Researchers at the University of Alberta (UAlberta), Canada, have designed a new type of nanoparticles from the earth-abundant elements phosphorus and zinc. Years in the making, the discovery could lead to inexpensive spray-on or printed photovoltaic thin films that bring solar power to remote off-grid locations or areas that face exceptionally energy costs, such as the Canadian North.
The research objective was to develop a synthetic method for making solution-processed colloidal zinc phosphide (Zn3P2) nanocrystals that dissolve into an ink for leightweight photovoltaic applications. “In this work we prepared ITO/zinc oxide/zinc phosphide/molybdenum oxide/Ag heterojunction devices,” says UAlberta postdoctoral fellow Erik Luber of the Faculty of Engineering. The zinc oxide and zinc phosphide layers, which in commercial production would be free from the manufacturing restrictions imposed on lead-based nanoparticles, were spin-coated from either a sol-gel or colloidal solution in this experiment.
The team chose the elemental constituents of zinc phosphides — zinc and phosphorus — because both are abundant in the earth's crust, have a low material extraction cost and are nontoxic. Moreover, Zn3P2 is expected to have a near ideal direct band gap of 1.5 eV, according to Luber, who is also the first author of the paper “Solution-Processed Zinc Phosphide (α-Zn3P2) Colloidal Semiconducting Nanocrystals for Thin Film Photovoltaic Applications,” published in ACS Nano.
“Much like other next-generation nanoparticle-based thin-film PV technologies, zinc phosphide offers the potential for lightweight solar modules, which is a critical requirement for shipping and installation costs in remote parts of the world,” the materials engineer says.
Though development and testing of the zinc phosphide thin films are still in their infancy, Luber and colleagues were able to fabricate prototype heterojunction devices from the nanoparticle ink that exhibited a rectification ratio of 600 and photosensitivity with an on/off ratio around 100 — both important suitability benchmarks for solar cell applications. “However, significant amount of work still needs be done to optimise the nanoparticle ink formulation and device architecture,” Luber admits. “Realistically,” the expert predicts the timescale until the commercialisation of spray-on zinc phosphide solar cells to be “roughly 10 years.”
In a future paper, UAlberta wants to describe yet a better synthetic method to improve the surface passivation of the Zn3P2 nanocrystals. “This will allow us to investigate the impact of surface passivation on electrical transport properties,” Luber says in conclusion.
Image: UAlberta researcher Jillian Buriak (center) worked with post-doctoral fellows Erik Luber (right) and Hosnay Mobarok to create nanoparticles that could lead to printable or spray-on solar cells. Courtesy of the University of Alberta