Direct correlation between the built-in potential and the power conversion efficiency of NFA organic solar cells

While it is known that too low built-in potential is detrimental to cells' performance, there is no consensus regarding the importance of maximizing the internal electric field or the built-in potential for achieving the highest power conversion efficiency of non-fullerene acceptor (NFA) organic solar cells. We use one of the prototypical NFA bulk heterojunction solar cells to show a direct correlation between the built-in potential, the open circuit voltage, the fill factor, and the device's efficiency. This is achieved using statistical analysis of devices made of two different cathodes (Ag, Mg) and a simple, intuitive model for the solar cell's current-voltage characteristics. Designing device structures with enhanced built-in potential (internal electric field) is crucial for surpassing the 20% efficiency limit.