Laser Experiment Reveals Efficiency Loss of Photocathodes

A green laser pulse initially excites the electrons in the copper oxide; just fractions of a second later, a second laser pulse (UV light) probes the energy of the excited electron. (Source: M. Künsting, HZB)

Solar cells and photo­cathodes made of copper oxide might in theory attain high efficiencies for solar energy conversion. In practice, however, large losses occur. Now a team at the Helmholtz Center Berlin HZB has been able to use a sophis­ticated femto­second laser experiment to determine where these losses take place: not so much at the interfaces, but instead far more in the interior of the crystalline material. These results provide indi­cations on how to improve copper oxide and other metal oxides for appli­cations as energy materials.

Copper oxide is a very promising candidate for future solar energy conversion: as a photo­cathode, the copper oxide might be able to use sunlight to electroly­tically split water and thus generate hydrogen, a fuel that can chemically store the energy of sunlight. Copper oxide has a band gap of 2 electron volts, which matches up very well with the energy spectrum of sunlight. Perfect copper oxide crystals should theore­tically be able to provide a voltage close to 1,5 volts when illu­minated with light. The material would thus be perfect as the top-most absorber in a photo­electro­chemical tandem cell for water splitting. A solar-to-hydrogen energy conversion effi­ciency of up to 18 per cent should be achievable. However, the actual values for the photo­voltage lie considerably below that value, insufficient to make copper oxide an efficient photo­cathode in a tandem cell for water splitting. Up to now, loss processes near the surface or at boundary layers have been mainly held responsible for this.

A team at the HZB Institute for Solar Fuels has now taken a closer look at these processes. The group received high-quality copper oxide single crystals from colleagues at the California Institute of Tech­nology (Caltech), then vapour-deposited an extremely thin, transparent layer of platinum on them. This platinum layer acts as a catalyst and increases the efficiency of water splitting. They examined these samples in the femto­second laser laboratory to learn what processes lead to the loss of charge carriers and in particular whether these losses occur in the interior of the single crystals or at the interface with the platinum.

A green laser pulse initially excited the electrons in the copper oxide; just fractions of a second later, a second laser pulse (UV light) measured the energy of the excited electron. The team was then able to identify the main mechanism of photo­voltage losses through this time-resolved two-photon photon emission spectro­scopy (tr-2PPE). “We observed that the excited electrons were very quickly bound in defect states that exist in large numbers in the band gap itself”, reports Mario Borgwardt, who is now continuing his work as a Humboldt fellow at Lawrence Berkeley National Labora­tory in the USA. The coordinator of the study, Dennis Friedrich, explains: “This happens on a time scale of less than one picosecond, i.e. extremely fast, especially compared to the time interval charge carriers need to diffuse from the interior of the crystal­line material to the surface.”

“We have very powerful experimental methods at the femto­second laser laboratory for analysing energy and dynamics of photo-excited electrons in semi­conductors. We were able to show for copper oxide that the losses hardly occur at the interfaces with platinum, but instead in the crystal itself”, says Rainer Eich­berger, initiator of the study and head of femtosecond spectro­scopy lab. “These new insights are our first contribution to the UniSysCat Excellence Cluster at the Technische Univer­sität Berlin, in which we are a partner”, emphasises Roel van de Krol, who heads the HZB Institute for Solar Fuels. UniSysCat focusses on catalytic processes that take place over very diverse time scales: while charge carriers react extremely quickly to excitations by light, chemical processes such as (electro)­catalysis require many orders of magnitude more time. An efficient photo­chemical conversion requires that both processes be optimised together. (Source: HZB)

Reference: M. Borgwardt et al.: Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes, Nat. Comm.; DOI: 10.1038/s41467-019-10143-x

Link: Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany

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