Semiconductor nanocrystals, known as quantum dots, hold promises for solar cells if only more of the generated charge would arrive at the connections.
Quantum dots are semiconductor crystals of a few micrometres in size. The invisibly small crystals are kept in a solution or they are fixed in a matrix to make a thin film.
The size of the crystals determines the colour of the medium they are in. The colour can be tuned all over the visible spectrum into the infrared.
When hit by a photon, quantum dots in films produce mobile charges: electrons and holes. This makes them an interesting medium for solar cells that can be printed on curved and flexible substrates or even on textiles.
These exciting promises are smothered by the modest efficiency of these devices, which is less than 10%. However, a decade ago it was as low as 1%.
The work that Dr. Simon Boehme has done at one of the labs of the Faculty of Applied Sciences aims to improve the efficiency of quantum dot solar films. He explained that once a photon has sparked an electron-hole pair, the challenge is to separate the charges and transport them to the opposite electrodes in order to generate a current.
But the path through a quantum dot film is littered with traps. Once a free charge drops into a trap, its energy is transferred to another charge or emitted as light, but the energy doesn’t contribute to the external current.
Applying a small voltage over the device by means of bathing it in an electrolyte solution achieves wonders for the charge transport. The voltage (less than 1 volt) fills up the traps with charge, so that photon-generated charges cross the traps unhindered to arrive at the electrodes. “It’s like when the tide comes into a dry fallen harbour and lifts all boats”, Boehme said.
He adds that the finding is not directly applicable to solar cells. “You don’t want an electrolyte solution on your roof. And the power you need to apply the voltage detriments the devices output.”
His second finding seems more practical. Boehme found out that the electron traps are on the outside of the quantum dots. Here long molecules or ligands connect the quantum dots to the bulk of the film. The occurrence of traps increases as ligands connect less to the quantum dot. He said: “We’re used to thinking of quantum dots as molecular balls, but it’s better to think of them as hairy balls. If hairs are missing, that’s where electron traps will occur.” Better chemical binding of the quantum dots should therefore result in higher efficiencies of the solar film. (JW)
Simon Christian Böhme, Charge injection, charge trapping and charge transfer in Quantum-Dot Solids, 3 March 2015, PhD thesis supervisors Prof. Laurens Siebbeles and Prof. Daniël Vanmaekelbergh (Utrecht University).
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