Zn_1-xSn_xO_y Buffer Layer Deposited by Chemical Bath Deposition for Low and Wide Band Gap (Ag,Cu)(In,Ga)Se₂ Solar Cells

Cu(In,Ga)Se₂ (CIGSe) solar cells with a tunable bandgap stand out as a promising technology for tandem applications. However, challenges arise, particularly in the performance of wide-bandgap CIGSe with a high [Ga]/([Ga]+[In]) ratio (GGI), which exhibits lower efficiency than anticipated, mostly related to losses in open-circuit voltage (V_oc). On the other hand, alloying the copper position with silver has shown improvement in the V_oc, related with a better conduction band offset between high-Ga (Ag,Cu)(In,Ga)Se₂ (ACIGSe) and the traditional CdS buffer layer, with a maximum PCE of 16.3% for an absorber band gap of 1.43 eV, without antireflective coating and RbF postdeposition treatment. [1]
Addressing the environmental concerns associated with Cd-based buffers, we recently developed a solution-based method for depositing thick ZTO (Zn_1-xSn_xO_y) buffer layers by ammonia-based chemical bath deposition (CBD). [2] This method allows for the tuneability of the ZTO bandgap (3.2–3.6 eV) by adjusting the [Sn]/([Sn]+[Zn]) ratio (TTZ). Here, we investigate the suitability of zinc tin oxide (ZTO), as a Cd-free alternative for both low-bandgap CIGSe and wide-bandgap (Ag,Cu)(In,Ga)Se₂ (ACIGSe) solar cells. Best ZTO-buffered devices exhibited competitive power conversion efficiencies (PCE) of 14% and 7% for low-bandgap and wide-bandgap absorbers, respectively. The optimization process revealed distinct optimal tin concentrations for the two absorbers, with 10% TTZ ZTO identified as optimal for the wide-bandgap samples, and 20% TTZ ZTO for the low-bandgap material. A pronounced decline in performance was observed for both materials when higher Sn content buffer layers were used, primarily resulting from losses in open-circuit voltage. These losses in ZTO-based devices might be linked to poor band alignment, providing a plausible explanation for the results obtained in the low-bandgap CIGSe sample. Further characterization of the full devices showed inhomogeneities at the interface between the absorber and buffer that impacts negatively the open-circuit voltage when they occupy most of the interface.
In summary, ZTO-based devices showcased promising photovoltaic performance with average PCE values of (12 ± 3) % [V_oc = (520 ± 34) mV; E_{g, CIGSe} = 1.03 eV] for low-bandgap CIGSe and (6 ± 1)% [V_oc = (0.62 ± 0.03) V; E_{g, ACIGSe} = 1.46 eV] for wide-bandgap ACIGSe. This emphasizes the ZTO's potential as a practical and non-toxic buffer layer, deposited by CBD, for diverse CIGSe solar cell applications.

References
[1] Keller, J., Aboulfadl, H., Stolt, L., Donzel-Gargand, O. and Edoff, M. (2022), Rubidium Fluoride Absorber Treatment for Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells. Sol. RRL, 6: 2200044. https://doi.org/10.1002/solr.202200044
[2] Garzón, D.A., Rossi, C., Khatri, I., Soggia, F., Çaha, I., Deepak, F.L., Colombara, D. and Sadewasser, S. (2023), Chemical Bath Deposition of Zn1−xSnxOy Films as Buffer Layers for Cu(In,Ga)Se2 Solar Cells. Sol. RRL, 7: 2300173. https://doi.org/10.1002/solr.202300173