High Efficiency SHJ/Poly-Si Hybrid Solar Cells

Abstract

High efficiency silicon heterojunction (SHJ) solar cells have already reached more than 26% (Yoshikawa et al., 2017) efficiency when tunnel oxide passivated contacts solar cells just broke the 26% barrier at the beginning of February 2018 (ISFH, 2018). For such devices, major losses occur at both the front and rear contacts where parasitic recombination can be very high. Carrier-selective contacts use a special design in order to build a barrier that would block one specific charge carrier and let the other one go through. A Hybrid solar cell is a combination between a heterojunction solar cell and a TOPCon device, featuring then carrier-selective contacts at both sides. In this thesis, a p-type TOPCon structure is implemented at the rear when the front contact is made of n-type amorphous silicon (a-Si:H).
Intrinsic amorphous silicon (a-Si:H) used as front passivation layer, is deposited on top of crystalline silicon and requires an interface with as few defects as possible to minimize the parasitic recombination velocity. A new pretreatment method studied in this thesis involves the growth of a silicon oxide (SiO2) on a crystalline silicon substrate, that will allow to get rid of most of the superficial defects after etching and before a-Si:H deposition. Lifetimes of up to 6 ms and saturation current density (J0) as low as 14 fA/cm2 can be reached with a 200 nm thick oxide.
As a-Si:H presents a very low lateral conductivity, a transparent conductive oxide (TCO) is needed to transport the charge carriers towards the front metal contacts. The resistivity of such a material should be as low as possible. While increasing the deposition temperature of Indium Tin Oxide (ITO), it has been possible to decrease the resistivity up to 2.5 ·10-4 Ω.cm at a temperature of 130°C, without reducing the optical properties of such a layer.
Finally, manufacturing defects are often introduced during the fabrication process, leading to some shunt losses (low shunt resistance). As we are fabricating several solar cells per wafer, it is most of the time necessary to cut them to get rid of the shunt before doing the measurements. This sensitive step could be avoided with a better isolation of each solar cell. Patterned ITO and metal have been developed in this thesis, allowing to reduce the shunt power losses in the range of 1-2% without the need to cut the cells.