H- and J-aggregation inspiring efficient solar conversion

Abstract

Studies of morphology of organic solar cells (OSC) in bulk heterojunction (BHJ) structures are usually focussed on nanoscale morphology. However, morphology at molecular level, such as aggregation type, may also have a profound influence on the performance of OSCs. It is found that H- and J-aggregation coexist in BTIC-CF₃-m and can be easily controlled by different additives. This provides a chance to study and gain a deeper insight into the role of the two aggregation types by directly comparing their effect on various photovoltaic performance parameters. Two common additives, 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO), support the formation of H- and J-aggregation, respectively and have different effects on the photovoltaic performance of OSCs. H-aggregation favors a higher open circuit voltage (V_oc), while J-aggregation favors a higher short circuit current (J_sc). Both of these aggregation types can improve the fill factor (FF). On the whole, both the improvement of H- and J-aggregation can enhance the power conversion efficiency (PCE) and the H-aggregation of BTIC-CF₃-m is more efficient in the system of PBDB-TF:BTIC-CF₃-m. Then, H- and J-aggregation are further tuned by solvent vapor annealing (SVA), and consequently the PCE is enhanced to 16.36% from its pristine value of 13.37%. It demonstrates that the morphology optimization, especially precise control of the H- and J-aggregations, is the key factor to further improving premium organic solar conversion systems.