Open tube chemical vapour deposition (CVD) is an alternative method for growing chalcopyrit thin films. It has been developed at the HMI in co-operation with the company Aixtron (Aachen). At the beginning of this study maximum efficiencies, obtained on solar cells based on CuGaSe₂ absorber films grown by a single stage open tube CVD-process, have been limited to 4,5 %. The device performance of thin film solar cells based on wide gap chalcopyrites such as CuGaSe₂ with a gap energy of E_G = 1.68 eV is mainly limited by a high density of deep defects in the bulk of the polycrystalline films. Therefore, a detailed knowledge of the defect structure of these materials is necessary for their improvement as active layers in solar cells

The radiative transitions of CuGaSe₂ single crystals and polycrystalline films with various compositions were studied by means of steady state photoluminescence spectroscopy. A model, describing all observed luminescence peaks is presented. It is based on three acceptor levels with activation energies of E(A1) = 60 meV, E(A2) = 100 meV and E(A3) = 243 meV, and two donor levels with activation energies of E(D1) = 12 meV and E(D2) = 237 meV, respectively. According to the analysis of the chemical reactions taking place during crystal/film growth, the third acceptor-like defect band A3 is attributed to either V_Ga or Cu_Ga, whereas the second donor D2 is attributed to Cu_i.

Based on this results a two stage process was developed for growing CVD-CuGaSe2 absorber layers, resulting in comparably good efficiencies of CuGaSe₂/CdS/ZnO solar cells above 7 % under standard conditions. A detailed sub-gap absorption analysis, applying - for the first time - both, photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) to chalcopyrit thin films, shows the suitability of this process for the reduction of the sub-gap absorption. In addition, a two layer defect model, observed in these CuGaSe₂ thin films, is proposed consisting of a bulk region, whose sub-gap absorption is strongly reduced in two stage grown samples and a highly defective near-surface region, whose sub-gap absorption does not change significantly during the second process stage.

The final part is intended to complete the picture of CVD-grown single and two stage CuGaSe₂ thin films by a detailed surface analysis applying XPS (X-ray photoelectron spectroscopy), UPS (ultraviolet photoelectron spectroscopy) and IPES (inverse photoelectron spectroscopy). A comparison of bulk composition, obtained by XRF (X-ray fluorescence), and surface composition, obtained by XPS line intensity analysis, reveals a nearly stoichiometric bulk while the surface is highly copper depleted. This effect is enhanced in the "two stage" samples. Combined UPS and IPES analysis of the valence band maxima and conduction band minima shows an increased surface band gap up to 2.2 eV, while optical transmission experiments reveal an optical band gap around 1.65 eV. The data can be explained by the proposed two layer model for CVD CuGaSe₂ consisting of a nearly stoichiometric bulk layer and a copper depleted surface layer.