Organic photovoltaics (OPVs) are a promising renewable energy technology due to their scalable manufacture and recent substantial gains in power conversion efficiency to 15.6% [1].  Commercialisation of OPVs will require similar step-changes in the lifetime to be made.  OPV layers are susceptible to physical or chemical degradation with different processes occurring at different aging times [2], which in turn are accelerated by water [3] and oxygen [4].  Recent work by Al-Busaidi et al [5] has shown the addition of an inert polymer, poly (methyl methacrylate) (PMMA) to a donor-acceptor blend, yielding Ternary OPVs, increases lifetime due to the PMMA scavenging water in the active later.  Here we show the impact of the Ternary blend morphology on both the lifetime and performance for two donor-acceptor systems: well-studied P3HT:PC₆₁BM and higher-performance PTB7:PC₇₁BM.

Ternary P3HT:PC₆₁BM:PMMA blends were shown to have better initial performance, and lifetimes more than double than P3HT:PC₆₁BM controls, depending upon the molecular weight of the PMMA. This in turn is shown to be due to a significant reduction in the rate at which J_sc degrades.  AFM studies showed that islands comprising PMMA form and increase in size with increasing weight percent (wt%) and molecular weight (M_W) of PMMA as shown in Figure 1, thus we can relate the distribution of PMMA to its lifetime-extending functionality.  

Processing additives such as DIO are a common tool to further control morphology, however, it was found that using DIO in P3HT:PC₆₁BM:PMMA and PTB7:PC₇₁BM:PMMA OPVs blends led to worse initial performance and lifetime than the relevant binary control. This is important since PTB7:PC₇₁BM OPVs are commonly processed with DIO to limit fullerene aggregation. The use of processing additives has been shown to have an impact on the chemical stability of PTB7 [6] and by adding PMMA this degradation appeared to be hastened. However, we show that processing the PTB7:PC₇₁BM:PMMA blends without DIO recovers the beneficial effects upon lifetime shown in the P3HT based devices.  Therefore, we have demonstrated that dilution of a donor-acceptor OPV blend with PMMA can increase lifetime, but that the normal range of morphology optimising tools may be limited.

[1]  Yuan et al., Joule 3, 1–12, 2019 [2]  W. Ma, et al., Adv. Funct. Mater., vol. 15, no. 10, pp. 1617–1622, 2005. [3]  C.H. Peters, et al., Adv. Energy Mater., 1491–494, 2011. [4]  A. Tournebize, et al., Chem. Mater. 25, 4522–4528, 2013. [5]  Al-Busaidi, Z., et al., Sol. Ener. Mater. and Sol. Cells 160: 101-106, 2017. [6]  Kettle, J., et al., Organic Electronics 39, 222-228, 2016.