Silicon solar cells are approaching their efficiency limit of 29% under the standard solar spectrum. In order to surpass this limit, a device is required that better manages the energy in each incoming energy packet (photon). One approach to this end is to split the energy of higher energy photons in two, such that two electron-hole pairs can be generated by one photon. This strategy has an upper limit of 45.9%. Organic Multiple Exciton Generation (OMEG) is executed by a photophysical process called singlet fission. A spin-0 (singlet) exciton is generated by a photon, and it decays into two spin-1 triplet excitons in a spin-conserving process. This talk will detail our progress towards developing OMEG augmented silicon solar cells (OMEGA-Si).

As the demand for renewable energy sources continues to rise, photovoltaic (PV) technology has emerged as a key player in the global energy issues. PV solar cells are specifically engineered to absorb solar photons effectively, yet only a fraction of the captured energy is converted into electricity. The remaining energy is dissipated as heat, causing solar modules to reach temperatures as high as 50-60 °C during real-world operation. However, this rise in temperature adversely affects both the power conversion efficiency and the lifespan of the solar cells. This work aims to provide an overview of the ongoing research and developments in the simulation of solar cells, thermal phenomena, and the modeling of radiative cooling in photovoltaics, highlighting the importance of accurate modeling for optimizing PV performance and addressing challenges in this domain. By developing a fully coupled opto-electro-thermal model, this research contributes to the advancement of solar cell design and optimization strategies.

Ab initio molecular dynamics (AIMD) simulations have been performed for different halide perovskites to investigate their response to low-energy radiation. The threshold displacement energy (Ed) is the minimum amount of transferred kinetic energy to an atom so that it generates a stable defect in the lattice of a particular material. The Ed is a critical physical parameter for simulating non-ionizing radiation damage in materials, the primary degrader of optoelectronic properties under radiation environments. AIMD allows us to probe atoms in different lattice directions and establish the Ed of the species in the halide perovskite. These efforts would allow for better simulations of the radiation hardness of materials.

The PV parameters of triple cation perovskite solar cells are studied focusing on the electro-optical properties and differences in performance at low and high temperatures. A parasitic barrier to carrier extraction at low temperatures causes a loss of performance at T < 200 K. Combined Intensity and temperature dependent measurements suggest that extraction across this parasitic interface is constrained by a combination of the binding energy of the excitons and thermionic emission. However, the performance of the device is restored at low intensity_ where the thermionic extraction rate exceeds the photocarrier generation.

The 2D Ruddlesden- Popper perovskite (EPEA)2PbI4 was investigated using temperature - and power -dependent PL and TA spectroscopy. This endeavor revealed the presence of multiple excitonic complexes in these materials, with signatures of carrier redistribution mediated by power and/or temperature, and the presence of extremely long-lived dark states in TA. These states appear to be light -induced defects which heal after illumination ceases. As such, they bring new insight into failure mechanisms and material design approaches in perovskite photovoltaics

Perovskite solar cells are one of the most actively studied next-generation solar cells. This is mainly because high power conversion efficiencies can be achieved even with simple solution-based fabrication processes. In addition, wide bandgap perovskite solar cells can be used as top sub-cells in multi-junction solar cells due to their easily tunable bandgap properties. On the other hand, colloidal quantum dots (CQDs), whose band gap depends on the quantum dot size, are one of the few options that are compatible with solution processes and can be employed as lower sub-cells. Here, we show the potential of both types for the construction of multi-junction solar cells. To this end, we constructed a wide bandgap perovskite solar cell that is ideal for monolithic 2-junction perovskite/GaAs solar cells. We also developed a colloidal quantum dot solar cell with infrared absorbing PbS CQDs and constructed a spectral splitting multi-junction solar cell as a proof of concept.

The standard method to measure subcell external quantum efficiency (EQE) for multi-junction photovoltaics (MJPV) uses light biasing to bring each subcell into current limitation. This method is suitable when each subcell absorbs in a different wavelength range. However, isolating individual subcells via light biasing is difficult for semitransparent subcells with overlapping absorptance, as in MJPV designed for monochromatic irradiance in power-by-light systems. For these cells, the standard measurement approach falls short. Here, we present an alternative technique that incorporates a negative bias voltage to overcome this limitation. We demonstrate subcell EQE measurements in MJPV devices with up to six GaAs subcells.

We present a universal model of broadband absorption in a slab of semiconductor. The theoretical framework, based on the description of multiple overlapping resonances in the frequency domain, has a very broad domain of validity. We derive simple analytical formulas for reference light-trapping models and for absorption upper bounds. Two light-trapping strategies are compared: multi-resonant absorption achieved with a sub-wavelength periodical pattern, and isotropic scattering obtained with random texturing. We provide an answer to the long-debated question of the best strategy for light-trapping in solar cells, and guidelines for the design of ultrathin solar cells. They apply to both silicon and thin-film solar cells. The new upper bounds on absorption presented in this work could be used to revisit the maximum efficiency of single-junction silicon solar cells.

We have developed a machine learning empowered computational framework to facilitate design space exploration for optoelectronic devices. In this work, we apply dimensionality reduction and clustering machine learning algorithms to identify optimal ten-junction C-band photonic power converter (PPC) designs. We outline our framework, design optimization procedure, calibrated optoelectronic model, and experimental calibration devices. We report on top performing device designs for on-substrate and flat back-reflector architectures. We comment on the design sensitivity for these PPCs and on the applicability of dimensionality reduction and clustering algorithms to assist in optoelectronic device design.

I will discuss the status of intermediate band (IB) solar cells and of a recently proposed hot carrier solar cell concept called valley photovoltaics (VPV). I will discuss the development of a trio of models, with varying computational cost, for understanding and designing advanced concept photovoltaics. I will describe Simudo, a Poisson/drift-diffusion solver for advanced PV, and use it to validate a semi-analytic model for IB devices, which allows rapid exploration of the design space. Including realistic nonradiative losses, efficient IBSCs will be easier to make with higher bandgap materials than are most commonly studied, and I will show experiments and design concepts for GaN-based IB devices. The VPV concept is that hot carriers can be maintained in and extracted from metastable valleys of the conduction band. Devices so far have JV curves with a strong S-shape and low power generation. We adapt Simudo to treat VPV and well reproduce experimental JV curves. I will discuss the various origins of the S-shape. I will introduce an equivalent circuit model for the S-shape and connect it to physics in the device, giving insight into the obstacles to achieving high efficiency.

The study of hot carrier dynamics in semiconductors, driven out of equilibrium by strong electrical and/or optical fields, is critical to the function of a broad range of microelectronic technologies. While the carriers in such materials typically equilibrate rapidly (on sub-100-fs time scales), efficiently reaching some “hot-carrier” temperature, electron-phonon energy exchange is generally a much slower process (occurring on ns scales) that limits the overall heating of such systems. Understanding the mechanisms of energy transfer via electron-phonon coupling is therefore critical to the quantitative design of advanced electronic/optoelectronic devices. In this presentation I will describe how fast (ns-duration) electrical pulsing of semiconductors can be used to un-derstand the nonequilibrium operation of a variety of electronic devices. These include charge-density-wave materials, in which the roles of field- and thermally-driven processes in resistive switching can be distinguished, 2D van der Waals heterostructures, in which the hot-carrier dynamics is impacted by Moire miniband formation, and novel solar absorbers that use inter-valley transfer for hot-carrier solar cells.

Type-II InAs/AlAsSb multi-quantum well (MQW) structures have seen usage in both quantum-cascade lasers and avalanche photodiodes. There has been recent interest in investigating this material system for next-generation photovoltaic applications, specifically hot carrier solar cells, due to the type-II offset spatially separating electrons and holes and the predicted high LO phonon lifetime. In order to successfully realize a hot carrier solar cell, the ultrafast relaxation process needs to be well understood. To investigate these effects, we simulated a MQW structure under both pulsed and continuous wave laser excitation with an Ensemble Monte Carlo (EMC) solver self-consistently coupled to a multi-valley non-parabolic Schrödinger/Poisson solver. The EMC includes intervalley scattering, carrier-carrier scattering, and nonequilibrium phonon effects.The EMC simulations show that the inhibited cooling is primarily due to a build up of LO phonons. We demonstrate good agreement with temperatures extracted via photoluminesce techniques.

The hot carrier dynamics of thermally stable triple halide perovskite solar cells are investigated through power dependent transient absorption (TA) measurements. The TA measurements from both front and back sides of the solar cells were done to better understand the cooling processes. After extracting the PV parameters of the solar cells from the J-V characteristics, the TA measurements were repeated in-operando conditions and under varying external biases to monitor the thermalization of the carriers in more practical conditions.

The optical response of 2D layered perovskites is composed of multiple equally-spaced spectral features, often interpreted as phonon replicas, separated by an energy Δ ≃ 15−40 meV, depending upon the compound. We show that the characteristic energy spacing, seen in both absorption and emission, is correlated with a substantial scattering response above ≃200 cm−1 (≃25 meV) observed in resonant Raman. This peculiar high-frequency signal, which dominates both Stokes and anti-Stokes regions of the scattering spectra, possess the characteristic spectral fingerprints of polarons. Notably, its spectral position is shifted away from the Rayleigh line, with a tail on the high energy side. The internal structure of the polaron consists of a series of equidistant signals separated by 25-32 cm−1 (3-4 meV), depending upon the compound, forming a polaron vibronic progression. The observed progression is characterized by a large Huang-Rhys factor (S >6) for all of the 2D layered perovskites investigated here, indicative of a strong charge carrier – lattice coupling.

III-V nanowire structures have shown promising results in mitigating hot carrier thermalization rates suitable for hot carrier solar cell applications. This effect is attributed to the spatial confinement of charged particles and the adjustment of material properties in these nanostructures. Furthermore, by designing vertically standing nanowires, it is possible to improve photo-absorption by increasing internal surface reflection. Investigating the properties of hot carriers in core-shell InGaAs nanowires has shown evidence for a strong diameter dependence of these nanostructures. Determining the origin of this effect provides valuable information for the development of efficient hot carrier absorbers for 3rd generation solar cells.

Machine learning (ML) is a powerful tool to accelerate the development of halide perovskite materials and devices. We apply ML models varying from echo state networks to statistical models to classify and predict physical properties such as hole transport layer electrical conductivity, halide perovskite photoluminescence response, the power conversion efficiency of photovoltaic devices, etc. Specifically, we use in situ environmental optical measurements to predict the optical behavior of Cs-FA perovskites for 50+ hours, upon materials’ exposure to moisture.

Bifacial photovoltaic devices harvest solar irradiance from their front and rear surfaces, boosting electrical power production per footprint area. Metal halide perovskite solar cells possess the potential to realize efficient bifacial thin-film photovoltaics owing to their outstanding optoelectronic properties and unique features of device physics. This talk will discuss device physics, design principles, fabrication, and measurement of highly efficient bifacial perovskite solar cells. Our technoeconomic analysis shows that bifacial perovskite solar cells can outperform their monofacial counterparts with higher energy yields and lower levelized costs of energy (LCOE) in future commercialization.

Stable and reliable optical power converting devices have been obtained for various Optical Wireless Power Transmission (OWPT) applications, including power-over-fiber (PoF) or power beaming uses. They are obtained using vertical multi-junction laser power converters (LPCs) based on the GaAs and InP material systems. The LPCs are high-performance photovoltaic (PV) devices typically optimized for a narrow wavelength range. Such Optical Power Converters (OPCs) enable several isolated electrical power or remoter power applications using high-power lasers for their input power. Broadcom’s vertical multijunction PV devices (VEHSA design) permit optical-to-electrical conversion with record efficiencies and output power capabilities. This presentation will review the recent developments in both, the GaAs-based and InP-based systems. For example, high-efficiency and high-power capabilities have recently been demonstrated at ~1480nm for the InP-based PT10-InGaAs/InP, which are designed with 10 InGaAs subcells lattice-matched to InP and connected with transparent tunnel junctions. These long-wavelength OPCs are capable of directly producing electrical output voltages in the 5 V range.

Photonic power converters (PPCs) are photovoltaic cells that convert monochromatic light into electric power. The impact of luminescent coupling (LC) on InGaAs-based PPCs is studied. Multi-junction PPCs are simulated using an experimentally validated drift-diffusion model, and the contribution of LC is quantified. Up to 85% of the photons emitted across the InGaAs layers are re-absorbed in the dual-junction device considered. This number increases to 96% when a back reflector is included due to improved light management. Interference effects produced by multiple reflections are examined as a function of the emission angle.

A Laser power converter (LPC) cell targeting 0.74 eV has been grown on an InP substrate using two lattice-matched GaInAs p/n junctions vertically stacked with a tunnel diode, resulting in double the output voltage and half of the output current. Historically, multi-junction devices like this have typically used Zinc doped p+GaInAs in the tunnel junction. However, this can be difficult to grow reliably by MOCVD and Zn diffusion can lead to lower tunnel diode performance. In this work, MicroLink demonstrates a multijunction GaInAs cell using a novel p+GaAsSb/n+InP tunnel junction from diffusion stable C doped p+GaAsSb. These LPC cells are also grown inverted and then processed using substrate etch back into thin film foils for added performance. This thin film LPC cell would improve thermal conductivity by a factor of 43 and specific power by a factor of 4.2 compared to series interconnected upright LPC designs.