Photocathodes Made Of Copper Oxide: Laser Experiment Shows Reasons For High Losses

Copper oxide could theoretically enable high efficiencies in solar cells or as a photocathode for solar energy conversion. Practically, however, it comes to big losses. Now a team at HZB was able to explain with a sophisticated femtosecond laser experiment where these losses take place: they occur less at the interfaces, but rather already inside the crystalline material. These results provide guidance to optimize copper oxide and other metal oxides for energy-use applications.

Copper oxide (Cu₂O) is a promising candidate for future solar energy conversion: As a photocathode, the semiconductor copper oxide could electrolytically split water with sunlight, creating the fuel hydrogen that chemically stores the energy of sunlight.

Theoretically great, practically not.
Single-crystalline copper oxide has a band gap of 2 electron volts, which fits very well with the solar energy spectrum. Perfect copper oxide crystals should theoretically provide a voltage near 1.5 volts under light irradiation. Thus, the material would be very well suited as a top absorber in a stack cell for the solar water splitting and should allow an efficiency (solar energy to chemical energy in hydrogen) of up to 18 percent. However, the real values ​​for the photovoltage are well below and are insufficient to efficiently use copper oxide as a photocathode in a stack cell for solar water splitting. So far, loss processes near the surface or at boundary layers have been held responsible.

Where do losses occur?
Now a team at the HZB Institute for Solar Fuels has taken a close look at these processes. They obtained high-quality Cu₂O single crystals from partners of the US research institute California Institute of Technology (Caltech) and additionally vapor-deposited them with a wafer-thin, transparent layer of platinum. This platinum layer acts as a catalyst and increases the efficiency of water splitting. They studied these samples in the femtosecond laser laboratory (1 fs = 10 ^-15 s) at HZB to find out which processes lead to the loss of the charge carriers and, in particular, whether these losses occur inside the single crystals or at the interface with the platinum.

Experiment in the femtosecond laser laboratory
For this, a first laser pulse in the visible green region excited the electrons in the Cu₂O; only a few seconds later, a second laser pulse (UV light) followed to measure the energy of the excited electron. With this time-resolved two-photon photoemission spectroscopy (tr-2PPE) they were able to identify the main mechanism of photovoltage loss. "We observed that the excited electrons bind very fast to defect states that exist in large numbers in the bandgap," says lead author Mario Borgwardt, who now continues his work as a Humboldt Fellow at the Lawrence Berkeley National Laboratory in the US. The coordinator of the study Dennis Friedrich states: "This happens on a time scale of less than one picosecond (1 ps = 10-12 s), ie extremely fast, especially in comparison to the time in which charges from the interior of the crystalline material can diffuse to the surface. "

Losses especially in the interior of the crystal
"At the HZB femtosecond laser laboratory, we have very powerful experimental methods for analyzing the energy and dynamics of photoexcited electrons in semiconductors. For copper oxide, we were able to show that the losses hardly occur at the interfaces to platinum, but in the crystal itself, "says the initiator of the study and head of femtosecond spectroscopy at HZB, Rainer Eichberger.

Contribution to the Cluster of Excellence UniSysCat
"With these new insights, we are making a first contribution to the Cluster of Excellence UniSysCat of the Technical University of Berlin, in which we are involved," emphasizes Roel van de Krol, who heads the HZB Institute for Solar Fuels. UniSysCat focuses on catalytic processes that take place on very different time scales: While charge carriers react extremely quickly to excitation by light (femtoseconds to picoseconds), chemical processes such as catalysis require many orders more time (milliseconds). For a successful photocatalysis, however, both processes must be optimized together. The present results, which are now published in the renowned journal Nature Communications, are an important step in this direction.