Admittance spectroscopy of CdTe∕CdS solar cells subjected to varied nitric-phosphoric etching conditions

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. In this work we investigate the electric and structural properties of CdTe/ CdS solar cells subjected to a nitric-phosphoric ͑NP͒ acid etching procedure, employed for the formation of a Te-rich layer before back contacting. The etching time is used as the only variable parameter in the study, while admittance spectroscopy is employed for the characterization of the cells' electric properties as well as for the analysis of the defect energy levels. Particular attention was also given to the characteristics of unetched devices and it is shown that despite the larger height of back-contact barrier such samples show well defined admittance spectra, as well as allow for extraction of as much as five defect levels in the range of 0.08-0.9 eV above the valence band. In contrast, admittance characteristics of the etched samples show a decrease of the number of the detectable trap levels with increasing etching time. ͑Hence it is usual for only one or two trap levels to be reported in the literature for finished devices.͒ The latter leads to the anomalous Arrhenius energy plots as well as the breakdown of low-frequency capacitance characteristics for samples etched with times larger than 30 s. The observed effects are attributed to physical thinning of the cells, the etching out of grain boundaries, and the tellurium enrichment of the CdTe surface by NP etching.We also perform analysis of the back-contact barrier height as extracted from dark I-V measurements at different temperatures. The dependence of this barrier height on NP etching time is compared with that of conversion efficiency, from which conclusions are drawn about both positive and negative effects of the nitric-phosphoric etch.

The Effects of Hydrogen Iodide Back Surface Treatment on CdTe Solar Cells

“…In contrast, the capacitance response from E a2 is bias independent (Figure c), which suggests that the charged defects contributing to the carrier transport are located in the bulk of the CdTe absorber. The HI‐etching leads to Te enrichment at GBs and a drop in GB resistance ( R GB ), which restrains the response of the bulk defects . Additionally, the capacitive properties of GBs may also be changed by additional doping and the electrical field of the depletion region, when the amount of enriched Te is large.…”

“…Wet‐chemical etching has been widely used to produce such a Te‐rich surface as well as to remove various oxides and/or chlorides that may form at the CdTe surface during device processing, such as the cadmium chloride (CdCl 2 ) mediated heat treatment procedure that is used to improve the electronic quality of the CdTe material. A variety of etching agents have been used to fabricate efficient CdTe solar cells, including nitric/phosphoric‐based acid (NP), potassium dichromate sulfuric acid (KD), and bromine in methanol (BM) . However, these etching agents contain toxic and/or hazardous materials which are not desirable due to manufacturing safety concerns.…”

Comparative study of trap densities of states in CdTe∕CdS solar cells

“…6,12,13 ͑It is important to note that according to Ref. 2 and 3, respectively, several important observations can be made.…”

“…6 Figure 1͑a͒ shows the capacitance-voltage data on a Mott-Schottky plot for samples 156 and 173, and Fig. The efficiency of this sample is = 9%, and open circuit voltage V oc = 0.6 V. For sample 156, no ZnO buffer layer was grown between ITO and CdS ͑ = 7.3% and V oc = 0.57 V͒.…”

Impedance spectroscopy of thin-film CdTe/CdS solar cells under varied illumination

“…18 These states were self-consistently incorporated into the total equivalent circuit of the device in dark. 1 The number of such deep states, however, can be arbitrary and depends on the sample growth and processing conditions.…”