Organic solar cells: A new look at traditional models

Traditional inorganic solar cell models, originating with the work of Shockley, are widely used in understanding bulk heterojunction (BHJ) organic solar cell response (organic solar cells are also referred to as organic photovoltaics, or OPVs). While these models can be useful, there are several key points of departure from traditional solar cell behavior. In this Perspective, we discuss three important areas: (1) geminate pair and bimolecular recombination, (2) effects of interfacial layers inserted between the electrodes and active layer, and (3) resistance effects. Since organic solar cell materials typically have large Coulombic exciton binding energies (e.g., similar to 0.3-0.5 eV), limited dissociation of photogenerated charge carriers can be a significant limitation in these cells that is not observed in traditional silicon solar cells. Additionally, the active layer morphology of BHJ organic solar cells allows free charge carriers to recombine before extraction from the cell, creating another photocurrent loss mechanism. Interfacial layers serve a unique role in BHJ organic solar cells; in addition to conventional functions such as photon transmission and charge injection, interfacial layers often act as blocking'' layers, ensuring that charge carriers are collected at their respective electrodes (i.e., electrons at the cathode and holes at the anode). Additionally, resistance effects in organic solar cells differ from traditional models in both field and cell area dependencies. Organic semiconductor mobilities and charge densities exhibit significant sensitivity to field strength, with mobility varying by similar to 10x over typical cell voltage test ranges (1 V). This creates the need for alternative models to describe cell internal resistance. Finally, resistance losses are also sensitive to cell area, due to the limited conductivities of the transparent electrode materials used. Therefore, accommodation of the above deviations from traditional models is imperative for the design and synthesis of new generation high efficiency organic solar cell materials.