Fine-tuning of the chemical structure of photoactive materials for highly efficient organic photovoltaics

The performance of organic photovoltaics is largely dependent on the balance of short-circuit current density (J(sc)) and opencircuit voltage (V-oc). For instance, the reduction of the active materials' optical bandgap, which increases the J(sc), would inevitably lead to a concomitant reduction in V-oc. Here, we demonstrate that careful tuning of the chemical structure of photoactive materials can enhance both J(sc), and V-oc simultaneously. Non-fullerene organic photovoltaics based on a well-matched materials combination exhibit a certified high power conversion efficiency of 12.25% on a device area of 1 cm(2). By combining Fouriertransform photocurrent spectroscopy and electroluminescence, we show the existence of a low but non-negligible charge transfer state as the possible origin of V-oc loss. This study highlights that the reduction of the bandgap to improve the efficiency requires a careful materials design to minimize non-radiative V-oc losses.