J. Poplawski

Spin signature of photoexcitations in sexithiophene

Abstract We present photoexcitation studies of vacuum-deposited neutral films of sexithiophene (6T) using photoinduced absorption spectroscopy and photoinduced absorption detected magnetic resonance (PADMR) techniques. We find evidence for photoinduced polarons (photoexcited ‘radical ions’) having spin 1/2, with two absorption bands at 0.80 and 1.54 eV, respectively, and with negative PADMR signal at g ≅2. Similar absorption bands are observed in lightly p-doped 6T, and are interpreted as due to 6T + radical cations. In addition, a photoinduced absorption band is found at about 1.1 eV, which is shorter lived and decreases faster with increasing temperature than the polaron bands. We identify this band as due to spinless bipolarons (photoexcited ‘di-ions’). A similar di-cation band has been identified in heavily p-doped 6T. Finally, we have identified triplet excitons ( S =1) with triplet-triplet transition energy at about 1.45 eV.

Absorption spectroscopy of charged excitations in α-sexithiophene: evidence for charge conjugation symmetry breaking

Abstract We have studied the absorption spectra of charged excitations in sexithiophene thin films induced by doping and photogeneration. Absorption bands at 0.80 and 1.54 eV are attributed to spin - 1 2 polarons, whereas a single absorption band at 1.1 eV is due to spinless bipolarons. Charge conjugation symmetry breaking is evident for both polarons and bipolarons.

Photoexcitation spectroscopy of poly(3-methylthiphene): Electronic bands and vibrational modes

Abstract Photoexcitation spectroscopy, including photoluminescence, resonant Raman scattering and photoinduced absorption in the energy range 0.1eV to 2.5eV, is reported for the conjugated polymer poly(3-methylthiophene). Photoinduced electronic bands and infrared active vibrations characteristic of bipolarons were observed. Amplitude mode analysis of the Raman and photoinduced infrared modes indicates for this non-degenerate ground state system a one-dimensional gap of ⋍2.2 eV and an extrinsic component of ⋍0.22 eV . Broad cw photoluminescence band with phonon side bands are observed below the band gap and is assigned to weakly bound excitons.

Photoexcited defects in poly(3-methylthiophene): Light induced electron spin resonance and photoinduced absorption

Abstract Light induced electron spin resonance (LESR) and photoinduced absorption (PA) are reported for poly(3-methylthiophene) (P3MT). The LESR is composed of a single line at g≅2 whose position, width and intensity are dependent on the exciting photon energy, ηωL. The LESR line intensity shows a sharp peak at 2.05 eV, where the line is the narrowest and it has the smallest g. The PA spectrum is composed of infrared active vibrations (IRAV) absorption bands and two midgap electronic bands, at 0.39 and 1.28 eV. The intensity of these three features as a function of ηωL is similar to each other and shows a sharp peak at 2.10 eV. Based on these observations, the possible spin-charge relationships for the photoinduced defects are discussed.

Light activated p-n junction device based on bilayer substituted polythiophene derivatives

Abstract A new photoinduced current rectifier based on an all-organic donor-acceptor bilayer substituted polythiophene derivative is described. Under visible and UV illumination (350–750 run), a p-n junction is formed leading to current rectification. Maximum photorectified current is obtained at ≅400 run, with a sharp decrease at shorter wavelengths. This sharp decrease indicates that photons with energy higher than 3.1 eV quench the light activation of this bilayer device.

Protonation-deprotonation effects on the electrooptics of bipyridine containing PPV derivatives

The electrooptic properties of novel bipyridine-containing PPV derivatives were found to be sensitive to the presence of sub-ppm concentration of different acids and bases. Upon exposure to ammonia (or HCl) vapors, the polymer film turns yellow (orange) and the electro- and photoluminescence peak at 580 (640) nm. Using controlled exposure, stable yellow/orange luminescent devices could be fabricated. In addition, reversible decrease (increase) of the effect of aggregation is observed in the electro- and photoinduced absorption, upon the deprotonation (protonation) process.

Photoconductivity of acceptor substituted poly(3-butyl)thiophene

Abstract We Have studied acceptor substituted poly(3-butyl)thiophene (PBT) in order to examine the usefulness of the acceptor as an intrinsic charge trap under laser excitations. We have found considerable enhancement of the slow decay component of the transient photoconductivity upon acceptor substitution, which is attributed to charge trapping.

Optical Excitations of Acceptor Substituted Poly-Thiophene Derivatives

We have studied acceptor substituted polythiophene derivatives in order to examine its usefulness as an intrinsic charge-trap under laser excitation. The polymers studied are the chemically prepared poly 3-Butylethylether Thiophene (BETh) and the copolymer 3-(4-nitrobenzene 1,2-ethen) Thiophene/BETh (1:3.5). We have established that the excitations which survive at long times in these polymers are: (a) bipolarons, which are doubly charged segments of the polymer accompanied by a significant bond alternation relaxation and having spin 0, and (b) triplet excitons, with zero charge and spin 1. When compared to unsubstituted polythiophene, it is noted that triplet excitons are such more efficiently photogenerated in these polythiophene derivatives

Optical excitations of poly(3-butylthiophene) and high electron affinity substituted poly(3-butylthiophene)

Abstract We have characterized the long lived photoexcitations in thin films of poly(3-butylthiophene) [PBT] and co-polymers made of PBT and acceptor substituted PBT, using photo-induced absorption (PA) and optically detected magnetic resonance PA (PADMR) spectroscopies. The PA spectra reveal two main absorption bands; one at 1.4-1.7 eV, which is composed of contributions from both S=1 and S = 1 2 species, and another at 0.5 eV which is due to S = 1 2 charged species. Our results are explained within a picture where the S=1 species are triplet excitons and the S = 1 2 species are photogenerated polarons.

Charge density waves and the complete Raman and photoinduced absorption spectra in polyacetylene copolymers t-[(C2H2)y(C2D2)1−y]x

Abstract The complicated spectra of resonance Raman scattering and photoinduced IR absorption of [(C2H2)y(C2D2)1−y]x are presented and analyzed in the framework of the amplitude mode model. All the features of the spectra are explained with the assumption of interaction between the amplitude modes and low intensity modes of the same symmetry which are found in the pure crystals as well.