Publisher’s Note: This paper, originally published on 23 September 2016, was replaced with a corrected/revised version on 21 December 2016. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance. Light concentration has proven beneficial for solar cells, most notably for highly efficient but expensive absorber materials using high concentrations and large scale optics. Here we investigate light concentration for cost efficient thinfilm solar cells which show nano- or microtextured absorbers. Our absorber material of choice is Cu(In,Ga)Se₂ (CIGSe) which has a proven stabilized record efficiency of 22.6% and which - despite being a polycrystalline thin-film material - is very tolerant to environmental influences. Taking a nanoscale approach, we concentrate light in the CIGSe absorber layer by integrating photonic nanostructures made from dielectric materials. The dielectric nanostructures give rise to resonant modes and field localization in their vicinity. Thus when inserted inside or adjacent to the absorber layer, absorption and efficiency enhancement are observed. In contrast to this internal absorption enhancement, external enhancement is exploited in the microscale approach: mm-sized lenses can be used to concentrate light onto CIGSe solar cells with lateral dimensions reduced down to the micrometer range. These micro solar cells come with the benefit of improved heat dissipation compared to the large scale concentrators and promise compact high efficiency devices. Both approaches of light concentration allow for reduction in material consumption by restricting the absorber dimension either vertically (ultra-thin absorbers for dielectric nanostructures) or horizontally (micro absorbers for concentrating lenses) and have significant potential for efficiency enhancement.

In this paper, high-efficiency STPV systems are investigated using spectrally selective absorber/emitter consisted of metal-dielectric multilayer and a GaSb TPV cell. A solar-thermophotovoltaic (STPV) system is expected to as highefficiency solar energy conversion using single-junction photovoltaic (PV) cells. However, the reached experimental system efficiency has been still low because spectral control of emitter is not sufficient. Narrowband thermal radiation from the emitter is effective for obtaining high-efficiency STPV systems, exceeding the Shockley-Queisser limit. From theoretical analysis, the narrowband thermal emitter can leads to obtain PV conversion efficiency over 45% at Qvalue= 30 and 1300K. The spectrally selective absorber/emitter was also investigated to obtain high ηPV. The ηPV = 23.5% was estimated by the fabricated emitter spectrum, which exceeds the Shockley-Queisser limit of 19.6% for a GaSb bandgap of 0.67 eV. The entire STPV system and the power generation tests were conducted using the fabricated absorber/emitter. The total system efficiency 4.9% at 1505K was obtained under an irradiance of 109 Wcm^-2.

Optical spectrum splitting systems that divide light between independent solar cells of different band gaps have received increasing attention in recent years as an alternative to expensive multijunction cells for high-efficiency PV. Most research, however, has focused on dichroic filters and other photonic structures that are expensive to manufacture. This has the effect of transferring the cost of the system from the PV cells to the optics. As a low-cost spectrum splitting approach we designed a prismatic lens that simultaneously splits and concentrates light and can be fabricated by injection molding. We present experimental results of a two-cell demonstration system, and calculations for low-cost configurations of commercial solar cells, enabled by the removal of lattice-matching requirements.

The hot Carrier Solar Cell (HCSC) allows the photon-induced hot carriers (the carriers with energy larger than the band gap) to be collected before they completely thermalise. The absorber of the HCSC should have a large phononic band gap to supress Klemens Decay, which results in a slow carrier cooling speed. In fact, a large phononic band gap likely exists in a binary compound whose constituent elements have a large mass ratio between each other. Binary hydrides with their overwhelming mass ratio of the constituent elements are important absorber candidates. Study on different types of binary hydrides as potential absorber candidates is presented in this paper. Many binary transition metal hydrides have reported theoretical or experimental phonon dispersion charts which show large phononic band gaps. Among these hydrides, the titanium hydride (TiH_X) is outstanding because of its low cost, easy fabrication process and is relatively inert to air and water. A TiH_X thin film is fabricated by directly hydrogenating an evaporated titanium thin film. Characterisation shows good crystal quality and the hydrogenation process is believed to be successful. Ultrafast transient absorption (TA) spectroscopy is used to study the electron cooling time of TiH_X. The result is very noisy due to the low absorption and transmission of the sample. The evolution of the TA curves has been explained by band to band transition using the calculated band structure of TiH₂. Though not reliable due to the high noise, decay time fitting at 700nm and 600nm shows a considerably slow carrier cooling speed of the sample.

The introduction of GaSb quantum dots (QDs) within a GaAs single junction solar cell is attracting increasing interest as a means of absorbing long wavelength photons to extend the photoresponse and increase the short-circuit current. The band alignment in this system is type-II, such that holes are localized within the GaSb QDs but there is no electron confinement. Compared to InAs QDs this produces a red-shift of the photoresponse which could increase the short-circuit current and improve carrier extraction. GaSb nanostructures grown by molecular beam epitaxy (MBE) tend to preferentially form quantum rings (QRs) which are less strained and contain fewer defects than the GaSb QDs, which means that they are more suitable for dense stacking in the active region of a solar cell to reduce the accumulation of internal strain and enhance light absorption. Here, we report the growth and fabrication of GaAs based p-i-n solar cells containing ten layers of GaSb QRs. They show extended long wavelength photoresponse into the near-IR up to 1400 nm and enhanced short-circuit current compared to the GaAs control cell due to absorption of low energy photons. Although enhancement of the short-circuit current was observed, the thermionic emission of holes was found to be insufficient for ideal operation at room temperature.

Hot carrier solar cells (HCSCs) have been proposed as devices, which can increase the conversion efficiency of a single junction solar cell above the Shockley-Queisser limit. For practical implementation of such systems, solar cells operating with efficient hot carrier extraction must circumvent two fundamental challenges: 1. Find an absorber material in which hot carriers are sustained either via inhibiting or circumventing phonon relaxation pathways; 2. Implement energy selective contacts in which only a narrow range of energy within the hot carrier distribution is extracted; thereby, reducing cooling losses in the contacts.

Here, type-II InAs/AlAs_0.16Sb_0.84 quantum-wells are investigated as a candidate system for hot carrier absorbers. Continuous wave power and temperature dependent photoluminescence measurements are presented that indicate: a transition in the dominant hot carrier relaxation process from conventional phonon-mediated carrier relaxation − below 90 K − to a regime where inhibited radiative recombination dominates the hot carrier relaxation − at higher temperatures¹. The reduction in the PL efficiency is strongly coupled to an increase in the hot carrier temperature extracted from the measurements. This behavior is attributed to a build-up of electrons in the QWs, which appears to inhibit electron-phonon relaxation².

Quantum Dots (QDs) have a broad applications in science and specifically in solar cell. Many research groups show that by adding QDs with lower bandgap respect to host material, the overall absorption of sun spectrum coverage will increase. Here, we propose using QDs with higher band gap respect to host material to improve efficiency of solar cell by improving quantum efficiency. GaAs solar cells have the highest efficiency in single junction solar cells. However, the absorption of GaAs is not good enough in wavelength lower than 550nm. AlSb can absorb shorter wavelength with higher absorption coefficient and also recombination rate should be lower because of higher bandgap of AlSb respect to GaAs. We embed AlSb QDs in GaAs solar cells and results show slight improvement in quantum efficiency and also in overall efficiency. Coverage of AlSb QDs has a direct impact on quality of AlSb QDs and efficiency of cell. In the higher coverage, intermixing between GaAs and AlSb causes to shift bandgap to lower value (having AlGaSb QDs instead of pure AlSb QDs). This intermixing decrease the Voc and overall efficiency of cell. In lower coverage, AlSb can survive from intermixing and overall performance of cell improves. Optimizing growth condition of AlSb QDs is a key point for this work. By using AlSb QDs, we can decrease the thickness of active layer of GaAs solar cells and have a thinner solar cell.

Operation of concentrated solar power receivers at higher temperatures (<700°C) would enable supercritical carbon dioxide (sCO2) power cycles for improved power cycle efficiencies (<50%) and cost-effective solar thermal power. Unfortunately, radiative losses at higher temperatures in conventional receivers can negatively impact the system efficiency gains. One approach to improve receiver thermal efficiency is to utilize selective coatings that enhance absorption across the visible solar spectrum while minimizing emission in the infrared to reduce radiative losses. Existing coatings, however, tend to degrade rapidly at elevated temperatures. In this paper, we report on the initial designs, fabrication, and characterization of spectrally selective metamaterial-based absorbers for high-temperature, high-thermal flux environments important for solarized sCO2 power cycles. Metamaterials are structured media whose optical properties are determined by sub-wavelength structural features instead of bulk material properties, providing unique solutions by decoupling the optical absorption spectrum from thermal stability requirements. The key enabling innovative concept proposed is the use of structured surfaces with spectral responses that can be tailored to optimize the absorption and retention of solar energy for a given temperature range. In this initial study we use Tungsten for its stability in expected harsh environments, compatibility with microfabrication techniques, and required optical performance. Our goal is to tailor the optical properties for high (near unity) absorptivity across the majority of the solar spectrum and over a broad range of incidence angles, and at the same time achieve negligible absorptivity in the near infrared to optimize the energy absorbed and retained. To this goal, we apply the recently developed concept of plasmonic Brewster angle to suitably designed nanostructured Tungsten surfaces. We predict that this will improve the receiver thermal efficiencies by at least 10% over current solar receivers.

The ability to downconvert (1 photon to 2 photons) and upconvert (2 photons to 1 photon) energy can boost solar energy conversion efficiencies by 30% or more. Downconversion can be accomplished through exciton fission, in which an initially created high energy singlet exciton spontaneously splits into a pair of lower energy triplet excitons. In organic semiconductors like tetracene and rubrene, the Frenkel character of the excitons leads to energetically separate singlet and triplet bands, providing an ideal set of energy levels for both processes to take place. In this talk, our efforts to understand the basic photophysics of singlet fission using time-resolved transient absorption, photoluminescence and magnetic field effects will be described. The role of molecular packing in controlling the fission rate will be emphasized. Upconversion occurs via the reverse process, where a pair of triplet excitons fuse into a high-energy singlet state. While most approaches to upconversion require a sensitizer to populate the dark triplet states, an alternate approach is to take advantage of low-energy intermolecular states in organic crystals to sensitize triplet states. We show that this process can be surprisingly efficient in certain molecular crystals, even in the absence of sensitizers. The exciton interactions responsible for this process are investigated using steady-state and time-resolved spectroscopy.

III-V compound semiconductor nanowire arrays are promising candidates for photovoltaics applications due to their high volumetric absorption. Uniform nanowire arrays exhibit high absorption at certain wavelengths due to strong coupling into lossy waveguide modes. Previously, simulations predicted near-unity, broadband absorption in sparse semiconductor nanowire arrays (<5% fill fraction) with multi-radii and tapered nanowire array designs [1]. Herein, we experimentally demonstrate near-unity broadband absorption in InP nanowire arrays via a scalable, epitaxy-free fabrication method, using nanoimprint lithography and ICP-RIE to define nanowire arrays in bulk InP wafers. In addition to mask pattern design (wire radius and spacing) and etch chemistry (wire taper), appropriate selection of a hard mask for the InP etch is critical to precise dimension control and reproducibility. Polymer-embedded wires are removed from the bulk InP substrate by a mechanical method that facilitates extensive reuse of a single bulk InP wafer to synthesize many polymer-embedded nanowire array thin films. Arrays containing multiple nanowire radii and tapered nanowires were successfully fabricated. For both designs, the polymer-embedded arrays achieved ~90% broadband absorption (λ=400-900 nm) in less than 100 nm planar equivalence of InP. The addition of a silver back reflector increased this broadband absorption to ~95%. The repeatable process of imprinting, etching and peeling to obtain many nanowire arrays from one single wafer represents an economical manufacturing route for high efficiency III-V photovoltaics. [1] K.T. Fountaine, C.G. Kendall, Harry A. Atwater, “Near-unity broadband absorption designs for semiconducting nanowire arrays via localized radial mode excitation,” Opt. Exp. (2014).

We have demonstrated broadband sensitization of Er³⁺-doped upconverters coupled with crystalline silicon (c-Si) solar cells by introducing Ni²⁺ co-dopants into ABO₃-type perovskite host materials such as La(Ga,Sc,In)O₃ and CaZrO₃. The Ni²⁺ sensitizers absorb 1.1−1.45 μm photons, which are not absorbed by either c-Si or Er³⁺, and transfer the energies to the Er³⁺ emitters. Thus, 1.1−1.45 μm photons are also upconverted to 0.98 μm photons, in addition to 1.45−1.6 μm photons that are directly absorbed by the Er³⁺. To compensate the charge imbalance caused by introducing divalent Ni²⁺ ions into the trivalent (Ga³⁺, Sc³⁺, and In³⁺) and tetravalent (Zr⁴⁺) sites, Nb⁵⁺ co-dopants were incorporated. Similarly, codoping with monovalent ions (Li⁺, Na⁺, K⁺) notably enhanced the upconversion emission when the Ca²⁺ sites were substituted with the Er³⁺ ions. These broadband-sensitive upconverters overcome the shortcoming of conventional Er³⁺- doped upconverters that only a small portion of the solar spectrum at around 1.55 μm is utilized. If all the photons in the Er³⁺ absorption band ranging from 1.45 μm to 1.6 μm were perfectly upconverted, the improvement in the short-circuit current density (JSC) would be 1.9 mA/cm² under the AM1.5G 1 sun solar illumination. The additional improvement for the broadband-sensitive upconverters developed here could be as high as 4.1 mA/cm² by utilizing 1.1−1.45 μm photons, thus totally 6.1 mA/cm². This corresponds to a significant gain in conversion efficiency (η) by 3.8% for c-Si solar cells with JSC = 40 mA/cm² and η = 25%. The architecture of the broadband sensitization opens the door toward the concept of the third-generation solar cells with high conversion efficiency and low cost.

Up-conversion for Si solar cells using Er phosphors has the twin problems of a very high concentration requirement and the very narrow bandwidth for absorption. An alternative is to use the equivalent circuit of an up-converter, two below bnadgap solar cells connected in series to pump an above band gap LED, as suggested by Trupke SOLMAT 2005, but not yet attempted experimentally. Results on the current enhancement in such devices are shown for absorption of below Si band gap light in long wavelength cells to pump a InGaAs LED so as to sensitise a Si solar cell.

We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites using a confocal microscopy setup with independent excitation and collection objectives. We observed regenerated PL emission at distances as far as 50 micrometers away from photoexcitation. We then made a scratch in the film to increase out-scattering and found that the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. This is caused by the sharp decay of the absorption coefficient at the band tail, which allows longer wavelength photons to travel further between emission and absorption events, originating charges far from excitation. We then built a lateral-contact solar cell with selective electron- and hole-collecting contacts, using a combination of photolitography and electrodeposition. We used these devices as a platform to study photocurrent propagation and found that charge extraction can be achieved well beyond 50 micrometers away from the excitation. We connect these two observations by comparing the decay in intensity of the recycled component of the PL (which is around 765 nm) with the decay in photocurrent. Taking into account that PL is proportional to the square of charge density, whilst photocurrent is proportional to charge density. Photon recycling leads to an increase in internal photon densities, which leads to a build-up of excited charges. This increases the split of quasi-Fermi levels and enhances the achievable open circuit voltage in a solar cell.

A crucial limit of solar devices is their inability to harvest the full solar spectrum. Currently, sensitized up-conversion based on triplet-tripled annihilation (STTA-UC) in bi-component organic systems is the most promising technique to recover sub-bandgap photons, showing good efficiencies also at excitation intensities comparable to the solar irradiance. In STTA-UC, high-energy light is generated through annihilation of metastable triplet states of molecules acting as emitters, which are populated via resonant energy transfer from a light-harvesting sensitizer. However, suitable sensitizers show narrow absorption bands, limiting the fraction of recoverable photons, therefore preventing the application of STTA-UC to real-world devices. Here we demonstrate how to overcome the described limit by using multiple sensitizers that work cooperatively to broaden the overall system absorption band. This is obtained using an additional sensitizer that transfers the extra harvested energy to the main one (sensitization of the sensitizer), or a set of properly designed complementary absorbing sensitizers all able to excite simultaneously the same emitter (multi-sensitizers). In both cases STTA-UC performances result strongly enhanced compared to the corresponding mono-sensitizer system, increasing the up-converted light intensity generated at AM 1.5 up to two times. Remarkably, by coupling our light converters to a DSSC we prove its operation by exploiting exclusively sub-bandgap photons. A detailed modeling of the photophysical processes involved in these complex systems allows us to draw the guidelines for the design of the next generation STTA-UC materials, encouraging their application to photovoltaic technologies.

Spectrum splitting is an approach to increasing the conversion efficiency of a photovoltaic (PV) system. Several methods can be used to perform this function which requires efficient spatial separation of different spectral bands of the incident solar radiation. In this paper several of holographic methods for implementing spectrum splitting are reviewed along with the benefits and disadvantages associated with each approach. The review indicates that a volume holographic lens has many advantages for spectrum splitting in terms of both power conversion efficiency and energy yield. A specific design for a volume holographic spectrum splitting lens is discussed for use with high bandgap InGaP and low bandgap silicon PV cells. The holographic lenses are modeled using rigorous coupled wave analysis, and the optical efficiency is evaluated using non-sequential raytracing. A proof-of-concept off-axis holographic lens is also recorded in dichromated gelatin film and the spectral diffraction efficiency of the hologram is measured with multiple laser sources across the diffracted spectral band. The experimental volume holographic lens (VHL) characteristics are compared to an ideal spectrum splitting filter in terms of power conversion efficiency and energy yield in environments with high direct normal incidence (DNI) illumination and high levels of diffuse illumination. The results show that the experimental VHL can achieve 62.5% of the ideal filter power conversion efficiency, 64.8% of the ideal filter DNI environment energy yield, and 57.7% of the ideal diffuse environment energy yield performance.

This paper presents a segmented parabolic concentrator employing holographic spectral filters that provide focusing and spectral bandwidth separation capability to the system. Strips of low band gap silicon photovoltaic (PV) cells are formed into a parabolic surface as shown by Holman et. al. [1]. The surface of the PV segments is covered with holographic elements formed in dichromated gelatin. The holographic elements are designed to transmit longer wavelengths to silicon cells, and to reflect short wavelength light towards a secondary collector where high-bandgap PV cells are mounted. The system can be optimized for different combinations of diffuse and direct solar illumination conditions for particular geographical locations by controlling the concentration ratio and filtering properties of the holographic elements. In addition, the reflectivity of the back contact of the silicon cells is used to increase the optical path length and light trapping. This potentially allows the use of thin film silicon for the low bandgap PV cell material. The optical design combines the focusing properties of the parabolic concentrator and the holographic element to control the concentration ratio and uniformity of the spectral distribution at the high bandgap cell location. The presentation concludes with a comparison of different spectrum splitting holographic filter materials for this application.

In this work a spectrum splitting micro-scale concentrating PV system is evaluated to increase the conversion efficiency of flat panel PV systems. In this approach, the dispersed spectrum splitting concentration systems is scaled down to a small size and structured in an array. The spectrum splitting configuration allows the use of separate single bandgap PV cells that increase spectral overlap with the incident solar spectrum. This results in an overall increase in the spectral conversion efficiency of the resulting system. In addition other benefits of the micro-scale PV system are retained such reduced PV cell material requirements, more versatile interconnect configurations, and lower heat rejection requirements that can lead to a lower cost system. The system proposed in this work consists of two cascaded off-axis holograms in combination with a micro lens array, and three types of PV cells. An aspherical lens design is made to minimize the dispersion so that higher concentration ratios can be achieved for a three-junction system. An analysis methodology is also developed to determine the optical efficiency of the resulting system, the characteristics of the dispersed spectrum, and the overall system conversion efficiency for a combination of three types of PV cells.

In this study the impact of outdoor temperature variations and solar illumination exposure on spectral filter material and holographic optical elements is examined. Although holographic components have been shown to be useful for solar spectrum splitting designs, relatively little quantitative data exist to demonstrate the extent to which these materials can withstand outdoor conditions. As researchers seek to investigate practical spectrum splitting designs, the environmental stability of holographic materials should be considered as an important factor. In the experiment presented, two holographic materials, Covestro Bayfol HX photopolymer and dichromated gelatin, and 3M reflective polymer filter materials are exposed to outdoor conditions for a period of several months. The environmental effect on absorption, spectral and angular bandwidth, peak efficiency, and Bragg matching conditions for the holograms are examined. Spectral bandwidth and transmittance of the 3M reflective filter material are also monitored. Holographic gratings are recorded, measured, and mounted on glass substrates and then sealed with a glass cover plate. The test samples are then mounted on a photovoltaic panel to simulate realistic temperature conditions and placed at an outdoor test facility in Tucson, Arizona. A duplicate set of holograms and 3M filter material is stored as a control group and periodically compared over the test period.

Concentrating Photo Voltaic (CPV) systems maximize energy harvested from the sun with multi-junction solar cells of less area, reducing related implementation costs and reaching energy production thresholds up to 38,9 %. Nowadays, CPV systems are generally implemented in solar energy farms in a permanent location, however, these systems could be used in other dynamic contexts, such as vehicles or portable devices. In this way, mechanical and geometrical parameters related to manipulation, transportation and installation should be carefully considered at the design stage. Besides, each condition of use presents different variables affecting these parameters. In all, there is not an established architecture for these systems, opening up the possibility of radically changing their use, geometry and components. Therefore, a concept of a methodical process for designing of CPV systems is proposed in order to predict their behavior in terms of implementation and energy production. This might allow the development of robust concepts that can be adapted to different context of use as required, providing an itinerant character and thus extending the field of implementation of these systems beyond a static use.

The relevant variables for the use of CPV systems are determined through experimentation considering the implementation of Fresnel lenses as light concentrators. This allows generating a structured design guide composed of different methods of measurement, selection and development. The methodical process is based on a perspective of functional modules considering needs, technical aspects and particular usage conditions of each design and it would provide appropriate guidelines in each circumstance.

The color of a crystalline silicon (c-Si) solar cell is mainly determined by its anti-reflective coating. This is a lambda/4 coating made from a transparent dielectric material. The thickness of the anti-reflective coating is optimized for maximal photocurrent generation, resulting in the typical blue or black colors of c-Si solar cells. However, for building-integrated photovoltaic (BiPV) applications the color of the solar cells is demanded to be tunable – ideally by a cheap and flexible coating process on standard (low cost) c-Si solar cells. Such a coating can be realized by applying plasmonic coloring which is a rapidly growing technology for high-quality color filtering and rendering for different fields of application (displays, imaging,…).

In this contribution, we present results of an approach for tuning the color of standard industrial c-Si solar cells that is based on coating them with metallic nano-particles. In particular, thin films (< 20 nm) of a metal (e.g., silver) were sputtered onto c-Si solar cells and thermally annealed subsequently. The sizes and the shapes of the nano-particles (characterized by SEM) were found to depend on the thickness of the deposited films and the surface roughness of the substrates/solar cells. With such an approach it is possible to tune the color of the standard c-Si cells from blue to green and brownish/red. The position of the resonance peak in the reflection spectrum was found to be almost independent from the angle of incidence. This low angular sensitivity is a clear advantage compared to alternative color tuning methods, for which additional dielectric thin films are deposited on c-Si solar cells.

We successfully demonstrated and reported the highest solar-to-hydrogen efficiency with crystalline silicon cells and Earth-abundant electrocatalysts under unconcentrated solar radiation. The combination of hetero-junction silicon cells and a 3D printed Platinum/Iridium-Oxide electrolyzer has been proven to work continuously for more than 24 hours in neutral environment, with a stable 13.5% solar-to-fuel efficiency. Since the hydrogen economy is expected to expand to a global scale, we demonstrated the same efficiency with an Earth-abundant electrolyzer based on Nickel in a basic medium. In both cases, electrolyzer and photovoltaic cells have been specifically sized for their characteristic curves to intersect at a stable operating point. This is foreseen to guarantee constant operation over the device lifetime without performance degradation. The next step is to lower the production cost of hydrogen by making use of medium range solar concentration. It permits to limit the photoabsorbing area, shown to be the cost-driver component. We have recently modeled a self-tracking solar concentrator, able to capture sunlight within the acceptance angle range +/-45°, implementing 3 custom lenses. The design allows a fully static device, avoiding the external tracker that was necessary in a previously demonstrated +/-16° angular range concentrator. We will show two self-tracking methods. The first one relies on thermal expansion whereas the second method relies on microfluidics.

One can convert a Luminescent Solar Concentrator (LSC) to an energy-harvesting display by scanning a laser beam on it. By incorporating a guest-host system of liquid crystal (LC) and dye materials in an LSC, the power of photoluminescence (PL) utilized for either display or energy-harvesting can be adjusted to the changes in ambient lighting conditions. We have measured basic characteristics of an LC/dye cell with twisted-nematic (TN) alignment. These are absorption of the laser light, PL radiation pattern, contrast of luminance, spreading of the PL generated by a narrow laser beam, and their dependencies on the bias. The results are similar to those of the LC/dye cell with antiparallel (AP) alignment with the following exceptions. First, absorption by the TN cell depends on the bias for both polarization components of the excitation light, while the AP cell exhibits a bias dependency only for the component polarized along the alignment direction. Second, the PL from the TN cell is mostly polarized along the alignment direction on the exit side of the cell while the PL from the AP cell is mostly polarized along its alignment direction. These observations can be attributed to the fact that the polarization plane of a linearly polarized light rotates as it propagated the TN-LC layer. For both AP and TN cells, low-intensity PL is observed from the whole cell surfaces. This can degrade the contrast of a displayed image. Bias application to the cell suppresses this effect.

One of the popular solar air conditioning technologies is desiccant air conditioning. Nonetheless, single stage desiccant air conditioning systems’ coefficient of performance (COP) are relatively low. Therefore, multi-stage solid desiccant air conditioning systems are recommended. In this paper, an integrated double-stage desiccant air conditioning systems and PV/T collector is suggested for hot and humid climates such as the UAE. The results for the PV/T implementation in the double-stage desiccant cooling system are assessed against the PV/T results for a single-stage desiccant air conditioning system. In order to provide a valid comparative evaluation between the single and double stage desiccant air conditioning systems, an identical PV/T module, in terms of dimensions, is incorporated into these systems. The overall required auxiliary air heating is abated by 46.0% from 386.8 MWh to 209.0 MWh by replacing the single stage desiccant air conditioning system with the proposed double stage configuration during June to October. Moreover, the overall averaged solar share during the investigated months for the single and double stage systems are 36.5% and 43.3%.

Dye sensitized solar cells (DSSCs) have shown promising results in the field of renewable energy owing to their low cost and portable features. In practical applications, their harvested energy could be stored in a supercapacitor once it exceeds the regular consumption. Various methods of manipulation of the active electrode have been examined to facilitate the energy storage of the system, whereas the counter electrode has always been known for its catalytic functionality and its contribution to the capacitive response of the device left a well-oriented study to be desired. In this work, the substitution of the platinum electrode with a specific porous electrode resulted in a supercapacitive behavior of the device. The photoactive electrode was fabricated using zinc oxide nanowires (ZnO) grown on a conductive transparent substrate with hydrothermal deposition method. The electrode was used to make a standard DSSC using a ruthenium dye, iodide/triiodide standard redox electrolyte, and a platinum counter electrode. The cyclic voltammetry (CV) study of the device showed a low capacitance with 350 mV open circuit voltage. Replacing the platinum counter electrode with a particularly designed porous paper-based carbon nanotube electrode resulted in a considerable difference in the CV response. A capacitive behavior was observed due to the large surface area of the counter electrode and the ZnO nanostructures on the photoactive electrode. Due to the large capacitance and relatively small photocurrent, the change in the open circuit voltage was limited. However, enhancement of the photocurrent could improve both the energy harvesting and charge storage in the device.

Energy storage is an essential ground for solar energy systems, particularly for the off-grid applications. Concurrent energy harvesting and charge storage in a photoactive supercapacitor has already been demonstrated. The key element in such a device is a conducting polymer which stores the charge via change in its redox states. In this work, we have studied the photoelectrochemical reactions in composites of polyaniline (PANI). We used the electrochemical deposition method for the polymer growth. The results of the current study indicate that the photo-reactivity of the materials largely depends on the electrolyte and the type of the dye molecule. Among different synthetic dyes, methylene blue has shown the strongest photoelectrochemical reaction in an HCl electrolyte. The cyclic voltammetry (CV) results showed that the amplitude of the redox peaks changes significantly upon illumination. The amount of stored charges in the polymer was estimated from the CV results. The results encourage the application of PANI for charge storage in a photoactive supercapacitor.

In this paper, different approaches are considered to calculate the cosine factor which is utilized in Campo code to expand the heliostat field layout and maximize its annual thermal output. Furthermore, three heliostat fields containing different number of mirrors are taken into consideration. Cosine factor is determined by considering instantaneous and time-average approaches. For instantaneous method, different design days and design hours are selected. For the time average method, daily time average, monthly time average, seasonally time average, and yearly time averaged cosine factor determinations are considered. Results indicate that instantaneous methods are more appropriate for small scale heliostat field optimization. Consequently, it is proposed to consider the design period as the second design variable to ensure the best outcome. For medium and large scale heliostat fields, selecting an appropriate design period is more important. Therefore, it is more reliable to select one of the recommended time average methods to optimize the field layout. Optimum annual weighted efficiency for heliostat fields (small, medium, and large) containing 350, 1460, and 3450 mirrors are 66.14%, 60.87%, and 54.04%, respectively.

The optical properties of Yb³⁺/Er³⁺ doped Y₂O₃ upconversion phosphor and the enhancement of efficiency of a-Si:H solar cell on incorporation of upconverter are investigated. The Y₂O₃ host material has high corrosion resistance, thermal stability, chemical stability, low toxicity and relatively low phonon energy (≈ 500 cm^-1). Y₂O₃:Yb³⁺ (x %): Er³⁺ (y %) upconversion nanophosphors with different dopant concentrations were synthesized via simple hydrothermal method followed by a heat treatment at 1200°C for 12 hrs. Highly crystalline, quasi-spherical, body centered cubic Y₂O₃ structure was obtained. The structure, phase and morphology of the nanocrystals were determined using x-ray diffraction and SEM. Following pumping at 980 nm two dominant emission bands were observed at about 550 nm(green) and 660 nm(red), corresponding to ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions respectively. The dependence of emission intensity on pump power shows that the mechanism involved is two photon absorption. The upconversion phosphor along with a binder is coupled behind the a-Si:H solar cell which absorbs transmitted sub-band-gap photons and emits back the upconverted visible light which can be absorbed by the solar cell. Under suitable intensity of illumination the solar cell short circuit current is found to be increased on adding the upconversion layer.

This study investigates the characteristics of luminescent solar concentrators (LSCs) with structured gratings. By creating the optical model, the characteristics of the proposed LSC were simulated. They consist of the analyses of different grating periods and dye combinations. The LSC devices were fabricated and verified. The results show that the simulation and experiment have good consistence.

This PDF file contains the front matter associated with SPIE Proceedings Volume 9937, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.