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 TE₂ 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 σₚ≤2.6×10^-18 cm² for 560 nm ≤λ≤660 nm. The accuracy of this data is unsurpassed by other techniques used for the unambiguous study of polaronic species in organic thin films.