CdTe Solar Cells in Substrate Configuration: Influence of Mo, Na, Cl and Mg Treatments to Promote Recrystallization in the Absorber Layer

Amirkhalili, Arezoo (2017) CdTe Solar Cells in Substrate Configuration: Influence of Mo, Na, Cl and Mg Treatments to Promote Recrystallization in the Absorber Layer. Doctoral thesis, Northumbria University.

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Abstract

Cadmium telluride (CdTe) is the leading thin-film photovoltaic material with efficiency up to 22.1 % (superstrate configuration). Devices developed in the substrate configuration allow roll-to-roll processing using lightweight and flexible substrates; however, exhibit a lower efficiency (13.6 % on glass substrate). This work focuses on the properties of metal-organic chemical vapour deposition (MOCVD) grown CdTe thin film and devices deposited on molybdenum (Mo) coated glass (substrate configuration). Particular attention is given to the role of sodium (Na) and chlorine (Cl).

A Cl-treatment was performed using either MOCVD-CdCl2 or solutionMgCl2 on MOCVD-grown CdTe Mo-coated glass before formation of the pn junction. Devices were completed with the deposition of windows layers (i-ZnO and ITO) and a front metal contact grid (Ni-Al).

Initially, the Mo properties and their effect on Na diffusion from the sodalime glass (SLG) were investigated. Sputtering conditions were set to achieve Mo layers with a nominal thickness of 1 µm and sheet resistance and resistivity ranging between 0.16 – 0.59 Ω/�and 16 – 66 µΩ·cm, respectively. The variations of resistivity are linked to the Na content in the Mo film: for a given deposition power, an increase in resistivity yields a lower Na diffusion in Mo.

Relying on the diffusion of Na from the SLG substrate is not suitable for flexible or metal substrates; therefore, a second approach was investigated using a thin sodium fluorine layer. It was observed that the inclusion of Na between the substrate and the Mo enlarges the CdTe grains following CdCl2 treatment by 50 % but weakens the adhesion to the substrate. The introduction of Na between the Mo and CdTe layer promotes the formation of large faceted grains for the as-grown films with an average grain size 10 times larger than in the case of Na-free deposition while maintaining a good adhesion to the substrate. The devices had a low efficiency (< 5 %) arising from weak J–V parameters despite the improved open circuit voltage.

The replacement of the CdCl2-treatment with MgCl2 was tested and this did not affect the morphology or structure of the films and showed limited efficiencies. An investigation in widening the window layer bandgap by addition of cadmium zinc sulfide was successful as improvements in the optical transmission at wavelength range of 400 < < 550 nm and the short circuit current were measured.

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