Interfacial Engineering for Organometal Halide Perovskite Solar Cells

• Main Authors: Yen-Chen Shih, 施彥辰
• Other Authors: King-Fu Lin
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/35xfby

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 106 === In this dissertation, we demonstrate the effects of interfacial engineering on the performance of perovskite solar cells (PSCs). Modification of TiO2 electron transporting layer with various amino acids strikingly affects the charge transfer efficiency at the TiO2/MAPbI3 interface, among which L-alanine modified cell exhibits the best power conversion efficiency with 30% enhancement. This study also shows that (110) plane of MAPbI3 organometal halide perovskite (OHP) crystallites tends to align in the direction perpendicular to the amino acid-modified TiO2 as observed from the grazing-incidence wide-angle X-ray scattering of thin MAPbI3 perovskite film. The results of electrochemical impedance spectroscopy (EIS) reveal less charge transfer resistance at the TiO2/MAPbI3 interface after modified with amino acids, which is also supported by the lower intensity of steady state photoluminescence (PL) and the reduced PL lifetime of perovskite. In addition, based on the PL measurement with excitation from different side of the sample, amino acid-modified samples show less surface trapping effect compared to the sample without modification, which may also facilitate the charge transfer efficiency at the interface. The results suggest that the preferential orientation of perovskite crystallites in the interface and the trap-passivation are the niche for better perovskite photovoltaic performance. Next, we report the behavior of charged species at compact TiO2/MAPbI3 interface with respect to electrode polarization in PSC devices, and comprehensively discuss the results obtained from open-circuit voltage (VOC) buildup and VOC decay measurements under illumination and in the dark, respectively, for the PSCs with PCBM inserting at TiO2/MAPbI3 interface with varied thickness. By combining with EIS, we propose a justified mechanism in an attempt to elucidate the dynamics of interfacial species with respect to the time and frequency domains. Our results demonstrate that the retarded VOC buildup and decay observed in PSC devices is related to the electrode polarization taking place as a consequence of bound charge formation in TiO2. Such bound charge is attracted by oppositely charged ion/vacancy accumulating at the OHP side. Besides, inserting a layer of PCBM at TiO2/MAPbI3 interface as a passivation layer can efficiently reduce current-voltage hysteresis of devices and alleviate the electrode polarization, which can be verified by the less dielectric constant response observed in low frequency regime from EIS analysis. This work opens up another window to investigate the interfacial issue of PSCs associated with passivation effect of fullerene, and help to further understand the impact of ion migration on photovoltaic performance. We further report the accumulated charged species in OHP and piled-up bound charges at TiO2 side are the origin of the increased capacitive response in low frequency regime for PSC device and thereby cause the current-voltage hysteresis during performance evaluation. Such hysteretic response detrimentally affects the reliability of performance evaluation but can be well suppressed by PCBM passivation and structural design. In addition, Ag nanoparticles can be embedded into compact TiO2 to facilitate the charge extraction and efficiently suppress the capacitance-induced current-voltage hysteresis during performance evaluation. Moreover, externally applied voltage and light power are found to influence the ion/vacancy generation. Hence these two factors should be taken into account during the performance evaluation for PSCs. Furthermore, an anomalous negative capacitance is observed from PSCs. By well interfacial engineering for PSC device such as PCBM passivation, structural design, and embedding Ag nanoparticle for TiO2 modification, the negative capacitance is remarkably suppressed. This implies that the negative capacitance should correlate with the back redistribution of accumulated ions/vacancies in OHP layer and the release of bound charges at TiO2.