Elucidation on Joule heating and its consequences on the performance of organic light emitting diodes

Current work presents a quantitative analysis of Joule heating by temperature measurements using infrared thermography and heat estimation of organic light emitting diodes (OLEDs) and their correlation with device life time. These temperature measurements were performed at 10, 20, 30, 40, and 50 mA/cm(2) current densities and studied with operational time. The temperature rise of the device has increased from 9.8 to 16.6 degrees C within 168 h at an operating current density of 40 mA/cm(2). This has been ascribed as due to the external contamination by water, oxygen, and dust particles as well as by internal heat generation. Encapsulation of the device avoids external degradation of OLEDs by preventing the destruction caused by these external contaminations. In this way, encapsulation has led to the decreased temperature rise of 12.4 degrees C within the duration of 168 h, which reflects the improved stability of the device. The temperature measured has been used to calculate the heat generated inside the device by solving the heat conduction equation using a transverse matrix approach. It has been found by these calculations that about 97%-98% of the power supplied to the device are converted into the heat for un-encapsulated device and results in rapid degradation of device with time, which in turns leads to the increase in operating voltage and decrease in luminous intensity with operational time. Proper encapsulation has reduced the heat generated inside the device by about 3%-4%, thereby, increasing the life time of the device. However, the glass encapsulation reduces the possibilities of the device cooling by heat convection to the atmosphere and prohibited the maximum utilization of encapsulation

Influence of chirp time-bandwidth on frequency modulated thermal wave imaging based materials characterization technique

Abstract An estimation of thermo-physical properties such as thermal diffusivity, thermal conductivity and heat capacity are extremely important for any practical and industrial applications. Contact free and fast measurement approaches using active infrared thermography principles have shown prolific results in this regard. The present study demonstrates frequency modulated thermal wave imaging (FMTWI) as a fast and efficient in-plane thermal diffusivity measurement technique. Here, a novel photo-thermal excitation signal in the form of a chirp is applied on the sample surface and the thermal response is monitored using an infrared (IR) camera. The in-plane thermal diffusivity of any self-sustaining sample can be measured using the multiple phase information extracted from a single run of the experiment. The applied excitation signal is characterized by the chirp bandwidth and time period which are also related to the noise-equivalent temperature (NET) of the IR camera used. The influences of these two parameters on the final thermal diffusivity results have been discussed. Experimental results from measurements done on standard anodic alumina (AAO) templates have been included

MAGeI₃-Based Multi-Dimensional Perovskite Solar Cells for Superior Stability and Efficiency

Perovskite solar cells (PSCs) have driven improvements in photovoltaic technology as a promising post-silicon photovoltaic technology. However, their decency in providing efficiency is quite intriguing but remains poor in stability. Advancement in lower dimensional technology indicates the shortcomings of 3D perovskite materials, which can be overcome by the introduction of 2D perovskites in an appropriate manner. Two-dimensional perovskites have piqued researchers’ interest in photovoltaic technology because of their remarkable structural and electrical properties which yield an increase in stability and enhance its light absorption properties. Therefore, 2D/3D multi-dimensional perovskite solar cells are expected to provide substantial stability and higher efficiency. In this study, 2D perovskite materials such as BA2MA2Pb3I10 and BA2MA2Pb4I13 were used as the capping layer on a 3D MAGeI3 layer to fulfil the mixed-dimensionality. The band alignments of both 2D and 3D perovskite were matched decently and other properties like defect tolerance and other IV characteristics on varying defect densities are provided in this study. Mixed-dimensional perovskite with n = 4 showed increased efficiency with respect to single 3D perovskite in decimals, yet is more stable in harsh environments

Conductive cooling in white organic light emitting diode for enhanced efficiency and life time

We demonstrate white organic light emitting diodes with enhanced efficiency (26.8 lm/W) and life time (similar to 11 000 h) by improved heat dissipation through encapsulation composed of a metal (Cu, Mo, and Al) and mica sheet joined using thermally conducting epoxy. Finite element simulation is used to find effectiveness of these encapsulations for heat transfer. Device temperature is reduced by about 50% with the encapsulation. This, consequently, has improved efficiency and life time by about 30% and 60%, respectively, with respect to glass encapsulation. Conductive cooling of device is suggested as the possible cause for this enhancement

Degradation of organic light emitting diode: Heat related issues and solutions

Degradation of organic light emitting diodes (OLEDs) is the most serious obstacle towards their commercialization. OLED degrades due to various internal and external mechanisms. External degradation is mainly caused due to the instability of low work function cathode, pin-hole formation during fabrication which provides a path for oxygen and moisture infiltration. Operation of OLED also leads to degradation with major causes being morphological instability of organic layers, trap formation, indium or oxygen diffusion from anode, interface deterioration etc. Heat generation in the OLED also acts as a source of degradation. Most of the heat is generated instantaneously upon biasing of OLED due to resistive or Joule heating as a consequence of high resistance of organic layers and non-radiative recombination. Generated heat can be reduced by reducing the effects of the generation sources such as improving conductivity of organic layers by doping, using additional layers to improve charge injection, employing emissive layers with low recombination losses etc. However, these ways can only reduce the heat up to a certain amount. To further improve the lifetime of OLED, the generated heat can be dissipated by employing heat sinks using either thermally conducting substrate or encapsulation etc. We present a review on OLED degradation with a particular focus on heat generation, its consequences and ways of reduction

Thermal diffusivity measurements of templated nanocomposite using infrared thermography

Electrodeposited bismuth telluride (Bi2Te3) nanowires using anodic alumina (AAO) templates show anisotropic thermal properties in a direction parallel and perpendicular to the nanowire/nanochannel axis. The present study reports thermal diffusivity measurement of templated Bi2Te3 nanowires in a direction perpendicular to the nanowire axis, using an active infrared thermography based noncontact technique. Measurements were performed on empty AAO templates of varying pore dimensions, AAO/Bi2Te3 nanocomposites and a single crystal Bi2Te3 sample. A first order lower bound model estimation showed nearly five-fold reduction of thermal conductivity in 200 nm Bi2Te3 nanowires as compared to the bulk values. (C) 2013 Elsevier B.V. All rights reserved