Optimization Of Photoactive Layers For High Performance Organic Photovoltaic Cells

Organic photovoltaic cell?OPVC?is one of the research hotspots in the field of photovoltaics for its advantages of light weight,low cost,wide source of materials,and solution processing.According to its different functions,OPVC can be classified into organic solar cell?OSC?and organic photodetector?OPD?.OSC is designed to convert solar energy into electrical energy,and the purpose of OPD is to convert optical signals into electrical signals.In recent years,by studying photovoltaic mechanism and improving fabrication process,OSC has achieved the power conversion efficiency?PCE?over 14%,and OPD has obtained the photo detectivity?D^*?of 1×10¹⁵ Jones.However,low device efficiency,low device longevity,and high-cost fabrication processing still ask for the further device optimization for the industrialization of OPVC.In this case,this thesis studied the photoactive layer optimization processes of two-step annealing,two-step fabrication,and ternary doping for improving the performance of OPVC,paving the way for the industrial fabrication of OPVC.1.Influence of two-step annealing on the performance of ternary OSC was studied.Based on poly?3-hexylthiophene-2,5-diyl??P3HT?:[6,6]-phenyl-C71 butyric acid methyl ester(PC₇₁BM):squaraine?SQ?ternary heterojunction photoactive layer,a two-step annealing technique,including solvent annealing and further thermal annealing,was introduced to optimize the morphology of photoactive layer.Studies have shown that two-step annealing was beneficial to the improvement of photoactive layer absorption,enhancement of P3HT crystallinity,optimazition of blend morphology,and the self-assembly of components in the photoactive layer,leading to the PCE enhancement from1.32%to 4.42%.This work provides a new idea for the morphological optimization of ternary photoactive layer.2.Influence of high mobility small molecule doping on the charge carrier behaviors of OSC was studied.The charge carrier behaviors in the photoactive layers were investigated by doping a high mobility small molecule of TIPS-pentacene into the poly[[4,8-bis[?2-ethylhexyl?oxy]benzo[1,2-b:4,5-b']dithio-phene-2,6-diyl][3-fluoro-2-[?2 ethylhexyl?carbonyl]thieno[3,4-b]-thiophenediyl]]?PTB7?:PC₇₁BM system.The results showed that TIPS-pentacene doping improved the hole mobility and hole lifetime,and decreased the charge recombination,leading to the enhanced PCE of 8.09%.This work indicates that high mobility small molecule doping is beneficial to the improvement of carrier transport in OSC.3.Influence of TIPS-pentacene doping on the performance of P3HT:PC₇₁BM OSC was studied.The effect of TIPS-pentacene doping into the P3HT:PC₇₁BM blends on its photovoltaic performance was studied,and the charge carrier mobility and film morphology of the TIPS-pentacene doped photoactive layer were also analyzed.The results showed that the doping of 0.6 wt%TIPS-pentacene can balance hole/electron transport,enhance P3HT crystallinity,and optimize blend morphology,resulting in the overall enhancement of open circuit voltage,short current density,and fill factor.This work broadens the application range of TIPS-pentacene doping in OSC.4.Influences of F?rster resonance energy transfer and post solvent treatment on the performance of OPD were studied.Based on the poly?diketopyrrolopyrrole-terthiophene??PDPP3T?:PTB7:PC₇₁BM ternary system,the F?rster resonance energy transfer from PTB7 to PDPP3T was studied,and a ternary OPD with wide response between ultraviolet?UV?and near-infrared regions was fabricated.The results showed that by introducing PTB7 into the PDPP3T:PC₇₁BM binary system,an enhanced D^*was obtained.In addition,the post solvent treatment inhibited the agglomeration of PC₇₁BM,reduced the film surface roughness,and decreased the dark current.This work provides a new idea for the preparation of high performance OPD with wide detection spectra.5.Influence of two-step processing on the performance of OPD was studied.The P3HT/PC₇₁BM photoactive layer was prepared by using a two-step processing method to reduce the dark current of OPD.By adjusting the ratio of mixed solvent of PC₇₁BM layer,the donor-acceptor interfacial morphology was optimized,and the corresponding photodetective performance was studied.The results showed that the penetration of PC₇₁BM into the predeposited P3HT layer formed an optimal three-phase morphology,which was beneficial to the reduced dark current and increased photocurrent,leading to a high D^*of 1.23×10¹² Jones.This work paves a way for the realization of high performance OPD.6.Influence of charge-transfer-featured material on the performance of organic bifunctional device with UV photodetective and electroluminescent?EL?properties was studied.A charge-transfer-featured material 6-[3,5-bis-[9-?4-t-butylphenyl?-9H-carbazol-3-yl]-phenoxy]-2-?4-t-butylphenyl?-benzo[de]isoquinoline-1,3-dione?CzPhONI?was used as the photoactive layer to fabricate the organic bifunctional device with UV photodetective and EL properties.The results showed that in the UV photodetective mode,the D^*was 1.5×10¹¹ Jones at-3 V.In EL mode,the device achieved a luminance of 1437cd/m² and a current efficiency of 0.3 cd/A.This work provides a design idea for the realization of new organic optoelectronic device.In summary,this thesis focused on the design and optimization of the photoactive layer in OPVC,and the influence of different processes,including solvent annealing,thermal annealing,ternary blending,and two-step fabrication,have been studied for the purpose of performance enhancement of OPVC,which laid a solid foundation for the preparation of high performance OPVC.