Novel D-A-π-A-Type Organic Dyes Containing a Ladderlike Dithienocyclopentacarbazole Donor for Effective Dye-Sensitized Solar Cells.

A novel ladderlike fused-ring donor, dithienocyclopentacarbazole (DTCC) derivative, is used to design and synthesize three novel donor-acceptor-π-acceptor-type organic dyes (C1-C3) via facile direct arylation reactions, in which the DTCC derivative substituted by four p-octyloxyphenyl groups is served as the electron donor and the carboxylic acid group is used as the electron acceptor or anchoring group. To fine-tune the optical, electrochemical, and photovoltaic properties of the three dyes, various auxiliary acceptors, including benzo[2,1,3]thiadiazole (BT), 5,6-difluorobenzo[2,1,3]thiadiazole (DFBT), and pyridal[2,1,3]thiadiazole (PT), are incorporated into the dye backbones. The results indicate that all of the three dyes exhibit strong light-capturing ability in the visible region and obtain relatively high molar extinction coefficients (>31 000 M-1 cm-1) due to their strong charge transfer (CT) from donor to acceptor. Moreover, theoretical model calculations demonstrate fully separated highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels for the three dyes, which is helpful for efficient charge separation and electron injection. Using the three dyes as sensitizers, conventional dye-sensitized solar cells (DSSCs) based on liquid iodide/triiodide redox electrolytes are fabricated. Our results indicate that the BT-containing dye C1 affords the highest power conversion efficiency of up to 6.75%, much higher than that of the DFBT-containing dye C2 (5.40%) and the PT-containing dye C3 (1.85%). To our knowledge, this is the first example reported in the literature where the DTCC unit has been used to develop novel organic dyes for DSSC applications. Our work unambiguously demonstrates that the ladderlike DTCC derivatives are the superb electron-donating blocks for the development of high-performance organic dyes.

Introduction

Dye-sensitized solar cells (DSSCs) as a kind of clear-energy device have attracted worldwide attention due to their great potential in low-cost fabrication of highly flexible, large-scale energy conversion devices.[1−7] Thanks to great efforts in innovative materials design and device engineering, DSSCs have exhibited much enhanced power conversion efficiency (PCE) recently.[1−7] As the key part of DSSCs, sensitizers play a crucial role in light capture, charge separation/recombination, and photovoltaic properties of the devices.[8−12] Generally, the sensitizers can be divided into two main categories: metal organic complexes and metal-free organic dyes. Although metal organic complexes, especially for zinc porphyrin[13−18] and ruthenium polypyridine,[19−21] have achieved superior photovoltaic performance relative to metal-free dyes, their intrinsic drawbacks, such as heavy metal toxicity, high cost, and difficulty in synthesis and purification,[2] would limit the further development. Recently, great interest has arisen to develop metal-free organic dyes owing to their tremendous advantages, including low production cost, tunable structure, and facile synthetic procedure.[22−24] So far, some metal-free sensitizers have obtained encouraging PCEs of above 12%,[25−27] which can be comparable to metal complex dyes. Undoubtedly, the development of novel sensitizers is one of the most promising approaches for further advances in photovoltaic performance as well as understanding the relationship between materials structure and performance.

As far as molecular design is concerned, current mainstream strategy for metal-free organic dyes is to construct donor−π–acceptor (D−π–A) or donor–acceptor−π–acceptor (D–A−π–A) configuration to ensure efficient light capture and strong intramolecular charge transfer.[28−32] Compared to D−π–A dyes, the properties of D–A−π–A-type organic dyes can be readily adjusted by inserting an auxiliary acceptor between the D and π units, which thereby endows them with more flexible photoelectric properties. Moreover, this delicate molecular modification can not only fine-tune energy-level structure and light response region but also facilitate electron transportation from D unit to anchor group.[24] On the basis of this strategy, current research interest for ideal D–A−π–A-type organic dyes mainly focuses on the modulation of π-conjugated aromatic donors,[33−37] π-bridges,[38−40] anchoring groups,[41] auxiliary acceptors,[42,43] as well as solubilizing side chains.[44−46] Among them, the exploitation of novel π-conjugated aromatic donors is considered to be the most versatile approach to develop large numbers of high-performance organic dyes. Hence, further development of novel aromatic donors for high-performance D–A−π–A-type organic dyes is worthy of being studied.

Due to their highly electron-donating nature and tunable modular structure, dithienocyclopentacarbazole (DTCC) derivatives have been widely utilized as the donor unit to develop a set of organic/polymeric semiconductors for organic solar cells (OSCs) and organic field-effect transistors (OFETs).[47−50] Since the first report on DTCC-based polymeric semiconductor by Hsu et al.,[47] a series of DTCC-like aromatic fused rings and their organic/polymeric semiconductors have sprung up subsequently.[50] Traditionally, they have been used as p-type semiconductors in OFETs and fullerene-based OSCs, which afford 0.001–0.14 cm2 V–1 s–1 hole mobilities and 3.7–4.6% PCE values.[47,48] Our recent work reveals that, by embedding two strongly electron-deficient 3-(dicyanomethylene)indian-1-one terminal groups, the first n-type DTCC-based organic semiconductor (DTCC-IC) has been successfully developed for nonfullerene OSCs,[51] which achieves an encouraging PCE of 6.0% and a high electron mobility of 2.17 × 10–3 cm2 V–1 s–1. Although great advances in OSCs and OFETs have been made for DTCC-based semiconductors recently, there is no DTCC-based organic dye reported to date that can be applied in DSSCs.

Compared to the parent carbazole unit, our developed DTCC derivative (see Figure ) exhibits much stronger electron-donating ability due to the incorporation of two indenothiophenene units into the molecular backbone; moreover, its unique 3D spatial structure (see Figure ) caused by four aryl side groups would be expected to reduce dye aggregation on TiO2 films and retard interfacial charge recombination. On the basis of the discussion mentioned above, we report the design, synthesis, and structural characterization of three D–A−π–A-type DTCC-based organic dyes (C1–C3, Figure ), in which the ladder-type DTCC derivative substituted by four p-octyloxyphenyl groups is used as the donor unit and the carboxylic acid group is used as the electron acceptor. To manipulate the optical, electrochemical, and photovoltaic properties of the three dyes, we use different electron-withdrawing units as the auxiliary acceptors in the three dyes, including benzo[2,1,3]thiadiazole (BT), 5,6-difluorobenzo[2,1,3]thiadiazole (DFBT), and pyridal[2,1,3]thiadiazole (PT). Investigation of the performance of DSSCs has demonstrated that the DSSC devices fabricated from the BT-containing dye C1 achieves the highest PCE value of up to 6.75%, which is much higher than that of the DFBT-containing dye C2 (5.40%) and the PT-containing dye C3 (1.85%).

Figure 1

Molecular structures of carbazole, DTCC, and DTCC-based organic dyes.

Results and Discussion

Synthesis and Characterization

Synthetic routes to the three DTCC-based organic dyes are illustrated in Scheme . Detailed synthetic procedures and characterization data are provided in Supporting Information. Compound 3 was prepared via a Suzuki coupling reaction of 9-octyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (1) and ethyl 2-bromothiophene-3-carboxylate (2) in high yields (75%). Subsequently, a double nucleophilic addition reaction of freshly prepared 4-(octyloxy)phenyllithium and compound 3 was performed to obtain an aromatic alcohol, which was then directly converted to the ladderlike dithienocyclopentacarbazole (DTCC) derivative via a Friedel–Crafts reaction. Additionally, a series of key intermediates (aryl halides, 9–11) were prepared via Suzuki coupling reactions of ethyl 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzoate (5) and compounds 6–8. After that, the DTCC donor was translated to the D–A−π–A-type intermediates (12–14) via direct arylation reactions (DARs) in moderate yields of ca. 40%. It should be mentioned that this versatile DAR approach makes our synthetic procedures more environmentally friendly and facile,[52−54] which enables direct coupling reaction between the DTCC donors and the aryl halides (9–11). Finally, the target organic dyes C1–C3 were obtained in high yields (>73%) by the hydrolysis of compounds 12–14. All of the dyes were purified by column chromatography on silica gel and then recrystallized from acetone. The chemical structures of all of the novel compounds were confirmed by 1H NMR, 13C NMR, and matrix-assisted laser desorption ionization time-of-flight (Figures S4–S34). Furthermore, the structural regioregularity of compound 11 was confirmed by 1H–1H nuclear Overhauser effect spectroscopy (Figure S22). The results reveal that no cross-correlation peaks can be observed between the PT (H1) and phenyl group (H2 and H3) protons (Figure S22), which thereby provides an evidence for the regioselective cross-coupling between compounds 8 and 5. We thus assume that the PT-containing dye C3 has a well-defined structural regioregularity.

Scheme 1

Synthetic Routes to the Three Dyes C1–C3

Reagents and conditions: (a) Pd(PPh3)Cl2, K2CO3, toluene/H2O, 90 °C; (b) 1-bromo-4-(octyloxy)benzene, tetrahydrofuran (THF), n-BuLi, −78 °C for 2 h and then 70 °C for 24 h; acetic acid/H2SO4, 85 °C, 4 h; (c) Pd(OAc)2, Cs2CO3, PivOH, toluene, 110 °C, 4 h; (d) KOH, THF/H2O (v/v, 3/1), 80 °C, 12 h.

Optical Properties

The UV–vis absorption spectra of the three dyes C1–C3 in diluted CHCl3 solutions (10–5 M) are presented in Figure a, and the relevant data are summarized in Table . As shown in Figure a, three dyes exhibit a similar absorption feature with two typical absorption bands in the ranges of 350–420 and 450–680 nm, respectively. The former bands can be attributed to a localized aromatic π–π* transitions, whereas the latter ones can be assigned to a charge transfer (CT) between DTCC donor and electron acceptor.[55−57] The CT peaks and the corresponding molar extinction coefficients (ε) are observed at 516 nm (ε = 3.46 × 104 M–1 cm–1) for C1, 509 nm (ε = 3.89 × 104 M–1 cm–1) for C2, and 568 nm (ε = 3.14 × 104 M–1 cm–1) for C3 (Figure S1). The results indicate that the incorporation of DTCC donor into the dye backbone can achieve high ε values (>3.14 × 104 M–1 cm–1). Therefore, these dyes are expected to efficiently capture light and enable high photocurrents in their DSSCs. Although DFBT unit has much stronger electron affinity as compared to BT unit, the CT peak of the DFBT-containing dye C2 still displays a slight blue shift by ca. 7 nm. This blue-shifted feature can be ascribed to the disruption of molecular planarity due to the introduction of two large fluoro substitutes into C2, thereby resulting in larger dihedral angle (−38.2°) relative to C1 (−35.1°), which is demonstrated in theoretical model calculations in Figure . Moreover, a significant red shift of about 52 nm is observed for the PT-containing dye C3, as a result of the strong CT transfer and the enhanced molecular planarity among the three dyes (Figure ). The normalized absorption spectra on TiO2 films are depicted in Figure b. Once anchored on TiO2 films, the CT peaks of C1, C2, and C3 are determined to be 512, 498, and 551 nm, respectively. In comparison to their solution spectra, all of the dyes show ca. 4–17 nm blue shift because of the deprotonation effect of the acid groups binding with TiO2.[11]

Figure 2

UV–vis absorption spectra of the three dyes (a) in CHCl3 (10–5 M) and (b) on TiO2 films.

Table 1

Photophysical and Electrochemical Properties of the Three Dyes C1–C3

	λabs (nm)
	ε (M–1 cm–1)
dye	π–π*	CT	λabsb (nm)	λemc (nm)	λint (nm)	E0–0 (eV)	Eox (eV)	Ered (eV)	Egap (eV)
C1	401	516	512	645	579	2.14	1.10	–1.04	0.54
	(48 400)	(34 600)
C2	398	509	498	642	572	2.17	1.17	–1.00	0.50
	(47 600)	(38 900)
C3	406	568	551	718	638	1.94	1.14	–0.80	0.30
	(59 100)	(31 400)

Maximum absorption in CHCl3 solution (10–5 M) at 25 °C.

Maximum absorption on TiO2 films.

Maximum emission in CHCl3 solution (10–5 M) at 25 °C. E0–0 was calculated as 1240/λintersection. Ered was calculated from Eox– E0–0. Egap is the energy gap between the Ered of dye and the conductive band (CB) level of TiO2.

Figure 3

Optimized molecular geometries and electron distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the dyes C1–C3.

Theoretical Model Calculations

To investigate the influence of various auxiliary acceptors on geometric configuration and electron distribution, we performed density functional theory (DFT) calculations at the B3LYP/6-31G* level. Herein, four octyloxy groups in the dyes C1–C3 were replaced by four methoxy groups to shorten calculation time. The optimized molecular structures, as depicted in Figure , reveal that the four aryl side groups locate outside of the main backbone, thereby forming a unique 3D backbone configuration. This 3D backbone feature is believed to suppress dye aggregation on TiO2 films and thus give high photovoltaic efficiency.[11] For the three dyes, relatively small twisted dihedral angles (<3.4°) can be observed between the DTCC unit and the adjacent auxiliary acceptors. Compared to C1 (35.1°) and C3 (4.7°), the DFBT-containing C2 exhibits larger dihedral angles (38.2°) between the DFBT unit and phenyl group because the backbone coplanarity of C2 is sharply disrupted by introducing two large fluoro substitutes. However, due to the weak steric effect between adjacent groups, the PT-containing dye C3 exhibits excellent backbone coplanarity with the smallest twisted dihedral angles of 4.7°, which is much smaller than those of the BT-containing dye C1 (35.1°) and the DFBT-containing dye C2 (38.2°). Although a good backbone coplanarity will facilitate charge transfer, aggregation of dyes on TiO2 films caused by coplanar backbone is also enhanced. Thus, this feature generally leads to strong electron recombination on the TiO2/dye/electrolyte interface, which thereby gives partial explanation for the lowest PCE values of the C3-based DSSCs. Moreover, as shown in Figure , for the HOMO orbitals of the three dyes C1–C3, the electrons mainly distribute on the DTCC donors and also partly extend to the auxiliary acceptors, whereas the LUMO orbitals dominantly delocalize over the acceptor and the π-linker groups. This fully separated HOMO and LUMO orbitals distribution can guarantee the efficient charge separation and electron injection.[11]

Electrochemical Properties

Cyclic voltammetry (CV) measurements were performed to study the electrochemical properties and energy-level structures of the dyes C1–C3. Typical CV curves and the corresponding energy-level diagram are provided in Figure . The relevant data are collected in Table . As shown in Figure a, three DTCC-based dyes exhibit one strong clear oxidation couple. We note that the first oxidation potentials (Eox) of the dyes C1–C3, which correspond to the HOMO energy levels, are determined to be 1.10, 1.17, and 1.14 V (vs normal hydrogen electrode (NHE)), respectively; it should be mentioned that all of these Eox values are externally calibrated by the ferrocenium/ferrocene (Fc+/Fc, see Figure S3) couple and addition of 0.63 V to the potential (vs Ag/AgCl).[11] The observed results reveal that the Eox values are much more positive than those of iodide/triiodide (I–/I3–) redox couple (0.4 V vs NHE, Figure b), which provides enough driving forces for dye regeneration from excited-state dye to ground-state dye via capturing electron from I–/I3– redox couple. On the other hand, the E0–0 energies of the dyes C1–C3, which were determined from the intersections of the normalized absorption and emission spectra, are calculated to be 2.14, 2.17, and 1.94 eV, respectively. According to the empirical equation of Eox – E0–0, the reduction potentials (Ered vs NHE) are determined to be −1.04, −1.00, and −0.80 V, respectively. Thus, for the three dyes, more negative Ered values are observed as compared to the conductive band (CB) of TiO2 (−0.5 V vs NHE, Figure a); moreover, the calculated energy-level differences (Egap) between Ered and CB of TiO2 are ca. 0.54, 0.50, and 0.30 eV, which are much larger than that of the theoretical requirement (Egap > 0.2 eV).[58] Therefore, we assume that three dye-based DSSCs would conduct an efficient electron injection from excited dye into TiO2 conductive band. However, the low-lying Ered value of C3 relative to C1 and C2 reveals that its driving force for electron injection may be less sufficient.

Figure 4

(a) CV curves recorded in CHCl3 solution. (b) Energy-level diagram of the dyes C1–C3.

Photovoltaic Performance

To investigate the photovoltaic properties of the dyes C1–C3, conventional liquid DSSCs were fabricated and tested using I–/I3– redox couple as the electrolyte. The detailed fabrication procedures are provided in Supporting Information. Figure a shows the typical photocurrent density–voltage (J–V) curves of the dyes C1–C3 measured under global AM 1.5G sunlight with irradiance of 100 mW cm–2 at 298 K. All of the device parameters are summarized in Table . For the BT-containing dye C1, the optimized DSSCs achieved an excellent PCE of 6.2%, along with a short-circuit current density (Jsc) of 12.98 mA cm–2, an open-circuit voltage (Voc) of 671 mV, and a fill factor (ff) of 0.71. Compared to C1, the DFBT-containing dye C2 afforded relatively lower PCE of 4.74% and recorded a disappointing Jsc of 10.44 mA cm–2, a slightly dropped Voc of 653 mV, and a comparable ff of 0.70. We assume that the reduced PCE value for C2 mainly results from a 12.2% decrease in Jsc value as compared to C1-based DSSCs. Although the PT-containing C3 obtained a broad absorption band relative to C1 and C2, the optimized C3-based DSSCs still gave quite low Jsc and Voc values (3.27 mA cm–2 and 562 mV, respectively), thereby leading to the lowest PCE of up to 1.42% among the three dyes. The reduced photovoltaic efficiency for C3 may partly result from its unexpected dye aggregation and electron recombination caused by its enhanced backbone coplanarity as demonstrated in the DFT calculation above. Another explanation presumably can be ascribed to its low-lying Ered (−0.80 V) and too small Egap (0.30 eV) between Ered and CB of TiO2, which make the electron injection from excited dye to CB of TiO2 much less efficient than those of C1 and C2. Upon coadsorbing with 5 mM chenodeoxycholic acid (CDCA), both Jsc and Voc values of the C1–C3-based DSSCs were further improved due to the suppression of dye aggregation and charge recombination. Among the three dyes, C1-based DSSCs exhibited the highest PCE of up to 6.75%.

Figure 5

(a) J–V curves and (b) incident photon-to-electron conversion efficiency (IPCE) spectra of the dyes C1–C3.

Table 2

Photovoltaic Performance of the Three Dyes with/without CDCA

dyes	Jsc (mA cm–2)	Voc (mV)	ff	PCE (%)
C1	12.98	671	0.71	6.20
C2	10.44	653	0.70	4.74
C3	3.27	562	0.77	1.42
C1 + CDCA	14.00	685	0.70	6.75
C2 + CDCA	10.91	685	0.72	5.40
C3 + CDCA	4.19	580	0.76	1.85

To further analyze Jsc value, we measured the incident photon-to-electron conversion efficiency (IPCE) spectra of the C1–C3-based DSSCs. As shown in Figure b, both dyes C1 and C2 exhibited strong and broad absorption response in the visible region with the absorption onsets extended to ca. 700 nm, which enables both DSSC devices to obtain strong light-capturing ability and high Jsc value. On the contrary, in the whole visible region, the C3-based DSSCs showed a quite weak absorption response with a maximum IPCE value of ca. 17%, thereby resulting in low Jsc values. On the other hand, in comparison to C2, the C1-based cells displayed higher and broader IPCE values and achieved IPCE values over 60% at 400–630 nm. In general, the IPCE spectra have a positive effect on Jsc and PCE of DSSCs; the highest IPCE values for C1 guarantee its highest Jsc and PCE values among the three dyes. After the addition of 5 mM CDCA, the IPCE values of the three dyes were slightly improved and hence leading to enhanced Jsc and PCE values compared to those of the DSSCs without CDCA.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) is a powerful technique to study interfacial charge transfer in devices.[11] Herein, to elucidate correlation of Voc, we performed the Nyquist plots and Bode plots of the dye cells in complete darkness with a forward bias of −0.70 V and a frequency region at 0.01–100 kHz, respectively. As shown in Figure a,c, two semicircles from left to right in the Nyquist plot are visible; the small semicircles represent the impedances of the charge transfer on the Pt electrode/electrolyte interface (RPt), whereas the larger semicircles are correlated to the interfacial charge transfer resistances (RCT) on the TiO2/dye/electrolyte interface. The larger semicircle in theory means the larger RCT, generally providing the slower electron recombination rate.[11] For the DSSCs without CDCA, the radius of the large semicircle increases in the order of C3 (20 Ω) < C2 (70 Ω) < C1 (92 Ω) (Figure a,c), suggesting that the electron recombination rate increases in the order of C1 < C2 < C3, which is consistent well with the Voc tendencies of C3 (562 mV) < C2 (653 mV) < C1 (671 mV). In comparison to the DSSCs without CDCA, the addition of CDCA has caused an increase of RCT at the same forward bias for the C1–C3-based DSSCs, thereby leading to enhanced Voc and PCE values.

Figure 6

Nyquist plots (a, c) and Bode plots (b, d) of C1–C3-based DSSCs with/without CDCA under a bias of −0.70 V in complete darkness.

The electron lifetime (τ) of the exited dyes can be associated with the peak frequency (f) at intermediate-frequency regions in Bode plots, which can be calculated from the equation of τ = 1/(2πf).[11] The smaller the charge recombination rate, the longer the electron lifetime is, generally leading to improved Voc and Jsc values. As shown in Bode plots (Figure b,d), the f values increase in the order of C1 (5.0 Hz) < C2 (5.8 Hz) < C3 (32 Hz). As a result, the τ values increase in the order of C3 (5.0 ms) < C2 (27.5 ms) < C1 (31.8 ms). On the basis of the results observed above, the best performance of the C1-based DSSCs mainly results from its strongest light-capturing ability and smallest charge recombination rate compared to the other two dyes. To further elucidate the effect of CDCA on photovoltaic performance, we also studied the Bode plots of the DSSCs with 5 mM CDCA. By comparison of the f values, we found that coabsorbing CDCA led to the reduced charge recombination rate due to the effective retard of dye aggregation, thereby providing higher photovoltaic performance than that of the DSSCs without CDCA.

Conclusions

In summary, for the first time, a ladderlike fused-ring donor (dithienocyclopentacarbazole (DTCC) derivative) has been used to design and synthesize three novel D–A−π–A-type organic dyes (C1, C2, and C3) via facile direct arylation reactions. We have demonstrated that the use of auxiliary acceptors (BT, DFBT, and PT) can effectively manipulate the optical, electrochemical, and photovoltaic performances of the three dyes. Moreover, strong D–A interaction endows the three dyes with strong intramolecular charge transfer and high molar extinction coefficients of above 3.14 × 104 M–1 cm–1, thereby ensuring their strong light-capturing ability. Investigation of DFT calculations reveals that the well-separated HOMO and LUMO orbitals of the three dyes are observed, which is helpful for efficient charge separation and electron injection. Although the PT-containing C3 obtains a broad absorption band relative to C1 and C2, larger charge recombination rate caused by dye aggregation in TiO2 led to quite low Jsc, Voc, and PCE values. Under AM 1.5G illumination, the BT-containing C1-based DSSCs afford the best photovoltaic performance with a Voc of 671 mV, a Jsc of 12.98 mA cm–2, an ff of 0.71, and a PCE value of 6.20%. Upon the addition of 5 mM CDCA, both the Jsc and Voc values are further improved due to the enhanced electron life of the devices, which thereby provides an enhanced PCE of 6.75%. Our findings indicate that the ladderlike DTCC derivative donors have great potential to design broad novel organic dyes for high-performance DSSCs.