In recent years, PV technologies have been developing rapidly. With respect to cells, the high-efficiency PERC, bifacial cell and black silicon, technologies have moved to mass production gradually, while N-type and heterojunction technologies have obtained footholds in the market. With respect to module, the double-glass, half-cell, multi-busbar and shingled-cell technologies have evolved to large-scale production. With respect to monocrystalline silicon wafer, many technological breakthroughs have been made,notably the size of the wafer itself.
Before 2010, monocrystalline silicon wafers were small-sized with 125mm width (f164mm silicon ingot diameter) and a few 156mm (f200mm) wafers. After 2010, 156mm wafers have occupied an increasingly bigger share and become the mainstream. 125mm P-type wafers were almost eliminated around 2014, with only some IBC or HIT cells. At the end of 2013, LONGi, Zhonghuan, Jinglong, Solargiga and Comtec jointly issued the standards for M1 (156.75-f205mm) and M2 (156.75-f210mm) wafers. Without changing the size of the module, M2 could increase the module power by more than 5Wp, rapidly becoming the mainstream and maintaining the status for several years. During that period, there were also a few M4 (161.7-f211mm) wafers on the market, the area of which was 5.7% larger than M2, and such wafers were mainly applied to N-type bifacial modules.
In the second half of 2018, due to intensified market competition, many enterprises turned their attention to silicon wafers again, hoping to increase the power of modules by expanding the size of silicon wafers to secure product competitiveness. One methodology is to copy the release of M2, continue to increase the width across the wafer, to 157mm, 157.25mm or 157.4mm for instance, without increasing the size of the module, but the increase in power obtained is limited, the requirement on production accuracy is increased, and the certification compatibility may be affected (e.g. failing to meet the creepage distance requirement of UL). Another methodology is to follow the route of increasing the width across the wafer from 125mm to 156mm, and increase the size of the module, such as 158.75mm pseudo-square wafer or square wafer (f223mm).The latter increases the wafer area by about 3%, which increases the power of a 60-cell module by nearly 10Wp; meanwhile, some N-type module manufacturers choose 161.7mm M4 wafers; some enterprises plan to launch 166mm wafers.
Now let’s take a look at why the wafer size is getting bigger and bigger.
1. From the perspective of production, the production rates of cells and modules (wafers/hour, modules/hour) are basically fixed, and the increase in the size of wafer can enhance the power of cells or modules produced per unit time, which can reduce the equipment, manpower and even other costs per Wp of the company, thereby reducing the manufacturing costs of cells and modules, especially when 125mm wafers are switched to 156mm wafers.
2. From the perspective of the cost of power station system, taking terrestrial power station as an example, under the same efficiency, the module obtains higher power due to bigger wafer size, while the number of modules in a string remains unchanged, as a result, the module efficiency on a single bracket increases accordingly, and the costs of bracket and pile foundation per Wp is reduced; when large modules have little effect on the transportation and installation speed, the installation efficiency of modules and brackets per Wp will be enhanced; as the capacity per array is determined by the inverter and can be deemed fixed, high-power modules can reduce the use of combiner boxes or string inverters, and the reduction in the use of brackets can reduce the footprint of the array (considering the front and back spacing and the left and right spacing of the brackets), and the reduction in the number of brackets and their footprint can reduce the use of power cables. It’s estimated that a 425Wp module using 166mm wafers can save the BOS cost by at least RMB0.05/Wp compared to the 380Wp module using M2 wafers (both of 72-cell type). If a tracker is used or in an overseas area where the labor cost is high, more BOS costs will be saved.
The above two points show that when the equipment production and transportation are not a problem, the wafer size should be as large as possible to save more cell and module costs and system BOS costs. For this reason, cadmium telluride thin film cell manufacturer First Solar directly increased the module size from the fourth-generation 1200*600mm to 2009*1232mm. The module area (near 2.5m2) and the weight (35kg) should be the limit values obtained after comprehensive analysis. For crystalline silicon modules, it’s necessary to take the opportunity of this industry change to adjust the size to a more stable and cost-effective one, just like the adjustment from 125mm to 156mm. According to a WeChat article titled “Monocrystalline is easier to realize large wafer size”, the main factor restraining wafers from becoming bigger is the diffusion furnace. To make the wafers bigger in a diffusion furnace with limited diameter, the pseudo-square monocrystalline silicon wafer should have certain advantages over the square monocrystalline silicon wafer.
In conclusion, big wafers can bring obvious value to the photovoltaic industry. Major enterprises should take this opportunity to determine a size that can be relatively stable for many yearsto reduce repeated investment in production line transformation and module certification expenses. The 166mm monocrystalline silicon wafer, as the maximum size compatible with all production lines, seems to be a good choice at the current stage.