Fritz Falk

Silicon Nanowire-Based Solar Cells on Glass: Synthesis, Optical Properties, and Cell Parameters

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%.

Laser Induced Crystallization of Amorphous Silicon Films on Glass for Thin Film Solar Cells

Two different methods of laser induced crystallization for preparing large grained polycrystalline silicon thin films on glass are reported. The first one is a lateral epitactic crystallization process following melting by an Ar+ laser. The second one is an explosive crystallization process. Both methods lead to crystal grains of several 10 μm in size. The films, 200 to 500 nm thick, may be used as a seed layer for an epitactic thickening process leading to solar cells.

PEDOT:PSS emitters on multicrystalline silicon thin-film absorbers for hybrid solar cells

We fabricated an efficient hybrid solar cell by spin coating poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) on planar multicrystalline Si (mc-Si) thin films. The only 5 μm thin Si absorber layers were prepared by diode laser crystallization of amorphous Si deposited by electron beam evaporation on glass. On these absorber layers, we studied the effect of SiOx and Al2O3 terminated Si surfaces. The short circuit density and power conversion efficiency (PCE) of the mc-Si/Al2O3/PEDOT:PSS solar cell increase from 20.6 to 25.4 mA/cm2 and from 7.3% to 10.3%, respectively, as compared to the mc-Si/SiOx/PEDOT:PSS cell. Al2O3 lowers the interface recombination and improves the adhesion of the polymer film on the hydrophobic mc-Si thin film. Open circuit voltages up to 604 mV were reached. This study demonstrates the highest PCE so far of a hybrid solar cell with a planar thin film Si absorber.

Preparation of single crystalline regions in amorphous silicon layers on glass by Ar+ laser irradiation

Abstract By melting amorphous silicon layers on glass by the beam of an Ar+ laser, large grained polycrystalline films as well as single crystalline regions at predefined positions were generated. If the layers are crystallized by scanning a circular laser beam at a rate of up to 5 cm/s the crystal size depends on the overlap between successive scanning traces. The lateral dimensions of the crystals exceed several 10 μm for an overlap slightly above 50%. Crystals with size dimensions of about 100 μm were produced by line scanning of a focused laser beam. Large single crystals were obtained by scanning a sickle-shaped or L-shaped beam profile. If the laser is switched on and off repeatedly, single crystalline regions are produced at predefined positions.

Preparation of thick polycrystalline silicon layers on glass by laser irradiation

Abstract For polycrystalline silicon thin film solar cells a silicon layer 50 μm thick is required consisting of grains 100 μm in diameter deposited on low cost glass substrate. We report on a preparation method combining plasma enhanced CVD of amorphous silicon and laser crystallization. We start from a-Si:H thin films 200 nm thick which are deposited on glass (Corning 7059) by a rf-CVD process. These films are irradiated by scanning with an Ar+ laser to result in crystals of several 10 μm in diameter. In order to increase the film thickness on this crystalline seed layer further amorphous silicon is deposited by the same CVD process at a rate of 20 nm/min. During the deposition the growing layer is irradiated by excimer laser pulses with about 300 mJ/cm2 at a repetition rate of less than 0.1 Hz. Each laser pulse melts the newly deposited amorphous layer down to the crystalline interface which acts as a homoepitactic substrate during resolidification. In this way the whole growing amorphous layer is converted to a polycrystal.

Silicon Nanowire Solar Cells With Radial p-n Heterojunction on Crystalline Silicon Thin Films: Light Trapping Properties

We present a concept for a core-shell silicon nanowire thin-film solar cell showing strong light trapping. Nanowires are wet chemically etched into a several micrometer-thick laser-crystallized silicon thin film on glass. The nanowires are equipped with an a-Si heteroemitter deposited as a shell around the nanowires by plasma-enhanced chemical vapor deposition to achieve a radial p-n heterojunction. The space between the nanowires is filled with ZnO:Al, acting as a transparent contact. Our core-shell nanowire solar cells reached an efficiency of 8.8%. The main emphasis of this study is on the optical properties of the nanowire solar cell system.

Deposition of SiC and AlN thin films by laser ablation

Abstract SiC and AlN thin films were deposited on silicon substrates by KrF excimer laser ablation from SiC and AlN targets. They were characterized by FTIR and Raman spectroscopy, X-ray diffraction, Auger electron spectroscopy, scanning and transmission electron microscopy. Improvements in crystalline SiC thin film deposition by laser ablation combined with laser surface activation were reported recently [M. Diegel, F. Falk, H. Hobert, R. Hergt, H. Stafast, Appl. Phys. A, Vol. 66, 1998, p. 183]. These investigations were extended by using an intermediate layer of AlN. Crystalline AlN films were deposited on Si at a substrate temperature of 750°C. Laser activation of the film surface during deposition deteriorates the AlN film properties. As next step SiC was deposited onto the AlN layers. At the relatively low temperature of 800°C an AlN buffer layer improves the quality of SiC films. Modellings of IR reflection spectra revealed that the SiC/AlN films on Si(111) can be simulated by optical methods. For both materials high deposition rates of approximately 50 nm/min at a moderate substrate temperature were obtained. Thus, film preparation for optical applications where a film thickness of some 100 nm is required seems to be realistic.

In-situ diagnostics for preparation of laser crystallized silicon films on glass for solar cells

Polycrystalline silicon thin film solar cells require coarse grained silicon layers on a glass substrate. The preparation starts with a layer of amorphous silicon some hundred nanometers thick. By laser crystallization it is converted into a seed layer consisting of grains several ten μm in size. We report on in situ diagnostics by time resolved reflection and transmission (TRRT) measurements during the preparation process. Joint diagnostics by different lasers and the comparison with optical and electron micrographs of the resulting films give unique information about the crystallization processes. Even if different processes occur in hydrogenated or hydrogen free amorphous silicon films during the heating induced by different irradiation parameters the results of crystallization are quite similar.

Electron backscatter diffraction on femtosecond laser sulfur hyperdoped silicon

This paper analyzes the impact of femtosecond laser pulse irradiation on the crystallinity of silicon wafers by means of electron backscatter diffraction (EBSD) measurements. EBSD based image quality maps and orientation imaging microscopy maps are correlated to the grade of the silicon crystallinity. We analyze the impact of accumulated net laser irradiation originating from a laser spot overlap that is necessary to process macroscopic areas, e.g., for sulfur doping of semiconductor devices. Furthermore, we demonstrate that post processing annealing recovers crystallinity and therefore allows fs-laser processed silicon to be used in semiconductor device manufacturing.

Temperature dependent optical properties of amorphous silicon for diode laser crystallization.

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.