Gayatri Chauhan

Trap Assisted Carrier Recombination in 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran Doped Bis[2-(2-hydroxyphenyl)bezoxazolate] Zinc

The energy transfer mechanism has been studied in 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyram (DCM) doped zinc complex bis[2-(2-hydroxyphenyl) bezoxazolate]zinc [Zn(hpb)2]. The photoluminescence of the zinc complex and the optical absorption of the dye molecule have been found to have a large overlap favoring Forster mechanism for energy transfer from the host to the dye molecule. The photoluminescence of the host–guest system has been assigned to Forster type energy transfer where as the electroluminescence has been found to be dominated by trap assisted recombination and subsequent decay of excitons. The charge carrier trapping has been supported by electrical transport studies.

Organic Light Emitting Diodes for White Light Emission

During the last few years, research based on energy saving technologies is being given high priority all over the world. General lighting is one area in which large quantity of electrical energy is being spend and substantial energy saving is possible by using energy saving technologies. Conventional light sources like incandescent filament lamps in which a major part of the energy is wasted as heat and is a less energy efficient technology is being phased out. Other technologies like gas filled electrical discharge lamps are more efficient but are polluting. Therefore there is a need for energy efficient and clean light source and solid state lighting is one of the ways to address the problem Organic light emitting diodes (OLED) is a new technology which has the potential to replace the existing lighting technologies. The attraction to organic semiconductors for lighting and display application has started during 1950-1960 because of the high fluorescence quantum efficiency exhibited by some organic molecules and their ability to generate a wide variety of colors. Study of electroluminescence (EL) in organic semiconductors have started in 1950s by Bernanose et.al (1953) using dispersed polymer films This was followed by the study of electroluminescence in anthracene single crystals by Pope et al (1963) and W.Helfrich et.al. (1965) who has studied the fundamental aspects of light generation in OLEDs. Since the single crystal based anthracence OLEDs fabricated by Pope et al (1963) were very thick and worked at very high voltages, the devices were not commercialized. In 1987, Tang and VanSlyke (1987) of Eastman Kodak has demonstrated a highly efficient multi layer OLED device based on vacuum evaporated aluminum tris 8-hydroxy quonoline (Alq3)as the emitter material. The device had different layers for hole transporting, electron transporting and light emission. Transparent Indium Tin Oxide (ITO) and aluminum metal were the anode and cathode respectively. Quantum efficiency and luminescence efficiency of 1% and 1lm/W respectively were considered enough for commercial application. This work has stimulated a very intense activity in the field of Organic electroluminescence. Numerous improvements in device structure and addition of more layers having different functionalities were incorporated and are now on the verge of commercialization. Further, the developments in conjugated polymers by Heeger, MacDiarmid, and Shirakawa in 10

White electroluminescence from hybrid organic inorganic LEDs based on thermally evaporated nanocrystals

Light-emitting diodes (LEDs) with a broad spectral emission have been fabricated using co-evaporated monolayers of cadmium sulfide (CdS) and cadmium telluride (CdTe) nanocrystals in a hybrid organic-inorganic structure. The nanocrystals of CdS and CdTe were grown by vacuum thermal evaporation technique. The incorporation of 50:50 ratio of CdS and CdTe resulted in a broad emission spectrum with CIE coordinates (0.22, 0.33). The fine tuning of the emission spectrum to achieve pure white light (CIE coordinates (0.30, 0.33)) was achieved by incorporating a small quantity of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyan (DCM) dye in the device structure. The study indicates a promising route towards more stable and efficient light-emitting devices for lighting applications.

Effect of sublimation on performance of CuPc: PTCDA bilayer organic solar cell

Silicon photovoltaic is the most commonly used commercial technology. However it has limitation in terms of efficiency and cost effectiveness. As such there is a need to search for alternate technologies for conversion of solar energy into electricity. Organic photovoltaic (OPV) is one of the emerging R&D areas in this direction. In OPV two approaches are being used for fabrication of solar cells i.e. using small molecule and polymers, respectively. In both the cases donor:acceptor (D/A) concept is being used for the fabrication of the device. For example in small molecule approach bilayer devices having D/A layer is used whereas in polymer solar cells D/A interpenetrating bulk hetrojunction is being used. Both the type of devices need improvement in their performance in terms of efficiency, stability, life time etc. Global efforts are being made to improve the performance of these devices by using new hetrojunction materials, interface layers, thermal treatments, purification/sublimation etc. The present work is also an attempt in this direction wherein we have improved the performance of small molecules based solar cell by the process of sublimation of the material used in device fabrication. The configuration of the device used in the present work was prepared as follows. Thin film of PEDOT: PSS was deposited on ITO substrate by spin casting. Subsequently copper(ll)phthalocyanine (CuPc) film (~45nm) (both without sublimation as well as with sublimation) was vacuum deposited in a vacuum ~ 10-6. Then on top of CuPc layer Perylen-3, 4,9,10-tetracarbonsaure-dianhydrid 3,4,9,10- Perylenetetracarboxylic dianhydride dianidride perilen-3,4:9,10-tetracarbossilico (PTCDA) film (~40 nm) was also vacuum deposited. Finally LiF and Al electrodes were vacuum deposited resulting in to a bilayer device having the configuration ITO/PEDOT: PSS/CuPc: PTCDA/LiF/AI. The device without sublimation of CuPc shows the solar cell parameters i.e. open circuit voltage (Voc),short circuit current density (Jsc), fill factor ( FF) and power conversion efficiency( eta) as 0.20 V, 2.8 times10- 6A/cm2, 0.32, and 2.2 times 10- 4% .respectively. However when the device is made in same configuration by sublimation of CuPc prior to its deposition a considerable improvement in the Jsc and eta of the device has been observed. In fact new electrical parameter obtained in the second case (sublimated/purified CuPc) being 0.12V, 52.6times 10- 6A/cm2, 0.25 and 13.2 times 10- 4% respectively. The improvement in the Jsc and overall eta of CuPc: PTCDA bilayer device has been attributed to the reduction in the defects/impurities in CuPc on sublimation.