Results and Discussion

1. MJQDSCs: High Efficiency, Low Cost Conclusions Results and Discussion • Multijunction solar cells, with multilayer structures and each layer fine-tuned to absorb and convert specific energy bands of sunlight, have been demonstrated to bypass the Shockley-Queisser Limit. • It is generally accepted that multijunction solar cells are the key to improving efficiency. However, constructing effective multijunction solar cells integrating many different materials can be prohibitively expensive. • An idea has been proposed that the same materials, with the help of varying sizes of quantum dots, can be used for different stacks. This is the idea of multijunction quantum dot solar cells (MJQDSCs). • Following is a schematic showing a MJQDSC. Photons of various energies (depicted as red, green, and blue rays) are absorbed by a particular layer of specifically-sized quantum dots, with their energy converted to electricity, depicted as electrons (e-). This ensures improved efficiency in solar cells. • Maximum 50.0%, 57.5%, 66.1% and 75.0%-efficiency 2,3, 5, and 9-junction lead sulfide (PbS) quantum dot solar cells, respectively, were designed using Monte Carlo simulation, up to 2.23 times conventional solar cell maximum efficiency of 33.7% and about 4 times efficiency of current commercial solar cells. • By combining quantum mechanical predictions and Monte Carlo simulation, the first ever novel model to design and optimize multijunction quantum dot solar cells was developed and tested to quickly design and optimize multijunction quantum dot solar cells, cutting design and testing time from months or even years to merely days or hours of computation. • This model has an open architecture capable of utilizing absorption properties either obtained theoretically or experimentally, enabling rapid calibration of data and refinement of predictive abilities in the future. Using a grid of 0.5 nm for PbS quantum dot diameter and Monte Carlo modeling as discussed above, various MJQDSCs were designed and evaluated. Their efficiencies were calculated and compared. Following is the progression of energy spectra for best designs of 2, 3, 5 and 9-layer MJQDSCs. With a fixed total thickness, the spectral change at each quantum dot layer were tracked and plotted. The highest-efficiency PbS MJQDSC designs identified with this model are listed below. Within these MJQDSC designs, a detailed breakdown of how each layer contributes was obtained through this model. As expected, since the total thickness is held constant for comparison, increasing the number of quantum dot layers leads to higher efficiency. Nearly 80% intrinsic efficiency is achievable with 9 QD stacks. Of course, the incremental improvement decreases as the number increases; diminishing return is observed. 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Researchers are in the exploration stage, focusing on understanding fundamental properties of quantum dots to utilize their properties. • This project attempts to find a new path to achieve breakthroughs. Colloidal PbS quantum dots, the best experimentally-studied low-cost system, are utilized. Absorption properties using quantum mechanical modeling are integrated using Monte Carlo simulation to predict photon and quantum dot interactions, which in turn are used to calculate the intrinsic solar cell efficiency to design MJQDSCs.