Photoinduced Electron Transfer and Photocurrent in Multicomponent

Jun 18, 2008 - Kimmo Kaunisto*, Tommi Vuorinen, Heidi Vahasalo, Vladimir Chukharev, Nikolai V. Tkachenko, Alexander Efimov, Antti Tolkki, Heli Lehtivu...
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10256

J. Phys. Chem. C 2008, 112, 10256–10265

Photoinduced Electron Transfer and Photocurrent in Multicomponent Organic Molecular Films Containing Oriented Porphyrin-Fullerene Dyad Kimmo Kaunisto,*,† Tommi Vuorinen,‡ Heidi Vahasalo,† Vladimir Chukharev,† Nikolai V. Tkachenko,† Alexander Efimov,† Antti Tolkki,† Heli Lehtivuori,† and Helge Lemmetyinen† Department of Chemistry and Bioengineering, Tampere UniVersity of Technology, P.O. Box 541, FI-33101 Tampere, and VTT Technical Research Centre of Finland, P.O. Box 1300, FI-33101 Tampere, Finland ReceiVed: January 12, 2008; ReVised Manuscript ReceiVed: April 15, 2008

Layers of poly(3-hexylthiophene), PHT, phenyl vinyl thiophene, PVT3, poly(p-phenylene-2,3′-bis(3,2′diphenyl)-quinoxaline-7-7′-diyl), PPQ, and covalently linked porphyrin-fullerene donor-acceptor dyad, P-F, were deposited as various multilayer films, which then were used to study photoinduced electron transfer and photocurrent generation. The aim of the research was to clarify functioning of different energy and electron donating and accepting layers in charge transfer processes, which were initially created in a film consisting of parallel P-F molecules. The reactions were studied by means of time-correlated and steady-state fluorescence, time-resolved photovoltage, and electrochemical photocurrent measurements. The longest-lived charge-separated state and the highest efficiency of photocurrent generation were obtained for the multilayer structure of PHT|PVT3|porphyrin-fullerene. Porphyrin-fullerene dyads deposited parallel as the Langmuir –Blodgett film transfer electrons from porphyrin to fullerene yielding radical cation and anion moieties, respectively. The dyad on a PHT layer induces electron donation from PHT to the porphyrin cation. When PVT3 is deposited between the PHT and the P-F layers, it promotes both energy and electron transfer to the porphyrin moiety of the dyad, retards the recombination of the primary charge-separated state, and thus increases the photocurrent generation. PPQ was used as an electron acceptor from the fullerene radical anion, causing an increased lifetime of the charge separation. 1. Introduction Organic photovoltaic devices have been under intensive research during the past few decades.1–4 Molecular systems capable of intramolecular photoinduced electron transfer (PET) are widely studied and efficient electron donor-acceptor (DA) pairs have been discovered.5–11 The most recent studies are related to the DA dyads where electron donor and acceptor moieties are covalently linked to each other.12–15 In particular, different types of porphyrin-fullerene dyads have gained a lot of attention and intramolecular charge separation (CS) in this kind of dyads has been reported.16–19 Fullerene and porphyrin are ideal building blocks for DA dyads because of their excellent electron-accepting and good electron-donating properties, respectively.20,21 Even more complex molecular structures are synthesized and shown to be capable of photoinduced CS.22–25 To study intra- and intermolecular CS in the solid state, the Langmuir-Blodgett (LB) technique can be used to manufacture highly ordered molecular monolayers from amphiphilic molecules and deposit them onto solid substrates.26 Even if the LB technique sets certain requirements for the molecules, it is a powerful method to create desired multilayer structures from compounds of interest. Orientation of DA molecules, and thus direction of the charge transfer (CT), can be controlled by direction of the LB layer deposition. * To whom correspondence should be addressed. E-mail: kimmo.kaunisto@ tut.fi. Phone: +358 3 3115 3627. Fax: +358 3 3115 2108. † Tampere University of Technology. ‡ VTT Technical Research Centre of Finland.

A molecular photovoltaic device can be considered as interacting molecular assemblies with different functions.27–31 The primary photoinduced CS between a donor and an acceptor is the fundamental process for the operation of the entire device. As a result of the primary CS, a radical cation and an anion are formed.32–34 The charges generated in the DA interface have to be separated from each other to prevent the charge recombination (CR) and delivered to the electric circuit.35,36 An electron transporting layer (ETL) is used to capture the electron from the radical anion and transfer it to an anode.37 The function of a hole transporting layer (HTL) is to induce a secondary electron transfer (ET) to the radical cation and evacuate the hole to a cathode.38,39 The function of an energy transfer (ENT) layer is to expand the absorption spectrum of the device and transfer its excitation energy to the DA pair, and thus increase efficiency of the primary CS. Careful analysis of these processes is vital for understanding the performance of the photovoltaic device. The knowledge of each process yields the right design parameters for manufacturing of efficient photovoltaic devices. In the present study, we focus especially on photoinduced CT processes in solid films and on their functions as photovoltaic devices. This is the first paper where orientation of a donor-acceptor pair has been utilized as an initiator for ET through multicomponent films, and also the first paper where the interlayer interactions have been studied systematically by using several photochemical and photophysical methods. Influences of different electron-donating and -accepting layers on the CT reactions in sandwich-like film structures were characterized mainly by means of the time-resolved Maxwell displace-

10.1021/jp8003008 CCC: $40.75  2008 American Chemical Society Published on Web 06/18/2008

Porphyrin-Fullerene Dyad Multilayer Films

Figure 1. Chemical structures and acronyms of the used compounds.

ment charge (TRMDC) and the electrochemical photocurrent (PC) methods. The function of each layer was identified by measuring several layer sequences with different methods and by comparing the results to each other in order to cross-check the conclusions about the layer functions. Thiophene derivative, PVT3, was used both as the ENT and the ET layer. An n-type semiconducting polymer, PPQ, was used as a secondary electron acceptor from the fullerene radical anion.40 The primary CS in the porphyrin-fullerene dyad8,13 and the hole conducting properties of poly(3-hexylthiophene), PHT, have been studied earlier.41,42 The secondary ET from PHT to porphyrin cation is known to occur with yield close to unity.38 2. Materials and Methods All sample preparation and measurements were carried out in air at room temperature. 2.1. Materials and Their Acronyms. Structures of the porphyrin-fullerene dyads ZnDHD6ee, P-F, and TBD6a, F-P, phenyl vinyl thiophene, PVT3, poly(3-hexylthiophene), PHT, and poly(p-phenylene-2,3′-bis(3,2′-diphenyl)-quinoxaline7-7′-diyl), PPQ, are presented in Figure 1. In following, when the functions of the molecular films are described, acronyms P-F and F-P are used for the dyads ZnDHD6ee and TBD6a, respectively, indicating the location of the hydrophilic end, first in the porphyrin (P) and then in

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10257 the fullerene (F) moiety. Because of their electron-donating and light-absorbing natures, the compounds PHT and PVT3 are denoted D and L, respectively, and the compound PPQ as A, because of its electron-accepting character. Synthesis43 and influence of the metallization44 of the used dyads are presented elsewhere. PHT was purchased from Sigma-Aldrich and was of 98.5% grade. Synthesis of PVT3 is described in Supporting Information. Polymer PPQ was received from the University of Potsdam.40,45 Chloroform of analytical grade was purchased from Merck and was used without further purification for solution preparation and as a spreading solvent. Octadecylamine (ODA) of 99% grade (Sigma) was used as a matrix compound for the LB film preparation. All the compounds used were dissolved in chloroform as stock solutions to have concentrations of ∼1 mg/ mL. Spreading solutions were diluted from the stock solutions to concentration of e1 mM prior to the film preparation. 2.2. Sample Preparation. Single- and double-compartment Langmuir troughs (LB 5000 and LB Minitrough, KSV Instruments) were used for the isotherm measurements and the film depositions. Surface pressure was controlled by a Wilhelmy plate that was monitored by a computer with software supplied by the manufacturer. The subphase was phosphate buffer containing 0.5 mM Na2HPO4 and 0.1 mM NaH2PO4 (pH ∼7) in Milli-Q water (Millipore Corporation). Temperature of the subphase was kept at 18 ( 0.5 °C during the film deposition with a thermostat. Monolayers were compressed at rates of 450 and 330 mm2/ min for the single- and the double-compartment troughs, respectively. For spectroscopic studies, films were deposited onto glass or quartz substrates cleaned by the standard procedure.26 For the TRMDC and the electrochemical PC measurements, films were deposited onto glass substrates covered by semitransparent indium tin oxide (ITO) electrode on one side. The ITO substrates were cleaned in an ultrasonic bath first with acetone and finally with chloroform. Prior to the film deposition, the glass/quartz and the ITO substrates were plasma-etched for 15 or 10 min in low-pressure nitrogen atmosphere, respectively. The porphyrin-fullerene dyads (P-F and F-P) were deposited with the porphyrin moiety toward the substrate (ITO|porphyrin-fullerene) in order to direct ET reactions from ITO to a second electrode. Electron and hole transporting layers were deposited to support the direction of the CS in the dyad. Orientation of the dyads and the direction of ET in the dyad monolayer are demonstrated by Vuorinen et al.13 The P-F concentration in the LB film preparation was 18 mol % in ODA. PHT (counted per monomer unit) and PVT3 concentrations were 60 and 70 mol % in ODA, respectively, unless otherwise mentioned. The layers of F-P and PPQ were deposited horizontally by the Langmuir-Schaeffer46 (LS) method with 100% concentrations. In Langmuir film on the water surface, both the P-F and the F-P compounds are oriented with the hydrophilic end, P in P-F and F in F-P, toward water. The LB deposition takes place with vertical lifting of solid substrate through the Langmuir film. In LS deposition, the solid substrate is led horizontally in contact with the molecular film, which then adsorbs onto the substrate. Deposition pressures and rates for the monolayers of P-F and PVT3 were 15 mN/m and 5 mm/min, but for PHT 20 mN/m and 4 mm/min, respectively, in both deposition directions. Drying time of the films after every water-to-air deposition was 10 min. Deposition pressures for the monolayers of F-P and PPQ were 11 and 5 mN/m, respectively. Film depositions by

10258 J. Phys. Chem. C, Vol. 112, No. 27, 2008 the LB and the LS methods were tested and confirmed by the absorption measurements prior to the actual sample preparation. A typical sample structure for the photovoltage (PV) measurements was ITO|ODA layers|photoactive layers|ODA layers|2nd electrode. A liquid indium-gallium (InGa) drop was used as second electrode in the PV experiments. Eleven or twelve ODA monolayers were used to insulate the photoactive molecules from the electrodes. Sample structure for the electrochemical PC measurements was similar, but without insulating ODA layers. Chart 1 shows a schematic energy level diagram of the studied photovoltaic devices. Oxidation potential of PVT3, together with oxidation and reduction potentials of P-F, was defined by the differential pulse voltametry (0.1 M TBAPF4 in benzonitrile by using ferrocene as a reference). 2.3. Microscopy. The atomic force microscopy (AFM) imaging was performed with a Nanoscope IIIa (Digital Instruments) in the tapping mode by using Veeco NanoprobeTM tips (model: RTESP). Scan rate was 2.0 Hz and resonance frequency for the tip 250 kHz. Thickness of the PVT3 film was determined by measuring step height of a single PVT3 layer with a Wyko NT1100 optical profilometer (Veeco) in the phase-shifting interferometry (PSI) mode with 20× objective. 2.4. Spectroscopic Methods. Steady-state absorption and fluorescence spectra were measured by using Shimadzu UV-2501PC and Fluorolog-3 (SPEX Inc.) spectrometers, respectively. Fluorescence spectra were measured at 90° to the excitation beam and were corrected by using a correction function supplied by the manufacturer. Fluorescence decays for the films of PVT3 and PVT3|porphyrin-fullerene in nanosecond and subnanosecond time domains were measured using a time-correlated single photon counting (TCSPC) system (PicoQuant GmBH) consisting of the PicoHarp 300 controller and PDL 800-B driver. The films were excited with pulsed diode laser head LDH-P-C-405B at 404 nm, and fluorescence decays were monitored at the wavelength of 510 nm (close to the fluorescence maximum of PVT3). The signals were detected with a Hamamatsu microchannel plate photomultiplier (R2809U). The time resolution of the TCSPC instrument is about 60 ps (fwhm of the instrument response function). 2.5. Photovoltage Measurements. Vectorial photoinduced electron transfer (VPET) was studied by using the timeresolved Maxwell displacement charge method.49–51 The measuring circuit and the measurement system are described in detailed elsewhere.13 Photoactive layers were insulated from the electrodes by the ODA layers. Thus, observed TRMDC signals were caused in the photoactive layers by the VPET perpendicular to the plane of the film. Signal amplitudes were negative when electrons move from ITO toward the InGa electrode and positive for the opposite direction. The TRMDC signals were measured in time domains much shorter than the instrumental time constant (∼10 ms) of the measurement circuit, i.e., in the PV mode. Thus, the signal amplitude is directly proportional to the number of CT states and to the distance of CS. Decay of the signal describes recombination of the CS states in the photoactive layers. A built-in electric field between the electrodes due to the work function differences of ITO (4.7 eV) and InGa (4.2 eV) can be canceled by an external bias voltage of -500 mV. Samples were excited by 10 ns laser pulses at the wavelength of 436 nm. The excitation energy densities varied between 0.18 and 0.23 mJ/cm2, which is lower than the saturation intensity of the dyads.13 Time resolution of the PV measurements was

Kaunisto et al. CHART 1: Schematic Energy Level Diagram of the Multicomponent Photovoltaic Cella

a Energy levels of ITO,47 PHT,48 PPQ,40 methyl viologen,4 and oxygen4 were taken from literature. *Reduction potential of PVT3 is determined by subtracting the band gap (∼3.0 eV, calculated from the absorption spectrum) from the oxidation potential.

about 20 ns determined by the excitation pulse width and the bandwidth of the amplifier. The primary CS in the dyad8 and the following secondary CT reactions are too fast to be timeresolved in the PV measurements. Reproducibility of the signal amplitude in the TRMDC measurements was about 20%. 2.6. Electrochemical Photocurrent Measurements. Electrochemical PC measurements were monitored by a voltage/ current measuring instrument (E5272A, Agilent Technologies). Measurements were performed in a three-electrode cell using ITO as a working electrode (area 0.28 cm2), a platinum wire as a counter electrode, and Ag/AgCl (sat. KCl) as a reference electrode. The ITO electrode was modified by depositing the photoactive molecular layers onto it. Anodic PCs were measured by using 1,1′-dimethyl-4,4′-bipyridinium dichloride (methyl viologen, MV2+) as an electron acceptor in aqueous (Milli-Q water) electrolytic solution. Methyl viologen supports the expected direction of the ET from porphyrin to fullerene (Chart 1). Concentration of MV2+ was chosen to be 5 mM.52 Supporting electrolyte was 100 mM KCl. Excitation light source was a Xe-arc lamp coupled with a monochromator used to select the excitation wavelength. Excitation energy density, chosen with a set of gray filters, altered between 0.39 and 0.45 mW/cm2. Samples were excited stepwise, and the current generation was monitored as a function of time. In addition, current-voltage (IV) curves and incident photon-to-current (IPCE) spectra were measured. Sign of PC is negative when electrons move from the working electrode toward the counter electrode, i.e., from ITO to Pt. 3. Results and Discussion 3.1. Film Preparation. The surface pressure-area isotherms (Figure 2) were recorded for the PVT3 Langmuir films at nine different PVT3 concentrations in the ODA matrix, ranging from 1% to 100% of PVT3. Between surface pressure values of 15 and 30 mN/m, a shoulder in the isotherm is observed for the 10 to 70 mol % films, indicating a slight phase transition. With

Porphyrin-Fullerene Dyad Multilayer Films

Figure 2. Isotherms of PVT3 mixed with the ODA matrix.

increasing PVT3 concentration, the transition shifts to higher surface pressures. For the mixed films, all the isotherms are steep with pronounced collapses. While the PVT3 concentration increases, the isotherms become smoother. A clear collapse point cannot be determined for the 100% PVT3 film. The shapes of the isotherms of 1, 5, and 10 mol % PVT3 films are similar to that of the pure ODA, but shifted to larger mean molecular areas (mma). The mma values decrease with increasing PVT3 concentration. This is surprise since PVT3 has larger molecular area than that of ODA (∼18.5 Å2 at 30 mN/m). Based on the molecular structure of PVT3, values of 620 and 80 Å2 are estimated for the areas of PVT3 plane and molecule projection, respectively. An explanation for the decreasing molecular area of mixed films is a plane-to-plane stacking of PVT3 molecules at the air-water interface. PVT3 molecules are hydrophobic and they prefer to aggregate rather than distribute homogeneously over the surface area. The limiting areas are extrapolated from the linear parts of the isotherms (at the beginning of the pressure rise) to zero surface pressure. The limiting area can be calculated with the equation Stotal ) x1 × (S1 - S2) + S2, where xi and Si are the molar fraction and the limiting area of the immiscible molecule in the mixed film, respectively. The obtained limiting areas as a function of the PVT3 fraction are shown in the inset of Figure 2. The limiting area of ODA is 22.4 Å2, which is close to that determined for pure ODA. The limiting area of PVT3 achieved from the fit is practically equal with that obtained by extrapolation for the 100% PVT3 film, 7.5 Å2. Taking into account the molecular area of PVT3, the determined limiting area suggests formation of the PVT3 molecule stacks with heights of more than ten molecules. The linear dependence of limiting areas from the PVT3 fraction together with the negative slope indicate that the PVT3 aggregation takes place even at the low PVT3 fractions. However, PVT3 has a clear influence to the isotherms, which rules out the possibility of PVT3 being squeezed out of the matrix on top of ODA layer during the compression of the Langmuir film. All observations suggest that the plane-to-plane stacking of PVT3 molecules takes place at the air-water interface. The molar fraction of PVT3 in the LB film was chosen to be 70 mol % on the basis of the LB deposition. It was a compromise between the highest surface coverage by PVT3 and the LB film transfer ratio. For fluorescence measurements, 40 mol % PVT3 was used. The LB film properties of the dyads13 and PHT41 have been studied earlier.

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10259 3.2. Atomic Force Microscopy. The AFM imaging was performed for a 70 mol % PVT3 film deposited onto a glass substrate (see Supporting Information SI 1). The film topography shows oval aggregates distributed unevenly over the PVT3rich area. The aggregates are identified as PVT3 stacks with maximum height about 40 nm. The dark flat areas in the AFM image are associated with matrix-rich domains. The coverage of PVT3 and ODA domains estimated from the AFM figure are roughly 90% and 10%, respectively. Thickness of the matrixrich area can be estimated to be about 2.5 nm based on the number of covalent bonds in ODA molecule. Thickness of the film cannot be determined on the basis of the AFM figure. The average thickness of 70 mol % PVT3 film was about 15 nm defined by the optical profilometer. Detailed surface analysis of the multilayer structures is not essential in the scope of this paper but is important as future research. 3.3. Spectroscopy. Films for the UV-vis absorption measurements were deposited onto clean quartz or a quartz substrate covered with three ODA layers. Glass substrates for the fluorescence measurements contained five or six ODA bottom layers. Absorption Measurements. Absorption spectra of the monolayers are shown in Figure 3a. The monolayers of PVT3, P-F, and PHT were deposited by the LB method from water-to-air. The layers of PPQ and F-P were deposited by the LS method. Absorbance of PVT3 depends linearly on the number of deposited layers as shown in Figure 3b. Absorption spectra of multilayer films correspond to the sum of their respective components indicating low ground-state interaction between the molecules of different layers. The absorption maxima of PVT3 in film and in toluene are 411 and 423 nm, respectively (Supporting Information SI 2). Molar absorption coefficient, ε, of PVT3 is 64 340 M-1 cm-1 at 423 nm in toluene. The observed blue-shift and the band broadening are moderate, but still indicate the H-aggregation53,54 of PVT3. This supports the assumption that PVT3 molecules are piled up in stacks in films. Fluorescence Measurements. The F-P dyad was used instead of the P-F for the fluorescence quenching studies due to its stronger fluorescence.13 The F-P can be deposited as 100% monolayer by the LS method and thus covers the PVT3 layer totally. The hydrophilic fullerene end of F-P (Figure 1) enables preparation of a PVT3|porphyrin-fullerene (or LP-F structure, if the acronyms defined in section 2.1 are used) film sequence by the LS method. Steady-state fluorescence spectra for the films of F-P, L (PVT3), and LP-F (PVT3|P-F) were measured by exciting at 400 nm (Supporting Information SI 3). The PVT3 film has fluorescence maximum at 508 nm. The F-P film has two fluorescence bands at 655 and 720 nm, which correspond to the characteristic emission bands of free base porphyrin. On the basis of steady-state emission, the fluorescence of PVT3 is quenched about 12% in the LP-F film (Supporting Information SI 3). Time-resolved fluorescence measurements for the films of PVT3 and PVT3|P-F (Supporting Information SI 4, Table S1) show that lifetimes of the fastest and second-fastest components of PVT3 fluorescence are quenched by the F-P layer about 7.5 and 1.5 times, respectively, indicating even more effective quenching than the steady-state fluorescence. 3.4. Photovoltage Measurements. The PETs in the F-P and the P-F monolayer systems have been studied previously.13 In the present study, the P-F was deposited both on the PVT3 layer (LP-F) and on the PHT|PVT3 double layer (DLP-F) in order to create both the ENT and the secondary ET from PVT3

10260 J. Phys. Chem. C, Vol. 112, No. 27, 2008

Figure 3. (a) Absorption spectra of the monolayers of used compounds. (b) Absorbances of 70 mol % PVT3 multilayer film at 411 nm as a function of layers. (c) Schematic illustration from the arrangement of the each layer in the films. Arrows show the direction of ET and ENT in the film.

to porphyrin. The monolayer of PPQ was deposited on the P-F (P-FA) in order to cause the ET from fullerene radical anion to PPQ. More complex film structures were studied by repeating sections of DP-F, LP-F, and DLP-F. Graphic illustration and schematic energy level diagram of the structures are shown in Figure 3c and Chart 1, respectively. Film structures DL and LD were used to study CT between PHT and PVT3. In earlier studies, PHT has been proven to be capable of efficient ET to porphyrin radical cation.38 The monolayers of P-F, PVT3, PHT, and PPQ were measured as a reference. The used excitation wavelength (436 nm) corresponds to the Soret band

Kaunisto et al. of zinc-porphyrin and is close to the maximum of PVT3 absorbance (Figure 3a). PV responses for the films of DL and LD at a bias voltage of -500 mV were measured (Supporting Information SI 5). Negative bias voltage was used to compensate the internal electric field caused by the electrodes. At the excitation wavelength, mainly PVT3 is excited (Figure 3a). The sign of the PV signal of DL is negative but positive for the LD structure proving that PHT donates electrons to PVT3. This observation is supported by the energy levels in Chart 1. On this basis, PVT3 was deposited between PHT and P-F (DLP-F) in order to direct the electron movement parallel in the whole film system. In the LP-F structure, the PV amplitude increases approximately 3-fold compared to the sum of the amplitudes of PVT3 and P-F monolayers (Table 1). The PV signals for the films of P-F and LP-F are presented in Figure 4. The PV enhancement of LP-F indicates an increased distance of charges55 and/or increased number of the CS states. PV signal recombination for LP-F is fast right after the excitation, as for the P-F film, but slows down in time, as can be stated from Figure 4a,b. The primary decay process immediately after the excitation is the recombination of the CS between fullerene and porphyrin. The signal of the dyad decreases close to zero value as a distinction of the LP-F. The PV decays of the films were fitted by a power law,56,57 U(t) ∼ t-b (Figure 4c). In logarithmic scales, signals thus appear linear and the b values are inversely proportional to the recombination rate of charges. The PV decay of the dyad monolayer follows the power law13 with a b value of ∼0.25, but deviates from linearity in complex films.38 In the present study, the PV signals were fitted with the power law, although in complex film structures, the linearity is poor. Decrease of the b values is, however, evident in complex structures. The half-times, t1/2, of the decays can be calculated from the b values and used to compare the CR rates of different films. The b and t1/2 values as well as the PV amplitudes of different film structures are presented in Table 1. PHT in contact with the porphyrin moiety of the dyad in the film sequence of DP-F causes secondary ET from PHT to the porphyrin cation.38 The maximum amplitudes of the structures DP-F and LP-F are 3.5 and 4.2 times higher than that of the dyad monolayer, respectively, and the b values are about 4-fold (Table 1). The half-times, t1/2, of DP-F and LP-F films are 4 orders of magnitude longer than that of the P-F monolayer (∼1 µs), indicating much slower overall CR. For the DP-F, the increased PV amplitude and slower CR are ascribed to the longer distance of CS due to the secondary ET from PHT to porphyrin cation.38 For the LP-F, the situation is more complicated, and it is difficult to distinguish ENT from secondary CT. Slow signal decay for the LP-F (Figure 4c) is caused by the delayed PV recombination of the PVT3 layer (Table 1). The increased PV amplitude indicates, at least partly, enhanced primary ET from porphyrin to fullerene. If electron and energy transfers are described as a projection of the consequence transfers on the axes perpendicular to the molecular films and the individual vertical steps contain also the preceding lateral movement of electrons and holes in particular layers, the whole processes can be presented as follows:

DP*-F f DP+-F- f D+P-F-

(1)

L*P-F f LP*-F f LP+-F- f L+P-F-

(2)

and

These reactions explain the increased amplitudes and delayed recombinations. Possible competing processes are not efficient

Porphyrin-Fullerene Dyad Multilayer Films

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10261

TABLE 1: Relative Photovoltage, PV, Amplitudes, Obtained b and t1/2 Values as Well as the Real and Relative Photocurrent, PC, Amplitudes of the Studied Film Sequencesa film structure ITO|P-F ITO|DP-F ITO|LP-F ITO|DLP-F ITO|(DP-F) × 2 ITO|(DP-F) × 3 ITO|(LP-F) × 2 ITO|(DLP-F) × 2 ITO|P-FA ITO|DP-FA ITO|LP-FA ITO|DLP-FA ITO|L ITO|D ITO|A

PV rel. amp.

b

1 3.5 4.2 8.4 19.4 9.9 17.4 0.9 10.1 9.3 11.3 0.6 0 0

t1/2, s

0.217 0.058 0.057 0.039 0.066 0.018 0.040 0.082 0.058 0.041 0.060 -

9.2 × 10 1.3 × 10-2 4.9 × 10-2 9.7 0.4 2.3 × 1016 1.6 1.9 × 10-3 4.0 × 10-2 6.8 2.8 × 10-2 -7

PC rel. amp

Isc (nA/mW)

1 2.1 7.8 7.8 3.1 2.2 5.8 4.1 0.2 1.4 3.8 2.8 4.3 0.6 0.3

90 190 700 690 280 200 520 370 20 130 340 250 390 50 30

CT eq. (1) (2) (3) (4) (5) (6)

a

Charge transfer, CT, equations refer to the equations in text. Insulating ODA layers are not shown for the PV samples. For PC samples, two PHT or PVT3 bottom layers (marked by one D or L) were necessary to use due to technical deposition reasons.

enough to prevent the main processes described here and can be left without more detailed consideration. At the excitation wavelength of 436 nm, the absorbances of PHT and PVT3 layers are about one-half and 6-fold compared to that of the dyad monolayer, respectively. The increased amplitudes are due to the fast and efficient secondary ET from PHT to P+ in reaction 1 and increased excitation of P due to the ENT from L*, followed by less efficient ET from PVT3 to P+ in reaction 2. ET from L or D to P+ is supported by the favorable HOMO levels of PHT, PVT3, and P (Chart 1). The long lifetimes of the CS states are caused by longer distances of the positive and negative charges. The most important result obtained from these experiments is that the PV amplitudes of the films DP-F and LP-F are higher than the PV sum of dyad monolayer and PHT or PVT3. This is a clear indication from the positive influences of PHT and PVT3 to the ET reactions in the films. The film sequence of DLP-F was deposited in order to study CT through the entire layer structure (eq3). PV responses of the DLP-F are shown in Figures 4a,c and 5. Addition of PHT increases the signal amplitude 2-fold and reduces the b value (prolonging the half-time to second time domain) compared to those of the LP-F structure (Table 1). The increased PV amplitude can be stated due to the longer distance of CS. Thus, movement of an electron through the whole film can be supposed: +

-

+

-

DL*P-F f DLP*-F f DLP -F f D LP-F

(3)

The PV signal of the DLP-F still increases 600 ns after the excitation. The smaller b and the longer t1/2 values demonstrate slower CR compared to that of LP-F (Figures 4b and 5). Slower decay indicates longer distance and lateral migrations of the charges in the donor and acceptor networks. Migration followed by an electron transfer to the next layer also explains the slow signal rise. After formation of the primary CS state, (DLP+F-), the secondary ET from PVT3 and/or PHT to the porphyrin radical cation occurs and yields the final state of D+LP-F-. On the basis of the energy diagram in Chart 1, one can expect ET through the layers. The prolonged lifetime of CS and the increased signal amplitude are indications of efficient electron movement through the whole layer structure of DLP-F. Recombination is hindered because back reaction can occur only when electrons and holes are located in vertical orientation. Obtained b values from the power law fit tend to decrease (together with increasing t1/2

values) with increasing number of functional layers, i.e., the CR slows down when more layers are added to the film. The b value obtained for the most complex structure, 2× (DLP-F), was even negative due to the very long rise time of the signal. A slow CR rate is vital for the performance of photovoltaic device, as it enables efficient charge collection to an external circuit. This kind of sandwich-type photovoltaic device, where every layer has its own purpose, is one possibility to construct a working organic solar cell. The PV amplitude of the PVT3 layer is 0.6 of that of the P-F monolayer (Table 1). PVT3 does not contain any specific electron donor or acceptor moieties, and the low PV signal originates from the charge redistribution in the PVT3 film after the excitation. Film sequences of 2 × (DP-F), 2 × (LP-F), and 2 × (DLPF) were used to study CT reactions between the individual segments. The 2 × (DP-F) structure enhances the PV amplitude approximately 6-fold compared to that of the single structure (Table 1). For the films of 2 × (LP-F) or 2 × (DLP-F), the amplitudes are only 2-fold, which indicates that two segments work independently without CT between each other. For the 2 × (DP-F), because of the good hole-conducting and electrondonating properties of PHT, the CT takes place between two segments that boost overall CS:

DP*-F|DP*-F f DP+-F-|DP+-F- f D+P-F-|D+P-F- f D+P-F|DP-F- (4) In the final state, the uppermost fullerene layer is negatively and the lowest PHT layer positively charged. CT between the individual segments of DP-F is nicely supported by the energy levels in Chart 1. Due to the good hole- and electron-conducting properties of PHT and fullerene layers, respectively, CT between different segments does not require porphyrin excitations in vertical positions. In the 2 × (LP-F) film, the PVT3 layer creates a screening between the fullerene anion and porphyrin cation, formed in the primary ET steps in different segments, due to the less effective conductivity and ET properties of PVT3.

L*P-F|L*P-F f LP*-F|LP*-F f LP+-F-|LP+-F- f L+P-F-|L+P-F- (5) A similar behavior can be expected for the 2 × (DLP-F) film and only 2-fold amplitude enhancement is achieved:

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DL*P-F|DL*P-F f DLP*-F|DLP*-F f DLP+-F-|DLP+-F- f D+LP-F-|D+LP-F- (6) The PV decay of the DP-F is faster than that of the 2 × (DP-F) or DLP-F, which is still faster than decay of the 2 × (DLP-F) (Table 1). This can be explained by the increased complexities of the films. The signal decay of the 2 × (LP-F) film is slower compared to that of the LP-F (Table 1), indicating weak interaction between the LP-F segments, which is supported also by the energy levels (Chart 1). The t1/2 value of the 2 × (LP-F) is unrealistic and can be left without further conclusions.

Figure 5. PV responses for the films of indicated structures at time scale of 300 µs.

Figure 4. PV responses for the films of indicated structures at (a) short and (b) long time scales, and (c) the responses in the double logarithmic plot together with the slopes obtained from the power law fit.

The film sequence of P-FA (A ) PPQ) was used to study secondary ET from fullerene radical anion to PPQ layer. On the basis of the LUMO levels of PPQ and F (Chart 1), ET from F- to A is not effective. PV amplitudes of the P-FA film and the P-F itself are approximately the same, but the signal decay is slower for the P-FA as can be noticed from the b and t1/2 values in Table 1. The half-time of CS state is increased, but not the distance between the charges or the number of the charges, indicating that the ET to the PPQ layer is not efficient enough to increase the amplitude. The amplitude is controlled by the fast recombination of the primary ET, but the decay by the PPQ layer. In film structures of DP-FA, LP-FA, and DLP-FA, signal amplitudes increase substantially and the recombination rates remain about the same compared to the structures in the absence of the PPQ layer (Table 1). Figure 5 shows the PV responses for the films of DLP-F, DLP-FA, DP-F, and DP-FA on a time scale of 300 µs. The signals of the films with PPQ are more intense immediately after the excitation, but decay considerably faster compared to the structures in the absence of PPQ. An important observation is that the signal amplitudes at 300 µs after the excitation are the same and thus independent of the PPQ layer. Thus, fullerene anions donate electrons to PPQ, but the recombination is fast and independent of the CT processes taking place in other parts of the film. Delayed CR of the primary CS in P-F (due to the secondary CT reactions discussed earlier) increases the probability of ET from F- to A, even if it is forbidden process by the LUMO levels of F and PPQ (see also discussion of the PC experiments). 3.5. Electrochemical Photocurrent Measurements. Series of molecular film structures, similar to those used in the PV experiments, were deposited directly onto ITO cathode for the PC measurements (Table 1). The excitation wavelength of 436 nm used corresponds to the maximum absorbance of zincporphyrin (Figure 3a). The excitation energy density was ∼0.40 mW/cm2. Table 1 presents the short-circuit PC amplitudes, Isc, and amplitudes relative to that of the P-F monolayer of the studied layer structures, taken from the on/off-measurements with zero bias voltage, divided by the excitation energy density (mW/cm2) and by the area of the working electrode (0.28 cm2). Figure 6 shows light-on/off PC responses for the films of P-F, DP-F, DLP-F, and LP-F. Energy levels of the devices (Chart

Porphyrin-Fullerene Dyad Multilayer Films

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Figure 7. IV curves of the PHT|PVT3|P-F film. Figure 6. PC responses for the films of indicated structures at the excitation wavelength of 436 nm.

1) support the electron movement from ITO to MV2+ as discussed earlier. Only PPQ seems to create a barrier for the charge movement in the films. The PC amplitudes increase with the number of the single functional layers as far as fullerene is the topmost layer (Table 1). The P-F monolayer has the lowest PC amplitude, 90 nA/ mW. The PHT on ITO under the dyad layer (DP-F structure) increases the PC generation roughly 2-fold. The LP-F film gives approximately eight times higher PC amplitude compared to that of the dyad monolayer, i.e., 700 nA/mW. The film structure DLP-F gives about the same PC amplitude as the LP-F even if it absorbs only half that of the LP-F at the excitation wavelength. When the absorbance is taken into account, the film structure DLP-F works most efficiently, i.e., it has the highest quantum yield for photocurrent. The absorbances of the DP-F and DLP-F films are about 2- and 7-fold compared to the dyad monolayer, respectively, and so are the PC enhancements. For the LP-F structure, the absorbance is 12 times and the PC enhancement 8 times higher compared to those of the P-F. The PC for the multilayer structures is higher than the sum of the PC amplitudes of each component alone (Table 1), i.e., the PC improvement is neither due to the possible penetration of the electrolytic solution into the film nor solely due to the increased absorbance of the multilayer film. Conclusions made based on the PV measurement and the assumptions behind the whole idea are proved to be working in the PC experiments as well. Increased absorbance of the films together with the longer-lived CS enhance the PC generation as was predicted. Comparing the DP-F and the 2 × (DP-F) films, the PC generation is enhanced 50%, but the PV signal about six times for the double structure (Table 1). The differences are due, according eq 4, to the long distance between electrons and holes in the final CS state, to increasing the PV amplitude, and to the increased resistance of the middle layers, which decreases the PC signal. The PC signal of the 3 × (DP-F) film is still lower than that of the similar double structure, for the same reason. For the 2 × (LP-F) film, the PC signal decreases 25%, but the PV signal increases about 2-fold compared to the corresponding single structure. This indicates, as previously stated, that both of the LP-F segments work independently and that PVT3 film acts as CT barrier between two P-F molecules. In

the 2 × (DLP-F) structure, the PC signal is reduced about 50%, but the PV signal is increased 2-fold compared to the single structure. Here, once more, the long distance between the cation radicals in the PHT layer and anion radicals in the fullerene layer creates the high PV amplitude (Table 1 and eq 6) and the isolating PVT3 layers between them reduce the PC signal. The films with PPQ on the top caused relatively high PV amplitudes for the structures with the most layers as is shown in Table 1. The result was completely opposite in the PC measurements. Addition of a PPQ layer decreased the PC generation substantially for every sample (Table 1). For example, the structure LP-FA caused 2-fold lower PC amplitude compared to a similar film in the absence of A. Taking into account the discussion in the PV section, the explanation is that diffusion of electrons in the PPQ layer is poor, i.e., its conductivity is too low for photovoltaic applications. 3.6. Electrochemical Current-Voltage Measurements and Action Spectra. Current-voltage (IV) curves for several films have been measured. As an example, the result for the DLP-F sample with the excitation wavelength of 436 nm is presented in Figure 7. The excitation energy density was ∼0.40 mW/cm2. The current in the IV curve is divided by the area of the working electrode (0.28 cm2). This film structure has a fill factor (FF) of 22%. When the open circuit voltage, Voc, of the P-F monolayer is roughly 80 mV, it is for the DLP-F film approximately 500 mV. In the fourth quadrant, the IV curve changes linearly as a function of the bias voltage, indicating the existence of unwanted serial resistance in the film.58 Figure 8 shows absorption and IPCE spectra for the films of P-F and DLP-F, measured with zero bias voltage. The IPCE spectrum of the DLP-F follows the absorption of the film, and the influences of PHT and PVT3 to the PC generation can be noticed. The influence of the dyad on the spectrum is hidden under the strong absorption of PVT3. It is clear that mainly PVT3 is responsible for the PC generation at the shorter wavelengths (from 400 to 500 nm), but at longer wavelengths (from 500 to 650 nm), influence of PHT dominates. This supports the function of PVT3 as an ENT material. However, a direct ET from excited PVT3 to fullerene and secondary ET from ground-state PVT3 to porphyrin cation are still possible processes (Chart 1). PHT broadens the absorption of the film structure and acts as an electron donor to the fullerene moiety of the dyad via PVT3 and porphyrin. The PC action spectrum of the DP-FA film (Supporting Information SI 6) resembles the absorption spectrum of the PPQ

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Kaunisto et al. well, by donating electrons to porphyrin radical cation. PHT works as an electron donor to PVT3. Both PHT and PVT3 are promising compounds, when combined with P-F, for the photovoltaic devices. The electron conducting properties of PPQ were found to be poor for the photovoltaic applications. Although the PV and the PC methods, when used separately, do not give quantitative information on the mechanisms and efficiencies regarding how the electrons move through multilayer films, combining the results obtained by using both methods simultaneously yields more quantitative information on intraand interlayer processes. This can be used in developing photovoltaic cells. By increasing film thicknesses, more photons can be collected and higher PC amplitudes are achieved. Optimization of the layer thicknesses and measurement conditions are the next steps to improve the PC generation in this kind of sandwich-like photovoltaic cell.

Figure 8. IPCE and absorption spectra for the films of P-F (inset) and PHT|PVT3|P-F.

TABLE 2: Values of External, ΦE, and Internal, ΦI, Quantum Yield as well as Short Circuit Photocurrent, Isc-iv, Open Circuit Voltage, Voc, Fill Factor, FF, and Power Conversion Efficiency, η, for the Samples of Indicated Film Structuresa film structure ITO|P-F ITO|LLP-F ITO|DLP-F

A

ΦE, %

Φ I, %

0.009 0.025 1.199 0.117 0.200 0.847 0.068 0.195 1.354

Isc-iv, nA/cm2 Voc, V 64.0 241.8 194.4

0.08 0.53 0.50

FF, %

η, %

32b 0.0005 20.2 0.0065 21.7 0.0053

a Ubias ) 0 V, λex ) 436 nm, Pex ∼ 0.40 mW/cm2, A ) absorbance. b Estimated value.

monolayer; i.e., the PPQ layer is mainly responsible for the PC generation, but is poorly conductive even in an excited state. PPQ is able to accept electrons from photoactive layers but does not transfer them to the electrode. It seems that PPQ is not usable for an electron acceptor layer in photovoltaic applications. External, ΦE, and internal, ΦI, quantum yields, fill factors, FF, open circuit voltages, Voc, power conversion efficiencies, η, and short-circuit PC values from IV curves, Isc-iv, are presented in Table 2 for the two most effective layer structures and for the P-F monolayer. The quantum yields are calculated from the Isc values in Table 2. DL film between ITO electrode and P-F layer increases internal quantum yield. For the LP-F film, the internal quantum yield decreases about 30%, but the external quantum yield increases 10-fold compared to that of the dyad monolayer. This supports the idea that PVT3 works as an energy donor to porphyrin. The best external quantum yield, ∼0.2%, was obtained for the films of LP-F and DLP-F. In this study, the highest achieved power conversion efficiency was ∼0.007%, obtained for the LP-F structure. 4. Conclusions The aim of the study was to understand the mutual function of different individual molecular layers on the CT efficiency and PC generation in porphyrin-fullerene-based organic multilayer films. Combining the monolayers of PHT(D), PVT3(L), and P-F to ordered film structures increases the PC efficiency compared to simpler films. Increased light absorption together with remarkably reduced recombination rate of the CS state enhance the PC generation. In the DLP-F film, PVT3 transfers excitation energy to porphyrin, but acts as an electron donor as

Acknowledgment. This work was supported by the Finnish National Graduate School in Nanoscience. The author expresses his gratitude to Professor Hiroshi Imahori (Fukui Institute for Fundamental Chemistry, Kyoto University) and Ms. Aiko Kira (Department of Molecular Engineering, Graduate School of Engineering, Kyoto University) for their kind AFM measurements. We thank Professor Schrader from the University of Potsdam for giving PPQ polymer to our studies. Supporting Information Available: Synthesis of PVT3; AFM image of the 70 mol % PVT3 film (Figure S1); normalized absorption spectra of PVT3 in toluene and in the LB film (Figure S2); fluorescence spectra for the films of PVT3, PVT3|P-F, and F-P (Figure S3); time-resolved fluorescence decays for the films of PVT3 and PVT3|P-F at the monitoring wavelength of 510 nm as well as slopes, lifetimes, and preexponential factors obtained from the three exponential fit (Figure S4, Table S1); PV signals for the films of PHT|PVT3 and PVT3|PHT with zero total electric field at the excitation wavelength of 436 nm (Figure S5); PC action and absorption spectra of the PHT|PHT|P-F|PPQ film together with absorption spectrum of the PPQ monolayer (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, W-B.; Xiang, H-F.; Xu, Z. X.; Yan, B. P.; Roy, V. A. L.; Che, C. M.; Lai, P. T. Appl. Phys. Lett. 2007, 91, 191109. (2) Guldi, D. M.; Rahman, G. M. A.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38, 871. (3) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (4) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (5) Araki, Y.; Chitta, R.; Sandanayaka, A. S. D.; Langenwalter, K.; Gadde, S.; Zandler, M. E.; Ito, O.; D’Souza, F. J. Phys. Chem. C 2008, 112, 2222. (6) D’Souza, F.; Chitta, R.; Sandanayaka, A. S. D.; Subbaiyana, N. K.; D’Souza, L.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2007, 129, 15865. (7) Indelli, M. T.; Chiorboli, C.; Flamigni, L.; De Cola, L.; Scandola, F. Inorg. Chem. 2007, 46, 5630. (8) Chukharev, V.; Tkachenko, N. V.; Efimov, A.; Guldi, D. M.; Hirsch, A.; Scheloske, M.; Lemmetyinen, H. J. Phys. Chem. B 2004, 108, 16377. (9) El-Khouly, M.; Ito, O.; Smith, P. M.; D′Souza, F. J. Photochem. Photobiol. C 2004, 5, 79. (10) Lapinski, A.; Graja, A.; Olejniczak, I.; Bogucki, A.; Imahori, H. Chem. Phys. 2004, 305, 277. (11) Kleverlaan, C. J.; Indelli, M. T.; Bignozzi, C. A.; Pavanin, L.; Scandola, F.; Hasselman, G. M.; Meyer, G. J. J. Am. Chem. Soc. 2000, 122, 2840. (12) Ito, F.; Ishibashi, Y.; Khan, S. R.; Miyasaka, H.; Kameyama, K.; Moriseu, M.; Satake, A.; Ogawa, K.; Kobuke, Y. J. Phys. Chem. A 2006, 110, 12734.

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