J. Fölsch

Microcrystalline Silicon-Germanium Alloys for Absorption Layers in Thin Film Solar Cells

Thin microcrystalline silicon-germanium films (μ-Si l.x Ge x :H) prepared by PECVD at 95 MHz have been investigated. The optical absorption of these films increases in the infrared spectral region with increasing germanium content. In addition to the shift of the indirect gap an increase of the absorption coefficient above the band edge is observed. The material shows high crystallinity and exhibits good structural quality similar to pure μ-Si:H films. The films are homogeneous on a macroscopic to a microscopic scale as confirmed by Raman spectroscopy and Electron Microscopy methods. p-i-n solar cells with pc-Si l-x Ge x :H i-layers have been prepared for the first time. An efficiency of η = 3.1 % under AM1.5 has been obtained for a cell with 150 nm thin i-layer.

AMORPHOUS SILICON BASED NIPIIN STRUCTURE FOR COLOR DETECTION

Hydrogenated amorphous silicon based nipiin three color detectors with a bias voltage controlled spectral response have been fabricated. These band-gap and mobility-lifetime product engineered structures employed as two terminal devices exhibit a dynamic range above 95 dB. The maximum of the spectral response shifts by variation of the applied voltage. Three linearly independent spectral response curves can be extracted to generate a red-green-blue signal. Conventional spatial color separation with optical filters is transferred into a voltage multiplexed read out sequence. Bias voltage switching under different monochromatic illumination and illumination switching-on transients for different bias voltages are carried out to investigate the time dependent behavior of the photocurrent. Based on these results optimization criteria to accelerate the transient behavior and to determine the maximum frame rate for color detection are presented.

Transient photocurrent response of a-Si:H based three-color nipin detector

Bandgap and defect engineered amorphous silicon based nipin photo diodes can be used as color detectors. Changing the applied voltage from -1.5 V to -0.6 V and 1.0 V shifts the responsivity from red, to green, to blue, respectively. Wavelength dependent voltage switching and switching-on the illumination experiments are carried out to investigate the transient behavior and to determine the frame rate for color detection. While the transient response after bias switching depends on the trapped charge in the device, the transients after switching-on the light is strongly influenced by the generation profile.

Four terminal color detector for digital signal processing

A novel color sensor realized by a four terminal device has been developed. The color detector based on three stacked pin diodes exhibits good spectral separation with maxima of the spectral response at 450, 565 and 640 nm, respectively. To discuss the sensor performance, optical simulations of the multi-layer structure are carried-out and compared with measured reflectance and spectral response data. Since the red, green and blue signal can be generated simultaneously at the same spatial detector position, the color moire effect can be prevented. Due to the vertical integration of the multi-layer system, the novel sensor is ideal for applications such as digital photographs or digital image processing.

Role of bandgap grading for the performance of a-SiGe:H based solar cells

The influence of bandgap grading and bandgap discontinuities on the performance of a-SiGe:H based multi-junction solar cells is investigated. Different types of bandgap grading of the i-layer, like linear profiling in asymmetrical v- and u-form, are studied with respect to their influence on the solar cell parameters under white, red and blue light illumination. Additionally, the cell series with different bandgap designs are light soaked under filtered red light for 300 h and 3000 h. Optimum performance is found for an asymmetrical v-shape with a band gap minimum close to the p/i interface. Using the concept of bandgap grading, a-Si:H/a-SiGe:H tandem and a-Si:H/a-Si: H/a-SiGe:H triple solar cells have been fabricated with efficiencies above 10%.

Defect distributions in a-SixGe1−x:H

Gap states in a-SiGe:H alloys were examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. In contrast to simple deconvolution methods our analysis uses occupation statistics and takes into account the condition of charge neutrality. The simulations yield information on the energy distribution and the charge state of the defects. The results reveal the coexistence of charged and neutral defects. The defect distributions are similar to those found in amorphous hydrogenated silicon. In the investigated range of compositions charged states dominate the defect density. Taking the position of the defect states as a reference level both band edges shift towards the defect states with decreasing band gap. In contrast to electron spin resonance measurements, no evidence for a distinction between Si-related and Ge-related defect states can be found in sub-bandgap absorption spectra.