Surface states and photovoltaic effects in CdSe quantum dot films

Photovoltaic effects in CdSe quantum dot (QD) films have been studied using surface photovoltage spectroscopy and complementary methods. The results show that, contrary to previous studies, nonnegligible electric fields can exist in QD films. As a result, driftlike currents must be considered, in addition to the well-known diffusionlike currents. However, it is found that the specific case of photovoltage sign reversal, observed after etching highly quantized CdSe QD films, is governed by diffusionlike transport. The latter is highly influenced by preferential trapping of one type of charge carrier. The preferential trapping is shown to be surface localized and is strongly ambient dependent. It is shown that the photovoltaic properties of these CdSe QD films are dominated by their surface state distribution.