A new approach for alkali incorporation in Cu₂ZnSnS₄ solar cells

Abstract

The addition of alkali elements has become mandatory for boosting solar cell performance in chalcogenide thin films based on kesterites (Cu2ZnSnS4, CZTS). A novel doping process is presented here, that consists in the incorporation of sodium or lithium during the deposition of the CdS buffer layer, followed by a Post-Deposition Annealing (PDA). As the doping route leads to more efficient devices in comparison with the undoped reference sample, the influence of PDA temperature was also investigated. Compositional profiling techniques (TOF-SIMS and GDOES) revealed a dependence of the alkaline distribution in kesterites with the PDA temperature. Although the doping process is effective in that it increases the alkaline concentration compared to the undoped sample, the compositional profiles indicate that a significant proportion of Li and Na remains "trapped" within the CdS layer. In the 200-300 ᵒC range the alkali profiles registered the higher concentration inside the kesterite. Despite this, an additional alkali accumulation close to the Mo/FTO substrate was found for all the samples, which is frequently related to alkali segregation at interfaces. The addition of both, lithium and sodium, improves the photovoltaic response compared to the undoped reference device. This is mainly explained by a substantial improvement in the open-circuit potential (Voc) of the cells, with best devices achieving efficiencies of 4.5% and 3% for lithium and sodium, respectively. SEM images depicted a “bilayer structure” with larger grains at the top and small grains at the bottom in all samples. Moreover, the calculated bandgap energies of the CZTS films account for changes in the crystallographic order-disorder of the kesterites, more related to the PDA treatment rather than alkali incorporation. Even if further optimization of the absorber synthesis and doping process will be required, this investigation allowed the evaluation of a novel strategy for alkali incorporation in kesterite based solar cells.

Fundamentals of thin film depth profiling by glow discharge optical emission spectroscopy

“…8 Valdés et al used GDOES (Horiba) to study alkali incorporation in Cu 2 ZnSnS 4 solar cells. 39 The authors reported the diameter of the hollow anode (4 mm) but did not reveal any other parameter. They deposited the Cu 2 ZnSnS 4 film onto the fluorine-doped tin oxide (ITO) glass and covered it with a CdS film.…”

“…Therefore, Valdés et al showed an important GDOES advantage, i.e., rapid depth profiling. 39 The authors also disclosed the deposition of about 20 nm thick molybdenum film on the glass substrate to suppress the diffusion of alkaline into the glass. The interfaces of Mo (either with ITO or Cu 2 ZnSnS 4 ) observed in the depth profiles, acquired by SIMS or GDOES, were narrow and practically overlapped.…”

Progress and Prospects with Cationic and Anionic Doping of Solution-Processed Kesterite Solar Absorbers: A Mini-Review

“…In order to overcome these defects, one of the possible strategies is elemental doping of the kesterite compound. Doping of kesterite films improves its properties by reducing the V oc deficit, passivating grain boundaries, increasing net hole carrier concentrations, and grain size. , Density functional theory (DFT) of several doped kesterites reveals the changes in the chemical bonding and its composition with undoped reference samples. − The doping mainly effect the bonding nature which causes the variation of the conduction band and valence band. Energy band shifting, broadening, and narrowing are observed due to doping .…”

Controlled Li Alloying by Postsynthesis Electrochemical Treatment of Cu2ZnSn(S, Se)4 Absorbers for Solar Cells

“…Predeposition processes include the use of an alkali fluoride precursor layer or an alkali-containing Mo back contact. Diffusion from the soda lime glass substrate is a major and often necessary supply source. , Alkali elements can also be incorporated during absorber deposition (e.g., coevaporation), or within the precursor layer in the case of solution-based absorber synthesis. , Postdeposition, or postsynthesis, treatments (PDT) are the third strategy for alkali incorporation, implemented by, e.g., deposition of an alkali fluoride layer, soaking the absorber in an alkali-containing solution or using so-called CdS-doping. , Diffusion of the alkali elements into the absorber is then typically activated by elevated temperatures …”

“…6,11 Postdeposition, or postsynthesis, treatments (PDT) are the third strategy for alkali incorporation, implemented by, e.g., deposition of an alkali fluoride layer, soaking the absorber in an alkali-containing solution or using so-called CdS-doping. 12,13 Diffusion of the alkali elements into the absorber is then typically activated by elevated temperatures. 11 The use of electrochemical methods in the field of chalcogenides has been introduced previously.…”