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A Closer Look into 2-Step Perovskite Conversion with X-Ray Scattering

The morphology of the hybrid perovskite material CH3NH3PbI3-xClx was investigated using simultaneous GISAXS and GIWAXS. The conclusions on the crystallization mechanism helped to improve recently developed hybrid perovskite solar cells’ performance.

J. Schlipf et al. , J. Phys. Chem. Lett. 6 (7), 1265 (2015).



The hybrid material CH3NH3PbI3-xClx with perovskite crystal structure has recently led to a revolution in thin film photovoltaics. Since its inception in 2009, solar cell power conversion efficiencies have risen up to 20°% now challenging conventional inorganic photovoltaic technologies as perovskite solar cells promise cheap production from abundant precursor materials. However, fundamental understanding of material properties and especially the correlation of crystallization dynamics and film morphology lack behind the rapid device development. Especially in the very simple planar cell architecture these cells suffer from performance losses due to unreproducible film morphologies. Thus, understanding and ultimately controlling the crystal morphology of perovskite thin films is of great importance to achieve a reproducible high photovoltaic performance.

A very versatile approach for the synthesis of such hybrid perovskites is a 2-step technique, where the perovskite is obtained by conversion from a crystalline precursor film, lead iodide (PbI2), by dipping into a solution containing the organic cations. It has been reported before that optimization of the PbI2 layer is of utter importance for good performance in photovoltaic devices.

We conducted the first successful grazing-incidence small angle X-ray scattering (GISAXS) measurements on thin films of a hybrid perovskite and its respective precursor while simultaneously monitoring the crystal phase with grazing-incidence wide angle X-ray scattering (GIWAXS). With the latter we confirmed the crystal structure for our thin films. In GISAXS we made use of the fact that an incident angle below the critical angle of the material yields surface-sensitive data while higher angles penetrate the entire film and give information on the bulk morphology. By horizontal line cuts we gather information on the lateral film morphology in the bulk and on the surface of PbI2 and perovskite thin films. Using a distorted wave Born approximation (DWBA) and assuming a local monodisperse domain distribution (LMA) we simulated the GISAXS data and extracted structure information. We find a close correlation of crystal sizes in the precursor and the perovskite films. As during the conversion to perovskite these films usually exhibit a significant volume expansion due to the incorporation of CH3NH3 into the inorganic framework, it can be concluded that crystal growth is confined laterally. In fact, by a Williamson-Hall analysis on powder X-ray diffraction measurements certain amount of crystal strain is confirmed for the perovskite film. Furthermore, from our measurements it is possible to get insight into the distribution of crystal sizes (see Figure 1). Thus, it is apparent that not only does the ratio of smaller to larger crystals increase during conversion in favor of smaller crystals, but also that this effect is more severe inside the bulk.
 

 

Figure 1. Crystal size distribution of the hybrid perovskite and its precursor PbI2 extracted from simulation of GISAXS data.


This leads to the following model of the conversion mechanism present: As the crystal growth is constrained laterally the volume expansion of the crystals during conversion of PbI2 to perovskite is mainly vertical. This, however, requires a mechanical redistribution in within the film, i.e. due to the strain larger crystals crack up into smaller units that accumulate closer to the substrate. On the surface, on the other hand, the concentration of organic compounds is higher and facilitates the complete reconstruction of the domains and smaller crystals are dissolved or incorporated into larger ones by Ostwald ripening.
 

 

Figure 2. Scheme depicting the assumed conversion mechanism from PbI2 precursor to perovskite.

In conclusion, our results help to elucidate the origin of the morphology in such-way prepared hybrid perovskite thin films and thus aim in the improvement of synthesis methods for this type of material which is important for reproducible high photovoltaic performance.

 

 

Retrieve article
A Closer Look into Two-Step Perovskite Conversion with X-ray Scattering;
J. Schlipf, P. Docampo, C.J. Schaffer, V. Körstgens, L. Bießmann, F. Hanusch, N. Giesbrecht, S. Bernstorff, T. Bein, P. Müller-Buschbaum;
J. Phys. Chem. Lett. 6 (7), 1265-1269 (2015).
10.1021/acs.jpclett.5b00329


Last Updated on Monday, 13 May 2019 15:19