Probing triplet transport in singlet fission materials can be challenging due to the presence of multiple diffusing species. We present a device-based method to measure the intrinsic triplet diffusion length (LD) in organic semiconductor thin films exhibiting singlet fission. Triplet states are optically injected into the singlet fission material of interest via energy transfer from an adjacent thin film characterized by strong spin-orbit coupling. Injected triplets migrate through the full thickness of the material before undergoing dissociation at a donor-acceptor interface. By modeling the ratio of injector and acceptor photocurrent as a function of layer thickness, the triplet LD is extracted separate from processes of unknown efficiency including singlet fission and diffusion. In considering three archetypical fission systems, a wide range is found for the triplet LD, ranging from 3.3 ± 0.4 nm for 5,12-bis((triisopropylsilyl)ethynyl)tetracene to 17.1 ± 1.3 nm for pentacene and 32.1 ± 2.6 nm for tetracene.
Bibliographical noteFunding Information:
This work was supported by National Science Foundation (NSF) Electronics, Photonics and Magnetic Devices under ECCS-1509121 and Solid-State and Materials Chemistry under DMR-1708177. T.Z. acknowledges support from a University of Minnesota Doctoral Dissertation Fellowship. R.J.H. acknowledges support from Ronald L. and Janet A. Christenson, a Leverhulme Trust Visiting Professorship at the University of Cambridge, and a Visiting Fellowship at Clare Hall, University of Cambridge. R.J.H. also thanks Dr. Simon Dowland, Dr. Aksahy Rao, and Prof. Richard Friend for useful discussions.