Current-injection organic semiconductor laser diode

Abstract

Disclosed is a current-injection type organic semiconductor laser diode including a pair of electrodes, an optical resonator structure having a grating with a groove completely removed, or a grating with an electrode exposed in the groove, and one or more organic layers including a light amplification layer composed of an organic semiconductor.

Description

[0001] This application is a divisional application. The international application number of the original application is PCT / JP2018 / 005362, the international application date is February 7, 2018, the Chinese national phase application number is 201880003730.3, and the invention title is "Current-injected organic semiconductor laser diode, manufacturing method and procedure thereof". Technical Field

[0002] This invention relates to a current-injection organic semiconductor laser diode and a method for manufacturing the same. The invention also relates to a procedure for designing the current-injection organic semiconductor laser diode. Background Technology

[0003] The performance of optically pumped organic semiconductor lasers (OSLs) has improved significantly over the past two decades, thanks to major advances in the development of high-gain organic semiconductor materials and the design of high-quality factor resonator structures. 1-5 Organic semiconductors offer advantages as laser gain media, including high photoluminescence (PL) quantum yield, large stimulated emission cross-section, wide emission spectrum in the visible region, and their chemical tunability and ease of fabrication. Recent advancements in low-threshold distributed feedback (DFB) OSLs have enabled optical pumping via electrically driven nanosecond pulsed inorganic light-emitting diodes, providing a pathway for novel compact, low-cost visible laser technologies. 6 However, the ultimate goal remains electrically driven organic semiconductor laser diodes (OSLDs). The realization of OSLDs will not only enable the complete integration of organic photonics and optoelectronic circuits, but will also open up new applications in spectroscopy, displays, medical devices (such as retinal displays, sensors, and photodynamic therapy devices), and LiFi telecommunications.

[0004] The main problems hindering lasing via direct electric pumping of organic semiconductors are optical losses from the electrical contacts and triplet and polaron losses occurring at high current densities. 4，5,7-9 As a method to address these fundamental loss problems, the following has been proposed: using a triplet quencher. 10-12 To suppress triplet absorption loss, singlet quenching based on singlet-triplet exciton annihilation, and reduce the active region of the device. 13 The goal is to spatially separate the exciton formation region from the region where exciton radiative decay occurs and to minimize the polaron quenching process. However, although this has already been achieved in organic light-emitting diodes (OLEDs) and optically pumped organic semiconductor DFB lasers... 5 Progress has been made, but current-injection OSLDs have not yet been definitively verified.

[0005] Patent Document 1 P1The implementation of a current-injection OSLD is described. According to the literature, the device was fabricated through the following steps: a 500 nm pitch grating (resonator) was formed on an ITO film, followed by a 250 nm thick hole transport layer of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) formed by vapor deposition. Then, a 100 nm thick light-emitting layer was further formed by spin-coating with a dichloromethane solution of aromatic polycarbonate resin. A 250 nm thick electron transport layer of 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole was formed by vapor deposition, and a 200 mm thick MgAg alloy layer was formed. The literature describes obtaining laser emission by applying a 30 V voltage to the device. However, in practice, when the dichloromethane solution of polycarbonate was spin-coated onto the TPD layer, the TPD layer dissolved, making it clear that the device could not be reproduced. Furthermore, the device has an organic hole transport layer and an organic electron transport layer, both 250 nm thick, as well as an organic light-emitting layer 100 nm thick, resulting in a very thick total organic layer thickness. Applying a 30 V DC current to a device with such a thick organic layer makes it impossible to obtain laser oscillation.

[0006] Other patent documents P2，P3 The possibility of realizing a current-injection OSLD is documented. However, these patent documents only provide a general description of current-injection OSLDs and do not mention any specific current-injection OSLDs confirmed to have laser oscillations.

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Publication 2004-186599

[0009] Patent Document 2: Japanese Patent Application Publication No. 10-321941

[0010] Patent Document 3: Japanese Patent Publication 2008-524870

[0011] Non-patent literature

[0012] Non-patent literature 1: Tessler, N., Denton, GJ & Friend, RH Lasing from conjugated-polymer microcavities. Nature 382, ​​695–697 (1996).

[0013] Non-patent literature 2: Kozlov, VG, V.,Burrows,PE&Forrest,SRLaseraction in organic semiconductor waveguide and double-heterostructure devices.Nature 389,362–364(1997).

[0014] Non-patent literature 3: Hide, F. et al., Semiconducting polymers: A new class of solid-state laser materials. Science 273, 1833 (1996).

[0015] Non-patent literature 4: Samuel, IDW & Turnbull, G.A. Organic semiconductor lasers. Chem. Rev. 107, 1272–1295 (2007).

[0016] Non-patent literature 5: Kuehne, AJC & Gather MC. Organic lasers: Recent developments on materials, device geometries and fabrication techniques. Chem. Rev. 116, 12823–12864 (2016).

[0017] Non-patent literature 6: Tsiminis, G. et al., Nanoimprinted organic semiconductor laserspumped by a light-emitting diode. Adv. Mater. 25, 2826–2830 (2013).

[0018] Non-patent literature 7: Baldo, MA, Holmes, RJ & Forrest, SR Prospects for electrically pumped organic lasers. Phys. Rev. B 66, 035321 (2002).

[0019] Non-patent literature 8: Bisri, SZ, Takenobu, T. & Iwasa, Y. The pursuit of electrically-driven organic semiconductor lasers. J. Mater. Chem. C 2, 2827–2836 (2014).

[0020] Non-patent literature 9: Samuel, IDW, Namdas, EB & Turnbull, GA How to recognize lasing. Nature Photon. 3, 546–549 (2009).

[0021] Non-patent document 10: Sandanayaka, ASD, et al., Improvement of the quasi-continuous-wave lasing properties in organic semiconductor lasers using oxygen as triplet quencher. Appl. Phys. Lett. 108, 223301 (2016).

[0022] Non-patent literature 11: Zhang, YF & Forrest, S. Existence of continuous-wave threshold for organic semiconductor lasers. Phys. Rev. B 84, 241301 (2011).

[0023] Non-patent literature 12: Schols, S. et al., Triplet excitation scavenging in films of conjugated polymers. Chem. Phys. Chem. 10, 1071–1076 (2009).

[0024] Non-patent document 13: Hayashi, K. et al., Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection / transport areato 50nm. Appl. Phys. Lett. 106, 093301 (2015).

[0025] Non-patent literature 14: C. et al., The influence of annihilation processes on the threshold current density of organic laser diodes. J.Appl.Phys.101,023107(2007).

[0026] Non-patent literature 15: Sandanayaka, ASD et al., Quasi-continuous-wave organic thinfilm distributed feedback laser. Adv. Opt. Mater. 4, 834–839 (2016).

[0027] Non-patent document 16: Aimono, T. et al., 100% fluorescence efficiency of 4,4'-bis[(N-carbazole)styryl]biphenyl in a solid film and the very low amplifiedspontaneous emission threshold. Appl. Phys. Lett. 86, 71110 (2005).

[0028] Non-patent literature 17: Sandanayaka, ASD et al., Toward continuous-wave operation of organic semiconductor lasers. Science Adv. 3, e1602570 (2017).

[0029] Non-patent document 18: Karnutsch, C. et al., Improved organic semiconductor lasers based on a mixed-order distributed feedback resonator design. Appl. Phys. Lett. 90, 131104 (2007).

[0030] Non-patent literature 19: Chénais, S. & Forget, S. Recent advances in solid-state organic lasers. Polym. Int. 61, 390–406 (2012).

[0031] Non-patent document 20: Yokoyama, D. et al., Spectrally narrow emissions at cutoffwavelength from edges of optically and electrically pumped anisotropicorganic films. J. Appl. Phys. 103, 123104 (2008).

[0032] Non-patent document 21: Yamamoto, H. et al., Amplified spontaneous emission underoptical pumping from an organic semiconductor laser structure equipped with transparent carrier injection electrodes. Appl. Phys. Lett. 84, 1401-1403 (2004).

[0033] Non-patent literature 22: Wallikewitz, BH et al., Lasing organic light-emitting diode. Adv. Mater. 22, 531-534 (2010).

[0034] Non-patent literature 23: Song, MH et al., Optically-pumped lasing in hybrid organic-inorganic light-emitting diodes. Adv. Funct. Mater. 19, 2130-2136 (2009).

[0035] Non-patent literature 24: Kim, SY et al., Organic light-emitting diodes with 30% external quantum efficiency based on horizontally oriented emitter. Adv. Funct. Mater. 23, 3896-3900 (2013).

[0036] Non-patent literature 25: Uoyama, H. et al., Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234-238 (2012).

[0037] Non-patent document 26: Matsushima, T. & Adachi, C. Suppression of excitonannihilation at high current densities in organic light-emitting dioderesulting from energy-level alignments of carrier transport layers. Appl. Phys. Lett. 92, 063306 (2008).

[0038] Non-patent literature 27: Kuwae, H. et al., Suppression of external quantum efficiency roll-off of nanopatterned organic light-emitting diodes at high currentdensities. J. Appl. Phys. 118, 155501 (2015).

[0039] Non-patent literature 28: Bisri, SZ et al., High mobility and luminescent efficiency inorganic single-crystal light-emitting transistors. Adv. Funct. Mater. 19, 1728–1735 (2009).

[0040] Non-patent literature 29: Tian, ​​Y. et al., Spectrally narrowed edge emission from organic light-emitting diodes. Appl. Phys. Lett. 91, 143504 (2007).

[0041] Non-patent literature 30: El-Nadi, L. et al., Organic thin film materials producing novel blue laser. Chem. Phys. Lett. 286, 9–14 (1998).

[0042] Non-patent literature 31: Wang, X., Wolfe, B. & Andrews, L. Emission spectra of group 13 metal atoms and indium hybrids in solid H2 and D2. J. Phys. Chem. A 108, 5169–5174 (2004).

[0043] Non-patent literature 32: Ribierre, JC et al., Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers. Appl. Phys. Lett. 91, 081108 (2007).

[0044] Non-patent literature 33: Schneider, D. et al., Ultrawide tuning range in doped organic solid-state lasers. Appl. Phys. Lett. 85, 1886–1888 (2004).

[0045] Non-patent literature 34: Murrawski, C., Leo, K. & Gather, MCEfficiency roll-off inorganic light-emitting diodes. Adv. Mater. 25, 6801–6827 (2013).

[0046] Non-patent literature 35: Nakanotani, H. et al., Spectrally narrow emission from organic films under continuous-wave excitation. Appl. Phys. Lett., 90, 231109 (2007).

[0047] Non-patent literature 36: Nakanotani, H., Sasabe, H. & Adachi, C. Singlet-singlet and singlelet-heat annihilations in fluorescence-based organic light-emittingdiodes under steady-state high current density. Appl. Phys. Lett., 86, 213506 (2005).

[0048] Non-patent literature 37: Nicolai, HT, Mandoc, MM & Blom, PWM Electron traps in semiconductor polymers: Exponential versus Gaussian trap distribution. Phys. Rev. B 83, 195204 (2011).

[0049] Non-patent literature 38: Connell, GAN, Camphausen, DL & Paul, W. Theory of Poole-Frenkel conduction in low-mobility semiconductors. Philos. Mag. 26, 541–551 (1972).

[0050] Non-patent literature 39: Pautmeier, L., Richert, R. & H. Poole-Frenkel behavior of charge transport in organic solids with off-diagonal disorder studied by Monte Carlo simulation. Synth. Met. 37, 271–281 (1990).

[0051] Non-patent literature 40: Pope, M. & Swenberg, CEE Electronic Processes in Organic Crystals and Polymers. New York: Oxford University Press, 1999.

[0052] Non-patent document 41: Setoguchi, Y. & Adachi, C. Suppression of roll-off characteristics of electroluminescence at high current densities in organic light emitting diodes by introducing reduced carrier injection barriers. J. Appl. Phys. 108, 064516 (2010). Summary of the Invention

[0053] Therefore, a laser oscillation current-injection OSLD has not yet been provided. The object of this invention is to provide a laser oscillation current-injection OSLD. Through in-depth research, the inventors have discovered that this object can be achieved through this invention. This invention provides a current-injection organic semiconductor laser diode comprising a pair of electrodes, an optical resonator structure, and one or more organic layers containing an optical amplification layer composed of organic semiconductors, and satisfying at least one of "2" to "16" below and / or having at least one of the following embodiments. This invention includes the following embodiments:

[0054] 1. A current-injection organic semiconductor laser diode, comprising a pair of electrodes, an optical resonator structure, and one or more organic layers containing an optical amplification layer composed of organic semiconductors, wherein during current injection, the current-injection organic semiconductor laser diode has sufficient overlap between the distribution of exciton density and the electric field intensity distribution of the resonant optical mode to emit laser light.

[0055] 2. The current-injection organic semiconductor laser diode according to item 1, wherein,

[0056] The optical resonator structure has a distributed feedback (DFB) structure.

[0057] 3. The current-injection organic semiconductor laser diode according to item 2, wherein,

[0058] The optical resonator structure consists of a second-order Bragg scattering region surrounded by a first-order Bragg scattering region.

[0059] 4. The current-injection organic semiconductor laser diode according to item 2, wherein,

[0060] The second-order Bragg scattering region and the first-order Bragg scattering region are alternately formed in the optical resonator structure.

[0061] 5. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein the number of the one or more organic layers is two or less.

[0062] 6. The current-injection organic semiconductor laser diode according to any one of claims 1 to 5, wherein,

[0063] The thickness of the light-amplifying layer is greater than 50% relative to the total thickness of the one or more organic layers.

[0064] 7. The current-injection organic semiconductor laser diode according to any one of claims 1 to 6, wherein the organic semiconductor contained in the optical amplification layer is amorphous.

[0065] 8. The current-injection organic semiconductor laser diode according to any one of claims 1 to 7, wherein the molecular weight of the organic semiconductor contained in the optical amplification layer is less than 1000.

[0066] 9. The current-injection organic semiconductor laser diode according to any one of claims 1 to 8, wherein the organic semiconductor contained in the optical amplification layer is a non-polymer.

[0067] 10. The current-injection organic semiconductor laser diode according to any one of claims 1 to 9, wherein the organic semiconductor contained in the optical amplification layer has at least one stilbene unit.

[0068] 11. The current-injection organic semiconductor laser diode according to any one of claims 1 to 10, wherein,

[0069] The organic semiconductor contained in the light amplification layer has at least one carbazole unit.

[0070] 12. The current-injection organic semiconductor laser diode according to any one of claims 1 to 11, wherein,

[0071] The organic semiconductor contained in the light amplification layer is 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz).

[0072] 13. The current-injection organic semiconductor laser diode according to any one of claims 1 to 12, having an electron injection layer as one of the organic layers.

[0073] 14. The current-injection organic semiconductor laser diode according to claim 13, wherein,

[0074] The electron-injected layer contains Cs.

[0075] 15. The current-injection organic semiconductor laser diode according to any one of claims 1 to 14, having a hole injection layer as an inorganic layer.

[0076] 16. The current-injection organic semiconductor laser diode according to claim 15, wherein,

[0077] The hole injection layer contains molybdenum oxide.

[0078] 17. A method for designing a current-injection organic semiconductor laser diode, the method comprising:

[0079] The steps involve selecting diode materials and designing the structure to increase the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode during current injection.

[0080] 18. A method for manufacturing a current-injection organic semiconductor laser diode, the method comprising:

[0081] Steps to evaluate the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode during current injection in a designed or existing diode;

[0082] A new diode is designed by changing at least one of the materials and structure of the diode, thereby increasing the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode during current injection; and

[0083] The steps for manufacturing the new diode.

[0084] 19. A current-injection organic semiconductor laser diode, manufactured by the method of 18.

[0085] 20. A procedure for designing a current-injection organic semiconductor laser diode, wherein the current-injection organic semiconductor laser diode is designed to increase the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode during current injection. Attached Figure Description

[0086] Figure 1 Organic semiconductor DFB laser diode structure. a) Schematic diagram of the organic laser diode. b, c) Laser micrographs (b) and SEM images (c) at 5,000× and 200,000× (inset) magnifications of the DFBSiO2 grating structure formed on the ITO top layer. d) SEM cross-sectional view of the complete OSLD. e) EDX cross-sectional view of the OSLD. Contrast was enhanced to improve the visibility of low-concentration Cs.

[0087] Figure 2 OSLD fabrication and structure. a) Schematic diagram of OSLD fabrication steps. b) Structure of the ITO-coated glass substrate used in this invention and the general structure of the DFB grating. Detailed values ​​of different grating parameters are shown in Table 1. c, d) EDX and SEM analysis of the hybrid-level DFB grating formed on the ITO top layer. These images demonstrate the possibility of achieving electrical contact with ITO.

[0088] Figure 3Electrical characteristics of electrically pumped organic semiconductor DFB lasers. a) Energy level diagram of the OSLD, representing the highest occupied molecular orbital and lowest unoccupied molecular orbital levels of organic materials and the work function of inorganic materials. b) Micrographs of the OSLD and a reference OLED under DC operation at 3.0 V. The lengths of the first-order and second-order grating regions are 1.68 μm and 1.12 μm, respectively. c, d) Current density-voltage (JV) characteristics (c) and η in the OLED and OSLD under pulsed operation (400 ns pulse width and 1 kHz repetition frequency). EQE -J characteristic (d).

[0089] Figure 4 Hole and electron transport in the organic layer. a, b) Structures of hole-only (a) and electron-only (b) devices used to evaluate transport. c) Representative current density-voltage (JV) characteristics of hole-only (HOD) and electron-only (EOD) devices under DC operation (black symbols) and pulsed operation (white symbols), expressed on logarithmic and linear scales (inset). Device area is 200 × 200 μm. These JV curves show good hole and electron transport in the high-voltage region of the laser diode fabricated in this invention. Due to hole current trapping limitation, electron current is higher than hole current at low voltages.

[0090] Figure 5 Characteristics of OSLDs with different DFB geometries. a) Micrographs of OSLDs operating at 3.0V DC. Micrographs were taken using the same magnification, and all gratings were extended vertically. b, c, d) Current density-voltage (JV) and η of the OSLD. EQE - Characteristics of J. e) Electroluminescence intensity and FWHM as functions of J. f) Emission spectrum collected in a direction perpendicular to the substrate plane as a function of J.

[0091] Figure 6 DC characteristics and emission spectra of the devices. a, b) Current density-voltage (JV) curves of OLED and OSLD measured under DC operation (a) and η EQE -J curve (b). c) PL spectrum of impurity-free BSBCz film (black line) and EL spectrum of OLED (red line) and OSLD below laser threshold (blue line).

[0092] Figure 7 Excitation characteristics of OSLD. a) Emission spectra of OSLD collected in a direction perpendicular to the substrate plane for different injection current densities. Current density higher than 3.5 kA / cm² -2At the laser emission wavelength, the device suffers severe degradation, resulting in a significant increase in background EL relative to the laser. b) Emission spectrum close to the laser threshold. c) Output intensity and FWHM as a function of current. d) Output power as a function of current. The inset is a photograph of the OSLD under 50V pulsed operation.

[0093] Figure 8 Characterization of OSLD emission. a) Emission spectra of OSLDs above the threshold measured at different polarization angles. The polarization intensity is stronger above the threshold (circles in the inset) than below the threshold (triangles in the inset). Here, 90° corresponds to the direction parallel to the groove of the DFB grating. b, c) CCD camera images (b) and cross-sections (c) show the spatial Gaussian distribution of the focused emission beam from the OSLD at different current densities. d, e) Unfocused beams of OSLDs operating above the threshold projected onto a screen.

[0094] Figure 9 Characteristics of an OSLD under optical pumping. a) Used for measurement Figure 8 b、 Figure 8 c) Test setup for beam distribution. b) Characteristics, near-field beam images, and cross-sections of the OSL under light excitation below (i), near (ii), and above (iii) thresholds (refer to the structures in Table 1). c) Characteristics, near-field beam images, and cross-sections of the OSLD-6 under light excitation below (iv), near (v), and above (v) thresholds (refer to the structures in Table 1). d) Far-field beam cross-sections of the OSL under light excitation above, near, and below the threshold, and a simulated far-field beam above the threshold. The inset above the threshold is the initial emission mode. e) Emission spectra collected in a direction perpendicular to the substrate plane of the OSLD-6 under light pumping with different light excitation densities. The steady-state photoluminescence spectrum of BSBCz on SiO2 with a grating is shown as a dashed line. f) Output intensity and FWHM of the OSLD-6 as a function of light excitation density. Excited by an N2 laser at 337 nm for 3.0 ns, with the device at ambient temperature. The slope efficiencies of optically pumped OSL (g, referring to the structure in Table 1), electrically pumped OSLD (h), and optically pumped OSLD-6 (i) are shown in g, h, and i. The input power of the optically pumped device is the power of the light source, incident on the organic thin film side of the OSL and the glass side of the OSLD-6.

[0095] Figure 10Absorption spectra of triplet, radical cations, and radical anions of BSBCz. a) Stimulated emission and triplet absorption cross-section spectra of BSBCz. Emission spectra of OSLs were measured from pure BSBCz films above the threshold. b) To investigate spectral overlap between components, absorption spectra of pure BSBCz films (50 nm, black) and composite films BSBCz:MoO3 and BSBCz:Cs (1:1 molar ratio, 50 nm; blue and red, respectively) were measured. Absorption spectra were measured using an absorption spectrometer (Lamda950, PerkinElmer). Steady-state PL spectra of pure BSBCz films (green) and representative laser emission spectra (orange) from optically pumped OSLs are also shown to indicate that polaron absorption in BSBCz-type OSLDs can be neglected.

[0096] Figure 11 Optical and electrical simulations. a) Experimental (symbols) and simulation (solid lines) JV curves for devices with only holes (blue circles), devices with only electrons (red squares), and bipolar devices (black triangles). (Compared with...) Figure 4 (a) Model parameters are extracted by fitting the unipolar device in the model, and these parameters are used to simulate the bipolar device. (b) The mobility calculated using the parameters extracted from the unipolar device (solid line) is known. 41 Comparison of the mobilities (symbols) of holes (blue) and electrons (red) in BSBCz. c) Experimental (symbol) and simulated (solid line) JV curves of OSLD. d) Schematic diagram of the OSLD structure used for calculation. e) At J = 500 mA / cm -2 Spatial distribution of recombination rate R in OSLD. f) Cross section of DFB device at y = 0.11 μm (e). g) Average exciton density as a function of current density in OSLD and OLED.

[0097] Figure 12 Simulation of OSLD. a) Spatial distribution S of exciton density. b) Electric field distribution of a passive DFB resonator at a resonant wavelength λ0 = 483 nm, extending to a structure including the first-order region. c) Modal gain as a function of current density. d) At J = 500 A / cm -2 The exciton density S(x,y) of a period in the second-order region and the optical mode |E(x,y)| 2 Spatial overlap between them. Model layers other than the raster as flat (see reference). Figure 11 d), and y = 0 corresponds to the boundary of BSBCz / MoO3. Detailed Implementation

[0098] The present invention will now be described in detail. The constituent elements are described below with reference to representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and examples. In this specification, the numerical range represented by "X to Y" indicates a range that includes the values ​​X and Y as the lower limit and upper limit, respectively.

[0099] All references to this document and the contents of PCT / JP2017 / 033366 are incorporated herein by reference.

[0100] The current-injection OSLD of the present invention includes at least one pair of electrodes, an optical resonator structure, and one or more organic layers comprising an optical amplification layer composed of organic semiconductors. The current-injection OSLD of the present invention has a structure in which the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode during current injection is sufficient to emit laser light. "A structure in which the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode during current injection is sufficient to emit laser light" is a structure capable of generating laser oscillation, and implies the selection and combination of materials and structures described below.

[0101] The structure and features of the present invention will be described in detail below.

[0102] (Optical magnification layer)

[0103] The light amplification layer constituting the current-injected OSLD of the present invention comprises an organic semiconductor compound containing carbon atoms but no metal atoms. The organic semiconductor compound preferably consists of one or more atoms selected from carbon, hydrogen, nitrogen, oxygen, sulfur, phosphorus, and boron atoms. For example, an organic semiconductor compound composed of carbon, hydrogen, and nitrogen atoms can be cited. Preferred examples of the organic semiconductor compound are compounds having at least one stilbene unit and a carbazole unit, and more preferred examples are compounds having both stilbene and carbazole units. The stilbene and carbazole units may be substituted with substituents such as alkyl groups, or may be unsubstituted. The organic semiconductor compound is preferably a non-polymer without repeating units. The molecular weight of the compound is preferably less than 1000, for example, less than 750. The light amplification layer may contain two or more organic semiconductor compounds, but preferably only one.

[0104] The organic semiconductor compound used in this invention can be selected from a laser-gain organic semiconductor compound that can generate laser oscillation when used as an organic light-emitting layer in a photoexcited organic semiconductor laser. One of the most preferred organic semiconductor compounds is 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) (see reference). Figure 1 (chemical structure in a) 15Because its combination possesses excellent optical and electrical properties, such as a threshold of amplified spontaneous emission (ASE) in thin films (0.30 μJ / cm² under 800 ps pulsed light excitation). -2 ) 16 Low, and with maximum external quantum efficiency (η) of electroluminescence (EL) EQE In over 2% of OLEDs, operation under a 5μs pulse can withstand up to 2.8kA / cm. -2 Injection of current density 13 Furthermore, lasers excited by high repetition rates of 80 MHz and long pulses of 30 ms have recently been used in optically pumped BSBCz-type DFB lasers. 17 This has been confirmed, likely because the triple absorption loss at the laser emission wavelength of the BSBCz film is extremely small. Besides BSBCz, compounds with the following ASE threshold, preferably 0.60 μJ / cm², can also be used, for example, when forming the same film as in Reference 16 and measured under 800 ps pulsed light excitation conditions. -2 The following is more preferably 0.50 μJ / cm. -2 Hereinafter, 0.40 μJ / cm is further preferred. -2 The following can also be used. A compound exhibiting the following durability when formed into a device identical to that in Document 13 and measured under 5 μs pulse operating conditions is preferably 1.5 kA / cm. -2 The above is preferred, and more preferably is 2.0 kA / cm. -2 The above is further optimized to 2.5 kA / cm. -2 above.

[0105] The thickness of the optical amplification layer constituting the current-injected OSLD of the present invention is preferably 80 to 350 nm, more preferably 100 to 300 nm, and even more preferably 150 to 250 nm.

[0106] (Other layers)

[0107] In addition to the optical amplification layer, the current-injection OSLD of the present invention may have an electron injection layer, a hole injection layer, and other layers. These may be organic layers or inorganic layers without organic materials. When the current-injection OSLD has two or more organic layers, it is preferable to have a laminated structure with only organic layers and no non-organic layers. In this case, the two or more organic layers may contain the same organic compound as the organic compound in the optical amplification layer. The fewer the number of heterointerfaces in the organic layers, the better the performance of the current-injection OSLD; therefore, the number of organic layers is preferably 3 or less, more preferably 2 or less, and most preferably 1. When the current-injection OSLD has two or more organic layers, the thickness of the optical amplification layer is preferably greater than 50% of the total thickness of the organic layers, more preferably greater than 60%, and even more preferably greater than 70%. When the current-injection OSLD has two or more organic layers, the total thickness of the organic layers may, for example, be 100 nm or more, 120 nm or more, or 170 nm or more, and may be 370 nm or less, 320 nm or less, or 270 nm or less. The refractive indices of the electron injection layer and the hole injection layer are preferably less than the refractive index of the optical amplification layer.

[0108] In the case of an electron injection layer, a substance that promotes electron injection into the optical amplification layer is added to the electron injection layer. In the case of holes, a substance that promotes hole injection into the optical amplification layer is added to the hole injection layer. These substances can be organic compounds or inorganic substances. For example, inorganic substances used in the electron injection layer include alkali metals such as Cs, and their concentration in the electron injection layer containing organic compounds can, for example, be greater than 1 wt% or 5 wt% or more, or 10 wt% or more, and can be less than 40 wt% or 30 wt%. The thickness of the electron injection layer can, for example, be greater than 3 nm, greater than 10 nm, or greater than 30 nm, and can be less than 100 nm, less than 80 nm, or less than 60 nm.

[0109] As a preferred embodiment of the present invention, a current-injection OSLD can be described, in which the electron injection layer and the optical amplification layer are organic layers, and the hole injection layer is an inorganic layer. The inorganic material constituting the hole injection layer includes metal oxides such as molybdenum oxide. The thickness of the hole injection layer can be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and can be less than 100 nm, less than 50 nm, or less than 20 nm.

[0110] (Optical resonator structure)

[0111] The current-injection OSLD of the present invention has an optical resonator structure. The optical resonator structure can be a one-dimensional resonator structure or a two-dimensional resonator structure. Examples of two-dimensional resonator structures include a ring resonator structure and a whispering-gallery mode optical resonator structure. Distributed feedback (DFB) structures and distributed Bragg reflector (DBR) structures can also be used. Regarding DFB, a hybrid-level DFB grating structure is preferred. That is, a hybrid structure of DFB grating structures with different orders relative to the laser emission wavelength is preferred. Specific examples include an optical resonator structure consisting of a second-order Bragg scattering region surrounded by a first-order Bragg scattering region, and a hybrid structure formed by alternating second-order and first-order scattering regions. For details regarding the preferred optical resonator structure, please refer to the following specific embodiments. The current-injection OSLD can also have an external optical resonator structure as the optical resonator structure.

[0112] For example, the optical resonator structure can preferably be formed on the electrodes. The materials constituting the optical resonator structure include insulating materials such as SiO2. For example, a grating structure is formed such that the depth of the grating is preferably less than 75 nm, more preferably selected from the range of 10 to 75 nm. The depth can be, for example, more than 40 nm, or less than 40 nm.

[0113] (electrode)

[0114] The current-injection OSLD of the present invention has a pair of electrodes. For light output, one electrode is preferably transparent. Regarding the electrode, electrode materials commonly used in the art can be appropriately selected considering their work function, etc. Preferred electrode materials include Ag, Al, Au, Cu, ITO, etc., but are not limited thereto.

[0115] (Preferred current-injection OSLD)

[0116] In current-injected OSLDs, excitons are generated by current excitation. The laser oscillation characteristics of the current-injected OSLD are improved by increasing the overlap between the distribution region of the generated exciton density and the electric field intensity distribution of the resonant optical mode. That is, laser oscillation characteristics can be improved when the exciton density overlaps with the optical resonant mode of the optical resonator structure. The exciton density distribution and the electric field intensity distribution of the resonant optical mode can be controlled by changing the structure and materials of the current-injected OSLD. For example, the distribution can be controlled by using a current-narrowing structure with a grating, etc., and by controlling the depth and period of the grating. The distribution can also be controlled by specifying or controlling the material and thickness of the optical amplification layer, and if an electron injection layer and a hole injection layer are present, their materials and thicknesses can also be specified or controlled. Furthermore, more precise distribution control can be achieved by considering the conditions employed in the simulation algorithm below. A preferred current-injected OSLD has an overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode during current injection that is greater than or equal to the overlap shown in the specific current-injected OSLD in the following embodiment.

[0117] In the current-injection OSLD of the present invention, the ratio of electron mobility to hole mobility in the organic optical gain layer is preferably controlled in the range of 1 / 10 to 10 / 1, more preferably in the range of 1 / 5 to 5 / 1, even more preferably in the range of 1 / 3 to 3 / 1, and even more preferably in the range of 1 / 2 to 2 / 1. By controlling the ratio within this range, the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode can be easily expanded.

[0118] In the current-injection OSLD of the present invention, the excitons generated by current excitation are preferably substantially non-annihilated. The exciton annihilation loss is preferably less than 10%, more preferably less than 5%, even more preferably less than 1%, further preferably less than 0.1%, even more preferably less than 0.01%, and most preferably 0%.

[0119] The current-injection OSLD of the present invention preferably has substantially no polaron absorption loss at the laser emission wavelength. In other words, there is preferably no substantial overlap between the polaron absorption spectrum and the emission spectrum of the organic semiconductor laser. The polaron absorption loss is preferably less than 10%, more preferably less than 5%, even more preferably less than 1%, further preferably less than 0.1%, even more preferably less than 0.01%, and most preferably 0%.

[0120] The oscillation wavelength of the current-injected OSLD of the present invention preferably does not substantially overlap with the absorption wavelength regions of excited states, free radical cations, or free radical anions. Absorption in these states may be caused by singlet-singlet, triplet-triplet, or polaron absorption. The absorption loss in the excited state is preferably less than 10%, more preferably less than 5%, even more preferably less than 1%, further preferably less than 0.1%, even more preferably less than 0.01%, and most preferably 0%.

[0121] The current-injection OSLD of the present invention preferably does not contain a triplet quencher.

[0122] (Method for fabricating an current-injection OSLD)

[0123] This invention also provides a method for manufacturing a current-injected OSLD, which designs and manufactures the OSLD to increase the overlap between the exciton density distribution generated by current excitation and the electric field intensity distribution of the resonant optical mode. During the design process, simulations are performed under various conditions, such as the depth and period of the grating, and the constituent materials and thicknesses of the optical amplification layer, electron injection layer, and hole injection layer, thereby evaluating the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode. As a result of the simulations under various conditions, a product that does not present manufacturing problems is selected from those evaluated as having increased overlap, and this selected product can be actually manufactured. Therefore, a current-injected OSLD with excellent laser oscillation characteristics can be effectively provided.

[0124] In the above design, a design program for current-injected OSLDs can be pre-created and used. This program has the function of expanding the overlap between the distribution of excitons generated by current excitation and the electric field intensity distribution of the resonant optical mode. This program can be stored on media such as hard disks and optical discs.

[0125] Furthermore, the present invention provides a method for improving the laser oscillation characteristics of a designed or existing current-injected OSLD. The method evaluates the overlap between the exciton density distribution and the electric field intensity distribution of the resonant optical mode in the designed or existing current-injected OSLD using a simulation algorithm. Even if the materials and structures of the overlapping distributions change, the same simulation algorithm is used for calculation. Therefore, it is possible to provide a current-injected OSLD with improved laser oscillation characteristics.

[0126] Preferred Implementation

[0127] The following will refer to Figure 1 The preferred embodiments shown in Figure a illustrate the present invention in detail. However, the scope of the present invention should not be limited by the following detailed description.

[0128] The performance of optically pumped organic semiconductor lasers (OSLs) has improved significantly over the past two decades, thanks to major advances in the development of high-gain organic semiconductor materials and the design of high-quality factor resonator structures. 1-5 Organic semiconductors offer advantages as laser gain media, including high photoluminescence (PL) quantum yield, large stimulated emission cross-section, wide emission spectrum in the visible region, and their chemical tunability and ease of fabrication. Recent advancements in low-threshold distributed feedback (DFB) OSLs have enabled optical pumping via electrically driven nanosecond pulsed inorganic light-emitting diodes, providing a pathway for novel compact, low-cost visible laser technologies. 6 However, the ultimate goal remains electrically driven organic semiconductor laser diodes (OSLDs). The realization of OSLDs will not only enable the complete integration of organic photonics and optoelectronic circuits, but will also open up new applications in spectroscopy, displays, medical devices (such as retinal displays, sensors, and photodynamic therapy devices), and LiFi telecommunications.

[0129] The main problems hindering lasing via direct electric pumping of organic semiconductors are optical losses from the electrical contacts and triplet and polaron losses occurring at high current densities. 4，5,7-9 As a method to address these fundamental loss problems, the following has been proposed: using a triplet quencher. 10-12 To suppress triplet absorption loss, singlet quenching based on singlet-triplet exciton annihilation, and reduce the active region of the device. 13 The aim is to spatially separate the exciton formation region and the region where exciton radiative decay occurs, and to minimize the polaron quenching process. However, although this has already been achieved in organic light-emitting diodes (OLEDs) and optically pumped organic semiconductor DFB lasers... 5 Progress has been made, but current-injection OSLDs have not yet been definitively verified.

[0130] Previous studies have shown that if the additional losses associated with electric pumping are to be completely suppressed, it may require more than several kA / cm. 2 Only with a current density that can OSLD laser emission be achieved 14 The molecule most likely to achieve OSLD is 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) (see reference). Figure 1 (chemical structure in a) 15 Because its combination possesses excellent optical and electrical properties, such as a threshold of amplified spontaneous emission (ASE) in thin films (0.30 μJ / cm² under 800 ps pulsed light excitation). -2 ) 16 Low, and with maximum external quantum efficiency (η) of electroluminescence (EL) EQEIn over 2% of OLEDs, operation under a 5μs pulse can withstand up to 2.8kA / cm. -2 Injection of current density 13 Furthermore, lasers excited by high repetition rates of 80 MHz and long pulses of 30 ms have recently been used in optically pumped BSBCz-type DFB lasers. 17 This has been confirmed, likely because the triple absorption loss at the laser emission wavelength of the BSBCz film is extremely small. Here, the inventors provide a complete characterization of a first embodiment of laser emission originating from a directly electrically excited organic semiconductor film and an inverted OLED structure with a DFB SiO2 grating integrated in the active region of the device, representing a BSBCz thin-film OSLD.

[0131] The structure and manufacturing process of the OSLD developed in this study are schematically shown below. Figure 1 a and Figure 2 (Detailed explanation of the experimental procedure, reference materials, and methods). A mixed-order DFB grating with an area of ​​30 × 90 μm was formed by etching a SiO2 sputtered layer on an indium tin oxide (ITO) glass substrate using electron beam lithography and reactive ion etching. Figure 1 b) An organic layer and a metal cathode are vacuum-deposited on the substrate to complete the device. The hybrid-order DFB grating designed by the inventors has first-order and second-order Bragg scattering regions that respectively provide strong optical feedback for laser emission and effective vertical output coupling. 17，18 Based on the Prague condition 4，19 mλ Bragg =2n eff Λ m Grating periods of 140 nm and 280 nm (Λ1 and Λ2) were selected for the first-order and second-order regions, respectively, where m is the diffraction order and λ is the lattice order. Bragg It is the Bragg wavelength, which is set as the maximum gain wavelength (477nm) in the well-known BSBCz. eff This is the effective refractive index of the gain medium, which is 1.70 in BSBCz. 20，21 The first group of devices (hereinafter referred to as OSLD) is characterized by individual length domains of 1.12 μm and 1.68 μm in the first-order and second-order DFB grating regions, respectively.

[0132] Figure 1 Scanning electron microscope (SEM) images in Figures c and d show that the fabricated DFB grating has a period of 140 ± 5 nm and 280 ± 5 nm, respectively, and a grating depth of approximately 65 ± 5 nm. Complete removal of the SiO2 layer in the etched area to expose the ITO is important for good electrical contact with the organic layer, and energy-dispersive X-ray spectroscopy (EDX) analysis was performed. Figure 2c) and d) were verified. SEM and EDX cross-sectional images of the complete OSLD are shown below. Figure 1 Figures d and e. The surface morphology of all layers exhibits a grating structure with a surface modulation depth of 50–60 nm. Although the interaction between the resonant laser mode and the electrode may reduce the quality factor of the feedback structure, this type of grating structure on the metal electrode also reduces the absorption loss of the mode introduced into the device. 22，23 .

[0133] The OSLD manufactured in this invention has the following characteristics: Figure 3 A simple inverted OLED structure with the energy levels shown in Figure a: ITO (100nm) / 20wt.%Cs:BSBCz (60nm) / BSBCz (150nm) / MoO3 (10nm) / Ag (10nm) / Al (90nm) (operational example). If Cs is doped into the BSBCz film near the ITO contact, electron injection into the organic layer is improved, and MoO3 is used as a hole injection layer. Figure 4 While the most efficient OLEDs typically employ multilayer structures to optimize charge balance24,25, charge can accumulate at organic heterojunctions26 under high current densities, which can potentially negatively impact device performance and stability. The OSLD fabricated in this invention contains only BSBCz as an organic semiconductor layer (light amplification layer) and is specifically designed to minimize the number of organic heterojunctions. A reference device (hereinafter referred to as an OLED) without a SiO2DFB grating was also fabricated in this invention to investigate the effect of the grating on EL performance.

[0134] Figure 3 b shows optical micrographs of the OSLD and reference OLED under DC operation at 3.0V. In addition to the aforementioned DFB gratings, five other DFB grating geometries (Table 1) were optimized and characterized in the OSLD. Although EL emission from the active region of the reference OLED is uniform, it can be observed that EL originates from the second-order DFB grating region in the OSLD, which is specifically designed to promote vertical light output coupling. Figure 3 b and Figure 5 A stronger launch. Figure 2 c and Figure 2 d shows the current density-voltage (JV) and η of OSLD and OLED under pulsed conditions (voltage pulse width 400 ns and repetition frequency 1 kHz) at ambient temperature. EQE -J feature, and in Figure 6The characteristics obtained under DC conditions are shown. While some current flows through the region above the SiO2 grating (approximately 20% according to simulations), the majority of the current flows through the region above the exposed ITO. For simplicity and consistency, the exposed ITO area was used in the current density calculations for all OSLDs, although this may result in a slight overestimation.

[0135] [Table 1] Parameters of different OSLD geometries

[0136]

[0137] Figure 2 The values ​​for the parameters for different grating geometries are shown, along with the total exposed ITO area A used to calculate the current density. OSL is a 200 nm thick BSBCz layer deposited on the grating on a fused silica substrate, excluding the contacts.

[0138] The maximum current density prior to breakdown of the reference OLED device was 6.6 A / cm² under DC operation. -2 Rise to 5.7 kA / cm under pulse operation -2 This is because the Joule heating under pulse operation is reduced. 13，27 Under DC operation, all devices exhibit a maximum η greater than 2% at low current densities. EQE And above 1A / cm -2 The high current density exhibits a strong efficiency roll-off, which may be due to thermal degradation of the device. On the other hand, the efficiency roll-off in OLEDs under pulsed operation ( Figure 3 c. Figure 3 d) Starting from above 110A / cm -2 The current density is consistent with previous reports. 13 Under pulsed operation, the efficiency roll-off in the OSLD is further suppressed, and η EQE Even a substantial increase of more than 200 A / cm -2 And it reaches a maximum value of 2.9%. η EQE Above 2.2 kA / cm -2 The rapid drop in current density may be due to thermal degradation of the device.

[0139] Although the EL spectrum of this OLED is similar to the steady-state PL spectrum of the impurity-free BSBCz film ( Figure 6 c) and does not change as a function of current density, but the EL spectrum from the glass surface of the OSLD shows narrowing of the spectral lines and an increase in current density under pulsed operation. Figure 7 a). Below 650 A / cm -2 The current density was observed at 478.0 nm, corresponding to the Bragg tilt angle of the stopband of the DFB grating. Figure 7(b) As the current density increases and exceeds this value, the spectral line narrows sharply at 480.3 nm, indicating the onset of laser emission. The intensity of the narrow emission peak is found to increase faster than the EL emission background, which may be related to the nonlinearity of stimulated emission.

[0140] The output intensity and full width at half maximum (FWHM) of the OSLD are set in... Figure 7 The output intensity is plotted as a function of current in c. While the steady-state PL spectrum of the impurity-free BSBCz film has a free current-weighted intensity (FWHM) of approximately 35 nm, the FWHM of the OSLD decreases to below 0.2 nm at high current densities, approaching the limit of the spectral resolution of the spectrometer used in this invention (0.17 nm for a wavelength range of 57 nm). The slope efficiency of the output intensity changes abruptly with increasing current and can be used to determine 600 A / cm². -2 The threshold is 8.1 mA. Above 4.0 kA / cm². -2 At this point, the output intensity decreases with increasing current, possibly due to the sharp temperature rise that leads to device breakdown, but the emission spectrum remains sharp. The aforementioned increase and subsequent decrease are related to η. EQE -J curve consistent. At a distance from the ITO glass substrate ( Figure 7 d) The maximum output power measured by the power meter placed 3cm in front of the OSLD is 3.3kA / cm. -2 The current density is 0.50 mW. These observed EL characteristics strongly suggest that optical amplification occurs at high current densities, and that electrically driven laser emission is achieved when the current density exceeds a threshold.

[0141] The beam polarization and shape were characterized to further demonstrate that this was laser emission. 9 The output beam of the OSLD follows the grating pattern expected in laser emission from a one-dimensional DFB. Figure 8 a) Perform strong linear polarization. Focus the spatial profile of the OSLD emission ( Figure 8 b and Figure 9 a) Clearly demonstrate the existence of a Gaussian beam with a diameter of approximately 0.1 mm. Figure 8 c) and demonstrated that the output beam from the OSLD exhibits excellent focusing properties above the laser emission threshold. Projecting the beam onto the screen produces a fan-shaped pattern that can be obtained in a one-dimensional DFB. Figure 8 d、 Figure 8 e). Rapid degradation above the threshold prevents interferometric measurements at this stage, but the inventors derived the formula L = λ peak 2 / FWHM estimates the coherence length (L) to be 1.1-1.3 mm in all devices of this invention, which is consistent with that of lasers, where λ peakIt is the peak wavelength. It degrades more slowly under photoexcitation, and the near-field beam pattern of similar device structures is independent of the presence or absence of electrodes. Figure 9 b、 Figure 9 c) is similar, which further indicates that the device can emit laser light. Furthermore, the far-field pattern under optical excitation is also consistent with laser emission. Figure 9 d).

[0142] Before an inventor claims rights to laser emission, the cause of the observation must exclude several phenomena that have been mistaken for laser emission in the past. 9 The emission from the OSLD of this invention is detected in a direction perpendicular to the substrate plane and exhibits distinct threshold behavior, thus the line narrowing starting from the edge emission of the wavelength-guided mode without optical amplification can be ignored. 20,28,29 While ASE appears similar to laser emission, the FWHM (<0.2 nm) in the OSLD of this invention is much narrower than the typical ASE linewidth of organic thin films (several nm), and is consistent with the typical FWHM (<1 nm) of optically pumped organic DFB lasers. 5 The very narrow emission spectrum obtained by accidentally stimulating atomic transitions in ITO was also mistakenly attributed to emission from the organic layer. 30 .However, Figure 7 The peak emission wavelength of OSLD a is 480.3 nm, therefore the emission could not originate from ITO, which has atomic spectral lines at 410.3, 451.3, and 468.5 nm. 31 .

[0143] If the laser emission is indeed from the DFB structure, the OSLD emission should exhibit resonator mode characteristics, and the output should be highly sensitive to any changes in the laser resonator. Therefore, OSLDs with different DFB geometries were fabricated and characterized, labeled OSLD-1 to OSLD-5 (Table 1). Figure 5 This confirms that the emission wavelength can be tuned as expected (common in optically pumped organic DFB lasers). 4,5,32,33 OSLDs, OSLD-1, OSLD-2, and OSLD-3 (480.3 nm, 479.6 nm, 480.5 nm, and 478.5 nm, respectively), with the same DFB grating period, exhibit almost identical laser emission peaks. Furthermore, OSLD-1, OSLD-2, and OSLD-3 all possess low minimum FWHM (0.20 nm, 0.20 nm, and 0.21 nm, respectively) and well-defined thresholds (1.2 kA / cm², respectively). -2 0.8kA / cm -2 and 1.1 kA / cm -2On the other hand, OSLD-4 and OSLD-5, with different DFB grating periods, achieved high performance at 459.0 nm, FWHM 0.25 nm, and threshold of 1.2 kA / cm, respectively. -2 (OSLD-4) and 501.7nm, FWHM 0.38nm and threshold 1.4kA / cm -2 (OSLD-5) shows the laser peak value. These results demonstrate that the laser emission wavelength is controlled by the DFB geometry.

[0144] To verify that the laser emission threshold of the electrically driven OSLD is consistent with that obtained by optical pumping, the laser characteristics of the optically pumped OSLD (OLSD-6) were measured through the ITO side using an N2 laser emitting 3.0 ns pulses (laser emission wavelength of 337 nm). Figure 9 e Figure 9 f). The laser peak value (481 nm) of the optically pumped OLSD-6 is consistent with that of the electrically pumped OSLD (480.3 nm). When only the power coupled to the device is considered (approximately 18% according to simulation), the measured laser threshold under optical pumping is approximately 77 Wcm. -2 Optimizing the layer thickness to minimize light loss due to the electrodes yields results comparable to never having two electrodes (30Wcm). -2 ) 17 The threshold gain obtained by optically pumped BSBCz-type DFB lasers is relatively small compared to the threshold increase. Assuming that OSLD-6 has no additional loss mechanism at high current densities, the threshold gain at 0.3 kA / cm² under optical pumping can be predicted from the threshold gain under optical pumping. 2 The laser threshold (detailed algorithm reference materials and methods). Therefore, the thresholds of OSLD and OSLD-2 (with the same grating period as OSLD-6) are higher than 0.6–0.8 kA / cm. -2 It is reasonable to observe laser emission under electric pumping at the same time. Furthermore, the slope efficiency ( Figure 9 g~ Figure 9 i) Similar under optical pumping and electric pumping (0.4% and 0.3%, respectively), but significantly higher in optically pumped devices without electrodes (6%).

[0145] These results indicate that, typically at high current densities 34 The additional losses occurring in OLEDs (including exciton annihilation, triplet and polaron absorption, high-field quenching, and Joule heating) are largely suppressed in BSBCz OSLDs. This is entirely consistent with the fact that no EL efficiency roll-off was observed in OSLDs under strong pulsed electrical excitation. The suppression of losses can be explained by the characteristics of BSBCz and the device. As mentioned above, the BSBCz film does not exhibit significant triplet losses ( Figure 10 a)35 Furthermore, reducing the active region of the device will decrease Joule-heat-assisted exciton quenching. 36 Based on measurements of the composite films of BSBCz:MoO3 and BSBCz:CS, the overlap between polaron absorption and emission spectra can be neglected for both radical cations and anions in BSBCz. Figure 10 b). In addition, although metal loss is a major problem in OLED structures, the DFB structure in the OSLD of the present invention reduces this loss by keeping the light away from the metal.

[0146] Electrical and optical simulations of the device were performed to further confirm current-injected laser emission in the OSLD. Figure 11 Using unipolar devices ( Figure 11 a, Figure 11 The carrier mobility extracted from the fitting of the experimental data in b) is consistent with the simulated and experimental characteristics. Figure 11 a, Figure 11 c. Figure 11 d) JV curves for devices with and without gratings, showing sufficient etching with good electrical contact with ITO in devices with gratings. Recombination rate distribution ( Figure 11 e Figure 11 f) represents the periodic variation within the device due to the periodic injection of electrons from the ITO electrode through the insulating SiO2 grating. Similar to recombination, the exciton density (S) is distributed throughout the thickness of the organic layer ( Figure 12 a) but mainly concentrated in regions where SiO2 does not obstruct the path from cathode to anode. OSLD and OLED ( Figure 11 The average exciton density is similar to that of the reference device, indicating that the high accumulation of excitons near SiO2 compensates for the low exciton density between the gratings (without injection regions).

[0147] The light output from the second-order grating is coupled and trapped in the ITO layer that forms the wavelength-guided loss. The light is captured at the calculated resonant wavelength λ0 = 483 nm by the OSLD and is clearly visible in the simulated electric field distribution E(x,y) of the optical field. Figure 12 b). The DFB resonator is characterized by a constraint factor Γ of 40% and a quality factor of 255, which is consistent with the use of equation λ. peak / FWHM from Figure 7 The calculated quality factor of 204 is consistent with that of b. The modal gain (g), which is an indicator of optical amplification in the laser mode, is... m The current density function is calculated from the overlap of the exciton density distribution and the optical field distribution (see Materials and Methods for details), where BSBCz 35 stimulated emission cross section σstim 2.8×10 - 16 cm 2 And in Figure 12 The second-order region is shown in c. (Above 500 A / cm) -2 The high modal gain and increased modal gain are consistent with observations of laser emission. The insulating DFB structure transmits through strong optical modes ( Figure 12 b) The localization of high exciton density inside and above the grating valley helps enhance coupling with the optical mode. Figure 12 a) The result obtained Figure 12 The value of J = 500 A / cm is shown in d. -2 High values.

[0148] In summary, this invention demonstrates that laser emission from current-driven organic semiconductors can be achieved by suppressing losses and enhancing coupling through appropriate design and selection of resonators and organic semiconductors. The laser emission described in this invention has been reproduced in multiple devices and has been thoroughly characterized to rule out other phenomena that might be mistaken for laser emission. This result provides strong evidence for the claim that this invention represents the first discovery of electrically pumped laser emission in organic semiconductors. The low loss of BSBCz is indispensable for laser emission; therefore, developing strategies for designing new laser molecules with similar or improved properties is the next important step. This invention creates new opportunities for organic photonics and lays the foundation for the future development of organic semiconductor laser diode technology, which is simple, inexpensive, tunable, and enables fully and directly integrated organic optoelectronic platforms.

[0149] Materials and methods

[0150] (Device manufacturing)

[0151] An indium tin oxide (ITO) coated glass substrate (100 nm thick ITO, Atsugi Micro Co.) was ultrasonically cleaned using neutral detergent, pure water, acetone, and isopropanol, followed by UV-ozone treatment. A 100 nm thick SiO2 layer, which will become a DFB grating, was sputtered onto the ITO coated glass substrate at 100 °C. The argon pressure during sputtering was 0.66 Pa. The RF power was set to 100 W. The substrate was cleaned again with isopropanol and ultrasonically, followed by UV-ozone treatment. The SiO2 surface was treated with hexamethyldisilazane (HMDS) as follows: spin-coated at 4,000 rpm for 15 seconds and annealed at 120 °C for 120 seconds. A resist layer with a thickness of approximately 70 nm, formed from ZEP520A-7 solution (ZEON Co.), was spin-coated onto the substrate at 4,000 rpm for 30 seconds and baked at 180 °C for 240 seconds.

[0152] Using the JBX-5500SC system (JEOL), at 0.1 nCcm -2 Electron beam lithography was used to create a grating pattern on the resist layer using the optimal dosage. After electron beam irradiation, the pattern was developed at room temperature with a developer solution (ZED-N50, ZEON Co.). The patterned resist layer was used as an etching mask, and the substrate was plasma etched with CHF3 using an EIS-200ERT etching system (ELIONIX). To completely remove the resist layer from the substrate, plasma etching with O2 was performed using a FA-1EA etching system (SAMCO). The etching conditions were optimized to completely remove SiO2 from the grooves in the DFB until ITO was exposed. The grating formed on the SiO2 surface was observed using SEM (SU8000, Hitachi., Ltd.). Figure 1 c). EDX analysis (at 6.0 kV, SU8000, Hitachi., Ltd.) was performed to confirm the complete removal of SiO2 from the DFB trenches. Figure 2 c. Figure 2 d). Cross-sections were measured using a cold field emission SEM (SU8200, Hitachi High-Technologies), an energy-dispersive X-ray spectrometer (XFlash FladQuad5060, Bruker Japan kk), and a focused ion beam system (FB-2100, Hitachi High-Technologies kk), and were obtained from SEM and EDX measurements by Kobe Steel, Ltd. Figure 1 d, Figure 1 e).

[0153] The DFB substrate was cleaned using conventional ultrasonic cleaning. Then, it was cleaned at 1.5 × 10⁻⁶. -4 Under a pressure of Pa, the total evaporation rate on the substrate is 0.1-0.2 nms. -1 Vacuum deposition of the organic layer and metal electrode via thermal evaporation resulted in an OSLD with an indium tin oxide (ITO) (100 nm) / 20 wt% BSBCz:Cs (60 nm) / BSBCz (150 nm) / MoO3 (10 nm) / Ag (10 nm) / Al (90 nm) structure. In addition to the DFB grating, the SiO2 layer on the ITO surface also acts as an insulator. Therefore, the current flow region of the OLED is limited to the DFB region where BSBCz and ITO are in direct contact. A reference OLED with an active region of 30 × 45 μm and the same current flow region was also fabricated.

[0154] (Device Characterization)

[0155] All devices were encapsulated in a nitrogen-filled glove box with glass covers and UV-cured epoxy resin to prevent any degradation caused by moisture and oxygen. The current density-voltage-η of the OSLD and OLED were measured at room temperature using an integrating sphere system (A10094, Hamamatsu Photonics kk). EQE (JV-η EQE DC characteristics. To measure the pulse, a pulse generator (NF, WF1945) was used to apply a rectangular pulse with a pulse width of 400 ns, a pulse period of 1 ms, a repetition frequency of 1 kHz, and a varying peak current to the device at ambient temperature. Using these conditions, the inventors could achieve a current of 1 kA / cm² before electrical breakdown. -2 (Approaching the threshold) 50 pulses, at 2kA / cm -2 The pulse duration is 20 pulses, at 3kA / cm -2 Ten pulses were applied to a normally operating OSLD. Approximately 500 devices were fabricated in this work, with a yield of about 5%. The JV-luminance characteristics under pulsed drive were measured using an amplifier (NF, HSA4101) and a photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics kk). The PMT response and the driving square wave signal were monitored on a multi-channel oscilloscope (Agilent Technologies, MSO6104A). η was calculated by dividing the number of photons calculated using the PMT-response EL intensity and correction factor by the number of injected electrons calculated using the current. EQE The output power was measured using a laser power meter (OPHIR Optronics Solution Ltd., StarLite 7Z01565).

[0156] To measure the spectrum, a multi-channel spectrometer (PMA-50, Hamamatsu Photonics kk) connected to an optical fiber and positioned 3 cm from the device collected the emitted laser light from both optically and electrically pumped OSLDs perpendicular to the device surface. The beam profile of the OSLD was examined using a CCD camera (WimCamD-LCM, DataRay). To determine the characteristics of the optically pumped OSLD-6 and OSL, pulsed excitation light from a nitrogen laser (NL100, N2 laser, Stanford Research System) was focused onto a 6 × 10⁻⁶ Ω·cm area of ​​the device through a lens and slit. -3 cm 2 The region was used. The excitation wavelength was 337 nm, the pulse width was 3 ns, and the repetition frequency was 20 Hz. The excitation light was incident on the device at approximately 20° relative to the normal to the device plane. A set of neutral density filters was used to control the excitation intensity. Figure 10 The spectrophotometer (FP-6500, JASCO Corporation) and Figure 6 Steady-state PL spectra were monitored using a spectrometer (PMA-50). Near-field patterns of OSL and OSLD-6 were captured using a laser beam analyzer (C9334-01, Hamamatsu Photonics) with near-field optics (A4859-06), and far-field patterns of OSL were captured using the same laser beam analyzer and near-field optics (A3267-11, Hamamatsu Photonics).

[0157] The lower limit of the electric laser threshold is determined from the optical threshold using the following equation.

[0158]

[0159] Among them, P th , λ, h, c, η out φ PL η EQE Let and e represent the optical pump threshold and wavelength, Planck's constant, the speed of light, the device output coupling efficiency, the photoluminescence quantum yield of BSBCz, the external quantum efficiency of BSBCz-type OSLDs, and the fundamental charge, respectively. The equation can be easily solved to show that the singlet rate generated under electrical excitation should be equal to that under P... th The singlet state rate is the same under photoexcitation at the same current density. This equation does not consider the additional loss mechanisms that occur under electroexcitation at high current densities. η is used in this invention. out 20% and φ PL 76% (Table 2). Inventor iterations in Figure 3 Several values ​​in d are in η EQE The optimal consistency between J and η, and ultimately η EQE The percentage is set at 2.1%. The second factor is to illustrate the fact that the inventors used only half of the exposed ITO area of ​​the total grating area when calculating the current density of the OSLD in this document.

[0160] (Device modeling and parameters)

[0161] Optical simulations of the resonant DFB cavity were performed using Comsol Multiphysics 5.2a software. The finite element method (FEM) in the RF module of Comsol was used to obtain solutions to the Helmholtz equations for each frequency. Each layer is represented by its complex refractive index and thickness. The computational domain was limited to a supercell consisting of a second-order grating surrounded by a first-order grating. Floquet periodic boundary conditions were applied to the transverse boundaries, while scattering boundary conditions were applied to the top and bottom domains. Only the TE mode was considered because the TM mode is blocked, as the TM mode suffers more loss than the TE mode (due to metal absorption).

[0162] Using Silvaco's Technology Computer Aided Design (TCAD) software, charge transport through the OSLD was described using a two-dimensional, time-independent drift-diffusion equation coupled to the Poisson equation and a continuity equation for charge carriers. Electron and hole concentrations were represented using a parabolic density of states (DOS) and Maxwell-Boltzmann statistics. A Gaussian distribution was used to model the trap distribution within the organic semiconductor. 37 Charge carrier mobility is considered to be field-dependent and has a Pool-Frenkel form. 38，39 In this model, energy disturbance is not considered; therefore, the inventors assume the validity of the Einstein relation and calculate the charge carrier diffusion coefficient from the charge carrier mobility. The recombination rate R is defined by the Langevin model. 40 The continuity equation for singlet excitons is solved by considering exciton diffusion, radiation, and non-radiation processes.

[0163] Fit experimental data for devices with only holes and devices with only electrons (reference) Figure 4 The energy map and structure were used to extract charge carrier mobility. The values ​​of the fitted mobility parameters and other input parameters used in the simulation are shown in Table 2. The extracted values ​​were used to simulate a bipolar OLED device with an ITO / 20wt%Cs:BSBCz(10nm) / BSBCz(190nm) / MoO3(10nm) / Al structure. The work function of the cathode (ITO / 20wt%Cs:BSBCz) was 2.6 eV, and the work function of the anode (MoO3 / Al) was 5.7 eV. The effect of the DFB grating on the OSLD electrical performance was calculated and compared with a reference device (without grating). Mode gain g m The following equation is used to obtain the optical mode intensity |E(x,y)| from S(x,y). 2 calculate:

[0164]

[0165] Where L is the cavity length (only in the second-order grating region), and d is the active film thickness.

[0166] Near-field and far-field patterns were simulated using the OptiFDTD software package (Optiwave). The FDTD method was used to simulate the near-field pattern. Based on these patterns, the Fraunhofer approximation was used to calculate the far-field pattern. A perfectly matched layer and periodic conditions were used as boundary conditions.

[0167] [Table 2] Parameters for optical and electrical simulations.

[0168]

[0169] ε r E is the relative permittivity of the material. HOMO and E LUMO These are the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. N HOMO and N LUMO It represents the density of HOMO and LUMO level states. N tp It is the total density of the traps, E tp It is an energy depth of traps above the HOMO level, σ TP μ is the width of the Gaussian distribution. n0 and μ p0 It is zero field mobility. F n0 and F p0 These are the characteristic electric fields of electrons and holes, respectively. k r It is the radiation decay constant, k nr It is the nonradiative decay constant. φ PL It is the photoluminescence quantum yield. L S This is the exciton diffusion length. As an approximation, the mobility of BSBCz:CS was set to be the same as that of BSBCz, which fits the experimental data well, so no further improvement in mobility was made.

Claims

1. A current-injection organic semiconductor laser diode comprising: a pair of electrodes, an optical resonator structure, and one or more organic layers including a light amplification layer composed of an organic semiconductor, the current-injection organic semiconductor laser diode having, during current injection, sufficient overlap between a distribution of exciton density and a distribution of electric field intensity of a resonant optical mode in an x-direction of an x,y plane of the light amplification layer to emit laser light, where y is a thickness direction of the organic light-emitting layer and x is a direction perpendicular to y, wherein the optical resonator structure has a grating with grooves completely removed, or a grating with electrodes exposed in the grooves.

2. The current-injection organic semiconductor laser diode according to claim 1, wherein the optical resonator structure has a distributed feedback (DFB) structure.

3. The current-injection organic semiconductor laser diode according to claim 2, wherein the optical resonator structure is composed of a second-order Bragg scattering region surrounded by a first-order Bragg scattering region.

4. The current-injection organic semiconductor laser diode according to claim 2, wherein the second-order Bragg scattering region and the first-order Bragg scattering region are alternately formed in the optical resonator structure.

5. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein the number of the one or more organic layers is two or less.

6. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein a thickness of the light amplification layer is greater than 50% relative to a total thickness of the one or more organic layers.

7. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein the organic semiconductor included in the light amplification layer is amorphous.

8. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein a molecular weight of the organic semiconductor included in the light amplification layer is less than 1000.

9. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein the organic semiconductor included in the light amplification layer is a non-polymer.

10. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein the organic semiconductor included in the light amplification layer has at least one stilbene unit.

11. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein the organic semiconductor included in the light amplification layer has at least one carbazole unit.

12. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, wherein the organic semiconductor included in the light amplification layer is 4,4'-bis[(N-carbazolyl)styryl]biphenyl (BSBCz).

13. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, having one of the organic layers as an electron injection layer.

14. The current-injection organic semiconductor laser diode according to claim 13, wherein, the electron injection layer contains Cs.

15. The current-injection organic semiconductor laser diode according to any one of claims 1 to 4, which has a hole injection layer as the inorganic layer.

16. The current-injection organic semiconductor laser diode according to claim 15, wherein, the hole injection layer contains molybdenum oxide.

Images