Processing additive suppresses phase separation in the active layer of organic photovoltaics based on naphthalene diimide

The development of non-fullerene electron acceptors for organic photovoltaics is gaining interest, as they offer the promise to overcome the light harvesting and energy tunability limitations of fullerenes. However, to fully take advantage of alternative acceptors, we must identify and achieve the needed morphologies within the active layer to maximize device performance. Here we demonstrate that the microstructure in the active layer of optimized poly(3-hexylthiophene)/naphthalene diimide devices resembles that of poly(3-hexylthiophene)/fullerene mixtures. Previously, we have reported on the synthesis of 2,6-dialkylaminonaphthalene diimides and found that the best performance was obtained with N,N′-di((thiophen-2-yl)methyl)-2,6-di(N-cyclohexylamino)-1,4,5,8-naphthalenetetracarboxydiimide (RF1). In this article, we show that suppressing the crystallization of both the donor and acceptor through the addition of 0.2% 1,8-diiodooctane (DIO) to the casting solution leads to finer morphologies in the active layer and a two-fold enhancement in the device efficiencies. Nevertheless, further increasing the DIO content of the casting solution leads to lower photocurrents and power conversion efficiencies, even though the morphology appears similar by energy-filtered TEM. We hypothesize that higher DIO content breaks up small RF1 aggregates, leading to suppression of charge separation. Continued development of novel non-fullerene acceptors must therefore take into consideration the balance between crystallization and aggregation of donors and acceptors for optimal performance.