Monitoring the Charge-Carrier-Occupied Density of States in Disordered Organic Semiconductors under Nonequilibrium Conditions Using Thermally Stimulated Luminescence Spectroscopy

Injected or photogenerated charge carriers in disordered organic semiconductors relax in energy to form an occupied density of states (ODOS) that is inherently difficult to probe. Thus little is known about nonequilibrium ODOS properties, although they are important in e.g. solar cells and LEDs. This work presents an optical technique for monitoring the ODOS distribution of relaxed charge carriers at low temperatures. The distribution that forms under nonequilibrium conditions is always narrower than that of the full DOS, in a universal ratio of about 2/3 that is reproduced by kinetic Monte Carlo simulations assuming spatially correlated disorder.