Study of electron-irradiated silicon thin films using transient photocurrent spectroscopy

S Reynolds,V Smirnov,O Astakhov

doi:10.1088/1742-6596/558/1/012001

S Reynolds, V Smirnov + Show 1 more

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https://doi.org/10.1088/1742-6596/558/1/012001

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Abstract

Electron irradiation of silicon thin films creates localised states, which degrade their opto-electronic properties. We present a series of transient photocurrent spectroscopy (TPC) measurements on electron-irradiated amorphous and microcrystalline silicon films, annealed at progressively increasing temperatures. This has enabled localised states associated with both dangling bonds and conduction band tails to be examined over a wide energy range. Trends in the evolution of the DOS following electron irradiation followed by isochronal annealing steps indicate reductions in the deep defect density, which correlate with spin density. We also find a steepening of the conduction band tail slope in amorphous silicon on annealing. Both defect density and tail slope may be restored close to as-prepared material values. Earlier CPM data are re-examined, and a similar trend in the valence band tail slope is indicated. Computer simulations predict that following e-irradiation changes in deep defect density primarily control solar cell performance, and will tend to obscure effects related to band tails.

Highlights

We present a series of transient photocurrent spectroscopy (TPC) measurements on electron-irradiated amorphous and microcrystalline silicon films, annealed at progressively increasing temperatures
Trends in the evolution of the density of states (DOS) following electron irradiation followed by isochronal annealing steps indicate reductions in the deep defect density, which correlate with spin density
It is found that the deep DOS detected by TPC increases in keeping with the spin density following e-irradiation, and decreases in a similar way after isochronal thermal annealing at progressively increasing temperatures

Summary

Introduction

The effects of energetic charged particles on films of hydrogenated amorphous silicon (a-Si:H) [1,2,3,4]. The effect of increased defect densities on the performance of thin-film silicon solar cells is predominantly a consequence of increased carrier recombination in the absorber layer, which reduces the short-circuit current density JSC, coupled with a reduction in electric field strength in certain regions due to increased trapped space-charge, which degrades the fill-factor FF. As well as improving our ability to predict performance of cells in space applications, defects created by laboratory e-irradiation may be used to gain a more general understanding of solar cell. By using annealing steps to control the defect density in the absorber layer, laboratory measurements of solar cell properties, coupled with computer simulations, enable a detailed optoelectronic model and material parameter sets to be developed [8, 9]. Results are linked to the performance of solar cells with varying defect distributions by computer simulations

Experimental

Results and discussion

Conclusions