4. C HAPTER 2. E LECTRONIC C OUPLING E NGINEERING FOR R EALIZING V ECTORIAL
4.2. R ESULTS AND D ISCUSSION
4.2.4. Transient-Absorption Spectroscopic Study
fs-TA measurements were performed to investigate charge injection efficiency and ultrafast kinetic in the DPP-based DSCs with redox-active iodine electrolytes. The fs-TA profiles of the DPP- based DSCs with TiO2 films are shown in Figure 4.19. After photoexcitation, the excited electrons in sensitizers are injected into TiO2 within two hundred femtoseconds. The appearance of a broad absorption for electrons in TiO2 conduction band is evidence of the fast charge injection process (Figure 4.20).30,39 Hence, oxidized DPP-based sensitizers are generated rapidly as evidence of appearing the characteristic absorption bands of the oxidized sensitizers at 800-900 nm (Figure 4.4a and 4.16).
Charge injection efficiency of the DPP-based sensitizers was calculated using the following equation:
ηinj = kinj/(kinj + kF),40 where kinj is the rate constant of charge injection and kF is rate constant of the excite-state relaxation. Because of kinj (~1013 s-1) and kF (109–1010 s-1), all the DPP-based sensitizers show nearly 100% charge injection efficiency. This high charge injection efficiency can be rationalized by the efficient ICT of the DPP-based sensitizers driven by high HD|DPP, as shown in Figure 4.8–4.11.23-
25
Figure 4.19. fs-TA profiles of the DPP-based DSCs with TiO2 films. Pump wavelengths were 590 nm for (a), 570 nm for (b), and 555 nm (c). The wavy features over 900 nm of the spectra are because of the Fabry–Pérot interference of the enclosed electrodes.
Figure 4.20. fs-TA profiles of the DPP-based DSCs with TiO2 films at NIR region. Pump wavelengths were 590 nm for (a), 570 nm for (b), and 555 nm (c) with the excitation fluence of 30 μJ/cm2.
Figure 4.21. fs-TA profiles of the DPP-based sensitizers in solution and in DSCs with Al2O3 films.
Pump wavelengths were 636 nm for (a), 590 nm for (b), 627 nm for (c), 585 nm for (d), 612 nm for (e), and 575 nm for (f) to excite photosensitizers. The dye solutions are 20 µM in CH3CN/CHCl3 and their ratio are 19:1 (v/v).
Figure 4.22. fs-TA profiles of bTPA-DPP-DMP. (a) In solution, (b) in DSCs with a Al2O3 film, and (c) with a TiO2 film. Pump wavelengths were 560 nm for both the solution and the DSC samples to excite sensitizer. The dye solutions are 20 µM in CH3CN/CHCl3 and their ratio are 19:1 (v/v).
Figure 4.23. fs-TA kinetic decays of the DPP-based sensitizers. (a) The DPP-based DSCs with TiO2
films, (b) with Al2O3 films, and (c) the DPP-based sensitizers in solution. The kinetic decays are taken at 1000 nm from the spectra in Figure 4.19–4.22. Solid lines are fitted exponential curves.
Table 4.4. Fit parameters for the fs-TA kinetic decays of the DPP-based DSCs with Al2O3 films and the DPP-based sensitizers in solution.
Phase Dye ΔA1
/mOD
τ1 /ps
ΔA2 /mOD
τ2 /ps
ΔA3 /mOD
τ3 /ps
<τavg> b) /ps
On Al2O3
DD-DPP-Ph 0.50
[76] a)
160 (±13)
0.16 [24]
610 (±164)
270 (±75)
DD-DPP-MP 0.73
[69]
100 (±8)
0.32 [31]
330 (±53)
170 (±31) DD-DPP-DMP 0.23
[31]
60 (±5)
0.52 [69]
570 (±18)
410 (±37) bTPA-DPP-DMP −0.34
[100]
2.5 (±0.2)
1.0 [58]
750 (±160)
0.74 [42]
1900 (±620)
1200 (±470)
In solution c)
DD-DPP-Ph 7.7
[37]
7.0 (±0.2)
6.0 [29]
140 (±11)
6.8 [34]
710 (±38)
280 (±23)
DD-DPP-MP 1.9
[21]
9.0 (±0.7)
4.0 [42]
180 (±18)
3.2 [36]
600 (±59)
300 (±38)
DD-DPP-DMP 0.5
[97]
720 (±192)
0.01
[3] ∞ d) 720 e)
(±192) bTPA-DPP-DMP 0.32
[67]
110 (±10)
0.16 [33]
1400 (±170)
540 (±82)
a) Fractional amplitude in %. b) Amplitude weight-averaged lifetime. c) 20 µM in CH3CN/CHCl3 and ratio is 19:1 (v/v). d) Much longer
The wavelength of 1000 nm was investigated to trace ultrafast BET kinetics. This wavelength (1000 nm) can selectively analyze photo-injected surface electrons in TiO2 with a broad absorption of 800–1100 nm, as shown in Figure 4.19 and 4.20 on the nanosecond timescale,35,41-43 while simultaneously reducing the contributions of signals of various indistinctness states over the visible region. Furthermore, the kinetic decay of 1000 nm shows mirror-like dynamics with the ground-state bleach of 600 nm, which is the recovery profile from oxidized-state sensitizers to ground-state sensitizers by recombination process with the surface electrons in TiO2 (Figure 4.24 and Table 4.5).
The transients extracted at 1000 nm are shown in Figure 4.23a, and their time constants are summarized in Table 4.6. The fastest decay (τ1) exhibits a time constant of hundreds of femtoseconds , corresponding to the ultrafast BET strongly Coulombic bounded electron-hole pairs.37 The subsequent decays of τ2 and τ3 correspond to the relatively slower ultrafast BET of weakly coupled electron-hole pairs.30-32,44 The decay timescales depend on the degree of HDPP|π, as follows: DD-DPP-DMP (τ1 = 2.0 ± 0.2 ps, τ2 = 70
± 29 ps, τ3; not resolved) > DD-DPP-MP (τ1 = 0.9 ± 0.1 ps, τ2 = 40 ± 2 ps, τ3 = 970 ± 1700 ps) > DD- DPP-Ph (τ1 = 0.4 ± 0.2 ps, τ2 = 9.0 ± 2.0 ps, τ3 = 60 ± 4.1 ps). It is noteworthy that the percentage of the slowest decay increases from DD-DPP-Ph (12%) to DD-DPP-MP (35%) to DD-DPP-DMP (53%), confirming that the weakened electronic coupling of photosensitizers reduces the charge recombination in the DPP-based DSCs.
Figure 4.24. fs-TA kinetic decays of the DSCs with DD-DPP-Ph. Upper and lower transients were taken at 1000 and 600 nm, respectively. Surface electrons (upper) and ground-state bleach (lower) are assigned to 1000 and 600 nm, respectively.
Table 4.5. Fit parameters for the fs-TA kinetic decays of the DSCs with DD-DPP-Ph and a TiO2
film monitored at 1000 and 600 nm.
λprobe
/nm
ΔA1
/mOD
τ1
/ps
ΔA2
/mOD
τ2
/ps
ΔA3
/mOD
τ3
/ps
ΔA4
/mOD
τ4
/ps
<τavg> b) /ps
1000 0.40
[16] a)
0.4 (±0.2)
0.55 [22]
9.0 (±2.0)
1.3 [50]
60 (±4)
0.30
[12] ∞ c) 40 d) (±20)
600 −0.94
[21]
0.6 (±0.2)
−0.53 [12]
10 (±3)
−2.4 [54]
60 (±7)
−0.57
[13] ∞ 40 d)
(±17)
a) Fractional amplitude in %. b) Amplitude weight-averaged lifetime. c) Much longer than our time window of 3 ns. d) Average time constant calculated without the large-error component.
Table 4.6. Fit parameters for the fs-TA kinetic decays of the DPP-based DSCs with TiO2 films.
Dye ΔA1
/mOD τ1
/ps
ΔA2
/mOD τ2
/ps
ΔA3
/mOD τ3
/ps
ΔA4
/mOD τ4
/ps
ΔA5
/mOD τ5
/ps
<τavg> b) /ps
DD-DPP-Ph 0.40
[16] a) 0.4 (±0.2)
0.55 [22]
9.0 (±2.0)
1.3 [50]
60 (±4)
0.30
[12] ∞ c) - - 5
(±3)
DD-DPP-MP 0.21
[15]
0.9 (±0.1)
0.33 [23]
40 (±2)
0.38 [27]
970 (±1700)
0.50
[35] ∞ −0.46
[−100]
5000 (±1000)
20 (±3) DD-DPP-DMP 0.35
[42]
2.0 (±0.2)
0.54 [58]
70
(±29) - - - - −0.40
[−100]
160 (±88)
40 (±19) bTPA-DPP-DMP −0.21
[40]
1.0 (±0.2)
−0.32 [40]
13 (±1)
1.1
[100] ∞ - - - - -
a) Fractional amplitude in %. b) Amplitude weight-averaged lifetime (components 1 and 2). c) Much longer than time window of 3 ns.
We performed ns-TA spectroscopy measurements of the DPP-based DSCs with iodine or redox- inactive inert electrolytes to investigate non-geminate BET and charge regeneration. The wavelength of the pump-pulse laser was 532 nm, which corresponds to the ICT region of the DPP-based sensitizers.
The repetition rate of the pulse laser was 10 Hz for the iodine electrolyte sample and 1.0 Hz for DSCs with the redox-inactive inert electrolyte. After photoexcitation, transient decay was monitored at 980 nm, which corresponds to the absorption of the oxidized-state DPP-based sensitizers. The transients of ns-TA were fitted with a single stretched exponential Kohlrausch–Williams–Watts (KWW) function:45,46
∆A (λ, t) = Ao(λ)e – (t/τ)β (4.2) where ΔA (λ, t) is the wavelength and time-dependent absorption differential amplitude, τ is the characteristic decay lifetime, and β is the stretch parameter related to the breadth of an underlying Lévy distribution of rate constants (β = 1 corresponding to a mono-exponential decay). The kinetic decays and corresponding time constant are summarized in Figure 4.25, 4.26, and Table 4.7.
Non-geminate BET is the recombination process between diffused electrons in TiO2 and holes of the oxidized-state sensitizers.36 Accordingly, the time constants of the non-geminate BET were measured by tracing the decay of the oxidized-state DPP-based sensitizers (Figure 4.25). The time
constants of the non-geminate BET also depend on the degree of HDPP|π, as follows: 31.8 ms for DD- DPP-DMP > 26.4 ms for DD-DPP-MP > 0.554 ms for DD-DPP-Ph, which indicates that the decrease in electronic coupling (HDPP|π) decreases the non-geminate BET. On the other hand, all the DPP-based sensitizers have similar charge regeneration dynamics in the iodine environment, as shown in Figure 4.26. The fs/ns-TA results confirm that the presented electronic coupling engineering strategy induces VET via fast charge injection and effective retardation of both ultrafast and non-geminate BET.
Figure 4.25. ns-TA kinetic decays of the DPP-based DSCs with the inert electrolyte. The transients were taken at 980 nm in ns-TA spectra.
Figure 4.26. ns-TA kinetic decays of the DPP-based DSCs. (a) ns-TA decay profiles of the DPP- based DSCs with the iodine electrolyte. (b) ns-TA decay profiles of the DSCs with bTPA-DPP-DMP and the iodine or inert electrolyte.
Table 4.7. Time constants of charge regeneration and non-geminate BET of the DPP-based DSCs.
Sensitizer τrec [ms] τreg [μs]
DD-DPP-Ph 0.554 493
DD-DPP-MP 26.4 212
DD-DPP-DMP 31.8 598
bTPA-DPP-DMP 0.981 13.3