The shift of threshold voltage in the FET measurement indicates a change of carrier concentration within the QD film after the cALD process. The sulfur enrichment mediates the
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stoichiometry of PbS QDs, which is considered responsible for tuning the fermi level of the QD ensemble.14,24 However, the decrease of doping concentration after 1 μL Na2S cALD treatment
even with compensated Pb:S ratios cannot be explained by this picture. In fact, there are reports claiming no net doping effect should be expected by adding doubly ionized sulfur, as the closed- shell S2- should not alter the valence band filling.22,25 Therefore, we suspect alternative chemical
origins induced by the sulfur enrichment that essentially causes the doping phenomena, and employ XPS for preliminary studies.
We have examined two film samples of PbS QDs by XPS, one pristine as reference and the other with 1 μL Na2S cALD treatment and plotted their Pb 4f and S 2s characteristics in Figure
3.6, respectively. By deconvoluting the XPS spectra, we identify spectral line of PbS and PbSO3
from the both the Pb and S region, while there are also lines assigned to S-C and S-S originated from the EDT ligands.26 No reasonable match is found for the additional peaks denoted as * and
** in the spectra in the NIST data base, and they are excluded for the concern of our study here. The atomic percentage of all composition is listed in Table 3.4.
After the 1 μL Na2S cALD treatment, the relative percentage of PbSO3 decreases in both
the Pb and S spectra. It is known that PbSO3 introduces shallow states close to the valence band
edge,27,28 which are possible to act as acceptors, so less PbSO3 formation after the cALD process
is consistent with lower doping observed in FET measurements. The PbS component detected in the S regime represents all Pb-S bonds including the ones in the nanocrystal lattice and those on the surface. A higher percentage of it relative to the PbSO3 in the cALD treated sample suggests
that the QD surface is enriched by sulfur but fewer sulfur atoms are transformed into PbSO3.
Hypothetically, when only a small number of S atoms are deposited onto the QD surface, the QDs are still surrounded by densely packed organic ligand shells, which block oxygen diffusion and prevent surface S atoms from oxidation. The remaining OLAM ligands from the cALD process as inferred from the XRD result can be barriers for carrier transport causing lower
85
mobility, if they are not fully exchanged by EDT. This assumption remains to be supported by further high resolution XPS and FTIR characterizations and other control experiments.
Figure 3.6 Pb 4f and S 2s XPS spectra of EDT-PbS QD films (a) (b) without cALD and (c) (d) after 1 μL Na2S cALD treatment.
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Table 3.4 Atomic percentage of components detected in XPS measurement
3.5 Conclusions
In this chapter, we describe a solution cALD approach utilizing Na2S in FA to enrich the
PbS QD surface with sulfur. This method is suitable for precisely tuning the stoichiometry of ultrasmall-sized PbS QDs from Pb rich till S rich while maintaining their colloidal stability by additional OLAM ligands. As the Pb:S ratio of QDs is decreased by the cALD process, we improve the hole mobility and concentration within the EDT capped PbS QD films and achieve enhanced photoconductivity from QDs treated by 3 μL of Na2S solution. To investigate the doping
mechanism and explain the opposite trend observed in FET characteristics with 1 μL Na2S cALD
treatment, we probe the surface composition of PbS QD films after EDT ligand exchange and speculate that the formation of PbSO3 is the chemical origin of p-type doping. Moving forward, we
will complete the correlation between oxidation states and doping concentration, study the effect of sulfur enrichment on PbS QD films as hole transport layers in the solar cell geometry. By increasing p-type doping of the EDT-PbS QD layer, we expect the Schottky barrier at the device back contact will be removed and band alignment will become more favorable for hole extraction, hence improving solar cell parameters including short-circuit current, open-circuit voltage and fill factor.
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CHAPTER 4 The Effect of Dielectric Environment on Doping Efficiency in Colloidal PbSe