Justin E. Cochran

Solution-processed nanostructured benzoporphyrin with polycarbonate binder for photovoltaics.

Figure 1 . (a) Chemical transformation of BP precursor (CP) to benzoporphyrin (BP) during annealing; (b) standard BP OPV device; (c) TGA analysis of PC and CP; (d) schematic of processing method used to form fi lms of BP for morphology analysis. Organic photovoltaics (OPV) based on bulk heterojunctions (BHJs) have effi ciencies above 7% [ 1 , 2 ] and can be processed from solution at potentially low cost. [ 3 ] BHJs comprise a blend of an electron donor and an electron acceptor in a phase separated morphology that provides a large interfacial area for photogeneration of electron-hole pairs. [ 4 ] The highest performance OPVs currently comprise blends of semiconducting polymers and fullerenes, such as [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). [ 4 ] Despite their performance in BHJs, semiconducting polymers often take many complicated steps to synthesize and may be diffi cult to purify. [ 5 , 6 ]

Two-dimensional GIWAXS reveals a transient crystal phase in solution-processed thermally converted tetrabenzoporphyrin.

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.