Front Heterojunction

Front Heterojunction

Front Heterojunction TMO/c-Si Solar Cells

Schematic cross-sectional diagram of TMO/n-type c-Si solar cell.
Here, a-SiC:H is used as reflector.
Transition metal oxides (TMOs) are abundant, non-toxic, and inexpensive. It can be deposited by thermal evaporation. Charge carriers can pass through TMO layer. It is used as permeable transport layer.

For electron transport layer, TiO2 or ZnO is used, and for hole transport layer, MoO3 or WO3 is used. In the a-Si:H/n-type c-Si HIT solar cells, p-type a-Si:H is used as emitter. Here, in TMO/n- c-Si solar cells, instead of p-type a-Si:H emitter, TMO is used as hole transport layer and therefore there is no p-n junction in the TMO/c-Si solar cell. TMO role is to pull out holes from n-type c-Si.

Fermi level in MoO3 is much lower than that of c-Si. When MoO3 and n-type c-Si forms a junction, electrons from n-type c-Si diffuse to MoO3, and hence MoO3 Fermi level comes up and c-Si Fermi level comes down and Fermi levels on both sides align with each other.

Band bending occurs at c-Si side and no band bending occurs at the MoO3 side because diffused electrons stay within few nanometers and band bending is negligible. Diffused electrons stay in a narrow range because MoO3 consists of high-level donor states. Entire deletion layer is on the c- Si side but not on the TMO side since there is no band bending in TMO.

Front Junction TMO/n-type c-Si Solar Cell: Principle of Operation

Light enters through ITO and charge carriers are created in the c-Si. TMO extract holes from c- Si. At the rear side, back surface field (BSF) is formed across the n+-a-Si:H/i-a-Si:H/low-doped n-c-Si interface and BSF points from bulk towards rear surface.

BSF lets electrons to reach rear surface, repel minority carriers (holes) towards bulk and hence rear surface recombination is reduced. From rear surface, electrons are collected by Ti/Al. To reflect unused light towards bulk, a-SiC:H is used as back reflector (BSF is explained in detail in the perovskite solar cell and front junction solar cell pages).


Transition metal oxides as hole-selective contacts in silicon heterojunctions solar cells

Solar Energy Materials and Solar Cells, 145 (2016) 109-115.

n- and p-type silicon solar cells with molybdenum oxide hole contacts

Energy Procedia, 77 (2015) 446-450.