We are interested in light-matter interaction, with a particular focus on many-body interactions such as polaron formation, exciton fission, and Auger recombination. These problems underlie a range of optoelectronic processes, including light emission, lasing, and solar energy conversion. A main research thrust in the latter is to explore hot carrier or multi-exciton harvesting to increase solar cell efficiency beyond the Shockley-Queisser limit. We employ state-of-the-art experimental techniques, many of them based on ultrafast lasers, to study interesting material systems (organic semiconductors, hybrid semiconductors, quantum confined materials, and 2D van der Waals semiconductors and interfaces, etc.). Our favorite tools/toys include, among others, time-resolved two-photon photoemission spectroscopy (TR-2PPE), femtosecond nonlinear optical spectroscopies, transient absorption and time-resolved fluorescence spectroscopies/microscopies, 2D electronic spectroscopies, and scanning probe spectroscopies. As examples, recent discoveries in our lab showed how an electron and a hole is bound by the Coulomb potential across an organic semiconductor interface, how an exciton can split into two via the singlet fission process, and how hybrid semiconductors can yield the highest possible quantum efficiency in light emission.
Haiming Zhu, Yongping Fu, Fei Meng, Xiaoxi Wu, Zizhou Gong, Qi Ding, Martin V. Gustafsson, M. Tuan Trinh, Song Jin, X.-Y. Zhu, “Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors,” Nature Mater. 2015, 14, 636-642.
Nicholas R. Monahan, Kristopher W. Williams, Bharat Kumar, Colin Nuckolls, X.-Y. Zhu, “Direct observation of entropy driven electron-hole pair separation at an organic semiconductor interface,” Phys. Rev. Lett. 2015, 114, 247003.
Wu, M. T. Trinh, D. Niesner, H. Zhu, Z. Norman, J. S. Owen, O. Yaffe, B. J. Kudisch, X.-Y. Zhu, “Trap States in Lead Iodide Perovskites,” J. Am. Chem. Soc. 2015, 137, 2089–2096.
Jailaubekov, A. P. Willard, J. Tritsch, W.-L. Chan, N. Sai, R. I. Gearba, L. G. Kaake, K. J. Williams, K. Leung, P. J. Rossky, X.-Y. Zhu, “Hot charge transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics,” Nature Mater. 2013, 12, 66-73
W.-L. Chan, M. Ligges, X.-Y. Zhu, “The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain,” Nature Chem. 2012, 4, 840-845.
W.-L. Chan, M. Ligges, A. Jailaubekov, L. Kaake, L. Miaja-Avila, X.-Y. Zhu, “Observing the Multi-Exciton State in Singlet Fission and Ensuing Ultrafast Multi-Electron Transfer,” Science 2011, 334, 1541-1545.
A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil, X.-Y. Zhu, “Hot electron transfer from semiconductor nanocrystals,” Science 2010, 328, 1543-1547.