Polycrystalline silicon films obtained by crystallization of amorphous silicon on aluminium based substrates for photovoltaic applications (original) (raw)
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Understanding phenomena of thin silicon film crystallization on aluminium substrates
The realization of crystalline silicon thin films on foreign substrates is an attractive alternative to the ingot casting aiming at a reduction in total costs. The purpose of this work is to form polycrystalline silicon films using the crystallization of amorphous silicon deposited on aluminum (Al) substrates. The Al-substrate is used as a catalyzer for silicon crystallization but also as a conductive substrate and as a back reflector for the photovoltaic cell. The crystallization of 1-5 μm thick amorphous silicon films were carried out at a temperature of 550°C and for duration times from 10 to 80 min. The crystallized silicon films were then characterized by Raman spectroscopy, scanning electron microscopy and by electron backscatter diffraction. The analysis show that the annealed layer is composed of two distinct layers: a thin polysilicon film located just above the Al substrate and on the top a thicker layer made of a mix of silicon and aluminum. The thickness of the polysilicon film is found to increase with the annealing time. The crystallization of 5 μm thick amorphous silicon during 80 min resulted in a 1 μm thick polysilicon layer composed of grains of few micrometers in size. The mechanisms of accelerated crystallization are discussed. Such polysilicon films can be used as a seed layer for the growth of a thicker absorbing silicon film for photovoltaic applications.
IEE Proceedings - Circuits, Devices and Systems, 2003
Thin film polycrystalline silicon solar cells on foreign substrates are viewed as one of the most promising approaches to cost reduction in photovoltaics. To enhance the quality of the film, the use of 'seeding layers' prior to deposition of active material is being investigated. It has been shown that a phenomenon suitable to create such a seeding layer is the aluminium-induced crystallisation of amorphous silicon. Previous work mainly considered glass as the substrate of choice, thereby introducing limitations on the deposition temperature. Results concerning the application of such a technique to ceramic substrates (allowing the use of high-temperature CVD) are described. Also, the first reported results of a solar cell made in silicon deposited on these seeding layers are presented.
2017
The effect of various annealing treatments on the structure properties of crystalline silicon (c-Si) produced by the inverted aluminium induced crystallization of amorphous silicon (a-Si) films was studied. The surface morphology and grain size of c-Si films were observed by optical microscope and scanning electron microscope. X-ray diffraction and Raman spectroscopy were used to study quantity of Si crystallization due to thermal annealing. Results showed that the c-Si with an average grain size of 54 nm in a (111) orientation was obtained by the thermal annealing at 350 o C for 1 h. Prolonged heat treatment improved Si crystallite quality and increased the average grain size.
EBSD analysis of polysilicon films formed by aluminium induced crystallization of amorphous silicon
Thin Solid Films, 2008
Among the methods for enlarging the grain size of polycrystalline silicon (poly-Si) thin films, aluminium induced crystallization (AIC) of amorphous silicon is considered to be a very promising approach. In the AIC process, a thin a-Si layer on top of an aluminium layer crystallizes at temperatures well below the eutectic temperature of the Al/Si system (T eu = 577°C). By means of electron backscattering diffraction (EBSD), we have mainly studied the effect of the aluminium layer quality varying the deposition system on the grain size, the defects and the preferential crystallographic orientation. We have found a strong correlation between the mean grain size and the size distribution with the Al deposition system and the surface quality. Furthermore, we show for the first time that more than 50% of the surface of the AIC films grown on alumina substrates are (103) preferentially oriented, instead of the commonly observed (100) preferential orientation. This may have important consequences for epitaxial thickening of the AIC layer into polysilicon absorber layers for solar cells.
2010
Large-grained, n + n-type polycrystalline silicon (poly-Si) films were obtained on alumina substrates by combining the aluminium induced crystallization (AIC) process of amorphous silicon and chemical vapour deposition (LPCVD) at high temperature (1000 1C) for the epitaxial thickening. The n + seed layer was obtained by phosphorus doping of the AIC layer. The electron backscattering diffraction (EBSD) technique was used for the crystallographic analysis of the poly-Si thin films. Seed layers with an average grain size of 7.6 mm were obtained on alumina substrates by exchange annealing at 475 1C for 6 h. Heterojunction emitter (HJE) solar cells were fabricated on such layers and their characteristics were monitored. IQE measurements show that n-type material based solar cells led to a much higher current collection over a large part of the spectrum compared to p-type cells. Accordingly a high effective diffusion length of about 2 mm for n-type heterojunction solar cells was obtained while it is about 0.9 mm for the p-type cell. As a result, the first n-type solar cells showed efficiencies above 5%, which is a very promising result considering that no optimization nor texturing have been applied so far.
Poly-Silicon Thin Films Prepared by Low Temperature Aluminum-Induced Crystallization
Modern Physics Letters B, 2001
P-type poly-Si thin films prepared by low temperature Aluminum-induced crystallization and doping are reported. The starting material was boron-doped a-Si:H prepared by PECVD on glass substrates. Aluminum layers with different thicknessess were evaporated on a-Si:H surface and conventional thermal annealing was performed at temperatures ranging from 300 to 550°C. XRD, SIMS, TEM and Hall effect measurements were carried out to characterize the annealed films. Results show that a-Si:H contacted with adequate Al could be crystallized at temperature as low as 300°C after annealing for 60 minutes. This material has high carrier concentration as well as high Hall mobility can be used as a p-layer or seed layer for thin film poly-Si solar cells. The technique reported here is compatible with PECVD process.
Metal induced crystallization of amorphous silicon for photovoltaic solar cells
Physics Procedia, 2011
A silicon thin-film technology could lead to less expensive modules by the use of less silicon material and by the implementation of monolithic module processes. A technology based on polycrystalline-silicon thin-films with a grain size between 1 μm and 1 mm (pc-Si), seems particularly promising since it combines the low-cost potential of a thin-film technology with the high efficiency potential of
Crystallization of P Type Amorphous Silicon (a-Si: H) by AIC Method: Effect of Aluminum Thickness
Silicon, 2019
In this work, we will study the crystallization of P type hydrogenated amorphous silicon (a-Si:H) by Aluminum Induced Crystallization technique (CIA) by varying the thickness of the aluminum films. We have deposited a 100 nm thickness of p-type a-Si:H layer on Corning glass substrates using PECVD technique. An aluminum layer with thickness ranging from 10 to 400 nm was thermally evaporated on the a-Si:H surface. The thermal annealing was performed in a conventional furnace at temperature of 550°C for 4 h in flowing N 2 ambient. The study of the crystallization of the Al/a-Si:H/Glass structure according the aluminum thickness was carried out by using Raman spectroscopy, X-rays diffraction and Hall Effect measurements. Raman results reveal the presence of the peaks between 510 and 520 cm −1 , which are close to the peak of crystallized Si (about 521 cm −1) proving the crystallization of all samples. The XRD measurements show the presence of the characteristic peaks of the crystalline silicon, thus the a-Si: H (p) layer was effectively crystallized by the AIC method in a short time. Through Hall measurements we found an improvement in electrical properties and an increase in dopant concentration (+ 5.3 10 14 to + 2.9 10 17 cm 2).
Process development of amorphous silicon/crystalline silicon solar cells
Solar Energy Materials and Solar Cells, 1997
We have already investigated some crucial limiting process steps of the amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell technology and some specific characterization tools of the ultrathin amorphous material used in devices. In this work, we focus our attention particularly on the technology of the ITO front contact fabrication, that also is used as an antireflective coating. It is pointed out that this layer acts as a barrier layer against the diffusion of metal during the annealing treatments of the front contact grid. The criteria of the selection of the metal to be used to obtain good performance of the grid and the deposition methods best suited to the purpose are shown. We were able to fabricate low temperature heterojunction solar cells based p-type Czochralski silicon, and a conversion efficiency of 14.7% on 3.8 cm 2 area was obtained without back surface field and texturization.
MRS Proceedings, 2006
Aluminum-induced crystallization of hydrogenated amorphous silicon was used to fabricate epitaxial silicon films through solid phase epitaxy. Silicon wafers of (100) orientation were used as the starting crystalline structure for the epitaxial thin film growth. A configuration of c-Si/Al/a-Si:H was used to produce these films through the phenomenon of layer inversion. A thin layer of aluminum (300 nm) was deposited on a silicon wafer by sputtering. On top of this layer, a 300 nm amorphous silicon film was deposited using plasma-enhanced chemical vapor deposition. After annealing the samples at 475°C for 40 minutes, a continuous film of crystalline silicon was formed on the silicon substrate. X-ray diffraction, scanning electron microscopy, and cross-sectional transmission electron microscopy were used to characterize the films. Auger depth profiling indicated the formation of a Si/Al mixed phase within the first few minutes of annealing. A proposed model of the growth mechanism is presented.