Quasi-Mono Standards: Paving the Way to Acceptance
Reprinted with permission from pv magazine
Quasi-mono standards: The SEMI standards for quasi-monocrystalline silicon wafers can create a clear pathway for market acceptance of the technology and facilitate growth. SEMI PV Group’s Stephan Raithel and LDK Solar’s Yuepeng Wan and Linyan Liu explore the potential of the technology and why standards could be so valuable.
When perusing the different show floors at any major PV trade show, it becomes quickly apparent that nearly every leading PV manufacturer has quasi-mono (also known as mono-like) products on the market. The overall goal is simple: to be able to reach the same efficiency as monosilicon (mono-Si) cells at the cost level of a multisilicon cell. In theory, of course, that sounds interesting. In reality however it is not that easy. Existing equipment can be used, but only after some modifications. Another challenge is to retain the required quality and purity during the crystallization process. The efficiency of the mono-like cells is definitely higher than multisilicon products – approximately between 0.5 and 1%.
Photo: Gary Meek
So far the industry has not joined forces to work on pre-competitive issues and therein cut down the time to market. At the moment the answer to the question “Hype or future commodity?” needs to be answered with a pragmatic, “maybe both.” At its current status, mono-like technology will not replace mono-Si, however it could be a new technology with a certain market share (the latest International Technology Roadmap for Photovoltaics predicts a market share of 40 percent by 2020, see www.itrpv.net). At the end, the costs per watt peak will be the deciding factor. SEMI’s China Standards Committee has created a task force to determine specifications for mono-like wafers. All leading manufacturers are involved and therefore a promising result in a short time frame is expected.
In today’s PV market, the majority of PV modules are made with crystalline silicon materials. There are generally two types of crystalline silicon wafers, monocrystalline (also called single crystal) silicon wafers which are generally produced with the Czochralski (CZ) method and multicrystalline silicon wafers, which are commonly made with directional solidification methods (also called the casting method) with many variations of crystal grow systems. Multicrystalline silicon wafers have gained the highest market share (more than 40 percent) due to their low cost of crystallization. The CZ monocrystalline silicon wafer is still holding its market share, especially in rooftop applications, due to its higher cell conversion efficiency. The difference in the cell conversion efficiency between the mono and multisilicon modules is typically about 1.5%. Such a difference is a result of the surface property (about 0.5%) and the bulk crystal quality.
The directional solidification method (or casting method) can also be used for growing single crystals. Examples can be found in the crystal growth of many different materials, such as sapphire. In fact, GT Solar’s directional solidification system (DSS) furnace, which is widely used for multicrystalline silicon wafer production, originated from the heat exchange method (HEM) furnace for single crystals of sapphire. In the early stage of development of the HEM DSS furnace for silicon crystals, the focus was initially on the single crystal of silicon, and of course with a single crystal seed at the bottom of the melt. The results exposed that some parts of the crystal weren’t single crystal (multicrystalline grains), and the cell conversion efficiency of the multi-section was close to that of the single section. The HEM furnace was eventually employed for producing multicrystalline silicon ingots, without putting seeds at the bottom of the crucible, simply and cost effectively, and it was further developed into the DSS furnace for multicrystalline silicon ingots.
The growth of single crystal silicon with the directional solidification (or casting) method and the application of the resulting mono (or quasi-mono) silicon wafers for solar cells was also carried out by numerous researchers in the past 20 years. C.P. Khattak first studied the reduction of grain boundaries in DSS by large grains growth. It was regarded as the most straightforward approach to increase crystal quality and consequently solar cell efficiency back in 1987. Then, in 2000, the control of undercooling for preferred grain boundaries was studied extensively in Japan on a laboratory scale. The first large-scale (pilot) application of quasi-mono wafers could be considered that which was implemented by BP Solar. As early as 2006, BP Solar branded its quasi-mono wafers and solar cells as “Mono2 TM.” However, the quasi-mono wafers didn’t receive much attention until 2011. As the market shifted increasingly towards higher efficiency solar modules, and solar cell technology advanced to a much higher level, the requirements for (click here to view the rest of the article from pv magazine.)