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Waiting for Act 2: What Lies Beyond Organic Light- Emitting Diode (OLED)

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Waiting for Act 2: What Lies Beyond Organic Light- Emitting Diode (OLED) Nanophotonics 2021; 10(1): 31–40 Opinionated article Stephen R. Forrest* Waiting for Act 2: what lies beyond organic light- emitting diode (OLED) displays for organic electronics? https://doi.org/10.1515/nanoph-2020-0322 astonishing success of organic light-emitting diodes Received June 9, 2020; accepted July 23, 2020; published online (OLEDs). This technology platform has launched a revo- August 24, 2020 lution in information displays and lighting, while moti- vating researchers worldwide to explore a vast variety of Abstract: Organic light-emitting diode (OLED) displays new materials with intriguing optical and electronic are now poised to be the dominant mobile display tech- properties that were never imagined in those early days of nology and are at the heart of the most attractive televisions discovery [2]. The fundamental discoveries encouraged the and electronic tablets on the market today. But this begs small community of researchers to consider if there were the question: what is the next big opportunity that will be any practical outcomes that could be achieved using addressed by organic electronics? We attempt to answer organic semiconductors. Some of the first devices to exploit this question based on the unique attributes of organic these “soft” materials were memories and solar cells. But electronic devices: their efficient optical absorption and compared to conventional semiconductor devices (most emission properties, their ability to be deposited on ultra- notably Si), the performance of organic devices was thin foldable, moldable and bendable substrates, the di- depressingly inferior, and worse, they did not last very versity of function due to the limitless palette of organic long. It was their lack of stability that has given rise to a materials and the low environmental impact of the mate- myth that persists up to the present day: organic devices rials and their means of fabrication. With these unique are inherently unstable. We will return to this issue below. qualities, organic electronics presents opportunities that range from lighting to solar cells to medical sensing. In this – paper, we consider the transformative changes to elec- Formally, an organic material is one that contains a carbon hydrogen bond. By this definition, fullerenes (e.g., C60), carbon nanotubes and tronic and photonic technologies that might yet be realized graphene are not organic compounds. But, more practically, it can be using these unconventional, soft semiconductor thin films. considered to be one of a class of carbon-rich compounds. In the context of this paper, the organic materials of interest in electronics Keywords: lighting; organic semiconductor; reliability; and photonics are semiconductors whose energy gaps are typically solar cell; thin film transistor. between 0.75 and 3.5 eV. The pace of these first, tentative steps in exploiting the 1 An introduction to organic unlimited variety of organic materials for optoelectronic electronics applications took an immense leap by the publication of two papers out of Eastman Kodak in 1986 and 1987. The first one, by C. W. Tang, announced the demonstration of The field of organic electronics, now in its 70th year since an organic solar cell with 1% solar to electrical power the identification of semiconducting properties of viola- conversion efficiency (PCE). The efficiency was not nthrone by Akamatsu and Inokuchi [1], has enjoyed an particularly high – that is not what made this demonstra- extended period of discovery of the characteristics of tion so notable [3]. What was different is that this was a disordered organic materials, ultimately leading to the device that mimicked an inorganic p–n junction by combining two different organic semiconductor layers, one an electron donor (D) and the other an acceptor (A), into a *Corresponding author: Stephen R. Forrest, Departments of Electrical fi – Engineering and Computer Science, Physics, and Materials Science bilayer cell. For the rst time, the current voltage charac- and Engineering, University of Michigan, Ann Arbor, MI 48104, USA, teristics showed nearly ideal rectifying characteristics that E-mail: [email protected] up to that time were only found in inorganic junction Open Access. © 2020 Stephen R. Forrest, published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 32 S.R. Forrest: What lies beyond OLED displays for organic electronics? Figure 1: Current–voltage characteristics of the bilayer organic Figure 2: A simplified organic light-emitting diode (OLED) structure photovoltaic cell shown schematically in the inset [3]. The chemicals indicating the contacts, electron transport layer (ETL), light emission used are the acceptor, 3,4,9,10-perylenetetracarboxylic bis- layer (EML) and hole transport layer (HTL). The EML typically com- benzimidazole (PTCBI), and the donor, copper phthalocyanine prises a conductive organic host doped at low density with an (CuPc). Indium tin oxide (ITO) serves as the transparent anode and emissive fluorescent or phosphorescent emitting molecule. The Ag as the cathode. The short circuit current (ISC), open circuit voltage entire device thickness is ∼100 nm. (VOC), power conversion efficiency (PCE) and fill factor (FF) under AM2 simulated illumination at 75 mW/cm2 intensity are indicated. Here, the maximum power generated by the cell is equal to the area in the achieved by LCDs. This situation changed dramatically with shaded rectangle and is given by PCE FF ⋅ I ⋅ V /P , where max SC OC sun the introduction of electrophosphorescence by Baldo et al. [5, P is the incident solar power intensity. sun 6] that almost immediately led to OLEDs with 100% internal fi fl diodes (see Figure 1). From that demonstration forward, all quantum ef ciency [7]. Brie y, molecular excited states, or organic solar cells have used the same basic D–A hetero- excitons, fall into two categories based on the spin of the excited electron: singlets and triplets. Singlets have odd junction (HJ) concept. symmetry under spin exchange, leading to rapid, fluorescent The second paper in 1987 was in many ways similar to emission by transitions to the ground state, which also has the solar cell. Again, C. W. Tang, this time in collaboration singlet symmetry. Singlet emission was the basis for the with Steven van Slyke announced the successful demon- earliest OLEDs. Triplets, on the other hand, have even sym- stration of a bilayer organic light-emitting diode (OLED) [4]. metry and hence are quantum mechanically forbidden to The OLED, like the solar cell, had a clear rectifying behavior. transition to the singlet ground state. However, the selection But most interestingly, it exhibited bright green emission rule that prevents their relaxation can be perturbed, leading due to exciton recombination on one of the molecules to slow and very inefficient phosphorescence – aprocessthat forming the bilayer, namely 8-hydroxyquinoline Aluminum is not interesting for display applications. Unfortunately, (Alq ) with an external quantum efficiency of 1%. This was 3 simple statistical arguments show that an injected electron inferior compared to inorganic semiconductors at that time into an organic medium will excite singlet states only 25% of fi ∼ based on GaAs or InP, but the very thin lms ( 100 nm) the time, with the other 75% of the electrons wasted on triplet comprising the device were grown on a glass substrate. state formation. This constraint is eliminated by the intro- fi Figure 2 shows a simpli ed, generic structure of modern duction of a heavy metal atom such as Pt or Ir into the OLEDs. Perhaps, if organics could only last long enough, luminescent molecule. The atom has a large orbital angular they would be the foundation of a new generation of dis- momentum, which when mixed with the electron spin in the plays that, at that time, was dominated by cathode ray tubes organic ligand, results in spin–orbitcouplingthatleadsto and the emerging liquid crystal displays (LCDs). violation of the spin selection rule. This is the process of While OLEDs looked promising (their colors could easily electrophosphorescence that can make every excited mo- be modified across the visible spectrum by implementing lecular state radiative, leading to 100% internal quantum only minor modifications to the chemical structures of the efficiency. With this innovation, the electrophosphorescent fluorescent emitting molecules or fluorophores), their life- OLEDs (PHOLEDs) became the most efficient light emitters times and efficiencies still fell short of what was already being known. S.R. Forrest: What lies beyond OLED displays for organic electronics? 33 Display manufacturers, notably Samsung in the Re- not easily accessed by incumbent and proven inorganic public of Korea, took immediate notice of benefits of semiconductors. After all, trying to displace an already electrophosphorescence, and the OLED display revolution served and mature market with an upstart technology is was off and running. In rapid succession, the Galaxy generally a fruitless exercise of catch-up, doomed to failure smartphone series featuring efficient, emissive and sur- from the start. Listed below are several defining charac- prisingly long lifetime OLED displays was introduced, teristics that point toward application spaces that might followed by LG Display producing large, ultrathin and best be filled by organic electronics: attractive OLED televisions. And now, Apple iPhones and 1. Optoelectronics: The very high absorption coefficients iWatches
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