S17_LightAbsorption

Introduction

Hydrogenated amorphous silicon (a-Si:H) is often used in photo-voltaic devices because it is cheaper and easier to manufacture than pure crystalline silicon (c-Si). While hydrogenation allows for higher conductivity, the conductivity of a-Si:H is reduced by several orders of magnitude after prolonged exposure to light, due to a process known as the Staebler-Wronski effect (SWE) [1].The conductivity is returned to the original value after annealing at a temperature of 450 K for over an hour. This makes a-Si:H a poor photo-voltaic source. It has been shown that thin films of mixed phase amorphous silicon (a/nc-Si:H) is resilient to SWE.

The goal of this project was to find conclusion about a/nc-Si:H thin films relation to the SWE. Specifically, the project showed that there is a relation between nanocrystalline fraction and the disorder of the film; a high disorder leads to a worse SWE. The Urbach energy is a measure of the number of defect states and therefore measures disorder. The Urbach energy was obtained via a CPM measurement of the absorption spectrum for eight samples of various nanocrystalline fraction.

Theory

The standard technique for measuring the absorption coefficient, taking the ratio of transmission signals with and without the sample, is unable to resolve values of alpha small enough to measure the Urbach energy. Instead the Constant photocurrent method (CPM) is used to determine the absorption spectrum. CPM can only determine alpha up to a constant factor. The absolute value of alpha was obtained via transmission data in the applicable region. Then the CPM spectrum was fitted to match the transmission spectrum in order to obtain an absolute value of alpha for the whole spectrum. PM relies on the fact that the absorption coefficient is simply proportional to inverse intensity when the photocurrent is held constant throughout measurement. This is shown to be true only in the photon energy range (1.1 to 2.1 eV) used in this project [2].

Apparatus

The apparatus used in the project is shown below. A tungsten lamp was used as a light source. A monochromator is placed in front of the lamp to control the photon energy. Films of a/nc-SiH were mounted inside a vacuum chamber. The current across the films were measured with an ammeter. A computer program controlled the power supplied to the lamp and a motor that adjust the wavelength of the monochromator. A LabView program took the CPM measurements. First the program selected a target photocurrent. It then slowly changes the wavelength of light by adjusting the monochromator. The photocurrent will have changed due to the photons having different energy, but the computer adjusted the power output to compensate. Once the power was such that the photocurrent returned to the target value, the wavelength is adjusted and the process repeats.

Conclusions

The data obtained from the CPM measurements allowed for computation of the Urbach energies of 8 sample. The Urbach energies were recorded and are listed in the table below. The crystalline fractions are rough estimates due to the fact that the full measurement was never taken. Upon fabrication of the samples, rough estimates of the crystalline fraction were made. The Urbach energies shown a clear difference between 0 and 15% however the exact nature of the increase is not known do to the fact that the exact crystalline fraction is not know.

References

[1] D. L. Staebler, C. R. Wronski, Applied Physics Letters 31 (1997), 292.

[2] G. Moddel , D. A. Anderson, W. Paul, Deviation of low-energy optical-absorption

spectra of a-Si:H from photoconductivity (Harvard University, Cambridge MA

1980), 1918-1920.