Determination of the Density of States on N-type Ptcdi-c13 Organic Thin-film Semiconductor
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Determination of the Density of States on N-type Ptcdi-c13 Organic Thin-film Semiconductor J. Puigdollers1,2, A. Marsal1, S. Galindo1, P. Carreras3, C. Voz1, J. Bertomeu3, R. Alcubilla1,2 1
Dept Enginyeria Electrònica, Univ. Politècnica Catalunya, Jordi Girona 1-3, Barcelona-08034, SPAIN 2 Center of Research in Nanoengineering, Univ. Politècnica Catalunya, Barcelona-08028, SPAIN 3 Dept Física Aplicada I Òptica, Univ. Barcelona, Avda Diagonal 647, Barcelona-08028 SPAIN ABSTRACT In this paper we study the density of states in n-type N,N’-ditridecylperylene-3,4,9,10tetracarboxylic diimide organic semiconductor using two different methods. The first one is based on the temperature dependence of the channel conductance in field-effect transistors. The second one is based on the subgap optical absorption coefficient measured using the Photothermal Deflection Spectroscopy technique. Both techniques allow estimating the distribution of localized states in the band gap of the semiconductor. INTRODUCTION Organic semiconductors have been successfully proposed to fabricate different electronic devices because of their potential in large area and low-cost applications. Moreover, organic semiconductors even allow the fabrication of flexible devices. Examples of these applications cover photovoltaic devices, thin-film transistors (TFT) and light-emitting diodes. The electrical characteristics of organic devices have improved significantly during the last years. Nowadays, the electrical performance of the bests organic thin-film transistors (OTFT) fabricated from small-molecule organic semiconductors is already comparable to that obtained with hydrogenated amorphous silicon (a-Si:H), the standard semiconductor used to fabricate TFTs for display applications. Nowadays, OTFTs with field-effect mobilities (µ) higher than 1cm2/V·s, threshold voltages (VT) around 0V and current on/off ratios over 106 can be obtained using organic semiconductors deposited on thermally grown silicon dioxide dielectrics [1]. Impressive achievements have been also obtained in the field of organic solar cells. Recently, Heliatek reported small-molecule organic solar cells with efficiency around 9.8 % on 1cm2 [2]. The optoelectronic properties of the aforementioned devices depend largely on the density of states (DOS) in the band gap of the active semiconductor layers. The DOS distribution in organic semiconductors determines the electrical transport, photosensitivity, doping efficiency and, at the end, impairs the device performance. The study of defects with states within the band gap is of great interest, since they act as traps and recombination centers with a strong influence in the electrical behaviour of the fabricated devices. Experimental information on the distribution of defects cannot be obtained in a straightforward way. Different techniques have been used for extracting information about the energy distribution of the DOS in thin-film semiconductors, among them we can mention ultraviolet photoelectron spectroscopy, Kelvin probe force microscopy
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