Exciplex Formation in Light Emitting Molecularly Doped Polymer Diodes Based on Polycarbonate:TPD:PBD Blends
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C O
CH3
n
(PC) Bisphenol-A-polycarbonate
N
N
CH3
CH3
(TPD) N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’ diamine N N t-Bu
O
(PBD) 2-(4-biphenyl)-5-(4-terbuthylphenyl)1,3,4-oxadiazole Figure1. Chemical structures of the materials used in the PL and EL measurements. Absorption spectra were recorded with a Perkin-Elmer Lambda9 UV/Vis /NIR spectrophotometer; PL and EL measurements were made using a SPEX Spectrofluorimeter. PL lifetimes were determined with an IBH single-photon-counting equipment (N2 lamp, excitation at 337 nm). RESULTS AND DISCUSSION Figure 2 shows the EL spectra of a SL PC(20%):TPD(40%):PBD(40%) OLED at different voltages and, for comparison, the absorption and PL spectra of films having the following composition: PC(25%):TPD(75%), PC(25%):PBD(75%) and PC(20%):TPD(40%):PBD(40%). The PL spectrum of the three component mixture is broad, almost structureless, with a maximum considerably red shifted with respect to the PL spectra of PC films doped with either PBD or TPD (both spectra are well structured with maximum at 370-380 nm and 400-420 nm respectively , and correspond with the PL spectra of the same compounds in diluted solutions). The high-energy region of the spectrum shows a small shoulder at 400 nm, which probably is due to monomeric PBD or TPD emission.
Absorbance
1.2
0.8
0.6
Abs
0.4 I
0.4
0.2
II
0.0
0.0 200
Intensity / a.u
1.2 PBD 75% TPD 75% 1.0 PC:TPD:PBD I EL(U=12V) II EL(U=18V) 0 . 8
PL
1.6
300
400
500
600
700
Wavelength / nm Figure 2 Absorbance and PL spectra of PC(25%):TPD(75%) [λexc=360nm], PC(25%):PBD(75%) [λexc=300nm], PC(20%):TPD(40%):PBD(40%) [λexc=360nm]. EL spectra of ITO/PC(20%):TPD(40%):PBD(40%)/Ca,Ag devices at different applied voltage : I = 12 V; II = 18 V. Fluorescence emission bands, red shifted if compared with the emission from the same materials in dilute solutions, often have been observed in organic film. Since our results exclude the emission from impurities , and the absorption spectrum of the film containing PBD and TPD is the sum of the absorption of the two components, without any additional band denoting the formation of a charge transfer complexbetween TPD and PBD in the ground state, we argue that the band with maximumat 460-470 nm is due to exciplex formation between TPD and PBD. The red shifted emission originates from an exciplex resulting from the interaction of excited electron donor (TPD)*or acceptor (PBD)* molecules with its unexcited counterpart PBD or TPD. The exciplex energy (Eexc) can be approximated by its value for exciplex formation in non-polar solvents, knowing the oxidation potential of TPD, Eox(TPD), and the reduction potential of PBD, Ered(PBD), according to the equation [6,7]: Eexc = Eox(TPD) – Ered(PBD) – 0.15 eV = (0.35 + 2.4 – 0.15) eV = 2.6 ± 0.1 eV which gives excellent agreement with experiment, using the oxidation potential of TPD≅0.35 V [8] and the reduction potential of PBD≅-2.4 V [9]. Generally Eexc is lower than donor or acceptor excited singlet energy. This behavior is not surprising, it
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