Theory predicts that periodic photonic nanostructures should outperform their random counterparts in trapping light in solar cells. However, the current certified world-record conversion efficiency for amorphous silicon thin-film solar cells, which strongly rely on light trapping, was achieved on the random pyramidal morphology of transparent zinc oxide electrodes. Based on insights from waveguide theory, we develop tailored periodic arrays of nanocavities on glass fabricated by nanosphere lithography, which enable a cell with a remarkable short-circuit current density of 17.1 mA/cm2 and a high initial efficiency of 10.9%. A direct comparison with a cell deposited on the random pyramidal morphology of state-of-the-art zinc oxide electrodes, replicated onto glass using nanoimprint lithography, demonstrates unambiguously that periodic structures rival random textures.

Nanopatterning has gained tremendous importance in the field of photovoltaics, as absorption of sunlight in solar cells can be enhanced drastically by proper engineering of photonic nanostructures [1][2][3][4][5][6][7][8] . However, despite intensive efforts, neither the ideal surface morphology nor the ideal scattering characteristics for optimum light trapping have been identified. Experimentally, a method capable of implementing arbitrarily designed surface morphologies directly into functional devices is desirable. Here, we establish a nanomoulding process that provides exactly such a platform, enabling precise, large-area, nanoscale patterning of functional zinc oxide films at low cost. We illustrate the application of nanomoulded zinc oxide films as transparent front electrodes in amorphous silicon solar cells, demonstrating excellent initial conversion efficiencies of 10.1%. In the quest to find the most efficient light-harvesting scheme, we anticipate that nanomoulding will catalyse the development and integration of exciting new nanophotonic structures.Zinc oxide (ZnO) is currently one of the key functional materials in advanced optoelectronic and photonic applications, including photovoltaics, due to its high transparency across the solar spectrum, excellent electrical properties, and the possibility to synthesize a rich variety of nanostructures [9][10][11][12] . Its abundance and non-toxicity are important additional criteria in view of the global large-scale deployment of photovoltaics. Approaches that have already been successfully employed to increase light trapping in solar modules on millions of square metres (see, for example, real-time solar panel counter at www.oerlikon.com/solar) include the growth of ZnO films with randomly oriented pyramids by means of chemical vapour deposition 13 and wet etching of crater-like structures into sputtered ZnO films 14 . The pyramidal morphology in particular has demonstrated outstanding light-trapping capabilities and has led to several certified world-record conversion efficiencies 15,16 . Solution-based methods have also been extensively investigated for the synthesis of nanopillar-type ZnO structures 9 . Although all these approaches provide a certain degree of freedom in designing the surface morphology of ZnO films, the basic feature morphology (pyramids, craters or pillars) is dictated by the underlying growth and etch kinetics. Nanomoulding completely frees ZnO films from morphological constraints imposed by nature, and allows one to transfer or replicate an arbitrary master structure made from an arbitrary (transparent or opaque) master material onto a transparent ZnO electrode (Fig. 1). Consequently, nanomoulding provides full flexibility in designing light-trapping structures, moving one step further towards the fundamental limits for photonic light trapping predicted by theory 17,18 . We also expect that our nanomoulding technique may be extended to other materials and deposition methods, further widening its range of applications.A sketch of the...

We demonstrate high-efficiency thin-film silicon solar cells with transparent nanotextured front electrodes fabricated via ultraviolet nanoimprint lithography on glass substrates. By replicating the morphology of state-of-the-art nanotextured zinc oxide front electrodes known for their exceptional light trapping properties, conversion efficiencies of up to 12.0% are achieved for micromorph tandem junction cells. Excellent light incoupling results in a remarkable summed short-circuit current density of 25.9 mA/cm 2 for amorphous top cell and microcrystalline bottom cell thicknesses of only 250 and 1100 nm, respectively. As efforts to maximize light harvesting continue, our study validates nanoimprinting as a versatile tool to investigate nanophotonic effects of a large variety of nanostructures directly on device performance.

ABSTRACT:The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 μm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm 2 /V/s), and the unequaled lightscattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.

Angle resolved measurements of the external quantum efficiency of n-i-p single junction amorphous solar cell deposited on a grating structure show clearly defined peaks of enhanced photocurrent in the weakly absorbing region between 1.6 and 2.15 eV. We explain these absorption phenomena and their angular variation with the excitation of guided modes via grating coupling. Calculation using an equivalent flat multilayer system permits to relate the theoretical values with the experimental data.