Article pubs.acs.org/JPCC
Evidence for “Slow” Electron Injection in Commercially Relevant DyeSensitized Solar Cells by vis−NIR and IR Pump−Probe Spectroscopy Mindaugas Juozapavicius,† Marius Kaucikas,‡ Stoichko D. Dimitrov,† Piers R. F. Barnes,§ Jasper J. van Thor,‡ and Brian C. O’Regan*,† †
Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom Division of Molecular Biosciences, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom § Department of Physics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ‡
S Supporting Information *
ABSTRACT: We present femtosecond to nanosecond transient absorption (TA) data on electron injection in dye-sensitized solar cells (DSSCs) fabricated with low volatility, commercially relevant electrolytes, with and without added lithium. Results are shown over an extended time range (300 fs−6.3 ns) and extended wavelength range (800−1400 nm) for both N719 and C106 dyes. Kinetics were measured on both TiO2 and noninjecting ZrO2. Using the latter, we have determined the spectra and absorption coefficient of N719* across the wavelength range. We find an isosbestic point in the TA spectra on TiO2 near 900 nm for all cells, existing from 1 ns. We show how measurements near this isosbestic point can give a false impression of uniformly femtosecond injection dynamics in DSSCs. Comparison of dynamics measured at 1200 nm with mid-IR transient absorption measured at 5100 nm confirms a majority proportion of slow (>10 ps) electron injection in these commercially relevant cells. We also comment on a recent publication which appears to directly contradict the results we present.
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INTRODUCTION Electron injection from a photoexcited dye to a metal oxide semiconductor is one of the key steps in light to power conversion in dye-sensitized solar cells. Reliable measurement of electron injection dynamics has been a subject of debate for over a decade now. Several different techniques have been employed to study injection kinetics: time-correlated single photon counting1,2 (TCSPC) and transient absorption spectroscopy (TAS) in the UV−vis,3−6 NIR,7,8 mid-IR,7,9,10 and THz regions.11,12 We have recently reported kinetics of electron injection from excited dyes to TiO2 utilizing visible pump/mid-IR probe TAS.10 In that work we specifically focused on electron injection in cells with the same electrolytes used to make the highest efficiency stable DSSCs. These we refer to as “commercially relevant” (CR) electrolytes. In this report, we present complementary vis−NIR TAS over a broad spectrum between ∼850−1400 nm, using the same CR electrolytes. Our newer results corroborate the conclusions we reached utilizing mid-IR spectroscopy. We find that electron injection from ruthenium polypyridyl dyes in CR electrolytes occurs over a broad time scale: 1500 ps. The data also suggest that successful analysis of injection kinetics using only vis and NIR TAS is possible only if one has very accurate extinction coefficients of all the transient species involved. Otherwise, quantification of electron injection is subject to considerable uncertainties. In light of these conclusions, we discuss briefly other recent attempts to measure electron injection that may have used similar CR electrolytes.13 © 2013 American Chemical Society
EXPERIMENTAL METHODS vis−NIR Femtosecond Transient Absorption Setup. Samples were excited with a pump light at 580 nm in the range of 46−90 μJ cm−2 (accurate energy indicated in the text). The pump light was generated by a commercially available optical parametric amplifier TOPAS (Light conversion) pumped by a Solstice Ti:sapphire regenerative amplifier (Newport Ltd.). Pump laser excitation was followed by a second broadband pulse (830−1450 nm) generated in a sapphire crystal. A HELIOS (Ultrafast systems) transient absorption spectrometer was used for recording the dynamics of the transient absorption spectra up to 6.5 ns with average 200 fs IRF. The pump pulse frequency was 500 Hz. Absorption at each time delay was recorded for 5 s (2500 pulses), and this averaging was repeated four times, scanning in both directions. The pump pulse was unfocused and had a fwhm of roughly 3 mm while the probe pulse was focused to an fwhm of
