Progress in Organic Coatings 58 (2007) 303–311 On the mechanism of electrical conduction in some new quaternary salts of bipyridine and indolizine pyridine in thin films L. Leontie a,∗ , I. Druta b , B. Furdui c , G.I. Rusu a a Faculty of Physics, “Al.I. Cuza” University, 11 Carol I Boulevard, 700506 Iasi, Romania b Faculty of Chemistry, “Al.I. Cuza” University, 11 Carol I Boulevard, 700506 Iasi, Romania c Faculty of Sciences, The University “Dunarea de Jos” of Galati, 47 Domneasca Street, 800008 Galati, Romania Received 11 July 2006; accepted 12 January 2007 Abstract The temperature dependences of dc electrical conductivity, σ, and Seebeck coefficient, S, for six recently synthesized quaternary salts of bipyridine and indolizine pyridine (GAL compounds) in thin films, have been investigated. The thin-film samples (d = 0.06–0.60 m) were deposited by spin-coating using dimethylformamide solutions, onto glass. XRD was used for structure investigations, while AFM technique, corroborated to optical microscopy, was used for the examination of surface morphology of samples. The present compounds behave as typical n-type polycrystalline semiconductors. The activation energy of electrical conduction, E, ranged between 1.61 and 1.73eV, while the ratio of charge carrier mobilities, b, laid in the range 1.08–1.14. By studying optical absorption direct band gaps ranged between 3.78 and 4.00 eV have been found. Some correlations between semiconducting parameters and molecular structure of the compounds were established. The model based on band gap representation can be conveniently used for the explanation of electronic transport in investigated compounds. The investigated compounds are also suitable for applications in thermistor manufacture. © 2007 Elsevier B.V. All rights reserved. Keywords: Organic semiconductors; Thin films; AFM; Electrical conductivity; Seebeck coefficient 1. Introduction During the past three decades, organic semiconductor thin films have gained constantly increasing attention due to both potential technological applications and academic interest [1–5]. Combining their semiconducting electronic properties with the processing advantages (ready shaping and manufacture), they are suitable for a series of large area applications ranging from photodetectors and sensors to portable computing, flexible smart cards and flat panel displays [6–9]. Organic semiconductors show attractive physical properties (band gap in the infrared-visible range, high charge carrier mobilities, enhanced electroluminescence), as well as chemi- cal, and mechanical features, that can be tuned by chemical modification [10]. Together with a high versatility, these charac- ∗ Corresponding author. Tel.: +40 232 201168; fax: +40 232 201201/201150. E-mail address: lleontie@uaic.ro (L. Leontie). teristics make the novel organic materials promising candidates for new generations of low cost ‘plastic’ electronic and opto- electronic devices: Schottky diodes [11], transistors [12–15], organic (polymer) light-emitting diodes [7,16,17], photodiodes [7], lasers [18], and photovoltaic devices [19–23]. The performance of these devices critically depends on the efficiency of the charge-transport processes within respective organic films. That is why in recent years, a great amount of research was devoted to understanding the basic charge- transport mechanisms in organic materials [24–30]. A large number of papers have shown that the electronic transfer and optical properties of these compounds (monomers, polymers, charge transfer complexes, etc.) strongly depend on their molecular structures. In this connection, it is very impor- tant to establish some correlations between chemical structure of the materials and the values of fundamental characteris- tic parameters (activation energy, concentrations and mobilities of charge carriers, etc.), which describe their semiconducting properties. 0300-9440/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2007.01.003