Periodic-Random Modulated Surface Textures For Efficient Light Trapping in Thin-Film Silicon Solar Cells

Abstract

Thin-film silicon solar cells make use of relatively thin layers of active material compared to wafer based solar cells. The main advantage of thin absorber layers is the possibility to fabricate flexible solar cells. However, due to limited absorber layer thickness and weak absorption coefficients at long wavelengths, light management techniques need to be implemented in order to increase the
device performance. Advanced substrate texturisation is one of the promising light management techniques that have drawn much attention recently. In state-of-the-art devices, randomly textured interfaces are often used to increase the optical performance. However, the introduction of periodic diffraction gratings resulted in world record conversion efficiencies for hydrogenated amorphous/nanocrystalline silicon (a-Si:H/nc-Si:H) tandem solar cells.

A different method of advanced texturisation is introducing modulated surface textures (MST). These MST structures combine various surface morphologies (random and/or periodic) to reach higher levels of light scattering over a broader wavelength range. Modulated surface textures based on large periodic gratings and small random textures have been successfully employed in single junction
nc-Si:H solar cells, resulting in a world record efficiency of 11.8 %. An MST structure based on large random features and small periodic gratings, however, has not been reported yet.

The aim of this work is to investigate the possibility of fabricating MST structures based on micro-textured glass with superimposed periodic gratings, and to obtain a functioning n-i-p nc-Si:H solar cell, based on an MST substrate. An optical and morphological analysis of randomly textured substrates, periodic gratings, and modulated surface textures was carried out. It was found that MST substrates can be fabricated using ITO induced, wet-etched glass substrates, and a photolithography process. MST structures resulted in light scattering into greater, less distinct angles, when compared to either one of the individual surface morphologies. Therefore, these structures show promising light scattering behaviour for application in nc-Si:H solar cells.

Hydrogenated nanocrystalline silicon solar cells were fabricated on randomly textured glass substrates and MST structures. Two-dimensional (2D) periodic gratings, with square and hexagonal lattice structures, were superimposed on randomly textured glass to obtain MST substrates. nc-Si:H solar cells based on hexagonal MST structures seemed to outperform their square lattice counterparts. A functioning device with an active area efficiency of 6.46 % and short-circuit current density (Jsc) of 19.74 mA/cm2 was fabricated. This work reports the first functioning nc-Si:H solar cell, based on a periodic-random modulated surface texture substrate. The best performing solar cell on randomly textured glass, however, exceeded this performance with an active area efficiency of 7.67 % and JSC of 23.2 mA/cm2. It can be concluded that functioning nc-Si:H solar cells can be fabricated on 2D MST substrates. Further research focused on the morphological optimisation of MST substrates could help to prevent defective material growth, to improve the electrical properties of future devices.