Investigation On Crystalline Silicon Surface Passivation Mechanism Of Al2O₃by Atomic Layer Deposition

Crystalline silicon solar cells play a very important role in the Photovoltaic area due to the high conversion efficiency. In order to improve the conversion efficiency of crystalline silicon solar cells, the anti-reflectance layers and the surface passivation layers need special deal. And Surface passivation is becoming more and more important in the silicon solar cells area. The general passivation materials include SiO2and SiNx and so on. However, high temperature during the growth of SiO2will degrade the lifetime of carriers in silicon and the positive charge in SiNx will induce a decrease of short-circuit current in cells. Several researchs show that, as the fixed negative charge, Al2O3films grown by atomic layer deposition (ALD) display a good passivation effect for silicon. In our paper, textured CZ crystalline silicon (c-Si) deposited with Al2O3films by ALD was investigated. Firstly, Al2O3films deposited under200℃,220℃,240℃,260℃,280℃,300℃were studied, with H2O and O3for the O source of Al2O3. The Al2O3films grown above240℃show the best passivation effect for both H2O and O3, measured by transient surface photovoltage (SPV). After annealing process, the SPV relaxation time increased to400μs. Additionally, Al2O3films deposited on different conductivity type of c-Si wafers were studied. The properties of Al2O3films were measured by field emission scanning electron microscopy (FESEM), XRD, Reflectance spectra and the effective lifetime (τeff) of minority carriers was measured by quasi-steady-state photoconductance (QSSPC). The measurement showed Al2O3(amorphous structure by XRD) films had little difference for the reflectance of p-and n-type c-Si. Through QSSPC, it is found that xen of p-and n-type was9.2us and11.1us, respectively. After annealing, Teff of p-and n-type increased to39.4us and28.2us, respectively. The origin of the more effective surface passivation of p-type textured c-Si is due to the fixed negative charge density and the decrease of defect density at the interface...