The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

Abstract

Open AccessCCS ChemistryMINI REVIEW1 Aug 2020The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes Chihaya Adachi and Atula S. D. Sandanayaka Chihaya Adachi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Google Scholar More articles by this author and Atula S. D. Sandanayaka *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Department of Physical Sciences and Technologies, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka, Belihuloya 70140 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000327 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated molecular and device architectural designs. In OLEDs, the development of thermally activated delayed fluorescence molecules has been intensively investigated recently. As a result, the internal quantum efficiency of OLEDs containing relatively simple aromatic compounds without precious metals has reached almost 100%. Furthermore, incorporating a distributed feedback resonator structure into the OLED architecture has yielded OSLDs that exhibit the features of current-pumped lasing. In this short review, the authors describe the recent paradigm shift from OLEDs to OSLDs, mainly from the perspective of materials innovation. Download figure Download PowerPoint Progress of Emitter Materials in Organic Light-Emitting Diodes In an organic light-emitting diode (OLED), electrons and holes are injected from the cathode and anode, respectively, into multiple organic layers with thicknesses of ∼100 nm and transported in these layers. The recombination of electrons and holes in the light-emitting layer generates excitons, which might deactivate radiatively, leading to light emission from the OLED. At the time of exciton generation, four eigenstates are formed from a combination of electrons and holes according to spin statistics (Figure 1).1 In this event, the excited singlet state with spin s = 0 is generated with a probability of 25% and the excited triplet states with s = 1 are generated with a probability of 75%. Singlet excitons are usually generated with a probability of almost 100% in the photoexcitation process, whereas triplet excitons are generated with a 75% probability in the electrical excitation process (Figure 2). Thus, achieving radiative deactivation of triplet excitons generated by electrical excitation is the key to realizing highly efficient OLEDs. However, most organic molecules are fluorescent materials that emit light from singlet excitons; emission from their triplet excited states is not usually observed at room temperature because of the competition of nonradiative deactivation. Therefore, triplet-state emission from fluorescent materials is typically limited to low temperatures, such as that of liquid nitrogen. OLED research started in the 1950s with single crystals of anthracene, which is a typical fluorescent molecule (first generation).2,3 Until around 1997, only fluorescent materials were used as light-emitting materials (Figure 3). It was then discovered that the excited triplet energy level of anthracene derivatives could be controlled systematically by introducing a wide variety of substituents, which led to the use of triplet–triplet upconversion (TTU) to raise their electroluminescence (EL) efficiency to higher than that of typical fluorescent molecules. At present, the external quantum efficiency (EQE) of TTU-based OLEDs is >10%, which exceeds the theoretical limit of fluorescence-based OLEDs, that is, the EQE = 5%. In addition, TTU-based emitters with durable molecular structures have been developed, resulted in their practical application in blue OLEDs. Figure 1 | Four eigenstates generated under current excitation. Statistically, the recombination of electrons and holes produces 25% excited singlets and 75% excited triplets. (a) Conceptual diagram of the four spin states. (b) Spin function. Download figure Download PowerPoint Since the early 1950s, it has been widely recognized from theoretical considerations that high EL efficiency can be obtained in OLEDs by using phosphorescence, which is direct luminescence from a triplet excited state. In the early 1990s, some phosphorescent materials such as keto-coumarin derivatives,4,5 Eu derivatives,6,7 and Tb derivatives7 were examined. However, the EQE of OLEDs containing these phosphorescent materials was much lower than that of fluorescent OLEDs. Then in the latter half of the 1990s, some organometallic complexes containing heavy metals such as Os, Au, Pt, and Ir were examined, aimed for OLED application. In fact, Ma and Che first demonstrated the feasibility of metal complexes to obtain high-efficiency OLEDs using Os(CN)2(PPh)3X,8 although their very first device showed a rather low EQE of <0.1%. This study initiated the examination of various luminescent materials, which revealed that PtOEP9 and Ir(ppy)310,11,12 showed great promise for use in OLEDs. Indeed, an internal quantum efficiency (IQE) of almost 100% was realized for OLEDs with Ir(ppy)3 derivatives and sophisticated device architectures,12 giving rise to second-generation luminescent materials. Then the molecular structure of Ir complexes was optimized considering device durability, resulting in current practical devices that operate in the green and red regions. However, Ir is inherently scarce and expensive. Furthermore, even after 15 years of research and development, it is still difficult to achieve highly stable blue phosphorescent OLEDs.13 Figure 3 | Progress of OLED light-emitting molecules: first generation (fluorescent molecules), second generation (phosphorescent molecules), and third generation (TADF). TTA is an extension of first-generation technology. Download figure Download PowerPoint In 2012, our research group reported a current-to-photon conversion efficiency of nearly 100% using advanced thermally activated delayed fluorescence (TADF) materials as third-generation luminescent materials,14 following our lead studies.15–17 To achieve efficient TADF, a small energy difference between the lowest singlet and triplet excited states (ΔEST) is needed to facilitate reverse intersystem crossing (RISC). In TADF, the RISC process is used as an emission light path (Figure 2). Moreover, the phenomenon of TADF itself was first confirmed in the 1930s,18 but the efficiency of upconversion was rather low, masking it as a possible OLED mechanism.19–21 However, focusing on precise molecular design with the aim of minimizing ΔEST has led to pure aromatic compounds with ΔEST as small as several hundreds of millielectronvolts with almost 100% upconversion efficiency. As a result, OLEDs with an IQE of 100% were realized.14 Figure 2 | Mechanisms of exciton generation under current excitation. (a) Conventional fluorescence and phosphorescence emission mechanisms under optical and electrical excitations. In case of fluorescence molecules, only 25% of electrically generated excitons contributes for light emission, while phosphorescence molecules can harvest 100% excitons for light emission via direct triplet exciton formation and indirect triplet formation through ISC. (b) TADF mechanism. In case of thermally activated delayed fluorescence (TADF) mechanism, both electrically generated singlet and triplet excitons contribute for prompt and delayed emissions, leading to 100% emission from the singlet state. ISC, intersystem crossing; RISC, reverse intersystem crossing; TADF, thermally activated delayed fluorescence; NRD, nonradiative decay process. Download figure Download PowerPoint So far, many reported TADF molecules comprise donor–acceptor (D–A) structures in which the electronic configurations of the ground and excited states are orthogonal to each other, like the n–π* transition but not π–π*. Thus, it is vital to understand the mechanism of effective spin upconversion in the TADF system. In the case of D–A-type TADF molecules, it has been well recognized that there are two major electronic states, such as locally excited (LE) and charge-transfer (CT) states, that form multiple energy levels depending on the molecular structures.14 A recent study clarified that LE and CT states could mix partially to form ψ(LE + CT) states. Upconversion from an excited triplet to an excited singlet state is a transition between different spin states, and according to the El-Sayed rule,22 a transition between triplet CT and singlet CT states or triplet LE and singlet LE states is a forbidden process when the wavefunctions of these states are composed of pure components. Thus, the transition between the same types of pure electronic states does not occur, but instead, as a mechanism to promote the triplet-to-singlet RISC transition, and a model was proposed in which the transition between the singlet CT and triplet CT states goes through an intermediate triplet LE transition state (Figure 4). Quantum chemical calculations have also revealed that in actual molecules pure CT and LE states do not exist, and in many cases, the electronic level is a mixture of CT and LE states.23–25 Furthermore, it has been pointed out that the presence of different CT levels, such as through-space and through-bond levels, plays an essential role in upconversion.26,27 Figure 4 | A possible mechanism of the electronic transition from the lowest triplet excited state to the lowest singlet excited state. Spin conversion from 3CT to 1CT occurs via 3LE. The CT state is based on the electronic transition from the donor site to the acceptor site in a molecule, and the LE state is the electronic state localized at the donor site. In practical devices, the mixing of CT and LE states occurs, promoting the RISC process. Download figure Download PowerPoint D–A compounds are considered the fundamental TADF structure for designing high-performance TADF molecules, and many such molecules have now been developed. It has also been clarified that high-performance TADF properties could be achieved using other novel molecular skeletons. In 2014, it was reported that an n–π*-type heptazine derivative without a D–A skeleton exhibited TADF properties.28 Although the photoluminescence quantum yield of guest–host thin films with the heptazine derivative was about 30%, its TADF lifetime was extremely short (about 250 ns). Furthermore, Hatakeyama et al.29–33 proposed a separation mechanism of the highest occupied and lowest unoccupied molecular orbitals using the charge-resonance effect, which yielded a high-performance TADF molecule. Since this molecule had a rigid molecular skeleton, it showed a very narrow emission spectrum with a full width at half maximum (FWHM) of 27 nm, making it an excellent candidate for display applications. Currently, the molecular skeletons of TADF materials include D–A type, charge-resonance type, multiple heterocycles utilizing the n–π* excited state, and proton transfer molecules.32 Therefore, a wide variety of molecular skeletons could be used to realize TADF, and it is expected that further molecular designs would be developed in the future. In this way, OLED research started with fluorescent molecules, progressed to the development of room-temperature phosphorescent molecules, and then rapidly evolved to focus on TADF molecules. Besides, very recent studies have demonstrated some novel conceptual light-emitting materials based on organic radical and organic–inorganic perovskite materials, which use triplet-to-triplet,33 doublet-to-doublet,34 and band-to-band transitions,35,36 respectively. Indeed, various developments are being made because of the high degree of freedom in the molecular design of organic molecules. Active Molecules for Organic Lasers Another attractive feature of organic light-emitting molecules is their ability to amplify light; that is, laser action. Since the first reports of lasing from organic materials using Eu complexes by Sorokin, Lankard, and Schafer more than 50 years ago,37–41 various molecular skeletons have been developed for this purpose. Research has centered on styrylamine-, coumarin-, and cyanine-based materials, keeping their application to liquid dye lasers in mind, and the number of such lasing materials exceeds tens of thousands.42 Especially since 1995, the development of materials for solid-state waveguide thin-film lasers has progressed along with that of OLED light-emitting molecules, and various molecular skeletons exhibiting low lasing thresholds have been reported.43–58 Figure 5 summarizes the lasing/amplified spontaneous emission (ASE) threshold of representative laser materials in solid films. It has been recognized that stilbene and fluorene units in both small molecules and polymers provide excellent lasing behaviors, indicating all possessing rigid backbones with high photoluminescent quantum yield (PLQY) and radiative decay rates. Actually, some reports have aimed to develop current injection lasers using organic materials.59,60 Because organic molecules exhibit strong concentration quenching, a thin solid film consisting of a few mol % of the laser molecules dispersed in a host matrix, that is, guest–host system, is used in such current injection lasers. Figure 5 | Correlation between the molecular structures of organic laser molecules and thresholds of ASE and lasing. Download figure Download PowerPoint Of these various molecular skeletons, it has been reported that laser molecules with a stilbene skeleton exhibit a low threshold value for ASE and lasing.61,62 In particular, 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) showed an ASE oscillation wavelength (λASE) of 461 nm in a thin-film waveguide structure with 6 wt % BSB-Cz: 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl CBP as the active layer and an ASE threshold (Eth) of 0.32 ± 0.1 μJ/cm2, which is extremely low (Figure 6).61 The fluorescence lifetime (τf) of this thin film was short (∼1.0 ns), its fluorescence quantum yield (Φf) reached almost 100%, and its radiative deactivation rate constant (kr) was large (1 × 109 s−1). Because Φf and τf of this film did not show temperature dependence from 5 to 300 K, nonradiative deactivation was suppressed entirely even at room temperature. λASE of BSB-Cz occurs near the 0–1 transition in its fluorescence emission spectrum, which suggests the slight self-absorption of the 0–0 transition. Here, λASE is discussed based on kr, the stimulated emission cross section (σem), and the absorption cross section (σABS). kr (kr = ΦPL/τf) is calculated from τf and the emission quantum efficiency (ΦPL) of each codeposited thin film. σem is calculated using the following formula,63,64 σ em ( λ ) = λ 4 E f ( λ ) 8 π n 2 ( λ ) c τ f (1) n f = ∫ E f ( λ ) d λ (2) Figure 6 | Laser oscillation characteristics and optical properties of a 6 wt % BSB-Cz:CBP thin film as an active layer. (a) Chemical structures of BSB-Cz and CBP as an active emitter and host, respectively. (b) Temperature dependence of the emission quantum efficiency and emission lifetime of the thin film. (c) Lasing oscillation spectrum. (d) Excitation power dependence of emission intensity. The threshold is around 0.32 μJ/cm2. Download figure Download PowerPoint In Eq. (1), Ef(λ) is the quantum yield distribution, and n is the refractive index. σABS54 was calculated using Eq. (3), in which n = 1.8. σ ABS , Sol ( λ ) = 1000 ɛ ( λ ) ln 10 N A (3)where ɛ(λ) is the molar extinction coefficient, and NA is Avogadro’s number. In the 6 wt % BSB-Cz:CBP thin film, a high value of σem = 2.7 × 10−16 cm2 was obtained. Furthermore, the effective stimulated emission cross section (σemeff) is the difference between σem and the cross section related to a loss (σABS and the singlet and triplet excited-state absorption cross sections, σSS and σTT, respectively), and is given by Eq. (4). σ emeff = σ em − ( σ ABS + σ SS + σ TT ) (4) Figure 7 shows the spectra of σeff and σABS and the excited-state absorption spectrum of a 6 wt % BSB-Cz:CBP coevaporated thin film. In BSB-Cz, the singlet excited-state absorption, triplet excited-state absorption, and ground-state absorption spectra do not have a large overlap with λASE. Thus, the 6 wt % BSB-Cz:CBP codeposited thin film has a high kr (i.e., a large σem), σABS as small as <10−19 cm2 at λASE, and an excited-state absorption. The absence of these absorptions provides a very large σemeff, leading to a very low Eth. Figure 7 | (a) Ground-state absorption spectrum (solid red line), fluorescence spectrum, laser oscillation spectrum (solid blue line), S–S absorption spectrum (blue circles), and T–T absorption spectrum (orange circles) of BSB-Cz. (b) Energy-level diagram of BSB-Cz. Download figure Download PowerPoint Laser Oscillation Characteristics Under Optical Excitation As described earlier, BSB-Cz is suitable for optical amplification because of its high Φf, low probability of intersystem crossing, and the absence of overlapping excited-state absorption in the λASE region.66 For laser oscillation, it is necessary to introduce an optical resonator structure; however, in an amorphous organic thin film with a thickness of several hundred nanometers, it is difficult to form an end face like in the case of an inorganic semiconductor crystal with a distributed Bragg reflector structure. Thus, for organic thin-film lasers, it is ideal for forming a distributed feedback (DFB) resonator structure, which could outcouple the emission perpendicular to the longitudinal direction of the device. Among DFB resonator structures, the mixed-order DFB structure, which has a primary feedback region that produces strong optical feedback and a secondary Bragg scattering region that allows light extraction, is suitable for organic thin-film lasers. In a DFB resonator structure, the Bragg condition is given by Eq. (5), m λ Bragg = 2 n eff Λ (5)where m is the diffraction order, λBragg is the Bragg wavelength, neff is the effective refractive index of the gain medium, and Λ is the grating period. Laser oscillation occurs when this condition is satisfied.25 Using the reported values of neff and λBragg for BSB-Cz, the optimum Λ for m = 1 and 2 in DFB laser devices are 140 and 280 nm, respectively. Figure 8 shows a DFB grating observed by scanning electron microscopy (SEM). The DFB grating was designed to possess a depth of 65 ± 5 nm and Λ of 140 ± 5 and 280 ± 5 nm. The primary and secondary DFB grating lengths were approximately 15.12 and 10.08 µm, respectively. By forming a 200 nm-thick BSB-Cz film on the grating by vacuum deposition, the surface morphology of the organic layer possessed a lattice structure with a surface modulation depth of 20–30 nm. Figure 9 shows the oscillation characteristics of a mixed-order DFB device under optical excitation. With increasing excitation intensity, the FWHM decreased remarkably, and at Eth = ∼0.2 μJ/cm2, laser oscillation occurred from the vicinity of the stopband at the central oscillation wavelength of 481 nm. In this mixed-order-type DFB structure, compared with those of devices with ASE and second-order DFB structures, Eth was decreased by about 1/3 and 1/2, respectively, demonstrating the superior performance of the mixed-order-type DFB structure. These results confirmed the light confinement effect of the mixed-order DFB structure with BSB-Cz. Figure 8 | (a) Schematic of a mixed-order DFB structure with first- and second-order gratings. (b, c) SEM images of the DFB structure with a 140-nm primary structure and 280-nm secondary structure. Download figure Download PowerPoint The limited overlap of the excited-state absorption, ground-state absorption, and emission spectra of BSB-Cz suggest the possibility to realize quasi-continuous-wave (qCW) laser oscillation. Figure 10 shows the qCW laser oscillation characteristics of a device with BSB-Cz. Continuous laser action was obtained even at a high frequency of 80 MHz. Besides, continuous laser action was observed even with a long pulse excitation of 800 µs to 30 ms.55 The optical gain and loss coefficient estimated from the ASE characteristics of the doped film (optical waveguide structure with a thickness of 200 nm) using the variable stripe method were 40 and 3 cm−1, respectively. These results confirmed that BSB-Cz is an attractive candidate for qCW lasers able to operate even under long-pulsed light excitation. Figure 10 | Quasi-CW lasing characteristics of mixed-order DFB structures. Streak images of the oscillation state when (a) the excitation frequency was changed from 0.01 to 80 MHz, and (b) the pulse width was 30 ms and 800 μs. (c) Excitation pulse-width dependence of the CW lasing threshold (Eth). The doped film exhibited a lower Eth than that of the neat film. CW, continuous wave. Download figure Download PowerPoint Development of Current-Pumped Organic Semiconductor Laser Devices With the successive advent of fluorescent molecules, phosphorescent molecules, and TADF molecules, OLEDs, capable of current-to-light conversion with the ultimate IQE of 100%, are now possible. At the same time, the realization of an organic semiconductor laser diode (OSLD) has long been a significant challenge in organic semiconductor research. Adachi68 proposed an OSLD using an Eu complex in the active layer in 1988, but over 30 years have passed since then. An organic semiconductor laser that operates by current excitation is expected to have great potential because of its low cost and tunable wavelength from the visible to the infrared region. Furthermore, such organic semiconductor lasers are attractive for use at the frontier of organic optoelectronics research, such as in future optical integrated circuits mounted on flexible substrates. Figure 9 | (a) Laser oscillation characteristics of the mixed-order DFB structure. (b) Dependence of oscillation intensity and FWHM on excitation intensity. (c) Spatial distribution of emission intensity around the threshold, showing good agreement with the calculated value. Download figure Download PowerPoint In 2019, our research group reported signs of lasing by current excitation.69 The device structure was based on that of a conventional OLED. To enable electrical excitation, the primary and secondary DFB structures in the aforementioned optical resonator and a fluorescent thin film of BSB-Cz as the organic semiconductor active layer were sandwiched between an indium thin oxide (ITO) anode and aluminum (Al) cathode (Figure 11). In this organic amorphous thin-film device, a thin-film structure is required to generate a high electric field strength of ∼106 V/cm for effective current injection/transport, and the total thickness of the organic active layer was limited to 210 nm. Furthermore, to form ohmic contacts with the electrode, the cathode side of the organic active layer was n-doped with Cs, and a 10-nm MoO3 layer was included on the anode side of the organic active layer to achieve p-type doping. Figure 11 | Schematic diagram of the structure of a current-injection OSLD. An ohmic contact was obtained using Cs-doped BSB-Cz on the cathode side (lower) and MoO3 layer inserted on the cathode side (upper). Download figure Download PowerPoint Conventional OLEDs are based on a double heterostructure to uniformly inject and transport electrons and holes and confine the generated excitons in the light-emitting layer. This structure tends to achieve high efficiency, and the EL emission efficiency of OLEDs did not decrease markedly up to current densities of about 1 A/cm2. However, an injection of 1 kA/cm2 is needed for laser oscillation. At such a high current density and various exciton deactivation processes facilitated by the heterointerface occur. Therefore, it is necessary to use a single-layer structure containing no heterointerface to both generate and deactivate excitons in the bulk of the light-emitting layer. Space-charge-limited current measurements revealed that the mobilities of both electrons and holes in the BSB-Cz layer were about 10−4 cm2/Vs. Thus, provided that there are ohmic contacts at the interfaces between the electrodes and the organic layer, the recombination site should be near the center of the BSB-Cz emission layer. In fact, the EQE–current density characteristics of the BSB-Cz-based device, fortunately, showed a constant EQE up to a high current density of ∼1 kA/cm2 without remarkable roll-off. A challenge facing organic molecules under current excitation is the presence of polaron (radical cation and radical anion) absorption. Because many organic molecules have a broad absorption spectrum in the radical state, the absorption caused by overlap with the oscillation wavelength inhibits laser oscillation. BSB-Cz shows strong polaron absorption around 600 to 1000 nm, which does not overlap with the emission spectrum near 460 nm. Therefore, BSB-Cz successfully avoids the overlap of the absorption from the ground state, excited singlet absorption, excited triplet absorption, and polaron absorption, satisfying the requirements to achieve excellent performance as a laser molecule for current excitation. A surface-emitting laser was constructed using a 6 wt % BSB-Cz:CBP codeposited thin film as an active layer, and as an optical resonator structure suitable for an OSLD, first- and second-order DFB structures were incorporated into an OLED device to resonate light. Figure 12 shows the laser oscillation characteristics of the device under the current excitation. From a current density of about 700 A/cm2, a narrow band and enhanced emission intensity were obtained. A sharp decrease in FWHM was observed with a clear threshold, and a spectral width of 0.2 nm or less was obtained. The current threshold was almost equal to the threshold value estimated under photoexcitation. The slope efficiency under current excitation was 0.31%, which was virtually the same as that under optical excitation (0.38%). In contrast, the slope efficiency of the device without a metal electrode was 6.2%, which suggests that the propagation loss caused by the metal electrode was large. Figure 12 | Lasing characteristics of a current-pumped OSLD. (a) Dependence of the oscillation spectrum near the threshold on current density. (b) Dependence of oscillation intensity and FWHM on current density. Download figure Download PowerPoint Despite the signs of laser oscillation, the current OSLD, unfortunately, displayed a very short device lifetime. Deterioration of OLEDs has been drastically inhibited by improving durability by thoroughly controlling oxygen and moisture and suppressing chemical deterioration of light-emitting molecules using proper device architectures. However, OLSDs require a very high current density, compared with that used for OLEDs. Also, the successful yield of OSLDs was ∼5% because of the difficulty of device fabrication procedures, including DFB microfabrication. In the future, along with the decrease of the laser threshold, the mechanism of degradation caused by the instability of the excited triplet states should be investigated. BSB-Cz is known to deteriorate because of its styryl group, which is relatively free to rotate.70 Also, it has been confirmed that the introduction of triplet scavengers into organic semiconductor laser structures dramatically improves the stability of lasing layers.71–74 In the future, in addition to investigating the triplet exciton deactivation mechanism, it is necessary to improve the stabil