Polymer Light-Emitting Diodes

CHAPTE R 9

Polymer Light-Emitting Diodes Dhanashree Moghe, Dinesh Kabra Department of Physics, Indian Institute of Technology Bombay, Mumbai, India

9.1 Introduction Since the first breakthrough by Tang and Slyke (1987), organic light-emitting diodes (OLEDs) have garnered global attention in academics and industry for wearable, healthcare, display, and solid-state lighting (Tang and Slyke, 1987; Vaynzof et al., 2012; Fyfe, 2009). Of the aforementioned applications, the latter two have been the major focus of the research. At present, almost 20% of the energy created is consumed by illumination, with the consumption expected to increase to 60% by 2030 (Johnson, 2007). Various sources of light, such as the incandescent bulb and compact fluorescent lamp (CFL), are available in the market today. While the latter cost four times more, they consume 20%–25% less than incandescent bulbs and last 10 times longer. Because CFLs contain about 5 mg of mercury, search for an alternative “green” technology is underway, paving the way for light-emitting diodes (LEDs) for commercial application. LEDs last 5–10 times longer than CFLs. Because LEDs are roughly 7–9 mm, several LEDs are required to generate sufficient power. The most common LEDs are generally gallium-based, monochromatic, and employed for traffic, automobile indicators, and exit signs (Thejokalyani and Dhoble, 2014). Certain organic semiconductors (OSCs) are another viable option for LEDs, as they offer band-gap tunability and large- or small-area fabricating techniques. OSCs are π-conjugated molecules that mainly consist of carbon and hydrogen, and may also include oxygen, nitrogen, sulfur, and other elements. Due to the low dielectric constant of organic materials (about 2–3.5), the photoexcited carriers are not free electrons and holes, but rather a bound electron-hole pair referred to as an exciton with binding energy 0.5–1.0 eV. While inorganic semiconductors exhibit band gap in the infrared (IR) region, most conducting polymers exhibit band gap in the ultraviolet– visible (UV–Vis) range (2–3 eV). Broadly, they can be classified as molecular crystals and amorphous films. Examples of the first include anthracene (or naphthalene), where each anthracene molecule forms a basis for crystal held together by Van der Waals interactions. Amorphous films consist of either small molecules or polymers. Polymers, which are chains of molecular-coupled repeat units, can be dissolved in most organic solvents and can be deposited by spin-coating, doctor blade, and inkjet printing, among other methods. Advanced Nanomaterials for Solar Cells and Light-Emitting Diodes. https://doi.org/10.1016/B978-0-12-813647-8.00009-6 © 2019 Elsevier Inc. All rights reserved.

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344  Chapter 9 The initial OLEDs were fabricated using small molecules (SM-OLEDs), where the emissive layer was deposited using a dry process such as vapor sublimation in vacuum through a shadow mask. The use of high-temperature vacuum deposition may render the thermal evaporation process difficult for roll-to-roll processing, thus limiting the deposition to smallarea displays only. In 1990, Burroughes et al. demonstrated the first polymer light-emitting diode (PLED) with external quantum efficiency (EQE) of about 0.05%, wherein the emissive layer consisted of wet or solution-processed polymer semiconductors (Burroughes et al., 1990). A year later, Braun et al. improved the solubility of the same polymer by chemical modification that raised the EQE to 1% (Braun and Heeger, 1991). The polymer films, processed at low temperature and pressure, were homogenous as well as robust in nature, which spurred the academic community to demonstrate processing techniques such as inkjet printing and screen printing (Zheng et al., 2013). By the end of the decade, the lowprocessing cost and possibility of large-area displays endowed PLEDs with the potential of being a disruptive technology that is attractive for industrial applications in display and lighting (Minshall et al., 2007; Holmes, 2003). Owing to dedicated research over more than two and a half decades, the highest luminous efficiency and EQE of PLEDs has now reached 61.6 cd A−1 and 17.8%, respectively, with a lifetime of tens of thousands of hours (Kabra et al., 2010; Sekine et al., 2014; Lee et al., 2014b). Being solution processable, there have been continuous efforts in both the academic and industrial communities to make multilayer PLEDs using electric polymerization and/or cross-linking approaches to deposit the layer underneath, or by using orthogonal solvent to deposit film. Such approaches have allowed successful demonstrations of white PLEDs with controlled color temperature. Along with academic research, parallel industrial efforts are being carried out at Cambridge Display Technology (CDT) Pvt. Ltd. in the United Kingdom; Sumitomo Chemical Groups, Tokyo; the Holst Center in Eindhoven, the Netherlands; Mitsubishi Chemical and Pioneer in Japan, and a few other institutions to develop highly efficient OLEDs or PLEDs. Typically, the luminance values of about 400 cd m−2, turn-on voltage of about 5 V, operating lifetime >5000 h, and shelf life of >5 years have been demonstrated by CDT with their OLEDs. Current solutionprocessed white OLEDs without light outcoupling have demonstrated luminance of 1000 cd m−2 and low turn-on voltage of 3.9 V, and a T70 (retention of 70% of the maximum luminance) lifetime exceeding 10,000 h.

9.1.1  PLED Structures A typical device architecture can be a hybrid structure consisting of polymer and nonorganic layers sandwiched between two metal electrodes or pure organic structure where all the emissive and transporting layer are organic materials. Furthermore, the device architecture can be divided into four categories: bottom-emitting conventional, bottom-emitting inverted, top-emitting conventional, and top-emitting inverted, with the first two structures shown in Fig. 9.1A (Chen et al., 2010). The terms bottom- and top-emitting refers to the layer through

Polymer Light-Emitting Diodes  345

Fig. 9.1 (A) The device architecture for bottom-emitting conventional and inverted for fluorescence and phosphorescent (host:guest) emitters. Chemical structure of (B) light-emitting polymers, (C) phosphorescent emitters, (D) hole-transport polymers, conjugate electrolyte, and (E) host polymer systems.

which the forward emission is collected. Conventional and inverted refer to the polarity of the charges, positive (hole) or negative (electrons), injected through the bottom electrode. In this discussion, we will focus only on bottom-emitting structures. For the first category (Fig. 9.1A), the bottom electrode is a transparent electrode deposited on a flexible [polyethylene terephthalate (PET)] or nonflexible (glass) substrate. In the laboratory setting, indium tin oxide (ITO) is the most popular choice for electrodes on a nonflexible substrate. ITO is an indium oxide, In2O3:Sn, doped with tin, and it is crystalline

346  Chapter 9 in nature. Stoichiometrically, In2O3 would have a 5-s conduction band of In with an edge (EC) of about 3.5 eV above the valence edge (EV) (Fan and Goodenough, 1977). Optimum doping of In2O3 (indium oxide) with Sn (donor) results in the Fermi level, which is expected to be very close to the conduction band. It is robust in nature, has low sheet resistance (a few tens Ω/sq), and high work function that is close to the highest occupied molecular orbital (HOMO) of most of the organic molecules (Kim et al., 1999;Wu et al., 1997). The next set of layers consist of hole-injection layers (HILs) or hole-transport layers (HTLs), emissive layers, and electron-injection layers (EIL) or electron-transport layers (ETL), followed by cathode metal-electrode. Often, high-conducting water-soluble poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS) is spin-coated as HIL, which smoothens the rough ITO surface and enhances good, uniform film forming capability of the polymer film. Although beneficial, the acidic nature of the PEDOT:PSS is detrimental to the lifetime of the device. Transparent conducting oxides such as NiO2, MoO3, WO3, or V2O5 were found to be a good replacement for PEDOT:PSS. Prominent metal cathodes such as Mg/Ag, Ca/ Al (−2.9 eV) Al have work function close to lowest unoccupied molecular orbital (LUMO) of most of the emissive layer. Often, a thin layer of wide band-gap material or an excitonblocking layer is deposited between the EML and the cathode. One such layer is LiF (band gap about 12 eV), which when deposited as thin layer (<1 nm) allows tunnelling of charge carriers and improves the charge injection. For ETL/EIL, compact transparent oxide layer as ETL/EIL such as zinc-oxide (c-ZnO), titanium oxide (c-TiO2), ZrO2 were proposed. These layers require high-annealing temperature after spin-coating to improve the conductivity and crystallinity of the layer, making them unsuitable to deposit onto a thin organic layer. Bottom-emitting inverted structures (right image in Fig. 9.1A) offer the solution, as the ETL/ EIL can be deposited on ITO and constitute the transparent bottom electrode, followed by the emissive layers. HILs can then be thermally deposited (MoO3) or solution processed (NiO). High-work-function gold (−5.4 eV) is the preferred top electrode, as it provides a low barrier for hole injection. The large work functions of MoO3 and NiO are also known to pin gold (Au) to the HOMO of the emissive layer lowering the hole-injection barrier (Kabra et al., 2010). Given the instability of the polymer, oxide layers, and oxidation of the electrode due to the ambient atmosphere, an impermeable encapsulation layer is required for any of these structures. The chemical structures of most well-known solution-processed light-emitting polymers, phosphorescent emitters and their hosts, and the HTLs for LEDs are shown in Fig. 9.1B–E. Based on the emitting material, PLEDs can be broadly classified as fluorescentor phosphorescent-based, depending on whether the emission originates from the singlet (S1→S0) or triplet (T1→S0) state.

9.2  PLEDs Using Fluorescent Polymers Emissive layers in a PLED can either be single, bilayer, or multiple-emitting layered (Jankus et al., 2014; Hung et al., 2012). The most explored single-emissive-layer PLEDs

Polymer Light-Emitting Diodes  347 are poly(p-phenylene) (PPP)-, polyfluorene (PF)-, and poly (p-phenylenevinylene) (PPV)based polymers. The emission takes place when the injected electron and holes recombine within a layer or at the interface with one of the injection layers. In general, the hole mobility of these polymers is higher than the electron mobility; hence, recombination takes place near the EIL/emissive layer interface (Kabra et al., 2010). Bilayer PLEDs, also referred as exciplex-based PLEDs, consist of a separate donor and an acceptor polymer layer. Herein, the recombination and emission occur at the donor-acceptor (DA) interface. Of the three polymer families, PFs are most popular due to their inherent stability against moisture due to their deep HOMO level (about 5.9 eV) (Nicolai et al., 2012). PFs are rigid-rod-shaped polymers consisting of rigid planar biphenyl units that are bridged by a carbon atom in position 9 (C-9). The bridge ensures a high degree of conjugation, and the C-9 atom provides an option for additional substituents to modify the polymer processibility and the interchain interactions in films without significantly altering the electronic structure of the individual chains. Suitable substituents enable solubility in common organic solvents, as well as the processing of high-quality thin films by spin-casting. The thermal stabilities of the homopolymers and copolymers are excellent with decomposition temperatures exceeding 400°C. PFs can be tailored to emit in green and red colors as well by adding additional acceptor units, which generally are thiophenes and benzothidazoles (Kabra and Narayan, 2007; Kabra et al., 2008; Lu et al., 2013).

9.2.1  Single Emissive Layer 9.2.1.1  PPV-based polymers and ladder-type (LPPP) polymers PPV-based polymers show PL and EL in the green-yellow region (Braun and Heeger, 1991; White et al., 2013). Because both types of luminescence are generated by exciton recombination, the identical spectra suggest that the same excited states are responsible for the emission, and minimal defects are introduced in PPV upon application of voltage (Brown et al., 1992). Previously, it was proposed that the presence of conjugation in PPV may extend the electron cloud, generating more pathways for excitons to reach the quenching centers. By synthesizing a conjugated/nonconjugated type system, such as a chain with different π-π* energy band-gap regions, the quenching sites were lowered, thereby increasing the device efficiency from 0.01% to 0.3% (Burn et al., 1992). Using a soluble dialkoxy derivative of PPV (MEH-PPV) with a band gap of about 2.1 eV (red), it was shown that the electron and hole barrier at the polymer-metal interface plays a critical role in the charge injection. For devices with an ITO/MEH-PPV/Ca architecture, the J-V characteristics showed that the charges from metal contact were injected via tunneling (Braun and Heeger, 1991). EL takes place when an injected electron, localized as a negative polaron in the upper state of the emissive molecular chain, recombines radiatively with a positive polaron (the lower state) to form a radiative neutral bipolaron. Alternatively, electrons can recombine in lower gap states that are filled with diffused positive polarons to form nonradiative neutral bipolarons.

348  Chapter 9 Thus, EL efficiency is limited by the competition between the radiative and nonradiative processes, as well as the barrier offsets at the interface. To identify the spatial origin of the luminance spectrum, the PL spectrum was measured from a highly aligned MEH-PPV chain in polyethylene. PL was observed to be highly polarized along the chain, suggesting that luminance centers are the intrachain excitons. Because the EL and PL spectra were identical, EL was also expected to originate from intrachain excitons. The barriers could be lowered by introduction of an HTL or ETL (White et al., 2013;Greenham et al., 1993; Gao et al., 1996). Devices with transport layers reported a high turn-on voltage (VON) between 15 and 20 V. High luminance intensity (500 cd m−2) was also reported at high drive voltages, with a maximum EQE of about 1%–2%. Thickness-dependent studies by Parker on single-emissive-layer MEH-PPV devices showed that the charge injection through anodes and cathodes can be controlled by changing the hole and electron barrier at the respective interfaces (Parker, 1994). Because of dependence of the current on the barrier height, small changes in height can lead to exponential changes in the current density. Further, the current density was found to be dependent on the electric field rather than on voltage. Changing the thickness of the emissive layer changed the operating voltage of the device, but not the VON. Here, the operating voltage is the voltage at which the current density starts rising drastically and the emission is visible to the naked eye. On the other hand, VON is the value of the smallest applied voltage at which the tunneling starts, along with emission. Ideally, the turn-on voltage will be equal to the band gap of the emissive layer and independent of the thickness of the emissive layer. For a PLED without any transport layer, the flat band was reached at about 1.8 eV, which is slightly less than the polymer band gap but close to the difference between the work functions of the two electrodes (ITO and Ca). To evaluate the role of MEH-PPV device efficiency as a function of its morphology, MEHPPV:PMMA samples have been analyzed (Iyengar et al., 2003). By varying the concentration of MEHPPV (5%–75%), it was observed that the polymer forms 300–900-nm domains within the insulating PMMA polymer. Beyond 50% MEH-PPV, a bicontinuous network is observed, with MEH-PPV surrounding isolated PMMA domains. Ladder-type polymers are an interesting class of two-dimensional (2D) polymers, with strong steric hindrances prohibiting conformational changes that lead to lower electron delocalization (Scherf, 1999). Such conjugated polymers, first prepared by Scherf and Mullen, had a high molecular mass of about 50,000, which implies about 150 phenyl rings in the main chain of the polymer, resulting in a chain length of 8–10 nm. Their absorption and emission spectra lie within 2.5–2.8 eV. Their high photoluminescence quantum yield (PLQY) in solution (about 90%) and solid state (40%) make conjugated polymers an ideal candidate for blue emitters in PLEDs reaching a high EL quantum efficiency of up to 1% and luminance efficiencies 0.25–0.5 cd A−1 (Jacob et al., 2004).

Polymer Light-Emitting Diodes  349 9.2.1.2  Polyfluorene-based PLEDs Blue-emitting PF One of the most challenging obstacles to realize the commercialization of display or white solid-state lighting is a stable, high-efficiency blue-emitting LED. F8-based, large-band-gap fluorescent materials are the preferred choice over the phosphorescence materials because of their simplicity in synthesis and high PLQY, as well as the fact that they offer device stability. The most prominent blue PFs [namely, F8 (poly(9,9-dihexylfluorene)), aryl-F8, and spirobifluorene] are shown in Fig. 9.1B. The initial PFs developed showed a broad absorption band ranging from 300–450 nm, arising from the distribution of the conjugation lengths, with a peak at about 390 nm in films. Strong PL with vibronics was observed in the range of 400– 500 nm. In the solution and film, the PLQY was 50%, with the sharp vibronics observed in solution assigned to the intrachain singlet transitions (Neher, 2001). Single-layer PF devices composed of ITO/poly(9,9-dihexylfluorene-2,7-diyl)(DHF)/Mg:In showed an EL maximum at 470 nm and a high VON of about 10 V (Ohmori et al., 1991). No efficiency or brightness was calculated, but high hole- and electron-injection barriers were expected at the semiconductormetal interface. A slightly modified PF, dioctyl-substituted PF, developed by Dow Chemical Corporation, was incorporated by Grice et al. in ITO/polyTPD/PFO/Mg:In (Grice et al., 1998). Introduction of an HTL [polymeric triphenyldiamine derivative (poly-TPD)] lowered the hole-injection barrier, leading to a VON of 5 V. The luminance and luminous efficiency were 0.25 cd A−1 and 0.04 lm W−1, respectively. Although the PLQY in the solid state was 55%, the maximum EQE was limited to 0.2% at 17.5 V. Thickness-dependent luminance efficiency measurements on the devices suggested the role of dipole-quenching effects that present an oscillating function of the distance of the emission zone from the cathode. Traditionally, a blue photoluminescence emitter device operation of F8 PLEDs introduces ketone defects and excimer formation, leading to a red shift of EL and enhancement in the green emissions. Further, a deep HOMO level and electron trap states introduced during synthesis limit the efficiency of the blue-emitting devices. Chemically, the C-9 position in PFs is prone to oxidation. Arylation of fluorenes at the C-9 position (Aryl-F8) suppresses reactivity at this position and improves the solubility of the material in organic solvents, all without changing the interchain and electronic properties (Abbel et al., 2009). Most important, by using a Spiro unit at the C-9 position, the oxidation and ketone defects can be suppressed to maintain the color quality of the blue LED (Kreuder et al., 1997). However, the deep HOMO level of Spiro-F8 created a high hole-injection barrier, limiting the luminous efficiency to single digits. More recently, the luminous efficiency about 6 cd A−1 (unoptimized outcoupling) has been demonstrated by doping Aryl-F8 [Mn=380 k and a polydispersity index (PDI) = 2.7] with a hole transporter, poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB), in a bottom-emitting, inverted device architecture

350  Chapter 9 (ITO/c-ZnO/Cs2CO3/Aryl-F81−x:TFBx/MoO3/Au) (Lu et al., 2012). Aryl units are attached at the C-9 position of the fluorene, which prevents the stacking of polymer chains and oxidation. Addition of a hole-transport polymer, TFB (poly(9,9-dioctylfluoreneco-N-(4-butylphenyl)diphenylamine), as shown in Fig. 9.1C, did not affect the optical properties of Aryl-F8 and the HOMO level (5.9 eV). No phase separation was observed in the blends, indicating that the changes in the device were not of a morphological nature. By the addition of different TFB concentrations (0.1%–1%) to Aryl-F8, the unwanted green emission could be suppressed. It was proposed that the higher HOMO of TFB (−5.3 eV) trapped holes. Being an HTL, it also lowered the electron mobility pathways and provided efficient recombination centers with holes. Overall, a good charge-carrier balance seems to have been achieved by the dopant, resulting in an increase in the current efficiency from 1 to 5.9 cd A−1. Green-emitting PFs

PF-based copolymers developed by Cavendish Display Technology (CDT) were of the DA type and emitted in blue, green, and red regions (Millard, 2000). Out of the three polymers, poly(9,9′-diotyfluorene-co-benzothiadiazole) (F8BT), as shown in Fig. 9.1, is the best choice for efficient green PLEDs due to its high PLQY in film (about 78%), high stability, and good transport properties (Kabra et al., 2008). The absorption onset of F8BT is at 550 nm, with peaks at 325 and 475 nm. The PL extends from 500 to 750 nm and peaks at 575 nm (Hou et al., 2003). When F8BT is used as guest in PF host material, ITO/PEDOT:PSS/PFO:F8BT/Ca devices showed VON of 3.5 V, reaching a luminance of 2600 cd m−2 at 0.8 MV cm−1 (9 V) and luminance efficiency of about 2.1 cd A−1 (Morgado et al., 2002). Inclusion of a PPV [poly(pphenylenevinylene, shown in Fig. 9.1B] as an HTL and a electron blocking layer (EBL) between PEDOT:PSS and F8BT lowered the VON (about 3 V) and the threshold current. At the same average field (0.8 MV cm−1 at 12.5 V), the current density was reduced, which increased the luminance and luminance efficiency to 3500 cd m−2 and 4.1 cd A−1, respectively. Here, PFO/Ca possessed a low barrier to electron injection, but PEDOT:PSS/PFO still possesses a large hole-injection barrier of about 0.7 eV at room temperature, acting as an excitonquenching site and thereby limiting PL and EL efficiency. Kima et al. introduced a thin layer of large-band-gap (>3.0 eV), triarylamine-based PF copolymer TFB interlayer, in order to prevent exciton quenching and reduce the decay channel (Kim et al., 2005; Kim et al., 2004). Compared to devices without TFB, devices with TFB have lower leakage current and VON. The EQE and luminance efficiency for devices with TFB increases seven and six times, respectively, as opposed to without TFB. EL spectra for both devices were the same, indicating emissions from F8BT only and negligible alternation in the average location of the emission zone. In spite of significant improvement in the EQE, the low-work-function metal cathodes (calcium and magnesium) are highly reactive and often coated with aluminum.

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As previously mentioned, bottom-emitting structures encompass robust metal oxides [compact (c-) or mesoporous(m-) ZnO, TiO2] that are not susceptible to oxidation and overcome the issues related to ETLs, HTLs, and cathode instability (Kabra et al., 2008). Consequently, a luminance efficiency of 3 cd A−1 and low VON (0.87 V) both could be achieved in the inverted structures. Two factors were dominant: first, the morphology of the compact layer combined with film-forming capabilities on c-ZnO and c-TiO2; and second, the barrier at the oxide/polymer interface. The compact metal oxide layers appear to be polycrystalline, with almost similar domain size (about 40 nm) and roughness (about 5 nm). Tappingmode atomic force microscopy (AFM) images of F8BT films spin-coated onto c-TiO2 and c-ZnO layers of quartz substrates show fewer pinholes for the more polar ZnO surface (Fig. 9.2), suggesting that the film-forming capability of F8BT on each surface is different. Consequently, uniform film on c-ZnO reduced the shunting paths and reduced the leakage current in devices. Smoother film also reduced the nonradiative recombination of excitons,

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Fig. 9.2 Current density versus voltage (J-V) and luminance versus voltage (L-V) characteristics of (A) c-TiO2 (B) c-ZnO-based F8BT/TFB bilayer devices. MoO3 was used as the hole-injecting layer for all devices, and the devices were operated in air, without any encapsulation. The tapping-mode AFM images of F8BT films spin-coated onto (C) c-TiO2 and (D) c-ZnO layers on quartz substrate. Inset shows the bare compact metal oxide layers on height scales of 0–10 nm (Kabra et al., 2008).

352  Chapter 9 resulting in better PL. Generally, ZnO is polar due to the presence of charge transfer (CT). Hence, the single-crystal ZnO surface is metallic in nature. It is postulated that the presence of Zn ions at the metallic surface may lead to a favorable shift in surface potential and improve the electron injection, lowering the VON. Introduction of a thin, cesium (Cs)- or barium (Ba)-based oxide interlayer further prevents quenching at the n-type oxide/polymer interface and acts as a hole-blocking layer (HBL) (Kabra et al., 2008; Lu et al., 2012). Devices with ITO/c-ZnO/Cs2CO3/F8BT/TFB/MoO3/ Au structure were fabricated that showed peak luminance efficiency of about 22.7 cd A−1 and EQE 7.3%.The role of c-ZnO/Cs2CO3 and MoO3 was clear from the electron and hole-only devices. The hole-only devices were made using PEDOT:PSS/F8BT/MoO3/Au architecture, and the thickness of F8BT ranges from 190 to 1130 nm. The J-V characteristics showed hole current injected from the MoO3 side so as to be five orders of magnitude more than that from the PEDOT:PSS side. There are two reasons for MoO3 being a good hole contact: First, MoO3 acts purely as a p-type semiconductor, and the work function correctly matches F8BT to enable barrierless hole injection. Second, the thin MoO3 layer (<10 nm) pulls electrons from the HOMO level of the F8BT, resulting in p-type doping at this interface, which in turn creates an ohmic contact between the F8BT and either the MoO3 or the Au. Electron-only devices fabricated as ITO/c-ZnO/Cs2CO3/F8BT/Ca/Al show similar electron-current densities for electrons injected from either electrodes, but the current is four orders of magnitude less than that achieved through MoO3 in the hole-only devices. Because electron-only devices without Cs2CO3 show further depreciation in the current, it is possible that annealing causes F8BT to be doped with Cs2CO3 (Fung et al., 2002, 2003). Overall, both the electron and hole densities are considered bulk limited rather than injection limited, with the mobilities being field dependent. For bipolar devices with Cs2CO3, the current is comparable to the current of hole-only devices, indicating that the current in bipolar devices is not limited by electron transport. It is observed that the operating voltage increases with the thickness of the F8BT layer, which is expected, as PLED devices are generally more dependent on electric field than on voltage. Interestingly, the offset between current turn-on and luminance turn-on is small, even for devices with thickness >1 μm, implying that injecting carriers recombine efficiently. The EL in bottom-emitting devices does not change with the thickness of the F8BT layer, indicating a broad recombination zone that remains closer to the cathode. For devices with ITO/c-ZnO/Ba(OH)2/F8BT/TFB/MoO3/Au, the current is lowered more than that for Cs2CO3. Thus, BaOH2 is more efficient at hole-blocking than Cs2CO3. Photovoltaic (PV) measurements show that devices with oxide interlayers have high VOC but low photocurrent, indicating that the interlayer prevents quenching (a hole from within F8BT with the injected electrons) and charge separation at the interface. The device geometry discussed here is advantageous in terms of scalable manufacturing, as thick emissive films provide conformity and reduce the occurrence of black spot, thus significantly increasing the yield.

Polymer Light-Emitting Diodes  353 In single-emitting layers, owing to single phase no nanomorphology, other than roughness, is expected. However, devices fabricated with a blend of emitting polymers show strong phase segregation because of the low entropy of mixing of various polymers. Blending of high-electron and high-hole transport material is often desirable, as it can bring about a charge-carrier balance. These blends can be further placed in two categories: one where both components are present in significant amounts, and the other, where one component is present in a small quantity as a dopant with the other component. In both cases, exciplex can be formed at the interface, but the final emission depends on the ratio of components and the CT or energy transfer dynamics of components within the blend. F8:F8BT (95:5) has been previously explored for PLEDs.At low F8BT concentrations, excitation of F8 moieties leads to efficient Forster energy transfer from F8 to F8BT. It is found that the copolymer is embedded in F8 rather than forming walls around it (Morgado et al., 2002). For 1:1 blend film excited at peak absorption of F8, scanning near-field microscopy images revealed incomplete energy transfer and the presence of each polymer within the other phase. Interestingly, the fluorescence of F8BT within the F8 phase was higher than that of the pure F8BT phase. Out of the excitons generated in F8-rich domains, 90% of excitons were transferred to F8BT within the same domain. A cross-sectional profile of blend films using deuterium nuclear reaction analysis shows that F8 films form a thin (about 10 nm) layer over both F8 and F8BT domains, such that F8 may have a crystalline fibril structure (about 10 nm width) or not, depending on the layer underneath. Furthermore, F8 emissions were stronger at the F8-F8BT interface as opposed to those from the center of an F8-rich domain. In summary, the blend consisted of F8 domains ranging from the micrometer- to nanometer-scale range, whose presence could explain the lapse in complete energy transfer from F8 to F8BT (Chappell et al., 2003). Yellow- and red-emitting PFs

Fig. 9.1B shows the chemical structures of the most well-studied red-emitting polymer MEH-PPV (poly(2-methoxy, poly(2-methoxy, 5-(2′-ethyl-hexoxy)-I,5-(2′-ethyl-hexoxy)-1, 4-phenylene4-phenylene-vinylene)), F8TBT (poly((9,9-dioctylfluorene)-2.7-diyl-alt[4,7-bis(3-hexylthien-f-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl))), and the yellow-emitting Super-yellow (Bolink et al., 2009). Compared with highly efficient green light-emitting PFs such as F8BT, derived from fluorene and benzothiadiazole, the LED device efficiency with red emitters is typically low, and acquiring high-efficiency red emissions remains a great challenge. CDT reported a red emitter with a high EQE of 3%, but the chemical structure was not disclosed. Electroluminescent devices of F8-DBT reached a maximum luminance of about 259 cd m−2 and EQE of about 1.4% when the F8:DBT unit concentration was 100:15 (Hou et al., 2002). The device efficiencies were further improved by the alkylation of side chains, which also enhanced solubility and modified the packing structure within the film. The alkyl substitution on the thiophene rings in F8TBT greatly reduced the interchain interaction by preventing close-packing and thereby reducing concentration quenching of F8TBT chains in comparison with its parent analog.

354  Chapter 9 The position of the alkyl chain also plays a critical role, as evidenced by studies on conventional F8TBT (with the alkyl side chain facing outward, called F8TBT-out), and F8TBT-in (with the alkyl side chain in different positions). The varied F8TBT-in (Fig. 9.1B), with two alkyl side chains inward, is likely to introduce more steric hindrance and further prevent the close-packing of the interchains of the polymer, as compared with F8TBT-out. The F8TBT-in copolymer is found to have enhanced photoluminescence quantum efficiency (PLQE) in the solid state to about 42%, as compared with F8TBT-out (11%). Consequently, F8TBT-in showed a luminance efficiency of 0.084 cd A−1, and luminance of about 960 cd m−2, which were five times higher than F8TBT-out (Lu et al., 2013). On the other hand, the Superyellow polymer belongs to the phenylenevinyelene family and surprisingly shows extremely high device efficiency (<10 cd A−1) compared to other PPV family-based polymers (Bolink et al., 2009; Lee et al., 2012) This polymer in general suffers from trap-limited charge transport for electrons.

9.2.2  Bilayer or Exciplex Devices Single active-layer PLEDs can suffer from charge-carrier imbalance, leading to lower recombination in the material. Emitting moieties can be capped with electron- or holetransporting materials for efficient transport (Hung et al., 2005). Alternatively, bilayers comprised of individual carriers or blends also could result in better carrier balance, and the emission is said to originate from the intermolecular CT state or exciplex. When two different moieties come in contact with each other, electron delocalization can take place over the whole system. When one molecule of the system is in an excited state while the other is in a ground state, the system is said to form an exciplex. Radiative relaxation of the exciplex leads to emissions that are red-shifted than the PL of the individual components. Although the oscillator strength of the emission dipole is less than that of fluorescence, color tuning and better charge balance are the advantages of exciplex-based PLEDs. PF-blended systems such as F8:PFB, F8:TFB, F8BT:PFB, and F8BT:TFB show interesting PV or LED properties, along with phaseseparated morphology (Arias et al., 2001; Morteani et al., 2003). The phase can be varied from micron to nanometer size, depending on the solvent and the spin-coated films’ processing time. From the device perspective, TFB as an interlayer or as a dopant works best to give good PLED efficiency (Kim et al., 2004, 2005; Lu et al., 2012). Bilayer systems based on siloles have shown exciplex PLQYs reaching 62%, VON about 4.5 V, and EL efficiency of 3.4% at 100 A m−2 (Palilis et al., 2003). Blue EL from devices fabricated from blends of fluorene (PFO) with poly(vinyl diphenylquinoline) (PVQ) shows enhanced efficiency over pristine fluorene. Further, the unwanted green emission is also suppressed in the EL spectrum of the blended samples. Evaluation of the morphology of the blends using tapping-mode AFM shows the presence of phase separation on the length scale of 100–500 nm, depending on the PFO to PVK ratio (Kulkarni and Jenekhe, 2003).

Polymer Light-Emitting Diodes  355

9.2.3  White Polymer Light-Emitting Diodes White emission can be obtained in two ways; by blending two or more polymer systems or by single polymer emission (Raja et al., 2008). Three-polymer blends containing red, green, and blue (RGB) light-emitting polymers are often utilized to generate white-light emissions. Twopolymer blends, such as those containing blue and orange light-emitting polymers or blends of two polymers and an organometallic complex as the phosphorescent emitter or blends consisting of two or three kinds of fluorescent dyes in an efficient blue light-emitting polymer matrix, are also utilized. The second approach, single polymer emission, involves careful synthesis of the polymers. But the devices with the single polymer are found to be more stable. In recent years, devices have been created with efficiencies reaching 50 lm W−1, which is now more than that of bulbs (15 lm W−1) and comparable to fluorescent bulbs in fixtures (30–40 lm W−1) (Kamtekar et al., 2010). White light is characterized by three factors: (1) the Commission Internationale d’Eclairage (CIE) coordinates (0.33, 0.33); (2) the color temperature (CT); and (3) the color rendering index (CRI), which measures how true the color of an object appears under illumination, compared to a reference source of illumination—that is, a black body source (CRI=100). As discussed in previous examples of PLEDs, the efficiency of OLEDs is reported as their EQE and candelas per amp (cd A−1). However, lighting sources are usually quoted in terms of power efficiency (or efficacy), in order to take into account the response of the human eye, which is sensitive to light only in the visible spectrum (400–700 nm) and with a maximum sensitivity to green light (555 nm). For an ideal, monochromatic green light source, the maximum efficacy is 683 lm W−1. For white light with CRIs of 90 and 100, the maximum value is 408 and 240 lm W−1, respectively. In anthracene-, PPV-, and polyaniline-based systems, used for white polymer light-emitting diodes (WPLEDs), the blue emission originates from the main moiety, while the green, red originates from the aggregates/excimers. These types of emission are not very stable with respect to voltage (Neher, 2001). The versatile nature of PF is well known with applications in WPLEDs as well. Inherently, PFs are ideal for blue emissions. In addition, the C-9 position of the fluorene unit provides the opportunity to tune the optoelectronic properties of these materials.

9.3  PLEDs Using Phosphorescence Materials Theoretically, white OLED panels could reach up to 248 lm W−1 (Sasabe and Kido, 2013). This efficiency can be reached by (1) increasing the internal efficiency to 100%, (2) lowering the operating voltage, and (3) improving the light-outcoupling efficiency. Phosphorescent emitters, such as fac-tris(2-phenylpyridine)iridium(III) [Ir(ppy)3] and iridium(III)bis[4,6(difluorophenyl)pyridinato-N,C′]-picolinate (FIrpic), have shown phosphorescent-internal QE

356  Chapter 9 of about 100% (Endo et al., 2008). To lower the operating voltage to the limit (about 1.95 V for 100 cd m−2), various ETLs, EILs, and host systems have been developed (Endo et al., 2008). The last factor of low light-output efficiency (about 20%) results from losses in the high refractive index of organic materials, optical confinement, and internal reflection within the device structure. Enhancement in the light-outcoupling was obtained by a low-index grid in the organic layer and lens-based systems, as will be briefly discussed in Section 9.4 of this chapter (Sun and Forrest, 2008)

9.3.1  Operation Mechanism of Phosphorescent PLED Formation of triplets occurs much more under electrical excitation than optical, and triplet quenching is very strong. To avoid quenching and achieve high PLQE, a phosphorescent emitter is dispersed in a suitable host material (Baldo et al., 1998). This host material must have suitable singlet-triplet energy levels such that efficient energy transfer can take place from host to guest. Phosphorescent PLEDs work via CT and charge-trap processes. In the presence of optical or electrical excitation, the holes are generated in HOMO and electrons in LUMO of the host and guest. The transfer of excitation from host to guest can occur via three processes: Förster transfer, Dexter transfer, and direct generation of excitations. Förster transfer is dominant when there is a significant overlap between the emission spectrum of the host and the absorption spectrum of the guest. It is also an induced dipole transfer, and so triplet transfer cannot take place. For Dexter transfer, the singlet and triplet energy of the host must match that of the guest for physical CT to take place. Consequently, the first two transfers are short-range processes that attenuate exponentially with distance. For application in electrophosphorescence, hosts are wide band-gap materials with side chains as charge transport moieties for efficient charge carriers. In a host-guest system, these three processes can take place simultaneously. However, the dominance of one process over the other can be decided by the offsets between the HOMO (LUMO) of the host and guest. If the HOMO-LUMO offset is small, energy transfer dominates the excitation of the guest. If one of the energy offsets (e.g., HOMO) is large, charge trapping is the primary way of populating the triplet state and enhancing phosphorescence. When offsets result in the formation of type II junction, the excitation has CT character that can quench emission.

9.3.2  Phosphorescent Hosts and Emitters Heavy metals such as platinum- and iridium-based ones have been the most common choice of emitters that are combined with a popular choice of host such as PVK or PFO. The presence of the heavy metal helps in the spin-orbit coupling, thereby increasing the intersystem crossing (ISC) from singlet to triplet. In these systems, the absorption spectrum

Polymer Light-Emitting Diodes  357 show bands related to π-π* and metal-to-ligand CT (MLCT). The lowest state can be either triplet π-π* or MLCT. Further, the chemical structure of the ligands can have strong effects on the HOMO-LUMO as well. For example, Ir(ppy)3 has a HOMO at 5.4 eV, and the addition of fluorine atoms deepens the HOMO to 5.8 eV. Phosphorescent emitters have been created with quantum efficiency in the range 10.3%–31% (Kim and Lee, 2014; Udagawa et al., 2014; Lee et al., 2016). and with a lifetime of about 3.6 times longer than that of conventional devices (Fukagawa et al., 2015). Here, the emissive layers were deposited using vacuum thermal evaporation, which cannot be applied to flexible substrate, inkjet printing, or screen printing. Production of wet-processed emitting layers remains a challenge, as solvent depositions often lead to the aggregation of small molecules within the thin film, hindering efficient energy transfer between the host and emitter molecule and subsequently quenching emissions. Solution-processed, green electrophosphorescent devices have been demonstrated by a spincoating blend of PVK, paradichlorobenzene (PBD), and Ir(mppy)3 in chlorobenzene solution with an ITO/PEDOT:PSS/light-emitting polymer layer/CsF/Al architecture (Choulis et al., 2005). With a thin-TFB, hole-transport interlayer (about 3 nm) on PEDOT:PSS (suitable for green-emitting layers) and PBD as ETLs, the device showed a respectable luminous efficacy of 50 lm W−1 at luminous efficiency of 55 cd A−1. The increase in these efficiencies compared to devices with no transport layers was attributed to improved hole injection at the TFB/ emitting layer interface, as the HOMO of TFB is about −5.3 eV, which is well aligned with that of Ir(ppy)3 (−5.4 eV) compared to the work function of PEDOT:PSS (−5.1 eV). Similarly, the LUMO of PBD aligns well with that of Ir(pyy)3 improving electron injection into Ir(pyy)3. All solution-processed phosphorescent PLEDs are tricky, as the solution deposition of ETL or HTL requires an orthogonal set of solutions (i.e., the upper-layer solvent must not dissolve the layer beneath it). Deposition of a thin film of heterocyclic conjugated polymers (quinoline) or PF-based electrolytes deposited from an organic acid solvent, as ETLs, were some of the options to overcome the issues (Earmme et al., 2010). However, ions in electrolyte can dope and create stability issues after depositing metal electrode. Wide band-gap ETL based on quinoline (band-gap ~ 3.4 eV) spin-coated from a formic acid/water solution utilized for blue host:FIrpic, phosphorescent organic light-emitting device (PhOLED) had a luminous efficiency of 28.3 cd A−1 at 2790 cd m−2 with an EQE of 15.5%. Alternatively, a mixture of different small-molecule electron-transport material in H2O:MeOH has been utilized (Ye et al., 2011). Elaborate studies were performed by Kim et al. on the roles of solvent and annealing temperature on the nanomorphology of solution-processed Ir(ppy)3:PVK (dopant:host) films (Kim et al., 2017). Four solvents were chosen—chlorobenzene (CB), tetrahydrofuran (THF), chloroform (CF), and dichloromethane (DCM)—to probe the aggregates at different weight percentages of the dopant in PVK. The surface topography images of the film showed needlelike aggregates for CB, needle and spherical for TFH, and spherical aggregates for CF-

358  Chapter 9 and DCM-processed aggregates of the Ir complex in a PVK matrix. The surface roughness (CB
9.4  Device Physics and Characterization 9.4.1  Physics of Devices The semiconductor polymer sandwiched between two metallic electrodes is considered as a metal-insulator-metal (MIM) diode. Extensive device characterization studies by Parker showed that the electronic structure of PLED can be modeled as a rigid band model if the carrier concentration in the films was low (about 1014 cm−3) (Parker, 1994). According to the model, it is possible to predict the voltage required for tunneling, and hence the light emissions. There is no tunneling in the zero-bias state. As the device forward bias increases, the flat-band condition is reached, at which point the tunneling starts. The height of the hole(electron-) barrier is taken as a difference between the work function of the anode (cathode) and the HOMO or π (LUMO or π*) level. If the barrier is close to 0.1 eV, the thermionic energy at room temperature (kBT) can help the charges overcome the barrier. For larger barriers, the injected charge carriers can tunnel the barrier by field emission tunneling from the anode (cathode) to the HOMO (LUMO) level of the polymer, if the electric field at the interface is significantly high. According to the Fowler-Nordheim tunneling theory,  −κ  I = F 2 exp    F 

(9.1)

where I is the current, F is the electric field strength, and κ is the parameter that depends on the shape of the barrier. For rigid-band models, the injected carriers can be thought of as traversing a triangular barrier at the interface. Then, the barrier shape constant can be given by

κ=

8πϕ 3 / 2 2 m∗ 3qh

where φ is the barrier height and m* is the effective mass of each hole.

(9.2)

Polymer Light-Emitting Diodes  359

9.4.2  Characterization of PLED 9.4.2.1  Electroluminescence and color coordinates Similar to the OLEDs discussed in previous chapters of this book, PLEDs are evaluated on the basis of a number of factors, such as power efficiency (cd A−1), efficacy (lm W−1), EQE (%), color coordinates, and device lifetime. The sensitivity of the human eye is different for different colors and different intensities of light. Thus, evaluation of PLEDs needs to be done in photometric units that is done in a photopic region (luminance is expressed in cd m−2). In this region, the human eye has maximum sensitivity at 555 nm. The various measurements have been described by Forrest et al. (2003). A fact to remember is that, in contrast to bare films, PLEDs are stacked structures that introduce interference because of the refractive index of each layer, and they can affect the outcoupling and external efficiencies. The staking of the structure results in a strong angular dependence of the luminance profile. Usually, PLEDs have at least one side (cathode or anode) that is transparent. Therefore, one ideally expects a Lambertian (i.e., an isotropic emitter) that emits equal numbers of photons (observed by observer) at the same solid angle and in the forward direction. However, a large number of photons get waveguided due to cavity and substrates, and hence are lost. Within photometry, the flux normalized to the surface is measured as illuminance (lm m−2). If the measurement takes into account angular variation, then at the surface, luminance or luminous intensity is measured as lm sr−1 m−2 or candela m−2. Because most of the application lies in the display, measurements in the forward direction (i.e., the direction perpendicular to the surface) are considered significant. For a Lambertian source with radius r, the intensity measured by a small-area detector at a far distance d subtends a small angle (Ω≤0.01 sr) that follows a cosine law. The signal from PLED can be obtained by comparing it with that from a calibrated lamp. A second method to avoid waveguide losses is to use a detector with a sufficiently large area than the PLED and placing it close to the PLED. The third method is to place the PLED in an integrating sphere and collect all the emitted photons using a fiber. However, the fiber response itself is not ideal, and the first two methods are considered suitable for correct intensity measurement. 9.4.2.2  The current-voltage-luminance characteristics A PLED is characterized by the current density (J in mA cm−2) and luminance (L in cd m−2) as a function of applied bias (V). For a F8BT device in the ITO/c-ZnO/Ba(OH)2/F8BT/ MoO3/Au structure (Fig. 9.3A), the J-V-L is shown in Fig. 9.3B (Dey et al., 2017). The semilogarithmic graph shows three distinct regions. Region 1 shows, rather than zero current, a leakage current J ∝ V, and it originates because of defects such as shunting paths or pinholes in the film. For V≫VBI, represented by region 3, electrons and holes are injected from an ohmic-like contact (i.e., there is an unlimited supply of charges through the electrode). Note that here, ohmic does not mean that J ∝ V. In this region, the current is limited only by the

360  Chapter 9

Fig. 9.3 (A) Energy level for ITO/c-ZnO/Ba(OH)2/F8BT/MoO3/Au structure. (B) J-V characteristics of ITO/c-ZnO/Ba(OH)2/F8BT/MoO3/Au showing the three regions (Dey et al., 2017).

buildup of space charge that it creates, which in turn shields the electric field at the electrode/ polymer interface. For a unipolar device, in this region, J-V can be fitted using Child’s law or the Mott-Gurney equation [J=(9∕8)εrε0μ(V−VBI)2∕d3] to extract the mobilities. The value at which the current (and luminance) start increasing beyond VBI is represented by region 2. In this intermediate regime, the gradient of the charge density creates a diffusion current, while the gradient of the electric potential creates a drift current in the opposite direction. When the diffusion current dominates, empirically, the net positive current increases exponentially as the bias given by the Shockley equation [J(V)=J0exp((qV∕ηkT)−1)], where η is the ideality factor that varies from 1 to 2 depending on trap- or free-carrier-assisted recombination (Kuik et al., 2011). Note that the Mott-Gurney equation is applicable to field independent mobility. Taking into account the Pool-Frenkel model, the SCLC is related to applied bias by 2  V − VBI ) ( V − VBI  9 J = ε 0ε r µ exp β   , where β is a parameter that describes the field d  8 d3  dependence effect (Fran and Simmons, 1967). J-V fitting of the pristine unipolar devices gave mobility of 0.5–1.8×10−6 cm2 V−1 s−1, depending on the thickness of the film (Kabra et al., 2010). Similarly, J-V fitting of TFB-doped F8BT unipolar devices gave mobility of 7.46– 1.32×10−6 cm2 V−1 s−1, suggesting that doped TFB lowers mobility by creating traps that helps increase the recombination and better device performance (as discussed earlier in this chapter, in Section 9.2.1.2) 9.4.2.3  Stability and roll-off Polymer-based LEDs mostly suffer from instability at PEDOT:PSS/emissive layer interfaces, cathode interfaces, and low thermal stability scenarios, while fluorene materials also suffer from keto defects (Lu et al., 2012; Lee et al., 2006; Bolink et al., 2007; Grisorio et al., 2011; Kim et al., 2003). Practical applications of PLED require operational lifetimes (50% of the maximum) ranging from 1000 h (about 41 days) to more than 10,000 h (>400 days

Polymer Light-Emitting Diodes  361 or 1 year) depending on the application (Haskal, 2002). Single-panel passive-matrix have been developed that run for more than 4 years, while the lifetime of yellow-green PPVbased matrix displays at 50% relative humidity (RH) was maximized to 400 h. Industrial applications require all pixels to have high operational lifetimes. The most common degradation mechanism was caused by loss of conjugation in the polymer, which resulted in a blue shift in the emission of the devices. Extensive studies on PPVderived polymers showed that the passage of electrons through the polymer film was the main issue rather than holes (Parker et al., 1999). The presence of singlet oxygen radicals is thought to attack the vinyl bonds, which eventually quenches the photoluminescence and electroluminescence. The lifetime (LT) of organic LEDs is found to follow power law dependence with respect to the current density J; that is, LT∝J−β, where typically 1.5<β<3 (Murawski et al., 2013). From the equation, if J is increases by an order of magnitude, the lifetime will be lowered by more than an order of magnitude. Apart from decrease in luminance over time, quantum efficiency roll-offs at high current densities is the most common issue encountered. EQE roll-off at high current densities is attributed to (1) imbalance between the numbers of holes and electrons in the emitting layer and (2) the nonradiative exciton quenching (Giebinkand and Forrest, 2008; Xie et al., 2017). A brief introduction to advanced methods to characterize exciton quenching will be presented in Section 9.5. 9.4.2.4  Ways to enhance output As stated previously, the EQE of the PLEDs can be given by EQE=γ×ηPLQY×ηS/T×ηout, and for a sample graph for the F8BT device depicted in Figs. 9.3 and 9.4A. In PLEDs, the output efficiency can be improved by the correct choice of solvent and annealing, which in turn affects the morphology of the active layer. Chemical modification of the emitting polymer with an ETL or HTL, which improves charge-carrier balance (γ) and introduces trap sites, which increases radiative recombination, can also improve the output efficiencies (Lim et al., 2005; Hung et al., 2005). In the last few years, ηS/T or singlet harvesting has been improved via thermally activated delayed fluorescence (TADF) to increase EQE (Yang et al., 2017; Adachi, 2014; Dias et al., 2017). The last factor in the equation, ηout, has been improved by physical modification of the device. Typically, from the emitter position, light emission is confined within 30 degrees with respect to the normal surface, and the energy contained in it is <20% of the total energy, as shown by the inset in Fig. 9.4B (Brütting et al., 2013). Losses in a stacked system take place due to refractive index mismatch [i.e., light has to travel from a high-refractive index material (glass) to air]. Thus, emitted light is waveguided in the organic layers, and the transparent electrode, and lost by residual absorption or edge emission, as shown by the inset in Fig. 9.4A. For a system where molecular dipoles are parallel to the substrate, the plasmonic, waveguide, and substrate emission losses account for about 70%, limiting the ηout value to about 23%, such as in the F8BT system (Dey et al., 2017).

362  Chapter 9 10 1

Coupling efficiency

EQE (%)

8 6 4 2 0

(A)

Plasmon

0.8

Waveguide

0.6 0.4

Substrate emission

0.2 air 0

0

200

400

600

800

1000 1200

Distance from MoO3 (nm)

200

0

Photodetector

d

400

800 600 J (A m–2)

1000

1200

Photodetector

d

(B) Fig. 9.4 (A) EQE of an F8BT device. Inset shows losses due to low-refractive index of the substrate and optical outcoupling efficiency of F8BT PLED in different layers. An oscillation in the fraction of the emitted power is observed for air and substrate emissions. The waveguided mode is farther from the metallic contact (MoO3/Au). Losses due to surface plasmon resonance are nearer the metallic contact. (B) The diagram on the right shows index-matching, semi-pherical lens on a device that reduces the substrate losses.

Alternatively, improvement in the light-extraction efficiency has been a major focus of research over the last several years. Light can be extracted from glass substrate by using index-matching macroscopic or microlens array (Fig. 9.4B), microcavity structures (partially radiating electrodes), and scattering particles (Sun and Forrest, 2008). Grating, distributed, or aperiodic Bragg reflectors such as the ripple-shaped nanostructure of ZnO have been utilized for F8BT devices to limit the emission cone and reduce waveguiding losses. Further, polar-solvent treatment of the ZnOby amine (NH2) and hydroxyl (OH) groups created a smother surface, along with an interfacial negative dipole that reduced the electron injection barrier between the active polymer layer and the ZnO layer, and promoted electron injection. The smoothened morphology in turn lowered the exciton quenching and promoted higher radiative recombination. Luminance efficiency as high as 61.6 cd A−1, a power efficiency of 19.4 lm W−1, and an EQE of 17.8% in PLEDs have been achieved in these structures.

Polymer Light-Emitting Diodes  363

9.5  Advanced Characterization Techniques 9.5.1  Transient Electroluminescent Steady-state measurement requires the application of continuous wave (CW) voltage for a sufficiently long time. This limits the maximum voltage applied in order to prevent device failure. Because there is no heat sink associated with a device, CW excitation also results in Joule-heating of pixels and can introduce spectral shifts due to the high current (Braun, 1993). It has been observed that in a CW drive beyond a certain current density, current-induced heating increases the temperature of the device, thereby enhancing the mobility of and current through the device. The excitations result in the center of the pixel being at least 20 K hotter than the remainder (Pinner et al., 1999). In comparison to CW excitations, a pulsed excitation lowers the Joule-heating effects and failure of the device as it is held at high voltage for a very short time, allowing one to probe high-field regions. Pulsed excitation techniques such as transient electroluminescent measurements have been widely reported for insight on charge transport processes in the devices; estimating electron and hole mobility from a single device, rather than a single carrier (e.g., hole-only devices), excitonic yields, and annihilation processes within an active device (Dey et al., 2017; Rothe et al., 2006). In this technique, the PLED is excited with a pulse generator with a rise time of a few nanoseconds over a range of operating frequencies. The voltage, current, and electroluminescence (EL) can be monitored using an oscilloscope. For PPV-based copolymers and PFs, the transient characteristics were applied to a range of device structures and cathodes (Dey et al., 2017; Pinner et al., 1999). Because the presence or the absence of an HTL controls the hole-injection barrier while the work function of the cathode controls the electron-injection barrier, different structures can create a hole- or electron-injection system and provide a possible source for extracting electron and hole mobilities. The EL time evolution has five distinct regions: (1) a delay time before the light is detected, (2) fast rise time when EL turns on, (3) slow rise time, (4) fast turn-off modulation to a nonzero EL, and (5) long exponential tail. Depending on the device architecture, regions (1), (2), and (3) can be used to extract mobilities, while region (5) has been extended to analyze annihilation processes.

9.5.2  Pump-Probe Method The EQE of a PLED is given by EQE=γ×ηPLQY×ηS/T×ηout, where γ is the charge-carrier balance; ηPLQY is the PLQY; ηS/T, singlet formation (25%); and ηout is the output coupling factor (about 0.22). The product of the first three terms on the right side is denoted by internal quantum efficiency (IQE) (i.e., IQE≈EQE/ηout). In the architecture used for F8BT systems (Fig. 9.3A), the observed EQE (about 9%) was significantly higher than the expected (3.85%) (Fig. 9.4A), leading to IQE of about 39% in spite of significant losses. As the IQEs of these F8BT PLEDs go beyond simple spin statistics, there should be an additional channel to boost the singlet yield, as reportedly has happened via triplet-triplet annihilation (TTA) in F8BT PLEDs. By using either an electrical or optical pump, or both, the pump-probe experiment

364  Chapter 9 allows one to monitor the EL or PL, or both, as well as the optical absorption of the excited species in the ITO/c-ZnO/Ba(OH)2/F8BT/MoO3/Au devices (Dey et al., 2017). For these studies, the device was simultaneously excited via a function generator (electrical) with different voltage heights and a pulsed laser diode (optical), as shown in Fig. 9.5A. The electrical pulse EL (with a width about 100 μs) will generate singlets and triplets in ratio 1:3. The optical probe (405 nm, pulse width 10 ns) is chosen so as to be close to the absorption maxima and is used as excitation for PL (i.e., to generate excess singlets and then triplets via ISC only). In this setup, the optical pump intensity should be sufficiently strong so that EL and PL intensity is almost the same. The optical excitation is delayed with respect to the electrical one, so that the EL reaches a constant value and optical probing is done in quasi-steady state. The EL and PL can be collected using a suitable band pass filter and oscilloscope. It was observed that the delayed and prompt ELs overlap, implying that the delayed EL originates from DF, which itself originates from TTA in the F8BT system.

AP

Long pass filter

D

To probe the triplet density during the electrical excitation of the PLED, the triplet-triplet absorption was obtained (Fig. 9.5B) by measuring the change in reflection (ΔR) using

0.6

Electrical pulsar

Device

1 0 DPL ´ 10–2

LD@ 405 nm pulsed optical pump beam

LD@ 780 nm CW probe beam

(A)

480 A m–2

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900 A m–2 1200 A m–2 0.2

0.0

(B)

0

50

100

200

250

Tn

TTA

s1

–1

ISC

320 A m–2

–2

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DPL = ((PL+EL)-(PL0+EL))

RISC T1

DEST

480 A m–2

–3

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–4

PF

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(C)

APD

DR (´ 10–3)

320 A m–2 PL+EL Band pass filter

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DF S0

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Fig. 9.5 (A) Schematic diagram of a dual-pump (electrical and optical) transient absorption spectroscopy setup. (B) Triplet absorption (Dey et al., 2017) at different current densities. (C) Singlet-exciton quenching (−ΔPL=((PLfield+EL)−(PL0−EL)) during the device operation versus time at different injected current densities (Dey et al., 2017). (D) Schematic showing various processes, such as RISC and TTA, that contribute to delayed fluorescence (DF).

Polymer Light-Emitting Diodes  365 a CW laser (780 nm). Further, at the end of the pulse width, the DF intensity decreases as the injected current (J) increases. This decrease in DF with respect to J can be due to singlet exciton annihilation in the presence of triplets and polarons. To study the effect of triplets and polaron-assisted annihilation processes on singlet excitons in operational F8BT PLEDs, an optical excitation of pulse width (10 ns), and delayed (about 60 μs) to the electrical pulse, was introduced. In comparison to the situation in which only an optical pulse excites the device and not an electrical pulse, a quenching of PL (Fig. 9.5C) was observed in the electrically pulsed device, implying that singlet-polaron annihilation (SPA) and singlet-triplet annihilation (STA) are the quenching processes in this device. When the same technique was applied to a unipolar device, no changes in the PL were observed, suggesting that SPA to be less dominant than STA in the devices. Overall, TTA and STA are considered to be the major emission mechanisms, while TPA and SPA were insignificant in F8BT PLEDs. Apart from TTA, reverse intersystem crossing (RISC) also can be another process responsible for the origin of DF. The role of RISC in conjugated polymers is currently under study.

9.6  Future Directions Exciting progress has been made in PLEDs, with research in organic electronics motivated by including them in wearables, display and lighting, and interactive packages (Guo et al., 2017; Wang et al., 2016). For interactive packages, such as by preparing a paper substrate that has been screen-printed with a low-cost, conductive, carbon-based track and integrating it with touch-pad, silicon (Si)-based CMOS circuitry, and PLED. Eventually, application in touchpads requires a multicolor-display along with a flexible substrate. Because the involvement of flexible substrates like paper, plastic, and even glass places restrictions on process temperature, all solution-processed PLEDs remain a major challenge in efficient device realization. Because each solution-processed layer has an ability to corrode the spin-coated layer underneath, recent research has focused on preventing the mixing of interlayers by cross-linkable HTL polymer, orthogonal solvents (Lee et al., 2014a, b, c; Aizawa et al., 2014; Kasparek and Blom, 2017). Elimination of reactive layers, calcium (Ca) and barium (Ba) as electrodes, by including conjugated polyelectrolytes as ETLs/HTLs is being considered as a vital option for all solution-processed PLEDs (Fang et al., 2011; Bao et al., 2015). The exciton/charge dynamics in these devices is still under study, as film morphology controls the interchain interaction, which in turn controls the dynamics (Kondakov, 2015). The outcomes of these research endeavors have been very encouraging, but further work in the direction of molecular engineering, device architecture, and carrier dynamics is still required to realize the full potentials of PLEDs.

Acknowledgment DM acknowledges IIT Bombay for the research fellowship.

366  Chapter 9

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