This PDF file contains the front matter associated with SPIE Proceedings Volume 11681

Introduction to SPIE Photonics West OPTO conference 11681: Physics, Simulation, and Photonic Engineering of Photovoltaic Devices X.

I will present different strategies to improve the efficiency and power yield of solar cells, both by reducing optical losses of the cell itself as well as by optical design of the surroundings. First, I will talk about effectively transparent contacts (ETCs), the only front contact technology eliminating close to all shading losses while providing high conductivity. Then, I will talk about spectro-angular solar irradiance and the search for ideal albedo materials which redirect light from the surroundings towards the solar modules. Last, free-space diffused light concentration based on the luminescent wave guide principle will be discussed.

We demonstrated recently a record 19.9%-efficient GaAs solar cell with an absorber thickness of only 200 nm. Our next step is to optimize the device to reach a 25% efficiency. In this contribution we will present our latest simulation and experimental results based on an extensive analysis of the optical and electrical losses. The benefits brought by the contacts optimization and the improvement of the nanostructured design at the rear side of the solar cell will be emphasized.

Ultra-thin (less than 100 nm thick) photovoltaics are proposed as an enabling technology for space power applications due to their intrinsic radiation tolerance. Outside of the Earth's atmosphere, spacecraft are bombarded with energetic electrons and protons which can cause dislocations in the lattice structure of their solar cell materials thus limiting mission lifetimes. Certain orbits that could be advantageous for imaging, security and network coverage of the Earth are currently inaccessible due to high levels of radiation making them inhospitable to space craft. Ultra-thin cells have superior radiation tolerance but lower optical absorption which necessitates the integration of a nanophotonic light-trapping structure. The first iteration of ultra-thin 80 nm absorber layer devices patterned by Displacement Talbot Lithography has shown promising electrical and optical performance.

Optimal light absorption is decisive in obtaining high-efficiency solar cells for which the current approach uses direct nanostructuring of its absorber layer. This has a detrimental impact on the electrical properties of the solar cell due to an increased surface recombination current (owing to enlarged surface area and surface defects) and due to direct patterning process itself. To mitigate these effects, this work theoretically explores a Transformation Optics (TrO) approach to map the nanopatterned textures onto its planar equivalent. This allows to achieve structures with the same optical functionality but with much improved electrical properties. Schwarz-Christoffel mappings are used for ensuring conformality of the maps thus leading to planar inhomogeneous dielectric-only materials. The practical aspects concerning a possible implementation are also discussed in this work, thus paving a way towards a novel approach for implementing light-trapping structures into solar cells.

One of the trends making its way through the Photovoltaics (PV) industry, is the search for new application possibilities. Cu(In,Ga)Se₂ (CIGS) thin film solar cells stand out due to their class leading power conversion efficiency of 23.35 %, flexibility, and low cost. The use of sub-μm ultrathin CIGS solar cells has been gaining prevalence, due to the reduction in material consumption and the manufacturing time. Precise CIGS finite-difference time-domain (FDTD) and 3D-drift diffusion baseline models were developed for the Lumerical suite and a 1D electrical model for SCAPS, allowing for an accurate description of the optoelectronic behavior and response of thin and ultrathin CIGS solar cells. As a result, it was possible to obtain accurate descriptions of the optoelectronic behavior of thin and ultrathin solar cells, and to perform an optical study and optimization of novel light management approaches, such as, random texturization, photonic nanostructures, plasmonic nanoparticles, among others. The developed light management architectures enabled to push the optical performance of an ultrathin solar cell and even surpass the performance of a thin film solar cell, enabling a short-circuit current enhancement of 6.15 mA/cm² over an ultrathin reference device, without any light management integrated.

The use of III-V and group IV compounds in the same heterostructure is of great interest for high performances solar cells under concentration. In fact, the combination of these III-V and group IV compounds can lead to interesting strategic bandgap choices and engineering to better match the absorption of the solar spectrum, and therefore better solar cell performance. The series-connected quad-junctions (4J) solar cell strategies have the potential to improve solar cells performance and therefore enable low-cost concentrator photovoltaic (CPV) systems, allowing lower levelized cost of electricity (LCOE) from a CPV system. This work presents the investigation of the performance of dual junction (2J) GaInP/GaAs that might be implemented as upper cells with group IV (SiGeSn) cells as bottom cells. The aim of this study is to validate the epitaxial structure and the fabrication process for future 4J cells development. Pitch is varied from 125 μm to 400 μm for two different size of cells, in order to optimize solar cells performance under concentration (X) In the range of 100X to 1000X. Solar cells demonstrated high fill factor (FF) values and ideality factors (n) approaching unity per subcell have been obtained in the range of 100X to 500X. A FF of 85% and 88% are obtained at a concentration of 1000X for the bigger and smaller cells respectively, for the narrowest pitch. These results close to the state-of-the-art are encouraging for the implementation of this 2J with IV bottom subcell for the purpose of high performance 4J.

Luminescence imaging has been adopted rapidly as efficient research and development tool for photovoltaic devices. We review applications of line scan photoluminescence imaging for high throughput non-contact end of line inspection of silicon solar cells, with particular emphasis on the correlation between metrics derived from photoluminescence images and the electrical performance parameters of solar cells. In the second part we review photoluminescence imaging methods for the inspection of crystalline silicon photovoltaic modules that rely on the sun as the sole excitation source and that can be performed outdoors on installed modules during normal system operation and in full daylight.

Doping thin films used for photovoltaic absorbers is both critical to maximize device voltage and challenging due to complex interactions between point defects in these materials. Such interactions can result in compensation of the intended dopant species, meaning that the active charge-carrier concentration is lower than the concentration incorporated dopants. Charge-carrier compensation is directly related to the open-circuit voltage (VOC) deficit, or magnitude of VOC relative to the theoretical limit. Understanding how the carrier concentration varies within thin-films is necessary to design material processing schedules to minimize this VOC deficit and produce more efficient devices. Unfortunately, measurements of the free carrier concentration are generally relevant at the device level and cannot resolve local differences. Resolving local doping differences in thin-films such as Cd(Se,Te), CZTS, and CIGSe requires techniques with micron or sub-micron spatial resolution due to the polycrystalline structure as well as intended and unintended composition variations in these materials. In this contribution, we show how simultaneous measurement of cathodoluminescence (CL) and electron-beam-induced current (EBIC) can be used to expose doping variations in Cd(Se,Te) thin-films. Simultaneous collection of these signals reveals unexpected differences in the electric field strength through the device thickness due to spatial variation in the carrier concentration.

Analyzing the photoluminescence (PL) maps of semiconductors complementarily in time and wavelength allows to derive their key optoelectronic and transport properties. Up to now, separate acquisitions along time or wavelength had to be acquired for time and wavelength so that a comprehensive study of the dynamics was out of reach. We developed a 4D imaging set-up that allows the simultaneous acquisition of spectral and temporal luminescence intensity with micrometric spatial resolution under the exact same experimental conditions. This novel set-up relies on single pixel imaging, an approach that enables the reconstruction of the spatial information recorded from a higher resolution non-imaging detector. The sample PL signal is spatially modulated with different patterns by a digital micro-mirror device¹. We make use of this technique for the first time with a streak camera as a detector, allowing to record the PL intensity decays and spectrum for each pixel with very high temporal (<100ps) and spectral resolutions (<1nm). A patent application has been filled. We demonstrate the use of this setup by characterizing III-V samples. We observe the spatial variations of a red shift occurring during the short time of the decay.

We expose here a new method to map various optoelectronic parameters of solar cells from absolutely calibrated voltage dependent electroluminescence imaging. The absolute calibration is derived from radiometric analysis of the setup coupled to a collection model of the minority carriers on the one hand and to the reciprocity existing between electroluminescence and quantum efficiency of the device on the other hand. The method is illustrated on a classical Al-BSF cast-mono silicon solar cell for which we map with good accuracy the diffusion length, dark recombination current, local voltage and lumped series resistance and analyze the results.

Short time carrier dynamics of transient photoluminescence decays contain valuable information on the optoelectronic properties of photovoltaic materials. We perform a theoretical analysis on short time dynamics to provide scaling laws for the time derivative of the transient photoluminescence signal as a function of both laser excitation power and wavelength . This innovative approach allowed us to extract in a simple and effective manner the external radiative recombination rate and was tested on different absorbers such as state-of-the-art triple cation mixed halide perovskite and III-V materials. Moreover, by coupling this analysis with the fitting of the whole PL decay, we have quantified different transport parameters and precisely estimated their uncertainties.

Solar energy conversion is a nonequilibrium process in which high temperature light is converted into low temperature electrical power. Yet, photovoltaic cells are usually treated in an equilibrium thermodynamics formalism, assuming quasi-equilibrium electron distributions. I will unravel how different timescales of interaction determine the applicability of quasi-equilibrium models in the photovoltaic concepts of multi-exciton generation, hot carrier, and intermediate band solar cells. In hot carrier solar cells, the sparsity of the solar flux plays a key role in establishing quasi-equilibrium, making the local density of optical states a critical parameter for enabling high efficiencies.

Low-cost, high-efficiency metal halide perovskite solar cells (PSC) are a promising alternative to Si photovoltaics, but poor stability currently precludes commercialization. We present a framework for accelerated PSC design using machine learning (ML) to identify optimal compositions, fabrication parameters, and device operating conditions. We present four examples showcasing our ML roadmap using various types of neural networks, applied to diverse problems such as forecasting time-series photoluminescence (PL) from perovskite thin films, projecting PSC power output and degradation over time, and predicting figures of merit from simple, high-throughput experimental procedures. Our paradigm informs the rational development of perovskite devices, providing an accelerated pathway to commercialization.

Two-dimensional V-trough concentrators are widely used for photovoltaic applications since they can be implemented using low-cost fabrication method thanks to the flat surfaces. However, there is a trade-off between the effective concentration ratio and the concentrator ability to collect and concentrate the light uniformly from wide acceptance angle. The concentrated light uniformity is important to avoid the formation of hot spots on the solar cell that reduce the cell efficiency or causing damage. In this work, we present a novel design methodology and a set of optimal designs for three-dimensional V-trough concentrators maximizing the uniformity of collected light and acceptance angle at a higher concertation ratio shown to be C2 with respect to the two-dimensional case with a concentration ratio of C. A theoretical model is presented and compared to non-sequential ray tracing showing a good agreement. The non-uniformity is calculated as the ratio between the standard deviation and the mean of the value intensity on the cell. The acceptance angle is calculated based on the edge ray principle. The analysis shows that three-dimensional V-troughs suffer from slightly higher non uniformity than two-dimensional ones for the same acceptance angle. Example of the results show that the nonuniformity for C = 1.5 is increased from 0.29 to 0.39 and for C = 2.5 is increased from 0.18 to 0.22. The proposed designs can be used for wide acceptance angle concentration and tracking-free solar cells.

Biological systems involved in photosynthesis have recently revealed nanoscale properties and robust quantum behavior, exhibiting photon-to-electron conversion efficiency close to one. Today it is believed that this record is offered by the assistance of the interaction with the vibrations of the surrounding protein scaffold. In this contribution, we propose to discuss potential technological alternative for mimicking such a synergistic mechanism, in a biologically-inspired two-branch molecular junction. We demonstrate that time-dependent external excitations may enhance the photocurrent inside the junction.

Loss in output power in multi-junction space photovoltaics occurs due to reduction in minority carrier diffusion length caused by on-orbit radiation damage of the crystal. One method to improve the radiation resistance of a multi-junction device is through thickness reduction of the various layers, increasing carrier collection at reduced diffusion length. However, this reduction in thickness can also result in current loss unless some type of mirror is used to increase the optical path length (OPL). In this talk, we will explore two options for increasing the OPL in the middle and bottom junction of a standard InGaP2/GaAs/In0.3Ga0.7As inverted metamorphic (IMM) solar cell using a chirped distributed Bragg reflector (DBR) between the GaAs and InGaAs cell and using a maskless texture at the back side of the InGaAs cell. The fabrication of these structures will be reviewed as well as device and radiation improvements afforded by application of the photonic structures.

In photovoltaic devices, thermalization of hot carriers generated by high energy photons is one of the major loss mechanisms, which limits the power conversion efficiency of solar cells. Hot carrier solar cells are proposed to increase the efficiency of this technology by suppressing phonon-mediated thermalization channels and extracting hot carriers isentropically. Therefore, designing hot carrier absorbers, which can inhibit electron-phonon interactions and provide conditions for the re-absorption of the energy of non-equilibrium phonons by (hot) carriers, is of significant importance in such devices. As a result, it is essential to understand hot carrier relaxation mechanisms via phonon-mediated pathways in the system. In this work, the properties of photo-generated hot carriers in an InGaAs multi-quantum well structure are studied via steady-state photoluminescence spectroscopy at various lattice temperatures and excitation powers. It is observed that by considering the contribution of thermalized power above the absorber band edge, it is possible to evaluate hot carrier thermalization mechanisms via determining the thermalization coefficient of the sample. It is seen that at lower lattice temperatures, the temperature difference between hot carriers and the lattice reduces, which is consistent with the increase of the quasi-Fermi level splitting for a given thermalized power at lower lattice temperatures. Finally, the spectral linewidth broadening of multiple optical transitions in the QW structure as a function of the thermalized power is investigated.

Hot-carrier solar cells could overcome the Shockley-Queisser limit by having electrons and holes at a higher temperature than the lattice. To generate these hot carriers under concentrated sunlight, the thermalization rate should be as low as possible. Our objective in this presentation is to quantify the influence of different thermalization mechanisms. We determine the carrier temperature in ultrathin GaAs absorbers using continuous-wave photoluminescence and identify distinct surface and volume thermalization contributions. We explain the origin of these contributions using theoretical models involving non-equilibrium LO phonon populations and thermionic emission. We implement these mechanisms in detailed balance calculations for further understanding.

This work describes the relative contribution of intervalley scattering and phonon bottleneck effects in type-II InAs/AlAsSb quantum well solar cells. Moreover, recent predictions also suggest that altering the QW to barrier thickness ratio in these structures enables control of the phonon scattering rate, and therefore hot carrier relaxation may be inhibited by design. Experimental analysis of these predictions is presented in solar cell architectures, as well as, their effects upon both the optical and electrical performance of these devices.

In this contribution we report on the progress innovative solar energy conversion technologies based on silicon.

Bifacial photovoltaics present a clean and cheaper alternative to diesel generators for high-latitude remote communities; however, solar cells are tested at air mass 1.5, while average air mass increases with increasing latitude. For example, Cambridge Bay (69ºN) has an irradiance-weighted average air mass of 3.1. We demonstrate improved efficiency of bifacial silicon heterojunction modules under high air mass spectra due to reduced incident UV light. We implement air mass correction in our bifacial PV modelling software, and we quantify the impact of air mass on energy yield for fixed-tilt and tracked systems in high latitude locations.

In this work, we evaluate the performance of photovoltaic (PV) power generation forecast using various hybrid deep-learning algorithms, including long and short term memory (LSTM), LSTM with an Autoencoder ( LSTM-Autoencoder ) and LSTM with an attention mechanism (LSTM-Attention). We show that the LSTM-Attention model is significantly more accurate in predicting the hourly power generation of a PV plant with 162kW capacity than the other two reference counterparts. After 100 epochs training, the model achieves a superior Root Mean Square Error (RMSE) below 0.01, Mean Absolute Error (MAE) below 0.005, the Absolute Deviation (AD) below 0.02, and the Mean Absolute Percantage Error (MAPE) is around 35%. Moreover, since the correlation coefficient is up to 92%, this hybrid model not only can be used in solar power generation prediction in PV plants, but potentially can also be extended to other renewable energy sources such as predicting wind power or tide power generation.

To develop efficient strategies for mitigating the elevated temperature-induced losses and improving the annual energy yield of solar cells and photovoltaic modules, thermal modelling is of utmost importance. In this contribution, we use rigorous Finite Element Method (FEM) simulations to investigate the steady-state spatial temperature distribution in commercial high-efficiency crystalline silicon PV modules, with particular focus aimed towards studying the impact of various influencing parameters. First, we investigate how heat conduction within an encapsulated solar cell operating at maximum power point is influenced by metallization and surface textures. Then, we study how the operating temperature is affected by the optical power density incident on the PV module and to what extent the natural convection, hence the cooling of the device, is influenced by changing the PV module inclination angle from 0° to 30°. Finally, the forced convection in form of wind is introduced. We demonstrate that forced convection has an even greater beneficial impact at higher wind speeds and larger PV module dimensions, since the transformation of laminar to turbulent wind flow that can occur above the surface of the module contributes to additional cooling.

Structured interfaces to enhance light trapping are standard concepts for silicon solar cells. Within this simulation study we investigated the influence of the refractive index of front side coupling structures on top of a crystalline silicon solar cell on the light trapping performance. Simulations were carried out both at cell level and for the complete module stack. The light trapping behavior can only be reliably assessed if the complete system is investigated. It could be shown that the refractive index of the light trapping structure strongly influences the light trapping behavior.

The behavior of solar cells is very often limited by inhomogeneously distributed nanoscale defects. This is the case throughout the entire lifecycle of the solar cell, from the distribution of elements and defects during solar cell growth as well as the charge-collection and recombination during operation, to degradation and failure mechanisms due to impurity diffusion, crack formation, and irradiation- and heat-induced cell damage This has been known for a while in the field of crystalline silicon, but inhomogeneities are far more abundant in polycrystalline materials, and are the limiting factor in thin-film solar cells where grain sizes are often on the order of the diffusion length. We will show that the high penetration of hard X-rays combined with the high sensitivity to elemental distribution, structure, and spatial resolution offers a unique avenue for highly correlative studies at the nanoscale. We will present results on Cu-doped CdTe, where carrier collection is directly correlated to the kinetics of Cu inside the absorber and the particular Cu-phases at the ZnTe/CdTe interface. We will complement the results with modeling using PVRD-FASP and PyCDTS.

Photonic power converters (PPCs) are multijunction photovoltaic devices that convert monochromatic light to electricity. We measure the current-density-dependent external radiative efficiency of representative single-junction GaAs PPCs and use the results in a multijunction detailed balance model to predict attainable device efficiencies in multi-junction GaAs PPCs. For fixed input optical power density, we determine the number of junctions that maximizes efficiency, including effects of luminescent coupling and series resistance. The optimal number of junctions ensures that the device operates close to its maximum radiative efficiency.

Photonic power converters (PPCs) are one of the main components of optical power transmission systems, generating electrical power via the photovoltaic effect. We simulate ultrathin PPCs designed for operating at the telecommunication wavelength of 1310 nm with 9 and 12 times thinner absorbing layers using cubic and pyramidal nanostructured back reflectors (BRs), respectively. While increasing efficiency by 13% (rel.) over conventional PPCs, results also show superior light trapping for pyramidal BR with twice the absorption of a simple double pass absorber layer of the same thickness and higher short-circuit current for pillar BR reaching 94% of an ideal Lambertian surface.

Though the sun is our most abundant source of energy, existing solar power production methods can generate electricity only while the sun is shining. To shift to a truly sustainable energy grid, new green energy production techniques that function at night must be developed. Here we consider a “nighttime photovoltaic cell” that harnesses electricity from the flow of heat between the earth and the cold night sky. We discuss thermoradiative photovoltaics, the physics driving this photovoltaic technique, and the theoretical limits of such a device. We conclude with an analysis of this novel photovoltaic concept’s practical limits and integrability.

After around 10 years of research, perovskite cells are one of the most promising technologies in the field of photovoltaic. Despite this, there are still hurdles to overcome, such as stability and inhomogeneities in upscale processes, before it can be commercialized. It is therefore necessary to find a protocol to provide a quality assessment of the perovskite cells and identify the type of defects present inside. Electroluminescence (EL) imaging is an ideal candidate to meet these requirements, as it allows defect detection through the application of a voltage/current. We performed electroluminescence characterization on perovskite cells and modules fabricated at IPVF and showing efficiencies between 14 and 18%. We observed unusual behaviors such as transient phenomena, cell extinction or alternating cell luminescence. To better understand these phenomena we modeled our modules using LTSpice. We managed to reproduce these experimental behaviors by varying parameters such as shunt resistance or recombination rate and observing their effect on modeled EL intensity. Moreover, we can also identify which type of defect is predominant according to the applied voltage. Thanks to this work, we determined a precise protocol to link certain electroluminescent behaviors of the module to a specific physical parameter failure and to their potentially related synthesis defect. To go further, we are working on a 2D version of the model that will allow us to a better understanding of the effect of local inhomogeneities inside a larger cell in larger modules.

Quantum dot solar cell (QDSC) has been introduced to stretch the quantum efficiency towards longer wavelength. This is an efficient way to harvest photons with sub band gap energy values. Still, these can absorb photons within a particular energy band, depending upon the size distribution and corresponding energy levels of such quantum dots. To increase the energy conversion efficiency in this near infrared region one can introduce electronic coupling or varying size distribution of quantum dots. In epitaxially grown III-As based solar cell has shown very promising results with In(Ga)As/GaAs quantum dots, embedded periodically in the active region. In the current study, we are showing a new hybrid way to use both Stranski-Krastanov (SK) and submonolayer (SML) quantum dots simultaneously for better near infrared energy harvesting. With a detailed investigation on photo-generated carrier dynamics, and correlating this with photovoltaic energy conversion efficiency, we have established this hybrid SK-SML QDSC as a superior configuration than its homogeneous counterpart (SK QDSC).

Passive radiative cooling has attracted great attention due to its capability to dissipate heat without energy consumption [1,2]. Here, we demonstrate a one dimensional photonic structure for high-performance daytime radiative cooling [3]. Structural parameters of the proposed photonic structure are optimized for both wavelength range of the solar and atmospheric transparency window simultaneously. The types of materials and thicknesses of up-to 10 layers of multilayer are optimized by genetic algorithms. We develop an objective function in the solar region to achieve high-performance daytime radiative cooling with a focus on minimizing solar absorption power. Among the four material candidates of SiO2, Si3N4, MgF2, and HfO2, proper materials are recommended and the best thickness are optimized for desired optical functionalities for daytime radiative cooling. The designed structures minimize the solar power absorbed while strongly emit thermal radiation at the atmospheric transparency window at 8-13 um wavelength region.

The introduction of light scattering structures for efficient absorption of incident illumination is essential in ultra-thin solar cells given their reduced optical path length. The growing interest in these devices demands attaining high efficiencies through the identification of optimal designs to maximise the absorption of incident photons. A pathway towards such high efficiencies is the implementation of transparent scattering structures to minimise parasitic losses. We study the performance of these structures by focusing on dielectric/high-band-gap semiconductor scattering layers in an ultra-thin (80 nm) GaAs solar cell. Comparisons with absorptive scattering layers are enabled by presenting data for an equivalent device with metal/dielectric structures. Following a previously reported light management optimisation method which is guided by the dispersion of the avail- able waveguide modes, we find an improved performance for the transparent scattering layers. Our study also compares the light absorption enhancement offered by transparent photonic crystal diffractive arrays to that of transparent quasi-random geometries which target the diffracted power to the most favourable waveguide modes in the device. We find the former designs to have a superior performance in our device of interest, and the latter to suffer from greater reflection losses. Finally, our results also demonstrate the effectiveness of the optimisation method used and its applicability to multiple device architectures for the design of high-efficiency photovoltaics.