Synergistic Study On Ultrafast Excited State Dynamics And Device Performance In Nonfullerene Based Organic Solar Cells

Organic solar cells（OSCs）have attracted great attention because of their special characteristics including adjustable light absorption,low cost,flexibility,solution process and scalable printing,showing broad application in the future.Recently,OSCs have achieved tremendous success with the power conversion efficiency（PCE）over19%,owing to the invention of nonfullerene acceptor（NFA）materials.However,the research on photophysical processes and device physics in nonfullerene organic solar cell is still very insufficient.The excited state dynamics process after light excitation largely determines the device performance,showing the potential and shortcomings of the materials as well.Compared to conventional fullerene acceptors,NFAs show remarkable advantages,including narrow bandgap,tunable energy levels,intense light absorption,strongπ-πstacking and crystalline,long exciton diffusion length.Therefore,it is very important to fully understand the synergy between the excited state properties of nonfullerene systems and device perfoemance,then reveal the photophysical mechanisms and regulation strategies.In this thesis,several typical NFA based heterojunction systems were studies by combining state-of-the-art ultrafast spectroscopy and device physics analysis.The mainpoints of this thesis are as follow:（1）Reavealing biphasic hole transfer behavior in high-efficiency NFA based OSCs with a minor ultrafast（<100 fs）interfacial process and a major diffusion-mediated hole transfer process until～100 ps,which depends strongly on phase segregation.Because of the interplay between charge transfer and transport,a compromised domain size of20–30 nm for NFAs shows the best performance.This study highlights the critical role of exciton diffusion and phase morphology in high-efficiency OSCs.（2）Reavealing the dominant ultrafast（～80 fs）and lossless F?rster resonance energy transfer（FRET）from photoexcited polymer donors to nonfullerene acceptors（Y6）.The FRET process with efficiency over 50%is followed by reverse hole transfer,bypassing direct electron transfer,becoming a more efficient charge generation channel in high-efficiency NFA based OSCs,especially for Y-series based systems.In striking contrast to fullerene OSCs,the synergistic two-step process facilitates spectral uniform photocurrent generation,also lower the non-radiative recombination energy loss by surpressing the formation of intermedia non-luminous charge transfer（CT）exciton.This study emphasizes the great promise of FRET and the central role of hole transfer,and provides new guidelines to engineer FRET for high-efficiency OSCs.（3）Revealing the formation of NFA triplet exciton from free carrier recombination as a main non-radiative recombination and energy loss channel.In contrast to the previous energetics point of view,we show side-chain fluorination in high-efficiency NFA OSCs has little effect on charge transfer energy landscapes but efficiently inhibits charge recombination to NFA triplet excitons,leading to higher V_OCand PCE.More importantly,our findings provide in NFA OSCs and summarize a new view of the NFA triplet energetics manipulation toward OSCs with lower energy loss and PCE exceeding20%.（4）Star narrow-bandgap nonfullerene acceptor BTP-e C9 was used in preparing NFA/PVSK double-layer heterojunction,providing a champion NFA/PVSK hybrid solar cell with PCE of over 22%.Here,we identify the ultrafast formation（～200 fs）of an intra-moiety excimer（i-EX）state,with the activating energy～0.064 e V.Moreover,we reveal the i-EX state acts as the intermediate for the hole transfer channel in NFA/PVSK double-layer heterojunction.This study enlarge the understanding of NFA（especially for Y6 series）excited state dynamics process and the hole transfer mechanism for high-efficiency NFA/PVSK system.