Thin films of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C14–PBTTT) exhibit a monolayer-terraced morphology that indicates a pronounced lamellar order with π-stacks of extended polymer chains. Previously this remarkable state of order was thought to be promoted by the interdigitation of alkyl side chains between the lamellae during cooling from the liquid-crystalline (LC) phase. Here we establish that the key to this ordering in fact is the formation of unentangled π-stacks of extended polymer chains in dilute solutions of chlorobenzene (CB) or 1,2-dichlorobenzene (o-DCB), which though routinely used as the “best” solvents are in fact borderline solvents. Film formation causes these π-stacks to deposit substantially oriented in the film plane, while the subsequent anneal and cool from LC phase accentuates this incipient order to develop the monolayer-terraced morphology. This mechanism is supported by the following lines of evidence. (i) Hydrodynamic and viscometry measurements respectively of the Kuhn segment length and Mark–Houwink–Sakurada exponent of PBTTT reveal that CB is a near-Θ solvent, and PBTTT is significantly stiffer than regioregular polythiophene. (ii) Solution-state UV–vis spectroscopy reveals an early coil → rod transition in highly dilute solutions, which gives rise to unentangled π-stacks. (iii) Solid-state UV–vis spectroscopy, atomic force microscopy and variable-angle spectroscopic ellipsometry together reveal the as-deposited π-stacks are already substantially oriented in the film plane. We further demonstrate that this monolayer-terraced morphology can also be induced in regioregular poly(3-hexylthiophene) films using a borderline solvent mixture of chlorobenzene and mesitylene, and in very dilute CB where the incipient π-stacks do not entangle. Therefore, this dilute π-stacking mechanism is general. Processing with a borderline solvent or solvent additive thus provides a general route to obtain superior supramolecular order in π-stackable conjugated polymers.

We have extended the well known bisfluorinated(phenyl azide) (bisFPA) methodology to develop an ionic bisFPA process suitable for photo‐crosslinking a wide variety of polyelectrolyte thin films. The crosslinking efficiencies (0.1–1.0 crosslink per photo‐reaction) are sufficiently high for the gel fraction to exceed 80 % for crosslinker concentrations of only a few weight %. This method is based on the photo‐induced formation of singlet nitrenes from FPAs and their insertion into unactivated C–H or other bonds, which thus general and not dependent on the presence of specific chemical functional groups. By derivatizing with ionic charge groups, we obtained ionic bisFPAs that can be properly dispersed into polyelectrolyte thin films. The sorbed moisture always present in these films however severely limits the photo‐crosslinking efficiency, apparently through nitrene protonation and intersystem crossing. This can be avoided by dehydration of the films, in some cases, to 130 °C for 10 min in nitrogen before photo‐exposure. We found that efficient photo‐crosslinking can then be achieved for polyelectrolytes even when they have nucleophilic groups. These include poly(styrenesulfonic acid) and their salts, poly(acrylic acid) and their salts, poly(dimethyldiallylammonium salts), as well as the electrically‐conducting poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonic acid) complex (PEDT:PSSH). We further demonstrate using this ionic bisFPA methodology both photo‐patterning and post‐deposition chemical modifications of polyelectrolyte thin films. This opens broad new possibilities in membrane, sensor and actuator technologies, as well as for organic semiconductor plastic electronics (such as field‐effect transistors) and polyelectrolyte‐based devices.

Fourier-transform infrared spectroscopy of two prototypical high-mobility polymer organic semiconductors (OSCs), regioregular poly(3-hexylthiophene) (rr-P3HT) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT), reveals photoinduced doping that involves both oxygen and water dissolved in the polymer matrix when exposed to light. The equilibrium concentration of water at room temperature and 60% relative humidity in these films is $2 Â 10 19 cm À3, and exists primarily as monomers, with a small population of dimers and trimers. Photo-excitation in room light ultimately generates a polaron density of the order of a few 10 17 cm À3, which is sufficient to degrade the saturation and 'on-off' characteristics of organic field-effect transistors, and the dark current of organic photovoltaics. The dopant anion has been identified primarily to be hydroxide ion species. This process occurs to a smaller extent in wet nitrogen, but even less in dry oxygen, which points to a key role of the dissolved water. The relative stability of PBTTT over rr-P3HT is found to be largely kinetic in origin, attributed to its higher crystallinity (X-ray diffraction crystallinity 27% vs 21% in rr-P3HT), and shorter pÁÁÁp stacking distance (3.64 Å vs 3.78 Å in rr-P3HT), which gives better moisture exclusion from its thiophene backbone.Understanding the degradation mechanisms of polymer organic semiconductor (OSC) devices is an essential step to develop more robust OSC systems and their devices, whether in light-emitting diodes (LEDs), field-effect transistors (FETs), or photovoltaics (PVs). Despite decades of research, detailed spectroscopic studies have seldom been reported, [1,2] primarily because of the considerable challenges to identify chemical transformations that occur on sub-1-mol % of repeat units in the thin films. Yet changes at these levels can be electronically significant. The typical carrier density for LED operation [3] is $1 Â 10 18 cm À3 , which is approximately 0.05 mol % of repeat units, assuming a unit molecular weight of 300 g mol À1 and density of 1.1 g cm À3. For organic FETs, doping at this level can open a parallel source-drain conduction path in the bulk that degrades the shut 'off' and saturation characteristics. [4,5] A simple performance figure-of-merit is the 'on-off' ratio, which for long-channel FETs with a small drain voltage V d and an off-state defined at a gate voltage (V g ) of 0 V is given by i on / i off ¼ mC ox (V g À V th )/(sd), where m is the carrier mobility, C ox is the gate-dielectric capacitance, (V g À V th ) is the effective gate voltage, s is the shunt conductance, and d is the film thickness. For m $ 0.1 cm 2 V À1 s À1 , d $ 30 nm, and typical values for the other parameters, we require s < 10 À8 S cm À1 to give an i on /i off ratio > 10 6

Franck–Condon absorption analysis reveals the existence of several aggregate states in poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) thin films which impact their recrystallization and the attainable field-effect mobility (μFET). Poor solvents (toluene and mixed-xylenes) lock in both disordered and well-ordered states that cannot be annealed away even in the liquid crystalline phase. This reduces μFET and increases mobility activation energies compared with films from good solvents (chlorobenzene and o-dichlorobenzene). Despite its poor solubility characteristics, PBTTT can be ink-jet printed in dilute chlorobenzene, and devices can be operated unencapsulated in ambient, in the dark (&gt;105cycles over several days) with only a moderate mobility loss.

The dependence of morphology and polymer-chain orientation of regioregular poly(3-hexylthiophene) (rrP3HT) thin films on processing conditions have been widely studied. However, their possible variation across the film thickness direction remains largely unknown. We report here a marked difference in the optical dielectric (n,k) spectra between the top and bottom interfaces of spin-cast (sc) rrP3HT films deposited from chlorobenzene solutions. These spectra were obtained from reflection variable-angle spectroscopic ellipsometry using a self-consistent graded optical model with self-imposed Kramers-Krönig consistency. The top interface shows a red-shifted absorption that is characteristic of better order than at the bottom, across a wide range of film thicknesses. This disparity diminishes in drop-cast (dc) and multipass inkjet-printed (ijp) films, and disappears in amorphous films such as those of polystyrene and of a green-emitting phenyl-substituted poly(p-phenylenevinylene). The (n,k) spectra also reveal that crystallinity increases across sc < dc < ijp films. This is supported by cross section scanning electron microscopy of the cleaved edges and measurement of the microroughness of both the film interfaces. Furthermore, optical anisotropy decreases across sc > dc > ijp films. Finally, near-edge X-ray absorption fine structure spectroscopy also shows the frontier chains in ijp and dc films are more isotropically oriented than those in sc films. These results suggest that semicrystalline conjugated polymer films can be produced far from equilibrium. This explains the marked variation in their (opto)electronic properties between the top and bottom surfaces that has sometimes been found depending on the film deposition method. In particular, an unusually pronounced crystallization is induced by ijp. We label this marked ijp-induced crystallization the "ijp morphology", which appears to be general, as it is found also in single-inkjet-droplet films. It appears also to be responsible for the lower field-effect mobility measured for ijp films deposited on a variety of linear and circular electrode arrays. This however can fortuitously be reversed by annealing in solvent vapor. As all films were deposited in the low Peclet-number regime, we can rule out surface skin formation. We attribute the extensive crystallization to the non-uniform drying of picoliter droplets, further promoted by repeated film swelling-deswelling cycles in multipass-ijp films.