Right now, silicon-based photovoltaics rule the production lines. That's good, in the sense that the silicon is cheap and abundant. But the form used in photovoltaic panels has to be exceptionally pure and processed heavily, which adds significantly to its cost. For that reason, research has continued into alternative materials for use in solar cells.

Based on the frequency that they appear in scientific journals, there's a class of substances that have materials scientists excited: perovskites. Originally named after a mineral, "perovskite" is now used to refer to any material that adopts the same crystalline structure as calcium titanium oxide.

Perovskites have some significant advantages, in that they can also be made from abundant and cheap elements, and many types of perovskite crystals will form spontaneously from a saturated solution. There are some downsides, however, as one of the best photovoltaic materials contains lead, which is toxic. Another problem is that one of the layers in perovskite cells tends to degrade rapidly in use. A just-published paper describes a new perovskite photovoltaic that, while still reliant on lead, gets rid of the problematic layer entirely.

All photovoltaics work according to the same basic principle: a photon strikes with enough energy to free an electron from an atom. This creates a free charge and a positively charged atom, termed a "hole." While the electron is obviously mobile, so is the hole, which can migrate as atoms steal electrons from their neighbors. In most cases, the electron and hole rapidly recombine. The challenge of making an efficient solar cell involves keeping this from happening before the electron can be harvested as electrical current.

The simplest way to do this is to separate the charge and hole as quickly as possible. In perovskite photovoltaics, this has traditionally involved a layer dedicated to moving the hole. Unfortunately, that layer has traditionally relied on an organic material that tends to degrade in sunlight, limiting the effective lifespan of the material.

The new perovskite material simply leaves the holes in place while clearing out the electrons quickly. The perovskite itself is formed in pores of a layer of titanium dioxide simply by infusing a solution into the pores and allowing crystals to grow in place. The crystals are formed of lead and iodine atoms, coordinated with methyl ammonium. The researchers found that spiking this solution with a bit of a second ammonia-based compound (ammoniumvaleric acid) increased crystal growth. They suggest that the second chemical coated the titanium dioxide, creating an environment that encouraged crystal formation.

This structure allows the electrons that are ejected from the perovskite to quickly move to the titanium dioxide, where they can be harvested. The key to keeping them from recombining with holes is a layer of zirconium dioxide, which has a band gap that keeps the electrons away from the layer where the holes gather.

The resulting solar cell isn't as efficient as silicon; it has a conversion efficiency of 12 percent, while silicon is above 20 percent. But the ability to lay down photovoltaic materials from solution makes perovskites pretty appealing, and all the materials involved are made from what the authors term "Earth abundant" elements. But the best news is that the material is durable, with performance being stable for more than 1,000 hours of operation, at which point the researchers stopped testing.

Science, 2014. DOI: 10.1126/science.1254763 (About DOIs).