Zn3P2 boasts a direct optical band gap of 1.5 eV, which is nearly ideal for absorbing light across all wavelengths of the solar spectrum and efficiently converting it into electrical current. What is more, zinc and phosphorus are abundant in the Earth’s crust, which makes them cheap, and these elements are non-toxic too.

Although researchers have previously fabricated efficient solar cells from bulk zinc phosphide, they relied on either high temperatures (greater than 850 °C) or complicated vacuum deposition methods. "In contrast, we have developed a zinc phosphide ‘ink’ to make solar cells from this semiconductor, something that could allow for large-scale and easy production using inexpensive techniques such as slot-die coating or spray coating," explains team member Erik Luber.

Making the colloidal zinc phosphide nanoparticles was not easy and took a lot of work, he says. "However, once we had figured this out, it turned out that the method itself was not complicated at all and easy to reproduce."

First, a solution of 1-octadene (a solvent) and tri-n-octylphosphine (a source of phosphorus) were mixed together at 100 °C. The solution was then heated to 320 °C, and dimethylzinc (a source of zinc) injected into the mix. "We then stirred this preparation for several hours, and ended up with the Zn3P2 nanoparticles," he told nanotechweb.org.

The particles appear to be very crystalline and have the α- Zn3P2 structure, he added. They are round in shape, approximately 8 nm in size and strongly absorb sunlight. They have a bandgap that is 0.5 eV larger than bulk Zn3P2 – something that is thought be a result of quantum confinement effects, but this hypothesis still needs to be confirmed.

Photosensitive heterojunction devices

The team then made heterojunction devices consisting of ITO/ZnO/ Zn3P2 /MoO3/Ag by depositing the active layer components (ZnO and Zn3P2) using a simple bench-top technique known as spin coating. Here, drops of solution are placed on a substrate that is then spun at high speeds to form a thin and uniform layer. "We can control the thickness of the Zn3P2 by using layer-by-layer deposition in which very thin layers of around 16 nm are deposited in a sequential fashion. This ensures that the films are uniform and free of large-sale defects," explained Luber.

According to the researchers, the devices are highly photosensitive (with an on/off ratio of 100) when compared to other nanoparticle semiconductor heterojunction devices, which could be the result of electric charges generated when the Zn3P2 nanoparticles absorb solar energy. The devices also have good rectification behaviour (allowing one-way flow of electric current) with a rectification ratio of 600. This could be thanks to the favourable energy offset between the ZnO and Zn3P2 layers, said Luber, and such one-way current flow is necessary when it comes to making an efficient solar cell device.

The Alberta team, led by Jillian Buriak, now plans to actually fabricate nanocrystalline solar cells from its Zn3P2 material. "In particular, we are developing a new method to synthesize Zn3P2 nanoparticles to improve the material’s surface properties and charge transport," revealed Luber. "We are also looking into alternative electron acceptor materials to Zn3P2."

The present research is detailed in ACS Nano DOI: 10.1021/nn4034234.

Further reading

Patterning nanostructures on etched silicon (Jun 2011)
Nano-heterojunctions improve solar cells (Jul 2012)
Heterostructures make better solar cells (Apr 2012)
Efficient silicon heterostructure LEDs unveiled (Dec 2009)
Thin-film solar: low-cost synthesis of CZTS nanocrystals (Jul 2011)