Energetic disorder induced leakage current in organic bulk heterojunction solar cells: comprehending the ultra-high open circuit voltage loss at low temperatures

TL;DR

This study investigates the cause of ultra-high open circuit voltage loss at low temperatures in organic bulk heterojunction solar cells, highlighting the role of energetic disorder and charge mobility in leakage currents.

Contribution

It introduces a device model incorporating energetic disorder and mobility effects to explain temperature-dependent $V_{oc}$ behavior in organic solar cells, supported by experimental comparison.

Findings

Low temperature reduces charge mobility, causing leakage currents in nonselective contacts.

Selective contacts maintain increasing $V_{oc}$ as temperature decreases.

Charge generation rate remains relatively unaffected by temperature variations.

Abstract

In organic bulk heterojunction solar cells, the open circuit voltage ($V_\mathrm{oc}$) suffers from an ultra-high loss at low temperatures. In this work we investigate the origin of the loss through calculating the $V_\mathrm{oc}-T$ plots with the device model method systematically and comparing it with experimentally observed ones. When the energetic disorder is incorporated into the model by considering the disorder-suppressed and temperature-dependent charge carrier mobilities, it is found that for nonselective contacts the $V_\mathrm{oc}$ reduces drastically under the low temperature regime, while for selective contacts the $V_\mathrm{oc}$ keeps increasing with the decreasing temperature. The main reason is revealed that as the temperature decreases, the reduced mobilities give rise to low charge extraction efficiency and small bimolecular recombination rate for the photogenerated charge carriers, so that in the former case they can be extracted from the wrong electrode to form a leakage current which counteracts the photocurrent and increases quickly with voltage, leading to the anomalous reduction of $V_\mathrm{oc}$. In addition, it is revealed that the charge generation rate is slow-varying with temperature and does not induce significant $V_\mathrm{oc}$ loss. This work also provides a comprehensive picture for the $V_\mathrm{oc}$ behavior under varying device working conditions.