High Efficiency Organic Photovoltaic Design: Charge Carrier Recombination and Resistance Limitations

High efficiency organic photovoltaic (OPV) design strategies are reported here based on models addressing charge carrier recombination and resistance constraints. Traditional solar cell models, originating with the work by Shockley, are widely used in understanding bulk heterojunction (BHJ) organic solar cell response. While these models are useful for evaluating OPVs, there are several key points of departure from traditional solar cell behavior. This dissertation addresses areas focused on photocurrent recombination and resistance loss characteristics. Resistance effects in organic solar cells differ from traditional models due to both field and cell area dependencies. Organic semiconductor mobilities and charge densities exhibit significant sensitivity to field strength, leading to unique resistance behavior. Resistance losses are also sensitive to cell area, due to the limited conductivities of electrode materials used. With these behaviors in mind it is shown here that OPVs are approaching the limit of resistance-based efficiency enhancement. Furthermore, recombination losses in organic solar cells are paramount. Since OPV materials typically have significant charge carrier binding energies (e.g., ∼0.3-0.5 eV), recombination of geminate photogenerated charge carriers can be a significant issue in these cells not observed in traditional silicon solar cells. Additionally, the morphology of BHJ organic solar cells allows for dissociated charge carriers to recombine before being extracted from the cell, creating another photocurrent loss mechanism. It is shown here that geminate electron-hole pair recombination has a major impact on limiting current state-of-the-art efficiencies to ∼7-8%. Achieving high efficiency OPVs (>10%) will require reducing exciton binding energies and increasing mobilities near the donor-acceptor interface. Traditional OPV design routes of reducing the optical bandgap (Eg ≈ 1.5 eV) to achieve high efficiencies are shown to be ineffective. OPVs also employ interfacial layers that serve a unique role in BHJ organic solar cells; in addition to usual functions like photon transmission and charge injection, interfacial layers often need to act as minority carrier "blocking" layers, ensuring that only majority carriers are collected at their respective electrode. Therefore, consideration of the above deviations from traditional models is imperative for the successful design and synthesis of new generation materials for high efficiency organic solar cells.