Production of high performance multi-crystalline silicon ingots for PV application by using contamination-free SixNy seed particles
Introduction
The silicon wafer production for photovoltaics is still dominated by multi-crystalline silicon which is produced by the directional solidification technique [1]. This is mainly caused by the emergence of the so called “high performance mc-silicon” or “HPM silicon” since 2011 [2], [3], which makes the multi material still attractive besides the high quality mono-crystalline material produced by the Czochralski (CZ) technique. Meanwhile the production method of the HPM silicon has been improved by replacing the common seeding approach, namely the use of a thick, 10–30 mm high silicon feedstock layer, by thin “functional layers” located directly at the crucible bottom. The benefits of this so called “HPM 2.0” approach are the easier process control at large scales in industrial production (full melting process) and the increased wafer yield due to the homogeneous seeding over the complete bottom area as well as the avoidance of a non-usable, porous silicon seeding layer up to 20 mm thickness.
For the foreign seed assisted growth of industrial HPM silicon ingots typically SiO2 based seeds in terms of slurries or pure particles are used [4], [5]. They are placed in the bottom region of the crucible above or underneath the Si3N4 release coating. The silicon ingots produced hereby show a “HPM-like” microstructure with small grain size and high random grain boundary fractions R at the bottom of the ingot in most cases. However, this seed type material has the disadvantage of a limited nucleation area due to the sintering behavior of SiO2. This is reducing the effective nucleation area when a certain particle density limit has been exceeded and therefore the particles are starting to bake together [4]. Another disadvantage is a higher risk of oxygen incorporation, shown e.g. by Zhang et al. [5] for industrial G5 scale or Hess et al. [6] for laboratory G1 dimensions. They observed a higher oxygen content (up to + 50%), especially in the lower parts of the ingots and a subsequent detrimental LID effect in the produced solar cells.
To overcome these problems, a new type of SixNy seed particles is proposed and evaluated for the industrial production of HPM 2.0 silicon ingots in the present work. For that purpose several G1 crystallization experiments (full melt approach) were carried out. The SixNy particle size, the introduced particle mass as well as the composition of the underlying Si3N4 releasing coating is varied systematically in this work. It will be shown that quite similar grain structure properties and cell efficiencies like in classical HPM silicon material can be achieved by the use and simple application of these particles, but without the disadvantage of additional (oxygen) contamination of the ingot and the loss of a feedstock layer.
Section snippets
Experimental setup and characterization
All crystallization experiments were carried out in a G1 crystallization furnace [7] allowing the growth of silicon ingots with dimensions of 220 × 220 × 130 mm3 and a weight of 15 kg. In all cases, polysilicon feedstock from Wacker was filled into standard fused silica crucibles coated with a high purity Si3N4 layer on the inner sides using Silzot® SQ Si3N4-powder from AlzChem were used. The Si3N4 coating is an inevitable component of the used crucible setups and is applied on the inner
Influence of the seed particle size and mass on the grain structure – Experiment series I and II
In the first experiment series I (experiment 1A – 1E, compare Table 1) the influence of the SixNy seed particle size on the random grain boundary fraction R has been investigated. The measured d50 values for the SixNy particles vary between 0.23 and 1.4 mm. Moreover for the silicon seeded HPM reference, the grain size values of the internal grain structure of the silicon chunks are used (0.07–0.27 mm [10]) for comparison hence it is known from literature that this is the most important
Conclusion
Within this work a new type of contamination-free SixNy seed particles for generating HPM silicon ingots was evaluated concerning its industrial application. By a systematic variation of the seed particle size, introduced seed particle mass or seed particle density at the crucible bottom it was shown, that only a small amount of seed particles providing a seed density of 120/cm2 is sufficient to generate HPM-like grain structures, meaning random grain boundary fractions R > 60%. Additionally,
Declaration of Competing Interest
The authors declare no conflicts of interest.
References (16)
- et al.
J. Crys. Growth
(2012) - et al.
J. Crys. Growth
(2016) - et al.
J. Crys. Growth
(2016) - et al.
J. Crys. Growth
(2010) - et al.
Acta Mater.
(2014) - et al.
J. Crys. Growth
(2016) - et al.
J. Crys. Growth
(2016) - et al.
J. Crys. Growth
(2017)
Cited by (7)
Impact of silicon melt infiltration on the quality of cast crystalline silicon
2021, Solar EnergyCitation Excerpt :This growth method can obtain uniform fine grains and a high proportion of random grain boundaries (GBs), which can significantly release thermal stress during the growth, thereby inhibiting the generation and propagation of dislocations (Lan et al., 2016; Stokkan et al., 2014). Afterwards, a series of improved methods were developed to shorten the length of red zone at the ingot bottom, such as the nucleation layer based on SiO2, Si3N4 or Si powder (Ding et al., 2016; Kupka et al., 2017; Schwanke et al., 2019; Lei et al., 2020). In recent years, cast mono-Si has also become an important development trend of casting crystalline silicon due to its low-cost and higher cell efficiency (Liu et al., 2020).
Growth of high-quality multi-crystalline silicon ingot by using Si particles embedded in the Si<inf>3</inf>N<inf>4</inf> layer
2020, Journal of Crystal GrowthCitation Excerpt :But the fraction of random grain boundaries of grains nucleated from SiC layers is obviously lower than that of SiO2 layers. S. Schwanke et al. [13] have studied the mc-Si ingots grown with different Si3N4 nucleation layers. It confirmed that the ingot with random grain boundary ratio over 60% could be obtained by using Si3N4 particles which have an appropriate size and distribution density.
Production of high performance multi-crystalline silicon ingot by using composite nucleant
2020, Journal of Crystal GrowthCitation Excerpt :The unmelted seeds will increase the red zone length at the ingot bottom and significantly reduce the ingot yield [11]. Then, some researchers used non-silicon materials that melting points higher than silicon as nucleation layers for nucleation [12–14], such as SiO2, SiC and Si3N4. The feedstocks in the crucible can be completely melted without worrying that the seeds at the bottom of crucible will be melted.