N.J. Ekins-Daukes, Imperial College London
Louise Hirst, U.S. Naval Research Laboratory
Richard King, Arizona State University
Bryce Richards, Karlsruhe Institute of Technology (KIT)
ES11.1: Hot Carrier Solar Cells I
Tuesday AM, April 18, 2017
PCC North, 200 Level, Room 221 AB
10:30 AM - *ES11.1.01
Where is the Heat Gone? Recent Progress in Coupled Photovoltaic and Thermoelectric Conversion Using Hot Carriers
Jean Francois Guillemoles 1 2 3 Show Abstract
1 IRDEP, CNRS, Chatou France, 2 , IPVF, Antony France, 3 RCAST, NextPV, Tokyo Japan
In typical solar cells, close to half of the incident power is lost to heat. We will show how this loss can be partially offset using nanostructures where dissipation from carrier cooling can be slowed down, and from carrier collection can be minimized.
11:00 AM - *ES11.1.02
Solar Rectifying Antennas—A New Distinct Paradigm for Solar Power Conversion
Jeffrey Gordon 1 Show Abstract
1 , Ben-Gurion University of the Negev, Sede Boqer Campus Israel
This talk will explore the tantalizing prospect of using rectifying antennas for solar power conversion – an approach fundamentally distinct from photovoltaic or solar thermal power converters principally because antennas harvest only coherent radiation. We evaluate the fundamental limits for this novel solar power conversion strategy from basic principles of optics and thermodynamics.
Rectifying antennas constitute an established, high-efficiency power conversion technology for radio-frequency, microwave and, most recently, terahertz frequencies. Although sunlight is commonly viewed as incoherent – therefore ostensibly not suitable for antenna collection – all radiation exhibits spatial coherence on a sufficiently small scale.
The latest advances in the theory and experimental confirmation of basic performance bounds for aperture antenna harvesting, based on the partial coherence of broadband solar radiation, will be reviewed. This includes the first direct measurement of the spatial coherence of sunlight. The ramifications for using optical concentrators that can effectively replace orders of magnitude of antenna and rectifier elements will be discussed.
Even if the antenna harvesting of unpolarized, wide-spectrum solar radiation can be tackled, the question remains whether an incoherent, continuous, broadband signal can be rectified at reasonable efficiencies. Accordingly, we derive a basic upper bound on the ability to rectify any continuous broadband spectrum – including blackbody (most notably solar) radiation and white noise - and thereby confirm the promising potential for high-efficiency solar rectification.
Finally, a fundamental thermodynamic limit for coherence-limited solar converters at arbitrary optical concentration is derived – a generalization of the eponymous Landsberg limit for solar power conversion. In the process, the device dimensions and potential efficiency for high-performance solar rectifying antennas are established.
11:30 AM - *ES11.1.03
Generating and Exploiting Hot Carriers in a Metallic Solar Cell
James Dimmock 1 2 , Matthias Kauer 1 , Paul Stavrinou 3 , N.J. Ekins-Daukes 2 Show Abstract
1 , Sharp Labs of Europe, Oxford United Kingdom, 2 , Imperial College London, London United Kingdom, 3 , University of Oxford, Oxford United Kingdom
The hot carrier solar cell is a device designed to exploit a temperature gradient in the electron population following illumination, the aim being to use the gradient as a driving force for electron extraction while limiting heat transfer. We have previously demonstrated hot carrier extraction from semiconductor-based devices but argued that such devices are limited by low total broadband absorption. The relatively fast electron cooling time in conventional semiconductors limits the absorbing layer thickness and consequently the total absorption.
One method to overcome this limitation is to use a metallic region as the light absorbing layer. By careful impedance matching a metallic film to air, a broadband absorption more than 95% over the full visible and NIR region is achievable in a film as thin as 10nm. We will review the structures that lead to this high absorption and describe two methods to extract the resulting excited electrons to form a heat driven photovoltaic cell: firstly extraction over a Schottky barrier and secondly extraction by tunnelling from the metal layer.
Two cells have been experimentally developed based on these different extraction principles, using a Chromium absorber on an AlGaAs/GaAs heterostructure. We will compare the characteristics of cells operating using these two extraction methods theoretically and experimentally to reveal their operating principles. In particular, dual-beam illumination of the samples at various paired wavelengths, and the resulting photocurrent and differential photocurrent, is used to provide evidence for hot carrier generation and exploitation in these structures.
ES11.2: Hot Carrier Solar Cells II
Tuesday PM, April 18, 2017
PCC North, 200 Level, Room 221 AB
1:30 PM - *ES11.2.01
Hot Carrier Cooling Mechanisms in Multiple Quantum Wells
Gavin Conibeer 1 Show Abstract
1 School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales, Australia
The Hot Carrier solar cell has the potential to yield a very high efficiency, well over 50% under 1 sun. Multiple quantum wells have been shown to have significantly slower hot carrier cooling rates than bulk material and are thus a promising candidate for hot carrier solar cell absorbers. However, the mechanism(s) by which hot carrier cooling is restricted is not clear. Presented is a systematic study of carrier cooling rates in GaAs/AlAs MQW with either varying barrier or well thickness. These allow a determination as to whether the mechanisms of either a reduction in hot carrier diffusion; a localisation of phonons emitted by hot carriers; or mini-gaps in the MQW phonon dispersion are responsible for reduced carrier cooling rates. Results will be placed in the context of use of MQW as absorbers in recent hot carrier cell results which show power generation by hot carriers.
2:00 PM - *ES11.2.02
Nonequilibrium Electron and Phonon Dynamics in Advanced Concept Solar Cells
Stephen Goodnick 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
The realization of advanced concept solar cells that circumvent the assumptions inherent in the Shockley-Queisser limit depends strongly on the competition between carrier energy relaxation processes to the lattice and high energy processes that do useful work. Nanostructured systems offer advantages in terms of reduced channels for energy relaxation in reduced dimensional systems, and the possibility of bandgap engineered structures for improved collection and charge separation. Here we use ensemble Monte Carlo simulation of electrons and holes to investigate the role of ultrafast carrier processes in the realization of two advanced concept devices, hot carrier capture and multi-exciton generation. The particle based simulation approach includes the electron-phonon scattering in quantum wells and quantum wires, intercarrier scattering, and nonequilibrium phonon effects.
In the case of hot carrier solar cell architectures, we have simulated carrier relaxation dynamics including the dynamical nonequilibrium population of optical phonons that are generated in III-V quantum well structures through primarily polar optical phonon emission, assuming a phenomenological anharmonic phonon decay time from an optical mode to two acoustic modes. With increasing phonon lifetime, simulation results show an evolution of energy loss dominated by the polar emission rate, to one dominated by the anharmonic decay time, depending on the injected carrier density. While typical III-V bulk materials have short anharmonic decay times, less that 10 ps, recent experimental work has indicated slow decay in ‘phononic bandgap’ materials like InN, where the separation of the optical and acoustic modes is large enough to suppress the dominant two phonon decay process.1
We also investigate the role of impact ionization in nanowires relative to multiexciton generation using an atomistic tight binding representation of the nanowires, and calculating the full multi-subband impact ionization rate from perturbation theory. The short time carrier dynamics in nanowires under varying photoexcitation conditions are investigated using a full band Cellular Monte Carlo (CMC) simulation based on an atomistic representation.2 The CMC also includes the scattering rates due to optical and acoustic phonons that tend to the bulk material scattering rates for larger nanowire widths. The effect of impact ionization is shown to be prominent smaller width nanowires due to the increase in the band gaps as the nanowire width decreases. The percentage of electrons undergoing an impact ionizing event, thereby creating multiple electron hole pairs, is also shown to drastically above the threshold given by twice the bandgap of the nanowire, showing a strong potential for multiexciton generation in such systems.
1. Y. Zhang et al., Applied Physics Letters 108, 131904 (2016)
2. R. Hathwar, M. Saraniti, and S. M. Goodnick, J. Appl. Phys. 120, 044307 (2016)er cooling are simulated.
2:30 PM - ES11.2.03
Type-II Quantum Well Absorbers—Candidate Systems for Hot Carrier Solar Cells
Hamidreza Esmaielpour 1 , Jinfeng Tang 1 , Vincent Whiteside 1 , Sangeetha Vijeyaragunathan 1 , Tetsuya Mishima 1 , Michael Santos 1 , Bin Wang 2 , Ian Sellers 1 Show Abstract
1 Homer L. Dodge Department of Physics & Astronomy, Nielsen Hall, University of Oklahoma, Norman, Oklahoma, United States, 2 School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma, United States
Hot carrier solar cells have been proposed as devices, which can increase the conversion efficiency of a single junction solar cell above the Shockley-Queisser limit. For practical implementation of such systems, hot carrier solar cells require absorber materials in which the high-energy (hot) photogenerated carriers are inhibited prior to thermalization; typically, by reducing the efficiency of phonon relaxation pathways. Here, the potential of using type-II quantum-wells is investigated as a candidate system for hot carrier absorbers. Continuous wave power and temperature dependent photoluminescence measurements are presented that show a transition in the dominant hot carrier relaxation process: from conventional phonon-mediated carrier relaxation at low temperature; to a regime where: radiative recombination dominates the process at elevated temperatures . A reduction in the PL intensity is strongly coupled to an increase in the hot carrier temperature extracted from the measurements. This behavior is attributed to a build-up of electrons in the type-II QWs facilitated by increased hole mobility at elevated temperatures; this is also supported by anomalous thermalization coefficients above 90 K despite the prevalence of phonon relaxation channels at elevated temperatures . Temperature dependent transmission measurements will also be presented to demonstrate the effect of the absorption with increasing optical excitation. In addition to the optical measurements described, our recent analysis of type-II QWs in p-i-n diode structures, and the associated effects upon photocurrent and voltage will, also be discussed.
 J. Tang, V. R. Whiteside, H. Esmaielpour, I. R. Sellers et al., Appl. Phys. Lett. 106, 061902 (2015)
 H. Esmaielpour, V. R. Whiteside, J. Tang, I. R. Sellers et al., Progress in PV: Res. & Apps., 24, 591-599 (2016)
2:45 PM -
ES11.3: Soalr Cell Optics I
Tuesday PM, April 18, 2017
PCC North, 200 Level, Room 221 AB
3:30 PM - *ES11.3.01
Multi-Resonant Light-Trapping in Ultrathin Solar Cells
Stephane Collin 1 Show Abstract
1 , CNRS, Marcoussis France
We propose a new paradigm for light trapping in solar cells. It aims at reaching nearly perfect optical absorption over a broad spectral range, with a number of resonant modes in periodical structures. We show that multi-resonant absorption can exceed the lambertian light-trapping limit. We present a simple theoretical model, numerical examples, and our latest experimental results with record efficiency achieved for ultrathin GaAs solar cells.
4:00 PM - ES11.3.02
Durable Broadband Graded-Index Fluoropolymer Antireflection Coatings for Plastic Optics
Baomin Wang 1 , Jared Price 1 , Noel Giebink 1 Show Abstract
1 , Pennsylvania State University, State College, Pennsylvania, United States
Anti-reflection (AR) coatings are crucial for minimizing optical loss in high efficiency concentrating photovoltaic (CPV) systems; however, a high performance, environmentally-robust strategy for plastic optics such as acrylic Fresnel lenses remains an ongoing challenge for the CPV community. Here, we demonstrate simple, two-layer fluoropolymer AR coatings that reduce the solar spectrum-averaged (400 < λ < 1600 nm) reflectance of acrylic plastic from ~3.9% to ~0.4% over a wide range of incidence angles. The coating layers are fabricated via glancing angle deposition in a thermal evaporator and exhibit controllable nanoporosity due to self-shadowing that enables their refractive index to be continuously tuned from n = 1.31 to n = 1.15 depending on the deposition angle. Owing to their fluoropolymer nature, the films exhibit very low dispersion and are transparent deep into the ultraviolet down to λ~200 nm.
The resulting AR coatings adhere strongly to common optical plastics such as polymethylmethacrylate and polycarbonate and survive repeated mechanical bend and compression cycles on substrates flexed to 1 cm radius. The coatings are also extremely hydrophobic, with a water contact angle >140° that supports anti-fogging and self-cleaning behavior, and they are unaffected by prolonged sonication in most organic solvents, acids, and bases. They exhibit no deterioration in AR performance after ten days of damp heat testing (T = 85° and RH = 85%) nor after one month of continuous (ongoing) rooftop outdoor exposure in central Pennsylvania. The coatings have been successfully applied to a variety of curved lens surfaces, with virtually no variation in AR performance for f-numbers as low as f/1. In particular, coating both sides of an f/2 acrylic Fresnel lens increases its solar spectrum-averaged transmittance from approximately 92% to 98%. Taken together, these results represent a significant development for plastic optics commonly used in CPV systems as well as more generally for broadband AR applications that demand extreme environmental, chemical, and mechanical durability.
4:15 PM - ES11.3.03
High Concentration Planar Microtracking Photovoltaic System Exceeding 30% Power Conversion Efficiency
Jared Price 1 , Alex Grede 1 , Baomin Wang 1 , Michael Lipski 1 , Brent Fisher 2 , Kyu-Tae Lee 3 , Xiaokun Ma 4 , Scott Burroughs 2 , Christopher Rahn 4 , John Rogers 3 , Noel Giebink 1 Show Abstract
1 Department of Electrical Engineering and Computer Science, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 , Semprius, Inc., Durham, North Carolina, United States, 3 Department of Materials Science and Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois, United States, 4 Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Concentrating photovoltaics (CPV) have long been pursued as a route to make ultra-high efficiency photovoltaic power economical; however, existing dual-axis tracked CPV is incompatible with deployment on rooftops and other constrained-space environments, which represent a significant fraction of worldwide photovoltaic installations. Embedded planar microtracking CPV, in which microscale photovoltaic cells translate relative to a fixed optical system have been proposed as a means to resolve this challenge and realize CPV in the form factor of standard photovoltaic panels.
Here, we demonstrate a fully automated prototype microtracking CPV system that operates with a 650 μm square triple junction photovoltaic cell at a geometric concentration, G=743x. In this system, the microcell translates laterally, sandwiched between a pair of refractive top and reflective bottom glass lenses in a catadioptric configuration that effectively eliminates Petzval curvature at the intermediate focal plane to enable >80% measured optical efficiency for solar incidence angles over a 140° full field of view. The overall concentrator stack is <1 cm thick and the system tracks over an entire day with a net lateral displacement of <2 cm. Tracking is accomplished by an algorithm that combines feed-forward coarse positioning based on the known solar position and feedback fine tuning based on the microcell output current. In outdoor testing on a clear sunny day at latitude tilt in central Pennsylvania, the system achieved >30% power conversion efficiency and generated >50% more energy per unit area over the entire day than an adjacent monocrystalline Si cell with 19% rated power conversion efficiency. These results affirm the technical feasibility of microtracking CPV to deliver high efficiency fixed-tilt rooftop solar panels at commercially relevant concentration ratios.
4:30 PM - ES11.3.04
Measuring and Exploiting Optical Anisotropies in Nanophotonic Photovoltaics
Jon Schuller 1 Show Abstract
1 , University of California, Santa Barbara, Santa Barbara, California, United States
Nanophotonic coatings and structures provide an attractive approach for enhancing light-matter interactions in advanced photovoltaics. A little-discussed aspect of photonic architectures is the presence of strong anisotropies in the electromagnetic field enhancements. These anisotropies are of particular importance in relation to nanostructured materials that possess intrinsic structure-dependent optical anisotropies. In this talk, we describe a novel class of momentum-resolved spectroscopies that provide new insight into structure-dependent optical properties of thin-film material and discuss approaches to exploit these effects in photovoltaic devices that incorporate nanophotonic enhancements.
Specifically, we use Fourier imaging techniques to measure or control the momentum distribution of in-coming or out-going light rays respectively. These techniques are applied to test-case organic photovoltaic materials that can be deposited with distinct morphologies depending on processing conditions. Using momentum-resolved photoluminescence, we determine the morphology-dependent orientation of transmission and absorption dipoles. We subsequently demonstrate the use of momentum-resolved reflectometry to perform “model-free” measurements of optical constants. The approach provides precise and accurate optical constants with quantified error estimates, obviating the complications associated with highly model-dependent, multi-parameter spectral fitting procedures used in ellipsometry. We conclude by describing ongoing efforts to exploit optical anisotropies to enhance light absorption in nanostructured photovoltaics.
4:45 PM - ES11.3.05
Dielectric Resonator-Based Antireflection Coatings
Dongheon Ha 1 2 3 , Chen Gong 2 4 , Marina Leite 2 4 , Jeremy Munday 2 3 Show Abstract
1 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland, United States, 3 Electrical and Computer Engineering, University of Maryland, College Park, Maryland, United States, 4 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States
To increase light absorption within solar cells, many researchers have investigated nanoscale surface patterning (e.g. nanocone, nanodome, black silicon, etc). These approaches improve current density through absorptivity enhancement. However, nanostructuring can cause a significant decrease in the open circuit voltage due to increased surface area and surface recombination, which reduces the total power conversion efficiency. Here, we introduce a novel antireflection coating based on SiO2 nanosphere arrays that leads to an absorptivity enhancement of ~20% without surface texturing of the active material . Because this coating does not require direct surface patterning, the solar cell’s optoelectronic response remains high. We present experimental measurements and finite difference and time domain (FDTD) calculations that show the absorptivity enhancement is due to the combination of excited whispering gallery-like modes within the nanospheres and thin-film interference effects. Nanoscale optoelectronic measurements are also performed on a Si solar cell containing a conventional Si3N4 antireflection coating with the addition of the SiO2 nanosphere array on top, and an additional ~5% photocurrent enhancement is achieved at a wavelength corresponding to whispering gallery-like mode excitation. From FDTD calculations, we also show that the wavelengths for mode excitation can be tuned by changing the diameter of the nanospheres and/or the spacing between the nanospheres. As this antireflection coating is made with the Meyer rod rolling technique, which is a scalable and room-temperature fabrication processes, this concept could be used as a replacement for conventional thin-film antireflection coatings that require complicated high-temperature vacuum deposition processes.
 Dongheon Ha, Chen Gong, Marina S. Leite, and Jeremy N. Munday, “Demonstration of resonance coupling in scalable dielectric microresonator coatings for photovoltaics,” ACS Applied Materials and Interfaces, 8 (37), 24536-24542, 2016 (Cover)
ES11.4: Poster Session I: Advanced PV Materials and Devices
Tuesday PM, April 18, 2017
Sheraton, Third Level, Phoenix Ballroom
8:00 PM - ES11.4.01
Alternative Low Cost Conductors for Photovoltaic Applications
Myong Jae Yoo 1 Show Abstract
1 , KETI, Seongnam Korea (the Republic of)
The current trend in Photovoltaic applications still use high cost silver powders for forming fingers and busbars. It is anticipated that screen printing technology will stay mainstream for metallization for front and rear side. Thus in order to increase the efficiency of PV cells current trend is focused on pushing screen printing to new levels. This requires reduced finger sizes of below 45 microns with line resistivity of 10-6 order with good contact resistivity as conductor material. Altogether with the above mentioned electrical and formability requirements lower cost per cell is required for competiveness in PV markets. In order to meet the market and technological requirements many alternative materials to silver for forming conductor materials has been investigated
In this study, copper alloy and silver coated copper has been used to formulate low cost conductor materials. Also to prevent oxidation of copper alloy and silver coated copper powders various types of additives was used and their effect on electrical properties was studied. The selected low cost conductive powder and additives was formulated into paste for screen printing process. Ethyl cellulose resin and appropriate dispersants was selected and thixotropic agents was used to optimize paste properties. Formulated pastes were printed on silicon wafers and finally at maximum firing temperatures of 800 celcius the conductive lines were formed and their properties investigated. So far we have been able to obtain line resistivity of 10-5 order with good contact with silicon wafer with line width of 50 micron. Further study is expected to obtain line resistivity comparable to conductors using silver powder with line width below 40 microns.
8:00 PM - ES11.4.02
Mathematical Modeling of Silicon Doping by Neutron Transmutation Doping Method for High Efficient Solar Cells
Valery Dshkhunyan 3 , Alexander Diakov 2 , Sergey Karabanov 1 , A. Kozlov 2 , Dmitry Markov 2 , Masahiro Hoshino 4 Show Abstract
3 , Solar Consult Ltd., Ryazan Russian Federation, 2 , Institute of Nuclear Materials JSC, Ekaterinburg Russian Federation, 1 , Ryazan State Radio Engineering University, Ryazan Russian Federation, 4 , Japan Semiconductor Engineering Consulting, Saitama Japan
It is known that n-Si solar cells have higher efficiency than p-Si solar cells. It is connected with the fact that n-Si is less sensitive to regular impurities and defects than p-Si. This results in increase of charge carrier lifetime and therefore, in solar cell efficiency growth.
One of the problems connected with n-Si application for solar cell production is the difficulty of using Czochralski method for growing n-Si ingots, uniform in structure. It is known that only 30-40% of a grown ingot has the necessary resistance which is determined by doping uniformity. By comparison: when p-Si growing, 60-70% of ingot has the required resistance.
The present paper examines the possibility of production of n-Si ingots, uniform in resistance, by neutron transmutation doping (NTD) method for photovoltaics using mathematical modeling method.
The NTD method is based on formation of phosphorus atoms in a silicon ingot at neutron irradiation of the ingot.
The provided calculation data are obtained by MCU RFFI/A accounting code with DLC/MCUDAT-1.0 constant library developed by Russian Research Center “Kurchatov Institute”. The MCU accounting code is used for solution of the neutron-transport equation by Monte-Carlo procedure on the basis of estimated nuclear data for arbitrary three-dimensional geometry systems.
The present paper provides the estimation of doping uniformity along the ingot length by NTD method; doping uniformity along the ingot radius by NTD method; dependence of doping degree on neutron fluence.
The performed research results are as follows:
1. Distribution of the doping level and silicon resistance after irradiation along the ingot length and radius is established;
2. Regularities of silicon resistance change depending on duration and intensity of irradiation are specified;
3. The economic efficiency of NTD process for n-Si solar cells production is estimated.
8:00 PM - ES11.4.03
Silver Coated Copper Paste for Low Cost Silicon Solar Cells
Sung Hyun Kim 1 , Myong Jae Yoo 1 , Ji-sun Park 1 , Byeong-tak Park 1 , Ye Ji Yoo 1 , Hyeong Jin Son 1 Show Abstract
1 , Korea Electronics Technology Institute, Seongnam-si Korea (the Republic of)
Despite recently achieved reductions in consumption, screen printed silver used in solar cell front side metallization is still an important cost driver in solar cell production. Thus, in order to meet the market needs many alternative materials and processes to silver for forming conductor materials has been investigated.
In this study, silver coated copper containing 9 wt% silver has been used to substitute for low cost conductor material for front side metallization of solar cells. Also, various types of additives were used to prevent oxidation of silver coated copper powders during the co-firing and their effect on electrical properties was studied. Ethyl cellulose resin and appropriate dispersants was used to formulate a paste. Formulated pastes were printed on silicon nitride coated silicon wafers using screen print and fired at maximum temperatures of 800 degrees centigrade. So far we have been able to obtain the resistivity of low 10-5 Ωcm with good contact with silicon wafer with line width of around 50 micron. Further study is expected to obtain low resistivity comparable to silver paste.
8:00 PM - ES11.4.04
Low Temperature, Combustion Reacted Al Doped ZnO Coating on Ag Nanowire Transparent Electrode for Flexible Solar Cells
Min Kyu Park 1 , Seung Min Han 1 Show Abstract
1 , KAIST, Daejeon Korea (the Republic of)
helped in enhancing the performance and the lifetime of the Ag nanowire based transparent electrode for a solar cell. Unlike the traditional sol-gel process, the oxide phase was efficiently formed at relatively low temperature of 200oC by utilizing emitted heat from combustion reaction. AZO thin film fabricated by the combustion reaction showed relatively low conductivity value of 4x10-2 S/cm, but is sufficient to serve the role of transporting the charge carriers from active layer to the Ag nanowire network. It should be noted that the low processing temperature of below 200oC is required for depositing AZO layer on top of already existing Ag nanowires that are on flexible polymeric substrates for flexible solar applications. Using the optimized reaction conditions, a uniform coating on top of the Ag nanowires was obtained and the chemical stability was also enhanced due to the presence of AZO layer, which acts as a diffusion barrier for oxygen. The Ag nanowire/AZO composite transparent electrode developed in this study was demonstrated to have excellent electrical and optical performance of 10ohm/sq and 90% at 550nm.
8:00 PM - ES11.4.05
Study of Electrical Properties Derived from Optical Constants for Transparent Composite Electrode
Aditya Yerramilli 1 , Terry Alford 1 Show Abstract
1 , Arizona State University, Scottsdale, Arizona, United States
Transparent conducting oxides (TCO) are elemental for several optoelectronic applications such as light emitting diodes, flat panel displays, solar cells, and organic light-emitting diodes. Transparent composite electrodes (TCEs) which are usually a combination of dielectric/metal/dielectric layers are a further advancement over TCO’s in the way of improving the optical and electrical properties by exploiting the interface between dielectric and metal layers. In this study, TCEs of TiO2/Ag/TiO2 were deposited via sputtering with varying thickness of Ag metal layer from 5–13 nm. A critical thickness of 9.5 nm is the necessary thickness needed to form a contiguous layer as opposed to isolated islands. Optical constants, refractive index (n) and extinction coefficient (k) are extracted via ellipsometry analysis of measured transmittance and reflectance of the sample. Using Drude Model equations for free electrons in metals, electrical properties (i.e., effective resistivity, carrier concentration, and mobility) are derived from the experimentally determined optical properties. These theoretically derived property values are more than 90% similar to measured resistivity, and 80% for measured carrier concentration and mobility values for three thickness values, at and above the Ag critical thickness. Investigating the results revealed that the derived results match the measured results at increasing wavelengths for increasing thickness. This can be concluded as a consequence of absorption being more effective than scattering for thicknesses higher than critical value at longer wavelengths.
8:00 PM - ES11.4.06
Hydrogenated Indium Oxide without Introduction of Water during Sputtering
Nathan Rodkey 1 , Mathieu Boccard 1 , Zachary Holman 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
High-mobility IO:H (hydrogenated indium oxide) relies on the introduction of water during the sputter process. However, water is technologically challenging to control and thus affects its large scale manufacturability. We demonstrate that by using molecular hydrogen and oxygen, IO:H films of similar electrical properties can be made. A sputter tool equipped with a residual gas analyzer monitors the partial pressure of hydrogen and water in the chamber to draw parallels between gas composition and electrical properties. It was found that a hydrogen pressure around 107 torr could yield mobilities as high as 100 cm2/Vs – post anneal. A simulation was then used to determine optimal finger-spacing, sheet resistance, and thickness for incorporating the film in a heterojunction structure. Cells employing these film characteristics were then made and compared to cells that employed a traditional ITO (indium tin oxide) layer as their transparent conducting oxide.
8:00 PM - ES11.4.07
Reduction of Zn2+/dye Complex Formation by Controlled Soaking Time in Dye Solution
Kichang Jung 1 , Yaqiong Li 1 , Minerva Uribe 1 , Alfredo Martinez-Morales 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
Crystal aggregation of the Zn2+/dye complex is a known issue for dye-sensitized solar cells (DSSCs) made of ZnO nanorods (NRs). The formation of Zn2+/dye complex impedes the effective electron transfer from the dye to the ZnO photoelectrode. Although ZnO possesses high electron mobility and low recombination rate, DSSCs using ZnO show relatively lower solar light to electric energy conversion efficiency than those devices using TiO2 due to the Zn2+/dye formation. The instability of ZnO in acidic dye solution is expected to induce the formation of Zn2+/dye complex because the bonding between Zn and O atoms are broken at the interface of the ZnO and the acidic dye solution. Eventually, Zn atoms are oxidized and released from the ZnO material, forming the Zn2+/dye complex, by combining with the dye molecules. In this research, we focused on reducing the formation of Zn2+/dye complex to improve the efficiency of DSSCs using ZnO NWs grown by chemical vapor deposition (CVD).
The ZnO NRs were synthesized on fluorine-doped tin oxide (FTO) substrate by CVD method in a tube furnace, using Zn powder as the precursor. The synthesis was conducted under 450°C with a mixture of oxygen and nitrogen gas (1:25 ratio by volume). The synthesized ZnO NRs were immersed into the N719 solution in ethanol, at room temperature. By adjusting the soaking time, the amount of Zn atoms released from the ZnO material was controlled. The electrolyte based on iodide/triiodide redox couple was used as a hole transport layer. The Pt on the FTO:glass substrate was used as a counter electrode. As a baseline reference, a device using TiO2 as its photoelectrode (formed by doctor blading with DSL18NR-T paste), was fabricated to compare against the efficiency of the ZnO devices.
The photovoltaic characterization of the fabricated DSSCs showed that the short circuit current of ZnO devices is relatively lower than that of TiO2, while the open circuit voltage was similar. Specifically, the ZnO cells where the immersed time was the longest showed the lowest short circuit current. From our results, we were able to confirm that the formation of Zn2+/dye complex between the ZnO NRs negatively impact the efficiency of the ZnO devices.
 Miroaki Horiuchi, J. Phys. Chem. B 2003, 107, 2570-2574
 Karin Westermark, J. Phys. Chem. B 2002, 106, 10102-10107
8:00 PM - ES11.4.08
Preventing Formation of Zn2+/dye Compound by Using CuO as a Protective Layer on ZnO Photoelectrode for Dye-Sensitized Solar Cells
Kichang Jung 1 , Yaqiong Li 1 , Minerva Uribe 1 , Alfredo Martinez-Morales 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
One of the biggest issues for dye-sensitized solar cells (DSSCs) using ZnO as the photoelectrode material is the dissolution of Zn2+ in the acidic dye solution (i.e. N719). The dissolved Zn2+ forms a Zn2+/dye compound which inhibits electron transport from the dye to the ZnO photoelectrode . The amount of formed compound increases with the soaking time of ZnO in the dye solution. In the literature, it has been reported that conversion efficiency decreases when the ZnO photoelectrode is soaked into the acidic dye solution for longer than 1 hour . However, the efficiency of ZnO-based DSSCs is also limited to the amount of dye molecules that can attach on the surface of ZnO. Therefore, the development of a ZnO hybrid material that can endure the acidic nature of the dye solution for a prolonged time, has the potential to increase efficiency of ZnO-based DSSCs.
In this work, ZnO photoelectrode is modified by a CuO layer to prevent Zn2+/dye compound formation at the interface. The CuO is synthesized by thermal oxidation. In order to confirm the effect of CuO layer, the DSSCs using ZnO and ZnO-CuO have been compared. The scanning electron microscope images show the formation of CuO nanoparticle on the ZnO surface. The X-ray diffraction results shows crystallized CuO is formed with  and [-111] direction preference. The current-voltage (I-V) characteristics of DSSCs using ZnO and ZnO-CuO with respect to various soaking time are analyzed. The I-V curve shows the relationship between soaking time and efficiency for the DSSCs using ZnO and ZnO-CuO.
 Ryuzi Katoh, J. Photochem. Photobiol, A 2004, 166, 69-74
 Karin Keis, Langmuir 2000, 16, 4688-4694
8:00 PM - ES11.4.09
Three-Dimensional Compositional Analysis of III-V Alloys with Clustering Behavior for Implementation in High Efficiency Photovoltaics
Nicole Kotulak 1 , Keith Knipling 1 , Louise Hirst 1 , Stephanie Tomasulo 1 , Josh Abell 1 , Maria Gonzalez 1 , Michael Yakes 1 , Jerry Meyer 1 , Robert Walters 1 Show Abstract
1 , U.S. Naval Research Laboratory, Washington, District of Columbia, United States
High efficiency, multi-junction photovoltaic devices demand precision in both device design and material quality in order to meet intended performance metrics. As targeted record efficiencies reach higher levels, new device designs require materials with tailored optical and electronic properties that will be able to perform the necessary functions within the device. By using lattice-matched semiconductor alloys composed of multiple elemental species, and by varying the composition of the constituents, the correct bandgap combinations for optimal photon absorption – or transparency – can be achieved. These engineered materials, however, can also prove challenging to grow due to the immiscibility of certain constituent species. This leads to defects incurred during growth, which can alter the material’s fundamental optical and electronic properties enough to negate the efficacy of implementing it within a device structure.
One manifestation of defect formation is the clustering of elements, for which the identification of size, shape, distribution, and composition can be used to understand not only the effect of clustering on the optical and electronic properties, but also how to mitigate said clustering during growth. In this work, we address the quaternary alloy InAlAsSb, which is intended for use in the high bandgap subcell of a multi-junction device lattice-matched to InP. This material has presented luminescence characteristics, as well as results of microscopy analyses, that indicate the existence of clusters within the material. Atom probe tomography (APT), a technique that allows for three-dimensional composition mapping of a material with sub-nm resolution, is well suited to the identification of such clusters. Using APT, the size and shape of the clusters can be identified in three dimensions. Reconstructions allow for the determination of any patterns for cluster formation throughout the bulk material, and identify cluster composition. All of this information can be used to facilitate an understanding of the growth mechanisms that lead to cluster formation in InAlAsSb. The method is also applicable to the study of other semiconductors exhibiting similar behavior.
8:00 PM - ES11.4.10
Design and Modeling of InGaN-Based Concentrator Solar Cells under High Temperature
Yi Fang 1 , Dragica Vasileska 1 , Stephen Goodnick 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
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 C). 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 eV 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, mechanical multi-junction solar cell designs and thick InGaN design with multiple tunnel junctions 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 C with an incident solar radiation concentration of 200 suns. Models are validated under the latest fabricated MQW (Multiple Quantum Well) devices for top cell. Optical management including back reflector and ARC have been implemented to reduce the front reflection and back transmission loss. Thus, we demonstrate that 2J tandem solar cells made in InGaN material system are very suitable for concentrated solar power hybrid system.
8:00 PM - ES11.4.11
The Band Gap Bowing of Thick InGaN Alloys for Photovoltaic Applications
Alec Fischer 1 , Joshua Williams 1 , Fernando Ponce 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
The first report on growth of InGaN alloys was published by Osamura back in 1972.1 Although the material was polycrystalline, Osamura determined that the band edge energy depended on the indium content nonlinearly. The nonlinear value is typically called bowing parameter and equals to the coefficient in the second-order term in the quadratic polynomial of the energy band gap as a function of indium content. It was not until 1995, one year after the first blue InGaN-based laser was developed that researchers refocused their attention on determining the bowing parameter across the whole InGaN compositional range. Unfortunately, poor quality and inaccurate determination of the indium composition and energy band gap resulted in a wide range of bowing values from 1 to 6 eV. In 2002, Wu et al. reported on a 0.7 eV band gap for InN which corrected the overestimated bowing parameter to about 1.43 eV.2 However, most of the published work have used X-ray diffraction (XRD) to determine the indium content in the InGaN film. This experimental technique is rather inaccurate as the strain state and Poisson ratio of the InGaN film is difficult to measure, and one typically assumes fully strained or fully relaxed layers to calculate the indium content in the film. Rutherford backscattered spectroscopy (RBS) measures the density of atoms of a crystal indifferent from the strain state of the film.
The determination of the band gap and the indium composition are crucial for accurately determining the bowing parameter. The use of the photoluminescence peak emission in InGaN films may underestimate of the real band gap due to the presence of large Stokes shifts across the whole composition range. Optical spectroscopy such as absorptance or photoreflectance have shown to be the most accurate way to determine the band gap.
In this talk, we report on the bowing parameter of high quality thick InGaN film measured by photoluminescence (emission) and optical spectroscopy (absorption), where the indium content was determined by RBS. The emission and absorption values give rise to two different bowing parameters which result from the Stokes shifts due to the potential fluctuations in the conduction and valence bands. The bowing parameter for unstrained InGaN films across the whole compositional range is determined by taking into account the degree of relaxation of the InGaN film with increasing lattice mismatch.
1 Osamura, et al., Solid State Communications 11, 617 (1972).
2 Wu, et al., Appl. Phys. Lett. 80, 4741 (2002).
8:00 PM - ES11.4.12
Defect-Tolerant BiI3 Films for Photovoltaic Applications
Yoon Myung 1 , Xing Huang 1 , Taehun Kim 1 , Rohan Mishra 1 , Parag Banerjee 1 Show Abstract
1 , Washington University in St. Louis, St. Louis, Missouri, United States
Bismuth triiodide (BiI3) has a band gap of 1.7 eV and a large dielectric constant, which makes it a promising solar absorber material in photovoltaic devices.1 However, there is limited understanding of the fundamental electronic transport and optical properties of BiI3, which is necessary to realize the potential of BiI3 as a solar absorber material.
In this work, we characterize fundamental electronic transport properties of BiI3 thin films. BiI3 thin films are formed using vapor phase transport deposition of BiI3 powder heated in a tube furnace set at 420 oC. The films are structurally characterized using UV-vis spectroscopy, X-ray diffraction, X-ray/ultra-violet photoelectron spectroscopy and aberration corrected scanning transmission electron microscopy confirming the presence of high quality BiI3 films with large grain sizes (> 100 µm).
Schottky junction based devices are formed by sandwiching BiI3 thin films between Au and indium tin oxide electrodes (ITO). Temperature-based current-voltage characteristics are measured and the barrier height between Au and BiI3 is established. While no conductance is detected in the dark, conductance under white light illumination as a function of temperature, yields the activation energy for charge conduction of 456 meV. This high value suggests charge transport is ionic. Thermal admittance spectroscopy is used to characterize the defect densities and energy position of the defects within the bandgap of the BiI3. Surprisingly, the admittance spectroscopy is unable to detect any defect states within the depletion width of the Schottky junction suggesting that BiI3 is highly defect tolerant.
First-principles density functional theory (DFT) calculations show Bi interstitials to be the constitutional defect with a low or negative formation energy, which suggests that they are formed spontaneously. However, the Bi interstitials do not show any deep level defect states consistent with the electrical measurements confirming the defect-tolerant behavior. The calculations also show that the heavy Bi interstitials have a small migration barrier of 0.25-0.75 eV depending on their charge state. Therfore, they can migrate easily within the layered structure of BiI3 on photexcitation, which confirms the experimentally observed ionic transport. Overall, our results show the promise of BiI3 as a defect-tolerant material for photodetector and solar cell applications.
1. Lehner, A. J., Wang, H., Fabini, D. H., Liman, C. D., Hébert, C.-A., Perry, E. E., Wang, M., Bazan, G. C., Chabinyc, M. L. & Seshadri, R. Electronic structure and photovoltaic application of BiI3. Applied Physics Letters 107, 131109, doi:10.1063/1.4932129 (2015).
8:00 PM - ES11.4.13
Fabrication of Three-Dimensional Hybrid Nanostructure-Embedded ITO and Its Application as a Transparent Electrode for High-Efficiency Solution Processable Organic Photovoltaic Device
Jeong Won Kim 1 , Hwan-Jin Jeon 1 , Chang-Lyoul Lee 2 , Chi Won Ahn 1 Show Abstract
1 , National NanoFab Center (NNFC), Daejeon Korea (the Republic of), 2 , Advanced Photonics Research Institute (APRI), Gwangju Korea (the Republic of)
In the work, 3D hybrid nanostructure-embedded ITO substrates were fabricated using a combination of secondary sputtering lithography and tilted deposition processes. These substrates were used as anodes in SP-OPVs. The fabricated 3D hybrid nanostructure-embedded ITO substrates maintained their inherent superior opto-electrical properties after the surface-structuring process. The SP-OPV prepared using the 3D hybrid nanostructure-embedded ITO displayed a 10% enhancement in the photovoltaic performance compared to the SP-OPV, which had been prepared with flat ITO anode. The enhancement in the photovoltaic performances resulted from (1) an improved charge transport efficiency through efficient interfacial connections between the hole collecting ITO electrode and the photo-active layer, (2) an improved charge collection (extraction) efficiency through good energy level matching between ITO and hole transporting layer due to the presence of thin Au islands, and (3) enhanced light absorption in the photo-active layer due to the light scattering and/or the surface plasmon effects by thin Au islands of 3D nanostructures.
8:00 PM - ES11.4.14
High Efficiency Small Molecular Solar Cells with New Fullerene-Free Acceptor
Min Jae Sung 1 , Yeon Hee Ha 1 , Hyosung Choi 2 , Soon-Ki Kwon 1 , Yun-Hi Kim 1 Show Abstract
1 , Gyeongsang National University, Jin Ju Korea (the Republic of), 2 , Hanyang University, Seoul Korea (the Republic of)
Solution-processed organic solar cells (OSCs) offer the attractive prospect of light-weight, low-cost and sustainable solar energy production. OSCs with the most promising bulk-heterojunction (BHJ) architecture have achieved power conversion efficiency (PCE) close to 11%. Meanwhile, fullerene-free OSCs have also been advancing by the need to detect alternative acceptors that overcome poor bandgap tenability, limited optical absorption, the high synthetic costs and morphological uncertainty of fullerene derivatives including such as phenyl-C61-butyric acid methyl ester (PC60BM) . Among the various classes of fullerene-free acceptors, Naphthalene diimide derivatives (NDIs) were some of the most and earliest common fullerene-free acceptors studied in BHJ OSCs because of their strong absorption ability, high electron mobility, and high environmental, thermal stability. Small molecules(SM) have distinct advantages such as significant high purity molecular structures, intermolecular arrangements generally known by easy synthesis and purification methods, crystallographic analysis, better batch-to-batch reproducibility, a high open circuit voltage (VOC) relatively high charge carrier mobility. In this study, we designed new class of acceptor-donor-acceptor-donor-acceptor(A-D-A-D-A) type SM acceptor with NDI as central core unit. It is expected that the new class NDI-SM acceptors will be efficient acceptors because they have high electron mobility, high electron affinity and stability, and appropriate LUMO energy level, which is similar with that of fullerene derivatives for efficient energy off-set between LUMO levels of donor and acceptor. The devices with NDI-SM acceptor exhibited the high fill factor of 0.69 and PCE of 3.07%
8:00 PM - ES11.4.15
Enhancement of Power Conversion Efficiency for A-D-A Type NDI-Based Small Molecule as Non-Fullerene Acceptor for Solution-Processed Organic Photovoltaics
Yeon Hee Ha 1 , Min Jae Sung 1 , Chan Eon Park 2 , Soon-Ki Kwon 1 , Yun-Hi Kim 1 Show Abstract
1 , Gyeongsang National University, Jinju Korea (the Republic of), 2 , POSTEC, Jinju Korea (the Republic of)
Chemical modification of fullerenes is limited and requires tedious purification procedures, developing non-fullerene electron-accepting and electron-transporting materials is desirable to enhance the performance of OPVs. Several groups have reported novel non-fullerene small molecules as electron acceptors in solution-processed OPVs. Naphthalene diimides (NDIs) are extensively utilized as dye materials, providing a unique variability in structure modification and a widely tunable absorption. Among the various p-conjugated building blocks utilized for organic optoelectronic devices, NDI is one of the most widely studied electron-accepting. We synthesized new non-fullerene acceptors with NDI as small molecules. A novel NDI-based small molecules are designed and synthesized by linking two NDI monomers via a four donor moiety. It is consisted of acceptor–donor– acceptor (A–D-A) type NDI building blocks with different donor abilities such as phenyl, xylene, bithiophene, tvt as donor unit. Organic photovoltaics by using NDI as the acceptors show a highest power conversion effi cency (PCE) of at least over 3%, which is the best result for NDI-based small molecular acceptors. We have optimized the morphology to enhancement.
8:00 PM - ES11.4.16
Improvement of Open Circuit Voltage by Adding the Cascade Materials in Ternary Bulk Heterojunction Photovoltaic Cells
Luyao Song 1 , Yaodong Guan 1 , Yajun Sun 1 , Yan Yu 1 , Jianfeng Zang 1 , Lei Ye 1 Show Abstract
1 School of Optical and Electronic Information, Huazhong University of Science and Technology, WuHan China
Searching alternatives to improve the attainable performance of organic photovoltaics is one of the most crucial issues toward renewable energy source. Here, the model ternary systems based on poly (3-hexylthiophene) (P3HT), [6, 6]-phenyl C61 butyric acid methyl ester (PC61BM) and a group of cyclopent[hi] aceanthrylenes (CPAs) derivatives, were designed to improve the existing conversion efficiency of P3HT/PC61BM solar cells. In all of these solar cells, although the ternary structures containing a group of cyclopent[hi]aceanthrylene derivatives as cascade materials in photoactive layer demonstrate the improvement of open-circuit voltage (VOC) compared to binary device of P3HT/PC61BM, the whole conversion efficiencies of these devices can not all be improved, thus indicating the different influence on the device metrics attributed by the different substituents around cyclopent[hi]aceanthrylene core. By extension, these results suggest that the ternary system with judicious selection of donor and acceptor components, which could be possible without sacrificing the simplicity of a single active layer processing step, can provide a potentially effective way to optimize the performance in BHJ solar cells.
8:00 PM - ES11.4.17
Effect of Chlorination for Efficient Non-Fullerene Polymer Solar Cells
Feng He 1 , Zhao Mu 1 , Pengjie Chao 1 Show Abstract
1 , South University of Science and Technology of China, Shenzhen China
Nowadays, while the inorganic based solar cells have already commercialized and matured for several years, the polymer based counterpart are still on their way to hit the 10% or even higher power conversion efficiency. Generally speaking, the power conversion efficiency is the multiply of open voltage, current density and fill factor. As is known to all that the absolute energy gap between the LUMO Levels of donor and acceptor should be no less than 0.3 eV. However, this thumb rule seemed having been challenged from recent reports. It should be an encouraging result if we can minimize the gap without deteriorates the charge dissociation, given the fact while the necessary gap provide the required driving force but at the same time it is one of the source of energy loss for larger open circuit voltage. In order to fulfill this aim, side chain decorating has been widely studied, so did the introducing of heteroatoms, such as fluorine. However, it is less studied the introducing of chlorine into the side chain. In this study, chlorination as an effective method to enlarge the open voltage was realized by chlorine substitution of either the electron rich or electron deficient part of PTB7-Th precursor. By combining this newly designed polymer with the widely used molecular ITIC, an evident enhancement of power conversion efficiency (PCE) up to 8.21% (no additive) with an open circuit voltage up to 1.03 V was realized, which is much higher than that of PTB7-Th counterpart (6.80%) with 0.25% DIO additive.
8:00 PM - ES11.4.18
Transparent Wide Bandgap Inorganic Halide Material for Hybrid Solar Cells
Karunakara Moorthy Boopathi 1 , Chintam Hanmandlu 1 , Chih-Wei Chu 1 Show Abstract
1 , Research Center for Applied Sciences, Academia Sinica, Taipei Taiwan
Low-cost semiconductor materials and facile fabrication routes for photovoltaic device have been crucial part of photovoltaic materials research. Here, solution processable wide bandgap lead iodide semiconductor with different organic polymers was investigated for hybrid photovoltaics. Tauc plot and X-ray diffraction showed, lead iodide having wide intrinsic bandgap of 2.6 eV, and exists in hexagonal crystalline structure with preferable orientation of (001) plane. Light intensity dependence of Voc and Jsc of device was studied to investigate the charge recombination mechanism. The power conversion efficiency of 2.1% was achieved while using opaque electrode (Ca/Al), 0.75% (bottom illumination) and 0.67% (top illumination) were achieved while using transparent electrode (BCP/Ag/MoO3). Using of wide bandgap light absorber materials opens up a new path for making efficient, transparent photovoltaic devices for building integrated window applications.
8:00 PM - ES11.4.19
Cation-Controlled Aggregation in Fluorene-Triarylamine Sulfonate Copolymers
Meilin Li 1 2 , Stefan Adams 1 Show Abstract
1 Materials Science and Engineering, National University of Singapore, Singapore Singapore, 2 , Solar Energy Research Institute of Singapore, Singapore Singapore
Among organic semiconductors, the requirement of solution processability has led to a focus on conjugated polyelectrolytes, which combine the ability to transport excitons and charge due to the conjugated backbone and variable band gap light absorption and fluorescence with water solubility and processability and hence structurally controlled charge transport via tuneable ionic interaction and aggregation.
Fluorene-triphenylamine copolymers (e.g. poly(9,9’-di-n-oc-tylfluorenealt-N-(4-butylphenyl)-diphenyl-amine), TFB) have attracted significant attention as effective hole transporting materials (HTMs) in bulk heterojunction organic solar cells (OSCs), perovskite solar cells and organic light-emitting diodes (OLEDs). In this computational study, we focus on the combination of a model sulfonated fluorene-triarylamine copolymer anion poly((N-(4-butylphenyl)-diphenylamine)-alt-fluorene 9,9’di-n-propane sulfonate), BAFS, with a wide range of monovalent cations. Ionic aggregation due to the interaction between sulfonate groups and cations results in unique optoelectronic properties. Moreover, the performance of TFB as the HTM can in principle be tuned by controlling the ionic interactions, because the hole conductivity depends on ionic aggregation. Detailed information about the local structure of ion clusters in such systems is, however, difficult to obtain from experiments, so that atomistic Molecular Dynamics (MD) simulations provide the most viable approach to probe the morphology of ion clusters at atomic level.
Systematic MD simulations of the model fluorene-triarylamine sulfonate with various monovalent cations are conducted and the resulting trajectories are analysed for each of the conjugated polyelectrolyte focusing on ion aggregation, as well as its influence on the static and dynamic polymer morphology and on the charge carrier mobilities that determine the performance of such HTMs in OSCs and OLEDs. A pronounced variation of the degree of cation clustering with the cation field strength is found to control polymer morphology, cation mobility and thereby the time evolution of the Coulomb energy landscape for hole transport.
8:00 PM - ES11.4.20
A PCBM-Assisted Perovskite Growth Process to Fabricate High Efficiency Semitransparent SolarCells
Chao Li 1 2 , Joseph Sleppy 1 , Jayan Thomas 1 2 Show Abstract
1 , University of Central Florida, Orlando, Florida, United States, 2 Nanoscience Technology Center, University of Central Florida, Orlando, Florida, United States
Developing highly efficient perovskite solar cells in a simple and rapid fashion will open the window for their
potential commercialization. Semi-transparent devices are of great interest due to their attractive
application in building integrated photovoltaics (BIPVs). In this study, efficient perovskite solar cells with
good transparency in the visible wavelength range have been developed by a facile and low-temperature
PCBM-assisted perovskite growth method. This method results in the formation of a perovskite-PCBM
hybrid material at the grain boundaries which is observed by EELS mapping and confirmed by steadystate
photoluminescence (PL) spectroscopy, transient photocurrent (TP) measurements and X-ray
photoelectron spectroscopy (XPS). The PCBM-assisted perovskite growth method involves fewer steps
and therefore is less expensive and time consuming than other similar methods. The semitransparent
solar cells developed using this method exhibited power conversion efficiency (PCE) ranging from 12% to
4% depending on the average visible transmittance (AVT) ranging from 3% to 35%. The as-fabricated
semitransparent perovskite solar cell with an active layer thickness of only about 150 nm, which is only
less than half the active layer thickness of a typical perovskite solar cell, provided a PCE as high as 9.1%
with an AVT of 18% and more than 12% with an AVT of 3%.
8:00 PM - ES11.4.21
Investigation of Thermal Donors and Other Bulk Defects in n-Type Czochralski-Grown Si for High Efficiency Solar Cells
Apoorva Srinivasa 1 , Pradeep Balaji 1 , André Augusto 1 , Stuart Bowden 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
N-type silicon is used for high efficiency Silicon Heterojunction (SHJ) devices. It can also be used for Silicon tandem cells to achieve greater efficiencies. Efficiency of such devices which operate at the maximum power point (MPP) depends heavily on surface and bulk lifetimes. N-type Czochralski silicon is prone to a range of impurity related defects which can substantially reduce the bulk lifetime. Lifetime losses due to bulk defects lead to considerable efficiency losses. Thermal donors due to high interstitial oxygen concentration are reported to be one such factor that can degrade the bulk lifetime. It is also noticed that a fraction of the Silicon ingot inherently demonstrates low lifetimes which results in low yield. In this paper we investigate defects causing degradation of n-type wafers resulting in low bulk lifetimes on standard commercial grade n-type Czochralski silicon. We also analyze the effects of formation and annihilation of thermal donors in wafers. We use Fourier transform infrared spectroscopy (FTIR), deep-level transient spectroscopy (DLTS), capacitance-voltage (C- V), and resistivity measurements for characterization of bulk defects. High temperature annealing, rapid thermal annealing (RTA) and gettering are performed in an effort to improve bulk lifetime, so that wafers with inherent low lifetimes, often deemed unsuitable, could be used to achieve high efficiencies. Furthermore, temperature- and injection-dependent lifetime spectroscopy will be used to reveal the mechanisms of lifetime improvement.
8:00 PM - ES11.4.22
Silicon Heterojunction Solar Cells with MoOx Hole Selective Contact
Hisham Nasser 1 3 , Gamze Kokbudak 1 4 , Rasit Turan 1 2 Show Abstract
1 , The Center for Solar Energy Research and Applications GUNAM, Ankara Turkey, 3 , Middle East Technical University METU, Ankara Turkey, 4 Micro and Nanotechnology Graduate Program, Middle East Technical University METU, Ankara Turkey, 2 Physics, Middle East Technical University METU, Ankara Turkey
Photovoltaics (PV) currently generate about 1% of global electricity. To increase this percentage, the price per watt-peak of solar cells must continue to fall, requiring reduced manufacturing cost or increased cell efficiencies. Currently, silicon based solar cells account for more than 90% of the global PV market, leveraging this magnificent market share is one of the main motivations for reducing the fabrication cost of crystalline silicon (cSi) solar cells while keeping high conversion efficiencies. Yet, the attained efficiency record of conventional cSi cells is unlikely to increase significantly in the foreseeable future due to practical Auger limits. For single-junction high temperature heavily doped cSi solar cells, the previous efficiency record of 25% has been established back in 1998. This record stood for the last 16 years until 2014, when Panasonic announced their back-contact silicon heterojunction cell (SHJ, or HIT, heterojunction with intrinsic amorphous silicon, a-Si:H) with a record efficiency of 25.6%. HIT based solar cells hold set of asymmetric carrier-selective heterocontacts the p- and n-type a-SiH. So far, all efficiency record obtained from cSi material, including SHJ, still contain doped-silicon layers, which require intricate deposition optimizations and impose high fabrication costs. An alternative low-cost fabrication route is to introduce dopant-free, selective carrier contacts in the Si cells design, which yields selective hole and electron collections through the negative- and positive-polarity contacts, respectively.
The key concept underlying this work is the application of thermally evaporated MoOx thin films as the emitter of n-type cSi absorber instead of high temperature thermally fabricated boron-doped (p+) silicon layer in conventional p-n junction cSi solar cell. We further investigate the potential of MoOx; inserted between the rear surface and Al contact, to be used as BSF at the rear of n-p cSi solar cell solar formed by the phosphorus diffusion into the p-type cSi.
Herein, we investigate the effect of varying thermally evaporated MoOx film thickness, post deposition treatment conditions, and the doping concentration of cSi wafer on the photovoltaic behavior of n-type Si heterojunction solar cells. The photovoltaic behavior of the fabrication Si heterojunction was optimized by inserting a passivation a-Si:H thin film of different thicknesses (less than 7 nm) between cSi wafer and MoOx.
N.J. Ekins-Daukes, Imperial College London
Louise Hirst, U.S. Naval Research Laboratory
Richard King, Arizona State University
Bryce Richards, Karlsruhe Institute of Technology (KIT)
ES11.5/ES14.5: Joint Session: Tandem Devices
Wednesday AM, April 19, 2017
PCC North, 200 Level, Room 221 ABC
8:00 AM - *ES11.5.01/ES14.5.01
Efficiency Potential and Recent Activities of High Efficiency and Si Tandem Solar Cells
Masafumi Yamaguchi 1 , Hiroyuki Yamada 3 , Yasuhiro Katsumata 2 Show Abstract
1 , Toyota Technological Institute, Nagoya Japan, 3 , NEDO, Kawasaki Japan, 2 , JST, Kawasaki Japan
The present status of R&D for various types of solar cells is presented by over viewing research and development projects for solar cells in Japan as the Project Leader of the previous PV R&D Project “High Performance Photovoltaic System Technology Development for the Future” under the NEDO (New Energy and Industrial Technology Development Organization of Japan) and as the Research Supervisor of the Research Filed “Creative Clean Energy Generation by using Solar Energy” under the JST (Japan Science and Technology Agency). Developments of high efficiency solar cells such as 44.4% (under concentration) and 37.9% (under 1-sun) InGaP/GaAs/InGaAs 3-junction solar cells by Sharp, 25.1% crystalline Si hetero-junction back-contact (HBC) solar cells by Sharp, 20.9% CIGS solar cells by Solar Frontier, and 11.9% dye-sensitized solar cells by Sharp have been demonstrated under the previous NEDO PV R&D Project. 15.0% efficiency has also been attained with 1cm2 Perovskite solar cell by NIMS under the JST Project. Most recently, 26.3% crystalline Si HBC solar cells by Kaneka, 22.3% CIGS solar cells by Solar Frontier, and 18.2% Perovskite solar cell by NIMS have been achieved under the present NEDO PV R&D Project.
In order to create future clean energy infra structures based on photovoltaics, further development of PV science and technology is necessary. One of most important R&D issues is to develop high efficiency and low cost solar cells. This paper also presents analytical results for efficiency potential of high-efficiency solar cells such as crystalline Si, GaAs, GaAs/Si, CIGSe and CdTe solar cells based on external radiative efficiency (ERE), open-circuit voltage loss and fill factor loss. Crystalline Si solar cells have efficiency potential of 28.5% by improvement in ERE from around 1% to 20%. GaAs and GaAs/Si cells have efficiency potential of 29.7% and 27.4% by improvements in ERE from 22.5% to 30% and from 0.1% to 1%, respectively. CIGSe and CdTe solar cells have potential efficiencies of 26.5% by improvements in ERE from 0.5% to 10% and from around 0.1% to 5%, respectively. Efficiency potential of future generation solar cells such as CZTS and CZTSSe, MQW and QD, Perovskite and Ferroelectric solar cells is also discussed. Recent activities of Si tandem solar cells such as III-V/Si tandem solar cells are also overviewed. Because III-V/Si tandem solar cells have great potential of high- efficiency and low-cost and are great candidate for Solar EV applications.
8:30 AM - *ES11.5.02/ES14.5.02
Development of High Gap Ge- and Si-Based Kesterite-Like Solar Cells for Tandem Applications
Guy Brammertz 1 , Sylvester Sahayaraj 1 , Zijian Huang 1 , Samaneh Ranjbar 1 , Bart Vermang 2 , Marc Meuris 1 , Jef Poortmans 2 Show Abstract
1 , imec - Division of IMOMEC, Leuven Belgium, 2 , imec, Leuven Belgium
Simulations show that for tandem solar cell applications with Si bottom layer cells, the top absorber band gap should ideally be around 1.8 eV1. Ge- and Si-based Kesterite-like materials such as Cu2ZnGe(S,Se)4 and Cu2Zn(Sn,Si)Se4 have band gap energies in that range. In the present contribution we have investigated the potential of these type of materials as solar cell absorbers. We have fabricated high gap Ge- and Si-based thin film absorber layers on Mo/glass substrates by a two step selenization or sulfurization process. First, we sequentially evaporate a metal layer stack on a Mo/glass substrate, followed by selenization or sulfurization in either a 10% H2Se in N2, 100% H2S or an elemental Se environment at temperatures varying from 460 to 560°C. Investigated materials in this work are Cu2ZnSiSe4, Cu2Zn(Sn,Si)Se4, Cu2SiS3, Cu8SiS6 and Cu2ZnGe(S,Se)4.
Our studies show that, despite a very remarkable crystallinity and photoluminescence response at 1.8 eV of the Cu8SiS6 absorber, none of the Si-based materials shows any photocurrents in a standard solar cell configuration with a CdS buffer layer.
On the other hand, Cu2ZnGe(S,Se)4 with a CdS buffer layer does show relatively good solar cell behavior with efficiencies in excess of 5 % and very good crystallinity of the absorber layer. Besides a still relatively large Voc deficit, the main limitation of the efficiency seems to be a large series resistance of the order of 5 Ohm cm2. A possible reason for the large series resistance could be a backside contact problem with the Mo and we have therefore studied the effect of different backside contacts such as TiN, TiW, Ti, Cr or Al on the solar cell efficiency. We have furthermore analyzed the devices using spectral response measurements, capacitance-voltage measurements as well as temperature dependent current-voltage and photoluminescence measurements, trying to gain some insight in the main recombination pathways in the solar cells.
1 T. P. White, N. N. Lal, and K. R. Catchpole, IEEE Journal of Photovoltaics 4, 208-214 (2014).
9:00 AM - ES11.5.03/ES14.5.03
NIR-Transparent Perovskite Solar Cell for Flexible All-Thin-Film Tandem Devices
Stefano Pisoni 1 , Fan Fu 1 , Thomas Feurer 1 , Stephan Buecheler 1 , Ayodhya Tiwari 1 Show Abstract
1 , EMPA, Dubendorf Switzerland
The outstanding photovoltaic properties and large bandgap of organometal halide perovskite solar cells make them attractive as top cells in tandem structures. We have already demonstrated 20.5% efficiency perovskite/Cu(In,Ga)Se2 (CIGS) tandem solar cells on rigid glass substrates in a four-terminal configuration. However, the use of flexible and lightweight foil as a substrate opens up the possibility for roll-to-roll manufacturing of high efficiency tandem devices in future. Here we report the development of NIR-transparent flexible perovskite solar cells for their application as top cells on highly efficient CIGS bottom cells, realizing flexible and lightweight tandem devices. Perovskite solar cells in planar heterojunction configuration were developed, avoiding the use of high-temperature sintered mesoporous structures. By employing an appropriate electron transport multilayer stack and suitable transparent conductive oxide materials, NIR-transparent flexible perovskite photovoltaic devices, with stabilized efficiency of 12.2% and NIR transmittance greater than 78%, were achieved in the preliminary development work. Flexible perovskite/CIGS tandem device with 18.2% efficiency was measured in 4-terminal configuration. In order to further improve tandem performances, current efforts are directed for achieving enhanced NIR-transparency and higher efficiency with perovskite layers of bandgap better matched for double-junction tandem devices with CIGS and CuInSe2 (CIS) bottom cells. Special emphasis is placed on the development of low temperature processing methods, including optimization of the two-step perovskite deposition, suitable for temperature-sensitive flexible substrates. The paper would present a comprehensive work on the development of high efficiency flexible perovskite/CIGS thin film tandem devices and discuss processing and performance related challenges.
9:15 AM - ES11.5.04/ES14.5.04
Infrared-Tuned Silicon Bottom Cell for 23.6%-Efficient Perovskite/Silicon Tandem
Zhengshan Yu 1 , Mathieu Boccard 1 , Peter Firth 1 , Kevin Bush 2 , Axel Palmstrom 2 , Stacey Bent 2 , Michael McGehee 2 , Zachary Holman 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States, 2 , Stanford University, Stanford, California, United States
Amorphous silicon / crystalline silicon heterojunction (SHJ) solar cell is an excellent bottom cell because of its high open-circuit voltage, which results from the separation of the highly recombination-active (ohmic) contacts from the silicon absorber bulk, and because of its dominant performance-loss mechanism under the standard solar spectrum—parasitic absorption of blue light in the front amorphous silicon (a-Si:H) layers—is irrelevant in tandems.
We report our design and fabrication of “IR-tuned SHJ cells” which are designed to serve as bottom cells in perovskite/silicon tandem applications. In this cell structure, the front surface was polished to facilitate perovskite deposition, its rear surface was textured to scatter infrared light. On the rear side, which accounts for the main IR light loss due to parasitic absorption in the rear TCO/metal stack, we insert porous, nanoparticulate films as low-refractive-index layers between silicon bulk and silver, and fabricate nanoparticle/silver rear reflectors. We vary the porosity and thus the refractive index (n =1.1–1.5) of the nanoparticle films, which are deposited by a controllable aerosol spray process, and investigate their effectiveness in reducing infrared parasitic absorption in the solar cells. Optical test structures incorporating films with the highest n exhibit an internal reflectance of over 99%, which means almost eliminating parasitic loss.
On top of this silicon cell, a cesium formamidinium lead halide perovskite cell was deposited with a new, nickel oxide hole contact. This cell also featured a buffer layer deposited by atomic layer deposition at its front side; this layer prevents sputter damage during deposition of the front transparent conductive oxide electrode.
Combing both sub-cells, we report a two-terminal monolithic perovskite/silicon tandem solar cell with an efficiency that exceeds that of either sub-cell and that of the record single-junction perovskite cell. This cell reached an efficiency of 23.6%, certified at NREL, and has been stable at that value for 500 hours of continuous illuminated operation.
9:30 AM - ES11.5.05/ES14.5.05
Large-Area Scalable Perovskite/Silicon Multi-Junction Solar Modules
Manoj Jaysankar 1 , Ulrich Paetzold 2 1 , Weiming Qiu 1 , Tamara Merckx 1 , Tom Aernouts 1 , Robert Gehlhaar 1 , Maarten Debucquoy 1 , Jef Poortmans 1 Show Abstract
1 , imec, Leuven Belgium, 2 , Karlsruhe Institute of Technology, Karslruhe Germany
Crystalline silicon (c-Si) photovoltaics is the dominant technology today for solar power generation. However, the power conversion efficiency of c-Si solar cells is approaching the theoretical limit. Recently, hybrid organic-inorganic perovskite based solar cells, owing to their remarkable lab-scale efficiency, bandgap tuneability, and low cost of fabrication, have attracted a great deal of attention. Moreover, multi-junction solar cells employing perovskite and c-Si solar cells bear the exciting potential to surpass the efficiency limit of market-leading single-junction c-Si solar cells besides being cost-efficient compared to other multi-junction solar cell technologies. However, scaling up this technology and maintaining high efficiency over large areas is challenging, as evidenced by the small-area (< 1.5 cm2) perovskite/Si multi-junction solar cells reported so far. For the economic viability of the perovskite/Si multi-junction technology, an efficient transition from lab-scale cells to industrial-scale modules is crucial.
In this work, we present four-terminal perovskite/c-Si multi-junction solar modules that are fully scalable to commercial solar module dimensions. We first demonstrate a module-on-cell architecture in which a semi-transparent methylammonium lead triiodide perovskite solar module is stacked onto an interdigitated back contact c-Si bottom solar cell of identical aperture area. By a detailed opto-electronic analysis we investigate the impact of the transparent electrodes and the design of the semi-transparent perovskite solar module on the overall performance of the four-terminal multi-junction solar module. With a combination of optimised transparent electrodes and efficient module design, our perovskite/c-Si multi-junction solar modules yield power conversion efficiencies of 23% on 4 cm2 aperture area. In a second demonstrator, we scale up the aperture area to 16 cm2 by going to a module-on-module architecture and achieve power conversion efficiencies of 20%. Both efficiencies represent record results for such sizes. Furthermore, by quantifying and addressing the various losses in our four-terminal solar modules, we demonstrate the feasibility of achieving perovskite/c-Si multi-junction solar modules that can outperform stand-alone c-Si solar cells. Our multi-junction solar module concept is compatible with industrially scalable fabrication techniques thus, enabling the production of high-performance large-area perovskite/silicon multi-junction solar modules.
9:45 AM - ES11.5.06/ES14.5.06
Study of Polycrystalline MgxCd1-xTe/MgyCd1-yTe Double Heterostructures for Tandem Solar Cell Applications
Calli Campbell 1 2 , Cheng-Ying Tsai 1 3 , Yong-Hang Zhang 1 3 Show Abstract
1 Center for Photonics Innovation, Arizona State University, Tempe, Arizona, United States, 2 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States, 3 School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona, United States
Polycrystalline cadmium telluride (CdTe) and silicon solar cells are the two most mature photovoltaic technologies, each with impressive single junction efficiency records (22.1% and 25.6% respectively) and relatively high manufacturability and economy. In order to achieve higher efficiencies than either of these technologies can achieve alone, a tandem configuration is necessary. Connecting cells in series is the most practical method from a fabrication standpoint, however limits the bandgaps which can be used together in order to achieve maximum efficiency. Detailed balance calculations determine that the ideal bandgap for a top subcell in series configuration with Si (Eg = 1.1 eV) is 1.7 eV, slightly higher that of CdTe (Eg = 1.5 eV). It is found that alloying CdTe with Mg shifts the bandgap up with relatively little Mg incorporation to achieve Eg = 1.7 eV. Previous work in the group growing bulk monocrystalline 1.7 eV Mg0.13Cd0.87Te/Mg0.5Cd0.5Te double heterostructures (DHs) by molecular beam epitaxy (MBE) on lattice matched InSb(001) substrates reveals thin films with high structural and optical quality. Solar cells featuring an n-type Mg0.13Cd0.87Te/Mg0.5Cd0.5Te absorber layer and a p-type a-Si hole contact reveal promising preliminary results including a current record efficiency of 11.2% and implied open circuit voltage of 1.3 V.
In this study, polycrystalline MgxCd1-xTe bulk layers and MgxCd1-xTe/MgyCd1-yTe double heterostructures will be grown in a molecular beam epitaxy (MBE) chamber for precise environmental control. Real-time in-situ analysis by Reflection High Energy Electron Diffraction (RHEED) and ex-situ structural and optical characterization, including X-ray diffraction and photoluminescence (PL) spectroscopy, will monitor the material samples as substrate temperature and material flux are varied. Polycrystalline double heterostructure samples will be grown to reveal if wider bandgap barriers reduce recombination and increase PL intensity in the polycrystalline bulk. Preliminary polycrystalline MgCdTe solar cells will be fabricated and electrically characterized.
ES11.6: Solar Cell Optics II
Wednesday AM, April 19, 2017
PCC North, 200 Level, Room 221 AB
10:30 AM - *ES11.6.01
Nanophotonic Control of Thermal Emissivity and Its Implication for Energy Applications
Shanhui Fan 1 , Linxiao Zhu 1 , Kaifeng Chen 1 , Wei Li 1 , Parthi Santhanam 1 , Aaswath Raman 1 Show Abstract
1 , Stanford University, Stanford, California, United States
Nanophotonic structures can be used to tailor thermal emission, and this capability has important implications for a variety of energy technology including photovoltaic applications. In this talk, I would show that the control of thermal emissivity of solar cells can used to reduce the operating temperature of the cell under the sun. We will also discuss the physics of photovoltatics for other energy applications such as solid state cooling.
11:00 AM - ES11.6.02
The Radiative Emissivity of Silicon Solar Cells—What is It, Where Does It Come From, and What Can We Do about It?
Alexander Mellor 1 , Alberto Riverola Lacasta 2 , Ilaria Guarracino 1 , Diego Alonso Alvarez 1 , Lourdes Ferre Llin 3 , Douglas Paul 3 , Daniel Chemisana 2 , Christos Markides 1 , N.J. Ekins-Daukes 1 Show Abstract
1 , Imperial College London, London United Kingdom, 2 , Universitat de Leida, Lleida Spain, 3 , University of Glasgow, Glasgow United Kingdom
Radiative-emissivity control is gaining increasing interest in photovoltaics (PV) as a means of increasing a solar cell’s operating efficiency by decreasing its operating temperature. There is also a converse application in hybrid photovoltaic-thermal (PV-T) generators, in which it may be desirable to decrease emissivity for improved thermal performance. The opportunities for emissivity control in PV therefore lie in both directions: up and down. However, there is very little in the literature on what the mid-IR radiative emissivity of today’s commercial silicon solar cells actually is. In this work, we undergo a combined experimental and theoretical study to explicitly quantify the emissivity of a commercial diffused-junction silicon solar cell and investigate its origins. It is shown that the emissivity is relatively high (over 80 % at 10 um), and originates from the highly-doped emitter and back surface field regions. Finally, we demonstrate a selective emissivity coating for solar cells that has the potential to significantly improve the thermal efficiency of PV-T generators, with minimal effect of the electrical efficiency.
The emissivity/absorptivity of commercial monocrystalline diffused-junction silicon solar cells was measured in the 200 nm – 20 um range via hemispherical reflection/transmission measurements using an integrating sphere and applying the Kirchhoff relation. The emissivity is shown to be over 75 % in the 8 – 13 um region, which corresponds to earth’s atmospheric transparency window, and also to the peak of the black-body spectrum at reasonable operating temperatures. The emissivity/absorptivity was also simulated using the OPTOS formalism to properly account for optical in-/out-coupling via the textured surfaces. Excellent agreement is observed between measurement and simulation. It is inferred from the simulation that the radiative emission originates in the highly-doped emitter and back surface field regions, and that this is effectively outcoupled by the surface texture.
In light of this result, we have developed a wavelength-selective low-emissivity ITO coating for silicon solar cells for use in evacuated PV-T generators. This coating is highly transmissive at solar wavelengths – allowing sunlight to enter the cell, but highly reflective at mid-IR wavelengths – trapping the thermal emission inside the cell. Optical measurements show that the coating reduces the mid-IR emissivity from 80 to 50 % at 10 um, which has the potential to significantly improve the thermal performance of this type of generator. Light IV and quantum efficiency measurements show that the coating reduces the electrical efficiency by 0.5 % absolute as a result of reduced optical transmission.
Ongoing work is being carried out to improve the films for reduced loss and reduced emissivity, and to investigate their effect on the thermal performance of evacuated PV-T generators.
11:15 AM - ES11.6.03
Near Perfect Transmittance Arising from Waveguide Modes in Printed Nanocone Arrays
Colton Bukowsky 1 , Pei-Ling Chen 2 , Hung-Jung Hsu 2 , Sisir Yalamanchili 1 , Katherine Fountaine 3 , Chuang-Chuang Tsai 2 , Harry Atwater 1 Show Abstract
1 , California Inst of Technology, Pasadena, California, United States, 2 , National Chiao Tung University, Hsinchu Taiwan, 3 Basic Research, Northrop Grumman Corporation, Redondo Beach, California, United States
Nanophotonic patterns have been shown to increase the absorption in thin-film photovoltaic devices beyond that of simply planar devices with equivalent, or sometimes greater, material usage. The concept of embedding nanophotonic elements into the active area of devices has been explored in depth as one strategy to increase the density of optical modes and/or coupling into the guided modes of absorbers with thicknesses on the order of hundreds of nanometers. Yet, previous work has shown that while internal patterning can enhance absorption, it is often at the cost of electronic quality. Another attractive alternative is to use nanophotonic elements in positions that are spatially decoupled from the absorber layer. Literature has shown that nanopatterning glass surfaces can decrease the reflection at the air/glass interface in photovoltaic devices. These works have used the effective medium approximation to explain the anti-reflection phenomenon. Distinct from these phenomena, our FDTD simulations identify sub-micron tall truncated nanocone structures as highly efficient anti-reflection structures for glass. We show that truncated nanocone arrays patterned onto the surface of glass can couple incident light into cylindrical waveguide modes that preferentially scatter light into the substrate, resulting in extremely low reflectance from glass across the visible range, lower than equivalent effective medium, graded index structures or other nanopatterned structures on the surface of glass. The dispersion of cylindrical waveguide modes depends only on the refractive index and radius of the cylinder, and thus deforming a cylinder into a cone shape results in a continuum of modes and gives rise to a spectrally broadband response. Herein we report on silica nanocones designed to couple light into glass via directed optical engineering of waveguide modes spanning the visible and near-infrared spectrum. These structures show greater than 99% transmission at all wavelengths in this spectrum, as well as insensitivity to both polarization and incident angle. We find from electromagnetic simulations that the reflectance of nanocone arrays is lower than the equivalent graded index, and are nearly equivalent to a perfect graded index layer. Additionally, nanocones are more easily realized than perfectly graded index layers, via substrate conformal nanoimprint lithography. We discuss the optical concepts in play and present experimental results for nanocone arrays on thin-film a-Si:H single junction and a-Si:H/a-SiGe:H tandem junction photovoltaic cells to demonstrate enhanced current due to anti-reflection.
11:30 AM - ES11.6.04
Microstructured Transparent Electrodes Utilizing Directed Total Internal Reflection
Pieter Kik 1 Show Abstract
1 , University of Central Florida, Orlando, Florida, United States
Recently a transparent interdigitated metallic electrode design was proposed that promised complete transparency at 50 percent metal areal coverage. Such electrode structure could significantly improve the responsivity of metal-semiconductor-metal photodetectors and the power efficiency select advanced photovoltaic devices. The structure utilizes electrode lines with a slanted top surface embedded in a refractive cover layer, producing total internal reflection (TIR) directed toward the electrode gap thereby virtually eliminating shadowing losses. Initial numerical simulation of a silver based catoptric electrode with metal covering fifty percent of the surface predicted 84% optical transmission of unpolarized light averaged over the entire visible optical spectrum at fifty percent silver areal coverage. This presentation discusses a new implementation of such an electrode, featuring electrode lines with curved top surfaces, analogous to an embedded ultracompact reflecting telescope design, with the role of the secondary mirror taken by TIR at the cover layer surface. It is shown that the need for TIR results in an analytic relation between the refractive index of the cover layer and the maximum allowed metal coverage for complete transmission. It is shown that complete transmission may be achieved at >75 percent metal coverage for a cover layer index of n=2. Numerical simulations of the transmission spectra of these new structures will be presented with 2 um wide electrode lines, taking into consideration realistic reflection losses at metal surfaces and realistic dispersive antireflection coatings.
 Catoptric electrodes: transparent metal electrodes using shaped surfaces, Pieter G. Kik, Opt. Lett. 39, 5114 (2014)
ES11.7: III-V Rapid Deposition
Wednesday PM, April 19, 2017
PCC North, 200 Level, Room 221 AB
1:45 PM - *ES11.7.01
III-V Nano-Epitaxial MOVPE for High-Efficiency and Low-Cost Solar Cells
Masakazu Sugiyama 1 Show Abstract
1 Department of Electrical Engineering and Information Systems, School of Engineering, The University of Tokyo, Tokyo Japan
III-V semiconductors are nowadays requested not only to be a booster of high efficiency solar cells but also cost-effective material. This imposes large challenges for metal-organic vapor-phase epitaxy (MOVPE) such as high growth rate, low material consumption and growth on foreign substrates such as silicon.
Epitaxial GaAs can be grown on GaAs substrates with both a higher growth rate and a lower V/III ratio than a typical growth conditions for high-end optical devices. Even though the step structure of the growth surface seems degraded with such an economical growth condition with V/III=1.5, the PV performance of a single-junction GaAs cell was never degraded unless the growth condition was too much aggressive. The problem is how to assess the boundary of the growth condition that induces degradation in PV performance. Rather than conventional electrical measurements, optical characterization such as the evaluation of external luminescence efficiency was proven to be more sensitive to the vital degradation of carrier transport properties in GaAs.
A counter-intuitive approach for low-cost PV devices with III-V semiconductors is the use of quantum structures. It is true that we need much more elaborate (and expensive) conditions for the growth of quantum structures than the growth of bulk layers, a PV cell including a well-designed quantum structure can be benefitted by the suppression of non-radiative recombination. A good candidate for such a purpose is shallow quantum wells. Carriers can be concentrated in quantum wells and have more chance of radiative recombination rather than non-radiative one because the former is more favored at higher carrier concentration. In this situation, even though crystal defects exist because of a too-economical growth condition and/or the growth on a lattice-mismatched substrate, photo-generated carriers are accumulated in quantum wells and are less susceptive to non-radiative recombination centers associated with the crystal defects. This mechanism can be mentioned as “local carrier concentration” in analogy to the efficiency gain under sunlight concentration. We have evidenced such an effect by evaluating external-luminescence efficiency of GaAs single-junction cells with and without quantum wells. This concept will provide another freedom of design for low-cost PV devices.
2:15 PM - ES11.7.03
Flexible III-V Solar Cells Using Single-Crystal-Like Materials Grown on Hastelloy Tapes
Sara Pouladi 1 , Mojtaba Asadirad 1 , Monika Rathi 1 , Pavel Dutta 1 , Yao Yao 1 , Ying Gao 1 , Shahab Shervin 1 , Seung Kyu Oh 1 , Venkat Selvamanickam 1 , Jae-Hyun Ryou 1 Show Abstract
1 , University of Houston, Houston, Texas, United States
Thin-film photovoltaics are emerging rapidly. They have several advantages such as light-weight, low cost, mechanical flexibility, and easy scalability. Conversely, maximum conversion efficiency was recorded for single crystal wafer-based III-V solar cells (SCs), while their applications have been limited primarily due to the high-cost for initial materials and lack of flexibility. In this study, we target to combine the advantages of both technologies. We employ single-crystal-like thin-film GaAs material directly grown on a metal tape to demonstrate low-cost, flexible, and high-efficiency SCs. This technology can bypass expensive single crystal wafer fabrication while offering flexibility and scalability. High-quality III-V semiconductor thin films have been developed on polycrystalline flexible metal tape, recently. This emerging technology was used to grow p-n junction diode thin films of nearly-single-crystalline GaAs as an active region through a transitional textured oxide buffer layers. A lateral device structure for the single junction GaAs SC was designed due to the presence of insulating oxide buffer layers. In order to model and optimize the SC device structure, a two-dimensional technology computer aided design (TCAD) was used by invoking the threading dislocation density (TDD), minority carrier lifetime and hall mobility of the grown semiconducting layers into the simulation. The device structure started by growing a relatively thick ~1 µm high-quality buffer of p-type GaAs on ~1-μm-thick Ge as the first epitaxially grown semiconductor layer on oxide buffers. Then, ~2-μm-thick Zn-doped Al0.2Ga0.8As epi-layer with a hole concentration of p~5×1018 cm-3 as back surface field (BSF) and etch stop layer followed by the GaAs p-n junction i.e., 1-μm-thick Zn-doped GaAs epi-layer with a hole concentration of p~1×1017 cm-3 as base layer and 100-nm-thick Si-doped GaAs epi-layer with an electron concentration of n~1×1018 cm-3 as an emitter layer. Ultimately, a window layer of highly doped n-type Al0.2Ga0.8As epi-layer is grown with a thickness of ~50 nm and concentration of ~5×1018 cm-3. A relatively thin ~ 90 nm Indium Tin oxide (ITO) layer is deposited to act as a current spreading layer and antireflection coating. Device fabrication consists of ITO etching, mesa formation and n- and p-contact metallization and annealing. The I-V characteristics of the fabricated device showed a proof-of-concept epitaxial GaAs thin film solar cell with an open-circuit voltage of 0.3 V and short circuit current of 13.2 mA/cm2, resulting in conversion efficiency of ~2% in AM1.5G condition. Comprehensive results will be further discussed.
ES11.8: Thin-Film PV
Jean Francois Guillemoles
Wednesday PM, April 19, 2017
PCC North, 200 Level, Room 221 AB
3:30 PM - ES11.8.01
Can “Photovoltaic” Halide-Perovskites be Ferroelectric? The Case of MAPbI3 and MAPbBr3
Yevgeny Rakita 1 , David Ehre 1 , Omri Bar-Elli 2 , Elena Meirzadeh 1 , Hadar Kaslasi 1 , Gary Hodes 1 , Igor Lubomirsky 1 , Dan Oron 2 , David Cahen 1 Show Abstract
1 Materials and Interfaces, Weizmann Institute of Science, Rehovot Israel, 2 Physics of Complex Systems, Weizmann Institute of Science, Rehovot Israel
Ferroelectricity, which is well known for oxide perovskites, has been suggested as a possible reason for the outstanding solar-to-electrical energy conversion of halide perovskites - especially methylammonium lead iodide and bromide (abbr. MAPbI3 and MAPbBr3). Low carrier recombination rate, high voltage efficiencies and an efficient exciton separation are some of the possible benefits of ferroelectric domains.
Contradicting experimental evidence based on structural diffraction-based characterizations, classical electric field vs polarization plots and piezoelectricity measurements, created confusion regarding the possible existence of ferroelectricity. To resolve this debate we decided to investigate the presence of pyroelectric currents, as their presence is a direct evidence for spontaneous polarization. Spontaneous polarization is a necessary condition for ferroelectricity, where for perovskites it is usually assumed to be sufficient.
Using the periodic temperature change (Chynoweth) method* under different surrounding temperature conditions, we investigate the polar nature of the different crystallographic phases of MAPbI3 and MAPbBr3. We show that the cubic phases of MAPbI3 (>330K) and MAPbBr3 (>236K) phase are clearly non-polar, which exclude any possible ferroelectric activity at these phases. The tetragonal phase, however, show (at least for MAPbI3: <330K) clear evidence of pyroelectric currents, which prove the polar nature of MAPbI3 at room-temperature. We continue our low-temperature investigations and will report our most recent results, including on the lower symmetry (orthorhombic) phases of these halide perovskites. Complimentary experimental, such as assessment of centrosymmetry using second-harmonic-generation, will be discussed.
*Lubomirsky, I. & Stafsudd, O. ; Rev. Sci. Instrum. 83, 051101 (2012).
3:45 PM - ES11.8.02
Cu2O/Si Heterojunction Based Carrier Selective Contact for Silicon Photovoltaics
Pramod Ravindra 1 , Rudra Mukherjee 1 , Sushobhan Avasthi 1 Show Abstract
1 , Indian Institute of Science, Bangalore India
Carrier-selective contacts allow either electron or hole to pass without hindrance, while they block the passage of the other type of carriers. This is made possible by a large band offset to block one type of carriers, and small offset that ensures a low barrier for transport of the other type. Such contacts can lead to fabrication of solar cells which are homojunction-free, reducing thermal budget and thus the total cost of production. Metal oxide thin films are interesting due to their abundance, low cost of deposition and ease of scalability. Electron-selective, hole-blocking oxide/Si heterojunctions have already been reported. However, there are very few reports of electron-blocking oxide/Si heterojunctions, due to lack of oxides which have high enough hole-conductivity and appropriate band alignments. In this talk, we demonstrate the use of a well-known oxide based absorber Cu2O, as an electron-blocking layer, using Si as a model absorber.
Carrier-selective contacts for solar cells need to be highly transparent to visible light to avoid parasitic light absorption. Traditionally, this has been achieved by selecting oxides with large bandgap but this is too restrictive. Considering the lack of p-type oxides with the good hole-conductivity and appropriate alignments, we suggest another way to achieve high-transparency: restrict the thickness of the oxide layer to below the absorption depth for visible light. The latter approach significantly increases the list of materials that can be used.
Cu2O films were deposited using room-temperature reactive RF sputtering – a simple and a scalable process. Using UV-visible spectroscopy, the bandgap was determined to be 2.25 eV. This means that bulk-Cu2O absorbs light in visible region, which is not desirable for a contact material. However, it was also determined that films with thickness <50 nm, would be transparent to wavelengths in visible region (>400 nm). Films deposited in this work are all 15 nm thick, which shows a transmittance of 74% at 400 nm and 94% at 550 nm.
Band-alignment was determined using a combination of UV-visible spectroscopy, X-Ray and UV-photoelectron spectroscopy. A small band offset of -0.2 eV was measured between the Cu2O and Si valence bands. This means that holes can pass from Si into Cu2O without a large barrier. A large band-offset of 0.9 eV was measured between Cu2O and Si conduction bands, which offers a large barrier for electrons to pass from Si to Cu2O, thus blocking them. Heterojunction solar cells with Al/Si/Cu2O/Au structure were fabricated using Al/Si/Au cells as control devices. Using a tunnel-oxide passivation layer to mitigate issues of sputtering damage and dangling bonds at the Si/Cu2O interface, we obtain an improvement of VOC from 302 mV to 528 mV and a two-fold improvement in JSC from 15.4 mA cm-2 to 32 mA cm-2, upon incorporating a Cu2O blocking layer. This proves that the Cu2O/Si heterojunction blocks electrons and improves cell performance.
4:00 PM - ES11.8.03
Selective Grain Boundary Etching to Improve on the Back-Contact of Polycrystalline CdTe Solar Cells
Sudhajit Misra 1 , Jeffery Aguiar 2 3 , Brian Devener 4 , Christos Ferekides 5 , Ana Kanevce 6 , Mike Scarpulla 1 2 Show Abstract
1 Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Material Science and Engineering, University of Utah, Salt Lake City, Utah, United States, 3 Fuel Design and Development, Idaho National Laboratory, Idaho Falls, Idaho, United States, 4 Surface Science and Analysis Laboratory, University of Utah, Salt Lake City, Utah, United States, 5 Electrical Engineering, University of South Florida, Salt Lake City, Utah, United States, 6 , National Renewable Energy Laboratory, Golden, Colorado, United States
Polycrystalline (px) CdTe is a thin-film photovoltaic that is the focus of ongoing research to devise class-leading solar cells. Recent work has suggested that CdTe has superior charge transport with excellent responses to grain boundary (GB) engineering. The crystallinity and stoichiometry of px-CdTe, however, presents unique challenges to measure device properties and relate those to the underlying microstructure and chemistry. Common to all these solar cells is efficiently collecting charge carriers at the back contact of px-CdTe, which is the focus of our current work.
Effective back contact and GB passivation, is pivotal for the development of record breaking thin-film photovoltaics (high-efficiency barriers >20%, open-circuit voltages >1000 mV), where resistance matching is crucial. CdTe solar cell back contact preparation includes a chemical etching process, which helps to obtain a Te-rich CdTe n-type back surface. In light of chemical etching processes, effective routes to back contact preparation, the roles of material microstructure, surfaces, defects, and interfaces need to be investigated in greater detail. To answer whether these parameters are the contributing factors, we have studied the same px-CdTe photovoltaic compound, after an Br:MeOH etching process, on device samples. We focus our efforts on understanding the nano-chemistry and micro-structure at the terminating surfaces and GBs in px-CdTe. Using scanning transmission electron microscopy, we observe a Te-rich layer at the GBs in the vicinity of the back contact, and on the surface of px-CdTe films after the device processing. We find these GBs are Te-rich and influence the inversion of back contact and GBs, which aids in better charge collection and higher current extraction from the back surface of CdTe. Solar cells prepared showed no rollover behavior, indicating improved back contact. Our results support the ideas that the difference in surface and grain boundary stoichiometry yielding a plausible improvement in device rollover.
4:15 PM - ES11.8.04
< 1g/W Solar Cells on Flexible Silicon Substrates
André Augusto 1 , Bill Dauksher 1 , Stuart Bowden 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
The use of thinner silicon wafers potential lowers the cost of solar cells, while keeping the efficiencies high. By combining suitable light trapping and high quality passivation the optimum thickness is estimated to be in the range of 10 – 80µm. The main efficiency gains from using thinner wafers is in the increase of the open-circuit voltages by increasing the injection level which the cell operates. Thinner wafers and high quality passivation are required as the open-circuit voltage increases as a function of the excess carrier density. Recently we have fabricated cells with open-circuit voltages over 760 mV with bandgap-voltage offset below 0.35 V (comparable with GaAs and GaInP). In this work we present the recombination mechanism at open-circuit voltage and at maximum power point for thin silicon heterojunction solar cells, in particular a roadmap to achieve 770 mV open-circuit voltages. Using test structures for optimum passivation we have achieved surface recombination velocities below 1 cm s-1 on cells manufacture on thin Cz wafers. According with our model, the Auger recombination is responsible for over 4/5 of the losses at open-circuit voltage injection level, however at the maximum power point injection level the Shockley-Read-Hall and the surface take over the recombination process representing 2/3 of the losses. Due to their thickness these cells are flexible, lightweight (<1g/W), and when encapsulated they can be bent to curvature radius of 4 cm, without any structural damage, or efficiency degradation.
ES11.9: Poster Session II: Hybrid
Wednesday PM, April 19, 2017
Sheraton, Third Level, Phoenix Ballroom
8:00 PM - ES11.9.01
Partially Passivated Micro-Pyramidal Silicon/PEDOT:PSS Hybrid Solar Cells with High Efficiency
Inyoung Choi 1 , Chanul Kim 1 , Myounghun Jeong 1 , Sungbum Kang 1 , Kyoung Jin Choi 1 Show Abstract
1 School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan Korea (the Republic of)
Organic/inorganic hybrid solar cells can be attractive owing to the synergetic advantages of the high carrier mobility and efficiency from inorganic semiconductors and the simple process and low cost from organic semiconductors. Especially in case of Si/PEDOT:PSS hybrid solar cells, PEDOT:PSS acts as a hole-transport layer with high transparency and conductivity and enhances the efficiency of the solar cell. However, when PEDOT:PSS is applied to the micro-pyramidal Si surface to improve the light absorption, the conformal coating becomes very challenging due to the networking of long polymer chains. Namely, PEDOT:PSS tends to cover only the tip areas of micro pyramids instead of whole Si surfaces including deep valleys of Si micro pyramids, which causes the degradation of the hybrid solar cells because the uncovered Si surface acts as surface recombination centers as well as they are not contributing the separation of photo-excited electron-hole pairs. In this work, we suggest a novel local SiN passivation technique around the valleys of Si micro pyramids. The SiN passivation layer was synthesized by plasma-enhanced chemical vapor deposition. For the local passivation, we spin-coated photoresist (PR) on a micro-pyramidal SiN/Si surface, followed by two-step soft and hard baking at 85 °C and 110 °C, respectively. During the soft baking, PR flows towards the valleys of micro pyramids resulting in non-uniform PR coating thickness between the tip and valley areas. Because of the non-uniform PR thickness, we could obtain a partially-passivated Si surface after a chemical etching of the SiN layer using a buffered-oxide etchant (BOE). The Si/PEDOT:PSS hybrid solar cells are fabricated by a spin-coating of PEDOT:PSS on a partially-passivated micro-pyramidal Si surface, followed by a annealing at 130 °C. The partially-passivated hybrid solar cells were demonstrated to have an enhanced power conversion efficiency, compared with the sample without the partial-passivation layer. This is due to the enhancement of minority carrier lifetime or lower surface recombination velocity as well as the anti-reflection effect of the remaining SiNx layer around the valley area of Si micro pyramids.
8:00 PM - ES11.9.02
Broadband Light Absorption in Perovskite Solar Cell Using Metamaterial Cross Grating Structures
Omar Abdelraouf 1 , Nageh Allam 1 Show Abstract
1 Energy Materials Laboratory (EML), Department of Physics, School of Sciences and Engineering, The American University in Cairo, Cairo Egypt
The efficiency of organic halide perovskite thin film solar cells has increased dramatically in the last decade and become a good alternative to conventional bulk silicon photovoltaic due to its low cost of used materials. However, most of research and development made in finding and enhancing optical and electrical properties of used materials. Finding a way for light manipulation and guiding in low thickness of thin film perovskite will enhance efficiency further more. Nanophotonic structures have showed a big possibility of controlling and concentrating light at low dimensions of other types of thin film solar cell.
In this paper, we study effect of using noble metallic “silver” metamaterial cross grating structures over surface of CH3NH3PbI3 perovskite layer. We showed theoretically the enhancements in light absorption and overall efficiency of metamaterial perovskite solar cell over the planar one by studying many nanoshapes and sweeping on dimensions targeting highest enhancements.
After that, we replace the silver by titanium nitride (TiN) and study the differences in performance between silver and TiN. TiN is a low cost material and exhibit plasmonic properties comparable to gold in the visible light spectrum. All theoretical simulations made in three dimensions (3D) using finite element method and compared to experimental published results, material properties has been taken from literature.
8:00 PM - ES11.9.03
Fabrication and Analysis of c-Si Solar Cells with Micro and Nano Holes Prepared by Reactive Ion Etching
Hayriye Altinoluk 1 2 , Rasit Turan 2 3 Show Abstract
1 Electrical and Electronics Engineering, Mugla Sitki Kocman University, Mugla Turkey, 2 Center for Solar Energy Research and Applications (GUNAM), Middle East Technical University, Ankara Turkey, 3 Physics, Middle East Technical University, Ankara Turkey
Surface texturing is an indispensable way of light trapping for solar cells. Beyond random pyramid texturing which is widely applied for standard wafer thicknesses around 150-170 µm, micro-nano holes are promising candidates on thin wafers for which pyramid texturing is not an option due to high amount of Si consumption during pyramid texturing process. Hole- texturing process of c-Si surface was done using reactive ion etching (RIE) method, where the diameter (d) of holes varied from micron to sub-micron level corresponding to ray optic and wave optic regimes respectively. Standard solar cell procedures were followed to fabricate on hole textured p-type Si wafers. Solar cell performance parameters and loss mechanisms were studied as a function of physical properties of the holes. A special attention was paid to the loss due to the increased surface area due to the hole formation on the surface. Under certain condition, we have obtained best cell efficiency of % 15.7 which is the highest value reported so far. In addition, angle resolved measurement were carried out to understand the effect of the incidence angle on the cell performance, which is crucial when they are used in the outdoor environment. For this purpose, a new current-voltage measurement system was built and used under 1-Sun illumination. We conclude that texturing Si surface with a hole pattern proves to be a promising approach for future solar cell application especially in the case of thin Si wafers.
8:00 PM - ES11.9.04
Theoretical Study of Heteroatom-Doped Carbon Nanomaterials as Effective Catalysts in Dye-Sensitized Solar Cells
Zhenghang Zhao 1 , Zhenhai Xia 1 Show Abstract
1 , University of North Texas, Denton, Texas, United States
Doped carbon nanomaterials can be used as a counter electrode in dye-sensitized solar cells (DSSCs) for triiodide reduction since they are inexpensive, abundant on earth and poisoning-free. Experimental studies on this aspect are impressive, however, theoretical studies are a lack of. In this paper, density functional theory calculations were performed on heteroatom-doped graphene as metal-free catalysts for triiodide reduction in DSSCs. Results showed the electrocatalytic performance of doped graphene is comparable to platinum and the hetero-edges facilitate the reactions even more. A design principle regarding the intrinsic properties of the catalysts is built which enables the prediction of the electrocatalytic performance of the catalysts. Finally, charge distribution analysis proved the positive charge of the carbon active site played an essential role in determining the electrochemical properties of doped graphene for triiodide reduction.
8:00 PM - ES11.9.05
Investigation of Deep-Level Defects Lateral Distribution in Active Layers of Multicrystalline Silicon Solar Cells
Vladimir Litvinov 1 , Alexander Ermachikhin 1 , Dmitry Kusakin 1 , Nikolay Vishnyakov 1 , Valery Gudzev 1 , Andrey Karabanov 2 , Sergey Karabanov 1 , Sergey Vikhrov 1 Show Abstract
1 , Ryazan State Radio Engineering University, Ryazan Russian Federation, 2 , Helios-Resource Ltd, Saransk Russian Federation
Short introductive summary
Solar cells (SCs) based on multicrystalline silicon (mc-Si) have such technical and economical advantages as acceptable lower production cost and relatively high and comparable efficiency of solar energy conversion into electricity to SCs based on monocrystalline silicon. Mc-Si is a naturally defective material due to the structural features. The defects or the deep centers (DCs) form deep energy levels (DLs) in the semiconductor band gap. DLs are the centers of free charge carriers recombination. The efficiency of SCs depends on DLs and their parameters such as spatial and lateral distribution and activation energies. In order to optimize the structure and technology of SCs and to obtain advanced parameters it is important to study DLs and their distribution peculiarities.
Purpose of the work
The study of the DL’s energy spectrum of defects and their distribution along the active layers of the SCs with different efficiencies is necessary to find the correlation of DL’s parameters and SC’s efficiency.
For the investigation of DLs in SCs the I-DLTS measurement system was developed. In addition the capacitance-voltage (C-V) characteristics were measured also.
Scientific innovation and relevance
A correlation between the total concentration of DCs in the active layer of SCs based on mc-Si and conversion efficiency is experimentally observed. The distribution peculiarities of DCs along the active layers of the SCs with different efficiencies were studied. The method for the SC’s I-DLTS-spectra measuring is developed.
A specially designed measuring system that allow to measure I-DLTS-spectra of barrier structures and SCs characterized by high leakage current, large area of the p-n-junction and relatively large electrical capacitance was used for investigation of the DLs parameters. C-V characteristics of SCs were measured by Agilent E4980A in order to obtain DCs concentration from I-DLTS-spectra and free charge carriers concentration.
Samples were pieces with areas 2-10 mm2 that cleaved from different parts of the SC’s surface. I-DLTS spectra were measured in DL’s recharge mode by applying a pulse voltage. The amplitude of the filling pulse voltage was chosen close to open circuit voltage of the SC. At this condition DLs in the active layer where electron-hole pairs are generated and recombined during SC operation are recharged. Detected DLs were hole traps with activation energies in the range 0.17-0.56 eV. The peculiarities of DL’s lateral distribution of concentration were analyzed. The investigated SCs have efficiencies from 10 to 20.4%.
The energy and concentration of deep level defects localized in mc-Si were measured. The deep-level defects lateral distribution in the active layer of multicrystalline silicon solar cells was investigated. The correlation between the total DL’s concentration in the active layer of mc-Si SCs and conversion efficiency was found.
8:00 PM - ES11.9.06
Amine-Based Interfacial Molecules as Versatile Interlayer for Inverted Polymer-Based Optoelectronic Devices
Seungjin Lee 1 , Myoung Hoon Song 1 Show Abstract
1 , UNIST, Ulsan Korea (the Republic of)
We demonstrate highly efficient inverted polymer-based light-emitting diodes (iPLEDs) and inverted polymer-based solar cells (iPSCs) by employing series of amine-based interfacial molecules (AIMs) containing various number of amine groups as a versatile interlayer. Our method provides a simple and efficient processing as well as effective tunability of energy level at the interface between the ZnO and the active layer depending on the number of amine groups of AIMs. As the number of amine groups increases, the work functions (WFs) of AIMs-modified ZnO gradually shift to the vacuum level, which reduces the energy barrier effectively between the ZnO and the active layer. Moreover, AIMs treatment passivates the defect sites of the ZnO and blocks the holes, leading to enhanced device performances in both iPLEDs and iPSCs. In particular, optimized iPLEDs with PEHA (6N) show the maximum luminance of 99,300 cd m-2, LE of 23.1 cd A-1, PE of 8.83 lm W-1 and EQE of 8.40 %, which are enhanced by approximately 30-fold, 32-fold, 38-fold and 30-fold, respectively, compared to the reference device. For iPSCs, optimized iPSCs with DETA (3N) showed the PCE of 9.04%, which is enhanced by roughly 30 %, compared to that of the reference devices. Furthermore, our approach is a promising method for high device performances in organic based optoelectronic devices.
8:00 PM - ES11.9.07
Ultrafast One-Step Deposition of Perovskite Thin Films by Atmospheric Plasma Curing—Towards a New Route of Synthesis for Efficient and Reliable Solar Cells
Florian Hilt 1 , Michael Hovish 1 , Nicholas Rolston 1 , Reinhold Dauskardt 1 Show Abstract
1 , Stanford University, Stanford, California, United States
Scalable processing solutions are desperately needed in order to expedite the production of perovskite solar cells (PSC) from the laboratory to the high volume manufacturing environment. In light of this goal, we have developed an atmospheric pressure plasma curing process which is capable of rapidly producing stable and photoactive perovskite material in ambient conditions. This innovative one-step method is based on an environmental-friendly technique that can easily be up-scaled to cost-effective roll-to-roll processes. This is the first demonstration of photoactive perovskite formation with the aid of an atmospheric pressure plasma discharge. Briefly, an ultrasonic spray nozzle is used to evenly distribute perovskite forming solution onto a surface. Then, an atmospheric pressure plasma discharge is scanned over the mixture, curing the perovskite. With this design, we are able to coat and fully cure perovskite material at a rate of 1 cm2/s, in strips as large as 25 cm2. Additionally, this process requires minute amounts of precursor solution along with a high conversion yield. For inverted MAPbI3 perovskite device, a power conversion efficiency of 13% was achieved.
Scanning electron microscopy (SEM) was used to compare the microstructure of typical solution-processed films to plasma-cured layers. The structure and chemical composition of the films were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to give insight into the stoichiometry and crystallinity of the films. Furthermore, J-V curves and external quantum efficiency (EQE) spectra of plasma-cured and solution-processed thin films were correlated, showing a 13%-power conversion efficiency achieved in for a MAPbI3 perovskite film deposited by the atmospheric plasma method. Finally, the cohesion of the perovskite thin films was assessed. It is shown that plasma-cured films exhibit better fracture resistance compared to solution-processed films, signifying significant gains in the reliability of this technology. We also show that reactive plasma species contribute to curing in two ways which are distinct from simple heating of the surface.
The reactive plasma species present in atmospheric pressure discharges may be used advantageously to optimize perovskite film chemistries and create scalable pathways towards mass production of PSCs.
8:00 PM - ES11.9.08
Multiscale Modeling of Silicon Heterojunction Solar Cells
Pradyumna Muralidharan 1 , Stephen Goodnick 1 , Dragica Vasileska 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
Silicon based single junction solar cell technology continued to make significant strides in the past year with new world record module efficiencies being reported for the Panasonic heterojunction with thin intrinsic layer (HIT) module (23.8%) and the SunPower rooftop silicon module (24.1%). The HIT cell which is comprised of amorphous silicon (a-Si) and crystalline silicon (c-Si) currently holds the world record efficiency (25.6%) for a silicon based single junction solar cell. Further improvement in this technology requires a rigorous understanding of the underlying physics of the device.
The device performance of a-Si and c-Si heterojunction solar cells depends heavily on the nature of transport at the hetero interface and defect assisted transport through the a-Si. Different microscopic processes dominate transport in different regions of the device and take place across widely varying time scales. In this work we present a multiscale model which utilizes different simulation methodologies to study physics in various regions of the device, namely, the Ensemble Monte Carlo (EMC), Kinetic Monte Carlo (KMC), and Drift Diffusion (DD) solvers. The EMC studies the behavior of the photogenerated carriers at the heterointerface; the KMC analyzes transport of the photogenerated carriers through the intrinsic amorphous silicon (i-a-Si) barrier layer; and the DD solver calculates current and other device properties in the low field regions of the cell. These solvers are then self consistently coupled to analyze device performance. Previously, our KMC simulations have shown that hopping is the main mode of transport through the i-a-Si, and the photogenerated carries are collected by defect emission rather that Poole - Frenkel emission or direct tunneling1. In addition, using EMC simulations we have shown that the photogenerated carriers exhibit non Maxwellian behavior at the heterointerface2.
This work specifically describes the self-consistent coupling of the DD and EMC solvers. By adding the EMC solver to the multiscale solver we are able to capture the high field behavior of the photogenerated carriers, and its affect on device parameters such as JSC, VOC, FF and efficiency.
1. P. Muralidharan, S. Bowden, S. M. Goodnick and D. Vasileska, "A kinetic Monte Carlo approach to study transport in amorphous silicon/crystalline silicon HIT cells", 42nd IEEE Photovoltaic Specialist Conference (PVSC),pp. 1-4, 2015.
2. P. Muralidharan, K. Ghosh, S.M. Goodnick and D. Vasileska, "Hot hole transport in a-Si/c-Si heterojunction solar cells", 40th IEEE Photovoltaics Specialists Conference (PVSC), pp 2519-2523, 2014.
8:00 PM - ES11.9.09
Do Electron and Hole Transport Layers Contribute Equally to Surface Photovoltage?
Sunil Kumar Samji 2 , Pravin Prakash Rathode 1 , Naresh Chandrasekaran 1 3 4 , Dinesh Kabra 1 Show Abstract
2 Center for Excellence in Nanoelectronics, Indian Institute of Technology (IIT B), Mumbai India, 1 Department of Physics, Indian Institute of Technology Bombay (IIT B), Mumbai India, 3 , IITB-Monash Research Academy, Mumbai India, 4 Department of Materials Science and Engineering, Monash University, Melbourne, Victoria, Australia
Halide perovskite based photovoltaic systems have sky rocketed their performance from around 3% to 22 % in five years and have proved a potential competent to other PV devices. In addition to surface coverage, millimeter sized grains, carrier life times of the order μs, selection of suitable electron and hole transport layers (ETL and HTL) also plays a key role in achieving high efficiency. Spatial resolution of the order of nanometers and energy accuracy to sub-millivolt accuracy has made Kelvin probe force microscopy (KPFM) one of the powerful techniques to carry out surface photovoltage studies to understand contribution each layer to surface photovoltage. The surface photovoltage measured is a result of charge separation at surface (by surface space charge region) as well as the interface (by the electric field between active layer and ETL or HTL). Nehoshtan et al.  have carried surface photovoltage studies on MAPbI3-xClx sandwiched between different electron and hole transport layers and reported that ETL contributes more than twice that of HTL. The fact that organic part of MAPbI3-xClx renders it thermally unstable makes the inorganic CsPbX3 halide perovskites more interesting. A high efficiency of 21%  reported on CsPbX3 mixed halide systems also makes it a more promising material.
In the present work surface photovoltage studies were carried out on CsPbBr3 films spin coated on ZnO/ITO and PEDOT: PSS/ITO substrate, Where ZnO is ETL and PEDOT: PSS is HTL. KPFM studies were carried out in dark as well under light illumination. Under illumination, electrons are carried away by ZnO in ZnO/ITO and holes will rise to the surface, hence surface photovoltage is observed to be positive in ZnO/ITO and vice-versa in PEODT: PSS/ITO. The magnitude of surface photovoltage is around 160 mV in case of CsPbBr3 deposited on PEDOT: PSS/ITO and 550 mV in case of ZnO/ITO, which clearly shows that contribution of ETL is more than thrice to the surface photovoltage and hence calls for an in depth study of charge separation mechanism, which will be discussed in detail.
1. Nehoshtan et al. J. Phys. chem.Lett., 5, 2014, 2408–2413.
2. Saliba et al.Energy Environ. Sci., 9, 2016, 1989-1997
8:00 PM - ES11.9.10
Three-Component Light Trapping Structure for Multi-Junction Solar Cells
Alexander Mellor 1 , Nicholas Hylton 1 , Stefan Maier 1 , N.J. Ekins-Daukes 1 Show Abstract
1 , Imperial College London, London United Kingdom
We present a composite light-trapping structure capable of significantly enhancing the photon absorption in any subcell of a multi-junction solar cell, based on coupling into waveguide modes by a periodic grating. This is a particularly complex problem, since, in general, any given subcell can be sandwiched between two other subcells. Therefore, conventional reflectors cannot be used to fully exploit the light trapping effect. A distributed Bragg reflector (DBR) can be used as a wavelength reflector between two subcells, but is ineffective for obliquely propagating photons and cannot therefore provide the desired light trapping alone. To solve this complex problem, we show that the combination of a DBR, a low-index transparent spacer layer, and a periodic grating with the correct period can excite guided modes and prevent these modes from leaking into the lower subcells, which would otherwise cause an efficiency loss. This allows the thickness of the target subcell to be reduced by a factor of 5without compromising photon absorption, which improves carrier collection, and therefore photocurrent.
The work focuses on using the composite structure to improve the radiation hardness of a InGaP/Ga(In)As/Ge space solar cell. In this context, it is shown via simulation that the Ga(In)As middle-cell thickness can be reduced from 3500 to 700 nm, whilst maintaining strong photon absorption, and that this leads to a significantly improved end-of-life photocurrent in the Ga(In)As middle cell. However, the design can in general be applied to a wide range of multi-junction solar cell types. We discuss the principles of operation of the design, as well as possible methods of its fabrication and integration into multi-junction solar cells.
8:00 PM - ES11.9.11
Surface Defect Analysis on n-Type Substrates Using Spatially Resolved Photoluminescence Images and TIDLS for High Performance Devices
Pradeep Balaji 1 , Apoorva Srinivasa 1 , André Augusto 1 , Stuart Bowden 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
Silicon heterojunction cells (SHJ) are now at efficiencies over 26%. One way to push efficiencies closer to 30% is to integrate SHJ cells as a bottom cell in tandem with wide band gap materials. The surface quality of such cells is important for reducing recombination current and achieving better interface between the top and bottom cells. SHJ cells are highly sensitive to defects and impuritiies at the surface, necessitating better quality control. Surface defects and contaminants are introduced during wafer sawing process. During saw damage removal and cleaning, metal impurities and surface defects propagate along the  direction of the substrate. It is noticed from PL imaging that, even after 50 μm of etching such defects/impurities are preserved. Processing at 80 oC during saw damage removal can be a reason that leads to diffusion of such impurities. In this paper we investigate the impact of these defects on device performance and try to identify the contaminants present on the surface. Steady-state Photoluminescence (PL) and temperature- and injection-dependent lifetime spectroscopy (TIDLS) can be used as effective characterization techniques for spatially analyzing defects and identifying the impurities present. Calibrated PL gives us spatially resolved lifetime, open circuit voltage (Voc) and saturation current density (Jo) images of cells at various stages of processing. This can be used to obtain spatially resolved injection-dependent lifetime curves. Subsequently, TIDLS can be used to identify the impurities. Furthermore, acid etching, pre-cleaning of wafers before undergoing saw damage removal and surface polishing at lower temperatures are being investigated for avoiding such migration of surface contaminants.
8:00 PM - ES11.9.12
High Performance of Perovskite Solar Cell with a High Temperature Annealing Process and Their Large Size Application
Dong Suk Kim 1 , Yimhyun Jo 1 Show Abstract
1 , Korea Institute of Energy Research, Ulsan Korea (the Republic of)
Organic-inorganic hybrid metal halide perovskite solar cells (PSCs) are attracting tremendous research interest due to their high solar-to-electric power conversion efficiency with a high possibility of cost-effective fabrication and certified power conversion efficiency now exceeding 22%. Although many effective methods for their application have been developed over the last decade, their practical transition to large-size devices have been restricted by difficulties in achieving high performance. Here we report on the development of a simple and cost-effective production method with high-temperature and short-time annealing processing to obtain uniform, smooth, and large-size grain domains of perovskite films over large areas. Solar cells fabricated using this processing technique, at 400 oC with an annealing time of 2 s, had a maximum power conversion efficiency exceeding 20% over a 0.1 cm2 active-area and 18% over a 1 cm2 active-area. We believe our approach will enable the realization of highly efficient large-area PCSs for practical development with a very simple and short time procedure. This simple method should lead the field towards the fabrication of uniform large-scale perovskite films, which are necessary for the production of high-efficiency solar cells that may also be applicable to several other material systems for more wide-spread practical deployment.
8:00 PM - ES11.9.13
Comparison of Donor and Acceptor Doped TiO2 Nanorods as a Mesoporous Scaffold of Perovskite Solar Cells
Fangda Yu 1 , Gill Sang Han 1 , Jung-Kun Lee 1 Show Abstract
1 Department of Mechanical Engineering and Material Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
The organic-inorganic halide perovskite solar cells (PSCs) are now a strong candidate for the next generation renewable energy device due to their high power conversion efficiency (PCE) and economic fabrication process. The mesoporous (mp)-TiO2 film is the most common type of electron transport layer (ETL) as a scaffold of PSCs. ETL extracts electrons from the perovskite layer and delivers them to the transparent conducting layer on the opposite side. However, the mp-TiO2 base PSCs have a hysteresis problem due to the surface defects of mp-TiO2. Defects trap photogenerated electrons and increase the recombination of electrons. There have been many studies to optimize appropriate ETL for high performance and less hysteresis in the PSCs through various materials, structure, and crystalline optimization. It has been reported that the surface chemistry of TiO2 causes the electron trapping at the TiO2/perovskite interface, which leads to the formation of a barrier for the electron transport. Therefore, the passivation methods of TiO2 surface have been applied including the Lewis bases, Li doping and the replacement by polymer ETL. These reduce the electron trapping and retard the hysteresis of J-V curves.
In this study, we compare the transport behavior of photogenerated electrons in pristine, donor and acceptor impurity doped TiO2 nanorods. Three kinds of single crystalline TiO2 nanorod arrays (pristine, Al doped and Nb doped) are fabricated by microwave assisted hydrothermal method. A doping concentration is in a range of 0~0.5% because excess doping changes the morphology and electric conductivity of the nanrods dramatically. The lengths of nanorods are controlled to 400-600nm. A two-step method is applied to fabrication of the perovskite solar cells. Nanrod based solar cells are characterized using multiple techniques such photovoltaic measurements, scanning electron microscopy (SEM), electron dispersive spectrum (EDS) x-ray diffraction (XRD), incident photon conversion efficiency (IPCE) and transient current method. Experimental results show that the acceptor doping reduces the trapping sites at TiO2/perovskite interface by compensating oxygen defects on the surface of the nanorods. This, in turn, reduces the hysteresis of the perovskite solar cells. In contrast, donor doping do not prevent the defect formation, though the solar cell performance increases due to the higher electric conductivity of the donor doped TiO2 nanorods.
8:00 PM - ES11.9.14
Correlation between Phase-Separated Morphology of Active Layer and Photovoltaic Properties in All-Polymer Solar Cells
Yuxiang Li 1 3 , Changyeon Lee 2 , Bumjoon Kim 2 , Han Young Woo 3 Show Abstract
1 , Pusan National University, Miryang Korea (the Republic of), 3 Department of Chemistry, Korea University, Seoul, Seoul, Korea (the Republic of), 2 , Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Due to the significantly reduced entropic contribution by two macromolecular species (polymer donor-polymer acceptor) that energetically disfavors their mixing, controlling the large phase separated bulk-heterojunction (BHJ) morphology in the active layer of all-PSCs remains a critical hurdle for obtaining high device efficiency.[2,3] Here, a straightforward relationship between donor/acceptor domain size and photovoltaic device performance in all-PSCs based on a model system of the PDFQx-3T:P(NDI2OD-T2) was investigated via varied the film-processing solvents (chloroform (CF), chlorobenzene (CB), o-dichlorobenzene (DCB) and p-xylene (XY)), thereby manipulating the phase separation of all-polymers blends with domain size in the range of 30-300nm. The highly volatile CF solvent enabled to form kinetically frozen morphologies with smaller domain sizes, whereas slow evaporation of CB and DCB allowed sufficient time for demixing of the polymers, leading to larger domain sizes. Decreased domain sizes of all-polymer blends were closely correlated with the significantly improved short-circuit current density (JSC) of devices from 7.91 to 10.58 mAcm-2, while the open-circuit voltage (0.80V) and fill factors (0.60) were unaffected. As a result, the CF-processed PDFQx-3T:P(NDI2OD-T2) film with the smallest average domain size of 30 nm had the highest PCE of 5.11% due to large interfacial areas and efficient exciton separation. The domain size does not solely influence the device characteristics; other morphological factor including the domain purity in D and A phases as well as the polymer orientation at D/A interface should be considered together for optimization of all-PSCs.
1. H. Kang, W. Lee, J. Oh, T. Kim, C. Lee, B. J. Kim, Acc. Chem. Res., 2016, DOI: 10.1021/acs.accounts.6b00347.
2. C. Lee, H. Kang, W. Lee, T. Kim, K.-H. Kim, H. Y. Woo, C. Wang, B. J. Kim, Adv. Mater., 2015, 27, 2466.
3. H. Kang, M. A. Uddin, C. Lee, K.-H. Kim, N. Thanh Luan, W. Lee, Y. Li, C. Wang, H. Y. Woo, B. J. Kim, J. Am. Chem. Soc. 2015, 137, 2359.
4. Y. Li, S. J. Ko, S. Y. Park, H. Choi, T. L. Nguyen, M. A. Uddin, T. Kim, S. Hwang, J. Y. Kim, H. Y. Woo, J. Mater. Chem. A, 2016, 4, 9967-9976.
5. C. Lee, Y. Li, W. Lee, Y. Lee, J. Choi, T. Kim, C, Wang, E. D. Gomez, H. Y. Woo, B. J. Kim, Macromolecules, 2016, 49, 5051–5058.
8:00 PM - ES11.9.15
Characterization of Titanium Nitride as Iron Diffusion Barrier for GaAs Thin Film Solar Cell on Steel
Saloni Chaurasia 1 , Srinivasan Raghavan 1 , Sushobhan Avasthi 1 Show Abstract
1 Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore, KA, India
Steel foil is an attractive substrate for thin film solar cells because of its low cost, stability at high temperatures, and flexibility. Unfortunately, efficiency of solar cells on steel is always poor . The problem is that at typical device fabrication temperatures (6000 C-9000 C), iron in steel diffuses in to the active layers of the solar cell.In the active layers, diffused Fe introduces mid-gap defect levels which increase carrier bulk recombination and hence reduce solar cell efficiency. In this paper, we report that sputtered TiN thin-films can be used to inhibit the iron diffusion by 105 times, enabling higher efficiency solar cells on steel.
TiN films are deposited using reactive pulsed DC sputtering on 100 micron thick low carbon steel substrate. The power density is 2.2 Wcm-2 , gas flows are20 sccm and 5sccm for Ar and N2 respectively, chamber pressure is 3.00 X 10-3 mbar, substrate temperature is3000 C and the TiN thickness is 600nm.
X-ray diffraction (XRD) measurements show that the deposited films is polycrystalline TiN, with (111), (200), & (310) peaks clearly visible. Other than TiN, peaks of steel substrate are also detected. Scanning electron microscopy (SEM) further confirms that the TiN films are polycrystalline with a columnar morphology .Density measured using X–ray reflectivity measurements (XRR) is 5.65g/cc, which compares well to the reported bulk value of 5.43g/cc. This shows that the polycrystalline TiN film is dense and non-porous.
To measure the Fe diffusion, TiN-film on steel were annealed at 6000C – 9000C. XRD of the annealed films show TiN peaks but with much lower intensity suggesting a more amorphous phase. SEM of the annealed films shows coalescence of columnar structure suggesting that the films are more amorphous .The diffusion of iron from the steel into the TiN film is estimated using dynamic secondary ion mass spectrometry (SIMS). The diffusion coefficient obtained at temperatures of 300 0C, 7000C and 9000C are 0.02 ± 0.001 x 10-14 cm-2 s-1, 0.15 ± 0.02 x 10-14 cm-2 s-1 and 0.89 ± 0.05 x 10-14 cm-2 s-1 respectively . These values compare favourably with previously reported value of Fe diffusivity in TiN . In comparison, diffusion of Fe in GaAs is ~105 times higher .Clearly TiN can act as an iron diffusion barrier for GaAs solar cells on steel. Integration of solar cells on this barrier-layer coated steel is underway.
 CIGS thin-film solar cells on steel substrates, Thin Solid Films, Volume 517, Issue 7, 2 February 2009, Pages 2415–2418.
Iron contamination in silicon solar cell production environments, 2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), Denver, CO, 2014, pp. 3479-3484.
Optimal deposition conditions of TiN barrier layers for the growth of vertically aligned carbon nanotubes onto metallic substrates, Appl. Phys. 42 ,2009,104002 (9pp).
 Diffusion And Solubility Of Electrically Active Iron Atoms In Gallium Arsenide , Russian Physics Journal,Vol. 51, No 11, 2008.
8:00 PM - ES11.9.16
Surface Passivation Quality of Ozone and Water-Based Al2O3 Film Grown by Atomic Layer Deposition for Silicon Solar Cell
Hyo Sik Chang 1 , Young Joon Cho 1 Show Abstract
1 , Chungnam National University, Daejeon Korea (the Republic of)
In this work we present the surface passivation properties of ozone-oxidized Al2O3 compared to water-oxidized Al2O3 films by using atomic layer deposition (ALD). Ozone-based Al2O3 sample show a better thermal stability after firing process. The surface passivation quality was determined directly after deposition, after anneal at 500oC in nitrogen ambient for 25 min and after firing at 850oC for several seconds. After deposition and activation annealing, we find significantly higher passivation quality for the water-based ALD-Al2O3 samples than for ozone based samples. After firing the surface passivation quality for ozone-based Al2O3 sample is slightly better because of less interfacial reaction. Thermal stability of ozone and water reactant samples was investigated passivation performance using Quasi-Steady-State photoconductance (QSSPC), interface defect density, XPS.
8:00 PM - ES11.9.17
Effects of Columnar Grain Boundaries on Thin Film Polycrystalline GaAs Solar Cells
Khushboo Kumari 1 , Sushobhan Avasthi 1 Show Abstract
1 Centre for Nanoscience and Engineering, Indian Institute of Science Bangalore, Bengaluru, KA, India
The state-of-the-art single crystalline GaAs solar cells have an efficiency of >29% while the best single junction multicrystalline GaAs solar cells have an efficiency of only 18.4 %. This reduction in efficiency is due to intra-grain defects as well as defects at the grain boundaries. In order to have a preliminary idea, solar cells with varying grain sizes and base thicknesses were simulated in two-dimensions using SILVACO ATLAS. For ease of analysis, the GaAs thin-films were assumed to have columnar grain structures. Thin-films with such an orientation have been shown to yield best films because of lower recombination losses. Here these grain boundaries were simulated as vertical interfaces with varying surface recombination velocities. In accordance with our expectation results show that the efficiency of solar cells increases with increase in grain size. However the gains in efficiency are limited for films with large grains (> 20 µm), because in large grains most of the carriers are generated too far from the grain-boundary defects. There is also an increase in efficiency as the base thickness is increased from 1 µm to 3 µm. This is because thicker GaAs films absorb more incident radiation. Furthermore, increase in base thickness to 5 µm does not increase the efficiency much as almost all the solar radiations are absorbed within 3 µm of GaAs solar cells. Next, the effect of intra-grain lifetimes was studied by changing the minority carrier lifetime of electrons in the bulk GaAs. As expected, higher lifetime increases the efficiency of the solar cell, but only to an extent. Once the diffusion lengths are comparable to the grain-size, the characteristics of the solar cells do not change any further. In order to understand the effects of these grain boundaries in further detail, simulations were carried out by varying width and density of states at these grain boundaries. These grain boundary effects also change with the doping levels in these grains. So the doping levels in these grains were varied and their effects on efficiency of solar cell were observed. We also simulated double junction solar cells with AlGaAs as the window layer. The effect of doping and thickness of AlGaAs window layer was also studied.
In conclusion, we simulated single junction and double junction thin-film GaAs solar cells with columnar grains to design better devices. 3-5 µm thick GaAs thin-films with grain size larger than 60 µm were found to have efficiency >19%. Addition of AlGaAs as window layer further improved this efficiency. These numbers can be used as guideline by materials growth community to fabricate better thin-film GaAs solar cells.
8:00 PM - ES11.9.18
HIT Solar Cell with V2Ox Window Layer
Erenn Ore 1 , Gehan Amaratunga 1 Show Abstract
1 Engineering, University of Cambridge, Cambridge United Kingdom
In the conventional crystalline silicon heterojunction solar cell with intrinsic thin layer (the HIT solar cell) structure, a p-doped thin film silicon or its alloy (pDTF-Si/A) is used as the hole collecting window layer. However, the parasitic absorbance in the pDTF-Si/A window layer, and the toxic, explosive diborane gas used for p-doping are limiting factors for achieving HIT cells with reduced processing costs and / or with higher efficiencies. In this work, pDTF-Si/A is replaced by V2Ox, which is deposited by a simple physical vapour deposition technique. Due to the wide band gap of V2Ox, the HIT solar cell with the V2Ox window layer generates a higher short current density than the reference conventional HIT cell under 1 sun, and achieves an open circuit voltage of 0.7 V. Furthermore, from the quasi steady state open circuit voltage measurements, the charge carrier lifetime and pseudo-efficiency values of the HIT solar cell with the V2Ox window layer indicate that this cell has the potential to outperform the conventional HIT cell in terms of the power conversation efficiency under the standard test conditions.
8:00 PM - ES11.9.19
Solution Process Synthesis of a TiOx Passivation Layer for Silicon-PEDOT:PSS Solar Cell
Yuqiang Liu 1 , Baoquan Sun 1 Show Abstract
1 , University, SuZhou China
Silicon hybrid solar cells that made of n-type silicon (Si) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) offered a considerable and practical solution to achieve high efficiency and low cost solar cells. However, the performances of Si-PEDOT:PSS solar cells were still jeopardized by silicon rear contact recombination losses. Here, titanium oxide (TiOx) layer from simple solution method was fabricated on silicon rear side to reduce performances losses. Minority carrier lifetime, transient photovoltage decay and external quantum efficiency measurements indicated that TiOx could effective suppress the recombination losses of silicon rear side, which reflected in the improvement of open circuit voltage (Voc) and long-wavelength photoresponse. Contact resistance and capacitance-voltage measurements further showed that resistance and rear adverse barrier height between silicon and aluminum (Al) could be reduced owing to TiOx layer. The highest recorded Voc of 643 mV was achieved for textured Si-PEDOT:PSS solar cell by inserting TiOx between rear side silicon and Al. Power conversion efficiency (PCE) of 14.2% solar cell was achieved, which was the record efficiency for nanostructured Si-organic heterojunction solar cell. The simple solution processed metal oxide layer provided a facile way for silicon surface passivation to achieve high performance solar cells.
8:00 PM - ES11.9.21
Ultra-Thin SiO2 Layers for Defect-Free Si/Oxide Carrier Selective Contacts
Rudra Mukherjee 1 , Pramod Ravindra 1 , Sushobhan Avasthi 1 Show Abstract
1 Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore, Karnataka, India
Several Si/oxide heterojunctions have been proposed as carrier-selective contacts for Si solar cells . These heterojunctions are especially useful for Si/perovskite tandem cells because the oxides can also serve as transport layers for perovskite subcell. The trap states at the Si surface cause carrier recombination, reducing the VOC of carrier-selective contacts. In this work, we present a tunnel-oxide that can be used to reduce interface recombination losses at carrier selective contact. The tunnel-oxide is used to improve the VOC of a Cu2O/Si heterojunction solar cell from 328mV to 528mV.
To quantify the surface recombination velocity(SRV) at the Si surface, the lifetime of minority carrier in Si was measured using microwave-detected photoconductivity (MDP). First, a 100 nm thermal oxide was grown on 1015 cm-3 n-Si(100) wafer. The measured minority carrier lifetime was 320 μs. Next, the top SiO2 layer was etched using buffered HF. After an RCA-clean, a tunnel-oxide was deposited via rapid thermal oxidation (RTO) at two conditions: 800C & 40s; and 650C & 40s. The tunnelling-oxide thickness, measured by ellipsometry, for the two conditions was 2 nm and 1.2 nm, respectively. For the 2nm and 1.2 nm thick oxides, the carrier lifetime and implied SRV were 80 us & 63 us; and 480 cm/s & 655 cm/s, respectively. These tunnelling oxides lies in the current path of a solar cell, so it is important that the voltage drop across these oxides be minimal. To estimate the resistive losses across the tunnel oxides, devices with Al/tunnel-oxide/Al structure were fabricated. At current-density of 25mA/cm2 (typical currents in solar cells at AM1.5), the contact resistance of the 2nm thick oxide is 102 Ωcm2. In comparison, the contact resistance of the 1.2nm thick oxide is only 9.5 Ωcm2. Based on the SRV and conductivity data, the 1.2nm thick tunnelling-oxide was selected for device-testing. Cu2O/Si heterojunction is a electron-blocking carrier-selective contact that has been reported before . Cu2O/Si heterojunction solar cells were fabricated with and without the tunnelling-oxide. Without the tunnelling-oxide, the Cu2O/Si heterojunction solar cells showed a VOC of only 328 mV, arguably due to the high density of Si defects at the Cu2O/Si interface. With the tunnel-oxide, the VOC of the Cu2O/Si heterojunction solar cells improved dramatically to 528 mV, confirming the passivating characteristics of the tunnel-oxide. In conclusion, we present RTO-grown tunnel-oxide with a Si surface recombination velocity of 480-655 cm/s and a contact resistance of 102-9.5 Ωcm2. The passivating tunnel-oxide reduces surface defects at Si/oxide carrier selective contacts, increasing the VOC of a Cu2O/Si heterojunction solar cell by 200mV.
 Hassan Imran, "Carrier-Selective NiO/Si and TiO2/Si Contacts for Silicon Heterojunction Solar Cells"
 Pramod Ravindra, Sushobhan Avasthi, "1EP4.11.32 Electron-Blocking Properties of Crystalline-Silicon/Cu2O Heterojunctions for Photovoltaics"
8:00 PM - ES11.9.22
High Temperature InGaN Solar Cells for Full Spectrum Solar Energy Harvesting
Ehsan Vadiee 1 , Heather McFavilen 2 , Alec Fischer 1 , Shirong Zhao 1 , Joshua Williams 1 , Stephen Goodnick 1 , Christiana Honsberg 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States, 2 , Photonitride Devices Inc, Tempe, Arizona, United States
There exists a continuing need for multi-junction solar cell devices for absorbing full solar spectrum. We address this need by suing InxGa1-xN based solar cells grown by Metalorganic Chemical Vapour Deposition (MOCVD) growth method. InxGa1-xN alloy has unique properties including a tunable bandgap that can cover the entire visible solar spectrum and a high absorption coefficient (×10 higher than GaAs). In addition, it has the capability to operate at high temperatures due to its high thermal and high radiation resistances.
In this work, we investigate the single junction InXGa1-XN/GaN MQW solar cells grown by MOCVD with different contact metallurgies to emphasis on the operation at temperatures, higher than 500 oC, with no permanent degradation. In order to improve the carrier transport in Mg doped GaN contact layer, different metal/indium-tin-oxide contact schemes were implemented. Electrical and optical characterizations have been also performed to analyze the contact quality. We found low ohmic contacts with high transmittance of 94% at λ~500 nm. The p-contact proves suitable for high temperature performances.
Temperature dependent external quantum efficiency and 1-sun light current-voltage measurements were performed for all samples. Correlations are made between different characterization methods to draw conclusions about device behavior in the high temperature regime. Photovoltaic performances including VOC, JSC, FF, and WOC are presented at different temperatures.
8:00 PM - ES11.9.23
Frequency Selective Thermal Extraction for High Efficiency Thermophotovoltaics
Zoila Jurado 1 , Junlong Kou 1 , Austin Minnich 1 Show Abstract
1 , California Institute of Technology, Pasadena, California, United States
The efficiency of thermophotovoltaic energy conversion is limited by the broad spectrum of thermal radiation due to thermal loss or poor absorption of low energy photons. Although spectral filters have been used to restrict the emitted radiation spectrum to increase efficiency, these filters also decrease the power density. Here, we demonstrate a route to achieve simultaneous spectral filtering and enhancement of radiative flux beyond the apparent blackbody limit of the emitter using thermal extraction [Nature Communications, 4, 1730 (2013)]. The passive device consists of a hemispherical dome composed of ZnSe covered with a frequency selective patterned surface in optical contact with an emitter. The frequency selective pattern is designed such that only wavelengths around the photovoltaic band gap are transmitted. The radiative flux in this bandwidth is enhanced beyond the apparent blackbody limit of the emitter due to the transmission of high wave vector modes in the emitter to the far-field, facilitated by the hemisphere. Our work demonstrates an approach to simultaneously achieve higher efficiency and increased power density of thermophotovoltaic energy conversion.
N.J. Ekins-Daukes, Imperial College London
Louise Hirst, U.S. Naval Research Laboratory
Richard King, Arizona State University
Bryce Richards, Karlsruhe Institute of Technology (KIT)
ES11.10: Singlet Fission and IBSC
Thursday AM, April 20, 2017
PCC North, 200 Level, Room 221 AB
8:45 AM - *ES11.10.01
Exploring Strategies for Charge or Energy Transfer from Molecules to Semiconductors after Singlet Fission
Justin Johnson 1 , Natalie Pace 1 2 , Dylan Arias 1 , Ye Yang 1 , Garry Rumbles 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States
Singlet fission is a process occurring in specific molecular systems that produces two electron-hole pairs for each absorbed photon. It has the potential to utilize higher energy solar photons (e.g., blue-green) more efficiently than conventional semiconductors, which typically allow highly excited charge carriers to undergo thermalization and thus loss of potential energy to phonons. After significant progress toward understanding the molecular design principles and photophysical mechanism of singlet fission, we have begun to consider methods for practical utilization. Depending on the desired scheme, singlet fission can serve in tandem with a conventional solar cell by donating either energy or charge, effectively doubling the photocurrent above the absorption onset of the singlet fission layer. We have investigated mono- and multilayers of pentacene, tetracene, and 1,3-diphenylisobenzofuran derivatives adsorbed to various bulk semiconductors, including wide band gap oxides and high-performing solar cell materials (e.g., TiO2 and perovskites). The molecular ordering within the layer itself influences the rate and yield of the singlet fission process, while the energy level alignment affects the energy or charge transfer scheme. These processes occur in parallel and in competition, thus control of the various rates is important for producing the desired outcome. We have used a variety of techniques to investigate the dynamics near and across the interface: pump-probe transient absorption and reflectivity through visible and infrared wavelengths, time-resolved photoluminescence, time-resolved microwave conductivity, and the detection of photocurrent. The former techniques are capable of monitoring the formation of triplets and their initial transfer across the interface, whereas the latter detect the yield of free charges produced. The trends observed outline a route toward the ideal set of molecules, semiconductors, and interfacial properties that best utilizes the advantages of singlet fission.
9:15 AM - ES11.10.02
A Solid-State Organic Intermediate Band Solar Cell with Electrically Integrated Triplet-Triplet Annihilation Up Conversion
YunHui Lin 3 , Marius Koch 1 , Alyssa Brigeman 2 , David Freeman 4 , Lianfeng Zhao 3 , Hugo Bronstein 4 , Noel Giebink 2 , Greg Scholes 1 , Barry Rand 3 Show Abstract
3 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 1 Chemistry, Princeton University, Princeton, New Jersey, United States, 2 Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States, 4 Chemistry, University College London, London United Kingdom
Conventional solar cells absorb photons with energy above the bandgap of the active layer while sub-bandgap photons are unharvested. One way to overcome this loss is to capture the low energy light in the triplet exciton state of a molecule capable of undergoing triplet-triplet annihilation upconversion (TTA-UC), which pools the energy of two triplet states into one high energy singlet state that can then be extracted as charge. This mechanism underlies the function of an organic intermediate band solar cell (IBSC). Despite being an attractive next-generation device concept with the same limiting efficiency as its close relative, the singlet fission solar cell, the organic IBSC has received very little attention.
Here, we report a solid-state organic IBSC that uses TTA-UC for enhanced photocurrent response to sub-bandgap light. Femtosecond resolution transient absorption spectroscopy and delayed fluorescence spectroscopy provide evidence for the triplet sensitization and upconversion mechanisms, while external quantum efficiency measurements in the presence of a broadband background light demonstrate the performance enhancements that are achievable in this device. The solid-state design introduced in this work improves upon aspects that have limited the efficiency gains of previously reported solution-based designs, and therefore serves as a compelling model for future research efforts in this area.
ES11.11: Multi-Junction PV
Diego Alonso Alvarez
Thursday AM, April 20, 2017
PCC North, 200 Level, Room 221 AB
10:00 AM - *ES11.11.01
Dilute Nitride Solar Cells—Technology Developments towards 50% Efficiency
Mircea Guina 1 , Antti Tukiainen 1 , Arto Aho 1 , Ville Polojarvi 1 Show Abstract
1 Optoelectronics Research Centre, Tampere University of Technology, Tampere Finland
Dilute nitride materials, i.e. family of GaInNAsSb/GaAs compounds, offer ideal band-gap and lattice characteristics for the development of tandem lattice-matched solar cells with more than 3 junctions. In terms of material quality and efficiency performance, molecular beam epitaxy (MBE) has recently emerged as the preferred alternative for the development of such multi-junction solar cells. However, despite the proven potential of this material system for demonstrating high-efficiency lattice-matched tandem cells, a certain level of pessimism remains in respect with the potential for large scale deployment of MBE as cost-effective manufacturing approach. While there are clear cost advantages for metal-organic chemical vapour deposition (MOCVD) processes, largely linked to the high growth rates and throughput, there are no fundamental limits that would prevent emergence of MBE as a cost-effective manufacturing technology for solar cells with 4 or more junctions.
From this general perspective, we review a new technology approach for the fabrication of multi-junction solar cells combining the best parts of the MOCVD and MBE techniques to realize high-efficiency tandem solar cells . As an implementation example, triple-junction GaInP/GaAs/GaInNAsSb (1 eV) solar cells exhibiting an efficiency of ~31% for 1-sun illumination are reported.
We also review recent developments concerning the fabrication of 4-junction dilute-nitride solar cells involving MBE-only processes. In particular, we focus on performance characteristics of solar cells incorporating two dilute nitride sub-junctions with absorption edge at 1.2 eV and 0.9 eV. Finally, a development outline towards demonstrating solar cells with efficiency of 50% is discussed.
 A. Tukiainen, A. Aho, G. Gori, V. Polojärvi, M. Casale, E. Greco, R. Isoaho, T Aho, M. Raappana, R. Campesato and M. Guina; High-efficiency GaInP/GaAs/GaInNAs solar cells grown by combined MBE-MOCVD technique; Progress in PV: Research. and Applications, 24, 7, p. 914-919 (2016)
10:30 AM - *ES11.11.02
Development of Epitaxial III-V/Si Multijunction Solar Cells
T. Grassman 1 , Daniel Chmielewski 1 , John Carlin 1 , Steven Ringel 1 Show Abstract
1 , The Ohio State University, Columbus, Ohio, United States
The marriage of high-performance III-V compound semiconductor materials and devices with low-cost, high-availability Si substrates has been a ‘holy grail’ of photovoltaics materials research for decades. Further, III-V/Si multijunction solar cells utilizing active Si sub-cells, rather than merely cheap Si substrates – a “Si-plus” architecture – hold the potential for high conversion efficiencies on par with pure III-V multijunction structures, but at substantially lower costs. One of the most promising approaches to this end is via direct heteroepitaxial GaP/Si integration and bandgap/lattice constant engineering via compositionally-graded (strain-graded) GaAsyP1-y and/or Ga1-xInxP alloys to reach III-V compositions with bandgaps that are ideally-matched to the underlying Si. Substantial work by our team at The Ohio State University has recently culminated in the demonstrations of the first monolithic epitaxial GaAsP/Si dual-junction and GaInP/GaAsP/Si triple-junction solar cells. These early-stage prototypes indicate clear pathways for optimization toward high-performance devices. This presentation will discuss these results, along with a brief overview of the prior lead-up work—GaP/Si integration, GaAsyP1-y metamorphic grading, Si bottom cell design and development, and metamorphic III-V upper cell and tunnel junction development.
11:00 AM - ES11.11.03
1-eV GaNAsSb Solar Cells Lattice-Matched to GaAs
Aymeric Maros 1 , Chaomin Zhang 1 , Nikolai Faleev 1 , Christiana Honsberg 1 , Richard King 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
Dilute nitride semiconductors have attracted a great deal of interest in the field of III-V multijunction cells due to their ability to be grown lattice-matched to GaAs or Ge while having a tunable bandgap in the range 0.8 – 1.4 eV. These properties make them ideal candidates for use in lattice-matched tandem solar cells with four junctions and higher . Monolithic lattice-matched tandems present advantages over other multijunction technologies as they do not require the use of metamorphic buffer layers or complex wafer-bonding processes, making them much more manufacturable than current efficiency record holders designs.
In this work we report on the growth, fabrication and characterization of 1-eV GaN0.024As0.905Sb0.071 solar cells lattice-matched to GaAs. Due to the low minority carrier diffusion length associated with these dilute nitride layers, both p-i-n and p-n configurations have been investigated. These GaNAsSb solar cells were grown by plasma-assisted molecular beam epitaxy at a growth temperature of 440°C and a growth rate of 1 µm/hr. The As/Ga and Sb/Ga beam equivalent pressure ratios were set to 10 and 0.16, respectively. GaAs p-i-n and p-n reference cells were also grown for comparison. No antireflective coating layers were applied.
The best GaNAsSb device was a p-n heterojunction with a 1-µm thick p-GaNAsSb base and a 200-nm thick n-GaAs emitter. The bandgap was 0.97 eV and the cell demonstrated a short circuit current density of 17.17 mA/cm2 with an open-circuit voltage of 0.365 V, corresponding to a bandgap-voltage offset Woc ≡ (Eg/q) – Voc of 0.605 V. It was found that GaAs cells grown under the exact same growth conditions as the GaNAsSb cells (i.e., the same growth temperature and V/III ratio) demonstrated similar bandgap-voltage offset, indicating that N-related defects are not the only defects responsible for the high recombination rate and high Woc in these dilute nitride cells. An analysis based on deep-level transient spectroscopy will be used to provide more insight into the type of traps and their density present in both the GaAs and GaNAsSb solar cells.
Additionally, bismuth has been shown to improve the optical quality of GaNAs(Bi) and GaInNAs(Bi) , similar to the surfactant effect of antimony during the growth of GaInNAs(Sb) . We propose here to use bismuth as a surfactant during the growth of GaNAsSb and will present the effect of adding a small flux of Bi during growth on the surface reconstruction, the N and Sb incorporation and on the structural and optical properties of the materials. The potential advantages of using Bi during the growth of GaNAsSbBi solar cells will be discussed.
 R. R. King et al., Prog. Photovolt. Res. Appl., vol. 20, no. 6, pp. 801–815, Sep. 2012.
 S. Tixier et al., J. Cryst. Growth, vol. 251, no. 1–4, pp. 449–454, Apr. 2003.
 H. B. Yuen et al., J. Appl. Phys., vol. 99, no. 9, p. 093504, 2006.
11:15 AM - ES11.11.04
Imaging Atomic Scale Clustering in III-V Semiconductor Alloys for Multi-Junction Photovoltaics
Louise Hirst 1 , Nicole Kotulak 1 , Stephanie Tomasulo 1 , Josh Abell 1 , Maria Gonzalez 2 , Michael Yakes 1 , Jerry Meyer 1 , Robert Walters 1 , Chengyu Song 3 , Petra Specht 4 , Peter Ercius 3 , Christian Kisielowski 3 Show Abstract
1 , U.S. Naval Research Laboratory, Washington, District of Columbia, United States, 2 , Sotera Defense Solutions, Inc., Annapolis Junction, Maryland, United States, 3 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 , University of California, Berkeley, California, United States
Quaternary alloys are critical to the development of high efficiency multi-junction photovoltaics. Emerging device designs, targeting record efficiencies, are increasingly complex and require combinations of materials with specific bandgaps and band alignments to achieve optimal solar absorption, as well as new types of tunnel junctions to transparently transport carriers between sub-cells and to create effective passivating layers. Materials with different lattice constants and bandgap combinations can be engineered by adjusting the relative compositions of the constituent elements in multi-species alloys. However, immiscibility of these elements can degrade the optical and electrical properties of the material, and photovoltaic devices are particularly sensitive to this degradation.
While the effects of cluster formation in these materials can be effectively characterized with luminescence methods, imaging the clusters themselves to determine the atomic arrangements which lead to the observed properties is a historically difficult challenge, primarily because of the complexity of electron microscope image interpretation and the potential for ambiguity. We have studied the novel quaternary alloy InAlAsSb as a candidate for a high bandgap subcell. Luminescence measurements indicate that non-uniformities in the material significantly degrade the optical properties of the solid. In this work we address the difficulties associated with imaging cluster formation in this material, employing quantitative image analysis to interpret atomic scale scanning transmission electron microscope (STEM) images. Through comparison with simulation we identify a non-random atomic arrangement within the alloy, which is consistent with the observed anomalous luminescence signatures. This work might be used to mitigate cluster formation in InAlAsSb and other prospective alloys for photovoltaic applications, as well as to identify the original seeding mechanisms for inhomogeneities in the materials and trends in the cluster formation as a function of growth parameters.
11:30 AM - ES11.11.05
Structural Properties of Si/GaAs Interfaces Fabricated by Surface-Activated Bonding at Room Temperature
Yutaka Ohno 1 , Jianbo Liang 2 , Naoteru Shigekawa 2 Show Abstract
1 , Tohoku University, Sendai Japan, 2 , Osaka City University, Osaka Japan
Tandem cells consisting of Si and III-V compounds are one of the promising candidates for next-generation solar cells due to their high conversion efficiencies, as well as their low cost, lightweight and mechanical robustness, in comparison with conventional Si and III–V multijunction cells. However, Si/III-V interfaces free from structural defects, by which a current flow is not disturbed, have been hardly fabricated by epitaxial deposition because of the differences in crystal lattice and thermal expansion coefficient. Recently, a surface-activated bonding (SAB) method at room temperature (RT), in which surfaces of substrates are activated by the fast atom beams of argon (Ar) prior to bonding, is applied to form Si/GaAs interfaces with a low interface resistance , and InGaP/GaAs/Si hybrid triple-junction cells with a high conversion efficiency above 26% are fabricated . The interface resistance varies depending on the SAB condition of Ar beam irradiation and post-growth annealing , even though the origin of the resistance is still unclear. Accordingly, a comprehensive knowledge of the electrical property at the interfaces depending on their atomistic structure is indispensable to establish the fabrication processes of high-efficiency tandem cells by optimizing the interface structure.
In the present work, p-Si/n-GaAs interfaces were fabricated at RT under a SAB condition , with the substrates of B-doped (100) p-Si (with a carrier concentration of 2x1014 cm-3) and Si-doped (100) n-GaAs (2x1016 cm-3). A part of them were then annealed at 473 K or 673 K for 1 min. Their structural property were determined by transmission electron microscopy (TEM).
An amorphous layer (about 3 nm thick) was formed at an as-bonded Si/GaAs interface . Plane-view TEM of the interface (i.e., TEM observation normal to the interface) revealed that, there was no structural defects expanding along the interface, such as cracks, void, and misfit dislocations. This would result in a low resistance at the interface . The Si surface on the interface was rather smooth, while the GaAs surface on the interface was strained presumably due to dimples (about 5 nm in size) introduced by Ar atom irradiation and dumps (about 20 nm in size) related to threading dislocations in GaAs. This results suggest that the Si/GaAs interface resistance may be reduced further by smoothing the GaAs surface via optimization of the SAB condition. Effects of post-growth annealing on the structural property will be discussed.
 J. Liang, L. Chai, S. Nishida, M. Morimoto, and N. Shigekawa, Jpn. J. Appl. Phys. 54 (2015) 030211.
 N. Shigekawa, J. Liang, R. Onitsuka, T. Agui, H. Juso, and T. Takamoto, Jpn. J. Appl. Phys. 54 (2015) 08KE03.
11:45 AM - ES11.11.06
The GaAs/GaAs/Si Two-Terminal Tandem Solar Cell—Current Matching for a Non-Ideal Bandgap Combination
Ian Marius Peters 1 2 , Zekun Ren 2 , Haohui Liu 3 , Zhe Liu 3 , Chuan Seng Tan 4 , Armin Aberle 3 5 , Tonio Buonassisi 1 2 Show Abstract
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , SMART, Singapore Singapore, 3 , SERIS, Singapore Singapore, 4 , NTU, Singapore Singapore, 5 , NUS, Singapore Singapore
Tandem solar cells are widely viewed as a next development step in photovoltaics due to their ability to enable fundamentally higher conversion efficiencies than single-junction devices. To reach these high efficiencies in practice, either of the used sub-cell technologies needs to be sufficiently advanced. GaAs and silicon mark the two materials with the highest single-junction efficiencies under non-concentrated sunlight. The bandgap pairing of these materials is, however, not ideal, which fundamentally limits the conversion efficiency that can be achieved with this combination. Furthermore, current matching the two cells, a requirement for an integrated two-terminal tandem, is challenging as the number of photons that can be absorbed in the GaAs top cell is roughly two thirds of that of the silicon bottom cell.
In this contribution we introduce the GaAs/GaAs/Si triple-junction concept. This concept solves the current matching problem by distributing the current generation in GaAs over two cells. The resulting current in each of the GaAs cells provides a good match to the generation in the silicon bottom cell, and therefore allows realizing a current matched triple-junction cell, in which each photon is used in the material in which it creates the highest PV efficiency. We have fabricated a prototype of the GaAs/GaAs double-junction solar cell with an efficiency of 17.8%. The main challenge of this structure is the need of a top cell with a small thickness of only 105 nm. We have used this junction to create a four-terminal GaAs/GaAs//Si solar cell with an efficiency of 20.4%.
To further establish this concept, we have performed a study to address implications of the trade-off between the non-ideal bandgap combination and the high material quality achieved with the two materials. The non-ideal bandgap pairing of Si (1.12 eV) and GaAs (1.42 eV) yields a radiative efficiency limit of 41.7%, significantly below the value of 45.1% for an ideal bandgap partner of silicon (~1.75 eV). The high material quality that has been achieved with GaAs and Si means, however, that a tandem made of these materials can potentially operate close to the ideal value. Using parameters extracted from current world record cells, we estimate the realistic efficiency potential for the two-terminal GaAs/GaAs/Si tandem cell at 33.0%. For comparison, a two-terminal tandem of InGaP (1.8 eV) and Si with best published parameters would only achieve 27.6%. Note that 29.8% were achieved with this material combination, though in a four-terminal configuration. However, recent results from industry indicate that the quality of InGaP has been improved beyond what is currently published.
In summary, the combination of GaAs and Si has the potential to achieve world record one-sun tandem solar cells. We have introduced the GaAs/GaAs/Si concept as a way to realize an integrated, current-matched two-terminal tandem solar cell.
ES11.12: Intermediate Band PV
Thursday PM, April 20, 2017
PCC North, 200 Level, Room 221 AB
1:30 PM - *ES11.12.01
Intermediate Band Solar Cells—Status and Future Directions
Jacob Krich 1 Show Abstract
1 , University of Ottawa, Ottawa, Ontario, Canada
Intermediate band (IB) materials are a novel class of materials that, like semiconductors, have a band gap but also have an extra set of allowed electronic levels entirely contained inside the semiconductor band gap, allowing sub-gap photon absorption. Solar cells made from such materials have the potential to radically improve photovoltaic efficiencies, similar to triple-junction cells. Current intermediate band devices are made from three classes of materials: quantum dots, highly-mismatched alloys, and hyperdoped semiconductors. None has simultaneously achieved high sub-gap absorption and sufficient carrier lifetime. I will describe theoretical and experimental work to understand carrier lifetimes and their impact on device efficiencies. I will introduce a figure of merit, which predicts the potential effectiveness of candidate IB materials in advance of device fabrication. This figure of merit captures the tradeoff between enhanced absorption and enhanced recombination within an IB material, and it suggests a path toward efficient IB materials. I will give examples of measurements of the figure of merit and demonstrate a method for theoretical predictions for new systems.
2:00 PM - *ES11.12.02
Multicolor Emission in Intermediate Band Solar Cell Materials
Wladyslaw Walukiewicz 1 2 Show Abstract
1 , Lawrence Berkeley National Lab, Berkeley, California, United States, 2 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
Highly mismatched alloys (HMAs) are semiconductors whose electronic energy band structure is determined by the band anticrossing interaction between localized states of minority component and extended states of the host semiconductor matrix. The interaction splits the conduction or the valence band leading to formation of an intermediate band making HMAs suitable for intermediate band solar cell (IBSC) applications. The key requirement for proper operation of an IBSC is a strong optical coupling between different bands of the light absorber. Here we present a direct observation of these optical transitions in two highly mismatched alloy systems, GaNAs in which the anticrossing interaction splits the conduction band and ZnOSe in which localized Se levels are responsible for the splitting of the valence band. We use electroluminescence (EL) to demonstrate optical transitions involving intermediate band in GaNAs IBSC structure and combination of time resolved photoluminescence (TRPL) and photoreflectance to show optical transition between conduction band and different valence subbands in ZnOSe alloy. The EL spectrum in GaNAs IBSC structure consists of two emission bands associated with transitions from conduction to the intermediate and from intermediate to the valence band. As expected the multicolor EL is observed for both forward and reverse bias conditions but only in the structure with the intermediate band electrically isolated from contacts. . Also we have measured a simultaneous three colour photoluminescence originating from the transitions between conduction and the valence band split by the anticrossing and the spin-orbit interactions in O-rich ZnOSe with up to 8 % Se. The transitions are confirmed by PR measurements and their dependence on the alloy composition is well explained by the valence band anticrossing model. The observation of the emission is possible because of longer than 1 nsec lifetimes. The results demonstrate feasibility of using highly mismatched alloys for solar power conversion applications.
2:30 PM - ES11.12.03
Lead Halide Perovskite-Based Intermediate Band Absorbers
Alex Martinson 1 , Matthew Sampson 1 , Ji-Sang Park 1 , Richard Schaller 1 , Maria Chan 1 Show Abstract
1 , Argonne National Laboratory, Argonne, Illinois, United States
Lead halide perovskites have proven to be a versatile class of visible light absorbers that allow rapid access to the long minority carrier lifetimes and diffusion lengths desirable for traditional single-junction photovoltaics. We explore the extent to which the attractive features of these semiconductors may be translated to a multi-level solar energy conversion system capable of exceeding the Shockley-Queisser limit through the identification of a substitutionally-doped intermediate band absorber. We computationally and experimentally explore the substitution of transition metals on the Pb site of MAPbX3 (MA = methylammonium, X = Br or Cl) to achieve a tunable density of states within the parent gap. Computational screening identified two substituted MAPbBr3 as promising intermediate band candidate materials, and the films were synthesized via conventional solution-based processing techniques. First-principles density functional theory (DFT) calculations support the existence of intermediate bands upon transition metal incorporation and enhanced sub-gap absorption, which are confirmed by UV-visible-NIR absorption spectroscopy. Furthermore, steady state and time-resolved PL studies reveal no sign of self-quenching, which suggest this class of materials to be promising for future intermediate band photovoltaics.
3:15 PM - *ES11.12.04
Challenges and Progress on the Development of Quantum Dot Intermediate Band Solar Cells
Yoshitaka Okada 1 , Yasushi Shoji 1 , Ryo Tamaki 1 Show Abstract
1 , University of Tokyo, Tokyo Japan
On-going progress and challenges on the development of quantum dot based intermediate band solar cells (QD-IBSC) are reviewed. For QD-IBSCs reported till today, the cells suffer from small absorption and low QD densities, lead to a drop of the open-circuit voltage VOC. VOC and hence the efficiency, however, has been shown to recover fast and the cell performance improves with increasing concentrated illumination of sunlight. The areal density of QDs inevitably has a direct influence on the generation and recombination processes via IB. For a common InAs/GaAs QD system, it is calculated that the net gain of photocurrent production can be achieved with 100 suns and higher for the QDs areal density of 1×1013 cm-2.
The carrier lifetime and occupancy rate in IB are other important parameters that directly affect the conversion efficiency of QD-IBSCs. These parameters are determined by the carrier recombination strength, and thermal and tunneling escape rates out of QDs. A long electron lifetime is obtained by controlling the recombination rate using a type-II QD heterostructure or ratchet structure. Embedding QDs with a high potential barrier can suppress the thermal carrier escape out of QDs, and introducing an electric field damping layer in the junction can reduce the field-assisted tunneling rates.
3:45 PM - *ES11.12.05
Control of Carrier Dynamics and Efficient Two-Step Photon Up-Conversion in Quantum-Dot Intermediate-Band Solar Cells
Takashi Kita 1 Show Abstract
1 Department of Electrical and Electronic Engineering, Kobe University, Kobe Japan
Many efforts have been made to realize high-efficiency solar cells (SCs) by breaking the conversion limit, and novel concepts have been proposed to improve the efficiency. One promising SC is the intermediate-band SC containing an additional parallel diode connection, which can reduce the transmission loss. The intermediate-band SC includes intermediate states in the band-gap. By absorbing a sub-band-gap photon, an electron transits from the valence band to the intermediate band. Upon absorbing another sub-band-gap photon, the electron is further excited into the conduction band. This two-step photon up-conversion process following the absorption of two below-gap photons produces additional photocurrent without degrading the photovoltage. Though theoretical predictions of the conversion efficiency of an ideal intermediate-band SC is extremely high, several issues, such as very weak intraband transition from the intermediate band to the conduction band and quick energy relaxation of the excited electrons into the intermediate state, push down the efficiency. Therefore, improving the second-excitation efficiency in the two-step photon up-conversion process strongly influences the conversion efficiency. As the second-excitation efficiency is a function of the electron density of the initial state in the intermediate state, long-lived electrons are needed to be provided. Here, we control carrier dynamics in InAs quantum-dot intermediate-band SC and tackle to improve the two-step photon up-conversion efficiency. The key in our study is carrier separation in the intermediates state. If excited electron and hole are separated in the intermediate state, the electron lifetime is extended, and, thereby, the second-excitation efficiency becomes strong. First, I talk about results of carrier separation effects in the intermediate band formed by InAs quantum-dot superlattice minibands on the two-step photon up-conversion process. Then, I present our recent progress of room-temperature operation of the two-step photon up-conversion. Finally, I would like to discuss a structure realizing more efficient up-conversion.
4:15 PM - ES11.12.06
High Efficiency "Quantum Ratchet" Intermediate Band Solar Cells
Anthony Vaquero-Stainer 1 , Nicholas Hylton 1 , Megumi Yoshida 1 , Andreas Pusch 1 , Kenneth Kennedy 2 , Edmund Clarke 2 , Saurahb Kumar 2 , Mark Frogley 3 , Gianfelice Cinque 3 , Ortwin Hess 1 , N.J. Ekins-Daukes 1 , Chris Phillips 1 Show Abstract
1 Physics, Imperial College London, London United Kingdom, 2 , Sheffield University, Sheffield United Kingdom, 3 , Diamon Light Source, Oxford United Kingdom
Solar cells typically operate with a single fixed bandgap energy material, fundamentally limited by the Shockley-Queisser limit of 31.0% at 1 sun, whereby below bandgap photons are not absorbed. An efficiency enhancement can in principle be made by introducing an electrically isolated intermediate band, which allows sequential absorption of below bandgap photons. This increases the theoretical limiting efficiency to 46.8%, however in real world devices, induces large recombination losses.
Another level - the "ratchet" level can be introduced as shown in figure 2, which is optically decoupled from the valence band and at a lower energy than the intermediate band, thus decreasing occupation in the intermediate band and dramatically extending the carrier lifetime. This increases the probability of a sequential absorption process and reduces recombination losses, resulting in an increase of the limiting efficiency of the cell to 48.5% at 1 sun. The efficiency improvement over the conventional IBSC can even be much larger for real world devices with non-unity absorptivity into and out of the IB, despite the small energy loss, delta E, from the intermediate band, caused by the irreversible ratchet stage.
One method is to realise this in a real device is to utilise a highly optimised, coupled quantum well super lattice to cascade the electrons through the quantum well sub bands in the conduction band via quantum mechanical tunnelling as shown in figure 2. Thus the electrons excited into the conduction band via absorption of a high energy photon will be spatially separated from the holes in the valence band. This reduces the wavefunction overlap which will dramatically extend their lifetime. The probability of a second, lower energy photon being absorbed is now increased, thus providing additional two photon photocurrent.
In the talk, we will present the latest two-photon photocurrent spectroscopy data from this device, including time resolved measurements with multiple pulsed sources (both from lasers and a synchrotron) which reveal the enhanced lifetimes achieved in the ratchet state.