Stearic acid mediated growth of edge-on oriented bilayer poly(3-hexylthiophene) Langmuir films

The growth and structural evolution of stearic acid (SA) blended poly(3-hexylthiophene) [P3HT] Langmuir and Langmuir-Blodgett (LB) films were studied using complimentary surface and interface sensitive techniques to understand the possibility of ordering and layering of promising charge carrier mobility polymers, at the air-water interface and on the transferred solid substrate. SA-induced and subsequent compression-induced transitions in P3HT structure, from aggregated-3D to soft-2D and from in-plane mixed to unmixed layer, are evident at low and high pressures, respectively. The blending of SA molecules enhances the amphiphilic character of P3HT, which reduces the extent of the out-of-plane aggregation to form edge-on oriented (EO) bottom side-chain folded-bilayer (f-BL) islands (of size ~60 nm) within SA monolayer (ML), of commensurate thickness (~2.6 nm). Further compression, gradually rejects the less hydrophilic f-BL islands from the mixed layer to form EO P3HT BL islands (of coverage in-tune with starting composition) on top of SA ML. The formation of nearly covered P3HT(BL)/SA(ML) structured film on solid substrate is evident for the first time, which (even of limited P3HT thickness) has immense importance in the device properties, as the current in the bottom-gated organic thin-film transistors is known to travel only within few ML region near gate-dielectric.

Structures of spin-coated and annealed monolayer and multilayer poly(3-dodecylthiophene) thin films

The effects of film thickness and post annealing on the edge-on ordering and grain-size of spin-coated P3DDT films, which are of immense importance for their better device performances, were investigated using complementary XR and AFM techniques.

In situ probing of the crystallization kinetics of rr-P3HT on single layer graphene as a function of temperature

In situ X-ray diffraction analysis of P3HT films during cooling down on both Si and G.

High-Pressure Solvent Vapor Annealing with a Benign Solvent To Rapidly Enhance the Performance of Organic Photovoltaics

A high-pressure solvent vapor annealing (HPSVA) treatment is suggested as an annealing process to rapidly achieve high-performance organic photovoltaics (OPVs); this process can be compatible with roll-to-roll processing methods and uses a benign solvent: acetone. Solvent vapor annealing can produce an advantageous vertical distribution in the active layer; however, conventional solvent vapor annealing is also time-consuming. To shorten the annealing time, high-pressure solvent vapor is exposed on the active layer of OPVs. Acetone is a nonsolvent for poly(3-hexylthiophene-2,5-diyl) (P3HT), but it can dissolve small amounts of 1-(3-methoxycarbonyl)-propyl-1,1-phenyl-(6,6)C61 (PCBM). Acetone vapor molecules can penetrate into the active layer under high vapor pressure conditions to alter the morphology. HPSVA induces a PCBM-rich phase near the cathode and facilitates the transport of free charge carriers to the electrode. Although P3HT is not soluble in acetone, locally rearranged P3HT crystallites are generated. The performance of OPV films was enhanced after HPSVA; the film treated at 30 kPa for 10 s showed optimum performance. Additionally, this HPSVA method could be adapted for mass production because the temporary exposure of films to high-pressure acetone vapor in ambient conditions also improved performance.

Poor solvent and thermal annealing induced ordered crystallites in poly(3-dodecylthiophene) films

The influence of poor solvent and thermal annealing, with their specific roles, in the crystalline ordering of P3DDT films, which is important to their performance as semiconducting materials, were investigated using complementary techniques.

Local structure of semicrystalline P3HT films probed by nanofocused coherent X-rays

Spatially resolved x-ray study of semicrystalline P3HT films reveals nanoscale inhomogeneity of the conjugated network, as well as structural variations induced by Au nanoparticles.

Enhancement of field-effect mobility due to structural ordering in poly(3-hexylthiophene) films by the dip-coating technique

Organic field-effect transistors (OFETs) were fabricated by depositing a regioregular poly(3-hexylthiophene) (P3HT) active layer using a dip-coating method. The field-effect mobility in OFETs depends on chain orientation and crystallinity and is related to direction and withdrawal speed with respect to the source/drain orientation. In this paper, how to control the structural and transport properties of P3HT films by coating parallel and perpendicular to the dipping direction is demonstrated. X-ray diffraction curves taken in the perpendicular direction exhibit a higher degree of crystalline ordering and edge-on conformation compared with those in the parallel direction; this finding correlates with the directional anisotropy of the OFET mobility. Both structural anisotropy and transport properties are enhanced upon thermal treatment.

Acceptor-Enhanced Local Order in Conjugated Polymer Films

Disorder in conjugated polymers is a general drawback that limits their use in organic electronics. We show that an archetypical conjugated polymer, MEH-PPV, enhances its local structural and electronic order upon addition of an electronic acceptor, trinitrofluorenone (TNF). First, acceptor addition in MEH-PPV results in a highly structured XRD pattern characteristic for semicrystalline conjugated polymers. Second, the surface roughness of the MEH-PPV films increases upon small acceptor addition, implying formation of crystalline nanodomains. Third, the low-frequency Raman features of the polymer are narrowed upon TNF addition and indicate decreased inhomogeneous broadening. Finally, the photoinduced absorption and surface photovoltage spectroscopy data show that photoexcited and dark polymer intragap electronic states assigned to deep defects disappear in the blend. We relate the enhanced order to formation of a charge-transfer complex between MEH-PPV and TNF in the electronic ground state. These findings may be of high importance to control structural properties as they demonstrate an approach to increasing the order of a conjugated polymer by using an acceptor additive.