In this contribution, we examine the use of cascaded energy heterostructure to control loss mechanisms in organic photovoltaic devices (OPVs). It has been proposed that introducing a third component between the donor and acceptor with carefully selected HOMO and LUMO can encourage the spatial separation of electron-hole pairs, and this reduce recombination [1]. Here, we specifically focus on the impact of ternary blend morphology on the effectiveness of the cascade on reducing recombination in a pair of OPV blend systems.

 We examine both PTB7:ICBA:PC₇₁BM and P3HT:ICBA:PC₇₁BM ternary blends, and compare these devices against PTB7:PC₇₁BM and P3HT:PC₇₁BM binary controls with and without 3% of 1,8-diiodooctane (DIO). The choice of these of materials sets give rise to a cascaded heterojunction structure, shown in Fig. 1. In the series of devices fabricated, the fraction of ICBA was varied from 0% (i.e. binary) to 30% to build a picture of how the ternary blend operates. We show that PTB7-based ternaries with 25% concentration of ICBA and DIO have a higher PCE due to an increase in external quantum efficiency (EQE) from 23% in the reference blend to 48% in the ternary blend at 600 nm respectively, notwithstanding the poor absorption of ICBA.  However, we only find this when DIO is added. The morphology of these blends were investigated by TEM and AFM. We find that, like PTB7:PC₇₁BM binary OPVs, that blends with DIO had significantly reduced fullerene aggregation compared to those without (Fig. 2 a and b). In contrast to PTB7, the P3HT-based ternaries showed reduced EQE as ICBA was added. TEM imaging showed that P3HT-based ternaries did not show as significant a degree of fullerene aggregation as the PTB7-based devices without DIO. We attribute this to the difference in the molecular morphology of PTB7- and P3HT-based blends, where the former has been shown to exhibit crystallites of a few molecules [2], while the latter produces intercalated mixed phases [3] (Fig. 2 c, d). Therefore, we hypothesize that ternary blends can improve OPV efficiency, provided that the smallest length scale of phase separation is in the order of a few nm. Thus, we show in this paper two important findings; that this technique can work and ternary OPVs can outperform binary blends, but that in order to do this the morphology of the ternary blend must be optimized by careful control of morphology.

[1]        Groves, C. (2013). Energy & Environmental Science 6(5): 1546-1551.

[2]        Collins, B. (2013). Adv. Energy Mater. (3)1: 65-74.

[3]        Miller, N.C. (2012). Nano Lett. 12(3): 1566-1570.