much smaller increase was measured when evaporation has stopped. During the deposition process, dopants are expected to carry thermal energy, allowing a fast immersion into the P3HT film. As a control experiment, and in order to verify that it is not a matter of surface doping, Mo(tfd-CO2Me)3 was evaporated on a cooled P3HT film and subsequently annealed. Since this experiment was also not done by the author, the reader is referred for details to Reiser et al.245 However, the experiment shows, that on a cooled substrate only vibrational modes attributed to neutral Mo(tfd-CO2Me)3 with the characteristic C=O stretching vibration at 1735 cm−1 are observed (beside a small initial charge transfer at the interface for 0.3 nm coverage). When the substrate warms up to room temperature, diffusion sets in at 5-10◦C and doping related absorp-tion features (IRAV modes + polaron) drastically changes the spectrum. These feature slowly increase and saturate at room temperature within several hours, much slower than those ob-served during thermal evaporation. The final saturated spectrum of doped P3HT is comparable for both samples, which are either kept at -20 ◦C or room temperature during dopant deposi-tion. With higher coverage further deposition of dopant molecules does not induce additional electron transfer, and presumably a dopant adlayer is formed. However, since IR spectroscopy probes the whole layer stack, the method is not able to distinguish the diffusion profile of neutral dopants, which is considered to be an important mechanism for device degradation.114 Here, XPS can distinguish the chemical composition at the surface and how it is related to diffusion.
is found for rra-P3HT in Figure 10.2c, although the amount of charged dopants is lower than for rr-P3HT (determined by the S 2p3/2 peak at ca. 162.5 eV). At a high coverage of Ld =15 nm, the S 2p intensity for rra-P3HT is larger than for rr-P3HT, suggesting that there are more neutral dopants accumulating on the surface in case of rra-P3HT. The position of the S 2p3/2 peak of P3HT, Mo(tfd-CO2Me)3 and Mo(tfd-CO2Me)·−3 is marked by a red dotted line in Figure 10.2. In order to study the diffusion and changes in dopant surface concentration, XPS measurements were performed after various waiting times with the sample being at room temperature. In Figure 10.2b, d the spectra are plotted separately after evaporation together with a spectrum recorded for a 10 h waiting time with the sample kept in UHV at room temperature. For low doping concentrations with a majority of charged dopants, there is no change in the amount of dopants found with waiting time. For a coverage of Ld =8 nm, a considerable decrease of the neutral dopant fraction is observed after 10 h, whereas the amount of Mo(tfd-CO2Me)·−3 at 162.5 eV mainly stays unperturbed. In fact, the shoulder towards lower binding energies does not decrease regardless of the type of P3HT or the dopant concentration for the probed infor-mation depth, even though an even distribution of dopants in the whole film is not expected.
In contrast to rra-P3HT, a significant decline in the amount of neutral dopants after 10 h can be identified for rr-P3HT and coverages Ld = 10 - 15 nm. This suggests that neutral dopants are more mobile than charged dopants, which are likely coulombically bound to the host ma-trix. For rra-P3HT, such a decrease is not seen above Ld =11nm and the total S 2p peak stays roughly constant for 10 h, which is dominated by neutral Mo(tfd-CO2Me)3, probably forming a dopant layer.
The changes in surface concentration as a function of time, which are due to neutral dopants, can be further quantified in Figure 10.3. From C 1s and F 1s core-level spectra, recorded to-gether with the S 2p emission in Figure 10.2, the molar ratio of dopants to P3HT monomers can be calculated. For this purpose, the background-subtracted core-level spectra have to be integrated and weighted with atomic sensitivity factors (ASF), which are specific to the spec-trometer and account for analyzer transmission, cross-section, and average electron mean free path, as described in Section 4.1. The correctly scaled intensity˜I is then derived from the mea-sured peak intensityIaccording to: ˜I =I/ASF and its ratios correspond to the ratio of respective atomic densities. However, because both dopant and matrix contain sulphur and carbon, the molar ratio nd/nm of dopantsnd to matrix monomers nm has to be inferred from rearranging:
˜I(F 1s)
˜I(C 1s, S 2p) = Nd(F)nd
Nd(C, S)nd+Nm(C, S)nm
(10.1) where Nd(C,F,S) and Nm(C,S) denote the number of atoms (C, F, S) in dopant and matrix monomers, respectively. Alternatively the most probable doping concentration can be calcu-lated using the formalism in Section 5. For very high doping concentration, Eq. 10.1 is not accurate and leads to big errors when the carbon intensity is mainly dominated by Mo(tfd-CO2Me)3 because of division close to zero. As a consequence, a peak component fit of the C 1s emission as depicted in Figure 9.2d is used instead, attributing the main C 1s line to P3HT plus dopant, whereas the components toward higher binding energies are only from Mo(tfd-CO2Me)3. Molar fractions in Figure 10.3 are then simply derived from molar rations nd/nm by:
nd
nd+nm = nd/nm
nd/nm+1 (10.2)
10.2 Photoelectron Spectroscopy after Deposition 75
~1nm t=0 ~1nm t=10h
P3HT Mo(tfd-CO2Me)3
Mo(tfd-CO2Me)3
~6nm t=0
~6nm t=10h Mo(tfd-CO2Me)3
Mo(tfd-CO2Me)3
-P3HT
4000
Intensity [a.u.]2000
166 165 164 163 162
Binding Energy [eV]
~1nm t=0 ~6nm t=0 ~8nm t=0 ~15nm t=0
rr-P3HT
P3HT
Mo(tfd-CO2Me)3
-Mo(tfd-CO2Me)3
166 165 164 163 162
Binding Energy [eV]
~15nm t=0 ~15nm t=10h
Mo(tfd-CO2Me)3
Mo(tfd-CO2Me)3
-P3HT
166 165 164 163 162
Binding Energy [eV]
~8nm t=0
~8nm t=10h Mo(tfd-CO2Me)3
Mo(tfd-CO2Me)3
-P3HT
a) S 2p b)
6000
4000
2000
Intensity [a.u.]
166 165 164 163 162
Binding Energy [eV]
~1nm t=0 ~8nm t=0 ~11nm t=0 ~15nm t=0
rra-P3HT
Mo(tfd-CO2Me)3
Mo(tfd-CO2Me)3
-P3HT
166 165 164 163 162
Binding Energy [eV]
~11nm t=0 ~11nm t=10h
P3HT Mo(tfd-CO2Me)3
Mo(tfd-CO2Me)3
-166 165 164 163 162
Binding Energy [eV]
~15nm t=0 ~15nm t=10h
P3HT Mo(tfd-CO2Me)3
Mo(tfd-CO2Me)3
~8nm t=0 ~8nm t=10h
P3HT Mo(tfd-CO2Me)3
Mo(tfd-CO2-Me)3
~1nm t=0 ~1nm t=10h
Mo(tfd-CO2Me)3
-P3HT Mo(tfd-CO2Me)3
c) S 2p d)
Figure 10.2:S 2p spectra for varying dopant coverage measured directly after dopant evapora-tion on rr-P3HT (a) and rra-P3HT (c). Each spectrum is remeasured after 10 hours for rr-PH3HT in (b) and rra-P3HT (d). For (b), (d) the solid lines show the spectra recorded directly after evaporation from (a), (c), and the dashed lines show the spectra recorded 10 h later. The assignment of the S 2p3/2 emission line for differ-ent chemical compondiffer-ents is marked by a red dashed line in each plot. The intensity scale in (a) and (b), (c) and (d) is similar but differs between (a) and (c). Adapted from Reiser et al.245
Finally, in Figure 10.3, the dopant fraction at the surface is plotted versus waiting time on a logscale for the various nominal dopant coverages, given by the nominal layer thickness Ld. Each sample represents one data series with a fixed deposited coverage encoded by colour. It can be seen that the decrease in surface concentration is continuous and not strictly exponential.
For low molar fractions of < 0.05, the surface concentrations seems to be constant with time and, provided that almost all Mo(tfd-CO2Me)3 is charged, this agrees with the hypothesis that charged dopants are less mobile than neutral dopants. For extremely high coverage there is also no change in the molar fraction, which can be explained by a saturated P3HT film and a dense dopant toplayer. On the other hand, an opaque dopant layer would be expected at a nominal coverage of Ld = 10 nm, if no diffusion had occurred (provided an IMPF of around
76 10 Diffusion in Sequentially Doped P3HT
3nm). However, the molar fraction shows there is indeed no dense adlayer of Mo(tfd-CO2Me)3 present at the surface (already at the first XPS measurement) up to a nominal dopant layer thickness of 18 nm for rr-P3HT and 14nm for rra-P3HT. This agrees with IR spectroscopy data, which find a diffusion into the P3HT film already during deposition. Further deposition beyond 18 nm will hold a molar fraction of one, meaning a pure dopant contribution on the surface. To understand the data in Figure 10.3 in more detail, a numerical model is constructed in the next section.
Ld[nm] Ld[nm]
(a) (b)
Figure 10.3:Molar fractions of Mo(tfd-CO2Me)3to P3HTmonomers for rr-P3HT (a) and rra-P3HT (b) as determined in the surface region. The color legend on the right side shows the approximate nominal layer thicknessLdof Mo(tfd-CO2Me)3as measured by the micro-balance. Relative errors for molar ratios can be estimated to be as high as 15%.