Vibronic States and Edge-On Oriented π-Stacking in Poly(3-alkylthiophene) Thin Films

Abstract

The influence of alkyl side-chain length [C n H2n+1 (A), where n = 6 (hexyl), 8 (octyl), 12 (dodecyl)] and thermal annealing (TA) and the definite roles of these in the quantity and quality of π-stacked crystallinity and edge-on orientated (EO) ordering of spin-coated poly­(3-alkylthiophene) [P3AT] thin films, which are of massive significance in their optoelectronic properties, were investigated using complementary optical absorption spectroscopy and X-ray reflectivity (XR) techniques. The energy-band diagram, corresponding to the vibronic levels, obtained from the optical absorption spectrum provides information about the percentage, planarity, local order, and average conjugation length of the crystalline aggregates, while the electron density profile obtained from the XR provides unique information about the EO ordering and its variation along the depth. A prominent EO ordering near the substrate with a gradually decreased ordering toward the top surface is observed in each film. An as-cast P3HT film of shorter side-chain (having a natural nucleation tendency and rigidity) shows a slightly higher crystallinity and weaker excitonic coupling, while as-cast P3DDT film (having longer side-chain flexibility) shows a slightly better EO ordering. After the TA, the planarity of the unfolded backbone improves in each film; however, the effect is maximum in the P3DDT film. The EO ordering also improves throughout the film (by a thermal energy-induced reorientation of crystallites) but more toward the top surface (by increasing the decay length), preserving the exponential decay nature. The P3HT film (with the better initial crystallinity and planarity) shows an appreciable improvement, while the P3DDT film (where the crystallinity increases and crystallites have a better reorientational ability) shows the maximum improvement in the EO ordering. The improvement of the crystallinity (in quantity and quality) and the EO ordering of P3AT in thin films (as active layers), especially near the film–substrate interfaces, are of immense importance for their better in-plane charge carrier mobility in a thin-film transistor.

Evolution of Edge-On Oriented Polymer Films Self-Assembled at the Air–Liquid Interface for High-Performance Electronic Device Applications

“…The 2D electronic dispersion due to intrachain π-conjugation and intermolecular coupling among the proximate monomer motifs contributes to these well-structured absorption spectra. Among the first two absorption peaks, the low energy vibronic shoulder, located at ∼600 nm for PBTTT and ∼589 nm for P3HT and PQT, respectively, is attributed to the 0–0 transition . The bathochromic spectral shift in the case of PBTTT film is mainly because of the higher resonance stabilization energy of linearly symmetrical thieno­[3,2- b ]­thiophene restricts the delocalization of electron clouds to single thiophene units and thus brings the HOMO (highest occupied molecular orbital) as well as LUMO (lowest unoccupied molecular orbital) to a lower energy level.…”

Group 16 conjugated polymers based on furan, thiophene, selenophene, and tellurophene

“…The edge-on orientation is generally thermodynamically preferred, but the orientation of crystallites largely depends on film processing conditions, and it is also known to be correlated with regioregularity and MW. [177][178][179][180] Devices incorporating polychalcogenophenes require a certain stacking directionality, depending on the specific application. For example, in OPVs charge transport is directed out-of-plane between electrodes which lay normal to the substrate, so a face-on crystallization on the substrate would be beneficial.…”

“…In OFETs, charge transport occurs in-plane, necessitating an edge-on orientation for optimal material charge transport properties. 180,181 Since charge transport occurs in the p-stacking direction in OFETs, greater (010) distances, as observed for heavier polychalcogenophenes, are detrimental to their charge carrier mobilities. However, other factors such as backbone planarity, conjugation length, and aggregation strength also play a large role in charge transport, and since heavier polychalcogenophenes notably suffer from poorer solubility, their lower thin film quality can also reduce observed carrier mobilities as well.…”

“…Thermal annealing, which promotes edge-on orientation, shows the greatest crystallinity improvement for longer side chain polymers. 180 Within donor/acceptor blends used in OPV bulk heterojunctions (BHJ), efficient nanophase separation is required for improved optoelectronic properties. For such blends, heavier polychalcogenophenes benefit the most from slow film drying as seen by finer polymer/fullerene mixing, while polythiophenes display similar properties in films prepared by both fast and slow drying.…”

“…Thermal annealing, which promotes edge-on orientation, shows the greatest crystallinity improvement for longer side chain polymers. 180…”