Pressure-induced Structural Phase Transition And The Electrical Transpotr Properties In Organic Semiconductor Alq3

Organic semiconductor material Alq3plays a crucial role in the applications oforganic light-emitting diodes (oleds), because of the higher electronic transmissioncapacity, which can plate into dense film under the vacuum condition, possessing abetter stability and higher fluorescence quantum efficiency. Thus, study of theelectrical transport properties of Alq3is rather important in expanding the applicationscope and optimizing the optoelectronic properties of organic semiconductor devices.In this paper, the structure and electrical properties of Alq3powders under highpressure have been studied by using a variety of in-situ electrical measurementtechnologies under high pressure based on diamond anvil cell. The results are shownas follows:First, high-pressure X-ray diffraction data shows that all the diffraction peaks hadright shift and the peak value also decreased with the increasing pressure.Simultaneously, several diffraction peaks merge into one peak even some diffractionpeaks disappeared. At2.5GPa, there is a new diffraction peak，which indicates thatnew structure has formed in this pressure range. Above15.0GPa, reduce the amountof the diffraction peaks, suggesting that the molecular crystals have transformed intoplane structure. After releasing pressure, all the diffraction peaks return to the statewhich after the structure phase transition, indicating the complannation is fullyreversible.Second, through high-pressure Raman scattering measurement of the Alq3powders,we have observed that all the Raman peaks have blue shifts, the peaks values becomesmaller and the spectral width broaden with increasing pressure. A new peak appearsat around2.8GPa, indicating that the structure phase transition occurs. Above11.2GPa, all the Raman peaks disappear, indicating that the sample undergoes atransformation from a three-dimensional structure to a plane structure. After releasingpressure, all the peaks return to the initial state, indicating that the complannation is fully reversible. This result is consistent with the results in high-pressure X-raydiffraction experiment.Third, high-pressure in-situ AC impedance spectrum measurement shows that grainconduction is dominant in the whole electrical transport process. There are two mainreasons for the variation of pressure-dependent resistances in the organic molecularcrystals:(i) with increasing pressure, the energy gaps between the highest occupiedmolecular orbital (HOMO) and the lowest unoccupied molecular orbital(LUMO) willdecrease, which strengthen the carriers transportation corresponding to the decrease inresistance;(ii) the increase in lattice defects will hinder the electric transportation,which leads to the increase of resistance. At the beginning of applying pressure, theobvious impedance decrease is because of the pressure-induced shrinkage of the bandgap. At2GPa, the impedance begins increasing, indicating that the structure phasetransition occurs. In the range of2~8.3GPa, the impedance increased gradually withpressure, because the lattice defects effect is dominant in the electrical transportprocess. In the range of8.3~15.0GPa, the result that the impedance decreased withthe pressure is probably caused by disappearance of lattice defects effect and thereduction of energy gap. Above15GPa, the sample suffers transformation from athree-dimensional structure to a plane structure. the impedance increased with thepressure.