Photogeneration and geminate recombination in amorphous silicon

Abstract

Photogeneration in undoped amorphous silicon (a-Si) has been investigated at photon energies €Ph from 3·1 to 1·3 eV, lying above and below the optical gap of the material (1·65 eV). The measurements were carried out on p-i-n junctions, deposited by the glow-discharge technique, in which the saturated (primary) photocurrent Jr under reverse bias is a reliable measure of the quantum efficiency of photogeneration, η. It is shown both by direct measurement of η and from the field dependence of Jr that at photon energies above 1·5 eV η closely approaches unity. Jr(E) is completely independent of the applied field for E between 20 and 150 kV cm⁻¹ which, according to the Onsager theory, gives a thermalization length _r0 ≳400 Å and η(20 kV cm⁻¹) ≳0·95. It is concluded that geminate recombination does not impose any fundamental limitation on photogeneration in a-Si at photon energies of interest in photovoltaic applications. At €Ph<l·5 eV, involving transitions into electron tail states, η decreases rapidly and Jr(E) shows a field dependence in agreement with the Onsager theory; geminate recombination thus plays a predominant role in this spectral range. The temperature dependence of Jr has been investigated and shows a rapid rise in activation energy for €Ph < 1·5 eV, suggesting that photogeneration involves a thermal activation step into the extended states. These results are discussed on the basis of a simple model.