The appeal of CFTS lies in its chemistry. Unlike CdTe, the current thin-film efficiency champion, CFTS contains no toxic cadmium, and unlike CIGS it requires no indium or gallium, elements so scarce in the Earth’s crust that they impose hard limits on terawatt-scale deployment. Every constituent of CFTS is plentiful: copper sits at roughly 50 parts per million in the crust, iron at a staggering 41,000 ppm, tin at 2.2 ppm, and sulfur at 260 ppm. The compound also brings strong intrinsic photovoltaic credentials, including an absorption coefficient above 10^4 per centimeter, an electron affinity of 3.3 electronvolts, and a bandgap near 1.6 electronvolts, close to the single-junction optimum. Yet despite years of study, experimental CFTS devices have remained stubbornly below the roughly 30 percent Shockley-Queisser limit, which is exactly why the authors turned to systematic simulation calibrated against real devices.

The experimental side of the work relied on chemical spray pyrolysis, one of the cheapest and most scalable thin-film deposition techniques in the photovoltaic toolbox. The researchers first sprayed a 100-nanometer CdS window layer onto fluorine-doped tin oxide glass at 300 degrees Celsius, then deposited p-type CFTS absorber layers of 245 and 510 nanometers at 250 degrees Celsius using a precursor solution with a Cu:Fe:Sn:S ratio of 2:1:1:5.2. Silver paste contacts completed the FTO/n-CdS/p-CFTS/Ag stack. X-ray analysis confirmed a tetragonal stannite phase for CFTS with an average crystallite size of about 18 nanometers and a hexagonal phase for CdS, while optical measurements yielded bandgaps of 1.54 and 2.44 electronvolts respectively, values well matched for efficient heterojunction operation.

When illuminated under a tungsten lamp at 20 milliwatts per square centimeter, the thicker 510-nanometer device clearly outperformed its thinner sibling. The 245-nanometer cell delivered an efficiency of 1.92 percent with an open-circuit voltage of 0.569 volts, a short-circuit current density of 1.28 milliamperes per square centimeter, and a fill factor of 53 percent. Doubling the absorber thickness to 510 nanometers lifted the efficiency to 5.23 percent, with the open-circuit voltage rising to 0.755 volts, the current to 2.5 milliamperes per square centimeter, and the fill factor to 56 percent. The physics behind the improvement is straightforward: a thicker absorber intercepts more photons, generates more electron-hole pairs, and gives carriers a longer diffusion path before they recombine, so more of them survive to contribute to photocurrent.

To understand how far this material system could ultimately go, the team turned to SCAPS-1D, a one-dimensional solar cell simulator developed at Ghent University that solves Poisson’s equation together with the drift-diffusion and continuity equations for electrons and holes. The software is particularly well suited to thin-film heterojunctions and has become a workhorse for modeling CIGS, CZTS, CdTe, and perovskite devices. What distinguishes this study is its grounding: the simulation input parameters were drawn from the team’s own structural, optical, and electrical characterization of spray-deposited films, and an interfacial layer between CFTS and CdS was explicitly modeled to capture the physics of the real junction. The authors also note an important caveat in comparing their numbers: the fabricated cells were measured at 20 milliwatts per square centimeter due to equipment constraints, while the simulations used the standard AM1.5G spectrum at 1000 watts per square meter, a fivefold difference in power density that naturally inflates simulated current and efficiency relative to the laboratory baseline.

The first and most consequential optimization concerned absorber thickness. Sweeping the CFTS layer from 0.5 to 6.0 micrometers, the team found that efficiency, open-circuit voltage, current density, and fill factor all climbed steadily up to 5.0 micrometers, reaching 23.53 percent, 1.32 volts, 24.27 milliamperes per square centimeter, and 73.28 percent respectively, before saturating. Beyond that point, additional thickness adds parasitic resistance and recombination without meaningful gains in absorption. The optimal 5.0-micrometer value represents a balance between photon capture and carrier collection, and although it is roughly ten times thicker than the experimental films, the authors frame it as a theoretical benchmark that future spray-pyrolysis work must approach through optimized deposition cycles while managing mechanical stress and processing time.

Bandgap and doping proved equally decisive. Varying the CFTS bandgap from 1.50 to 1.70 electronvolts, with electron affinity held fixed in accordance with the common anion rule for sulfide semiconductors, raised the open-circuit voltage and fill factor but depressed the current and overall efficiency, because a wider gap forfeits long-wavelength photons. A bandgap of 1.525 electronvolts emerged as the optimum. Raising the shallow acceptor density in the absorber from 10^18 to 10^19 per cubic centimeter strengthened the built-in electric field at the junction, boosting efficiency from 20.85 to 23.93 percent by improving the separation and transport of photogenerated carriers, with only a negligible effect on short-circuit current. Intriguingly, the CdS electron transport layer proved largely insensitive to its own thickness, bandgap, and donor density; the team settled on 0.2 micrometers and a donor density of 10^18 per cubic centimeter as pragmatic, cost-conscious choices.

Contour mapping of paired parameter sweeps added further nuance. The highest open-circuit voltage of 1.34 volts appeared with the thinnest CdS window and the thickest absorber, since a thicker CdS layer partially absorbs light and erodes voltage, while current density saturated beyond roughly 4 micrometers of absorber regardless of window thickness. The parasitic resistances told a sharper story: series resistance, which the experiments measured at 99 to 105 ohm-square-centimeters, must fall to around 4 ohm-square-centimeters to unlock high performance, while shunt resistance in the 500 to 1000 ohm-square-centimeter range proved adequate. Temperature simulations from 300 to 400 kelvin showed the fill factor actually improving with heat, from 72.78 to 83.69 percent, while the open-circuit voltage declined as recombination accelerated, with efficiency peaking at 24.73 percent near 360 kelvin before falling.

With all parameters tuned, the final simulated device achieved an efficiency of 25.52 percent, an open-circuit voltage of 1.17 volts, a short-circuit current density of 26.89 milliamperes per square centimeter, and a fill factor of 81.25 percent, comfortably above the 19.28 percent benchmark previously reported for CFTS-based cells and competitive with recent numerical results for related quaternary sulfide and perovskite-derived systems. The quantum efficiency curve showed strong response across wavelengths shorter than 780 nanometers, with the gently curved edges attributable to front and back surface recombination. The authors are candid about the model’s limits, however: grain boundary recombination in polycrystalline spray-deposited films, possible secondary phases such as copper sulfide or tin sulfide that create shunt paths, and non-uniform thickness and defect distributions all fall outside what a one-dimensional model can capture.

Even so, the study delivers something the field has lacked: a parameter-by-parameter bridge from a real, cheaply made 5.23 percent device to a 25.52 percent theoretical design, with the dominant losses identified as excessive series resistance, interface recombination, and an absorber far too thin to harvest the full spectrum. For manufacturers weighing the next generation of thin-film photovoltaics, the message is that CFTS deserves renewed attention precisely because its raw materials are cheap, abundant, and benign, and because the losses separating today’s devices from state-of-the-art performance are engineering problems, not fundamental ones. If experimental teams can thicken their absorbers, tame their contact resistances, and suppress interfacial defects along the lines this simulation prescribes, earth-abundant CFTS could graduate from laboratory curiosity to a serious contender in the global race for affordable solar energy.

Subject of Research: Numerical and experimental optimization of earth-abundant CFTS/CdS thin-film heterojunction solar cells using SCAPS-1D

Article Title: Numerical investigation and experimental validation of CFTS/CdS heterojunction solar cells via structural and material parameter optimization using SCAPS-1D

Article References: Yadav, A. A., Londhe, R. R., & Singh, V. N. (2026). Numerical investigation and experimental validation of CFTS/CdS heterojunction solar cells via structural and material parameter optimization using SCAPS-1D. Discover Electrochemistry, 3(1), Article 31. https://doi.org/10.1007/s44373-026-00119-0

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00119-0

Keywords: CFTS, solar cells, SCAPS-1D, thin-film photovoltaics, spray pyrolysis, CdS buffer layer, earth-abundant materials, heterojunction, power conversion efficiency, chalcogenide semiconductors, absorber thickness, carrier concentration