Enhanced absorption in thin crystalline silicon films for solar cells by nanoimprint lithography Christos Trompoukis *1, 2 , Aline Herman 3 , Ounsi El Daif 1 , Valerie Depauw 1 , Dries Van Gestel 1 , Kris Van Nieuwenhuysen 1 , Ivan Gordon 1 , Olivier Deparis 3 , Jef Poortmans 1, 2 1 Imec, Kapeldreef 75, B-3001 Belgium 2 Katholieke Universiteit Leuven, Departement Elektrotechniek – ESAT, Kasteelpark Arenberg 10, B-3001 Heverlee (Leuven), Belgium 3 Solid-State Physics Laboratory, Facultés Universitaires Notre-Dame de la Paix (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium ABSTRACT Two dimensional (2D) periodic photonic nanostructures, fabricated by nanoimprint lithography (NIL) and dry etching on the front surface of crystalline silicon (c-Si) layers, are investigated experimentally and theoretically in order to characterize their optical properties and demonstrate their relevance to photovoltaic (PV) applications. Nanoimprint lithography is performed on c-Si wafers and ultra-thin c-Si films with various thicknesses. A comparison with state-of- the-art front side texturing with an antireflection coating is made. The 2D periodic photonic nanostructures result in an enhanced light absorption in the photoactive material. The results are validated through simulations based on Rigorous Coupled Wave Analysis (RCWA). The nanoimprinted substrates result in a similar absorption compared to the state-of- the-art random pyramid texturing while consuming less than a micron of photoactive material. In contrast to the random pyramid texturing, the nanopatterning exhibits a robust performance for a wide range of incident angles up to 70°. The light trapping mechanism we propose is based on the combination of a graded index effect and the diffraction of light inside the photoactive layer at high angles. Keywords: Light-trapping, light-management, photonic nanostructures, diffraction, crystalline silicon solar cells, thin- film silicon solar cells, anti-reflection texturing 1. INTRODUCTION Silicon-based photovoltaic (PV) technology requires improvements in both cost reduction and efficiency in order to compete with cheaper renewable technologies. Although crystalline silicon (c-Si) PV technology yields the most efficient single junction-based module, it is more expensive than other renewable energy technologies 1 with the cost of silicon driving the price levels 2 . Even though lower cost PV technologies are available, limitations are imposed either by the material toxicity, scarcity and availability (CdTe and CuIn(Ga)Se) or by the low energy-conversion efficiency of the cells (amorphous and nano-crystalline silicon). A way to decrease cost, avoiding the aforementioned limitations, would be to use less c-Si material. Films of a few micrometers of c-Si can be either transferred or deposited epitaxially on foreign substrates. However, such ultra-thin photoactive layers result in a significantly reduced optical absorption at the near-infrared region of the solar spectrum due to the indirect band-gap of c-Si, yielding a low absorption coefficient. Loosing part of the solar spectrum is a major efficiency limiting factor. Historically, phenomena based on geometrical optics have been used to improve the absorption in the thick active layer yielding efficiencies up to 25% while the theoretically calculated upper limit indicates values around 33% 3-4 . However, in ultra-thin technologies of a few microns, conventional light * christos.trompoukis@imec.be; phone +32 16281525