Transient Photocurrent를 이용한 유기 LB 막에서 광 유발 전자전달현상에 대한 연구

조 경상, 최 정우, 이 원홍, 송 남웅*, 김 동호*

서강대학교 화학공학과, 한국 표준과학연구원 분광그룹*

Transient Photocurrent  Based on Photoinduced Electron Transfer in Hetero LB Film

K.S. Cho, J.-W. Choi, W.H. Lee, N.W. Song*, D. Kim*

Department of Chemical Engineering, Sogang University

Spectroscopy Laboratory, Korea Research Institute of Standards and Science*

INTRODUCTION

Various artificial molecular devices have been fabricated by mimicking the electron transport function of biological photosynthesis [1,2]. One of the characteristics of photosynthesis applicable to the artificial molecular electronic device is the photoswitching effect induced by exciting electrons under light illumination, which initiates electron transfer and enables photodiode effect by transferring electrons unidirectionally from electron donors to acceptors due to their redox potential differences. In the photoinduced electron transfer, excited electrons irradiated by light are transferred by way of charge separation, charge shift and charge transfer process, sequentially. To investigate the photoinduced electron transfer in hetero LB film, the measurement of time-resolved fluorescence decay and transient photocurrent study are needed. By time-resolved fluorescence measurement, charge separation rate can be measured and by transient photocurrent, charge shift and charge transfer rate can be measured. The authors have been performed the time-resolved fluorescence study in the electron donor/acceptor LB film [3]. In that study, the effects of distance of functional groups and redox potential difference between donor and acceptor on the photoinduced electron transfer rate have been investigated.

In this work, to investigate the dynamic process of electron transfer and to observe the performance of hetero LB film as a photoswitching device, the transient photocurrent experiment are performed using flavin as a sensitizer, ferrocene as a donor and TCNQ and viologen as an electron acceptor. Based on the transient photocurrent data, charge transfer dynamics of hetero LB films are investigated.

EXPERIMENTAL DETAILS

1. Materials

Three kinds of functional materials were used. Ferrocene octadecyl amine (Ferrocene), 7,8-dimethyl-10-dodecyl isoalloxazine (flavin), N-docosilquinolinium TCNQ (TCNQ), N-Allyl-N'-[3-propylamido-N",N"-di(n-octadecyl)]-4, 4'-bipyridium dibromide (viologen) were used as donor, sensitizer and acceptors, respectively. The deposition of LB films was carried out using a circular Langmuir trough (Type 2022, Nima Tech., England). The substrate (bottom electrode) is ITO glass and top electrode is vacuum evaporated aluminium.

2. Photodiode and photoswitching experiment

An exciting input light was generated with xenon lamp system (150W, Oriel Co., USA), and then the photocurrent produced from the molecular device was detected through a current-voltage measurement unit (SMU Model 216, Keithley, USA).

3. Transient photocurrent measurement

A light pulse from a Q-switched Nd:YAG-dye laser (FWHM: 6 ns, wavelength: 355 nm, repetition rate: 10 Hz) was introduced to excite the Flavin molecules. With fast electronics using a V/V amplifier of 300 MHz frequency (Model SR445, stanford research) and a storage oscilloscope of 500 ps resolution (2Gs/s, model 54616B, Hewlett-Packard), the interlayer photocarrier movement was detected as voltage built up between electrodes which form a 50 □ strip line geometry in order to acquire signals with high time resolution.

Fig. 1. Schematic illustration of a transient photocurrent measurement system

RESULTS AND DISCUSSION

The photocurrent-time response with the irradiation by a xenon lamp was shown in Fig. 1. When a forward bias was applied in accordance with the energy level profile in the MIM device, a stable photocurrent was generated. With repeated step illumination, the reproducible photocurrent was generated accordingly. The photocurrents were very stable and level of responses was consistent during the repeated cycle over 30 minutes. The results indicated that the photoswitching function of the MIM device was achieved. When reverse bias was applied, photocurrent was much smaller than that of the forward bias case. In the proposed molecular device, the photo-induced unidirectional flow of electron could be achieved due to the redox potential difference as well as electronic coupling between the functional molecules. To confirm the photodiode property of the MIM device, photocurrent (I) - voltage (V) characteristic was observed. As shown in Fig. 3, the difference between photocurrent and darkcurrent is ca. 2 nA at +1 V applied voltage, and the slope of the curve at forward bias is increased exponentially like rectifying effect. So the proposed device has effective photodiode property.

To find the electron transport mechanism and switching effect of the device, transient photocurrent was measured. Identification of the mechanism for photocarrier generation in the molecular device is of great is importance in order to understand the dynamical behavior of photoelectric properties based on photoinduced electron transfer process. The action spectrum of the photocurrent of the molecular device coincides well with the absorption spectrum of the flavin monolayers, which means that the initial process of carrier generation is photoexcitation of flavin. Fig. 3 and 4 shows transient photocurrents obtained by homo (flavin) and hetero (flavin/TCNQ) LB films. For these two samples, time constants for the initial decrease (or increase) of the transient photocurrent are found to be virtually identical to that of the rise of the excitation pulse (nanosecond order). This result means that the initial decrease (or increase) of the photocurrents is due to the charge separation process, the time constant of which is of sub-nanosecond order and that the time constant of the initial decrease (or increase) of the photocurrent is observed to be of nanosecond order because of the experimental time resolution. In the previous study using the time-resolved fluorescence [2], it was confirmed that charge separation process is picosecond order.

The solid line in Fig. 3 and 4 denote fitted results. Suffix 1 and 2 denote the direction from the heterojunction locus to the electrode attached to the TCNQ monolayers and to the flavin monolayers, respectively. The transient photocurrent of the molecular device involves elementary electron transfer process: (1) photoexcitation of flavin, (2) charge separation from excited flavin to TCNQ at the molecular heterojunction, (3) charge shift in monolayers, (4) charge transfer between monolayers and an electrode, and (5) discharge of the carriers. The photoexcitation and charge separation rate can be measured by time-resolve fluorescence measurement. It was found that the time constant of flavin homo LB film and flavin/TCNQ hetero LB film was measured 400 ps and 300 ps, respectively. The initial fall of photocurrent in the homo LB film, and rise of the photocurrent in the hetero LB film related with the charge separation rate. But due to the time resolution of electronics and input laser light pulse width, it is impossible to measure less than 2 ns time resolution. Charge separation rate occurs less than ns level so to find the charge separation rate using transient photocurrent is impossible but it was found that the charge separation occurred less than ns level based on photocurrent experiment result. The time constant of decay of transient photocurrent was found th be 5.46 □s (□1) and 77 ns (□2) for homo and hetero LB films, respectively. This is the charge shift and transport rate in the LB film. The charge shift and transport of hetero LB film is much faster than homo LB film due to the redox potential difference of flavin and TCNQ. The measured time constants of charge separation and charge shift in the homo and hetero LB film can be used as a basic data to find the electron transport mechanism of LB film molecular device.

REFERENCES

[1]. J.W. Choi, G.Y. Jung, S.Y. Oh, W.H. Lee and D.M. Shin, Thin Solid Films, 284/285, 876(1996).

[2]. J.W. Choi, J. Y. Bae, J. Min, K. S. Cho and W.H. Lee, Secsors and Materials, 8(8), 493(1996).

[3]. K.S. Cho, J.W. Choi, W.H. Lee, N.W. Song and D. Kim, accepted in Thin Solid Films, 1998.