The effect of light soaking on crystalline silicon surface passivation by atomic layer deposited Al2O3

Abstract

The effect of light soaking of crystalline silicon wafer lifetime samples surface passivated by thermal atomic layer deposited (ALD) Al2O3 is investigated in this paper. Contrary to other passivation materials used in solar cell applications (i.e., SiO2, SiNx), using thermal ALD Al2O3, an increase in effective carrier lifetime after light soaking under standard testing conditions is observed for both p-type (∼45%) and n-type (∼60%) FZ c-Si lifetime samples. After light soaking and storing the samples in a dark and dry environment, the effective lifetime decreases again and practically returns to the value before light soaking. The rate of lifetime decrease after light soaking is significantly slower than the rate of lifetime increase by light soaking. To investigate the underlying mechanism, corona charge experiments are carried out on p-type c-Si samples before and after light soaking. The results indicate that the negative fixed charge density Qf present in the Al2O3 films increases due to the light soaking, which results in an improved field-effect passivation. Numerical calculations also confirm that the improved field-effect passivation is the main contributor for the increased effective lifetime after light soaking. To further understand the light soaking phenomenon, a kinetic model—a charge trapping/de-trapping model—is proposed to explain the time dependent behavior of the lifetime increase/decrease observed under/after light soaking. The trap model fits the experimental results very well. The observed light enhanced passivation for ALD Al2O3 passivated c-Si is of technological relevance, because solar cell devices operate under illumination, thus an increase in solar cell efficiency due to light soaking can be expected.

Light-induced performance increase of silicon heterojunction solar cells

“…42,43 The trapping of the electron results in an increase of negative fixed charge density in the Al 2 O 3 film. 44 To test if this expression can be translated into the context of the a-Si:H/c-Si interface, we assume only one type of charge trap present (that may be associated with the silicon dangling bond).…”

Light-induced enhancement of the minority carrier lifetime in boron-doped Czochralski silicon passivated by doped silicon nitride

“…The reason was interpreted by field effect passivation induced by the negative fixed charge in the ALD Al 2 O 3 [21]. In order to verify the applicability of the explanation for the LIE phenomenon in our case, an Al/ P-doped SiN x /p-Si/Al metal insulator semiconductor (MIS) capacitor structure was prepared for high-frequency dark C-V curves measurements using a Semiconductor Characterization System (Keithley (4200-SCS)).…”

Effect of low thermal budget annealing on surface passivation of silicon by ALD based aluminum oxide films

“…1,[7][8][9][10] A well passivated surface reduces the recombination of photo-generated carriers in the vicinity of the two surfaces and improves the cell performance parameters. [11][12][13][14] Thermally grown silicon oxide (SiO 2 ), silicon nitride (a-SiN x :H) and amorphous silicon (a-Si:H) are commonly used for surface passivation of c-Si solar cells. [15][16][17][18][19][20][21] Aluminum oxide (Al 2 O 3 ), a dielectric, has excellent surface passivation properties on c-Si of both conductivity type materials.…”

“…[7][8][9] Recently Al 2 O 3 films have been used to passivate the silicon surface to achieve high efficiency solar cells. [11][12][13][14] There are several methods for Al 2 O 3 film deposition, e.g., sputtering, 22 atmospheric pressure chemical vapor deposition, 23 RF magnetron sputtering 24 and plasma enhanced chemical vapour deposition, 25,26 etc. Atomic layer deposition (ALD) has been proven to be a valuable technique for the growth of Al 2 O 3 thin films.…”