Evaluation and characterization of efficient organic optoelectronic materials and devices
Principal supervisor: Professor So Shu Kong ; Thesis submitted to the Department of Physics
With the progression towards lighter but larger-display self-sustainable mobile devices, device eﬃciency becomes increasingly important, owing to the higher power display consumption but at the same time more limitation on the size and volume of energy storage. In this thesis, selected aspects regarding to eﬃciency of three types of optoelectronic devices, indoor photovoltaics (IPVs), perovskite thin-ﬁlm transistors (TFTs) and organic light-emitting diodes (OLEDs) have been investigated. IPVs can make oﬀ-grid devices self-sustainable by harvesting ambient light energy. Its weak irradiance necessitates high-eﬃciency IPVs to generate suﬃcient power. Our work addresses the need of knowing the limit of the device parameters for correct evaluation and understanding the eﬃciency loss for developing clinical tactics. We delivered a general scheme for evaluating the limiting eﬃciency and the corresponding device parameters of IPVs under various lights, illuminance and material bandgap. In contrast to the AM1.5G conditions, a maximum power conversion eﬃciency (PCE) of 51-57 % can be achieved under the optimal bandgap of 1.82-1.96 eV. We also propose using the second thickness peak of interference instead of the ﬁrst as a better optimal absorber thickness after identifying the ﬁnite absorption as the major source of eﬃciency loss. The work provides insights for device evaluation and material design for eﬃcient IPV devices. The novel hybrid organic-inorganic perovskites have gained enormous research interest for its various excellent optoelectronic properties such as high mobility. TFT as an alternative application to the majorly focused photovoltaics is realized in this work. There are few reports on perovskite TFTs due to wetting issues. By employing polymethacrylates with ester groups and aromatic substituents which provide polar and cation-π interactions with the Pb2+ ions, quality ﬁlms could be fabricated with large crystals and high electron mobility in TFTs. We further improved the performance by resolving interfacial mixing between the perovskite and the polymer using the crosslinkable SU-8, achieving the highest mobility of 1.05 cm2 V−1 s−1. Subsequently, we cured the grain boundaries using methylamine solvent vapor annealing, suppressing the TFT subthreshold swing. The work provides a map for the improvement of perovskite TFTs. It has been revealed that molecular orientations of the emitters in OLEDs with the transition dipole moment lying in plane enhances light outcoupling eﬃciency. Multiple experimental techniques are needed to provide complementary orientation information and their physical origin. Here, we propose using TFT to probe the orientation of the phosphorescent emitters. Homoleptic fac-Ir(ppy)3 and heteroleptic trans-Ir(ppy)2(acac) and trans-Ir(ppy)2(tmd) were deposited on polystyrene (PS) and SiO2 substrates. Compared to the PS surface inducing isotropic orientation as the control, trans-Ir(ppy)2(acac) and trans-Ir(ppy)2(tmd) possessed decreased carrier mobilities on SiO2. With the study of initial ﬁlm growth, we infer that preferred orientation induced by the polar SiO2 surface led to an increase in energetic disorder in the well-stacked trans-Ir(ppy)2(acac) and hopping distance in the amorphous trans-Ir(ppy)2(tmd). The highly symmetric fac-Ir(ppy)3 remained its isotropic orientation despite the dipolar interaction. Surprisingly, the TFT technique gives much higher sensitivity to surface-induced orientation, and thus may potentially serve as a unique electrical probe for molecular orientation.