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

At the heart of this challenge is the complex chemistry and physics of multinary semiconductor materials like CZTSSe. Unlike simpler binary or ternary compounds, these materials consist of four or more elements whose interactions determine critical properties such as bandgap, carrier mobility, and defect formation. Consequently, the synthesis routes and formation pathways exert profound influence on the ultimate device performance. Recent advances have spotlighted the synthesis stage, particularly the design and use of molecular inks, as a pivotal aspect of kesterite fabrication that can unlock higher efficiencies.

Molecular inks are precursor solutions containing metal complexes and chalcogen sources that, upon deposition and thermal processing, form the kesterite thin film. This approach enables finer control over elemental distribution and uniformity at the nanoscale, which is indispensable for producing defect-minimized absorber layers. By tailoring the chemical state of these inks—through the choice of ligands, solvent environment, and precursor ratios—researchers can influence nucleation dynamics and crystallization pathways. This precise control mitigates the formation of detrimental point and extended defects, which historically limited photovoltaic performance by acting as recombination centers.

One of the notable breakthroughs reported in recent research is the crossing of the 15% efficiency threshold using molecular ink-based synthesis. This milestone signifies a critical step towards making kesterite solar cells viable competitors to established thin-film technologies like CdTe and CIGS. Achieving this level of performance required not only optimization of the ink chemistry but also a deep understanding of the post-deposition annealing and crystallization kinetics. Controlling these parameters allowed for the deliberate engineering of grain boundaries and the reduction of secondary phases, which often impair charge transport and extraction.

A central focus of the latest studies centers on defect chemistry in CZTSSe films. Unlike single-element semiconductors, multinary compounds are prone to complex defect configurations due to their multiple constituent atoms. The interplay between copper, zinc, tin, sulfur, and selenium can generate intrinsic defects that act as electron or hole traps. The molecular ink strategy aids in managing this complexity by ensuring homogeneous precursor mixing and facilitating optimal stoichiometry control. Such advancements directly translate to improved open-circuit voltage (Voc) and fill factor (FF) metrics in finished solar cells.

The synthesis temperature and atmosphere also play decisive roles in the quality of the kesterite absorber layers. High-temperature annealing under controlled environments promotes grain growth and defect passivation but can also risk the evaporation or segregation of volatile components. Fine-tuning these conditions in combination with molecular ink chemistry has allowed researchers to circumvent these drawbacks, preserving the desirable phase purity and enhancing device stability. Understanding these thermodynamic and kinetic processes at a granular level is vital for replicating laboratory successes at industry-relevant scales.

Furthermore, the use of molecular inks paves the way for low-cost, scalable fabrication techniques compatible with large-area substrates and roll-to-roll manufacturing. This aspect is critical for the commercial viability of kesterite photovoltaics, as it promises the reduction of material wastage and energy input during synthesis. Compared to vacuum-based deposition techniques common in other thin-film photovoltaics, ink-based methods present an attractive alternative that aligns with sustainable manufacturing goals.

The evolution of CZTSSe solar cells is also marked by the integration of sophisticated characterization tools that elucidate the material’s microstructure and electronic properties. Techniques such as time-resolved photoluminescence, scanning transmission electron microscopy, and X-ray diffraction mapping provide insights into defect distribution, phase segregation, and carrier dynamics. These analyses have been instrumental in refining molecular ink formulations and processing protocols, leading to solar cells with enhanced electron lifetimes and mobility.

Future directions in kesterite research, inspired by the molecular ink paradigm, include the exploration of novel ligands and solvent systems that further improve precursor solubility and stability. Some efforts are focused on incorporating additives that passivate defects or promote preferential crystallographic orientations to improve charge transport. Additionally, the development of multi-step annealing and selenization processes tailored to the ink chemistry offers pathways to engineer absorber layers with superior optoelectronic quality.

Moreover, understanding the fundamental thermodynamic principles governing the formation of secondary phases remains a critical research area. Unwanted phases such as ZnSe, Cu₂SnSe₃, or SnS can both consume active materials and create electronic barriers at interfaces. Molecular ink strategies enable dynamic compositional adjustments during synthesis, potentially minimizing these phases and optimizing absorber homogeneity. This fine balance between precursor chemistry and final film properties is key to pushing efficiencies beyond the current limits.

Beyond photovoltaic applications, the insights gained from the study of molecular ink chemistry and formation pathways in multinary semiconductors have broader implications. Similar methodologies could be applied to other emerging materials systems for optoelectronics, thermoelectrics, or photocatalysis. The foundational understanding of how precursor chemistry influences crystallization and defect landscapes could accelerate the discovery and optimization of materials with complex elemental compositions.

In conclusion, the breakthrough achievements in kesterite solar cells owe much to the meticulous control over precursor chemistry afforded by molecular inks. This synthesis pathway offers a robust platform for addressing the longstanding challenges in CZTSSe photovoltaic technology, including defect mitigation, phase purity, and large-scale manufacturability. As research continues to harness these advantages, the prospect of affordable, efficient, and environmentally benign solar energy conversion via kesterite cells appears increasingly within reach.

The path forward involves not only continued refinement of molecular ink formulations but also innovative device architectures and interface engineering to maximize power conversion efficiencies. Coupling these advances with computational modeling and machine learning could further accelerate the optimization process, tailoring synthesis parameters for custom applications. The confluence of chemistry, materials science, and engineering in this interdisciplinary effort is emblematic of the future of sustainable energy research.

Ultimately, the story of kesterite solar cells exemplifies how fundamental chemistry and careful materials design converge to solve complex technological challenges. As these solar cells edge closer to commercial viability, their success will represent a triumph of both scientific ingenuity and practical innovation, enabling a cleaner energy future powered by Earth-abundant materials.

Subject of Research: Synthesis and formation pathways of high-efficiency kesterite solar cells through molecular ink chemistry.

Article Title: Formation pathway of high-efficiency kesterite solar cells fabricated through molecular ink chemistry.

Article References:
Jimenez-Arguijo, A., Gong, Y., Caño, I. et al. Formation pathway of high-efficiency kesterite solar cells fabricated through molecular ink chemistry. Nat Energy (2026). https://doi.org/10.1038/s41560-025-01900-y

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41560-025-01900-y