A Photovoltaic Fiber Design for Smart Textile

A Photovoltaic Fiber Design for Smart Textile

(Parte 2 de 3)

Figure 5Absorption spectra for solutions of P3HT:PCBM and MDMO-PPV:PCBM in chlorobenzene.

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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 structures, it was assumed that they may be used as a kind of textile surface for clothes or coverings and so half of the fiber was illuminated and considered for characterization. In experiments, the photoactive layer on the P fiber absorbed the light at different angles due to its circular cross section (Figure 6). The current density versus voltage characteristics 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 Table1, the highest values of the photovoltaic parameters are obtained with an open-circuit voltage of 360 mV, a short-circuit current density of 0.1 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 opencircuit 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 opencircuit voltage, short-circuit current density, current, and voltage at the maximum power point under an illumination of 100mW/cm2 (under AM 1.5G conditions), for both devices are shown in linear I–V curves in Figure 8. As can be

Table 1Photoelectrical characteristics of photovoltaic fibres having different photoactive layers (MDMO-PPV:PCBM and P3HT:PCBM).

Type of photoactive materialVoc (mV)Isc (mA/cm2)F (%)η (%) * 10–3 P3HT:PCBM 360 0.1 24.5 10

Figure 7I–V curves of (a) P3HT:PCBM and (b) MDMO-PPV:PCBM-based photovoltaic fibers, lighting through the cathode direction.

Figure 6Schematic demonstration of measurement for photovoltaic characteristics of photovoltaic fibers.

A Photovoltaic Fiber Design for Smart Textiles A. Bedeloglu et al.7TRJ seen from Figure 8, for the P3HT:PCBM-based photovoltaic fiber the maximum power (Pmax) was obtained as ~0.1 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 0.13mA/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 photovoltaic 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 P fiber was taken with 300x magnification. 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 interface 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 thicknesses of the PEDOT:PSS in P3HT:PCBM and the MDMOPPV:PCBM-based solar cells are approximately 366 nm and 458 nm, respectively. Generally, the short-circuit current 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 combinations is too small to absorb all photons within the absorption 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 8Linear I–V curves demonstrating Isc, Voc, Impp, and Vmpp of P3HT:PCBM- (a) and MDMO-PPV:PCBM- (b) based photovoltaic fibers.

Figure 9Surface of the photovoltaic fiber after coating a photoactive layer (P3HT:PCBM).

8Textile Research Journal 0(0)TRJTRJ increases, the electrical field and the number of charge carriers decrease and so, a decrease in the external quantum efficiency of the devices is observed. Short-circuit current density is inversely proportional to the thickness of the photo-active layer. Therefore, due to limited charge, transport higher currents cannot be obtained in general [3,34]. As can be seen from Figures 7, 8, and 10 and Table 1, the thickness of the active layer plays an important role in determining the electrical characteristics of the device. Lower electrical results (Table 1, Figures 7 and 8) are probably due to the thickness of the light-absorbing layers. The optimum thickness is required to provide both maximum light absorption and maximum charge collection, at the same time. Thus, optimizing the thickness of the photoactive layer in photovoltaic fibers provides the possibility to increase the power conversion efficiency of a polymer-based solar cell. The thickness of the layers for optimal photovoltaic fibers can be controlled by solution concentration and dipping time.

In conventional solar cells, since the LiF/Al cathode is ca.100 nm, light cannot enter into the device through the top electrode. Therefore, an ITO-coated glass layer having a transmission of 90% is used at the bottom (ITO as anode) of this kind of solar cell. However, in order to produce a flexible textile structure, a transparent and a flexible outer electrode of about 10 nm (40% of transmission) was utilized in this study, although transmission was lower than that with ITO. By using these flexible layers, P fiber-based organic solar cells can be curled and crimped without losing any photovoltaic performance from their structure.

After the manufacturing of the P monofilament, the processes given in Figure 1 are suggested for achieving the

Figure 10SEM pictures of P photovoltaic fibers (a) H1: P3HT:PCBM H2: PEDOT:PSS and (b) MDMO-PV:PCBM, H2: PEDOT:PSS.

Figure 11Possible manufacturing processes for photovoltaic fiber.

A Photovoltaic Fiber Design for Smart Textiles A. Bedeloglu et al.9TRJ photovoltaic effect on the fibers. This schematic drawing, showing possible photovoltaic fiber production processes in the industrial scale, demonstrates the liquid solution and drying treatments that are already used in textile finishing processes. Therefore, when a polymer-based organic photovoltaic fiber is produced with high efficiency in the future, the adaptation of fabrication stages into commercialization will not be so troublesome.

In the manufacturing scheme, there are also anti-reflective and protective layer coating steps that will be considered in our future studies. A protective layer will save the organic material from moisture and oxygen. The anti-reflective layer in solar cells can obstruct the reflection of light and also contribute to the device performance. This is an important point that should be overcome in the case of largescale production of photovoltaic fibers. The manufactured photovoltaic fibers may be also utilized in the manufacture of yarn by spinning and then, a fabric by weaving or knitting processes. While the bobbins are being prepared from photovoltaic fibers for subsequent textile manufacturing processes (spinning, knitting, or weaving), the mechanical forces involved in the weaving, knitting, and cleaning steps may possibly damage the coating layers of the photovoltaic fibers. These delicate structures can be protected by using special layers and techniques to develop usable textile products.

To enhance the power conversion efficiency of the photovoltaic fiber, existing materials and techniques need to be improved. In particular, the optical band gap of the polymers used as the active layer in organic solar cells is very important. Generally, the best bulk heterojunction devices based on widely studied P3HT:PCBM materials are active for wavelengths between 350 and 650 nm (Figure 5). Polymers with narrow band gaps can absorb more light at longer wavelengths, such as infra-red or near-infra-red, and consequently enhance the device efficiency. Low band gap polymers (<1.8 eV) can be an alternative for better power efficiency in the future, if they are sufficiently flexible and efficient for textile applications [35,36].

Conclusions

In this study, the development of photovoltaic P fibers using two different kinds of polymer-based photoactive materials was reported. The approach is especially beneficial for photovoltaic textiles, when non-transparent materials are used as substrates of the photovoltaic devices. This study showed that the highly conductive PEDOT:PSS solution and the semi-transparent metallic layer can be used successfully as the anode and cathode of the organic solar cells, respectively, in the case of flexible devices. The maximum open-circuit voltage values for these fibers were obtained as 360 mV and 300 mV for P3HT:PCBM and MDMO-PPV:PCBM- based devices, respectively. In addition, the organic photo- voltaic fiber based on MDMO-PPV:PCBM yielded the maximum short-circuit current density with 0.27 mA/cm2. Although these flexible and cylindrical-shaped devices produced modest power conversion efficiencies due to the materials and modified preparation techniques of solar cells, using a flexible P fiber as a substrate and a semi-transparent metal layer as the cathode shows promise for future wearable photovoltaic textile applications. For further optimization of the commercial-scale production that enhances the optimized power conversion efficiency of the photovoltaic fibers, some alternatives are to reduce the thickness of the light-absorbing layer and the PEDOT:PSS layer, to use low band-gap materials, infrared light activated materials which would generate power at night, ultraviolet (UV) light selective materials, and to apply anti-reflective and protective nano-coatings. After optimization, this photovoltaic fiber design can be produced in the industry with techniques similar to the textile manufacturing steps and may be used to manufacture smart textiles.

Acknowledgements

One of the authors, Ayse Bedeloglu, acknowledges The Scientific and Technological Research Council of Turkey (TÜBİTAK) for the grants with reference numbers “21” and “2214”. Dr. Robert Koeppe and Philipp Stadler from the LIOS and Serap Gunes and Nimet Canli Yilmaz from Yildiz Technical University are also gratefully acknowledged for fruitful discussions.

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(Parte 2 de 3)

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