More Like this

1. Efficient White Polymer Light‐Emitting Diode (WPLED) Based on Single‐Component Eu 3+ –Tb 3+ ‐Containing Metallopolymer

   https://doi.org/10.1002/adom.201900776  

   Fu, Guorui ; Liu, Lin ; Li, Wentao ; He, Yani ; Miao, Tiezheng ; Lü, Xingqiang ; He, Hongshan ( August 2019 , Advanced Optical Materials)

   Despite the excellent physical properties of single‐component Eu³⁺–Tb³⁺‐containing metallopolymers, the development of their flexible white polymer light‐emitting diodes (WPLEDs) for portable full‐color flat displays remains a formidable challenge. Herein, the WPLEDs from a metallopolymerPoly(NVK‐co‐2‐co‐7)are reported, in which [Eu(DBM)₃(4‐vp‐PBI)] (2) and [Tb(tba‐PMP)₃(4‐vp‐PBI)] (7) with different localized circumstances are grafted into poly(N‐vinyl‐carbarzole) (PVK). In this design, both Dexter and Förster energy transfers occur, which endow a photoluminescent quantum yield up to 22.3% of the straightforward high‐quality white‐lights. Contributing from the stepwise alignment of frontier molecular orbitals ofPoly(NVK‐co‐2‐co‐7)as the emitting layer in combination with CBP‐ and BCP‐assisted carrier‐transports, a reliable WPLED with the record‐renewed electroluminescent performance (L^Max= 388.0 cd m⁻², η_c^Max= 31.1 cd A⁻¹, η_p^Max= 15.0 lm W⁻¹, η_EQE^Max= 18.1%, and weak efficiency‐roll‐off) among previous organo‐Ln³⁺‐based white organic light‐emitting diodes/WPLEDs is achieved. This finding renders a single‐component Eu³⁺–Tb³⁺‐containing metallopolymers a potential new platform to cost‐effective flexible WPLEDs for practical applications.

    

   more » « less
2. Efficient Red Light Emitting Diodes Based on a Zero‐Dimensional Organic Antimony Halide Hybrid

   https://doi.org/10.1002/adma.202209417  

   Liu, He ; Shonde, Tunde Blessed ; Gonzalez, Fabiola ; Olasupo, Oluwadara Joshua ; Lee, Sujin ; Luong, Derek ; Lin, Xinsong ; Vellore Winfred, J. S. Raaj ; Lochner, Eric ; Fatima, Iqra ; et al ( January 2023 , Advanced Materials)

   Zero‐dimensional (0D) organic metal halide hybrids (OMHHs) have recently emerged as a new class of light emitting materials with exceptional color tunability. While near‐unity photoluminescence quantum efficiencies (PLQEs) are routinely obtained for a large number of 0D OMHHs, it remains challenging to apply them as emitter for electrically driven light emitting diodes (LEDs), largely due to the low conductivity of wide bandgap organic cations. Here, the development of a new OMHH, triphenyl(9‐phenyl‐9H‐carbazol‐3‐yl) phosphonium antimony bromide (TPPcarzSbBr₄), as emitter for efficient LEDs, which consists of semiconducting organic cations (TPPcarz⁺) and light emitting antimony bromide anions (Sb₂Br₈²⁻), is reported. By replacing one of the phenyl groups in a well‐known tetraphenylphosphonium cation (TPP⁺) with an electroactive phenylcarbazole group, a semiconducting TPPcarz⁺cation is developed for the preparation of red emitting 0D TPPcarzSbBr₄single crystals with a high PLQE of 93.8%. With solution processed TPPcarzSbBr₄thin films (PLQE of 86.1%) as light emitting layer, red LEDs are fabricated to exhibit an external quantum efficiency (EQE) of 5.12%, a peak luminance of 5957 cd m⁻², and a current efficiency of 14.2 cd A⁻¹, which are the best values reported to date for electroluminescence devices based on 0D OMHHs.

    

   more » « less
3. RTD Light Emission around 1550 nm with IQE up to 6% at 300 K

   https://doi.org/10.1109/DRC50226.2020.9135175  

   Brown, E. R. ; Zhang, W-D. ; Fakhimi, P. ; Growden, T. A. ; Berger, P.R. ( June 2020 , IEEE Device Research Conference (DRC), Columbus, OH (June 22-24, 2020))

   Resonant tunneling diodes (RTDs) have come full-circle in the past 10 years after their demonstration in the early 1990s as the fastest room-temperature semiconductor oscillator, displaying experimental results up to 712 GHz and fmax values exceeding 1.0 THz [1]. Now the RTD is once again the preeminent electronic oscillator above 1.0 THz and is being implemented as a coherent source [2] and a self-oscillating mixer [3], amongst other applications. This paper concerns RTD electroluminescence – an effect that has been studied very little in the past 30+ years of RTD development, and not at room temperature. We present experiments and modeling of an n-type In0.53Ga0.47As/AlAs double-barrier RTD operating as a cross-gap light emitter at ~300K. The MBE-growth stack is shown in Fig. 1(a). A 15-μm-diam-mesa device was defined by standard planar processing including a top annular ohmic contact with a 5-μm-diam pinhole in the center to couple out enough of the internal emission for accurate free-space power measurements [4]. The emission spectra have the behavior displayed in Fig. 1(b), parameterized by bias voltage (VB). The long wavelength emission edge is at  = 1684 nm - close to the In0.53Ga0.47As bandgap energy of Ug ≈ 0.75 eV at 300 K. The spectral peaks for VB = 2.8 and 3.0 V both occur around  = 1550 nm (h = 0.75 eV), so blue-shifted relative to the peak of the “ideal”, bulk InGaAs emission spectrum shown in Fig. 1(b) [5]. These results are consistent with the model displayed in Fig. 1(c), whereby the broad emission peak is attributed to the radiative recombination between electrons accumulated on the emitter side, and holes generated on the emitter side by interband tunneling with current density Jinter. The blue-shifted main peak is attributed to the quantum-size effect on the emitter side, which creates a radiative recombination rate RN,2 comparable to the band-edge cross-gap rate RN,1. Further support for this model is provided by the shorter wavelength and weaker emission peak shown in Fig. 1(b) around = 1148 nm. Our quantum mechanical calculations attribute this to radiative recombination RR,3 in the RTD quantum well between the electron ground-state level E1,e, and the hole level E1,h. To further test the model and estimate quantum efficiencies, we conducted optical power measurements using a large-area Ge photodiode located ≈3 mm away from the RTD pinhole, and having spectral response between 800 and 1800 nm with a peak responsivity of ≈0.85 A/W at  =1550 nm. Simultaneous I-V and L-V plots were obtained and are plotted in Fig. 2(a) with positive bias on the top contact (emitter on the bottom). The I-V curve displays a pronounced NDR region having a current peak-to-valley current ratio of 10.7 (typical for In0.53Ga0.47As RTDs). The external quantum efficiency (EQE) was calculated from EQE = e∙IP/(∙IE∙h) where IP is the photodiode dc current and IE the RTD current. The plot of EQE is shown in Fig. 2(b) where we see a very rapid rise with VB, but a maximum value (at VB= 3.0 V) of only ≈2×10-5. To extract the internal quantum efficiency (IQE), we use the expression EQE= c ∙i ∙r ≡ c∙IQE where ci, and r are the optical-coupling, electrical-injection, and radiative recombination efficiencies, respectively [6]. Our separate optical calculations yield c≈3.4×10-4 (limited primarily by the small pinhole) from which we obtain the curve of IQE plotted in Fig. 2(b) (right-hand scale). The maximum value of IQE (again at VB = 3.0 V) is 6.0%. From the implicit definition of IQE in terms of i and r given above, and the fact that the recombination efficiency in In0.53Ga0.47As is likely limited by Auger scattering, this result for IQE suggests that i might be significantly high. To estimate i, we have used the experimental total current of Fig. 2(a), the Kane two-band model of interband tunneling [7] computed in conjunction with a solution to Poisson’s equation across the entire structure, and a rate-equation model of Auger recombination on the emitter side [6] assuming a free-electron density of 2×1018 cm3. We focus on the high-bias regime above VB = 2.5 V of Fig. 2(a) where most of the interband tunneling should occur in the depletion region on the collector side [Jinter,2 in Fig. 1(c)]. And because of the high-quality of the InGaAs/AlAs heterostructure (very few traps or deep levels), most of the holes should reach the emitter side by some combination of drift, diffusion, and tunneling through the valence-band double barriers (Type-I offset) between InGaAs and AlAs. The computed interband current density Jinter is shown in Fig. 3(a) along with the total current density Jtot. At the maximum Jinter (at VB=3.0 V) of 7.4×102 A/cm2, we get i = Jinter/Jtot = 0.18, which is surprisingly high considering there is no p-type doping in the device. When combined with the Auger-limited r of 0.41 and c ≈ 3.4×10-4, we find a model value of IQE = 7.4% in good agreement with experiment. This leads to the model values for EQE plotted in Fig. 2(b) - also in good agreement with experiment. Finally, we address the high Jinter and consider a possible universal nature of the light-emission mechanism. Fig. 3(b) shows the tunneling probability T according to the Kane two-band model in the three materials, In0.53Ga0.47As, GaAs, and GaN, following our observation of a similar electroluminescence mechanism in GaN/AlN RTDs (due to strong polarization field of wurtzite structures) [8]. The expression is Tinter = (2/9)∙exp[(-2 ∙Ug 2 ∙me)/(2h∙P∙E)], where Ug is the bandgap energy, P is the valence-to-conduction-band momentum matrix element, and E is the electric field. Values for the highest calculated internal E fields for the InGaAs and GaN are also shown, indicating that Tinter in those structures approaches values of ~10-5. As shown, a GaAs RTD would require an internal field of ~6×105 V/cm, which is rarely realized in standard GaAs RTDs, perhaps explaining why there have been few if any reports of room-temperature electroluminescence in the GaAs devices. [1] E.R. Brown,et al., Appl. Phys. Lett., vol. 58, 2291, 1991. [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [2] M. Feiginov et al., Appl. Phys. Lett., 99, 233506, 2011. [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [3] Y. Nishida et al., Nature Sci. Reports, 9, 18125, 2019. [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [4] P. Fakhimi, et al., 2019 DRC Conference Digest. [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). 

   more » « less
4. Electrospun Tri‐Cation Perovskite Nanofibers for Infrared Photodetection

   https://doi.org/10.1002/adfm.202207326  

   Kim, Min‐Woo ; Yuan, Yihang ; Jeong, Sehee ; Chong, Jenny ; Mølnås, Håvard ; Alaei, Aida ; Cleveland, Iver J. ; Liu, Na ; Ma, Yichen ; Strauf, Stefan ; et al ( August 2022 , Advanced Functional Materials)

   Tri‐cation (Cs⁺/CH₃NH₃⁺/CH(NH₂)₂⁺) and dual‐anion (Br^–/I^–) perovskites are promising light absorbers for inexpensive infrared (IR) photodetectors but degrade under prolonged IR exposure. Here, stable IR photodetectors based on electrospun tri‐cation perovskite fibers infiltrated with hole‐transporting π‐conjugated small molecule 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9‐spirobifluorene (Spiro‐OMeTAD) are demonstrated. These hybrid perovskite photodetectors operate at a low bias of 5 V and exhibit ultra‐high gains with external quantum efficiencies (EQEs) as high as 3009%, decreasing slightly to ≈2770% after 3 months in air. These EQE values are almost ten times larger than those measured for photodetectors comprising bilayer perovskite/Spiro‐OMeTAD films. A high density of charge traps on electrospun fiber surfaces gives rise to a photomultiplication effect in which photogenerated holes can travel through the active layer multiple times before recombining with trapped electrons. Time‐resolved photoluminescence and conductive atomic force microscopy mapping reveal the improved performance of electrospun fibers to originate from the significantly enhanced interfacial surface area between the perovskite and Spiro‐OMeTAD compared to bilayers. As a solution‐based, scalable and continuous method of depositing perovskite layers, electrospinning thus presents a promising strategy for the inexpensive fabrication of high‐performance IR photodetectors for applications ranging from information technology to imaging.

    

   more » « less
5. Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

   https://doi.org/10.1002/adma.201904114  

   Wang, Miaosheng ; Huang, Yi‐Hsuan ; Lin, Kai‐Siang ; Yeh, Tzu‐Hung ; Duan, Jiashun ; Ko, Tzu‐Yu ; Liu, Shun‐Wei ; Wong, Ken‐Tsung ; Hu, Bin ( September 2019 , Advanced Materials)

   Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge‐transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light‐emitting diodes are reported. Magneto‐photoluminescence studies reveal that the triplet‐to‐singlet conversion in exciplexes involves an artificially generated spin‐orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge‐transfer occurs with forming electric dipoles (D^+•→A^−•), providing the ionic polarization to generate SOC in exciplexes. By having different singlet‐triplet energy differences (ΔE_ST) in 9,9′‐diphenyl‐9H,9′H‐3,3′‐bicarbazole (BCzPh):3′,3′″,3′″″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(([1,1′‐biphenyl]‐3‐carbonitrile)) (CN‐T2T) (ΔE_ST= 30 meV) and BCzPh:bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methyl‐pyrimidine (B3PYMPM) (ΔE_ST= 130 meV) exciplexes, the SOC generated by the intermolecular charge‐transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE_max(21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE_ST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D^+•→A^−•) size, enhanced SOC, increased orbital polarization, and decreased ΔE_STcan simultaneously occur to cooperatively operate the triplet‐to‐singlet conversion.

    

   more » « less