Solar-weighted Reflectance Research Articles

A photoluminescent hydrogen-bonded biomass aerogel for sustainable radiative cooling.

Passive radiant cooling is a potentially sustainable thermal management strategy amid escalating global climate change. However, petrochemical-derived cooling materials often face efficiency challenges owing to the absorption of sunlight. We present an intrinsic photoluminescent biomass aerogel, which has a visible light reflectance exceeding 100%, that yields a large cooling effect. We discovered that DNA and gelatin aggregation into an ordered layered aerogel achieves a solar-weighted reflectance of 104.0% in visible light regions through fluorescence and phosphorescence. The cooling effect can reduce ambient temperatures by 16.0°C under high solar irradiance. In addition, the aerogel, efficiently produced at scale through water-welding, displays high reparability, recyclability, and biodegradability, completing an environmentally conscious life cycle. This biomass photoluminescence material is another tool for designing next-generation sustainable cooling materials.

Anisotropic thermally superinsulating boron nitride composite aerogel for building thermal management

Thermally insulating materials are commonly used to reduce energy consumption in buildings. Most commercial products possess only low thermal conductivities but poor insulating capabilities in the daytime with little sunlight reflectance and thermal emittance. It is challenging to achieve all traits in the same material. Herein, anisotropic boron nitride nanosheet (BNNS)/polyvinyl alcohol composite aerogels are developed using the unidirectional freeze-casting technique. Benefitting from the aligned porous structure, the composite aerogel with an optimal BNNS content exhibits a combination of an ultralow TC of 20.3 mW/mK in the through-thickness direction, a high solar-weighted reflectance of 95.0 % over the whole sunlight wavelength and a high emittance of above 93 % within the atmospheric transparency window. These exceptional thermo-optical properties enable the composite aerogel to maintain the interior temperature much cooler than commercially available foams, making them promising candidates as superinsulating envelopes for energy saving in buildings towards carbon neutrality.

Excellent omnidirectional light trapping properties of inverted micro-pyramid structured silicon by copper catalyzed chemical etching

Nano/micro structuring of silicon (Si) attracts a lot of research interests for minimizing the reflective losses (>30%) of Si surface limiting the solar cell performance majorly. Here, arrays of inverted micro-pyramids Si (IP–Si) have been fabricated on large area of the solar-grade, as-cut Si wafers by a simple, one-step copper (Cu) catalyzed chemical etching (CuCCE) in aqueous etch bath of 5 mM-Cu(NO3)2/4.6M-HF/0.55M − H2O2 at 50 °C. Influence of etching time is investigated under constant etch bath parameters and starting surface properties of the Si wafers. The CuCCE mechanism is compared with the standard alkali (aq. KOH) texturing of Si(100) wafers. The formation of inverted structure is attributed to Cu nanoparticles catalyzed localized anisotropic as well as isotropic etching of Si(100) as compared to only anisotropic etching in the KOH texturing. The IP-Si surface offers a superior light trapping ability in broad spectral range compared to the conventional KOH based random upright micro-pyramids (UP–Si). The IP-Si surface could achieve solar weighted reflectance (SWR) as low as <7% in 400–1000 nm spectral range as compared to ∼14% of the UP-Si surfaces. This has been attributed to higher percentage of triple bounce interaction of the incident light with Si surface offered by the IP-Si. Further, the IP-Si offers enhanced omnidirectional light trapping for wide angle of incidence (AOI) of light (−40° to +40°) as compared to that in UP-Si. The enhanced omnidirectional light trapping of the IP-Si and its mechanism has further been explained by finite difference time domain (FDTD) simulation. Further, potential of IP-Si for enhanced solar cell performance is proposed on the basis of theoretical calculation of total photocurrent for planar, UP-Si and IP-Si surfaces. A simple, economic yet effective process for large area fabrication of the inverted micro/nano-pyramid arrays of Si with enhanced and broad band omnidirectional light trapping properties has been shown which has potential for efficient Si solar cell application.

Fabrication of black GaAs by maskless inductively coupled plasma etching in Cl2/BCl3/O2/Ar chemistry

Textured GaAs surfaces with near-zero reflectance are promising for the antireflective application of photovoltaic and optoelectronic devices. Such surfaces can be described as “black GaAs” attributing to their superior light-trapping capability. In this paper, black GaAs with grasslike nanowire bunches was fabricated in a maskless inductively coupled plasma (ICP) etching manner. Cl2/BCl3/O2/Ar chemistry was selected as the feed gas in the ICP system. Undoped black GaAs with solar-weighted reflectance (SWR) of 1.01% and doped black GaAs with SWR of 2.56% were obtained. In the study, the surface morphology and antireflection behavior of the textured GaAs were found to vary at different etching time, power conditions, and O2 contents. Combined with the surface element analyses, the etching mechanism of the maskless process was proposed. It is considered as a competing mechanism for the removal of the GaAs material by Cl-based gases and the passivation of the surface by O radicals. This maskless texturing method has the advantages of simplicity, scalability, and efficiency. The black GaAs of such patterns has prospects in photodetectors, solar cells, and flexible devices.

Embossed Mie resonator arrays composed of compacted TiO2 nanoparticles for broadband anti-reflection in solar cells

Mie resonator arrays formed by embossing titanium dioxide (TiO2) nanoparticles (NPs) from solution are investigated as optical coatings for anti-reflection applications. Compacted nanoparticle assemblies offer unique possibilities to tailor the effective refractive index (RI). Here, we demonstrate a simple table-top, low pressure, and low temperature method to fabricate structured optical coatings. TiO2 nanostructures in the form of nanodisks support Mie resonances in the visible wavelength spectrum and exhibit strong forward scattering into the high index substrates, making them suitable as broadband anti-reflection coatings for solar cells. TiO2 NP-based nanodisk arrays are designed, fabricated, and characterized regarding their anti-reflection properties on Si, GaAs, and InP substrates and solar cells. Detailed finite-difference time-domain simulations are performed to optimize the TiO2 NP-based Mie resonator arrays for the broadband anti-reflection as well as to explain the measured reflectance spectra. The solar-weighted reflectance is used as a figure of merit (FoM). TiO2 nanodisk arrays on Si show a FoM of ~ 7% in the 400–1,100 nm wavelength spectrum; similar values are obtained for GaAs and InP substrates. TiO2 nanodisk arrays embossed directly on prefabricated planar single-junction Si, GaAs, and InP solar cells result in an appreciable increase (~ 1.3 times) in the short-circuit current densities.

Synthesis of silicon nanowalls exhibiting excellent antireflectivity and near super-hydrophobicity

Silicon nanowalls have been fabricated on top of p-type silicon (100) wafer using a cost-effective metal assisted chemical etching method. The results obtained from the scanning electron and transmission electron microscopies as well as X-ray diffraction studies have shown that the silicon nanowalls are vertically aligned, single crystalline and exhibit the same crystallographic orientation of the base silicon wafer. It is shown from the spectral reflectance measurements that the silicon nanowalls atop the Si wafer produce very low reflectance signals with a solar-weighted reflectance (Rsw) value of 1.48% over the entire wavelength region of interest for crystalline Si solar cell (220–1107 nm). This observation is in direct contrast with the polished Si wafer that yields a high (Rsw) value of 44%. In addition, the wetting contact angle measurements reveal that the silicon wafer surface structured with silicon nanowalls becomes near super-hydrophobic with excellent water repellent properties.

Formation of silver nanoparticles and their application for suppressing surface reflection of n-type silicon

In this paper silver-assisted chemical etching (SACE) was studied to increase roughness and reduce light reflection from the surface of n-type (100) mono-crystalline silicon. SACE process includes two basic steps which are silver deposition and chemical etching step. Simple electroless silver deposition step using mixture of AgNO3 and HF was utilized to form silver nanoparticles onto n-type silicon surface. Following chemical etching step was carried out by immersing samples in mixture of HF and H2O2. Concentration of AgNO3 in eletroless deposition step and concentration of HF in chemical etching step were investigated to get minimum reflectance. The surface of sample was characterized by scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and sample reflectance was measured at the wavelength range from 350–1100 nm. The results show that reflectance depends on AgNO3 concentration in electroless deposition step and HF concentration in etching step and the minimum solar-weighted reflectance is 1.74%. This method can be considered as simple, inexpensive method to effectively increase roughness and suppress light reflection for photovoltaic, optoelectronic, sensor, photonic applications.

Single-layer, anti-reflective thin films of porous MgF2 for solar thermal applications

Single-layer, anti-reflective coatings prepared via the sol–gel method can be scaled to large-area applications (e.g. solar collectors). In the specific case of evacuated tube solar thermal collectors, there are often three glass–air or glass–vacuum interfaces which can cause reflection. Thus, in the absence of any anti-reflective technology (i.e. most commercial products), a steep 15% or 22% reflective loss (e.g. transmission ~0.92N, where N = 2 or 3) occurs in evacuated tube solar collectors. Porous MgF2-based materials have been shown to provide low refractive index as a single layer, but they have not yet been applied to solar thermal collector technology—which requires elevated temperature stability and spectrum transmission. Importantly, these should be low cost at the m2 scale. Further, since solar technologies are considered ‘green’, the materials used in their manufacturing should be carefully considered. In this study, we have employed trifluoroacetic acid (instead of hydrofluoric acid) as the fluorine source to produce single-layer porous MgF2 coatings. These coatings were designed and modeled with OpenFilters, fabricated with dip-coating, and characterized with ellipsometetry, x-ray diffraction, Raman spectroscopy, and focused ion beam/transmission electron microscopy to validate the OpenFilters model. For high transmission substrates, the dip coated films produced in this study can reach a solar-weighted reflectance of 2.3%. Additionally, to test if these films are suitable for solar thermal collectors, durability tests were conducted, which revealed acceptable heat resistance. However, without a water-resistant outer layer, the films did not pass an ASTM D870-09 water immersion test. Overall, this study presents a low-cost approach to developing anti-reflective coatings which are suitable for large-area solar thermal applications.

Communication—Black GaAs with Sub-Wavelength Nanostructures Fabricated via Lithography-Free Metal-Assisted Chemical Etching

A practical nanofabrication process is detailed for the generation of black GaAs. Discontinuous films of Au nanoparticles are electrodeposited onto GaAs substrates to catalyze site-specific etching in a solution consisting of KMnO4 and HF via the metal-assisted chemical etching. This provides a solution-based and lithography-free method for fabricating sub-wavelength nanostructure arrays that exhibit solar-weighted reflectance approaching 4%. This two-step benchtop process can be performed at room-temperature without lithography, vacuum instrumentation, or epitaxy, providing a high-throughput nanotexturing approach for photovoltaics applications.

Potential of chemical rounding for the performance enhancement of pyramid textured p-type emitters and bifacial n-PERT Si cells

We have investigated the effects of chemical rounding (CR) on the surface passivation and/or antireflection performance of AlOx- and AlOx/SiNx:H stack-passivated pyramid textured p+-emitters with two different boron doping concentrations, and on the performance of bifacial n-PERT Si solar cells with a front pyramid textured p+-emitter. From experimental results, we found that chemical rounding markedly enhances the passivation performance of AlOx layers on pyramid textured p+-emitters, and the level of performance enhancement strongly depends on boron doping concentration. Meanwhile, chemical rounding increases solar-weighted reflectance (RSW) from ∼2.5 to ∼3.7% for the AlOx/SiNx:H stack-passivated pyramid textured p+-emitters after 200-sec chemical rounding. Consequently, compared to non-rounded bifacial n-PERT Si cells, the short circuit current density Jsc of 200-sec-rounded bifacial n-PERT Si cells with ∼60 and ∼100 Ω/sq p+-emitters is reduced by 0.8 and 0.6 mA/cm2, respectively under front p+-emitter side illumination. However, the loss in the short circuit current density Jsc is fully offset by the increased fill factor FF by 0.8 and 1.5% for the 200-sec-rounded cells with ∼60 and ∼100 Ω/sq p+-emitters, respectively. In particular, the cell efficiency of the 200-sec-rounded cells with a ∼100 Ω/sq p+-emitter is enhanced as a result, compared to that of the non-rounded cells. Based on our results, it could be expected that the cell efficiency of bifacial n-PERT Si cells would be improved without additional complicated and costly processes if chemical rounding and boron doping processes can be properly optimized.

Low reflecting hierarchically textured silicon by silver assisted chemical etching for potential solar cell application

Present study reports fabrication of silicon nanowires over micro-textured Si substrates. Silver assisted electroless chemical etching route has been adopted for fabrication of the nanowires. Influence of HF concentration on the formation kinetics has been investigated by using scanning electron microscopy. The hierarchical binary structures have been able to reduce solar weighted reflectance (SWR) to <3% in broad spectral range (300-1100 nm), from 38% SWR of the polished silicon without any additional coating. Such reduction in reflectivity is obtained for <1 µm nanowires length. It is observed that at relatively lower concentration of HF the nanowires are preferably formed only in <100> direction. However, with increase in the concentration, etching in both <100> and non-<100> directions such as <111> also occurs on the micro-pyramids of similar dimensions and results in the binary structures with slanted silicon nanowires on 3-dimansional micro-pyramids.

Nano/micro dual-textured antireflective subwavelength structures in anisotropically etched GaAs.

Light trapping by surface texturing is widely used to improve the performance of optoelectronic devices. In this Letter, we demonstrate nano/micro dual-scale textured GaAs by integrating triangular GaAs by orientation-dependent wet etching and subwavelength nanoholes by metal-assisted chemical etching (MacEtch). This is the first report on nano/micro dual-scale textured GaAs. The reflectance was adjusted by controlling the aspect ratio of the nanoholes by varying the MacEtch duration. The combination of the microstructure and subwavelength structures significantly reduced the solar-weighted reflectance of a bare GaAs substrate by 72%.

Enhancement of Light Absorption in Photovoltaic Devices using Textured Polydimethylsiloxane Stickers.

We designed and fabricated a random-size inverted-pyramid-structured polydimethylsiloxane (RSIPS-PDMS) sticker to enhance the light absorption of solar cells and thus increase their efficiency. The fabricated sticker was laminated onto bare glass and crystalline silicon (c-Si) surfaces; consequently, low solar-weighted reflectance values were obtained for these surfaces (6.88 and 17.2%, respectively). In addition, we found that incident light was refracted at the PDMS-air interface of each RSIPS, which redirected the incident power and significantly increased the optical path length in the RSIPS-PDMS sticker which was 14.7% greater than that in a flat-PDMS sticker. Moreover, we investigated power reflection and propagation through the RSIPS-PDMS sticker using a finite-difference time-domain method. By attaching an RSIPS-PDMS sticker onto both an organic solar cell (OSC) based on a glass substrate and a c-Si solar cell, the power conversion efficiency of the OSC and the c-Si solar cell were increased from 8.57 to 8.94% and from 16.2 to 17.9%, respectively. Thus, the RSIPS-PDMS sticker is expected to be universally applicable to the surfaces of solar cells to enhance their light absorption.

Fabrication and optical characterization of hybrid antireflective structures with zinc oxide nanorods/micro pyramidal silicon for photovoltaic applications

We investigated the hybrid antireflective (AR) structures with zinc oxide nanorods (ZnO NRs) grown on micro pyramidal silicon (MP-Si) structures. The MP-Si structures were fabricated by a simple and cost-effective anisotropic wet chemical etching technique under different ratios between potassium hydroxide (KOH) and isopropyl alcohol (IPA). For the MP-Si structures etched at 10:9 vol% of KOH and IPA, the relatively low average reflectance (Ravg) value of 14.5% was obtained in the wavelength range of 300-1100 nm. Its solar weighted reflectance (SWR) value was also estimated to be 12.6% in the wavelength range of 400-1100 nm. The ZnO NRs were hydrothermally grown on the MP-Si structures at various solution concentrations. For the ZnO NRs (25 mM)/MP-Si, the Ravg and SWR values were further decreased to 3.6% and 3.8%, respectively. The omnidirectional AR behavior was also observed in the wide incident angle range of 20-70°. The hybrid ZnO NRs/MP-Si AR structures revealed a superhydrophilic surface with water contact angles of < 5 o.

AuAg bimetallic nonalloyed nanoparticles on a periodically nanostructured GaAs substrate for enhancing light trapping.

We present a light trapping structure consisting of AuAg bimetallic nonalloyed nanoparticles (BNNPs) on cone-shaped GaAs subwavelength structures (SWSs), combining the advantages of plasmonic structures and SWSs for GaAs-based solar cell applications. To obtain efficient light trapping in solar cells, the optical properties' dependence on the size and composition of the Ag and Au metal nanoparticles was systematically investigated. Cone-shaped GaAs SWSs with AuAg BNNPs formed from an Au film of 12 nm and an Ag film of 10 nm exhibited the extremely low average reflectance (R(avg)) of 2.43% and the solar-weighted reflectance (SWR) of 2.38%, compared to that of a bare GaAs substrate (R(avg), 37.50%; SWR, 36.72%) in the wavelength range of 300 to 870 nm.

Morphology based statistical analysis of nanosecond pulsed laser texturing of the multicrystalline silicon

In this paper, we investigate the surface morphology of the textured multicrystalline silicon with nanosecond (ns) Nd: YVO4 laser (wavelength of 1064 nm, repetition rate of 10 KHz, and pulse duration of 14 ns). Various surface topographies have been achieved with different laser as well as irradiation parameters. The textured area average roughness and depth have been statistically analyzed through ANOVA test, which could evaluate the significance and effectiveness of the adopted design of experiment. This research work is based on three control factors: Laser fluence, laser pulse overlap percentage, and number of irradiations. The statistical assessments were conducted based on roughness and depth values measured by optical interferometry. The effect of roughness and depth on solar weighted reflectance (SWR) was analyzed and significant reduction in SWR with increase in Ra was observed. In addition, time and energy consumption, which are highly significant in the industrial applications, have been investigated.

Antireflection silicon structures with hydrophobic property fabricated by three-beam laser interference

This paper demonstrates antireflective structures on silicon wafer surfaces with hydrophobic property fabricated by three-beam laser interference. In this work, a three-beam laser interference system was set up to generate periodic micro–nano hole structures with hexagonal distributions. Compared with the existing technologies, the array of hexagonally-distributed hole structures fabricated by three-beam laser interference reveals a design guideline to achieve considerably low solar-weighted reflectance (SWR) in the wavelength range of 300–780nm. The resulting periodic hexagonally-distributed hole structures have shown extremely low SWR (1.86%) and relatively large contact angle (140°) providing with a self-cleaning capability on the solar cell surface.

Three-dimensional nanotub submicrometer diffraction gratings for solar cells.

Diffraction gratings are a promising approach for reducing reflection and achieving light-trapping in solar cells. Using square lattices as a base structure, we investigate a novel bi-periodic nanotub three-dimensional grating structure and compare it with established textured structures for thin-film and wafer applications. For wafer application, simulations show that optimal AR coated nanotubs demonstrated solar weighted reflectance (SWR) of 2% compared to AR coated square pyramids with values 1.9%. Nanotubs also show SWR below 8% for polar angles to 60°. Simulated short-circuit current thin-film cells with nanotubs using smaller dimensions show higher yields (3-6 mA/cm2 average) compared to square pyramids. For periods greater than 700 nm at aspect ratios of 0.7 and greater, nanotubs have reduced current attributed to the increased planar surface area of the nanotub base, and evident in increased SWR. A simple nanoimprint lithography process was employed in experiments to define a square array of circular holes, utilizing a polydimethylsiloxane (PDMS) stamp applied onto a sol-gel imprint resist. The underlying silicon was then wet etched to produce the nanotub textures of 200 nm height and 513 nm period. AR coated nanotub wafers were produced via plasma enhanced chemical vapor deposition (PECVD), with an experimental and theoretical SWR of 6.4% and 5.4%, respectively.

The Effect of a Thermotropic Material on the Optical Efficiency and Stagnation Temperature of a Polymer Flat Plate Solar Collector

Solar hot water and space heating systems constructed of commodity polymers have the potential to reduce the initial cost of solar thermal systems. However, a polymer absorber must be prevented from exceeding its maximum service temperature during stagnation. Here, the addition of a thermotropic material to the surface of the absorber is considered. The thermotropic layer provides passive overheat protection by switching from high transmittance during normal operation to high reflectance if the temperature of the absorber becomes too high. A one dimensional model of a glazed, flat-plate collector with a polymer absorber and thermotropic material is used to determine the effects of the optical properties of the thermotropic material on the optical efficiency and the stagnation temperature of a collector. A key result is identification of the reflectance in the translucent state required to provide overheat protection for potential polymer absorber materials. For example, a thermotropic material in its translucent state should have a solar-weighted reflectance greater than or equal to 52% to protect a polypropylene absorber which has a maximum service temperature of 115 °C.

Efficiency improvement of III–V GaAs solar cells using biomimetic TiO2 subwavelength structures with wide-angle and broadband antireflection properties

The power conversion efficiency (PCE) improvement of III–V gallium arsenide (GaAs) single-junction (SJ) solar cells with titanium dioxide (TiO2) subwavelength structures (SWSs) as antireflection coatings (ARCs) is reported. TiO2 SWSs are fabricated by dry etching using dotted gold nanopatterns prepared with glancing angle deposition and thermal dewetting processes. Their optical reflection properties are studied in the wavelength range of 350–900nm at incident light angles (θi) of 3–80°, together with the theoretical prediction by a rigorous coupled-wave analysis. For SWSs on the 50nm-thick TiO2 layer/GaAs substrate, a lower solar weighted reflectance (SWR) of ~6.2% is obtained compared to that (i.e., SWR ~9.5%) of the TiO2 single-layer ARC (SLARC). By employing the TiO2 SWSs into the III–V GaAs SJ solar cell, the increased short circuit current density (Jsc) of 24.82mA/cm2 is achieved, thus leading to an improved PCE of 19.66% at θi=0° (i.e., Jsc=18.38 and 23.65mA/cm2 and PCE=14.74 and 18.98% for the solar cells with bare surface and TiO2 SLARC, respectively). For incident angle-dependent device characteristics, the solar cell with the TiO2 SWSs also exhibits superior performance over a wide θi range of 20–80°. Additionally, after the encapsulation, its Jsc and PCE values are slightly increased to 26.15mA/cm2 and 20.73%, respectively. For the stability, there are no significant variations in Jsc and PCE after about 50 days.