EAGER: Transparent electrode device architecture for high efficiency tandem colloidal quantum dot photovoltaics

Mount Holyoke College, South Hadley MA

Investigators

Abstract

Abstract Nontechnical The emergence of lightweight, flexible, efficient, and affordable solar cell modules could revolutionize energy generation from the sun. Among the contending technologies, photovoltaics employing lead sulfide nanocrystalline films have been increasing in efficiency at one of the most rapid paces ever seen. However, two aspects limit the efficiency of these cells: how far electrons can move through the lead sulfide film and how many material defects exist in the nanocrystals. Tandem solar cells (multiple solar cells grown on top of one another) can circumvent these limitations because multiple thin cells can be stacked to achieve strong absorption across the whole cell. Surprisingly, previous attempts at fabricating two-layer tandem photovoltaics using lead sulfide nanocrystals have not achieved the dramatic gains in efficiency that would be expected. This project presents modeling data showing that at least five layers of cells must be stacked before significant enhancements in efficiency will be observed. A proof-of-principal device will be fabricated consisting of two solar cells connected with a transparent conducting metal oxide layer. The research will take place at Mount Holyoke College, a women's undergraduate college with a remarkable history of educating women in the sciences. Technical The PI will fabricate a lead sulfide colloidal quantum dot photovoltaic in a tandem structure in order to enhance absorption in the critical long-wavelength region of the solar spectrum. Dramatic gains in the power conversion efficiency of colloidal quantum dot photovoltaics have been achieved over the past decade, but many of the best devices still suffer from low absorption in the infrared, a consequence of weak oscillator strength at the first excitonic transition peak, often resulting in more than a 50% reduction in absorption alone. The mismatch between the carrier diffusion length (ranging around 100 nm) and the thickness needed to absorb an appreciable amount of the solar spectrum (ranging around 500 nm) accounts for the shortfall in absorption. Preliminary modeling demonstrates that a five-junction tandem structure can achieve full absorption while allowing the maximum thickness of the lead sulfide layer in each sub cell to stay within 100 nm, achieving a straightforward, attainable pathway to realistic efficiencies approaching 19% and theoretical efficiencies approaching 28%. Tandem structures are an effective method of increasing absorption already employed in small molecule organic PV and elsewhere, but never before have had researchers assembled the tools needed to effectively construct five or more tandem junctions. The PI will fabricate a proof-of-principle tandem structure by employing a custom low-damage sputtering technique for the deposition of metal oxide transport layers and transparent conductors at the recombination zone, an integrated fiber-optic spectrophotometer for accurate absorption measurements coupled with careful optical modeling, an interconnected glovebox system for seamless device fabrication, and a revolutionary Thermo-reflectance imaging technique to map current flow and electric field non-uniformities.