Organometallic Halide Perovskite / Barium Di-Silicide Thin-Film Double-Junction Solar Cells
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.
1M. A. Green, Prog. Photovolt: Res. Appl. 9, 123 (2001).
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 - EE3.5.08
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 .
 Y. Ye , L. Dai , J. Mater. Chem. 2012 , 22 , 24224 .
 E. Shi , L. Zhang , Z. Li , P. Li , Y. Shang , Y. Jia , J. Wei , K. Wang , H. Zhu , D. Wu , S. Zhang , A. Cao , Sci. Rep. 2012 , 2 , 884 .
 E. Shi , H. Li , L. Yang , L. Zhang , Z. Li , P. Li , Y. Shang , S. Wu , X. Li , J. Wei , K. Wang , H. Zhu , D. Wu , Y. Fang , A. Cao , Nano Lett. 2013 , 13 , 1776 .
E. Shi , H. Li , L. Yang , L. Zhang , Z. Li , P. Li , Y. Shang , S. Wu , X. Li ,J. Wei , K. Wang , H. Zhu , D. Wu , Y. Fang , A. Cao , Nano Lett. 2013 ,13 , 1776 .
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 AM, 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 exemplified by the mantra: “A great solar cell also needs to be a great LED”. It is essential to remove the original semiconductor substrate, which absorbs luminescence, and to replace it with a high reflectivity mirror.
In thermo-photovoltaics, high energy photons from a thermal source are converted to electricity. The question is what to do about the majority of low energy infrared photons? It was recognized that the semiconductor band-edge itself can provide excellent spectral filtering for thermophotovoltaics, efficiently reflecting the unused infrared radiation back to the heat source. Exactly those low energy photons that fail to produce an electron-hole pair, are the photons that need to be recycled.
Thus the effort to reflect band-edge luminescence in solar cells has serendipitously created the technology to reflect all infrared wavelengths, which can revolutionize thermo-photovoltaics. We have never before had such high rear reflectivity for sub-bandgap radiation, permitting step-function spectral control of the unused infrared photons for the first time. This enables conversion from heat[ii] to electricity with >50% efficiency. Such a lightweight “engine” can provide power to electric cars, aerial vehicles, spacecraft, homes, and stationary power plants.
[i] O. D. Miller, Eli Yablonovitch, and S. R. Kurtz, “Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit”, IEEE J. Photovoltaics, vol. 2, pp. 303-311 (2012). DOI: 10.1109/JPHOTOV.2012.2198434
[ii] The heat source can be combustion, radio-activity, or solar thermal.
3:15 PM - *EE3.8.02/NT1.8.02
Controlling both Solar and Thermal Spectra for Solar Cell Applications
Shanhui Fan 1
1 Stanford Univ Stanford United States,Show Abstract
We show that the use of photonic structures, which allows control of both solar and thermal radiation spectra, has important implications for various aspects of solar energy conversion, including voltage, current and cooling considerations.
3:45 PM - EE3.8.03/NT1.8.03
Highly Conductive Ag Nanowire Meta-Electrodes Improve Silicon Heterojunction Solar Cells
Mark Knight 1,Jorik Van De Groep 1,Paula Bronsveld 2,Wim Sinke 1,Albert Polman 1
1 FOM Institute AMOLF Amsterdam Netherlands,2 Energy research Centre of the Netherlands Petten Netherlands2 Energy research Centre of the Netherlands Petten Netherlands,1 FOM Institute AMOLF Amsterdam NetherlandsShow Abstract
Silicon heterojunction (SHJ) solar cells, where the crystalline Si is passivated by thin layers of a-Si:H, have attracted significant interest due to their record voltages. However, reflection from macroscopic metallic ‘fingers’ has constrained current generation – and efficiency – in front contacted cells. These fingers are essential for harvesting electrons due to poor lateral transport in indium tin oxide (ITO), which is both the most common transparent conductive electrode (TCE) and limited by a fundamental tradeoff between transmission and conductance. Since the optimal finger spacing depends on TCE conductivity, the efficiency of SHJ cells is married to the quality of the TCE layer.
In this presentation we introduce a hybrid TCE which decouples the optical and electrical functionalities, and enables the independent optimization of each function. The geometry consists of Ag nanowires (80 nm wide, 120 nm tall) arranged in a sparse square grid, fabricated on top of an ultrathin ITO layer to transport charge in the interstitial regions. The nanowires are conformally coated with Si3N4, providing both environmental stability and an antireflection coating free from the interband and free carrier losses in ITO. For cells with fingers spaced by the standard 2 mm, replacing ITO with Si3N4 reduces optical loss by 1.1 mA cm-2.
We apply this nanostructured hybrid electrode to large-area (4 cm2) untextured SHJ solar cells using substrate-conformal imprint lithography (SCIL), a fabrication method that enables rapid, wafer-scale fabrication of nanowire (NW) arrays with full geometric control. The SCIL fabrication process does not damage the passivating a-Si:H layer, with consistent values for Voc measured on cells with and without nanowire modification. Optically, the nanowire networks exhibit broadband anomalous transmission due to detuning of the plasmonic nanowire resonances from the solar spectrum, shading 30% less light than the NW area coverage. Electrically, the square grids of nanowires have measured sheet resistances of 4.0, 7.2, and 15.0 Ω/sq. for pitches of 1, 2, and 4 µm, respectively. This is a significant (10x) improvement relative to an industrially processed ITO electrode with a measured 150 Ω/sq. sheet resistance. Due to the improved transmission and conductivity, the hybrid electrode enables an increase in finger pitch from 2 to 5 mm, reducing shading. For the champion hybrid electrode SHJ cell we measure a Jsc enhancement of 1.4 mA cm-2 (32.7 to 34.1 mA cm-2) with a simultaneous increase in FF (0.622 to 0.670), yielding an absolute efficiency enhancement of 2.2% (13.8% to 16.0%).
This demonstration of an engineered ‘meta-electrode’ within the electrical and optical environment of high performance SHJ solar cells shows that large-area nanostructuring provides practical pathway to increased performance and a reduced dependence on rare metals, especially indium.
EE3.9: Novel Solar Absorbers
Thursday PM, March 31, 2016
PCC North, 100 Level, Room 123
4:30 PM - *EE3.9.01
Adduct Approach for High Efficiency Perovskite Solar Cells
Nam-Gyu Park 1
1 Sungkyunkwan Univ Suwon Korea (the Republic of),Show Abstract
In this talk, an effective methodology for high quality perovskite layer is presented, which is Lewis acid-based adduct chemistry. In the solution process to form the perovskite layer, PbI2 and CH3NH3I or HC(NH2)2I are dissolved in polar aprotic solvents. Since polar aprotic solvents bear oxygen, sulfur or nitrogen, they can act as Lewis base. In addition, the main group compound PbI2 is known to be Lewis acid. Thus PbI2 has a chance to form adduct by reacting with Lewis base. Using the dative bonding characteristics in the adduct, crystal growth and morphology can be controlled. We have successfully fabricated the highly reproducible CH3NH3PbI3 perovskite solar cells with PCE as high as 19.7% via adduct of PbI2 with oxygen-donor N,N’-dimethyl sulfoxide. Formation of adduct is confirmed by FTIR, where stretching vibration of S=O is shift to lower wavenumber. This adduct approach is extended to formamidinium lead iodide, in which HC(NH2)2PbI3 with large grain, high crystallinity and long-lived carrier life time is successfully fabricated via adduct of PbI2 with sulfur-donor thiourea. The proposed Lewis acid-base adduct approach is expected to be a promising method for single crystal growth on the conductive substrate.
5:00 PM - EE3.9.02
The Ultimate Efficiency of Organolead Halide Perovskite Solar Cells Limited by Auger Processes
Ibraheem Almansouri 1,Anita Ho-Baillie 2,Martin Green 2
1 Institute Center for Energy (iEnergy), Department of Electrical Engineering and Computer Science Masdar Institute Abu Dhabi United Arab Emirates,2 Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable Energy Engineering University of New South Wales Sydney 2052 AustraliaShow Abstract
The key to improving the conversion efficiency of perovskite solar cells lie in the identification and control of different limiting factors. Both intrinsic and extrinsic losses are shown here to be detrimental on conversion efficiency well below the thermodynamic limit. The effect of radiative and Auger, for the first time, recombination processes as intrinsic losses are shown. Additionally, the impact of light concentration, important in Auger limited devices is investigated. The extrinsic losses are shown to impose severely bounds on efficiency limits. Thus, this work presents the possible approaches in achieving performance beyond what is currently demonstrated in the highest efficient perovskite solar cells and the implications on perovskite/silicon(Si) tandems.
5:15 PM - EE3.9.03
Semi-Transparent Perovskite Solar Cell with >80% Transparent Sputtered Front and Rear Electrodes for a Four-Terminal Tandem
The Duong 1,Niraj Lal 1,Daniel Jacobs 1,Shakir Rahman 1,Heping Shen 1,Klaus Weber 1,Thomas White 1,Kylie Catchpole 1
1 Research School of Engineering Australian National University Canberra Australia,Show Abstract
Perovskite on silicon tandem is a promising method to achieve large-area high-efficiency solar cells. In a tandem configuration, perovskite solar cells require two transparent contacts. Through detailed power loss analysis of electrical and optical losses, we examine optimum contact parameters and outline directions for the development of future transparent contacts for tandem cells. A semi-transparent perovskite cell is fabricated with steady-state efficiency exceeding 12% and broadband transmittance of >80%, using optimized sputtered indium tin oxide front and rear transparent contacts. Our semi-transparent cell exhibits much less hysteresis compared to the opaque cell. A four-terminal perovskite on silicon tandem efficiency of more than 20% is achieved, and we identify clear pathways to exceed the current single silicon cell record of 25.6%.
5:30 PM - *EE3.9.04
Quantum Ratchet Intermediate Band Solar Cells
Nicholas Hylton 1,Ture Hinrichsen 1,Anthony Vaquero-Stainer 1,Megumi Yoshida 1,Andreas Pusch 1,Ortwin Hess 1,Chris Phillips 1,Ned Ekins-Daukes 1
1 Imperial College London London United Kingdom,Show Abstract
Intermediate band solar cells have been proposed as one route towards high efficiency photovoltaics by capturing low energy photons via intermediate states in the forbidden gap. This ability to harvest a broader swathe of the solar spectrum while maintaining high open circuit voltage in a single junction device is highly desirable. It promises to extend the fundamental efficiency limit beyond that of the conventional Schockley-Queisser analysis; however intermediate devices have to date exhibited low efficiencies due to rapid carrier recombination, leading to short lifetimes in the intermediate states.
The short lifetime of excitations in these intermediate states prevents efficient sequential absorption and also leads to excitations in the conduction band recombining via this route. We recently proposed to make use of a possible energetic splitting of the intermediate band into higher energy states that are efficiently connected to the valence band and states with lower energy, known as the ratchet band, which are disconnected from the valence band. This increases the lifetime of carriers in intermediate states, limiting recombination losses and promoting sequential absorption. Our limiting efficiency calculations have shown that such a ratchet could increase the efficiency of an IBSC further beyond the efficiency of conventional IBSCs, even if recombination is dominated by radiative processes.
A direct analogy of (and one way to realise) this type of intermediate band mechanism is photoluminescence upconversion, whereby two or more low energy photons are converted into a high energy photon. It has been shown that upconversion occurs at III-V interfaces, including the interface between layers of GaAs and InGaP. In this system photogenerated carriers in the GaAs become trapped at quasi-stable states and can be further excited into the InGaP resulting in luminescence at energies higher than that of the excitation. To be able to take advantage of this phenomenon in a photovoltaic device a better understanding of the upconversion mechanism is required and will enable the design of structures with higher efficiencies. To this end we have performed a detailed spectroscopic study of upconversion photoluminescence at such an interface to elucidate the nature of the processes involved.
In this contribution we will present the concept of a quantum ratchet applied to intermediate band solar cells and discuss its advantages over the conventional IBSC concept. We will also show how upconversion at III-V interfaces may be used to realise this type of solar cell, as well as surveying other candidate systems.
EE3.10: Poster Session III: Novel Solar Energy Harvesting Concepts
Thursday PM, March 31, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - EE3.10.01
Transformation of Near-Infrared Alloyed Semiconductor Nanocrystals on Water Surface
Terefe Habteyes 1,Bijesh Kafle 1,Tefera Tesema 1
1 Univ of New Mexico Albuquerque United States,Show Abstract
Owing to their broader wavelength tunability of alloyed semiconductor nanocrystals than binary nanocrystals, there is increasing interest in utilizing them for various important applications including biological imaging, solar cells and photo-detectors. A large variety of alloyed semiconductor nanocrysals have been successfully synthesized but study of their behavior under different environmental exposure has been lacking. Here, we report the transformation of near-infrared (NIR) alloyed CdSeTe quantum dots (QDs) to red-emitting QDs on water surface for the first time. When the NIR QDs with 840 nm emission wavelength and 75 nm spectral linewidth are spread on water surface for about 20 minutes and transferred to an oxide coated silicon wafer using a Langmuir-Blodgett procedure, an ~20 nm blue-shift and an ~8 nm linewidth broadening are observed, which is attributed to surface chemical reaction. More interestingly, two prominent new sharp emission peaks are observed at ~630 nm and ~660 nm emission wavelengths that have ~23 nm and ~39 nm spectral linewidths, respectively. Based on detailed analysis of the photoluminescence and Raman spectra, the 630 nm and 660 nm emission peaks are assigned to CdSe and CdTe core binary nanocrystals. At the water-air interface, the QDs are effectively separated from the stabilizing excess surfactant molecules, resulting in self-assembled and aggregated QDs that are confined between the water surface and the surfactant layer. Chemical potential gradient, in and out diffusion of Se and Te atoms, nucleation and interparticle interactions are suggested as possible mechanisms to transform lattice strained metastable ternary nanocrystals to the binary constituents. This work highlights the importance of post-synthesis studies of alloyed nanocrystals to understand their behaviors under different environmental conditions so that their surface chemical properties and stability can be improved for various technological applications.
9:00 PM - EE3.10.02
Band Edge Electronic Structure of Doped-ZnS as a Photocatalyst for Hydrogen Production
Fran Kurnia 1,Judy Hart 2
1 School Materials Science and Engineering University of New South Wales Sydney Australia,1 School Materials Science and Engineering University of New South Wales Sydney Australia,2 Integrated Materials Design Centre University of New South Wales Sydney AustraliaShow Abstract
Zinc sulfide (ZnS) is an important II-VI semiconductor photocatalyst because of its high rate of photoexcited charge carrier generation, which is related to its direct band-gap (~ 3.6 eV). However, to enable efficient photocatalysis of water splitting at visible-light wavelengths, the band edges of ZnS should be tailored to give a band gap of ~ 2.0 eV. Doping is one of the most effective approaches to extending the absorption edge of ZnS to the visible-light range. Instead of doping with a single impurity, codoping can also be used to modify the electronic structure in a desired way. In this work, we present the results of first-principles calculations, using a hybrid functional for the exchange-correlation energy, of codoped ZnS. An anion (C, N, or P) and a transition metal were substituted into ZnS lattice, which leads to the introduction of dopant states in the band gap. It has been found that (Cu,N)-codoped ZnS can narrow the band gap by 42%, thus giving a material that could be useful for hydrogen production under sunlight. Furthermore, the results of our calculations show that (Co,P)-codoped ZnS is very promising for overall water-splitting as it shows direct and indirect electronic transitions of 3.28 eV and 2.70 eV, respectively. The electronic states introduced by the dopants may be able to trap photoinduced electrons, which could prolong the lifetime of the charge carriers. Both band gap reduction and minimization of the recombination rate are important factors for efficient light-to-current conversion in photocatalysts, and so achieving these affects with the codopants proposed in this work could lead to a remarkable enhancement in the photocatalytic activity of ZnS. To assist the synthesis of the proposed ZnS-based photocatalysts, we have analyzed the formation energies of the codoped systems. All these findings are crucial for the development of efficient non-oxide photocatalysts and for understanding the synergistic effects of codopants.
 J.-F. Reber, K. Meier, J. Phys. Chem. 88, 5903 (1984).
 K. Maeda, J. Photoch. Photobio. C: Photochem. Rev. 12, 237 (2011).
 F. Kurnia, J. N. Hart, ChemPhysChem 16, 2397 (2015).
9:00 PM - EE3.10.03
Photocatalytic Performance of Cuprous Oxide-Decorated Graphene-Like Nanosheets
Po-Ya Chang 1,Yu-Min Sung 1,I-Hsiang Tseng 1
1 Feng Chia University Taichung Taiwan,Show Abstract
A series of two dimensional graphene-like nanosheets were synthesized as photocatalysts for photocatalytic reduction of carbon dioxide under visible light illumination. In-house graphene oxide (GO) was suspended in ammonia solution to obtain N-doping graphene oxide (N-GO) after hydrothermal process. On the other hand, graphitic carbon nitride (g-CN) was synthesized by directly heating melamine to 550 oC. Then, photo- or chemical-reduction process was applied to deposit silver or cuprous oxide nanoparticles on the surface of these nanosheets to improve the efficiency of charge transfer. Spin trap technique combined with the electron paramagnetic resonance (EPR) spectroscopy was employed to evaluate the type and concentration of photoinduced radicals from each sample under UVB illumination. The greatest number of hydroxyl radicals was generated from the surface of N-GO under UVB irradiation. N-GO also exhibited the highest photoreduction rate of methylene blue (MB) solution with the concentration of 15 ppm under visible light illumination. On the other hand, no spin adducts formed from illuminated g-CN suspension and g-CN was unable to decolorize MB under the same condition. We believed that the hydrophobicity and poor charge transfer efficiency of g-CN constrain the formation of hydroxyl radicals and limit the ability to decolorize MB. The presence of silver or cuprous oxide on nanosheets prolongs the life-time of photo-induced radicals, especially for g-CN. In addition, the deposition process modified the chemical structure of g-CN and consequently strengthened the interaction between MB and g-CN. Preliminary results suggest that the conversion of CO2 and the selectivity of products can be tuned by verifying the components of nanosheets as well as the amounts and types of nanoparticles. The kinetics and mechanism of photocatalytic reduction of CO2 over graphene-like nanosheets are in progress in order to more efficiently convert CO2 into valuable products by harvesting solar energy.
9:00 PM - EE3.10.04
Photovoltaic Power of Single-Walled Carbon Nanotube Films by Interface Structures of Different Film Thickness
Yoshinori Sato 2,Hajime Sakakibara 1,Kenichi Motomiya 1,Kazuyuki Tohji 1
1 Tohoku Univ Sendai Japan,2 Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research Shinshu University Matsumoto Japan,1 Tohoku Univ Sendai JapanShow Abstract
Single-walled carbon nanotubes (SWCNTs) possess spike-like DOS derived from one-dimensional van Hove singularity. Furthermore, SWCNT shows metallic or semiconducting DOS as a result of its chiral index. The optical transitions of SWCNTs arise between matching van Hove singularities in conduction and valence bands. In particular, semiconducting SWCNTs have two absorption peaks in the near-infrared light region, whereas metallic SWCNTs have one absorption peak in the visible light region. Omari et al. illuminated infrared light on part of 90%-enriched semiconducting SWCNT thin films. They reported that the photovoltage appeared only for asymmetric illuminations by the photothermovoltaic effect, which is explained on the basis of a photogenerated heat flow model, and the photo-thermoelectric power was attributed to the temperature gradient of both ends of the film. In general, the thermal conductivity of individual SWCNTs is high and ranges from 3000 to 5000 W/mK (at 25 °C) due to ballistic phonon transport. On the other hand, the thermal conductivity of randomly aggregated SWCNT films shows a low value of around 35 W/mK (at 25 °C).
The low thermal conductivity of randomly aggregated SWCNT films is due to phonons being scattered at the interface between nanotubes, where phonon transport is inhibited. In addition, theoretical considerations indicate that interface thermal resistance arises at the interface between nanotubes. Therefore, numerous tube-tube junctions are due to dominant barriers to thermal transport in the SWCNT films. In contrast, π-electrons are easily able to transport between nanotubes by π-electron hopping. Thus, the difference between π-electron mobility and phonon mobility is considered to occur at the interface of the SWCNT film with different film thickness. Here, we produced SWCNT films with thin/thick interface structures in an effort to improve the photothermovoltaic effect by utilizing the low thermal and high electronic conductivities of SWCNT aggregates, and also investigated their photovoltages.
9:00 PM - EE3.10.05
Effect of Chemical Cleaning before Deposition of Cat-CVD Passivation Films to Realize Extremely Low Surface Recombination Velocity on Textured Structure
Cong Nguyen 1,Koichi Koyama 1,Shigeki Terashima 1,Chikao Okamoto 2,Shuichiro Sugiyama 2,Keisuke Ohdaira 1,Hideki Matsumura 1
1 JAIST (Japan Adv. Inst. Sci. amp; Tech.) Nomi Japan,2 SHARP Corporation Katsuragi JapanShow Abstract
Silicon-nitride (SiNx)/amorphous-silicon (a-Si) stacked passivation on flat crystalline silicon (c-Si), prepared by plasma-damage-less Cat-CVD (Hot-Wire CVD), realizes extremely low surface recombination velocity lower than 1 cm/s. Ultra-high resolution scanning transmission electron microscope (STEM) image reveals that the width of interface layer between a-Si and c-Si is as small as 0.6 nm when a-Si is prepared by Cat-CVD, but larger than 1.8 nm when it is prepared by the conventional plasma enhanced CVD (PECVD). This suggests that the plasma damages c-Si surface during a-Si deposition, leading to rough interface. However, at the same time, it is difficult to obtain really clean c-Si surface, without suffering from any contamination. PECVD may be effective to destroy such contaminants and remove them from c-Si surface. Consequently, the passivation ability of PECVD is not so different from Cat-CVD, although the superiority of Cat-CVD is apparent from STEM observation.
Thus, in the present work, we concentrate on development of chemical cleaning process to remove any contaminants from c-Si surface prior to deposition of Cat-CVD films. At first, the effect of dipping c-Si in hot acid solution is studied, and next, the effect of adding surfactants into HF and H2O2 solutions for adjusting hydrophobicity of chemical solutions to make chemical reactions uniform at the surface. Optical reflectivity of textured structures is unchanged after chemical cleaning. Finally, it is succeeded to obtain Cat-CVD SiNx/a-Si passivation which realizes carrier life time up to 11.2 ms on flat c-Si substrates with bulk carrier life times around 11.5 ms, and about 8 ms on 170 mm-thick textured c-Si used for solar cells. This means that the maximum surface recombination velocity as low as 1.1 cm/s, evaluated under the assumption of no carrier recombination in bulk, is successfully obtained for textured c-Si by proper chemical cleaning and film deposition by Cat-CVD.
9:00 PM - EE3.10.06
Graphene Quantum Dots/Metal Oxide Hybrid Photoelectrodes for Efficient Solar Energy Conversion
Radwa Shedeed 1,Ahmed Shehata 1,Nageh Allam 1
1 Energy Laboratory Lab (EML) The American Univ in Cairo Cairo Egypt,Show Abstract
With the looming energy crisis in the world, especially in my country Egypt, and the depletion of petroleum and natural-gas reserves, searching for sources of alternate fuel becomes very urgent. To this end, direct solar energy conversion into electricity or into storable fuels offers a promising solution. One of the most critical issues is the development of a suitable photoelectrodes with high efficiency and long-term durability in different environments; such as an aqueous media in case of fuel production or hot and dusty environment in case of electricity production. Despite the present efforts, the capital cost as well as the stability of the material is still inconvenient for large-scale
applications. Therefore, the work aims at constructing high performance, affordable, and air-stable inorganic photoelectrochemical devices based on graphene quantum dots/metal oxide hybrid nanoarchitectures, which are abundant and environmentally benign, enabling long-term and scalable solar energy conversion and storage. The research has included materials design, fabrication, characterization and device assembly and testing.
The first step in my work was fabrication of uniquely advantageous one-dimensional (1D) nanoarchitectures of various metal oxide electrodes (specifically TiO2 and ZrO2) based on the literature, by improved low temperature method. Then, the second step is, synthesis of graphene quantum dots with the desired size using Hummer’s method and chemical vapor deposition method. After synthesis of graphene quantum dots, Investigation the best way(s) to homogenously anchor the graphene quantum dots to the optimized metal oxide nanomaterials without any cracking or stability issues has been done. The expected effect of various graphene quantum dots on the efficiency of the fabricated electrodes upon their use in photoelectrochemical has been understood. Then,the research has identfied the best hybrid electrode assemblies (graphene quantum dots/metal oxide) and experimentally investi