The photovoltaic performance of CuGaSe2 thin-film solar cells is limited by their relatively low open circuit voltage and by poor carrier collection that reduces the short circuit current of these devices. In addition, the photocurrent exhibits a strong voltage dependence that additionally degrades the fill factor of the solar cell. This contribution provides an experimental investigation of the spectral quantum efficiency (QE) of CuGaSe2 solar cells with and without bias voltage. Experimentally, we find two different types of current collection in CuGaSe2 solar cells. In the first case, application of reverse bias voltage leads to an increase of the QE in the long wavelength regime. This behaviour results from the increase of carrier collection by increasing the space charge region. Other cells, though having as well a low response in the long wavelength regime of the QE spectra, do not exhibit a strong voltage dependence of carrier collection. In these cells, a part of the absorber material appears not to contribute to the photocurrent at all. (C) 2003 Elsevier B.V. All rights reserved.