Interface Engineering For Efficient And Stable Planar Perovskite Solar Cells

Organic and perovskite solar cells（OSCs and PVSCs）,as a new generation of solar cells,have the advantages of solution processing and low cost,and great progress has been made in terms of power conversion efficiencies（PCEs）and device stabilities in recent years.In these devices,the interfacial properties between active layers and cathode/anode electrodes can determine the overall performances of the solar cells,which refer to charge extraction,charge transport,charge recombination and trap states.How to realize the efficient and stable unidirectional transport of photogenerated carriers across the interface between semiconductor and electrode is one of important scientific issues in novel photovoltaic systems.Concentrated on the topic of charge transport at the interface,we have developed fullerene-based electron transporting materials as well as the application of self-assembled monolayer（SAM）and polymer modification,to optimize interfacial level alignment,defect passivation and suppress the interfacial interaction,and it provides feasible ideas and practical guidance for the realization of efficient and stable perovskite-based photovoltaic devices.This thesis consists of seven chapters.Chapter I is an introduction of the principles,developments,interface engineering and research proposals for organic and perovskite solar cells;Chapters II to VI respectively the detailed researches on interfacial materials and interface modification to improve the performances of solar cells;Chapter VII is the summary and prospect of the thesis.In Chapter II,self-doped fullerene interfacial molecules（FPPI,Bis-FPPI,Bis-FIMG and Bis-FITG）were designed and synthesized to obtain high-efficiency organic and perovskite solar cells under mild solution processing conditions.The self-doped and highly conductive fullerenes can be obtained via anion-induced n-doping of intramolecular electron transfer between Lewis basic halogen anions andπ-acid fullerenes.Polar fullerene materials as cathode interfacial materials,with the advantages of high conductivity,work function tunability and orthogonal solvent processability,not only enable good device performances of OSCs and PVSCs,but also show device performance insensitive to the interlayer thickness.In Chapter Ⅲ,two Bingel fullerene interface molecules PCP and MCM with similar chemicalstructures were designed and synthesized for achieving thick-film perovskite solar cells with efficiencies beyond 19%with a planar absorber layer over 1 micrometer.Fourier-transform infrared（FTIR）spectroscopies and coordination titration experiments were used to explore molecular interactions between fullerene materials and perovskites（anion-πand Lewis acid-base interaction）on charge extraction and recombination at the heterogeneous interface,as well as the influences of hysteresis phenomena and photovoltaic efficiencies.In particular,the weak O-Pb²⁺Lewis base-acid interaction between MCM and perovskite can passivate interface defects,resulting in effective electron extraction and low J-V hysteresis.However,the strong N-Pb²⁺coordination will cause interfacial charge accumulation due to energy misalignment at the perovskite/PCP interface,leading to an obvious hysteresis.In the first two works,we designed and developed novel electron transporting materials,followed by the optimization of HTLs.In Chapters IV and V,the p-doping of potassium halides（KF,KCl,KBr and KI）were used to prepare conductive nickel oxide films,and the surface properties were studied.It was found that the doped HTLs perform effective hole transport and low surface defect,thereby achieving MAPb I₃ based perovskite solar cells with PCEs of over20%.And the NiO_x film was further modified with triphenylamine SAM（TPAB and TPAA）to passivate its surface defects and reduce carrier recombination,and the adsorption of SAM molecules on the NiO_x surface was analyzed by FTIR and X-ray photoelectron spectroscopy（XPS）.Meanwhile,SAM modification can realize the energy alignment,carrier extraction and transport,and achieve a high PCE of 21.35%in MAPb I₃-based perovskite solar cells.In Chapter Ⅵ,high-mobility conjugated polymers,PB2T-E and PB2T-E/A,were inserted into the perovskite/NiO_x interface to effectively cure the instable heterointerface,and obtain the champion PVSC module（18.57 cm²）with a PCE of 16.25%.The polymer can block the photochemical degradation reaction of NiO_x and perovskite.P-E/A-N based modules have good stabilities under light illumination（2000 h at 70℃）and UV irradiation（162.4 k W.h,over 10times of reference doses）from the IEC61215 standard.