Understanding Open-Circuit Voltage Loss through the Density of States in Organic Bulk Heterojunction Solar Cells

The field of organic photovoltaics has recently produced highly efficient single-junction cells with power conversion efficiency >10%, yet the open-circuit voltage (V-OC) remains relatively low in many high performing systems. An accurate picture of the density of states (DOS) in working solar cells is crucial to understanding the sources of voltage loss, but remains difficult to obtain experimentally. Here, the tail of the DOS is characterized in a number of small molecule bulk heterojunction solar cells from the charge density dependence of V-OC, and is directly compared to the disorder present within donor and acceptor components as measured by Kelvin probe. Using these DOS distributions, the total energy loss relative to the charge transfer state energy (E-CT)ranging from approximate to 0.5 to 0.7 eVis divided into contributions from energetic disorder and from charge recombination, and the extent to which these factors limit the V-OC is assessed.