Morphology Optimization For High Performance Organic Nonfullerene Solar Cells

In recent years,the efficiency of organic solar cells(OSCs)has developed rapidly and is approaching 20%.The challenges of the current development mainly focus on the film morphology regulation and long-term device stability.In this paper,we concentrate on the morphology control of OSCs and investigate the morphological characteristics of high performance organic non-fullerene solar cells(ONSCs)from two perspectives:molecular chemical structure modification-chlorine substitution and film treatmentsvolatile solid additives,respectively.The main investigations are as follows:First,the effects of chlorine substitution on material properties,device performance,and film morphology were systematically investigated in benchmark all-PSC blending of donor PBDB-T and acceptor N2200 to fully understand the correlation between molecular structure,film morphology,and photovoltaic performance.Due to the stronger electronwithdrawal ability of Cl atoms,the chlorine(Cl)-substituted polymers show blue-shifted absorption and down-shifted energy levels.Besides,the chlorinated polymers exhibit non-covalent bonding,resulting in the changes of polymer aggregation behavior during the film-forming process,which further tunes the phase separation between the donor and acceptor polymers.When applied to all-PSCs,the PBDB-T:N2200-based devices outputted a maximum efficiency of 8.44%with an appropriate blend morphology and high filling factor.And the devices based on chlorinated all-PSCs yield higher opencircuit voltage approaching 1.0 V,however,decreased short-circuit current density.There are close correlations between the changes in device performance,material properties due to chlorine substitution and film morphology regulation.We believe these results would provide more insights into the understanding of the correlation between molecular structure,morphology,and device performance in all-PSCs.Second,ONSCs have made unprecedented progress in the past five years,with efficiency approaching 20%.However,morphology optimization of ONSCs has been proven to be particularly challenging relative to the classical fullerene-based devices.In this section,we report a novel volatile solid additive,2-hydroxy-4methoxybenzophenone(2-HM),for achieving high efficiency ONSCs.2-HM functions as a universal morphology-directing agent for several well-known PM6:Y6-series nonfullerenes blends,viz.PM6:Y6,PM6:BTP-eC9,and PM6:L8-BO,leading to the best efficiency of 18.85%at the forefront of reported binary ONSCs.Volatile solid additives have been reported recently,while the intrinsic kinetic processes are still unclear.Therefore,we employed a set of in-situ and ex-situ characterizations to first illustrate the molecule-aggregate-domain transition dynamic process assisted by the volatile solid additive.More specifically,we unlock the role of 2-HM in individual donor PM6 and acceptor Y6 systems,and further investigated the function of 2-HM in the modulating the morphology of PM6:Y6 bulk heterojunction blends for enhanced photovoltaic performance.We believe the achievement brings not only a deep insight into the molecular ordering of the donor and acceptor in BHJ,the film microscopic morphology,and device performance,but also provide a theoretical basis for the design of new highperformance volatile solid additives.In summary,we have explored the morphological characteristics of highperformance ONSCs from two perspectives,molecular chemical structural modification-chlorine substitution and film treatments-volatile solid additives,respectively,and investigated in depth the morphological properties of the blend films in terms of molecular structure design,molecular aggregation,and crystallization,which provide new insights for understanding the connection between the film microstructure and device performance of ONSCs.