- Nano Express
- Open Access
The Fabrication of Nanoimprinted P3HT Nanograting by Patterned ETFE Mold at Room Temperature and Its Application for Solar Cell
© Ding et al. 2016
- Received: 17 March 2016
- Accepted: 13 May 2016
- Published: 20 May 2016
Nanoimprinting lithography (NIL) is investigated as a promising method to define nanostructure; however, finding a practical method to achieve large area patterning of conjugated polymer remains a challenge. We demonstrate here that a simple and cost-effective technique is proposed to fabricate the nanoimprinted P3HT nanograting by solvent-assisted room temperature NIL (SART-NIL) method with patterned ETFE film as mold. The patterned ETFE template is produced by embossing ETFE film into a patterned silicon master and is used as template to transfer nanogratings during the SART-NIL process. It indicates that highly reproducible and well-controlled P3HT nanograting film is obtained successfully with feature size of nanogratings ranging from 130 to 700 nm, due to the flexibility, stiffness, and low surface energy of ETFE mold. Moreover, the SART-NIL method using ETFE mold is able to fabricate nanogratings but not to induce the change of molecular orientation within conjugated polymer. The conducting ability of P3HT nanograting in the vertical direction is also not damaged after patterning. Finally, we further apply P3HT nanograting for the fabrication of active layer of OBHJ solar cell device, to investigate the morphology role presented by ETFE mold in device performance. The device performance of OBHJ solar cell is preferential to that of PBHJ device obviously.
- ETFE film
- Nanoimprint lithography
- P3HT nanograting
- Solar cell
Conjugated polymers have been the focus of polymer and material science research in the past decades due to its excellent electricity, magnetic, and optic properties [1, 2]. Significant research efforts have been devoted to indicate that it is necessary to pattern the conjugated polymer film in pursuit of improving the performance of organic optoelectronic devices or requiring for special material processing . Different patterning methods of conjugated polymer, either bottom-up or top-down techniques, have been employed to perform in the performance optimization and device fabrication, such as self-assembly, template, and irradiation patterning technique [4–6]. However, finding a practical method to achieve large-scale industrial patterning of conjugated polymer remains a challenge to now.
Among these techniques, nanoimprinting lithography (NIL) is investigated as a promising method to define nanostructures due to its high resolution, effective cost, and simple process [7, 8]. Thermal NIL technique is mainly able to transfer the patterns from the mold to resist materials by forced mechanical deformation and achieve patterned structure on the resist surface. Therefore, NIL is a typical template patterning method and has shown great potential for shaping functional soft materials into nanostructures. For example, thin films of semiconducting polymers [9–12], ferroelectric polymers [13, 14], and proteins  have recently been directly shaped to obtain desirous nanostructures with well-defined morphology. NIL is a cost-effective patterning method because a template can be used repeatedly without losing its reproducibility. Thus, like the typical template patterning technique, the control of nanoimprint mold is very significant in the fabrication of polymer pattern by NIL technique. Some traditional molds with high resolution over a large area, such as silicon mold or anodic aluminum oxide mold, are reported to apply; however, they are usually time-consuming, complicated process, simple fragility and crush or easy to be deformed, not to meet the demand of different commerce purpose. Thus, future template fabrication techniques which possess cost-effective and simple processing method will be the next pursuits significantly and a long way in this field will go for challenge.
The fluoro-polymer ethylene(tetrafluoroethylene) (ETFE) is a copolymer of ethylene and tetrafluoroethylene and has been attracting more and more attention due to its excellent property . ETFE material possesses novel toughness, flexibility, and relatively high stiffness ability. Furthermore, ETFE film also bears a high melt point above 250 °C, which renders them useful in the thermal stability application. Thus, the copolymer is widely used in aerospace, nuclear utilization, and solar exploitation areas . Correspondingly, ETFE films are intensively explored on the molecular parameters and microstructures [18, 19], thermal and crystallization behaviors [20, 21], and rheology property researches [16, 22]. Recently, Chen et al.  indicated that the flexible UV transparent ETFE mold could be fabricated by the hot embossing with the HSQ master template and further used to fabricate the OTFT device. The performance of the device was improved greatly, and the process was suitable for the roll-to-roll processing of flexible electronics. Barbero et al.  showed that ETFE thin film could be employed to fabricate the high-resolution nanoimprinting mold with a robust and reusable property. Densely packed nanostructures down to 12 nm into a wide range of various polymers were able to gain, and this high resolution is mainly dependent on the template’s mechanical stability and resistance to distortion at high pressure and temperature. The successful patterning techniques of a variety of technological important polymers have been proposed by employing the patterned ETFE film as mold; however, a comprehensive understanding of patterning by ETFE film mold remains unknown and is still a challenge, especially for the patterning formation of conjugated polymer by ETFE mold and its application.
In addition, room temperature NIL has been proposed to obtain desired patterning of conjugated polymer thin films because conjugated polymer material is inclined to easily oxidize and decompose at elevated temperatures, potentially detrimental to the property and application of conjugated polymer [5, 9, 25]. Therefore, it is also meaningful at room temperature to fabricate the nanoimprinting patterning of conjugated polymer by patterned ETFE template. Then, an ordered bulk heterojunction (OBHJ) morphology consisting of vertically aligned conjugated polymer nanostructure surrounded by the acceptor materials is important to gain high performance of solar cell device [25, 26]. NIL technique was investigated as a promising method to achieve this nanostructure morphology within the OBHJ solar cell and was able to enhance the device performance significantly [27–29]. Therefore, it is necessary to fabricate the pattern structure of conjugated polymer using patterned ETFE mold and further employ this technique to prepare the OBHJ solar cell for application, to investigate the morphology role presented by the ETFE mold in device performance.
In this paper, the patterned ETFE template was produced by embossing ETFE film into a patterned silicon master and solvent-assisted room temperature NIL (SART-NIL) with patterned ETFE mold was employed as a means to fabricate nanoimprinted nanopattern on the surface of poly(3-hexylthiophene) (P3HT) thin film. Then, the technique is applied to fabricate the active layer of OBHJ solar cell with P3HT as donor and PCBM as acceptor. We aim to report that a simple and cost-effective technique on the fabrication of nanoimprinted P3HT nanograting using patterned ETFE film as a mold is able to achieve the preparation and its application of patterned P3HT thin film at room temperature.
Conjugated polymer P3HT (M w 50 000 g mol−1; regioregularity 98 %) and PCBM (purity 99.5 %) were obtained from Rieke Metals Inc. and Solenne B. V. Co., respectively.
Simple and cost-effective technique on the fabrication of patterned ETFE mold was produced by the conventional thermal imprinting method. The patterned silicon master face was laid against the surface of ETFE thin film (100-μm thickness), and then exerted pressure (70 bar) at 240 °C and held for 15 min according to nanoimprinter system (Obducat, Eitre 3). Before releasing the pressure, the stacks were evacuated to solidify ETFE thin film and temperature was cooled down to room temperature (23 °C). After removing the silicon master, the patterned ETFE thin film was achieved successfully.
Solvent-assisted room temperature NIL (SART-NIL) with patterned ETFE mold was employed as a means to fabricate nanoimprinted nanopattern. The P3HT thin films were obtained by spin coating (1500 rpm) from chlorobenzene solution (20 mg ml−1) onto substrate. After spin coating for 10 s, the polymer films were immediately transferred to nanoimprinter system (Obducat, Eitre 3) and covered with patterned ETFE film. The nanoimprinting lithography process was performed under pressure (50 bar) at room temperature (23 °C) and held for 15 min. After the patterned ETFE thin film separated, the P3HT nanograting film was obtained.
The organic solar cells were fabricated with P3HT and PCBM as donor and acceptor materials, respectively. ITO-coated glass was washed with deionized water, ethanol, acetone, and isopropyl alcohol. After the glass was dried, PEDOT:PSS (about 30-nm thickness) was spin cast onto the ITO surface treated with ultraviolet ozone. Then, the whole substrates were annealed at 125 °C for 20 min in air. Nanostructured P3HT surfaces were prepared by the SART-NIL method described above. Then, PCBM in dichloromethane solution (10 mg ml−1) was spin coated (900 rpm) onto the top of patterned P3HT film under ambient atmosphere for 60 s. For the contrast devices, the planar bulk heterojunction (PBHJ) solar cell was also fabricated by spin coating PCBM onto the unimprinted P3HT thin film. In the end, the devices were completed by evaporating a LiF layer (0.8-nm thickness) protected by aluminum electrode (100-nm thickness) at a base pressure of 4 × 10−4 Pa. The effective photovoltaic area was 10 mm2.
The morphology of samples was investigated by scanning electron microscopy (SEM, Hitachi S-4800), operated voltage at 15 kV. Contact angle measurements were performed using a tensiometer (SL200C, Kino of American Company). Grazing incidence wide-angle X-ray diffraction (GIWAXD) measurements were performed at the BL14B1 beam line at the Shanghai Synchrotron Radiation Facility in China. The wavelength and the incident angle of the X-ray beam are 0.12398 nm and 0.18°, respectively. The platinum/iridium-coated cantilevers (0.2 N/m force constant from nanosensors) were employed for the conducting atomic force microscopy (C-AFM) (MFP-3D-SA, Asylum Research) measurements and the bias voltage between the ITO substrate and conducting cantilever (V bias) was 1.2 V under the atmosphere environment and at room temperature. UV absorption spectrum was performed using UV3600 spectrometer (Shimadzu) in the transmission geometry mode. Current-voltage characteristics of solar cells were measured under illumination of white light (100 mW cm−2) from a Hg-Xe lamp filtered by a Newport 81094 Air Mass Filter, using a Gwinstek SFG-1023 source meter. The EQE measurement was carried out with monochromatic light from Hg-Xe lamp (Newport 67005) and monochromator (Oriel, Cornerstone 260). The response was recorded as the voltage over a 50 Ω resistance, using a lock-in amplifier (Newport 70104 Merlin).
Next, SART-NIL method with patterned ETFE mold is employed as a means to fabricate nanoimprinted grating on the surface of P3HT thin film, as shown in Scheme 1. SART-NIL is a simple and cost-effective technique to fabricate the nanopattern of conjugated polymer at room temperature . Room temperature NIL was proposed to obtain desired patterning of conjugated polymer films since conjugated polymer is incline to easily oxidize and decompose at elevated temperatures, potentially detrimental to the property and application of polymer [5, 9, 25]. The SART-NIL process consists of preparing P3HT thin films by spin coating for very short time and immediate nanoimprinting at room temperature. The residual solvent in the P3HT thin film resulting from short-time spin coating is able to lower the glass transition temperature and the viscosity of polymers during the NIL process, beneficial to the mobility of polymer molecule. It was reported that the high resolution was achieved due to the ETFE mold mechanical stability and resistance to distortion even at high pressures and high temperatures . Therefore, under the effect of residual solvent, pressure, and the stiffness of the ETFE mold, the polymer P3HT is able to flow into the nanocavities of patterned ETFE mold during NIL process at room temperature. After removing the ETFE thin film, the nanoimprinting P3HT pattern on the surface of polymer thin film is obtained conveniently. In fact, a balance between stiffness and flexibility is present in the ETFE material. During the demolding, the ETFE mold is bent and gradually separated from the polymer surface using only a low force . The ETFE mold does not break or deform and thus can be reused again for the employment to fabricate the P3HT nanostructure film for several times. Therefore, it is significant to employ this cost-effective and simple SART-NIL method using patterned ETFE film as a mold to fabricate the nanostructure of conjugated polymer.
The device performance parameters of solar cells under the illumination
The whole curves exhibit a broad response covering 300–700-nm wavelength, as shown in Fig. 7a. It indicates that the EQE spectra of the devices show a similar shape to their absorption spectra of active layer used in the device, showing both components (P3HT and PCBM) contributing to the photocurrent. Moreover, the EQE curve of OBHJ solar cell is obviously enhanced compared to that of PBHJ solar cell. As discussed above, the molecular orientation and vertical conducting property of nanograting P3HT film are compared to unimprinted film. Therefore, the enhancement of EQE spectra of the devices may be due to optical absorption enhancement, interfacial area increase and bi-continuous pathway within device structure.
The devices show similar open circuit voltage values (0.55 and 0.56 V), indicating that current generation from exciton dissociation at the interfaces between P3HT nanograting arrays and PCBM occurs in the both devices. In contrast to the similar open circuit voltage values, the PSCs based on the OBHJ film show significantly improved Jsc (from 3.49 to 6.55 mA cm−2) due to the formation of bi-continuous transportation pathways (P3HT nanograting arrays and filled PCBM) by the SART-NIL technique with ETFE mold. Considering about the mechanism of bulk heterojunction solar cell, the improvement of circuit of solar cell device based on OBHJ film may be determined by many effects. Firstly, the fabrication of nanoimprinted P3HT nanograting can be ensured by this cost-effective and simple SART-NIL method using patterned ETFE film as mold. Uniform P3HT nanograting arrays in the OBHJ film can effectively raise the interfacial area and thus efficiently increase the exciton dissociation efficiency, leading to the Jsc improvement. Then, the enhanced light absorption intensity of P3HT nanograting arrays based on OBHJ film is able to absorb more photons from sunlight and further contributes to the enhancement of Jsc. Finally, compared to the device based on PBHJ film, the exciton within the device based on OBHJ film is able to more easily reach the interface between P3HT and PCBM due to the shorter pathway of exciton diffusion and the higher efficiency of exciton transportation, leading to facilitate the photocurrent generation. In addition, the improvement of exciton dissociation efficiency due to the increase of interfacial area is also able to reduce the exciton recombination rate during exciton dissociation process and thus it is certain to improve the FF value of device based on OBHJ active layer. In all, the whole device performance of OBHJ solar cell is preferential to that of PBHJ device obviously, especially in the Jsc, FF, and PCE. Therefore, it confirms that the SART-NIL method using patterned ETFE film as mold is an effective technique to fabricate an ideal active layer bearing bi-continuous pathways and significantly improve the solar cell performance.
It is noted that we aim here to report a simple and cost-effective technique on the fabrication of nanoimprinted P3HT nanograting film by SART-NIL method with patterned ETFE film as mold and its application for active layer of OBHJ solar cell device. The efficiency of OBHJ solar cell device here does not show even higher value anticipated (here 2.18 %); however, it is not our desire to gain the absolute value of solar cell efficiency. The low efficiency of solar cell based on OBHJ active layer film may be limited by many factors of device fabrication process, such as the feature size of nanograting structure or under an ambient atmosphere.
In all, in this paper, we report a simple and cost-effective technique on the fabrication of nanoimprinted P3HT nanograting film by SART-NIL method with patterned ETFE film as mold and its application for active layer of OBHJ solar cell device. The patterned ETFE template is produced by embossing ETFE film into a patterned silicon master and SART-NIL method with patterned ETFE film as mold is employed as a means to fabricate nanoimprinted grating on the surface of P3HT thin film. It indicates that the highly reproducible and well-controlled P3HT nanograting film is obtained successfully with feature size of nanogratings ranging from 130 to 700 nm, due to the flexibility, stiffness, and low surface energy of patterned ETFE mold. Moreover, it is suggested that the SART-NIL method using ETFE film as mold is able to fabricate nanostructures but not to induce the change of molecular orientation within the conjugated polymer. The conducting performance of P3HT thin film in the vertical direction is also not damaged after patterning. Finally, we further apply the nanoimprinted P3HT nanograting film for the fabrication of OBHJ solar cell using SART-NIL method, to investigate the morphology role presented by the ETFE mold in device performance. P3HT and PCBM are used as donor and acceptor materials, respectively. It indicates that the whole device performance of OBHJ solar cell is enhanced obviously, especially in the Jsc, FF, and PCE.
This work was financially supported by the National Natural Science Foundation of China (No. 21504028), Natural Science Foundation of Educational Committee of Anhui Province (KJ2015A119), the Collaborative Innovation Center of Advanced Functional Composites in Anhui Province, and the BL14B1 beam line at the Shanghai Synchrotron Radiation Facility in China.
GD carried out the experiments, participated in the sequence alignment, and drafted the manuscript. XL and QC participated in the preparation of the solar cell device. QC, KW, ZH, and JL were involved in the SEM, GIWAXD, C-AFM, and performance analysis of the devices. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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