Loi Group Topics
On this page we highlight a few of the topics that we work on. In general our group researches novel materials for solar cell & microelectronics applications. In our case these materials have in common that they are solution processable. This property holds the promise of cheap production methods with a low energy demand.
Our group focusses on a number of subjects:
- The properties of organic semiconductors and organic/organic interfaces and their application in optoelectronic devices
- Physical and optoelectronic properties of carbon nanotubes and hybrids systems
- Fabrication of hybrid optoelectronic devices composed by inorganic nanocrystals and organic molecules
A few examples of the projects within our group:
Selective dispersing of semiconducting carbon nanotubes
Single-walled carbon nanotubes (SWNTs) are among the most promising materials for the future optoelectronics beyond silicon, due to its extraordinary high carrier mobility and high mechanical, thermal, and chemical stability. However, the coexistence of semiconducting and metallic species in the as-grown SWNTs remains a big challenge for its applications. Our research group has advanced in the separation of semiconducting SWNTs using polymer-wrapping method, and study their photophysical properties as well as their applications in transistors and solar cells. We employed different types of conjugated polymers, joint collaboration with Ullrich Scherf groups at Wuppertal University, to selectively dispersed semiconducting SWNTs, aiming for a better understanding of the physical selection mechanism of this technique and further obtaining high purity and high extraction yield. (Widi Gomulya)
Photoelectrochemical (PEC) water splitting
Semiconductor nanocrystals (NCs) and semiconducting single-walled carbon nanotubes (SWNT) have emerged as candidates for light harvesting materials for photovoltaics devices that convert sunlight into electricity. However, since sunlight is an intermittent energy source, a buffer medium, which guarantees a continuous supply of energy, is necessary. The major drawback of electrical energy is the difficulties to store it. Hydrogen is an ideal energy buffer medium and energy carrier because: (1) the raw material (water) for producing hydrogen is abundant; (2) it can be converted into electricity by using fuel cells without producing pollutants 1; (3) H2 storage has progressed rapidly in the last decade. 2 The cell that can directly convert the solar energy into hydrogen is called photoelectrochemical water splitting device. In this project we study the way to harness sunlight and convert it to hydrogen by s-SWNT, NCs and organic-inorganic hybrids. 3 (Lai-Hung Lai)
- B. C. Steele and A. Heinzel, Nature, 2001, 414, 345-352.
- K. J. Jeon, H. R. Moon, A. M. Ruminski, B. Jiang, C. Kisielowski, R. Bardhan and J. J. Urban, Nat. Mater., 2011, 10, 286-290.
- L. H. Lai, W. Gomulya, M. Berghuis, L. Protesescu, R. J. Detz, J. N. Reek, M. V. Kovalenko and M. A. Loi, ACS Appl. Mat. Interfaces, 2015, 7, 19083-19090.
Colloidal Quantum Dots
Colloidal quantum dots (CQDs), are a promising and versatile class of material for opto-electronic applications. The optical bandgap can be varied throughout the near-infrared (0.8-2µm) range simply by changing the size of the nanocrystals. This flexibility makes them interesting as IR-photodetectors and emitters, and also makes them ideal candidates for the active layer of solar cells. Using solution-based processing techniques, we can stack organized arrays of these QDs into a thin film; the electronic properties of which we can control by changing the surface chemistry of QD, for example by changing the ligands, or by putting a shell around them. This gives us a large toolbox to explore the physics of these artificial solids. In our group, we focus mainly on the following topics using CQDs. (Mark Speirs)
- Controlling the surface chemistry via inorganic or organic ligand exchange, both in thin film and in solution.
- Controlling the self-assembly of the CQDs to form highly ordered superlattices.
- Fabricating transistors to explore the electronic properties and the transport mechanisms of the CQD solids.
- Fabricating solar cells with various structures and deposition methods.
Over the past five years, intensive research efforts have been devoted to improving the power conversion efficiency (PCE) of the organomteal halide perovskite solar cells by developing strategies for perovskite film growth, the device structure, optimizing the interfacial layers, the composition in perovskite film, which have brought fruitful results with PCE increased to over 20%. Accompanied with the sky-rocketing increase in the performance are several key questions unresolved, such as hysteresis and slow photo-conductivity response phenomenon in perovskite solar cells.
We investigate how the bulk and interfacial properties of perovskite film influence the device performance by control the processing condition and optimizing the interfacial materials. Combining SEM/AFM measurements with steady state and time resolved photoluminescence study, the variation of device performance are correlated with the bulk and interfacial properties of perovskite film. (Shuyan Shao)
Optical spectroscopy is a very powerful, non-destructive tool to investigate excited state dynamics following photoexcitation in a broad range of semiconducting materials. We are able to carry out absorption spectroscopy, steady-state and time-resolved photoluminescence spectroscopy, pump-probe measurements and confocal microscopy on a range of materials which can be either in solution, as films, single crystals or in working devices such as solar cells and transistors. From these measurements, we gain information about fundamental electronic processes, interfaces (if any) as well as the (electronic) environment.
Our research interests include:
(1) Charge generation and transfer processes in organic semiconductors
(2) Excited state dynamics in electronically coupled colloidal nanocrystals using different ligands
(3) Assessing the efficiency of polymer-assisted separation of carbon nanotubes and
(4) Hybrid perovskites, where for example we can study changes in the dynamics as a function of composition (of say, the organic cation).