Hybrid Perovskite Solarcell Materials

In 2009 a new perovskite solar cell material was discovered. The efficiency of this material improved faster than for any other material and currently the efficiency is above 20% for research solar cells. Existing knowledge surely aided the fast improvement, but the mechanism for the high efficiency is still not well understood. The material consists of an unusual combination of inorganic ions of lead and iodine forming cavities inhabited by organic methylammonium ions. In particular, the role of the organic ions is unclear. Claims that the ions form an ordered ferroelectric phase, somewhat similar to the ordering of spins that leads to magnetism in iron, have been put forward. The suggestion is that between such ordered domains highways for electrons and holes will be formed effectively preventing recombination of the two, which would lead to loss of the harvested energy. Less drastic mechanisms have been put forward as well, but so far it is unclear, why the methylammonium is needed. We studied the dynamics in these materials and discovered that the methylammonium move on two timescales. First of all they quickly rock a little bit back and fourth around their minimum energy positions along the crystal axes. Secondly, they make big orientational jumps of 90°, which happens every 3 picosecond on average.

Key references:

Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites A. A. Bakulin, O. Selig, H. J. Bakker, Y. L. A. Rezus, C. Müller, R. Lovrincic, Z. Sun, Z. Chen, A. Walsh, J. M. Frost, and T. L. C. Jansen, J. Phys. Chem. Lett. 6 3663-3669 (2015)