Perovskite solar cells (PSC) are a favorable candidate for next-generation solar systems with efficiencies comparable to Si photovoltaics, but their long-term stability must be proven prior to commercialization. However, traditional trial-and-error approaches to PSC screening, development, and stability testing are slow and labor-intensive. In this Perspective, we present a survey of how machine learning (ML) and autonomous experimentation provide additional toolkits to gain physical understanding while accelerating practical device advancement. We propose a roadmap for applying ML to PSC research at all stages of design (compositional selection, perovskite material synthesis and testing, and full device evaluation). We also provide an overview of relevant concepts and baseline models that apply ML to diverse materials problems, demonstrating its broad relevance while highlighting promising research directions and associated challenges. Finally, we discuss our outlook for an integrated pipeline that encompasses all design stages and presents a path to commercialization.

1 of 6) 1600568 dielectric function. Alloying of these noble metals has been applied to tune the material dielectric function, where the LSPR can be modulated progressively from the UV (pure Ag) to the NIR (pure Au). [12][13][14][15] Thus, metallic nanostructures composed of Ag-Au can enable the rational design of building blocks for different applications, such as metamaterials, [16,17] hot carrier devices, [18] light absorption improvement in photovoltaics, [19,20] colored glasses, [21] displays, [22,23] and catalysis. [24] To date, different fabrication techniques have been successfully utilized to realize Ag x Au 1−x alloyed NPs. They can be formed by colloidal synthesis via the reduction of precursors containing metals in solution, [25,26] and by the sequential pulsed laser deposition of Ag and Au targets, [27,28] which can yield large amounts of NPs with narrow size distribution. However, the overall size of the NPs cannot be varied beyond 150 nm. [29] Alternatively, nanolithographic methods enable full control of NPs size, shape, and distribution. [13] Nevertheless, this technique is constrained to specific applications due to its high cost and very limited scalability. The dewetting of metallic thin films has also been used to fabricate pure [21,[30][31][32] and alloyed [19] metal NPs. In this simple and effective fabrication route, a very thin layer of metal (<50 nm) is initially deposited onto a substrate. Then, when the thin-film sample is annealed under a controlled environment (oxygen free), surface diffusion takes place and results in the formation of nanostructures to minimize the energy of the system. [33][34][35][36] This method has been particularly useful for optoelectronic devices, where these metallic NPs act as light scattering centers that ultimately increase light absorption within the semiconductor. [4,37] In this work, we fabricate fully alloyed Ag x Au 1−x NPs with controlled chemical composition by dewetting thin films and characterize their optical response at the macro-and nano-scale. Surprisingly, we find that the NPs' distribution heavily depends on the thin-film chemical composition, irrespective of the original film thickness. Simultaneously, we measure a shift of the LSPR due to the NPs' composition variation, which defines their optical response. We map the elemental distribution of Ag and Au and confirm that the NPs are fully alloyed, forming a solid solution at the nanoscale. To further illustrate how the chemical composition affects the material optical response, we perform a detailed analysis of the optical characteristics of fully alloyed Ag 0.5 Au 0.5 nanostructures in the visible range of the spectrum. For that, we combine spectrally dependent NSOM measurements and finite-difference time-domain (FDTD) simulations to locally resolve the optical response of individual NPs. Our results of the near-field light-matter interactions for Ag 0.5 Au 0.5 nanostructures reveal an electric field enhancement of 30 times in the visible range of the spectrum under the NPs Combining meta...

Photocurrent spectroscopy of intersubband transitions in GaInAsN/(Al)GaAs asymmetric quantum well infrared photodetectors

single, multiple, or broadband frequency of the electromagnetic spectrum. The ultrahigh light absorption is obtained due to an impedance match between the material and the medium. [9] Here, the electric and magnetic resonances are designed so that the bulk effective impedance is equal to the one of the free space (air or vacuum). As a result, most of the incident light is absorbed and the reflection is negligible. Salisbury [10] and Dallenbach [11] first idealized classical absorbers to operate in the microwave range of the electromagnetic spectrum. The former included a resistive layer located at a quarter wavelength from a metallic substrate while the latter consisted of a dielectric layer on top of a metallic substrate.In the past two decades, advances in nanofabrication have given rise to nanostructures with controlled geometry, recently inspiring the design of metamaterials and metasurfaces for superabsorbers. [12] With the flexibility of tuning nanostructures' geometry and periodicity, such absorbers have been demonstrated in the visible, [13,14] near-infrared (NIR), [15] mid-infrared (MIR), [16] and far-infrared (FIR) [17,18] frequency ranges, proving to be a powerful approach for producing optical responses that are not feasible by any conventional material. However, the cost of the current fabrication methods limits their commercial applications. For instance, metasurfaces consisting of arrays of nanostructures on a dielectric surface are difficult to manufacture through physical deposition approaches in large scale even by using state-of-the-art bottomup nanolithography methods.To overcome the scalability constraints of the fabrication methods currently implemented for metasurfaces, the use of thin films in superabsorbers has been explored for a broad range of the spectrum, extending from visible to the FIR. [19][20][21][22][23][24][25][26][27][28] The Dallenbach configuration provides significant benefits regarding the fabrication of ultrathin, planar, omnidirectional, and polarization independent structures with very high absorption. Recently, it was reported that more than 98% of the normally incident light could be absorbed in an ultrathin layer of Ge on top of Ag at a wavelength (λ) of 625 nm, decreasing to 80% for incident angles up to 66° for both polarizations. [21] In addition, highly doped Si has been used as a metallic-like substrate under a thin Ge layer to absorb light in the MIR, where the doping concentration in Si was the knob to engineer its Superabsorbers based on metasurfaces have recently enabled the control of light at the nanoscale in unprecedented ways. Nevertheless, the sub-wavelength features needed to modify the absorption band usually require complex fabrication methods, such as electron-beam lithography. To overcome the scalability limitations associated with the fabrication of metallic nanostructures, engineering the optical response of superabsorbers by metal alloying is proposed, instead of tuning the geometry/size of the nanoscale building blocks. The superior performance...

CONSPECTUS: Metallic materials with engineered optical properties have the potential to enhance the performance of energy harvesting and storage devices operating at the macroand nanoscale, such as solar cells, photocatalysts, water splitting, and hydrogen storage systems. For both thin films and subwavelength nanostructures, upon illumination, the coherent oscillation of charge carriers at the interface with a dielectric material gives rise to resonances named surface plasmon polariton (SPP) and localized surface plasmon resonance (LSPR), respectively. These resonances result in unique light absorption, scattering, and transmission responses over the electromagnetic spectrum, which, in turn, can be exploited to tailor the behavior of active metallic components in optoelectronic devices containing Ag, Au, Cu, Al, Mg, among other metals. The wavelength in which the resonances occur primarily depends on the metal itself (i.e., the dielectric function or permittivity), the dielectric medium surrounding the metals, and the size, geometry, and periodicity of the metallic nanostructures. Nevertheless, the aforementioned parameters allow a limited modulation of both SPP and LSPR over a narrow window of frequencies. To overcome this constraint, we have proposed and realized the alloying of metals via physical deposition methods as a paradigm to almost arbitrarily tuning their optical behavior in the UV-NIR, which leads to permittivity values currently not available. Our approach offers an additional knob, chemical composition, to engineer light-matter interactions in metallic materials. This Account highlights recent progress in using alloying as a pathway to control the optical behavior of metallic thin films and nanostructures for energy harvesting and storage applications, including (photo)catalysis, photovoltaics, superabsorbers, hydrogen storage, among other systems. We choose to primarily focus on the optical properties of the metallic mixtures and in their near-to far-field responses in the UV-NIR range of the spectrum as they represent key parameters for materials' selection for the devices above. By alloying, it is possible to obtain metallic materials with LSPR not available for pure metals, which can enable the further control of the electromagnetic spectrum. First, we discuss how the permittivity of binary mixtures of coinage metals (Au, Ag, and Cu) can be tailored based on the chemical composition of their pure counterparts. Second, we present how novel metallic materials can be designed through band structure engineering through density functional theory (DFT), a paradigm that could benefit from artificial intelligence methods. Concerning alloyed thin films, we discuss the promise of earthabundant metals and provide an example of the superior performance of AlCu in superabsorbers. In the realm of nanostructures, we focus the discussion on physical deposition methods, where we provide a detailed analysis of how chemical composition can affect the far-and near-field responses of metallic building blocks. Fina...

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We report the optical control of localized charge on positioned quantum dots in an electro-photo-sensitive memristor. Interband absorption processes in the quantum dot barrier matrix lead to photo-generated electron-hole-pairs that, depending on the applied bias voltage, charge or discharge the quantum dots and hence decrease or increase the conductance. Wavelength-dependent conductance control is observed by illumination with red and infrared light, which leads to charging via interband and discharging via intraband absorption, respectively. The presented memristor enables optical conductance control and may thus be considered for sensory applications in artificial neural networks as light-sensitive synapses or optically tunable memories. a) Corresponding

We have achieved conditions to obtain optical memory effects in semiconductor nanostructures. The system is based on strained InP quantum wires where the tuning of the heavy-light valence band splitting has allowed the existence of two independent optical channels with correlated and uncorrelated excitation and light-emission processes. The presence of an optical channel that preserves the excitation memory is unambiguously corroborated by photoluminescence measurements of free-standing quantum wires under different configurations of the incoming and outgoing light polarizations in various samples. High-resolution transmission electron microscopy and electron diffraction indicate the presence of strain effects in the optical response. By using this effect and under certain growth conditions, we have shown that the optical recombination is mediated by relaxation processes with different natures: one a Markov and another with a non-Markovian signature. Resonance intersubband light-heavy hole transitions assisted by optical phonons provide the desired mechanism for the correlated non-Markovian carrier relaxation process. A multiband calculation for strained InP quantum wires was developed to account for the description of the character of the valence band states and gives quantitative support for light hole-heavy hole transitions assisted by optical phonons.