Date Published: May 05, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Min Wang, Pengsha Ma, Min Yin, Linfeng Lu, Yinyue Lin, Xiaoyuan Chen, Wei Jia, Xinmin Cao, Paichun Chang, Dongdong Li.
Antireflection (AR) at the interface between the air and incident window material is paramount to boost the performance of photovoltaic devices. 3D nanostructures have attracted tremendous interest to reduce reflection, while the structure is vulnerable to the harsh outdoor environment. Thus the AR film with improved mechanical property is desirable in an industrial application. Herein, a scalable production of flexible AR films is proposed with microsized structures by roll‐to‐roll imprinting process, which possesses hydrophobic property and much improved robustness. The AR films can be potentially used for a wide range of photovoltaic devices whether based on rigid or flexible substrates. As a demonstration, the AR films are integrated with commercial Si‐based triple‐junction thin film solar cells. The AR film works as an effective tool to control the light travel path and utilize the light inward more efficiently by exciting hybrid optical modes, which results in a broadband and omnidirectional enhanced performance.
The efficient light management is of special interest in improving the performance of photovoltaic (PV) devices.1, 2, 3, 4 A variety of state‐of‐the‐art light management strategies to increase the optical path length have been developed by random or periodic micro/nanostructures, which could enable the scattering effect, surface plasmonic resonance, and photonic modes.5, 6, 7, 8, 9, 10, 11 In order to reduce the reflection of incident light, antireflection (AR) coating has been also widely used on the surface of solar cells12, 13 and their modules.14, 15
The flat ETFE films with a thickness of 75 µm serve as the starting material. Figure1a shows the photograph of an ETFE film prototype with a triangular prism texture imprinted. The textured area is 20 cm wide showing by the yellow dash lines that can be further widened by modifying the R2R machine. The film shows a hazy look surface resulted from its triangular structure refracting the light. Figure 1b represents the scanning electron microscope (SEM) image of the film surface showing the line‐space triangular prism structure with the pitch and height of 25 and 5 µm, respectively. The production of stripe‐like structures is much easier than the 3D matrix in view of both the roller molds fabrication during CNC machining process and the demolding effect in the imprinting process. In order to fabricate the trapezoid prism structure, the triangular microstructured film was subsequently hot pressed under 0.3 MPa and 160 °C for 15 min in a homemade double‐chamber setup as illustrated in Figure 1c. One can find a morphology evolution from triangular prism to trapezoid structure with the pitch and height of 25 and 2 µm, respectively (Figure 1d). The trapezoid prism structure with a lower aspect ratio is beneficial to obtain better mechanical durability. Moreover, the hot pressing process is compatible with the lamination process that is widely used for the encapsulation of solar panels.34, 35 The insets in Figure 1b,d show corresponding 3D schematics, which define line‐space prisms parallel to the y‐axis. For the subsequent comparison, the nanopillar structure was also transferred from the nanostructured nickel mold through planar hot embossing process with the pitch size, height, and diameter of 1 µm, 500 and 500 nm, respectively (Figure 1e).
In summary, we have demonstrated large‐area flexible AR films with microprism structures by a versatile R2R imprinting process. Large‐area microscale trapezoid structure can be easily transferred through roller molding and hot pressing fabrication resulting in reduced cost and prolonged the service life of the mold. The AR films delivered broadband suppression of light reflectance together with improved hydrophobic property that can be easily integrated with flexible and rigid solar cells. The solar cell with the presence of AR film demonstrates a 5.5% energy generation enhancement over the device with flat encapsulation film. The AR film shows attractive application prospects with much enhanced mechanical strength and can be easily utilized on many commercial solar panels.
Fabrication of AR Films with R2R Process: The AR films are fabricated by an R2R imprinting equipment, as shown in Figure S6a (Supporting Information). A copper‐coated steel roller mold (Figure S6b, Supporting Information) was made by CNC lathe technology, whose surface had triangular prisms microstructure (pitch = 25 µm and height = 5 µm). The roller mold was used as the driver roller, where a heater was inserted into its core. The microstructure was transferred from the rollers to ETFE films at 120 °C with a pressure of 0.5 MPa. After cooling down, the polymer with microstructure demolds from the roller. In order to realize the trapezoid prism structure, a hot pressing process was subsequently carried out in a homemade vessel with a heater (Figure 1c). The triangular prism film served as the starting material, which is sandwiched in between two flat polyimide (PI) films. The upper PI film also served as a separator to isolate the chamber into two parts. The hot pressing was conducted where the elevated temperature (160 °C) was achieved by a well‐controlled heater. The applied pressure (0.3 MPa) was realized by vacuum pumping the bottom chamber and injection of high‐pressure nitrogen gas into the upper chamber.
The authors declare no conflict of interest.