The efficiency of an organic photovoltaic (OPV) device is determined by the shape of its current-voltage (J-V) curve. Previous studies showed that the J-V curve for the best-studied OPV system, P3HT:PCBM, is determined mainly by non-geminate recombination where free electrons and holes recombine before being collected, thereby reducing output current. In this thesis we study non-geminate recombination experimentally, as well as charge collection efficiency and rates of geminate recombination, in several polymer:fullerene material systems. The aim is to determine limits to the performance of OPV devices and to quantify the recombination losses.
The first two experimental chapters investigate P3HT:PCBM devices. The first presents an analysis of non-geminate recombination, and the application of temperature dependent measurements and new transient techniques to probe the energetic distribution of trap states within the semiconductor. The second reconciles two apparently contradictory experimental results, namely, the highly non-linear dependence of non-geminate recombination rate upon charge density and the linear dependence of corrected photocurrent on light intensity.
In the third experimental chapter of this thesis both geminate and non-geminate recombination processes are studied and quantified in several material systems. Specifically we study the extent to which the two recombination mechanisms can impact upon the generation, collection and recombination of charges in the devices and we relate this directly to the fill-factor of the devices.
In the final experimental chapter, we study the effect of electronic doping space-charge accumulation upon the electrostatics of a device and hence on non-geminate recombination and charge collection. Through optical and electronic modelling we show that the doping of the photovoltaic active layer causes the accumulation of space-charge, which in turn alters the electric field within the solar cell, reducing the electric field driving collection of the minority carriers and consequently reducing charge collection.