Novel electrical contact to improve solar cell stability

Today’s solar cells show a lot of promise as an efficient and stable source of renewable energy, but their electrical contacts suffer from a ‘trade-off’ relationship between surface passivation and conductivity. Japanese researchers have now developed a new type of electrical contact that can overcome this problem, with their results published in the journal ACS Applied Nano Materials.

The most recent type of commercial photovoltaic cell (solar cell) uses stacked layers of crystalline silicon (c-Si) and an ultrathin layer of silicon oxide (SiO_x) to form an electrical contact. The SiO_x is used as a ‘passivating’ film — an unreactive layer that improves the performance, reliability and stability of the device. But that does not mean that simply increasing the thickness of this passivating layer will lead to improved solar cells. SiO_x is an electrical insulator and there is a trade-off relationship between passivation and the conductivity of the electrical contact in solar cells.

Now, researchers from Nagoya University and The University of Tokyo have developed a novel SiO_x layer that simultaneously allows high passivation and improved conductivity. Known as a nanocrystalling transport path in ultrathin dielectrics for reinforcing passivating contact (NATURE contact), the electrical contact consists of three-layer structures made up of a layer of silicon nanoparticles sandwiched between two layers of oxygen-rich SiO_x.

“You can think of a passivating film as a big wall with gates in it,” said research co-leader Assistant Professor Kazuhiro Gotoh, from Nagoya University. “In the NATURE contact, the big wall is the SiO_x layer and the gates are Si nanocrystals.”

The conductivity of the electrical contact in solar cells is dependent on the formation of a ‘carrier pathway’ for the transport of electronic charges. The formation of this electrical pathway is dependent on a high-temperature treatment called ‘annealing’.

Previous research has shown that SiO_x contacts that contain silicon nanoparticles as a carrier pathway can achieve good electrical properties. In the NATURE contact, the annealing process leads to the formation of very small silicon nanocrystals in the passivation layer that are nearly spherical in shape. The diameter of these nanocrystals corresponds to the thickness of the passivation layer. Thus, by controlling the annealing conditions, the diameter and subsequent thickness of the passivation layer can be adjusted.

The research team fabricated NATURE contacts and then subjected them to varying annealing conditions. Upon studying the contacts with transmission electron microscopy, they discovered that silicon nanocrystals were formed in the contact at an annealing temperature of 750°C. They also investigated the electrical properties of the contact. They saw that compared to existing contacts such as the tunnel oxide passivating contact (TOPCon) or polysilicon on the oxide (POLO) contacts, the NATURE had comparable values of contact resistance and ‘recombination current’ — a phenomenon that causes current and voltage losses in solar cells and decreases their efficiency.

“The NATURE contact overcomes the trade-off relationship between the protective ability and conductivity of passivating films,” Dr Gotoh said. “This development will lead to the realisation of future building-integrated photovoltaics (BIPV) and vehicle-integrated photovoltaics (VIPV) and help us achieve zero-energy buildings and solar cars in future decarbonised societies.”

Image credit: ACS Publications.

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