Open-circuit voltage in organic solar cells

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.