ARTICLE pubs.acs.org/JPCC
Coexistence of Femtosecond- and Nonelectron-Injecting Dyes in Dye-Sensitized Solar Cells: Inhomogeniety Limits the Efficiency Kenji Sunahara,†,‡ Akihiro Furube,*,†,‡ Ryuzi Katoh,‡ Shogo Mori,§ Matthew J. Griffith,|| Gordon G. Wallace,|| Pawel Wagner,|| David L. Officer,|| and Attila J. Mozer|| †
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan § Department of Fine Materials Engineering, Shinshu University, Nagano, 386-8567, Japan Intelligent Polymer Research Institute, ARC Centre for Excellence for Electromaterials Science, University of Wollongong, Wollongong 2522, Australia
)
‡
bS Supporting Information ABSTRACT: We performed a detailed and quantitative spectroscopic study of the electron injection dynamics for porphyrin as one of organic dyes at an adequate level to discuss the dyesensitized solar cell performance. The electron injection kinetics and the electron injection yield for dye-sensitized TiO2 electrodes in redox-containing electrolytes were measured by femtosecond transient absorption and picosecond fluorescence spectroscopy. By comparing the dynamics of two of the most studied porphyrins with those of a Ru complex (N719), we have directly elucidated that the short-circuit current for the porphyrin-sensitized solar cells is limited by the presence of excited dyes that are quenched in the subnanosecond time range without competing with the electron injection process, even though both porphyrins shows faster injection processes within the picosecond time range than N719. Therefore, it was clearly indicated the electron injection efficiency was mainly limited by the inhomogeniety, which should be carefully considered for further development of organic dye-sensitized solar cells.
’ INTRODUCTION The dye-sensitized solar cell (DSSC)1 is one of the most attractive next generation solar cells because of its potential lowcost, variable architecture, color, and flexibility. To realize this potential, DSSCs without precious metal-containing sensitizers are needed. Recently such organic dye-sensitized solar cells have shown a dramatic improvement in efficiency up to 10 to 11%, using a zinc porphyrin or a triphenylamine-based dye.2,3 In contrast, whereas Ru complexes have been proven to be the most efficient sensitizers,4 their efficiency has not been much improved in the last several years.5 Therefore, the potential of organic dyes seems comparable to that of Ru complexes. In general, the operational principles of the DSSC are explained by successive electron transfer processes. To achieve high device performance, each electron transfer process must be ∼100% efficient. The electron injection process from an excited dye to the nanoporous semiconductor is the first and, therefore, very important process of charge separation. It has been widely studied from the perspective of dye sensitizers,6 12 semiconductor electrodes,13 16 and environmental conditions.17 21 The electron injection within 100 fs as well as on the 10 100 ps scale has been observed for Ru complexes and organic dyes, indicating that in both cases electron injection is inhomogeneous in nature.6,22 We have reported that organic dyes and porphyrin dyes show ultrafast electron injection kinetics, but their electron injection r 2011 American Chemical Society
quantum yields are still not unity efficient.23,24 Also, some previous works for organic dyes have indicated femtosecond electron injection, but there was picosecond recombination.8,11 In contrast with Ru complexes, these observations make us think that such inhomogeneity causes some barriers against the better device performance for organic dye-based DSSCs. The mechanism is not fully clear because there is a lack of discussion based on the quantitative evaluation of such an inhomogeneous electron injection process. Actually, understanding of ultrafast dynamics for organic dye-based DSSCs has been proceeding slower than the case of Ru complexes. Most studies for the electron injection dynamics have employed time-resolved transient absorption (TA), fluorescence spectroscopy, or both. Time-resolved fluorescence spectroscopy is technically easier than TA, but only emissive excited states of sensitizers can be detected. In general, one may not know whether fluorescence decay is due to electron injection or other quenching processes. Quantitative application of this technique for electron injection yields is limited.25 TA spectroscopy observes dyes in both the excited and oxidized states as well as the injected electron when a wide range of probe wavelength (visible to IR) is employed.23,26 Therefore, a quantitative analysis is more reliable. Received: September 27, 2011 Published: October 04, 2011 22084
dx.doi.org/10.1021/jp2093109 | J. Phys. Chem. C 2011, 115, 22084–22088
The Journal of Physical Chemistry C Furthermore, it is common fact that the efficiency of DSSC is very sensitive to the condition for the interface among TiO2, dye, and electrolyte. However, the evaluation of electron injection kinetics under normal device operating conditions is technically difficult and has been reported in only a few papers.21,27,28 This actually makes it difficult to understand how the injection rates relate to the solar cell performance. In this Article, we report the direct evaluation of electron injection kinetics and yield for a porphyrin-sensitized TiO2 electrode in redox-containing electrolytes, which were simultaneously measured by femtosecond TA spectroscopy in the visible-to-IR range and picosecond fluorescence spectroscopy. The quantitative evaluation based on the electron injection yield shows a good agreement with the device performance. Our results are explained by the presence of non-electron-injecting dyes on the electrode surface and clearly indicate that the inhomogeniety limits the photocurrent efficiency. This finding demonstrates that to improve DSSC performance using organic dyes in the future, it is essential to consider inhomogeneous electron injection processes.
’ EXPERIMENTAL SECTION We employed N71929 as a reference sample, which is wellknown as one of the most efficient Ru complex sensitizers. We employed two of the most studied porphyrin sensitizers. The one referred to as GD230 is one of the most efficient porphyrin sensitizers for DSSCs. Another porphyrin P25131 has also been frequently investigated, and its electron injection kinetics have been evaluated by photoluminescence lifetime experiments.32 Porphyrin structures are shown in Supporting Information (Section S1). The HOMO/LUMO energy level for each dye was experimentally evaluated by following equation: ELUMO = EHOMO + E0 0. EHOMO is the HOMO energy potential of the ground state obtained from a photoelectron emission yield spectrum. E0 0 is the energy of the relaxed excited state from the ground state, which was obtained from an onset of fluorescence spectrum. (See the Supporting Information S2 for details.) For N719, GD2, and P251, the LUMO levels are 3.6, 3.4, and 3.5 eV versus vacuum, respectively. GD2 and P251 have higher LUMO levels than N719, making them energetically more favorable for electron injection from the excited dye to the semiconductor electrode. A femtosecond TA spectroscopic system with the light source of an amplified Ti:sapphire laser (Spectra-Physics, Hurricane) combined with two optical parametric amplifiers was employed, the detail of which was already described.26 The instrumental time response was ∼250 fs. The repetition rate was varied between 1 kHz and 20 Hz to check some accumulation effect in this measurement, and it was found to be negligible for the TA kinetics. A streak camera (Hamamatsu, StreakScope C4334) was employed to measure fluorescence spectra and decay, the detail of which was already described.26 The time resolution was 30 ps. Steady fluorescence was constructed by the time integration of the measured spectra. In both of the time-resolved measurements, the excitation wavelength was 532 nm and the ambient temperature was 295 K. ’ RESULTS AND DISCUSSION Figure 1 shows the TA kinetics for N719-, GD2-, and P251sensitized TiO2 in redox-containing electrolytes probed at 3440 nm wavelength. Two electrolytes were used; one is denoted as
ARTICLE
Figure 1. TA kinetics probed at 3440 nm for N719, GD2 and P251 on TiO2 with the acetonitrile electrolytes including LiI 0.7 M and I2 0.05 M (denoted as LiI0.7M), and LiI 0.1 M, DMPImI 0.6 M, tBP 0.5 M, and I2 0.05 M (denoted as LiI0.1M). The bold lines are fits to single exponential functions. TA signals were corrected by the ground absorption fraction (1 10 A).
LiI0.7M [0.7 M lithium iodide (LiI), and 0.05 M I2 in acetonitrile], and the other is denoted as LiI0.1M [0.1 M lithium iodide (LiI), 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPImI), 0.5 M 4-tert-butylpyridine (tBP), and 0.05 M I2 in acetonitrile]. The samples were excited at 532 nm with the same intensity (1 μJ in the focus size of ∼0.3 mm). Photons at 3440 nm are primarily absorbed by electrons in TiO2. Therefore, electron injection from the photoexcited dye to TiO2 is directly monitored. The TA signals were corrected by dividing by the absorption fraction, 1 10 A, where A is the ground-state absorbance of dye-sensitized TiO2 film at the pump wavelength.33 As a result, the TA signals for each dye can be directly compared as the electron injection efficiency. It is important to note that the samples without dye showed very small TA signals, indicating that the I3 excitation34 was negligible. For N719-sensitized film with LiI0.7M, the electron injection kinetics showed both fast injection within 250 fs as well as slow injection of a ∼50 ps time constant, which is consistent with previous results.21 The GD2-sensitized film gave faster electron injection kinetics than that of N719 under the same electrolyte, with a
