Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PICOPROP (Photo Induced Collective Properties of Hybrid Halide Perovskites)

Teaser

The recent discovery of the organo-inorganic perovskite CH3NH3PbI3 as very efficient material in photoelectric conversion is multifaceted: it turns out that this compound is promising not only in photovoltaics, but it is lasing, it gives bright light emitting diodes, promising...

Summary

The recent discovery of the organo-inorganic perovskite CH3NH3PbI3 as very efficient material in photoelectric conversion is multifaceted: it turns out that this compound is promising not only in photovoltaics, but it is lasing, it gives bright light emitting diodes, promising in water splitting and we are persuaded that it can play an important role in basic sciences, as well.
We have recently realized that under white light illumination the photoelectrons, due to their very long recombination time, stay in the conduction band and the resistivity of a single crystal shows a metallic behavior. If the lifetime is sufficiently long and the density of these excited carrier is high enough they could condense into a Fermi sea. The project’s goal is to realize this highly unusual state and to document its properties by magneto-transport and spectroscopic techniques. We will check in our model compound the long-sought superconductivity of photo-excited carriers, extensively searched for in cuprates, if we could stabilize it by fine tuning the interactions by hydrostatic pressure under constant illumination.
The availability of high quality samples is primordial for this program. It turns out that CH3NH3PbI3 is ideal compound, it seems to be almost free of charged defects (its room temperature resistance is 5 orders of magnitude higher than that of Phosphorus doped Silicon at 1013 cm-3 doping concentration) and we can grow excellent single crystals of it. Furthermore, it has a flexibility in material design: one can vary all the constituents, and even the dimensionality by making layered materials with the main chemical motifs. A special effort will be devoted to tune the spin-orbit coupling by different elements, since this could be at the origin of the long recombination time of the photo-electrons.
We suspect that the highly tunable, clean and disorder-free doping obtained by shining light on these ionic crystals opens a new era in material discovery.

Work performed

The important part of the project is to synthesize high quality single crystals at different scales, from nano sized quantum dots to mm sized single crystals was achieved. Furthermore, in the search to understand the high efficiency in photon to electricity conversion of the CH3NH3PbI3 we have produced a large number of other compounds, among them some of them are done for the first time. These are:
1. Ethylammonium lead iodide (New)
2. Propylammonium lead iodide
3. Butylammonium lead iodide
4. Octadecylamine lead iodide
5. Melamine lead iodide (New)
6. Triethylammonium lead iodide
7. Formamidine lead iodide
8. 3-Aminopyridinium lead iodide
9. Ethylenediammonium lead iodide (New)
10. Methylammonium bismuth bromide

In the control of the material processing we have discovered an important step which is the graphoepitaxial liquid-solid growth of nanowires of the photovoltaic compound CH3NH3PbI3 in open nanofluidic channels. These structures were essential for producing a very high sensitivity photodetector which was achieved by depositing the photovoltaic nanowire on graphene and carbon nanotubes.

The intrinsic d.c. electrical resistivity - measurable on single crystals only – is often the quantity first revealing the properties of a given material. In the case of CH3NH3PbI3 perovskite measuring under white light we have shown that illumination provides insight into the coexistence of extended and shallow localized states. The former ones dominate the electrical conduction while the latter, coming from neutral defects, serve as a long-lifetime charge carrier reservoir accessible for charge transport by thermal excitation. Remarkably, in the best crystals the electrical resistivity shows a metallic behaviour under illumination up to room temperature, giving a new dimension to the material in basic physical studies.

We reported the synthesis of a ferromagnetic photovoltaic CH3NH3(Mn:Pb)I3 material in which the photo-excited electrons rapidly melt the local magnetic order through the Ruderman–Kittel–Kasuya–Yosida interactions without heating up the spin system. Our finding offers an alternative, very simple and efficient way of optical spin control, and opens an avenue for applications in low-power, light controlling magnetic devices.

Final results

1. Spatial positioning of nanocrystal building blocks on a solid surface is a prerequisite for assembling individual nanoparticles into functional devices. We reported on the graphoepitaxial liquid-
solid growth of nanowires of the photovoltaic compound CH3NH3PbI3 in open nanofluidic channels. The presently discovered crystallization leads to the fabrication of mm2-sized surfaces composed of perovskite nanowires having controlled sizes, cross-sectional shapes, aspect ratios and orientation which have not been achieved thus far by other deposition methods. The automation of this general strategy paves the way towards fabrication of wafer-scale perovskite nanowire thin films well-suited for various optoelectronic devices, e.g. solar cells, lasers, light-emitting diodes and photodetectors.

2. Showing that the photoconductivity in high quality single crystals is metallic in CH3NH3PbI3 is also a unique report in this field.

3. We have synthesized few new photovoltaic perovskites which were not khown before.

4. The synthesis of a ferromagnetic photovoltaic CH3NH3(Mn:Pb)I3 material in which the photo-excited electrons rapidly melt the local magnetic order is the first report in which magnetism was melted by low intensity light due to electronic interactions. Our finding offers an alternative, very simple and efficient way of optical spin control, and opens an avenue for applications in low-power, light controlling magnetic devices.