Erasing efficiency-limiting structural defects in high performance multicrystalline Si wafers

REC Solar produces raw materials for silicon solar cells using a proprietary and unique production process based on the recycling of fines from the wafer sawing process, and is the only one in the world to take this former waste material back to solar grade silicon, at a competitive production cost. The process is world-leading in terms of energy consumption and carbon footprint. It is based on metallurgical purification techniques, and this means that the product contains traces of the dopant elements boron and phosphorus. The ERASE project started in 2019, and aimed to demonstrate that the solar efficiency gap between multicrystalline and monocrystalline solar cells can be closed by deactivating electrically active defects in multicrystalline silicon by heat treatment and hydrogenation at wafer and cell level, and to develop industrially relevant methods for this. The project developed new methods for (i) detecting minor changes in the electrical activity of structural defects and (ii) semi-automated analysis of larger sample areas to establish necessary statistics. In 2020, a test program was conducted that examined many different product and process dimensions to identify those that have the greatest impact on defect deactivation. The conclusion of the work was that, despite a positive response on grain boundaries and certain types of defects, we found no clear path to a breakthrough that deactivates electrically active defects sufficiently to close the gap in solar efficiency between multicrystalline and monocrystalline solar cells. During the project, two major changes took place in the solar cell market: Multi was completely outcompeted by mono, and gallium has completely replaced boron as a dopant element in p-type cells. A new shift towards n-type cells is expected in the next few years. From 2021, REC only produces raw materials for mono. Based on these trends, we applied for a change of the focus of the ERASE project in 2021, towards studies of defects that are related to the interaction between dopant elements in wafers and hydrogen from the cell process, and which are activated in solar cells under field conditions. These degradation processes are potentially a bigger problem for mono than for multi, but in the industry today they are handled with methods where different combinations of light, electric current and temperature deactivate the defects (regeneration) so that their impact is minimized. Study of these mechanisms is an active field of academic research, and so far it has not been possible to identify the structure of the defect that is responsible, apart from the fact that hydrogen from the cell process is involved. Earlier in the ERASE project, it was shown that multicrystalline solar cells produced with compensated silicon (which contains both p-type and n-type doping elements) degrade differently from cells produced with uncompensated material. This means that the processes for regenerating / deactivating performance-limiting defects must be adapted to the raw material properties. Since REC supplies a compensated material that differs from industry standards, a detailed understanding of how mechanisms and kinetics depend on the degree of compensation is crucial for optimal utilization and market acceptance. Another implication is that the standardized tests used to measure degradation can give misleading results for cells made of compensated material. The experimental work in 2021 has focused on measuring time series of charge carrier lifetime under conditions that accelerate degradation and regeneration. These are time consuming experiments that can last for weeks. The experiments were performed with monocrystalline samples with variation of dopant elements (Ga, B, P), degree of compensation, resistivity, pre-treatment (anti-reflection coating, firing, emitter) and process conditions (temperature, light intensity, time). The complex interactions between multiple factors will require considerably more experimental work before a full understanding of the mechanisms has been achieved. We therefore want to continue the work in a new IPN project ("REFORM"). The most important result achieved so far is that compensated Ga-doped and n-type wafers can be regenerated to the same level as reference wafers. This means that it is possible to use REC's solar cell silicon in current cell technologies without compromising performance, and thus contribute to a further reduction of the climate footprint of solar energy. The result is partially reproduced in full scale in commercial solar cell lines. The project has worked closely with FME Susoltech, which has developed a new and unique method for cryogenic FTIR that can detect hydrogen states in silicon during the regeneration process. This can prove to be an important tool for clarifying the structure of the defect (s) responsible for degradation.

During the first half of the project, we focused on gettering and passivating defects in mc-Si. Lately mono has completely overtaken multi in the market. REC Solar has undergone a major technology change and now produces feedstock for mono, causing the ERASE project to reorient toward studies of performance limiting defects in mono silicon cells, with focus on aspects related to the simultaneous presence of several dopants. The results have several impacts: - Knowledge from the project has improved technical support to REC Solar customers - Production at REC Solar has been optimized for a product that minimizes degradation phenomena - REC feedstock can now be used in mono ingots, with performance comparable to conventional feedstock - Increasing market acceptance of REC Solar feedstock reduces PV industry CO2 emissions - REC Solar's process provides a solution to a sizeable waste problem (cutting fines from wafer slicing)

The current efficiencies of commercial PERC-type solar cells are up to ~23.5% for mono-Si and ~22% for HPMC-Si. A gap of about 1.5 percentage points in absolute efficiency still exists between the two technologies. After a large effort in process development of the ingot crystallization phase , the carrier lifetime within large grains in REC Solar’s HPMC-Si is now comparable to that of mono-Si wafers. Structural defects like grain boundaries,where two regions with different crystal orientation meet, and dislocations, i.e. imperfections in the crystal, are, however, still present and reduce the carrier lifetimes in the HPMC-Si wafers, and therefore also the performance of solar cells made from these wafers. The detrimental recombination activity can, however, be manipulated with high temperature processing and hydrogen injection, e.g. during emitter in-diffusion and contact firing in the solar cell production process. The PERC process has not been optimized with the aim of reducing the recombination activity of structural defects in HPMC-Si. Therefore, we aim to increase the efficiency of solar cells made using high-performance multicrystalline Si by erasing detrimental structural defects in the material using both existing and novel processing steps specifically designed for defect engineering in HPMC-Si wafers .