Performance Optimization Of PTB7:PC₇₁BM-based Organic Photovoltaic Devices By Solvent Vapor-assisted Annealing Process

Organic photovoltaic devices（OPVs）have received extensive attention due to their comprehensive advantages of environmental protection,low energy consumption,lightweight,low cost,flexibility,solution processing,and large-area printing preparation.At present,the power conversion efficiency（PCE）of OPVs has exceeded 18%,but there is still a big gap with the high efficiency required for commercial production.In order to further improve the performance,improvements need to be made from three aspects:material design,device structure,and manufacturing process.Among them,the morphology of the active layer is a key factor,the efficiency of exciton dissociation and carrier transport can be effectively improved by adjusting the crystallinity,π-πstacking degree and surface roughness of the active layer.This paper uses a low-cost,easy-to-operate solvent vapor annealing process（SVA）to optimize OPVs based on the PTB7:PC₇₁BM system,and successfully increases the PCE from7.97%to 9.64%,and the physical mechanism is analyzed in detail.Subsequently,the solvent vapor-assisted annealing process（TA+SVA）was continuously applied to the system,and positive performance optimization results（8.86%）were also obtained,and made a further in-depth study on the relationship between annealing treatments in different sequences and device performance parameters.The main research work is as follows:1.Optimize the standard device based on PTB7:PC₇₁BM,including the control of the active layer thickness,the deposition thickness and rate of the electron transport layer.The study found that when the thickness of the active layer is 85 nm,the deposition thickness is0.7 nm,and deposition rate is 0.03（?）/s of the electron transport layer,the best performance（7.96%）is obtained.2.Optimize standard devices by SVA,including the selection of annealing solvent,the adjustment of annealing time and temperature,and the orientation of the active layer.The study found that the efficiency of the device treated with chlorobenzene SVA was the best（9.64%）,followed device treated with methanol SVA（9.23%）,and finally device treated with chloroform SVA（8.95%）.Compared with the standard device,the performance was increased by 21%,16%and 12%,respectively.3.Through a series of characterization tests on the devices treated with SVA of different polarities,the surface morphology,absorption performance,crystallinity and carrier mobility of the active layer were comparatively studied,and the results showed that the active layer with chlorobenzene SVA has a relatively higher light absorption capacity,thus improving the J_SC;the suitable size of the donor and acceptor phase areas intersect each other to form a network structure,which is more conducive to the extraction and transport of charges,thus improving the FF,and the more balanced electron and hole mobility makes the J_SC and FF synchronously improved,and finally achieved an excellent performance of 9.64%.4.The solvent vapor-assisted annealing process（TA+SVA,SVA+TA）with different annealing sequence is applied to the PTB7:PC₇₁BM system.The performance test results show that the device with SVA+TA treated has better efficiency（8.86%）.Further characterization tests were performed on the devices with different annealing treatments.The study showed that the annealing sequence has an effect on the morphology,crystallinity and light absorption properties of the active layer,and ultimately affects the PCE.