Extremely thin absorber (ETA) solar cells can theoretically provide higher efficiencies than planar cells for low quality absorbers, but only if their nanostructured morphology and interfaces are properly controlled. We report on the dependence of photovoltaic performance on absorber and buffer layer thickness in ETA solar cells containing CdSe-coated ZnO nanowires with CdS buffer. The optimal thickness of electrodeposited CdSe absorber was similar to 50 - 80 nm, as determined using planar cells. Thicker films lead to poor charge separation efficiency due to bulk recombination, while thinner films suffered from poor light absorption and increased interfacial recombination. In ETA cells, the addition of a sub-5 nm CdS buffer layer significantly reduced interfacial recombination and improved fill factor and open circuit voltage. CdS prevents dissolution of the ZnO during CdSe deposition and improves interface quality. With appropriate selection of absorber thickness and interfacial treatment, ETA cells had efficiencies that were 30% larger and short circuit current densities that were two times larger than optimized planar cells. This work presents a rational approach toward designing ETA cell morphology, and it demonstrates a test case where the ETA architecture provides superior performance compared to planar cells made with the same materials and processing conditions. (C) 2014 Elsevier Ltd. All rights reserved.