To enhance efficiencies of quantum dot CdSe/TiO(2) based solar cells, understanding of the space charge at the CdSe/TiO(2) interface is crucial. In this paper, the presence of a shallow acceptor in the CdSe quantum dots is found by means of a detailed impedance and Mott-Schottky (C(-2)-phi) study. Furthermore, it is clearly shown that this acceptor density decreases strongly with increasing quantum dot size. The presence of these defect states may give rise to Auger recombination in small quantum dots and therewith decrease the efficiency of quantum-dot-sensitized solar cells.

Type transformation in CuInSe2 and CuInS2 solar cells is an important issue with far reaching consequences. In the present study, the presence of a p-n homojunction inside CuInS2 in a TiO2∕CuInS2 device is revealed with a detailed impedance spectroscopy and capacitance study. A n-type CuInS2 film with a thickness of 40nm is found at the TiO2 (n-type)/CuInS2 (p-type) interface. The effective donor density of this n-type film is 2×1017cm−3 at 400K and is higher than the effective acceptor density in the remaining p-type CuInS2, being 4×1016cm−3 at 400K. Both densities decrease upon increasing the temperature. This is explained by the activation of a CuIn″ acceptor state in n-type CuInS2 and a thermally activated hole trap in p-type CuInS2.

To elucidate minority carrier transport in CdSe quantum dot films, a detailed DLTS study on TiO 2 /CdSe quantum dot heterojunctions is performed. Long transient times are found, which are related to tunneling instead of the thermal emission of electrons. Surprisingly, the transient times increase with increasing temperature, which is possibly related to thermal expansion of the tunnel barrier width between quantum dots. This effect can give rise to unexpected behavior of quantum dot devices.

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