Abstract
Combating climate change is not an avoidable mandate but a survival quest for all of us. The Paris agreement’s ambitious goal — keeping global warming below 1.5 °C — can only be met with decarbonization of the world. That is, a fundamental shift to a new economy is imperative. The so-called “hydrogen economy” is an alternative to our fossil fuel-based energy system. The realization of the hydrogen economy requires cost-efficient carbon-neutral solutions to hydrogen production. Photoelectrochemical (PEC) water splitting is considered as a viable pathway to produce hydrogen fuel using solar energy. The PEC water splitting process harvests solar energy by a light-absorbing semiconducting material and generates electrochemical potential to split water into hydrogen and oxygen. It is important to develop cheap and efficient semiconductor electrodes to deploy the technology at a greater scale.
This thesis investigates tin sulfide (SnS) as a semiconductor photocathode material for hydrogen generation from water. SnS has suitable optoelectronic properties for PV applications: a narrow band gap of ~1.3 eV, a large absorption coefficient (≥104 cm-1), and suitable free carrier concentration around 1017-1018 cm–3. Also SnS contains only non-toxic and earth-abundant elements. Although some studies of SnS have demonstrated the production of high quality SnS thin films, the slow and energy intensive process hinders device fabrication at a larger scale.
In this thesis, a solution-phase deposition technique was adopted to deposit SnS thin films. Molecular inks are molecular precursor solutions in the form of a single phase: homogenous liquids and not slurries, suspensions or colloidal dispersions of nanoparticles. The homogenous precursor solutions ultimately can lead to uniform deposition of precursor layers and singlephase chalcogenide films. Two molecular ink systems were found to be effective to form a molecular ink for SnS: an amine-thiol solvent mixture and a thiourea solution system. Investigation on both inks with respect to their dissolution mechanisms and phase recovery processes is reported in Chapter 3. Single crystal XRD was measured to investigate the nature of tin-thiourea compounds in the ink. TGA and DTA analysis was performed to investigate the phase recovery mechanism from the ink. SnS thin films were successfully deposited by spin coating the inks on substrates followed by a heat treatment at 350 °C in an inert atmosphere. It was shown that the molecular ink approach is versatile to modify the film morphology by modifying the ink formula. Types of solvents, chalcogen source precursor, and substrate influenced the film morphology.
In Chapter 4, the deposited SnS thin films were tested as a photocathode for PEC water splitting. Bare SnS photocathodes showed photocathodic currents but with a substantial overpotential. Three different approaches were adopted to improve the PEC performance of SnS based PEC cells: (1) applying a catalyst to the surface to decrease the overpotential, (2) forming a pn junction with In2S3 or Ga2O3 to improve the photovoltage, and (3) annealing in an H2S atmosphere to promote grain growth. Pt catalyst decreased the overpotential and improved the onset potential by ca. 100 mV. One of the main achievements of this thesis is a unique pn junction formation of SnS/Ga2O3, exhibiting improved onset potential at 0.25 V vs RHE. In addition, the SnS/Ga2O3 junction was stable over 30 min of operation at 0 V vs RHE. However, improving the anchoring of the catalyst to the TiO2 surface is essential to maintain the steady state operation of the device. Surprisingly, H2S annealing was found to oxidize the surface of SnS into SnS2, confirmed by XRD without any noticeable grain growth. The poor onset potential of SnS/In2S3/TiO2/Pt device fabricated with H2S annealed SnS photoabsorber is likely due to an unfavorable pn junction formation between SnS and SnS2.
In chapter 5, the synthesis of BiSI via low temperature sulfurization of BiOI was successfully demonstrated. Deposition of BiSI thin film was achieved by adopting an electrochemical method for BiOI thin film deposition. It was observed that the phase transformation between BiOI and BiSI is reversible under mild temperatures by changing the annealing atmosphere. The PEC performance of BiSI for Na2SO3 oxidation showed that the material is photoactive but susceptible to photocorrosion. TiO2 was deposited to protect BiSI layer but charge transport was hindered, despite the presence of a defect band in the TiO2. In summary, this thesis provides understanding of the molecular ink chemistry for SnS, which can be extended to other chalcogenide materials, and suggests a pn junction formation with an optimal conduction band offset is one of the key solutions to improve the photovoltage.
Suh, Jihye