A Photovoltaic Fiber Design for Smart Textile

A Photovoltaic Fiber Design for Smart Textile

(Parte 1 de 3)

Textile Research JournalArticle

Textile Research Journal Vol 0(0): 1–10 DOI: 10.17/0040517509352520© The Author(s), 2009. Reprints and permissions: http://www.sagepub.co.u k/journalsPermissions.nav

A Photovoltaic Fiber Design for Smart Textiles

Ayse (Celik) Bedeloglu1

Dokuz Eylül University, Textile Engineering Department, 35160, Buca, Izmir, Turkey

Ali Demir

Istanbul Technical University, School of Textile Technologies and Design, Istanbul, Turkey

Yalcin Bozkurt

Dokuz Eylül University, Textile Engineering Department, 35160, Buca, Izmir, Turkey

Niyazi Serdar Sariciftci

Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University of Linz, A-4040 Linz, Austria

Today, energy is an important requirement for both industrial and daily life, as well as political, economical, and military issues between countries. While the energy demand is constantly increasing every day, existing energy resources are limited and slowly coming to an end. Due to all of these conditions, researchers are directed to develop new energy sources which are abundant, inexpensive, and environmentally friendly. Solar energy, which is limitless, clean, and renewable, can meet these needs of mankind.

The solar cells, which directly convert sunlight into electrical energy, are very interesting structures for energy generation. In particular, polymer-based organic solar cell materials have the advantages of low price and ease of operation in comparison with silicon-based solar cells. Organic semiconductors, such as conductive polymers, dyes, pigments, and liquid crystals, can be manufactured cheaply and used in organic solar cell constructions easily. In the manufacturing process of organic solar cells, thin films are prepared utilizing specific techniques, such as vacuum evaporation, solution processing, printing [1,2], or nanofiber formation [3] and electrospinning [4] at room temperatures. Dipping, spin coating, doctor blading, and printing techniques are mostly utilized for manufacturing organic solar cells based on conjugated polymers [1].1

A conventional organic solar cell (Figure 1) consists of a transparent conductive bottom electrode, e.g., indium tin oxide (ITO) (approximately 120 nm), a poly(3,4-ethylenedioxythiophene:poly(styrene sulfonic acid) (PEDOT:PSS) layer facilitating the hole injection and surface smoothness, an organic photoactive layer to absorb the light, and a metal electrode (approximately 100 nm) to collect charges on the top of the device. In addition, it has to be mentioned that all of these conventional solar cell materials are mainly developed on rigid substrates, such as glass, which are heavy, fragile, and inflexible, and which also have problems of storage and transport [5].

AbstractIn this paper, the active photovoltaic fibers consisting of nano-layers of polymer-based organic compounds are presented. A flexible solar cell, including a polymer-based anode, two different nano-materials in bulk heterojunction blends as the light absorbing materials, and a semi-transparent cathode to collect the electrons, was formed by coating these materials onto flexible polypropylene (P) fibers layer by layer, respectively, to produce electricity. Photovoltaic performances of the fibers were analyzed by measuring current versus voltage characteristics under AM1.5 conditions. The maximum value obtained as the shortcircuit current density of photovoltaic fibers was 0.27 mA/cm2. The fabrication issues and also possible smart textile applications of these photovoltaic fibers were discussed.

Key wordspolymer-based organic solar cell, photovoltaic fiber, smart textile, smart fiber

Textile Research Journal OnlineFirst, published on October 29, 2009 as doi:10.17/0040517509352520

2Textile Research Journal 0(0)TRJTRJ

Smart Textiles and Photovoltaic Applications

Both conventional and technical textiles are indispensible products for human daily life with various functions. Research and development activities in the field of textiles are running parallel to the advances in smart materials, which sense all relevant environmental stimuli (electrical, chemical, mechanical, magnetic, optical, etc.) and evaluate, react, or sometimes adapt to those conditions [5]. Smart materials may be in the form of phase-changing materials, chromic materials, shape-memory polymers and alloys, piezo materials, and light-emitting diodes, as well as photovoltaic materials. For example, smart photovoltaic textiles can produce power for electronic devices [6], such as mobile phones, IPods, pocket computers, etc. by collecting sunlight with nano-based materials.

There are limited scientific studies and few commercial applications of wearable solar cells based on inorganic materials [6–1]. In fact, the patching process, which is generally prevailed to develop wearable photovoltaics, may not always meet consumer demands, such as flexibility, comfort, and ease of cleaning. Although, there are some studies about flat textiles integrated with organic solar cells [12,13], photovoltaic fibers may form energy-harvesting textile structures in any shape and structure. Therefore, some researches have been conducted to develop fiberbased solar cells using inorganic materials, photochemical reactions, etc. [14–17].

In the scientific literature, there are also a few patents, projects [18–20], and research papers [21,2] about fibershaped organic solar cells. To obtain photovoltaic fibers, both polymer and small molecule-based light-absorbing layers were used in previous studies. In one of these stud- ies, the optical fibers, which are not flexible, were coated with poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (P3HT:PCBM)-based photoactive materials. While the light was travelling through the optical fiber and generating hole–electron pairs, the 100 nm top metal electrode (which does not let the light transmit from outside) was used to collect the electrons [21]. In addition, in another study [2] that used small molecule-based materials in an organic active layer of the fiber-shaped solar cell, all layers were deposited onto polyimide coated silica fibers using the thermal evaporation technique in a vacuum. A semitransparent top electrode that let the light enter the device was used and the fibers were rotating during the process in the mentioned study.

In organic solar cells, the most widely used transparent hole collecting electrode material is ITO. However, besides being an expensive material due to the low availability of indium, ITO requires expensive vacuum deposition techniques and high temperatures to guarantee highly conductive transparent layers. The advantages of the application of transparent flexible plastic substrates are restricted due to the thermal and mechanical damages of the ITO deposition process. There are some ITO-free alternative approaches, such as using carbon nanotube (CNT) layers or different kinds of PEDOT:PSS and its mixtures [23–27], or using a metallic layer [28] to perform as a hole-collecting electrode. Therefore, in order to realize polymer-based solar cells, which are completely flexible, and to substitute the ITO layer, this paper focuses on the highly conductive PEDOT:PSS solution as a polymer anode that is more convenient for textile substrates in terms of flexibility, material cost, and fabrication processes compared with ITO material.

In this study, the structure and properties of the photovoltaic fiber converting sunlight into electricity are described [29]. The sun’s rays entered into the photoactive layer of photovoltaic fibre by passing through a semi-transparent cathode which is very thin outer electrode consisting of ca. 10 nm of lithium fluoride/aluminum (LiF/Al) layers. The materials and techniques used to fabricate the photovoltaic fibres are explained and experimental results are presented. The maximum short-circuit current density was obtained as 0.27 mA/cm2. Here, the advantages of photovoltaic fibers and their main diversities from conventional solar cells are also explained and a possible approach to continuous photovoltaic fiber manufacturing is suggested.

Experimental details

Preparation of Photovoltaic Fiber Structure

Photovoltaic fibers were prepared using the PEDOT:PSS layer, the photoactive layer, and a metal-based electrode (Figure 2) [29]. Firstly, a substrate was prepared using a flexible polypropylene (P) monofilament (obtained from

Figure 1Schematic drawing of a conventional polymerbased organic solar cell on ITO-coated glass-based substrate.

A Photovoltaic Fiber Design for Smart Textiles A. Bedeloglu et al.3TRJ

SUNJUT, Turkey) with a diameter of 0.59 m to form the photovoltaic fiber. The non-transparent material and nonconductive monofilament was cut in order to obtain certain length pieces (5 cm long). Then, the fibers were gently cleaned of industrial and environmental contaminants using methanol, iso-propanol, and distilled water, respectively, and dried in nitrogen flow.

In the next step, the solution of highly conductive PEDOT: PSS (Baytron PH 500), which is a doped conjugated poly- mer with high hole conductivity [30], was prepared as the anode. The chemical structure of PEDOT:PSS is given in Figure 3(a). A PEDOT:PSS mixture was prepared by adding approximately 5% dimethylsulfoxide (DMSO) (Sigma- Aldrich) and approximately 0.1% Triton X-100 (Sigma- Aldrich) to improve conductivity and adhesion to the surface of the P fiber, respectively, and stirred for 24 hours. Then, the fibers were dip coated with PEDOT:PSS mixture one by one and dried at 50C for 3 hours; the samples were stored under the vacuum (in a nitrogen environment) for about 24 hours. Conventional organic solar cells prepared on ITO-coated glass substrates are generally heated after being coated with a PEDOT:PSS layer (>100°C) to achieve complete drying. However, common textile-based substrates, such as the P fibers used in this study, are not stable at these temperatures. So, fiber solar cells were processed at lower temperatures. For thermal treatment, a temperature of 50°C and a longer period of time (3 hours) were enough for complete drying of the PEDOT:PSS solution.

In the third step, two types of photoactive materials were prepared and coated with a similar way to the nano-coating of PEDOT:PSS. To achieve this, a blend of P3HT (Rieke Specialty Polymers), as the conjugated polymer, and phenyl C61 butyric acid methyl ester (PCBM) (Nano-C) materials were prepared by dissolving P3HT and PCBM with the ratio of 1:0.8 in chlorobenzene. In the meantime, a blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) (Merck) and PCBM were

Figure 2Schematic drawing of a photovoltaic fiber.

Figure 3Chemical structures of (a) PEDOT:PSS, (b) P3HT, (c) MDMOPPV, and (d) PCBM.

4Textile Research Journal 0(0)TRJTRJ dissolved with a ratio of 1:4 in chlorobenzene. The chemical structures of P3HT, MDMO-PPV, and PCBM are given in Figure 3(b), (c), and (d), respectively. PEDOT:PSS-coated fibers were coated with the solutions of light-absorbing materials by dipping the fibers one by one in the solution. Then, the samples were stored at 50C for 15 minutes under vacuum. The conductive top metal electrode was the last layer of the photovoltaic fiber structure. Rectangular masks (2 × 8 mm2) were used during the deposition of metal layers onto the fiber-based solar cells. The fibers were placed in the middle of the holes of the mask and the rest of the holes were covered according to fiber diameter. After the samples were inserted into the evaporation cabin in a glove box (MBraun), transparent metal layers consisting of 0.7 nm LiF and 10 nm Al were deposited on top of the fibers, using the thermal evaporation technique, in a vacuum that is about 5 × 10–6 mbar. The thickness of the metallic layer was controlled and measured by quartz crystal in the cabin of the evaporation machine. The evaporation rate was changed between 0.01–0.2 nm per second. After deposition of the metal layers, before the photoelectrical measurements were carried out in the glove box, small drops of silver paint were placed onto the electrodes of the photovoltaic fibers in order to develop contacts for better charge conduction. A brief description about the manufacturing procedure of a prototype photovoltaic fiber is given schematically in Figure 4.

Characterization of the Photovoltaic Devices

The electrical performances of photovoltaic fibers were characterized in an inert argon environment inside a glove box system (MBraun). All current–voltage (I–V) characteristics of the photovoltaic devices were measured with a Keithley 236 source measure unit in the dark and under simulated AM1.5 global solar conditions at an intensity of 100 mW cm–2. The solar simulator source (K.H. Steuernagel Lichttechnik GmbH) was calibrated using a standard crystalline silicon diode. Photovoltaic fibers were illuminated through the cathode side. I–V characteristics were measured immediately, the same day, after the photovoltaic fibers were prepared. Photovoltaic devices are generally characterized by the short-circuit current (Isc), the open-circuit voltage (Voc),

Figure 4Schematic description of the preparation of a photovoltaic P fiber.

A Photovoltaic Fiber Design for Smart Textiles A. Bedeloglu et al.5TRJ and the fill factor (F). The photovoltaic power conversion efficiency (η) of a solar cell is defined as the ratio between the maximum electrical power (Pmax) and the incident optical power and is determined by [1]

In Equation (1), the short-circuit current (Isc) is the maximum current that can run through the cell. The open- circuit voltage (Voc) depends on the highest occupied molecular orbital level of the donor (p-type semiconductor quasi

Fermi level) and the lowest unoccupied molecular orbital level of the acceptor (n-type semiconductor quasi Fermi level), linearly. Pin is the incident light power density. F, the fill-factor, is calculated by dividing Pmax by the multipli- cation of Isc and Voc and this can be explained by the following equation [1]:

In the Equation (2), Vmpp and Impp represent, respectively the voltage and the current at the maximum power point

(MPP), where the product of the voltage and current is maximized [1].

The ultraviolet-visible absorption spectra of the solid thin films were obtained using a Varian Carry 3G UV-Visible spectrophotometer. The thin films for the measurements were prepared by the spin-coating technique (Spincoater obtained from Specialty Coating Systems Inc. model P6700) on microscope glasses from chlorobenzene solutions containing 10 mg of P3HT and 8 mg of PCBM (in the case of 1:0.8)/ml and 4.5 mg of MDMO-PPV and 18 mg of PCBM (in the case of 1:4)/ml. The absorption spectra for these thin films are given Figure 5.

Both morphology studies and thickness measurement of layers of photovoltaic fibers were performed by scanning electron microscopy (SEM) (LEO Supra 35).

Results and Discussion

Generally, textile-based materials manufactured in fiber or tape forms are colored, not completely transparent. Therefore, these kinds of structures take the light from their outer surface. In this study, considering non-transparent P monofilament as the substrate of photovoltaic fiber, a semi-transparent top electrode (approximately 10 nm (10 + 0.7 nm)), through which light can be transmitted, was used as cathode. The ITO layer was not used in photovoltaic fiber formation because of the disadvantages of ITO material in terms of brittleness, high cost, and application problems in textiles. The PEDOT:PSS layer, having good conductivity, flexibility, and an easy coating process, was used successfully to substitute the ITO layer as the anode in this organic photovoltaic fiber formation.

Among the polyolefins, P is one of the most interesting thermoplastic materials due to its beneficial properties, such as low price and balanced properties and the ability to be recycled. However, poor bondability due to the low surface energy of P has limited the widespread use of these materials. Therefore, surface modification of these polymers is required. Various surface treatments are used to improve the adhesion of coatings to P surfaces [31]. Among these methods, using Triton X-100, which is a water-soluble, liquid, and non-ionic surfactant, can be a simple and effective way to improve the wettability of polymer surfaces. In our study, the P monofilament became highly hydrophilic and was coated with polymeric anode when exposed to a PEDOT:PSS mixture consisting of Triton X-100 mixture for about 5seconds.

To achieve a highly efficient photovoltaic device, solar radiation needs to be efficiently absorbed. In this type of solar cell the absorption of light causes electron hole pairs, which are split into free carriers at the interface between the donor and the acceptor material. Ultraviolet-visible absorption spectra for thin films of P3HT:PCBM (in 1:0.8 wt/wt ratio) and MDMO-PPV:PCBM (in 1:4 wt/wt ratio) are given in Figure 5. As can be seen from here, the absorption band in the visible range is because of the band-gap absorption of the polymer, while the increase of the absorption for wavelengths shorter than 400 nm is a superposition of the absorption of the polymer and PCBM. As the thickness is the same for both films on the glass, it can be concluded that P3HT:PCBM-based thin film showed better absorption than that of MDMO-PPV:PCBM within the visible range of wavelength (400–700 nm). However, this was reversed below 400 nm due to the fact that the MDMO-PPV:PCBM η Isc Voc F×

F Impp Vmpp×

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