The review article describes the composition, working, and benefits of the electrodynamic screen (EDS) film, a self-cleaning surface technology that can be retrofitted onto solar and thermal energy collectors. The EDS film avoids the use of water and robotic parts that are the common cleaning techniques used in solar/thermal power plants and thus emerges as a viable and scalable solution to the soiling problem faced recurrently by these plants. The article summarizes different experiments conducted to improve the efficiency of the EDS film in terms of reflectivity and performance. Field test results are also included to underscore the success of the EDS film operation.

Dust build-up or soiling on thermal and solar energy collector surfaces is a major problem and its cleaning is a major issue for solar energy conversion. Here, a self-cleaning technology is described as a scalable and viable solution to clear the surfaces. EDS film technology does not require water, manual labor, or moving parts to function, and the power needed to operate EDS is almost negligible and can be derived from the harvesting device itself. The EDS films thereby help mitigate the energy loss caused by soiling in solar and thermal harvesting systems. An EDS film with reflective or transparent electrodes can be retrofitted on concentrated solar power mirrors and on photovoltaic (PV) panels to sustain and aid their unhindered reflection and absorption of incident sunlight, respectively. We report experiments and describe methods used to increase the reflectivity of the electrodes of an EDS film. Results obtained from lab test setups and field test units that define the functionality, reflectivity, and stability of the electrodes on the EDS films are also presented. Field test results that compare and report the performance of PV panel output current over long periods of testing, with and without EDS films are also discussed. Test results from 3-month outdoor testing, which demonstrate recovery back to >95% of the pristine system, after decrease to 80–90% before EDS film activation, are also shown.

Traditional solar power applications largely avoid using the infrared spectrum. Nevertheless, this region makes up about 45% of the solar power spectrum and therefore represents an untapped resource. Temperature control of buildings represents a significant cost for both businesses and private consumers. We are interested in developing thermochromic materials for building coatings to help moderate solar infrared absorption and thereby offset temperature control costs for buildings. Our initial effort in this study has been to characterize materials which might represent starting points for our research. We previously designed and 3D-printed an optical test platform to perform reflectance measurements with an ultraviolet-visible-near infrared spectrometer over a spectral range from 200-1000nm. The test platform temperature can be adjusted in real time using Peltier modules. In this study, a sample of microencapsulated 7-anilino-3-diethylamino-6-methyl fluoran was studied by diffuse reflectance spectroscopy from 15-40 degrees Celsius. Scanning electron microscopy was used to characterize the dye particles. Temperature and spectral data were monitored while the sample temperature was adjusted. The visible diffuse reflectance from the sample increased from around 15% below the transition to more than 40% above the transition. A modification of this fluoran which extends the switching behavior into the infrared may be viable for passive thermo-optical switching in building coatings.

Novel metrology tool for in-situ characterization of surfaces semiconductor solar cells (both silicon and compound), and Light Emitting Device diffusers is presented. The tool measures the total integrated scattering when measuring forward, or back-reflection at very large angles of incidence. The tool is insensitive to vibrations and stray light. We discuss polarization resolved data and characterize our technique using NIST traceable standards. We discuss it’s applications to semiconductor manufacturing.

There is great interest in improving TiO2’s photocatalytic activity in the visible portion of electromagnetic spectrum. Recent work has shown that co-doping mesoporous TiO2 microparticles with a transition metal and nitrogen, hereby designated as (M,N) TiO2, significantly increases its visible light absorption. However, the hydrothermal method used to produce the microparticles creates a wide distribution in the size of the microparticles, which could affect the absorption properties. Recently, it has become possible to produce monodisperse, mesoporous TiO2 microparticles with engineered sizes using a hybrid sol-gel/hydrothermal technique. Further, it has also been shown that the size of monodisperse TiO2 microparticles affects the the photocatalytic activity.

This study investigated whether using mondodisperse (M,N) TiO2 microparticles would further increase visible-light absorption for (M,N)TiO2. The first-row transition metals chosen for this study - Mn, Fe, Co, Ni, and Cu – have been characterized in the earlier (M,N) TiO2 UV-vis study, which was used as a baseline. The doping levels of the transition metals samples were set at the 2.5 percent level previously shown to be optimum for photocatalytic activity.

Optical pump-probe studies of cubic crystalline Ge2Sb2Te5/GaSb(001) have previously shown that the amplitude of a coherent optical phonon (COP) with frequency of 3.4 THz observed in the anisotropic reflectance (AR) signal exhibits a four-fold dependence upon the polarization of the probe beam. The appearance of the mode in the AR signal but not the reflectance (R) signal, and the dependence upon probe polarization, both suggest a three-dimensional mode character. Confirmation that this mode indeed has three-dimensional character, similar to the Raman inactive T2 mode in the pristine rock salt structure, is highly important in understanding the structure of the crystalline phase of Ge2Sb2Te5 that has important applications within data storage technology. A phonon of the same frequency has been observed in an epitaxial Ge2Sb2Te5/InAs(111) structure, suggesting that this phonon is indeed characteristic of epitaxial cubic GST. A theory, which considers the symmetry of the Raman tensor for a particular phonon mode, is used to predict the dependence of R and AR signal amplitude upon pump and probe polarization for the T2 mode of a (111) facet of the putative rock-salt structure.

The microwave-induced magnetoresistance oscillations are exhibited by the GaAs/AlGaAs two dimensional electron system (2DES) under microwave and terahertz photo-excitation at liquid helium temperatures. Such oscillations are presently understood in terms of various theories. In order to identify the relative physical contributions, we have concurrently examined magnetotransport and microwave reflection from the 2DES. For the reflection measurements, a sensitive microwave detector was assimilated into the standard experimental setup. Here, we correlate changes in reflection with the concurrent transport response of the photo-excited 2DES.

Investigation of optical absorption in ∼25μm thick, monocrystalline silicon (Si) substrates obtained from a novel exfoliation technique is done by fabricating solar cells with single heterojunction architecture (without using intrinsic amorphous silicon layer) with diffused back junction and local back contact. The ease of process flow and the rugged and flexible nature of the substrates due to thick metal backing enables use of various light-trapping and optical absorption enhancement schemes traditionally practiced in the industry for thicker (>120μm) substrates. Optical measurement of solar cells using antireflective coating, texturing on both surfaces, and back surface dielectric/metal stack as mirror to reflect the long wavelength light from the back surface show a very low front surface reflectance of 4.6% in the broadband spectrum (300nm-1200nm). The illuminated current voltage (IV) and external quantum efficiency (EQE) measurement of such solar cell shows a high integrated current density of 34.4mA/cm2, which implies significant internal photon reflection. Our best cell with intrinsic amorphous silicon (i-a-Si) layer with only rear surface textured shows an efficiency of 14.9%. EQE data shows improved blue response and current density due to better front surface passivation. Simulations suggest that with optimized light trapping and surface passivation, such thin c-Si cells can reach efficiencies >20%.

The surface plasmon enhanced transmission of light though a plasmonic crystal provides a novel approach for fabricating an optical modulator. The extraordinary transmission passing though these patterned metallic films is very sensitive to the surface dielectric environment. In this study, hexagonal lattice plasmonic crystals were fabricated with a self-assembly technique. Arrays of gold nano-holes or bumps with 500/600 nm periodicity were used to test the sensitivity of plasmon resonance wavelength for liquids and polymers with different dielectric constants. A nonlinear optical polymer P3HT coated onto the plasmonic crystal and pumped with 475 nm laser was found to modulate the transmission of a normally incident red light at 670 nm.

We describe two techniques dedicated to observe and study the heating of structured materials like micro and nanowires and multilayered compounds. The techniques are thermally modulated fluorescence and thermoreflectance. Thermally modulated fluorescence allows mapping the heating of devices with a sub-wavelength lateral resolution. Thermoreflectance allows deeper physical investigations and can be directly used to determine the thermal conductivity and diffusivity of layered structures. In particular, we will show that by thermally modulating a surface by a point-like source, we are able to determine such quantities for several geometries, taking into account the nature of the substrate (conductive or not) as well as the interface quality between the layers. The experimental results, measured on aluminum thin films of variable thickness and on vanadium dioxide layers are corroborated by an analytical model that analyzes both the amplitude and the phase of the lateral heat diffusion in the structure

Thin film silicon solar cells, consisting of an epitaxially grown active layer on a low quality highly doped silicon substrate, incorporate many attractive features usually associated with their sister cells based on bulk silicon. However, the efficiency of the current epitaxial semi-industrial screen printed cells is limited to 11-12% mainly due to optical shortcomings. This paper will give an overview of our work aimed at tackling the 2 most important problems: (i) Finding and implementing an adequate front surface texture and (ii) the simulation, fabrication and incorporation of an intermediate reflector.

The former issue has been addressed by the development of plasma texturing based on halogen species. This method allows us to fulfil the sometimes contradictory requirements for the textured surface, i.e. a uniform and reduced reflection, a strong lambertian character to scatter the light and a limited removal of silicon. It will be shown that the scattering efficiency is dependent on both the wavelength of the impinging light and on the silicon removal during the texturing process.

The second and main issue of this work is the limited absorption volume of the epitaxial layer. To resolve this drawback, an intermediate reflector is placed at the epi/substrate interface to enhance the path length of the low energy photons through the epi-layer. In practice, a multi-layer porous silicon stack is created by electrochemical anodization of the substrate. The reflection at the epi/reflector/substrate interface is a combination of several different effects including a Bragg mirror and Total Internal Reflection (TIR). Measurements of the external reflectance as well as extraction of the internal reflection parameters are used to clarify the issue. Advanced structures, including chirped porous silicon stacks, are introduced. Finally, the benefits of the reflector on the level of the epitaxial silicon solar cell are analysed. Efficiencies close to 14% are obtained for epitaxial cells incorporating an advanced porous Si reflector.

Thermoreflectance is an non contact optical method using the local reflectivity variations induced by heating to infer temperature mappings, and can be conducted at virtually any wavelength. In the visible, the technique is now well established. It can probe temperatures through several micrometers of transparent encapsulation layers, with sub-micron spatial resolution and 100 mK thermal resolution. In the ultraviolet range, dielectric encapsulation layers are opaque and thermoreflectance gives access to the surface temperature. In the near infrared, thermoreflectance is an interesting solution to examine chips turned upside down, since these wavelengths can penetrate through silicon substrates and give access to the temperature of the active layers themselves. Here, we explore the possibilities of each wavelength range and detail the CCD-based thermal imaging tools dedicated to the high resolution inspection of integrated circuits.

By reduction of SiO2 opal in atmosphere of nitrogen (80%)-helium (15%)-hydrogen (5%) gas mixture at various temperatures we tuned the dielectric contrast ratio and band gap of photonic crystal (PC) up to 10 %. The SEM images dose not show any detectible change in lattice parameters, whereas the elemental analysis shows remarkable decrease of oxygen content. The shift of reflectance spectra toward the long wavelength and increase of transmission in infrared region confirm the redaction of silica and partial transformation to the silicon.

Non-absorbing mirror (NAM) lasers, also known as window lasers, were developed to increase catastrophic optical damage level of the devices. In this work we present the analysis of the laser diodes (LD) front facet temperature distribution. Micro-thermoreflectance technique is used to perform a detailed temperature maps of the operating LDs. Such a technique gives us temperature maps with a high spatial resolution equal 0.6μm. We demonstrate micro thermoreflectance as a perfect tool to determine real temperature distribution of the operating laser diode front facet.