# Recent Research Progress in Surface Ligand Exchange of PbS Quantum Dots for Solar Cell Application
## 2 Solid-State Ligand Exchange
### Carrier Mobility of PbS CQDs Depending on Ligand Species  
Carrier mobility in the CQD thin film is also one of the important factors that determine the performance of the CQD solar cell, and many studies have been performed to improve carrier transport characteristics in PbSe and PbS CQD thin films. Prior to this study, 1,4-benzenedithiol (BDT) and EDT were generally used as short-chain ligands for replacing the native ligands of as-cast PbSe and PbS CQD thin film. These short-chain ligands enabled improved carrier transport in the PbSe and PbS CQD thin film. However, these ligands still had insufficient properties for application to optoelectronic and photovoltaic devices, because the S-S distance is still \(\sim\)6.4 A scale in the case of PbS CQD with BDT ligands [17]. In the thiol-treated case, the \(\sim\)100 nm scale carrier diffusion length results in low quantum efficiency. Additionally, BDT- and EDT-treated PbSe and PbS CQD thin films are vulnerable to oxidation, which results in the ligand loss of CQDs [8; 18; 19]. In 2010, [PERSON] et al. used carboxylic acid-based short-chain organic ligands to overcome the issues originating from BDT- and EDT-based solid-state ligand exchange processes [20]. An FET device was fabricated to measure the carrier transport characteristics of carboxylic acid ligand-treated PbSe and PbS CQD thin film. These PbSe and PbS CQD thin films showed not only good carrier transport characteristics but also better stability in ambient conditions. In BDT- and EDT-treated CQD thin films, carrier mobility is immediately decreased when exposed in ambient conditions. However, carboxylic acid ligand-treated PbSe and PbS CQD thin films showed increased mobility in ambient conditions for 72 h (Figure 3). This phenomenon, which also originated from a two-stage oxidation mechanism, resulted in PbSe and PbS CQD thin films showing 10-30 times higher carrier mobility compared to the previous PbSe and PbS CQD thin film. The initial adsorption of water and oxygen provides better surface passivation, reducing the surface trap states and increasing the carrier mobility. Over time, gradually-hydrolyzed carboxylate ligands enable the penetration of oxygen to PbSe and PbS CQDs, leading to a decrease in carrier mobility.  
Herein, for the CQD FET, it is revealed that the carboxylic acid ligand-treated PbSe and PbS CQD thin film enhances carrier transport characteristics with increased stability in ambient conditions. From this, it is expected that adequate carboxylic acid-based ligand treatment for the PbSe and PbS CQD thin film, rather than the BDT or EDT, could lead to further enhancement of the PbSe and PbS CQD solar cell performance.  
Figure 2: Comparison of large- (**a**–**c**) and small-sized (**d**–**f**) lead sulfide (PbS) CQDs (**a**). (**b**) Absorption change of large and small PbS CQDs dispersed in octane solution under ambient as a function of storing time. (**c,d**) XPS analysis of large and small PbS CQDs showing S 2p spectra of as-fabricated film (black), annealed-film at 90 \({}^{\circ}\)C in air for 10 min (red) and for 90 min (cyan). (**e,f**) Schematic of large and small PbS CQDs described by red and yellow spheres, and cyan tails connected to sulfur atoms. Small PbS NCs are more resistant to oxidation compared to large PbS NCs. Reprinted from reference [10]. Copyright 2010 American Chemical Society.