The Staebler-Wronski Effect: New Physical Approaches and Insights as a Route to Reveal its Origin
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1245-A14-02
The Staebler-Wronski effect: new physical approaches and insights as a route to reveal its origin A.H.M. Smets1,2,4, C.R. Wronski3, M. Zeman4 and M.C.M. van de Sanden1 1
Eindhoven University of Technology, the Netherlands, 2National Institute of Advanced Industrial Science and Technology, Japan, 3Pennsylvania State University, USA, 4Delft University of Technology, the Netherlands Abstract In the recent years more and more theoretical and experimental evidence have been found that the hydrogen bonded to silicon in dense hydrogenated amorphous silicon (a-Si:H) predominantly resides in hydrogenated divacancies. In this contribution we will philosophize about the option that the small fraction of divacancies, missing at least one of its bonded hydrogen, may correspond to some of the native and metastable defect states of a-Si:H. We will discuss that such defect entities are an interesting basis for new and alternative views on the origin of the SWE. Introduction Recent experimental studies have revealed two crucial features of the StaeblerWronski effect (SWE) which up to now did not receive any attention in the models proposed to explain its mechanism. First, using charge deep-level transient spectroscopy (Q-DLTS) [1] at Delft University of Technology and dual beam photoconductivity (DBP) [2] analysis at Penn State, it has been shown that hydrogenated amorphous silicon (aSi:H) has at least three native gap states related to defects. Under light soaking the three distributions increase with distinctly different kinetics for the creation of the metastable states as well as annealing characteristics. Both groups have identified three defects states with a broad density distribution, here for clarity denominated state A, B and C. The peak energies of the Gaussian distributions from DBP are [2]: for the A states 0.05 eV above midgap; for the B states 0.095 eV below midgap; and for the C states 0.39 eV below midgap. It was found that that state A and B are efficient electron recombination centers which dominate the photoconductivity in the protocrystalline a-Si:H films [3]. The C states positioned further from midgap on the other hand are inefficient electron, but very efficient hole recombination centers [3] and are the metastable defects which dominate the degradation in the solar cells under 1 sun illumination. Secondly, Wronski and coworkers [4,5] showed that it is far from straightforward to reveal the correct kinetics of the SWE based on the evolution of defects during light-soaking. Their detailed study on films and cells showed that the contribution of the initial defect density obscures the real appearance of the SWE during the time scales of typical light-soaking experiments. After correction for the contribution of the initial defects, the evolution of meta-stable defect states A and B for protocrystalline silicon show a scaling of ~Gt1/2 under one sun illumination (with G being the generation rate and t the time) during the initial two hours of light soaking in contrast to the commonly reported ~G2/3
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