Real-time in-situ observation of morphological changes in organic bulk-heterojunction solar cells by means of capacitance measurements

We present a method to directly study temperature induced structural and morphological changes in the active layer of organic solar cells by capacitance determination. At sufficiently high frequencies the change in device capacitance with temperature is dominated by the expansion of the organic layer. Variations in the expansion coefficient or the permittivity are visible in the capacitive response. We have studied thin film devices based on poly(3- hexyl)thiophene (P3HT) as well as organic solar cells made from blends of P3HT with a fullerene derivative. Temperature induced transitions are correlated to structural changes in the polymer (e.g., glass transition) as well as morphological changes of the blend; the morphology transition driven by phase separation sets in around 120°C. The advantage of our capacitance method is substantiated as we have demonstrated that confinement due to the substrate and the top electrode alters the effect of temperature on thin films as compared to the bulk material.