We have explored the effect of a range of different cathode materials on the power conversion efficiency of organic (polymer) solar cells based on a blend of the conjugated polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) with the fullerene acceptor PC70BM. We use a transfer matrix reflectivity model to quantify the optical properties of the cathode and the device structure on its operational efficiency and compare this with the results of experimental measurements. We show that both optical and electrical effects play a role in determining overall device efficiency through their impact on short-circuit current, open circuit voltage and fill-factor. We use our model to demonstrate that devices composed of a thin (60–70 nm) active semiconductor layer and a composite cathode composed of a 5 nm thick layer of calcium capped by aluminium combine low optical loss and improved charge extraction and optimised power conversion efficiency.