Device engineering for improving the performance of organic light-emitting devices and organic photovoltaic devices

Abstract

This thesis focuses on the optimization of OLEDs and OPVs via device engineering. Particular attention is paid to two aspects: i) the employment of different metal-doped electron-transporting layers or host systems for OLEDs; ii) the utilization of different exciton blocking layers or charge-transporting layers for improving the photovoltaic responses of OPVs. In the first part, the optical properties of different metal doped thin films were investigated. The optical bandgap of metal-doped Alq3 film is found to be strongly dependent on the choice of metal dopant, in which a good linear relationship between the optical bandgap and metal electronegativity has been obtained. These findings suggested that metal electronegativity is an appropriate property for describing the metal dependence of the optical properties of organic thin films. To improve the EQE of the OLED devices, various host materials doped into the Alq3-based films was attempted. Substantial increase in ηPL of over 22 % for Alq3 doped films was found together with the slight shift of photoluminescence spectral maxima towards shorter wavelength of 515–521 nm and with narrowed FWHM of 100–103 nm for the neat Alq3 counterparts. OLEDs based on Alq3:mCP demonstrated high efficiencies of 7.8 cd A-1, 7.4 lm W-1 and 2.5 %. The substantial reduction of excimeric luminescence quenchers, more efficient host to Alq3 guest energy transfer, and reduced number of Alq3 phases distributed towards unity might account for the improvement in the film properties. In the second part, the relationship among the properties of photoactive layer, device structure, and the photovoltaic responses of OPV devices has also been systematically studied. The influences of non-absorbing hole-transporting and electron-transporting materials on the performance of the modified bulk heterojunction OPV devices have been studied. Six representative materials were employed as donor and doped into fullerene acceptor. Interestingly, the incorporation of hole-transporting donor material could dramatically improve the photovoltaic responses, in which a high PCE of up to 4.44 % has been realized for the TAPC-doped devices, an order of magnitude higher than that of the C70-only device. In sharp contrast, the performance of the device would deteriorate when an electron-transporting material is used as donor. ΔEHOMO was found to play crucial role in determining the VOC. The VOC is strongly dependent on the HOMO level of the donor and an optimal VOC has been obtained for ΔEHOMO of ~0.4 eV. Two series of OPV devices with EBLs were fabricated to study the relation between the physical properties of EBLs and the photovoltaic responses of OPV devices. With the insertion of EBLs, all photovoltaic responses such as JS and VOC were found to be dramatically increased, leading to a concomitant increase in the PCE of the OPV devices. These were ascribed to a reduced bimolecular recombination, as indicated by a smaller ideality factor and saturated dark current density. The VOC was found to be linearly increased with the interface offset energy, in which the linearity factor (S1) is dependent on the choice of EBLs.