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Organic Electronic Devices
Bias-Dependent Normal and Inverted J–V Hysteresis in Perovskite Solar Cells Fan Wu, Behzad Bahrami, Ke Chen, Sally Mabrouk, Rajesh Pathak, Yanhua Tong, Xiaoyi Li, Tiansheng Zhang, Ronghua Jian, and Qiquan Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07298 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018
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ACS Applied Materials & Interfaces
Bias-Dependent Normal and Inverted J–V Hysteresis in Perovskite Solar Cells Fan Wu*a,b, Behzad Bahramia, Ke Chena, Sally Mabrouka, Rajesh Pathaka, Yanhua Tongc, Xiaoyi Lib, Tiansheng Zhangb, Ronghua Jianb, Qiquan Qiao*a a
Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD 57007, United States;
b
School of Science and Key Lab of Optoelectronic Materials and Devices, Huzhou University, Huzhou, Zhejiang Province 313000, People’s Republic of China;
c
Department of Material Chemistry, Huzhou University, Huzhou, Zhejiang Province 313000, People’s Republic of China
*Corresponding authors: (F.W.) Tel: 0086-572-2321593; Email:
[email protected] (Q.Q.) Tel: 001-605-866-4526; E-mail:
[email protected] Abstract Perovskite solar cells (PSCs) typically exhibit hysteresis in current density−voltage (J−V) measurements. The most common type of J−V hysteresis in PSCs is normal hysteresis, in which the performance in the reverse scan is better than that in the forward scan. However, inverted hysteresis also exists, in which the reverse scan performance is worse than in the forward scan; this hysteresis, however, is significantly less well studied. In this work, we show that the hysteresis decreases when the sweep rate is decreased only in cases involving a small bias range, and it does not decrease with large bias range. Under large forward bias and slowing sweep rate, we observe enhanced normal hysteresis or inverted hysteresis in PSCs. Moreover, the degree of normal and inverted hysteresis can be adjusted by varying the bias. Here, we hypothesize that the tunable hysteresis is derived from the different distribution of ionic defects (VI and VMA) at the electron (hole) transport layer/perovskite interface due to ionic movement in perovskite layer under the different bias scanning conditions. This conclusion is confirmed using Kelvin probe force microscopy with different bias voltages and scanning rates, which shows surface potential hysteresis based on ionic-migration-related Fermi level shifting in perovskite films and agrees with the tunable J−V hysteresis hypothesis. Moreover, the increased time response in the milliseconds region in open-circuit voltage decay after J−V scanning further corroborates the mechanism of ionic migration under bias. Our work provides new insights into the ionic movement hypothesis for the J−V hysteresis in PSCs. Keywords: Perovskite solar cells; Inverted J−V hysteresis; Ionic defects migration; Kelvin probe force microscope; Surface potential 1
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1. Introduction
Perovskite solar cells (PSCs) based on organometal halide perovskite MAPbI3 (MA=CH3NH3) materials have advanced significantly in the recent years.1–3 Despite this progress, however, a considerable anomalous mismatch, known as hysteresis, exists between the forward scanning (FS, from short-circuit to open-circuit) and reverse scanning(RS, from open-circuit to short-circuit) current density (J)−voltage (V) curves of the PSCs.4–6 Nowadays, hysteresis in the J−V response is one of the major issues that complicate the use of PSCs, since the hysteresis phenomenon can lead to inaccurate conversion efficiency, and thus its reliability and stability during practical application become questionable.7 Understanding the hysteresis origins and its effects on photovoltaic performance will be critical to advancing the practical applications of PSCs further; however, to date, the origin of hysteresis has not been clarified.8 At present, many possible theories explaining the origin of hysteresis in PSCs have been proposed, involving ferroelectricity,9 trapping/de-trapping of charge carriers,10 capacitive effects,11 and vacancy-assisted ionic migration.12 Nevertheless, the underlying mechanism is currently still under debate and there is no unified conclusion. In a stereotypical J−V hysteresis in PSCs, the RS performance is better than the FS one—in this work, this behavior is defined as normal hysteresis.4–12 Unexpectedly, abnormal inverted hysteresis can also occur in PSCs, in which the RS result exhibits worse performance than the FS under certain circumstances. For example, Tress et al. reported13 the inverted J−V hysteresis exists in mixed-cation mixed-halide PSCs; meanwhile, the authors also observed inverted J−V hysteresis in MAPbI3 devices with a mesoporous TiO2 scaffold covered with a thin insulating Al2O3 shell. Rong et al. obtained14 hysteresis-normal, hysteresis-free, and hysteresis-inverted PSCs through adjusting the spray deposition cycles for the c-TiO2 layer and UV-ozone treatment. Shen et al. found15 that the accumulation of ions at the MAPbI3/TiO2 interface, caused by biasing devices above built-in potential, leads to inverted hysteresis. Nemnes et al. observed16 inverted J−V hysteresis in CH3NH3PbI3-xClx based PSCs using a negative pre-poling treatment before the J–V measurement. In these works, inverted hysteresis was obtained by either a material treatment during fabrication or by pre-poling the device before the J−V measurement. However, no study has examined whether 2
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hysteresis-normal, hysteresis-free, and hysteresis-inverted PSCs could be obtained with a pristine PSC with no pre-treatment, simply by adjusting the scanning bias range and scanning sweep rate. The majority of studies reported to date have focused on studying the effects of sweep rate on hysteresis with a fixed bias range (normally, the FS bias
