Highly Sensitive Determination of the Polaron-Induced Optical Absorption of Organic Charge-Transport Materials

We examine polaron-induced absorption in organic transport materials using a highly sensitive measurement technique. A hole only device is embedded into a low-loss TE(2) waveguide structure, and the current induced change of the waveguide absorption is measured. The exemplary study of 2,2('),7,7(')-tetrakis(N,N-diphenylamine)-9,9(')-spiro-bifluorene (S-TAD) reveals a very low polaron absorption cross section of sigma(p)<= 2.6x10(-18) cm(2) for 560 nm <=lambda <= 660 nm. The accuracy of this data is unsurpassed by other techniques used for the unambiguous study of polaronic species in organic thin films.