Photovoltaic Effect in Self-Assembled Molecular Monolayers on Gold

Article pubs.acs.org/JPCC

Photovoltaic Effect in Self-Assembled Molecular Monolayers on Gold: Influence of Orbital Energy Level Alignment on Short-Circuit Current Generation Ratheesh K. Vijayaraghavan,† Fatemeh Gholamrezaie,† and Stefan C.J. Meskers*,†,⊥ †

Molecular Materials and Nanosystems and ⊥Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands S Supporting Information *

ABSTRACT: Photoinduced current generation at metal−organic monolayer interfaces is observed upon photoexcitation of a monolayer of hemicyanine molecules chemically adsorbed onto a gold electrode. A series of hemicyanines is investigated that bind to the gold via a thiol moiety, in an orientation such that the acceptor moiety of the hemicyanine is closer to the metal than its donor part. The quantum yield of short-circuit photocurrent generation in a diode using a liquid electrolyte as second contact, correlates with the strength of the donor moiety of the dyes. Modeling of the photocurrent generation using Marcus theory indicates that the net photocurrent results from asymmetry in the electron transfer rates of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with the electrodes. The quantum efficiency of short-circuit photocurrent generation decreases for the HOMO levels of the hemicyanine going deeper below the Fermi-level of the metal. The deeper HOMO level provides a larger driving force for back electron transfer from the metal to the photo-oxidized molecule and suppresses current generation in favor of quenching of the excited state.

■

INTRODUCTION

surface. Hemicyanine dyes feature a strongly coupled donor and acceptor unit and an intense optical absorption in the visible region of the spectrum that has considerable charge transfer character. In a classical description, the charge transfer transitions involve transfer of a electron from the donor to the acceptor moiety.24 Current rectification in SAMs of hemicyanine molecules (in the ground state) was reported by Ashwell et al.25−27 Current rectification28 is one of the requirements for net photocurrent generation in a diode under short circuit conditions. The organization of the hemicyanine SAM is such that the acceptor moiety of the hemicyanine is closer to the metal interface than the donor unit. The built-in potential within the molecules associated with the donor−acceptor character of the dye, in combination with asymmetry in the electrical contacts may be expected to drive photoinduced charge generation (see Scheme 1). We remark that the unidirectional orientation of the

The metal−organic interface is of prime importance in organic electronics.1−4 At the interface, charges are injected into (or extracted from) the molecular material via the electron transfer processes.5,6 Also at the metal−organic interface quenching of excited states in the molecular material occurs. The quenching induces losses in organic light emitting diodes7,8 and photovoltaic devices.9 Self-assembled monolayers (SAM)10,11 of π-conjugated molecules on a metal surface provide a model system to study photophysical and photoelectrochemical processes at the interface between a metal and a molecular material. Recently π-conjugated, self-assembled monolayers, have come under intense investigation,3 mainly because of prospects for molecular electronics.12 For instance, with the use of π-conjugated molecular monolayers, field-effect transistors13−15 and rectifiers16−20 have been realized. Photovoltaic effects in monolayers on metal surfaces involving molecules with weakly coupled donor−acceptor moieties have been reported.21−23 Here we investigate the photocurrent generation in SAMs of hemicyanine molecules with donor−acceptor structure on a gold © 2013 American Chemical Society

Received: May 30, 2013 Revised: July 20, 2013 Published: July 22, 2013 16820

dx.doi.org/10.1021/jp4053242 | J. Phys. Chem. C 2013, 117, 16820−16829

The Journal of Physical Chemistry C

Article

Scheme 1. (a) Schematic Representation of Experimental Procedure; (b) Mechanism of Current Generation; and (c) Quenching of Excitations

Scheme 2. Chemical Structures of the Hemicyanine Molecules under Investigation

excitation depends on the distribution of the molecular highest occupied molecular orbital (HOMO) and lowesst unoccupied molecular orbital (LUMO) levels in space and in energy.

■

EXPERIMENTAL SECTION Chemical Synthesis. The molecules under investigation were synthesized via a base mediated condensation reaction of a suitably substituted benzaldehyde derivative with picolinium or quinolinium salt as described earlier.25a,b All compounds were purified by column chromatography on silica using chloroform: methanol (2%−20%) as the eluent and further characterized by 1 H, 13C NMR, and mass spectral analysis. Detailed synthetic procedures and characterization data are provided in the Supporting Information. Junction Fabrication. Electrochemical cells for photocurrent measurements, whose architecture is depicted schematically in Scheme 1a, were constructed using a cathode consisting of a 100 nm thick layer of gold on glass deposited via vacuum sublimation. To improve adhesion of the gold, a thin layer of Cr (5 nm) was deposited on to the glass before deposition of Au. A transparent indium tin oxide (ITO) electrode was used in combination with an electrolyte solution containing 30 mM hydroquinone and 1 M KCl in water. The quinone acts as secondary redox mediator, facilitating the transfer of positive charges (holes) to the transparent top electrode. The separation between the two electrodes was adjusted to 6 μm, using silicon dioxide spacers. The photocurrent and photovoltage were measured using a lock-in amplifier. All measurements were carried out in nitrogen atmosphere.

dyes in the SAM makes the monolayer polar in the crystallographic sense. Polar materials have a built-in electrical field that can result in generation of electrical currents upon excitation by heat (pyroelectricity29−31) or, in some cases, by visible light (bulk photovoltaic effect32−34). There is experimental evidence that a strong electric field in the dielectric layer near a metal−dielectric interface can drive separation of charges and result in photocurrent generation.35−37 Thus, in SAMs of polar molecules on metal surface, the built-in electrical potential in the polar molecules provides a tool to promote directed, excited state electron transfer and current generation (see Scheme 1b). The polar organization helps to reduce back electron transfer from the metal to the photo-oxidized molecule that leads, effectively, to quenching of the excited state (see Scheme 1c). We investigate the quantum yield of photocurrent generation in the hemicyanine SAM under short circuit conditions by systematically changing the electronic structure of the hemicyanine chromophore in the self-assembled monolayer (see Scheme 2). The general structure of the hemicyanine molecules used is donor-π-acceptor-spacer-thiol with the thiol functionality to promote chemisorption on gold surface. To obtain a set of molecules with distinct excited state and electrochemical properties, the strength of the donor group in the hemicyanine is varied systematically from N,N′-dimethylamino to fluoro. The hemicyanine SAMs on gold are made into a diode using as second electrical contact, a liquid electrolyte involving the hydroquinone(HQ)/quinone(Q) couple as redox mediator, and a transparent conducting oxide as electrode (Scheme 1). The largest photocurrent generation efficiencies are observed for the molecules with the strongest donor group. We find that the balance between photocurrent generation and quenching of the

■

RESULTS AND DISCUSSION Optical Properties of Hemicyanine Dyes. Figure 1 shows the optical absorption spectra of the series of hemicyanine molecules depicted in Scheme 2 in dilute chloroform solution. The position of the lowest electronic absorption band is sensitive to the substitution on the donor moiety and shifts to the lower photon energy region with increasing donor character of the pphenyl substituent.24 The Amino-Quin molecule features the most red-shifted absorption (2.2 eV), while F-Pyr has the most blue-shifted π−π* absorption band in the UV spectral range, 16821



Article

pyridinium series, the emission spectrum shifts bathochromically with increasing solvent polarity (see Table 1), consistent with dipolar relaxation of polar solvent molecules around the excited state. The magnitude of the solvatochromic shifts in the pyridinium series are comparable, indicating that the changes in dipole moment between ground and excited state are similar in these molecules. The quinoline based molecules, Amino-Quin and OH-Quin, show a small hypsochromic shift of the fluorescence maximum. The excited state lifetime of these molecules in solution is in the subnanosecond time range. Such relatively short lifetimes are characteristic for this class of dye molecules. For most of the molecules in this series, the lifetimes in chloroform and methanol are similar (see Table 1). For Amino-Quin, an increase of the excited state lifetime to 0.3 ns in MeOH compared to 0.2 ns in chloroform, indicates additional dielectric relaxation beyond the nuclear relaxation associated with the Franck−Condon factors. The fluorescence quantum yield is largest for Amino-Quin (0.21) and Amino-Pyr (0.20) and smallest for F-Pyr (0.03) indicating rapid nonradiative decay for the latter compound. HOMO and LUMO. To investigate the electronic structure of the hemicyanine molecules, we show in Figure 3 the HOMO and LUMO orbitals of Amino-Pyr and F-Pyr, calculated using the B3LYP/6-31G(p,d) functional and adopting a molecular geometry for the cations from single crystal X-ray diffraction experiments reported before.39 The alkanethiol moiety is left out, since it would hardly influence electronic structure of the chromophore. As expected, for Amino-Pyr, the HOMO is mainly localized on the N,N′-dimethylamino group, while the LUMO level is localized near the pyridinium unit. For the case of F-Pyr, similar orbitals are obtained. Transfer of an electron from HOMO to LUMO upon photoexcitation, will enhance the πelectron density at the heterocyclic acceptor moiety near the gold substrate. Cyclic voltammetry on the hemicyanines in dichloromethane was performed to determine oxidation and reduction potentials, and further, to estimate the energies of HOMO and LUMO. Reversible oxidation and reduction processes could be observed for all molecules and the corresponding redox potentials and calculated energy levels are summarized in Table 1. The energy of the HOMO level varies widely and correlates with the strength of the electron donating p-subsituted phenyl moiety. In contrast, the energy of the LUMO is found to be almost constant throughout the hemicyanine series under investigation, at about 3.8 eV. This is in agreement with the fact the molecules contain acceptor parts of very similar strength (quinolinium or pyridinium). The LUMO level of ∼3.8 eV is significantly higher than the Fermi level of gold (5.1 eV) indicating that for all dyes photoinduced electron transfer from the LUMO to the gold electrode is energetically favorable. The reduction potential for surface-bound molecules could be determined directly from cyclic voltammetry using monolayer modified gold as the working electrode40 in 0.1 M solution of TBAPF6 in acetonitrile as the electrolyte. The reduction potentials of all hemicyanine molecules shift to more negative values when assembled in the monolayer as compared to the molecularly dissolved solution (see Table 1). This corresponding upward shift in the LUMO levels, amounting to about 0.2 eV, is predominantly due to packing effects of the molecules in the monolayer.41 The binding of alkanethiols to the gold surface is known to lower the workfunction of gold.42 The latter effect would cause a shift of the reduction potentials to less negative values, in contrast to the experimental observations. The

Figure 1. Normalized absorption spectra of the hemicyanine molecules in chloroform.

centered at 3.4 eV (see Figure 1). The main absorption band of the hemicyanines in the visible region corresponds to the transition from the groundstate to the lowest excited singlet state and has pronounced charge transfer (CT) character. The CT nature of the excited state is further evidenced by negative solvatochromism, that is, a shift of spectral maximum of absorption to higher photon energies with increasing solvent polarity (Figure 2). The negative solvatochromism indicates that

Figure 2. Normalized Absorption and Photoluminescence spectra in chloroform (red) and methanol (blue) of Amino-Pyr (top) and F-Pyr (bottom).

the dipole moment of the molecule in the ground state is larger than that in the Franck−Condon excited state,38 consistent with transfer of electron density from the p-phenyl moiety to the quinolinium/pyridinium group upon photoexcitation. All hemicyanine molecules show fluorescence with moderate quantum yield (see Table 1). For Amino-Pyr, the photon energy corresponding to maximum photoluminescence (PL) intensity EPL max is centered at 1.91 eV in methanol, while in chloroform it is at around 1.97 eV. Figure 2 also displays the PL spectra of the pyridium based dyes with smallest (Amino-Pyr) and largest (F-Pyr) S0−S1 gap in chloroform and in methanol. Within the 16822



Article

Table 1. Summary of Spectroscopic and Electrochemical Properties of Hemicyanine Molecules solution compd AminoQuin OH-Quin Amino-Pyr OMe-Pyr OH-Pyr Br-Pyr F-Pyr

a EAbs max (eV)

10−4 × εb (M−1cm−1)

c EPL max (eV)

ΔEAbsd (eV)

2.19

6.3

1.82

2.86 2.45 3.07 3.13 3.37 3.43

3.1 5.8 4.1 2.8 2.4 2.1

2.30 2.05 3.07 2.48 2.96 2.98

SAM

ΔEPLe (eV)

ΦPLf (−)

τg (ns)

Eredh (V)

Eoxh (V)

HOMO (eV)

LUMO (eV)

Eredi (V)

LUMOi (eV)

0.052

0.027

0.21

0.2

−1.03

0.82

−5.62

−3.77

−1.33

−3.47

0.059 0.064 0.122 0.039 0.104 0.087

0.013 −0.092 −0.078 −0.092 −0.089 −0.097

0.08 0.20 0.19 0.04 0.08 0.03

0.2 0.3 0.2 0.2 0.2 0.2

−1.00 −1.01 −1.00 −1.01 −0.98 −0.95

0.96 0.76 1.03 0.96 1.21 1.36

−5.76 −5.56 −5.83 −5.76 −6.01 −6.16

−3.80 −3.79 −3.80 −3.70 −3.82 −3.85

−1.17 −1.32 −1.14 −1.25 −1.16 −1.10

−3.63 −3.48 −3.66 −3.55 −3.64 −3.70

EmaxAbs photon energy of maximal absorbance in CHCl3. bMolar decadic extinction coefficient. cPhoton energy of maximal PL intensity. dΔEAbs =EmaxAbsmethanol −EmaxAbschloroform. eCorresponding quantity of photoluminescence. fLuminescence quantum yield in chloroform, estimated σ: ± 0.02 ΦPL. gExcited state lifetime of the hemicyanine. hVersus Fc/Fc+ in dichloromethane σ: ± 0.02 V. iVersus Fc/Fc+ on SAM in acetonitrile σ: ± 0.03 V. a

Figure 3. Calculated HOMO and LUMO molecular orbitals (B3LYP/6-31G(p,d)) for Amino-Pyr (left) and F-Pyr (right).

coverage as a function of time and I0 and I(t) are the redox currents of the bare gold electrode and the SAM-modified gold electrode at time t, respectively, resulting from an exchange of electrons with an aqueous KCl/K3[Fe(CN)6] solution (0.5 M/5 mM). By analyzing the surface coverage data at different growth times of the monolayer, the rate constant of the entire adsorption process on the gold surface could be acquired. Figure 4A shows the redox currents of the gold working electrode with the ferricyanide, after immersing the gold substrate in a solution of deprotected Amino-Quin for 0, 10, 40, 120, 360, 1080, and 2880 min. The current was found to decrease gradually with increasing immersion time in the monomer solution. For sufficiently long immersion times, practically no redox currents were observed anymore in the voltage range corresponding to the half potential of the cyanide complex. This indicates formation of pinhole free monolayers on the gold substrate. The kinetics of growth of the Amino-Quin monolayer on gold electrode is represented in Figure 4B and follows the Lagergren first-order equation,43 Γ(t) = 1 − exp(−kt). The rate constant of the adsorption, k = 3.9 × 10−4 s−1, was obtained for solutions with 1.0 mM concentration of hemicyanine in methanol (Figure 4B). The density of hemicyanine molecules in the SAM on the gold electrode, could be calculated from the first reduction peak of the hemicyanine in the cyclic voltammogram, using the surface modified gold electrode as the working electrode. Assuming single electron reduction of the molecule, the integrated current gives the number of molecules chemically adsorbed onto the surface. An average surface density of 8.3 × 10−10 mol/cm2 was obtained, indicating that the molecules are tightly packed on the gold surface with an average surface area of 20 Å2/molecule.

electrochemical measurements on the monolayers on the gold working electrode also allow us to determine the surface coverage of the metal by the hemicyanine molecules (see below). Preparation and the Kinetics of Monolayer Formation. Dense monolayers are grown from solution containing hemicyanine molecules in 1.0 mM concentration after in situ deprotection of the thioester groups.25 The optimized hydrolysis conditions used in monolayer formation were 1 equiv of 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) as base with 1 equiv of tri-n-butyl-phosphine in methanol or THF solutions of hemicyanine (PF6−). The mixture was stirred for 1 h to yield 75% of free thiols, which was sufficient to obtain a dense monolayer on gold substrates. The relative concentration of free thiols, unreacted thioacetates, and/or disulfides can be determined from the distinct nuclear magnetic resonance signals of the methylene hydrogen atoms adjacent to the sulfur atom (see Supporting Information for detailed explanation). Incidentally, we found that for hemicyanines with I− as counterion, the stability of the molecules under the basic conditions was poor. In the presence of mineral bases like KOH or organic base like triethylamine, methanol/THF solutions of hemicyanines turned colorless irreversibly, within 3 h from starting the deprotection of the hemicyanine molecules (Supporting Information for detailed analytical data). Upon counterion exchange with PF6−, and using a bulky base, the stability of the molecules under basic conditions was found to improve, and the hydrolyzed product was formed in significant yield. The monolayer formation kinetics and surface coverage of monolayers were investigated by cyclic voltammetry. The surface coverage (Γ) of the SAM-modified gold electrode can be determined from Γ(t) = 1 − I(t)/I0, where Γ(t) is the surface 16823



Article

Figure 5. Absorption spectra of Amino-Pyr in chloroform (−∇−), monolayer on 5 nm Au (−○−) and monolayer on SiO2 (−☆−, scaled down by × 2/3).

illuminating the monolayer through the transparent top contact with mechanically modulated light from a laser source and using lock-in detection under short circuit conditions. The direction of the photocurrent from the monolayer could be unambiguously assigned by comparing it with the current flowing from a standard P3HT/[60]PCBM solar cell under the same illumination conditions. Photoexcitation of the monolayer induces net transfer of electrons from the dye molecules into the underlying gold electrode. Scheme 1 explains the proposed mechanism of the HQ/Q redox couple-mediated photocurrent/ photovoltage generation in SAM solar cells and the corresponding energy level diagram of the entire photocurrent generation process in the whole device. Control experiments performed include diodes (i) without liquid electrolyte (to exclude the possibility of a thermovoltage), (ii) absence of liquid redox mediator (to ensure the proper role of redox mediator), and (iii) devices with redox electrolyte but absence of SAMs. In all these control experiments the anodic photocurrent was found to be negligible. The quantum yield of photocurrent generation was calculated from the following equation:

Figure 4. (A) Cyclic voltammogram of KCl/K3[Fe(CN)6] using bare/ SAM modified gold electrodes as working electrode at various time intervals of monolayer growth of Amino-Quin and (B) corresponding kinetic plot for the layer formation process.

X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical composition of SAMs on gold and to estimate their thickness. The chemical state and the atomic concentrations of the elements present were determined. The relative concentrations of the elements per unit area are presented in the Supporting Information, Table S1. The intensities of the peaks in XPS are consistent with the formation of a closely packed layer on the surface of the metal. The optical absorption spectrum of the hemicyanine SAMs could be obtained using substrates with a thin, semitransparent gold layer (5 nm Au with 3 nm Cr adhesion layer on glass). For Amino-Pyr, the absorption spectrum of the monolayer is illustrated in Figure 5. In comparison with the absorption of the same dye in chloroform solution, we notice a redshift of the onset of absorption, combined with a blue shift of the maximum of absorption and a broadening of the band. These features are indicative of the formation of H-aggregates of dye molecules and are compatible with closely packed, two-dimensional arrays of dye molecules with their transition dipole moment perpendicular to the gold surface.44 For the hemicyanine chromophore the transition dipole moment is oriented along the long axis of the molecule. To verify that the absorption features of the monolayers on gold are indeed due to aggregation and do not result from interaction with the gold, corresponding hemicyanine dyes with trimethoxysilane functionality instead of a thiol were synthesized (see Supporting Information). The trimethoxysilane derivatives form monolayers on quartz with optical characteristics closely resembling the Haggregate type properties of the monolayers on gold (see Figure 5). Photocurrent and Photovoltage. Diodes could be fabricated from the hemicyanine SAMs on gold by complementing them with a liquid electrolyte top contact (0.1 M KCl, 30 mM hydroquinone and a transparant ITO covered glass plate). A stable and spontaneous anodic photocurrent can be measured by

Φ = Jsc /(qeI0(1 − 10−A))

(1)

where Jsc is the short circuit current density, qe is the elementary charge, A the absorbance, I0 is the incident photon flux calculated as W/Ephoton with W being the power output of the laser. The factor (1−10−A) represents the fraction of the incoming beam that is absorbed by the monolayer with dimensionless absorbance A. Jsc/qe represents the flux of electrons that enter the external circuit after coming out of the diode; I0 (1−10−A)) gives the flux of photons that will be absorbed. The ratio of these fluxes, each expressed in units of reciprocal time and area, gives a dimensionless number, Φ. Φ represents the efficiency of conversion of absorbed photons into electrons in the external circuit, provided that the whole active area of the diode is illuminated with constant flux, or, alternatively, that the current density is calculated taking into account the illuminated surface area. Table 2 summarizes the comparative photovoltaic performance of the devices prepared from all the hemicyanine molecular monolayers with Q/HQ redox electrolyte. For the determination of the quantum yield for photocurrent generation, the power of the incident photon beam was kept at 33 mW. The maximum efficiency was obtained for the Amino-Quin and Amino-Pyr. 16824



Article

consistent with all relevant elementary charge generation and charge transfer steps in the photovoltaic action taking place at rates >103 s−1. The other compounds gave a similar frequency dependence of photocurrent and photovoltage. For the hemicyanine monolayers on gold, both the magnitude of the photocurrent and of the photovoltage increase upon an increase in the intensity of the modulated laser beam. For Amino-Quin the dependence of photocurrent and photovoltage on intensity is illustrated in Figure 6B. The photocurrent varies from 16 nA at 20 mW laser power and approaches a saturation value of ∼90 nA for powers exceeding 90 mW. The corresponding change in photovoltage is from 36 μV to 85 μV. The saturation in the photocurrent shows that at high illumination intensities additional loss mechanisms become active. The photocurrent and photovoltage action spectra were investigated on monolayer devices. As an example, Figure 7

Table 2. Photovoltaic Performance of Diodes with General Structure Au/SAM/HQ/ITO under 33 mW Illumination with Mechanically Modulated Intensity at 133 Hz. Surface Coverage Γ of SAM on Gold, Short Circuit Current Density JSC, Open Circuit Voltage VOC, and Internal Quantum efficiency Φ compound AminoQuin OH-Quin AminoPyr OMe-Pyr OH-Pyr Br-Pyr F-Pyr a

1010 × Γ (mol/cm2)a

photon energy (eV)

8.2

1.96

8.1 9.4 9.4 8.2 9.1 9.3

JSC (μA/cm2)b

VOC (μV)

Φ (%)c

8.4

32

0.12

2.54 2.28

2.9 6.7

11 28

0.07 0.08

3.10 3.10 3.31 3.31

1.1 2.6 0.04 0.008

6 14 0.3 0.04

0.05 0.06 0.004 0.0007

Estimated σ: 0.05 × Γ. bσ: 0.05 × JSC. cσ: 0.2 × Φ.

Apart from a spontaneous photocurrent, the hemicyanine SAMS can also generate an open circuit photovoltage (VOC, see Table 2), that can be detected using modulated light and lock-in detection with high input impedance. As an example, Figure 6A

Figure 7. Photocurrent and photovoltage action spectrum for Au/ Amino-Quin/HQ/ITO diodes (illumination power, 30 mW; 133 Hz modulation). The solid line indicates the absorption spectra of AminoQuin SAM on 5 nm gold.

describes the wavelength-dependent photocurrent and photovoltage that generated in the case of Amino-Quin monolayer devices. The maximum photocurrent and photovoltage modulation are obtained for 1.96 eV photon energy, which is close to the absorption maximum of the SAM. Surprisingly, the measurements show an isolated secondary maximum near the local maximum of ca. 2.5 eV photon. We have currently no explanation for this band but we expect that it may be related to the excitation of surface plasmons on the gold film. Kinetic Model. In Figure 8B, we summarize graphically the quantum efficiencies for short-circuit photocurrent generation in the SAM of the seven hemicyanine dyes. For comparison, Figure 8A shows the energies of HOMO and LUMO as determined from electrochemistry in solution. In a comparison of the two data sets, a correlation between the energy of the HOMO and the quantum efficiency becomes apparent. For instance, the molecule with the lowest HOMO energy (F-Pyr) gives the lowest quantum yield for photocurrent generation. To explain the origin of the photovoltaic effect in the monolayer and the correlation of the quantum efficiency for photocurrent generation, we propose the energy level diagram in Scheme 3. Here we model optical excitation of the hemicyanine as an excitation of an electron from HOMO to LUMO. Optical excitation occurs with a rate constant I0 that is proportional to the illumination intensity. The excited state of the hemicyanine may decay by back electron transfer from LUMO to HOMO, with a

Figure 6. (A) Frequency dependency of photocurrent and photovoltage generated Au/Amino-Quin/HQ/ITO devices, 2.42 eV, 30 mW excitation; (B) variation in photocurrent and photovoltage with the incident power of photon flux for the same devices: 2.42 eV, 133 Hz. Lines joining the data points serve to guide the eye.

describes the variation in photocurrent and photovoltage of the Amino-Quin monolayer as a function of the modulation frequency of the illumination. In the 30 Hz to 3.3 kHz frequency range, only relatively small variations in JSC and VOC are found, 16825



Article

current and the quantum efficiency of current generation can be derived easily. Because the LUMO is more localized near the acceptor moiety of the hemicyanine and the HOMO is more localized near the donor functionality (see Figure 3), the matrix element VAAu describing the electronic coupling between the LUMO and gold electrode can be expected to be larger than VDAu describing coupling between the HOMO and gold electrode. In the modeling we adopt a value for VAAu = 3 cm−1, a value determined experimentally for electron transfer in a SAM between gold and ferrocene across an alkyl spacer comprising 11 single bonds.18 At the other contact we assume VAQ/VDQ = VDAu/ VAAu and include a common scaling factor kQ for the rates for electron transfer involving the quinone electrolytic contact. The hemicyanine dye is modeled as a donor−acceptor junction with an acceptor (LUMO) level with electron energy EA and a donor (HOMO) level with energy ED .The electron transfer rate between the Au electrode and acceptor unit A is taken as the usual Marcus rate for electron transfer between a metal electrode and a molecule:45 kAuA = AAuA

⎡ (λ + E − ε − ε)2 ⎤ dε A Au ⎥ × f (ε) 4λkBT kBT ⎦

∫ exp⎢⎣−

(2) Figure 8. (A) HOMO−LUMO energy level diagram of the hemicyanine molecules. (B) Experimentally observed and simulated quantum efficiency for short-circuit photocurrent generation in Au/ SAM/HQ/ITO junctions (see also Table 3).

where εAu denotes the Fermi energy of the gold electrode (in eV), EA is the energy of the LUMO orbital on the (acceptor part of the) hemicayanine, λ is the reorganization energy (0.85 eV), f(ε) is the Fermi-Dirac distribution (dimensionless, εF = 0) and ε an integration variable (in eV). A (in s−1) denotes a rate, given by

Scheme 3. Schematic Orbital Energy Diagram for the Au/ SAM/HQ Junction with Rate Constants for Electron Transfer, Hole Transfer, and Photoexcitationa

AAuA

⎛ π ⎞1/2 ⎟⎜ = ρ |VAuA| ⎜ ⎟ ⎝ ℏ ⎠⎝ λkBT ⎠ 2 ⎛ kBT ⎞

(3)

where VAuA is a quantum mechanical matrix element (in eV), and ρ is a density of states (0.3 eV−145) kAuA = AAuA

⎡ (λ − E + ε + ε)2 ⎤ dε A Au ⎥ × f (− ε) 4λkBT kBT ⎦

∫ exp⎢⎣−

(4)

a

EAu refers to the Fermi level of the gold electrode; EQ refers to the formal potential of the quinone/hydroquinone electrolytic contact.

+

The rate constant for transfer of a hole (h ) from the electrode to the HOMO orbital on the (donor part of the) hemicyanine can be expressed as

rate kAD that is taken to be proportional to the inverse excited state lifetime of the hemicyanine (τ−1 = (0.2 ns)−1). Alternatively, in the diode the excited state may also decay via transfer of an electron from the LUMO to either the gold electrode (kAAu) or to the hydroquinone electrolytic contact (kAQ). Lastly, the excited state may decay via hole transfer from the HOMO to either gold (kDAu) or electrolytic contact (kDQ). Also the reverse electrochemical processes can be included. For completeness, spontaneous thermal excitation of the dye molecules will also be included. The rate for this process (kDA) is almost negligible, yet its inclusion ensures full microscopic reversibility and detailed balance. We assume Marcus rates for the electron (hole) transfer with the electrical contacts and assume oxidation and reduction to be independent processes. The elementary electron transfer steps in Scheme 3 give rise to four different electronic states of the hemicyanine in the junction: ground, excited, oxidized, and reduced (labeled respectively G, A, B, and C). The relative population of these four states at shortcircuit conditions (EAu = EQ) can be computed as function of the excitation rate I0. Knowing the population of the four different states, the magnitude of the steady-state, short-circuit photo-

kAuD(DAu) = AAuD

⎡ (λ ∓ E ± ε ± ε)2 ⎤ dε D Au ⎥ × f (∓ ε) 4λkBT kBT ⎦ (5)

∫ exp⎢⎣−

The rate AAuD is related to the matrix element VAuD analogous to eq 3 above. Because the HOMO and LUMO orbitals on the hemicyanine are located in different regions of space we assume that the matrix elements VAuD and VAuA differ in magnitude. In our simulations VAuA is a fixed parameter (3 cm−1, see above), VAuD is adjustable. The redox reaction of the hemicyanine with the quinine/ hydroquinone liquid electrolyte are modeled using Marcus rates: ⎡ (λ ± E ∓ ε )2 ⎤ A Q ⎥ k QA(AQ) = AAuDkQ exp⎢ − 4λkBT ⎢⎣ ⎥⎦ ⎡ (λ ∓ E ± ε )2 ⎤ D Q ⎥ k QD(DQ) = AAuA kQ exp⎢ − 4λkBT ⎢⎣ ⎥⎦ 16826

(6)



Article

where εQ equals −qeVQ with VQ the formal potential of the quinone/hydroquinone liquid electrolyte. On the basis of the assumption that the LUMO orbital couples more strongly to the gold electrode than the HOMO orbital (|VAuA| > |VAuD|), one would expect an opposite relation for the magnitude of the coupling constant for the HOMO and LUMO with the liquid electrolyte contact (|VQD| > |VQA|). Here we assume that |VQD|/| VQA| = |VAuA|/|VAuD| and furthermore that AQD = kQAAuA and AQA = kQAAuD (in s−1) . Here kQ is a dimensionless number, accounting for differences in the distance between redox centers and the contact as well as differences in the density of states. kQ is an adjustible parameter is our simulations. For simplicity we assume the same value for the reorganization energy, λ = 0.5 eV, for all possible electron transfer processes. The rate for internal electron transfer between reduced acceptor and oxidized donor, kAD = 5 × 109 s, is taken equal to the inverse excited state lifetime of the hemicyanine dyes. To obey detailed balance, the rate for spontaneous excitation of an electron from HOMO to LUMO is set to ⎡ (E − E D) ⎤ kDA = kAD exp⎢ − A ⎥ kBT ⎦ ⎣

expressed in terms of elementary electron transfer processes as indicated in Scheme 3 in the following way: kCG = kAAu + kAQ kGC = kAuA + kQA kAC = kDQ + kDAu kCA = k QD + kAuD kAB = kAAu + kAQ = kCG kBA = kAuA + kQA = kGC kGB = kAuD + kQD = kCA kBG = kDAu + kDQ = kAC kAG = kAD = τ −1 kGA = kDA + I0

The chemical equilibria between the four states of the acceptor− donor junction can now be expressed in matrix form:

(7)

The rate for internal electron transfer from donor to acceptor under illumination equals kDA + I0 with I0 denoting the rate of optical excitation. From the density of molecules in the monolayer and the absorbance we calculate a rate for optical excitation, I0, of 17 s−1 for each molecule under illumination with a typical illumination intensity of 4 × 103 W/m2 of light at a wavelength of 513 nm. To calculate net electrical (photo)currents, we introduce in Scheme 4 various rate constants for interconversion between the four different states of an acceptor−donor molecular junction in electrical contact with the electrodes (G, C, A, and B). The rates describing interconversion between these four states can be

The system of coupled equations is subject to the condition that the sum of the populations of all four states G, A, B, and C of the junction must add up to the total molecular site density, G0, that is eq 10. Because of the eq 10 condition, the matrix eq 9 can be simplified to eq 11. G0 = G + A + B + C

⎡ − (k + k + k ) k − k ⎤−1⎡ k ⎤ −kGC ⎡ C′ ⎤ CG CA GC AC GC ⎢ ⎥ ⎢ GC ⎥ ⎢ ⎥ ⎥ ⎢ kGA ⎥ −(kAC + kAG + kAB + kGA) kBA − kGA ⎢ A′⎥ ≅ −⎢⎢ kCA − kGA ⎥ ⎢ ⎥ ⎢⎣ B′ ⎥⎦ ⎢⎣−kGB −(kBG + kBA + kGB)⎥⎦ ⎣ kGB ⎦ kAB − kGB

where C′ = C/G0; A′ = A/G0; and B′ = B/G0. The relative population of the ground state follows from G′ = G /G0 = 1 − C′ − A′ − B′ k C′ + kAGA′ + kBGB′ = CG I0

(8)

(10)

(11)

In the modeling, the quantum efficiency was calculated at an excitation rate I0 = 17 s−1 and was found to be independent of the excitation rate for excitation rates that are small compared to the excited state lifetime of the hemicyanine molecules I0 ≪ kAD ≈ 108 s−1. The reorganization energy λ in the Marcus rates are the same for all electron transfer processes (0.5 eV), and the energies of HOMO and LUMO levels are determined from the electrochemistry in solution. Parameters used in the modeling are summarized in the Table 3. In the modeling parameters VAuA, ρ, and kAD, were kept constant. Parameters VAuD and kQ were

(12)

The electrical current density (in units of (qe G0 s−1)) as measured under state conditions near the Au electrode can now be expressed as the difference of a hole and an electron current density:

Scheme 4. Schematic State Diagram for an Acceptor−Donor Molecule

Je = kAAu(C′ + A′) − kAuA(G′ + B′) Jh = kDAu(B′ + A′) − kAuD(G′ + C′) J = Jh − Je = (kDAu − kAAu)A′ − (kAuD − kAuA)G′ + (kDAu + kAuA)B′ − (kAuD + kAuA)C′

(13)

Finally, by dividing the short circuit current density J by the excitation rate we obtain the quantum efficiency Φ = J/I0 16827



Article

Notes

Table 3. Parameters Used in the Kinetic Simulation of ShortCircuit Photocurrents parameter

value

fixed/adjustible

VAuA VAuD kQ ρ kAD λ

3 cm−1 2.5 cm−1 0.042 0.3 eV−1 5 × 109 s−1 0.5 eV

f a a f f f

The authors declare no competing financial interest.

■

REFERENCES

(1) Braun, S.; Salaneck, W. R.; Fahlman, M. Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 2009, 21, 1450−1472. (2) Hwang, J.; Wan, A.; Kahn, A. Energetics of Metal−Organic Interfaces: New Experiments and Assessment of the Field. Mater. Sci. Eng. Rep. 2009, 64, 1−31. (3) Scott, J. C. Metal−Organic Interface and Charge Injection in Organic Electronic Devices. J. Vac. Sci. Technol. A 2003, 21, 521−531. (4) Arkhipov, V. I.; Emelianova, E. V.; Tak, Y. H.; Bässler, H. Charge Injection into Light-Emitting Diodes: Theory and Experiment. J. Appl. Phys. 1998, 84, 848−857. (5) Lindstrom, C. D.; Zhu, X.-Y. Photoinduced Electron Transfer at Molecule−Metal Interfaces. Chem. Rev. 2006, 106, 4281−4300. (6) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; et al. Charge Transfer on the Nanoscale: Current Status. J. Phys. Chem. B 2003, 107, 6668−6697. (7) Vogel, M.; Doka, S.; Breyer, Ch.; M. Lux-Steiner, Ch.; Fostiropoulos, K. On the Function of a Bathocuproine Buffer Layer in Organic Photovoltaic Cells. Appl. Phys. Lett. 2006, 89, 163501/1−3. (8) Kuhnke, K.; Becker, R.; Epple, M.; Kern, K. C-60 Exciton Quenching Near Metal Surfaces. Phys. Rev. Lett. 1997, 79, 3246−3249. (9) Burin, A. L.; Ratner, M. A. Exciton Migration and Cathode Quenching in Organic Light Emitting Diodes. J. Phys. Chem. A 2000, 104, 4704−4710. (10) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1169. (11) Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533−1554. (12) Halik, M.; Hirsch, A. The Potential of Molecular Self-Assembled Monolayers in Organic Electronic Devices. Adv. Mater. 2011, 23, 2689− 2695. (13) Smits, E. C. P.; Mathijssen, S. G. J.; van Hal, P. A.; Setayesh, S.; Geuns, T. C. T.; Mutsaers, K. A. H. A.; Cantatore, E.; Wondergem, H. J.; Werzer, O.; Resel, R.; et al. Bottom-up Organic Integrated Circuits. Nature 2008, 455, 956−959. (14) Hutchins, D. O.; Acton, O.; Weidner, T.; Cernetic, N.; Baio, J. E.; Ting, G.; Castner, D. G.; Ma, H.; Jen, A. K.-Y. Spin Cast Self-Assembled Monolayer Field Effect Transistors. Org. Electron. 2012, 13, 464−468. (15) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. SelfAssembly: Molecular Self-Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin-Film Transistor Applications. Adv. Mater. 2009, 21, 1407−1433. (16) Honciuc, A.; Metzger, R. M.; Gong, A.; Spangler, C. W. Elastic and Inelastic Electron Tunneling Spectroscopy of a New Rectifying Monolayer. J. Am. Chem. Soc. 2007, 129, 8310−8319. (17) McCreery, R. L. Molecular Electronic Junctions. Chem. Mater. 2004, 16, 4477−4496. (18) Jaiswal, A.; Amaresh, R. R.; Lakshmikantham, M. V.; Honciuc, A.; Cava, M. P.; Metzger, R. M. Electrical Rectification in a Monolayer of Zwitterions Assembled by Either Physisorption or Chemisorption. Langmuir 2003, 19, 9043−9050. (19) Salomon, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes, V. B.; Frisbie, C. D. Comparison of Electronic Transport Measurements on Organic Molecules. Adv. Mater. 2003, 15, 1881−1890.

■

CONCLUSION Self-assembled monolayers of hemicyanine molecules on a gold surface show a photovoltaic effect in a Au/SAM/liquid junction. The photocurrent generation efficiencies correlate with the electron donating strength of the p-phenyl substituent of the hemicyanine, namely, the HOMO energy of the hemicyanines. The quantum efficiency is maximal when the driving force for the unwanted back electron transfer from gold to HOMO orbital is minimized with respect to the driving forces for the desired forward electron transfer. Maximization of the photovoltaic energy conversion efficiency, necessarily requires minimization of the driving force for the electron transfer at the electron injecting contact. Results presented indicate that lowering the driving force for forward electron transfer while avoiding the unwanted back electron transfer, may be realized in monolayers on metal with a large difference in electronic couplings V between metal/ LUMO and metal/HOMO. This may be realized using chromophoric groups in with the LUMO and HOMO that are spatially separated in space as far as possible. ASSOCIATED CONTENT

S Supporting Information *

Detailed synthetic procedures and characterization, SAM characterization by AFM, XPS, reflectometry. This material is available free of charge via the Internet at http://pubs.acs.org.

■

ACKNOWLEDGMENTS

This work is part of the Joint Solar Programme (JSP) of Hyet Solar and the Stichting voor Fundamenteel Onderzoek der Materie FOM, which is part of The Netherlands Organisation for Scientific Research (NWO). The authors also acknowledge Ralf Bovee for the MALDI measurements.

varied such that the mean square error between experimental and predicted efficiencies was minimal. Using the kinetic model described above, we can simulate the photocurrent efficiency for the different hemicyanines. Simulated rates are shown in Figure 8B. As can be seen, the dependence of current generation efficiency and the energy of the HOMO can be reproduced. The best fit of the model with only VDAu and kQ as adjustable parameters is obtained for VDAu = 2.5 cm−1 and kQ = 0.042 (see Table 3). The modeling indicates that asymmetry in the electronic coupling of LUMO and HOMO with the electrodes can result in a net flow of electrons into the gold electrode and holes to the hydroquinone electrolytic contact, as depicted schematically in Scheme 1B. The asymmetry results from differences in the matrix elements V describing the electronic coupling and from differences in driving force for transport of holes and electrons to the electrodes. For molecules with deep lying HOMO such as F-pyr, the driving force for transfer of a hole from HOMO level in the photooxidized hemicyanine to the gold electrode is expected to approach the rate for transfer of electrons from the LUMO to gold. This opens an efficient loss mechanism consisting of fast forward and fast backward electron transfer of the hemicyanine with the gold electrode (see Scheme 1C).

■

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +31-40-2473723. 16828



Article

Effect Transistors by Self-Assembled Monolayers of Alkanethiols. J. Mater. Chem. 2007, 17, 1947−1953. (43) Marczewski, A. W. Analysis of Kinetic Langmuir Model. Part I: Integrated Kinetic Langmuir Equation (IKL): A New Complete Analytical Solution of the Langmuir Rate Equation. Langmuir 2010, 26, 15229−15238. (44) Gholamrezaie, F.; Kirkus, M.; Mathijssen, S. G. J.; De Leeuw, D. M.; Meskers, S. C. J. Photophysics of Self-Assembled Monolayers of a πConjugated Quinquethiophene Derivative. J. Phys. Chem. A 2012, 116, 7645−7650. (45) Weber, K.; Huckett, L.; Creager, S. Long-Range Electronic Coupling between Ferrocene and Gold in Alkanethiolate-based Monolayers on Electrodes. J. Phys. Chem. B 1997, 101, 8286−8291.

(20) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronic Device. Science 1999, 286, 1550−1551. (21) Imahori, H.; Fukuzumi, S. Porphyrin Monolayer-Modified Gold Clusters as Photoactive Materials. Adv. Mater. 2001, 13, 1197−1199. (22) Imahori, H.; Fukuzumi, S. Porphyrin- and Fullerene-Based Molecular Photovoltaic Devices. Adv. Mater. 2004, 14, 525−536. (23) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. Photovoltaic Properties of SelfAssembled Monolayers of Porphyrins and Porphyrin−Fullerene Dyads on ITO and Gold Surfaces. J. Am. Chem. Soc. 2003, 125, 9129−9139. (24) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G. B. Cyanines during the 1990s: A Review. Chem. Rev. 2000, 100, 1973− 2011. (25) Ashwell, G. J.; Tyrrell, W. D.; Whittam, A. J. Molecular Rectification: Self-Assembled Monolayers in which Donor−(πBridge)−Acceptor Moieties Are Centrally Located and Symmetrically Coupled to Both Gold Electrodes. J. Am. Chem. Soc. 2004, 126, 7102− 7110. (26) Ashwell, G. J.; Tyrrell, W. D.; Whittam, A. J. Molecular Rectification: Self-Assembled Monolayers of a Donor−(π-Bridge)− Acceptor Chromophore Connected via a Truncated Au−S−(CH2)3 Bridge. J. Mater. Chem. 2003, 13, 2855−2857. (27) Ashwell, G. J.; Chwialkowska, A.; Hermann, L. R. Rectifying Au− S−CnH2n−P3CNQ Derivatives. J. Mater. Chem. 2004, 14, 2848−2851. (28) Metzger, R. M. Unimolecular Electronics. J. Mater. Chem. 2008, 18, 4364−4396. (29) Bauer, S. Poled Polymers for Sensors and Photonic Applications. J. Appl. Phys. 1996, 80, 5531−5558. (30) Kocot, A.; Wrzalik, R.; Vij, J. K.; Zentel, R. Pyroelectric and Electro-Optical Effects in the SmC* Phase of a Polysiloxane Liquid Crystal. J. Appl. Phys. 1994, 75, 728−733. (31) Naciri, J.; Shenoy, D. K.; Keller, P.; Gray, S.; Crandall, K.; Shashidhar, S. Synthesis and Pyroelectric Properties of Novel Ferroelectric Organosiloxane Liquid Crystalline Materials. Chem. Mater. 2002, 14, 5134−5139. (32) Koch, W. T. H.; Munser, R.; Ruppel, W.; Wurfel, P. Bulk Photovoltaic Effect in BaTiO3. Solid State Commun. 1975, 17, 847−850. (33) Glass, A. M.; Von der Linde, D.; Auston, D. H.; Negran, T. J. High-Voltage Bulk Photovoltaic Effect and the Photorefractive Process in LiNbO3. J. Electron. Mater. 1975, 4, 915−943. (34) Festl, H. G.; Hertel, P.; Krätzig, E.; Von Baltz, R. Investigations of the Photovoltaic Tensor in Doped LiNbO3. Phys. Status Solidi B 1982, 113, 157−164. (35) Tan, Y.; Tatsuma, T. Mechanisms and Applications of PlasmonInduced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632−7637. (36) McFarland, E. W.; Tang, J. A Photovoltaic Device Structure Based on Internal Electron Emission. Nature 2003, 421, 616−618. (37) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to NearInfrared Wavelength Using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031−2036. (38) Reichhardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319−2358. (39) Marder, S. R.; Perry, J. W.; Yakymyshyd, C. P. Organic Salts with Large Second-Order Optical Nonlinearities. Chem. Mater. 1994, 6, 1137−1147. (40) Eckermann, A. L.; Feld, D. J.; Shaw, J. A.; Meade, T. J. Electrochemistry of Redox-Active Self-Assembled Monolayers. Coord. Chem. Rev. 2010, 254, 1769−1802. (41) Polaske, N. W.; Lin, H. C.; Tang, A.; Mayukh, M.; Oquendo, L. E.; Green, J. T.; Ratcliff, E. L.; Armstrong, N. R.; Saavedra, S. S.; McGrath, D. V. Phosphonic Acid Functionalized Asymmetric Phthalocyanines: Synthesis, Modification of Indium Tin Oxide, and Charge Transfer. Langmuir 2011, 27, 14900−14909. (42) Asadi, K.; Gholamrezaie, F.; Smits, E. C. P.; Blom, P. W. M.; de Boer, B. Manipulation of Charge Carrier Injection into Organic Field16829


Photovoltaic Effect in Self-Assembled Molecular Monolayers on Gold

Recommend Documents