Nanocrystals promise big benefits in future solar cell development

Paul Boughton

US Los Alamos National Laboratory scientists have discovered that a phenomenon called carrier multiplicationin which semiconductor nanocrystals respond to photons by producing multiple electronsis applicable to a broader array of materials than previously thought.

The discovery increases the potential for the use of nanoscrystals as solar cell materials to produce higher electrical outputs than current solar cells.

In papers published recently in the journals Nature Physics and Applied Physics Lettersthe scientists demonstrate that carrier multiplication is not unique to lead selenide nanocrystalsbut also occurs with very high efficiency in nanocrystals of other compositionssuch as cadmium selenide.

In additionthese new results shed light on the mechanism for carrier multiplicationwhich likely occurs via the instantaneous photoexcitation of multiple electrons. Such a process has never been observed in macroscopic materials and it explicitly relies on the novel physics of the nanoscale size regime.

According to Richard Schallera Los Alamos scientist on the team: “Our research of carrier multiplication in previous years was really focused on analysing the response of lead selenide nanocrystals to very short laser pulses. We discovered that the absorption of a single photon could produce two or even three excited electrons. We knewsomewhat instinctivelythat carrier multiplication was probably not confined to lead selenidebut we needed to pursue the question.”

Lead project scientist Victor Klimov explains: “Carrier multiplication actually relies upon very strong interactions between electrons squeezed within the tiny volume of a nanoscale semiconductor particle. That is why it is the particle sizenot its compositionthat mostly determines the efficiency of the effect. In nanosize crystalsstrong electron-electron interactions make a high-energy electron unstable. This electron only exists in its so-called ‘virtual state’ for an instant before rapidly transforming into a more stable state comprising two or more electrons” (Fig. 1).

The Los Alamos findings point toward practical photovoltaic technologies that may utilise such traditional solar cell materials as cadmium telluridewhich is very similar to cadmium selenide.

Other interesting opportunities may also be associated with the use of carrier multiplication in solar-fuel technologies and specificallythe production of hydrogen by photo-catalytic water splitting. The latter process requires four electrons per water molecule and its efficiency can be dramatically enhanced if these multiple electrons can be produced via a single-photon absorption event.

Next generation solar panels

In a separate developmentShell has signed a memorandum of understanding with Saint-Gobain VitrageSaint-Gobain’s flat glass companyto produce next-generation solar panels.

At its plant in GelsenkirchenGermanyShell has developed a new generation of photovoltaïc panel based on Copper Indium Selenide (CIS) thin film deposited on glass (Fig. 2). This panel does not incorporate silicon wafers. According to Shellit has been successfully tested and offers record efficiencyespecially under low illumination

Saint-Gobain Vitrage brings its experience and in-depth knowledge of thin-film layer deposits and glass transformationdeveloped after several years operating in the automotive and building markets.


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