Two-Dimensional GIWAXS Reveals a Transient Crystal Phase in Solution-Processed Thermally Converted Tetrabenzoporphyrin

Abstract

Thermally convertible organic materials are useful for the fabrication of multilayered thin film electronic devices such as solar cells. However, substantial changes in molecular ordering can occur during the conversion process that may lead to multiple polymorphs having differing electronic properties. In-situ grazing incidence wide-angle X-ray scattering with 2-D detection (2-D GIWAXS) was used to study the changes in the thin film crystal structure, texture, and crystallite size of a convertible small-molecule electron donor, tetrabenzoporphyrin (BP), during thermal conversion from the precursor bicycloporphyrin (CP) and the resulting crystal-crystal phase transition from a metastable phase (phase I) to a stable phase (phase II). The annealing temperature and the presence of an underlying BP layer both affect the phase-transition behavior. Phase II has a much weaker degree of crystalline texture than phase I, attributed to changes in molecular packing to achieve a herringbone arrangement. The unit cell for phase I was determined by electron diffraction and GIWAXS, and the thin film structure of phase II matched the previously determined bulk structure. The texture of crystallites in phase II was characterized by the simulation of the GIWAXS pattern. Transmission electron microscopy revealed differences in the morphology, grain size, and film coverage of the two polymorphs. Peak shape analysis with corrections for geometric smearing and paracrystalline disorder showed an increase in crystallite size from phase I to phase II. These results demonstrate the utility of in-situ 2-D GIWAXS in revealing polymorphic phases during the structural transition of thermally convertible organic semiconductors, the presence of which may impact the performance of solar cells.

Substrate‐Induced and Thin‐Film Phases: Polymorphism of Organic Materials on Surfaces

“…For certain compounds like pentacene ( 1 ), C 8 O‐BTBT‐OC 8 ( 12 ) or Pc ( 16 ), the SIP is transitory in nature, i.e., it transforms to the bulk structure either by aging or when initiated by an external stimulant such as temperature or solvent vapor. A better understanding of the phenomenon could likely be obtained by in situ studies; recent in‐situ X‐ray scattering studies show that they may be a powerful and useful tool to study SIPs . Moreover, the growth mechanisms of the SIPs could also be monitored by in‐situ imaging techniques .…”

GIWAXS-SIIRkit: scattering intensity, indexing and refraction calculation toolkit for grazing-incidence wide-angle X-ray scattering of organic materials

“…For similar reasons, GIWAXS has also become popular in understanding perovskite thin films (Lilliu et al, 2016;Chen et al, 2018;Zhong et al, 2018). Other nanoscale thin-film materials benefit from GIWAXS as well: a few examples include polyethylene (Sasaki et al, 2007), porphyrins (Liman et al, 2013;Kim, Lee et al, 2012), block copolymers (Liu et al, 2012;Lin et al, 2014) and nanoparticlebased superlattices (Corricelli et al, 2014).…”

Structural stability of naphthyl end-capped oligothiophenes in organic field-effect transistors measured by grazing-incidence X-ray diffraction in operando

“…As the performance of OFETs depends on the thin-film structures, various aspects and processing steps of conjugated oligomers and polymers have been studied by GIXRD including alignment [24], phase behavior [25,26], solution shearing [27], solvent vapour annealing [28], roll-to-roll processing [29], doctor-blading [30] and spray-processing in real time [31]. However, apart from the GIXRD studies of Liscio and co-workers [13] the GIXRD literature of OFETs in operando is less comprehensive.…”