Materials

High-efficiency silicon solar cells require silicon wafers of high electrical quality as the base material. One advantage of n-type compared with p-type doped silicon is the smaller impact of many metal impurities on the electrical material quality. This applies especially to n-type multicrystalline silicon ingots produced by the directional solidification process, with dissolved metal impurities typically introduced by the crucible system.

GCL-Poly Energy Holding has placed a bid of US$150 million for the polysilicon assets of bankrupt renewable energy firm SunEdison via the US bankruptcy court dealing with the Chapter 11 proceedings.

Most high-efficiency solar cells are fabricated from monocrystalline Czochralski (Cz) silicon (Si) wafers because of the high quality of the material. Despite the considerable heritage from microelectronics, the Cz-Si substrate quality can still limit cell performance.

With the PV industry continually pushing for ever-higher silicon solar cell efficiencies, the requirements on the electronic quality of the bulk material are becoming more stringent. Advanced characterization of silicon ingots after cutting into bricks allows early quality control and immediate feedback in crystal growth, thereby facilitating shorter R&D cycles, higher yield, lower cost and higher product quality in mass manufacturing.

Because the wire itself is the dominant cost in diamond wire sawing, economics dictate that the wire life must be prolonged. This paper presents recent progress made in real-time non-contact monitoring of diamond wire using the resonant vibration (RV) characteristics of the wire. Additionally, a theoretical framework is presented which shows that the characteristics of the resonance curve do not change at speeds above 500m/s. As a result, this technology is expected to be able to meet the increasing demands of monitoring diamond wire wear during sawing as the wire speed continues to increase in the coming years.

During the severe plummet of PV prices that took place during 2008–2012 as a result of overcapacity, the polysilicon sector suffered a major adjustment of costs and capacity to face the reduction in prices and the mismatch between demand and supply. In 2012 that significant drop in prices provoked the bankruptcy of many polysilicon producers, with only the large and efficient players still surviving. However, there was also an impact on the (at that time) promising and immature industry of metallurgical purification of metal silicon, also known as upgraded metallurgical-grade silicon (UMG-Si). The strong selling point of UMG-Si producers – the production costs – was no longer an asset, leaving UMG-Si with nothing but its weakness – the quality. The generation costs for solar energy are currently comparable to those for conventional fuels. The solar industry is self-sustaining and is not dependent on government subsidies. In this current situation, the industry requires an updated comparison between the two main routes of silicon purification and their products, which is the aim of this paper.

With the transition of the cell structure from aluminium back-surface field (Al-BSF) to passivated emitter and rear cell (PERC), the efficiency of multicrystalline silicon solar cells becomes more and more sensitive to variations in electrical material quality. Moreover, the variety of multicrystalline materials has increased with the introduction of high-performance multicrystalline silicon. For these reasons, a reliable and verifiable assessment of the electrical material quality of multicrystalline wafers gains importance: to this end, a rating procedure based on photoluminescence imaging has been developed.

The mechanical strength of monocrystalline and multicrystalline silicon wafers is mainly dictated by the cracks induced during the wire-sawing process. Different sawing technologies, such as diamond-wire- or slurry-based processes, lead to different strength behaviours of as-cut wafers. Furthermore, the strength is strongly influenced by texturization, and at this stage can be interpreted as the basic strength of a solar cell. The metallization and firing processes determine the final strength and reliability of a solar cell, with the metallization contacts being the root cause of breakage of solar cells, depending on the particular cell concept. This paper gives a comprehensive overview of the typical ranges of strength for as-cut wafers, textured wafers and solar cells, for the two different sawing technologies. Around 100 batches with 4,253 samples were evaluated in the study.

Extended crystal defects, such as grain boundaries and dislocations, have long been considered the main factors limiting the performance of multicrystalline (mc-Si) silicon solar cells. However, because the detrimental effects of these crystal defects are reduced as a result of improvements in the solidification process as well as in the feedstock and crucible quality, the degradation caused by boron–oxygen complexes is expected to be of increasing importance. Light-induced degradation (LID) occurs in both p- and n-type crystalline silicon solar cells that contain both boron and oxygen. Because of the fundamental differences in the solidification processes, mc-Si silicon contains less oxygen than Czochralski silicon; nevertheless, the oxygen content in mc-Si silicon is still sufficient to cause degradation, although to a lesser extent than in the case of Czochralski silicon. Whereas B–O-related degradation of 0.5 to 1% abs. can be found in Czochralski cells, the degradation in conventional mc-Si cells is limited to around 0.1 to 0.2% abs.

A shift from free-abrasive/steel wire sawing to fixed-abrasive diamond wire sawing is expected to take place in the PV cell manufacturing industry, with 2018 being the anticipated pivotal point for market dominance. This shift is due to several key advantages of diamond wire sawing, such as higher throughput, less wire per wafer, no slurry and the possibility of kerf recycling. However, in order for diamond wire sawing to realize its promise as the next-generation workhorse for the slicing of silicon PV wafers, inherent fundamental challenges must be properly identified and successfully addressed by the PV industry. As a first step to increasing the current collective understanding of the critical needs/challenges of diamond wire sawing, the c-Si programme of the U.S. PVMC held a workshop on July 8th, 2014 in San Francisco, California. One of the key products of this workshop was an extensive list of short- and long-term challenges. This article expands on some of the most important challenges identified at the workshop through the collective discussions and dialogue among a variety of PV industry experts and stakeholders.