Luminance degradation and recovery studies in Alq3 based Organic light emitting diodes (original) (raw)
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Luminance degradation and recovery studies in AIq3 based Organic light emitting diodes
2011
Though organic light emitting diodes are being commercialized in many applications, issues relating to lifetime and degradation remain as fundamental concerns limiting performance. A coherent understanding of degradation mechanisms is yet to emerge. We focus on intrinsic degradation of high quality Alq3 based diodes due to electrical stressing. We monitor progressive luminance degradation and recovery by introducing well defined relaxation time windows in the current stress cycles. The method helps to clearly distinguish between recoverable and permanent degradation systematically. The voltage shift due to degradation and recovery is also monitored as a function of time. Further, we introduce a method of reconstructing the transients of the recoverable part using progressive isolated current pulses as a probe. The recovery of degradation is related to the charging and discharging of the traps in the device and our method provides a technique of measuring significant parameters of trapping through luminance transients. The origin and distinguishing features of the two types of degradation are discussed.
Organic semiconductor devices and materials have matured sufficiently to be limited by intrinsic degradation processes which are as yet not understood well. We use high quality Alq 3 based organic light emitting diodes to study the rate processes involved in degradation due to electrical stressing and its auto-recovery. The method involves interspersing degradation due to electrical pulsing with variable relaxation windows to monitor time evolution of loss and recovery of luminescence. The corresponding rate processes for permanent and auto-recoverable degradation is discussed on the basis of charging and discharging of traps, and a phenomenological model based on metastability in configuration-coordinate diagram is proposed.
Effect of driving method on the degradation of organic light emitting diodes
Synthetic Metals, 2003
Lifetime testing results are reported for organic light emitting diodes (OLEDs) having the structure ITO (anode)/N,N-diphenyl-N,Nbis(3-methylphenyl)-1,1-diphenyl-4,4-diamine (TPD)/tris-(8-hydroxyquinoline)aluminum (AlQ)/Al (cathode) and operated using dc and pulsed waveforms for comparison. In ambient atmosphere non-encapsulated devices show a lifetime of about 70 h in pulsed operation at an initial luminance of 500 cd/m 2 , almost four times longer than in dc operation. A fast initial decay of luminance is observed for dc operation. It is most probably due to a combination of Joule heating and mobile ionic impurities migration within the OLED structure under the continuous forward electric field. We study also the transient electroluminescence (EL) and report the observation of a short duration spike in the leading edge of the EL response. Its origin is attributed to the temporal shift of the recombination zone in the AlQ layer from the bulk to the interface with TPD subsequent the application of a voltage step.
Charge transport properties in electrically aged organic light-emitting diodes
Organic light-emitting diodes (OLEDs) are a rapidly developing technology with high innovation potential for displays, lighting, and further applications. Critical issues for current research remain the extension of lifetimes and, related to this aim, a deeper understanding of degradation processes in OLEDs. In the present work, we used the charge-based deep level transient spectroscopy technique to investigate changes in the charge transport in OLEDs induced by electrical aging. Both trap states capturing electrons and holes, respectively, were detected. Temperature-dependent measurements enabled estimating the depth of the trap states for electrons. Comparison of spectra of electrically aged and undriven devices revealed that aging seemed to increase the density of traps but did not lead to the occurrence of new types of trap states. Furthermore, experiments with different voltage pulses to fill the charge carrier trap states allowed conclusions on age-induced changes of the injection barrier for holes. V C 2013 American Institute of Physics. [http://dx.
Degradation of organic light-emitting diodes under different environment at high drive conditions
Organic Electronics, 2007
The origin of dark spots, crucial for understanding the degradation mechanisms in organic light-emitting diodes (OLEDs), is typically attributed to the penetration of moisture, oxygen and other active atmospheric agents. Employing scanning X-ray photoelectron spectromicroscopy we have investigated the morphology and chemical composition of degraded micro-areas created in OLEDs under different environment. The same confined degradation events, involving decomposition of the ITO film and organic layer and oxidation and delamination of the Al cathode were observed even for devices grown in situ and operated in ultra-high vacuum at pressures lower than 10 À9 mbar. Our results provide unambiguously prove that 'uncontrollable' imperfections in the fabricated structures are the major cause for ignition of degradation events, whereas external causes related to the air ambient act as efficient promoters.
Sudden death of organic light-emitting diodes
Organic Electronics, 2015
The degradation in light output of an Organic Light Emitting Diode (OLED) has been studied extensively and has been explained by different mechanisms, such as formation of chemical defects or electrical traps and by thermally induced inter-diffusion of dopants. However, there is an overlooked type of degradation, where the light output decreases rapidly with time. This catastrophic failure can often be attributed to a hard electrical short due to local defects. Here, we show that this ''sudden death'' can also occur in the absence of a hard electrical short. We investigate this phenomenon by current-voltage characteristics and small-signal impedance measurements on typical OLEDs with a LiF cathode interlayer. We show that in a short period of time the built-in voltage of the diode vanishes; the J-V characteristics become symmetric. The origin is a dramatic increase in the work-function of the LiF interlayer. The interlayer changes from an electron-injecting contact to a quasi-Ohmic hole-injecting contact. The pristine bipolar diode does not become electrically shorted, but suddenly transforms into a unipolar hole-only diode. Upon applying a high voltage the original diode is restored, manifesting the dynamic switching of the LiF contact.
Organic Electronics, 2016
AC-driven organic light-emitting diodes (OLEDs) can overcome some reliability-related drawbacks to traditional DC-OLEDs. They imply the use of insulating layers in the device. In this work OLEDs containing an insulating layer of poly (methyl methacrylate) (PMMA) in storing charges with the thickness of 2, 6, 8, 10 nm have been prepared and investigated. The emission mechanisms of the device are analyzed considering transient and AC electroluminescence (EL) measurements. We show that charges are stored in the PMMA layer as surface charges and bulk charges. The former contribute to the occurrence of EL spike after the driving pulse with a decay tail for about 80 ms to 10nm PMMA device, and the latter can be released to emit light under reversed voltage more than 2 V because they are immobile unless under the stronger reversed field. Stored charges commonly are harmful for the performance of OLEDs devices due to quenching, nonradiative transition and the thermal energy accumulation even degradation. Whereas when operating under alternating current (AC) stress, we not only obtain the EL peak from injection charges under forward voltage, but also obtain another peak below the built-in voltage 4.3 V, whose peak point lies in À0.2 V. It turns to originate from stored charges so they become beneficial instead of weakening EL for reusing to produce light. All the results pave the way to realize AC driven OLEDs devices using these stored charges, to uncover the AC EL mechanisms and to improve their EL performance.
Degradation in Organic Light Emitting Diodes
2009
The objective is to fabricate organic light emitting diode and to study its degradation process in atmosphere condition in which PFO as an emitting material and PEDOT:PSS as a hole injecting material were used on ITO substrate. Thus degradation process of the OLED was studied upon its current-voltage characteristic. By fabricating this OLED and obtaining blue light and analysis of
Approaches for Long Lifetime Organic Light Emitting Diodes
Advanced Science, 2020
Organic light emitting diodes (OLEDs) have been well known for their potential usage in the lighting and display industry. The device efficiency and lifetime have improved considerably in the last three decades. However, for commercial applications, operational lifetime still lies as one of the looming challenges. In this review paper, an in-depth description of the various factors which affect OLED lifetime, and the related solutions is attempted to be consolidated. Notably, all the known intrinsic and extrinsic degradation phenomena and failure mechanisms, which include the presence of dark spot, high heat during device operation, substrate fracture, downgrading luminance, moisture attack, oxidation, corrosion, electron induced migrations, photochemical degradation, electrochemical degradation, electric breakdown, thermomechanical failures, thermal breakdown/degradation, and presence of impurities within the materials and evaporator chamber are reviewed. Light is also shed on the materials and device structures which are developed in order to obtain along with developed materials and device structures to obtain stable devices. It is believed that the theme of this report, summarizing the knowledge of mechanisms allied with OLED degradation, would be contributory in developing better-quality OLED materials and, accordingly, longer lifespan devices.