New Methods of Texturing Crystalline Silicon For Multi-Junction Solar Cell Applications

Abstract

In this thesis, two new types of crystalline silicon texturization were developed, eventually aimed to be used in a triple junction solar cell from monocrystalline silicon and two thin film silicon alloy layers. The bottom of the two thin film silicon layers is made of hydrogenated nanocrystalline silicon (nc-Si:H). The current problem with this technology is that the adhesion of the nc-Si:H to a flat surface of crystalline silicon is not always ideal. This can be improved by texturing the monocrystalline silicon. This also improves the optical efficiency. The standard pyramidal texture method of crystalline silicon has proven to be too sharp which leads to cracks in the layer of nc-Si:H during deposition. The two investigated texturing methods are a periodic hexagonal texture with semispherical walls created using photolithography and a random texture of craters using a sacrificial layer of polycrystalline silicon. For the hexagonal texture, a photolithography process was designed and tested on the ability to grow crack-free nc-Si:H. It was found that a texture period of 3 µm was most suited for the desired nc-Si:H layer thickness of 3-3.5 µm. For the sacrificial layer process, the effect of the different parameters on the crater size was investigated. These tests showed that an amorphous silicon (a-Si) layer thickness of 1-2 µm gives the largest craters. The ideal implantation energy is dependent on both the implanted ion and the a-Si layer thickness. It was also found that the crater size increases with increasing dopant concentration. An anneal temperature of 950 °C for 60 minutes was determined to result in the largest craters. Furthermore, argon implantation using an energy of 250 keV in 1 or 2 µm of a-Si showed the biggest promise in terms of crater size, with an average hole diameter of around 320 nm. In terms of optical performance, the hexagonal texture results in a slightly lower mean reflectance over a wavelength range of 300 to 1200 nm. The angular distribution of the reflectance showed that the reflectance of the hexagonal texture was very dependent on the measured angle and this angular dependence varies for different wavelengths. This indicates good light scattering of this texture. The results for the large craters show light scattering at approximately the same level as the photolithography samples, but more gradual over different angles. The best hexagonal and sacrificial layer textures were used to create SHJ's. Unfortunately, the minority charge carrier lifetime measurements of these SHJ's showed that the deposited passivation layers were likely to be too thin, which resulted in very low lifetimes. Consequently, the efficiency of the sacrificial layer SHJ's was only 0.1%. The efficiency of the hexagonal texture SHJ's were much higher, at 10.0%. Still, this efficiency was lower than anticipated. The reflectance measurements of these solar cells showed that the average reflectance of the SHJ's using the photolithography texture was 13.4% with TCO. This is lower than the reflectance of the sacrificial layer SHJ, which was at 14.4% with TCO.