A cheap method for producing solar cells from nanowires

Ways must be found to manufacture solar cells from cheap components through cheap chemical processes and low energy consumption, and they must be able to convert sunlight into electricity in an efficient and competitive manner

Solar cells (or photovoltaics) represent one of the most likely technologies for providing a completely clean and practically inexhaustible source of energy that can satisfy humanity's subsistence needs. However, in order for this dream to become a reality, ways must be found for the production of solar cells from cheap components through cheap and low-energy chemical processes, and they must be able to convert sunlight into electricity in an efficient and competitive manner.

A research team from the Lawrence Berkeley National Laboratory, which belongs to the US Department of Energy, has now succeeded in achieving two of these three requirements and is working feverishly on the third requirement.

Researcher Peidong Yang, a chemist from the Department of Materials Science at the laboratory, led the research that resulted in the development of a solution-based method for the production of solar cells based on the structure of a core/shell nanowire solar cells using cadmium sulfide semiconductors. for the core) and copper sulfide (for the shell). These nanowire-based solar cells are cheap and simple to prepare, and have superior properties over conventional flat-structure solar cells. In addition, the new nanowires exhibited an energy conversion efficiency of 5-4 percent, which is a competitive value for flat solar cells.

"This is the first time ever that a solution-based cation exchange chemical method has been used to fabricate core/shell nanowires composed of high-quality single crystals of cadmium sulfide and copper sulfide," notes the principal investigator. "Our achievement, combined with the increased light absorption we previously demonstrated for nanowire arrays, indicates that core/shell nanowires are indeed a promising component in the development of future solar cell technology." The research findings were published in the scientific journal Nature Nanotechnology.

Normal solar cells that exist on the market today consist of surfaces of extremely pure zoran single crystals that require the use of a thickness of 100 micrometers of this very expensive material in order to absorb a sufficient amount of sunlight. Moreover, the high level of purification required makes the production of even the simplest flat solar cell a complex, energy-intensive and expensive process.

A particularly promising alternative would be semiconductor nanowires - one-dimensional strips of materials that are one-thousandth of the thickness of a single human hair, but their length may even reach a millimeter. Solar cells based on nanowires provide a number of advantages over conventional flat solar cells, including charge separation and better energy collection capabilities, and in addition, they can be made from common materials rather than alloys that require complex and expensive processing. However, to date, the low efficiency of nanowire-based solar cells has overwhelmed their advantages.

"Previously developed nanowire-based solar cells had an efficiency that was lower than their flat counterparts," notes the lead researcher. "Possible reasons for this poor performance include surface reconnection and limited control of pn junction quality when high-temperature plating processes are used."

The basis of all solar cells on the market today is two separate layers of materials, one with an abundance of electrons that functions as a negative pole, and one with an abundance of electron holes (positively charged energy spaces) that functions as the positive pole. When photons are absorbed from the sun, their energy is used to create electron-hole pairs, which are separated in the next step at the pn junction (the interface between the two layers) and collected as electricity.

About a year ago, while working with Zorn, the researchers developed an inexpensive way to convert the pn junctions found in regular solar cells into a circular pn junction where a layer of n-type Zorn forms a shell around a p-type Zorn nanowire core. This geometry effectively turned each of the individual nanowires into a photovoltaic cell and greatly improved the light-absorbing capabilities of zorn-based photovoltaic thin films. Now, the researchers have taken this approach to create core/shell nanowires using cadmium sulfide and copper sulfide, but this time using solution chemistry, which does not require high temperature conditions.
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