Experimental 123

5.2.1

Open-cellular organic thin films

Synthesis of the 3DOM inverse open-cellular structures from co-deposition is a two step process: (i) vertical or convective self-assembly of “small” PS spheres in the presence of the organic semiconductor fillers, to form thin periodic sheets; and (ii) removal of the PS nanosphere template.

Vertical self-assembly by co-deposition

PS sphere latexes with mean particle diameters of 96 nm, 78 nm and 35 nm were prepared with polydispersities of 0.010, 0.014 and 0.086 (5.2 %, 5.9 % and 9.9 % relative standard deviation) synthesised in (R8), (R1) and (R5) respectively from Chapter 3 (Table 5.1).

Table 5.1 Summary of the different nanosphere characteristics employed in vertical co-deposition from

(R8), (R1) and (R5) (Chapter 3). Average diam. (dry) [nm] Standard deviation (dry) [nm] Relative standard deviation (dry) [%] Z-average diameter (DLS) [nm] PDI React. (Chapt. 3) 96 5.0 5.2 115 0.014 (R8) 78 4.6 5.9 100 0.010 (R1) 35 3.5 9.9 61 0.086 (R5)

Typically, 0.10-0.30 mL of latex was added to 15 mL of water (pH 11) containing 0.02-0.15 mg mL-1 of pre-dissolved water-soluble polymeric semiconductor, PTEBS, or the molecular semiconductor, TSCuPc. The latex volume fractions were varied between 0.015 % and 0.080 %. In case of the very small particles of 35 nm in diameter latex volumes of up to 4 ml were added due to the very low initial solid content of nanospheres (<10-3 _{%) from the quenched reaction (R5). Composite films of close-packed, self-}

Chapter 5: 3D interdigitated organic D-A composite structures and OPV devices assembled monodisperse PS spheres infilled with water-soluble organic semiconductors were fabricated in a single-step process (co-deposition). The growth was adapted to vertical self-assembly conditions reported by McLachlan et al.[196, 202]

Ordered colloidal composite thin films were grown on either 12 x 75 mm glass slides (VWR International) or 12 x 36 mm ITO coated glass substrates, which were immersed in the appropriate colloidal dispersion in glass vials (Figure 5.1). All substrates were cleaned following the standard method. Glass substrates were used to optimise the growth parameters prior to ITO substrate use. Typical dispersion volumes were 15 mL in glass vials with dimensions of 25 mm in diameter and 100 mm in height. After immersing the substrate in a blend of colloidal dispersion and organic semiconductor, the structures were grown in a temperature-stable incubator at 60 ˚C ± 0.4˚C and a relative humidity (RH) <20 %. Under these growth conditions an empirical balance between solvent evaporation and particle sedimentation was found.[189, 202] _{The temperature was} monitored by either a digital thermometer or a data logger (Dickson, TM325) with remote probe for RH and temperature measurement. The samples were usually kept in the incubator for up to 3 days until the drying process was completed.

PS sphere template removal

The PS sphere templates were selectively removed from the composite structure by direct exposure of the sample to vapour from refluxing tetrahydrofurane (THF) for 20 to 30 minutes. For hot solvent vapour sphere removal a reflux apparatus was set up with a round-bottom flask as a solvent reservoir, a column and a water-cooled condenser with a sample holder placed in the vapour stream at the height of optimal solvent condensation. Penetration of pure solvent vapour into the composite thin film, condensation and dissolution of the PS spheres followed by gravity-induced draining of the polymer solution resulted in the formation of well-defined 3DOM organic thin films. This process is equivalent to continuous washing in high purity warm solvent, although it requires only a very small amount of solvent and the degree of template removal is simply a function of exposure time. After successful sphere removal the samples were dried for 10 minutes at 80 ˚C under inert atmosphere to remove remaining solvents. The resulting 3DOM structures were analysed by SEM.

5.2.2

Fabrication of 3D nanostructured composite devices

For 3D interpenetrating nanostructured D-A composite OPV devices pre- fabricated open-cellular thin films of PTEBS and TSCuPc on ITO from different template sphere diameters were used. The second infiltration by the acceptor material was performed from solution with either PCBM or C60 dissolved in 1,2-dichlorobenzene (5 to 20 mg mL-1_{) under inert atmosphere. The acceptor material solutions were prepared and} stirred under N2 for at least 24 hours and filtered (0.2 µm) to prior use. The infiltration of the 3D open-cellular thin films was performed by different methods. A first method employed one simple dipping step in a dichlorobenzene solution of C60 (20 mg mL-1). A second method involved drop-casting (20 mg mL-1 C60 or 10 mg mL-1 PCBM) with a penetration time of 2 minutes followed by spin-coating at 700 rpm. The third method was based on drop-casting of solution (5 mg mL-1 PCBM) followed by controlled drying. After initial solvent evaporation at room temperature for 10 minutes the films were slowly heated up and dried at 120 ˚C for 20 minutes. In order to complete the device an additional C60 buffer layer of 40 nm followed by a BCP layer of 7 nm and a thicker Al electrode of ca. 200 nm thickness were grown on top of the generated composite structures by vacuum deposition. Thick Al electrodes are deposited to flatten surface inhomogeneities to provide sufficient contact coverage. The device top contact area is 0.06 cm2. Film and device analysis included J-V device characterisation as well as SEM for structure and morphology analysis.

In addition to the devices presented in Chapter 4, planar reference devices with solution processed acceptor layer were fabricated using optimised PTEBS (5 mg mL-1) layers as a basis for C60 and PCBM solution deposition. C60 and PCBM solutions (20 mg mL-1) were spin coated onto the pre-deposited PTEBS film at 2000 rpm for 2 minutes followed by drying at room temperature and elevated temperature of 120 ˚C for 20 minutes under inert atmosphere. The devices were completed by the vacuum deposition of 7 nm of BCP and Al. The fabrication of heat and solvent treated reference devices (I1) and (I2) it is reported in Chapter 4.

Chapter 5: 3D interdigitated organic D-A composite structures and OPV devices