Absolute Photoluminescence Research Articles

Surface recombination measurements on III–V candidate materials for nanostructure light-emitting diodes

Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes.

A Novel Series of Efficient Thiophene-Based Light-Emitting Conjugated Polymers and Application in Polymer Light-Emitting Diodes

A novel series of soluble conjugated polymers, poly[1,4-bis(4-alkyl-2-thienyl)-2,5-disubstituted phenylene]s, with different substituents on the phenylene ring (hydrogen, methyl group, or decyloxy group) and on the thiophene ring (n-hexyl chain or cyclohexyl group) were synthesized through FeCl 3 -oxidative polymerization. The structures of the polymers were fully verified by FT-IR, 1 H and 13 C NMR, and element analysis. The high regioregularity of head-to-head linkage between two adjacent thiophene rings in the polymers was demonstrated. The electronic spectra of the polymers can be tuned by changing the substitution on both the phenylene and thiophene rings, emitting blue to green light. The electronic structures of the polymers can also be tuned by different substitutions, as demonstrated by electrochemical measurements (cyclic voltammetry). The absolute photoluminescence (PL) efficiencies of the polymers in neat films, which can be remarkably effected by the substitution on both the phenylene ring and the thiophene ring, can be up to 29%, much higher than the PL efficiencies in other polythiophene-based light-emitting materials. It was demonstrated that inserting substituted phenylene rings into the backbone of polythiophene is a useful approach to improve the PL efficiency of thiophene-based conjugated polymers. Blue to green electroluminescence (EL) was realized from some of the new polymers in single-layered light-emitting diodes with the configuration of ITO/polymer film/Ca or Al. One green light-emitting diode fabricated from the green-emissive polymer with the highest PL efficiency gave rise to the best EL performance, turning on at ∼8V and reaching an external EL quantum efficiency of 0.1%.

The synthesis and characterization of an efficient green electroluminescent conjugated polymer: poly[2,7-bis(4-hexylthienyl)-9,9-dihexylfluorene

A novel soluble electroluminescent (EL) material, poly[(4-hexylthiophene-2,5-diyl)(9,9-dihexyl-9H-fluoren-2,7-ylene )(4-hexylthiophene-2,5-diyl)] (PHTDHFHT), with high absolute photoluminescence (PL) quantum efficiency (32 ± 3%) compared with other polythiophene derivatives, was synthesized and a green EL is observed.

A novel triarylamine-based conjugated polymer and its unusual light-emitting properties

A new soluble triphenylamine-based conjugated polymer, poly[N-(4′-butylphenyl)imino(1,1′∶4′,1 ″-terphenyl-4′4″-ylene)], (PBPITP), has been synthesized, which emits blue photoluminescence (PL) with high absolute PL quantum efficiency (45 ± 3%) but exhibits red electroluminescence (red shifted by as much as 157 nm from the PL spectrum).

Photoluminescence quantum yield of pure and molecularly doped organic solid films

We present measurements of the absolute photoluminescence (PL) quantum yield, φPL, for a wide variety of organic compounds in solid films, pure and molecularly doped with strongly fluorescent materials. The procedure, which uses an integrating sphere, does not entail comparison to other standards, and provides accurate measure of the photoluminescence efficiency for submicron thick films, prepared by high vacuum vapor deposition. Host materials include N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4-4′-diamine (TPD), a common hole transport material for light emitting diodes, tris (8-hydroxyquinolinolato) aluminum (III) (Alq3) and its methyl derivative, Almq3, two aluminum chelates used as electron transport and/or green emitting materials. Dopants include tetraphenylnapthacene (rubrene) and N,N′-diethyl quinacridone (DEQ). Doping results in a substantial increase (∼a factor 2–4) of φPL in comparison with that of the pure host. For instance, measured φPL increases from 0.25 and 0.42 for pure Alq3 and Almq3, respectively, to near unity upon doping with rubrene at a concentration of ∼1 mol %. The above data are discussed within the framework of Förster energy transfer from host to guest.

High-efficiency oligothiopene-based light-emitting diodes

We report investigations of the photoluminescence (PL) and electroluminescence (EL) of a thiophene oligomer for which we have devised a variety of substitutions aimed at enhancing the solid-state efficiency. We find that the absolute PL quantum efficiency in the solid state is up 37% for both powders or spin-coated thin films of the compound. The material thus becomes competitive for applications in organic light-emitting diodes (LEDs). EL efficiencies up to 1.2 cd/A are demonstrated in LEDs prepared with indium–tin–oxide and Ca–Al electrodes.

Photophysical characterisation of poly(p-pyridine)

We report measurements of the photophysical behaviour of poly(p-pyridine) (PPY), including thin film absolute photoluminescence quantum yield, solid and solution state photoluminescence properties. The photoluminescence quantum yield of PPY is stable in air consistent with its high oxidative stability. The large stokes shift and further shift in emission in the solid state will be explained in terms of ring torsion and ring distortion in the excited state.

Vacuum UV photoelectric yield and photoluminescence in CsCl and CsBr

Absolute photoelectric total yield and photoluminescence of CsCl and CsBr are studied in a vacuum UV region (5–30 eV). Temperature dependences of the yield and the luminescence are examined in detail with an interest in the critical relation between the band gap energy and Auger process in these materials. A complementary relation between the photoelectric yield and Auger-process-free luminescence is well demonstrated in the temperature dependences of these phenomena.

Luminescence properties of poly(p-phenylenevinylene): Role of the conversion temperature on the photoluminescence and electroluminescence efficiencies

We have investigated the luminescence properties of poly(p-phenylenevinylene) (PPV) prepared via the standard precursor route as a function of the conversion temperature in the range 170–270 °C. In particular, we have determined the absolute photoluminescence (PL) efficiencies of PPV thin films prepared on quartz or indium–tin oxide (ITO) coated glass substrates and found that the dependence on conversion temperature is different, depending on the type of substrate. The optical data show that heating at 170 °C for 10 h is sufficient to achieve full conversion. For PPV on quartz, a further increase of the temperature induces a decrease of the PL efficiency, whereas for PPV on ITO the PL efficiency shows a nonmonotonic dependence on the conversion temperature, with a maximum for conversion at about 205 °C. We discuss this behavior with reference to the interplay between the decrease in concentration of PL-quenching impurities (formed by reaction of the conversion byproducts and ITO) and an increase of exciton quenching efficiency due to polymer oxidation and/or crystallization, with increasing conversion temperature. We have also investigated the dependence on conversion temperature of the electroluminescence (EL) efficiency of single-layer ITO/PPV/Ca light-emitting diodes (LEDs) and of two-layer LEDs where an electron-transport/hole-blocking layer [namely 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole blended with polystyrene] is inserted between the PPV and the calcium cathode. The EL efficiency for the single-layer devices increases monotonically with increasing conversion temperature, whereas it decreases for the two-layer diodes. This complex behavior is due to the combined effects of the conversion temperature on the luminescence and injection/transport processes in PPV and is consistent with an interpretation based on a hole mobility which decreases with increasing conversion temperature.

Measurements of the absolute external luminescence quantum efficiency in ZnSe/ZnMgSSe multiple quantum wells as a function of temperature

We report on a novel experimental setup using a miniature integrating sphere fitted into a cryostat to measure spectrally resolved the absolute external photoluminescence quantum efficiency ηlum in the temperature range from 6 K to beyond room temperature. Direct access to ηlum is achieved by an uncomplicated method to calibrate the spatially integrated emission spectra absolutely. The fraction of light coupled out of the sphere into the detection system is in good approximation independent of the emitted wavelength and the emission characteristics of the investigated samples, due to their smallness compared with the total sphere area. This fact is proved by theoretical calculations for the sphere throughput and the irradiance within the sphere. To demonstrate the capabilities of this setup, we investigate in high quality ZnSe/ZnMgSSe 10× multiple quantum wells, the temperature dependence of the quantum efficiency, simultaneously for the quantum well and the barrier luminescence. Within a thermal activation model, three processes are found to contribute to the thermal quenching of ηlum of the quantum well luminescence.

Efficient photo and electroluminescence of regioregular poly(alkylthiophene)s

We report absolute photoluminescence (PL) and electroluminescence quantum efficiencies for thin films and polymer light-emitting diodes prepared with regioregular (that is, exclusively head-to-head/tail-to-tail conformation and/or exclusively head-to-tail conformation) poly(alkylthiophenes). In particular, we find a maximum PL quantum efficiency of ∼11% for the head-to-head/tail-to-tail conformers, an order of magnitude higher than for the head-to-tail conformers. The results show that chemical conformation plays a crucial role in determining the material electronic structure and, hence, the relevant optical properties. The results are significant to the wide field of conjugated, synthetic, light-emitting materials, and hence to the development of large-area organic displays.

Luminescence peculiarities of Tl2ZnI4 crystals

X-ray and photoluminescence emission spectra as well as excitation and reflection spectra of Tl2ZnI4 crystals have been researched at T=77–293 K. Eex=3.82 eV is the exciton peak n=1 position. Crystal photoluminescence is concentrated in two bands at 2.71 and 2.48 eV. The value of the absolute photoluminescence light output has been measured to be ηk=0.70 at 77 K. At low temperatures, the Tl2ZnI4 crystal is an effective transformer of high-energy emission.

Measurement of time dependent quantum yield of poly( p-pyridine) and poly( p-phenylenevinylene)

In this paper we report the absolute photoluminescence quantum yield of poly( p-pyridine) (PPy), which is a promising electroluminescent polymer. The influence of vacuum and air on the quantum yield of the PPy is investigated. We find a significant decay of the efficiency under vacuum whereas it is stable in air. We compare the results with those obtained on poly( p-phenylenevinylene)(PPV).

Electrical and luminescent properties of double-layer oligomeric/ polymeric light-emitting diodes

We report the use of α-sexithiophene (T6) thin films sublimed onto glass substrates coated with indium-tin oxide in light-emitting diodes. Absolute photoluminescence quantum efficiencies were found to be in the range 10 −2–10 −3%, and indicate that T6 should be used as a hole injector into an emissive layer, rather than as a luminescent layer. We have fabricated double-layer organic light-emitting diodes where a cyano-substituted derivative of poly( p-phenylenevinylene) (PPV), MEH-CN-PPV, was spun on top of the T6 layer prior to evaporation of Ca-Al cathodes. Turn-on voltages for electroluminescence of about 3 V were found in structures with total thickness between 160 and 230 nm, while internal quantum efficiencies were up to 0.4%, i.e. at least ten times less than those measured in comparative devices where a PPV hole-injecting layer was used in place of T6. The substantial difference is interpreted on the basis of a significant lowering of the barrier to electron ejection from the luminescent layer into the hole-injecting layer when passing from PPV to T6. This allows inefficient recombination to take place in T6, as confirmed by the electroluminescence spectra.

Measurement of absolute photoluminescence quantum efficiencies in conjugated polymers

Measurements of absolute photoluminescence (PL) efficiencies have been performed for solid films of several conjugated polymers commonly used for electroluminescence. In poly( p-phenylenevinylene) (PPV), a PL efficiency of 0.27 is measured in samples which show an initial PL decay time-constant of 320 ps. These values indicate that photoexcitation in PPV produces intra-chain singlet excitons with a high quantum yield. The PL efficiencies of derivatives of PPV have been investigated, and efficiencies in excess of 0.4 have been measured for cyano-substituted PPVs.

Ultrahigh spontaneous emission quantum efficiency, 99.7% internally and 72% externally, from AlGaAs/GaAs/AlGaAs double heterostructures

Optically thin AlGaAs/GaAs/AlGaAs double heterostructures, (5000 Å), are floated off their substrates by the epitaxial liftoff technique and mounted on various high reflectivity surfaces. From the absolute photoluminescence intensity, we measure internal and external quantum efficiencies of 99.7% and 72%, respectively. High spontaneous emission quantum efficiency, is important for photon number squeezed light, diode lasers, single-mode light-emitting-diodes, optical interconnects, and solar cells.

Properties and Strain Relaxation below Room Temperature of Epitaxial Pbse and Pb(Se,Te) on Fluoride-Covered Silicon Substrates

ABSTRACTNarrow-gap PbSe and PbSe1−xTex films were grown by MBE on 3n-Si(111) wafers. The layers are used for IR-sensor array applications. Epitaxy was achieved by using an intermediate CaF2/BaF2 bilayer to overcome the large difference in lattice constants (14%) and thermal expansion coefficients (700%). While PbSe is rather soft, the hardness increases with x (x<0.5) in PbSe1−xTex. Thermal stresses are relieved by dislocation glide on (100)-planes, leading to slip lines on the surface. The surface morphology and step heights were studied with the scanning tunneling microscopy technique, and a model for the resulting (111)-surface structure is presented. The value of the remaining strain was determined with precision Rutherford backscattering spectrometry absolute angle measurements, X-ray diffraction and photoluminescence in the temperature range down to 10 K. It is found that no appreciable strain builds up even at cryogenic temperatures and after many temperature cycles. In contrast to PbSe, ternary PbSe1−xTex layers are strained elastically below room temperature owing to solid solution strengthening, and plastic deformation occurs only at higher temperatures.