Waltzing molecules for high-efficiency photovoltaics

Every hour the Sun hits Earth with enough energy to power all the human activity on the planet for an entire year. Harvesting only a small fraction of that energy would give us practically limitless, renewable power. One strategy to achieve this goal is to convert incoming energy into electrical power by means of photovoltaic (PV) devices. In these, light creates a pair of positively and negatively charged carriers (an electron-hole pair) that, once separated, produces an electrical current. The process is possible in virtually any semiconductor as long as the threshold for light absorption falls within the solar light emission spectrum.

However, the efficiency of the energy conversion is dramatically different in the various semiconductors, and the choice of semiconductor is affected equally by several other properties. Some of these properties are intrinsic to the material, such as the robustness to prolonged light exposure, or the presence of nontoxic and easy-to-recycle elements. Others are associated with the production pipeline, namely compatibility and ease of integration with other technologies. Finally, there are properties that have market-driven requirements, such as low cost. When all of these factors are taken into account, it is not surprising that very few PV materials have ever made it to the market, despite a worldwide effort spanning more than four decades. At present, 90% of PV modules are based on silicon (Si).

The recent synthesis of hybrid organic/inorganic halide perovskite compounds has raised expectations that a new revolutionary class of PV materials finally exists.¹ These are compounds with the standard AMX₃ perovskite structure, where X is a halide (iodine, I, or bromine, Br), M is lead (Pb), and the remaining cation position, A, is taken by an organic molecule, in the most common case methylammonium (CH₃NH₃⁺) (see Figure 1). Such compounds enable the growth of high-efficiency solar cells cheaply from the liquid phase,² and the cell's efficiency can be tuned by controlling the structural order and composition of the compounds.³ Particularly intriguing is that we may achieve high efficiencies even for planar cells,⁴ indicating that charge separation can occur in the hybrid perovskite absorber and that efficient charge diffusion takes place for both electrons and holes.⁵ The efficiency of these materials has increased rapidly, from an initial 3% to around 21% currently.

Figure 1. Structures of the cubic phase of perovskite CH₃NH₃PbI₃ (methylammonium lead iodide) for two different orientations of the CH₃NH₃⁺ cations, namely along (111) (a) and (011) (b). The views along the xy and xz planes are shown in the upper and lower panels, respectively. The PbI₃ octahedrons are gray with the iodine ions (I⁻) in purple at the corner.

Despite such explosive progress, the microscopic reason behind the high performance of these compounds has remained elusive. Recently, we demonstrated that it is the interplay between the organic and inorganic components that does the trick.⁶ Using state-of-the-art electronic structure calculations, we separated the organic molecules' contribution to the light absorption properties from that of the PbI₃ lattice, and we discovered that the two are very different.

In this halide perovskite, the PbI₃ inorganic lattice, whose electronic structure dominates both the bottom of the conduction band and the top of the valence band, absorbs light. The same lattice provides the pathway for the photogenerated charges to move within the crystal with mobilities that in general are quite poor compared with that of silicon. These features are no different from what usually happens in standard PV semiconductors. The molecules, however, play a very unusual role, which is found only here: they can dance!

Methylammonium (and other molecules tested so far) cannot absorb light, and is highly transparent to most of the light in the solar spectrum. However, these molecules can rotate almost freely within the crystal because there is a large space at their crystallographic site and their interaction with the inorganic part of the lattice is dominated by hydrogen bonds and is weak. As such, at room temperature these molecules constantly rotate with a timescale on the order of a few picoseconds. During their rotation they produce two fundamental effects on the inorganic lattice: they exert strain on the octahedral PbI₃ cage, and they generate a disordered electrostatic environment due to the changing orientation of their electric dipoles. Both these effects modify the electronic structure of the inorganic lattice in a dynamical way on a picosecond timescale. This reduces the binding energy of the photogenerated electron-hole pairs and prolongs their lifetime, boosting the material's efficiency.

In summary, we have described the subtle interaction between the organic molecules (which can ‘dance’) and the light-absorbing inorganic lattice in hybrid perovskites, to further the understanding of how these compounds may enable improved PV efficiency. This research will inform our future work developing high-performance materials for solar cells.