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 Voc 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 Ev + 0.64 eV which is attributed to the formation of VCd-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 Ev + 0.64 eV trap and cell Voc, 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.

Surface photovoltage phenomena: theory, experiment, and applications

“…Another venue in which SPS is frequently used is the evaluation of the effect of various treatments, such as electron [101] or ion [349,352] bombardment, etching [352,354], annealing [383,384], lightsoaking [304,306], etc., on semiconductor surfaces. Brillson [363,364,372,373,375,376] has pioneered the use of SPS in tandem with chemical analytical tools in order to assess the impact of surface chemistry on surface electronic properties.…”

Electronic structure, structural and optical properties of thermally evaporated CdTe thin films

“…For fabrication of the CdTe films a variety of preparation techniques have been employed such as vacuum deposition [14][15][16], electro deposition [17], molecular beam epitaxy [18], metal-organic chemical vapor deposition [19], closed-space sublimation [20], and screen-printing [21]. The vacuum evaporation method has some advantages such as: the amount of impurities included in the growing layer will be minimized, the tendency to form oxides will be considerably reduced and finally straight line propagation will occur from the source to substrate.…”

Processing and modeling issues for thin-film solar cell devices. Final report

“…For example, films which are fabricated from sintered powder slurries are processed with CdC1, additives at temperatures of 600-700°C to encourage grain growth and film densification r422-4251. Other devices, such as those deposited by physical vapor deposition (PVD) and molecular beam epitaxy, receive a lower temperature treatment to achieve efficiencies in the 11 to 13% range [426,427]. They are coated with a CdCl,:CH,OH solution, dried, then heattreated at 400°C.…”