Please use this identifier to cite or link to this item: https://doi.org/10.1109/TED.2009.2036787
Title: Origin of different dependences of open-circuit voltage on the electrodes in layered and bulk heterojunction organic photovoltaic cells
Authors: Zhang, C. 
Tong, S.-W. 
Jiang, C.-Y.
Kang, E.-T. 
Chan, D.S.H. 
Zhu, C. 
Keywords: Ambipolar carrier distribution
Electrode dependence
Light injection
Open-circuit voltage
Organic solar cells
Issue Date: Feb-2010
Citation: Zhang, C., Tong, S.-W., Jiang, C.-Y., Kang, E.-T., Chan, D.S.H., Zhu, C. (2010-02). Origin of different dependences of open-circuit voltage on the electrodes in layered and bulk heterojunction organic photovoltaic cells. IEEE Transactions on Electron Devices 57 (2) : 397-405. ScholarBank@NUS Repository. https://doi.org/10.1109/TED.2009.2036787
Abstract: Experimental results show that the VOC of layered heterojunction (HJ) organic photovoltaic (PV) cells behaves with a very weak dependence on the electrodes. However, the VOC of bulk HJ PV cells behaves with a strong dependence on the electrodes. In this paper, an explanation for the different behaviors of VOC on the electrodes is proposed. It is found that the VOC of the two types of PV cells follows the same mechanism and is mainly determined by the light-injected carriers at the donor/acceptor (D/A) interface and the electrodes. However, the distinct device structures make the boundary conditions in layered and bulk HJ PV cells different, which leads to the different dependences of VOC on the electrodes. The layered HJ PV cells have geometrically flat D/A and metal/organic (M/O) interfaces (the interface near the electrode), which makes the effective thickness from the D/A interface to the M/O interface large. Thus, there is a low electric field at the M/O interface and, then, a very small barrier lowering. Under this condition, the light-injected carriers at the D/A interface tend to pin the Fermi level of the electrodes. As a result, VOC shows only a very weak dependence on the work function of the electrodes. However, the formation of the interpenetrating network in bulk HJ PV cells greatly decreases the D and A domain dimensions and induces the ambipolar carrier distribution in the blend layer. This will cause very large barrier lowering at the M/O interface when there is a high barrier. Under this condition, the light-injected carriers at the D/A interface can no longer pin the electrode Fermi level. Thus, a strong dependence of VOC on the electrodes for bulk HJ PV cells is observed. © 2009 IEEE.
Source Title: IEEE Transactions on Electron Devices
URI: http://scholarbank.nus.edu.sg/handle/10635/82852
ISSN: 00189383
DOI: 10.1109/TED.2009.2036787
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