Polycrystalline semiconductor nanofilm electrodes are emerging as easy-to-prepare, low-cost and environmentally-friendly alternatives to conventional photovoltaic solar cells. Despite these advantages, nanofilm electrodes still suffer shortcomings such as instability to photo-corrosion and low cell performance. Various strategies have been proposed to enhance solar cell performance. Heterojunction solar cells were reported [1]. Ternary and quaternary compounds were examined, such as CuInGaS (CIGS) materials which showed promising future, but unfortunately, they involve high cost and questionable constituents such as In and Ga. Attempts to lower cost and save environment, by minimizing the CIGS layer thickness, have been investigated. In some studies, the CuZnSnSSe (CZTSSe) material was proposed as an additional layer to a very thin CIGS layer [2]. Fortunately, the low cost and environmentally friendly CZTSSe layer behaved as a complementary absorber layer, and together with the thin CIGS layer gave improved absorbance. Researchers also examined tandem cells involving CIGS nanofilm electrodes, using two (or more) cells together. In these systems, the top sub-cell involved a wider band gap material to absorb at short wavelengths, while the bottom sub-cell involved narrower materials. In these systems, it was possible to absorb incident radiations of wide range wavelengths. High performance cells were studied, but this area is still under active research aiming at increased sort circuit photocurrent (J_SC) and open circuit photovoltage (V_OC) [3]. Other areas of film electrodes involve dye sensitized solar cells (DSSCs), where wide band gap semiconductor materials are coated with dyes to absorb in the visible range. Tremendous publications were made on various aspects of DSSCs, by varying the semiconductor (using TiO₂, ZnO, ZnTiO₃, and other materials), the dye (metal-based dyes, metal-free dyes and natural dyes) and the counter electrodes [4]. Other nanofilm electrode, such as metal chalcogenides (MX: M is Cd, Cu, Zn; X is S, Se, Te or even O).    In metal chalcogenides, with middle to narrow band gap values (1.5 - 2.5 eV), a number of simple methods were attempted.  The first method involved careful choice of annealing temperature that suits the type of the film material, followed by careful choice of cooling rate. Based on earlier reports, semiconductors with medium band gap values, that suit visible solar light, may not be thermodynamically stable. Method of annealing, as continuous vs. pulse heating, should be examined.  Annealing enhances semiconductor film properties (crystallinity, particle size, sintering, carrier mobility and overall cell performance) as annealing lowers crystallite imperfections and brings metastable atoms back to stable positions [5-10]. Cooling rate is another strategy to improve the solar cell performance. Slow cooling may be needed to improve the semiconductor characteristics. However, if annealed at higher temperatures, the film should be brought back to room temperature rapidly or quenched. Our earlier results in the area will be discussed. Suitability of these methods to CIGS/CTZSSe and to DSSCs is under investigation and will be mentioned in this presentation. In the third strategy, charged ions are attached to the semiconductor surface, at the interface between the film electrode and the solution redox couple. Depending on their density, the charged species affected the flat band potential by up to 300 mV, and behaved as charge transfer catalysts at the interface. The short circuit current was thus improved. Moreover, quick release of holes in the space charge layer and stabilized the n-semiconductor surface to corrosion. All such advantages can be gained by attaching the proper electro-active materials to the proper SC electrode. Combining annealing temperature and cooling rate control, with attaching charge species, has been examined here.  Conversion efficiency values of 4.4, 8.0, 15.0 and 18.0% have been observed from CdSe, CdTe, CuS and CuSe film electrodes, respectively. Such values have not been reported for pristine metal chalcogenide film electrodes before. Applying these strategies to other film electrode soar cells, such as tandem cells and DSSCs will be discussed in this presentation, which will highlight a critical survey of results observed throughout the last 25 years, as compared to other literature. A new model to explain the efficiency and stability enhancement will also be rigorously presented.  Future prospects of this work will also be discussed.

Conference Title

New Perspectives in solid-state and materials research - from fundamentals to applications -

Conference Country

Germany

Conference Date

Feb. 3, 2025 - Feb. 5, 2025

Conference Sponsor

Max-Planck Institute

Additional Info

Conference Website