Central to this study is the precise determination of carrier concentrations within the Sb2(S,Se)3 absorber layers, a critical factor governing device performance. By deploying capacitance–voltage (C–V) profiling under variable voltage biases, the researchers established depletion widths of 317 nm for the conventional thermal (CT) sample and 262 nm for the sodium sulfide (SS) treated sample at zero bias. Intriguingly, these measured depletion widths corroborate well with the physical thicknesses observed via cross-sectional transmission electron microscopy (TEM), providing direct evidence that the absorber layers in both samples are entirely depleted. This insight is pivotal as full depletion ensures that the electrical characteristics measured genuinely represent the intrinsic properties of the absorber material rather than extrinsic interface effects.

Pushing the boundaries further, the experimental strategy incorporated the application of a positive bias (0.4 V) during C–V studies, deliberately shifting the depletion boundary deeper into the bulk absorber. This approach allowed the extraction of a more representative net carrier concentration intrinsic to the Sb2(S,Se)3 bulk material. Notably, the SS sample exhibited a substantial increase in carrier density, rising from 4.83 × 10^16 cm^−3 in the CT counterpart to 1.72 × 10^17 cm^−3. The augmentation is attributed to the effective suppression of deep-level defects facilitated by the sodium sulfide additive, which mitigates charge trapping and non-radiative recombination pathways that are notoriously detrimental to charge carrier lifetimes and transport.

This enhanced carrier profile directly translates to a decrease in the series resistance of the Sb2(S,Se)3 layer, fundamentally improving the hole transport dynamics within the absorber. The sodium sulfide treatment also eradicates an unfavorable valence band maximum (VBM) gradient, effectively flattening the band alignment and eliminating potential barriers that impede carrier flow. Collectively, these factors catalyze a remarkable uplift in the fill factor (FF) from 66.09% in CT devices to 69.02% in SS devices, underscoring the critical role of intrinsic material properties in optimizing photovoltaic performance.

Supplementing the electrical measurements, external quantum efficiency (EQE) spectra reveal nuanced improvements across the solar spectrum. At shorter wavelengths—where photons primarily generate carriers close to the CdS/Sb2(S,Se)3 interface—the elevated hole collection efficiency in SS devices emphasizes the beneficial impact of the improved band alignment and reduced bulk defects. Since holes act as minority carriers within the n-type Sb2(S,Se)3 absorber, their ability to traverse the entirety of the absorber layer without recombination is paramount. The accomplished near-interface control, combined with bulk defect passivation, substantially raises the EQE response.

Equally striking is the enhancement at longer wavelengths, where photons penetrate deeply into the absorber layer. Here, the suppression of deep-level recombination centers diminishes carrier losses, allowing more photogenerated electrons and holes to contribute to the photocurrent. The integrated short-circuit current density (Jsc) ascended from 22.35 mA cm^−2 in CT devices to an impressive 24.54 mA cm^−2 in SS devices, consistent with the Jsc values measured under standard AM 1.5G illumination conditions. This coherence between EQE integration and direct current–voltage (J–V) characterization affirms the reliability and reproducibility of the device metrics.

Importantly, while the Sb2(S,Se)3 absorber layer in SS devices is marginally thinner—an alteration typically associated with some voltage penalty—the enhanced material quality and improved electrical properties counterbalance this effect. As a result, the open-circuit voltage (Voc) exhibits negligible detriment, enabling the overall efficiency to attain a new milestone with a certified PCE exceeding 10%. This breakthrough is attributed primarily to the precise tuning of hydrothermal reaction kinetics via sodium sulfide, which refines the crystallinity, stoichiometry, and interface energetics of the absorber layer.

The implications of this research extend beyond efficiency metrics alone. The batch fabrication and statistical analysis of multiple devices revealed consistent trends across Voc, Jsc, FF, and PCE parameters, underscoring the repeatability and scalability of the sodium sulfide additive strategy. Such reproducibility addresses a central challenge in thin-film solar cell manufacturing—device-to-device variability—and hints at viable pathways toward commercial viability.

Fundamentally, this work illuminates the intricate interplay between material synthesis conditions, electronic properties, and device performance. By regulating the hydrothermal reaction environment, the team successfully reduced the density of sub-bandgap defect states—long considered a barrier to high-efficiency antimony chalcogenide photovoltaics. These defects serve as non-radiative recombination centers, trapping charges and hampering transport. Their suppression translates into lower series resistance and enhanced charge carrier mobility, culminating in devices with superior fill factors and quantum efficiency.

Moreover, the elimination of a detrimental valence band offset mitigates energetic barriers at critical interfaces, facilitating more effective hole extraction. This modification is particularly significant given the multi-layered nature of the devices, where interfacial band alignment dictates carrier dynamics and overall diode behavior. The work thus underscores the necessity of holistic optimization, encompassing bulk absorber quality and interface engineering.

This technical progression arrives within a broader context of the quest for cost-effective, earth-abundant, and scalable photovoltaic technologies. Sb2(S,Se)3—composed of environmentally benign constituents—has emerged as a promising candidate. However, the pathway to commercialization has been impeded by challenges in controlling defect chemistry and maintaining suitable band alignment under practical fabrication conditions. The sodium sulfide approach offers a compelling solution, combining straightforward chemical modulation with profound material benefits.

Beyond the immediate technological impact, this research is emblematic of advanced characterization techniques driving deep insights into photovoltaic material behavior. The adept use of capacitance–voltage profiling coupled with cross-sectional microscopy bridges structural and electronic perspectives, facilitating a comprehensive understanding of absorber layer dynamics under operational biases.

Equally, the correlation between device performance improvements and meticulous modification of synthesis chemistry emphasizes the potential of chemical additives as powerful levers in photovoltaic research. This approach transcends mere doping—it reflects a nuanced control over nucleation, growth, and defect passivation mechanisms during the hydrothermal process, delivering a paradigm for future exploration across a variety of chalcogenide and emerging photovoltaic materials.

In conclusion, the introduction of sodium sulfide into the hydrothermal synthesis of Sb2(S,Se)3 solar cells heralds a new era of efficiency and device stability, reaching a certified PCE of 10.7%. This work not only motivates further research into additive-driven defect management but also charts a path toward the scalable production of high-performance, environmentally sustainable solar absorbers. As the global demand for renewable energy escalates, advances of this caliber embolden the prospect of affordable, efficient solar electricity generation rooted in innovative materials science and chemistry.

Subject of Research:
Photovoltaic performance enhancement of Sb2(S,Se)3 solar cells through hydrothermal synthesis modification using sodium sulfide additives.

Article Title:
Regulation of hydrothermal reaction kinetics with sodium sulfide for certified 10.7% efficiency Sb2(S,Se)3 solar cells.

Article References:
Qian, C., Sun, K., Huang, J. et al. Regulation of hydrothermal reaction kinetics with sodium sulfide for certified 10.7% efficiency Sb2(S,Se)3 solar cells. Nat Energy (2026). https://doi.org/10.1038/s41560-025-01952-0

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41560-025-01952-0

Keywords:
Sb2(S,Se)3 solar cells, carrier concentration, hydrothermal synthesis, sodium sulfide additive, defect passivation, capacitance-voltage measurement, depletion region, band alignment, external quantum efficiency, power conversion efficiency, thin-film photovoltaics.

Harald Ade, the Goodnight Innovation Distinguished Professor of Physics at North Carolina State University, emphasizes the importance of finely tuning the mixing behavior of polymer:SMA blends for achieving optimal solar cell efficiency and stability. In the realm of conventional commodity polymers, mixing behavior is more straightforward, but organic semiconductors introduce an array of complexities due to their intricate molecular structures. Ade points out that before this study, little attention had been given to the phase behavior inherent in these solar composites, presenting a significant gap in the field of materials science.

To grasp the nuances of this research, the team meticulously derived binary phase diagrams for over fifty distinct polymer:SMA composites. A binary phase diagram serves as a map that elucidates the relationship between temperature variations and the tendency of two materials to mix or separate. Given that the operational efficacy of solar cells hinges on the mixing behavior of their constituent materials, these diagrams serve a critical role in predicting the stability and overall performance of photovoltaic devices.

Typically, one would expect that increasing temperature promotes greater mixing of materials. However, the findings reveal a counterintuitive phenomenon: in about half of the blends studied, the components exhibited separation with rising temperatures and a propensity to mix as temperatures dropped. This intriguing behavior is characterized as “re-entrant” phase diagrams, a concept that proposes a material can experience multiple phase transitions upon changing temperature before reverting to its initial state.

Jasper Michels, a staff scientist at the Max Planck Institute for Polymer Research and co-author of the study, elaborates on the significance of this discovery. He notes that the highly complex molecular configurations present in organic semiconductors contribute to their rich phase behavior, which necessitates an extension of classical models for polymer blends. The research highlights the necessity to include additional parameters to adequately represent the behavior of these solar composites, furthering the understanding of their phase transitions and stability.

At the heart of the investigation was the concept of free volume within the composites, a critical factor that dictates how materials respond to thermal changes. Free volume relates to the space within a material that allows it to contract or expand with temperature variations. Furthermore, understanding the glass transition temperature—the point at which a material transitions into a rigid, non-crystalline state—was pivotal to the study. This uncharted territory into the relationship between glass transition and phase diagram shapes has unveiled how organic semiconductor blends behave uniquely compared to traditional materials.

The glass transition temperature has a notable influence on the phase diagrams, suggesting the dominant role of configurational entropy in these polymer:SMA composites. Understanding how these transitions occur offers a fresh perspective on mixing behavior that has, until now, been overlooked. By integrating this element into their model, the researchers have developed a comprehensive framework that qualitatively aligns with experimental data and observations.

As the team reflects on their findings, they express optimism that this deeper understanding of mixing behavior will serve as a resource for future studies. Ade points out that the prevailing model for mixing has typically relied on two components: disorder and interaction. However, organic semiconductors introduce further complexities tied to molecular characteristics, which may affect both efficiency and stability at smaller scales. This finding can spur further investigation into optimizing the design of materials for solar applications.

The implications of the research extend beyond mere academic curiosity. The burgeoning interest in sustainable energy solutions makes the advancement of highly efficient solar technologies a critical goal. The insights gathered from this study can lead to significant breakthroughs in the materials used for solar cells, potentially resulting in lower costs and improved performance, thereby accelerating the transition to renewable energy sources.

Moreover, the findings were not borne out of isolation; they represent a concerted international effort, showcasing the collaboration of experts from diverse research institutions. Zhengxing Peng, a former Ph.D. student at NC State, served as the lead author, along with contributions from other researchers, including postdoctoral researcher Masoud Ghasemi. The study has also garnered recognition in esteemed scientific circles, with its publication in the high-impact journal “Nature Materials.”

The study’s funding support from prominent institutions, such as the Office of Naval Research and the Max Planck Society, underscores the importance of this research in advancing the scientific knowledge frontier. The collaboration not only illustrates the potential for advancements in solar technology but also highlights the synergy that comes from interdisciplinary partnerships in tackling complex global challenges like climate change.

In conclusion, the work undertaken by this research team opens new avenues for materials science and renewable energy technologies alike. By illuminating the complex phase behaviors of organic solar composites, they provide a roadmap for future developments. As the world pivots towards sustainable energy solutions, such research is vital for cultivating the next generation of solar cell materials, pushing the boundaries of efficiency, stability, and overall performance.

Subject of Research: Phase behavior of polymeric and small molecular acceptors in organic photovoltaics
Article Title: Re-entrant phase behaviour of organic semiconductors
News Publication Date: September 8, 2025
Web References: Nature Materials
References: Not applicable
Image Credits: Not applicable

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