Three dimensional a-Si:H thin-film solar cells with silver nano-rod back electrodes
Introduction
Silicon based thin-film solar cells (Si-TFSC) have been a promising candidate for low cost, flexible and semi-transparent photovoltaic devices. However, their low conversion efficiency and stability issues, Staebler–Wronski Effect (SWE) in amorphous silicon [1], have prevented wide spread usage and industry expansion of Si-TFSC, and many efforts have been thus performed in order to overcome the efficiency limitation of Si-TFSC. Among them, light trapping management are especially crucial in Si-TFSC; because thickness scaling of the intrinsic amorphous silicon (a-Si:H) provides reduction in SWE, and harnessing Si-TFSC a broadband light trapping technology enables absorber thickness scaling without photo-current degradation. In superstrate p–i–n configurations, transparent conducting oxides (TCO) with random roughness or pyramid-shaped surface were shown to give light trapping enhancement [2], [3], [4]; and in substrate-type n–i–p devices, thick silver (Ag) back electrodes with rough surface on plastic or stainless steel substrate have been demonstrated to enhance light trapping [5], [6]. These enhancements based on rough TCO or metal electrodes have provided sound technologies for light management, however, the need for further improvement of efficiency and SWE-free realization of Si-TFSC drives further development of light trapping technology. Light scattering at textured electrodes is experimentally smaller than Lambertian limit where ideal random texture is assumed, but in a certain periodically textured geometry, it is theoretically predicted that this limit can be overcome due to surface plasmon resonances [7]. This theoretical background requires the fabrication technology of appropriately patterned electrodes compatible with low cost mass production processes. Recently, a wide range of light trapping structures with periodic nano-patterns and wires such as nano-holes [8], nano-cones [9], photonic crystals [10], nano-particles [11] and wires [12] have been reported. While solar cells employing light trapping structures have improved short circuit current density (Jsc) due to the enhanced light absorption, the open circuit voltage (Voc) and fill factor (FF) degrade compared to flat cells by which overall improvement of cell efficiency is not so high. Their fabrication processes are not well compatible with current thin-film solar cell process. In this paper, we report three-dimensional (3-D) amorphous silicon (a-Si:H) solar cells with Ag nano-rods formed by nano imprint lithography (NIL) which is low cost, large area and roll-to-roll process compatible [11], [12], [13]. The device performances of 3-D and flat control solar cells are compared as functions of physical geometry and optical properties. Possible mechanisms for degradation of Voc and FF in 3-D solar cells are also addressed in this paper.
Section snippets
Experimental
We have formed Ag nano-rods on glass substrate (Corning) by well-known NIL processes [13], [14]. Fig. 1 describes fabrication sequences of Ag nano-rods, where representative cross-sectional structures and SEM images are shown. Lift-off resist (LOR) and imprint resin are formed by spin-coatings (Fig. 1(a)). Silver layers are deposited by thermal evaporation at room temperature (Fig. 1(a) and (e)). After imprinting with PDMS mold, LOR layer was removed by reactive ion etching process with O2
Results and discussion
Fig. 2(b) and (c) shows SEM images of a-Si:H solar cells with different Ag electrodes, where the geometries of a-Si:H solar cells with Ag nano-rod back electrodes with height of 300 nm are totally different with cells prepared on SnO2:F with micro-sized roughness. While solar cells prepared on flat Ag and micro-rough SnO2:F (Fig. 2(d)) have almost two dimensional (2-D) flat geometry, Ag nano-rod provides 3-D shape like nano-domes due to the conformal coverage of Ag nano-rods by ZnO:Al, a-Si:H
Conclusion
We have demonstrated 3-D a-Si:H solar cells with high light harvesting effect by incorporating Ag nano-rod back electrode. The 3-D solar cells have 54% increase in Jsc and 45% in conversion efficiency without any deterioration of Voc over the flat solar cells. The Jsc enhancement is mostly contributed by the enhanced absorption of incident light within a-Si:H thin-films by the effective antireflection and enhanced light trapping effect from Ag nano-rods. This 3-D geometry can also improve Jsc
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (project no. 2009-0090912) and a research grant from Hankyong National University in the year of 2013.
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