Dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene based organic sensitizers for dye-sensitized solar cells

Abstract

We report two novel D–π–A type organic dyes with a coplanar dithieno[2,3-d;2′,3′-d′]benzo[1,2 b;4,5-b′]dithiophene (DTBDT) as π-spacer for dye-sensitized solar cells. A best device performance with a power conversion efficiency of 6.32% is achieved, making DTBDT unit a promising building block for design of organic sensitizers.

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Dye-sensitized solar cells (DSSCs), as one of the most promising photovoltaic technologies, have attracted sustained attention over the past decades because of their potential in low-cost solar-to-electricity conversion.^1,2 Sensitizers play a critical role in light harvesting and electron injection and thereby affect the power conversion efficiency (PCE) of the DSSCs.³ Compared to expensive ruthenium complexes, metal-free organic dye sensitizers promise modest fabrication costs and grand flexibility in molecular tailoring.⁴

A donor–π spacer–acceptor (D–π–A) structure has been commonly exploited to lower the band gap and tune the molecular absorption for attaining panchromatic light-harvesting, relying on efficient intramolecular charge transfer (ICT).⁵ Upon manipulating the three components of this chromophore one can optimize the performance of the DSSCs.⁶ To date, a strong electron-poor unit such as cyanoacrylic acid bearing an anchoring group toward the TiO₂ surface is widely applied as the acceptor moiety,⁷ while electron-rich units such as aromatic amines,^8,9 carbazoles,¹⁰ and coumarins¹¹ are mostly adopted as donors. In addition to these, it is equally significant to judiciously modify the π spacer for modulating properties of the organic dyes. Many conjugated building blocks have been introduced as bridges between donor and acceptor units, for instance, oligoene,¹² oligothiophene,^13,14 thieno[3,2-b]thiophene,¹⁵ cyclopentadithiophenes,^16,17 dithieno[3,2-b:2′,3′-d]silole,⁸ dithieno[3,2-b:2′,3′-d]pyrrole,¹⁸ benzo[1,2-b:4,5-b′]dithiophene,¹⁹ indacenodithiophene,^20,21 and ladder-type pentaphenylene.²² Among these linkers, fused heteroacenes (see examples in Fig. S1†) possess good π-conjugation, increased coplanarity, and strong rigidification. It has proven that these features facilitate bathochromic and hyperchromic absorptions of organic dyes, leading to improved PCEs as compared to unplanar counterparts. The dyes containing coplanar spacers composed of three fused rings have been reported providing PCEs exceeding 9%.^18b Coplanar building blocks with longer fused rings for DSSC dyes are relatively rare.^20–22 Therefore, from the view of material development it still keeps interesting to prepare new organic sensitizers containing coplanar π-spacers with large-conjugation-length for DSSC application.

Dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT) is an analogue of pentacene with four benzene rings replaced by thiophenes, showing excellent coplanarity and π-conjugation as well as electron-rich characteristics. In recent years, it has been applied in high-mobility organic field-effect transistors (OFETs),^23,24 highly sensitive ammonia sensors,²⁵ and also been used as building block of semiconducting polymers.^26,27 These works inspired us to exploit it as a π-spacer to yield new organic dyes for DSSCs. Herein, we report two new D–π–A type organic sensitizers (Fig. 1) using coplanar DTBDT as the π-spacer, together with cyanoacrylic acid and triphenylamine derivative as acceptor and donor, respectively. Both sensitizers provide remarkable PCEs with a best value of 6.32% obtained from DTBDT2-based DSSC devices, indicating potential of the DTBDT-based compounds as DSSC dyes since further improvements of device efficiency could be achieved by modifying the DTBDT core. These two new dyes are prepared according to Scheme 1 and the detailed synthesis is described in the ESI.†

Fig. 1 Chemical structures of the two DTBDT-bridged organic sensitizers.

Scheme 1 Synthetic route for sensitizers DTBDT1 and DTBDT2. Reagents and conditions: (i) 2-tributyltinthiophene, Pd(PPh₃)₂Cl₂, DMF, 100 °C; (ii) (1) Eaton's reagent, r.t., 48 h; (2) H₂O, 60 °C, 30 min; (iii) (1) NBS, CHCl₃/AcOH, r.t., 12 h; (2) POCl₃, DMF, 1,2-dichloroethane, 80 °C, 12 h; (iv) pinacol ester of D (D = N,N-bis(4-(2-ethylhexyloxy)phenyl)aniline for DTBDT1, D = N,N-bis(2′,4′-dihexyloxybiphenyl-4-yl)aniline for DTBDT2), Pd(PPh₃)₄, aq. K₂CO₃ (2 M), aliquat 336, toluene, 90 °C, 12 h; (v) cyanoacetic acid, piperidine, CHCl₃, reflux, 12 h.

Fig. 2 depicts the UV-vis absorption and photoluminescence spectra of the dyes in chloroform solution (10⁻⁵ M). Both dyes display broad absorption bands ranging from 250 to 600 nm with high molar extinction coefficients (ε) of 3.67 × 10⁴ M⁻¹ cm⁻¹ for DTBDT1 and 3.48 × 10⁴ M⁻¹ cm⁻¹ for DTBDT2. The absorption band in the high-energy region corresponds to the π–π* transition of the whole D–π–A conjugated backbone while the one between 400 and 600 nm can be attributed to the ICT from the donor to the acceptor. Compared to DTBDT1 exhibiting an absorption maximum (λ_max) at 472 nm, DTBDT2 reveals a red-shifted λ_max at 485 nm, which can be explained by the stronger electron-donating ability of the donor unit leading to a stronger photoinduced ICT. This feature is also reflected in the fluorescence spectra by a slight red-shift of the charge transfer emission maximum. In addition, a shoulder around 575 nm shows up for both dyes, which can be traced back to the emission from the locally excited state, with higher intensity for the DTBDT1.²¹

Fig. 2 UV-vis absorption and photoluminescence spectra of organic dyes DTBDT1 and DTBDT2 in chloroform.

Cyclic voltammetry (CV) was carried out to investigate the electrochemical properties of the dyes. As shown in Fig. S2,† both dyes exhibit one reversible oxidation wave at low potential ascribed to the removal of an electron from the triphenylamine moiety and additional quasi-reversible oxidation waves at higher potential attributed to the contribution from the DTBDT π-spacer. The first oxidation potential (E_ox) of the DTBDT1 is lower than that of DTBDT2. This is due to the more delocalized π-conjugation caused by the introduction of two additional benzenes in DTBDT2.¹⁹ The HOMO levels of both dyes estimated from the onset of the E_ox are more positive than that of Co(II/III)(bpy)₃ redox couples (0.56 V vs. NHE), which is necessary to ensure that the neutral dye is effectively regenerated after being oxidized.²² The LUMO levels are calculated from the HOMO levels and the zero–zero excitation energy (E_0–0 = 2.14 eV for both dyes) determined from the onset of the absorption spectra to be −2.84 eV for DTBDT1 and −3.02 eV for DTBDT2. The values are sufficiently more negative than the conduction band edge of TiO₂ (−0.5 V vs. NHE), in favor of efficient electron injection from the excited dye onto the TiO₂ electrode.¹⁷

Density functional theory (DFT) was employed to optimize the geometries and calculate the frontier molecular orbitals of the two dyes. The calculation reveals the coplanar structure of the DTBDT bridge and the electron distributions in the HOMO and LUMO levels of the dyes as illustrated in Fig. 3. HOMOs extend from the triphenylamine donor to the DTBDT π-spacer, while LUMOs are mainly localized from the cyanoacetic acid acceptor to its adjacent π-spacer. Such an electronic distribution will facilitate electron injection from the excited dye to the TiO₂ electrode.

Fig. 3 Calculated frontier molecular orbitals of organic dyes DTBDT1 and DTBDT2 at B3LYP/6-31G* level.

Both DTBDT1 and DTBDT2 were applied in DSSCs as photosensitizers in conjunction with the Co(II/III)(bpy)₃ redox couple as a TFSI salt. The cobalt complex redox shuttle was chosen since it yielded the highest open circuit potential (V_OC) hence the best DSSC efficiency so far.^28–30 The solar cell performances are summarized in Table 1 and the corresponding J–V curves are shown in Fig. 4a as measured at 100 mW cm⁻² AM 1.5G. Clearly, DTBDT2 yields better PCE as compared to DTBDT1, owing to the higher V_OC and particularly to the more superior short circuit current density (J_SC). The significantly enhanced J_SC of about 4 mA cm⁻² for the DTBDT2 cell most probably originates from the amplified incident photon to current conversion efficiency IPCE, as illustrated in Fig. 4b. This phenomenon is in agreement with the observed red shift of the DTBDT2 absorption relative to DTBDT1, as triggered by the stronger donor group in the former dye.

Table 1 DSSC performances of the dyes measured at 100 mW cm⁻² AM 1.5G. PCE shown in maximum value with deviation of 0.5%

Dye	VOC (V)	JSC (mA cm^−2)	FF	PCE (%)
DTBDT1	0.65	8.90	0.70	4.05
DTBDT2	0.73	12.73	0.68	6.32

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Fig. 4 (a) J–V curves and (b) IPCE spectra for DSSCs based on studied dyes.

In addition, the V_OC of the DTBDT2-based device is 80 mV higher than that based on DTBDT1. In order to explain this observation, photovoltage and photocurrent transient measurements were performed with the aim to investigate the differences in electron lifetimes within the TiO₂ photoanode and possible conduction band shifts in the TiO₂ layer of the DSSCs. As Fig. 5a highlights, electron lifetimes of the DTBDT2 cells are superior in that comparison, thus accounting for the higher V_OC. Another source of V_OC alterations can stem from changes in the TiO₂ conduction bands. As Fig. 5b reveals, the changes in V_OC with the TiO₂ film capacitances are almost identical, implying that the conduction bands within the DSSCs of both sensitizers are very similar. Therefore, the measured variation in V_OC must result from a more suppressed charge recombination with Co(III)(bpy)₃ when utilizing the DTBDT2 dye. This can be explained by the more bulky donor of DTBDT2, in this way more effectively blocking the Co(III)(bpy)₃ to approach the TiO₂ surface at which recombination takes place.³¹ The efficiency we obtained from the DTBDT2 is comparable with those of indacenodithiophene-bridged dyes with PCEs of 6–7%.²¹ Both DTBDT and indacenodithiophene are composed of five fused rings. These dyes have slightly lower PCEs than those containing shorter fused spacers^15,18 but higher ones than dyes with longer fused spacers.²² All these spacer units possess coplanar structures and same backbone curvatures. It may thus imply that the conjugated length of coplanar π-spacers takes effect on the device efficiency, although different processing conditions cannot be excluded. This is worthy of further investigations.

Fig. 5 (a) Electron lifetimes within the TiO₂ film and (b) shifts in V_OC in dependence of TiO₂ layer capacitance of the two DTBDT dyes.

In summary, we present two DTBDT-bridged organic sensitizers for DSSCs. The coplanar and electron-rich nature of the DTBDT spacer favors efficient ICT from donor to acceptor, which can shift the absorption to long wavelength and improve PCE of the dyes. The two title chromophores show broad absorption bands between 250 and 600 nm with a more red-shifted absorption from DTBDT2 than DTBDT1. When applied in DSSCs with the cobalt complex redox electrolyte, the dye DTBDT2 provides a better PCE of 6.32% than the DTBDT1 thanks to higher V_OC and J_SC of the former. It should be noted that the rigid central DTBDT core is not substituted by any groups, which can cause undesirable aggregation between dye molecules. Thus, there is still room to further improve the performance of the DTBDT-based DSSCs by introducing substitutions either at the central benzene or at the outer thiophenes of the DTBDT unit. This work thus points toward the DTBDT unit as a promising π-spacer for new organic DSSC sensitizers.

Acknowledgements

This work was financially supported by the graduate school IRTG 1404 – “Self-organized Materials for Optoelectronics”. X.G. gratefully acknowledges the Alexander von Humboldt Stiftung for granting a research fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, characterization, NMR and HRMS spectra of the DTBDT dyes, and fabrication details of the DSSC devices. See DOI: 10.1039/c4ra11873a

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