ABSTRACT
We carry out the structural design of photonic crystals (PCs) using sensitivity analysis for enhancing optical absorption of thin film microcrystalline silicon (µc-Si) solar cells. In this paper, we employ a model which includes absorption of not only the thin film µc-Si, but also the transparent conductive oxide and metal back reflector for design accuracy. We carry out structural design for this model using sensitivity analysis which maximizes optical absorption in µc-Si layer. As a result, we succeed in obtaining the maximum short circuit current density of 25.2 mA/cm2 for thin film (600-nm thick) µc-Si solar cells (1.4-fold increase compared to the case without a PC).
ABSTRACT
We investigate the improvement of the conversion efficiency of ultra-thin (~500nm-thick) microcrystalline silicon (µc-Si) solar cells incorporating photonic-crystal structures, where light absorption is strongly enhanced by the multiple resonant modes in the photonic crystal. We focus on the quality of the intrinsic µc-Si layer deposited on the substrate, which is structured to form a photonic crystal at its upper surface with a period of several hundred nanometers. We first study the crystalline quality from the viewpoint of the crystalline fraction and show that the efficiency can be improved when the deposition conditions for the µc-Si layer are tuned to give an almost constant crystalline fraction of ~50% across the entire film. We then study the influence of the photonic-crystal structure on the crystalline quality. From transmission-electron microscope images, we show that the collision of µc-Si grains growing at different angles occurs when a photonic-crystal structure with an angular surface is used; this can be suppressed by introducing a rounded surface structure. As a result, we demonstrate an efficiency of 8.7% in a ~500-nm thick, homo-junction µc-Si solar cell, which has only ~1/4 the thickness of typical µc-Si solar cells. We also discuss the possibility of further improving the efficiency by performing calculations that focus on the absorption characteristics of the fabricated cell structure.
ABSTRACT
We numerically investigate broadband optical absorption enhancement in thin, 400-nm thick microcrystalline silicon (µc-Si) photovoltaic devices by photonic crystals (PCs). We realize absorption enhancement by coupling the light from the free space to the large area resonant modes at the photonic band-edge induced by the photonic crystals. We show that multiple photonic band-edge modes can be produced by higher order modes in the vertical direction of the Si photovoltaic layer, which can enhance the absorption on multiple wavelengths. Moreover, we reveal that the photonic superlattice structure can produce more photonic band-edge modes that lead to further optical absorption. The absorption average in wavelengths of 500-1000 nm weighted to the solar spectrum (AM 1.5) increases almost twice: from 33% without photonic crystal to 58% with a 4 × 4 period superlattice photonic crystal; our result outperforms the Lambertian textured structure.
