This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with Cu3SnS4 modes at the surface region while spectra measured close to the back region show peaks from ZnS and MoS2. Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.

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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.

Kesterite solar cells are at a crossroads, and a significant breakthrough in performance is needed for this technology to stay relevant in the upcoming years. In this work, we propose to follow the proven strategy of band engineering to assist charge carrier collection taking inspiration from chalcopyrite solar cells. Using a process based on a combination of metallic precursor sputtering and chalcogen-reactive annealing, we achieve controlled cationic substitutions by partly replacing Sn by Ge, hence tailoring several rear band gap grading profiles along the absorber thickness. A complete set of results is presented, with samples ranging from pure Sn to pure Ge compounds. The formation of a rear band gap grading is determined through different characterization techniques, specifically through a combination of glow discharge optical emission and Auger spectroscopies with an advanced multiwavelength Raman spectroscopy analysis carried out at the front and back (rear) sides of the films using a lift-off process. As such, a preferential Ge enrichment toward the back of the absorber is unequivocally demonstrated in kesterite absorbers and further applied to complete devices for deliberately generating distinct rear band gap profiles, leading to an efficient back surface field that potentially enhances the carrier selectivity of the back interface. The electrical analysis of the complete devices shows a complex interplay between the benefits of band gap grading and possible Ge-related defects in the absorber. Using optimized synthesis conditions, an absolute increase in efficiency (compared to the Ge-free reference) is obtained for the record device (η > 9%) without any antireflective coating or metallic grid. This performance enhancement is mostly ascribed to the presence of a drift electric field assisting in the carrier collection while preventing back side recombination. These results confirm the possibility of generating back band gap grading in kesterite solar cells and open the way to further development of the kesterite photovoltaic technology toward higher efficiencies through tailored band gap engineering.

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