Symposium EE3 : Materials and Devices for Full Spectrum Solar Energy Harvesting

A lateral multi-junction photovoltaic array has potential to harvest solar energy more efficiently than the tandem solar cell as it eliminates the need for current matching and epitaxial compatibility of the PV materials. A key technology to realize a lateral design and parallel connection of multi-junction PVs is to spectrally split solar spectrum in the lateral direction using a photoresist thin film imprinted with a micron-scale surface texture. Here, we present our progress in the design and fabrication of such optical thin film for full spectrum splitting. Adjoint-based gradient-descent algorithm was used to design diffractive optical elements capable of laterally splitting the visible and near-infrared bands of the solar spectrum in the far field. Prototype thin films were then fabricated by direct laser writing technique to produce the designed micro structures on the film surface. The final film has a minimum lateral feature size of 5 µm and a maximum step height of 2.7 µm. A mean spectral splitting efficiency of 69.5% in the 380-970 nm range was measured on this element under normal incidence. This is slightly lower than our design target of 79.1% spectral splitting efficiency. However, after considering the fabrication errors of those micron scale diffractive optical elements, excellent agreement between the fabricated prototype and numerical design was obtained. To investigate the ability of existing spectral-splitting systems to collect diffuse sunlight, we study our design’s performance under off-axis incidence angles. Future work is still needed to understand the limits of diffuse light collection. Our results indicate these thin-film structures have the potential for full spectrum solar energy harvesting using a lateral device geometry.

For quantum dot (QD) silicon (Si) based tandem solar cells, silicon germanium (SiGe) QDs can be applied as absorber layers of the bottom cell. After having achieved QD Si particles in VHFPECVD [1] it is our endeavor to fabricate SiGe QDs in a similar configuration. There is sparse report on nanoparticles (NPs) of alloys produced in a gas phase, particularly SiGe NPs. The difficulty in fabricating SiGe alloy QD is the segregation of the two phases and according to molecular dynamic studies Ge segregates to the shell leaving behind a Si rich core [2]. The advantage of SiGe is the comparatively larger Bohr’s radius compared to Si [3], thus allowing larger QDs to satisfy quantum confinement.
We show our success in producing SiGe alloy NPs in the gas phase, in a 60MHz VHF PECVD process using a shower head electrode and a multihole grounded upper electrode through which the NPs are pulled to the substrate, placed behind the grounded electrode, by a combination of thermophoresis effect and directional gas flow facilitated by a pump. SiH₄+GeH₄+ Ar is used as precursor gas mixture.
TEM and HAADF-STEM images reveal that there are two types of particles; particles in the 100-300 nm range with a cauliflower shape and more spherical small particles of 10-30 nanometer size, each type consisting of crystallites of ~10nm, as estimated from grazing incidence XRD spectra for the NPs with estimated compositions of Si(78%)Ge(22%) and Si(64%)Ge(36%). EDS spectra revealed that all the particles contain both Si and Ge, baring a few particles with only Ge. EDS mappings showed that whereas small particles have a constant Si:Ge ratio across the sample, indicating homogeneous SiGe alloy formation, the cauliflower type structure has a gradient in Si:Ge ratio. The presence of Si and Ge in SiGe alloy form is confirmed by Raman spectroscopy where the Si-Ge transverse optic (TO) vibration is identified [4]. The shift of the Si-Si TO peak to lower wavenumbers also lends support to this argument. The monotonic shift of Si-Si TO peak to lower wavenumbers with increasing GeH₄/SiH₄ gas flow ratios indicates that Ge incorporation in the alloy can be varied by changing GeH₄ flow, allowing precise band gap control. The production of these SiGe NPs are in comparable gas phase condition as Si nanoparticle, whereas nc-SiGe film growth on a substrate requires a much higher hydrogen dilution compared to nc-Si film. This clearly indicates the difference in growth mechanism between the gas phase particles and films; in the former the nucleation occurs due to local heating facilitated by hydrogen abstraction and charge recombinations whereas in the latter nucleation is surface diffusion mediated where the low diffusion coefficient of GeHx radicals limits the nucleation.
1. A.Mohan et al, J. Phys. D: Appl. Phys. 48 (2015) 375201, 2. A. D. Zdetsis et al, J. Math. Chem. 46 (2009) 942, 3. G. Gu, et al, J. Appl. Phys., 90 (2001) 5747, 4. J.K.Rath et al, Sol. Energy Mat. Sol. Cells 74 (2002) 553.

Hybrid full spectrum solar systems (FSSS) designed to capture and convert the full solar wavelength spectrum use hybrid solar photovoltaic/thermodynamic cycles that require low thermal exergy loss systems capable of transferring high thermal energy rates and fluxes with very low temperature differentials and losses. One approach to achieving this capability are high-heat-flux reflux boiling systems that take advantage of high heat transfer boiling and condensation mechanisms. Advanced solar systems are also intermittent by their nature and their electrical generation is often out-of-phase with electric utility power demand, and their required power system cycling reduces efficiency, performance (dispatchability), lifetime, and reliability. High-temperature thermal energy storage (TES) at 300-600°C enables these reflux boiling systems to simultaneously store thermal energy internally to increase the energy dispatchability of the associated solar system, as this can increase the power generation profile by several hours (up to 6-10 hours) per day. Many TES phase change materials (PCM’s) exist including KNO₃, NaNO₃, LiBr/KBr, MgCl₂/NaCl/KCl, Zn/Mg, and CuCl/NaCl, which have various operating melting points and different latent heats of fusion. Common, cost effective TES PCM's are FeCl₂/NaCl/KCl mixtures, whose phase change temperature can be varied and controlled by simple composition adjustments. This paper presents and discusses unique "temperature-staged" thermal energy storage configurations using these TES materials and analysis of such systems integrated into high-heat-flux reflux boiling systems. In this specific application, the TES materials are designed to operate at staged temperatures surrounding an operating design point near 350°C, while providing 18 kW of source heat transfer to operate a thermoacoustic power system during off-sun conditions (e.g., temporary cloud conditions, after sun-down). The TES system being located directly within the reflux boiling working fluid helps ensure efficient heat transfer with minimal thermal exergy losses by controlling the heat transfer at several incrementally higher temperature levels. It also allows a lighter weight, more compact system and a higher performance (lower thermal exergy) system as the point of heat transfer is in direct contact with the working fluid. This system also provides a "thermal switch" feature as the highest temperature TES serves as a safety-enhancing thermal storage point that provides more recovery and reaction time to any undesirable thermal transients emanating from unanticipated equipment failures, process anomalies, or overall cooling losses or disconnections in a hybrid FSSS. This paper will discuss relevant configurations, and critical thermal models of the TES configurations, which show the inherent minimization of thermal exergy during critical heat transfers within the configurations and systems envisioned.

This presentation explores the prospect of applying molecular engineering concepts to design suitable combinations of chromophores for dye-sensitized solar cell (DSC) applications that exhibit panchromatic light absorption. Such a form of engineering is important since a lack of suitable dyes is stifling the progress of DSC technology. The conceptual idea involves making (narrow-band) laser dyes DSC-active by chemical modification, while maintaining their key structural property attributes that are attractive to DSC applications, and broadening their optical bandwidth. This molecular engineering follows a step-wise approach. Firstly, molecular structures and optical absorption properties are determined for a series of parent laser dyes. These reveal structure-property relationships which define the prerequisites for computational molecular design of DSC dyes; the nature of their molecular architecture (D-π-A) and intramolecular charge transfer. Secondly, new DSC dyes are computationally designed by the in silico addition of a carboxylic acid anchor at various chemical substitution points in the parent laser dyes. The resulting frontier molecular orbital energy levels are then compared with the conduction band edge of a TiO₂ DSC photoanode and the redox potential of two electrolyte options I^-/I₃^- and Co^(II/III)tris(bipyridyl). Considering the complementary light absorption characteristics of these computationally designed dyes against those of a range of existing dyes, one can identify judicious combinations of dyes that, when embedded into a DSC device via dye co-sensitization, can yield a panchromatic response. We illustrate this molecular engineering workflow via two examples [1,2], whereby the results suggest promise for these computationally designed dyes when they are suitably paired as co-sensitizers for DSC applications.

References
[1] S. L. Bayliss, J. M. Cole, P. G. Waddell, S. McKechnie, X. Liu, “Predicting solar-cell dyes for co-sensitization”, J.
Phys. Chem. C 118 (2014) 14082–14090
[2] F. A. Y. N. Schroeder, J. M. Cole, P. G. Waddell, S. McKechnie, “Transforming benzophenoxazine laser dyes into chromophores for dye-sensitized solar cells: a molecular engineering approach” Advanced Energy Materials 5 (2015) 1401728.

When coupled with miniaturized concentrating optics, high performance, multijunction solar microcells provide a promising, scalable path to cost effective solar energy conversion in terrestrial settings. This talk describes two recent advances in this technology. The first involves strategies for harvesting diffuse illumination, with demonstrated ability to improve the module level efficiencies by more than 2% absolute, even in geographic regions that offer the highest levels of direct normal irradiance. The second is in the development of broadband antireflection surfaces for use on planar glass substrates and on tightly curved concentrating microlenses. The results enable exceptional optical throughput in telescopic concentrators with magnification factors that reach as high as 1600x.

Spectrum splitting photovoltaics provide a pathway to unprecedented efficiency and power production. We report here a spectrum splitting module design and prototype realizing high efficiency. We have demonstrated a polyhedral specular reflector (PSR), a series of dichroic filters embedded at a 45° angle to the light path in a solid dielectric optical train, achieving record spectrum splitting efficiency of 90.1%. Dielectric stack filters were vacuum deposited on 45° oriented parallelepipeds. Parallelepipeds were assembled into a PSR using thin polydimethylsiloxane (PDMS) layers as a broadband transparent adhesive. Splitting efficiency was characterized using a supercontinuum laser coupled to a monochromator for spectrally resolved transmission measured via silicon and germanium photodiodes.

The splitting optic is used to sequentially reflect bands of decreasing frequency onto a set of seven custom-designed, MOCVD-grown III-V alloy subcells fabricated with epitaxial liftoff and lattice-matched to InP or GaAs substrates. In0.53Ga0.43As, In0.71Ga0.29As0.62P0.38, In0.87Ga0.13As0.28P0.72, GaAs, Al0.1Ga0.9As, In0.48Ga0.52P and Al0.2In0.48Ga0.32P subcells span 0.74-2.1 eV. Totally internally reflective light pipe concentrators provide 100 suns total concentration in between the PSR and the subcells. We prototyped an optical concentrating and splitting element demonstrating 80.3% optical efficiency. A broadband antireflection coating was attached to the input face of the PSR using PDMS, and lightpipes were similarly adhered to the output faces. Major losses result from misalignment to the PSR during assembly and light pipe surface scattering. At the subcell design specification, for which the highest EREs assumed are experimental results, demonstrated optical efficiency corresponds to 42.3% module efficiency, clearly showing the potential for spectrum splitting modules of unprecedented efficiency. We are currently prototyping an integrated optoelectronic module with III-V alloy subcells and off-the-shelf power combination to demonstrate record two-terminal module efficiency. Performance of an integrated prototype will be reported.

Characterized optical performance matches our detailed simulations well. Modified detailed balance calculations accounting for the external radiative efficiency (ERE) of subcell materials were used in combination with optical ray-tracing to determine splitting and overall optical efficiencies of 92.0% and 89.9%, respectively. Individual subcell contact optimizations for inverted square contact geometry were performed using a 3D distributed equivalent circuit SPICE model. Interconnection and power combination losses were calculated to yield an overall electrical efficiency of 96.7%. Thermal simulations in COMSOL of passively cooled modules show less than half the temperature rise of a monolithic multijunction at equal cell size and concentration. Coupling these models together yields 45.6% module efficiency.

Our group has recently developed optically-transparent, thermally-insulating silica aerogels, which allow for the design of a novel, Solar-Thermal Aerogel Receiver (STAR). A transparent aerogel layer placed on top of a high-temperature solar absorber will insulate it from thermal losses while still allowing sunlight to be absorbed. The STAR achieves efficient operation in the absence of vacuum, which allows receiver geometries to move away from traditional vacuum tubes. This flexibility in receiver geometry makes STAR well suited for pairing with linear Fresnel reflector collection optics, which has better land use area and cheaper costs than the traditional parabolic trough collectors. Modeling indicates that STAR paired with linear Fresnel reflectors could produce electricity at 75% of the cost of state-of-the-art vacuum tube receivers paired with parabolic trough collectors. This work was funded by Advanced Research Projects Agency - Energy (ARPA-E) under award number DE-AR0000471.

While there is a growing need to increase solar cell efficiencies and reduce the cost per watt, reported efficiencies are still well below the thermodynamic limit of photovoltaic energy conversion. The major factor that affects the efficiency (by more than 40\%) is the lack of absorption or thermalization of electrons. To improve absorption, existing approaches, till date, are focused on combining multiple materials in the form of heterostructures. Material choices and mechanically realizable thicknesses are thus bounded by the maximum allowable mismatch-induced lattice strain. Although mechanics forms the backbone of the architecture, exploration of fundamental mechanisms are confined mainly to the ideas of material science and photovoltaic design. This talk will show the application of a physics-based mechanistic approach to engineer absorption by exploiting compositional and deformation heterogeneity within the layers as well as within the fields of alloy quantum dots. Results show pragmatic pathways for pushing the mechanical limits of heterostructure formation and for providing unprecedented material choices to construct cost effective solutions to solar energy. Using a parallel multiscale computational framework that combines density functional theory, k.p method and the finite element calculations, the work shows that heterogeneous distribution of composition and strain fields can lead to substantial confinement in alloy quantum dots. Subsequently alloy quantum dots that are much larger (on the order of 50 nm) in size -- compared to their single crystalline counterparts (which are on the order of 5 nm) -- can still provide significant confinement. The findings uncover new fundamental insights for confinement that are unattainable under conventional homogenization approximation of material properties.

We introduce a new tandem solar energy converter that employs a “PVMirror” as a concentrating mirror, spectrum splitter, and high-efficiency light-to-electricity converter. A PVMirror is a full-aperture curved photovoltaic module that either has planar PV cells with specular rear reflectors or includes a spectrum-splitting dichroic mirror. In either case, a portion of the spectrum is absorbed in the cells in the PVMirror and converted to electricity, and the remainder of the spectrum is reflected. As the PVMirror is curved, the reflected light arrives at a common focus, at which a second solar energy converter intended to receive concentrated light is placed. This receiver may be another PV cell with a bandgap that is complementary to that of the cells in the PVMirror, or it may be a non-PV converter such as a thermal absorber plumbed to a heat engine.

The PVMirror design is advantageous in several ways. First, like all four-terminal tandems, the cells (assuming the receiver is a PV cell) do not need to be current matched, lattice matched, or process compatible. Second, the PVMirror receives one-sun illumination whereas the receiver is under concentration. This means that the tandem collects some diffuse light (that absorbed in the PVMirror) and that an expensive technology (e.g., III-V cells) can be potentially coupled with a less expensive technology (e.g., silicon), provided the expensive cell is placed at the focus. Third, the “top” cell (that with the wider bandgap) can be placed either as the PVMirror so that the incident light hits it first and sub-bandgap light is reflected to the receiver, or it can be placed as the receiver, in which case the PVMirror necessarily requires a dichroic mirror that reflects shorter-wavelength photons.

We present our work on trough PVMirrors incorporating monocrystalline silicon solar cells and a dichroic mirror. The dichroic mirror forms the heart of these PVMirrors, as it splits the spectrum, reflecting visible (and ideally infrared) light while transmitting the near-infrared light that is efficiently converted by the silicon cells. We have characterized 48-layer SiO₂/TiO₂ Bragg reflectors deposited on planar silicon solar cells and on curved glass troughs, as well as two dichroic mirror freestanding polymer films, and found that the films combine high performance (low absorption, defined transmission and reflection bands) and low cost. PVMirror prototypes in which these films are laminated to glass with our best near-infrared-tuned silicon solar cells have been tested outdoors on a tracker with both PV and thermal receivers. A PV-PV tandem with a GaAs receiver demonstrated an efficiency of 28.5% with respect to the DNI, and a PV-CSP tandem with a thermal receiver achieved 10.3% global-to-AC-electricity conversion and, simultaneously, 41.2% global-to-heat conversion. These initial results open up many new opportunities for materials development that will be highlighted in the presentation.

In the last few years, polymer solar cells (PSCs) have been gained a significant interest due to its high throughput using simple solution method and their adaptability to roll-to-roll process. The power conversion efficiency (PCE) of single junction PSCs is already reached to almost ~11% in lab condition owing to the development of new donor polymers, controlling aggregation and morphology as well as most importantly proper interfacial engineering [1-3]. Despite remarkable progress in PCE of PSCs, it is still lagging in the electrical performances as compared to the commercialized silicon solar cells. In order to tackle this situation, few approaches are available particularly tandem and ternary PSCs to absorb more photon and to improve the performances but these techniques are complex in terms of fabrication and also have low lifetime yield. For commercialization of such promising technology further improvements in PCE is needed without sacrificing its device stability and other electrical parameters.
In this article, we demonstrate an optical technique to proficiently harvest more photons in the photoactive layer of inverted PSCs. Herein; mold transfer processed periodic microstructured scattering layer was incorporated in the PSC device. A fabricated inverted PSC with photoactive layer of low bandgap polymer poly[4,8-bis(5-2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl) -3-fluorothieno [3,4-b]thiophene-)-2-carboxylate-2-6-diyl)](PTB7-Th) and phenyl-C₆₁-butyric-acid-methyl-ester (PC₆₀BM) revealed an enhanced PCE of 8.60%, which is 14% higher than the reference device. Particularly, our light trapping approach shows more than 15% improvement in photocurrent without compromising dark electrical properties of inverted PSC. Such significant improvement in solar cell performances is associated with reduction in reflection and improved scattering at the glass surface. Our optical approach not only harvest more light in the photoactive layer but also not shown any effect on the device resistance. Both theoretical and experimental optical results are well supported with the device results. We believe that this simple optical approach will be applicable to the photocurrent improvement of future PSCs.

Acknowledgment
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MSET) (NRF-2009-0093323).

References
1. Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat. Com. 5, 5293 (2014).
2. J. M. Lee, J. Lim, N. lee, H. I. Park, K. E. Lee, T, Jeon, S. A. Nam, J. Kim, J. Shin, and S. O. Kim, Adv. Mater., 27,1519 (2015).
3. S. H. Liao, H. J. Jhuo, P. N. Yeh, Y. S. Cheng, Y. L. Li, Y. H. Lee, S. Sharma, S. A. Chen, Sci Rep. 4, 6813(2015).

For the high performance of organic photovoltaics (OPVs), additive solvent such as 1,8-diiooctane (DIO) has been introduced for forming an appropriate morphology of photoactive layer through its selective solubility of acceptor. However, it was reported that additive solvent still remains in the photoactive film during device fabrication due to its high boiling point, and its residue reduces controllability of the device performance. In addition, we found that another oblivious issue that the residue accelerates chemical oxidation of photoactive materials and consequently leads to the degradation of device performance, which is more severe in the OPV devices printed in ambient condition. Here, we demonstrated a novel co-solvent system for the efficient and stable OPVs through ambient printing process. We found that our co-solvent not only induced appropriate morphology for high performance of OPVs comparable to that by common solvent (DIO), but was easily removed during ambient fabrication which prevents the chemical oxidation of photoactive materials. Furthermore, our approach is generally applicable to various donor materials. We fabricated reproducible and efficient OPVs by using our co-solvent system with doctor blade, which showed the best performance (8.4 %) of OPV devices printed in ambient condition. We envision that our finding will be useful for the fabrication of highly efficient organic electronics through ambient printing.

Tetrabenzo triaza corroles (Tbcs) are a family of organic molecules that are structurally related to the more commonly used organic semiconductors, phthalocyanines (Pcs).^1-2 Tbcs can be made by a relatively simple chemical reduction reaction of a Pc. Tbcs exhibit properties that are significantly different from Pcs including an upward shift in their frontier orbitals, HOMO and LUMO, and a red-shift in their Soret-bands allowing it to absorb light in the blue region of the solar spectrum. Nonetheless, the chemistry of Tbcs is relatively unexplored and the application of Tbcs is first being reported by our group. Since our group is interested in Pc-like molecules for organic electronic applications, we began by incorporating oxy phosphorus Tbc derivative (POTbc) in organic solar cells (OSC) with the commonly used materials α-sexithiophene and C₆₀ to prove/explore the functionality of POTbc as both an electron donating material and as an electron accepting material. Moreover, POTbc exhibits a light absorption behavior that is complimentary to the absorbance of the emerging highly adapted electron acceptors boron subphthalocyanines (BsubPc)^3-5 to cover the visible light between 400 nm and 750 nm. POTbc was utilized as an electron donor with Cl-Cl₆BsubPcs as an electron acceptor to yield a black OSC and an attempt to reach %T=0 in the visible light region. This presentation will outline our current and up to date activities in this area with a focus on whether we can achieve %T=0 for a black Pc based OSC.


1.Raboui, H.; Al-Amar, M.; Abdelrahman, A. I.; Bender, T. P., Axially phenoxylated aluminum phthalocyanines and their application in organic photovoltaic cells. RSC Advances 2015, 5 (57), 45731-45739.
2.Williams, G.; Sutty, S.; Klenkler, R.; Aziz, H., Renewed interest in metal phthalocyanine donors for small molecule organic solar cells. Sol. Energy Mater. Sol. Cells 2014, 124, 217-226.
3.Cnops, K.; Zango, G.; Genoe, J.; Heremans, P.; Martinez-Diaz, M. V.; Torres, T.; Cheyns, D., Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells. J. Am. Chem. Soc. 2015, 137 (28), 8991-7.
4.Cnops, K.; Rand, B. P.; Cheyns, D.; Verreet, B.; Empl, M. A.; Heremans, P., 8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer. Nat. Commun. 2014, 5, 3406.
5.Beaumont, N.; Castrucci, J. S.; Sullivan, P.; Morse, G. E.; Paton, A. S.; Lu, Z. H.; Bender, T. P.; Jones, T. S., Acceptor properties of boron subphthalocyanines in fullerene free photovoltaics. J. Phys. Chem. C 2014, 118 (27), 14813-14823.

The fabrication of organic photovoltaic modules via printing techniques has been the greatest challenge that must be overcome for their commercial manufacture. Current module architecture, based on a monolithic geometry consisting of serially interconnecting stripe-patterned sub-cells with finite widths, requires highly sophisticated patterning processes, which significantly increase the complexity of printing production lines, and causes serious module efficiency drops arising from the so-called ‘aperture loss’ in series connection regions. Herein, we demonstrate an innovative module structure which can simultaneously reduce both patterning processes and aperture loss. By using a charge recombination feature occurring at contacts between electron/hole transport layers, we devise a new series connection method that facilitates module fabrication without patterning the charge transport layers. With the successive deposition of the component layers using slot-die and doctor blade printing techniques, we successfully demonstrated high-efficient PSCs modules with PCE of 7.3 % in 4.15 cm², and 6.7 % in large sized 16.6 cm² under Air Mass (AM) 1.5 condition.

Photovoltaic cells can only utilize a limited region of the solar spectrum due to the discrete bandgap of the materials used. In contrast, solar thermal receivers have near-uniform absorption over the entire solar spectrum. The methodology developed here takes advantages of these two features and aims to generate both electrical and thermal energy by using the full solar spectrum in a hybrid concentrated photovoltaic (CPV)/ concentrated solar thermal power (CSP) system. In this system, an infrared transparent CPV module acts as a spectrum splitter, dividing solar radiation into two parts. Here we refer to them as in-band and out-of-band light. The in-band light is converted to electricity while the out-of-band light passes through to the thermal receiver.
In this presentation, we focus on the optical and electrical design of the spectrum splitting CPV cells and module. Our CPV module consists of five layers: a transparent superstrate, encapsulant, solar cell, adhesive and transparent substrate. Device modelling is used to predict the in-band power conversion efficiency (PCE) and a transfer matrix style approach is applied to study the optical transmission in each layer under a wide range of incident angles (0-45°). Several strategies are employed to get more solar radiation transmitted to the cell (in-band) and through the module (out-of-band) while keeping the electrical losses in a reasonable range. First of all, four different anti-reflection coatings (ARC) are designed for interfaces between air and superstrate, encapsulant and cell, cell and adhesive and substrate and air. The final ARC designs are the optimized combination of SiO₂, Al₂O₃, ZnS and MgF₂ optical thin films with reflection 3.1%, 2.7%, 0.9% and 1.8% respectively. Also, different contact grid width, height/width ratio and grid pitch for both sides of the solar cell are studied to optimize the design and the tradeoff between shadowing and series resistance. The design for the front side is 6μm x 6μm grid fingers with grid pitch of 110μm, as well as a back side design with 10μm width, 10μm height grid fingers with grid pitch of 250μm. Different doping levels for the GaAs wafer from 1.0E16 to 1.0E18 cm^-3 are studied under a wide temperature range (30-150 °C) to optimize sheet resistance, as well as the free carrier absorption, which is a key factor that effects out-of-band light absorption in the solar cell. The results show that GaAs wafers with doping level from 5.0E16 to 5.0E17 cm^-3 can satisfy our requirement. Our spectrum splitting transmissive CPV cell shows PCE of 49.3% for in-band light under 500X concentration ratio at temperature 70°C when the electrode series resistance is 0.027 ohm.cm² from both the front side and back side contact grids. The CPV module featuring this cell has optical transmission of 75.4% of out-of-band light through the cells and to the thermal receiver. We are now prototyping this work and will present our latest modeling and experimental results.

Gas Technology Institute (GTI), together with its partners, University of California at Merced (UC Merced) and Microlink Devices Inc. (Microlink) are developing a Hybrid Solar System to deliver variable electricity and on demand heat. The technology uses novel secondary optics in a solar receiver to achieve high efficiency at high temperature, collects heat in particles instead of silicone oil for high temperature and low fire danger, stores heat in particles instead of molten salt for low cost, and uses reflective liftoff cooled photovoltaic cells (PV) on the secondary reflector to raise exergy efficiency. This is a disruptive technology solution that can exceed DOE’s FOCUS program objectives. The team has addressed a number of critical risks during a proof-of-principle phase in separate tests on the collector and the particle heat transport and storage media:
Demonstrating 365°C receiver outlet and about 40% thermal efficiency on sun with Therminol

No particle degradation, heat transfer and pressure drop impacts for over 4,000 heating cooling cycles (100°C to 650°C) representing 11 year system operation

Estimated system pumping loss below 1% of thermal output at 500°C

Estimated thermal system costs of $16/kWhr for powder that are comparable to $20/kWhr for molten salt and significantly below $53/kWhr for Therminol.

The Hybrid Solar System technology is capable to deliver a high exergy efficiency FOCUS system without the added cost for a spectrum splitting element like a dichroic mirror. It revolutionizes parabolic trough Concentrated Solar Power (CSP) with higher temperature and higher efficiency receiver, delivers significant enhancement to established, bankable trough technology, and exceeds the heliostat power tower molten salt temperature limit. The use of inert particles for Heat Transfer Fluid make parabolic trough safe near population centers and valuable for industrial facilities at even higher temperatures than today. The Hybrid Solar System technology can open the microgrid market for dispatchable electricity and expand the use of solar energy in the industrial process heating market.

Aerogels are well known for their thermally insulating properties due to their nano-porous structure and low solid volume fraction. As recently demonstrated by our group and others, aerogels made from silica can also be fabricated with high solar transmittance by tuning the production process. These combined properties allow for using visibly-transparent aerogels to improve the efficiency of solar-thermal energy conversion. In particular, a transparent aerogel placed on top of an absorbing surface transmits sunlight to the absorber while simultaneously thermally insulating the absorber, which necessarily operates at high temperature. A combination of these effects allows for improved thermal efficiency of the receiver, and may also help to decrease the cost and complexity of the overall system as it allows the receiver to operate efficiently at lower solar concentrations without vacuum. We report the optical and thermal properties (> 95% solar transmittance and