Simulating Phase Separation during Spin Coating of a Polymer-Fullerene Blend: A Joint Computational and Experimental Investigation

During spin coating of the photoactive layer of a bulk heterojunction organic solar cell, phase separation between the donor (D) and acceptor (A) components is triggered by solvent evaporation. The morphology of the resulting layer is one of the main determinants of the device efficiency and critically depends on processing conditions such as the spinning speed, D-A mixing ratio, and choice of solvents. It is crucial to understand how these conditions influence the nanostructure of the photoactive layer. Optical experiments have a limited spatial resolution and cannot probe the short length scales of phase separation. In this work, we present three-dimensional simulations of evaporation-induced phase separation in a diketopyrrolopyrrole-fullerene D-A blend, where we derive the simulation parameters from in situ laser interference and contact angle experiments. Depending on the drying rate, phase separation initiates in different regions of the thinning film. From a linear stability analysis, we estimate the early stage length scale of phase separation and compare it with simulations. The normalized drying rate is shown to be the key parameter. The experimentally found power law dependence of the characteristic length scale of phase separation on this parameter is reproduced with a matching exponent.