Microscopic Description of Light Induced Defects in Amorphous Silicon Solar Cells

Using a combination of quantum and classical computational approaches, we model the electronic structure in amorphous silicon in order to gain an understanding of the microscopic atomic configurations responsible for light-induced degradation of solar cells. We demonstrate that regions of strained silicon bonds could be as important as dangling bonds for creating traps for charge carriers. Further, our results show that defects are preferentially formed when a region in the amorphous silicon contains both a hole and a light-induced excitation. These results are consistent with the puzzling dependencies on temperature, time, and pressure observed experimentally.

Figure 1

Hole trap depths as a difference of total energies for 128 216-atom bond networks referenced to the network at (0,0), with dangling-bond states (triangles) separated from systems with all fourfold coordinated Si atoms (circles). While a dangling bond is usually a trap, there are strained bonds that trap holes very strongly.

Figure 2

Hole state in $a\mathrm{\text{−}}\mathrm{Si}$ with a dangling bond (A) and without (B). The amount of strain increases from white to red (dark grey). In both cases, the hole orbital (blue or black) is centered around a region of strained silicon atoms. Sample B is lower in energy for a hole by 0.63 eV in our calculations. The dangling bond in sample A is not able to confine the hole, which is therefore also located on other strained regions. The highly strained atoms on the right side of sample B does not attract the hole state because there are unstrained bonds surrounding them, which would cause the hole to be in a highly confined, energetically unfavorable state. The atoms are colored according to their energy in the Tersoff [32] effective potential.

Figure 3

Diffusion Monte Carlo energies as a sample of $a\mathrm{\text{−}}\mathrm{Si}$ undergoes a bond switch process in: ■ ground state, ◆ positively charged ground state, ▲ positively charged excited state, ● neutral excited state. The highlighted atoms are exchanging their bonds. The lines are guides to the eye.

Figure 4

Top: Hole trap depth as a function of anneal barrier. The line is an average of the trap depth for the surrounding region, with error bars indicating the rms deviations. Bottom: The anneal barrier distribution (values are from DFT with a uniform correction from DMC calculations).