The problem company is solving:
According to Shockley and Queisser, the maximum efficiency of a single-junction solar cell for the conversion of unconcentrated solar radiation is 31%.
- In multi-junction solar cells with five or more junctions the ultimate efficiency may be greater than 70%.
- However, current technology enables to produce only triple-junction cells with the maximum conversion efficiency about 40% for concentrator cells.
- Technological limitations are determined by the need to match crystalline lattices, thermal expansion coefficients, and, the most difficult, by the need to match all photoinduced currents in the cascade of heterojunctions.
The suggested solution of this problem:
Quantum dot (QD) structures have the potential to overcome Shockley-Queisser limit for single- junction solar cells due to their ability to enhance light absorption via multiple energy levels introduced by quantum dots and extend the absorption edge into the infrared (IR) range.
- Up to now the most efforts were concentrated on the quantum-dot solar cells with the intermediate band, which is formed from discrete QD levels due to strong tunneling between QDs.
- Theoretical calculations predict that the intermediate band solar cell can provide a maximum efficiency about 65%.
- However, intensive experimental efforts to improve intermediate band solar cells show very limited success: an increase in photovoltaic efficiency has not exceeded a few percent.
- The limitations of this solar cell design are well understood: together with harvesting of IR radiation QDs enhance the inverse photoelectron processes, i.e. relaxation and recombination processes. In the best case scenario the QD-enhanced IR harvesting just compensates the QD-related recombination and relaxation losses.
Recently we have proposed a novel approach to design QD solar cells, which allows us to enhance solar radiation harvesting and to suppress capture of photoelectrons into QDs, and, in this way, to minimize the recombination losses.
- Even the first-pass qualitative optimization of the solar cell design has allowed for 50% increase of power efficiency in InAs/GaAs solar cells due to quantum dots with built-in charge (Q-BIC).
- Using this approach we plan to develop scientific, engineering, and technological basis for Q-BIC solar cells with potential conversion efficiency of ~50%.
- We plan to optimize photoelectron kinetics by engineering potential barriers around QDs and minimizing the photoelectron capture from the wetting layer in Q-BIC devices.
- We are confident that such optimization will allow us to enhance IR conversion and to double the conversion efficiency of Q-BIC test devices with respect to identical reference cells (solar cell devices without dots and with uncharged dots show practically the same efficiency).
- Q-BIC solar cells have strong potential to overcome limitations of modern photovoltaic technologies.
- Well-developed fabrication technology of QD structures combined with deep understanding of basic electron processes and numerous possibilities for incorporating various three-dimensional potential barriers strongly encourage investigations aimed at advanced photovoltaic applications of these structures.