The Relationship of CdS/CdTe Cell Band Profiles to J-V Characteristics and Bias-Dependent Quantum Efficiency

The built-in voltage (V_bi) and the charged electronic state (ES) distribution in a solar cell determine its conduction and valence band profiles. Changes in the charge state of the ES give rise to J-V curve anomalies like cross-over, roll-over, and, in some cases, long J-V and capacitance transient effects. CdTe is highly compensated containing deep donor- and acceptor-like ES, with larger densities than the shallow acceptor density. For close compensation, the charge density in the depletion layer can be so low that V_bi is determined by the front and back contact work functions. In that case the cells must be analyzed in terms of an n/i/p junction model. AMPS models of two extreme profiles are discussed here as illustrations: an n/i/p junction, where V_bi is mainly supported by charge at the contacts, and an n/p junction, where V_bi is supported by charge in the bulk CdTe within the absorber