Efficiency Optimization of Pyramid-Textured Crystalline Solar Cells with Metal-Assisted Chemical Etching
Authors: Su Vin-Cent , Liu Yan-Chun , Kuan Chieh-Hsiung 
Published: February 2020

1. Introduction

Crystalline silicon solar cells are the mainstream technology for the photovoltaic market. Due to the great prospect of next-generation green energy, their module efficiency requires further improvement. Moreover, the continuous cost reduction is another big issue for crystalline silicon solar cells. Recently, the one-step metal-assisted etching (MAE) method for improving the efficiency of crystalline silicon solar cells has been drawn much attention because it increases both of the light ravel distance and the number of scattering events. The MAE method also meets the requirement of industrial-scale fabrication regarding the large-scale and large-area production in a controlled manner. However, the systematic study on the relationship between the photovoltaic performance and the corresponding pyramid-textured surface under various MAE duration have been less reported. In this paper, the efficiency optimization for pyramid-textured crystalline silicon solar cells with MAE is reported. In addition, we have also incorporated the MAE method as a batch process into a standard crystalline silicon solar cell fabrication line so as to verify the accuracy of experimental results.

2. Fabrication Process

The fabrication process of pyramid-textured crystalline silicon solar cells with MAE is illustrated in Fig. 1. At first, the standard clean process was applied on the Czochralski-grown 200-µm-thick P-type crystalline silicon wafers with dimensions of 156mm × 156mm of which resistivity is 1-5 Ω∙cm. After that, the pyramid-textured silicon surface was performed in hot sodium hydroxide (NaOH) solution, followed by the MAE method. In brief, HF and silver nitrate (AgNO3) for 5, 15, 25, 35, 45, and 55 seconds. At the end of the process, the associated wafers were cleaned using nitric acid (HNO3) to remove residual Ag nanoparticles (NPs). Next, the n-type emitter layer and a silicon nitride layer as an anti-reflection (AR) coating was deposited via plasma-enhanced chemical vapor deposition (PECVD). Finally, the front and rear electrodes were printed and co-fired, accompanying with the formation of back-surface field (BSF).

3. Experimental Results and Discussion

The surface morphology of the hierarchical structures as a function of the etching duration without removal of Ag NPs as shown in Fig. 2(a)-(b) with the instrument of FEI Dual-Beam NOVA 600i Focused Ion Beam. As the etching duration increases, the SEM images show that the size and density of Ag NPs keep increasing until the larger Ag NPs start sinking into pyramid-textured surface. The pyramid-textured silicon structures tend to collapse while the etching time exceeds 45 sec. To investigate the effect of various MAE-based etching durations on the performance of pyramid-textured crystalline silicon solar cells, Fig. 3 shows the efficiency and the associated fill factor in relation to various etching duration. As shown in the figure, efficiency and fill factor increase up to a specific etching duration after which it slightly decreases, and then drop significantly with the etching time longer than 45 sec. Fig. 4 demonstrates the corresponding Isc and Voc versus etching duration. As confirmed in the figure, Isc and Voc also shows enhancement with an increase in etching time by up to 35 seconds, and then decrease dramatically. This could be reasoned by the initial destruction of pyramid-textured surface from the longer sinking track of Ag NPs. In addition, the higher series resistance can be another evidence regarding the surface destruction as seen in the inlet of Fig. 4. The series resistance apparently increases after the etching duration of 35 seconds, which discoveries the collapsed pyramid-textured silicon surface as well.

4.
Discussion

From the analysis of efficiency, fill factor, Isc, Voc, and series resistance, the hook-liked trends in relation to the etching duration can be summarized as follows. The density and size of Ag NPs increase at the early increment of etching duration, which leads to stronger light scattering and higher light absorbance. Nevertheless, with continuously increasing the etching duration, the surface area of hierarchical structures increase rapidly due to the longer sinking track of Ag NPs. This results too many broken / dangling Si bonds generated on the pyramid-textured surface, given that the reduced efficiency of devices is acquired. In the respect of suppressing surface destruction, the reduction of AgNO3 concentration experiment is indicated in the Fig. 5. It is expected that the maximum of efficiency and fill factor for the crystalline silicon solar cells occurs at higher etching duration due to the lower AgNO3 concentration. However, the further optimization for device performance during the etching time between 45 and 55 seconds is needed. The proposed results report the successful implementation of one-step MAE method on the standard industrial fabrication process for pyramid-textured crystalline silicon solar cells.

Figure 1