Balzer Frank, Breuer Tobias, Witte Gregor, Schiek Manuela
SDU Centre for Photonics Engineering, University of Southern Denmark, Sønderborg DK-6400, Denmark.
Department of Physics, Philipps University of Marburg, Marburg D-35032, Germany.
Langmuir. 2022 Aug 2;38(30):9266-9277. doi: 10.1021/acs.langmuir.2c01023. Epub 2022 Jul 20.
Controlling the polymorph formation in organic semiconductor thin films by the choice of processing parameters is a key factor for targeted device performance. Small molecular semiconductors such as the prototypical anilino squaraine compound with branched butyl chains as terminal functionalization (SQIB) allow both solution and vapor phase deposition methods. SQIB has been considered for various photovoltaic applications mainly as amorphous isotropic thin films due to its broad absorption within the visible to deep-red spectral range. The two known crystalline polymorphs adopting a monoclinic and orthorhombic crystal phase show characteristic Frenkel excitonic spectral signatures of overall H-type and J-type aggregates, respectively, with additional pronounced Davydov splitting. This gives a recognizable polarized optical response of crystalline thin films suitable for identification of the polymorphs. Both phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation in spin-casted thin films depending on subsequent thermal annealing. By contrast, upon vapor deposition on dielectric and conductive substrates, such as silicon dioxide, potassium chloride, graphene, and gold, the polymorph expression depends basically on the choice of growth substrate. The same pronounced out-of-plane orientation is adopted in all crystalline cases, but with a surface templated in-plane alignment in case of crystalline substrates. Strikingly, the amorphous isotropic thin films obtained by vapor deposition cannot be crystallized by thermal postannealing, which is a key feature for the spin-casted thin films, here monitored by polarized in situ microscopy. Combining X-ray diffraction, atomic force microscopy, ellipsometry, and polarized spectro-microscopy, we identify the processing-dependent evolution of the crystal phases, correlating morphology and molecular orientations within the textured SQIB films.
通过选择加工参数来控制有机半导体薄膜中的多晶型物形成是实现目标器件性能的关键因素。小分子半导体，如具有支链丁基链作为末端官能化的典型苯胺基方酸化合物（SQIB），既可以采用溶液法也可以采用气相沉积法。由于SQIB在可见光到深红色光谱范围内具有广泛的吸收，它主要被认为适用于各种光伏应用中的非晶各向同性薄膜。已知的两种结晶多晶型物分别采用单斜晶相和正交晶相，显示出整体H型和J型聚集体的特征弗伦克尔激子光谱特征，并伴有明显的达维多夫分裂。这使得结晶薄膜具有可识别的偏振光学响应，适用于多晶型物的识别。在旋涂薄膜中，这两个相在随后的热退火作用下，都呈现出强烈的面外择优取向和相当随机的面内取向。相比之下，在诸如二氧化硅、氯化钾、石墨烯和金等介电和导电衬底上进行气相沉积时，多晶型物的表现基本上取决于生长衬底的选择。在所有结晶情况下都采用相同明显的面外取向，但在结晶衬底的情况下，面内取向是由表面模板化的。引人注目的是，通过气相沉积获得的非晶各向同性薄膜不能通过热退火结晶，而这是旋涂薄膜的一个关键特征，在这里通过偏振原位显微镜进行监测。结合X射线衍射、原子力显微镜、椭偏仪和偏振光谱显微镜，我们确定了晶相的加工依赖性演变，将织构化SQIB薄膜中的形态与分子取向相关联。
