Study of Carrier Generation in Phthalocyanines by Time-Resolved Fluorescence

Abstract

We have studied the influence of electric field on fluorescence in particles of photoconductive TiOPc (I), TiOPc (IV), HOGaPc, and x-H2Pc dispersed in a polymer matrix. Electric field induced quenching of both the integrated and time-resolved fluorescence were measured. Time-resolved fluorescence decays were analyzed by fitting the data to a sum of two exponentials, representing the fast and slow fluorescence components. For HOGaPc, TiOPc(I), and TiOPc (IV), the fast fluorescence component exhibits both amplitude and lifetime quenching. These results indicate that carrier generation in HOGaPc, TiOPc(I), and TiOPc (IV) originates from both relaxed and nonrelaxed intrinsic excited singlet states, while the trapped excitons do not lead to significant carrier production. In contrast, for x-H2Pc, significant amplitude quenching of the fast component is observed only at high field, and the trapped excitons are an important source of photogenerated carriers. This indicates that x-H2Pc possesses at least some bulk-sensitized photocarrier generation. All of the phthalocyanines studied exhibited a quadratic dependence of integrated fluorescence quenching on electric field, indicating the existence of a neutral carrier precursor state.

Temperature dependence of exciton and charge carrier dynamics in organic thin films

“…At either temperature no changes were seen in absorption spectra at the same electric fields, which rules out the Stark effect. 57 From the integrated PL spectra measured in the presence and in the absence of electric field (I PL (E) and I PL (0), respectively), we calculated the parameter Q, which quantifies the amount of the PL quenching, using 57,59-61…”

“…5 The electric field dependence of Q can help identify the nature of the exciton; for example, linear (quadratic) electric field dependence of Q has been reported in charge-transfer (neutral) excitons dissociating into free carriers. 57,58 The dependence of Q on the electric field observed in ADT-TES-F films at low temperatures and at electric fields of above ~10 5 V/cm is considerably stronger than quadratic and can be better described by a one-dimensional model for tunneling through a parabolic potential barrier:…”

“…The observed changes in a 1 indicate that most nonradiative recombination responsible for strong temperature dependence of the PL QY occurred at shorter time scales than the time resolution of our PL lifetime measurements (~200 ps). 57,58 Based on the observations above, the PL lifetimes τ 1 and τ 2 may be tentatively assigned to excitons that sample the entire thermally accessible portion of the exciton bandwidth and only the top part of it, respectively (Sec. IV.A, Fig.11).…”

Electromodulation of photoluminescence in vacuum-evaporated films of fac-tris(2-phenylpyridine)iridium(III)

“…In this context we recall also the recent transient EML experiments where PL decays affected by external electric fields are measured in TCTA:Ir(ppy) 3 [11] and PVK:Ir(ppy) 3 [47] systems. In contrast to stationary measurements, the transient EML experiments allow to distinct amplitude-type quenching (when population of excited states is decreased) from rate-type quenching (when the lifetime of emissive state is reduced) [48,49]. Interestingly, both in TCTA:Ir(ppy) 3 and PVK:Ir(ppy) 3 systems the decay time of Ir(ppy) 3 emission was independent of electric field and the amplitudetype PL quenching was observed.…”

Delayed electroluminescence: a new tool for studies of OLED fundamental properties

“…If, on the other hand, the delayed light emission is produced by triplet-triplet annihilation process, we may expect some change of the delayed light emission caused by electric field induced quenching of the excited state emission (process well documented in many organic photoconductors). 6,7 This quenching, however, will be much more pronounced for the singlet state as in general it has a higher energy than a triplet state and therefore is more easily dissociated by external electric field. Removal of reverse bias will therefore be also expected to lead to an increase of delayed light emission, as electric field induced singlet quenching process is eliminated.…”