Transparent Cellulose Nanofibrils Composites with Two-layer Delignified Rotary-cutting Poplar Veneers (0°-layer and 90°-layer) for Light Acquisition of Solar Cell

Our transparent cellulose nanofibrils composites (TCNC) directly from rotary-cutting poplar veneer (RPV) whose lignin can be easily stripped by our treatment. This TCNC is prepared by stripping lignin of original RPV and infiltrating epoxy resin (ER) into delignified RPV. This TCNC with two-layer delignified RPVs whose grains perpendicular (0/90°) to each other, which were solidified on solar cell while infiltrating ER. This TCNC with high transmittance (~90%), high haze (~90%), and equal refractive index fluctuation. Comparing with epoxy resin (ER), this TCNC can enhance open circuit voltage (VOC) from 1.16 to ~1.36 and short circuit density (JSC) from 30 to ~34 for the solar cell, and can enhance test fore from 0.155 kN to ~0.185 kN and displacement from 43.6 mm to ~52.5 mm.

In our this work (Table 1), the lignin of RPV was stripped by hydrothermal treatment in sodium hypochlorite (NaClO) solution, and impregnation treatment 1 in ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) solution, and impregnation treatment 2 in sodium hypochlorite (NaClO) solution. Epoxy resin (ER) and its hardener were infiltrated into the two-layer delignified RPVs in this work. To compare with our previous work, this TCNC with more high transmittance (~90%), high haze (~90%) and equal refractive index fluctuation.

Results and Discussion
Cell wall contents of RPV before and after delignification. Fourier transform infrared spectroscopy (FTIR) was used to investigate the changes of its cell wall contents from original RPV to delignified RPV by using FTIR-850 (Gangdong, Tianjin, China). In the FTIR spectrum, the band at 1505 cm −1 is aromatic compounds (phenolic hydroxy groups) and is attributed to aromatic skeleton vibrations from lignin 15,27,30 . The bands at 1235 cm −1 and 1735 cm −1 are characteristic of hemicelluloses and C=O functional group respectively 27,[31][32][33] . Comparing with original RPV and delignified RPV of previous work (ref. 27 ), the peaks at 1505 cm −1 , 1235 cm −1 and 1735 cm −1 have disappeared in delignified RPV of this work, proving that lignin, hemicellulose and C=O functional group have been stripped from original RPV in our this work (Fig. 2). As Table 2 shows, the absolute-drying weight of original RPV (60 mm × 60 mm × 3 mm) is about 2.124~2.381 g, and the absolute-drying weight of delignified RPV (60 mm × 60 mm × 3 mm) is about 1.041~1.164 g. After delignification, the absolute-drying weight of delignified RPV was about 50% of original RPV.

Microstructure of TCNC.
ER is a kind of index-matching polymer for delignified wood, and transmittance of delignified wood can be developed by infiltrating ER 13 . Before and after ER infiltration, delignified RPV and TCNC were cut from its radial direction and longitudinal direction, these sections were examined by using Quanta 450 scanning electron microscopy (FEI, US). Figure 3(a-d) are SEM images of radial direction and longitudinal direction from delignified RPV and TCNC, respectively. In Fig. 3, graphical illustration and SEM images indicate that the microstructure of TCNC is well-infiltrated and well-preserved by ER.  www.nature.com/scientificreports www.nature.com/scientificreports/ Optical properties of TCNC for light acquisition of solar cell. In TCNC, its cellulose nanofibrils network and its lumen are the main pathway of optical transmittance. Modification of the wood cell wall will help to tune the light scattering properties of its material, and introducing strong scattering, resulting in diffused luminescence from embedded quantum dots 15,16,27 . The optical haze of TCNC is due to its nature-structural anisotropy and its light scattering properties.
Transmittance and haze were obtained by using WGT-S transmittance and haze tester (SGIC, Shanghai, China). Figure 4(a,b) shows that our TCNC with high transmittance of ~90%, high haze of ~90%. When TCNC be in contact with the substrate whose colored shape can be clearly seen, and when it be took 5 mm above the substrate whose colored shape becomes very fuzzy. S130C photodiode power sensor (Thorlabs, US) was used to record the scattered light intensity distribution in both the x and y directions on the surface of TCNC. Figure 4(c) indicates that this TCNC with almost equal refractive index fluctuation in the x and y direction. In our previous work, TWC with one-layer delignified RPV, that has anisotropic light diffraction and lower refractive index fluctuation in the direction of aligned cellulose fibers 27 . Our this TCNC with two-layer delignified RPVs whose grains perpendicular (0/90°) to each other, that making its refractive index fluctuation of the x direction close to the y direction.
According to its high transmittance, high haze and equal refractive index fluctuation, TCNC is superior transparent layers for light acquisition of solar cell, which as Fig. 4(d) shows. The electrical properties of solar cell mainly includes open circuit voltage (VOC) and short circuit density (JSC) 8 , and the current density-voltage curves of solar cell with ER and with TCNC were obtained by using CS310H electrochemical workstation (CorrTest, Wuhan, China). Figure 4(d) and Table 3 indicate that TCNC improving the light acquisition of solar cell to compare with ER, and enhancing the solar cell's VOC from 1.16 to ~1.36 and its JSC from 30 to ~34.
Mechanical characteristics of TCNC. ER that is a kind of current material for surface of solar cell at present, but our TCNC has better tensile strength than ER. Figure 5(a) indicates that TCNC has almost equal tensile strength from longitudinal directions in 0°-layer and 90°-layer. Comparing with ER (60 mm × 60 mm × 3 mm), the test fore of TCNC (60 mm × 60 mm × 3 mm) can enhance from 0.155 kN to ~0.185 kN, and its displacement can enhance from 43.6 mm to ~52.5 mm, which as Fig. 5(b) and Table 4 show. The tensile strength was tested by using the tester of mechanical property SmartTest (Joyrun, China). TCNC can meet more flexible shape for solar cell to compare with ER.

Conclusions
For improving practicability of TWC in light acquisition of solar cell, we have basically mastered a kind of method of preparing TWC from original rotary-cutting poplar veneer. Our TCNC with high transmittance (~90%), high haze (~90%), and almost close refractive index fluctuation, which can enhance VOC from 1.16 to ~1.36 and JSC from 30 to ~34 for the solar cell to compare with ER. Although ER being a kind of current material for surface of solar cell at present, however, comparing with ER, our TCNC can enhance test fore from 0.155 kN to ~0.185 kN  www.nature.com/scientificreports www.nature.com/scientificreports/ and displacement from 43.6 mm to ~52.5 mm, which can meet more flexible shape for solar cell. Furthermore, our future work will pay more attention to reduce the time cost and the resource consumption in preparation of TCNC, and to improve the quality of TCNC for the light acquisition of solar cell.
Stripping lignin of original RPV. The step 1 of delignification is hydrothermal treatment that boiling the sample of original RPV in the NaClO solution (0.405 mol L −1 in deionized water) for about 3 h at 130-160 °C. Then, the RPV sample was took out from the solution and its chemicals was removed by rinsing in hot distilled water. The step 2 of delignification is impregnation treatment 1 that immersing the RPV sample in the (NH 4 ) 2 S 2 O 8 solution (1.1 mol L −1 in deionized water) for about 72 h at 15-25 °C. Then, the chemicals of sample was also removed by rinsing in hot distilled water. The step 3 of delignification is impregnation treatment 2 that immersing the RPV sample in the NaClO solution (0.81 mol L −1 in deionized water) for about 24 h at 15-25 °C until its color has disappeared. After stripping lignin, the delignified RPV was preserved in C 2 H 6 O.
Infiltrating ER into delignified RPV and solidifying it on solar cell. First, the delignified RPV was attached to the surface of the sample of solar cell by C 2 H 6 O. Second, a kind of liquid resin was prepared by mixing ER and its hardener at a ratio of 3 to 1 (ER 45 ml, its hardener 15 ml), and this liquid resin (60 ml) was covered on the delignified RPV. Then, this liquid resin was filled into the delignified RPV by vacuumizing in RV-620-2 vacuum reactor (YBIF, Shanghai, China) at 25-30 °C. All the above processes should be completed within 30 min. After first layer of delignified RPV (0°-layer RPV) solidifying on solar cell for about 24 h at 25-30 °C, second layer of delignified RPV (90°-layer RPV) was solidified on 0°-layer RPV by repeating the above processes.   Table 4. Test fore and displacement from ER or TCNC, respectively.