An all-in-one Ag_2Se-based flexible solar-thermoelectric generator with photothermal integration

Domain: physics
Published: 2026-02-02
Editorial outcome: minor_revision

Abstract
Flexible solar-thermoelectric generators hold great promise for efficient solar energy harvesting and power supply in wearable electronics. However, the achievement of strong photothermal and thermoelectric performance simultaneously within a single material remains a significant challenge. Here, we propose a fully integrated solar-thermoelectric generator that directly employs Ag2Se thermoelectric thin films as the light-absorbing terminal, combined with a bottom infrared-reflective layer and surface visible anti-reflective coating. This multilayer architecture enables solar-selective absorption and enhances the photothermal conversion efficiency of Ag2Se up to 87.6%, while demonstrating good generalizability to other narrow-bandgap thermoelectric films. The resulting ring-shaped flexible generator delivers a maximum temperature difference and power density of 19.6 K and 0.17 μW cm-2 under 1-sun irradiation and exhibits a stable peak output power of ~1 μW under prolonged outdoor sunlight. These results highlight an effective strategy for high-efficiency solar-thermoelectric generators design and broaden the application potential of narrow-bandgap thermoelectric thin films in photothermal energy conversion.

Introduction
Solar-thermoelectric generators (STEGs) can efficiently convert the heat generated by sunlight into electricity, making them highly promising for applications in solar power generation, photodetection, and self-powered wearable electronics. Conventional STEGs are typically composed of thermoelectric (TE) modules integrated with photothermal (PT) conversion systems1. Despite extensive research efforts over several decades, the record solar-to-electricity conversion efficiency of STEGs based on bulk thermoelectric materials remains at ~9.6%2,3. In comparison, a significant gap still exists between STEGs and other emerging solar-energy conversion technologies, such as thermophotovoltaic (TPV) cells, which can achieve efficiencies exceeding 35% owing to the high operational temperature of thermal emitters4,5 and the utilization of highly efficient PV cells6–8. Consequently, STEGs still have a considerable distance to go before they can be considered viable for large-scale power generation applications. However, STEGs based on flexible thermoelectric materials offer distinct advantages, including facile fabrication, low costs, and adaptability to curved surfaces. Crucially, flexible STEGs prioritize consistent energy output rather than peak efficiency, which is a trade-off well suited to the requirements of low-power electronics and sensor networks. Such attributes render them highly suitable for wearable applications, where energy reliability and mechanical adaptability are essential.

According to the configuration, STEGs can be categorized into discrete and integrated types9. Discrete STEGs typically require an additional PT layer, such as a metal-nanoparticle coating or a metal-ceramic multilayer selective absorber,3,10 to achieve photothermal conversion. However, the insulating barrier layer located between the PT layer and TE materials introduces thermal loss, limiting the achievable temperature gradient. Additionally, the inclusion of a multilayered PT layer increases the complexity of the device fabrication process. In contrast, integrated STEGs utilize the TE materials as the light-absorbing terminal, thereby reducing the interface and simplifying the fabrication process. Nevertheless, this integrated strategy requires TE materials with strong intrinsic light absorption. Materials such as carbon nanotubes11,12 and organic semiconductors (e.g., PEDOT)13–15 exhibit broadband absorption and efficient light-to-heat conversion, but their relatively low TE performance restricts overall device output and makes simultaneous optimization of PT and TE properties challenging.

Wearable applications require higher room-temperature (RT) TE performance. Although n-type Bi2Te2.7Se0.3, p-type Bi0.5Sb1.5Te3, and emerging Mg3Bi2-based materials18,19. With superior TE performances at RT can be considered as potential candidates, Ag2Se is particularly well-suited for flexible and wearable thermoelectric applications due to its balanced combination of high TE performance at RT20,21, mechanical compliance22, and environmental stability. In recent years, flexible Ag2Se films have been fabricated using various processing routes, including magnetron sputtering23, vacuum filtration24, direct selenization25, and printing techniques26,27. Among these, magnetron sputtering is particularly advantageous, as it allows for precise control of film thickness, ensures excellent layer uniformity, and facilitates good reproducibility with large-area fabrication, compatible with existing industrial production lines.

A variety of high-performance Ag2Se-based flexible TE devices have been extensively investigated24,26,28,29, demonstrating their great potential for efficient energy conversion and wearable power generation. By integrating various PT conversion layers such as carbon black film30, TiO2 coating27,31, and polypyrrole (PPy)32, Ag2Se flexible TE devices can effectively achieve photo-thermal-electrical conversion for photothermal power generation and light detection. In addition, the narrow bandgap (~0.15 eV) endows Ag2Se film with broad-spectrum light absorption characteristics33, making it a promising candidate for integrated STEGs. Recent studies have demonstrated the feasibility of Ag2Se as a photothermal terminal in flexible STEGs9,34–36. Specifically, thermoelectric devices based on Ag2Se/Nylon composite films, without requiring additional PT conversion layers or heat sink components, achieved an open-circuit voltage (Uoc) of 8.8 mV under 1 sun irradiation9. However, the achieved Uoc remains modest relative to that of most flexible STEGs (Supplementary Table 1). The absence of strategies to further improve the photothermal performance of the Ag2Se-based films has constrained the overall output performance of the corresponding STEGs.

In this study, we designed a multilayer PT conversion terminal to realize an integrated STEG. As shown in Fig. 1a, the PT terminal directly employs Ag2Se film as the light-absorbing layer, utilizing its narrow bandgap to achieve broadband solar absorption. Photothermal conversion is accomplished through non-radiative recombination processes. By integrating an infrared-reflective metallic layer at the bottom and a dual anti-reflection coating on the surface, the structure enables high solar absorption while suppressing infrared emission, thereby maintaining an elevated operating temperature. The flexible STEG using this multilayer design as the photothermal terminal exhibited a maximum temperature difference of 19.6 K. The output power density reached 0.17 μW cm-2 under a light irradiance of 100 mW cm-2 (Fig. 1b), which is superior to recently reported flexible photo-thermoelectric devices11,30,37–39 (Fig. 1c). More importantly, this strategy is generalizable to other narrow-bandgap thermoelectric thin films, providing a versatile approach to extend the applicability of thermoelectric materials.

Results
In this work, Ag2Se films were deposited on flexible polyimide (PI) substrate using magnetron sputtering, and the TE performance was optimized by adjusting the stoichiometry of the target (Supplementary Figs. 1, 2). The specific experimental scheme is shown in the Supplementary Note 1. As the Se content in the target increases, as shown in Fig. 2a, the carrier concentration (nH) decreases from 6.3 × 1018 cm−3 for Ag2Se film to 4.8 × 1018 cm−3 for Ag2Se1.02 film (with 2 at.% excess Se in the sputtering target). In contrast, the Hall mobility (μH) of the film is significantly enhanced. Notably, the Ag2Se1.02 film exhibits a room-temperature μH as high as 1414 cm2 V−1 s−1, surpassing those reported for most Ag2Se-based flexible thermoelectric films24,29,40. Such high μH primarily arises from the high orientation and large grain size (Supplementary Fig. 3), which lowers the grain-boundary barrier height and grain-boundary density (Supplementary Fig. 4), thereby weakening carrier scattering41–43. Despite the decrease in nH, the room-temperature (~300 K) electrical conductivity (σ) of the film increases with Se content due to the enhanced μH (Fig. 2b). The Seebeck coefficient (S) increased with increasing Se content from −132 μV K−1 to −152 μV K−1 (Fig. 2c). As shown in Fig. 2d, the effective mass of Ag2Se1+x films remains nearly constant at 0.28 mₑ, suggesting that the added Se has little influence on the band structure. The observed increase in S thus primarily originates from the reduced nH. It should be noted that when the Se content was excessively high, as in the Ag2Se1.03 sample, the nH shows an upward trend, while both the preferred orientation factor (Supplementary Fig. 3b) and in-plane mobility (Fig. 2a) decrease significantly. This degradation in film quality is likely due to the precipitation of elemental Se. Finally, when 2 at% of additional Se was added to the target, the room-temperature power factor (PF) of the film reached 25.7 μW cm−1 K−2 (Fig. 2e), comparable to state-of-the-art flexible Ag2Se films (Supplementary Table 2)23–26,28,29,40,42,44–47. Moreover, the film exhibits a resistance change of less than 2.5% after 1000 bending cycles at a bending strain of 1% (Fig. 2f), demonstrating low excellent mechanical flexibility (experimental details are provided in the Supplementary Note 2 and Figs. 5−7)

The photothermal performance of the pristine Ag2Se (hereafter referring to the TE performance optimized Ag2Se1.02 film) flexible film was evaluated by monitoring its temperature rise under Xe lamp illumination. Details of the measurement are provided in the Materials and Methods section. Upon light illumination, the photons with energies higher than the bandgap generate above-bandgap electron-hole pairs in the Ag2Se film. The excited carriers subsequently relax to the band edges, releasing the excess energy as heat through thermalization14,48. The temperature of the pristine film (including PI substrate) rose rapidly as soon as the Xe lamp was turned on, and stabilized at 62 °C after 70 s under a light irradiance of 100 mW cm−2 (Supplementary Fig. 8a). Moreover, the maximum stable temperature (Tmax) exhibited thickness-independent behavior (Supplementary Fig. 8b). In addition to Ag2Se, similar Tmax values (60–70 °C) were also observed in other flexible narrow-bandgap thermoelectric films, such as Bi2Te3, Sb2Te3, and Cu2Se (Supplementary Fig. 9). However, the achieved Tmax remains lower than that of blackbody-like PT films12,49. To further enhance the photothermal conversion efficiency of the Ag2Se film, we adopted a multilayer design inspired by selective-absorption coatings.

As shown in Fig. 3a, the overall structure of the multilayer designed Ag2Se (MD-Ag2Se) film consists of a metallic base layer, an intermediate absorption layer, and a surface antireflection layer. Both Ag and W are well-known infrared thermal radiation barrier materials50–52. At the bottom of the structure, they serve as infrared reflectors, effectively reducing the infrared radiation of the structured film. Meanwhile, the reflective property of the metal layer enhances the absorption and utilization of infrared light by the Ag2Se absorption layer, thereby improving the photothermal conversion efficiency. Moreover, due to its high thermal stability, the W layer also functions as an effective diffusion barrier, preventing interdiffusion between Ag and Ag2Se during the annealing process. To further enhance overall absorption, a double-layer antireflection coating was fabricated using low-refractive-index Al2O3 and SiO2 thin films. The gradient refractive index induced by this bilayer structure promotes destructive interference of reflected light, thereby suppressing surface reflection and enhancing the overall light absorption of the film53.

The microstructure of MD-Ag2Se thin films was characterized in detail by transmission electron microscopy (TEM). From the TEM images in Fig. 3b, the interfaces between the layers are sharp, and the film has uniform thickness. The magnified view of region 1 in Fig. 3b reveals that the dual antireflection layers are amorphous, with the thicknesses of Al2O3 and SiO2 measuring ~20 nm and ~35 nm, respectively. Beneath the antireflection layer, the Ag2Se film exhibits high crystallinity. The HRTEM image of region 2 in Fig. 3b reveals a lattice spacing of 0.78 nm, characteristic of the orthorhombic β-Ag2Se (001) plane (Fig. 3d), consistent with the XRD results. Selected-area electron diffraction (SAED) patterns obtained from different regions of the Ag2Se layer (Fig. 3d-e) exhibit sharp, well-defined diffraction spots, confirming the film’s uniformity and high crystallinity, which are key factors contributing to its high in-plane carrier mobility. The SAED image of the bottom metallic layer (Fig. 3f) is indexed to cubic Ag. EDS mapping illustrates the elemental distribution across the multilayer structure (Fig. 3g, h), where the Ag signal intensity changes across the W layer, reflecting the presence of Ag2Se above and the Ag film below. The line-scan profiles exhibit sharp transitions at the interface (Supplementary Fig. 10), demonstrating the absence of elemental diffusion between the two layers and attesting to the effectiveness of the W layer as a diffusion barrier.

The optical images reveal that the MD-Ag2Se film exhibits a notably darker surface appearance compared to the intrinsic film (Supplementary Fig. 11). This distinction corresponds to a substantial increase in average absorptance of the structured film in the UV-Vis-NIR range (250-2500 nm), rising from 63.6% to 84.3% (Fig. 4a). The MD-Ag2Se film also exhibits high reflectance in the infrared range of 2.5-18 μm (Supplementary Fig. 12), indicating low heat loss. Simulations of electric field distribution (Fig. 4b) reveal that the multilayer design enhances absorption across the solar spectrum, verifying its pronounced anti-reflection behavior. Under Xe-lamp irradiation at 1 sun, the Tmax reaches 85 °C, representing a 1.37-fold enhancement relative to the pristine Ag2Se film (Supplementary Fig. 8a). As the Xe lamp intensity increases from 0.25 to 2.0 suns, the Tmax sharply increases from 47.2 °C to 145.7 °C (Fig. 4c), surpassing many photothermal materials such as carbon-based materials, conjugated polymers, MXene 2D materials, etc. (Supplementary Table 3)12,14,37,38,49,54–59, and approaching the performance of commercial selective solar absorber films60. Moreover, the Tmax demonstrates a high degree of linearity with the light intensity, indicating its potential for use in light intensity detection (inset of Fig. 4c).

The photothermal conversion efficiency (η) of the films was evaluated based on the following equation48,61,62:

η = hs(Tmax - Tsurr) / IAλ

Where h is the heat-transfer coefficient, s is the surface area of the film, Tsurr is the ambient temperature, Tmax is the maximum equilibrium temperature of the film, I is the solar irradiance, Aλ is the absorptance of the PT film (details of the calculation are given in Supplementary Note 3 and Fig. 13). As shown in Fig. 4d, the η of the flexible MD-Ag2Se film was calculated to be 87.6%, more than twice of the pristine film (~31.2%). Figure 4e presents the temperature variations under light illumination (“on” and “off” states), indicating excellent photothermal stability and repeatability of the MD-Ag2Se film. Notably, the film reaches its peak temperature within 70 s, significantly faster than those reported for state-of-the-art solar thermal conversion materials49,56,58,63,64. This rapid response is further demonstrated in infrared thermal images of a structured Ag2Se film patterned into “HBU” under 1 sun irradiation (Fig. 4f).

The photothermal response of the structured Ag2Se film was continuously monitored in outdoor conditions in Baoding (China) on 7 May 2025 under cloudless and sunny conditions, with temperatures ranging from 15 to 28 °C, moderate humidity (45%), and low wind speed (≤ 2.5 ms−1). The experimental apparatus, schematically illustrated in Supplementary Fig. 14, mainly comprises a polystyrene box coated with aluminum foil to block external radiation. Figure 4g shows that the temperature variation of the samples consistently follows the solar irradiance. The Tmax of the MD-Ag2Se film reached 93 °C during the peak solar irradiance period (11:30 a.m.–1:30 p.m.), in contrast to 67 °C for the pristine film. The outdoor test results were superior to those obtained under Xe lamp illumination, likely due to the larger illuminated area that heated the entire test setup. Similarly, cyclic bending tests were conducted to evaluate the mechanical properties of the MD-Ag2Se film. As shown in Fig. 4h, after 10,000 bending cycles with a radius of 3.75 mm (1% strain), the photothermal performance (as indicated by the Tmax) exhibited negligible change, indicating excellent flexibility (Supplementary Fig. 15). In addition, after 1000 cycles of bending, the relative resistance change of the multilayer film was less than 1% (Supplementary Fig. 16). Such remarkable flexibility renders the film highly promising for applications in the field of wearable electronics.

To further validate the generality of this multilayer design strategy, similar structured modifications were applied to other representative narrow-bandgap thermoelectric thin films, such as Bi2Te3, Sb2Te3, and Cu2Se. Upon structural optimization, all films exhibited a significantly enhanced Tmax that increased linearly with light intensity (Supplementary Fig. 17). Under Xe lamp illumination, the Tmax was typically 20–30 °C higher than that of the pristine films (Supplementary Fig. 9). Notably, the Tmax of the structured Cu2Se film reached 99 °C under 1 sun and further increased to 160 °C when the light intensity reached 2 suns. These results demonstrate the general applicability of our strategy in enhancing the photothermal performance of narrow-bandgap thermoelectric thin films.

To systematically verify the universality of this design, we fabricated a flexible ring-shaped STEG containing nine pairs n-type Ag2Se and p-type Sb2Te3 TE legs. The TE performance of the Sb2Te3 film is shown in Supplementary Fig. 18. Prior to device assembly, finite element simulations were performed to optimize the overall device configuration. Owing to the disparity in TE properties between the n- and p-type legs, the output performance of the device is highly sensitive to the relative cross-section dimension65. As shown in Fig. 5a, a thickness ratio of 1.2 between the n-type and p-type legs yielded the highest output power. Further simulations examined the effect of the ratio (r) between the photothermal terminal length (l1) and the thermoelectric leg length (l2). As shown in Fig. 5b, increasing the proportion of the photothermal terminal caused a gradual decrease in the Uoc. This reduction occurs because expansion of the photothermal region shortens the non-illuminated segment, while the heat dissipation capacity of the cold end remains unchanged, leading to a smaller steady-state ΔT. The Pmax of the device is governed by the Uoc and the internal resistance (Rin) of the sector-shaped TE leg as follows:

Pmax = Uoc2 / 4Rin

The Uoc is associated with the ΔT, demonstrating a decreasing trend as the length ratio r increases (Supplementary Fig. 19a). The Rin of the sector-shaped TE leg can be calculated as follows:

Rin = (ρ / dθ) * ln(r2 / r1) = (ρ / dθ) * ln(l2 + r1 / r1)

where ρ is the electrical resistivity, d is the thickness of the TE films, θ, r1, and r2 denote the sector angle, inner radius, and outer radius of the sector-shaped TE leg, respectively (Supplementary Fig. 19b). The increasing r decreases l2, thereby reducing the Rin. Hence, both ΔT and Rin are functions of the leg-length ratio r. Consequently, Pmax initially increases with r and then decreases, reaching a maximum at r = 0.28. The final geometric configuration and the optical graph of the device were shown in Fig. 5c and Supplementary Fig. 20, and fabrication details were provided in the Materials and Methods section and Supplementary Fig. 21.

Device performance was evaluated under simulated sunlight. During the test, only the central photothermal terminal was illuminated (Supplementary Fig. 22). Upon irradiation at 1 sun, the Uoc rose rapidly, reaching 49 mV within 60 s (Fig. 5d), in good agreement with the simulation results (Supplementary Fig. 23). The characteristic rise and fall times, corresponding to 10% and 90% amplitude, were 18.2 s and 24.6 s, respectively (Supplementary Fig. 24). When the light intensity increased to 1.5 and 2 suns, Uoc further increased to 66 mV and 79 mV, respectively. Under 1 sun irradiation, the measured ΔT of 19.6 K was consistent with theoretical predictions based on the Seebeck coefficient and temperature difference (Supplementary Fig. 25). The STEG demonstrated stable behavior, nearly identical voltage responses, over five consecutive on/off light cycles under 1 sun irradiation (Fig. 5e). Figure 5f presents the I-U and I-P characteristics of the STEG under Xe lamp irradiation with varying light intensities from 1 sun to 2 suns, where Pmax increased proportionally with light intensity. The corresponding maximum output powers were 1.2, 2.1, and 3.0 μW under irradiation of 1, 1.5, and 2 suns, respectively. The maximum output power density (PDmax) was then calculated by dividing Pmax by the total surface area of the device. As shown in Fig. 1c, the PDmax reached 0.17 μW cm−2 under 1 sun irradiation and further increased to 0.42 μW cm−2 when the light intensity rose to 2 suns, surpassing most previously reported flexible photo-thermoelectric devices, such as fiber-based STEGs37–39, printed Ag2Se film-based STEG30, and carbon nanotube-based STEGs11.

A full-day outdoor test was carried out on 10 May 2025 in Baoding, China, from 8:00 to 18:00 to evaluate the real-world output performance of the ring-shaped STEG under natural sunlight. The experimental setup and weather conditions are presented in Supplementary Fig. 26. Notably, the device achieved a peak output power of approximately 1 μW at noon with the maximal solar irradiance (Fig. 5g). Further experiments revealed that the ΔT varied synchronously with the solar irradiance, resulting in a change in the Uoc and the Pmax (Supplementary Fig. 27-28).

To demonstrate integration into wearables, the ring-shaped STEG was mounted on a sun hat and a backpack, and its output performance was tested at noon on 21 September 2025 in Baoding, with an ambient temperature of ~24 °C and a wind speed of ≤ 0.5 ms−1. A solar radiometer and a multimeter were used to measure the solar irradiance and Uoc, respectively. As shown in Fig. 5h, the STEG delivered a Uoc of 14.5 mV under a sunlight irradiance of 525 W m−2 when attached to the sun hat. When it was further sewn onto a backpack, its Uoc increased to 23.7 mV under a light irradiance of 657 W m−2. Further studies indicated that both ΔT and Uoc exhibited small fluctuations under continuous wind disturbance. However, when the wind speed was low, a relatively high Uoc was maintained, whereas higher wind speeds led to a reduction in Uoc (Supplementary Fig. 29). The variations of ΔT and Uoc follow an identical trend. Nevertheless, such environmental perturbations can be effectively mitigated in future practical applications through improved system-level packaging and protection strategies.

These results demonstrate that the integrated STEG architecture proposed in this work offers significant advantages over conventional discrete devices and systems employing organic photothermal films. The structure enables strong coupling between PT and TE conversion processes within a unified platform, there by improving heat utilization efficiency. Notably, in comparison to common flexible PT materials such as CNT11,12 and PEDOT13 films, the utilization of Ag2Se film results in significantly improved PF of 25.7 μW cm−1 K−2. In addition, the entire device fabrication relies solely on magnetron sputtering, which is highly compatible with MEMS fabrication and photolithography techniques, offering a scalable route toward wafer-scale production of STEGs. The integrated device delivers excellent output performance under outdoor conditions, enabling it to power wearable health-monitoring sensors (e.g., pulse or sweat-rate sensors). Moreover, its linear photothermal response and light-induced Uoc characteristics indicate substantial potential for applications in real-time outdoor light intensity sensing.

In summary, we presented a design strategy for integrated flexible solar-thermoelectric devices that combines the strong broadband absorption of narrow-bandgap thermoelectric thin films with a multilayer selective-absorption architecture. The multilayer-designed Ag2Se flexible films exhibited temperature rises exceeding 85 °C under both Xe lamp and outdoor sunlight illumination. Correspondingly, the photothermal conversion efficiency was enhanced from 31.2% for the pristine film to 87.6% for the multilayer-designed film. The as-prepared ring-shaped Ag2Se-based STEG fabricated using this strategy generated a large ΔT of 19.6 K, and delivered a maximum output power density of 0.17 μW cm−2 under 1 sun irradiation, outperforming most reported flexible STEGs. Moreover, it achieved an output power of ~1 μW under outdoor conditions and generated an open-circuit voltage of 14.5–23.7 mV when integrated with external equipment. These findings demonstrate the broad applicability of our strategy and highlight its potential to accelerate the practical deployment of high-performance, flexible STEGs for energy harvesting.

Methods
Fabrication of Se excess Ag2Se targets
Polycrystalline Ag2Se ingots were synthesized by sealing stoichiometric amounts of high-purity (≥99.99%) Ag and (≥99.99%) Se chunks in an evacuated quartz ampoule. The ampoule was gradually heated to 1323 K over a period of more than 10 hours and maintained at this temperature for 12 hours to ensure complete melting and homogenization. Subsequently, the melt was allowed to cool naturally within the furnace by shutting off the power supply. The resulting ingots were then cut into small pieces (less than 3 mm3). Subsequently, Se particles with specific excess atomic ratios of 1 at%, 2 at%, and 3 at% were introduced, and the mixture was ground into fine powder under ambient conditions using a pulverizer. The powder was subsequently consolidated into a dense sputtering target with a diameter of 50.8 mm and a thickness of 4 mm by spark plasma sintering (SPS) at 573 K under a uniaxial pressure of 30 MPa.

Deposition of flexible Ag2Se films
Ag2Se1+x (x=0, 0.01, 0.02, 0.03) targets were connected to a direct current (DC) power source for sputtering. Prior to deposition, PI substrates with a thickness of 75 μm were sequentially cleaned using ultrasonic baths in acetone and ethanol for 20 minutes each. During film deposition, the Ag2Se targets were sputtered at a power of 100 W, and the Ar pressure was fixed at 0.4 Pa. The resulting films were designated Ag2Se, Ag2Se1.01, Ag2Se1.02, and Ag2Se1.03, corresponding to the compositions of the sputtering targets. To improve crystallinity, all deposited films were subsequently annealed in a nitrogen atmosphere at 448 K for 30 minutes.

Characterization
The Seebeck coefficient (S) and electrical conductivity (σ) were simultaneously measured by a commercial system (ZEM-3, Advance-Riko). The measurement uncertainty was 3% for electrical conductivity and 5% for the Seebeck coefficient. Hall coefficients (RH) were measured using the van der Pauw method with a four-probe configuration under a reversible magnetic field of 1.5 T and an electrical current of 10 mA. The Hall carrier concentration (nH) was calculated via nH = 1/(eRH). The absorption spectra were measured using a UV–Vis–NIR spectrophotometer (Agilent, Cary 5000). The cross-sectional morphology of the MD-Ag2Se film was characterized using a transmission electron microscope (JEOL ARM 200F.).

Device fabrication
The fabrication process of the ring-shaped STEG, illustrated in Supplementary Fig. 21, involved several critical steps. First, the inner electrode, composed of an Ag/W bilayer, was deposited onto the substrate via magnetron sputtering. Subsequently, p-type Sb2Te3 legs were deposited and annealed under vacuum at 573 K for 1 h to improve crystallinity. Next, the n-type Ag2Se film was deposited and annealed at 448 K for 30 min. An Al2O3/SiO2 double-layer anti-reflection coating was then deposited on the central area. Finally, an Ag film was deposited as the outer electrode to connect the n- and p-type thermoelectric legs in series. To further enhance the effective temperature difference, the substrate regions not covered by the thermoelectric materials and electrodes were selectively removed. An optimized co-sputtering process was employed to prepare the Sb2Te3 thin films. The fabrication parameters for each layer and thermoelectric leg are provided in Supplementary Table 4.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors suggested and developed multilayered designed Ag2Se integrated structure and well compared pristine flexible film. Also the authors have shown ring-shaped STEG and its performance under outdoor environment. This manuscript presented a novel all-in-one structural design which is interesting but, the overall performance as a generator device appears insufficient and lacks some information. I would have comment for the manuscript to provide additional supporting information and for further strengthen. Following are the comments or questions in this manuscript.\n- The title of the manuscript should be revised to better highlight the integrated structural feature of this work.\n- In the Introduction, it would be helpful to provide further explanation on the advantages of PT, TE integration beyond the reduction fo the interfacial losses. And also there's lack of elaboration on the potential applications or intended uses of the generator. Additionally, including a comparison of the Ag2Se fabrication processing methos as an application will help better understanding the contents and related research background.\n- In the Results and Discussion section, authoor described resistance change depends on the bending cycle. For multilayered Ag2Se case, presinting the resistance change could better demonstrate the interfacial stability comparing the pristine film. In addition, through S.I. Fig. 10., the slope of the resistance change depends on bending cycyling appears to vary depending on the substrate thickness, please explain why this behavior is affected by the thickness even it is same material.\n- As is well known, the TEG performance is strongly influenced by the temperature difference, which is directly proportional to the power output. The explanation on how the temperatrue difference and resulting power output vary with the leg length ratio appears insufficient, so it would be helpful to provide a more detailed discussion.\n- Also the temperature difference data from the test under the outdoor performance should be shown to verify whether the power output is indeed affected by the temperature difference.\n- When the device was integrated into the actual wearable hat and bag, was the environment affected by wind or not? Since the device was not packaged and the outdoor experiment was conducted with a PE film covering in insulated box, it would be helpful to provide detailed environmental data and experimental methods. This would make it easier to understand the observed power output trends.\n- End of the Results and discusstion section, if the author demonstrate the briefly highlight the advantages of this technology and discuss its potential applications, it will help for butter quality of the manuscript.\n- All the photographs in Figures, the scale bar is missing. Including it would greatly improve the clarity and understanding of the images.", "reviewer_2": "The manuscript presented the fabrication and characterization of AgSe-based flexible solar thermoelectric generators. The approach is promising, and the manuscript is logically organized into sections. The results are remarkable compared to previously reported thermoelectric generators, however have a long way to be considered for real power utilities. This should be clear in several places at the main text of the manuscript, as indicated in the attached document and compared with theoretical and reported experimental values in the literature. Further minor presentation issues should be revised as indicated in attachment. All comments should be carefully addressed with line by line responses, before the manuscript is further considered.", "reviewer_3": "The paper under review proposes a thin-film structure made of silver selenide, which is comprehensively demonstrated to act as a photothermal absorber and thermoelectric generator. An interesting characterization of the active materials has been performed together with the reasons supporting their choice. The proposed converter ring-shaped structure is interesting and novel. The testing under outdoor conditions strengthens the achieved results for a proof-of-concept demonstration of microwatt-range output power.\nTherefore, in my opinion paper is interesting and novel, and it seems to have comprehensively characterized the material, designed a novel device structure, tested its performance, and probed its possible wearable application.\nI have identified the following two weak points, which I suggest the Authors address:\n1) Although Fig. 1a and 1c can be considered self-explanatory, the caption describes them poorly. The Authors are requested to explain all the elements therein reported and specify explicitly the related citations to which their technology is compared.\n2) Fig. 3a does not seem to be coherent with the other reported structural, morphological, and compositional characterizations. I suggest shifting it (into Fig. 4?)."}, "author_response": "Dear Editor,\nWe are very grateful for your immediate attention to our submission. We also appreciate\nthe useful comments from the reviewers, which help us further enhance the quality of\nour manuscript. We have revised the manuscript point-by-point according to the\nreviewers’ comments and provided revisions wherever necessary. Now we hope the\nrevised version is satisfactory for acceptance. In the following, we summarize the\nreviewers’ comments and our responses point-by-point (blue color).\nQian Zhang\nAuthors' response to the reviewers\nREVIEWER COMMENTS\nReviewer #1:\n(Remarks to the Author): In this manuscript, the authors suggested and developed\nmultilayered designed Ag Se integrated structure and well compared pristine flexible\n2\nfilm. Also, the authors have shown ring-shaped STEG and its performance under\noutdoor environment. This manuscript presented a novel all-in-one structural design\nwhich is interesting but, the overall performance as a generator device appears\ninsufficient and lacks some information. I would have comment for the manuscript to\nprovide additional supporting information and for further strengthen. Following are\nthe comments or questions in this manuscript.\nResponse:\nWe sincerely appreciate your positive evaluation of our work and are grateful for\nyour constructive comments, which have been invaluable in improving the quality of\nour manuscript. Our point-by-point responses to the specific comments are provided\nbelow.\n1) The title of the manuscript should be revised to better highlight the integrated\nstructural feature of this work.\nResponse:\nThank you for your suggestion. To better highlight the integrated structural\nfeatures of this work, we have revised the title of the manuscript as follow:\n“An all-in-one Ag Se-based flexible solar-thermoelectric generator with\n2\nphotothermal integration”\n2) In the introduction, it would be helpful to provide further explanation on the\nadvantages of PT, TE integration beyond the reduction of the interfacial losses. And\nalso, there's lack of elaboration on the potential applications or intended uses of the\ngenerator. Additionally, including a comparison of the Ag Se fabrication processing\n2\nmethods as an application will help better understanding the contents and related\nresearch background.\nResponse：\nWe appreciate your insightful comments. In the revised manuscript, we further\nclarify the origin of the thermal loss at the interfaces of the integrated Solar-\nThermoelectric Generators (STEGs), and highlight the advantages of integrating the\nphotothermal and thermoelectric layers. The integration minimizes interface issues and\nsimplifies the device fabrication process. Additionally, we have expanded the\ndiscussion on potential applications and intended uses of the device.\nRegarding the preparation of Ag Se films, we have employed magnetron\n2\nsputtering, which enables precise thickness control and excellent film uniformity. These\nfeatures provide irreplaceable advantages for the fabrication of multilayer optical thin-\nfilm structures compared to alternative methods such as vacuum filtration, direct\nselenization, and screen-printing processes. At the end of the manuscript, we\nspecifically emphasize the compatibility of magnetron sputtering with standard\nsemiconductor device fabrication processes, including micromachining and\nphotolithography, and highlight its potential for high‑density integrated device\nfabrication, which further underscores the advantages of the fabrication approach used\nin this work.\nRevisions to the main text are shown below.\nOn page 3: “…the insulating barrier layer located between the PT layer and TE\nmaterials introduces thermal loss, limiting the achievable temperature gradient.\nAdditionally, the inclusion of a multilayered PT layer increases the complexity of the\ndevice fabrication process. In contrast, integrated STEGs utilize the TE materials as the\nlight-absorbing terminal, thereby reducing interface and simplifying the fabrication\nprocess. Nevertheless, this integrated strategy requires TE materials with strong\nintrinsic light absorption.…”\nOn page 3-4: “…In recent years, flexible Ag Se films have been fabricated using\n2\nvarious processing routes, including magnetron sputtering24, vacuum filtration25, direct\nselenization26, and printing techniques27, 28. Among these, magnetron sputtering is\nparticularly advantageous, as it allows for precise control of film thickness, ensures\nexcellent layer uniformity, and facilitates good reproducibility with large-area\nfabrication, compatible with existing industrial production lines.”\nOn page 16: “…The integrated device delivers excellent output performance under\noutdoor conditions, enabling power supply for wearable health-monitoring sensors (e.g.,\npulse or sweat-rate sensors). Moreover, its linear photothermal response and light-\ninduced U characteristics indicate its substantial potential for applications in real-time\noc\noutdoor light intensity sensing…”\n3) In the Results and Discussion section, author described resistance change depends\non the bending cycle. For multilayered Ag Se case, presenting the resistance change\n2\ncould better demonstrate the interfacial stability comparing the pristine film. In\naddition, through S.I. Fig. 10., the slope of the resistance change depends on bending\ncycling appears to vary depending on the substrate thickness, please explain why this\nbehavior is affected by the thickness even it is same material.\nResponse：\nWe appreciate your insightful comments. To validate the resistance stability of the\nmultilayered Ag Se films, we conducted additional bending tests. Since the top surface\n2\nof the multilayered Ag Se is covered by an insulating oxide layer, electrode windows\n2\nwere reserved for resistance measurements, as shown in Figure R1a. We maintained a\nbending radius of 3.75 mm, consistent with that of the intrinsic Ag Se film. Owing to\n2\nthe presence of the top protective layer, the overall resistance of the bent sample\nremained between 1.4 and 1.5 Ω, and the relative resistance increase was within 1%\neven after 1000 bending cycles (Figure R1b). Such excellent mechanical robustness of\nthe multilayer film can be attributed not only to the stable interfacial bonding but also\nto the encapsulation effect provided by the surface anti-reflection layer, which is\nconsistent with our previous report1.\nFigure R1. Bending test of the multilayer Ag Se thin films. a Photograph of the\n2\nmultilayered Ag Se film and bending test setup. b Resistance and the relative resistance\n2\nchange of the multilayered Ag Se film as a function of bending cycles.\nmay arise from measurement uncertainty. Due to the large difference between the\nsubstrate and the film thickness, for samples with different substrate thicknesses (d ),\nsub\nthe bending radii (R ) corresponding to the identical applied strain (ε) have the\nbent\nfollowing relationship2:\nd R (1)\nsub bent\nIn principle, when the strain in the thin film is fixed, the resistance change should be\nindependent of the substrate thickness. To accurately characterize this behavior,\nmultiple sets of samples with different substrate thicknesses were fabricated and\nsubjected to cyclic bending tests. As shown in Figure R2, although some fluctuations\nin the slope of the resistance change are observed, the relative resistance increase of all\nsamples remains within the range of 2% to 2.5%. These results indicate that, under a\nfixed strain condition, the mechanical performance of the thin film is only weakly\naffected by the substrate thickness. We have updated the bending test data in the revised\nmanuscript.\nFigure R2. Mechanical properties of Ag Se/PI films with different substrate\n2\nthicknesses. a-c Bending states of films with substrate thicknesses of 25 μm, 75 μm,\nand 100 μm, respectively. d-f Relative resistance variation of films deposited on 25, 75,\nand 100 μm-thick substrates during cyclic bending tests under 1% strain.\nRevisions to the Results and Conclusion are shown below.\nOn page 12: “…In addition, after 1,000 cycles of bending, the relative resistance\nchange of the multilayer film was less than 1% (Supplementary Fig. 16)….”\nOn page 6: “…Moreover, the film exhibits a resistance change of less than 2.5% after\n1000 bending cycles at a bending strain of 1%....”\nReferences\n1. Hou S, et al. Encapsulated Ag Se-based flexible thermoelectric generator with\n2\nremarkable performance. Mater Today Phys 38, 101276 (2023).\n2. Fukuda K, et al. A bending test protocol for characterizing the mechanical\nperformance of flexible photovoltaics. Nature Energy 9, 1335-1343 (2024).\n4) As is well known, the TEG performance is strongly influenced by the temperature\ndifference, which is directly proportional to the power output. The explanation on how\nthe temperature difference and resulting power output vary with the leg length ratio\nappears insufficient, so it would be helpful to provide a more detailed discussion.\nResponse：\nWe sincerely appreciate your valuable comments and apologize for the error in the\ninset of Fig. 5b, where the actual length of l during the simulated does not include the\n2\nouter electrode. We corrected this labeling error in the revised manuscript. Regarding\nthe effect of the length ratio (r=l /l ) on the temperature difference (ΔT), we carried out\n1 2\nthe simulation as follows. As shown in Figure R3a, the ΔT decreases with the increase\nof r, and the maximum ΔT reaches ~26 K.\n28\n24\n20\n16\n12\n8\n0.0 0.1 0.2 0.3 0.4 0.5 0.6\nFigure R3. a Simulated ΔT as a function of the leg-length ratio r. b Schematic diagram\nused to calculate the internal resistance of the sector-shaped TE leg.\nThe maximum output power P of the device is governed by the open-circuit\nmax\nvoltage (U ) and the internal resistance (R ) of the sector-shaped TE leg as follows:\noc in\n(2)\nThe U is associated with the ΔT, demonstrating a decreasing trend as the length ratio\noc\nr increases (Figure R3b). The internal resistance R of the sector-shaped TE leg can be\nin\ncalculated as follows:\n)K(\nTD\na b\nl\nr\nl 2\n1\nr = l /l PT layer\n1 2 θ\nr\nU 2\nP  oc\nmax 4R\nin\n r  l r\nR  ln 2  ln 2 1 (3)\nin d r d r\n1 1\nwhere ρ is the electrical resistivity, d is the thickness of the TE films, θ, r , and r denote\n1 2\nthe sector angle, inner radius and outer radius of the sector-shaped TE leg, respectively\n(Figure R3a). The increasing r decreases l , thereby reducing the R . Hence, both ΔT\n2 in\nand R are functions of the leg-length ratio r. Consequently, P initially increases\nin max\nwith r and then decreases, reaching a maximum at r = 0.28.\nAccordingly, we have provided a more detailed discussion of the dependence of\nΔT and P on the leg-length ratio r in the revised manuscript and the Supplementary\nmax\nInformation.\nRevisions to the Results and Conclusion are shown below.\nOn page 13: “…The P of the device is governed by the U and the internal\nmax oc\nresistance (R ) of the sector-shaped TE leg as follows:\nin\nU 2\nP  oc (2)\nmax 4R\nin\nThe U is associated with the ΔT, demonstrating a decreasing trend as the length ratio\noc\nr increases (Supplementary Fig. 19a). The R of the sector-shaped TE leg can be\nin\ncalculated as follows:\n r  l r\nR  ln 2  ln 2 1 (3)\nin d r d r\n1 1\nwhere ρ is the electrical resistivity, d is the thickness of the TE films, θ, r , and r denote\n1 2\nthe sector angle, inner radius, and outer radius of the sector-shaped TE leg, respectively\n(Supplementary Fig. 19b). The increasing r decreases l , thereby reducing the R .\n2 in\nHence, both ΔT and R are functions of the leg-length ratio r. Consequently, P\nin max\ninitially increases with r and then decreases, reaching a maximum at r = 0.28.… ”\n5) Also the temperature difference data from the test under the outdoor performance\nshould be shown to verify whether the power output is indeed affected by the\ntemperature difference.\nResponse：\nThanks for your instructive suggestions. In the initial experiments, the ΔT between\nthe photothermal hot end and the cold end of the device was not recorded. To directly\nverify the correlation between the temperature difference and both the U and P of\noc out\nthe STEG, we conducted an additional set of experiments to simultaneously monitor\nthe ΔT and U . As shown in Figure R4a-b, an ultrafine thermocouple with the diameter\noc\nof 0.1 mm was attached to the back sides of both the photothermal and cold ends to\nrecord the temperature difference, while a digital source meter was used to record the\nU and the P at different time intervals (Figure R4c). The results shown in Figure\noc max\nR5 clearly demonstrate that the ΔT varies consistently with the solar intensity, and both\nthe U and P follow the same trend. We have added a brief discussion of this ΔT-\noc max\ndependent behavior in the revised manuscript, and the corresponding experimental\ninformation and updated figures have been included in the Supplementary Information.\nFigure R4. Temperature difference and performance test of the STEG. a\nPhotograph of the ultrafine thermocouples. b Position of the thermocouples. c Outdoor\nperformance testing setup of the STEG. The whole system includes a solar radiometer,\na thermometer, a digital source meter, a solar tracking system, and a computer. The\nenlarged picture shows an aluminum foil-coated foam used as a shading layer to restrict\nincident sunlight exclusively to the photothermal structure. The solar tracking system\nis used to test the light intensity and the device output performance.\nFigure R5. Daily variations in output performance and temperature difference of\nthe STEG packed in a foam box on the roof of a building in Baoding, China, on 21\nNovember 2025. The weather was sunny, with a maximum and average ambient\ntemperature of 16.3 °C and 5.9 °C, respectively, and wind speeds ranging from 0.3 to\n3.0 m s⁻1.\nRevisions to the Results and Conclusion are shown below.\nOn page 15: “…Further experiments revealed that the ΔT varied synchronously with\nthe solar irradiance, resulting in a change in the U and the P (Supplementary Fig.\noc max\n27-28).…”\n6) When the device was integrated into the actual wearable hat and bag, was the\nenvironment affected by wind or not? Since the device was not packaged and the\noutdoor experiment was conducted with a PE film covering in insulated box, it would\nbe helpful to provide detailed environmental data and experimental methods. This\nwould make it easier to understand the observed power output trends.\nResponse：\nWe appreciate your comment regarding the environmental influence during the\noutdoor testing. The outdoor tests reported in this work were conducted under low\nwind-speed (≤0.5 m s-1) conditions, resulting in a negligible influence of wind on the\ndevice. In the revised version, we have provided detailed environmental data and a more\ncomprehensive description of the outdoor experiment.\nHowever, to further address your concern, we conducted additional experiments\nto evaluate the influence of wind on the ΔT and U of the STEG integrated into an\noc\nactual wearable bag. The outdoor tests were carried out in Baoding, China, on 20\nNovember 2025, under sunny weather conditions. The maximum and average ambient\ntemperatures were 13.5 °C and 6.1 °C, respectively, with wind speeds ranging from 0.3\nto 2.5 m s⁻¹. The experimental setup, shown in Figure R6a, includes a solar radiometer,\na source meter connected to a computer for recording the U , a temperature logger for\noc\nmeasuring the ΔT, an anemometer for monitoring wind speed, and a portable power\nbank. The measurement of ΔT was performed using the same configuration as that\nshown in Figure R4a and b.\nThe results in Figure R6b indicate that the U varies synchronously with ΔT,\noc\nexhibiting a consistent trend whereby U increases with increasing ΔT. When the wind\noc\nspeed is lower than 0.5 m s-1, both ΔT and U remain relatively stable at higher values.\noc\nAs the wind speed increases, both ΔT and U decrease simultaneously. Notably, higher\noc\nwind speeds enhance convective heat loss on the photothermal side, resulting in a\nreduced ΔT and, consequently, a lower U . Nevertheless, such environmental\noc\nperturbations can be effectively mitigated in future practical applications through\nimproved system-level packaging and protection strategies.\nDT\n16\n12\n8\n4\n11:04 11:06 11:28 11:30\nTime\nFigure R6. Outdoor output performance of the STEG integrated on a backpack.\na The photograph of the experimental setup, which includes a solar radiometer, a source\nmeter connected to a computer for recording the U , a temperature logger for\noc\nmeasuring the ΔT, an anemometer for monitoring wind speed, and a portable power\nbank. b Time-dependent solar irradiance, temperature difference, and open-circuit\nvoltage. The tests were conducted in Baoding, China, on 20 November 2025, under\nsunny weather conditions. During the measurement, the maximum and average ambient\ntemperatures were recorded as 13.5 °C and 6.1 °C, respectively, with wind speeds\nvarying between 0.3 and 2.5 m s-1.\nRevisions to the main text are shown below.\n)K(\nTD\n30\n25\n20\n15\n)Vm(\nU\nco\n1000\na b\nAnemometer 800\nSolar irradiance W m-2\nTemperaturelogger\nComputer\nWind speed ≥ 2 m s-1\nDigitalsourcemeter Wind speed\n≤0.5 m s-1\nPortable power bank\nOn page 15: “…and its output performance was tested at noon on 21 September 2025\nin Baoding, with ambient temperature of ~24℃ and wind speed of ≤0.5 m s-1. A solar\nradiometer and a multimeter were used to measure the solar irradiance and U ,\noc\nrespectively. As shown in Fig. 5h, the STEG…”\nOn page 15: “…Further studies indicated that both ΔT and U exhibited small\noc\nfluctuations under continuous wind disturbance. However, when the wind speed was\nlow, a relatively high U is maintained, whereas higher wind speeds leaded to a\noc\nreduction in U (Supplementary Fig. 29). The variations of ΔT and U follow an\noc oc\nidentical trend. Nevertheless, such environmental perturbations can be effectively\nmitigated in future practical applications through improved system-level packaging and\nprotection strategies.…”\n7) End of the Results and discussion section, if the author demonstrates the briefly\nhighlight the advantages of this technology and discuss its potential applications, it will\nhelp for butter quality of the manuscript.\nResponse：\nWe sincerely appreciate your helpful suggestion. In the revised manuscript, we\nhave added a concluding paragraph to highlight the advantages of this technology and\nto discuss its potential application scenarios.\nRevisions to the Results and Conclusion are shown below.\nOn page 15-16: “These results demonstrate that the integrated STEG architecture\nproposed in this work offers significant advantages over conventional discrete devices\nand systems employing organic photothermal films. The structure enables strong\ncoupling between PT and TE conversion processes within a unified platform, thereby\nimproving heat utilization efficiency. Notably, in comparison to common flexible PT\nmaterials such as CNT12,13 and PEDOT14 films, the utilization of Ag Se film results in\n2\nsignificantly improved PF of 25.7 μW cm-1 K-2. In addition, the entire device\nfabrication relies solely on magnetron sputtering, which is highly compatible with\nMEMS fabrication and photolithography techniques, offering a scalable route toward\nwafer-scale production of STEGs. The integrated device delivers excellent output\nperformance under outdoor conditions, enabling it to power wearable health-\nmonitoring sensors (e.g., pulse or sweat-rate sensors). Moreover, its linear photothermal\nresponse and light-induced U characteristics indicate substantial potential for\noc\napplications in real-time outdoor light intensity sensing.”\n8) All the photographs in Figures, the scale bar is missing. Including it would greatly\nimprove the clarity and understanding of the images.\nResponse：\nWe apologize for this oversight. In the revised manuscript, scale bars have been\nincluded in all relevant figures. The specific figures include Figure 2f, Figures 4f and\n4h in the main text, and Figs. 1, 6, 7, 11, 16, 20, and 27 in the Supplementary\nInformation.\nReviewer #2 (Remarks to the Author):\nThe manuscript presented the fabrication and characterization of AgSe-based flexible\nsolar thermoelectric generators. The approach is promising, and the manuscript is\nlogically organized into sections. The results are remarkable compared to previously\nreported thermoelectric generators, however have a long way to be considered for real\npower utilities. This should be clear in several places at the main text of the manuscript,\nas indicated in the attached document and compared with theoretical and reported\nexperimental values in the literature. Further minor presentation issues should be\nrevised as indicated in attachment. All comments should be carefully addressed with\nline by line responses, before the manuscript is further considered.\nResponse：\nWe sincerely appreciate your positive evaluation and constructive feedback on our\nmanuscript. We fully agree with your critical assessment that, although our device\ndemonstrates competitive performance within the field of flexible thermoelectric\ngenerators, substantial advancements are still required before such technology can be\nconsidered for real-world power applications. In response to your valuable feedback,\nwe have incorporated a discussion of these limitations in several sections of the main\ntext, including the Introduction, Results and Discussion sections. Furthermore, we have\nincluded a comprehensive comparison between our experimental findings and both\ntheoretical and reported experimental values from the literature. All minor presentation\nissues highlighted in the attached document have also been carefully addressed and\nrevised. Our detailed, point-by-point responses are provided below.\n1) Do the authors mean visible anti-reflection? Please clarify and rephrase to avoid\nrepetitive words. (Line 27)\nResponse：\nWe apologize for the ambiguity in our original statement. The anti-reflection layer\nis specifically designed to operate primarily in the visible wavelength range. As shown\nin Figure R7, the introduction of the anti-reflection layer significantly enhances light\nabsorption, particularly in the visible band. In the revised manuscript, we have\nexplicitly clarified the effective wavelength range of the anti-reflection effect to ensure\nclarity and avoid potential misunderstanding. In addition, to clearly distinguish the\nfunctions of the two layers, we have provided a detailed description of the relative\npositions of the infrared reflective layer and the visible anti-reflection layer within the\nphotothermal structure.\nFigure R8. UV-Vis-NIR absorption spectra of the pristine and MD-Ag Se films.\nOn page 2: “…combined with a bottom infrared-reflective layer and surface visible\nanti-reflective coating....”\n2) What is the solar-to-current conversion efficiency of STEG? Please be transparent\nto the reader, by stating typical and maximum efficiency of STEG and comparing to\nemerging solar energy technologies such as TPV generators, see:\nhttps://doi.org/10.1016/j.solmat.2025.114023\nhttps://doi.org/10.1016/j.solmat.2016.08.018\nand 3rd generation PV solar cells, see\nhttps://doi.org/10.1002/pip.3599\nhttps://doi.org/10.1016/j.solmat.2015.04.020\nhttps://doi.org/10.1002/adom.201400402 (Line 39-43)\nResponse：\nThank you for your constructive comments. The record solar-to-current\nconversion efficiency of STEGs based on bulk thermoelectric materials has stands at\n~9.6 % 3, 4. In contrast, thermophotovoltaic (TPV) can achieve efficiencies exceeding\n35% ascribed to the high operational temperature of thermal emitters 5,6 and the\nutilization of highly efficient PV cells 7,8,9. Although the development of STEG\ntechnology has not yet achieved a notable breakthrough, the flexible STEG devices\ncapable of providing stable and sustained power have considerable potential for\npowering wearable sensors. As stated in the manuscript: “Crucially, flexible STEGs\nprioritize consistent energy output rather than peak efficiency, which is a trade-off well\nsuited to the requirements of low-power electronics and sensor networks. Such\nattributes render them highly suitable for wearable applications, where energy\nreliability and mechanical adaptability are essential.”\nIn the revised manuscript we have highlighted the current limitations of STEG\ntechnology and provided comparisons with emerging solar energy technologies.\nRevisions to the Introduction are shown below.\nOn page 2: “... Despite extensive research efforts over several decades, the record solar-\nto-electricity conversion efficiency of STEGs based on bulk thermoelectric materials\nremains at ~9.6%2, 3. In comparison, a significant gap still exists between STEGs and\nother emerging solar-energy conversion technologies, such as thermophotovoltaic\n(TPV) cells, which can achieve efficiencies exceeding 35% owing to the high\noperational temperature of thermal emitters4, 5 and the utilization of highly efficient PV\ncells 6, 7, 8. Consequently, STEGs still have a considerable distance to go before they can\nbe considered viable for large-scale power generation applications. However,...”\nReferences\n3. Kraemer D, et al. Concentrating solar thermoelectric generators with a peak\nefficiency of 7.4%. Nature Energy 1, 1-8 (2016).\n4. Kashyap RK, Pillai PP. Plasmonic Nanoparticles boost solar-to-electricity\ngeneration at ambient conditions. Nano Lett 24, 5585-5592 (2024).\n5. Mishra BR, Vossier A, Revol I, Almuneau G, Vaillon R. Insight into cooling\nrequirements for thermophotovoltaic devices. Sol Energy Mater Sol Cells 296,\n114023 (2026).\n6. Piness-Sommer M, Braun A, Katz EA, Gordon JM. Ultra-compact combustion-\ndriven high-efficiency thermophotovoltaic generators. Sol Energy Mater Sol Cells\n157, 953-959 (2016).\n7. Marques‐Hueso J, Peretti R, Abargues R, Richards BS, Seassal C, Martínez‐\nPastor JP. Photonic crystal-driven spectral concentration for upconversion\nphotovoltaics. Advanced Optical Materials 3, 568-574 (2014).\n8. Esmaielpour H, Lombez L, Giteau M, Guillemoles JF, Suchet D. Impact of\nexcitation energy on hot carrier properties in InGaAs multi-quantum well structure.\nProgress in Photovoltaics: Research and Applications 30, 1354-1362 (2022).\n9. Arnaoutakis GE, et al. Enhanced energy conversion of up-conversion solar cells\nby the integration of compound parabolic concentrating optics. Sol Energy Mater\nSol Cells 140, 217-223 (2015).\n3) Is AgSe the only TE material? A rationale and brief comparison with other TPV\nmaterials, see https://doi.org/10.1016/j.solmat.2025.113636\nhttps://doi.org/10.1063/1.4931428 is missing from the manuscript to provide a clear\nintroduction to the reader. (Line 62-63)\nResponse：\nThanks for your suggestion. Besides Ag Se, n-type Bi Te Se , p-type\n2 2 2.7 0.3\nBi Sb Te 10, 11 and emerging Mg Bi -based materials12, 13 with superior TE\n0.5 1.5 3 3 2\nperformances at room temperature can be utilized as potential candidates. However,\nAg Se presents several advantages that make it particularly attractive for flexible and\n2\nwearable TE applications. Its room-temperature TE performance is comparable to that\nof BiTe-based alloys, while its inherently low Young’s modulus and non-layered crystal\nstructure endow it with superior mechanical flexibility14. Moreover, Ag Se is free of the\n2\nmildly toxic Te and exhibits much better environmental stability than Mg-based TE\nmaterials. These combined features explain its widespread use and relevance in flexible\nTE device research.\nWe have supplemented the introduction with additional background to clarify the\nrationale for choosing Ag Se and have incorporated the two suggested references to\n2\nhelp readers better understand the context and significance of our study.\nRevisions to the Introduction are shown below.\nOn page 3: “Wearable applications require higher room-temperature (RT) TE\nperformance. Although n-type Bi Te Se , p-type Bi Sb Te 17, 18 and emerging\n2 2.7 0.3 0.5 1.5 3\nMg Bi -based materials 19, 20 with superior TE performances at RT can be considered as\n3 2\npotential candidates, Ag Se is particularly well-suited for flexible and wearable\n2\nthermoelectric applications due to its balanced combination of high TE performance at\nRT 21, 22, mechanical compliance23, and environmental stability….”\nReferences\n10. Vossier A, Blandre E, Vaillon R. Hybridising photovoltaics and thermoelectrics: A\ndetailed-balance analysis. Sol Energy Mater Sol Cells 289, (2025).\n11. Beeri O, Rotem O, Hazan E, Katz EA, Braun A, Gelbstein Y. Hybrid photovoltaic-\nthermoelectric system for concentrated solar energy conversion: Experimental\nrealization and modeling. J Appl Phys 118, (2015).\n12. Mao J, et al. High Thermoelectric cooling performance of n-type Mg Bi -based\n3 2\nmaterials. Science 365, 495-498 (2019).\n13. Zhao P, et al. Plasticity in single-crystalline Mg Bi thermoelectric material.\n3 2\nNature, (2024).\n14. Lin S, et al. Revealing the promising near-room-temperature thermoelectric\nperformance in Ag Se single crystals. J Materiomics 9, 754-761 (2023).\n2\n4) The reader will wonder, is 8.8 mV a high-performance value? Please explain by\ncomparing to theoretical and record values for TEGs. (Line73-75)\nResponse：\nThank you for your valuable suggestions. The open-circuit voltage (U ) of 8.8\noc\nmV is not a record value when compared with devices employing a larger number of\nthermoelectric legs or operating with an additional PT layer, as shown in Table 1 below.\nThis mainly arises from the limited light-absorption efficiency of the pristine Ag Se\n2\nfilm, which restricts the achievable ΔT between the hot and cold ends. Therefore, it is\nnecessary to enhance the light absorption capacity through integrated design and further\nimprove the effective temperature difference of the STEG under solar irradiation. In the\nrevised manuscript, we pointed out the current research limitations, listed the typical\nU values for STEG reported in recent literature, and added the corresponding table to\noc\nthe Supplementary Information.\nRevisions to the Introduction are shown below.\nOn page 4: “…the achieved U remains modest relative to that of most flexible STEGs\noc\n(Supplementary Table 1).”\nSupplementary Table 1. A summary of the open-circuit voltage of recently reported\nflexible STEGs12, 13, 14, 15, 16, 17, 18, 19, 20, 21. (TE: thermoelectric PT: photothermal SWCNT:\nsingle-walled carbon nanotubes)\nTE materials N of TE legs PT matraites Light intensity U (mV) Ref\noc\nSWCNT 36 pairs SWCNT 75 mW cm-2 52.4 12\nSWCNT 24 Commercial 100 mW cm-2 41.6 13\ncoating\nPrinted Ag Se 8 carbon-based 100 mW cm-2 16.1 14\n2\nfilm\nPEDOT:PSS 3 PEDOT:PSS film 186.9 mW cm-2 0.8 15\nfilm\nCu Te/Ag Te 8 Ag Te nanowires 61.4 mW cm-2 8.9 16\n1.75 2 2\nCNT-film 28 CNT-film 80 mW cm-2 49 17\nAg Se/MXene 12 TiO 120 mW cm-2 5 18\n2 2\nTE fabrics 20 PEDOT:PSS 100 mW cm-2 20.14 19\nTE fabrics 81 PT membrane 100 mW cm-2 78 20\nAg Se/Nylon 4 Ag Se 100 mW cm-2 8.8 21\n2 2\n5) At room temperature 300K. (Line 112-113)\nResponse：\nWe apologize for the lack of a specific temperature value for room temperature.\nSince the ambient temperature during the thermoelectric measurements of different\nsamples was not strictly constant and fluctuated within 300 ± 2 K, we have clarified\nthis point in the revised manuscript by indicating the room temperature as “~300 K”.\nRevisions to the Introduction are shown below.\nOn page 6: “…room-temperature (~300 K) electrical conductivity (σ) of the film\nincreases with Se…”\n6) The bending process cannot be clearly seen. A clear schematic with reference and\nexplanation of the photo in supplemental, can address concern. (Line 128-134)\nResponse：\nWe appreciate your comments. To address this concern, we have added a detailed\nphotograph of the bending test setup in Figure 2h. In addition, we have included the\ncorresponding photograph in Supplementary Information, along with detailed\nannotations and explanations to clarify the procedure used for the bending test.\nRevisions to the Supplementary Information are shown below.\nOn Supplementary Fig. 6.:\nSupplementary Fig. 6. Schematic and experimental setup for the bending test of\nthe flexible Ag Se film. a Schematic illustration of the bending process. b Photograph\n2\nof the Ag Se film for the bending test. A 100-nm-thick Ag film is used as the electrode\n2\nto ensure ohmic contact between the film and the Cu wires. c Experimental setup\nshowing the film in its initial flat state, with Cu wires connected for resistance\nmeasurements. d Film in the bent state.\n7) A brief description and equation of the PCE is required to distinguish from power\nconversion efficiency. This can be added in the figure or its caption. (Line241-242)\nResponse：\nWe sincerely thank you for this valuable and insightful suggestion. To clearly\ndistinguish the photothermal conversion efficiency (PCE) from the power conversion\nefficiency, we have added a brief description and the corresponding equation of PCE in\nthe revised manuscript. Furthermore, the photothermal conversion efficiency is denoted\nby the symbol η instead of the abbreviation PCE to avoid potential confusion and to\nenhance clarity. Based on this definition, the calculated photothermal conversion\nefficiencies of the pristine and MD-Ag Se films are 31.2% and 87.6%, respectively.\nefficiency from the photothermal conversion efficiency.\nRevisions to the Results and Discussion are shown below.\nOn page 10: “The photothermal conversion efficiency (η) of the films was evaluated\nbased on the following equation49, 62, 63:\nhsT T \n max surr (1)\nIA\n\nwhere h is the heat-transfer coefficient, s is the surface area of the film, T is the\nsurr\nambient temperature, T is the maximum equilibrium temperature of the film, I is the\nmax\nsolar irradiance, A is the absorptance of the PT film (details of the calculation are given\nλ\nin Supplementary Note 3 and Fig. 13).…”\nReviewer #3 (Remarks to the Author):\nThe paper under review proposes a thin-film structure made of silver selenide, which is\ncomprehensively demonstrated to act as a photothermal absorber and thermoelectric\ngenerator. An interesting characterization of the active materials has been performed\ntogether with the reasons supporting their choice. The proposed converter ring-shaped\nstructure is interesting and novel. The testing under outdoor conditions strengthens the\nachieved results for a proof-of-concept demonstration of microwatt-range output power.\nTherefore, in my opinion paper is interesting and novel, and it seems to have\ncomprehensively characterized the material, designed a novel device structure, tested\nits performance, and probed its possible wearable application.\nI have identified the following two weak points, which I suggest the Authors address:\nResponse：\nThank you for reviewing our manuscript and providing positive feedback. We\nappreciate your favorable comments regarding the material characterization, device\nstructure design, outdoor testing, and potential wearable applications. We have\ncarefully considered your comments and fully agree with your assessment. Our point-\nby-point responses to the specific comments are provided below.\n1) Although Fig. 1a and 1c can be considered self-explanatory, the caption describes\nthem poorly. The Authors are requested to explain all the elements therein reported and\nspecify explicitly the related citations to which their technology is compared.\nResponse：\nWe appreciate your constructive suggestions regarding improving the figure\ncaptions. In the revised manuscript, we have provided a more detailed explanation of\nall elements shown in Fig. 1a, including the working mechanism of the multilayer\nstructure. In addition, we have clarified the references used for comparison in Fig. 1c\nby explicitly describing the corresponding studies and citing their reference numbers in\nthe caption.\nRevisions to Figure 1 are shown below.\na visible\nultraviolet infrared\nNarrow bandgap TE films\nwide Eg narrow Eg\nλ\nconduction band\nelectron\ndefect\nhole\nvalence band\nFigure 1. Overview of the design and performance of the integrated STEG. a\nSchematic of the ring-shaped STEG integrated with a multilayer photothermal structure.\nThe right panel shows the working mechanism of the structured design, highlighting\nthe broadband absorption enabled by narrow-bandgap materials, the non-radiative\nrecombination-driven photothermal conversion mechanism, and the selective\nabsorption characteristics of the multilayer structure. b Schematic diagram of the actual\nmeasurement condition of the STEG. c Comparison of output power density of the all-\nin-one STEG in this work and others, such as CNT12, printed Ag Se film31 and TE\n2\nfabrics38, 39, 40 based STEGs.\n2) Fig. 3a does not seem to be coherent with the other reported structural,\nmorphological, and compositional characterizations. I suggest shifting it (into Fig. 4?).\nResponse：\nWe appreciate your insightful suggestion. Following your recommendation, we\nhave reorganized the figure layout to improve consistency and readability. Specifically,\nsince Figure 4c already presents the temperature curve of the structured Ag Se film\n2\nunder 1 sun illumination, we have moved the visual comparison between the structured\n）%（ecnatcelfeR\nb\nXe lamp\nelectrode\nVoltage\nCurrent\nPower density: 0.17 μWcm-2& 1 sun\nheat 0.42 & 2 suns c\nRC-layer 0.29 & 1.5 suns This work\n0.17 & 1 sun\n6E-7 & 1 sun TE layer\n1.9E-5 & 1 sun TE fabrics\nBarrier\n3.2E-4 & 1 sun\nIR-layer 2.4E-6 & 1 sun Printed Ag2Se based\nPI 0.12 & 1 sun CNT-based\n0 2.5 8 20 0.32 & 2 suns\nWavelength (μm)\nPower density (μW cm-2)\nand pristine films to the Supplementary Fig. 8a. Correspondingly, Figure 3 has been\nreformatted.\nRevisions to Figure 3 are shown below.\nFigure 3. Schematic and microstructure of MD-Ag Se film. a Schematic of the MD-\n2 2\nEnlarged view of region 1 in c, showing the thickness of the double anti-reflection layer.\nd High-resolution image and selected-area electron diffraction pattern of region 2 in b,\nwhere the zone axis is identified as [100]. e SAED pattern of region 1 in b, where the\nzone axis is determined to be [010]. f SAED pattern of the Ag layer. g EDS-mapping\nimage of the entire cross-section. h Distribution of Si, Al, Ag, W, and Se elements. Scale\nbar is 500 nm."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have thoroughly addressed the previous comments, and the revised manuscript is well written and organized. The responses significantly improve the clarity and overall understanding of the work. I have no further comments.\nThank you.", "reviewer_2": "The authors applied detailed revisions to the line-by-line concerns of the review. The concerns of the review have been successfully addressed. As a result, the manuscript has been considerably improved and is now a more clear body of work to be published.", "reviewer_3": "After a careful reading of the paper, this current version meets all my suggestions and considerations. Therefore, in my opinion, it can be published."}, "author_response": "Authors' response to the reviewers\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors have thoroughly addressed the previous comments, and the revised\nmanuscript is well written and organized. The responses significantly improve the\nclarity and overall understanding of the work. I have no further comments.\nThank you.\nResponse:\nWe sincerely thank the reviewer for the positive comments and appreciate your time\nand constructive evaluation.\nReviewer #2 (Remarks to the Author):\nThe authors applied detailed revisions to the line-by-line concerns of the review. The\nconcerns of the review have been successfully addressed. As a result, the manuscript\nhas been considerably improved and is now a more clear body of work to be published.\nResponse:\nWe are grateful for the reviewer’s careful reading and helpful suggestions, which have\nimproved the manuscript.\nReviewer #3 (Remarks to the Author):\nAfter a careful reading of the paper, this current version meets all my suggestions and\nconsiderations. Therefore, in my opinion, it can be published.\nResponse:\nWe thank the reviewer for your kind assessment and for confirming that the manuscript\nis suitable for publication."}]

Decision letter