1. Introduction

Rising energy prices are making alternative energy sources increasingly attractive. However, a major drawback of semiconductor-based solar cells is their low efficiency, which is unavoidable [1–7]; innovations in polymers and organic materials science have advanced the production of cheaper, more efficient organic solar cells [8, 9].

During the last decade, many approaches have been proposed for polymer-based solar cell fabrication, such as screen printing, doctor blading, inkjet printing, and spray deposition [8, 10–12]. These techniques are low cost, and the fabricated polymer-based solar cells are lightweight and flexible, leading to reduced fabrication and installation costs. Although these techniques have some advantages, they still suffer from certain limitations, such as poor light absorption [8, 13]. Power conversion efficiency is important in order to compete with the more conventional solar cell fabrication based on silicon and semiconductor materials.

Surface texturing is one of the well-known solutions for increasing light absorption of solar cells. During the last few decades, a variety of techniques have been introduced by researchers for solar cell surface texturing [14–17]. Among these methods, laser texturing is a non-contact technique, allowing a great deal of flexibility in defining surface texture. It can be utilized on a wide range of semiconductor materials, and can lead to an alternative solution for solar cell surface texturing [7, 18–20]. However, the current laser texturing methods cannot be applied on polymer-based solar cells because of some compatibility issues, such as low melting temperature and the ablation threshold of polymer and organic materials in the interaction with laser pulses.

In this article, a new method is proposed for laser surface texturing of polymer substrate in nanoscale: a combination of high frequency and ultra-short laser pulses enables us to control the average surface temperature and energy influence below the ablation threshold, which can lead to fabricating a 3D nanofibrous layer of polymer on top of the substrate. This technique is single step, and the 3D nanofibrous layer is generated on the substrate without any additive materials. The light spectrometry results show a significant enhancement in light absorption. The result is an efficient solar cell that performs well in terms of light absorption. Finally, the effect of laser parameters has been investigated in order to optimize the light absorption.

2. Experimental setup

All polymer substrates were prepared with Sylgard-184, an elastomeric PDMS kit manufactured by Dow Corning. The PDMS prepolymer (SYLGARD 184 Silicone Elastomer Kit) was dissolved in the curing agent in a 1:10 weight ratio. The mixture was poured on top of the silicon wafer substrate; this was followed by spinning the wafer at a spindle speed of 400 rpm for 40 s, which yielded an approximate coating thickness of 700 μm. The samples were baked for 40 minutes at 100 C and left at room temperature for 24 hours until they become completely solidified. Nano-texturing process of polymer samples was carried out using a diode-pumped, Yb-doped femtosecond laser system which can radiate laser pulses with the central wavelength of 1064 nm in the range of 214 femtoseconds to 3.5 picoseconds. The pulse frequency could be variable from 200 kHz to 26 MHz with an average laser power of 12 W. In this experiment, the samples were processed with femtosecond laser pulses at 26 MHz frequency with an average laser power of 4, 7 and 10 W. The laser beam diameter was 4.5 mm, which was expanded to 9 mm using two UV fused silica plano-convex (f = −100) and a plano-concave (f = 200) lenses. Before entering into a galvoscanner, the diameter of the laser beam was reduced to 8 mm by using an iris diaphragm.

A 2D galvoscanner was used to focus the laser beam onto the substrate with a scanning speed of 50 mm/s, controlled by EZCAD^© software. The focused spot diameter of the laser on the substrate was calculated to be around 10.5 µm. The PDMS samples were irradiated with laser pulses at different power; the irradiated samples were then examined under a scanning electron microscope (SEM), transmission electron microscope (TEM) and Energy Dispersive X-ray (EDX) spectroscopy as well as an optical spectrometer to analyze the properties of the 3D polymer nanofibers generated on the substrate. The results are presented and discussed in the following sections.

3. Results and discussion

Figure 1 illustrates three different morphologies obtained with an average laser power of 4, 7 and 10 W at a pulse duration of 214 fs, a 26 MHz frequency and a scanning speed of 50 mm/s. Previous results show that for achieving a nanofibrous structure of semiconductor and metallic materials, megahertz frequency laser pulses at femtosecond range are required [21, 22].

 

Fig. 1 SEM Images of the irradiated areas at a) 4 W, b) 7 w and c) 10 W.

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As shown in Fig. 1, by increasing the laser power from 4 W to 10 W, the surface morphologies varied significantly; in the case of 4 W power in Fig. 1(a), we observed dense 3D microstructures of irradiated polymers with a diameter of 40 μm; by increasing the power to 7 W, the diameter of the 3D microfibers reduced to 10 μm with a more permeable structure, and finally at 10 W, the microstructures vanished completely, and only a flat surface of polymer covered by 3D nanofibrous structures could be observed in SEM images in Fig. 1(c).

In Fig. 2, EDX results show the presence of silicon, oxygen and carbon with comparable concentrations for three indicated elements in irradiated areas at 4, 7 and 10 W; this result excluded the possibility of a different compound formation at different powers.

 

Fig. 2 EDX results of the irradiated areas at a) 4 W, b) 7 W and c) 10 W.

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Figure 3 shows close-up images of the irradiated area at 4, 7 and 10 W; Figs. 3(a) and 3(b) illustrate that in both cases of 4 and 7 W laser power, the 3D microfibrous structures have spongy and porous structures made of nanofibrous materials. By increasing the laser power to 7 W, the 3D microstructures become thinned and more permeable, and finally at 10 W, no microstructure is observed in SEM image in Fig. 3(c), and we have a surface coated by a layer of 3D nanofibers induced by laser pulses.

 

Fig. 3 Detailed SEM Images of the irradiated areas at a) 4 W, b) 7 w and c) 10 W.

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The surface morphology and material properties of the synthesized structures are influenced by laser parameters. Figures 2 and 3 illustrate the SEM images of the synthesized nanofibrous structures of polymer induced by 214 fs laser pulses at 26 MHz with different pulse energies. A TEM image of the nanofibers generated by laser pulses with an average power of 10 W, presented in Fig. 4, shows that they are comprised of nanoparticles in which form interconnected chains in nanoscale. The diameter of the generated nanofibers is around 30 nm.

 

Fig. 4 TEM Image of the generated nanofibers at 214 fs, 26 MHz and 10 W.

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In order to verify the elemental composition of the produced nanofibers, an EDX analysis was conducted. In Fig. 5, the EDX results clearly show the presence of silicon, carbon and oxygen in nanostructured materials, which are the main elements in the PDMS chemical structure.

 

Fig. 5 EDX result of the generated nanofibers with laser power of 10 W.

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The method of nanofiber generation by laser ablation is achieved by heating the target material above its boiling temperature induced by laser pulses, followed by rapid cooling once the laser pulses stop. When ablation of the target material is carried out in a background gas environment or in ambient air, the presence of the air/gas causes the redeposition of the ablated material onto the target surface this does not take place for laser ablation in a vacuum [22, 23]

To verify the precision of the experimental results observed in the SEM figures, analytical methods were used to investigate the effects of the laser parameters on the nanofiber generation process. In this approach [24–26], it is presumed that the laser energy is absorbed in a layer much thinner than the penetration depth of the heat wave, thus the one-dimensional heat conduction equation can be estimated by:

Here, $a=k/C_p{\rho }_0$, a is thermal diffusion, C_p is specific heat, k is the heat conduction, finally, ${\rho }_0$ is the material density. Also, it is assumed that the laser pulse profile is in a rectangular shape with the step-like rise and fall [26], thus:

where, t_p is pulse duration, I_a is the absorbed laser light intensity estimated by [27]:

where, R is reflection coefficient, P is average power, f is frequency and d is spot diameter.

The maximum surface temperature, T_max, occurs at the end of the laser pulse; thus the surface temperature at the center of spot area on the substrate during the laser pulse (t<t_p) is calculated by$T(t)=\sqrt{t/t_p}{T}_{\max }$; T_min is the minimum temperature at the beginning of the next laser pulse which is given by$T_{\min }=\alpha T_{\max }$. Where α is equal to $\alpha =\sqrt{t_p/t_{pp}}$(t_pp is pulse interval and given by$t_{pp}=1/f$). The maximum and minimum surface temperatures for n laser pulses can be nearly calculated by:

The average surface temperature after n pulses is estimated with:

According to [28], thermal conductivity, k, is 0.15 W/mK, $C_P$, specific heat is 1.46 kJ/kgK and PDMS density is 965 kg/m³. The absorption coefficient of the substrate was assumed to be around 0.3 and the effective number of pulses was calculated to be 6850 by$N_{eff}=\sqrt{\pi /2}fd/v$, where v is 50 mm/s (scanning speed).

Although the calculated temperature may contain large errors due to the assumptions that have been made, the tendency of the surface temperature in relation to the pulse number and power is quite clear, and this can be used to explain the phenomenon observed in the experimental results. The computed results in Fig. 6 show that the surface temperature increases with the accumulation of number of pulses and reaches saturation. The increases in surface temperature at the power of 4 and 7 W are around 332 and 580 °K, which leads to the formation of 3D microstructures on the substrate surface of polymer. However, given enough numbers of pulses (saturation), the laser fluence is increased by increasing the laser pulse power. The 3D micro- and nanostructures are results of particle aggregation taking place in the process of vapor condensation by the collision of the nucleus. To generate nanofibrous structures, a critical amount of laser energy should be injected to the substrate in order to form the plume, and a continuous influx of laser pulses is required in order to keep the plume at the proper level of temperature and density. As a result, the generation of nanofibers is not feasible at lower laser power and pulse numbers, since at lower pulse energy, a very dense microstructure would be generated instead of 3D nanofibrous structures [29, 30]

 

Fig. 6 Computed temperature rise on the substrate at a) 4 W, b) 7 w and c) 10 W.

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In order to verify the optical properties of the 3D micro- and nanostructures induced by laser pulses at different laser powers, the reflection spectrum was measured using USB2000 + RAD spectroradiometer (Ocean Optics, Dunedin, Florida, USA) with a resolution of 0.35 nm. The measurement was conducted for wavelengths in the range of 530–1100 nm, with 1 nm increments. As shown in Fig. 7, the reflection of the prepared samples at wavelengths of 500–1000 nm decreases by increasing the laser power. This is due to the fact that at higher laser power, more energy is transferred to the polymer substrate. This results in an increase in the volume of plume and nucleus density as well as the number of evaporated particles from the surface, which leads to an increase in the deposition rate of generated nanostructures. Conversely, at lower laser power, the ablation process leads to the generation of a thinner layer of microstructures rather than a thick layer of 3D nanofibers [29, 30]. The proposed method suggests considerable promise for the synthesis of 3D nanofibrous structures from organic materials to develop new functional compound materials for solar cell and photovoltaic applications.

 

Fig. 7 Results of light spectroscopy of the synthesized structures at various laser power.

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4. Conclusion

This work presented a laser-based approach to synthesizing micro/nanofibrous structures from PDMS as an organic material. The experiments conducted at different pulse energies and the morphological analyses by SEM and TEM showed that by altering the laser pulse fluence, different 3D structures from micro to nano with different porosities and structures could be generated. In addition, an analytical approach was used in order to investigate the effect of laser parameters on surface temperature and plume generation. Finally, light spectroscopy analyses confirmed that the laser-induced 3D nanofiber layer results in an increase of the light scattering and better light absorption of the irradiated samples, and it was found that the synthesized 3D structures of polymer generate a better light absorption coefficient when they are generated at higher pulse powers. This method is single step and there is no need to add any materials in the fabrication process. It suggests a promising step towards engineering 3D nanofibrous platforms from polymers and organic materials, which can have very broad practical applications in a variety of areas, specifically in fabrication of polymer solar cell and photovoltaic devices with higher light absorption efficiency.