Loss of photocurrent efficiency in low mobility semiconductors: Analytic approach to space charge effects

We derive an analytic expression for the photocurrent efficiency as a function of the optical excitation power within the framework of space charge limit of Mott and Gurney [Electronic Processes in Ionic Crystals (Oxford University Press, London, 1940)]. This complements the approach based on charge recombination and we show that the two give similar expressions. Namely, in low mobility and intrinsic semiconductors (as conjugated polymers) based photocell, where recombination follows Langevin's expression, the onsets of space charge and of charge recombination coincide. The analysis shows that the onset of space charge or recombination depends only on the slow carrier mobility value and do not require imbalanced mobility values. (c) 2006 American Institute of Physics.