Large-area MoS(2) atomic layers are synthesized on SiO(2) substrates by chemical vapor deposition using MoO(3) and S powders as the reactants. Optical, microscopic and electrical measurements suggest that the synthetic process leads to the growth of MoS(2) monolayer. The TEM images verify that the synthesized MoS(2) sheets are highly crystalline.

Perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies, exhibiting rapidly rising efficiencies. This is likely to continue to rise, but in the development of these solar cells there are unusual characteristics that have arisen, specifically an anomalous hysteresis in the current-voltage curves. We identify this phenomenon and show some examples of factors that make the hysteresis more or less extreme. We also demonstrate stabilized power output under working conditions and suggest that this is a useful parameter to present, alongside the current-voltage scan derived power conversion efficiency. We hypothesize three possible origins of the effect and discuss its implications on device efficiency and future research directions. Understanding and resolving the hysteresis is essential for further progress and is likely to lead to a further step improvement in performance.

Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown significant progress recently, o ering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both di cult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room-temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance as being due to spontaneous freecarrier generation following light absorption. Additionally, we determine the excitonic reduced e ective mass to be 0.104m e (where m e is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties.T he recent rapid development of perovskite solar cells is revolutionizing the photovoltaic research field, with the latest certified power conversion efficiencies reaching over 20% (ref. 1). Initially developed from the concept of the nanostructured excitonic solar cell where there is no requirement for longrange charge or exciton diffusion 1-8 , it has now become clear that owing to the remarkable properties of the inorganic-organic perovskite family of materials ABX 3 (A = CH 3 NH 3 + ; B = Pb 2+ ; and X = Cl − , I − and/or Br − ) these cells 2-10 are capable of operating in a comparable configuration and with comparable performance to the best inorganic semiconductors 7,9,10 , where the solid absorber layer is sandwiched between n-and p-type charge selective contacts in a planar heterojunction configuration 7,8 . Despite this success, several fundamental properties of the organic lead tri-halide perovskites remain controversial and poorly known. In particular the binding energy of the excitons (R * ), bound electron-hole pairs that are the primary photoexcited species created in the absorption process, is vital to understanding the way that the cells function. The operating mechanisms depend on what fraction of excitons dissociate in the bulk material, giving rise to free-charge transport, or what fraction need to be dissociated at heterojunctions within the cells. Knowledge of the true exciton binding energy is also crucial for interpreting spectroscopic measurements based on these materials, such as time-resolved spectroscopy. Values for R * reported in the literature cover a broad range from 2 to 55 meV (refs 11-17), with the larger values being initially adopted and a growing number of reports suggesti...

We demonstrate four- and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2-electron volt band-gap perovskite, FACsSnPbI, that can deliver 14.8% efficiency. By combining this material with a wider-band gap FACsPb(IBr) material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with >1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable "all-perovskite" thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.

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