The transition from bipolaron to triplet-polaron magnetoresistance in a single layer organic semiconductor device

We measure the current–voltage–luminescence (I–V–L) and Magneto-Conductance (MC) response of a poly(3-hexyl-thiophene) (P3HT) based device (Au/P3HT(300 nm)/Al) in forward and reverse bias. In reverse bias (<1 V), the negative MC is described by a single non-Lorentzian function, consistent with the bipolaron theory. In forward bias, the transition from negative saturation MC (low bias) to positive saturation MC (high bias) occurs when the current density exceeds ∼10⁻² A cm⁻² and coincides with electroluminescence. Under these conditions the triplet density (∼10¹⁵ cm⁻³) becomes comparable to the hole density (∼10¹⁶ cm⁻³), consistent with the triplet-polaron interaction theory. From the current density dependence of the MC we conclude that in forward bias both mechanisms must be occurring simultaneously, within a given device, and that the overall sign of the MC results from competition between the two mechanisms.     

Annealing effects on the electrical properties of spin coated poly (3-hexylthiophene) (P3HT) thin films

Poly (3-hexylthiophene-2, 5-diyl) (P3HT) thin films were prepared using spin coating technique and the effect of annealing on the bias switching for memory applications were studied. Due to annealing, the threshold voltage for switching was reduced considerably. In bias switching, threshold voltage was least for the sample annealed at 100 °C. Addition of phenyl-C₆₁-butyric acid-methyl-ester (PCBM) into P3HT also reduced the threshold voltage. It was also found that the devices with gold (Au) top electrode switched at a lower threshold voltage compared to their aluminium (Al) counterparts.     

Phospholipid film in electrolyte-gated organic field-effect transistors

A totally innovative electrolyte-gated field effect transistor, embedding a phospholipid film at the interface between the organic semiconductor and the gating solution, is described. The electronic properties of OFETs including a phospholipid film are studied in both pure water and in an electrolyte solution and compared to those of an OFET with the organic semiconductor directly in contact with the gating solution. In addition, to investigate the role of the lipid layers in the charge polarization process and quantify the field-effect mobility, impedance spectroscopy was employed. The results indicate that the integration of the biological film minimizes the penetration of ions into the organic semiconductor thus leading to a capacitive operational mode as opposed to an electrochemical one. The OFETs operate at low voltages with a field-effect mobility in the 10⁻³ cm² V⁻¹ s⁻¹ range and an on/off current ratio of 10³. This achievement opens perspectives to the development of FET biosensors potentially capable to operate in direct contact with physiological fluids.     

Stabilization of poly(3-hexylthiophene)/PCBM morphology by hydroxyl group end-functionalized P3HT and its application to polymer solar cells

A new concept to stabilize the morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blend through H-bond formation by using a hydroxyl group end-functionalized P3HT (HOC-P3HT-COH) as a compatibilizer is presented. Domain size of the PCBM crystals in the annealed P3HT/PCBM film is diminished with addition of HOC-P3HT-COH. Surface roughness of the P3HT/PCBM film also becomes smoother with addition of HOC-P3HT-COH. Thermal stability of solar cell device is improved significantly through the H-bond formation between HOC-P3HT-COH and PCBM. A high performance and thermal stable polymer solar cell with 4.06% power conversion efficiency under AM1.5G irradiation is fabricated with 5% HOC-P3HT-COH in P3HT/PCBM layer.     

Memory stabilities and mechanisms of organic bistable devices with a phase-separated poly(methylmethacrylate)/poly(3-hexylthiophene) hybrid layer

Organic bistable devices (OBDs) with a poly(methylmethacrylate) (PMMA)/poly(3-hexylthiophene) (P3HT) hybrid layer, acting as a charge storage region, formed by using a vertical phase self-separation method were fabricated. The current–voltage curves of the Al/P3HT/PMMA/indium-tin-oxide devices exhibited current bistabilities with a maximum ON/OFF ratio of 1 × 10⁴. The write-read-erase-read sequence results demonstrated the switching characteristics of the OBDs. The cycling endurance number of the ON/OFF switching for the OBD was above 1 × 10⁵. The memory characteristics of the OBDs were attributed to trapping and detrapping processes of electrons into and out of the P3HT/PMMA heterointerfaces.     

Photovoltaic device performance of highly regioregular fluorinated poly(3-hexylthiophene)

Highly regioregular poly(3-hexylthiophene) derivatives with varying degrees of fluorine substitution on the thiophene moieties have been demonstrated in photovoltaic devices and characterized using ultraviolet and inverse photoelectron spectroscopy. As fluorine content is increased, an increase in ionization energy of 0.3 eV is observed for 50% fluorination compared to non-fluorinated poly(3-hexylthiophene). The electron affinity is observed to increase to a lesser extent with increased fluorination, consistent with a systematic increase in the optical bandgaps of up to 0.12 eV. Bulk heterojunction photovoltaic devices made from polymer:PC₆₁BM blends achieve power conversion efficiencies of 3%, however film morphologies measured using atomic force microscopy indicate that strong phase separation with increasing fluorination limits device performance. UV–vis spectra of thin films of the fluorinated materials exhibit a long tail in the red, extending to longer wavelengths than non-fluorinated poly(3-hexylthiophene). Photovoltaic devices similarly exhibit non-zero quantum efficiency in this region. This behavior has been attributed to a low energy, interchain charge transfer state.     

Nanoscale photovoltaic characteristics of single quantum dot hybridized with poly(3-hexylthiophene)

Hybrids consisting of CdSe/ZnS quantum dot (QD) as a core and thiol-group functionalized poly(3-hexylthiophene) (P3HT) as a shell were fabricated using the ligand-exchange method. We clearly observed the photovoltaic characteristics of a single QD-P3HT hybrid by using conducting atomic force microscopy. Monochromatic power conversion efficiency drastically increased with an increase in the molecular weight (Mw) of P3HT, suggesting sufficient photoinduced charge transfer between the QD and highly ordered P3HT chains. The nanoscale photoluminescence (PL) intensity for a single QD considerably decreased with increasing Mw of P3HT owing to charge transfer effects. On the basis of time-resolved PL and transient absorption spectra measurements of the QD-P3HT hybrids, we deduced that the exciton lifetimes of the QD were reduced with higher-Mw P3HT hybrids, and photobleaching was observed. The measured nanoscale optical characteristics of the single QD-P3HT hybrids support their distinct photovoltaic behaviors.     

Laser-induced crystallization and conformation control of poly(3-hexylthiophene) for improving the performance of organic solar cells

Femto-second laser irradiation on P3HT:PCBM solutions have been demonstrated to have a significant impact on the conformational structures and photovoltaic performance of the resultant thin films. The crystallinity and edge-on/face-on conformations of P3HT and the aggregation of PCBM can be manipulated by controlling the wavelength (400–800 nm) and illumination duration (1–3 h) of the lasers. Grazing incidence wide- and small-angle X-ray scattering (GIWAXS and GISAXS) have been simultaneously utilized to characterize the nanostructures of the P3HT:PCBM blend films spin-cast from pristine and laser-irradiated solutions. The results show that the crystallinity, π-π* stacking and face-on conformations of P3HT can be enhanced as a result of the laser irradiation at 500 nm for 3 h. Furthermore, the diffusion and aggregation of PCBM molecules are suppressed by the photo-induced dimerization, as evidenced by the Raman spectra of the films cast from laser-irradiated PCBM solutions. The time-resolved fluorescence decay profiles show the charge transfer efficiency is improved, which may correlate to the supramolecular ordering of the polythiophene chains and the optimized phase separation in P3HT:PCBM composite. In the P3HT:PCBM active layer of the organic solar cells, more efficient charge transport and fine interpenetrating networks can be achieved due to the improved conformational microstructures. Consequently, the short-circuit current densities and power conversion efficiencies can be enhanced in organic solar cells based on the laser-irradiation processed P3HT:PCBM solutions.     

High mobility organic field-effect transistors based on defect-free regioregular poly(3-hexylthiophene-2,5-diyl)

We report on organic field-effect transistors (OFETs) prepared using defect free (100% regioregular) poly(3-hexylthiophene-2,5-diyl) (DF-P3HT) as semiconductor and cross-linked poly(vinyl alcohol) (cr-PVA) as gate insulator. High field-effect mobility (μ_FET) of 1.2 cm² V⁻¹ s⁻¹ is obtained and attributed to the absence of regioregularity defects. These transistors have transconductance of 0.35 μS and the DF-P3HT film shows larger crystallites (∼80 Å) than a highly regioregular (>98%) material (∼32 Å). Devices with increased μ_FET (2.8 cm² V⁻¹ s⁻¹) could be obtained at the expense of the On-Off current ratio, which was reduced by one order of magnitude, when poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) treatment was applied to the dielectric surface. Our results suggest that the interaction of charged sites at the dielectric surface with regioregularity defects of the P3HT is an important factor degrading μ_FET even at very low concentration of regioregularity defects.     

A comprehensive review on poly(3-alkylthiophene)-based crystalline structures,protocols and electronic applications

Poly (3-alkylthiophenes) (P3ATs) particularly poly (3-hexylthiophene) (P3HT) have vigorous tendency to crystallize into the ordered morphologies, thereby reflect the efficient electrical and optical characteristics. Here, the remarkable crystalline structures of P3ATs comprising single crystals, nanofibers, nano/microwires, nanoribbons, micelles, nanowhiskers, thin films, spherulites, etc. are thoroughly reviewed. In each category, the principle aspects like preparation methodologies (e.g., self-seeding, shearing, electrospinning, self-organization, epitaxial solidification, etc.), micro/nanostructures, orientation and optical properties are meticulously brought into consideration. Applications of the ordered P3AT structures in solar cells, organic field-effect transistors, and sensors as well as the theoretical studies are also highlighted. Furthermore, the polymorphism, sol-gel, nanoparticles, liquid crystalline morphology, and two-dimensional sheets are reviewed.     

Blocking layer optimisation of poly(3-hexylthiopene) based Solid State Dye Sensitized Solar Cells

The optimisation study of the fabrication of a compact TiO₂ blocking layer (via Spray Pyrolysis Deposition) for poly(3-hexylthiopene) (P3HT) for Solid State Dye Sensitized Solar Cells (SDSCs) is reported. We used a novel spray TiO₂ precursor solution composition obtained by adding acetylacetone to a conventional formulation (Diisopropoxytitanium bis(acetylacetonate) in ethanol). By Scanning Electron Microscopy a TiO₂ layer with compact morphology and thickness of around 100 nm is shown. Through a Tafel plot analysis an enhancement of the device diode-like behaviour induced by the acetylacetone blocking layer respect to the conventional one is observed. Significantly, the device fabricated with the acetylacetone blocking layer shows an overall increment of the cell performance with respect to the cell with the conventional one (ΔJ_sc/J_sc = +13.8%, ΔFF/FF = +39.7%, ΔPCE/PCE = +55.6%). A conversion efficiency optimum is found for 15 successive spray cycles where the diode-like behaviour of the acetylacetone blocking layer is more effective. Over three batches of cells (fabricated with P3HT and dye D35) an average conversion efficiency value of 3.9% (under a class A sun simulator with 1 sun A.M. 1.5 illumination conditions) was measured. From the best cell we fabricated a conversion efficiency value of 4.5% was extracted. This represents a significant increment with respect to previously reported values for P3HT/dye D35 based SDSCs.     

Polarized photoluminescence and electroluminescence in oriented films of regioregular poly(3-alkylthiophenes)

We report polarized photoluminescence (PL) and electroluminescence (EL) from regioregular poly(3-alkylthiophene) thin films oriented by means of the rubbing technique. With a proper thermal annealing of the rubbed films we have increased the anisotropy reaching values up to 8 both in PL and EL. We have evidenced that the orientation in the rubbing direction is higher for the longest conjugation segments than for the less conjugated ones. Moreover polarized PL study demonstrates that emission takes place after migration of the excitation from shorter, less oriented, to longer, more oriented conjugation segments.     

Effects of semiconductor/dielectric interfacial properties on the electrical performance of top-gate organic transistors

We investigated the effects of varying the properties of the interface between a semiconductor P3HT layer and a dielectric Cytop™ layer on the performances of the resulting transistor devices by comparing the mobilities of devices prepared with bottom gate/bottom contact or top gate/bottom contact architectures. The reduced channel roughness that arose from the thermal annealing step dramatically enhanced the field-effect mobility, yielding the highest mobility yet obtained for a top-gate transistor: 0.12 cm²/V s. High-performance OFETs may be fabricated by controlling the channel roughness and the properties of the interface between the semiconductor and the gate dielectric.     

Role of electron blocking and trapping layers in transport characterization of a photovoltaic polymer poly(3-hexylthiophene)

Hole injection and transport in films (300-350 nm) of poly(3-hexylthiophene) (P3HT) were investigated by dark-injection space-charge-limited current (DI-SCLC) technique. For samples with a nominally hole-only configuration of anode/P3HT/Au, the DI current transients depart significantly from the theory, and the signals cannot be used for reliable carrier mobility extraction. The origin of the departure can be attributed to electron leakage from the Au cathode. We outline a means of suppressing electron leakage by inserting an interlayer between the P3HT and the cathode. This interlayer has dual functions of blocking and trapping electrons. Using this interlayer, we obtain well-defined DI-SCLC signals for reliable carrier mobility determination. With a suitable interlayer to suppress undesirable carrier injection and transport, DI-SCLC technique should find broad applications in the transport characterization of narrow gap photovoltaic polymers.     

High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths

The device characteristics of top-gate field-effect transistors (FETs) based on typical polymer semiconductor regioregular poly(3-alkylthiophenes) (P3ATs) with different alkyl chain lengths are investigated. High field-effect mobilities of ∼10⁻² cm²/Vs are obtained irrespective of alkyl chain length even when polymer gate insulators with different dielectric constants (2.1–3.9) are used. This is attributed to the spontaneous formation of highly ordered edge-on lamellar structures at the surface of P3AT thin films that are the channel regions in top-gate FETs. In addition, top-gate P3AT FETs containing different gate insulators exhibit high operational stability, with low threshold voltage shifts of <0.5 V following prolonged gate bias stress, which is comparable to that of hydrogenated amorphous silicon thin film transistors.     

The interplay of electronic structure,molecular orientation and charge transport in organic semiconductors: Poly(thiophene) and poly(bithiophene)

Ultrathin films of poly(thiophene) (PT) and poly(bithiophene) (PBT) were prepared by electrochemical route using ionic liquid (BFEE) as medium and electrolyte. Distinct morphologies and electrical properties were observed in these materials. To evaluate its response in photovoltaics, these films were used as active layer in bilayer geometry solar cells with the electron acceptor molecule C₆₀. The best performance was observed for PT films. In order to probe the differences in molecular dynamics and structural order, ultrafast electron dynamics in the low-femtosecond regime was evaluated by resonant Auger spectroscopy using the core–hole clock method at the sulfur K absorption edge. Electron delocalization times for the different polymeric films were derived as a function of the excitation energy. Photoabsorption measurements were conducted and molecular orientation derived. These results corroborated with the morphology found for these films and thus the performance of PT and PBT in the devices, and with the proposed conduction mechanism.     

Thermal and optical properties of nano/micro single crystals and nanofibers obtained from semiconductive-dielectric poly(3-hexylthiophene) block copolymers

Optical and thermal characteristics of single crystals and nanofibers prepared from poly(3-hexylthiophene) (P3HT)-based homopolymers and block copolymers were deeply studied in toluene, xylene, and anisole. The free exciton bandwidth of aggregates (W) decreased with increasing the ratio of A_0–0/A_0–1 and, consequently, the conjugated length and the intrachain ordering enhanced. In these structures, the intrachain interactions were dominant interactions. The peaks of A_0–0, A_0–1 and A_0–2 appeared from longer wavelengths towards shorter ones in ultraviolet-visible (UV–Vis) spectra. The effective parameters on the aggregation type consisted of solvent quality, P3HT backbone length, and growth method. In the single crystals prepared from the longer conjugations, the P3HT₄₈₈₀₀ backbones acted as J-aggregate (1.21<A_0–0/A_0–1<2.42 and −283<W (meV) <−56). The P3HT₄₈₈₀₀ nanofibers grown from toluene and xylene were J-aggregate, but the ones grown from anisole were H-aggregate. The short and middle P3HT conjugated backbones acted as H-aggregate in both single crystals and nanofibers (A_0–0/A_0–1<1). The crystallinity, as a fingerprint of highly ordered structures, was higher in J-aggregate structures compared to H-aggregate ones. The single crystals, due to having a highly ordered configuration, represented a higher red-shifted absorbance, melting enthalpy (∆H_m=35.85 J/g), and crystallinity (x_c=97%) compared to the corresponding nanofibers (∆H_m=25.91 J/g and x_c=70%). The crystallinity and chain ordering of homopolymer single crystals were also higher than those detected for the block copolymer ones.     

Photo stability enhancement of Poly(3-hexylthiophene)-PCBM nanocomposites by addition of multi walled carbon nanotubes under ambient conditions

Poly(3-hexylthiophene)-Phenyl-C61-butyric acid methyl ester (P3HT–PCBM) composites find wide application in optoelectronic devices, especially bulk-hetero junction (BHJ) solar cells. These composites, even though could give efficient polymer solar cells with ∼4–5% power conversion efficiencies (PCE), a major problem of photo stability is associated with it and remains unsolved. P3HT–PCBM composite was found to be degrading on irradiation with ultraviolet radiation or a solar simulator providing AM1.5G illumination (1000 W m^–2, 72 ± 2 °C or 330 W m⁻², 25 °C), in presence of oxygen and moisture. Here, we have studied the photo stability of P3HT–PCBM under ambient conditions and showed that a new ternary composite, P3HT–PCBM–MWCNT (multi walled carbon nanotube) has superior photo stability even on extended UV–Vis exposure. A total of 7% (w/w) PCBM and 3% (w/w) MWCNT with respect to P3HT resulted in optimum stability. UV–Visible and fluorescence spectral analysis have been used to study the photo stability, both in solution state and solid/film state. Transmission electron micrograph (TEM) along with selected area electron diffraction (SAED) pattern and Field Emission Scanning Electron Microscopy (FE-SEM) micrographs have been used to show the well coating of MWCNT on P3HT–PCBM composite. Since MWCNT is one of the very important carbon based nanomaterial with several supreme characteristics, this new ternary composite has great importance for optoelectronic applications.     

Modeling of optimum size and shape for high photovoltaic performance of poly(3-hexylthiophene) nanopore in interdigitated bilayer organic solar cells

The interdigitated design for donor–acceptor in solar cell has been studied in some detail, but the optimum size and shape leading to direct enhancement in nanopore (or nanopillar) structure is still not well understood. Here, we demonstrate a modeling method to forecast the optimum size and shape for poly(3-hexylthiophene) (P3HT) nanopores in interdigitated P3HT: [6, 6]-phenyl C₆₁ butyric acid methyl ester (PCBM) photovoltaic device, based on experimental results of P3HT:PCBM bilayer solar cell. In our analysis, the energy generated at unit nanopore is supposed to the same as the one generated at infinite point of P3HT:PCBM bilayer solar cell with variable layer thickness. A definitive function in terms of a radius of unit nanopore with various shapes is established, substituting a regression function derived from the results of power conversion efficiency in bilayer solar cell. Interpreting the function, we finally showed that the effective radius for P3HT nanopores with rectangular or cylinder, cut-cone, cone shape should be less than 135, 53, 2 nm respectively.     

Nanoscale imaging of dense fiber morphology and local electrical response in conductive regioregular poly(3-hexylthiophene)

We present a thermal annealing method for producing micrometer long and dense semi-crystalline fiber structures in poly(3-hexylthiophene) (P3HT) films. The thin film topology and local electrical properties are investigated by conductive atomic force microscopy (c-AFM), and three-dimensional device models are used to illustrate the effects of current spreading, anisotropic mobility and traps. In-plane current flow along high mobility fibers provides a mechanism for c-AFM contrast in fibrous films. The comparison of c-AFM results between the annealed and non-annealed P3HT films suggests that trap effects are pronounced in disordered P3HT thin films prior to thermal annealing. The methodologies we demonstrate here on the archetypal P3HT film can be generalized for understanding the correlation between structure and local electrical properties in a variety of polymer and polymer nanocomposite systems.