The effects of CdCl2 on the electronic properties of molecular-beam epitaxially grown CdTe/CdS heterojunction solar cells

Abstract

Significant improvements in CdTe/CdS solar cell efficiency are commonly observed as a result of a postdeposition CdCl₂ dip followed by a 400 °C heat treatment during cell processing which increases CdTe grain size. In this paper, we investigate the electronic mechanisms responsible for CdCl₂‐induced improvement in cell performance along with possible performance‐limiting defects resulting from this process in molecular‐beam epitaxy‐grown polycrystalline CdTe/CdS solar cells. Current density‐voltage‐temperature (J‐V‐T) analysis revealed that the CdCl₂ treatment changes the dominant current transport mechanism from interface recombination/tunneling to depletion region recombination, suggesting a decrease in the density and dominance of interface states due to the CdCl₂ treatment. It is shown that the change in transport mechanism is associated with (a) an increase in heterojunction barrier height from 0.56 to 0.85 eV, (b) a decrease in dark leakage current from 4.7×10⁻⁷ A/cm² to 2.6×10⁻⁹ A/cm² and, (c) an increase in cell V_oc from 385 to 720 mV. The CdCl₂ also improved the optical response of the cell. Substantial increases in the surface photovoltage and quantum efficiency accompanied by a decrease in the bias dependence of the spectral response in the CdCl₂‐treated structures indicate that the CdCl₂ treatment improves carrier collection from the bulk as well as across the heterointerface. However, deep level transient spectroscopy measurements detected a hole trap within the CdTe depletion region of the CdCl₂‐treated devices at E_v + 0.64 eV which is attributed to the formation of V_Cd‐related defects during the annealing process after the CdCl₂ dip. J‐V‐T analysis demonstrated that this trap is the probable source of dominant recombination in the CdCl₂‐treated cells. An inverse correlation was found between the density of the E_v + 0.64 eV trap and cell V_oc, suggesting that the heat treatment with CdCl₂ may eventually limit the CdTe/CdS cell performance unless the formation of this defect complex is controlled or eliminated.