Graded-Bandgap Solar Cells Using All-Electrodeposited ZnS, CdS and CdTe Thin-Films

Abstract

Abstract:A 3-layer graded-bandgap solar cell with glass/FTO/ZnS/CdS/CdTe/Au structure has been fabricated using all-electrodeposited ZnS, CdS and CdTe thin layers. The three semiconductor layers were electrodeposited using a two-electrode system for process simplification. The incorporation of a wide bandgap amorphous ZnS as a buffer/window layer to form glass/FTO/ZnS/CdS/CdTe/Au solar cell resulted in the formation of this 3-layer graded-bandgap device structure. This has yielded corresponding improvement in all the solar cell parameters resulting in a conversion efficiency >10% under AM1.5 illumination conditions at room temperature, compared to the 8.0% efficiency of a 2-layer glass/FTO/CdS/CdTe/Au reference solar cell structure. These results demonstrate the advantages of the multi-layer graded-bandgap device architecture over the conventional 2-layer structure. In addition, they demonstrate the effective application of the two-electrode system as a simplification to the conventional three-electrode system in the electrodeposition of semiconductors with the elimination of the reference electrode as a possible impurity source.

The effects of anode material type on the optoelectronic properties of electroplated CdTe thin films and the implications for photovoltaic application

“…This is possible since only one defect level (1.39 eV) is present in this CdTe and close to the valence band edge therefore making Fermi level pinning highly probable at this level. This particular device structure of g/FTO/n-CdS/n-CdTe/Au n-n heterojunction + Schottky barrier is known to produce solar cells with high short-circuit current density as observed here, due to a possible combination of two depletion regions at both the n-n heterojunction and at the Schottky junction [19,20,32]. This situation creates a combined wide depletion region in the device.…”

“…This ensures that equal thickness of CdTe is deposited. This is possible because for a given deposition time (t), the theoretical thickness (T) of an electrodeposited semiconductor film according to the Faraday's equation (T = MtJ/nFρ) is known to be directly proportional to the deposition current density (J) [11,18] with the rest of the parameters [molecular mass (M), number of electrons involved in the formation (n), Faraday's constant (F) and density of the material (ρ)] being constants for any given material. In one set of the films deposited, graphite rod was used as anode material while platinum plate was used in the other set in order to study the effect of each of the anode materials on the properties of the CdTe thin films electroplated.…”

“…Another set of cyclic voltammetry was carried out using the graphite and platinum anodes in turn in order to determine the range of deposition potentials for CdTe using the different anodes. Full details of the electrodeposition of CdTe thin films using graphite anode and platinum anode have been reported recently [6,18]. The cathodic electrodeposition of two sets of CdTe thin films (of four samples per set), at different voltages, was carried out on glass/FTO (g/FTO) substrate using the same Gill AC Potentiostat/Galvanostat in two-electrode configuration.…”

“…For the complete solar cell fabrication, the CdTe surfaces were etched with acidified K 2 Cr 2 O 7 and NaOH+Na 2 S 2 O 3 , successively rinsed in de-ionised water each time and finally dried in a stream of N 2 . Then Au metal contacts were evaporated onto the etched CdTe surfaces in a vacuum chamber as described previously [19,20]. Performance of the devices was assessed using Keithley 619 Electrometer/multimeter under AM 1.5 condition for comparison.…”

Effect of the inclusion of galium in normal cadmium chloride treatment on electrical properties OF CdS/CdTe solar cell

“…Both the CCT and GCT treated glass/FTO/n-CdS/n-CdTe/p-CdTe layers were allowed to air-dry before heat treatment. The heat treatment was performed at 430˚C for 20 minutes in air atmosphere for samples undergoing each treatment based on previously optimised conditions [4], [23]- [25]. The CCT and GCT treated glass/FTO/n-CdS/n-CdTe/p-CdTe layers were then rinsed in DI water and dried in a stream of nitrogen afterwards.…”