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Gigantic Relevance of Twisted Intramolecular Charge Transfer for Organic Dyes Used in Solar Cells Ahmed M. El-Zohry, and Martin Karlsson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08326 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018
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The Journal of Physical Chemistry
Gigantic Relevance of Twisted Intramolecular Charge Transfer for Organic Dyes Used in Solar Cells. Ahmed M. El-Zohrya*, Martin Karlssonb a
Department of Chemistry, Ångström Laboratories, Uppsala University, Box 523, SE-75120 Uppsala, Sweden.
b
Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30, SE-10044 Stockholm, Sweden. *
[email protected] Abstract Within this work, we emphasis on the importance of Twisted Intramolecular Charge Transfer (TICT) process in organic dyes based on triphenyl amine moiety to achieve high performance in dye sensitized solar cells. Through the comparison between two recent made dyes, L1 and L1Fc, on different semiconductors (TiO2, and ZrO2), we could spectrally and dynamically detect for the first time the formation of TICT state for L1 on ZrO2 after localized charge transfer (LCT) state population, and an electron injection process from TICT state on TiO2. However, in the excited L1Fc, the ultrafast electron transfer from ferrocene (Fc) moiety to the L1 unit quenched the formation of TICT state in L1Fc on semiconductors, leading to an electron injection process from LCT state. The electron injection from TICT state in L1 associated with structural rearrangements on TiO2 leads to slow recombination process and an efficiency improvement of ca. 325%, in comparison to solar cells based on L1Fc dye, in which TICT state formation is hindered. Similar electron dynamics are obtained for L1 on TiO2 upon physically hindering the TICT process by adding polymer matrix. The presence of TICT state for L1 dye aids to reconstruct the kinetic profile for these dyes on semiconductor surfaces, and redesign organic dyes for higher efficiency in solar cells. Introduction Large-scale motions of organic dyes used in Dye Sensitized Solar Cells (DSSCs) have been overlooked for a long time. The adsorbed dye on a low band gap semiconductor is understood to absorb the incident light and inject an electron to the conduction band (CB) of the SC with a quantum yield (QY) close to unity in various systems.1 Traditionally, the dye aggregation problem was charged for the low efficiency for most of the tested dyes, in which the QY for electron injection is lower than one.2 On the other hand, the few reports that highlighted these large-scale motions, discussed the competence between these motions and the electron injection process.3-7 For example, the isomerization process has been investigated for various adsorbed organic dyes on different SCs, and it was suggested before to minimize or block such a process to avoid the formation of isomers with low performance and to increase the electron injection QY.3-5, 8 However, it has been recently discussed that other large-scale motions such as the Twisted Intramolecular Charge Transfer (TICT) could enhance the performance of organic dyes due to the decrease in the back-electron transfer or electron recombination QY.9-10 TICT is a known phenomenon in solution for various molecular systems that are especially based on Donor-Acceptor (D-A) strategy.11 The influence of TICT 1 ACS Paragon Plus Environment
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process has been previously discussed by indirect measurements through efficiency measurements of coumarine dyes.10, 12-13 These coumarine dyes show TICT and LCT emissions in solutions,12-13 and display different transient signals under various pHs10, 13. However, in the current work, we could assign the existence of TICT process of adsorbed organic dyes on semiconductors, and the influence of TICT on both electron recombination, and electron injection processes. These findings have been proven by measuring ultrafast vibrational/electronical transient signals of injected electrons in the CBs of low band gap semiconductor, TiO2, and on high band gap semiconductor, ZrO2. These electrons are injected from recent-made dyes, L1 and L1Fc, in which these dyes were based on the triphenyl amine moiety as a donor unit.9, 14 Triphenyl amine moiety is an important member in the arylamine family of organic dyes used in DSSCs, and is a matter of interest for various studies due to their low cost and high performance in DSSCs.15-16 Arylamine moiety was used in the most best performing dyes such as SM315 dye that recorded the maximum efficiency until the moment (13%).17 The L1Fc dye was synthesized to investigate the possibility of reducing the oxidized triphenyl amine moiety, after electron injection process, by the attached strong reductive Fc moiety aiming to obtain high photocurrent.9 However, it turned out that L1Fc has a very low photocurrent in comparison to L1 due to fast electron recombination in the former dye.9 The exact causes for this fast electron recombination in L1Fc were not known, nevertheless, this work illustrates the influence of TICT process on electron injection and electron recombination for these dyes. In addition, TICT process is believed to be existing in many organic dyes used in DSSCs, and the understanding of the current dynamics for L1 and L1Fc dyes will certainly enhance the strategic plans for building more efficient dyes in DSSCs. Experimental 1. Steady-state measurements Absorption spectra were recorded using a Varian Cary 5000, and emission measurements were performed using a Horiba Jobin Yvon Fluorolog. Details about the instruments have been described before.3 2. Chemicals The synthesis procedures of the dyes (L1, L1Fc and L1Fc2) were recently published.9 Acetonitrile was of spectroscopic grade purchased from Sigma-Aldrich. 3. Transient absorption (TA) The TA setups using infrared/visible probes were used in this study. The detailed specifications of the instrumentation have been described earlier.18-19 Briefly, excitation wavelength of 440 nm was employed upon using the IR probe, and 400 nm with the visible probe. The average excitation power ranged from 200 to 350 mW at repetition rate of 1 kHz. Results and Discussion 2 ACS Paragon Plus Environment
The L1 dye is a simple design of triphenylamine organic dyes used for solar cell applications.14, 20 The excited state of L1 dye in solution shows two emissive states; the first emissive one is from localized charge transfer state (LCT), and the second one is from twisted intra molecular charge transfer state (TICT), see LCT (ߣா௦௦ ~ 565 nm) and TICT ௫ 5, 9 ா௦௦ (ߣ௫ ~ 680 nm) in MeCN for L1 dye at Figure 2A. This gives an energy shift of ca. -1 2990 cm , between the LCT and TICT states in MeCN. A chemically modified dye, L1Fc, with an extra ferrocene moiety, shows emission solely form the LCT state in solution (Figure 2A), due to a fast charge transfer from the ferrocene moiety to the triphenylamine group within time constant of 50 fs that has been assigned previously.9 This makes the two dyes, L1 and L1Fc, potential candidates for exploring the influence of TICT state on charge dynamics in DSSCs, more specifically on low band-gap semiconductor, TiO2. L1 L1Fc
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Figure 1: Chemical structures of dyes used herein, L1 and L1Fc. (A) Normalized absorption, emission, and excitation spectra for L1, and L1Fc in MeCN, in which both dyes have concentrations of ca. 15 µM. Emission spectra were recorded at excitation wavelength of 470 nm, LCT and TICT emission bands are shown for the L1 dye solely.
Figure 2 shows the normalized absorption and emission spectra of L1 on TiO2. The absorption spectrum presented is broader and red shifted (λabs~ 500 nm) than the ones for L1 in solution (λabs~ 475 nm in acetonitrile, see Figure 2)5. These differences have been assigned to the presence of aggregation of L1 on TiO2 surfaces.3 The presence of aggregations can be confirmed by measuring the excitation spectrum at emission of 700 nm of L1 on TiO2 (Figure 2), in which the narrower bands are due to non-aggregated forms of L1 on TiO2. Also, the position of the absorption spectrum of adsorbed L1 on TiO2 is similar to the one for the reported dimer form of L1 in polar solvents.5, 21 The similarity between the absorption spectra of adsorbed L1 and L1Fc on TiO2 and the dimer species confirms the increase in the dye’s dipole strength upon adsorption. Interestingly, upon physically covering the L1/TiO2 film by polar-plastic polymer (PMMA), the absorption spectrum has not been changed much except some broadening and slight red-shift. However, the measured emission spectra for L1/TiO2 before and after using PMMA is quite different (see Figure 2). If the polarity of PMMA22 would affect the measured emission data, one might expect that the emission data would shift 3 ACS Paragon Plus Environment
The Journal of Physical Chemistry
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to the red-side. However, the emission position for L1/TiO2 film that is maximized at ca. 625 nm, has been blue-shifted to ca. 575 nm. This 50 nm difference in emission maxima highlights the influence of the PMMA on the emissive state of L1 on TiO2, which are likely connected to the observed emission states in Figure 1, and discussed previously5, 9. In addition, the emission of adsorbed L1Fc dye on TiO2 is maximized ca. at 535 nm, as shown in Figure 2B, giving a difference in energy of ca. 2700 cm-1, with respect to the emission observed in the L1 case. This energy difference is similar to the one measured for those dyes in MeCN, Figure 1. Thus, the emissive state of L1/TiO2 is attributed to the TICT state, while the LCT state is assigned for the L1Fc/TiO2. An intermediate situation is expected upon using PMMA that hinders partially the torsional motions of L1 dye on TiO2.
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Figure 2: (A) Normalized absorption, excitation, and emission spectra of L1 dye on TiO2 in the absence and presence of PMMA. (B) Comparison between normalized emission data for L1, PMMA/L1, and L1Fc on TiO2, see the text for more discussion.
To investigate the electron dynamics of L1/TiO2 film in the absence and presence of PMMA, the fs-TA in the infrared region has been used. This measurement was done by exciting L1 in the visible range (~ 410 nm), and probing electrons in the CB of TiO2 around ~ 4900 nm.9 Figure 3 shows the 2D-color plot for electrons in the CB of L1 in the CB of TiO2 from 4600 to 5200 nm. The extracted kinetic trace of L1/TiO2 shows two processes, an electron injection process of ca. 440 fs, and a slow recombination process of more than 10 ns (see Figure 3B and Table 1).
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Figure 3: (A) False-2D color plot of electrons absorption in the CB of TiO2 after exciting the adsorbed L1 dyes in the presence of PMMA on top of adsorbed L1 dye, using fs-TA in the infrared region. (B) Normalized kinetic traces for the electrons absorption inside the CB of TiO2 from different dyes and under different conditions as shown in the legend.
However, the same conditions have been repeated again and compared to the presence of plastic polymer matrix (PMMA) on the top of adsorbed L1 on TiO2. The extracted kinetic trace shows different dynamics, in which the electron injection is a bit faster, ca. 220 fs, as well as the charge recombination process as shown in Table 1. Comparing these results with L1Fc on TiO2, in which TICT process is completely chemically blocked5, 9, the electron injection process has a time constant of 110 fs and the recombination process was relatively faster as well than L1, see Table 1. Seeing these results in Table 1 and Figure 2, the electron dynamics in both cases, L1Fc/TiO2 and PMMA/L1/TiO2, are very similar. Thus, the presence of PMMA is safely assigned to physically hindering the TICT process of L1 or similar dye, if present on semiconductors surfaces. For L1Fc/TiO2 and PMMA/L1/TiO2, the electron injection takes place from the LCT state as the TICT state is hindered (Table 1). For L1/TiO2, the electron injection can take place from either the LCT or TICT state; however, the electron injection is slower than the former cases with a time constant of 440 fs (Table 1). The transformation of the LCT to TICT state of L1 in solution (acetonitrile) happens in ca. 250 fs5, similar to other triphenyl-methane dyes using ultrafast transient IR probe.23 Nevertheless, this transformation is expected to be slower on semiconductor surfaces. To measure the time needed for converting LCT to TICT on surfaces, ZrO2, a high band-gap semiconductor blocking the electron injection process to happen, was used for L1 and L1Fc dye adsorption. Table 1: Extracted time constants from kinetic traces shown in Figure 3. Lifetimes are shown in ps with corresponding amplitudes in percent between parentheses.
Dye L1 L1/PMMA L1Fc
Electron injection 0.44 0.22 0.11
--4.5 (21) -----
Electron recombination --530 (16) Long τ (84) 35 (23) 430 (16) Long τ (40) 17 (30) 225 (36) Long τ (34)
Figure 4 shows changes in the 2D-transient spectra in the visible region for L1 on ZrO2 at early times with time constant of ca. 500 fs, as shown via the decay associated spectra (DAS) for the fitted data. This time constant has a corresponding red-shifted spectrum maximized at 5 ACS Paragon Plus Environment
The Journal of Physical Chemistry
575 and 675 nm. This spectrum is changed to a blue-shifted spectrum maximized at 550 nm within 500 fs (Figure 4). These changes have been attributed to the formation of TICT state of L1 on ZrO2 similarly to TICT formation of L1 in acetonitrile.5 In this case, the former spectrum represents the LCT state that is directly populated and the latter one corresponds to the TICT state. To the best of our knowledge, this is the first spectral evidence for the TICT formation for adsorbed organic dyes on semiconductor surfaces. The subsequent time constants, 10 and 240 ps, represent the twisted ground state11, and the recovery to the ground state of L1/ZrO2, respectively. As discussed previously, TICT state was not found for L1Fc in solution, so, it is expected also that it is not present on semiconductor surfaces.5 Reusing the global fitting procedure for L1Fc on ZrO2 reveals of four time constants of 260 fs, 2.65 ps, 50 ps, and ns component, with no spectral changes (Figure 4). The first time component represents the electron transfer from the Fc to the L1*, forming L1(-)—Fc(+).9 This radical ion pair formation is expected to followed by a vibrational relaxation process24, which is represented by the second time constant, 2.65 ps. The last components, 50 ps, and ns component are assumed to characterize partial geometrical rearrangements of L1(-)—Fc(+) and the formation of hot ground state species, respectively. The formation of hot ground species is expected due to the negative absorption present of the forth component in the DAS for L1Fc on ZrO2 (Figure 4).
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Figure 4: 2D-false color plots for visible transient absorption profiles for the L1 and L1Fc dyes adsorbed on ZrO2 (Top). Decay associated spectra extracted from the kinetic profile using global fit analysis for L1 on ZrO2 (A) and L1Fc on ZrO2 (B), at which various time constants were used for fitting the obtained data (Bottom). More description for the presented data are shown in the text.
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The Journal of Physical Chemistry
Using the above information, one can rebuild the kinetic model for different processes present in the adsorbed L1 and L1Fc dyes on TiO2 and ZrO2. Going back to dyes on TiO2, the observed lifetime for the electron injection of L1 on TiO2 is slower than L1Fc even when the TICT process is blocked in the later dye. Even though the transformation of LCT to TICT state on TiO2 is missing, this can be understood in the scope of TICT state formation. Originally, molecules showing TICT process are based on D-A molecular structure and the TICT formation is proportionally relies on the strength of D and A moieties.11 Upon adsorption of dyes on TiO2, the CB vibrational states of TiO2 overlap with the vibrational levels of the excited state for the adsorbed dye, which is similar to the donor-acceptor systems in solution.1, 10 This overlap would increase the strength for the TICT state formation on TiO2 and the time constant required would be faster than on ZrO2, i.e.,
