Effect of Energetic Disorder on the Open-Circuit Voltage in Organic Bulk Heterojunction Composites

TL;DR

This study investigates how energetic disorder affects the open-circuit voltage and transient photovoltage in organic bulk heterojunction composites, revealing linear dependence on light intensity and the influence of trap state dispersion.

Contribution

It introduces a detailed analysis of the impact of energetic disorder on $V_{OC}$ using the multiple trapping model, highlighting the role of the dispersion parameter $oldsymbol{eta}$.

Findings

$V_{OC}$ depends linearly on the logarithm of light intensity.

The photovoltage decay follows a pseudo-first-order process under continuous irradiation.

The power-law dependence of photovoltage on light intensity has an exponent of $1/(1+eta)$, reducing to 1/2 without disorder.

Abstract

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.