Flexible Nanocone Anti-Reflection Films for High-Efficiency Photovoltaics

A photovoltaic device is essentially a solar energy harvesting device converting the incoming photons to charge carriers. Therefore, the capability of capturing the incident photons is one of the key characteristics of a solar cell device. Since the reflectance loss of light leads to inefficient utilization of the incident photon, various anti-reflection (AR) schemes have been developed to achieve high efficiency solar cell devices. Conventionally, quarter-wavelength (λ/4) AR coatings have been widely used on the front surface of photovoltaic devices/modules. However, its effectiveness typically has wavelength and incident angle dependence, and high quality AR coating relies on chemical or physical deposition processes which increase the production cost. Meanwhile, nano/microstructures have been discovered with broadband light trapping capability which can significantly suppress device front surface reflection. Hence, a variety of nano/microstructures, such as nano/micro-pyramids, nanowires, nanopillars, nanocones, nanodomes, nanospheres, and so forth, have been extensively studied with different photonic materials, such as Si, Ge, CdTe, and Cu(In, Ga)Se (CIGS), etc. Although these structures have demonstrated appealing performance on photon management, many of them have been fabricated with costly and/or destructive methods, such as lithographic and wet/dry etching approach. And these approaches may not be necessarily applicable for thin film photovoltaics, especially for flexible applications. On the other hand, fabricating active photovoltaic materials into nano/microstructures introduces defects and the increased surface recombination, hence it needs to be carefully designed and performed. In this work, we have utilized a facile molding process to fabricate flexible plastic AR films with three-dimensional nanocone arrays on the front surface. The geometry of the nanocones, i. e. pitch and height, can be precisely controlled by tuning the structure of the inverse nanocone mold fabricated with anodization in conjunction with nanoimprint. The AR films can be readily attached to flat substrates, such as glass and Si, without adhesive glue. Therefore, their effectiveness has been examined on high efficiency CdTe thin film solar cells fabricated on glass substrates. The optical reflectance measurements and simulations have shown that the nanocone structure can significantly reduce the reflectance of the glass window above the CdTe light absorbing layer, resulting in appreciable device performance improvement confirmed by both current-voltage characteristics and quantum efficiency measurements. Furthermore, it was found that the improved AR effect can be observed with oblique light incident angle, which is highly beneficial for practical deployment of photovoltaic panels. Particularly for the studied high performance CdTe solar cell devices, it was found that over 1 kWh/m² daily electrical energy output can be achieved with the nanocone AR film, indicating ~7% overall enhancement over the same device without the nanocone AR film. Besides intriguing AR property, it was also discovered that the nanocone structures are superhydrophobic with high water contact angle. This effect suggests the plastic AR film can possess self-cleaning function which is favorable for solar cells/modules deployed in dusty environment.