Trap Assisted Carrier Recombination in 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran Doped Bis[2-(2-hydroxyphenyl)bezoxazolate] Zinc Virendra Kumar RAI, Ritu SRIVASTAVA  , Gayatri CHAUHAN, Kanchan SAXENA, Suresh CHAND, and M. N. KAMALASANAN Centre for Organic Electronics, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-110012, India (Received October 5, 2007; accepted January 14, 2008; published online May 16, 2008) 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 Fo ¨rster mechanism for energy transfer from the host to the dye molecule. The photoluminescence of the host–guest system has been assigned to Fo ¨rster 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. [DOI: 10.1143/JJAP.47.3408] KEYWORDS: electroluminescence, photoluminescence, energy transfer 1. Introduction Doping of organic semiconductors to vary their electronic and photo-physical properties is one of the thrust areas of research in organic electronics. Use of different emitter materials for obtaining different colors is always not advantageous as some materials of desired emission colors may have lower luminous efficiencies, lower stabilities and poor electrical properties. Doping of stable and luminous efficient materials can be done with small amount of dyes to tune the emission of the material to desired colors without effecting much change in their electrical properties. Electro- luminescence (EL) in such cases can be exclusively from the dye molecule 1–4) as most of the energy is transferred from the host material to the dye by various energy transfer mechanisms like Fo ¨rster energy transfer, Dexter transfer or trap assisted carrier recombination. 1) Doping with phosphor- escent dyes have shown very high efficiency as the triplet excitons of the hosts can also transfer their energy to the dye molecules which otherwise are wasted in the normal fluorescence process. 5) Dye doping has also shown to slow the crystallization of the host matrix thereby increasing EL device lifetime. 6) For the successful use of this technique well matched guest–host systems are to be selected. Zinc based complexes are ideal choice as host materials because of their easy synthesis procedure and broad spectral features. Further bis[2-(2-hydroxyphenyl)bezoxazolate] zinc [Zn(hpb) 2 ] is a material with good thermal stability and excellent luminescence properties. It has a photolumines- cence (PL) maximum at 478 nm and spectral width of 110 nm (FWHM). These properties are ideal for the fabrication of white light emitting organic LEDs with proper doping. In the present paper we have synthesized an electro- luminescent material Zn(hpb) 2 and used it as a matrix to dope 4-(dicyanomethylene)-2-methyl-6-(4-dimethylamino- styryl)-4H-pyran (DCM) dye in it and studied its PL and EL properties at various dye concentrations. We found that the system satisfies the conditions for Fo ¨rster energy transfer and the PL emission of the dye doped matrix was dominated by this process where as the EL emission was dominated by trap assisted recombination of charge carriers at the dye molecules. 2. Experiment Zn(hpb) 2 was synthesized in a similar way as reported earlier. 7) EL devices were fabricated on glass substrates coated with 140 nm thick indium tin oxide (ITO) having a sheet resistance of 20 /Ã and optical transmittance of over 85%. These substrates were photo lithographically patterned having 5 mm strip and sequentially cleaned in deionized water, acetone, trichloroethylene and isopropyl alcohol for 20 min each in an ultrasonic bath and dried in an oven under flowing nitrogen gas. Prior to organic film deposition, the ITO surface was treated with oxygen plasma for 5 min to increase its work function. On to the substrate, the hole transport layer and the emitting layers were deposited sequentially under high vacuum (1  10 5 Torr) at a deposition rate of 0.1 nm/s. Thickness of the deposited layers were measured in situ by a quartz crystal monitor. N, N-diphenyl- N 0 N 0 -bis(1-naphthyl)-1,1 0 -biphenyl-4,4 0 -di- amine (-NPD; 40 nm, Sigma Aldrich) was used as hole transport layer, Zn(hpb) 2 doped with DCM dye (35 nm, x%, Sigma Aldrich) as emitting layer, LiF (1 nm, Merck) was used as electron injection layer and aluminum (150 nm) as the cathode metal (Cerac). The emitter layer has been evaporated from a molybdenum boat containing a premixed mixture of Zn(hpb) 2 and DCM dye in the desired ratio. The stoichiometry of the deposited films were checked by depositing same thickness of the material one after another from the same boat and examining the films using optical absorption spectroscopy. The variation in guest–host ratio was found less than 10 wt % in all the samples. For EL studies organic light emitting diodes (OLEDs) were prepared in pixel form. The size of each pixel was 5  5 mm 2 and device configuration was ITO/-NPD/Zn(hpb) 2 +DCM (x%)/LiF/Al. The device optimization have been achieved by fabricating four devices: A, B, C, and D with different concentration (x) of DCM dye in Zn(hpb) 2 matrix, that is, x ¼ 0, 0.01, 0.10, and 1.00 wt % respectively. The EL spectrum has been measured with a high resolution  E-mail address: ritu@mail.nplindia.ernet.in Japanese Journal of Applied Physics Vol. 47, No. 5, 2008, pp. 3408–3411 #2008 The Japan Society of Applied Physics 3408