Push–pull type alkoxy-wrapped N-annulated perylenes for dye-sensitized solar cells

Abstract

Three push–pull type, alkoxy-wrapped N-annulated perylene based sensitizers with different electron-accepting moieties were designed and synthesized. All the three dyes exhibited broad and intense absorption in the visible region. Co(II)/(III) based dye-sensitized solar cells showed overall power conversion efficiencies up to 8.38% for QB5 under the 100 mW cm⁻², simulated AM1.5G sunlight. The detailed optical, electrochemical, photovoltaic measurements and theoretic calculations were conducted and discussed to understand the relationship between the structure and properties.

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Introduction

Dye-sensitized solar cells (DSCs) have drawn intensive interest since the seminal demonstration in 1991 due to their low production cost, good conversion efficiency, simple fabrication and environmental concerns.¹ Development of push–pull type organic dyes with appropriate energy level alignment and good light-harvesting capabilities was one of the crucial strategies to achieve high performance devices.² Considering of the rarity and high cost of the ruthenium metal, Ru-free organic dyes have caught much attention.³ Great progress has been achieved by using porphyrin based sensitizers and power conversion efficiency (PCE) has been further improved to 13%.⁴ In addition, researchers spent more and more efforts on metal-free organic dyes because of the wide availability and flexibility.⁵

Our attention was laid on a new perylene derivative, the so-called N-annulated perylene (NP), in which a nitrogen atom is annulated at the bay position. As well-known, perylene and its derivatives are widely used dyes and pigments in a variety of applications due to their outstanding chemical, thermal and photochemical stability as well as excellent photophysical properties.⁶ Compared to normal perylene, modification (e.g., alkylation) can be done on the amine site of NP to improve the solubility and suppress aggregation, and regio-selective functionalization at the peri-positions can be easily done. Moreover, the higher electron density of NP leads to new opto-electronic properties due to electron-donating character of amine. All these factors make NP a superior building block for the design of organic sensitizers in photovoltaic devices. Recently, our group first demonstrated that NP can be used as an electron donor as well as a π-spacer in high performance DSCs.⁷ Meanwhile, P. Wang and Z. Wang's groups also showed very good results by employing NP as a basic building block for organic sensitizers.⁸

In this work, we designed and synthesized three new alkoxyl-chain wrapped NP sensitizers (QB4–QB6 shown in Fig. 1) to further explore the fundamental structure–physical property–device performance relationships. The design is based on the following considerations: (1) NP with bulky o-alkoxy-substituted phenyl group rather than flexible alkyl chain on amine site was chosen to suppress the problematic dye aggregation, to eliminate charge recombination at the TiO₂ interface, and to improve the dye solubility;⁹ (2) triphenylamine (TPA) with long alkoxyl chain was chosen as the donor because of its excellent electron donating character as well as good solubility;² (3) carbon–carbon triple bond was inserted as a π bridge between the TPA and NP (QB4 and QB5) or between the NP and the acceptor moieties (QB5) in order to extend the π-conjugation and to form a rigid structure;¹⁰ (4) benzoic acid was chosen as acceptor for QB4 (ref. 4) while for QB5, an electron-deficient benzothiadiazole (BT) moiety was inserted to facilitate intramolecular charge separation as well as to tailor the light-harvesting property;¹¹ (5) for QB6, cyclopentadithiophene (CPDT) together with cynoacetic acid was used as electron acceptor part because CPDT has been proved to be an efficient building block for many metal-free sensitizers,¹² and it also allows us to compare with QB4 and QB5 in this work and another dye (QB2) in our previous work.⁹

Fig. 1 Chemical structures of sensitizers QB4, QB5 and QB6.

Results and discussion

Synthesis of materials

The synthetic route for the dyes QB4, QB5 and QB6 is outlined in Scheme 1. The NP building blocks 1 and 4 were synthesized according to our previous report.⁹ Sonogashira coupling between 1 and terminal alkynes 2 and 3 in one step gave the QB4 in 30% yield. Compound 5 was synthesized in 70% yield by selective bromination of 4 with one equivalent of N-bromosuccinimide (NBS). Then iodination of 5 with N-iodosuccinimide (NIS) resulted in compound 6 in 60% yield. The key intermediate compound 7 was synthesized by Sonogashira coupling reaction between 2 and 6 in 45% yield. Another Sonogashira coupling reaction between 7 and compound 8 ¹³ gave the target compound QB5 in 35% yield. Pd-Catalysed borylation of 7 with pinacolborane followed by Suzuki coupling with compound 9 (ref. 11) afforded the aldehyde 10 in 50% yield for two steps. QB6 was then prepared by Knoevenagel condensation of compound 10 with cyanoacetic acid in 87% yield. All the dyes QB4, QB5 and QB6 are black powder and soluble in normal organic solvents such as dichloromethane (DCM), chloroform and tetrahydrofuran (THF). They were thoroughly characterized by ¹H NMR, ¹³C NMR, and high resolution mass spectrometry (HR MS) (ESI†).

Scheme 1 Synthetic route of sensitizers QB4–QB6. Conditions: (a) Pa₂(dba)₃/AsPh₃, THF/Et₃N, 75 °C; (b) NBS, CHCl₃, 0 °C; (c) NIS/TFA (one drop), CH₂Cl₂; (d) Pd(PPh₃)₂Cl₂/CuI, THF/Et₃N, 75 °C; (e) (i) pinacolborane, Pd(PPh₃)₂Cl₂/Et₃N/1,2-dichloroethane, 90 °C; (ii) Pd(PPh₃)₄/K₂CO₃, toluene, reflux; (f) cyanoacetic acid, CHCl₃/piperidine, reflux.

Optical properties

The absorption spectra of QB4, QB5 and QB6 in DCM are shown in Fig. 2 and the corresponding data are collected in Table 1. QB4 exhibits an intense and broad absorption band with peak at 507 nm (ε = 4.11 × 10⁴ M⁻¹ cm⁻¹). Compared to QB4, when BT unit was inserted between the NP and benzoic acid moieties, the absorption spectrum of QB5 becomes even broader, and the absorption maximum is bathochromically shifted by 25 nm from 507 to 532 nm (ε = 4.47 × 10⁴ M⁻¹ cm⁻¹), while the absorption onset is remarkably red-shifted by about 100 nm. This suggests the presence of the electron-withdrawing BT unit can further increase the intramolecular donor–acceptor interaction and enhance the light-harvesting capability. QB6 shows an intense band covering the major part of visible region with absorption maximum at 525 nm (ε = 4.86 × 10⁴ M⁻¹ cm⁻¹). The absorption band shape of QB6 is quite different from that of QB4 and QB5 due to a different acceptor part. Interestingly, when QB6 was compared with its analogue QB2 in which the carbon–carbon triple bond is missed,⁹ the absorption is blue shifted by 25 nm. Such a change may be due to the increase of the distance between the donor and acceptor unit in QB6. UV-vis absorption spectra of three organic dyes on TiO₂ transparent film are also shown in Fig. S1 (ESI†). The absorption maximum of the dyes QB4, QB5 and QB6 is hypsochromically shifted by 23, 48 and 6 nm on TiO₂ film in comparison to that in solution, which could be attributed to either the formation of H-aggregates¹⁴ or deprotonation of carboxylic acid.¹⁵

Fig. 2 UV-vis absorption spectra of QB4 (black), QB5 (red) and QB6 (blue) in DCM.

Table 1 Summary of optical and electrochemical properties of QB4, QB5 and QB6

Dye	λabs (nm)	ε (M^−1 cm^−1)	E^optg (eV)	E^ox1/2 (V)	E^red1/2 (V)	HOMO^a (eV)	LUMO^a (eV)	E^ECg (eV)
a The HOMO and LUMO energy levels were determined from the onset of first oxidation and reduction wave vs. Fc^+/Fc, respectively.
QB4	501	41 100	2.21	0.34, 0.58	−1.47	−5.10	−3.46	1.64
QB5	532	44 700	1.95	0.36, 0.57	−1.27, −1.48, −1.72	−5.12	−3.52	1.60
QB6	525	48 600	1.88	0.37, 0.50	−1.34	−5.09	−3.51	1.58

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Electrochemical properties and DFT calculations

The electrochemical properties of QB4, QB5 and QB6 were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deoxygenated DCM solution containing 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF₆) as supporting electrolyte (Fig. 3 and S2 in ESI†). Multiple quasi-reversible or irreversible redox waves were observed for all three compounds (see the list of half-wave potentials E^ox_1/2 (anodic scan) and E^red_1/2 (cathodic scan) in Table 1). The HOMO and LUMO energy levels were estimated according to equations: HOMO = −[4.8 + E^onset_ox] eV, and LUMO = −[4.8 + E^onset_red] eV, where E^onset_ox and E^onset_red are the onset of the first oxidation and reduction wave, respectively (vs. Fc⁺/Fc).¹⁶ The HOMO and LUMO energy levels of QB4–QB6 were determined to be −5.10, −5.12, −5.09 eV (HOMO) and −3.46, −3.52, −3.51 eV (LUMO), respectively (Table 1). The electrochemical energy gaps were calculated accordingly to be 1.64, 1.60, and 1.58 eV for QB4–QB6. The trend is in agreement with the optical energy gaps.

Fig. 3 Cyclic voltammograms of QB4–QB6 in dry DCM.

To gain better understanding of the molecular structure and electron distribution, density functional theory (DFT) calculations were conducted at the B3LYP/6-31G* level. Fig. 4 shows their optimized molecular structures and frontier molecular orbital profiles together with the calculated energy levels of HOMOs and LUMOs (Fig. 4 and Tables S1–S3 in ESI†). For QB4, the HOMO is delocalized along both the donor group and the NP core, while the LUMO is delocalized through the NP core and the acceptor group. Compared to QB4, QB5 and QB6 exhibit even more segregated HOMO and LUMO with a proper overlap. Such a spatially well-separated orbital distribution is desirable for efficient intramolecular charge separation and fast injection of photo-excited electron into the conduction band of TiO₂ via the carboxylic group adsorbed on the TiO₂ surface. Time-dependent DFT calculations also predicted that QB5 and QB6 showed more red-shifted spectra than that of QB4 (Tables S4–S6 and Fig. S3–S5 in ESI†), and this trend is well in agreement with the experimental data.

Fig. 4 Calculated HOMO and LUMO profiles and energy levels of QB4, QB5 and QB6 (B3LYP/6-31G*). Hydrogen atoms are omitted for clarity.

Photovoltaic properties

Although N-annulated perylene with bulky phenyl group as core is used, due to the high planarity of the sensitizers favored by the presence of the perylene spacer and triple bond linker, a fast intramolecular charge transfer should be expected as well as an unfavorable π–π stacking during the uptake process, which may lead to intermolecular quenching or back transfer of the injected electrons from the TiO₂ conduction band.^17d In this case, the addition of chenodeoxycholic acid (CDCA) to the dye solutions could be useful because the CDCA can insert itself between the sensitizers minimizing detrimental dye aggregation effects.¹⁷ In addition, the use of a co-adsorbent such as CDCA ensures a uniform coverage of the inorganic semiconductor, thereby lowering the probability of recombination. Meanwhile, CDCA also could lower the amount of sensitizers loaded on the working electrode. On this basis, the use of CDCA as co-adsorbent has been evaluated: we prepared solutions of the dyes with or without 30 mM CDCA and studied the influence of the amount of additive on photovoltaic parameters and on the dye loading, which are shown in Table 2. The efficiencies of QB4–6 without the presence of CDCA were measured to be 4.71% (V_oc = 0.82 V, J_sc = 7.96 mA cm⁻², ff = 0.72), 5.65% (V_oc = 0.78 V, J_sc = 10.24 mA cm⁻², ff = 0.71) and 4.27% (V_oc = 0.75 V, J_sc = 8.77 mA cm⁻², ff = 0.65), respectively (Fig. S6 in ESI†). With the addition of 30 mM CDCA the performances of the QB4, QB5 and QB6 sensitized devices were significantly improved. In particular, the short-circuit photocurrent density (J_sc) augmented from 7.92 to 8.73, from 10.24 to 14.49, and from 8.77 to 13.30 mA cm⁻² for QB4, QB5 and QB6 based cells, respectively, resulting from large enhancement of charge generation efficiencies, although the dye loading density (c_m) was decreased from 2.74 to 1.94, 3.78 to 2.61, 2.19 to 1.77, 10⁻⁸ mol cm⁻² μm⁻¹, respectively. The relatively low dye loading of QB6 may due to the larger molecular size and the floppy feature of the C=C double bond which might tilt the molecules adsorbed on TiO₂ film.¹⁰ The V_oc of QB4–QB6 was slightly changed from 0.82 to 0.76 V, 0.78 to 0.78 V and 0.75 to 0.79 V, respectively. On the whole, PCEs of 5.14%, 8.38%, and 7.78% have been achieved with QB4, QB5 and QB6 sensitized devices. The action spectrum of incident photon-to-electron conversion efficiency (IPCE) and the photocurrent density–voltage (J–V) curves based on the three sensitizers in the presence of 30 mM CDCA are illustrated in Fig. 5. Remarkably broad and high IPCE covering most of the visible region were observed for all dyes, which is in consistence with their absorption spectra. The IPCE spectra of QB4 dye showed a maximum of ∼85% at 450 nm, slightly higher with respect to the ∼80% obtained for the QB5-based device at 450 nm and to the plateau at 75%, ranging from 450 to 550 nm, observed for QB6. Meanwhile, the onset of IPCE was largely red-shifted from 650 nm for QB4 to 770 nm for QB5 and QB6. All these factors confirm the superior photovoltaic properties of the QB4–QB6 dyes.

Table 2 Photovoltaic parameters of cells measured at simulated AM1.5 sunlight

Cell name	η/%	JEQE^a/mA cm^−2	Jsc/mA cm^−2	Voc/V	ff	cm/10^−8 mol cm^−2 μm^−1
a JEQE was derived by wavelength integration of the product of the standard AM1.5 emission spectrum (ASTM G173-03) and the EQEs measured at the short circuit. The validity of measured photovoltaic parameters was evaluated by comparing the calculated JEQE value with the experimentally measured Jsc value.b Co-adsorbed with CDCA (30 mM).
QB4	4.71	7.92	7.96	0.82	0.72	2.74
QB4 + CDCA	5.14^b	8.32	8.73	0.76	0.78	1.94
QB5	5.65	10.35	10.24	0.78	0.71	3.78
QB5 + CDCA	8.38^b	14.43	14.49	0.78	0.74	2.61
QB6	4.27	8.74	8.77	0.75	0.65	2.19
QB6 + CDCA	7.78^b	13.06	13.30	0.79	0.74	1.77

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Fig. 5 (a) Photocurrent action spectra and (b) current–voltage characteristics recorded for DSCs devices fabricated with the dyes QB4–QB6 in the presence of 30 mM CDCA.

To further understand the dye structure–device performance relationship, we conducted further physical measurements of QB4–QB6 cells in their better testing condition. For a short-circuit DSC device, it is known that the J_sc is measured at the condition of a considerable low electron density in the nanocrystalline titania film and the charge recombination flux is significantly reduced. Thereby, the measured J_sc is roughly proportional to the photocarrier generation flux. The validity of this analysis motivated us to compare the dye structure correlated V_oc variation at a given J_sc by measuring J–V curves under various light intensities and plotting V_oc as a function of J_sc (Fig. 6a). It is noted that at a given J_sc, the QB6 cell exhibits a higher V_oc than QB5, while QB4 gives the lowest V_oc value.

Fig. 6 (a) Open-circuit photovoltage plotted as a function of short-circuit photocurrent density. (b) Plots of open-circuit photovoltage versus extracted charge. (c) Plots of lifetime of photoinjected electrons in titania as a function extracted charge.

It is widely recognized that for a fixed redox electrolyte in DSCs, the rise or fall of V_oc is determined by the titania electron quasi-Fermi-level (E_F,n), which intrinsically stems from a change of titania conduction band edge (E_c) and/or a variation of titania electron density.¹⁸ At a given flux of photocarrier generation, the titania electron density is determined by the interfacial recombination rate of titania electrons with electron accepting species in electrolytes and/or dye cations. Thereby charge extraction (CE) and transient photovoltage decay (TPD) measurements¹⁹ were further carried out to understand the electronic origins of the aforesaid V_oc fluctuation. As shown in Fig. 6b, the cells made with QB4, QB5 and QB6 give similar extracted charge (Q) at the same potential bias V_oc, suggesting a fixed conduction-band edge of titania with respect to the electrolyte Fermi-level. In Fig. S6d,† similar phenomena was found without CDCA. As shown in Fig. S6e,† without CDCA, at a given charge Q, charge recombination lifetime (τ) increases in the order of the QB4 > QB5 > QB6. However, with CDCA, as Fig. 6c presents, at a given charge Q, charge recombination lifetime (τ) increases in the order of the QB6 > QB5 > QB4, accounting for the change of V_oc and aforementioned photovoltage tendency at a given J_sc.

Conclusions

In summary, we have synthesized three new push–pull type dyes (QB4–QB6) by using the alkoxy-wrapped N-annulated perylene unit as the π-bridge and triple bond as π-spacer. Varying the acceptor unit in the general selected structure, all dyes exhibited remarkable solar-to-energy conversion efficiencies resulting from favourable light-harvesting capacity and high absorptivity. Due to the high planarity of the structure, the addition of CDCA additive helps the efficiency of the final device by minimizing detrimental dye aggregation effects. In fact, all dyes show a better PCE value in the presence of CDCA co-adsorbent. In particular, QB5 based device showed PCE as high as 8.38%, highlighting that the coplanar NP unit is a good building block for the design of sensitizers for high performance DSCs. New structures can be engineered by selecting different donor units and anchoring groups, as partners of the coplanar NP, in order to gain deeper insight into the potentialities of this new class.

Experiments

Materials and instruments

All reagents and starting materials were obtained from commercial suppliers and used without further purification unless otherwise noted. Anhydrous toluene and THF were distilled from sodium-benzophenone immediately prior to use. The ¹H NMR and ¹³C NMR spectra were recorded in solution of CDCl₃ or THF-d₈ on Bruker DPX 300 or DRX 500 NMR spectrometers with tetramethylsilane (TMS) as the internal standard. The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet. MALDI-TOF or APCI mass spectrometry was used to confirm the mass of compound. The electrochemical measurements were carried out in anhydrous methylene chloride with 0.1 M TBAPF₆ as the supporting electrolyte at room temperature under the protection of nitrogen. A gold stick was used as working electrode, platinum wire was used as counting electrode, and Ag/AgCl (3 M KCl solution) was used as reference electrode. The potential was externally calibrated against the ferrocene/ferrocenium couple. The absorption spectra of dyes in solutions and sensitized ∼3 μm thick films were measured by a Shimadzu UV-1700 spectrometer. The solvents used for UV-vis measurements are of HPLC grade (Merck). The synthetic details of QB4, QB5 and QB6 are described in the ESI.†

DSC fabrication and characterization

A 4.5 μm-thick, transparent layer of 25 nm-sized TiO₂ particles was first screen-printed on FTO glass (Nippon Sheet Glass, Solar, 4 mm thick) and further coated with a 5.0 μm-thick second layer of scattering titania particles (WER4-O, Dyesol) to produce a bilayer titania film, which was used later as the negative electrode of a DSC. The preparation procedures of TiO₂ nanocrystals and paste for screen-printing were reported in a previous paper.¹⁶ The film thickness was monitored with a bench-top Ambios XP-1 stylus profilometer. After sintering at 500 °C and cooling to room temperature, a circular titania electrode (∼0.28 cm²) was stained by immersing it overnight into a solution of 150 μM dye dissolved in a binary solvent of chloroform and ethanol (volume ratio, 1/4). The dye-coated titania electrode was then rinsed with acetonitrile and dried by air flow, and was further assembled with a thermally platinized FTO positive electrode by a 25 mm-thick Surlyn (DuPont) hot-melt gasket and sealed up by heating. The internal space was perfused with an electrolyte with the aid of a vacuum-back-filling system. The infiltrated Co-bpy electrolyte is composed of 0.25 M tris(2,2′-bipyridine)cobalt(II)di[bis-(trifluoromethanesulfonyl)imide], 0.05 M tris(2,2′-bipyridine)cobalt(III)tris[bis(trifluoromethanesulfonyl)imide], 0.5 M tBP, and 0.1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in acetonitrile.

Transient photoelectrical experiments were measured with an Autolab-PGSTAT302N electrochemical workstation. The steady and perturbing lights on the photoanode side of a testing cell were supplied with white and red light-emitting diodes, respectively. We used the red light to generate a photovoltage perturbation near the open-circuit photovoltage of a testing cell under a certain white light and measured the voltage decay process thereafter. The modulated photovoltage by the red pulse of a testing cell was below 5 mV. The electron lifetime can be obtained by fitting a stretched exponential function to the photovoltage decay. The electron density was estimated by the charge extraction method. A testing cell was first kept at open circuit under white light and subsequently the white light was turned off upon switching the cell from open circuit to short circuit to record the resulting current transient, and the electron density was obtained by current integration.

Acknowledgements

J. W. and J. Z. acknowledge financial support from A*STAR-DST joint grant (IMRE/14-2C0239) and MOE Tier 2 grant (MOE2011-T2-2-130). P. W. thanks the National Science Foundation of China (No. 91233206) and the National 973 Program (2015CB932204) for financial support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: The synthetic procedures and characterization data of QB4, QB5 and QB6; DFT calculation details. See DOI: 10.1039/c6ra18221f

‡ These authors contributed equally to this work.

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