Secondly, we examined the concentration dependence of the performance of CA7-sensitized solar cell by dilution of the sensitizer with a spacer, deoxycholic acid (DCA). Surprisingly, the performance was enhanced from that at 100% by the initial dilution to 70% and even after the later dilution to 30%. The concentration dependence of the IPCE profile and the electronic absorption spectrum suggested changes in the form of singlet excitation of the sensitizer on the TiO2 layer. We suspected that ‘singlet-triplet annihilation’ due to the aggregate formation is the key in suppressing the photocurrent and conversion efficiency before dilution.
Finally, we prepared a set of four sensitizers having different polarizabilities and, as a result, different tendency of aggregated formation, and examined changes in the photocurrent and conversion efficiency of the fabricated solar cells, depending on the dye concentration and the light intensity. The most aggregate-forming dye exhibited the enhancement of performance by lowering the concentration and the light intensity, supporting the idea of singlet-triplet annihilation. The details will be described below.
2.1. Mechanisms of Electron Injection and Charge Recombination Generating Radical Cation and Triplet Species
Conjugation-length dependence of photocurrent and conversion efficiency of RA- and CA-sensitized solar cells. Figure 5a
shows the I–V
curves of solar cells using the set of sensitizers [10
]. The short-circuit photocurrent density (Jsc
) is in the order, RA5 < CA6 < CA7 > CA8 > CA9 > CA11 > CA13, whereas the open-circuit photovoltage (Voc
) is in the order, RA5 > CA6 > CA7 > CA8, and CA8, CA9, CA11 and CA13 exhibit similar values.
Presumably, the coverage on the surface of TiO2
layer should be better-organized in the shorter-chain RA5, CA6 and CA7 sensitizers in the complete all-trans
configuration; the longer-chain sensitizers tend to form cis
isomers, as well. Open-circuit photovoltage (Voc
) in Figure 5
must reflect this situation. Figures 6a
and b present the conjugation-length dependence of short-circuit current density (Jsc
, hereafter called ‘photocurrent’) and solar energy-to-electricity conversion efficiency (η
, called ‘conversion efficiency’). Both photocurrent and conversion efficiency are at the maximum in CA7; they decline toward the shorter-chain in the order, CA6 and RA5, and also toward the longer-chain in the order, CA8, CA9, CA11 and CA13. The relevant parameters of solar cells and the one-electron oxidation potentials of the sensitizers are listed in Table 1
in Supporting Information of Ref. [10
The excited-state dynamics of RA and CAs bound to TiO2 nanoparticles in suspension.
To understand the mechanism giving rise to the above dependence of photocurrent and conversion efficiency on n
, we examined the excited-state dynamics of the set of sensitizers (except for CA13) bound to TiO2
nanoparticles in suspension by subpicosecond and submicrosecond pump-probe spectroscopy [11
]: Figure 7
shows an energy diagram for the π-conjugated chains of RA and CAs with n
= 5~13: The linear dependence of the optically-active 1Bu+
state, as a function of 1/(2n
+ 1), was determined by conventional electronic-absorption spectroscopy. The linear dependence of the optically-forbidden 1Bu−
states was transferred from those of bacterial Cars (n
= 9~13) determined by the measurement of resonance-Raman excitation profiles [1
] (Figure 1
); the energies for CA8~RA5 were extrapolation of the linear relations. According to the state ordering, after excitation to the 1Bu+
state by the absorption of photon, (i) RA5, CA6, CA7 and CA8 are expected to internally convert, in the order, 1Bu+
(the ground state), (ii) CA9 and CA10, in the order, 1Bu+
and (iii) CA11, in the order, 1Bu+
On the basis of the above set of energy levels and internal conversion processes, we analyzed, by means of singular-value-decomposition (SVD) followed by global fitting, the time-resolved data matrices for the set of RA5~CA11 sensitizers free in solution and bound to TiO2 nanoparticles in suspension.
presents the results for free in solution, including the species-associated-difference-spectra (SADS) shown in the top panels and the time-dependent changes in population shown in the third panels: In RA 5, rapid transformation from the 1Bu+
to the 2Ag−
state followed by the generation of radical cation (D0•+
) is observed. In CA6~CA8, rapid 1Bu+
transformation followed by the slow decay of the 2Ag−
state is observed; here, no generation of D0•+
is seen. In the SADS of CA9 and CA11, clear transformation from the 1Bu+
to the 2Ag−
state is not seen in the visible region, but rapid transformation from the 1Bu+
to the 1Bu−
state and that from the 1Bu+
to the 3Ag−
state, respectively, are seen in the near-infrared region. Their spectral patterns agreed with those of the 1Bu−
states of neurosporene (n
= 9) and lycopene (n
= 11), respectively [12
]. The time-dependent changes in population for CA9 shows extremely-rapid 1Bu+
transformation followed by the slower 1Bu−
transformation, whereas those for CA11, extremely-rapid 1Bu+
transformation followed by the slower 3Ag−
The results for RA5~CA11 bound to TiO2
nanoparticles in suspension are also shown in Figure 8
(the second and fourth panels): The singlet-excited states generated by the photo-excitation of the sensitizers bound to TiO2
were basically the same as those generated free in solution. The most conspicuous difference in the excited-state dynamics, in the bound state, is that the transient absorptions of the triplet (T1
) and the radical-cation (D0•+
) states appear immediately after electron injection. The former transient absorptions agree in energy with those of the T1
states obtained by anthracene-sensitized photo-excitation, whereas the latter transient absorptions, with the stationary-state absorptions of radical cation obtained electrochemically (see the spectral lines shown in the second panels). The generation of the apparent D0•+
state, however, drastically influences the dynamics of singlet-excited states: In RA5~CA8, the generation of the D0•+
state substantially accelerates the decay of both the 1Bu+
states, showing efficient electron injection from these excited states into TiO2
. In CA9 and CA11, on the other hand, it accelerates the decay of not
state, showing electron injection only from the latter.
presents the internal-conversion and electron-injection pathways and the relevant time constants for the free and bound states. Table 1
lists the electron-injection efficiencies through the 1Bu+
channels and a sum of the two for the set of RA and CAs, which were calculated by the use of those time constants.
The conjugation-length dependence of the total electron-injection efficiency (Φ
) is depicted in Figure 6c
. The highest efficiency in CA7 (almost unity) and the decline toward CA11 can be explained nicely in terms of electron-injection efficiency. The results definitely indicate that the decline toward the longer-chain, i.e.
, CA7 > CA8 > CA9 > CA11, reflects the intrinsic excited-state dynamics of the Car conjugated chain. However, the decline toward CA6 and RA5 is left unexplained. Table 2
shows that the values of one electron-oxidation potential systematical lowers with n
, a trend which predicts the electron-injection efficiency monotonically increasing with n
all the way from n
= 5 to 11, which is contrary to the observation.
We have observed the generation of ‘the D0•+ + T1 state’ just by transient absorptions, which does not decay at all in the ps time scale. Therefore, we do not know, at this moment, what we now call ‘the D0•+ + T1 state’ is either ‘a combined D0•+ + T1 state’ or ‘a mixture of the D0•+ state and the T1 state’. We have applied submicrosecond pump-probe spectroscopy to examine the later stages after excitation.
shows the results of the SVD and global-fitting analysis of submicrosecond time-resolved data matrices for the four shorter-chain RA and CAs. Here, a relaxation mechanism, including the splitting of a combined D0•+
state into a pair of the D0•+
states, has been nicely explained. The first SADS (upper panels) show that the T1
population ratio in the combined
state increases toward RA5. Consistently, the time-dependent changes in population (lower panels) show that the ratio of the split
species also increases toward RA5.
lists the quantum yields for the D0•+
) calculated by the use of the relevant time constants. The efficiency of electron injection (ϕD
) gradually declines toward RA5. This trend partially
solves the above-mentioned contradiction in the dependence on n
shown in Figure 6
, (a) and (b) vs
Finally, we will propose the mechanisms of charge-separation and charge-recombination, which generates the radical-cation and triplet species of RA and CAs on the surface of TiO2
nanoparticles: Figure 11
presents the energies of the singlet, triplet and redox states of RA5 and CA6~CA11 in reference to the conduction-band edge (CBE) of TiO2
. Importantly, the energy gap between the CBE and the T1
levels is the smallest in RA5 and systematically increases toward CA11, which explains the decreasing order of the triplet generation mentioned above.
proposes the excited-state dynamics in a typical CA that is bound to TiO2
: (i) Process 0
: Upon absorption of photon, electron is transferred to a higher singlet level (S1
). (ii) Process 1
: Electron injection takes place to generate a charge-separated state having a singlet character on the boundary. (iii) 12
: Electron is transferred further into TiO2
to form a stable charge-separated state. (iv) 6
: the reverse electron transfer followed by charge recombination takes place to relax into the ground state. This is a series of changes among the singlet-excited and redox states having a singlet character.
Now, we will consider the generation of the triplet-excited and radicalcation states both having a triplet character: (v) Process 12 → 33: When there is a strong spin-orbit coupling in the charge-separated state having the singlet character, it can transform, by the inversion of spin, into the charge-separated state having a triplet character. When the energy gap between the CBE and the T1 levels is small, the resultant charge-separated state can transform further into a charge-transfer complex (33) consisting of the charge separated (TiO2−–CA(D0•+)) state and a neutral (TiO2–CA(T1)) state. This is exactly what we called ‘the combined D0•+ + T1 state’ (vide supra), because the former component gives rise to the radical-cation electronic absorption, whereas the latter component, the T1-state electronic absorption of CA.
, the relative contribution of the T1
-state CA becomes larger when the energy gap between the CBE of TiO2
and the T1
states of CA becomes smaller (see Figure 11
); this is actually evidenced by the SADS of the D0•+
state (see Figure 10
). This charge-transfer complex can split into two independent components as follows: (vi) 33
: It transforms into the pure D0•+
state of CA, the lifetime of which can be very long when the electron is trapped far from the surface of TiO2
particles in suspension. (vii) 33
: it can transform into the T1
state of CA, which decays with an intrinsic T1
Summary: The mechanisms of electron injection immediately after excitation to the 1Bu+(S1) state and charge recombination of the TiO2−–Car (D0•+) pair to form triplet Car, after the intersystem crossing and the formation of charge-transfer complex, have been revealed by the analysis of the ps and μs time-resolved data obtained by pump-probe spectroscopy of RA and CAs bound to TiO2 nanoparticles in suspension. The conjugation-length (n) dependence of the initial excited-state dynamics has nicely explained the photocurrent and conversion efficiency of solar cells using the RA and CA sensitizers, i.e., the maximum at n = 7 and the decline toward n = 11. On the other hand, the decline toward n = 5 has been explained partially in terms of the triplet generation at later stages.
2.2. Mechanisms of Singlet-Triplet Annihilation Suppressing Photocurrent and Conversion Efficiency
Dependence of photocurrent and conversion efficiency on the dye concentration in CA7-sensitized solar cells: a possible mechanism of singlet-triplet annihilation. Figure 5b
(shown at the beginning of Section 2.1) presents the I
curves of CA7-sensitized solar cells, when the sensitizer was diluted with a spacer, deoxycholic acid (DCA) [10
]. Table 2
in Supporting Information of Ref. [10
] lists the relevant parameters showing the performance of CA7-sensitized solar cells at different dye concentrations. Figure 13a
shows the concentration dependence of Jsc
. Both parameters exhibit consistent but unique concentration dependence, which can be characterized as follows: (i) At 100%, these values are medium among the values at all the different concentrations. (ii) On going from 100% to 90%, the values exhibit a sudden drop. (iii) Then, they increase up to a maximum at 70%. (iv) From 70% down to 30%, the values gradually decrease. (v) Below 30%, they decrease steeply toward the values at 10%.
The consistent changes not only
in photocurrent and conversion efficiency shown in Figure 13 but also
in the IPCE profile (action spectrum) and the electronic absorption spectrum (see Ref. [10
]) strongly suggest changes in the form of singlet excitation with the turning points at 90%, 70% and 30%. We propose four different forms of excitation based on Figure 14
, where the dye molecules (○) are diluted with the spacer molecules (•): (i) At 100%, a coherent excitonic excitation takes place in an aggregate of dye molecules (we call this ‘coherent delocalized excitation’). (ii) At 90%, this excitation is destroyed by a small number of spacer molecules that function as defects. (iii) At 70%, a localized excitation on a single molecule can migrate from one to another. This ‘migrating excitation’ must become most efficient when the dye concentration becomes around 2/3, because branched routes for the migrating excitation are formed. (iv) At 30%, the dye molecules become isolated being intervened by a larger number of spacer molecules. This ‘isolated excitation’ must become the largest in number when the dye concentration becomes around 1/3.
Based on the above three different types of singlet excitation on the TiO2
layer and the generation of the triplet state as an intrinsic property of CAs bound to TiO2
(see Section 2.1), we propose a possible mechanism to explain the unique concentration dependence of photocurrent and conversion efficiency in the fabricated CA7-sensitized solar cell (see Figure 13a
): (i) In a coherent delocalized excitation at 100%, there is a good chance that such widely-expanded excitation reaches a dye molecule in the T1
state to cause the singlet-triplet annihilation. (ii) In partially-destroyed delocalized excitation at 90%, the advantage of the widely-expanded coherent excitation in electron injection is lost to suppress electron injection, but there is still a chance of collision between ‘an expanded delocalized excitation’ and a localized triplet excitation to annihilate the former. (iii) In a localized excitation migrating along one of the branched routes at 70%, there is a much less chance of collision with a triplet excitation unless it is located on the particular route. (iv) In an isolated singlet excitation, there is no chance of collision with an isolated triplet excitation. Then, the photocurrent and conversion efficiency decrease linearly with the decreasing number of dye molecules excited.
The relative photocurrent (rJsc
) and conversion efficiency (rη
) are depicted in Figure 13b
(see the caption for their definition). Their concentration dependence indicates that the changes in the singlet excitation take place continuously, and the relative performance (rJsc
) becomes systematically enhanced until 9~10 times on going from the first to the last form of singlet excitation.
Summary: The dependence of the photocurrent and conversion efficiency of the CA7-sensitizerd solar cell on the dye concentration has been explained in terms of changes in the form of singlet excitation of the sensitizer molecules on the surface of TiO2 layer, i.e., the coherent delocalized excitation → the localized migrating excitation → the isolated excitation. There is a good chance of substantial enhancement of performance, if we succeeded in achieving only the localized excitation, keeping the total number of excited-state dye molecules the same.
The substantially reduced performance at the 100% dye concentration is ascribable to the singlet-triplet annihilation reaction. Therefore, the decrease in the photocurrent and conversion efficiency of solar cells from the CA7 sensitizer toward the RA5 sensitizer (see Figure 6a
and b) can now be explained by the effect of singlet-triplet annihilation among the sensitizer molecules on the surface of the TiO2
layer, in addition to the effect of the increasing triplet generation described in Section 2.1.
Dependence of conversion efficiency on dye concentration and light intensity in solar cells using polyene sensitizers having different polarizabilities
. Scheme 15 shows the structures of four different polyene sensitizers that were used for fabricating the solar cells [15
]. The common skeleton of the sensitizers is the benzene ring connected to a polyene (n
= 6), to the end of which the carboxyl group is attached (ϕ
-6-CA); to the opposite end of the benzene ring the MeO-, (MeO)3
- or Me2
N- electron-donating groups is attached to realize the electron push-pull relation in the latter set of sensitizers.
The set of polyene sensitizers are named ϕ-6-CA, MeO-ϕ-6-CA, (MeO)3-ϕ-6-CA and Me2N-ϕ-6-CA as shown in the figure; the polarizability of polyene to enhance van der Waals intermolecular interaction to form aggregates is supposed to increase in this order. Actually, the transition-dipole moment calculated by the use of molar extinction coefficient (ɛ) was in the order, 14.2, 15.1, 15.2 and 15.6 D, and the tendency of aggregate formation judged by the blue-shift of the 1Bu+ absorption was in the same order (data not shown).
shows the concentration dependence of the I
curves of solar cells using the above set of sensitizers. In the least-polarizable sensitizer, ϕ
-6-CA, the photocurrent (Jsc
) is the highest at 100% and monotonously decreases toward the lower concentration. In the most-polarizable sensitizer, Me2
-6-CA, on the other hand, the photocurrent is the lowest at 100% and monotonously increases toward the lower concentration. The latter change is contrary to our expectation, and can be explained only in terms of singlet-triplet annihilation. At 100%, the delocalized excitonic excitation should be generated due to the aggregate formation, which can be readily annihilated by collision with the triplet species within the expanded, excitonically-excited region. The chance of this singlet-triplet annihilation must become smaller by lowering the dye concentration.
shows the dependence of the I
curves of the solar cells on the light intensity at two different dye concentrations (5% and 100%). In the least-polarizable sensitizer, ϕ
-6-CA, the photocurrent decreases with the lowering light intensity. On the other hand, in the most-polarizable sensitizer, Me2
-6-CA, the photocurrent increases, instead. The latter change is contrary to our expectation, and can be explained in terms of singlet-triplet annihilation, because the generation of both the singlet and triplet excitation must become suppressed at the lower light intensity.
plots the concentration dependence of conversion efficiency (η
) for the set of polyene sensitizers. In the least-polarizable sensitizer, ϕ
-6-CA, the conversion efficiency monotonously decreases, while in the most-polarizable sensitizer, Me2
-6-CA, it monotonously increases with the lowering dye concentration. In the second-least polarizable sensitizer, MeO-ϕ
-6-CA, conversion efficiency exhibits the maximum at 70%, while in the second-most polarizable sensitizer, (MeO)3
-6-CA, it exhibits the maximum at 5%.
in Supporting Information of Ref. [15
] lists the values of (i) conversion efficiency (η
), (ii) conversion efficiency scaled to the concentration (sη
), and (iii) the ratio of scaled conversion efficiency in reference to that at 100% (rη
). The concentration dependence of the rη
values are depicted in Figure 8b
. Interestingly, the relative conversion efficiency (rη
) at 5% is in the order, Me2
-6-CA > (MeO)3
-6-CA > MeO-ϕ
-6-CA > ϕ
-6-CA, in agreement with the decreasing order of polarizability of the sensitizers.
Summary: The absence or presence of singlet-triplet annihilation has been demonstrated by lowering the dye concentration and the light intensity in solar cells by the use of the four sensitizers having the increasing polarizability and, as a result, the increasing tendency of aggregate formation. The least polarizable (the least aggregate-forming) sensitizer gave rise to the decreasing conversion efficiency with the decreasing dye concentration and light intensity, whereas the most polarizable (the most aggregate-forming) sensitizer gave rise to the increasing conversion efficiency with the decreasing dye concentration and light intensity. The four different patterns, in the dependence on the dye concentration and the light intensity, can be used as a standard to examine the degree of aggregate formation and the absence and presence of singlet-triplet annihilation of a new sensitizer.