Solution-processed two-dimensional transition metal dichalcogenides for solar energy conversion

The layered semiconducting transition metal dichalcogenides (TMDs), such as MoS2, WS2, MoSe2, and WSe2, are promising light absorbers for solar energy conversion applications due to their superior optoelectronic properties and chemical stability, however severe recombination at grain boundaries and edge defects limits practical application. This drawback can be overcome by preparing atomically-thin TMD layers, however, the lack of controllable and economically feasible methods to fabricate large-area thin films need to be developed. This work aims at addressing the challenges in the fabrication of two-dimensional (2D) TMD thin films using solution based approaches, and further investigates the roles of morphological and structural factors on the performance of TMD based photoelectrodes. In chapter 2, we advance the exfoliation and surface modification procedures for concentrated and stable flake dispersions, which enable charge carrier mobility and conductivity studies in multi-flake electronic devices. Then chapter 3 describes a novel thin film fabrication method based on TMD nanoflake self-assembly at liquid/liquid interface. The resulting large-area 2D films with controlled flake alignment are superior to aggregated films fabricated by traditional methods, and therefore enable efficient charge harvesting in 2D WSe2 photoelectrodes. In chapter 4, the size and thickness of solvent exfoliated flakes are engineered to demonstrate the critical effects of flake edges on photogenerated charge transport and recombination in photoelectrochemical devices, via both experimental and simulated routes. Moreover, chapter 4 shows significantly improved internal quantum yields (IQE) up to 50-60% achieved by an edge passivation strategy. This flake edge limitation is further confirmed in solar water splitting in chapter 5, while internal defects originating from the starting materials are also proven inducing considerable efficiency lose in passivated TMD electrodes. For WSe2 photocathodes eliminated both internal and edge defects and with an optimized Pt-Cu co-catalyst, photocurrent up to 4.0 mA cm–2 (at 0V vs RHE) and IQE up to 52 % under standard illumination is established for solar-to-hydrogen conversion. Finally, in chapter 6 we further gain insights into the photogenerated charge separation at hybrid heterojunctions formed by exfoliated TMD flakes and an organic semiconductor.