Photoinduced electron transfer in a MIM device composed of ferrocene-flavin-viologen-TCNQ molecular heterojunctions

Jeong-Woo Choia,*, Sung Wook Chunga, Se Yong Oha, Won Hong Leea, Dong Myung Shinb

aDepartment of Chemical Engineering, Sogang University, P.O. Box 1142, Seoul 100-611, South KoreabDepartment of Chemical Engineering, Hong Ik University, Seoul 121-791, South Korea

Abstract

A molecular photodiode was fabricated with the hetero-type Langmuir–Blodgett (LB) film consisting of two electron acceptors (A1, A2),a sensitizer (S) and an electron donor (D).N-Docosylquinolinium TCNQ,N-Allyl- N′-[3-propylamido-N′′ ,N′′ -di(n-octadecyl)]-4,4′-bipyr-idinium dibromide, 7,8-dimethyl-10-dodecyl isoalloxazine and 1-1′ferrocene-N-dioctadecyl carboxamide were used as A2, A1, S and Dunits, respectively. One-way electron transfer from D to S was observed with cyclic voltammetry for the modified electrode containingcomplex S/D LB layers. Also, one-way electron transfer in S/A1, S/A2 and S/A1/A2 was observed based on the fluorescence quenchingmeasurement in the complex LB layers. By aligning hetero-type LB films of A2/A1/S/D four units on ITO glass with an aluminum thin film,a molecular photodiode with the metal/insulator/metal (MIM) structure was constructed. Due to excitation by irradiation with a 460 nmmonochromatic light source, the photoinduced unidirectional flow of electrons in the MIM device could be achieved and was detected asphotocurrents. The direction of energy flow was in accordance with the energy level profile across the LB films. The photoswitchingfunction was achieved and the rectifying characteristic was observed in the molecular device. 1998 Elsevier Science S.A. All rightsreserved

Keywords:Langmuir–Blodgett film; Molecular photodiode; Metal/insulator/metal device; Photocurrent

1. Introduction

In the initial process of photosynthesis, a biological elec-tron transfer system, photoelectric conversion occurs andthen long-range electron transfer takes place very efficientlyin one direction through the biomolecules [1]. The specificenergy and electron transfer takes place on a molecularscale due to the redox potential difference as well as theelectron transfer property of functional molecules, espe-cially the electron acceptor, sensitizer and electron donor[2].

Various artificial molecular devices have been fabricatedby mimicking the electron transport function of biologicalphotosynthesis [2–5]. The electrochemical photodiodeconsisting of Langmuir–Blodgett (LB) films of three func-tional molecules or an aligned triad on the electrode whichworked in electrolyte solution have been made [3]. Studiesof electron transfer between the electrode and the excited

dye molecules were carried out. The metal/insulator/metal(MIM) structured devices consisting of hetero-type LBfilms of an electron donor, a sensitizer and an electronacceptor were fabricated and photoinduced electron transferwas investigated [2,4,5].

In the present paper, the MIM structured device was fab-ricated with the hetero-type LB films consisting of TCNQ,viologen, flavin and ferrocene derivatives, which are thesecond electron acceptor (A2), the first electron acceptor(A1), a sensitizer (S) and an electron donor (D), respectively.One-way electron transfer in D/S hetero-type LB films andS/A1/A2 LB films was evaluated using the cyclic voltamme-try and the fluorescence measurement system, respectively.Molecules of four functional materials were arranged onITO glass regularly in space normal to the electrode surface,e.g. A2/A1/S/D, by the LB method. Finally by depositingaluminum on the hetero-type LB films, a molecular devicewith MIM structure was constructed. Photocurrent proper-ties of the MIM structured device were investigated to eval-uate the direction of electron transfer and photoswitchingfunction.

Thin Solid Films 327–329 (1998) 671–675

0040-6090/98/$ - see front matter 1998 Elsevier Science S.A. All rights reservedPII S0040-6090(98)00738-X

* Corresponding author. Tel.: +82 2 7058480; fax: +82 2 7110439;e-mail: jwchoi@ccs.sogang.ac.kr

2. Experimental details

2.1. Materials

Synthesis and characterization ofN-DocosylquinoliniumTCNQ and 1-1′-ferrocene-N-dioctadecyl carboxamide havebeen published [6,7].N-Allyl- N′-[3-propylamido-N′′ ,N′′ -di(n-octadecyl)1–4,4′-bipyridinium dibromide and 7,8-dimethyl-10-dodecyl isoalloxazine were synthesizedaccording to methods described previously [8,9]. Thedeposition of LB films was carried out with a circular Lang-muir trough (Nima Tech, Coventry, UK). From the preli-minary works, the target pressures for optimal dipping ofTCNQ (A2), viologen (Al), flavin (S), and ferrocene (D)were determined as 45, 37, 39 and 30 mN/m, respectively.Based on the transfer ratio, the types of LB films for A2, Al,S, and D were determined as Z, Y, Y, and Z type, respec-tively, and the resulting transfer ratio of each material wasapproximately 100% during the deposition procedure. Alu-minum used as a top electrode was deposited by vacuumevaporation with slow rate to minimize the film damage.

2.2. Cyclic voltammetry

Cyclic voltammetry was carried out at 25°C with a CV-75potentiostat (BAS, West Lafayette, IN). A three-electrodesystem composed of the working (Pt plate), reference (Ag/AgCl) and counter (Pt wire) electrode was used. The work-ing electrode was a Pt plate deposited with LB films (fourlayers) of each material for the measurement of redoxpotential. The one-way electron transfer from D to S wasmeasured by cyclic voltammetry. The modified workingelectrode was a Pt plate deposited by two layers of S andtwo layers of D in sequence.

2.3. Fluorescence measurement

The photoinduced electron transfer from excited S layersto A layers was evaluated by measuring the steady-statefluorescence quenching. A nitrogen laser (LSI LaserScience, Newton, MA) emitting 337 nm wavelength wasused as the light source and a photodiode array detector(Oriel, Startfort, CT) was used for the detection of thesteady-state fluorescence intensity.

2.4. Photocurrent measurement

The molecular photodiode was fabricated with the hetero-type LB films consisting of A2, Al, S and D units, respec-tively. A2, A1, S and D materials were arranged on ITO glassregularly in space normal to the electrode surface by the LBmethod. By depositing aluminum onto the hetero-type LBfilms, a molecular device with the MIM structure was con-structed. A schematic diagram of the apparatus for thephotocurrent measurement was shown in Fig. 1. An inputexciting light of wavelength 460 nm was generated with axenon lamp system. The photocurrent was detected througha current-voltage amplifier, analog/digital (A/D) converterand personal computer.

3. Results and discussions

3.1. Analysis of cyclic voltammograms

Based on the ellipsometry results, the film thickness ofA2, A1, S, and D LB films (four layers for each material)deposited onto a Pt plate was estimated as about 152, 120,65 and 168 A˚ , respectively. During the positive potentialsweep of D, the anode current reached a maximum at 0.563V (vs. Ag/AgCl electrode) (data not shown). This indicatesthat the concentration of oxidized D was maximized at0.563 V. When the cyclic direction was reversed, the oxi-dized form of D was reduced back to the original startingmaterial at 0.448 V. The redox potential of D was thereforedetermined as 0.51 V. The cyclic voltammogram of A2

under the Pt electrode showed two reversible peaks. Thetwo maximum cathodic peaks indicate the reduction of neu-tral A2 to A2

−1, followed by, at more cathodic peak poten-tials, the reduction of A2

−1 to A2−2. For the reduction of

neutral form to anion form, the formal redox potential was0.31 V. Based on the cyclic voltammogram of Al and S, theredox potential was found to be−0.30 and−0.33 V, respec-tively.

Based on the above results, the energy diagram of fourunits and the expected direction of photoinduced electrontransfer were depicted in Fig. 2. Due to the energy level, thephotoexcitation in the sensitizer should occur and then thephotoinduced electron should be transferred from the elec-tron donor to sensitizer while charge separation occursbetween them. Also the effective and efficient charge

Fig. 1. Schematic illustration of a hetero-type cell and experimental con-figuration: (1) 150 W xenon lamp; (2) 460 nm filter; (3) shield box; (4)MIM device; (5) current–voltage converter; (6) A/D converter; (7) perso-nal computer.

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separation can be achieved by attaching another electronacceptor to the sensitizer in such a way as to prevent rapidelectron return [10]. Since the redox potential levels of Sand Al were very close, it could be considered that theelectron transfer was initially favored between S and Al,and then electron hopping occurred between Al and A2.As the charge separation distance between the ionsincreases, electron return becomes unfavorable, whichresults in long-lived ion pairs which are needed to increasethe efficiency of the artificial photodiode.

3.2. One-way electron transfer

Cyclic voltammetry with a modified electrode of Pt/S/Dstructure was carried out as shown in Fig. 3. In the firstcycle, the oxidation peak was occurred at around 1.5 Vduring the positive potential sweep while the reductionpeak was not clearly occurred during the negative sweep.During the positive sweep, the electrons are fairly welltransferred from the LB films to the Pt plate (working elec-trode) since the energy level of D is higher than that of S(ground state), and thereby the oxidation peak is occurred.On the other hand, during the negative sweep, the electrons

are rarely transferred from the Pt plate to the LB films due tothe energy barrier since the energy level of S (ground state)is lower than that of D, and thereby the reduction peak hasnot obviously occurred. In the second, third, and fourthcycles, similar results were obtained except that the peakvalue of oxidation was gradually decreased as the sweepcycle was repeated. It can be explained that the attenuationof the peak value was due to the gradual decrease of theamount of the electrons to be transferred from D layers tothe Pt plate through S layers as the sweep cycle wasrepeated. From these results, it can be suggested that theelectron transfer between S/D hetero-type LB films wasunidirectional.

The photoexcitation of 460 nm light on S/A1 and S/A2

hetero-type LB films was carried out and the fluorescencespectra were shown in Fig. 4a,b. The fluorescence spectra ofS LB films shows that emitting fluorescence peak existed at

Fig. 2. Energy diagram of the A2/A2/S/D molecular system.

Fig. 3. Cyclic voltammogram of hetero-type LB films of S/D: curve a, thefirst cycle; curve b, the second cycle, curve c, the third cycle; curve d, thefourth cycle.

Fig. 4. (a) Fluorescence spectra of hetero-type LB films of S/A1: curve a, S(12 layers); curve b, S (12 layers)/Al (six layers); curve c, S (12 layers)/A1

(12 layers); curve d, S (12 layers)/A1 (18 layers). (b) Fluorescence spectraof heterotype LB films of S/A2: curve a, S (12 layers); curve b, S (12layers)/A2 (six layers); curve c, S (12 layers)/A2 (12 layers); curve d, S (12layers)/A2 (18 layers).

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560 nm. As the numbers of Al or A2 layers in hetero-type LBfilms were increased, the peak at 560 nm was decreaseddrastically. This result indicates that the fluorescencequenching started and then the charge separation betweenS and A1 or between S and A2 in hetero-type LB filmsfollowed [11].

The photoexcitation of 460 nm light on S/A1/A2 hetero-type LB films was carried out and the fluorescence spec-tra were shown in Fig. 5a,b. When the number of A2

layers were fixed, the peak was decreased as the numberof A1 layers was increased in Fig. 5a. When the numberof Al layers were fixed, a decrease of peak was observedin Fig. 5b. These results suggest that the photoexcitedelectron in the S molecules was transferred to the Al,A2 molecules and successive charge separation could beachieved.

3.3. Photocurrent response

Fig. 6a shows the photocurrent-time response with theirradiation of a 460 nm monochromatic light by a xenonlamp system. The photocurrent of the MIM device consist-ing of A2/A1/S/D was generated by light excitation. When aforward bias was applied in accordance with the energylevel profile in the MIM device, a stable photocurrent wasgenerated. With repeated step illumination, the reproduciblephotocurrent was generated accordingly. The photocurrentswere very stable and the level of responses was consistentduring the repeated cycle over a 30-min period. The resultsindicate that the photoswitching function of the MIM devicewas achieved. In the proposed molecular device, the photo-induced unidirectional flow of electrons could be achieveddue to the redox potential difference as well as electroniccoupling between the functional molecules. As shown inFig. 6a, it was also observed that the intensity of the photo-current was dependent on the external bias voltage. As theexternal bias voltage was increased, higher photocurrentswere generated. As shown in Fig. 6b, the rectifying char-

Fig. 5. (a) Fluorescence spectra of hetero-type LB films of S/A1/A2 withthe fixed number of A2 layer: curve a, S (12 layers); curve b, S (12 layers)/A1 (six layers)/A2 (12 layers); curve c, S (12 layers)/A1 (12 layers)/A2 (12layers); curve d, S (12 layers)/A1 (18 layers)/A2 (12 layers). (b) Fluores-cence spectra of hetero-type LB films of S/A1/A2 with the fixed number ofAl layer: curve a, S (12 layers); curve b, S (12 layers)/A1 (12 layers)/A2 (sixlayers); curve c, S (12 layers)/A1 (12 layers)/A2 (12 layers); curve d, S (12layers)/A1 (12 layers)/A2 (18 layers).

Fig. 6. (a) Photocurrent-time curves of the MIM device to the variousforward bias voltages: curve a, 3.5 V; curve b, 2.5 V; curve c, 1.5 V;curve d, 0.5 V. (b)I–V characteristics of the MIM device: (W) photocur-rent; (X) dark-current.

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acteristic was also observed from the current-voltage (I–V)measurement. When a 460 nm monochromatic light wasirradiated, the photocurrent was generated with the appro-priate bias voltage. On the other hand, the current was notgenerated in the dark environment (without light irradiation)even though the bias voltage was increased. From theseresults, it can be concluded that the diode characteristicsof the proposed device was verified and the proposed mole-cular array mimicking photosynthetic reaction center can beusefully applied as a model system for the development ofthe molecular photodiode.

Acknowledgements

This work was supported by the Research Fund on Opti-cal Technology Project (F-15, 1996) of the Korean Ministryof Science and Technology. Acknowledgment is also madeto Dr. S. Isoda and Mr. S. Ueyama at the Advanced Tech-nology R&D Center of Mitsubishi Electronics for their help-ful discussions.

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