Study of Surface Passivation and Charge Transport Barriers in DASH Solar Cell

Dopant-free asymmetric heterocontact (DASH) solar cells have recently shown the potential for high efficiency with no toxic dopant gases. In this paper, we simulate both the hole [Ag/IOH/MoO
_x
/a-Si:H(i)] and electron (a-Si:H(i)/TiO
_x
/LiF
_x
/Al) heterocontacts and study the surface passivation and charge transport mechanism using Sentaurus TCAD. Our simulation predicts that the intrinsic amorphous silicon layer [a-Si:H(i)] is mainly required to improve the field-effect passivation at the hole contact (>100 mV increase in V
_oc
) and to improve the chemical passivation at the electron contact. However, it comes at the cost of degradation in fill factor (FF). To overcome the FF loss, high work function material (MoO
_x
WF ~ > 5.5 eV) is required at the hole contact and low work function material (Al/LiF
_x
WF ~ <; 4. 2 eV) is required at the electron contact. For our simulated structure, barriers at the hole contact lead to selectivity loss (V
_oc
- iV
_oc
) in addition to FF loss, whereas at the electron contact, affecting only the FF unless there is a strong chemical passivation loss. This is attributed to the strong hole blocking nature of the TiOx film. This feature is used to predict the nature of the internal transport barriers by combining the terminal characteristics, surface photovoltage, and selectivity loss, which we believe will be helpful for further progress in this arena.