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Abstract: Optical spacer layers based on titaniumalkoxide precursor solutions were prepared by spin-coating on top of bulk heterojunction layers based on poly-3-hexylthiophene (P3HT) and phenyl-C61-butyric acid methylester (PCBM). Models and experiment have shown that the performance of polymer solar cells can improve upon application of an optical spacer by shifting the maximum of the electrical field vector of the incident light into the active layer. This avoids the so called “dead zone” close to the reflective electrode. We demonstrate a simple linear model that can be used to predict the intensity variations of the electrical field vector of the incident light through a multilayer structure. Central to our study is the thickness of the optical layer and we find that it is critical to control the optical spacer thickness on the actual active layer employed. X-ray reflectometry allows for the simultaneous determination of the active layer thickness and of the optical spacer layer. [Copyright &y& Elsevier]