Study of Carrier Generation in Titanyl Phthalocyanine (TiOPc) by Electric-Field-Induced Quenching of Integrated and Time-Resolved Fluorescence

Abstract

We have studied the influence of electric field and humidity on photoconductivity and fluorescence in particles of highly photoconductive Y-form TiOPc dispersed in a poly(vinylbutyral) polymer matrix. Both integrated and time-resolved fluorescence quenching by electric field were measured. Integrated fluorescence quenching showed a linear dependence at low applied fields and a linear correlation with carrier generation efficiency. Time-resolved fluorescence decays were analyzed by fitting the data to a sum of two exponentials representing fast and slow fluorescence components. Qualitative features of fluorescence decay were the same for low and high humidity levels. The amplitude and lifetime changes of the two fluorescence components upon the application of the electric field are in agreement with theory which describes energy transfer between free and trapped exciton states. These results indicate that carrier generation in Y-TiOPc originates from both relaxed and nonrelaxed intrinsic excited singlet states, while the trapped excitons do not lead to significant carrier production. The field dependence of integrated fluorescence quenching supports the existence of a carrier precursor state with charge-transfer character.

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

“…(4) at temperatures above ~210 K. In addition, no changes in the PL lifetimes were observed as a function of applied electric field at either temperature, and the electric field-induced changes in the PL emission were mostly incorporated in those of the amplitude a 1 , thus occurring within ~200 ps, limited by the time resolution of our PL lifetime measurements. 58 As there was no indication of an increase in the free carrier yield accompanying the PL quenching (discussed below), 60 we attribute the fast PL quenching to electric field-enhanced nonradiative recombination, as previously observed in conjugated polymers, 64 which favorably competes with the thermally-activated nonradiative recombination at low temperatures (Sec. IV.A, Fig.11).…”

“…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).…”

Organic Photovoltaic Cells Based On Solvent‐Annealed, Textured Titanyl Phthalocyanine/C60 Heterojunctions

“…720 nm in as-deposited thin films. [1,4,[29][30][31][32][33][34][35][36][37][38][39] These Pcs, however, are easily converted to polymorphs, which extends their activity out to ca. 900-1000 nm.…”

“…[29][30][31][32][33][34][35][36][37][38][39] The near-IR photoelectrical activities of TiOPc and VOPc as crystalline dispersions in polymer hosts and as vacuum deposited thin films, have been extensively studied. [29][30][31][32][33][34] The broad visible to near-IR Q-band spectrum in the Phase II polymorph of TiOPc ( Fig. 1) arises from a combination of exciton splitting due to cofacial aggregation of the Pc, and from charge transfer character in the absorbance band of this polymorph.…”