Origin of domain structure in perovskite hybrid piezoelectric responses

Detailed for the first time in 1987 and successfully applied since 2009 in photovoltaic (PV) devices,1.2 Methylammonium lead iodide (MAPbI3) crystallizes in a tetragonal perovskite structure at room temperature.3

Due to the fact that perovskite materials generally exhibit ferroic and MAPbI properties3 has an additional central polar methylammonium ion (2.3 D),4 researchers have debated at length a potential ferroelectric and ferroelastic nature of the material.5–7

Ferroelectricity arises from a change in crystal symmetry when a ferroelectric phase transition occurs; this is where the loss of central symmetry leads to a sudden electrical polarization.

To reduce electrostatic energy, the crystal forms polarization domains aligned in parallel. Depending on the orientation of the domain polarization with respect to the walls between different domains, the walls can conduct a bound load.

This load can be compensated by free load supports fitted to the 2D conductive plates.8 For MAPbI3, theorists indicated that domain walls with alternating charges could act as selective charge carrier pathways, reducing recombination in the PV material.5

Ferroelasticity is attributed to a change in the crystal system, for example, from a cubic structure to a tetragonal structure during a ferroelastic phase transition.

The unit cell distortion establishes an internal stress in the material which decreases by the formation of domains with fluctuating unit cell orientations.8

Structural anomalies at the domain walls among ferroelastic domains can create a local loss of central symmetry and, therefore, produce domain wall polarization.9 For MAPbI3, theorists have suggested a similar effect which is advantageous for the directed extraction of charge carriers.ten

Classification and characterization of ferroelectric domains requires a contact mode atomic force microscopy (AFM) technique – Piezoelectric Force Microscopy (PFM) – which resolves local electromechanical responses with nanometric resolution.11.12

In PFM, the surface of a ferroelectric sample is scanned using a conductive tip mounted on a cantilever while introducing an AC voltage between the tip and a counter electrode below the sample. Since all ferroelectric materials are piezoelectric, the applied alternating voltage applies a periodic strain of the sample, known as the piezoelectric response (Figure 1a).

Depending on the orientation of the polarization in the areas below the tip, for the areas with a polarization perpendicular to the sample surface, the piezo response is either in phase or out of phase by 180 ° with the applied AC voltage.11

The PFM introduces low frequency alternating voltages in its basic configuration. This technique, called non-resonance PFM, is less vulnerable to topographic crosstalk because it works away from the cantilever contact resonance (3 to 5 times the free resonance).

However, materials with inherently brittle piezoelectric response or thin films may require an increase in signal-to-noise ratio by selecting an AC voltage near or at the contact resonance of the cantilever.11

Although this advanced single-frequency resonance PFM technique improves the sensitivity to the piezoelectric response, the detected signal becomes more vulnerable to crosstalk, for example, from the topography and mechanics of the sample.

The frequency of the contact resonance is highly dependent on stable, unmodified tip-to-sample contact, which is difficult to achieve when scanning, especially on samples with a lot of topography or heterogeneous nanomechanics as expected on domains of ferroelastic nature.13

Figure 1. a) Schematic working principle of piezoelectric force microscopy (PFM), where an alternating voltage introduces periodic oscillation of the sample, i.e. the piezoelectric response, during a contact mode scan. The piezoelectric response is detected by deflecting the optical beam using a superluminescent diode and a position sensitive photodetector. b) Amplitude of the contact resonance CR1 at frequency f1 in the frequency spectrum of the AC excitation with the two side bands SB generated at f1± f1m used for dual frequency resonance tracking (DFRT). Image Credit: Park Systems Europe

Dual Frequency Resonance Tracking (DFRT), available by combining a Park Systems NX10 AFM with an HF2 locking amplifier from Zurich Instruments (see Application Note 56), helps differentiate between true electromechanical signal contributions and Mechanical or topographic crosstalk using additional feedback that follows contact resonance.

The HF2 produces sidebands to the left and right of the contact resonance at frequencies determined by the bandwidth at the maximum half of the contact resonance for DFRT (Figure 1b).

The feedback contrasts the amplitudes of the two sidebands continuously and changes the frequency of the AC voltage to maintain a constant amplitude ratio during the PFM sweep.

Thus, DFRT not only adds another signal channel for local visualization of the contact resonance distribution, which enables qualitative resolution of mechanical heterogeneities,13 but also allows a significant reduction in crosstalk

The piezo response was imaged on a MAPbI3 film via DFRT and resolved an out-of-plane domain with specific PFM amplitude and phase signals as well as a corresponding offset of the contact resonant frequency for this application note.

The frequency signal demonstrates a mechanical comparison between the domain and the bulk grain, indicating a ferroelastic character of the domain.

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The references

  1. Poglitsch, A. & Weber, D. Dynamic disorder of methylammonium (II) trihaloplombates observed by millimeter wave spectroscopy. J. Chem. Phys. 87, 6373-6378 (1987).
  2. Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometallic halide perovskites as visible light sensitizers for photovoltaic cells. Jam. Chem. Soc. 131, 6050-6051 (2009).
  3. Stoumpos, CC, Malliakas, CD & Kanatzidis, MG Perovskites semiconductor tin and lead iodide with organic cations: phase transitions, high mobilities and photoluminescent properties in the near infrared. Inorg. Chem. 52, 9019–9038 (2013).
  4. Wilson, JN, Frost, JM, Wallace, SK & Walsh, A. Dielectric and ferroic properties of metal halide perovskites. APL Mater. seven, 10901 (2019).
  5. Frost, JM et al. Atomic origins of high performance in hybrid halide perovskite solar cells. Nano Lett. 14, 2584-2590 (2014).
  6. Röhm, H., Leonhard, T., Hoffmann, MJ & Colsmann, A. Ferroelectric domains in thin films of perovskite with lead iodide and methylammonium. Energy Approx. Sci. ten, 950-955 (2017).
  7. Hermès, IM et al. Ferroelastic fingerprints in lead methylammonium iodide perovskite. J. Phys. Chem. VS 120, (2016).
  8. Tagantsev, AK, Cross, LE & Fousek, J. Domains in ferroic crystals and thin films. 13, (Springer, 2010).
  9. Zubko, P., Catalan, G., Buckley, A., Welche, PRL & Scott, JF Stress gradient induced polarization in single crystals of SrTiO 3. Phys. Rev. Lett. 99, 167601 (2007).
  10. Warwick, AR, Íñiguez, J., Haynes, PD & Bristowe, NC Investigation of the first principles of ferroelastic twins in halide perovskites. J. Phys. Chem. Lett. ten, 1416-1421 (2019).
  11. Soergel, E. Piezoresponse force microscopy (PFM). J. Phys. D Appl. Phys. 44, 464003 (2011).
  12. Güthner, P. & Dransfeld, K. Local pole of ferroelectric polymers by scanning force microscopy. Appl. Phys. Lett. 61, 1137-1139 (1992).
  13. Rodriguez, BJ, Callahan, C., Kalinin, S. V & Proksch, R. Atomic force microscopy with dual frequency resonance tracking. Nanotechnology 18, 475504 (2007).

This information has been obtained, reviewed and adapted from documents provided by Park Systems Europe.

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