Bulk-heterojunction morphology control during spin coating: Modelling diffusion assisted phase separation

We demonstrate the use of controlling the ambient during spin coating to finely control the solvent evaporation rate in order to obtain varying degree of phase separation in P3HT:PCBM blend films and study the effect on device performance. To understand the basic phenomenon driving the phase separation, modelling study has been performed, which show that the phase separation is due to PCBM diffusion towards the nucleation site once its solubility limit is crossed during spin coating process.