In spintronics, electrons’ spin degree of freedom plays a key role in transmitting, processing and storing information, revolutionising the current electronics based technology. Organic semiconductors (OSCs) are being investigated as an emerging class of materials for electronics and spintronics, not only because of their ease of low-temperature solution processing, but also because of their unique functional properties, such as a long spin lifetime that reflects their light-atom, mainly carbon-based, composition. We are investigating fundamental physics such as spin relaxation, spin transport, spin statistic and spin-to-charge conversion in organic semiconductors.
Spin current phenomena in OSCs
A pure spin current is a flow of electron spin angular momentum without a simultaneous flow of charge current. We have demonstrated for the first time that polaron, charge carriers in π-conjugated organic materials, can carrier pure spin currents over hundreds of nanometers, with extremely long spin lifetimes up to a millisecond1. Apart from offering technological benefits, such as reduced heat dissipation, pure spin currents also allow the observation of new physics, such as the conversion of a spin current into a transverse charge current, inverse spin Hall effect. Recently, we have reported the first observation of the inverse spin Hall effect in a highly doped, conducting polymer2 .
Spin-dependent recombination in optoelectronic materials and devices
In organic semiconductors, spin plays a critical role in determining which recombination pathways are possible. For example, an excitation in a spin triplet configuration cannot directly couple to the ground state which is a spin singlet. Manipulation of these spin states provides a way to directly probe these spin-dependent recombination processes, and explore the rich spin physics of organic semiconductors. We have demonstrated a new approach to controlling the spin statistics of recombining charges by spin polarizing carriers after injection using high magnetic fields and low temperatures, allowing us to observe drastic changes in device current when spin triplet electron-hole pairs are selectively populated3. Additionally, we have explored the spin-dependent recombination of triplet excitons formed by the process of singlet fission, in which two triplet excitons are formed from one singlet exciton. Using optically detected magnetic resonance we have been able to investigate the competing recombination pathways which present themselves to these triplet states4.
Charge transport probed by magnetic field
Magnetotransport such as Hall effect and magnetoresisistance measurements is powerful way to address the charge transport physics in organic semiconductors. We have revealed an ideal, free-electron-like charge transport in small molecule5, as opposed to the prediction of hopping transport. A combination of magnetotransport measurements, optical spectroscopy, and theoretical simulations elucidates structure-property relationships at a molecular level
S. Watanabe, K. Ando, K. Kang, S. Mooser, Y. Vaynzof, H. Kurebayashi, E. Saitoh, and H. Sirringhaus “Polaron spin current transport in organic semiconductors” Nature Physics 10, 308 (2014).
A. Ando, S. Watanabe, S. Mooser, E. Saitoh, and H. Sirringhaus “Solution-processed organic spin charge converter” Nature Materials, 12 622 (2013)
S. L. Bayliss, A. D. Chepelianskii, A. Sepe, B. J. Walker, B. Ehrler, M. J. Bruzek, J. E. Anthony and N. C. Greenham “Geminate and nongeminate recombination of triplet excitons formed by singlet fission” Physical Review Letters 112, 238701 (2014).
J. Wang, A. D. Chepelianskii, F. Gao, and N. C. Greenham “Control of exciton spin statistics through spin polarization in organic optoelectronic devices” Nature Communications 3, 1191 (2012)
J. F. Chang, T. Sakanoue, Y. Olivier, T. Uemura, M. B. Dufourg-Madec, S. G. Yeates, J. Cornil, J. Takeya, A. Troisi and H. Sirringhaus “Hall-effect measurements probing the degree of charge-carrier delocalization in solution-processed crystalline molecular semiconductors” Physical Review Letters 107, 06601 (2012).