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The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley-Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

The efficiency of perovskite-based solar cells has increased dramatically over the past decade to as high as 25%, making them very attractive for commercial use. Vapor deposition is a promising technique that potentially enables fabrication of perovskite solar cells on large areas. However, to implement a large-scale deposition method, understanding and controlling the specific growth mechanisms are essential for the reproducible fabrication of high-quality layers. Here, we study the structural and optoelectronic kinetics of MAPbI

We investigate hole-selective passivating contacts that consist of an interfacial layer of silicon oxide (SiOx) and a layer of boron-doped SiCx(p). The fabrication process of these contacts involves an annealing step at temperatures above 750 degrees C which crystallizes the initially amorphous layer and diffuses dopants across the interfacial oxide into the wafer to facilitate charge transport, but it can also disrupt the SiOx layer necessary for wafer-surface passivation. To investigate the transport mechanism of the charge carriers through the selective contact and its changes during the annealing process, we utilize various characterization methods, such as transmission electron microscopy, micro Raman spectroscopy, and conductive atomic force microscopy. Combining the latter with a sequential removal of material, we assemble a tomographic reconstruction of the crystallized layer that reveals the presence of preferential vertical transport channels.

Metal-halide perovskites feature very low deep-defect densities, thereby enabling high operating voltages at the solar cell level. Here, by precise extraction of their absorption spectra, we find t...

The heterostructures of two-dimensional (2D) and three-dimensional (3D) materials represent one of the focal points of current nanotechnology research and development. From an application perspective, the possibility of a direct integration of active 2D layers with exceptional optoelectronic and mechanical properties into the existing semiconductor manufacturing processes is extremely appealing. However, for this purpose, 2D materials should ideally be grown directly on 3D substrates to avoid the transferring step, which induces damage and contamination of the 2D layer. Alternatively, when such an approach is difficult&mdash, as is the case of graphene on noncatalytic substrates such as Si&mdash, inverted structures can be created, where the 3D material is deposited onto the 2D substrate. In the present work, we investigated the possibility of using plasma-enhanced chemical vapor deposition (PECVD) to deposit amorphous hydrogenated Si (a-Si:H) onto graphene resting on a catalytic copper foil. The resulting stacks created at different Si deposition temperatures were investigated by the combination of Raman spectroscopy (to quantify the damage and to estimate the change in resistivity of graphene), temperature-dependent dark conductivity, and constant photocurrent measurements (to monitor the changes in the electronic properties of a-Si:H). The results indicate that the optimum is 100 C deposition temperature, where the graphene still retains most of its properties and the a-Si:H layer presents high-quality, device-ready characteristics.

Plasmon-enhanced absorption, often considered as a promising solution for efficient light trapping in thin film silicon solar cells, suffers from pronounced optical losses i.e. parasitic absorption, which do not contribute to the obtainable photocurrent. Direct measurements of such losses are therefore essential to optimize the design of plasmonic nanostructures and supporting layers. Importantly, contributions of useful and parasitic absorption cannot be measured separately with commonly used optical spectrophotometry. In this study we apply a novel strategy consisting in a combination of photocurrent and photothermal spectroscopic techniques to experimentally quantify the trade-off between useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating self-assembled silver nanoparticle arrays located at their rear side. The highly sensitive photothermal technique accounts for all absorption processes that result in a generation of heat i.e. total absorption while the photocurrent spectroscopy accounts only for the photons absorbed in the μc-Si:H layer which generate photocarriers i.e. useful absorption [1]. We demonstrate that for 0.9 μm thick μc-Si:H film the optical losses resulting from the plasmonic light trapping are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. For the films deposited on nanoparticle arrays coupled with a flat silver mirror (plasmonic back reflector), we achieved a significant broadband enhancement of the useful absorption resulting from both surface texturing and plasmonic scattering, and achieving 91% of the theoretical Lambertian limit of absorption. [1] S. Morawiec et al. Experimental Quantification of Useful and Parasitic Absorption of Light in Plasmon-Enhanced Thin Silicon Films for Solar Cells Application. Scientific Reports 6 (2016)

We present Schottky junction solar cell composed of graphene transferred onto hydrogenated amorphous and microcrystalline silicon, a low-cost alternative to well-explored crystalline silicon. We demonstrated sample with open-circuit voltage of 445 mV, a remarkable value for undoped graphene-based solar cell. Photovoltaic characteristics of this sample remained stable over 11 months and could be further improved by doping. The graphene/silicon junctions were characterized by current-voltage curves obtained locally by conductive atomic force microscopy (C-AFM) and macroscopically by standard solar simulator. Very good correlation between both independent measurements proved C-AFM as highly useful tool for photovoltaic characterization on nano- and micrometer scale.

Optical absorptance spectroscopy of polycrystalline CH3NH3PbI3 films usually indicates the presence of a PbI2 phase, either as a preparation residue or due to film degradation, but gives no insight on how this may affect electrical properties. Here, we apply photocurrent spectroscopy to both perovskite solar cells and coplanar-contacted layers at various stages of degradation. In both cases, we find that the presence of a PbI2 phase restricts charge-carrier transport, suggesting that PbI2 encapsulates CH3NH3PbI3 grains. We also find that PbI2 injects holes into the CH3NH3PbI3 grains, increasing the apparent photosensitivity of PbI2. This phenomenon, known as modulation doping, is absent in the photocurrent spectra of solar cells, where holes and electrons have to be collected in pairs. This interpretation provides insights into the photogeneration and carrier transport in dual-phase perovskites.

KLuS2 single crystal samples undoped and doped with a low (5 %) or high (38 %) concentration of trivalent thulium or ytterbium ions were synthesized in the form of thin hexagonal plates. The low phonon energy ( ∼ 220 cm−1) was confirmed using Raman spectroscopy. Samples were studied also by using optical absorption and emission as well as electron paramagnetic resonance spectroscopy and the observed spectral features are discussed. In particular, the 5 % Yb doped samples demonstrate homogeneous dopant distribution inside the material over the lutetium sites. No other signals either from potassium site or the exchange coupled ions were detected. For infrared transitions the luminescence kinetic was determined. Significantly broad infrared emission bands (at 1.4, 1.8 and 2.3 μm for Tm3+, 1 μm for Yb3+) and long decay times (e.g. 0.4 ms from 3H4 to 19 ms from 3F4 levels of Tm3+, 1.4 ms from 2F5/2 level of Yb3+) make those materials perspective for future laser development.

Friction and wear resistance are important factors in almost all branches of industry, as they affect lifetime, power consumption and other properties of all mechanical tools and machines. Diamond-like carbon (DLC) and derived metal-carbon composite (WC:H) layers are known to substantially improve both of these factors and frequently find their use in tribological applications. One of the desired properties of the tribological layers is a smooth surface. However, microscopic analysis of the WC:H layers shows a frequent occurrence of a certain type of structural defects that have major influence on the layer roughness. The defects are conically shaped objects penetrating the layer all the way down to the substrate, being covered with a spherical cap on the top of the layer. The lateral dimensions of the defects vary from several hundreds of nanometers to a few micrometers. To study these defects we prepared several layers at various conditions and analysed them by different methods, such as SEM, FIB, EDX, AFM and Raman spectroscopy. We will discuss the structure of the defects and influence of the preparation conditions on their growth to find the way to control their occurence in the layer.

Radial junction solar cells with only ~100 nm thin amorphous Si absorber layer deposited on Si nanowires can be prepared by a relatively simple and low-cost thin film technology. Metal assisted Si nanowire growth leads to a disorder in nanowire orientations, lengths and shapes, which is then preserved by the conformal absorber layer. Interestingly, high conversion efficiencies are reached in spite of the disorder. In this contribution we describe microscopic methods aiming at exploring the role of structural disorder on the local electronic properties of radial junction cells. A method for locating the same nanostructure in different microscopes, using the nanoindentation marks for orientation on the sample, is described. Indents can be easily located by optical microscopy, scanning electron microscopes or scanning probe microscopes. Groups of three indents arranged in triangles can serve as coordinate systems for triangulation on samples, enabling correlative microscopy even in instruments which were not designed for it. This approach also enables localization of the same positions on samples after repeated mounting in various microscopes with a precision better than 50 nm. This is possible even on samples without any structural features, as demonstrated for flat silicon thin films prepared by solid phase crystallization, for which we have correlated crystallographic maps from electron backscattering diffraction and conductivity maps by atomic force microscopy. The technique allows observing the same locations before and after technological steps, as shown for the hot wire chemical vapor deposition of carbon nanowalls.

We apply a combination of photocurrent and photothermal spectroscopic techniques to experimentally quantify the trade-off between useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (?c-Si:H) films incorporating self-assembled silver nanoparticle arrays, located at the rear side, for improved light trapping via resonant plasmonic scattering. The photothermal technique is used to measure the total absorptance while the photocurrent spectroscopy accounts only for the photons absorbed in the ?c-Si:H layer (useful absorptance); therefore, the method allows for independent quantification of the useful and parasitic absorptance of the plasmonic (or any other) light trapping structure [1]. We demonstrate that for 0.9 ?m thick ?c-Si:H film the optical losses resulting from the plasmonic light trapping are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. For the films deposited on nanoparticle arrays coupled with a flat silver mirror (plasmonic back reflector), we achieved a significant broadband enhancement of the useful absorption with an average useful absorption of 43% and an average parasitic absorption of 19% over 400?1100 nm wavelength range, achieving 91% of the theoretical Lambertian limit of absorption.

A combination of photocurrent and photothermal spectroscopic techniques is applied to experimentally quantify the useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating optimized metal nanoparticle arrays, located at the rear surface, for improved light trapping via resonant plasmonic scattering. The photothermal technique accounts for the total absorptance and the photocurrent signal accounts only for the photons absorbed in the μc-Si:H layer (useful absorptance); therefore, the method allows for independent quantification of the useful and parasitic absorptance of the plasmonic (or any other) light trapping structure. We demonstrate that with a 0.9 μm thick absorber layer the optical losses related to the plasmonic light trapping in the whole structure are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. An average useful absorption of 43% and an average parasitic absorption of 19% over 400–1100 nm wavelength range is measured for μc-Si:H films deposited on optimized self-assembled Ag nanoparticles coupled with a flat mirror (plasmonic back reflector). For this sample, we demonstrate a significant broadband enhancement of the useful absorption resulting in the achievement of 91% of the maximum theoretical Lambertian limit of absorption.

Here, the authors demonstrate the fabrication of silicon nanowire array based solar cells consisting of crystalline Si cores and amorphous a-Si:H layers using a single pump-down plasma-enhanced chemical vapor deposition (PECVD) process. The authors investigate the influence of geometry, physical dimensions, and doping of the nanowire arrays on the solar cell performance. The authors show that the length and thickness of the nanowire radial NIP junction have a significant influence on the current density–voltage characteristics and external quantum efficiency of the cells. The efficiency of radial solar cells is found to be mainly driven by the photogenerated current which is the largest when the thickness of the absorber film is approximately one half of the NIP nanowire length. Doping of the nanowires done by adding phosphine gas in situ in the PECVD growth has affected both the axial and radial nanowire growth rate, leading to the nanowire growth reduction in both directions at higher doping concentrati...

Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri-iodide (CH3NH3PbI3), tribromide (CH3NH3PbBr3), and mixed iodide–bromide (CH3NH3PbI3–xBrx) organic–inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH3NH3PbI3 layers and discuss the distinct Raman bands that develop during moisture-induced degradation. We also prove unambiguously that the final degradation products contain pure PbI2. Moreover, we describe CH3NH3PbI3–xBrx Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to-bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio.

Solar cells with radial junctions based on silicon nanowires were investigated using correlative microscopy in order to determine the nature and origin of previously reported inhomogeneity of their electronic properties. For correlating various microscopy techniques, we have prepared sets of three Vickers type nanoindents arranged in a right triangle with 20 µm sides as marks of local frame of reference. Due to the shape of the indents (squares with 4 µm sides and clear diagonals) the position of the marks can be located with high precision by various microscopes. This makes it possible to correlate the results from scanning electron microscopy, Kelvin probe force microscopy and conductive atomic force microscopy techniques on the same place of the sample, with a precision down to individual nanowires, obtaining new information about the electronic inhomogeneity. Using the conductive AFM we analyzed the growth process of silicon nanowires step by step in order to find possible origins of the local (photo)current variations.

The influence of the phosphorus pentoxide (P2O5) content on the material properties of Er,Yb-doped potassium-lanthanum phosphate glass was studied. Glass samples of the following nominal composition 35.0K2O-6.8Yb2O3-8.0La2O3-0.2Er2O3-50.0P2O5 (in mol%) were prepared from starting materials mixed with five additional amounts of P2O5 (0, 7.5, 15.0, 30.0, and 45.0 mol% related to the nominal glass composition). The P2O5 addition influence on properties of prepared glasses was studied using Raman, absorption, and fluorescence spectroscopy. The glass residual IR absorption and Judd–Ofelt intensity parameters together with absorption and emission cross sections were estimated. The results showed the increasing polymerization of glass and the P─O bond shortening with P2O5 content increase. The spectroscopy of Er3+ and Yb3+ ions was affected only marginally by the glass composition. It was found that fluorescence decay time corresponding to upper-laser-level I413/2 increased with the decrease of P2O5 content in the glasses, which was related to increasing OH− contamination of the glass. The laser action at 1.53 μm under 975-nm pulsed laser diode pumping was successfully demonstrated. Low threshold and laser slope efficiency up to 21% in respect to absorbed pumping power were obtained.