Measurements of Light-Induced Degradation in A-Si,Ge:H,F Alloys
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MEASUREMENTS OF LIGHT-INDUCED DEGRADATION IN a-Si,Ge:H,F ALLOYS,*) J. Kolodzey,a) S. Aljishi, Z E. Smithb) V. Chu, R. Schwarz and S. Wagner, Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544. ABSTRACT The effects of illumination on the optical and electronic properties of narrow gap hydrogenated and fluorinated amorphous Si-Ge (a-Sij_..Ge.:H,F) alloys have been evaluated. A series of alloys with optical gaps ranging from 1.30 eV to 1.64 eV has been light soaked at -14 sun intensity for 354 hours. Measurements of sub-gap absorption, photo- and dark conductivities and dark conductivity activation energy were made on alloys in the annealed and the light-soaked states. The results indicate that samples with optical gaps Z 1.4 eV degrade significantly. The 1.3 eV sample shows no degradation in its optical or electronic properties except for a factor of 5 increase in the dark conductivity. INTRODUCTION Illuminated photovoltaic cells made of hydrogenated amorphous silicon (a-Si:H) suffer a gradual performance degradation known as the Staebler-Wronski (SW) effect. 1, 2 This lightinduced degradation appears as an increase in the metastable defect density with a resulting decrease in the photoconductivity. In unalloyed a-Si:H the defects can be created by photons with energies down to 1.2 eV. 3 They are detectable by electron spin resonance (ESR), by the 5 4 intensity dependence of the photoconductivity, and by the sub-gap optical absorption. , 6 These defects can be removed by annealing at (typically) 160' C for 45 min. Reports of the SW effect in narrow gap alloys of a-Si:H with Ge (a-Si,Ge:H) indicate that the degradation is less than in unalloyed a-Si:H. 7,8,9, 10 The smaller SW effect has been attributed to a high density of intrinsic defects which masks additional photo-generated defects. The dependence of the defect generation rate on alloy composition may be obscured by different initial defect densities. Light soaking should therefore be maintained for a sufficiently long time so that the changes in defect density exceed initial defect density variations among samples. In terms of recent models, 9 narrow gap alloys may lack sufficient band tail-to-band tail recombination energy to create metastable defects, or perhaps the intrinsic defect density is so high that efficient tail-to-gap state recombination shunts the defect-creating tail-to-tail recombination. Both causes will result in a reduced SW effect. This may result in a cutoff Eopt below which SW degradation will not occur. EXPERIMENTAL PROCEDURES The diode-type deposition station is turbomolecular-pumped with all-metal seals and a load lock and was described elsewhere. 11, 12 The a-Si1 _xGex:H alloys were prepared by the decomposition of SiF 4,GeF 4 and H2 in a radio frequency (RF) glow discharge. The RF (13.56 MHz) power density was 250 mW-cm- 2 , the gas flow rates: 28 standard cubic centimeters/ minute (sccm) SiF 4, 0 to 0.7 sccm GeF 4 and 4.6 sccm H2, the deposition pressure 14.4 Pa (0.108 Torr) and the substrate tempe
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