Monocrystalline silicon remains the number one material of choice for harnessing solar energy due to its natural abundance and superior electronic properties. Thinner solar cells are important for many applications such as powering flexible electronics. While thin silicon is lighter, its photon absorption is low thus limiting how thin the solar cell can be made without compromising efficiency. In order to realize ultra-thin (less than 30 micron) monocrystalline silicon solar cells, a combined photon and electron transport management process needs to be established that will achieve the theoretical efficiency limit of about 33 percent. This EArly-Grant for Exploratory Research (EAGER) award will investigate the science behind a unified design approach in order to maximize collective photon-electron harvesting. The research will implement a unified photon-electron harvesting mechanism to achieve greater than 20 percent energy conversion efficiency in a 25-micron thick monocrystalline silicon wafer, which is about 7 times thinner than present conventional cells. This will result in enormous cost, weight and material savings as well as enabling flexible solar modules. To fabricate these modules, a large area nanoimprinting technique will be employed that is shown to reach molecular level of resolution and reproducibility, which are needed for efficient photon and electron transport inside the thin-film.
For many years, the photovoltaic community has studied carrier life time, surface passivation and recombination mechanisms to improve efficiencies in silicon solar cells. Similarly, the optics community has extensively studied various light trapping mechanisms to maximize photon absorption. Evidently, both electronic and photonic concepts have followed very independent and somewhat mutually exclusive paths. A unified photon-electron harvesting method is needed in order to maximize solar cell efficiency. Light trapping mechanisms, whether based on photonic or plasmonic effects, create local "hot-spots" of high absorption, which significantly modulate the charge carrier generation rate across the wafer. However, all present cell architectures assume uniform generation of charge carriers, which fails to take advantage of such light trapping and hence, even with high photon absorption, the cell electrical efficiency remains low. These are some of the reasons that to date there has been no demonstration of high efficiency in thin-film solar cells. This inter-disciplinary research work will employ a nanoimprinted large area light trapping system in conjunction with multi-functional composite passivation and anti-reflection layer, a graded doping based efficient charge separation and an interdigitated electron collection system to maximize combined photon-electron harvesting in order to create high efficiency thin-film solar cells approaching the Shockley-Queisser limit of 33 percent.
