Opto-electronic properties of ITO/CuPc/NiPc/Al and ITO/NiPc/CuPc/Al double junction cells

Document Type : Original Article

Authors

1 St.Cloud State University, 720 Fourth Avenue South St.Cloud, MN 56301-4498, USA.

2 GIK Institute of Engineering Science and Technology, Topi 23640, Swabi, N.W.F.P, Pakistan.

Abstract

Abstract:
Thin organic semiconductor films of copper phthalocynanine (CuPc), Nickel
Phthalocyanine (NiPc) and a metal (Al) were deposited on a conductive glass (ITO)
substrate using vacuum evaporation technique to fabricate ITO/CuPc/NiPc/Al and a
ITO/NiPc/CuPc/Al cells. These structures have double junction: heterojunctions (CuPc-
NiPc, and NiPc-CuPc) and Schottky barrier (CuPc-Al, and NiPc-Al) junctions. This
study investigates the effect of the thickness on the properties of the cells (The
thicknesses of each organic semiconductor films in the three cells were equal to 25 nm,
50 nm and 75 nm.). The effective surface area of the samples was equal to 80 mm2
(8x10 mm2). The cells were carried through an annealing process at 150oC for 2 hrs and
its effect on the properties of the cells was studied. The voltage-current characteristics
under dark conditions as well as under filament lamp illumination were investigated.
The open-circuit voltage (Voc) and short circuit current (Isc) illumination relationships,
the dependences of photo-capacitance (Cph) on illumination and under modulated Red
LED light frequency were studied. The absorbance of double CuPc/NiPc layer in the
visible-IR spectrum was also examined. It was observed that the I-V characteristics
rectification ratios was upto 13; the ratio of the photo- to dark-currents were in the range
of 3-6; Voc and Isc reached 400 mV and 0.2 􀈝A respectively and the relative value of Cph
increased by 50 % under illumination of 2000 lx. The frequency response of the Voc, Isc
and Cph was investigated in the range 0-100 kHz. The Voc (DC) and Cph showed a
frequency response up to 100 kHz, whereas Voc(AC)’s response was up to 100 Hz
respectively. It was found that the overall performance of the ITO/NiPc/CuPc/Al cells is
better than the ITO/CuPc/NiPc/Al cells. Using the experimental data, the charge
transport mechanism in the double junction structures are discussed and the energy-banddiagram and an equivalent circuit were developed.

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