Field quenching in photoconductive CdS as possible reason to enhance Voc and FF in thin-film solar cells

Field-quenching via Frenkel-Poole excitation of Coulomb attractive hole traps limits the field in the CdS part of the junctions to 50 kV/cm. This is far below the field in typical other pn-junctions of thin-film semiconductors, which exceeds 100 kV/cm and approaches tunneling fields that make junctions leaky, hence reduces both V-oc and FF. Field-quenched CdS may become electronically inverted, thereby providing a possibility that the junction of the CdS/CdTe cell may extend into the CdS, with the cell becoming a hetero structure. With field quenching a region of negative differential conductivity is created causing a high-field domain that prevents the maximum electric field in the junction to exceed 50 kV/cm, avoiding tunneling, hence electron leakage through it. A preliminary band model of this cell is proposed. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim