Broad spectral-response organic D–A–π–A sensitizer with pyridine-diketopyrrolopyrrole unit for dye-sensitized solar cells

Abstract

In this work, four D–A–π–A sensitizers PDPP-I–IV based on pyridine-flanked DPP moieties (PDPP) were designed and synthesized for dye-sensitized solar cells. Remarkably, the incorporated electron-withdrawing unit of pyridine-flanked DPP improves the light-harvesting ability and modifies the electrochemical and absorption properties, generating a broader IPCE wavelength responding region. The electrochemical experiments and time-resolved photoluminescence measurements indicate the ability of electron-injection into the TiO₂ conductive band from the excited sensitizer. The transient absorption spectra were measured to investigate the feasibility of the dynamics for oxidized-state sensitizer regeneration. The IPCE spectra demonstrate the broad spectral response region of these sensitizers. Especially, the IPCE of PDPP-III reached the near infrared (NIR) region (>800 nm) with the highest short-circuit current of 16.17 mA cm⁻² in these sensitizers. Furthermore, the electrochemical impedance spectroscopy (EIS) experiments suggest that the electron-lifetime and charge recombination resistance increased when attaching the stereo substituted groups (R²) on the PDPP moiety, resulting in a higher open-circuit voltage (V_oc). It can be found that PDPP-II based DSSCs with liquid electrolyte exhibited the highest V_oc (523 mV) and power conversion efficiency (PCE) of 5.26%.

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Introduction

Dye-sensitized solar cells (DSSCs) have been under development for decades following the pioneering work by Grätzel and co-workers with a Ru-complex sensitizer.¹ A more effective sensitizer is undoubtedly one of the most important components for higher power conversion efficiency (PCE) because of its critical function in light harvesting and electron injection. The majority of efficient sensitizers are zinc porphyrin and Ru-complex dyes, with PCEs up to 13%.^2–6 However, the large-scale application of Ru- or Zn-complexes in DSSCs has been limited due to the shortage of Ru-resources and the high cost of purification.^7–10 As alternative sensitizers, metal-free organic dyes have attracted more attention owing to their tunable absorption properties, high molar extinction coefficients, and relatively low cost of purification.^11–13 Recently a few reports have shown PCE = 12.0–13% (ref. 14–16) with single metal-free organic dyes and the Co(II/III) redox electrolyte.

The traditional metal-free dyes have been focused on when designing π-conjugated spacer units which can increase the optical absorption and facilitate electron transfer.^17–19 Extending the π-conjugated configuration in dye molecular between the donor and acceptor can easily tune the absorption spectrum and energy band.^20,21 However, it has been found that the extending π-conjugated system always brought a lower stability of these dyes.²¹ Tian et al. has reported that employing the donor–acceptor–π-bridge–acceptor (D–A–π–A) electron framework was a promising strategy for effective sensitizer design,^21–23 resulting in more red-shifted absorption and higher stability than their analogues.

Diketopyrrolopyrrole (DPP) flanked by phenyl and thienyl groups has been applied in organic solar cell, owing to the high molar absorbance coefficient and stability.^24–29 However, the electron donating ability of phenyl and thienyl side groups resulted in the electron-rich nature of these moieties and offset the electron-withdrawing effect of the DPP core. The optical properties may be limited by this effect. To further extend the optical response, having more electron-deficient conjugate groups being connected on the DPP core would be desirable.^30–32 Intrigued by this question, we introduced the pyridine as a flanking group for DPP.³² Compared with the phenyl and thienyl side groups, pyridine-flanked DPP (PDPP) is a more electron deficient group because of the electron-withdraw ability of the imine (–C=N–) group in pyridine, thus the introduction of PDPP in molecular can low the frontier molecular orbital energy levels.³³ And also, due to no hydrogen beside the nitrogen of pyridine, the steric hindrance between DPP core and flanked group can be reduced significantly, resulting in more planar molecular.^30,31 In this work, we made the pyridine as flanked group in DPP (PDPP) and utilized the PDPP as additional electron-acceptor (A) to synthesis a series of D–A–π–A framework sensitizers, containing triphenylamine and its derivatives as donor moiety (D), cyanoacrylic acid moiety as electron acceptor (A) and anchoring group (PDPP-I–IV, shown in Fig. 1). Finally, four new sensitizers were applied in DSSC, and the corresponding properties were also presented.

Fig. 1 The molecular structures of PDPP-I–IV.

Experimental section

Materials

The tetrahydrofuran (THF) was dried by calcium hydride then distilled under normal pressure. The conductive glasses was fluorine tin oxide (F:SnO₂, transmission > 90%)-coated glass (12–14 Ω per square, TEC 15, U.S.A.). 1,3-Dimethylimidazoliumiodide (DMII) and acetonitrile were purchased from Aldrich. The I₂ and LiI were from Alfa Aesar. All other solvents and chemicals contained in this work were of reagent grade and reacted without further purification unless other-wise noted.

Characterization

¹H NMR and ¹³C NMR spectra were tested on a Brücker spectrometer (400 MHz). HRMS (MALDI-TOF) spectra were performed with a GCTeTOF and LTQ Orbitrap XL Mass Spectrometer. The UV-vis spectra were obtained on a U3900H UV-vis spectrophotometer (Hitachi, Japan). The time-resolved photoluminescence measurements (TR-PL) were recorded on a LaserStrobe Time-Resolved Spectrofluorometer (Photon Technology International (Canada) Inc.) with a USHIO xenon lamp source, a GL-302 high-resolution dye laser (lifetimes 100 ps to 50 ms, excited by a Nitrogen laser) and a 914 photomutiplier detection system. The TR-PL signal was excited at 480 nm laser. Transient absorption spectra were carried out using a LKS80 spectrometer (Applied Photophysics Ltd.) with a tunable Nd:YAG-Laser System NT341A (with a 4 ns pulse duration and a 5 Hz repetition rate), and attenuated to 120–157 μJ cm⁻². The signals were excited at 500 nm laser. The cyclic voltammetry measurement (CV) was employed with a CHI660d electrochemical analyzer (CH Instruments, Inc., China) using a normal three-electrode cell with a Pt working electrode, a Pt wire counter electrode, and a regular calomel reference electrode in saturated KCl solution. 0.1 M tetrabutylammonium hexafluorophosphate acted as supporting electrolyte, the scan rate was 50 mV s⁻¹. Photovoltaic measurements J–V curves employed a using a 3A grade AM 1.5 solar simulator with a Keithley model 2400 digital source meter. The photocurrent action spectra were measured as a IPCE test system consist of a xenon lamp (69911, Newport, USA), by a Si photodetector (71675, Newport, USA). The EIS measurements were tested on an AUTOLAB analyzer (PGSTAT 302N, Metrohm, Switzerland), with −0.49 V voltage bias and frequency from 100 mHz to 1000 kHz.

Device fabrication

The TiO₂ photoanodes were prepared according to the literature.³⁴ The double layer TiO₂ films (∼12 μm) were obtained on the FTO-coated glasses. And then, the TiO₂ films were sintering at 510 °C for 30 min. After cooling to the room temperature, the films were treated with 40 mM TiCl₄ aqueous solution at 70 °C for 30 min, following by sintering at 450 °C for 30 min in air. And then cooling to 80 °C, these films absorbed the PDPP sensitizers in THF (3 × 10⁻⁴ mol L⁻¹) solution for 12 h. After this, these photoanodes were rinsed with THF and ethanol and then dried. The area of electrodes was 0.25 cm². The Pt counter electrodes were prepared with 5 mM H₂PtCl₆ solution in isopropanol depositing on FTO glasses, then pyrolysed at 410 °C for 20 min. The DSSCs were fabricated by sealing the sensitizer-absorbed photoanodes and Pt counter electrodes with a 45 μm thermal adhesive film (Surlyn, U.S.A.). The liquid electrolyte (0.05 M I₂, 0.1 M LiI, 0.6 M DMII in acetonitrile) was injected between these electrodes.

Results and discussion

Synthesis

The pyridine-flanked DPP (PDPP) was synthesized referred to the literature.³⁵ Two-step Suzuki coupling reaction with PDPP unit were synthesized to afford the corresponding aldehyde intermediates (7–10). And then, the intermediates were united with cyanoacetic acid through Knoevenagel condensation to form D–A–π–A sensitizers (PDPP-I–IV), shown in Scheme 1. The octyl (R¹) and its geometrical isomer (R²) on PDPP unit can improve the solubility and form a tightly packed insulating monolayer blocking the electron recombination between electrolyte ions and the TiO₂. All the key intermediates and four D–A–π–A target dyes (PDPP-I–IV) were characterized by standard spectroscopic methods. The detailed synthesis procedures of these PDPP sensitizers are shown in ESI.†

Scheme 1 Synthesis of the PDPP sensitizers: (i) DMF, K₂CO₃, 125 °C for 50 min; (ii) K₂CO₃, Pd(PPh₃)₄, THF, 90 °C for 16 h; (iii) K₂CO₃, Pd(PPh₃)₄, THF, 90 °C for 16 h; (iv) CH₃COONH₄, CH₃COOH/THF (2 : 1, v/v), reflux for 2.5 h.

Photophysical and electrochemical properties

Fig. 2 shows the absorption spectra of four dyes in tetrahydrofuran (THF) solution (3 × 10⁻⁵ M) and on TiO₂ films. The corresponding data are summarized in Table 1. The UV-Vis spectrum in solution exhibits two major prominent bands for these PDPP dyes, appearing at 350–410 nm and 540–650 nm. The former band can be attributed to the localized aromatic π–π* exciting from donor moiety to PDPP unit. All these sensitizers can be observed the intramolecular charge transfer band (ICT) at 579, 564, 588 and 580 nm for PDPP-I, PDPP-II, PDPP-III and PDPP-IV respectively (listed in Table 1). Obviously, PDPP-III and PDPP-IV with two methoxyl groups substituent triphenylamine on the donor enhance the electron delocalization over the sensitizer molecule, resulting in a bathochromic spectrum responding. The bathochromic shift spectra by 9 nm and 16 nm are observed and also the absorption thresholds are red-shift when introducing the methoxyl groups in donor moiety (compared to PDPP-I and PDPP-II). It suggests that the strong electron donor in sensitizer is beneficial for optical harvesting. The replacing n-octyl (R¹) with its geometrical isomer (R²) in PDPP acceptor unit (shown in Scheme 1) shows slightly hypsochromic absorption spectra (15 nm for PDPP-II compared to PDPP-I, 8 nm for PDPP-IV compared to PDPP-III). When the stereo substituted group was directly attached to the PDPP moiety, the stereo hindrance of R² disrupts the molecular skeleton planarity, leading to the poor coplanarity and electron delocalization over the sensitizer molecular and indicating the blue-shift absorption spectra. The corresponding maximum extinction coefficient of PDPP-I–IV are 2.91 × 10⁴, 2.18 × 10⁴, 3.12 × 10⁴ and 3.29 × 10⁴ M⁻¹ cm⁻¹, respectively (compared with conventional ruthenium-complex N₃, 1.42 × 10⁴ M⁻¹ cm⁻¹ at 534 nm (ref. 36 and 37)). The more effective extinction coefficient allows thinner nanocrystalline film, resulting in a decreased film mechanical strength, effective light-harvesting and low probability for electron-recombination.³⁸

Fig. 2 The absorption spectra of these sensitizers: (a) in THF solution and (b) attached on TiO₂ films.

Table 1 Optical and electrochemical properties of four sensitizers

Dye	λmax^a/nm	λmax^b/nm	E0–0^e (eV)	ED/D+^d (V)	ED*/D+^f (V)
	(ε/10^4 M^−1 cm^−1)	Load^c (/10^−8 mol cm^−2)
a Absorption maximum in THF solution (3 × 10^−5 M).b Absorption maximum on TiO2 film.c The dye loads are calculated from absorbance data of the sensitized TiO2 electrodes.^39d ED/D+ HOMO potentials measured vs. Fc^+/Fc were converted to those vs. the normal hydrogen electrode (NHE) by addition of +0.63 V.e E0–0 is estimated from the absorption thresholds in THF solution.f ED*/D+ is estimated by subtracting E0–0 to ED/D+.
PDPP-I	579(2.91)	567(4.64)	1.94	1.12	−0.82
PDPP-II	564(2.28)	558(5.61)	1.90	1.10	−0.80
PDPP-III	588(3.12)	571(4.43)	1.86	0.92	−0.94
PDPP-IV	580(3.29)	565(4.10)	1.85	0.92	−0.93

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To accurately investigate the optical response, the absorption spectra of thin transparent TiO₂ films (4 μm thickness) loaded with these sensitizers were measured (shown in Fig. 2). Generally, in comparison with the spectra in THF solution, the maximum absorption peaks of these entire dyes exhibit a slightly hypsochromic shift by 12, 6, 17 and 15 nm, respectively. The blue-shifted absorption spectra of these dyes may be attributed to the H-aggregation and deprotonating of the carboxylic acid on films.⁴⁰ Notably, when anchoring on TiO₂ films, the absorption onsets of these sensitizers are extended over 25 nm compared to the absorption spectra thresholds in THF solution, which is beneficial for light-harvesting in solar cells.

To assess the ability of electron-injection from excited sensitizers into conductive band (E_cb) of TiO₂ and electron-regeneration from electrolyte, the cyclic voltammetry measurement (CV) was employed to evaluate the redox potential. The data are shown in Fig. 3 and Table 1. All the four dyes exhibit more negative LUMO (the lowest unoccupied molecular orbital) levels (−0.82, −0.80, −0.93 and −0.94 V vs. NHE for PDPP-I–IV, respectively) than the conductive band of TiO₂ (−0.5 V vs. NHE), which suggests that the electron of excited dyes can inject into conductive band of TiO₂ effectively for these dyes. And also, more positive HOMO (the highest occupied molecular orbital) levels of four dyes (1.94, 1.90, 1.84 and 1.85 V vs. NHE) than the iodine/iodide redox potential value (0.4 V vs. NHE) indicate thermodynamic feasibility for dyes' regeneration. Observed from these electrochemical data, powerful donor group results in more negative LUMO level and strong driving force, indicating more effective electron injection from excited dyes into conductive band of TiO₂.

Fig. 3 The cyclic voltammetry plots of dyes PDPP-I–IV (a) and the HOMO and LUMO levels of the PDPP dyes (b).

Time-resolved photoluminescence spectra (TR-PL) and transient absorption spectrum

In order to study the direct evidence of charge transfer at the dye/TiO₂ interface, the time-resolved photoluminescence spectra (TR-PL) were measured for PDPP-I–IV (shown in Fig. 4).²⁵ Theoretically, when the sensitizer absorbed on TiO₂ film, the excited sensitizer (S*) injected electron into the E_cb of TiO₂ extremely rapidly, resulting in a short excited state (S*) fluorescence lifetime (τ) (eqn (1)). In a pure solvent, the excited electrons have no other possibility than recombining with the oxidized sensitizers cations, indicating a longer excited state (S*) fluorescence lifetime.⁴¹ As shown in Fig. 4, all the sensitizers exhibit comparable spectra features for TR-PL. The corresponding PL decay times are listed in figures, indicating the lifetime (τ) of excited sensitizer (S*). In a diluted solvent, the sensitizers PDPP-I–IV show excition lifetimes at nanosecond (10⁻⁹ s) time scale (τ = 5.63, 3.83, 3.18 and 1.37 ns, respectively). Considering the dynamics on TiO₂ film, a much faster PL decay could be observed (PL decay time at picosecond 10⁻¹² s time scale, τ < 200, 150, 200, 150 ps for PDPP-I–IV), qualitatively suggesting that the excited sensitizers could inject the electron into the E_cb of TiO₂.^42,43 Combined with the LUMO levels, the time-resolved photoluminescence spectra show the thermodynamic and dynamic feasibility for excited dyes' injection.

S*–TiO₂ → S⁺–TiO₂ + e_cb⁻ (in TiO₂) (1)

S⁺–TiO₂ + e_cb⁻ (in TiO₂) → S–TiO₂ (2)

S⁺–TiO₂ + E (electrolyte) → S–TiO₂ + E⁺ (3)

Fig. 4 The time-resolved photoluminescence spectra of PDPP-I–IV and them on TiO₂ film: excited at 480 nm, monitored at 645 nm (in THF for PDPP-I–IV) and 710, 690, 715, 700 nm (on films for PDPP-I–IV, respectively).

After the ultrafast electron injection of excited sensitizers,⁴² two charge transfer processes, the injected electron (e_cb⁻) recombination with oxidized state sensitizer (S⁺) (eqn (2)) and the regeneration of oxidized state sensitizer (S⁺) from redox electrolyte (eqn (3)).⁴⁴ Transient absorption spectrum measurements were carried out to observer the S⁺ regeneration and e_cb⁻ recombination processes. Fig. 5(a) displays the time proprieties of transient absorption decay signal measured at 750 nm on TiO₂ films for these sensitizers in absence of redox electrolyte (I/I₃⁻), corresponding to the e_cb⁻ recombination. These decay traces can be fitted with the single exponential decay process. The corresponding decay times are listed in Table 2. Without the redox electrolyte, the oxidized state sensitizers (S⁺) have the dynamics feasibility for recombination with injected electron e_cb⁻, with micro-second temporal scale decay times (τ_rec) of 9.49, 8.68, 8.30 and 21 μs for PDPP-I–IV. The recombination rate constants of back-electron transfer are 1.05 × 10⁵, 1.15 × 10⁵, 1.20 × 10⁵ and 4.76 × 10⁴ s⁻¹, respectively. In the presence of the redox electrolyte, the oxidized state sensitizers (S⁺) could renascence from the I⁻/I₃⁻, resulting a much faster decay process of the oxidized state sensitizer (Fig. 5(b)). The fitting single exponential decay processes suggest the rate constants are 3.47 × 10⁶, 2.42 × 10⁶, 3.48 × 10⁶ and 1.46 × 10⁶ s⁻¹ for these sensitizers, giving significantly fast nano-second temporal scale regeneration decay times (τ_reg) of 288, 414, 287 and 683 ns. The yield of interception by redox electrolyte could be estimated to 96.9%, 95.2%, 96.5% and 96.7% respectively. These results indicate that the regeneration of oxidized state sensitizers form electrolyte was significantly fast and effective, and the sensitizers (S⁺) cations could be intercepted by the redox mediator (I⁻/I₃⁻).

Fig. 5 The transient absorbance decays of the excited sensitizers (S⁺) PDPP-I–IV using nanosecond flash photolysis (excited at 500 nm, probed at 750 nm): (a) adsorbed on 4 mm nanocrystalline TiO₂ film without redox electrolyte (no iodide electrolyte); (b) absorbed on film in the presence of the redox electrolyte under the same condition.

Table 2 Transient absorbance decay times with (τ_reg, regeneration) and without (τ_rec, recombination) redox electrolyte

	PDPP-I	PDPP-II	PDPP-III	PDPP-IV
τrec	9.49 μs	8.68 μs	8.30 μs	21 μs
τreg	288 ns	414 ns	287 ns	683 ns

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Photovoltaic device performance

These DSSCs performances of all the dyes were tested under AM 1.5G irradiation (100 mW cm⁻²). The incident photon-to-electron conversion efficiency (IPCE) spectra are shown in Fig. 6. Generally, the IPCE spectra of the DSSCs based on these sensitizers are consistent with the absorption spectra. The wavelength responding region of four sensitizers is about 350–720 nm, meaning that the devices could efficiently convert visible light to photocurrent in this region. All the sensitizers exhibit the threshold responding wavelength above >735 nm, resulting from the introduction of electron-withdraw pyridine-flanked DPP moiety. Especially, the onset wavelength of PDPP-III IPCE responding spectrum is extended to the nearly infrared (NIR) region (>800 nm),⁴⁵ which is beneficial for light-harvesting. Compared with PDPP-I and PDPP-II, the sensitizers PDPP-III and PDPP-IV with stronger electron donor group show more extended responding range due to more negative LUMO level and stronger electron-injection ability. The IPCE spectrum of PDPP-II and PDPP-IV maintain much higher IPCE values than those of PDPP-I and PDPP-III. This may be ascribed to a much lower intermolecular charge transfer caused by the stereo hindrance between sensitizer molecular when the substituted group octyl (R¹) was replaced by its stereo isomer (R²) (shown in Scheme 1).²¹

Fig. 6 IPCE action spectra for these PDPP-based DSSCs.

Fig. 7(a) shows the photovoltaic performance of the four sensitizers (data listed in Table 3). According to IPCE responding spectra, The devices based on PDPP-III and PDPP-IV exhibit higher and broader absorption response, generating much higher short-circuit current (J_sc: 16.17, 16.15 mA cm⁻² for PDPP-III, PDPP-IV than 14.31, 14.72 mA cm⁻² for PDPP-I, PDPP-II). This suggests that powerful electron-donor is beneficial for electron injection and increasing photo-current. The weaker intermolecular interactions demonstrate higher IPCE values for PDPP-II and PDPP-IV with the stereo substituted groups. However, the hypsochromic responding wavelength neutralizes the higher value for PDPP-IV, leading to almost the same short-circuit photo-current for PDPP-III and PDPP-IV. The hypsochromic IPCE spectrum of PDPP-II has less influence compared to PDPP-IV, indicating a higher J_sc for PDPP-II. Consistent with the spectra of IPCE, the J_sc of four sensitizers increases with this order of PDPP-I < PDPP-II < PDPP-IV ≈ PDPP-II. Meanwhile the devices based on PDPP-II and PDPP-IV with stereo substituted group (R²) show higher open-circuit voltage (V_oc) than the PDPP-I and PDPP-III cells: 523 mV (PDPP-II) > 495 mV (PDPP-I), 464 mV (PDPP-IV) > 425 mV (PDPP-III). It may be reasonable that the two stereo substituted groups decrease the dye's aggregation and help to form the blocking-layer which avoided the I₃⁻ ions approaching the TiO₂ surface, increased the electron-lifetime and V_oc. Moreover the sensitizers PDPP-III and PDPP-IV with two methoxy groups in electron-donor exhibit relatively lower open-circuit voltage. This can be ascribed to higher dark current for these sensitizers (shown in Fig. 7(b)), resulting in increasing electron-recombination and lower V_oc. Overall, the stereo substituted groups in pyridine-flanked DPP moiety decrease the electron recombination and intermolecular interaction, and is beneficial for high open-circuit voltage and IPCE value. The introduction of methoxy groups in PDPP dyes supports the broadly spectral response and electron injection, suggesting to high short-circuit current. Finally, the DSSC based on PDPP-II was optimized to best efficiency 5.26%. Under the same conditions, the efficiencies of cells based PDPP-I, PDPP-III and PDPP-IV are 4.80%, 4.59% and 5.10%, respectively.

Fig. 7 J–V curves of these sensitizers: (a) under AM 1.5G illumination (100 mW cm⁻²) and (b) under dark condition.

Table 3 Performance parameters of DSSCs based on PDPP-I–IV

Dye	Voc/V	Jsc mA cm^−2	ff/%	η/%
PDPP-I	0.494	14.31	68	4.80
PDPP-II	0.523	14.72	68	5.26
PDPP-III	0.425	16.17	67	4.59
PDPP-IV	0.464	16.15	68	5.10

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For long-term stability measurement, we fabricated PDPP-II based DSSCs with ionic-liquid gel electrolytes (0.1 M iodine, 0.1 M lithium iodide and 0.5 M benzimidazole in 1-propyl-3-methyl imidazolium iodide). Photovoltaic performances were listed in Fig. S1 (in ESI†), indicating J_sc values of 12.86 mA cm⁻², V_oc of 512 mV and ff of 0.65, corresponding to the η value of 4.28%. Meanwhile, the high stable DSSCs based on PDPP-II with ionic-liquid electrolyte have been successfully realized. As shown in Fig. S2,† the photovoltaic performances of PDPP-II based DSSCs were recorded over a period of 1000 h. The overall efficiency remained at 92% of the initial value after 1000 h of visible-light soaking. This demonstrated that the sensitizer on TiO₂ surface remained intact after long time light soaking.

Electron lifetime and EIS measurements

To study the electron recombination of these sensitizers, the Electrochemical Impedance Spectroscopy (EIS) was employed under −0.49 V bias applied voltage in the dark. The Nyquist plot and equivalent circuit are shown in Fig. 8. The parameters are listed in Table 4. In the equivalent circuit, R_s, R_CE, and R_rec represent series resistance (the beginning of the Nyquist plot), counter electrode (the first inconspicuous semicircle in Nyquist plot) and charge-transfer resistances at the dye/TiO₂/electrolyte interface (the middle semicircle), respectively. As shown in Fig. 8, the R_s and R_CE show almost the same values for these four sensitizers, because of the same electrode materials and electrolyte. The R_rec of PDPP-I–IV, corresponding to the middle semicircle in Nyquist plot, are 122.9, 168.7, 46.1 and 84.3 Ω, respectively. The electron lifetime at the dye/TiO₂/electrolyte interface can be calculated to 24.4, 27.2, 15.2 and 21.1 ms for PDPP-I–IV. The larger resistance of R_rec and longer lifetime indicate low probability of electron recombination, generating the improving V_oc. Among sensitizers, PDPP-II exhibits the highest V_oc, ascribed to the longest electron lifetime. The EIS measurements support observed shift in the V_oc for these sensitizers under standard global AM 1.5 illumination.

Fig. 8 Impedance spectra of these devices based on sensitizers PDPP-I–IV: Nyquist plots and the equivalent circuits. The lines of Nyquist show theoretical fits using the equivalent circuits.

Table 4 Parameters obtained by fitting the impedance spectra of the DSSCs with sensitizers

	RS/Ω	RCE/Ω	Rrec/Ω	τe/ms
PDPP-I	1.98	14.25	122.9	24.4
PDPP-II	2.08	16.69	168.7	27.2
PDPP-III	2.2	14.52	46.13	15.2
PDPP-IV	2.9	15.36	84.31	21.1

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Conclusion

In summary, we designed and synthesized four D–A–π–A sensitizers PDPP-I–IV, with pyridine-flanked DPP moiety as additional acceptor. The results indicate that the introduction of electron-withdraw pyridine-flanked DPP moiety is an effective method to improve the light-harvesting ability and modify the electrochemical and absorption properties. Meanwhile the stereo substituted group (R²) in pyridine-flanked DPP moiety avoids the electron recombination and intermolecular interaction, increasing the open-circuit voltage and IPCE values. The introduction of methoxy group in PDPP dyes improves the broadly spectral response and electron injection. Time-resolved photoluminescence spectra (TR-PL) reveal efficient injection of electrons at the TiO₂/dye interface for the excited sensitizers (S*). Transient absorption spectra show dynamically faster regeneration process for these dyes. Especially, the threshold wavelength of PDPP-III IPCE responding spectrum is extended to the nearly infrared (NIR) region (>800 nm), reaching the highest short-circuit current 16.17 mA cm⁻² in these sensitizers. The best overall light-to-electricity conversion efficiency is 5.26% (J_sc = 14.72 mA cm⁻², V_oc = 523 mV, ff = 0.68) based on PDPP-II sensitizer.

Acknowledgements

This work was supported by Natural Science Foundation of Anhui Province (No. 1508085SMF224) and the National High Technology Research and Development Program of China (No. 2015AA050602). We also thank the National Natural Science Foundation of China under Grant no. 21273242 and State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (Grant No. LAPS14012) for their support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27288b

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