Effect Of Charge Transfer Exciton Behavior On The Photocurrent And Photovoltage Of Organic Solar Cells

Photovoltaic is a clean,renewable,low-cost energy technic,playing a key role in solving serious global problem about resource and environment.Organic solar cells is part of photovoltaic,which possess several advantages such as multiple source of materials,light weight,solution-processed and flexibility.In the past 20 years,the power conversion efficiency（PCE）of organic solar cells has been significant improved,mainly owing to the material innovation and device optomization,with the state-of-the-art reported so far exceeding 14%.In early stage,chemists devoted themselves to design novel low bandgap donor materials,which can broaden absortion spectrum,aiming to improve the short-circuit current of device.Over the past three years,chemists focus on develop the optical and electric property of non-fullerene acceptor based on the star acceptor ITIC,providing new ideas for fabricating highly efficient non-fullerene solar cells.In the aspect of device optimization,research staff improve the deivce performance mainly through interface modification and controlling morphology such as thermal annealing,solvent vapor annealing and incorporation of additives.Not only employing novel naterials,but also control the morphology of buck-heterojunction can be related to the behavior change of charge transfer excitons.In the second chapter of this paper,we study the relationship of morphology,excitions behavior and device performance in small-molecule-based solar cells upon solvent vapor annealing.The crystallinity and phase separation of DPPZnPor-TBO/PC₆₁BM increase coordinate with solvent vapor annealing time,resulting in higher dissociation rate of charge transfer excitons in the donor/acceptor interface and significantly suppressing the geminate recombination.Due to the enhanced crystallinity of donor and acceptor,the free hole and electron can get longer and more efficient transpor channel,resulting in improvement of short-circuit current and fill factorIn the third chapter of this paper,we demonstrated highly efficient OSCs based on a blend of PC₆₁BM and a newly developed conjugated small molecule（DPPEZnP-THD）with a high PCE up to 9.41%,as obtained through SVA treatment.The optimized devices（SVA devices）show significant improvement in short-circuit density and FF but suffer from remarkable reduction in open-circuit voltage.Measurements of EL due to the emission of the charge transfer state and photovoltaic EQE measurement revealed that the reduction of V_OC in the SVA device stems from a combination of increased radiative recombination loss（～0.04 V）and increased nonradiative recombination loss（～0.07 V）in the active layer,despite the SVA device showing suppressed geminate recombination.Therefore,to further improve the device performance of OSCs,minimizing nonradiative recombination losses and/or suppressing energetic disorder in the active layer are needed.In the fourth chapter of this paper,we achieved power conversion efficiency（PCE）of11.4%(V_OC=0.92 V,J_SC=17.3 mA·cm^-2,FF=71.6%)non-fullerene solar cells based on PBDT-T:IT-M bulk-heterojunction.Meanwhile,the PCE of fullerene solar cells is only 5.66%(V_OC=0.80 V,J_SC=12.3 mA·cm^-2,FF=57.5%)based on PBDB-T:PC₇₁BM bulk-heterojunction.To go insight into the origin of energy loss based on fullerene and non-fullerene solar cells,we carried out the eletroluminescence spectrum to determine the energy of charge transfer state.The energy loss of charge transfer excitons separation(q?E_cs)for PBDB-T:PC₇₁BM and PBDT-T:IT-M solar cells are 0.48 eV and 0.32 eV,respectively.In additions,we calculated the eletroluminescence external quantum efficiency(EQE_EL)of PBDB-T:PC₇₁BM and PBDT-T:IT-M solar cells.The energy loss of non-radiative recombination for PBDB-T:PC₇₁BM and PBDT-T:IT-M solar cells are 0.34 eV and 0.25 eV,respectively.