p-type emitters covered with a thin CdS layer show a substantial improvement of Voc and FF

A number of p-type emitters of solar cells, covered with a thin, copper-doped CdS layer, show a significant increase in open circuit voltage and fill factor. This is caused by field limitation in the junction to 50 kV/cm, and induced by field quenching in photo-conducting CdS. As a result, the junction field increases when the bias is reduced from forward toward Voc; however, it cannot increase above the critical field for the onset of field quenching. Consequently, electron leakage through the junction interface is reduced, which could otherwise occur in uncoated solar cells when the junction field approaches tunneling fields. In addition, with increasing quenching the Fermi level moves further away from the conduction band, causing the bands at the junction interface to disconnect. As a consequence of field quenching, the CdS enters a range of negative differential conductivity, creating a high-field domain that absorbs increased voltage in the reverse bias range without increasing the junction field. The doping density with Cu is critical; it is self-adjusted by limited solubility of Cu in CdS and is extremely difficult to realize homogeneously for any other known semiconductor cover layer.