Annett Gawlik

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.

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.

Varying the layer structure in multicrystalline LLC-silicon thin-film solar cells

Multicrystalline silicon thin-film solar cells with grains exceeding 100 μm were prepared by layered laser crystallization. The layer system is generated in two steps. In the first step a multicrystalline seed layer is fabricated on a low cost glass substrate. This is achieved by depositing a-Si followed by scanning a diode laser beam for crystallization. In a second step this seed layer is epitaxially thickened by electron beam evaporation of a-Si combined with repeatedly applying pulses of an excimer laser. p⁺pn⁺ and p⁺nn⁺ superstrate cells with 2 μm thick absorber were prepared with different doping levels and different thickness of the seed layer. Without reflector these cells, after hydrogen passivation, delivered V<inf>oc</inf> up to 514 mV and I<inf>sc</inf> of 17.5 mA/cm² if deposited directly onto the glass substrate. With an additional SiN<inf>x</inf> antireflection layer I<inf>sc</inf> reached 20 mA/cm².

Experimental setup for investigating silicon solid phase crystallization at high temperatures

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(2)-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.

Multicrystalline silicon thin film solar cells based on a two-step liquid phase laser crystallization process

We present a technology for preparing multi-crystalline silicon thin film solar cells based on laser crystallization. The technology makes use of high rate electron beam evaporation of amorphous silicon and of liquid phase crystallization by scanning the beam of a line focus high power diode laser. The resulting several μm thick absorber of the solar cell consists of grains sized up to mm. On top of the absorber an epitaxial emitter is prepared by excimer laser crystallization. Our solar cells reached an open circuit voltage of 550 mV and an efficiency of 7.8 %.

Influence of intermediate layers on the surface condition of laser crystallized silicon thin films and solar cell performance

The intermediate layer (IL) between glass substrate and silicon plays a significant role in the optimization of multicrystalline liquid phase crystallized silicon thin film solar cells on glass. This study deals with the influence of the IL on the surface condition and the required chemical surface treatment of the crystallized silicon (mc-Si), which is of particular interest for a-Si:H heterojunction thin film solar cells. Two types of IL were investigated: sputtered silicon nitride (SiN) and a layer stack consisting of silicon nitride and silicon oxide (SiN/SiO). X-ray photoelectron spectroscopy measurements revealed the formation of silicon oxynitride (SiOxNy) or silicon oxide (SiO2) layers at the surface of the mc-Si after liquid phase crystallization on SiN or SiN/SiO, respectively. We propose that SiOxNy formation is governed by dissolving nitrogen from the SiN layer in the silicon melt, which segregates at the crystallization front during crystallization. This process is successfully hindered, when...

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

Fabrication of self-assembled spherical Gold Particles by pulsed UV Laser Treatment

We report on the fabrication of spherical Au spheres by pulsed laser treatment using a KrF excimer laser (248 nm, 25 ns) under ambient conditions as a fast and high throughput fabrication technique. The presented experiments were realized using initial Au layers of 100 nm thickness deposited on optically transparent and low cost Borofloat glass or single-crystalline SrTiO3 substrates, respectively. High (111)-orientation and smoothness (RMS ≈ 1 nm) are the properties of the deposited Au layers before laser treatment. After laser treatment, spheres with size distribution ranging from hundreds of nanometers up to several micrometers were produced. Single-particle scattering spectra with distinct plasmonic resonance peaks are presented to reveal the critical role of optimal irradiation parameters in the process of laser induced particle self-assembly. The variation of irradiation parameters like fluence and number of laser pulses influences the melting, dewetting and solidification process of the Au layers and thus the formation of extremely well shaped spherical particles. The gold layers on Borofloat glass and SrTiO3 are found to show a slightly different behavior under laser treatment. We also discuss the effect of substrates.

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 ...

Progress in single crystalline silicon nanowire solar cells

Silicon nanowire carpets were grown from gold nanotemplates by the Vapor Liquid Solid (VLS) process. As substrates silicon wafers, laser crystallized silicon thin films on glass, or TCO on glass were used. The Au nanotemplates act, in a thermal CVD process, as a catalyst to decompose silane at about 500°C. The result is a carpet formed of perfect single crystalline nanowires several 10 to 200 nm in diameter and several μm in length.