The orientation of the constituent molecules in organic thin film devices can affect significantly their performance due to the highly anisotropic nature of π-conjugated molecules. We report here an angle dependent x-ray absorption study of the control of such molecular orientation using well-ordered interlayers for the case of a bilayer heterojunction of chloroaluminum phthalocyanine (ClAlPc) and C 60. Furthermore, the orientation-dependent energy level alignment of the same bilayer heterojunction has been measured in detail using synchrotron radiation-excited photoelectron spectroscopy. Regardless of the orientation of the organic interlayer, we find that the subsequent ClAlPc tilt angle improves the π-π interaction at the interface, thus leading to an improved short-circuit current in photovoltaic devices based on ClAlPc/C 60. The use of the interlayers does not change the effective band gap at the ClAlPc/C 60 heterointerface, resulting in no change in open-circuit voltage.
Bibliographical noteFunding Information:
This work was supported in part by the NSF under Grant No. CHE-0807368. The NSLS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Financial support from the EPSRC, UK is also acknowledged through the SUPERGEN Excitonic Solar Cell Consortium programme.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy (miscellaneous)