In Chapter 3 we examine alternate redox mediators to optimize the regeneration of the oxidized sensitizer while maximizing open circuit voltage Voc.Bipyrazine based ruthenium sensitizers
Introduction to Dye-sensitized Solar Cells
Composition of a dye-sensitized solar Cell ccsesesssessesssscssssssesssssscssssessscsasenes 3
The most widely investigated type of dye-sensitized solar cell (commonly referred to as the Gratzel cell) is comprised of four major parts: (1) a molecular sensitizer dye attached to a (2) nanocrystalline semiconductor substrate immersed in a (3) redox mediator electrolyte containing a (4) platinum coated counter electrode Understanding the properties of these four components is crucial when striving for high solar energy conversion efficiency The first three of these are discussed in some detail bellow.
The dyes most commonly used for DSSC are ruthenium based polypyridyl complexes These dyes have a metal-to-ligand charge-tranfer (MLCT) excited states. The most efficient nanocrystalline solar cell to date produced utilizes cis- di(thiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxyllic acid)ruthenium(I) (N3 dye) as the sensitizer (Figure 1.1).'” An important factor determining the effectiveness of a dye as a sensitizer is the spectral overlap between the absorbance of the dye and the solar radiation reaching the earth Significant research has been done to engineer dyes that capture light throughout the visible and near infrared spectrum !”"" œ=œ
Figure 1.1 Structure of cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4°-dicarboxylic acid)ruthenium(II) sensitizer Commonly referred to as N3 dye.
The formal reduction potentials (E°) of ground, reduced and excited state of the sensitizer (S) are also important Molecules can be tailored to have these potentials energetically match the semiconductor substrate and redox mediator The reduction potentials of the ground (E°(S*”)) and reduced (E°(S)) states can be measured directly on the semiconductor interface using common electrochemical techniques “* The thermally equilibrated excited state reduction (E°(S*”)) and oxidation (E°(S)) potentials are calculated by the Rehm-Weller equations (1.1 and 1.2, respectively)’:
E°(S"")=E°(S"")+AG, 12 here AG;; is the Gibbs free energy stored in the thermally equilibrated excited state The value of 4G; is often estimated to be the high-energy onset of the corrected photoluminescence spectrum.'°
Previous research has shown that excited sensitizers bound to semiconductor surfaces inject electrons on pico- to femto second time scales, competing with vibrational relaxation.'” This observation is important, as chromophores with excited state reduction potentials E*(S ””) more positive than the conduction band edge can participate in interfacial electron transfer processes when vibrationally hot excited states are involved.
A less frequently studied group of sensitizers are tetrapyrrole macrocycles.
Porphyrins, phthalocyanines, chlorins: and other porphyrinic compounds are ideal sensitizers for low surface area substrates, as their large extinction coefficients allow for sufficient light absorption with low geometrical surface coverage Light absorption can be further improved by creating linear, semirigid, chromophore arrays from these compounds Research by Lindsey, Bocian and Donohoe showed extremely efficient singlet excited-state energy transfer from one porphyrin to the next through various linkers between the chromophores.'*°”° Patten et al modeled such energy transfer for long arrays of various arrangements and explained how energy conversion and shuttling processes in these macromolecules can be optimized.”’ The large light harvesting efficiency of these molecules allows for the use of less porous or even planar semiconductor electrodes Such electrodes would have an advantage in solid-state dye- sensitized solar cell applications where interpenetration of the substrate by a hole conductor is crucial Furthermore, planar single crystal semiconductors can be characterized by impedance spectroscopy to quantify flat-band potentials in different environments Details of these measurements are explained under the semiconductors section The benefits of a large extinction coefficient porphyrin dye attached to a planar semiconductor substrate have been exploited in Chapter 2.
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator It is characterized by a forbidden energy gap (Epg) between the valence band maximum (VB), and the conduction band minimum (CB) A material with a band gap of < 4 eV is considered a semiconductor The potential energy (E) distribution of the electrons in a semiconductor is described by the Fermi-Dirac function
(Equation 1.3)” Ị ƒŒ)=——mxy l+e # 13 where & is the Boltzmann constant and 7 is the absolute temperature Ey is the Fermi level, defined as the energy level at which the probability of finding an electron is 1⁄2.” For pure undoped semiconductors, the Fermi level is half way between the valence band and the conduction band.” A semiconductor under irradiation is not at thermodynamic equilibrium but achieves a steady-state condition under which Ey is referred to as quasi Fermi level.
Inclusion of electron rich or deficient impurities can significantly change the electronic configuration of semiconductors For example, in the case of an n-type semiconductor, doping by an electron rich impurity creates filled energy levels close to the valence band of the semiconductor Thermal excitation at room temperature promotes electrons into the conduction band with an efficiency close to unity As a result, the electron density in the conduction band equals the density of dopant atoms, Np.
The Fermi energy thus shifts as the equilibrium potential of electrons has increased The new position of Ey can be calculated by equation 1.4:
D where Ecg is the conduction band energy minimum and Neg is the density of acceptor states in the conduction band.
The position of the Fermi level can also be potentiostatically controlled Under ideal conditions, the energies of the valence and conduction band-edge positions are fixed on the semiconductor surface When a potential is applied, electron transfer occurs until the Fermi level of the semiconductor bulk equilibrates with the set potential As a result of the potential drop at the semiconductor surface a space charge region of width W is created between the fixed band-edge positions and potentiostatically controlled bulk The width of the space charge region due to an applied potential (V) can be calculated using yW = [2ee,V -Vp) 1.5 where Np is the doping level, € is the dielectric constant in the direction normal to the equation 1.5: surface, € is the permittivity of free space and gq is the charge on an electron.” This equation predicts that the width of the space charge region is inversely proportional to the square root of the doping level Therefore, lightly doped semiconductors have wider space charge layers than highly doped ones The same is true for semiconductors with low and high dielectric constants Figure 1.2 illustrates the band structure of a semiconductor electrode under bias.” The potential distribution most relevant to dye- sensitized solar cells is depletion (Figure 1.2B) Under these conditions the CB and the
VB lies at a lower potential in the bulk than at the surface, with a gradual change in between, shown by the band bending (Fig 1.2B) This potential distribution shuttles the injected electrons away from the semiconductor/dye interface, therefore reducing the chance of recombination reactions It is worth noting that depletion does not play a significant role in most nanostructured materials, as the small particle size does not allow for a significant space charge layer.
Figure 1.2 Potential distribution for n-type semiconductors Flat band (A) and depletion (B).
Determination of the flat band potential is crucial in designing a dye-sensitized solar cell To do this, the most commonly used technique is the Mott-Schottky analysis of impedance spectroscopy data.’* These measurements are done in the dark, without the chromophore on the surface Therefore, any shift in the flat band potential due to illumination or sensitization are unaccounted for The Mott-Schottky treatment of impedance data relates the space charge capacitance (C) to the potential applied (V) to the semiconductor electrode (equation 1.6):
The assumptions made when using the Mott-Schottky equation are discussed in the literature.”> The flat band potential can be determined from the intercept of a J/C vs applied potential plot By rearranging equation 1.4 the conduction band edge energy can be calculated if the donor density is known (equation 1.7).
1.2.3 Semiconductors in dye-sensitized solar cells: TiO; and SnO;
Dye-sensitized regenerative solar cells are based on the sensitization of wide band gap semiconductors One of the most promising semiconductors for such a purpose is intrinsically n-doped TiO2 Titania is a widely used semiconductor, due to its excellent chemical stability, optical transmittance in the visible region, and its inexpensive production cost.
The energy gap between the valence band (VB) and the conduction band (CB) of TiO; in the rutile phase is 3.26 eV, corresponding to light absorption of around 380 nm.”° The more commonly used nanocrystalline anatase TiO has similar electronic and absorption properties As this absorption only covers a small part of the solar radiation spectrum, a solar cell solely based on the band gap absorption of TiO would not result in efficient conversion of solar energy Nevertheless, the large band gap facilitates the formation of a wide depletion layer since the semiconductor can be biased at more positive potential without causing carrier inversion Thus the spatial separation of electrons from the oxidized dye on the surface can be enhanced.
Dye-sensitization Mechanisim .- -‹ cu eg 14
The steps involved in the light to electrical energy conversion in a greatly simplified dye sensitized solar cell are shown in Scheme 1.1 Light absorption by the
“S” creates an excited state “S” with a rate constant #ị If the newly created sensitizer state is a sufficient reductant, interfacial electron transfer can occur with a rate constant ky The oxidized dye “S”” is then reduced by an electron donor (D) in the surrounding electrolyte solution, which in turn is regenerated at the platinum counter electrode The open circuit voltage (V,.) of this regenerative process under illumination (ignoring ohmic losses) is given by the difference between the quasi-Fermi level of the FTO and the formal potential of the electrolyte.
An alternative mechanism for sensitization has also been proposed In this mechanism the excited dye is first reduced by an electron donor creating an even stronger reducing agent, which then injects an electron into the conduction band of TiO) This alternative mechanism would allow sensitization by dyes with excited state reduction potentials below the conduction band edge.*°
A | Pt Liên ka 7X e8 k Ki fa k3 1 V oc s
5 ke Am q9/† © Ny Mwnu mm nai
Scheme 1.1 Processes involved in the dye sensitization of wide band gap semiconductors in a regenerative solar cell
Characterization of dye-sensitized solar celẽs - 6 ỏc G1 ve se 17
The most important characteristic of a solar cell is its global efficiency In other words, the percentage of the solar radiation reaching the earth’s surface that is captured as electrical energy Power output under solar radiation can be obtained from the current- voltage response of the cell Figure 1.4 shows a typical current voltage curve for an N3 sensitized TiO electrode under quasi-monochromatic light This curve was obtained by gradually changing the resistance in the external circuit from open circuit (infinite resistance) to short circuit (no resistance) conditions The optimal power produced by the cell is at the power point (pp), where the product ofthe photocurrent and voltage is at maximum The current-voltage curve can also be used to determine the fill factor (FF) of the solar cell, Equation 1.8.
Fill factors of unity are ideal, in practice however, values below 0.8 are typical.
Deviation from ideal is due to losses encountered during the photon to electron conversion process, such as charge recombination and ohmic losses Using the fill factor
(FF), short circuit photocurrent (i,.) and the open circuit voltage (Voc) the efficiency (7) of the solar cell can be calculated using equation 1.9: a 1.9 where l;aa is the intensity of radiation For the data shown in Figure 1.4, ƒ¿¿= 530 mV, ise
= 81 HA/cmẺ, FF = 0.71 and nso = 3.6 %.
Figure 1.4 Current-voltage curve for N3 sensitized TiO electrode immersed in an electrolyte comprised of 0.6 M 3-butyl-1-methyl imidazolium hexaforophosphate, 0.5 M tert-butylpyridine, 0.6 M Lil and 0.05 M Ip in 1:1 valeronitrile:acetonitrile Quasi monochromatic radiation centered at 500 nm was used.
Another common characterization technique for dye-sensitized solar cells is to measure the incident photon-to-current efficiency (IPCE) In contrast to global efficiencies, which are measured under sunlight or simulated sunlight, IPCEs are usually measured as a function of the excitation wavelength The efficiency of incident photon- to-current conversion at a specific wavelength is proportional to the product of the light harvesting efficiency (LHE), electron injection quantum yield (®), and charge collection efficiency (7) at that wavelength (Equation 1.10):
IPCE’s are often reported as a fraction between 0 and 1 or as a percentage from 0-100%.
For example, a dye-sensitized solar cell with an IPCE of 0.8 at a given wavelength converts 80% of the incident photons to electrons in the external circuit To a first approximation, the electron injection yield and the collection efficiency are wavelength independent However, the sensitizer LHE changes as a function of wavelength, and therefore IPCE’s usually track the absorptance spectrum of the dye IPCE as a function of wavelength is determined by first measuring the photocurrent action spectra (photocurrent as a function of wavelength) then applying equation 1.11.
A(nm) Power of incident light (W) IPCE = 1.11
Here the Power of incident light is measured with a calibrated photodiode.
Optimization of dye-sensitized solar ceẽẽS .- -s- s6 G xxx se ssa 20 IS ố Ầ 22
To achieve optimal performance in dye sensitized solar cells, it is essential to optimize the processes responsible for photocurrent generation These include absorption (Ai), electron injection (&›), and the regeneration of electron donors in electrolyte (k4) as shown on Scheme 1.1 Furthermore, competing processes such as radiative and non- radiative decay of the excited senzitizer (4.1), recombination of the injected electrons with the oxidized dye (k3) or the oxidized donors have to be minimized In an operational cell this is rarely the case As a result, reduced open circuit voltage and thus a less efficient cell is observed The diode equation (1.12) models the essential physics of the DSSC’s and correctly predicts a 59 mV increase in Voc for every order of magnitude decrease in electron recombination. r-(ol sa] 42
Here Jin; is the flux of charge resulting from sensitized injection, mạ; is the concentration of electrons on the TiO, surface, ke; is the rate of back electron transfer and [A]; is the concentration of acceptors (i.e, oxidized donors and sensitizers).!° Grọtzel and Frank have reported the use of Lewis bases, such as pyridine and amines, to improve open circuit voltage.'°'’ However, it is yet unknown whether this improvement is achieved by blocking back electron transfer, or by shifting the flat band potential of TiO, to more negative values.
The following chapters in this thesis are aimed at optimizing the performance of the dye-sensitized solar cell Chapter 2 describes a study towards understanding the mediators to optimize the regeneration of oxidized sensitizer while maximizing open circuit voltage (Voc) In Chapter 4 the interaction of iodide with a ruthenium polypyridyl sensitizer is studied to elucidate on the workings of the iodide/tri-iodide redox couple A more specific interaction between iodide and a zinc tetraphenylporphyrin sensitizer is shown to improve the light harvesting efficiency of the DSSC in Chapter 5.
1 Meyer, G J Journal of Chemical Education 1997, 74(6), 652-656.
2 Meyer, T J Accounts of Chemical Research 1989, 22(5), 163-170.
3 Chapin, D M.; Fuller, C S.; Pearson, G L Journal of Applied Physics
4 Mcevoy, A J.; Gratzel, M Solar Energy Materials and Solar Cells 1994,
7 Gerische.h; Michelbe.me; Rebentro.f, Tributsc.h Electrochimica Acta
8 Gerischer, H Pure and Applied Chemistry 1980, 52(12), 2649-2667.
10 Nazeeruddin, M K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.;
Liska, P.; Vlachopoulos, N.; Gratzel, M Journal of the American Chemical Society 1993, 115(14), 6382-6390.
Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G B.; Bignozzi, C A.; Gratzel, M Journal of the American Chemical Society 2001, 123(8), 1613-1624.
13 Argazzi, R.; Bignozzi, C A.; Hasselmann, G M.; Meyer, G J Inorganic
14 Bard, A J.; Faulkner, L R Electrochemical Methods: Fundamentals and
Applications; Wiley: New York, NY, 1980.
15 Rehm, D.; Weller, A Jsrael Journal of Chemistry 1970, 8(2), 259-&.
16 _ Lakowicz, J R Principles of Fluorescence Spectroscopy; Second Edition ed Kluwer Academic/Plenum Publishers: New York, 1999.
Schlichtrérl, S.Y Huang Future Generation Photovoltaic Technologies; American Institute of Physics: Woodbury, New York, 1997; p 145.
Wagner, R W.; Johnson, T E.; Lindsey, J S Journal of the American
Hsiao, J S.; Krueger, B P.; Wagner, R W.; Johnson, T E.; Delaney, J. K.; Mauzerall, D C.; Fleming, G R.; Lindsey, J S.; Bocian, D F.;
Donohoe, R J Journal of the American Chemical Society 1996, 118(45), 11181-11193.
Seth, J.; Palaniappan, V.; Wagner, R W.; Johnson, T E.; Lindsey, J S.; Bocian, D F Journal of the American Chemical Society 1996, 118(45), 11194-11207.
Van Patten, P G.; Shreve, A P.; Lindsey, J S.; Donohoe, R J Journal of Physical Chemistry B 1998, 102(21), 4209-4216.
Sze, S M Physics of Semiconductor Devices; Wiley: New York, NY, 1981.
Finklea, H O Semiconductor Electrodes; Elsevier Science Publishers B. V.: New York, NY, 1988.
Macdonald, J R Impedance Spectroscopy: Emphasizing Solid Materials and Systems; Wiley: New York, NY, 1987.
Gomes, W P.; Cardon, F Progress in Surface Science 1982, 12(2), 155- 215.
Kiebooms, R.; Vanderzande, D.; Kienberger, F.; Schindler, H Synthetic Metals 2001, 125(3), 279-287.
Clark, W D K.; Sutin, N Journal of the American Chemical Society
Gratzel, M Journal of Photochemistry and Photobiology C-
Hagfeldt, A.; Gratzel, M Accounts of Chemical Research 2000, 33(5), 269-277.
Gratze, M.; Moser, J E Electron Transfer in ChemistryWiley-VCH: Weinheim, Germany, 2001; Vol V, pp 587-644.
Bergeron, B V.; Meyer, G J Photovoltaics for the 2lset Century IThe
Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gratzel, M Journal of the American Chemical Society 1988, 110(4), 1216-1220.
Gregg, B A.; Pichot, F.; Ferrere, S.; Fields, C L Journal of Physical Chemistry B 2001, 105(7), 1422-1429.
Argazzi, R.; Bignozzi, C A.; Heimer, T A.; Castellano, F N.; Meyer, G.
Gratzel, M Journal of Physical Chemistry B 2001, 105(43), 10461-10464. Bergeron, B V.; Meyer, G J Journal of Physical Chemistry B 2003,
Influence of Surface Protonation on the Sensitization
Introduction .c.csccssessssssssccsccccesssssessssessssesssssscessessssesssecsssesessavsscavsesscsseassaeaees 25
Dye-sensitized photoelectrochemical cells have been the subject of considerable interest over the past three decades.'” The accepted mechanism for dye-sensitization 5 involves the excitation of a sensitizer followed by interfacial electron transfer, or electron
‘268 In regenerative dye-sensitized injection, into a semiconductor acceptor state. photoelectrochemical cells, the oxidized sensitizer is reduced by a donor present in the electrolyte, which is in turn reduced at the counter electrode Thus, photocurrent and photovoltage are generated under sub-bandgap illumination, while no net chemistry occurs The development of efficient dye-sensitized solar cells using single-crystal electrodes was precluded by poor light harvesting efficiencies and low photocurrent densities To improve the light harvesting efficiency, Grọtzel and coworkers developed high surface area, nanocrystalline dye-sensitized metal oxide films.”'? In 1991, O’Regan and Gratzel reported a global power conversion efficiency of ~7% for a dye-sensitized nanocrystalline TiO, electrode under simulated AM 1.5 solar illumination.’ The efficiency has since been optimized to 10.69%.'*"5
The cation concentration at the semiconductor-electrolyte interface profoundly influences the efficiency of dye-sensitized photoelectrochemical cells."° Cations have 16 been found to affect many parameters, including the energetics of the semiconductor and
23-26 sensitizer,'”?? the efficiency and dynamics of electron injection,”? and the kinetics of charge transport.”"? Understanding and controlling cation effects is important with regard to maximizing the efficiency of dye-sensitized solar cells |
In the early dye-sensitization literature, several reports described pH-dependent sensitized photocurrents at single-crystal metal oxide electrodes.”*' The effect was attributed to the well-known Nernstian dependence of the conduction band edge potential on the electrolyte proton concentration,'”!? and the resulting pH dependence of the driving force for electron injection Appendix 1 is a preliminary report on pH-dependent sensitized photocurrents for porphyrin-modified, low-surface-area TiO, films.** This surprising result could not easily be explained by pH-induced shifts of the TiO, conduction band, because electron injection was thermodynamically favorable over the entire pH range through which the photocurrent onset occurred Instead, the result was attributed to the influence of surface protonation-deprotonation equilibria on the stabilization of injected electrons in low-surface-area TiO, films.
In this chapter, a continuation of the short study presented in Appendix 1 is reported We have characterized the sensitization of nanocrystalline and low-surface- area TiO, films with porphyrins 1-4 (Chart 2.1) using photoelectrochemical and spectroscopic techniques The interfacial proton concentration was found to exert similar effects on the sensitization efficiency of nanocrystalline TiO, and low-surface-area TiO,.Our findings have important implications for optimizing the efficiency of nanocrystalline dye-sensitized photoelectrochemical cells.
Chart 2.1 Structures of the porphyrin sensitizers, 1: M = Zn(II), X = CO;H; 2: M 2H, X = CO;H; 3: M = Pt(), X = CO;H; 4: M = Zn(II), X = PO;H;.
Experimental Section ccsssssccsscssssssssescsessssscsessesssssssssssssssevscsessessessasasvaseds 28
Materials Porphyrin synthesis Porphyrins 1, 2, and 4 were obtained from Prof. J.S Lindsey’s laboratory.**** Porphyrin 3 was synthesized by Pt”* insertion into the free base porphyrin (2), following the method of Mink et al.Š
Semiconductor film preparation Single-crystal rutile TiO, (100) was obtained from Princeton Scientific Crystals were reduced by heating at 600°C for one hour in a
10/90 H,/N, atmosphere Low-surface-area TiO, films were prepared by adaptation of the method of Ting et al.“” Elemental Ti was sputter-deposited onto conductive FTO- coated glass slides (8-20 Q/square, Libby Owens Ford) at room temperature under 10 mtorr Ar atmosphere at 50 W Optimal film thickness was achieved with 2-6 min sputtering time Ti films were oxidized to TiO by heating at 450 °C in air for 30-60 min.
Nanocrystalline TiO, and ZrO, films on glass or FTO substrates were prepared as previously described.**°
Surface attachment reactions TiO, films were exposed to solutions of 1-4 in toluene or THF for 12-15 hours Upon removal from porphyrin solutions, derivatized slides were rinsed thoroughly with the derivatizing solvent Surface coverages were determined from the Soret and Q band absorbances of derivatized films.
Scanning electron microscopy SEM images were obtained with a JEOL scanning electron microscope Secondary electron images were acquired at 2 - 10 kV.
Electrochemistry Impedance spectroscopy Impedance and capacitance of low- surface-area TiO./FTO samples were measured using a Stanford Research SR530 lock-in amplifier (LIA) and a PAR Model 173 potentiostat/galvanostat All measurements were performed in a three-electrode configuration using a low-surface-area TiO,/FTO working electrode, Pt mesh counter electrode, and Ag/AgCl.) reference electrode The electrolyte was aqueous 0.1 M Na,B,O,, the pH of which was adjusted with HCl and NaOH The geometry of the electrochemical cell was the same throughout the experiments, enforcing a minimal distance between electrodes and a 0.28 cm’ working electrode area.
Using a LabView program, the LIA and potentiostat were programmed to obtain frequency vs impedance data and potential vs capacitance data The LIA was used to generate both the reference signal and the modulating signal (3 mV peak to peak) of known frequency "The modulating signal was added onto the potential generated by the potentiostat The current to voltage output of the potentiostat was connected to the voltage input of the LIA The LIA was then used to measure the real and imaginary part of the current in the electrochemical cell as a function of either frequency or potential. The real and imaginary current impedance (Z) and capacitance (C) were calculated by the program Complex plane plots of the impedance ({Im(Z)| vs Re(Z)), Bode plots
(og({ZÙ vs log(œ)), and C vs potential plots were also produced by the LabView program |
Photocurrent onset potentials Band gap photocurrents were measured in a 3- electrode, single compartment cell with a low-surface-area TiO,/FTO working electrode, Pt-coated FTO counter electrode, and Ag/AgCl reference electrode The electrolyte was aqueous 0.1 M Na;B,O;, the pH of which was adjusted with HCl and NaOH Linear sweep voltammograms (V = 10 mV/s) were measured in the dark and under chopped
(320 Hz) illumination by the full spectral output of a Xe lamp Photocurrents were measured by lock-in technique with a Stanford Research Systems SR530 lock-in amplifier.
Cyclic voltammetry Cyclic voltammetry was performed with a Bioanalytical Systems CV-50 potentiostat A 3-electrode, single-compartment cell was used, with a porphyrin-derivatized TiO, working electrode, Pt mesh counter electrode, and Ag/AgCl) reference electrode The electrolyte was 0.1 M tetrabutylammonium perchlorate in CH3CN or 0.1 M Na2B,O; in water The pH of the aqueous electrolyte was adjusted with HCl and NaOH.
Sensitized photocurrent measurement Photocurrents were measured using one of two setups (1) A 3-electrode sandwich cell was employed, with a porphyrin-modified TiO,/FTO working electrode, Pt-coated FTO counter electrode, and Ag pseudoreference electrode The electrolyte was 0.1 M aqueous Na,B,O, with 0.05 M hydroquinone as an electron donor The electrolyte pH was adjusted with HCl and NaOH The working electrode was illuminated through the backside using a Schoeffel CPS 255HR 450 W Xe lamp output through a Kratos GM252 monochromator Current was measured with a
PAR 173 potentiostat output to a Houston Instruments 2000 X-Y recorder Photocurrent action spectra were measured at constant positive bias at potentials corresponding to saturation sensitized photocurrent, as determined from linear sweep voltammetry measurements under chopped (~1 Hz) Soret band illumination (In linear sweep experiments, potential was controlled with a PAR 175 universal programmer.) (2) A 2- electrode sandwich cell was employed, with a porphyrin-modified TiO./FTO working electrode and Pt-coated FTO counter electrode The working electrode was illuminated through the backside using a Spectra-Physics Xe lamp output through an Oriel Cornerstone 4m monochromator Current was measured by a Keithley 617 electrometer.
Spectroscopy Static absorption and emission UV/vis absorption spectra were obtained with a Hewlett-Packard 8453 diode array spectrophotometer Emission spectra were obtained with a Spex Fluorolog fluorimeter Porphyrin-modified films were placed diagonally in a 1 cm cuvet, and front-face emission was measured.
Transient absorption spectroscopy Transient absorption data were acquired as described previously.“ The frequency-tripled, 355 nm output of a Q-switched Nd:YAG laser (Continuum Surelite) was Raman-shifted with Hyp, yielding pulsed 417 nm irradiation (~8 ns, 5-10 mJ/cm’, 1 Hz) Sensitizer-derivatized semiconductor films were placed in a 1 cm quartz cuvet at a 45° angle to the pump beam The cuvet was stoppered, and the surrounding aqueous electrolyte solution was purged with Ar or N; The pump beam was expanded to ~1.5 cm diameter to ensure uniform illumination of the sensitizer- derivatized films The probe source was a pulsed 150 W Xe lamp (Applied Photophysics) focused on the semiconductor film Transmitted light was output through a Spex 1702-04 monochromator and detected with a Hamamatsu R928 PMT The pump and probe beams were orthogonal The PMT signal was digitized by a LeCroy 9450 oscilloscope .
TiO, morphology Two types of TiO, film were prepared: “low-surface-area” films from thermal oxidation of sputter-deposited titantum, and nanocrystalline films from sol-gel deposition Scanning electron micrographs of low-surface-area TiO, films (Figures
2.la-c) reveal that the morphology of the films depended on the sputter deposition timescale At short deposition times (< 4 min), the TiO, films consisted of relatively large particles with dimensions on the order of 100 nm Some additional surface roughness on the individual TiO, particles is apparent The morphology of these films corresponded closely to that of their FTO substrates Increasing the sputter deposition time resulted in increased film thickness and increased surface area through the formation of smaller TiO, particles The film prepared by oxidation of 30-minute sputtered titanium consisted of aggregates of TiO, with features on an approximately 10 nm length scale The nanocrystalline TiO, film also consisted of aggregates of particles with roughly 10 nm dimensions (Figure 2.14).
Sensitizer adsorption Porphyrins 1-4 were adsorbed to TiO, surfaces from solution Equilibrium binding was well-described by the Langmuir adsorption isotherm model (Figure 2.2).“° Porphyrins were adsorbed to low-surface-area TiO, films (2-6 min sputter deposition time) with surface adduct formation constants (Kaa) of 10° M” and saturation surface coverages (Ta) of 4.0 x 10° mol/cmử For nanocrystalline TiO, films,
Attempts to adsorb the tetramesityl analog of 1 to TiO, yielded no measurable surface coverage, suggesting that 1-4 are anchored to the surface through carboxylic acid or phosphonic acid functional groups When low-surface-area TiO, films derivatized with
1, 2, or 3 were immersed in organic solvents in which the porphyrins were soluble, rapid and nearly complete desorption occurred Low-surface-area TiO, derivatized with 4,which contains the phosphonic acid surface-attachment group, underwent little or no desorption in organic solvents For all four porphyrins, very little desorption occurred from nanocrystalline TiO; films immersed in organic solvents. Ỉ
X50 000 TỦÙnhh ý 1 8.2mm 30.000 100nm WD 53mm
References .ccscesccsssctscecccessesscssscsscesscessessscssesaeecssseucensesseecesuscsassssvesesessceesenses 58
4 Kalyanasundaram, K.; Grọtzel, M Coord Chem Rev 1998, 77, 347-414.
5 Qu, P.; Meyer, G J In Electron Transfer in Chemistry; Balzani, V., Ed.;
John Wiley and Sons: New York, 2001; Vol IV, pp 355-411.
7 Gleria, M.; Memming, R Z Phys Chem (Munich) 1975, 98, 303-316.
9 Desilvestro, J.; Grọtzel, M.; Kavan, L.; Moser, J J Am Chem Soc 1985,
11 Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gritzel, M J Am Chem.
12 Liska, P.; Vlachopoulos, N.; Nazeeruddin, M K.; Comte, P.; Gratzel, M.
14 Nazeeruddin, M K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Miiller, E.;
Liska, P.; Vlachopoulos, N.; Gratzel, M J Am Chem Soc 1993, 115, 6382-6390.
15 Gratzel, M In Future Generation Photovoltaic Technologies: AIP
Conference Proceedings; McConnell, R D., Ed.; NREL: Denver, CO, 1997.
16 Watson, D F.; Meyer, G J Coord Chem Rev., In press.
17 Finklea, H O In Semiconductor Electrodes; Finklea, H O., Ed.; Elsevier:
Watanabe, T.; Fujishima, A.; Honda, K Bull Chem Soc Jpn 1976, 49, 355-358.
Rothenberger, G.; Fitzmaurice, D.; Grọtzel, M J Phys Chem 1992, 96, 5983-5986.
Redmond, G.; Fitzmaurice, D J Phys Chem 1993, 97, 1426-1430. Boschloo, G.; Fitzmaurice, D J Phys Chem B 1999, 103, 7860-7868.
Liu, Y.; Hagfeldt, A.; Xiao, X.-R.; Lindquist, S.-E Sol Energy Mater.
Kelly, C A.; Farzad, F.; Thompson, D W.; Stipkala, J M.; Meyer, G J. Langmuir 1999, 75, 7047-7054.
Tachibana, Y.; Haque, S A.; Mercer, I P.; Moser, J E.; Klug, D R.; Durrant, J R J Phys Chem B 2001, 105, 7424-7431.
Asbury, J B.; Anderson, N A.; Hao, E.; Ai, X.; Lian, T J Phys Chem B
Kopidakis, N.; Schiff, E A.; Park, N.-G.; van de Lagemaat, J.; Frank, A.
Kambe, S.; Nakade, S.; Kitamura, T.; Wada, K:: Yanagida, S J Phys.
Watanabe, T.; Fujishima, A.; Tatsuoki, O.; Honda, K.-i Bull Chem Soc. Jpn 1976, 49, 8-11.
Clark, W D K.; Sutin, N J Am Chem Soc 1977, 99, 4676-4682.
Watson, D F.; Marton, A.; Stux, A M.; Meyer, G J J Phys Chem B
Lindsey, J S.; Prathapan, S.; Johnson, T E.; Wagner, R W Tetrahedron
Muthukumaran, K.; Loewe, R S.; Ambroise, A.; Tamaru, S.-i; Li, Q.; Mathur, G.; Bocian, D F.; Misra, V.; Lindsey, J S J Org Chem 2004,
Mink, L M.; Neitzel, M L.; Bellomy, L M.; Falvo, R E.; Boggess, R. K.; Trainum, B T.; Yeaman, P Polyhedron 1997, 16, 2809-2817.
Ting, C.-C.; Chen, S.-Y.; Liu, D.-M J Appl Phys 2000, 88, 4628-4633.
Ting, C.-C.; Chen, S.-Y.; Liu, D.-M Thin Solid Films 2002, 402, 290- 295.
Heimer, T A.; D'Arcangelis, S T.; Farzad, F.; Stipkala, J M.; Meyer, G.
Bonhôte, P.; Gogniat, E.; Tingry, S.; Barbé, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Grọtzel, M J Phys Chem B 1998, 102, 1498- 1507.
Chirvony, V S.; van Hoek, A.; Galievsky, V A.; Sazanovich, I V.; Schaafsma, T J.; Holten, D J Phys Chem B 2000, 104, 9909-9917. Mott, N F Proc R Soc London, A 1939, 171, 27.
Gomes, W P.; Cardon, F Z Phys Chem (Munich) 1973, 86, 330-334. Frank, S N.; Bard, A J J Am Chem Soc 1975, 97, 7427-7433.
Zaban, A.; Ferrere, S.; Gregg, B A J Phys Chem B 1998, 102, 452- 460.
Tokel-Takvoryan, N E.; Bard, A J Chem Phys Lett 1974, 25, 235-238. Rehm, D.; Weller, A Isr J Chem 1970, 8, 259-271.
Bailey, S I.; Ritchie, I M.; Hewgill, F R J Chem Soc., Perkin Trans 2
Sửdergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S.-E J Phys Chem.
Rodriguez, J.; Kirmaier, C.; Holten, D Am Chem Soc 1989, 111,6500-6506.
Pekkarinen, L.; Linschitz, H J Am Chem Soc 1960, 82, 2407-2411.
Felton, R H In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol 5, pp 53-125.
Finklea, H O In Semiconductor Electrodes; Finklea, H O., Ed.; Elsevier: New York, 1988; pp 1-42.
Watanabe, T.; Fujishima, A.; Honda, K.-I Chem Lett 1974, 897-900.
Nozik, A J Annu Rev Phys Chem 1978, 29, 189-222.
Gerischer, H In Physical Chemistry: An Advanced Treatise; Eyring, H., Henderson, D., Jost, W., Eds.; Academic Press: New York, 1970; Vol. 9A, pp 463-542.
Stanienda, A.; Bieble, G Z Phys Chem (Munich) 1967, 52, 254-275. Giraudeau, A.; Callot, H J.; Gross, M Inorg Chem 1979, 18, 201-206.
Schindler, P W.; Gamsjọger, H Discuss Faraday Soc 1971, 52, 286- 288.
Ghosh, H N.; Asbury, J B.; Weng, Y.; Lian, T J Phys Chem B 1998,
Weng, Y.; Wang, Y.-Q.; Asbury, J B.; Ghosh, H N.; Lian, T J Phys. Chem B 2000, 104, 93-104.
Hao, E.; Anderson, N A.; Asbury, J B.; Lian, T J Phys Chem B 2002,
Wang, Y.; Hang, K.; Anderson, N A.; Lian, T J Phys Chem B 2003,
Hannappel, T.; Burfeindt, B.; Storck, W.; Willig, F Phys Chem B
Dye-Sensitized SnO; Electrodes with Iodide and
Introduction ` 63 3.2 EXp€riimerifAè ôch Hà TH HT TH ng TT TH TH ve 67 3.3 R.€SUẽIS HH HH HT TT TT TH Ho TH Co ch ng gà 69
Efficient conversion of sun light to electrical power is an important goal.’
Sensitized nanocrystalline (anatase) TiO, thin films have renewed interest in molecular approaches to this objective.”° Regenerative photoelectrochemical cells based on these materials with an organic electrolyte based on the triiodide/iodide, I;/T, redox mediators, have yielded global conversion efficiencies of 10.6 % under simulated AM 1.5 solar irradiance conditions.’ It is often found that absorbed photons are converted to electrons near quantitatively, while only a small fraction of the free energy stored in the sensitizer excited-state contributes to open circuit photovoltage (Vạ„„).” In other words, the kinetics ` of the dye-sensitized solar cells appear to be well optimized while the thermodynamics are not.
A significant fraction of the wasted free energy in these cells can be attributed to the use of one redox mediator, I;/T, regardless of the molecular sensitizer Ideally, a redox mediator should be optimized for each particular molecular sensitizer used within the cell Alternative redox mediators, such as quinone/hydroquinone,’ Br/Br, Fc*/Fc
(where Fc is ferrocene),” PTZ*/PTZ (where PTZ is phenothiazine),° as well as solid-state hole conductors,’*” have been far less effective for energy conversion Recent reports of cobalt-based mediators are particularly encouraging, but thus far compare well to I;/T only at irradiances below one sun.* Very recently, a high 7.5% efficiency solar cell was reported based on the (SeCN),/SeCN” redox mediator in solvent-free ionic liquid electrolyte.’
Oskam et al reported details on the use of the pseudohalide redox mediators
(SeCN),/SeCN and (SCN)/SCN in dye-sensitized TiO, (anatase) solar cells in acetonitrile.'° With cis-Ru"(dcbH,),(NCS),, where dcbH, is 4,4’-(CO,H),-2,2’- bipyridine, as the sensitizer, low photocurrent efficiencies were observed Despite having more positive formal reduction potentials than that of I;/T, only a slight increase in Vụ. was measured using (SeCN),/SeCN,, and the cell potential obtained using (SCN),/SCN” was considerably worse Transient spectroscopic studies revealed that the excited-state electron injection yields were high and independent of the redox mediator employed.
However, sluggish oxidation rates of the pseudohalides by cis-Ru”(dcbH,);(NCS),* allowed a significant fraction of the injected electrons to recombine with the oxidized sensitizer thereby decreasing the solar conversion efficiency.
Ruthenium sensitizers based on bipyrazine (bpz) and 2,3-dipyridyl pyrazine (dpp) ligands with Ru’ reduction potentials that are ~ 550 to 900 mV more positive than thatHƯN of cis-Ru”(dcbH;);(NCS);, were independently prepared and tested, Scheme 3.1.!° While these sensitizers absorb light less efficiently in the red region, the hope was that the increase in V,, would compensate for this and higher power conversion yields would be realized However, these photo-excited sensitizers did not inject electrons into TiO, with high quantum yields In some cases, even the reduced sensitizers, ie Ru(dcbH;)(bpz )(bpz)”, did not transfer electrons to TiO, effectively, indicating that neither the excited- or the reduced-state were sufficiently strong reducing agents to inject electrons into the
In the present work, we have turned to nanocrystalline SnO, thin films The conduction band-edge of single crystal SnO, is reported to be ~ 500 mV more positive than that of single crystal rutile TiO, in aqueous electrolytes.'' There exists experimental evidence that this is also true for the nanocrystalline SnO,.'? Even though, the aqueous conduction band-edge given in Scheme 3.1 indicate unfavorable thermodynamics for injection from Ru(deeb);(dpp)”"” and Ru(deeb),(bpz)’", we emphasize that injection may occur from ‘hot’ excited states and that the ‘acceptor states’ relevant to nanocrystalline metal oxides may have an exponential density of unfilled states rather than the abrupt edge indicated in Scheme 3.1.’
L - 1,59 V Rulfqeeb);(đpp) 77" r - 1.73 V Ru”""(deeb)z(bpz) *7”7*
Scheme 3.1 The conduction band-edge energy of single crystal SnO; (Ecp at pH = 1) is shown on the left hand side with the ground- and excited-state reduction potentials of the ruthenium sensitizers, and the reduction potentials of the mediators in acetonitrile on the right All potentials are vs SCE.
Therefore, our expectation was that Ru(deeb);(dpp)”“” and Ru(deeb);(bpz)”” would inject electrons into SnO, more effectively than into TiO; The positive Ru potentials should also result in more rapid oxidation of the pseudohalides and iodide, and produce higher photocurrent efficiencies These expectations were in fact realized. Furthermore, we discovered conditions where regenerative solar cells with the pseudohalides yielded comparable and even slightly higher photocurrents and open circuit photovoltages than did iodide We also report evidence for static (k > 10° s") oxidation of iodide within dye-sensitized solar cells.
Materials The sensitizers Ru(deeb)(bpy),(PF,),, Ru(deeb),(dpp)(PF,)>, and
Ru(deeb);(bpz)(PE¿);, were available from previous studies.'” Reagents were obtained as follows: CH;CN (Burdick and Jackson, BioSyn grade); Pb(SCN), (Aldrich, 99.5%); Br, (Aldrich, 99.5%); NaI (Aldrich, 99.5%); I, (Aldrich, 99.8%); NaSCN (Alfa Aesar, 98%); KSeCN (Alfa Aesar, 98.5%); LiClO, (Aldrich, 99.99%); poly(ethylene glycol) reacted with Bisphenol A diglycidyl ether (Aldrich, M, ~ 15,000); NaHCO, (EM Science, 99.7%), NaOH (EM Science, 97%); microscope slides (VWR); and FTO (Pilkington, 7 Q/square) Commercially available SnO, as a 15% aqueous colloidal solution of 15 nm particles was used as received from Alfa Aesar.
SnO, Thin Film Preparation Poly(ethylene glycol), 0.6 g, and 0.5 ml of a pH
11 NaHCO,,,,/NaOH,,,) solution were combined and dissolved with 10 mL of colloidalSnO;„„ and stirred for ~ 2 hours This solution was applied to an fluorine doped tin oxide (FTO) or microscope slide with an adhesive tape mask, and spin coated at 2000 rpm for 12 seconds using a Laurell Technologies corporation WS-400A-6NPP/LITE spinner The tape was removed after 30 min of air-drying, and the films were heated at
450 °C for 30 min under a slow flow of oxygen The ruthenium compounds were attached to SnO, by surface reactions in acetonitrile solutions containing ~ 100 mM sensitizer for 24 hours.
Absorbance Steady-state absorption measurements were made on a Hewlett- Packard 8453 diode array spectrophotometer Transient absorption spectra were acquired as previously described.’
Photoluminescence Steady-state photoluminescence (SSPL) spectra were obtained with a Spex Fluorolog that had been calibrated with a NIST standard tungsten- halogen lamp * Time-resolved measurements were obtained as previously described."
Solar Cell Electrolyte The electrolyte solutions were freshly prepared immediately before each set of experiments as previously described '
Photoelectrochemistry The photoanode consisted of colloidal SnO, deposited upon an FTO substrate sensitized with either Ru(deeb)(bpy);(PFạ);, Ru(deeb),(dpp)(PF,)., or Ru(deeb),(bpz)(PF,), The counter electrode consisted of an FTO substrate with a thin
Pt coating Measurements were performed in a two-electrode cell at room temperature using the I, /I redox mediator, and at 0 °C for the (SeCN);/SeCN and (SCN),/SCN’ redox mediators, with preparations discussed elsewhere.'° A 150 W Xe lamp coupled to a f/2
0.2 m McPherson monochromator was used for incident photon-to-current efficiency (IPCE) measurements The 488 nm laser line of an Innova Argon Ion Laser was used as the excitation source for photocurrent density vs photovoltage, and irradiance dependent measurements Plasma lines were removed using a 488 nm notch filter, and the flux attenuated using a beam collimator Incident irradiances were measured using an $370
UDT Optometer Photocurrents and photovoltages were obtained using a Keithley 617 electrometer and a Keithley 199 System DMM/Scanner, respectively.
Impedance Spectroscopy Impedance and capacitance of FTO/SnO, samples were measured using a Stanford Research SR530 lock-in amplifier (LIA) and a PAR
Model 173 potentiostat/galvanostat All measurements were done in a three-electrode configuration using a Pt gauze counter electrode, a 10 mM AgNO,/Ag reference electrode in acetonitrile and an FTO/SnO, sample as the working electrode The geometry of the electrochemical cell was the same throughout the experiments, enforcing a minimal distance between electrodes and a 0.28 cm’ working electrode area.