Enhanced Mobility-Lifetime Products in PbS Colloidal Quantum Dot Photovoltaics

Abstract

Colloidal quantum dot (CQD) photovoltaics offer a promising approach to harvest the near-IR region of the solar spectrum, where half of the sun's power reaching the earth resides. High external quantum efficiencies have been obtained in the visible region in lead chalcogenide CQD photovoltaics. However, the corresponding efficiencies for band gap radiation in the near-infrared lag behind because the thickness of CQD photovoltaic layers from which charge carriers can be extracted is limited by short carrier diffusion lengths. Here, we investigate, using a combination of electrical and optical characterization techniques, ligand passivation strategies aimed at tuning the density and energetic distribution of charge trap states at PbS nanocrystal surfaces. Electrical and optical measurements reveal a more than 7-fold enhancement of the mobility-lifetime product of PbS CQD films treated with 3-mercaptopropionic acid (MPA) in comparison to traditional organic passivation strategies that have been examined in the literature. We show by direct head-to-head comparison that the greater mobility-lifetime products of MPA-treated devices enable markedly greater short-circuit current and higher power conversion efficiency under AM1.5 illumination. Our findings highlight the importance of selecting ligand treatment strategies capable of passivating a diversity of surface states to enable shallower and lower density trap distributions for better transport and more efficient CQD solar cells.

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

“…13,15,33,34 It has been reported that MPA passivates the QD surface more completely than EDT. 7,8,21 Our SPS results on PbS-MPA ( Figure 1B), however, reveal the presence of IGS at nearly the same level as those in the PbS-EDT, indicating that the IGS are not due to incomplete ligand passivation. We further performed SPS on ARTICLE ZHANG ET AL.…”

An Enhanced UV–Vis–NIR an d Flexible Photodetector Based on Electrospun ZnO Nanowire Array/PbS Quantum Dots Film Heterostructure

“…However, the spatial separation induced by the long oleic acid (OA) ligand that was used to cap QDs34 probably degrade the charge transfer between QDs and NWs. Therefore, it is necessary to exchange the long ligand with short one, such as EDT, tetrabutyl ammonium iodide (TBAI) 35, 36, 37, 38. Herein, 1–2 mL EDT solution was dropped on the device to replace the long ligand of OA for about 2–3 min, see the schematic illustration in Figure 1c.…”

Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy

“…[15] Using optical pump-probe spectroscopy, the groups of Asbury and Sargent observed a broad photoinduced absorption (PIA) around 0.3 eV with a long lifetime (still visible after 200 µs) for a range of ligands, including EDT and MPA, and attributed this observation to surface trap states. [16,17] Bakulin et al finally reported two distributions, independent of each other, at activation energies of 0.2 and 0.3-0.5 eV for EDT and MPA capped PbS, distinguishable by their lifetimes. [18] The corruption of a clean bandgap via ligand exchanging was furthermore related to the emergence of photoluminescence (PL) at energies below the direct bandgap transition.…”