Recombination in compensated crystalline silicon for solar cells

Abstract

Deliberate compensation of crystalline silicon results in a decrease in the equilibrium carrier concentration, which leads to an increased carrier lifetime for the intrinsic recombination processes of Auger and radiative recombination. We present modeling which reveals that compensation also often leads to a significant increase in lifetime for recombination through defects via the Shockley-Read-Hall mechanism, a conclusion which is confirmed experimentally for the case of interstitial iron in p-type silicon. We show that the increased Shockley-Read-Hall lifetime can result from either an injection-level effect for deep levels, or from a Fermi-level effect for shallower levels. For cases where the defect exhibits no injection dependence of the carrier lifetime, compensation does not lead to an increased lifetime. Further modeling demonstrates that in certain cases, the lifetime increase can be expected to significantly outweigh the competing reductions in carrier mobilities and net doping, resulting in an improved short-circuit current, open-circuit voltage, and solar cell efficiency.

Impact of compensation on the boron and oxygen-related degradation of upgraded metallurgical-grade silicon solar cells

“…This illustrates well the positive outcome of compensation on carrier lifetime before lightinduced degradation, which was previously reported and analyzed by several groups [30][31][32].…”

“…Both ingots Cz#U1 and CZ#U2 having the same [B] profile, they also have the same BO defect density profile. Since the BO recombination center is a deep level with a capture cross section for electrons which does not differ strongly from that for holes [33] (capture cross section ratio of about 10 for the BO defect against around 1000 for interstitial iron), its recombination activity does not vary much with the majority carrier density [34].…”

A Contactless Method for Determining the Carrier Mobility Sum in Silicon Wafers

“…Compensation doping has been found to be an efficient way to control ingot resistivity when using solar-grade silicon feedstocks [21]- [23]. Carrier recombination, which is being driven by the net doping rather than the sum of the dopant concentrations, can be improved by compensation [24]. However, the mobility is driven by the sum of the dopant concentrations rather than the net doping [25].…”

Impact of incomplete ionization of dopants on the electrical properties of compensated p-type silicon

“…The equilibrium carrier density (p 0 ) in the base of crystalline silicon (Si) solar cell devices has a great influence on their performance. Especially in boron (B) and phosphorus (P) compensated Si, such as low-cost upgraded metallurgicalgrade (UMG) Si, 1 p 0 has been found both by theoretical 2,3 and experimental studies 2,[4][5][6][7] to be a key parameter to optimize the material's electrical properties and the device performance. Most of the recently published papers 3,6,[8][9][10][11] dealing with compensated Si consider p 0 to be equal to the net doping N A -N D , implicitly assuming that all the dopants are ionized at room temperature (T).…”