Baixe A Photovoltaic Fiber Design for Smart Textile e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! Textile Research Journal Article Textile Research Journal Vol 0(0): 1–10 DOI: 10.1177/0040517509352520 © The Author(s), 2009. Reprints and permissions: http://www.sagepub.co.uk/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 indus- trial and daily life, as well as political, economical, and mil- itary 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 environmen- tally friendly. Solar energy, which is limitless, clean, and renewable, can meet these needs of mankind. The solar cells, which directly convert sunlight into elec- trical 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 opera- tion in comparison with silicon-based solar cells. Organic semiconductors, such as conductive polymers, dyes, pig- ments, 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 nano- fiber formation [3] and electrospinning [4] at room temper- atures. 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-ethylene- dioxythiophene: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 men- tioned 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 prob- lems of storage and transport [5]. Abstract In 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 differ- ent nano-materials in bulk heterojunction blends as the light absorbing materials, and a semi-trans- parent cathode to collect the electrons, was formed by coating these materials onto flexible polypro- pylene (PP) fibers layer by layer, respectively, to produce electricity. Photovoltaic performances of the fibers were analyzed by measuring current versus voltage characteristics under AM1.5 con- ditions. The maximum value obtained as the short- circuit current density of photovoltaic fibers was 0.27 mA/cm2. The fabrication issues and also possi- ble smart textile applications of these photovoltaic fibers were discussed. Key words polymer-based organic solar cell, photovoltaic fiber, smart textile, smart fiber 1 Corresponding author: tel: +90 2324127712; fax: +90 2324127750; e-mail: ayse.celik@deu.edu.tr Textile Research Journal OnlineFirst, published on October 29, 2009 as doi:10.1177/0040517509352520 2 Textile Research Journal 0(0)TRJ 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 tex- tiles 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 photo- voltaic 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 [6–11]. 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 fiber- based 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, 22] about fiber- shaped 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 mate- rials. 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 out- side) was used to collect the electrons [21]. In addition, in another study [22] that used small molecule-based materi- als in an organic active layer of the fiber-shaped solar cell, all layers were deposited onto polyimide coated silica fib- ers 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 tech- niques and high temperatures to guarantee highly conduc- tive 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 elec- trode. 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 con- venient 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 photo- voltaic 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 pre- sented. The maximum short-circuit current density was obtained as 0.27 mA/cm2. Here, the advantages of photo- voltaic 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 (PP) monofilament (obtained from Figure 1 Schematic drawing of a conventional polymer- based organic solar cell on ITO-coated glass-based sub- strate. A Photovoltaic Fiber Design for Smart Textiles A. Bedeloglu et al. 5 TRJ and the fill factor (FF). The photovoltaic power conversion efficiency (η) of a solar cell is defined as the ratio between the maximum electrical power (Pmax) and the incident opti- cal power and is determined by [1] (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 molec- ular 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. FF, the fill-factor, is calculated by dividing Pmax by the multipli- cation of Isc and Voc and this can be explained by the fol- lowing equation [1]: (2) 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 con- taining 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. There- fore, these kinds of structures take the light from their outer surface. In this study, considering non-transparent PP 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 photo- voltaic fiber formation because of the disadvantages of ITO material in terms of brittleness, high cost, and applica- tion 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, PP is one of the most interest- ing 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 sur- face energy of PP 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 PP surfaces [31]. Among these methods, using Triton X-100, which is a water-soluble, liq- uid, and non-ionic surfactant, can be a simple and effective way to improve the wettability of polymer surfaces. In our study, the PP 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 5 seconds. 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 absorp- tion 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 absorp- tion 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 FF×× Pin ---------------------------------= FF Impp Vmpp× Isc Voc× ---------------------------= Figure 5 Absorption spectra for solutions of P3HT:PCBM and MDMO-PPV:PCBM in chlorobenzene. 6 Textile Research Journal 0(0)TRJ solution contains a higher concentration of PCBM, which has higher absorption below 400 nm. Active areas for photovoltaic fibers were between 4 and 10 mm2. When we consider the photovoltaic fiber struc- tures, it was assumed that they may be used as a kind of tex- tile surface for clothes or coverings and so half of the fiber was illuminated and considered for characterization. In experiments, the photoactive layer on the PP fiber absorbed the light at different angles due to its circular cross section (Figure 6). The current density versus voltage characteris- tics of the photovoltaic fibers consisting of P3HT:PCBM and MDMO-PPV:PCBM blends are given in Table 1 and demonstrated in Figures 7 and 8. As can be seen from Table 1, the highest values of the photovoltaic parameters are obtained with an open-circuit voltage of 360 mV, a short-circuit current density of 0.11 mA/cm2, a fill factor of 24.5%, and a power conversion efficiency of 0.010% from P3HT:PCBM-based photovoltaic fibers. In addition, the MDMO-PPV:PCBM-based photovoltaic fiber gives an open- circuit voltage of 300 mV, a short-circuit current density of 0.27 mA/cm2, a fill factor of 26%, and a power conversion efficiency of 0.021%. As can be seen from these results, the power conversion efficiency of the MDMO-PPV:PCBM- based photovoltaic fiber was higher than the P3HT:PCBM- based photovoltaic fiber. The semi-logarithmic I–V curves in Figure 7 demonstrate the current density versus voltage behavior of photovoltaic fibers in the dark and under simulated light. In addition, the characteristics of photovoltaic fibers, including open- circuit voltage, short-circuit current density, current, and voltage at the maximum power point under an illumination of 100 mW/cm2 (under AM 1.5G conditions), for both devices are shown in linear I–V curves in Figure 8. As can be Table 1 Photoelectrical characteristics of photovoltaic fibres having different photoactive layers (MDMO-PPV:PCBM and P3HT:PCBM). Type of photoactive material Voc (mV) Isc (mA/cm 2) FF (%) η (%) * 10–3 P3HT:PCBM 360 0.11 24.5 10 MDMO-PPV:PCBM 300 0.27 26.0 21 Figure 7 I–V curves of (a) P3HT:PCBM and (b) MDMO-PPV:PCBM-based photovoltaic fibers, lighting through the cathode direction. Figure 6 Schematic demonstration of measurement for photovoltaic characteristics of photovoltaic fibers. A Photovoltaic Fiber Design for Smart Textiles A. Bedeloglu et al. 7 TRJ seen from Figure 8, for the P3HT:PCBM-based photovoltaic fiber the maximum power (Pmax) was obtained as ~0.11 W, when values of voltage (Vmpp) and current (Impp) at the maximum power point were 0.18 V and 0.06 mA/cm2. In addition, for the MDMO-PPV:PCBM-based photovoltaic fiber, the maximum power was obtained as ~0.21 W, when the values of voltage and current at the maximum power point were 0.16 V and 0.13 mA/cm2, respectively. The characteristic intersections with the abscissa and ordinate are the open-circuit voltage (Voc) and the short-circuit cur- rent (Isc), respectively. The maximum power output (Pmax) is determined by the point where the product of the voltage and current is highest. The maximum power output was obtained higher with the MDMO-PPV:PCBM-based pho- tovoltaic fiber, and a higher power conversion efficiency was also obtained. The surface photo of the photovoltaic fiber after being coated with the light-absorbing layer is shown in Figure 9. The top view of the PP fiber was taken with 300x magnifi- cation. SEM photographs of the photovoltaic fibers are given in Figures 10(a) and (b). SEM analysis shows that the layers of the photovoltaic fibers were more clearly visible with 50000x magnification. The thicknesses of the layers can be also seen from photographs, showing a bright inter- face line between the polymer anode and the photoactive layer. The metal electrode layer is too thin (~10 nm) to be seen on the fiber surface of these images. The film thick- nesses of the PEDOT:PSS in P3HT:PCBM and the MDMO- PPV:PCBM-based solar cells are approximately 366 nm and 458 nm, respectively. Generally, the short-circuit cur- rent density decreases slightly with increasing thickness of the PEDOT:PSS [32] in solar cells. In this study, thickness measurements of the PEDOT:PSS layer demonstrate that PEDOT:PSS layers are much thicker in both types of devices. An optimization of this layer can increase the short-circuit current density, resulting an improvement in power conversion efficiency. In addition, a PEDOT:PSS layer as a buffer layer between the bottom electrode and the light-absorbing layer reduces the risk of short circuiting in the case of thin (less than 100 nm) blend layers. The thickness of the photoactive layers in P3HT:PCBM and MDMO-PPV:PCBM-based solar cells were approximately 302 nm and 313 nm, respectively (see Figure 10). In organic solar cells, the external quantum efficiency of the devices depends on the absorption and charge carrier mobility. The optimal layer thickness for many material combina- tions is too small to absorb all photons within the absorp- tion bands. A thick film can absorb more light compared with a thinner film. On the other hand, the film thickness is limited by a low-charge carrier. As the film thickness Figure 8 Linear I–V curves demonstrating Isc, Voc, Impp, and Vmpp of P3HT:PCBM- (a) and MDMO-PPV:PCBM- (b) based pho- tovoltaic fibers. Figure 9 Surface of the photovoltaic fiber after coating a photoactive layer (P3HT:PCBM).