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Low-temperature preparation of crystallized graphite nanofibers for high performance perovskite solar cells
Solar Energy 193 (2019) 205–211
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Low-temperature preparation of crystallized graphite nanofibers for high performance perovskite solar cells ⁎
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Lijun Yanga, , Pan Yanga, Jingchuan Wanga, Yawei Haoa, Yintao Lib, Hong Linc, , Zhao Xiaochonga,b a
Institute of Materials, Chinese Academy of Engineering Physics, Jiangyou 621908, China State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, China c State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphite nanofibers Electrospinning Graphitization temperature Perovskite solar cell Charge transfer
Integration of two-dimensional graphite nanosheets into one-dimensional graphite nanofibers is essential for deep application of graphite materials. Here, we report preparation of crystallized graphite nanofibers by nozzleless electrospinning and subsequent low-temperature heat treatment (500 °C). Depending on this graphitized one-dimensional nanostructure, we first apply the graphite nanofibers as scaffold for perovskite solar cells (PSCs), which would provide a charge transfer highway in the light absorb layer and then improve the performance of devices. A high power conversion efficiency of 18.23% was recorded for the graphite nanofibers based PSC with high fill factor of 76%.
1. Introduction Carbon materials, such as graphite, carbon nanosheet, carbon nanofibers, carbon nanotube et al. have attracted intense attention as available and inexpensive materials due to its excellent in plane mechanical, thermal and electrical properties, which has been wide applied in catalysis, optoelectronics, gas separation and memory storage (Ricke et al., 2017; Ricciardulli et al., 2018; Koren et al., 2015; Abdelkader et al., 2015; Hou et al., 2019; Luo et al., 2018; Wu et al., 2019; Ma et al., 2019; Ma et al., 2019; Zhao et al., 2019). These excellent properties may be amplified if planar graphite can be designed to be one-dimensional fiber-shape nanostructure (Zhu et al., 2019; Gu et al., 2019; Le et al., 2019; Gong et al., 2019; Idrees et al., 2019; Du et al., 2019; Kirubasankar et al., 2018; Xie et al., 2018; Dong et al., 2018). Typical techniques used to produce graphite nanofibers or carbonaceous one-dimensional nanomaterials involve chemical vapor deposition (CVD), solid-phase synthesis and electrospinning (Lai et al., 2016; Wang et al., 2016; Ren et al., 2015; Liu et al., 2017; Nishihara et al., 2017; Chen et al., 2018; Li et al., 2017). Electrospinning with subsequent thermal treatment, involving stabilization and carbonization processes, is a simple, cost-effective and versatile approach to produce carbon nanofibers from polymer-based precursor solutions (Wang et al., 2016). However, so far, most carbon nanofibers produced by electrospinning exhibits relatively poor graphitic structures,
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consequently, non-optimial properties (Liu et al., 2019; Yang et al., 2019; Ren et al., 2019; Lin et al., 2019; Lin et al., 2019; Ma et al., 2019; Li et al., 2019; Wang et al., 2019). Increasing carbonization temperature (1200–3000 °C) is an effect approach to improve graphitization of carbon nanofibers (Wang et al., 2016; Chen et al., 2017; Mao et al., 2013; Wang et al., 2012; Zussman et al., 2006; Chen et al., 2014). However, such high temperature requires specialized, expensive equipment that limits economic manufacturing and its applications to some low melting-point substrates for electronic devices. Another strategy for improving graphitization of carbon nanofibers is incorporating oriented inclusions (or called “templates”) that can increase the alignment of the polymer chains inside the spun-polymer nanofibers (precursor nanofibers) (Chae et al., 2007; Wang et al., 2017; Hashmi et al., 2017; Hashmi et al., 2017; Hou et al., 2005; Maitra et al., 2012; Papkov et al., 2013). Carbon nanotube has been used as nanoscale inclusion for several types of fibers, and it indeed increased chains alingnment, however, the improvements were modest, and at some architectures, carbon nanotube ends sticked out of carbon nanofibers (Chae et al., 2007; Wang et al., 2017; Hashmi et al., 2017). Dzenis et al. incorporated a small amount of graphene oxide in precursor solution and showed improved graphitic order and orientation in carbon nanofibers, however, the carbonization temperature was still higher than 800 °C (Papkov et al., 2013). Therefore, fabrication of carbon nanofibers with quality graphitic structures at low carbonization
Corresponding authors. E-mail addresses: [email protected] (L. Yang), [email protected] (H. Lin), [email protected] (X. Zhao).
https://doi.org/10.1016/j.solener.2019.09.065 Received 4 April 2019; Received in revised form 11 August 2019; Accepted 17 September 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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MeOTAD, 28.8 μL of 4-tert-butylpyridine, 17.5 μL of lithium bis(trifluor-omethylsulphonyl)imide in acetonitrile in 0.99 mL chorobenzene. The HTL coated substrates were kept overnight before depositing the counter-electrode. Finally, a 100 nm thick Au were thermally evaporated on top of these devices through the shadow mask with an effective area of 0.12 cm2.
temperatures is a great challenge. Here, we show that crystallized graphite nanofibers with high quality can be produced at significant yield by nozzle-less electrospinning and following low graphitization temperature (500 °C). Incorporating one-dimensional nanostructure materials or carbon based materials, such as TiO2 nanofibers, carbon nanotubes/TiO2 nanofibers, carbon nanotubes, fullerene, etc., in photoelectrodes to improve the electron extract and transfer properties of solar cells have been widely reported (Viculis et al., 2003; Luo et al., 2011; Ramli et al., 2016; Kojima et al., 2009; Lee et al., 2012; Saliba et al., 2016; Bush et al., 2017; Yang and Leung, 2013; Batmunkh et al., 2017; Ramos et al., 2015; Han et al., 2015; Habisreutinger et al., 2017; Yang and Leung, 2011; Lee et al., 2015; Habisreutinger et al., 2014; Abrusci et al., 2013; Batmunkh et al., 2015; Yang et al., 2019; Li et al., 2016). Benefiting from crystallized one-dimensional nanostructure, graphite nanofibers should be suitable for using as electrodes for perovskite solar cells (PSCs), especially for 1D (Li et al., 2019; Wan et al., 2019; Wu et al., 2019). Therefore, in this paper, high quality graphite nanofibers have been utilized as scaffold for PSCs to improve electron transfer property and then enhance performance of devices.
2.3. Characterization
2. Experimental
The morphology of nanofibers were examined by atomic force microscopy (AFM, NT-MDT Spectra), scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscope (TEM, Hitachi HT7700) with 200 kV acceleration voltage. The crystal structure of nanofibers was measured by Raman spectra (Renishaw invia) and X-ray diffraction (XRD, Bruker D8) with Cu-Kα radiation with 2θ range of 1060o. The stability was performed on thermogravimetric differential scanning calorimetry (TG-DSC, Netzsch STA 449F3) under a N2 thermobalance with heating rate of 10 K min−1. The time-resolved photoluminescence (TRPL) was characterized by fluorescence spectrophotometer (Edinburgh FSL980). The photovoltaic characterization was carried out using a Keithley 2400 digital source meter under illumination of AM1.5G 100 mW cm−2 from a solar simulator.
2.1. Nanofiber synthesis
3. Results and discussion
Graphite powder (1 g, Alfa) was dissolved in DMF (10 mL) and dispersed by ultrasonic cell disruption for 12 h to obtain graphite nanosheets (solution 1). Polyvinylpyrrolidone (PVP, 1 g, Mw = 1,300,000, Aldrich Co.) and polyacrylonitrile (PAN, 1 g, Mw = 150,000, Aldrich Co.) were dissolved in DMF (10 mL) and stirred for 4 h (solution 2). Then, the precursor for electrospinning was prepared by mixed solution 1 (10 mL) with solution 2 (10 mL) and stirred for 30 min. Using nozzle-less apparatus, electrospinning was carried out by applying a high positive voltage of 60 kV over a collector distance of 15 cm. The electrospun nanofibers for characterization were collected in aluminum foil. The obtained nanofibers were stabilized in air at 250 °C for 1 h and then carbonized under Ar atmosphere at 500 °C for 2 h with heating rate of 5 °C min−1.
As shown, synthesis of graphite nanofibers by nozzle-less electrospinning is illustrated in Fig. 1. First of all, through high-energy sonication, graphite powders partially separated in connected graphite nanosheets with diameter around 2 µm and thickness about 6 nm, as confirmed by AFM in Fig. 2. And then, PVP, PAN, graphite nanosheets were dissolved in DMF as precursor solution for electrospinning. PVP and PAN were chosen as carriers to adjust the viscosity of precursor solution, graphite nanosheets function as the main carbon source. Graphite nanosheets within precursor solution were rolled to be onedimensional graphite/PVP/PAN composite precursor nanofibers during electrospinning. In order to fix thermal treatment program and explore thermal stability of precursor nanofibers, thermogravimetric-differential scanning calorimetry (TG-DSC) was carried out from room temperature to 800 °C. From TG-DSC curves (Fig. 3), it can be seen that there was almost no change in weight and heat beyond of 500 °C for the precursor nanofibers. According to these results, graphite nanofibers were obtained by oxidized the graphite/PVP/PAN precursor fibers at 250 °C in air for 2 h (stabilized) and then carbonized at 500 °C under Ar atmosphere for 2 h (Hou et al., 2018). Fig. 4a is SEM image of graphite nanofibers which exhibit a long and straight fibrous morphology with diameter ranging from 100 to 300 nm (average diameter around 200 nm). Taking a closer examination, there are some loosen parts (circled area) in the nanofibers, which would be further investigated by TEM. Fig. 4b–d are TEM images of the straight, loosen and end parts of graphite nanofibers, respectively. From the loosen (Fig. 4c) and end part (Fig. 4d), we can clearly see the laminar structure, which indicating graphite nanofibers were rolled from planar graphite nanosheets under assistance of elongation electrostatic forces. Similar phenomenon has been observed in exfoliated graphene sheets that could be bended and folded into various shapes and able to roll into scrolls under certain conditions (Wang et al., 2017; Hashmi et al., 2017). Taking in-depth insight into structure of graphite nanofiber, XRD and Raman spectra were carried out. XRD pattern of graphite powder and graphite nanofibers is shown in Fig. 5a. The very sharp diffraction peaks at 26.5° assigned to the graphite crystallographic plane (0 0 2). (1 0 0) and (0 0 4) planes can been observed either. Hence, graphite nanofibers with high graphitization degree can be obtained by technique proposed here at low temperature of 500 °C, which is much lower than conventional carbonization temperature. Raman spectroscopy was carried out to obtain more detailed structure information of graphite
2.2. Solar cells fabrication Unless stated otherwise, all the fabrication processes were taken in the glove-box. Devices were fabricated on the laser etched FTO glass substrates with a sheet resistance of 10–15 Ω square−1. The substrates were cleaned by ultrasonic in soap water, deionization water, acetone, 2-propanol, and subjected to an UV-ozone treatment for 30 min. An approximate 30 nm thick c-TiO2 was spun-coated on the substrates by using a 0.15 M TIP ethanol solution at a speed of 3000 rpm for 30 s. Subsequently, the sample was calcinated at 450 °C for 2 h. After calcination, graphite nanofibers were electrospun on c-TiO2 and stabilized in air at 250 °C for 1 h and carbonized under Ar atmosphere at 500 °C for 2 h with heating rate of 5 °C min−1. The thickness of graphite scaffold was determined by electrospinning time. Here, the electrospinning time was about 1 min with a layer of graphite nanofibers. PbI2 was dissolved in DMF at a concentration of 461 mg mL−1 under stirring at 70 °C, which was maintained during the entire fabrication process. After filtration with 0.25 μm pore size filter, the PbI2 layer was deposited by spin-coating on the graphite nanofibers scaffold layer with 1500 rpm for 30 s, and dried at 70 °C for 30 min. Subsequently upon cooling down to room temperature, the PbI2 coated substrates were immersed in the organic part CH3NH3I solution (30 mg mL−1 in 2-propanol) for 90 s to complete the reaction. Finally, a rinsed step with 2-propanol was required for removing the excess organic fraction and the resulting sample was dried at 90 °C for 30 min. The hole-transport layer (HTL) was subsequently deposited by spin-coating at a speed of 4000 rpm for 30 s. The HTL solution was prepared by dissolving 79 mg of Spiro206
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Fig. 1. Schematic illustration of the nozzle-less electrospinning fabrication of graphite nanofibers.
nanofibers (Fig. 5b). Graphite powder and graphite nanosheets were also characterized for comparison. Raman spectra of pristine graphite powder, as expected, shows a prominent G-band peak at around 1580 cm−1 related to ordered graphitic structures, corresponding to sp2 hybridized carbon, and this band is correlated to the (0 0 2) diffraction peak in the XRD pattern. Graphite nanosheets exhibits the similar behavior, the dominate G-band. The spectrum of graphite nanofibers also reveals the dominate G-band and a very week D-band (around 1350 cm−1). The D-band corresponding to defects or disorder in the sp2 hybridized carbon. It can be seen that the half width at half maximum (HWHM) of G-band of graphite nanofibers is increased a little comparing with graphite powder. Therefore, it can be conclude that graphite nanofibers show an ordered graphite crystallites with a little disordered carbonaceous components. The Raman results are in good agreement with XRD and these results reveal that graphite nanofibers possesses a graphitic character. Crystallized graphite nanofibers should be suitable use as electron transfer high-way in PSCs due to their one-dimensional nanostructure. Therefore, graphite nanofibers were utilized as scaffold for PSCs. The fabrication procedure of graphite nanofibers scaffold based PSCs was displayed in Fig. 6a. A layer of graphite nanofibers scaffold was electrospun on TiO2 compact layer (c-TiO2), and then perovskite layer (CH3NH3PbI3, MAPbI3) was deposited on this scaffold, after that, hole transport layer (Spiro-MeOTAD) and Au electrode was deposited in turn. In order to investigate the effect of graphite nanofibers scaffold on charge-transport dynamics in PSCs, conductive AFM (c-AFM) was carried out. Fig. 6b is AFM topological images (left) along with the corresponding c-AFM images (right) for perovskite films with graphite nanofibers scaffold on FTO glass. Darker contrast in c-AFM indicating more conducting current flow through the perovskite layer, which revealed that graphite nanofibers provide a pathway for charge conductance. At the same time, time resolved photoluminescence (TRPL) spectrum of a series of perovskite films with m-TiO2 and graphite nanofibers scaffolds are also adopted. The PL decay monitored at the peak emission (770 nm) with 375 nm excitation (Ti: sapphire laser) are shown in Fig. 6c, where samples of FTO/c-TiO2/graphite nanofibers/ MAPbI3 and FTO/c-TiO2/mesoporous TiO2 (m-TiO2)/MAPbI3 were prepared and tested under front illumination. The effect of graphite nanofibers scaffold may affect the carrier dynamics, therefore, perovskite thin film with and without graphite nanofibers scaffold were prepared for comparison. The fast decay component τ1 can be ascribed to the quenching of free carriers from perovskite to m- TiO2 or graphite
Fig. 2. AFM image of graphite nanosheets.
Fig. 3. TG-DSC curves of graphite/PVP/PAN precursor nanofibers.
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Fig. 4. (a) SEM image of electrospun graphite nanofibers; (b), (c) and (d), TEM images of different parts of graphite nanofibers.
perovskite, which could benefit to performance of PSCs. The current density (J)-voltage (V) characteristics of PSCs devices with graphite nanofibers and m-TiO2 scaffolds are shown in Fig. 6d and their performance parameters are inserted. The efficiency of 18.23% was obtained for the device with graphite nanofibers scaffold, 21% improvement in performance compare with device with m-TiO2 scaffold. The current density and open voltage almost maintained the same. The improved efficiency mainly due to the improved fill factor (FF), which increased from 63% to 76%, 20.6% enhancement. In order to investigate the reproducibility of the photovoltaic performance, 20 cells for each condition were made and measured. The graphite nanofibers
nanofibers (Yang et al., 2019; Li et al., 2016; Hou et al., 2018; Qin et al., 2018; Ponseca et al., 2014; Xing et al., 2013; Stolterfoht et al., 2018). From the fitted results, it can be seen that τ1 in graphite nanofibers scaffold based perovskite thin film were slower than that in m-TiO2 scaffold, which represent improved charge transportation in FTO/cTiO2/graphite nanofibers/MAPbI3. The fast decay indicates improved charge transportation, which associated with the appearance of graphite nanofibers. Therefore, it is apparent that graphite nanofibers provide a pathway for charge conductance that benefit to performance of PSCs. The graphite nanofibers provide a charge transfer highway in
Fig. 5. (a) XRD pattern of graphite powder and graphite nanofibers; (b) Raman spectrum of graphite powder, graphite nanofibers and graphite nanosheets. 208
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Fig. 6. (a) Fabrication sequence of graphite nanofibers scaffold photoanode based PSCs and corresponding SEM images; (b) AFM (left) and c-AFM (right) of perovskite layer with graphite nanofibers scaffold; (c) TRPL spectrua of MAPbI3 with m-TiO2 and graphite nanofibers scaffolds; (d) J-V curves of perovskite solar cells based on graphite nanofibers scaffold.
evolution of other important photovoltaic parameters can also be observed in Fig. 7. These include the degradation of Jsc and FF over time, which are the major reasons for decrease of PCE.
scaffold based PSCs showed an average PCE of 17.8% with open voltage (Voc) of 1.01 V, short current density (Jsc) of 23.42 mA cm2 and FF of 75%. The longitudinal follow up on the photovoltaic parameters of devices with m-TiO2 and graphite nanofibers scaffold can be found in Fig. 7. These devices are encapsulated simply by the epoxy glue covered with glass slides and maintained in air at room temperature and over 60 RH%. The devices with graphite nanofibers scaffold shows around 37% decrease in PCE over 500 h, and the performance of perovskite solar cell with m-TiO2 scaffold decreases about 50% after 500 h. Besides PCE,
4. Conclusions In summary, we have demonstrated a simple and low temperature method to fabricate high quality graphite nanofibers from graphite powder and they can be efficient applied as scaffold in PSCs. The graphitized one dimensional graphite nanofibers provide a charge transfer 209
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Fig. 7. Stability on performance of perovskite solar cells with m-TiO2 and graphite nanofibers scaffold exposed in air at room temperature and over 60 RH %
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Low-temperature preparation of crystallized graphite nanofibers for high performance perovskite solar cells ⁎
T
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Lijun Yanga, , Pan Yanga, Jingchuan Wanga, Yawei Haoa, Yintao Lib, Hong Linc, , Zhao Xiaochonga,b a
Institute of Materials, Chinese Academy of Engineering Physics, Jiangyou 621908, China State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, China c State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphite nanofibers Electrospinning Graphitization temperature Perovskite solar cell Charge transfer
Integration of two-dimensional graphite nanosheets into one-dimensional graphite nanofibers is essential for deep application of graphite materials. Here, we report preparation of crystallized graphite nanofibers by nozzleless electrospinning and subsequent low-temperature heat treatment (500 °C). Depending on this graphitized one-dimensional nanostructure, we first apply the graphite nanofibers as scaffold for perovskite solar cells (PSCs), which would provide a charge transfer highway in the light absorb layer and then improve the performance of devices. A high power conversion efficiency of 18.23% was recorded for the graphite nanofibers based PSC with high fill factor of 76%.
1. Introduction Carbon materials, such as graphite, carbon nanosheet, carbon nanofibers, carbon nanotube et al. have attracted intense attention as available and inexpensive materials due to its excellent in plane mechanical, thermal and electrical properties, which has been wide applied in catalysis, optoelectronics, gas separation and memory storage (Ricke et al., 2017; Ricciardulli et al., 2018; Koren et al., 2015; Abdelkader et al., 2015; Hou et al., 2019; Luo et al., 2018; Wu et al., 2019; Ma et al., 2019; Ma et al., 2019; Zhao et al., 2019). These excellent properties may be amplified if planar graphite can be designed to be one-dimensional fiber-shape nanostructure (Zhu et al., 2019; Gu et al., 2019; Le et al., 2019; Gong et al., 2019; Idrees et al., 2019; Du et al., 2019; Kirubasankar et al., 2018; Xie et al., 2018; Dong et al., 2018). Typical techniques used to produce graphite nanofibers or carbonaceous one-dimensional nanomaterials involve chemical vapor deposition (CVD), solid-phase synthesis and electrospinning (Lai et al., 2016; Wang et al., 2016; Ren et al., 2015; Liu et al., 2017; Nishihara et al., 2017; Chen et al., 2018; Li et al., 2017). Electrospinning with subsequent thermal treatment, involving stabilization and carbonization processes, is a simple, cost-effective and versatile approach to produce carbon nanofibers from polymer-based precursor solutions (Wang et al., 2016). However, so far, most carbon nanofibers produced by electrospinning exhibits relatively poor graphitic structures,
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consequently, non-optimial properties (Liu et al., 2019; Yang et al., 2019; Ren et al., 2019; Lin et al., 2019; Lin et al., 2019; Ma et al., 2019; Li et al., 2019; Wang et al., 2019). Increasing carbonization temperature (1200–3000 °C) is an effect approach to improve graphitization of carbon nanofibers (Wang et al., 2016; Chen et al., 2017; Mao et al., 2013; Wang et al., 2012; Zussman et al., 2006; Chen et al., 2014). However, such high temperature requires specialized, expensive equipment that limits economic manufacturing and its applications to some low melting-point substrates for electronic devices. Another strategy for improving graphitization of carbon nanofibers is incorporating oriented inclusions (or called “templates”) that can increase the alignment of the polymer chains inside the spun-polymer nanofibers (precursor nanofibers) (Chae et al., 2007; Wang et al., 2017; Hashmi et al., 2017; Hashmi et al., 2017; Hou et al., 2005; Maitra et al., 2012; Papkov et al., 2013). Carbon nanotube has been used as nanoscale inclusion for several types of fibers, and it indeed increased chains alingnment, however, the improvements were modest, and at some architectures, carbon nanotube ends sticked out of carbon nanofibers (Chae et al., 2007; Wang et al., 2017; Hashmi et al., 2017). Dzenis et al. incorporated a small amount of graphene oxide in precursor solution and showed improved graphitic order and orientation in carbon nanofibers, however, the carbonization temperature was still higher than 800 °C (Papkov et al., 2013). Therefore, fabrication of carbon nanofibers with quality graphitic structures at low carbonization
Corresponding authors. E-mail addresses: [email protected] (L. Yang), [email protected] (H. Lin), [email protected] (X. Zhao).
https://doi.org/10.1016/j.solener.2019.09.065 Received 4 April 2019; Received in revised form 11 August 2019; Accepted 17 September 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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MeOTAD, 28.8 μL of 4-tert-butylpyridine, 17.5 μL of lithium bis(trifluor-omethylsulphonyl)imide in acetonitrile in 0.99 mL chorobenzene. The HTL coated substrates were kept overnight before depositing the counter-electrode. Finally, a 100 nm thick Au were thermally evaporated on top of these devices through the shadow mask with an effective area of 0.12 cm2.
temperatures is a great challenge. Here, we show that crystallized graphite nanofibers with high quality can be produced at significant yield by nozzle-less electrospinning and following low graphitization temperature (500 °C). Incorporating one-dimensional nanostructure materials or carbon based materials, such as TiO2 nanofibers, carbon nanotubes/TiO2 nanofibers, carbon nanotubes, fullerene, etc., in photoelectrodes to improve the electron extract and transfer properties of solar cells have been widely reported (Viculis et al., 2003; Luo et al., 2011; Ramli et al., 2016; Kojima et al., 2009; Lee et al., 2012; Saliba et al., 2016; Bush et al., 2017; Yang and Leung, 2013; Batmunkh et al., 2017; Ramos et al., 2015; Han et al., 2015; Habisreutinger et al., 2017; Yang and Leung, 2011; Lee et al., 2015; Habisreutinger et al., 2014; Abrusci et al., 2013; Batmunkh et al., 2015; Yang et al., 2019; Li et al., 2016). Benefiting from crystallized one-dimensional nanostructure, graphite nanofibers should be suitable for using as electrodes for perovskite solar cells (PSCs), especially for 1D (Li et al., 2019; Wan et al., 2019; Wu et al., 2019). Therefore, in this paper, high quality graphite nanofibers have been utilized as scaffold for PSCs to improve electron transfer property and then enhance performance of devices.
2.3. Characterization
2. Experimental
The morphology of nanofibers were examined by atomic force microscopy (AFM, NT-MDT Spectra), scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscope (TEM, Hitachi HT7700) with 200 kV acceleration voltage. The crystal structure of nanofibers was measured by Raman spectra (Renishaw invia) and X-ray diffraction (XRD, Bruker D8) with Cu-Kα radiation with 2θ range of 1060o. The stability was performed on thermogravimetric differential scanning calorimetry (TG-DSC, Netzsch STA 449F3) under a N2 thermobalance with heating rate of 10 K min−1. The time-resolved photoluminescence (TRPL) was characterized by fluorescence spectrophotometer (Edinburgh FSL980). The photovoltaic characterization was carried out using a Keithley 2400 digital source meter under illumination of AM1.5G 100 mW cm−2 from a solar simulator.
2.1. Nanofiber synthesis
3. Results and discussion
Graphite powder (1 g, Alfa) was dissolved in DMF (10 mL) and dispersed by ultrasonic cell disruption for 12 h to obtain graphite nanosheets (solution 1). Polyvinylpyrrolidone (PVP, 1 g, Mw = 1,300,000, Aldrich Co.) and polyacrylonitrile (PAN, 1 g, Mw = 150,000, Aldrich Co.) were dissolved in DMF (10 mL) and stirred for 4 h (solution 2). Then, the precursor for electrospinning was prepared by mixed solution 1 (10 mL) with solution 2 (10 mL) and stirred for 30 min. Using nozzle-less apparatus, electrospinning was carried out by applying a high positive voltage of 60 kV over a collector distance of 15 cm. The electrospun nanofibers for characterization were collected in aluminum foil. The obtained nanofibers were stabilized in air at 250 °C for 1 h and then carbonized under Ar atmosphere at 500 °C for 2 h with heating rate of 5 °C min−1.
As shown, synthesis of graphite nanofibers by nozzle-less electrospinning is illustrated in Fig. 1. First of all, through high-energy sonication, graphite powders partially separated in connected graphite nanosheets with diameter around 2 µm and thickness about 6 nm, as confirmed by AFM in Fig. 2. And then, PVP, PAN, graphite nanosheets were dissolved in DMF as precursor solution for electrospinning. PVP and PAN were chosen as carriers to adjust the viscosity of precursor solution, graphite nanosheets function as the main carbon source. Graphite nanosheets within precursor solution were rolled to be onedimensional graphite/PVP/PAN composite precursor nanofibers during electrospinning. In order to fix thermal treatment program and explore thermal stability of precursor nanofibers, thermogravimetric-differential scanning calorimetry (TG-DSC) was carried out from room temperature to 800 °C. From TG-DSC curves (Fig. 3), it can be seen that there was almost no change in weight and heat beyond of 500 °C for the precursor nanofibers. According to these results, graphite nanofibers were obtained by oxidized the graphite/PVP/PAN precursor fibers at 250 °C in air for 2 h (stabilized) and then carbonized at 500 °C under Ar atmosphere for 2 h (Hou et al., 2018). Fig. 4a is SEM image of graphite nanofibers which exhibit a long and straight fibrous morphology with diameter ranging from 100 to 300 nm (average diameter around 200 nm). Taking a closer examination, there are some loosen parts (circled area) in the nanofibers, which would be further investigated by TEM. Fig. 4b–d are TEM images of the straight, loosen and end parts of graphite nanofibers, respectively. From the loosen (Fig. 4c) and end part (Fig. 4d), we can clearly see the laminar structure, which indicating graphite nanofibers were rolled from planar graphite nanosheets under assistance of elongation electrostatic forces. Similar phenomenon has been observed in exfoliated graphene sheets that could be bended and folded into various shapes and able to roll into scrolls under certain conditions (Wang et al., 2017; Hashmi et al., 2017). Taking in-depth insight into structure of graphite nanofiber, XRD and Raman spectra were carried out. XRD pattern of graphite powder and graphite nanofibers is shown in Fig. 5a. The very sharp diffraction peaks at 26.5° assigned to the graphite crystallographic plane (0 0 2). (1 0 0) and (0 0 4) planes can been observed either. Hence, graphite nanofibers with high graphitization degree can be obtained by technique proposed here at low temperature of 500 °C, which is much lower than conventional carbonization temperature. Raman spectroscopy was carried out to obtain more detailed structure information of graphite
2.2. Solar cells fabrication Unless stated otherwise, all the fabrication processes were taken in the glove-box. Devices were fabricated on the laser etched FTO glass substrates with a sheet resistance of 10–15 Ω square−1. The substrates were cleaned by ultrasonic in soap water, deionization water, acetone, 2-propanol, and subjected to an UV-ozone treatment for 30 min. An approximate 30 nm thick c-TiO2 was spun-coated on the substrates by using a 0.15 M TIP ethanol solution at a speed of 3000 rpm for 30 s. Subsequently, the sample was calcinated at 450 °C for 2 h. After calcination, graphite nanofibers were electrospun on c-TiO2 and stabilized in air at 250 °C for 1 h and carbonized under Ar atmosphere at 500 °C for 2 h with heating rate of 5 °C min−1. The thickness of graphite scaffold was determined by electrospinning time. Here, the electrospinning time was about 1 min with a layer of graphite nanofibers. PbI2 was dissolved in DMF at a concentration of 461 mg mL−1 under stirring at 70 °C, which was maintained during the entire fabrication process. After filtration with 0.25 μm pore size filter, the PbI2 layer was deposited by spin-coating on the graphite nanofibers scaffold layer with 1500 rpm for 30 s, and dried at 70 °C for 30 min. Subsequently upon cooling down to room temperature, the PbI2 coated substrates were immersed in the organic part CH3NH3I solution (30 mg mL−1 in 2-propanol) for 90 s to complete the reaction. Finally, a rinsed step with 2-propanol was required for removing the excess organic fraction and the resulting sample was dried at 90 °C for 30 min. The hole-transport layer (HTL) was subsequently deposited by spin-coating at a speed of 4000 rpm for 30 s. The HTL solution was prepared by dissolving 79 mg of Spiro206
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Fig. 1. Schematic illustration of the nozzle-less electrospinning fabrication of graphite nanofibers.
nanofibers (Fig. 5b). Graphite powder and graphite nanosheets were also characterized for comparison. Raman spectra of pristine graphite powder, as expected, shows a prominent G-band peak at around 1580 cm−1 related to ordered graphitic structures, corresponding to sp2 hybridized carbon, and this band is correlated to the (0 0 2) diffraction peak in the XRD pattern. Graphite nanosheets exhibits the similar behavior, the dominate G-band. The spectrum of graphite nanofibers also reveals the dominate G-band and a very week D-band (around 1350 cm−1). The D-band corresponding to defects or disorder in the sp2 hybridized carbon. It can be seen that the half width at half maximum (HWHM) of G-band of graphite nanofibers is increased a little comparing with graphite powder. Therefore, it can be conclude that graphite nanofibers show an ordered graphite crystallites with a little disordered carbonaceous components. The Raman results are in good agreement with XRD and these results reveal that graphite nanofibers possesses a graphitic character. Crystallized graphite nanofibers should be suitable use as electron transfer high-way in PSCs due to their one-dimensional nanostructure. Therefore, graphite nanofibers were utilized as scaffold for PSCs. The fabrication procedure of graphite nanofibers scaffold based PSCs was displayed in Fig. 6a. A layer of graphite nanofibers scaffold was electrospun on TiO2 compact layer (c-TiO2), and then perovskite layer (CH3NH3PbI3, MAPbI3) was deposited on this scaffold, after that, hole transport layer (Spiro-MeOTAD) and Au electrode was deposited in turn. In order to investigate the effect of graphite nanofibers scaffold on charge-transport dynamics in PSCs, conductive AFM (c-AFM) was carried out. Fig. 6b is AFM topological images (left) along with the corresponding c-AFM images (right) for perovskite films with graphite nanofibers scaffold on FTO glass. Darker contrast in c-AFM indicating more conducting current flow through the perovskite layer, which revealed that graphite nanofibers provide a pathway for charge conductance. At the same time, time resolved photoluminescence (TRPL) spectrum of a series of perovskite films with m-TiO2 and graphite nanofibers scaffolds are also adopted. The PL decay monitored at the peak emission (770 nm) with 375 nm excitation (Ti: sapphire laser) are shown in Fig. 6c, where samples of FTO/c-TiO2/graphite nanofibers/ MAPbI3 and FTO/c-TiO2/mesoporous TiO2 (m-TiO2)/MAPbI3 were prepared and tested under front illumination. The effect of graphite nanofibers scaffold may affect the carrier dynamics, therefore, perovskite thin film with and without graphite nanofibers scaffold were prepared for comparison. The fast decay component τ1 can be ascribed to the quenching of free carriers from perovskite to m- TiO2 or graphite
Fig. 2. AFM image of graphite nanosheets.
Fig. 3. TG-DSC curves of graphite/PVP/PAN precursor nanofibers.
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Fig. 4. (a) SEM image of electrospun graphite nanofibers; (b), (c) and (d), TEM images of different parts of graphite nanofibers.
perovskite, which could benefit to performance of PSCs. The current density (J)-voltage (V) characteristics of PSCs devices with graphite nanofibers and m-TiO2 scaffolds are shown in Fig. 6d and their performance parameters are inserted. The efficiency of 18.23% was obtained for the device with graphite nanofibers scaffold, 21% improvement in performance compare with device with m-TiO2 scaffold. The current density and open voltage almost maintained the same. The improved efficiency mainly due to the improved fill factor (FF), which increased from 63% to 76%, 20.6% enhancement. In order to investigate the reproducibility of the photovoltaic performance, 20 cells for each condition were made and measured. The graphite nanofibers
nanofibers (Yang et al., 2019; Li et al., 2016; Hou et al., 2018; Qin et al., 2018; Ponseca et al., 2014; Xing et al., 2013; Stolterfoht et al., 2018). From the fitted results, it can be seen that τ1 in graphite nanofibers scaffold based perovskite thin film were slower than that in m-TiO2 scaffold, which represent improved charge transportation in FTO/cTiO2/graphite nanofibers/MAPbI3. The fast decay indicates improved charge transportation, which associated with the appearance of graphite nanofibers. Therefore, it is apparent that graphite nanofibers provide a pathway for charge conductance that benefit to performance of PSCs. The graphite nanofibers provide a charge transfer highway in
Fig. 5. (a) XRD pattern of graphite powder and graphite nanofibers; (b) Raman spectrum of graphite powder, graphite nanofibers and graphite nanosheets. 208
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Fig. 6. (a) Fabrication sequence of graphite nanofibers scaffold photoanode based PSCs and corresponding SEM images; (b) AFM (left) and c-AFM (right) of perovskite layer with graphite nanofibers scaffold; (c) TRPL spectrua of MAPbI3 with m-TiO2 and graphite nanofibers scaffolds; (d) J-V curves of perovskite solar cells based on graphite nanofibers scaffold.
evolution of other important photovoltaic parameters can also be observed in Fig. 7. These include the degradation of Jsc and FF over time, which are the major reasons for decrease of PCE.
scaffold based PSCs showed an average PCE of 17.8% with open voltage (Voc) of 1.01 V, short current density (Jsc) of 23.42 mA cm2 and FF of 75%. The longitudinal follow up on the photovoltaic parameters of devices with m-TiO2 and graphite nanofibers scaffold can be found in Fig. 7. These devices are encapsulated simply by the epoxy glue covered with glass slides and maintained in air at room temperature and over 60 RH%. The devices with graphite nanofibers scaffold shows around 37% decrease in PCE over 500 h, and the performance of perovskite solar cell with m-TiO2 scaffold decreases about 50% after 500 h. Besides PCE,
4. Conclusions In summary, we have demonstrated a simple and low temperature method to fabricate high quality graphite nanofibers from graphite powder and they can be efficient applied as scaffold in PSCs. The graphitized one dimensional graphite nanofibers provide a charge transfer 209
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Fig. 7. Stability on performance of perovskite solar cells with m-TiO2 and graphite nanofibers scaffold exposed in air at room temperature and over 60 RH %
highway in perovskite layer and then improve the performance of PSCs devices. A high efficiency of 18.23% was obtained for the device with the graphite nanofibers scaffold, 21% improvement in performance compare with device with m-TiO2 scaffold due to prolonged carrier lifetime and reduced recombination in perovskite film associated with the appearance of graphite nanofibers. Moreover, this graphite nanofiber is also a promising candidate for fabricating flexible fiber-shaped devices. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements This work was supported by the National Natural Science Foundation of China (51602293 and 1705168), and the Open Project of State Key Laboratory of Environment-friendly Energy Materials (18kfhg06). References Abdelkader, A.M., Cooper, A.J., Dryfe, R.A.W., Kinlocha, I.A., 2015. How to get between the sheets: a review of recent works on the electrochemical exfoliation of graphene materials from bulk graphite. Nanoscale 7, 6944–6956. Abrusci, A., Stranks, S.D., Docampo, P., Yip, H.L., Jen, A.K.Y., Snaith, H.J., 2013. Highperformance perovskite-polymer hybrid solar cells via electronic coupling with fullerene monolayers. Nano Lett. 13, 3124–3128. Batmunkh, M., Shearer, C.J., Biggs, M.J., Shapter, J.G., 2015. Nanocarbons for mesoscopic perovskite solar cells. J. Mater. Chem. A 3, 9020–9031. Batmunkh, M., Macdonald, T.J., Shearer, C.J., Bat-Erdene, M., Wang, Y., Biggs, M.J., Parkin, I.P., Nann, T., Shapter, J.G., 2017. Carbon nanotubes in TiO2 nanofiber photoelectrodes for high performance perovskite solar cells. Adv. Sci. 4, 1600504. Bush, K.A., Palmstrom, A.F., Yu, Z.J., Boccard, M., Cheacharoen, R., Mailoa, J.P., McMeekin, D.P., Hoye, R.L.Z., Bailie, C.D., Leijtens, T., Peters, I.M., Minichetti, M.C., Rolston, N., Prasanna, R., Sofia, S., Harwood, D., Ma, W., Moghadam, F., Snaith, H.J., Buonassisi, T., Holman, Z.C., Bent, S.F., McGehee, M.D., 2017. 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nat. Energy 2, 17009. Chae, H.G., Minus, M.L., Rasheed, A., Kumar, S., 2007. Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers. Polymer 48, 3781–3789. Chen, Y., Li, X., Zhou, X., Yao, H., Huang, H., Maiad, Y.W., Zhou, L., 2014. Hollowtunneled graphitic carbon nanofibers through Ni-diffusion-induced graphitization as high-performance anode materials. Energy Environ. Sci. 7, 2689–2696. Chen, L.F., Lu, Y., Yu, L., Lou, X.W., 2017. Designed formation of hollow particle-based nitrogen-doped carbon nanofibers for high-performance supercapacitors. Energy Environ. Sci. 10, 1777–1783. Chen, Y.M., Yu, B.Z., Miao, Y.Q., Gao, F., Jing, G.Y., Fan, H.M., 2018. Pushing the cycling stability limit of hierarchical metal oxide core/shell nanoarrays pseudocapacitor
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