Fabrication and Characterization of Mo-Free Cu2ZnSn(S, Se) 2 Solar Cells from Elemental Precursors: The Role of Selenium Addition

Technical Summary: Fabrication and Characterization of Mo-Free Cu2ZnSn(S, Se)2 Solar Cells from Elemental Precursors

Problem Statement
The global energy landscape remains heavily dependent on fossil fuels, necessitating a shift toward renewable sources like solar energy. While silicon-based solar cells dominate the market, third-generation thin-film technologies using abundant, non-toxic elements offer a sustainable alternative. Specifically, Kesterite Cu2ZnSn(S,Se)2 (CZTS) is a promising absorber material due to its optimal bandgap (1.0–1.5 eV) and high absorption coefficient. However, the commercialization of CZTS solar cells faces challenges, including the reliance on expensive or rare materials (such as Molybdenum for back contacts and Indium/Gallium for other thin films) and the complexity of vacuum-based fabrication processes. Furthermore, existing CZTS devices often suffer from low efficiency due to secondary phases, recombination losses, and difficulties in forming high-quality p-n junctions. This research addresses the need for a low-cost, non-vacuum fabrication method using locally sourced elemental precursors to produce Molybdenum-free (Mo-free) CZTS solar cells with improved performance.

Methodology
The study employed a solution-process (sol-gel) method to fabricate solar cells with a novel structure: ITO/TiO2/Cu2ZnSn(S, Se)2/HTM/Pt. This design eliminates the need for a Molybdenum back contact and a toxic Cadmium Sulfide (CdS) buffer layer, replacing the latter with a Hole-Transport Material (HTM).

• Precursors: Four samples were synthesized using two types of precursors:
  • Local (L): Elemental powders of Copper (Cu), Zinc (Zn), Tin (Sn), and Sulfur (S).
  • Standard (N): Chemical salts (e.g., Cu(CH3CO2)2·H2O, Zn(CH3CO2)2·2H2O, SnCl2·2H2O, SC(NH2)2) and Selenium (Se).
  • Selenium was added to create CZTSSe variants to investigate the role of Se addition.
• Fabrication Process:
  1. Absorber Synthesis: Precursors were dissolved in 2-methoxyethanol, heated, and filtered. Trimethylamine (TMA) and monoethylamine (MEA) were added as surfactants.
  2. Deposition: The absorber solution was spin-coated onto TiO2/ITO substrates, preheated at 170°C, and calcined at 500°C for 30 minutes in an ambient atmosphere (without H2S sulfurization).
  3. Cell Assembly: A quasi-solid-state electrolyte (HTM) was applied, followed by a Platinum (Pt) counter electrode.
• Characterization: The study utilized X-ray Diffraction (XRD) with the Reference Intensity Ratio (RiR) method to analyze crystallinity and secondary phases; Scanning Electron Microscopy (SEM) for morphology; UV-Vis spectroscopy for optical properties (bandgap, absorbance); and J-V measurements under AM 1.5 illumination to determine photovoltaic performance.

Key Contributions

• Novel Device Architecture: This work reports the first fabrication of a complete solar cell structure (ITO/TiO2/CZTSSe/HTM/Pt) using a Mo-free conductive layer and a Cd-free buffer layer, fabricated entirely via non-vacuum solution processing.
• Local Material Utilization: The study demonstrates the feasibility of using locally sourced elemental powders (Cu, Zn, Sn, S) from Indonesia as viable precursors for high-performance solar absorbers, reducing dependence on imported materials.
• Se Addition Analysis: The research systematically evaluates the impact of Selenium addition on crystallinity, morphology, and electronic properties, comparing local elemental precursors against standard chemical precursors.

Results

• Crystallinity and Phase Purity: All samples exhibited crystallinity between 17.40% and 31.97%, with the remainder being amorphous due to the lack of H2S sulfurization. The CZTSSe N (Standard + Se) sample showed the lowest secondary phase percentage (16.2%) and the smallest deviation (16.0% error) from the ideal non-stoichiometric Cu-poor/Zn-rich reference ratio (Cu2.1Zn1.25SnS2Se2).
• Morphology: The addition of Selenium significantly reduced grain size. For standard precursors, grain size decreased from 32.8 nm (CZTS N) to 10.7 nm (CZTSSe N). For local precursors, it decreased from 195.9 nm to 98.1 nm. The smaller grain size in CZTSSe N facilitated better adsorption onto the porous TiO2 surface.
• Optical Properties: Selenium addition caused a redshift in absorption peaks and reduced the bandgap (Eg) by 0.08–0.28 eV. CZTSSe N achieved the smallest bandgap of 1.16 eV. Consequently, it exhibited the highest Light Harvesting Efficiency (LHE) of 88.12% across the 200–1100 nm range.
• Photovoltaic Performance:
  • CZTSSe N achieved the highest power conversion efficiency (0.166%) among all samples, driven by the highest Open Circuit Voltage (Voc = 0.42 V) and Fill Factor (FF = 50.5%).
  • CZTS L (Local, no Se) produced the highest Short Circuit Current Density (Jsc = 0.972 mA/cm²), attributed to low secondary phases of Cu2S and ZnS.
  • The efficiency of the local material-based CZTSSe L (0.153%) was noted to be close (92%) to that of the standard CZTSSe N.

Significance and Claims
The paper claims that the successful synthesis of Mo-free CZTSSe solar cells using local elemental precursors validates a pathway toward low-cost, environmentally friendly solar energy production in Indonesia. The study highlights that while the absolute efficiency (0.166%) is currently low compared to commercial standards, the CZTSSe N sample demonstrates the most favorable electronic and structural properties (smallest bandgap, lowest secondary phase, highest LHE) due to the synergistic effect of Selenium addition and precise stoichiometric control.

The authors emphasize that the primary limitations of the current study are the high amorphous content resulting from the absence of H2S sulfurization and the formation of secondary phases (Cu2Sn(S/Se)3, Sn(S/Se)) which hinder electron transfer. The work serves as a foundational step, proving that locally sourced materials can form functional kesterite structures and that optimizing the synthesis parameters (such as sulfurization and precursor mixing) could significantly enhance future performance. The research contributes to the development of independent solar cell manufacturing capabilities in Indonesia, utilizing abundant local mineral resources.