Richard Auer

Electroluminescence imaging of organic photovoltaic modules

We report on electroluminescence (EL) imaging of organic photovoltaic cells and modules with poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester as semiconductor layer. The dominant EL emission is found in a spectral regime between 1200 and 1400 nm and is identified as the radiative decay of the charge transfer complex formed between the polymer and the fullerene. Electroluminescence emission from the pristine compounds is either much weaker or completely absent. Overall, electroluminescence imaging is shown to give valuable information on the defects but also on the performance of organic solar modules.

Impact of moisture on PV module encapsulants

Water vapor is one of the major reasons for corrosion and aging within photovoltaic modules. We investigated different encapsulants for photovoltaic devices in respect of their water vapor transmission rate (WVTR), diffusion profile and saturation concentration for varied climatic conditions (temperature and relative air humidity). For measuring the WVTR a gravimetric testing procedure was used, the diffusion profile was detected by infrared-absorption-spectroscopy in the wavelength range of 1.7 μm to 2.9 μm. The tested materials are a fast-cure ethylene vinyl acetate (EVA fc), a poly vinyl butyral (PVB), a thermoplastic polyurethane (TPU), an ionomer (ION) and a thermoplastic silicone (TSI). It was ascertained that the thermoplastics foils (TPU and TSI) have the highest WVTR, the ionomer the lowest. The saturation concentration of PVB was the highest at all, followed by TPU and EVA. The silicone and the ionomer store practically no water.

Crystalline thin-film Si cells from layer transfer using porous Si (PSI-process)

We fabricate thin-film Si cells by photolithography-free layer transfer processes. The confirmed power conversion efficiency of a 25.5 /spl mu/m-thick cell that is textured with random upright pyramids is 15.4 %. Upright pyramids are, however, difficult to form in ultra-thin Si films. We therefore introduce an alternative process that yields random inverted pyramids on the front surface without chemical etching of the thin epitaxial film. This novel process applies an epitaxial emitter to speed up the fabrication process when compared to a thermally diffused emitter.

Spatial Uniformity of Large-Area Silicon Layers (43 × 43 cm2) Grown by Convection-Assisted Chemical Vapor Deposition

Silicon layers have been grown by convection-assisted chemical vapor deposition (CoCVD) on assembled substrates with a rectangular area of 43 cmtimes43 cm. First cells have been produced from these epitaxially grown layers with efficiencies exceeding 11%. The layers were investigated with respect to layer thickness, dopant incorporation and uniformity within the deposited area. The experiments show that the spatial distribution of the Si-layer growth rate in vapor flow direction depends strongly on the tilt angle of the reactor. At an optimum angle, a variation of less than plusmn10% in layer thickness was obtained. Profiles of dopant incorporation correlate with growth rate profiles. The effect of a non-uniform absorber thickness on a typical photovoltaic device was calculated

Monolithically series-connected crystalline Si wafer cells for portable electronic devices

A simple process sequence for the fabrication of solar mini-modules from crystalline Si wafers for portable electronic devices, e.g. wrist watches, is presented. We apply a process, that we originally developed for modules from thin-film crystalline Si, to Si wafers for the first time. Its main features are the single-side-single-step (4S)-metallization and a busbar-only contacting scheme. First prototypes consisting of six series-connected cells show efficiencies up to 11% and an open circuit voltage of 3.624 V when measured under one sun AM1.5G-illumination. Fill factor and short circuit current density are 77% and 23.6 mA/cm/sup 2/, respectively. A multitude of mini-modules is processed on a single wafer in parallel.

Material Properties of Laser-Welded Thin Silicon Foils

An extended monocrystalline silicon base foil offers a great opportunity to combine low-cost production with high efficiency silicon solar cells on a large scale. By overcoming the area restriction of ingot-based monocrystalline silicon wafer production, costs could be decreased to thin film solar cell range. The extended monocrystalline silicon base foil consists of several individual thin silicon wafers which are welded together. A comparison of three different approaches to weld 50 μm thin silicon foils is investigated here: (1) laser spot welding with low constant feed speed, (2) laser line welding, and (3) keyhole welding. Cross-sections are prepared and analyzed by electron backscatter diffraction (EBSD) to reveal changes in the crystal structure at the welding side after laser irradiation. The treatment leads to the appearance of new grains and boundaries. The induced internal stress, using the three different laser welding processes, was investigated by micro-Raman analysis. We conclude that the keyhole welding process is the most favorable to produce thin silicon foils.

Electrical Characterization of Silicon Layers Grown by Convection-Assisted Chemical Vapour Deposition (CoCVD)

In this work, we present the electrical properties of thin epitaxial silicon layers grown by convection-assisted chemical vapour deposition (CoCVD). Mono-crystalline (100)-orientated Fz silicon wafers were used as test substrates. The thin layers were investigated by four-point-probing and Hall-measurement. Structural information was obtained by chemical etching with a Secco etchant. First solar cells with efficiencies of 11% were manufactured on 8.5mum-thick Si film and characterized