Scientists at Delft University of Technology in the Netherlands have fabricated an n-type silicon heterojunction (SHJ) solar cell based on a hole-transport layer (HTL) made with transition metal oxide (TMO) thin films.

In the paper “Universal interface engineering method for applying transition metal oxides in silicon heterojunction solar cell,” published in Solar Energy Materials and Solar Cells, the researchers said that TMO materials have a wide bandgap and are used in heterojunction PV devices to increase their short-circuit current.

“This work presents a successful application of interface engineering methods to enhance the performance of SHJ solar cells using polytungstate (WOx) and vanadium suboxide (V2Ox) thin films,” the research's lead author, Liqi Qao, told pv magazine. “Through precise control of the oxygen content in the TMO films, we mitigated the detrimental interface reactions between the TMOs and the silicon passivation layers, resulting in improved solar cell efficiencies.”

The scientists selected these two materials as they “establish” a balance between the amount of oxygen vacancies and their capacity for selective transport. They applied a plasma treatment with boron (PTB) to alleviate the interface reaction of TMO with the substrate via interface engineering.

This treatment reportedly modifies the oxygen content in TMO films, thus improving their electronic properties.

The group built a WOx-based cell and a V2Ox-based device. In the first cell, the thickness of the WOx film was 2 nm, while the V2Ox layer used in the second device was 2 nm thick.

The 2 cm × 2 cm devices were both based on 4-inch float-zone (FZ) wafers processed in tetramethylammonium hydroxide (TMAH) solution, hydrogenated amorphous silicon (a-Si:H) stacks, and an anti-reflective coating based on magnesium fluoride (MgF2).

Tested under standard illumination conditions, the WOx-based cell was able to reach a power conversion efficiency of 23.30% and a fill factor of 80.80%. The other device achieved values of 22.04% and 74.88%, respectively.

“Our TMO-based FJ-SHJ [front junction silicon heterojunction] solar cells’ results reveal that the PTB is a method that creates an optimal surface condition for the deposition of TMOs and achieves a desirable equilibrium between the quantity of defects and carrier transport of the film, leading to the enhanced performance of TMO-based SHJ solar cells,” the researchers concluded.