Conjugated Polymer Thin Films Research Articles

Electrochemical Formation of Stable p-i-n Junction in Conjugated Polymer Thin Films

Stable p-i-n junction was formed in polymer thin films by a facile electrochemical method. The polymer thin films contained an alkoxyphenyl substituted poly(1,4-phenylene vinylene), a cross-linkable liquid trimethylolpropane trimethacrylate (TMPTMA), and lithium trifluoromethane sulfonate at a weight ratio of 20:10:1. The thin films were sandwiched between an indium tin oxide anode and an aluminum cathode. During initial charging at 12 V, a p-i-n junction was formed in a similar fashion as reported in the polymer light-emitting electrochemical cells except that the junction did not relax during subsequent idling without external bias. TMPTMA was responsible in conducting the ions (dopants) during the formation of the junction. The junction stabilized as TMPTMA cured and lost its ionic conductivity. This mechanism is supported by the measurements of the electroluminescence behavior, the photovoltaic response and the AC impedance of the charged devices. The formation of the stable p-i-n junction improves the electroluminescence performance and photovoltaic efficiency compared to similar devices without such a junction. The polymer p-i-n junction devices exhibited current-voltage response characteristic of light-emitting diodes with turn-on time<5 mS. The luminance efficiency was 3.0 cd/A, and brightness was 450 Cd/m2 at 10 V. The same junction devices under 100 mW/cm(-2) AM1.5 solar illumination had an open-circuit voltage of 1.5 V and a short circuit current of 6 microA/cm2 with an over 460% enhancement on efficiency compared to the device without the junction.

Molecular Orientation and Field-effect Transistors of a Rigid Rod Conjugated Polymer Thin Films

Molecular orientation in thin films of a rigid rod conjugated polymer, a derivative of poly(para-phenylene ethynylene)s with linear side chains and thioacetyl end groups, was investigated by reflection-absorption infrared spectroscopy and X-ray diffraction technique. The results indicated that TA-PPE molecules tended to align with their backbone planes perpendicular to substrates, that is, with an "edge-on" molecular orientation in the films. Such molecular orientation is favorable for the efficient carrier transport in two-dimensional direction in the polymer films (i.e., via both the intrachain and interchain), so that high performance organic field-effect transistors were fabricated with hole mobility at around approximately 4.3 x 10(-3) cm(2)/Vs.

Interface morphologies and magnetization characteristics of Co 70Fe 30 thin films deposited on conjugated polymer thin films

The surface and interface morphology and magnetization characteristics of Co 70Fe 30 thin films deposited on bare glass and p-Si/SiO 2 substrates and on conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films on such substrates have been studied by atomic force microscopy and magneto-optic Kerr effect. It was found that the average absolute magnitude of the coercive field of Co 70Fe 30 correlates with the roughness of the underlayer prior to Co 70Fe 30 deposition. P3HT deposited on p-Si/SiO 2 substrates possesses an increased surface roughness as compared to the p-Si/SiO 2 surface, but displays a decreased surface roughness as compared to the one of a bare glass substrate.

Bulk Electroluminescence from Conjugated Polymer Thin Films via the Formation of Gold Nanoislands

Bulk Electroluminescence from Conjugated Polymer Thin Films via the Formation of Gold Nanoislands

Anisotropic optical constants of electroluminescent conjugated polymer thin films determined by variable-angle spectroscopic ellipsometry

This article reports on in-plane and out-of-plane refractive index (n) and extinction coefficient (k) values measured using variable-angle spectroscopic ellipsometry for poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) thin films of different molecular weights (Mn=9−255 kg∕mol), both in the pristine and annealed states. The in-plane n and k values are generally larger than the out-of-plane values for all pristine films leading to a measurable optical anisotropy which becomes much stronger as F8BT molecular weight increases. This indicates that polymer chains lie preferentially in the plane of the substrate and this configuration is more energetically favorable for longer polymer chains. Upon annealing, a larger reduction in kout-of-plane than in kin-plane is measured leading to a further increase in optical anisotropy. A redistribution of oscillator strengths and a broadening toward lower energies in absorption spectra are also observed indicating significant restructuring of F8BT chains upon annealing.

Domain Formation in a Conjugated Polymer Thin Film

Domain Formation in a Conjugated Polymer Thin Film

STM-induced light emission of conjugated polymer thin film in different chain aggregations

The STM-induced luminescence (STML) of a conjugated polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)- p-phenylene vinylene] (MEHPPV) with or without annealing on substrate of Ag(1 1 1) were investigated in air at room temperature. The surface morphologies of pure Ag(1 1 1), MEHPPV thin film on Ag(1 1 1) with or without annealing were modified by STM scans under higher positive or negative sample biases. The experimental results show that for MEHPPV film without annealing, the STML intensity increases after aggregates formation induced by STM scans. However, for annealed MEHPPV film, the negative bias scans decrease the STML intensity. The chain aggregations among MEHPPV molecules are believed to be the key factor for the behavior.

Temperature treatment of semiconducting polymers: An X-ray reflectivity study

The influence of temperature treatment on three types of conjugated polymer thin films, named poly(3-hexylthiophene) (P3HT), polyarylamine (PAA) and octylfluorene–bithiophene copolymer (F8T2) is studied. A detailed knowledge of the film morphology and crystalline structure is important since the performance of organic thin film transistors is extremely sensitive to small changes of morphology and structure. Samples are prepared via a spin-casting process on thermal oxidized silicon wafers. The influence of heat treatment in the range from ambient temperatures to 600 K is studied with specular X-ray reflectivity, X-ray diffraction and differential scanning calorimetry. The morphological parameters like layer thicknesses, electron densities and roughnesses of the interface and of the surface are calculated from the XRR measurements. The maximum change of layer thickness due to heat treatment is 15% of the initial layer thicknesses. The maximum variation of the mean electron densities are about 20% and the rms surface roughness vary from 2 Å up to 20 Å as a result of annealing. Interface roughness show variation of about 1 Å. Strong variations of the morphological parameters next to phase transition temperatures are observed as well.

Solvent Effects on Chain Orientation and Interchain π‐Interaction in Conjugated Polymer Thin Films: Direct Measurements of the Air and Substrate Interfaces by Near‐Edge X‐ray Absorption Spectroscopy

The orientation and packing of semiconducting polymer chains at film interfaces crucially affects charge injection and transport at those interfaces. Although X-ray diffraction techniques have been used extensively as a structural probe, they are not particularly sensitive to these interfaces. Near-edge X-ray absorption fine-structure spectroscopy (NEXAFS), on the other hand, is sensitive to surface orientation and has been applied to investigate polymer structures, for example, in polyimide, [1] polyfluorene, [2] and polythiophene. [3] Here we show that NEXAFS band shapes and intensities are in fact also sensitive to the intermolecular p-interaction in regioregular poly(3-hexylthiophene) (rreg-P3HT) films. NEXAFS spectra of the same material deposited from different solvents and processing conditions are substantially different. This indicates a far more extensive breakdown of the “building-block” picture than previously reported, [4,5] allowing p-interactions to be studied. The surface sensitivity has recently been established to be ca. 2.5 nm for the total electron yield in rregP3HT, [6] which is highly relevant to interfacial charge transport and surface-induced self-organization. Rreg-P3HT thin films were deposited from a wide range of solvent qualities, and both the air and substrate interfaces were studied. Delamination was used to expose the substrate interface. We found that: i) the thiophene (Th)-ring orientation at both these interfaces was disordered and showed only a weak preference for the out-of-plane (oop) stacking favorable for field-effect mobility (lFET); ii) the hexyl side chains were also disordered; and iii) the interchain p-interaction was strongly dependent on the deposition solvent quality, and varied in opposite directions between the air and substrate interfaces, being generally better at the substrate than the air interface. Direct imaging by friction-mode atomic force microscopy (AFM) revealed the expected presence of a mosaic surface morphology with a finer length scale than surface topography. Transistor lFET measurements suggested that the high lFETof rreg-P3HT is still limited by heterogeneity, which we attribute to the distribution of interface domains of varying orientation and/or packing. This ability to directly measure chain segmental orientation and interaction at the interfaces will open the way to their systematic optimization for even higher device performance. The self-organization behavior of polymers at interfaces is of significant scientific and technological interest. Much of the structural knowledge of conjugated polymers such as rregP3HT, a prototype high-lFET polymer, has come from wide

Thickness-Dependent Thermal Transition Temperatures in Thin Conjugated Polymer Films

We report the effects that geometrical confinement has on both the glass transition temperature, Tg, and the crystalline phase transition temperature, Tc, for two widely studied conjugated polymers...

Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

Interchain interactions have a profound effect on the optical as well as charge transport properties of conjugated polymer thin films. In contrast to oligomeric model systems in solution-deposited polymer thin films the study of such effects is complicated by the complex microstructure. We present here a detailed study of interchain interaction effects on both charged polarons as well as neutral excitons in highly crystalline, high-mobility poly-3-hexylthiophene (P3HT) as a function of molecular weight. We find experimental evidence for reduced exciton bandwidth and increased polaron delocalization with increasing conjugation length and crystalline quality. From comparative studies of field-effect transistor characteristics, film morphology, and optical properties our study provides a microscopic understanding of the factors which limit the charge transport in P3HT to field-effect mobilities around $0.1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$, and which will need to be addressed to improve mobility further.

Highly Effective Water-Soluble Fluorescence Quenchers of Conjugated Polymer Thin Films in Aqueous Environments

Guy D. Joly, Lars Geiger, Steven E. Kooi, andTimothy M. Swager*Department of Chemistry, Massachusetts Institute ofTechnology, 77 Massachusetts AVenue,Cambridge, Massachusetts 02139ReceiVed July 18, 2006ReVised Manuscript ReceiVed August 15, 2006Conjugated polymers have found excellent use as sensorymaterials.

Large Damage Threshold and Small Electron Escape Depth in X-ray Absorption Spectroscopy of a Conjugated Polymer Thin Film

The information depth of near-edge X-ray absorption fine structure spectroscopy in the total electron yield mode (TEY-NEXAFS) is given by the escape depth of the TEY electrons z(TEY). This is determined by the effective ranges both of the inelastically scattered secondary electrons and of the primary excited electron before they thermalize below the vacuum level. For regioregular poly(3-hexylthiophene) (rreg-P3HT) thin films, we have measured the total electron emission efficiency to be 0.028 +/- 0.005 e/ph at an incident photon energy of 320 eV. The range of the primary electron was computed using optical dielectric-loss theory to be 7.5 nm. The range of the secondary electrons was then found by modeling to be 3.0 nm. This gives z(TEY) to be 2.5 nm, which is considerably less than the often-assumed value of 10 nm in the literature. It is also considerably smaller than the computed electron-electron scattering inelastic mean free path in the material, which suggests the predominance of electron-phonon scattering. Thus, TEY-NEXAFS has sufficient surface sensitivity to probe the frontier molecular layers of these organic conjugated polymers. In a second aspect of this report, the rreg-P3HT films have been characterized by in-situ core and valence photoemission spectroscopies and by ex-situ microattenuated total-reflection vibrational spectroscopy as a function of irradiation dose. No damage was observed in composition, bonding, orientation, and surface morphology under typical TEY-NEXAFS spectral acquisition conditions. For an integrated TEY that exceeds 2 x 10(-3) C cm(-2), however, the material degrades via alkyl side-chain dehydrogenation to unsaturated units, cross linking, ring opening of the backbone, and sulfur extrusion. Given that secondary electrons are the dominant cause of radiation damage, this exposure threshold measured by integrated TEY should also be valid at other X-ray energies.

Anisotropic optical properties in electroluminescent conjugated polymers based on grazing angle photoluminescence measurements

Grazing angle photoluminescence (GPL) originates from a waveguided light emitted at grazing angle to the substrate due to the total internal reflections, and the light emission is polarized with enhanced intensity at selective mode wavelength. GPL measurements reveal the optical anisotropy of luminescent conjugated polymers, in particular, the alignment of emitting dipoles from which emission occurs, in contrast to spectroscopic ellipsometry measurements that give the anisotropy in the absorption. Based on the GPL emission intensities and spectra, we investigate the anisotropic optical properties in electroluminescent poly(9,9'-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer thin films of different molecular weights (M(n) = 9-255 kg/mol), both in the pristine and annealed states. The optical anisotropy in F8BT films generally increases with molecular weight, suggesting that higher molecular weight polymers with longer chains are more likely to lie in-plane to the substrate. Upon annealing, high molecular weight F8BT films show even a higher degree of anisotropy, in contrast to low molecular weight F8BT films that become more isotropic. Annealing causes the polymer chains to rearrange and adopt a configuration in which the interchain exciton migration to better ordered low energy (LE) emissive states is strongly suppressed. We observe that the emissive states in F8BT are strongly affected by the local polymer chain arrangement, producing the less ordered high energy (HE) emissive states near the substrate interface where there is a higher degree of chain disorder and the LE states in the bulk of the film. When spin coated onto a quartz substrate precoated with a poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) layer, films of F8BT show severe luminescence quenching near the PEDOT:PSS interface for both the LE and HE emissive states, but a selective quenching of the LE states in the bulk of the film. These observations have important implications for fabricating efficient electronic devices using conjugated polymers as an active material, since the performance of these devices will strongly depend on anisotropic optical properties of electroluminescent conjugated polymers.

Preparation of plasma poly(1-isoquinolinecarbonitrile) thin film and its ultrafast nonlinear optical property

How to get a uniform, defect-free, and reproducible conjugated polymer thin films is now becoming the main fabrication problem for the practical application of these materials as the fast switches and modulators in opto-electronic devices. In this research, a novel plasma-polymerized 1-isoquinolinecarbonitrile (PPIQCN) thin film was prepared by plasma polymerization under different glow discharge conditions. The effect of the discharge power on the chemical structure and surface compositions of the deposited PPIQCN films was investigated by Fourier transform infrared (FTIR), UV-Visible absorption spectra and X-ray photoelectron spectroscopy (XPS). The results show that a high retention of the aromatic ring structure of the starting monomer in the deposited plasma films is obtained when a low discharge power of 10 W was used during film formation. In the case of higher discharge power of 30 W, more severe monomer molecular fragmentation can be observed, which results in a decrease in the effective conjugation length of PPIQCN film. The morphology characterized by atomic force microscopy (AFM) indicates that a fine, homogenous PPIQCN film could be obtained under a relatively low discharge power. A femtosecond time-resolved optical Kerr effect technique at a wavelength of 820 nm has been applied to investigate the third-order nonlinearity of the plasma PPIQCN film. For the first time, a non-resonant optical Kerr effect and ultrafast response of the PPIQCN film was observed.

Ruptured conjugated polymer thin films formed during spin coating

AbstractThe formation of ruptured poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenyl vinylene] (MEH–PPV) thin films coated from undried tetrahydrofuran (THF) solutions was investigated. Because of the incompatibility of water and MEH–PPV, the polymer films coated from THF/water solutions showed a ruptured film structure. In the photoluminescence (PL) spectra of the polymer thin films, the ruptured polymer films showed a redshifted emission in comparison with continuous polymer thin films. According to a comparison of the PL spectra of polymer solutions and films, MEH–PPV in THF showed a coil–cylinder transition during precipitation from solution. Because of the incompatibility of water and MEH–PPV, an increase in the water content could increase the ratio of polymer chains in the cylinder conformation, resulting in a redshifted emission for the films. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 79–84, 2006

Selective growth of conjugated polymer thin films by the vapor deposition polymerization

We present a selective growth technique for polymer thin films using patterned hydrophobic/hydrophilic surface treatments. On a glass substrate surface, where a hydrophobic treatment was applied using hexamethyl–disilazane, a hydrophilic SiO thin film was deposited through a metal mask by the vacuum evaporation to form a hydrophilic/hydrophobic pattern. A poly-azomethine (poly-AM) film that is a conjugated polymer was grown on the surface by the vapor deposition polymerization using p-phenylenediamine and terephthalaldehyde as reactive source monomers. The poly-AM thin film was selectively grown on the SiO region, that is, on the hydrophilic region, without growth on the hydrophobic region.

On the optical anisotropy of conjugated polymer thin films

In this paper the problem of optical anisotropies in conjugated polymer thin films is revisited and the results in the literature are critically reappraised. Several models for upper bounds on the different types of anisotropies are developed and these are compared with experiment. We analyze both in-plane/out-of-plane and in-plane anisotropies in spin coated and oriented films, respectively. For the in-plane/out-of-plane anisotropy in spin coated films we study the reasons for the large disparity amongst the results reported in the literature, and propose the method of interference enhancement variable angle spectroscopic ellipsometry to retrieve this anisotropy more accurately. Experimental results are reported for poly(9,9-dihexyl fluorene) (spin coated), poly(9,9-dioctylfluorene) (both spin coated and uniaxially aligned), and poly(9,9-dioctylfluorene-co-benzothiadiazole) (both spin coated and uniaxially aligned). Our results concerning the measurement of optical anisotropy can be applied equally well to other molecular systems, such as self-assembled monolayers or Langmuir--Blodgett films.

Absorption and luminescence characteristics of optically thin conjugated polymer films

Poly[(2-methoxy,5-octoxy)1,4-phenylenevinylene] (MO-PPV) optically thin films have been prepared from their chloroform solutions of different concentrations. Then their UV–Vis absorption (Abs), photoluminescence (PL), and selective-excitation photoluminescence (SEPL) spectra have been measured at room temperature. The position of S0 → S1 absorption peak shows red shift and broadened relative to that in MO-PPV solution form due to solidation effect, while no relative shifts between the absorption spectra of these film samples are observed. A “long-wavelength tail” appears in Abs spectra which should attribute to the disorder morphology of these thin films. A long-wavelength emission component near 630 nm has been identified as S2 → S0 vibronic band through Gaussian decomposition method and confirmed by below-gap PL and selective-excitation PL experiments. This absorption tail and the second vibronic band cannot be observed in thick films. The PL efficiency of MEH-PPV thin film and that of MO-PPV thin film are also compared and explained by the side substituent effect, which can be proved both by theoretical simulation of interchain interactions and by comparing their morphology in film forms. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient Exciton Quenching by Hole Polarons in the Conjugated Polymer MEH‐PPV

AbstractRecently published near field scanning optical measurements (NSOM) on the conjugated polymer MEH‐PPV exhibited a strong dependence of the photoluminescence intensity on the applied electric fields at the NSOM tip. The observed effect is apparently due to exciton quenching by hole polarons. In the present paper, a model “single carrier” electro‐modulated‐photoluminescence device is used to further explore the exciton quenching effect of hole polarons in MEH‐PPV. Hole polarons, created by charge injection from an ITO electrode, are observed to dramatically quench the photoluminescence intensity of MEH‐PPV. The Stern‐Volmer quenching efficiency of a hole polaron in conjugated polymer thin films was measured to be 390 nm3. This value, and other data presented herein, are consistent with the published NSOM photoluminescence modulation measurements and offer further evidence that hole polarons are efficient photoluminescence quenchers in MEH‐PPV.