Study On The Interfacial Properties Of Perovskite Solar Cells By In-situ Electrochemical Impedance Spectroscopy Analysis

Perovskite solar cells(PSCs)have become the forefront of research in the field of photovoltaics because of their low cost,simple process,and high power conversion efficiency(PCE).After more than a decade of rapid development,the recorded PCE of PSCs has reached 25.5%,which is comparable to silicon-based solar cells.At present,the technology has stood at an important position in industrialization application,and is expected to provide transformative technology support for the development and efficient use of clean solar energy.In thin-film solar cells,the interface characteristics between the various functional layers determine the important dynamic processes such as charge separation,transmission,and recombination,and directly affect the device efficiency.So,there are many scientific problems at the interfaces of the device.The numerous interfaces in PSCs,coupled with the organicinorganic hybrid and ionic conductor of perovskite,bring about complex and variable interfacial properties.At present,people are short of effective methods for the study of PSCs interface electrical characteristics,especially the lack of understanding of the electrical characteristics of several important interfaces,which is an important challenge to understand the physical mechanism of PSCs photoelectric conversion.Electrochemical impedance spectrum(EIS)has become an effective method of analyzing multi-interface electrical processes based on differences in response frequency between different interfacial features.Based on this,this paper studies the important interfacial electrical characteristics and evolution behavior of PSCs under the light,bias,and temperature by EIS.The main research contents are as follows:(1)The evolution and reversibility of the electrical characteristics of the planar PSCs interfaces under the external electric field were studied by in-situ EIS,and the changes of resistance and capacitance in the PSCs were calculated by the analysis of equivalent circuits.The results show that the bias induces anions and cations migrate in perovskite,and cations accumulate at the interface between perovskite and electron transport layer and anions accumulate at the interface between perovskite and hole transmission layer,which hinders the transfer of interfacial charges on both sides of perovskite,forming a reverse electric field hindering further ion migration.After about 4 h of bias,the evolution of electrical properties in PSCs reaches a balanced state,and the reverse electric field hinders the transfer of photongenerated carriers at the interface,resulting in the PCE decreased to 70%.In addition,the EIS curves are fully restored and the PCE is restored to the initial 95%after the device is placed at 7 hours,indicating that the evolution of this interface characteristic is partially reversible.According to the above evolution,we established a charge dynamics model which reasonably explains the evolution and physical mechanism of PSCs interfacial electrical properties under the external electric field.(2)The evolution of the interface characteristics of the planar structure PSCs under the coeffect of temperature and bias is studied by in-situ EIS.The results show that the elevated temperature promotes the ion migration in perovskite,and causes the loss of salt additives in the hole transport layer,and the interfacial transfer resistance of PSCs is triple the initial value,causing the PCE to decrease to 84%.On this basis,we also found that the long-term co-effect of temperature and bias induced the slow decomposition of perovskite from the crystal boundary,which in turn destroyed the crystallization of perovskite and its interfaces on both sides,resulting in the degradation of the device's photoelectric performance.After 2 hours on the co-effect of temperature and bias,PCE remains only 12%of the original value.Based on the cross-section of the PSCs by SEM,a degradation mechanism of PSCs is constructed,which successfully explains the physical mechanism of PSCs photoelectric conversion performance degeneration under the co-effect of temperature and bias.(3)For the abnormal negative capacitance phenomenon affecting the PCE of planar structure PSCs,we systematically studied the influence of device structure and test conditions on EIS results.combining theoretical analysis of semiconductor energy band,we put forward a physical mechanism of the origin of negative capacitance.The results show that under the illumination and open voltage,there is a negative capacitance phenomenon in the medium frequency region of EIS results,and the interface between FTO and perovskite is the cause of negative capacitance.The lighting condition causes the perovskite quasi-Fermi splitting,the perovskite interface can be over-bended under the open voltage,and a large number of holes accumulate at the interface,resulting in the formation of an anti-type layer between the perovskite and the FTO interface,causing the local charge properties to reverse,thus inducing the emergence of the phenomenon of negative capacitance in the medium frequency zone.