Classical CIS thin films solar cells contains a thin film of CdS. To avoid its toxicity and make cadmium-free buffer layers solar cells, we tried to make some surface modification of CIS. The electrochemical approach was defined for wet treatments and ALD was choosen from dry interface engineering. Electrochemical modifications were already investigated succesfully, and also ALD work led us to achieved the world record with vapor phase cadmium free processed CIS cells, with 16.4 % in 2003^[1].

In a second time, we would to complete the CIS cell with ZnOi/ ZnO:Al as window layer. The window layer has to respect two major parameters, the sheet resistivity and the optical transmittance. ALD ZnO films and doped ZnO films were optimized for CIS absorber devices. This process is well known method to prepare high quality multilayer films of compound materials like II-VI semiconductors, and provided a good way for the fabrication of photovoltaic devices. This is a soft method with a good control of the thickness, which allows obtaining homogeneous films with a good covering. We obtained films with good homogeneity and good covering properties as shown by the SEM views. By varying the number of pulses of Trimethyl aluminium we have been able to change the concentration of aluminium into the films and improved the conductivity of the films we used for complete the CIS cells. The resistivity of the films can be tuned from about 10^-2 to about 2.10^-3 W.cm. The films we obtained are highly transparent shown by transmission curves. This will allow us to complete devices.

Electrical performances of the cell were characterized by current-voltage measurements and Quantum efficiency (QE) characteristics. Complete cells with CdS and sputtered ZnO gives efficiencies of 12 - 13%. The first surface modified cadmium free results (basic treatment of CIS absorber) are lower (about 10%).

[1]: N. Naghavi, S. Spiering, M. Powalla, B. Canava, D. Lincot, Prog. Photovoltaics 11 (7), 2003, 437.

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