Visible Light Sensitization of Titanium Dioxide with Self-Organized

Mar 23, 1999 - Free base 5,10,15,20-tetrakis(4-n-octylphenyl) porphyrin (H2TOPP) belongs to a class of self-organizing porphyrins. Since its LUMO lies...
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J. Phys. Chem. B 1999, 103, 2702-2708

Visible Light Sensitization of Titanium Dioxide with Self-Organized Porphyrins: Organic P-I-N Solar Cells Jeannette Wienke and Tjeerd J. Schaafsma Agricultural UniVersity of Wageningen, Department of Molecular Physics, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands

Albert Goossens* Delft UniVersity of Technology, Laboratory for Inorganic Chemistry, Julianalaan 136, 2628 BL Delft, The Netherlands ReceiVed: July 13, 1998; In Final Form: January 10, 1999

Free base 5,10,15,20-tetrakis(4-n-octylphenyl) porphyrin (H2TOPP) belongs to a class of self-organizing porphyrins. Since its LUMO lies above the conduction band of titanium dioxide (TiO2) and its visible light absorption is very strong, sensitization of TiO2 with H2TOPP thin films is possible. After spin-coating this porphyrin onto n-type TiO2, the Fermi-level of H2TOPP is measured, from which it is established that it behaves as intrinsic semiconductor, i.e., donor and acceptor densities, if present, compensate each other. Thin films of zinc 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (ZnTCPP) are also investigated and show profound p-type character. Moreover, the LUMO of ZnTCPP is located about 0.4 eV above that of H2TOPP making an organic based heterostructure p-i-n solar cell possible. In this cell, H2TOPP is sandwiched between n-type TiO2 and p-type ZnTCPP. The LUMO positions of these porphyrins is such that unidirectional energy transfer from ZnTCPP to H2TOPP occurs. By carefully comparing the photocurrent action spectra with the absorption spectra, it could be established that a built-in field in the p-i-n structure is beneficial for solarenergy conversion.

Introduction Study of the fundamental properties of organic semiconductors toward their possible use in photoconducting thin films, light emitting diodes, optical switches, plastic electronics, and photovoltaic solar cells has been a challenging research topic for several decades.1 Indeed the prospect that the group of inorganic semiconductors will once be supplemented by a collection of novel organic materials attracts the imagination of many scientists. Within this innovative field of research, this paper is focused on the development of organic photovoltaic solar cells. Previous investigations on solar cells based on molecular semiconductors exposed a wealth of features that organic systems offer.2-12 However, despite the versatility of organic compounds and the invested effort, energy conversion efficiencies are still limited to about 1%. There are several complications which hinder effective lightto-electricity conversion. Upon exciting molecular semiconductors, bound electron-hole pairs (excitons) rather than free electrons and free holes are generated. To overcome the electron-hole binding energy, a local potential fluctuation such as an interface, an impurity, or a trap is required. Therefore, efficient energy conversion demands long- range energy migration. In addition, once separated, the free electron and the free hole must be transported without energy loss. Both energy migration and charge migration can be improved by minimizing the concentration of impurities and by maximizing the internal order thereby reducing the number of energy * To whom all correspondence should be addressed. E-mail: a.goossens@ stm.tudelft.nl.

barriers that excitions and free charges encounter within their lifetime. Preferably, a well-ordered arrangement of the organic molecules, approaching the crystalline properties of inorganic semiconductors, has to be realized. An attractive possibility has been introduced by Gregg13,14 who used discotic mesomorphic porphyrins with self-organizing properties. In these materials, large aromatic macrocycles spontaneously arrange in long linear columnar stacks facilitated by peripheral substitution of long hydrocarbon chains. It could be demonstrated that the degree of long-range order directly affects the photoconversion properties in terms of charge and exciton migration.14 An other well-known approach to deal with the transport of free electrons and free holes is the formation of a built-in potential by which charge carriers that enter the space charge region are accelerated. In Schottky barriers and p-n junctions, the formation of a space charge region occurs spontaneously and establishes an electrostatic electric field inside the device. Unfortunately, in organic p-n devices the thickness of the space charge region as well as the minority carrier diffusion length are negligeable compared to the thickness required for full light absorption.5-7,9-11 Since exciton dissociation preferably takes place at an interface, the nature of the interface between electrode and organic material has a profound influence on the performance of a photovoltaic cell. Charge separation at an interface can be mastered by an efficient bend bending,15 the formation of an accelerating built-in potential in so-called Schottky-type cells,2-4,16 or profitable interfacial kinetics.73 The photovoltaic cell, described in this paper, includes two of the above-mentioned strategies to increase the energy conversion efficiency. Besides the use of a self-organizing

10.1021/jp9829851 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/23/1999

Organic P-I-N Solar Cells porphyrin, a built-in potential is created to enhance the charge carrier transport. In contrast to previous arrangements, the created electrical field is made to extend over the entire selforganized porphyrin film. This approach leads to an organic based p-i-n heterostructure which has proven its value in amorphous silicon solar cells.17 Previous attempts to produce a p-i-n structure in organic systems by sandwiching a coevaporated mixture of a p- and n-type dye between doped dyes have not been very successful.18,19 Here a different strategy is followed. A self-organized free base 5,10,15,20-tetrakis(4-noctylphenyl) porphyrin (H2TOPP) is prepared to act as an intrinsic film and is sandwiched between a titanium dioxide (ntype) thin film and a zinc 5,10,15,20-tetra (4-carboxyphenyl) porphyin (ZnTCPP) (p-type) thin film. The anionic ZnTCPP has been recognized before as an excellent sensitizer for titanium dioxide.20-22 In our organic p-i-n heterostructure, the following course of events is anticipated. Optical excitation creates bound electron-hole pairs in the H2TOPP and the ZnTCPP thin films. The binding energy of the excitons is too large for spontaneous exciton dissociation in the bulk. Instead, exciton dissociation and subsequent charge carrier generation predominantly occur at the TiO2|H2TOPP and at the H2TOPP|ZnTCPP interfaces. The molecular order in the self-organized H2TOPP film enhances the exciton diffusion length such that excitons collected at the interface predominantly originate from the H2TOPP film. After the electron transfer between the LUMO of H2TOPP and the conduction band of TiO2 has taken place, the hole in the HOMO of H2TOPP is accelerated toward the H2TOPP|ZnTCPP interface pulled in the right direction by the built-in field. Currently, the proper way to interpret optoelectronic phenomena in molecular semiconductors is still under discussion. One possible approach is to take the conventional semiconductor concepts as a starting point and try to extend these into the realm of molecular systems. This approach certainly has its limitations since band theory is on the verge of its break down when electron localization is strong. Yet, in this paper, concepts such as valence and conduction bands, n- and p-type conduction, and Fermi-level are used without further explanation. It is shown that the experiments are adequately described when making use of these concepts. This, however, does not exclude the possible use of other types of explanations. Instead of using delocalized electronic states, an approach based on localized electronic states has also proven its value. Possibly, reality is somewhere between since electronic states may be spread out over only a few molecules, in which case neither fully delocalized nor fully localized electronic wave functions are adequate. Experimental Aspects The purity of zinc 5,10,15,20-tetra(4-carboxyphenyl) porphyin (ZnTCPP, Midcentury Chemicals) was checked by thin-layer chromatography and absorption spectroscopy. Free base 5,10,15,20-tetrakis(4-n-octylphenyl) porphyrin (H2TOPP) was synthesized following the reported procedure by Adler et al.23 The molecular structure of the porphyrins is given in the inset of Figure 1. The anatase phase of titanium dioxide TiO2 was synthesized by metal organic chemical vapor deposition (MOCVD) using titanium tetra-isopropoxide (Fluka) and oxygen as reactant gases. Deposition took place on ITO substrates (Glastron, 50 Ω square) at a reactor pressure of 1 bar and a substrate temperature of 400 °C. Atomic force microscopy reveals that the surface roughness of the ITO substrates is about 20 nm; ITO columns of 20 nm height stick out of the plane. On top of this, a 200 nm thick TiO2 is deposited with MOCVD.

J. Phys. Chem. B, Vol. 103, No. 14, 1999 2703

Figure 1. Absorption spectra of ZnTCPP on TiO2. The monolayer absorption (thin line) has a blue-shifted Soret band at 430 nm compared with the absorption of a spin-coated ZnTCPP film from 3 mM solution (thick line). In the inset, the structures of the used ZnTCPP and H2TOPP molecules are shown.

This film has a roughness of about the same magnitude as that of the underlying ITO substrate, i.e., small anatase TiO2 crystals protrude 20 nm out of the plane. Before adsorption of the porphyrins, the substrates were cleaned in a Decon soap solution, rinsed in distilled water and ethanol, and dried with purified nitrogen. Porphyrin dyes were applied to TiO2 by spin coating at 3000 rotations per minute using chloroform and methanol as solvents for H2TOPP and ZnTCPP, respectively. By varying the porphyrin concentration the thickness of the porphyrin films could be controlled. In case of double layers, ZnTCPP solution is spin coated on top of the methanol-rinsed H2TOPP film or H2TOPP solution on top of a chloroform-rinsed ZnTCPP film. This choice of solvents excludes mixing of the two porphyrin thin films which would destroy the heterostructure device. To remove the solvent and to enhance self-organization of the H2TOPP film (see below), the spin-coated layers were dried at 120 °C for 30 min. Absorption spectra were recorded with a spectrophotometer (Varian, model CARY 5E) equipped with a diffuse reflection sphere to correct for reflection losses. For photoelectric measurements a back contact was provided by a mercury drop with an effective area of 0.78 mm2 allowing adequate photoelectric characterization of the organic thin films with thicknesses down to 2 nm.24 Samples were irradiated with focused light from a 150 W Xenon lamp (Spectral Energy), which passed through a monochromator (Spectral Energy, model GM 252), an appropriate cut-off filter, and an electronic shutter (Uniblitz). In all cases irradiation is directed through the glass support and transfers the ITO and TiO2 coatings before reaching the porpyrin films. Current-voltage (i-V) plots and photocurrent action spectra were recorded using a potentiostat (Autolab, model PGSTAT 10). In all experiments, the mercury contact was connected to the working electrode. Current-voltage scans were recorded at 5 mV s-1 using 13 mW cm-2 light with a wavelength of 440 nm. Rutherford back scattering (RBS) was carried out in a vacuum chamber at 10-6 mbar connected to a 3 MeV Van de Graaff generator.25 The experiments were performed with 2 MeV He+ ions. The incident beam had an angle of 70° to the surface normal and the detector was positioned at a scattering angle of 170°.26 The Kelvin probe technique is a noncontacting method to measure the potential difference between a reference probe and a sample surface that has been brought in close proximity (0.5

2704 J. Phys. Chem. B, Vol. 103, No. 14, 1999 mm) to the probe.27 Using an electrostatic voltmeter (Trek, model 320B) equipped with a highly sensitive probe (model 3250) the contact potential difference (CPD) could be derived with an accuracy of 100 mV. The work function (being the Fermi level energy, EF) of the TiO2 samples was derived from the measured CPD of TiO2 versus the reference work function of gold being 5.1 ( 0.1 eV. A sputtered thin gold film on glass served as reference standard. The total error in work function with respect to vacuum can be up to 0.2 V. CW irradiation of the TiO2 samples with 350 nm light (1 W m-2) generates a surface photovoltage. When the light intensity reaches the level where saturation of the photovoltage is observed, TiO2 is brought into the flat band situation. By comparing the work functions with and without 350 nm irradiation, the band bending inside TiO2 can be measured with 100 mV accuracy.

Wienke et al.

Figure 2. Photocurrent action spectrum of TiO2 sensitized with a film of seven monolayers of ZnTCPP.

Results and Discussion TiO2 sensitized by ZnTCPP. The absorption spectrum of ZnTCPP in solution shows one Soret band at 420 nm and two Q bands at 556 nm, Qy(0,1), and 594 nm, Qy(0,2) (not shown). After spin-coating ZnTCPP on TiO2 a red shift of the absorption bands and a thickness-dependent change of the Soret band shape are observed (Figure 1). The absorption maxima are now located at 440 nm for the Soret band and at 562 and 605 nm for the Q bands. To derive the average thickness of the spin-coated films, a ZnTCPP monolayer has been prepared by rinsing the spin-coated substrate in methanol.28 By comparing the Qy(0,1) band absorbance of the spin-coated film with that of the monolayer and using the Lambert-Beer law, spin-coated ZnTCPP films from 3 mM methanol solution are found to have a coverage of seven monolayers. In all the photoelectric experiments a ZnTCPP film of seven monolayers thick is used. Monolayer coverage of ZnTCPP corresponds to 4.5 × 1013 molecules cm-2; seven monolayers contain 3.15 × 1014 molecules cm-2. Intimate contact between the ZnTCPP layer and titanium dioxide is crucial for effective charge injection into the conduction band of TiO2, and a pinhole-free ZnTCPP film is required. While a monolayer shows short circuiting, a ZnTCPP film spin-coated from 3 mM solutions (seven monolayers thick) is homogeneous and virtually without interfering pinholes when contact is made with a mercury drop. Mercury drop contacts on 20 nm rough TiO2 surfaces covered with only seven monolayers are free from shunting because the surface tension of mercury is so high that it prevents the metal to fill in the small nanometer sized holes in the organic film. Only when the size of the pinholes approaches that of the contact area, i.e., 0.78 mm2, short circuiting will occur. When this happened the mercury drop was moved away to a different location. As is shown in Figure 2, the photocurrent action spectrum of TiO2 sensitized by ZnTCPP matches the absorption spectrum of the chromophore very well. Information about the electrostatic interaction between the organic compound and TiO2 can be obtained by recording the dark current-voltage characteristics. In particular, from i-V scans the presence of an internal electric field can be deduced.172 Depending on the work function difference between either one of the electrodes (Hg and TiO2) and the porphyrin layers ohmic or blocking contacts are possible. For an ohmic contact the dark current is a linear function of the applied voltage; a blocking contact (barrier) is characterized by an asymmetrical dark i-V curve with an approximately constant current in the reverse bias range. As is presented in Figure 3 (thin line), the dark i-V curves of TiO2|ZnTCPP|Hg cells show a diode-like characteristic. We therefore conclude

Figure 3. Current-voltage characteristics of a TiO2|ZnTCPP (7 monolayers) |Hg cell. In the dark (thin line), with 440 nm irradiation (thick line).

that a blocking contact is formed between ZnTCPP and TiO2 or Hg. Formation of a built-in electric field at the TiO2|ZnTCPP interface can be evidenced by measuring changes in the work function of TiO2 after applying a ZnTCPP film using the Kelvin probe technique.29 The Fermi energy EF for uncoated titanium dioxide is 5.1 ( 0.2 eV and shifts to 5.6 ( 0.2 eV after adsorption of ZnTCPP. Therefore, at the TiO2|ZnTCPP interface, space-charge formation takes place in which TiO2 obtains a positive and ZnTCPP a negative charge. The formation of built-in electric fields in organic thin films is still not fully understood. The following paragraph addresses some implications of this observation; in a previous paper a more elaborate discussion on the issue has been given.29-30 Upon excitation with 440 nm light, diode like i-V curves are obtained with open circuit voltages (VOC) of 0.6 V, short circuit currents (ISC) of 0.1 mA cm-2 and a fill factor (FF) of 0.75 (Figure 3, thick line). The calculated IPCE is 2.2% from which a monochromatic quantum efficiency of about 14% is derived when corrections for reflection and transmission losses are made. The negative value for VOC indicates that the porphyrin film acts as donor and injects electrons into the conduction band of TiO2 upon excitation. To understand this behavior, the energy of the ZnTCPP electron donating level (LUMO) is compared to the position of the conduction band of TiO2 using a semiempirical model.31 Interfacial exciton dissociation occurs if the exciton bond, having an energy of 0.20.4 eV, can be broken. This energy barrier can be crossed by thermal excitation or by tunneling. In either case, the initial state is a bound electron in the LUMO and the final state is a delocalized electron in the conduction band of TiO2. Together

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with the orbital overlap, it is the energy difference between the initial and final states that determine the electron-transfer rate. Therefore, we compare the energy of the LUMO with the conduction band minimum to determine whether electron transfer can occur or not. The ionization potential IC of a solid organic compound can be derived from the half-wave oxidation potential EOX1/2 by using

IC ) EOX1/2 + (4.6 ( 0.1)

(1)

where the constant 4.6 ( 0.1 eV equals the free-energy difference between an electron in a normal hydrogen reference electrode and the same electron in a vacuum at infinity.32 The half-wave oxidation potential of ZnTCPP in solution is 0.8 ( 0.1 eV (NHE),20 and with the aid of eq 1, IC is calculated to be 5.4 ( 0.2 eV, which equals the HOMO energy level on the absolute energy scale. Subtracting the HOMO-LUMO energy difference of 2.0 eV 33 (corresponding to the S0-S1 energy difference) the LUMO position of ZnTCPP is found at 3.4 ( 0.2 eV. It is good to realize that the energy values for the HOMO and LUMO are derived from oxidation potentials of solvated molecules which may differ from those of thin films. Here we assume these to be the same which may introduce an additional error of 0.1 V. The Fermi level of ZnTCPP, as determined with the Kelvin probe technique, is 5.6 ( 0.2 eV, and when comparing this value with the HOMO energy level it appears that for ZnTCPP both levels coincide within the experimental accuracy being 5.5 ( 0.1 eV. While it may seem that the Fermi level is lower than the HOMO this cannot be the case. It is also contradicted by the absorption spectra. The reader is urged to note the limited experimental accuracy of absolute Kelvin-probe measurements and oxidation potentials. But even so, it is save to conclude that the Fermi level is near to the HOMO. In semiconductor language this implies that spin-coated ZnTCPP fims act as a p-type semiconductor. While the chemical nature of the electron acceptors remain obscure, it is postulated that oxygen incorporation and coordination of the metal center occurs in ZnTCPP thin films. In our previous investigations29 the Fermi level of zinc tetra(N-methyl4-pyridinium) porphyrin (ZnTMPyP) has also been found to approach the oxidation level (HOMO) while the Fermi level in the metal-free equivalent (H2TMPyP) is near the center of the HOMO-LUMO gap. Here with a different zinc porphyrin a similar observation is made. It is well-known that zinc porphyrins have a tendency to form complexes. The completely filled d-shell of zinc favors a 5-fold ligandation; inside the porphyrin ring four positions are occupied and the fifth position remains vacant. It is this position that may interact with ambient oxygen which is abundantly present since spin-coating, processing, and the experiments are all performed in air. Oxygen has a large electronegativity and will act as electron acceptor which confers p-type character to zinc containing porphyrins. The conduction band edge of TiO2 has also been derived from Kelvin probe measurements. Using 350 nm excitation a saturated photovoltage of 0.9 eV is detected. The flatband potential is obtained by subtracting the photovoltage from the Fermi-level position in the dark, i.e., 5.6-0.9 ) 4.7 ((0.2) eV. The saturated photovoltage can be equated with the band bending because at the flat band potential the electron-hole recombination rate equals their generation rate. The absorption coefficient of anatase TiO2 at 350 nm is 3 × 106 m-1 21 which implies a characteristic penetration depth of 300 nm. Only with the aid of an internal electric field the minority carriers (holes) are able to cross this distance. Since all minority carriers created outside the space

Figure 4. Absorption spectra of H2TOPP applied onto TiO2 by spincoating a 4 mmolar solution. Prior to heating (open circles) and after heating at 130 °C (solid line).

charge region recombine, the maximum photovoltage equals the band bending. Because our TiO2 films are not intentionally doped, the difference between the Fermi level and the conduction band in the field free region is about 0.3 eV. From this, the position of the conduction band minimum at the surface, which is equal to the electron affinity (EA), is calculated to be 4.7-0.3 ) 4.4 ( 0.2 eV. The LUMO of ZnTCPP lies at 3.4 (0.2 eV and is about 1 eV above the conduction band edge of TiO2, making efficient electron injection possible. The TiO2|ZnTCPP photocurrent voltage scans show a remarkably high fill factor of 0.75, it reflects the strong tendency of TiO2|ZnTCPP to generate unidirectional electron transfer. TiO2 sensitized by H2TOPP. The absorption spectrum of H2TOPP in dilute chloroform shows a strong Soret band absorption at 420 nm and four characteristic Q bands between 500 and 650 nm. The spectrum of the film on quartz substrates reveals a red shift of the Soret band to 440 nm, which is largely caused by excitonic interactions.34 H2TOPP films belong to a group of self-organizing materials. Making use of the properties of the liquid crystalline phase in which the porphyrin stacks have a sufficient degree of motional freedom, they spontaneously reorganize into an ordered structure. After heating a spin-coated H2TOPP film above the phase transition temperature of 120 °C, a change from the crystalline phase to the mesophase can be observed via a change of the Soret band region of the absorption spectrum35 which is shown in Figure 4. Linear dichroism and ESR orientation studies disclose the formation of a nonrandom film,33 although it is likely that the spin-coating procedure imposes limitation on the spatial extend of selforganization14. To determine the coverage of H2TOPP, solutions of different concentrations have been spin coated onto TiO2. Because a monolayer reference is not available in this case, a different method is demanded to determine the coverage of H2TOPP. Rutherford back scattering (RBS) has now been used for calibration of the film coverage expressed in molecules per square centimeter.256 Since only heavy atoms can be detected by RBS, the free-base porphyrin is replaced by CuTOPP assuming that spin-coating of CuTOPP resembles that of H2TOPP. A linear relationship between the concentration and the coverage holds. To demonstrate sensitization of TiO2 by H2TOPP, photocurrent action spectra of TiO2|H2TOPP|Hg cells are shown in Figure 5b and can be compared with the absorption spectrum of Figure 5a. In both figures, a maximum in the Soret band region and the four Q bands are characteristic for H2TOPP. As can be derived from Figure 6 (thick line), the TiO2|H2TOPP|Hg

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Wienke et al.

Figure 7. Photocurrent-voltage characteristics of a TiO2|H2TOPP|Hg cell (thin line) and a TiO2|H2TOPP|ZnTCPP|Hg cell (thick line).

Figure 5. (a) Absorption spectra of TiO2 sensitized with a H2TOPP film (thin line) and sensitized with a H2TOPP|ZnTCPP heterojunction film (thick line). Both spectra are normalized on the Qy(0,1) band at 531 nm. (b) Photocurrent action spectra of TiO2 sensitized with a H2TOPP film (thin line) and sensitized with a H2TOPP|ZnTCPP heterojunction film (thick line). Both spectra are normalized on the Qy(0,1) band at 531 nm.

Figure 6. Current-voltage characteristics of a TiO2|H2TOPP|Hg cell. In the dark, thin line (left-hand axis); with 440 nm irradiation, thick line (right-hand axis).

cells have the following characteristics: ISC ) 38 µA cm-2, VOC ) 0.59 V, FF ) 0.25 and the IPCE ) 1.7%. The poor fill factor of 0.25 contrasts with of that TiO2|ZnTCPP|Hg cells and indicates ohmic losses. Short circuit photocurrents of the TiO2|H2TOPP|Hg cells are up to 40 µA cm-2 depending on the H2TOPP film thickness and the treatment of the TiO2 substrates; an optimum is found when 4 mM porphyrin solutions are used. Following the same line of reasoning as above, the electron donating LUMO of H2TOPP is found at 3.9 ( 0.2 eV, and its HOMO at 5.8 ( 0.2 eV. The Fermi level of the TiO2|H2TOPP is located at 5.05 (0.2 eV, which is nearly identical to that of virgin TiO2. Accordingly, the Fermi level of H2TOPP lies about

halfway the HOMO and LUMO levels, which indicates that H2TOPP films are to be considered intrinsic, i.e., electron donors and acceptors, if present, compensate each other. The conduction band minimum of TiO2 at the surface, i.e., the electron affinity, is 3.9 ( 0.2 eV, yielding an energy difference with the H2TOPP LUMO of 0.0 (0.2 eV. Again the reader should notice the experimental error. Within the experimental accuracy it is save to conclude that there is a small but still sufficient driving force to allow for electron injection from the LUMO of H2TOPP into the conduction band of TiO2. When compared with ZnTCPP, the free-energy difference acting as the driving force is reduced by about 0.9 eV, which explains the much smaller photocurrent quantum efficiency of only 5%. TiO2-Sensitized by Combinations of H2TOPP and ZnTCPP. Since both H2TOPP and ZnTCPP are sensitizers of TiO2 electrodes, the combined action of these two of porphyrins has also been explored. The LUMO of both porphyrins lies above the conduction band of TiO2, but there is a 0.9 eV difference between the excess energies (ELUMO - EC) which implies a much larger driving force for electron injection from ZnTCPP into TiO2 compared to that of H2TOPP. The lower LUMO of H2TOPP with respect to that of ZnTCPP provides the possibility to obtain a sequential energy decrease from the LUMO of ZnTCPP via that of H2TOPP to the conduction band of TiO2. With this in mind a film of H2TOPP is sandwiched between ZnTCPP and TiO2. Similar to inorganic p-i-n devices an electric field is expected inside the H2TOPP film because of the differences in work function between p-type ZnTCPP and n-type TiO2. First, a TiO2|H2TOPP|ZnTCPP|Hg cell with a rather thin H2TOPP layer is investigated. Next, the thickness of the H2TOPP film in this cell is increased to ensure that, by virtue of the filter effect, Soret band irradiation (440 nm) is unable to excite ZnTCPP. Finally, the sequence of the porphyrin films is reversed using the same thick H2TOPP films to ascertain that pinholes in the H2TOPP film do not interfere with the experiments. A 3 mM ZnTCPP solution (3.15 × 1014 molecules cm-2) is spin coated on top of a 9.0 × 1014 molecules cm-2 H2TOPP film obtained from spin coating a 4 mM solution, which yields a TiO2|H2TOPP|ZnTCPP|Hg photoelectrode. When compared with the single junction TiO2|H2TOPP|Hg cell, the photo i-V curve, as being shown in Figure 7, changes significantly: the open circuit voltage VOC changes from 0.6 V to 0.65V, the short circuit increases from 38 to 85 µA cm-2, the fill factor increases from 0.25 to 0.51, and the IPCE at 440 nm increases from 1.5 to 3%. The absorption spectrum of this organic heterostructure is shown in Figure 5a (thick line) and reveals that, compared to the single H2TOPP film (thin line), the contribution of

Organic P-I-N Solar Cells

Figure 8. Photocurrent action spectra of TiO2 sensitized with both a thick H2TOPP film (2.8 × 1015 molecules cm-2) and a thin ZnTCPP (3.15 × 1014 molecules cm-2) film. Normal configuration (thick line), reversed configuration (thin line). Both spectra are normalized on the Qy(0,1) band at 531 nm.

ZnTCPP is manifested by a Soret band broadening and an enhanced Q band absorption at 555 and 595 nm. Broadening of the Soret band absorption in the TiO2|H2TOPP|ZnTCPP|Hg cell is reflected in the photocurrent action spectrum shown in Figure 5b (thick line) which now covers a wider range of the solar spectrum. The effect of the built-in electric field can be separated from the additional light absorbance in the heterostructure by using a thick H2TOPP film which absorbs all the light at 440 nm. In the “normal configuration” the H2TOPP is sandwiched between TiO2 and ZnTCPP and in the “reversed configuration” ZnTCPP is sandwiched between TiO2 and H2TOPP. In these experiments H2TOPP films are spin coated from a 12 mM chloroform solution and have 2.8 × 1015 molecules cm-2; ZnTCPP films are obtained from a 3 mM methanol solution (3.15 × 1014 molecules cm-2). The absorbance of thick H2TOPP films at 440 nm is 2.5; the transmission is only 0.3%. While a small exciton flux still reaches the ZnTCPP|H2TOPP we assume that this will only slightly distort the experiment. The photocurrent action spectrum of the normal configuration is presented in Figure 8 (thick line) and shows the filter effect; only H2TOPP contributes to the photocurrent. Excitons are generated exclusively in the H2TOPP film relatively near to the TiO2|H2TOPP interface; only exciton dissociation at that interface occurs. The thick H2TOPP film induces large ohmic losses leading to small photocurrents and a poor fill factors, i.e., VOC ) 0.83V, ISC )17 µA cm-2, FF ) 0.25, IPCE ) 0.4%. Nevertheless, the current-voltage scans show that, in the normal configuration also with thick H2TOPP films, an increased photocurrent is detected after applying ZnTCPP. In this case, ZnTCPP remains in the ground state and the H2TOPP|ZnTCPP interface is inactive with regard to exciton dissociation. ZnTCPP is only involved by its ability to form a p-n junction with TiO2. To further investigate whether our hypothesis that a built-in electric field is responsible for the observed increase of the photocurrent generation process, a cell with a reversed sequence of the porphyrins has been investigated as well. When the sequence of the dyes is changed by applying a H2TOPP film on top of a ZnTCPP film, electric field formation is not expected in the H2TOPP region. Furthermore, since the LUMO of H2TOPP lies below that of ZnTCPP, photoexcited electrons in the LUMO of H2TOPP are unable to enter the LUMO of ZnTCPP and are therefore deprived of their ability to jump into the conduction band of TiO2. Substantially lower photocurrents are anticipated because H2TOPP cannot contribute to the photovoltaic effect. Figure 8 also shows the photocurrent action spectrum in the reversed configuration (thin line). It should be

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Figure 9. Photocurrent-voltage characteristics of a TiO2|ZnTCPP|Hg cell (thick line, left- hand axis) and a TiO2|ZnTCPP|H2TOPP|Hg cell (thin line, right-hand axis).

TABLE 1: Energy Positions of TiO2, ZnTCPP Films on TiO2, and H2TOPP Films on TiO2 Fermi level energy/eV band bending/eV TiO2 conduction band/eV HOMO level/eV LUMO level/eV type of conduction

ZnTCPP/TiO2

H2TOPP/TiO2

5.6 ( 0.2 0.9 ( 0.1 4.4 ( 0.2 5.4 ( 0.2 3.4 ( 0.2 p-type

5.05 ( 0.2 0.9 ( 0.1 3.9 ( 0.2 5.8 ( 0.2 3.9 ( 0.2 intrinsic

noted that both spectra in Figure 8 are normalized to each other at the Qy(0,1) band position; the photocurrent response in the normal configuration is 2 orders of magnitude larger than that of the reversed configuration. In Figure 9 (thin line, right-hand axis) it is shown that the reversed TiO2|ZnTCPP|H2TOPP|Hg cell only generates 1 µA cm-2 photocurrent, which is 2 orders of magnitude lower than the performance of TiO2|ZnTCPP|Hg cells, Figure 9 (thick line, left-hand axis). In the TiO2|ZnTCPP|H2TOPP|Hg cell excitation of the ZnTCPP film does not lead to electron injection into the conduction band of TiO2, but to a competing electron injection into the LUMO of H2TOPP. Quenching of the excited state of ZnTCPP by H2TOPP is found to be very fast and competes effectively with electron injection from ZnTCPP into TiO2. The hole in the HOMO of ZnTCPP cannot be transferred into the valence band of TiO2 because this band is much lower in energy. Accordingly, a stationary photocurrent in the reverse direction (electrons flowing into the Hg contact) cannot occur. Only a small fraction of the excited ZnTCPP molecules is able to inject an electron into the conduction band of TiO2. Subsequently the hole flows into the H2TOPP film and a small photocurrent is observed. Conclusions The relevant energy levels of TiO2 and the two types of porphyrins are collected in Table 1. From this table it can be concluded that H2TOPP acts like an intrinsic semiconductor and that ZnTCPP acts as p-type semiconductor. ZnTCPP and H2TOPP have HOMO-LUMO gaps of 2.0 and 1.9 eV, respectively. The LUMO position of the two porphyrins relative to the conduction band minimum of TiO2 allow electron injection from the molecular excited state. Figure 10 shows the two band diagrams. By sandwiching the self-organized H2TOPP between TiO2 and p-type ZnTCPP the fill factor and the IPCE increase significantly with respect to a cell in which ZnTCPP is absent which is attributed to the influence of an electric field extending from ZnTCPP across H2TOPP. Even when applying a thick H2-

2708 J. Phys. Chem. B, Vol. 103, No. 14, 1999

Figure 10. Energy diagrams of TiO2|ZnTCPP and TiO2|H2TOPP heterostructures.

Figure 11. Energy diagram of a TiO2|ZnTCPP|H2TOPP heterostructure resembling an n-i-p semiconductor device.

TOPP film which absorbes virtually all the light such that ZnTCPP remains in the ground state, a positive effect of the applied ZnTCPP p-type film is detected. The schematic band diagram in this case is shown in Figure 11. When switching from the normal configuration to the reversed configuration, the originally strong sensitization activity of the ZnTCPP is almost fully quenched by the H2TOPP film. In this case the excited state of ZnTCPP is quenched by electron injection into the H2TOPP film. Acknowledgment. The authors acknowledge Dr. Ellen Moons for performing the Kelvin-probe experiments and Dr.Tom Maree for Rutherford back scattering analysis. Dr. Jan Kroon is kindly acknowledged for critically reading the manuscripts and his stimulating interest in this study. One of the authors (A.G.) wishes to thank The Netherlands Academy of Arts and Sciences (KNAW) for his fellowship. This study is financially supported by The Netherlands Agency for Energy and the Environment (NOVEM). References and Notes (1) Simon, J.; Andre´, J.-J.; Molecular Semiconductors; SpringerVerlag: Berlin, 1985. (2) Morel, D. L.; Ghosh, A. K.; Feng, T.; Stogryn, E. L.; Purwin, P. E.; Shaw, R. F.; Fishman, C. Appl. Phys. Lett. 1978, 32, 495.

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