Joachim Bergmann

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.

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.

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.

A new technology for crystalline silicon thin film solar cells on glass based on laser crystallization

A technology is proposed to prepare crystalline silicon thin film solar cells on glass as a superstrate. In a first step an a-Si:H layer is deposited by PECVD onto borosilicate glass. By scanning an Ar/sup +/-laser beam, this layer is crystallized with grains several 10 /spl mu/m in size and is at the same time p/sup +/-doped by boron from the glass so that a transparent electrode layer is formed. In the next step further a-Si is deposited and repeatedly irradiated by an excimer laser during deposition. In this way, a p-absorber layer is grown epitaxially from the underlying electrode which is acting as a seed layer. Finally, by excimer laser doping, a n/sup +/-emitter is fabricated to result in a p/sup +/-p-n/sup +/-layer sequence. Onto the silicon, a metal is deposited as the second electrode acting as a back reflector. Results on the characterization of the different layers are presented with the emphasis on crystallographic and chemical properties. Challenges in preparing the proposed layer sequence are discussed.

Optimization of layered laser crystallization for thin-film crystalline silicon solar cells

Layered laser crystallization during PECVD of a-Si:H is a new and advantageous method to deposit c-Si films onto glass with a rate of 10 ( As � 1 . This new technology consists of two laserinduced crystal growth steps: A seed layer is prepared from a-Si:H by an overlapped scanning of an Ar + laser beam. Then the seed is repeatedly thickened by melting of newly deposited aSi:H on top of the c-Si with KrF laser pulses. Various deposition parameters were matched together to process a layer with crystallites 100mm in size. p + pn + -o r n + np + -junctions were deposited in one chamber and in one run. VOC of 530 mV was achieved. r 2002 Elsevier Science B.V. All rights reserved.

Laser induced crystallization: a method for preparing silicon thin film solar cells

The preparation of coarse grained silicon thin films on glass suited for solar cells is investigated. The authors start from amorphous hydrogenated silicon thin films several 100 nm thick deposited on glass. Different laser crystallization processes are discussed. By an Ar/sup +/ laser scan, the amorphous layer is melted and recrystallized to grains several 10 /spl mu/m in size. Alternatively an explosive crystallization process was applied in which the film is preheated to 1000/spl deg/C by an Ar/sup +/ laser. The explosive crystallization is induced by an additional Nd:YAG laser pulse. In this process, at any position, the melt exists only for some ns. Grains of several /spl mu/m in length were produced. The films were thickened to several /spl mu/m by simultaneous deposition of further a-Si:H and in situ epitactic crystallization applying repeated excimer laser pulses.

Multicrystalline Silicon Thin Film Solar Cells Based on Laser Crystallized Layers on Glass

Multicrystalline silicon thin film cells were prepared by the LLC (layered laser crystallization) process on a glass superstrate. In this process an a-Si layer is crystallized by a cw laser resulting in a seed layer with grains exceeding 100 mum in size. Subsequently this seed layer is epitaxially thickened by simultaneous deposition of a-Si and crystallization by repeated pulses of an excimer laser. Then a phosphorus doped emitter is added. In this paper cells which are prepared by a single chamber PECVD laboratory type process are compared to devices prepared by an industrially relevant multi-chamber process based on high power diode laser crystallization and high rate electron beam evaporation, respectively. Crystallization with the diode laser resulted in significantly larger grains. However, solar cell performance does not quite reach the values of cells prepared by the laboratory process

Photovoltaic properties of polycrystalline silicon films thickened by laser melting

During RF CVD of a-Si:H onto glass an Ar/sup +/ laser beam was scanned to crystallize a seed layer. Subsequently every 20 nm new deposited a-Si:H was molten by one light pulse from a KrF laser, so that the laser processing did not take any additional time. A highly conducting p/sup +/ region close to the glass as transparent electrode and a p/sup +/pn junction within the film were deposited without a doping gas. REM, TEM images and XRD measurements confirmed an epitaxial growth of large crystallites. The self structured silicon/glass interface and film surface show increased scattering which supports light absorption. Transients of open circuit voltage after a pulsed generation by UV light at the p/sup +/, as well as at the n side, were contactless sensed to evaluate the PV quality of films as deposited. A lifetime of more than 5 /spl mu/s and a diffusion length for holes of more than 25 /spl mu/m were determined.

Multicrystalline LLC-silicon thin film cells on glass

In a one chamber process multicrystalline silicon thin film solar cells with crystallites in the range of 10 to more than 100 /spl mu/m were deposited on uncoated glass by layered laser crystallization (LLC). During PECVD deposition of a-Si:H, laser crystallization was performed in the deposition chamber. A 400 nm thick seed layer simultaneously acting as transparent electrode was crystallized by scanning an Ar/sup +/-laser beam. Epitaxial thickening by applying repeated pulses of an KrF excimer laser was performed during further a-Si:H deposition. p/sup +/-p-n/sup +/ cells with 3 /spl mu/m thick absorber without reflector and without antireflection coating showed V/sub oc/ = 425 mV, I/sub sc/ = 9.8 mA/cm/sup 2/, FF = 55%, and n = 2.3%.

Temperature and hydrogen diffusion length in hydrogenated amorphous silicon films on glass while scanning with a continuous wave laser at 532 nm wavelength

Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring the silicon melting point and with calculated data using a physical model as well as published work. Since D-H interdiffusion measures hydrogen diffusion length and temperature within the silicon films by a memory effect, the method is capable of determining both quantities precisely also in multilayer structures, as is demonstrated for films underneath metal contacts. Several applications are discussed. Employing literature data of laser-induced temperature rise, laser scanning is used to measure the H diffusion coefficient at T > 500 °C in a-Si:H. The model-based high temperature hydrogen diffusion parameters are confirmed with important implications for the understanding of hydrogen diffusion in the amorphous silicon material.Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring ...