Research Article

Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency

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Science  04 Nov 2011:
Vol. 334, Issue 6056, pp. 629-634
DOI: 10.1126/science.1209688

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Abstract

The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, we report mesoscopic solar cells that incorporate a Co(II/III)tris(bipyridyl)–based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific molecular design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocrystalline titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 volt. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another organic dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight.

Dye-sensitized solar cells (DSCs) have recently received great attention because of their ease of fabrication and cost-effectiveness compared with silicon (Si)–based photovoltaic devices (1). So far, the best conversion efficiencies have been obtained with ruthenium-based dyes used together with the iodide/triiodide redox couple (210). Upon optical excitation, the dye injects an electron into the conduction band of a nanocrystalline film of a wide band gap oxide, such as titanium dioxide (TiO2), and is subsequently regenerated back to the ground state by electron donation from a redox couple present in the electrolyte. In this fashion, the DSC achieves the separation of light harvesting and charge generation from charge carrier transport, whereas all other known photovoltaic devices, including Si cells, perform both operations simultaneously. This imposes stringent demands on the puritiy of the semiconductor—99.9999% for solar grade Si—resulting in high material cost and long energy payback times. The separation of the functions of light absorption and carrier transport achieved by the DSC has many advantages, key points being a large choice of light absorber and charge transport materials, which can be applied by simple screen printing methods and do not have to undergo costly purification or doping treatments. Although the DSC already outperforms its competitors in ambient light and indoor conditions, its validated solar-to-electric power-conversion efficiency (PCE) under standard air mass 1.5 (AM 1.5) reporting conditions (1000 W/m2 solar light intensity and 298 K) of 11.1% (2) is still a factor of 2 below that of Si solar cells. (The air mass number expresses the ratio of the path length of the solar light in the atmosphere over that corresponding to vertical position of the sun. At air mass 1.5, the sun is at an angle of 48.19° with regard to the vertical axes).

Because of an excessive loss of voltage during the dye-regeneration reaction, the use of the iodide/triiodide electrolyte as a redox shuttle limits the attainable open-circuit potential, Voc, to 0.7 to 0.8 V and is thus a drawback of current DSC embodiments. This has prevented substantial gains in PCE over the last 5 years (11). To further improve the PCE, development of redox mediators exhibiting higher reduction potentials than that of I3 is warranted; Co(II/III)tris(bipyridyl) complexes do have this property. However, such one-electron, outer-sphere redox couples often yield shorter electron lifetimes when used in the DSC. This faster interfacial charge recombination, when compared with iodide/triiodide–based redox electrolytes (1218), lowers the photovoltage and reduces the efficiency of charge collection, decreasing the short circuit photocurrent density (Jsc) and hence the PCE (1921). Recent evidence suggests that the introduction of long-chain alkyloxy groups in the dye structure may retard the unwanted charge recombination process (22). Donor-π-bridge-acceptor (D-π-A) sensitizers, endowed with such groups, recently reached Voc values exceeding 0.8 V when used with Co(II/III)tris(bipyridyl) redox electrolytes. However, the PCEs of these devices remained in the 6.7 to 9.6% range because of their insufficient solar light harvesting, resulting in low photocurrents (2224). This rationale has inspired us to synthesize a D-π-A zinc (Zn) porphyrin dye, designated YD2-o-C8, which absorbs light over the whole visible range and is endowed with long-chain alkoxy groups so as to impair interfacial back electron transfer reaction. When used in conjuntion with cobalt polypyridyl–based redox electrolytes, YD2-o-C8 achieves conversion efficiencies exceeding those obtained with today’s best ruthenium sensitizers.

Inspired by the important role that porphyrins play in natural photosynthesis, researchers have tested numerous derivatives of this substrate class as sensitizers for DSCs, but conversion efficiencies obtained so far have largely remained below 8% (2533). An exception is the class of D-π-A porphyrin dyes, such as YD2, which has reached a PCE of 11% when used with iodide/triiodide redox electrolyte (34). A diarylamine group attached to the porphyrin ring acts as an electron donor, and an ethynylbenzoic acid moiety serves as an acceptor, anchoring the dye to the titania surface. The unique feature of these sensitizers is that the porphyrin chromophore itself constitutes the π-bridge of the D-π-A structure (3540). The judiciously tailored variant of YD2 that we report here,YD2-o-C8 (Fig. 1), incorporates two octyloxy groups in the ortho positions of each meso-phenyl ring, producing a striking amelioration of the photo-induced charge separation in DSCs using Co(II/III)tris(bipyridyl)–based redox electrolyte. High PCE values reaching 11.9% have been achieved with this molecular photovoltaic system, which produces a Voc of 965 mV, a Jsc of 17.3 mA/cm2, and a fill factor (FF) of 0.71 under standard AM 1.5 sunlight at 995 W/m2 intensity. The cosensitization of YD2-o-C8 with the previously prepared organic D-π-A dye, coded Y123, yielded an efficiency of 12.3% when used in conjunction with the Co(II/III)tris(bipyridyl)–based redox electrolyte. The PCE even exceeds 13% under AM 1.5 solar light of 500 Wm−2 intensity.

Fig. 1

The molecular structures of the (left) YD2 and (right) YD2-o-C8 porphyrin dyes.

Molecular properties and photovoltaic performance. The detailed synthetic procedure for preparing the YD2-o-C8 porphyrin is described in the supporting online material (SOM). Absorption spectra of these two porphyrin dyes measured in tetrahydrofuran (THF) solvent are shown in fig. S1, and table S1 lists the corresponding spectral properties. Both porphyrins exhibit maxima in the 400 to 500 nm and 550 to 750 nm ranges, corresponding to the Soret and Q bands, respectively, with similar molar absorption coefficients. Nevertheless, the Soret band of YD2-o-C8 is slightly red-shifted, and its Q band is narrower as compared with the YD2 spectrum. This is ascribed to the electronic effect of the two electron-donating dioctyloxy substituents introduced in ortho-position of the meso-phenyl ring. The emission behavior for the two sensitizers matches their absorption spectra, the fluorescence maximum of YD2-o-C8 being blue-shifted by 13 nm relative to that of YD2. The oxidation potentials of the two dyes were determined by means of cyclic voltammetry (CV). YD2-o-C8 shows two oxidation waves at half-wave potential (E1/2) = +0.82 and +1.37 V versus normal hydrogen electrode (NHE) (fig. S2). These values are lower than the corresponding potentials of YD2 (table S1) because of the electron-donating substituents on the meso-phenyl rings. The 190-mV negative shift of the reduction potential of YD2-o-C8 corresponds to a lifting of the lowest unoccupied molecular orbital (LUMO) level and is a manifestation of the same effect.

To gain further insight into the electron density distribution within the frontier and other close-lying orbitals, we performed quantum chemical calculations on the YD2-o-C8 porphyrin using density functional theory (DFT) at the B3LYP/6-31G(d) level [Spartan 08 package (Wavefunction, Irvine, CA)] and compared them with the calculated electronic structure of the YD2 dye (fig. S3). In the ground state, the electron density of YD2 for the highest occupied molecular orbital (HOMO) and HOMO-1 is shared by the diphenylamine donor moiety and the π-system of the porphyrin ring. Introduction of the strongly electron-donating group (octyloxy) at the ortho-positions of phenyl rings, attached at the meso-positions of porphyrin core in YD2-o-C8, increases the electronic density on the porphyrin π-system over tert-butyl groups present at the HOMO and HOMO-1 level of the YD2 dye. Thus, for YD2-o-C8 there is a considerable electronic coupling between the alkoxy groups and the porphyrin core, lifting the LUMO level higher than that of the YD2 dye. The predicted effect is an increase of the HOMO-LUMO gap of YD2-o-C8 compared with YD2 dye, which is in good agreement with the experimental observations.

Photovoltaic experiments were conducted with an electrolyte by using the Co(II/III)tris(bipyridyl) tetracyanoborate complexes as redox couple. The optimized electrolyte composition, designated AY1, consists of 0.165 M [CoII(bpy)3](B(CN)4)2, 0.045 M [CoIII(bpy)3](B(CN)4)3 along with 0.8 M tert-butyl pyridine (TBP), and 0.1 M LiClO4 in acetonitrile as a solvent. Details on the optimization of the concentration for the electrolyte components are presented in fig. S4. Using the Nernst equation and standard potential (Eo) = 0.57 V versus NHE for the experimentally determined (23) standard potential of the Co(II/III)tris(bipyridyl) tetracyanoborate couple, the oxidation potential for this electrolyte is derived as 0.535 V versus NHE.

Performance in a Co(II/III)tris(bipyridyl)–mediated solar cell. Photocurrent density versus voltage (J-V) is shown in Fig. 2A; the curves were measured under standard photovoltaic reporting conditions (AM 1.5 global sunlight at 1000 W/m2 and a temperature of 298 K) for cells with YD2-o-C8 or YD2 sensitizers used in conjunction with the AY1 electrolyte. YD2-o-C8 gives a PCE of 11.9% compared with only 8.4% for the reference dye YD2. The key observation here is that a subtle modification of the porphyrin structure induces strikingly large Jsc and Voc improvements. This performance advantage is maintained over a large range of light intensities, ranging from 10% to full solar intensity (Table 1).

Fig. 2

(A) Comparison of the J-V curves of YD2-o-C8– and YD2-sensitized solar cells under full AM 1.5 solar intensity. (B) Incident photon-to–electric current conversion efficiencies as a function of wavelength for the two dyes (YD2-o-C8, red; YD2, blue). The TiO2 film thickness is a 5-μm transparent layer and on top a 5-μm scattering layer.

Table 1

Detailed photovoltaic parameters of the devices made with the dyes YD2 and YD2-o-C8 and cobalt-based AY1 electrolyte at different light intensities. Pin, incident intensity of AM1.5 solar light.

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The incident monochromatic photon-to–electric current conversion efficiency (IPCE) as a function of wavelength and J-V curves for the same YD2 and YD2-o-C8 devices is presented in Fig. 2. YD2-o-C8 shows impressively high IPCE values over the whole visible wavelength range, exceeding 80% from 450 nm up to 680 nm, except for a narrow dip in the spectrum around 530 nm. The spectral response of the photocurrent obtained with the YD2 cell shows similar features, with peaks at around 460 and 650 nm. However, the absolute IPCE values are 20 to 30% lower for YD2 as compared with YD2-o-C8, despite its higher molar extinction coefficient, which is in keeping with the lower short-circuit photocurrent densities observed for YD2 in Fig. 2A. The Jsc values obtained from calculating the overlap integral of the IPCE spectrum with the standard AM 1.5 global spectral solar photon flux were 17.3 and 14.9 mA/cm2 for YD2-o-C8 and YD2, respectively. These figures agree within 2% with the measured Jsc, showing that any spectral mismatch between the simulated and true AM 1.5 solar emission is very small with the solar simulator used in our experiments.

The four factors determining the IPCE are the light-harvesting efficiency (LHE), the quantum yield of electron injection from the excited sensitizer into the TiO2 conduction band (Φinj), the efficiency for dye regeneration (ηreg), and the collection efficiency of the photo-generated charge carriers (ηcoll):IPCE = LHE Φinj ηreg ηcoll(1) The charge carrier collection efficiency is determined in turn by the time constant for transport (τtrans) and recombination (τrec) of the conduction band electrons injected into the nanocrystalline TiO2 film:ηcoll = 1/(1 + τtrans/τrec) (2)We separately examined the four efficiency parameters of Eq. 1 in order to elucidate the reasons for the much higher IPCE values obtained with YD2-o-C8 as compared with the YD2 porphyrin. The LHE of a 6-μm-thick transparent TiO2 film loaded with a monolayer of the sensitizer is shown in Fig. 3. The LHE for the two porphyrins is very similar, the Q-band for YD2-o-C8 being slightly shifted to the blue with respect to that of YD2, which matches the behavior of the absorption spectra. The LHE reaches close to 100% near the Soret and Q bands, even for the 6-μm-thick transparent TiO2 film. The spectral domain where all the photons are captured by the dye-sensitized mesoscopic TiO2 film is further broadened for the 10-μm-thick double layer used in Fig. 2 because of the increase of the optical path length by light scattering from the top particle layer. This rules out that a change in the LHE causes the difference in the IPCE and Jsc values for the two porphyrins (34).

Fig. 3

LHE as a function of wavelength for the YD2 and YD2-o-C8 porphyrins adsorbed at the surface of a 6-μm-thick nanocrystalline TiO2 film.

Dividing the IPCE by the LHE gives the absorbed photon-to–electric current generation efficiency (APCE), presenting the true quantum yield for electric current generation from light. The APCE values in the wavelength range from 420 to 700 nm are between 80 and 100%, and 60 to 90% for YD2-o-C8 and YD2, respectively (Fig. 4). This indicates that the electron injection, dye regeneration, or charge carrier collection is less efficient for the latter than for the former sensitizer when used in conjunction with the Co(II/III)tris(bipyridyl) tetracyanoborate complexes as redox couple.

Fig. 4

APCE as a function of wavelength for the YD2 and YD2-o-C8 porphyrin adsorbed at the surface of a 6-μm-thick nanocrystalline TiO2 film in contact with Co(II/III)tris(bipyridyl)–based electrolyte.

We performed time-resolved luminescence experiments to unravel any differences in the dynamics of photo-induced electron injection between the two porphyrins. In fig. S5, we compare emission data in THF solution and for YD2-o-C8– or YD2-covered nanocrystalline TiO2 films in contact with the same Co(II/III)tris(bipyridyl)–based redox electrolyte used in the photovoltaic experiments. In solution, the fluorescence lifetimes for YD2-o-C8 and YD2 are 1.5 and 1.2 ns, respectively, whereas the emission for both sensitizers is strongly quenched in contact with TiO2. Data fitting, including reconvolution of the emission signal over the ~1 ns excitation pulse length, indicates that the lifetime of the excited singlet state of the two porphyrins in the adsorbed state is at most 100 ps. These experiments do not reveal any significant difference in the fluorescence kinetics of the two porphyrins. There is also little doubt that the very rapid quenching of luminescence leads in both cases to near-quantitative charge injection from their excited singlet state to the conduction band of the TiO2 nanocrystals, as implied by the very high APCE values observed for YD2-o-C8 in Fig. 4 and the near-unity IPCE values for YD2 obtained with iodide/triiodide–based redox electrolytes.

The data obtained from intensity-modulated photo-induced absorption (PIA) measurements (41) shown in the top row of fig. S6 confirm the occurrence of oxidative quenching after light excitation of the two porphyrins adsorbed on the nanocrystalline TiO2 film. The absorption peaks at 550, 800, and 1400 nm reveal the formation of porphyrin radical cations by electron injection from the excited state in the conduction band of the titania. The two PIA spectra in the lower row show that these features disappear completely in the presence of the Co(II/III)tris(bipyridyl) electrolyte, indicating complete regeneration of the YD2 or YD2-o-C8 porphyrins through electron donation from the Co(II) form of the redox couple.

Because the LHE as well as the quantum yield for electron injection and regeneration are very high for both dyes, we reasoned that the lower IPCE values for YD2 as compared with YD2-o-C8, reflect differences in the efficiency for carrier collection between the two porphyrin sensitizers.

Inhibition of back electron transfer. We used transient photocurrent and photovoltage decay measurements to compare the rates of interfacial recombination of electrons from the TiO2 conduction band to CoIIItris(bipyridyl) (42, 43). The electron lifetime as a function of Voc for YD2 and YD2-o-C8 is plotted in Fig. 5A. The Voc was adjusted by varying the intensity of the bias light impinging on the cell. The electron lifetime is 2 to 10 times longer for YD2-o-C8– than for YD2-sensitized TiO2 films, the difference becoming larger as the Voc increases. One plausible explanation of this observation is that at open circuit, electrons in the conduction band of TiO2 (eTiO2) reacts faster with CoIIItris(bipyridyl) for films loaded with the latter than with the former dye.

Fig. 5

Electron (A) lifetime and (B) capacitance determined with photocurrent and photovoltage decay measurements of devices with YD2 and YD2-o-C8 dyes.

It appears that the specific molecular structure of YD2-o-C8—in particular, the presence of the four octyloxy groups—reduces the recombination rate, most likely by inhibiting the access by CoIIItris(bipyridyl) to the TiO2 surface. The distance dependence of the back reaction is in accordance with the semiclassical electron transfer theory (44, 45).

The lower rate of electron recapture by CoIIItris(bipyridyl) for YD2-o-C8–sensitized nanocrystalline TiO2 films allows very high open-circuit voltages to be realized, with this sensitizer reaching a value of 965 mV in full sunlight without sacrificing short circuit photocurrent or fill factor. The voltage advantage of YD2-o-C8 over YD2 is maintained at lower light levels, down to 3 W/m2 solar intensity (fig. S7). The CoII/IIItris(bipyridyl) electrolyte yields slopes of 90 mV/decade and 80 mV/decade, corresponding to ideality factors m = 1.52 and 1.35 in the diode equation for YD2-o-C8 and YD2 dyes, respectively. This suggests that surface-trapping states participate in the back-electron transfer reaction, which is similar to the findings by Hagfeldt and co-workers (22).

It may be argued that the higher Voc observed with YD2-o-C8 with respect to YD2 arises from a larger dipole moment of the former as compared with the latter sensitizer in the adsorbed state, causing an upward shift of the conduction band of TiO2. This would displace the trap-state distribution function toward higher energies, rendering the density of occupied states (DOS) at a given forward bias voltage lower for YD2-o-C8 than for YD2. To check this possibility, the DOS of the films loaded with sensitizer was determined from transient photocurrent decay measurements (42, 43). The chemical capacitance Cμ for the YD2-o-C8– and YD2-sensitized films as a function of Voc are compared in Fig. 5B. Cμ is directly proportional to the DOS: Cμ = q(e) DOS, where q(e) is the charge of one electron. The Cμ values are very similar for the two sensitizers, with small differences being noted only at Voc > 0.8 V. This rules out any substantial contribution of a conduction band shift to the observed decrease in the back-electron transfer rate.

Using YD2-o-C8 as a sensitizer, we compared the photovoltaic performance of CoII/IIItris(bipyridyl) with that of a I/I3–based redox electrolyte using again a (5+5) μm double-layer TiO2 film. The concentrations of I and I3 in this electrolyte, designated AY2, were identical to those of the Co2+ and Co3+ tris(bipyridyl) in AY1; the other additives, such as TBP and LiClO4, were also maintained at the same levels, as was the acetonitrile solvent. The results presented in table S2 and fig. S8 show that the photovoltaic performance of devices using the CoII/IIItris(bipyridyl) electrolyte is far superior to that of the I/I3–based devices. The Voc and Jsc values increase by 193 mV and 2.3 mA/cm2, respectively, producing a 58% gain of the PCE from 7.6 to 11.9%. To evaluate the photovoltaic performance of the newly designed YD2-o-C8 dye under conditions that are optimal for I/I3–based DSCs, a thicker double-layer TiO2 film of (12+5) μm was applied along with the Z959 standard volatile electrolyte [1.0 M 1,3-dimethylimidazolium iodide (DMII), 0.03 M iodine, 0.1 M guanidinium thiocyanate, and 0.5 M tert-butylpyridine in a mixture of valeronitrile/acetonitrile (15:85 v/v)]. Under these conditions, the PCE reached 9.4% (detailed photovoltaic parameters are in table S2). These results confirm that for YD2-o-C8, the CoII/IIItris(bipyridyl) complex outperforms the I/I3–based redox electrolyte, even under optimal conditions for the latter.

Despite having excellent light-harvesting properties, the YD2-o-C8 dye lacks absorption in the 480 to 630 nm range, as evidenced by the IPCE spectra in Figs. 2B and 6B. The dip in the green spectral region reduces the JSC and hence the PCE. To avoid this loss, we used a cosensitizer coded Y123 (a D-π-A dye whose structure is shown in fig. S9) that possesses a complementary absorption spectrum to YD2-o-C8. Y123 exhibits an absorption maximum at 532 nm in THF with an extinction coefficient of 53,000 M−1 cm−1 (23). This coincides with the minimum in the spectral IPCE response of the YD2-o-C8 dye, the absorption maximum of which is at 644 nm (ε = 31,200 M−1 cm−1).

Fig. 6

(A) J-V characteristics of a YD2-o-C8/Y123 cosensitized DSC, measured under standard AM 1.5 global sunlight under various light intensities. The molar YD2-o-C8/Y123 ratio in the dye solution was 7. (B) Spectral response of the IPCE for YD2-o-C8 (red dots), Y123 (blue triangles), and YD2-o-C8/Y123 cosensitized nanocrystalline TiO2 films (black squares)

Enhanced performance from adding a co-sensitizer. The cosensitized films achieved better photovoltaic performance than that of solar cells using a single dye. The photocurrent J-V curves measured at three different light intensities, using the AY1 redox electrolyte, are shown in Fig. 6A. The photovoltaic performance parameters are listed in Table 2. Increased short-circuit current density and FF are observed for the cosensitized films compared with YD2-o-C8 alone. The cumulative increases of Jsc and FF give rise to an efficiency of 12.3% at AM 1.5 global full sun using the CoII/IIItris(bipyridyl)–based redox electrolyte. The PCE reached an even higher value of 13.1% at 509 W/m2 solar intensity (50.9% sun). These results were independently verified by repeating the photovoltaic characterization of the cells in the Photovoltaic Laboratory at the Institute of Micro Technique (IMT), Neuchâtel, Switzerland. The Wacom high-precision class AAA solar simulator system available at the IMT Photovoltaic Laboratory very closely mimics the solar spectrum in the absorption range of the cosensitized solar cells in the range of 350 to 750 nm. This avoids any substantial spectral mismatch between the simulated and true AM 1.5 solar light source. Results shown in fig. S10 and table S3 fully confirm the PCE measurements carried out in our own laboratory.

Table 2

Detailed photovoltaic parameters of the devices made with AY1 electrolyte and YD2-o-C8 dye cosensitized with Y123 dye at different light intensities.

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The IPCE spectra of devices made with the individual and combined dyes are shown in Fig. 6B. The Jsc value obtained from integrating the product of the IPCE spectrum with the AM 1.5 global spectral solar photon flux was 18.3 mA/cm2. This value is greater than the measured Jsc value (17.6 mA/cm2) at full sunlight. The difference can be attributed to mass transport limitations at full sunlight, which limit the photocurrent. This effect is corroborated by the photocurrent transient measurements by using an on/off modulation of the incident light (46). The data reported in fig. S11 show a spike in the photocurrent, which reaches a lower stationary value after a few seconds of illumination time. Our goal is to eliminate this small Jsc loss so as to attain a strictly linear response of the photocurrent up to full solar intensity. Such linear behavior already can be achieved by reducing the porphyrin content in the YD2-o-C8/Y123–containing staining solution from 7:1 to 4:1, as shown in table S4. This indicates that the mass transport limitation is also partly caused by the bulky nature of the YD2-o-C8 dye. Because the increase in Y123 content in the dye mixture also reduces the Voc value, the linear photocurrent response does not contribute to the PCE at full sunlight.

The YD2-o-C8/Y123 cosensitized nano-crystalline TiO2 film exhibit an impressive panchromatic photocurrent response over the whole visible range, achieving incident photon-to-electron conversion efficiencies of >90% in a large wavelength domain below 700 nm. Although the co-sensitization increases, the Jsc, the Voc decreases by about 30 mV with respect to films sensitized by YD2-o-C8 alone. Transient photo-voltage decay measurements giving access to the electron lifetime were used to examine the reason for this decrease. The plots of electron lifetime versus open-circuit potential in fig. S12 show that the incorporation of Y123 reduced the lifetime of the photo-generated electrons in the TiO2 conduction band compared with YD2-o-C8 alone.

Continued exposure of YD2-o-C8–sensitized solar cells for 220 hours to full sunlight at 30°C showed only a 10 to 15% decrease of the overall efficiency over this extended light-soaking period. This shows that the photosystem is robust and that the small decline is probably caused by losses of the volatile acetonitrile solvent.

The present results provide a fertile base for further investigation, which will focus on achieving linear response of the photocurrent to full sunlight, enabling PCE values above 13% to be reached under standard reporting conditions. Further challenges to be tackled are the use replacement of the volatile electrolyte by nonvolatile ionic liquid-based systems, as well as solid-state hole conductors. Last, new anchoring groups will be introduced in the porphyrin in order to avoid any desorption on long-term heat and light exposure, along with functional groups increasing the near-infrared response of the sensitizer.

Supporting Online Material

www.sciencemag.org/cgi/content/full/334/6056/629/DC1

Materials and Methods

Figs. S1 to S12

Tables S1 to S4

References (47, 48)

References and Notes

  1. Acknowledgments: E.W.-G.D. and C.Y.Y. acknowledge the financial support from the National Science Council of Taiwan and Ministry of Education of Taiwan. M.G. thanks the European Research Council (ERC) for an Advanced Research Grant (ARG 247404) funded under the “Mesolight” project. Financial support under the European community’s 7th FWP for project 227057 (INNOVASOL) and under a grant from the Dayton U.S. Air force Research Laboratory is gratefully acknowledged. M.K.N. thanks World Class University program, Photovoltaic Materials, Department of Material Chemistry, Korea University, Chungnam 339-700, Korea, which is funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea (R31-2008-000-10035-0). We thank R. Humphry-Baker for fruitful discussions as well as optical measurements and M. J. M. Bonnet-Eymard from the IMT Photovoltaic Laboratory for assistance with the cell photovoltaic performance measurements.
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