Micro-Raman analysis of reverse bias stressed dye-sensitized solar cells

Abstract

The degradation mechanisms of Reverse Bias (RB) stressed Dye Solar Cells (DSCs), sensitized with cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (N719, Red Dye) and with cis-dicyano-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) (Ru505, Orange Dye) have been studied by means of resonance micro-Raman and UV-Vis spectroscopy. For N719 sensitized devices, the visible degradation induced by the stress tests involves both electrolytic solution and the sensitizer: the electrolyte suffers gas bubble formation and loss of solvent, while the dye cannot be regenerated and undergoes irreversible chemical changes. Confocal Raman imaging and UV-Vis absorption spectra confirmed that in regions where the electrolyte was absent, the detachment of the thiocyanate ligand (SCN⁻) from the dye is favored. On the other hand, measurements carried out on DSCs realized with the bis-cyano dye (Ru505) do not show dye modifications during the RB stress. We also clarify that the apparent N719 dye bleaching in particular zones of the cell active area, is not related to dye desorption from the TiO₂ layer, but to loss of solvent and to dye chemical changes, which are responsible for a characteristic blue shift in the absorption spectrum.

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Introduction

In recent years, synergy between chemistry, physics and engineering has allowed the development of nanoscale materials and the fabrication of devices such as DSCs, which could open the way to new application of photovoltaic energy.^1,2

After the first paper published by Grätzel and O'Regan in 1991,³ DSCs received great interest and nowadays more than 3000 publications and 2000 patents have been disclosed.^4,5 Initially much effort was addressed to the building and characterization of small area solar cells, but progressively the attention shifted towards larger devices, trying to solve all problems related to the dimension scaling up and to their industrialization.⁶

A crucial factor for the industrial development of DSCs is the device stability, which can be limited by dye oxidation, UV light exposure, increased temperature, and by particular bias conditions. It is well known that high temperatures, in conjunction with electrolytes containing water as impurity or incorrectly balanced in their chemical components, may induce a strong cell degradation.⁷ In order to improve DSC technology, there is therefore the need for a basic understanding of the different degradation processes^8–10 and for the definition of a protocol for accelerated long-term stability tests.

Up-scaling of DSCs requires to interconnect in series and parallel a number of cells to form a module which can give the desired output voltage and power. Cells in modules can have different performances due to extrinsic phenomena such as shadowing, leading to current mismatch. From the stability point of view, the mismatch can force the least performing cells to work under Reverse Bias (RB) and to behave as a dissipating load. This prolonged stress condition may thus cause degradation of performance and gas formation inside the cell¹¹ up to cell failure.

A number of contributions have dealt with the RB problem and most of them have considered solar devices based on the bis-thiocyanato ruthenium dye, N719.^12–15 Reduction of cell efficiency has been correlated in general to the loss of coordinated NCS⁻ ligands.¹³ Wheatley et al. analyzed reverse biased DSCs applying various constant voltages to the cell (up to 3000 mV) and observing the induced phenomena by Raman spectroscopy and spectroelectrochemical analysis.¹²

Analogous studies allowed to correlate the loss of cell efficiency to the depletion of the coordinated thiocyanate ligand.^13–15

We have now extended our investigation towards molecular dyes which could show, in principle, an higher stability towards the RB aging and we report here the results of a comparative study on DCSs devices sensitized with two dyes: cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (N719, Red Dye) and cis-dicyano-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) (Ru505, Orange Dye) (Fig. 1a). The study has been performed by using Resonance Raman (RR) and UV-Vis spectroscopy.

Fig. 1 (a) Molecular structure of the two sensitizers used to analyze the contribution of the ancillary ligands SCN⁻ or C≡N in the regeneration processes, and their respective fresh cells. The inner area of the sealing layer was 10 × 50 mm², larger than the active area of the photoelectrode (8 × 40 mm²) in order to leave a transparent region where the electrolyte alone could be monitored (“Electrolyte monitor window”). (b) Schematic representation of the charge transfer processes under dark in a complete DSC for positive voltages at TiO₂ (RB). (c) A totally aged cell after Reverse Bias stress sensitized with (left) N719 (stress time 1735 h) and (right) Ru505 (stress time 3500 h) dye. Note the electrolyte leakage and discoloration patterns. Two different dark and light zones are evident due mainly to presence or absence of degraded electrolyte solution.

Results and discussion

RB stress tests were performed in the dark forcing a current of 120 mA, which is about three times the short circuit current (I_SC) of the cell under 1 SUN illumination condition. In fact, when in a several series-interconnected cells module, one of them is shadowed, this is forced to work under reverse bias voltage for a wide range of module operating points; the worst case occurs for small loads at the module's I_SC (about 45 mA). In this working conditions,¹¹ device's degradation take the form of a gradual increase of anodic operating voltage (V_RB) and accelerates strongly when the reverse biased cells starts working in diffusion-limited current I_lim conditions.

When I_lim approaches the current forced on the reverse biased cell rapidly gas bubble formation becomes visible in the devices leading to the complete device's failure.

In this work, we applied a current of about 120 mA as an accelerated stress test verifying the same degradation phenomena already experimented in our previous studies at 45 and 90 mA.¹¹

As previously shown¹⁴ the power dissipated in DSCs working in this RB condition is small and the induced temperature variation (<5 °C), measured by means of an infrared camera, is not sufficient to lead the electrolyte solvent or possible traces of water to boil.

Thus, we exclude that gas evolution inside the cell could have a temperature-dependent origin.

When a DSC is forced to work under reverse bias conditions (positive potentials at TiO₂) the electric charge follows the path from Photo Electrode (PE) to Counter-Electrode (CE) through the external circuit. Electrons are then transferred from the CE to the electrolyte via reduction of triiodide (I₃⁻) to iodide (I⁻). Concentration gradients drive ions diffusion to the PE, where charge transfer occurs at the electrolyte/TCO and electrolyte/dye/TCO interface (Fig. 1b). The former occurs for low reverse potentials (V_RB) through oxidation of I⁻ to I₃⁻, while the latter occurs by holes percolation through oxidized dye molecules for V_RB higher than a threshold voltage (V_th) which value depends on the dye used. The complete and detailed explanation about charge transfer mechanisms was treated in a previous publication.¹⁴

Cells subjected to prolonged RB stress undergo physical and electrical changes such as to cause V_RB to shift towards more positive values.

In RB stress two main mechanisms lead to progressive increase of V_RB: the increase of anodic threshold voltage V_th and the decrease of the diffusion-limited current (I_lim).

V_th is strongly dependent on dye chemical structure and influences conduction in RB regime, while I_lim is proportional to I₃⁻ concentration.

TCO/dye/electrolyte interface plays a crucial role in charge transfer, while titania is not conductive and acts as insulator layer.^16–19 So charge transfer in RB regime is strongly related to dye molecular structure. In the case of ruthenium complexes N719, Z907, N3 and Ru505 the HOMO energy is significantly affected by the nature of the ancillary ligands: CN⁻ ligands induce a positive shift of the oxidation potential of ca. 300 mV in comparison to SCN⁻ ligands, causing V_th to increase.²⁰

Instead, the effect of the substitution of carboxylic groups with hydrophobic alkyl chains (N719 vs. Z907) on V_th is a weak change of about 30 mV;¹⁴ thus a significant difference in RB regime between different thiocyanate based ruthenium dye is not expected.

On the other hand, it's useful to compare the behavior of a typical SCN⁻ based ruthenium dye (N719) and a well-known CN⁻ based ruthenium dye (Ru505).

Since the two realized devices differ only in the dye molecular structure, the strong difference in degradation behavior, is imputable to different stability between cyanide and thiocyanate substituted ruthenium complexes.^7,20–24

It could be reasonable to assume that also alterations of the dye molecules can occur. Recording I–V curve during the stress and observing the V_th shift, it is possible to monitor changes in oxidation potential of the dye, which reflects changes occurring in dye molecular structure and in particular in its HOMO level. As shown in the next sections, it is possible to assert that important changes involve only N719 dye chromophores, while Ru505 does not show evidences of chemical changes.

Fig. 1c shows the effects of the prolonged stress in two devices. Here, when V_RB reaches (2.4 ± 0.15) V for N719 sensitized cell and (4.2 ± 0.15) V for Ru505 sensitized cell, the electrolyte suffered serious degradation as it can be noted from the darkened spots out of the TiO₂/dye area. In fact, as a consequence of bubbling, the pressure inside the device increases markedly causing the loss of adhesion of the sealant (even the breaking of the glass in one case) and, hence, the electrolyte leakage from the active area. This is also witnessed by the yellowing of the sealant in some areas. As soon as these phenomena occur, the cell almost immediately loses its functionality.

At the beginning of the RB stress, before the cell enters in the final bubbling regime, two main effects occur involving simultaneously, electrolyte and dye: (i) a concentration gradient of ions, causing I₃⁻ and I⁻ to be concentrated mainly at the PE and at the CE respectively, (ii) the regeneration of oxidized dye (D⁺) is impaired by the depletion of I⁻, while the formation of the ionic assembly between the oxidized dye (D⁺) and I₃⁻ ([D⁺–I₃⁻]) is favoured. The last point will be detailed in the next section. Before any change in the electrolyte solution appears, electrochemical dye oxidation over the entire active area could be assumed uniform.

When RB voltage increases and a partial leak of electrolyte occurs, some active areas of the cell become electrolyte free (light zones of Fig. 1c); on the opposite, zones where the electrolyte is still present are indicated as dark zones. In conclusion, the RB stress on DSCs produces light and dark zones which can be clearly observed and investigated.¹⁴

Absorbance of fresh and stressed DSCs

Fig. 2 displays the UV-Vis absorption spectra of the fresh and aged N719 sensitized cell compared to the absorption spectra of fresh and degraded electrolyte. These latter are acquired in correspondence to the “electrolyte monitor window”; this area is obtained fabricating the sealing layer larger than the active area of the photoelectrode in order to leave a transparent region where the electrolyte alone could be monitored (Fig. 1).

Fig. 2 Normalized absorption spectra of a typical N719 sensitized cell: fresh cell (a), aged cell dark zone (b), aged cell light zone (c) acquired on active area, compared with fresh electrolyte (d) and degraded electrolyte (e) acquired on electrolyte monitor window.

We note that fresh electrolyte absorption (Fig. 2 curve d) contributes significantly to the total absorption of the fresh cell (Fig. 2 curve a) only in the low wavelength region (λ < 470 nm), thus it is possible to monitor specific changes involving dye molecular structure by analyzing the spectral variations that occur at higher wavelengths.

The absorbance spectrum of a fresh N719 cell shows characteristic bands at 430 and 530 nm, which are assigned to metal-to-ligand charge transfer transitions involving dπ Ru(II) orbitals and the π* orbitals of the bipyiridine-carboxylate ligand.

The absorption spectra of RB stressed N719 sensitized cell have been acquired for both dark and light zones (Fig. 2 curve b and c) shown in Fig. 1c. The absorption spectra of fresh and RB stressed cells in dark zone exhibit a similar shape if we consider the contribution in the absorbance spectra of the degraded electrolyte with respect to the fresh one (Fig. 2 curves d and e). It is thus reasonable to assume that most of dye molecules in the dark zone still retain dye's original properties. On the other hand, the absorption spectrum recorded in correspondence of the faded zone of the stressed cells shows a visible blue shift of ca. 30 nm, although no well resolved bands could be distinguished from the featureless spectrum of the degraded dye, clearly indicating that, in the lighter zone, chemical changes involve most of the dye molecules.

A similar analysis was performed also for Ru505 sensitized cells, as reported in Fig. 3.

Fig. 3 Normalized Ru505 sensitized cell absorbance spectra: fresh cell (a), aged cell dark zone (b), aged cell light zone (c) acquired on active area, compared with fresh electrolyte (d) and degraded electrolyte (e) acquired on electrolyte monitor window.

It is worthy to note, that the absorbance spectrum of the stressed device in the light zone nicely overlaps with the spectrum of the fresh active area. In addition, if we consider the contribution of the absorption spectrum of the degraded electrolyte (Fig. 3 curve e), still present in this area, also the absorbance spectrum of the dark region overlaps with that of the fresh cell one. On the contrary of what happens with N719, it is therefore reasonable to assume that under RB stress the Ru505 dye doesn't undergo irreversible chemical changes.

Resonance Raman scattering of freshly prepared DSCs

Fig. 4 shows the micro-Raman spectra of a fresh cell sensitized with N719 dye, acquired by focusing the laser beam (λ = 514 nm) on the PE (Fig. 4a) and on the CE (Fig. 4b) in open circuit condition. The Raman spectra can be divided into four frequency regions, with assignments largely based on previous studies.^25,26

Fig. 4 In the upper panel, micro-Raman spectra of a fresh cell sensitized with N719 dye, acquired by focusing the laser beam (λ = 514 nm) on the photo-electrode PE (a) and on the counter-electrode CE (b); the inset in (a) panel shows the Raman bands at 2102 cm⁻¹ and 2125 cm⁻¹ stemming from thiocyanate groups that remain coordinated to dye molecules and respectively when the vibrations are free or coupled to I₃⁻ ions or I₂. In the lower panel, micro-Raman spectra of a fresh cell sensitized with Ru505 dye, acquired by focusing the laser beam (λ = 488 nm) on the PE (c) and on the CE (d). The inset in (c) panel shows the Raman bands of symmetric and antisymmetric cyanide stretching modes at 2087 and at 2112 cm⁻¹. All spectra are acquired by 1800 lines per mm grating.

(a) 140–700 cm⁻¹ dominated by an intense Raman band at 145 cm⁻¹ corresponding to the overlap between the most intense TiO₂ bending mode (O–Ti–O) and the contribution of vibrational modes from 1-propyl-3-methylimidazoliumiodide (PMII), present in the electrolyte. The first I₃⁻ overtone at 224 cm⁻¹ of the symmetric stretching mode (ν_S) is also evident in this spectral region. The peak at 168 cm⁻¹ can be confidently assigned to the symmetric I₃⁻ vibration in a [D⁺–I₃⁻] complex²⁷ where the dye is in its oxidized form. Formation of the oxidized dye in these experimental condition, without any applied bias, is due to laser beam excitation at 514 nm. The possible assignment of this band to the I₂SCN vibration²⁵ can be ruled out since the same Raman band is observed with dye molecules lacking of the SCN⁻ ligand;²⁷ moreover the I₂SCN vibrations due to guanidinium thiocyanate, present in the electrolyte solution, are reported at 2175 cm⁻¹.²⁸

(b) 700–2000 cm⁻¹, where the main bipyridine vibrations of the dye molecules were observed at 700, 1022, 1102, 1262, 1292, 1470, 1540, and 1608 cm⁻¹. The band at 1733 cm⁻¹ is assigned to the stretching of the carboxylic function at the bipyridine ligand which provide an ester type linkage with the TiO₂ surface.²⁹ The low intensity of this band highlights a very good adsorption of dye molecules on the TiO₂ surface.³⁰

(c) 2000–2200 cm⁻¹, displaying the vibrations at 2102 and 2125 cm⁻¹ stemming from SCN⁻ ligands that remain coordinated to N719 dye molecules (inset in Fig. 4a),³¹ and the I₂SCN vibration in the electrolyte at 2175 cm⁻¹.

(d) 2200–3000 cm⁻¹, where the vibration of the nitrile group from methoxy propionitrile at 2250 cm⁻¹ and the CH vibrations at 2831 and at 2936 cm⁻¹ from both electrolyte and dye are shown.³²

Raman spectra on a fresh cell sensitized with Ru505 dye were acquired at a different wavelength (488 nm) in order to reduce the marked fluorescence (Fig. 4c and d). The labelled peaks in the figure are close to those of the N719 sensitized device. Symmetric and antisymmetric cyanide stretching modes are observed in a shoulder at 2087 and a peak at 2112 cm⁻¹ (inset in Fig. 4c).

Resonance Raman scattering of stressed DSCs

Raman spectra acquired focusing the laser beam at the PE, in correspondence of light and dark zone, resulting from the RB stress of N719 and Ru505 sensitized cells, are reported in Fig. 5.

Fig. 5 Comparison between Raman spectra of stressed N719 (λ = 514 nm) and Ru505 (λ = 488 nm) sensitized cells, recorded by focusing on dark (a) (c) and light zones (b) (d) of the photoelectrode. All spectra are acquired by 600 lines per mm grating.

Fig. 5 shows that for N719, the band at 168 cm⁻¹, corresponding to the symmetric stretching vibration ν(I–I) of the triiodide in the chemically stable [D⁺–I₃⁻] complex, is greatly increased in the light zone with respect to the dark zone, while the increase is less sensitive in the case of Ru505. This correlation can be clearly seen in the Raman images of Fig. 6, displaying the intensity maps of the 168 cm⁻¹ band on an area of 0.16 mm² which includes both dark and light zones, and on the optical images of the same regions.

Fig. 6 168 cm⁻¹ Raman band intensity map on RB stressed: N719 sensitized cell (a) and, microscopic detail of scanned area (b); Ru505 sensitized cell (c) and microscopic detail of the scanned area (d).

These results indicate that formation of [D⁺–I₃⁻] is prevalent in the light zone and is maximized in the case of N719. From Fig. 6 it can be estimated a concentration ratio between the two regions of 2 for N719 and of 1.3 for Ru505, which are consistent with the intensity of the 168 peaks in Fig. 5.

The dye molecules examined in our study (N719 and Ru505) only differ for the type of ancillary ligand (either CN⁻ or NCS⁻) coordinated to ruthenium centre and not for the type of chromophoric ligand (H₂DCB) bearing the carboxylic anchoring group which binds to the TiO₂ surface via bidentate or bridging linkage.³⁰ So it is reasonable to assume that the different behavior of the two dyes under RB stress cannot be ascribed to the phenomena of desorption of the dye that should necessarily be similar, given the identical interaction mode with the TiO₂ surface.

Current-voltage (I–V) curves

To clarify the different behaviour observed for the two dyes under RB stress, we have recorded current–voltage (I–V) curves for anodic potential under 1 Sun illumination conditions and in dark conditions at two different stress stages, at time t = 0 h and after 714 h, well before the strong degradation of the electrolyte resulting in gas evolution (t > 1730 h). The devices degradation effects could be monitored through the modification of some specific electrical parameters under 1 Sun illumination, such as short circuit current density (j_SC) and open circuit voltage (V_OC).

Several authors^33–36 correlated DSC electrical parameters with the physical and chemical properties affecting the overall performances of the device.

Thus, j_SC is given by the light harvesting efficiency of the dye, its capability to inject electrons into TiO₂ conduction band (CB), and the ability of the semiconductor to transport them to the collecting electrode; V_OC instead, within the thermodynamic limit, is given by the difference of the conduction band of the TiO₂ (E_CB) and the redox potential of the electrolyte (E_CB–E_redox).

Recently Raga et al.,³⁴ by means of impedance spectroscopy measurements on DSCs samples realized with different electrolytes, deduced a strong V_OC dependence from recombination resistance (R_rec) which itself exponentially depends on E_CB–E_redox.

By analyzing the I–V curves (here not reported) under 1 Sun illumination after 714 hours of RB stress test, we recorded a significant decrease in V_OC, more evident for Ru505 sensitized devices (about −7.8%) in comparison to N719 one (about −5.8%) while j_SC enhanced of about +6% in both devices.

Changes in V_OC are strongly related to E_CB–E_redox energy difference. In a prolonged RB stress, the electrolyte suffers a progressive decrease of triiodide concentration shifting E_redox³⁷ towards the E_CB level and reducing V_OC.

On the opposite, an increase in energy difference between the Fermi level of electrons accumulated in the TiO₂ CB (E_Fn) and E_redox, leads to an increase of charge transfer resistance R_rec at the TiO₂/electrolyte interface; this implies that the rate constant k_rec for back electron transfer (electrons recombining with I₃⁻ ions) decreases and consequently V_OC value would undergo a significant increase.^37,38

In our experiments, we recorded an overall decrease in V_OC values after RB stress, but more relevant for the Ru505 device than for the N719 one. The lower decrease in V_OC value after stress for the N719 sensitized devices in comparison to Ru505 one, could arise from the different concentration of oxidized dye molecules. In fact, fast reaction of D⁺ with the redox couple implies that the concentration of D⁺ is low so that no significant reaction is observed between the injected electron and D⁺.³⁹ Thus, due to the severe oxidative degradation of N719⁴⁰ an increased population of oxidized dye molecules is present, and the electrostatic [D⁺–I₃⁻] complex formation is favoured, leading to a reduction of k_rec and to an increased V_OC.

The increase in j_SC for both devices after 714 hours of RB stress test, could be related to the increase of R_rec that leads to a better electron collection efficiency and a larger electron lifetime.⁴¹

On the other hand, the analysis of I–V curves recorded in dark condition can give us useful information about chemical modification suffered by dye molecules and electrolyte; these curves are reported in Fig. 7 for both dyes and show a reduction of current as a function of the stress time.

Fig. 7 Current–voltage curves for anodic potential acquired in dark condition on N719 sensitized cell: before the stress beginning curve (a) and after 714 hours of stress curve (b). The same curves are acquired on Ru505 sensitized cell at the same time steps and plotted as curve (c) and curve (d).

This is ascribed to the depletion of I⁻ from the PE, which is paralleled by a depletion of I₃⁻ at the counter electrode interface, resulting in a lower efficiency of dye regeneration.³² In RB conditions, the anodic current is mainly limited by the oxidation rate of I⁻ at the exposed TCO/electrolyte interface. Given that TiO₂ is insulating under positive polarization, and that the oxidation overpotential of I⁻ at the bare FTO electrode is high, it is reasonable to assume that the oxidation of I⁻ ions is mainly mediated by oxidized dye molecules adsorbed on TiO₂ or electronically coupled with the FTO substrate. In addition, self-exchange electron transfer reactions between dye molecules bound to FTO and to the titania nanoparticles may also occur, activating a cascade of electron transfer events that concur to the generation of an anodic current under reverse polarization.^14,19 Thus, the electrochemical stress induces at PEs an increased concentration of I₃⁻ and of oxidized dye molecules favouring the formation of the [D⁺–I₃⁻] adduct, that inhibits efficient regeneration, resulting in the formation of a permanent population of dyes in their oxidized state.¹¹ Raman imaging of Fig. 6 confirms indeed the presence of [D⁺–I₃⁻] species in stressed devices based either on the N719 and Ru505 dye. Nevertheless there are meaningful differences in the degradative pathways of the cells based on the two different dyes, which can be more easily understood by analyzing the I–V characteristics of the cells under RB stress in dark condition (Fig. 7).

In fact, the temporal evolution of the I–V curves is quite different for the two dyes, the N719 cell showing a consistent positive shift (ca. 300 mV, Fig. 7 curve a and curve b) of the threshold voltage (V_th) after 714 h of RB ageing, whereas the Ru505 cell shows essentially no variation in V_th. This fact is consistent with previous studies on the oxidative degradation of N719 (ref. 40) and of the corresponding acidic form,⁴² showing that upon oxidation of the complexes one SCN⁻ ligands can be initially released and substituted by the nitrile solvent S, which in our experimental conditions corresponds to 3-metoxypropionitrile. Coordination of the nitrile solvent produces an anodic shift of the oxidation potential of the solvated molecule Ru(II)(S)NCS, causing a blue shift of the metal to ligand charge transfer band in the absorption spectrum (Fig. 2), a strong decrease of the current and a further increase of the overpotential at the PE.

On the contrary, Ru505, does not easily release the strongly coordinated CN⁻ ligands, even under prolonged electrochemical stress, in agreement with its quasi-reversible electrochemical behavior. Thus, no variation in the overpotential for iodide oxidation is expected, as confirmed by the fact that V_th does not changes during the RB stress as shown in Fig. 7 (curve c and curve d).

When prolonged RB stress (>1730 h) causes gas evolution, the electrolyte is removed from certain areas of the photoanode, giving rise to the so called light zones where the dye molecules cannot be further regenerated. In the case of the N719 sensitized cell, formation of the positively charged solvent ruthenium complex contributes, together with oxidized N719 molecules,²³ to the accumulation of the [D⁺–I₃⁻], which explain the enhanced intensity of the Raman band at 168 cm⁻¹ compared to darker zones where reduction of the oxidized dye favourably competes with its decomposition.

It should be noted that decomposition of N719 may also involve formation of the S bound L₂Ru(SCN)(NCS) linkage isomer as well as of the mono or bis-cyanide species, L₂Ru(NCS)(CN), L₂Ru(CN)₂.⁴² The presence of an S bound isomer (Ru–SCN) can be ruled out since it should be revealed by the appearance in the Raman spectrum of a cyanide stretching modes at 2050 cm⁻¹. On the contrary the spectral evolutions in Fig. 2 curve c shows an absorption spectrum for the aged light zone resembling that of the bis-cyano complex in Fig. 3 curve a, and analogous IV curves are observed when the current voltage of N719, after stress, is compared with that of Ru505 in Fig. 7. However the detailed inspection of the Raman region 2000–2200 cm⁻¹ doesn't allow to draw a clear conclusion since cyanide stretching for L₂Ru(CN)₂ are expected at ca. 2100 cm⁻¹, and the corresponding bands are probably hidden by those of the coordinated NCS. In fact, in Fig. 8, where Raman spectra acquired on light and dark zone for RB stressed N719 cells are displayed, only a remarkable decrease of a broad band at 2125 cm⁻¹, paralleled by an intensity increase at 2130 cm⁻¹, are observed, testifying that colour differences between dark and light zones cannot be entirely attributed to the presence or absence of degraded electrolyte but also to loss of coordinated NCS or to the formation of Ru(III) intermediates finally leading to Ru(II)(CN), both processes producing a blue shift of the absorption spectrum. As to the intensity increase at 2130 cm⁻¹, this could arise from the cyanide stretching of NCS or CN coordinated to Ru(III) centers, as discussed for a series of polynuclear bypyridine complexes containing Ru(III)CN moieties.⁴⁵ In order to clarify this point a series of calculations at DFT-B3LYP level have been performed by considering both reduced and oxidized forms of the two sensitizers N719 and Ru505, by assuming a complete dissociation of the carboxylic function which is thought to occur upon interaction with the TiO₂ surface⁴⁴ and in agreement with our findings. The calculations have shown for Ru(II)(NCS)₂ and Ru(III)(NCS)₂, of N719, CN stretching frequencies of 2128, 2139 cm⁻¹ and 2055, 2108 cm⁻¹, respectively; while for Ru(II)(CN)₂ and Ru(III)(CN)₂, of Ru505, cyanide stretching at 2074, 2084 cm⁻¹ and 2097, 2103 cm⁻¹ were respectively calculated (Fig. 9). The fact that upon oxidation the CN stretching mode of N719 undergoes a downward shift in frequency is most probably due to electron back donation from the sulfur bound to the NC moiety (inset Fig. 9). In the case of Ru505 a theoretical increase in ν(CN) frequency of ca. 23 cm⁻¹ is observed upon oxidation to Ru(III). This upward shift was also experimentally observed by previous studies on several cyano substituted model complexes.⁴³ Thus, independently by the absolute frequency values obtained by the computations, the appearance of an evident higher frequency band at ca. 2130 cm⁻¹ can be reasonably motivated by the formation, under reverse bias, of a permanent population of Ru(III)(CN)₂ which cannot be regenerated in the light zones due to the absence of electrolyte.²³

Fig. 8 Comparison between RB stressed N719 cells high frequencies region Raman spectra acquired on light (a) and dark (b) zones.

Fig. 9 Computed vibrational frequencies of CN and NCS groups in ruthenium dyes: (A) Ru(II)-505; (B) Ru(II)-N719; (C) Ru(III)-505; (D) Ru(III)-N719. Inset: isodensity plots of the highest occupied molecular orbitals involved in the Ru–CN (A–C) and Ru–NCS (B–D) bonds.

The computed vibrational frequencies to the CN and NCs stretching as well as the relevant molecular orbitals computed at the equilibrium geometries for both Ru505 and N719 in their reduced Ru(II) and oxidized Ru(III) form have been reported in Fig. 9.

Experimental

Masterplate fabrication

Five 3.6 cm² cells in a masterplate configuration^10,45 were fabricated with transparent nano-porous titanium dioxide (TiO₂) paste (DSL 18NR-T) screen-printed through a 43T mesh screen onto fluorine-doped tin oxide (FTO) coated glass substrate (NSG TEC 8/from Pilkington). After 15 min drying at 80 °C, TiO₂ films were sintered at 525 °C for 30 min. The resulting thickness of the TiO₂ films was measured with a surface profilometer Dektak 150 from Veeco and was observed to be 11 μm for all the samples. TiO₂ electrodes were dyed for 16 h into a 0.3 mM ethanol solution of N719 or of Ru505, prepared according to previously reported procedures,⁴⁶ to obtain Photo Electrodes (PEs) of the same size (Fig. 1a). After dyeing, the PE were rinsed with ethanol in order to remove the excess dye before the assembling process.

Platinised Counter-Electrodes (CEs) were made by screen-printing a platinum paste (4000 SC from Chimet) through a 100T mesh screen onto FTO coated glass substrate. The CEs were dried at 80 °C and then sintered at 525 °C for 30 min.

The two electrodes were laminated with Bynel from Solaronix. After the hot-melting step, the distance between the two electrodes was measured to be about 45 μm.

The used electrolyte was the commercial Iodine-based electrolyte HSE (High Stability Electrolyte) from Dyesol. Silver busbars were used as current collectors.

Reverse bias ageing

RB stress tests were performed in the dark forcing a current of 120 mA, which is about three times the short circuit current (I_SC) of the cell under 1 SUN illumination condition. The stress was maintained for 1735 h in the N719 sensitized device and for 3500 h in the Ru505 sensitized device. The current was applied utilizing a DC power supply Agilent E3649A. In this stress condition we define the reverse bias voltage V_RB as the operating reverse bias applied to the cell fixing a constant current equal to three times its short circuit current at 1 SUN,¹⁴ while the threshold voltage V_th as the minimum reverse voltage needed to induce a significant reverse current, V_RB > V_th.

Raman and UV-Vis spectroscopy

Raman spectra and images were performed using a Jobin–Yvon–Horiba micro-Raman system (LabRAM ARAMIS) equipped with Ar⁺ ion laser (514 nm/488 nm) as excitation source (100 mW). The Horiba micro-spectrometer is coupled with a confocal microscope that allows the spatial resolution of the sample through detector pinhole aperture.

The cut-off from the notch filters in the spectrometer is less than 120 cm⁻¹. The laser light reached the sample surface at normal incidence by means of ultra long working distance (50×) objective with 10.5 mm focal distance. The scattered radiation was collected in a backscattering geometry. Subtraction of the fluorescence background on the Raman spectra was performed by a polynomial fitting, while spectral de-convolution was carried out by nonlinear least-squares fitting of the Raman peaks to a mixture of Lorentzian and Gaussian line shapes, providing the peak position, width, height, and integrated intensity of each Raman band. The spectrometer is equipped with four diffraction gratings of 300, 600, 1200 and 1800 lines per mm coupled to a CCD camera.

The UV-Vis absorption of the investigated molecules was recorded using UV-Vis 2550 Spectrophotometer from Shimadzu. The spectra were collected over a scan range from 400 to 700 nm at a scan rate of 480 nm min⁻¹ and a resolution of 1 nm.

Raman spectroscopy allows to carry out non-invasive and non-destructive investigations (i.e. it does not require cell disassembly) with a very high sensitivity to the detection of localized chemical changes within the cell. In order to prevent direct laser induced degradation of the cell components^30,47 some precautions must be taken during the measurements. In our set-up, the time for the signal collection was set to 8 seconds with a laser power of about 2.6 mW but we controlled this power with a filter, so the power density reached on the sample is about 0.26 mW μm⁻².

Current–voltage I–V characteristics

Current–voltage I–V characteristics under AM1.5 G solar simulator Solar Constant from KHS at 1000 W m⁻² (1 Sun), calibrated with a Skye SKS 1110 sensor were measured through a Keithley 2420 Source Meter.

The dark I–V characteristics were performed under dark conditions at room temperature using an Autolab 302N Modular Potentiostat from Metrohm in the two-electrodes configuration.

Computational studies

Frequency calculations were carried out in vacuo on the optimized structures of [Ru(II)(DCB)₂(NCS)₂]⁴⁻ and [Ru(III)(DCB)₂(NCS)₂]³⁻ and on [Ru(II)(DCB)₂(CN)₂]⁴⁻ and [Ru(III)(DCB)₂(CN)₂]³⁻ (DCB = 2,2′-bipyridyl-4,4′-dicarboxyate) in order to analyze the vibrational modes of N719 and Ru505 in their reduced and oxidized form respectively. The calculations were carried out using restricted (singlet ground state in Ru(II) complexes) and unrestricted (doublet ground state in Ru(III) complexes) DFT approach by using the hybrid B3LYP functional and the LANL2DZ basis set implemented in Gaussian 09.⁴⁸

Conclusions

We have investigated the stability performances of DSCs based on two different molecular sensitizer, N719 and Ru505, under prolonged Reverse Bias stress using Resonance Raman and UV-Vis spectroscopy. Stress tests were performed at constant current of 120 mA for several hours until final devices breakdown. This stress induces a non-uniform irreversible degradation of the cell's components across the active area with the appearance of colour changes (light and dark zone formation), in part related to electrolyte degradation and to non-homogeneous loss of the solvent. Raman and UV-Vis spectroscopy measurements on fresh and stressed solar cells showed that the specific aging mechanisms depends on the molecular structure of the dye.⁴⁹

For N719, the absence of electrolyte in the light zone prevents dye regeneration and, consequently, promotes the detachment of SCN⁻ groups from ruthenium. This translates in an increased overpotential for I⁻ oxidation and in the formation of a relevant population of [D⁺–I₃⁻] adducts, observable by monitoring the intensity increase of the 168 cm⁻¹ Raman band. The structural change is also confirmed by a blue shift of absorption spectrum in the light zone compared to dark ones. The spectral changes are either consistent with the formation of a solvent Ru complex where the methoxypropionitrile substitutes at least one NCS group as well as with the formation of a bis-cyanide complex where both NCS are converted to CN.

On the contrary, under RB stress, the Ru505 molecule does not undergo to relevant changes of the chemical structure across the cell active area, due to the stronger coordination to ruthenium of CN⁻ with respect to the SCN⁻ ligand.

Acknowledgements

We acknowledge the Regione Lazio “Polo Solare Organico Regione Lazio”, and PRIN 2012 project “DSSC X” for financial support of this work. The authors would like to thank Prof. P. Falaras and Prof. A.G. Kontos, Institute of Physical Chemistry NCSR “Demokritos”, and Prof. R. Paolesse, Department of Chemistry of University of Rome “Tor Vergata” for useful discussions.

Notes and references

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