PCCP PAPER. Inverted planar NH 2 CHQNH 2 PbI 3 perovskite solar cells with 13.56% efficiency via low temperature processing.

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1 PCCP PAPER View Article Online View Journal Cite this: DOI: /c5cp02705e Inverted planar NH 2 CHQNH 2 PbI 3 perovskite solar cells with 13.56% efficiency via low temperature processing Da-Xing Yuan, a Adam Gorka, b Mei-Feng Xu, a Zhao-Kui Wang* a and Liang-Sheng Liao* a Received 11th May 2015, Accepted 10th June 2015 DOI: /c5cp02705e In this work, NH 2 CHQNH 2 PbI 3 (FAPbI 3 ) was employed for light harvesting in inverted planer perovskite solar cells for the first time. Except for the silver cathode, all layers were solution-processed under or below 140 1C. The effect of the annealing process on device performance was investigated. The FAPbI 3 solar cells based on a slowed-down annealing shows superior performance compared to the CH 3 NH 3 PbI 3 (MAPbI 3 )-based devices, especially for the short circuit current density. A power conversion efficiency of 13.56% was obtained with high short circuit current density of ma cm 2. This work paves the way for low-temperature fabrication of efficient inverted planer structure FAPbI 3 perovskite solar cells. Introduction Since organo-metal halide perovskite was first employed for light harvesting in solar cells in 2009, 1 much effort has been made to improve device performance, including introducing solid state hole conductors, 2 modifying the fabrication process 3 10 and the annealing process, structure engineering, interface engineering and so on. So far, the highest performance has overcome 20%. 31 However, most work was based on the CH 3 NH 3 PbI 3 (or CH 3 NH 3 PbI 3 x Cl x ) system, in which its energy bandgap (B1.55 ev) is beyond the ideal bandgap of photovoltaic materials ( ev). What s more, the CH 3 NH 3 PbI 3 system was reported to have a very low phase transition temperature, which will strongly influence the device stability. 32 Recently, a brand new perovskite system, known as NH 2 CHQ NH 2 PbI 3 (FAPbI 3 ) perovskite, has drawn much attention, since it has superior properties compared with the CH 3 NH 3 PbI 3 (MAPbI 3 ) system, such as extended absorption range, higher phase transition temperature and better photostability. 36 a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu , China. zkwang@suda.edu.cn, lsliao@suda.edu.cn; Tel: b Department of Physics & Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, N2L 3G1, Ontario, Canada Electronic supplementary information (ESI) available: EQE spectra of the onestep annealed and slowed-down annealed FAPbI 3 perovskite solar cells, when PCBM acts as an electron transporting layer. J V curve of a representative modified FAPbI 3 device tested under forward and reverse bias, respectively. J V curve of a representative slowed-down annealed FAPbI 3 based solar cell tested every 10 min in air under continuous light illumination. Photovoltaic parameters of some representative modified FAPbI 3 perovskite solar cells. See DOI: /c5cp02705e By introducing a small amount of hydroiodic acid (HI) to the FAPbI 3 precursor solution, Snaith et al. obtained a compact and uniform one-step spin-coating processed FAPbI 3 perovskite layer, and a high performance of 14.9% in power conversion efficiency (PCE) was realized. Park and co-workers 36 achieved a current density voltage ( J V) hysteresis-free and photostable FAPbI 3 perovskite solar cell based on a mesoporous structure by using a sequential deposition technique. Zhao et al. reported that FAPbI 3 perovskite films exhibit a very pure crystalline phase with a strong (110) preferred orientation when a new precursor compound of HPbI 3 was introduced. 37 However, all related works were based on a conventional structure, in which condensed or mesoporous TiO 2 acts as the bottom electron extraction layer and allows for a pretty high temperature ( C). Such a high temperature is not suitable for low cost and flexibility. In this work, FAPbI 3 was employed as the light harvester in inverted planer perovskite solar cells for the first time. Except for the silver cathode, all layers were solution-processed at or below 140 1C. With a slowed-down annealing process, the FAPbI 3 perovskite layer showed high crystallinity, large grain size and full surface coverage. A power conversion efficiency of 13.56% was obtained with a high short circuit current density of ma cm 2. Experimental section Device fabrication The NH 2 CHQNH 2 PbI 3 (FAPbI 3 ) based perovskite solar cell with the device structure of ITO/PEDOT:PSS/FAPbI 3 /PCBM/BCP/Ag (Fig. 1) was fabricated according to the following steps. Firstly, This journal is the Owner Societies 2015 Phys. Chem. Chem. Phys.

2 View Article Online Paper PCCP Fig. 1 (a) Device structure of the FAPbI 3 -based perovskite solar cell. (b) Crystal structure of the FAPbI 3 perovskite. pre-cleaned ITO-coated glass substrates were treated by ultraviolet-ozone for 15 min. The poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, Clevious AI 4083) layer was deposited by spin-coating at 4000 rpm for 40 s and annealed at 140 1C for 15 min in air. Then the substrates were transferred into a nitrogen filled glovebox. The FAPbI 3 precursor solution (40 wt%), prepared by dissolving NH 2 CHQNH 2 I (Xi an Polymer Light Technology Corp.) and PbI 2 (Alfa Aesar) in N,N-dimethylformamide (DMF) solvent with a molar ratio of 1 : 1, was spin coated on the PEDOT:PSS layer following the fast deposition crystallization procedure as previously reported. 6 Then the FAPbI 3 precursor solution was annealed to form black FAPbI 3 perovskite. After cooling down to room temperature, PC 60 BM (20 mg ml 1 in chlorobenzene) was spin-coated on the perovskite layer at 2000 rpm, followed by drop casting an interfacial layer solution of BCP (0.5 mg ml 1 in anhydrous ethanol) at 4000 rpm without further annealing. Finally, devices were transferred into the thermal evaporation system (OMV-FS300) for silver cathode evaporation. The active area of the devices (7.25 mm 2 )wasdefined through a shadow mask. The MAPbI 3 -based solar cell with the same device structure was also fabricated for comparison. Measurements and characterization Current density voltage ( J V) characteristics of perovskite solar cells were measured in air using a programmable Keithley 2400 source meter under AM 1.5G solar irradiation at 100 mw cm 2 (Newport, Class AAA solar simulator, 94023A-U). The light intensity was calibrated by a certified Oriel Reference Cell (91150 V) and verified with an NREL calibrated Hamamatsu S diode. The external quantum efficiency (EQE) was measured by a certified IPCE instrument (Zolix Instruments, Inc., Solar Cell Scan 100). The scanning electron microscope (SEM) images were obtained from a field emission scanning electron microscope (FEI Quanta 200). The ultraviolet-visible spectroscopy (UV-vis) spectra were achieved on a Perkin Elmer model Lambda 750 instrument. X-ray diffraction (XRD) patterns were collected on an analytical (Empyrean) apparatus. The steady-state photoluminescence spectra and timeresolved photoluminescence were measured by utilizing Horiba Jobin-Yvon LabRAM HR800 and a single photon counting spectrometer, which was combined with the Fluorolog-3 spectrofluorometer (Horiba-FM-2015), respectively. A 625 nm laser source was used in the time resolved PL measurement. Results and discussion Fig. 2(a) shows the X-ray diffraction patterns of ITO/PEDOT:PSS, MAPbI 3 and FAPbI 3 perovskites on an ITO/PEDOT:PSS surface. Highly oriented crystallinity was observed in the FAPbI 3 perovskite. Its main diffraction peaks shift toward lower degrees compared with the MAPbI 3 system, due to the replacement of the smaller organic CH 3 NH 3 + (MA) cation with the larger NH 2 CHQNH 2 + (FA) cation. The zoomed in X-ray diffraction patterns between 12 and 16 degrees (Fig. 2(b)) shows a much more obvious change. In the FAPbI 3 system, peaks labeled with a # are assigned to the Fig. 2 (a) X-ray diffraction patterns of ITO/PEDOT:PSS, MAPbI 3 and FAPbI 3 perovskites on the ITO/PEDOT:PSS surface. (b) The zoomed in X-ray diffraction patterns between 12 and 16 degrees for the modified MAPbI 3 and FAPbI 3 perovskites, respectively. Phys. Chem. Chem. Phys. This journal is the Owner Societies 2015

3 View Article Online PCCP Paper Fig. 3 SEM images of the (a) one-step annealed and (b) slowed-down annealed FAPbI 3 perovskite films on the ITO/PEDOT:PSS surface, respectively. ITO/PEDOT:PSS substrate, and other peaks are assigned to the labelled reflections from a tetragonal perovskite lattice with cell parameters a = b = 8.99 Å and c = 11.0 Å. These results are in good agreement with the previously reported black phase of FAPbI 3. 38,39 In addition, no PbI 2 peaks can be observed. It indicates that FAPbI 3 perovskite also has a pure crystalline phase. The morphology of the perovskite layer is pretty important for the device performance. Two annealing methods were conducted to improve the morphology of the FAPbI 3 layer, the one-step annealing process (140 1C/20 min) and the slowed-down annealing process (100 1C/20 min, 120 1C/20 min and 140 1C/20 min). The SEM images of the one-step annealed and slowed-down annealed FAPbI 3 perovskites on the ITO/PEDOT:PSS surface were shown in Fig. 3(a) and (b). Compact films with full surface coverage were obtained for both methods. Noticeably, the slowed-down annealed film shows a much larger grain size, which will obviously result in fewer grain boundaries. We fabricated both one-step annealed and slowed-down annealed FAPbI 3 perovskite solar cells employing PCBM as the electron transporting layer. The related current density voltage ( J V) curves and photovoltaic parameters of the perovskite solar cells measured under AM 1.5G solar illumination at 100 mw cm 2 are shown in Fig. 4(a) and Table 1. The EQE spectra were also tested, as shown in Fig. S1 (ESI ). Obviously, higher performance Table 1 Photovoltaic parameters of the inverted planar structure FAPbI 3 based perovskite solar cells with different configurations a Configurations FAPbI 3 (one-step annealing)/pcbm FAPbI 3 (slowed-down annealing)/pcbm FAPbI 3 (slowed-down annealing)/icba J sc (ma cm 2 ) V oc (V) Fill factor PCE (%) a All the photovoltaic parameters are the average of a batch of twelve devices. and EQE value were achieved for the slowed-down annealed device. The enhancement was ascribed to less carrier recombination and better charge extraction, probably as a result of there being fewer grain boundaries. To further explain the related mechanisms, we measured the steady-state photoluminescence (PL) spectra of the one-step annealed and slowed-down annealed FAPbI 3 perovskites on the ITO/PEDOT:PSS surface, respectively. As shown in Fig. 4(b), the slowed-down annealed FAPbI 3 perovskite showed enhanced PL intensity compared to the one-step annealed FAPbI 3 perovskite. It implies that the non-radiative decay is significantly suppressed through our slowed-down annealing process. The time-resolved Fig. 4 (a) J V curve of the one-step and slowed-down annealed FAPbI 3 perovskite solar cells employing PCBM and ICBA as electron transporting layers, respectively. (b) The steady-state photoluminescence spectra and (c) time-resolved photoluminescence of one-step annealed and slowed-down annealed FAPbI 3 perovskites on the ITO/PEDOT:PSS surface, respectively. This journal is the Owner Societies 2015 Phys. Chem. Chem. Phys.

4 View Article Online Paper PCCP Fig. 5 (a) Current density voltage (J V) curves of the perovskite solar cells measured under AM 1.5G solar illumination at 100 mw cm 2. (b) The external quantum efficiency (EQE) spectra of the MAPbI 3 and FAPbI 3 based devices. (c) UV-vis absorption of MAPbI 3 and FAPbI 3 perovskites on the ITO/PEDOT:PSS surface, respectively. Inset figure shows the related zoom of the absorption between 700 and 850 nm. PL (TRPL) was also measured, as shown in Fig. 4(c). The sloweddown annealed FAPbI 3 perovskite gave a lifetime of approximately 59 ns, whereas the slowed-down annealing process increased the lifetime to 112 ns. The PL and TRPL measurements showed that the slowed-down annealing process significantly suppressed the non-radiative recombination channels and increased the PL lifetime, which will obviously reduce the number of crystal defects, resulting in fewer carrier recombinations and better charge extraction. We also fabricated devices employing indene-c 60 bisadduct (ICBA) as an electron transporting layer (ETL). The fabrication conditions of ICBA are the same as with PCBM. The related current density voltage ( J V) curves and photovoltaic parameters are shown in Fig. 4(a) and Table 1. We find that poor performance was achieved when ICBA acted as the ETL, probably as a result of there being higher trap densities of states (tdos) in these devices. 40 This result indicates that electron transporting materials can obviously influence the performance of the FAPbI 3 based perovskite solar cell. We can investigate superior electron and hole transporting materials to further improve device performances. The modified FAPbI 3 -based device with the modified MAPbI 3 -based device were further fabricated for comparison. The J V curves of the FAPbI 3 and MAPbI 3 based solar cells measured under AM 1.5G solar illumination at 100 mw cm 2 are shown in Fig. 5(a). Their related photovoltaic parameters are summarized in Table 2. Both devices showed comparable open circuit voltage (V oc ) and fill factor (FF). The reference device (MAPbI 3 ) shows an average PCE of 10.59%, while the FAPbI 3 -based device shows a higher performance, with an average PCE of 12.56%. An increased PCE was ascribed to the Table 2 Photovoltaic parameters of the inverted planar structure MAPbI 3 and FAPbI 3 based perovskite solar cells measured under AM 1.5G solar illumination at 100 mw cm 2 J sc J sc Solar cells (ma cm 2 ) Average (ma cm 2 ) Highest V oc (V) Fill factor (%) PCE (%) Average PCE (%) Highest MAPbI FAPbI obvious enhancement of the short circuit current density (from ma cm 2 to ma cm 2 on average). Fig. 5(b) shows the EQE spectra of the MAPbI 3 and FAPbI 3 based devices. We observed that the MAPbI 3 -based device generates photocurrent up to 800 nm, while the FAPbI 3 -based device generates photocurrent up to 840 nm. The FAPbI 3 -based device showed higher EQE spectra compared to the MAPbI 3 - based device on the whole, which will obviously result in higher short circuit current density. The UV-vis absorption properties of both modified MAPbI 3 and FAPbI 3 perovskites on the ITO/PEDOT:PSS surface are shown in Fig. 5(c). The zoom of the absorption for both perovskites between 700 and 850 nm is shown in the inset Fig. 5(c). We can clearly see that the absorption cut-off edges for both MAPbI 3 and FAPbI 3 perovskites are consistent with their EQE photocurrent generation cut-off edges, respectively. The absorption cut-off edge of the MAPbI 3 system was set at 800 nm, while the FAPbI 3 system was set at 840 nm. Compared with the MAPbI 3 system, the FAPbI 3 perovskite film shows stronger absorption and a broadened absorption range, which will obviously enhance the short circuit current density. Due the superior properties of slowed-down annealed FAPbI 3 perovskite, such as high crystallinity, large grain size, compact film with full surface coverage, stronger absorption and broadened absorption range, an excellent PCE of 13.56% was obtained, with a short circuit current density ( J sc ) of ma cm 2, open circuit voltage (V oc ) of 0.89 and fill factor (FF) of 0.71, under 100 mw cm 2 AM 1.5 illumination (Fig. 6(a)). From the PCE histogram of 40 devices (Fig. 6(b)), we can see that our slowed-down annealed FAPbI 3 perovskite solar cell also showed very good reproducibility. Here, we noticed that our slowed-down annealed FAPbI 3 perovskite solar cell shows obvious hysteresis properties as well (Fig. S2, ESI ). The origin of the anomalous hysteresis in perovskite solar cells is not clearly understood at present The photostability of our modified FAPbI 3 based solar cells was also evaluated as shown in Fig. S3 (ESI ). When the device was tested under continuous illumination, the performance did not show an obvious decrease. Phys. Chem. Chem. Phys. This journal is the Owner Societies 2015

5 View Article Online PCCP Paper Fig. 6 (a) J V curve of the champion FAPbI 3 -based device. (b) PCE histogram of 40 devices for the modified FAPbI 3 perovskite solar cells. Conclusion In summary, we have demonstrated an inverted planar structure perovskite solar cell with a power conversion efficiency of 13.56% by using the FAPbI 3 as a light harvester for the first time. Except for the silver cathode, all layers were solution-processed under or below 140 1C. We investigated the effect of the annealing process on device performance. When the slowed-down annealing process was conducted, the FAPbI 3 perovskite layer showed high crystallinity, large grain size and full surface coverage. Our slowed-down annealed FAPbI 3 -based device shows superior performance to the modified MAPbI 3 -based solar cell, especially for the short circuit current density. This work paves the way for low-temperature fabrication of efficient inverted planer structure FAPbI 3 perovskite solar cells. Acknowledgements We acknowledge financial support from the Natural Science Foundation of China (No and ) and from the Natural Science Foundation of Jiangsu Province (No. BK ). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology, and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References 1 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Gratzel and N.-G. Park, Sci. Rep., 2012, 2, J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Gratzel, Nature, 2013, 499, M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, Q. Chen, H. Zhou, Z. Hong, S. Luo, H. S. Duan, H. H. Wang, Y. Liu, G. Li and Y. Yang, J. Am. Chem. Soc., 2014, 136, M.Xiao,F.Huang,W.Huang,Y.Dkhissi,Y.Zhu,J.Etheridge, A. Gray-Weale, U. Bach, Y. B. Cheng and L. Spiccia, Angew. Chem., Int. Ed., 2014, 126, Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang, Y. Gao and J. Huang, Energy Environ. Sci., 2014, 7, N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and S. I. Seok, Nat. Mater., 2014, 13, C. W. Chen, H. W. Kang, S. Y. Hsiao, P. F. Yang, K. M. Chiang and H. W. Lin, Adv. Mater., 2014, 26, W. Nie, H. Tsai, R. Asadpour, J. C. Blancon, A. J. Neukirch, G. Gupta, J. J. Crochet, M. Chhowalla, S. Tretiak, M. A. Alam, H. L. Wang and A. D. Mohite, Science, 2015, 347, G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and H. J. Snaith, Adv. Funct. Mater., 2014, 24, R. Kang, J. E. Kim, J. S. Yeo, S. Lee, Y. J. Jeon and D. Y. Kim, J. Phys. Chem. C, 2014, 118, J. You, Y. Yang, Z. Hong, T. B. Song, L. Meng, Y. Liu, C. Jiang, H. Zhou, W.-H. Chang, G. Li and Y. Yang, Appl. Phys. Lett., 2014, 105, Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan and J. Huang, Adv. Mater., 2014, 26, Z. Ren, A. Ng, Q. Shen, H. C. Gokkaya, J. Wang, L. Yang, W. K. Yiu, G. Bai, A. B. Djurišić, W. W. Leung, J. Hao, W. K. Chan and C. Surya, Sci. Rep., 2014, 4, F. X. Xie, D. Zhang, H. Su, X. Ren, K. S. Wong, M. Gratzel and W. C. H. Choy, ACS Nano, 2015, 9, M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338, M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M. Graetzel, M. K. Nazeeruddin and H. J. Bolink, Nat. Photonics, 2014, 8, P. W. Liang, C. Y. Liao, C. C. Chueh, F. Zuo, S. T. Williams, X. K. Xin, J. Lin and A. K. Y. Jen, Adv. Mater., 2014, 26, P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon and H. J. Snaith, Nat. Commun., 2013, 4, S. Bai, Z. Wu, X. Wu, Y. Jin, N. Zhao, Z. Chen, Q. Mei, X. Wang, Z. Ye, T. Song, R. Liu, S. T. Lee and B. Sun, Nano Res., 2014, 7, H. Zhou, Q. Chen, G. Li, S. Luo, T. B. Song, H. S. Duan, Z. Hong, J. You, Y. Liu and Y. Yang, Science, 2014, 345, Q. Wang, Y. Shao, Q. Dong, Z. Xiao, Y. Yuan and J. Huang, Energy Environ. Sci., 2014, 7, This journal is the Owner Societies 2015 Phys. Chem. Chem. Phys.

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7 Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics. This journal is the Owner Societies 2015 Supporting Information Inverted Planar NH 2 CH=NH 2 PbI 3 Perovskite Solar Cells With 13.56% Efficiency via Low Temperature Processing Da-Xing Yuan, a Adam Gorka, b Mei-Feng Xu, a Zhao-Kui Wang,* a and Liang-Sheng Liao* a a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu , China. b Department of Physics & Astronomy, University of Waterloo. 200 University Avenue West, Waterloo, N2L 3G1, Ontario, Canada. *Corresponding author. Tel.: zkwang@suda.edu.cn; lsliao@suda.edu.cn 1

8 Fig. S1 EQE spectra of the one-step annealed and slow-down annealed FAPbI 3 perovskite solar cells, when PCBM act as electron transporting layer. 2

9 Fig. S2 J-V curve of a representative modified FAPbI 3 device tested under forward and reverse bias, respectively. Our FAPbI 3 based device also showed obvious hysteresis. 3

10 Fig. S3 J-V curve of a representative slow-down annealed FAPbI 3 based solar cells tested every 10min in air (humidity:~45%, temperature:~26 ) under continuous light illumination. 4

11 Fig. S4 Key parameters changing with illumination time. 5

12 Table S1 Photovoltaic parameters of some representative modified FAPbI 3 perovskite solar cells. Jsc (ma/cm 2 ) Voc (V) Fill Factor (%) PCE (%)