Comprehensive photoluminescence study of chlorine activated polycrystalline cadmium telluride layers

Abstract

Temperature and power dependent photoluminescence (PL) measurements were employed in order to study defects in close-space-sublimation grown polycrystalline cadmium telluride layers that had been activated with different chlorine containing compounds. The samples were either measured as-grown or after thermal treatment in an oxygen containing ambient with and without the chlorine containing compounds such as cadmium chloride, hydrochloric acid, and sodium chloride. The as-grown sample is discussed in detail, in order to then demonstrate the changes in the PL spectra induced by the postdeposition treatments. A deep level transition at 1.32 eV was observed in the as-grown sample which can be correlated with cadmium vacancies. Due to postdeposition treatments this deep level transition disappears and a broad band correlated with A-centers arises instead at about 1.43 eV. Another transition band at 1.479 eV in the as-grown sample is not influenced by any postdeposition treatment. Furthermore, by processing solar cells out of the respective samples, the PL results can be related to the solar cell parameters.

Explanation of red spectral shifts at CdTe grain boundaries

“…For the CdCl 2 -treated sample, the band associated with A-centers is present, but with a low intensity relative to the features at higher energies. Emission from the Z-band was not found in the global spectrum or the individual spectra in the spectrum images, which is consistent with the results of Kraft et al 5 Figure 3.1 Global spectra for the as-deposited and CdCl 2 -treated films. At each Photon Energy, the intensity of a global spectrum is the sum of the intensities for the spectra collected at each pixel in a 256 × 256 pixel SEM image.…”

“…The importance of the Z-band to CdTe solar cells was recently highlighted in a PL study of close-spaced sublimation (CSS)-deposited thin-films by Kraft et al 5 In that study, the Z-band was observed before the CdCl 2 treatment. After the treatment, the Z-band luminescence believed to be introduced during CSS film growth, 9 this transformation of defects may contribute to the order-of-magnitude increase in minority-carrier lifetime and large improvement in device performance that occur as a result of the CdCl 2 treatment.…”

“…5 As these charge carriers move through the crystal, they induce a polarization of the ion cores by attracting oppositely charged ions and repelling similarly charged ions. This is shown schematically in Figure 2.5.…”

“…5 This means that the usual Hamiltonian for the free electrons (or holes) in a semiconductor ( Here and * are the phonon destruction and creation operators; is the vibrational frequency of the LO phonons (in the long-wavelength limit); 0 is the static dielectric constant;…”

“…In deriving the Hamiltonian, Fröhlich considered polarons with a spatial extent that is much greater the lattice spacing (the "continuum approximation") and he neglected relativistic and spin effects. 5 The parameter is called the focused into a spectrograph where the light is dispersed into spectra. Spectra are detected by a cryogenically cooled CCD camera.…”