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Manufacturing products that are of the highest possible quality is key to DeVilbiss Healthcare. The latest Compact 525 oxygen concentrator is designed and built to meet the high demands of patients and cost challenges faced by service providers.In commitment to our quality aims, we have invested in new manufacturing processes and innovative production techniques. The new black, Compact 525 is now manufactured at our dedicated oxygen therapy facility in Pennsylvania, USA, which has given us the control to deliver a reliable, high quality, more eco-friendly oxygen concentrator.Delivering up to 5 litres per minute with high oxygen concentration across all flow rates and adopting the latest developments in the reduction of sound and power consumption, the latest Compact 525 increases patient comfort and lowers operating costs.

GodCare Oxygen concentrator adopts international advanced oil-free lubricate air compressor technology and gas separation technology , it uses PSA gas separation to separates oxygen from air, improving the concentration up to 94% and supplying oxygen continuously.

This product has numbers of feature, such as long continues work time, simple operation, easy to use, long lifetime, low power consumption, low noise, etc. Its operation cost is very low and even 1 kilowatt hour is sufficient for up to 3 hours continuous working, so it is perfect for family healthcare using, and its compact and small-sized design is able to provide you the ideal portable healthcare equipment for outdoor travel.

## top 5 fuel efficient 150cc bikes in india: best mileage bikes

The Indian two wheeler maret is abuzz with a lot of motorcycles, starting from the entry-level commuter bikes to out and out performance motorcycles. Fuel efficiency of a motorcycle has been one of the most important factors in India while buying a new bike in the Indian two-wheeler market. In order to capture these buyers, two wheelers manufacturers in India are launching 150cc bikes that are stylish, powerful and provide good fuel efficiency. Here, we bring you a list of top 5 most fuel efficient bikes in India in the 150cc segment.

Hero XtremeWith fuel efficiency of 62Kmpl this is the most popular bike from Hero. It has a 149.2cc single cylinder engine that churns our 14.2bhp at 8500rpm.Priced at INR 71,925 (ex-showroom Delhi) for self start disc brake alloy wheel and INR 75,025 (ex-showroom) for self start front and rear disc alloy wheel we think this bike is good value for money.

Bajaj Pulsar 150The best and the oldest bikes in domestic market is the Bajaj Pulsar 150. It has a 149cc engine from the DTS-i engine family and produces 14.85 bhp at 9,000 rpm and 12.5 Nm at 6,500 rpm. With a mileage of 60kmpl. Priced at INR 70,757(ex-showroom Delhi) the Pulsar 150 is powered by a Stylish looks, good performance and affordable pricing made her famous among youngster and daily commuters

Bajaj Discover 150With 72Kmpl fuel efficiency it is the most fuel efficient 150cc bike available in India and that too within affordable pricing of INR 51,720 (ex-showroom Delhi). Powered by a 144.8cc, single cylinder engine which produces 14.3 bhp at 8,500 rpm and 12.75 Nm at 6,500 rpm, it is launched in two variants both offering the same mileage.

Honda CB Unicorn 160Unicorn comes with a 163cc engine that produces 14.5bhp and 14.61Nm torque. Though company claimed its fuel efficiency to be 64Kmpl it actually gives around 60kmpl. It is the most comfortable 160cc bikes on Indian roads and with pricing of INR 77,119 (ex-showroom Delhi) it is light on your wallet for every day ride.

Suzuki GixxerOne of the most successful bikes from Suzuki, Gixxer is that has a great acceleration and power in 150cc segment. The bike has a 154.9cc engine, single cylinder, air cooled, single stroke engine that pumps out 14.8Ps power at 8,000 rpm and 14Nm of peak torque at 6,000 rpm. The bike is available with a price tag of INR 84,634 (ex-showroom Delhi).

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## large stokes shift and high efficiency luminescent solar concentrator incorporated with cuins 2 /zns quantum dots | scientific reports

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Luminescent solar concentrator (LSC) incorporated with quantum dots (QDs) have been widely regarded as one of the most important development trends of cost-effective solar energy. In this study, for the first time we report a new QDs-LSC integrated with heavy metal free CuInS2/ZnS core/shell QDs with large Stokes shift and high optical efficiency. The as-prepared CuInS2/ZnS QDs possess advantages of high photoluminescence quantum yield of 81% and large Stocks shift more than 150nm. The optical efficiency of CuInS2/ZnS QDs-LSC reaches as high as 26.5%. Moreover, the power conversion efficiency of the QDs-LSC-PV device reaches more than 3 folds to that of pure PMMA-PV device. Furthermore, the PV device is able to harvest 4.91 folds solar energy with the assistance of this new CuInS2/ZnS QDs-LSC for the same size c-Si PV cell. The results demonstrate that this new CuInS2/ZnS QDs-LSC provides a promising way for the high efficiency, nonhazardous and low cost solar energy.

Lowering the cost of generating per unit power is one of the most important issues in global solar photovoltaic (PV) energy technology today in which fundamental researches toward reaching high conversion efficiency goes hand in hand with those on lowering production cost1,2,3. Concentrating sunlight is considered as an important way to decrease the cost of PV electricity generation, since the rise of the energy density will reduce the usage area of PV cells dramatically. Furthermore, conversion of the incident solar spectrum to material related absorption wavelength ranges which have efficiently PV effect would greatly increase the efficiency of PV cells. Therefore, the concept of luminescent solar concentrator (LSC) was proposed in solar energy conversion to serve the above beneficial purposes.

LSC was initially suggested in the late 1970s which had potential to enhance the economic viability of solar energy4,5,6,7. Basically it consists of a highly transparent planar sheet incorporated with suitable luminophores, which including fluorescent organic dye molecules or inorganic quantum dots (QDs) that absorb the incident sunlight and re-emit it at longer wavelength which could be absorbed by PV cells more efficiently. LSC could concentrate both direct and diffuse incident sunlight and then guide towards the side edges through the total internal reflection (TIR). This design is attractive when it is implemented with high efficiency compound semiconductor PV cells at the edges of LSC, which named LSC-PV device. LSC-PV device could reduce the cost of solar energy by not only allowing replacement of expensive large area PV cells with cheaper solar-harvesting antennae coupled to the small ones, but also reaching high conversion efficiency. Whats more, it can convert high energy photons, which less absorbed by PV cells, into low energy photons, which more absorbed by PV cells, by the luminophores and further improve the conversion efficiency of PV cells. Figure 1(a) shows schematic of QDs-LSC edge-attached with PV cell and illustrates the principle of the device. This system can be integrated into the electronic displays, solar windows as well as other glazing systems8. Therefore, it will be a cost-effective alternative to optics-based solar concentration systems. Figure 1(b) illustrates three major behaviors of light in the QDs-LSC device, including light converting by QDs, Rayleigh scattering by nanoscale particles (QDs) and light guiding due to the TIR.

(a) Schematic of the conventional flat QDs-LSC-PV device and illustrates the principle of the device. (b) Three behaviors of light in this device: light converting, Rayleigh scattering and light guiding.

In recent years, many efforts have been made to increase the efficiency of LSCs, especially focusing on many promising luminophores for applications in LSCs. Liu et al. demonstrated that multiple organic dyes doped into LSC would enhance the absorption and efficiency9. Tummeltshammer et al. reported a new way to brighten LSC through homeotropic alignment and Frster resonance energy transfer between organic dyes and liquid crystals10. Organic dyes doped LSC was intensively investigated until researches highlighted the limited properties of organic dyes, such as narrow absorption band, poor photo-stability and large reabsorption. QDs doped LSCs have advantages over organic dyes since QDs possess a larger absorption region than that of dyes and are able to tune the absorption and emission spectra simply by adjusting core diameter and, being crystalline semiconductors, they are more stable and less degradable than organic dyes. Meinardi et al. introduced Stokes-shift-engineered core/shell CdSe/CdS QDs which allow absorption and emission tunable across the entire solar spectra for incorporation in LSC11. Wilton et al. reported using PbSe QDs as the active fluorescent material and the self-absorption could be reduced by utilizing Frster resonance energy transfer between two different sizes of PbSe QDs12. Bradshaw et al. proposed to minimize reabsorption in large scale LSC by the doped Cd1-xCuxSe QDs13. However, the wide employment of heave metal such as Cd and Pb in QDs-LSC devices would be harmful to the environment as well as to our human beings.

It is true that different applications demand specifically tailored luminophores, the achievements of commercial viability in various LSC configurations require new luminophores that possess high efficiency, low reabsorption and nontoxicity. CuInS2/ZnS core/shell QDs possess high optical absorption coefficient facilitating extensive utilization of the solar spectrum, large Stokes shift, broadband luminescence, superior stability under solar radiation and direct band gap of 1.5eV which overlaps well with solar spectrum as well as absence of toxic elements14,15,16,17. In this study, we designed to assess the viability of heavy metal ions free CuInS2/ZnS core/shell QDs as the luminophores for LSC device for the first time to the best of our knowledge. A new LSC integrated with CuInS2/ZnS core/shell QDs with large Stokes shift and high optical efficiency was proposed. Moreover, the power conversion efficiency (PCE) of the PV cell attached at the side of the LSC at simulated sunlight also increased significantly comparing to that of PV cell with pure polymethyl methacrylate (PMMA) as light guider.

Colloidal CuInS2/ZnS core/shell QDs were synthesized via one-pot method. By using the air-stable non-coordinating solvent paraffin liquid to slow down the reaction rate, a spherical shape and nearly monodispersed CuInS2/ZnS QDs with the average size of 4.00.2nm were obtained as shown in the Transmission Electron Microscopy (TEM) image, Fig. 2(a) and High Resolution TEM (HRTEM) image, Fig. 2(b). The X-Ray Diffraction (XRD) pattern of CuInS2 and CuInS2/ZnS QDs are illustrated in Fig. 2(c). The main peaks of CuInS2 (CIS) are observed to move towards larger angel for CuInS2/ZnS (CIS/ZnS) indicating the well inorganic alloyed and coating18,19.

Figure 3(a) shows the normalized absorption and emission spectra of CuInS2/ZnS QDs in chloroform solution. The emitting peak of CuInS2/ZnS QDs is observed at 550nm and the full width at half maximum (FWHM) is about 125nm. Moreover, the photoluminescence quantum yield (PL QY) of the QDs reaches to 81%. More importantly, combining with the PL spectrum and absorption spectrum, CuInS2/ZnS QDs are considered to possess weak self-absorption due to the core-shell structure of QDs20,21 and the Stocks shift reaches as large as more than 150nm. We can find from Fig. 4(b) that, for the CuInS2/ZnS QDs, the emission wavelength is mainly dominated by the CuInS2 core and Zn-CuInS2 nanocrystals, while the absorption wavelength is mainly dominated by the ZnS shell with wider band gap since it has much larger amount than that of core material. In addition, Fig. 3(b) demonstrates that CuInS2/ZnS QDs are able to convert light with wavelength less than 450nm into light with wavelength around 550nm effectively, which could be absorbed by the c-Si PV cells more efficiently.

(a) Normalized absorption and emission spectra of CuInS2/ZnS QDs. (b) The relationships of AM 1.5G spectrum, c-Si PV cells responsive spectrum and the absorption as well as emission spectra of CuInS2/ZnS QDs.

(a) Normalized emission spectra of CuInS2 core QDs and CuInS2/ZnS core/shell QDs. (b) Photoluminescence emission mechanism of CuInS2/ZnS core/shell QDs. (c) Schematic of the changes of energy gap with the addition of Zn2+.

The emission wavelength was observed to blue shift from 623nm for CuInS2 QDs to 550nm for CuInS2/ZnS QDs as shown in Fig. 4(a), while the PL QY value was consequently improved from 23% for pure CuInS2 QDs to 81% for CuInS2/ZnS QDs with ZnS shell coating. This phenomenon is partially different from the typical binary QDs, such as CdSe for inorganic coating22,23,24, especially for the significant blue shift of emission wavelength after ZnS shell coating. Typical binary QDs inorganic coating mechanism is shown in Fig. 4(b). Wider band gap materials, such as ZnS, are always acting as shell by epitaxial growth on cores with lower valence band and higher conduction band than that of core materials to obtain the core-shell structures. The exciton will be well confined in the cores band gap as shown in Fig. 4(b) and the coated QDs is no longer sensitive to longer wavelength photos due to the interaction of outer shell absorption character in this core-shell structure. Moreover, with the inorganic coating, the surface defects states, such as dangling bands, surface imperfections, etc., will be removed efficiently which is beneficial to the exciton radiate recombination and resulting in the increase of PL QY value of QDs. For CuInS2/ZnS QDs, besides the benefits of inorganic shell coating for typical binary QDs such as low self-absorption and high PL QY, the ion exchange effect is implemented to explain the blue shift phenomenon showing in Fig. 4(c). The Zn2+ ions are used to form the ZnS inorganic shell and incorporating into to CuInS2 lattice structure to replace the Cu+ and In3+ and therefore reduce the core region resulting in the blue shift since the quantum size effect. Meanwhile, the gradual Zn2+ incorporating brings alloying effect by the formation of Zn-CuInS2 nanocrystals which possess a broader emission band as shown in Fig. 4(c). Increasing amount of Zn2+ incorporating into CuInS2 results in emission band enlargement and emission peak blue-shift for the final CuInS2/ZnS QDs18,25,26.

In order to evaluate the performance of these QDs in the application of LSC device, we have fabricated a CuInS2/ZnS QDs-LSC prototype sized as 22mm22mm3mm, yielding a geometric gain (denoted as G, the surface area of the top face divided by the surface area of the edges) of 1.83, by incorporating the CuInS2/ZnS QDs into the PMMA matrix by an in-situ polymerization method described in the experimental section. Figure 5 presents the photographs of the CuInS2/ZnS QDs-LSC devices under daylight and ultraviolet (UV) light, where the left sample is pure PMMA plate and the right sample is the LSC with QDs incorporation. The LSC reveals good transparency under daylight and luminescence performance under UV light.

Figure 6 presents the absorption and emission spectra of the CuInS2/ZnS QDs in solution and PMMA matrix. The QDs-LSC has successfully maintained the emission peak of original QDs. In addition, the decease of FWHM (about 8nm) for QDs from solution to solid was observed due to the circumstance alteration. As known from literature, CuInS2 QDs intrinsically possess broad emission due to not only the size distribution but also the distinct lattice vibration27. We assume that the vibration behavior would be more restricted in solid PMMA matrix than that in solution resulting in the narrower FWHM for QDs-LSCs PL emission to that of QDs in liquid.

Moreover, the PL QY of QDs-PMMA composite was measured as 56% which was somewhat lower than that of QDs in solution possibly due to the alterations of QDs concentration from dilute to dense and circumstance from liquid to solid. QDs luminescence properties had successfully maintained to a large extant during the chemical polymerization. The good performances of CuInS2/ZnS QDs in PMMA demonstrate they have very potential to be applied in LSC device.

The behavior of a QDs-LSC device under different excitation wavelength is usually to be evaluated in terms of its optical efficiency , which defines as the number of photons emitted from sides of QDs-LSC device (Nem) divided by the total number of photons absorbed by the QDs-LSC device (Nab) as Equation 1 as follows:

An integrating sphere system is adopted to measure the spectrum as well as the optical power of the light emitted from and absorbed by the QDs-LSC device so as to obtain . The calculation of is defined as Equations 2 and 3 as follows:

where L1 is the number of excitation photons while no sample is in the integrating sphere and L2 and P2 are the number of excitation photons that not absorbed and the number of emission photons that emitted from the sample while it is not directly illuminated by the excitation light, respectively. Similarly, L3 and P3 are the number of excitation photons that not absorbed and the number of emission photons that emitted from the sample while it is directly illuminated by the excitation light, respectively.

The values of as a function of excitation wavelengths are shown in Fig. 7, which have been measured using an integrating sphere with diameter of 120mm. The process of measurement can be summarized by two steps. Firstly, the total optical efficiency from all surfaces of CuInS2/ZnS QDs-LSC device was measured, defined as 1. Secondly, black carbon paint was used to cover the edges of the device such that the light could only be emitted from the top and bottom faces of the device and the optical efficiency 2 was then measured again as previously stated. By subtracting 2 from 1, the final optical efficiency (=12) was obtained, defined here as the fraction of the emission from the edges alone.

As we can see from Fig. 7, the value of is dependent on the excitation wavelengths and the maximum value of locates at 460nm (fitting curves) reaching as high as 26.5%. These are comprehensive results affected by several factors simultaneously, including different responses of CuInS2/ZnS QDs excited by excitation light with different wavelengths, different absorption of the PMMA matrix of LSC as well as different light scattering behavior of nanoparticles (e.g. QDs) against different wavelengths. Moreover, the maximum response point is near 460nm which indicates that the QDs-LSC device will be most efficient for light converting and light guiding near the specified wavelength from the sunlight.

In order to well perform the new CuInS2/ZnS QDs-LSC, we assembled two LSC-PV devices, QDs-LSC-PV and pure PMMA-PV, as shown in Fig. 8(b). The CuInS2/ZnS QDs-LSC and the pure-PMMA plate possess the same size and are integrated with the same commercial c-Si PV cell with the given PCE of 13%. These devices were illuminated by a solar simulator with an air mass 1.5 global illumination (AM 1.5G, 100mW/cm2) during experiments. Photocurrent density-voltage (JV) curve is a very important characterization to evaluate the performance of one PV device. The JV curves as well as other important parameters, including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and PCE, of these two different LSC-PV devices are provided in Fig. 8(a) and Table 1. We can find that the photocurrent density increases significantly as the incorporation of CuInS2/ZnS QDs. In details, the values of Voc and Jsc increase from 0.72V and 7.2mA/cm2 to 0.91V and 14.8mA/cm2 of the same c-Si PV cell combined with pure PMMA and CuInS2/ZnS QDs-LSC respectively. More importantly, the PCE of LSC-PV device has been increased from 2.73% for pure PMMA to 8.71% for CuInS2/ZnS QDs-LSC as much as more than 3 folds. The performance enhancement is mainly due to the addition of CuInS2/ZnS QDs-LSC which absorbs light with short wavelength and emits light with long wavelength that is more sensitive for the c-Si PV cell. Moreover, though the PCE value of the new CuInS2/ZnS QDs-LSC-PV device (8.71%) is lower than that of commercial PV cell (13%), the solar harvest area is enlarged for 7.33 folds to the same size c-Si PV cell, which means we can harvest 4.91 folds solar energy with the assistance of the CuInS2/ZnS QDs-LSC for the same size c-Si PV cell. In other words, only much smaller size c-Si PV cell is needed by using CuInS2/ZnS QDs-LSC to generate the same electrical power, which will reduce the cost of solar photovoltaic system dramatically.

(a) JV characteristics of c-Si PV cells combined with pure PMMA as well as CuInS2/ZnS QDs-LSC with the same size of 22mm22mm3mm for light harvest under AM 1.5G illuminated. (b) Different schematic of LSC-PV devices.

In this research, a new type of QDs-LSC integrated with heavy metal free CuInS2/ZnS core/shell QDs with large Stokes shift (larger than 150nm) and high photoluminescence quantum yield (81%) has been proposed for the first time. Performance both of CuInS2/ZnS QDs and its related QDs-LSC are described and analyzed in detail. The optical efficiency of the new CuInS2/ZnS QDs-LSC reaches as high as 26.5%. Moreover, the power conversion efficiency of the c-Si PV cell attached at the side of the LSC increases as much as more than 3 folds from 2.73% for pure PMMA-PV device to 8.71% for the CuInS2/ZnS QDs-LSC-PV device. Furthermore, the PV device is able to harvest 4.91 folds solar energy with the assistance of the new CuInS2/ZnS QDs-LSC for the same size c-Si PV cell. This new CuInS2/ZnS QDs-LSC provides an effective way for the high efficiency, nonhazardous and low cost solar energy.

Copper () iodide (CuI, 99.999%), indium () acetate (In(OAc)3, 99.99%), zinc stearate (Zn(St)2, 1012% Zn basis), 1-dodecanethiol (DDT, 98%), methyl methacrylate (MMA, 99%), 2, 2-azobis(2-methylpropionitrile) (AIBN, 99%), paraffin liquid, n-hexane, chloroform and absolute ethanol were used as raw materials. All chemicals were used as received without any further purification.

All synthesizes were performed in the non-coordinating solvent paraffin liquid under an argon atmosphere using the standard Schlenk techniques. CuI (0.25mmol), In(OAc)3 (1mmol), DDT (10mL) were mixed with paraffin liquid (10mL) in a 50mL three-neck flask and then degassed at 120C for 30min and then Ar-purged. The mixture was then heated to 250C for 3min. With the temperature increased, the color of the reaction solution gradually changed from slight green to transparent orange, red and finally brownish red and the CuInS2 core were synthesized.

Subsequently, for the synthesis of CuInS2/ZnS core/shell QDs, Zn(St)2 (16mmol), DDT (8mL) and paraffin liquid (16mL) were mixed and added to the core solution under argon atmosphere and then the mixture was heated to 260C and maintained for 120min to obtain final core-shell QDs. The QDs were purified and stored for further use.

A practical device of efficient CuInS2/ZnS-LSC requires the incorporation of QDs into high optical quality transparent matrix, such as polymethyl methacrylate (PMMA). The optical-grade PMMA is typically produced by the bulk polymerization of MMA in the presence of thermal radical initiators, such as az-compounds and peroxides, which carried out in a thermostatic water bath. To generate homogeneous and transparent nanocomposites, it is necessary to transfer the nanocrystals into the monomer solution to form to a stable and homogeneous dispersion before the process of polymerization. For thermal polymerization of QD-PMMA, the process was characterized by two steps, called pre-polymerization and post-polymerization.

Firstly, MMA (20mL) monomer and AIBN (0.2%wt/wt with respect to MMA) were added to a 50mL beaker and kept stirring until the AIBN was dissolved completely. Then the mixture was transferred into a 50mL three-neck round-bottom flask. Subsequently, CuInS2/ZnS QDs chloroform solution was dropwise added into the flask and the mixture was homogeneously dispersed by the ultrasound treatment. After that, the flask was placed into the thermostatic water bath at 70C for the desired reaction time and cooled to room temperature when the mixture reached certain viscosity. Then, the viscous liquid was introduced into the casting mould. Secondly, the casting mould was placed in the vacuum oven at 45C and kept at this condition for 24h. Finally, the nanocomposite was heated to 70C overnight. Then the resulting composite was cut into squares and polished for optical measurement so that LSC device was obtained.

The PMMA-QDs composite was obtained, tailored and polished to 22mm22mm3mm bulk sharp to achieve QDs-LSC. The sunlight receiving panel of c-Si solar cell was pasted on one of 22mm3mm faces. One 22mm22mm face of QDs-LSC was exposured under the solar simulate light source for further tests.

The High Resolution Transmission Electron Microscopy (HRTEM) was carried out on a JEOL JEM-2100F (Cs) microscope operating at 200kV. The ultraviolet-visible (UV-Vis) absorption spectra were measured using a TU-1901 UV-Vis spec-trophotometer over the scan range 250800nm and a resolution of 1.0nm. The excitation and emission spectra were carried out using a FluoroSENS-9000 photoluminescence spectrophotometer with a static xenon lamp (150W) as an excitation source. The PL QY of QDs was performed using a quantum yield measurement system (FluoroSENS-9000 photoluminescence spectrophotometer) with a 150W xenon lamp coupled to a monochromator for wavelength discrimination, a 120mm integrating sphere as sample chamber and a multichannel analyzer for signal detection. Solar simulator with an air mass 1.5 global illumination (AM 1.5G, 100mW/cm2) and a Keithley 2400 source meter were used for the JV characterization. All the measurements were carried out at room temperature.

How to cite this article: Li, C. et al. Large Stokes Shift and High Efficiency Luminescent Solar Concentrator Incorporated with CuInS2/ZnS Quantum Dots. Sci. Rep. 5, 17777; doi: 10.1038/srep17777 (2015).

Tummeltshammer, C., Taylor, A., Kenyon, A. J. & Papakonstantinou, I. Homeotropic alignment and Frster resonance energy transfer: The way to a brighter luminescent solar concentrator. J. Appl. Phys. 116, 173103 (2014).

Chuang, P. H., Lin, C. C. & Liu, R. S. Emission-tunable CuInS2/ZnS quantum dots: structure, optical properties and application in white light-emitting diodes with high color rendering index. ACS Appl. Mater. Inter. 6, 1537915387 (2014).

Chen, W., Cao, W. Q., Hao, J. J. & Wang, K. Synthesis of high-quality and efficient quantum dots with inorganic surface passivation in a modified phosphine-free method. Mater. Lett. 139, 98100 (2015).

Hao, J. J., Zhou, J. & Zhang, C.-y. A tri-n-octylphosphine-assisted successive ionic layer adsorption and reaction method to synthesize multilayered core-shell CdSe-ZnS quantum dots with extremely high quantum yield. Chem. Comm. 49, 63466348 (2013).

Wang, X., Li, W. & Sun, K. Stable efficient CdSe/CdS/ZnS core/multi-shell nanophosphors fabricated through a phosphine-free route for white light-emitting-diodes with high color rendering properties. J. Mater. Chem. 21, 85588565 (2011).

Zhong, H., Bai, Z. & Zou, B. Tuning the Luminescence Properties of Colloidal I-III-VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 3, 31673175 (2012).

Castro, S. L., Bailey, S. G., Raffaelle, R. P., Banger, K. K. & Hepp, A. F. Synthesis and characterization of colloidal CuInS2 nanoparticles from a molecular single-source precursor. J. Phy. Chem. B. 108, 1242912435 (2004).

This work was supported by National Natural Science Foundation of China (Grant No. 51402148 and No. 11304147), Guangdong High Tech Project (Grant No. 2014A010105005 and 2014TQ01C494), Shenzhen Nanshan Innovation Project (Grant No. KC2014JSQN0011A) and SUSTC Foundation (Grant No. FRG-SUSTC1501A-48 and No. FRG-SUSTC1501A-67).

K.W. conceived the idea and concepts and guided the research. C.L. fabricated the QDs-LSC-PV device and conducted the optical and electrical performance measurements. W.C. synthesized the quantum dots and performed the characterization of the QDs-LSC-PV device. D.W. provided important suggestions in preparing the quantum dots. D.Q. and Z.Z. assisted for the fabrication and characterization of device. J.H. and J.Q. performed the characterization of quantum dots. Z.H. and Y.L. provided some suggestions in data analysis and discussion. C.L., W.C. and K.W. wrote the manuscript. All authors have discussed and reviewed the manuscript and given their approval to the final version of this manuscript.

Li, C., Chen, W., Wu, D. et al. Large Stokes Shift and High Efficiency Luminescent Solar Concentrator Incorporated with CuInS2/ZnS Quantum Dots. Sci Rep 5, 17777 (2016). https://doi.org/10.1038/srep17777

## new process that harnesses heat energy could double efficiency of solar cells

Photovoltaic solar cells convert light energy from the sun into electricity. Although significant strides have been made in increasing the efficiency of photovoltaic technology, they usually only result in incremental increases. Researchers at Stanford University have come up with a way that could more than double the efficiency of existing solar cell technology and potentially reduce the costs of solar energy production enough for it to compete with oil as an energy source. Instead of relying solely on photons, the new process, called photon enhanced thermionic emission, or PETE, simultaneously combines the light and heat of solar radiation to generate electricity.

Unlike photovoltaic technology currently used in solar panels which becomes less efficient as the temperature rises the new process excels at higher temperatures. The Stanford engineers who discovered it say the process promises to surpass the efficiency of existing photovoltaic and thermal conversion technologies. And the materials needed to build a device to make the process work are cheap and easily available, meaning the power that comes from it will be affordable.

"This is really a conceptual breakthrough, a new energy conversion process, not just a new material or a slightly different tweak," said Nick Melosh, an assistant professor of materials science and engineering, who led the research group. "It is actually something fundamentally different about how you can harvest energy."

Most photovoltaic cells, such as those used in rooftop solar panels, use the semiconducting material silicon to convert the energy from photons of light to electricity. But the cells can only use a portion of the light spectrum, with the rest just generating heat. This heat from unused sunlight and inefficiencies in the cells themselves account for a loss of more than 50 percent of the initial solar energy reaching the cell.

The researchers knew that if this wasted heat energy could somehow be harvested, solar cells could be much more efficient. The problem has been that high temperatures are necessary to power heat-based conversion systems, yet solar cell efficiency rapidly decreases at higher temperatures. Until now, no one had come up with a way to wed thermal and solar cell conversion technologies.Melosh's group figured out that by coating a piece of semiconducting material with a thin layer of the metal cesium, it made the material able to use both light and heat to generate electricity.

"What we've demonstrated is a new physical process that is not based on standard photovoltaic mechanisms, but can give you a photovoltaic-like response at very high temperatures," Melosh said. "In fact, it works better at higher temperatures. The higher the better."

Because PETE performs best at temperatures well in excess of what a rooftop solar panel would reach, the devices will work best in solar concentrators such as parabolic dishes, which can get as hot as 800 C. Dishes are used in large solar farms similar to those proposed for the Mojave Desert in Southern California and usually include a thermal conversion mechanism as part of their design, which offers another opportunity for PETE to help generate electricity as well as minimize costs by meshing with existing technology."The light would come in and hit our PETE device first, where we would take advantage of both the incident light and the heat that it produces, and then we would dump the waste heat to their existing thermal conversion systems," Melosh said. "So the PETE process has two really big benefits in energy production over normal technology."

Photovoltaic systems never get hot enough for their waste heat to be useful in thermal energy conversion, but the high temperatures at which PETE performs are perfect for generating usable high-temperature waste heat. Melosh calculates the PETE process can get to 50 percent efficiency or more under solar concentration, but if combined with a thermal conversion cycle, could reach 55 or even 60 percent almost triple the efficiency of existing systems.The team would like to design the devices so they could be easily bolted on to existing systems, thereby making conversion relatively inexpensive.

The researchers used a gallium nitride semiconductor in the "proof of concept" tests. The efficiency they achieved in their testing was well below what they have calculated PETE's potential efficiency to be which they had anticipated. But they used gallium nitride because it was the only material that had shown indications of being able to withstand the high temperature range they were interested in and still have the PETE process occur.With the right material most likely a semiconductor such as gallium arsenide, which is used in a host of common household electronics the actual efficiency of the process could reach up to the 50 or 60 percent the researchers have calculated. They are already exploring other materials that might work.

"For each device, we are figuring something like a 6-inch wafer of actual material is all that is needed," Melosh said. "So the material cost in this is not really an issue for us, unlike the way it is for large solar panels of silicon."

"The PETE process could really give the feasibility of solar power a big boost," Melosh said. "Even if we don't achieve perfect efficiency, let's say we give a 10 percent boost to the efficiency of solar conversion, going from 20 percent efficiency to 30 percent, that is still a 50 percent increase overall."

## a spectrally tunable dielectric subwavelength grating based broadband planar light concentrator | scientific reports

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Energy consumption of buildings is increasing at a rapid pace due to urbanization, while net-zero energy buildings offer a green and sustainable solution. However, limited rooftop availability on multi-story buildings poses a challenge for large-scale integration of photovoltaics. Conventional silicon solar panels block visible light making them unfeasible to cover all thesurfaces of a building. Here, we demonstrate a novel dielectric grating based planar light concentrator. We integrate this functional device onto a window glass transmitting visible light while simultaneously guiding near infrared (NIR) portion of sunlight to edges of the glass window where it is converted to electricity by a photovoltaic cell. Gratings are designed to guide NIR region and realize polarization independent performance. Experimentally, we observe 0.72% optical guiding efficiency in the NIR region (7001000nm), transmitting majority of the visible portion for natural room lighting. Integrating solar cell at the window edge, we find an electrical conversion efficiency of about 0.65% of NIR light with a 25 mm2 prototype. Major losses are coupling and guiding losses arising from non-uniformity in fabrication over a large area. Such a functional window combining energy generation, natural room lighting and heat load reduction could mitigate urban heat island effect in modern cities.

The development of efficient optical concentrators to deliver a functional and economic solution for renewable power harvesting has focused mainly on solar photovoltaic and solar thermal power applications. Prominent methods utilize reflection (parabolic dish1,2), refraction (Fresnel lens3,4,5) or a combination of both reflective and refractive optics such as a compound parabola and prism6. Existing concentrators such as the Fresnel reflector, parabolic dish and light funnel7 require complex opto-mechanical frame works including moving lenses/mirrors to guide/manipulate the direction of light. However, these designs require sophisticated solar tracking, which make them bulky and expensive systems.

In contrast, advances in planar concentrators such as Luminescent Solar Concentrators (LSC)8 (either using dyes9,10 or quantum dots11,12,13), holographic concentrators14,15,16 and micro-optic concentrators17,18,19 demonstrate a low-cost and minimally tracking approach for solar light concentration. LSCs consist of dyes or quantum dots deposited on a transparent glass substrate10, which absorb incident solar energy over a certain band of wavelengths and reradiate in a different band (either down-shifted or up-shifted). Subsequently, the reradiated light is guided to the edges through the substrate. For instance, reported LSCs have demonstrated a power conversion efficiency of 2.85%20 (using silicon quantum dots over an area of 144cm2), 2.8%21 (using CdSe quantum dots over an area of 15.4cm2 with attached reflectors and solar cells) and an external quantum efficiency of ~3%22 (using CdSe and CdZnS quantum dots for a length of 100cm). Major challenges of LSCs are degradation of dyes, self-absorption and low conversion efficiency8,22,23,24. Moreover, LSCs reradiatethe light in all directions resulting in reduced coupling into the guided modes of the substrate. Additionally, in the current designs8,25,26 a relatively narrow band of the incoming solar radiation is harnessed (full width at half maximum, FWHM <200nm).

A planar holographic concentrator is a potential technology to overcome these challenges. This system has shown power generation improvements of about 25% in solar cells27. However, holographic concentrators partially block visible light due to the placement of solar cells in the direct light path. Micro-optic concentrator is another class of planar concentrators in which a micro-lens and a diffuser is utilized to guide incoming sunlight to the waveguide. This technology can achieve higher optical concentration, as it utilizes multiple lenses to focus the light and couple to waveguides with rather low angular acceptance (<2)17, thus requiring bulky and precise solar tracking infrastructure. Hotspot formation here results in accelerated degradation of solar cells28 and absence of wavelength selectivity causes thermalization losses thus reducing the overall conversion efficiency.

While conceptually promising, there are several challenges in the existing optical concentration technologies such as precise tracking, maintenance requirements, lower lifetime of opto-mechanical moving components, absence of spectrally selective light guiding, etc. Grating based designs are one of the approaches to realize a planar concentrator to overcome most of these challenges29,30,31. There a various High Contrast Grating (HCG) based designs such as aperiodic metasurfaces as a planar focusing element32, a two-dimensional metasurface as planar micro-lenses30 and a metal-based Fabry-Perot cavity solar cell33. In this work, we explore a subwavelength grating to realize a planar concentrator. Gratings diffract the incoming light into the glass substrate and can be designed to couple the light of desired wavelengths to first order transmission with highest efficiency by eliminating zero order transmission/reflection. Subwavelength gratings diffract the light at steeper angles over a broad wavelength range with high efficiency and this ensures guiding of desired wavelength band in the glass substrate. Grating based approach has the potential to eliminate the longer wavelengths (below bandgap) of sunlight reaching the solar cell which otherwise would not contribute to solar photovoltaic conversion and causes solar cell heating. The desired spectral selectivity of diffraction gratings can be tuned by optimizing the grating geometry parameters: grating period, duty cycle and thickness for a chosen material.

Planar light concentrators offer significant benefits when used for building integrated photovoltaics. Modern dwellings such as skyscrapers are often covered with glass and hence trap the radiation thereby imposing additional heat load to the cooling systems and increase the temperature in the surrounding areas, commonly known as urban heat island effect. Recent studies claim that urban heat island effect contributes to 30% of climate warming34 and can lead to an annual mean air temperature rise up to 13C and as high as 12C35 in the evenings. Here, we propose and demonstrate a planar light concentrator using a silicon nitride subwavelength grating (Fig.1) for conversion of NIR light to electricity, which has potential to be a smart power window. This planar concentrator is a multi-spectral light filter (Fig.1(a)). It diffracts NIR radiation at steep angles into the window glass substrate where it is guided towards its edges and is converted to electricity using an edge-integrated solar cell. Simultaneously, it reflects thermally parasitic infrared radiation away from the building, which reduces the air conditioning load, while transmitting the visible light to enter the building, providing natural room lighting (Fig.1(b)).

Proposed building integrated photovoltaics system based on a dielectric grating for broadband spectral filtering forming an optical concentrator. (a) Concept and schematic of the spectrally tunable grating based planar solar concentrator. The layer thickness is denoted as tl, grating thickness denoted as tg, the width of grating denoted as a, is the grating period and a/ is the duty cycle of grating. (b) The distinct spectral regions in the solar irradiance: IR (>1000nm) with 240W/m2, NIR in the wavelength range of 7001000nm with an irradiance of 270W/m2 (operating range of the grating spectral concentrator shown as red arrows), the visible region from 400700nm with 430W/m2 (transmitted) and <400nm corresponds to the UV region of AM1.5G spectrum48.

Our grating design is based on a selectively etched silicon nitride gratings on top of a glass substrate (Fig.1(a)). Geometric design parameters defining spectral light guiding, reflection and transmission coefficients are grating period (), duty cycle (DC), layer thickness (tl) and grating thickness (tg). For incident radiation with Transverse Electric (TE) polarization, the electric field is parallel to the grating bars while for Transverse Magnetic (TM) polarization, the magnetic field is parallel to the grating bars. Optimization enables the incident light of desired wavelengths to be diffracted into single transmission order29, thus effectively guiding the light to the window edges.

The dielectric grating concentrator consists of a transparent glass substrate with high refractive index coating (silicon nitride) followed by a grating of the same coating material (Fig.1(a)). The grating is optimized to diffract the NIR portion of the incoming spectrum into the substrate. The angular space of the planar concentrator is defined by the grating diffraction condition:

where, $${n}_{{sub}}$$: refractive index of the substrate, $${\theta }_{i}$$: the incident angle, $$\lambda$$: wavelength of the incident light, $${\rm{\Lambda }}$$: grating period, $$m$$: diffraction order, and $${\theta }_{m}$$: diffraction angle.

Equation (1) shows that for a subwavelength grating, the higher diffraction orders contributing to reflection and transmission losses can be eliminated and guiding in the desired wavelength range is maximized. Effective guiding in the glass substrate is achieved by exciting first order diffraction modes that are guided inside the glass substrate towards the PV cell. For example, for a glass substrate (with refractive index of 1.5) and a grating period of 680nm, the visible region (400700nm) diffracts in the angular range of 23.143.3. The diffraction angle for NIR wavelength range of 7001000nm is greater than the critical angle (41.8) for glass/air interface, hence results in effective guiding into the glass substrate as desired here. The glass substrate acts as a multimode waveguide with different modes propagating at different angles (Eq.1).

However, Eqs1 and 2 do not provide any insight regarding the diffraction efficiency of the grating structure except the angular distribution of different diffraction orders. Moreover, Eq.2 does not show the relation of geometric concentration ratio and Cmax. Hence, FDTD simulations are carried out to obtain the optical guiding efficiency and transmission to map-out the geometric parameter space of the grating based light concentrator. We note that these simulations are also carried out with the actual grating profile obtained from cross-sectional Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) images to evaluate the performance of the fabricated sample (Fig.3(d)).

The choice of materials and geometry directly affects the grating performance. Rigorous Coupled Wave Analysis (RCWA) is used to optimize the grating structure and material. Materials such as silicon, titanium dioxide, silicon dioxide, zinc oxide and silicon nitride on a glass substrate are used in estimating the diffraction efficiencies and angles of the desired wavelength region. RCWA approximation considers the device structure as infinitely periodic but the actual structure is finite, hence we used the Finite Difference Time Domain (FDTD) method instead to evaluate the guiding efficiency (Fig.3). However, FDTD simulation for actual dimension is computationally intensive and hence we have used a scaled down model (15 times) with the same geometric concentration ratio of 2.5 i.e. a substrate dimension of width 330m and thickness 66m. The device structure is optimized with silicon nitride as the grating material since the fabrication process is well developed compared to other materials for our structural dimensions and silicon nitride has comparatively high refractive index (>2)37,38,39, which allows for better optimization of diffraction, reflection and transmission40. In our case, the reflection in the NIR region, should be minimal to reduce optical loss. In addition to this, silicon nitride shows minimal optical absorption in the visible and NIR regions41, has low sensitivity to temperature variations41 and is widely used in state-of-the-art CMOS foundries42.

Numerical (FDTD) optimization of the grating parameters. (a,b) Optimization of grating parameters (DC, tl and tg) for a polarization tolerant structure. Polarization tolerant planar concentrator guiding spectra as a function of grating thickness for TE and TM respectively at tl=90nm. At a grating thickness of 350nm (coarse optimization), the guiding efficiency spectrum for both TE and TM are similar. (c) TE & TM incidence guiding efficiency spectrum for grating parameters of =680nm, DC=0.5, tl=90nm & tg=340nm, the guiding efficiency (g) for TE and TM are 29.7% and 26.9% in the wavelength range of 7001000nm respectively yielding an overall efficiency of 28.3%. (d) Guiding efficiency spectrum of the fabricated grating profile obtained from cross-sectional FIB/SEM is used to estimate the effect of non-rectangular profile and sidewall roughness (=680nm, tg=280nm, tl=120nm & DC=0.5). The inset shows the measured profile (top) and simulated profile (bottom) after profile extraction using image processing. The results show slight decrease in guiding efficiency compared to the ideal case due to the change in profile.

The visible light transmission efficiency (VT -Eq.S1) and guiding efficiency (g -Eq.S2) are the two key metrics used to evaluate the performance of planar concentrators (Fig.2). These efficiencies are sensitive to the grating period, duty cycle and silicon nitride thickness. Visible light transmission accounts for the natural room lighting and guiding efficiency accounts for the generated electricity from NIR light. One-dimensional grating structures are mostly polarization dependent i.e. the guiding and reflection/transmission efficiencies vary with polarization of incident radiation. Hence, high guiding efficiency can be achieved in one of the polarizations, while it is lower for the other. However, sunlight is unpolarized in nature, so it is desired to have a minimal polarization dependence and hence an overall higher guiding efficiency can be achieved resulting in higher electrical output. Moreover, the guided wavelength and bandwidth can be tuned further to match any solar cell material bandgap towards improving the PV conversion efficiency (i.e. reduced non-radiative recombination) by adjusting the grating parameters. Here, we consider conventional silicon PV cells and aim for a guided spectral range of 7001000nm where the Si-PV conversion efficiency is high43.

Silicon nitride grating on glass can be tuned for polarization dependent or independent light guiding. In this paper, we mainly focus on minimally polarization dependent structures i.e. similar guiding spectra for both TE and TM polarized incidences, for energy conversion using solar cells. The simulation results shown in Fig.2 considers surface normal incidence. The spectral grating thickness shows a parameter space where TE (Fig.2(a)) and TM (Fig.2(b)) spectra show matching guiding efficiency, with values of 29.7% and 26.9% for TE and TM incidences respectively (average of 28.3% efficiency) (Fig.2(c)). The optimized grating parameters are DC=0.5, tl=90nm, tg=340nm & =680nm. The angular tolerance studies are carried out for the optimized design, the studies show that an angle tolerance of >100 (Fig.S8(a)) when the incidence angle is varied in the direction parallel (yz-plane in Fig.1(a)) to the grating bars and 27 (Fig.S8(b)) in the perpendicular direction (xy-plane in Fig.1(a)). A detailed temperature tolerance study is also carried out and the results are presented in section 10 of supplementary information, based on the study the variation in grating width and grating thickness will be maximum of 0.06nm for an operating temperature variation of 50C. For a grating width/thickness variation of 10nm the combined guiding efficiency decrease is less than 1% (Fig.S9). Thus, ensuring the temperature stability of the planar concentrator in the actual operating conditions.

The grating profile has rectangular edges but the measured profile using FIB/SEM shows round edges, which affects the spectral guiding efficiency moderately. For the simulations of measured profile, a DC of 0.5 is considered as the grating widths are non-uniform across the 5mm sample and the DC varies in the range of 0.310.69 (Fig.S3). The grating simulation parameters of =680nm, tg=284nm, tl=120nm areused, which gives a minimal polarization dependence close to 935nm wavelength which is observed in the experiments (Fig.4(a)) as well. The simulation results for the rounded edges show slight TM to TE polarization discrepancy of 5.7% (Fig.2(d)).

The dielectric gratings are fabricated with electron beam lithography over a large area of 25mm2 (5mm5 mm - fabrication process is given in Fig.S1). Reactive ion etching process is used as a pattern transfer onto the silicon nitride using electron beam resist polymethyl methacrylate (PMMA) as a soft mask and chromium layer as a hard mask. The grating period of the structure is relatively uniform over this area (Figs.3(a) and 3(b)) and we obtain the height profile via atomic force microscope (Fig.3(c)). We find that the sidewalls are relatively straight, however, the surface roughness on the grating has a root mean square value of 5.78nm calculated from the results of atomic force microscopy (Fig.S6). To gain further insights into the profile of the grating, we have performed a cross-section using FIB milling and imaged it with a scanning electron microscope (Fig.3(d)). Here, we find that the gratings have rounded edges, as compared to the targeted sharp edges, this affects the guiding efficiency by a few percentage and polarization tolerance, as discussed above (Fig.2(d)).

Grating fabrication and characterization. (a) Optical microscope image of electron beam lithography fabricated grating at lower magnification. (b) Optical microscope image of electron beam lithography fabricated grating at a higher magnification. Gratings are fabricated over a large area of 25 mm2 using electron beam lithography followed by reactive ion etching. (c) Atomic force microscope image of the electron beam fabricated sample after etching shows a grating thickness of 276nm. (d) Cross-sectional FIB/SEM image of the fabricated sample, this grating profile and parameters are used in the simulation to estimate the effect of profile variation due to the etching process.

Here, we show experiments demonstrating the conceptual functionality of the power window (Fig.4). We observe a peak in the guiding spectrum at 935nm (using measurement setup Fig.S4), which is independent of polarization as designed (Fig.4(a)). The beam spot size used in this measurement is ~4mm in diameter so that the scattering from the edges can be avoided. The polarization independency requires two uncoupled guided waves to be simultaneously excited44. This excitation happens only for particular combinations of grating parameters45. Moreover, the experimental results for TE and TM incidences show similar guiding spectrum (TE - 0.78%, TM - 0.75% & unpolarized - 0.72%). While these guiding efficiencies appear low as compared to the numerical results (Fig.2(c)), they can be attributed to various losses in the system including material absorption, optical guiding loss, scattering loss due to roughness, non-uniformity of duty cycle and grating thickness across the sample. Even when there is low optical scattering in the substrate, longer optical path lengths can lead to higher scattering loss. For the guided modes, the optical path is longer for lower wavelengths and vice versa. Additionally, impurities in materials may cause additional optical scattering losses in a propagating mode. Moreover, the average visible light transmission for the fabricated sample is 64.9% and the transmission spectrum is shown in Fig.S12.

Optical characterization and performance evaluation of fabricated silicon nitride planar concentrator. (a) Guiding efficiency of the e-beam fabricated sample of dimensions 5mm5mm. The combined guiding efficiency for both the edges for TE, TM & unpolarized light are 0.78%, 0.75% & 0.72%, respectively in the wavelength range of 7001000nm (beam diameter of 4mm). (b) Measured guided spectra as a function of distance from the edge close to the integrating sphere. (c) Average guiding efficiency as a function of distance from the edge close to the integrating sphere. The exponential decay in the guiding efficiency suggests guiding loss in the system (dots represent experimental data points and solid line represent curve fit using Eq.3). (d) IV curve with attached silicon solar cell on to one edge of the planar concentrator and the efficiencies reported are doubled accordingly. Reference sample is a glass coated with 400nm thick silicon nitride.

We use a technique similar to the cutback technique known from optoelectronics to investigate the effective optical loss (Fig.4(b,c)). For effective loss evaluation, the guiding efficiency is measured as a function of distance from the edge of the substrate facing the integrating sphere with a beam diameter of ~1mm. Indeed, we find a declining efficiency with distance to the sample edge. This can be understood from a multitude of effects such as material absorption, scattering and diffraction at silicon nitride layer/grating interface. These losses play a major role in the performance of the planar concentrator. We attempt to model the losses in the system with an assumption of mainly two components, a linear and an exponential loss component (Eq.3). The linear component is mainly due to scattering centers because of defects and impurities46 of the substrate and the exponential component is due to the material absorption and guiding loss. From Beer- Lamberts law, it is understood that the absorption has an exponential relation with the optical path length. The guiding efficiency also contributes to the exponential component. However, the material (glass) absorption losses are very low (average absorption coefficient in NIR is 2104mm1 - Fig.S7). The guiding efficiency spectra (Fig.4(b)) and average guiding efficiency (Fig.4(c)) as a function of distance to the solar concentration edge show that the planar light concentrator has high losses. The measured data points and guiding efficiency model (curve fitting of the measured data points Eq.3) shown in Fig.4(c) is used to estimate the losses.

Fitting our distance-related optical powers at the sample edge with Eq.3 (i.e. only considering absorption, guiding and scattering losses), we find a reduction of about 87% of the initial optical power due to losses (average propagation length of 2.5mm). Out of these losses, the guiding loss arises from our design, which can be minimized through optimization. The main loss contributors in the exponential component of Eq.3 are guiding and absorption losses; for every 1.4mm of propagated distance the guiding efficiency becomes 1/e, i.e. the attenuation length. The absorption loss is almost negligible and major contribution is from the guiding loss i.e. out-coupling loss of the guided light in the glass substrate at the silicon nitride/glass interface. However, scattered light can also contribute to the loss. For the calculation (Fig.4(c)), we have assumed that the main contributor to the scattering loss is from the linear term in Eq.3. The scattering loss mainly arises from the glass substrate material impurities and defects, which can be minimized47. Thus, if at least the scattering losses were eliminated, the guiding efficiency would be 26% higherthanthe experimentally measured value i.e. 0.91% (Fig.4(a)). Multiplication factor is obtained by the ratio of scattering excluded case to that of included for a distance 2.5mm (i.e. x=3mm) using Eq.3.

The performance of smart power window prototype is tested by integrating a silicon solar cell at the edge of the grating sample that captures the guided spectral power. The IV characteristics of prototype (Fig.4(a)) shows that the short circuit current is 11 times more than that of the controlled sample. Similarly the open circuit voltage is 120mV higher than that of the controlled sample. The controlled sample is a glass substrate coated with 400nm silicon nitride (the same thickness of silicon nitride is used as in the grating structure). The silicon solar cell is diced into an area of 16 mm2 and covered with a copper sheet with black coating (to reduce unwanted reflections) to match thearea of 7 mm2 equivalentto the edge area of the samples. We note that, this will introduce additional parasitic shading losses from the solar cell, which may adversely affect the performance of the system. This prototype showsa conversion efficiency of 0.65% under the solar simulator with an irradiance of 780W/m2. The solar cell converts more than 80% of the incoming radiation, as there is lower thermalization loss in the guided spectrum at NIR wavelengths. If the scattering losses are minimized, the electrical efficiency in the NIR wavelength range can be increased to 0.82%. The planar concentrator scalability is discussed in detail in the supplementary information section 11. The results are promising, for a glass window of size 30cm30cm with a thickness of 15mm (geometric concentration ratio of 10), the NIR guiding efficiency of >10% and electrical power conversionof 8% can be achieved.

In an ideal case (rectangular gratings with sharp edges), the average optical guiding efficiency can as high as about 28% and the corresponding electrical conversion efficiency in the NIR region (7001000nm) of about 17% (Fig.2(c)). Even though, the experimental efficiencies are low compared with simulations as explained above, the lower efficiency is mainly due to the fabrication and material related imperfections; by further optimization of the fabrication process to realize large area uniform gratings and by reduction of the scattering losses and optimization of the grating design parameters, the optical guiding and electrical conversions of the smart power window can be further improved in the future.

We demonstrated a transparent planar concentrator using a dielectric grating integrated onto a glass substrate. This system has a unique potential of generating electrical power from the NIR portion of the sunlight incident on the windows of modern buildings. The dielectric grating functions as a spectral filter on the incoming sunlight, which allows visible light to pass through the glass windows but guides the NIR light towards the glass edge for efficient PV conversion. We have demonstrated a prototype with an area of 25 mm2. The integrated device shows an optical guiding efficiency of 0.72% and solar cell conversion efficiency of 0.65% in the wavelength range of 7001000nm with an average visible light transmission of 64.9% in the wavelength range of 400700nm. Under the scenario of zero scattering losses, the guiding efficiency can reach to 0.91% and the solar cell conversion efficiency of 0.82%. The conversion efficiency of the device is not limited by the concept but with challenges in large area fabrication. As such, this planar concentrator prototypeis well suited for building integrated photovoltaic applications and can become an integral part of future on-site electrical power generation, heating and cooling infrastructure for modern buildings. Such a device in principle can be fabricated over a large area using cost-effective techniques such asroll to roll interference lithography, stepper lithography and roll to roll nano-imprint lithography.

Rigorous Coupled Wave Analysis (RCWA) based software from Grating Solver Development Co. (GSolver V5.2) is used to calculate the diffraction efficiencies of various material combinations and as a preliminary tool in choosing the materials. RCWA considers an infinitely periodic structure and yields only the diffraction efficiencies of various diffraction orders. Finite Difference Time Domain (FDTD) simulation methodology from Lumerical (FDTD Solutions 2018b R1 -v8.20.1634) is used to design a finite device and to obtain guiding efficiency at the edge of the substrate. The optimization process aims at maximizing the guiding efficiency followed by minimizing the polarization dependency. Simulations were carried out using a non-uniform mesh with a mesh accuracy of three. All the simulations consider two-dimensional geometry with PML and symmetric/anti-symmetric boundary conditions. The simulation results show Fabry-Perot resonance as the substrate thickness is very small. To eliminate this, smoothening is carried out on the obtained spectra. The material refractive index for silicon nitride utilizes the ellipsometer measured data (Fig.S2) and that for glass substrate is from Palik database.

We developed a large area Electron Beam Lithography (EBL) process to fabricate silicon nitride grating on glass using the Raith VOYAGER EBL tool. After cleaning the glass substrate, it is coated with 400nm of silicon nitride by using plasma enhanced chemical vapor deposition (Oxford PlasmaPro 100 PECVD) at a substrate temperature of 50C with gases such as silane (15 sccm) and nitrogen (12 sccm) at an inductively coupled plasma power of 500W. Silicon nitride coated sample is characterized using surface profilometer (Bruker Dektak XT Profilometer) and ellipsometer (J. A. Woollam M-2000 DI Spectroscopic Ellipsometer) for obtaining thickness and refractive index parameters. The photoresist used for the EBL step (PMMA) is thin and shows poor selectivity during the reactive ion etch process of silicon nitride. An additional 30nm thin chromium layer is sputter deposited (Denton Vacuum Discovery 550 Sputtering System) on the wafer that is used as the etch mask for silicon nitride. The optimized device geometry design (Fig.2) is then used for the fabrication of the grating prototype. Using the EBL exposure, an area of 5mm5mm grating is fabricated on a 1mm thick glass substrate. PMMA is spin coated at 4000rpm for 45seconds and subsequently baked at 180C for 2minutes. The EBL tool uses an electron high tension of 30keV with an aperture of 30m at a dosage of 400C/cm2. The chromium layer is then wet etched with PMMA as a masking layer using CR7. After removal of PMMA with acetone, RIE etch is carried out to etch the grating features on silicon nitride (CHF3 - 50 sccm and O2 - 5 sccm, RF power: 1000W, operating pressure: 5106Torr using Unaxis 790 Reactive Ion Etcher). The etch depth is characterized by AFM (Bruker Dimension FastScan Atomic Force Microscope) and FIBSEM (FEI Helios FIB SEM).

The optical measurements are carried out to obtain transmission, reflection and guiding efficiency. The results for electron beam fabricated sample are shown in Fig.4. The setup uses a fibre coupled halogen lamp from Ocean Optics HL 2000 followed by collimating optics with chromatic aberration correction. The light guided through the glass substrate is collected at the edge using an integrating sphere (ISP-R from Labsphere) and fed to the spectrometer (Ocean Optics USB2000).

Solar cell IV characterization is carried out using Sol-3A solar simulator from Oriel Instruments at AM1.5G with an illumination of 780W/m2. IV characteristics are measured using a Keithley 2440 source meter.The electrical measurements are carried out with solar cell at one edge. The result shown in Table 1 is doubled as in actual case solar cells will be integrated on both the edges.

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Ludman, J. et al. Holographic solar concentrator for terrestrial photovoltaics. in Photovoltaic Energy Conversion, 1994., Conference Record of the Twenty Fourth. IEEE Photovoltaic Specialists Conference - 1994, 1994 IEEE First World Conference on 1, 12081215 vol.1 (1994).

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The author A.E would like to acknowledge IUSSTF for providing BASE fellowship. A.E. and M.V.N.S.G. would like to thank the Council of Scientific and Industrial Research (CSIR) for the award of Senior Research Fellowship.

B.P. conceived the original idea. A.E., M.V.N.S.G. and M.T. contributed towards simulation and characterization (Figures 2 and 4). M.T. and R.M. contributed towards fabrication and metrology (Figure 3). V.S. and B.P. supervised the project. All authors discussed the results and prepared the manuscript.

Elikkottil, A., Tahersima, M.H., Gupta, M.V.N.S. et al. A Spectrally Tunable Dielectric Subwavelength Grating based Broadband Planar Light Concentrator. Sci Rep 9, 11723 (2019). https://doi.org/10.1038/s41598-019-48025-3

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