Origin of open circuit voltage in planar and bulk heterojunction organic thin-film photovoltaics depending on doped transport layers

The aim of this article is to investigate the origin of the open circuit voltage (V-oc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C-60 as acceptor material and an oligophenyl-derivative 4,4'-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the V-oc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, V-oc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, V-oc, is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C-60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a V-oc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced V-oc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed. (C) 2008 American Institute of Physics.