Surface and bulk passivation of multicrystalline silicon solar cells by ...
Surface and bulk passivation of multicrystalline silicon solar cells by ... Surface and bulk passivation of multicrystalline silicon solar cells by ...
39 Figure 3.1 Band-to band recombination in a direct band-gap semiconductor [76]. Band-to-band transition is typically also a radiative transition in direct-bandgap semiconductors such as GaAs and is technically exploited in light-emitting diodes (LEDs). 3.1.1.2 Trap-Assisted Recombination. Trap-assisted recombination occurs when an electron falls into a "trap" — i.e., an energy level within the bandgap caused by the presence of a foreign atom or a structural defect. Once the trap is filled, it cannot accept another electron. The electron occupying the trap, in a second step, moves into an empty valence band state, thereby completing the recombination process. One can envision this process as a two-step transition of an electron from the conduction band to the valence band or as the annihilation of the electron and hole, which meet each other in the trap. This process is often referred to as Shockley-Read-Hall (SRH) recombination [77, 78]. Figure 3.2 shows the forbidden gap of a semiconductor that has several types of impurity levels. Those near the midgap position, Ε, are labeled as recombination centers. Also shown are levels that are designated as electron traps and hole traps. These lie near the conduction band and the valence band, respectively.
40 Figure 3.2 Schematic diagram of impurity-related energy levels within the forbidden gap of a semiconductor. Levels are labeled as to whether the defect is likely to be a trap or a recombination center according to the SRH model. 3.1.1.3 Auger Recombination. Auger recombination is a process in which an electron and a hole recombine in a band-to-band transition, but now the resulting energy is given off to another electron or hole instead of emitting a photon. Hence, this recombination process involves three charge carriers. The third excited carrier returns to its initial energy state by emitting phonons. 3.1.2 Surface Recombination 3.1.2.1 Fundamentals. Recombination at surfaces and interfaces can have a significant impact on the behavior of semiconductor devices. This is because surfaces and interfaces typically contain a large number of recombination centers. These centers are due to the abrupt termination of the semiconductor crystal, which leaves non-saturated (`dangling') bonds resulting in a large density of defects (surface/interface states). In addition, the
- Page 7 and 8: APPROVAL PAGE SURFACE AND BULK PASS
- Page 9 and 10: Chuan Li, B.L. Sopori, P. Rupnowski
- Page 11 and 12: ACKNOWLEDGEMENT The work presented
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- Page 41 and 42: 22 reflectance of polished Si can b
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- Page 47 and 48: 28 Figure 2.5 Deposition of SiΝ :
- Page 49 and 50: 30 Figure 2.6 shows the dependence
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- Page 53 and 54: 34 2.5 Bulk Passivation of Si by Si
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- Page 70 and 71: 51 Na, sigma_n, sigma_p: enter x.xx
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39<br />
Figure 3.1 B<strong>and</strong>-to b<strong>and</strong> recombination in a direct b<strong>and</strong>-gap semiconductor [76].<br />
B<strong>and</strong>-to-b<strong>and</strong> transition is typically also a radiative transition in direct-b<strong>and</strong>gap<br />
semiconductors such as GaAs <strong>and</strong> is technically exploited in light-emitting diodes<br />
(LEDs).<br />
3.1.1.2 Trap-Assisted Recombination. Trap-assisted recombination occurs when an<br />
electron falls into a "trap" — i.e., an energy level within the b<strong>and</strong>gap caused <strong>by</strong> the<br />
presence <strong>of</strong> a foreign atom or a structural defect. Once the trap is filled, it cannot accept<br />
another electron. The electron occupying the trap, in a second step, moves into an empty<br />
valence b<strong>and</strong> state, there<strong>by</strong> completing the recombination process. One can envision this<br />
process as a two-step transition <strong>of</strong> an electron from the conduction b<strong>and</strong> to the valence<br />
b<strong>and</strong> or as the annihilation <strong>of</strong> the electron <strong>and</strong> hole, which meet each other in the trap.<br />
This process is <strong>of</strong>ten referred to as Shockley-Read-Hall (SRH) recombination [77, 78].<br />
Figure 3.2 shows the forbidden gap <strong>of</strong> a semiconductor that has several types <strong>of</strong><br />
impurity levels. Those near the midgap position, Ε, are labeled as recombination centers.<br />
Also shown are levels that are designated as electron traps <strong>and</strong> hole traps. These lie near<br />
the conduction b<strong>and</strong> <strong>and</strong> the valence b<strong>and</strong>, respectively.
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