The spectral dependence of the primary (reverse) and secondary (forward) photocurrents, Jr and Jf, in Schottky barrier diodes made from glow discharge deposited a-Si : H is studied in the energy range 0.8 - 3.1 eV using a two beam experimental set-up. Below 1.5 eV significant differences are observed between the spectra of Jf and Jr and also between diodes made from n-type and p-type material. They are interpreted as arising from differences between the generation rates of mobile electrons and mobile holes caused by the asymmetry of the density of gap states