Probing the Microstructure of Methylammonium Lead Iodide Perovskite Solar Cells

Abstract

The microstructure of absorber layers is pivotally important for all thin‐film solar technologies. Using electron backscattered diffraction (EBSD), the crystal orientation in methylammonium lead iodide thin films with submicrometer resolution is reported. For the vast majority of (110) oriented grains, the c‐axis of the perovskite unit cell is oriented in‐plane. Although some adjacent grains exhibit the same in‐plane horizontal orientation of the c‐axis, no universal horizontal orientation of the c‐axis within the sample plane exists. The (110) crystal orientation correlates with an in‐plane orientation of the ferroelectric polarization as investigated by vertical and lateral piezoresponse force microscopy (PFM). The individual grains with different crystal orientations that exhibit different ferroelectric patterns and surface potentials are identified. The strong correlation between crystal orientation and ferroelectric polarization allows conclusions to be drawn about the microstructure from PFM measurements and, likewise, the ferroelectric polarization to be derived from crystallographic observations by EBSD.

Ferroelectric Properties of Perovskite Thin Films and Their Implications for Solar Energy Conversion

“…By measuring lateral (torsion of the cantilever) or vertical (deflection) movements of the AFM tip near the respective resonance frequencies, lateral (in-plane, L-PFM) and vertical (out-of-plane, V-PFM) crystal responses can further be distinguished. [21] The weak domain pattern in V-PFM may stem from a vertical deflection of the scanning cantilever by the lateral deformation of the crystal (buckling effects). However, Vorpahl et al have demonstrated that dual-resonance tracking PFM obtains similar results to sf-PFM data.…”

“…The corresponding L-PFM phase images typically show a distinct 180° phase contrast between stripes. [21] Therefore, these stripes represent crystal domains of alternating polarization. As depicted in Figure 3d, on the vast majority of grains, these domains are hardly visible in V-PFM which lets us conclude that the direction of polarization in these grains is oriented in-plane.…”

“…A multitude of measurement techniques was employed in order to probe ferroic properties of single crystal samples, [5,6] pressed powder pellets, [7] and polycrystalline thin-films [8] of OMH perovskites. Techniques that probe large areas of specimens such as X-ray diffraction (XRD), [21] impedance spectroscopy, [22,23] and J-V characterization [5,7] added a rather spatially averaged picture of the samples' microstructure. Some of these studies revealed ordered domains within crystal grains, but no consensus has been reached about the interpretation of their ferroic and crystallographic origins.…”

“…Scanning microscopy techniques such as atomic force microscopy (AFM), [9][10][11][12] scanning electron microscopy (SEM), [13] and tunneling electron microscopy (TEM) [14] were extensively used in order to reveal the microstructure of OMH-perovskite thinfilm samples with subgrain resolution. Techniques that probe large areas of specimens such as X-ray diffraction (XRD), [21] impedance spectroscopy, [22,23] and J-V characterization [5,7] added a rather spatially averaged picture of the samples' microstructure. Claims included piezoelectricity, [15][16][17] pyroelectricity, [5] ferroelasticity, [9,12,18] antiferroelectricity [19] as well as ferroelectricity, [10,11,15,16,20] and the seemingly contradictory reports on these ferroic properties have heated up the discussion.…”

“…Claims included piezoelectricity, [15][16][17] pyroelectricity, [5] ferroelasticity, [9,12,18] antiferroelectricity [19] as well as ferroelectricity, [10,11,15,16,20] and the seemingly contradictory reports on these ferroic properties have heated up the discussion. Techniques that probe large areas of specimens such as X-ray diffraction (XRD), [21] impedance spectroscopy, [22,23] and J-V characterization [5,7] added a rather spatially averaged picture of the samples' microstructure. For example, second harmonic generation (SHG) measurements were employed in order to identify whether or not OMH perovskites form polar crystals, but their interpretation led to ambiguous results.…”