To assist the engineering of novel silicon heterojunctions, numerical device simulations (Sentaurus TCAD) are used to improve knowledge regarding relevant heterojunction and thin film properties. This knowledge is necessary to understand limitations of current contacts and allow systematical optimization. With the focus on metal oxide based hole contacts, it is shown that for an ideal hole extraction from the c-Si absorber via the a-Si buffer and the transition metal oxide (TMO) into the external metal electrode, two conditions have to be fulfilled: A.) An induced c-Si pn-junction with a high p/n ratio is needed which is provided by a high metal oxide work function. B.) At the TMO/a-Si interface an efficient hole extraction into the n-type TMO must be guaranteed by band-to-band or trap-assisted tunneling. If A or/and B are not fulfilled the standard pn-junction theory is violated which results in power losses caused by insufficient hole selectivity.

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