Temperature‐Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers

Abstract

Previous investigations of the field‐effect mobility in poly(3‐hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium‐MW: Mn = 7 200 g mol−1; high‐MW: Mn = 27 000 g mol−1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and organic field‐effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra‐grain mobility depends only weakly on MW (being in the order of 10−2 cm2 V−1 s−1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature‐dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium‐MW P3HT undergo a “pre‐melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short‐chain P3HT is strongly inhibited by the presence of disordered domains, while in high‐MW P3HT the low‐mobility disordered zones are bridged via inter‐crystalline molecular connections.

Temperature dependence of exciton and charge carrier dynamics in organic thin films

“…5 As the temperature decreased, σ abs slightly decreased (Fig.2), reaching a value of ~370 cm -1 (0.046 eV) at 100 K (for the A 1 band). This suggests that line broadening in the absorption spectrum of ADT-TES-F films in the studied temperature range is dominated by a weakly temperature-dependent static disorder, which has a considerable contribution from solid-state defects such as temperature-dependent due to several factors; 36 for example, in P3dHT films W decreased from 0.22 eV to 0.14 eV as the temperature decreased from 120°C to 30°C, 36 due to temperature-induced changes in the spatial correlation length l 0 . 30,41 From the data in Fig.2(a), we obtained W abs ≈ 0.06 eV at 298 K, 0.05 eV at 225 K, and 0.013 eV at 100…”

“…9,26 At 298 K (room temperature), a red-shift of the film spectrum with respect to that of isolated ADT-TES-F molecules, D ≈ 870 cm -1 (~0.108 eV) in Fig.2(a), was observed and attributed to enhanced Coulomb interaction of the molecule with its surrounding environment and exchange interactions between translationally equivalent molecules. 5,[27][28][29][30] 26,34,36 and attributed to freezing out of the torsional and librational modes, which promotes exciton delocalization, as well as to a thermal contraction of the lattice leading to an increase in nonresonant intermolecular interactions (which would increase the parameter D in Fig.2(a)). 30 In ADT-TES-F films, these effects were relatively strong, as the peak red-shift rate of ~2×10 -4 eV/K in ADT-TES-F films was a factor of ~2-5 higher than that in, for example, conjugated polymers 26 and PTCDA crystals.…”

Structure and morphology control in thin films of regioregular poly(3‐hexylthiophene)

“…However, different groups have shown that a strong correlation exists between charge transport and nanomorphology/ crystallinity of the P3HT films [41][42][43][44][45][46][47][48][49] which depend crucially on the thin-film processing conditions. The morphology and crystallinity of P3HT thin films is affected by (i) the thin-film preparation method (drop-casting, spin coating, inkjet printing), (ii) the nature of the substrates and (iii) the macromolecular parameters, for example, average molecular weight, regioregularity and polydispersity (PDI) of the polymer.…”

High-mobility solution-processed copper phthalocyanine-based organic field-effect transistors

“…Furthermore, molecular tailoring of organic compounds via chemical modification can achieve high field-effect performance of the active semiconducting layers. For example, fused acene [18], oligothiophene [19], tetrathiafulvalene derivatives [20] and conjugated polymers [21] exhibit field-effect mobilities much larger than 0.1 cm 2 V −1 s −1 . However, the chemical and electrical stabilities of these OFETs, and consequent degradation in on-current, threshold voltage and field-effect mobility under air exposure remain a problem.…”