ON THE CORRECTED PHOTOCURRENT OF ORGANIIC BULK HETEROJUNCTIION SOLAR CELLS

Abstract

The measured photocurrent of a solar cell may be considered the sum of a photogenerated current due solely to the influx of photons, and a photovoltage-induced current due to carrier injection at the electrodes. Correcting the measured photocurrent for the injected current yields the voltage dependence of the photogenerated current alone. This corrected photocurrent can provide valuable insight into the processes governing the behaviour of a solar cell, yet is seldom measured or discussed within the community. In this dissertation, an original experimental technique designed specifically for the reliable measurement of the corrected photocurrent is described, with the intent of applying it to organic bulk-heterojunction solar cells. Solar cells based on a number of donor-acceptor combinations were investigated. Using the experimental technique developed here, corrected photocurrent-voltage characteristics exhibiting remarkably anti-symmetric profiles were obtained and subsequently rationalised with a simple physical model. From the perspective of this model, the nature of charge extraction at the electrodes – and how this is affected by processes such as thermal annealing – was examined. Finally, a new low band-gap, small-molecule acceptor material was used in bulk-heterojunction solar cells, and shown to promising photovoltaic performance. Interestingly, these devices exhibited anomalous current-voltage characteristics, which, on closer examination, could be explained by an electric field dependence in the photogeneration rate. Throughout this work, particular attention was given to how these findings may be used to improve device efficiencies.