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. 2021 Mar 4;6(10):6817–6823. doi: 10.1021/acsomega.0c05887

Comparative Studies on the Structure–Performance Relationships of Phenothiazine-Based Organic Dyes for Dye-Sensitized Solar Cells

Shengzhong Li †, Jingwen He †, Huiyun Jiang ‡, Shu Mei †, Zhenguang Hu †, Xiangfei Kong †, Miao Yang ∥, Yongzhen Wu ‡,*, Shuhua Zhang §,*, Haijun Tan †,*
PMCID: PMC7970489  PMID: 33748595

Abstract

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A D–A−π–A dye (PTZ-5) has been synthesized by introducing a benzothiadiazole (BTD) unit as an auxiliary acceptor in a phenothiazine-based D−π–A dye(PTZ-3) to broaden its spectral response range and improve the device performance. Photophysical properties indicate that the inclusion of BTD in the PTZ-5 effectively red-shifted the absorption spectra by reducing the Egap. However, the device measurements show that the open-circuit voltage (Voc) of PTZ-5 cell (640 mV) is obviously lower than that of the PTZ-3 cell (710 mV). This results in a poor photoelectric conversion efficiency (PCE) (4.43%) compared to that of PTZ-3 cell (5.53%). Through further comparative analysis, we found that the introduction of BTD increases the dihedral angle between the D and A unit, which can reduce the efficiency of intramolecular charge transfer (ICT), lead to a less qCT and lower molar extinction coefficient of PTZ-5. In addition, the ESI test found that the lifetime of the electrons in the PTZ-5 cell is shorter. These are the main factors for the above unexpected result of PCE. Our studies bring new insights into the development of phenothiazine-based highly efficient dye-sensitized solar cells (DSSCs).

Introduction

Dye-sensitized solar cells (DSSCs) have received massive attention due to their simple structure and low cost.1 Novel and highly efficient dyes play a decisive role in developing efficient DSSCs. Since the pioneering work of Grätzel,1 numerous researchers have done a lot of research work in dye development.2−5 At present, thousands of dyes have been applied to DSSCs, mainly concentrated in porphyrin,6−8 ruthenium complexes,9,10 and metal-free pure organic dyes.11−20 As early as in 2015, Kakiage et al. reported a fantastic photoelectric conversion efficiency (PCE) of 14.34% using co-sensitization of silyl-anchor-based organic dye (ADEKA-1) and carboxy-anchor-based organic dye (LEG-4).21 In 2019, Sun et al. designed and synthesized organic dye ZL003, which achieved a relatively high PCE of 13.6% due to minimal energy losses on molecular excitation.22 Recently, Kim et al. have also shown a striking PCE of 14.2% by co-sensitizing novel thieno-[3,2-b]indole-based organic dye and a porphyrin sensitizer.23

Compared with metal complex dyes, metal-free pure organic dyes have the advantages of high molar extinction coefficient, easy structure modification, and adjustable spectral response range.24−26 The incipient design of organic dyes for DSSC is based on the donor−π–conjugated-bridge-acceptor (D−π–A) structure.27,28 The push–pull structure is formed between the electron donor and the acceptor, which facilitates the injection of electrons. However, the limitations of spectral response are still the deficiency of D−π–A dye molecules. In 2011, Zhu et al. innovatively proposed the concept of donor-acceptor−π–bridge-acceptor (D–A−π–A).15,29−33 They introduced an auxiliary acceptor such as benzotriazole, quinoxaline, benzothiadiazole (BTD), and benzoxadiazole to the traditional D−π–A-type molecules, which can reduce the molecular optical band gap and broaden the spectral response range.34−37

Inspired by the above works. Here, we have synthesized two phenothiazine-based organic dyes, PTZ-3 (D−π–A) and PTZ-5 (D–A−π–A). As shown in Figure 1, compared to PTZ-3, a benzothiadiazole was added as an auxiliary acceptor to PTZ-5 to broaden the spectral response range and improve the device performance. We also investigated the structure–performance relationships of these two dyes. The relationships between the structure and the performance gleaned from our study bring new insight into the further research of phenothiazine-based organic dyes in DSSCs.

Figure 1.

Figure 1

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Molecular structures of the PTZ-3 and PTZ-5.

Results and Discussion

Synthesis

The synthetic route of PTZ-3 and PTZ-5 are shown in Scheme S1. These two dyes were synthesized by C–N coupling, brominating, Suzuki cross-coupling, and Knoevenagel reactions. All of the solvents, original materials, and reagents were purchased commercially and used as received without further purification. The specific synthesis steps of compound 1 refer to our previous work.38 The synthetic details and characteristic results for the intermediates and target dyes are provided in the Supporting Information (SI).

Photophysical and Electrochemical Properties

Figure 2 shows the UV–vis absorption spectra of PTZ-3 and PTZ-5 dissolved in tetrahydrofuran (THF). We found that the spectral response of PTZ-5 is significantly red-shifted to 57 nm, which is probably due to the auxiliary acceptor presence compared with that of PTZ-3. Furthermore, PTZ-3 exhibits a higher molar extinction coefficient compared with PTZ-5, which were 62.3 × 103 and 38 × 103 M–1·cm–1, respectively. The UV–vis absorption spectra and the steady-state fluorescence emission spectra were processed using a normalized method, as shown in Figure 3. The spectral intersections of the dye molecules PTZ-3 and PTZ-5 are at positions of 503 and 544 nm, respectively. According to the transition band gap formula: E0–0 = 1240/λ, the calculated band gap width (E0–0) is 2.5 and 2.28 eV, respectively. The results demonstrate that the introduction of benzothiadiazole unit leads to a smaller band gap and a broader spectral response range.

Figure 2.

Figure 2

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UV–vis absorption spectra of PTZ-3 and PTZ-5 in THF.

Figure 3.

Figure 3

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Normalized electronic absorption and emission spectra of PTZ-3 and PTZ-5 in THF.

The adaptation of energy level between the dye molecules, semiconductor conduction band (CB), and the redox electrolyte is the basis for the manufacture of DSSCs. The ground-state oxidation–reduction potential (ED/D+) of the two dyes can be derived from the cyclic voltammetry (CV) curve (Figure 4), which were measured in THF with the Fc/Fc+ redox couple as a reference. The ED/D+ for PTZ-3 and PTZ-5 are 0.23 and 0.21 V, respectively. The excited state redox potential (ED*/D+) of the dye molecules PTZ-3 and PTZ-5 are obtained according to the formula (ED*/D+ = ED/D+ – E0–0/e) are −2.27 and −2.07 V, respectively (no entropy changes are considered during light excitation). Table 1 summarizes the relevant parameters.

Figure 4.

Figure 4

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Cyclic voltammetry of PTZ-3 and PTZ-5 in THF.

Table 1. Photophysical and Electrochemical Data of PTZ-3 and PTZ-5.

dye λmaxabsa (nm) εmaxabsa (103 M–1 cm–1) λmaxpla (nm) E0–0b (eV) ED/D+c (V) ED*/D+d (V)
PTZ-3 449 62.3 558 2.5 0.23 –2.27
PTZ-5 506 38 635 2.28 0.21 –2.07
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a

The maximum absorption wavelength (λmaxabs), the maximum molar absorption coefficient (εmax), and the maximum emission wavelength (λmaxpl) were derived from the steady-state absorption–emission spectra of the dye in THF. The molar absorption coefficient (ε) were calculated by the equation ε = A/(cl), where A is the absorbance, c is the concentration in moles per liter, and l is the path length in centimeters.

b

The 0–0 transition energy (E0–0) was estimated from the intersection of the normalized absorption and the emission spectra.

c

The ground-state redox electricity (ED/D+) was referenced to Fc/Fc+.

d

The excited redox potential (ED*/D+) was derived from the formula ED*/D+ = ED/D+ – E0–0/e without considering the entropy change in the light excitation process.

Theoretical Calculation

The intrinsic electronic structures of the dyes such as the distribution of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) affect the intramolecular charge transfer (ICT) and excitation characteristics. As shown in Figure 5, the HOMOs of PTZ-3 and PTZ-5 are distributed on the donor units, while the LUMOs are primarily localized on the π–A or A−π–A units. The simulated UV–vis absorption spectra of the dyes in the THF solvent are presented in Figure 6. There are two distinct absorption bands in the UV and visible regions. The absorption from 200 to 400 nm can be described by π–π* electronic transitions and the range 400–700 nm corresponds to the ICT process from the donor unit (HOMO) to the acceptor unit (LUMO) of dyes. The calculated maximum absorption wavelengths of PTZ-3 and PTZ-5 are 446 and 504 nm, respectively. This is quite consistent with the results of the spectral test. Table 2 shows the values for the specific parameters of these two dyes. Notably, the calculated oscillator strengths (f) of the S0 → S1 for dye PTZ-5 is lower than that of dye PTZ-3.

Figure 5.

Figure 5

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Electron cloud distribution of PTZ-3 and PTZ-5 at the B3LYP/6-31G(d,p) level with Gaussian 09.

Figure 6.

Figure 6

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Absorption spectra of PTZ-3 and PTZ-5 at the time-dependent TD-B3LYP/6-31G(d) level with Gaussian 09.

Table 2. Calculated Excitation Energies, λmax, Oscillator Strengths (f), and Main Transition Assignments for PTZ-3 and PTZ-5.

dye state transition assignmentsa E (eV)[λmax (nm)] f
PTZ-3 S0 → S1 60.1% H → L, 24% H – 1 → L 2.78[446] 1.1068
  S0 → S2 38% H → L, 33% H – 1 → L + 1, 12% H – 1 → L + 2 4.27[290] 0.3627
PTZ-5 S0 → S1 51% H → L, 19% H – 1 → L 2.46[504] 0.8936
  S0 → S2 57.5% H → L + 2, 27.5% H – 2 → L 3.62[343] 0.2115
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a

H = HOMO, L = LUMO, H – 1 = HOMO – 1, L + 1 = LUMO + 1, H – 2 = HOMO – 2, and L + 2 = LUMO + 2.

Then, we analyzed the intermolecular charge transfer (ICT) characteristics for further exploration. As shown in Figure 7 and Table 3, the introduction of BTD on the one hand can improve the H index (the half sum of the centroid axis length along the D–A direction) of PTZ-5 to strengthen the hole and electron transfer and on the other hand can slightly reduce the molecular dCT by reducing the Egap, which is also reflected in the hole (blue)–electron (green) distribution map (Δρ) and charge density difference map (centroids). It is a reasonable diagnostic index to quantify through the space transitions, which denotes a substantial charge separation if it has a positive value.39 However, the t of PTZ-5 is a negative number. Also, it can be seen from the optimized structure from Figure 7 that the introduction of BTD increases the dihedral angle between the D–A units and affects the effective transfer of excitation charges from D to A (HOMO to LUMO), thereby reducing the qCT (amount of transfer electrons) of PTZ-5. This produced a hypochromic effect and resulted in a lower molar extinction coefficient of PTZ-5. This eventually affects the performance of the device.

Figure 7.

Figure 7

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Simulated optimized structure and computed hole (blue)–electron (green) distribution map (Δρ) and charge density difference map (centroids) of PTZ-3 and PTZ-5.

Table 3. Computed Charge Transfer Parameters of PTZ-3 and PTZ-5 in the THF Solvent Predicted PBE0/6-311G(d)/SMD Level of Theory.

dyes dCT (Å)a qCT (e–)b t (Å)c H (Å)d
PTZ-3 4.771 0.5551 1.077 3.694
PTZ-5 3.901 0.439 –0.541 4.442
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a

dCT (in Å): length of charge transfer.

b

qCT (in e–): amount of transfer electrons.

c

t (in Å): extent of charge separation.

d

H (in Å): half sum of the centroid axis length along the D–A direction.

Photovoltaic Performance

Figure 8 shows the incident photon-to-current conversion efficiency (IPCE) of the devices. The results reveal that the IPCE of the PTZ-3 cell are higher than that of PTZ-5 in the range of 300–560 nm. Also, the IPCE maximum value of PTZ-5 cell is 58%, which is significantly lower than that of the PTZ-3 cell (78%). This phenomenon is probably due to the higher molar extinction coefficient for PTZ-3 than that for PTZ-5 in this region. Nevertheless, the IPCE of the PTZ-5 cell has a broadened IPCE spectrum response than the PTZ-3 cell, with the maximum reaching 800 nm, which is probably due to the introduction of benzothiadiazole, and exhibits a broadened absorption spectrum for PTZ-5.

Figure 8.

Figure 8

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Incident photon-to-electron conversion efficiency (IPCE) spectra of PTZ-3 cell and PTZ-5 cell.

The photocurrent density–voltage (J–V) characteristics were recorded under simulated AM 1.5 G illumination, and the charts are given in Figure 9 and Table 4. The results reveal that the open-circuit voltage (Voc, 710 mV) of the PTZ-3 cell is significantly higher than that of PTZ-5 (Voc, 640 mV). In the end, the photoelectric conversion efficiency (PCE) of the PTZ-3 cell is 5.35%, which is higher than that of PTZ-5 (PCE = 4.43%). This result is consistent with the previous IPCE analysis.

Figure 9.

Figure 9

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J–V characteristics measured under irradiation of 100 mW·cm–2 simulated AM 1.5 sunlight.

Table 4. Photovoltaic Parameters of Cells Measured at an Irradiation of 100 mW·cm–2, Simulated AM 1.5 Sunlight.

dyes Voc (mV) Jsc (mA·cm–2) FF% PCE%
PTZ-3 710 10.80 69.36 5.35
PTZ-5 640 10.06 69.10 4.43
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Then, the electrochemical impedance spectroscopy (EIS) analysis was carried out in the dark with a series of bias voltages around their Voc values to gain insights into the differences in Voc of these two devices. In general, the Voc was determined by the quasi-Fermi level (EF,n) of TiO2 and the potentials of the electrolyte. Since we have a fixed electrolyte in this work, the conduction band (CB) position and the injected electron density in TiO2 will be the main factors that affect Voc. It can be seen from Figure 10, at a fixed potential, the capacitance values of PTZ-3- and PTZ-5-based devices are almost the same, suggesting a negligible influence of the CB shift on Voc variation. Ultimately, we found that the electron lifetime, which can reflect the injected electron density was the real main factor for determining Voc. As shown in Figure 10, under the same bias voltage, the electron lifetime of PTZ-3 cell was significantly longer than that of the PTZ-5 cell, which is in line with the Voc difference.

Figure 10.

Figure 10

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TiO2 capacitance and electron lifetime as a function of potential based on the PTZ-3 cell and the PTZ-5 cell.

Therefore, we consider that the lower qCT and molar extinction coefficient of dye and shorter electron lifetime in the device were the critical reasons for the poor performance of the PTZ-5 cell.

Conclusions

In summary, we have done comparative research about the influence of benzothiadiazole unit on the properties of phenothiazine-based DSSCs. The photophysical measurements and the theoretical calculations show that the introduction of benzothiadiazole unit to the D−π–A-based dye molecules can broaden the absorption spectrum and strength the hole and electron transfer. However, the device performance is not improved, and the PCE is even reduced. This unexpected result is related to the ICT characteristics of dyes and the electronic lifetime of the devices. The lower qCT and molar extinction coefficient of dye and shorter electron lifetime in the device were the critical reasons for the poor performance of the PTZ-5 cell. In short, the introduction of benzothiadiazole can improve the spectral response range of the original D−π–A-type dye. However, it is not guaranteed that the photoelectric conversion efficiency will improve. It is necessary to comprehensively investigate the structure of dye and the performance of the device, such as the ICT process of dye and the lifetime of the excited electrons. Our studies thus provide important references for future research on the molecular engineering of phenothiazine-based highly efficient DSSCs.

Experimental Section

Characterization

1H NMR was obtained using a Bruker AM 500 spectrometer. Electronic absorption spectra and emission spectra were recorded on Agilent G1103A and Perkin Elmer LS55 luminescence spectrophotometers, respectively. The cyclic voltammetry (CV) measurements were performed using a computer-controlled CHI660C electrochemical workstation with a three-electrode system. All potentials were reported against the ferrocene/ferrocenium (Fc/Fc+) reference.38

Theoretical Calculation

The optimization molecular ground-state geometry and absorption spectra of the PTZ-3 and PTZ-5 were obtained in the THF using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculation at B3LYP/6-311G(d,p)//B3LYP/6-311G(d) level with Gaussian 09 software, respectively. The detailed methods were described in the previous article.17,18,40−42

Device Assembly and Measurements

After treatment, the square TiO2 electrodes were then immersed into a 300 μM solution of the PTZ-3 and PTZ-5 dyes in a mixture of CHCl3 and EtOH (3:7, v/v) for 12 h. The seal uses a 45 mm thick Bynel (DuPont) hot melt gasket to fill the electrolyte into the interior space through a vacuum backfill system. The osmotic electrolyte consisted of 0.6 M dimethylpropylimidazolium iodide, 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile. The detailed processes of device fabrication were very similar to that in the previous articles.43 Under standard AM 1.5 simulated solar irradiation (WXS155S-10), photocurrent density–voltage (J–V) curves of the solar cell devices were measured by Keithley 2400 Source Meter Instruments. Monochromatic incident photon-to-current conversion efficiency (IPCE) spectra measurement were recorded by a Newport-74125 system (Newport Instruments). Electrochemical impedance spectroscopy (EIS) was measured with a two-electrode system in the dark by Electrochemical Workstation (Zahner IM6e).

Acknowledgments

All authors acknowledge the financial support of this work from the National Natural Science Foundation of China (NSFC 21663010), the Natural Science Foundation of Guangxi (2015GXNSFBA139029), and the special funding for distinguished experts from Guangxi Zhuang Autonomous Region.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c05887.

  • Synthetic details and characteristic results for intermediates and target dyes (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c05887_si_001.pdf (151KB, pdf)

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