Developing design criteria for organic solar cells using well-absorbing non-fullerene acceptors

In recent years, efficiencies of bulk heterojunction solar cells have risen substantially mostly due to the development of well-absorbing small molecules that replace fullerenes as the acceptor molecule. The improved light absorption due to the combination of two strongly absorbing molecules raises the question, how to best combine the absorption onsets of the donor and acceptor molecule to maximize efficiency. By using numerical simulations, we explain under which circumstances complementary absorption or overlapping absorption bands of the two molecules will be more beneficial for efficiency. Only when mobility and lifetime of charge carriers are sufficiently high to allow sufficient charge collection for layer thicknesses around the second interference maximum, a combination of complementary absorbing molecules is more efficient. For smaller thicknesses, a blend of molecules with the same absorption onset achieves higher efficiencies.