An inverted a-Si:H/c-Si hetero-junction for solar energy conversion

A new structure to circumvent the problem of light absorption in dead layers (i.e., highly doped emitters) is investigated: an inversion of the conventional a-S:H/c-Si solar cell hetero-structure, i.e., illumination of the backside, the c-Si part of the junction. This has the advantage that only a small part of the excitation light reaches the a-Si:H emitter. However, in the simplest configuration the lateral collection of excess majority carriers is less easy and a higher series resistance therefore limits the current. The longer path of excess-charge carrier pairs to the junction also causes higher demands on the minority carrier lifetime than in the conventional configuration. It will be shown that a thin (70 nm) Si3N4 film offers an efficient electrical passivation as an antireffective coating. The first inverted Si3N4/ n c-Si/p a-Si:H solar cells were characterized by a moderate efficiency of about 11%. Most detrimental appeared to be the high series resistance of the device. Modelling enabled the causes of this effect to be localized and new ways of contacting the c-Si base decreasing the series resistance will be proposed. Also the contacting of the c-Si part of the junction will be discussed in view of an optimal absorption of the incident radiation. (c) 2006 Elsevier B.V. All rights reserved.