Evaluating Charge Carrier Transport and Surface States in CuFeO₂ Photocathodes

Abstract

Interest in delafossite (CuFeO2) as a candidate p-type photocathode for photoelectrochemical (PEC) solar fuel production has recently been increasing, mainly due to its excellent stability in aqueous environments and favorable light absorption properties. However, its PEC performance has remained poor for reasons that have not yet been specifically determined. Herein, we report a detailed investigation on sol–gel-processed CuFeO2 with a range of spectroscopic, PEC, and microscopy techniques aimed at unraveling the material properties governing photogenerated charge carrier harvesting in this v. An analysis of the bulk transport properties using microwave conductivity measurements reveals a good charge carrier mobility (0.2 cm2 V–1 s–1) and a relatively long lifetime (200 ns) for photogenerated charge carriers. Conversely, systematic PEC measurements with varied redox systems reveal the existence of a high density of surface states (1014 cm–2) positioned 0.35 eV above the conduction band, inducing Fermi level pinning at the semiconductor–liquid junction. X-ray photoelectron spectroscopy suggests the presence of a thin layer of metal hydroxide at the surface of the material. These surface states were found to behave as electron traps, correlated with an inversion of polarity at the surface of the semiconductor, and thereby promoting charge recombination and limiting the photovoltage developed at the junction. These findings suggest that if the detrimental effects of the surface states can be eliminated, CuFeO2 would provide a sufficiently high photovoltage to be combined with other solution-processed and stable photoanodes into an easily scalable tandem PEC cell.

Pinning of the Fermi Level in CuFeO2 by Polaron Formation Limiting the Photovoltage for Photochemical Water Splitting

“…Figure a shows the range of Fermi level positions that were obtained after calcining the 2H CuFeO 2 pellets and after establishing the interfaces of 2H CuFeO 2 with water and ITO. According to literature, the VBM of 3R CuFeO 2 is situated between 1.0 –1.4 eV versus RHE and the CBM should be within −0.1 to −0.5 eV versus RHE . For 2H CuFeO 2 a similar valence band position can be assumed due to the similarity of the VBM of Cu‐delafossites in general as well of that of different polytypes .…”

“…The inability of any of the heterostructured 2H CuFeO 2 powders to produce high amounts of hydrogen through sacrificial water splitting is a remarkable phenomenon. Recently, Prévot et al showed through a set of photoelectrochemical measurements with varied redox systems that 3R CuFeO 2 thin films suffer from Fermi level pinning at a defect level positioned at about 0.7 eV versus normal hydrogen electrode (NHE) . According to the authors the Fermi level pinning was due to surface states caused by a metal hydroxide surface layer.…”

“…Interface experiments are mostly performed with thin film substrates to avoid sample charging during XPS measurements, which may happen for weakly conductive semiconductors. However, since 3R CuFeO 2 reportedly has a relatively good conductivity, we assumed that pellets made from our bare 2H CuFeO 2 powders, could be used as substrates. In Figure S4 of the Supporting Information the X‐ray diffractogram of 2H CuFeO 2 powder and pellet are compared.…”

“…Because 2H CuFeO 2 cannot be easily synthesized, only 3R CuFeO 2 has been studied extensively so far. The suitability of 3R CuFeO 2 for water reduction stems from its appropriate properties such as its reduced bandgap of 1.5 eV, its suitable conduction band minimum, which allows for water reduction, its good carrier mobility of about 0.2 cm 2 V −1 s −1 , and because it is composed of earth‐abundant elements.…”

Boosting Visible Light Harvesting in p‐Type Ternary Oxides for Solar‐to‐Hydrogen Conversion Using Inverse Opal Structure

“…[4][5][6][7]46] To investigate the charge transport/transfer behaviors for both IO and planar CuFeO 2 -based photocathodes, electrochemical impedance spectroscopy (EIS) measurements were carried out in argon-purged 1 m NaOH electrolyte under simulated solar illumination as shown in Figure S10 (Supporting Information). [9][10][11][12][13] Surprisingly, unlike the planar CuFeO 2 , the IO CuFeO 2 /C 60 /CoFe LDH photocathode shows no loss of quantum efficiency as the incident photon energy decreases from 400 to 700 nm, which has not been observed for ternary oxide-based photocathodes. According to the diffusion-recombination model, the resistance R 1 obtained from the semicircular corresponds to the resistance for the charge transport and transfer.…”