Investigating and controlling charge carrier behavior in p-type delafossite CuFeO₂ photocathodes for solar fuel production

Humanity is facing the challenge of a global-scale energy transition over the next decades, where solar energy is set to play a crucial role. With the immense amount of sunlight reaching the surface or the Earth, and constantly decreasing manufacturing cost, photovoltaic technologies are on their way to replace unsustainable electricity sources. However, finding efficient ways to store solar energy over long periods, while necessary due to the intermittent nature of daylight, remains a challenge. A possible sustainable way to overcome this challenge is to convert solar energy into chemical energy inside a solar fuel, such as solar hydrogen. To accomplish this conversion, photoelectrochemical tandem devices, composed of semiconducting n-type photoanode and p-type photocathode, are predicted to achieve high efficiencies for low manufacturing costs. The original work presented in this thesis deals with the investigation and advancement of a novel photocathode material: p-type delafossite CuFeO2. The attractive structural and opto-electronic properties of the material are presented, along with a new sol-gel processing route to produce thin-film CuFeO2 photocathodes. Then, promising photoelectrochemical features a good flat-band potential, and excellent stability but also limitations low photocurrents and photovoltage are identified. Poor charge carrier transport limitations are addressed by extrinsic doping with oxygen and magnesium on one hand, and a host-guest approach whereby a CuAlO2/CuFeO2 composite electrode is created on the other hand. These approaches lead to significant improvement in photocurrents, reaching ca. 2.5 mA.cm-2 in the presence of an electron scavenger. A more in-depth study of the material is then presented, concluding to the presence of a high density of surface states, causing Fermi level pinning at the CuFeO2/water junction. These states are characterized and identified as the primary cause for the limited photoelectrochemical performances of CuFeO2 photocathodes, and their presence is addressed by surface modification of the photoelectrode. Two oxide overlayers based on Al-doped ZnO/TiO2 and amorphous gallium oxide show promising preliminary results, especially when combined with electrocatalytic Pt nanoparticles, The best-performing photoelectrode produces a phototocurrent as high as 2 mA.cm-2 for water reduction.