This PDF file contains the front matter associated with SPIE Proceedings Volume 8256, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Upconversion (UC) of sub-band-gap photons can increase solar cell efficiencies. Up to now, the achieved efficiencies are too low, to make UC relevant for photovoltaics. Therefore, additional means of increasing UC efficiency are necessary. In this paper, we investigate both metal and dielectric photonic nanostructures for this purpose. The theoretical analysis is based on a rate equation model that describes the UC dynamics in β-NaYF₄ : 20% Er³⁺. The model considers ground state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multi phonon relaxation. For one, this model is coupled with results of Mie theory and exact electrodynamic theory calculations of plasmon resonance in gold nanoparticles. The effects of a 200 nm gold nanoparticle on the local field density and on the transition rates within in the upconverter are considered. Calculations are performed in high resolution for a three dimensional simulation volume. Furthermore, the effect of changed local fields in the proximity of grating waveguide dielectric nanostructure is investigated. For this purpose FDTD simulation models of such structures are coupled with the rate equation model of the upconverter. The results suggest that both metal nanoparticles and dielectric nanostructures can increase UC efficiency.

We propose an approach for enhancing the absorption of thin-film amorphous silicon solar cells using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin planar cell. We numerically demonstrate this enhancement using 3D full field finite difference time domain simulations and 3D finite element device physics simulations of a nanosphere array above a thin-film amorphous silicon solar cell structure featuring back reflector and anti-reflection coating. In addition, we use the full field finite difference time domain results as input to finite element device physics simulations to demonstrate that the enhanced absorption contributes to the current extracted from the device. We study the influence of a multi-sized array of spheres, compare spheres and domes and propose an analytical model based on the temporal coupled mode theory.

The problem of light trapping in thin film solar cells is one of mode coupling into a lossy waveguide structure. This paper therefore derives the full-field solution for electromagnetic wave propagation in a three-layer dielectric system with arbitrary complex indices of refraction. The functional form of the eigenvalue equation is identical to that of the strictly lossless case, but with complex-valued propagation constants rather than real. Lossy mode propagation is then explored in the context of photovoltaics by modeling a thin film solar cell made of amorphous silicon with an aluminum back contact.

Application of up-conversion in photovoltaic is limited by the up-conversion efficiency of materials. We propose to use a realistic plasmonic structure proposed in the literature to exceed this limitation. Erbium doped yttrium fluoride thin layer has been elaborated by ALD at 250°C to be used in such a structure as active up-converter material. Up-conversion properties of samples have been characterized using a confocal microscope. Samples, having a thickness below 100 nm, deposited onto a gold mirror, exhibit an up-conversion visible with naked eyes. The measurement of the enhancement factor of structures associated with the plasmonic resonnator has been performed by luminescence cartography. From these data, total enhancement factor up to 35 has been achieved by the use of the plasmonic structures, as compared to a layer deposited on a bare glass substrate.

Improvement of efficiency for crystalline silicon (c-Si) with nanopillar arrays (NPAs) solar cell was demonstrated by deployment of CdS quantum dots (QDs). The NPAs was fabricated by colloidal lithography of self-assembled polystyrene (PS) nanospheres with a 600 nm in size and reactive-ion etching techniques, and then a colloidal CdS QDs with a concentration of 5 mg/mL was spun on the surface of c-Si with NPAs solar cell. Under a simulated one-sun condition, the device with CdS QDs shows a 33% improvement of power conversion efficiency, compared with the one without QDs. Additionally, we also found that the device with CdS QDs shows a 32% reduction in electrical resistance, compared with the one without QDs solar cell, under an ultraviolet (UV) light of 355nm illumination. This reduced electrical resistance can directly contribute to our fill-factor (FF) enhancement. For further investigation, the excitation spectrum of photoluminescence (PL), absorbance spectrum, current-voltage (I-V) characteristics, reflectance and external quantum efficiency (EQE) of the device were measured and analyzed. Based on the spectral response and optical measurement, we believe that CdS QDs not only have the capability for photon down-conversion in ultraviolet region, but also provide extra antireflection capability.

Light trapping techniques such as textured interfaces and highly reflective back contacts are important to thin-film solar cells. Scattering at rough interfaces inside a solar cell leads to enhanced absorption due to an increased optical path length in the active layers, which is generally characterized by a haze ratio. In this work, we demonstrate the measured haze characteristics of indium tin oxide nano-whiskers deposited on an ITO-coated glass substrate. A theoretical model based on a modified Mie theory is also employed to analyze the scattering effects of nano-whiskers. Instead of spherical model, a cylindrical condition is imposed to better fit the shapes of the whiskers. The calculated haze-ratio of an ITO whisker layer matches the measurement closely.

Using material systems displaying a band offset only on the conduction (GaAs/(In)GaAsN) or valence (GaAs/GaAsSb(N)) band, we offer device designs that rely on intra-subband thermal transitions accompanied by resonant tunneling to adjacent wells, which greatly accelerates the carrier escape process. Typically, photo-excited carriers in the well regions need about several nanoseconds to make their way out of the well, but a proper design of energy states in successive quantum wells can reduce this escape time to few picoseconds, leading to reduced recombination and higher carrier collection. Using a solar cell modeling program based on the drift-diffusion framework, we show that quantum well solar cells displaying such thermo-tunneling carrier escape process can substantially surpass the efficiency limit of their bulk counterpart.

While radiative recombination is a well-known intrinsic loss mechanism in photovoltaic devices, nonradiative recombination mechanisms typically dominate compound semiconductor diode currents and limit the performance of even state-of-the-art devices. However, recent advances in device structure design have allowed quantum well structures to begin reaching the radiative limits of dark current operation. In this work, a novel extended heterojunction structure is employed in InGaAs quantum well devices to reduce non-radiative recombination and expose the limiting n=1 radiative component of the diode current. Short circuit current versus open circuit voltage curves derived from illuminated currentvoltage measurements indicate that the underlying dark diode currents of the InGaAs quantum well devices vary with well thickness and emission energy. Analysis of the extracted n=1 saturation current densities indicate that these high-voltage InGaAs quantum well devices are operating in a regime of suppressed radiative recombination.

A quantum-well suparlattice cell, in which In_0.13Ga_0.86As (4.7 nm) / GaAs_0.57P_0.43 (3.1 nm) strain-balanced quantum wells are inserted in the intrinsic region of a GaAs pin cell, has been implemented by metalorganic vapor-phase epitaxy (MOVPE) and has exhibited an enhanced short-circuit current density, with an increment of 3.0 mA/cm² and a minimal drop in open-circuit voltage (0.03 V) compared to a pin cell without the superlattice. The collection efficiency of photocarriers, which are generated in a cell upon the irradiation of monochromatic light, to an external circuit has been evaluated for both the superlattice cell and a conventional quantum-well cell with thicker wells and barriers. This carrier collection efficiency is was above 0.95 for the superlattice cell, regardless of a wavelength and an external bias, while the value for the quantum-well cell degraded to be below 0.8 at a large forward bias, which evidenced superior carrier transport with the help of tunneling through the thin barriers. With such a fast electron-hole separation in the superlattice, photo-current generation by two-step photon absorption has been observed, using the electron ground state of the superlattice as an intermediate band.

Silicon nanocrystal quantum dots in a dielectric matrix form a material with higher band gap than silicon, but still compatible with silicon technology. So far, devices using silicon nanocrystals have been realized either on silicon wafers, or using in-situ doping in the superlattice deposition which may hinder the nanocrystal formation. In this paper, a vertical PIN device is presented which allows to investigated the electrical and photovoltaic properties of nanocrystal quantum dot layers. The device structure circumvents any influence of a substrate wafer or dopants and provides full flexibility in the material choice of both, i.e. electron and hole, contacts. Furthermore, not-high-temperature stable contact materials can be applied. Devices have been realized using SiC/Si nanocrystal multilayers as the i-region and doped a-SixC1-x:H layers as electron and hole contacts. First devices show open-circuit voltage of up to 400mV.

In this work, we present a Quantum Dot Intermediate Band Solar Cell (QD-IBSC) photogeneration model that is based on detailed balance principles. The 3D Schrödinger equation is solved for a regimented array of cubic quantum dots known as a Quantum Dot Crystal (QDC). Energy levels used in the simulation are derived from the dispersion relation. We consider only the dispersion relation along the [100] quasi-crystallographic direction. Absorption coefficients used were assumed to be constant and non-overlapping for each energy transition. Various JV curves were simulated for different dot sizes for the InAs_0.9N_0.1/GaAs_0.98Sb_0.02 dot/host system. This material system was chosen due to its property of a negligible valance band offset. The negligible valance band offset offers more feasibility for the isolation of the intermediate band. Simulations were done under a non-concentrated 6000K black body spectrum at a cell temperature of 300K. Performance parameters for each IV curve were calculated in order to ascertain the effect of dot size on performance from a fundamental level. Results show that for a fixed dot separation of 2nm, cell efficiency increases to 36.7% as the dot size is increased to 3.5 nm, but begins to decrease for larger dot sizes.

Cu(In,Ga)Se₂ microcells are photovoltaic devices of increased efficiency and low semiconductor consumption. They show an increase in efficiency due to concentrated illumination up to more than ×100, which is a breakthrough as thin films were previously limited to low concentration applications (about 10 suns). New measurements, made under concentrated natural solar illumination are presented, which confirm the conclusions of laser experiments. We also extend our approach to an other direction, that of using thin Cu(In,Ga)Se₂ layers. This reduces further the volume of the solar cells and gives an insight in the effect of thickness as a key parameter controlling the performances of thin film microcells. On thinner microcells, optimum efficiencies are reached at illumination intensities over ×400. Due to their favorable architecture, microcells present efficient resistive and thermal management, leading to gains in efficiency and material usage.

With much effort devoted to the improvement of material and electrical designs, high-quality GaAs single-junction solar cell performance is getting close to its theoretical limit. To further improve device performance, it is critical to find the optimal optical designs for single-junction solar cells. In this work, planar single-junction solar cells are investigated using a semi-analytical model, where combinations of smooth, textured, non-reflective, and reflective surfaces are explored. Statistical ray tracing is used to obtain the optical properties of planar structures and the impact of critical design parameters such as junction thickness, together with material quality and solar concentration on the device performance is analyzed. The combination of textured and reflective surfaces shows the best performance by effectively increasing the photon and carrier densities, which leads to higher open-circuit voltages and conversion efficiencies. It is expected that the GaAs single-junction cells can practically achieve ~30% conversion efficiency under one sun AM1.5G, with optimal optical structures, the state-of-art material quality, and properly designed doping profile. Even higher efficiency of ~38% is possible via concentration of 1000 suns.

To realize high efficiency solar cells, new concepts beyond the Shockley-Queisser limit are widely investigated. The intermediate band solar cell (IBSC) is one of the candidate concepts. From the importance of device physics, we have developed a device simulator for IBSCs. For device simulation of IBSC, the Poisson equation, carrier continuity equations of electrons in the conduction band (CB) and the valance band (VB) and balanced equation of IB state electrons must be solved self-consistently. The simulation methods can clarify the intrinsic device behavior of IBSCs which cannot be investigated by the detailed balance model. For example, by the existence of electrons trapped in IB states, electrostatic potential along the depth direction of the solar cells is strongly modified from the equilibrium under illumination of sunlight. This potential change is strongly related to its absorption property of sunlight. And the doping to IB region can enhance short circuit current density via IB states. Under larger concentration, this doping effect is decreased by the photofilling effects in the radiative limit. Absorption coefficients of each band-to-band transition are decided by the semiconductor materials and fundamental physics. These limitations make the different spectra and values from ideal treatments and decide the maximum efficiency of the IBSC. In this work, we present the fundamental properties and suggestions to approach the high efficiency IBSC operations as a device.

We present a hybrid thermodynamic model for multijunction solar cells with intermediate bands that demonstrates possible improvements to conventional multijunction photovoltaic systems. Applying this model to selected tandem cell structures shows that the performance of such hybrid solar cells is enhanced and that multiple transitions from intermediate bands can reduce the number of material stacks and boost overall efficiency. We demonstrate the results of detailed simulations for multiple numbers of stacks of hybrid multijunction solar cells. And, we can choose proper materials to compose intermediate band for each junction. Furthermore, we suggest other alternative hybrid solar cell systems to absorb moderate photon energy range and find appropriate materials for hybrid solar cells.

A III-V/Si metamorphic epitaxy approach to achieve multi-junction solar cells having nearly ideal optical partitioning of the solar spectrum is described. Following our previously-established methodology for the growth of defect-free GaP on Si(100) substrates and demonstrations of heteroepitaxially integrated III-V-on-Si photovoltaics via GaAs_yP_1-y metamorphic buffers, we discuss work undertaken on the further development and refinement of these processes and materials, with the goal of minimization of threading dislocation densities in order to enable high-performance solar cells. A substantial, non-trivial increase in growth temperature and general improvement of growth conditions and designs has been achieved for both the heterovalent GaP/Si epitaxial integration process and the GaAs_yP_1-y compositional grading. Improved dislocation glide and significantly more efficient epitaxial relaxation is found for the GaP/Si system, while enhanced dislocation glide dynamics in the metamorphic GaAs_yP_1-y buffer system is demonstrated by the evolution of new epitaxial tilt characteristics.

This work uses simulations to predict the performance of InAlAsSb solar cells for use as the top cell of triple junction cells lattice matched to InP. The InP-based material system has the potential to achieve extremely high efficiencies due the availability of lattice matched materials close to the ideal bandgaps for solar energy conversion. The band-parameters, optical properties and minority carrier transport properties are modeled based on literature data for the InAlAsSb quaternary, and an analytical drift-diffusion model is used to realistically predict the solar cell performance.

Based on Crosslight APSYS, we have made 2D simulation of dual and triple junction solar cells based on CdZnTe and CdTe material system on Si substrate with tunnel junctions. The basic physical quantities like band diagram, optical absorption and generation for these solar cells, and external quantum efficiency for individual subcell junctions of triple junction solar cells are obtained. Current matching analyses and multi-sun concentration simulation are also performed. The modeling shows efficiency 28.85% (one sun AM1.5G) for CdZnTe/Si dual junction solar cells and efficiency 34.92% (one sun AM1.5G) and maximum 39.09% (multi-sun concentration around 500-700 suns) for CdZnTe/CdTe/Si triple junction solar cells. The presented results indicate that the dual and triple junction solar cells with II-VI CdZnTe and CdTe on Si can achieve efficiency comparable to those III-V based compound on Ge substrate.

Third generation concepts in photovoltaic devices depend critically on the dynamics of ultrafast carrier relaxation and electron-phonon interactions on very short times scales in nanostructures such as quantum wells, wires and dots. Hot carrier solar cells in particular depend on the reduction in the energy relaxation rate in an absorber material, where hot carriers are extracted through energy selective contacts. Here we investigate the short time carrier relaxation in quantum well, hot electron solar cells under varying photoexcitation conditions using ensemble Monte Carlo (EMC) simulation coupled with rate equation models, to understand the limiting factors affecting cell performance. In particular, we focus on the potential role of hot phonons in reducing the energy loss rate in order to achieve sufficient carrier temperature for efficient performance.

Hot carrier solar cells have a fundamental efficiency limit well in excess of single junction devices. Developing a hot carrier absorber material, which exhibits sufficiently slow carrier cooling to maintain a hot carrier population under realistic levels of solar concentration is a key challenge in developing real-world hot carrier devices. We propose strain-balanced In_0.25GaAs/GaAsP_0.33 quantum wells as a suitable absorber material and present continuous-wave photoluminescence spectroscopy of this structure. Samples were optimised with deep wells and the GaAs surface buffer layer was reduced in thickness to maximise photon absorption in the well region. The effect of well thickness on carrier distribution temperature was also investigated. An enhanced hot carrier effect was observed in the optimised structures and a hot carrier distribution temperature was measured in the thick well (14 nm) sample under photon flux density equivalent to 1000 Suns concentration.

Third generation photovoltaic approaches aim to use multiple energy level approaches to circumvent the Schockley- Queisser limit but to still allow use of thin film approaches. Hence they offer significant potential to reduce cost per Watt and move solar cell technologies towards the levels necessary to achieve LCOE values that give grid parity. The Hot Carrier solar cell is a Third Generation device that aims to tackle the carrier thermalisation loss after absorption of above band-gap photons. It is theoretically capable of extremely high efficiencies, 65% under one sun, very close to the maximum thermodynamic limit. However, it relies on slowing the rate of carrier cooling in the absorber from ps to ns. This very tough challenge can perhaps be addressed through nanostructures and modulation of phonon dispersions. The mechanisms of carrier cooling are discussed and methods to interrupt this process investigated to give a list of properties required of an absorber material. Quantum well or nano-well structures and large mass difference compounds with phonon band gaps are discussed in the context of enhancing phonon bottleneck and hence slowing carrier cooling. Materials for these structures are discussed and potential combined structures to maximize phonon bottleneck and slow carrier cooling are suggested.

While multiple exciton generation (MEG) is known to occur more efficiently in semiconductor nanocrystals than in the bulk, the required energy threshold prevents visible photons from being utilized. We report two-color pump-probe measurements demonstrating a two-fold increase in the MEG efficiency of solution samples of PbSe quasi onedimensional nanorods over zero-dimensional nanocrystals to a value of 0.78, where 1 is the largest efficiency possible. This improvement is accompanied by a reduction of the MEG threshold energy to 2.28E_g, which allows visible photons to participate in MEG. This approaches the theoretical limit for the threshold energy of 2E_g imposed by energy conservation. Detailed balance calculations show that, unlike nanocrystals, photovoltaic cells based on PbSe nanorods can use MEG to improve power conversion efficiencies, particularly when used in conjunction with solar concentrators.

Non-destructive, non-invasive measurement and monitoring tools, such as spectroscopic ellipsometry (SE), are needed at all scales of thin film photovoltaic (PV) technology development - from the nanometer scale that describes the electronic and physical structure of materials fabricated in research laboratories to the gigawatt scale that requires the large-area uniformity of materials made in mass production. In the research laboratory, real time SE during materials fabrication has provided insights into the structural and electronic property evolution, and dielectric functions of the thin films. Here we will present such results for the growth of thin film PV materials of various compositions and properties, including those used in the demanding I-III-VI₂ solar cell technology. In the PV manufacturing plant, mapping SE can be used to evaluate the uniformity of thickness and properties of the layers in full scale PV panels and even in completed modules. Here we will present representative results for the uniformity of CdS, the window layer in CdTe PV technology, which reached 1.4 GW annual production in 2010.

We analyze photoluminescence (PL) and electroluminescence (EL) using a hyperspectral imager that records spectrally resolved luminescence images of a GaAs solar cell. Thanks to the absolute calibration, we first investigate the reciprocity relations between Solar Cell and LED and determine the External Quantum Efficiency (EQE) from EL images for a specific range of voltage. Spatial variations are observed due to series resistance effect that we can evaluate. Second, the PL experiment allows us to plot the recombination current at a given spatial location versus the quasi Fermi level splitting at the same location. Indeed, under reasonable assumptions a link can be done with the classical plot of the short circuit current versus the open circuit voltage. We therefore can optically determine optoelectronic properties such as the saturation currents. The assumptions made in this experiment are discussed in order to correctly investigate polycrystalline solar cells in the future where strong lateral variations exist.

The operation of solar cells incorporating multiple repeat units of InAs quantum dot structures, as well as those with and without δ-doping of 4 and 8 electrons per quantum dot, were studied. Room temperature measurements of these samples revealed high quality devices, but insignificant differences between δ-doped samples and undoped samples. An IR-pumped quantum efficiency measurement was performed at 6 K to probe the extraction of quantum confined carriers through a two-photon process while shutting off phonon-assisted extraction. No two-photon signal rose above the noise, but additional sub-bandgap illuminated IV curves revealed current generation in the quantum dot devices, suggesting the dominant carrier removal mechanism is through tunneling. Finally, dark-diode data was taken and fit to determine ideality factor as a function of temperature. Control devices had an overall larger ideality, while QD devices exhibited variations as a function of temperature, which were attributed to kinetic barriers in the first QD layers, as well as possible Auger recombination at very low temperature.

A pulsed voltage bias method is proposed to eliminate the measurement artifacts of external quantum efficiency (EQE) of multi-junction solar cells. Under the DC voltage and light biases in the EQE measurements, the output current and voltage drops on the subcells under the chopped monochromatic light are affected by the low shunt resistances of the Ge subcells, which cause the EQE measurement artifacts for InGaP/InGaAs/Ge triple junction solar cells. A pulsed voltage bias superimposed on the DC voltage and light biases is used to properly control the output current and subcell voltages to eliminate the measurement artifacts. SPICE simulation confirms that the proposed method completely removes the measurement artifacts.

This study addresses the potential of different approaches to improve the generated current density in ultrathin Cu(In,Ga)Se₂ (CIGSe) based solar cells down to 0.1 μm. Advanced photon management, involving both absorption enhancement and reflection reduction in the absorber, is studied. In this contribution, the three main approaches used are: - The reduction of the CIGSe thickness by chemical etching which combines thickness reduction and smoothing effect on the absorber. - Optical management by front contact engineering and by the replacement of the back contact by the "lift-off" of CIGSe layer from the Mo layer and the deposition of a new reflective back contact. - Application of plasmonic structures to CIGSe solar cells enabling light confinement at the subwavelength scale.

Solar concentrator systems rely on focusing a large collecting aperture onto a small one where a high efficiency solar cell is placed. The main drawback of concentrator systems is the need to track the direction of the sun. We report on the development of a trackfree planar solar concentrator which employs a self-adaptive light responsive mechanism. The working mechanism is based on optical trapping reflective particles dispersed into a liquid waveguide. The trapping effect experienced by the reflective particle at the focus of the collecting aperture effectively forces the particle to follow the direction of sunlight. The role of the reflective particle is to couple the cone of focused sunlight into a waveguide by total internal reflection (TIR) towards a solar cell placed at its edges. We report on preliminary experiments on metallic reflective particles and on gas-filled hollow glass particles that exhibit reflective properties by total internal reflection. We show promising results on vapor bubbles generated by the focused light which couple nearly 100% of the incoming light into waveguiding modes. The generated bubbles are stable and track the focal point.

Due to the energy crisis, the principle of green energy gains popularity. This leads the increasing interest in renewable energy such as solar energy. Thus, how to collect the sunlight for indoor illumination becomes our ultimate target. With the environmental awareness increasing, we use the nature light as the light source. Then we start to devote the development of solar collecting system. The Natural Light Guiding System includes three parts, collecting, transmitting and lighting part. The idea of our solar collecting system design is a concept for combining the buildings with a combination of collecting modules. Therefore, we can use it anyplace where the sunlight can directly impinges on buildings with collecting elements. In the meantime, while collecting the sunlight with high efficiency, we can transmit the sunlight into indoor through shorter distance zone by light pipe where we needs the light. We proposed a novel design including disk-type collective lens module. With the design, we can let the incident light and exit light be parallel and compressed. By the parallel and compressed design, we make every output light become compressed in the proposed optical structure. In this way, we can increase the ratio about light compression, get the better efficiency and let the energy distribution more uniform for indoor illumination. By the definition of "KPI" as an performance index about light density as following: lm/(mm)², the simulation results show that the proposed Concentrator is 40,000,000 KPI much better than the 800,000 KPI measured from the traditional ones.

In our simulation of reflection losses for 1D and 2D subwavelength dielectric grating, surface texturing was done while comparing reflection losses with various incident angles for photovoltaic materials like Si and III-Vs GaAs. Transfer matrix formalism is modeled by treating each grating's effective refractive index as being composed of several layers of varying refractive indexes. Discrete parameterization on intervals with different profiles such as 1D rectangles and triangle, as well as 2D pyramids and hemispheres are used to minimize power reflected for black body radiation. This simulation treats each layer to be uniform, which requires the texturing to be in the subwavelength region. We compared the reflection loss and incident angle dependence for dielectric layers, dielectric gratings, and the combination of both dielectric layers and gratings, and found that with gratings, reflection losses are less dependent on incident angle. By optimizing the texturing and design parameters, we can obtain reflection losses around 1% for spectral range of solar cell with a very small increase in incidence angle.

The nipi photovoltaic device is a doping superlattice-based device, that uses iterative n-type / intrinsic / p-type / intrinsically doped GaAs layers to minimize the effect of minority carrier diffusion length. Following photon absorption, carriers are quickly swept vertically by drift into majority doped layers. Carriers are collected in the lateral contacts via diffusion through the doped superlattice layers. Epitaxial regrowth is used to form selective lateral contacts in v-grooves that are etched into the superlattice layers. Testing was completed to improve the epitaxial regrowth process used, where an improvement in the morphology of the regrown material was demonstrated by adjusting the growth parameters. Devices have been fabricated, and the effects of varying the cell size and grid finger spacing have been studied. The competing effects of series resistance which increases as the grid finger spacing increases and shunt resistance which decreases as the finger spacing decrease have to be balance to optimize the efficiency for the design. Although an additional shunt path was created between the contacts, a one sun efficiency of 3.42% was achieved. The development of a fabrication process makes way for the use of the nipi device to be used in conjunction with quantum dots to increase subband absorption and potentially realize an intermediate band solar cell.

QD size, uniformity and density in InAs/GaAsSb material system for increasing Sb content are studied using Atomic Force Microscopy (AFM). AFM results show that QD density and uniformity improve with Sb content increase. The improvement of QD uniformity is ensured by the narrowing of the analysis of AFM scans. To obtain minimum VBO, InAs/GaAsSb with various Sb compositions is investigated by PL and TRPL measurements. PL data shows a blue-shift as excitation power increases as evidence of a type II band structure. Since the PL peak of 8 and 13 % Sb samples did not shift while that of 15 % Sb sample is blue-shifted with increasing the excitation power it is concluded that InAs QDs/GaAs0.86Sb0.14 would have minimum valence band offset. This tendency is supported by the change of a carrier lifetime estimated from TRPL data

Bulk, lattice-matched InGaAsSbN material has been grown by metal organic vapor phase epitaxy (MOVPE) for applications in concentrated multi-junction solar cells. By optimizing the growth conditions for high Sb and As partial pressures, we achieved background hole concentrations as low as 2 x 10¹⁸ cm^-3. After thermal annealing, the background hole concentration increased from 2x10¹⁸ to 2 x 1019 cm^-3, although PL intensity increased by a factor of 7. We recently grew single junction (1eV) solar cells incorporating dilute-nitride materials and devices were fabricated and characterized for solar cell application. Performance characteristics of these cells without anti-reflection coating included the efficiency of 4.25% under the AM1.5 (air mass) direct illumination, V_oc of 0.7 V, and a spectral response extended to longer wavelength compared with GaAs cells.

The dilute nitride GaInNAs(Sb) alloy system is challenging to grow and defects can cause short diffusion lengths and high background doping densities. Despite these difficulties, extremely high cell efficiencies have recently been achieved in multi-junction solar cells utilising 1 eV GaInNAs absorber layers. This study aims to highlight the tradeoffs between the electrical and optical characteristics related to the performance of GaInNAs(Sb) diode structures grown by molecular beam epitaxy , with band gaps ranging from 0.90 to 1.04 eV. Post-growth annealing was necessary in some instances to reduce the background doping and dark current densities. The incorporation of Sb into GaInNAs has enabled the possibility of producing a dilute nitride cell with a band gap lower than 0.80 eV, although with an increased dark current.

This paper studies the feasibility of using GaN/InGaN quantum dot as the Intermediate Band Solar Cell. Different dot sizes are compared and the result shows significant differences due to the quantum confinement strength. The band structure and transition rate in the quantum dot are calculated. For the smaller quantum dot, the efficiency is much higher because of the larger separation of IB band to conduction band. However, the contribution of intermediate bands is small and the bottle neck is found as the low transition rate between IBs and bulk state.

Photovoltaics continue to be the primary source of electrical power for most near-Sun space missions. The desire to enhance or enable new space missions through higher efficiency, increased specific power (W/kg), increased volumetric power density (W/m³) and improved radiation resistance, along with decreased costs, continues to push development of novel solar cell and array technologies. To meet present and future space power requirements, gallium arsenide based multijunction solar cells, thin-film solar cells, and more novel technologies such as intermediate bandgap devices are being pursued. These efforts have resulted in a continual advancement in performance, but new paradigms will be required to continue that performance trend. As cell efficiency increases, other cell and power system characteristics may become more important, namely cost and environmental durability as well as power system survivability. Opportunities for high performance photovoltaics continue to expand for both space and terrestrial applications.

Using drift-diffusion model and considering experimental III-V material parameters, AM0 efficiencies of lattice-matched multijunction solar cells have been calculated and the effects of dislocations and radiation damage have been analyzed. Ultrathin multi-junction devices perform better in presence of dislocations or/and radiation harsh environment compared to conventional thick multijunction devices. Our results show that device design optimization of Ga_0.51In_0.49P/GaAs multijunction devices leads to an improvement in EOL efficiency from 4.8%, for the conventional thick device design, to 12.7%, for the EOL optimized thin devices. In addition, an optimized defect free lattice matched Ga_0.51In_0.49P/GaAs solar cell under 10¹⁶cm^-2 1Mev equivalent electron fluence is shown to give an EOL efficiency of 12.7%; while a Ga_0.51In_0.49P/GaAs solar cell with 10⁸ cm^-2 dislocation density under 10¹⁶cm^-2 electron fluence gives an EOL efficiency of 12.3%. The results suggest that by optimizing the device design, we can obtain nearly the same EOL efficiencies for high dislocation metamorphic solar cells and defect filtered metamorphic multijunction solar cells. The findings relax the need for thick or graded buffer used for defect filtering in metamorphic devices. It is found that device design optimization allows highly dislocated devices to be nearly as efficient as defect free devices for space applications.

Radiation tolerance of quantum dot (QD) enhanced solar cells has been measured and modeled. GaAs solar cells enhanced with 10, 20, 40, 60, and 100X layers of strain compensated QDs are compared to baseline devices without QDs. Radiation resistance of the QD layers is higher than the bulk material. Increasing the number of QD layers does not lead to a systematic decrease in QD response throughout the course of radiation exposure. Additionally, InGaP/(In)GaAs/Ge triple junction solar cells with and without 10 layers of strain compensated QDs in the (In)GaAs triple junction solar cells are analyzed. Triple junction solar cells with QDs have a better resistance to Voc degradation but these samples have a degradation in Isc that leads to lower radiation resistance for power output.

We theoretically and experimentally study the structuration of organic solar cells in the shape of photonic crystal slabs. Using a Finite Difference Time Domain (FDTD) method, we investigate the double structuration of the PEDOT:PSS layer and the metallic electrode. By taking advantage of the optical properties of photonic crystals slabs, we show the possibility to couple Bloch modes with very low group velocities in the active layer of the cells. Such Bloch modes, also called slow Bloch modes (SBMs), allow increasing the lifetime of photons within the active layer. We show that an absorption gain ranging between 4% and 11% is possible according to the band gap of the organic material. Finally, we present experimental demonstration performed using nanoimprint to directly pattern the standard organic semiconductor P3HT :PCBM blend in thin film form in the shape of a photonic crystal able to couple SBMs.

Semiconductor Quantum Dots (QDs) are promising materials for photovoltaic applications because they can be engineered to absorb light from visible to near infrared and single absorbed photons can generate multiple excitons. However, these materials suffer from low carrier mobility, which severely limits the prospects of efficient charge extraction and carrier transport. We take advantage of the optical properties of QDs and overcome their drawback by using a hybrid photovoltaic device. This photovoltaic configuration exploits the absorption of solar photons in the QDs and the transfer of excitons from the QDs to a silicon p-n junction. We study the Resonance Energy Transfer (RET) mechanism to inject excitons from the QDs into the depletion layer of a silicon p-n junction. Lead sulphide (PbS) nanocrystals are deposited onto the silicon substrate and the efficiency of Resonance Energy Transfer (RET) from the PbS nanoparticles to bulk silicon is investigated. We study the efficiency of this transfer channel between the PbS nanocrystals and silicon by varying their separation distance. These results demonstrate RET from colloidal quantum dots to bulk silicon. Temperature measurements are also presented and show that the RET efficiency is as high as 44% at room temperature. Such a hybrid photovoltaic device makes a potentially inexpensive scheme for achieving highefficiency and low-cost solar-cell platforms.

In this work we describe how to model the efficiency of solar cells with novel metamaterial coatings optimized for light harvesting. Full device modeling is implemented using optical and electrical simulations. As a proof of concept, we simulate the operation of a metamaterial contact on a first generation monocrystalline silicon solar cell. We compare device characteristics and efficiencies to standard antireflective coatings applied to a grid contact cell. The effects of the metamaterial contact on silicon solar cell efficiencies is discussed for PN junction and metal-insulator-semiconductor cell structures. It is found that the metal-insulator-semiconductor solar cell designed performs better than the PN junction cell.

In this paper, we discuss the concept of an efficient infrared upconverting phosphor as an energy converting material that could potentially improve the efficiency of Si solar cells in bifacial configuration. Basic spectroscopic studies of Yb and Er-doped La₂O₂S phosphor was reported with particular attention to its upconversion properties under 1550 nm excitation. Different concentrations of phosphors were synthesized by solid state flux fusion method. The phosphor powders were well crystallized in a hexagonal shape with an average size 300-400 nm. The most efficient upconverting sample (1%Yb: 9% Er doped La₂O₂S) was also studied under the illumination with infrared (IR) broad band spectrum above 1000 nm. Our measurements show that even with an excitation power density of 0.159 W/cm2 using a tungsten halogen lamp the material shows efficient upconversion corroborating the fact that the present phosphors could be potential candidates for improving the efficiency of the present Si solar cells.

Concentrated photovoltaic (PV) technology represents a growing market in the field of terrestrial solar energy production. As the demand for renewable energy technologies increases, further importance is placed upon the modeling, design, and simulation of these systems. Given the U.S. Air Force cultural shift towards energy awareness and conservation, several concentrated PV systems have been installed on Air Force installations across the country. However, there has been a dearth of research within the Air Force devoted to understanding these systems in order to possibly improve the existing technologies. This research presents a new model for a simple concentrated PV system. This model accurately determines the steady state operating temperature as a function of the concentration factor for the optical part of the concentrated PV system, in order to calculate the optimum concentration that maximizes power output and efficiency. The dynamic thermal model derived is validated experimentally using a commercial polysilicon solar cell, and is shown to accurately predict the steady state temperature and ideal concentration factor.

The voltage degradation of GaInP/GaAs/Ge triple-junction solar cells after exposure to proton irradiation is analyzed using electroluminescence (EL) measurements. It is shown that EL measurements in combination with the reciprocity relationship allow accurate determination of the degradation of the open-circuit voltage (V_oc) of each individual subcell. The impact of different proton energies on the voltage degradation of each subcell is analyzed. For solar cells exposed to extremely high radiation levels, a correlation between the degradation of the quantum efficiency of the Ge subcell and its EL properties is presented.

The solar energy industry strives to produce more and more efficient and yet cost effective photovoltaic (PV) panels. Integration of specific micro/nano optical structures on the top surface of the PV panels is one of the efficient ways to increase their PV performance through enhancing light trapping and in-coupling. In this study, periodic triangular gratings (PTGs) in polymethyl methacrylate (PMMA) were numerically simulated and optimized. The goal of this study is to enhance the ability of solar panels to convert maximum obtainable amount of solar energy by improving the optical in-coupling of light to PV material. Initial optical simulation results shown that a flat PV panel (without any enhancing micro-optical structures) exhibits an average incident light power of 0.327 W over a range of the incident light angles between 15º and 90º. Introduction of the PTG allows capturing the incoming sunlight and reflecting it back onto the PV material for a second or more chances for absorption and conversion into electricity. The light trapping and redirection is achieved through the total internal reflection (TIR) phenomenon. Geometry of the PTG was initially optimized with respect to an incident sunlight orientation of 15º, 30º, 45º, 60º, 75º, and 90º. Optical performance of the particular optimized PTGs was analyzed over daylight conditions and several optical parameters, such as average incident power and intensity, were calculated when sunlight orientation angle was changing from 15º to 90º. By adding the PTG optimized for 15º incidents light, an average incident power of 0.342 W was achieved (4.6% improvement of optical performance). Functional PTG prototypes were fabricated with optical surface quality (below 10 nm Ra). The simulation results allow understanding how the overall daytime photovoltaic performance of solar panels can be improved.