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J. Phys. Chem. C 2009, 113, 9412–9422
Prolonged Light and Thermal Stress Effects on Industrial Dye-Sensitized Solar Cells: A Micro-Raman Investigation on the Long-Term Stability of Aged Cells Vlassis Likodimos, Thomas Stergiopoulos, and Polycarpos Falaras* Institute of Physical Chemistry, NCSR Demokritos, 153 10, Aghia ParaskeVi Attikis, Athens, Greece
Ravi Harikisun, Johann Desilvestro, and Gavin Tulloch Dyesol Limited Company, Queanbeyan, NSW 2620, Australia ReceiVed: February 9, 2009; ReVised Manuscript ReceiVed: April 8, 2009
Micro-Raman spectroscopy is applied to investigate the long-term stability of industrial dye-sensitized solar cells under prolonged light soaking and thermal stress following continuous illumination over 6450 h at 55-60 °C. The Raman spectral characteristics of the individual cell components have been investigated using two excitation wavelengths in the visible and near-infrared range allowing us to assess the microstructure of the TiO2/conducting glass photoelectrode, the chemical bonding of the hydrophobic Ru(II)-polypyridyl dye complex on the mesoporous TiO2 film, and the electrolyte composition. Comparative ex situ resonance Raman measurements on fresh and aged cells indicate minor differences in the vibrational characteristics of the triiodide, dye molecules, and the triiodide/dye charge transfer adduct at the electrode/electrolyte interface upon aging, confirming the absence of any distinct chemical modification that could create instability. In situ Raman experiments implemented via the application of a polarization bias reveal a less pronounced potential dependence of both the electrolyte and the dye Raman response for the aged cells. These features together with the intensity reduction and broadening of the anatase Raman modes imply that the chemical stability of the cell interfaces is accompanied by a modification of the interfacial electric field on the TiO2/dye/electrolyte junction after long-term light and thermal stress. Introduction Mesoscopic dye solar cells (DSCs) have been attracting much attention due to their unique potential as a viable alternative to current silicon-based photovoltaics that offers the prospect of efficient, clean, and low-cost solar energy conversion.1-3 Mimicking the principles of natural photosynthesis for the separation of light-harvesting and charge carrier transport, the operation mechanism of DSCs relies upon the synergistic function of the three major cell components, the light-absorbing antenna-sensitizer [the most effective to date being Ru(II)polypyridyl complexes], the electron-transporting system involving a wide band gap semiconductor of mesoporous nanocrystalline structure (typically the anatase phase of TiO2), and the electrolyte/hole-transporter subsystem (the most frequent being the I-/I3- redox couple).2 Systematic studies of the DSCs’ components and their complex interdependence have established marked efficiency improvements of the underlying physicochemical processes, including fast electron injection from the photoexcited state of the dye into the conduction band of TiO2 with near unity photonto-electron conversion efficiency, rapid dye regeneration, weak interfacial recombination losses, as well as efficient collection of the photogenerated electrons and hole transport at the counter electrode through the redox couple, leading to power conversion efficiencies over 11% under standard air mass AM 1.5 solar illumination.4 However, apart from the requirement of high device efficiency, long-term stability of the DSC operation is a key aspect in view of practical applications.5 DSCs have been * Corresponding author. Tel: +30-210-6503644. Fax: +30-210-6511766. E-mail:
[email protected].
accordingly tested under light soaking as well as thermal and humidity stress conditions that cause destructive side reactions involving the excited or the oxidized state of the sensitizer as well as the evaporation of organic solvents.6-14 Molecular engineering of the sensitizer focused on the development of high molar extinction coefficient ruthenium polypyridyl complexes endowed with hydrophobic properties in combination with robust, nonvolatile electrolytes, as well as hermetic sealing methods that have been recently realized, paving the way toward high photovoltaic performance and stability under accelerated aging tests.15-21 However, up to now most stability tests have been realized on small active area DSCs.14 Recognizing the need for systematic aging experiments on devices assembled in a production line with commercially available materials compatible with scalable industrial processes that would reinforce the building integration application prospects of DSC technology, industrial test cells were developed and adequate long-term stability tests were performed.21 Thus, despite the significant progress in the long-term performance of these industrial DSC devices (efficiencies ranging from 7 to >8% at 0.33 sun and 5.8% to >7% at 1 sun),21 detailed spectroscopic information on aging phenomena, besides monitoring through current-voltage characteristics and electrochemical impedance spectroscopy, has been limited, though extremely useful for detecting and clarifying degradation mechanisms.21-23 To better understand limiting factors and optimize the DSC device, micro-Raman spectroscopy has been proved to be a sensitive experimental technique offering high spectral and spatial resolution to probe the vibrational properties and coordinative interactions of the individual cell components and the corresponding interfaces and
10.1021/jp901185f CCC: $40.75 2009 American Chemical Society Published on Web 05/04/2009
Long-Term Stability of Aged Solar Cells most importantly investigate in situ the DSC response under operating conditions via the application of an external polarization bias.23 On the basis of the resonance Raman effect, the Raman signal of a single dye monolayer chemisorbed on the semiconductor surface is greatly enhanced. The technique has been successfully applied to resolve the dye anchoring on nanocrystalline TiO2 films used in DSCs24-31 and more recently to study variations of the interfacial electron transfer mechanism on colloidal TiO2 nanoparticles in different solvents.32-34 Moreover, in situ micro-Raman measurements on DSCs under real photocurrent conditions has been shown to be a powerful tool for monitoring the photoelectrode/electrolyte interface by the variation of the Raman spectra as a function of applied potential, thereby providing insight into the dye-redox couple interactions.23,27,29,35-38 In particular, detailed investigations by both micro- and macro-Raman spectroscopy on DSCs sensitized by various polypyridyl-based Ru(II) complexes bearing different functional anchoring groups (-COOH, -PO3H2) have systematically identified the presence of strongly bias dependent Raman bands in the low wavenumber region that stem from the vibrations of triiodide electrostatically bound to the oxidized form of the dye, providing spectroscopic evidence for the formation of an intermediate complex at the photoelectrode surface.23,38 Although a few long-term stability studies have been recently reported using mainly chromatography methods,39,40 an in situ spectroscopic investigation of possible degradation products inside a DSC device under operation has never been attempted. In this work, resonance micro-Raman spectroscopy is exploited as a site-specific spectroscopic probe to evaluate the stability of industrial DSCs under prolonged exposure to light soaking and thermal stress, involving continuous illumination over 6450 h at 55-60 °C. Micro-Raman measurements at different excitation wavelengths are initially employed to determine the vibrational properties of the individual cell components, including the TiO2/conducting glass photoelectrode, the hydrophobic ruthenium dye, and its chemisorption on the mesoporous TiO2 film, as well as the electrolyte components. Subsequently, resonance Raman is used to resolve aging effects in the fresh and aged DSC test devices under both ex situ and in situ (applying a polarization bias) conditions, by monitoring variations of the surface chemistry of the cell interfaces and the bias dependence of both the electrolyte and the sensitizing dye Raman response. The absence of any distinct chemical modification/degradation process on industrial DSC devices after prolonged light and thermal stresses is thus established for the first time in the literature by a direct Raman investigation of the cell components and interfaces. Experimental Section Test DSCs were fabricated on the basis of transparent TiO2 photoelectrodes prepared by screen printing of TiO2 paste (transparent titania paste DSL 18NR-T, Dyesol Ltd.) onto F-doped SnO2 (FTO) conducting glass. The TiO2 film was dried at ∼80 °C and then fired at 525 °C for ∼30 min to remove any organics and form a porous film structure. The thickness of the TiO2 film was 12 µm.41 The TiO2 film was immersed into a 1:1 volume ratio of acetonitrile:tert-butyl alcohol solution of the heteroleptic polypyridyl ruthenium complex cis-RuLL′ (SCN)2 (L ) 2,2′-bipyridyl-4,4′-dicarboxylic acid, L′ ) 4,4′-dinonyl2,2′-bipyridyl) (Z907, Dyesol Ltd.), for 16 h, which has shown very promising long-term stability at elevated temperatures.15 The dye-coated TiO2 film was then rinsed with absolute ethanol to remove any excess dye before cell assembly. The platinized
J. Phys. Chem. C, Vol. 113, No. 21, 2009 9413 counter electrode was prepared by screen-printable platinum paste (Platinum Paste PT1, Dyesol Ltd.) onto FTO conducting glass, followed by firing at 510 °C for ∼25 min. DSCs were assembled by sealing the counter electrode and the dye-coated photoanode with a thermal plastic spacer (50 µm thermoplastic gasket TPS 065093-50, Dyesol Ltd.) in a test cell assembly machine (TCAM, Dyesol Ltd.). A redox electrolyte (EL-HSE, Dyesol) was introduced into the cell through a 2 mm hole sandblasted in the counter electrode. The hole was subsequently sealed with aluminum foil covered with a layer of thermoplastic (Bynel) using a heat press. The active area of the DSCs is 0.88 cm2. Continuous visible light soaking under thermal stress was implemented by subjecting the DSCs to solar simulated light of ∼0.8 sun using a high-pressure sodium lamp for over 6450 h at temperatures of 55-60 °C (aged cells). The efficiency of aged cells at 1 sun was 13% lower compared to that of the fresh cells, mainly because of lower short circuit current.21,42 Micro-Raman spectra were measured in the backscattering configuration using a Renishaw inVia Reflex microscope with an Ar+ ion laser (514.5 nm) and a high-power near-infrared (NIR) diode laser (785 nm) as excitation sources. The spectrometer is equipped with two diffraction gratings of 1800 and 1200 lines/mm together with a holographic notch and a dielectric edge Rayleigh rejection filter with cutoffs at approximately 130 and 100 cm-1 for the 514.5 and 785 nm laser lines, respectively. Raman measurements down to 17 cm-1 from the laser line were performed by the use of the near excitation tunable filter (NEXT). The laser line was focused either on the sample surface using the 50× (numerical aperture NA ) 0.75) objective for the various DSC components or a long working distance (8 mm) 50× (NA ) 0.55) objective of a Leica DMLM microscope for the solution spectra of the DSCs components sealed in quartz cells and the assembled DSC devices at a wide range of laser powers. In situ photoelectrochemical Raman measurements were performed by biasing the test cells with a homemade potentiostat working in the two-electrode mode, which also was used to record the resulting photocurrent under laser illumination. The experiments were carried out in the voltage range of +0.4 to -0.8 V vs Pt, which includes both the I-V characteristic’s region and the potential regime where the cell consumes power to generate a current. No variation of the cell performance and efficiency parameters was detected during and after Raman investigation on both fresh and aged cells. Figure 1 depicts a schematic presentation of the setup for the in situ resonance Raman measurements on DSCs. The frequency shifts were calibrated by an internal Si reference. Subtraction of the luminescence background on the Raman spectra has been performed by polynomial fitting and/or cubic spline interpolation routines, while spectral deconvolution has been carried out by nonlinear least-squares fitting of the Raman peaks to a mixture of Lorentzian and Gaussian line shapes, formally a pseudoVoight function, providing the peak position, width, height, and integrated intensity of each Raman band. For comparison, the Raman characteristics of the parent 2-fold deprotonated polypyridyl ruthenium complex cis-RuL2(SCN)2 (L ) 2,2′-bipyridyl4,4′-dicarboxylic acid) (N719), which does not contain the hydrophobic alkyl chains of the Z907 dye, were investigated. An anatase TiO2 film prepared by the doctor blade technique using Ti-Nanoxide 300 (Solaronix) paste containing about 20% wt of 400 nm sized TiO2 anatase particles was used as a reference. The corresponding Raman spectrum is characteristic of bulk anatase.
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Figure 1. Schematic diagram of the micro-Raman setup for the in situ photoelectrochemical Raman characterization of the test DSC devices under operating conditions.
Results and Discussion 3.1. DSC Components. Figure 2a shows the micro-Raman spectrum of the TiO2/FTO photoelectrodes at 514.5 nm, in comparison with a reference anatase TiO2 film. Both samples exhibit the characteristic Raman-active phonon modes of the 19 -I41/amd) of the anatase TiO2. Speciftetragonal structure (D4h ically, the TiO2/FTO sample exhibits the three Eg anatase modes at about 144, 198, and 639 cm-1, the B1g mode at 398, and one at ∼519 cm-1, stemming from the superposition of the two close lying A1g and B1g modes, unresolved at room temperature.43 In addition, two relatively weak and broad bands are observed at about 315 and 796 cm-1 (marked with asterisks), which may arise from the disorder-induced or two-phonon scattering and the first overtone of the B1g mode at about 398 cm-1, respectively.44 No trace of the vibrational bands of the rutile TiO2 phase44 could be identified on the TiO2/FTO sample, confirming the presence of crystalline anatase. Furthermore, appreciable broadening and shift of the anatase Raman peaks is observed for the TiO2/FTO electrode, compared to the anatase reference sample, as shown in Figure 2a. This indicates the presence of size effects caused by the spatial confinement of optical phonons arising from the absence of periodicity beyond the particle dimension that relaxes the q ) 0 selection rule in nanosized systems.45 The underlying optical phonon confinement mechanism should be discriminated from quantum size effects, which are restricted to very low sizes (
