Dye-Sensitized Solar Cells Based on the Principles and Materials of Photosynthesis: Mechanisms of Suppression and Enhancement of Photocurrent and Conversion Efficiency
Abstract
:1. Introduction
1.1. Photosynthetic Pigments
1.2. How to Apply the Principles of Photosynthesis to DSSCs
2. Polyene Sensitizers
2.1. Mechanisms of Electron Injection and Charge Recombination Generating Radical Cation and Triplet Species
2.2. Mechanisms of Singlet-Triplet Annihilation Suppressing Photocurrent and Conversion Efficiency
3. Pheophorbide Sensitizers Combined with Polyene Spacers
3.1. Mechanisms of Electron Transfer from Carotenoid Spacers to Pheophorbide a Sensitizer
3.2. Pheophorbide–Car adduct: Energy Transfer and Electron Transfer from Car to Phe Moiety
4. Bacteriochlorin, Chlorin and Porphyrin Sensitizers
4.1. Pheophorbide Sensitizers Having Bacteriochlorin, Chlorin and Porphyrin Skeletons
4.2. Chl c (Mg-Pheophorbide) Sensitizers Having Porphyrin Skelton
4.3. Pheophorbide and Metal-Pheophorbide Sensitizers Having Chlorin and Porphyrin Macrocycles
- The absorption spectra of the sensitizers (in Figure 45) show that the major light absorption through the Soret bands is highly competitive in the a-type + a-type pair, complementary rather than competitive in the a-type + b-type pair, and absolutely complementary in the a-type + c-type pair. Therefore, the highest enhancement in the a-type + b-type co-sensitization and the next highest enhancement in the a-type + c-type co-sensitization can be rationalized in terms of complementary absorption not by the Qx and Qy levels but by the Soret levels.
- The combination of the a-type sensitizer having the carboxyl group in the y-direction and the b-type or c-type sensitizer having the carboxyl group in the x-direction should give rise to the highest enhancement of photocurrent and conversion efficiency, because of the minimum interference of the transition dipoles between the pair of co-sensitizers. Polarization and electron-injection along the orthogonal directions must prevent the interference between the intermolecular transition dipole–transition dipole interactions that can trigger intermolecular energy transfer and the resultant dissipation of the singlet energy.
- The different pathways of internal conversion, Soret → Qx → Qy in the a-type sensitizer and Soret → Qy in the b-type or c-type sensitizer may also prevent interaction in the internal-conversion processes because of the different time scales of internal conversion.
5. Conclusions and Future Perspective
5.1. Conclusions
- By the use of a set of RA and CA sensitizers (n = 5~13), the dependence of photocurrent and conversion efficiency of DSSC on the conjugation-length of the sensitizer was determined to be, in the order, RA5 < CA6 < CA7 > CA8 > CA9 > CA11 > CA13. For comparison, the electron-injection efficiencies for RA5–CA11 bound to TiO2 nanoparticles in suspension were determined by means of subpicosecond time-resolved pump-probe spectroscopy. The maximum for CA7 and the decline toward CA11 were explained in terms of excited-state dynamics of the sensitizers. On the other hand, the decline toward RA5 was explained by the increasing efficiency of triplet generation and, as a result, the enhanced singlet-triplet annihilation due to the aggregate formation of the dye sensitizers on the TiO2 surface.
- Excited-state dynamics including the formation of a charge-transfer complex, what we call ‘the combined D0•+ + T1 state’, consisting of a charge-separated (TiO2−–CA (D0•+) and a neutral (TiO2–CA (T1)) states, and its subsequent splitting into the D0•+ plus T1 Car species, was identified by subpicosecond and microsecond time-resolved pump-probe spectroscopy, respectively.
- The mechanism of singlet-triplet annihilation to suppress the photocurrent and conversion efficiency was first identified by their dependence on the dye concentration in the CA7-sensitized solar cell. This mechanism was confirmed by the use of sensitizers having the increasing transition-dipole moments and, as a result, the increasing trend of aggregate formation. The least polarizable (the least aggregate-forming) sensitizer gave rise to the decreasing conversion efficiency, whereas the most polarizable (the most aggregate-forming) sensitizer gave rise to the increasing conversion efficiency, both with the decreasing dye concentration and light intensity.
- Sets of bacterial (n = 9~13) and plant (n = 8~11) Cars were used as redox spacers for the Phe a–sensitized solar cell. The idea behind this attempt is to induce electron transfer from Car to Phe a radical cation (Phe a•+) to stabilize the charge-separated TiO2−– Car•+ state to prevent immediate charge recombination of the TiO2−–Phe a•+ pair. Rapid electron injection into TiO2 to generate Phe a•+ (20–40 fs) followed by electron transfer from bacterial Cars to Phe a•+ (200–240 ps) was evidenced by subpicosecond pump-probe spectroscopy of each Phe a−bacterial Car pair bound to TiO2 nanoparticles in suspension. Among the two set of Cars, β-carotene having the lowest one-electron oxidation potential (Eox = 0.61 V) exhibited the maximum enhancement of conversion efficiency (η = 3.4 → 4.2%). In the above mixture of Car and Phe a, no singlet-energy transfer was observed. However, in Phe–Car adduct sensitizer, both singlet-energy transfer and electron transfer from the Car to the Phe moiety were identified in the solar cell. No sign of singlet-triplet annihilation due to aggregate formation was seen in this particular sensitizer.
- In a set of Phe sensitizers having the chlorin and porphyrin macrocycles, photocurrent (Jsc) was found to be the functions of the integrated Qy absorption and one-electron oxidation potential (Eox). Phe c2 having the highest one-electron oxidation potential (Eox = 1.33 V) exhibited the lowest conversion efficiency (η = 1.1%) among the Phe sensitizers. On the other hand, Chl c2 (Mg-Phe c2) having low one-electron oxidation potential (Eox = 1.06 V) exhibited the highest conversion efficiency (η = 4.6%) among all the sensitizers we have tested. The extremely-low conversion efficiency in Phe c2 was ascribed to the high Eox value and electron injection via the Qy level, whereas the high conversion efficiency in Chl c2 was ascribed to the low Eox value and electron injection via the Soret level, which is stabilizer by the absence of the Qx level.
- By co-sensitization using the Phe a and Chl c2 sensitizers of the second-best and the best performance, we have succeeded in enhancing the photocurrent and conversion efficiency to 14.0 mA·cm−2 and η = 5.4%, respectively. The enhancement was ascribed to the supplementary light absorption, the orthogonal directions of transition-dipoles and the independent internal conversion processes between the pair of sensitizers.
5.2. Future Perspective
- Pump-probe subpicosecond time-resolved spectroscopy of the single Car sensitizer as well as the Chl a sensitizer plus Car redox spacer, both bound to TiO2 nanoparticles in suspension, has turned out to be very powerful in elucidating the initial electron-injection and electron-transfer mechanisms, respectively. This technique should be applied to determine the mechanisms of excitation, energy-transfer and electron injection in each chlorin or porphyrin sensitizer as well as the pairs of these sensitizers used for co-sensitization.
- In the case of the well-characterized CA and RA sensitizers, it is time to start pump-probe time-resolved spectroscopy of fabricated DSSCs, in various time regions, to elucidate the real electron flow processes in the cell.
- To establish the mechanism of singlet-triplet annihilation, which is a key issue to enhance the performance of DSSCs in general, other spectroscopic methods such as time-resolved fluorescence (up-conversion or Kerr-gate) and Raman, in the subpicosecond time region, will be most useful (see Ref. [9], for example).
6. Relevant Work by Other Investigators
Acknowledgments
- Notice: The clarity of large figures may depend on the computer and the print-out conditions.
References and Notes
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RA5 | CA6 | CA7 | CA8 | CA9 | CA11 | |
---|---|---|---|---|---|---|
1Bu+ channel | 0.04 | 0.31 | 0.46 | 0.31 | 0.60 | 0.29 |
2Ag− channel | 0.88 | 0.61 | 0.52 | 0.63 | — | — |
Sum | 0.92 | 0.92 | 0.98 | 0.94 | 0.60 | 0.29 |
RA5 | CA6 | CA7 | CA8 | CA9 | CA11 | |
---|---|---|---|---|---|---|
Eox (vs. Ag/AgCl) | 1.08 | 0.97 | 0.87 | 0.80 | 0.77 | 0.71 |
RA5-TiO2 | CA6-TiO2 | CA7-TiO2 | CA8-TiO2 | |
---|---|---|---|---|
kd−1(μs) | 34 | 22 | 9.4 | 5.9 |
kt−1 (μs) | 3.1 | 2.7 | 2.1 | 2.0 |
kt0−1 (μs) | 22 | 18 | 12 | 9.0 |
kd0−1 (μs) | ~50 | ~150 | ~150 | ~150 |
ϕD (%) | 8 | 11 | 18 | 25 |
ϕT (%) | 92 | 89 | 82 | 75 |
Time constant | no Car | +Neu | +Sph | +Lyc | +Ahr | +Spx |
---|---|---|---|---|---|---|
Phe a• + rise | 0.03 | 0.04 | 0.03 | 0.03 | 0.02 | 0.02 |
Phe a• + decay | 338 | 203 | 210 | 220 | 235 | 241 |
Principal and Co-sensitizers | Voc/V | FF | Jsc (Av) r Jsc/mA·cm−2 | η (Av) rη/% | S | Eox/Vvs | |||||
---|---|---|---|---|---|---|---|---|---|---|---|
Phe a (a-type) a-type | 0.56 | 0.68 | 9.0 | 3.4 | 1.16 | ||||||
Mg-Phe a | 0.51 | 0.70 | 4.4 | 1.6 | 0.79 | ||||||
co-sensitization | 0.50 | 0.69 | 5.6 | (6.7) | 0.83 | 1.9 | (2.5) | 0.79 | 0.8 | 41 | |
Phe y | 0.49 | 0.70 | 5.2 | 1.8 | 1.19 | ||||||
co-sensitization b-type | 0.50 | 0.68 | 6.8 | (7.1) | 0.36 | 2.3 | (2.6) | 0.92 | 0.9 | 62 | |
Phe b | 0.53 | 0.70 | 4.6 | 1.7 | 1.24 | ||||||
co-sensitization c-type | 0.57 | 0.68 | 10.9 | (6.8) | 1.60 | 4.3 | (2.6) | 1.65 | 1.6 | 39 | |
Zn-Phe c1 | 0.62 | 0.63 | 10.4 | 4.0 | 1.16 | ||||||
co-sensitization | 0.60 | 0.69 | 11.9 | (9.7) | 1.23 | 5.0 | (3.7) | 1.35 | 1.3 | 80 | |
Mg-Phe c2 (Chl c2) | 0.58 | 0.66 | 9.9 | 3.8 | 1.06 | ||||||
co-sensitization | 0.60 | 0.64 | 14.0 | (9.5) | 1.47 | 5.4 | (3.6) | 1.50 | 1.5 | 95 |
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Koyama, Y.; Miki, T.; Wang, X.-F.; Nagae, H. Dye-Sensitized Solar Cells Based on the Principles and Materials of Photosynthesis: Mechanisms of Suppression and Enhancement of Photocurrent and Conversion Efficiency. Int. J. Mol. Sci. 2009, 10, 4575-4622. https://doi.org/10.3390/ijms10114575
Koyama Y, Miki T, Wang X-F, Nagae H. Dye-Sensitized Solar Cells Based on the Principles and Materials of Photosynthesis: Mechanisms of Suppression and Enhancement of Photocurrent and Conversion Efficiency. International Journal of Molecular Sciences. 2009; 10(11):4575-4622. https://doi.org/10.3390/ijms10114575
Chicago/Turabian StyleKoyama, Yasushi, Takeshi Miki, Xiao-Feng Wang, and Hiroyoshi Nagae. 2009. "Dye-Sensitized Solar Cells Based on the Principles and Materials of Photosynthesis: Mechanisms of Suppression and Enhancement of Photocurrent and Conversion Efficiency" International Journal of Molecular Sciences 10, no. 11: 4575-4622. https://doi.org/10.3390/ijms10114575