PbS Nanocrystal Surface Oxidation for Trap State Elimination
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Solution Overview
Problem
The performance of nanocrystal (NC) thin films in optoelectronic devices is limited by sub-bandgap states, also known as trap states, which reduce charge carrier lifetimes and diffusion lengths, and are thought to arise from under-charged Pb atoms in ligand-exchanged PbS NC films.
Innovation Solution
Chemical oxidation of the surface of PbS nanocrystals using oxidizing agents such as 1,4-benzoquinone or pyruvic acid reduces the density of trap states by modifying the oxidation state of Pb atoms, thereby reducing the density of sub-bandgap states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ligand exchange is performed on PbS nanocrystals to improve surface passivation, then surface passivation is improved, but sub-bandgap states are generated that reduce charge carrier lifetimes and diffusion lengths
Solution Approach 1:
The patent changes the oxidation state parameter of Pb atoms on the nanocrystal surface from under-charged (+2) to over-charged (+3 or +4) through chemical oxidation treatment. This parameter change eliminates the sub-bandgap states responsible for charge carrier trapping while preserving the surface passivation benefits of ligand exchange, thereby extending charge carrier lifetime and diffusion length.
2Reliability
If ligand exchange is performed on PbS nanocrystals, then surface passivation is improved, but trap state density increases reducing charge carrier diffusion lengths
Solution Approach 1:
The patent applies chemical oxidation to change the oxidation state parameter of surface Pb atoms, transforming them from under-charged to over-charged states. This eliminates trap states that would otherwise reduce charge carrier diffusion lengths, allowing carriers to diffuse farther while maintaining the surface passivation achieved through ligand exchange.
3Reliability
If chemical oxidation is applied to increase Pb oxidation state, then trap state density is reduced, but additional processing steps are required
Solution Approach 1:
The patent employs strong oxidizing agents such as iodine (I2), iodine/triiodide (I2/I3-), and other oxidants to rapidly increase the oxidation state of surface Pb atoms. This accelerated oxidation approach effectively reduces trap state density with controlled processing, achieving the desired electronic structure modification through targeted chemical treatment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach achieves a 40-fold reduction in trap state density, improving the performance of NC thin films by eliminating emissive sub-bandgap states and enhancing charge carrier diffusion lengths.
Implementation Method 1
contacting a surface of the nanocrystal with an oxidizing agent
Data Source
AI summary
The size-dependent band-gap tunability and solution processability of nanocrystals (NCs) make them attractive candidates for optoelectronic applications. One factor that presently limits the device performance of NC thin films is sub-bandgap states, also referred to as trap states. Trap states can be controlled by surface treatment of the nanocrystals.