Fullerene solubility-current density relationship in polymer solar cells

During the last decade polymer solar cells have undergone a steady increase in overall device efficiency. To date, essential efficiency improvements of polymer-fullerene solar cells require the development of new materials. Whilst most research efforts aim at an improved or spectrally extended absorption of the donor polymer, not so much attention has been paid to the fullerene properties themselves. We have investigated a number of structurally related fullerenes, in order to study the relationship between chemical structure and resulting polymer-fullerene bulk heterojunction photovoltaic properties. Our study reveals a clear connection between the fullerene solubility as material property on one hand and the solar cells short circuit photocurrent on the other hand. The tendency of the less soluble fullerene derivates to aggregate was accounted for smaller current densities in the respective solar cells. Once a minimum solubility of approx. 25mg/ml in chlorobenzene was overcome by the fullerene derivative the short circuit current density reached a plateau, of about 8-10 mA/cm(2). Thus the solubility of the fullerene derivative directly influences the blend morphology and displays an important parameter for efficient polymer-fullerene bulk heterojunction solar cell operation.