Density-of-state engineering of conjugated polymers by implanting partial two-dimensional conjugation: Broad absorption and high performing organic photovoltaics

Abstract

Uniform and broad absorption characteristics are fundamental for improving the light-harvesting capabilities of organic photovoltaics (OPVs). A partial two-dimensional (2D) conjugated structure is formed by introducing additional conjugation in the transverse direction of the main backbone of conjugated polymers (CPs). This alters the bimodal-shaped absorption spectrum of typical donor–acceptor CPs to a trimodal-shaped profile by modulating the density of energy states, enabling uniform absorption characteristics in the visible region. The uniform and broad absorption properties of CPs can be confirmed by the proportional improvement in the short-circuit current density (JSC) and external quantum efficiency (EQE) in OPVs. In particular, when a CP with modified absorption characteristics is blended with a non-fullerene acceptor (Y6) that can absorb light in the near-infrared (NIR) region, the CP:Y6 blend exhibits a uniform light-harvesting capability from the visible to NIR regions through complementary absorption between the CP and Y6. This results in a promising power-conversion efficiency (PCE) of 16.13 % with a JSC of 27.83 mA·cm−2 and fill factor (FF) of 75.22 %. Finally, the morphological effects that determine the charge-recombination properties of the CP:Y6 blends are systematically characterized. The results confirm that the film morphology, in which small domains are uniformly formed, partly contributes to FF enhancement.