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Thin amorphous Silicon Films

In order to decrease recombination at the amorphous/crystalline silicon heterojunction and thereby increase solar cell efficiency, it is important to obtain information on the Fermi level position and the distribution of recombination active defects in the band gap of the ultra-thin undoped and doped a-Si:H films which are used in such cells as buffer and emitter-/BSF layers, respectively. Photoelectron spectroscopy with excitation in the near-UV spectral range (NUV-PES) can provide such information, especially in the so-called “Constant Final State“ mode.

enlarged view

Fig. 1: Density of occupied states in 10nm thin a-Si:H films with varying doping, as measured with near-UV photoelectron spectroscopy in the “Constant Final State“ mode and normalized to N(E-EV=0)=2×1021 cm-3 eV-1. EV was obtained by fitting an a-Si:H model density of states to the measured data. Arrows mark the position of the Fermi energies EF.