Simulation of absorption, photogeneration, and carrier extraction in nanostructure-based and ultra-thin film solar cell devices beyond the classical picture
In this paper, an optoelectronic device simulation framework valid for arbitrary spatial variation of electronic potentials and optical modes, and for transport regimes ranging from ballistic to di usive, is used to study non- local photon absorption, photocurrent generation and carrier extraction in ultra-thin lm and nanostructure- based solar cell devices at the radiative limit. Among the e ects that are revealed by the microscopic approach and which are inaccessible to macroscopic models is the impact of structure, doping or bias induced nanoscale potential variations on the local photogeneration rate and the photocarrier transport regime.
The effects of electric field on InGaAs quantum well i-region placement in InAlGaAs solar cells
Christopher G. Bailey;
Matthew P. Lumb;
David V. Forbes;
Michael K. Yakes;
Seth M. Hubbard;
Louise C. Hirst;
Joseph G. Tischler
Cory D. Cress;
Phillip P. Jenkins;
Robert J. Walters
InGaAs quantum well / InAlGaAs barrier solar cells were grown and tested in order to evaluate their solar cell performance. These samples were grown with five layers of QWs at varying depths in the intrinsic region of the n-i-p devices. An external quantum efficiency measurement was used to determine the sub-bandgap spectral responsivity, and showed efficient absorption and collection beyond the bulk material bandedge, from 1280 to 1580 nm. Simulations were performed to evaluate electric field strength as a function of depth and a resonant excitation short-circuit current density measurement was then used to characterize the samples with varied quantum well depths. The electric field acting on carriers, photoexcited into the quantum wells, impacts on the probability of those carriers contributing to the measured short-circuit current. We observe the simulated dependence of carrier collection on electric field in these devices, with a 29% increase in relative carrier collection efficiency between the sample experiencing the highest versus the lowest electric field.
Effects of electric field on thermal and tunneling carrier escape in InAs/GaAs quantum dot solar cells
David V. Forbes;
Seth M. Hubbard;
Paul J. Roland;
Randy J. Ellingson
The effects of electric field on carrier escape in InAs/GaAs quantum dots embedded in a p-i-n solar cell structures have been studied by quantum efficiency. Via band structure simulation, effective barrier height of carriers inside QDs is reduced with increasing local electric field, so tunneling and thermal escape are enhanced. At 300K, when electric field intensity is below 40kV/cm, thermal escape is dominant in all confined states in QDs; when electric field intensity is above 40kV/cm, tunneling is dominant in shallow confined states and thermal escape is dominant in the ground state of QDs.
Preparation and study of 2-D semiconductors with Dirac type bands due to the honeycomb nanogeometry
M. P. Boneschanscher;
J. J. Geuchies;
C. Morais Smith;
W. H. Evers;
The interest in 2-dimensional systems with a honeycomb lattice and related Dirac-type electronic bands has exceeded the prototype graphene1. Currently, 2-dimensional atomic2,3 and nanoscale4-8 systems are extensively investigated in the search for materials with novel electronic properties that can be tailored by geometry. The immediate question that arises is how to fabricate 2-D semiconductors that have a honeycomb nanogeometry, and as a consequence of that, display a Dirac-type band structure? Here, we show that atomically coherent honeycomb superlattices of rocksalt (PbSe, PbTe) and zincblende (CdSe, CdTe) semiconductors can be obtained by nanocrystal self-assembly and facet-to-facet atomic bonding, and subsequent cation exchange. We present a extended structural analysis of atomically coherent 2-D honeycomb structures that were recently obtained with self-assembly and facet-to-facet bonding9. We show that this process may in principle lead to three different types of honeycomb structures, one with a graphene type-, and two others with a silicene-type structure. Using TEM, electron diffraction, STM and GISAXS it is convincingly shown that the structures are from the silicene-type. In the second part of this work, we describe the electronic structure of graphene-type and silicene type honeycomb semiconductors. We present the results of advanced electronic structure calculations using the sp3d5s* atomistic tight-binding method10. For simplicity, we focus on semiconductors with a simple and single conduction band for the native bulk semiconductor. When the 3-D geometry is changed into 2-D honeycomb, a conduction band structure transformation to two types of Dirac cones, one for S- and one for P-orbitals, is observed. The width of the bands depends on the honeycomb period and the coupling between the nanocrystals. Furthermore, there is a dispersionless P-orbital band, which also forms a landmark of the honeycomb structure. The effects of considerable intrinsic spin-orbit coupling are briefly considered. For heavy-element compounds such as CdTe, strong intrinsic spin-‐orbit coupling opens a non-trivial gap at the P-orbital Dirac point, leading to a quantum Spin Hall effect10-12. Our work shows that well known semiconductor crystals, known for centuries, can lead to systems with entirely new electronic properties, by the simple action of nanogeometry. It can be foreseen that such structures will play a key role in future opto-electronic applications, provided that they can be fabricated in a straightforward way.
Group IV clathrates: synthesis, optoelectonic properties, and photovoltaic applications
Aaron D. Martinez;
Lauryn L. Baranowski;
Nicholas P. Brawand;
Carolyn A. Koh;
Mark T. Lusk;
Eric S. Toberer;
Adele C. Tamboli
Group IV clathrates are a unique class of guest/framework type compounds that are considered potential candidates for a wide range of applications (superconductors to semiconductors). To date, most of the research on group IV clathrates has focused heavily on thermoelectric applications. Recently, these materials have attracted attention as a result of their direct, wide band gaps for possible use in photovoltaic applications. Additionally, framework alloying has been shown to result in tunable band gaps. In this review, we discuss the current work and future opportunities concerning the synthesis and optical characterization of group IV clathrates for optoelectronics applications.
Upconverter materials and upconversion solar-cell devices: simulation and characterization with broad solar spectrum illumination
K. W. Krämer;
B. S. Richards;
J. C. Goldschmidt
Upconverter materials and upconverter solar devices were recently investigated with broad-band excitation revealing the great potential of upconversion to enhance the efficiency of solar cell at comparatively low solar concentration factors. In this work first attempts are made to simulate the behavior of the upconverter β-NaYF4 doped with Er3+ under broad-band excitation. An existing model was adapted to account for the lower absorption of broader excitation spectra. While the same trends as observed for the experiments were found in the simulation, the absolute values are fairly different. This makes an upconversion model that specifically considers the line shape function of the ground state absorption indispensable to achieve accurate simulations of upconverter materials and upconverter solar cell devices with broadband excitations, such as the solar radiation.
Cutoff wavelength optimization for high-efficiency split spectrum photovoltaics
Thomas E. Vandervelde
Split spectrum photovoltaics, where incident light is divided onto multiple cells on the basis of wavelength, are
an exciting recent development in the solar energy field. This technology has the potential to exceed record conversion
efficiencies by utilizing a large number of p-n junctions while mitigating the constraints that plague monolithic cells:
lattice matching and current matching. Each cell in a split spectrum system can have a different lattice constant
(allowing for more combinations of materials) and to have different operating currents (allowing for more combinations
of band spacing).
In this work, we examine a split spectrum system utilizing a single spectrum splitting device (a dichroic filter)
to divide the solar spectrum onto two cells. Whereas many split spectrum designs use numerous filters to direct light
onto single junction cells, in this system each cell is composed of multiple active junctions. Each cell is then tailored to
absorb a portion of the solar spectrum. The combination of the two cells allows for four, five, or more active junctions
while maintaining lattice and current matching conditions in each cell.
A number of different cutoff frequencies for the dichroic filter are examined. Each cutoff frequency
corresponds to its own combination of ideal band placements for both the shorter and longer wavelength cells. Materials
corresponding to those band placements are examined to determine if any combinations can satisfy lattice matching
parameters; designs which do are then simulated using TCAD Sentaurus.
Interdependence of reabsorption and internal energy losses in luminescent solar concentrators
Stephen M. Kuebler
As a complementary device to photovoltaic (PV) cells, luminescent solar concentrators (LSCs) can reduce the cost of solar energy by replacing the expensive PV material with inexpensive energy-harvesting plastic or glass matrix. However, due to its low efficiency, LSCs are still not commercially viable. The low efficiency is due to the various losses associated with light harvesting and trapping. Most of these losses come from reabsorption and escape of reemitted energy from the LSC device. State-of-the-art LSC technology focuses on decreasing reabsorption loss by employing luminophores with a large Stokes shift. But these materials typically have low quantum yield. Increasing the Stokes shift of the luminophore reduces reabsorption but introduces substantial loss due to low quantum yield and the Stokes shift of the re-emitted photons. The interdependence of these losses is studied computationally using a ray-tracing model that accounts for reabsorption, Stokes shift, escape cone loss, and matrix loss. It is shown that using high Stokesshift luminophores does not give the highest energy efficiency. Higher energy efficiency is obtained by optimizing the Stokes shift. Even greater performance can be achieved by employing high-quantum-yield dyes with intermediate Stokes shift. LSC devices based on this approach could be nearly twice as efficient as those based on conventional luminophores, such as Rhodamine B.
Impact of light management on photovoltaic characteristics of GaAs solar cells with photonic crystals and quasi-photonic crystals
In detailed balance model, the efficiency of single-junction solar cells can be potentially as high as 33.5% under AM 1.5G illumination. However the best state-of-the-art devices are still far lower than those figures, even the electronic quality is nearly perfect. Therefore the efficiency gap should stem from the light management inside solar cells. Recently, external radiation efficiency (ηext) derived from detailed balance model is emphasized to evaluate light management and photon recycling, which aggregates the loss of backward emission into substrate and non-radiative recombination. This factor can be highly relevant to the cell’s performance, especially open-circuit voltage (Voc), and maximizing Voc is generally considered as the last mile to approach ultra-high efficiency limit. In this work, we try to quantify the Voc enhancement in GaAs solar cells by enhancing light extraction. The simulation tools are RCWA simulation and photon recycling model NREL developed recently. The top structures we simulate here are TiO2 cones arranged in three PC/QPC lattices. After our calculation, the QPC 12-folds symmetry can make the biggest Voc enhancement 11.21meV compared with bare one, and the structure also possess extraordinary omni-directional anti-reflection ability for maintaining high Jsc. Our results also show that using this way to enhance Voc is especially suitable for cells with ordinary material quality. Therefore, the requests of ideal top structures for solar cells’ use are not only near-perfect anti-reflection, but the ability to maximize light extraction if no feature of angular filter exists.
Optical phonon decay in cubic semiconductors: a hot carrier solar cell picture
In the framework of hot-carrier solar cell absorber material design, we revisit the LO-phonon decay processes in a wide variety of III-V and group IV binary semiconductors. We present a detailed description of the two-phonon final states, from the exact dispersion relation calculated within the Density Functional Perturbation Theory formalism. We focus on the relation between Klemens surfaces features and atomic mass differences, and the importance of the Ridley channels in some group IV binaries.
Hot-carrier solar cell spectral insensitivity: Why develop the hot-carrier solar cell when we have multi-junction devices?
Louise C. Hirst;
Matthew P. Lumb;
Simon P. Philipps;
Andreas W. Bett
Robert J. Walters
The hot carrier solar cell (HCSC) offers one route to high efficiency solar energy conversion and has similar
fundamental limiting efficiency to multi-junction (MJ) solar cells however, the HCSC is at a much earlier stage
of development. We discuss the unique features of the HCSC which distinguish it from other PV technologies,
providing motivation for development.
We consider the potential for a low concentration hot-carrier enhanced single-junction solar cell, enabled
by field enhancing cell architectures. To support this we experimentally show that changing sample geometry
to increase carrier density, while keeping phononic and electronic properties constant, substantially reduces
hot-carrier themalization coefficient. Such a scheme might have similar applications to todays high efficiency
single-junction devices while allowing from some intrinsic efficiency enhancement.
We also use spectral data simulated using SMARTS to identify HCSC spectral insensitivity relative to MJ
devices. Spectral insensitivity increases annual energy yield relative to laboratory test efficiency, reducing the
cost of PV power generation. There are also several practical advantages: a single device design will operate
optimally in a variety of locations and solar power stations are less reliant of accurate, long-range atmospheric
simulation to achieve energy yield targets.
Silicon wafer-based tandem cells: The ultimate photovoltaic solution?
Martin A. Green
Recent large price reductions with wafer-based cells have increased the difficulty of dislodging silicon solar cell
technology from its dominant market position. With market leaders expected to be manufacturing modules above 16%
efficiency at $0.36/Watt by 2017, even the cost per unit area ($60-$70/m2) will be difficult for any thin-film photovoltaic technology to significantly undercut. This may make dislodgement likely only by appreciably higher energy conversion efficiency approaches. A silicon wafer-based cell able to capitalize on on-going cost reductions within the mainstream industry, but with an appreciably higher than present efficiency, might therefore provide the ultimate PV solution. With average selling prices of 156 mm quasi-square monocrystalline Si photovoltaic wafers recently approaching $1 (per
wafer), wafers now provide clean, low cost templates for overgrowth of thin, wider bandgap high performance cells,
nearly doubling silicon’s ultimate efficiency potential. The range of possible Si-based tandem approaches is reviewed
together with recent results and ultimate prospects.
Modeling intermediate band solar cells: a roadmap to high efficiency
Jacob J. Krich;
Anna H. Trojnar;
Alexandre W. Walker
Intermediate band (IB) photovoltaics have the potential to be highly efficient and cost effective solar cells. When the IB concept was proposed in 1997, there were no known intermediate band materials. In recent years, great progress has been made in developing materials with intermediate bands, though power conversion efficiencies have remained low. To understand the material requirements to increase IB device efficiencies, we must develop good models for their behavior under bias and illumination. To evaluate potential IB materials, we present a figure of merit, consisting of parameters that can be measured without solar cell fabrication. We present a new model for IB devices, including the behavior of their junctions with n- and p-type semiconductors. Using a depletion approximation, we present analytic approximations for the boundary conditions of the minority carrier diffusion equations. We compare the analytic results to Synopsys Sentaurus device models. We use this model to find the optimal thickness of the IB region based on material parameters. For sufficiently poor IB materials, the optimal thickness is zero – i.e., the device is more efficient without the IB material at all. We show the minimum value of the figure of merit required for an IB to improve the efficiency of a device, providing a clear goal for the quality of future IB materials.
Degradation modeling of InGaP/GaAs/Ge triple-junction solar cells irradiated by protons
S. I. Maximenko
M. P. Lumb;
S. R. Messenger;
P. P. Jenkins;
R. J. Walters
Experimental results on triple-junction solar cells irradiated by 3 MeV proton irradiation to very high damage levels are presented. The minority carrier transport properties were obtained through quantum efficiency and EBIC measurements and an analytical drift-diffusion solver was used in understanding the results for different degradation levels where multiple damage mechanisms are evident.
Enhancement of radiation tolerance with the use of a doping superlattice solar cell
Michael A. Slocum
David V. Forbes;
Seth M. Hubbard
Solar cells utilizing doping superlattices in the active region of the device have been proposed as an alternative design to increase radiation hardness. Multiple diodes are connected together in parallel, where each diode can be as thin or thick as the design requires. Thinning the doped layers reduces the diffusion length requirements ensuring efficient carrier collection and maintenance of short circuit current. Experimental comparisons between nipi and a conventional pin solar cells that were irradiated with 1 MeV electrons at fluences from 4x1014 to 2x1015 e−/cm2 show much more efficient maintenance of efficiency for the nipi design, maintaining nearly 100% efficiency up to a final dose of 2x1015 e−/cm2. Further simulations have indicated that the efficient maintenance of voltage and fill factor are likely due to traps created in the nipi solar cell during the fabrication process. Beginning of life voltage and efficiency values can be improved significantly by limiting the trap density, while this has a minor impact on the efficiency comparison between a nipi and conventional device with respect to radiation.
Conductive space solar cell coverglass replacement technology
Zach S. Levin;
David M. Wilt;
A flexible space solar cell coverglass replacement called Pseudomorphic Glass (PMG) has been under investigation in hopes of providing a robust, flexible, high transmissivity replacement for conventional coverglass. PMG is composed of conventional cover glass and/or fused silica in the form of small spheres incorporated in a variety of polymer matrices. The glass spheres provide the primary radiation protection and the polymer matrix provides the mechanical integrity. PMG development has recently focused on technologies for providing the electrical conductivity required to dissipate environmental charging, even in the presence of electric propulsion plumes.
Demonstration and future potential of a self-tracking phase change actuator
State-of-the-art concentrators use free-space optics to concentrate sunlight onto photovoltaic cells. To achieve high concentration factors it is necessary to track the sun’s position. In current systems, mechanical actuators keep the focal spot in the solar cell. Planar concentrators have recently emerged, which use a waveguide slab to concentrate the sunlight. Here we demonstrate the development of a phase-change actuator (PCA) for self-adaptive tracking. The demonstrated mechanism is light-responsive and provides self-adaptive light concentration in a planar waveguide while maintaining efficient concentration over an angular range of +/- 16 degrees. The proposed system consists of a lens array to focus the sunlight, a waveguide slab acting as a concentrator, a dichroic prism membrane, splitting the solar spectrum into a visible (VIS) and infrared (IR) part, and the phase-change actuator. The actuator undergoes a phase change upon absorption of the IR light and vertically expands, creating a coupling feature upon contact with the waveguide. Visible light is then reflected off the prism membrane and efficiently coupled into the waveguide. As the focus spot moves, so does the coupling feature due to the light responsiveness of the actuator. We show an experimental proof-of concept prototype, highlighting the desired features of the concept. This is then further expanded by simulations of a full system achieving high effective concentrations (>100X) and first experimental results expanding the prototype to a full system.
Design of sub-wavelength dielectric antireflective grading for multijunction concentrator photovolatics
In III-V concentrator applications, sunlight is focused with wide angular distribution that limits the effectiveness of conventional thin-film AR coatings. Furthermore the transmission properties are generally degraded non-uniformly over the electromagnetic spectrum, which in the case of multi-junction solar cells leads to additional sub-cell current matching related losses. Here, and in an attempt to identify a better alternative to the conventional planar layer ARCs for III-V multi-junction concentrator cells in case of with/without protective cover glass in conjunction with wide optical aperture angles, a systematic analysis of design parameters and angular dependent antireflective properties of dielectric gratings has been undertaken, through the implementation of sub-wavelength 2D pyramidal gratings of ZnS and TiO2. The study indicated limited improvement for devices operated with SiO2 like cover glass. In the absence of SiO2 like cover glass, the evaluation indicated that reflection-loss related current losses can be reduced by 2-3 fold compared to their doublelayer ARC counterparts. i.e. for a 3J metamorphic device this lead to a current improvement of 0.7 mA/cm2 per concentration for a 60 degree aperture angles
Cu(In,Ga)Se2 mesa microdiodes: study of edge recombination and behaviour under concentrated sunlight
In order to develop photovoltaic devices with increased efficiency using less rare semiconductor materials, the
concentrating approach is applied on Cu(In,Ga)Se2 thin film devices. For this purpose, Cu(In,Ga)Se2 microcells with a
mesa design are fabricated. The influence of the edge recombination signal is analyzed. It is found that with an
appropriate etching procedure, devices as small as 50x50 μm do not experience edge recombination efficiency
limitations. Under concentration, significant Voc gains are seen, leading to an absolute efficiency increase of two points
Ag nanoparticle-blended plasmonic organic solar cells: performance enhancement or detraction?
Tze Chien Sum
The blending of metallic nanoparticles into the active layer of organic solar cells in a bid to enhance their light
absorption and device performance has led to controversial reports of both efficiency enhancement and degradation.
Herein, through comprehensive transient absorption spectroscopy, we present clear evidence of traps being responsible
for performance degradation of poly (3-hexylthiophene): [6,6]-phenyl-C 61-butyric acid methyl ester organic
photovoltaic devices incorporated with oleylamine-capped silver nanoparticles. Although the presence of the metallic
nanoparticles leads to more excitons being generated in the active layer, higher losses suffered by the polaron population
through trap-assisted recombination strongly limits the device performance. Device modeling based on a single mid-gap
trap state introduced by the AgNPs can well reproduce the current-voltage curves of the plasmonic organic solar cells –
in agreement with the transient absorption findings. These new insights into the photophysics and charge dynamics of
plasmonic organic solar cells would help resolve the existing controversy and provide clear guidelines for device design
Hybrid bulk heterojunction solar cells based on low band gap polymers and CdSe nanocrystals
Solar energy converters based on organic semiconductors are inexpensive, can be layered onto flexible surfaces, and
show great promise for photovoltaics. In bulk heterojunction polymer solar cells, charges are separated at the interface of
two materials, an electron donor and an electron acceptor. Typically, only the donor effectively absorbs light. Therefore,
the use of an acceptor with a wide absorption spectrum and high extinction coefficient and charge mobility should
increase the efficiency of bulk heterojunction polymer solar cells. Semiconductor nanocrystals (quantum dots and rods)
are good candidate acceptors for these solar cells. Recently, most progress in the development of bulk heterojunction
polymer solar cells was achieved using PCBM, a traditional fullerene acceptor, and two low band gap polymers, poly[N-
9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and poly
(PTB7). Therefore, the possibility of combining these polymers with semiconductor nanocrystals
Here, we present the first comparison of solar cells based on PCDTBT and PTB7 where CdSe quantum dots serve as
acceptors. We have found that PTB7-based cells are more efficient than PCDTBT-based ones. The efficiency also
strongly depends on the nanocrystal size. An increase in the QD diameter from 5 to 10 nm causes a more than fourfold
increase in the cell efficiency. This is determined by the relationship between the nanoparticle size and energy spectrum,
its pattern clearly demonstrating how the mutual positions of the donor and acceptor levels affect the solar cell
efficiency. These results will help to develop novel, improved nanohybrid components of solar cells based on organic
semiconductors and semiconductor nanocrystals.
Analytical modeling of III-V solar cells close to the fundamental limit
Matthew P. Lumb;
Myles A. Steiner;
John F. Geisz;
Robert J. Walters
A highly effective strategy of photon management is to use a back surface reflector. In this work, we present a full analytical model incorporating effects from both the modified generation function and photon recycling in GaAs solar cells with a BSR. We discuss the impact of doping concentration, non-radiative recombination, solar cell dimensions and BSR reflectivity on the efficiency, and compare the prediction of the device models to experimental data measured on GaAs devices. We use the model to predict the performance of alternative III-V materials, such as InP, comparing the predicted performance to state-of-the-art GaAs solar cells.
Towards numerical simulation of nonhomogeneous thin-film silicon solar cells
Tom H. Anderson;
Tom G. Mackay
We have developed an algorithm for the numerical simulation of the electrical and optical properties of a thin-film silicon solar cell. The intrinsic layer in the p-i-n solar cell is nonhomogeneous in the thickness direction. This nonhomogeneity is to be engineered via variations in the composition of the amorphous silicon. A layer of a transparent conducting oxide is welded to the p layer and the n layer is backed by a periodically corrugated metallic back reflector. The nonhomogeneous intrinsic layer may trap the incident light better than a homogeneous layer and increase the generation rate of electron-hole pairs. The periodically corrugated metallic back reflector can excite surface plasmon-polariton waves as well as waveguide modes. The generation rate of electron-hole pairs is computed using the rigorous coupledwave approach and the drift-diffusion model is used for the computation of the current density-voltage characteristics of the solar cell.
Modeling, design and experimental results for high efficiency multi-junction solar cells lattice matched to InP
Matthew P. Lumb;
Michael K. Yakes;
Joseph G. Tischler
Christopher G. Bailey;
Jerry R. Meyer
Louise C. Hirst;
Kenneth J. Schmieder;
Sergio I. Molina;
Francisco P. Delgado;
Jessica G. J. Adams;
Nicholas J. Ekins-Daukes
Robert J. Walters
The high conversion efficiencies demonstrated by multi-junction solar cells over the past three decades have made them indispensable for use in space and are very attractive for terrestrial concentrator applications. The multi-junction technology consistently displays efficiency values in excess of 30%, with record highs of 37.8% under 1 sun conditions and over 44% under concentration. However, as material quality in current III-V multi-junction technology reaches practical limits, more sophisticated structures will be required to further improve on these efficiency values. In a collaborative effort amongst several institutions we have developed a novel multi-junction solar cell design that has the potential to reach the 50% conversion efficiency value. Our design consists of a three junction cell grown on InP substrates which achieves the optimal bandgaps for solar energy conversion using lattice matched materials. In this work, we present the progress in the different subcells comprising this multi-junction structure. For the top cell, InAlAsSb quaternary material is studied. For the middle, InGaAlAs and InGaAsP materials and devices are considered and for the bottom, a multi-quantum well structure lattice matched to InP for fine bandgap tunability for placement in an InGaAs cell is demonstrated.
Ga-rich GaxIn1-xP solar cells on Si with 1.95 eV bandgap for ideal III-V/Si photovoltaics
T. J. Grassman;
J. A. Carlin;
D. J. Chmielewski;
S. A. Ringel
Theoretical models for III-V compound multijunction solar cells show that solar cells with bandgaps of 1.95-2.3 eV are needed to create ideal optical partitioning of the solar spectrum for device architectures containing three, four and more junctions. For III-V solar cells integrated with an active Si sub-cell, GaInP alloys in the Ga-rich regime are ideal since direct bandgaps of up to ~ 2.25 eV are achieved at lattice constants that can be integrated with appropriate GaAsP, SiGe and Si materials, with efficiencies of almost 50% being predicted using practical solar cell models under concentrated sunlight. Here we report on Ga-rich, lattice-mismatched Ga0.57In0.43P sub-cell prototypes with a bandgap of 1.95 eV grown on tensile step-graded metamorphic GaAsyP1-y buffers on GaAs substrates. The goal is to create a high bandgap top cell for integration with Si-based III-V/Si triple-junction devices. Excellent carrier collection efficiency was measured via internal quantum efficiency measurements and with their design being targeted for multijunction implementation (i.e. they are too thin for single junction cells), initial cell results are encouraging. The first generation of identical 1.95 eV cells on Si were fabricated as well, with efficiencies for these large bandgap, thin single junction cells ranging from 7% on Si to 11% on GaAs without antireflection coatings, systematically tracking the change in defect density as a function of growth substrate.
Detailed physics based modeling of triple-junction InGaP/GaAs/Ge solar cell
Space exploration missions and space electronic equipment require improvements in solar cell efficiency and radiation hardness. Triple-junction photovoltaic (TJ PV) cell is one of the most widely used PV for space missions due to it high efficiency. A proper models and simulation techniques are needed to speed-up the development on novel solar cell devices and reduce the related expenses. In this paper we have developed a detailed 3D TCAD model of a TJ PV cell, and calibrated the various (not accurately known) physical parameters to match experimental data, such as dark and light JV, external quantum efficiency (EQE) . A detailed model of triple-junction InGaP/GaAs/Ge solar cell has been developed and implemented in CFDRC’s 3D NanoTCAD simulator. The model schematic, materials, layer thicknesses, doping levels and meshing are discussed. This triple-junction model is based on the experimental measurements of a Spectrolab triple-junction cell by  with material layer thicknesses provided by Rochester Institute of Technology . This model of the triple-junction solar cell is primarily intended to simulate the external quantum efficiency, JV and other characteristics of a physical cell. Simulation results of light JV characteristics and EQE are presented. The calculated performance parameters compare well against measured experimental data . Photovoltaic performance parameters (Jsc, Voc, Jm, Vm, FF, and Efficiency) can also be simulated using the presented model. This TCAD model is to be used to design an enhanced TJ PV with increased efficiency and radiation tolerance. Keywords: photovoltaic cell, triple-junction, numerical modeling, TCAD, dark and light JV.
Variable temperature carrier dynamics in bulk (In)GaAsNSb materials grown by MOVPE for multi-junction solar cells
Steven C. Moss;
Tae Wan Kim
Luke J. Mawst
Thomas F. Kuech
III-V multi-junction solar cells are typically based on a triple-junction design that consists of an InGaP top junction, a GaAs middle junction, and a bottom junction that employs a 1 – 1.25 eV material grown on GaAs substrates. The most promising 1 – 1.25 eV material that is currently under extensive investigation is bulk dilute nitride such as (In)GaAsNSb lattice matched to GaAs substrates. The approach utilizing dilute nitrides has a great potential to achieve high performance triple-junction solar cells as recently demonstrated by Wiemer, et al., who achieved a record efficiency of 43.5% from multi-junction solar cells including MBE-grown dilute nitride materials . Although MOVPE is a preferred technique over MBE for III-V multi-junction solar cell manufacturing, MOVPEgrown dilute nitride research is at its infancy compared to MBE-grown dilute nitride. In particular, carrier dynamics studies are indispensible in the optimization of MOVPE materials growth parameters to obtain improved solar cell performance. For the present study, we employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in MOVPE-grown bulk dilute nitride InGaAsN materials (Eg = 1 – 1.25 eV at RT) lattice matched to GaAs substrates. In contrast to our earlier samples that showed high background C doping densities, our current samples grown using different metalorganic precursors at higher growth temperatures showed a significantly reduced background doping density of ~ 1017 /cm3. We studied carrier dynamics in (In)GaAsNSb double heterostructures (DH) with different N compositions at room temperature. Post-growth annealing yielded significant improvements in carrier lifetimes of (In)GaAsNSb double heterostructure (DH) samples. Carrier dynamics at various temperatures between 10 K and RT were also studied from (In)GaAsNSb DH samples including those samples grown on different orientation substrates.
Silicon solar cell enhancement by plasmonic silver nanocubes
W. N. Ye
Our paper presents a detailed numerical simulation and experimental study of the efficiency enhancement gained by optimizing metal nanocubes incorporated on the surface of silicon solar cells. We investigate the effects of nanoparticle size, surface coverage and spacer layer thickness on solar absorption and cell efficiency. The fabrication of nanocubes on solar cells is also presented, with the trends observed in simulation verified through experimental data. Testing reveals that nanocubes show worse performance than nanospheres when sitting directly on the silicon substrate; however, enhancement exceeds that of nanospheres when particles are placed on an optimized spacer layer of SiO2, for reasonable surface coverages of up to 25%. Our analysis shows that for a large range of particle sizes, 60 - 100nm, enhancement in light absorption remains at a high level, near the optimum. This suggests a high level of fabrication tolerance which is important due to the chemical growth mechanism used for nanocube synthesis, as it consistently produces nanocubes in that range. Further, we note that efficiency enhancement by nanocubes is influenced by particle size, surface coverage, and spacer layer thickness much differently than that for a spherical geometry, thus our study focuses on the optimization of the nanocube parameters. We show that 80nm nanocubes on a 25nm SiO2 spacer layer realize ~ 24% enhancement in light absorption compared to an identical particle-free cell. Finally, we present both the numerical and experimental results for silicon solar cells coated with nanocube arrays.
Optimum feature size of randomly textured glass substrates for maximum scattering inside thin-film silicon solar cells
Armin G. Aberle;
Optimization of light scattering by designing proper randomly textured surfaces is one of the important issues when
designing thin-film silicon solar cell structures. The wavelength region that needs to be scattered depends on the
absorber material and the thickness of the solar cell. The optimum morphology of the textured substrate can be defined
regarding the wavelength range intended for scattering. Good scattering is experimentally achieved by optimizing the
fabrication process of the randomly textured substrate. However, optimum morphological parameters have not been
analytically formulated. In this work we develop the morphological criteria for optimum light scattering in a-Si:H solar
cells using Aluminum Induced Texture (AIT) glass superstrates. Transmission haze is widely used as an evaluating
factor for scattering properties. Haze can be easily measured for the substrate/air interface. However, the relevant
scattering properties are those in the absorber material. These properties cannot be measured directly, but can be
predicted by an appropriate model. The simple model for haze calculation based on scalar scattering theory cannot
correctly estimate the haze value because it only considers the root mean square (RMS) roughness of the textured
surface, which does not contain information about lateral feature size. In addition, the opening angel of the haze
measurement is not considered in the equation. In this work, we demonstrate that the power spectral density (PSD)
function of the randomly textured surface can provide the missing information in the haze equation. A general
formulation for calculating the lateral feature size based on the PSD function is presented. We use this calculated haze
value based on PSD to find the optimum lateral feature size for scattering a specific wavelength into the desired material.
The optimum lateral feature size for scattering 620-nm light, which is weakly absorbed in a-Si:H, is shown to be 100 nm.
Simulation of the scattering effect of randomly textured surfaces on the efficiency of thin film tandem solar cell
Thin film silicon solar cells are optimized to increase their efficiency. One technique to obtain higher efficiency
is to increase path length of light using textured surfaces. The impact of these layers on efficiency is usually
studied using experimental methods. This requires building of a solar cell and is time consuming and prone to
error. Simulation is used to predict light scattering effects in large domains with textured layers. We studied
these effects using a conformal finite integration technique (FIT) that efficiently simulates complex geometries
with surface roughness. The simulated external quantum efficiency EQE for a solar cell with a μc-Si:H and aSi:H
layers with surface roughness are presented.
Optical scattering by anodized aluminum oxide for light management in thin film photovoltaics
We investigate the optical scattering properties of self-assembled nanoporous anodized aluminum oxide (AAO) films, and propose integrating AAO as a backscattering layer for light management in thin film photovoltaics. Angle selective scattering and direction of light to extreme, near-horizontal angles can enable new functionality for semitransparent PV window coatings, allowing improved absorption of direct sunlight without sacrificing transparency in the normal direction. Scattering to extreme angles can also be exploited to aid light trapping in thin epitaxial semiconductor absorbers, without texturing.
InP-based nano solar cells
Light trapping enhancement is a major research field in photovoltaics. Scarce and expensive resources for
semiconductor material drive the research on light management in thin absorber layer. This paper reviews some of the
known techniques, from back reflector to nanophotonic technologies such as nanowires or plasmonic-enhanced
photovoltaic devices. Light trapping enhancement can reach ~100 fold and experimental demonstrations of device
exceeding the ray optics limits have been reported.
Dedicated nanoantenna elements for vertical nanorods in plasmonic photovoltaics
Michael A. Fiddy
We investigate a novel light conversion scheme in nanostructures for the highly demanding field of plasmonic solar cells. In our study, we incorporate vertical nanorods made of semiconductor materials, which are coupled optically to plasmonic nanoantennas for optimal absorption of sunlight. Utilizing the unique properties of localized surface plasmon resonances, we create dedicated nanoantenna elements such that the emission pattern is effectively directed toward the absorber material. In our approach, we use a computational finite element method to investigate the effects of size and shape of metallic nanoparticles to obtain an asymmetric radiation pattern that matches the geometry of our design.
Extending the operational wavelength of thermophotovoltaic devices via superlattice and barrier engineering
Abigail S. Licht;
Dante F. DeMeo
J. B. Rodriguez;
Thomas E. Vandervelde
In this paper, we investigate extending the operational wavelength of thermophotovoltaic diodes. Our calculations demonstrate that employing a barrier structure can reduce the diffusion current by several orders of magnitude, reducing dark current and improving the overall function of the TPV diode for room temperature operation. We first investigated GaSb/InAs type–II superlattice structures with monovalent barriers targeting wavelength cut-offs of five microns. Simulations were used to optimize the band structure energy levels for superlattice materials and to align the energy bands between different layers in the device. We examine the difference in IV curves between barrier and non-barrier structures for a five micron (Eg=0.248 eV) diode with a barrier of 300 meV.
Design strategy for low e windows with effective insulation
Mike P. C. Watts
Optimal window glass assemblies have been developed for three use cases, when the average outside temperature is greater or less than the target indoor temperature. These assemblies have 2× better insulation than a standard double glazed window. They were developed by identifying insulation strategies for each of the 4 energy bands that transfer heat through windows. The insulation strategies were identified through analytical models for each energy band. The strategies were then applied to glasses in the DOE International Glass Data Base and evaluated using the industry standard modelling programs. These evaluations provided a systematic method to developed optimal glass assemblies.
The optimization of textured a-Si:H solar cells with a fully three-dimensional simulation
This article studied the a-Si:H solar cell with a randomly rough surface for high-power conversion efficiency. A full 3D numerical modeling program developed by our group including 3D FD-TD for optics and 3D Poisson and drift-diffusion solver for electronic simulation are used to model the characteristics of a textured solar cell. The balance between the optical and electrical performance of the a-Si:H solar cell is studied in this work. For model verification, a solar cell with high 9.23 % power conversion efficiency is used to examine the model and parameters. This article figured out the electrical limit for the a-Si:H solar cell and studied the influence of different roughness scales.
On the light trapping mechanism in silicon solar cells with backside diffraction gratings
In this numerical study, we investigate the light trapping mechanism in silicon solar cells with backside diffraction gratings. In order to obtain a clearer view on the physical mechanisms underlying the light trapping we employ a simulation scheme that combines ray tracing with rigorous coupled wave analysis (RCWA). This combined simulation approach treats the light propagation inside the silicon absorber layer incoherently and averages out Fabry-Perot resonances, which otherwise would obscure characteristic humps in the absorption spectra that are directly related to light trapping effect of the diffraction gratings. We provide an in-depth explanation for the origin of these characteristic humps and their interrelationship with the silicon absorber thickness. A major benefit of this combined RCWA/ray tracing approach compared to the fully electromagnetic simulation methods RCWA and finite difference time domain (FDTD) is the more efficient use of computational power accompanied by a gain in simulation precision, in particular for cells with an absorber thicker than 10 μm.
Beryllium implant activation and damage recovery study in n-type GaSb
Emma J. Renteria;
D. M. Shima;
Ayse J. Muniz;
L. F. Lester
Damage induced by the implantation of beryllium in n-type GaSb and its removal by Rapid Thermal Annealing (RTA) are studied in detail by Atomic Force Microscopy (AFM), Cross Sectional Transmission Electron Microscopy (XTEM) and Energy Dispersive X-ray Spectroscopy (EDS). RTA has been implemented with different times and temperatures in order to optimize ion activation and to avoid Sb outdiffusion during the process. Results indicate a lattice quality that is close to pristine GaSb for samples annealed at 600 °C for 10s using a thick Si3N4 capping layer. Electrical response of the implanted diodes is measured and characterized as function of different annealing conditions.
Cheap and efficient plasmonic solar cell
Ahmed E. Khalifa;
Mohamed A. Swillam
Plasmonic solar cell is a very promising structure for high efficient solar cell application. It has some unique
characteristics that allow high energy localization and higher solar energy absorption. Most of the proposed designs are
based on using noble metals such as gold and silver to achieve the plasmonic effect. These metals are, however,
expensive and increase the cost of the solar cell. Thus, the need to propose novel and cheap material with plasmonic like
effect is of prime importance. In this work we demonstrate the applications of TiN that has good plasmonic like effect
over wide bandwidth. A detailed comparative study of TiN and silver in an optimized design is presented, and we report
comparable TiN field localization and light scattering effects. In addition, TiN is more compatible with the CMOS
fabrication technology than the conventional plasmonic metals, which can even ease the integration with other
optoelectric devices. Should the electrical performance be further studied and optimized, the overall efficiency of the
solar cell can be maintained and/or enhanced and total cost/watt dramatically reduced.