Routes Forward for Evaporated Small Molecule Organic Solar Cells*

Organic solar cells (OSCs) fabricated via thermal evaporation in vacuum are highly relevant for industrial manufacturing, but are less investigated than polymer-based OSCs. Additionally, there is a lack of reported viable non-fullerene acceptor (NFA), which constituted a step change for solution processed OSCs. We perform extensive optoelectronic characterization on co-evaporated blends to quantify voltage losses and charge transport – probed via sensitive EQE and CELIV - and link them to blend phase separation - probed via SIMS interdiffusion experiments as well as X-ray and neutron scattering methods. Example systems include ZnPc:C60 and DCV5T-Me:C60 as they show either good charge collection or reduced voltage losses.

Our experiments demonstrate that charge collection suffers from low mobility and give evidence whether deposition kinetics or thermodynamic equilibrium factors are the root cause. We find that voltage losses in evaporated donor:C60 reach 100mV lower than in polymer:PCBM but 200mV higher than in polymer:NFA systems. We obtain low voltage losses by combining evaporated donors with solution-processed NFAs demonstrating the potential of NFAs for evaporated OSCs. Our work gives an overview of today’s evaporated OSCs and highlights paths for further improvements.