Effect of Molecular Weight and Annealing of Poly(3‐hexylthiophene)s on the Performance of Organic Field‐Effect Transistors

Abstract

The optical, structural, and electrical properties of thin layers made from poly(3‐hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater. 2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X‐ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 150 °C. However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue‐shift of the solid‐state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low‐molecular weight P3HT layers.

Photohole Screening Effects in Polythiophenes with Pendant Groups

“…In the absence of structural and chemical defects, the experimental and calculated HOMO-LUMO gaps here are very different from the values observed in optical spectroscopy for P3HT (about 2.1-2.5 eV 3,9,26,27,30,34 ). Even taking into account the fact that Coulombic intramolecular interaction effects occur with optical excitations, this value is much smaller than the HOMO-LUMO gap obtained in our combined photoemission and inverse photoemission studies as well as in the groundstate molecular orbital calculations.…”

“…10 There are structural defects that occur, in the absence of any chemical defect, due to disorder in the molecular structure. For example, the mobilities 9 in organic field-effect transistors at elevated temperature were correlated with structural distortions in the poly-(3-hexylthiophene-2,5 diyl) layer of the transistor 1 , but the only "defect" considered was disorder in the relative orientation of the adjacent aromatic rings and associated pendant groups.…”

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

“…This high tendency to crystallize adds to their ability to phase separate into well defined heterojunction morphology when blended with electron acceptors like [6,6]-phenyl C61-butyric acid methyl ester (PCBM).…”

“…4 In transistor studies of P3HT films over a range of MWs, it has been shown that higher MW P3HT has better quality crystallites which are more co-aligned and connected, possibly by individual polymer chains spanning between crystallites 5 leading to high field effect transistor (FET) mobility. Low MW polymer exhibits reduced transistor performance, which has been explained either by grain boundary trapping effects at lower MW, 4 or enhanced out-of-plane backbone twisting, 6 which results in a reduction in backbone planarity and a resulting decrease in effective conjugation length and the efficiency of charge hopping. Solution studies upon higher MW P3HT have also shown that physical entanglements can initiate polymer aggregation in some solvents, which then leads to gellation.…”

“…C 60 itself exhibits low solubility in common solvents, and therefore a variety of functionalisation approaches have been explored, both to improve solubility and to further tune the electronic properties. One of the first soluble derivatives to be investigated, and subsequently one of the most widely used is [6,6]-phenyl C61-butyric acid methyl ester (abbreviated as PC 61 BM or, more commonly, PCBM, Fig. 5).…”