V_OC variation with different molecular weight fractions in highly efficient organic photovoltaic bulk heterojunctions

Tuning molecular weight fractions is one of the most important strategies for optimizing the morphology of bulk heterojunctions (BHJs) to obtain high-performance organic solar cells. Here, we investigate the effects of molecular weight fractions on the performance of state-of-the-art PM6:Y6 BHJ solar cells. A remarkable blueshift in the photoluminescence spectra can be observed with the introduction of the PM6 component. The electroluminescent and morphological analyses indicate that this observation can be attributed to increased charge-transfer state energy and an intermolecular interaction between donor and acceptor, leading to open circuit voltage (V-OC) variations in the photovoltaic device. The transient absorption spectra confirm that the optimized donor:acceptor (D:A) ratio can suppress the generation of polarons and achieve high efficiency exciton dissociation. The results of atomic force microscopy and neutron reflectometry reveal that higher acceptor contents promote the formation of a desirable interpenetrating network and enable high fill factors and short-circuit currents to be obtained.