Temperature-dependent Electroluminescence Research Articles

Temperature dependence of electron mobility, electroluminescence and photoluminescence of Alq3 in OLED

The correlation of electroluminescence (EL), photoluminescence (PL) and electron mobility were investigated over temperature from 60 to 300 K in small-molecule organic light emitting diode (OLED) structures. The devices consisted of ITO/PEDOT(50 nm)/TPD(50 nm)/Alq3(60 nm)/LiF(1 nm)/Al(90 nm), and were fabricated with high-vacuum sublimation/evaporation in a cross-linked configuration. Electron mobility was measured using an ac analysis of the device optical modulation characteristics, while PL and EL were measured by measuring optical power out at fixed pump power of 1 mW, and analysis of dc brightness–voltage (L–V) characteristics, respectively. PL intensity and mobility had a clear maximum at around 220 K, while EL efficiency was constant below 220 K and decrease monotonically above. The reason for the temperature dependent EL, PL and electron mobility behaviour will be discussed.

Interband dot-to-well transitions in InAs∕InGaAs dots in a well probed via photocurrent and electroluminescence spectroscopy

Hole states of InAs∕InGaAs dots in a well are optically probed by observing interband–quantum well (QW)–quantum dot (QD) transitions through photocurrent and electroluminescence (EL) spectroscopy. We find multiple sharp peaks with spacing in the range of 10–20meV in between the expected QD and the QW signals. The spacing and the observed temperature dependent EL suggest that the features are brought about by transitions between the QW electron ground state and the QD hole states. The data extracted from these transitions provide experimental values for the QD hole confinement energies, which can potentially aid in the verification of theoretical models in QD structures.

Sb-doped p-ZnO∕Ga-doped n-ZnO homojunction ultraviolet light emitting diodes

ZnO p-n homojunction light emitting diodes were fabricated based on p-type Sb-doped ZnO∕n-type Ga-doped ZnO thin films. Low resistivity Au∕NiO and Au∕Ti contacts were formed on top of p-type and n-type ZnO layers, respectively. Au∕NiO contacts on p-type ZnO exhibited a low specific resistivity of 7.4×10−4Ωcm2. The light emitting diodes yielded strong near-band-edge emissions in temperature-dependent and injection current-dependent electroluminescence measurements.

Temperature dependent behaviour of 340 nm light emitting diodes incorporating a gallium nitride interlayer

Ultra-violet light emitting diodes with an increased efficiency have been produced. The increase in efficiency was brought about by the introduction of a thin GaN layer between the AlN buffer and the subsequent AlGaN layers. The GaN interlayer causes a reduction in the number of threading dislocations that propagate through the ultra-violet light emitting structure. Temperature dependent electroluminescence measurements show an improved performance at temperatures up to 400 K.

Quantized level transitions and modification in InGaN∕GaN multiple quantum wells

A detailed study of emission mechanism is performed in undoped and Mg-doped InGaN∕GaN multiple quantum wells (MQWs) by means of injection-current- and temperature-dependent electroluminescence measurements. Two emission peaks corresponding to the recombination in InGaN quantum well are observed at high injection-current level in both MQWs. According to the emission behaviors with increasing injection current and decreasing temperature, in conjunction with the numerical calculations, these two peaks are tentatively assigned to be the interband transitions from the first quantized electron level to the first and second quantized heavy-hole levels (1e-1hh and 1e-2hh), respectively. Moreover, the energy separation of the interband transitions is reduced from 200to130meV by Mg dopant, which indicates that the quantized levels have been modified as a result of weakening of the polarization field.

Physical properties and efficiency of GaNP light emitting diodes

Ga N P ∕ Ga P is promising for yellow-amber-red light emitting diodes (LEDs). In this study, pressure and temperature dependent electroluminescence and photocurrent measurements on bulk GaP∕GaN0.006P0.994∕GaP LED structures are presented. Below ∼110K, emission is observed from several localized nitrogen states. At room temperature, the band-edge energy increases weakly with pressure at a rate of +1.6meV∕kbar, substantially lower than the Γ band gap of GaP (+9.5meV∕kbar). Thus, despite the multiplicity of nitrogen levels, the band anticrossing model reasonably describes this system based on an average of the nitrogen states. Furthermore, carrier leakage into the X minima of GaP reduces the efficiency in GaNP-LEDs with increasing pressure.

Quantum-well and localized state emissions in AlInGaN deep ultraviolet light-emitting diodes

Injection current and temperature dependence of electroluminescence (EL) is investigated in AlInGaN deep untraviolet light-emitting diodes. Two EL bands with different behaviors are observed. The high-energy band (P1) shows a monotonous redshift and an amazing increase of intensity with increasing current, however, a “U”-shaped shift and a saturation of intensity at high current are measured for the low-energy band (P2). Accordingly, P1 and P2 are attributed to emissions from quantum-well and localized states, respectively, with P1 dominant at high current and high temperature and P2 the main emission mechanism under low temperature and low current. Modeled data based on the theory of random population for localized states in quantum wells taking into account self-heating effect agree well with the experimental results.

Electroluminescence analysis of high efficiency Cu(In,Ga)Se2 solar cells

We compare the electroluminescence (EL) of polycrystalline ZnO∕CdS∕Cu(In,Ga)Se2 heterojunction solar cells with similar band gaps but different open circuit voltages, indicating a difference in the electronic quality of the absorber. Temperature dependent EL measurements reveal that all cells feature transitions from donor-acceptor pair recombination at lower temperature to band to band recombination at higher temperatures. However, the less efficient cells show a longer transition range with donor-acceptor pair recombination still apparent at room temperature. We find further that the part of the room temperature spectrum that is due to band to band transitions in the respective cells is relatively broader than expected from a direct semiconductor with a homogeneous band gap. We analyze this spectral broadening by a model that accounts for band gap fluctuations of the absorber material. The experimental results show that the dominant part of this spectral broadening results from the intentional band gap grading and not from stochastic band gap fluctuations. We show further that the spectral EL emission is linked to the photovoltaic external quantum efficiency by electro-optical reciprocity. In a similar way, the external EL quantum efficiency is related to the open circuit voltage of the device. We verify experimentally that the difference between radiative and measured open circuit voltage determines the EL external quantum efficiency of the solar cell. The best Cu(In,Ga)Se2 solar cell reaches an external light emitting diode quantum efficiency of around 0.1%.

Largely variable electroluminescence efficiency with current and temperature in a blue (In, Ga)N multiple-quantum-well diode

Electroluminescence (EL) efficiency of a blue In0.3Ga0.7N multiple-quantum-well diode has been investigated as a function of current between 0.001 and 20mA at various temperatures (20–300K). The low-temperature EL quenching previously observed below 100K at a driving current of 20mA does not occur at 0.001mA and is found to be strongly dependent on the current level. Largely variable temperature-dependent EL quantum efficiency with current suggests that the injected carrier capture efficiency by radiative recombination centers plays a decisive role for determination of the EL efficiency under the forward bias condition.

Temperature-dependent electroluminescence spectra of organic light emitting diodes based on thermally evaporated bis-imido-phenylene vinylene derivative

This letter reports on the effect of temperature on the electroluminescence spectrum of organic light-emitting diodes based on a low molecular weight compound. Bis-imido-phenylene vinylene derivative has been chosen as green-emitting molecule because of its similarity with poly-phenylene-vinylene derivatives. The electroluminescence spectra are found softly dependent on the degree of crystallinity of the layer and strongly dependent on the temperature. Electroluminescence spectra are fitted with multiple Gaussian peaks corresponding to phonon replica. With increasing temperature a blueshift of the zero-phonon line is observed. This phenomenon, commonly observed with poly-phenylene-vinylene derivatives, is usually explained as a reduction of the effective conjugation length. Such a model cannot be used with small molecules due to a finite conjugation length. A model of thermally activated statistic occupation of excited states is used to fully describe the experimental data. The results definitely rule out any significant role of a reduction of the conjugation length by increasing the temperature. It is also found that increasing the degree of order of the molecular morphology induces a reduction of the width of density of excited states.

Electroluminescence from Cu(In,Ga)Se2 Thin-film Solar Cells

AbstractWe compare the electroluminescence (EL) of three polycrystalline ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells with similar bandgaps but different open circuit voltages, indicating a difference in the electronic quality of the absorber. Temperature dependent electroluminescence measurements reveal that all cells feature transitions from donor-acceptor pair recombination at lower temperatures to band to band recombination at higher temperatures. However, the less efficient cells show a longer transition range with donor-acceptor pair recombination still apparent at room temperature. The thus broadened room temperature luminescence is one effect which reduces the open circuit voltage of the devices below the Shockley-Queisser-limit. The other effect is the existence of non-radiative recombination currents, which determine the efficiency of the device as light emitting diode. To quantify the open circuit voltage losses, we use reciprocity relations between electroluminescent and photovoltaic action of solar cells, which allow us to predict the light emitting diode efficiency. Measurements support the theory and show that Cu(In,Ga)Se2 solar cells reach external LED efficiencies approaching.

Temperature dependence of electroluminescence from silicon p-i-n light-emitting diodes

The temperature dependence of electroluminescence from silicon p-i-n light-emitting diodes with a layer of β-FeSi2 particles inserted in intrinsic silicon was investigated. Anomalous blueshift of the peak energy and enhanced electroluminescence intensity of the silicon band-edge emission were observed at temperatures from 50to200K. The electroluminescence intensity was enhanced due to longer diffusion paths of the injected electrons at elevated temperature, as well as thermal escape of the electrons from the β-FeSi2 particles. The low peak energy compared to that from bulk silicon at low temperature is due to the bound electron-hole pairs induced by the strain potential at the interface between silicon and β-FeSi2 particles. The blueshift of the peak is ascribed to the transition of bound electron-hole pairs into free excitons at elevated temperature. Room temperature electroluminescence from such a silicon light-emitting diode can be obtained at a low current density of 0.3A∕cm2.

Observations of electrical and luminescence anomalies in InGaN∕GaN blue light-emitting diodes

Unique correlations between the electrical and optical characteristics of InGaN∕(In)GaN multiple quantum-well light-emitting diodes (LEDs) were investigated over a broad range of temperatures. The dependence of nonunity ideality factors extracted from the current-voltage analysis on temperature determines the carrier-transport mechanisms in the heterodevices. The pseudotemperatures To for the LEDs with multiquantum barriers and with GaN barriers were found to be 945 and 1385K, respectively, at temperatures of 180–300K while having values of 1195 and 2720K below about 180K. Correspondingly, the temperature-dependent electroluminescence observations suggest that the To anomaly caused the spectral intensity to deteriorate.

Temperature-dependent electroluminescence of AlGaN-based UV LEDs

The electrical and optical characteristics of AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) (265-365 nm) at elevated temperatures (25/spl deg/C-175/spl deg/C) were investigated, and compared to those of InGaN-based visible LEDs (400-465 nm). Strong carrier localization and localized-state emission were retained in the InGaN LEDs up to 175/spl deg/C, leading to temperature-independent emission intensity at low-energy tails. The deep-UV LEDs, however, showed dominant band-edge emission, much smaller alloy broadening, and weaker localization effects. The optical power of the UV LEDs decreased much more rapidly with increasing temperature. The characteristic temperature was in the range of 31-73 K, and decreased with increasing Al content in the active region. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the UV LEDs and suggest that increasing carrier-confining potentials will provide a critical means to improve their radiative efficiencies.

Interplay of external and internal field effects on radiative recombination efficiency in InGaN quantum well diodes

Electroluminescence (EL) and photoluminescence (PL) properties have been investigated of the high-brightness green InGaN single quantum well (SQW) diode over a wide temperature range (T = 15–300 K) and as a function of injection current level. When the necessary forward bias conditions to get a certain current level are different, it is found that the anomalous temperature-dependent EL efficiency varies quite differently. That is, when the current is low and thus the forward driving voltage is small, the EL quenching observed below 100 K for high injection current levels is less significant or even absent due to the efficient carrier capture. This finding is consistent with decrease of the PL efficiency with increasing the bias over +2.5 V. These results indicate that the EL efficiency is significantly influenced by the interplay of internal and external field effects on the carrier capture and escape processes in addition to the localization phenomena caused by In fluctuations in the SQW layer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of electroluminescence from Si-based light emitting diodes with β-FeSi 2 particles active region

Electroluminescence (EL) properties of Si-based light emitting diodes with β-FeSi 2 particles active region grown by reactive deposition epitaxy are investigated. EL intensity of β-FeSi 2 particles versus excitation current densities has different behaviors at 8, 77 K and room temperature, respectively. The EL peak energy shifted from 0.81 to 0.83 eV at 77 K with the increase of current density from 1 to 70 A/cm 2. Temperature dependence of the peak energy can be well fitted by semi-empirical Varshni's law with the parameters of α=4.34 e–4 eV/K and β=110 K. These results indicate that the EL emission originates from the band-to-band transition with the band gap energy of 0.824 eV at 0 K.

Temperature-dependent carrier-transport and light-emission processes in a phosphorescent organic light-emitting device

We report on carrier-transport, electroluminescence, and photoluminescence processes in a phosphorescent organic light-emitting device based on fac tris(2-phenylpyridine) iridium doped 4,4′-N,N′-dicarbazole-biphenyl over a wide temperature range from 5 to 295 K. The current flow could be described in terms of a space-charge-limited current with an exponential trap distribution at temperatures above 150 K. The electroluminescence intensity gradually decreased with decreasing temperature, whereas the photoluminescence intensity was almost independent of temperature. The difference between the temperature-dependent electroluminescence and photoluminescence intensities suggests that the carrier mobility depends on the electric field and the temperature.

Resonance-tunneling-assisted emission enhancement in green light-emitting diodes with nanocraters formed in InGaN∕GaN quantum-well active layers

Data are presented on the resonant tunneling-related abrupt redshift observed in the temperature-dependent electroluminescence spectra of high-brightness InGaN∕GaN multi-quantum-well green light-emitting diodes (LEDs). It is found that the redshift arises mostly between 120 and 150K, and brighter LEDs yield larger redshifts. These results are well explained by the proposed nanocrater model which comprises a Ga-rich quantum barrier surrounding the In-rich quantum-dot-like localized state. Intensity analysis manifests that the resonant tunneling from the quantum-well to the nanocrater-shaped localized states induces such an abrupt energy shift and enhances the room-temperature emission.

Electroluminescence and photoluminescence excitation study of asymmetric coupledGaAs∕AlxGa1−xAsV-groove quantum wires

We have studied systems of asymmetric coupled GaAs V-groove quantum wires p-i-n diodes, in which the electrons and holes are injected into different wires. Despite the use of a 7 nm thick AlGaAs tunnel barrier, and although we demonstrate that holes tunnel more slowly than electrons, we observe a very efficient and fast tunneling between the quantum wires (QWR's). We attribute this to the achievement of a resonance between hole levels in the two QWR's. Photoluminescence excitation experiments are compared with accurate calculations of the excitonic absorption yielding a level of carrier transfer of nearly 100%. Temperature-dependent electroluminescence exhibits clear effects of tunneling up to room temperature but cannot distinguish separate electron/hole tunneling from exciton tunneling.

Experimental and theoretical study of the electroluminescence temperature dependence of iron disilicide light-emitting devices

In this paper, experimental results of a study on iron disilicide light-emitting diodes will be presented. A comparison will be made between the luminescence characteristics of standard iron disilicide diodes and diodes that have been augmented by the use of dislocation engineering, using dislocation loops, to provide spatial localisation of the injected carriers. The improvements observed will be discussed and the results kinetically modelled using a rate equation approach.