Two-dimensional Nanophotonic Crystals For Advanced Light Trapping In Crystalline Silicon Thin Film Solar Cells

Crystalline silicon thin film(c-Si TF) solar cells with an active layer thickness of a few micrometers may provide a viable pathway for further sustainable development of photovoltaic technology, because of its potentials in cost reduction and efficiency improvement. However, the performance of such cells is largely constrained by the deteriorated light absorption of the ultra-thin photoactive material. To explore a suitable light trapping scheme for c-Si TF solar cells, novel two-dimensional(2D) nanophotonic crystals are designed to efficiently coupling the freely propagated sunlight into the active c-Si TFs via two typical ways: minimized reflection by impedance matching and enhanced scattering by waveguide modes. The main works include the following aspects:(1) We report an efficient light-trapping strategy in c-Si TFs(~20 μm in thickness) that utilizes 2D arrays of inverted-nanopyramid(INP) as surface texturing. Three types of INP arrays with typical periodicities of 300, 670, and 1400 nm, either on front, rear or both surface of the c-Si TFs, are fabricated by scalable colloidal lithography and anisotropic wet etch technique. With the extra aid of antireflection coating, the sufficient optical absorption of 20 μm-thick c-Si with a double-sided 1400 nm-INP arrays yields a photo-current density of 39.86 mA/cm2, which is about 76% higher than the flat counterpart(22.63 mA/cm2) and is only 3% lower than the value of Lambertian limit(41.10 mA/cm2).(2) A high throughput surface texturing process for optical and optoelectric devices based on a large-area self-assembly of nanospheres via a low-cost micro-propulsive injection(MPI) method is presented. The novel MPI process enables the formation of a well-organized monolayer of hexagonally arranged nanosphere arrays with tunable periodicity directly on the water surface, which is then transferred onto the preset substrates. This process can readily reach a throughput of 3000 wafers/h, which is compatible with the high volume photovoltaic manufacturing, thereby presenting a highly versatile platform for the fabrication of periodic nanotexturing on device surfaces.(3) To investigate the impact of INP arrays on the photovoltaic performance of solar cells, simple-fabricated c-Si/PEDOT:PSS heterojunction solar cells are chosen. With the regular geometric characteristics of INP arrays, c-Si substrates are deliberately textured into INP arrays with designed fill factors, thereby presenting a highly versatile platform for construction and investigation of textured c-Si/PEDOT:PSS solar cells with tunable junction area. We found that the increases in junction area could be directly reflected in the decreases in series resistance and thus improving in fill factor of the hybrid solar cells. Combining with the increased short circuit current density(Jsc) from enhanced light absorption, an enhancement of 23.7% in efficiency is achieved in the hybrid solar cells with 1.47 times junction area, in comparison with the reference one.