ITO/PEDOT:PSS

Same as arylamine DSA derivatives used as guest materials in blue OLEDs, Geise et al. synthesized a series of alkyloxy-substituted biphenyl vinyl compounds as dopant materials [236]. These authors studied OLEDs fabricated using PVK as a host polymer and holetransporting PBD as a codopant with a PLED structure of ITO/PEDOT-PSS/PVK 230VBP PBD/LiF/Al, which gave an optimized QE of 0.7% and brightness of 1600 cd/m2 at 100 mA/cm2 with emission peak at 450 nm. 

The impedance of PLEDs can be viewed in many different formats G-B, R-X, or as a function of frequency (see Figure 10.6). The diode in this example was ITO/PEDOT PSS/Alkoxy-PPV/Ca/Al, measured from 102 to 107Hz, with forward bias from 0 to 5 V. 

FIGURE 7. Current-voltage characteristic of the device ITO/PEDOT PSS/polymer 30 PCBM/A1 under illumination with simulated AM 1.5 solar light. 

Polymers la and lh were fabricated into electroluminescent devices A and B, respectively, with the following configuration ITO/PEDOT PSS/polymer/Ca/Al. 

Device structure ITO/PEDOT PSS/polymer(Pl—P3)/BCP/Alq3/LiF/Al, where the polymer (PI—P3) is an emitting layer. bVoa is the turn-on voltage. 

Fig. 1.33. Semi-logarithmic plot of the current/voltage curves of a PET/ITO/PEDOT-PSS/OPV4 C60/A1 photovoltaic cell in which OPV4-C60 is the active material. Open squares represent dark curve and solid squares were recorded under about 65 mW cm-2 white light illumination...

Fig. 3.17. J-V characteristics of the ITO/PEDOT PSS/MEH-PPV/Au diode at 240 K. The thickness of the active layer is 120 nm. The symbols represent die experimental data. The dash-dot line represents the ohmic region due to thermally generated and background carriers. The dashed line represents die calculated values using the conventional equation (3.42) with a zero Schottky barrier, while die dotted curve represents the calculated values using Eq. (3.46) with a Schottky barrier = 0.1 eV. At lower voltages below die point D, the plot of Eqs. (3.46) and (3.42) are practically identical. The values of die parameters used in this representation are N0 = 1019 cm-3, Tc = 400 K, e = 3, e0 = 8.85x 1(T14 F/cm, fi = 7x 1(T5 cm2/Vs, iVv = 2x 1019 cm-3 and Hh = 4.5 x 1018 cm-3 [44],...

FIG. 3.21. J-V characteristics of the conducting organic polymer diode ITO/PEDOT PSS/MEH-PPV/Au on the log-log scale at different temperatures, where the thickness of MEH-PPV is 65 nm. The symbols represent the experimental data, while the solid lines are the calculated values using Eq. (3.42) at the corresponding temperatures [53],... 

We prepared another diode with the same structure ITO/PEDOT PSS/MEH-PPV/Au, but different thickness of MEH-PPV. The thickness of MEH-PPV in this sample was 65 nm. The experimental J-V characteristics of this diode at 300, 243 and 210 K are shown by symbols in Fig. 3.23. As expected the conventional power law equa-... 

FIG. 3.35. Experimental J—V characteristics of an ITO/PEDOT PSS/PCBM/Au injection limited electron current (triangles) and calculated space charge limited hole current in OC4C10-PPV (circles) for a thickness of L = 170 nm and temperature T = 290 K. The inserted figure represents die device band diagram under the flat band condition of a bulk heterojunction solar cell using Au as a top electrode [65]. 

Carter and coworkers studied how side-chain branching in PFs affects device performance with and without an additional HTL of cross-linkable polymer 2 [ 19]. They found that the device efficiency is affected more by the position of the exciton recombination zone than by variations of polymer morphology induced by side-chain branching, which mainly controls the relative emission between vibrational energy levels and has a minimal effect on polymer charge transport properties. For double-layer devices (ITO/PEDOT PSS/2/3,4, or 5/Ca), a typical brightness of 100 cdm 2 at 0.8 MV cm-1, maximum luminance of 10 000 cd m-2 at 1.5 MV cm x, and device efficiencies between 1.3 and 1.8 cd A 1 for 3 and 5 branching can be achieved. 

Lee and Hwang synthesized a PF with aryl side chains 6 via the Yamamoto coupling reaction [20]. The device fabricated therewith (ITO/PEDOT PSS/6/ Ca/Al) emits blue light with suppressed long tail emission but gives low performance with maximum efficiency of 0.03 cd A-1 and maximum luminance of 820 cd nr2. Note that the device could bear considerably high current density (>1.5 A cm-2). 

Meerholz and coworkers incorporated the HTM (triphenyldiamine derivative) into the backbone of SPF to elevate the device efficiency through promoting hole injection/transport properties [24]. The cross-linkable oxetane-functionalized SPF derivatives 10-12 were also synthesized to realize full color display via spin-coating processes. The resulting EL devices (ITO/PEDOT PSS/10, 11, or 12/Ca/Ag) showed maximum efficiencies of 2.9, 7.0, and 1.0 cd A-1 for blue, green, and red emissions, respectively. 

Here, TPA acts as soluble group and prevents the formation of aggregates and, in addition, can lower the hole-injection barrier between PF and ITO anode due to the closer Ip of TPA (5.3 eV) to the work function of the ITO anode (5 eV). However, for the device configuration (ITO/PEDOT PSS/14/Ba), this TPA-containing PF, 14, did not improve device performance as compared to PFO or 3. This was attributed to the lower PLQE of 14 film (22%) than that of PFO film (50%) and the hole-trap property of TPA. In addition, its EL spectrum exhibited a pale blue emission (main peak at 428 nm) with CIE coordinates of (0.184, 0.159). 

On the other hand, Scherf and coworkers used N,N-bis(4-methylphenyl)-N-phenylamine as the end-capper to yield the polymer 16 (end-capper concentration 3 mol%) its single-layer device (ITO/PEDOT PSS/16/Ca/Al) exhibited maximum luminance of 1600cdm 2 and efficiency of 1.1 cdA-1, with CIE coordinates (0.15, 0.08) at 8.5 V [29]. This efficiency is higher than that of non-end-capped polymer 3 (that is, bromine exists at each chain end) by one order of magnitude and was attributed to an efficient hole trapping at the... 

For PFs with HTM grafting as side chain, the alternating copolymer 18 with electron-deficient moiety (4-ferf-butylphcnyl-l,3,4-oxadiazole) functionalized fluorene and monomer of PFO was synthesized by Shu and coworkers [31]. The device with the configuration ITO/PEDOT PSS/18/Ca/Ag showed improved performance turn-on voltage of 5.3 V (defined as voltage needed for brightness of 1 cdm-2), maximum brightness 2770 cdm-2 at 10.8 V, and maximum external quantum efficiency 0.52% at 537 cdm-2 rela-... 

Another electron-deficient moiety, quinoline, was introduced into PF 19 by Su et al. [32]. However, the EL device performance (with the structure ITO/PEDOT PSS/19/l,3,5-tris-(M-phenylbenzimidazol-2-yl)benzene (TPBI)/ Mg Ag) remains low, with the maximum external quantum efficiency of 0.8%, maximum luminance of 1121 cd m-2 and high turn-on voltage of 7.2 V. Therefore, the introduction of electron-deficient moieties (either 4-ferf-butylphenyl-1,3,4-oxadiazol e or quinoline) cannot provide efficient EL performance, probably due to the large hole-injection barrier. 

Another example of bipolar PF, 23, was also reported by Shu and coworkers [37]. They claimed that the bulky hole-transport Cz-derivative provides more effective hole injection than 18. The blue EL device, ITO/PEDOT PSS/23/TPBI/Mg Ag, exhibits a maximum external quantum efficiency of 1.0%, maximum luminance of 1070 cdm 2, and turn-on voltage of 4.5 V. 

As mentioned in Sect. 2.2.1.3 [33], we proposed that a trace amount of /3-phase, induced by the use of an electron-deficient moiety (TAZ) as an end-capper for PFO, can improve device performance to give a better blue purity. Following the idea of /3-phase formation, we further proposed a novel simple physical method to generate /3-phase at a content of up to 1.32% in a PFO film spin-coated on a substrate (the remaining part is amorphous phase) by immersing it in a mixed solvent/non-solvent (tetrahydrofuran/methanol) for a few seconds [45]. The device based on PFO with 1.32% / -phase (ITO/PEDOT PSS/emitting polymer/CsF/Al) has a dramatically enhanced device efficiency and an improved blue-color purity of 3.85 cd A-1 (external quantum efficiency, 3.33%) and CIE of x+y = 0.283 (less than the limit of... 

After that, Cao and coworkers synthesized a series of copolymers derived from DOF and narrow-band-gap 4,7-di-2-thienyl-2,l,3-benzothiadiazole (DBT) with various feed ratios of DOF/DBT [58]. The highest quantum efficiency and brightness were only 1.4% and 259 cdm2, respectively, from the device based on the copolymer with 15% DBT content (ITO/PEDOT PSS/27/Ba/Al). As DBT content increases from 1 to 35%, the EL emission peak shifts from 628 nm to 674 nm, indicating that the copolymers are promising candidates as pure-red emitters though the performance needs to be further improved. 

Another PF copolymer 28, composed of alternating DOF and thieno-[3,2-b]thiophene, was reported by Iim et al. [59]. The incorporation of electron-rich thieno- [ 3,2- b ] thiophene moiety in the polymer backbone led to lower HOMO (5.38 eV) and higher LUMO (2.4 eV) levels than those of PFO (HOMO 5.8 eV, LUMO 2.12 eV [12]). The device ITO/PEDOT PSS/28/IiF/Al exhibited a pure green emission with a peak at 515 nm and CIE coordinates of (0.29, 0.63), which is very close to the standard green required by the National Television System Committee (NTSC) (0.26, 0.65). In addition, the maximum brightness and efficiency of this device were 970 cdm 2 and 0.32 cd A-1, respectively. 

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