Enhancing Exciton Diffusion Length Provides New Opportunities for Organic Photovoltaics

Organic semiconductors can potentially revolutionize solar cell technology by offering very thin, lightweight and flexible modules for outdoor and indoor power generation. Light absorption in organic semiconductors generates a bound electron-hole pair (exciton) which needs to travel to the interface between electron donor and acceptor materials to dissociate into charge carriers. Because the exciton diffusion length in organic semiconductors is typically much shorter than the light absorption depth (~100 nm), planar donor-acceptor heterojunctions are inefficient and most effort has been dedicated to optimization of bulk heterojunctions with nanoscale phase separation. In this Perspective we review recent findings and new approaches to increase the exciton diffusion length and discuss how these improvements can benefit environmentally friendly production of solar modules using organic nanoparticles or graded heterojunctions obtained by sequential deposition of electron donor and acceptor.