This PDF file contains the front matter associated with SPIE Proceedings Volume 10913, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Time-resolved fluorescence imaging (TR-FLIM) allows to obtain time-resolved photoluminescence maps with a micrometric resolution. We showed last year that wide-field illumination with TR-FLIM allows to observe lateral transport to a more defective zone. This study will be focused on point and structured illumination, extending the breadth of this characterization technique to bare & homogeneous absorbers. We explore this new possibility by presenting applications for a wide range of semi-conductors. By treating the datacubes with algorithms solving 2D/3D drift-diffusion equations, we derive optoelectronic properties representative for lateral diffusion and local recombination properties.

Cesium lead halide perovskite materials have recently attracted attention in view of their optical and electronic properties which make them excellent candidates for potential applications in lasers, light emitting diodes and photodetectors. In this work, we provide the experimental and theoretical evidence for sequential photon absorption/re-emission in CsPbBr3 perovskite microwires. Using two-photon excitation, we recorded PL lifetimes and emission spectra as a function of the lateral distance between PL excitation and collection positions along the microwire, with separations exceeding 100 µm. As the propagation length increases, the PL spectrum develops a new emission peak that is red-shifted by 20 nm from the main emission and is accompanied by the appearance of the well-resolved rise times in the PL kinetics. We undertake quantitative modeling that accounts for bimolecular recombination and photon recycling within the microwire waveguide, and find that it is sufficient to account for the observed decay modifications. The model relies on a high radiative efficiency in CsPbBr3 perovskite microwires to explain the photon recycling observed. Such findings provide crucial information about the potential impact of photon recycling and waveguide trapping on optoelectronic properties of cesium lead halide perovskite materials

This work is focused on understanding the role that ferroelectric domains in methylammonium lead halide perovskite (MAPbI3) on the one hand and grain boundaries on the other can have on the performance of solar cells built from this material. We study 2D and 3D systems considering different polarization domain patterns, inspired by measurement data, by proposing a polarization model based on the knowledge of the crystalline structure, symmetry considerations and electrical simulations. Structures with grains are constructed from SEM data. We compute charge carrier transport by solving a drift-diffusion model, in which the Poisson equation for the electrostatic potential calculation explicitly includes the polarization field. The effects of grain boundaries are simulated by considering different types of trap states at the boundaries. We show that the presence of polarization domains has a strong impact on charge separation, thus leading to a decrease of recombination losses and formation of current pathways at domain interfaces. Specifically, the decrease of Shockley-Read Hall recombination losses improves the open-circuit voltage, while the low resistivity current pathways lead to improved transport and an increase of the short-circuit current. The achieved results demonstrate that the presence of ordered ferroelectric domains, even with weak magnitude of polarization, can actually affect the performance of the solar cell in terms of enhanced power conversion. Moreover, from the comparison between our results and experimental IV characteristics of MAPb(I,Cl)3 devices we conclude that the polarization model proposed can effectively reproduce the solar cell operation.

Semiconductor heterostructures are used in high-efficiency solar cells and in other electronic devices. Solar cells can’t reach thermodynamic efficiency limits in part due to the charge carrier recombination, and efforts are applied to understand and reduce recombination. We describe a novel experimental approach to identify and quantify recombination losses at semiconductor interfaces. Using time-resolved two-photon excitation microscopy, we generate carriers at well-defined absorber depths and find that the red spectral shift of the photoluminescence (PL) emission can be used as a “spectroscopic ruler” to identify recombination depth up to 30 μm. We apply this analysis to quantify Shockley-Read-Hall recombination at the buried CdTe/CdTe interface, where 15 μm thick epitaxial CdTe is grown by the molecular beam epitaxy on the single crystal CdTe substrate. We also measure luminescent coupling between the GaInP and GaAs layers in heterostructures grown by the metal-organic chemical vapor deposition. Our results resolve important limitations for accurate 3D charge carrier lifetime tomography. Earlier we analyzed recombination due to extended defects and grain boundaries with the lateral resolution sufficient to resolve such features (approximately 0.5 μm), but interpretation of the carrier lifetime microscopy data for buried interfaces and buried semiconductor layers was a challenge. Using methods described here, the axial (z) coordinate for the PL microscopy measurements becomes as well defined as the lateral (x, y) coordinates, enabling accurate 3D identification and analysis of the charge carrier recombination locations in semiconductor heterostructures.

Alkali post-deposition treatments significantly improve the performance of CuInGaSe2 (CIGS) devices, but there is still room for improvement. Here, we investigate the effects of potassium fluoride alkali post-deposition treatment on the defect chemistry and recombination at grain boundaries and grain interiors using temperature- and injection-dependent cathodoluminescence (CL) spectrum imaging. We study CIGS thin films grown on alkali-free sapphire substrates to isolate the effects of alkali treatment from alkali metals that can diffuse from standard soda-lime glass substrates. We find that alkali treatment affects the energy and temperature dependence of the luminescence peaks, as well as the defect activation energies. CL spectrum images reveal that the luminescence transitions at grain boundaries have a distinct power dependence after alkali treatment and substantially different defect chemistry. This work shows that temperatureand injection-dependence CL spectrum images can provide unique insight into the defect chemistry and recombination behavior of CIGS thin films.

The optical and electrical properties of planar optoelectronic devices are well known, but their fully self-consistent modeling has remained a serious challenge. At the same time, the improving device fabrication capabilities and shrinking device sizes make it possible to reach higher efficiencies and develop totally new device applications. Success in this context, however, requires sophisticated device modeling frameworks, such as fully self-consistent models of optical and electrical characteristics. In this article, we explore the predictions provided by the recently introduced interference radiative transfer (IRT) model and apply it to a simplified double-diode structure presently used to study the possibility of electroluminescent cooling. The purpose of this proof-of-principle study is to show that the IRT model is straightforward to implement once one has access to the dyadic Green's functions, and that it produces solutions that satisfy the more general quantized fluctuational electrodynamics framework. We examine the photon numbers, propagating optical intensities and net radiative recombination rates from the IRT model solved by assuming a constant quasi-Fermi level separation in the active region. We find that they behave qualitatively as expected for the chosen device structure. However, the results also exhibit waveoptical characteristics, as e.g. the propagating intensity depends non-monotonously on the propagation angle due to constructive and destructive interferences. Based on the results, the IRT model offers a promising way to self-consistently combine the modeling of photon and charge carrier dynamics, also fully accounting for all interference effects.

Hot-carrier solar cells (HCSC) can potentially overcome the Shockley-Queisser limit, by having carriers at a higher temperature than the lattice. To this end, the carriers need to thermalize slower than power is generated by absorbing photons. In thin films, a hot-carrier distribution can only be achieved with very high incident power, by saturating the thermalization channels. Ultra-thin absorbers have a smaller thermalization rate, due to fewer channels. However, they typically absorb only a limited amount of light, which prevents them from reaching high efficiencies. Light trapping is an excellent way to increase significantly the amount of light absorbed in an ultra-thin material. Yet, studies on the coupling between light trapping and hot carriers are still lacking, due to the complexity of the whole system. We analyze numerically and experimentally how light trapping can enable high-efficiency HCSC. This manuscript presents the progress towards the experimental demonstration of the enhancement of the hot-carrier effect with light trapping. 280 nm-thick devices have successfully been reported on a gold mirror using epitaxial lift-off (ELO) and gold-gold bonding. These devices have been characterized by photoluminescence spectroscopy. Hot carriers with a temperature 37 K above lattice temperature were measured, in accordance with theoretical predictions. We are now working towards the ELO of absorbers 10 times thinner, on which we will implement light trapping to increase the carrier temperature.

In single junction solar cells a large part of the incident energy ends up as heat which limits their maximum achievable efficiency. Thus the achievement of maximum power conversion efficiencies relies on complex multijunction devices. Here we show the possibility to harvest the available solar energy using hot carrier devices and evidence a positive contribution of the hot carrier effect on photovoltaic performances. We investigated a semiconductor heterostructure based on a single InGaAsP quantum well using quantitative optoelectrical characterization, especially luminance measurements. The quantitative thermodynamic study of the hot carrier population allows us to discuss the hot carrier contribution to the solar cell performance. We demonstrate that voltage and current are enhanced due to the presence of the hot carrier population in the quantum well. These experimental results substantiate the potential of increasing photovoltaic performances in the hot carrier regime. Moreover, by developing a suitable analytic theoretical framework, we show how to obtain separate (hot) temperatures of electrons and holes from photoluminescence spectra analysis. The individual thermalization coefficients of each carrier type are also discussed. The method developed in this article paves the way towards the design of new energy harvesting devices and to the development of advanced characterization tools. Finaly, to increase the PV performance enhancement and reduce the concentration factor, an optimize design is investigated.

The Hot Carrier solar cell has the potential to yield a very high efficiency, well over 50% under 1 sun. Multiple quantum wells have been shown to have significantly slower hot carrier cooling rates than bulk material and are thus a promising candidate for hot carrier solar cell absorbers. However, the mechanism(s) by which hot carrier cooling is restricted is not clear. Presented is a systematic study of carrier cooling rates in GaAs/AlAs MQW with either varying barrier or varying well thickness. These allow a determination as to whether the mechanisms of either a reduction in hot carrier diffusion; a localisation of phonons emitted by hot carriers; or mini-gaps in the MQW phonon dispersion are responsible for reduced carrier cooling rates. Initial devices fabricated using MQW as hot carrier absorbers indicate promising photovoltaic performance which result from collection of hot carriers.

For the full realization of the practical potential of III-V multi-junction devices that incorporate a dilute nitride 1-1.25 eV sub-cell (<40% 1 sun, and <50% at 500X and above), our group, over the past years, have focused on dilute nitride-based devices where the degraded minority carrier diffusion length has a minimal impact on the device performance. We have shown that the incorporation of resonantly coupled GaAsN/GaAs multi-uantum wells in the intrinsic region of p-i-n GaAs cells allows both a significant sub-GaAs-bandgap photon harvesting while maintaining a high open circuit voltage with Woc. Here, in order to gain a better understanding of photo-generated carrier escape and recombination mechanisms in these devices and further optimize the performance, we examine the optical and electrical properties of such devices including with periodic MQWs using various characterization techniques such as: photoluminescence (PL), modulated photo-reflectance (PR), photo-current (PC) as well as current-voltage (IV) measurements under dark or illuminated conditions. The temperature dependent PL analysis enables us to modulate and freezes the carrier thermalization phenomena having extracted activation energies reveal interesting details about carrier escape, intra-cells coupling, and recombination sequences. Extracted electronic temperature of the carriers (about 900K at 300K lattice temperature) from PL measurement reveal more interesting phenomenon on carrier escape and recombination mechanism in both periodic and RTT MQWs solar cell.

In order to accurately characterize the photoluminescence from an InAs/AlAsSb multi-quantum well hot carrier absorber, the band structure is generated with an 8 band k·p model utilizing the Naval Research Laboratory’s MultiBands® software tool. The simulated spectra for transitions between the lowest energy electron sub-band and the four lowest hole sub-bands are computed from the optical matrix elements and the calculated band structure. In depth temperature dependent simulations for absorption and photogenerated recombination of electron-hole carriers are compared with the experimental spectra. There is close agreement between simulated and observed spectra in particular, the room temperature e1-hh1 simulated transition energy of 805 meV nearly matches the 798 meV transition energy of the experimental photoluminescence spectra. Also, the expected energy separations between local maxima (p1-p2) in the simulated/experimental spectra have a difference of just 2 meV. The model has a valence band offset of 63 meV which is in general agreement with photoluminescence feature that suggests a valence band offset of 70 meV. To analyze the ‘hot’ carriers, the photoluminescence spectra is evaluated with three different methods, a linear fit to the high energy portion of the spectra and two methods which utilize either an equilibrium or non-equilibrium generalized Planck relation to fit the whole spectrum. The non-equilibrium fit enables individual carrier temperatures for both holes and electrons. This results in two very different carrier temperatures for holes and electrons: where the hole temperature, Th, is nearly equal to the lattice temperature, TL; while, the electron temperature, Te, is ‘hot’.

In photovoltaics, recent advances in light trapping open the way toward a decrease of the absorber thickness by more than one order of magnitude. It could enable significant material savings and cost reduction of silicon and thin-film solar cells, and may constitute a breakthrough for the development of hot-carrier solar cells. First, we review recent achievements of ultrathin GaAs and CIGS solar cells. Second, we describe 200nm-thick GaAs solar cells with a nanostructured back mirror and demonstrate a record 19.9% efficiency. Third, we present the development of ultrathin and low-cost CIGS solar cells with a back mirror.

Diffraction gratings have emerged as one of the main strategies for effective light trapping in thin-film solar cells. The simulation of such photonic structures requires computationally intensive 2D or 3D full-wave approaches, which are therefore unfeasible for computer-aided design purposes. This would be even more challenging in view of performing self-consistent coupling with electronic transport models to fully account for carrier collection and carrier-photon interactions. In this work this problem has been addressed by means of a novel and computationally efficient multiphysics approach for coupled electrical-optical simulations, based on the multimodal scattering matrix formalism, wherein the grating is modeled by a scattering matrix that can be easily derived from simulations performed by rigorous coupled wave analysis.

Active solar concentrators attract significant interest in photovoltaic (PV) research activity since they can substantially reduce the area of PV cells while still collecting significant amount of solar energy via large aperture collecting optics. Solar concentrators include lenses or curved mirrors directing light from the sun into a smaller spatial spot falling on the PV cell. However, the main problem of active concentrators, severely limiting their practicality, is the high cost and low angular accuracy of sun tracking apparatuses. Specifically, tracking of the sun in existing concentrators is currently done through elaborate and expensive mechanical/optical systems, which exhibit lower performance over time and require energy input by themselves. In this paper we develop a novel active solar concentrator without any mechanical tracking. We aim to accomplish this goal through designing tunable prisms via novel chemical system comprising nanoparticles (NPs), specifically gold (Au) nanorods and silica NPs, embedded in semi-rigid transparent sol-gel matrixes, and placed within an electrical field. Changing the electrical field changes the partial distribution of the NPs and yields spatial gradient of refraction index, affecting the direction of the collected optical rays and allows their directing towards the PV cell according to the movement of the sun. In the paper we present the design and the realization of the first prototype as well as its preliminary experimental characterization.

In order to reduce costs, the solar cell industry is aiming at producing ever thinner solar cells. Structuring the surfaces of optically thin solar cells is important for avoiding excessive transmission-related losses and, hence, to maintain or increase their efficiency. Light trapping leading to longer optical path lengths within the solar cells is a well established field of research. In addition to this, other possible benefits of structured surfaces have been proposed. It has been suggested that nanostructures on the surface of thin solar cells function as resonators, inducing electric-field resonances that enhance absorption in the the energy-converting material. Further, coupling of electric field resonances in periodically structured solar cells may couple with each other thereby increasing the absorption of energy. A deeper understanding of the nature of the energy-conversion enhancement in surface-structured and thin solar cells would allow to design more targeted structures. Generally, efficiency enhancement may be evaluated by investigating the electric field and optimizing the optical generation rate. Here, we establish a model system consisting of multilayered solar cells in order to study resonances and coupling of resonances in a one-dimensional system. We show that resonances in energyconverting and non-energy converting layers exist. The coupling of resonances in the non-energy converting material and the energy-converting material is only possible for certain parameter ranges of thickness of the energy converting material and the imaginary part of the refractive index. We evaluate the resonances and the coupling of resonances in different thin-film systems and show how they affect the total absorption of energy in the energy converting layer. We show how resonances in non-absorbing layers can contribute to increasing the resonances in the absorbing layers. We optimize the parameters of the multilayered thin-film systems to achieve an increase in the amount of the absorbed energy. The optimization is also evaluated for an experimentally realizable thin-film solar cell.

Today, the record in photovoltaic (PV) conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, especially the expensive III-V substrates. As an alternative, a hybrid solar cell was proposed by LaPierre et al.1 The cell geometry, which combines a 2D Si bottom-cell with a nanowire (NW) top-cell in a tandem device, presents a theoretical efficiency record of 34% when the top-cell band gap lies around 1.7 eV[1],[2]. In this work, we report the elaboration, nanoscale characterization and device fabrication of solar cells based on axial junction GaAsP NWs. Organized GaAsP NWs were grown on patterned SiO2/Si(111) substrates by MBE. Junction was axially created during the growth by incorporating different doping impurities (Be for p- and Si for n-doping). In-situ surface passivation using a radial GaP shell was applied to reduce non-radiative recombinations on surface states[3]. Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases were used to probe the electrical properties and the generation patterns of individual NWs. The doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. Macroscopic devices based on NW arrays were fabricated by dielectric encapsulation and ITO contacting. Top view EBIC analyses were applied to probe the device homogeneity. References [1] R.R. LaPierre et al., J. Appl. Phys. 110 (2011), 014310. [2] S. Bu et al., Appl. Phys. Lett. 102 (2013), 031106. [3] C. Himwas et al., Nanotechnology. 28 (2017), 495707.

Liquid glass, a photo-curable amorphous silica nanocomposite, recently demonstrated groundbreaking capabilities as a transparent fused silica glass that can be structured in arbitrary geometries [1,2]. The ability to process high-quality glass like a polymer, including the use of 3D printing techniques, opens up new routes to integrate optical microstructures for improved light harvesting in solar module architectures [3]. Optical microstructures increase the power conversion efficiency of solar modules by improved light in-coupling or by guiding light into the active area of solar modules. We investigate freeform surface cloaks that effectively increase the active area of solar modules [4] and micro-cones that reduce front side reflection and trap light in solar modules [5]. The first prototypes of encapsulated freeform surface cloaks have demonstrated a significant increase in generated current density of around 6% relative [6]. Yet, embedding of freeform surface cloaks into the architecture of conventional solar modules relies on encapsulation with various polymer layers and a glass cover by plasma bonding. In this contribution, we will present the direct integration of optical microstructures, represented by the above outlined concepts, into transparent fused silica glass covers. This approach provides both a higher optical quality of the module encapsulation and an improved compatibility of optical microstructures with the fabrication process of common solar modules. In summary, we demonstrate a new route for integrating optical microstructures into the architecture of solar modules by the example of embedded freeform surface cloaks and micro-cones. This highlights the great opportunities 3D shaping of liquid glass brings to the world of photovoltaics. References: [1] F. Kotz, K. Arnold, W. Bauer, D. Schild, N. Keller, K. Sachsenheimer, T. M. Nargang, C. Richter, D. Helmer, and B. E. Rapp, "Three-dimensional printing of transparent fused silica glass," Nature 544, 337–339 (2017). [2] F. Kotz, K. Plewa, W. Bauer, N. Schneider, N. Keller, T. Nargang, D. Helmer, K. Sachsenheimer, M. Schäfer, M. Worgull, C. Greiner, C. Richter, and B. E. Rapp, "Liquid Glass: A Facile Soft Replication Method for Structuring Glass," Adv. Mater. 28, 4646–4650 (2016). [3] F. Kotz, N. Schneider, A. Striegel, A. Wolfschläger, N. Keller, M. Worgull, W. Bauer, D. Schild, M. Milich, C. Greiner, D. Helmer, and B. E. Rapp, "Glassomer—Processing Fused Silica Glass Like a Polymer," Adv. Mater. 30, 1–5 (2018). [4] M. F. Schumann, M. Langenhorst, M. Smeets, K. Ding, U. W. Paetzold, and M. Wegener, "All-Angle Invisibility Cloaking of Contact Fingers on Solar Cells by Refractive Free-Form Surfaces," Adv. Opt. Mater. 5, 1700164 (2017). [5] S. Dottermusch, R. Schmager, E. Klampaftis, S. Paetel, O. Kiowski, K. Ding, B. S. Richards, and U. W. Paetzold, "Micro-cone textures for improved light in-coupling and retroreflective light-trapping at the front surface of solar modules," in subsmission (2018). [6] M. Langenhorst, M. F. Schumann, S. Paetel, R. Schmager, U. Lemmer, B. S. Richards, M. Wegener, and U. W. Paetzold, "Freeform surface invisibility cloaking of interconnection lines in thin-film photovoltaic modules," Sol. Energy Mater. Sol. Cells 182, 294–301 (2018).

High-efficiency solar cell devices characterized by extremely high open-circuit voltage (VOC) values have shown that the traditional constraints imposed by extrinsic recombination processes will be eventually surpassed at some time in the foreseeable future. The accurate evaluation of Auger lifetime and its temperature-dependence are thus fundamental not only for the correct interpretation of effective carrier lifetime data, but also for the simulation of device performance, especially when these are deployed in the field, where the operating conditions can greatly vary from the standard testing conditions. In this work, we present the Auger lifetime across a range of temperatures from 300 to 500 K and a range of injection level from 5 x 1014 to 1 x 1016 cm-3 showing that, in stark opposition with what generally accepted, a strong increment of the lifetime values happens at high temperatures. Based on these results, we discuss the ambipolar Auger coefficient in the high injection range and propose a parameterization for its temperature dependence in agreement with a model previously presented in literature. Finally, we evaluate the intrinsic-limited implied voltage (iV) within the same range of injection level and temperature, and show that the evaluated strong increment of Auger lifetime counteracts the typical drop of high-efficiency solar cells performance with high temperature.

III-V solar cell cost reduction and direct III-V/Si integration can both be realized by depositing a thin layer of high-quality Ge on relatively low-cost Si substrates. However, direct epitaxial growth of Ge on Si substrates is difficult due to the 4% lattice mismatch between the film and the substrate. Threading dislocations (TDs) introduced within the Ge layer have a detrimental effect on device performances. The goal of this research is to address the perennial need to minimize the defect density of Ge epilayers grown on a Si substrate. We seek to accommodate the effects of the lattice mismatch by introducing a porous Si interface layer to intercept dislocations and prevent them from reaching the active layers of the device. The porous Si layer is formed through dislocation-selective electrochemical deep etching and thermal annealing. The porous layer created beneath the top Ge layer can both act as dislocation traps and as a soft compliant substrate, which displays high flexibility. Transmission electron microscopy (TEM) analysis of the Ge/porous Si interface shows that the lattice mismatch strain of the Ge films was almost relaxed. The surface roughness of this modified Ge/Si substrate has been reduced using chemical mechanical polishing (CMP) process to fulfil the requirements for epitaxy of III-V alloys. Finally, we present simulation results exploring the effect of threading dislocations on device performance.

Black silicon processing is a promising research area to improve optical properties of silicon solar cells. Currently, RIE method is used at cryogenic temperature because it enables a very good control of shapes of nano-structures but working at cryogenic temperature in a clean room can be an issue. In order to produce black silicon under realistic industrial conditions, room temperature process has to be achieved. We present a study aiming at etching silicon wafer surfaces using “Room Temperature SF₆/O₂ Reactive Ion Etching” (RT-RIE).

Photovoltaic (PV) device development is much more expensive and time consuming than the development of the absorber layer alone. The presentation focuses on two methods that can be used to rapidly assess and develop PV absorber materials independent of device development. The absorber material properties of quasi-Fermi level splitting and carrier diffusion length under steady effective one-Sun illumination are indicators of a material’s ability to achieve high open circuit voltage and short circuit current. These two material properties can be rapidly and simultaneously assessed with steady-state absolute intensity photoluminescence and photoconductivity measurements when combined with theory. As a result, these methods are extremely useful for predicting the quality and stability of PV materials prior to PV device development. The presentation will summarize the methods, discuss their strengths and weaknesses, and compare photoluminescence and photoconductivity results with device performance for a wide range of hybrid perovskite compositions of various bandgaps along with conventional PV materials CuInSe2, CuInGaSe2, and CuZnSnSe4.

Transport and recombination of excess carriers in an InGaAs solar cell are investigated by using time-resolved photoemission spectroscopy. We found that photovoltage rises and decays over 820 ps and 980 ps, respectively, at the pump fluence of 0.16 μJ/cm². This result shows that charge separation and recombination occur in a close time scale while charge separation is substantially faster than recombination in a GaAs solar cell which we studied in the previous study. This implies that the InGaAs cell suffers from higher non-radiative recombination loss. We also analyze the limiting factor of the temporal resolution for the present technique. The temporal resolution can be improved by employing a light source with a higher photon energy while its benefit is not drastic. Alternative methods for the improvement are discussed. In addition, time-resolved photoluminescence spectroscopy was performed in order to compare the two time-resolved techniques. The photoluminescence decay of a GaAs cell shows a fast decay at a weak photo-injection level, which becomes slower at higher injection levels as observed in previous studies.

Cathodoluminescence (CL) measurements can be applied to assess grain-boundary (GB) and grain-interior (GI) recombination in thin-film solar cell materials and made quantitative if we can develop CL models that account for material and measurement complexities. Recently, we developed a three-dimensional numerical CL model, based in MATLAB, that simulates the GI CL intensity as a function of four parameters: grain size, GI lifetime, and GB and surface recombination velocities. The model assumes that GB electrostatic potentials are screened by the high excesscarrier densities used in the CL measurement such that transport is governed by ambipolar diffusion. Here, we develop models to address directly GB potentials and their effects on these measurements. First, we transfer the MATLAB-based model to COMSOL software, and then introduce shallow donors to the GBs to produce potentials. We also develop a two-dimensional model in COMSOL to simulate CL GB contrast with GB potentials. Simulations indicate that GB potentials can increase or decrease CL intensities relative to the zero-potential case. However, the high electron-beam currents typically applied in CL measurements minimize the impact of GB potentials.

InGaP top cell often limits the efficiency of multi-junction solar cells. Its efficiency is expected to increase by introducing strain-balanced In_1-xGa_xP/In_1-yGa_yP multiple-quantum-wells since the concentration of photoexcited carriers into the wells enhances radiative recombination. To keep carrier collection efficiency to be similar to the case of a bulk InGaP cell, effective mobilities of carriers through the cascaded quantum wells are evaluated. The critical mobilities for maintaining good carrier transport as a photovoltaic device are discussed, which provides a general design principle for the multiplequantum- well architecture.

The unavoidable presence of the wetting layer (WL) in Stranski-Krastanov quantum dots (QD) has typically a negative impact on the performance of QD solar cells. In this work, a simple method to engineer the WL of InAs/GaAs QD solar cells is investigated. In particular, we show that covering the QDs at high GaAs capping rates reduces In-Ga intermixing and, therefore, In redistribution from the QDs to the WL. This results not only in larger QDs, but also in thinner WLs, with larger quantum confinement energies and reduced potential barriers for electrons and holes. Carrier trapping by the WLs and subsequent recombination is therefore reduced, resulting in larger photocurrent values of the QD solar cells under short circuit conditions.

This work investigates 1.7eV- 1.9 eV devices lattice matched to silicon where a p-i-n GaP solar cell, fabricated with an i-region that comprises a plurality of resonantly coupled quantum well of GaAsPN/GaP and investigates the evolution of the performance of these devices operating in tandem configuration with variety of advanced high efficiency silicon solar cells (i.e. HIT, PERC, PERT etc.).The band structures, evolution of bandgaps, and confinement energies are calculated using eight band k.p Hamiltonian that combines a Band Anti-Crossing model accounting for the incorporation of dilute amounts of nitrogen in the host matrix of a Kane-like semiconductor. The confinement energies are derived using a transfer matrix formalism and an envelope function approximation. Considering all possible electron/hole transitions, the optical absorption for coupled quantum well material systems are evaluated using the Fermi golden rule. Next, the performances of the targeted devices are analyzed within the framework of drift-diffusion model that incorporates realistic parameters extracted from past experiment along with demonstrated spectral response and I-V characteristic of record Si bottom cells. The study then explores the parameter design space and illumination conditions (AM0, AM1.5 and concentration) and identifies optimal parameters for achieving highest possible efficiency for each type of Si bottom cell. The study indicates the potential for 1 sun efficiencies exceeding 33% and 35% for series-connected two terminals and 4 terminal tandems respectively.

We present a concept of nonequilibrium solar cell, heat recovery solar cell (HERC cell), with its theoretical efficiency exceeding the detailed-balance limit. HERC cell uses an absorber hotter than electrodes, in terms of the lattice temperatures, and carrier-energy selecting layers in front of the electrodes. As being different from hot carrier solar cells, HERC cell does not require fast carrier extraction within the thermalization time and therefore the concept can be used to improve Si solar cells. Thermoelectric voltage produced by the temperature difference recovers the reduction due to the internal voltage drop in the hot absorber and improves the open-circuit voltage as a whole.

Thermophotovoltaic (TPV) cells based on narrow bandgap interband cascade (IC) structures with discrete type-II (T2) InAs/GaSb superlattice (SL) absorbers are a relatively new type of device for converting radiant infrared photons into electricity. By taking advantage of the broken-gap alignment in a T2 heterostructure, these quantum-engineered IC TPV structures have great flexibility to tailor the bandgap and facilitate carrier transport through interband tunneling with multiple stages for high open-circuit voltage and collection efficiency. Here, we present an investigation of narrow-bandgap (~0.2 eV at 300 K) TPV devices with a varying number of cascade stages and different absorber thicknesses. By comparing the characteristics of five TPV structures with a single absorber or multiple discrete absorbers, it is clearly demonstrated that the device performance of a conventional single-absorber TPV cell is limited mainly by the small collection efficiency associated with a relatively short diffusion length. Furthermore, this work revealed that multi-stage IC TPV structures with thin individual absorbers can circumvent the diffusion length limitation and can achieve a collection efficiency approaching 100% for photo-generated carriers. It is shown that the open-circuit voltage approximately scales with the number of cascade stages, verifying the effectiveness of cascade action. Additionally, the open-circuit voltage, the output power and power conversion efficiency can be significantly increased in IC TPV devices compared to the conventional single-absorber TPV structure. These results have further validated the potential and advantages of narrow bandgap IC structures for TPV cells.

Integrating both electrochemical solar cells (harvesting energy) and supercapacitors (energy storage) into a single device is unquestionably one of the great challenges nowadays. There has been an extended research in the design and construction of integrated solar energy harvesting and storage systems that can simultaneously capture and store various forms of energies from nature. Here, we successfully designed, fabricated and characterized a compact and monolithically photoelectrochemical device combining a polyvinyl alcohol (PVA)/hydrochloric acid (HCl)- based gel electrolyte, multi-walled carbon nanotube (MWCNTs), and fluorine doped tin oxide (FTO) as counter and working electrodes, respectively. The combination device can act either as an independent solar cell, a supercapacitor, or as a solar cell/supercapacitor device. In this structure, energy harvesting takes place only at the working electrode (WE) that made of a thin film composite of a conducting polymer (i.e. Polyaniline, PANI) and synthetic dyes materials that coat on the FTO surface by electrochemical deposition technique. The energy storage occurs in both WE and counter electrode (CE) that made of (MWCNTs) in addition to the gel electrolyte materials. Different synthetic dyes have been used such as Methylene Blue (MB), Methyl Orange (MO), and Prussian Blue (PB). Among them, MB has shown the strongest photoelectrochemical reaction in HCl-PVA gel electrolyte. The cyclic voltammetry was used to show the effect of PANI/synthetic dyes on the cell, and impedance spectroscopy demonstrated the effect of surface modification of MWCNTs on the performance of the CE.

An ultra-broadband, nearly polarization-independent absorber is studied numerically, in which an Fe square- patch is periodically placed on Fe-SiO₂ multilayer stacks. Special emphasis is placed on the determination of the patch width, since its optimum width significantly differs from that previously found in a 2D structure. A somewhat wider width leads to a broader bandwidth than that for the 2D structure. Further calculations reveal that the proposed structure operates even for oblique incidence, although the absorption gradually decreases as the angle of incidence is increased. Comparison with a different metal-insulator structure also shows that a comparable performance is obtained for Ti-Al₂O₃.

Nowadays, the efficiency improvement of the silicon solar cells (SCs) is a hot research topic in photovoltaic industry. Silicon nanowires (Si-NWs) offer a promising candidate for low-cost SCs with high-efficiency due to their unique optical and electrical properties. The Si-NWs can achieve highly efficient light trapping with reduced cost. In this paper, a novel design of asymmetric tapered nanocone Si-NWs is introduced and analyzed by 3D finite difference time domain (FDTD) method. The particle swarm optimization (PSO) technique is also used to optimize the geometrical parameters of the suggested design to maximize the optical efficiency of the suggested SC. The asymmetric Si-NWs offer an ultimate efficiency of 39.6 % with an improvement of 24 % relative to the conventional symmetric nanocone Si-NWs.

In this paper, a modified nanopyramid solar cell (SC) is introduced and numerically analyzed. The finite difference time domain (FDTD) method is used for computing the optical efficiency of the suggested design. The modified nanopyramid SC consists of an upper tapered nanopyramid part and lower nano-rectangular unit. The geometrical parameters of the proposed design are studied to maximize the optical absorption and hence the ultimate efficiency of the reported SC. The modified structure provides an optical ultimate efficiency and short circuit current density (J_sc) of 39.6 % and 32.4 mA/cm² with improvement of 24.1% and 22.2 %, respectively over the conventional thin film (TF) nanopyramid SC. Further, the p-i-n axial junctions of the suggested SC exhibits open circuit voltage (V_oc) of 0.57 volt, J_SC of 28.42 mA/cm² and power conversion efficiency (PCE) of 13.3% which are better than 0.559 volt, 19.6 mA/cm² and 8.95%, respectively of the conventional nanopyramid TF-SC. This enhancement is mainly attributed to the combination between higher order modes generated by lower rectangle unit with lower order modes supported by the upper tapered nanopyramid.

Photometers use correction filters to adjust spectral responsivity of sensors so that the combined spectral responsivity approximates the responsivity of the human eye V(λ). However, the combination of these components is hardware based, and the quality of the photometer depends on this combination. We propose a meter that uses an RGB sensor, a LED and an artificial neural network that transforms the output of the sensor into luminous transmittance, without the need of a filter. The ANN was trained and validated with two different spectra datasets and generated results with error values below 3%. The methodology presents an option for a meter with calibration that depends only on a software. This allows the development of a low cost and compact photometer.

Development in solar photovoltaic (PV) technology has made it possible to manufacture curved or shaped panels. However, little research has been done on the topic of solar resource for curved surfaces. This project aims to develop a numerical program to estimate solar resource for a curved cylindrical panel on Earth and Mars. Numerical calculations of solar resource were performed through MATLAB using Typical Meteorological Year (TMY) empirical irradiance data for New York City. This data was used as inputs for the code and the solar resource for a cylindrical panel of different curvatures and orientation was calculated using the MATLAB program. The cylindrical surface will be discretized into segments of flat surfaces. The isotropic diffused sky solar irradiance model was then used to calculate total solar resource for the given surface. It was found that as the curvature of the panel increased, the total solar resource per unit surface area decreased while the total solar resource per unit footprint area, which is the area an object occupies on a horizontal surface, increased. In addition to quantifying the performance of a curved surface on Earth and Mars, this work shows the potential of highly efficient non-tracking curved surfaces for collecting solar resource in volume limited situations such as space travel or urban applications. The resource estimation algorithm can also be used to estimate solar resource for commercial applications and system sizing.

The introduction of flexible solar cells embedded in fabrics motivates the search for more efficient solar cell designs than flat panels. The optimal configuration of solar cells should receive the maximal flux density of sunlight rays over the course of a year. There may also be spatial restrictions which only allow the cells to cover an arbitrary roof or area and surrounding structures which cast shadows in that area. So, it is difficult to analytically find the most efficient way to cover an arbitrary surface on Earth with solar cells. The genetic algorithm was used to find the optimal geometry for solar cells that have constant footprints at various latitudes. Random configurations of solar cells covering a constant area evolved into efficient configurations under the guidance of chosen selection, crossover, and mutation mechanisms. The results allow us to cover arbitrary roofs or areas as efficiently as possible, which greatly increases the value of solar energy.

Quantum dots have attractive potential for multiple junction and intermediate band solar cells. However, the device level modeling of quantum dot based solar cells is a challenging task, since it inherently requires multiscale approaches combining nano- and micro-scale descriptions at an affordable computational cost. In this work quantum dot solar cells are studied by means of a multiscale model based on a semi-classical transport-Poisson framework enriched by a proper treatment of the quantum dot dynamics. The impact of a few design and physical parameters is investigated, providing better understanding of experimental results reported in literature.

The photogeneration current of solar cells can be enhanced by light management with surface structures. For solar cells with thin absorbing layers, such as optically thin solar cells, it is especially crucial to take advantage of this fact. The general idea is to maximize the path length of light rays in the absorber. For instance, assuming normal incidence, before entering the energy-converting material, the light rays need to be directed away from the incident direction in order to maximize their path length, and therefore the absorption, in the energy-converting material of a optically thin solar cell. In the field of chaotic scattering it is well known that trajectories that approach the invariant set of a chaotic scatterer may spend a very long time inside of the scatterer before they leave. The invariant set, also called the chaotic repeller in this case, contains all rays of infinite length that never enter or leave the region of the scatterer. If chaotic repellers exist in a system, a chaotic dynamics is present in the scatterer. Chaotic scattering dynamics is interesting in the context of surface-structured solar cells, since the topology of the shape can imply the existence of the invariant set of infinitely long-lived trajectories. On this basis, we investigate an elliptical dome structure placed on top of an optically thin absorbing film, a system inspired by the chaotic Bunimovich stadium. A classical ray-tracing program has been developed to classify the scattering dynamics and to evaluate the absorption efficiency, modeled with Beer-Lambert’s law.

We present Indium-rich InGaN thin-film solar cells containing plasmonic and dielectric nanostructures such as Ag and ITO nanopillars. Finite-difference time-domain (FDTD) simulations were carried out for solar cells containing these nanostructures on the back side and on the front side of the solar cells, and an improvement in the performance of the solar cells was compared for the different geometries and sizes of these nanostructures. In order to develop highefficiency InGaN solar cells, the indium content in the InGaN active layer needs to be increased in order to cover the large solar spectral range. Recently, several reports have demonstrated the growth of single-crystalline Indium-rich InGaN alloys without phase separation by controlling the growth temperature and the pressure. Our FDTD simulation results demonstrate that the Ag nanostructures on the back side of the solar cell lead to an enhanced surface plasmonbased scattering mostly for longer wavelengths of light including band edge of active material, while the ITO nanostructures on the front side lead to enhanced scattering of a middle wavelength range from 450 nm to 700 nm. Hence, a combination of Ag and ITO nanostructures leads to a significant broadband absorption enhancement in the active-medium of the solar cells which in turn leads to a significant enhancement (~ 25 %) in the short circuit current density (J_sc) of these solar cells.