Molecular Engineering for Enhanced Charge Transfer in Thin-Film

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Molecular engineering for Enhanced Charge Transfer in Thin Film Photoanode Jeong Soo Kim, Byung-Man Kim, Un-Young Kim, HyeonOh Shin, Jung Seung Nam, Deok-Ho Roh, and Tae-Hyuk Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08098 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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ACS Applied Materials & Interfaces

Molecular engineering for Enhanced Charge Transfer in Thin Film Photoanode Jeong Soo Kim†,§, Byung-Man Kim‡,§, Un-Young Kim†, HyeonOh Shin†, Jung Seung Nam†, DeokHo Roh†, Tae-Hyuk Kwon*,†,‡

†Department

of Chemistry, Ulsan National Institute of Science and Technology, Ulsan, 689-798,

Republic of Korea

‡School

of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology,

Ulsan, 689-798, Republic of Korea

KEYWORDS: Molecular engineering, Bonding effect, Intramolecular charge transfer, Charge injection, Self-aggregation

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ABSTRACT: We developed three types of dithieno[3,2-b;2′,3′-d]thiophene (DTT)-based organic sensitizers for high-performance thin photoactive TiO2 films and investigate the simple but powerful molecular engineering of different types of bonding between the triarylamine electron donor and the conjugated DTT π-bridge by the introduction of single, double, and triple bonds. As a result, with only 1.3-µm transparent and 2.5-µm TiO2 scattering layers, the triple bond sensitizer (T-DAHTDTT) shows the highest power conversion efficiency (η = 8.4%; VOC = 0.73 V, JSC = 15.4 mA·cm−2 and FF = 0.75) in an iodine electrolyte system under one solar illumination (AM 1.5, 1000 W·m−2), followed by the single bond sensitizer (S-DAHTDTT) (η = 7.6%) and the double bond sensitizer (D-DAHTDTT) (η = 6.4%). We suggest that the superior performance of T-DAHTDTT comes from enhanced intramolecular charge transfer (ICT) induced by the triple bond. Consequently, T-DAHTDTT exhibits the most active photoelectron injection and charge transport on a TiO2 film during operation, which leads to the highest photocurrent density among the systems studied. We analyzed these correlations mainly in terms of charge injection efficiency, level of photo-charge storage, and charge transport kinetics. This study suggests that the molecular engineering of a triple bond between the electron donor and π-bridge of a sensitizer increases the performance of DSC with a thin photoactive film by enhancing not only JSC through improved ICT, but also VOC through the evenly distributed sensitizer surface coverage.

1. INTRODUCTION As third generation photovoltaic devices, dye-sensitized solar cells (DSCs) have received considerable attention in terms of their flexibilities, transparencies, and colorful displays, as well as high solar power conversion efficiencies (PCEs) at low manufacturing costs.1-4 To realize a flexible DSC, a thin film is indispensable for mechanical stability and low cost, but a thin TiO2 active layer results in lower efficiency because of the limited amount of loaded dye.5-7 For example, ruthenium complex-based 2

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dyes, like N719, require a TiO2 film that is over 10-µm thick in order to achieve more than 10% PCE with an iodine electrolyte system.8 In addition, organic sensitizers that employ porphyrin core units, such as YD2, required an 11.5-µm thick TiO2 active layer to achieve 8.8% PCE with an iodine electrolyte system, while a much lower PCE (5.6%) was recorded with a thin TiO2 active layer (2.4µm).9 Recently, a fused-thiophene organic dye based on improved molecular engineering of core unit recorded 10.2% PCE. However, the fused-thiophene organic dye also required 12-µm thick TiO2 film for the high performance, which still does not meet the requirement for the commercialization of flexible DSCs.10 Although sensitizers play very important roles in this trade-off between TiO2 film thickness and efficiency, there has been little research into dyes with high extinction coefficients as well as high efficiencies that are suitable for thin film devices.11 To achieve high performance from a thin TiO2 active layer (~1-µm), the following molecular engineering strategies for the structures of sensitizers need to be considered: (1) planarity for high extinction coefficients, (2) low aggregation to prevent the self-quenching of photo-excited excitons, and (3) strong intramolecular charge transfer (ICT) for efficient charge injection and broad absorption. So far, however, little attention has been given to the types of bonding between the electron donor and the conjugated π-bridge that satisfy all of abovementioned conditions. In this study, we developed three types of dithieno[3,2-b;2′,3′-d]thiophene (DTT)-based organic D-π-A sensitizers, with single, double, and triple bonds between the triarylamine electron donor and conjugated DTT π-bridge, as shown in Figure 1a. Depending on the bonding type, these molecules predominantly showed differences in charge injection capacity and degree of aggregation, which have a major influence on device performance. While the planar π-conjugated structure found in the double bond sensitizer (2-cyano-3-(5-(6-(5-(2-(4-(diphenylamino)phenyl)vinyl)2'-3'-hexylthienyl)-dithieno[3,2-b;2',3'-d]thien-2-yl)-4-hexylthien-2-yl)acrylic acid, D-DAHTDTT,) is most favorable for light-harvesting ability, molecules of D-DAHTDTT tend to agglomerate together due to intense π-π stacking on the surface of the TiO2 film. As a result, it exhibited a poor PCE of 6.4% 3



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with an open-circuit voltage (VOC) of 0.65 V, a photocurrent density (JSC) of 13.3 mA cm−2, and a fill factor (FF) of 0.73 in a thin TiO2 film (1.3-µm transparent and 2.5-µm scattering layers). To retard dye aggregation, a direct single bond was introduced between the donor and the DTT π-bridge. The calculated dihedral angle between the donor and the DTT π-bridge in the single bond sensitizer (2cyano-3-(5-(6-(5-(2-(4-(diphenylamino)phenyl)-2'-3'-hexylthienyl))-dithieno[3,2-b;2',3'-d]thien-2-yl)-4hexylthien-2-yl)acrylic acid, S-DAHTDTT) was the largest in the series, at 25°, as shown in Figure 1b and Table S1, resulting in steric hindrance and much less self-aggregation on the TiO2 surface compared to the other dyes, in turn leading to a highly enhanced VOC value (0.75 V). In order to further enhance ICT while preventing the severe aggregation, a triple bond was introduced between the donor and the DTT π-bridge. The triple bond could provide a pathway for enhanced ICT because of the proper planar dihedral angle (4.3°) between the donor and DTT π-bridge, as well as the extended π-conjugation. Moreover, it is less self-aggregating on TiO2 film than the double bonded analog considering the smaller photoluminescence (PL) loss factor. Consequently, the triple bond sensitizer (2-cyano-3-(5-(6-(5-(2-(4(diphenylamino)phenyl)ethynyl)-2'-3'-hexylthiophenyl)-dithieno[3,2-b;2',3'-d]thiophen-2-yl)-4hexylthiophen-2-yl)acrylic acid, T-DAHTDTT) led to active photoelectron injection by enhancing ICT, so that highly concentrated electrons diffuse within the TiO2 network. This resulted in the highest JSC (15.4 mA cm−2), enabling the T-DAHTDTT sensitizer to record the best PCE (8.4%) on a very thin active layer (1.3-µm) with the iodine electrolyte system. To understand the increased JSC, we studied factors such as light-harvesting efficiency (ηlh), charge injection efficiency (ηinj), and charge collection efficiency (ηcc) using UV/vis absorption spectroscopy, time-correlated single photon counting (TCSPC), and controlled intensity modulated photo spectroscopy (CIMPS).11 Accordingly, we found that ηinj contributed the most to the highly enhanced JSC value derived from the improved ICT pathway involving the triple bond. To support the relationship between the ICT and ηinj, we further investigated the level of photo-charge storage and charge transport on the TiO2 film. 4


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2. Result and Discussion 2.1. Synthesis The D-DAHTDTT sensitizer and 2,6-bis[2′-(3′-hexylthienyl)]dithieno[3,2-b;2′,3′d]thiophene 1 were prepared following previously reported literature procedures.12 The syntheses of SDAHTDTT and T-DAHTDTT are shown in Scheme 1. The bromo-aldehyde 3 was synthesized by the Vilsmeier-Haack

formylation

of

2,6-bis[2′-(3′-hexylthienyl)]dithieno[3,2-b;2′,3′-d]thiophene

1,

followed by bromination with N-bromosuccinimide (NBS) to afford the bromide 2. The boronate 5 was synthesized by the NBS bromination of triphenylamine, followed by borylation with n-butyl lithium and phenylboronic acid pinacol ester; 5 was further reacted with 3 by Suzuki coupling to give aldehyde 8. The terminal alkyne 7 was simply prepared by the Syferth-Gilbert procedure using the Bestman-Ohira reagent following formylation of triphenylamine; it was then reacted with 3 by Sonogashira coupling to produce aldehyde 9.13 Finally, carboxylic acids 10 and 11 were obtained via the Knövenagel condensation reaction of aldehydes 8 and 9, respectively, with cyanoacetic acid in the presence of piperidine. All intermediates and the target sensitizers were characterized by 1H and

13

C NMR, and

high-resolution mass spectroscopy(HR-MS). Their corresponding spectra are presented in the Supporting Information. 2.2 Optoelectronic Properties When a sensitizer is photo-excited, an electron is transferred from the donor to the acceptor through the conjugated structure. Therefore, altering the type of bonding between the donor and π-bridge influences the optical properties of the sensitizer. Figure 2a displays the molecular extinction coefficient spectra of the three DTT sensitizers in CH2Cl2, and the corresponding data are summarized in Table 1. The absorbance spectra of the sensitizers show two similar salient bands in the 300–400 nm and 400–550 nm regions. The former is caused by the aromatic π-π* transition of the triarylamine group, while the latter is induced by ICT from the triarylamine donor to the cyanoacetic acid acceptor. In the ICT-absorption spectral region, it is noteworthy that T-DAHTDTT (ε 5



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= 5.86 × 104 M−1 cm−1) exhibits the highest (λmax) molar extinction coefficient compared to SDAHTDTT (ε = 4.74 × 104 M−1 cm−1) and the π-conjugated D-DAHTDTT (ε = 5.25 × 104 M−1 cm−1). Moreover, on TiO2 films, the order of absorbance intensity is identical to that in solution, as shown in Figure 2b. In addition, D-DAHTDTT exhibited the most red-shifted absorbance spectrum on the film, followed by T-DAHTDTT and S-DAHTDTT. This observation is a result of the degree of intermolecular aggregation, as evidenced by TCSPC analysis in the charge injection efficiency section. We performed cyclic voltammetry (CV) to determine the energy of the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) of each DTT sensitizer, as shown Figure S1a–S1b and Table 1. The ground oxidation potential (Eox) is 0.84 V for S-DAHTDTT, 0.85 V for D-DAHTDTT, and 0.93 V for T-DAHTDTT (vs NHE; in this article all potentials are related to NHE). The excited-state potential was estimated by subtracting the transition energy (E0-0) from Eox. The excited-state potentials of the DTT sensitizers were found to be –1.47 V for S-DAHTDTT, –1.37 V for D-DAHTDTT, and –1.45 V for T-DAHTDTT. These potentials are sufficient for charge injection from the photo-excited sensitizers into the TiO2 conduction band (ca. –0.5 V). The charge excitation behavior of all three DTT sensitizers was anticipated by density functional theory (DFT) calculations using the B3LYP functional. The lowest excited states were characterized by time-dependent DFT (TDDFT) at the same level. All calculations were performed with the Gaussian 09 software.14 The frontier molecular orbitals (FMOs) of all sensitizers are shown in Figure S2 and the results from the timeindependent approach are summarized in Table S2. The frontier orbitals of the ground states of all sensitizers are mainly distributed from the triarylamine donor to the DTT π-bridge. In comparison, the FMOs of the excited states are predominantly localized from the DTT π-bridge to the cyanoacrylic acid acceptor. Accordingly, the excellent ICTs of all sensitizers, from the donor to the acceptor, are clearly predicted by DFT, and the trend in the calculated HOMO energies are well matched to the experimental oxidation potentials obtained from CV. We also compared photo-stability of DAHTDTT sensitizers 6


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through their film absorbance spectra according to duration of light soaking under one sun condition. As shown in Figure S3, S-DAHTDTT shows negligible photo-degradation after 60 min of light soaking, whereas D-DAHTDTT suffers from photo-degradation due to its photo-isomerization. 2.3 Device Performance The DTT sensitizers were applied to the DSCs as thin (1.3-µm) active layers. For preparing the photoactive TiO2 films, screen-printed mesoporous TiO2 films were immersed in a 0.2 mM dye solution in CHCl3/EtOH (v/v, 7:3) with 10 mM chenodeoxycholic acid (CDCA) as the co-adsorbate, in the dark, at room temperature for 5 h. Photo electrodes were assembled with Pt-coated counter electrodes into sandwich configurations, followed by the injection of electrolytes composed of 0.055 M I2, 0.025 M LiI, 0.05 M guanidine thiocyanate, 0.6 M DMPII and 0.5 M 4-tert-butylpyridine in a mixture of acetonitrile and valeronitrile (v/v, 85:15). Figure 3a depicts the current-voltage (J-V) curves of each device measured under one solar illumination (AM 1.5), and all performance parameters are listed in Table 2. With a thin 1.3-µm active layer, T-DAHTDTT reached the best PCE (η = 8.4%), corresponding to a VOC of 0.73 V, JSC of 15.4 mA·cm−2, and FF of 0.75. To the best of our knowledge, this is one of the best performances of a sensitizer on a thin TiO2 film (1.3-µm transparent layer) using the iodine electrolyte system. The next highest PCE (η = 7.6% with a VOC of 0.75 V, JSC of 14.2 mA·cm−2, and FF of 0.72) was achieved by S-DAHTDTT. The highest VOC for S-DAHTDTT suggests that this dye has the largest torsion angle among the dyes under investigation, leading to the lowest level of aggregation on the TiO2 film, and hence, the lowest charge recombination rate. This result is in good agreement with the transient PL and dark electrochemical impedance spectroscopy (EIS) results, which are further discussed later in this article. On the other hand, D-DAHTDTT exhibits the lowest PCE (η = 6.4% with a VOC of 0.65 V, JSC of 13.3 mA·cm−2, and FF of 0.73) because of severe π-π stacking. SDAHTDTT and D-DAHTDTT show the trade-off relationship between a conjugated planar structure for efficient light-harvesting and π-π stacking on the TiO2 film.15 In other words, molecular engineering of the type of bonding between the donor and the π-bridge is effective for thin TiO2 film DSCs, 7



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resulting in the PCE ordering: T-DAHTDTT (8.4%) > S-DAHTDTT (7.6%) > D-DAHTDTT (6.4%). To understand the PCE trend, further in-depth analyses will be discussed in the following parts. 2.3.1. Incident Photon to Current Conversion Efficiency (IPCE) In Figure 3b, the incident photon to current conversion efficiency (IPCE) trend is shown to correspond to the JSC trend over a broad spectral range (~600 nm); this mainly contributes to improving the PCE. According to equation 1, the JSC is determined from the light-harvesting efficiency (ηlh), charge-injection efficiency (ηinj), and the charge-collection efficiency (ηcc): =

(1)

, where q is the elementary charge and I0 is the incident photon flux.16 By analyzing these factors (ηlh, ηinj and ηcc), we can determine which factor is the most influential on device performance and understand various device phenomena resulting from it. 2.3.2. Charge Injection Efficiency (ηinj) Generally, ηinj is affected by two factors: (1) the energy gap between the excited state of the sensitizer and the conduction band of the metal oxide, and (2) the degree of dye aggregation on the film.17 However, based on our results, ICT also plays a critical chargeinjection role. To confirm our hypothesis, we evaluated the ηinj of each sensitizer through TCSPC technique.18 We prepared two types of dummy cell based on porous zirconium dioxide (ZrO2) and TiO2 films, which were sandwich-type cells containing an inert acetonitrile solution between the dyed metal oxide on fluorine-doped tin oxide (FTO), and a pure FTO counterpart. Figure 4a shows the principle for determining ηinj. In the case of the ZrO2-based dummy cell, the conduction band edge of ZrO2, which is higher than the lowest unoccupied molecular orbital (LUMO) level of the sensitizers, prevents photoelectron injection from the excited sensitizers into the ZrO2. On the contrary, the conduction band edge of TiO2 is much lower than the LUMO level of the sensitizers and allows smooth photoelectron injection into the TiO2. Therefore, the difference in the PL decay in

8


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each case provides information on the charge injection ability (Figure 4b–4d). The ηinj of each sensitizer was determined according to equation 2: = − ⁄

(2)

, where τTiO2 is the exciton lifetime on the dyed TiO2 film and τZrO2 is that of the dyed ZrO2 film (see Table S3 for details). As expected from the discussion above, T-DAHTDTT provides the highest ηinj (94%), followed by S-DAHTDTT (89%) and D-DAHTDTT (74%), which are well matched with the JSC trend. Furthermore, we calculated ηinj using another approach introduced by Barnes et al. (Figure S4); ηinj was quantitatively estimated by external quantum efficiency (ηEQE), absorption coefficient (α), electron diffusion length (Ln), and active layer thickness (d), as shown in Equation S1.19 The calculated ηinj trend is identical to that determined experimentally by TCSPC. Therefore, from the findings detailed above, we can conclude that the high JSC of T-DAHTDTT is mainly attributable to enhanced ICT leading to an efficient charge injection rate. On the other hand, the most agglomerated D-DAHTDTT clearly exhibits the lowest ηinj because of the fastest self-quenching induced by excessive dye aggregation as discussed in section 2.3.5. To further investigate the impact of enhanced ICT on device performance, we measured the electrical charge stored in the TiO2 film after exposure to light. Firstly, DSCs were illuminated in the open-circuit state to photo-charge the TiO2 film. In this situation, photo-injected electrons cannot reach the external circuit, only repeating the injection and recombination processes. After sufficient time had passed to achieve charge equilibrium, the light was switched off and the DSCs were concurrently short circuited to extract the stored charge. The purpose of this experiment was to find out which sensitizer supplies the most photo-charge during irradiation. Actually, it was difficult to determine only the photo-injected charge using the standard electrolyte due to the undesirable charge loss arising from the charge recombination process with the oxidized redox ions. For this reason, we determined the levels of stored charge as functions of electrolyte concentration, as summarized in Table 3 (see Figure S5 for spectra). 9



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Figure 5a and 5b display the extracted charges (Qext) as functions of time for electrolyte concentrations of 100% and 1%, together with VOC. The initially observed solid line corresponds to VOC generated under irradiation. After 5 seconds of irradiation when the light is turned off, the concurrent broken line corresponds to Qext. With 100% electrolyte, S-DAHTDTT (41.01 µC) provides a slightly higher Qext than T-DAHTDTT (40.33 µC), but a much higher Qext than D-DAHTDTT (24.31 µC), which is well matched to the chemical capacitance (Cµ) trend acquired from the EIS measurements (Table 3 and Figure 5c). This observation is the result of the slowest charge recombination corresponding to SDAHTDTT-based DSCs as discussed previously. However, the Qext order observed for S-DAHTDTT and T-DAHTDTT was inverted with the dilute electrolytes (0 – 50%). In these cases, the impact of charge recombination becomes more and more negligible with decreasing electrolyte concentration, and the contribution of the photo-injected charge to Qext dominates. Consequently, T-DAHTDTT, with the most active charge injection, always provides the highest Qext with the diluted electrolytes. This is clear evidence of the effect of enhanced ICT, through the triple bond, on device performance. Considering that charge transport in a TiO2 network occurs through electron diffusion, the injection of active photoelectrons must accelerate charge transport. In light of this, we investigated electron diffusion in the TiO2 network under irradiation in terms of charge transport resistance (Rtran) and charge transport time (τd). Rtran generally appears in the near middle frequency range (from 102 to 103 Hz) in a Nyquist diagram.20 Values of Rtran were estimated using the transmission line model and are summarized in Table 3.21 As expected, T-DAHTDTT showed the smallest Rtran (1.7 Ω), followed by S-DAHTDTT (2.6 Ω), and D-DAHTDTT (4.7 Ω). This result is more reliable when considered in conjunction with the τd trend. CIMPS was employed to measure τd values, which are presented as functions of incident photon flux (λ = 463 nm) in Figure 5d; T-DAHTDTT exhibited the fastest charge transport, followed by S-DAHTDTT and D-DAHTDTT. Note that the results obtained for ηinj, Qext, and τd share a common connection and clearly support our hypothesis about a correlation between enhanced ICT and JSC. 10


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2.3.3. Charge Collection Efficiency (ηcc) The ηcc was also considered a candidate factor for determining the JSC trend. The ηcc is closely related to both τd and charge recombination time (τrec) because the photo-injected electrons are prone to inevitable recombination processes with the oxidized redox ions or oxidized dyes before reaching the charge collector; = − ⁄ .22 Accordingly, τd and τrec were measured using CIMPS in the short-circuit and open-circuit states, respectively. Plots of τd, τrec, and resulting ηcc, as functions of incident photon flux (λ = 463 nm), are shown in Figure S6, and the corresponding data are summarized in Table S4. As stated above, T-DAHTDTT exhibited the shortest τd in the device, while the longest τrec was provided by S-DAHTDTT because of low self-aggregation. As a result, S-DAHTDTT and T-DAHTDTT exhibit the same ηcc (avg. 0.98), However, the ηcc (avg. 0.94) of D-DAHTDTT was lower, which is attributed a higher rate of charge recombination due to severe dye aggregation on the TiO2 surface. 2.3.4. Light Harvesting Efficiency (ηlh) The ηlh is determined by the molar extinction coefficient and the level of dye-loading in the projected area ( = − = − ).23 All sensitizers exhibited favorable near infrared (IR) absorptions, with light-harvesting efficiencies in the order: D-DAHTDTT > T-DAHTDTT > S-DAHTDTT, as shown in Figure S7. This trend is in good accordance with that of the absorbance spectra on the TiO2 films (Figure 2b). Although D-DAHTDTT shows a slightly higher ηlh compared to that of T-DAHTDTT over 510 to 690 nm, T-DAHTDTT provides higher IPCE values over a wide range of the visible region (up to 610 nm), as shown in Figure 3b. This result clearly demonstrates that the slightly lower value of ηlh for T-DAHTDTT is completely compensated for by the other factors, ηinj and ηcc. 2.3.5. Open-Circuit Voltage (VOC) As one of the influential factors on VOC, we investigated the degree of dye aggregation. A number of groups have suggested highly concentrated dye monolayer on the solid state adsorbents compared with state in the bulk solution.17,

24

Closely neighboring dye

monomers might produce quenched excition lifetime due to their fast charge seperation.25-26 Based on 11



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this point, we suggest the method for characterizing the degree of dye aggregation on the film. Comparing PL lifetime of dye monolayer on ZrO2 film with that in the bulk solution, we determine PL loss factor (δPL) that is closely associated with the degree of dye aggregation as following equation 3:

!

= − ⁄"

(3)

, where τSol is the exciton lifetime in the bulk dye solution. As mentioned above, the ordering of δPL reveals which sensitizer is likely to agglomerate on the TiO2 film. Figures S8a and S8b display transient PL signals extracted from the ZrO2-based dummy cells and 20 µM dye-dissolved dichloromethane solution, respectively. We summarized corresponding fitting data in Table S3. The calculated δPL values are: D-DAHTDTT (0.76) > T-DAHTDTT (0.49) > S-DAHTDTT (0.21), which reflects the order of dye aggregation. These results suggest that S-DAHTDTT effectively prevents self-aggregation compared to the other dyes, presumably because it possesses the largest torsion angle. The degree of aggregation directly influences the VOC of the device because sensitizers that are more agglomerating make the TiO2 surface more exposed to oxidized redox mediators (I3−) and thus charge recombination processes. To demonstrate this, EIS measurements were conducted in the dark condition;27 Figure S8c displays the Nyquist diagram for each device. The largest charge recombination resistance of 354.5 Ω was produced by S-DAHTDTT, followed by T-DAHTDTT (139.9 Ω), and D-DAHTDTT (57.1 Ω), which is in good accordance with the trend in VOC (0.75 V for S-DAHTDTT, 0.73 V for TDAHTDTT, and 0.65 V for D-DAHTDTT). These findings suggest that the S-DAHTDTT sensitizer might form the most evenly distributed dye monolayer on the TiO2 surface, minimizing the surface area in contact with I3− ions in the electrolyte. As a result, the reduced charge recombination rate of SDAHTDTT led to the highest VOC among these dyes;20 this is also clearly demonstrated by the trend in charge recombination times measured in the dark EIS experiments: S-DAHTDTT (20.1 ms) > TDAHTDTT (11.3 ms) > D-DAHTDTT (3.2 ms). In addition, there is no huge difference of calculated dipole moment between dyes as shown in Table S5. In other words, the position of dyed TiO2 CB edge 12


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might be similar each other. Note that all DAHTDTT sensitizers show similar dye loading amount on the TiO2 film (Table 2) because they have the same DTT π-bridge backbone with hexyl group, which mainly determines the dye adsorption mode. Therefore, we point out that such huge difference in the dye aggregation and charge recombination might originate from the different bonding type between the triarylamine donor and the DTT π-bridge. In-depth studies are being undertaken to clarify the more exact picture of correlation between dye aggregation and molecular structure.

3. Conclusions

We successfully designed and synthesized three organic sensitizers based on the planar dithienothiophene (DTT) π-bridge, by modifying the type of bonding between the donor and the bridge, to give S-DAHTDTT, D-DAHTDTT, and T-DAHTDTT as high-performing materials for use in thin film DSCs. The triple bond in T-DAHTDTT is beneficial to the molar extinction coefficient and charge injection via enhanced ICT. Furthermore, T-DAHTDTT self-aggregates less than the double-bonded DDAHTDTT. As a result, T-DAHTDTT exhibits a superior photocurrent density (JSC), resulting in the best PCE of 8.4% (VOC = 0.73 V, JSC = 15.4 mA· cm−2 and FF = 0.75) in the series, with only a 1.3-µm active layer under one solar illumination (AM 1.5, 1000 W·m-2), because of active charge injection and, hence, fast charge transport through TiO2 network. By molecular engineering the type of bonding, a material exhibiting one of the best performances on a ~1-µm-thick active layer, in the iodine electrolyte system, was developed. In addition, the single bond in S-DAHTDTT leads to a highly enhanced VOC, approximately 100 mV higher than that of D-DAHTDTT, because of alleviated dye aggregation and, as a consequence, reduced charge recombination. This results in S-DAHTDTT exhibiting the second highest PCE (7.6%), while D-DAHTDTT exhibited the poorest performance (6.4%) due to severe selfaggregation. Ultimately, the molecular engineering of the triple bond between the donor and π-bridge is

13



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crucial in enhancing ICT and the mitigation of the self-aggregation. These results open a new design strategy

for

high

performance

sensitizers

in

thin

film

DSCs.

ASSOCIATED CONTENT Supporting Information Material characterization, cyclic voltammetry, DFT calculations, and detailed processes for electrochemical and photophysical analyses. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Author Contributions §

J.S.K and B.-M.K. contributed equally to this work.

Notes

The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support provided by the Ulsan National Institute of Science and Technology (Research Fund 1.170003.01), the National Research Foundation of Korea (Grant 2016R1A2B4009239), and the Korea Institute of Energy Technology Evaluation and Planning (No. 20143030011560), and the Technology Development Program to Solve Climate Change of the NRF funded by the Ministry of Science, ICT and Future Planning (2016M1A2A2940910). 14


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Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16, Wallingford, CT, 2016. (15) Liu, J.; Zhou, D.; Xu, M.; Jing, X.; Wang, P., The Structure-Property Relationship of Organic Dyes in Mesoscopic Titania Solar Cells: Only One Double-Bond Difference. Energy Environ. Sci. 2011, 4, 3545-3551. (16) Guo, M.; Li, M.; Dai, Y.; Shen, W.; Peng, J.; Zhu, C.; Lin, S. H.; He, R., Exploring the Role of Varied-length Spacers in Charge Transfer: a Theoretical Investigation on Pyrimidine-Bridged Porphyrin Dyes. RSC Adv. 2013, 3, 17515-17526. (17) Katoh, R.; Furube, A., Electron Injection Efficiency in Dye-Sensitized Solar Cells. J. Photochem. Photobiol., C 2014, 20, 1-16. (18) Eom, Y. K.; Choi, I. T.; Kang, S. H.; Lee, J.; Kim, J.; Ju, M. J.; Kim, H. K., Thieno[3,2‐ b][1]benzothiophene Derivative as a New π‐Bridge Unit in D–π–A Structural Organic Sensitizers with Over 10.47% Efficiency for Dye‐Sensitized Solar Cells. Adv. Energy Mater. 2015, 5, 1500300. (19) Barnes, P. R. F.; Anderson, A. Y.; Koops, S. E.; Durrant, J. R.; O’Regan, B. C., Electron Injection Efficiency and Diffusion Length in Dye-Sensitized Solar Cells Derived from Incident Photon Conversion Efficiency Measurements. J. Phys. Chem. C 2009, 113, 1126-1136. (20) Wang, M.; Chen, P.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M., The Influence of Charge Transport and Recombination on the Performance of Dye-Sensitized Solar Cells. ChemPhysChem 2009, 10, 290-299. 17



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)


(b)

Figure 1. (a) Chemical structures of S-DAHTDTT, D-DAHTDTT and T-DAHTDTT. All sensitizers have flat structures and are identical except for the type of bond between the diarylamine donor and DTT core unit. (b) DFT-optimized ground-state geometries of all DAHTDTT sensitizers with key dihedral angles indicated. The values of α°, β°, and γ° in the ground and oxidized state are listed in Table S1.

19



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Scheme 1. Synthesis routes to DAHTDTT-based sensitizers.

20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

Figure 2. UV-vis absorption spectra of the three dyes in: (a) solution, and (b) film.

21



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Table 1. Summary of Photo-Physical Data for DAHTDTT-Based Sensitizers.

Abs.[a] 4

-1

-1

(ε*10 M cm )

EOX[b] vs

E0-0[c] vs

EOX – E0-0 vs

NHE (V)

NHE (V)

NHE (V)

HOMO

HOMO

/LUMO[d]

/LUMO[e]

(eV)

(eV)

S-DAHTDTT

441 (4.74)

0.84

2.31

–1.47

-5.34/-3.03

-5.14/-3.06

D-DAHTDTT

439 (5.25)

0.85

2.22

–1.37

-5.35/-3.13

-5.05/-3.06

T-DAHTDTT

434 (5.86)

0.93

2.38

–1.45

-5.43/-3.05

-5.19/-3.07

All the spectra were measured in 20 µM CH2Cl2 solution at 298 K. [b] The onset point in anodic curve was used for estimating ground state oxidation potential. [c]The transition energy of each dye was estimated from the onset point in absorbance spectrum. All the potential were represented versus normal hydrogen electrode by adding Fc+/Fc redox potential (+0.63 V vs NHE). [d] Experimental HOMO and LUMO from CV. [e] Calculated HOMO and LUMO from DFT.

[a]

22


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(a)

(b)

Figure 3. (a) J-V curves of DSCs based on the DAHTDTT sensitizers. (b) Incident photon to current conversion efficiencies of the DAHTDTT sensitizers on a 1.3-µm active layer.

23



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Table 2. Photovoltaic Parameters of DSCs Based on DAHTDTT Sensitizers. Film thickness (µm) S-DAHTDTT D-DAHTDTT T-DAHTDTT

1.3 + 2.5

VOC

JSC −2

η[a]

ηinj[b]

(%)

(%)

Dye loading

FF

−8

mol cm-2)

(V)

(mA cm )

0.75

14.2

0.72

7.6

89

3.98

0.65

13.3

0.73

6.4

74

5.04

0.73

15.4

0.75

8.4

94

4.09

(× 10

[a]

The power conversion efficiency was calculated as follows: $ = %&' · )*' · ++⁄,-. , where Pin is the incident light intensity (1000 W·m-2), VOC is the open circuit voltage, JSC is the photocurrent density and FF is the fill factor. When measuring, a black mask (0.5 × 0.5 cm2) was attached on the DSCs for preventing the overestimation. All dye solutions were composed of 0.2 mM DAHTDTT-based sensitizers in CHCl3 with 10 mM chenodeoxycholic acid (CDCA) as co-adsorbate. [b]Charge injection efficiencies of sensitizers were evaluated from Equation 2 ( $-.0 = 1 − 2 3-&4 ⁄256&4 ). Each exciton lifetime was measured by fitting transient PL spectra (Figure 4b–4d), followed by equalization of the whole spectra.


24

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(a)

(c)

(d)

Figure 4. (a) The principle of ηinj evaluation. Excited photoelectrons in the dyed ZrO2 layer selfrelax through radiative emission (left). In contrast, excited photoelectrons in the dye smoothly transfer to the conduction band of the TiO2, leading to considerable quenching of radiative emission (right). PL decay signals from dyed metal oxide layer: (b) S-DAHTDTT, (c) DDAHTDTT, and (d) T-DAHTDTT.


25


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(a)

(b)

(c)

(d)

Page 26 of 28

Figure 5. Extracted charges (broken lines) as functions of time and electrolyte concentration: (a) 100%, and (b) 1%, together with VOC (solid line). In Figure a-b, solid symbols correspond to VOC at the open circuit state, while blank symbols correspond to Qext at the short circuit state. (c) Nyquist diagrams of the DSCs according to sensitizer. The inset shows the equivalent circuit used for fitting. EIS measurements were conducted under one solar conditions, giving AC modulations (10 mV) superimposed on constant forward biases (−VOC) with frequencies ranging from 106 to 10−1 Hz. (d) Charge transport times as functions of incident photon flux (λ = 463 nm). The fastest charge transport observed for T-DAHTDTT results from the most active photoelectron injection.


26

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Table 3. Fitting Parameters of EIS Measurement and the Extracted Charge Amount from DAHTDTT-Based DSCs Depending on the Electrolyte Concentration. Qext[a] (µC) RS

Rdl

Rtran

Rct

Cµ

(Ω)

(Ω)

(Ω)

(Ω)

(µF)

0%

100% 1%

10%

50%

(Inert)

(Standard)

S-DAHTDTT

5.2

5.1

2.6

18.8

145.3

8.23

24.35

27.69

27.33

41.01

D-DAHTDTT

5.6

3.5

4.7

23.7

81.6

6.83

16.32

17.95

22.19

24.31

T-DAHTDTT

6.2

5.8

1.7

12.4

137.2

9.42

28.44

35.01

32.26

40.33

[a]

Charge extraction was performed in shielded black box using monochromatic light source (λmax = 503 nm, 100 W −2 m ). Photoactive objects, DSCs, were illuminated under open circuit state for 5 s, followed by switching rapidly to short circuit state for charge extraction with discharging current of 10 µA.


27


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Table of Contents


28

Molecular Engineering for Enhanced Charge Transfer in Thin-Film

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