Effects of CdCl2 treatment on deep levels in CdTe and their implications on thin film solar cells: a comprehensive photoluminescence study

Abstract

This work is aimed at studying defect level distributions in the bandgap of CdTe thin films, used for solar cell development. In particular, the effects of CdCl 2 treatment on the defect levels are the main objectives of this research. Four different CdTe thin films were electroplated using three different Cd-precursors (CdSO 4 , Cd(NO 3 ) 2 and CdCl 2 ), and bulk CdTe wafers purchased from industry (Eagle Pitcher and University Wafers in US) were studied using low temperature photoluminescence. The finger prints of defects, 0.55 eV below the conduction band down to the valence band edge were investigated. In all of the CdTe layers, four electron trap levels were observed with varying intensities but at very similar energy positions, indicating that the origin of these defects are mainly from native defects. CdCl 2 treatment and annealing eliminates two defect levels and the mid-gap recombination centres are reduced drastically by this processing step. The optical bandgap of all four as-deposited CdTe layers is ~1.50 eV, and reduces to ~1.47 eV after CdCl 2 treatment. The material grown using the CdCl 2 precursor seems to produce CdTe material with the cleanest bandgap, most probably due to the built-in CdCl 2 treatment while growing the material.

Unravelling complex nature of CdS/CdTe based thin film solar cells

“…The detailed results of this study are reported in a recent publication [55]. One example is shown in Fig.…”

“…This should be completely removed in order to produce high performance devices. These defects are also related to the Te-richness [55,56], and therefore to reduce these defects, material should be grown with Cd-richness, without Te-precipitation.…”

“…Brominemethanol, nitric acid and phosphoric acid containing solutions (NP etch) are good examples for this type of etchants. 6 a PL spectra of as-deposited CdTe layers, grown from CdCl 2 precursor and the effects of subsequent CdCl 2 treatment in two steps [55]. b PL spectra of bulk CdTe wafers and electroplated CdTe after CdCl 2 treatment and c the representation of defects distribution in asmade and CdCl 2 treated CdTe thin films.…”

“…b PL spectra of bulk CdTe wafers and electroplated CdTe after CdCl 2 treatment and c the representation of defects distribution in asmade and CdCl 2 treated CdTe thin films. Note the drastic removal of defects from the bandgap of CdTe [55] On the other hand alkaline solutions preferably removes non-metallic element, Te and leave Cd-rich CdTe surface. NaOH, KOH, hydrazene hydrate etc are good examples of these etchants.…”

Analysis of electrodeposited CdTe thin films grown using cadmium chloride precursor for applications in solar cells

“…Substitution of Cl into Te sites makes the material n-type by acting as a shallow donor [13]. However, there are experimental evidence of the formation of a defect level at 1.39 eV below the conduction band during CdCl 2 treatment [27]. This shows that during the CdCl 2 treatment, acceptor-like defect is also formed closer to the top of the valence band and this leads to the formation of p-type doping of CdTe.…”