What is all this hoopla around perovskite solar cells? Why aren't they on the market yet if they're so great?

NREL best research cell efficiency chart

Perovskite solar cells are a recent phenomenon. They've experienced an unprecendented rise in record cell efficiency and seem to naturally have fewer issues with defects than many other competing technologies. They also seem amenable to low-cost production techniques. That being said, many issues need to be overcome in order for them to be commercialized.

"Perovskite unit cell" by Sevhab - Own work. Licensed under CC BY-SA 4.0
via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:
Perovskite_unit_cell.png#/media/File:Perovskite_unit_cell.png

Perovskites are based around the perovskite crystal structure, as shown above. This structure is repeated in all directions to form a crystal. Unfortunately, most of the high-performance cells contain lead but, since these are thin films, the amount of lead is pretty small and it is (relatively) safely stored beneath protective coatings. The word "halogen" refers to atoms in the second to last (17th, or 7A) column in the Periodic Table including F, Cl, Br, I, and At. In many of these cells, the halogen used is usually iodine (I), although chlorine (Cl) and bromine (Br) are also common. Methylammonium is a small organic ion.
This perovskite material is the part of the cell that absorbs the light. Electron and hole transport layers on either side of this material selectively remove charge carriers that have been created when light is absorbed.
Perovskite solar cells have a ways to go before becoming a marketable product. Solar cells on the market today have 25+ year warranties. These perovskite cells currently have issues with durability. Presently, they decompose when exposed to water, including water vapor in the air, so careful sealing is required (they actually turn yellow). Furthermore, when held under conditions similar to those of device operation (constant forward bias), the ions start to move and collect at either side of the device, causing performance degradation. The internal field cancels the field created by the light-induced field. Below is a plot of J-V curves taken with different voltage sweep rates. You can see that there are serious reproducibility issues under conditions useful for actual solar cell use.

W. Tress et al.,  Energy Environ. Sci., 2015, 8, 995-1004. The curve on the left represents actual J-V outputs, whereas the curve on the right shows J-V outputs normalized at -0.2 V. You can see the hysteresis in these curves. Hysteresis means that the curve made by varying the x-axis in one direction is different from the curve made by varying the x-axis in another direction.

These cells need to be more durable in order to be practical. While promising, more research is needed to get them to this point. They do represent a promising technology that definitely deserve the hype and excitement!

To learn more check out:
Will perovskite solar cells live up to their promise?
Perovskite solar cells could beat the efficiency of silicon
To succeed, solar perovskites need to escape the ivory tower