Roland Steim

Interface materials for organic solar cells

The progress in the development and understanding of interfacial materials for organic photovoltaics (OPV) is reviewed. The proper choice of interface materials is a must for highly efficient and stable OPV devices and has become a significant part of the OPV research today. Interface materials are either non-conducting, semiconducting or conducting layers which not only provide selective contacts for carriers of one sort, but can also determine the polarity of OPV devices, affect the open-circuit voltage, and act as optical spacers or protective layers. In this review both inorganic and organic interface materials are discussed with respect to their function in the OPV device.

Interface modification for highly efficient organic photovoltaics

We present highly efficient inverted polymer:fullerene bulk-heterojunction solar cells by incorporation of a nanoscale organic interfacial layer between the indium tin oxide (ITO) and the metal oxide electron-conducting layer. We demonstrate that stacking of solution-processed organic and metal oxide interfacial layers gives highly charged selective low ohmic cathodes. The incorporation of a polyoxyethylene tridecyl ether interfacial layer between ITO and solution-processed titanium oxide (TiOx) raised the power conversion efficiency of inverted organic photovoltaics to 3.6%, an improvement of around 15% in their performance over comparable devices without the organic interfacial layer.

Formation and impact of hot spots on the performance of organic photovoltaic cells

The failure mechanisms of organic solar cells under reverse bias conditions were investigated. Localized inhomogenities, so-called “hot spots,” leading to increased leakage currents under reverse bias, were identified as the dominant origin for failure. The intensity of hot spots does increase with the duration under reverse bias voltage. Cells with a higher leakage current density (i.e., >100 μA/cm−2 at −1 V) have a significant higher probability for dominant failure, while devices with low leakage current densities show less degradation under reverse bias stressing.