Tandem or multijunction photovoltaic devices cells offer the clearest path to high efficiency and high areal energy density solar energy conversion. Theoretically and at the laboratory scale, increasing the number of junctions is a simple way to maximize the amount of electricity that can be produced from a small-area device. However, there are multiple approaches to electrically and optically interconnecting the sub-cells in a tandem stack that have different trade-offs in terms of efficiency, cost, and manufacturability. Three terminal (3T) tandems have attracted a great deal of interest at the laboratory scale for their high potential efficiencies and polarity-changing interconnections. However, the coupled nature of 3T devices adds a degree of complexity to the devices themselves and the ways that their performance can be measured and reported. In this talk, I will discuss the recent progress in the field of 3T tandems, including our recently proposed taxonomy for naming 3T devices, experimental demonstrations, robust measurement approaches, and interconnecting 3T cells into strings.

Transition metal dichalcogenides such as MoS2 and WSe2 are excellent candidates for the development of ultrathin solar cells due to their layered crystalline structure and their extremely high light absorption coefficients. In this talk we review the technological challenges associated with the fabrication of solar cells based on quasi 2D-crystals of MoS2 (crystals with thickness between ∼ 5 and 100 nm). We show that it is possible to achieve a large open-circuit voltage (∼ 1 V) in MoS2 homojunction devices under concentrated light and discuss the impact of surface effects, contact technology, and optical design. In the last part of the presentation we talk about the potential use of this emerging technology for the fabrication of semitransparent power-generating windows.

This conference presentation was prepared for Photonics West 2023.

Climate change and its many associated impacts are among the most serious and pressing global challenges. Photovoltaics (PV) is instrumental in the mitigation of CO2 through the generation of low carbon electricity. However, the goal of limiting global warming to 1.5°C requires additional approaches. This paper presents how PV surfaces can reverse the Earth’s radiative imbalance caused by increasing atmospheric greenhouse gases and thereby stabilize global temperatures. The benefits are realized by: (1) high effective albedo between 30 – 40% (2) maximizing thermally emitted radiation; and (3) active infrared emission averaging 300 W/m2 through atmospheric wavelength windows, e.g., at 1.5-1.75 micron. With such PV surfaces, we show that 25 TW of PV can reverse or mitigate global warming.

We review our recent advances in the simulation of perovskite-based tandem solar cells, including fundamental model developments regarding both, the optics and the charge transport in such devices. This includes features such as the detailed balance-compliant dipole radiation model for emission in re-absorbing media and its application to photon recycling and luminescent coupling in perovskite-based tandem solar cells, as well as the analysis of the electrical interconnection of sub-cells via recombination junctions.

High efficiency III-V multijunction solar cells require multiple alloys with optimal bandgaps and excellent material quality. However, many useful alloys are lattice-mismatched to available substrates, and require strain-engineering tricks to enable efficient use. To this extent, we have developed low-dislocation density metamorphic material and optically-thick stress-balanced superlattices, enabling access to a wide variety of III-V alloys. Combined with transparent tunnel junctions, these components allow optimizing multijunction devices for a variety of applications. We have designed multijunction cells for the G173G-terrestrial and AM0-space spectra with record 39.5% and 34.2% efficiency, respectively, and also thermophotovoltaic cells for blackbody spectra that reach over 40% TPV-efficiency. Finally, using these PV components, we demonstrate a multijunction LED with high quantum efficiency.

In this paper we discuss the emerging applications of photovoltaics for laser-based optical wireless power transfer. In particular, we focus on how key factors impact the design of the system, including wavelength, power range, operating temperature and environment and their impact on the choice of laser and photovoltaic cells. Two examples are considered: the first using bespoke InP-based photovoltaics targeting 1550 nm operation for eye-safe terrestrial applications, and the second concerning the use of visible and near-IR laser illumination of solar cells for satellites in low Earth orbit. The challenges and opportunities associated with each application will be discussed.

Photonic power converters designed to operate in the telecommunications O-band were measured under non-uniform 1319 nm laser illumination. Two device architectures were studied, based on lattice-matched InGaAsP on an InP substrate and lattice-mismatched InGaAs grown on GaAs using a metamorphic buffer. The maximum measured efficiencies were 52.9% and 48.8% for the lattice-matched and -mismatched designs respectively. Both 5.4-mm2 devices were insensitive to the incident laser spot size for input powers of < 250 mW and exhibited better performance for larger spot sizes with more uniform illumination profiles at higher powers.

In this work, we incorporate 50 InGaAs quantum wells (QWs) into the bottom junction of an InGaP/GaAs dual junction solar cell. Tensile GaAsP is used to compensate the compressive wells, enabling high quality growth confirmed by high resolution XRD. A distributed Bragg reflector (DBR) is grown below the device, centered on the QW absorptive region for improved optical path length in the QWs. These structures enable an increase of 2.6 mA/cm² from sub-gap absorption with a minimal loss in voltage (11 mV) compared to a control device without QWs or DBR, providing an absolute efficiency increase of 3.6% under AM0.

This conference presentation was prepared for Photonics West 2023.

To mitigate perovskites’ degradation, there have been a pressing need to identify the effects of environmental stressors on material physical behavior and device performance. We implement high-throughput environmental photoluminescence (PL) to interrogate the response of Cs-FA perovskites with a range of chemical composition while exposed to temperature and relative humidity cycles. These measurements are used as input when comparing how machine learning methods can be realized to forecast material response. We quantitatively compare linear regression, Echo State Network (ESN), and Auto-Regressive Integrated Moving Average with eXogenous regressors (ARIMAX).

The development of advanced photovoltaic devices, including those that might overcome the single junction efficiency limit, as well as the design of new materials, rely heavily on advanced characterization techniques. Among all the existing methods, optical ones are well suited to probe quantitatively optoelectronic properties, and luminescence-based ones feature preeminently for this purpose. We here present the use of multidimensional imaging techniques that record spatially (with up to 2 µm spatial resolution), spectrally (5 nm), and time-resolved (50 ps) luminescence images. We will discuss the benefits and challenges of looking into energy conversion systems from a multidimensional perspective. We will use some examples, mostly drawn from halide perovskite and III-V materials and device, which will help revisit questions related to efficient transport and conversion in solar cells.

This study presents the results of attaching colloidal Cesium Perovskites quantum dots Spin Coated on Silicon Solar Cells to Improve efficiency. Colloidal perovskite quantum dots of cesium lead were prepared with Bromine and Iodine. The photoluminescent of the quantum dots can be tuned by QD size. The important parameter in controlling the QD size is the temperature of the precursor solution. Our preliminary studies show that a combination of CsPb Br3 and CsPbI3 layers can be added as absorbing layers on the top silicon solar cells, which resulted in an enhancement of silicon conversion efficiency of about 18%.

The effects of spectral albedo on bifacial silicon heterojunction photovoltaic cell performance is explored in six locations in North America using an optoelectronic drift-diffusion model. We model seven spectral albedos using the scaled rear irradiance method and compare to broadband values. Cell performance varies geographically, with the maximum efficiency of 22.6% calculated for Cambridge Bay (69°N) with snow ground cover, and maximum output power of 216 W/m2 for Mexico City (20°N) with white sand. Neglecting spectral effects of albedo can under or over-estimate power by >2%, which can significantly impact system-level energy yield.

Ultra-thin devices offer desirable properties including lightweight flexible form factors for system integration; reduced materials for lower cost sustainable production; and intrinsic tolerance to particle radiation, making them compelling candidates for space power applications. As thickness is reduced light absorption also reduces, necessitating the use of light management approaches, such as integrated rear surface planar reflectors, random or quasi-random scattering surfaces, and nanophotonic gratings. The relative merits of these approaches and different regimes in which they might be desirable are discussed along with correlative device and simulation results for their application to ultra-thin GaAs photovoltaics.

We discuss issues and factors that affect the device performance of narrow bandgap photovoltaic (PV) cells. Narrow bandgap interband cascade PV devices are presented as a promising route to circumvent or alleviate several limiting factors in conventional narrow bandgap PV cells. Some modelling and preliminary experimental results will be presented. Also, remaining issues and an outlook will be discussed.

Hot Carrier Solar Cells (HCSCs) are a proposed next-generation photovoltaic technology for overcoming the single-gap efficiency limit. Here, our latest work in developing protocols for effective hot carrier extraction and field aided scattering within the framework of valley photovoltaics (VP) will be presented. A study of various absorber/selective barrier material combinations provides insight into current bottlenecks towards the realization of a VP HCSC, and how these might be circumvented using several complementary experimental techniques.

Very-low bandgap thermophotovoltaic (TPV) cells (with ~0.25 eV bandgap) aiming at harvesting photons from the mid-infrared spectrum, have yet to operate at ambient temperature. Often requiring cooling down to 100K, the power consumption for maintaining such temperature can be treated as swimming against the efficiency tide. We propose in this study to adapt infrared photodetectors based on Ga-free type-II InAs/InAsSb superlattice (T2SL) barrier structure, into TPV cells, and assess their performances. Such structures have already demonstrated higher operating temperature as photodetectors, and could give promising results similar to existing devices based on InAs/GaSb T2SL while getting rid of Ga-native defects problematics.

This conference presentation was prepared for Photonics West 2023.

The paper discusses a quantum transport model that includes explicitly the phonon-mediated intervalley photocarrier transfer in InAs/AlAsSb heterostructures, enabling the simulation-based assessment of hot-carrier extraction via the valley-photovoltaic effect. The model resolves both, intra- and intervalley scattering rates and valley-specific currents, and is applied at short circuit conditions and under forward bias voltage. In both cases, the impact of intervalley charge transfer and L-valley transport on photocarrier collection is evaluated. Special consideration is given to the contacting of the L-valley states for efficient charge extraction, and to the carrier temperature of the steady-state population that dominates the dark current at the radiative limit.

Metal halide perovskites are emerging as an intriguing class of solution-based semiconductors with significant potential for photovoltaics. In this study we investigate the triple halide perovskite (FA,Cs)Pb(I,Br,Cl)3, which is known to be amongst the most stable metal halide perovskite systems available. This stability allows a comprehensive study of the hot carrier dynamics to be assessed under steady-state conditions at high fluence and various temperatures in a device structure.Here, we show measurements support the presence of hot carriers in the device in advance of any negative effects due to halide segregation or decomposition.

The last decade has witnessed a rapid growth in the use of in situ and operando techniques, in which materials and devices are probed under conditions which resemble as closely as possible those used under real operating environments, with time-resolved measurements increasingly being made as well. Electron beam techniques and Kelvin probe force microscopy have led the way as powerful tools to study charge separation and recombination at the nanoscale. However, despite their strengths, there are several limitations imposed by their penetration depth, and sample preparation requirements that limit their ability to suitably represent the full system under study. In this talk I will cover how X-rays are an ideal way to noninvasively obtain information from full devices demanding little or no specimen preparation even under working conditions. Workhorse techniques such as X-ray photoelectron spectroscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF), and multi-dimensional imaging can be coupled with X-ray beam–induced current (XBIC) to study the effect of structure and composition inside a device. Furthermore, because the interaction of X-rays with matter is highly energy-dependent around the absorption edges of the probed atoms, energy-dependent X-ray studies can be used to probe changes in the local atomic environments during operation with high sensitivity. The field of operando science is growing rapidly and offers tremendous opportunities to uncover the relationship between structure, properties, composition and performance at the nanoscale for optoelectronic materials and devices.

Terrestrial deployments of III-V solar cells require cell manufacturing costs well below $10/WDC in order to gain widespread adoption for a variety of use-cases. To achieve these targets, the cost of substrates, processing, and epitaxial growth must significantly decrease from today’s levels. In this work, we present Sonic Lift-off (SLO) of operational single junction GaAs cells as an avenue to enable multiple substrate re-use for subsequent growth, with minimal re-polishing, to drive down substrate costs dramatically.