We develop a systematic approach of quantifying spin-orbit coupling (SOC) and a rigorous theory of carrier spin relaxation caused by the SOC in disordered organic solids. The SOC mixes up- and down-spin in the polaron states and can be characterized by an admixture parameter. The spin relaxation rate is found to be proportional to the carrierhopping rate, or equivalently, carrier mobility. The spin diffusion length depends on the spin mixing and hopping distance but is insensitive to the carrier mobility. The SOCs in tris-(8-hydroxyquinoline) aluminum (Alq₃) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu 3d orbitals in the latter.The theory quantitatively explains the recent measured spin diffusion lengths in Alq₃ from muon spin rotation and in CuPc from spin-polarized two-photon photoemission.

Date Published: 22 February 2012
PDF: 5 pages
Proc. SPIE 8333, Photonics and Optoelectronics Meetings (POEM) 2011: Optoelectronic Devices and Integration, 83331A (22 February 2012); doi: 10.1117/12.919390

Published in SPIE Proceedings Vol. 8333:
Photonics and Optoelectronics Meetings (POEM) 2011: Optoelectronic Devices and Integration
Erich Kasper; Jinzhong Yu; Xun Li; Xinliang Zhang; Jinsong Xia; Junhao Chu; Zhijiang Dong; Bin Hu; Yan Shen, Editor(s)