A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

A comprehensive review of the literature on electron transport materials (ETMs) used to enhance the performance of organic light-emitting diodes (OLEDs) is presented. The structure−property−performance relationships of many classes of ETMs, both small-molecule- and polymer-based, that have been widely used to improve OLED performance through control of charge injection, transport, and recombination are highlighted. The molecular architecture, electronic structure (electron affinity and ionization potential), thin film processing, thermal stability, morphology, and electron mobility of diverse organic ETMs are discussed and related to their effectiveness in improving OLED performance (efficiency, brightness, and drive voltage). Some issues relating to the experimental procedures for the estimation of relevant material properties such as electron affinity and electron mobility are discussed. The design of multifunctional electroluminescent polymers whereby light emission and electron- and hole-transport pro...

Electron Transport Materials for Organic Light-Emitting Diodes

We thank the University of St Andrews for support. EZ-C thanks the Leverhulme Trust for financial support (RPG-2016-047). and the EPSRC (EP/P010482/1) for financial support.

/pdf/purely-organic-thermally-activated-delayed-fluorescence-1n0nk7e6ke.pdf

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes.

The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.

/pdf/on-the-origin-of-the-open-circuit-voltage-of-polymer-1foc4cxh8l.pdf

On the origin of the open-circuit voltage of polymer–fullerene solar cells

Here, a general experimental method to determine the energy ECT of intermolecular charge-transfer (CT) states in electron donor–acceptor (D–A) blends from ground state absorption and electrochemical measurements is proposed. This CT energy is calibrated against the photon energy of maximum CT luminescence from selected D–A blends to correct for a constant Coulombic term. It is shown that ECT correlates linearly with the open-circuit voltage (Voc) of photovoltaic devices in D–A blends via eVoc = ECT − 0.5 eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S1 with energy Eg) in the blend to the CT state (S1 → CT) occurs when Eg − ECT > 0.1 eV. Additionally, it is shown that subsequent charge recombination from the CT state to the lowest triplet excited state (ET) of D or A (CT → T1) can occur when ECT − ET > 0.1 eV. From these relations, it is concluded that in D–A blends optimized for photovoltaic action: i) the maximum attainable Voc is ultimately set by the optical band gap (eVoc = Eg − 0.6 eV) and ii) the singlet–triplet energy gap should be ΔEST < 0.2 eV to prevent recombination to the triplet state. These favorable conditions have not yet been met in conjugated materials and set the stage for further developments in this area.

/pdf/the-energy-of-charge-transfer-states-in-electron-donor-1o56tcr0yh.pdf

The energy of charge-transfer states in electron donor-acceptor blends : insight into the energy losses in organic solar cells

Organic light emitting diodes (OLEDs) have attracted much interest because of their particular low driving voltage, convenient design of the device structures, possible control of emission band, and low fabrication cost compared with any other devices. Since the discovery of electroluminescence (EL) from a conjugated polymer, main-chain conducting polymers such as poly(plenylene vinylene) (PPV), poly(p-phenylene) (PPP), their copolymers and soluble derivatives, has been studied intensively for the possible application as large-area light emitting diodes (LEDs). However, PPV has the disadvantage that electron injection is much more difficult than hole injection. Using additional organic charge-transporting layers, and adjusting the energy band of the polymer by introduction of electronwithdrawing groups attached to the polymer backbone were reported to overcome the imbalance of charge carrier injection or mobility. The introduction of electron-withdrawing groups onto the arylene rings or the vinyl groups of PPV lowers the HOMO and LUMO energy levels of the polymer, thereby permitting the use of a higher work function metal in the LED device. Numerous derivatives of PPV have been reported with electron-withdrawing substituents such as halide, cyano, trifluoromethyl, or methylsulfonylphenyl on the arylene rings. In our previous work, we reported poly(p-phenylenedifluorovinylene) (PPDFV) and poly(2-dimethyloctylsilyl-p-phenylenedifluorovinylene) (DMOS-PPDFV) with fluoro groups as electron withdrawing substituents on vinylene units of conjugated polymers, which showed the high performance of OLEDs. In the present paper, we report the synthesis and electroluminescent properties of the copolymers with p-phenylenedifluorovinylene and 2-dimethyloctylsilyl-p-phenylenevinylene units. The resulting EL polymers were synthesized by the Gilch polymerization method.

/pdf/polymer-light-emitting-diodes-based-on-ppdfv-copolymers-with-1v5aw5lepu.pdf

Polymer light-emitting diodes based on PPDFV copolymers with fluoro groups in vinylene units

New light-emitting PCPP derivatives, poly(4,4-bis{4-[(2-ethylhexyl)oxy]phenyl}-4 H -cyclopenta[ def ]phen- anthrene)
(BEHP-PCPP), with stabilized blue emission is reported. The OLED with the configuration of ITO/PEDOT:PSS/BEHPPCPP/
Ca/Al generates EL emission with maximum peak at 430 nm, low turn-on voltage (9V), and CIE coordinates ( x 
= 0.19, y = 0.15) for the blue color without any filtering. The maximum brightness of the OLEDs using BEHP-PCPP
was 1034 cd/m 2 at 15 V. The maximum luminescence efficiency of the polymer LEDs with BEHP-PCPP was 0.21
cd/A at 617 mA/cm 2 .

New conjugated polymers with stabilized blue emission

New fluorin-containing electroluminescent polymers in vinylene units, poly(2-dimethyldodecylsilyl-p-phenylenedifluorovinylene)
(DMDS-PPDFV), have been synthesized by GILCH polymerization. This polymer has been used
as the electroluminescent layers in light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). DMDS-PPDFV shows
PL around λ max = 455 nm (exciting wavelength, 365 nm) and green EL around λ max =489nm. Attachment of two the
fluoro groups were introduced to poly(2-dimethydodecylsilyl-1,4-phenylenevinylene) (DMDS-PPV) to give DMDSPPDFV
in an attempt to increase the electron affinity of the parent polymer.

Jinwoo Kim

Papers

Polymer light-emitting diodes based on PPDFV copolymers with fluoro groups in vinylene units

New conjugated polymers with stabilized blue emission

Stability of electroluminescent conjugated with fluoro groups in vinylene units