Fabrication of 20.19% Efficient Single-Crystalline Silicon Solar Cell with Inverted Pyramid Microstructure
This paper reports inverted pyramid microstructure-based single-crystalline silicon (sc-Si) solar cell with a conversion efficiency up to 20.19% in standard size of 156.75 × 156.75 mm2. The inverted pyramid microstructures were fabricated jointly by metal-assisted chemical etching process (MACE) with ultra-low concentration of silver ions and optimized alkaline anisotropic texturing process. And the inverted pyramid sizes were controlled by changing the parameters in both MACE and alkaline anisotropic texturing. Regarding passivation efficiency, the textured sc-Si with normal reflectivity of 9.2% and inverted pyramid size of 1 μm was used to fabricate solar cells. The best batch of solar cells showed a 0.19% higher of conversion efficiency and a 0.22 mA cm−2 improvement in short-circuit current density, and the excellent photoelectric property surpasses that of the same structure solar cell reported before. This technology shows great potential to be an alternative for large-scale production of high efficient sc-Si solar cells in the future.
KeywordsInverted pyramid sc-Si solar cell Metal-assisted chemical etching Alkaline anisotropic texturing
Atomic layer deposition
External quantum efficiency
Finite difference time domain
Internal quantum efficiency
Short-circuit current density
Metal-assisted chemical etching
Plasma-enhanced chemical vapor deposition
Reactive ion etching
Scanning electron microscope
Single-crystalline silicon (sc-Si) solar cell has long dominated the solar cell market owing to its high photoelectric conversion efficiency and comprehensive performance [1, 2, 3, 4, 5]. However, the advantage of comprehensive quality over other crystalline and noncrystalline silicon solar cell has gradually diminished, due to the rapid development of diamond wire sawing technique, advanced passivation technique, and other type solar cells [6, 7, 8, 9, 10, 11, 12, 13]. As reported in practical production, sc-Si solar wafers with upright pyramid structure fabricated in plant production have a mean reflectivity of 10–12%, which almost has reached the limit of one-step alkaline chemical texturing technique . The improvement in photoelectric conversion efficiency gained little from modulation of upright pyramid structure. In order to change this situation, the improvement in conversion efficiency may be probably continued by fabricate new light-trapping structure such as black silicon . The black silicon technique can be used to modify surface with extremely low reflectivity and high light absorption . Due to its ultra-low reflectivity (near 0.3%) in the ultraviolet visible and near infrared region which benefits efficiency improvement, black silicon solar cell has become a very promising direction of conventional sc-Si solar cell . Thus, the conversion efficiency of sc-Si solar cell can be further improved from the perspective of black silicon.
The black silicon technique has immediately become a research hotspot since its discovery in 1995 . There are three dominant techniques based on nanostructure fabrication: femtosecond laser technique, reactive ion etching (RIE), and metal-assisted chemical etching (MACE) [16, 18, 19]. Given the compatibility of current sc-Si solar cell technology and cost, MACE is the optimal solution to replace conventional alkaline texturing technology . The great light-trapping ability of MACE-fabricated black silicon is beneficial to improve photoelectric conversion efficiency of sc-Si solar cells. However, a lower reflectivity of black silicon corresponds to more nanostructures, which would enlarge surface defect area and accelerate indirect recombination of photo-generated carriers, thereby restraining the photoelectric conversion efficiency .
Many pertinent works have been done to solve the problem above. Specifically, the conversion efficiency of sc-Si solar cell can be enhanced by either optimizing the surface structure for light trapping or improving the passivation technique [20, 22]. Savin et al. introduced atomic layer deposition (ALD) into the passivation process and combined it with the interdigitated back contact crystalline silicon solar cells, and the solar cell conversion efficiency reached 22.1% . Despite the improvement of conversion efficiency, however, the application into large-scale industrial production was still limited by despairing costs. RIE-fabricated black silicon could significantly increase light-trapping ability, but the investment in hardware equipments was large which made it hard to be applied in mass production or less competitive against wet chemical texturing technology. The inverted pyramid structure obtained low surface area and great light-absorbing ability [24, 25, 26]. Stapf et al. used mixed solution of hydrogen peroxide (H2O2), hydrofluoric acid (HF), and hydrochloric acid (HCl) to texture sc-Si, and random inverted pyramid structures were accessed, but the light-trapping ability of inverted pyramid structure was still under investigation . The mechanism of MACE (metal = Au, Cu, and Fe) has been explored, and its application in crystalline silicon surface texturation is also studied [28, 29, 30, 31, 32, 33, 34]. However, the concentrations of metal ions in MACE ever reported, applied for crystalline silicon solar cells, were very high, which disobeyed the increasingly harder environmental protection policies and cost too much. Moreover, the texturation fabricated in MACE reported before was mostly explored to generate nanostructures as much as possible for light-absorbing ability rather than practical application. It was rarely reported about black silicon technique with low cost, which obtained potential in plant production. Our team introduced MACE with Ag nanoparticles into sc-Si texturing process at a low cost and optimize the MACE process by using specific etching additive, which reduced the concentration of Ag ion to two orders-of-magnitude lower than ever reported . Furthermore, the required temperature of alkaline anisotropic texturing process was relatively lower than that in industrial production.
In this work, the optimized MACE technique was introduced into post rinse treatment of sc-Si solar cell, which promoted the photoelectric performance. Black silicon solar cells with inverted pyramid structure manufactured in bulk were accessed, which the conversion efficiency was up to 20.19%. Meanwhile, the formation mechanism of inverted pyramid structure was studied. As expected, black silicon solar cell with inverted pyramid microstructure showed a great potential in large-scale industrial production.
Diamond wire sawing (100)-oriented P-type sc-Si wafers (200 ± 20 μm thick, 1–3 Ω cm) with standard solar cell size of 156.75 × 156.75 mm2 were used in this experiment. The wafers were rinsed in an aqueous solution consisting of NaOH (AR) and H2O2 (30 wt.%) to remove surface impurities and then rinsed in ultra-pure water. In the MACE process, firstly, wafers were immersed in an aqueous solution containing HF (0.2 M) and AgNO3 (3 × 10−5 M) at 25 °C. Then, nanoporous silicon structures were fabricated when the silicon wafers coated with Ag nanoparticles were etched in the mixed acid solution of H2O2 (3.13 M) and HF (2.46 M) for 3 min, which contained 0.1% commercial additive (C, Nanjing Natural Mew Material Co. Ltd., China). The wafers with nanoporous structures were rinsed in ammonia water (0.1 M) with H2O2 (0.1 M) for 5 min to remove residual Ag nanoparticles. After being rinsed in ultra-pure water, nanoporous silicon structures were modified in an aqueous solution of NaOH (0.003 M) and 0.4% commercial additive (A, Nanjing Natural Mew Material Co. Ltd., China) at 60 °C. Finally, the industrial process for sc-Si solar cells was to produce inverted pyramid solar cells. The detailed steps were phosphorus element diffusion to form p-n junction emitters, acid etching to remove phospho silicate glass, plasma-enhanced chemical vapor deposition (PECVD) to deposit SiNx antireflection layer, and screen printing to metallize bottom/top electrodes.
The sc-Si surface morphology was observed under cold field emission scanning electron microscope (SEM; Hitachi S-4800, Japan). The sizes of sc-Si surface microstructure were measured on a Zeta 3D metrology system. The optical reflectance index from 300 to 1000 nm was measured by a UV-VIS and NIR spectrophotometer (UV-3101PC, Japan, with an integrating sphere). The SiNx film was measured by film thickness measurement system (Filmetrics, F20-UV, USA). The internal/external quantum efficiency and photovoltaic conversion efficiency of sc-Si solar cells were measured by Enlitech QE-R and PVIV-411V systems, respectively.
Results and Discussion
Figure 1a clearly shows that white sediment was deposited into the sc-Si substrate, which was verified by an energy-dispersive spectrometer (EDS: inset in Fig. 1a) to be Ag nanoparticles. The reduced Ag nanoparticles substituted silicon where the oxidizing reaction happened and deposited on the silicon substrate. Ag nanoparticles of 15 nm in diameter were distributed evenly and densely with the presence of 5 ppm AgNO3 (Fig. 1a). However, with 10 ppm AgNO3 or higher concentration, the diameters of Ag nanoparticles increased unevenly (Fig. 1b, c). The diameter of regional Ag nanoparticles in Fig. 1b increased to 80 nm, and that in Fig. 1c was up to 100 nm. SEM images in Fig. 1d–f show the Ag nanoparticles deposited for 2, 4, and 6 min, respectively, at which was 5 ppm AgNO3 and 25 °C. It illustrates that shape of Ag sediment changed much and became irregular (varied from one dimension to two dimensions) with deposition time being prolonged. Moreover, these stick-shaped Ag nanoparticles (around 130 nm in length) deposited on the sc-Si surface irregularly by time delaying, which destroyed the uniformity of Ag nanoparticle distribution. In summary, we propose the Ag ion concentration at 5 ppm and deposition time for 2 min at room temperature.
Comparison of box resistance in P-N injection and PECVD effect between inverted and upright pyramid structures
Box resistance in P-N injection
Thickness of SiNx film/nm
Inverted pyramid sc-Si
Upright pyramid sc-Si
Comparison of electrical properties of inverted and upright pyramid sc-Si solar cells
Upright pyramid Si
Inverted pyramid Si
In summary, the sc-Si with inverted pyramid microstructure fabricated by modulated alkaline texturing combined with optimized MACE showed great potential in optimizing both optical reflectivity and microstructure size compared with any other texturing technologies. The conversion efficiency of sc-Si solar cells with inverted pyramid structure designed with the size of 1 μm reached 20.19%, and short-circuit current density of solar cell was up to 38.47 mA cm−2. Predictably, cell property will be improved if the optimization of inverted structure or texturation technology continues.
The study was supported by funding of Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the National Natural Science Foundation of China (Project No. 21071081). The computational resources generously provided by the High-Performance Computing Center of Nanjing Tech University are greatly appreciated.
This research was supported by the National Natural Science Foundation of China (Project No. 21071081).
Availability of data and materials
The datasets supporting the conclusions of this article are included within the article (and its Additional file 1).
CYZ gave the idea and designed and performed the experiment, data processing, and manuscript drafting. LZC did the performance characterization of the samples. YJZ gave help in the theoretical modeling simulation. ZSG modified the manuscript writing. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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