Identifying the optimum composition in organic solar cells comprising non-fullerene electron acceptors

We explore the inter-relationship between the phase behavior and photovoltaic performance for two blend systems comprising poly(3-n-hexylthiophene-2,5-diyl) (P3HT) as the electron donating moiety and two newly developed small molecule non-fullerene electron acceptors. Binary non-equilibrium temperature/composition phase diagrams of the two systems are prepared from differential scanning calorimetry (DSC) thermograms of blends of different compositions. The phase behavior is correlated with the optoelectronic performance of corresponding binaries in bulk heterojunction (BHJ) solar cells. The thermal and optoelectronic blend characterization is supported with optical microscopy and specular X-ray diffraction (sXRD) experiments. For both electron-accepting compounds the composition yielding the maximum photocurrent generation in devices was found to be in the hypoeutectic regime, i.e. at compositions that are shifted from the eutectic towards the small molecule rich region in the phase diagrams. We demonstrate that measuring the thermal properties of the blends is useful for rapid component ratio optimization and the evaluation of unexplored materials combinations.