PHYSICAL SCIENCE 2017 © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). Anomalous photovoltaic effect in organic-inorganic hybrid perovskite solar cells Yongbo Yuan, 1,2 Tao Li, 3 Qi Wang, 1 Jie Xing, 1 Alexei Gruverman, 3 Jinsong Huang 1 * Organic-inorganic hybrid perovskites (OIHPs) have been demonstrated to be highly successful photovoltaic materials yielding very-high-efficiency solar cells. We report the room temperature observation of an anomalous photovoltaic (APV) effect in lateral structure OIHP devices manifested by the device’s open-circuit voltage (V OC ) that is much larger than the bandgap of OIHPs. The persistent V OC is proportional to the electrode spacing, resembling that of ferroelectric photovoltaic devices. However, the APV effect in OIHP devices is not caused by ferroelectricity. The APV effect can be explained by the formation of tunneling junctions randomly dispersed in the polycrystalline films, which allows the accumulation of photovoltage at a macroscopic level. The formation of internal tunneling junctions as a result of ion migration is visualized with Kelvin probe force microscopy scanning. This observation points out a new avenue for the formation of large and continuously tunable V OC without being limited by the materials’ bandgap. INTRODUCTION Organic-inorganic hybrid perovskite (OIHP) materials have been revo- lutionizing the photovoltaics field in recent years with their use in high- efficiency solar cells (with power conversion efficiency exceeding 22%) and low-cost potential (1–10). Meanwhile, the development of other OIHP-based devices, such as lasers (11, 12), high-gain photodetectors (13, 14), light-emitting diodes (15), and transistors (16), is also in full swing. It is widely wondered whether OIHP materials will continue to generate breakthroughs in the optoelectronics field with their intriguing electronic and optoelectronic properties. Currently, there is a consensus that OIHP materials are a group of soft materials with high electronic and ionic conduction due to their relatively loosely bonded crystal structure (17–21). Investigations focused on making full use of this unusual property have been recently performed (22). In addition, a more insightful understanding of the photovoltaic process in OIHP materials is crucial to further improving the photocurrent and photo- voltage output of OIHP solar cells (23, 24). Here, we report the observation of an anomalous photovoltaic (APV) effect in lateral structure OIHP solar cells. Investigations focused on the APV effect in some inorganic materials, especially ferroelectrics, have attracted an upsurge in interest over the past decade because of its completely different working mechanisms and output characteristics compared to those of traditional p-n junction solar cells (25–29). To date, several semiquantitative or phenomenological models have been established to explain the APV effect, which can be classified into two major types: (i) the intrinsic noncentrosymmetry in bulk materials and (ii) the granularity of the polycrystalline materials (26, 30, 31). The former type of APV mechanism principally demands a very low con- ductivity in photovoltaic materials (25), whereas the latter mechanism does not suffer from a similar limitation. However, to date, most models for the granularity mechanism (for example, the Dember effect model, the structure transition model, and the p-n junction array model) are speculative because the corresponding formation mechanisms have not been proven. Hence, determining the origins of the open-circuit voltage (V OC ) that is larger than the materials’ bandgap with experimental ev- idence is of great academic interest (25). RESULTS The OIHP solar cells used in this study had a symmetric lateral structure of Au/MAPbBr 3 (or MAPbI 3 and CsPbBr 3 )/Au, where the photoactive layer and metallic electrodes were deposited directly on a glass substrate (Fig. 1A). The as-made OIHP solar cells did not show any photovoltaic effect because of the symmetric lateral structure (17, 19). After the lateral MAPbBr 3 device was electrically poled by a moderate electrical field of 0.3 to 0.5 V/mm at room temperature (RT) for 1 to 2 min, a large V OC of about 1.1 to 1.3 V was obtained (Fig. 1A) from the MAPbBr 3 devices, with an electrode spacing of 50 mm, which is close to the best V OC (1.4 V) reported for vertical structure MAPbBr 3 solar cells (32, 33). The gener- ation of this large V OC in a device with symmetrical electrodes can be ascribed to the in situ formation of a p-i-n or p-n structure in the MAPbBr 3 film induced by ion migration and space-charge doping, as illustrated in Fig. 1A (middle) (19, 33). The short-circuit current (I SC ) is ~300 pA for devices with an active area of 50 mm × 1 mm under an illumination intensity of 25 mW/cm 2 . The device’s V OC showed a linear relationship with the poling bias, as shown in Fig. 1 (B and C). Aston- ishingly, when the lateral MAPbBr 3 device was poled by a larger elec- trical field of 5 V/mm for 1 to 2 min, the V OC further increased markedly to 7.4 eV, which is much larger than the bandgap (2.3 eV) of MAPbBr 3 . Meanwhile, the photocurrent increased slightly to 490 pA (Fig. 1B). In lateral devices with a MAPbI 3 photoactive layer, no V OC larger than the bandgap was observed at RT, which is consistent with our previous study (17, 19). Nevertheless, a similar over-bandgap V OC of 2.6 V was observed when the poling temperature was elevated to be around 330 K (fig. S1). The anomalously large V OC in the OIHP materials was found to increase with the spacing between the two electrodes (Fig. 1, C and D) when the poling electrical field was fixed at 5 V/ mm. A large V OC of 14.9 V was found in MAPbBr 3 solar cells with an electrode spacing of 100 mm after an electrical poling at 5 V/mm for 1 to 2 min. There is a threshold at an electrode spacing of 8 mm, below which V OC exceeding the bandgap cannot be found, even at a high poling field of 5 V/mm. This is the first time that OIHP solar cells with a V OC larger than the bandgap have been observed, which needs a mechanism for interpreta- tion. An APV effect dependent on electrode spacing has been frequently observed in some materials that lack a center of symmetry, generally 1 Department of Mechanical and Materials Engineering, University of Nebraska–Lincoln, Lincoln, NE 68588–0656, USA. 2 Hunan Key Laboratory of Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, P. R. China. 3 Department of Physics and Astronomy, University of Nebraska–Lincoln, Lincoln, NE 68588–0299, USA. *Corresponding author. Email: jhuang2@unl.edu SCIENCE ADVANCES | RESEARCH ARTICLE Yuan et al., Sci. Adv. 2017; 3 : e1602164 17 March 2017 1 of 7