Vacuum-Deposited Biternary Organic Photovoltaics

Ternary blend organic photovoltaics (OPVs) have been introduced to improve solar spectral absorption and reduce energy losses beyond that of binary blend OPVs, but the difficulties in simultaneously optimizing the morphology of three molecular components result in devices that have generally exhibited performance inferior to that of analogous binary OPVs. Here, we introduce a small molecule-based biternary OPV comprising two individual, vacuum-deposited binary bulk heterojunctions fused at a planar junction without component intermixing. In contrast to previous reports where the open circuit voltage (V_OC) of a conventional, blended ternary cell lies between those of the individual binaries, the V_OC of the biternary OPV corresponds to one of the constituent binaries, depending on the order in which they are stacked relative to the anode. Additionally, dipole-induced energy-level realignment between the two binary segments necessary to achieve maximum efficiency is observed only when using donor-acceptor-acceptor' dipolar donors in the photoactive heterojunctions. The optimized biternary OPV shows improved performance as compared to its two constituent binary OPVs, achieving a power conversion efficiency of 10.6 ± 0.3% under AM 1.5G 1 sun (100 mW/cm²) simulated illumination with V_OC = 0.94 ± 0.01 V, a short circuit current density of 16.0 ± 0.5 mA cm^-2, and a fill factor of 0.70 ± 0.01.