rXXXX American Chemical Society A dx.doi.org/10.1021/jp2093109 | J. Phys. Chem. C XXXX, XXX, 000–000 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 b S Supporting Information ’ INTRODUCTION The dye-sensitized solar cell (DSSC) 1 is one of the most attractive next generation solar cells because of its potential low- cost, 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 con- trast, 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, 6À12 semiconductor electrodes, 13À16 and environmental conditions. 17À21 The electron injection within 100 fs as well as on the 10À100 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 quantum yields are still not unity efficient. 23,24 Also, some pre- vious works for organic dyes have indicated femtosecond elec- tron 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 mechan- ism 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 spectros- copy, or both. Time-resolved fluorescence spectroscopy is tech- nically easier than TA, but only emissive excited states of sen- sitizers 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 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 dye- sensitized solar cell performance. The electron injection kinetics and the electron injection yield for dye-sensitized TiO 2 electrodes in redox-containing electrolytes were measured by femto- second 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.