Donor and trap densities in CdS polycrystalline evaporated layers

For Cu 2 S-CdS photocells it is particularly important to have CdS layers with low trap densities. Therefore the traps must be characterised with maximum precision. This can be accomplished by measurement of the transient capacitance, a method that is applied here to Au-CdS layer Schottky diodes. In the CdS layers considered here, the trap density is higher than the donor density. A given energy level can act as a donor as well as a trap level. The D level (E T =0.25 eV) associated with complex sulphur vacancies is the dominant trap near room temperature and its capture cross section is much higher in CdS polycrystalline layers than in CdS single crystals.