Absorption enhancement in amorphous silicon photonic crystals for thin film photovoltaic solar cells
Ounsi El Daif, Emmanuel Drouard, Yeonsang Park, Alain Fave, Anne Kaminski, Mustapha Lemiti, Xavier Letartre, Pierre Viktorovitch, Sungmo Ahn, Heonsu Jeon, Christian Seassal
We report on very high enhancement of thin layer's absorption through band-engineering of a photonic crystal structure. We realized amorphous silicon (aSi) photonic crystals, where slow light modes improve absorption efficiency. We show through simulation that an increase of the absorption by a factor of 1.5 is expected for a film of aSi. The proposal is then validated by an experimental demonstration, showing an important increase of the absorption of a layer of aSi over a spectral range of 0.32-0.76 microns.
Silicon in a cubic diamond structure currently plays a significant role in the photovoltaic industry. However, the intrinsic band structures of crystalline silicon restrict its sunlight conversion efficiency. Recently, a clathrate-like Si-24 has been successfully synthesized, which has a quasi-direct bandgap and sheds light on silicon-based photovoltaics. Here, we proposed a two-step crystal structure search method based on first-principles calculations and explored silicon clathrate structures extensively. First, the guest-host compounds were searched at high pressure, and then, the porous gu…
The preferentially (220) orientated polycrystalline silicon films have been fabricated by performing photon-involved rapid thermal annealing upon amorphous silicon films. In contrast, conventional thermal annealing of the silicon amorphous films results in the preference of (111) orientated crystallization. This difference reveals the overlooked impaction of photons upon silicon atoms, which results in the preferential orientation of lattice plans with the smallest density of atoms on their projection plane rather than the lowest energies.
Normal mode decomposition of atomic vibrations has been used to provide microscopic under-standing of thermal transport in amorphous solids for decades. In normal mode methods, it is naturally assumed that atoms vibrate around their equilibrium positions and that individual normal modes are the fundamental vibrational excitations transporting heat. With the abundance of predictions from normal mode methods and experimental measurements now available, we care-fully analyze these calculations in amorphous silicon, a model amorphous solid. We find a number of discrepancies, suggesting that treati…
The Effect of Hybrid Photovoltaic Thermal Device Operating Conditions on Intrinsic Layer Thickness Optimization of Hydrogenated Amorphous Silicon Solar Cells
Computational searching and screening of new functional materials exploiting earth abundant elements can accelerate developments of their energy applications. Based on a state-of-the-art materials search algorithm and ab initio calculations, we demonstrate a recently suggested stable silicon oxide with a layered structure (Si$_{3}$O) as an ideal photovoltaic material. With many-body first-principles approaches, the monolayer and layered bulk of Si$_{3}$O show direct quasiparticle gaps of 1.85 eV and 1.25 eV, respectively, while an optical gap of about 1.2 eV is nearly independent of the number…
Locally, the atomic structure in well annealed amorphous silicon appears similar to that of crystalline silicon. We address here the question whether a point defect, specifically a vacancy, in amorphous silicon also resembles that in the crystal. From density functional theory calculations of a large number of nearly defect free configurations, relaxed after an atom has been removed, we conclude that there is little similarity. The analysis is based on formation energy, relaxation energy, bond lengths, bond angles, Voronoï volume, coordination, atomic charge and electronic gap states. All thes…
We prepared thin layers of amorphous silicon by deposition of a liquid-phase polysilane precurser on glass substrate. Raman scattering provides evidence for residual tensile stress in the silicon, which is evaluated quantitatively. Under treatment with hydrofluoric acid, this stress leads to spiral cracks in the silicon. We explain the process of crack formation and examine this phenomenon both analytically and numerically, the latter with the finite element method (FEM). The FEM yields the geometry correction factor for such spiral cracks in terms of the Griffith criterion. This allows for th…
This study provides a new insight into the processes which occur when thin film hydrogenated amorphous silicon is subjected to UV laser irradiation in the presence of oxygen. It achieves this by observing the effects of subjecting the films to progressively increasing laser radiation doses. This reveals that an array of nuclei is first created, leading to the formation of a well-defined crystalline network, consistent with the structure of silicon oxide. Further irradiation results in the formation of cone-like structures on the crystalline network, due to silicon-oxygen bond breakage and migr…
The thermally-induced structural evolution of silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) was investigated by X-ray photoelectron and absorption spectroscopy, as well as molecular dynamics (MD) simulations. The spectroscopic results indicate that the introduction of Si and O in hydrogenated amorphous carbon (a-C:H) increases the activation energy for the conversion of sp3- to sp2-bonded C. MD simulations indicate that the higher thermal stability of a-C:H:Si:O compared to a-C:H derives from the lower fraction of strained C-C sp3 bonds in a-C:H:Si:O.
We have studied the effects of oxygen on hydrogenated amorphous/crystalline silicon films in terms of their structural and optical properties. Different hydrogenated silicon oxide (SiO:H) and silicon (Si:H) films are fabricated between microcrystalline and amorphous transition region. X-ray diffraction, Raman, FTIR and UV-Vis emission spectrometry have been used to characterize different films. A comparison of the results with those of different types of films like hydrogenated amorphous silicon oxide (a-SiO:H), hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon ($μ$c-Si:H) f…
Experimentally Constrained Molecular Relaxation: The case of hydrogenated amorphous silicon
我们已经将我们的实验约束分子弛豫技术（P. Biswas { \it et al}，Phys. Rev. B { \bf 71} 54204（2005））扩展到氢化非晶硅：构建了具有7.4%氢的540原子模型和具有22%氢的611原子模型。从一个随机构型出发，利用物理相关约束、{ \it ab initio}相互作用和实验静态结构因子，我们构建了氢化非晶硅的真实模型。我们的模型证实了存在一个高频局域带的振动态密度由于Si-H振动，已观察到在最近的振动瞬态光栅测量等离子体增强化学气相沉积薄膜的氢化非晶硅。
On the basis of density functional calculations and using Bader's atom in molecule theory, this article presents quantitative microscopic analyses on the bonding properties of amorphous silicon (a-Si) which could reflect in the observable mechanical and electronic behaviors of this material. In addition, the occurrence and strength of quantum confinement of charge carriers in a composition of silicon crystal nano slabs (SiNSs) embedded in hydrogenated a-Si (a-Si:H) semiconductor are studied. It is shown that the strongest confinement effect happens for Si slabs limited in [100] direction. The …
The evolution is presented of infrared transmission spectra of hydrogenated and chlorinated amorphous silicon films versus the chlorine concentration and the annealing temperature. As the chlorine content increases, an increase of the 2100 cm-1 band intensity is observed. We associate this effect with the existence of SiH2CI2, mixed configurations. The Sic1 stretching band, located at 545 cm-1 in unannealed samples, shifts continuously up to 590 cm-1 as the annealing temperature increases. This displacement is attributed to a predominance of SiCI2, species around 500°C and their transformation…
The dynamics of charge carriers in amorphous semiconductors fundamentally differ from those in crystalline semiconductors due to the lack of long-range order and the high defect density. Despite intensive technology-driven research interests and the existence of well-established experimental techniques, such as photoconductivity time-of-flight and ultrafast optical measurements, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hyd…
Graphene-silicon Schottky junction (GSJ) which has the potential for large-scale manufacturing and integration can bring new opportunities to Schottky solar cells for photovoltaic (PV) power conversion. However, the essential power conversion limitation for these devices lies in the small open-circuit voltage ($V_{oc}$), which depends on the Schottky barrier height (SBH). In this study, we introduce an electromechanical method based on the flexoelectric effect to enhance the PV efficiency in GSJ. By atomic force microscope (AFM) tip-based indentation and in situ current measurement, the curren…
We apply a new method "force enhanced atomic refinement" (FEAR) to create a computer model of amorphous silicon (a-Si), based upon the highly precise X-ray diffraction experiments of Laaziri et al. The logic underlying our calculation is to estimate the structure of a real sample a-Si using experimental data and chemical information included in a non-biased way, starting from random coordinates. The model is in close agreement with experiment and also sits at a suitable minimum energy according to density functional calculations. In agreement with experiments, we find a small concentration of …
Amorphous silicon films prepared by electron beam evaporation have systematically and substantially greater atomic density for higher thickness, higher growth temperature, and slower deposition rate, reaching the density of crystalline Si when films of thickness greater than ~300 nm are grown at 425 $^{\circ}$C and at <1 $Å$/sec. A combination of spectroscopic techniques provide insight into atomic disorder, local strains, dangling bonds, and nanovoids. Electron diffraction shows that the short-range order of the amorphous silicon is similar at all growth temperatures, but fluctuation electron…
Specific heat measurements from 2 to 300 K of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large excess specific heat at low temperature, significantly larger than the Debye specific heat calculated from the sound velocity. The as-prepared films have a Schottky anomaly that is associated with metastable hydrogen in the amorphous network, as well as large linear and excess cubic term commonly associated with tunneling two-level systems in amorphous solids. Annealing at 200 °C, a temperature that enables hydrogen mobility but not evaporation, irreversibly …
Several amorphous silicon structures were generated using a classical molecular dynamics (MD) protocol of melting and quenching with different quenching rates. An analysis of the calculated electronic properties of these structures revealed that the midgap state density of a-Si which is of interest for solar cell and thin film transistor applications can be correlated to bond angle standard deviation. We also found that this parameter can strongly determine the excess energy of a-Si, which is an important criteria in theoretically generating realistic atomic structures of a-Si.
Atomistic Origin of Urbach Tails in Amorphous Silicon
指数带边已经在各种材料中观察到，包括晶体和非晶体。在本文中，我们推断这些尾巴在非晶和有缺陷的晶体硅的结构起源的直接计算与电流{ \it ab initio}方法。我们发现，指数尾出现在放松模型的金刚石与适当的点缺陷。在非晶硅（a-Si）中，我们发现网络中存在短键和长键的结构丝，并且在两个带尾的极端边缘附近的尾态也是连续的，在结构丝上有许多局域化。我们连接的存在下，这两个灯丝系统的缺陷和/或拓扑紊乱的存在下，结构松弛。
Amorphous silicon contains tunneling two-level systems, which are the dominant energy loss mechanisms for amorphous solids at low temperatures. These two-level systems affect both mechanical and electromagnetic oscillators and are believed to produce thermal and electromagnetic noise and energy loss. However, it is unclear whether the two-level systems that dominate mechanical and dielectric losses are the same; the former relies on phonon-TLS coupling, with an elastic field coupling constant, $γ$, while the latter depends on a TLS dipole moment, $p_0$, which couples to the electromagnetic fie…
Amorphous materials are coming within reach of realistic computer simulations, but new approaches are needed to fully understand their intricate atomic structures. Here, we show how machine-learning (ML)-based techniques can give new, quantitative chemical insight into the atomic-scale structure of amorphous silicon (a-Si). Based on a similarity function ("kernel"), we define a structural metric that unifies the description of nearest- and next-nearest-neighbor environments in the amorphous state. We apply this to an ensemble of a-Si networks, generated in melt-quench simulations with an ML-ba…
Understanding the structural origins of the properties of amorphous materials remains one of the most important challenges in structural science. In this study we demonstrate that local 'structural simplicity', embodied by the degree to which atomic environments within a material are similar to each other, is powerful concept for rationalising the structure of canonical amorphous material amorphous silicon (a-Si). We show, by restraining a reverse Monte Carlo refinement against pair distribution function (PDF) data to be simpler, that the simplest model consistent with the PDF is a continuous …
