Developers push c-Si efficiency toward 20% with help from narrower interconnect

Photovoltaic cells are getting steadily more efficient, and even the small area taken up by interconnect is shrinking to get more electrons flowing. Some of these developments were discussed at the recent Photovoltaic Specialists Conference in Philadelphia and at the accompanying PV America exhibition.

by Bob Haavind, editor-at-large, Photovoltaic World

Photovoltaic cells are getting steadily more efficient, and even the small area taken up by interconnect is shrinking to get more electrons flowing. Some of these developments were discussed at the recent Photovoltaic Specialists Conference in Philadelphia and at the accompanying PV America exhibition.

An average efficiency of 19.1% for p-type crystalline silicon (c-Si) is achieved in SunTech’s new Pluto cells, according to Martin Green, executive research director at the U. of New South Wales’ (Australia) ARC Photovoltaics Centre of Excellence, and he claimed Pluto 2 coming later this year should boost this further to 20%-21%. One key to improved efficiency, he explained, is the use of 10µm interconnect lines compared to 100µm in present commercial cells. These narrower traces are achieved without the use of costly optical lithography, he stated. Although details were not presented, he said that screen printing, the basic process used for commercial PV cells, is employed.

Even incremental improvements in efficiency are of great interest for solar panels, according to Andy London, global business manager for Heraeus’s photovoltaic business unit, West Conshohocken, PA, which was set up at the beginning of this year. Since solar panels will operate for 25 years or more, even improvements in conversion efficiency of 0.2% or 0.3% will get an enthusiastic response from users, he said.  Currently, typical efficiencies are commercial c-Si cells are 16% for polysilicon and 17% for monosilicon, according to London, with 19%-20% in the labs.

Almost all of the silver paste used for interconnect on present commercial cells comes from DuPont, with a uniform formulation. Heraeus has developed an alternative approach in which each silver paste is customized for individual wafers. London explained that substrates from each vendor differ because of variations such as in phosphorous doping or nitride coatings. Heraeus gets 100 wafers from a vendor and then optimizes the additives in its silver paste to get improved performance, such as lower resistivity and better adhesion, which can add to conversion efficiency. The silver paste is screen-printed onto the surface of a wafer, front and back, and heated to 800-850°C for two seconds to melt through the SiNx coating and make ohmic contact to the phosphorous-doped silicon below. Traces are typically 100µm wide, with 80µm coming soon and 50µm in R&D, according to London. The total market for silver paste is about $200M per year, with a CAGR of 40% a year, he said; the Heraeus share is only in the single-digit range, but he noted that the company has a target of 20% by 2011.

Meanwhile, the industry awaits the coming introduction of the higher-efficiency Pluto 2 cells later this year. SunTech, headquartered in the New District of Wuxi, China, was founded by Shi Zhengrong, one of the many graduates of Dr. Greene’s photovoltaics and solar energy program at the U. of New South Wales. Since there are few jobs locally, many graduates have gone on to become entrepreneurs or executives in the solar industry around the world, he explained.

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