III. Results and discussion 40
9. Dyes 105
9.2. Influence of the anchoring group
9.2.2. Dye/Spiro-MeOTAD interface
Lineup
The energetic lineups of both dyes to nc-TiO2 are shown in Figure 9.15. The results are equal and, therefore, the lineups are the same. The nc-TiO2/dye interfaces do not show a band bending in neither of the layers. Also no interface dipole is concluded, as the work functions (4.3 eV) are the same for TiO2, ID224 and ID94.
The calculated optical band gap of ID224 and ID94 (2.1 eV for both) obtained from the project partner BASF is the difference of the HOMO and LUMO onset. The band gap was calculated according to Equation 8.1, which transforms the optical band gap into the band gap which would be measured by photoelectron spectroscopy and inverse photoelectron spectroscopy. Due to the band gap of 3.4 eV, the theoretical binding energy positions of the LUMO is 1.3 eV above the the Fermi level and thus 1.1 eV above the conduction band minimum of TiO2. Again the value of the conduction band maximum was taken from Subsection 7.1.4 as the determination of the valence band maximum of TiO2 is very difficult due to the underestimation of minor valence band emissions and the interference of TiO2gap-states.
Figure 9.15.:The band diagrams of the TiO2/ID224 and the TiO2/ID94 interfaces as formed after drop-casting of ID224 and ID94. The LUMO maximum positions of ID224 and ID94 are determined by the calculated energy gap of the dyes (3.4 eV) and result 1.3 eV above the Fermi level. The offsets of the conduction band minimum of TiO2 and the LUMO of ID224 and ID94 are 1.1 eV, whereas the work functions of all materials are the same, which indicates no interface dipole.
Valence band
As discussed in the previous subsection, the valence band spectrum labeled as "ID224" is a superposi-tion of the ID224 spectrum and the TiO2 substrate spectrum labeled as "nc-TiO2 " due to either a thin or not closed dye layer. Moreover, the HOMO emission of ID224 can be decomposed into a HOMO (2.1 eV) and a HOMO-1 (2.7 eV) molecular orbital. Because of the thin or not closed dye layer, a TiO2 gap state contribution is found at ca. 1.3 eV which is attributed to VO gap-states as shown in Subsection 7.1.4.
The features of the nc-TiO2 sample and the drop-casted ID224 are damped with increasing Spiro-MeOTAD overlayer thickness. The Ti3p emission does not shift, which indicates that no band bending takes place in the nc-TiO2 substrate. The features of Spiro-MeOTAD grow in intensity with increasing overlayer thickness. The spectrum of the thick and dense layer of Spiro-MeOTAD reached at 137 Å orig-inates from the TiO2/Spiro-MeOTAD interface experiment of the same measuring period (Subsection 8.1.1). As the HOMO emissions and the valence band structures of ID224 and Spiro-MeOTAD overlap, the spectra with Spiro-MeOTAD thicknesses of 1.2 and 2 Å are difficult to evaluate.
The secondary edges are taken from other experiments at the beamline TGM7. The work functions of the nc-TiO2 and the dye ID224 are both 4.3 eV. In contrast to that, at the second interface, ID224/Spiro-MeOTAD, a work function difference of ca. 0.2 eV is found. This difference is concluded to form an interface dipole of the same magnitude.
Intensity [a.u.]
7 6 5 4 3 2 1 0
Kinetic energy [eV]
SE
hQ = 90eV norm. to max.
nc-TiO2 ID224 Spiro-MeOTAD
Intensity [a.u.]
50 40 30 20 10 0
Binding energy [eV]
VB region
hQ = 90eV
nc-TiO2 (Int/2) ID224 1.2Å 2Å 137Å
Intensity [a.u.]
3 2 1 0
Binding energy [eV]
HOMO region
hQ = 90eV
nc-TiO2 ID224 1.2Å
2Å 137Å(Int/2)
I II
III
Figure 9.16.:The valence band and the detailed HOMO region of the nanocrystalline TiO2 substrate, the drop-casted ID224 and the stepwise evaporated Spiro-MeOTAD (formal thickness of the Spiro-MeOTAD layer is indicated), recorded with a photon energy of 90 eV. The secondary edges of nc-TiO2 (4.3 eV), ID224 (4.3 eV) and Spiro-MeOTAD (4.1 eV) recorded at a photon energy of 50 eV at the TGM7 beam-line are taken from other experiments. After the drop-casting of ID224, the TiO2 features are not completely damped. The HOMO emission of ID224 is decomposed into HOMO (I) at 2.1 eV, HOMO-1 (II) at 2.7 eV and the V0 gap state (III) of TiO2 at ca. 1.3 eV. With increasing Spiro-MeOTAD thickness the features of this molecule evolve.
Core levels
The detailed C1s, N1s, O1s and S2p core level spectra, which were measured with a photon energy of 360, 450, 600 and 210 eV, respectively, are shown in Figure 9.17. Again, the spectra of the "thick"
Spiro-MeOTAD layer with a thickness of 137 Å are borrowed from the TiO2/Spiro-MeOTAD interface ex-periment of the same measuring period (compare to Subsection 8.1.1).
After drop-casting of ID224 no shift in the O1s emission maximum at 530.7 eV is observed. This maxi-mum is attributed to TiO2. Strong C1s, N1s and S2p emissions at 285.7, 400.7 and 164.1 eV, respectively,
arise upon drop-casting. Those emissions are caused only by the organic dye and are composed of the emissions of the different bonds of the respective kind of atom in the molecule. In contrast, the O1s emission is a combination of emissions originating from both nc-TiO2 and the dye.
Intensity [a.u.]
292 288 284 Binding energy [eV]
C1s
hQ = 360eV
nc-TiO2 ID224 1.2Å 2Å 137Å
Intensity [a.u.]
404 402 400 398 Binding energy [eV]
N1s
hQ = 450eV
nc-TiO2 ID224 1.2Å
2Å 137Å
Intensity [a.u.]
538 536 534 532 530 528 Binding energy [eV]
O1s
hQ = 600eV
nc-TiO2 ID224 1.2Å 2Å 137Å
Intensity [a.u.]
167 166 165 164 163 Binding energy [eV]
S2p
hQ = 210eV
nc-TiO2 ID224
1.2Å 2Å
137Å
Figure 9.17.:The C1s (top left), N1s (top right), O1s (bottom left) and S2p (bottom right) core level spectra of (from the bottom up) nc-TiO2, drop-casted ID224 and stepwise evaporated Spiro-MeOTAD.
According to the S2p emission (S2p3/2 at 164.1 eV), which can be solely attributed to the dye, no shift is observed (indicated with a line); the O1s emission at 534.1 eV, which can be attributed to Spiro-MeOTAD (compare to the O1s emission of evaporated Spiro-Spiro-MeOTAD in Figure 8.3), show no shift ei-ther (the position of the emission of Spiro-MeOTAD is indicated with a line). C1s spectra were recorded at a photon energy of 360 eV, N1s at 450 eV, O1s at 600 eV and S2p at 210 eV.
The S2p core level, which is only present in the dye and, therefore, indicates solely the energetic shift of the dye, shows no shift in the dye during Spiro-MeOTAD deposition. The emission with the highest binding energy in the O1s detailed spectrum at 534.1 eV which is attributed to Spiro-MeOTAD (see the O1s emission of the TiO2/Spiro-MeOTAD interface experiment in Figure 8.3) shows no shift either. The other core levels, C1s and N1s, consist of overlapping emissions of both molecules and, therefore, are neglected here for evaluation.
Ti2p / gap-states
Figure 9.18 shows the Ti2p emission of each step of the TiO2/ID224/Spiro-MeOTAD experiment. The energetic positions of the Ti2p1/2(465.1 eV) and Ti2p3/2 (459.3 eV) do not change either after the drop-casting of ID224 or during the evaporation of Spiro-MeOTAD. As seen in the previous subsection, the normalized Ti2p spectra show a small amount of reduced Ti3+ species, which can be seen as a low
bind-ing energy shoulder in the spectra. As mentioned in previous subsections, the reduced titanium species is accompanied by the appearance of TiO2 gap-states and vice versa. The reduced species is damped but not quenched after the depositionof the dye and the first two deposition steps of Spiro-MeOTAD. Besides the gap-states, the region shows the HOMO emission of ID224. Therefore, an interaction of either the HOMO of the dye or the HOMO of Spiro-MeOTAD with the TiO2gap-states can be excluded.
!
!
!
"
"
Intensity [a.u.]
468 464 460 456 Binding energy [eV]
Ti2p
hQ = 600eV norm. to max.
and in BE
Intensity [a.u.]
4 3 2 1 0
Binding energy [eV]
GS region
hQ = 465eV Ti2p-Ti3d resonance
nc-TiO2 ID224
1.2Å 2Å 137Å
Figure 9.18.:The Ti2p core level spectra, the normalized Ti2p spectra and the gap-states region measured in Ti2p-Ti3d resonance of (from the bottom up) nc-TiO2, drop-casted ID224 and stepwise evaporated Spiro-MeOTAD. No shift can be observed in the Ti2p spectra but a small amount of reduced Ti3+species in the normalized spectra. The resonance measurements show no complete damping of the TiO2 gap-states for the dye and the first two Spiro-MeOTAD deposition steps. The detailed Ti2p and gap-gap-states region spectra were recorded with a photon energy of 600 and 465 eV, respectively.
Lineup
The results of the experiment are summarized in the energetic lineup of the nc-TiO2 /ID224/Spiro-MeOTAD sequence in Figure 9.19. The nc-TiO2/ID224 interface, which was shown and discussed in the previous subsection, does not show a shift in either of the layers and no interface dipole. The bind-ing energy position of the LUMO is 1.3 eV above the Fermi level and thus 1.1 eV above the conduction band minimum of TiO2. The energetic offset is very similar to the obtained offset for the dye ID504 (1.0 eV see Subsection 9.1.1). The offset is important for the function of the DSSC as an offset in the other direction would cause a barrier for charge transfer. The LUMO position of the dye has a system-atic error as it was not measured by inverse photoemission spectroscopy and has a certain energetic extension which is unknown. If one consider the extension of the HOMO (ca. 1.1 eV in both directions) and the optical gap (2.0 eV) which is the energetic difference between HOMO and LUMO onset, the LUMO onset of the dye ends up at 1.0 eV above the Fermi level. Hence, the onset is 0.8 eV above the conduction band minimum of TiO2, which means that all photo-excited electrons in the dye do not have to overcome a barrier, and the extension of the LUMO (1.0 eV) is the same as the extension of the HOMO.
At the second interface, ID224/Spiro-MeOTAD, an interface dipole of 0.2 eV is assumed because of the work function difference. The binding energy position of the HOMO of ID224 is 0.2 eV below the one of Spiro-MeOTAD and thus comparable to the offset of the dye ID504 (0.3 eV see Subsection 9.1.1), but matches with the superposition of HOMO and HOMO-1 of Spiro-MeOTAD. In both cases, the electron injection should be very efficient as the electrons do not have to overcome this energetic offset but the transfer is driven by this overpotential. Nevertheless, this overpotential is a built-in loss potential, which is why the energetic HOMO position of the dye ID224 fits better to the HOMO of Spiro-MeOTAD.
The loss mechanism of the photo-excited electron via a charge transfer to the LUMO of Spiro-MeOTAD
(2.9 eV above the Fermi level, see Subsection 8.1.3) should be negligible as the offset of 1.6 eV is too high forefficient charge transfer.
Figure 9.19.:The band diagram of the TiO2/ID224/Spiro-MeOTAD sequence as formed by drop-casting of ID224 and subsequently PVD of Spiro-MeOTAD. The LUMO maximum position of ID224 (1.3 eV) is determined by the calculated energy gap of the dye (3.4 eV). The offset of the conduction band minimum of TiO2 and the LUMO of ID224 is 1.1 eV, whereas the work functions of both materials are the same, which indicates no interface dipole. At the interface to Spiro-MeOTAD no band bending is observed and an interface dipole of 0.2 eV is concluded from the work function difference. The lineup of the Spiro-MeOTAD HOMO maximum to the HOMO maximum of ID224 is 0.2 eV.