Construction of Transparent Cellulose-Based Nanocomposite Papers

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Construction of Transparent Cellulose-Based Nanocomposite Papers and Potential Application in Flexible Solar Cells Qiaoyun Cheng, Dongdong Ye, Weitao Yang, Shuhua Zhang, Hongzheng Chen, Chunyu Chang, and Lina Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01599 • Publication Date (Web): 06 May 2018 Downloaded from http://pubs.acs.org on May 6, 2018

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ACS Sustainable Chemistry & Engineering

Construction of Transparent Cellulose-Based Nanocomposite Papers and Potential Application in Flexible Solar Cells Qiaoyun Cheng1, Dongdong Ye1, Weitao Yang2, Shuhua Zhang2, Hongzheng Chen2, Chunyu Chang1, 3∗, Lina Zhang1, 4∗

1 College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China 2 MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon Materials, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China 3 Suzhou Institute of Wuhan University, Wuhan University, Suzhou, 215123, China 4 School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

*To whom correspondence should be addressed. Phone: +86-27-87216311. Fax: +86-27-68762005. E-mail: [email protected] (L. Zhang); [email protected] (C. Chang).

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ABSTRACT The flexible electronics are developing rapidly due to the promising applications in displays, sensors, and energy conversion fields. To explore biodegradable, light-weight and flexible thin film electronics, O-(2, 3-dihydroxypropyl) cellulose (DHPC) was synthesized by homogeneous etherification of cellulose in 7 wt% NaOH/12 wt% urea aqueous solution without extra catalyst. DHPC exhibited a high level of transparency, outstanding ductility, and good adhesiveness, but poor mechanical properties. Thus, stiff tunicate cellulose nanocrystals (TCNCs) was introduced to construct tough nanocomposite papers. The reinforcement of nanocomposite papers was well predicted by a percolating model, indicating the formation of the network of TCNCs. On the basis of the excellent interfacial compatibility between TCNCs and DHPC, supported by atomic force microscope (AFM) mapping, the nanocomposite papers exhibited smooth surface, high transparency, as well as satisfactory mechanical properties, which was suitable for the construction of flexible polymer solar cells. Tin-doped indium oxide (ITO) could be directly coated on the adhesive transparent paper with any glue as electrode, and the power conversion efficiency (PCE) of the resulting flexible inverted polymer solar cells was 4.98%, suggesting its potential application as biodegradable and wearable electronics or optoelectronics. This work is important for developing the clean energy by using sustainable materials derived from renewable resources.

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Keywords: O-(2, 3-dihydroxypropyl) cellulose, tunicate cellulose nanocrystals, transparent paper, interface compatibility, flexible solar cells.

INTRODUCTION The pursuit of human sustainable development and non-polluting energy promotes the rapid development and research in the utilization of inexhaustible and renewable solar energy.1,

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On the other hand, the manufacture of portable, wearable, light

weight, transparent, flexible electronic and solar energy conversion devices become a new trend for the blooming electro-technology field.3, 4 Designing and fabricating flexible electronic devices need to choose an appropriate substrate for the first step. The synthetic polymer such as polyimide5, poly(ethylene terephthalate)6, 7, and poly(ethylene naphthalate)8 have been frequently-used as substrates for flexible electronic and optoelectronic devices owing to their low cost, light weight, and good mechanical properties. Considering the utilization of petroleum-based plastic was against the human sustainable and non-polluting development, renewable and biodegradable materials such as cellulose would be an environmentally attractive and potential alternatives for green devices.9-11 Cellulose is the most abundant natural polymer resource. Regular paper, made of cellulose fibers with diameters of ~ 20 µm, have been explored as a substrate for solar cells.12-14 However, the device performance has been limited by the high surface roughness and porosity of paper that effect the coating process.15, 16 To solve the intrinsic problems, the surface roughness can be decreased by introducing buffer layers or lamination under high stress.17, 18 The vapor delivery without solvent casting 3



can also avoid the disadvantages because of no wettability or surface tension effects on rough paper substrates.19 Since the fiber diameter is much larger than the wavelength of visible light, traditional papers are basically opaque, precluding their use in the fabrication of transparent devices. Recently, cellulose-based transparent papers have been used as substrate materials for flexible electronic and photoelectric devices such as sensor15, antenna20, 21, transistors16, 22, diodes22, solar cells23-26. It not only takes the advantages of ordinary paper, but also has higher optical transparency, lower roughness, and even superior mechanical properties due to higher packing density of fibers.27 O-(2, 3-dihydroxypropyl) cellulose (DHPC), a kind of cellulose derivatives, can be synthesized by homogeneous etherification of cellulose in NaOH/urea aqueous solution without extra catalyst.28 The DHPC papers with smooth surface have been fabricated, exhibiting a high level of transparency, outstanding ductility, and high adhesiveness.29 However, low tensile strength and toughness of DHPC papers limit their application as the substrates for flexible solar cell. In recent years, all-cellulose nanocomposites have emerged which can overcome the critical problem of filler-matrix interfacial compatibility for composite materials by using chemically similar cellulosic materials as both filler and matrix.30-33 Cellulose nanocrystals (CNCs) have been widely investigated for the reinforcement of polymeric composites due to their intrinsic low density and high stiffness.34 Especially, tunicate cellulose nanocrystals (TCNCs) isolated from the mantles of sessile sea creatures, are advantageous for the fabrication of high performance nanocomposites, owing to their 4


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higher aspect ratio and modulus in comparison with CNCs from other sources.35, 36 Higher aspect ratio of the nanofillers benefited both formation of percolating network and stress transfer in the composite materials, while high modulus can improve the stiffness of composite materials.37, 38 The high aspect ratio and rigidity of TCNCs have been investigated in our previous work39, showing its advantage as nanofillers. It has reported recently that the plastic waste was classified as hazardous, and eight million tons of plastic is dumped at sea each year, leading to serious pollution.40, 41 The sustainable polymers derived from renewable resources have been used as feedstocks for the manufacture of a variety of materials and products.42 Thus, it was worthwhile endeavor to fabricate the flexible transparent nanocomposite papers from renewable sources, which were used as solar cell substrates in the clean energy field. As shown in Figure 1, our strategy was sustainable both in utilizing renewable resources (seafood waste and cellulose pulp) and protecting marine environment, leading to the ecological benign cycle. Herein, the soft matrix DHPC was reinforced with rigid percolating TCNC network to obtain transparent cellulose-based paper for flexible solar cells. The high stiffness of nanocomposites was attributed to the strong interactions between the surface hydroxyl groups of TCNCs.43, 44 The interfacial compatibility of fillers/matrix and mechanical properties were characterized by atomic force microscope (AFM) mapping technology. The excellent interface compatibility between fillers and matrix ensured that the mechanical properties of nanocomposite papers were enhanced while maintaining high transparency. Tin-doped indium oxide (ITO) coated cellulose-based 5



transparent papers exhibited stable electrical conductivity under repeated binding, and the PCE of resulting flexible solar cells was up to 4.98%. Therefore, the new flexible transparent nanocomposite papers fabricated from cellulose were used successfully as solar cell substrates, which is important for sustainable development of chemical and engineering.

Figure 1. Scheme to describe the "green" conversion from renewable resources to flexible solar cell substrates without marine pollution, showing the ecological benign cycle.

EXPERIMENTAL SECTION Materials Tunicate (Halocynthia roretzi Drasche) was purchased from Weihai Evergreen Marine science and technology Co. Ltd (Shandong, China) and used as raw material. Weight-average molecular weight (Mw) of tunicate cellulose was measured to be 5.9×105, and the crystallinity was 89% (Figure S1). The X-ray diffraction (XRD) 6


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patterns and solid-state 13C NMR spectra of tunicate cellulose demonstrated nearly all Iβ structure. Cotton linter (α-cellulose > 93%, Mw = 9.2×104) obtained from Hubei chemical fiber Co. Ltd was used to synthesize cellulose derivative, O-(2, 3-dihydroxypropyl)

cellulose

(DHPC).

Electron

donor

material

Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[ (2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7) and electron acceptor material [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) were purchased from 1-material Chemscitech and Aldrich, respectively, and used as received. Glycidol (2, 3-epoxy-1-propanol) and other reagents were analytical-grade purchased from Shanghai Chemical Agents Co. Ltd. Preparation of DHPC/TCNCs nanocomposite papers The tunicate cellulose nanocrystals (TCNCs) suspension was prepared after sulfuric acid hydrolysis for 2 h at 70 °C, following a modified approach in previous work39. After sulfuric acid hydrolysis, ribbon-like TCNCs were obtained as fillers of nanocomposite. DHPC were synthesized by homogeneous etherification of cellulose in 7 wt% NaOH/12 wt% urea aqueous solution without extra catalyst by homogeneous etherification according to the reported protocols.29 For the preparation of DHPC/TCNCs nanocomposite papers, TCNCs suspension (3 wt%) and DHPC aqueous solution (4 wt%) was blended in accordance with a certain proportion. After removing air bubbles by centrifugation, the mixed solution was cast in glass mould and evaporated at 40 °C for 24 h to obtain DHPC/TCNCs nanocomposite papers which were denoted as P0, P5, P10, P15, and P20, with TCNCs contents of 0 wt %, 5 7



wt %, 10 wt %, 15 wt %, and 20 wt %, respectively. Fabrication of polymer solar cells After the P10 were cleaned with acetone and dried using clean N2 gas, a thin layer of ITO (80 nm) was deposited using magnetron sputtering as the cathode. The average roughness of P10-ITO was measured to be 4.4 nm. The cathode was cleaned by acetone and dried using clean N2 gas again, then treated in a UV-ozone machine for 20 min. The finally inverted device structure was P10 (100 µm)/ITO (80 nm)/ ZnO (30nm)/ PTB7-PC71BM (1:1.5 by weight, 100 nm)/MoO3 (10 nm)/ Ag (100 nm). The procedure of deposition except ITO layer was according to the published literature.45 The active area of the device was defined as approximately 24 mm 2 according to the overlap of the ITO cathode and Ag anode. Characterizations The dimensions of TCNCs and their distributions in DHPC matrix were examined by transmission electron microscopy (TEM) using a JEM-2010 microscope (JEOL, Japan). The samples were prepared by evaporating a drop of TCNCs dispersion (0.05 wt%) on a carbon-coated copper grid (TCNCs samples), and slicing for ultra-thin sections (P10). Scanning electron micrograph (SEM) measurements were carried out on a HITACHI 5-4800 microscope (Tokyo, Japan) at an accelerating voltage of 5 kV. The papers were frozen in liquid nitrogen, fractured immediately, and then freeze dried. Both the surface and cross-section of samples were sputtered with gold, and then observed and photographed. Atomic force microscope (AFM) mapping of nanocomposite was performed on CypherTM 8


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S (Asylum Research) using contact mode under ambient condition at 20 °C. The sample was fixed on a silicon wafer and scanned using silicon nitride probe (RTESP-300, BRUKER) with a nominal spring constant of 40 N m-1. The actual spring constant was measured by a thermal tuning method. The area of 2×2 µm2 was scanned with the resolution was 256×256 pixels by using a tip with the radius of 8 nm. The elastic modulus was obtained by fitting the Johnson-Kendall-Robert (JKR) model. The papers were cut into width of 1 cm and the transmittances of papers with a thickness of 100 µm were measured over the wavelength range of 200 ~ 800 nm by using a double beam ultraviolet-visible spectrophotometer (UV-6100PCS, China). Dynamic mechanical analysis (DMA, TA instrument Q800 series) were performed in tension mode with an oscillation frequency of 1 Hz, a static force of 10 mN, an oscillation amplitude of 10 µm, and an automatic tension setting of 125%. The temperature ranged from -90 °C to 150 °C, and the heating rate was 3 °C min-1. The test specimen was a thin rectangular strip (20 mm×5 mm×0.2 mm) cut from a portion of casting papers stored in dryer. The similar samples were subjected to a tensile mechanical test by using an Electromechanical Universal Testing Machine CMT6503 at 25 °C and 75 % RH with an elongation rate of 2 mm·min-1. Five strips were tested and the average values were adopted for the specific tensile data. Bursting strength of papers were measured on DRK109 (Drick, China), according to ISO2758, at an air inflation speed of 170 mL min-1. The papers were stored under constant humidity (75 % RH) at 25 °C before 9



measurements. The sheet resistance was tested by 4-point Probes Resistivity measurement System. The current density-voltage (J-V) curve was recorded with Keithley 2400 source unit under AM 1.5G illumination using a solar simulator (Enlitech, Taiwan, China). The external quantum efficiency (EQE) data was obtained using a solar-cell spectral-response measurement system QE-R3011 (Enlitech, Taiwan, China).

RESULTS AND DISCUSSION Fabrication and structure of cellulose-based nanocomposite papers

Figure 2. Schematic illustration of fabrication cellulose-based nanocomposite papers (a), TEM image of TCNCs (b), TEM image (c) and photograph (d) of P10, optical transmittance under UV-Vis light of neat DHPC and nanocomposite papers (e).

The formation and structure of flexible transparent nanocomposite papers were shown in Figure 2a. O-(2, 3-dihydroxypropyl) cellulose (DHPC) with the 10


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dihydroxypropyl groups molar substitution (MSdhp) of 12.6 (Figure S2) was synthesized successfully by homogeneous etherification in the cellulose solution.28 However, the DHPC papers were weak, thus the stiff tunicate cellulose nanocrystals (TCNCs) were introduced to improve their mechanical properties. As shown in Figure 2b, TCNCs exhibited ribbon-like morphologies with 10 ~ 30 nm in width (W) and 0.4 ~ 3 µm in length (L). The average width and length of TCNCs were estimated to be 20 nm and 1.396 µm, respectively. The average aspect ratio (L/W) was calculated to be 72, the high aspect ratio of TCNCs was mainly attributed the high crystalline fraction of tunicate cellulose. The interaction between DHPC and TCNCs in the mixed solution was investigated by rheological experiment (Figure S3). The storage modulus G’ exceeded the loss modulus G” after mixing TCNCs into DHPC solution, while G’ and the shear viscosity of solution increased with the increase of TCNC contents, indicating the existence of DHPC/TCNC

and

TCNC/TCNC

interaction.

The

prominent

interfacial

compatibility in the nanocomposites benefited from the hydrogen bonding interactions between TCNCs and DHPC, where both of them contained abundant hydroxyl function groups (Figure 2a), resulting in good dispersion of TCNC fillers in DHPC matrix and high transparency of nanocomposites (Figure 2d). The transmittance (Tr) of all papers raised with an increase of wavelength from 200 to 800 nm, undergoing a rapid growth and then toward the maximum values. The Tr value of neat DHPC paper (P0) was 92% at 550 nm, whereas that of P10 slightly decreased to be 85% because of the incorporation of TCNCs (Figure 2e). 11



Figure 3. SEM images of the surface (a-d) and cross-section (e-h) of DHPC/TCNC papers: P0 (a, e), P5 (b, f), P10 (c, g) and P20 (d, h).

To further evaluate the morphology of TCNCs in the nanocomposites, TEM was employed to distinguish the filler and matrix. Figure 2c shows the TEM image of P10 after ultrathin section processing. The TCNCs were dispersed well in the DHPC matrix without obvious aggregation. Furthermore, the morphology and distribution of TCNCs in the nanocomposites were also investigated by SEM. Different from porous and rough surface of conventional cellulose paper, the surface of P0 and composite papers was compact and smooth, which gave an edge to fabricate electronic devices (Figure 3). Many clear white spots could be observed in nanocomposites, which could be assigned to the morphology of TCNCs. The white spots observed in SEM dramatically increased with the increase of the TCNC content in the nanocomposites. The sizes of TCNCs emerged in 12


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cross-section of composite papers were obviously smaller than those in the surface. This phenomenon could be explained by the alignment of TCNCs during the fracture of papers for their cross-section observation. Obviously, the TCNCs were homogeneously distributed in the DHPC matrix, revealing excellent interface interaction between the TCNC fillers and the cellulose-based matrix when the TCNC content was lower than 15% (Figure 3 and Figure S4). However, obvious aggregation in P20 was appeared in both the surface and cross-section images. As mentioned above, TCNCs were cellulose Iβ and showed high crystallinity and rigid characteristic, whereas DHPC was soft with amorphous structure. Major diffraction peaks of TCNCs at 14.9°, 16.7°, 22.9°, 34.5°, corresponding to the cellulose Iβ crystallographic planes 1ī0, 110, 200, and 004, respectively, appeared in the nanocomposites. The intensity of crystalline peaks increased with an increase of TCNC contents (Figure S5a), indicating that the crystalline structure of TCNCs were well maintained in the nanocomposite papers. Meanwhile, strong hydrogen bonding formed in the interface between TCNCs and DHPC (Figure S5b). Interfacial compatibility of TCNC and DHPC in nanocomposite papers

13



Figure 4. AFM height image (a, d), adhesive force (b, e) and elastic modulus (c, f) maps for P0 (a-c) and P10 (d-f).

The mechanical properties and structure of DHPC and nanocomposite papers were measured by AFM mapping technology. From AFM height images (Figure 4a, 4d), both P0 and P10 displayed smooth surfaces from the height image, indicating homogeneous incorporation of TCNCs into DHPC matrix. As seen in Figure 4b, neat DHPC sample had adhesion of ~ 30 nN, revealing the adhesive property of matrix. In Figure 4e, DHPC as soft polymeric matrix with higher adhesion (~ 30 nN) could be distinguished, whereas TCNCs acted as rigid fillers with lower adhesion (~ -30 nN). The smooth transition region with adhesion of ~ 0 nN was identified as the interface between DHPC and TCNCs, indicating their good interface compatibility in nanocomposites. Despite the addition of TCNCs with lower adhesion, the P10 was still adhesive from full view. Therefore, the conductive layer of solar cell would stick well on the surface of P10. The elastic modulus distribution map verified the results 14


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of elastic modulus distribution of nanocomposite paper (Figure 4f). Three domains with different colors in elastic modulus map could be observed. The green domain reflected high elastic modulus, which could be attributed to the contribution of rigid TCNCs, while the blue domain showed low elastic modulus belonged to the region of DHPC which was higher than that from P0 in Figure 4c (620 MPa). Importantly, the red domain with elastic modulus of ~ 1.4 GPa was assigned to the region of interface between TCNCs and DHPC. The elastic modulus of TCNCs domain was lower than that of TCNCs measured by DMA in Figure 5a at 20 °C (11.9 GPa), whereas the elastic modulus of DHPC domain was higher than that of DHPC measured by DMA at 20 °C (0.15 GPa), indicating that DHPC enwrapped TCNCs in both measured domains. The average elastic modulus (~ 1.4 GPa) of P10 from AFM results was comparable to that (0.73 GPa) measured by DMA. These results demonstrated that excellent interface compatibility between TCNCs and DHPC resulted in ameliorative mechanical properties of P10, while remaining smooth and adhesive surface. Mechanical properties of nanocomposite papers The linear mechanical behaviours of neat DHPC and nanocomposite papers were analysed by DMA. Figure 5a gives the evaluation of storage modulus (E′) of samples versus temperature. The tensile modulus of P0 slightly decreased with the temperature below Tg (~ 20 °C)29 in the glassy state. Nanocomposite papers containing 5 to 20 wt% TCNCs exhibited a significant increase in E′ compared to the neat DHPC paper. At 0 °C, E′ increased from 0.55 GPa for P0 to 2.21 GPa for P15, indicating that P15 had a 4-fold increase in the tensile storage modulus below 15



Tg. A more dramatic reinforcement effect was observed above Tg (80 °C), where E′ was increased from 0.05 GPa for P0 up to 0.65 GPa for P15, indicating a 13-fold increase. After the slippage of DHPC chains (150 °C, Figure S6), P15 represented a 165-fold increase in the tensile storage modulus from 3.1 MPa to 512.2 MPa. These results supported strongly that TCNCs contributed to the enhancement of the rigidity. Furthermore, the thermal decomposition temperature of samples slightly shifted to higher temperature after incorporation of TCNC contents, due to the strong interfacial interaction between fillers and matrix (Figure S7).

Figure 5. Tensile storage moduli (E′) as a function of temperature measured by DMA 16


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(a), E′ at 45 °C as a function of volume fraction of TCNCs and the dashed line shows the values predicted by the percolation models (b), stress-strain curves (c), and bursting index as a function of TCNC contents (d).

To further understand the effect of TCNCs content on the mechanical properties of nanocomposites, we investigated the experimental data with respect to a percolating model.38 The tensile storage modulus of the composites E′ could be expressed according to equation (1)

(2) where ψ is the volume fraction of the stiff percolating network for the load transfer which can be calculated by equation (2), Xr is the volume fraction of TCNCs as shown in Table S1, Xc is the critical percolation volume fraction calculated by 0.7/A, where A is the aspect ratio of TCNCs, and E′s and E′r were the storage moduli of soft DHPC and rigid TCNCs paper, respectively. The aspect ratio of TCNCs was determined by TEM images to be 72. E′s was measured by DMA to be 61.7 MPa for neat DHPC paper (45 °C), while E′r was also estimated by DMA to be 11.7 GPa for solution casted TCNCs paper (45 °C). Figure 5b shows that the experimental E′ values of samples approximated the line which describes the percolating model. It indicated the formation of the percolating network of TCNCs through their strong hydrogen bonding interactions. The final experimental E′ value of nanocomposites (Xr = 0.188) 17



was higher than the predictions from percolating model. It could be explained by that the strong interactions between the aggregated TCNCs and DHPC resulted in the formation of filler-bound matrix polymer layer around the TCNCs, which would effectively increase the volume fraction of the rigid phase, leading to an increase of E′.46 The nonlinear elastic behavior of the papers was observed from stress-strain curves, as shown in Figure 5c. The tensile strength for P0 was extremely low (0.72 MPa), whereas its elongation at break (εb) was about 160% indicating high ductility. After incorporating TCNCs, nanocomposite papers displayed significant increase in tensile strength (σb). However, the εb of nanocomposite papers gradually decreased with the increasing of TCNC contents. Because the higher stresses were employed in the tensile test than that in DMA, the toughness related to DHPC/TCNCs interface interactions and dispersion quality of TCNCs in the DHPC matrix became important.47 Fortunately, the toughness of nanocomposites, which was calculated by the area under stress-strain curves, was great improved after reinforced with TCNCs. For example, the toughness of P5 was more than 3 times higher than that of P0, while the toughness of P10 was 8.5 times that of P0. The trend was that the soft and ductile DHPC papers converted to rigid and stiff nanocomposite papers after incorporation of TCNCs. The cellulose nanocrystals from other sources (cotton linter and microcrystalline cellulose) with lower aspect ratio (~ 10, Table S2) than TCNCs are less glamorous as reinforced fillers, as shown in Figure S8. It indicated that the higher aspect ratio benefited the stress transfer in the nanocomposites in accordance with the 18


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results of Eichhorn and co-workers from Raman spectroscopy.37 The impact resistance of the papers was characterized by bursting testing. Figure 5d shows the bursting index (BI) of papers with different TCNC contents. The impact resistance of P0 was significantly improved after incorporation of TCNCs. For example, the BI value of P0 was 0.94 kPa·m2.g-1, which increased to be 2.74 for the P10. In the view of above results, the toughness, tensile strength, tensile modulus, and impact resistance of nanocomposite papers were improved significantly by introducing TCNCs. The reinforcement was attributed to the formation of percolating TCNC network, where the stress was assumed to be transferred primarily through TCNC/TCNC interactions. Application of the transparent nanocomposite papers

Figure 6. The structure of inverted polymer solar cell (a), the sheet resistance of P10 deposited with ITO under repeated bending at bend radius of 2 mm (b), J-V 19



characteristic curve under 1,000 W m-2 AM 1.5G illumination (c), photograph (inset of d), and EQE spectrum (d) of solar cell.

Since the transparent cellulose-based papers have the advantages of the biodegradability, reproducibility, light-weight, low-cost, smooth and adhesive surface and good mechanical flexibility, it was suitable to fabricate the flexible solar cell as substrates. Considering mechanical properties and transparency of the nanocomposite papers, the P10 was selected as substrates. The architecture of inverted polymer solar cell is shown in Figure 6a. The papers were directly coated with 80 nm conductive ITO layer to form a transparent cathode (Figure 2e). The sheet resistance of cathode was measured to be 20 Ω/□, and the electric conductivity was changed hardly under repeated bending to a very small radius of 2 mm, as a result of the excellent adhesiveness of the transparent paper (Figure 6b). Device parameters such as open-circuit voltage (Voc), fill factor (FF), short-circuit current density (Jsc) and power conversion efficiency (PCE) were deduced from the J–V characteristics curve in Figure 6c. The polymer solar cell exhibited a PCE of 4.98% with a Voc of 0.71 V, a FF of 51.37%, and a Jsc of 13.65 mA cm-2. The external quantum efficiency (EQE) curve showed the photo-responses of the device could reach 42% (Figure 6d). The Tr value in the range of 300 ~ 400 nm was lower than 53%, it would cause loss of the EQE at corresponding wavelength. The PCE of the polymer solar cells based on rigid glass with the same photoactive layer and structure was optimized to be ~ 7%.48, 49 The performance of the flexible polymer solar cells on cellulose-based substrate was 20


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comparable to that on glass and could further improved by optimizing layer thicknesses and process parameters. More significantly, the flexible paper was derived from renewable and biodegradable natural polymer, following the concept of the sustainable development.

CONCLUSIONS The transparent and flexible cellulose-based nanocomposite papers were successfully fabricated to be used as solar cell substrate. O-2, 3-dihydroxypropyl cellulose (DHPC) synthesized by homogeneous etherification was transparent and flexible but poor of mechanical properties, which was reinforced significantly by introducing rigid tunicate cellulose nanocrystals (TCNCs) with an aspect ratio of 72. As a result of the similar polyhydroxyl structure which could form strong hydrogen bonding interactions, TCNCs were homogeneously immobilized in DHPC matrix when the content was below 15%. The interfacial compatibility between TCNCs and DHPC as well as elastic modulus distribution map were measured by AFM mapping technology, indicating that the excellent compatibility resulted in ameliorative mechanical properties, remaining smooth and adhesive surface of DHPC10. The toughness, tensile strength, tensile modulus and impact resistance of the nanocomposite papers were enhanced significantly by introducing TCNCs. The reinforcement was attributed to the formation of percolating TCNC network, which was well predicted by a percolating model. The nanocomposites could be directly deposited transparent conductive material without further treatment. The electrical 21



conductivity of the electrode was stable under repeated binding. Importantly, the flexible cellulose-based polymer solar cells exhibited a PCE of 4.98%, showing enormous potential for the optoelectronic devices. The fabrication of clean energy devices from renewable natural polymer conforms to the principle of sustainable development.

ACKNOWLEDGEMENTS This work was supported by the Major Program of Natural Science Foundation of China (21334005), the Major International (Regional) Joint Research Project (21620102004), the National Natural Science Foundation of China (21304021), and Jiangsu Province Science Foundation for Youths (BK20150382).

ASSOCIATED CONTENT Supporting Information. The physical parameters of papers, the size of CNCs from different sources, SEC-LLS of methylcellulose (methylation of tunicate cellulose), gas chromatography trace of the methylcellulose after total hydrolysis and acetylation, solid-state 13C NMR spectrum and XRD patterns of TC and TCNCs, FTIR spectra of pDHPC and DHPC, rheology of DHPC solution, SEM images of P10, XRD patterns and FTIR spectra of neat DHPC paper and nanocomposite papers, loss tangent tanδ of neat DHPC paper and nanocomposite papers, thermal analysis of nanocomposite papers, TEM image of CNCs from MCC and cotton and stress-strain curves for P10 reinforced with CNCs from different source. 22


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PTB7:PC71BM Photovoltaic Cells by Means of Impedance Spectroscopy. Solar Energy Materials & Solar Cells 2015, 144, DOI 10.1016/j.solmat.2015.09.050.

TOC

The flexible transparent nanocomposite papers fabricated from renewable sources were used as polymer solar cell substrates for developing the clean energy, leading to the ecological benign cycle.

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Construction of Transparent Cellulose-Based Nanocomposite Papers

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