Core-shell Colloidal Quantum Dots For Photovoltaics With Improved Open Circuit Voltage



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Over the past decades, research efforts have been dedicated to solution processed photovoltaics based on colloidal quantum dots (CQD). Due to its tunable band-gaps, good stability in air and earth-abundant nature, lead sulfide (PbS) quantum dots have become competing materials for the production of low-cost flexible solar cells. Although the power conversion efficiency of CQD solar cell has increased rapidly in the past 10 years and reached certified efficiency of 16.6% in 2018, it is not comparable to silicon and perovskite solar cells with the power conversion efficiency of 27.6% and 25.2%. One challenge that limits the performance of CQD solar cell is the incomplete absorption of the solar spectrum, resulting from the short diffusion length restricting the thickness of CQD layer to below 100 nm and could possibly solved by integrat- ing multiple layers into a tandem structure. Another challenge is the open circuit circuit voltage (Voc) and its deficit. High carrier recombination rates in PbS CQD photovoltaics can result in a reduction in Voc. Surface states and sub-gap states are thought to increase recombination which leads to Voc loss. Here we present a method of forming an oxide shell on the CQD surface capped with native oleic acid ligands prior to the deposition of the CQD film and ligand swap. The core-shell QDs exhibit a narrowing in size distribution and the resulting devices yield improved Voc. In addition, films of core-shell QDs are more resilient to damage incurred during sputter deposition of overlaying films. Because the thickness and uniformity of the oxide shell can be precisely controlled, a natural balance between trap passivation and charge transport can be achieved.



colloidal quantum dots, open-circuit voltage, photovoltaics