In a remarkable stride toward advancing solar technology, researchers have unveiled a groundbreaking illumination-assisted annealing technique that revitalizes the performance potential of selenium (Se) solar cells. Selenium, known historically as the earliest photovoltaic material, has long been overshadowed by newer compounds despite possessing an inherently advantageous wide bandgap of approximately 1.9 eV. This intrinsic property makes Se an exceptionally promising candidate for use as a top-cell absorber in tandem solar cell assemblies and indoor photovoltaic applications. Nonetheless, the broader exploitation of selenium in solar cells has been hindered due to significant non-radiative recombination losses, largely attributed to the small grain size (~500 nm) typical of Se films produced by traditional thermal annealing methods.
The new illumination-assisted annealing process represents a paradigm shift by enabling photo-induced crystallization of Se films at ambient temperatures, effectively suppressing the detrimental dewetting phenomenon that previously limited film quality. By initially irradiating the selenium film under carefully controlled light exposure, the researchers facilitate the growth of substantially larger crystalline grains, achieving an average grain size of approximately 2.7 μm—over five times larger than conventional films. This enhancement in grain structure translates directly into fewer trap states where charge carriers could recombine non-radiatively, a chief mechanism that undercuts cell efficiency.
Following this photo-induced crystallization, the films undergo a subsequent thermal annealing step to consolidate their enhanced microstructural and electronic properties. The collaborative effect of these sequential processes results in selenium films with remarkably reduced trap-state density, measured to be on the order of 6.9 × 10¹⁴ cm⁻³, and notably extended carrier lifetimes reaching 22.9 nanoseconds. These metrics signify a substantial reduction in recombination centers and improved charge transport, both critical factors in lifting the limiting efficiency barriers that have stalled selenium photovoltaics for decades.
Harnessing these advanced films, the research team has demonstrated certified power conversion efficiencies in selenium solar cells exceeding 10%, with a record-setting open-circuit voltage of 1.03 V. This represents a historic milestone for Se photovoltaics, positioning them competitively not only as candidates for tandem solar cells but also in emerging applications under indoor light environments where wide bandgap materials excel. The open-circuit voltage figure is particularly noteworthy as it surpasses 1 volt, an indicator of photovoltaic quality previously unattainable for selenium-based devices.
Additionally, the durability of the fabricated selenium solar cells is equally impressive. Under rigorous operational conditions—sustained maximum power point tracking for 1,000 hours in ambient air without any encapsulation—the devices showed negligible performance degradation. This robustness highlights selenium’s intrinsic material stability advantage, promising long-term reliability and cost-effective deployment in real-world environments where encapsulation can add complexity and expense.
The illumination-assisted crystallization technique addresses one of the long-standing material synthesis challenges in selenium photovoltaics. Conventional thermal annealing methods often promote rapid dewetting of selenium films, resulting in discontinuous, polycrystalline layers with defective interfaces and small grain sizes. By contrast, light irradiation triggers localized heating and photo-chemical effects while maintaining a controlled ambient temperature, allowing selenium atoms to rearrange into larger, well-oriented crystalline domains. This nuanced control over the microstructure is what underlies the substantial improvements in electronic properties and device performance.
This breakthrough holds particularly exciting implications for tandem solar cells, where wide bandgap absorbers are employed to capture the high-energy portion of the solar spectrum while lower bandgap materials capture the remainder, maximizing the overall efficiency beyond single-junction limits. Selenium’s bandgap near 1.9 eV matches the ideal top-cell absorber bandgap that can be paired with silicon or other narrow-gap materials in tandem architectures. With this renewed processing approach, selenium’s historical photovoltaic legacy is revitalized with cutting-edge performance metrics suitable for modern sustainable energy solutions.
The researchers also highlight selenium’s potential in indoor photovoltaic applications, where illumination conditions differ significantly from sunlight. The wide bandgap and high open-circuit voltage enable selenium cells to operate efficiently under artificial light sources, expanding their utility in powering low-consumption electronics, Internet of Things (IoT) devices, and other ambient-light energy harvesting scenarios. The film stability and low trap density evident under these ambient conditions underscore selenium’s versatility as a solar material adaptable to diverse technological needs.
Scientifically, this work reveals new fundamental insights into crystallization dynamics under photo-assisted annealing conditions. The interplay between photon-induced atomic mobility and minimal thermal input fosters a unique material evolution pathway unprecedented in conventional semiconductor manufacturing. This discovery paves the way for further exploration of light-assisted processing in other semiconductor systems, which may yield similarly transformative material and device advancements.
Beyond efficiency gains, the simplicity and scalability of the illumination-assisted annealing technique also bear significant technological promise. The ambient temperature crystallization step reduces energy input requirements compared to high-temperature processes, making it more environmentally friendly and cost-effective. These attributes satisfy critical criteria for industrial viability, potentially facilitating the widespread commercial adoption of selenium-based photovoltaics, which have historically faced integration challenges despite selenium’s advantageous optoelectronic properties.
In summary, this research heralds a new era for selenium photovoltaics by merging time-honored material qualities with innovative light-driven processing techniques. By surpassing the 10% efficiency threshold and delivering a high open-circuit voltage alongside enduring device stability, selenium is repositioned from an early photovoltaic relic to a state-of-the-art contender. This advancement promises to enrich the portfolio of wide bandgap materials accessible for next-generation solar energy harvesting, offering pathways to both enhanced performance tandem configurations and resilient indoor power sources.
As the global race to harness renewable energy intensifies, breakthroughs like this underscore the value of revisiting and revitalizing classic materials with modern scientific ingenuity. Selenium’s transformation through illumination-assisted annealing demonstrates that old photovoltaic champions still have untapped potential to shape the future landscape of sustainable energy technology.
The implications for the photovoltaic industry are profound, as improving cell efficiency while reducing processing temperatures and costs aligns with market demands for competitive, scalable solutions. Moreover, the long-term stability shown by these selenium solar cells under ambient conditions bodes well for their deployment in diverse environmental settings, from urban rooftops to remote indoor applications.
Future research directions spurred by these findings may include detailed exploration of the photo-induced crystallization mechanism at atomic scales, optimization of annealing parameters for maximum grain size control, and integration of selenium films into tandem devices with complementary absorbers. Researchers may also investigate encapsulation strategies tailored to selenium’s unique chemistry to extend operational lifetimes even further.
This novel annealing strategy revitalizes selenium’s photovoltaic prospects by harnessing the synergistic effects of light and heat, setting a new benchmark in the field. The seamless combination of fundamental material science with innovative process engineering exemplifies how interdisciplinary approaches continue to drive solar cell technologies toward ever higher efficiencies, sustainability, and commercial viability.
With certified efficiencies exceeding 10% and a robust open-circuit voltage above 1 V, selenium solar cells leap into practical relevance for the first time in decades. As the sunlight-harvesting power of this ancient semiconductor is rekindled via illumination-assisted annealing, the door opens for new photovoltaic architectures and applications that could accelerate the global transition to clean energy.
Subject of Research: Selenium solar cells and photo-assisted annealing techniques for efficiency and stability enhancement.
Article Title: Illumination-assisted annealing enables selenium solar cells with open-circuit voltage over 1 V and efficiency exceeding 10%.
Article References: Wen, X., Li, Z., Lu, W. et al. Illumination-assisted annealing enables selenium solar cells with open-circuit voltage over 1 V and efficiency exceeding 10%. Nat Energy (2026). https://doi.org/10.1038/s41560-025-01939-x
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41560-025-01939-x
Keywords: Selenium photovoltaics, illumination-assisted annealing, photo-induced crystallization, wide bandgap solar cells, tandem solar cells, indoor photovoltaics, power conversion efficiency, open-circuit voltage, carrier lifetime, trap state density, solar cell stability
