Electroabsorption studies of organic p-i-n solar cells:Increase of the built-in voltage by higher doping concentration in the hole transport layer

The built-in voltage in solar cells has a significant influence on the extraction of photogenerated charge carriers. For small molecule organic solar cells based on the p-i-n structure, we investigate the dependence of the built-in voltage on the work function of both the hole transport layer and the electrode material. The model system investigated here consists of a planar heterojunction with N, N, N', N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as donor and Buckminster Fullerene (C-60) as acceptor material. A higher concentration of the dopant C60F36 in the hole transport layer induces a shift of the work function towards the transport level. The resulting increase of the built-in voltage is studied using electroabsorption spectroscopy, measuring the change in absorption (Stark effect) caused by an externally applied electric field. An evaluation of these electroabsorption spectra as a function of the applied DC voltage enables the direct measurement of the built-in voltage. It is also shown that an increased built-in voltage does lead to a larger short-circuit current as well as a larger fill factor. (C) 2013 Elsevier B.V. All rights reserved.