Milan Stolka

Photovoltaic and photoconductive properties of aluminum/poly(p-phenylene vinylene) interfaces

Photoconductive and photovoltaic properties of Al/PPV/ITO sandwich devices were investigated by measuring steady-state photocurrents resulting from illumination through the Al electrode. A built-in potential (Vbi) was detected at the Al/PPV interface. The voltage dependence of the photocurrent in the vicinity of Vbi was measured at 1 mW/cm2 of incident illumination to give an open-circuit voltage and a short-circuit current of 1.2 V and 6 × 10−7 A/cm2, respectively. The dependences of the short-circuit current on excitation wavelength and illumination intensity are presented and the C-V characteristics of the Al/PPV interface are analyzed. The quantum collection efficiency decreased from 5% to 1% as the intensity of illumination increased from 10−5 to 1 mW/cm2. The photovoltaic conversion power efficiency was about 0.07% for intensities approaching 1 mW/cm2. The Vbi value was accounted for by surface band-bending at the Al/PPV interface.

Electronic transport in silicon backbone polymers

Abstract Results of recent time-of-flight transport measurements on the silicon backbone polymer poly(methylphenylsilylene) (PMPS) carried out over broad ranges of fields and temperatures described in much greater detail than heretofore. The relative effect of a series of chemical dopants in reducing the hole drift mobility of PMPS is correlated with their respective ionization potentials. Doping effects can be completely understood on the basis of a simple model which is described. Transport in PMPS manifests the key features observed in a broad class of polysilylenes containing aromatic and/or aliphatic side group substituents including those completely devoid of π electrons. Side-group substituents in saturated carbon backbone polymers such as poly(N-vinylcarbazole) completely dominate the transport process which is hopping between the carbazole chromophores. In contrast with this behaviour, comparative measurements indicate that side-group substituents in polysilylenes play a secondary role. The compl...

Common features in the transport behaviour of diverse glassy solids: Exploring the role of disorder

Abstract Key results of time-of-flight drift mobility measurements on silicon and germanium backbone polymers are reviewed. Transport in these polymers displays a convoluted and yet familiar pattern of electric field and temperature dependences similar in remarkable detail to behaviour already reported for a very diverse collection of disordered molecular solids. The latter, while varied in composition and morphology, all share at least two common features, namely firstly transport controlled by interaction with localized states which are either hopping sites or shallow traps in communication with a band and secondly disorder. We explore in this paper the application of what is arguably the simplest model (derived from Monte Carlo simulation) of transport incorporating these characteristics. Consideration will be restricted to hopping among a spatially regular array of sites made energetically inequivalent by disorder. Thus, neither positional disorder nor site relaxation is explicitly considered. Some re...

The origin of dispersion of transiting charge carriers in molecular l y doped polymers

Abstract The dispersion of transiting charge carriers in organic solids is analysed by examining the normalized time-of-flight current signals obtained over a wide range of electric fields, sample thicknesses and temperatures. Carriers injected into polycarbonate molecularly doped with N, N′-diphenyl-N, N-bis(3-methylphenyl)-[1, 1′-biphenyl]-4, 4′-diamine reach dynamic equilibrium with the environment in a small fraction of the overall transit time. The carrier dispersion then appears to follow the time-dependent Gaussian statistics with a broader Gaussian bandwidth observed at higher electric fields. The Gaussian dispersion arises from the off-diagonal disorder, in agreement with the prediction of Marshalls Monte Carlo simulation. The existence of a finite field-independent dispersion at very long transit times and the observed narrowing of the dispersion with increasing temperature suggest that diagonal disorder (a distribution of hopping site energies) also plays an important role in controlling the c...

Space-Charge-Limited Charge Injection From Ito/Ppv Into A Trap-Free Molecularly Doped Polymer

We describe bilayer structures comprised of a poly ( p -phenylene vinylene) (PPV) layer and a trap-free diaryldiamine (TPD) doped in polycarbonate (PC) layer, sandwiched between indium-tin-oxide (ITO) and aluminum (Al) contacts. Two critical phenomena in the operation of polymer based electroluminescent devices, interface injection and carrier range, are investigated. It is established that the ITO/PPV contact is capable of sustaining dark current under trap-free space-charge-limited (TFSCL) conditions into a hole transporting TPD:PC layer. TFSCL currents are not observed in devices without the PPV layer. Upon increasing the thickness of the PPV layer a deviation from the TFSCL regime is observed which is attributed to trapping of the injected holes within PPV. These observations suggest a novel method for estimating the trapping Mobility-lifetime product μτ for holes in PPV. By this means we estimate μτ ∼ 10 −9 cm 2 /V.

Behavior of an ideal injecting contact on a trap‐free polymer

Solution coatable insulators capable of unipolar photoinjected carrier transport with negligible loss of transiting charge to deep traps are required for the fabrication of organic electrophotographic receptors. Molecularly designed polymeric insulators with these characteristics can now be routinely synthesized. Such trap‐free polymer films provide a unique venue for the study of contact and interface behavior. A test for distinguishing ohmic from emission limited contact behavior which exploits the availability of these polymers is described. The test involves direct comparison of dark injection transients excited by application of a voltage step to the contact under investigation, with small signal time‐of‐flight transients photoexcited by laser pulse irradiation through a semitransparent blocking contact on the opposite face of the same specimen film.

Redox migration mechanism of charge transport in molecularly doped polymers

Abstract A simple model of hopping transport in a molecularly doped polymer is described based on a small electric-field perturbation of diffusional (random walk) electron self-exchange reactions in the solid state. A master expression is derived which quantitatively accounts for the electrical field, temperature and concentration dependence of the hole mobility in the N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine/polycarbonate system in terms of fundamental electron exchange parameters employed in modern (semi-classical) electron-transfer theory. Charge mobility which is simply related to electron hopping between neighbouring oxidized/reduced sites is assumed and shown to be normally thermally activated. The electric-field dependence of mobility is derived by modifying the zero-field Arrhenius rate expression for charge diffusion in a manner analogous to the derivation of the overpotential (electric field) dependence of charge transfer at an electrified interface. The intersite dist...

Hole transport in solid solutions of substituted triarylmethanes in bisphenol-A-polycarbonate

The time-of-flight and xerographic discharge techniques have been used to study electronic transport in films of dialkylamine-substituted triarylmethanes in bisphenol-A-polycarbonate binder. The hole mobility has been measured as a function of applied electric field, concentration of the molecules, and temperature. The results indicate that charge transport occurs by hopping among localized states associated with the molecule. In chemical terms, the hole transport involves a series of one-electron oxidation-reduction steps involving the organic molecules. The influence of some substituent groups on the electronic transport in these aromatic diamines has also been studied.

Polymers with photoconductive properties

Photoconductivity of organic polymers is reviewed. The current views on the mechanism of charge carrier generation and transport in sensitized and unsensitized polymers are presented. The known photoconductive polymers are categorized according to their structure into three basic groups: polymers with high degree of conjugation in the main chain, polymers with condensed aromatic groups and polymers with substituted arylamino functional groups.

Electronic transport in glassy Si-backbone polymers

Abstract Polysilylenes, polymers built on a saturated silicon backbone, represent a new class of dielectric materials capable of transporting injected holes. Charge transport in poly(methylphenylsilylene), a typical representative of these polymers, is thermally activated and non-dispersive over a wide range of temperatures. The carrier drift mobility is high for organic glasses, in excess of 10 −4 cm 2 /V.s. at E > 10 5 V/cm, and filed dependent at T g . The charge transport in polysilylenes has most of the characteristics of hopping trasport among discrete states, but it is insensitive to the nature of the side groups (aromatic or aliphatic) and the molecular weight of the polymer. The proposed mechanism of transport is that of hopping among rather short segments of the silicon chain.