Spin States-related Photovoltaic Processes In Non-fullerene Organic Solar Cells

In recent years,thanks to the development of non-fullerene acceptor?NFA?materials,the design and synthesis of compatible donors,as well as some relevant research on interfacial engineering,organic bulk heterojunction solar cells?OSCs?have received tremendous attention with rapid development.Nevertheless,such development in the spin-and photo-physics have lagged behind,and there are still lack of researches and understandings on the potentially spin-related photovoltaic processes.Thus,in the present thesis,some highly efficient and stable NFA based organic bulk heterojunction solar cells were designed and fabricated.With assists of the spin-sensitive magneto-photocurrent?MPC?,magneto-electroluminescence?MEL?and electron paramagnetic resonance?EPR?spectroscopic techniques,we have studied the spin-related photovoltaic processes in NFA based organic solar cells.The first task of the master thesis is based on the NFA star material such as ITIC.PBDB-T and PBPD-Th donor materials that have complementary absorption abilities with respect to ITIC are selected in order to form organic BHJwith ITIC.Both binary and ternary organic BHJ solar cells with the common structure of ITO/Zn O/BHJ/Mo O₃/Ag have been designed and fabricated.Spin-sensitive MPC and MEL measurements suggest that ITIC always exhibits large magnetic field effects with negative signs in the ambient temperature.The effect has been attributed to the exciton charge reaction.However,both positive and negative signs have been detected in those binary and ternary organic blends at small and large fields respectively.The results elucidate that the mutual combination of the spin-dependent polaron pair dissociation and exciton charge reaction play decisive role in the photovoltaic process.Further,with photo-induced EPR studies,the full width at half maximum?FWHM?of the line-shape for the positive magneto-photocurrent is firmly associated to the magnitude of the effective g-factor.The present study may shed new light on the deep understanding of spin-dependent photo-physical process in NFA based solar cells for organic opto-spintronic developments.This work has been published as an inside frontcover article in the Journal of Solar RRL,please refer to Fenggui Zhao,et al.Solar RRL 2019,3,1900063 for more details.The second task of the master thesis is based on PBDB-T-SF:IT-4F BHJ solar cells,to incorporate low-dimensional methylammonium lead triiodine?MAPb I₃?perovskite nanowires?Pe NWs?at the interface of the electron transport layer Sn O₂ and the organic blend PBDB-T-SF:IT-4F for the surface decoration.Such interface functions well for the improvement of photovoltaic performance for NF-OSCs of the structure ITO?glass?/Sn O₂/Pe NWs/BHJ/Mo O₃/Ag.Experimental results indicate that the electron-hole dissociation,charge extraction and photo-absorption ability of the NF-OSCs can be improved significantly,and the final energy conversion efficiency of the device?PCE?increased from 9.53%to 10.72%.The inside generation of photocurrent is explored by the MPC method.Finally,the NF-OSCs based on Pe NWs interface modification exhibit more than 80%PCE even after 20 days.This work has been published in Fenggui Zhao,et al.ACS Applied Materials&Interfaces 2020,12,5120-5127.The third task of the master thesis is about the design and fabrication of highly efficiently semitransparent NFA based organic BHJ solar cells.In the work,an ultra-narrow bandgap NFA material such as IEICO-4F and the organic donor material PTB7-Th are selected as the BHJ opto-absorption layer.The solar cell has a structure of ITO/Zn O/BHJ/Mo O₃/Au/Ag.As a comparison,an opaque NF-OSCs are prepared.The optimal PCE of the opaque-NF-OSCs device is 10.75%,while,the open-circuit voltage(V_oc),the short circuit current density(J_sc)and the fill factor?FF?are 0.714 V,23.63m A/cm²,63.8%respectively.More importantly,the semitransparent NF-OSCs have a high average visible light transmittance?AVT?of 26.8%,the PCE of this device can reach8.73%,while,V_oc=0.715 V,J_sc=19.29 m A/cm²,and FF=63.3%.The performance of the semitransparent NF-OSCs device has reached a considerable level among analogous devices reported in literatures.This paper has 36 figures,9 tables,and 178 references.