Studying the Effect of High Substrate Temperature on the Microstructure of Vacuum Evaporated TAPC: C60 Organic Solar Thin Films

Organic solar cells (OSCs), also known as organic photovoltaics (OPVs), are an emerging solar cell technology composed of carbon-based, organic molecules, which convert energy from the sun into electricity. Key for their performance is the microstructure of the light-absorbing organic bulk heterojunction. To study this, organic solar films composed of both fullerene C-60 as electron acceptor and different mole percentages of di-[4-(N,N-di-p-tolyl-amino)-phenyl]-cyclohexane (TAPC) as electron donor were evaporated in vacuum in different mixing ratios (5, 50 and 95 mol%) on an ITO-coated glass substrate held at room temperature and at 110 degrees C. The microstructure of the C-60: TAPC heterojunction was studied by grazing incidence wide angle X-ray scattering to understand the effect of substrate heating. By increasing the substrate temperature from ambient to 110 degrees C, it was found that no significant change was observed in the crystal size for the C-60: TAPC concentrations investigated in this study. In addition to the variation done in the substrate temperature, the variation of the mole percent of the donor (TAPC) was studied to conclude the effect of both the substrate temperature and the donor concentration on the microstructure of the OSC films. Bragg peaks were attributed to C-60 in the pure C-60 sample and in the blend with low donor mole percentage (5%), but the C-60 peaks became nondiscernible when the donor mole percentage was increased to 50% and above, showing that TAPC interrupted the formation of C-60 crystals.