  We study bulk recombination in a medium with energetic disorder by using the
multiple trapping (MT) model under continuous irradiation with excitation
light. Charge densities are shown to obey power law as a function of the
intensity of excitation light and the exponent is related to the dispersion
parameter $\alpha$ of the MT model. The theory is applied to analyze charge
densities in bulk heterojunction photovoltaic cells measured by light-induced
electron spin resonance (LESR). The values of the exponent are consistent with
those obtained from the transient decay of charge densities after pulsed
excitations.We show that the value of the exponent is robust against addition
of Gaussian trapping energy distribution due to shallow trap states. We also
show that open-circuit voltage ($V_{\rm OC}$) depends linearly on logarithm of
the light intensity and the slope is equal to $k_B T/e$ when only shallow trap
states are present. The same linear relation holds with a slope larger than
$k_B T/e$ in the presence of an exponential trapping energy distribution due to
deep trap states.
