Influence of seed layer morphology on the epitaxial growth of polycrystalline-silicon solar cells (original) (raw)
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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.
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.
Polycrystalline silicon on glass thin-film solar cell research at UNSW the seed layer concept
Clinical Nutrition Supplements, 2003
A novel seed layer-based poly-Si solar cell concept on glass-ALICIA (aluminium-induced crystallisation, ion-assisted deposition)-is presently being developed at the University of New South Wales (UNSW). The first key feature of ALICIA solar cells is a large-grained p/sup +/-doped poly-Si seed layer made on the glass by means of aluminium-induced crystallisation (AIC) of amorphous silicon. The other key feature is the
Thin Solid Films, 2008
Polycrystalline silicon (grain size~0.1-100 μm) solar cells on foreign substrates are a promising approach for the next generation silicon solar cells. Aluminum-induced crystallization AIC in combination with epitaxy is a possible way to obtain such absorber layers. It is believed that Si islands present on the surface of AIC seed layers have a negative effect on the epitaxy. The removal of these islands could therefore lead to an increased absorber layer quality and solar cell performance. In this paper, we present a selective island removal procedure based on the Al layer already present after AIC annealing. By selecting an etchant which removes Si at least as fast as Al (in this paper plasma etching using SF 6 ), the Al layer acts as a perfectly aligned etching mask for the fully developed islands.
Thin Solid Films, 2017
The fabrication of crystalline silicon thin films on foreign substrates is an attractive and alternative approach to the ingot casting aiming to the reduction of the total costs of photovoltaic cells and modules. The purpose of this work is to describe the CRYSTALSI process which aims at forming polycrystalline silicon films thanks to the thermal crystallization of amorphous silicon layer deposited on aluminium based substrates. The latest are used as a catalyzer for silicon crystallization but also as a back metal contact and reflector for photovoltaic solar cells. Two types of aluminium substrates were applied in these studies: a pure aluminium substrate (99.7% purity) and a silicon rich aluminium substrate containing about 12% of silicon. Silicon thicknesses between 1 and 10 μm were deposited and then annealed at temperatures of 490°C, 520°C and 550°C and for duration times from 5 min to 12 h. The crystallized silicon films were then characterized by Raman spectroscopy, by scanning electron microscopy and by electron backscatter diffraction. The analyses show that the resulting annealed film is composed of two distinct layers: a thin polycrystalline silicon film located just above the substrate and a thicker layer made of a mixture of silicon and aluminium. Contrary to the case of the pure aluminium substrate, the silicon rich aluminium substrate allow to obtain thick and continuous polycrystalline silicon layers due to a controlled diffusion of the silicon within the substrate. As a result, the crystallization at 550°C of 5 μm thick amorphous silicon on silicon rich aluminium substrate led to the formation of a thick polycrystalline silicon layer composed of grains of few micrometers in size. A low activation energy of about 2 eV is extracted suggesting that the silicon rich aluminium substrate is a catalyzer for the crystallization of amorphous silicon. As for the AIC process, it can be noticed that the limiting step of the CRYSTALSI process is the diffusion of the silicon in the aluminium. A chemical etching using a HNO 3 , HF, H 2 O (72.5 ml/1.5 ml/28 ml) solution is found to be appropriate to remove the residual top layer, in order to have access to the polycrystalline silicon layer. This work demonstrates that the CRYSTALSI process can lead to the formation of polysilicon films that can serve as a seed layer for the growth of a thicker absorbing silicon film for photovoltaic applications.
Progress in Photovoltaics: Research and Applications, 2014
We fabricate thin epitaxial crystal silicon solar cells on display glass and fused silica substrates overcoated with a silicon seed layer. To confirm the quality of hot-wire chemical vapor deposition epitaxy, we grow a 2-μm-thick absorber on a (100) monocrystalline Si layer transfer seed on display glass and achieve 6.5% efficiency with an open circuit voltage (V OC ) of 586 mV without light-trapping features. This device enables the evaluation of seed layers on display glass. Using polycrystalline seeds formed from amorphous silicon by laser-induced mixed phase solidification (MPS) and electron beam crystallization, we demonstrate 2.9%, 476 mV (MPS) and 4.1%, 551 mV (electron beam crystallization) solar cells. Grain boundaries likely limit the solar cell grown on the MPS seed layer, and we establish an upper bound for the grain boundary recombination velocity (S GB ) of 1.6x10 4 cm/s.
Journal of Applied Physics, 2009
Polycrystalline silicon ͑pc-Si͒ thin-films with a grain size in the range of 0.1-100 m grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization ͑AIC͒ of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current ͑EBIC͒ measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason for the grain size independent device performance. Besides dislocations and stacking faults, also ⌺3 boundaries were electrically active as shown by combining electron backscattered diffraction with EBIC measurements. The electrical activity of the defects is probably triggered by impurity decoration. Plasma hydrogenation changed the electrical behavior of the defects, as seen by photoluminescence, but the defects were not completely passivated as shown by EBIC measurements. In order to reveal the origin of the defects, cross section transmission electron microscopy measurements were done showing that the intragrain defects are already present in the AIC seed layer and get copied into the epitaxial layer during epitaxial growth. The same types of intragrain defects were found in layers made on different substrates ͑alumina ceramic, glass ceramic, and oxidized silicon wafer͒ from which we conclude that intragrain defects are not related to the relatively rough alumina ceramic substrates often used in combination with high temperature epitaxy. Further improvement of the material quality, and hence device performance, is therefore not simply achieved by increasing the grain size, but the intragrain quality of the material also needs to be taken into account. For pc-Si layers based on AIC and epitaxial growth, the seed layer has a crucial impact on the intragrain defect formation.
Epitaxial thickening of AIC poly-Si seed layers on glass by solid phase epitaxy
Journal of Crystal Growth, 2005
A novel method for producing high-quality polycrystalline silicon (poly-Si) films on glass by means of solid phase epitaxy (SPE) of evaporated amorphous silicon on aluminium-induced crystallisation (AIC) poly-Si seed layers is introduced. Optical transmission microscope, Raman, UV reflectance spectroscopy and cross-sectional transmission electron microscope measurements show consistently that a transfer of the crystal properties of the AIC poly-Si seed layer into the crystallised amorphous silicon layer has been achieved. A 1-sun open-circuit voltage of 337 mV is realised with a hydrogenated SPE/AIC p-n junction device, which is a promising result considering the early stage of process development. The SPE/AIC method appears well suited for the fabrication of poly-Si thin-film solar cells on glass and, due to the high crystal quality and the much larger average grain size, could lead to improved energy conversion efficiencies compared to Si solar cells made by solid phase crystallisation. r
Epitaxial Growth of Silicon Thin Films for Solar Cells
2008
Crystalline silicon thin film solar cells on glass substrates are a low cost alternative to silicon wafer cells. As an alternative to a simple furnace annealing step in which a-Si is converted to c-Si with 1 µm grains, an epitaxial crystal growth process is presented here. First a seed layer is prepared on glass by diode laser crystallization of an a-Si layer on glass to result in 100 µm grains. Then a-Si is deposited on top of the seed which is converted to c-Si by epitaxial growth. A 1.1 µm thick c-Si layer with 100 µm grains was produced in this way. The paper presents details of the epitaxial growth process.
International Journal of Photoenergy, 2014
Vapour-phase and solid-phase epitaxy are used for thickening of a solid-phase crystallised silicon seed layer on glass. Crosssectional transmission microscope images confirm that a transfer of crystallographic information has taken place from the seed layer into the epilayers. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy reveal that the density of planar defects (mainly on {111} plains) in the vapour-phase epitaxial sample is much higher than in the solid-phase epitaxial sample. These planar defects can act as recombination centres for free-charge carriers. Consequently, PC1D modelled minority carrier diffusion length in the vapour-phase grown epilayer is 50% shorter than that in the solid-phase grown epilayer. As a result, a solar cell grown by solid-phase epitaxy achieves open circuit voltage of 468 mV, short circuit current of 9.17 mA/cm 2 , and photovoltaic conversion efficiency at 2.75% which are all higher than those of the solar cell grown by vapour-phase epitaxy on the same seed layer, 400 mV, 7.28 mA/cm 2 , 1.69%, respectively. It proves that solid-phase epitaxy is more suitable for the solar cell growth on the solid-phase crystallised silicon seed layer than vapour-phase epitaxy.