Laminated Carbon Nanotubes for the Facile Fabrication of Cost

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Laminated Carbon Nanotubes for the Facile Fabrication of Cost-Effective Polymer Solar Cells Abid Ali, Mehmet Kazici, Sinem Bozar, Bahadir Keskin, Murat Kaleli, Syed Mujtaba Shah, and Serap Günes ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00345 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Energy Materials

Laminated Carbon Nanotubes for the Facile Fabrication of Cost-Effective Polymer Solar Cells Abid Ali,a,b* Mehmet Kazici,a Sinem Bozar,a Bahadir Keskin,c Murat Kaleli,d Syed Mujtaba Shah,b Serap Gunesa* a

Department of Physics, Faculty of Arts and Science, Yildiz Technical University, Istanbul, 34210, Turkey. b c

Department of Chemistry, Quaid-i-Azam University Islamabad, 45320, Pakistan.

Department of Chemistry, Faculty of Arts and Science, Yildiz Technical University, Istanbul, 34210, Turkey.

d

Department of Physics, Faculty of Art and Sciences, Süleyman Demirel University, 32260 Isparta, Turkey. *Corresponding Author: [email protected], & [email protected]

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Abstract In this study, we fabricate a metal and vacuum-free electrode composed of aligned multiwall carbon nanotubes (MWCNTs) as top electrode in inverted type polymer solar cells. Simply drawn MWCNTs sheets from vertically grown arrays were directly transferred over the top layer (PEDOT:PSS) of device as anode material. Without any post treatment, devices exhibited a short circuit current density (Jsc) of 7.53 mAcm-2 which is 78% of that of devices employing thermally deposited silver (Jsc= 9.69 mAcm-2) based anode. Scanning electron microscopy (SEM) images of the device (top and cross-sectional view) show the successful incorporation of MWCNTs sheets over the surface of the device without any damage. Thickness of the MWCNTs sheets as top electrode are also investigated, and it has been demonstrated that by increasing the sheets thickness, conductance of the electrode boosted up which showed improvement for the device performance. Flexible in nature, vacuum free and stable with environmental risks may trigger this idea a versatile approach towards cost effective roll to roll production of environmental friendly polymer solar cells. Key words: Laminated carbon nanotubes, Vacuum-free, Cost-effective, Electrode fabrication, Flexible polymer solar cells


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1. Introduction Polymer solar cells with superior properties like flexibility, environmental friendly, solid state and easy fabrication are attaining significant attention for scale up production among all other renewable energy sources.1-4 Economical manufacturing and long-term stability with facile fabrication are the key challenges in order to realise this emerging field for scale up production. Typically, polymer solar cells consist of indium tin oxide (ITO) coated glass as anode and thermally evaporated metals like Al,5-6 Ag7-9 and Au10 as cathode electrode respectively. Thermal deposition of these metals over the device surface under high vacuum is expensive and complicated procedure which is the key obstacle for the cost effective polymer solar cells. Alternative materials with compatible work function and better charge collection would be a milestone to commercialize this technology with low cost and versatility for portable electronics. Many efforts have been done to explore the new materials to replace thermally deposited metal based electrodes for facetious processing. Frederik Krebs and his co-workers used silver grid11-14 and graphitic silver paste15 as top electrode for vacuum free roll to roll production of polymer solar cells. Different methods have also been reported for vacuum free power conversion devices based on polymer photoactive materials with simple solution processed PEDOT:PSS.16-17 Recently, carbon based nanomaterials like carbon nanotubes, graphene, fullerenes and carbon fiber have been widely used in energy harvesting and storage devices.18-23 Carbon based nanomaterials with better electrical conductivity, convenient fabrication and versatile manipulation would be the promising candidates for metal free electrodes in energy devices.18, 24-28 These carbon nanomaterials and their composites with different polymers extensively employed in photovoltaics with admirable advantages.29 Carbon nanotubes,30-31 silver nanowires32-33 and graphene with its composites materials23, 34-37 are used as top electrode without thermal annealing in polymer based solar cells. Most of these reported methods have still complex procedure and employ 3 ACS Paragon Plus Environment


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expensive materials like silver or gold with stability issues. Herein, we reported a facile approach towards vacuum free procedure based on unique semi-transparent MWCNTs sheets as top electrode with versatile pattern. Simply drawn MWCNTs sheets from vertically aligned arrays were directly transferred over the surface of device as top electrode for both rigid and flexible polyethylene terephthalate (PET) base ITO substrate. Highly aligned MWCNTs with excellent conductance (149 ohms per square (Ω/□) sheet resistance for 10 sheets) replaced the thermally evaporated silver deposited as top electrode with reasonable efficiency in inverted type polymer solar cells. Unique patterned carbon based nanomaterials have high surface area with average number density of ≈1011 cm-2 and weight density of ≈8.4 mg/cm3.38 Highly conductive, low cost, easy to fabricate, ideal for roll to roll production, flexible in nature and stable to environment reflect these kind of materials could be better replacement for hectic processing metal (Al, Ag, Au) based electrode materials in polymer solar cells. Transparency of these MWCNTs sheets (85% light can transmit through one sheet of MWCNTs) as top electrode is another impressive advantage which allow these kind of devices to harvest energy from both top and bottom side illumination. These semi-transparent MWCNTs sheets are also highly flexible in nature which could lead these materials as benchmark for the applications in portable electronics and e-textiles. 2. Experimental 2.1 Growth of MWCNTs arrays Highly aligned MWCNTs were synthesised by chemically vapour deposition (CVD) method.39 Briefly a glass tube furnace was used for the flow of ethylene as precursor gas and hydrogen and argon as carrier gasses at 740 ºC with the flow rate of 90, 30 and 400 sccm (Standard cubic centimetre per minute) respectively. Iron (Fe) nanoparticles were used as catalyst to grow the highly aligned MWCNTs arrays. Aluminium oxide (Al2O3) as preventing layer (5 nm) first deposited over a silicon substrate and then iron (Fe) thin film (1.5 nm)


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deposited over Al2O3 by electron-beam evaporation methods. Upon annealing at high temperature under inert conditions, iron thin film break down and converted into nanoparticles which acted as catalyst for the growth of aligned patterned MWCNTs arrays. These vertically aligned arrays were pulled as sheets with a sharp edge blade and were directly transferred over the surface of device as top electrode in inverted type polymer solar cell. 2.2 ZnO nanoparticles synthesis and cell fabrication ZnO nanoparticles were synthesised by the simple sol-gel method via a previous reported method.40 Zinc acetate di-hydrate [Zn(CH3COO)2].2H2O sigma-Aldrich, 99.9%, 1 g) was dissolved in 10 ml of 2-methoxyethanol (Sigma-Alrich 99.98%) and then ethanolamine (Sigma-Aldrich 99.8% 0.28 g) was added slowly and the mixture was vigorously stirred for 12 hours in air. ITO coated glass and PET were first rinsed with deionised water at room temperature and then at 60 ºC for one hour in water and then rewashed with acetone and isopropanol under constant stirring. A sol-gel ZnO precursor solution was spun coated with 5000 RPM for 30 s (~42 nm). The film was pre-heated on a hot plate at 150 ºC and then into furnace at 290 ºC for 10 minutes each. After cooling to room temperature ZnO nanoparticles modified ITO glass was transferred into nitrogen filled glove box and a mixture of P3HT:PCBM (30:20 mg/ml in chlorobenzene) as a photoactive layer was spin coated with 800 RPM for 30 seconds. Polymer active layer was then heated inside the glove box at 120 ºC for 5 minutes. PEDOT:PSS as a hole transport material was later coated over the active layer. For PET based ITO glass, first ZnO nanoparticles synthesised41 and then spin coated over the surface of PET/ITO with annealing at 120 ºC for 15 minutes (all other same procedure). MWCNTs sheet were pulled out from the arrays and directly transferred over the device of the devices. Methanol was sprayed over MWCNTs sheet/s for the better contact with bottom material. For standard polymer solar cell, silver was thermally evaporated at 5 ACS Paragon Plus Environment


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very low pressure (10-5 mbar) and deposited over the surface of device (~100nm) with a shadow mask. 2.3 Characterization of the solar cell Solar cell efficiency of the device was measured using Keithley 2400 source meter under AM 1.5 illumination (100 mWcm-2) (Abet solar simulator). Sheets resistances of the MWCNTs also measured by Keithley 2400 using two probe method. Light intensity was calibrated using a standard silicon solar cell. Incident photon-to-current efficiency (IPCE) was measured by using external Quantum efficiency system of Newport Instruments. SEM images were taken with Quanta FEG250, Bruker. Transparency of the MWCNTs sheets characterized by UVVisible spectroscopy (Shimadzu-2450) and absorption values were checked out at 550 nm. Gamry Interface 1000E Potentiostat was used to measure the electrochemical impedance spectroscopy. Active areas of the solar cells were measured by digital Vernier caliper (Macrona). 3. Results and Discussion Schematic illustration and optical image of the inverted type polymer solar cell with MWCNTs sheets as top electrode are given in Fig. 1(a, b). n-type ZnO layer over ITO glass substrate acts as an electron transport layer while p-type PEDOT:PSS facilitate the transport of holes through cathode and anode electrodes. Different layers in device (Glass substrate/ ITO (~100 nm)/ ZnO (~42)/ P3HT:PCBM (~215 nm)/ PEDOT:PSS/MWCNT (~20 nm for one sheet) with approximate thicknesses are also shown in cross sectional SEM image in Fig. 1(c). Alignment of the MWCNTs arrays maintained their integrity after the direct lamination as shown in Fig. 1(d). Energy Dispersive X-ray Spectroscopy (EDS) for devices with different layers are also elaborated in supporting information (Fig.S1). ZnO layer was successfully deposited over ITO glass surface with moderate temperature thermal annealing.


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Fig. 1. (a, b) Schematic illustration and optical image of inverted type polymer solar cell with MWCNTs sheets as top electrode (c) cross sectional and (d) top view of the device with laminated MWCNTs electrode via SEM. Scanning electron microscopy (SEM) for ZnO layer shows the uniformity of the particles with ~100 nm range of particle size given in Fig. 2a and S3. Simply vertically aligned MWCNTs arrays (Fig. 2c and 2d) with unique patterns were pulled by a sharp edge blade as sheets (Fig. 2e) and transferred over the device surface as anode electrode (Fig. 2b). Alignment of MWCNTs put these materials far superior to collect the charges more effectively as compared to powder MWCNTs.42-43 Organic solvent like methanol or ethanol was used as spray to enhance the surface contact between the MWCNT and PEDOT:PSS for the better cell performance. In current device configuration the interface of p-type thiophene based polymer P3HT and n-type fullerene derivative PC61BM form a p-n junction at which 7 ACS Paragon Plus Environment


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the exciton break down to positive (hole) and negative (electron) charge carrier. Electrons penetrate through ZnO layer and travel via ITO electrode while holes move toward the MWCNTs sheets facilitated by the PEDOT:PSS. Highly conductive with compatible work function, (-4.8 eV for MWCNTs) MWCNTs play an important role in the collection of positive charge carriers. MWCNTs used here as sheets are also highly flexible in nature as shown in Fig. 2f over the surface of a plastic substrate. This property could lead the current idea towards a very fascinating source for the role to role printed organic photovoltaic. Moreover, moderate temperature annealing for a short time interval (10 minutes) over a flexible electrode and vacuum free processing also feasible to commercialize this emerging technology for portable devices.


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Fig. 2. SEM images of (a) ZnO nanoparticles (b) MWCNT sheet over device surface (c, d) low and high magnified SEM images of vertically aligned MWCNT arrays (e, f) optical images of MWCNT sheet extracted from arrays and MWCNTs sheet on a plastic substrate with bending. Comparative current-voltage characteristics for photovoltaic performance of the device based on silver metal and MWCNTs arrays are given in Fig. 3a. Short circuit current density (Jsc) for the device with silver electrode was 9.69 mAcm-2, whereas MWCNTs based electrode device exhibited a Jsc of 7.53 mAcm-2 which is almost 78% of that of the silver based standard device. Devices involving MWCNTs exhibited a Jsc of 7.53 mAcm-2 with open 9 ACS Paragon Plus Environment


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circuit voltage (Voc) of 0.57 V and a fill factor (FF) of 0.34 which led to a power conversion efficiency (PCE) of 1.46% whereas devices consisting of silver electrodes (standard cell) exhibited a Jsc of 9.69 mAcm-2, Voc of 0.54 V and a fill factor (FF) of 0.44 which led to a PCE of 2.25%. We also observed that sheet thickness of MWCNTs has a significant impact on the overall performance of the device. From one to ten sheets of MWCNTs current density, fill factor, open circuit voltage and PCE improved up to a reasonable extent. Devices with one sheet of MWCNTs (~20 nm thick) revealed a poor performance of (0.26% PCE) with Jsc of 2.27 mAcm-2, Voc of 0.54 and FF of 0.21. Increasing the number of sheets from three to six improved the Jsc, Voc, FF and PCE from 4.29 to 7.53 mAcm-2, 0.55 to 0.57 V, 0.24 to 0.34 and 0.57 to 1.47%, respectively. Photovoltaic parameters for different number of sheets are also organised in the Table 1. Table 1. Photovoltaic parameters of polymer solar cells with different sheet thicknesses of MWCNTs and silver as top electrodes MWCNT Sheets

Voc (V)

Jsc (mAcm-2)

FF

PCE (%)

1

0.54

2.27

0.21

0.26

3

0.55

4.29

0.24

0.57

6

0.57

7.53

0.34

1.46

10

0.57

7.30

0.34

1.42

Ag

0.53

9.69

0.44

2.25

Further increment in sheet thickness (~116 nm thick shown in Fig. S5) did not reveal more progress in cell performance with almost similar FF and Voc. Same MWCNTs sheets were also used for flexible PET/ITO substrates to put this idea for the beneficial approach towards plastic based flexible and portable devices. Devices fabricated on PET/ITO substrates exhibited a Jsc of 6.2 mAcm-2, Voc of 0.54 V and a FF of 0.29 leading to a PCE of ca. 1 %. Efficiency was well maintained after 100 times bending of the device which showed that the 10 ACS Paragon Plus Environment

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top electrode material (MWCNTs) is much flexible in nature and stable with bending. Device bending and the efficiency ratio decline are given in the Fig. 6 (a, b). These results indicate that MWCNTs could be used as a top electrode for inverted type flexible solar cells to facilitate the vacuum free production.

Fig. 3. J-V curves for (a) comparative behaviour of device MWCNTs and silver metal as top electrode (b) effect of MWCNTs sheets thickness (c) top and bottom side illumination for three sheets of MWCNTs (d) IPCE performance of device with different sheet thickness of MWCNT. Certainly, 35 % efficiency dropped down with the laminated MWCNTs as top electrode as compared to the standard thermally evaporated silver based electrode. This reduction in the device performance could be compensated with the most hectic and expensive process (thermally evaporated silver or aluminium metal) process for facile production of polymer solar cells. On the other hand, photoactive polymer used in this study was P3HT, which



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shows a maximum efficiency of ~3.5 %. New class of photoactive polymers with more efficient donor (PC71BM) like PCDTBT/ PC71BM44 and PTBT7/ PC71BM45 exhibited high efficiency and lead this idea with more than 5 % efficiency towards metal and vacuum-free device. Another interesting device fabrication is tandem polymer solar cells which also boosted the efficiency near 10 %46-47 for organic photovoltaics. MWCNTs as top electrode could be also used in this type of device with more than 6 % expected efficiency. The other method to improve the cell performance is the surface modification of MWCNTs sheets to get more diffusivity on the surface of device and it could increase the fill factor and ultimately the cell performance. Beside the vacuum free production of polymer based photovoltaics, one more impressive advantage for these MWCNTs materials is their transparency. With optimization of sheet thickness, we can take advantage of light penetration through MWCNTs and illuminate the device from any side. Almost 85% of incident light could be transmit from one sheet of MWCNTs as is shown in Fig. 4(a, c), which exhibited the same performance with top (MWCNTs) or bottom (ITO/ZnO) side illumination (Fig S2.) Further increment in the sheet thickness led to the reduction in transmittance which in turn reduces the cell performance for top side illumination. With increasing the sheet thickness transmittance reduce from 85% to 18% (shown in Fig. 4a) from one to ten sheets of MWCNT, respectively. Optical images of different sheet thicknesses are displayed in Fig. 4c which sketched the distinct observation about their transparency with sheet thickness. The capital letters “MWCNT” was written on a piece of paper and was used to prove the transparency of the MWCNTs under different sheet/s. When using one sheet the word is very clear and with 10 sheets it’s difficult to read it properly (see Fig. 5c).


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Fig. 4. (a) UV-Visible transmittance spectra (b) I-V characteristics and (c) optical images over the glass surface for 1 to 10 sheet/s of MWCNTs (d) Transmittance and sheet conductance behaviour of MWCNTs with sheets thickness. Electrochemical impedance spectroscopy also reveals very fruitful information about the inner charge transfer behaviour for the different layers in the device. Two semicircles in Nyquist (Fig. 5) plot depict the charge transfer behaviour and equivalent resistance for the device. First semicircle at high frequency region represents the interfacial hole transport from the PEDOT:PSS/MWCNTs while high frequency region semicircle controlled by the equivalent interfacial resistance between different layers of the overall device (ITO/ZnO/P3HT:PCBM).48-49 First semicircle for silver based electrode started from low resistance value (35 Ω) as compared to MWCNTs (410 Ω) which indicated that metal based electrodes are much conductive as compared to carbon based nanomaterials. Diameter of first semicircle for MWCNTs is smaller (147 Ω) as compared to silver based electrode (460 Ω) which may indicate the better work function with more compatibility for the hole transport at 13 ACS Paragon Plus Environment


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the interface of PEDOT:PSS/MWCNTs. This is also in agreement with the little enhancement in Voc in case of J-V curves of MWCNTs based devices as compared to Ag based devices. Second semicircle which indicated the equivalent resistance of device, also revealed that the overall efficiency for silver based electrode is better with ideal conductivity and deep penetration of the thermally deposited metal onto the surface of device.

Fig. 5. Electrochemical impedance spectroscopy of inverted type polymer solar cells with MWCNTs and silver as top electrode. Frequencies were ranged from 0.1 to 1 MHz with 0.1 V DC voltage under dark. Metallic vapours, dumped over the surface of photovoltaic device have much better charge collection which depressed the charge recombination in photoactive layer. This phenomenon caused to reduce the second semicircle and finally equivalent impedance. So, the overall resistance for the device with MWCNTs sheets as top electrode get more value and hence the lesser will be the performance with reduction in fill factor as well as short circuit current density. Along with transparency, conductance is another important parameter for the electrode materials. Here we checked out the sheet resistance of MWCNTs as sheets by Ohmic behaviour of I-V curve. Sheet resistance (Rs) decreases from 1138 Ω/□ to 149 Ω/□ for 14 ACS Paragon Plus Environment

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one and ten sheets of MWCNTs respectively. Carbon based materials as sheets with lower resistance lead to a better charge collection and ultimately boosted the performance for the device. Table S1 in supporting information elaborated the percentage transmittance (T %) and sheet resistances (Rsh) for 1 to 10 sheets of MWCNTs. Almost 7 times increment was observed in sheets conductance (Siemen.Ohms) from one sheet (0.88 mS.□) to 10 sheets (6.70 mS.□). Devices fabrication with metals like Ag, Al or Au as based top electrodes is not only an expensive procedure but also its rigid in nature and can be easily damaged while bending. Many efforts had been reported for the flexible energy devices in this regard and modified carbon nanotubes50-51, graphene52-54 and silver nanowire33, 55-56 were used for the flexible and semi-transparent polymer based solar cells. Semi-transparent materials have another advantage in which light could be harvested from both sides of the device which provide and additional edge. All the devices made by the materials mentioned above need an extra labour work like surface modification, expensive silver based materials and also durability impact made them less attractive for the mass production. MWCNTs as sheets extracted from arrays have excellent flexibility (shown in Fig. 2f and 6b.) and very stable performance after the bending. Directly transfer over the surface of the device without any post treatment made this idea very charming for the flexible electrode in organic electronics. Along with flexibility these carbon nanomaterials another advantage over the silver based or other metal based materials which is the inert behaviour towards the environment like humidity, atmospheric oxygen and other toxic gases for the polymer active layer. Here we also employed the same MWCNTs as top electrode for the plastic based (PET/ITO) substrate which is another enormous advantage for this unique patterned highly flexible material. Efficiency of the plastic based devices was well maintained (above 90%) after 100 times bending as shown in Fig. 6 without any damage in top electrode material. 15 ACS Paragon Plus Environment


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Fig. 6. (a) Optical image of bend PET/ITO based inverted polymer solar cell with MWCNTs as top electrode (b) efficiency ratio after different bending cycles (c) J-V curves of the device with time and (d) it efficiency ratio with respect to different time intervals. Environmental impact is almost ideal for carbon based nanomaterials as compared to metal based electrodes. MWCNTs sheets are very passive with environmental effects such as oxygen and moisture. From the J-V curves it was shown that PCE is still preserved after 5 days. The device was kept five days in nitrogen filled glove box and tested after each day. Later, the same device was saved under ordinary conditions and its efficiency was measured each day. Efficiency was well retained above 80% with 10 days’ time period (Fig. 6d, 6c). 4. Conclusion In summary, we first time established a facile approach towards vacuum free MWCNTs based materials as top electrode in inverted type polymer solar cells. Devices based on the MWCNTs sheets show comparable performance with standard devices with


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silver as top electrode. Unique pattern MWCNTs also successfully employed as anode materials for PET/ITO based solar cells which would be a benchmark for the next generation photovoltaic devices. Further optimization of MWCNTs arrays like chemical doping or surface modification could be the next step to improve the FF and ultimately better performance is expected. Low cost carbon based nanomaterials with excellent performance and better stability after bending made these materials as promising candidates for the scale up production of polymer photovoltaic. Acknowledgment This work was supported by “The Scientific and Technological Research Council of Turkey” (TÜBĐTAK-2216 research grant number 53325897-216.01-157281) and Higher Education Commission, Pakistan (Research grant number I-8/HEC/HRD/20143435) for the financial support. Author also acknowledge to Prof. Huisheng Peng for the approval of his research visit in State Key Laboratory of Molecular Engineering of Polymers, Laboratory of Advance Materials, Fudan University, Shanghai, 200438, China to provide the facilities for the growth of MWCNTs arrays. ASSOCIATED CONTENT Supporting information available: EDX of the polymer solar cell device, SEM images of ZnO nanoparticles over ITO glass, J-V curves for one sheet of MWCNTs, tabulated data for the different sheets of MWCNTs, SEM images of the MWCNTs arrays and sheets, cross sectional images of the device with MWCNTs thickness, J-V curve for the device with best performance.



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Graphical Abstract


Laminated Carbon Nanotubes for the Facile Fabrication of Cost

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