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that some of the efficiencies reported in the literature have been articially enhancedby hysteretic eects, and that these solarcells would not demonstrate the reported efficiency if they were held at a steady voltage for an extended period of time. Jeon and colleagues investigated the behaviourof the layers deposited with their methodby reporting currentvoltage curves in the forward and reverse scanning directions. They demonstrated that hysteresis canbe avoided by inltrating the perovskiteinto a mesostructured scaold of titania (Fig.2b). However, it is not clear why the mesostructured titania reduces the hysteresis with respect to the perovskite lms deposited on at substrates; further investigation ofthe role of the perovskite/titania interface as well as of the dynamics of the charge carriers in these architectures is needed to fully understand this anomalous behaviour.
Although the results reported by Jeon and colleagues provide encouraging solutions to deposit uniform perovskite lms and construct hysteresis-free solar cells, other issues remain to be addressed before perovskite solar cells can be turned into a game-changing technology. Througha combination of optimization strategies aimed at ne-tuning the perovskite chemical composition, improving perovskite lm quality, tuning the energy levels of the charge-extraction materials used in thecell, and harvesting a broader part of the solar spectrum by using stacked cells, the power-conversion efficiency has the potential to soar past 20%, thus making perovskites competitive with the silicon solar cells that currently dominate the market. Bearing this in mind, efficiency limitations are unlikelyto be the problem that slows the progress of perovskite solar cells towards commercial viability. Long-term stability, however, still presents a potential barrier for the success of perovskite semiconductors. Metalhalogen bonds tend to be weak and can sometimes
be broken when a photon is absorbed. This property is helpful for making photographic lm, but not desirable for solar cells. We already know that these perovskites are not stable in the presence of humidity as they are hygroscopic, but we do not know how long they can last when properly encapsulated. Close behind stability are toxicity concerns; the general populace is unlikely to embracea water-soluble lead compound for use in solar panels. Because of these concerns, the coming year will likely bring signicant research on perovskite stability and the development of non-toxic perovskites some of...