Effect of Multidonor and Insertion Position of a Chromophore on the Photovoltaic Properties of Phenoxazine Dyes.

Research and development of new organic semiconductor materials can never be terminated because any structural fine-tuning may result in an important impact on its application performance, although the effect may be negative in many cases. Herein, we designed and synthesized a series of phenoxazine-based dyes, YH1, YH2, YH3, and YH4, whose absorption spectrum, electrochemical cyclic voltammetry, theoretical calculation, dye-sensitized solar cell photovoltaic characteristics, and electrochemical AC impedance are used to analyze the photophysical, electrochemical, and photovoltaic performance of the materials, aiming to study the effect of multidonor and adjustment of the chromophore insertion position on their photovoltaic performance. When donor triphenylamine is added at the end of YH1 and YH3, the absorption spectrum and photovoltaic performance of dyes YH2 and YH4 improved a little. The improvement is much greater when the chromophore (ethylenedioxy)thiophene in YH1 and YH2 is adjusted and inserted on the other side of phenoxazine and the energy conversion efficiencies (photon-to-current conversion efficiency) of the resulting dyes YH3 and YH4 reach 8.02 and 8.97%, respectively, which are 23 and 25% higher than those of YH1 and YH2, respectively. Although the improvement may be because of factors such as the dihedral angle, the result will undoubtedly provide some reference for the future study of the relationship between the structure and performance of organic dyes.

Introduction

Since the invention of the first an class="Chemical">polythiophenen> field-effect transistor in 1986,[1] development of new organic semiconductor materials and research on devices and manufacturing processes have never been terminated. Nowadays, because of the wide-ranging sources, easily controllable structure, low cost, flexibility, and ease of manufacture, many devices such as organic thin-film transistors,[1] organic electroluminescent devices,[2] organic solar cells,[3,4] organic sensors,[5] and organic memory[6] based on organic semiconductor materials have made amazing progress. Since first reported by O’Regan and Gräzel in 1991,[7] dye-sensitized solar cells (DSCs), as one of the photo-to-electric conversion devices based on organic semiconductor materials, have attracted considerable and sustained attention because of their ease of fabrication, high efficiency, and cost-effectiveness.[8−10] As one of the crucial components in DSCs, great efforts have been paid to the design and synthesis of new photosensitizers from the past to present: n class="Chemical">metal complexes such as ruthenium polypyridine[8,11−13] and zinc porphyrin[11,14,15] and metal-free organic sensitizers such as perylene,[16] benzo-[1,2-b:4,5-b′]dithiophene,[17−19] dithieno[3,2-b:2′,3′-d]pyrrole,[20,21] thiophene fluorene,[22−24] isoindigo,[25,26] coumarin,[27,28] and indacenodithiophene[29] and its derivatives.[30−33] So far, impressive photovoltaic performances have been obtained using zinc porphyrin dye-based DSCs, which showed a power conversion efficiency (PCE) exceeding 13%.[15] In recent years, limited by the use of precious metals, high-cost synthesis, and environmental issues, the development of metal organic dyes has become increasingly difficult, whereas pure organic sensitizers have been intensively investigated because of the robust availability, ease of structural tuning, and generally high molar extinction coefficients as well as high performance. Most of the design concepts of organic dyes are subordinated to D−π–A configuration because the D−π–A configuration can effectively increase the intramolecular electron transfer potential energy difference and guarantee the molecular adsorption attitude on the surface of TiO2.[34] In the D−π–A configuration of a dye, aromatic amine moieties such as triphenylamine (TPA) and phenoxazine (POZ) are the desired candidates for being an electron donor, cyanacrylic acid is widely employed as an electron acceptor, and benzoheterocyclic moieties and thiophene and its derivatives have been generally used as a π-conjugated bridge in the molecular design of organic dyes.[35]

Appropriate tailoring of D−π–A configuration, such as increasing the conjugate length or increasing or decreasing certain moieties to suit the special needs of DSC devices, has always been one of the important men class="Chemical">ans of designing high-performance organic dyes.[36] For example, extended and enlarged π-conjugated bridge is one of the strategies to broaden the absorption spectra and improve the Jsc of DSC,[37] but the results are often unsatisfactory, for the extension of the π-conjugated system often results in poor Voc because of an class="Disease">dye aggregation as well as electron recombination issues; therefore, a long alkyl chain is often introduced into the donor moieties to prohibit n class="Disease">dye aggregation and electron recombination.[38] However, in the structural regulation of D−π–A type organic dyes, some fine-tuning of the structures, such as making some moieties cyclized or changing the position of certain moieties, still lacks systematic research on the effect on DSC photovoltaic performance.

Herein, we use an class="Chemical">TPA or an class="Chemical">POZ as the donor, (ethylenedioxy)thiophene (EDOT) or thiophene as the π-bridge, and the classic moiety cyanoacetic acid as the anchor group and combine appropriate molecular tailoring strategy to design and synthesize four POZ-based dyes YH1, YH2, YH3, and YH4 through Stille coupling, Suzuki coupling, Knoevenagel condensation, and other classic reactions. We attempt to investigate the effect of multidonor and structural fine-tuning on the photophysical, electrochemical, and photovoltaic properties of the DSCs. Preliminary test results show that when adding a donor (TPA) to YH1 and YH3, the absorption spectra and photovoltaic performance of dyes YH2 and YH4 improved: compared with YH1 (Voc = 0.72 V, Jsc = 12.60 mA·cm–2, FF = 0.72, η = 6.53%), the PCE of YH2 (Voc = 0.76 V, Jsc = 12.93 mA·cm–2, FF = 0.73, η = 7.17%) increased by 9.8%; under the same conditions, the PCE of YH4 (Voc = 0.77 V, Jsc = 15.96 mA·cm–2, FF = 0.73, η = 8.97%) increased by about 11.2% compared with YH3 (Voc = 0.73 V, Jsc = 15.48 mA·cm–2, FF = 0.71, η = 8.02%) (FF is the fill factor). While we fine-tune the structures of YH1 and YH2 and move the insertion position of the chromophore EDOT (π-bridge) from the left to the right of POZ, the photovoltaic performance of the resulting compounds YH3 and YH4 had an amazing improvement: compared with YH1, the PCE of its isomer YH3 is increased by 23%, and compared with YH2, the PCE of its isomer YH4 is increased by 25%. By analyzing the photovoltaic data of four dyes, we find that this improvement is mainly due to the higher Jsc of YH3 and YH4. In addition, we find that YH2 and YH4 obtained a relatively high Voc. The electrochemical AC impedance test shows that the increase in Voc is due to the increased electron lifetime of YH2 and YH4; from the perspective of molecular structure, the reason may be that the introduction of TPA inhibits the electron return at the dye/electrolyte interface (Figure ).

Figure 1

Molecular structures of YH1, YH2, YH3, and YH4.

Molecular structures of an class="Chemical">YH1, an class="Chemical">YH2, YH3, and YH4.

Result and Discussion

Material and Synthesis

The raw materials, catalyst, and reagents were purchased or preprocessed as described. n class="Chemical">an class="Chemical">Cyanoacetic acid, ann> class="Chemical">N-bromosuccinimide (NBS), K2CO3, ultradry tetrahydrofuran (THF), and piperidine were purchased form Saan Chemical Technology (Shanghai) Co., Ltd. Palladium Catalyst Pd (PPh3)4 was purchased from Sigma-Aldrich Co., Ltd. Other reagents and solvents such as toluene, CH2Cl2, CHCl3, and acetonitrile (AN) were purchased from the National Drug Standards and used directly without additional treatment. The fluorine-doped tin oxide (FTO) electrode and the liquid electrolyte required for the preparation of DSCs were prepared according to the previous work of our group.[39] Intermediates 2, 3, 6, and 8 were purchased from Suna Tech Inc and used directly without other treatment. Compounds 1 and 11 were synthesized according to the corresponding literature in our previous works.[21,40] The other intermediates and dyes were synthesized according to the traditional organic reactions, such as Stille coupling, Suzuki coupling, and Knoevenagel condensation with good yields. The synthesis routes are illustrated in Scheme , and the detailed synthesis procedures are described as follows. Intermediates 4, 5, 7, 9, 10, and 12 were verified by 1H nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS). All dyes were verified by 1H NMR and ESI-MS as well as 13C NMR.

Scheme 1

Synthetic Routes for POZ Sensitizers of YH1, YH2, YH3, and YH4

Synthesis of Compound 4

To a solution of 1 (2.14 g, 11.2 mmol) and an class="Chemical">2-(tributylstannyl)-3,4-(ethylenedioxy)thiophene (7.834 g, 13.4 mmol) in an class="Chemical">toluene (80 mL), Pd(PPh3)2Cl2 (1.62 g, 1.4 mmol) was added. The mixture was refluxed at 110 °C for 24 h under argon. The crude compound was extracted into ethyl acetate, washed with brine and water, and dried over anhydrous sodium sulfate. After removing the solvent under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60–90 °C, 1/4, v/v) on silica gel to yield a yellow powder (5.25 g, 86% yield). mp 152–156 °C, ESI-MS m/z: 545.17. 1H NMR (CDCl3, 400 MHz), (TMS, ppm): δ 9.86 (s, 1H), 7.66–7.65 (d, J = 6.5 Hz, 1H), 7.28–7.36 (m, 2H), 7.18 (s, 2H), 7.10 (s, 1H), 6.88–6.92 (m, 2H), 6.58 (s, 1H), 4.41–4.48 (m, 4H), 4.29–4.26 (m, 2H), 1.72–1.82 (m, 1H), 1.28–1.58 (m, 8H), 0.85–0.90 (t, J = 6.8 Hz, 6H).

Synthesis of Compound 5

The synthesis of compound 5 used the same procedure as that for 4, yielding a red powder as the desired product 5 (0.55 g, 78% yield). mp 164–166 °C, ESI-MS m/z: 1045.41. 1H NMR (an class="Chemical">CDCl3, 400 MHz, ppm): δ 9.83 (s, 1H), 7.68 (d, J = 6.8 Hz, 2H), 7.27 (s, 2H), 7.22 (d, J = 11.8 Hz, 4H), 7.11 (d, J = 11.1 Hz, 2H), 6.91 (d, J = 11.3 Hz, 2H), 6.60 (d, J = 11.2 Hz, 2H), 6.50 (d, J = 11.3 Hz, 2H), 6.47 (d, J = 11.5 Hz, 2H), 6.45 (d, J = 11.3 Hz, 2H), 4.28–4.35 (t, 4H), 4.03–4.07 (d, J = 9.6 Hz, 4H), 2.95–3.05 (m, 2H), 1.83–1.94 (m, 3H), 1.56–1.75 (m, 6H), 1.25–1.33 (m, 18H), 0.85–0.90 (m, 18H).

Synthesis of Compound 7

To a cold solution of 6 (1.623 g, 6.43 mmol) in an class="Chemical">THF (80 mL) was added an class="Chemical">NBS (0.366 g, 6.43 mmol) at 0 °C. The reaction mixture was warmed to room temperature and stirred for 12 h. After THF was removed by evaporation under vacuum, the reaction mixture was extracted with dichloromethane and washed with water, and the organic layer was dried over anhydrous sodium sulfate. After removing the solvent under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60–90 °C, 1/4, v/v) on silica gel to yield a yellow solid (1.92 g, 90% yield). mp 130–133 °C; ESI-MS m/z: 331.93. 1H NMR (CDCl3, 400 MHz), (TMS, ppm): δ 9.86 (s, 1H), 7.65 (d, J = 6.6 Hz, 1H), 7.20 (d, J = 6.8 Hz, 1H), 4.40–4.34 (t, 4H).

Synthesis of Compound 9

To a suspended solution of 7 (3.310 g, 10 mmol), 8 (6.300 g, 15 mmol), and an class="Chemical">potassium carbonate aqueous solution (4.140 g, 2 M) in an class="Chemical">THF (80 mL) was added Pd(PPh3)4 (0.35 g, 0.3 mmol) under argon. The reaction mixture was refluxed at 78 °C for 24 h. After THF was removed by evaporation under vacuum, the reaction mixture was extracted with dichloromethane, washed with brine and water, and dried over anhydrous sodium sulfate. After removing the solvent under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60–90 °C, 1/4, v/v) on silica gel to yield an orange red solid (3.80 g, 70% yield). mp 156–160 °C; ESI-MS m/z: 545.20. 1H NMR (400 MHz, CDCl3, TMS, ppm): δ 9.85 (s, 1H), 7.65 (d, J = 6.5 Hz, 1H), 7.38 (d, J = 6.8 Hz, 1H), 7.15–6.95 (m, 5H), 6.95 (d, J = 11.8 Hz, 2H), 4.44–4.38 (t, 4H), 2.12 (s, 1H), 0.88–1.25 (m, 16H).

Synthesis of Compound 10

The synthesis of compound 10 used the same procedure as that for 7, yielding a red powder as the desired product 5 (3.12 g, 83% yield). mp 165–168 °C; ESI-MS m/z: 625.05. 1H NMR (an class="Chemical">CDCl3, 400 MHz), (TMS, ppm): δ 9.85 (s, 1H), 7.85 (d, J = 6.5 Hz, 1H), 7.37 (d, J = 6.8 Hz, 1H), 7.12 (t, 3H), 6.95 (d, J = 11.8 Hz, 2H), 6.82 (s, J = 11.3 Hz, 1H), 4.35–4.30 (t, 4H), 2.12 (s, 1H), 0.88–1.25 (m, 16H).

Synthesis of Compound 12

The synthesis of compound 12 used the same procedure as that for 9, yielding a dark red powder as the desired product 12 (0.68 g, 65% yield). mp 153–157 °C; ESI-MS m/z: 1044.54. 1H NMR (an class="Chemical">CDCl3, 400 MHz, ppm): δ 9.83 (s, 1H), 7.68 (d, J = 6.8 Hz, 2H), 7.27 (s, J = 11.5 Hz, 2H), 7.22 (d, J = 11.3 Hz, 4H), 7.11 (d, J = 11.3 Hz, 2H), 6.91 (d, J = 11.8 Hz, 2H), 6.60 (d, J = 11.5 Hz, 2H), 6.50 (d, J = 11.3 Hz, 2H), 6.47 (d, J = 11.8 Hz, 2H), 6.45 (d, J = 11.8 Hz, 2H), 3.44 (t, 4H), 2.43 (d, J = 11.9 Hz, 4H), 1.83 (m, 4H), 1.56 (m, 7H), 1.33 (m, 12H), 1.25 (m, 6H), 0.90 (m, 18H).

Synthesis of Dye YH1

To a stirred solution of compound 4 (0.273 g, 0.5 mmol) and an class="Chemical">cyanoacetic acid (0.128 g, 1.5 mmol) in an class="Chemical">chloroform (20 mL) was added piperidine (0.298 g, 3.5 mmol). The reaction mixture was refluxed under argon for 12 h and then acidified with 2 M hydrochloric acid aqueous solution (40 mL). The crude product was extracted into chloroform, washed with water, and dried over anhydrous sodium sulfate. After removing the solvent under reduced pressure, the residue was purified by flash chromatography, with chloroform and methanol/chloroform (1/10, v/v) in turn as the eluent to yield a red-black powder (0.25 g, 83% yield). ESI-MS m/z: calcd for C34H32N2O5S2, 612.1833; found, 611.1831 ([M – H]+). Anal. Calcd for C34H32N2O5S2: C, 66.62; H, 5.28; N, 4.53. Found: C, 66.64; H, 5.23; N, 4.55. 1H NMR (400 MHz, TMS, DMSO, ppm): 12.15 (s, 1H), 8.41 (s, 1H), 8.12 (d, J = 6.8 Hz, 1H), 8.07 (d, J = 6.5 Hz, 1H), 7.46–7.48 (s, 1H), 7.34–7.35 (s, 1H), 6.95–7.08 (d, J = 11.3 Hz, 2H), 6.73–6.78 (d, J = 11.5 Hz, 2H), 6.56 (s, 1H), 4.41–4.48 (t, J = 6.5 Hz, 4H), 3.52–3.86 (d, J = 11.9, 2H), 1.70–1.78 (m, 1H), 1.25–1.55 (m, 8H), 0.83–0.88 (t, J = 6.8 Hz, 6H). 13C NMR (400 MHz, DMSO): δ 159.83, 153.43, 147.76, 143.84, 143.45, 141.51, 139.23, 137.72, 136.44, 129.41, 128.32, 126.32, 122.65, 122.55, 120.11, 119.56, 119.36, 117.47, 117.35, 113.65, 113.02, 112.25, 110.15, 92.74, 64.88, 65.23, 37.58, 34.93, 32.62, 30.38, 28.33, 25.63, 23.89, 22.36, 14.45, 11.68.

Synthesis of Dye YH2

The synthesis of dye an class="Chemical">YH2 used the same procedure as that for dye an class="Chemical">YH1, yielding a dark red powder as the desired product YH2 (0.45 g, 81% yield). ESI-MS m/z: calcd for C68H77N3O7S2, 1112.4743; found, 11111.4746 ([M – H]+). Anal. Calcd for C68H77N3O7S2: C, 73.48; H, 7.05; N, 3.76. Found: C, 73.43; H, 7.06; N, 3.78. 1H NMR (CDCl3, 400 MHz, TMS, ppm): δ 11.85 (s, 1H), 8.28 (d, J = 6.5 Hz, 1H), 8.07 (d, J = 6.6 Hz, 1H), 7.46 (s, 2H), 7.12–7.16 (t, 4H), 6.88–6.91 (d, J = 11.3 Hz, 6H), 6.69–6.75 (d, J = 11.5 Hz, 6H), 4.25–4.28 (d, J = 6.5 Hz, 4H), 4.01–4.06 (d, J = 6.5 Hz, 4H), 2.90–3.11 (d, J = 6.9 Hz, 2H), 1.71–1.98 (m, 3H), 1.25–1.78 (m, 24H), 0.80–0.91 (t, 18H). 13C NMR (400 MHz, DMSO): δ 160.35, 159.63, 152.74, 148.06, 147.06, 145.89, 145.75, 137.62, 135.58, 130.73, 129.47, 128.96, 128.74, 128.36, 128.03, 127.33, 126.46, 125.75, 124.72, 124.45, 123.68, 123.05, 122.69, 121.43, 121.23, 120.81, 120.42, 119.86, 119.47, 118.52, 115.65, 115.02, 114.45, 114.03, 107.87, 102.91, 92.34, 75.69, 65.88, 40.36, 38.68, 37.33, 37.25, 35.73, 34.83, 31.65, 30.39, 27.38, 26.98, 25.86, 25.73, 23.75, 22.73, 14.12, 11.66.

Synthesis of Dye YH3

The synthesis of dye an class="Chemical">YH3 used the same procedure as that for dye an class="Chemical">YH1, yielding a dark red powder as the desired product YH3 (0.27 g, 88% yield). ESI-MS m/z: calcd for C34H32N2O5S2, 612.76; found, 611.73 ([M – H]+). Anal. Calcd for C34H32N2O5S2: C, 66.63; H, 5.28; N, 4.55. Found: C, 66.65; H, 5.26; N, 4.52. 1H NMR (400 MHz, DMSO, TMS, ppm): δ 12.15 (s, 1H), 8.08 (d, J = 6.8 Hz, 1H), 8.04 (d, J = 6.6 Hz, 1H), 7.16 (s, 1H), 7.05–7.08 (m, 4H), 6.89–6.95 (d, J = 11.6 Hz, 2H) 4.43 (t, 4H), 2.95–3.16 (m, 2H), 1.76 (m, 1H), 1.30–1.53 (m, 8H), 0.80–0.89 (m, 6H). 13C NMR (400 MHz, DMSO): δ 158.39, 148.63, 140.91, 132.36, 130.73, 122.44, 122.35, 122.12, 118.47, 117.52, 112.65, 111.82, 111.45, 110.63, 92.74, 63.18, 38.68, 35.73, 34.83, 31.65, 27.38, 25.73, 23.75, 14.12, 11.66.

Synthesis of Dye YH4

The synthesis of dye an class="Chemical">YH4 used the same procedure as that for dye an class="Chemical">YH1, yielding a dark red powder as the desired product YH4 (0.43 g, 78% yield). ESI-MS m/z: calcd for C68H77N3O7S2, 1112.4743; found, 11111.4741 ([M – H]+). Anal. Calcd for C68H77N3O7S2: C, 73.48; H, 7.05; N, 3.76. Found: C, 73.45; H, 7.03; N, 3.75. 1H NMR (400 MHz, DMSO, TMS, ppm): 11.85 (s, 1H), 8.11 (d, J = 6.6 Hz, 1H), 7.86 (s, 2H), 7.36 (d, J = 6.6 Hz, 1H), 6.98–7.15 (t, 4H), 6.78–6.91 (d, J = 11.5 Hz, 6H), 6.60–6.71 (d, J = 11.86 Hz, 6H), 4.45–4.48 (d, J = 6.5 Hz, 4H), 3.95–4.01 (d, J = 6.5 Hz, 4H), 2.94–3.10 (d, J = 6.9 Hz, 2H), 1.70–2.03 (m, 3H), 1.20–1.74 (m, 24H), 0.82–0.95 (t, 18H). 13C NMR (400 MHz, DMSO): δ 160.03, 152.43, 148.77, 147.76, 144.57, 143.01, 141.23, 138.59, 138.44, 137.62, 137.41, 135.58, 131.88, 131.81, 131.56, 131.36, 130.73, 129.47, 123.53, 123.46, 123.25, 122.72, 122.46, 121.81, 121.75, 121.56, 121.48, 121.42, 120.21, 117.87, 117.52, 116.65, 116.62, 116.25, 115.93, 115.87, 114.91, 92.04, 75.81, 65.79, 40.39, 38.56, 37.56, 37.63, 35.63, 34.81, 31.62, 30.36, 27.34, 27.02, 25.81, 25.75, 23.78, 22.66, 14.22, 11.14.

Photophysical, Electrochemical Properties, and Computational Analysis

Electronic absorption spectra of dyes n class="Chemical">an class="Chemical">YH1, ann> class="Chemical">YH2, YH3, and YH3 were measured on an UV-2600 spectrometer in THF with a concentration of 20 μg/L. Emission spectra were recorded with a Perkin Elmer LS55 luminescence spectrometer using the same concentration. Figure illustrates the absorption characteristics of the dyes. Note that the absorption range of the four dyes covered between 300 and 600 nm, similar to most aromatic amine dyes, and contained two distinct characteristic absorption peaks: The weaker absorption peak at 300–400 nm in the near ultraviolet region belonged to the typical aromatic π–π* electron transition absorption peak. There was a strong absorption peak in the visible light range 400–600. According to the structural characteristics of aromatic amine dyes, which belong to the intramolecular electron transfer absorption peak (ICT), it is also an important driving force for the charge separation of the exciton which is generated by the excitation of the dye.

Figure 2

UV–vis absorption spectra recorded on a UV-2600 spectrometer in a solution of THF.

UV–vis absorption spectra recorded on a UV-2600 spectrometer in a solution of an class="Chemical">THF.

The absorption spectra have obvious red shifts in the order of an class="Chemical">YH1, an class="Chemical">YH2, YH4, and YH3. The maximum absorption wavelengths (λmaxabs) in the visible region were 464 nm for YH1, 474 nm for YH2, 486 nm for YH3, and 480 nm for YH4 and exhibited a relatively high molar extinction coefficient (εmaxabs), which were 1.45 × 104 M–1 cm–1 for YH1, 2.85 × 104 M–1 cm–1 for YH2, 2.14 × 104 M–1 cm–1 for YH3, and 2.92 × 104 M–1 cm–1 for YH4. In addition, note that although the structure of the four dyes was similar, the absorption spectral characteristics of the four dyes were significantly different: first, dyes YH2 and YH4 were red-shifted or the molar extinction coefficient increased because of the introduction of donor TPA, which was well known to all. Second, when EDOT was located on the right side of POZ, the constructed dyes YH3 and YH4 showed red-shifted absorption spectra, and a significantly enhanced ICT peak suggested that the tailoring strategy in this work can effectively improve the dye’s ability to absorb light in the visible region and is expected to achieve higher photocurrent density.

Surprisingly, compared to an class="Chemical">YH1n> and n class="Chemical">YH2, the π–π* electron transition absorption peak of dyes YH3 and an class="Chemical">YH4 showed significant difference. In the range of 300–400 nm, the absorption peak of YH3 was relatively weakened, while that of YH4 was significantly enhanced. Analysis of the optimized molecular structure of theoretical calculations (Figure ) revealed that the electron cloud distribution at the highest occupied molecular orbital (HOMO) level had a good overlap with the lowest occupied molecular orbital (LUMO) level of YH3; hence, it is speculated that it had better intramolecular electron transfer characteristics, which made the ICT absorption peak significantly enhanced. However, YH4 has a large twist angle between the TPA and POZ units, resulting in a poor overlap between the HOMO and LUMO state electron clouds and a relatively enhanced π–π* electron transition absorption peak.

Figure 4

Optimized structures and electron cloud distribution of dyes performed at the B3LYP/6-31G(d,p) level with Gaussian 09.

The measurement of energy level was one of the important parameters to judge whether a dye has a matching energy level to produce photocurrent in the DSC. In orgn class="Chemical">anic semiconductor materials, as shown in Figure , cyclic voltammetry (CV) curves combined with normalized ultraviolet (UV)-fluorescence spectra (Figure A) were often used to measure the energy level of dyes because we often cannot accurately obtain the reduction potential by the electrochemical CV test. Figure A illustrates the normalized UV-fluorescence spectra of an class="Chemical">YH1, n class="Chemical">YH2, YH3, and YH4, and the maximum emission wavelengths of the four dyes at 617, 631, 642, and 640 nm could be obtained from the photoluminescence (PL) curves, respectively. According to the wavelength corresponding to the intersection point of the normalized UV-fluorescence spectrum and the empirical formula 1240/λ, the optical band gaps of the four dyes were 2.30, 2.23, 2.16, and 2.19 eV. Figure B shows the CV test using a three-electrode system with 0.1 M tetrabutylammonium hexafluorophosphate in dimethylformamide solution as the supporting electrolyte and ferrocene as the internal standard. The initial oxidation potentials of the four dyes were 0.65, 0.56, 0.40, and 0.49 V, respectively, which were obtained by the tangent method. Then, the HOMO levels were −5.45, −5.36, −5.20, and −5.29 eV for YH1, YH2, YH3, and YH4, respectively, calculated according to formula , and the LUMO energy levels of the four dyes were 3.15, 3.13, 3.04, and 3.10 eV, respectively, calculated by the formula LUMO = HOMO – Eg. The detailed parameters are listed in Table .

Figure 3

(A) Normalized absorption and emission spectra of the sensitizers and (B) CV curves tested under an electrolyte of 0.1 mol/L Bu4NPF6 in THF solution, measured on a CHI660C electrochemical workstation with a three-electrode electrochemical cell: glassy carbon as the working electrode, Ag/AgCl (sat. KCl) as the reference electrode calibrated by ferrocene/ferrocenium (Fc/Fc+), and Pt as the counter electrode.

Table 1

Photophysical, Electrochemical, and Energy Level Parameters of YH1, YH2, YH3, and YH4

dye	<keep-together>λmaxabs/nma</keep-together>	<keep-together>εmaxabs/104 M–1 cm–1a</keep-together>	<keep-together>λmaxpl/nma</keep-together>	HOMOcal./eV	LUMOcal./eV	E0–0cal.	<keep-together>Eoxonset/Vb</keep-together>	HOMO/eVc	LUMO/eVc	Eg/eVd
YH1	464	1.45	617	–4.89	–2.55	2.34	0.65	–5.45	–3.15	2.30
YH2	474	2.85	631	–4.83	–2.51	2.31	0.56	–5.36	–3.13	2.23
YH3	486	2.14	642	–4.47	–2.52	1.96	0.40	–5.20	–3.04	2.16
YH4	480	2.92	640	–4.55	–2.48	2.07	0.49	–5.29	–3.10	2.19

The maximum absorption wavelength and maximum molar absorption coefficient and PL maximum wavelength were derived from the static electronic absorption and emission spectra in THF solution.

The ground-state redox potential was tested using Fc/Fc+ (EFc/Fc = 0.36 V) as the interior label, and Eoxonset was obtained using the tangent method of CV curves.

The HOMO level was calculated by formula , and the LUMO level was estimated by equation LUMO = HOMO – Eg without considering any entropy change during light excitation.

The band gap (Eg) was estimated from the intersection points of normalized absorption and emission spectra.

(A) Normalized absorption and emission spectra of the sensitizers and (B) CV curves tested under an electrolyte of 0.1 mol/L an class="Chemical">Bu4NPF6 in n class="Chemical">THF solution, measured on a CHI660C electrochemical workstation with a three-electrode electrochemical cell: glassy carbon as the working electrode, Ag/AgCl (sat. KCl) as the reference electrode calibrated by ferrocene/ferrocenium (Fc/Fc+), and Pt as the counter electrode.

The maximum absorption wavelength and maximum molar absorption coefficient and PL maximum wavelength were derived from the static electronic absorption and emission spectra in an class="Chemical">THF solution.

The ground-state redox potential was tested using Fc/an class="Chemical">Fc+ (EFc/Fc = 0.36 V) as the interior label, and Eoxonset was obtained using the tangent method of CV curves.

The HOMO level was calculated by formula , and the an class="Chemical">LUMO level was estimated by equation an class="Chemical">LUMO = HOMO – Eg without considering any entropy change during light excitation.

According to Table , note that the an class="Chemical">LUMOn> energy levels of the four dyes were higher than the n class="Chemical">LUMO energy level of titanium dioxide (−4.00 eV), and the HOMO energy levels were lower than the HOMO energy level of the iodide ion electrolyte (−4.6 eV), which guaranteed the charge separation and dye regeneration in the DSC device. In addition, when adding an class="Chemical">TPA units on the head of YH1 and YH3, the HOMO and LUMO energy levels of dyes YH2 and YH4 increased slightly; while changing the position of EDOT, the energy levels of dyes YH3 and YH4 showed a significant difference: the HOMO and LUMO increased and the band gap narrowed, which were also reflected in their absorption spectrum characteristics.

The DFT algorithm of Gaussian software cn class="Chemical">an be used to optimize the structure, simulate the distribution of the electron cloud of the dye, and calculate the theoretical energy level, which helped to further analyze the relationship between the structure and performance of the dyes. In this work, an class="Chemical">YH1, n class="Chemical">YH2, YH3, and YH4 were calculated on the B3LYP/6-31G(d,p) level. Figure shows the optimized structure and front-line orbit distribution of the four dyes, and the detailed energy level data are listed in Table . As it is shown in the distribution of front-line orbits, the HOMO levels of YH1 and YH3 have a good overlap with the LUMO energy level, while for YH2 and YH4, they were slightly worse, which may be related to their larger dihedral angle; especially in YH4, the dihedral angle between TPA and POZ reached 45°, which also explains to a certain extent why the π–π* transition peak in the absorption spectrum of YH4 was extremely strong. The GaussView software was used to quantify the frontal orbital distribution to obtain the theoretical frontal orbital energy level of the dyes, and then the theoretical energy level of the dyes was obtained through energy unit conversion (hartree to eV). As shown in Table , it can be seen that the theoretical HOMO energy levels of the four dyes matched well the iodine electrolyte energy level, and the LUMO energy level also matched that of titanium dioxide. In addition, note that although the order was consistent with the actual energy level, the theoretical energy levels were higher than the actual energy level, which may be related to the solvent effect.

Photovoltaic Performance of DSCs

The preparation of DSC devices based on an class="Chemical">YH1, an class="Chemical">YH2, YH3, and YH4, including the postprocessing and the use of electrolyte, was in accordance with our previous work, with no extra treatment.[21] TiO2 mesoporous thin film electrode was sensitized in the solution of THF/AN (v/v, 1/3) at a concentration of 150 μM based on the four dyes. Figure A illustrates the incident photon-to-current conversion efficiency (IPCE) characteristic curves of the four dyes measured on a Zolix DCS300PA Data System and a Zolix SCS100-Omni-λ3007 monochromator equipped with a 300 W xenon lamp. Note that the red-shifted onset wavelength of photocurrent response with the YH2 and YH4 dyes compared to YH1 and YH3 was generally consistent with absorption measurements. In addition, it can be found that the maximum IPCE of the four dyes exceeds 80%, indicating that this series of dyes have a high photoelectric response capability, but unexpectedly the IPCE value does not match the absorption spectrum. In order to explain the cause of this phenomenon, the dye loading of the four dyes was tested. The test method was according to our previous work,[39] and the test results are listed in Table . It can be seen that the adsorption amounts of YH1 and YH3 were 2.86 × 10–8 and 3.01 × 10–8 mol·cm–2, which were much larger than the adsorption amounts of 2.69 × 10–8 and 2.72 × 10–8 mol·cm–2 for YH2 and YH4, which may be the main reason for the larger IPCE values of YH1 and YH3. By analyzing the structure of the four dyes, it is found that YH1 and YH3 have smaller molecular weights, so the molecular volumes were smaller than those of YH2 and YH4, resulting in the larger adsorption amounts of YH1 and YH3.

Figure 5

(A) IPCE spectra and (B) J–V curves of the dyes YH1, YH2, YH3, and YH4 based DSCs measured under irradiation of 100 mW cm–2 simulated AM 1.5 sunlight.

Table 2

Photovoltaic Parameters of Dyes YH1, YH2, YH3, and YH4 Based DSCs, under Simulated Sunlight Irradiation of AM 1.5 (100 mW cm–2)

dye	adsorption amount (10–8 mol·cm–2)	IPCEmax/%	Voc/mV	Jsc/mA·cm–2	FF	η %a
YH1	2.86	85.1	0.72	12.60	0.72	6.53
YH2	2.69	86.0	0.76	12.93	0.73	7.17
YH3	3.01	86.6	0.73	15.48	0.71	8.02
YH4	2.72	86.2	0.77	15.96	0.73	8.97

The validity of our photovoltaic data was confirmed by comparing the calculated Jsc via wavelength integration of the product of the standard AM 1.5 emission spectrum (ASTM G173-03) and the measured IPCE spectra with the experimental Jsc, showing less than 5% error. Also note that all our cells show a linear dependence of photocurrent on the light intensity.

(A) an class="Chemical">IPCE spectra and (B) J–V curves of the dyes n class="Chemical">YH1, YH2, YH3, and YH4 based DSCs measured under irradiation of 100 mW cm–2 simulated AM 1.5 sunlight.

The validity of our photovoltaic data was confirmed by comparing the calculated Jsc via wavelength integration of the product of the standard n class="Chemical">an class="Species">AM 1.5 emission spectrum (ASTM G173-03) ann>d the measured an class="Chemical">IPCE spectra with the experimental Jsc, showing less than 5% error. Also note that all our cells show a linear dependence of photocurrent on the light intensity.

Figure B illustrates the J–V curve of DSCs based on dyes an class="Chemical">YH1, an class="Chemical">YH2, YH3, and YH4 measured on a Keithley 2450 source meter and a Zolix SS150 solar simulator under a simulated solar irradiation condition of AM 1.5G (100 mW cm–2). The detailed data are listed in Table . Note that when adding a donor TPA for YH1, the Voc of dye YH2 increased slightly from 0.72 to 0.76 V, Jsc increased from 12.60 to 12.93 mA·cm–2, and the energy conversion efficiency (η) increased from 6.53 to 7.17%. These results suggest that the introduction of donor TPA can indeed improve the photovoltaic characteristics of the dye, which has been confirmed in our previous work.[21,39] When introducing the EDOT units to the right of the POZ unit, the Voc of YH3 and YH4 was slightly increased to 0.73 and 0.77 V, while the Jsc significantly increased to 15.48 and 15.96 mA·cm–2, and the energy conversion efficiency finally reached 8.02 and 8.97%, respectively, indicating that appropriate molecular tailoring strategies can effectively improve the photovoltaic performance of the dye. In addition, note that the FF of the four dyes was 0.72, 0.73, 0.71, and 0.73 for YH1, YH2, YH3, and YH4, respectively, indicating that the adsorption attitude and electron injection efficiency of the dyes have achieved the desired effect.

Electrochemical Impedance Spectroscopy Studies

To elucidate the relationship between the charge-transfer process and photovoltaic properties of the DSCs, electrochemical impedance spectroscopy (EIS) based on the DSC devices of the four dyes were measured in the dark. We fitted the DSCs’ EIS diagram into a suitable circuit and AC impedance spectra by ZView fitting software; the fitted AC impedance spectra is shown in Figure , and the corresponding parameters are listed in Table . In order to make the circuit closest to the actual situation inside the cell, we designed the inner surface of the DSC as the equivalent circuit diagram in the upper right corner of Figure . The resistance Rs was equivalent to the external circuit and the an class="Chemical">FTO/n class="Chemical">TiO2 interface, which was the smallest semicircle on the impedance spectrum and almost indistinguishable. The resistance Rce was equivalent to the resistance of the electrolyte/counter electrode interface, corresponding to the small semicircle in the high-frequency region on the impedance spectrum, which represented the charge-transfer impedance from the electrolyte to the counter electrode interface. The resistance Rrec was equivalent to the TiO2/dye/electrolyte interface resistance, which corresponds to the large semicircle in the low-frequency region on the impedance spectrum, indicating the charge recombination impedance from TiO2 to the interface of the dye and the electrolyte. The CPE2 value of the constant phase angle element can be obtained by fitting the resistance Rrec and the interface capacitance. Corresponding parameters are listed in Table .

Figure 6

Nyquist plots for DSCs based on YH1, YH2, YH3, and YH4 recorded on a RST5200 electrochemical workstation in the dark.

Table 3

Parameters Obtained by Fitting the Impedance Spectra of the DSCs with YH1, YH2, YH3, and YH4

dye	Rce (Ω cm–2)	Rrec (Ω cm–2)	CPE2 (μF)	τr (ms)
YH1	43.2	102.3	1083	110.8
YH2	51.3	128.7	1336	171.9
YH3	54.6	109.5	1108	121.3
YH4	53.5	133.6	1398	186.7

Nyquist plots for DSCs based on an class="Chemical">YH1, an class="Chemical">YH2, YH3, and YH4 recorded on a RST5200 electrochemical workstation in the dark.

Since all dyes were based on the same electrolyte, counter electrode, and device technology, the Rce values of the four DSCs had little difference, which were 43.2, 51.3, 54.6, n class="Chemical">and 53.5 Ω cm–2, respectively. In addition, it also showed that the difference of resistance in the electrolyte/electrode interface of the DSC based on four dyes was small, indicating a desired device packaging process. However, the impedance (Rrec) as well as the positive phase angle element (CPE2) of the interface an class="Chemical">TiO2/dye/electrolyte showed great difference, which were 102.3 Ω cm–2 and 1083 μF for n class="Chemical">YH1, 128.7 Ω cm–2 and 1336 μF for YH2, 109.5 Ω cm–2 and 1008 μF for YH3, and 133.6 Ω cm–2 and 1398 μF for YH4. Note that the Rrec and CPE2 of YH2 and YH4 were larger than those of YH1 and YH3, suggesting that the former two dyes obtained a higher electron density on the TiO2 interface, which was mainly reflected by the Voc of the device. The above-mentioned device photovoltaic test results confirmed the inference.

Generally speaking, the high electron density on the an class="Chemical">photoanoden> of the device implies a higher Voc, and it can be proved by the electron lifetime (τr) on the n class="Chemical">photoanode of the cell. In the impedance test of DSCs, the electron lifetime of the photoanode can be roughly estimated by formula .

According to formula , the electron lifetimes of the four dyes were calculated as 110.8, 171.9, 121.3, and 186.7 ms. n class="Chemical">Note that the electron lifetimes of the an class="Chemical">YH2 and ann> class="Chemical">YH4 based devices were slightly higher than YH1 and YH3 based devices, which confirmed that YH2 and YH4 obtained a larger Voc. Analyzing from the perspective of molecular structure, it may be that the introduction of the TPA unit increased the molecular volume of YH2 and YH4, and in addition, the hindrance of the longer alkyl chain on the TPA unit inhibited the dark current of the dye/electrolyte interface to a certain extent. Hence, the electron density on the surface of the photoanode increased, and the electron lifetime increased, resulting in the higher open-circuit voltage of YH2 and YH4. Through the impedance test, we also found that the additional donor has a certain positive effect on the Voc of the DSC, while the interchanged position of the EDOT unit had a positive impact on the Jsc of the DSC.

Conclusions

In summary, in this work, we designed and synthesized four n class="Chemical">an class="Chemical">POZ dyes ann> class="Chemical">YH1, YH2, YH3, and YH4 by using appropriate molecular tailoring methods. The main purpose of the design was to investigate the influence of introduction of additional donors and changing the position of the chromophore EDOT on the absorption spectrum, energy level, and photovoltaic performance of the organic dye. The absorption spectrum test in solution suggested that when adding a donor (TPA) at the end of YH1 and YH3, the absorption spectrum and molar extinction coefficient of the dyes YH2 and YH4 were somewhat improved. Nevertheless, when inserting the chromophore EDOT to the right of POZ, the absorption spectra of dyes YH3 and YH4 were obviously red-shifted and the molar extinction coefficient were obviously increased. CV tests showed that after inserting a donor to YH1 and YH3, the HOMO levels of YH2 and YH4 were slightly increased, and the band gaps were slightly narrowed, while the change of the insertion position of the chromophore EDOT significantly improved the absorption spectrum and band gaps of YH3 and YH4. The photovoltaic characteristic tests suggested that the introduction of donor TPA only slightly increased the open-circuit voltage as well as the short-circuit current of YH2 and YH4. While changing the position of the chromophore EDOT, YH3 and YH4 obtained a significant increase of short-circuit currents, resulting in a significant improvement of the energy conversion efficiency. The EIS test suggested that the increase of the open-circuit voltage of YH2 and YH4 was due to the dilation of the electron lifetime of the photoanode. From the perspective of molecular structure, the reason might be that the introduction of TPA effectively inhibited the electron return at the dye/electrolyte interface. Finally, we found that although the design strategy of introducing multidonors have somewhat improved the absorption spectrum and photovoltaic characteristics of the dye, the change of the insertion position of EDOT was the main reason for the significant improvement of the photovoltaic characteristics of YH3 and YH4..