Dichromophoric Zinc Porphyrins: Filling the Absorption Gap

Article pubs.acs.org/JPCC

Dichromophoric Zinc Porphyrins: Filling the Absorption Gap between the Soret and Q Bands Long Zhao,† Pawel Wagner,† Holly van der Salm,‡ Tracey M. Clarke,† Keith C. Gordon,‡ Shogo Mori,§ and Attila J. Mozer*,† †

ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Wollongong, New South Wales 2522, Australia ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago, Dunedin, 9016 New Zealand § Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan S Supporting Information *

ABSTRACT: Porphyrins are some of the most studied chromophores employed in photo-electrochemical energy conversion devices. However, the molar extinction coefficient of most simple porphyrins is small within the 450−550 nm wavelength region, referred to here as the absorption gap, which limits the light harvesting efficiency of thin photoelectrodes. The purpose of this work is to fill the absorption gap by covalently attaching additional chromophores with complementary absorption in the 450−550 nm wavelength region. To this end, three carbazolefused thiophene-substituted zinc porphyrin dyes were synthesized, and their photophysical properties were investigated using UV−vis absorption, photoluminescence, resonance Raman, and electrochemical methods, supported by density functional theory calculations. All three dyes showed much-improved light harvesting up to 550 nm when attached to TiO2 photoelectrodes, resulting in doubling the short circuit current of dye-sensitized solar cells using the Co2+/Co3+ electrolyte. The highest power conversion efficiency of 4.7% was achieved using dithieno[3,2-b:2′,3′-d]thiophene attached to carbazole as the additional chromophore. All three carbazole-fused thiophene dichromophoric porphyrin dyes studied have attained increased electron lifetimes contributing to their higher open circuit voltage (VOC) compared to that of a simple porphyrin. Absorbed photon to collected electron efficiency together with charge extraction studies suggests that the performance of the carbazole-fused thiophene dyes is limited by electron injection.

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Attaching electron donors such as N-annulated perylene,8 functionalized diphenylamine,9,10 fluorenyl substitution,11 and cyclic aromatic hydrocarbons,12−14 and varying the electron acceptor species on the porphyrin core3,15−21 resulted in smaller energy gaps between the HOMO and the LUMO. However, the open circuit voltages (VOC) using these systems were often low, possibly due to increased tendency for the dyes to aggregate. Furthermore, increasing the π-conjugation is a proposed contributing factor to reduced electron lifetimes due to stronger London forces between the chromophores and the polarizable redox mediator molecules increasing their concentration near the TiO2 surface.22 While extending the light absorption range to the near-infrared part of the solar spectrum provides the most benefits to device efficiency, the additional complexity of the organic synthesis and purification may limit the commercial use of these dyes in large-scale deployment.

INTRODUCTION Porphyrins exhibit some of the highest molar extinction coefficients with values of up to 5 × 105 M−1 cm−1 at their Soret band.1 Molar extinction coefficients of the Q bands are typically an order of magnitude lower but still sufficient to absorb nearly all incident photons when thick photoelectrodes (up to 15 μm) with large surface area are used.2,3 Because of their chemical, thermal, and photophysical robustness, porphyrins are some of the most frequently studied and highly efficient sensitizers for dye-sensitized solar cells (DSSCs).4−6 The two main limitations of most simple metal porphyrins are their relatively high energy band gaps with limited absorption beyond 700 nm and the lack of significant absorption between the Soret and Q bands (typically from 450 to 550 nm), both contributing to lower absorbed photon current densities of the solar spectrum. Significant effort to lower the porphyrin highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) band gap, for example, by extending the π-conjugation of the porphyrin chromophore using the donor−acceptor dye design concept, has resulted in improved short circuit current density (JSC) up to 18.1 mA cm−2.7 © XXXX American Chemical Society

Received: January 6, 2015 Revised: February 17, 2015

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DOI: 10.1021/acs.jpcc.5b00147 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

here, also brings about its own challenges controlling dye aggregation and maintaining efficient photon to electron conversion processes. Cascaded electron transfer42,43 as well as energy transfer36,41 from the “antenna” chromophores to the electron injector chromophore are both possible mechanisms, which is similar to the three-dimensional (3D) light harvesting in natural light harvesting systems. The benefit of the multichromophoric approach was demonstrated using a series of carbazole-substituted porphyrins with a nonconjugated phenylethenyl linker between the chromophores (CZPs).39 An improved electron lifetime was attributed to a steric blocking effect by the additional chromophores and the lack of increased dispersion forces of the nonconjugated dye design.39 Despite the minimal improvement of light harvesting of visible photons by the carbazole side chain, a simultaneous increase in short circuit current and open circuit voltage leading up to 6.2% power conversion efficiency (4.4% for the simple porphyrin) has been demonstrated with I−/I3− redox couple. Using this same design platform, in this work, the light harvesting is extended into the absorption gap of the porphyrin dyes by introducing three fused thiophene units, namely, thieno[3,2-b]thiophene, dithieno[3,2b:2′,3′-d]thiophene, and 4,8-dicecyloxybenzo[1,2-b:4,5-b′]dithiophene (structures of the compounds are shown in Scheme 1). These units are frequently used in photoelectric conversion devices, particularly in low bandgap semiconducting polymers.44−49 Light absorption of a series of carbazole-fused thiophene oligomers was reported to be in the 250−450 nm wavelength range.50,51 The thiophenes used here are fused, leading to a more rigid structure and planarization of the backbone, which promotes π-electron delocalization and electronic coupling between the carbazole and fused thiophene units.52,53 UV−vis absorption shows that carbazole-fused thiophene units on the porphyrin core lead to enhanced light harvesting up to 550 nm. Density functional theory (DFT) calculations and resonance Raman spectroscopy suggest the lack of strong coupling between the two chromophores; the new absorption band is primarily attributed to photon absorption by the carbazole-fused thiophene side chain. The benefit of increased light harvesting is demonstrated by fabricating efficient photovoltaic devices using the Co2+/Co3+ electrolyte with power conversion efficiencies of up to 4.7%. This efficiency is nearly twice as large as for the unsubstituted porphyrin under the same experimental conditions. Detailed absorbed photon to collected electron and charge transport and recombination studies demonstrate that despite extensive optimization of the dye bath solution and the addition of a coadsorber charge injection losses still limit the performance, providing the scope for further improvements using the multi-chromophoric dye design aimed at filling the porphyrin absorption gap.

A second, lower impact, but potentially simpler, option to increase the light harvesting efficiency of porphyrin-sensitized solar cells is to fill the absorption gap between the Soret and Q bands. As Figure 1 shows conceptually, increasing the incident

Figure 1. Proposed concept of filling the absorption gap using the multi-chromophric molecular design. The power conversion values, calculated as JSC × VOC × FF, is predicted to increase by 30% to up to 7.5% by increasing the IPCE in the 450−550 nm wavelength range from the typical 30−80%. The IPCE in Soret and Q bands was kept at 80%. JSC was calculated by multiplying the IPCE values with the integrated AM 1.5 solar spectrum for each wavelength interval. The VOC and FF were assumed to be 750 mV and 0.75, respectively.

photon to current conversion efficiency (IPCE) from the typical 30−80% within the 450−550 nm wavelength region would result in a 30% increase in power conversion efficiency (assuming VOC = 750 mV and FF = 0.75 and remains constant). The coadsorption method or as often referred to as the “dye cocktail” approach is a rather simple procedure, when two or more dyes with complementary photon absorption are mixed in solution to prepare photoelectrodes.23−25 Rather unexpectedly, on more than one occasion, synergistic effects are observed in some cosensitized systems, when not only the short circuit current but also the open circuit voltage is increased.10,21,26−31 The synergistic effect could be related to improved injection as well as improved electron lifetime. The weakness of the coadsorption approach is the requirement of even thicker photoelectrodes, since in a simple approximation each chromophore in a cosensitized system is diluted requiring the increase of the total surface area to compensate for the dilution effect. The use of energy relay dyes, where the additional chromophores are dissolved in the redox mediator containing solution, has also been demonstrated.32,33 The proposed mechanism involved energy transfer from the relay dye to the TiO2-bound electron injecting dye. This design overcomes the dilution problem of the coadsorption approach; however, it introduces its own challenges in achieving a high quantum yield of energy transfer between the solvated chromophores and the electron injector, as well as limiting excited state quenching by the redox mediator in the solution and avoiding photodegradation.34,35 An alternate approach is to covalently link additional chromophores to the porphyrin core. This approach is often referred to as the “antenna effect” or referred here as multichromophoric dye approach.6,36−41 The main benefit of the multi-chromophoric design is that in principle, the concentration of the individual chromophores on the TiO2 surface would not decrease as long as a linearly branched design is used, where the additional chromophores are attached to the porphyrin close to perpendicular arrangement to the TiO2 surface.37 The multi-chromophoric design, as will be demonstrated

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EXPERIMENTAL SECTION I. Chemical Structure, Synthesis, and Characterization. The synthetic procedure is displayed Scheme 1. The carbazole phosphonium salt (1) was reacted, in a Wittig reaction, with dialdehydes (2a−c) using DBU as a base. The products (3a−c) were mixtures of E and Z isomers and used without further isomerization for the subsequent reactions. The yields of the reactions were moderate. The free aldehyde group was almost quantitatively reduced to the corresponding alcohols (4a−c) with sodium borohydride and converted to phosphonium salts (5a−c) with triphenylphosphonium hydrobromide. The resulting salts were reacted with porphyrin dialdehyde (6), and B


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The Journal of Physical Chemistry C Scheme 1. Synthesis of the Investigated Compounds

atoms except Zn, for which LANL2DZ was used, was utilized for structural, vibrational frequency mode, and electronic transitions (TD-DFT) calculations; the CAM-B3LYP functional was also used for TD-DFT calculations but did not give any improvement. The mean absolute deviation (MAD) between experimental and calculated Raman bands was less than 10 cm−1 in all cases, indicating the calculation is satisfactory.55−59 FT-Raman spectra were measured on solid samples using a 1064 nm Nd:YAG laser with approximately 50 mW power. Raman scattering was detected with a Bruker Equinox-55 Fourier transform interferometer equipped with a FRA106/5 Raman accessory and a D418T liquid nitrogen cooled Germanium detector, controlled with the Bruker OPUS v5.5 software package. Spectral resolution was 4 cm−1, and an average of 256 scans was used. Resonance Raman spectroscopy was performed on solutions with 1 mM in dichloromethane (DCM, 99%, Sigma-Aldrich).

the resulting isomer mixtures were isomerized with trifluoroacetic acid to pure E, E moieties (7a−c; not shown at the scheme). The reaction yields were moderate. The resulting porphyrin dyads were reacted with cyanoacetic acid in a Knoevenagel reaction to cyanoacrylic acids and in one-pot conversion into the corresponding zinc complexes (8a−c). The reaction yields were quantitative. For comparison, three carbazole-fused thiophene moieties (9a−c) were also prepared (Scheme 1) alongside with a simple porphyrin 10 without carbazole-fused thiophene substitution. The synthesis of compound 10 was reported previously.39 A more detailed synthetic procedure and the chemical analysis of the compounds are shown in SI A. Density Functional Theory. Gaussian 0954 was used to model the compounds with density functional theory. To decrease computational cost, the alkyl chains were modeled as methyl groups. The B3LYP functional with a 6-31G(d) basis set on all C


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The Journal of Physical Chemistry C The experimental procedures were described elsewhere.60−62 Excitation wavelengths 351, 406, and 413 nm were obtained from an Innova I-302 krypton ion laser (Coherent Inc.), 448 nm from a diode-pumped laser (CrystaLaser), and 457 and 488 nm from an Innova Sabre argon ion laser (Coherent Inc.), using powers of approximately 20−50 mW. In some cases, collection of spectra was not possible due to sample fluorescence. UV−visible (UV−vis) absorption spectroscopy was performed in dimethylformamide (DMF, 99.99%, Honeywell) solution using a Shimadzu UV-3600 spectrophotometer at room temperature. One cm length Quartz cuvettes and 1 μM solutions were used to measure the absorbance. Photoluminescence spectroscopy was recorded with a fluorescence (Fluorolog FL3-221, Horiba) spectrometer at room temperature. The emitted light was detected with a single photon counting interface (FluoroHub). The same solutions and 1 cm Quartz cuvette with UV−vis measurements were employed. The slit width was 3 nm, and the integral time was 0.1 s to get modest numbers of Counts Per Second (CPS). Differential pulse voltammetry (DPV) was measured with a three-electrode system using an eDAQ (BVI) instrument. A Pt wire with the surface area of 6.6 mm2 was employed as the working electrode, a slice of Pt mesh as the counter electrode and a pseudo Ag/AgCl wire as the reference electrode. The solution was used 0.5 mM of the compound and 0.1 M of the tetrabutylammonium perchlorate (TBAP, 99.0%, Fluka) supporting electrolyte, respectively, in DMF. DMF was dried by going through a column of activated alumina and purged with argon for 30 min to remove oxygen. The Ag/AgCl potential was calibrated using 1 mM freshly prepared ferrocene/ ferrocenium couple (Fc/Fc+, 98%, Aldrich) in the same solution. All the measurements were carried out in a glovebag (Atmosbag, Sigma-Aldrich). II. Dye-Sensitized Solar Cells (DSSCs) Fabrication and Characterization. Both photoanode and counter electrode used fluorine doped tin oxide glass (FTO glass, 2.2 mm, 7 Ω/square, TEC) as the substrate. The fabrication of the photoanode was comprised of a compact TiO2 layer (titanium diisopropoxide bis(acetylacetonate), 75% in isopropanol, Aldrich), a mesoporous TiO2 layer and TiCl4 solution (99.0%, Sigma-Aldrich) post-treatment. Mesoporous TiO2 layer was deposited using screen-printing of a transparent TiO2 layer (18-NRT, Dyesol) and a scattering layer (WER2-O, Dyesol) for preparing thick TiO2 electrodes. The counter electrode was fabricated using 10 mM platinic acid (H2PtCl6, 38% Pt, SigmaAldrich) in ethanol by a thermal decomposition method.39 The TiO2/FTO glass substrates, with 4 mm × 4 mm active area, were immersed into a mixed dye solution with 0.2 mM dyes for 1.5 h to achieve dye-sensitization. The mixed solvent was ethanol (99.8%, Sigma-Aldrich): toluene (99.9%, RCI Labscan) = 1:1 (vol) with 0.1 mM chenodeoxycholic acid (CDCA, Solaronix) as coadsorber. DSSCs were prepared using a Co2+/Co3+ electrolyte. The Co2+/Co3+ electrolyte consisted of 0.22 M Co2+/0.05 M Co3+ (cobalt tris(4,4′-dimethyl-2,2′-bipyridine), synthesized in house, chemical structures in Figure S9, Supporting Information), 0.05 M LiClO4 (95%, Aldrich), 0.1 M tert-butylpyridine (t-BP, 96%, Aldrich) and 1 mM CDCA in acetonitrile (AN, 99.8%, Sigma-Aldrich). For comparison, DSSCs sensitized using MK2 dye (95%, Sigma-Aldrich) were fabricated. The dye solution was 0.2 mM in AN: toluene = 1:1 (vol) with 6 h dye-sensitization.

Current density−voltage (J-V) measurements were performed using a simulated 100 mW cm−2 AM 1.5G solar simulator (TriSOL, OAI) coupled to a Keithley 2400 source measure unit. A 20 min light soaking and three times J-V measurements were applied to the finished devices before the last recording. A 6 mm × 6 mm mask was employed during the measurements. Incident photon-to-current conversion efficiency (IPCE) was recorded using a QEX10 quantum efficiency measurement system (PV measurements). Both the entrance and exit slit widths for the monochromator were 6 nm, resulting in a 0.8 mm × 0.8 mm beam dimensions which is smaller than the active area of the devices. DC mode was chosen for both calibration and measurements. The photocurrent response of the devices was recorded in 5 nm wavelength steps. Most of the default settings for the delay and measurements were used except the delay after changing wavelength was increased to 50 ms. The measured currents were referenced to a calibrated Si diode (THORLABS, LMR1/M). Light harvesting efficiency (LHE) was calculated from the UV−vis absorbance of the sensitizers on 2.9 μm TiO2 films.37,63 Small amount of electrolyte was used during the measurement to better match the UV−vis absorption measurement conditions to device operating conditions. Absorbed photon-to-current conversion efficiency (APCE) was calculated using eq 1 APCE (λ) =

IPCE (λ) = E inj (λ)Ecoll (λ) LHE (λ)

(1)

where Einj is electron injection efficiency and Ecoll is charge collection efficiency. The amount of dye loading on TiO2 was determined using 2.9 μm TiO2 films. 0.1 M Tetra-butylammonium hydroxide solution (TBAOH, 40 wt % in water, Fluka) in DMF was used to desorb the sensitizers from the surface of the TiO2 films. Stepped light-induced measurements of photocurrent and photovoltage (SLIM-PCV) and charge extraction were performed with a 635 nm diode laser as the light source. Devices after 20 min of light soaking followed by J−V measurements as described above were used for these measurements. A multimeter (ADCMT 7461A) was used to record the small photocurrent or photovoltage perturbations trigged by a Labview program. The electron lifetime (τ) as well as the matched photovoltage had been obtained during the photovoltage decay under an open circuit condition. The diffusion coefficient (D) coupled with the matched photocurrent had been obtained during the photocurrent decay under a short circuit condition.64 Electron density (ED) matched with τ and photovoltage was obtained after extracting the electrons using a nanosecond switch (AsamaLab) from illumination to dark at the open circuit condition. Electron diffusion length (L) was obtained after the aforementioned parameters (τ, D, and ED) and the ED at short circuit condition (ED_sc). The electron lifetime at short circuit condition (τ_sc) was calculated by fitting the log τ versus log(ED) plot and introduced ED_sc into the fitted mathematic equation. Therefore, the L was calculated by eq 2. L=

D τ_sc

(2)

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RESULTS I. UV−vis Absorption. Figure 2 shows the molar extinction coefficients (ε) of the compounds dissolved in dimethylformamide D


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Figure 3. Photoluminescence (PL) spectra of (a) the carbazole-fused thiophene chromophores and (b) the porphyrins on 280 nm excitation in DMF with 1 μM concentration at room temperature. The second harmonic of the excitation wavelength at 560 nm was deleted (indicated by a star) from the spectra.

Figure 2. Molar extinction coefficient of (a) the carbazole-fused thiophene chromophores and (b) the porphyrins measured in DMF.

(DMF). The wavelength of the maximum absorption (λabs max) and their ε values are listed in Table 1. The carbazole-fused thiophene chromophores 9a−c show a photon absorption band in the 320−450 nm wavelength range, with the tail of absorption extending to 490 nm (Figure 2a). The maximum ε values are in the range of (3−6) × 104 M−1 cm−1, which are similar or slightly larger than that of the porphyrin absorption in the Q bands. The absorption band of 9b containing the dithieno[3,2-b:2′,3′-d]thiophene unit is slightly red-shifted compared to that of 9a. 9c shows a higher energy absorption tail (450 nm) compared to both 9a and 9b (490 nm). All three carbazole-fused thiophene substituted porphyrins 8a−c show extended light absorption at the red-edge of the Soret band, increased ε values in the 300−400 nm wavelength region and in the Q-band region when compared to the simple porphryin 10 (Figure 2b). 8c shows a higher ε value, while both 8a and 8b show 20−35% lower values at the peak of the Soret band compared to 10. The absorption feature at the red-edge of Soret band in 8a−c is significantly more intense than the absorption of the side chain chromophore or the porphyrin at these wavelengths alone, although the onset of absorption matches that of the side chain chromophores reasonably well (see Table 1). II. Photoluminescence. Figure 3a,b shows the photoluminescence (PL) of the carbazole-fused thiophene chromophores and the porphyrin dyes dissolved in DMF and

illuminated at 280 nm, respectively. At 280 nm, both the porphyrin core and the carbazole-fused thiophene chromophores are photoexcited. The carbazole-fused thiophene chromophores 9a−c typically emit photons featuring a weak band centered at ∼380 nm and a stronger band in the 440− 500 nm wavelength range (Figure 3a). The main PL band of 9b and 9c are red-shifted in relative to that of 9a, which is consistent with their slightly red-shifted UV−vis absorption spectra (Figure 2a). The emission of the carbazole-fused thiophene side chains is efficiently quenched when covalently linked to the porphyrin core (Figure 3b), except for 8b, in which some emission from the dithieno[3,2-b:2′,3′-d]thiophene chromophore was observed. Weak emission was observed in the higher energy 380 nm band for 8a. On the other hand, emission of the porphyrin core without a significant wavelength shift, but two to three times increased intensity (under the same experimental conditions) is observed at 615 and 658 nm, quantified by the increased PL peak area (Table 1). Emission spectra of the compounds photoexcited at 480 nm (excitation of the carbazole-fused thiophene chromophores) and at 560 nm (excitation of the porphyrin core) are very similar showing only emission bands attributed to the porphyrin core (Figure S1). The lowest excited state energy gap E0−0 was determined as the intersection of the onset of normalized

Table 1. Summary of Spectral and Electrochemical Data of the Compounds λmax (nm) dye

λonset (nm) of abs

9a 9b 9c 8a 8b 8c 10

470 489 447 502 514 496 451

a

abs 298, 298, 298, 430, 430, 430, 428,

381 397 371, 562, 562, 562, 561,

392, 413 606 606 606 603

peak potential vs Fc/Fc+ PL 375, 375, 386, 616, 615, 615, 612,

437 454 462 658 658 658 658

ε (10 M 5

0.19, 0.24, 0.22, 3.10, 2.59, 4.49, 3.97,

0.27 0.34 0.40, 0.22, 0.20, 0.33, 0.17,

−1

−1

cm )

0.46, 0.39 0.15 0.14 0.23 0.09

b

PL peak area 4.48 4.21 2.23 5.24 4.94 7.99 1.94

× × × × × × ×

107 107 107 106 106 106 106

E0−0c

(eV)

2.96 2.87 2.92 2.05 2.05 2.05 2.06

ERe/V

EOx/V

−2 −1.99 −1.97 −1.97

0.35 0.28 0.33, 0.54 0.31, 0.82 0.38, 0.87 0.32, 0.86 0.3, 0.59, 0.85

ΔE/eVd

2.31 2.37 2.29 2.27

a The porphyrins 8a−c and 10 used the Soret band onset absorption wavelength (λonset). bCalculated by integrating the PL intensity (CPS) versus wavelength (400−650 nm for 9a−c while 600−700 nm for 8a−c and 10). cCalculated from the wavelength at the intersection of PL spectra with the corresponding abs spectra. dCalculated by the equation of ΔE = EOxfirst − ERefirst.

E


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Electrochemical band gaps of the porphyrin dyes are also shown in Table 1, which were found to be 0.2−0.3 eV larger than the values obtained using optical spectroscopy. IV. DFT Calculations and Resonance Raman Spectroscopy. DFT calculations predict that for the porphyrins with carbazole-fused thiophene side chains, frontier molecular orbitals (Figures 5 and S4) H-2, H-1, L+1, and L+2 are

absorption and normalized emission spectra and is displayed in Table 1. E0−0 for all porphyrin dyes is very similar (2.05 eV), whereas the carbazole-fused thiophene chromophores possess larger energy gaps of around 2.9 eV. III. Electrochemical Properties. Figure 4a,b shows the oxidation and reduction reactions of the compounds measured

Figure 4. Differential pulse voltammograms of the investigated compounds, (a) positive scan; (b) negative scan. Sample concentration: 0.5 mM; solvent: DMF; supporting electrolyte: TBAP; Pt wire surface area: 6.6 mm2; scan rate: 100 mV s−1.

by differential pulse voltammetry (DPV), respectively. Peak potentials in the DPV plots are listed in Table 1. The first oxidation potentials (anodic sweeps) of the carbazole-fused thiophene side chains 9a−c and the simple porphyrin core 10 are similar within 100 mV (EOxfirst = 0.30 to 0.38 V vs Fc/Fc+ depending on the compound). 9c containing the 4,8-dicecyloxybenzo[1,2-b:4,5-b′]dithiophene unit shows a clear second oxidation reaction, while a featureless tail is observed in both 9a and 9b at potentials above 0.3 V. The simple porphyrin 10 shows three anodic peaks, with the second and the third oxidation reactions showing approximately half the current response of the first oxidation peak. The dichromophoric porphyrin dyes 8a−c show a broader first oxidation peak compared to 10. The first oxidation peaks are not a simple superposition of the side chain chromophore and porphyrin oxidation peaks. This is more evident from the DPV plots shown in Figure S2, where the recorded current− voltage curves for the side chain chromophore, the porphyrin core and the dichromophoric dyes are overlapped. The onset of the oxidation of the dichromophoric dyes is shifted by approximately 100 mV to lower potentials compared to the porphyrin core alone. The second oxidation peaks of 8a−c are found at similar potentials (0.85 V vs Fc/Fc+). Figure 4b shows (cathodic sweeps) that only the porphyrin core is reduced in the applied potential window of up to −2.2 V vs Fc/Fc+. The reduction potentials of 8a−c are quite similar to that of 10, i.e., −1.97 to −2.00 V vs Fc/Fc+, with some smaller changes in the shape, but not in the potential range or current magnitude upon addition of the carbazole-fused thiophene side chains.

Figure 5. Calculated frontier molecular orbitals of 8c.

predominantly based on the porphyrin core. These resemble the traditional a1u, a2u, and eg orbitals of Gouterman’s 4-orbital model.65 The HOMO is based on the whole side chain, and L+3 on the fused-thiophene portion of this, while the LUMO is predominantly located on the cyanocarboxylate TiO2 binding group (Figures 5 and S4). TD-DFT calculations show transitions between porphyrin MOs in the Soret band region (Table 2). The HOMO to L+3 transition can essentially be described as a mixture of carbazole to fused-thiophene (as the carbazole loses electron density) and thiophene π, π* transitions. The Q-band region is also predicted to show porphyrin-based transitions, which is consistent with what is known about porphyrin behavior. MO energies are relatively similar between the compounds, with a few exceptions, most significantly for L+3 (Figure 6). The oxidized species were also optimized with DFT, and this showed that the hole (cation singly occupied molecular orbital, SOMO) was localized on the carbazole-fused thiophene side chain. The carbazole-fused thiophene side chains were also investigated, F


Article 4,0 (−4) 82,60 (−22) 2,39 (+37) H-2→L+1 (40%), H-1→LUMO (38%) 0.109 72,1 (−71) 6,25 (+19) 0,73 (+73)

Figure 7. Resonance Raman spectra of 8c measured in CH2Cl2 at excitation wavelengths across the Soret band and new transition, with approximately 1 mM concentration. FT Raman spectra were recorded on solid samples. Solvent bands are marked *.

the Soret band, spectra are dominated by bands at 1600 and 1615 cm−1 (8a); 1600 and 1620 cm−1 (8b); and 1576 and 1621 cm−1 (8c). These are vibrations of the cyanocarboxylate and the carbazole unit, respectively. At 406 and 413 nm, the Soret band is directly probed, and spectra are dominated by porphyrin bands at 1002 (ν6), 1074 (ν9), 1235 (ν1), 1356 (ν4), and 1551 (ν2) cm−1. The enhancement of porphyrin bands in the Soret region is consistent with transitions that are both expected and predicted by TD-DFT. Finally, to the red of the Soret band, carbazole-fused thiophene side chain bands are again enhanced, but these are either different bands or in different intensity ratios to the blue edge. For example, 8a shows bands at 1181 (porphyrin or thiophene), 1417 (delocalized side chain), and 1497 (delocalized) cm−1, as well as the 1600 and 1615 cm−1 modes (Figure S5). The enhancement of these side chain modes is consistent with the

0.236 634

H-2→L+1 (11), H-1→LUMO (80) HOMO→LUMO (93) 0.261 577 560 605

2.29 475

0.839 374

475

0.929 373 430

10,1 (−9) 83,34 (−49) 3,65 (+62)

22,0 (−22)

558

4.47 413 22,15 (−7) 71,73 (+2) 7,11 (+4) 0,1 (+1)

9,7 (−2) 48,28 (−20) 43,48 (+5) 1,17 (+16)

4,0 (−4)

H-2→L+1 (15%), HOMO→LUMO(13%), HOMO→L+3 (50%)

25,29 (4)

12,2 (−10)

17,10 (−7) 57,47 (−10)

4,0 (−4)

0,15 (+15)

thiophene

12,1 (−11)

porphyrin

83,57 (−26)

binder

1,42 (+41) H-2→LUMO (57%), H-1→L+1 (30%), HOMO→L+1 (12%)

CI (% contribution) f

1.21 369 11,22 (+11) 85,57 (−28)

thiophene

1,16 (+15)

2,4 (+2)

calc/ nm

all showing strong HOMO to LUMO transitions as being lowest in energy, in which the HOMO is delocalized over the thiophene and carbazole, and the LUMO is localized to the thiophene. Resonance Raman spectra collected across the Soret band region showed three different patterns of band enhancement (Figure 7). First, at 351 nm excitation, which is to the blue of

porphyrin

carbazole

Figure 6. Calculated energy levels of frontier molecular orbitals.

binder CI (% contribution) f calc/nm expt/nm

change in Mulliken population B3LYP

Table 2. Experimental and Calculated Electronic Absorption Data for 8c

H-4→L+1 (10), H-2→L+2 (24), H-2→L+3 (23), H-1→L+1 (20) H-4→L+2 (28), H-4→L+3 (12), H-2→L+1 (15), HOMO→L+4 (10) HOMO→L+3 (92)

CAM-B3LYP

Change in Mulliken population

carbazole


G


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The Journal of Physical Chemistry C broadening of the Soret band due to transitions between side chain MOs. 8c shows even more bands enhanced to the red of the Soret band, namely, those at 1125, 1299, 1312, and 1576 cm−1 (Figure 7), but these still have side-chain character and thus are consistent with the same transition type. The nature of vibrational modes is determined by comparison with the spectra of the carbazole-fused thiophene components alone (Figure S6) and DFT frequency calculations. V. Photovoltaic Performance. Optimization of DSSCs fabrication was carried out first by determining the most appropriate solvent for dye-sensitization, followed by optimizing the TiO2 thickness using an I−/I3− electrolyte (Table S3). The best solvent was found to be ethanol: toluene = 1:1 (vol.) with 0.1 mM chenodeoxycholic acid (CDCA), which showed increased open circuit voltage (VOC) and slightly enhanced short circuit current density (JSC) for most dyes. Increasing the CDCA concentration beyond 0.1 mM had no significant effect on the DSSC performance. Using 7.4 μm thick TiO2 electrodes achieved higher JSC and VOC compared to both thinner (2.9 μm) and thicker (17.5 μm) films. The best fabrication conditions obtained using the I−/I3− electrolyte (power conversion efficiency of 5.5%) were further optimized using the Co2+/ Co3+ redox couple (cobalt tris(4,4′-dimethyl-2,2′-bipyridine), chemical structures in Figure S9). Reducing the concentration of Co2+/Co3+ by half resulted in much lower fill factors (FFs). Adding CDCA to the Co2+/Co3+ electrolyte resulted in more than 10% increase in JSC and slightly decreased VOC (Table S4). The optimized conditions were found to be ethanol/toluene = 1:1 (vol) with 0.1 mM CDCA as solvent, 7.4 μm thick TiO2 electrodes and the Co2+/Co3+ electrolyte comprised of 0.22 M Co2+, 0.05 M Co3+, 0.05 M LiClO4, 0.1 M tert-butylpyridine (t-BP) and 1 mM CDCA in acetonitrile (AN). The current density−voltage (J−V) curves comprising the porphyrin dyes 8a−c and 10 and the optimized Co2+/Co3+ electrolyte are shown in Figure 8a,b and using thin (2.9 ± 0.1 μm) and thick (7.4 ± 0.5 μm) TiO2 films, respectively. The best results are displayed, while Tables S5 and S6 in the Supporting Information show the performance of all devices prepared, showing good reproducibility. For comparison, the performance of DSSCs prepared using commercial MK2 dye (2-cyano-3-[5‴-(9-ethyl-9H-carbazol-3-yl)-3′,3″,3‴,4-tetra-nhexyl-[2,2′,5′,2″,5″,2‴]-quater thiophen-5-yl] acrylic acid) as a reference is also shown in Tables S5 and S6. All dichromophoric porphyrins 8a−c show increased JSC and higher VOC values compared to that of the single porphyrin 10 under the same experimental conditions (Figure 8a). The highest efficiency 8b shows a 40% increase in JSC and a 100 mV higher VOC compared to 10, leading to an overall 70% higher power conversion efficiency (η). 8c shows the lowest performance among the dichromophoric dyes with 5.84 mA cm−2, which is 20% lower than that of 8a and 8b. FFs are quite similar among 8b, 8c, and 10 (0.68), while that of 8a is lower (0.62). The solar cell performance trend is similar when using thick TiO2 films (Figure 8b). The difference in JSC between dichromophoric porphyrins 8a−c, and the simple porphyrin 10 is even larger compared to that of thin films. For example, 8b (9.25 mA cm−2) attains a 55% increase in JSC compared to 10 (5.95 mA cm−2). The VOC increase using 8a−c is retained compared to 10 using thick TiO2 films, and the FF does not show much variation. The best η is achieved by 8b showing ∼4.7%, which is nearly twice as much as the performance of the simple porphyrin 10 using the same conditions. In comparison,

Figure 8. Current density−voltage (J−V) curves of DSSCs with the porphyrin dyes fabricated using (a) 2.9 ± 0.1 μm transparent and (b) 7.4 ± 0.5 μm with 5 μm transparent +2.5 μm scattering TiO2 under AM 1.5 illumination (solid lines) and in the dark (dashed lines). The inset shows the photovoltaic performance for each device.

MK2-sensitized solar cells outperform all porphyrin dyes both in JSC and VOC reaching a maximum η of 5.7% using 7.4 ± 0.5 μm TiO2 films (Table S5). VI. Incident Photon-to-Current Conversion Efficiency. Incident photon-to-current conversion efficiency (IPCE), light harvesting efficiency (LHE), and absorbed photon-to-current conversion efficiency (APCE) measurements are displayed in Figure 9 using 2.9 μm transparent TiO2 electrodes. Compared to the simple porphyrin 10, the dichromophoric porphyrins 8a−c show enhanced IPCE at most wavelengths except the peak of Soret band at 400−440 nm (Figure 9a). The main difference among 8a−c arises from the differences in IPCE values above 450 nm. 8b shows a broader IPCE in the Soret band compared to that of 8a and 8c, i.e., better filling of the absorption band between the Soret and Q bands. The dichromophoric porphyrins achieve less peak IPCE values (55%) compared to that of 10 (60%) at the maximum IPCE in the Soret band. 8b shows the highest IPCE at the Q bands, followed by 8a, 8c, and 10. The dichromophoric porphyrins 8a−c show 40−80 nm broadened light harvesting efficiency within the absorption gap between the Soret and Q bands and more than 10% increased LHE values in the Q bands compared to 10 (Figure 9b). 8b shows the most extended spectrum among dichromophoric porphyrins. These features mirror the trends in the UV−vis absorption spectra in solution (Figure 2), but with a noticeable broadening when bound to the TiO2 films. Dye loading on TiO2 films is shown in the inset of Figure 9c. Although the amount of 8c bound to the TiO2 is less compared to 8a and 8b, the LHE spectra at Soret and Q bands are unaffected. The APCE peak values of dichromophoric porphyrins 8a−c at the Soret band are close to 75% (Figure 9c), whereas the Q bands show slightly lower efficiencies (8a: 55%, 8b: 60%, and H


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Figure 10. (a) Electron lifetime and (c) diffusion coefficient versus short circuit current density density, (b) electron lifetime and (d) open circuit voltage versus electron density for the porphyrin dyes and MK2 (two samples for each dye). Film thickness: 2.9 ± 0.1 μm.

compared to 8b, an 80 mV downward shift in the VOC versus log ED plot and lower electron diffusion coefficient at matched JSC. Figure 11 shows the calculated electron diffusion length (L) versus the electron density measured at short circuit conditions

Figure 9. (a) IPCE, (b) LHE, and (c) APCE spectra of the porphyrin dyes on thin TiO2 films. The inset of (c) shows the dye loading (Γ). Film thickness: 2.9 ± 0.1 μm; the unit of Γ is mol cm−3. Figure 11. Diffusion length (L) versus electron density at short circuit (ED_sc) of dichromophoric porphyrins 8a−c and 10 using 3.1 μm TiO2 films.

8c: 50%). In comparison, 10 shows 10% higher peak APCE values than any of the dichromophoric porphyrins. APCE values of 10 in Q bands are similar to 8b and slightly higher than 8a and 8c. VII. Electron Lifetime, Diffusion Coefficient, and Diffusion Length. Figure 10 shows the results obtained from stepped light-induced measurements of photocurrent and photovoltage (SLIM-PCV) and charge extraction measurements. The electron lifetime (τ) is plotted versus both short circuit current density (JSC) and electron density (ED) (Figure 10a,b). DSSCs fabricated using the dichromophoric porphyrins 8a−c obtained higher electron lifetimes compared to that of the simple porphyrin 10 at matched short circuit current density (Figure 10a) or at matched electron density (Figure 10b). 8b showed the longest τ among the dichromophoric porphyrins, more than an order of magnitude longer than 10 at matched electron density ED = 3 × 1017 cm−3. The electron diffusion coefficient (D) is very similar at matched JSC (Figure 10c). The relation between VOC and log ED in Figure 10d also shows a very similar slope and y-axis intercept. MK2-sensitized solar cells used in this work as a reference, demonstrate an order of magnitude longer electron lifetime

(ED_sc) using a charge extraction method. All porphyrins show decreased L with increasing ED_sc, which is similar to the trend reported previously for carbazole-substituted porphyrin dyes.39 Charge collection efficiencies (Ecoll) at the shortest L calculated using eq 3 are 94% for 8a, 96% for 8b, 94% for 8c, and 90% for 10, respectively.66,67 Ecoll = tanh(ω/L)/(ω/L)

(3)

where ω is the thickness of the TiO2 film.

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DISCUSSION The aim of synthesizing a series of dichromophoric carbazolefused thiophene porphyrin dyes was to fill the light absorption gap between the Soret and Q bands of the porphyrin. This aim has been largely, albeit not completely, achieved by covalently attaching thieno[3,2-b]thiophene, dithieno[3,2-b:2′,3′-d]thiophene and 4,8-dicecyloxybenzo[1,2-b:4,5-b′]dithiophene units using a phenylethylene linker at the meso-position of the porphyrin core. I


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Table 3. Contribution to Frontier Molecular Orbitals from Each Component Group, Listed in the Order: Binding Group, Porphyrin, Thiophene, Carbazole compound

L+3

L+2

L+1

LUMO

HOMO

H-1

H-2

8a 8b 8c

1,9,71,19 1,11,72,15 1,8,76,16

25,64,9,1 25,62,12,1 26,66,8,1

1,99,1,0 1,99,1,0 1,99,1,0

73,25,1,0 73,25,1,0 73,25,1,0

0,5,63,32 0,4,65,31 0,3,74,23

4,84,7,5 4,84,7,5 4,86,7,3

0,99,0,0 0,99,0,0 0,99,0,0

the four porphyrins studied. It can be unequivocally determined that buckling does not occur, as the ν4 band does not shift. For efficient charge photogeneration, photoexcitation from the side chain chromophore needs to be efficiently channelled to the electron injecting chromophore either by cascaded electron or energy transfer. Given the energy diagrams of the dichromophoric porphyrins drawn using the electrochemical and photophysical data (Figure S3), both mechanisms are possible. The quenched side chain and enhanced porphyrin emission (Figure 3) suggest that Förster resonance energy transfer (FRET) may be operable.69−71 Indeed, the broad emission from the carbazole-fused thiophene chromophores has some overlapping wavelength regions with the Soret and Q bands absorption of the porphyrin core. Internal conversion, as an alternative mechanism to energy transfer, is unlikely due to the lack of strong electronic coupling between the side chain chromophores and the porphyrin core. Either way, the IPCE/APCE measurements of 8a−c indicate efficient injection and charge collection from wavelength regions attributed to the carbazole-fused thiophene side chain absorption, although with APCE values 10−20% lower than that of the Soret band. Further photoluminescence lifetime and femtosecond transient absorption studies are needed to determine the exact charge separation mechanism. Importantly, it is demonstrated the filling of the absorption gap leads to a much improved device efficiency using the Co2+/Co3+ electrolyte, typically requiring thin TiO2 films due to shorter electron lifetime. Considering our concept in Figure 1, we have achieved a nearly 2-fold increase in JSC employing 8b. The increased JSC is clearly attributed to the increased light harvesting efficiency (Figure 9b), although the scale of improvement is not as much as predicted using the simple calculation in Figure 1. This is mainly due to the incomplete filling of the absorption gap, as well as some other losses that will be discussed below. Despite our best effort to optimize the dye uptake conditions and using CDCA as a coadsorber (Supporting Information SI E), the APCE values are 25−40% lower than the maximum achievable 100%. Microsecond to millisecond transient absorption measurements in both the absence and the presence of the Co2+/Co3+ redox couple indicated that the regeneration of the dye cation is more than 2 orders of magnitude faster than the recombination of the dye cation with the electrons in TiO2, which suggests a close to 100% regeneration yield of 8a−c (not shown). The lower APCE efficiency may originate from lower injection and/or charge collection efficiencies. We have shown previously that dye-sensitized solar cells fabricated using the simple porphyrin 10 and I−/I3− electrolyte suffer from charge injection losses attributed to dye aggregation and/or the presence of noninjecting dyes on the TiO2 surface.39 This limitation is also evident when the Co2+/Co3+ electrolyte is used as the maximum APCE in the Soret band is only 80%. Calculations of the electron diffusion length and the observation that JSC does not increase significantly between 2.9 and 7.4 μm

All three dichromophoric porphyrins show extended light absorption compared to 10. The enhancement in the UV part of the spectrum (280−415 nm) can be attributed to absorption by the carbazole fused-thiophene chromophores, while the additional absorption within the 435−500 nm region, which is also present in the carbazole-fused thiophene units, is enhanced in the dichromophoric dyes. Structures optimized with DFT calculations show that the inclusion of the carbazole-fused thiophene side chain does not alter the dihedral angle between the porphyrin and the side chain (by comparison to 10), which is approximately 65 degrees for both the cyanocarboxylate and carbazole-fused thiophene groups. The porphyrin core has a cyanocarboxylate group, and therefore the LUMO is effectively unchanged between the dyes; the inclusion of the carbazolefused thiophene side chains alters the nature of L+3 and H-2, and therefore changes their energy. Resonance Raman spectroscopy and TD-DFT calculations suggest that the 435−500 nm absorption is due to electronic transitions within the carbazole-fused thiophene side chain, in particular, H-3 or HOMO to L+3 transitions. Although the isolated side chains (thiophene and carbazole) do not absorb to such low energy, it is expected that the HOMO−LUMO gap of the side chain moiety is reduced by attachment to the porphyrin, as the conjugation is extended by the addition of a phenyl ring and a vinyl linker. This is supported by DFTcalculated orbital energies (Figure S7), where equivalent orbitals (LUMO of side chain, L+3 of porphyrin, the HOMO remains the same) indeed show a reduction in energy when joined to the porphyrin core. The lower onset of oxidation of 8a−c in Figure 4a provides further support to this argument. If each atom in a structure is assigned as belonging to one of four groups: binding group, porphyrin, thiophene, and carbazole; then the calculated MOs may be divided into percentage contributions from each group in a Mulliken population analysis (Figure S8D). This is shown for the frontier molecular orbitals of 8a−c in Table 3. With respect to the mixing of MOs, most are based either on the porphyrin and binding unit or on the carbazole-fused thiophene side chain. In all cases, L+3, L+2, and H-1 show the most mixing, but this is very similar between the three compounds (Table 3 and Figure S8). This suggests a small amount of electronic communication is possible. Enhanced UV−vis absorption as well as the slight shift of the onset of oxidation of the carbazolefused thiophene in 8a−c (Figure S2) also suggest that weak electronic communication exists between the two chromophores. The most significant difference is that 8c shows a greater contribution from the thiophene unit to the HOMO and less from the carbazole than 8a and 8b. The maxima emission wavelengths (λFL max) do not show significant difference between the dichromophoric porphyrins and 10, suggesting that the electronic structure of the lower excited state of the dichromophoric porphyrins is similar to that of 10.68 Resonance Raman supports this, as the ν4 band, which is often called the core marker band because it reflects changes due to distortion of the porphyrin ring, is not shifted between J


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The Journal of Physical Chemistry C TiO2 films suggest that in addition to injection losses, charge collection losses may also limit the JSC obtained using 10. This is evident in the lower APCE values obtained using 10 using thicker films (Figure S10). The origin of the reproducible, 5−10% variation in APCE values among 8a−c in the Q bands using ∼3 μm TiO2 films (Figure S10) is discussed below. The APCE is limited by the electron injection efficiency. The calculated diffusion lengths using 8a−c are longer than using 10 due to the increased electron lifetime, which, alongside the increased JSC with increasing TiO2 film thickness, suggest that charge collection losses may contribute only a few percent to the decreased APCE (Figure S10). It is an important result of this work that the electron lifetime of all dichromophoric porphyrin dyes has improved compared to 10, which leads to larger open circuit voltages providing additional contributions to improved device performance beyond increased light harvesting. This simultaneous improvement of JSC and VOC is similar to the previous report of carbazole-substituted porphyrin dyes,39 attributed to enhanced blocking effect by the bulky dichromophoric porphyrin structure. Even still, the electron lifetime of 8a−c is an order of magnitude lower than that of MK2-sensitized solar cells, suggesting there is much more room for improvement. The lower injection efficiency may originate from the presence of noninjecting dyes,72 a too small difference between 73 or a fast the lowest excited state energy (Efirst Re ) and the ECB component of the recombination kinetics between conduction band electrons and dye cations.74−76 Dye cation recombination was checked using a nanosecond (ns) transient absorption setup, showing no decline of the dye cation absorption signal on the 1−100 ns time scales (not shown), suggesting that dye cation recombination is not limiting the APCE. However, recombination on a much faster, picosecond time scales is not ruled out. To check the effect of energy difference between the Efirst Re and the ECB, DSSCs sensitized with 8a−c without the addition of t-BP to the electrolyte were prepared (Supporting Information SI E). t-BP was shown to negatively shift the TiO2 conduction band bottom edge and hence removal of t-BP from the electrolyte is expected to increase the difference between 77−80 the Efirst This is indeed what we have also Re and the ECB. observed (Figure S11) showing a 130 mV positive shift of the VOC versus log ED plot when t-BP is removed. The removal of t-BP also resulted in increased JSC, IPCE, and APCE (Figures S12 and S13) to almost the same level as the MK2-sensitized solar cells. This suggests that the injection limitation in Figure 9c of 8a−c is indeed largely originates from a TiO2 CB bottom edge position that is too high for efficient injection using the optimized electrolyte. The estimated energy difference between the first reduction potential ERefirst and the TiO2 CB bottom edge ECB is 0.78 eV (74.9 kJ mol−1) for 8a−c (Figure SI3). The first reduction potential of MK2 is approximately 0.20−0.25 V more negative compared to 8a−c, which together with the positively shifted ECB of the MK2-sensitized solar cells (Figure 10d) using the same electrolyte is consistent with the above explanation. We note that the reduction potentials of 8a−c were measured in DMF, while MK2 was measured in a dichloromethane (DCM) solution due to solubility limitations of 8a−c in DCM. The VOC is much lower when no-tBP was used due to the positive shift of the quasi-Fermi levels of TiO2 electrons, so overall, the power conversion efficiency has not improved. We note that the APCE of 8c is reproducibly 5−10% lower than 8a and 8b even when no t-BP is used (Figure S13).

The origin of this difference is not yet clear. The dye uptake of 8c is only half amount of 8a−b (Figure 9 inset), which may also explain its lowest electron lifetime among the series (Figure 10). Low dye loading may originate from weaker dye binding to TiO2, which in turn may lead to the presence of noninjecting dye on the surface.72 Noninjecting dyes contribute to light absorption, but do not inject electrons efficiently due to weak electronic coupling with the acceptors states in the TiO2. These above results highlight the additional complexities involved in using the multi-chromophore approach, requiring elaborate optimization of the preparation conditions, for example, dye uptake conditions, use of coadsorber to reduce aggregation and electrolyte composition. The results, however, demonstrate with clarity that filling the absorption gap between the Soret and Q bands of the porphyrin by using carbazole-fused thiophene units covalently attached to the porphyrin core is a viable strategy to increase JSC, electron lifetime, and consequently VOC.

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CONCLUSION The light absorption gap between the porphyrin Soret and Q bands is filled by attaching three different carbazole-fused thiophene chromophores to the zinc porphyrin core via a phenylethenyl linker. UV−vis absorption, photoluminescence, and Raman spectroscopy, coupled with TD-DFT calculations, suggest the additional absorption at the red edge of the Soret band is due to electronic transitions between molecular orbitals located on the carbazole-fused thiophene side chain. Both short circuit current and open circuit voltage have increased using this series of carbazole-fused thiophene substituted porphyrins in dye-sensitized solar cells using the Co2+/Co3+ redox electrolyte compared to using a simple porphyrin. The 2-fold increased JSC is attributed to the enhanced light harvesting efficiency above 450 nm and, to a lesser extent, to increased charge collection efficiency using thicker TiO2 films. The increased VOC is attributed to the increased electron lifetime. The highest performance using a dichromophoric porphyrin in this study was 4.7%, which is lower than that obtained using MK2, but almost twice as much as using a simple porphyrin. These results provide important insights into the design of multi-chromophoric porphyrin dyes for efficient dye-sensitized solar cells using thinner TiO2 electrodes, where maximizing light harvesting efficiency is paramount. An important result is APCE values close to 100% originating from light absorption from the carbazole-fused thiophene sides chains within the 450−550 nm wavelength region, suggesting the promise of the multi-chromophoric approach as a general design strategy. Under optimized electrolyte conditions maximizing the power output of the solar cell, APCE values lower than 100% are measured, which is attributed to a first reduction potential that is too low for efficient electron injection.

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ASSOCIATED CONTENT

S Supporting Information *

Material synthesis, photoluminescence spectra, differential pulse voltammetry, energy diagram, detailed DFT calculations and resonance Raman spectroscopy, detailed photovoltaic characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +61242981429. K


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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.J.M., K.C.G., P.W., S.M., and T.M.C. acknowledge support from the Australian Research Council (ARC) through Discovery Project no. DP110102201 and ARC Centre of Excellence for Electromaterials Science. A.J.M. acknowledges funding for Australian Research Fellowship (ARF). A.J.M. and P.W. acknowledges support from ANFF. H.V.D.S. and K.C.G. acknowledge the University of Otago and the MacDiarmid Institute for Advanced Materials and Nanotechnology for funding.

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