Abstract
Abstract Considerable efforts have been devoted to this study on interface of ITO, conjugated material layer, and cathode, which is thought to be one of the most important factors that can determine device performance of polymer light-emitting diodes (PLEDs). For example, a self-assembled monolayer (SAM), a CFx film formed by plasma polymerization of CHF3 on ITO, and a novel dual-functional composite layer having superior hole blocking effect along with promising electron transport (HB-ET) capability all have introduced in this study. Besides, the failure mechanism in PLEDs with different cathode has also been investigated. We demonstrate that introducing a SAM derived from 1,1,1,3,3,3-hexamethyldisilazane (HMDS) on an indium-tin oxide (ITO) anode surface for the device, (ITO/SAM/poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV)/Ca/Al), can lead to an improvement in maximum device efficiency from 2.0 to 3.9 cd/A and maximum brightness from 33,000 to 34,400 cd/m2. The improvement results from a hole blocking effect and better wetting by converting the ITO surface from hydrophilic to hydrophobic provided by the SAM. We demonstrate that introducing a thin CFx film formed by plasma polymerization of CHF3 on an indium-tin oxide (ITO) anode surface for the polymer light-emitting diode, (ITO/CFx/MEH-PPV/Ca/Al), can lead to a high device performance (5.1 cd/A and 24 000 cd/m2). The high device performance is attributed to a better balance between hole and electron fluxes, resulting from a formation of interfacial dipole at the CFx/MEH-PPV interface, which provides a hole blocking effect and an enhancement of electron/hole recombination. We demonstrate that three CFx layers (CFx(20 W), CFx(35 W), and CFx(50 W)) formed by plasma polymerization of CHF3 based on different radio frequency (rf) power, 20, 35, and 50 W on an indium-tin oxide surface for poly(9,9-dioctylfluorene) (PFO)-based light-emitting diodes can control the enhanced amount of hole injection due to they have different ionization potential (IP), 5.7, 5.6, and 5.3 eV, respectively. In addition, we found that introducing a CFx(35 W) layer for the device, (ITO/CFx(35 W)/PFO/CsF/Ca/Al), can lead to a high device performance (3.1 cd/A and 8 400 cd/m2). This is attributed to a better balance between hole and electron fluxes, resulting from a decrease in hole injection barrier proved by ultraviolet photoelectron spectroscopy and scanning surface potential microscopy. We demonstrate that a high-performance white polymer light-emitting diodes (WPLEDs) based on a single emission layer with injection barrier-free interface can be achieved using a blend of the poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT) and MEH-PPV as the dopant, the amipolar poly(9,9-bis(N-carbazole-decyl)-fluorene) (Cz75PF) as the host, and the CFx and CsF/Ca as the anode and cathode, respectively. We show that the device, ITO/CFx/Cz75PF+0.26 wt% F8BT+0.52 wt% MEH-PPV/CsF/Ca/Al, achieves excellent performance with maximum current efficiency 8.6 cd/A, maximum brightness, 35 000 cd/m2, and CIE coordinates of (0.32, 0.34). These coordinates are very close to the CIE coordinates for pure white light, (0.33, 0.33), and result from a superior balance between hole and electron fluxes and a proper dopant to host ratio. For the first time, we demonstrate a novel dual-functional composite layer having superior hole blocking effect along with promising electron transport (HB-ET) capability for the two model electroluminescent polymers, PFO and MEH-PPV. The dual-functional composite layer is composed of a nonconjugated polymer, (such as polyethyleneoxide (PEO)) or a conjugated molecular material, (such as TPBI) as well as an inorganic salt (such as Cs2CO3), which allows an effective recombination of electrons and holes and results in a high device performance. Furthermore, the composite bilayer cathode Cs2CO3(2 nm)/Ca(3 nm) is also introduced, which can enhance the electron injection dramatically. For PFO device, introducing PEO doped with 20 wt % Cs2CO3 as HB-ET layer gives the maximum brightness (Bmax) 27 000 cd/m2 and current efficiency (ηmax) 3.5 cd/A with the pure blue emission at CIEx,y (0.16, 0.07), where the pure blue emission is defined as x＋y≦0.3. For MEH-PPV device, introducing TPBI doped with 10 wt % Cs2CO3 by thermal co-evaporation as HB-ET layer provides Bmax 62 000 cd/m2 and ηmax 5.7 cd/A, equivalent to the external quantum efficiency (ηext) 2.2 %. To the best of our knowledge, these performance results are the best among those reported PLEDs using PFOor MEH-PPV as the emitting layer, where no further treatment is made such as dipping method in our previous report. This study also shows a new observation of a failure mechanism having many bright spots formed in Cs-based devices during their operation, where different cathode, CsF, Cs2CO3, CsF/Ca, or Cs2CO3/Ca is used. These bright spots are caused by Cs self-agglomeration due to its low melting point 28.5 ℃ (301.5 K) and result in a uniform emission light and a faster decay of brightness, as determined by high resolution optical microscopy and life-time testing under a high vacuum operational environment (＜ 5 × 10-7 Torr). As a result, introducing Cs compound as the cathode provides high device performance but poor stability in long-term operation, indicating that it is not acceptable for industrialization of OLEDs and PLEDs, even when encapsulated under very good conditions. In summary, we have succeeded in using different electrode surface and interface modifications (SAM, CFx, and HB-ET) to improve the device performance of PLEDs. The procedures of modifications we present here are very easy, indicating that we can use these methods applying to the industry. Besides, we have shown that the stability and thickness of cathode have strong correlation between device performance and stability, which provides a important information for industrialization of OLEDs and PLEDs.