As the most abundant renewable natural polymer on earth, cellulose is regarded as a primary material for the material and the chemical industry. Cellulose is a long-chain polymer composed of D-glucose units connected by β-1,4-glycosidic bonds and has many advantages such as low cost, sustainability, biocompatibility, and biodegradability [20, 21]. The molecular chains in cellulose include ether bonds, hydroxyl groups, carbon-carbon bonds, and carbon-hydrogen bonds, which do not absorb sunlight in the visible range, allowing light to pass through cellulose and make it colorless [22]. Therefore, cellulose can interact with light in various ways, enabling the creation of transparent and tunable haze materials. Considering the challenge of shortened lifetime in PSCs under prolonged UV exposure [12, 23], here, based on the existing papermaking industry platform, a transparent, hazy, and UV-blocking paper was designed and prepared by using CMC-Na and TA as functional fillers (Fig. 1).
The results of ATR-FTIR confirmed the chemical bonding interactions between cellulose molecules, as depicted in Fig. S1. In the case of CE paper, distinct -OH stretching vibration shows at 3566 cm− 1. Peaks at 1411 cm− 1 and 1016 cm− 1 correspond to -CH2 bending and C-O stretching vibrations, respectively [24]. For the CMC film, peaks appeared at 1583 cm− 1 and 1409 cm− 1 which attributed to the asymmetric and symmetric stretching of COO- ions, respectively [25]. Furthermore, other characteristic peaks associated with CMC are observable in all samples. Peaks at 2883 cm− 1 and 1100–1150 cm− 1 can be attributed to C-H stretching and bending modes in the CMC backbone, respectively. The broad band observed between 3000 and 3700 cm− 1 is attributed to O-H stretching [26]. In the spectrum of TA, peaks related to C = O stretching vibrations appears at 1705 cm− 1, while the aromatic vibrations are observed in the range of 1415–1645 cm− 1. Other peaks include 1305 cm− 1 (out-of-plane vibration of O-H) and prominent features in the fingerprint region at 1019–1185 cm− 1 (substituted benzene ring vibrations) and 754 cm− 1 (in-plane vibration of O-H) [24, 27].
After impregnating CMC into the CE paper, the peak at 3566 cm− 1 shifts to 3390 cm− 1. After adding TA, the peak red shifted, indicating the formation of hydrogen bonds between the phenolic -OH groups of TA. The hydrogen bonding interactions leads to the enhancement of vibration energy, causing the disappearance of the C-O peak at 1693 cm− 1 in the FTH paper, further confirming the presence of hydrogen bonds between TA and cellulose [28–31].
To better understand the mechanisms behind the transparency and haze of the paper, microstructure, porosity, and crystallinity were analyzed. The size and distribution of internal pores within the paper can lead to different scattering behaviors, ranging from ballistic scattering (isolated and small air inclusions) to multiple scattering (cellulose fiber scattering elements under the high pore density) [32]. For CE paper, visible light is not significantly absorbed due to the difference in refractive index between air and cellulose fibers, allowing opacity to be achieved through scattering and reflection at the interface [33].
Porosity detection of the paper samples was performed by using a mercury porosimeter (Fig. 2b). A large number of microsized pores were detected in the range of 10 − 1 µm. Light is refracted when passing through two different media, leading to substantial light scattering within the voids and the CE paper [34]. In comparison, the number of macropores in TH and FTH paper was only 1/7 and 1/10 of the CE paper, respectively, which significantly reduced refraction. Therefore, light undergoes a transition from ballistic scattering to intermediate scattering when encountering smaller pores, resulting in the properties of high transparency and haze.
SEM images show the surface and cross-sectional pore structures of the paper samples (Fig. 2b-g). The CE paper exhibited a loose porous structure, which is mainly due to the removal of hemicellulose and lignin from the cell walls [30]. TH and FTH paper prepared by impregnating with CMC and CMC/TA solutions exhibited relatively smaller pore sizes and denser structures, which was attributed to CMC filling, and higher crosslinking density between TA, cellulose, and CMC. These results are consistent with FTIR [31, 35].
Figure 2h shows XRD spectra of the paper samples. The spectrum of CMC film exhibited a strong peak at 2θ = 22.70, indicating the semi-crystalline mode of polysaccharide chains [36]. This result was also confirmed by the polarized light microscopy image, where the crystallized regions of the CMC film were small and almost invisible (Fig. 2i). Therefore, the CMC film has a weak light scattering ability. The XRD spectrum of TA displayed a broad peak centered around 2θ = 26.1°corresponding to the amorphous nature of TA [37]. Since the a minimal amount of TA in FTH films, the characteristic peak of TA is not obvious.
The original cellulose, TH, and FTH paper all exhibit the crystal structure of cellulose I. The diffraction peaks at 2θ = 14.8°, 16.3°, and 22.6° are attributed to the (110) and (200) planes [38]. Combining the polarizing light microscopy images, changes in refractive index contrast may lead to altered light scattering states (Fig. 2j-l) [32]. It can be seen that the CE paper contains a large number of crystalline regions, while in the TH and FTH papers prepared with CMC impregnation, a considerable amount of semi-crystalline micrometer-scale fibers are intertwined in an irregular manner. The refractive indices of the crystalline region (luminescent area) and the non-crystalline region (dark area) of cellulose are 1.584 and 1.532, respectively [39]. Although the refractive index difference is only 0.052, the presence of multiple interfaces results in strong forward scattering. Therefore, the randomly distributed softwood fibers in the paper play a significant role in achieving high transmittance haze.
Based on the experimental results, the mechanism underlying the transparency and haze of paper is depicted in Fig. 2m. When the light beam irradiates the surface of the paper, interface reflection and refraction occur initially. The refracted light incident on the pores undergoes light scattering, resulting in the film exhibiting haze. Cellulose itself has light scattering ability due to variations in diameter and the presence of crystalline and amorphous regions. When light strikes the cellulose fibers, it likewise leads to light scattering.
To quantitatively evaluate the transparency and haze, the paper samples were analyzed with different weights ranging from 20 g/m² to 42.5 g/m². As shown in Fig. 3a and Fig. S2-S3, as the basis weight increases, the transmittance gradually decreases while the haze gradually increases. Paper substrates with a basis weight above 40 g/m² exhibit haze levels approaching 100%. Although it is theoretically possible to achieve both high transparency and haze employing higher basis weight paper, the augmented fiber intertwining density creates difficulties in impregnation and prevents obtaining a uniform impregnation (Fig. S4). The 20 g/m² paper substrate with the highest transparency and a haze of over 90% was selected for the following experiments.
To minimize the pore space, the sizes of cellulose fiber were reduced from the micron to the nanoscale. As pore size decreases, light scattering decreases, resulting in enhanced transparency [18, 19, 33]. To investigate the effect of the impregnation amount of CMC solution (1 wt%) on transparency and haze, experiments with CMC impregnation levels ranging from 8–28% relative to the paper substrate were conducted (Fig. 3b and Fig. S5-S6). The results revealed that as the impregnation amount increases, the transparency of TH paper first increases and then decreases. The voids within the paper substrate gradually filled and the volume of internal voids decreased, resulting in a transition from a multiple scattering mechanism to an intermediate scattering mechanism [32]. Scattering is a holistic phenomenon, thickness also holds a significant influence on the outcome [32]. As the impregnation amount continues to increase, the thickness paper increases, resulting in a reduction in light transmittance. The haze of TH paper gradually decreases with the increase of impregnation amount, but did not reach a level comparable to that of the CMC film (Fig. 3d). The haze here is affected by Mie scattering caused by the diameter of cellulose fibers (α = 2πr/λ, where r is the diameter of the scattering object, λ is the incident wavelength and α ranges from 0.1 to 100) [22]. According to the Mie scattering equation, the paper substrates with a diameter of 30.9 µm exhibited strong forward scattering (Table S1). In addition, as the impregnation amount increases, the voids in the paper are gradually filled and even reach the nanoscale. This is the reason for the reduction in haze for TH paper but not to the level of the CMC film.
The results of UV-visible transmittance confirmed the UV-blocking capability of TA (Fig. S7). The addition of TA affected the UV-blocking efficiency and light transmittance of the paper, prompting us to investigate the addition of tannic acid in FTH paper (Fig. S8). When the addition of TA is 0.7%, FTH paper exhibited a light transmittance close to 86.7% in the visible light range and a stronger UV-blocking effect. Therefore, an addition of 0.7% tannic acid was selected to prepare the FTH paper.
Figure 3c and d show that CMC film exhibits high transparency and low haze in the visible light range, while the CE paper substrate possesses low transparency and high haze. When the CMC film is lifted 3 cm away from the pattern, "Tianjin University of Science and Technology" on the background becomes clearly visible (Fig. 3e and f). For the CE paper, when placed closely to the pattern, the text on the background appears blurry. When lifted 3 cm, the content on the back becomes almost indiscernible. In contrast, the text on the background is clearly visible and remains distinguishable when the TH paper is placed close and lifted 3 cm to the pattern, respectively. These phenomena indicated the high transparency and the high haze characteristic of TH paper. FTH paper has similar optical properties to TH paper, with a transmittance of 86.83% at 600nm and a haze of 71.45% (Fig. 3c and d). In this case, the high transmittance is due to light scattering rather than direct transmission [40]. Compared to TH paper, FTH paper exhibits an effective UV shielding ability. Figure 3m can intuitively see that FTH can effectively block UV light so that the anti-counterfeiting marks on the banknote do not show fluorescence.
In addition to its superior optical performance, the FTH paper also demonstrates good aging stability, mechanical properties, flexural resistance, and water resistance. Prolonged exposure of the transparent paper to sunlight often reduces their light transmittance, affecting the efficiency of PSCs. In order to evaluate the stability of the paper in practical applications, a 100-hour light soaking aging experiment was conducted. Figure 4a and b show that the light transmittance and haze of the samples remain stable after UV aging. Pure cellulose contains fewer chromophoric and auxochromic groups and possesses a higher degree of crystallinity, thus exhibiting excellent UV aging resistance. The slight changes in light transmittance and haze may be due to the presence of small amounts of lignin with chromophoric groups in the pulp, which may be oxidized under UV irradiation [40].
The mechanical properties and flexural resistance of materials play a crucial role in their practical applications. In this regard, we conducted mechanical performance testing on the composite films before and after aging. After UV aging, all the paper samples exhibited a slight decrease in stress and a slight increase in strain. FTH paper demonstrates the best mechanical stress, which could be due to the strong hydrogen bonds between cellulose and CMC, while the added TA enhances interfiber binding [31, 41]. Figure 4d demonstrates the outstanding bending resistance of FTH paper, which can withstand up to 2156 bending cycles, which is 154 and 5.78 times that of regular printing paper and CE paper, respectively. The FTH paper can be easily folded into the shape of a Kawasaki rose and maintains its integrity when unfolding. This highlights the advantages of FTH paper in terms of its good mechanical properties and durability, making it suitable for applications that require frequent folding and bending, such as foldable solar cells [42, 43].
The initial water contact angle (WCA) of our FTH paper measures 78.7°, significantly higher than that of regular paper (49.5°) (Fig. 5a). FTH cartons filled with liquid can maintain their original shape without leaking and fading, showing their excellent waterproof properties. When the dyed CE paper and FTH paper were immersed in water, the CE paper curled and lost color in the water, while FTH paper retained its shape and color (Fig. 5b). These phenomenon can be attributed to two factors: First, the protonation of CMC-Na into CMC-H forms additional physical crosslinking between H3O+, -COOH, and -OH through hydrogen bonding interactions [35]. Second, compared with TH paper, the water contact angle of FTH paper further increases due to the hydrogen bonding interactions between TA and cellulose fibers (Fig. 5c).
Based on the above basic properties, we deduce that FTH paper with with high transparency, high haze, UV-blocking, UV aging stability, water resistance, and mechanical performance. This combination of attributes positions it as a promising candidate for widespread applications in areas such as solar cells, windows, displays, and beyond. Figure 6a and b illustrate the structure of PSCs with a fluorine-doped tin dioxide (FTO)/TiO2/perovskite/spiro-OMeTAD/Au architecture. Our FTH paper is directly attached to the surface of FTO to improve light path length and UV stability (Fig. 6b). The current density-voltage (J-V) curves and the electrical performance (PCE, Voc, FF, Jsc) show that the PCE of FTH paper-coated PSCs was 20.26%, which is a slight decrease than the TH paper (with a PCE of 20.8%, Fig. 6c). This result could be due to the slight decrease in light intensity caused by the UV resistance of FTH paper. It is obvious from the curves that the short-circuit current of PSCs with FTH paper decreased.
In addition, the UV stability of PSCs was evaluated through a UV aging test, which was performed for 100 h under 253 nm UV lamp irradiation at a power density of 384.61 mW cm− 2. Notably, the UV aging test was conducted on both bare perovskite films and devices without any encapsulation and the UV intensity is approximately four times that of AM1.5 solar radiation (around 100 mW cm− 2 UV). The results show that FTH paper-coating significantly improved the stability of PSCs (Fig. 6d). After 100 h of UV irradiation, the device with FTH paper still retained 81% of its initial PCE, while that with TH paper and control group only retained 72% and 55% of their initial PCE, respectively.