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J. Phys. Chem. B 2001, 105, 9524-9532
Charge Transfer and Recombination Kinetics at Electrodes of Molecular Semiconductors Investigated by Intensity Modulated Photocurrent Spectroscopy Torsten Oekermann,† Derck Schlettwein,* and Nils I. Jaeger Institut fu¨ r Angewandte und Physikalische Chemie, UniVersita¨ t Bremen, Fachbereich 2 (Biologie/Chemie), Postfach 330 440, D-28334 Bremen, Germany ReceiVed: February 28, 2001; In Final Form: June 30, 2001
Vapor-deposited thin films of phthalocyaninatozinc(II) (PcZn), hexadecafluorophthalocyaninatozinc(II) (F16PcZn), and N,N′-dimethyl perylene tetracarboxylic acid diimide (MePTCDI) were investigated by electrochemical impedance spectroscopy (EIS), photocurrent transient measurements in the millisecond-regime, and by intensity modulated photocurrent spectroscopy (IMPS). Interfacial states which act as traps and recombination sites (surface states) were detected. Quantitative kinetic data could be obtained from IMPS for p-type PcZn, where light-induced electron transfer to Fe(CN)63- and p-benzoquinone was found to occur mainly from the LUMO to adsorbed reactant molecules, whereas transfer from surface states plays a minor role. This was found to be opposite in the electron transfer from PcZn to oxygen which occurred mainly via surface states. F16PcZn was found to behave as a compensated n-type semiconductor after storage in air. Surface states were detected which can be occupied by photogenerated electrons and led to their partial subsequent transfer to the electrolyte. Also found were near-surface states which can be occupied by photogenerated holes but which do not lead to subsequent charge transfer to the electrolyte. At MePTCDI, another n-type material, adsorption of electroactive species from the electrolyte not only led to light-induced charge transfer to the adsorbed reactant but also to the reversible generation of additional surface traps. The results are rationalized by the rates of competing reactions, and implications for the use of such films in chemical sensors and organic photovoltaics are discussed.
Introduction The electrical and electrochemical properties of molecular semiconductors1 are of interest for many current2 and future3,4 applications of these substances. Especially the photoelectrochemical properties of molecular semiconductors have recently regained interest in view of their possible application in photovoltaic cells. Phthalocyanines (Pc) and perylene pigments in particular are of interest for this application, due to their high absorption coefficients for visible light, their chemical and thermal stabilities,5-8 and their commercial availability. These classes of materials already have been tested successfully for applications in all-solid state9-14 and electrochemical15,16 photovoltaic cells as well as in dye-sensitized solar cells.17-21 Another possible application of phthalocyanines and perylene pigments is their use in sensing layers of gas sensors, which are based on a change in the conductivity of the films under the influence of gas molecules adsorbed on the film surface or diffusing into the films.22-26 Especially in view of applications in photovoltaics and sensors, a thorough knowledge of the surface properties of the thin films is important to understand and further improve their performance. Dynamic photoelectrochemical methods have proved to be proper tools for the investigation of semiconductor surfaces.27 Photocurrent transient methods28-34 and intensity modulated photocurrent spectroscopy (IMPS),27,29,35-41 where * Corresponding author. E-mail:
[email protected]. Present address: Physical Chemistry 1, Department of Chemistry, University of Oldenburg, Postfach 2503, D- 26111 Oldenburg, Germany. † Present address: Graduate School of Engineering, Environmental and Renewable Energie Systems (ERES) Division, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan. E-mail:
[email protected].
the working electrode is illuminated with sinusoidal modulated light, have been used for this purpose in particular. While the use of IMPS was almost limited to inorganic semiconductors and dye sensitized inorganic electrodes so far, photocurrent transient measurements had already successfully been employed to characterize thin films of phthalocyanines and perylene pigments.34,42-44 In the photocurrent transients, the charging and discharging of surface states could be observed. It was found that charge transfer to the electrolyte can occur from surface states as well as from the HOMO or LUMO. An adsorption step of the electroactive species in the electrolyte is necessary to enable charge transfer, which was seen in a saturation of the steady-state photocurrents and a corresponding decrease of the charging and discharging currents with increasing reactant concentration in the electrolyte. The presence of surface states could also be shown by partial Fermi-level pinning.34 In this paper we report about IMPS measurements which have been performed at thin film electrodes of molecular semiconductors to study their photoelectrochemical kinetics in a more quantitative way. Phthalocyaninatozinc(II) (PcZn) was chosen as a typical p-type molecular semiconductor,6,45-49 N,N′dimethyl perylene tetracarboxylic acid diimide (MePTCDI) as a typical n-type molecular semiconductor.48-51 Combinations of these two materials have already been investigated in solidstate organic heterojunctions8,51 and photoelectrochemical cells.16 To investigate the influence of electron withdrawing groups in the Pc molecule, hexadecafluorophthalocyaninatozinc(II) (F16PcZn) was included in the investigation. It was shown before, that electron-withdrawing substituents in Pc lead to a
10.1021/jp0107661 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/07/2001
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change from p-type to n-type behavior, which is caused by a stabilization in the energetic positions of the frontier orbitals.8,44,52-54 IMPS measurements have been performed in aqueous electrolytes using different electroactive species in various concentrations to study the respective changes in the charge-transfer rates. The present IMPS measurements are compared with photocurrent transients measured under the same conditions and discussed with respect to a quantitative model considering charge transfer via both frontier orbitals and surface states. Experimental Section PcZn was purchased from Aldrich, MePTCDI was obtained from Hoechst and F16PcZn was synthesized as described earlier.54 All pigments were purified by zone sublimation. Other chemicals were purchased in analytical grade and used without further purification. ITO (indium tin oxide) with an average transmission of 85-90% in the visible region and a surface resistance of 5 Ω cm-2 was obtained from Flachglas and cut into pieces of 1 × 1.5 cm, which were contacted by a glass covered copper wire using conductive adhesive Ecobond 57C (Grace). The connections and a part of the ITO were sealed with Araldite Rapid epoxy resin (Ciby Geigy), leaving a square of 1 × 1 cm free ITO area. The thin films were vapor deposited on the prepared ITO electrodes at 10-5 Torr, 25 °C, and a deposition rate of 0.05 nm s-1. The film thickness was 100 nm, as controlled by the change in resonance frequency of a calibrated quartz crystal during evaporation. Atomic force microscopy (AFM) in the resonant mode of a NTMDT SMENA revealed nanoparticulate films that consisted of particles with an average diameter of 100 nm, which is in the range of the film thickness. Under the mentioned conditions, films of reproducible structure were obtained for each given material. This was detected by a constant relative alignment of chromophores as concluded from constant optical absorption spectra of the electrodes, which were recorded using a Perkin-Elmer Lambda 2 spectrometer. If not mentioned otherwise, electrodes exposed to air for at least 15 days after preparation were used in the electrochemical and photoelectrochemical experiments. After this time period both the density of surface states and the doping level of the electrodes were shown to remain constant.34 The electrochemical and photoelectrochemical measurements were performed at 25 °C in aqueous solutions containing 1 mol l-1 KCl, deaerated by purging the cell with nitrogen. All photoelectrochemical experiments were carried out potentiostatically in a 100 mL glass cell with a quartz window, a saturated calomel reference electrode, a platinum counter electrode, and a molecular semiconductor thin film electrode as the working electrode. Photocurrents were calculated as the difference between the overall current under illumination and the dark current. For photocurrent transient measurements the light source was an Oriel 1000 W Xe arc lamp equipped with a water filter to avoid IR radiation. White light with an intensity of 250 mW cm-2 (measured with a Kipp & Zonen CA1-754399 thermopile), which led to uniform illumination of the active electrode area, was used in these experiments. Illumination of the electrode was controlled using a Prontor magnetic E/40 mechanical shutter. The shutter needs 10 ms to reach a completely open position as measured with a calibrated Siemens SFH 291 silicon-PIN-photodiode. The transients were measured using a Jaissle bi-potentiostat-galvanostat with a rise time of 100 µs, connected to a Hameg HM 305 oscilloscope. The transients obtained by the oscilloscope were recorded by a personal computer using Hameg SP91-2 software.
Figure 1. Schematic representation of the photoelectrochemical processes at the surface of a p-type molecular semiconductor electrode, including the charging and discharging of surface states.34 The ki indicate rate constants.
For IMPS, high-intensity LEDs were used as light sources. PcZn and F16PcZn electrodes were illuminated with a red LED (626 nm) from Toshiba (TLRH 190 P), which led to a light intensity of 1 mW cm-2 at the electrode. For MePTCDI a green GaP LED (520 nm) from Marl (SPG 41510) was used, which led to a light intensity of 0.1 mW cm-2. An amplifier had been built for both LEDs which enabled the modulation of the light intensities with frequencies up to 100 kHz. The light intensities were modulated sinusoidally by (10%, as measured with the Siemens SFH 291 silicon-PIN-photodiode. An EG&G 7220 lock-in amplifier (frequency range 10-3 Hz-120 kHz) with function generator was used to control the modulation of the light as well as to measure the amplitude and the phase shift of the photocurrents. The photocurrents were measured potentiostatically using a Jaissle IMP 88 potentiostat (rise time 25 V/10-6 s, phase shift
