We used both cw and transient spectroscopies for studying the optical properties and photoexcitations in the low bandgap
copolymer PTB7 that has been used in organic photovoltaic applications (OPV). Surprisingly we observed two
primary photoexcitations that are generated within ~150 fs (our time resolution); we identify them as singlet exciton (S1)
and triplet-pair (1TT). The singlet exciton has been considered to be the only primary photoexcitation in regular π-conjugated polymers and is related with a transient absorption band that peaks at an energy value close to the exciton
binding energy (~0.4 eV in PTB7). The TT pair is a novel photoexcitation species in low band-gap π-conjugated
copolymers. It has an absorption band close to that of isolated triplet exciton, and may readily dissociate at the donoracceptor
interfaces in the PTB7/fullerene blend. This finding may explain the underlying mechanism for the high
obtained power conversion efficiency in OPV devices based on the PTB7 copolymer.
Extensive research effort is devoted to the investigation on synthesis, physical property of
conjugated molecules with pi-electronic systems. These molecules exhibit interesting electrical
and optical properties useful for field effect transistor, electrical conductivity, optical
nonlinearity, photovoltaic and electroluminescent properties.[1-6] Conjugated diblock
copolymer possessing a rod-rod structure will be rigid rods, which dictate their assembling
behaviors. Since the structure of the conjugated blocks can be modified with different functional
groups, such as donor and acceptor groups, the electronic properties of the resulting diblock
copolymers can be engineered to behave like semiconductor p-n junctions, offering opportunity
to observe rectification effects. These properties are closely related to photovoltaic effect and
can find application in solar energy harvesting. In this paper, we describe several molecular
systems that show clear rectification effect. [9-12]
KEYWORDS: Molecules, Luminescence, Absorption, Picosecond phenomena, Polymers, Upconversion, Electron transport, Solar cells, Quantum efficiency, Energy transfer
In a recent study, it has been shown that organic photovoltaic (OPV) solar cells consisting of polymers with certain
stoichiometric ratios of alkyl thiophene:thieno[3,4-b]thiophene monomeric units in random sequences, when combined
with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), may have potentials for creating more efficient devices. Such a
potential enhancement is mainly due to the light harvesting in most of the visible and near infrared region by these low
band-gap polymers. However, very little is known about the photoinduced energy/electron transfer and transport within
these copolymers. It is important to understand both the ultrafast interactions between these two monomeric units when
they are linked in the copolymers and their interactions with the electron acceptor PCBM in order to determine the
transport mechanisms in these systems, and then to create the architectures that optimize electronic transport properties.
Therefore, three oligomer molecules have been synthesized to model the local interactions in the copolymers, each of
which consists of a thieno[3,4-b] thiophene derivative at its center linked with two alkyl oligothiophene side units. The
alkyl oligothiophene units for the three molecules are 2, 4, or 8 units in length. By performing transient absorption and
fluorescence upconversion measurements, the nature of the early exciton diffusion and energy transfer between these
different units is elucidated.
A highlight of the recent advances in the study of fully functionalized organic photorefractive materials based on polymers, oligomers and small organic molecules is presented.
We report a new photorefractive polymer that contains an ionic tri(bispyridyl) ruthenium complex as the charge generating species, a conjugated polymer backbone as the charge transporting channel and a nonlinear optical chromophore. The ruthenium complex was introduced to utilize its efficient light induced metal-to-ligand charge transfer process. This polymer shows greatly enhanced photorefractive performance; a large net optical gain of about 200 cm-1 was obtained at a zero external electric field.
The nonlinear optical properties of a series of substituted thiophene oligomers have been investigated using the Z-scan technique. As the length of the thiophene increases from the monomer to an oligomer containing nine thiophene units, the absorption spectrum shifts to longer wavelength. This shift in wavelength moves the absorption band into resonance with the laser wavelength and the nonlinear optical properties of these materials change. In the samples where there is no linear absorption at the laser wavelength, the nonlinear absorption appears to originate from a two-photon absorption process. The nonlinear refraction observed is dominated by the contributions of the solvent. In the samples where there is linear absorption at the laser wavelength, several competing mechanisms appear to be responsible for the nonlinear response observed. At low laser irradiance, saturable absorption is observed. As the laser irradiance is increased, a decrease in the transmitted energy is observed. The nonlinear refraction in these materials is now large and negative.
A general approach to the synthesis of second-order nonlinear optical (NLO) polymides exhibiting high thermal stability has been developed. Several selected NLO chromophores have been incorporated into the polyimide backbone. Detailed physical studies showed that these polymers are very promising for practical applications. Three of these polyimides are soluble in common orgainc solvents, offering the ease of processing. High glass temperatures, around 230 degrees C, ensured a long term NLO stability at elevated temperatures, such as 150 degrees C. Low optical loss was observed for those soluble polymers. The synthetic approach is also versatile and will allow the syntheses of many other functional polymers.
Conjugated polymers are of great interest due to their unique structure, interesting physical properties, and potential applications in electronic or opto-electronic devices. Recently, the synthesis of soluble and processable conjugated polymers with different functionalities has become an active research area. We have been exploring palladium-catalyzed reactions (the Stille and the Heck reactions) for preparing functionalized, conjugated polymers. These palladium catalyzed reactions have the advantage of mild reaction conditions and high yields. Different types of conjugated polymers with different properties and applications, such as liquid crystalline conjugated polymers, photoactive metalloporphyrin-containing polymers, and conjugated photorefractive polymers have been synthesized.
Several novel photorefractive polymers based on the charge transfer properties in photosynthetic model compounds were synthesized. These polymers contain a prophyrin moiety, a second order nonlinear optical chromophore covalently linked to a polyimide backbone. High thermal stability of the second order nonlinear effect were observed at elevated temperatures. An asymmetric optical energy exchange was detected in two beam coupling experiments, indicating the photorefractive nature of the polymers. Large optical gains were deduced at zero-external electric filed (20 cm-1).
To manifest a photorefractive effect, the polymer must possess a photocharge generator, a charge transporter, a charge trapping center and a nonlinear optical chromophore. We have synthesized two types of novel photorefractive (PR) polymers. The type I PR polymers contain the NLO chromophore, the charge generator and the transporting compound covalently linked to the polymer backbone. The type II photorefractive polymers contain a conjugated backbone and a second order NLO chromophore. The design idea behind the structure of the type II polymers is the expectation that they will possess photorefractivity. Since the conjugated backbone absorbs photons in the visible region and is photoconductive, it is expected to play the triple roles of charge generator, charge transporter and backbone. Thus, the four functionalities necessary to manifest the PR effect will exist simultaneously in a single polymer.
The nonlinear and linear optical properties of a group of polymers containing DR19 are reviewed. The thermal setting polymers have demonstrated large NLO coefficient and long term poling stability. Dry processed micro-patterning of the index of refraction and the birefringence has been demonstrated with sub-micron resolution.
Optical nonlinearities can arise as the result of intense electromagnetic radiation fields inducing either changes in electron or nuclear configurations. Indeed, as is discussed in this paper, several mechanisms, including mechanisms depending upon electron-phonon coupling, may be elicited from the same material. The precise contribution that a given mechanism makes to observed optical nonlinearity is often dependent upon pulse conditions employed in transient nonlinear optical experiments. The ability to control optical nonlinearity by pulse conditions is demonstrated and analyzed for a high symmetry ladder polymer where contributions from coherent parametric mixing, excitons, and bipolarons are observed. The different timescales associated with various mechanisms for index of refraction and absorption changes are discussed. The utilization of photo-induced changes occurring on widely different timescales is demonstrated in the realization of efficient second harmonic generation by quasi- phase matching. The role of chemical synthesis in engineering multi-functional materials is discussed.
Multifunctional properties of nonlinear optical chromophores are discussed both in terms of a given chromophore exhibiting more than one type or mechanism of optical nonlinearity and in terms of a chromophore exhibiting useful auxiliary properties. For materials exhibiting more than one type of mechanism of optical nonlinearity, the concept of pulse-controlled optical nonlinearity is introduced and discussed. An analogy is drawn to multidimensional nuclear magnetic resonance studies which are useful in systematically elucidating excited state dynamics. Practically, pulsed control of optical nonlinearity provides a means of enhancing and modulating nonlinear optical phenomena. The photochemical reactivity of nonlinear optical chromophores is discussed in terms of fabricating ordered lattices appropriate for the development of integrated circuits and the realization of specific effects such as quasi-phase matching in second harmonic generation.
This paper presents the study of the photoinduced refractive index change and birefringence in a nonlinear optical polymer --polyester with disperse red 19 side groups. Polyester with disperse red 19 side groups showed sizable second-order nonlinear optical properties when poled under intense electric field. In addition, under ultra-violet or short wavelength visible light illumination, large photoinduced refractive index changes were measured in the near infrared region. Furthermore, with linearly polarized light illumination large photoinduced birefringence was also exhibited in the thin films of this polymer. Some device structures, such as birefringent gratings, thin film waveplates, and optical waveguides, were generated by photo exposure method. A simple theoretical model is also presented to investigate the relation between the photoinduced birefringence and the refractive index change.
Second order nonlinear optical (NLO) polymers containing NLO moieties with large optical nonlinearities and a cross-linkable unit have been synthesized using difunctionalized disperse red dye. The enhanced second order NLO coefficients, X exp (2) of 250 pm/v for the first polymer and 500 pm/v for the second, were obtained at 532 nm using the corona poling method. The introduction of flexible chains into the polymer backbone is found to cause a decrease in glass transition temperature and, hence, a decrease in the stability of the second-order NLO effects.
Third order nonlinear optical properties of organic ladder copolymer (POL) system is studied
using degenerate four-wave mixing with picosecond laser pulse. Both the real and imaginary part
of the third order nonlinear susceptibility (3) were determined by a new phase conjugate
interferometric method over the wavelength range of 532 - 720 nm. From the space symmetry and
wavelength dependence of -) we attribute the observed nonlinearity to the nonlinear
photoexcitation of bipolarion states in this ladder copolymer system.
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