Physics, Simulation, and Photonic Engineering of Photovoltaic Devices XIV

The recent advent of robust, refractory photonic materials such as plasmonic ceramics, specifically, transition metal nitrides (TMNs) and MXenes is currently driving the development of durable, compact, chip-compatible devices for sustainable energy, harsh-environment sensing, information technology, aerospace, chemical and oil & gas industries. These materials offer high-temperature and chemical stability, great tailorability of their optical properties, strong plasmonic behavior, optical nonlinearities, and high photothermal conversion efficiencies. In this talk, I discuss advanced machine-learning-assisted photonic designs, materials optimization, and fabrication approaches for the development of efficient thermophotovoltaic (TPV) systems, lightsail spacecrafts, and high-T sensors utilizing TMN metasurfaces. The development of environmentally-friendly, large-scale fabrication techniques will be discussed, and the emphasis will be put on novel machine-learning-driven design frameworks that leverage the emerging quantum solvers for meta-device optimization and bridge the areas of materials engineering, photonic design, and quantum technologies.

One historical limitation of CdTe PV devices has been the open circuit voltage (VOC) deficit (difference between theoretical maximum VOC and measured values). While non-radiative recombination at extended structural defects and interfaces negatively impacts VOC, intentionally adding impurities (dopants) to increase the concentration of free carriers in the photovoltaic (PV) absorber material can increase the VOC. In this contribution we present a detailed analysis of the Sb doped CdTe and the Cd(Se,Te) alloy system via electron beam induced current (EBIC) measurements and the presence as well as spatial distribution of defect levels through cathodoluminescence spectrum imaging (CLSI).

Vibrational properties and lattice dynamics are fundamental to photovoltaic device performances in semiconductors. Metal-halide perovskites show intriguing dynamics in Raman spectra, where an extremely broad response is visible towards low frequencies, and the origin of this phenomenon is still under debate within the research community. In this work, we employed both ultra-low frequency (THz) Raman and THz-TDS (IR) spectroscopies on a range of metal halides to gain a more comprehensive understanding of this phenomenon. We discuss and assess a few hypotheses from the literature regarding the cause of this phenomenon. We propose that such central Raman response stems from an interplay between broad lattice modes in soft metal-halide semiconductors and the Bose-Einstein population factor. This work highlights the complexities in vibrational spectroscopies for low-energy lattice dynamics for metal halides for photovoltaic applications.

Over the years, luminescence spectroscopy has become fundamental for analyzing the photophysical properties of various samples, from organic molecules to semiconductor materials and photovoltaic (PV) devices. Detection sensitivity is crucial for handling weakly luminescent samples and short measurement times. Single photon counting data acquisition increases sensitivity and dynamic range, ideal for weak photoluminescence (PL) measurement. Steady-state luminescence spectroscopy provides insights but offers only a partial picture. Time-resolved PL spectroscopy reveals deeper photophysical processes, and spatial information gives a comprehensive understanding. Acquiring time-resolved data at regions of interest helps infer structural-to-photophysical relationships, crucial for optimizing PV materials. Combining time-resolved microscopy and PL spectroscopy offers a powerful tool for characterizing PV materials. This combination maps phenomena like time-resolved PL, carrier diffusion, and wavelength-dependent emission. We also discuss the MicroTime 100 microscope with FlexWave software for imaging weakly emitting PV materials.

While metal-halide perovskites (MHPs) appear promising for space power due to their radiation tolerance, I will challenge this perception based on their structural fragility. I will present a framework where radiation causes displacement damage in MHPs, but a combination of lattice softness, electron-phonon coupling, and low thermal conductivity enables instantaneous defect healing. However, we believe this recovery, driven by electronic ionization and the resulting lattice heat, may also make MHPs vulnerable to high temperatures and ionizing radiation. I will outline key research areas needed to fully evaluate their suitability for space applications.

In a metal halide perovskite solar cell, photovoltaic action is a result of the heterojunctions formed by the perovskite layer and the charge transport layers (CTLs) on either side. As the fabrication of perovskite layers has improved, the cell performance has become increasingly limited by the properties of the perovskite-CTL interfaces. Here we show that charge carrier processes at these heterojunctions can be studied by ultrafast optical and THz spectroscopy studies on perovskite-CTL bilayer heterostructures, and can be interpreted using numerical simulations of carrier transport including the Coulomb force. Despite their good performance in devices, we find that C60 and PCBM layers have large rate constants for cross-interface recombination that is limiting their performance. Conversely, while Spiro-OMeTAD has slow hole extraction, it does not increase the recombination rate at the perovskite’s surface, likely contributing to its success in photovoltaic devices.

Halide perovskites are strong contenders in becoming the next leading photovoltaic and light emitting technology. However, the dynamic nature of trap states in response to environmental stressors can alter the behavior of charge carriers in ways not well understood. Here, we investigate the impact of temperature and relative humidity on the minority carrier lifetimes in FASnI3-yBry family of compositions, by means of in situ time-resolved photoluminescence. We track the carrier dynamics as a function of time, environmental conditions, and excitation parameters to determine the impact of compositional variability on formation and energetics of trap states. Our results reveal that Sn-based perovskites show photostability from 10-60 oC at relative humidity below 10%. Furthermore, we observe shorter lifetimes in Br-rich films, suggesting lower crystallinity and higher density of trap states and self-doping compared to I-rich films.

We use a custom-built, high throughput, in situ photoluminescence (PL) characterization setup to collect sufficient data to train a variety of machine learning (ML) models and to quantify the degradation of several different perovskite compositions. We compare forecasts of PL of several CsyFA(1-y)Pb(BrxI(1-x))3 perovskites in response to temperature cycling with extreme gradient boosting (XGBoost) algorithms. These models enable predictions of PL figures of merit (peak location, area, intensity, and full-width half max (FWHM)), with up to 98% accuracy. Furthermore, we create a generalized model that can predict the PL behavior of ten compositions unseen during model training. The relative feature importance and correlations between environmental inputs and optical performance show that composition dominates sample degradation.

Perovskite Solar Cells (PSCs) are among the prime candidate technologies for next-generation PV applications where the shortcomings of silicon PV are manifested, due to their potential performance, tunable and adaptable properties. Both intrinsic and extrinsic factors still hinder their promotion from lab-scale to industry-scale and their subsequent commercial success. The study of reverse-bias stability in wide-bandgap semi-transparent FAPbBr3 PSCs is hereby presented. The observed degradation appears as a shunt-like phenomenon and pertains to the open-circuit voltage, while short-circuit current remains unaffected. These outcomes lead to an interpretation which involves the temporary relocation of ions and corresponding vacancies form the bulk of the perovskite material towards opposite interfaces. This generates a severe compensation of the energy band bending across the active layer, thus causing a temporary compensation of the internal potential and a shunt-like charge-transfer across the PVK/ETL interface; after removing the reverse-bias, this mechanism eventually ceases, and full recovery occurs.

Most efficient Photovoltaic Laser Power Converters (PVLPCs) are approaching to efficiencies of 70% but produce power densities of only a few W/cm2. In the pursuit of higher output power densities, here we revisit the PVLPC design guidelines and propose GaAs devices optimized for 840-860 nm laser light. We incorporate rear heterojunction (RHJ) structures for both the top and middle subcells combined with a front homojunction (FJ) structure for the bottom subcell. Following these guidelines, we demonstrate a 3J-PVLPC which supplies an output power density of 21.3 W/cm2 at a voltage of ~3.7 V with a conversion efficiency of 66.5 ± 1.7 % at 25ºC. Simulations indicate that this 3J-PVLPC could supply a power density of 65 W/cm2 while maintaining an efficiency of 65% when using a higher power laser providing 100 W/cm2. The pending issue of a characterization method in order to achieve conversion efficiencies which allow a real comparison among different devices is also treated. Aspects such as the laser spectrum and its stability, the uniformity of the laser light spot, and the PVLPC temperature management approach emerges as crucial in the quest of a characterization standard for PVLPC

This paper contains progress on LPCs material growth, including high-performance tunnel junction (p++-AlGaAs/n++-AlGaAs, p++-AlGaAs/n++-InGaP, p++-GaAs/i-InGaAs/n++-GaAs), GaAs/InGaAs metamorphic buffer layer. As well as progress in LPCs device research, including the device performance and reliability of 808 nm LPCs (single-junction, six-junction, ten-junction), 1064 nm LPCs (GaAs/InGaAs metamorphic, InP-based) . We used 808 nm LPCs with different junctions and areas for simultaneous wireless information and power transfer(SWPT) application. We successfully realized a power transmission of 1.59 watts and an information transmission rate of 200 kbps with the 10-junctionLPCs as the receiver.

The effective realization of long-distance energy transmission without the need for cables is closer than ever, thanks to advancements in optical wireless power transfer (OWPT) technology and improvements in photovoltaic laser power converter (PVLPC) performance. Although current state-of-the-art GaAs-based PVLPCs hold the highest reported efficiencies, theoretical studies suggest that wider bandgap semiconductors are more suitable for monochromatic PV conversion under high-power illuminations (~100 Wcm-2), where series resistance losses become a significant concern. In this sense, GaInP offers a solution by generating less current and reducing such losses. Our newly developed GaInP-based PVLPC demonstrated a 53.5% efficiency at 10 Wcm-2 and maintained 42.3% efficiency at 60 Wcm-2, showing better performance compared to existing GaInP PVLPCs. This progress marks a significant step towards PVLPCs exhibiting unprecedented performance under intense irradiances.

Dual-use power converter cells use both solar and laser energy simultaneously to generate current, an application relevant to space and terrestrial industries. This research explores two types of solar cells designed to convert both 1070 nm laser light and the solar spectrum into electricity: a single-junction cell and a triple-junction cell featuring a third contact to carry the extra current generated from the laser. A key component in both designs is a 1.1 eV p-n junction optimized for the 1070 nm laser. We have demonstrated 53% efficient conversion of 1070nm laser light into electricity in a single-junction metamorphic GaInAs PV device with antireflection coating. Insights from resistance modeling and varying the metamorphic buffer thickness further optimized the triple-junction cell performance.

Optical wireless power transfer (OWPT) technology is emerging as a suitable method for transmitting kilowatts of power over kilometers, ideal for challenging environments like the deep ocean and outer space. However, its overall efficiency is currently limited to around 20%, mainly due to the poor performance of photovoltaic laser power converters (PVLPCs), particularly at high power densities (≥ 100 Wcm-2). Previous research has identified InGaN as a promising material to improve performances at high power levels, with preliminary results showing efficiencies of up to 76% at 100 Wcm-2 and over 70% at 1000 Wcm-2. InGaN-based PVLPCs could achieve efficiencies of 70% for 10 km in atmosphere and 50% for 20 m underwater transmissions. These findings suggest the potential for developing PVLPCs with unprecedented efficiencies at intense illuminations.

Emerging applications in laser power conversion (LPC) and thermophotovoltaics (TPV) require high efficiency photoconverters that are finely tuned to the laser source or emitter temperature utilized in each specific application. III-V semiconductors span a wide range of bandgaps relevant to these applications and, in this presentation, we will discuss the metamorphic epitaxy of device solutions that access a wide range of bandgaps with very high performance despite the limited number of substrate platforms available. First, we will describe the design and growth of multijunction TPV converters optimized for thermal battery applications with emitter temperatures of 2000 °C. Next, we will describe the growth of 0.6 eV GaInAs devices designed for lower temperature TPV applications such as waste heat recovery near 1000 °C. Lastly, we will demonstrate 0.74 eV GaInAs laser power converters for 1550 nm laser power conversion. We will present devices grown lattice-matched to InP with near 50% conversion efficiency, and then metamorphic solutions grown on GaAs utilizing both GaInP and AlGaInAs compositionally graded buffers which have the potential for lower cost and increased scalability.

This study proposes a strategy to recycle the substrate used to fabricate efficient III-V solar cells, in order to reduce their cost, through the use of a two-dimensional material-based layer transfer method. This approach relies on the insertion of graphene, onto the substrate prior to the epitaxial growth of the III-V material. The objective is to take advantage of the weak Van der Waals bonds at the graphene layer plane to enable the epitaxially grown III-V material to be detached from the native substrate, which could be re-used later. We will present the optimization of the graphene transfer method and the epitaxy conditions, with a careful investigation of the intermediate and final products. We will show how graphene patterning can be optimized to favor the nucleation of III-V material, and further lateral growth towards planar layers.

A middle-contact three terminal architecture could pave the way for cost-effective and high energy yield perovskite/silicon tandem solar cells. However, it poses challenges because of the potential reduction in active area and the need for a highly transparent and conductive middle electrode, and smooth and defect free perovskite films. We present the development of such tandems, focusing on the perovskite composition and the optimization of the interfacial electrode layer in terms of electrical and optical properties and fabrication. The use of overlapping layouts for front and middle contact grids, as customary in microelectronics, is anticipated to conserve the active area.

Phosphorus(P)-doped ZnTe thin films were grown by molecular beam epitaxy (MBE) using cracked Zn3P2 source where P4 molecular beam from Zn3P2 is cracked to P2 molecular beam to enhance the P incorporation into ZnTe. The secondary ion mass spectroscopy (SIMS) analyses of P-doped ZnTe films indicate the linear increase of P concentration with increasing Zn3P2 flux from 8.3 x 10^16 cm^-3 to 4.8 x 10^17 cm^-3. Thus, a good controllability of P concentration using the cracking cell was achieved. A heterojunction solar cell was also fabricated using the P-doped ZnTe as the absorber with n-ZnS as the window layer. The higher open circuit voltage (Voc) of 0.82 V for n-ZnS/P-doped ZnTe/p-ZnTe solar cell was obtained due to the insertion of P-doped ZnTe layer with a moderate hole concentration.

We present Raman characterization of multijunction InP/InGaP axially heterostructured NWs with tandem cell configuration. The Raman measurements are done in micro-Raman and tip enhanced Raman spectroscopy (TERS) modes. The light NW interaction in both cases is modelled by finite element methods using COMSOL multiphysics. The results are analyzed in terms of this interaction, and the different spatial resolution. Information about the composition, crystal phase, and doping was obtained of the different sectors of the axially heterostructured NW, n-i-p InP bottom cell, p+InGaP/ n+InP tunnel diode, and n-i-p InGaP top cell.

Near-field thermophotovoltaics offer a key pathway to generate high power from a variety of low temperature thermal energy sources including industrial waste heat and solar-thermal. We present an experimental large area near-field device which leverages photon tunneling effects to yield a 25-fold increase over the same cell in a far-field configuration, for a total generation of 1.22 mW at 460 C. We additionally model several device design changes to improve the total output power.

Significant progress in thermophotovoltaics has led to record conversion efficiencies greater than 40% in recent years. This achievement can be attributed to better material quality, higher temperatures, and higher sub-bandgap reflectance. However, inconsistencies in efficiency trends across temperatures makes predicting optimal bandgaps difficult. The paper presents design rules and performance predictions for thermophotovoltaics cells and has been validated using past experimental data. Our results present the best bandgap energies for best performance at different emitter temperatures.

Thermophotovoltaics (TPVs) offer the opportunity to surpass convention photovoltaic (PV) efficiencies by utilizing an emitter structure which preferentially radiates in-band photons. For the emitters to be practically implemented on large scales, they must be thermally stable and optical tunable structures which achieve high cell power density to emitted power density ratios. We demonstrate thin film emitters as a potential structure for TPV systems with simulated power conversion ratios exceeding 50%. Using the optical and thermal stability data of 53 materials with melting temperatures above 2,000 °C, we computationally assess the device performance of coating-substrate bilayers operating at 1,800 °C paired with PV materials of varying bandgap. Variations of the structure including trilayer thin films and tandem junctions are used to increase device performance by improving the spectral shape and usage.

We presented a modeling strategy aiming for a comparative analysis of heat generation and dissipation in a perovskite and an a-Si solar cell by coupling the wave optics, semiconductor, and heat transfer modules of COMSOL Multiphysics software in the 3-D wizard. The model is first verified by comparing it with pre-existing results generated by different modeling methods in the literature. Hereafter for each type of cell, the study focused on examining the influence of major material and structural parameters such as bandgap, trap density, absorption, electrical and thermal properties, etc., on a certain heat loss mechanism. With a promising demonstration of this modeling strategy based on several major heat loss mechanisms, the model will be further expanded to include a more complete set of heat losses thus providing more insightful and precise guidance on a-Si and perovskite solar cell design based on thermal loss management and recycling.

Third-generation thin-film solar chalcogenide solar cells are highly attractive because of their exceptional optoelectronic properties, low fabrication costs and less toxic constituting elements but have very low efficiency. The widely used buffer layers are usually cadmium and indium based compounds, which are costly and heavily toxic. Herein, we propose a novel Ag2BaTiSe4-based solar cell and SnS2 as a buffer layer. The simulated structure is SnS2/ Ag2BaTiSe4/Metal Contact, and the performance of the proposed solar cell was analysed using the Solar Cell Capacitance Simulator (SCAPS-1D) program. To investigate the impact of parasitic resistance, both Series resistance and Shunt resistance were varied from 2 Ω to 10 Ω. The simulation results indicate that with the increase in series resistance, the device performance declined, while it improved with an increase in shunt resistance. The temperature was also varied from 300K to 500K and all the parameters declined rapidly. The results as a whole suggest that the Ag2BaTiSe4 could be a practical alternative for fabricating nontoxic high-performance solar cells

There are simulation studies on copper bismuth oxide (CuBi2O4) based solar cells (SCs) using SCAPS-1D. However, all are in ideal conditions which are far away from realistic values. In this work, simulation studies have been performed for thin film SCs with a configuration of Al/FTO/CdS/CuBi2O4/Cu2O/Au, where CuBi2O4, CdS, Cu2O, FTO, Al, and Au act as absorber layer, electron transport layer, hole transport layer, transparent electrode, front contact and back contact respectively, using SCAPS 1D in nonideal conditions such as parasitic resistance, reflection losses, and recombination. The ideal conditions show a photoconversion efficiency (PCE) of 29.14% while in the non-ideal conditions, PCE falls to 2.12%. By optimizing the thickness of Cu2O, and the band gap of CuBi2O4 the PCE can enhanced to 7.73%

The integration of photovoltaic (PV) systems with agricultural practices, particularly greenhouses, presents an innovative approach to addressing global energy and food security challenges. This study introduces a novel application of photonic InP nanopillars for agrivoltaic systems, optimizing light management for both photosynthesis and photovoltaic energy generation. The nanopillars are designed to transmit light across the Extended Photosynthetically Active Radiation (e-PAR) range (380–750 nm), essential for plant growth, while converting non-essential wavelengths into electrical energy. Using the COMSOL Wave Optics module, the nanopillar geometry was modeled and validated against established data, ensuring accurate simulation of optical properties. The results demonstrated precise tuning of light transmission and energy harvesting, showcasing the potential of nanopillar-based PV systems to revolutionize greenhouse designs. This work advances agrivoltaic technology by integrating nanophotonics for sustainable agriculture and energy.

This work investigates the collimation of diffuse irradiance in free space and its potential to enhance photovoltaic (PV) yield. We will present the fundamental principles that enable diffuse irradiance collimation and its practical realization using free-space luminescent solar concentrators. These devices employ polymer waveguides embedded with luminophores, featuring a Lambertian rear reflector for photon recycling and randomization, along with a spectro-angular notch filter that allows light to be accepted from all angles while directing down-converted light into a targeted escape cone. Additionally, we will highlight various application scenarios and their corresponding PV yield improvements. Notably, our results show that under certain conditions, winter energy yield can be significantly enhanced.

Accurate energy yield modelling of photovoltaic systems is essential for the design, financial analysis, and monitoring of solar photovoltaic plants. For bifacial photovoltaic applications, models must also offer robust rear-side irradiance algorithms. Yet, bifacial photovoltaic irradiance models have to be sufficiently validated for east-west vertically oriented systems, where direct beam solar irradiation swaps at solar noon. We validate 4 bifacial irradiance models with field data collected in Golden, Colorado and Fairbanks, Alaska for east-west vertical, north-south vertical, and south-tilted arrays. The 4 bifacial irradiance models used are the System Advisor Model, Bifacial_radiance, bifacialVF, and DUET.

Color aesthetics of photovoltaic modules is essential fordesign-sensitive applications, like building integrated photovoltaics (BIPV). Distributed Bragg reflector-based color filters modify the appearance of silicon solar cells. This study extends the aesthetic evaluation to emerging perovskite solar cells—typically grey or brown—by equipping them with a color filter. In this contribution we present an numerical optimization study which assesses parameters such as number of layers, total color filter height, materials choices and texture roughness for color appearance and efficiency within realistic manufacturing constraints. We use Bayesian optimization approaches for color filter design and focus on objective function definition. Finally, we present a selection of high performance ed perovskite solar cells, demonstrating the potential of our algorithm to tailor a customized color design.

Hot carrier solar cells are a proposed solar cell architecture in which the photo-generated carriers are collected before they thermalize to the lattice temperature. This would allow for efficiencies surpassing that of conventional solar cells, exceeding the Shockley-Queisser limit. Reduced carrier cooling rates have been observed in various low-dimensional systems. However, effectively extracting the hot carriers has thus far proved difficult. In some confined systems, a hot phonon bottleneck can inhibit carrier thermalization as the rate of absorption and emission of longitudinal optical (LO) phonons approach each other. However, because the photo-excited carriers actually interact with a range of phonons instead of a singular phonon mode, the picture is more complicated. We present simulation results in which we investigate the evolution and steady-state behavior of the LO phonon modes using Monte Carlo simulations of InAs/AlAsSb hot carrier solar cells. We discuss the values of LO phonon lifetimes needed to match experimental results based on physical principles.

The sun's energy, harvested via photovoltaic cells during the day, is inaccessible at night, leading to reliance on non-sustainable power sources. This presentation explores passive radiative cooling as a solution to this gap. By coupling with the cold night sky, we introduce a scalable method to convert the Earth's radiative heat into electrical and mechanical power. We demonstrate applications in health and agriculture and analyze global potential using NASA climate data. Additionally, we discuss device scalability, optimizations, and extraterrestrial applications. Finally, we propose a geo-engineering strategy to increase Earth's thermal emission, aiming to stabilize or cool the planet and mitigate climate change by enhancing radiative heat emission by 1 W/m².

Thermal radiation is a key aspect of solar cell thermal management. In this work we study, through detailed balance and multiphysics simulations, the thermal behavior of multi-junction solar cells and the impact of different radiative cooling designs on their achievable efficiency. We discuss the influence of the mid-infrared emissivity of the semiconductors constituting the cell and possible encapsulating materials, with the goal of evaluating the performance improvements achievable with an ideal thermal emitter.

Nanostructured materials, such as nanowires, can enhance the effects of hot carriers by confining them in small areas. This local confinement can create phonon-bottleneck effects, or hot phonons, and increase the rates of Auger recombination, leading to robust hot carrier effects in these nanostructures. Determining the properties of hot carriers in core-shell InGaAs/InAlAs nanowires has revealed a strong correlation between hot carrier effects and the rates of Auger recombination. Exploring the impact of Auger heating on slowing the rates of hot carrier thermalization provides valuable insights for designing efficient hot carrier absorbers for 3rd generation photovoltaic solar cells.

Adding QWs to the intrinsic region of the limiting GaAs subcell of a standard triple junction InGaP/GaAs/Ge solar cell enables absorption of sub-band edge photons, lowering the bandgap of the middle cell. We have shown this as an effective strategy to increase the GaAs middle cell current (compared to a control device without QWs) by nearly 2.5 mA/cm2 using 50-pairs of strain balanced quantum wells and a distributed Bragg reflector (DBR). Despite excellent beginning of life (BOL) performance, these cells showed some variability in performance at end of life (EOL) after irradiation with 1MeV electrons to a dose of 1×1015 cm-2. The Jsc remaining factor (RF) of the QW devise was 93% of the BOL value, while the control devices gave RFs of 85%. The Voc RFs for the QW and control device were 86% and 92%, respectively. In this talk, we will demonstrate the development of single-junction GaAs QW solar cells to improve the Voc RF by control of the QW design parameters. Carrier extraction and recombination will be investigated by changing the GaAsP strain balancing barrier composition We will also investigate the QW placement in the i-region with respect to background doping.

High energy carrier transfer and extraction are presented in a new generation of Al0.16Ga0.86As/GaAs valley photovoltaic solar cells that have been designed using complex band structure calculations focused on enabling hot carrier extraction from the upper valleys of the GaAs PV absorber. Thus, limiting carrier extraction issues due to the mismatch in energy and momentum across the valleys of the absorber and extraction layer.

A suite of temperature dependent optoelectronic measurements is presented on high quality GaInAs/GaAsP MQW solar cells to investigate the transport properties of charge carriers in these devices and to provide a fuller understanding of the operating principles of these record efficiency structures. Here, it will be shown that these measurements indicate very different operations to simple p-n junction principles typically proposed to describe the behavior of these structures.