Structure-performance Relationship For High Efficiency Non-fullerene Organic Solar Cells

Renewable energy resources will play a vital role in energy supplies in the future and become an emergent issue for human to solve.As a photovoltaic technique that can directly convert the solar energy into electricity,solar cell attracts more and more concerns around the world.Although inorganic solar cells occupy the photovoltaic market mainly at present due to their high power conversion efficiency（PCE）and long lifetime,compared with them,organic photovoltaics（OPVs）also show some advantages,such as light weight,good flexibility,solution-processed,easy manufacture and semitransparency.Thus,OPV has been a research hotspot both in the academic and industrial fields.With developments in the noval donor and acceptor materials and the device engineering,so far,the PCEs of single-junction organic solar cells have been improved to around 12%¹5%,which makes this technique more and more promising for practical applications.However,for applications,OPVs are required with not only high PCE,but also its longevity under continueous illumination and environment-friendly processing.In this thesis,based on non-fullerene acceptors（NFA）,we focus on developing efficient device engineering methods to improve the PCE of OPVs,revealing the key factors that influence the photo-stability of non-fullerene OPVs and finally exploring an effective way to improve their photo-stability.All works in this thesis can be divided into the following three parts:In chapter 3,we found that the PCE of conventional cells based on the PTB7-Th donor and IDT-BT-R acceptor are better than those of the corresponding inverted cells.Considering the better stability of inverted solar cells,we try to develop effective device engineering methods to improve the performance of the inverted cells based on the PTB7-Th:IDT-BT-R system.Thermal annealing of the active layer was found to optimize the phase separation morphology of the PTB7-Th:IDT-BT-R blend,improve the electron mobility and thus suppress the free charge recombination.In addition,interface modification of ZnO by the conjugated polymer PFN-ox can passivate the trap states of ZnO surface and enhance the charge extraction efficiency.Finally,combining these two strategies,we successfully improved the PCE of PTB7-Th:IDT-BT-R based solar cell from 7.39%to 9.39%,which was among the high level at that time.Our work indicates that device engineering is also important for improving the efficiency of OPVs even at the era of noval NFAs.In chapter 4,using the PTB7-Th as donor and ITIC,IDIC and EH-IDT as acceptors,which are really popular in the field of non-fullerene acceptors,we studied the relationship between NFA materials structure and devices’photo-stability.After long-time photo-aging,PTB7-Th:ITIC and PTB7-Th:IDIC based solar cells were found to lose their performance dramatically,mainly from the decreasing J_SC and FF.Particularly,there was a significant burn-in loss in the PTB7-Th:ITIC based solar cell.Measurements and analysis reveal that,the large Flory-Huggins interaction parameter between PTB7-Th and ITIC or IDIC means their poor miscibility,which led to severe phase separation of the active layers and thus increasing trap-assisted recombination and performance loss under continueous illumination.Based on another system of PTB7-Th and EH-IDT that was found to have good miscibility,we fabricated a non-fullerene organic solar cell with a PCE of 9.17%and lifetime over 2100hours,from non-halogen solvent of o-xylene and diphenyl ether additive.Here,we propose that the Flory-Huggins interaction parameter between donor and NFA materials should be a reliable factor to evaluate the photo-stability of non-fullerene OPVs.In chapter 5,based on the conclusion from chapter 4,we explored an efficient method to enhance the photo-stability of high-performance non-fullerene OPVs by ternary blend strategy.The PTB7-Th:IEICO-4F system,which has been reported to give high PCE in literature,was found to has big Flory-Huggins interaction parameter and poor miscibility but another NFA（EH-IDT）has good miscibility with the PTB7-Th donor.Then,we added 20 wt%of EH-IDT into the PTB7-Th:IEICO-4F system to fabricate a ternary solar cell,which was found to improve the device lifetime by three times without sacrificing the PCE.As a result,the ternary solar cell showed a PCE approaching 10%and T₈₀ beyond 3500 hours,which was among the best overall performance of non-fullerene OPVs.Finally,we tried this method on another high-efficient system of PBDB-T-2F:IT-4F and also doubled the device photo-stability.All above results indicates that adding a third component with better miscibility to fabricate ternary solar cells is an effective strategy to improve the photo-stability of non-fullerene OPVs.