Yun-Wen Chen

The quick rising records of energy conversion efficiency in applying Hybrid Halide Perovskites to solar cells open a new avenue in materials science recently. The interesting combination of organic cation (e.g. methylammonium (MA) molecule) and inorganic framework (e.g. PbI6) results in small effective masses of excited electrons and holes and small bandgap (~1.6 eV), which are believed to be the origins of high energy conversion efficiency. To improve the efficiency, several strategies including replacing/substituting organic cation, inorganic cation and anion, applying external pressure, and electric field are suggested. However, there are still more details in atomic level have not been investigated to well understand the fundamental properties of this type of materials. Take the most studied prototype, MApbI3 as an example, the inherent dipoles on MA molecules and hydrogen bonds existing between MA molecules and PbI6 framework catch academic interests recently. The electric-hysteresis, ferroelectric properties, and the materials' performance may correlated to each other. Currently we are modeling different phases of to see the possibility of using E-field to tune the bandgap, etc. On the other hand, we also try to investigate the possible reason for abrupt enhancing bandgap of MApbI3 at high pressure.

The next study direction will be the interface problems existing in fabricating solar cells like defect effects, MApbI3/HTM, ETM (hole, electron transport materials). We are going to perform large scale DFT modelings and check the chemical, physical properties, and other related statistics at interfaces.

Improving the new hydrogen storage using porous frameworks

To realize the hydrogen economy, developing cheap and safe hydrogen storage systems will be one of the main obstacles to overcome. Hydrogen is selected to be a possible replacement for fossil fuels because of its high energy density per weight. However, hydrogen is hard to store in a small volume for applications of using it as fuel in vehicle. Both traditional ways like compressing and liquefying hydrogen need to expend a large amount of energy of hydrogen chemical energy. Solid hydrides and porous materials are two kinds of systems proposed in recent decades. Although systems using solid hydrides have small volume; however, they are usually too heavy and they adsorb hydrogen too strong that hydrogen only can be released at very high temperature. On the other hand, ordinary porous materials usually physisorb hydrogen and can only keep hydrogen in system at very low temperature like 77K. And another shortcoming of porous materials is low volumetric density; ie. the volume will be too big to store enough hydrogen as fuel to drive a car traveling 300 Km.

Several strategies were proposed to improve the performance of using porous materials to store hydrogen. Doping transition metal atoms can adjust hydrogen adsorption energy for proper operating temperature. Different combination of building blocks can adjust the volumetric and gravimetric densities of a porous framework. With the tools of computational modeling, our recent study is to find out is it possible to build a feasible framework for hydrogen storage using silsesquioxane cages, phenyl group molecules, and transitional metals as building moieties.

Investigating the suitable materials for photocatalytic water splitting

Computational model of water splitting processes at

GaN surface decorated with a Pt4 cluster

  The crisis of upcoming global petroleum shortage stimulates the investigations of possible renewable energy resources for human’s future usage. Harvesting sunlight energy through photocatalytic water splitting is one of the methods believed to be feasible in mass production for getting hydrogen gas. Photocatalysts like TiO2, (Ga1-xZnx)N1-xOx, SrTiO3 … combining with various co-catalysts were studied experimentally and theoretically to look for the most efficient ways of transforming sunlight energy into chemical energy. However, there are still lots of rooms for improving the overall performance after many tries and errors in recent few decades. The understanding of its detail mechanism is essential for the design of a stable and efficient sunlight harvesting system. Nevertheless, in many cases the atomic level details are inaccessible for limited resolution in experiments. With the help of computational modeling, we try to investigate what actually happened in photocatalytic water splitting processes in terms of charge transfer, reaction energy barriers, density of states, and etc. With the power of parallel computing cross many processors, modeling complicated systems which are closer to experimental setup becomes possible.

    One of our study interests is to investigate the underneath mechanism in water splitting and hydrogen gas production. At the same time, we also aim at exploring suitable materials for photocatalysts and co-catalysts. We have studied the water adsorption behaviors on GaN polar (0001), non-polar (1010) surfaces and also the effect of ZnO doping on surfaces. Some results are published in 2013. Recently, we are focusing on the character of co-catalyst in water splitting processes and also investigating some new materials, like perovskites, for this application.