Silicon heterojunction solar cell hits 23.5% efficiency with new hole-selective contact

Scientists at Switzerland’s École Polytechnique Fédérale de Lausanne (EPFL) claim to have reached 23.5% efficiency with a crystalline silicon heterojunction solar cell using molybdenum oxide as the hole-selective contact.

The researchers said molybdenum oxide (MoO_x) – which has been used as a carrier-selective layer and crystalline silicon substrate in organic, inorganic and thin film solar cells due to its high work function and band gap – was used in a device structured in such a way to enable it to fully benefit from its improved transparency compared to p-type hydrogenated amorphous silicon. “MoO_x can rival traditional contacting schemes in spite of its lower level of optimization,” the scientists said.

The thinner, the better

The EPFL team found the tiniest layers of the two compounds resulted in the best performance, as the thin contact stacks allowed for high selectivity, passivation and good charge-carrier transport. “The highest efficiency of this series was obtained for a 4 nm-thick MoO_x layer and a 6 nm-thick intrinsic hydrogenated amorphous silicon ((i)a-Si:H) [layer],” the researchers wrote.

Photoluminescence imaging and high-illumination volatile organic compound measurements demonstrated the efficacy of the thinner layers, the scientists added.

Protecting passivation

In their view, a 4 nm-thick MoO_x film is thick enough to protect the passivation provided by the intrinsic hydrogenated amorphous silicon layer from sputtering damage while also maintaining sufficient selectivity to extract holes.

“Given that results with the thinnest (i)a-Si:H are quite spread, we processed another 4nm MoO_x cell with [a] thicker (i)a-Si:H layer of 8 nm, and … obtained a certified efficiency of 23.5%, with a remarkable fill factor [of] close to 82%,” the EPFL team stated. “This record efficiency for such [a] solar cell was obtained without any masking step or photolithography and used a low-temperature Ag [silver], screen-printed metallization, inducing around 3% of shadowing.”

The researchers are now planning to further reduce the thickness of the molybdenum oxide layer.

“In addition, investigating the industrial feasibility and long term stability of such devices will be required before any large scale deployment of this technology can be envisaged,” they added.

The findings of the research are described in the paper 23.5%-efficient silicon heterojunction silicon solar cell using molybdenum oxide as hole-selective contact, published in Nano Energy and on the ScienceDirect website.