Conductive polymer work functions

Summary of Sample Parameters Pristine Conducting Polymer Work Function ( ), and Hole Injection Barrier AE and Work Function < p after Deposition of 2.5-nm Pentacene, Interface Dipole ID... 

Inverted Device Structures The conventional device structure for PSCs is indium tin oxide (ITO)/PEDOT PSS/polymer blend/Al, where a conductive high-work-function PEDOTPSS layer is used for anode contact, and a low-work-function metal as the cathode. Both the PEDOTPSS layer and the low-work-function metal cathode can cause the degradation of PSCs [110-112]. The acidic PEDOTPSS was reported to etch the ITO and cause interface instability through indium diffusion into the polymer active layer. Low-work-fiinction metals, such as calcium and aluminum, are easily oxidized when exposed to air, increasing the series resistance at the metal/BHJ interface and degrading device performance. 

However, this should pave the way for conducting polymers to be used as an alternative soluhon. Today, the major challenge for scientists working in the field of conduchve polymer-based products is to overcome any existing problems such that they may offer easily processed, environmentally friendly and stable conductive polymer-based functional coatings for general use. 

Gurlo A, Sahm M, Oprea A, Barsan N, Weimar U (2004) A p- to n-transition on a-Fe Oj-based thick film sensors studied by conductance and work function change measurements. Sens Actuators B 102 291-298 Harbeck M (2005) New appUcatious of organic polymers in chemical gas sensors. PhD Thesis, University of Tibingen, Germany... 

Blom et al. [85] stated that the l/V characteristics in LEDs based on ITO/di-alkoxy-PPVs/Ca are determined by the bulk conductivity and not by the charge carrier injection, which is attributed to the low barrier heights at the interface ITO/PPV and PPV/Ca. They observed that the current flow in so called hole-only devices [80], where the work function of electrodes are close to the valence band of the polymer, with 1TO and Au as the electrodes, depends quadratically on the voltage in a logl/logV plot and can be described with following equation, which is characteristic for a space-charge-limitcd current (SCL) flow (s. Fig. 9-26) ... 

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

Meanwhile, the R-R coupling (see Sect. 2.2) has evidently found general acceptance as the main reaction path for the electropolymerization of conducting polymers The ionic character of the coupling species explains why polar additives such as anions or solvents with high permittivity accelerate the rate of polymerization and function as catalysts. Thus, electropolymerization of pyrrole is catalyzed in CHjCN by bromide ions or in aqueous solution by 4,5-dihydro-1,3-benzenedisulfonic acid The electrocatalytic influence of water has been known since the work... 

The surface oxidation products dete ted by the decrease in contact angle upon photolysis of PVCa films may dominate the photoconductivity of t. is polymer. Work is underway to confirm this relatio. ship and measure surface conductivity simultaneously with bulk conductivity as a function of photodegradation. 

Fig. 15 Simplified schematic representation of the electronic energy levels in a single-layer PLED. CB and VB are the conduction hand and valence hand, respectively, of the semiconducting polymer, which correspond to the ionization potential (IP) and electron affinity (EA) relative to vacuum level (EV). The work functions for anode (and cathode (

The energy reference in each case for the measurements described above is the fermi level and although the exact location of this level in relation to the valence and conduction bands is generally unknown for polymers, as we have noted under the conditions of X-ray irradiation it is possible for an insulator to be in electrical contact with the spectrometer i.e. their fermi levels are the same. Despite the difficulties associated with defining an analytical expression for the fermi level of an insulator, the use of the fermi level as energy reference is operationally convenient. If the work function of the insulator is known we may calculate the binding energy with respect to the vacuum level. 

Suppose that the metal and the semiconductor are both electrically neutral and separated from each other. Since the metal at the electron-injecting contact is assumed to have a low work function, the Fermi energy of the metal lies above that of the semiconductor, close to its conduction band. If the metal and the semiconductor are connected electrically, electrons will flow from the metal to the polymer in order to establish equilibrium, which is obtained when the Fermi energies of the two materials are aligned. Because of this flow of charge, the materials are no longer neutral after electric... 

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