Department of Physics, National University of Singapore, Lower Kent Ridge Road, S117542 Singapore.
Langmuir. 2010 Oct 5;26(19):15494-507. doi: 10.1021/la1013494.
The dependence of morphology and polymer-chain orientation of regioregular poly(3-hexylthiophene) (rrP3HT) thin films on processing conditions have been widely studied. However, their possible variation across the film thickness direction remains largely unknown. We report here a marked difference in the optical dielectric (n,k) spectra between the top and bottom interfaces of spin-cast (sc) rrP3HT films deposited from chlorobenzene solutions. These spectra were obtained from reflection variable-angle spectroscopic ellipsometry using a self-consistent graded optical model with self-imposed Kramers-Krönig consistency. The top interface shows a red-shifted absorption that is characteristic of better order than at the bottom, across a wide range of film thicknesses. This disparity diminishes in drop-cast (dc) and multipass inkjet-printed (ijp) films, and disappears in amorphous films such as those of polystyrene and of a green-emitting phenyl-substituted poly(p-phenylenevinylene). The (n,k) spectra also reveal that crystallinity increases across sc < dc < ijp films. This is supported by cross section scanning electron microscopy of the cleaved edges and measurement of the microroughness of both the film interfaces. Furthermore, optical anisotropy decreases across sc > dc > ijp films. Finally, near-edge X-ray absorption fine structure spectroscopy also shows the frontier chains in ijp and dc films are more isotropically oriented than those in sc films. These results suggest that semicrystalline conjugated polymer films can be produced far from equilibrium. This explains the marked variation in their (opto)electronic properties between the top and bottom surfaces that has sometimes been found depending on the film deposition method. In particular, an unusually pronounced crystallization is induced by ijp. We label this marked ijp-induced crystallization the "ijp morphology", which appears to be general, as it is found also in single-inkjet-droplet films. It appears also to be responsible for the lower field-effect mobility measured for ijp films deposited on a variety of linear and circular electrode arrays. This however can fortuitously be reversed by annealing in solvent vapor. As all films were deposited in the low Peclet-number regime, we can rule out surface skin formation. We attribute the extensive crystallization to the non-uniform drying of picoliter droplets, further promoted by repeated film swelling-deswelling cycles in multipass-ijp films.
对形态和聚合物链取向的依赖 （rrP3HT） 薄膜的各向异性的处理条件的研究已经很广泛。然而，它们在薄膜厚度方向上的可能变化仍然很大程度上未知。我们在这里报道的旋涂（ sc ） rrP3HT 薄膜的顶部和底部界面之间的光学介电常数（ n ， k ）谱有明显的差异，这些光谱是使用具有自强加的克来默-克龙尼格一致性的自洽渐变光学模型从反射变角光谱椭圆偏振测量中获得的。顶部界面显示出吸收的红移，这表明其有序性比底部好，这在很宽的薄膜厚度范围内都是如此。这种差异在滴涂（ dc ）和多次喷墨印刷（ ijp ）薄膜中减小，并且在无定形薄膜如聚苯乙烯和绿色发射的苯基取代聚（对苯撑乙烯）中消失。（ n ， k ）谱还表明，结晶度在 sc 〈 dc 〈 ijp 薄膜中增加。这得到了从 cleaved edges 的横截面扫描电子显微镜和薄膜界面的 microroughness 的测量的支持。此外，光学各向异性在 sc 〉 dc 〉 ijp 薄膜中减小。最后，近边 X 射线吸收精细结构光谱也表明，在 ijp 和 dc 薄膜中的前沿链比在 sc 薄膜中更各向同性取向。这些结果表明，半结晶共轭聚合物薄膜可以在远离平衡的情况下产生。这解释了在顶部和底部表面之间发现的有时取决于薄膜沉积方法的其（光电）电子性质的明显变化。特别是，喷墨印刷诱导了显著的结晶。我们将这种明显的喷墨印刷诱导的结晶标记为“ ijp 形态”，它似乎是普遍的，因为它也存在于单喷墨液滴薄膜中。它似乎也负责为在各种线性和圆形电极阵列上沉积的 ijp 薄膜测量的较低的场效应迁移率。然而，这可以通过在溶剂蒸气中的退火来幸运地逆转。由于所有薄膜都是在低 Peclet 数范围内沉积的，因此我们可以排除表面表皮的形成。我们将广泛的结晶归因于 picoliter 液滴的不均匀干燥，在多次 ijp 薄膜中进一步促进了重复的薄膜溶胀-去溶胀循环。
