We report the local UV-Vis absorption behaviour of single crystals of conjugated poly-3-hexylthiophene (P3HT), obtained by crystallization in dilute solutions at elevated temperatures based on a self-seeding approach and characterized by high internal structural order. Conjugated polymers represent promising active systems for a variety of optoelectronic applications, such as solar cells, 1 light emitting diodes, 2 eld-effect transistors, 3-5 etc. However, the performance of these materials in such devices has so far oen remained poor, mostly due to the difficulties in controlling molecular conformations, structural packing, and morphology 6,7 of these polymers. In order to correctly interpret and nally to improve the functional behavior of conjugated polymers, it is thus necessary to unravel the relationship between the light absorption properties and molecular conformation or polymer microstructure. As a suitable model system for a systematic study of such structure-optoelectronic property relations, we have recently identied highly ordered single crystals of conjugated poly-3-hexylthiophene (P3HT) of weight average molecular weight of 26 400 g mol À1 , characterized by a unique molecular conformation on all length-scales. 8 They were obtained via crystallization in solution by a self-seeding technique. These crystals are composed of closely packed, p-p stacked (p-p distance of $0.33 nm) fully extended chains which are oriented perpendicular to the substrate. 8

In this work, we report a comparative analysis of the infrared and Raman spectra of octa(3-hexylthiophene) (3HT) 8 , trideca(3-hexylthiophene) (3HT) 13 , and poly(3-hexylthiophene) P3HT recorded in various phases, namely, amorphous, semicrystalline, polycrystalline and single crystal. We have based our analysis on the spectra of the (3HT) 8 single crystal (whose structure has been determined by selected area electron diffraction) taken as reference and on the results of DFT calculations and molecular vibrational dynamics. New and precise spectroscopic markers of the molecular structures show the existence of three phases, namely: hairy (phase 1), ordered (phase 2), and disordered/amorphous (phase 3). Conceptually, the identified markers can be used for the molecular structure analysis of other similar systems.

The photophysical properties of a phenylsubstituted poly(thiophene), poly(3-(2,5-dioctylphenyl)-thiophene) (PDOPT), were studied as a function of polarization and degree of orientation of the crystalline structure. Under well-chosen controlled conditions, largesized spherulitic crystals of PDOPT were successfully prepared from the melt. From polarized optical microscopy and X-ray diffraction, the molecular orientation of PDOPT within the spherulite was determined, indicating that the fastest growth direction of the spherulite was the a-axis. This implied that crystallization of PDOPT was directed by the packing of the side chains rather than the backbones, which are significantly separated. As the crystalline lamellae were all radially oriented, the local absorbance strongly depended on the polarization of the incoming light. Compared to randomly oriented crystals in a quenched and thus rapidly crystallized sample, PDOPT spherulites displayed red-shifted absorption and emission spectra, combined with a reduced photoluminescence quantum yield. Even for these markedly separated polymer backbones (1.47 nm), the reduced photoluminescence suggests an enhancement of interchain interactions of highly ordered bulky substituted polythiophene induced by crystallization.T he optoelectronic properties of conjugated polymers are of importance for their application in organic electronic devices. 1 These properties are considerably determined by both the conformation of the chain-like molecules (intrachain arrangements of chromophores) and the way these molecules pack in the solid state (interchain arrangements of chromophores). In thin films of conjugated polymers prepared by standard techniques like spin coating, interchain energy transport is usually more rapid than intrachain transport. 2 Thus, the corresponding optoelectronic properties depend sensitively on the extent of interchain interactions. 3 It is widely accepted that interchain interactions in thin films are effectively influenced by chain aggregation and film morphology. Consequently, the relevant photophysical and electronic properties can be controlled by appropriate processing conditions. 4,5 Most previous studies have focused on conjugated polymers with short backbone stacking distances (<5 Å), such as in alkylsubstituted poly(thiophene)s (P3ATs) 5,6 or poly(phenylenevinylene)s (PPVs). 7,8 In these materials, interchain interactions are mainly determined by the overlap of π-orbitals, i.e., π−π interactions. To control interchain interactions, conjugated polymers with bulky side chains have been developed. 9 Bulky side chains can separate the polymer backbones from each other and thus may have a profound influence on the final properties of these materials. 9,10 In particular, the separation of main chains can strongly improve the photoluminescence (PL) quantum yield (Q y ) of conjugated systems. 11 Up to now, however, only a few reports have discussed the relationships between morphology and optoelectronic properties of conjugated systems having a large main c...

No abstract

No abstract

In this study, the fingerprint of the acid concentration during the hydrolysis process on the optical band gap of cellulose nanocrystals (CNCs) has been systematically studied. The CNCs have been prepared using hydrochloric acid at a hydrolysis temperature of 50°C and at a constant hydrolysis time of 4 hours but with varying hydrochloric cid concentrations of 5%, 10% and 15%. The crystalline structure and phase identification of the CNCs have been studied using XRD technique. UV-Vis Spectroscopy has been done and the optical band gap energy calculated by performing the Tauc’s plot. From the study, the grain size has been found to decrease with acid concentration while the band gap energy has been found to increase with increasing acid concentration. Further, the optical band gaps of the CNCs have been found to decrease with the increase in crystallite size. This shrinkage of the band gap has been attributed to the increased impurity concentration leading to the narrowing of the band gap due to the emerging of the impurity band formed by the overlapped impurity states

No abstract