Crystallization of Amorphous Silicon via Excimer Laser Annealing and Evaluation of Its Passivation Properties

Sanchari Chowdhury,Jinsu Park,Junsin Yi,Youngkuk Kim,Younghyun Cho,Eun-Chel Cho,Jaemin Kim,Sehyeon Kim

doi:10.3390/en13133335

Abstract

The crystallization of hydrogenated amorphous silicon (a-Si:H) is essential for improving solar cell efficiency. In this study, we analyzed the crystallization of a-Si:H via excimer laser annealing (ELA) and compared this process with conventional thermal annealing. ELA prevents thermal damage to the substrate while maintaining the melting point temperature. Here, we used xenon monochloride (XeCl), krypton fluoride (KrF), and deep ultra-violet (UV) lasers with wavelengths of 308, 248, and 266 nm, respectively. Laser energy densities and shot counts were varied during ELA for a-Si:H films between 20 and 80 nm thick. All the samples were subjected to forming gas annealing to eliminate the dangling bonds in the film. The ELA samples were compared with samples subjected to thermal annealing performed at 850–950 °C for a-Si:H films of the same thickness. The crystallinity obtained via deep UV laser annealing was similar to that obtained using conventional thermal annealing. The optimal passivation property was achieved when crystallizing a 20 nm thick a-Si:H layer using the XeCl excimer laser at an energy density of 430 mJ/cm2. Thus, deep UV laser annealing exhibits potential for the crystallization of a-Si:H films for TOPCon cell fabrication, as compared to conventional thermal annealing.

Highlights

Silicon is a formidable material in the electronic and photovoltaic industry due to its outstanding electrical performance and abundancy in nature [1,2]
A previous study showed that Raman spectra of amorphous silicon (a-Si):H and Polycrystalline silicon (poly-Si) are located around 480 cm−1 and 500–515 cm−1, respectively [25,26]
The experimental results indicated that the excimer laser annealing (ELA) process conditions need to be further optimized to reduce manufacturing costs and enable the large-scale application of this technique for efficient production. This experimental study focused on the crystallization of an a-Si:H layer using ELA instead of the conventional thermal annealing to prevent damage to the sample caused by the high-temperature process

Summary

Introduction

Silicon is a formidable material in the electronic and photovoltaic industry due to its outstanding electrical performance and abundancy in nature [1,2]. Polycrystalline silicon (poly-Si) thin films have been widely used in thin film transistors to achieve high definition display in large scale [3]. Poly-Si plays important role as a passivating layer in high efficiency crystalline silicon (c-Si) solar cells. The crystallization of amorphous silicon (a-Si) has been achieved using various methods such as thermal annealing, solid-phase crystallization, metal-induced crystallization, sequential lateral solidification, and excimer laser annealing (ELA) [4,5,6,7]. A combination of inert gases, such as Ar, Kr, and Xe, and reactive gases such as F2 and Cl2 , can be used as the laser gas in ELA. Excimer lasers are advantageous because they are generated under bias and high pressures, which can stop the surface damage caused by high-temperature process. The gas molecules in a Energies 2020, 13, 3335; doi:10.3390/en13133335 www.mdpi.com/journal/energies

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Conclusion