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Already exhibiting solar to electrical power conversion efficiencies of over 17%, organic-inorganic lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicology issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While organic-inorganic tin halide perovskites have shown good semiconducting behaviour, the instability of tin in its 2+ oxidation state has thus far proved to be an overwhelming challenge. Here, we report the first completely lead-free, CH3NH 3SnI3 perovskite solar cell processed on a mesoporous TiO2 scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap. This journal is © the Partner Organisations 2014.

(Chemical Equation Presented) Organic dyes for solid-state sensitized solar cells are commonly prepared by combining an electron-withdrawing group (acceptor, A) on one side of a conjugated π-spacer and one or more electrondonating groups (donor, D) on the other side. It is shown that without an electron donor group a signifi cantly higher opencircuit voltage is achieved, while maintaining the short-circuit current.

To date, there have been a plethora of reports on different means to fabricate organic–inorganic metal halide perovskite thin films; however, the inorganic starting materials have been limited to halide-based anions. Here we study the role of the anions in the perovskite solution and their influence upon perovskite crystal growth, film formation and device performance. We find that by using a non-halide lead source (​lead acetate) instead of ​lead chloride or iodide, the perovskite crystal growth is much faster, which allows us to obtain ultrasmooth and almost pinhole-free perovskite films by a simple one-step solution coating with only a few minutes annealing. This synthesis leads to improved device performance in planar heterojunction architectures and answers a critical question as to the role of the anion and excess organic component during crystallization. Our work paves the way to tune the crystal growth kinetics by simple chemistry.

The solid-state dye-sensitized solar cell (DSSC) was introduced to overcome inherent manufacturing and instability issues of the electrolyte-based DSSC and progress has been made to deliver high photovoltaic efficiencies at low cost. However, despite 15 years research and development, there still remains no clear demonstration of long-term stability. Here, solid-state DSSCs are subjected to the severe aging conditions of continuous illumination at an elevated temperature. A fast deterioration in performance is observed for devices encapsulated in the absence of oxygen. The photovoltaic performance recovers when re-exposed to air. This reversible behavior is attributed to three related processes: i) the creation of light and oxygen sensitive electronic shunting paths between TiO2 and the top metal electrode, ii) increased recombination at the TiO2/organic interface, and iii) the creation of deep electron traps that reduce the photocurrent. The device deterioration is remedied by the formation of an insulating alumino-silicate shell around the TiO2 nanocrystals, which reduces interfacial recombination, and the introduction of an insulating mesoporous SiO2 buffer layer between the top electrode and TiO2, which acts as a permanent insulating barrier between the TiO2 and the metal electrode, preventing shunting. Encapsulated solid-state dye-sensitized solar cells (ssDSSCs) show a reversible and quick deterioration in performance while aging under inert atmosphere. This is attributed to the activation of deep traps and the change in Schottky barrier at the TiO2 surface. This is remedied by forming an insulating alumino-silicate "shell" around the TiO2 nanocrystals, and by introducing an insulating mesoporous SiO2 "buffer layer" between the top electrode and TiO2. © 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Reversible photo-induced performance deterioration is observed in mesoporous TiO2-containing devices in an inert environment. This phenomenon is correlated with the activation of deep trap sites due to astoichiometry of the metal oxide. Interestingly, in air, these defects can be passivated by oxygen adsorption. These results show that the doping of TiO2 with aluminium has a striking impact upon the density of sub-gap states and enhances the conductivity by orders of magnitude. Dye-sensitized and perovskite solar cells employing Al-doped TiO2 have increased device efficiencies and significantly enhanced operational device stability in inert atmospheres. This performance and stability enhancement is attributed to the substitutional incorporation of Al in the anatase lattice, "permanently" passivating electronic trap sites in the bulk and at the surface of the TiO2. © 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.