John M. Warman

Bio: John M. Warman is an academic researcher from Delft University of Technology. The author has contributed to research in topics: Conductivity & Excited state. The author has an hindex of 58, co-authored 251 publications receiving 10465 citations. Previous affiliations of John M. Warman include University of Amsterdam.

Contactless determination of the photoconductivity action spectrum, exciton diffusion length and charge separation efficiency in polythiophene-sensitized TiO2 bilayers

Abstract: The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study photoinduced charge separation in bilayers consisting of a smooth, transparent, 80 nm thick layer of anatase TiO2 onto which poly(3-hexylthiophene) (P3HT) sensitizer layers have been spin-coated. Interfacial charge separation, resulting from excitation of the polymer in the visible, is found to persist well into the millisecond time domain. Photoconductivity action spectra have been measured between 420 and 700 nm for P3HT layer thicknesses, L, from ∼2 to 200 nm. Using this electrodeless technique, the bilayers could be irradiated from either the polymer (“front”) or semiconductor (“back”) side. On front-side irradiation at 540 nm (close to the absorption maximum of the polymer), the efficiency of charge separation per incident photon (IPCSE) initially increased to a maximum value of 0.8% for L ≈ 10 nm. For thicker layers the IPCSE gradually decreased, eventually to 0.1% for L ≈ 170 nm. On back-side irradi...

Excited-state dipole moments of dual fluorescent 4-(dialkylamino)benzonitriles: influence of alkyl chain length and effective solvent polarity

Abstract: Singlet excited-state dipole moments of a number of aminobenzonitriles have been determined in cyclohexane, benzene, and 1,4-dioxane, using time-resolved microwave conductivity (TRMC) and fluorescence spectroscopy techniques. For the 4-(dialkylamino)benzonitriles (methyl, ethyl, propyl, and decyl) intramolecular charge transfer (ICT) occurs in the excited singlet state even in the nonpolar solvent cyclohexane

Visible light sensitisation of titanium dioxide using a phenylene vinylene polymer

Abstract: The photophysical properties of thin films of poly[2-methoxy-5-(2′-ethyl-hexyloxy), para -phenylene vinylene] (MEH-PPV) on TiO 2 substrates have been investigated. Evidence for dissociation of excitons at the MEH-PPV/TiO 2 interface and electron injection into the conduction band of TiO 2 has been found from current/voltage characteristics. The open circuit voltage is to 0.92 V for these cells and an energy conversion efficiency of 0.15% is found for white light. Capacitance measurements follow the Mott–Schottky relationship and reveal an acceptor density of N A ≈9×10 17 cm −3 for the polymer layers. From analysis of the luminescence as a function of the film thickness, an exciton diffusion length of 20±3 nm for MEH-PPV is derived.

Glossary of Terms used in Photochemistry

Abstract: This chapter presents a Glossary that was prepared by the Photochemistry Commission of the Organic Chemistry Division of the International Union of Pure and Applied Chemistry during the period 1978–1985 The purpose of the glossary is to provide definitions of terms and symbols commonly used in the field in order to achieve consensus on the adoption of some definitions and on the abandonment of inadequate terms

Highly mobile electrons and holes on isolated chains of the semiconducting polymer poly(phenylene vinylene)

Abstract: The nature of the charge carriers in ‘conducting’ polymers is of considerable interest at present1,2, largely on the basis of the technological potential of these materials for use as the semiconducting layer in field-effect transistors (FETs) and the emissive layer in light-emitting diodes3 (LEDs). One of the main outstanding questions concerns the relative importance of intra- versus inter-chain charge transfer in determining the overall rate of charge transport. Here we apply the pulse-radiolysis time-resolved microwave conductivity technique4 to dilute solutions of a soluble dialkoxy derivative of the semiconducting polymer poly(phenylene vinylene), PPV, by which means we determine the one-dimensional intra-chain mobilities of electrons and holes on isolated polymer chains free from inter-chain interactions. The values so obtained—0.5 and 0.2 cm2 V−1 s−1 respectively—are considerably larger than the mobilities measured previously for bulk PPV-based materials5,6,7,8,9. This suggests that considerable improvement in the performance characteristics (in particular switching time and maximum current) of organic FET and LED devices should be possible if material purity and structural order can be better controlled.

Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3

Abstract: Low-temperature solution-processed photovoltaics suffer from low efficiencies because of poor exciton or electron-hole diffusion lengths (typically about 10 nanometers). Recent reports of highly efficient CH3NH3PbI3-based solar cells in a broad range of configurations raise a compelling case for understanding the fundamental photophysical mechanisms in these materials. By applying femtosecond transient optical spectroscopy to bilayers that interface this perovskite with either selective-electron or selective-hole extraction materials, we have uncovered concrete evidence of balanced long-range electron-hole diffusion lengths of at least 100 nanometers in solution-processed CH3NH3PbI3. The high photoconversion efficiencies of these systems stem from the comparable optical absorption length and charge-carrier diffusion lengths, transcending the traditional constraints of solution-processed semiconductors.

Electroluminescence in conjugated polymers

Abstract: Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Charge transport in organic semiconductors.

Abstract: 2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

Abstract: Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Abstract: 6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916