Organic-on-inorganic semiconductor heterojunctions: Energy-band discontinuities, quasi-Fermi levels, and carrier velocities

Organic-on-inorganic semiconductor heterojunctions (OI-HJs) exhibit rectification whereby the current-voltage characteristics are limited by the properties of the inorganic semiconductor substrate and the magnitude of the energy barrier at the heterointerface. In this paper we calculate the potential distribution and the quasi-Fermi level energy (or imref) across the OI diode bulk. Both ohmic as well as space-charge-limited conduction regimes of the organic thin film are considered. Previous work considered the OI-HJ to be similar to a Schottky, metal-semiconductor contact. While this can give a good approximation to OI-HJ transport processes under some bias regimes, it results in a misleading picture of the position of the imrefs under reverse bias, as well as errors in measurements of the band discontinuity energy at the OI-HJ. Unlike Schottky contacts, the imref in the OI diode is flat throughout the substrate under both low forward and reverse biases. These results are used to calculate carrier velocities within the organic film. The hole velocity is in the range of 100-2000 cm/s under reverse bias and is as high as 10⁵ cm/s under forward bias. Experimental measurements of the energy-band discontinuities are presented that are in agreement with the predictions of the current-voltage model.