TITLE:
Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy
AUTHORS:
Ryoji Maeda, Hiroshi Tokairin, Hidetsugu Ikeda, Jun-Ichi Takahashi, Yuji Yoshida
KEYWORDS:
Organic Photovoltaic Cells, Impedance Spectroscopy, Modulus Analysis
JOURNAL NAME:
Advances in Materials Physics and Chemistry,
Vol.5 No.1,
January
2,
2015
ABSTRACT: The carrier
dynamics in organic photovoltaic (OPV) cells were investigated by impedance spectroscopy.
We introduced a novel impedance spectrum representation called dynamic modulus
plot (DMP), which allowed us to observe the layer-to-layer carrier injection
behavior graphically. In this work, the impedance responses were characterized
in the N,N’-diphenyl-N,N’-di-m-tolyl- 4,4’-diaminobiphenyl (TPD)/C60 p-n heterostructured OPV cells against applied voltages. The dependence of
impedance responses on the layer thickness revealed a constant internal
electric field that disturbed the carrier transport within the OPV cells. We applied this technique to new donor
materials, in which thiophene units were inserted to the center of TPD. By
increasing the number of thiophene units in TPD the fill-factor (FF) improved
from 33% to 59%, which increased the power conversion efficiency (PCE). Based
on the DMP analysis, we assigned the improvement in device performance to the
reduction of the internal electric field.