Innovative and sustainable solar cells based on abundant elements on the earth’s crust

Currently, thin-film technology facilitates the production of high-efficiency solar cells at costs that are competitive with traditional Silicon technology. However, the development of these devices has largely overlooked the abundance of constituent elements found in the Earth’s crust. For instance, Indium, Gallium, and Tellurium, are considered rare elements, as noted by the European Community. To address this concern, researchers are investigating new compounds made of abundant elements, aiming to create solar cells that are sustainable in the long term. The literature features studies on many innovative materials including WSe3, Cu2ZnSnSe4, SnSe2, Sb2Se3, and FeS2, which are being explored as absorber layers in thin-film solar cells. In particular, antimony selenide (Sb2Se3) is emerging as one of the most promising candidate materials for use as an absorber for thin-film solar cells, thanks to its non-toxic nature and the abundance of its constituent elements in the Earth’s crust. Sb2Se3 exhibits exceptional properties, including an optimal energy gap that theoretically allows a maximum efficiency of 33%, alongside a high absorption coefficient in the visible spectrum. In recent years, significant advancements have been made, leading to a maximum photovoltaic conversion efficiency (PCE) exceeding 10%. However, this is still considerably below its theoretical limit. The limitations in PCE can be attributed to several factors, particularly the high degree of anisotropy exhibited by this material. Key challenges include the difficulty in controlling the growth of Sb2Se3 grains along the crystallographic direction necessary for optimal charge transport, the interaction and band-offset with different n-type partners depending on whether the cell architecture is substrate- or superstrate-based, and the challenge of achieving a back-contact that fulfills the requirements for ohmicity. In this PhD thesis, an effort was made to the primary limiting factors affecting the ability to surpass the photovoltaic efficiency record by examining Sb2Se3-based solar cells configured in a superstrate arrangement. From a crystallographic perspective, the structural characterization of Sb2Se3 films revealed that their crystalline quality and preferred orientation are significantly influenced by the choice of window layer, which act as substrate for the Sb2Se3 growth. To gain a deeper understanding of the growth mechanism, Sb2Se3 thin films were deposited via close-spaced sublimation (CSS) onto five different window layers: CdS, CdS:F, CdSe, As2S3, and ZnCdS. Solar cells based on Sb2Se3, fabricated in a superstrate configuration using these various substrates, clearly demonstrate the impact of the preferential orientation of Sb2Se3 on photovoltaic performance. The synthesis of antimony selenide can be accomplished using various low-cost techniques that are easily scalable for industrial applications, such as Chemical Bath Deposition, Close-Spaced Sublimation, Vapor Transport Deposition, Rapid Thermal Evaporation, Ion Vapor Deposition, and Radio-Frequency Magnetron Sputtering. This versatility is largely attributed to the absence of polymorphism and a relatively low melting point (885 K). However, once the substrate is established, the growth of Sb2Se3 is significantly influenced by its propensity to form defects and stoichiometric deviations. This thesis includes an in-depth study on the CSS growth of Sb2Se3. In particular, it has been demonstrated how the Ar counterpressure introduced into the growth chamber has a substantial impact on the material’s stoichiometry and preferred growth orientation. Another critical factor in surpassing the 10% PCE threshold is the exploration of various materials to achieve an ohmic contact with low resistivity, for use as a back contact. This thesis proposes an innovative compound based on Fe, S, and O elements. Given the extreme abundance of these elements in the Earth’s crust and their non-toxicity nature, the synthesis technique used is also straightforward. Specifically, the Fe-S-O thin film is deposited at room temperature using radio-frequency magnetron sputtering. XRD and Raman analyses revealed that the material is composed of two phases: Fe3O4 in both orthorhombic and cubic forms, and FeS in the troilite phase. This material establishes an effective ohmic contact on the antimony selenide thin film, yielding a contact resistivity of 0.8 Ω⋅cm², as determined from the current-voltage characteristics of fully assembled Sb2Se3-based solar cells. After three months of monitoring the photovoltaic parameters, a negligible average variation was observed. Another important factor, related to the CBO between the n-type window material and antimony selenide, is the phenomenon known as the Voc deficit. Indeed, literature reports indicate that solar cell consistently exhibit an open-circuit voltage significantly lower than the theoretical. In this thesis, a comprehensive analysis of an Sb2Se3 solar cell in the typical superstrate configuration was conducted. By investigating of current density-voltage characteristics as functions of temperature and wavelength, along with capacitance-voltage measurements and admittance spectroscopy, we attribute the observed low open-circuit voltage to the presence of a potential barrier within the absorber material particularly near the junction interface. In conclusion, this thesis presents a comprehensive investigation into the realization and performance optimization of Sb2Se3-based solar cells. Emphasizing low-cost and scalable techniques such as Close-Spaced Sublimation and sputtering, we explore the influence of growth conditions, including argon counterpressure, on material stoichiometry and preferred crystallographic orientation. The development of an innovative Fe-S-O compound as a back contact material demonstrates significant promise, achieving an ohmic contact with low resistivity and maintaining stable photovoltaic parameters over time. Furthermore, our analysis reveals the critical role of the conduction band offset and the associated Voc deficit in limiting the open-circuit voltage of Sb2Se3 solar cells. By elucidating the underlying mechanisms affecting device performance, this work contributes valuable insights toward enhancing the efficiency of Sb2Se3-based solar technologies, paving the way for future advancements in the field.