Open-circuit voltage in organic solar cells

Abstract

Open-circuit voltage (V_OC) is the maximum voltage a solar cell can provide to an external circuit, which is derived from the splitting of hole and electron quasi-Fermi levels. In crystalline Si solar cells, the effective density of states at the bottom (top) of the conduction (valence) band is constant, and the quasi-Fermi level can be directly calculated via the Fermi–Dirac distribution. However, in organic materials, similar to amorphous Si, disorder induces gap tail states. Relaxation of carriers into these tail states brings the electron quasi-Fermi level down and the hole quasi-Fermi level up, and hence reduces V_OC. Furthermore, carrier recombination of various kinds can cause additional loss of V_OC. This article reviews the research progress in understanding the origin of V_OC in organic solar cells. In particular, the dependence of V_OC on four important factors, namely temperature, light intensity, work function of the electrode and material microstructure are discussed based on the model of density of states. Techniques to enhance V_OC are also briefly introduced and their mechanisms are analysed.