By combining data from temperature- and injection-dependent lifetime spectroscopy (TDLS and IDLS) measured by means of the microwave-detected photoconductance decay technique and the quasi-steady state photoconductance technique, respectively, the exact electronic structure of the metastable defect in standard boron-doped Czochralski (Cz) silicon has been determined. A detailed Shockley–Read–Hall analysis of the entire TDLS curve reveals that the Cz-specific defect acts as an attractive Coulomb center [σn(T)=σn0T−2] which is localized in the upper band-gap half at EC−Et=0.41 eV and has an electron/hole capture cross section ratio k=σn/σp=9.3. The accuracy of this determination manifests itself by the fact that the corresponding IDLS curve can be simulated with the same parameter set.
Several combinations of oxidation and phosphorus diffusion processes suitable for silicon solar cell processing were applied to solar grade multicrystalline silicon. This resulted in drastic changes of the minority carrier lifetime. The effect of extended light exposure of the samples was measured with injection level dependent lifetime spectroscopy. This revealed iron as a contaminant source present in non-treated samples which could significantly be reduced by an appropriate phosphorus diffusion. To monitor the changes with a high spatial resolution the Carrier Density Imaging (CDI) technique was applied showing distinct differences between oxidations and diffusions.
Depending on the specific impurities and defect spectrum of a silicon material, the minority carrier lifetime can react diversely to Rapid Thermal Processing (RTP). We have measured the lifetime of silicon materials before and after RTP and have diffused solar cells either by RTP or by conventional quartz tube furnace processing (CFP). Our investigations show that the lifetime of Fz-Si can be preserved during RTP resulting in up to 18.7 % efficient solar cells. For 1.4 Ωcm PV-grade Cz-Si, the stable lifetime after light degradation could be improved by up to 60 % by RTP. In the case of EFG-Si, the same average cell efficiency was obtained with RTP diffusion as with the industrial reference diffusion process. However, in the case of block-cast mc-Si, the lifetime decreases with increasing diffusion temperature indicating that P-diffusion in the second range might provide insufficient gettering.
Oxygen-rich crystalline silicon materials doped with boron are plagued by the presence of a well-known carrier-induced defect, usually triggered by illumination. Despite its importance in photovoltaic materials, the chemical make-up of the defect remains unclear. In this paper we examine whether the presence of excess silicon self-interstitials, introduced by ion-implantation, affects the formation of the defects under illumination. The results reveal that there is no discernible change in the carrier-induced defect concentration, although there is evidence for other defects caused by interactions between interstitials and oxygen. The insensitivity of the carrier-induced defect formation to the presence of silicon interstitials suggests that neither interstitials themselves, nor species heavily affected by their presence (such as interstitial boron), are likely to be involved in the defect structure, consistent with recent theoretical modelling.
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