Optimization of Highly Ordered DDAP Films for FET Devices

In order to characterize the electrical properties of DDAP layers with highly ordered crystallites, thin films on standardized organic field effect transistor (OFET) substrates from Fraunhofer237 _{were prepared using the optimized intercalation conditions}

described above.l _{These substrates are pre}–patterned with Au contacts forming channels

of varying width (2.5 m, 5.0 m, 10 m, and 20 m) for the fabrication of bottom contact transistors. The spin_{–coated PbI}2 precursor films crystallized in a different morphology on

different areas of the substrate, as can be seen on the SEM images in Figure 4.9. On the SiO2

surface (top left), the film was porous and identical to the films grown on glass substrates under the same conditions (Figure 4.5). On top of the Au contacts of the OFET substrate (top right), however, a compact morphology of the PbI2 film was found. This dense

k _{additional information on phase II and intercalated H2O is given in chapter 7.2.3.1 (page 120).} l _{PbI2 films dried at 25 °C; dip–coating at 50 °C; under ambient atmosphere.}

arrangement of crystals seems to grow also over the edges of the Au contacts into the FET channels (bottom left: 5m channel, bottom right: 10 m channel).m _{It is apparent that the}

nature of the substrate influences the growth of the crystals to a large extent, which is in agreement with what Fan et al. reported on thermally evaporated PbI2 films.228

Figure 4.9. SEM images of spin–coated PbI2 thin films on Fraunhofer standard OFET substrates, different

areas of the substrate. Top left: on SiO2 surface; top right: on a gold contact; bottom left: 5 m channel;

bottom right: 10 m channel.

In the subsequent intercalation step, the inhomogeneity of the PbI2 precursor films

on OFET substrates also affected the growth of the final DDAP film. SEM images revealed large inhomogeneous DDAP crystallites on the Au contacts, while there were defects and holes in the channels and the films were also disrupted on the plain SiO2 surface around

the channels (Figure 4.10).m _{In short, the patterning of Fraunhofer standard OFET}

substrates was found to be unfavorable for the preparation of smooth and continuous DDAP films by the intercalation procedure.

Figure 4.10. SEM images of a DDAP film on Fraunhofer standard OFET substrate prepared by intercalation procedure. Left: overview of a block of channels shows disruptions of the film. Right: closer

view shows defects and holes in the channels.

m _{XRD patterns of PbI2 and DDAP films on OFET substrates shown here were the same as for the films on glass,}

As an alternative to bottom_{–contact FET devices, the electrical properties of thin} semiconducting films could also be characterized in a top_{–contact geometry.}238 _The

advantage of the latter is that films can be fabricated on plain Si_{–wafers that do not} contain any pre_{–patterning, which might adversely affect their morphology. In the} following, experiments towards optimizing the film thickness and continuity of DDAP layers for top_{–contact FET devices were carried out.}

Figure 4.11. SEM images of surfaces (top) and cross–sections (bottom) of a DDAP film on Si–SiO2 wafer

before (left) and after annealing (right).

For application in devices films are required to be compact and smooth with minimal gaps (grain boundaries) between the crystallites. In order to improve these properties annealing experiments of DDAP filmsn _{were carried out. Figure 4.11 shows SEM}

images illustrating the morphology differences of a film before (left) and after annealing at 100 °C, followed by slow cooling to room temperature (right). After annealing the film is more compact and homogeneous, as compared to before annealing, where well_–defined crystallites separated by grain boundaries are found. As introduced previously, DDAP undergoes two phase transitions, a first one (associated with a conformational change of the organic cations) at 42 °C, and a second one (associated with a quasi_{–melting of the} organic layer) at 70 °C.172 _{The quasi}–melting of the organic cations allows for the

morphology change observed in the annealed films. XRD spectra showed that the desired preferential orientation of the crystallites was preserved after annealing (Figure 7.7, page 126).

A further improvement for the application of DDAP films in devices was achieved by reducing their film thickness. As described previously, PbI2 films with a thickness of

n _{on Si–SiO2 wafer substrates; prepared from porous PbI2 film at a dip–coating temperature of 50 °C inside}

≈ 100 nm resulted in a thickness of ≈ 1000 nm of DDAP film after intercalation. Thus, in order to reduce the thickness of DDAP films to ≈ 100 nm, the thickness of PbI2 precursor

films had to be adjusted to ≈ 10 nm, which was achieved straightforwardly by lowering the concentration of the spin_{–coating solution. Figure 4.12 (top left) shows that the thin PbI}2

films still have the desired porous morphology. For the intercalation of DDAI into these very thin PbI2 films also the concentration of the dip–coating solution had to be adjusted,

as the conversion is completed much faster. The very short reaction times (on the order of a few seconds) would be difficult to control manually and, as described before, films are known to suffer from damage by abrasion when remaining in the dip_{–coating solution} after conversion is completed.224 _{When the dip}–coating solution was diluted by a factor of

10, the conversion rate could be slowed down to achieve homogeneous DDAP films with a thickness o_{f ≈ 100 nm (Figure 4.12). XRD patterns of these films showed intense (00ℓ)} reflexes indicating a highly ordered arrangement of crystallites (Figure 7.7, page 126).

Figure 4.12. SEM images of surfaces (top) and cross–sections (bottom) of PbI2 films (left) and DDAP films

(right) with reduced film thickness for device application.

To summarize, DDAP films were optimized for the fabrication of bottom_–contact FET devices. The film morphology could be considerably improved by annealing and the thic_{kness was reduced to ≈ 100 nm by adjusting the concentrations of the precursor} solutions. Fabrication of FET devices is currently ongoing.

4.3 _{NIR–Cyanine Cations in Organic–Inorganic Hybrid}