3:15 PM - EE3.1.03
Gas Phase Grown Silicon Germanium Nanocrystals by VHF PECVD
Jatindra Rath 1,Akshatha Mohan 1,Ruud Schropp 3,M. A. Verheijen 3,M Kaiser 2,I Poulios 1
1 Utrecht Univ Eindhoven Netherlands,3 Eindhoven University of Technology Eindhoen Netherlands2 Philips Innovation Services Eindhoen Netherlands,3 Eindhoven University of Technology Eindhoen Netherlands2 Philips Innovation Services Eindhoen NetherlandsShow Abstract
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  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 . The advantage of SiGe is the comparatively larger Bohr’s radius compared to Si , 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 . 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.
3:30 PM - EE3.1.04
Temperature-Staged Thermal Energy Storage Enabling Low Thermal Exergy Loss Reflux Boiling in Full Spectrum Solar Systems
Terry Hendricks 1,Bill Nesmith 1,Juan Cepeda-Rizo 1,Jonathan Grandidier 1
1 NASA Jet Propulsion Lab Pasadena United States,Show Abstract
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.
5:15 PM - EE3.1.08
A Solar-Thermal Aerogel Receiver (STAR) for Cost-Effective Electricity Generation
Lee Weinstein 1,Sungwoo Yang 1,Lin Zhao 1,Bikram Bhatia 1,Elise Strobach 1,David Bierman 1,Thomas Cooper 1,Laureen Meroueh 1,Svetlana B. Boriskina 1,Evelyn Wang 1,Gang Chen 1
1 MIT Cambridge United States,Show Abstract
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.
5:45 PM - EE3.1.10
PVMirror: A New Technology for Tandem Solar Cells and Hybrid Solar Converters
Zachary Holman 1,Zhengshan Yu 1,Kathryn Fisher 1,Xiaodong Meng 1,Mariana Bertoni 1,Lennon Reinhart 2,Jeffrey Jon Mrkonich 2,Justin Hyatt 2,Roger Angel 2
1 Arizona State University Tempe United States,2 University of Arizona Tucson United StatesShow Abstract
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.
9:00 PM - EE3.2.02
Periodic Microstructures for Light-in-Coupling in Inverted Polymer Solar Cells
Raju Lampande 1,Gyeong Woo Kim 1,Min Jin Kim 1,Jang Hyuk Kwon 1
1 Department of Information Display, Kyung Hee University Seoul Korea (the Republic of),Show Abstract
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.
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MSET) (NRF-2009-0093323).
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).
9:00 PM - EE3.2.05
Complementary Absorption Engineering Using Oxo Phosphorus Tetrabenzotriazacorrole and Boron Subphthalocyanine; a Potential Path to Black Bilayer Organic Photovoltaics
Hasan Raboui 1,Emmanuel Thibau 3,David Josey 1,Zheng-Hong Lu 3,Timothy Bender 3
1 Chemical Engineering and Applied Chemistry University of Toronto Toronto Canada,3 Materials Science and Engineering University of Toronto Toronto Canada1 Chemical Engineering and Applied Chemistry University of Toronto Toronto Canada,2 Chemistry University of Toronto Toronto Canada,3 Materials Science and Engineering University of Toronto Toronto CanadaShow Abstract
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.
9:00 PM - EE3.2.06
A Novel Series Connection Design for Large-Area Printed Polymer Solar Cell Modules with Power Conversion Efficiency Exceeding 7%
Soonil Hong 1,Hongkyu Kang 1,Geunjin Kim 1,Seonkyu Lee 1,Seok Kim 1,Jong-Hoon Lee 1,Jinho Lee 1,Minjin Lee 1,Junghwan Kim 1,Jae-Ryoung Kim 1,Hyungcheol Back 1,Kwanghee Lee 1
1 Gwangju Institute and Science and Engineering Gwangju Korea (the Republic of),Show Abstract
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.
9:00 PM - EE3.2.07
Optimization of Optical and Electrical Properties of a Transmissive Spectrum Splitting Multi-Junction Solar Module for a Hybrid CPV/CSP system
Yaping Ji 1,Qi Xu 1,Adam Ollanik 1,James Ermer 2,Matthew Escarra 1
1 Tulane Univ New Orleans United States,2 Boeing-Spectrolab Sylmar United StatesShow Abstract
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.
9:00 PM - EE3.2.09
Transparent Aerogels for Efficient Solar-Thermal Energy Conversion
Sungwoo Yang 1,Lin Zhao 1,Bikram Bhatia 1,Elise Strobach 1,Lee Weinstein 1,David Bierman 1,Thomas Cooper 1,Svetlana Boriskina 1,Gang Chen 1,Evelyn Wang 1
1 Massachusetts Institute of Technology Cambridge United States,Show Abstract
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
9:00 PM - EE3.2.10
Spectrally Selective and Thermally Enduring Co3O4 Nanoflower
Lizzie Caldwell 1,Tae Kyoung Kim 1,Bryan VanSaders 2,Alireza Kargar 1,Dongwun Chun 1,Li Chen 2,Qian Ma 2,Renkun Chen 1,Sungho Jin 1,Zhaowei Liu 2
1 Mechanical and Aerospace Engineering University of California, San Diego La Jolla United States,2 Electrical and Computer Engineering University of California, San Diego La Jolla United StatesShow Abstract
Spectrally selective materials can transmit or absorb favorable wavelengths in the electromagnetic spectrum, and reflect unwanted wavelengths. When spectrally selective materials become nano-sized, beneficial properties can become enhanced. Spectrally selective nanomaterials have a wide variety of applications, including energy efficient windows, radiative cooling, glare-reducing mirrors, and solar-to-thermal energy conversion. Concentrated solar power, which uses solar-to-thermal energy conversion for steam-powered generators, needs spectrally-selective materials that can absorb sunlight and withstand high temperatures for long periods of time.
Developing spectrally selective nano-sized materials that can absorb sunlight while enduring high temperatures is a challenging task, but they can be successfully synthesized with careful engineering. Research studies over the previous decades have produced nanoparticles, nanorods, and nanowires , but our team has recently designed a unique and symmetrical flower-like Co3O4 nanostructure. This Co3O4 “nanoflower” is spectrally selective and its “petals” have fantastic light-trapping properties; however, it is fragile, and degrades at high temperatures. When coated with Al2O3 via atomic layer deposition, the Co3O4 nanoflower structure maintains its distinctive shape, even after long-term high-temperature testing. In this talk, we will also analyze the solar-to-thermal energy conversion efficiency before and after long-term thermal testing.
 Feynman, R. Plenty of Room at the Bottom. American Physical Society, Pasadena, 1959.
 Caskey, G. et al (Donnelly Corporation) Spectrally selective mirror and method for making same. US Patent 5,179,471, 1990.
 Granqvist, C.G. Spectrally Selective Surfaces for Heating and Cooling Applications; SPIE Press: Bellingham, WA, 1989.
 Dienerowitz, M. Optical manipulation of nanoparticles: a review. Journal of Nanophotonics, Vol. 2, 021875, 2008.
9:00 PM - EE3.2.11
Core/Shell Quantum Dot Embedded Spectral Converting Layers for the Efficiency Enhancement of Thin-Film Solar Cells
Kyu-Sung Lee 2,Yoo Jeong Lee 2,Sun Jin Yun 2
1 Electronics and Telecommunications Research Institute (ETRI) Daejeon Korea (the Republic of),2 Department of Advanced Device Engineering University of Science and Technology (UST) Daejeon Korea (the Republic of),Show Abstract
Spectral conversions such as up-conversion and down-conversion (including down-shift) have shown power conversion efficiency improvement of solar cells by expanding the use of solar spectrum into the absorber. Fluorescent particles are mostly conducted for up-conversion phenomena to utilize infrared regions. On the other hand, quantum dot (QD) is suggested as a strong candidate for down-conversion from ultra-violet (UV) or short visible wavelengths to longer visible regions. Core/shell structured QDs have shown higher quantum yield than single core QDs due to better chemical and electronic stability. In this work, cadmium selenide (CdSe)/ zinc sulfide (ZnS) QDs (440 nm emission) embedded in various substances are obtained as spectral converting layers to improve the device performance of amorphous silicon thin film solar cells with the increase of light intensity from 0.9 sun to 2.7 sun.
Core/shell QDs in toluene are dispersed with thin inorganic and thick elastomer solutions, and then spin-casted on glass-side of amorphous silicon solar cells. Under 1.0 sun condition, power conversion efficiencies with spectral converting layers are slightly increased as 7.32%, 7.18%, and 7.26% for pristine QDs, inorganic, and elastomer matrices, comparing to 7.12% of solar cells without spectral conversion layer, respectively. However, spectral converting layer with CdSe/ZnS QDs embedded in the thick elastomer matrix improves the efficiency of thin film solar cell up to 9.02% comparing to 8.30% without QDs under 1.2 sun illuminations. Thus, down-shift effect of core/shell QDs embedded in transparent matrix with proper light intensity and thickness would enhance the device performances of solar cells by altering the solar spectrum from rarely-used UV into viable visible ranges.
9:00 PM - EE3.2.12
Controlled Fabrication of Nanotube Arrays and Nanohole Arrays through Electrodeposition for High Efficiency Solar Energy Conversion
Wipula Liyanage 1,Chuang Qu 2,Edward Kinzel 2,Manashi Nath 1
1 Department of Chemistry Missouri University of Science and Technology Rolla United States,2 Department of Mechanical and Aerospace Engineering Missouri University of Science and Technology Rolla United StatesShow Abstract
Implementing light management techniques such as light-trapping and enhanced light scattering to improve the efficiency of traditional solar cells has become a common goal of current photovoltaic research. On the other hand, nanostructuring the photoabsorber layer in the form of nanorods, nanowires, nanoparticles and especially, nanotubes has also shown promising results for improved photo conversion efficiency which has the potential for further development since this method allows manipulating the photo absorbance in nanometer scale. Although conceptually this method is very attractive, growing of nanostructures such as nanotube arrays over a region of interest with a specific size is still a significant challenge. Therefore, a simple method for the fabrication of nanotube arrays with controlled physical parameters would provide a ground for further studies of structure property relationships in this advancing field. We demonstrate a simple, yet reproducible approach for the fabrication of vertically aligned nanotube arrays with specific parameters such as tube wall thickness, diameter and the distance between adjacent tubes through electrodeposition of photoactive material on lithographically patterned nanoelectrodes. The nanoelectrode pattern of interest was defined on fluorine doped tin oxide coated glass, which was used as a typical conducting substrate, by electron beam lithography and the photoactive material of interest (CdTe, CuInSe2, etc) was electrodeposited to grow the nanotube arrays. The capability to fabricate uniform nanotube arrays with fine-tuned physical parameters makes this method unique. A CdS layer was fabricated on top of as-grown nanotube arrays by chemical bath deposition to create a p-n junction. Photoelectrochemical characterization showed that the fabricated nanodevice can produce a photocurrent density which exceeds that produced by a thin film device by at least a factor of three. These results will be further compared with the photocurrent densities obtained by thin film devices with nanohole arrays fabricated on top of the photovoltaic material by an innovative technique for enhanced light scattering.
9:00 PM - EE3.2.13
Refractory Plasmonics for Efficient Thin-Film Solar Cells
Ayman Selmy 1,Nageh Allam 1,Moamen Soliman 1
1 Dept of Physics, 1127 SSE Bldg The American University in Cairo 11835, New Cairo Egypt,Show Abstract
Plasmonic thin-film solar cells have recently gained a great research interests due to the tunability of plasmonic nanoparticles with efficient light trapping and I-V characteristics. Although noble metals have shown great physical and optical properties in the visible region of the solar spectrum, the main drawback of using such materials is their cost and unavailability for large scale production and integration.
In this paper, we show the possibility to replace Au and Ag with refractory plasmonics, especially transition metal nitrides/oxynitrides such as TiN. The main advantage of TiN is the low cost that may allow mass production as well as availability over noble metals. The similarity in optical and electrical properties between TiN and noble metals (such as Au or Ag) makes this replacement possible.
The plasmonic effect of TiN across the scattering and absorption cross-section is demonstrated using COMSOL Multiphysics. We then showed the I-V characteristics for a thin-film solar cell with TiN nanoparticles on top. Afterwards, multiple sizes of the nanoparticles are investigated, targeting the best scattering and coupling-to-substrate efficiency, and hence better overall efficiency. Moreover, different TiN nanoshapes have been synthesized and tested.
Xing Sheng, Tsinghua University
Matthew Escarra, Tulane University
Anita Ho-Baillie, The University of New South Wales
Matthew Lumb, U.S. Naval Research Laboratory and The George Washington University
EE3.3: Novel Multijunction Solar Cells I
Wednesday AM, March 30, 2016
PCC North, 100 Level, Room 123
9:30 AM - *EE3.3.01
Metamorphic and Bonded Four-Junction III-V Solar Cells
Myles Steiner 1
1 National Renewable Energy Laboratory Golden United States,Show Abstract
Four junction III-V solar cells have demonstrated the highest efficiencies of any solar cell, exceeding 38% at one-sun and 45% under concentrated illumination, and absorb photons over the wide wavelength range of 300-1800 nm. In order to achieve the optimal bandgap combination, four junction solar cells typically require the integration of materials with different lattice constants within a single device. This integration is accomplished either through metamorphic epitaxy, or by bonding materials grown on different substrates. In the case of the inverted metamorphic multijunction (IMM) solar cell, integrating the four junctions into a monolithic structure requires careful engineering of a compositionally graded buffer layer to change the lattice constant, in order to grow the lower bandgap InGaAs junctions while maintaining a low threading dislocation density. We have demonstrated that lattice-mismatched material with a wide bandgap range can be grown with very high material quality, allowing a near-ideal bandgap combination within a monolithic device. While more complex to process, the bonded architecture utilizes subcells that are lattice-matched to their growth substrates, and so naturally have a low threading dislocation density and do not require graded buffers. These devices can have higher internal radiative efficiencies in the various junctions, photon recycling enhancements due to low-index intermediate optical layers, and can possibly operate as four-terminal rather than two-terminal devices. We have fabricated four-junction devices of both architectures and will compare the performances, looking at how the appropriate choice of materials and architectures can provide high internal radiative efficiencies for each junction, enhanced photon recycling, and enhanced efficiencies.
10:00 AM - EE3.3.02
A New Self-Assembly Printing Method to Simplify Tandem Organic Photovoltaics with Four-Layer Structure
Seok Kim 1,Hongkyu Kang 1,Jinho Lee 1,Kwanghee Lee 1
1 Gwangju Institute of Sci and Tech Gwangju Korea (the Republic of),Show Abstract
Despite recent dramatic enhancements in power conversion efficiencies (PCEs) over 10%, complex tandem architectures consisting of six or more component layers cause serious difficulties in manufacturing tandem organic solar cells via high-throughput printing technologies. Here, we report an innovative printing method to simplify tandem structures by using an organic nanocomposite containing interfacial and photoactive materials. This nanocomposite creates a simultaneously printed bilayer with interfacial and photoactive layers through vertical self-organization, which reduces processing steps for tandem fabrication. Moreover, we demonstrate that reducing the molecular weight of the interfacial material in the nanocomposite can induce more efficient self-assembly by alleviating polymer chain entanglements with a photoactive polymer, thereby producing four-layered tandem structures with a high PCE of 9.1%. Our approach offers both an ultimate solution for printed tandem photovoltaics and a universal method for producing various organic electronics.
10:15 AM - EE3.3.03
Development of GaAsSb(N)/GaAs Heterostructures for 1 eV Solar Cell Applications
Aymeric Maros 1,Hongen Xie 1,Fernando Ponce 1,Nikolai Faleev 1,Richard King 1,Christiana Honsberg 1
1 Arizona State University Tempe United States,Show Abstract
The GaAsSbN dilute-nitride alloy makes an ideal candidate for use in multi-junction solar cells since it can be grown lattice-matched to GaAs with a band gap of 1 eV. It offers an interesting alternative to the most commonly studied InGaAsN for several reasons. GaAsSb presents a stronger band gap bowing than InGaAs which means that lower band gap can be achieved with fewer amount of nitrogen in dilute-nitride alloys. This is an attractive characteristic since N incorporation is often associated with N-related defects that degrade the material quality and hence, the device performance. Furthermore by replacing In with Sb, Ga atoms can only bond with group V elements therefore avoiding the formation of In-N defects which have been shown to reduce the minority carrier lifetime in InGaAsN materials.
In this work, we focus first on the growth optimization of the GaAsSb and GaAsN alloys to ensure good control of the Sb and N compositions independently. A number of GaAsSb/GaAs and GaAsN/GaAs heterostructures were grown by molecular beam epitaxy (MBE) on GaAs (001) wafers with various Sb and N compositions using different growth parameters. High resolution X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to evaluate the structural quality while photoluminescence (PL) spectroscopy was used as a function of temperature and excitation power to assess the optical properties of these structures.
Carrier localization effects, characterized by the S-shape behavior of the PL peak energy, were observed in GaAsSb alloys at low temperatures and were found to strongly increase with the Sb composition. Under low excitation power, two competing PL peaks were observed in the temperature range 30 – 80 K. The first peak located on the lower energy side is attributed to localized states present within the density of states as a result of random fluctuations of the Sb composition. The second high energy peak corresponds to the recombination of free carriers. The effect of the growth temperature on these compositional fluctuations was investigated and showed that reducing the growth temperature below 460 C fully eliminated the S-shape behavior of the PL indicating better uniformity of the Sb composition. Careful investigation of the RHEED pattern during growth of the GaAsN structures indicated that the partial pressure in the MBE growth chamber was the main parameter controlling the transition from 3D to 2D growth mode. After ensuring that 2D growth was maintained, the nitrogen composition was calibrated and found to be mainly dependent on the N2 flux and the plasma power.
Once the optimum growth conditions were found to accurately control the compositions and material quality of both materials, GaAsSbN bulk and quantum well structures were grown with the objective of reaching the desired 1 eV band gap. Preliminary results based on XRD, TEM and PL will be presented in the final paper.
10:30 AM - EE3.3.04
Modeling Multijunction Solar Cells by wxAMPS 3.0
Yiming Liu 1,Angus Rockett 2,Benny Lassen 1,Morten Madsen 1
1 University of Southern Denmark Sønderborg Denmark,2 University of Illinois Urbana United StatesShow Abstract
The tunneling junction plays a significant role in determining the performance of multijunction solar cells, and an appropriate description of its behaviors is a prerequisite for the solar cell simulator that is aimed to reproduce real device characteristics of the tandem solar cells. With incorporating the trap-assisted tunneling mechanism, wxAMPS of previous version has been employed on modeling the tandem solar cells mainly based on amorphous Si thin films. In an updated version of wxAMPS 3.0, the band-to-band tunneling mechanism has also been integrated into the physical model, which allows the code more suitable to simulate III-V multijunction solar cells as well as inorganic/organic tandem solar cells. In the electric model, the carrier mobilities are treated to be electric-field dependent, and the band-gap narrowing effect is also considered. The optical model has also been updated by utilizing the Transfer Matrix Method, which considers the internal reflection and transmission between internal layers.
wxAMPS 3.0 is able to model the current-voltage (JV) characteristics of a whole tandem device, and can also show the JV behaviors of each component sub-cell to assist the analysis of current matching. The simulation of External Quantum Efficiency of the total device and component cells is also supported. Several features, such as photon recycling, are to be developed in the future version. wxAMPS 3.0 also provides a more accurate approach for modeling the contact behaviors when the band-to-band mechanism plays a key role in forming the Ohmic contact.
10:45 AM - EE3.3.05
Dual-Junction Solar Cells for High Efficiency at Elevated Temperature
Minjoo Lee 1,Emmett Perl 2,Daehwan Jung 1,Joseph Faucher 1,John Simon 2,Clay McPheeters 3,Paul Sharps 3,Myles Steiner 2,Daniel Friedman 2
1 Yale Univ New Haven United States,2 NREL Golden United States3 SolAero Technologies Albuquerque United StatesShow Abstract
In this talk, I will describe our team’s multi-disciplinary efforts to demonstrate solar cells operating under high optical concentration at 400°C. Such high-T cells may play a crucial role in realizing full-spectrum solar energy generation systems that combine the low-cost thermal energy storage of concentrating solar power with the high efficiency of concentrator photovoltaics. Although cell performance is necessarily compromised by operation at such high temperatures, our modeling nonetheless suggests that efficiencies exceeding 25% are practically achievable using a combination of high optical concentration and appropriately designed dual-junction cells.
Numerous device design and materials processing changes are necessary to realize solar cells operating under such extreme conditions, from the bandgaps of the subcells to the contact metallizations. To date, we have shown that the internal quantum efficiency of both GaAs and (Al)GaInP cells stay in the range of 75-90% from 25-400°C over a wide spectral range, resulting in high short-circuit current density. We have also experimentally proven that the dark current in III-V solar cells can be accurately predicted over a wide temperature range and that ideal diode behavior is retained at 400°C. Under ~1000 sun illumination, open-circuit voltage values exceeding 600 mV and 900 mV have been achieved for GaAs and (Al)GaInP cells at 400°C, respectively. Recently, we have shown that tunnel junctions remain operational at 400°C, enabling the demonstration of dual-junction devices with clear voltage addition between the two subcells. I will conclude by discussing future challenges and opportunities for solar cells capable of efficient operation at such elevated temperatures.
11:30 AM - *EE3.3.06
New Materials and Devices for One-Sun Multijunction Photovoltaics
Richard King 1
1 Arizona State University Tempe United States,Show Abstract
Multijunction solar cells are the first photovoltaic technology to surpass single-junction Shockley-Queisser theoretical efficiency limits, and represent the highest efficiency of any solar cell technology. Many challenges remain to develop new semiconductors with lower rates of carrier recombination at the bandgaps needed to push toward still higher efficiencies. Semiconductor materials that are inherently tolerant to recombination at grain boundaries and other defects are particularly intriguing as they hold the promise to allow low-cost growth methods for flat-plate multijunction cells.
Work to identify unifying trends among defects in widely differing types of semiconductors, and to identify the fundamental principles that result in remarkably low minority-carrier recombination activity at defects in some types of semiconductor materials will be discussed in the talk. Earlier observations – for example, that electronic states of hydrogen in III-V and II-VI semiconductors are nearly constant with respect to the vacuum level, and that some defect energy levels in chalcopyrite I-III-VI semiconductors show a strong similarity with respect to the vacuum level even for materials with widely varying bandgap – reveal a profound, unifying aspect of these materials that influences recombination at the surface and at bulk defects. Research findings here may be extendable to other families of semiconductors as well, including II-VI semiconductors, kesterite semiconductors such as Cu2ZnSn(S,Se)4, and perovskite semiconductors such as CH3NH3PbX3 (where X = Cl, Br, I) which are showing promise for thin-film solar cells.
A variety of one-sun multijunction cells based on low-cost semiconductors for flat-plate terrestrial modules will be discussed. One-sun multijunction cells made from such materials could have much higher efficiency than their single-junction counterparts, and would have a game changing effect on the economics of solar electricity.
12:00 PM - EE3.3.07
Virtual Ge Substrates with Low Threading Dislocation Density Produced by CW Laser Induced Recrystallization
Ziheng Liu 1,Xiaojing Hao 1,Jialiang Huang 1,Anita Ho-Baillie 1,Martin Green 1
1 University of New South Wales Sydney Australia,Show Abstract
Ge wafers have been employed as substrates for high efficiency III–V multi-junction solar cells because of the almost ideal lattice match. However, the cost of Ge wafers is high and the Ge bandgap is too small which limits its contribution to the overall voltage output. Since Si has much higher bandgap and lower cost than Ge, using epitaxial Ge on Si as virtual substrates for III–V material growth is a promising solution. Si could be employed as the bottom cell and therefore boost the overall voltage output.
In this work we deposit the epitaxial Ge films on Si by sputtering which is an inexpensive process without the requirement of ultra-high vacuum. Sputtering also avoids the use of toxic gas and is potentially capable for fabricating large area films. Only very thin layers of Ge can be grown defect free on Si substrates owing to the 4.2% lattice mismatch between Si and Ge. Such layers are biaxially strained to adapt its lattice constant to that of the underlying Si substrate. When layer thickness is above the critical thickness, dislocations will nucleate with misfit segments near the interface and threading segments running through the layer to the surface. The threading dislocations density (TDD) might be very high for a fully relaxed Ge layer directly grown on top of Si substrate. In this work, the grown Ge layers have to be beyond the critical thickness for fully strain relaxation to work as virtual substrates.
One way to reduce TDD is gradually reducing the lattice mismatch via graded SiGe buffer although the overall buffer layer of more than 10 um thick is required. Another method of reducing the TDD is via thermal annealing after the direct pure Ge deposition on Si in which high temperature and long annealing are required. Dislocation blocking by substrate patterning is another approach to reduce the TDD in Ge which requires expensive and low throughout nano-meter level masking and patterning. Compared with abovementioned techniques, the CW laser annealing offers a simple, fast and low-cost alternative to effectively reduce the TDD in Ge film without the requirement of thick buffer layers.
By laser scanning the sample, the Ge layer is melted and recrystallized laterally following the laser beam at a high speed. Thanks to the lateral recrystallization, Ge acts as the regrowth seed instead of Si. This changes the mechanism from Ge/Si hetero-epitaxy with lattice mismatch to Ge/Ge homo-epitaxy. In addition, the high recrystallization speed results in the vacancies super-saturation which will force the dislocations propagate to the surface and be eliminated. After CW laser induced recrystallization, the TDD of Ge films can be reduced by three orders of magnitude to 106 cm which may be suitable for fabrication of high efficiency III-V solar cells. Raman spectroscopy, X-ray diffraction and Transmission Electron Microscopy measurements are employed to investigate the TDD reduction of the recrystallized Ge layers.
12:15 PM - EE3.3.08
GaSb-Based Solar Cells for Full Spectrum Energy Harvesting
Matthew Lumb 1,Shawn Mack 1,Maria Gonzalez 3,Kenneth Schmieder 1,Matt Meitl 4,Scott Burroughs 4,Behrang Hamadani 5,Robert Walters 1
2 The George Washington University Washington United States,1 NRL London United Kingdom,1 NRL London United Kingdom3 Sotera Defense Solutions Crofton United States4 Semprius Inc. Durham United States5 NIST Gaithersburg United StatesShow Abstract
Approximately 99% of the direct irradiance from the sun reaching the surface of Earth is contained within the spectral region between 300 nm and 2500 nm, and the most efficient solar energy conversion system would ideally be able to capture all of these photons and convert them into useful power output. In this paper, we show for the first time, a mechanically stacked, four junction solar cell including a narrow bandgap InGaAsSb cell, capable of harvesting the entire solar spectrum.
Multi-junction (MJ) solar cells split the spectrum between subcells connected in electrical series, thereby dramatically reducing the thermalization loss for any given photogenerated carrier, and the success of this approach is reflected by the world record conversion efficiency which has been held by MJ solar cells for the last 20 years. However, the optimal bandgaps for 2J and 3J solar cells do not encompass the entire solar spectrum due to the increased thermalization loss associated with the narrow bandgap absorber, and mechanically stacked architectures with >3 junctions thus far have focused on GaAs/InP substrates, which is limited to 0.74 eV using conventional lattice matched alloys.
As the number of subcells is increased to four and beyond, the ideal bandgaps extend to approximately 0.5 eV for the lowest energy absorber. In order to achieve this bandgap using high quality III-V materials, InGaAsSb lattice-matched to GaSb is the ideal candidate. In this paper, we describe in detail the design, growth, processing and characterization of InGaAsSb solar cells for concentrator photovoltaics applications. A combination of realistic device models and high-precision MBE growth have been used to achieve ultra-high performance InGaAsSb cells with the ideal bandgap of ~0.5 eV. Using commercially available, high efficiency GaAs-based multi-junction solar cells, current-matched four junction cells have been realized by heterogeneous integration of the GaAs-based cell and the GaSb-based cell. The heterogeneous integration was performed using transfer printing, a commercial assembly technique capable of the deterministic transfer of microscale devices onto non-native surfaces with micrometer precision. These microcells are the first demonstration of a concentrator PV cell which can harvest the entire solar spectrum and have conversion efficiencies close to world record performance.
12:30 PM - *EE3.3.09
Room-Temperature Wafer Bonded Multi-Junction Solar Cell Grown by Solid State Molecular Beam Epitaxy
Shulong Lu 1,Shiro Uchida 2
1 Nano Device SINANO, Chinese Academy of Sciences Suzhou China,2 Chiba Institute of Technology Chiba JapanShow Abstract
The GaInP/GaAs tandem cell on GaAs substrate and InGaAsP/InGaAs tandem cell on InP substrate were grown separately by all-solid state molecular beam epitaxy. Room-temperature direct wafer bonding technique was used to integrate these sub-cells to GaInP/GaAs/InGaAsP/InGaAs four-junction solar cell, which resulted in an abrupt interface with low resistance and high optical transmission. The current matching design and the determination of the base layer thickness of each cell were investigated. An efficiency of 42% at 230 suns of the four-junction solar cell demonstrates the great potential of the room-temperature wafer bonding technique.
EE3.4: Novel Multijunction Solar Cells II
Wednesday PM, March 30, 2016
PCC North, 100 Level, Room 123
2:30 PM - *EE3.4.01
Silicon-Based Tandem Solar Cells#xD;
Martin Green 1
1 Univ of New South Wales Sydney Australia,Show Abstract
Improved solar cell efficiency is the key to ongoing photovoltaic cost reduction, particularly as economies of scale propel module-manufacturing costs towards largely immutable basic material costs and installation cost begins dominating total system costs. Although silicon solar cell and module efficiency has improved at a steady rate of about 0.3% absolute/year, there is a natural barrier at 25% efficiency that has only been surpassed by rear junction approaches, with these only practical for high quality, monocrystalline silicon wafers. To progress further, there is a need to go beyond the performance of a single junction cell design. The only established way of doing this is by the tandem cell approach, whereby cells of increasing bandgap are stacked on top of one another, allowing the partitioning of incident light into successively absorbed spectral bands. Given the commercial dominance of silicon technology, the size of associated supporting industry, the ongoing cost reductions and the opportunity for initial market introduction as a premium silicon product, a silicon tandem cell seems like a key contender for the technology to drive the ongoing development of the industry. A range of silicon tandem cell options are described, ranging from growing III-V cells on silicon to taking advantage of the recent emergence of organic-inorganic perovskites, with some properties that make them particularly well-suited to this role and others that are problematic.
3:00 PM - EE3.4.02
Perovskite/Crystalline Silicon Tandem Solar Cells: Light Management and Performance Optimization
Jeremie Werner 1,Arnaud Walter 1,Soo-Jin Moon 2,Johannes Peter Seif 1,Ching-Hsun Weng 1,Luc Fesquet 1,Nicolas Tetreault 2,Philipp Loeper 1,Sylvain Nicolay 2,Stefaan De Wolf 1,Bjoern Niesen 1,Christophe Ballif 2
1 PV-Lab/IMT Ecole Polytechnique Fédérale de Lausanne (EPFL) Neuchâtel Switzerland,2 PV-Center CSEM Neuchâtel Switzerland1 PV-Lab/IMT Ecole Polytechnique Fédérale de Lausanne (EPFL) Neuchâtel Switzerland,2 PV-Center CSEM Neuchâtel SwitzerlandShow Abstract
As crystalline silicon (c-Si) solar cells are approaching the “practical” efficiency limit of 26%, novel solutions have to be found to increase the competitiveness of Si photovoltaics compared to conventional energy sources. One of the most promising approaches lies in combining market-proven c-Si solar cell technology with a low-cost wide-bandgap top cell to form a tandem device. Organic-inorganic halide perovskite solar cells are particularly attractive candidates for top cells, showing high efficiencies with simple and cost-effective device fabrication. Calculations have shown an efficiency potential of perovskite/c-Si tandem solar cells beyond 30% at reasonable cost.
Here, we present and compare the two most promising tandem configurations: Four-terminal tandem devices where both sub-cells are processed independently and then mechanically stacked on top of each other and two-terminal, monolithic tandems where the top cell is directly grown on the bottom cell. Monolithic tandem cells have the advantage of fewer transport and electrode layers, resulting in superior optical performance. Contrastingly, mechanically stacked tandems offer more process freedom concerning surface texture and thermal budget.
We show four-terminal tandem with total efficiency of 24%, as well as monolithic tandem cells with a record efficiency of more than 20%, for a cell size of >1 cm2, fabricated with low-temperature processes compatible with high-efficiency silicon heterojunction bottom cells. We show the influence of the bottom cell on the device optics and performance by comparing flat, one-side textured and double-side textured silicon heterojunction cells. We then demonstrate how each layer thickness must be optimized to maximize but also match the current between the two sub-cells, resulting in a tandem cell with a short-circuit current density of 17 mA/cm2 and an open-circuit voltage of 1.7 V. Finally, we show how anti-reflective coatings can boost the performance on flat tandem cells for both monolithic and four-terminal configurations.
In summary, we demonstrate highly efficient perovskite/crystalline silicon monolithic and four-terminal tandem solar cells with efficiencies beyond 20% and show several light management strategies that are crucial to further improve the device performance.
3:15 PM - EE3.4.03
Silicon Hetero-Junction Solar Cells with Excellent Infrared Response for Tandem Applications
Zhengshan Yu 1,Kathryn Fisher 1,Michael Bernstein 1,Jianwei Shi 1,Mathieu Boccard 1,Zachary Holman 1
1 Arizona State University Tempe United States,Show Abstract
Single-junction silicon solar cells are approaching their terminal efficiency: a 25.6%-efficient record heterojunction cell was announced by Panasonic in 2014 and SunPower is making 25%-efficient solar cells in a production line. These cells are sneaking up on the 29.4% theoretical limit for silicon and are unlikely to substantially exceed a practical efficiency limit of 26%. To surpass these limits, silicon-based tandem solar cells are promising for two reasons: First, silicon has the near-optimum bandgap for a bottom cell for two- or three-junction tandem solar cells1; second, given its existing GW-scale production capacity and cheap manufacturing cost, silicon is well positioned for tandem commercialization.
Several groups have published silicon tandem results, among which NREL tops the efficiency chart at 27% with an InGaP cell stacked on top of a diffused-junction silicon solar cell2. This tandem would be most improved by a tuned silicon bottom cell: A better red light response and >700 mV VOC is required to meet the 30% tandem efficiency target.
Silicon heterojunction (SHJ) technology is well known for its high VOC as the metal contacts are electronically separated from the absorber, and we previously demonstrated record infrared internal quantum efficiency with this technology by mitigating parasitic absorption in the transparent conductive oxide (TCO) and metal layers3. Here we report our recent progress on the design and fabrication of “IR SHJ cells” which are designed to serve as bottom cells in tandem applications.
In this cell structure, the front TCO layer, which acts as an anti-reflection coating, is tuned to maximize transmission of 700–1100 nm light that will be incident on the bottom cell. The front TCO layer is also made to have a lower electron density than in a one-sun cell, which decreases IR parasitic absorption without compromising lateral transport because the cell operates at approximately half-sun equivalent. The back side of the IR SHJ cell consists of a highly transparent TCO layer and a silver reflector, which traps IR light in the silicon and minimizes the parasitic absorption both in the TCO and metal. Quantum efficiency measurements show that our IR SHJ cell has better response than a one-sun SHJ cell for wavelengths above 700 nm. The VOC of the IR SHJ cell is 722 mV at one-sun illumination and 705 mV at the half-sun illumination that is nominally seen by a bottom cell in a tandem structure. The FF is 77.9% measured at half-sun illumination. The spectral efficiency, defined as JSC(λ)VOCFF shows that for the range of 700–1100 nm, the AM1.5G-weighted efficiency of the IR SHJ cell at half-sun illumination is 36%.
1 M. A. Green, Philos T R Soc A 371 (1996) (2013).
2 Stephanie Essig, Scott Ward, Myles A Steiner, Daniel J Friedman, John F Geisz, Paul Stradins, and David L Young, Energy Procedia 77, 464 (2015).
3 Z. C. Holman et al J Appl Phys 113 (1) (2013).
3:30 PM - *EE3.4.04
Recent Advances in Earth-Abundant and Tandem Photovoltaics
Tonio Buonassisi 1
1 MIT Cambridge United States,Show Abstract
This talk presents the latest advances from the MIT PVLab in Earth-abundant thin-film photovoltaics (PV), tandem devices, and cost modeling. Several novel candidate PV materials, many containing Bi(3+), have been identified on the basis of possessing band structures potentially supportive of large minority-carrier lifetimes. Second, we have recently demonstrated several new tandem device architectures that enable higher device performance with Earth-abundant materials. Third, our recent cost-modeling efforts in tandem devices and capital expenses indicate a potential path for tandem devices to gain traction in the flat-panel market.
4:30 PM - *EE3.4.05
Increased Spectrum Utilisation with GaAsP/SiGe Solar Cells Grown on Silicon Substrates
Allen Barnett 1,Li Wang 1,Brianna Conrad 1,Anastasia Soeriyada 1,Xin Zhao 1,Dun Li 1,Martin Diaz 1,Anhony Lochtefeld 2,Andrew Gerger 3,Ivan Perez-Wurfl 1
1 UNSW Australia Sydney Australia,2 AmberWave Inc Salem United States3 SolAero Technologies, Corp. Albuquerque United StatesShow Abstract
Use of the full spectrum of solar power has led to the highest solar to electricity conversion efficiencies. These high efficiency solar cells are made of III-V materials and have reached efficiencies close to 45% but their high cost has limited their terrestrial applications. One solution is to grow the III-V solar cells on silicon. This can combine the affordability of silicon with the high efficiency of the III-V materials and can lead to new high value, cost-effective utilization of solar energy.
Demonstrating a high performance tandem solar cell grown on silicon requires achieving a high open-circuit voltage in the top solar cell. Optimum performance for a top cell that is epitaxially grown on the bottom cell in a tandem structure requires a relatively low threading dislocation density (TDD), as this type of defect is known to suppress the open circuit voltage (Voc) of the solar cell. A low TDD can be realized by effective lattice-matching of the top and bottom cells. This lattice-matched structure enables high performance from the III-V top cell while maintaining some of the cost advantages of silicon solar cells. The SiGe graded buffer allows for lattice matching of the top and bottom cell while providing a low dislocation interface between the silicon substrate and the device layers.
We will discuss the design, structure and analysis of GaAsP/Si Ge on silicon solar cells. Our design has 18 active and passive layers. Electrical and optical characterization of the material is conducted to analyse each layer of the structure and verify the processing at each fabrication step. Photolithography provides all necessary processing requirements with few disadvantages and is the primary process used to fabricate these tandem device structures. Due to the complexity of this structure, we have developed a suitable five-layer photolithography mask system The methodology and approach will be described.
The GaAsP/SiGe solar cells have reached a measured efficiency of 20.6% under one sun and 24.5% under 20X concentration. Analysis of these results based on the product of the best parameters shows efficiency potential of 26% under one sun, 32.6% at 20X and 35.6% at 400X.
5:00 PM - EE3.4.06
The Developments of Gallium Phosphide Thin-Film and Fabrications for Interdigitated Back-Contact Heterojunction Silicon Photovoltaic Devices
Jongwon Lee 1,Chaomin Zhang 1,Nikolai Faleev 1,Christiana Honsberg 1
1 School of Electrical, Computer and Energy Engineering Arizona State University Tempe United States,Show Abstract
Interdigitated back contact (IBC) hetero-junction silicon solar cells are currently attracting photovoltaic (PV) devices structures due to absorption all of light from the entire front-surface. All contacts are at the rear side of wafer, no shading and improved current are observed that the optical properties of IBC solar cell can enhance the characteristics of solar cells. Therefore, achieving high conversion efficiencies of IBC structures can reduce the cost. Thus, good surface passivation of silicon is important for good performances of IBC PV devices. Therefore, we have developed a couple of approaches for IBC heterojunction silicon solar cells with appropriate materials and fabrications. Further, the material selection of front-surface is also significant to improve the electrical and optical properties of IBC PV devices. For instance, an amorphous silicon is the well-developed material for current IBC or heterojunction silicon solar cells. Recently, gallium phosphide (GaP) is good wide-bandgap materials to absorb sun-light efficiently for heterojunction solar cells. For good passivation, it requires low interface defects between layers. Therefore, we have researched the development of wide-bandgap materials such as GaP for good thin-film. Further, the physical layout design for IBC cell is also important to optimize the electrical parameters such as resistance and conversion efficiencies. In this paper, we present the characteristics for GaP IBC silicon PV devices in comparison to conventional IBC amorphous silicon/silicon heterojunction solar cells for future PV devices.
5:15 PM - EE3.4.07
Two-Terminal Hybrid Tandem Solar Cells Comprising Dye-Sensitized and Si Solar Cells
MinJi Im 1,Jeong Kwon 2,Sang Hyuk Won 1,Sungbum Kang 1,Min Joo Park 1,Jae Cheol Shin 3,Jong Hyeok Park 2,Kyoung Jin Choi 1
1 UNIST Ulsan Korea (the Republic of),2 Dept. of Chem.amp; Biomol. Eng Yonsei University Seoul Korea (the Republic of)3 Department of Physics Yeungnam University Yeungnam Korea (the Republic of)Show Abstract
Tandem solar cells (SCs) have been studied in order to harvest the fullest possible range of the solar spectrum. However, conventional technologies such as III-V compound semiconductor or a-Si/c-Si based tandem SCs cannot meet the high efficiency and low cost simultaneously. In this work, we demonstrate a hybrid tandem solar architecture comprising a DSSC(~1.7eV)/c-Si SCs(~1.1eV), which can be fabricated using solution processes and has a high efficiency. In order for a DSSC to be used as a top cell in a tandem SC, it should have a high transmittance at the long wavelength range (800 ~ 1100 nm) without compromising the cell efficiency. For this, we adopted a high-transparency cobalt-based electrolyte instead of conventional iodine-based one. In addition, the TiO2 photoanode was etched using HCl-HF acid to further decrease the size of nano-particles and correspondingly increase the transmittance of long-wavelength light. The highly-transparent DSSC, fabricated using these two techniques, has an enhanced transmittance at 800 nm from 55 to 70% with a high efficiency of 11.4%. In a tandem SC, the so-called junction layer, electrically connecting between a top and bottom cell, has critical role of recombining electrons and holes injected from both cells. And also, the junction layer should be electrically conductive to minimize resistance loss and optically transparent to minimize sacrificing photovoltaic performance in both cells. We have investigated two types of junction layers, ITO/Pt and ITO/PEDOT. The ITO/PEDOT layer was proven to be more effective because the band alignment of PEDOT is more appropriate between ITO and cobalt electrolyte compared to conventional Pt layer. The tandem SC with the ITO/PEDOT junction layer has much higher fill factor of 72.94% compared with 67.7% of that with Pt layer without degrading Voc and Jsc. The fully-optimized hybrid tandem solar cell efficiency was measured to be (1.34V)(15.0mA/cm2)(0.724) =14.5%.
5:30 PM - EE3.4.08
Optimization of Radial Junction p-i-n GaAs NWs for Top-Cell Integration on Silicon
Natasa Vulic 2,Dmitry Mikulik 1,Esther Alarcon-Llado 1,Federico Matteini 1,Gozde Tuetuencueoglu 1,Heidi Potts 1,Stephen Goodnick 2,Anna Fontcuberta i Morral 1
1 Laboratory of Semiconductor Materials Ecole Polytechnique Fédérale de Lausanne Lausanne Switzerland,2 School of Electrical, Computer, and Energy Engineering Arizona State University Tempe United States,1 Laboratory of Semiconductor Materials Ecole Polytechnique Fédérale de Lausanne Lausanne Switzerland2 School of Electrical, Computer, and Energy Engineering Arizona State University Tempe United StatesShow Abstract
In this study, we aim to optimize the radial p-i-n junction of self-catalyzed GaAs nanowires (NWs) grown by molecular beam epitaxy on Si (111) substrates. While planar GaAs single-junction solar cells are approaching the Shockley-Queisser limit, it has been demonstrated that a standing single-NW GaAs radial p-i-n junction solar cell can exceed that limit. To increase efficiency, while maintaining low cost, various approaches of designing III-V on Si tandems have been suggested. So far, due to lattice mismatch between III-Vs and Si, such structures can only be realized through (i) bonding related techniques or (ii) direct growth of NWs on Si substrates. Our GaAs NW arrays can be integrated in a multijunction solar cell with silicon either by monolithically integrating them or in a mechanically stacked configuration by embedding them in PDMS.
We are presently trying to optimize the carrier extraction efficiency of individual radial p-i-n GaAs NWs grown in such an array using optical and electrical techniques. In particular, we want to ensure that increased generation of carriers is accompanied by their effective separation at the junction and collection at the contacts. For our study, individual NWs are removed from an array of GaAs NWs grown catalyst-free by molecular beam epitaxy (MBE) on Si (111) substrates. We first passivate the wire surface through atomic layer deposition (ALD) of aluminum oxide (Al2O3) or by growing an AlGaAs shell around the vertical wires. The wires are then detached from the substrate and dispersed onto a polished Si wafer with an insulating layer of silicon dioxide. About half of the wires are individually contacted using E-beam lithography. We measure non-contacted nanowires using time-resolved photo-luminescence (TRPL), in order to analyze the quality of surface passivation through the radiative recombination lifetime. On contacted nanowires, we perform 4-pt probe resistivity measurements to determine the quality of the doping along the wire. Preliminary results indicate a gradient in doping of our p-GaAs wires, increasing from the tip to the bottom of the nanowire. We follow this with measurements using an in-situ electroluminescence set-up, equipped with 100x magnification objective and a CCD camera to further observe variations in doping and/or surface quality along the nanowire. Finally, we perform electron beam-induced current (EBIC) measurements to study the ability of the GaAs p-i-n junction to effectively separate and extract the carriers as a function of position along the wire.
5:45 PM - EE3.4.09
Organometallic Halide Perovskite / Barium Di-Silicide Thin-Film Double-Junction Solar Cells
Olindo Isabella 1,Miroslav Zeman 1
1 Delft Univ of Technology Delft Netherlands,Show Abstract
Multi-junction solar cells optimize spectrum utilization, limiting thermalization and transmission losses. Thus they can show efficiencies beyond the Shockley-Queisser limit of single-junction devices. Nowadays crystalline silicon (c-Si) solar cells dominate the photovoltaic market. As this technology has come close to its practical limit (26%), double-junction solar cells with c-Si bottom cell have become attractive. However, the investigation of better performing thin-film materials with low manufacturing costs is always of great interest. Orthorhombic barium di-silicide (BaSi2) has appealing opto-electrical properties, is an abundant and inexpensive material and can constitute an alternative to c-Si. In this contribution, we first report the optical potential of BaSi2-based single-junction cell and then propose thin-film double-junction solar cell based on organometallic halide perovskite (CH3NH3PbI3) as top absorber and BaSi2 as bottom absorber.
Our analysis was carried out with 3-D optical modelling. Layers’ optical properties were characterized via SE, RT and/or PDS. Measurement of BaSi2 film sputtered on c-Si substrate from University of Tsukuba (Japan) resulted in its complex refractive index and band-gap energy (Eg-BaSi2 = 1.25 eV). BaSi2 absorption coefficient (α) was also found to be several times higher than the one of c-Si (e.g. at E – Eg = 0.5 eV, αBaSi2 = 4.6 x 104 cm-1 and αc-Si = 0.1 x 104 cm-1). As for the perovskite-based top cell, we used the optical properties carried out by PV-LAB EPFL in Neuchâtel (Switzerland).
Previously we showed that decoupled front-back periodic textures applied to ultra-thin c-Si single-junction solar cells outperform the well-known 4n2 limit with an implied photocurrent density (Jph-cSi) of 36.0 mA/cm2. The same cell configuration was used in this work, but substituting the 2-µm thick c-Si absorber with 1-µm thick BaSi2 absorber. We obtained Jph-BaSi2 = 40.5 mA/cm2, which (i) also exceeds the 4n2 limit calculated for the same thickness and absorber, (ii) is higher than the short-circuit current density of state-of-the-art hetero-junction c-Si device (39.5 mA/cm2) employing ~100-μm thick wafer and (iii) demonstrates that BaSi2 is suitable as ultra-thin bottom cell in a double-junction architecture.
We studied then a monolithic double-junction solar cell (from illumination side): IOH (60 nm) / Spiro-OMeTAD (100 nm) / CH3NH3PbI3 (350 nm) / TiO2 (10 nm) / GZO (60 nm) / p-type polySi (20 nm) / BaSi2 (1000 nm) / n-type polySi (20 nm) / GZO (70 nm) / Ag (300 nm). This model is realistic in structure, deployed materials, thicknesses and processing thermal budget. We obtained a slightly bottom-limited device (Jph-CH3NH3PbI3 = 19.84 mA/cm2 versus Jph-BaSi2 = 18.81 mA/cm2) for a Jph-total = 38.65 mA/cm2. Considering the approach of University of Ljubljana (Slovenia) and state-of-the-art top and bottom cells, we can predict an efficiency of 27.6% for this
EE3.5: Poster Session II: Novel Absorbers—Inorganics
Thursday AM, March 31, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - EE3.5.01
Tight-Binding Implementation of the Valence Band Anticrossing Model for High Efficiency Solar Cell Materials
Yongjie Zou 1,Stephen Goodnick 1
1 Electrical Engineering Arizona State University Tempe United States,Show Abstract
There are two principle energy loss mechanisms in solar cells: one is the thermalization of carriers generated by above-bandgap absorption, another is due to the transparency to sub-bandgap photons.1 Multijunction solar cells are a proven approach for reducing these losses, and thus overcome the Shockley-Queisser limit.2 To increase the efficiency of the standard triple-junction InGaP/GaAs/Ge solar cell, detailed-balanced studies have suggested inserting an absorber with a bandgap of 0.9~1.2 eV.3,4 The absorber should also have a lattice constant between those of GaAs and Ge for high quality material grown by epitaxial techniques.
Large red shifts of the bandgap after incorporation of small amounts of N into GaAs was first reported in 1992.5 Dilute amounts of Sb has also been found to have similar effect on GaAs, and it can compensate the change in lattice constant due to introduction of dilute N. Therefore, GaNAsSb is a good candidate to achieve the aforementioned requirement for higher-efficiency multijunction solar cells.
To understand and engineer the physical properties of GaNAsSb, knowledge of the band structure is critically important. A band anticrossing (BAC) model was previously developed to successfully explain the reduction in bandgap and increase in effective mass of these highly-mismatched materials.6,7 This model has been implemented with the k●p method,8 a perturbation theory that can well describe energy bands at small k’s. The tight-binding (TB) method can be used to generate more accurate full band structures. It can also explicitly incorporate the BAC model by adding relevant orbitals for the dilute elements. By adding only two new parameters to the sp3d5s* TB model, Shtinkov et al. were able to reproduce and extend the change in the conduction band due to the interaction between host states and the localized N states to the entire Brillouin zone.
Here we implement for the first time to our knowledge, the BAC model within the TB sp3d5s* scheme for the valence band to account for the effects of dilute Sb on GaAs, and combine both the conduction- and valence-BAC for GaNAsSb materials lattice-matched to GaAs. The calculated band structures of pure GaNAsSb and GaNAsSb/GaAs superlattices will be shown, which are important for the full band device simulation and design of high-efficiency multijunction devices.
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2W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961).
3A. S. Brown and M. A. Green, Phys. E 14, 96 (2002).
4S. P. Bremner, M. Y. Levy, and C. B. Honsberg, Prog. Photovolt: Res. Appl. 16, 225 (2008).
5M. Weyers, M. Sato, and H. Ando, Jpn. J. Appl. Phys. 31, 853 (1992).
6W. Shan, W. Walukiewicz, and J. W. Ager III, Phys. Rev. Let. 82, 1221 (1999).
7J. Wu, W. Shan, and W. Walukiewicz, Semicond. Sci. Technol. 17, 860 (2002).
8K. Alberi, J. Wu, W. Walukiewicz, K. M. Yu, O. D. Dubon, S. P. Watkins, C. X. Wang, X. Liu, Y.-J. Cho, and J. Furdyna, Phys. Rev. B 75, 045203 (2007).
9:00 PM - EE3.5.02
2.55eV InGaN Quantum Well Solar Cell Operating at 450C and Varied Concentration
Heather McFavilen 1,Ding Ding 1,Josh Williams 2,Alec Fischer 2,Steven Young 1,Aymeric Maros 2,Yi Fang 2,Hongen Xie 2,Dragica Vasileska 2,Chantal Arena 1,Fernando Ponce 2,Christiana Honsberg 2,Stephen Goodnick 2
1 Soitec Phoenix Labs, Inc. Tempe United States,2 Arizona State University Tempe United StatesShow Abstract
InGaN is an attractive material for terrestrial and extraterrestrial solar cell applications. InGaN is thermally stable, radiatively hard, has a tunable bandgap across the solar spectrum, and has a relatively high absorption coefficient. InGaN challenges include poor material quality with higher indium incorporation and higher thicknesses. This paper focuses specifically on one of two junctions of a tandem InGaN solar cell (the top cell) to be used in a hybrid solar electric – solar thermal system, where the electricity generated by the InGaN dual junction cell will help offset the costs of operating a solar thermal power plant. The intended placement of the InGaN solar cells will be on the outside of pre-existing fluid transfer tubes in a solar thermal system. This placement of the cells requires an ability to withstand up to 450C. The parabolic concentration of the solar thermal system is ~50suns. The InGaN top cell discussed here has a band gap of ~2.55eV at 450C. Voc, FF, EQE, Jsc, and absorption measurements were measured at 450C with 1X and 300X concentration and extrapolated to 50X.
9:00 PM - EE3.5.03
Bandgap Engineering of Hydrogenated Amorphous Silicon Carbide
Jorge Guerra 2,Albrecht Winnacker 2,Roland Weingaertner 1
1 Pontificia Universidad Catolica del Peru Lima Peru,2 WW6 University of Erlangen Nuremberg Erlangen Germany,2 WW6 University of Erlangen Nuremberg Erlangen Germany1 Pontificia Universidad Catolica del Peru Lima PeruShow Abstract
Hydrogenated amorphous silicon carbide (a-SiC:H) films are well suited for several potential applications due to their structural and optoelectronic properties [1, 2]. This material exhibits a large thermal conductivity and excellent chemical and mechanical stability . Additionally, its wide bandgap can be tailored from 1.8 eV to 3.0 eV by changing the carbon content while retaining its ability to be doped n and p type [4, 5]. In particular, a-SiC:H is an attractive photo-electrode material for photo-electrochemical devices which offer a low-cost, renewable and non-polluting path for hydrogen production. However, the increase of carbon in a-Si1-xCx:H systems not only broadens the bandgap but introduces disorder-induced localization states near the band edges, thus increasing the extent of the Urbach tails in the forbidden gap [3-6]. Furthermore, the Tauc-gap and iso-absorption bandgap are sensitive to both the mobility band edges and Urbach tails and therefore are not a good measure of the true bandgap of an amorphous material.
In the present work, we investigate a-SiC:H thin films grown under different hydrogen dilution conditions by radio frequency magnetron sputtering and after post-deposition thermal annealing treatments at different temperatures. The optical bandgap is enhanced by the incorporation of hydrogen in the deposition process atmosphere. After a critical annealing temperature the optical bandgap shrinks. This behavior is typically attributed to the depletion and thermally induced out-diffusion of hydrogen. However, we report that this behavior also occurs in the non-hydrogenated case. We demonstrate that the hydrogen dilution does not have a significant effect on the Urbach energy but rather on the mobility edges of the a-SiC:H thin films even after thermally induced hydrogen out-diffusion. This feature is observed in the enhancement of the Urbach focus energy when we increase the hydrogen incorporation and independently of the annealing treatments. In order to support the latter result we propose a simple model based on band thermal fluctuations from which the Urbach energy and the band-edge are calculated [7, 8]. We compare this model with the traditional Tauc and Urbach rule equations. The main result is the capability to tune the bandgap of a-SiC:H through the variation of the hydrogen dilution during the deposition process instead of modifying the stoichiometry and therefore allowing the quenching of the Urbach energy by thermal annealing treatments only.
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9:00 PM - EE3.5.04
Optical Properties of Graded Composition CdS1-xSex Thin-Films Electrochemically Deposited
Carlos Pereyra 1,Andrea Viscarret 1,Carla Baez Aguilera 3,Gonzalo Riveros 2,Francisco Martin 4,Jose Ramos-Barrado 4,Enrique Dalchiele 1,Ricardo Marotti 1
1 Univ de la Republica Montevideo Uruguay,2 Universidad de Valparaíso Valparaíso Chile,3 Universidad de Santiago de Chile Santiago de Chile Chile2 Universidad de Valparaíso Valparaíso Chile4 Universidad de Málaga Málaga SpainShow Abstract
The optical properties of graded composition (GC) CdS1-xSex thin films electrochemically prepared were studied with the goal of obtaining a spatial variation of the optical properties, like bandgap energy (Eg) and refraction index, from the ones of CdS to those of CdSe. These variable gap semiconductors were proposed for increasing solar cell performance, mainly due to the appearance of a quasi-electric field (like open circuit voltage and short circuit current) . Recently there have been various reports on such GC structures in CdxZn1-xTe , CuIn(S1-xSex)2  and Cu(In1-xGax)Se2 .
The samples were electrodeposited potentiostatically from mixed solutions with different volumetric fractions of the precursors onto Fluorine doped Tin Oxide (FTO)/glass substrate. Single composition (SC) thin films of the ternary alloy obtained for the different mixtures (varying [S]/([S]+[Se]) ratio) were first studied. The GC thin films were deposited layer by layer from CdS to CdSe (in the same electrolytic baths than the previous ones), leaving exposed regions of the partial deposition for the characterizations. Present work studies their optical properties measured by transmittance. Analytical procedures were used to mathematically substract the interference of FTO/glass (which changes for each sample) and correct the zero absorptance for Eg determination.
The SC samples for high S contents clearly show a step absorption edge which red shifts as Se content is increased. However, the absorption edge becomes smoother for higher Se content. It is attributed to the appearance of multiple optical transitions close to the absorption edge, due to the increase of the split-off energy. When this is taken in consideration, the lowest laying Eg can be found to shift from 2.49 eV to 1.71 eV. This is in agreement with the accepted values of Eg for CdS and CdSe which are 2.48 eV and 1.73 eV, respectively.
The transmittance of the GC samples monotonically decreases with the number of layers, indicating an enhanced absorption as in a multigap solar cell. The absorptance spectra redshift and become smoother as the number of layers increases, reaching an almost linear dependence [1, 2] between the Eg of the binary alloy materials. For energies above 2.5 eV the absorptance still resemblance the sharp behavior of CdS. As the spectra of the ternary SC samples are also smoother than that of CdS, the optical properties of each layer (in the GC samples) were studied. The Eg obtained are lower than the ones obtained for the SC samples in more than 100 meV. Therefore this is also an indication of the increased absorption of the GC material and that each layer does not behaves as the corresponding SC sample.
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 – O. de Melo et al., Solar Energy Materials and Solar Cells 138 (2015) 17.
 – J. López García et al., Materials Chemistry and Physics 160 (2015) 237.
 – B. J. Babu et al., Materials Chemistry and Physics 162 (2015) 59.
9:00 PM - EE3.5.06
Characterization of Solar Absorptance Properties of AlxTi1-xN Multi Layer Coatings Produced by Reactive Magnetron Sputtering Technique
Serdar Ozbay 1,Sinan Akkaya 1,Kursat Kazmanli 1,Mustafa Urgen 1
1 Istanbul Technical University Istanbul Turkey,Show Abstract
In this study, solar absorptance properties of Al-Ti-N multilayer coatings for solar energy harvesting applications were characterized. The coatings were produced by reactive magnetron sputtering technique. AlxTi1-xN/AlyTi1-yN multi layer coatings were deposited on copper substrates as absorber and anti-reflection layers, respectively. Altering the aluminum contents of the layers, the optical properties of the layers changed and higher aluminum containing layers showed higher absorptance. On the other hand, AlN phase containing layers exhibited anti-reflection behavior. The solar absorptance and thermal emittance of the optimized solar selective coating were 0.98 and 0.05 (at 65°C), respectively. Thermal stability of the optimized multi-layer coatings was also investigated. The phases in the coatings were further identified by XPS technique.
9:00 PM - EE3.5.07
An Investigation of Transition Metal Oxides Window Layer for Thin-Film Amorphous Silicon Solar Cells
Liang Fang 1,Jie Mu 1,Bao Min Wang 1,Wei Gao 1,Bao Zhang 1,Hui Gao 1
1 Tianjin Itian Solar Tech Co. Ltd. Tianjin China,Show Abstract
Transition metal oxides (TMOs), such as tungsten oxide , molybdenum oxide, and vanadium oxide, have been adopted as window layer of Superstrate amorphous silicon (a-Si:H) solar cells， and the devices performance better than optimized reference cells with p-a-SiC window layer have been achieved . Thermally evaporated TMOs are promising to replace state of the art window layer materials due to excellent optical and electrical properties. Furthermore, the process control is very simple. In this paper, we discuss the influence of TMO materials properties on devices performance, and the influence of deposition rate on the optical and electronic properties of TMO films are studied based on the X-ray photoemission analysis (XPS) measurement, and the devices performances were analyzed. Design criterions in selecting suitable TMO materials and optimize materials properties are discussed in detail.
In this study, Asahi U type glass (glass/SnO2) was used as substrate to deposit devices. Vanadium oxide (V2Ox), WOx, and MoOx films were deposited by a thermal evaporator under a vacuum of 2×10-6 Torr with V2O5 source (Aldrich, 99.9%), WO2.9 source (Alfa Aesar, 99.99%) and MoO3 (Alfa Aesar, 99.5%), respectively. The substrate temperate was kept constant at 20 C°. The thickness of the film was measured by an in situ quartz monitor, which is calibrated by spectroscopic ellipsometry (SE). A two-chamber system was used for fabricating a-Si based solar cells, which consists of a photo-assisted chemical vapor deposition chamber for the p- and n-layers, an plasma enhanced chemical vapor deposition chamber for the intrinsic (i) layer. The fabricated devices have a structure of glass/SnO2/TMO (10 nm)/i-a-Si (500 nm)/n-a-Si (40 nm)/Al, and the cell areas were 0.092 cm2. 10-nm-thick was the optimized thickness for TCO window layer to realize best device performances. To understand the essential criteria for TMO window layer, and dependence of devices performance on materials properties, four kinds of substrates were prepared to fabricate devices: glass/SnO2, glass/SnO2/V2Ox, glass/SnO2/WOx, and glass/SnO2/MoOx, respectively. All the samples have the same air exposure time to eliminate the influence of air contamination. The detailed deposition conditions appeared in elsewhere.
In conclusion, the material properties required for the TMO window layer can be summarized as follows: Wide optical band gap Eopt to reduce parasitic loss, and a high WF to sustain a high built-in voltage Vbi. However, the high WF alone cannot guarantee devices performance, the composition of TMOs is also critical in determining the devices performance, which can be controlled by changing deposition rate.
9:00 PM - EE3.5.08
Strain Relaxation and Defect Evolution in Low-Indium-Content InxGa1-xN Films (x=0.07, 0.12 and 0.15)
Hongen Xie 2,Shuo Wang 1,Alec Fischer 1,Heather McFavilen 3,Fernando Ponce 1
1 Department of Physics Arizona State University Tempe United States,2 School for Engineering of Matter, Transport, and Energy Arizona State University Tempe United States,1 Department of Physics Arizona State University Tempe United States3 Soitec Phoenix Labs Tempe United StatesShow Abstract
A photovoltaic thermal hybrid solar cell is being developed for a energy conversion efficiency higher than the Shockley-Queisser limit, consisting of two junctions with different bandgaps operating on top of a thermal collector at 450 C under solar radiation focused by a concentrator. InxGa1-xN thin films are used due to their wide bandgap range and their stability at high temperatures. Good quality InxGa1-xN layers with high indium content have been achieved by molecular beam epitaxy. We report here on the structure properties of InxGa1-xN epitaxial layers with lower indium content and a larger bandgap (2.65eV at 450 C).
Due to the lattice mismatch between InxGa1-xN and GaN, strain relaxation takes place when the thickness of the epilayer exceeds a critical thickness. Several InxGa1-xN/GaN heterostructures were grown by metalorganic chemical vapor deposition with varying indium composition and layer thickness in order to understand the evolution of the defects during strain relaxation of the epilayer. The composition of the InGaN layers was determined by X-ray diffraction and the structure properties were investigated by transmission electron microscopy. For a 50 nm InxGa1-xN (x=0.15) layer, no relaxation is observed except for a-type threading dislocations from GaN with b=1/3
9:00 PM - EE3.5.09
Minimization of Recombination and Transport Losses at the GaP/Si Heterointerface in GaAsP/Si Tandem Solar Cell
Mehdi Leilaeioun 1,Zachary Holman 1,Kevin Nay Yaung 2,Minjoo Lee 2
1 EEE Arizona State Univ Tempe United States,2 Electrical department Yale University New Haven United StatesShow Abstract
GaAsP/Si tandem solar cell would be promising to reach to an efficiency of 30%, due to the specific characteristics of its GaAsP (~77% As) top cell with a direct and tunable bandgap which can be grown on a transparent, compositionally graded buffer on a GaP/Si template. The bottom cell, on the other hand, is based on SHJ device structure. Compared to a standard, it’s back surface consist of the traditional structure while its front surface is to be in contact with a III-V material (GaP). A recombination junction is to be formed between the GaAsP top cell and the Si bottom cell. The epitaxial layer of GaP deposited straight on silicon thus have to collect photogenerated electrons from the silicon wafer and transfer these to the recombination junction. Several challenges at this heterointerface are tackled, as it is nearly unstudied.
First, though structural defects in the III-V layer can be measured and quantified, their role in promoting recombination of photogenerated carriers in silicon is unknown: some structural defects might have no recombination activity while some others a tremendous recombination activity, with energy levels in the middle of the bandgap. We developed a procedure to etch, clean, and passivate (with intrinsic a-Si:H) the back surface of the wafer without damaging the GaP. We will assess the nature of the recombination-active defects at the interface through lifetime measurements under varied injection levels and temperature. Minority-carrier-lifetime measurements performed for different wafer thicknesses indicated that (1) the wafer bulk lifetime was not degraded by the GaP growth, which validates the concept of direct growth of GaP on silicon for a high-efficiency tandem device, and (2) that the combination of the n+ epitaxial silicon layer and GaP layer prevented front surface recombination. Plotting lifetime vs. wafer thickness is useful for teasing out bulk and surface recombination, and we found that Seff = 250 cm/s fits the data well. This is a very promising starting point for this project since the measured lifetimes correspond to an implied Voc of 620 mV—the highest that we are aware of for a silicon wafer passivated with GaP.
Second, transport loss at the GaP/Si heterointerface has been also minimized. Once a high internal voltage in the silicon wafer is achieved through low recombination, this high voltage has to be transferred to the selective contacts on each side of the device. The challenge lies in the conduction band offset between Si and GaP, estimated to be 0.25 eV, which will have to be overcome by electrons. Though this value is lower than the valence-band discontinuity for a-Si:H/Si (0.4 eV), trap-assisted transport has been shown to play an important role in that particular case, and no or few trap states are expected in the high-quality epitaxial GaP compared to a-Si:H. We’ll investigate the transport at this interface by growing n-type GaP on n-type Si and forming Ohmic contacts to the GaP layer.
9:00 PM - EE3.5.10
Polymer Embedded Silicon Microwires for Colorless, Transparent, Flexible Solar Cells
Sungbum Kang 1,Min Joo Park 1,sanghyuk Won 1,Kyoung Jin Choi 1
1 UNIST Ulsan Korea (the Republic of),Show Abstract
Transparent solar cells have potential applications such as building integrated photovoltaics (BIPV) and photovoltaic chargers for portable electronics. Several groups reported transparent solar cells technologies based on perovskite, organic, or dye-sensitized solar cells, taking advantage of relatively their high energy bandgaps. Unfortunately, those cells suffer from low efficiencies (h 3 diffusion process, making a core-shell type of n/p junction. Finally, the MWs array was embedded with BCB microwires are lifted off from the silicon substrate, followed by top and bottom ohmic metallization using transparent conducting oxides and silver nanowires. The SiMWs-polymer composite solar cell has an efficiency of ~ 5% and transparency of ~ 10% with color rendering index close to 100. In this presentation, optical simulation as a function of the diameter and pitch between microwires by COMSOL wave optics module and the trade-off relationship of transmittance and efficiency will be also included.
9:00 PM - EE3.5.11
Bendable CdTe/CdS Thin-Film Solar Cells on Ultra-Thin Flexible Glass Substrates
Eun Woo Cho 1,Hyomin Park 1,Yoonmook Kang 1,Donghwan Kim 1,Jihyun Kim 1
1 Korea university Seoul Korea (the Republic of),Show Abstract
Solar cells have been explored as future energy devices. Especially, Cadmium telluride (CdTe) is a promising candidate material for fabrication of solar cells due to its optimum energy band gap (~ 1.5 eV). Also, CdTe solar cells have many advantages such as its lowest unit cost for generating electricity and good stability. Until now, the best efficiencies for CdTe solar cells have been obtained from conventional superstrated structure of CdTe solar cells, with glass as its substrate. However, using conventional glass as substrate presents issues of heavy weight and rigid structure. Therefore, in this study, ultra-thin glass is used as a flexible substrate to fabricate light weight, bendable CdS/CdTe thin films solar cells with a superstrate configuration, allowing it to be applicable in various fields. Then it is followed by optimization of post-deposition process for enhancement in conversion efficiency of CdTe solar cells.
To optimize the procedure for fabricating thin-film high performance bendable CdTe solar cells, processing sequence was investigated. The standardized post-deposition processes are CdCl2 activation heat treatment, followed by Nitric-phosphoric (NP) etching step. In this study, the focus was on the NP etch treatment procedure, which is commonly used to remove native oxide (TeO2) layer on the surface of CdTe thin film, and consequently improve the efficiency of CdS/CdTe solar cells. To explore the extent of NP etching effect, three separate post-deposition treatment procedures were carried out: 1) no NP etch; 2) pre-NP etch prior to CdCl2 treatment, and post-NP etch after CdCl2 activation step; 3) only post-NP etch. The effects of each process on the flexible CdS/CdTe solar cells were investigated by comparing photovoltaic properties. The details of the result will be presented at the conference.
9:00 PM - EE3.5.12
Self-Deposition of Pt Nanoparticles on Graphene Woven Fabrics for Improving the Efficiency of GWF/n-Si Solar Cells
Xinyu Tan 2,Zhe Kang 2,Hongwei Zhu 3
1 College of Materials and Chemical Engineering China Three Gorges University Yichang China,2 CTGU Collaborative Innovation Center for Magneto-electronic Industry amp; Research Institute for New Energy China Three Gorges University Yichang China,2 CTGU Collaborative Innovation Center for Magneto-electronic Industry amp; Research Institute for New Energy China Three Gorges University Yichang China3 Department of Materials Science and Engineering Tsinghua University Beijing ChinaShow Abstract
Silicon based solar cells have drawn wide attention in the photovoltaic market. Recently, carbon/ Si and graphene/Si solar cells have been developing rapidly with their convenient assembly and their impressively high power conversion efficiency (PCE).
The efficiency of carbon/Si solar cells can be improve by depositing metal nanoparticles, doping with chemicals and decorating antireflective. Although the antireflective coating is an effective method to improve the PCE of solar cells, the characteristics of n-Si and graphene still dominate the efficiency of solar cells by regulating the built-in electric field and internal resistance of solar cells. We concentrate more on the metal nanoparticles deposition to optimize the characteristics of graphene and Graphene Woven Fabrics (GWF).
We demonstrated a self-deposition method to deposit Pt Nanoparticle on GWF to enhance the efficiency of the GWF/n-Si solar cells. Self-deposition consists of photo-assisted deposition and electrochemical deposition. Depositing Pt nanoparticle on GWF found to be effective method for reducing sheet resistance and improving the work function of GWF. In the case of enhancing the PCE of GFW/n-Si solar cells, 10 min was found to be the proper time to deposit the Pt nanoparticle by self-deposition method with 10mM chloroplatinic acid. The efficiency of GWF/Si solar cell was increased from 4.10% to 7.95% by being deposited with Pt nanoparticle. The efficiency can be further promoted to 10.29% after coating with solid electrolyte.
 C. X. Guo , G. H. Guai , C. M. Li , Adv. Energy Mater. 2011 , 1 , 448 .
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9:00 PM - EE3.5.13
Amorphous Silicon Photovoltaic Modules on Flexible Plastic Substrates
Yuri Vygranenko 1,Miguel Fernandes 2,Manuela Vieira 2,Andrei Sazonov 3,Paula Louro Antunes 1
1 CTS-UNINOVA Caparica Portugal,1 CTS-UNINOVA Caparica Portugal,2 Electronics, Telecommunications and Computer Engineering Department ISEL Lisbon Portugal3 Electrical and Computer Engineering Department University of Waterloo Waterloo CanadaShow Abstract
Solar cells on lightweight and flexible substrates have advantages over the glass- or wafer-based photovoltaic devices in both terrestrial and space applications. Here, we report on a monolithic 10 cm × 10 cm area PV module integrating an array of 72 a-Si:H n-i-p cells on a 100 μm thick polyethylene naphtalate (PEN) substrate. The n-i-p stack was deposited using a PECVD system at 150 oC substrate temperature. To improve the fabricated device performance trough design optimization, a two-dimension distributed circuit model of the photovoltaic cell was developed. The circuit simulator SPICE was used to calculate current and potential distributions in a network of sub-cell circuits, and also to map Joule losses in the front TCO electrode and the metal grid. Experimental results show that the shunt leakage is one of the factors reducing the device performance. Current-voltage characteristics of individual a-Si:H p-i-n cells were measured and analyzed to estimate the variation of shunt resistances. Using the LBIC technique, the presence of multiple shunts in the n-i-p cell was detected. The Joule losses due to shunts were also estimated by modeling of the photovoltaic cell with multiple shunts. To understand the nature of electrical shunts, the change in the surface roughness of all device layers was analyzed throughout fabrication process. It is found that surface defects in plastic foils, which are thermally induced during the device fabrication, form microscopic pinholes filled with the highly conductive top electrode material. The modification in device design and fabrication steps is proposed to reduce the shunt leakage.
9:00 PM - EE3.5.14
Structural and Device Investigations of GaSb Based Solar Cell for Full Spectrum Solar Energy Harvesting
Ehsan Vadiee 1,Nikolai Faleev 1,Ganesh Balakrishnan 2,Fernando Ponce 1,Christiana Honsberg 1
1 Arizona State University Tempe United States,2 University of New Mexico Albuquerque United StatesShow Abstract
There exists a continuing need for multi junction solar cell devices for absorbing full solar spectrum. We address this need with using GaSb based solar cells grown directly on semi-insulator GaAs (001) substrates by Molecular Beam Epitaxy (MBE). HRXRD, TEM, PL and AFM have been performed to investigate the structural properties and material quality. To control device properties, GaAs-based solar cells were compared to devices grown on the GaSb substrates.
Different AlGaSb/GaSb and AlGaAsSb/GaSb structures are grown for studying the accommodation of elastic strain and defect creation. These structures are also compared to specify different types of crystalline defects in epitaxial layers and reveal the defect density on the interface and in the volume. This helps to evaluate material and future device qualities. Complex analysis of XRD and TEM results allows to specify the extent of relaxation of elastic stress in each individual epitaxial layer, and hence, determine type, density, and spatial distribution of preferable crystalline defects. Crystalline defects created due to accommodation of the initial elastic strain in different epitaxial layers are the main concern for growth of solar cell heterostructures. Growth defects can significantly degrade the structural, optical and electrical properties of solar cells. To fully understand the correlation between crystalline defects and solar cell performance, the process of stress relaxation and defect creation must be perfectly investigated. In high-strained epitaxial structures (GaSb/GaAs), elastic strain will be fully accommodated in the first few monolayers by creation of pure edge dislocations localized at the interface with the periodicity of (|b|/exx) along  and [1 0] directions. In lower strained case (AlGaAsSb/GaSb), initial elastic strain will be partially accommodated by formation of 60° dislocation loops (DL) at the interface and in the volume.
We have experimentally investigated the feasibility of using different GaSb alloys on GaAs and GaSb substrates for multijunction solar cell purposes. The active regions include AlGaAsSb and AlGaSb with different Al compositions optimized by detailed balance analysis . The external quantum efficiency confirms the extended absorption of solar spectrum in different active regions coinciding with the photoluminescence results. The short-circuit current of cells are expected to be 5% to 10% less than the GaSb-based references. In addition, the cells on GaAs substrates maintain less than 15% difference in spectral response to those of the control cells over a large range of wavelengths. Under solar simulation the Al0.14GaAsSb on GaSb exhibits open-circuit voltage of 0.563 V. The cost-savings and scalability offered by GaAs substrates could potentially outweigh the reduction in performance.
 W. Shockley and H. Queisser, 'Detailed Balance Limit of Efficiency of p-n Junction Solar Cells', J. Appl. Phys., vol. 32, no. 3, p. 510, 1961.
9:00 PM - EE3.5.15
Electrical Defect Characterization of 0.5 eV InGaAsSb Solar Cells
Kenneth Schmieder 1,Matthew Lumb 2,Maria Gonzalez 3,Shawn Mack 1,Robert Walters 1
1 US Naval Research Laboratory Washington United States,1 US Naval Research Laboratory Washington United States,2 George Washington University Washington United States1 US Naval Research Laboratory Washington United States,3 Sotera Defense Solutions Annapolis Junction United StatesShow Abstract
InGaAsSb, lattice-matched to GaSb, is a promising alloy for full spectrum energy harvesting. This narrow-bandgap material can achieve efficient photoabsorption at wavelengths out to 2500 nm, making it an ideal candidate for the bottom junction solar cell in an advanced multijunction architecture. In this work, 0.5 eV InGaAsSb devices have been grown via Molecular Beam Epitaxy in order to investigate the nature of defects and the limitations they impose on carrier lifetime and solar cell performance. These investigations are carried out using Deep-Level Transient Spectroscopy (DLTS) in order to identify trap signatures and how they are affected by growth conditions. Subsequently, trap signature information is input into an analytical drift-diffusion model in order to identify the upper-limit of solar cell performance given present material quality.
9:00 PM - EE3.5.16
Molecular Architecturing for Tailoring Optical, Electrochemical and Photovoltaic Properties
Vinila Nellisserry Viswanathan 1,Praveen Ramamurthy 1
1 Indian Inst of Science Bangalore India,Show Abstract
The development of p type polymers with smaller band gap and suitable HOMO-LUMO energy level is crucial in improving the power conversion efficiency of organic photovoltaics. Donor-acceptor-donor architectured polymers were thus extensively dominated in the library of donor material for solar cells. Considering the important criteria for a polymer to have application in organic photovoltaics, a few D-A-D architectured low band gap polymers were designed. Benzothiadiazole, a strong acceptor and Flourene- a fused planar molecule with two long alkyl chains, impart solubility as the donor. The acceptor moiety is coupled with two thiophenes to increase the conjugation and thus broaden the absorption spectra. Substitution on the polymer backbone will change the properties of polymers. Hence, have done a study on the effect of substitution on polymer properties by substituting with a strong electron withdrawing fluorine groups on polymer backbone. Since the size of fluorine is small, the planarity of the polymer backbone will not get disturbed. The torsion angles of backbone obtained from DFT are close to 1800, shows the planar structure of polymer back bone. The polymers show broad absorption and highly planar structure which will enhance the hole mobility along the polymer backbone. The HOMO-LUMO levels could be tuned by the substitution, which can change the optoelectronic properties of the organic conjugated polymer.
Xing Sheng, Tsinghua University
Matthew Escarra, Tulane University
Anita Ho-Baillie, The University of New South Wales
Matthew Lumb, U.S. Naval Research Laboratory and The George Washington University
EE3.6: Solar Concentrator Systems
Thursday AM, March 31, 2016
PCC North, 100 Level, Room 123
9:00 AM - *EE3.6.01
Hybrids of Photovoltaic Cells and High-T Thermal Collection That Maximize Exergy Collection to Solve the Impending Renewable Energy Storage Problem
Howard Branz 1
1 Branz Technology Partners Boulder United States,Show Abstract
Photovoltaic (PV) solar energy systems are unlikely to economically supply much more than 10% of the world's electricity without a dramatic reduction in the cost of electricity storage, due to the problem of diurnal and weather-related variability in PV production. Although 10 - 15% PV penetration into the global electricity supply represents an enormous market opportunity for PV, lowering carbon emissions from electricity generation to acceptable levels may also require new hybrid solar energy converter systems. These hybrid converters integrate PV cells with the collection of concentrated solar heat at high temperatures between about 200 and 600 °C.1 This solar heat can be stored as sensible heat in molten salts or as the phase-change of materials, and then converted to electricity. The total cost of storage and generation is far lower per electric kWh than even optimistic projections for the cost of storing electricity in advanced batteries, pumped hydroelectric or compressed air.1 Here we describe the physical principles that underpin technical opportunities for hybrid solar converters to lower the cost of collection of high-temperature solar heat, including approaches for: 1) novel spectrum-splitting optical and photonic systems that collect infra-red and/or ultraviolet photons for heat while still providing PV cells the wavelengths they convert most efficiently; 2) modified concentrating PV cells for use with spectrum splitting optics; and 3) concentrator PV cells that can operate at 300 to 400°C to enable capture of PV losses as high-exergy heat. Hybrid approaches can increase the economic viability of concentrating solar power (CSP) systems while preserving CSP’s fundamental value of providing dispatchable electricity from stored heat. Promising technology examples will be drawn from the FOCUS Program, funded in 2014 with over $30M by the U.S. Department of Energy’s Advanced Research Projects Agency - Energy (ARPA-E). Metrics for hybrid solar converters are complicated by their co-generation of heat and electricity: we proposed that optimizing the exergy, rather than the energy, from these hybrid systems will optimize the value of the electricity they generate once PV satisfies much of the daytime energy requirements in a particular electricity market.1
1. H.M. Branz, W. Regan, K.J. Gerst, J.B. Borak, E.L. Santori, “Hybrid solar converters for maximum exergy and inexpensive dispatchable electricity,” Energy & Environmental Science, DOI: 10.1039/c5ee01998b, 2015.
9:30 AM - *EE3.6.02
Concentrating Solar Power Research and Development under the SunShot Initiative
Joseph Stekli 1,Levi Irwin 1
1 US DOE Alexandria United States,Show Abstract
As the world moves to generating electricity from renewable sources, research and development of new renewable technologies is key to driving down the costs of these technologies. The SunShot Initiative, which began in 2011, has taken an approach driving solar technologies to economic parity and has aligned all of the research efforts under the Initiative towards the goal of achieving a cost of 6 cents per kilowatt-hour, without subsidy. This discussion will focus on current research and development activities taking place in the field of Concentrating Solar Power (CSP) as well as future opportunities for research and development within the field. The talk will focus specifically on the optics and receiver work the program performs, but will also touch upon the other systems within a concentrating solar power plant so as to provide some understanding of the constraints imposed upon the solar field and receiver by the other systems.
10:00 AM - EE3.6.03
A Low LCOE Spectrum Splitting Multijunction Solar Module
John Lloyd 1,Cristofer Flowers 1,Sunita Darbe 1,Carissa Eisler 1,Harry Atwater 1
1 California Institute of Technology Pasadena United States,Show Abstract
We present here a design for a low levelized cost of electricity (LCOE) spectrum splitting module capable of conversion efficiencies in excess of 37% for AM1.5D that was developed using coupled ray-tracing and external radiative efficiency-adjusted detailed balance models, with insights from a ground up cost model. This design features three to five single-junction III-V subcells tiled from highest to lowest bandgap along a solid parallelepiped under a solid primary concentrating optic, either a compound parabolic concentrator or solid refractive optic. A specific micro-optical design following these principles is presented with a module thickness of 1 cm, primary concentration of 116x, four 240 um subcells, and a module efficiency of 37%. The subcells are InGaAsP, GaAs, InGaP, and AlInGaP alloys fabricated via epitaxial lift-off from either InP or GaAs wafers. Finally, proof-of-concept measurements on a scale prototype of the proposed parallelepiped receiver are presented.
To minimize the cost of relatively expensive III-V subcells, some concentration (>100x) is required which necessitates using only the direct portion of the solar spectrum. A cost model was developed to identify cost drivers and opportunities for cost reduction of this spectrum splitting module design, and several key insights from that model informed the module design. First, dichroic filters, utilized in a similar spectrum splitting module configuration, drive the cost upward due to integration and complexity rather than component cost, and thus dichroic filters were abandoned in favor of a single solid optical path with better manufacturability utilizing the cells themselves as reflective filters. This choice to use the photovoltaic cells as filters means they must close pack along the parallelepiped, eliminating the possibility of secondary concentrators to reduce cell area further. Thus, in order to cost effectively utilize high quality epitaxially lifted off single junction devices, which offer both the highest single junction cell efficiencies as well as the greatest sub-bandgap reflection, the number of cells was limited to five or fewer, and the primary concentration is driven towards order 100x.
Lateral splitting of broadband solar radiation onto multiple absorbers with different bandgaps has been recognized as a pathway towards solar energy conversion efficiencies in excess of the limits of single junction photovoltaic technologies and capacity factors greater than traditional multijunction architectures. As commercial Si cell efficiencies approach their material limits, the higher efficiencies possible via spectrum splitting offer a potential pathway towards a lower levelized cost of electricity, but only if their cost and complexity are sufficiently low.
10:15 AM - EE3.6.04
Numerical Simulation of InGaN-Based High Temperature Concentrator Solar Cells
Yi Fang 1,Dragica Vasileska 2,Stephen Goodnick 2
1 Department of Physics Arizona State University Tempe United States,2 School of Electrical, Computer and Energy Engineering Arizona State University Tempe United StatesShow Abstract
To improve the efficiency of concentrated solar power hybrid system, a photovoltaic (PV) solar cell with high efficiency and operated at high temperatures is needed. In that regard, InGaN material system provides a platform for high temperature PV solar cells since nitride based optoelectronics are demonstrated to operate at high temperatures (>400 degrees Celsius). The direct and tunable band gap of InGaN semiconductor offers a unique opportunity to develop high efficiency solar cells. Band gap of the InGaN semiconductor can vary from 0.65 to 3.42 eV, which covers a broad solar spectrum from near-infrared to near-ultraviolet wavelength region. This work involves TCAD simulation and optimization for InGaN solar cell at high temperature. Monolithic and mechanical multi-junction solar cell designs are investigated, and show promising efficiency under light trapping. We also introduce a step layer at hetero-interface to relax band offset and polarization, which is more practical compared with Indium composition grading layer for the sake of fabrication. Theoretical conversion efficiency of the best devices are larger than 26% at 450 degrees Celsius with an incident solar radiation concentration of 200 suns. Thus, we demonstrate that 2J tandem solar cells made in InGaN material system are very suitable for concentrated solar power hybrid system.
10:30 AM - *EE3.6.05
Luminescent and Microtracking Concentration for Rooftop CPV
Noel Giebink 1
1 The Pennsylvania State University University Park United States,Show Abstract
Sunlight is a diffuse energy resource and thus all methods of solar energy conversion and use by society share one feature in common – concentration. Optical concentration offers a route to lower the cost of high efficiency multi-junction photovoltaics, but this typically requires bulky mechanical tracking that is incompatible with rooftop installation and on geometric optics that cannot harvest the diffuse solar component. This talk will focus on recent developments in quasi-static microtracking and luminescent solar concentration that address these respective challenges.
Whereas étendue conservation limits geometric concentration of diffuse light in a dielectric slab depending on its refractive index to ~5x, luminescent concentration has the potential to reach higher concentration ratio >100x. We are exploring a new opportunity to boost luminescent concentrator performance by photonically controlling the luminescent étendue, leveraging highly directional emission within the framework of nonimaging optics to demonstrate >3x secondary geometric gain for applications ranging from photovoltaics to scintillator-based radiation detection.
Recent efforts in high efficiency concentrating photovoltaics (CPV) will also be discussed, focusing on a new paradigm that combines microscale solar cells with wide-angle microtracking to enable >200x concentration ratio CPV panels < 1 cm thick that accomplish full-day tracking at fixed latitude tilt with < 1 cm lateral translation. This approach is experimentally validated outdoors for a small-scale panel prototype featuring 3D-printed plastic lenslet arrays and GaAs microcell photovoltaics, representing a step toward the goal of embedded CPV systems that can be integrated on building rooftops in the form factor of standard fixed panel PV.
EE3.7: Solar Thermal Systems
Thursday PM, March 31, 2016
PCC North, 100 Level, Room 123
11:30 AM - *EE3.7.01
The Arpa-e Focus Research Program for Hybrid Photovoltaic-Thermal Solar Electricity: Rationales and Architectures
Eric Schiff 1,James Zahler 1
1 Advanced Research Projects Agency - Energy Washington United States,Show Abstract
Solar electricity generated directly by photovoltaic modules can be produced at a cost that’s less than $0.10/kWh in utility-scale installations. This cost is below some spot prices paid by utilities for additional electricity, and the cost is steadily falling. However, as the total capacity for this type of solar electricity increases, oversupply when the sun is shining sets in, and the market price declines. Limited by this effect, direct solar electricity is typically considered to have a potential market share of around 10%. Solar energy’s percentage of the entire electricity generation portfolio is likely to increase further only if inexpensive technologies can be developed to store solar energy for at least several hours before its use as electricity.
In the summer of 2013, the Advanced Research Projects Agency – Energy (ARPA-E) announced its “Full-spectrum Optimized Conversion and Utilization of Sunlight“ (FOCUS) research program. Thirteen projects were funded and commenced in the summer of 2014. The program itself is a wager based on two strong assumptions: (i) that storage of solar energy as heat, with later conversion to electricity in heat engines, will prove to be the most successful storage strategy, and (ii) that harnessing part of the solar spectrum for direct electricity generation using photovoltaic modules, with the remainder used to generate heat, will result in the highest-efficiency, lowest-cost solar energy conversion systems featuring thermal energy storage (1). Howard Branz, the ARPA-E program director who initiated the FOCUS program, summarized the second approach as “no photon left behind”.
This presentation will first review these strategic assumptions three years after FOCUS was announced. Does the recent announcement of a "gigafactory" for lithium batteries require revision of the assumption of thermal storage? And is it ultimately cheaper to wed photovoltaics with thermal storage than it is to just build two side-by-side plants, one using photovoltaics and the other based on concentrating solar power (CSP)? The second subject of the presentation will be the architecture of hybrid thermal-photovoltaic solar electricity generation. The ongoing FOCUS projects provide examples of several architectures.
(1) “Hybrid solar converters for maximum exergy and inexpensive dispatchable electricity”, Howard M. Branz, William Regan, Kacy J. Gerst, J. Brian Borak, and Elizabeth A. Santori, Energy & Environmental Science (2015), DOI: 10.1039/c5ee01998b .
12:00 PM - EE3.7.02
Spectrum Splitting Concentrated Photovoltaic Module Design for a Hybrid Photovoltaic-Photothermal System
Qi Xu 1,Yaping Ji 1,Adam Ollanik 1,Nicholas Farrar-Foley 1,Vince Romanin 2,Pete Lynn 2,Danny Codd 3,James Ermer 4,Matthew Escarra 1
1 Tulane Univ New Orleans United States,2 Otherlab San Francisco United States3 University of San Diego San Diego United States4 Boeing-Spectrolab Sylmar United StatesShow Abstract
A hybrid solar energy conversion system, utilizing a combination of concentrated photovoltaic (CPV) and photothermal conversion processes, can significantly increase the efficiency and ease of utilization of the incident broadband solar spectrum by producing electricity as well as dispatchable thermal energy. The PV cell is the most expensive component in the system, however concentrating approaches may offer cost benefits by reducing the amount of PV area required to convert a given amount of solar power to electrical power, all with enhanced efficiency. In our design, we utilize our photovoltaic module to efficiently divide the solar spectrum between the ultraviolet-visible portion (converted directly to electricity in the module) and the infrared portion (which passes through to a thermal receiver), all with high efficiency and minimal incident angle sensitivity. However, accompanied with increasing concentration levels on the module is also a potential rise in cell temperature, which is an undesirable effect as it may reduce the cell efficiency and could lead to module breakdown. Therefore, it is necessary to provide cooling solutions to reduce the cell temperature to maintain reasonable system efficiency and reliability.
In this work we propose a prototype design of a hybrid photovoltaic-photothermal system and the spectrum splitting CPV module at the core of it. We present numerical results based on Finite Elemental Method (FEM) analysis. The CPV module in this hybrid system, which employs III-V triple-junction solar cells, can convert the in-band light directly to electricity, while the thermal receiver will receive and store the energy from the out-of-band light as heat. The geometrical concentration ratio is 500X and the module consists of 49 individual sub cells. According to our simulations, the spectrum splitting CPV module can perform with overall power conversion efficiency exceeding 43% for in-band light, and a transmission efficiency of over 75% for out-of-band light under a standard AM1.5D solar spectrum. Designs will be shown illustrating that the maximal operating temperature of the CPV module can be controlled below 110°C with a passive cooling system, all while maintaining high transmissivity. We also investigate how to effectively control cell temperatures with active cooling and how to deal with non-ideal optics, including light spot wandering due to tracking error and dish roughness and shape errors. Moreover, we have developed a novel CPV circuit design to minimize power losses from current or voltage mismatch in our cells due to changing illumination conditions. Finally, we evaluate the overall performance of the hybrid system and analyze the costs and potential markets, showing that our system has potential economic advantage compared to a PV with battery storage system. We are now prototyping this module and system design and will present our latest experimental results as well.
12:15 PM - EE3.7.03
Semiconductor-Dielectric Selective Absorbers for Solar Thermal Energy Conversion
Nate Thomas 1,Austin Minnich 1
1 California Inst of Tech Pasadena United States,Show Abstract
Spectrally selective absorbers that absorb visible light yet do not emit infrared light are key to achieving high efficiency in solar thermal applications. However, available selective absorbers achieve at best around 200°C under unconcentrated sunlight due to high thermal losses via infrared (IR) emission, limiting the applications of solar thermal energy conversion. Here, we report photonic structures composed of thin films of semiconductors and dielectrics for high temperature, unconcentrated solar thermal applications. Our selective surface exhibits hemispherical IR emittance of 4% and average solar absorptance of 87% for an unprecedented absorption-to-emission ratio of 24. Such low IR emittance is critical for reaching the high temperatures relevant for industrial processes under single sun illumination.
12:30 PM - EE3.7.04
Full Spectrum Collection of Concentrated Solar Energy Using PV Coupled with Selective Filtration Utilizing Nanoparticles
Todd Otanicar 1,Drew DeJarnette 1,Nick Brekke 1,Ebrima Tunkara 2,Ken Roberts 2,Parameswar Harikumar 3
1 Department of Mechanical Engineering University of Tulsa Tulsa United States,2 Department of Chemistry University of Tulsa Tulsa United States3 Department of Physics University of Tulsa Tulsa United StatesShow Abstract
Hybrid solar receivers utilizing both photovoltaic cells and thermal collectors are capable of collecting the entire solar spectrum for use in energy systems. Such systems provide efficient solar energy conversion using PV in addition to dispatchability through thermal storage by incorporating a thermal collector in conjunction with the PV. Proposed hybrid systems typically invoke spectrum splitting so to redirect photons optimized for PV electric conversion to a cell while non-PV efficient photons are directed to a thermal absorber. This work discusses a hybrid system with a selective solar filter using a suspended nanoparticle fluid to directly absorb non-PV photons. Non-absorbed photons pass through the filter and impact the PV. Choice of nanoparticles in the fluid allow specific wavelengths to be absorbed and transmitted. Nanoparticles were chosen based on optimization simulations for a bandpass filter to a cSi solar cell. The synthesized fluid has been experimentally characterized to show the effects of high temperature on nanoparticle stability and optical properties. Thermodynamic modeling of the system indicates the solar to electric efficiency of the total system is 23.2% if all thermal energy is immediately converted to electricity through an organic Rankine cycle. However, high temperature generation could also be used for industrial process heat at a specific temperature by changing parameters such as absorbed energy and flow rates. Further, a protoype is being developed with 14x concentration to demonstrate the technology on-sun with initial testing targeted for the 2nd quarter of 2015. Overall, the hybrid nanoparticle filter concentrating solar collector can be modified to fit a variety of applications through easily changeable parameters in the system.
12:45 PM - EE3.7.05
A Hybrid CPV-CSP System to Fully Utilize the Solar Spectrum
Wei Pan 1
1 Sharp Labs of America Camas United States,Show Abstract
Conventional concentrate solar power (trough) system (CSP) can collect the full solar spectrum into heat. Part of the heat energy can be stored for later use (dispatchable energy) and part of heat energy can drive a turbine engine to generate electricity (variable energy). However, the annualized overall solar-electricity efficiency for CSP is low, about 15%. The concentrate photovoltaic (CPV) can convert part of solar spectrum with relative high efficiency, about 30% annualized. However, it does not have storage capability, i.e. CPV technology lacks dispatchability. Use of chemical energy storage would add cost significantly.
In this paper, a hybrid CPV-CSP system that addresses both solar-electricity conversion efficiency as well as solar energy dispatchability is presented. In this hybrid system, a conventional CSP trough collector is modified to adapt a hyperbolic dichroic mirror for spectrum splitting: visible and near IR spectra are reflected by the dichroic mirror and refocused onto specially designed double junction CPV modules; while UV and longer wavelength solar spectra are transmitted to the heat collector of CSP. With the spectra splitting, part of solar spectra are converted into electricity with very high efficiency (>50%) through CPV. The rest of solar spectra are collected as heat for thermal storage – maintaining the dispatchability of a CSP. Thus, the hybrid system will not only have higher solar energy conversion efficiency but also solar energy storage capability.
Key challenges of such a hybrid system are the optical design of CPV module to maintain high concentration ratio and the dichroic coating design that compensates skew angles (angels between sun irradiance and the trough normal) induced dichroic blue shift and maintains junction current balance in the CPV cells. In this paper, novel CPV module and dichroic designs are discussed. The annualized energy output for the hybrid system is also modeled out and compared to that of a conventional CSP system as well as a c-Si PV system.
This R&D project is supported by ARPA-E through FOCUS program. The research team includes Sharp Labs of America, Inc., University of Arizona, University of Missouri, and Solargenix LLC.
EE3.8/NT1.8: Joint Session: Recent Developments in Optoelectronics and Photovoltaics
Thursday PM, March 31, 2016
PCC North, 100 Level, Room 129 A
2:45 PM - *EE3.8.01/NT1.8.01
Optoelectronics: Is There Anything It Cannot Do; Can Opto-Electronics Provide the Motive Power for Future Vehicles
Vidya Ganapati 1,T. Xiao 1,Eli Yablonovitch 1
1 Electrical Engineering and Computer Sciences Dept. University of California, Berkeley Berkeley United States,Show Abstract
A new scientific principle[i] has produced record-breaking solar cells. This is exemplif