Photoluminescent delocalized excitons in donor polymers facilitate efficient charge generation for high-performance organic photovoltaics

Efficient delocalization of photo-generated excitons is a key to improving the charge-separation efficiencies in state-of-the-art organic photovoltaic (OPV) absorber. While the delocalization in non-fullerene acceptors has been widely studied, we expand the scope by studying the properties of the conjugated polymer donor D18 on both the material and device levels. Combining optical spectroscopy, X-ray diffraction, and simulation, we show that D18 exhibits stronger π-π interactions and interchain packing compared to classic donor polymers, as well as higher external photoluminescence quantum efficiency (~26%). Using picosecond transient absorption spectroscopy and streak camera photoluminescence measurements, we show that the initial D18 excitons form delocalized intermediates, which decay radiatively with high efficiency in neat films. In single-component OPV cells based on D18, these intermediate excitations can be harvested with an internal quantum efficiency >30%, while in blends with acceptor Y6 they provide a pathway to free charge generation that partially bypasses performance-limiting charge-transfer states at the D18:Y6 interface. Our study demonstrates that donor polymers can be further optimized using similar design strategies that have been successful for non-fullerene acceptors, opening the door to even higher OPV efficiencies.