DOI: 10.1002/cphc.201200715 Ultrafast Photodynamics of the Indoline Dye D149 Adsorbed to Porous ZnO in Dye-Sensitized Solar Cells Egmont Rohwer, [a] Christoph Richter, [b] Nadine Heming, [a] Kerstin Strauch, [b] Christian Litwinski, [c] Tebello Nyokong, [c] Derck Schlettwein, [b] and Heinrich Schwoerer* [a] 1. Introduction Research in the field of dye-sensitized solar cells (DSCs) has been driven by a desire to develop versatile, sustainable and affordable sources of electrical energy. [1, 2] The working electro- des of these solar cells consist of a layer of dye molecules, which act as electron donators upon irradiation with light, and of a semiconducting surface to which the dye molecules are attached, acting as an electron acceptor that can harvest elec- trons efficiently. The dye absorbs sunlight and, if the transition is energetically favourable and if there is sufficient coupling between the density of states in the dye’s excited states and the conduction band of the semiconductor, the photo-excited electron is injected into the semiconductor. The electrical cir- cuit can be closed through a counter electrode and an electro- lyte, which reduces the oxidized state of the dye molecules after the initial electron transfer and is in turn regenerated at the counter electrode. Under full AM1.5 solar illumination, state-of-the-art DSCs based on nanoparticulate porous TiO 2 as semiconductor with sensitization by a combination of dyes and contacted by a Co-complex redox electrolyte can deliver an open-circuit photovoltage of up to 0.935 V, a short-circuit photocurrent of 17.66 mA cm 2 at a fill factor of 0.74, yielding a power conversion efficiency of 12.3 %. [3] For cells based on bi- pyridine Ru II complexes adsorbed to TiO 2 and contacted by an iodine-based electrolyte, a similarly attractive solar-light-to- electricity conversion efficiency (h) of 12 % for small cells and about 9 % for minimodules have been reported. [2] In order to reach such values, a perfect interplay of semiconductor, dye and electrolyte is needed with optimization of rate constants of injection and electron transport as opposed to recombina- tion. Despite still lower efficiency when compared with classi- cal wafer-based solar cells but also with modern thin-film devi- ces, DSCs remain of technical interest because of the expected low energy-payback times (EPT), the applicability of abundant materials and extended design possibilities beyond those ac- cessible with traditional solar cells. [4] The working electrode of the most efficient DSCs consists of a porous TiO 2 semiconducting layer to which a layer of sensi- tizer is adsorbed. The preparation of this porous TiO 2 layer re- quires high temperature annealing (> 450 8C) hindering the use of cheap flexible light-weight substrates like plastics, which could open up a large number of new applications. Sin- tering by itself and its implication to use glass substrates also suppress further options to significantly decrease the EPT. ZnO represents an attractive alternative semiconductor ma- terial in DSCs with a similar band gap and an even higher carri- er mobility compared with TiO 2 . [5, 6] In recent years different techniques have been developed to deposit porous ZnO layers that do not require sintering at high temperatures. An attrac- tive method to grow ZnO films with high roughness factors is electrochemically induced crystallization of the ZnO film in the presence of structure-directing agents (SDAs), for example, eosin Y, followed by desorption of the SDA and subsequent ad- sorption of a sensitizer from solution. [7, 8] This allows facile access of the sensitizer molecule to the ZnO pores. Organic dyes as alternatives to Ru complexes have advantages in their We investigate the ultrafast dynamics of the photoinduced electron transfer between surface-adsorbed indoline D149 dye and porous ZnO as used in the working electrodes of dye-sen- sitized solar cells. Transient absorption spectroscopy was con- ducted on the dye in solution, on solid state samples and for the latter in contact to a I  /I 3  redox electrolyte typical for dye- sensitized solar cells to elucidate the effect of each component in the observed dynamics. D149 in a solution of 1:1 acetonitrile and tert-butyl alcohol shows excited-state lifetimes of 300  50 ps. This signature is severely quenched when D149 is ad- sorbed to ZnO, with the fastest component of the decay trace measured at 150  20 fs due to the charge-transfer mechanism. Absorption bands of the oxidized dye molecule were investi- gated to determine regeneration times which are in excess of 1 ns. The addition of the redox electrolyte to the system results in faster regeneration times, of the order of 1 ns. [a] E. Rohwer, N. Heming, Prof. H. Schwoerer Laser Research Institute Stellenbosch University Private Bag X1, Matieland, 7602 (South Africa) Fax: (+ 27) 21 808 3385 E-mail: heso@sun.ac.za [b] C. Richter, K. Strauch, Prof. D. Schlettwein Institute of Applied Physics Justus-Liebig-University Heinrich-Buff-Ring 16, 35392, Gießen (Germany) [c] Dr. C. Litwinski, Prof. T. Nyokong Department of Chemistry Rhodes University Cnr of University and Artillery Roads, Grahamstown, 6139 (South Africa)  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2013, 14, 132 – 139 132 CHEMPHYSCHEM ARTICLES