# Tunable Bandgap in Cobalt-Doped FeS2 Thin Films for Enhanced Solar Cell Performance

## TL;DR

Cobalt-doped FeS2 thin films with tunable bandgaps and improved solar cell performance were created using chemical bath deposition and annealing.

## Contribution

The study introduces a novel method to synthesize Co-doped FeS2 thin films with tunable bandgaps and enhanced optoelectronic properties.

## Key findings

- Cobalt-doped FeS2 thin films showed strong visible and near-infrared absorption and tunable direct bandgaps (1.14 to 0.96 eV).
- Annealing increased conductivity by four orders of magnitude and stabilized p-type behavior via Co2+ acceptor states.
- The films exhibited compact quasi-spherical nanoparticle morphologies and improved crystallinity after annealing.

## Abstract

Cobalt-doped iron disulfide (FeS2) thin films were synthesized via chemical bath deposition (CBD) followed by annealing at 450 °C, yielding phase-pure pyrite structures with multifunctional properties. A deposition temperature of 95 °C is critical for promoting Co incorporation, suppressing sulphur vacancies, and achieving structural stabilization of the film. After annealing, the dendritic morphologies transformed into compact quasi-spherical nanoparticles (~100 nm), which enhanced the crystallinity and optoelectronic performance of the films. The films exhibited strong absorption (>50%) in the visible and near-infrared regions and tunable direct bandgaps (1.14 to 0.96 eV, within the optimal range for single-junction solar cells. Electrical characterization revealed a fourth-order increase in conductivity after annealing (up to 4.78 Ω−1 cm−1) and confirmed stable p-type behavior associated with Co2+-induced acceptor states and defect passivation. These results demonstrate that CBD enabled the fabrication of Co-doped FeS2 thin films with synergistic structural, electrical, and optical properties. The integration of earth-abundant elements and tunable electronic properties makes these films promising absorber materials for the next-generation photovoltaic devices.