CdCl2 Activation Research Articles

Effect of in situ UHV CdCl 2-activation on the electronic properties of CdTe thin film solar cells

To reach reasonable conversion efficiencies of approximately 10% and above with CdTe thin film solar cells an activation step involving chlorine at elevated temperatures seems to be necessary before back contact formation. This activation process has been simulated in an ultrahigh-vacuum (UHV) system. Solar cells with a maximum efficiency of 9.1% have been prepared using this process. In addition the effect of the CdCl 2 activation process on the electronic properties of each solar cell layer, SnO 2, CdS and CdTe has been investigated in situ using photoelectron spectroscopy. The effects of the activation on the Fermi level position of all investigated layers is presented and discussed.

Band energy diagram of CdTe thin film solar cells

The knowledge of band energy diagrams of solar cells is essential for a fundamental understanding of their function. We have used photoelectron spectroscopy (PES) as a powerful tool for a systematic study of the formation of interfaces of CdTe solar cells in which the different layers CdS/SnO 2, CdTe/CdS and Te/CdTe are deposited step by step by thermal evaporation in model experiments. The results of these studies show that in contrast to other investigations the energy converting heterojunction is not a simple n-CdS/p-CdTe contact. Although depth profiling reveals a homogeneously intrinsic CdTe bulk layer, contact formation and CdCl 2-activation are assumed to form an n–i–p CdTe absorber. Such non-ideal conditions may strongly affect optimization processes of conversion efficiencies. The main limitations are evidently due to back-contact formation. Our results do not confirm that the electrochemically formed Te layer produces a good ohmic contact between the CdTe layer and the metallic back contact. From model experiments, we assume the formation of a tunneling contact, instead.