μc-Si Materials Research Articles

Plasmonic-enhanced microcrystalline silicon solar cells

This paper describes the enhanced performance of the microcrystalline silicon (µc-Si) thin-film solar cells due to the incorporation of plasmonic nanostructures. Finite-difference time-domain numerical modeling is used to calculate the optical properties of the solar cells such as the absorption and the short-circuit current density ( J s c ). In this paper, two-dimensional(2D) periodic arrays of various geometries of plasmonic nanostructures such as nano-rings, nano-discs, nano-hemispheres, and nano-cubes are employed on the back side of the solar cells to increase the absorption for the longer wavelengths of the incident light, where most of the photons remain unharvested due to extremely low absorption coefficients of µc-Si material. These plasmonic nanostructures are compared in terms of solar cell performance, and it is found that the 2D periodic arrays of nano-rings show significant absorption enhancement at multiple wavelengths, thereby leading to a substantial enhancement in the J s c . An enhancement of 35% in the J s c is obtained when a 2D periodic array of plasmonic nano-rings is present on the back side of the solar cell, which is higher than that of the µc-Si solar cells having arrays of plasmonic nanostructures of other geometries (i.e., nano-discs, nano-cubes, nano-hemispheres). It is observed that the enhancement in the absorption is attributed to the enhanced electromagnetic fields in the active layer due to several localized surface plasmon modes that are excited in the plasmonic nanostructures at different wavelengths. Moreover, the effect of the thickness of the spacer layer (the layer between the metal back-reflector and plasmonic nanostructure arrays) on the performance of different plasmonic solar cells is examined.

Progress in High Conversion Efficiency a-Si/μc-Si Tandem Solar Cells and Modules

ABSTRACTA fabrication technology for high-quality, device-grade microcrystalline silicon (μc-Si) thin film with a higher deposition rate has been required to reduce the production cost of amorphous silicon (a-Si)/μc-Si tandem modules. Localized Plasma Confinement CVD (LPC-CVD) has been proposed as one solution to this problem. It was found that this CVD is suitable for the deposition of high crystalline fractions and (220) orientation in the development of small-to-medium-size substrates. Since then, we have been developing high-rate deposition technology for production-size substrates by using the essence of LPC-CVD and evaluation techniques for μc-Si materials and plasma. A stabilized module efficiency of 11.1% was reported with a very high deposition rate on a production-size substrate. To improve conversion efficiency, we have been focusing on elemental technologies as well as high-rate deposition technology. Stabilized conversion efficiency of 12.2% for small-size cells (1 cm2) and stabilized module conversion efficiency of 10.7% for production-size substrates were achieved.

Hot-wire chemical vapor-deposited microcrystalline silicon in single and tandem n–i–p solar cells

This paper examines the potential of μc-Si deposited by hot-wire chemical vapor deposition (HWCVD) at a constant hydrogen dilution of 0.95, as absorber layer in n–i–p thin film solar cells. For a series of single junction cells onto plain stainless steel, the highest open circuit voltage (Voc) and Fill Factor (FF) were 0.54V and 0.72. From J–V measurements at reduced light intensities, the change of the AM1.5-normalized FF values with light intensity was found to depend on the μc-Si i-layer thickness. This is attributed to the thickness evolution of the i-layer crystallinity, which influences the cell properties, particularly for i-layers thinner than 2.0μm. For thicker layers the measurements clearly show drift-driven charge carrier collection. Tandem cells showed FF values as high as 0.77, which indicates the very high optoelectronic quality of the μc-Si material. A 2.0μm cell on a textured back reflector shows an efficiency of 7.8%.

Tandem and triple junction silicon thin film solar cells with intrinsic layers prepared by hot-wire CVD

Hot-wire CVD (HWCVD) can be applied for the deposition of various types of silicon films. At Utrecht University intrinsic layers of high optoelectronic quality have been successfully used as the absorber layers in thin film solar cells. This paper presents results for a proto-Si/μc-Si cell and a proto-Si/μc-Si/μc-Si triple junction cell with its μc–μc middle-bottom tandem cell, all deposited in nip configuration onto plain stainless steel. The proto-Si/μc-Si tandem had a good V oc of 1.38 V and a high FF of 0.75. Its efficiency of 7.3% was, considering the absence of a back reflector, adequate. The μc–μc tandem cell had a good V oc of 1.04 V and a very high FF of 0.77. Since both tandem cells were current-limited by the bottom cell, the high FF-values indicate that our μc-Si material is of very high quality. The triple cell had a lower FF of 0.69 and its V oc of 1.77 V was lower than the sum of the middle–bottom tandem and the top cell. Filtered light-IV measurements on the μc–μc tandem cell indicated that the losses in FF and V oc observed for the triple cell are, at least partly, a result of the optical absorption by the proto-Si top cell.

Assessment of the Use of Microcrystalline Silicon Materials Grown at Rates Near 15 Å/s as i-layer Material for Single and Multi-Junction Solar Cells

ABSTRACTA microwave-based technique has been used to prepare microcrystalline Si (µc-Si) materials rates near 15 Å/s. The use of these materials as intrinsic layers (i-layers) for single and multi-junction devices has been assessed. Since the high deposition rates allow for fabrication of the required thicker µc-Si i-layers in a similar amount of time to that used for high quality a-SiGe i-layers (rates of 1-3 Å/s), the materials are attractive, low cost replacements for a-SiGe bottom cell i-layers in a-Si/a-SiGe and a-Si/a-SiGe/a-SiGe multi-junction cells. Single-junction nip, a-Si/µc-Si and a-Si/a-SiGe/µc-Si devices have been fabricated. For these devices, the doped and amorphous layers were deposited using conventional rf glow discharge processes and deposition equipment separate from that used to fabricate the µc-Si materials. 7.0% efficiencies have been achieved for single-junction devices while pre-light soaked 9.8 and 11.4% efficiencies have been achieved for the tandem and triple-junction devices, respectively. The single-junction devices exhibit a degradation of only 0-2% after long term (1000 hrs.) of light soaking demonstrating a high degree of stability. Based on the present status, the µc-Si material prepared at high rates qualifies as a reasonable candidate for the i-layer of a bottom cell of a triple-junction device. Improvements in the performance, particularly the FF, will be needed before use in single-junction and tandem devices can be considered.

Xps Studies of Hydrogen and Oxygen Bondinguy Configurations in Hydrogenated Microcrystalline Silicon Materials Produced by Neutron Irradiation

ABSTRACTThe bonding of Si atoms in μc-Si:H thin films has been investigated using X-ray photoelectron spectroscopy (XPS) in conjunction with infra-red spectroscopy (IR), secondary ion mass spectroscopy (SIMS) and analytical electron microscopy (TEM/EDX/EELS) data. By using a-Si:H and single crystal silicon as reference samples, structural and hydrogen effects could be assessed, since both a-Si:H and μc-Si:H have a similar concentration of hydrogen based on the N15 hydrogen profiling data, but different structures. On the other hand, single crystal silicon and μc-Si:H both have a diamond cubic structure based on electron diffraction data, but single crystal silicon contains little or no hydrogen except adsorbed at the surface. Based on the XPS and IR data, charge transfer of the Si2p core level towards a deep lying level was observed in the μc-Si:H material. IR measurements snowed a large amount of hydrogen was located in the grain boundaries. The charge transfer is mainly due to a change in the hydrogen bonding configuration. A well bonded oxide is formed in the μc-Si:H material near the surface with an almost complete absence of the Si3+ intermediate oxide state. The presence of a large amount of hydrogen (25 at.%) even at a high volume fraction (70%) of μc-phase may limit the oxidation and promote better oxide formation. The variation of the quality of the oxide/μc interface could be a possible explanation for the different photoluminescence observed by various groups for porous μc-Si material.