[show abstract][hide abstract] ABSTRACT: This study demonstrates an innovative approach to clean a polycrystalline seed layer surface for solid phase epitaxy of amorphous silicon (a-Si). The excimer laser cleaning (ELC) makes use of in situ excimer laser irradiation during the first stages of a-Si deposition leading to melting of the as deposited a-Si and parts of the seed layer. The increased diffusion in liquid silicon allows for “smearing out” of surface contamination species. After liquid phase epitaxy is finished, further a-Si is deposited and crystallized by solid phase epitaxy. Thanks to the “smearing out,” the interface between the crystalline and amorphous silicon exhibits less contamination locally. SIMS measurements demonstrate a reduced carbon concentration at the seed layer surface after ELC. Numerical simulations, taking into account heat transfer and temperature dependent diffusion, support the reduction of carbon concentration. The simulations agree very well with experimental results. To get optimal solid phase growth conditions, the a-Si deposition temperature has to be below 200 °C. Above 200 °C deposition temperature, defective growth occurs resulting in poor crystallinity or even in nonepitaxial growth.
physica status solidi (a) 12/2013; 210(12):2729–2735. · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Polycrystalline silicon thin-film solar cells produced by continuous-wave diode-laser crystallization at the University of New South Wales were recently reported to have reached a conversion efficiency above 10%. One drawback of these cells, however, was that they exhibited efficiency degradation within several hours after the cell fabrication was completed. In this work we show that by applying laser firing to the rear point contacts of the solar cells, it is possible to stabilize and even to enhance the performance of these devices. Our investigation indicates that it is the poor quality of the contact between the aluminum and the silicon absorber that causes the cell degradation and offers an elegant and industrial-compatible process to improve the cell performance. This is the first time that the laser firing process, initially developed for alloying an aluminum layer through a dielectric layer on crystalline silicon wafer solar cells, is being applied to polycrystalline silicon thin-film solar cells.
Solar Energy Materials and Solar Cells 11/2013; 120(Part B):521-525. · 4.63 Impact Factor
[show abstract][hide abstract] ABSTRACT: An experimental setup is presented to measure and interpret the solid phase crystallization of amorphous silicon thin films on glass at very high temperatures of about 800°C. Molybdenum-SiO<sub>2</sub>-silicon film stacks were irradiated by a diode laser with a well-shaped top hat profile. From the relevant thermal and optical parameters of the system the temperature evolution can be calculated accurately. A time evolution of the laser power was applied which leads to a temperature constant in time in the center of the sample. Such a process will allow the observation and interpretation of solid phase crystallization in terms of nucleation and growth in further work.
[show abstract][hide abstract] ABSTRACT: Multicrystalline silicon films up to 2 μm thick with grain sizes up to 100 μm were prepared on glass substrates by laser crystallization followed by solid phase epitaxy of electron beam deposited amorphous silicon (a-Si) at 600 °C. The dependence of the epitaxial growth rate on the crystallographic orientation was investigated. While grains with <1 0 0> orientation with respect to the surface normal show the highest growth rate, <1 1 1>-grains tend to grow the slowest. Furthermore, we studied the kinetics of the solid phase growth depending on the deposition conditions of a-Si. For this purpose we implemented a simple measurement system that determines the transmittance of the c-Si/a-Si layer stack during furnace annealing at a wavelength of 808 nm. Fastest growth is obtained for a-Si deposited at highest rates at a temperature of 300 °C. Further increase of the deposition temperature prevents epitaxy. Interface cleaning deserves particular care since contaminations at the interface lead to a retardation time for solid phase epitaxy.
Journal of Crystal Growth 02/2013; 364:164–168. · 1.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: The temperature dependent optical parameters n and k of amorphous silicon deposited by electron beam evaporation were determined at the wavelength of 808 nm. This was achieved by fitting an optical model of the layer system to reflection values of a fs-laser beam. From n(T) and k(T) the absorption of a-Si layers as depending on thickness and temperature were calculated for this diode laser wavelength. By heating the layers to 600 °C the absorption can be increased by a factor of 4 as compared to room temperature, which allows for diode laser crystallization of layers down to 80 nm in thickness.
[show abstract][hide abstract] ABSTRACT: High temperature solid phase epitaxial crystallization of amorphous silicon layers prepared by electron beam evaporation is investigated. By using a continuous wave diode laser for heating the films rapidly (in milliseconds to seconds) this method is suitable on glass substrates with low temperature resistance. Therefore, the method is an economically advantageous technique of producing absorber layers for thin film solar cells. For the experiments 500 nm of amorphous silicon was deposited on two different configurations of substrates. In the first one monocrystalline wafers of three different crystallographic orientations were used. In the second one a polycrystalline seed layer prepared on borosilicate glass served as substrate. The crystallization process was monitored in situ by time resolved reflectivity measurements. Depending on the crystal orientation 2 s to 3 s was needed for complete solid phase epitaxial crystallization of the amorphous films. The evolution of temperature during crystallization was simulated numerically.
Thin Solid Films 08/2012; 520:7087-7092. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Multicrystalline silicon thin films with large grains (100 μm) on glass are useful for thin film solar cells and for thin film transistors. These films can be prepared by diode laser irradiation of amorphous silicon films, which previously had been deposited by PECVD or electron beam evaporation. In this paper, the applicability of sputtered amorphous silicon thin films on glass for diode laser crystallization is investigated. For this process the sputter gas content in the films and their optical properties, particularly the absorption for the diode laser wavelength of 808 nm, are crucial. The sputtering parameters gas pressure and power are optimized for increasing the optical absorption and for decreasing the sputter gas incorporation. It is demonstrated that if using optimum deposition parameters diode laser crystallization leads to large grained multicrystalline silicon thin films. The crystallized layers show, different to previous results, a preferred (100)-orientation.
[show abstract][hide abstract] ABSTRACT: Core-shell silicon nanowire (SiNW) solar cells with an a-Si heterojunction were prepared on SiNW arrays, which were etched into n-type silicon wafers or into n-doped multicrystalline silicon thin films on glass substrates. A stack of intrinsic and p-doped hydrogenated a-Si was deposited as a shell around the SiNWs by PECVD, acting as a heteroemitter of the solar cells. Finally a TCO layer consisting of aluminum doped zinc oxide was deposited on top of the a-Si by atomic layer deposition. In a mesa-structured solar cell (area 7 mm2) an open circuit voltage of 476 mV and an efficiency of 7.3% were achieved under AM 1.5 illumination. Electron beam induced current measurements show clear evidence that most of the photo-current comes from the thin SiNW layer.
[show abstract][hide abstract] ABSTRACT: Silicon nanowires have been introduced into P3HT:PCBM solar cells, resulting in hybrid organic/inorganic solar cells. A cell efficiency of 4.2% has been achieved, which is a relative improvement of 10% compared to a reference cell produced without nanowires. This increase in cell performance is possibly due to an enhancement of the electron transport properties imposed by the silicon nanowires. In this paper, we present a novel approach for introducing the nanowires by mixing them into the polymer blend and subsequently coating the polymer/nanowire blend onto a substrate. This new onset may represent a viable pathway to producing nanowire-enhanced polymer solar cells in a reel to reel process.
[show abstract][hide abstract] ABSTRACT: Grain size, grain boundary population, orientation distribution and lattice defects of polycrystalline silicon thin films are investigated by electron backscatter diffraction (EBSD). The silicon thin films are produced by a combination of diode laser melt-mediated crystallization of an amorphous silicon seed layer and epitaxial thickening of the seed layer by solid phase epitaxy (SPE). The combined laser-SPE process delivers grains exceeding several 10 μm of width and far larger than 100 μm in length. Strong lattice rotations between 10 and 50° from one side of the grain to the other are observed within the larger grains of the film. The misorientation axes are well aligned with the direction of movement of the laser. The intragranular misorientation is associated both with geometrically necessary dislocations and low angle boundaries, which can serve as recombination centres for electron-hole pairs. Since the lateral grain size is up to two orders of magnitude larger than the film thickness, the high dislocation density could become an important factor reducing the solar cell performance.
Thin Solid Films 01/2010; 519(1):58-63. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Silicon nanowire (SiNW)-based solar cells on glass substrates have been fabricated by wet electroless chemical etching (using silver nitrate and hydrofluoric acid) of 2.7 microm multicrystalline p(+)nn(+) doped silicon layers thereby creating the nanowire structure. Low reflectance (<10%, at 300-800 nm) and a strong broadband optical absorption (>90% at 500 nm) have been measured. The highest open-circuit voltage (V(oc)) and short-circuit current density (J(sc)) for AM1.5 illumination were 450 mV and 40 mA/cm(2), respectively at a maximum power conversion efficiency of 4.4%.
[show abstract][hide abstract] ABSTRACT: The fabrication of silicon nanowire-based solar cells on silicon wafers and on multicrystalline silicon thin films on glass is described. The nanowires show a strong broadband optical absorption, which makes them an interesting candidate to serve as an absorber in solar cells. The operation of a solar cell is demonstrated with n-doped nanowires grown on a p-doped silicon wafer. From a partially illuminated area of 0.6 cm(2) open-circuit voltages in the range of 230-280 mV and a short-circuit current density of 2 mA cm(-2) were obtained.
[show abstract][hide abstract] ABSTRACT: Silicon nanowires grown by the vapor-liquid-solid (VLS) mechanism catalyzed by gold show gold caps (droplets) approximately 20-500 nm in diameter with a half spherical towards almost spherical shape. These gold droplets are well suited to exploit the surface-enhanced Raman scattering (SERS) effect and could be used for tip-enhanced Raman spectroscopy (TERS). The gold droplet of a nanowire attached to an atomic force microscopy (AFM) tip could locally enhance the Raman signal and increase the spatial resolution. Used as a SERS template, an ensemble of self-organizing nanowires grown bottom up on a silicon substrate could allow highly sensitive signal-enhanced Raman spectroscopy of materials that show a characteristic Raman signature. A combination of a nanowire-based TERS probe and a nanowire-based SERS substrate promises optimized signal enhancement so that the detection of highly dilute species, even single molecules or single bacteria or DNA strands, and other soft matter is within reach. Potential applications of this novel nanowire-based SERS and TERS solution lie in the fields of biomedical and life sciences, as well as security and solid-state research such as silicon technology.
[show abstract][hide abstract] ABSTRACT: Residual mechanical stress is currently among the most important parameters for improving processing yields and it will become increasingly important in view of the targeted decrease in the wafer or thin film thicknesses to reduce processing costs. A technique that is intensively used for analysing stress states at the micrometer scale in semiconductors is micro-Raman spectroscopy. Several stress-tensor components can be obtained separately using polarized laser light for the µ-Raman measurements or average stresses when using unpolarized light. We have found inhomogeneous stress distributions including local peak compressive and tensile stresses at (sub)grain boundaries and in areas with high concentrations of extended lattice defects. These stresses most probably form during the different heating and cooling processes of the multicrytalline silicon block and ribbon casting or during the laser melting and re-crystallization of amorphous silicon layers on thermally mismatched glass substrates. The possible mechanisms of stress formation and relaxation are discussed.