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Abstract
Cu2ZnSnS4 (CZTS) solar cells are a promising potential replacement for Cu2(In, Ga)Se2 (CIGS) based photovoltaics. An advantage is that these substitute the rare metal indium with the earth abundant elements zinc and tin. CZTS absorbers demonstrate a high absorption coefficient of 104 cm-1 and a direct band gap of 1.5 eV, on the edge of the optimum range of 1.0 to 1.5 eV, which is suitable for solar cell applications.
Instability of Mo back contacts in CZTS solar cells is observed, as the reaction between the Mo back contact and the CZTS absorber is thermodynamically favourable. Intermediate layers of Ag and ZnO are deposited at the CZTS/Mo interface in order to address this issue. The addition of an Ag intermediate layer reduces the amount and size of voids in the vicinity of the Mo back contact. Additionally, the presence of a ZnO intermediate layer reduces the voids and MoS2 thickness at the CZTS/Mo interface. Consequently, the Ag intermediate layer was found to improve experimental efficiency from 2.31% to 4.42%. The ZnO intermediate layer enhances this efficiency from 1.13% to 4.3%, for controls prepared differently in this case.
Rapid thermal annealing (RTA) treatment of the Mo back contact is applied to improve CZTS device performance, which facilitates the diffusion of Na into the CZTS absorber. This promotion of Na diffusion results in smoother CZTS surface morphology, and reduces both voids at the CZTS/CdS interface and cracks in the ZnO window layer. Na also is known to effectively passivate the grain boundaries and reduce associated recombination. Consequently, the RTA treatment of the Mo back contact boosts the efficiency from 1.03% to 3.14%, for controls prepared differently in this case.
The effect of vacuum thermal annealing on a Mo bilayer structure is also investigated. Vacuum thermal annealing treatment reduces the sheet resistance and improves the adhesion of as-deposited Mo bilayer films. Both the grain size and density of Mo films increase after vacuum thermal annealing treatment. A Mo bilayer film with low sheet resistance of 0.132 Ω/• and strong adhesion was fabricated in our work, which is close to the state of the art sheet resistance in the range of 0.12 to 0.14 Ω/•.