Determination of the Density of States in Semiconductors from Transient Photoconductivity

This paper discusses the techniques we have developed to analyse the photo-response of amorphous semiconductors to transient optical excitation to determine the energy distribution of gap states (DOS). We highlight the difficulties arising from a direct ‘single point’ approach using the instantaneous transient photocurrent i(t) when there is significant structure in the DOS or in the presence of recombination. Frequency domain methods which involve measurement of the amplitude I(ω ) and phase φ(ω ) of the steady ac photocurrent in response to ac excitation are shown to overcome these problems, but include other, experimental shortcomings. The paper describes a solution which combines the best features of time and frequency domain methods. The method involves numerical Fourier transformation of the time sampled impulse response i(tk) into a complex ac response (ωn), followed by analysis for gap-state distribution. Essentially, the method succeeds since it folds in information from the whole of the measured response for each energy considered. Other advantageous features include applicability to semiconductors exhibiting dispersive or non-dispersive transport, and to pre- and post- recombination regimes of the transient photocurrent, without modification. The features of this analytical method are demonstrated with computer simulated and experimental transient photocurrent data for a-Si:H.