Synthesis and characterizations of nanostructured films for efficient solar energy conversion
by Shuang Xiao
Ph.D. Nano Science and Technology
i, xx, 146 pages : illustrations ; 30 cm
The clean and sustainable supply of global energy is arguably the most important scientific and technical challenge facing humankind in the past decades. Sunlight, as the largest resource of renewable energy, is a desirable general approach to address this issue. Among various utilization routes, solar driven water splitting technique stands out as it can directly convert solar energy into the storable and transportable chemical compound. However, present solar water splitting devices could hardly reach high power conversion efficiency and low cost at the same time, which seriously restricts their practical applications. In this thesis, I studied both the low-cost hybrid organic/inorganic perovskite solar cells (PSCs) and the earth abundant materials based photoelectrochemical water spl...[ Read more ]
The clean and sustainable supply of global energy is arguably the most important scientific and technical challenge facing humankind in the past decades. Sunlight, as the largest resource of renewable energy, is a desirable general approach to address this issue. Among various utilization routes, solar driven water splitting technique stands out as it can directly convert solar energy into the storable and transportable chemical compound. However, present solar water splitting devices could hardly reach high power conversion efficiency and low cost at the same time, which seriously restricts their practical applications. In this thesis, I studied both the low-cost hybrid organic/inorganic perovskite solar cells (PSCs) and the earth abundant materials based photoelectrochemical water splitting devices to establish a self-powered solar driven water splitting system.
The structure is important to the carrier transport and recombination in PSCs. In the first attempt, I designed a type of hierarchical dual scaffolds to enhancing the charge separation and collection efficiency of semitransparent (ST) PSCs. The dual scaffolds consist of a quasi-mesoscopic inorganic (TiO2) layer and a percolating organic (PCBM)
manifold throughout the capped or filled perovskite bulk. It is demonstrated that the soft PCBM scaffold affords efficient charge separation due to the formation of a penetrating network intimately interfaced with perovskite crystals, meanwhile the quasi-mesoporous hard TiO2 scaffold strongly based on the substrate further offers a continuous electron transport. As the result, the ST-PSCs based on the ultrathin perovskite layer (~100 nm) with the dual-scaffolds have achieved an internal quantum efficiency of approximately 100%, boosting the device efficiency to 12.32%. Furthermore, the real ST-PSCs fabricated by replacing the Ag electrode with a PEDOT:PSS transparent electrode have reached an efficiency of 8.21% with an average visible transmittance of 23%, placing among the highest performing devices of the kind reported to date. Apart from the scaffold, the crystal domain structure is also important. In pursuit of the large grain size, I have identified a MA2Pb3I8(DMSO)2 intermediate phase formed during the solvent vapor annealing process of MAPbI3 in DMSO atmosphere and located the reaction sites at perovskite grain boundaries by observing and rationalizing the growth of nanorods of the intermediate nanorods. This enabled us to propose and validate an intermediate assisted grain-coarsening model, which highlights the activation energy reduction for grain boundary migration. Leveraging this mechanism, I used MABr/DMSO mixed vapor to further enhance grain boundary migration kinetics and successfully obtained even larger grains, leading to an impressive improvement in power conversion efficiency (17.64%) relative to the pristine PSCs (15.13%). The revelation of grain boundary migration assisted grain growth provides a guide for the future development of polycrystalline perovskite thin film solar cells.
In the solar fuels technology, the development of a PCE device is also crucial. To understand the structure dependent properties of mesoporous BiVO4 film, I systematically study the origin of the changed photoelectrochemical performance under FTO-side illumination (F-illumination) than that under BiVO4-side illumination (B-illumination) with different thickness. Via intensity-modulated photocurrent spectroscopy in conjunction with modeling simulation of electron diffusion inside
mesoporous BiVO4 films with different thicknesses, I find that the F-illumination is more tolerant to recombination than the B-illumination, leading to a higher charge separation efficiency of the former. Specifically, I have identified a trap-free electron transport region of BiVO4 vicinal to the FTO substrate and a trap-limited transport region farther away under F-illumination, whereas only a trap-limited transport exists under B-illumination. Simulated results accord well with the experimental data and further provide a deep insight of the detailed electron transport behavior: it is the higher electron density in the region proximal to the FTO under F-illumination that led to the greater recombination tolerance than under B-illumination. Such a photo-generated electron transport characteristic in mesoporous films is expected to be common for other semiconductors and will inspire practicable strategies for designing high efficiency semiconductor nanostructure based photoelectrochemical devices. To exploit the previous findings, I develop a cost-effective stamping method to fabricate inverse nanocone array (ICA) substrates for supporting nanoporous Mo-doped bismuth vanadate (BiVO4) films. By utilizing this architecture, the incompatibility of its light absorption capability with the short carrier transport length could be addressed. The ICAs show a remarkable light trapping effect as the intensive light absorption region is advantageously shifted from the top of the active layer on the planar substrate to the bottom surrounded by the ICA, where charge separation is efficient. The ICA supported Mo:BiVO4 photoanode with co-catalyst delivers a photocurrent of 6.01 mA cm-2 at 1.23 V versus reversible hydrogen electrode. I also demonstrate the PEC cell in tandem with a tailor-made, bandgap-adjustable perovskite solar cell achieves a self-powered solar-to-hydrogen (STH) efficiency of over 7%.