- Email: [email protected]
Constructing binary electron transport layer with cascade energy level alignment for efficient CsPbI2Br solar cells
Journal Pre-proof Constructing Binary Electron Transport Layer with Cascade Energy Level Alignment for Efficient CsPbI2Br Solar Cells Cheng Chen, Cheng Wu, Xingdong Ding, Yi Tian, Mengmeng Zheng, Ming Cheng, Hui Xu, Zhiwen Jin, Liming Ding PII:
S2211-2855(20)30162-2
DOI:
https://doi.org/10.1016/j.nanoen.2020.104604
Reference:
NANOEN 104604
To appear in:
Nano Energy
Received Date: 16 December 2019 Revised Date:
20 January 2020
Accepted Date: 10 February 2020
Please cite this article as: C. Chen, C. Wu, X. Ding, Y. Tian, M. Zheng, M. Cheng, H. Xu, Z. Jin, L. Ding, Constructing Binary Electron Transport Layer with Cascade Energy Level Alignment for Efficient CsPbI2Br Solar Cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104604. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Graphical Abstract A binary electron transport layer (ETL), consisting of traditional electron transport material (ETM) P61CBM and a newly developed non-fullerene ETM SFX-PDI2, is proposed and firstly applied in inverted CsPbI2Br perovskite solar cell (PSC), achieving an impressive efficiency of 15.12% due to the better film morphology and electron extraction and transport properties of the binary ETL.
Constructing Binary Electron Transport Layer with Cascade Energy Level Alignment for Efficient CsPbI2Br Solar Cells Cheng Chen,a, d Cheng Wu,a Xingdong Ding,a Yi Tian,a Mengmeng Zheng,a Ming Chenga* Hui Xu,a Zhiwen Jin,c Liming Ding b* a
Institute for Energy Research, Key Laboratory of Zhenjiang, Jiangsu University, Zhenjiang 212013, China * E-mail: [email protected] b Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China * E-mail: [email protected] c School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China d State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Keywords: perovskite solar cells; electron transport materials; perylene diimide
Abstract Recently, all-inorganic perovskite solar cell (PSC) has attracted an increasing research attention due to its excellent thermal stability and suitable band gap for tandem devices. Herein, a highly efficient all-inorganic PSC with CsPbI2Br as light absorption material is assembled by employing a binary electron transport layer (ETL), which consists of PC61BM and a new designed non-fullerene electron transport material SFX-PDI2. The formed binary system primely implements in the form of a thin, uniform and full coverage ETL, and possesses more excellent electron extraction and transport properties than both pristine PC61BM and pristine SFX-PDI2. As a result, the binary ETL based PSC with CsPbI2Br as light harvesting material achieved an efficiency as high as 15.12%, which is among the highest performance for inverted planar structure all-inorganic PSC.
1
Main text During the past decade, due to the facile tunable absorption range, large absorption coefficient and excellent charge carrier separation and transport properties, lead halide perovskite materials with crystal structure of ABX3 had attracted extensive attention, and showed broad application prospects in energy conversion, photodetector, thermoelectric and electronic devices. [1-12] To date, perovskite solar cell (PSC) based on this type light-harvesting materials is one of the fastest growing photovoltaic technologies, with certified power conversion efficiencies (PCEs) skyrocketed over 25% in 2019. However, the intrinsically volatile and thermally unstable nature of organic cation make the organic-inorganic hybrid PSC suffer from poor stability, limiting its future commercial application. Recently, many research works manifested that simply replacing the organic A site [methyl ammonium (MA) or formamidine (FA)] with cesium (Cs) could greatly enhance the thermal stability, making the all-inorganic cesium lead halide (CsPbX3) perovskites become promising candidates for highly efficient and stable solar cells. [13-34] Similar with organic-inorganic counterpart, all-inorganic PSCs also have two different device architectures, including the conventional n-i-p and inverted p-i-n configurations. For n-i-p structured all-inorganic PSCs, although a record PCE of 18.4% has been achieved, the hysteresis behavior of all-inorganic PSCs was not stressed. [19, 27, 35] Compared with the n-i-p PSCs, the p-i-n PSCs tend to exhibit less hysteresis effect. However, the PCE of p-i-n structured all-inorganic PSCs is still lagging (13.3%) and much lower than that of n-i-p devices. [23, 36, 37] Therefore, great efforts need to be made to improve device efficiency. The first way is optimizing the perovskite crystal components and crystal growth process to get high quality CsPbX3 film. The other way is developing novel charge transport materials with high charge carrier mobility and more appropriate energy levels, favoring charge transport and minimize energy loss. [38, 39] Fullerene and its derivatives are commonly used electron transport materials 2
(ETMs) in the inverted PSCs, among which [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) is the most prevalent one. [40-43] The extraordinary photovoltaic performance of PC61BM based PSCs is regarded as the result of the effective trap passivation and fast electron extraction of PC61BM. However, PC61BM is far from perfect electron transport layer (ETL) because of the poor film-forming, ordinary electron mobility and conductivity properties and poor phase stability in ambient condition. [44] To get a continuous PC61BM film, the thickness of film is inevitably required to be more than 100 nm, which would increase the series resistance of PSCs, leading to poor photovoltaic performance. Hence, a key challenge in the further is reducing the pinholes and morphological defects of ETL without greatly increasing the thickness and series resistance of ETL. One of effective strategies is blending of two ETMs to construct binary ETL system, which has been successfully implemented in the organic solar cell (OSC) field. [45-48] On the one hand, blending of two ETMs could efficiently increase the ETM solution viscosity and prevent the de-wetting behavior of ETM solution on perovskite film, correspondingly, obtaining a good film-forming property. On the other hand, two ETMs might form film in different ways and complement the insufficiency of each other, in turn, improving the electron extraction and transport properties of binary ETL. Based on these concerns, novel ETMs with distinctly different configuration from PC61BM and high electron mobility are urgently desired. Apart from fullerene derivatives, lots of n-type materials, such as ITIC analogues, hexaazatrinaphthylene (HATNA) derivatives, and perylene diimide (PDI) derivatives were reported as alternatives. [49-55] Thereinto, PDI derivatives have received widespread attention due to the high conductivity and charge carrier mobility. When applied into organic-inorganic hybrid PSCs, PDI-based materials have been proved to be beneficial for enhancing photovoltaic performance of devices, including passivating perovskite surface trap and preventing the perovskite decomposition. [56-58] However, PDI-based ETMs have been rarely applied in all-inorganic PSCs. In this context, by connecting spiro[fluorene-9,9'-xanthene] (SFX) core building 3
block with two PDI units, we designed and synthesized a novel non-fullerene ETM, termed SFX-PDI2, which further served as one component of the binary ETL system to tune the morphology, charge transport and collection characteristics of the whole ETL, complementing the insufficiency of the other component PC61BM. The formed binary ETL system was systematically studied with optical spectroscopy, photoluminescence (PL), space charge limit current (SCLC), scanning electron microscope (SEM), etc. The optimal binary ETMs-based PSC with CsPbI2Br as light-harvesting material exhibits a high PCE of 15.12%, which is significantly over performed than devices based on either PC61BM (12.91%) or SFX-PDI2 (11.74%). The greatly improved photovoltaic performance of the binary ETMs-based PSC benefits from the synergistic effects of ETMs PC61BM and SFX-PDI2.
Figure 1 a) Schematic device structure of PSC and chemical structures of the binary ETMs, b) Cross-sectional SEM image of binary ETL-based PSC, c) Energy level alignment of main components in PSC Figure 1a illuminates the schematic structure of PSC in this study, wherein the detailed chemical structures of the binary ETMs are listed. To prevent PDI units from 4
forming excessively large aggregates, SFX core building block with orthogonal three-dimensional configuration was targetedly selected. SFX-PDI2 can be easily synthesized trough Suzuki coupling reaction with the yield of 68% (see Scheme S1). The structure was fully characterized by NMR and high-resolution mass spectrum. The detailed molecular configuration and charge density distribution were investigated by using density functional theory (DFT) calculation. As shown in Figure S1, SFX-PDI2 shows twisted non-planar structure, with a dihedral angle of 117o. The highest occupied molecular orbital (HOMO) of SFX-PDI2 distributes throughout the whole molecular skeleton, while the lowest unoccupied molecular orbital (LUMO) transfers to peripheral PDI units, indicating a good charge carrier transport property. The energy levels of SFX-PDI2 were roughly evaluated to be -3.96 V for LUMO energy level and -5.94 V for HOMO energy level, respectively (see Figure S2). CsPbI2Br, SFX-PDI2 and PC61BM form gradient energy level alignment, as shown in Figure 1c, which is facilitated the synergetic electron transport. To investigate the potential of binary ETL system for highly efficient all-inorganic PSCs, devices with a configuration of ITO/NiO/CsPbI2Br/binary ETL/Bathocuproine (BCP)/Ag were fabricated, as shown in Figure 1b. BCP was introduced as a thin buffer layer to improve the contact of ETL and Ag electrode. The effect of weight ratios of SFX-PDI2 to PC61BM on photovoltaic performances was systemically examined, as shown in Figure 2a and 2b. The formed binary ETL system indeed showed significantly positive effects on device performance and the optimal blend weight ratio of the SFX-PDI2 relative to PC61BM is defined to be 10% (see Table S1). The PSCs employing binary ETL systems with low weight ratio of SFX-PDI2 (5%, 10% and 15%) showed obviously higher PCEs than both the pristine PC61BM (12.91%) and the pristine SFX-PDI2 (11.74%). The enhancements of PCEs can be mainly attributed to the higher electron mobility and better film morphology of formed binary ETLs (see Figure S3, S4 and Table S2). Under the optimized conditions (PC61BM:SFX-PDI2 ETL, weight ratio 9:1; 3000 rpm), the formed binary ETL exhibited the highest electron mobility of 2.42 × 10-4 cm2·V-1·s-1 and good 5
conductivity, correspondingly, the highest PCE of 15.12% with an open-circuit voltage (Voc) of 1.21 V, a short-circuit current density (Jsc) of 15.72 mA·cm-2 and a fill factor (FF) of 79.5% was achieved, showed substantial increments of 17.1% and 28.8% compared with the devices with pristine PC61BM and pristine SFX-PDI2 ETL, respectively. The hysteresis behavior and stabilized current density at the maximum power output point of binary ETL based PSC were also studied. As shown in Figure 2c, no noticeable hysteresis was detected. The steady-state current density was calculated to be 14.75 mA·cm-2, further confirming the excellent photovoltaic performances of our devices (see Figure 2e). Moreover, the binary ETL based PSCs also showed good reproducibility, with an average PCE of 14.28±0.47% and 50% devices performed over average value (see Figure 2f, Table S3-5).
6
Figure 2 a) J-V characteristic of PSCs based on ETLs with different weight ratio of SFX-PDI2, b) The changing trend of PCE at different weight ratios of SFX-PDI2 in binary ETLs; c) J-V characteristic of PSCs based on optimized binary ETL (90% PC61BM:10% SFX-PDI2) with different scan directions (OC: open-circuit; SC: short-circuit), d) IPCE spectra of PSCs containing PC61BM, SFX-PDI2 and 90% PC61BM:10% SFX-PDI2 as ETMs, e) steady-state current density and PCE of PSC based on optimized binary ETL (90% PC61BM:10% SFX-PDI2) at maximum power output point, f) Repeatability statistic data of PSC based on different ETLs
Figure 3 SEM images of top-view film morphology of a) CsPbI2Br, b) CsPbI2Br/PC61BM, c) CsPbI2Br/SFX-PDI2, and d) CsPbI2Br/PC61BM:SFX-PDI2; and AFM height images of different films e) CsPbI2Br, f) CsPbI2Br/PC61BM, g) CsPbI2Br/SFX-PDI2, and h) CsPbI2Br/PC61BM:SFX-PDI2
The improved PCE of binary ETL based PSCs may result from the following two factors: better film formation property and film quality with the aid of SFX-PDI2, and more efficient electron extraction and transport of PC61BM:SFX-PDI2 binary ETL. To confirm our hypothesis and better understand the relationship between the ETL morphology and device performances, we firstly probed the features of ETL on perovskite surface by using scanning electron microscope (SEM) and atomic force microscope (AFM). As shown in Figure 3, the CsPbI2Br film showed a uniform and dense morphology. For the PC61BM thin film, it failed to fully cover the perovskite surface, with obvious pin-holes and bared perovskite detected. For SFX-PDI2 thin 7
film, the coverage on the perovskite is even lower due to the rapid crystallization and easy aggregation of PDI materials. Interestingly, when blending the PC61BM and SFX-PDI2 together to form a binary ETL system (keep the same total weight as pristine PC61BM and pristine SFX-PDI2, 20 mg/mL, with the same coating technology), the weight ratio of SFX-PDI2 shows significant impacts on film formation properties of formed binary ETLs. As shown in Figure S4, with addition of small amount of SFX-PDI2, the film is smooth and uniform. No aggregated dots and wrinkles were detected. While further increasing the weight ratios of SFX-PDI2, the films become plicate. With 25% SFX-PDI2, the coverage of formed binary ETL is very low, which is similar with pure SFX-PDI2, indicating the film formation property of SFX-PDI2 has a predominant effect on the film-forming properties of binary ETL in high weight ratio case. In optimized conditions, the roughness (R) of binary ETL significantly decrease (see Figure 3e-h) and the perovskite layer was almost fully covered, preventing the perovskite from intimate contact with Ag electrode, in turn successfully suppressing the leakage currents (see Figure 4a). The improved Jsc and FF could result from the more homogeneous and fully-covered ETL and smaller series resistance (see Table S1).
8
Figure 4 a) Dark current density of PSCs containing different ETLs, b) TRPL spectra of the glass/Al2O3/CsPbI2Br and glass/Al2O3/CsPbI2Br/ETMs (PC61BM, SFX-PDI2 or PC61BM:SFX-PDI2), c) the dependence of Voc on light intensity for PSCs containing different ETLs, d) stability of different ETLs based CsPbI2Br PSCs
To further manifest the detailed affecting factors for the enhanced PCE, the incident photon-to-current conversion efficiency (IPCE) for PC61BM, SFX-PDI2 and binary ETMs-based PSCs were measured as shown in Figure 2d. The IPCE values of the binary ETMs-based PSC was obviously higher than those of pristine PC61BM and pristine SFX-PDI2 counterparts in the entire visible range, indicating a synergistic favorable effect on the photoelectric conversion properties of binary ETL system. From the IPCE spectra, the integrated current densities were calculated to be 14.61, 13.78 and 15.49 mA·cm-2 for PC61BM, SFX-PDI2 and 90% PC61BM:10% SFX-PDI2, respectively, consisting well with the J-V measurements. Considering the influence factor for IPCE, the higher IPCE values obtained by binary HTMs-based PSCs were mainly due to the increased charge transport and collection efficiencies. 9
This deduction is further supported by time-resolved photoluminescence (TRPL) measurements (see Figure 4b). In our case, to facilitate the growth of perovskite crystals, Al2O3 film on FTO was employed as substrate. The PL lifetimes of CSPbI2Br/PC61BM,
CSPbI2Br/SFX-PDI2
and
CSPbI2Br/90%
PC61BM:10%SFX-PDI2 were extracted by a biexponential decay function. τ1 is assigned to the extraction time of ETL on the nanoseconds, and τ2 represents the charge recombination time in the radiative channel. In the case of pristine PC61BM, τ1 and τ2 were calculated to be 3.75 ns (79%) and 21.5 ns (21%), with an average τ of 7.48 ns. Under the same conditions, pristine SFX-PDI2 based devices exhibited longer decay time [4.29 ns (78%) and 24.1 ns (22%)], indicating inferior charge transport and extraction properties than PC61BM. When the binary ETMs PC61BM:10%SFX-PDI2 was used as the ETL, the sample showed the shortest PL quenching lifetime of 5.86 ns (τ1: 2.84 ns, 83%; τ2: 20.6 ns, 17%). The shorter τ1 for PC61BM:10%SFX-PDI2 based sample suggested that the binary ETL system fully guarantees the efficient electron extraction from perovskite without any serious recombination at the perovskite/ETM interfaces, correspondingly, the higher Voc is obtained. The results above were also confirmed by measurements of the dependence of the Voc on the light intensity (see Figure 4c). The 90% PC61BM:10%SFX-PDI2 based PSC showed a slope of 1.29 kT/q, suggesting trace of recombination traps in the 90% PC61BM:10% SFX-PDI2 based device. Comparatively, the pristine PC61BM and pristine SFX-PDI2 based samples showed much stronger dependence on the light intensity, with slopes of 1.47kT/q and 1.76 kt/q, respectively. These results demonstrate that the binary ETL system successfully prevents the charge recombination and facilitates the charge transport. As a crucial parameter, the stability of PSC based on the binary ETL was further evaluated and devices based on PC61BM and SFX-PDI2 were employed as references (see Figure 4d and S5). The PSCs were stored in the ambient condition (the humidity of 40-50%) without capsulation. For the PC61BM based PSCs, the PCE sharply dropped from 12.91% to 8.31% after 30 days storage. With addition of 10% 10
SFX-PDI2 in ETL, the stability was slightly improved, maintaining 73.6% of initial PCE. The enhancement of stability mainly causes by improved hydrophobicity of binary ETL (see Figure S6). As for SFX-PDI2 based PSC, the stability is much better than PC61BM and binary ETL based devices. Therefore, for the binary ETL based PSC, the phase instability and hydrophilia of PC61BM are decisive factors for device degradation. In summary, a binary ETL system, consisting of traditional ETM PC61BM and a newly designed non-fullerene ETM SFX-PDI2, was successfully proposed and firstly applied into the highly efficient inverted all-inorganic PSC. The addition component of SFX-PDI2 not only improved the charge extraction and transport properties, but also facilitated the formation of homogeneous ETL film and enhanced the ETL coverage rate on the perovskite surface, correspondingly, effectively restricting the charge recombination at perovskite/ETL interface. Under optimized conditions, the binary ETL based PSC showed an efficiency as high as 15.12%, which is about 17.1% and 28.8% higher than that of both the pristine PC61BM and pristine SFX-PDI2 based devices. This work hopefully supplies an effective and convenient way for advancing the performance of the all-inorganic PSCs.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grants 21805114, 21905119), Natural Science Foundation of Jiangsu province (BK20180867, BK20180869), China Postdoctoral Science Foundation (2019M651741), Top talents in Jiangsu province
(XNY066),
the
Jiangsu
University
Foundation
(17JDG032,
17JDG031), High-tech Research Key laboratory of Zhenjiang (SS2018002), the State Key Laboratory of Fine Chemicals (KF1902), the high-performance computing platform of Jiangsu University, the Priority Academic Program Development of Jiangsu Higher Education Institutions. L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and 11
the National Natural Science Foundation of China (51773045, 21572041, 21772030 and 51922032) for financial support.
Supporting Information The experimental details, characterization of SFX-PDI2, and fabrication of perovskite solar cell, conductivity and electron mobility measurements, stability measurements of PSCs, water contact angle measurements supplied as Supporting Information.
Reference [1] S. Wang, Y. Jiang, Emilio J. Juarez-Perez, Luis K. Ono, Y. Qi, Nature Energy, 2 (2016) 16195. [2] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.-S. Lim, J.A. Chang, Y.H. Lee, H.-j. Kim, A. Sarkar, M.K. Nazeeruddin, M. Grätzel, S.I. Seok, Nature Photonics, 7 (2013) 486. [3] J. Huang, Y. Yuan, Y. Shao, Y. Yan, Nat. Rev. Mater., 2 (2017) 17042. [4] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Science, 342 (2013) 341-344. [5] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gratzel, S. Mhaisalkar, T.C. Sum, Science, 342 (2013) 344-347. [6] H.S. Jung, N.G. Park, Small, 11 (2015) 10-25. [7] Q. Jiang, L. Zhang, HaolinWang, X. Yang, J. Meng, H. Liu, Z. Yin, JinliangWu, X. Zhang, J. You, Nature Energy, 2 (2016) 16177. [8] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Nature, 517 (2015) 476-480. [9] C.H. Lin, B. Cheng, T.Y. Li, J.R.D. Retamal, T.C. Wei, H.C. Fu, X. Fang, J.H. He, ACS Nano, 13 (2019) 1168-1176. [10] W. Yang, J. Chen, Y. Zhang, Y. Zhang, J.H. He, X. Fang, Adv. Funct. Mater., 29 (2019) 1808182. 12
[11] A.M. Alamri, S. Leung, M. Vaseem, A. Shamim, J.-H. He, IEEE Trans. Elect. Dev., 66 (2019) 2657-2661. [12] Y. Zhang, W. Xu, X. Xu, J. Cai, W. Yang, X. Fang, J. Phys. Chem. Lett., 10 (2019) 836-841. [13] D. Bai, H. Bian, Z. Jin, H. Wang, L. Meng, Q. Wang, S. Liu, Nano Energy, 52 (2018) 408-415. [14] W. Chen, H. Chen, G. Xu, R. Xue, S. Wang, Y. Li, Y. Li, Joule, 3 (2019) 191-204. [15] Z. Fang, L. Liu, Z. Zhang, S. Yang, F. Liu, M. Liu, L. Ding, Science Bulletin, 64 (2019) 507-510. [16] Z. Fang, X. Meng, C. Zuo, D. Li, Z. Xiao, C. Yi, M. Wang, Z. Jin, S. Yang, L. Ding, Science Bulletin, 64 (2019) 1743-1746. [17] Z. Guo, S. Zhao, A. Liu, Y. Kamata, S. Teo, S. Yang, Z. Xu, S. Hayase, T. Ma, ACS Appl. Mater. Interfaces, 11 (2019) 19994-20003. [18] K. Huang, Y. Peng, Y. Gao, J. Shi, H. Li, X. Mo, H. Huang, Y. Gao, L. Ding, J. Yang, Advanced Energy Materials, 9 (2019) 1901419. [19] X. Jia, C. Zuo, S. Tao, K. Sun, Y. Zhao, S. Yang, M. Cheng, M. Wang, Y. Yuan, J. Yang, F. Gao, G. Xing, Z. Wei, L. Zhang, H.-L. Yip, M. Liu, Q. Shen, L. Yin, L. Han, S. Liu, L. Wang, J. Luo, H. Tan, Z. Jin, L. Ding, Science Bulletin, 64 (2019) 1532-1539. [20] H. Jiang, J. Feng, H. Zhao, G. Li, G. Yin, Y. Han, F. Yan, Z. Liu, S.F. Liu, Adv. Sci., 5 (2018) 1801117. [21] D.H. Kim, J.H. Heo, S.H. Im, ACS. Appl. Mater. Interfaces, 11 (2019) 19123-19131. [22] C.F.J. Lau, M. Zhang, X. Deng, J. Zheng, J. Bing, Q. Ma, J. Kim, L. Hu, M.A. Green, S. Huang, A. Ho-Baillie, ACS Energy Lett., 2 (2017) 2319-2325. [23] C. Liu, W. Li, C. Zhang, Y. Ma, J. Fan, Y. Mai, J. Am. Chem. Soc., 140 (2018) 3825-3828. [24] J. Lu, S.-C. Chen, Q. Zheng, Science China Chem., 62 (2019) 1044-1050. [25] J. Tian, Q. Xue, X. Tang, Y. Chen, N. Li, Z. Hu, T. Shi, X. Wang, F. Huang, C.J. 13
Brabec, H.L. Yip, Y. Cao, Adv. Mater., 31 (2019) e1901152. [26] S. Wang, Y. Hua, M. Wang, F. Liu, L. Ding, Materials Chemistry Frontiers, 3 (2019) 1139-1142. [27] Y. Wang, T. Zhang, M. Kan, Y. Zhao, J Am Chem Soc, 140 (2018) 12345-12348. [28] L. Yan, Q. Xue, M. Liu, Z. Zhu, J. Tian, Z. Li, Z. Chen, Z. Chen, H. Yan, H.L. Yip, Y. Cao, Adv. Mater., (2018) e1802509. [29] S. Yang, H. Zhao, M. Wu, S. Yuan, Y. Han, Z. Liu, K. Guo, S. Liu, S. Yang, H. Zhao, S. Yuan, Y. Han, Z. Liu, S. Liu, M. Wu, K. Guo, Solar Energy Mater. and Solar Cells, 201 (2019) 110052. [30] G. Yin, H. Zhao, H. Jiang, S. Yuan, T. Niu, K. Zhao, Z. Liu, S.F. Liu, Adv. Func. Mater., 28 (2018) 1803269. [31] Q. Zeng, L. Liu, Z. Xiao, F. Liu, Y. Hua, Y. Yuan, L. Ding, Science Bulletin, 64 (2019) 885-887. [32] Q. Zeng, X. Zhang, C. Liu, T. Feng, Z. Chen, W. Zhang, W. Zheng, H. Zhang, B. Yang, Solar RRL, 3 (2019) 1800239. [33] L. Zhou, X. Guo, Z. Lin, J. Ma, J. Su, Z. Hu, C. Zhang, S. Liu, J. Chang, Y. Hao, Nano Energy, 60 (2019) 583-590. [34] C. Zuo, H.J. Bolink, H. Han, J. Huang, D. Cahen, L. Ding, Adv Sci (Weinh), 3 (2016) 1500324. [35] Y. Wang, M.I. Dar, L.K. Ono, T. Zhang, M. Kan, Y. Li, L. Zhang, X. Wang, Y. Yang, X. Gao, Y. Qi, M. Grätzel, Y. Zhao, Science, 365 (2019) 591-595. [36] T. Wu, Y. Wang, Z. Dai, D. Cui, T. Wang, X. Meng, E. Bi, X. Yang, L. Han, Adv. Mater., 31 (2019) e1900605. [37] C. Liu, W. Li, H. Li, H. Wang, C. Zhang, Y. Yang, X. Gao, Q. Xue, H.-L. Yip, J. Fan, R.E.I. Schropp, Y. Mai, Adv. Energy Mater., 9 (2019) 1803572. [38] S. Das, M.J. Hossain, S.-F. Leung, A. Lenox, Y. Jung, K. Davis, J.-H. He, T. Roy, Nano Energy, 58 (2019) 47-56. [39] H.-P. Wang, D. Periyanagounder, A.-C. Li, J.-H. He, IEEE Access, 7 (2019) 19395-19400. 14
[40] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Grätzel, L. Han, Science, 350 (2015) 6263. [41] C. Cui, Y. Li, Y. Li, Adv. Energy Mater., 7 (2017) 1601251. [42] G. Kakavelakis, T. Maksudov, D. Konios, I. Paradisanos, G. Kioseoglou, E. Stratakis, E. Kymakis, Adv. Energy Mater., 7 (2017) 1602120. [43] Z. Zhou, X. Li, M. Cai, F. Xie, Y. Wu, Z. Lan, X. Yang, Y. Qiang, A. Islam, L. Han, Adv. Energy Mater., 7 (2017) 1700763. [44] Y.H. Seo, J.S. Yeo, N. Myoung, S.Y. Yim, M. Kang, D.Y. Kim, S.I. Na, ACS applied materials & interfaces, 8 (2016) 12822-12829. [45] Q. An, F. Zhang, J. Zhang, W. Tang, Z. Deng, B. Hu, Energy & Environmental Science, 9 (2016) 281-322. [46] L. Nian, K. Gao, F. Liu, Y. Kan, X. Jiang, L. Liu, Z. Xie, X. Peng, T.P. Russell, Y. Ma, Adv Mater, 28 (2016) 8184-8190. [47] Z. Xiao, X. Jia, L. Ding, Science Bulletin, 62 (2017) 1562-1564. [48] G. Zhang, K. Zhang, Q. Yin, X.F. Jiang, Z. Wang, J. Xin, W. Ma, H. Yan, F. Huang, Y. Cao, Journal of the American Chemical Society, 139 (2017) 2387-2395. [49] Y. Liu, J.Y.L. Lai, S. Chen, Y. Li, K. Jiang, J. Zhao, Z. Li, H. Hu, T. Ma, H. Lin, J. Liu, J. Zhang, F. Huang, D. Yu, H. Yan, J. Mater. Chem. A, 3 (2015) 13632-13636. [50] Y. Liu, C. Mu, K. Jiang, J. Zhao, Y. Li, L. Zhang, Z. Li, J.Y. Lai, H. Hu, T. Ma, R. Hu, D. Yu, X. Huang, B.Z. Tang, H. Yan, Adv. Mater., 27 (2015) 1015-1020. [51] C.B. Nielsen, S. Holliday, H.Y. Chen, S.J. Cryer, I. McCulloch, Acc. Chem. Res., 48 (2015) 2803-2812. [52] J. Lee, R. Singh, D.H. Sin, H.G. Kim, K.C. Song, K. Cho, Adv. Mater., 28 (2016) 69-76. [53] F. Liu, Z. Zhou, C. Zhang, T. Vergote, H. Fan, F. Liu, X. Zhu, J. Am. Chem. Soc., 138 (2016) 15523-15526. [54] D. Zhao, Z. Zhu, M.Y. Kuo, C.C. Chueh, A.K. Jen, Angew. Chem. Int. Ed., 55 (2016) 8999-9003. [55] C. Chen, H. Li, X. Ding, M. Cheng, H. Li, L. Xu, F. Qiao, H. Li, L. Sun, ACS 15
applied materials & interfaces, 10 (2018) 38970-38977. [56] K. Jiang, F. Wu, H. Yu, Y. Yao, G. Zhang, L. Zhu, H. Yan, J. Mater. Chem. A, 6 (2018) 16868-16873. [57] K. Jiang, F. Wu, L. Zhu, H. Yan, ACS Appl. Mater. Interfaces, 10 (2018) 36549-36555. [58] Z. Luo, F. Wu, T. Zhang, X. Zeng, Y. Xiao, T. Liu, C. Zhong, X. Lu, L. Zhu, S. Yang, C. Yang, Angew. Chem. Int. Ed., 58 (2019) 8520-8525.
Cheng Chen received her Ph.D degree in 2014 from the State Key Laboratory of Fine Chemicals at Dalian University of Technology (DUT). Then she worked as a post-doc in KTH Royal Institute of Technology, hosted by Prof. Lars Kloo. In June 2017, she joined Jiangsu University as a full professor. Her research is mainly focused on new functional materials for dye-sensitized solar cells, perovskite solar cells and organic solar cells.
Cheng Wu received his Bachelor's degree from Chaohu University in 2018. Now he is a master student supervised by Prof. Cheng Chen at Jiangsu University. His current research interest mainly focuses on developing low-cost and highly efficient organic interlayer for perovskite solar cells.
Xingdong Ding received his Bachelor's degree from Chuzhou 16
University in 2017. Now he is a master student supervised by Prof. Ming Cheng at Jiangsu University. His current research interest mainly focuses on developing low-cost and highly efficient hole transport materials for perovskite solar cells.
Yi Tian is currently a full-time researcher in the Institute for Energy Research, Jiangsu University. He received his PhD degree in chemistry from Department of Science, Osaka University. Then he moved to Tohoku University and National University of Singapore as a post-doctoral fellow in 2015 and 2017, respectively. His research mainly focuses on design and synthesis of organic conjugated materials and their application in perovskite solar cells
Mengmeng Zheng received her Bachelor's degree from Qiqihar University in 2018. Now she is a master student supervised by Prof. Ming Cheng at Jiangsu University. Her research interest mainly focuses on developing highly efficient and stable non-fullerene electron transport materials for perovskite solar cells.
Ming Cheng received his PhD degree from Dalian University of Technology in 2014. After that, he went to KTH Royal Institute of Technology in Sweden for postdoctoral research under the host of Prof. Licheng Sun. In June 2017, he joined Jiangsu University as a full professor. Currently, his research interests mainly focus on perovskite solar cells.
Hui Xu received his Ph.D. degree in environmental engineering from Jiangsu University. He has joined Prof. Pulickel M. 17
Ajayan's group as a visiting scholar from October. 2015 to April 2016 at Rice University. He is currently a full professor at Jiangsu University. His current research focuses on nanomaterials (especially 2D nanosheets) for energy and environmental applications.
Zhiwen Jin is a professor with the School of Physical Science and Technology, Lanzhou University. His research interests include inorganic semiconductor materials, thin-film photoelectric devices and device physics, particularly inorganic perovskite solar cells. He received the B.S. degree from Lanzhou University in 2011 and Ph.D. degree from Institute of Chemistry, Chinese Academy of Sciences in 2016. And he joined Lanzhou University in 2018. Liming Ding got his PhD degree from University of Science and Technology of China. He started his research on OSCs and PLEDs in Olle Inganäs Lab in 1998. Later on, he worked with Frank Karasz and Tom Russell at PSE, UMASS Amherst. He joined Konarka as a Senior Scientist in 2008. In 2010, he joined National Center for Nanoscience and Technology as a Full Professor. Currently, his research interests include perovskite solar cells, organic solar cells and photodetectors.
18
Highlight For the first time, the binary electron transport layer was proposed and successfully applied in inverted full-inorganic perovskite solar cell. The binary electron transport layer based CsPbI2Br perovskite solar cell obtained impressive PCE of 15.12%. There is nearly no hysteresis for the binary electron transport layer based CsPbI2Br perovskite solar cell.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S2211-2855(20)30162-2
DOI:
https://doi.org/10.1016/j.nanoen.2020.104604
Reference:
NANOEN 104604
To appear in:
Nano Energy
Received Date: 16 December 2019 Revised Date:
20 January 2020
Accepted Date: 10 February 2020
Please cite this article as: C. Chen, C. Wu, X. Ding, Y. Tian, M. Zheng, M. Cheng, H. Xu, Z. Jin, L. Ding, Constructing Binary Electron Transport Layer with Cascade Energy Level Alignment for Efficient CsPbI2Br Solar Cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104604. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Graphical Abstract A binary electron transport layer (ETL), consisting of traditional electron transport material (ETM) P61CBM and a newly developed non-fullerene ETM SFX-PDI2, is proposed and firstly applied in inverted CsPbI2Br perovskite solar cell (PSC), achieving an impressive efficiency of 15.12% due to the better film morphology and electron extraction and transport properties of the binary ETL.
Constructing Binary Electron Transport Layer with Cascade Energy Level Alignment for Efficient CsPbI2Br Solar Cells Cheng Chen,a, d Cheng Wu,a Xingdong Ding,a Yi Tian,a Mengmeng Zheng,a Ming Chenga* Hui Xu,a Zhiwen Jin,c Liming Ding b* a
Institute for Energy Research, Key Laboratory of Zhenjiang, Jiangsu University, Zhenjiang 212013, China * E-mail: [email protected] b Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China * E-mail: [email protected] c School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China d State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Keywords: perovskite solar cells; electron transport materials; perylene diimide
Abstract Recently, all-inorganic perovskite solar cell (PSC) has attracted an increasing research attention due to its excellent thermal stability and suitable band gap for tandem devices. Herein, a highly efficient all-inorganic PSC with CsPbI2Br as light absorption material is assembled by employing a binary electron transport layer (ETL), which consists of PC61BM and a new designed non-fullerene electron transport material SFX-PDI2. The formed binary system primely implements in the form of a thin, uniform and full coverage ETL, and possesses more excellent electron extraction and transport properties than both pristine PC61BM and pristine SFX-PDI2. As a result, the binary ETL based PSC with CsPbI2Br as light harvesting material achieved an efficiency as high as 15.12%, which is among the highest performance for inverted planar structure all-inorganic PSC.
1
Main text During the past decade, due to the facile tunable absorption range, large absorption coefficient and excellent charge carrier separation and transport properties, lead halide perovskite materials with crystal structure of ABX3 had attracted extensive attention, and showed broad application prospects in energy conversion, photodetector, thermoelectric and electronic devices. [1-12] To date, perovskite solar cell (PSC) based on this type light-harvesting materials is one of the fastest growing photovoltaic technologies, with certified power conversion efficiencies (PCEs) skyrocketed over 25% in 2019. However, the intrinsically volatile and thermally unstable nature of organic cation make the organic-inorganic hybrid PSC suffer from poor stability, limiting its future commercial application. Recently, many research works manifested that simply replacing the organic A site [methyl ammonium (MA) or formamidine (FA)] with cesium (Cs) could greatly enhance the thermal stability, making the all-inorganic cesium lead halide (CsPbX3) perovskites become promising candidates for highly efficient and stable solar cells. [13-34] Similar with organic-inorganic counterpart, all-inorganic PSCs also have two different device architectures, including the conventional n-i-p and inverted p-i-n configurations. For n-i-p structured all-inorganic PSCs, although a record PCE of 18.4% has been achieved, the hysteresis behavior of all-inorganic PSCs was not stressed. [19, 27, 35] Compared with the n-i-p PSCs, the p-i-n PSCs tend to exhibit less hysteresis effect. However, the PCE of p-i-n structured all-inorganic PSCs is still lagging (13.3%) and much lower than that of n-i-p devices. [23, 36, 37] Therefore, great efforts need to be made to improve device efficiency. The first way is optimizing the perovskite crystal components and crystal growth process to get high quality CsPbX3 film. The other way is developing novel charge transport materials with high charge carrier mobility and more appropriate energy levels, favoring charge transport and minimize energy loss. [38, 39] Fullerene and its derivatives are commonly used electron transport materials 2
(ETMs) in the inverted PSCs, among which [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) is the most prevalent one. [40-43] The extraordinary photovoltaic performance of PC61BM based PSCs is regarded as the result of the effective trap passivation and fast electron extraction of PC61BM. However, PC61BM is far from perfect electron transport layer (ETL) because of the poor film-forming, ordinary electron mobility and conductivity properties and poor phase stability in ambient condition. [44] To get a continuous PC61BM film, the thickness of film is inevitably required to be more than 100 nm, which would increase the series resistance of PSCs, leading to poor photovoltaic performance. Hence, a key challenge in the further is reducing the pinholes and morphological defects of ETL without greatly increasing the thickness and series resistance of ETL. One of effective strategies is blending of two ETMs to construct binary ETL system, which has been successfully implemented in the organic solar cell (OSC) field. [45-48] On the one hand, blending of two ETMs could efficiently increase the ETM solution viscosity and prevent the de-wetting behavior of ETM solution on perovskite film, correspondingly, obtaining a good film-forming property. On the other hand, two ETMs might form film in different ways and complement the insufficiency of each other, in turn, improving the electron extraction and transport properties of binary ETL. Based on these concerns, novel ETMs with distinctly different configuration from PC61BM and high electron mobility are urgently desired. Apart from fullerene derivatives, lots of n-type materials, such as ITIC analogues, hexaazatrinaphthylene (HATNA) derivatives, and perylene diimide (PDI) derivatives were reported as alternatives. [49-55] Thereinto, PDI derivatives have received widespread attention due to the high conductivity and charge carrier mobility. When applied into organic-inorganic hybrid PSCs, PDI-based materials have been proved to be beneficial for enhancing photovoltaic performance of devices, including passivating perovskite surface trap and preventing the perovskite decomposition. [56-58] However, PDI-based ETMs have been rarely applied in all-inorganic PSCs. In this context, by connecting spiro[fluorene-9,9'-xanthene] (SFX) core building 3
block with two PDI units, we designed and synthesized a novel non-fullerene ETM, termed SFX-PDI2, which further served as one component of the binary ETL system to tune the morphology, charge transport and collection characteristics of the whole ETL, complementing the insufficiency of the other component PC61BM. The formed binary ETL system was systematically studied with optical spectroscopy, photoluminescence (PL), space charge limit current (SCLC), scanning electron microscope (SEM), etc. The optimal binary ETMs-based PSC with CsPbI2Br as light-harvesting material exhibits a high PCE of 15.12%, which is significantly over performed than devices based on either PC61BM (12.91%) or SFX-PDI2 (11.74%). The greatly improved photovoltaic performance of the binary ETMs-based PSC benefits from the synergistic effects of ETMs PC61BM and SFX-PDI2.
Figure 1 a) Schematic device structure of PSC and chemical structures of the binary ETMs, b) Cross-sectional SEM image of binary ETL-based PSC, c) Energy level alignment of main components in PSC Figure 1a illuminates the schematic structure of PSC in this study, wherein the detailed chemical structures of the binary ETMs are listed. To prevent PDI units from 4
forming excessively large aggregates, SFX core building block with orthogonal three-dimensional configuration was targetedly selected. SFX-PDI2 can be easily synthesized trough Suzuki coupling reaction with the yield of 68% (see Scheme S1). The structure was fully characterized by NMR and high-resolution mass spectrum. The detailed molecular configuration and charge density distribution were investigated by using density functional theory (DFT) calculation. As shown in Figure S1, SFX-PDI2 shows twisted non-planar structure, with a dihedral angle of 117o. The highest occupied molecular orbital (HOMO) of SFX-PDI2 distributes throughout the whole molecular skeleton, while the lowest unoccupied molecular orbital (LUMO) transfers to peripheral PDI units, indicating a good charge carrier transport property. The energy levels of SFX-PDI2 were roughly evaluated to be -3.96 V for LUMO energy level and -5.94 V for HOMO energy level, respectively (see Figure S2). CsPbI2Br, SFX-PDI2 and PC61BM form gradient energy level alignment, as shown in Figure 1c, which is facilitated the synergetic electron transport. To investigate the potential of binary ETL system for highly efficient all-inorganic PSCs, devices with a configuration of ITO/NiO/CsPbI2Br/binary ETL/Bathocuproine (BCP)/Ag were fabricated, as shown in Figure 1b. BCP was introduced as a thin buffer layer to improve the contact of ETL and Ag electrode. The effect of weight ratios of SFX-PDI2 to PC61BM on photovoltaic performances was systemically examined, as shown in Figure 2a and 2b. The formed binary ETL system indeed showed significantly positive effects on device performance and the optimal blend weight ratio of the SFX-PDI2 relative to PC61BM is defined to be 10% (see Table S1). The PSCs employing binary ETL systems with low weight ratio of SFX-PDI2 (5%, 10% and 15%) showed obviously higher PCEs than both the pristine PC61BM (12.91%) and the pristine SFX-PDI2 (11.74%). The enhancements of PCEs can be mainly attributed to the higher electron mobility and better film morphology of formed binary ETLs (see Figure S3, S4 and Table S2). Under the optimized conditions (PC61BM:SFX-PDI2 ETL, weight ratio 9:1; 3000 rpm), the formed binary ETL exhibited the highest electron mobility of 2.42 × 10-4 cm2·V-1·s-1 and good 5
conductivity, correspondingly, the highest PCE of 15.12% with an open-circuit voltage (Voc) of 1.21 V, a short-circuit current density (Jsc) of 15.72 mA·cm-2 and a fill factor (FF) of 79.5% was achieved, showed substantial increments of 17.1% and 28.8% compared with the devices with pristine PC61BM and pristine SFX-PDI2 ETL, respectively. The hysteresis behavior and stabilized current density at the maximum power output point of binary ETL based PSC were also studied. As shown in Figure 2c, no noticeable hysteresis was detected. The steady-state current density was calculated to be 14.75 mA·cm-2, further confirming the excellent photovoltaic performances of our devices (see Figure 2e). Moreover, the binary ETL based PSCs also showed good reproducibility, with an average PCE of 14.28±0.47% and 50% devices performed over average value (see Figure 2f, Table S3-5).
6
Figure 2 a) J-V characteristic of PSCs based on ETLs with different weight ratio of SFX-PDI2, b) The changing trend of PCE at different weight ratios of SFX-PDI2 in binary ETLs; c) J-V characteristic of PSCs based on optimized binary ETL (90% PC61BM:10% SFX-PDI2) with different scan directions (OC: open-circuit; SC: short-circuit), d) IPCE spectra of PSCs containing PC61BM, SFX-PDI2 and 90% PC61BM:10% SFX-PDI2 as ETMs, e) steady-state current density and PCE of PSC based on optimized binary ETL (90% PC61BM:10% SFX-PDI2) at maximum power output point, f) Repeatability statistic data of PSC based on different ETLs
Figure 3 SEM images of top-view film morphology of a) CsPbI2Br, b) CsPbI2Br/PC61BM, c) CsPbI2Br/SFX-PDI2, and d) CsPbI2Br/PC61BM:SFX-PDI2; and AFM height images of different films e) CsPbI2Br, f) CsPbI2Br/PC61BM, g) CsPbI2Br/SFX-PDI2, and h) CsPbI2Br/PC61BM:SFX-PDI2
The improved PCE of binary ETL based PSCs may result from the following two factors: better film formation property and film quality with the aid of SFX-PDI2, and more efficient electron extraction and transport of PC61BM:SFX-PDI2 binary ETL. To confirm our hypothesis and better understand the relationship between the ETL morphology and device performances, we firstly probed the features of ETL on perovskite surface by using scanning electron microscope (SEM) and atomic force microscope (AFM). As shown in Figure 3, the CsPbI2Br film showed a uniform and dense morphology. For the PC61BM thin film, it failed to fully cover the perovskite surface, with obvious pin-holes and bared perovskite detected. For SFX-PDI2 thin 7
film, the coverage on the perovskite is even lower due to the rapid crystallization and easy aggregation of PDI materials. Interestingly, when blending the PC61BM and SFX-PDI2 together to form a binary ETL system (keep the same total weight as pristine PC61BM and pristine SFX-PDI2, 20 mg/mL, with the same coating technology), the weight ratio of SFX-PDI2 shows significant impacts on film formation properties of formed binary ETLs. As shown in Figure S4, with addition of small amount of SFX-PDI2, the film is smooth and uniform. No aggregated dots and wrinkles were detected. While further increasing the weight ratios of SFX-PDI2, the films become plicate. With 25% SFX-PDI2, the coverage of formed binary ETL is very low, which is similar with pure SFX-PDI2, indicating the film formation property of SFX-PDI2 has a predominant effect on the film-forming properties of binary ETL in high weight ratio case. In optimized conditions, the roughness (R) of binary ETL significantly decrease (see Figure 3e-h) and the perovskite layer was almost fully covered, preventing the perovskite from intimate contact with Ag electrode, in turn successfully suppressing the leakage currents (see Figure 4a). The improved Jsc and FF could result from the more homogeneous and fully-covered ETL and smaller series resistance (see Table S1).
8
Figure 4 a) Dark current density of PSCs containing different ETLs, b) TRPL spectra of the glass/Al2O3/CsPbI2Br and glass/Al2O3/CsPbI2Br/ETMs (PC61BM, SFX-PDI2 or PC61BM:SFX-PDI2), c) the dependence of Voc on light intensity for PSCs containing different ETLs, d) stability of different ETLs based CsPbI2Br PSCs
To further manifest the detailed affecting factors for the enhanced PCE, the incident photon-to-current conversion efficiency (IPCE) for PC61BM, SFX-PDI2 and binary ETMs-based PSCs were measured as shown in Figure 2d. The IPCE values of the binary ETMs-based PSC was obviously higher than those of pristine PC61BM and pristine SFX-PDI2 counterparts in the entire visible range, indicating a synergistic favorable effect on the photoelectric conversion properties of binary ETL system. From the IPCE spectra, the integrated current densities were calculated to be 14.61, 13.78 and 15.49 mA·cm-2 for PC61BM, SFX-PDI2 and 90% PC61BM:10% SFX-PDI2, respectively, consisting well with the J-V measurements. Considering the influence factor for IPCE, the higher IPCE values obtained by binary HTMs-based PSCs were mainly due to the increased charge transport and collection efficiencies. 9
This deduction is further supported by time-resolved photoluminescence (TRPL) measurements (see Figure 4b). In our case, to facilitate the growth of perovskite crystals, Al2O3 film on FTO was employed as substrate. The PL lifetimes of CSPbI2Br/PC61BM,
CSPbI2Br/SFX-PDI2
and
CSPbI2Br/90%
PC61BM:10%SFX-PDI2 were extracted by a biexponential decay function. τ1 is assigned to the extraction time of ETL on the nanoseconds, and τ2 represents the charge recombination time in the radiative channel. In the case of pristine PC61BM, τ1 and τ2 were calculated to be 3.75 ns (79%) and 21.5 ns (21%), with an average τ of 7.48 ns. Under the same conditions, pristine SFX-PDI2 based devices exhibited longer decay time [4.29 ns (78%) and 24.1 ns (22%)], indicating inferior charge transport and extraction properties than PC61BM. When the binary ETMs PC61BM:10%SFX-PDI2 was used as the ETL, the sample showed the shortest PL quenching lifetime of 5.86 ns (τ1: 2.84 ns, 83%; τ2: 20.6 ns, 17%). The shorter τ1 for PC61BM:10%SFX-PDI2 based sample suggested that the binary ETL system fully guarantees the efficient electron extraction from perovskite without any serious recombination at the perovskite/ETM interfaces, correspondingly, the higher Voc is obtained. The results above were also confirmed by measurements of the dependence of the Voc on the light intensity (see Figure 4c). The 90% PC61BM:10%SFX-PDI2 based PSC showed a slope of 1.29 kT/q, suggesting trace of recombination traps in the 90% PC61BM:10% SFX-PDI2 based device. Comparatively, the pristine PC61BM and pristine SFX-PDI2 based samples showed much stronger dependence on the light intensity, with slopes of 1.47kT/q and 1.76 kt/q, respectively. These results demonstrate that the binary ETL system successfully prevents the charge recombination and facilitates the charge transport. As a crucial parameter, the stability of PSC based on the binary ETL was further evaluated and devices based on PC61BM and SFX-PDI2 were employed as references (see Figure 4d and S5). The PSCs were stored in the ambient condition (the humidity of 40-50%) without capsulation. For the PC61BM based PSCs, the PCE sharply dropped from 12.91% to 8.31% after 30 days storage. With addition of 10% 10
SFX-PDI2 in ETL, the stability was slightly improved, maintaining 73.6% of initial PCE. The enhancement of stability mainly causes by improved hydrophobicity of binary ETL (see Figure S6). As for SFX-PDI2 based PSC, the stability is much better than PC61BM and binary ETL based devices. Therefore, for the binary ETL based PSC, the phase instability and hydrophilia of PC61BM are decisive factors for device degradation. In summary, a binary ETL system, consisting of traditional ETM PC61BM and a newly designed non-fullerene ETM SFX-PDI2, was successfully proposed and firstly applied into the highly efficient inverted all-inorganic PSC. The addition component of SFX-PDI2 not only improved the charge extraction and transport properties, but also facilitated the formation of homogeneous ETL film and enhanced the ETL coverage rate on the perovskite surface, correspondingly, effectively restricting the charge recombination at perovskite/ETL interface. Under optimized conditions, the binary ETL based PSC showed an efficiency as high as 15.12%, which is about 17.1% and 28.8% higher than that of both the pristine PC61BM and pristine SFX-PDI2 based devices. This work hopefully supplies an effective and convenient way for advancing the performance of the all-inorganic PSCs.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grants 21805114, 21905119), Natural Science Foundation of Jiangsu province (BK20180867, BK20180869), China Postdoctoral Science Foundation (2019M651741), Top talents in Jiangsu province
(XNY066),
the
Jiangsu
University
Foundation
(17JDG032,
17JDG031), High-tech Research Key laboratory of Zhenjiang (SS2018002), the State Key Laboratory of Fine Chemicals (KF1902), the high-performance computing platform of Jiangsu University, the Priority Academic Program Development of Jiangsu Higher Education Institutions. L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and 11
the National Natural Science Foundation of China (51773045, 21572041, 21772030 and 51922032) for financial support.
Supporting Information The experimental details, characterization of SFX-PDI2, and fabrication of perovskite solar cell, conductivity and electron mobility measurements, stability measurements of PSCs, water contact angle measurements supplied as Supporting Information.
Reference [1] S. Wang, Y. Jiang, Emilio J. Juarez-Perez, Luis K. Ono, Y. Qi, Nature Energy, 2 (2016) 16195. [2] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.-S. Lim, J.A. Chang, Y.H. Lee, H.-j. Kim, A. Sarkar, M.K. Nazeeruddin, M. Grätzel, S.I. Seok, Nature Photonics, 7 (2013) 486. [3] J. Huang, Y. Yuan, Y. Shao, Y. Yan, Nat. Rev. Mater., 2 (2017) 17042. [4] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Science, 342 (2013) 341-344. [5] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gratzel, S. Mhaisalkar, T.C. Sum, Science, 342 (2013) 344-347. [6] H.S. Jung, N.G. Park, Small, 11 (2015) 10-25. [7] Q. Jiang, L. Zhang, HaolinWang, X. Yang, J. Meng, H. Liu, Z. Yin, JinliangWu, X. Zhang, J. You, Nature Energy, 2 (2016) 16177. [8] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Nature, 517 (2015) 476-480. [9] C.H. Lin, B. Cheng, T.Y. Li, J.R.D. Retamal, T.C. Wei, H.C. Fu, X. Fang, J.H. He, ACS Nano, 13 (2019) 1168-1176. [10] W. Yang, J. Chen, Y. Zhang, Y. Zhang, J.H. He, X. Fang, Adv. Funct. Mater., 29 (2019) 1808182. 12
[11] A.M. Alamri, S. Leung, M. Vaseem, A. Shamim, J.-H. He, IEEE Trans. Elect. Dev., 66 (2019) 2657-2661. [12] Y. Zhang, W. Xu, X. Xu, J. Cai, W. Yang, X. Fang, J. Phys. Chem. Lett., 10 (2019) 836-841. [13] D. Bai, H. Bian, Z. Jin, H. Wang, L. Meng, Q. Wang, S. Liu, Nano Energy, 52 (2018) 408-415. [14] W. Chen, H. Chen, G. Xu, R. Xue, S. Wang, Y. Li, Y. Li, Joule, 3 (2019) 191-204. [15] Z. Fang, L. Liu, Z. Zhang, S. Yang, F. Liu, M. Liu, L. Ding, Science Bulletin, 64 (2019) 507-510. [16] Z. Fang, X. Meng, C. Zuo, D. Li, Z. Xiao, C. Yi, M. Wang, Z. Jin, S. Yang, L. Ding, Science Bulletin, 64 (2019) 1743-1746. [17] Z. Guo, S. Zhao, A. Liu, Y. Kamata, S. Teo, S. Yang, Z. Xu, S. Hayase, T. Ma, ACS Appl. Mater. Interfaces, 11 (2019) 19994-20003. [18] K. Huang, Y. Peng, Y. Gao, J. Shi, H. Li, X. Mo, H. Huang, Y. Gao, L. Ding, J. Yang, Advanced Energy Materials, 9 (2019) 1901419. [19] X. Jia, C. Zuo, S. Tao, K. Sun, Y. Zhao, S. Yang, M. Cheng, M. Wang, Y. Yuan, J. Yang, F. Gao, G. Xing, Z. Wei, L. Zhang, H.-L. Yip, M. Liu, Q. Shen, L. Yin, L. Han, S. Liu, L. Wang, J. Luo, H. Tan, Z. Jin, L. Ding, Science Bulletin, 64 (2019) 1532-1539. [20] H. Jiang, J. Feng, H. Zhao, G. Li, G. Yin, Y. Han, F. Yan, Z. Liu, S.F. Liu, Adv. Sci., 5 (2018) 1801117. [21] D.H. Kim, J.H. Heo, S.H. Im, ACS. Appl. Mater. Interfaces, 11 (2019) 19123-19131. [22] C.F.J. Lau, M. Zhang, X. Deng, J. Zheng, J. Bing, Q. Ma, J. Kim, L. Hu, M.A. Green, S. Huang, A. Ho-Baillie, ACS Energy Lett., 2 (2017) 2319-2325. [23] C. Liu, W. Li, C. Zhang, Y. Ma, J. Fan, Y. Mai, J. Am. Chem. Soc., 140 (2018) 3825-3828. [24] J. Lu, S.-C. Chen, Q. Zheng, Science China Chem., 62 (2019) 1044-1050. [25] J. Tian, Q. Xue, X. Tang, Y. Chen, N. Li, Z. Hu, T. Shi, X. Wang, F. Huang, C.J. 13
Brabec, H.L. Yip, Y. Cao, Adv. Mater., 31 (2019) e1901152. [26] S. Wang, Y. Hua, M. Wang, F. Liu, L. Ding, Materials Chemistry Frontiers, 3 (2019) 1139-1142. [27] Y. Wang, T. Zhang, M. Kan, Y. Zhao, J Am Chem Soc, 140 (2018) 12345-12348. [28] L. Yan, Q. Xue, M. Liu, Z. Zhu, J. Tian, Z. Li, Z. Chen, Z. Chen, H. Yan, H.L. Yip, Y. Cao, Adv. Mater., (2018) e1802509. [29] S. Yang, H. Zhao, M. Wu, S. Yuan, Y. Han, Z. Liu, K. Guo, S. Liu, S. Yang, H. Zhao, S. Yuan, Y. Han, Z. Liu, S. Liu, M. Wu, K. Guo, Solar Energy Mater. and Solar Cells, 201 (2019) 110052. [30] G. Yin, H. Zhao, H. Jiang, S. Yuan, T. Niu, K. Zhao, Z. Liu, S.F. Liu, Adv. Func. Mater., 28 (2018) 1803269. [31] Q. Zeng, L. Liu, Z. Xiao, F. Liu, Y. Hua, Y. Yuan, L. Ding, Science Bulletin, 64 (2019) 885-887. [32] Q. Zeng, X. Zhang, C. Liu, T. Feng, Z. Chen, W. Zhang, W. Zheng, H. Zhang, B. Yang, Solar RRL, 3 (2019) 1800239. [33] L. Zhou, X. Guo, Z. Lin, J. Ma, J. Su, Z. Hu, C. Zhang, S. Liu, J. Chang, Y. Hao, Nano Energy, 60 (2019) 583-590. [34] C. Zuo, H.J. Bolink, H. Han, J. Huang, D. Cahen, L. Ding, Adv Sci (Weinh), 3 (2016) 1500324. [35] Y. Wang, M.I. Dar, L.K. Ono, T. Zhang, M. Kan, Y. Li, L. Zhang, X. Wang, Y. Yang, X. Gao, Y. Qi, M. Grätzel, Y. Zhao, Science, 365 (2019) 591-595. [36] T. Wu, Y. Wang, Z. Dai, D. Cui, T. Wang, X. Meng, E. Bi, X. Yang, L. Han, Adv. Mater., 31 (2019) e1900605. [37] C. Liu, W. Li, H. Li, H. Wang, C. Zhang, Y. Yang, X. Gao, Q. Xue, H.-L. Yip, J. Fan, R.E.I. Schropp, Y. Mai, Adv. Energy Mater., 9 (2019) 1803572. [38] S. Das, M.J. Hossain, S.-F. Leung, A. Lenox, Y. Jung, K. Davis, J.-H. He, T. Roy, Nano Energy, 58 (2019) 47-56. [39] H.-P. Wang, D. Periyanagounder, A.-C. Li, J.-H. He, IEEE Access, 7 (2019) 19395-19400. 14
[40] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Grätzel, L. Han, Science, 350 (2015) 6263. [41] C. Cui, Y. Li, Y. Li, Adv. Energy Mater., 7 (2017) 1601251. [42] G. Kakavelakis, T. Maksudov, D. Konios, I. Paradisanos, G. Kioseoglou, E. Stratakis, E. Kymakis, Adv. Energy Mater., 7 (2017) 1602120. [43] Z. Zhou, X. Li, M. Cai, F. Xie, Y. Wu, Z. Lan, X. Yang, Y. Qiang, A. Islam, L. Han, Adv. Energy Mater., 7 (2017) 1700763. [44] Y.H. Seo, J.S. Yeo, N. Myoung, S.Y. Yim, M. Kang, D.Y. Kim, S.I. Na, ACS applied materials & interfaces, 8 (2016) 12822-12829. [45] Q. An, F. Zhang, J. Zhang, W. Tang, Z. Deng, B. Hu, Energy & Environmental Science, 9 (2016) 281-322. [46] L. Nian, K. Gao, F. Liu, Y. Kan, X. Jiang, L. Liu, Z. Xie, X. Peng, T.P. Russell, Y. Ma, Adv Mater, 28 (2016) 8184-8190. [47] Z. Xiao, X. Jia, L. Ding, Science Bulletin, 62 (2017) 1562-1564. [48] G. Zhang, K. Zhang, Q. Yin, X.F. Jiang, Z. Wang, J. Xin, W. Ma, H. Yan, F. Huang, Y. Cao, Journal of the American Chemical Society, 139 (2017) 2387-2395. [49] Y. Liu, J.Y.L. Lai, S. Chen, Y. Li, K. Jiang, J. Zhao, Z. Li, H. Hu, T. Ma, H. Lin, J. Liu, J. Zhang, F. Huang, D. Yu, H. Yan, J. Mater. Chem. A, 3 (2015) 13632-13636. [50] Y. Liu, C. Mu, K. Jiang, J. Zhao, Y. Li, L. Zhang, Z. Li, J.Y. Lai, H. Hu, T. Ma, R. Hu, D. Yu, X. Huang, B.Z. Tang, H. Yan, Adv. Mater., 27 (2015) 1015-1020. [51] C.B. Nielsen, S. Holliday, H.Y. Chen, S.J. Cryer, I. McCulloch, Acc. Chem. Res., 48 (2015) 2803-2812. [52] J. Lee, R. Singh, D.H. Sin, H.G. Kim, K.C. Song, K. Cho, Adv. Mater., 28 (2016) 69-76. [53] F. Liu, Z. Zhou, C. Zhang, T. Vergote, H. Fan, F. Liu, X. Zhu, J. Am. Chem. Soc., 138 (2016) 15523-15526. [54] D. Zhao, Z. Zhu, M.Y. Kuo, C.C. Chueh, A.K. Jen, Angew. Chem. Int. Ed., 55 (2016) 8999-9003. [55] C. Chen, H. Li, X. Ding, M. Cheng, H. Li, L. Xu, F. Qiao, H. Li, L. Sun, ACS 15
applied materials & interfaces, 10 (2018) 38970-38977. [56] K. Jiang, F. Wu, H. Yu, Y. Yao, G. Zhang, L. Zhu, H. Yan, J. Mater. Chem. A, 6 (2018) 16868-16873. [57] K. Jiang, F. Wu, L. Zhu, H. Yan, ACS Appl. Mater. Interfaces, 10 (2018) 36549-36555. [58] Z. Luo, F. Wu, T. Zhang, X. Zeng, Y. Xiao, T. Liu, C. Zhong, X. Lu, L. Zhu, S. Yang, C. Yang, Angew. Chem. Int. Ed., 58 (2019) 8520-8525.
Cheng Chen received her Ph.D degree in 2014 from the State Key Laboratory of Fine Chemicals at Dalian University of Technology (DUT). Then she worked as a post-doc in KTH Royal Institute of Technology, hosted by Prof. Lars Kloo. In June 2017, she joined Jiangsu University as a full professor. Her research is mainly focused on new functional materials for dye-sensitized solar cells, perovskite solar cells and organic solar cells.
Cheng Wu received his Bachelor's degree from Chaohu University in 2018. Now he is a master student supervised by Prof. Cheng Chen at Jiangsu University. His current research interest mainly focuses on developing low-cost and highly efficient organic interlayer for perovskite solar cells.
Xingdong Ding received his Bachelor's degree from Chuzhou 16
University in 2017. Now he is a master student supervised by Prof. Ming Cheng at Jiangsu University. His current research interest mainly focuses on developing low-cost and highly efficient hole transport materials for perovskite solar cells.
Yi Tian is currently a full-time researcher in the Institute for Energy Research, Jiangsu University. He received his PhD degree in chemistry from Department of Science, Osaka University. Then he moved to Tohoku University and National University of Singapore as a post-doctoral fellow in 2015 and 2017, respectively. His research mainly focuses on design and synthesis of organic conjugated materials and their application in perovskite solar cells
Mengmeng Zheng received her Bachelor's degree from Qiqihar University in 2018. Now she is a master student supervised by Prof. Ming Cheng at Jiangsu University. Her research interest mainly focuses on developing highly efficient and stable non-fullerene electron transport materials for perovskite solar cells.
Ming Cheng received his PhD degree from Dalian University of Technology in 2014. After that, he went to KTH Royal Institute of Technology in Sweden for postdoctoral research under the host of Prof. Licheng Sun. In June 2017, he joined Jiangsu University as a full professor. Currently, his research interests mainly focus on perovskite solar cells.
Hui Xu received his Ph.D. degree in environmental engineering from Jiangsu University. He has joined Prof. Pulickel M. 17
Ajayan's group as a visiting scholar from October. 2015 to April 2016 at Rice University. He is currently a full professor at Jiangsu University. His current research focuses on nanomaterials (especially 2D nanosheets) for energy and environmental applications.
Zhiwen Jin is a professor with the School of Physical Science and Technology, Lanzhou University. His research interests include inorganic semiconductor materials, thin-film photoelectric devices and device physics, particularly inorganic perovskite solar cells. He received the B.S. degree from Lanzhou University in 2011 and Ph.D. degree from Institute of Chemistry, Chinese Academy of Sciences in 2016. And he joined Lanzhou University in 2018. Liming Ding got his PhD degree from University of Science and Technology of China. He started his research on OSCs and PLEDs in Olle Inganäs Lab in 1998. Later on, he worked with Frank Karasz and Tom Russell at PSE, UMASS Amherst. He joined Konarka as a Senior Scientist in 2008. In 2010, he joined National Center for Nanoscience and Technology as a Full Professor. Currently, his research interests include perovskite solar cells, organic solar cells and photodetectors.
18
Highlight For the first time, the binary electron transport layer was proposed and successfully applied in inverted full-inorganic perovskite solar cell. The binary electron transport layer based CsPbI2Br perovskite solar cell obtained impressive PCE of 15.12%. There is nearly no hysteresis for the binary electron transport layer based CsPbI2Br perovskite solar cell.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: