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In this article we present an alternative approach for the fabrication of silicon nanocrystals (Si–nc) prepared ex situ of the silicon dioxide (SiO[sub 2]) host matrix. The Si–nc are scratched from porous silicon layers and incorporated into a host spin-on-glass SiO[sub 2] based matrix. High-resolution transmission electron microscopy and Raman spectroscopy revealed Si–nc of 2–5 nm size. These nanocrystallites exhibit visible room temperature photoluminescence (PL) with a maximum at about 700 nm. The presence of the dopant in the host matrix is shown to induce a blueshift of the PL maxima due to modified surface states of the Si–nc. This approach allows the fabrication of self-supporting samples with very high Si–nc concentrations. A bright photoluminescence at room temperature is obtained on such materials. Finally, strong indication of optical gain at room temperature is shown for samples with high Si–nc concentrations in a phosphorus doped sol gel host matrix. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We discuss results of surface photovoltage (U) measurements for d = 10 µm thick layers of undoped hydrogenated microcrystalline silicon (µc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient a and a large penetration depth α[sup -1], a photovoltage peak appears on a curve U= U(α). This peak is located at α...d[sup -1] and its occurrence depends critically on the substrate material. The peak is present in a µc-Si:H film grown on p + crystalline silicon (c-Si), on the other hand it is missing in the µc-Si:H samples grown on n[sup +] c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface µc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge regions. The peak can be observed only when n is below a certain threshold value n[sub th]. In the simplified case when d/L ... 1 and when the thicknesses of the upper and lower depletion layers can be neglected, we have found n[sub th] ≈ 0.135 or 1, depending on the orientation of the top and bottom barriers to each other. However, the magnitude of the peak increases exponentially with further lowering of n. Therefore, the surface photovoltage method is suitable not only for evaluating the minority carrier diffusion length L, but also for detecting the occurrence and properties of the bottom space charge region in thin film solar cells. [ABSTRACT FROM AUTHOR]

Porous silicon grains embedded in the phosphorus doped SiO2 matrix exhibit improved photoluminesce properties and better stability in comparison with native porous silicon samples. We have tested this material for the presence of room temperature optical amplification under femtosecond (100 fs, 395 nm) excitation. Combined variable stripe length and shifted excitation spot experiments reveal positive optical gain, the net modal gain coefficient reaching 25 cm-1 at a pump intensity of 1.1 W/cm2 (mean power). The gain spectrum is broad (full width at half maximum ∼130 nm), peaked at ∼650 nm, and is slightly blueshifted with regard to the standard photoluminescence emission. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nanocrystals have a great potential for future materials with tunable bandgap, due to their optical properties that are related with the material used, their sizes and their surface termination. Here, we concentrate on the silicon-tin alloy for photovoltaic applications due to their bandgap, lower than bulk Si, and also the possibility to activate direct band to band transition for high tin concentration. We synthesized silicon-tin alloy nanocrystals (SiSn-NCs) with diameter of about 2-3 nm by confined plasma technique employing a femtosecond laser irradiation on amorphous silicon-tin substrate submerged in liquid media. The tin concentration is estimated to be [Formula: see text], being the highest Sn concentration for SiSn-NCs reported so far. Our SiSn-NCs have a well-defined zinc-blend structure and, contrary to pure tin NCs, also an excellent thermal stability comparable to highly stable silicon NCs. We demonstrate by means of high resolution synchrotron XRD analysis (SPring 8) that the SiSn-NCs remain stable from room temperature up to [Formula: see text] with a relatively small expansion of the crystal lattice. The high thermal stability observed experimentally is rationalized by means of first-principle calculations., (© 2023. The Author(s).)

We discuss the electronic properties of quantum-confined nanocrystals. In particular, we show how, starting from the discrete molecular states of small nanocrystals, an approximate band structure (quasi-band structure) emerges with increasing particle size. Finite temperature is found to broaden the discrete states in energy space forming even for nanocrystals in the quantum-confinement regime quasi-continuous bands in k-space. This bands can be, to a certain extend, interpreted along the lines of standard band structure theory, while taking also finite size and surface effects into account. We discuss this on various prototypical nanocrystal systems., (© 2023. The Author(s).)

We observe the extraction of carriers excited between two types of bands in the perovskite oxide, Sr-deficient strontium niobate (Sr 0.9 NbO 3 ). Sr 0.9 NbO 3 exhibits metallic behaviour and high conductivity, whilst also displaying broad absorption across the ultraviolet, visible, and near-infrared spectral regions, making it an attractive material for solar energy conversion. Furthermore, the optoelectronic properties of strontium niobate can easily be tuned by varying the Sr fraction or through doping. Sr-deficient strontium niobate exhibits a split conduction band, which enables two types of optical transitions: intraband and interband. However, whether such carriers can be extracted from an unusual material as such remains unproven. In this report, we have overcome the immense challenge of photocarrier extraction by fabricating an extremely thin absorber layer of Sr 0.9 NbO 3 nanoparticles. These findings open up great opportunities to harvest a very broad solar spectral absorption range with reduced recombination losses.

The knowledge of band edges in nanocrystals (NCs) and quantum-confined systems is important for band alignment in technologically significant applications such as water purification, decomposition of organic compounds, water splitting, and solar cells. While the band energy diagram of bulk silicon carbides (SiCs) has been studied extensively for decades, very little is known about its evolution in SiC NCs. Moreover, the interplay between quantum confinement and surface chemistry gives rise to unusual electronic properties and remains barely understood. Here, we report for the first time the complete band energy diagram of SiC NCs synthesized such that they span the regime from strong to intermediate to weak quantum confinement. The absolute positions of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals show clear size dependence. While the HOMO level follows the expected behavior for quantum-confined electronic states, the LUMO energy shifts below the bulk conduction band minimum, which cannot be explained by a simple quantum confinement caused by the size effect. We show that this effect is a result of the interplay between quantum confinement and the formation of surface states due to partial and site-selective oxygen passivation.

Organometal trihalide perovskite solar cells have attracted monumental attention in recent years. Today's best devices, based on a three-dimensional perovskite structure of corner-sharing PbI 6 octahedra, are unstable, toxic, and display hysteresis in current-voltage measurements. We present zero-dimensional organic-inorganic hybrid solar cells based on methylammonium iodo bismuthate (CH 3 NH 3 ) 3 (Bi 2 I 9 ) (MABI) comprising a Bi 2 I 9 bioctahedra and observe very low hysteresis for scan rates in the broad range of 150 mV s -1 to 1500 mV s -1 without any interfacial layer engineering. We confirm good stability for devices produced and stored in open air without humidity control. The MABI structure can also accommodate silicon nanocrystals, leading to an enhancement in the short-circuit current. Through the material MABI, we demonstrate a promising alternative to the organometal trihalide perovskite class and present a model material for future composite third-generation photovoltaics.