Two strategies to enhance efficiency of PbS quantum dot solar cells: Removing surface organic ligands and configuring a bilayer heterojunction with a new conjugated polymer

We present two novel techniques for improving the efficiency of PbS quantum dot (QD) solar cells. First, plasma was applied to QD film with the aim of removing surface organic ligands, and then the chemical and optical properties of the QDs were investigated. Second, a thin layer of conjugated polymer was then deposited on top of the plasma-treated PbS QD film as a transportation layer for holes. The charge separation and subsequent transfer dynamics were examined, as were the resultant photovoltaic characteristics, according to the kind of polymer used. The developed device, which comprises a bilayer heterojunction of plasma-treated PbS QDs and poly[2,6-(4,4'-bis(2-ethylhexyl) dithieno[3,2-b:2',3'-d] silole)-alt-4,7(2,1,3-benzothiadiazole)] (PSBTBT), showed not only broad-range absorption of the solar spectrum, but also high charge transfer efficiency prior to recombination. This results in a largely increased power conversion efficiency (PCE) of 1.76%, compared to the 0.7% value of a PbS QD-only device not subjected to plasma treatment. This indicates that the proposed techniques are very useful for improving the efficiency of inorganic QD-based solar cells. (C) 2013 Elsevier B.V. All rights reserved.