Low Surface Reflectance Research Articles

Nanoporous black multi-crystalline silicon solar cells: realization of low reflectance and explanation of high recombination loss

Nanoporous black silicon (nb–Si) structures with/without saw damage removal (SDR) on solar-grade multi-crystalline silicon substrates have been formed by simple Ag-induced chemical etching. The nb–Si shows a unique morphology of nano-scale holes on micron-scale patterns with low reflectance (<5%). The photovoltaic properties of nb–Si solar cells show that SDR process prior to Ag-induced chemical etching is an effective way to lower recombination and ohmic losses, resulting in significant efficiency enhancement of 26.0%. To further reduce the recombination loss for the nb–Si solar cells with ∼10% efficiency, a two-layer emitter model has been introduced to explain the reduced emitter diffusion length that significantly lower spectral response. This suggests that removing residual Ag nanoparticles completely might be the key approach to enhance the spectral response of nb–Si solar cells with very low surface reflectance, thus increasing the final conversion efficiency.

Range imaging: a new method for high-resolution topographic measurements in small- and medium-scale field sites

Topographic measurements are essential for the study of earth surface processes. Three-dimensional data have been conventionally obtained through terrestrial laser scanning or photogrammetric methods. However, particularly in steep and rough terrain, high-resolution field measurements remain challenging and often require new creative approaches. In this paper, range imaging is evaluated as an alternative method for obtaining surface data in such complex environments. Range imaging is an emerging time-of-flight technology, using phase shift measurements on a multi-pixel sensor to generate a distance image of a surface. Its suitability for field measurements has yet not been tested. We found ambient light and surface reflectivity to be the main factors affecting error in distance measurements. Low-reflectivity surfaces and strong illumination contrasts under direct exposure to sunlight lead to noisy distance measurements. However, regardless of lighting conditions, the accuracy of range imaging was markedly improved by averaging multiple images of the same scene. For medium ambient lighting (shade) and a light-coloured surface the measurement uncertainty was approximately 9 mm. To further test the suitability of range imaging for field applications we measured a reach of a steep mountain stream with a horizontal resolution of approximately 1 cm (in the focal plane of the camera), allowing for the interpolation of a digital elevation model on a 2 cm grid. Comparison with an elevation model obtained from terrestrial laser scanning for the same site revealed that both models show similar degrees of topographic detail. Despite limitations in measurement range and accuracy, particularly at bright ambient lighting, range imaging offers three-dimensional data in real time and video mode without the need of post-processing. Therefore, range imaging is a useful complement or alternative to existing methods for high-resolution measurements in small- to medium-scale field sites. Copyright © 2012 John Wiley & Sons, Ltd.

Design of Lighting System for Surface Mine Projects

Indian mining illumination standard is based on incident light without mentioning about uniformity ratio, but International Commission on Illumination (CIE), Austria insists uniformity ratio. In a mine with low surface reflectance, the surrounding illumination level is lower than the minimum lighting standard as specified by various regulatory bodies. The surface brightness further reduces in wet surface condition, and results in poor visibility. In the present study, design was made both in mineral and overburden benches based on the minimum acceptable reflected light and the reflected uniformity ratio. For the purpose of comparison of various types of lighting systems, a stretch of 1.0 km long haul road was considered. The design was attempted with five different types of luminaries. Lamp mounting heights were varied at five steps, namely 8, 10, 12, 14 and 16 m. Design under wet condition incurs an excess cost 9.4 % for mineral bench and 50 % for overburden bench haul roads. Designing under wet surface condition ensures the minimum light level even under the worst condition of surface reflectivity, with marginal increase in cost.

Formation of a Low Reflective Surface on Silicon Solar Cells by Chemical Treatment Using Ag-Assisted Electroless Etching

The Ag-assisted electroless etching of p-type silicon substrate in HF/H2O2 solution at room temperature was investigated. The porous silicon layer was formed in a mixed solution of H2O2 and HF by using screen-printed Ag front electrodes as the catalyst. And influence of the different concentration etching solution (HF and AgNO3) on the porous silicon layer was study by scanning electron microscopy (SEM). Through investigation of the track of catalyst particles, it was shown that Ag really catalyses the etching of silicon underneath Ag particle.

Design of Lighting System for Surface Mine Projects

Indian mining illumination standard is based on incident light without mentioning about uniformity ratio. But CIE (International Commission on Illumination, Austria) stresses upon uniformity ratio as well. In a mine because of low surface reflectance, the surrounding illumination level is much lower than the minimum lighting standard, as specified by various regulatory bodies. The surface brightness further reduces in wet surface condition, resulting in poor visibility. In the present study, design has been made both in mineral and overburden benches based on minimum acceptable reflected light and reflected uniformity ratio. For the sake of comparison of various types of lighting systems, a stretch of 1.0 km long haul road was considered and design was attempted with five different types of luminaries. Lamp mounting heights have been varied at five steps, namely 8, 10, 12, 14 and 16 m. Design under wet condition would incur an excess cost of about 9.4 % for mineral bench and about 50 % for overburden bench haul roads under study. Thus designing under wet surface condition would ensure minimum light level even under worst condition of surface reflectivity, with marginal increase in cost.

Seed-assisted cast quasi-single crystalline silicon for photovoltaic application: Towards high efficiency and low cost silicon solar cells

We have demonstrated the seed-assisted cast quasi-single crystalline (QSC) silicon technique to achieve high efficiency solar cells with low cost. Compared to multicrystalline (mc) silicon, the QSC silicon has better material properties, having higher minority carrier lifetime and fewer grain boundaries and dislocations. Furthermore, the 〈100〉 oriented QSC silicon can achieve a lower surface reflectance using alkaline texturing. Based on these two factors, the efficiency of the QSC silicon solar cells with the industrial size has been improved by up to 1% absolutely from the mc-Si counterparts. Compared to the Czochralski (CZ) silicon solar cells, the QSC cells have slightly lower efficiency but high productivity and negligible light-induced degradation. These results suggest a great potential of the QSC silicon applied in photovoltaic industry as the next generation substrate. To make the QSC silicon more competitive in industry, further efforts should be focused on the recycling of seed crystals, the coverage of mono-region and the control of structural defects.

Carbon monoxide from shortwave infrared reflectance measurements: A new retrieval approach for clear sky and partially cloudy atmospheres

The GMES atmospheric services include global and European air quality monitoring and forecasting which require near real time delivery of atmospheric CO abundances. To achieve this, a numerically efficient retrieval approach for operational data processing is needed to derive CO column densities from shortwave infrared measurements in the 2.3μm band of the Sentinel 5 missions and its Precursor mission. The expected performance of both spectrometers will allow for clear-sky CO column retrievals over land with a precision of ≤10% and an overall accuracy of ≤15% even for background CO abundance and low surface reflection in the shortwave infrared spectral range. In this context, we present a new algorithm approach of the retrieval of CO from shortwave infrared measurements in clear sky and partially cloudy atmospheres over land and ocean surfaces. The algorithm employs simplified radiative transfer, where the model atmosphere is separated in a clear sky part, and a part which is bounded below by an elevated Lambertian reflector to account for atmospheric scattering by clouds and aerosols. Within the inversion scheme, Tikhonov regularization is used to determine, for each individual measurement, not only the vertically integrated CO column density and its retrieval error, but also the column averaging kernel. For the retrieval, a prior estimate of methane abundance is used to characterize the light path by retrieving effective cloud parameters from the shortwave infrared band itself. A performance analysis shows that, for a single cloud layer in the middle and lower troposphere, the bias on the CO retrieval due to the Lambertian cloud model is less than 2–3%. The effect of boundary layer aerosols can also be treated with similar accuracy. In contrast, the presence of elevated dust plumes above bright surfaces or a single layer cirrus cloud causes significant errors and, in these cases, a reasonably low retrieval bias can only be achieved for an optical depth in the shortwave infrared spectral range lower than 0.4. Another relevant error source for the CO retrieval algorithm is given by the prior uncertainty of methane. It is found that a 5% uncertainty in the methane column density causes biases of 3–9% on the retrieved CO column, depending on cloud fraction.

Enhanced transmittance and hydrophilicity of nanostructured glass substrates with antireflective properties using disordered gold nanopatterns

We fabricated surface nanostructures with different pillar and cone shapes on glass substrates using thermally dewetted gold (Au) nanoparticles as etch masks by dry etching. Their optical total transmittance characteristics, together with theoretical predictions using rigorous coupled-wave analysis simulation, and wetting behaviors were investigated. The nanostructured glass substrates strongly enhanced the surface transmission compared to the flat glass substrate. The glass nanocones with a linearly graded effective refractive index profile exhibited better transmission properties than the glass nanopillars due to the lower surface reflectance, thus leading to higher average transmittance with increasing their height. For the glass nanocones with a period of 106 ± 39 nm at the Au film thickness of 5 nm, the higher average total transmittance (Tave) and solar weighted transmittance (SWT) of ~95.5 and ~95.8% at wavelengths of 300-1100 nm and the lower contact angle (θc) of 31° were obtained compared to the flat glass substrate (i.e., Tave~92.7%, SWT~92.7%, and θc~65°). The calculated total transmittance results showed a similar tendency to the experimental results.

Multi-scale surface texture to improve blue response of nanoporous black silicon solar cells

We characterize the optical and carrier-collection physics of multi-scale textured p-type black Si solar cells with conversion efficiency of 17.1%. The multi-scale texture is achieved by combining density-graded nanoporous layer made by metal-assisted etching with micron-scale pyramid texture. We found that (1) reducing the thickness of nanostructured Si layer improves the short-wavelength spectral response and (2) multi-scale texture permits thinning of the nanostructured layer while maintaining low surface reflection. We have reduced the nanostructured layer thickness by 60% while retaining a solar-spectrum-averaged black Si reflectance of less than 2%. Spectral response at 450 nm has improved from 57% to 71%.

Fabrication of Highly Ordered Silicon Nanowire Arrays With Controllable Sidewall Profiles for Achieving Low-Surface Reflection

A novel and simple approach is demonstrated for fabricating silicon nanowire arrays (SNWAs) with controllable sidewall profiles. A single-step deep-reactive-ion etching (SDRIE) is used to transfer the holography patterned photoresist template to silicon or silicon-on-insulator substrates. With the SDRIE etching process, scalloping of the sidewalls can be avoided while reserving the high-mask selectivity over resist and high-etching rate. The sidewall angle of resultant patterns can be adjusted by tuning the composition of the gas mixture of the process. A modified-SDRIE process with a linearly changed gas flow is further developed to extend its capability. A post-high-energy argon plasma treatment is used to create sharp tips on the top of SNWAs and to increase the filling factor. Broadband antireflective (AR) window with a low reflectivity can be realized from tall SNWAs with high-filling factor. Depositing silicon dioxide over SNWAs can further enhance the AR performance. The position and bandwidth of the AR window can be controlled by tuning the SNWA parameters.

Aerosol radiative effects over an urban location and a remote site in western India: Seasonal variability

Seasonal variability in radiative effects due to aerosols in the shortwave and longwave regions is studied utilizing measured aerosol optical properties and models over two distinct environments, namely, Ahmedabad (urban, industrialised location) and Gurushikhar (high altitude remote site) in western India. Top of the atmosphere (TOA) forcing in shortwave varies from positive to negative over Ahmedabad, while over Gurushikhar it is negative throughout the year because of lower surface reflectance and higher single scattering albedo (SSA). Aerosol radiative forcing at the surface (SFC) is >−30 Wm −2 over Ahmedabad. In Gurushikhar SFC forcing increases from winter (−7 Wm −2 ) to premonsoon (−14 Wm −2 ), which is attributed to the increase in the amount of dust aerosol loading. Aerosol radiative forcing in the atmosphere (ATM) peaks during postmonsoon with a value of 54 Wm −2 over Ahmedabad, while ATM warming over Gurushikhar peaks during premonsoon (9 Wm −2 ). Shortwave heating rate is >0.9 K d −1 over Ahmedabad while it is an order of magnitude lower over Gurushikhar. Aerosols produce a positive TOA and SFC forcing in the longwave and cool the atmosphere. Anthropogenic influence is evident from the aerosol radiative effects obtained over Ahmedabad and Gurushikhar. The abundance of fine (coarse) mode aerosols over Ahmedabad (Gurushikhar) results in higher (lower) aerosol optical depth, lower (higher) SSA, and higher (lower) aerosol radiative forcing. Owing to the dissimilar nature of shortwave and longwave radiative effects of aerosols, when coarse mode aerosols are abundant over a region, longwave radiative effects can balance shortwave atmospheric warming produced by aerosols. ► Top of the atmosphere forcing varies from positive to negative over Ahmedabad while it is negative over Gurushikhar. ► Abundance of fine mode absorbing aerosols give rise to higher atmospheric warming in Ahmedabad. ► Shortwave heating rate is an order of magnitude lower over Gurushikhar than Ahmedabad. ► Longwave cooling balances the shortwave atmospheric warming when coarse mode aerosols are abundant.

Using One-Step, Dual-Side Nanoimprint Lithography to Fabricate Low-Cost, Highly Flexible Wave Plates Exhibiting Broadband Antireflection

In this study, we used dual-side nanoimprint lithography to prepare sub-wavelength periodical grating (SPG)–based wave plates on flexible substrates. Without employing any etching, we directly imprinted SPGs on both sides of a polycarbonate (PC) substrate through a one-step imprinting process. This dual-side nanoimprint lithography process dramatically decreases the typical imprinting pressure required in conventional resist-based nanoimprint lithography. With this low-cost formation of PC-based wave plates, any degree of phase retardation could be obtained merely by stacking a set of wave plates with designed depths and working wavelengths. Our SPG-based wave plates exhibited the excellent broadband antireflection properties of sub-wavelength structures possessing refractive index gradients on both sides of the PC substrates. The high flexibility, low cost, and low surface reflection of these dual-side SPG-based wave plates make them attractive alternatives to commonly used birefringent crystal–based wave plates, suggesting that this phase retardation technique has promising potential for use in the development of next-generation flexible optoelectronic devices and systems.

Ultralow reflectance metal surfaces by ultrafast laser texturing

We report the fabrication of textured metal surfaces by ultrafast laser processing and its optical property characterization by measurement of reflection and scattering as a function of angle and wavelength. The ultrafast laser textured metal surface reflection combined with scattering can be reduced to low values of &#x223C;3% over a broad spectral and angular range. Furthermore, it is shown that surface optical reflection and scattering can be varied by parameters like laser fluence, number of laser pulses, and coating the textured surfaces with suitable absorbers. It is also shown that ultrafast laser textured metal surfaces can have low surface reflection and scattering properties even at high temperatures (&#x2264;500&#x2009;&#xB0;C) of operation.

Measurements and analysis of antireflection coatings reflectivity related to external cavity lasers

Facets of semiconductor optical amplifiers (SOA) designed for external cavity lasers must be coated with an antireflection (AR) film of high quality and extremely low reflectance. Therefore measurements of facet reflectance play a crucial role in the fabrication of such AR coating. The reflectance can be estimated by studying the optical power reflected from the Fabry–Perot cavity formed for that purpose by the examined SOA facet and the end of a single-mode fibre. We have made analysis of practical suitability of such measurement method. Theoretical calculations show that, for the low reflectance coatings, losses due to light coupling into optical fibre cannot be omitted in the analysis of the experimental results. To verify this conclusion a theoretical model was tested for a low reflectance surface and we have found that the relative error of the measurements supported by the theoretical model is on the order of 8%.

Correcting Airborne Laser Scanning Intensity Data for Automatic Gain Control Effect

The intensity data recorded by airborne laser scanning (ALS) systems are useful for several applications, e.g., automatic point classification, change detection, and environmental studies. Before the intensity values can be used for any specific application, it has to be calibrated for atmospheric effect, range, energy loss, and incidence angle. Some ALS systems use automatic gain control (AGC). AGC is useful for getting laser returns even from low-reflectance surfaces (e.g., dark roofs), but it also changes the recorded intensity during the data acquisition, even within one surface type. This means that the same asphalt road might have totally different intensity values depending on the surrounding environment, which has affected the state of the AGC level. Therefore, it is important to correct the intensity values to neglect the effect of AGC in order to be able to get a normalized intensity value, which is only affected by the target characteristics. A first approach to correct the intensity values for AGC is reported in this letter. The same area was flown with AGC on and off, which allowed the modeling to take place. The results showed that the model produces values that agreed with an R2 of 0.76 to the intensities obtained when AGC was turned off.

Low Reflectance Surface Observed on InP Porous Structures after Photoelectrochemical Etching

Extremely low reflectance was obtained from InP porous nanostructures in UV, visible, and near-infrared light ranges. The reflectance strongly depended on the surface morphology of the porous structures prepared by the electrochemical process, and the lowest reflectance of 0.1% in the visible light range was obtained from a sample after the irregular top layer was completely removed. Large anodic photocurrents were obtained on the InP porous structures that had low reflectance surfaces with deeper pores.

Realization of an extremely low reflectance surface based on InP porous nanostructures for application to photoelectrochemical solar cells

Extremely low reflectance was obtained from InP porous nanostructures in UV, visible, and near-infrared ranges. Porous samples were electrochemically prepared on which 130-nm-diameter nanopores were formed in a straight, vertical direction and were laterally separated by 50-nm-thick InP nanowalls. The reflectance strongly depended on the surface morphology. The lowest reflectance of 0.1% in the visible light range was obtained after the irregular top layer had been completely removed. Superior photoelectrochemical properties were obtained on the InP porous structures due to two unique features: the large surface area inside pores, and the large photon absorption enhanced on the low reflectance surface.

Simulation of Present-Day and Twenty-First-Century Energy Budgets of the Southern Oceans

Abstract The energy budget of the modern-day Southern Hemisphere is poorly simulated in both state-of-the-art reanalyses and coupled global climate models. The ocean-dominated Southern Hemisphere has low surface reflectivity and therefore its albedo is particularly sensitive to cloud cover. In modern-day climates, mainly because of systematic deficiencies in cloud and albedo at mid- and high latitudes, too much solar radiation enters the ocean. Along with too little radiation absorbed at lower latitudes because of clouds that are too bright, unrealistically weak poleward transports of energy by both the ocean and atmosphere are generally simulated in the Southern Hemisphere. This implies too little baroclinic eddy development and deficient activity in storm tracks. However, projections into the future by coupled climate models indicate that the Southern Ocean features a robust and unique increase in albedo, related to clouds, in association with an intensification and poleward shift in storm tracks that is not observed at any other latitude. Such an increase in cloud may be untenable in nature, as it is likely precluded by the present-day ubiquitous cloud cover that models fail to capture. There is also a remarkably strong relationship between the projected changes in clouds and the simulated current-day cloud errors. The model equilibrium climate sensitivity is also significantly negatively correlated with the Southern Hemisphere energy errors, and only the more sensitive models are in the range of observations. As a result, questions loom large about how the Southern Hemisphere will actually change as global warming progresses, and a better simulation of the modern-day climate is an essential first step.

Mt. Etna aerosol optical thickness from MIVIS images

This work focuses on the evaluation of Aerosol Optical Thickness (AOT) in Mt. Etna volcano area starting from the analysis of MIVIS VIS images. MIVIS images and ancillary data (atmospheric profiles, photometric measurements, atmospheric infrared radiances, surface temperatures, ground reflectances, SO2 abundances) were collected during the «Sicily ’97» campaign. Data elaboration was performed with extensive use of 6S radiative transfer model, determining optical thickness with an inversion algorithm that uses atmospheric vertical profile, ground reflectance data and radiance measured by the first MIVIS spectrometer (channels 1-20; range 0.44-0.82 n). Ground reflectance is the most problematic parameter for the algorithm. In order to have a low and ‘uniform’ surface reflectance, only pixels located at an altitude between 2000-3000 m a.s.l. were analysed. At this altitude,AOT is very low during non-eruptive periods: at Torre del Filosofo (2920 m a.s.l.) on June 16th 1997, during one MIVIS flight, AOT at 0.55 n was 0.19. The uncertainty about ground reflectance produces significant errors on volcanic background AOT, and in some cases the error is up to 100%. The developed algorithm worked well on volcanic plume, allowing us to determine the plume related pixels’AOT. High plume AOT values minimize the problems deriving from reflectance uncertainty. Plume optical thickness shows values included in a range from 0.5 to 1.0. The plume AOT map of Mt. Etna volcano, derived from a MIVIS image of June 16th 1997, is presented.

High Density n-Si/n-TiO2Core/Shell Nanowire Arrays with Enhanced Photoactivity

There are currently great needs to develop low-cost inorganic materials that can efficiently perform solar water splitting as photoelectrolysis of water into hydrogen and oxygen has significant potential to provide clean energy. We investigate the Si/TiO(2) nanowire heterostructures to determine their potential for the photooxidation of water. We observed that highly dense Si/TiO(2) core/shell nanowire arrays enhanced the photocurrent by 2.5 times compared to planar Si/TiO(2) structure due to their low reflectance and high surface area. We also showed that n-Si/n-TiO(2) nanowire arrays exhibited a larger photocurrent and open circuit voltage than p-Si/n-TiO(2) nanowires due to a barrier at the heterojunction.