The simplest single-junction solar cell is composed of the following four structural components. (This is the basic structure with a maximum theoretical ‘efficiency’ of 30%.)
- A transparent conductive oxide (TCO) that serves as the front electrical contact and transmits the light into the ‘emitter/absorber’ layers.
- An electrically active ‘emitter’ layer that creates the ‘charge pump’ that drives the electric current after the light is converted to a positive/negative charge pair.
- An electrically active absorber layer that captures (absorbs) the sunlight hitting the cell.
- A back contact, usually metallic, to provide electrical connection.
Uriel Solar, by combining two solar cells one on top of the other, has created a tandem or two-junction solar cell structure with maximum theoretical efficiency of 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 of 1.12 eV (electron volts), and a Cadmium Zinc Telluride (CdZnTe) top solar cell with zinc content ‘tuned’ to produce a wide band-gap material with band-gap energy of 1.74 eV.
Our 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’s advantages arise from:
- Higher panel efficiency: Uriel’s CdZnTe Hybrid Tandem technology can realistically produce 27-30% panel efficiency at low cost per peak watt, completely changing the economics of solar PV.
- Controllable activation of the solar cell junction: Current thin-film technology does not have the ability to controllably activate or dope the material p-type and n-type. Uriel is able to controllably activate its alloys both p-type and n-type. This results in a more reliable yield and higher overall performance.
- Transparent back contact: All current thin-film and silicon technologies use a non-transparent back contact. The transparent back contact is a critical component in a tandem solar cell. Uriel has already developed a transparent back contact for our Hybrid Tandem technology.
- Reduction of absorber layer thickness: Uriel can realistically achieve an absorber layer thickness between 2 and 3 microns. A thinner layer lowers production costs by faster deposition throughput and less material consumption.
- Superior production deposition sources: Production machines already exist whose basic architecture allows for rapid market entry of Uriel’s solar panel technology.
Uriel Solar expects to demonstrate a high efficiency single-crystal CdTe solar cell in collaboration with NREL in the near term. This will be followed by the adoption of this technology to a high efficiency Silicon-CdTe tandem solar cell in a pre-production format.
