A new model for PCBM phase segregation in P3HT:PCBM blends

The phase segregation in P3HT:PCBM blend films has been investigated from an experimental and theoretical viewpoint. Optical microscopy, atomic force microscopy, scanning electron microscopy and X-ray diffraction show that thermal annealing of P3HT:PCBM blend films leads to the formation of PCBM aggregates. These aggregates are composed of dense randomly packed ∼50 nm PCBM crystallites with an overall aggregate density of ∼0.85 g cm⁻³. By applying the critical radius of nucleation for PCBM and the Stokes-Einstein equation for mobility of PCBM in a P3HT matrix, a model is developed which explains the formation of both crystallites and aggregates.     

Effect of [6,6]‐phenyl‐C61‐butyric acid methyl ester on the morphology of poly(3‐hexylthiophene) film

The change of morphology of poly(3‐hexylthiophene) (P3HT) film as a result of blending with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) was studied using a freeze‐dry method. A porous structure was observed as the P3HT/PCBM solution was freeze‐dried. The pore size decreased as the proportion of PCBM increased in the P3HT/PCBM blended film. Additionally, the freeze‐dried P3HT/PCBM film was more resistant to the formation of PCBM crystals than that prepared by a spin‐coating method during the thermal annealing process. Homogeneous distribution of PCBM in the freeze‐dried P3HT/PCBM film was the main reason for the reduction of large PCBM crystal formation. Copyright © 2011 Society of Chemical Industry     

Investigation of P3HT/n-Si heterojunction using surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) was used to investigate the interactions of the interface between regioregular poly(3-hexylthiophene) (P3HT) and n-type single crystalline silicon. The SPS responses of silicon and the P3HT/n-Si heterojunction caused by band to band transition of silicon are 30 mV and 160 mV respectively. The band-bending in the silicon side of the P3HT/n-Si structure is larger than that of bare n-Si. The density of the interface states of the P3HT/n-Si heterojunction increased significantly after the deposition of P3HT. Based on the contact potential difference (CPD) transient results, charge transport and separation processes are fast in the silicon substrate and slow in the P3HT layer respectively.     

Enhancement in crystallinity of poly(3-hexylthiophene) thin films prepared by low-temperature drop casting

Low-temperature casting of high molecular weight (HMW > 40 kDa) regioregular poly(3-hexylthiophene) (P3HT) is demonstrated to yield highly crystalline P3HT thin films with well defined and preferentially edge-on oriented [(100) contact plane] nanocrystallites on the pure and silanized Si/SiO₂ substrates. Transmission electron microscopy and X-ray diffraction provide evidence for the increase in preferential edge-on orientation of P3HT-conjugated backbone while decreasing the film preparation temperature below room temperature. The optimum growth temperature is about −12°C for the given concentration (2 mg/mL in CHCl₃). The reduced evaporation rate of the solvent results in a better selection of the thermodynamically stable orientation of nanocrystallites onto the substrate. The degree of preferential orientation and size of the crystallites can be further increased on n-octadecyltrichlorosilane-treated SiO₂ substrates as well as by annealing the films at 200°C for 1 h. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Ternary Solvent System to Control the Morphology of Active Blend in Inverted Organic Solar Cells

Bulk heterojunction blend of poly(3-hexylthiophene)–[6,6]-phenyl-C₆₁-butyric acid methyl ester prepared using the ternary solvent mixture was used as the active layer of an inverted organic solar cell. The ternary solvent mixture consisted of a good solvent such as ortho-dichlorobenzene and two marginal solvents such as cyclohexanone and toluene offering limited solubility to the poly(3-hexylthiophene) component. Power conversion efficiency of inverted device was found to decrease from 2.74?±?0.05% in the unmodified device to 2.64?±?0.07% in the modified device. UV–visible measurement revealed less efficient photoabsorption in the mixed cosolvent-casted active layer due to insufficient and disordered crystallization of poly(3-hexylthiophene) domains.     

Fabrication of a planar water gated organic field effect transistor using a hydrophilic polythiophene for improved digital inverter performance

A planar water gated OFET (WG-OFET) structure is fabricated by patterning gate, source and drain electrodes on the same plane at the same time. Transistor output characteristics of this novel structure employing commercial regioregular poly(3-hexylthiophene) (rr-P3HT) as polymer semiconductor and deionized (DI) water as gate dielectric show successful field effect transistor operation with an on–off current ratio of 43 A/A and transconductance of 2.5 μA/V. These output characteristics are improved using P3HT functionalized with poly(ethylene glycol) (PEG) (P3HT-co-P3PEGT), which is more hydrophilic, leading to on–off ratio of 130 A/A and transconductance of 3.9 μA/V. Utilization of 100 mM NaCl solution instead of DI water significantly increases the on–off ratio to 141 A/A and transconductance to 7.17 μA/V for commercial P3HT and to 217 A/A and to 11.9 μA/V for P3HT-co-P3PEGT. Finally, transistors with improved transconductances are used to build digital inverters with improved characteristics. Gain of the inverters employing P3HT and P3HT-co-P3PEGT are measured as 2.9 V/V and 10.3 V/V, respectively, with 100 mM NaCl solution.     

Controlling the morphology of poly(3‐hexylthiophene)/methanofullerene film through a dynamic‐cooling and freeze‐drying process

A dynamic‐cooling and freeze‐drying (DCFD) process has been applied to the fabrication of polymer solar cells. The dynamic‐cooling process allows poly(3‐hexylthiophene) molecules to aggregate in solution into a more organized structure during the cooling process; the freeze‐drying process prevents severe agglomeration of [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) during the solvent removing process. Application of these two processes to the preparation of the poly(3‐hexylthiophene)/methanofullerene photoactive layer results in an enhanced poly(3‐hexylthiophene) aggregation and smaller PCBM agglomerates. Devices fabricated using the DCFD process generate 14% more in current density than those prepared by the spin‐coating process under AM1.5G illumination. © 2015 Society of Chemical Industry     

Poly(3-hexylthiophene) and hexafluoro-2-propanol-substituted polysiloxane based OFETs as a sensor for explosive vapor detection

The organic field effect transistors (OFETs) with regioregular poly 3-hexylthiophene (rr-P3HT) and hexafluoro-2-propanol-substituted polysiloxane (SXFA) as an organic layer, have been used for detection of explosive vapors with excellent sensitivity of less than 70 ppt for 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and less than 100 ppt for 2,4,6-trinitrotoluene (TNT). The sensor response (% change in saturation current) was found to be 125 ± 10% for TNT and 90 ± 10% for RDX. It was also observed that the incorporation of Cu^II tetraphenylporphyrin (CuTPP) into rr-P3HT/SXFA matrix resulted in an improved selectivity for the vapors of nitro based analytes (TNT, RDX and DNB) as compared to the vapors of non explosive oxidizing agents such as nitrobenzene (NB), benzoquinone (BQ) and benzophenone (BP). This is attributed to the increased binding of the vapors containing nitro compound to the thin films due to the presence of CuTTP. Spin coated thin films were further characterized by Atomic Force Microscopy (AFM) and Electrostatic Force Microscopy (EFM).