A novel thin-film single-offset superlattice photovoltaic technology with superior performance and low fabrication cost through use of pulsed laser deposition (PLD) manufacturing methods and lower cost materials.
Despite substantial advances in solar energy technology over the past 30 years - and an associated 10-fold reduction in cost - it accounts for less than 2% of the world's current energy use. This low adoption rate is primarily due to the persistent high cost of photovoltaics (PVs) when compared to fossil fuels. To remove the current roadblocks to getting the cost of PVs down, Dr. Judy Wu and her team have invented a novel single-offset superlattice PV (SOS PV) that has superior performance and low-cost.
Thin film solar cells.
Dr. Wu's invention is based on a superlattice structure in which the second photoactive semiconductor has a higher but comparable band-gap to the first one- a single offset superlattice PV. The key concept of the SOS PVs is the modulation generated on valance or conduction band, or both, which leads to higher exciton generation and higher charge mobility facilitated by quantum tunneling through the superlattice. Although several materials are possibilities with this invention, CdTe is an ideal PV photoactive material since its band-gap is ~1.5 eV, near the theoretical power conversion limit, and it is low cost.
The benefits of Dr. Wu's discovery include enhanced photocurrent; a thinner cell, which means lower cost of materials and waste treatment; extended charge carrier lifetime; improved power conversion efficiency; ease and flexibility in fabrication with respect to the variety of materials that can be used; and enabled use of high-performance, lower-cost materials. While 3rd gen PVs work on III-V semiconductors, this invention is applicable to all II-VI semiconductors.
Third generation PVs involve significant waste from photo energy converting to heat. Moreover, there are two major manufacturing challenges with 3rd gen PVs. First, it is very difficult to select two compatible photoactive materials with perfectly matching crystalline lattices to form strain-free or defect-free multilayer structures. Second, chemical contamination across the interface between the two selected photoactive layers is a major challenge.
Dr. Wu's SOS PVs overcome these weaknesses of both current and 3rd gen PVs. Her experiments have observed: 1) enhanced photocurrent as compared to the reference PVs; and 2) 300% extended charge carrier lifetime as compared to the reference PVs.
In order to overcome the difficulties in manufacturing CdTe-based II-VI semiconductor superlattice PVs, the pulsed laser deposition (PLD) method was employed. The unique advantages of PLD include the ease of changing targets and precise control of the thickness of the component layers. This invention is regarded as the first successful use of PLD for fabrication of superlattices with CdTe-based materials.
This invention is not limited to just II-VI semiconductors - it may apply to any semiconductor system. It could also be applied to converting solar energy to electricity, fuel, and other forms of energy.