Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)

Abstract

We introduce a version of the cw photomodulation technique, measured far from the steady state, for obtaining the quantum efficiency, , of long-lived photoexcitations in -conjugated polymers. We apply this technique to films of a ladder-type poly͑para-phenylene͒ ͓mLPPP͔ for studying the photogeneration action spectra, (E), and recombination kinetics of photogenerated neutral and charged excitations such as singlet and triplet excitons and charged polarons. Whereas the (E) spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap (Ӎ2.6 eV), both triplet and polaron (E) spectra show, in addition, a monotonous rise at higher E. The rise for triplets is explained by singlet exciton fission into triplet pairs, and from a model fit we get the triplet exciton energy (Ӎ1.6 eV). For polarons this rise is modeled by an electron intersegment tunneling process. The electroabsorption spectrum is also measured and analyzed in terms of Stark shift of the lowest lying exciton, 1B u , and enhanced oscillator strength of the important mA g exciton. A consistent picture for the lowest excited state energy levels and optical transitions in the neutral ͑singlet and triplet͒ and charged manifolds is presented. From both the exciton binding energy of Ӎ0.6 eV and the singlet-triplet energy splitting of Ӎ1 eV, we conclude that the e-e interaction in mLPPP is relatively strong. Our results are in good agreement with recent ab initio band structure calculations for several -conjugated polymers. ͓S0163-1829͑99͒13531-8͔

Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

“…The spectrum clearly shows that the absorption feature of the triplet exciton at 1.3 eV and that of the polaron at 1.9 and 2.1 eV are both reduced at the field for the spin-1 2 resonance as observed for measurements taken in an S-band microwave cavity. 9 As expected, for the same microwave power, 27 band at 1.9 eV, is much stronger at the X band, where it is approximately 6%, than at the S band, where it was found to be about 3% at a similar microwave power. ␦T/T exhibits a power-law dependence similar to that of the polaron PLDMR ⌬I PL /I PL :…”

“…22 However, as mentioned above, recent PADMR results have shown that as one reduces the population of polarons at the spin-decrease in the TE population. 9 Since an enhancement of the TE population is a key requirement for the observation of an enhancement in I PL due to triplet-triplet annihilation, this second mechanism is ruled out.…”

“…PA-detected magnetic resonance ͑PADMR͒ measurements show that at the spin-1 2 field-forresonance the polaron and TE populations decrease. 9,20 Therefore, PL-detected magnetic resonance ͑PLDMR͒, which typically includes a PL-enhancing ͑i.e., positive͒ spin-1 2 polaron resonance, 21 is an ideal tool to probe the influence of the decreasing polaron population on the SE's. However, two mechanisms have been suggested to account for this resonance: ͑a͒ polarons acting as quenching centers for SE's; hence a reduced polaron population leads to a reduced rate of SE quenching, and consequently an enhanced PL intensity (I PL ) ͑Ref.…”

“…11͒ are nonradiatively quenched by polarons, 12 similar to quenching by defects such as carbonyl groups. 13 In contrast to molecular crystals such as para-hexaphenyl, 14 conjugated polymers exhibit strong polaronic photoinduced absorption ͑PA͒ bands, 9,15 so there is a need to quantify the interaction of polarons and SE's. Moreover, single crystals of small -conjugated molecules such as tetracene, which do not contain grain boundaries defects that stabilize polarons, 16 have very recently led to electrically pumped organic solid-state lasers.…”

“…4,5 From previous studies on m-LPPP a comprehensive picture of the generation of polarons [6][7][8] and triplet excitons 8,9 ͑TE's͒ from singlet excitons ͑SE's͒ was drawn. It is well known that polarons, as found in OLED's upon injection of carriers, 10 recombine to form SE's, which generate the electroluminescence ͑EL͒.…”