Interface modification of CdTe thin film solar cells by CdCl2-activation

In thin film solar cell production several materials are subsequently deposited onto a glass substrate. The interface properties between the different layers are important for the opto-electrical performance of the solar cell device. CdTe thin film solar cells are currently produced using a layer sequence of CdTe/CdS/SnO2/ITO/glass. In order to reach reasonable conversion efficiencies the device has to be activated in a CdCl2 atmosphere. Finally, the back contact is prepared. The influence of the activation step on the solar cell is still not understood in detail. Therefore in, this study model experiments have been carried out in which US and CdTe thin films with a thickness of 100 nm have been deposited by thermal evaporation onto ITO/SnO2-coated glass substrates in an UHV system. The influence of the CdCl2-activation step on the morphology, chemistry and band alignment of the interfaces has been investigated with atomic force microscopy and sputter depth profiling using X-ray photoelectron spectroscopy. A change in surface morphology due to the CdCl2-activation has only been found for the CdTe layer, while the SnO2 and US films are unaffected. It can be shown that the activation step leads to diffusion processes at both interfaces. For the CdS/CdTe interface an interdiffusion of US and CdTe takes place. At the SnO2/CdS interface Cd diffuses into the SnO2 layer with a constant amount of approximately 5% Cd in the whole SnO2 layer. The diffusion and interdiffusion changes the electronic properties of the interfaces. A strong n-type doping for all semiconductor films is observed after the activation. (C) 2003 Elsevier Science B.V. All rights reserved.