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.