transport processes and recombination dynamics of the PKSC devices with different perovskite layers. It is clearly indicated that the difference between the electrochemical impedance spectra of FAPbI3 and FA0.7MA0.3PbI3 at different forward biases is attributed to the recombination processes [28-30], which is associated with the 15
recombination resistance (Rrec). An equivalent circuit was proposed to fit the measured data, as shown in Fig.7c using Z-View software. As shown in Fig. 7d, the recombination resistance is closely related with the applied bias voltage for the FAPbI3 and FA0.7MA0.3PbI3 based PKSCs. It is can seen that the recombination resistance of the FA0.7MA0.3PbI3 based PKSCs is higher than that of the FAPbI3 based PKSC in the applied bias voltage range, which indicates that a poorer recombination occurs at the interfaces of TiO2/FAPbI3/HTM based PKSCs than those of TiO2/FA1-xMAxPbI3/HTM based PKSCs. The results are in a good agreement with the previous reported work [31]. Because in this study, we used the same FTO layer, electron transport layer and HTM layer, and also the Au film of both devices were fabricated in the same process, the increase in the value of Rrec should be attributed to the higher coverage and faster carrier transfer rate of the FA0.7MA0.3PbI3 perovskite films. To check the reproducibility of photovoltaic performance, 20 devices with the same composition were fabricated and tested, the statistical data of the photovoltaic parameters are listed in Table S2 in the supporting information and plotted in Fig.S3. It can be seen that the data show a good reproducibility with relatively low standard deviations. The air stability of unsealed devices with different perovskite 16
films was also evaluated. These devices were exposed in the ambient environment at 25 ℃ with a relative humidity 50%-60%. Their performance variation is summarized in Fig.8. Apparently, the FA0.7MA0.3PbI3 based PKSCs show a better stability than those of the FAPbI3 and MAPbI3 based PKSCs in the air. As discussed above, the FA0.7MA0.3PbI3 sample has a higher surface coverage and pure α-phase structure without PbI2 residue and non-perovskite polymorph, thus it can decrease the sensitivity to the moisture and oxygen, Based on these benefuts, the FA0.7MA0.3PbI3 devices have a superior stability than both the FAPbI3 and MAPbI3 based devices in the air [32-33].
3. Conclusions In summary, we have fabricated FA1-xMAxPbI3 mesoscopic solar cells using a conventional two-step method. Effect of chemical composition on the photovoltaic performance and stability of the mixed-organic-cation perovskite was studied in details. It shows that the film morphology, crystallinity, and optical and electrical properties of FA1-xMAxPbI3 are significantly different when the x value is changed. Highly uniform and crystalline films with a pure α-phase structure have been achieved for the FA0.7MA0.3PbI3 sample 17
and PKSCs based on this film produced a maximum PCE of 16.0% with a stabilized PCE output of about 15.5%. Furthermore, the stability of the FA0.7MA0.3PbI3 devices is also improved compared with those of both the FAPbI3 and MAPbI3 devices. The unsealed FA0.7MA0.3PbI3 devices exposed in ambient condition could maintain the performance almost without any PCE loss for about 20 days. These results indicate that the content of MA+ plays a crucial role during the fabrication of FA+-based perovskite solar cell. Further optimization of the FA0.7MA0.3PbI3 devices is under investigation.
Acknowledgements This work is co-financed by National Natural Science Foundation of China (Grant no. 51572098 and 51272080), National Basic Research Program of China (Grant no. 2013CB632500), Natural Science Foundation of Hubei Province (Grant no. 2015CFB432), Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology (Grant no. 2016-KF-5), Technology innovation fund project of Huazhong University of Science and Technology Innovation Research College. The technical assistance from the Analytical and Testing Center of HUST is likewise gratefully acknowledged.
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Nano letters, 2014, 14(6): 3608-3616. [17] Zhou Y, Yang M, Kwun J, et al. Intercalation crystallization of phase-pure α-HC(NH2)2PbI3 upon microstructurally engineered PbI2 thin films for planar perovskite solar cells[J]. Nanoscale, 2016. [18] Wang F, Yu H, Xu H, et al. HPbI3: A New Precursor Compound for Highly Efficient Solution‐Processed Perovskite Solar Cells[J]. Advanced Functional Materials, 2015, 25(7): 1120-1126. [19] Lv S, Pang S, Zhou Y, et al. One-step, solution-processed formamidinium lead trihalide (FAPbI(3-x)Clx) for mesoscopic perovskite–polymer solar cells[J]. Physical Chemistry Chemical Physics, 2014, 16(36): 19206-19211. [20] Wang Z, Zhou Y, Pang S, et al. Additive-Modulated Evolution of HC(NH2)2PbI3 Black Polymorph for Mesoscopic Perovskite Solar Cells[J]. Chemistry of Materials, 2015, 27(20): 7149-7155. [21] Zuo L, Gu Z, Ye T, et al. Enhanced photovoltaic performance of CH3NH3PbI3 perovskite solar cells through interfacial engineering using self-assembling monolayer[J]. Journal of the American Chemical Society, 2015, 137(7): 2674-2679. [22] Lü X, Mou X, Wu J, et al. Improved-Performance Dye-Sensitized Solar Cells Using Nb-Doped TiO2 Electrodes: Efficient Electron Injection and Transfer[J]. Advanced Functional Materials, 2010, 20(3): 509-515. [23] Deng Y, Peng E, Shao Y, et al. Scalable fabrication of efficient organolead trihalide perovskite solar cells with doctor-bladed active layers[J]. Energy & Environmental Science, 2015, 8(5): 1544-1550. [24] Han Q, Bae S H, Sun P, et al. Single Crystal Formamidinium Lead Iodide (FAPbI3): Insight into the Structural, Optical, and Electrical Properties[J]. Advanced Materials, 2016. [25] Hu L, Wang W, Liu H, et al. PbS colloidal quantum dots as an effective hole transporter for planar heterojunction perovskite solar cells[J]. Journal of Materials Chemistry A, 2015, 3(2): 515-518. [26] Sun S, Salim T, Mathews N, et al. The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells[J]. Energy & Environmental Science, 2014, 7(1): 399-407. [27] Hou Y, Yang J, Jiang Q, et al. Enhancement of photovoltaic performance of perovskite solar cells by modification of the interface between the perovskite and mesoporous TiO 2 film[J]. Solar Energy Materials and Solar Cells, 2016, 155: 101-107. [28] Jeon N J, Lee J, Noh J H, et al. Efficient inorganic–organic hybrid perovskite solar cells based on pyrene arylamine derivatives as hole-transporting materials[J]. Journal of the American Chemical Society, 2013, 135(51): 19087-19090. [29] Heo J H, Im S H, Noh J H, et al. Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors[J]. Nature Photonics, 2013, 7(6): 486-491. [30] Krishnamoorthy T, Kunwu F, Boix P P, et al. A swivel-cruciform thiophene based hole-transporting material for efficient perovskite solar cells[J]. Journal of Materials Chemistry A, 2014, 2(18): 6305-6309. 20
[31] Zhu L, Xiao J, Shi J, et al. Efficient CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP hole-transporting layers[J]. Nano Research, 2015, 8(4): 1116-1127. [32] Berhe T A, Su W N, Chen C H, et al. Organometal halide perovskite solar cells: degradation and stability[J]. Energy & Environmental Science, 2016, 9(2): 323-356. [33] Yusoff A R M, Nazeeruddin M K. Organohalide Lead Perovskites for Photovoltaic Applications[J]. The journal of physical chemistry letters, 2016, 7(5): 851-866. .
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Fig.1. (a) X-ray diffraction patterns of the FA1-xMAxPbI3 perovskite thin films (x=0, 0.1, 0.2, 0.3, 0.4, 0.5). (b) Magnified X-ray diffraction patterns between 10 and 20° for the FA1-xMAxPbI3 22
perovskites (x=0, 0.1, 0.2, 0.3, 0.4, 0.5).
Fig.2. Top-view FESEM images of the FA1-xMAxPbI3 perovskite thin films: (a) x=0, (b) x=0.1, (c) x=0.2, (d) x=0.3, (e) x=0.4, and (f) x=0.5.
23
Fig.3. UV-vis absorption spectra of the FA1-xMAxPbI3 perovskite thin films prepared at various x values.
24
Fig.4. (a) Photocurrent density-voltage (J-V) characteristics of 25
mesoscopic perovskite solar cells based on the FA1-xMAxPbI3 films. (b) Cross-sectional SEM image of a typical FA0.7MA0.3PbI3-based mesoscopic perovskite solar cell.
26
Fig.5. (a) Steady-state PCE and photocurrent density at maximum power point as a function of time. (b) IPCE spectrum of the 27
champion PKSC.
Fig.6. The steady-state PL spectra of FA1-xMAxPbI3 perovskite thin films.
28
Fig.7. Nyquist plots of the devices with FAPbI3 (a) and FA0.7MA0.3PbI3 (b) perovskite thin films tested in the dark over different forward biases. (c) Equivalent circuit for fitting curves. (d) Plots of the recombination resistance (Rrec) for FAPbI3 and FA0.7MA0.3PbI3 perovskite solar cells, respectively.
29
Fig.8. Environmental stability test of the unsealed perovskite solar cells. Variation of normalized PCE of MAPbI3, FAPbI3 and FA0.7MA0.3PbI3 perovskite solar cells stored in air (humidity: 50-60%, temperature: 25 ℃) as a function of time.
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S0013-4686(17)31457-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.07.040 EA 29857
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
15-2-2017 1-5-2017 6-7-2017
Please cite this article as: Xin Li, Junyou Yang, Qinghui Jiang, Weijing Chu, Dan Zhang, Zhiwei Zhou, Yangyang Ren, Jiwu Xin, Enhanced photovoltaic performance and stability in mixed-cation perovskite solar cells via compositional modulation, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced photovoltaic performance and stability in mixed-cation perovskite solar cells via compositional modulation
Xin Li1,2, Junyou Yang1,2*, Qinghui Jiang1,2, Weijing Chu1,2, Dan Zhang1,2, Zhiwei Zhou1,2, Yangyang Ren1,2, Jiwu Xin1,2
1. State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, P.R. China 2. Shenzhen Institute of Huazhong University of Science & Technology, Shenzhen 51800, P.R. China *To whom correspondence should be addressed, email: [email protected]
Graphical abstract
1
Highlights A pure phase of FA1-xMAxPbI3 perovskite FA1-xMAxPbI3 was prepared. Discussing effect of chemical composition and morphology on the photovoltaic performance in detail. The PCE of PKSC based on FA0.7MA0.3PbI3 reaches up to 16.0%. The long term stability and reproducibility is obtained.
Abstract Preparation of high-quality and pure phase mixed-organic-cation FA1-xMAxPbI3 perovskite film is a big challenge, especially on a porous oxide scaffold for mesoscopic perovskite solar cells. In this study, we fabricated a pure phase of mixed-organic-cation perovskite FA1-xMAxPbI3 simply using a conventional two-step method. Effect of chemical composition and morphology on the photovoltaic performance and stability of the mixed-organic-cation perovskite has been studied in details. Results show that the FA1-xMAxPbI3 perovskite thin films with a FA+:MA+ molar ratio of 0.7:0.3 demonstrate the best photovoltaic performance. Mesoscopic perovskite solar cells fabricated using these FA0.7MA0.3PbI3 thin films yield a power conversion efficiency (PCE) up to 16.0%. 2
Moreover, the devices present a superior stability and the unsealed devices exposed in ambient condition (25 ℃, 50%-60% relative humidity) can maintain their photoelectric performance without apparent PCE loss for up to 20 days.
Keywords: Compositional modulation; Mixed organic cation; Enhanced photoelectric performance; Long term stability; Perovskite solar cells.
1. Introduction Organolead halide perovskite solar cells (PKSCs) have received much attention over the last several years, predominantly driven by the inevitable and huge demands for photoelectric active materials with high absorption coefficient, broad absorption range, large charge carriers mobility, suitable direct band gap, long carrier diffusion length, facile solution fabrication process and outstanding power conversion efficiency (PCE) [1-5]. Nowadays, the highest certified power conversion efficiency of PKSCs has reached over 22.1% [6], and has been regarded as one of the best candidates for replacing the conventional Si-based solar cells. The chemical formula of perovskite is ABX3, where A is the organic cation (A=CH3NH2+ or HC(CH2)2+), B is the metal cation (B=Pb2+ or Sn2+) 3
and X is the halide anion (X=I-, Br- or Cl-) [1-5]. Up to now, most of the reported PKSCs are based on the CH3NH3PbI3 perovskite (MAPbI3). Although the MAPbI3 has excellent photovoltaic properties with a bandgap of ~1.55 eV, it is less stable and decomposes rapidly even at a mediate elevated temperature less than 100 ℃once exposed in the air [7-9]. Comparatively, the HC(NH2)2PbI3 perovskite (FAPbI3) is relatively more stable, but it is less studied yet. In general, the FAPbI3 has two possible crystal structures at ambient temperature [10-12]. The first one is α-FAPbI3 phase (space group P3m1), which is black with good photoelectric properties, and the second one is δ-FAPbI3 phase (space group P63mc), which is yellow with an indirect bandgap of 2.48 eV. In comparison with MAPbI3, α-FAPbI3 has a suitable direct bandgap of about 1.47 eV and superior properties [12-15], such as extended absorption range [10-13], higher phase conversion and photostability [12]. However, preparation technique of high-quality FAPbI3 perovskite films is still a big challenge, especially for those based on a mesoscopic scaffold of oxides, e.g. mesoporous TiO2 films. On one hand, the ion size of FA+ (2.79 Å) is larger than that of the MA+ (2.70 Å), and thus the intercalation FA+ cation into the PbX6 octahedral framework is kinetically more difficult, which may result in some unreacted PbI2 4
residues [16-17]. On the other hand, compared with the single phase homomorphic MAPbI3, it is hard to prepare a pure α-FAPbI3 phase because of the formation of some δ-FAPbI3 phases accompanying along the crystallization process of α-FAPbI3 in the thin film. In theory, the δ-FAPbI3 phase can transform to the photoactive α-FAPbI3 phase at an elevated temperature (>150 ℃) [10, 13, 15, 18]. However, it is not easy for δ-FAPbI3 to be fully transformed into α-FAPbI3 within the scaffold layer probably because of the stress from the substrate, the residual PbI2 and related δ-FAPbI3 may degrade the photoelectric performance and affect its successful application for solar cells [5]. Zhou et al. demonstrated that the microstructurally engineered PbI2 thin films with porous and low crystallized microstructures are the most favorable precursors for uniform-coverage, pure phase α-FAPbI3 perovskite thin films, by which a planar perovskite solar cell was fabricated and a PCE of 13.8% had been achieved [17]. However, it is difficult to apply the same method to fabricate a mesoporous TiO2 scaffold. Pellet et al. [14] reported an improved PCE 13.4% for the PKSCs using mixed cation lead iodide perovskites (FA1-xMAxPbI3), but the ratio of FA+ to MA+ in the FA1-xMAxPbI3 perovskite is not yet optimized. Moreover, the long term stability of the FA1-xMAxPbI3 perovskite solar cells has not been confirmed yet. 5
Herein, we fabricated a single phase of mixed-organic-cation perovskite FA1-xMAxPbI3 on a mesoporous TiO2 substrate simply using a conventional two-step method. Effects of chemical composition on the photovoltaic performance and long term stability of the mixed-organic-cation perovskite have been studied in details. Due to the synergistic effect of the mixed-organic-cation, a maximum PCE of 16.0% has been achieved in the FA0.7MA0.3PbI3 perovskite solar cells. Meanwhile, the devices showed a good stability, and could maintain the performance without apparent PCE loss when they were exposed in ambient condition (25 ℃, 50%-60% relative humidity, without sealing) for about 20 days.
2. Experimental 2.1. Materials Unless otherwise stated, all of materials were purchased from Alfa Aesar. The process procedures for the MAI and FAI were previously reported [5]. The FAI was firstly prepared by mixing 30 ml hydroiodic acid (57% in water, Aladdin) and 15 g formamidine acetate (Sigma-Aldrich) at 0 ℃ for 2 h with stirring. The precipitates were obtained by evaporating the solutions at 65 ℃ for 1 h. The white products were dissolved in ethanol, recrystallized using 6
diethyl ether, and finally dried at 60 ℃ in a vacuum oven for 24 h. Preparation of the MAI was followed the similar procedures. 2.2. Solar cell fabrication FTO glass substrate was etched using Zn powder and hydrochloric acid. The etched substrate was then cleaned with ethanol, acetone, and deionized water, followed by drying with dry air. Subsequently, a 30 nm-thick TiO2 compact layer (c-TiO2) was spray coated onto the hot FTO glass (450 ℃) using 0.2 M solution of Ti(IV) bis(ethyl acetoacetate)-diisopropoxide in 1-butanol and subsequently annealed at 450 ℃ for 1 h. A mesoporous TiO2 (m-TiO2) layer was deposited by spin coating at 3500 rpm for 60 s using a TiO2 paste (Dyesol 18NRT) diluted in ethanol (1:3 with ethanol by weight). The TiO2 films were then annealed at 500 ℃ for 30 min. PbI2 was dissolved in N,N-dimethylformamide at a concentration of 462 mg mL-1 (1 M), and then spin-coated on the TiO2 coated FTO at 3000 rpm for 60 s, followed by drying at 70 ℃ for 30 min. The films were dipped in a mixed solution of FAI and MAI with different molar ratios (x=0.1, 0.2, 0.3, 0.4, 0.5) in 2-propanol (10 mg mL-1) for 5 min, rinsed with 2-propanol, and dried at 160 ℃ for 15 min. A solution of hole-transporting material (HTM) was prepared by dissolving 72.3 mg spiro-MeOTAD in 1 ml chlorobenzene, to which 28.8 μL 4-tert-butylpyridine and 17.5 μL LiN(CF3SO2)2N (LITSFI) solution 7
(520 mg LITSFI in 1 mL acetonitrile) were added. The HTM layer was deposited onto the FTO glass by spin-coating (3000 rpm, 60 s). Finally, a 80 nm Au electrode was thermally-evaporated to complete the solar cells. 2.3. Material and device characterization Crystalline structures of the films were analyzed using X-ray diffraction (XRD) with a Philip X’Pert PRO X-ray diffractometer (Cu Kα irradiation (λ=1.5406 Å)). The cross-sectional and top-view morphologies of thin films were observed using a Nova NanoSEM 450 scanning electron microscope (SEM, FEI Company). The light absorption measurement was performed using a UV-vis spectrophotometer (Lambda 950, PerkinElmer). The photocurrent density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under one-sun AM 1.5G (100 mW cm-2) illumination with a solar light simulator (Oriel, Model 71675-71580). Photoluminescence (PL excitated at 532 nm) spectra were obtained with a LabRM HR800 spectrometer (Horiba Jobin Yvon Company). The incident photon conversion efficiency (IPCE) was measured with a 150 W xenon lamp (Oriel) fitted with a monochromator (Cornerstone 74004) using a monochromatic light source. The impedance measurements were performed using a potentiostat (IviumStat 10800, Ivium Technologies) in a dark environment. The 8
frequency range was from 1 MHz to 100 mHz.
3.Results and discussion Fig.1a and b depict XRD patterns of the FA1-xMAxPbI3 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) perovskites with different organic cation ratios. As shown in Fig.1a, it can be seen that α-phase is retained in all the samples irrespective of the increase of methylammonium cation concentration. Moreover, there are PbI2 residues in the FA1-xMAxPbI3 (x=0.1, 0.2, 0.4, 0.5) samples except in the FA0.7MA0.3PbI3 sample. The FAPbI3 sample presents the highest intensity of PbI2 peaks, and the relative intensity of PbI2 (001) peak decreases while the intensity ratio of the FA1-xMAxPbI3 (110) peak to PbI2 (001) peak increases gradually with the replacement of FA+ by MA+ to form the FA1-xMAxPbI3 (x=0.1, 0.2) solid solution. It is noted that a pure α-phase FA0.7MA0.3PbI3 perovskite has been obtained without any unreacted PbI2 and δ-FAPbI3 phases when the molar ratio reaches to x=0.3. However, further increasing the molar ratio to x=0.4 and 0.5 results in reappearance of the PbI2 residue, indicating that the stability of FA1-xMAxPbI3 perovskite decreases when x >0.3. As shown in Fig.1b, the (110) peak of the perovskites shifts towards the high angle side with the increase of x [19-21]. Due to the smaller size of MA+ than the FA+ cation, the replacement of FA+ with MA+ 9
results in the lattice shrinkage of perovskite and thus the peak shifts to the higher angle side according to the Bragg equation 2d sinθ = λ [22]. Fig.2a-f show the top-view FESEM images of the perovskite films with different ratios of organic cations. The concentration of MA+ in FA1-xMAxPbI3, which is used to enhance the stability of the α-FAPbI3, has a significant influence on the morphology of the final perovskite films. Due to the larger size of FA+ (2.79 Å), it is kinetically more difficult for the FA+ cations to intercalate into the PbX6 octahedral framework, which may result in some unreacted PbI2 residues. In addition, it is not easy for δ-FAPbI3 to fully transform to α-FAPbI3 within the mesoporous TiO2 scaffold layer due to the stress and space limitation. Therefore, there are some PbI2 residues in the XRD pattern and lots of pinholes in the SEM image of the nominal FAPbI3 sample. As is well known, the surface morphology of the FAPbI3 perovskite layer is primarily affected by the reaction kinetics during its formation [5, 23-24], the addition of MAI can infulence the reaction kinetics and thus greatly influence the final morphology of the perovskite films. Therefore, the morphology and coverage of FA1-xMAxPbI3 perovskite films formed on the mesoporous TiO2 (mp-TiO2) scaffolds are significantly changed compared with pure FAPbI3 when the concentration of MA+ 10
is increased from x=0.1 to x=0.5. For FA1-xMAxPbI3 solid solutions, due to the smaller size of MA+ (2.70 Å) than FA+ (2.79 Å), MA+ is easier to intercalate the PbI6 octahedral framework to form primary MAPbI3 perovskite during the crystallization process, and the primary MAPbI3 phase will then be the nuclei in the subsequent crystallization of FA1-xMAxPbI3 solid solution. Therefore, the residue of PbI2 decreases and the fraction of FA1-xMAxPbI3 solid solution increases with the increase of x from 0 to 0.3, and the surface quality of FA1-xMAxPbI3 solid solution also improves accordingly, and the FA0.7MA0.3PbI3 sample presents a single phase perovskite structure with a homogeneous particle size and without evident voids or pinholes. Further increasing the nominal MA+ content x to 0.4 or 0.5, large amount of primary MAPbI3 perovskite can be formed accordingly. Some of primary MAPbI3 decomposes to PbI2 before the formation of FA1-xMAxPbI3 solid solution because of its unstability at a high annealing temperature of 160oC. That is why PbI2 residue peak reappears in the corresponding XRD patterns, and there are many pin-holes in the SEM morphology of FA1-xMAxPbI3 samples. The low magnified images of all the samples are shown in the supporting information (Fig.S1). Therefore, to achieve a high quality FA1-xMAxPbI3 perovskite film, the ratio of MA+/FA+ in the mixed solution of FAI and MAI is crucial. The microstructure results 11
from SEM observation are consistent with the results of XRD, both of which confirm that FA1-xMAxPbI3 with x=0.3 is the appropriate composition to acquire a pure FA1-xMAxPbI3 perovskite structure with a good surface quality. Fig.3 shows the UV-vis absorption spectra of the FA1-xMAxPbI3 thin films. When the MA+ content is increased from x=0.1 to 0.3, the absorption intensity is enhanced in the shorter wavelength range between 500 nm and 750 nm. However, with further increase of the MA+ content to x=0.4 and 0.5, the absorption intensity decreases dramatically, especially for the x=0.5 sample. As is well known, the UV-vis absorbance ability is closely related to the phase and microstructures of the FA1-xMAxPbI3 perovskite thin films. As demonstrated by the above XRD and SEM results, the FA0.7MA0.3PbI3 sample has a single perovskite phase and good surface quality/coverage on the mesoporous TiO2, therefore, it shows the highest UV-vis absorbance among the samples in the range of 500-750 nm. Fig.4a shows the J-V curves of devices based on the FA1-xMAxPbI3 films with MA+ content ranging from x=0.1 to x=0.5. Fig.4b shows a cross-sectional SEM image of a typical mesoscopic FA0.7MA0.3PbI3 perovskite solar cell, which has a cross-structure of FTO/c-TiO2 (~30 nm)/mesoscopic TiO2 (~300 nm)/FA0.7MA0.3PbI3 12
perovskite/HTM/Au. The FA0.7MA0.3PbI3 perovskite infiltrates into the mesoscopic TiO2 and forms a perovskite overlayer about 200 nm in thickness on the top of mesoscopic TiO2 (Fig.4b). Therefore, the FA0.7MA0.3PbI3 based device has a good structural and chemical affinity between the mp-TiO2/FA0.7MA0.3PbI3 perovskite and HTM layer. The details of the photovoltaic parameters were calculated and are listed in Table S1. The maximum power conversion efficiency of 16.0% is achieved in the device based on the FA0.7MA0.3PbI3 with a short-circuit of 22.6 mA cm-2, an open-circuit voltage of 1.03 V, and a fill factor of 0.68. These good photovoltaic parameters are attributed to the formation of a pure α-phase, a good surface quality/coverage of the FA0.7MA0.3PbI3 sample, as demonstrated by the XRD and SEM results shown above. However, the device’s efficiency decreases with further increases of x from 0.3 to 0.5 or decreasing the MA+ content from x=0.3 to 0. The decreases in the Voc, Jsc and FF values are mainly attributed to the increased recombination of the charge carriers in FA1-xMAxPbI3 (x=0.1, 0.2, 0.4, 0.5) due to the increase of PbI2 phase and the decrease of surface quality and coverage, as verified from results of XRD and SEM. The partial coverage of the perovskite thin film may lead to a direct contact between the hole transporting layer (HTM) and TiO2 contact layer, forming an internal shunt thus resulting in a lower Voc 13
value. The changing trends of Jsc values in all devices are also in good agreements with the UV-vis absorption spectra shown in Fig.3. The device based on the pure phase perovskite yields higher values of Jsc and Voc, indicating efficient charge generation/transport and a better perovskite/ETL junction for the completely converted FA0.7MA0.3PbI3 α-phase. The J-V curves of devices using pure α-phase FA0.7MA0.3PbI3 thin film under the forward and reverse scans at a rate of 10 mV per step are shown in Fig.S2. The maximum PCE of 16.0% is achieved in the reverse scan. Under the forward scan, the value of Voc is stable and the values of Jsc and FF are slightly changed. It indicates that the device has a less hysteresis. As shown in Fig.5b, a stable photocurrent of 20.39 mA cm-2 is obtained, and the steady-state PCE of the device is ~15.5%, which lies between the values extracted from J-V curves measured in the reverse and forward scan directions. The integrated short-circuit photocurrent density (Jsc) values obtained from IPCE spectra is 21.98 mA cm-2 as shown in Fig.5c, which is in good consistence with the Jsc of 22.6 mA cm-2 from the J-V measurements. The PL spectra are effective to be used to explore the recombination properties of light-excited electrons and holes in semiconductors and are related to the morphology of the perovskite 14
and the electronic coupling with the charge carrier quenching layer [21, 25-27]. Fig.6 shows the PL spectra of FA1-xMAxPbI3 layers deposited on mesoporous TiO2 samples. It can be seen that the emission peaks of all samples are located at around 800 nm (corresponding to an excitation wavelength of 532 nm). However, the PL intensity of the samples vary a lot and decrease from FAPbI3 to FA0.7MA0.3PbI3 (from x=0 to x=0.3) and then increases from FA0.7MA0.3PbI3 to FA0.5MA0.5PbI3 (from x=0.3 to x=0.5). The FA0.7MA0.3PbI3 sample presents the lowest peak intensity, indicating a minimum recombination and linking with the best photovoltaic performance. In contrast, the FA0.5MA0.5PbI3 sample exhibits the highest PL signal and thus a higher carrier recombination rate than the other samples. The results of the PL spectra are also consistent with the J-V curves as shown in the Fig.4a. The Nyquist plots of the device in the dark over different forward biases with the FAPbI3 and FA0.7MA0.3PbI3 as the light absorber were obtained and are shown in Fig.7a and 7b, which reveal the interfacial charge transport processes and recombination dynamics of the PKSC devices with different perovskite layers. It is clearly indicated that the difference between the electrochemical impedance spectra of FAPbI3 and FA0.7MA0.3PbI3 at different forward biases is attributed to the recombination processes [28-30], which is associated with the 15
recombination resistance (Rrec). An equivalent circuit was proposed to fit the measured data, as shown in Fig.7c using Z-View software. As shown in Fig. 7d, the recombination resistance is closely related with the applied bias voltage for the FAPbI3 and FA0.7MA0.3PbI3 based PKSCs. It is can seen that the recombination resistance of the FA0.7MA0.3PbI3 based PKSCs is higher than that of the FAPbI3 based PKSC in the applied bias voltage range, which indicates that a poorer recombination occurs at the interfaces of TiO2/FAPbI3/HTM based PKSCs than those of TiO2/FA1-xMAxPbI3/HTM based PKSCs. The results are in a good agreement with the previous reported work [31]. Because in this study, we used the same FTO layer, electron transport layer and HTM layer, and also the Au film of both devices were fabricated in the same process, the increase in the value of Rrec should be attributed to the higher coverage and faster carrier transfer rate of the FA0.7MA0.3PbI3 perovskite films. To check the reproducibility of photovoltaic performance, 20 devices with the same composition were fabricated and tested, the statistical data of the photovoltaic parameters are listed in Table S2 in the supporting information and plotted in Fig.S3. It can be seen that the data show a good reproducibility with relatively low standard deviations. The air stability of unsealed devices with different perovskite 16
films was also evaluated. These devices were exposed in the ambient environment at 25 ℃ with a relative humidity 50%-60%. Their performance variation is summarized in Fig.8. Apparently, the FA0.7MA0.3PbI3 based PKSCs show a better stability than those of the FAPbI3 and MAPbI3 based PKSCs in the air. As discussed above, the FA0.7MA0.3PbI3 sample has a higher surface coverage and pure α-phase structure without PbI2 residue and non-perovskite polymorph, thus it can decrease the sensitivity to the moisture and oxygen, Based on these benefuts, the FA0.7MA0.3PbI3 devices have a superior stability than both the FAPbI3 and MAPbI3 based devices in the air [32-33].
3. Conclusions In summary, we have fabricated FA1-xMAxPbI3 mesoscopic solar cells using a conventional two-step method. Effect of chemical composition on the photovoltaic performance and stability of the mixed-organic-cation perovskite was studied in details. It shows that the film morphology, crystallinity, and optical and electrical properties of FA1-xMAxPbI3 are significantly different when the x value is changed. Highly uniform and crystalline films with a pure α-phase structure have been achieved for the FA0.7MA0.3PbI3 sample 17
and PKSCs based on this film produced a maximum PCE of 16.0% with a stabilized PCE output of about 15.5%. Furthermore, the stability of the FA0.7MA0.3PbI3 devices is also improved compared with those of both the FAPbI3 and MAPbI3 devices. The unsealed FA0.7MA0.3PbI3 devices exposed in ambient condition could maintain the performance almost without any PCE loss for about 20 days. These results indicate that the content of MA+ plays a crucial role during the fabrication of FA+-based perovskite solar cell. Further optimization of the FA0.7MA0.3PbI3 devices is under investigation.
Acknowledgements This work is co-financed by National Natural Science Foundation of China (Grant no. 51572098 and 51272080), National Basic Research Program of China (Grant no. 2013CB632500), Natural Science Foundation of Hubei Province (Grant no. 2015CFB432), Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology (Grant no. 2016-KF-5), Technology innovation fund project of Huazhong University of Science and Technology Innovation Research College. The technical assistance from the Analytical and Testing Center of HUST is likewise gratefully acknowledged.
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Fig.1. (a) X-ray diffraction patterns of the FA1-xMAxPbI3 perovskite thin films (x=0, 0.1, 0.2, 0.3, 0.4, 0.5). (b) Magnified X-ray diffraction patterns between 10 and 20° for the FA1-xMAxPbI3 22
perovskites (x=0, 0.1, 0.2, 0.3, 0.4, 0.5).
Fig.2. Top-view FESEM images of the FA1-xMAxPbI3 perovskite thin films: (a) x=0, (b) x=0.1, (c) x=0.2, (d) x=0.3, (e) x=0.4, and (f) x=0.5.
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Fig.3. UV-vis absorption spectra of the FA1-xMAxPbI3 perovskite thin films prepared at various x values.
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Fig.4. (a) Photocurrent density-voltage (J-V) characteristics of 25
mesoscopic perovskite solar cells based on the FA1-xMAxPbI3 films. (b) Cross-sectional SEM image of a typical FA0.7MA0.3PbI3-based mesoscopic perovskite solar cell.
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Fig.5. (a) Steady-state PCE and photocurrent density at maximum power point as a function of time. (b) IPCE spectrum of the 27
champion PKSC.
Fig.6. The steady-state PL spectra of FA1-xMAxPbI3 perovskite thin films.
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Fig.7. Nyquist plots of the devices with FAPbI3 (a) and FA0.7MA0.3PbI3 (b) perovskite thin films tested in the dark over different forward biases. (c) Equivalent circuit for fitting curves. (d) Plots of the recombination resistance (Rrec) for FAPbI3 and FA0.7MA0.3PbI3 perovskite solar cells, respectively.
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Fig.8. Environmental stability test of the unsealed perovskite solar cells. Variation of normalized PCE of MAPbI3, FAPbI3 and FA0.7MA0.3PbI3 perovskite solar cells stored in air (humidity: 50-60%, temperature: 25 ℃) as a function of time.
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