17784
J. Phys. Chem. B 2006, 110, 17784-17789
Electrochemically Deposited Organic Luminescent Films: The Effects of Deposition Parameters on Morphologies and Luminescent Efficiency of Films Mao Li, Shi Tang, Fangzhong Shen, Meirong Liu, Weijie Xie, Hong Xia, Linlin Liu, Leilei Tian, Zengqi Xie, Ping Lu, Muddasir Hanif, Dan Lu, Gang Cheng, and Yuguang Ma* Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin UniVersity, Changchun 130012, China ReceiVed: May 22, 2006; In Final Form: July 7, 2006
The electropolymerization behaviors of an electroactive and luminescent compound TCPC as precursor are studied. The resultant electrochemical deposition (ED) films are characterized by cyclic voltammetry (CV), UV-vis, fluorescence spectra, scanning electron mictroscopy (SEM), and atomic force microscopy (AFM). Under the CV mode with potential range of -0.5 to 0.85 V vs Ag/Ag+, the coupling reactions between the carbazole units of TCPC are very efficient, while the fluorescent trifluorene segment in TCPC is chemically inert in this potential range, which results in a highly fluorescent film formation on indium tin oxide (ITO) electrode. The deposition parameters for preparing the TCPC-based ED films are optimized, and the best ED film gives the fluorescence efficiency of 45.5% with surface roughness of 2.8 nm and morphologic stability as heating to 180 °C. The light-emitting devices (LEDs) using this ED film as light emitting layer with structure ITO/ED film (∼100 nm)/Ba/Al achieve maximum luminescence and external quantum efficiency of 4224 cd/m2 at17 V and 0.72% at11.5 V, respectively, which are better than the device using TCPC spincoating films as emitting layer. The technique provides a facile route toward a patternable luminescent film and device because such luminescent ED films can be manipulatively deposited on the electrified electrode.
1. Introduction
CHART 1: Chemical Structure of the TCPC
Generally, electrochemical synthesis of conjugated polymers exhibit electrical conductivity and charge transport,1-4 while there is very little attention to optimize the optical properties of the emitting layer for organic light-emitting diodes (OLEDs). This is due to a variety of factors, including structural defects and doped counterions present in the electrodeposition (ED) films,5-7 which strongly quench the fluorescence. Previously, the important electroluminescent polymers poly(p-phenylene) and poly(p-phenylenevinylene) were electrochemically polymerized using cyclic voltammetry (CV) on the ITO for OLEDs, but the device performance was very poor due to the very weak fluorescence of ED films.8-11 Recently, Advincula and coworkers12 reported the luminescent thin films of conjugated polymers on a flat conducting substrate using the electropolymerization between pendant electroactive carbazole units in a polyfluorene precursor. The large difference in oxidation potential between polyfluorene and the carbazole units ensures the electropolymerization of the pendant carbazole groups without affecting the polymer backbone. The fluorescence from these ED films were observed, but the emission exhibited the low-energy band at 530 nm and seemed not very strong, likely due to the aggregation of polyfluorene chain and structural defects, e.g., ketonic and charge doped species.6 To enhance the emissive efficiency of ED films, we have designed and synthesized an electroactive and structurally welldefined compound TCPC (Chart 1). The compound TCPC composed of a trifluorene backbone and peripheral carbazole units, where the trifluorene backbone with strong blue fluorescence plays the role of luminescence and peripheral carbazole * Corresponding author. E-mail:
[email protected].
units with coupling activity under anodic oxidation play the role of the crossing site in electropolymerization. We report the studies for the electropolymerization behaviors of TCPC, the optical properties of resultant ED films, and the effects of electrochemical deposition conditions on the ED film properties. The ED films obtained under optimized conditions show high fluorescence and smooth surface morphology, which can be used as an emitting layer for OLEDs. Herein, we report the results. 2. Experimental Section 2.1. Materials. The electropolymerizable precursor compound TCPC is synthesized by the Suzuki coupling reaction and completely characterized by NMR, FT-IR, and elemental analysis (see ESI). 2.2. General Electrochemical Experiments. Supporting electrolytes tetrabutylammonium hexafluorophosphate (TBAPF6) and tetrabutylammonium tetrafluoroborate (TBABF4) were purchased from Aldrich and dried for 24 h under vacuum before use. Solvents for electrochemical experiments were the mixture of acetonitrile and CH2Cl2 (v/v ) 3/2), which were carefully purified and purged by dry nitrogen prior to the electrochemical measurements.
10.1021/jp0631230 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/19/2006
Electrochemically Deposited Organic Luminescent Films
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17785
Figure 1. CV of the TCPC recorded using ITO as the working electrode, TBABF4 as supporting electrolyte at scan rate of 50 mV/s.
The cyclic voltammety (CV) experiments are performed using a standard one-compartment, three-electrode electrochemical cell given by a BAS 100B/W, Bioanalytical Systems. In all cases, potential refers to the system Ag/0.1 M AgNO3 in acetonitrile calibrated versus the ferrocene redox couple. The ITO (∼1 cm2) was used as the working electrode and titanium metal as the counter electrode (area: ∼3 cm2). In the electrochemical experiment, TCPC (1 mg/mL) and TBABF4 or TBAPF6 (0.1 M) in a mixture of acetonitrile and CH2Cl2 (v/v ) 3/2) was used as electrolyte solution. After ED process, the resulting ED film is then washed with acetonitrile to remove any unreactive precursors. 2.3. Instruments for Characterization of ED Precursor and ED Films. The 1H NMR spectra were recorded on AVANCZ 500 spectrometers at 298 K by utilizing deuterated chloroform (CDCl3) or dimethyl sulfoxide (DMSO-d6) as solvent and tetramethylsilane (TMS) as standard. The compounds were characterized by Flash EA 1112, CHNS-O elemental analysis instrument. FTIR spectra were recorded on Perkin-Elmer spectrophotometer in the 400-4000 cm-1 region using a powdered sample in a KBr plate. The LC/TOF/MS mass spectra were recorded using an Applied Biosystems QSTAR instrument. UV-vis absorption spectra were recorded on a UV-3100 spectrophotometer. Fluorescence measurements were carried out with RF-5301PC. Atomic force microscopy (AFM) images were taken in air and ambient conditions by tapping mode using a Nanoscope II, Digital Instrument system equipped with a 5 × 5 µm2 scanner and a silicon nitride tip. The field emission scanning electron microscope (FESEM) images were taken by JSM-6700F. 3. Results and Discussion 3.1. Electrochemical Behavior of TCPC. The electropolymerizable precursor compound TCPC was characterized first by CV using a standard one-compartment, three-electrode electrochemical cell in acetonitrile/CH2Cl2 at room temperature under nitrogen with a scanning rate of 50 mV/s. Figure 1 shows the CV of TCPC in first CV scan. From the CV in the potential sweep between 0 and 1.15 V, two oxidation peaks were observed at 0.87 and 1.04 V, respectively. The oxidation peak in relatively low potential of 0.87 V, which has an oxidation onset potential of 0.79 V, is attributed to the oxidation of carbazole units in TCPC, as the existing research for electrochemical behaviors of carbazole and its derivatives have shown the oxidation and onset potentials at the same positions.7,13 The oxidation species of carbazyl radical cation can react with each other to form dimeric carbazyl easily, which is the essence of present studies of electrochemically deposited films of TCPC. The oxidation
Figure 2. CV for TCPC recorded for 10 scan cycles using ITO as the working electrode, TBABF4 and TBAPF6 as supporting electrolytes respectively at Scan rate of 50 mV/s.
peak in a relatively high potential of 1.04 V is attributed to oxidation of the trimer fluorene units in TCPC.14-16 On subsequent scans in the potential range from 1.15 to 0 V, a reduction peak was observed at low potential of +0.51 V, which corresponds to the formation of neutral dimeric carbazyl from dimeric carbazyl cation.7,13 The investigation for the first cycle CV of TCPC demonstrated that (1) the ring-ring coupling reaction as anodic oxidation of carbazyl is very efficient, because no reduction peak relative to monomeric carbazyl radical cation is observed, (2) if the anodic potentials are controlled in the range 0.79 V (onset of carbazyl oxidation) to 0.85 V (oxidation peak of carbazyl), the coupling reaction between carbazyl units may occur and the trimer fluorene backbone (fluorescence units) will not be affected as the oxidation of trifluorene backbone occurs at more positive potential (1.04 V), which is very important to obtain a high fluorescent electrodeposited film, although even at low anodic potentials (
