Ultrathin Amorphous Silicon Research Articles

Thin film silicon solar cells with non-simple integral period ratio between front and back gratings

We propose an ultra-thin amorphous silicon solar cell with dual-interface gratings of non-simple integral period ratios. The non-simple integral period ratio structure (NIPRS) shows a broad absorption band compared to the simple integral period ratio structure (SIPRS). Both the front and back gratings couple light into different optical modes in wavelength ranges that do not overlap. An integrated absorption of 67.2% is achieved when the thickness of the active layer is 100 nm.

Optical bandgap of ultra-thin amorphous silicon films deposited on crystalline silicon by PECVD

An optical study based on spectroscopic ellipsometry, performed on ultrathin hydrogenated amorphous silicon (a-Si:H) layers, is presented in this work. Ultrathin layers of intrinsic amorphous silicon have been deposited on n-type mono-crystalline silicon (c-Si) wafers by plasma enhanced chemical vapor deposition (PECVD). The layer thicknesses along with their optical properties –including their refractive index and optical loss- were characterized by spectroscopic ellipsometry (SE) in a wavelength range from 250 nm to 850 nm. The data was fitted to a Tauc-Lorentz optical model and the fitting parameters were extracted and used to compute the refractive index, extinction coefficient and optical bandgap. Furthermore, the a-Si:H film grown on silicon was etched at a controlled rate using a TMAH solution prepared at room temperature. The optical properties along with the Tauc-Lorentz fitting parameters were extracted from the model as the film thickness was reduced. The etch rate for ultrathin a-Si:H layers in TMAH at room temperature was found to slow down drastically as the c-Si interface is approached. From the Tauc-Lorentz parameters obtained from SE, it was found that the a-Si film exhibited properties that evolved with thickness suggesting that the deposited film is non-homogeneous across its depth. It was also found that the degree of crystallinity and optical (Tauc) bandgap increased as the layers were reduced in thickness and coming closer to the c-Si substrate interface, suggesting the presence of nano-structured clusters mixed into the amorphous phase for the region close to the crystalline silicon substrate. Further results from Atomic Force Microscopy and Transmission Electron Microscopy confirmed the presence of an interfacial transitional layer between the amorphous film and the underlying substrate showing silicon nano-crystalline enclosures that can lead to quantum confinement effects. Quantum confinement is suggested to be the cause of the observed increase in the optical bandgap of a-Si:H films close to the a-Si:H/cSi interface.

Cascading metallic gratings for broadband absorption enhancement in ultrathin plasmonic solar cells

The incorporation of plasmonic nanostructures in the thin-film solar cells (TFSCs) is a promising route to harvest light into the nanoscale active layer. However, the light trapping scheme based on the plasmonic effects intrinsically presents narrow-band resonant enhancement of light absorption. Here we demonstrate that by cascading metal nanogratings with different sizes atop the TFSCs, broadband absorption enhancement can be realized by simultaneously exciting multiple localized surface plasmon resonances and inducing strong coupling between the plasmonic modes and photonic modes. As a proof of concept, we demonstrate of 66.5% in the photocurrent in an ultrathin amorphous silicon TFSC with two-dimensional cascaded gratings over the reference cell without gratings.

Decorative power generating panels creating angle insensitive transmissive colors

We present ultra-thin (6 to 31 nm) undoped amorphous silicon/organic hybrid solar cell structure, which can transmit desired color of light. The transmitted colors show great angular tolerance due to the negligible optical phase associated with light propagating in ultra-thin amorphous silicon (a-Si) layers. We achieved the power conversion efficiency of the hybrid cells up to 2 %; and demonstrated that most of the absorbed photons in the undoped a-Si layer contributed to the extracted electric charges due to the suppressed electron-hole recombination in the ultra-thin a-Si layer. We also show the resonance is invariant with respect to the angle of incidence up to ±70° regardless of the polarization of the incident light. Our exploration provides a design to realize energy harvesting colored photovoltaic panels for innovative applications.

Analysis of the outdoor performance and efficiency of two grid connected photovoltaic systems in northern Italy

This paper analyzes and compares the actual performance of two grid connected photovoltaic plants, which are of similar size but based on different modules technologies. The facilities are located on the roofs of two buildings of Area Science Park in the site of synchrotron ELETTRA in Basovizza (Trieste). The first system is equipped with modules of mono crystalline silicon wafer surrounded by ultra-thin amorphous silicon layers (hetero-junction with intrinsic thin layer), facing south and tilted at 30°, the second is equipped with mono crystalline silicon modules oriented 35° west and inclined at 10°. The set of data analyzed in this paper has been systematically acquired from October 15th 2011 to October 14th 2012 by means of a dedicated monitoring system.The aim of this study is to analyze the actual performances of the plants and their yearlong evolution, separating the effects of the variability of environmental conditions from those due to the variability of the performance of panels and electrical components. Particular attention is given to the influence that irradiance and temperature have on the efficiency of the system.An analysis methodology was applied to appropriately filter, classify and normalize the data in order to identify the modules temperature coefficients as a function of irradiance and to compare their actual efficiency with those declared by the manufacturer. Acquired data and mathematical correlations will allow the development of accurate simulation models, useful for assessing the actual profitability of similar installations. The latter can be obtained through the main performance indices of the systems that are calculated and reported in this paper for the period of observation.

Ultrathin Metal/Amorphous-Silicon/Metal Diode for Bipolar RRAM Selector Applications

We propose a novel metal/silicon/metal (MSM) selector using ultrathin undoped amorphous silicon (a-Si) for resistive-RAM selector application. The new selector behaves as a bidirectional diode, showing a high current drive (~2.2 MA/cm)2, high selectivity (~240 for 1/2 bias), fast switching speed , and excellent endurance ( at target drive current). The doping-free a-Si structure alleviates the dopant-induced variability concerns for ultrascaled devices and eliminates the need for a dopant-activation anneal. Circuit simulations show feasibility of 1-Mb array, with over 25% read margin and 70% write margin, when using the new MSM structure as a selector for a HfO2-based resistive switching memory element.

Hetero-structured lumpy nanoparticle conformal structure for high absorbance of ultrathin film amorphous silicon solar cells

We present a concept for enhancing the absorbance of amorphous-silicon solar cells by using hetero-structured nanoparticles consisting of dielectric core particles combined with small metallic surface nanoparticles half embedded in the core to harness both the scattering effect and the near field light concentration. Through optimising key parameters, including the relative distance of the nanoparticles to the solar cell, the radius ratio of the core to the surface nanoparticles, and the refractive index of the core particles, the short circuit current density in a 20 nm nanoparticle-integrated active layer is equivalent to that in a 300 nm flat active layer.

Ultra-broadband performance enhancement of thin-film amorphous silicon solar cells with conformal zig–zag configuration

An ultrathin amorphous silicon solar cell with conformal zig-zag nanoconfiguration is studied from both light-trapping and light-conversion perspectives. The design improves the front antireflection property, optimizes the rear metallic reflector, and elongates the optical path inside the photoactive layer. Compared to conventional nanoconfigurations, this system shows significant absorption enhancement in the whole amorphous silicon band and exhibits extremely low sensitivity to light polarization. The nano-optimization indicates that the short-circuit current density (light-conversion efficiency) of the 200-nm-thick solar cell can be 16.88 mA/cm² (13.38%), showing an enhancement factor of 32.90% (33.53%) from the planar system.

Multi-resonant absorption in ultra-thin silicon solar cells with metallic nanowires

We propose a design to confine light absorption in flat and ultra-thin amorphous silicon solar cells with a one-dimensional silver grating embedded in the front window of the cell. We show numerically that multi-resonant light trapping is achieved in both TE and TM polarizations. Each resonance is analyzed in detail and modeled by Fabry-Perot resonances or guided modes via grating coupling. This approach is generalized to a complete amorphous silicon solar cell, with the additional degrees of freedom provided by the buffer layers. These results could guide the design of resonant structures for optimized ultra-thin solar cells.

Nanopatterned front contact for broadband absorption in ultra-thin amorphous silicon solar cells

Broadband light trapping is numerically demonstrated in ultra-thin solar cells composed of a flat amorphous silicon absorber layer deposited on a silver mirror. A one-dimensional silver array is used to enhance light absorption in the visible spectral range with low polarization and angle dependencies. In addition, the metallic nanowires play the role of transparent electrodes. We predict a short-circuit current density of 14.6 mA/cm2 for a solar cell with a 90 nm-thick amorphous silicon absorber layer.

Using spacer layers to control metal and semiconductor absorption in ultrathin solar cells with plasmonic substrates

We systematically explore the performance of ultrathin amorphous silicon solar cells integrated on plasmonic substrates of several different morphologies. Angle-resolved reflectance, external quantum efficiency measurements, and finite-difference time-domain simulations highlight the importance of the spacer layer in determining the mode profiles to which light can couple. Coupling mechanisms are found to strongly differ between periodic silver nanovoid arrays and randomly textured silver substrates. Tailoring the spacer thickness leads to 50% higher quantum efficiencies and short-circuit current densities by tuning the coupling between the near-field and trapped modes with enhanced optical path lengths. The balance of absorption for the plasmonic near field at the metal/semiconductor interface is analytically derived for a broad range of leading photovoltaic materials. This yields key design principles for plasmonic thin-film solar cells, predicting strong near-field enhancement only for CdTe, CuInGaSe2, and organic polymer devices.

Dynamical process of KrF pulsed excimer laser crystallization of ultrathin amorphous silicon films to form Si nano-dots

Molecular dynamics (MD) simulations based on the Tersoff potential have been developed to study the laser-induced crystallization of amorphous silicon (a-Si) film with ultrathin thickness to form size-controllable Si nano-dots. The influences of laser fluence and a-Si film thickness on the crystallization process were discussed. Classic nucleation theory was used to explain the results of the MD simulations. The constrain effect of a-Si films thickness on the formation of Si nano-dots was evaluated accordingly.

Influence of ultrathin amorphous silicon layers on the nucleation of microcrystalline silicon films under hydrogen plasma treatment

Microstructures of phosphorus-doped hydrogenated microcrystalline silicon (μc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition are certainly dependent on the thickness of the H2 plasma-treated amorphous silicon (a-Si:H) layers. An ultrathin H-treated a-Si:H layer is beneficial in obtaining a very thin μc-Si:H film with high conductivity. Experimental results indicate that H2 plasma treatment induces the occurrence of high-pressure H2 in microvoids and causes compressive stress inside the ultrathin a-Si:H layers, thereby enhancing the generation of strained Si–Si bonds and nucleation sites and consequently accelerating the nucleation of μc-Si:H films.

Surface deformation of amorphous silicon thin film on elastomeric substrate

Stiff thin layers on compliant substrates can generate various surface structures using equi-biaxial stress caused by large thermal expansion rate differences. We investigated the detailed understanding on the evolution of self-assembled wrinkle patterns of ultra-thin amorphous silicon (a-Si) layers on polydimethylsiloxane substrate. It turns out that the generation of various wrinkle patterns depends on the position of their orientation, film thicknesses, mechanical properties of the a-Si films, and the amount of pre-strain. The various self-assembled patterns include one-dimensional wavy patterns, randomly ordered two-dimensional structured patterns, and herringbone structures. The self-assembled wrinkles can be characterized by the wavelength and amplitude of the distinct structures: the amplitudes of the various patterns increase as the amount of pre-strain increases, while the wavelengths remain constant within our experimental ranges. The experimental results of the wavelengths and amplitudes for the wavy structured patterns of 270-nm-thick a-Si layer are in good agreement with the theoretical solutions of the single crystalline silicon (c-Si) model, which implies that the theoretical modeling of the deformation of c-Si film can be expandable to the case of a-Si film deformations.

Molecular dynamics simulations of pulsed laser crystallization of amorphous silicon ultrathin films

Laser crystallization of amorphous Si thin films is one of reliable method of preparing nanocrystalline silicon with high density and controllable size. In the present work, molecular dynamics simulation based on Tersoff potential was used to study the laser crystallization process of ultrathin amorphous silicon film (2.7 nm) on amorphous silicon nitride substrate. The influence of laser fluences on the crystallization and formation of nanocrystalline Si was investigated. It was found that there exists a laser fluence window in which nucleation and growth of nanocrystalline Si can be realized, which is in agreement with our previous experimental observations. The nucleation and growth processes in microscopic scale were simulated and the size of formed nanocrystalline Si was limited in both vertical and lateral directions by the film thickness and the formation of grain boundaries.

Hot electron effect in nanoscopically thin photovoltaic junctions

The open circuit voltage in ultrathin amorphous silicon solar cells is found to increase with light energy (frequency), due to extraction of hot electrons. The ultrathin nature of these junctions also leads to large internal electric fields, yielding reduced recombination and increased current. A simple phenomenological argument provides a qualitative understanding of these effects and gives guidelines for designing future, high-efficiency, hot electron solar cells.

Photoresponse enhancement in the near infrared wavelength range of ultrathin amorphous silicon photosensitive devices by integration of silver nanoparticles

We have investigated the contribution of localized surface plasmon polaritons (LSPPs) in silver nanoparticles with radii smaller than 20 nm to the photocurrent of ultrathin photosensitive devices based on amorphous silicon. An increased light absorption and an enhanced photocurrent are found for wavelengths between 600 nm and 1150 nm in presence of nanoparticles. As amorphous silicon absorbs light efficiently only at wavelengths up to 750 nm, the increased photocurrent in the near infrared range is explained in terms of LSPP-induced photoemission of electrons within and in close vicinity of the nanoparticles.

Enhanced infrared response of ultra thin amorphous silicon photosensitive devices with Ag nanoparticles

Small silver (Ag) nanoparticles (with diameter smaller than 50 nm) can lead to a resonant light absorption accompanied by an enhanced electromagnetic field in their vicinity when they are irradiated by light due to so-called localized surface plasmon. In order to study the influence of metal nanoparticles in thin-film silicon solar cells in more detail, a photosensitive test structure based on a-Si:H and consisting of a TCO/(nanoparticles)/i/n/TCO layer stack was realized. A higher quantum efficiency of the test structure is achieved for wavelengths longer than 800 nm compared to the structure without nanoparticles. As a-Si:H only efficiently absorbs light for wavelenghts of up to 800 nm, the enhanced photocurrent cannot be explained by improved light absorption in a-Si:H.

Behaviour of TEM metal grids during in-situ heating experiments

The stability of Ni, Cu, Mo and Au transmission electron microscope (TEM) grids coated with ultra-thin amorphous carbon (α -C) or silicon monoxide film is examined by in-situ heating up to a temperature in the range 500–850 °C in a transmission electron microscope. It is demonstrated that some grids can generate nano-particles either due to the surface diffusion of metal atoms on amorphous film or due to the metal evaporation/redeposition. The emergence of nano-particles can complicate experimental observations, particularly in in-situ heating studies of dynamic behaviours of nano-materials in TEM. The most widely used Cu grid covered with amorphous carbon is unstable, and numerous Cu nano-particles start to form once the heating temperature reaches 600 °C. In the case of Ni grid covered with α -C film, a large number of Ni nano-crystals occur immediately when the temperature approaches 600 °C, accompanied by the graphitization of amorphous carbon. In contrast, both Mo and Au grids covered with α -C film exhibit good stability at elevated temperature, for instance, up to 680 and 850 °C for Mo and Au, respectively, and any other metal nano-particles are detected. Cu grid covered Si monoxide thin film is stable up to 550 °C, but Si nano-crystals appear under intensive electron beam. The generated nano-particles are well characterized by spectroscopic techniques (EDXS/EELS) and high-resolution TEM. The mechanism of nano-particle formation is addressed based on the interactions between the metal grid and the amorphous carbon film and on the sublimation of metal.

Nonvolatile Memory Characteristics with Embedded Hemispherical Silicon Nanocrystals

Silicon nanocrystal-embedded memories were fabricated by using the thermal agglomeration of an ultrathin (1.5–1.8 nm) amorphous silicon (a-Si) film. The a-Si was deposited on a 4-nm tunnel-oxide layer by electron beam evaporation and subsequently annealed in situ at 850 °C for 5 min. Hemispherical Si nanocrystals were self-assembled, and nonvolatile memories were fabricated after depositing a 17-nm control-oxide layer. A threshold voltage window of 0.9 V was achieved under write/erase (W/E) voltages of ±10 V, and good endurance characteristics up to 104 cycles were exhibited. Increasing W/E voltages created a large memory window (>2.7 V), and the retention characteristics showed little temperature dependence up to 85 °C.