Guang Fu Zheng

Investigation of polycrystalline silicon deposition on glass substrates

Abstract Borosilicate glass has been chosen as substrate for solution growth of silicon due to its potential role as the superstrate of a solar module. Several approaches has been used to deposit polycrystalline silicon layer from solution on to glass substrates, namely, silicon particle seeded growth, growth on α-silicon coated glass substrates and growth on bare glass from the solutions containing Al or Mg. Large grain polycrystalline silicon crystals have been grown on glass substrates previously seeded by silicon particles. Continuous silicon thin films have been deposited onto α-silicon coated glass substrates. For the third approach, impinging growth of silicon layers has been obtained on sandblasted glass substrates from solutions containing Al and Mg at temperatures below 600°C, if the concentration of Al and Mg in the solution and the contact time between the solution and the substrate are appropriately adjusted. Large grain continuous silicon thin films with an area of 10 cm2 have also been groown on glass substrates at temperatures around the softening points of the glass.

16.4% efficient, thin active layer silicon solar cell grown by liquid phase epitaxy

An energy conversion efficiency of 16.4% is reported for a silicon solar cell of 4.11 cm2 total area with a thin active layer of 32 μm grown by liquid phase epitaxy (LPE). This is the highest ever total area efficiency for a cell of this type and is due to a number of improvements over earlier reported results. The thin active layer was grown by LPE on an inactive silicon substrate from an indium solution in a 20% hydrogen/argon forming gas mixture ambient rather than pure hydrogen. Higher current density and efficiency than previously reported for similar cell structures have been achieved by employing microgroove texturing of the front surface, a very shallow (0.25 μm) and high sheet resistivity (220 Ω□) top surface phosphorus diffusion, an optimized ZnS/MgF2 double layer antireflection coating on top of a 200A thick, high quality passivation SiO2 layer, a large aspect ratio (0.45) for the metal contacts, and a graded doping level within the 32 μm thick LPE active layer. The effect of the improved techniques on the cell performance and the properties of the thin active layers are discussed.

The effects of solvent and dopant impurities on the performance of LPE silicon solar cells

Abstract This paper reports the effect of solvent and dopant impurities on the performance of LPE silicon solar cells. For LPE layers grown from Sn and In based solutions and having similar surface morphology and resistivity, the performance of solar cells made on LPE layers grown from In was always higher than that of cells made on LPE layers grown using Sn as solvent. Consistently higher performance was also obtained from solar cells fabricated upon Ga-doped LPE layers than from cells made on Al-doped LPE silicon. The best cell was fabricated upon a Ga-doped LPE layer grown from In solution and had a total area efficiency of 16.4% confirmed by Sandia measurements. The observed phenomena are explained on the basis of Hall mobilities and minority carrier lifetimes of LPE layers grown from different solutions, and also the oxidation difference of these layers during cell processing.

Thin film silicon solar cells on glass by substrate thinning

Abstract We report on the fabrication of thin film Si solar cells on glass by substrate thinning. We use thin Si films grown on thick Si substrates by either liquid phase epitaxy or chemical vapour deposition. A novel solar cell device fabrication process is then applied to the structure, in which the Si is thinned down to 20–30 μm leaving the grown Si film as the majority of the active material of the structure. We obtain a conversion efficiency of 14.4% for such a thin film Si solar cell on glass.

17.6% efficient multilayer thin-film silicon solar cells deposited on heavily doped silicon substrates

We report new results for multilayer thin-film silicon solar cells deposited onto electronically inert, heavily doped crystalline silicon substrates. The n-p-n-p-n active layers of a total thickness of 17 μm combined with a 15-μm thick p+-type buffer layer were deposited by chemical vapour deposition epitaxially onto a 1019 cm−3 doped Czochralski-grown silicon substrate. The cells fabricated using these layers exhibit an energy conversion efficiency of up to 17.6%, as measured by Sandia National Laboratories, which is the highest efficiency ever achieved for a thin-film silicon cell deposited onto such an electronically inert crystallographic template. An open-circuit voltage of 664.2 mV is also reported, the highest ever for a cell on such substrates.

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

Abstract A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p++-type CZ silicon substrates. Growth was performed from In Ga solutions, and in a purified Ar 4% H 2 forming gas ambient, rather than pure H2. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm2. Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.

The growth and properties of liquid phase epitaxial silicon in a forming gas ambient

This paper reports the liquid phase epitaxial (LPE) growth of silicon in a purified ArH2 (> 10% H2) forming gas ambient and the electronic properties of the thin films. The epi-layer morphologies on both (111) and (100) oriented silicon substrates are very similar to those of epi-layers grown from pure hydrogen except that there are some shallow pits appearing on the epi-layer surface. These pits are believed to originate from the contaminants on the substrate surface. Hall mobilities of the LPE layers match quite well with the mobilities of LPE layers and bulk crystals reported by other groups. Thin film silicon solar cells fabricated from LPE layers grown on very heavily doped substrates have demonstrated an efficiency of 16.4% indicating that the LPE layers were of quite good quality.

High-efficiency drift-field thin-film silicon solar cells by liquid-phase epitaxy and substrate thinning

Silicon films of 20-35 /spl mu/m thickness were grown by LPE on electrically inactive heavily doped p/sup ++/-type CZ silicon substrates from In/Ga solutions. A Ga doping gradient was introduced throughout the film to produce a drift-field in the base of the solar cell, thus enhancing the effective minority-carrier diffusion length and increasing the long-wavelength response. An independently confirmed efficiency of 16.4% was achieved on an LPE drift-field thin-film silicon solar cell with total cell area of 4.11 cm/sup 2/. Substrate thinning, combined with light-trapping, is demonstrated to improve the long-wavelength response of cells and further increase the efficiency of the LPE cells. Cell efficiency increased by a factor of up to 23.7% when thinned to a total cell thickness of 30 /spl mu/m.

Very low light-reflection from the surface of incidence of a silicon solar cell

This paper reports very low light-reflection from the surface of incidence of a silicon solar cell. The measured hemispherical front surface light-reflectance of our epi thin film silicon solar cell is 1% over the optimum wavelength range 560 to 860 nm and below 2% in the range 440 to 960 nm. These reflectances are the lowest ever achieved for any silicon solar cell. The low reflection has resulted in a 7.9 mA/cm2 higher current density and a 4.5% higher efficiency than those of our best thin film silicon solar cell prior to the optimisation described.

High-efficiency CVD multi-layer thin-film silicon solar cells

We present new results for high-efficiency multilayer thin-film silicon solar cells deposited by CVD (chemical vapour deposition) onto electrically inactive p/sup ++/-type CZ silicon substrates. The cell isolation and the substrate thinning applied to the cells can enhance the short-circuit current density, open-circuit voltage, the fill-factor, and hence achieve higher energy-conversion efficiency. For n-p-n-p-n active layers of a total thickness of 17-/spl mu/m combined with a 15-/spl mu/m-thick p/sup +/-type buffer layer, the resulting CVD multi-layer thin-film silicon solar cells exhibit a V/sub oc/ of 664.2 mV, a J/sub sc/ of 32.9 mA/cm/sup 2/, a FF of 82.5%, and an energy-conversion efficiency of 17.6%, as measured by Sandia National Laboratories, which is the highest efficiency ever reported for thin-film silicon solar cells deposited onto an inert substrate.