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Long-term stability is a requisite for the widespread adoption and commercialization of perovskite solar cells (PSCs). Encapsulation constitutes one of the most promising ways to extend devices for lifetime without noticeably sacrificing the high power conversion efficiencies that make this technology attractive. Among encapsulation strategies, the most investigated methods are as follows: (1) glass-to-glass encapsulation, (2) polymer encapsulation, and (3) inorganic thin film encapsulation (TFE). In particular, the use of UV-, heat-, water-, and/or oxygen-resistant thin films to encapsulate PSCs is a new and promising strategy for extending devices for lifetime. Thin films can be deposited directly onto the PSC, as in TFE, or can be used in conjunction with glass-to-glass and polymer encapsulation to effectively prevent the photo-, thermal-, oxygen-, and moisture-induced degradation of the perovskite. This chapter will outline perovskite degradation mechanisms and provide a summary of the progress made to-date in the encapsulation of PSCs, with a particular focus on the most recent and promising advances that employ thin films. Additionally, the strengths and limitations of TFE approaches will be identified and contrasted against existing encapsulation strategies. Finally, possible directions for future research that can further enhance encapsulation effectiveness and extend PSC for lifetimes towards the 25-year target will be proposed.

Accurately controlling light emission using nano- and microstructured lenses and antennas is an active field of research. Dielectrics are especially attractive lens materials due to their low optical losses over a broad bandwidth. In this work we measure highly directional light emission from patterned quantum dots (QDs) aligned underneath all-dielectric nanostructured microlenses. The lenses are designed with an evolutionary algorithm and have a theoretical directivity of 160. The fabricated structures demonstrate an experimental full directivity of 61 ± 3, three times higher than what has been estimated before, with a beaming half-angle of 2.6°. This high value compared to previous works is achieved via three mechanisms. First, direct electron beam patterning of QD emitters and alignment markers allowed for more localized emission and better emitter-lens alignment. Second, the lens fabrication was refined to minimize distortions between the designed shape and the final structure. Finally, a new measurement technique was developed that combines integrating sphere microscopy with Fourier microscopy. This enables complete directivity measurements, contrary to other reported values, which are typically only partial directivities or estimates of the full directivity that rely partly on simulations. The experimentally measured values of the complete directivity were higher than predicted by combining simulations with partial directivity measurements. High directivity was obtained from three different materials (cadmium-selenide-based QDs and two lead halide perovskite materials), emitting at 520, 620, and 700 nm, by scaling the lens size according to the emission wavelength.

Photosynthetic microorganisms are important to the Earth’s ecosystem, since about half of the atmospheric oxygen is produced by photosynthesis. Microalgae and photosynthetic bacteria are also utilized in a wide range of industries in photobioreactors. In order to have better control over photobioreactors under various operating conditions, it is necessary to accurately characterize the propagation of light in the reactor. Theoretical methods are able to calculate the optical properties of microorganisms through the solution of Maxwell’s equations of electromagnetic wave theory. To solve Maxwell’s equations, various methods can be used including Lorenz-Mie, T-Matrix, Finite-Difference Time-Domain (FDTD), and Volume Integral methods. Most theoretical methods predict the optical properties of microorganisms by Lorenz-Mie theory. Lorenz–Mie theory is applicable for homogeneous and spherical particles, homogeneous concentric spheres, or coated spheres. This work seeks to determine the suitability of the commonly used homogenous-sphere, coated-sphere, and heterogenous-sphere approximation by simulating the optical behavior of photosynthetic microorganism (Chlamydomonas reinhardtii) using FDTD and an accurate geometric model. Here, each of the key cell organelles will be included in the model with the appropriate optical properties specified. These results allow for a more accurate optical model to be developed while studying the effects of different growth regimes.

Tandem photovoltaics, combining absorber layers with two distinct band gap energies into a single device, provide a practical solution to reduce thermalization losses in solar energy conversion. Traditionally, tandem devices have been assembled using two-terminal (2-T) or four-terminal (4-T) configurations; the 2-T limits the tandem performance due to the series connection requiring current matching, while the standard 4-T configuration requires at least three transparent electrical contacts, which reduce the total collected power due to unavoidable parasitic absorption. Here, we introduce a novel architecture based on a nanoscale back-contact for a thin-film top cell in a three terminal (3-T) configuration. Using coupled optical-electrical modeling, we optimize this architecture for a planar perovskite-silicon tandem, highlighting the roles of nanoscale contacts to reduce the required perovskite electronic quality. For example, with an 18% planar silicon base cell, the 3-T back contact design can reach a 32.9% tandem efficiency with a 10 μm diffusion length perovskite material. Using the same perovskite quality, the 4-T and 2-T configurations only reach 30.2% and 24.8%, respectively. We also confirm that the same 3-T efficiency advantage applies when using 25% efficient textured silicon base cells, where the tandems reach 35.2% and 32.8% efficiency for the 3-T, and 4-T configurations, respectively. Furthermore, because our design is based on the individual subcells being back-contacted, further improvements can be readily made by optimizing the front surface, which is left free for additional antireflective coating, light trapping, surface passivation, and photoluminescence outcoupling enhancements.

Algorithmic design holds promise of dramatically improving the efficiency of nanophotonic components. Herein we explore two aspects related to the configuration of evolutionary algorithms, as well as the design of new structures to enhance absorption and emission directivity of perovskite nanocrystals.

Three-dimensional (3D) characterization of nanomaterials is traditionally performed by either cross-sectional milling with a focused ion beam (FIB), or transmission electron microscope tomography. While these techniques can produce high quality reconstructions, they are destructive, or require thin samples, often suspended on support membranes. Here, we demonstrate a complementary technique allowing non-destructive investigation of the 3D structure of samples on bulk substrates. This is performed by imaging backscattered electron (BSE) emission at multiple primary beam energies - as the penetration depth of primary electrons is proportional to the beam energy, depth information can be obtained through variations in the beam acceleration. The detected signal however consists of a mixture of the penetrated layers, meaning the structure's three-dimensional geometry can only be retrieved after deconvolving the BSE emission profile from the observed BSE images. This work demonstrates this novel approach by applying a blind source separation deconvolution algorithm to multi-energy acquired BSE images. The deconvolution can thereby allow a 3D reconstruction to be produced from the acquired images of an arbitrary sample, showing qualitative agreement with the true depth structure, as verified through FIB cross-sectional imaging.

Nanophotonics is becoming invaluable for an expanding range of applications, from controlling the spontaneous emission rate and the directionality of quantum emitters, to reducing material requirements of solar cells by an order of magnitude. These effects are highly dependent on the near field of the nanostructure, which constitutes the evanescent fields from propagating and resonant localized modes. Although the interactions between quantum emitters and nanophotonic structures are increasingly well understood theoretically, directly imaging these interactions experimentally remains challenging. Here we demonstrate a photoactivated localization microscopy-based technique to image emitter-nanostructure interactions. For a 75 nm diameter silicon nanowire, we directly observe a confluence of emission rate enhancement, directivity modification and guided mode excitation, with strong interaction at scales up to 13 times the nanowire diameter. Furthermore, through analytical modelling we distinguish the relative contribution of these effects, as well as their dependence on emitter orientation., Light-matter interactions with single quantum emitters are generally difficult to measure with both high-resolution and a large field of view. Here, Johlin et al. develop far-field super-resolution fluorescence methods to map near-field emitter-nanostructure interactions over several microns.

The cell-to-module efficiency gap observed in a-Si/c-Si heterojunction solar cells is a key challenge to the broad adoption of this technology. To systematically address this issue, we describe an end-to-end modeling framework to explore the implications of process and device variations at the module level. First, a process model is developed to connect the a-Si deposition parameters to the material properties. Next, a physics-based device model is presented; the model uses the thermionic emission/diffusion theory to capture the essential features of photocurrent and diode injection current. Using the process and device models, the effects of process conditions on cell performance are explored. Finally, the performance of the module, as a function of device and process parameters, is explored to establish the cell-to-module efficiency gap. The insights developed through this process-to-module modeling framework will help close the cell-to-module efficiency gap of this commercially promising technology.

In photovoltaic devices, the bulk disorder introduced by grain boundaries (GBs) in polycrystalline silicon is generally considered to be detrimental to the physical stability and electronic transport of the bulk material. However, at the extremum of disorder, amorphous silicon is known to have a beneficially increased band gap and enhanced optical absorption. This study is focused on understanding and utilizing the nature of the most commonly encountered Σ3 GBs, in an attempt to balance incorporation of the advantageous properties of amorphous silicon while avoiding the degraded electronic transport of a fully amorphous system. A combination of theoretical methods is employed to understand the impact of ordered Σ3 GBs on the material properties and full-device photovoltaic performance.

Strain in a material induces shifts in vibrational frequencies, which is a probe of the nature of the vibrations and interatomic potentials, and can be used to map local stress/strain distributions via Raman microscopy. This method is standard for crystalline silicon devices, but due to lack of calibration relations, it has not been applied to amorphous materials such as hydrogenated amorphous silicon (a-Si:H), a widely studied material for thin-film photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H \ab initio under different strains $\epsilon$ and found peak shifts $\Delta \omega = \left( -460 \pm 10\ \mathrm{cm}^{-1} \right) {\rm Tr}\ \epsilon$. This proportionality to the trace of the strain is the general form for isotropic amorphous vibrational modes, as we show by symmetry analysis and explicit computation. We also performed Raman measurements under strain and found a consistent coefficient of $-510 \pm 120\ \mathrm{cm}^{-1}$. These results demonstrate that a reliable calibration for the Raman/strain relation can be achieved even for the broad peaks of an amorphous material, with similar accuracy and precision as for crystalline materials., Comment: 12 pages, 3 figures + supplementary 8 pages, 4 figures

The cell-to-panel efficiency gap observed in a-Si/c-Si heterojunction solar cells is one of the key challenges of this technology. To systematically address this issue, we describe an end-to-end modeling framework to explore the implications of process and device variation at the module level. First, a process model is developed to connect the a-Si deposition parameters to the device parameters. Next, a physics based device model is presented which captures the essential features of photo-current and diode injection current using the thermionic-diffusion theory. Using the process and device models, the effects of process conditions on cell performance are explored. Finally, the performance of the panel as a function of device and process parameters is explored to establish panel limits. The insights developed through this process-to-panel modeling framework will improve the understanding of the cell-to-panel efficiency gap of this commercially promising cell technology., United States. Department of Energy. Solar Energy Research Institute, National Science Foundation (U.S.). Nano-Engineered Electronic Device Simulation

Coupling perovskite and silicon solar cells in a tandem configuration is considered an attractive method to increase conversion efficiency beyond the single-junction Shockley-Queisser limit. While a mechanically-stacked perovskite/silicon tandem solar cell has been demonstrated, a method to electrically couple perovskite and silicon solar cell in a monolithic configuration has not been demonstrated. In this contribution, we design and demonstrate a working monolithic perovskite/silicon tandem solar cell, enabled by a silicon tunnel junction, with a VOC of 1.58 V. We further discuss possible efficiency loss mechanisms and mitigation strategies.

With the advent of high-bandgap perovskites, the opportunity now exists to make tandems with perovskites on top of silicon. We have prototyped a mechanically stacked tandem, achieving 17.9% certified efficiency using a perovskite cell with a silver nanowire mesh electrode. We have also prototyped a monolithically integrated tandem on silicon, with the two subcells electronically connected by band-to-band tunneling in the silicon. The primary challenges to propelling perovskite/silicon tandems into a high-efficiency (>25%) regime are spiro-OMeTAD parasitic absorption, perovskite crystal quality, stability of a perovskite with a 1.8 eV bandgap, perovskite environmental stability, and transparent electrode quality and stability.

In this work we aim to connect atomic structure to observed mobility through a series of experiments, leveraging computational simulations to correlate atomic-level phenomena with empirical mobility trends, providing insight into the shifting regimes of prevalent defects. The work is conducted in three main thrusts: First, through experimental measurements of a-Si:H hole mobility over a wide range of deposition conditions, we correlate the mobility with independent measurements of film intrinsic stress, and hydrogen concentration and bonding configuration. We show that the peak mobility values occur at moderately compressive values of stress, away from theextremaofeitherhydrogenbondingconcentration.Second, by extending our previous computational study of structural defects in a-Si:H to include variations in both biaxial stress and hydrogen concentration, we are able to hypothesize on the sources of defects in the various experimental conditions of the material, as well as design experiments to validate thesetheories.Finally,wedeconvolutethestressandhydrogen contents in our as-deposited films and thereby provide experimental evidence to corroborate our computational theories as to the dominant defects and their sources in the continuum of experimental samples produced.

Genetic programming is used to identify the structural features most strongly associated with hole traps in hydrogenated nanocrystalline silicon with very low crystalline volume fraction. The genetic programming algorithm reveals that hole traps are most strongly associated with local structures within the amorphous region in which a single hydrogen atom is bound to two silicon atoms (bridge bonds), near fivefold coordinated silicon (floating bonds), or where there is a particularly dense cluster of many silicon atoms. Based on these results, we propose a mechanism by which deep hole traps associated with bridge bonds may contribute to the Staebler-Wronski effect.

The adoption of amorphous silicon based solar cells has been drastically hindered by the low efficiency of these devices, which is mainly due to a low hole mobility. It has been shown that using both crystallized and amorphous silicon layers in solar cells leads to an enhancement of the device performance. In this study the crystallization of a-Si prepared by PECVD under various growth conditions has been investigated. The growth stresses in the films are determined by measuring the curvature change of the silicon substrate before and after film deposition. Localized crystallization is induced by exposing a-Si films to focused 532 nm laser beam of power ranging from 0.08 to 8 mW. The crystallization process is monitored by recording the Raman spectra after various exposures. The results suggest that growth stresses in the films affect the minimum laser power (threshold power). In addition, a detailed analysis of the width and position of the Raman signal indicates that the silicon grains in the crystallized regions are of few nm diameter.

Hydrogenated amorphous silicon (a-Si:H) refers to a broad class of atomic configurations, sharing a lack of long-range order, but varying significantly in material properties, including optical constants, porosity, hydrogen content, and intrinsic stress. It has long been known that deposition conditions affect microstructure, but much work remains to uncover the correlation between these parameters and their influence on electrical, mechanical, and optical properties critical for high-performance a-Si:H photovoltaic devices. We synthesize and augment several previous models of deposition phenomena and ion bombardment, developing a refined model correlating plasma-enhanced chemical vapor deposition conditions (pressure and discharge power and frequency) to the development of intrinsic stress in thin films. As predicted by the model presented herein, we observe that film compressive stress varies nearly linearly with bombarding ion momentum and with a (\ensuremath{-}1/4) power dependence on deposition pressure, that tensile stress is proportional to a reduction in film porosity, and the net film intrinsic stress results from a balance between these two forces. We observe the hydrogen-bonding configuration to evolve with increasing ion momentum, shifting from a void-dominated configuration to a silicon-monohydride configuration. Through this enhanced understanding of the structure-property-process relation of a-Si:H films, improved tunability of optical, mechanical, structural, and electronic properties should be achievable.

Low hole mobility severely limits the conversion efficiencies of amorphous silicon (a-Si) solar cells. Previously it has been proposed that carrier mobility can be improved by introducing certain types of stress into thin films. In this work we explore a range of deposition conditions allowing the formation of intrinsic stresses varying from −924 MPa compressive to 386 MPa tensile. We then discuss the origins of these stresses due to ion bombardment, presenting a model correlating our deposition parameters with our observed stress measurements. In doing so we elucidate the non-linear relationship between deposition pressure and the films intrinsic stress.

With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a VOC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.

While low hole mobilities limit the current collection and efficiency of hydrogenated amorphous silicon (a-Si:H) photovoltaic devices, attempts to improve mobility of the material directly have stagnated. Herein, we explore a method of utilizing nanostructuring of a-Si:H devices to allow for improved hole collection in thick absorber layers. This is achieved by etching an array of 150 nm diameter holes into intrinsic a-Si:H and then coating the structured material with p-type a-Si:H and a conformal zinc oxide transparent conducting layer. The inclusion of these nanoholes yields relative power conversion efficiency (PCE) increases of ∼45%, from 7.2 to 10.4% PCE for small area devices. Comparisons of optical properties, time-of-flight mobility measurements, and internal quantum efficiency spectra indicate this efficiency is indeed likely occurring from an improved collection pathway provided by the nanostructuring of the devices. Finally, we estimate that through modest optimizations of the design and fabrication, PCEs of beyond 13% should be obtainable for similar devices.