On the optical anisotropy of conjugated polymer thin films

In this paper the problem of optical anisotropies in conjugated polymer thin films is revisited and the results in the literature are critically reappraised. Several models for upper bounds on the different types of anisotropies are developed and these are compared with experiment. We analyze both in-plane/out-of-plane and in-plane anisotropies in spin coated and oriented films, respectively. For the in-plane/out-of-plane anisotropy in spin coated films we study the reasons for the large disparity amongst the results reported in the literature, and propose the method of interference enhancement variable angle spectroscopic ellipsometry to retrieve this anisotropy more accurately. Experimental results are reported for poly(9,9-dihexyl fluorene) (spin coated), poly(9,9-dioctylfluorene) (both spin coated and uniaxially aligned), and poly(9,9-dioctylfluorene-co-benzothiadiazole) (both spin coated and uniaxially aligned). Our results concerning the measurement of optical anisotropy can be applied equally well to other molecular systems, such as self-assembled monolayers or Langmuir--Blodgett films.