Energy and Electron Transfer in Multilayer Films Containing Porphyrin

Feb 6, 2009 - Kimmo Kaunisto*, Vladimir Chukharev, Nikolai V. Tkachenko, Alexander .... Paola Vivo , Tommi Vuorinen , Vladimir Chukharev , Antti Tolkk...
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J. Phys. Chem. C 2009, 113, 3819–3825

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Energy and Electron Transfer in Multilayer Films Containing Porphyrin-Fullerene Dyad Kimmo Kaunisto,* Vladimir Chukharev, Nikolai V. Tkachenko, Alexander Efimov, and Helge Lemmetyinen Department of Chemistry and Bioengineering, Tampere UniVersity of Technology, P.O. Box 541, FI-33101 Tampere, Finland ReceiVed: September 8, 2008; ReVised Manuscript ReceiVed: December 15, 2008

Photoinduced interlayer energy and electron transfer from a thiophene derivative, PVT3, to a porphyrin-fullerene dyad, P-F, was demonstrated. The laser flash photolysis method was utilized to characterize photoinduced processes in layered thin films constructed by the Langmuir-Blodgett and spin-coating techniques. Poly(3hexylthiophene), PHT, was used as an electron-donating layer to the dyad and PVT3. Electron transfer through a multilayer film with a PHT-PVT3-P-F layer sequence was shown, yielding the final charge-separated state where the positive charges are located in the PHT network and the electrons in the fullerene sublayer. The crucial role of excited P-F for the overall charge transfer efficiency of a bilayer film containing a phthalocyanine derivative, ZnPH4, as an electron-donating moiety to P-F was demonstrated. The lifetime of the electrical signal of the charge separation was shown to be prolonged compared to that of the optical signal. 1. Introduction Organic thin film photovoltaic applications have received a lot of research interest, and many such devices have been introduced.1-13 The power conversion efficiency of organic solar cells has increased steadily,14-16 and devices with efficiency exceeding 6% have been reported.17,18 Photophysical interaction between distinct molecular networks should be determined in detail to further enhance the efficiency and to construct a device with high performance. Photoinduced electron transfer (ET) has been demonstrated for many electron donor-acceptor (DA) pairs in solutions.19-21 Specifically, porphyrin and phthalocyanine derivatives have been widely studied as electron donors to fullerene.22-25 Charge and energy transfer kinetics for most of the DA pairs are, however, still unknown in the solid state. To study photoinduced intra- and interlayer processes, molecular films should be immobilized on a solid substrate. The spin-coating and Langmuir-Blodgett26 (LB) techniques can be used to construct multilayer films with desired layer sequences. The lifetime of charge transfer (CT) in the solid state27 is increased compared to that in solution, and thus spectroscopic methods with long time resolution (up to milliseconds) are needed. Absorption flash photolysis is one measurement method that can be utilized to monitor relatively long-lived transient states in molecular systems. In the present research, we focus on the spectroscopic characterization of the photoinduced interlayer energy and electron transfer reactions in sandwichlike organic thin films. This is the first paper in which the absorption laser flash photolysis method is used to characterize photoinduced processes taking place in complex multilayer films. Results from different layer sequences were compared to each other in order to determine reactions in the films. * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +358 3 3115 3627. Fax: +358 3 3115 2108.

A thiophene derivative, PVT3, was utilized both as an energy (light-absorbing layer) and as an electron transfer material to a porphyrin-fullerene dyad, P-F. A phthalocyanine derivative, ZnPH4, and a hole-conducting polymer poly(3-hexylthiophene), PHT, were used as electron donors to P-F and PVT3. Layered thin film samples with desired layer sequences were prepared by the LB and spin-coating methods. The primary ET from porphyrin to fullerene in P-F,27-29 as well as the secondary ET from PHT to the porphyrin cation of the dyad,27,30 has been demonstrated earlier. 2. Experimental Methods Samples were prepared and stored in air at room temperature. 2.1. Materials. Molecular structures of covalently linked electron DA porphyrin-fullerene dyad, P-F, phthalocyanine derivative, ZnPH4, regioregular poly(3-hexylthiophene), PHT, and thiophene derivative, PVT3, are shown in Figure 1. When the film structures are described in the following text, P-F is used to denote the dyad indicating the location of the porphyrin (P) and fullerene (F) moiety. Compounds PHT, ZnPH4, and PVT3 are denoted as D (for electron Donor), E (for Electron donor), and L (for Light absorber), respectively. The syntheses of P-F,31 ZnPH4,32 and PVT333 are described elsewhere. Polymer PHT was purchased from Rieke Metals and was 98.5% grade. Octadecylamine, ODA, was 99% grade (Sigma) and was used as a matrix compound for the LB film preparation. Chloroform (Merck) and methanol (Baker) of analytical grade were used for solution preparation and as spreading solvents. Compounds for LB deposition were dissolved in chloroform to a concentration of about 1 mg/mL. Spreading solutions were diluted from stock solutions to a concentration of e1 mM prior to LB film preparation. 2.2. Sample Preparation. Films were deposited on quartz substrates (0.5 mm × 10 mm × 35 mm) with both ends polished to 45° angles. The substrates were cleaned using a standard procedure26 and plasma etched 15 min in a low-pressure nitrogen atmosphere prior to LB film deposition. Three or five ODA bottom layers were deposited prior to the active layers.

10.1021/jp807962j CCC: $40.75  2009 American Chemical Society Published on Web 02/06/2009

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Figure 1. Chemical structures of the compounds.

CHART 1: Schematic Energy Level Diagram of the Studied Compounds

Substrates for the spin coating were used without plasma treatment and ODA bottom layers. Single- and double-compartment Langmuir troughs (LB minitrough and LB minialternate, KSV Instruments) were used for LB film deposition. Surface pressure was controlled by a Wilhelmy plate that was monitored by computer software supplied by the manufacturer. An aqueous phosphate buffer (pH ∼7) at a temperature of 18 ( 0.5 °C was used as a subphase. Monolayers were compressed at rates of 450 and 330 mm2/ min for the single- and double-compartment troughs, respectively. In the LB films, P-F, PVT3, and PHT (counted per monomer unit) concentrations were 10, 70, and 60 mol % in ODA, respectively. Deposition pressure for the P-F and PVT3 monolayers was 15 mN/m and for the PHT monolayer was 20 mN/m. Deposition rates were 5 mm/min for both directions. A drying time of 10 min was used after every water-to-air deposition. A 100% ZnPH4 layer was spin-coated (WS-400A-6NPP, Laurell) from a stock solution of 3.3 mg/mL in methanol with a rotation speed of 2500 rpm. The P-F monolayer was deposited with the porphyrin moiety toward the substrate (quartz-porphyrin-fullerene) in the film structures. Electron-donating layers (including PVT3) were deposited to support the charge separation (CS) started by the dyad. The layer arrangement of each compound with respect to others in the model samples is illustrated elsewhere.33,34 A schematic energy level diagram of the compounds is shown in Chart 1. Energy levels of PHT35 were taken from the literature. Redox potentials of P-F and ZnPH4, as well as the oxidation potential of PVT3, were defined by differential pulse voltametry (0.1 M TBAPF4 in benzonitrile with ferrocene as a reference). The reduction potential of PVT3 was determined by subtracting the band gap (about 3.0 eV from the absorption spectrum) from the oxidation potential. 2.3. Spectroscopy. 2.3.1. Steady-State Absorption. Absorption spectra of the films were recorded by a Shimadzu absorption spectrophotometer (UV-3600). The same samples were used in steady-state and time-resolved measurements.

Kaunisto et al. 2.3.2. Time-Resolved Absorption. Micro- and millisecond time-resolved absorption laser flash photolysis experiments were performed in a total internal reflection mode to increase sample absorption. A film absorbance (A) from 0.04 to 0.07 was needed to reach a total absorbance close to unity after about 20 internal reflections. A flash photolysis instrument (LFP-111, Luzchem), controlled by the software supplied by the manufacturer, was modified for the total internal reflection setup. A xenon lamp (66921, Newport) was used as a probe light source. The measurement system is described in detail elsewhere.27 Excitation took place with 10 ns laser pulses (broadened by two cylindrical lenses to cover the probed part of the sample) at excitation wavelengths of 355, 430, and 720 nm. Transient absorption decays at a 1 ms time scale were measured with 10 or 20 nm steps (averaged 50 or 100 times) in the absorption range from 450 to 880 nm in order to determine the timeresolved absorption spectra of the films. Decays at a 10 ms time scale (averaged 100 times) were used to compare the decay kinetics of the films. Measurements were performed in a nitrogen atmosphere at room temperature. Deviations in the excitation energy density and number of reflections in the films make a direct comparison between the signal amplitudes of different samples impossible unless stated otherwise. 3. Results and Discussion Photoinduced energy and electron transfer from PVT3 to P-F was studied by depositing PVT3 adjacent to the porphyrin moiety of the dyad. The ET system was further extended by introducing PHT as an electron donor to PVT3. In addition, phthalocyanine was studied as an antenna layer to the dyad instead of PVT3. The P-F monolayer has previously been studied extensively.27 The secondary electron donation from PHT, PVT3, and ZnPH4 to the porphyrin cation (P+) is supported by the higher HOMO (highest occupied molecular orbital) levels of the donors compared to the oxidation potential of the dyad (Chart 1). This level is free in the porphyrin cation moiety of the dyad in the CS state. Similarly, the higher HOMO levels of the next electron donor with respect to the previous one in the layer sequence support ET through the multilayer structures. The higher LUMO (lowest unoccupied molecular orbital) levels of the donors compared to that of the dyad support electron donation from excited donors to the fullerene. The fullerene radical anion has a transient absorption band36 around 1000 nm, but this range could not be used because of photomultiplier sensitivity. 3.1. Energy and Electron Transfer from the Thiophene Derivative to the Porphyrin-Fullerene Dyad. Energy and electron transfer from PVT3 to the dyad was studied by measuring a PVT3-P-F (or the LP-F structure) film and using the P-F and PVT3 (L) single layers as a reference. The excitation wavelengths used were 355 and 430 nm with energy densities of about 0.4 and 0.9 mJ/cm2, respectively. When the samples were excited at 355 nm, transient signal amplitudes of the samples were comparable to each other because of the same measurement conditions. The absorption spectra of the films prepared for flash photolysis with excitation at 355 and 430 nm are shown in Figures 2 and 3, respectively. Two P-F and PVT3 monolayers (one on each side of the substrate) were used on samples with excitation at 355 nm. In the case of the 430 nm excitation, monolayers were on one side of the substrate. The spectra of

Electron Transfer in Films Containing P-F

Figure 2. Absorption spectra of the PVT3, PVT3-P-F, and P-F films prepared for flash photolysis with excitation at 355 nm.

Figure 3. Absorption spectra for films composed of P-F, PVT3, and PHT layers prepared for flash photolysis with excitation at 430 nm. The PHT sample contains two monolayers separated by three ODA monolayers.

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3821 At the excitation wavelength of 355 nm, mostly PVT3 is photoexcited (Figure 2). Thus, no transient bands of P-F should appear, as demonstrated for the dyad monolayer in Figure 4, unless PVT3 transfers its excitation energy to the dyad. Because ET from porphyrin to fullerene (yielding the porphyrin cation and fullerene anion moieties) is known to take place in the excited P-F monolayer,28 the interlayer ET from PVT3 to the porphyrin radical cation could be expected in the LP-F film. The transient spectrum of the LP-F film is not a sum of its components, even if the shape is dominated by the absorption of the PVT3 triplet at 620 nm, demonstrating a photoinduced interaction between the molecular layers of PVT3 and P-F (Figure 4). The transient absorption bands of the LP-F film at 490 and 660-780 nm are not observed for the L single layer and are much stronger than those of the P-F monolayer. Both bands are assigned to the porphyrin cation.27 The weak additional absorption band of the LP-F film at 820 nm is assigned to the PVT3 cation.37-39 The fine structure of the transient absorption of the porphyrin cation (Q-band bleaching27) above 500 nm is hidden under the strong absorption of the PVT3 triplet but can be covered by the absorption of the PVT3 cation as well. Observed transient bands of the porphyrin cation on the spectrum of the LP-F film demonstrate an interlayer ener gy transfer from PVT3 to the dyad, which is then followed by the primary ET from the porphyrin to the fullerene, yielding the porphyrin cation moiety. In addition, the transient band of the PVT3 cation on the LP-F film spectrum shows the interlayer ET from PVT3 to P-F. If electron and energy transfers are described in films as a projection of the transfers on the axes perpendicular to the molecular layers, and the individual vertical steps contain the lateral migration of charges as well, the photoinduced reactions at the excitation wavelength of 355 nm can be presented as follows33

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

Figure 4. Time-resolved absorption spectra of the PVT3-P-F, PVT3, and P-F films with excitation at 355 nm. The transient spectrum of the PVT3-P-F film with excitation at 430 nm is shown for reference. The spectra of the bilayer films are multiplied by the factors shown in the legend.

the films with PHT (D) will be discussed later. The absorption maximum of P-F at 430 nm corresponds to the porphyrin Soret band. Absorption spectra of the PVT3 monolayers differ from each other by their amplitude showing weak reproducibility of LB deposition. Because of their nonamphiphilic nature, PVT3 molecules do not form an ideal monolayer on a subphase.33 Therefore, molecule aggregation in each monolayer differs slightly from one another causing absorption differences. Spectra of the films are sums of their corresponding components (taking into account the differences in the PVT3 spectra), indicating successful film deposition and weak ground-state interactions between the molecules of distinct layers (Figures 2 and 3). The time-resolved absorption spectra of the LP-F, L, and P-F films at a 5 µs delay with excitation at 355 nm are shown in Figure 4. Broad transient absorption of the PVT3 single layer with the maximum at 620 nm, as well as the weak absorption shoulder at 500 nm, is characteristic for the triplet state of thiophene oligomers.37,38

(1)

In the final transient state, charges are located in PVT3 (holes) and fullerene (electrons) networks. The transient absorption of the PVT3 triplet is quenched about two times in the presence of the dyad monolayer (Figure 4), when excited at 355 nm. Quenching cannot be explained solely by the 50% higher absorption (Figure 2) of the L single layer and thus supports the hypothesis of an interaction between the molecular layers after photoexcitation. The triplet state of PVT3 is quenched mainly by the interlayer energy transfer from excited PVT3 to the P-F monolayer. The transient absorption spectrum of the LP-F film with excitation at 430 nm is presented in Figure 4 for comparison with the 355 nm excitation. PVT3 and P-F are strongly excited when the excitation wavelength of 430 nm is used (Figure 2). The increased absorption above 650 nm and the weaker band of the porphyrin cation at 490 nm are the most obvious differences compared to the transient spectrum for excitation at 355 nm. The increased absorption above 650 nm is assigned mainly to the PVT3 cation but may be affected by the porphyrin cation absorption as well. Thus, in the case of dyad photoexcitation at 430 nm, the secondary ET from PVT3 to the porphyrin cation after the primary CS in the dyad (reaction 2) is more pronounced

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

(2)

Excitation in L*P*-F is not obligatory on the neighboring L and P-F but within the migration distance. In the final transient

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Figure 5. Normalized absorption decays of the PHT-PVT3-P-F, PVT3, and PVT3-P-F films for monitoring at 720 nm, when excited at 430 nm. Fast parts of the decays are normalized to unity, but for clarity of the slow parts, the vertical scale is cut at 0.5.

TABLE 1: Time Constants (τi) and Pre-Exponential Factors (ai) Obtained from Two Exponential Fitsa a1 τ1 (µs) a2 τ2 (µs) τave

L

LP-F

DLP-F

E

EP-F

0.0015 14.2 0.0003 1400 208

0.0027 10.1 0.0004 904 129

0.0029 13.5 0.0010 1250 323

0.0010 18.0 0.0002 892 141

0.0011 15.7 0.0002 533 93

a Transient absorption decays at 720 nm for the films composed of P-F, PVT3, and PHT layers, and transient absorption decays at 620 nm for the films with ZnPH4. The excitation wavelength was 430 nm. The weighted mean average time constant (τave) of the film decays is presented for comparison.

state, most of the positive charges are located in the PVT3 network and the electrons are in the fullerene sublayer. Figure 5 shows the normalized transient absorption decays of the DLP-F, LP-F, and L films for monitoring at 720 nm, when excited at 430 nm. The DLP-F decay will be discussed in detail later. Transient absorption decay of the P-F27 monolayer is much faster than those presented in Figure 5. Rate constants determined from the biexponential fit of the decays at a 1 ms time scale for the studied film structures are shown in Table 1. The average (weighted mean) time constants, τave, of the decays can be used to compare the transient absorption decay rates of the films. Because of the high noise level of the signals, the fitting error was rather high, causing inaccuracy of the rate constants. As shown in Figure 5, the transient state of PVT3 is quenched by the P-F monolayer causing faster decay in the LP-F film immediately after excitation. This can be concluded also from the decreased time constants of the LP-F film in Table 1. Despite the fast decay immediately after excitation, the transient absorption for the LP-F film is, even if the amplitude is low, longer-lived that that of the L single layer. The decreased τave of the bilayer film can be explained by the quenched PVT3 triplet state causing faster time constants in the short time scale. Taking into account the conclusions made based on the timeresolved spectra, the decreased lifetime of the fast component and the long-lived transient absorption in the LP-F film can be assigned to the energy and electron transfer from PVT3 to the dyad, respectively. The longest-lived transient state in the bilayer film is assigned to the interlayer CS state, L+P-F-. The increased lifetime of the CT (compared to that of the P-F monolayer) is ascribed to the longer distance of the CS and to lateral migration of the charges in the corresponding molecular networks. Previously, energy and electron transfers from PVT3 to the porphyrin-fullerene dyad were studied with the time-resolved emission and photovoltage methods without actual knowledge

Figure 6. (a) Time-resolved absorption spectra for films composed of P-F, PVT3, and PHT layers with excitation at 430 nm. (b) Normalized spectra of the films.

of the photoinduced processes in the film.33 The conclusions made based on flash photolysis measurements are supported by the results from the emission and photovoltage experiments. 3.2. Electron Transfer in the Multilayer Film. The ET system described previously was extended by introducing the PHT monolayer as the electron donor to the PVT3 and P-F films. Photoinduced CT reactions in a multilayer film were studied by measuring a PHT-PVT3-P-F (DLP-F) layer sequence and by using PHT (D), PVT3 (L), PVT3-P-F (LPF), and PHT-PVT3 (DL) films as references. In all cases, the layers were only on one side of the substrate. In the D film, two PHT monolayers were separated from each other by three ODA layers. The absorption spectra of the films are shown in Figure 3, and that of the dyad film (two P-F monolayers) is shown in Figure 2. Excitation took place at 430 nm with energy densities of about 0.7 and 2.1 mJ/cm2 for D and DLP-F films, respectively, and at about 0.9 mJ/cm2 for other samples. Figure 6a shows the time-resolved absorption spectra of the LP-F, DL, DLP-F, L, and D films at a 5 µs delay. The same spectra normalized at 630 nm are presented in Figure 6b. Absolute amplitudes of the transient spectra are not directly comparable to each other because of the different measurement conditions. Thus, normalized spectra are used for comparison. Clear changes in the transient spectra are observed when more functional layers are added to the film structure, even if the absorption of the PVT3 triplet at 620 nm dominates the spectral shape. The transient spectrum of the D film shows weak groundstate bleaching and transient absorption below and above 600 nm, respectively, which are both assigned to the formation of the PHT triplet state (Figure 6a). The transient absorption of the DL film is increased above 650 nm and decreased at wavelengths below 620 nm compared to that of the L single layer (Figure 6b). These differences are too strong to be explained only by formation of the PHT triplet in the D single layer. The transient spectrum of DL is not a sum of its individual

Electron Transfer in Films Containing P-F

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components, demonstrating a photoinduced interaction between the PHT and PVT3 layers. With the results from previous timeresolved photovoltage33 and absorption27 experiments, the increased transient absorption of the double structure above 650 nm is assigned to the PHT cation and PVT3 anion, which are difficult to resolve from each other. Similarly, the decreased transient absorption below 620 nm is assigned to PHT and PVT3 ground-state bleaching caused by the formation of corresponding cation and anion moieties. The previous results obtained from the photovoltage experiments are now supported by flash photolysis measurements of similar films, demonstrating the interlayer ET from PHT to PVT3 (eq 3)

DL* f D+L-

Figure 7. Absorption spectra of ZnPH4 and ZnPH4-P-F films.

(3)

In the final transient state, positive charges are located in the PHT layer and the electrons are in the PVT3 network. Deposition of the P-F monolayer onto the DL film (to construct the DLP-F structure) has a clear influence on the transient spectrum as can be seen in Figure 6. Transient absorption of the DLP-F film above 650 nm is decreased compared to that of the DL and LP-F structures referring to the missing absorption of the PVT3 anion and cation moieties, respectively. PVT3 acts as an energy and electron donor to the dyad as concluded earlier but recombines back to the ground state via the interlayer ET from PHT and, thus, causes the decreased absorption above 650 nm. The increased transient absorption intensity of the DLP-F film above 650 nm, if compared to that of the L single layer, is now assigned only to the PHT cation.27 As discussed previously, the decreased absorption below 620 nm is assigned to the PHT ground-state bleaching that is caused by the formation of the PHT cation in the ET reaction from PHT to PVT3 or P-F. Only transient absorptions of the PHT cation and the PVT3 triplet can be resolved from the spectrum of the DLP-F film as could be expected if ET through the entire film structure takes place (reaction 4).33

DL*P*-F f DL*P+-F- f DL+P-F- f D+LP-F- (4) To repeat, excitation in DL*P*-F is not obligatory on the neighboring L and P-F. The spectroscopic results together with those from the photovoltage33 measurements of similar films demonstrate the CT through the entire multilayer structure. This leads to the final transient CS state of D+LP-F-, where positive charges are located in the PHT layer and negative charges in the fullerene sublayer. The normalized absorption decay of the DLP-F film is shown in Figure 5 together with the decays of the LP-F and L films. The decay profile of this most complex film structure is slower than those of the simpler ones indicating the prolonged lifetime of the final transient state (D+LP-F-) in the multilayer film. The increased transient lifetime in the DLP-F film is also supported by the highest obtained τave of the studied samples (Table 1). The prolonged lifetime of the CS is assigned to the lateral migration of charges in the PHT (holes) and fullerene (electrons) networks, as well as with the longer distance between the charges. 3.3. Phthalocyanine as the Electron Donor to the Porphyrin-Fullerene Dyad. In addition to the already described systems, phthalocyanine was studied as an antenna layer to the dyad instead of PVT3. Photoinduced ET between the ZnPH4 and P-F layers was studied by measuring a ZnPH4-P-F (EP-F) structure and by using the single layers of P-F and ZnPH4 (E) as references. In all cases, the layers were only on one side of the substrate. Excitation wavelengths

Figure 8. Time-resolved absorption spectra of ZnPH4-P-F, ZnPH4, and P-F films at delay times of 5 and 150 µs (inset) with excitation at 430 nm. The line with crosses represents the sum of the ZnPH4 and P-F spectra.

of 430 nm (porphyrin and phthalocyanine Soret bands) and 720 nm (phthalocyanine Q-band) with energy densities of about 0.9 and 1.6 mJ/cm2, respectively, were used to study the photoinduced processes. Experimental conditions were similar for all films, and measurement accuracy was within 15% for the samples with the same excitation wavelength. The absorption spectra of the E and EP-F films are presented in Figure 7, and that of the dyad film (two P-F monolayers) is presented in Figure 2. The absorption maximum of ZnPH4 at 710 nm corresponds to the phthalocyanine Q-band. ZnPH4 layers were spin-coated, and thus slight differences in the absorption spectra were obtained. The spectrum of the bilayer film is equal to a sum of its components (taking into account differences in ZnPH4 absorption), showing successful film deposition and weak ground-state interaction between the molecules of distinct layers. Compared to the EP-F film, absorption of the E single layer is 19% higher at 430 nm but 14% lower at 720 nm. The time-resolved absorption spectra of the EP-F, E, and P-F films at a 5 µs delay with excitation at 430 nm, where both porphyrin and phthalocyanine absorb, are shown in Figure 8. The transient spectrum of the EP-F film is not a sum of its corresponding components, demonstrating interlayer interaction between ZnPH4 and P-F layers after photoexcitation. Absorption bleaching of the E single layer below 550 nm and at 750 nm corresponds to the ground-state absorption bands of phthalocyanine (Figure 7). The transient absorption of the E layer at 620 nm is assigned to the triplet state of ZnPH4. If the groundstate absorption differences of the films are taken into account, absorption bleaching at 750 nm is stronger for the EP-F structure. The increased transient absorption of the EP-F film below 620 nm is the most obvious difference between the spectra of the films with and without the dyad monolayer. Absorption

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below 620 nm is assigned mainly to the ZnPH4 cation because it is not observed for the E single layer, and it does not correspond to the transient absorption of the porphyrin cation. Formation of the ZnPH4 anion is ruled out on the basis of previous photovoltage measurements.34 The interlayer ET from ZnPH4 to P-F is now demonstrated by the spectroscopic method. When excited at 430 nm, the possible CT reactions in the bilayer film can be presented as follows34

EP*-F f EP+-F- f E+P-F+ -

+

-

E*P-F f E P -F f E P-F

(5) (6)

In the final transient CS state the positive charges are located in the ZnPH4 layer and the negative charges in the fullerene network. The time-resolved absorption spectra of the films at a 150 µs delay time with excitation at 430 nm are presented in the inset of Figure 8. Transient states of the E and P-F films are decayed close to zero at this time. The transient bands of the EP-F structure are, however, still intense 150 µs after the excitation, demonstrating the increased lifetime of the final transient state (E+P-F-) in the bilayer system. The transient absorption below 620 nm is now assigned mostly to the ZnPH4 cation. The time-resolved absorption spectra of the E and EP-F films at a 5 µs delay with excitation at 720 nm, where only ZnPH4 is excited, are shown in SI 1 of the Supporting Information. The transient spectra of both films are almost identical, and minor deviations originate from the differences in ground-state absorption. This indicates that there is no strong interaction between the layers when only ZnPH4 is excited. In the photovoltage measurements,34 inefficient ET from ZnPH4 to P-F was demonstrated when the sample was excited at the phthalocyanine Soret band around 700 nm, indicating a higher sensitivity of the electric method. No indications of the ZnPH4 cation or any long-lived transient states are observed on the transient spectrum of the EP-F film neither at a 150 µs delay time (inset of Figure SI 1 of the Supporting Information). Reaction 6 can therefore be concluded as inefficient, and thus reaction 5 is the main ET process from ZnPH4 to P-F. The normalized absorption decays of the EP-F and E films at 620 nm with excitation at 430 nm are shown in Figure SI 2 of the Supporting Information. Decay profiles of the films are similar, and no indication of the long-lived transient state in the EP-F film is observed in this case. Slightly higher time constants of the E film in Table 1 can be explained by the inaccuracy of fitting and will be left without further discussion. On the basis of the decay curves, transient states in the films with ZnPH4 recombine relatively fast compared to the transient states in the films with PHT or PVT3 (Figure 5, Table 1). This correlates well with conclusions made from the previous photovoltage measurements of similar films.34 The transient absorption of the ZnPH4 cation and the prolonged lifetime of the final transient state (compared to the E single layer) are observed for the EP-F film when excited at 430 nm, but not if the excitation at 720 nm is used. This demonstrates the crucial influence of the excited dyad for the overall CT efficiency in the bilayer cell with ZnPH4. The interlayer ET from excited ZnPH4 to fullerene via the porphyrin moiety of the ground-state dyad is inefficient. Therefore, the ZnPH4 cations are formed mainly in the secondary ET reactions from ZnPH4 to the porphyrin cation (P+) after the primary ET from porphyrin to fullerene. Conclusions made from the spectroscopic measurements are supported by the results from the previous time-resolved photovoltage experiments.34

Figure 9. Normalized photovoltage (electrical) and flash photolysis (optical) signal decays of PVT3-P-F and P-F (inset) films.

3.4. Comparison between Optical and Electrical Signals. Photoinduced intra- and interlayer CT in similarly layered molecular thin films have been studied by the time-resolved electrical photovoltage33,34 and optical flash photolysis27 techniques, as discussed earlier. Both methods are used to follow the final transient CS state in the films, and thus lifetimes of the optical and electrical signals should correlate with each other. The lifetime of the spectroscopic signal is, however, shorter than that of the photovoltage one.27,33 The normalized electrical30,33 and optical27 signals of the LP-F and P-F films are compared in Figure 9, demonstrating the shorter lifetime of the spectroscopic signal. The same phenomenon was observed for all of the studied film structures. Halflives for the electrical and optical signals of the P-F monolayer are about 17 and 14 µs, and for those of the LP-F film about 100 and 6 µs, respectively. Even if the half-lives for the dyad monolayer are close to equal, the lifetime of the electrical signal is evidently longer because of its long-lived tail (Figure 9). The shorter lifetime of an optical signal can be explained by different measurement principles. The photovoltage signal is sensitive to the number of positive and negative charges and to the distance between them, whereas the spectroscopic signal is affected by the existence of molecular transient states. Thus, all charges in the film have influence on the electrical signal, even if they are less pronounced in spectroscopic methods. Long-lived electron-hole pairs in the studied systems are the result of charge hopping in the corresponding molecular networks that may be followed by charge ejection into the molecular environment, known as “free” charges.27 Free charges are not bound to any specific molecular moieties, and thus they are spectroscopically less pronounced, yielding the decreased lifetimes of the optical signals. Observed long-lived photovoltage responses (especially extremely long-lived tails of the photovoltage signals) are caused by this kind of free electron-hole pairs. The intramolecular CS in the P-F monolayer results in similar electrical and optical responses, whereas for the film with the secondary electron donor (PVT3-P-F), the optical signal has a 17-times shorter half-life compared to that of the electrical signal (Figure 9). Thus, charge hopping and ejection as free charges is more pronounced in complex films with long CS distances and efficient charge migration in the molecular layers. The intramolecular ET in the dyad monolayer with fast CT recombination and weak charge migration yields similar electrical and optical responses, as could be assumed. 4. Conclusions Layered molecular thin films were constructed by using LB and spin-coating methods in order to study energy and electron

Electron Transfer in Films Containing P-F transfer in multicomponent film structures. Photoinduced interlayer processes were characterized by the laser flash photolysis method on micro- to millisecond time scales. The energy transfer from excited PVT3 (L) to the porphyrinfullerene dyad, as well as the secondary electron transfer from PVT3 to the porphyrin cation (P+) after the primary electron transfer from porphyrin to fullerene, was demonstrated. The charge transfer through the multicomponent film structure of DLP-F was shown, yielding the final transient charge-separated state of D+LP-F-, where positive charges are located in the PHT layer and negative charges in the fullerene sublayer. The crucial role of the excited dyad for the overall charge transfer efficiency in the bilayer system containing the P-F and ZnPH4 (E) layers was demonstrated. The interlayer electron transfer from ZnPH4 to the porphyrin cation (P+) was efficient, yielding the final transient charge-separated state of E+P-F-, where the positive charges are located in the ZnPH4 layer and the electrons in the fullerene network. All the conclusions made based on the spectroscopic flash photolysis measurements are supported by the results from previous time-resolved electrical photovoltage measurements.33,34 The lifetime of an electrical signal was found to be longer than that of the optical signal, even if the same phenomenon (final transient charge-separated state of the films) was monitored. The difference in lifetimes is explained by the charge ejection into the molecular environment as the final transient state of the systems, yielding the long-lived free charges that affect the electrical signal but are less pronounced in spectroscopic methods. Acknowledgment. This work was supported by the Finnish National Graduate School of Nanoscience. Supporting Information Available: Time-resolved absorption spectra of E and EP-F films at delay times of 5 and 150 µs with excitation at 720 nm (Figure SI 1) and normalized absorption decays of E and EP-F films for monitoring at 620 nm with excitation at 430 nm (Figure SI 2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Currie, M. J.; Mapel, J. K.; Heidel, T. D.; Goffri, S.; Baldo, M. A. Science 2008, 321, 226. (2) Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (3) Zhang, C.; Tong, S. W.; Jiang, C.; Kang, E. T.; Chan, D. S. H.; Zhu, C. Appl. Phys. Lett. 2008, 92, 083310. (4) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324. (5) Kietzke, T. AdV. OptoElectron. 2007, 1. (6) Reyes-Reyes, M.; Lo´pez-Sandoval, R.; Liu, J.; Carroll, D. L. Sol. Energy Mater. Sol. Cells 2007, 91, 1478. (7) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. ReV. 2007, 107, 1233. (8) Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2006, 128, 8108.

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