Cu2ZnSnSe4 kesterite compounds are some of the most promising materials for low-cost thin-film photovoltaics. However, the synthesis of absorbers for high-performing devices is still a complex issue. So far, the best devices rely on absorbers grown in a Zn-rich and Cu-poor environment. These off-stoichiometric conditions favor the presence of a ZnSe secondary phase, which has been proved to be highly detrimental for device performance. Therefore, an effective method for the selective removal of this phase is important. Previous attempts to remove this phase by using acidic etching or highly toxic organic compounds have been reported but so far with moderate impact on device performance. Herein, a new oxidizing route to ensure efficient removal of ZnSe is presented based on treatment with a mixture of an oxidizing agent and a mineral acid followed by treatment in an aqueous Na2S solution. Three different oxidizing agents were tested: H2O2, KMnO4, and K2Cr2O7, combined with different concentrations of H2SO4. With all of these agents Se(2-) from the ZnSe surface phase is selectively oxidized to Se(0), forming an elemental Se phase, which is removed with the subsequent etching in Na2S. Using KMnO4 in a H2SO4-based medium, a large improvement on the conversion efficiency of the devices is observed, related to an improvement of all the optoelectronic parameters of the cells. Improvement of short-circuit current density (J(sc)) and series resistance is directly related to the selective etching of the ZnSe surface phase, which has a demonstrated current-blocking effect. In addition, a significant improvement of open-circuit voltage (V(oc)), shunt resistance (R(sh)), and fill factor (FF) are attributed to a passivation effect of the kesterite absorber surface resulting from the chemical processes, an effect that likely leads to a reduction of nonradiative-recombination states density and a subsequent improvement of the p-n junction.

Pentenary Cu2ZnSn(S(y)Se(1-y))4 (kesterite) photovoltaic absorbers are synthesized by a one-step annealing process from copper-poor and zinc-rich precursor metallic stacks prepared by direct-current magnetron sputtering deposition. Depending on the chalcogen source--mixtures of sulfur and selenium powders, or selenium disulfide--as well as the annealing temperature and pressure, this simple methodology permits the tuning of the absorber composition from sulfur-rich to selenium-rich in one single annealing process. The impact of the thermal treatment variables on chalcogenide incorporation is investigated. The effect of the S/(S+Se) compositional ratio on the structural and morphological properties of the as-grown films, and the optoelectronic parameters of solar cells fabricated using these absorber films is studied. Using this single-step sulfo-selenization method, pentenary kesterite-based devices with conversion efficiencies up to 4.4 % are obtained.

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The increasing importance of the Cu(In,Ga)Se2 based thin films photovoltaic industry claims for the development of new assessment and monitoring tools to answer the needs existing in the improvement of the control of the processes involved in the production of solar cells modules. In this frame, a strong interest has been given to the development methodologies for the assessment of the CIGS absorber, nevertheless advanced optical tools for the characterization of the other layers in the solar cells are still missing. In this work, we report a non‐destructive optical methodology based on resonant Raman concepts that has been developed for the characterization of Al doped ZnO layers (AZO) that are used as window layer in Cu(In,Ga)Se2 solar cells. Doping the ZnO layer with Al leads to the presence of a characteristic defect induced band at 510 cm−1 spectral region. The correlation of the relative intensity of this band with the resistivity of the layers provides a fast and reliable tool for their electrical monitoring. Analysis of solar cells fabricated with layers of different conductivities has allowed demonstration at cell level of the proposed methodology for the determination of efficiency losses related to degradation of the resistivity of the AZO layers.