The crystals are around 4nm across, and the innovation is in preserving them at that size without ruining their properties, according to researchers Richard Brutchey and David Webber, both of the University of Southern California’s Dornsife College of Letters, Arts and Sciences.
“Brutchey and Webber solved one of the key problems of printable solar cells: how to create a stable liquid that also conducts electricity,” said the University. “In the past, organic ligand molecules were attached to the nanocrystals to keep them stable and to prevent them from sticking together. These molecules also insulated the crystals, making the whole thing terrible at conducting electricity.”
The researchers discovered a synthetic ligand that stabilises the crystals (1,2,3,4-thiatriazole-5-thiolate anion), and does not insulate them.
In certain polar solvents, the particles are (and therefore ink would be) stable for over two weeks and, once printed, can be processed at under 100°C to form films of CdSe thiocyanate.
“With a relatively low-temperature process, the method also allows for the possibility that solar cells can be printed onto plastic instead of glass without any issues with melting,” said the university.
“While the commercialisation of this technology is still years away, we see a clear path forward toward integrating this into the next generation of solar cell technologies,” added Brutchey.
The work is described in Dalton Transactions, an international journal for inorganic chemistry.
CdSe is a II-VI semiconductor, whose cadmium makes it poisonous.
Brutchey’s research group is looking for alternatives.
“We are focusing our synthetic efforts on IV-VI, III-VI, I-III-VI and I-IV-VI semiconductors, which have not been explored to as great an extent as II-VI semiconductors and are considerably less toxic,” said the group. “We are using these semiconductor nanocrystals to fabricate hybrid inorganic/organic solar cells using earth abundant (and nontoxic) photovoltaic materials.”
His groups aim is to gain comprehensive control over nanocrystals.
“Our goal is to develop new, low-temperature solution-phase synthesis routes to functional nanocrystals using rational chemical design principles. We aim to obtain hierarchical synthetic control over the structure of nanocrystals, from the core of the particle – including its composition, size, and shape – to its surface chemistry via modification with organic functionality and metal complexes.”