1 Introduction

Dye-sensitized solar cells (DSSCs) have been intensively studied and researched during the last decade as a promising 3rd solar cell generation due to their potential low cost manufacturing based on printing technology [1, 2]. The main stream of the research has been focusing on development of materials which would enhance the conversion efficiency, simplify the production of DSSC and assure their long-lifetime. The front photoactive electrode of a DSSC is a transparent conductive oxide (TCO) glass coated with nanoporous TiO2 sensitized with a monolayer of a Ruthenium complex dye, while the counter electrode is a TCO glass coated with a thin layer of platinum. The gap between the two electrodes is filled with an electrolyte containing an iodide/tri-iodide (I−/I3 −) redox couple [3]. Under illumination, the dye molecules are excited and initial charge separation occurs by injection of an electron from the dye into the conduction band of the TiO2. This electron is then transported to the external load via the nanostructured TiO2 and the front TCO.

Nanocrystalline TiO2 in anatase modification with a band gap of ~3.2 eV has been identified as the most appropriate material to use, since its conduction band lies just beneath the LUMO level of the ruthenium complex dye. In order to realize high efficient DSSC the following requirements must be met: good solar light harnessing, diminished loss reactions of photogenerated electrons with the tri-iodide species present in the electrolyte, and good electron transport within TiO2 layer and between TiO2 grains and TCO substrate. To satisfy the requirements a “sponge” like structure with the following characteristic apply for TiO2 layer: the layer thickness between 7 and 14 μm [4]; high inner surface area available for dye molecules to be attached; appropriate porosity of the layer assuring electrolyte penetration through the layer; good connections between the TiO2 grains; and good adhesion to the TCO. Therefore the optimization of the TiO2 layer morphology towards “sponge like” structure described above is a prerequisite for the realization of a high efficient DSSC [5–7].

At present, efficiencies of more than 11% can be obtained for DSSCs using electrolytes based on volatile organic solvents [8] The usage of these volatile organic electrolytes limits a large scale implementation of this technology due the poor long-term stability of the cells and the necessity of a complex sealing process [9]. Therefore, ionic liquids (ILs) are used as alternatives for volatile organic solvents due to their unique properties, such as negligible vapour pressure, excellent electrochemical and thermal stability, and high ionic conductivity. Slower physical transport processes in ILs as in conventional electrolytes due to the comparative high viscosity of ILs to some extent also limit the performance of DSSC. The highest efficiency for a small area DSSC (~0.2 cm2) using an IL electrolyte, reported so far, is 8.2% [10]. However, for their realization the three or fourfold layer deposition of TiO2 was needed [11]. The TCO was firstly treated with an aqueous solution of TiCl4 at 70 °C for 30 min in order to make a thin compact TiO2 layer assuring a good mechanical contact between the following printed TiO2 layer and the TCO substrate. In the second step a transparent layer consisting of 20 nm size TiO2 particles was screen printed on the TiCl4-treated TCO and additionally coated with the third screen printed layer of 400 nm light scattering TiO2 particles. Furthermore, the layers were gradually heated to 500 °C in order to achieve nanostructured porosity of the TiO2 layer with a high surface area. The fourth step was treating the triple layer with an ethanol solution of TiCl4 to improve the connectedness of the grains present in the thick nanoporous layer [11]. The preparation procedure of fourfold TiO2 layer is complex, since different TiO2 precursors as well as different deposition techniques are required. Therefore our aim was to simplify the preparation of TiO2 layer in such a way that solely a deposition of a single TiO2 paste followed by annealing of the layer would be sufficient to realize a highly efficient photoactive layer.

Some attempts have been already made to introduce Titanium alkoxide based sol into the TiO2 paste in order to act as an inorganic binder [12–15]. The results confirmed improved adhesion of the layers as well as improved connectedness of the crystalline grains, while the porosity of the layers and therefore their surface area has been diminished. We have focused on the development of a new TiO2 paste formulation that would unify the requirements mentioned above. Intensive research of our group in this field has already produced relevant results [16, 17] using a new paste formulation developed by applying the Pechini type sol–gel method [18]. The Pechini method is based on the preparation of a polymeric net (sol), which is achieved by mixing ethylene glycol with citric acid in which the metal alkoxide e.g. titanium isopropoxide is dissolved [18]. Titanium ions form mono and bi-dentate bonds with side carboxylic groups of the polyester [19, 20]. The paste was prepared with the addition of commercial TiO2 nanoparticles to the sol. The comparison study between the “standard” TiO2 paste based on terpineoel and methylcellulose [21] and the paste based on Pechini method has been reported [16]. The results showed an improvement of DSSC efficiency for up to 190% if the Pechini based TiO2 paste was used. The main reasons were higher dye loading and improved electron transport through the TiO2 layer.

In this paper we present systematic study of the Pechini based TiO2 paste formulation by varying the molar ratio of precursors in the paste and report the influence of paste composition on layer morphology and consequently on the performance of DSSC. Additionally, we try to evaluate and correlate the morphology of the TiO2 layer with the dye loading of layers and performance of DSSCs as well as to correlate the effectiveness of dye molecules with “improved” morphology of TiO2 layers. The layers have been sensitized with a ruthenium complex dye in order to determine the amount of the dye molecules adsorbed on the layer as well as to built the DSSCs and evaluate their efficiency. The morphology of the TiO2 layers, characterized by scanning electron microscopy, is presented as well as the adhesion of the TiO2 layers to the conductive substrate. The infrared spectroscopy was used to study the Pechini sol formulations used for the preparation of pastes.

2 Experimental

2.1 Preparation of TiO2 pastes and layers

The first step in the preparation of the TiO2 paste was the synthesis of a polyester-based titanium sol using a precursor molar ratio of 1:X:4X [titanium isopropoxide:citric acid:ethylene glycol, X = 1, 3, 4, 5, 6, 7, 8, 10, 12, 15]. The sols are marked with the number corresponding to the X value. The sol was prepared by heating ethylene glycol (Riedel–de Haen) to 60 °C and during stirring the titanium isopropoxide (Fluka) was added. Finally, the corresponding amount of citric acid (Fluka) was added and the temperature increased to 90 °C. The solution was stirred at this temperature until it turned clear. The paste was prepared by mixing the TiO2 powder (P25, Degussa) and sol in a mortar grinder (Retsch, RM200) for 3 h. The molar ratio between the TiO2 powder and titanium isopropoxide in the paste formulation was kept constant 7:1.

All pastes were deposited on a transparent conductive electrode (TCO) i.e. fluorine-doped SnO2 on glass substrate, using the “doctor blade” technique. Layers were annealed at 450 °C for 1 h. Afterwards, the layer thickness was determined by surface profilometer (Taylor-Hobson Ltd.) and the thicknesses of the layers are presented in Table 1. The pastes and layers are marked with numbers according to the sol used.

Table 1 The layer thickness (d), the amount of the dye molecules attached to the surface of TiO2 layers (n(dye)/V(layer)), short circuit current (J sc ) and conversion efficiency (η) of the DSSCs evaluated under STC (100 mW/cm2, 25 °C, AM 1.5G) and the effectiveness of the dye molecules [n(dye)/V(layer)]/J sc attached to the TiO2 determined for different TiO2 layers marked with numbers from 1 to 15, where the number X corresponds to the molar ratio between the polyester and titanium iso-propoxide in the sol/paste
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2.2 Production and characterisation of DSSC

The TiO2 layers were immersed in an ethanol solution of the Ruthenium complex dye (Ru(2,2′bipyridyl-4,4′dicarboxylate)2 (NCS)2, Solaronix) for 12 h. For a counter electrode, platinum (thickness ~5 nm) was sputtered onto a TCO glass. Both electrodes were sealed with a 25 μm thick polymer foil (Surlyn, DuPont) that acts also as a spacer between the electrodes. After sealing, the electrolyte was injected through two holes pre-drilled into the counter electrode. The electrolyte used for systematic study was a binary ionic liquid mixture of 1-ethyl-3-methyl-imidazolium dicyanamide (EMI-DCA, University of Erlangen; viscosity: 21 mPa s at 25 °C [22]) and 1-propyl-3-methyl-imidazolium iodide ionic liquid (PMII, Iolitec; viscosity: 1,620 mPa s measured at 20 °C) in a molar ratio 3:2 with 0.032 M of I2 (Fluka). For each type of the TiO2 layers, three identical samples of DSSCs each with an active area of 0.5 cm2 were assembled. Before characterization, the cells were stored in the dark at open circuit conditions for 24 h to allow the electrolyte to penetrate the TiO2 pores. Tungsten-halogen lamps were used as a light source as they mimic the black body spectrum with a colour temperature of 3,200 K. We calculated the short circuit current mismatch parameter and in conjunction with a calibrated c-Si reference solar cell determined the level of standard (1 sun, 100 mW/cm2) irradiance. During irradiance and characterization, the cells were covered with a black mask fitting the active area of the cell [23]. To set the cell temperature to standard test conditions STC (25 °C), the temperature was stabilized with a cooling/heating setup based on Peltier element designed for solar cell characterization [23, 24]. Current–voltage characteristics (I/V) were measured using a Keithley 238 source meter by applying voltage and measuring current. The I/V measurements were scanned from 0 (short circuit condition) to 0.9 V (beyond open circuit voltage), with 10 mV steps. Before characterization, each cell was placed on a white paper used as back reflector placed on a copper plate.

2.3 Instrumental and measuring techniques

IR spectra of the polyester, TiO2 sols, pastes, layers and TiO2 powder were recorded using a FT-IR Perkin Elmer 2000 system spectrometer. For the sols and pastes a diamond ATR cell equipped with KRS-5 lenses was used, while the powders were pressed into the pellets. In the case of layer, the layer was scratched from glass substrate to obtain the powder.

The particle size and the surface morphology of the TiO2 layers were analysed with Hitachi S 4700 scanning electron microscope (SEM).

To obtain a qualitative impression of the adhesion between TiO2 layer and the TCO substrate, the cross-cut test was applied according to the ISO 2409 standard. In the cross-cut test, two sets of cuts are made perpendicular to each other leading to a network of small squares. Then an adhesive tape is stuck on the network and pulled off by hand at a take-off angle of 60° with respect to the sample surface and almost with a constant force. Percent of squared surface crumbled from the edge of the squares, is a measure of the adhesion quality. According to ISO 2409, the quality of adhesion is ranked by different numbers ranging from 0 to 5. Ranking number of the cross-cut adhesion can vary from 0-excellent, followed by 1-very good, 2-good, 3-moderate, 4-poor and finally 5 as very poor.

The amount of the dye molecules adsorbed on the TiO2 layer, which correlates with the active surface area of the TiO2 layer, was determined with UV–Vis spectroscopy [25]. The Ruthenium complex based dye was desorbed from the TiO2 layers (marked as layer X, X = 1, 3, 4, 5, 6, 7, 8, 10, 12 and 15) with 0.02 M NaOH (Merck), the dye solution has been diluted with 0.02 M NaOH to 50 ml before obtaining UV–VIS spectra. In order to determine the amount of the dye adsorbed on the TiO2 layers, firstly a calibration curve was made for different dye solutions varying in the dye concentration (5.0 × 10−7 M, 1.0 × 10−6 M, 5.0 × 10−6 M, 1.0 × 10−5 M, 5.0 × 10−5 M, 1.0 × 10−4 M) in 0.02 M NaOH. The measurements were scanned in decrements of 5 nm from 600 to 320 nm, where two peaks (370 and 500 nm) characteristic for the dye are present in the spectrum. For the calibration curve and the evaluation of the samples concentration the absorbance of the second peak of the dye i.e. at 500 nm has been considered.

3 Results and discussion

In our study ten different TiO2 layers have been deposited from the corresponding pastes, which were developed by applying the Pechini type sol–gel method. The layers are marked as X, X = 1, 3, 4, 5, 6, 7, 8, 10, 12, 15; the number X is related to the molar ratio between titanium isopropoxide, citric acid and ethylene glycol (1:X:4X) in the paste. The structure and morphology as well as cross-cut test of sintered TiO2 layers have been researched. For better clarity, in this paper some results are presented only for three cases, where X = 1 and 15 present limit cases and 8 presents the middle case, showing the best results concerning DSSC performance.

3.1 Structure and morphology of the sols, TiO2 pastes and layers IR spectra

In the first step of the TiO2 paste preparation, polymeric net (sol) is prepared by mixing titanium isopropoxide and citric acid with ethylene glycol. We have kept the molar ratio of titanium isopropoxide, citric acid and ethylene glycol being 1:X:4X constant, while varying X value between 1 and 15 (X = 1, 3, 4, 5, 6, 7, 8, 10, 12, 15). Firstly, citric acid and ethylene glycol form the polyester, while the titanium ions attach to the side carboxylic groups of the polyester via mono and bidentate bonds forming a polymeric net (sol). Figure 1 shows the infrared spectra of the precursors of the TiO2 pastes, i.e. polyester and sols 1, 4, 8 and 15. The IR spectrum of the polyester is characterised with strong band at 1,710 cm−1 related to C=O stretching mode of the polyester (Fig. 1, curve e). The IR spectra of the sols show a blue shift of the C=O band from 1,712 to 1,720 cm−1 for sols 15 and 1, respectively, which could be an indication that complexation of titanium ions takes place, while a decrease of intensity with decreasing X is associated with a decrease of the polyester content in the sol. The complexation of titanium ions can be better analyzed by vibrations at 1,640, 1,550 and 1,380 cm−1. The band at 1,640 cm−1 is characteristic for a COO− stretching mode for a monodentate complex and the bands at 1,550 cm−1 and at 1,380 cm−1 are related to a COO− stretching mode for a bidentate complex, as reported by Leite et al. [19]. For the sol X = 1 the IR spectrum (Fig. 1, curve a) reveals the presence of strong bands at 1,640, 1,550 and 1,380 cm−1 corresponding to the mono and bidentate complex formation between COO− and titanium ions. An increase of the polyester in the sol (increase of X value) favours monodentate complexation. The intensity of the IR bands characteristic for the COO− stretching mode in the case of complexation is accordingly smaller with increasing X value, the bands are in the case of sol X > 3 noticed only as shoulders of the main bands of the polyester (Fig. 1), because the content of Ti ions involved in complexation process in the sol is decreasing with X.

Fig. 1
figure 1

The IR spectra of different TiO2 sols; sol 1 (curve a), sol 4 (curve b), sol 8 (curve c), sol 15 (curve d) and polyester (curve e) formed with citric acid and ethylene glycol

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Fig. 2
figure 2

The IR spectra of paste 4 (curve a), sol 4 (curve b), TiO2 nanopowder P25 (curve c) and layer 4 (curve d)

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In the second step TiO2 nanoparticles are added to the sol to make the paste. IR analysis revealed that there is no interaction between TiO2 nanopowder and sol, since the IR spectrum of the paste is a sum of the IR spectrum of the sol and TiO2 nanopowder, as shown for the sol 4 and paste 4 (Fig. 2, curve a, b and c). The IR spectrum of the TiO2 layer 4 measured after the annealing process at 450 °C confirms that all organic compounds of the paste burn out (Fig. 2, curve d) and the spectrum is almost identical to the IR spectrum of the TiO2 Degussa P25 nanopowder (Fig. 2, curve c).

3.2 Scanning electron microscopy (SEM)

The SEM graphs showing top view-surface morphologies of TiO2 layers 1, 8, and 15 are presented in Fig. 3. There are plenty of cracks with 1–4 μm in width noticed on the surface of layer 1 (Fig. 3a), while uniform and smooth surface with almost no cracks is characteristic for layers 8 and 15 (Fig. 3b, c). Figure 4 shows the cross-section of the active electrode consisting of TiO2 layer deposited on TCO glass substrate (from left to right: glass, TCO, TiO2 layer) for layer 1, 8 and 15. The thickness of the layer 1 (Fig. 4a) is 23.7 μm being about twice as thick as layer 8 (Fig. 4b), while the thickness of the layer 15 is around 5 μm (Fig. 4c), despite the fact that the same deposition technique was used for the preparation of all three layers. The main reason is higher content of the polyester in the paste for the paste with higher X value which burns out during sintering process. The layers 8 and 15 are well attached to the TCO coated glass substrate as it can be seen on Fig. 4b, c.

Fig. 3
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The SEM micrographs showing top view of layer 1 (a), layer 8 (b) and layer 15 (c)

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Fig. 4
figure 4

The SEM micrographs (cross section) of layer 1 (a), layer 8 (b) and layer 15 (c). The layers were deposited on the TCO-coated glass substrate (from left to right: glass, TCO and TiO2)

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More detailed and magnified insight view of the layers morphology is shown on Fig. 5. It is evident that all the layers consist of homogeneously distributed spherical TiO2 grains (~20 nm). The SEM micrographs of layers 8 and 15 (Fig. 5b, c) show an increase in porosity when compared with layer 1 (Fig. 5a). These findings confirm that during sintering of paste at 450 °C for 1 h, the sol, which is obtained by the Pechini sol–gel method, exothermally decomposes [26], as a result a “sponge” like TiO2 network is formed. An increase of the polyester in the paste (i.e. an increase of X value), results in increased porosity of the layer. Nevertheless, this effect is not well pronounced for the paste 1, while in this case the polyester content in the paste is low.

Fig. 5
figure 5

The SEM micrographs showing insight view of layer 1 (a), layer 8 (b) and layer 15 (c)

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3.3 Cross-cut test

Figure 6 shows three different TiO2 layers (marked as layer 1, 8 and 15) after cross-cut testing. According to ISO 2490, the adhesion quality of the layer 1 (Fig. 6a) is ranked by 3. The coating has flaked along the edges of the cuts partly in large ribbons. A cross-cut area not significantly greater than 35% is affected. In Fig. 6b, c can be seen that the layers 8 and 15 are compact and uniform after testing which reveals that the adhesion quality of the layer 8 and 15 is very good with the outcome of the cross-cut testing being 1. In these two cases the edges of the cuts are smooth, with hardly noticeable flakes of the coating at the intersection. Generally, the adhesion quality of the layers 8 and 15 is very good and comparable, while the layer 1 is not well attached to the TCO glass, which is in agreement with the SEM graphs (Fig. 4). The results show that in order to assure good adhesion of the TiO2 layer with the substrate the value X in molar ratio between titanium isopropoxide, citric acid and ethylene glycol in sols must exceed 4. (1:X:4X).

Fig. 6
figure 6

The photos of layer 1 (a), layer 8 (b) and layer 15 (c) taken after the cross-cut test

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3.4 Dye loading and dye sensitized solar cell

The thickness (d) of the layers, the amount of dye molecules attached to the surface of TiO2 layers per volume of the TiO2 layer [n(dye)/V(layer)] together with the short circuit current (J sc ) and conversion efficiency (η) of the corresponding DSSCs evaluated under standard test conditions (STC i.e. 100 mW/cm2, 25 °C, AM1.5G), and the effectiveness of the dye molecules [n(dye)/V(layer)]/J sc attached to the TiO2 are presented in Table 1 for all the layers marked from 1 to 15. The number of the layer (X) is related to the molar ratio between the polyester and titanium iso-propoxide in the sol/paste.

The thickness of the layers decreases with increasing the amount of polyester in the paste (value X), as shown in Table 1. The reason is that during annealing process decomposition of the polyester took place. For the layer 1 the lowest amount of attached dye molecules per volume of the layer n(dye)/V(layer) i.e. 6.32 × 10−5 mol/cm3 was found, the value increases with increasing X till layer 8, which has the highest value being 1.24 × 10−4 mol/cm3 (Table 1). For the layers X > 8 the amount of attached dye molecules per volume (n(dye)/V(layer)) again decreases and for the layer 15 reaches the value of 1.08 × 10−4 mol/cm3 (Table 1). The highest dye loading, discovered for layer 8, could be correlated with the highest surface area of the layer. This effect could be explained with the fact that an increase of the polyester content in the paste, which decomposes during the annealing process, increases the porosity of the layers that is correlated with the surface area of the TiO2 layer available for binding of the dye molecules. On contrary, a small decrease of the n(dye)/V(layer) value for the layers X > 8 (Table 1) is difficult to explain, but the reason could be that the structure of TiO2 layers became too soft and starts to collapse during the annealing process which is in accordance also with smaller thickness of the layers. The normalized results of the amount of dye molecules per volume n(dye)/V(layer) with the maximum value [n(dye)/V(layer)]max, i.e. 1.24 × 10−4 mol/cm3 for all layers are presented on Fig. 7a.

Fig. 7
figure 7

The normalized values of the amount of the dye molecules attached on different layers per layer volume n(dye)/V(layer) (a), the normalized J sc (b) and η values (c) of DSSCs assembled with different layers

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In order to evaluate the influence of different morphology and dye loading of the studied TiO2 layers on the performance of DSSCs, the dye sensitized TiO2 layers were used to assemble ten sets of DSSCs with the EMI-DCA/PMII molar ratio 3:2, 0.032 M I2 electrolyte. The performance of the DSSCs was evaluated under STC. Typical I/V characteristics for layer 1, 8 and 15 are shown in Fig. 8, while the values of short circuit current densities (J sc ) and efficiency (η) determined under STC for all layers are gathered in Table 1. The highest value of J sc was 13.8 mA/cm2 achieved with the layer 8 for which also the highest amount of dye molecules per volume (n(dye)/V(layer)) attached to its surface was found. Figure 7b shows the comparison of the current generation for examined TiO2 layers, while the measured values of J sc have been normalized with the maximum value of J sc achieved with layer 8, i.e. 13.83 mA/cm2. The η of DSSCs obtained with different TiO2 layers are presented in Table 1, while relative comparison of η is presented in Fig. 7c. In Fig. 7c the normalized values of η with the maximum value of η max obtained with layer 8 are shown for all the layers studied.

Fig. 8
figure 8

Current to voltage characteristics of DSSCs (IL electrolyte: EMI-DCA/PMII molar ratio 3:2, 0.032 M I2) assembled with layer 1, layer 8 and layer 15. The maximum power point of each DSSC is indicated with a full circle

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As evident, the layer 8, which was made from the paste based on polyester titanium sol using a precursor molar ratio of 1:8:32, posses the highest dye loading and achieves the highest performance of DSSC with the efficiency of 6.6% when using binary ionic liquid mixture of 1-ethyl-3-methyl-imidazolium dicyanamide and 1-propyl-3-methyl-imidazolium iodide. However with this kind of TiO2 layers the DSSCs with efficiency up to 10.2% for acetonitrile and 7.3% for an optimised ionic liquid electrolyte have been realised [17].

Additionally, very important parameter is also the effectiveness of the dye molecules [n(dye)/V(layer)]/J sc attached to the TiO2 layer, which tells us the amount of dye molecules needed to be adsorbed on the layer to generate J sc of 1 mA/cm2. The results calculated by dividing the amount of dye molecules per layer volume n(dye)/V(layer) with the J sc for all the layers examined are summarized in Table 1. They show that almost the same number of dye molecules needs to be adsorbed on layer 5, 6, 7, 10 and 12 to generate a J sc of 1 mA/cm2. This value is slightly lower for layer 8 (Table 1), which means that the lowest amount of dye is needed to sensitize the layer 8. Additionally, the connectedness of the TiO2 nanoparticles defining the electron transport in TiO2 layer could be directly correlated with the effectiveness of the dye molecules (n(dye)/V(layer)/J sc ), since the short circuit current of DSSC measured during illumination depends on the amount of dye molecules attached to the layer as well as on good transport of electrons through the TiO2 layer. The short circuit current is the difference between the number of generated electrons and the ones that undergo recombination reaction. While the number of generated electrons is directly correlated with the number of attached dye molecules the number of recombined electrons could be correlated with the connectedness of the TiO2 nanoparticles, taking into account that other parameters/materials in DSSC are not varied.

4 Conclusions

The TiO2 paste formulation based on Pechini sol–gel method and commercial nanocrystalline TiO2 powder has been developed. The TiO2 layers show good adhesion with the conductive substrate, and exhibit high dye loading that can be correlated with the large surface area. The results confirmed that the variation of the amount of the polyester in the TiO2 paste allows the optimisation of the TiO2 layer morphology which is essential for the production of highly efficient DSSC.

The most suitable molar ratio between titanium isopropoxide, citric acid and ethylene glycol in the paste concerning the DSSC performance was found to be 1:8:32. The layers made with this paste i.e. layers 8 exhibit the highest dye-loading and the DSSCs assembled show the highest efficiency. The efficiency of DSSC with the photoactive area of 0.5 cm2 and the electrolyte mixture of EMI-DCA/PMII when measured at 25 °C under 1 sun illumination (100 mW/cm2) was 6.6%.

Additionally the amount of dye molecules attached to the TiO2 layer per volume n(dye)/V(layer) and [n(dye)/V(layer)]/J sc values have been identified as good parameters to evaluate the quality of the TiO2 layer in DSSCs.