Temperature Dependence of Ideality Factors in Organic Solar Cells and the Relation to Radiative Efficiency

The value and temperature dependence of the ideality factor provides essential information about the dominant recombination route in solar cells. This study presents experimental results of accurate ideality factor determination for representative organic photovoltaic cells (OPV) evaluated at different temperatures over a large current density regime. It is noted that standard dark I-V curves strongly deviate from those obtained by evaluations based on short circuit current density (J(SC))-open circuit voltage (V-OC) pairs. This is attributed to the applied external voltage in a dark I-V measurement not being representative of internal chemical potential, particularly at lower temperatures. Complementary electroluminescence measurements attest that the current density dependence of the ability of the solar cell to emit light is better correlated to the series resistance free ideality factor. For the studied set of OPV devices it is observed that the ideality factors are quite low, and with very weak temperature dependence. The J(SC)-V-OC method to determine ideality factors further allows good estimates of activation energies as well as recombination current prefactors J(00). The findings imply that the principal OPV non-radiative recombination mechanism is not recombination of free carriers with trapped carriers in an exponential density of tail states as previously reported.