ABN and AF4BN blended with PCBM

then start to grow within minutes. This process predominantly occurs at high concentrations of ABN and is mediated by residual solvent in the film. This can be verified by comparing an annealed or vacuum treated film of similar concentration, which microscopically displays a neat film. The annealed film is further stable, at least for the observed period of a few hours, and does not develop any further crystalline structures. The few already formed crystals do not grow significantly. On the nano-scale, the morphology is of course completely unknown and may be composed of large clusters of ABN molecules.

Results, UV-Vis Absorption Spectroscopy: From the individual UV-Vis absorption spectra it could be presumed that all significant spectral features of PCBM and ABN are overlapping. how-ever, as it can bee inferred from Figure 12.8, they are not distinctive in the blends. Interestingly the PCBM’sπ-π^∗ transition¹T_1u→¹A_g at ca. 335 nm decreases in the activated films.^268,270A similar fading of characteristic absorption bands was also observed for the ABN bands at 300 -350 nm of the pure materials since without host matrix present, the ABN molecules are likely to react with each other. This may be an indicator that PCBM undergoes a chemical reaction with ABN after UV activation. For pure PCBM a similar exposure with UV light leads only to a slight decrease in visibility of the¹T_1u →¹A_g transition^267,268at ca. 335 nm, which may be related to polymerization or partial oxidation. However, there are no additional absorption bands arising in the blended films upon activation, see Figure 12.8. In any case, UV-triggered reactions of the PCBM blends can be verified by UV-Vis absorption spectroscopy that is likely occurring between PCBM and ABN or F4ABN.

Figure 12.8:The Figure shows UV-Vis absorption spectra of ABN (a) and AF4BN (b) for the blends with PCBM at a concentration of 1 molar ratio (MR) for ABN and 0.5 MR for AF4BN. The activated layers are represented by a blue line. The spectra of PCBM and ABN or F4ABN are plotted for comparison. Different absolute absorption values can correlate with varying film thickness.

Results, IR-spectroscopy: Well defined films of high mixing ratio of ABN and F4ABN with PCBM have been investigated with IR spectroscopy by H. Mager and are plotted in Figure 12.9. The observed vibrational modes can be directly attributed to the spectra of pure PCBM and ABN or F4ABN in Figure 12.5 implying that there is no particular interaction between the

96 12 Anchoring and Activation

molecules before activation. Absorption strength and precise peak position of vibrational modes can slightly change as the azide stretching vibration of ABN at 2110 cm⁻¹ in Table B.1 has shifted to 2113 cm⁻¹. Furthermore the C-O stretching vibration of PCBM at 1737 cm⁻¹ is re-duced in intensity compared to pure PCBM. Characteristic vibrational modes of ABN at 1282 cm⁻¹ connected to the functional azide group can be found in the blended films as well. Similar observations can be made for F4ABN in PCBM, for example the F4ABN absorptions bands at 1495 cm⁻¹ and 2120 cm⁻¹ can be identified in spectra of Figure 12.9. Relative intensity ratios of vibrational modes are preserved in the blended films. If the blends are exposed to UV light in the glove-box an activation of the azide group can be verified. Vibrational modes linked to the functional azide group at 2110 cm⁻¹ and 1282 cm⁻¹ for ABN as well as at 1495 cm⁻¹ and 2120 cm⁻¹ for F4ABN are vanishing during UV exposure. PCBM related bands like the C-O stretching vibration at 1737 cm⁻¹ remain unchanged after activation, which impedes making definite predictions or statements about azide reaction and immobilization.

Figure 12.9:Relative transmission spectra of ABN blended with PCBM (a) and F4ABN in (b).

The same film after UV activation is added in each sub-figure in blue. Important peaks are marked in the figure and can be compared with Table B.1. Each sample is remeasured after 22h UHV.The spectra have been measured and provided by H.

Mager (Univ. Heidelberg).¹⁰⁰

12.1 Spectroscopic studies on a model system 97

By integrating the absorption peak of the azide stretching vibration at around 2110 cm⁻¹ the transformed ABN/F4ABN fraction can be estimated. After UV exposure for 5 min with an UV-power of 0.24 mW cm⁻² about 47% of ABN and 67% of F4ABN molecules are activated.

The converted fraction is much smaller compared to pure ABN films or mixed with P3HT. Since PCBM is strongly absorbing around 250 nm, it is believed that a higher UV-dose is required since PCBM is partially absorbing UV light to be used for activation. A full transformation of the azide is achieved after an additional 10 min exposure at 0.85 mW cm⁻². To test stability and volatility of ABN in PCBM the blended films are exposed to UHV conditions and measured again with IR spectroscopy to track ABN content. A decrease in ABN concentration is indeed observed for the non-activated films but corresponding IR spectra remain unchanged for activated films. This observation agrees with the following XPS measurements done under similar conditions. If it is assumed that only a negligible amount of azide is transformed at room temperature, one can use the strong absorption at around 2110 cm⁻¹to quantify ABN content. Under this assumption 19% of non-activated ABN has left the PCBM film in 2 hours and about 40 % after 22 hours showing significant desorption. Similar values are obtained for F4ABN.

Results, Photoemission: In contrast to the pure azide films, blends with PCBM can be prop-erly characterized and analysed by XPS and UPS in various concentrations. In Figure 12.10 the spectra of PCBM mixed with ABN are plotted at a molar ratio ranging form 0.3 to 1 molar ratio (MR) before and after activation. The C 1s and O 1s emission is governed by PCBM and changes in these spectra are only observed at higher concentration of ABN after activation. Nitrogen ex-clusively occurs in ABN/f4ABN and can be used as a marker for activation. Before activating the film with UV-light, the N 1s spectrum clearly shows two distinct nitrogen species at an intensity ratio of 2:1.^179,185In analogue to the pure ABN films the N 1s component at ca. 404.5 eV is at-tributed to the central electron deficient nitrogen atom and the main peak at 402 eV is assigned to the lateral nitrogen atoms.¹⁸⁹ The outer nitrogen atoms of the azide group differ slightly in binding energy, which can be anticipated by a little broader emission at 402 eV. Interestingly, the overall N 1s intensity before activation does not depend strongly on the concentration of ABN prepared in solution. In fact, after calculating the molar ratio of ABN according to Eq.

5.3 in Section 5.1 from the combined core-level spectra in Figure 12.10, it is found that the remaining ABN content is significantly lower. After activation, however, the ABN content agrees with the expected concentration assuming that the azide group has transformed into nitrene and released molecular nitrogen. The higher amount of ABN in the activated film can also be identified in the increase of the O 1s peak component towards lower binding energies stemming from C-O-C bonds present in the ABN molecule. For pure PCBM a small increase in the O 1s signal is found, mediated by UV-light which is due to an oxidation at the surface from residual oxygen in the glovebox. A decrease in the C 1s peak and a shoulder towards higher binding energies was observed for the sample with highest ABN content after activation. Furthermore, upon activation the peak component at 404.5 eV vanishes and a broader peak at about 399.5 eV emerges which is attributed to the formation of carbon-nitrogen bonds and consequently to a successful activation of the ABN molecule. From the N 1s emission one finds a complete ABN conversion for a dose of 1 J cm⁻², which agrees with results obtained from IR spectroscopy. The higher amount of ABN after activation can be assigned to an increase in stability of ABN in the film under vacuum conditions and therefore to a reduced volatility. Though it is not possible to determine if or how the ABN is connected with the PCBM matrix. The XPS core level spectra of F4ABN blended with PCBM are very similar to ABN mixed with PCBM (see Figure 12.10).

98 12 Anchoring and Activation

Figure 12.10:Core-level emission spectra of ABN in (a) - (c) and F4ABN in (d) - (g) blended with PCBM for various concentrations ranging from a molar ratio (MR) of 0.3 to 1.0 with respect to PCBM. The activated films are plotted in blue. The legend for all graphs can be found in the bottom. In (i) and (j) the secondary electron cut-off (XPS) is depicted for ABN and F4ABN, respectively. Corresponding UPS spectra are shown in (j) and (k).

12.1 Spectroscopic studies on a model system 99

The interpretation of the O 1s and N 1s emission series can be carried out as for F4ABN, which in principle possesses the same anchor group and anchoring mechanism. Correspondingly, a transformation of the azide group upon activation and an overall increase in F4ABN content is observed in the activated films. A larger F4ABN content in the activated films is attributed to a successful activation that prevents F4ABN molecules to leave the surface in UHV conditions since it is likely connected to neighbouring molecules. An indicator for this correlation is the much large N 1s peak at about 401 eV, which originates from C-N bonds and follows the initial concentration in intensity. In addition, the strong F 1s emission serves as a direct tracer for F4ABN molecules in the blend. Without activation only a small amount of F4ABN is detected which rarely depends on the F4ABN concentration in solution. In the detailed spectra of Figure 12.11, a peak analysis has been carried out to highlight changes in the N 1s emission upon activation. In the N 1s spectrum of the non-activated ABN sample a small peak component at 400.5 eV can be interpreted as a partial decay of the azide prior UV-activation.

Figure 12.11:Fits of the N1s core level spectra of the PCBM blends with ABN in (a) and F4ABN in (b). The activated films are given in (c) and (d) for ABN and F4ABN, respectively.

Single components are plotted in red and the total fit in blue.

A closer look at the F 1s spectrum for the non-activated films reveals a peak component at 689.5 eV which is not observed after activation for higher F4ABN content at comparable intensity ratios. If the azide group is exchanged with a N-H or C-H bond a strong shift of the F 1s level seems unlikely. A possible explanation involves intermolecular interactions. A first simulation provides indication for a shift of the F 1s core-level of 1 eV provided the fluorinated ring systems are directly stacking. If the F4ABN molecules are disordered within PCBM the F 1s emission is expected at 688 eV. This may be an explanation for the observed peak component at 698.5 eV for the non-activated PCBM:F4ABN films.

100 12 Anchoring and Activation