Hybrid passivated colloidal quantum dot solids

Abstract

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.

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.…”

Quantifying Angular Correlations between the Atomic Lattice and the Superlattice of Nanocrystals Assembled with Directional Linking

“…34 Since the optoelectronic properties of PbS NC ensembles bear many opportunities for applications in solar cells or photodetectors, a number of ligand exchange procedures with small organic or inorganic molecules as well as single atom passivation strategies have been developed, all of which greatly increase the carrier mobilities within the SL of NCs. 28,33,[35][36][37][38][39][40][41][42][43][44] Due to the short interparticle spacing imposed by these ligands, structural coherence is mostly lost in such superlattices, but in rare cases it has been demonstrated that significant long-range order and even mesocrystallinity can be preserved. 25,35,45 However, a persisting problem of these protocols is that they are prone to introduce defects in the superlattice structure with some degree of granularity and significantly smaller grain sizes, which poses difficulties in determining the angular correlation with a meaningful statistical distribution.…”

Multiple exciton generation in quantum dot-based solar cells

“…TiO 2 ), show ultrafast, hot electron transfer to the MO [93]. Considering the similar device architecture in common QD-based PV devices where a MO is used as electron acceptor [80,94,95], this transfer constitutes another potential hot carrier decay channel competing with MEG and hot carrier cooling (see Figure 2). …”

“…On a more general level, surface trapped carriers are highly localised and are therefore challenging to extract during solar cell operation [79,80]. Additionally, charge transport properties are intimately linked to the interparticle coupling constant within the QD film [81].…”

“…These strategies use for such surface passivation either a ligand sphere of organic [55,82,83] and/or inorganic molecules [49,80,84] [82,83,89]. While QDs passivated with dithiol ligands showed high charge-carrier mobilities [90,91] and competitive device performances in solar cells, no evidence for MEG could be found in devices [26,60].…”