Direct Observation of Ferroelectricity and Charge Separation Process in CH3NH3PbI3 Perovskite Solar Cell

Abstract

In recent years, organometallic trihalide perovskites has attracted more and more attention for the application of solar cells. In particular, methylammonium (MA) lead triiodide (CH3NH3PbI3 or MAPbI3) has attracted the most attention and undergoes rapid development since it was first applied as light absorber in mesoscopic solar cells. Up to now, it has been demonstrated that the power conversion efficiency of MAPbI3 based solar cells can reach 20%, which is directly related to the favorable direct band gap, high carrier mobilities and long carrier diffusion length. To further improve the performance of MAPbI3 based solar cell, researchers have been focused on studying the otpical, electronic, and optoelectronic properties of MAPbI3 perovskites extensively. However, there is not a systematic study of its ferroelectric properties and the correlation between its ferroelectricity and its optoelectronic properties. Here, we report our direct observation of the ferroelectric domain structure in MAPbI3 thin film using piezoresponse force microscope (PFM). A strong evidence of the intrinsic ferroelectricity is that the ferroelectric domain structure has no correlation to its crystal domain structure, as shown in figure 1. Besides, we also observed the characteristic butterfly loop and hysteresis loop from pointwise experiment, to further verify the ferroelectricity. In addition to using PFM, we also directly observed the charge separation process by measuring the surface potential with Kelvin probe force microscope (KPFM). Under laser illumination when the charge carriers are generated, the surface potential will either decrease or increase depending on the device structure (p-i-n or n-i-p), which can be probed with KPFM and correlated with the surface topography as well. Our preliminary study shows that with AFM’s capability of localized probing, both the ferroelectric property and the optoelectronic properties can be measured and correlated to the structure, which can help us better understand the role of ferroelectricity in the high performance of organometallic trihalide perovskites. Figure 1