2 – Research

The Nature of Electronic Transport in The Array of Colloidal Quantum Dots

Devising method to scrutinize and improve carrier mobilities in colloidal quantum dot solar cells and field-effect transistors. Successfully enhanced charge carrier mobilities in ambipolar field-effect transistors of PbS nanocrystals by five-orders of magnitude to a record high value through the optimization of the colloidal quantum dots cleanliness, ligand selection, transistor architectures and the use of electric-double layer gating. Highly efficient solar cell based on PbS nanocrystals achieving 5.2% was demonstrated, despite the device structure was a very thin Schottky solar cell device with only 150 nm active layer. Focus on the surface modification and better ordering of the superlattice of colloidal quantum dot solids is being performed in order to investigate and reduce the charge carrier traps.

(S. Z. Bisri, et al. Advanced Materials 2013) (C. Piliego, S. Z. Bisri, et al. Energy and Environmental Science 2013)

Electronic Transport in Networks of Carbon Nanotubes

Fabricating solution-processed random-network carbon nanotubes field-effect transistors and measuring their ambipolar transports. Through purification and enrichment of semiconducting carbon nanotubes using polymer wrapping, record high ambipolar mobilities and on/off ratio were achieved for nanotube network transistors. The influence of carbon nanotube diameters on carrier transport under electric-double layer gating was investigated for the first time. A multidisciplinary approach was performed to investigate the capability of polymer wrapping technique to select different diameters of CNT, including the utilization of ultrafast photoluminescence spectroscopy, molecular dynamics simulation, as well as the comparison of device performance.

(S.Z. Bisri, et al. Adv. Mater. 2012) (W. Gomulya, S.Z. Bisri, et al. Adv. Mater. 2013)

Organic Semiconductor Laser

Investigating light amplification and gain in organic semiconductors, in particular organic single crystals, using ultrafast photopump excitation to estimate the required exciton density for laser action. Devising optical resonators: Fabry-Perot cavity, optical directional coupler, distributed Bragg reflector (DBR) microcavity, and distributed feedback (DFB) gratings, to lower the laser threshold of the organic single crystal system. Through the utilization of ambipolar light-emitting transistors, current-induced spectrally-narrowed emission was observed for the first time, which can become a precursor for lasing action. Investigation of the optical characteristics of the organic single crystals by using ultrafast photoluminescence spectroscopy is also being conducted.

(S.Z. Bisri, et al. Adv. Funct. Mater., 2009; S.Z. Bisri, et al. Scientific Reports, 2012).

Ambipolar Light-Emitting Transistors of Organic Single Crystals

Developed methods to fabricate ambipolar light-emitting transistors of organic single crystals and achieved record highest co-existing intensity and carrier mobilities. By using ambipolar light-emitting transistors as surface sensitive probes, the junction structure of the device was revealed as a p-i-n homojunction, and explained the origin of the stability of efficiency in this kind of device. Record efficiency stability at high current density (>10 kA/cm2) was achieved in ambipolar light-emitting transistor of organic single crystals, paving ways for the exploitation of this device for pursuing the current-driven organic laser realization. A multicolor ambipolar light-emitting transistors was also demonstrated owing to the unique dipole orientation of the molecules inside the single crystal.

(T. Takenobu, S.Z. Bisri, et al. Phys. Rev. Lett. 2008; S.Z. Bisri, et al. Adv. Mater. 2011) (Y. Yomogida, S. Z. Bisri, et al. Organic Electronics 2013).

The Fundamental Nature of Electronic Transport in Organic Single Crystal Transistors

Improved electron transport through the utilization of alkaline elec­trodes, multiple-step crystal purifications, and crystal/insulator interface modifications. Record high electron mobility in ambipolar field-effect transistors was achieved for rubrene single crystals and the electron trapping mechanism was investigated. Investigation of charge carrier injection from ferromagnetic electrode into the organic single crystal was also performed with an aim to achieve effective spin injection into organic single crystals.

(S.Z. Bisri, et al. Jpn. J. Appl. Phys. 2007; S.Z. Bisri, et al. Appl. Phys. Lett. 2010) (Y. Kitamura, S. Z. Bisri, et al. Appl. Phys. Lett. 2011).

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