The Fabrication of Ordered Bulk Heterojunction Solar Cell by Nanoimprinting Lithography Method Using Patterned Silk Fibroin Mold at Room Temperature
© Ding et al. 2015
Received: 6 October 2015
Accepted: 14 December 2015
Published: 23 December 2015
The performance of organic solar cell is greatly determined by the nanoscale heterojunction morphology, and finding a practical method to achieve advantageous nanostructure remains a challenge. We demonstrate here that ordered bulk heterojunction (OBHJ) solar cell can be fabricated assisted by a simple, cost-effective nanoimprinting lithography method using patterned silk fibroin film mold at room temperature. The P3HT nanogratings were achieved by nanoimprinting lithography (NIL) process, and phenyl-C61-butyric acid methyl ester (PCBM) was spin-coated on the top of P3HT nanogratings. The conducting capacity of P3HT nanograting film has little difference compared with the unimprinted film in the vertical direction, due to the same edge-on chain alignment. However, it can be found that the fabrication of OBHJ nanostructure using room temperature NIL technique with patterned silk fibroin mold is able to promote optical absorption, interfacial area, and bicontinuous pathway. Therefore, the ordered heterojunction morphology plays an important part in improving device performance due to efficient exciton diffusion, dissociation, and reducing charge recombination rate.
The organic solar cells based on the bulk heterojunction (BHJ) have received considerable attention as an attractive alternative to silicon photovoltaic cell as they have achieved favorable characteristics, such as low cost, flexibility, and simple process [1, 2]. As long as there is light absorption, the excitons are generated, diffused, and dissociated at the interface between donor and acceptor materials and then transported to their respective electrodes, forming the external circuit . Therefore, the performance of organic solar cell is greatly determined by the nanoscale heterojunction morphology within active layer. One of the ideal structures in active layer is to construct an ordered bulk heterojunction (OBHJ) morphology consisting of vertically bicontinuous and interdigitized heterojunction between donor and acceptor materials, to enable both efficient exciton separation and transport [4–6].
Despite OBHJ nanostructure morphology contributes to the solar cell performance and a comprehensive understanding of fundamental principle, finding a practical method to achieve this nanostructure remains a challenge to now. Several techniques, such as polymer nanowires, block copolymer, and nanoimprinting lithography (NIL), have been reported to fabricate OBHJ solar cell [7–9]. Among these techniques, thermal NIL is investigated as a promising method to define nanostructures due to its high resolution, effective cost, and simple process [10, 11]. NIL method is able to replicate the nanostructures defined on a hard mold into some soft materials, such as semiconducting polymers [12–19], ferroelectric polymers [20, 21], and proteins [22, 23]. Therefore, the control of nanoimprint mold is of great importance in the fabrication of OBHJ solar cell 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, have complicated process, have simple fragility, crush or deformed easily, and do not to meet the need of commercial application. It is desired to seek a cost-effective and simple process method for mold fabrication.
Silk fibroin film from the Bombyx mori silkworm has attracted considerable interest owing to its biological, mechanical, and optical properties [24–27]. Silk fibroin film is able to be easily patterned by several techniques [22, 28] and has been applied to biocompatible and degradable electronic or photonic devices [29, 30]. In nanoimprinting process, the application of heat and pressure to a patterned silk fibroin mold, which is fabricated by control of the water content and beta sheet crystallinity within silk film, can be accomplished to transfer nanostructure to other soft materials . Silk fibroin film was chosen for the nanoimprinting mold mainly due to its advantageous material properties as well as simple and inexpensive production process [31, 32]. The Bombyx mori silkworm which is used to produce patterned silk fibroin film comes from broad source and is cheap. Silk fibroin film is able to be easily patterned, and the fabrication process is simple and convenient, fully meet to the demand of large area production. Then, there is little interaction between silk film and conjugated polymer, for example, polymer P3HT, and thus, there is no need for special surface treating of patterned silk mold to facilitate template separation after nanoimprinting. Super stiffness and high modulus are present in the silk fibers, and it can guarantee to bear more pressure during NIL process and the three-dimensional size stability of nanostructure within silk film. Furthermore, compared to the conventional silicon or glass molds, the silk mold is not easy to be crushed and allows for direct conformal imprinting on curved surface, due to the flexibility of silk film material.
In addition, some conjugated polymer materials are easily oxidized and decomposed at elevated temperatures , potentially detrimental to the organic semiconducting device performance. To overcome the drawback of thermal NIL applied to conjugated polymers, room temperature NIL has been proposed to obtain desired nanostructures in conjugated polymer thin films [5, 12]. Therefore, it is significant at room temperature to produce the nanostructures of conjugated polymer by NIL method. The fabrication of OBHJ solar cell assisted by NIL technique with patterned silk fibroin film as nanoimprint mold, however, is rarely investigated, especially using the NIL process at room temperature.
In this study, we employ the patterned silk fibroin film as a template and room temperature NIL as a method to fabricate the active layer of OBHJ solar cell with P3HT as donor and phenyl-C61-butyric acid methyl ester (PCBM) as acceptor. We aim to report that the fabrication of OBHJ solar cell can be achieved by using the patterned silk fibroin film at room temperature. The influence of OBHJ active layer nanostructure fabricated by this convenient NIL method on the device performance of solar cell is investigated in details. Notably, the discussion of molecular orientation of polymer P3HT nanograting film is also investigated.
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.
The purified silk fibroin solution was drop cast onto a clean silicon sheet (2 × 2 cm). The production of the silk fibroin solution had been carried out as the previously published protocols . The patterned polydimethylsiloxane (PDMS) film was laid against the silk fibroin solution surface without exerting any pressure and dried for 24 h at room temperature. After removing the patterned PDMS film, the patterned silk film on the surface of silicon was treated with methanol solution (volume, 90 %) for about 5 h to induce β-sheet transition, leading to the patterned silk film water insoluble. The patterned silk fibroin films were subsequently dried for at least 24 h under vacuum.
The organic solar cells were fabricated with P3HT and PCBM as donor and acceptor materials, respectively. Indium tin oxide (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. The P3HT thin films were obtained by spin coating (1600 rpm) from chlorobenzene solution (20 mg ml−1) onto PEDOT:PSS-coated ITO/glass substrate. After spin coating for 10 s, the polymer films were immediately transferred to a nanoimprinter system (Obducat, Eitre 3) and covered with patterned silk film. The nanoimprinting lithography process was performed under pressure (60 bar) at room temperature (23 °C) and held for 15 min. After the patterned silk film separated, the P3HT nanograting film was obtained. Then, PCBM in dichloromethane solution (10 mg ml−1) was spin-coated (800 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 12 mm2.
The morphology of samples was shown with scanning electron microscopy (SEM, Hitachi S-4800), with an operating voltage of 15 kV. Grazing incidence wide angle X-rays 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. Data conversion to q space was obtained by calibration using LaB6 powder. 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 (Vbias) 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 external quantum efficiency (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). All the measurement processes were carried out under ambient atmosphere and at room temperature.
Results and Discussion
It is noted that the patterned silk film on the surface of silicon is treated with methanol solution to make the patterned silk film water insoluble and can be used at atmospheric environment. Then, there is little interaction between silk film and conjugated polymer P3HT. Therefore, the silk mold can be reused again after the employment to fabricate the P3HT nanostructure film.
In addition, the periodic nanostructure is able to enhance light trapping and thus increases the optical absorption by increasing the optical pass length relatively. Therefore, the enhancement of UV-vis absorption intensity can be attributed to light trapping. Of course, the UV-vis absorption intensity can be promoted by simply increasing the thickness of P3HT layer. However, the same effect as the ordered bulk heterojunction solar cell cannot be obtained by the simply increasing the thickness of P3HT layer and without the nanostructure fabricated by the NIL method. There are three reasons. First of all, although the UV-vis absorption intensity is enhanced due to the increase of P3HT layer thickness, the efficiency of the excitons transporting to the corresponding electrodes is reduced obviously due to the increase of exciton transporting pathway, and thus the device current is also declined. Second, the enhancement of UV-vis absorption intensity is also limited only by increasing the thickness of P3HT layer. Third, the whole fabrication process of organic solar cell is complicated, and here, we aim to report that the fabrication of OBHJ solar cell can be achieved by using the patterned silk fibroin film at room temperature. The result indicates the periodic nanostructure fabricated by NIL method is able to enhance the UV-vis absorption intensity and it is not our goal to promote the UV-vis absorption intensity only. Therefore, it indicates that the fabrication of OBHJ nanostructure using NIL technique with silk fibroin mold at room temperature is able to promote the UV-vis absorption intensity, beneficial to the solar cell performance.
The device performance parameters of solar cells under the illumination
V OC (V)
J SC (mA cm-2)
It indicates that the solar cell based on the OBHJ active layer film shows a significant improvement of J SC (from 4.15 to 7.14 mA cm−2) due to the formation of bicontinuous transportation pathways within OBHJ film by the NIL process using silk mold. Considering about the mechanism of bulk heterojunction solar cell, the enhancement of J SC of device based on OBHJ film may be determined by many factors. One reason for the circuit improvement is attributed to the enhanced light absorption intensity of P3HT nanograting within OBHJ active layer as discussed above, and thus, the device is able to absorb more photons to produce exciton. The second reason for the circuit increase may be attributed to the addition of interfacial area between the donor and acceptor materials. The aligned P3HT nanograting morphology within OBHJ active layer film fabricated by NIL process is able to effectively raise the interfacial area and in the end can efficiently increase the exciton dissociation efficiency, leading to the J SC improvement. Third, with the fabrication of nanograting by NIL method, the exciton 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. Therefore, the P3HT nanograting fabrication by NIL process using silk mold at room temperature significantly contributes to the J SC improvement of OBHJ solar cell. Furthermore, the improving exciton dissociation efficiency due to the increased interfacial area between P3HT and PCBM is able to reduce the exciton recombination rate during exciton dissociation process, and thus it is inevitable to enhance the FF value of OBHJ solar cell.
As shown in Fig. 7b, the EQE curve of OBHJ solar cell is clearly enhanced compared to the curve of PBHJ solar cell, and the whole curves exhibit a broad response covering 300–700 nm wavelength. In addition, the EQE spectra of the devices resemble its absorption spectra of active layer used in the device, showing both components (P3HT and PCBM) contributing to the photocurrent. Although the molecular orientation and vertical conducting property of nanograting P3HT film are compared to unimprinted film, the ordered heterojunction morphology plays an important part in improving device performance due to optical absorption enhancement, interfacial area increase and bicontinuous pathway. The aligned P3HT nanograting structure of active layer can contribute to exciton diffusion and dissociation and reduction of charge recombination rate. Therefore, J SC, FF and PCE of solar cell based on OBHJ film show a larger value compared to PBHJ solar cell. In all, these improved performances firmly confirm that the NIL method using patterned silk fibroin mold at room temperature is an effective technique to fabricate an ideal active layer bearing bicontinuous pathways and significantly improve the solar cell performance.
It indicates that although the solar cell fabricated by NIL process using patterned silk fibroin mold shows two times PCE than that of controlled PBHJ device, the efficiency does not show even higher value anticipated. However, in this study, we aim to report that the fabrication of OBHJ solar cell can be achieved by NIL process using the patterned silk fibroin film at room temperature and to investigate the influence of OBHJ active layer nanostructure fabricated by this convenient NIL method on the device performance of solar cell. Therefore, the absolute value seeking of solar cell efficiency is not our target. The limited efficiency of solar cell based on OBHJ active layer film may be determined by these factors: (a) the feature size with ~250 nm is much larger than the typical exciton diffusion length (10~20 nm) and limits more efficient excitons dissociation; (b) the fabrication process is performed under room temperature, and there is no any thermal history; thus the crystallinity of P3HT material is finite, which may affect the charge transport; (c) the device fabrication and characteristics testing process are all carried out under ambient atmosphere and not protected by any inert gas or in the glove box, which may reduce the device performance.
In summary, we demonstrated here that solar cell device based on OBHJ film can be fabricated by a simple, cost-effective nanoimprinting lithography method using nanopatterned silk fibroin mold at room temperature. The preparation of patterned silk fibroin mold, consisting of the drop-casting of silk fibroin solution and the formation of silk nanostructure film replicated from patterned PDMS film, was a simple and practical process. P3HT and PCBM were chosen as donor and acceptor materials, respectively. The P3HT nanogratings were achieved by pressing patterned silk fibroin mold against P3HT film at room temperature, and PCBM was spin-coated on the top of P3HT nanogratings. It indicated that edge-on chain alignment dominated in P3HT nanograting film, which may be due to unconfinement of P3HT rod-like crystals. Therefore, it showed that the NIL process with patterned silk fibroin mold at room temperature was not able to damage the charge transportation along the direction perpendicular to the substrate. However, the fabrication of OBHJ nanostructure was able to promote the UV-vis absorption intensity, beneficial to the solar cell performance. Furthermore, the ordered heterojunction morphology played an important part in improving device performance due to optical absorption enhancement, interfacial area increase, and bicontinuous pathway. Therefore, J SC, FF and PCE of solar cell based on OBHJ film were enhanced because the aligned P3HT nanograting structure of active layer contributed to exciton diffusion and dissociation and reducing charge recombination rate.
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.
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