KCN Chemical Etching of van der Waals Sb2Se3 Thin Films Synthesized at Low Temperature Leads to Inverted Surface Polarity and Improved Solar Cell Efficiency.

Recent developments in Sb2Se3 van der Waals material as an absorber candidate for thin film photovoltaic applications have demonstrated the importance of surface management for improving the conversion efficiency of this technology. Sb2Se3 thin films' versatility in delivering good efficiencies in both superstrate and substrate configurations, coupled with a compatibility with various low-temperature deposition techniques (below 500 °C and often below 350 °C), makes them highly attractive for advanced photovoltaic applications. This study presents a comparative analysis of the most effective chemical etchings developed for related thin film chalcogenide technologies to identify and understand the most appropriate surface chemical treatments for Sb2Se3 in substrate configuration, synthesized using a sequential process at very low temperatures (320 °C). Eight different chemical etchings were tested and investigated, and the results show that only KCN-based solutions lead to an improvement in the solar cell's performance, primarily due to an increase in the fill factor. Surface analysis of the samples shows that KCN etching produces very Sb-rich surfaces that do not affect the properties of the bulk. It is proposed that this Sb-rich interface inverts the surface polarity, creating a "buried junction" with CdS, thereby explaining the improvement of the fill factor of the devices, as confirmed by device modeling. The results of this study underscore the importance of surface management in low-temperature synthesized Sb2Se3 absorbers, where Sb-rich interfaces are crucial for achieving high-efficiency devices. This research contributes to ongoing efforts to improve the performance of Sb2Se3 thin film photovoltaic technology and could pave the way for the development of more efficient solar cells with optimized interfaces.

Does Sb2Se3 Admit Nonstoichiometric Conditions? How Modifying the Overall Se Content Affects the Structural, Optical, and Optoelectronic Properties of Sb2Se3 Thin Films.

Sb2Se3 is a quasi-one-dimensional (1D) semiconductor, which has shown great promise in photovoltaics. However, its performance is currently limited by a high Voc deficit. Therefore, it is necessary to explore new strategies to minimize the formation of intrinsic defects and thus unlock the absorber's whole potential. It has been reported that tuning the Se/Sb relative content could enable a selective control of the defects. Furthermore, recent experimental evidence has shown that moderate Se excess enhances the photovoltaic performance; however, it is not yet clear whether this excess has been incorporated into the structure. In this work, a series of Sb2Se3 thin films have been prepared imposing different nominal compositions (from Sb-rich to Se-rich) and then have been thoroughly characterized using compositional, structural, and optical analysis techniques. Hence, it is shown that Sb2Se3 does not allow an extended range of nonstoichiometric conditions. Instead, any Sb or Se excesses are compensated in the form of secondary phases. Also, a correlation has been found between operating under Se-rich conditions and an improvement in the crystalline orientation, which is likely related to the formation of a MoSe2 phase in the back interface. Finally, this study shows new utilities of Raman, X-ray diffraction, and photothermal deflection spectroscopy combination techniques to examine the structural properties of Sb2Se3, especially how well-oriented the material is.

Is It Possible To Develop Complex S-Se Graded Band Gap Profiles in Kesterite-Based Solar Cells?

This work presents the development of a novel chalcogenization process for the fabrication of Cu2ZnSn(S,Se)4 (CZTSSe or kesterite)-based solar cells that enable the generation of sharp graded anionic compositional profiles with high S content at the top and low S content at the bottom. This is achieved through the optimization of the annealing parameters including the study of several sulfur sources with different predicted reactivities (elemental S, thiourea, SnS, and SeS2). As a result, depending on the sulfur source employed, devices with superficially localized maximum sulfur content between 50 and 20% within the charge depletion zone and between 10 and 30% toward the bulk material are obtained. This complex graded structure is confirmed and characterized by combining multiwavelength depth-resolved Raman spectroscopy measurements together with in-depth Auger electron spectroscopy and X-ray fluorescence. In addition, the devices fabricated with such graded band gap absorbers are shown to be fully functional with conversion efficiencies around 9% and with improved VOC deficit values that correlate with the presence of a gradient. These results represent one step forward toward anionic band gap grading in kesterite solar cells.

Resonant Raman scattering based approaches for the quantitative assessment of nanometric ZnMgO layers in high efficiency chalcogenide solar cells.

This work reports a detailed resonant Raman scattering analysis of ZnMgO solid solution nanometric layers that are being developed for high efficiency chalcogenide solar cells. This includes layers with thicknesses below 100 nm and compositions corresponding to Zn/(Zn + Mg) content rations in the range between 0% and 30%. The vibrational characterization of the layers grown with different compositions and thicknesses has allowed deepening in the knowledge of the sensitivity of the different Raman spectral features on the characteristics of the layers, corroborating the viability of resonant Raman scattering based techniques for their non-destructive quantitative assessment. This has included a deeper analysis of different experimental approaches for the quantitative assessment of the layer thickness, based on (a) the analysis of the intensity of the ZnMgO main Raman peak; (b) the evaluation of the changes of the intensity of the main Raman peak from the subjacent layer located below the ZnMgO one; and (c) the study of the changes in the relative intensity of the first to second/third order ZnMgO peaks. In all these cases, the implications related to the presence of quantum confinement effects in the nanocrystalline layers grown with different thicknesses have been discussed and evaluated.

ZnSe etching of Zn-rich Cu2ZnSnSe4: an oxidation route for improved solar-cell efficiency.

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.