Several spectroscopic methods were applied to study the characteristic properties of the electronic excitations in thin films of regioregular and regiorandom polythiophene polymers. In the regioregular polymers, which form two-dimensional lamellar structures, increased interchain coupling strongly influences the traditional one-dimensional electronic properties of the polymer chains. The photogenerated charge excitations (polarons) show two-dimensional delocalization that results in a relatively small polaronic energy, multiple absorption bands in the gap where the lowest energy band becomes dominant, and associated infrared active vibrations with reverse absorption bands caused by electron-vibration interferences. The relatively weak absorption bands of the delocalized polaron in the visible and near-infrared spectral ranges may help to achieve laser action in nanocrystalline polymer devices using current injection.
Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly‐3‐alkyl thiophene with regioregular order, which forms self‐assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain‐like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long‐lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long‐lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano‐type interference mechanism, using the amplitude mode model.
We have measured the ratio, r = σS/σT of the formation cross section, σ of singlet (σS) and triplet (σT ) excitons from oppositely charged polarons in a large variety of π-conjugated oligomer and polymer films, using the photoinduced absorption and optically detected magnetic resonance spectroscopies. The ratio r is directly related to the singlet exciton yield, which in turn determines the maximum electroluminescence quantum efficiency in organic light emitting diodes (OLED). We discovered that r increases with the conjugation length, CL; in fact a universal dependence exists in which r −1 depends linearly on CL −1 , irrespective of the chain backbone structure. These results indicate that π-conjugated polymers have a clear advantage over small molecules in OLED applications.The efficiency of fluorescence-based organic light emitting diodes (OLED) is determined by the fraction of injected electrons (e) and holes (h) that recombine to form emissive spin-singlet excitons, rather than the nonemissive spin-triplet excitons. If the process by which these excitons form were spin-independent, then the maximum quantum efficiency, η max of OLEDs would be limited to 25% [1], which is the statistical limit. The reason for the 25% statistical limit is that the combination of two spin-1/2 particles gives four possible total spin states; three of which have total spin 1, only one is a singlet state. But recent reports have indicated that η max in π-conjugated OLEDs ranges between 22% to 63% [2-6], and the reason for this variation is under investigation. In particular the dependence of η max on the conjugation length (CL) was recently tested by measuring η max in a monomer and related polymer, and the possibility that η max increases with the CL in π-conjugated materials was advanced [6].For systems which are light emitting the quantum efficiency, η EL for electro-luminescence (EL), is η EL = η 1 η 2 η 3 , where η 1 is the singlet emission quantum efficiency, η 2 is the fraction of the total number of excitons that are singlets, and η 3 is the probability that the injected e and h find each other to form e-h pairs [7]. Since both η 1 and η 3 < 1, it follows that η EL < η 2 = η max . We have recently developed [5] a spectroscopic technique based on photoinduced absorption (PA) and PA-detected magnetic resonance (PADMR) spectroscopies, which allows direct measurement of the ratio r = σ S /σ T of the formation cross-section, σ of singlet (σ S ) and triplet (σ T ) excitons from oppositely charged polarons in films of π-conjugated materials. In the limiting case that the spin relaxation rates are much faster than the exciton formation rates from e-h polaron pairs we showed that η max = (1 + 3r. Thus the study of the ratio r in organic materials also provides information about η max in OLEDs. Using this spectroscopic technique we could infer r [5], and consequently η max in a variety of π-conjugated materials without the need to fabricate OLEDs based on the particular material as the active layer. We found that r depends on the ma...
We observed photoinduced quantum interference antiresonances between several discrete infrared-active vibrations and the lower-polaron continuous absorption band in a series of pi-conjugated polymer films having superior planar orders, where the polaron transition energy is relatively small. The photoinduced Fano-type antiresonances are well explained by extending the amplitude mode model beyond the adiabatic limit. The agreement between the data and the model confirms the presence of a continuous electronic band above the polaron state. We show that high frequency modes are strongly coupled to electrons, with implications for superconductivity.
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