fullerene

Cent. Eur. J. Eng. • 2(2) • 2012 • 248-252 DOI: 10.2478/s13531-011-0069-7

Central European Journal of Engineering Effects of germanium addition to copper phthalocyanine/fullerene-based solar cells Research article Takeo Oku∗ , Kazuma Kumada, Atsushi Suzuki, Kenji Kikuchi Department of Materials Science, The University of Shiga Prefecture, Hassaka 2500, Hikone, Shiga 522-8533, Japan

Received 05 October 2011; accepted 11 February 2012 Abstract: Copper phthalocyanine/fullerene-based solar cells were fabricated, and the electronic and optical properties were investigated. Effects of germanium addition to the solar cells were also investigated, which resulted in increase of power conversion efficiencies of the solar cells. Nanostructures of the solar cells were investigated by transmission electron microscopy and electron diffraction, which indicated formation of Ge compound nanoparticles in the copper phthalocyanine layers. Energy levels of the solar cells were discussed from the present analysis data. Keywords: Thin films • Chemical synthesis • Solar cells • Semiconductor • Nanostructure

© Versita sp. z o.o.

1.

Introduction

Carbon-based nanostructures such as C60 , giant fullerenes, nanocapsules, onions, nanopolyhedra, cones, cubes, and nanotubes have been reported and investigated [1, 2]. These carbon (C) nanomaterials with hollow cage structures show different physical properties, and have the potential of studying materials of low dimensionality within an isolated environment. By controlling of the size, layer numbers, helicity, compositions, and included clusters, the cluster-included C nanocage structures with band-gap energy of 0–1.7 eV and nonmagnetism are expected to show various electronic, optical, and magnetic properties such as Coulomb blockade, photoluminescence, and superparamagnetism.

∗

E-mail: [email protected]

Recently, C60 -based polymer/fullerene solar cells have been investigated and reported [3, 4]. These organic solar cells have a potential for utility in lightweight, flexible, inexpensive and large-scale solar cells [5–7]. However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants. One of the improvements is donor-acceptor (DA) proximity in the devices by using blends of donor-like and acceptor-like molecules or polymers, which is called DA bulk-heterojunction solar cells [8–10]. The purpose of the present work is to fabricate and characterize fullerene-based solar cells. In the present work, copper tetrakis (4-cumylphenoxy) phthalocyanine (Tc-CuPc) was used for p-type semiconductors, and fullerene (C60 ) was used for n-type semiconductors. In addition, germanium (IV) bromide (GeBr4 ) was added to the solar cells for formation of Ge-based quantum dots to increase the photovoltaic efficiencies. Device structures were produced, and efficiencies, optical absorption and nanostructures were investigated.

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T. Oku, K. Kumada, A. Suzuki, K. Kikuchi

Figure 1.

2.

Structure of (a) bulk heterojunction and (b) heterojunction solar cells.

Experimental procedures

A thin layer of polyethylenedioxythiophen doped with polystyrene-sulfonic acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated on pre-cleaned indium tin oxide (ITO) glass plates (Geomatec Co., Ltd., ∼ 10 Ω/). The PEDOT:PSS has a role as an electron blocking layer for hole transport. Then, semiconductor layers were prepared on a PEDOT layer by spin coating using a mixed solution of C60 (Material Technologies Research, 99.98%), Tc-CuPc (Sigma Aldrich, 97%) and GeBr4 (Sigma Aldrich) in 1 ml o-dichlorobenzene. Weight ratio of Tc-CuPc:C60 was 1:8 (2 mg: 16 mg), and 0.03 ml of GeBr4 was added into the solution. The thickness of the blended device was approximately 150 nm. A schematic diagram of the Tc-CuPc:C60 bulk heterojunction solar cells is shown in Fig. 1(a). Heterojunction solar cells with a Tc-CuPc/C60 structure were also fabricated as shown in Fig. 1(b), and the C60 layers were prepared by vacuum evaporation. To increase efficiencies, GeBr4 was also added in the TcCuPc layers for both structures in Fig. 1. After annealing at 100°C for 30 min in N2 atmosphere, aluminum (Al) metal contacts with a thickness of 100 nm were evaporated as a top electrode. Current density-voltage (J-V) characteristics (Hokuto Denko Corp., HSV-100) of the solar cells were measured both in the dark and under illumination at 100 mW/cm2 by using an AM 1.5 solar simulator (San-ei Electric, XES301S) in N2 atmosphere. The solar cells were illuminated through the side of the ITO substrates, and the illuminated area is 0.16 cm2 . Optical absorption of the solar cells was investigated by means of UV-visible spectroscopy (Hitachi U-4100). Transmission electron microscope (TEM) observation was carried out by a 200 kV TEM (Hitachi H-8100).

Figure 2.

3.

Measured J-V characteristic of Tc-CuPc:Ge:C60 and TcCuPc:C60 bulkheterojunction solar cells under illumination.

Results and discussion

Measured J-V characteristic of Tc-CuPc:Ge:C60 and TcCuPc:C60 bulk heterojunction (BHJ) solar cells under illumination is shown in Fig. 2. The bulk heterojunction indicates a one layered composite structures with p- and n-type semiconductors, which is denoted as Tc-CuPc:C60 . The common heterojunction (HJ) solar cell that has separated two layers was also investigated for comparison, which is denoted as Tc-CuPc/C60 . Measured parameters of the present solar cells are summarized as listed in Table 1. Power conversion efficiency, fill factor, short-circuit current density and open-circuit voltage are denoted as η, FF, JSC , and VOC , respectively. As shown in Fig. 2 and Table 1, open-circuit voltages of Tc-CuPc:C60 and Tc-CuPc/C60 were fairly increased by GeBr4 addition, and slight increases were also observed for short-circuit current density for both structures. Table 1.

Measured parameters of the present bulk heterojunction (BHJ) and heterojunction (HJ) solar cells.

Layered structure CuPc:C60 (BHJ) CuPc:Ge:C60 (BHJ) CuPc/C60 (HJ) CuPc:Ge/C60 (HJ)

VOC (V) JSC (mAcm−2 ) 0.037 0.18 0.073 0.32

0.020 0.024 0.10 0.20

FF

η (%)

0.25 0.24 0.26 0.25

1.9 × 10−4 1.0 × 10−3 1.9 × 10−3 1.6 × 10−2

Fig. 3 shows optical absorption of Tc-CuPc:Ge:C60 and TcCuPc:C60 bulk heterojunction solar cells. The Tc-CuPc:Ge: C60 structure provided higher photo-absorption in the range of 500 to 1200 nm (which corresponds to 2.5 and 1.0 eV, respectively), compared to the Tc-CuPc:C60 structure. An


Effects of germanium addition to copper phthalocyanine/fullerene-based solar cells

Figure 3.

Absorbance spectrum of Tc-CuPc:Ge:C60 and Tc-CuPc:C60 bulk heterojunction structure.

energy gap between HOMO and LUMO for C60 is 1.7 eV, which corresponds to absorbance of 730 nm [11]. A TEM image of the Tc-CuPc:Ge:C60 bulk heterojunction layer is shown in Fig. 4(a). Nanoparticles consisting of a Ge element that have the largest atomic number in the present solar cells are observed in the Tc-CuPc layer. An enlarged TEM image is shown in Fig. 4(b), and lattice fringes of Tc-CuPc are observed. The nanoparticle with Ge compounds is denoted as Ge comp. An electron diffraction pattern of the Tc-CuPc:Ge:C60 bulk heterojunction layer is shown in Fig. 4(c), and many diffraction spots and rings corresponding to C60 111, 220 and 311 were observed, which indicates microcrystalline structures of C60 [12, 13]. Dispersion of Ge-based nanoparticles is effective for optical absorption in the range of 500 to 1200 nm. Although Ge has a bandgap energy of 0.7 eV, optical absorption was observed in the range of 2.5 and 1.0 eV in the present work, which would be due to Ge compound formation and nano-dispersion effect of the nanoparticles, as reported in the previous work [14]. It is necessary to control the microstructure of the bulk heterojunction layer in solar cells [5]. Inter-penetrating donor-acceptor network has a large interfacial area, which would be effective charge generation. Since microstructure of Tc-CuPc and C60 were disordered, recombination of electrons of C60 and holes of Tc-CuPc would occur. Therefore, the ordered column-like structure would be suitable for carrier transport. If continuous nanocomposite structures are perpendicular to the thin films, it is believed that the recombination of electrons and holes could be avoided, and the conversion efficiency of the solar cells would increase.

An energy level diagram of the Tc-CuPc:Ge/C60 solar cell is summarized as shown in Fig. 5. Previously reported values [10, 15–19] were used for the energy levels in the figures by adjusting to the present work. Energy barrier would exist near the semiconductor/metal interface. Electronic charge-transfer separation was caused by light irradiation from the ITO substrate side. Electrons are transported to an Al electrode, and holes are transported to an ITO substrate. VOC of organic solar cells seems to be related with the energy gap between HOMO of donor molecule and LUMO of accepter one. Thus, control of the energy levels is important to increase the efficiency. By formation of Ge-based nanoparticles [14], the energy gap of Tc-CuPc would decrease, which resulted in increase of optical absorption. Quantum dot solar cells including intermediate band structures are one of the candidates of high efficiency solar cells [20–22]. In the present work, efficiencies of the solar cells increased by the formation of Ge-based nanoparticles. The present solution technique is very simple and cost effective methods for the formation of nanoparticles. To improve the efficiencies, arrangement of the quantum dots and the control of size distribution are necessary. Combination of the present solar cells and copper oxide nanomaterials with various direct band gaps might be also effective for increase of efficiencies [23]. The performances of the present solar cells would also be due to the nanoscale structures, and the control and structure should be investigated further.

4.

Conclusions

Tc-CuPc/C60 solar cells were fabricated, and the electronic and optical properties were investigated. By addition of GeBr4 to the Tc-CuPc/C60 solar cells, the open circuit voltages were increased, which results in increase of efficiencies of the solar cells. Nanostructure analysis of the solar cells by TEM indicated formation of Ge compound nanoparticles in the Tc-CuPc layers. These Ge-based nanoparticles would generate carriers, and they were transported to the Tc-CuPc layers. From the present JV measurements, optical absorption and structure analysis, energy levels of the solar cells were discussed. Optimization of GeBr4 addition to metal phthalocyanine/C60 -based solar cells would improve the efficiencies of solar cells.

Acknowledgments The authors would like to thank N. Kakuta, A. Kawashima, S. Kikuchi and S. Yoshida for experimental help and useful advices.


T. Oku, K. Kumada, A. Suzuki, K. Kikuchi

Figure 4.

Figure 5.

(a) TEM image, (b) enlarged image and (c) electron diffraction pattern of Tc-CuPc:Ge:C60 bulk heterojunction layer.

Energy level diagram of Tc-CuPc:Ge/C60 solar cell.

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