Enhanced charge collection in polymer photovoltaic cells by using an ethanol-soluble conjugated polyfluorene as cathode buffer layer

We report enhanced polymer photovoltaic (PV) cells by utilizing ethanol-soluble conjugated poly (9, 9-bis (6'-diethoxylphosphorylhexyl) fluorene) (PF-EP) as a buffer layer between the active layer consisting of poly(3-hexylthiophene)/[6, 6]-phenyl C61-butyric acid methyl ester blend and the Al cathode. Compared to the control PV cell with Al cathode, the introduction of PF-EP effectively increases the shunt resistance and improves the photo-generated charge collection since the slightly thicker semi-conducting PF-EP layer may restrain the penetration of Al atoms into the active layer that may result in increased leakage current and quench photo-generated excitons. The power conversion efficiency is increased ca. 8% compared to the post-annealed cell with Al cathode.

Achieving perpendicular alignment of rigid polythiophene backbones to the substrate by using solvent-vapor treatment

The rigid backbone of the poly(3-butylthiophene) molecule adopts a perpendicular orientation with respect to the substrate by using a solvent-vapor treatment (see figure). Small and closely contacting spherulites instead of conventional whisker-like crystals are achieved. This could be utilized to improve charge-carrier mobility particularly in the direction normal to the film plane by designing and constructing thick crystalline domains in the functional layer.

Enhanced electrical conductivity of highly crystalline polythiophene/insulating-polymer composite

Poly(3-butylthiophene) (P3BT)/insulating-polymer composites with high electrical conductivity have been prepared directly from the solution. These composites exhibit much higher conductivity compared to pure P3BT with the same preparation method provided that P3BT content is higher than 10 wt %. Morphological studies on both the pure P3BT and the composites with insulating polymer show that P3BT highly crystallizes and develops into whisker-like crystals. These nanowires are homogeneously distributed within the insulating polymer matrix and form conductive networks, which provide both extremely large interface area between conjugated polymer and insulating polymer matrix and highly efficient conductive channels through out the whole composite. In contrast, the conductivity enhancement of P3HT/PS composite is not so obvious and drops down immediately with increased PS content due mainly to the absence of highly crystalline whisker-like crystals and much larger scale phase separation between the components. The results presented here could further illuminate the origin of conductivity formation in organic semiconducting composites and promote applications of these polymer semiconductor/insulator composites in the fields of organic (opto-)electronics, electromagnetic shielding, and antistatic materials.

Nanoscale electronic properties of conjugated polymers and nanocrystals

The objective of this thesis is the exploration and characterisation of the nanoscale electronic properties of conjugated polymers and nanocrystals. In Chapter 2, the first application of conducting-probe atomic force microscopy (CP-AFM)-based displacement-voltage (z-V) spectroscopy to local measurement of electronic properties of conjugated polymer thin films is reported. Charge injection thresholds along with corresponding single particle gap and exciton binding energies are determined for a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] thin film. By performing measurements across a grid of locations on the film, a series of exciton binding energy distributions are identified. The variation in measured exciton binding energies is in contrast to the smoothness of the film suggesting that the variation may be attributable to differences in the nano-environment of the polymer molecules within the film at each measurement location. In Chapter 3, the CP-AFM-based z-V spectroscopy method is extended for the first time to local, room temperature measurements of the Coulomb blockade voltage thresholds arising from sequential single electron charging of 28 kDa Au nanocrystal arrays. The fluid-like properties of the nanocrystal arrays enable reproducible formation of nanoscale probe-array-substrate junctions, allowing the influence of background charge on the electronic properties of the array to be identified. CP-AFM also allows complementary topography and phase data to be acquired before and after spectroscopy measurements, enabling comparison of local array morphology with local measurements of the Coulomb blockade thresholds. In Chapter 4, melt-assisted template wetting is applied for the first time to massively parallel fabrication of poly-(3-hexylthiophene) nanowires. The structural characteristics of the wires are first presented. Two-terminal electrical measurements of individual nanowires, utilising a CP-AFM tip as the source electrode, are then used to obtain the intrinsic nanowire resistivity and the total nanowire-electrode contact resistance subsequently allowing single nanowire hole mobility and mean nanowire-electrode barrier height values to be estimated. In Chapter 5, solution-assisted template wetting is used for fabrication of fluorene-dithiophene co-polymer nanowires. The structural characteristics of these wires are also presented. Two-terminal electrical measurements of individual nanowires indicate barrier formation at the nanowire-electrode interfaces and measured resistivity values suggest doping of the nanowires, possibly due to air exposure. The first report of single conjugated polymer nanowires as ultra-miniature photodetectors is presented, with single wire devices yielding external quantum efficiencies ~ 0.1 % and responsivities ~ 0.4 mA/W under monochromatic illumination.

Separating interface state response from parasitic effects in conductance measurements on organic metal-insulator-semiconductor capacitors

A simple model is developed for the admittance of a metal-insulator-semiconductor (MIS) capacitor which includes the effect of a guard ring surrounding the Ohmic contact to the semiconductor. The model predicts most of the features observed in a MIS capacitor fabricated using regioregular poly(3-hexylthiophene) as the active semiconductor and polysilsesquioxane as the gate insulator. In particular, it shows that when the capacitor is driven into accumulation, the parasitic transistor formed by the guard ring and Ohmic contact can give rise to an additional feature in the admittance-voltage plot that could be mistaken for interface states. When this artifact and underlying losses in the bulk semiconductor are accounted for, the remaining experimental feature, a peak in the loss-voltage plot when the capacitor is in depletion, is identified as an interface (or near interface) state of density of similar to 4 x 10(10) cm(-2) eV(-1). Application of the model shows that exposure of a vacuum-annealed device to laboratory air produces a rapid change in the doping density in the channel region of the parasitic transistor but only slow changes in the bulk semiconductor covered by the gold Ohmic contact. (C) 2008 American Institute of Physics.

Detection of volatile organic compounds using a polythiophene derivative

Conjugated polymers have been subject of great interest in the recent literature from both fundamental point of view and applied science perspective. Among the several types of conjugated polymers used in recent investigations, polythiophene and its derivatives have attracted considerable attention over the past 20 years due to their high mobility and other remarkable solid-state properties. They have potential applications in many fields, such as microelectronic devices, catalysts, organic field-effect transistors, chemical sensors, and biosensors. They have been studied as gas and volatile organic compounds (VOCs) sensors using different principles or transduction techniques, such as optical absorption, conductivity, and capacitance measurements. In this work, we report on the fabrication of gas sensors based on a conducting polymer on an interdigitated gold electrode. We use as active layer of the sensor a polythiophene derivative: poly (3-hexylthiophene) (P3HT) and analyzed its conductivity as response for exposure to dynamic flow of saturated vapors of six VOCs [n-hexane, toluene, chloroform, dichloromethane, methanol, and tetrahydrofuran (THE)]. Different responses were obtained upon exposure to all VOCs, THF gave the higher response while methanol the lower response. The influence of moisture on the measurements was also evaluated. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fabrication of coaxial TiO2/Sb2S3 nanowire hybrids for efficient nanostructured organic- inorganic thin film photovoltaics

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Modelling polymers with side chains: MEH-PPV and P3HT

Modelling polymers with side chains is always a challenge once the degrees of freedom are very high. In this study, we present a successful methodology to model poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly[3-hexylthiophene] (P3HT) in solutions, taking into account the influence of side chains on the polymer conformation. Molecular dynamics and semi-empirical quantum mechanical methods were used for structure optimisation and evaluation of optical properties. The methodology allows to describe structural and optical characteristics of the polymers in a satisfactory way, as well as to evaluate some usual simplifications adopted for modelling these systems. Effective conjugation lengths of 8-14.6 and 21 monomers were obtained for MEH-PPV and P3HT, respectively, in accordance with experimental findings. In addition, anti/syn conformations of these polymers could be predicted based on intrinsic interactions of the lateral branches. © 2013 Copyright Taylor and Francis Group, LLC.

Fabricação e caracterização de filmes Langmuir e Langmuir-Blodgett de derivados do politiofeno

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Efeito da adição de moléculas anfifílicas na formação de filmes Langmuir e Langmuir-Blodgett de derivados alquilados do politiofeno: aplicação em sensores

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Structure of P3HT in the solid state

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Effect of the length of alkyl side chains in the electronic structure of conjugated polymers

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Desenvolvimento de dispositivos eletrônico-orgânicos e sua aplicação como sensores em meio aquoso

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Molecular design of new P3HT derivatives: Adjusting electronic energy levels for blends with PCBM

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)