  The photoluminescence decay in amorphous semiconductors is described by power
law $t^{-delta}$ at long times. The power-law decay of photoluminescence at
long times is commonly observed but recent experiments have revealed that the
exponent, $delta sim 1.2-1.3$, is smaller than the value 1.5 predicted from a
geminate recombination model assuming normal diffusion. Transient currents
observed in the time-of-flight experiments are highly dispersive characterized
by the disorder parameter $alpha$ smaller than 1. Geminate recombination rate
should be influenced by the dispersive transport of charge carriers. In this
paper we derive the simple relation, $delta = 1+ alpha/2 $. Not only the
exponent but also the amplitude of the decay calculated in this study is
consistent with measured photoluminescence in a-Si:H.
