Roll-to-Roll Fabrication of Large Scale and Regular Arrays of Three-Dimensional Nanospikes for High Efficiency and Flexible Photovoltaics

Thin film photovoltaics (PVs) are highly attractive solutions due to the remarkably light weight and excellent flexibility which enables a wide range of applications. Nevertheless, due to utilization of ultra-thin material with limited light absorption capability and typically low crystalline quality, poor performance of thin film solar cells has placed them in a disadvantageous situation competing with crystalline Si based solar cells. Recent studies have revealed that nanostructured substrates/templates can benefit performance of thin film solar cells, via introducing advanced light management schemes, such as photonic and/or plasmonic light trapping, and unique device design to improve minority carrier collection efficiency.^1-2 Although a number of different types of nanostructures, such as nanodome, nanocoax, nanorod and nanopillars, etc., have been proven effective in this regard, there is still a lack of fundamental understanding on how morphological and structural change will affect the trade-off between photon absorption and carrier collection in a thin film solar cell device.^3-4This is in fact crucial in order to develop rational design guidelines for high efficiency solar cell devices. Moreover, from a practical standpoint, a cost-effective and scalable scheme to fabricate desirable nanostructures on flexible substrates is of an urgent need. In this work, a low-cost process has been developed to fabricate perfectly ordered arrays of 3-D NSPs on flexible Al substrates. These 3-D NSPs were formed with a unique defect nanoengineering approach in conjunction with scalable Al anodization, and they have well controlled geometries, i. e. pitch and height. This type of 3-D nanostructure provides an excellent platform to investigate the aforementioned trade-off between light absorption enhancement with the 3-D architecture and the change of carrier collection property in a thin film solar cell. Specifically, p-i-n junction amorphous-Si (a-Si) thin films were used as model materials deposited on 3-D structures with different combinations of height and pitch to implement 3-D solar cell devices. Then systematic characterizations on device optical absorption and electrical performance were carried out, assisted with optical and device simulations in order to gain further understanding.  Through systematic experiments and simulations, an optimal 3-D structure was identified to achieve a good balance between light absorption and carrier collection, yielding as high as 43% improvement on power conversion efficiency from a planar counterpart. Furthermore, a roll-to-roll compatible nanostructure fabrication process was successfully demonstrated and 3-D flexible thin film solar cells were fabricated in a relatively large scale with respectable efficiency. These investigations have not only exemplified the critical necessity of the coupled optical/electrical design for device performance optimization and shown a set of generic guidelines, but also demonstrated a highly practical and scalable technological platform to develop a new generation of high efficiency thin film solar cells based on various materials.

REFERENCES AND NOTES

1. Leung, S. –F.; Gu, L.; Zhang, Q.; Tsui, K.-H.; Shieh, J. –M.; Shen, C.-H.; Hsiao, T. –H.; Hsu, C.-H.; Lu L.; Li, D.; Lin Q.; Fan, Z.; Roll-to-roll fabrication of large scale and regular arrays of three-dimensional nanospikes for high efficiency and flexible photovoltaics, Sci. Rep., 2014, 4, 4243.

2. Huang, H.; Lu, L.; Wang, J.; Yang, J.; Leung, S. –F.; Wang, Y.; Chen, D.; Chen, X.; Shen, G.; Li, D.; Fan, Z.; Performance Enhancement of Thin-Film Amorphous Silicon Solar Cells with Low-cost Nanodent Plasmonic Substrates, Energy Environ. Sci., 2013, 6 (10), 2965 - 2971

3. Leung, S. -F.; Yu, M.; Lin, Q.; Kwon, K.; Ching, K. L.; Gu, L.; Yu, K.; Fan, Z. Efficient Photon Capturing with Ordered Three-Dimensional Nanowell Arrays. Nano Lett. 2012, 12, 3682-3689.

4. Lin, Q.; Hua, B.; Leung, S.; Duan, X.; Fan, Z. Efficient Light Absorption with Integrated Nanopillar/Nanowell Arrays for Three-Dimensional Thin-Film Photovoltaic Applications. ACS Nano 2013, 7, 2725-2732.