Re-entrant phase behaviour of organic semiconductors.

The number of polymeric and small-molecular acceptors for organic photovoltaics has exploded in the past decade. As a result, physical insights and efforts aiming at elucidating the coupling between composition and behaviour are required more than ever. Here we present an encompassing study into the phase behaviour of 55 polymer:small-molecular acceptor blends, pivotal in determining device performance and stability. Many of these exhibit non-trivial behaviour, which cannot be understood by conventional mixing theory. Interestingly, the phase diagrams are subject to variations in glass transition temperature, strongly suggesting an important role of configurational entropy. We present an extended model for the mixing free energy, accounting for a temperature dependence of free volume and configurational freedom. The phase behaviour can be roughly categorized in terms of the ratio of the monomeric volumes of the individual components. The model qualitatively reproduces all experimental observations and poses a viable starting point for assisting development.