  Under open-circuit condition, the current is not extracted and the
photogenerated carriers in principle disappear only by recombination. We study
the open-circuit voltage $V_{\rm OC}$ and transient photovoltage under the
effect of bulk recombination in a medium with energetic disorder by using the
multiple trapping (MT) model. The key parameter in the MT model is the
dispersion parameter $\alpha$ given by the ratio of thermal energy to the
characteristic energy of trap states. We show that $V_{\rm OC}$ depends
linearly on the logarithm of the light intensity and the slope depends on the
$\alpha$ of the MT model. Under the continuous irradiation of light, the
photovoltage response to the weak perturbation by a pulsed light obeys
pseudo-first-order decay. The rate as a function of $V_{\rm OC}$ is independent
of the dispersion parameter. However, it obeys the power law as a function of
light intensity, and the exponent is given by $1/(1+\alpha)$, which reduces to
1/2 in the absence of energetic disorder.

Materials Science