Serap Günes

Fabrication and characterization of solution-processed methanofullerene-based organic field-effect transistors

The fabrication and characterization of high-mobility, n-channel organic field-effect transistors (OFET) based on methanofullerene [6,6]-phenyl C61-butyric acid methyl ester using various organic insulators as gate dielectrics is presented. Gate dielectrics not only influence the morphology of the active semiconductor, but also the distribution of the localized states at the semiconductor-dielectric interface. Spin-coated organic dielectrics with very smooth surfaces provide a well-defined interface for the formation of high quality organic semiconductor films. The charge transport and mobility in these OFET devices strongly depend on the choice of the gate dielectric. The electron mobilities obtained are in the range of 0.05-0.2 cm2 V-1 s-1. Most of the OFETs fabricated using organic dielectrics exhibit an inherent hysteresis due to charge trapping at the semiconductor-dielectric interface. Devices with a polymeric electret as gate dielectric show a very large and metastable hysteresis in its transfer characteristics. The observed hysteresis is found to be temperature dependent and has been used to develop a bistable memory element.

Correlation between morphology and ambipolar transport in organic field-effect transistors

Attaining ambipolar charge transport in organic field-effect transistors (OFET) is highly desirable from both fundamental understanding and application points of view. We present the results of an approach to obtain ambipolar OFET with an active layer of organic semiconductor blends using semiconducting polymers in composite with fullerene derivatives. Clear features of forming the super position of both hole and electron-enhanced channels for an applied gate field are observed. The present studies suggest a strong correlation of thin-film nanomorphology and ambipolar transport in field-effect devices.

Polymer Solar Cells

Polymer solar cells, a highly innovative research area for the last decade until today, are currently maturing with respect to understanding of their fundamental processes of operation. The increasing interest of the scientific community is well reflected by the-every year-dynamically rising number of publications. This chapter presents an overview of the developments in organic photovoltaics employing conjugated polymers as active materials in the photoconversion process. Here the focus is on differentiating between the various material systems applied today: polymer-fullerene, polymer-polymer, polymer-nanoparticle hybrids, and polymer-carbon nanotube combinations are reviewed comprehensively.

Vacuum-free processed bulk heterojunction solar cells with E-GaIn cathode as an alternative to Al electrode

In this paper, the photovoltaic characteristics of bulk heterojunction solar cells employing an eutectic gallium–indium (EGaIn) alloy as a top metal contact which was coated by a simple and inexpensive brush-painting was investigated. The overall solar cell fabrication procedure was vacuum-free. As references, regular organic bulk heterojunction solar cells employing thermally evaporated Aluminum as a top metal contact were also fabricated. Inserting the ZnO layer between the active layer and the cathode electrodes (Al and EGaIn) improved the photovoltaic performance of the herein investigated devices. The power conversion efficiencies with and without EGaIn top electrodes were rather comparable. Hence, we have shown that the EGaIn, which is liquid at room temperature, can be used as a cathode. It allows an easy and rapid device fabrication that can be implemented through a vacuum free process.

A green neutral state donor–acceptor copolymer for organic solar cells

We report on the photophysical and photovoltaic properties of a low band gap polymer bearing a quinoxaline moiety, poly(2,3-bis(3,4-bis(decyloxy)phenyl)-5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)quinoxaline) PDOPEQ, as an electron donor in bulk heterojunction solar cells blended with the acceptor 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM). Devices were composed of PDOPEQ and varying amounts of PCBM (1:1, 1:2, 1:3, 1:4 w–w ratio). The components were spun cast from chlorobenzene (CB) and characterized by measuring current–voltage characteristics under simulated AM 1.5 conditions. The devices with 1:3 polymer to PCBM ratio exhibited short circuit current density (Jsc) of 0.8 mA cm−2, an open circuit voltage (Voc) of 0.2 V, and a fill factor (FF) of 0.3. Incident photon to current efficiency (IPCE) is also reported. The IPCE spectrum spans from 400 nm to 800 nm and exhibits a photocurrent contribution of ca. 5.5% at around 400 nm. The nanoscale morphology was investigated with atomic force microscopy (AFM). Photoinduced absorption spectroscopy confirms the photoinduced charge transfer in such donor acceptor blends.

Photoelectrochemical cells based on emeraldine base form of polyaniline

Photoelectrochemical cells (PECs) have been fabricated using the emeraldine base form of polyaniline (EB) as a photoactive material and Eu2+/Eu3+ redox couple in methanol as an electrolyte. A PEC with a structure: Glass/ITO/EB/electrolyte/Pt produces an open-circuit voltage (VOC) of -0.132 V and a short-circuit current (ISC) of 0.64 µA cm-2 under 50 mW cm-2 white light illumination from Xe lamp. In an effort to increase the photoresponse, a PEC with a structure: Glass/ITO/EB:Nc-TiO2/Electrolyte/Pt has been devised in which a composite film of EB and nanocrystalline TiO2 (Nc-TiO2) is used as a photoactive material. The cell shows a VOC of -0.205 V and an ISC of 105 µA cm-2 when illuminated under the same conditions.

Photo-induced phenomena in organic field-effect phototransistors based on conjugated polymer/fullerene blends and organic dielectric

Photo-induced phenomena were investigated in photoresposive organic field-effect transistors (photOFETs) based on conjugated polymer/fullerene solid-state mixtures as active semiconductor layer and divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as gate dielectrics. The devices were characterized both in under dark showing n-type transistor behaviour with linear and saturated mobility of 1.7 x 10-3 cm2/Vs and 2.7 x 10-2 cm2/Vs respectively, and under white light illumination condition, where large shifts in the threshold voltage in the transfer characteristics were obtained. A typical phototransistor behaviour in a wide range of illumination intensities are observed in these devices.

Metal-free polymer/MWCNT composite fiber as an efficient counter electrode in fiber shape dye-sensitized solar cells.

UNLABELLED Highly aligned multiwall carbon nanotubes (MWCNT) as fiber were modified with a conducting polymer via a simple dip coating method. Modified MWCNT exhibited admirable improvement in electrocatalytic activity for the reduction of tri-iodide in dye sensitized solar cells. Scanning electron microscopy images confirm the successful deposition of polymer on MWCNT. Cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy studies were carried out to investigate the inner mechanism for the charge transfer behaviour. Results from bare and modified electrodes revealed that the MWCNT/(poly (3,4-ethylene dioxythiophene):poly(styrene sulfonate) ( PEDOT PSS) composite electrode is much better at catalysing the [Formula: see text] redox couple compared to the pristine fiber electrode. The photoelectric conversion efficiency of 5.03% for the modified MWCNT electrodes was comparable with that of the conventional Pt-based electrode. The scientific results of this study reveal that MWCNT/PEDOT:PSS may be a better choice for the replacement of cost intensive electrode materials such as platinum. Good performance even after bending up to 90° and in-series connection to enhance the output voltage were also successfully achieved, highlighting the practical application of this novel device.

Solid State Dye Sensitized Solar Cells Using Poly (3-hexylthiophene) as Hole Transport Material

Solid state dye sensitized solar cells have been fabricated using a ruthenium dye complex, RuL2(NCS)/TBA(2:2) (where L= 2,2-bipyridyl-4,4-dicarboxylic acid; TBA= tetrabutylammonium), as a sensitizer and poly(3-hexylthiophene) (P3HT) as a hole transporting layer with TiO2 electrodes as electron transporting layers. We have studied the photovoltaic properties of P3HT/dye coated TiO2 electrodes and P3HT coated TiO2 electrodes with and without using compact TiO2 layers. The compact TiO2 layer improved the photovoltaic performance of the devices.

Theoretical and experimental investigations of the 2-(4-chlorophenyl)-3-{[5-(2-cyano-2-phenylethenyl)]furan-2-yl}acrylonitrile molecule as a potential acceptor in organic solar cells.

A novel soluble asymmetric acrylonitrile derivative, 2-(4-Chlorophenyl)-3-{[5-(2-cyano-2-phenylethenyl)]furan-2-yl}acrylonitrile (CPCPFA, 3) was synthesized in three steps by Knoevenagel condensation. The structure of the CPCPFA was characterized using UV-vis, FTIR, (1)H NMR, (13)C NMR, and LC-MS. CPCPFA was evaluated as an electron acceptor in bulk heterojunction organic solar cells. Its optical and electronic properties as well as photovoltaic performance were investigated.