Per I. Widenborg

Polycrystalline Silicon Thin-Film Solar Cells on AIT-Textured Glass Superstrates

A new glass texturing method (AIT—aluminium-induced texturisation) has recently been developed by our group. In the present work, the potential of this method is explored by fabricating PLASMA poly-Si thin-film solar cells on glass superstrates that were textured with the AIT method. Using an interdigitated metallisation scheme with a full-area Al rear contact, PLASMA cells with an efficiency of up to 7% are realised. This promising result shows that the AIT glass texturing method is fully compatible with the fabrication of poly-Si thin-film solar cells on glass using solid phase crystallisation (SPC) of PECVD-deposited amorphous silicon precursor diodes. As such, there are now two distinctly different glass texturing methods—the AIT method and CSG Solars glass bead method—that are known to be capable of producing efficient SPC poly-Si thin-film solar cells on glass.

Integration of β-FeSi2 with poly-Si on glass for thin-film photovoltaic applications

Aluminum-alloyed polycrystalline p-type β-phase iron disilicide p-β-FeSi2(Al) films with different thicknesses are successfully integrated with n-type polycrystalline silicon films on glass for thin-film solar cell applications. A sharp and high-quality interface is formed between 49 nm thick β-FeSi2(Al) and poly-Si, through the formation of a thin layer (∼7 nm) of Al-doped p+ epitaxial Si. The quality of the interface between poly-Si and p-β-FeSi2(Al) is found to degrade with increasing p-β-FeSi2(Al) thickness. An ∼5 nm thick amorphous layer is observed at the interface for the 145 nm thick p-β-FeSi2(Al) layer. The structural and photovoltaic characteristics of the p-type β-FeSi2/p+ Si/n− Si/n+ Si solar cell samples are investigated in detail. For a sample annealed at 650 °C, a one-Sun open-circuit voltage of 320 mV and pseudo fill factor of 67% are obtained, using an ∼49 nm p-type β-FeSi2 film on n-type poly-Si. The efficiency of the investigated solar cell structure decreases with increasing annealing temperature and thickness of the β-FeSi2 film.

Impact of the n+ emitter layer on the structural and electrical properties of p-type polycrystalline silicon thin-film solar cells

The effect of the phosphine (PH3) flow rate on the doping profile, in particular the peak doping concentration of the n+ emitter layer, of solid phase crystallised polycrystalline silicon thin-film solar cells on glass is investigated by electrochemical capacitance-voltage profiling. The peak n+ layer doping is found to increase with increasing PH3 gas flow, resulting in a shift of the p-n junction location towards the centre of the diode. The impact of the PH3 flow rate on the crystal quality of the poly-Si films is analysed using ultraviolet (UV) reflectance and UV/visible Raman spectroscopy. The impact of the PH3 flow rate on the efficiency of poly-Si thin-film solar cells is investigated using electrical measurements. An improvement in the efficiency by 46% and a pseudo energy conversion efficiency of 5% was obtained through precise control of the flow rate at an intermediate n+ emitter layer doping concentration of 1.0 × 1019 cm−3. The best fabricated poly-Si thin-film solar cell is also found to hav...

Identification of geometrically necessary dislocations in solid phase crystallized poly-Si

In this work, the presence of geometrically necessary dislocations (GNDs) in polycrystalline silicon (poly-Si) thin films was detected, suggesting that plastic deformation occurs during the solid phase crystallization (SPC) process of amorphous silicon (a-Si:H). Electron backscatter diffraction was used to characterize dislocations in SPC poly-Si thin films. The elevated temperatures during SPC allow the GNDs to rearrange into arrays, forming low angle grain boundaries. We found that GNDs start forming in poly-Si grains with sizes >∼3 μm, suggesting that larger grains are more defective. GNDs are extra defects in addition to the existing statistically stored dislocations that form during grain growth and hence more care needs to be taken to minimize the formation of GNDs.

Study of large-grained n-type polycrystalline silicon thin films made by the solid phase crystallization method

N-type polycrystalline silicon (poly-Si) films with large grains exceeding 30 μm in width are successfully prepared by the solid phase crystallization (SPC) technique on glass through a control of the PH₃ gas flow rate. The grains in the poly-Si films are investigated by electron backscatter diffraction (EBSD) and are found to be randomly oriented, whereby the average grain size ranges from 4.3 to 18 μm. The grain size in the poly-Si film increases with increasing PH₃ gas flow rate. The impact of the PH₃ flow rate on the crystal quality and the electronic properties of the poly-Si thin-film solar cells are systematically investigated using UV reflectance and Hall effect measurements. A Hall mobility of about 71.5 cm²/Vs for n⁺ doped poly-Si films with a carrier concentration of 2.3×10¹⁹ cm^-3 is obtained.

Medium range order engineering in amorphous silicon thin films for solid phase crystallization

In recent years, it has been recognized that medium range ordering (MRO) in amorphous silicon (a-Si:H) plays a role in controlling its solid phase crystallization (SPC) behavior. Information on the MRO can be obtained from the width of the first X-ray diffraction (XRD) peak of a-Si:H centered around 2θ = 27.5°. The broader the full width half maximum (FWHM) of the first XRD peak, the less ordered the a-Si:H material in the medium range length scale (up to 5 nm). In this work, it was found that the FWHM of the first XRD peak changes with the pressure used during the deposition of a-Si:H. A threshold SPC behavior was observed as a function of the a-Si:H deposition pressure and a good correlation between the SPC behavior and the a-Si:H XRD peak width was found. Results in this study indicate that higher MRO in a-Si:H led to faster SPC rates and smaller grain sizes, suggesting the presence of relatively active and high density of nucleation sites. High angle annular dark field scanning transmission electron m...

Static Large-Area Hydrogenation of Polycrystalline Silicon Thin-Film Solar Cells on Glass Using a Linear Microwave Plasma Source

Hydrogenation of polycrystalline silicon thin-film solar cells on glass is performed to improve the open-circuit voltage <i>V</i>_oc of the devices. The hydrogenation process is performed using linear microwave plasma sources that are capable of generating a uniform hydrogen-argon plasma over a large area. The substrate is fixed (i.e., does not move) during the hydrogenation process. The optical emission intensities from the hydrogen-argon plasma are recorded at two wavelengths using an optical emission spectroscopy system and are used to study the impact of several process parameters. The impact of these process parameters on the devices <i>V</i>_oc is also studied. We demonstrate that this plasma reactor is able to hydrogenate samples with a size of up to 400 cm². The uniformity of the hydrogenation process is evaluated by measuring the 1-sun <i>V</i>_oc with a suns-<i>V</i>_oc tester, giving voltage variations of less than ±3%. The highest average <i>V</i>_oc achieved in this study is 465 mV on a 10 cm × 10 cm textured sample and 428 mV on a 20 cm × 20 cm textured sample. In addition, secondary ion mass spectroscopy is used to measure the hydrogen concentration in the poly-Si films, giving an average concentration of about 6 × 10¹⁹ cm^-3.

Investigation of Interdigitated Metallization Patterns for Polycrystalline Silicon Thin-Film Solar Cells on Glass

In this study, we evaluate the impact of interdigitated metallization patterns on the 1-sun performance of poly-Si thin-film solar cells on glass. We implement a model of the solar cell with an interdigitated metallization pattern in the simulation software Silvaco Atlas. Simulation and experimental results show consistently that the dominant factor for the performance of the metallization pattern is its impact on the current generation capability of the solar cell. For this reason, we study the effect of a variation of the emitter finger pitch, rear contact size, and carrier lifetime on current generation. For a pitch between 500 and 600 μm, the solar cell efficiency varies by less than 0.2% absolute. We also find that the optimum emitter finger pitch does not depend strongly on the charge carrier lifetime in the absorber layer.

Synthesis and characterization of large-grain solid-phase crystallized polycrystalline silicon thin films

n-type polycrystalline silicon (poly-Si) films with very large grains, exceeding 30 μm in width, and with high Hall mobility of about 71.5 cm2/V s are successfully prepared by the solid-phase crystallization technique on glass through the control of the PH3 (2% in H2)/SiH4 gas flow ratio. The effect of this gas flow ratio on the electronic and structural quality of the n-type poly-Si thin film is systematically investigated using Hall effect measurements, Raman microscopy, and electron backscatter diffraction (EBSD), respectively. The poly-Si grains are found to be randomly oriented, whereby the average area weighted grain size is found to increase from 4.3 to 18 μm with increase of the PH3 (2% in H2)/SiH4 gas flow ratio. The stress in the poly-Si thin films is found to increase above 900 MPa when the PH3 (2% in H2)/SiH4 gas flow ratio is increased from 0.025 to 0.45. Finally, high-resolution transmission electron microscopy, high angle annular dark field-scanning tunneling microscopy, and EBSD are used t...

Aluminum induced glass texturing process on borosilicate and soda-lime glass superstrates for thin-film solar cells

In this paper, the aluminum induced texturing (AIT) method for glass sheets is applied to two types of glass used in the thin-film PV industry for superstrate modules: soda-lime glass and borosilicate glass. Upon texturing one surface of the glass sheet with the AIT method, an aluminum-doped ZnO (AZO) film is deposited onto the textured glass surface using magnetron sputtering. The properties of the textured glass sheets are investigated using scanning electron microscopy. Then the optical and electrical properties of the AZO-coated AIT glass sheets are analyzed.