Effects of Ligands on Charge Generation and Recombination in Hybrid Polymer/Quantum Dot Solar Cells

Control of quantum dot surface chemistry offers a direct approach to tune the molecular interface between donor and acceptor constituents in hybrid bulk heterojunction photovoltaics incorporating organic semiconductors and colloidal quantum dots. We investigate the effects of altering the quantum dot surface chemistry via ligand exchange in blends of PbS quantum dots with the conjugated polymer poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b')-dithiophene-2,6-diyl) (2-((do decyloxy) carbonyl)thieno (3,4-b) thiophenediy1)) (PTB1). We study organic ligands with both thiol and carboxylic acid functional groups including 1,2-ethanedithiol (EDT), 3-mercaptopropionic acid (MPA), and malonic acid (MA), in addition to inorganic halide ions such as tetrabutylammonium iodide (TBAI). We show that the different ligand treatments influence hybrid solar cell efficiency primarily through changes in open-circuit voltage (V-oc) and fill factor (FF). We use photoinduced absorption (PIA) spectroscopy to probe the generation of long-lived polarons resulting from charge transfer between the donor and acceptor constituents. We further characterize the recombination dynamics in the hybrid devices using transient photovoltage (TPV) and charge extraction (CE) techniques. Both methods show that ligand exchange with MPA yields superior device performance by promoting longer carrier recombination lifetimes under open-circuit conditions.