Radiative and nonradiative recombination in polymerlikea−C:Hfilms

Abstract

Localized electronic states corresponding to \ensuremath{\pi} electrons in hydrogenated amorphous carbon have been investigated using photoluminescence (PL) and optical-absorption spectroscopies. Carbon films contain \ensuremath{\pi}-bonded ``grains'' with different configurations which spatially confine photogenerated electron-hole pairs. A model of PL in $a\ensuremath{-}\mathrm{C}:\mathrm{H},$ including excitonlike radiative recombination and electron tunneling to nonradiative acceptor centers at constant energy, is proposed to explain the following results. The broad PL emission spectra are composed of three bands (at 2.20--2.40 eV, 2.65 eV, and 2.95 eV, independent of the excitation energy) arising from different structural units containing $\mathrm{C}\mathrm{=}\mathrm{C}$ double bonds. Resonance features have been evidenced in the PL excitation spectra and attributed to excitation of confined electron-hole pairs similar to confined exciton optical transitions. Excitonic behavior is consistent with the redshift of peak (b) excitation resonance observed as a function of increasing gap (decreasing \ensuremath{\varepsilon}). Relative values of the radiative rates deduced from PL efficiency indicate that the exciton radius has the same order of magnitude for all three emissive centers. Nonradiative recombination cannot be explained by multiphonon emission processes. The dependence of PL efficiency on the density of acceptor sites (deduced from $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ optical transitions) can rather be described by electron tunneling from a confined exciton toward nonradiative localized ${\ensuremath{\pi}}^{*}$ states arising from distorted \ensuremath{\pi}-bonded sites. This mechanism accounts for the anticorrelation of the PL efficiency with the optical gap, the decrease of the decay time as a function of emission energy, and the relative enhancement of the blue emission (2.95-eV peak) as a function of decreasing optical gap. In contrast, the band-tail model proposed earlier leads to unrealistically low values of the electron Bohr radius.