1 Introduction

In general, solar cells represent a large family of photovoltaic devices, which can be constructed on inorganic or organic semiconductor materials as well as their various combinations and utilize for the direct conversion of sunlight energy to electricity [1]. Among them, dye-sensitized solar cells (DSSCs), reported for the first time by Grätzel and co-workers in 1991 [2], are a class of low-cost solar energy-to-electrical power conversion devices, wherein roles of light absorption and charge carrier transport are separated between different components of their photoelectrochemical system [3]. Indeed, further, DSSC devices are a promising alternative to the conventional Si-based solar cells due to their easy fabrication process as well as wide opportunities for the modification of their components, e.g., dye sensitizer or electrolyte system, affording to improve their stability and power conversion efficiency (PCE) [4]. Thus, the elaboration and study of new sensitizers that are one of the key components of DSSC, have attracted a considerable attention of many researchers in recent years as an appropriate way to form high-performance DSSC devices [5,6,7,8]. In this field, Ru(II)-based complex compounds were initially suggested as the effective sensitizers for DSSCs [2, 3]. However, Ru(II)-based sensitizers are limited for practical use due to their expensive, difficulty in synthesis and purification, and toxicity both for humans and the environment [5]. In turn, metal-free organic dyes, typically having donor-π-linker-acceptor (D-π-A) structure, in DSSCs demonstrated PCE and fill factor values comparable with the ruthenium dyes [5, 7, 9]. Moreover, metal-free organic dyes have a number of advantages for DSSC applications, such as high molar extinction coefficients, tunable optical and electrochemical properties, low cost of their synthesis due to the use of easy to perform procedures, and inexpensive chemicals [10]. Therefore, metal-free sensitizers are of important objectives for organic synthesis, since their use in DSSCs is one of the perspective ways to develop devices with improved characteristics [7, 11, 12]. It should be noted that many π-conjugated organic compounds, containing thiophene and pyrrole rings in the structure, have been declared as effective materials for organic photovoltaics, including DSSCs [13,14,15]. Among them, metal-free dyes, bearing electron-rich carbazole or thieno[3,2-b]indole (TI) ring system as a donor part in their push–pull structure, have been successfully applied in the construction of high-performance DSSC devices [16,17,18]. In this context, we have previously described an effective route for the synthesis of TI-based dyes IS 1–4, shown in Scheme 1 and their primary application as photosensitizers in DSSCs [19].

Scheme 1
scheme 1

The structure of TI-based dyes IS 1–10

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Herein, we wish to present the expanded series of dyes IS, containing two numbers of TI-based sensitizers, namely IS 1–5 and IS 6–10, bearing 2-cyanoacrylic acid and 5-(methylene)barbituric acid as an acceptor-anchoring group, respectively (Scheme 1). In this work, we have studied for the first time the optical and electrochemical properties, adsorption ability of these dyes as well as their application for the manufacture of DSSCs using commercially available TiO2-coated photoanodes.

2 Experimental section

The synthesis of dyes IS and common information on the analytical equipment and methods used are described in Supplementary Information.

3 Results and discussion

3.1 Thermal properties of dyes IS 4 and IS 9

The thermal gravimetric analysis curves of the synthesized dyes IS 4 and IS 9 are shown in Fig. 1. It can be seen that the analyzed IS 4 decomposes in three steps, in contrast to IS 9. At the first step at 210–300 °C, IS 4 exhibits a weight loss of 7% which is attributed to the elimination of the -CO2H fragment, because only CO2 is present in the gas phase (m/z = 44, [CO2]+, m/z = 12, [C]+).

Fig. 1
figure 1

Thermogravimetric analysis of the dyes IS 4 and IS 9

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Further, decomposition occurs in two steps due to the loss of 26% at 300–410 °C and the loss of 27% at 410–500 °C, may be attributed to the decomposition of the dye molecule into fragments and their oxidation. The gaseous phase consisted mostly of corresponding products: m/z = 78, [C6H6] + , m/z = 52 [C4H4]+ and [C3H3N]+, m/z = 26, [CN] + , m/z = 15, [CH3]+, m/z = 30, [C2H6]+, m/z = 44, [CO2]+, m/z = 64 [SO2]+, m/z = 48 [SO]+, and m/z = 18 [H2O]+. A similar tendency was observed for IS 9.

Nonetheless, synthesized dyes IS 4 and IS 9 display relatively high decomposition temperatures of 276.6 °C and 324.5 °C in the air with 5% weight loss, respectively. These data show that the thermal stability of the obtained dyes is good enough for applications in DSSCs.

3.2 Optical and electronic properties of dyes IS 1–10

The absorption and normalized emission spectra of the dyes IS 1–10 in CHCl3 are shown in Fig. 2, the corresponding data are summarized in Table 1. The main part of the electronic absorption of these dyes IS 1–10 carried out in the visible region at 400–610 nm, which is related to intramolecular charge transfer (πD-π*A) from the donor part of thieno[3,2-b]indole to the acceptor part of 2-cyanoacrylic acid or 5-(methylene)barbituric acid. The red shifts of the absorption and fluorescence maximum for 5-(methylene)barbituric dyes IS 6–10 are approximately 56 nm in comparison to 2-cyanoacrylic dyes IS 1–5, respectively. A similar situation was observed for carbazole-based D-π-A dyes [20]. Thus, it can be assumed that the electron-accepting ability of 5-(methylene)barbituric acid unit is stronger than that of 2-cyanoacrylic acid.

Fig. 2
figure 2

a The absorption and normalized emission spectra of the dyes IS 1–5 and b IS 6–10

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Table 1 Optical and electronic properties of IS 1–10
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The spectra of the dyes IS 4 and IS 9 upon absorption on the surface of the TiO2 film are shown in Fig. 3. Dye IS 4 exhibits a hypsochromic shift of 9 nm in comparison to solution in CHCl3, which can be attributed to the deprotonation of the carboxylic group in the acceptor-anchoring group of 2-cyanoacrylic acid [21]. On the other hand, the absorption of IS 9 anchored on the TiO2 film exhibits a bathochromic shift of 38 nm indicating that has a stronger tendency to form J-type aggregation [22, 23]. Despite the large values of the absorption maximum and extinction coefficient for the dyes IS 6–10, their photovoltaic performance did not show a significant improvement, as reported below.

Fig. 3
figure 3

The normalized absorption spectra of the dyes IS 4 and IS 9 anchored on TiO2 films

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3.3 Adsorption properties of organic dyes on TiO2

To study the adsorption ability of the synthesized dyes on the TiO2 surface, FTIR spectra were recorded for free dyes powder IS 4 and IS 9 and these dyes adsorbed TiO2 nanoparticles (Fig. 4). The C≡N frequencies (2214 cm−1) were not changed in the spectra-free dyes powder IS 4 and adsorbed TiO2. In this way, no interaction occurred between the C≡N moieties and TiO2 during the adsorption. The O–H (3095 cm−1) and C=O (1680 cm−1) frequencies in the neat sample disappeared or shifted after dye adsorption, respectively. Therefore, the anchoring of IS 4 dyes on the TiO2 surface occurs through the carboxylate group and has the character of chemical adsorption. While bands for dye IS 9 after adsorption were slightly shifted around 5–10 cm−1, compared with the dye powder. This change can be assigned to the physical interactions on the TiO2 surface.

Fig. 4
figure 4

FTIR spectra of dye IS 4 and IS 9 powder and dye IS 4, IS 9 adsorbed on TiO2 nanoparticles

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3.4 Electrochemical properties of the dyes IS 4 and IS 9

The electrochemical properties of the dyes IS 4 and IS 9 were studied by cyclic voltammetry. The cyclic voltammograms are shown in Fig. 5, which demonstrate quasi-reversible oxidation and reduction processes for IS 4 whereas IS 9 exhibits quasi-reversible oxidation and electrochemically inactive in a wide range of cathodic potentials. On the base of the redox potentials dyes IS 4 and IS 9, HOMO and LUMO energy levels were calculated. The related data are listed in Table 2.

Fig. 5
figure 5

Cyclic voltammogram on GC electrode for IS 4 and IS 9

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Table 2 Electrochemical properties of the IS 4 and IS 9 dyes
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To estimate the feasibility of electron injection and regeneration processes, we performed the comparison of the dye’s energy levels with those of the TiO2 conduction band and redox electrolyte.

As shown in Fig. 6, the HOMO energy levels of dyes IS 4 and IS 9 are located lower than the potential of I−/I3− redox couple (− 4.9 eV), which provides possible regeneration of oxidized dye molecules after injection of excited electrons into TiO2 electrode. In turn, the latter process is energetically permitted for each of these dyes because their LUMO energy levels are located higher than the conduction edge of the TiO2 (− 3.9 eV). Thus, dyes IS 4 and IS 9 are potentially appropriate as photosensitizers for DSSCs.

Fig. 6
figure 6

Schematic energy levels of dyes IS 4 and IS 9, a nanocrystalline TiO2 electrode, and I3−/I− redox electrolyte

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3.5 Photoelectrochemical characteristic of TiO2-based photoanodes with dyes IS 1–5 and IS 9

To compare the photoelectrochemical properties of TiO2-based photoanodes sensitized with dyes IS 1–5 and IS 9, we used a 3-electrode PECC-2 cell, which provides the equal experimental conditions for the photoelectrochemical test, and as a mediator, the standard system I−/I3− (0.5 M LiI + 0.05 M I2) in acetonitrile. The structure of the IS dyes determines the efficiency of light absorption by molecules, their adsorption on the TiO2 surface, and electron injection, and also affects the recombination losses and the efficiency of electron collection. The integral photoelectrochemical activity of photoanodes sensitized with dyes IS 1–5 and IS 9 was obtained under illumination with a solar spectrum simulator with a power of 100 mW cm−2 are shown in Fig. 7, the corresponding data are summarized in Table 3. It is noticeable that the photovoltaic parameters of TiO2-based photoanodes sensitized with dyes depend on the nature of the acceptor-anchor part of the IS dyes. Dye IS 4, bearing 2-cyanoacrylic acid as an acceptor-anchoring group exhibits a PCE of 3.06% (Jsc = 7.72 mA cm−2, Voc = 0.69 V, and FF = 0.57), whereas IS 9, bearing 5-(methylene)barbituric acid displays very low PCE of 0.58% (Jsc = 1.37 mA cm−2, Voc = 0.61 V and FF = 0.69), respectively.

Fig. 7
figure 7

Photocurrent vs the voltage for the PECC of TiO2-based photoanodes with dyes IS 1–5 and IS 9

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Table 3 Main photoelectrochemical characteristics of TiO2-based photoanodes with dyes IS 1–5 and IS 9 with a mediator system 0.5 M LiI + 0.05 M I2 in acetonitrile
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The spectral dependences of the external quantum efficiency are shown in Fig. 8. It is obvious that the main spectral range of operation of photoanodes sensitized with dyes IS 1–5 and IS 9 falls within the wavelength range of 365–650 nm. The maximum values of IPCE for all photoanodes are observed in the range 450–470 nm. In the all-spectral range, the IPCE values for photoanodes based on IS 1–5 sensitizers are 5–6 times higher than for IS 9. The IPCE data correspond to the main current–voltage characteristics. The IPCE spectrum is simultaneously influenced by several factors: light collection efficiency, electron injection, and carrier collection efficiency. In the case of IS 1–5, due to the presence of the 2-cyanoacrylic acid as acceptor-anchoring group, the dye molecules are reliably chemisorbed on the TiO2 surface, which ensures a high surface concentration of dye molecules and efficient light absorption by the photoanode. Apparently, 5-(methylene)barbituric acid part of the molecule IS 9 is not a sure anchoring group and cannot provide good adsorption of this dye on TiO2 surface. As a result, dye molecules are bound to the TiO2 surface only due to physical adsorption. This leads to a reduced concentration of dye IS 9 per unit TiO2 surface and, as a consequence, insufficiently effective light absorption by the photoanode. The presence of the 5-(methylene)barbituric acid acceptor-anchoring group in dye IS 9 can also obstruct the injection of electrons.

Fig. 8
figure 8

IPCE spectra of TiO2-based photoanodes with dyes IS 1–5 and IS 9

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As noted, the anchoring function of the acceptor-anchoring groups in the thieno[3,2-b]indole dyes can affect the recombination and collection efficiency of electrons. This can be qualitatively judged by the transient of the photopotential. The transients of the photopotential for photoanodes IS 4 and IS 9 in the dark and under illumination with a 100 mW cm−2 solar simulator are shown in Fig. 9.

Fig. 9
figure 9

Time dependence of photoanode potential for PECC with photoanode based on sensitizers IS 4 and IS 9 adsorbed on titanium dioxide in the dark and on exposure to light of power 100 mW cm−2

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As can be seen from Fig. 9, under dark conditions, the potentials of the photoanodes are close. Under illumination with simulated sunlight, the potential of each photoanode quickly shifts to the cathode region. The cathodic shift increases on going from IS 9 to IS 4. This is consistent with the corresponding increase in Voc in Table 3. When the light is turned off, the potential of photoanodes IS 4 relaxes more slowly to the dark potential as compared to IS 9. This indicates an increase in the lifetime of electrons going from IS 9 to IS 4.

The recombination losses and the collection efficiency of electrons were quantified by modulation photocurrent spectroscopy (IMPS) and photovoltage spectroscopy (IMVS). The IMPS and IMVS spectra obtained under illumination with monochromatic light (λ = 453 nm, which coincides with the maximum value of IPCE) are shown in Figs. 10 and 11. It can be seen that the real component of the IMPS spectrum is higher for IS 1–5 and significantly decreases on going to IS 9, which is consistent with the corresponding characteristics of short-circuit photocurrent density.

Fig. 10
figure 10

Photocurrent spectra (IMPS) for PECC of TiO2-based photoanodes with dyes IS 1–5 and IS 9

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Fig. 11
figure 11

Photopotential spectra (IMVS) for PECC of TiO2-based photoanodes with dyes IS 1–5 and IS 9

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The electron transfer time (τtr) was estimated from the IMPS spectra using the equation:

$${\tau }_{\text{tr}}= \frac{1}{{2\pi f}_{\text{tr}}},$$
(1)

where ftr is the characteristic minimum of the frequency of the imaginary component of the IMPS.

The electron lifetime (τrec) was estimated from the IMVS spectra using the equation:

$${\uptau }_{\mathrm{rec}}= \frac{1}{{2{\pi {\rm f}}}_{\mathrm{rec}}},$$
(2)

where frec is the minimum frequency of the imaginary component of the IMPS.

The charge collection efficiency (ηcc) was determined by the equation:

$${\eta }_{\mathrm{cc}}=1- \frac{{\tau }_{\mathrm{tr}}}{{\tau }_{\mathrm{rec}}}.$$
(3)

The obtained values τtr,τrec, and ηcc are given in Table 4. It is noticeable that the value of τtr slightly depends on the nature of the dye. The lifetimes of an electron largely depend on the nature of the dye and increase significantly on going from IS 9 to IS 1–5. It is also seen from the table that the electron lifetimes in the case of IS 1–5 are 20–30 times longer than the corresponding transit times, which provides an electron collection efficiency of about 95–97%. In the case of IS 9, the electron collection efficiency is 20% lower.

Table 4 Data from IMPS and IMVS spectra for of TiO2-based photoanodes with dyes IS 1–5 and IS 9 with a mediator system 0.5 M LiI + 0.05 M I2 in acetonitrile
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Taking into account that the thickness of the active TiO2 layer (L) is 10–15 μm, the electron diffusion coefficient (D) was calculated using the equation:

$$D=\frac{{{L}}^{2}}{{2.35\cdot \uptau }_{\mathrm{tr}}}.$$
(4)

The values of the electron diffusion coefficient obtained in this way were (1/2)·10–5 cm2 s−1.

Knowing D and τrec, the effective electron diffusion length (Ld) was determined by equation:

$${{L}}_{\mathrm{d}}= \sqrt{D\cdot {\uptau }_{\mathrm{rec}}}.$$
(5)

The calculated Ld values for photoanodes based on IS 1–5 dyes were 4–6 μm. Thus, the value of Ld is approximately half of the thickness of the active layer of photoanodes. This is additional evidence that, when photoanodes with sensitizers IS 1–5 function, electrons can effectively collect on the electrode before they recombine.

3.6 Photovoltaic performance of DSSCs based on dyes IS 1–10

To test the performance of the dyes IS 1–10 under realistic conditions, we have manufactured laboratory samples of DSSCs. The electrolyte used in photoelectrochemical measurements was replaced with a mixture of 3-propyl-1-methylimidazolium iodide (0.6 M), lithium iodide (0.1 M), iodine (0.05 M), and 4-(tert-butyl)pyridine (0.5 M) in 3-methoxypropionitrile. This modification was made since 3-methoxypropionitrile has a higher boiling point and less volatility than acetonitrile. Thus, it can be used in real-life DSSC operating conditions. Also, Ru(II)-based complex compound N719 was used as the reference dye. The J/V characteristics obtained at 166 mV s−1 for all fabricated DSSCs are shown in Fig. 12. The photovoltaic results are summarized in Table 5.

Fig. 12
figure 12

J/V curves of DSSCs based on IS 1–10 and N719

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Table 5 Photovoltaic performance of the fabricated DSSCs based on IS 1–10 and N719
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The overall efficiencies of the DSSCs based on dyes IS 1–5, bearing 2-cyanoacrylic acid as an acceptor-anchoring group were superior to that of the device constructed using dyes IS 6–10, bearing 5-(methylene)barbituric acid as an acceptor-anchoring group, which is consistent with the results described above. Undoubtedly, the DSSC based on IS 4 demonstrated the highest PCE of 3.02% among these dyes IS 1–10 (Jsc = 7.59 mA cm−2, Voc = 0.62 V and FF = 0.64), reaching ∼ 60% of the standard cell based on N719.

One of the features for DSSCs is the different shapes of the J/V characteristics obtained in the forward and reverse modes of voltage variation [24]. This is a hysteresis phenomenon varying with scan direction, external bias, and sweep rate during the J/V measurements [25]. The hysteresis behaviors of DSSCs based on IS 4 demonstrated by comparing the J/V curves of the forward and reverse scans are shown in Fig. 13.

Fig. 13
figure 13

Reverse and forward scan J/V curves for best performing DSSCs based on IS 4 with a 15, b 166, and c 560 mV s−1

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The hysteresis index (HI) is used to quantitatively describe this phenomenon, in particular, to gage the severity degree of the gap between the forward and reverse J/V curves. In this work, HI was calculated accordingly equation [26]:

$${{H}}_{\rm index}= \frac{{\mathrm{PCE}}_{R}-{\mathrm{PCE}}_{F}}{{\mathrm{PCE}}_{R}},$$
(6)

where PCER and PCEF are PCE obtained from the reverse and forward scanning directions, respectively.

The photovoltaic parameters including the calculated hysteresis index obtained for different scan rates are listed in Table 6. The photocurrent, FF, and PCE of investigated DSSCs for the forward scan are being higher than those from the reverse scan. So, the hysteresis index has a positive value for all measurements, which allows it to be identified as normal hysteresis [26].

Table 6 Hysteresis index for DSSC based on IS 4
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Also, it should be noted that HI increases with increasing scan rate, which is typical for these devices [27, 28] as opposed to planar hybrid perovskite solar cells [29]. The change in hysteresis with these external test conditions might be related to multiple factors such as the electric field, electron diffusion, cell structure, and moreover. The mechanisms of the hysteresis phenomena have been described in several works [25, 27, 28] and not considered here. However, we suppose that the main reason of hysteresis obtained for these cells has been attributed to the presence of capacitance in the electrode–TiO2–dye–electrolyte–electrode structure and the charge accumulation that are indirectly confirmed by the results of our previous study [30]. Reducing hysteresis leads to improving the overall photovoltaic performance of DSSCs. However, the capacitive elements cannot be fully removed as they are inherent properties of the corresponding components of DSSCs.

4 Conclusions

In summary, we have successfully prepared a wide series of metal-free D-π-A dyes (IS 1–10), bearing thieno[3,2-b]indole core as an electron-donating part and 2-cyanoacrylic acid (IS 1–5) and 5-(methylene)barbituric acid (IS 6–10) as electron acceptor-anchoring part and used these dyes as photosensitizers for DSSCs. In regard to the basic optical characteristics of the obtained dyes, IS 6–10 showed generally higher extinction coefficients compared to IS 1–5. In addition, the absorption maxima of dyes IS 6–10 were red shifted in comparison to the absorption maxima of dyes IS 1–5. Nevertheless, DSSCs based on dyes IS 6–10 exhibited low PCE values at 0.20–0.32%, probably due to the poor binding ability of 5-(methylene)barbituric acid group in the structure of dyes IS 6–10 to the TiO2 surface, which caused to decrease overall sensitization activity of these dyes. In contrast to this, DSSCs based on dyes IS 1–5, bearing 2-cyanoacrylic acid as an anchoring group, showed PCE values at 2.25–3.02%. However, taking into account optical characteristics of dyes IS 6–10, further modification of their structure by addition of the functional substituents, bearing good anchoring function, e.g., –CO2H group, at acceptor part of 5-(methylene)barbituric acid, can be a promising way to obtain photosensitizers with the improved properties.