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.

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.

We discuss the electronic and optical properties of semiconducting nanoparticles in the quantum-confinement regime and how they can be modeled accurately using first-principles calculations. We address how band-like features emerge from finite electronic states with increasing particle size and how we can control the characteristics of the electronic band structure by means of surface chemistry and alloying. In particular, we are focusing on technological important issues such as bandgap engineering of both magnitude of the bandgap as well as transition type (indirect vs. direct bandgap) and process stability of nanoparticles. We discuss this for several type of semiconducting nano particles composed of environmentally friendly materials, namely we discuss: (i) the origin of the experimentally observed change of the bandgap silicon (Si) nanoparticles upon functionalization with OH groups. We show that the decisive factor here is not as previously proposed surface-strain but charge transfer from the OH groups to the surface electronic states of the particle. (ii) How surface states control the optical properties of silicon carbide (SiC) nano particles, which can lead to in terms of quantum-confinement counter intuitive level diagrams We correlate the frontier electronic states to the measured absolute positions of the conductance band minimum and valance band maximum. (iii) Transition from indirect to direct bandgaps in ultrasmall tin (Sn) and alloyed silicon-tin (SixSn1-x) nanoparticles. Ultrasmall tin nanoparticles are found to have indeed a direct bandgap as recent measurements have proposed but its comparable small bandgap make applications difficult. We show that also for high enough Sn concentration binary silicon-ton alloyed nanoparticles can change their bandgap characteristic from indirect to direct, however the required tin concentration is larger than what one would expect from what is know for bulk silicon-tin. (iv) Lastly, we also investigate the thermal stability of silicon and tin nanoparticles which is especially important for high temperature processing necessary for in cooperating the nanoparticles into existing silicon technologies such as photovoltaic devices. We show that depending on the nanoparticles crystal structure they can remain stable up to several hundred degrees Celsius.

Passivating antireflection coating of crystalline silicon has been experimentally studied using a three-layered stack, consisting of an i/n a-Si:H/SiN trilayer. The passivation property is characterized by the minority carrier lifetime, which shows greater than or similar to 1 ms for the i/n a-Si:H/SiN trilayer prepared at a temperature as low as 250 degrees C. The antireflection property is confirmed by a reflection of light of. 2.0% at 550 nm and a low reflection in a wide range of visible and near-infrared regions, which is adequate for solar cell application, particularly in backcontact structure. The roles of each layer are discussed in terms of the chemical and field-effect passivation as well as the antireflection property. The optoelectronic properties of a neat SiN layer are also discussed to achieve a stable and reliable antireflection performance under the low-temperature growth conditions.

Here we discuss the synthesis paths and optoelectronic properties in the context of alpha tin (α-Sn) phase and silicon/tin (SiSn) alloyed nanocrystals (NCs). We discuss the possibilities and benefits of stabilizing α-Sn in larger sized nanocrystals by alloying with silicon. We demonstrate that plasma-based processes allow the synthesis of stable elemental α-Sn NCs and also alloyed α-Sn based SiSn NCs with quantum confinement size and extraordinarily high Sn concentrations: i.e. SiSn NCs fabricated by confined plasma from femtosecond (fs) laser in liquid media with a high tin concentration of approximately 17%. We also discuss the experimental and exact theoretical calculation determination of the thermal stability of the α-Sn based nanocrystals.

Nanocrystals in the regime between molecules and bulk give rise to unique electronic properties. Here, a thorough study focusing on quantum‐confined nanocrystals (NCs) is provided. At the level of density functional theory an approximate (quasi) band structure which addresses both the molecular and bulk aspects of finite‐sized NCs is calculated. In particular, how band‐like features emerge with increasing particle diameter is shown. The quasiband structure is used to discuss technological‐relevant direct bandgap NCs. It is found that ultrasmall Sn NCs have a direct bandgap in their at‐nanoscale‐stable α‐phase and for high enough Sn concentration (≈41%) alloyed Si–Sn NCs transition from indirect to direct bandgap semiconductors. The calculations strongly support recent experiments suggesting a direct bandgap for these systems. For a quantitative comparison many‐body GW + Bethe–Salpeter equation (BSE) calculations are performed. The predicted optical gaps are close to the experimental data and the calculated absorbance spectra compare well with the corresponding measurements.

We investigate the role of the silicon surface and orientation on double‐sided TOPCon solar cells properties. The solar cells of front and rear, (p) and (n), poly‐Si/SiO x stack, fabricated on polished surfaces oriented 〈100〉 and 〈111〉 and pyramid textured surfaces, are characterized as a function of the thickness of an ultrathin SiO x layer, controlled at atomic scale from one‐ to four‐cycle atomic layer deposition (ALD). Our findings underline that the optimized thickness of the ultrathin SiO x is about 1.1 ± 0.1 nm, corresponding to a two‐cycle ALD, regardless of the surface and orientation of the c‐Si substrate. The open‐circuit voltage is about 10 mV higher on the polished 〈100〉‐oriented surface, associated with lower defect density at the interface of SiO x /c‐Si. On the other hand, the contact resistance is much lower, about 0.45 Ω/cm 2 , on the polished 〈111〉‐oriented surface. On textured surfaces, we demonstrate a photoconversion efficiency of 19.1% for the double‐sided TOPCon structure strictly for a SiO x thickness with two‐cycle ALD, which underlines the importance of the ALD technique for the precise control of the ultrathin SiO x thickness on textured surface.

Double-sided, front and rear, tunnel oxide passivated contact (TOPCon) of crystalline silicon (c-Si) solar cells on textured wafer is presented. The double-sided TOPCon structure is composed of (p) poly-Si/SiOx/(n) c-Si/SiOx/(n) poly-Si, where the silicon oxide (SiOx) layer is formed by atomic layer deposition (ALD). With a 0.6 +/- 0.1 nm-thick SiOx layer, an 18.8 %-efficiency solar cell is fabricated, where an excellent short circuit current above 39 mA/cm(2) is confirmed for a front grid structure. The solar cell performance is improved by optimizing the boron and hydrogen diffusion profiles near the p-side SiOx/c-Si interface.

The chemical and electronic structures in the near‐surface region of Cu(In,Ga)Se2 thin‐film solar cell absorbers are investigated using nondestructive soft and hard X‐ray photoelectron spectroscopy. In addition to a pronounced surface Cu‐depletion, the [Ga]/([In]+[Ga]) composition indicates that the topmost surface is Ga‐poor (or In‐rich). For the studied depth region, common depth profiling techniques generally fail to provide reliable information and, thus, the near‐surface chemical and electronic structure profiles are often overlooked. The relation between the observed near‐surface elemental compositions and the derived electronic properties of the absorber material is discussed. It is found that the surface band gap energy crucially depends on the Cu‐deficiency of the absorber surface and suggests that it is, in this region, only secondarily determined by the [Ga]/([In]+[Ga]) ratio.

Cyclic dehydrogenation-rehydrogenation (D-R) experiments are performed on thin oxide passivated contact (TOPCon) p-type poly-Si/SiOx stack to investigate the role of hydrogen atoms for crystalline silicon solar cell. The dehydrogenation steps are performed by thermal annealing at 450 degrees C for 1h under vacuum condition, and the rehydrogenation steps by hydrogen plasma treatment (HPT) at 300 degrees C for 1min. The ultrathin SiOx layer of 0.8 +/- 0.1 nm is realized by atomic layer deposition (ALD) technique allowing a precise control at atomic scale. The effective lifetime is enhanced to 1.4 ms by the HPT. The D-R method underlines the contribution of H atoms to the c-Si surface passivation that diffuses into the SiOx/c-Si interface. Besides, the lifetime recovery is well described by a stretched exponential function that indicates the dispersive nature of the p-type poly-Si/SiOx stack.

Cyclic dehydrogenation-rehydrogenation experiments have been performed on p-type poly-Si/SiOx stack to study the roles of hydrogen (H) atoms for crystalline silicon (c-Si) surface passivation. The effective lifetime of minority carriers is enhanced to 1.4 ms by the hydrogenation of hydrogen plasma treatment (HPT) at 300 degrees C. The dehydrogenation by thermal annealing at 450 degrees C decreases the lifetime to similar to 0.6 ms, which is recovered by a consecutive HPT, underlining the c-Si surface passivation by H atoms diffused into the SiOx/c-Si interface. The lifetime recovery follows a stretched exponential function, indicating the dispersive nature of the p-type poly-Si/SiOx stack. (C) 2019 The Japan Society of Applied Physics

A stacked layer of ultrathin hydrogenated silicon oxide (SiOx:H) and hydrogenated amorphous silicon (a-Si:H) has been developed to passivate the crystalline silicon (c-Si) surface (see graphical abstract). Silicon oxide has the advantage of excellent optical and passivation properties. The SiOx:H layer is deposited on the c-Si surface by atomic layer deposition (ALD), with its thickness precisely controlled below 2 nm. The a-Si:H layer is deposited on the SiOx:H layer by plasma-enhanced chemical vapor deposition (PECVD) with a specific doping property, i.e. intrinsic, n- or p-type. The samples are then annealed in the range of 100 degrees C to 950 degrees C to study the fundamental passivation properties. We find that a combination of an ultrathin SiOx:H and (p) a-Si:H layers shows a favorable passivation compared to a neat (p) a-Si:H layer. The effective minority carrier lifetime, measured by quasi steady-state photoconductance (QSSPC), is similar to 0.5 ms after low temperature annealing at 300 degrees C. The passivation property is discussed in terms of hydrogen concentration, bond configurations, stoichiometry x of SiOx:H, and material microstructure, characterized by Fourier transform infra-red (FTIR) and Raman spectroscopy. It is suggested that a reorganization of both the SiOx:H, and the (p) a-Si:H layers, associated with hydrogen diffusion, plays an important role in improving the passivation.

The feasibility of using surfactant-free semiconducting silicon-tin alloy nanocrystals with quantum confinement effect for photovoltaics is demonstrated. Synthetized nanoparticles with average of 3 nm in diameter and optical bandgap of 0.81 eV at room temperature were obtained. X-ray diffraction measurements with synchrotron radiation have confirmed a Si0.88Sn0.12 alloyed composition that corresponds to a ratio of about eight Si atoms for every Sn atom. Fourier transform infrared spectroscopy revealed a reduced surface oxygen concentration compared with elemental silicon nanocrystals. Furthermore the potential of silicon-tin nanocrystals as a photovoltaic material is assessed and an enhancement of the solar cells performance is demonstrated due to the extended spectral range and increased absorption. In particular, the short circuit current density has shown improvements as the concentration of silicon-tin nanocrystals is increased. (C) 2018 Elsevier Ltd. All rights reserved.

In this paper, a superior-quality InN/p-GaN interface grown using pulsed metalorganic vapor-phase epitaxy (MOVPE) is demonstrated. The InN/p-GaN heterojunction interface based on high-quality InN (electron concentration 5.19 × 1018 cm-3 and mobility 980 cm²/(V s)) showed good rectifying behavior. The heterojunction depletion region width was estimated to be 22.8 nm and showed the ability for charge carrier extraction without external electrical field (unbiased). Under reverse bias, the external quantum efficiency (EQE) in the blue spectral region (300⁻550 nm) can be enhanced significantly and exceeds unity. Avalanche and carrier multiplication phenomena were used to interpret the exclusive photoelectric features of the InN/p-GaN heterojunction behavior.