Article pubs.acs.org/JPCC
Long-Term Thermal Stability of Liquid Dye Solar Cells Athanassios G. Kontos,*,† Thomas Stergiopoulos,† Vlassis Likodimos,† Damion Milliken,‡ Hans Desilvesto,‡ Gavin Tulloch,‡ and Polycarpos Falaras*,† †
Division of Physical Chemistry, Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, NCSR Demokritos, 153 10, Aghia Paraskevi Attikis, Athens, Greece ‡ Greatcell Solar S.A., c/o BDO S.A., Biopôle, 1066 Epalinges, Switzerland
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S Supporting Information *
ABSTRACT: Laboratory-size dye solar cells (DSCs), based on industrially feasible materials and processes employing liquid electrolytes, have been developed. Cells based on two electrolyte solvents with different physical properties were subjected to thermal stress test at 80 °C for 2000 h in the dark to monitor their long-term thermal stability. The DSCs incorporating a methoxypropionitrile (MPN)-based electrolyte presented a severe efficiency loss at 1 sun AM 1.5G of more than 70% upon thermal aging, while the solar cells using tetraglyme (TG) as a high boiling point solvent attained a promising stability with only 20% loss of performance. To better understand the above behavior, systematic experiments, including optical microscopy, linear sweep voltammetry, UV−vis absorption, electrochemical impedance, and Raman spectroscopies were conducted. Virtually no dye degradation/desorption, electrolyte decomposition, semiconductor passivation, or loss of cathode activity could be identified. For the MPN-based cells, a sharp decrease in the short-circuit photocurrent was observed at high illumination intensities following thermal stress, attributed to charge-transfer limitations due to severe triiodide loss, verified by different experimental techniques. These degradation effects were efficiently mitigated by replacing MPN with the high-boiling-point solvent in the electrolyte.
1. INTRODUCTION Dye solar cells (DSCs) are foreseen as a highly promising technology in the solar cell market due to their distinct advantages over conventional photovoltaics in terms of low manufacturing cost and use of environmentally friendly materials.1 Furthermore, DSCs outperform classical PV systems in specific application fields, especially under nonideal illumination, such as experienced with building integrated photovoltaics in areas with relatively high levels of diffuse solar radiation. Additional benefits are compatibility with flexible substrates, offering low-cost roll-to-roll manufacturability, tunable transparency (in the case of substrates such as glass or polymer) and color as well as particularly short energy payback times and avoidance of materials that are either highly toxic such as Cd or based on rapidly decreasing natural resources such as In or Te.2 Up to now DSCs have reached efficiencies of ∼12% under standard AM 1.5 conditions using either iodide/triiodide (I−/ I3−)-3 or Co2+/Co3+-based liquid electrolytes,4 and most of the scientific and technological issues including economic aspects of large scale batch and roll-to roll production of DSC components have been successfully addressed, placing DSC technology at the verge of commercialization. However, DSC stability under certain outdoor conditions, such as operating temperatures in excess of 70 °C, remains a challenge that needs to be effectively confronted for some of the applications. © 2013 American Chemical Society
Innovative sealing methods have been accordingly developed, aiming at securing hermetic packaging of the DSCs under all possible operating conditions,5,6 while several tests have been applied to assess long-term performance and stability of DSCs, including UV irradiation treatment, reverse bias, humidity, as well as light and thermal stress. Among these, UV irradiation is at the origin of destructive side reactions,7 which, nevertheless, can be avoided through UV-protecting filters, for example, applied to the outer surface of the solar cell.8 Moisture provides a path for desorption of hydrophilic dyes,5,9 while prolonged reverse bias stress may also lead to cell breakdown.10 DSCs employing robust dyes combined with nonvolatile electrolytes readily sustained their performance after 1000 h of continuous solar light (1 sun) illumination,11 while short-term light soaking was occasionally reported to improve the DSC electrical parameters.12,13 Extensive tests carried out at Dyesol demonstrated an impressive long-term stability of DSCs over 25 600 h of continuous light soaking at 55−60 °C under resistive load, with a maximum loss of power conversion efficiency of only 17% at 1 sun and no evidence of significant dye degradation, loss of iodine, or decrease in the Pt-based counter electrode electrocatalytic activity.14 Furthermore, Received: January 3, 2013 Revised: March 31, 2013 Published: April 5, 2013 8636
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based stabilized electrolytes were used with either MPN (99+%, Fluka) or TG (99%, Sigma-Aldrich). Comparing the physical properties of the two solvents, TG has much higher boiling point (bp = 275 °C), viscosity (ξ = 3.3 mPa s at 25 °C), and donor number (DN = 24 as for diglyme and triglyme) relative to MPN with (bp = 165 °C, ξ = 1.1 mPa s, and DN = 15.4). Both electrolyte systems contained 1-propyl-3-methylimidazolium iodide (PMII, >99%, Merck), iodine (I2, 99.8%, Aldrich), guanidinium thiocyanate (99.9%, Fluka), and benzimidazole (Aldrich, 98%). Note that no Li salt was added to the above mixtures due to generally decreased performance stability when LiI is added to the electrolyte solution.26 A first batch of three cells for each electrolyte was illuminated using metal halide lamps corresponding to a light intensity of >0.8 sun for 2000 h with resistors applied to maintain near-maximum power point (MPP) conditions. These cells, denoted hereafter as MPN-ref and TG-ref cells, respectively, did not present any deterioration in their electrical parameters, having virtually the same efficiency as fresh cells. Another batch of three cells for each electrolyte was thermally stored at 80 °C for 2000 h in the dark under open−circuit conditions, denoted hereafter as MPN-aged and TG-aged cells, respectively. 2.2. Electrical and Electrochemical Measurements. Current−voltage (J−V) measurements were performed by illuminating the DSCs (active area: 0.88 cm2) from the photoelectrode side using a large black metal mask with an aperture area of 0.15 cm2. A Xe lamp working in combination with AM 1.5G and 400 nm (UV) cutoff optical filters (Oriel) was employed to provide solar simulated light (1 sun, 1000 W/ m2). Illumination of the cells under lower light intensity was implemented with the addition of neutral density filters. J−V characteristics in the range of 0.1 to 1 sun were obtained using linear sweep voltammetry (LSV) at a scan speed of 50 mV/s on an Autolab PGSTAT-30 potentiostat. LSV experiments were also conducted under illumination and dark with an extended cell voltage range from −1.5 to +1.5 V, including both forward and reverse bias conditions, to study the behavior of limiting current upon possible changes of redox shuttle concentrations.12,27 Monochromatic incident photon-to-collected electron conversion efficiency (IPCE) was measured using an Oriel 1/8 monochromator for dispersing the light in an area of 0.5 cm2. A Thorlabs silicon photodiode was used for the calibration of the IPCE spectra. EIS measurements were carried out using the Autolab PGSTAT-30 potentiostat and its built-in frequency response analyzer (FRA) under dark conditions and at applied voltages near MPP and Voc (i.e., in a potential window from −0.3 to −0.8 V with a step potential of 0.05 V). Intensity modulated photovoltage spectroscopy (IMVS) was applied to extract apparent electron lifetimes.28 For this purpose, a sinusoidally modulated light beam of a red (640 nm) light emitting diode was used. 2.3. Spectroscopic Methods. UV−visible diffuse reflectance spectra were recorded on a 3200 Hitachi spectrophotometer equipped with a 60 mm integrating sphere using BaSO4 as a reference. Micro-Raman spectra were recorded in the backscattering configuration using a Renishaw inVia Reflex system equipped with an Ar+ ion laser at 514.5 nm excitation. The scattered light is filtered by a dielectric edge Rayleigh rejection filter with cutoff at 100 cm−1 and analyzed with a 1800 lines/mm diffraction grating. A 2.5 mW laser beam was focused on the sample surface using the long working distance (8 mm) objective with magnification 50× of a Leica DMLM microscope. The nominal spot size of the laser beam was ∼1 μm, and
analytical investigations of fresh and aged DSCs (6450 h continuous light soaking at 55−60 °C under resistive load) by micro-Raman spectroscopy confirmed the absence of any distinct chemical modifications of the DSC components and interfaces that could undermine device stability.15 However, stability of DSCs subjected to long-term (≥1,000 h) thermal aging at elevated temperatures (>80 °C) remains demanding. Largely varying results have been reported in the literature up to now. Marked deterioration of the cell performance is observed by utilizing electrolytes with conventional organic solvents (such as acetonitrile or methoxypropionitrile) and possibly lower quality seals.16,17 Promising thermal stability has been reported at 85 °C for DSCs employing an ionic-liquid -based electrolyte18 or gelled electrolytes19,20 and also in devices sealed by glass frits.6 Industrial DSCs at Dyesol stored in dark under open-circuit conditions for over 1000 h at 80 °C resulted in a 22−31% decrease in cell performance with ionic-liquid- or solvent-based electrolyte systems, respectively.14 Under identical aging conditions, DSCs based on a metal-free organic sensitizer attained 20% degradation of photovoltaic efficiency.21 Significant drop of Jsc has been frequently identified as the main reason underlying the performance degradation of thermally stressed DSCs at 80−85 °C, attributed to iodine depletion, volatile electrolyte’s components deterioration, dye degradation or desorption,16 and electrolyte leakage.17 Other causes for loss of cell performance have been identified such as positive charge accumulation on the TiO2 photoelectrode14 or degradation of the counter electrode under 60 °C thermal stress.22 It has to be pointed out, however, that the latter study was based on cells containing LiI and sealed by Surlyn only. Thus the seals may not have been sufficiently hermetic, permitting the addition of H2O to their electrolyte solution and resulting in more sluggish electrokinetic behavior of Pt. Particular attention has been recently devoted to dye degradation effects due to ligand exchange at elevated temperatures, impeding the performance of DSCs.23−25 Extensive -SCN ligands substitution in Ru bipyridyl sensitizers (i.e., N3, N719, and Z907) by additives of the liquid electrolyte such as 4-tert-butyl pyridine, at temperatures above 80 °C, has been put forward to account for the deterioration of photovoltaic performance, especially the pronounced decline of Jsc for thermally stressed DSCs. From the above analysis, it is evident that the hightemperature domain is the most crucial area for stability of DSC performance. In this work, the long-term thermal stability of laboratory-sized DSCs based on industrially feasible materials and processes and using electrolytes with two different organic solvents, namely, methoxypropionitrile (MPN) and tetraglyme (TG), selected for their high boiling point has been systematically investigated upon prolonged thermal stress at 80 °C for 2000 h in the dark. A combination of electrochemical, photoelectrochemical, optical, and spectroscopic techniques was applied to identify the degradation mechanisms and develop solutions toward DSCs stable under prolonged thermal stress at high temperature.
2. EXPERIMENTAL SECTION 2.1. DSC Construction and Aging. The cells were manufactured using standard Dyesol materials, that is, TEC15 glass, DSL 18NR-AO titania paste with both TiCl4 underlayer and overlayer, N719 dye, PT1 platinum paste, thermoplastic primary and hermetic secondary seals.15 Iodine8637
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the power density on the sample was controlled to 0.25 mW/ μm2 with a filter. Spectral analysis was carried out by nonlinear least-squares fitting of the Raman peaks to a mixture of Lorentzian and Gaussian line shapes.
3. RESULTS AND DISCUSSION 3.1. Solar Cells Performance. J−V curves at AM 1.5G were recorded (Figure S1 in the Supporting Information) to obtain the electrical characteristics of the cells, namely, opencircuit voltage, Voc, short-circuit photocurrent, Jsc, fill factor, FF, and power conversion efficiency, η. Mean values of the above parameters and standard errors are summarized in Table 1. The Table 1. Electrical Parameters (Mean Values and Standard Errors) of the Reference and Aged DSCs Derived from J−V Curves Recorded under 1 sun AM 1.5G Illumination electrolyte MPN-ref MPN-aged TG-ref TG-aged
Jsc (mA·cm−2) 12.9 3.2 12.1 10.9
± ± ± ±
0.8 0.1 0.3 1.0
Voc (V) 0.58 0.61 0.56 0.54
± ± ± ±
0.10 0.15 0.20 0.15
η (%)
FF 0.70 0.75 0.56 0.51
± ± ± ±
0.04 0.10 0.02 0.04
4.9 1.5 3.7 2.9
± ± ± ±
0.1 0.1 0.2 0.2
Figure 1. Short-circuit photocurrent (Jsc) plotted against the light power density (Pin) for the MPN and TG solar cells. A linear trend line is shown as a guide to the eye for the MPN-ref cells only.
systems employed by many academic researchers. A marked saturation at maximum photocurrent of 3 mA/cm2 was observed for the MPN-aged cells above 100 W/m2. The TGaged cells retained a linear Jsc versus Pin dependence up to 0.5 sun and only deviated from linearity at the highest light intensity, verifying their appreciably enhanced stability to thermal stress. Moreover, at the lowest irradiation density (i.e., at 0.1 sun), all cells presented quite similar Jsc values (as shown in Table 2),
corresponding parameters for all 12 tested cells are given in Table S1 in the Supporting Information, following the protocol for J−V data presentation proposed in the literature.29 Reference MPN-based solar cells, which were light soaked for 2000 h under artificial light close to 1 sun at 55 °C, provided the highest performance for all electrical characteristics, leading to a power conversion efficiency of η = 4.9%. It should be noted that these cells were not optimized toward highest efficiency. The corresponding solar cells using the electrolyte based on the TG solvent showed lower efficiency (3.7%) due to the reduction of all electrical parameters, mainly resulting from the decrease in the FF value from 0.70 to 0.56. This decrease in FF is ascribed to the more significant diffusion polarization with TG due to its much higher viscosity (3.3 mPa s at 25 °C) compared with that of MPN (1.1 mPa s at 25 °C). Thermal stress at 80 °C over 2000 h lead to an important decline of the photovoltaic performance of the cells using the MPN high-performance electrolyte (>70% loss when assessed at the highest light level of 1 sun), the severe deterioration of the MPN-aged cells arising mainly from a sharp drop (75%) of Jsc from 12.9 to 3.2 mAcm−2. Despite their compromised initial performance, the TG-aged cells exhibited significantly higher thermal stability (∼20% efficiency loss), with a relatively weak reduction of the short-circuit photocurrent Jsc (ca. 11%). This result is very important, indicating that use of an alternative electrolyte based on a high boiling solvent can significantly increase device lifetime upon thermal aging. (Lifetime refers to the expected time for which solar cells retain some measure of performance, often 80% of their initial efficiency.30) Jsc Dependence on the Incident Light Intensity Deviations from Linearity. Taking into account the photovoltaic behavior of the aged cells and to identify possible reasons for the Jsc decrease upon thermal aging, the short-circuit photocurrent dependence was plotted as a function of the incident light power (Pin) in the range of 0.1 to 1 sun (Figure 1). Strong divergence of Jsc versus Pin from linearity was identified for the MPN-aged cells, although small deviations from perfect linearity can be noticed even for the reference cells, probably due to the relatively high viscosities of both electrolytes used when compared with acetonitrile-based
Table 2. Electrical Parameters of the Reference and Aged MPN and TG DSCs Derived by Recording J−V Curves under 0.1 sun (AM 1.5G) Illumination electrolyte
Jsc (mAcm−2)
Voc (V)
FF
η (%)
MPN-ref MPN-aged TG-ref TG-aged
1.8 1.7 1.8 1.6
0.51 0.51 0.47 0.46
0.66 0.60 0.65 0.64
6.1 5.0 5.5 4.7
accompanied by only a relatively small efficiency reduction on the order of 20% for MPN and 15% for the TG cells, respectively. This is a particularly important result that could positively affect device calendar lifetime, especially for lower light applications such as for DSC on the side of a building in an area with generally hazy or cloudy conditions or for indoor usage. The drastic decline of the short-circuit photocurrents, occurring at high light intensities for the MPN-aged cells only, suggests limited mass transport of triiodide in the electrolyte through the TiO2 mesoporous network31,32 or limited dye regeneration by the electrolyte27 under conditions of severe electron injection inside the semiconductor (i.e., strong light illumination). Triiodide Transfer Limitations in the Linear Sweep Voltammograms. To identify the reason of linearity deviations, LSV measurements were conducted on the cells by sweeping the voltage through forward and reverse bias regions.12,33 Figure 2 displays the corresponding J−V curves registered between −1.5 and +1.5 V under 1 sun illumination. 8638
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Figure 3. J−V curves of typical MPN and TG-aged solar cells before and after application of a +1.0 V reverse bias.
Figure 2. Linear sweep voltammograms under 1 sun illumination, showing characteristic current saturation at reverse bias for the thermally aged cells, more distinct for the MPN-aged cells.
this direction, Mastroianni et al.10 showed evidence of H2 formation at reverse bias around 1.65 V. Transfer Limitation Influence in the IPCE Spectra. Transfer limitation due to triiodide loss was further evidenced by recording the IPCE spectra of the various cells under low power densities, 10−30 W/m2 (Figure 4). The shape of the
Current saturation at reverse bias is clearly observed for all cells, with the lowest limitation by triiodide diffusion34,35 obtained for the MPN-ref samples. For the aged MPN cells, one can observe only a very small step from short-circuit current to the current at cell voltages higher than +0.75 V (where the current became saturated); this result highlights the fact that the diffusion-limited current (JDL) is very close to Jsc and diffusion limitation could be a significant reason for the large photocurrent decrease, observed upon thermal aging for the MPN-cells. The diffusion-limited current was reduced by more than 90% while the corresponding Jsc was decreased only by 75% (Table 1). This rather unexpected phenomenon has already been reported in the literature36 and was assigned to loss of triiodide that simultaneously decreases diffusion and recombination current. Loss of redox shuttle was detrimental to MPN cell performance at 1 sun, although it was much less crucial for the cell operation under conditions of low light illumination where short-circuit photocurrents are considerably below the diffusion-limiting currents. For TG cells, the limiting current upon thermal aging was also reduced, implying that triiodide was also lost from these cells, although to a much lower percentage in comparison with the corresponding MPN cells. Temporal Solar Cells Recovery after Applying Reverse Bias. Asghar et al observed that MPN-aged cells partially recovered their initial photocurrent after sweeping the potential from forward to reverse bias.33 To investigate this effect, the aged cells were subjected to +1.0 V reverse bias for 2 min. (Further variation of the bias duration did not affect the cells’ behavior.) The TG-DSCs did not show any alteration on their photovoltaic performance upon reverse bias application (Figure 3). However, the MPN-aged cells presented a striking recovery of their Jsc very close to the value obtained before thermal aging. (See Table 1.) This phenomenon could be attributed to the regeneration of I3− charge carriers in the electrolyte7 via application of reverse bias.37 However, the observed recovery was only transient because after a few hours without applying the reverse bias, both photocurrent and cell efficiency returned to their initial values. At present, we do not know the electrochemical mechanism that leads to reformation of I3− under concomitant only partially reversible reduction of some electrolyte species or even an impurity such as O2 or H2O. In
Figure 4. IPCE spectra of the MPN- and TG-based DSCs.
IPCE profiles was very similar for both MPN and TG-ref cells, presenting IPCE values close to 70% at ∼530 nm, in agreement with their similar Jsc values presented in Tables 1 and 2. Furthermore, the TG-aged cells presented a relatively small, 5− 10% overall reduction, in accord with the corresponding photocurrent values, and an increase in the IPCE values in the 400−430 nm range for the aged cells (slight increase for the TG-aged and more significant profile modification for the MPN-aged cells) in comparison with the corresponding reference cells. This effect is attributed to the higher amount of incident light on the photoelectrode after loss of I3−, which absorbs in this spectral range. MPN-aged cells presented much lower IPCE values (∼40% lower at 530 nm) than those attained by the MPN-ref cells, reflecting the drop of the corresponding photocurrent density upon thermal aging. By integrating the product of the incident photon flux density at low illumination (i.e., at 0.3 sun) and the 8639
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IPCE values (obtained also at low power) over the 350−800 nm wavelength regime, the calculated short-circuit photocurrents closely matched the experimental Jsc values for all cells (within a maximum deviation of 10%). Extrapolation of the above calculations for high-illumination conditions (1 sun AM1.5G) predicts well the measured photocurrents for TG-ref and TG-aged and also for the MPN-ref cells. However, in the case of the MPN-aged cells, the corresponding integration resulted in a photocurrent of 7.5 mA·cm−2, far above the experimentally recorded value of 3.2 mA·cm−2 (Table 1). This large difference is due to large deviation from linearity of the Jsc versus light intensity dependence as a result of severe diffusion limitation. 3.2. Detection of Triiodide Loss in Thermally Stressed Cells. After 2000 h of 80 °C storage, all test cells were carefully inspected for visual differences as a result of thermal stress. Optical observation of the cells indicated that following thermal aging, the distinctly yellow electrolyte color (of the reference cells) tends to completely (for MPN) or significantly (for TGbased cells) vanish (Figure 5). Triiodide loss was further
Figure 6. Absorbance spectra of MPN and TG based reference and thermally aged DSCs.
The most striking feature in the optical spectra of the aged cells was a reduction in the absorbance in the 380−450 nm range. This behavior was particularly pronounced for the MPN-based cells, indicating drastic I3− depletion.38 Decrease in the absorbance in the same range was also observed in the TGbased cells, although to a much lower extent. 3.3. Electron Dynamics after Thermal Stress. Electrochemical impedance spectroscopy (EIS)39 was applied to explore the role of the solvents in the thermally aged cells and examine the effects of the partial loss of redox shuttle components on the electron dynamics (recombination, conduction band edge shifts) governing the cell operation as well as on the different interfaces inside the cell, which determine the effective charge transfer. EIS spectra, recorded in the dark for a large potential window of 0 to −0.8 V, presented in all cases three standard depressed semicircles.40 The equivalent circuit used to fit the spectra at potentials near Voc, where TiO2 is sufficiently conductive and transport resistance is negligible, was a simplified form of the transmission line model.41 Specifically, it consisted of an ohmic resistance (Rs ≈ 10 Ohms for all of the studied cells, mainly due to the resistance or the two TEC substrates) in series with two RC circuits representing the Pt/electrolyte and TiO2/electrolyte interfaces, respectively, and a finite Warburg impedance representing ionic diffusion in the electrolyte. More accurate fitting was obtained by employing a constant phase element (CPE) in the equivalent circuit instead of a simple capacitance (C).34 Influence of Triiodide Loss on the Diffusion Resistance. First, we focused on the diffusion resistance (Rdif) derived by the proper fitting of the EIS spectra obtained at potentials where the lowest frequency feature (assigned to triiodide diffusion) was clearly resolved (Figure S2 in the Supporting Information). Rdif was largely increased in the case of the MPN electrolyte (from about 1 up to more than 60 Ohms at applied potentials approaching Voc upon thermal aging) due to the severe loss of triiodide. In comparison, the corresponding diffusion resistance for the TG electrolyte was also increased but by a lower factor (from ∼15 to
