The paper reports on the effects of a proton irradiation campaign on a series of thin-film silicon solar cells (single-and double-junction). The effect of subsequent thermal annealing on solar cells degraded by proton irradiation is investigated. A low-temperature annealing behaviour can be observed (at temperatures around 100 to 160 C) for microcrystalline silicon solar cells. To further explore this effect, a second proton irradiation campaign has been carried out, but this time on microcrystalline silicon layers. The effect of proton irradiation and subsequent thermal annealing on the optical and electronic properties of microcrystalline silicon is, thus, thoroughly investigated.
Microcrystalline silicon (μc-Si:H) layers deposited by the very high-frequency-glow discharge technique at a radio-frequency excitation of 70 MHz are observed to be basically slightly 〈n〉 type. By doping (so-called ‘‘microdoping’’) with boron in the gas phase volume part per million (vppm) range, compensated material could be obtained. The influence of this doping on the electronic transport properties is documented. A pronounced onset of the boron incorporation into the films measured by secondary-ion-mass spectrometry is observed around 3 vppm (B2H6/SiH4), together with marked changes in the electrical properties. The compensated film obtained for a microdoping of about 1 vppm shows the lowest dark conductivity [3×10−8 (Ω cm)−1], the highest activation energy (517 meV), and, finally, the highest photoconductive gain of 6×103 (photo/dark current ratio). Depending on the value of the activation energy (the critical value is ≊0.2 eV), two different transport models are identified, corresponding to ‘‘Meyer–Neldel’’ or ‘‘anti-Meyer–Neldel’’ behavior. As for light-induced degradation, the compensated film exhibits better stability than undoped films. Finally, the use of slightly boron doped μc-Si:H as photovoltaically active material will be discussed.
A 7.7 % single junction cell efficiency for an entirely microcrystalline silicon (µc-Si:H) device has recently been reported by our group [1]. This was achieved by applying the purifier technique, a technique which is indeed easier to handle than the earlier used "microdoping" approach. The purpose of the present paper is twofold: First to show in detail the impact on device performance when a gas purifier is used; and second to illustrate that the deposition rate of the active, absorbing i-layer can be increased from the former 1.55 Å/s up to 4.3 Å/s while still maintaining reasonable device performances. In the latter case a first n-i-p solar cell structure on an aluminium sheet could be fabricated with an efficiency of 4.9 %.
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