Changes in the serial resistance Of a- Si, a-/μc-Si and mono-crystalline PV modules during the year
The changes in the serial resistance Rs of amorphous (a- Si) and amorphous/microcrystalline (a-/μc-Si) tandem silicon-based solar cells is a criterion to analyze the process of light -induced degradation and thermal healing of defects described by the Staebler - Wronski effect. Measurements under laboratory conditions show that the serial resistance increases during the light-induced degradation. Our annual measurement of a- Si, a-/μc-Si and mono-crystalline (c-Si) solar cells under field test conditions will verify this and it will improve the possibility of predicting the behavior of thin-film silicon photovoltaic cells. For a year, the complete current-voltage curve was measured every minute, recorded and analyzed. This data has been transferred into the equivalent circuit diagram for solar cells.
Defect relaxation in amorphous silicon: Stretched exponentials, the Meyer-Neldel rule, and the Staebler-Wronski effect.
Annealing and production of metastable defects in disordered solids is explained quantitatively with a model in which defect relaxation is a local phenomenon. The stretched-exponential time dependence of defect relaxation and the Meyer-Neldel rule for the relaxation-time constant are natural consequences of this model. The results are obtained by using an exponential distribution of activation barriers for transitions between the two states of the local defect. The model, applied to data in hydrogenated amorphous silicon, a-Si:H, gives an exponential distribution of barriers with a characteristic temperature of 220 \ifmmode^\circ\else\textdegree\fi{}C, roughly equal to the accepted freeze-in temperature for defect distributions in a-Si:H. The model explains that long degradation times convert defects with higher barriers and this results in longer annealing times. The microscopic models of the metastable defects in a-Si:H, weak-bond breaking and carrier trapping by charged dangling bonds, are discussed in the framework of this defect-relaxation model.
Electronic properties of substitutionally doped amorphous Si and Ge
Abstract It is shown that substitutional doping of an amorphous semiconductor is possible and can provide control of the electronic properties over a wide range. a-Si and Ge specimens have been prepared by the decomposition of silane (or germane) in a radio-frequency (r.f.) glow discharge. Doping is achieved by adding carefully measured amounts of phosphine or diborane, between 5 × 10−6 and 10−2 parts per volume, to obtain n- or p-type specimens. The room temperature conductivity of doped a-Si specimens can be controlled reproducibly over about 10 orders of magnitude, which corresponds to a movement of the Fermi level of 1·2 eV. Ion probe analysis on phosphorus doped specimens indicates that about half the phosphine molecules in the gaseous mixture introduce a phosphorus atom into the Si random network; it is estimated that 30–40% of these will act as substitutional donors. The results also show that the number of incorporated phosphorus atoms saturates at about 3 × 1019 cm−3, roughly equal to the number ...