An investigation of photocurrent loss due to reflectance and absorption in CdTe/CdS heterojunction solar cells

Development of high-efficiency, thin-film CdTe solar cells. Final subcontract report, 1 February 1992--30 November 1995

“…This etching process can leave a Cd deficient surface [8], and yet we observe a Cd pile-up in the cell, therefore, it is reasonable to conclude that the metallization process tends to enhance the Cd/Te ratio near the CdCdTe interface. This increase in the Ccl/Te ratio suggests a Cd out-diffusion or Te depletion near the interface.…”

“…These includes (a) work function 4, where q4 is the energy needed to move an electron from the Fermi level to the vacuum level; (b) electron affinity x, where qx is the energy needed to move an electron from the conduction band minimum to the vacuum level; (c) AEc and AEv are the conduction and valence band edge discontinuities, respectively. The equilibrium energy band diagram for the heterojunction system can be easily determined by applying two simple rules: (a) the Fermi level is flat throughout the junction and (b) the bulk energy band diagrams must be recovered far from the junction [7,8,9]. Figure 2.3 illustrates how the ideal p-CdTe/n-CdS heterojunctions energy band diagram can be determined from these d e s by assuming no interface charge.…”

“…Besides the optimum bandgap of 1.5 eV, CdTe is also a direct bandgap semiconductor with a high absorption coefficient (> 1 x lo4 cm-') for photons above the bandgap energies ( Figure 1.2) [8]. Therefore, 1-2 pm thick CdTe is sufficient to absorb more than 90% of solar spectrum.…”

“…Due to the large lattice mismatch at the GaAs/CdTe interface (lattice constant a=5.653 A for GaAs, while a=6.481 A for the CdTe bulk), the CdTe material at the CdTe/GaAs interface is expected to be more highly strained than the CdTe material at the free interface. This could result in smaller lattice spacing in this CdTe film compared to the bulk CdTe, in order to release the lattice mismatch [7,8]. The Bragg angles 8 for the (400) peaks should be 33.014' for GaAs'and 28.375" for CdTe, a difference of 4.639' .…”

“…Thus, we conclude that because of the reduction of the conduits for impurity migration, CdC1, treatment reduces defect density and improves the cell performance. Notice that several investigators have found that the CdCl, treatment promotes the grain growth [8,9,10], which should reduce S, and enhance the film quality and cell efficiency. In this study, we did not observe a significant change in the CdTe grain size, instead we saw a change in the S,, value.…”