Infrared Electroabsorption Spectra in Amorphous Silicon Solar Cells
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ABSTRACT We report measurements of the infrared spectrum detected by modulating the reverse-bias voltage across amorphous silicon pin solar cells and Schottky barrier diodes. We find a band with a peak energy of 0.8 eV. The existence of this band has not, to our knowledge, been reported previously. The strength of the infrared band depends linearly upon applied bias, as opposed to the quadratic dependence for interband electroabsorption in amorphous silicon. The band's peak energy agrees fairly well with the known optical transition energies for dangling bond defects, but the linear dependence on bias and the magnitude of the signal are surprising if interpreted using an analogy to interband electroabsorption. A model based on absorption by defects near the n/i interface of the diodes accounts well for the infrared spectrum.
INTRODUCTION Electroabsorption is the change in optical absorption due the application of a voltage bias across a sample. It is thus a form of modulation spectroscopy, and can offer insights not readily available from more direct optical measurements. Electroabsorption was first developed to study the band structure of crystalline solids. In amorphous semiconductors electroabsorption measurements corresponding to interband optical transitions indicate that electric fields warp the
wavefunctions of nearly localized electronic states near mobility edges [1], the effect is quadratic in the applied electric field. Investigators have explored the relationship between electroabsorption and the mobilities that govern photocarrier transport in the bands [2] as well as the possibility of light-soaking effects on electronic states at the bandedges [3] There has been relatively little work done on electroabsorption corresponding to defectrelated optical properties. The defect-related analog of the interband effect just described would be several orders of magnitude weaker, since the absorption coefficient for transitions involving mid-band gap states is far lower than the interband absorption, and would presumably share its quadratic dependence upon electric field. While this type of electroabsorption has not been observed, defect-related electromodulation was reported by Eggert and Paul [4] in forwardbiased a-Si:H diodes. In this case, electric-field induced warping of electronic states is no longer responsible for the electromodulation, and the effect is not quadratic in electric field. Changes in defect occupancy in the intrinsic layer due to injection of space-charge by forward bias lead to the readily observable changes in infrared optical properties. The measurements are closely related to sub-bandgap photomodulation measurements, which have been extensively studied in the last twenty years [5] a Permanent address:
Department of Physics, Korea University, Chungnam 399-800, South Korea.
457 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
In the present work we present measurements of sub-bandgap optical properties in reverse biased diodes; injection currents are negligible
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