Temperature‐dependent current‐voltage and lightsoaking measurements on Cu(In,Ga)Se2 solar cells with ALD‐Zn1‐xMgxO buffer layers

Abstract

AbstractIn this paper, lightsoaking and temperature‐dependent current‐voltage (JVT) measurements on Cu(In,Ga)Se2 solar cells with atomic layer deposited Zn1‐xMgxO buffer layers are presented. A range of Mg concentrations are used, from pure ZnO (x=0) to 26% Mg (x=0·26). Since this kind of solar cells exhibit strong metastable behaviour, lightsoaking is needed prior to the JVT‐measurements to enable fitting of these to the one‐diode model. The most prominent effect of lightsoaking cells with Mg‐rich buffer layers is an increased fill factor, while the effect on cells with pure ZnO buffer is mainly to increase Voc·. The activation energy is extracted from JVT‐measurement data by applying three different methods and the ideality factors are fitted to two different models of temperature‐dependence. A buffer layer consisting either of ZnO or Zn1‐xMgxO with a minor Mg content gives solar cells dominated by interface recombination, which probably can be related to a negative conduction band offset. A relatively high Mg content in the buffer layer (x=0·21) leads to solar cells dominated by recombination in the space charge region. The recombination is interpreted as being tunnelling‐enhanced. The situation in between these Mg concentrations is less clear. Before lightsoaking, the sample with x=0·12 has the highest efficiency of 15·3%, while after lightsoaking the sample with x=0·21 holds the best efficiency, 16·1%, exceeding the value for the CdS reference. The Jsc values of the Zn1‐xMgxO cells surpass that of the reference due to the larger bandgap of Zn1‐xMgxO compared to CdS. Copyright © 2009 John Wiley & Sons, Ltd.