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Transition metal oxides (TMOs) are of increasing importance for many applications reaching from thin-film transistors and non-volatile memory to novel contact layers in photovoltaics. Due to their tunable electrical properties and high transparency, TMOs are also promising candidates as contact layers in silicon heterojunction solar cells already leading to cell efficiencies of about 22%. However, the current extraction of charge carriers via these thin contact layers is still not fully understood. To assist the engineering of novel silicon heterojunctions, numerical device simulations are used to improve knowledge regarding relevant heterojunction and thin film properties. The efficient current extraction from a silicon absorber is investigated with Sentaurus TCAD for a TMO-based hole contact. It is shown that for an ideal hole extraction from the induced crystalline silicon pn-junction via the amorphous silicon buffer and the TMO into the external metal electrode, two requirements have to be fulfilled: (A) A sufficiently high TMO work function is needed to ensure a high hole conductivity (via a high charge carrier ratio p/n) in the induced pn-junction within the silicon absorber. (B) Extraction of those holes into the TMO calls for efficient trap-assisted tunneling. Experimental evidence for a limitation of hole extraction by (A) and (B) is given for a variety of TMO based hole contacts using molybdenum oxide. [ABSTRACT FROM AUTHOR]

The nanostructured surface - also called black silicon (b-Si) - is a promising texture for solar cells because of its extremely low reflectance combined with low surface recombination obtained with atomic layer deposited (ALD) thin films. However, the challenges in keeping the excellent optical properties and passivation in further processing have not been addressed before. Here we study especially the applicability of the ALD passivation on highly boron doped emitters that is present in crystalline silicon solar cells. The results show that the nanostructured boron emitters can be passivated efficiently using ALD Al2O3 reaching emitter saturation current densities as low as 51 fA/cm2. Furthermore, reflectance values less than 0.5% after processing show that the different process steps are not detrimental for the low reflectance of b-Si. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]