Shedding light on new frontiers of solar cell semiconductors

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IBM sets world record for photovoltaic energy conversion efficiency with earth-abundant materials
Editor’s note: This article is by Teodor Todorov and David Mitzi, IBM Research photovoltaic scientists.
by David Mitzi, Teodor K. Todorov, Jiang Tang, Santanu Bag, Oki Gunawan, Tayfun Gokmen, Yu Zhu, David B. Mitzi
Energy from the sun reaching the earth’s surface amounts to several thousand times our global consumption of electricity. Yet electricity from photovoltaic (PV) solar cells currently contributes significantly less than one percent of worldwide production. Of the numerous existing PV technologies, none so far have combined the virtues of being highly efficient, cheaply scalable and made with abundantly available materials.
IBM’s Materials Science team has partnered with Solar Frontier, Tokyo Ohka Kogyo (TOK) and DelSolar to develop an efficient and affordable PV cell made of abundant natural materials. So far, the tests of our Cu2ZnSn(S,Se)4 (made of readily available copper, zinc, and tin, and referred to as CZTS) thin-film devices have achieved a world-record PV solar-to-electric power conversion efficiency of 11.1 percent (10 percent better than any previous reports) for this class of semiconductors. And it can be manufactured by simple ink-based techniques such as printing or casting.
What makes CZTS better
Currently, the most widespread PV semiconductors, made of crystalline silicon, are abundant and highly efficient. They’re in panels used for everything from home electricity to the International Space Station. However, they have extremely high material purity requirements (>99.9999 percent!), and the wafers are typically cut from large solid ingots and wired in series to form PV modules – making it expensive and difficult to upscale. 

Photos of IBM’s CZTS Solar Cell Device.

Other thin-film chalcogenide materials used in PV cells, such as Cu(In,Ga)(SSe)2 (CIGS) and CdTe, have been developed to a performance level close to that of silicon, with inherently more scalable processing. They are directly deposited on large-area, low-cost substrates such as glass, metal or plastic foil. While CIGS and CdTe are easy to integrate into buildings and consumer products, their compounds contain rare and expensive elements that increase cost and limit their manufacturing levels to less than 100 Gigawatts per year (worldwide continuous electricity consumption is 15 Terawatts – 150 times greater than the level of what these CIGS can produce).
Our CZTS PV cells could potentially yield up to 500 GW/year – getting closer to the Terawatt levels of renewable electricity the planet needs. 
The focus of our joint-development team remains to further increase this device efficiency and transfer the technology to environmentally-friendly, high-throughput industrial manufacturing. The hope is that within several years this new class of photovoltaic materials will begin to contribute to the wider availability of lower-cost solar electricity. 
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