Abstract
Plasma-enhanced atom layer deposition (PEALD) can deposit denser films than those prepared by thermal ALD. But the improvement on thickness uniformity and the decrease of defect density of the films deposited by PEALD need further research. A PEALD process from trimethyl-aluminum (TMA) and oxygen plasma was investigated to study the influence of the conditions with different plasma powers and deposition temperatures on uniformity and growth rate. The thickness and refractive index of films were measured by ellipsometry, and the passivation effect of alumina on n-type silicon before and after annealing was measured by microwave photoconductivity decay method. Also, the effects of deposition temperature and annealing temperature on effective minority carrier lifetime were investigated. Capacitance-voltage and conductance-voltage measurements were used to investigate the interface defect density of state (Dit) of Al2O3/Si. Finally, Al diffusion P+ emitter on n-type silicon was passivated by PEALD Al2O3 films. The conclusion is that the condition of lower substrate temperature accelerates the growth of films and that the condition of lower plasma power controls the films’ uniformity. The annealing temperature is higher for samples prepared at lower substrate temperature in order to get the better surface passivation effects. Heavier doping concentration of Al increased passivation quality after annealing by the effective minority carrier lifetime up to 100 μs.
Highlights
As the crystalline silicon solar cell industry matures, minority carrier lifetime increases and the thickness of wafer decreases, causing front-surface passivation quality approaching the limit of the theoretical calculation
Uniformity becomes worse on 80 W plasma power than on 45 W
Uniformity of Al2O3 was even better when deposited on 45 W plasma power than that deposited on 80 W plasma power
Summary
As the crystalline silicon solar cell industry matures, minority carrier lifetime increases and the thickness of wafer decreases, causing front-surface passivation quality approaching the limit of the theoretical calculation. While the back of the solar cell is Al back-surface field (Al-BSF), surface recombination increases significantly. To decay back-surface minority carrier recombination is a major solution in all to increase incident photon-toelectron conversion efficiency (IPCE). Al2O3 deposition on back surface was proven to be effective on passivation. The result of simulation and calculation by PC1D concludes that using alumina to passivate back surface could increase IPCE of c-Si solar cell over 1% [1]. The preparation process of Al2O3 can be classified to atom
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