By combining two solar cells one on top of the other, referred to as the top solar cell and the bottom solar cell, you create a tandem or two-junction solar cell structure with maximum theoretical efficiency 40%. To achieve the highest efficiency, the two solar cells must have specific band-gap energies, a property of the semiconductor material of each cell. The maximum efficiency can be achieved with a silicon solar cell as the bottom cell, with fixed band-gap energy 1.12 eV, and a CdZnTe top solar cell with zinc content tuned to produce a wide band-gap material with band-gap energy 1.74 eV.
The top cell absorbs the blue light from the sun at a higher operational voltage because the voltage is directly related to the band-gap energy; the larger the band-gap energy the larger the achievable voltage. The bottom cell absorbs the red light from the sun at a lower operational voltage. The total power generated from the two pieces is greater than the total power that can be generated from each one alone.
Over the last decade, the solar industry has perfected the TCO layer of the four components discussed above. Uriel Solar has spent several years working to perfect the other three components for a high performance, wide band-gap energy top solar cell:
The first problem Uriel Solar has solved is the creation of a suitable transparent back contact for the top cell so that the red light could transmit unimpeded into the silicon cell underneath, while providing good electrical contact to the absorber layer.
The second problem Uriel Solar has solved is the development of a high performance p-type CdZnTe absorber layer. That material needs to absorb the blue light and efficiently transport the electric current generated by the blue light to the front and back contacts. It must also pass the red light unhindered.
Uriel Solar is now completing the third and final component, the n-type emitter. Candidate materials are identified and the work is underway.
Uriel Solar expects to demonstrate a high efficiency tandem solar cell by end of 2016/early 2017. This will be followed by transfer of the technology to a production deposition platform, already in early design.