This PDF file contains the front matter associated with SPIE Proceedings Volume 11996, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

Tandem solar cells made of organometal halide perovskite and crystalline silicon cells are one of the most promising routes towards high efficiency low cost photovoltaics. Among the possible architectures, monolithic three-terminal tandems hold the promise of the highest energy/cost figure of merit, by combining the advantage of two- and four-terminal approaches. Recently, three-terminal perovskite/silicon tandems have been reported, based on interdigitated back contact heterojunction silicon cells. Alternative solutions that can be integrated with double-sided contact silicon cells are worth to be investigated in view of their higher compatibility with industrial mass production. In this work, we present a simulation-based proof-of-concept of PVK/Si threeterminal tandem cells that use a heterostructure bipolar transistor structure. The extra terminal is implemented at the common selective layer between the perovskite and silicon subcells, avoiding the use of any recombination layer or tunneling junction. We demonstrate promising device performance through physics-based simulations preliminarily validated against experimental data of other perovskite/silicon tandem technologies reported in literature.

This paper presents theoretical models which consider various quantum processes of interactions between photons, carriers, and phonons in hot-carrier solar cells. Explicit formulas are developed to calculate carrier interband generation/recombination and intraband heating/cooling rates. The paper will focus particularly on energy relaxation issues regarding optical-phonon lifetime, nonequilibrium optical phonons, acoustic-phonon heat transport, and electron-hole energy transfer via intraband/interband electron-hole scattering and phonon sharing. It will discuss possible energy-selective contacts for facilitating hot-carrier extraction and preventing cool-carrier leakage. Their associated effects of extraction cooling and spectral-hole burning will also be investigated.

This theoretical work shows that ultra-thin InGaAs solar cells can have the operation of a hot carrier solar cell. Considering a quantum modeling of the electronic transport we show that the open circuit voltage V_oc increases with an energy-selective contact considered between the absorber and the reservoir. Moreover, we do not observe the feared corresponding current degradation. The V_oc improvement agrees with a simple and general expression based on the isentropic carrier extraction, confirming the link between the voltage and the carrier temperature. Concerning the current, as already shown in a precedent work, if carriers are confined in the absorber the current across an energy-selective contact is of the same order of magnitude as that obtained without selectivity. This advantageous behavior is explained by the hybridation of states in the absorber and in the reservoir.

Maintaining photon absorption in optically thin III-V multijunction space photovoltaics can be accomplished by integrating back surface reflectors (BSR) to increase the photon path length inside the thin solar cell and enhance the photogenerated current. This research investigates the integration of textured BSRs with thin-film 1-eV InGaAs solar cells to improve radiation tolerance and maintain device performance of thin-film inverted metamorphic (IMM) solar cells. The developed textures include surface treatments using reactive-ion etching (RIE) and in situ processing during the epitaxial growth of the solar cells. The textured layers achieve higher surface roughness than the pre-textured surface, indicating angular photon scattering. The InGaAs solar cells with textured BSRs show an increase in the short-circuit current density compared to the flat BSR with no degradation in the open-circuit voltage. The planar and RIE BSR solar cells result in lifetime enhancement factors of 2.4 and 3.6, respectively, indicating an increased photon path length due to the textured reflector. Improving the photon path length in the ultrathin InGaAs solar cells can be accomplished by using a low-index total internal reflection layer between the textured semiconductor and mirror. The results and discussion provided in this work support the integration of 600 nm-thick InGaAs subcells into a standard IMM design to achieve highly efficient and radiation-hardened space photovoltaic devices.

Radiative cooling is an attractive concept for future sustainable energy strategies, as it might enable passive cooling of buildings and photovoltaic systems, hence facilitating energy savings by boosting performance and lifespan. The key idea is the adoption of materials that strongly emit thermal radiation in the atmosphere transparency window (wavelengths between 8 and 13 μm) as cooling layers. Significant progress in the field of metamaterials has enabled the realization of dielectric photonic structures with properties matching radiative cooling requirements and capable of going below ambient temperature. However, these structures are rather expensive and appear unsuitable for today’s large-scale manufacturing. In the present work, we have studied radiative cooling applied to Shockley-Queisser solar cells by exploring alternative materials, namely cementitious phases, which exhibit the required properties while being low-cost and scalable. We have determined their emission behavior by electromagnetic simulations and estimated the corresponding solar cell operating temperature by means of a detailed-balance model. The results have been benchmarked against the current state-of-the-art and hint at the possible realization of a new class of radiative coolers based on cheap and scalable cementitious materials.

III/V semiconductor solar cells feature the highest photon conversion efficiencies (PCE), but they are still too expensive for terrestrial application. Conventional nanowire (NW) solar cells already partially resolve this issue since they can be grown on a silicon substrate and feature a low filling factor (the ratio of the 180 nm NW diameter to 500 nm NW pitch, squared). We take the next step by depositing PMMA micro-lenses with a diameter of 6 μm on top of a NW-array with the same 6 μm pitch, allowing to reduce the material consumption by more than 3 orders of magnitude. According to our FDTD simulations, the material consumption can even be further decreased by reducing the NW length with a factor of 2 down to 1 μm, since the lens is focusing the solar radiation near the top of the nanowires. We also expect a significantly increased V_oc due to an increased internal radiative efficiency (IRE) at a higher excitation power. Preliminary measurement show an increase in V_oc of at least 50 mV for randomly positioned microlenses on top of a dense NW array with 0.5 μm pitch.

Concentrator photovoltaic (CPV) technologies provide the highest photovoltaic conversion efficiency but remain too expensive for very large scale development. Reduction of the dimension (micro-CPV) is a promising approach towards cost reduction but necessitates sub-millimeter-scale high efficiency solar cells. In this paper, we review the challenges faced by sub-millimeter-scale solar cells for application in micro-CPV. We show that plasma etching processes are necessary to fabricate sub-millimeter-scale high-efficiency solar cells to avoid a waste of material in the isolation and dicing lines. We also show that despite the cell performance is known to degrade when the dimension of the cell is downscaled, this degradation can be negligible when optimized etching and passivation processes are used and when the cell operates under high concentration (<500x). The through-cell via contact architecture is a promising approach to avoid bus bars on the front side and therefore optimize the wafer usage and minimize dark current. Combining all these solutions, we claim that sub-millimeter-scale high efficiency solar cells as small as 0.01 mm² can be fabricated with more than 90% of wafer material used for photovoltaic conversion and without performance degradation when operating under 1,000x concentration compared to 1 mm² solar cells operating under 500x concentration. Challenges on characterization and in-line metrology remain to be solved and manufacturing lines need now to be adapted to provide commercial solutions for micro-CPV.

A lossless solar cell operating at the Shockley-Queisser (S-Q) limit generates an open-circuit voltage (V_OC) equal to the radiative limit. At V_OC, the highly directional beam of photons from the sun is absorbed and subsequently externally reemitted into a 4π solid angle, providing a large photon entropy loss. In our research we study the performance of a nanowire solar cell that can beat the S-Q limit and approach the 46.7% ultimate limit by placing a plano-convex lens on top of each nanowire. We have shown numerically that a 2 μm long InP tapered nanowire with the top radius of 83 nm and a tapering angle of 1.2 degrees shows a high photon escape probability of 42% due to an adiabatic expansion of the fundamental HE₁₁ mode which is then collimated using a plano-convex lens with a diameter of 8 μm. Both effects cause the increase of the open-circuit voltage of the solar cell by 159 mV above the radiative limit which is just 154 mV below the ultimate limit. The lens concept is also studied for a planar solar cell from the thermodynamics point view in terms of local entropy generation within the cell due to absorption/emission processes and is planned to be extended to a nanowire geometry.

Non-uniform irradiance on the rear side of bifacial photovoltaic (PV) modules causes electrical mismatch between cells and energy loss across the module. Racking structures increase this non-uniformity through shadows and reflections that vary throughout the day. However, commercial software typically use constant values to estimate mismatch losses in annual simulations. We investigate the impact of torque tube shading and reflection on rear side irradiance mismatch in bifacial PV modules in one-in-portrait (1P) and two-in-portrait (2P) horizontal single-axis trackers with a range of ground albedos over a typical meteorological year in Livermore, California, USA. Irradiance simulations use a version of bifacial_radiance, the National Renewable Energy Laboratory’s python wrapper for the RADIANCE ray tracing software, which we modified for arbitrary 2D irradiance sampling of the module(s) under investigation. For a torque tube reflectivity of 0.745, torque tube reflection accounts for 3.0% and 5.5% of the annual rear insolation in 1P and 2P configurations, respectively, for a 0.2 albedo; or 2.9% and 3.1% for a 0.6 albedo. Torque tube reflection decreases annual rear insolation mismatch from 11.8% to 10.7% in 1P configurations, and from 11.5% to 9.8% in 2P configurations with 0.2 albedo. Similarly, with 0.6 albedo, annual rear insolation mismatch decreases from 12.6% to 11.6% in 1P configurations, and from 11.9% to 10.4% in 2P configurations. However, we demonstrate that annual figures are insufficient for capturing the impact of torque tube reflection; seasonal and diurnal variations must also be considered.

Solar irradiance variability due to stochastic cloud dynamics can cause unwanted fluctuations in the output voltage of photovoltaic (PV) modules. These dynamics must in particular be understood at very-short and short time scales if grid interconnection and generation/load balance requirements are to be maintained for PV distributed across the grid edge. Using a recently-created database for Ottawa, Canada, a 6-month longitudinal study was conducted with a specific focus on cloud dynamics. A spectral pyranometer was used to derive full-range spectral and broadband global horizontal irradiance under all sky conditions every 250 ms. Exploiting the infrared (IR) measurement channel of this software-augmented multi-filter radiometer allowed the cloud dynamics to be probed across time scales ranging from the subsecond to minutes. Seven distinct sky conditions were self-consistently determined without sky imaging. Probability distributions, established via kernel density estimates (KDE), allowed the statistical dependence of these conditions on the spectral clear-sky index to be found. The stochastic nature of the spectral irradiance variability was probed using spectral clear-sky index increments, over time steps that were found to span three distinct variability regimes.

V-trough concentrators are usually implemented in low-concentration ratios photovoltaics for its low-cost and ease of manufacturing. The three-dimensional (3-D) configuration using four mirrors offers higher effective concentration ratios than the conventional two-dimensional v-trough concentrators. However, the skew rays hitting the photovoltaic cell in the 3-D case leads to an increase in the intensity non-uniformity and the formation of hot spots. One method to smooth incident rays on a receiver aperture is to add a diffuser in the light path such that the light incident on a diffuser is scattered in all directions following Lambertian distribution. In this work, we present a compound concentrator based on 3-D v-trough embedding a transmission diffuser to improve the illumination uniformity and reduce the hot spots. A concentrator with input and output apertures of 7.5 cm and 5 cm, respectively, and a length of 4.8 cm is proposed. Using ray tracing, the optimal position of diffuser inside the concentrator was located at a distance of 4 cm from the input aperture. The tilting angle effect on the effective concentration and the illumination non-uniformity were analyzed. The illumination nonuniformity is decreased by 33 %, while the effective concentration ratio is decreased by 17 %. The structure was realized using low-cost commercial flat mirror segments and tracing paper diffuser and was used for solar collection in a normal incidence case. The illumination profile of the cell was recorded before and after installing the diffuser showing an improvement of 41 % in non-uniformity and reduction in the effective concentration by 22 % only.

Our previously reported 17.8 % efficiency InP nanowire solar cell1 showed a short-circuit current 𝐼_𝑠𝑐 of 29.3 𝑚𝐴 𝑐𝑚², which is not far from the theoretical maximum 𝐼_𝑠𝑐 = 34.6 𝑚𝐴 𝑐𝑚², but the loss in the open circuit voltage with respect to the radiative limit still amounted to 272 mV. To avoid this loss and reach the radiative limit we have to increase both the internal radiative efficiency 𝜂int PL and the photon escape probability 𝑃𝑒𝑠𝑐 towards unity, as shown by the last term in Eq. 1. 𝑉_OC = 𝑉_oc Ultimate − 𝑘B𝑇 𝑞 |𝑙𝑛 𝜀in 𝜀out |− 𝑘B𝑇 𝑞 |𝑙𝑛(𝜂int PL𝑃𝑒𝑠𝑐)| (1) We report top-down etched InP nanowires intended to both optimize the amount of light outcoupling as well as the directionality of the emitted light. The photon entropy loss is governed by the 𝑙𝑛 𝜀in 𝜀out term, which is responsible for a 300 mV loss in the open circuit voltage. To circumvent this loss, we need to redirect all the emitted photoluminescence from the cell back to the sun (ε_𝑖𝑛 = ε_𝑜𝑢𝑡), For this purpose, we have fabricated PMMA microlenses by using a reflow process, which can be precisely positioned with respect to the InP nanowires.

As one of the most sustainable alternatives regarding environmental impact, cost-effectiveness, and social integration, solar energy is expected to become an ever more ubiquitous part of our intricate human world. Dropping prices in photovoltaics that can harvest clean energy in a decentralized, safe, and modular manner are making it more viable for solar devices to be implemented in complex environments, such as urban settings. These scenes involve more constrained and dynamic conditions, encouraging the use of solar devices that can adopt arbitrary positions and personalized tracking behaviors to make the most of available resources. In modeling the incoming solar radiation for such conditions, some common simplifying assumptions may be too limiting, in particular, not considering the anisotropic nature of diffuse shadows. We develop a variety of shadow modeling approaches for all anisotropic components of the radiation; four approaches for Beam radiation and three for Diffuse components. Through thousands of simulations in urban scenes of varying complexity, these approaches are tested, characterized, and compared in terms of accuracy, precision, run-time efficiency, and practicality. Critical trade-offs are revealed between accuracy and run-time as a function of the type of approach and resolution. Our characterizations support the development and selection of modeling frameworks that are better suited to the application. This may be useful in the design, optimization, control, and forecasting of more widely adopted solar harvesting in the challenging human environment.

In order to maximize the power output of a solar cell, the front surface metallization must efficiently transmit the solar cell generated power while minimizing the shadowing losses arising from the grid pattern area. In this work, the sheet resistivity of the emitter and finger spacing aspects are neglected and instead, the optimization of finger and busbar geometry is considered analytically to minimize the total power losses. It is found that linearly tapered (triangular) contacts are the most efficient geometry and that the position of the busbar highly influences the maximum cell output. To study the influence of the optimized contacts on the performance of a solar cell, a shadow mask was designed, and boron emitter n-type silicon solar cells were fabricated with the optimized contacts.

In this work, we present a comprehensive individual modeling approach to investigate the optical properties of a Perovskite solar cell by the ellipsometry analysis of a Methylammonium Lead Iodide (MAPbI3) perovskite/PEDOT:PSS/ITO film stack on a glass substrate. The absorption coefficient obtained from the model was compared to the ultraviolet-visible (UV-Vis) spectroscopy measured absorption spectra, while the optical constants are compared with values reported in literature. We propose that spectroscopic ellipsometry characterization can be used at the different stages of the PSCs fabrication process in order to understand the different mechanisms that impact the final performance of a photovoltaic device.