Back Cover (Phys. Status Solidi A 11/2009)



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CdS/CdTe solar cells have been known to be excellent photovoltaic converters for more than three decades. The fact that the presence of a very thin cover layer of n-type CdS improves the performance of the solar cells is long known but still little is known about the mechanism that permits such dramatic improvement, not only in CdTe but in many other CIS-type cells. The article by Karl W. Böer (p. 2665) discusses the most important reasons for such a behavior. The most important effect is that CdS limits the field in a junction by field quenching of its photoconductivity due to Frenkel—Poole excitation of holes that were trapped in Coulomb-attractive slow recombination centers. This field quenching can also electronically invert the CdS layer close to a junction to a p-type semiconductor. Field quenching forces the Fermi level to shift below the middle of the band gap, causing the conduction bands to disconnect, with the CdS band shifting up and reducing substantially electron leakage through the junction. The inside back cover picture shows the suggested band model for the junction part of the CdS/CdTe solar cell for three different bias conditions: red — reverse, black — open circuit, and green — forward bias.