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A4.08 - Nanolayer Stacks of -Si:H, AlO and ZnO as Passivating Selective Hole Contacts in Silicon Heterojunction Solar Cells 
April 23, 2014   11:00am - 11:15am

Traditional silicon heterojunction (SHJ) solar cells are known to achieve excellent open circuit voltages due to their high level of surface passivation, yet the amorphous silicon (-Si:H) layers at the front contact limit the short circuit current due to parasitic absorption in the -Si:H. Therefore, we have proposed stacks of AlO and ZnO as an alternative to the -Si:H(i)/-Si:H(p)/TCO stack that is currently being used in SHJ solar cells [1]. Just like -Si:H, AlO is known for its excellent ability to passivate -Si surfaces. It is also known that there exists a high density of negative fixed charge at the AlO/-Si interface. Therefore, when AlO is deposited on -type -Si, a hole inversion layer is formed at the interface, forming a junction. This junction is used to form the hole-selective contact. To extract the carriers and to provide lateral conductivity, ZnO is deposited on a thin (~1 nm) film of AlO using atomic layer deposition (ALD). The AlO film should be thin enough to allow the holes from the -Si to tunnel through and recombine with electrons from the ZnO, analogous to the tunneling recombination mechanism in traditional SHJ solar cells [2]. To assess the potential of AlO/ZnO stacks as hole-selective contacts on -Si, the surface passivation was investigated as well as the tunneling recombination current. It was found that for an AlO thickness of ~1 nm the tunneling recombination is efficient enough for application in solar cells. It was also found that for AlO thicknesses of < 3 nm, the surface passivation is significantly reduced by the presence of the ZnO. To improve the surface passivation of stacks with 1 nm of AlO, a thin film of 3 nm -Si:H was inserted between the -Si and the AlO to reduce the interface defect density, significantly reducing the surface recombination velocity [3]. Furthermore, results of theoretical modeling are provided to indicate directions for further improvement of selectivity of the stacks and the efficiency of the tunneling recombination.References:[1] D. Garcia-Alonso, S. Smit, S. Bordihn, W.M.M. Kessels, "Silicon passivation and tunneling contact formation by atomic layer deposited AlO/ZnO stacks", Semicond. Sci. Technol. 28 (2013) 082002.[2] S. De Wolf, A. Descoeudres, Z.C. Holman and C. Ballif, "High-efficiency Silicon Heterojunction Solar Cells: A Review", Green 2(1), 7-24 (2012).[3] S. Smit, D. Garcia-Alonso, S. Bordihn, M.S. Hanssen, W.M.M. Kessels, Metal-oxide-based hole-selective tunneling contacts for crystalline silicon solar cells, Sol. Energy Mater. Sol. Cells. (2013).

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