Low-Temperature Presynthesized Crystalline Tin ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 14922−14929

www.acsami.org

Low-Temperature Presynthesized Crystalline Tin Oxide for Efficient Flexible Perovskite Solar Cells and Modules Tongle Bu,†,§ Shengwei Shi,†,§ Jing Li,† Yifan Liu,† Jielin Shi,† Li Chen,† Xueping Liu,† Junhao Qiu,† Zhiliang Ku,† Yong Peng,† Jie Zhong,*,† Yi-Bing Cheng,†,‡ and Fuzhi Huang*,† †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China ‡ Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia S Supporting Information *

ABSTRACT: Organic−inorganic metal halide perovskite solar cells (PSCs) have been emerging as one of the most promising next generation photovoltaic technologies with a breakthrough power conversion efficiency (PCE) over 22%. However, aiming for commercialization, it still encounters challenges for the largescale module fabrication, especially for flexible devices which have attracted intensive attention recently. Low-temperature processed high-performance electrontransporting layers (ETLs) are still difficult. Herein, we present a facile lowtemperature synthesis of crystalline SnO2 nanocrystals (NCs) as efficient ETLs for flexible PSCs including modules. Through thermal and UV-ozone treatments of the SnO2 ETLs, the electron transporting resistance of the ETLs and the charge recombination at the interface of ETL/perovskite were decreased. Thus, the hysteresis-free highly efficient rigid and flexible PSCs were obtained with PCEs of 19.20 and 16.47%, respectively. Finally, a 5 × 5 cm2 flexible PSC module with a PCE of 12.31% (12.22% for forward scan and 12.40% for reverse scan) was fabricated with the optimized perovskite/ETL interface. Thus, employing presynthesized SnO2 NCs to fabricate ETLs has showed promising for future manufacturing. KEYWORDS: tin oxide, interface optimization, hysteresis, flexible perovskite solar cells, modules

■

flexible PSM with PCE over 8% recently based on a vacuum evaporation of C60; this low PCE and expensive process would retard the commercial application.19 In the other hand, inorganic semiconductors, such as TiO2, SnO2, and ZnO, are more promising, despite the difficulty in low-temperature processing.20−22 For example, Dagar et al. employed a lowtemperature processed SnO2/meso-TiO2 composite film as the electron transporting layer (ETL) and obtained a PCE of 8.8% for a large area (12 cm2, no mask) flexible PSM.23 As recent reports demonstrated, SnO2 has shown superior performances such as high conductivity, proper energy band, and good ultraviolet (UV) resistance. Thus, SnO2 has been widely studied in PSCs. There are various routes to fabricate SnO2 ETLs, such as atomic layer deposition (ALD), spin coating, and chemical bath deposition (CBD).24−26 Recently, Wang et al. have reports a low-temperature plasma-enhanced ALD-processed SnO2 ETL sand achieved a high PCE over 18% with the water vapor treatment, although the device still had non-negligible hysteresis.21 However, a wet chemical processed SnO2 has more advantages as it does not involve a vacuum

INTRODUCTION Hybrid organic−inorganic metal halide perovskite materials with their outstanding properties, such as moderate band gap, high light extinction coefficient, long charge diffusion length, and high charge mobility, have emerged as very promising photovoltaic materials and attracted more and more attention.1−3 Since it was first used as light absorber in solar cells, the up to date certificated power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been over 22% just in few years.4 The feasibility of solution and low-temperature processability makes it possible to fabricate flexible devices,5−7 which opens more applications compared to silicon or GaAs solar cells. However, before the real commercial application, there are still some fatal problems that need to be highly concerned such as hysteresis, stability, and up-scaling.8−12 In particularly, for flexible PSCs, the low-temperature process is vital because the flexible plastic substrate cannot stand high temperature even over 150 °C.13,14 Although organic semiconductors, such as phenyl-C61-butyric acid methyl ester, poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine), poly(3-hexylthiophene), and spiro-OMeTAD, can be directly applied in flexible PSCs, they are still quite expensive and somehow unstable, especially for large area perovskite solar modules (PSMs).15−18 Li et al. reported a large area (16 cm2, no mask) © 2018 American Chemical Society

Received: February 12, 2018 Accepted: April 10, 2018 Published: April 10, 2018 14922

DOI: 10.1021/acsami.8b02624 ACS Appl. Mater. Interfaces 2018, 10, 14922−14929

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the facile synthesis route for SnO2 NCs, (b) TEM image of the synthetic SnO2 NCs, and (c) XRD pattern of the prepared SnO2 NCs.

Figure 2. (a)Transmittance spectra and (b) conductivity of different-temperature sintered SnO2 films. (c) Schematic of a PSC with the structure FTO/SnO2/perovskite/spiro-OMeTAD/Au. (d) J−V curves of different-temperature sintered SnO2 ETLs based PSCs.

released acid during the hydrolysis in turn induces the slow hydrolysis process to form the uniform SnO2 NCs. When the presynthesized SnO2 NCs were employed for the ETLs, after thermal and UV-ozone (UVO) treatment, a high efficiency over 19% and hysteresis-free PSC on glass substrates can be easily achieved via a novel quadruple cation (KCsFAMA, FA: HC( NH)NH3+, MA: CH3NH3+) perovskite absorber.32 Meanwhile, flexible PSCs employing this low-temperature sintered and UVO-treated SnO2 ETLs can be prepared with a high efficiency of 16.5% also without hysteresis. In addition, a hysteresis-free 5 × 5 cm2 (aperture area: 10 cm2) flexible PSM with a PCE of 12.40% (reverse scan, RS) and 12.22% (forward scan, FS) is also obtained and showed excellent bending durability. The SnO2 NCs prepared via such a simple process can be regarded as promising ETLs and have great potential in future commercial application.

process and is a promising route for future low-cost production. For example, SnO2 ETLs can be spin-coated with precursor solutions such as SnCl4·5H2O and SnCl2·2H2O; however, the prepared films normally need to be sintered at a higher temperature usually at 180 °C that is not suitable for the flexible plastic substrate. Thus, a UV annealing process was employed to facilitate the annealing of such films at a low temperature for the flexible PSCs.23 Also, CBD methods with the addition of acids to control the hydrolysis rate of SnO2 have been employed. Unfortunately, the flexible conducting substrate (indium tin oxide) would be etched by the acids.25,27,28 Therefore, using of presynthesized SnO2 nanocrystals (NCs) to prepare the films under low temperature (150 °C) is a smart route to fabricate efficient ETLs for the flexible PSCs. There are a lot of ways to synthesize SnO2 NCs; Zhu et al. have reported highly crystalline SnO2 NCs via a hydrothermal method at 200 °C.29 Asdim et al. also reported microwave synthesis of SnO2 NCs with an average size over 10 nm.30 Liu et al. developed a dual-fuel combustion method to create a milder exothermic process allowing the growth of uniform SnO2 films at low temperatures.31 Herein, we obtained uniform SnO2 NCs with small size (