We study the effect of S enrichment on the doping and transport properties of PbS QD films by fabricating FET devices. On heavily n-doped Si wafers with thermally grown 250 nm SiO2,
we spin coat PbS QDs after the solution cALD process using different concentrations of the Na2S
solution. A standard EDT solid-state ligand exchange is then applied on the QD film as commonly described in literature to remove remaining OLAM and oleic acid ligands on the QD surface,2 and
Au contacts are evaporated to complete the device.
All PbS QD FET devices show higher currents as the gate voltage is swept to negative values, characteristic of predominantly p-type semiconductor materials [Figure 3.4]. However, the overall current experiences a decrease when the QDs are treated by a 1 μL of Na2S solution
before increasing for all of the higher Na2S concentrations. At a fixed gate voltage higher than the
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the channel material. Since the device current in the linear regime is too small, we extract hole mobility μ and VT from the ID-VG characteristics in the saturation regime to distinguish the effect of
S enrichment on them. In the saturation regime, the current follows:
where Cox is the oxide capacitance per unit area, W and L are the channel width and length
respectively, VG is gate bias applied and VT is threshold voltage. By linearly fitting the ID1/2 vs. VG
curve, we obtain μ from the slope and VT from the intercept and plot them as a function of Pb:S
stoichiometry in Figure 3.4b. Except for the data point at 1 μL Na2S treatment, as the system is
made more S-rich, the hole mobility increases and VT shifts positively, indicating enhanced carrier
transport and p-type doping. This trend is consistent with literature report, where the improved hole mobility with increasing S content is attributed to improved interdot electronic coupling in the PbS QD films, as the valence band density of states is increased by reduction of interparticle distance or densification of the QD film.22 We further calculate the change of carrier concentration
Δp induced by the S enrichment according to the relationship:23
where TQD is the thickness of the active QD film which is the Debye length.14 Based on TQD= 20
nm, we obtain Δp~4x1016 ΔVT. Combining this relationship and the simulated crystal structures in
Figure 3.3d, we can estimate the additional hole concentration and the number of sulfur atoms added to introduce it at a given Pb:S. For example, by adding 91 sulfur atoms on the surface of individual PbS QDs through cALD, we achieve Pb:S~1 in the QD dispersion, and ~2x1016 cm-3
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Figure 3.4 (a) ID-VG characteristics of EDT capped PbS QD thin-film FETs (black), after 1(red), 3 (green), 10
(blue) and 30 (cyan) μL of Na2S treatments. (b) Hole mobility and threshold voltage extracted from the FET
characteristics as a function of QD stoichiometry.
Motivated by improving the hole transport layer of PbS QD solar cells, we study how the photoresponse is influenced by the cALD process. We fabricate photoconductors on glass substrates using the same surface treatments to prepare the QDs before depositing their films, as used for FETs on Au bottom contacts. Current across the two terminals is collected at an
electrical field of 500 V/cm both in the dark and under illumination by a 647-nm single-wavelength laser at 6.4 mW/m2. The photoconductivity, or photocurrent at a constant electrical field, is directly
proportional to the product of the lifetime τ and mobility μ of photogenerated majority carriers. And the τ∙μ is proportional to the majority carrier diffusion length.
After the 1 μL Na2S cALD treatment, both the dark current and photocurrents decrease
[Figure 3.5, black and red]. The lower current in the dark is consistent with the FET measurement. Since the mobility is also reduced, it is not conclusive about the change in carrier lifetime from the decrease of photocurrent along in this case. However, the QD film treated by 3 μL of Na2S
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green]. Because the mobility is not altered significantly by this cALD condition confirmed by both the dark current here and FET measurements, the higher current under illumination is mainly attributed to extended carrier lifetime. After stored in ambient environment for 9 days, the
samples are remeasured to be investigated for their air stability [Figure 3.5b]. The reference EDT- PbS QD film becomes apparently less conductive both in the dark and under illumination. Its absorption peak shifts from 990 to 890 nm [Figure 3.5c], indicating oxidation of QDs, which is responsible for poor carrier transport. The cALD treated QDs are yet less affected by the prolonged air exposure, showing higher dark and photocurrents than the reference. Their absorption peaks exhibit smaller blue shifts, from 990 to 920 nm, probably due to better passivation of surface Pb through the deposition of S atoms.
Figure 3.5 Photoconductivity measurements on (a) day 1 and (b) day 10 of EDT capped PbS QD films (black), with 1 (red) and 3 (green) μL of Na2S cALD treatments. (c) Corresponding absorption spectra of the
three samples on day 1 (solid) and 10 (dashed).