FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com in terms of the first two attributes, the stability of these devices remains unproven. [1–3] For solid-state dye sensitized (ssDSSC) [4] and perovskite sensitized solar cells (PSSC) [5] non-stoichiometry induced defects in TiO 2 appears to be a limiting factor for two of the key param- eters; efficiency and more importantly the long-term photo-stability. Limitation in charge transport is an important factor for device performance, which has been studied extensively in context of ssDSSCs. Recently the emergence of perovskite- based solar cells (PSSCs) has led to a marked increase in solid-state device per- formance, to up to 15%. In the perovskite devices, replacing the electron acceptor TiO 2 with an insulating alumina scaffold has played an important part in the increase in open-circuit voltage. [1] This has been rationalized by the presence of deep electron traps in the non-stoichiometric TiO 2 reducing the splitting of the quasi Fermi levels under illumina- tion. The same mechanism is likely to affect ssDSSCs, but in contrast to perovskite solar cells in which organometal trihalide perovskites are a light absorbing charge transporter, ssDSSCs rely on the metal oxide for charge transport, ruling out the use of alumina or other insulating scaffolds. [1] In ssDSSCs, the dye is regenerated from its oxidised state within a few hundred picoseconds, orders of magnitude faster than in the iodide/triiodide-based liquid electrolyte cells, where dye regeneration occurs on the microsecond time scale. [6] These extremely rapid regeneration dynamics should play an important role in improved long-term stability of the dye in the ssDSSC, since the dye is most likely to degrade in its charged oxidized state. [7] However, to realize long-term stability of DSSCs it is paramount to protect the organic components of the device from oxidation by oxygen, moisture and other oxidizing agents, by encapsulation in inert environment. We have discovered a critical instability of mesoporous TiO 2 based devices; when they are encapsulated in an inert atmosphere and exposed to sunlight, a quick loss in device performance is observed. [8,9] Encouragingly though, the cells recuperate to their initial performance when the encapsulation is broken, exposing them to air. Here, we discuss in detail the nature of the titania surface chemistry in the presence of oxygen and light, and its role in Performance and Stability Enhancement of Dye-Sensitized and Perovskite Solar Cells by Al Doping of TiO 2 Sandeep K. Pathak, A. Abate, P. Ruckdeschel, B. Roose, Karl C. Gödel, Yana Vaynzof, Aditya Santhala, Shun-Ichiro Watanabe, Derek J. Hollman, Nakita Noel, Alessandro Sepe, Ullrich Wiesner, Richard Friend, Henry J. Snaith,* and Ullrich Steiner* Reversible photo-induced performance deterioration is observed in mesoporous TiO 2 -containing devices in an inert environment. This phenom- enon is correlated with the activation of deep trap sites due to astoichiom- etry of the metal oxide. Interestingly, in air, these defects can be passivated by oxygen adsorption. These results show that the doping of TiO 2 with aluminium has a striking impact upon the density of sub-gap states and enhances the conductivity by orders of magnitude. Dye-sensitized and per- ovskite solar cells employing Al-doped TiO 2 have increased device efficiencies and significantly enhanced operational device stability in inert atmospheres. This performance and stability enhancement is attributed to the substitu- tional incorporation of Al in the anatase lattice, “permanently” passivating electronic trap sites in the bulk and at the surface of the TiO 2 . DOI: 10.1002/adfm.201401658 Dr. S. K. Pathak, P. Ruckdeschel, B. Roose, K. C. Gödel, Dr. Y. Vaynzof, A. Santhala, Dr. S.-I. Watanabe, Dr. A. Sepe, Prof. R. H. Friend, Prof. U. Steiner Cavendish Laboratory Department of Physics University of Cambridge JJ Thomson Avenue CB3 0HE, UK E-mail: h.snaith1@physics.ox.ac.uk Dr. S. K. Pathak, Dr. A. Abate, D. J. Hollman, N. Noel, Prof. H. J. Snaith Clarendon Laboratory Department of Physics University of Oxford Parks road, Oxford OX1 3PU, UK E-mail: ullrich.steiner@unifr.ch Prof. U. Wiesner Material Science and Engineering University of Cornell 214 Bard Hall, Ithaca, NY 14853-1501, USA Prof. U. Steiner Adolphe Merkle Institute Chemin des Verdiers CH-1700, Fribourg, Switzerland 1. Introduction The commercial viability of a photovoltaic technology replacing single-crystal silicon solar cells relies on three essential attrib- utes: cost, performance and lifetime. While dye-sensitized and more recently perovskite solar cell are highly promising Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201401658 www.afm-journal.de www.MaterialsViews.com