Tönu Pullerits, professor of chemical physics at Lund University, first of all notes that, though ubiquitously used among solar researchers, the term perovskite is a somewhat “sloppy” way of naming a specific crystal structure with an elemental content of ABC_3 — the currently very popular family of materials where A is a small organic molecule methyl ammonium (or similar), B is Pb (can be Sn or Bi too) and C is I, Cl or Br.
Perovskite’s not so popular issues are its instability, particularly its detrimental sensitivity to moisture. This new study could yield a solution.
“In our work, we have used very thin — in the order of 1 nm — plates of the material, where the outer layer is hydrophobic and protects it from moisture,” Pullerits reports. The problem however is that this protective layer also reduces conductivity. Thus, the multiple stacked layers needed to achieve adequate light absorption, simultaneously preserving efficient electron transport, would indeed prove challenging. “Therefore, one needs plates that would self-organize themselves to stand on edge, so that electrons can move along the plate from one contact to the other without needing to jump many times,” explains Pullerits, adding that in an ideal system the layers would in fact directly contact the electrodes.
A unique approach
The result of this study is unique in two ways: Firstly, a protective layer of such molecules to shield the material from moisture had not been used before and, according to the professor, is yielding the best results in their study. Secondly, Pullerits says, “The way we demonstrated the self-organization via laser spectroscopy, more precisely via time-resolved THz spectroscopy is unique. This has not been done so far.”
Furthermore, Pullerits and his colleagues where able to demonstrate that their perovskite plates even self-organized at room temperature. “Earlier analogous studies needed to apply annealing to obtain the effect,” he says.
Solving perovskite’s biggest problem
“We have made a serious step towards realizing stable perovskite solar cells,” says the expert. “If one can make self-protected perovskite material, the demands for encapsulation of the cells would be much easier to meet.” In the long run, he believes, this should lead to cheaper solar cells.
A surprising outcome
Pullerits and his team experimented with two different materials for the protective outer layer: One had been used before and the other one was a similar molecule but had a branched structure. “We first thought that it will lead to more disordered organization, but apparently, as far as standing up on the edge is concerned, the system is better organized,” the researcher reports.
Pullerits points out there are still many unanswered basic research questions around perovskites. “We still do not even fully understand how a system that has so many defects can make efficient solar cells,” he says. Questions his team seeks to answer as they carry on with this research endeavor include, for example, why these plates self-organize the way they do in the first place. Another, more practical question is how to achieve better control over these plates that form. “Right now, we have a mixture of the plates of various thicknesses,” he says. “I would expect that better control of the plates would make more efficient solar cells, but we do not know. Plenty to do.”
Results of the research are detailed in the article “Tailoring Organic Cation of 2D Air-Stable Organometal Halide Perovskites for Highly Efficient Planar Solar Cells,” published in Advanced Energy Materials.
Figure 1: Photographs of MAPbI3 (A), HC (n-BA)2(MA)3Pb4I13 (B), HC (isoBA)2(MA)3Pb4I13 (C), RT (n-BA)2(MA)3Pb4I13 (D) and RT (iso-BA)2(MA)3Pb4I13 (E) thin films on glass substrate over time without encapsulation, which are stored in an environmental chamber at 20 °C with a RH of 60%.
Written by Sandra Henderson, Research Editor, Solar Novus Today