Chapter 1. Introduction
1.2 Organic-Inorganic Hybrid Optoelectronic Devices
OLEDs have attracted considerable interest as the potential next generation lighting and display.
However, high device performance can be only obtained using multilayer devices. Moreover, these devices are composed of air-sensitive charge transport layer and metal electrodes, leading to inferior device stability8-10. The encapsulation process should be required to prevent degradation of device. High device performance and stability of OLEDs are required to enter successfully display and lighting market.
1.2.1 Conventional Structure
The conventional structure of optoelectronic devices mostly consists of an indium tin oxide (ITO) as transparent anode, a poly(3,4-ethylenedioxythiophene):poly-styrene sulfonate (PEDOT:PSS) as HTL, an active layer (emitting materials and absorbing materials) and a low work function calcium (Ca), barium (Ba) and aluminum (Al) as metal cathode. Schematic of the conventional device structure is shown in Figure 1.10. The conventional-structured optoelectronic devices show good device
performance because of small energy barrier by matching energy levels of each layer. However, PEDOT:PSS causes corrosion of ITO due to its acidic characteristics, leading to degraded device stability. Low work function metal electrodes such as Ca, Ba and Al can be easily oxidized under ambient condition. The conventional-structured optoelectronic devices have inferior device stability, which requires strict encapsulation to enhance device stability. To overcome stability issue of the conventional-structured devices, reactive HTLs and low work function metals should be replaced by introducing inverted structure with stable inorganic metal oxide layers as charge transport layers and high work function metal electrodes such as silver (Ag) and gold (Au).
Figure 1.10. Schematic of the conventional device structure
1.2.2 Inverted Structure
The inverted structure of optoelectronic devices is mostly composed of an ITO as transparent cathode, inorganic metal oxide as ETL, active layer (emitting materials and absorbing materials), inorganic metal oxide as HTL and high work function metal as anode. Schematic of the conventional device structure is shown in Figure 1.11. Metal oxides as charge transport layers have been introduced as an alternative to reactive materials due to high transparency, high conductivity and good air stability. Generally, zinc oxide (ZnO), titanium oxide (TiOx) and tin oxide (SnO2) are employed as an electron transport layers and Molybdenum oxide (MoO3), nickel oxide (NiOx) and vanadium oxide (V2O5) are introduced as a hole transport layer in the inverted-structured optoelectronic devices. The inverted-structured optoelectronic devices exhibit excellent device stability by using air-stable metal oxides as charge transport layers and air-stable high work function metal as cathode15-18. However, device performance of organic-inorganic hybrid optoelectronic device is not good because of large energy barrier between metal oxide as electron transport layer and active layer. Moreover, solution-processed metal oxide as a
charge transport layer unavoidably contains undesired defects such as oxygen vacancy that causes leakage current and luminescence quenching. In order to overcome the issues of device performance, interfacial engineering between metal oxide and active layer should be required to improve device performance by enhancing charge injection/transport with blocking opposite and passivating defect sites of metal oxide.
Figure 1.11. Schematic of the inverted device structure
Steven et al. have reported new method to extend device stability of polymer-based solar cells. The device stability of unencapsulated inverted polymer solar cells were compared to conventional polymer solar cells by employing ZnO as electron transport layer19. The inverted-structured devices exhibited much better stability than the conventional structured device under ambient condition, retaining over 80% of initial efficiency until 40 days, as shown in Figure 1.12. The enhanced device stability is considered due to using air-stable ZnO as electron transport layer. The conventional-structured polymer solar cell using Al as electrode was very unstable, showing the reduction less than half of initial value within one day and completely degradation within 5 days. In contrast, all the inverted-structured devices revealed the high stability under ambient condition. These devices showed no remarkable performance changes until 40 days, indicating the degradation of the electrode are the main reason for the decreases in performance.
Figure 1.12. Device performance of conventional and inverted-structured polymer solar cells without encapsulation under ambient conditions. (a) Normalized PCE, (b) JSC, (c) VOC, and (f) FF.
Bolink et al. have reported the inverted-structured polymer-based LEDs by introducing TiO2 as electron transport layer deposited on transparent cathode, MoO3 as hole transport layer and high work function metal of Au as anode20. Unfortunately, performance of inverted-structured devices is rather row compared to conventional structured devices. The light-emitting material, poly(9,9- dioctylflourene-co-benzoghiadiazole) (F8BT) is p-type semiconductor, which means that hole conductivity is much higher than electron conductivity. Moreover, hole injection from MoO3/Au to F8BT layer is considerably efficient while electron injection barrier between TiO2 and F8BT is rather high, which leads to unbalanced charge injection and transport, as shown in Figure 1.13. To improve device efficiency, electron transport layer should have deep valence band level to block majority hole carriers, which results in enhanced recombination yields with reduced leakage current.
Figure 1.13. (a) Current density versus voltage characteristics for an ITO/TiO2/F8BT/Au (squares) and an ITO/TiO2/F8BT/MoO3/Au (triangles) devices. (b) Luminance versus voltage characteristics for an ITO/TiO2/F8BT/MoO3/Au (triangles) and an ITO/PEDOT/F8BT/Ba/Ag (circles) devices.