Electron Spin Resonance Spectroscopic Study of Electronic Charge Transport in an Aromatic Diamine

Article abstract of DOI:10.1021/j100379a016

Electron spin resonance (ESR) spectra have been obtained for the radical cation of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-4,4'-diamine (TPD) in dichloromethane solution, in solid amorphous films, and in TPD/polycarbonate films doped with tris(p-bromophenyl)ammonium hexachloroantimonate or HNO3.By use of ESR the mediation of electronic charge (hole) transport via the TPD+ radical cation has been observed in the TPD films in the absence of an applied field.Arrhenius activation parameters were calculated for charge transport from the ESR data, giving E_A = 10 +/- 2 kJ/mol Α = (1.7 +/- 1) * 10¹⁰ s^-1, and a rate constant at 300 K of (3.1 +/-1) * 10⁸ s^-1.The value of E_A is one-half that from time-of-flight (TOF) measurements extrapolated to zero field, while the rate constant is a factor of 10 smaller, and Α is a factor of 1000 smaller.The differences can be understood in terms of the compensation effect due to the presence of residual solvent, and the ion pairing of an TPD+ ions with dopant counterions.In TPD/polycarbonate films the rate of hole transport is too slow to produce significant changes in the ESR spectrum.The rate of charge transport is thus less than 2*10⁸ s^-1 in these films, consistent with TOF data.In dichloromethane solution, an E_A of 9.8 kJ/mol was observed with ESR.Extrapolating the solution data to solid TPD gives a hole transport rate of 9.75 * 10⁹ s^-1, a factor of 3 higher than that from TOF data.The lower E_a and higher rate in dichloromethane solution are consistent with the effect of the higher dielectric constant compared to an TPD film.This suggests that the rate-determing step for hole transport is the same in solution as it is in the solid state.