Advanced Functional Materials and Devices (AFMD) Group

Abstract:

An equivalent circuit to characterize energy distribution of trap states present in organic semiconductors by impedance spectroscopy is proposed. We analyze the impedance spectra of a small-molecule organic solar cell and observe the contribution of trap states at low frequencies. Starting from previously reported equivalent circuits and a theoretical model based on the integration of a general traps distribution, we develop an equivalent circuit, which is able to describe the energetic distribution of trap states typically observed in organic semiconductors. The experimental data can be reproduced by our equivalent circuit, and we estimate a density of trap states in a Zn-phthalocyanine:C60 bulk heterojunction to be about 1.9 0.6 10 16 cm -3 eV -1. © 2012 American Institute of Physics.

Abstract:

Recombination contacts play an important role in highly efficient organic tandem solar cells. We present a photoelectron spectroscopy study on contact systems that have previously been shown to work efficiently as recombination contacts. Here, the conversion of an electron current into a hole current is realized either by insertion of gold clusters or by a highly doped pn-junction. From the measured energy level alignments, we show that the working principles of these two approaches are significantly different. For gold clusters, the recombination current is promoted by an accumulation of charge carriers, while for doped pn-junctions, it is achieved by tunneling through a depletion layer. © 2012 American Institute of Physics.

Abstract:

2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I) was synthesized and employed as a strong n-type dopant for fullerene C(60), a well-known n-channel semiconductor. The coevaporated thin films showed a maximum conductivity of 5.5 S/cm at a doping concentration of 8.0 wt% (14 mol%), which is the highest value reported to date for molecular n-type conductors. o-MeO-DMBI-I can be stored and handled in air for extended periods without degradation and is thus promising for various organic electronic devices.