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In ultrathin Cu(In,Ga)Se-2(CIGS) film solar cells, the CdS/CIGS interface may become one of the limiting factors for efficiency. The first step toward reducing the impact of this problem could be a surface treatment process to improve the quality of the front interface. The purpose of this study is to have a better understanding of the effect of wet chemical surface treatment, using ammonium sulfide ((NH4)(2)S), on CIGS thin film layers with different Cu/(Ga + In) (CGI) ratios. Herein, photoluminescence (PL) and time-resolved PL (TRPL) studies are conducted on bare CIGS, ammonium sulfide-treated CIGS thin films, and samples with CdS. In bare CIGS, CGI ratio-dependent changes in PL are observed on both a low-energy (defect related transition) and a high-energy peak (band-to-band transition). After the surface treatment, the PL maximum increases by factors ranging from 4 to 11 depending on the CGI ratio, accompanied by a slower decay. Trends with similar improvement as in the PL study are observed in the performance of the solar cells. It is shown that the impact of the surface treatment is beneficial independently of the CGI ratio of the absorber layers. In all cases, the treatment is shown to improve the efficiency. This work received funding from the European Union's H2020 research and innovation program under grant agreement No. 715027. Buldu, DG (corresponding author), Hasselt Univ Partner Solliance, Inst Mat Res IMO, Agoralaan Gebouw H, B-3590 Diepenbeek, Belgium ; Imec Div IMOMEC Partner Solliance, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; EnergyVille, Thorpk,Poort Genk 8310 & 8320, B-3600 Genk, Belgium. Dilara.Gokcen.Buldu@imec.be

In this study, an ultra-thin MoO3 layer synthesized by a solution-based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10 nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open-circuit voltage compared to the reference solar cells. Temperature dependent current density–voltage measurement indicated that the activation energy (EA) of the main recombination is higher (∼ 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where EA ∼ 770 meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved.

Herein, using the spin-coating process as a sodium pre-deposition technique for Cu(In,Ga)Se-2, ultrathin film solar cells is discussed. Throughout this study, several challenges are detected to overcome, such as unevenly deposited salt layers and unplanned clusters (even islands) of sodium salts due to accumulation. The solar cell performances with the optimal and nonoptimal amounts of sodium are shared to assess the effect of varying sodium amounts on performance. The hyperspectral photoluminescence (PL) mapping and imaging and reflection measurements are also used to emphasize the importance of homogeneity of the sodium layer. All the authors thank to Laura-Isabelle Dion-Bertrand (Photon etc.) for hyperspectral measurement. This work received funding from the European Union's H2020 research and innovation program under Grant Agreement no. 715027.