The Electrical Transpotr Properties Of Organic Semiconductors Tetracene And Pentacene Under High Pressure

Organic semiconductors have being attracted considerable attention for variousapplications including the light-emitting diode, monitor, field-effect transistor andphotovoltaic cells devices. Pentacene and tetracene are two typical numbers oforganic materials. They show relatively higher carrier mobility than others, and are ofbroad research interest in application of electronic and optical device. Both of themcrystallize triclinic crystal structures at ambient condition which display polymorphicstate properties as same as other organic semiconductors. Due to the similarities anddifferences between pentacene and tetracene in some aspects, it is necessary to besystematically and extensively researched.In this paper, we integrated the film deposition with photolithographicmicrofabrication technique to fabricate a microcircuit on the diamond anvil cell(DAC), by a parallel plate electrodes and van der Pauw electrode，we carried out asystematic study on the pressure dependent electrical transportation properties ofpentacene and tetracene by in situ alternate current (AC) impedance spectroscopytechniques and direct current (DC) electrical resistivity measurement in a diamondanvil cell up. A series of electrical transport parameters of them are shown under highpressure including pressure and temperature dependences of electrical resistivity,resistance, relaxation frequency, relaxation activation energy and permittivity. Theexperimental results are as follows:1. By measuring pentacene under high pressure using in-situ AC impedancespectra, we found: in the whole transport process, the contribution of grain resistanceis dominated in the total resistance. At5.5GPa the contribution of grain boundaryresistance becomes very weak and can be ignored, which is related to theaccomplished C-H phase transition of pentacene at5GPa, and this indicates the phasetransition of crysta structure would have a great influence on the properties of the grain and grain boundary. Electronic and structural phase transition induced bypressure will generate new defects, vacancies and dangling bonds between theinterfaces, which changes the carrier concentration and carrier scattering mechanisms,and leading to the discontinuous change of grain boundary resistance. The resistanceand relaxation frequency discontinuous changes occurs again at17.4GPa, thisindicates a loss of molecular identity within the structure above17.4GPa where anamorphous product is produced due to the formation of saturated C···C bonds, whichis lead to internal electrical transport mechanism changed. The amorphous characterin pentacene could be related to the presence of static disorder through the chargetransport process. We described the charge hopping mechanism by Marcus chargetransport theory. Above17.4GPa, the pressure dependence of the relaxationactivation energy is21.7meV/GPa, indicating that the charge carrier transportbecomes difficult, and the carrier concentration decreases, this corresponds with theresistance trends.2. We found pentacene shows a characteristic of semiconductor from ambientpressure to28.3GPa through the temperature dependence of DC electrical resistivitymeasurements at several pressures. In our measured temperature region (80K-290K),carrier concentration increases with the increasing temperature leading to theresistivity decreases. Our work is coincide with previous findings: pentacene powdersamples becoming metallic requires higher pressure.3. By the high-pressure impedance spectroscopy measurements, we have studiedthe electrical transport properties of tetracene under high pressure. The results showthat both the grain resistance and the grain boundary resistance have contribution tothe whole electrical transport process, but grain resistance plays a dominant role.Below19GPa, grain resistance decreases with increasing pressure, while the grainboundary resistance increases with increasing pressure. When the pressure comes upto19GPa, grain and grain boundary resistance appear discontinuous change, andreversed changes occur. The relative permittivity also displays discontinuous changeat19GPa. And we believe that at19GPa molecule structure could be showamorphous state, and the molecule structure is changed, thus resulting in a change ofthe transmission mechanism.