High-field domains in CdS adjacent to a junction of p-type solar cells

Abstract

A thin cover layer (150 Å preferred) of copper-doped CdS, when applied on top of any p-type solar cell, can connect this cell directly to an electron-blocking electrode without a pn-junction and increases the open circuit voltage close to its theoretical value; in the example of a CdS/CdTe cell, it increases Voc to its extrapolated value at T = 0 K of the band gap of 1.45 eV. This is caused by a high-field domain that is attached to the junction and limits the field to below tunneling to prevent junction leakage and connects to the CdS that has turned p-type. The large Debye length exceeding the thickness of the CdS forces a direct connection to the electron-blocking cathode with holes tunneling into the metal. The difference of junction-attached high-field domains to the electrode-attached domains, which were described earlier, are given and the consequences are delineated by increasing the conversion efficiency from 8% to 16% in CdTe, while also causing some series resistance limitation. The effect of the added CdS layer is discussed by drawing a to-scale model of the CdS/CdTe solar cell from all experimentally available data and the assumption of the continuity of the hole current. A small jump of the valence band downward is caused by interface recombination. The assistance of high-field domains in CdS is also exemplified by the results of an extremely simple production procedure of the CdS/Cu2S solar cells.