Pairing Non‐Fullerene Acceptors With the Right Polymer: Impact of Phase Morphology and Short‐Range Transport on Charge Generation

Organic photovoltaics based on non‐fullerene acceptors (NFAs) have demonstrated high and rapidly increasing power conversion efficiency in recent years. Yet, the requirements for pairing NFAs with an optimal donor polymer remain unclear. To explore the desirable properties of polymers when combining them with NFAs, we herein study a representative NFA molecule combined with three different polymers. To mimic different phase morphologies, we compare bilayers to optimized blends with a three‐phase morphology (neat donor and acceptor regions coexisting with intermixed domains) and to ‘dilute‐donor’ and ‘dilute‐acceptor’ systems (completely intermixed scenario). The solar cell performance is related to the free charge generation efficiency probed by ultrafast transient and electro‐modulated absorption spectroscopy. We find that the free charge generation in the optimal polymer:NFA pair is highly resilient to the phase morphology. This blend benefits from a favorable kinetic competition between fast transport‐assisted charge separation (related to locally ordered polymer chains) and slow intrinsic recombination (related to a high‐lying charge transfer state, according to the energy gap law). These findings guide the molecular design of polymeric donors for efficient charge generation in NFA‐based photovoltaic blends.