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Morphological stability

Roar Søndergaard

A major breakthrough within PSC was made with the introduction of the bulk heterojunction active layer. With the bulk heterojunction it is possible to realize a much greater interfacial area between the donor and the acceptor material than for the bilayer structureDOI:10.1126/science.270.5243.1789DOI:10.1038/376498a0, see Figure 1. The distribution of the donor and acceptor material in the three dimensional bulk defines the morphology. Since the lifetime and the diffusion length of excitons in organic materials is limited, the increased interfacial area, which in ideal case represent a bi-contiguous and interpenetrating network of both the donor and the acceptor material throughout the active layer, improves the generation of free charge carriers leading to an improved device performance compared to the bilayer heterojunction.DOI:10.1016/j.progpolymsci.2013.07.010

Figure 1. Schematic illustration of the active layer in a PSC showing from the left: a bilayer heterojunction and a bulk heterojunction structure.

Generally high performance PSC devices rely on highly optimized bulk heterojunction morphology in order to obtain extraordinary charge separation and charge transport properties. Several different factors and processing methods have shown to affect the final active layer morphology, such as: donor-acceptor blend ratio, regionregularity, molecular weight, processing solvent, additive and annealing conditions.DOI:10.1002/adma.200802854 But the optimized morphology is in general not in thermodynamic equilibrium and will therefore evolve over time when exposed to elevated temperatures. This can be observed as the growth of PCBM acceptor crystallites leading to destruction of the optimal morphology causing a decrease in the device performance, see Figure 2.

Figure 2. Optical micrographs (200 x 260 mm2) of P3HT:PCBM films before (left) and after annealing at 140 ˚C for 24 h (right).

In order to preserve the high device performance a stabilization of the optimal morphology is requested. Several approaches have been applied in order to immobilize the materials and hereby inhibit the growth of PCBM domains.DOI:10.1002/macp.201300076 Several approaches have focused on functionalization of the polymer side-chains by incorporating different groups that can be activated after processing of the active layer. One such functional group is tertiary esters that by thermal or acid treatment of the processed layer can be cleaved off. The residual carboxylic acid groups then forms hydrogen bonds giving a rigid structure. An example of this is the polymer poly[3-(2-methyl-2-hexyl)-oxycarbonylbithiophene] (P3MHOCT) that has tertiary ester side chains. When heated to approximately 200 ˚C the alkyl part of the tertiary ester is cleaved of as a volatile alkene leaving the polymer P3CT with the corresponding carboxylic acid groups. Further heating to around 300 ˚C decarboxylation will occur as CO2 is eliminated giving the pure polythiophene polymer, PT, see Figure 3. Both P3CT and PT have shown enhanced morphological stability as well as improved device stability because of the absence of side chains which are known to be initiators to several degradation pathways.DOI:10.1016/j.solmat.2007.11.008DOI:10.1002/adma.200601828

Figure 3. Thermal cleaving of the tertiary ester side chains.DOI:10.1002/macp.201300076.

Because of the elevated temperature needed for the thermo-cleavage of side chains the pathway is not suited for the use of flexible plastic substrates which can normally only support up to around 150 °C. As an alternative to the traditional heating method using an oven, which heats both the substrate and the tertiary polymer film, Helgesen et al.DOI:10.1039/C2PY20429K have applied light induced heat generation to initiate the thermo-cleavage. The advantage of using light lies in the fact that thin films of light absorbing materials can be selectively heated to high temperature using short-duration pulses with high power density that are rapid enough to avoid heat buildup on the substrate hereby leaving the thermally fragile substrates unaffected.

Another pathway to locking the morphology is to incorporate a cross-linkable group like azide,DOI:10.1021/cm1018184DOI:10.1021/cm203058pDOI:10.1021/ma3001725 alkyl-bromide,DOI:10.1002/adfm.200900043DOI:10.1002/adma.201004743 vinyl,DOI:10.1021/ja100236bDOI:10.1016/j.orgel.2010.11.021 and oxetaneDOI:10.1039/c1jm10195a into the polymer. Only a small amount of active groups are needed in the final polymer in order to obtain effective crosslinking. The cross-linkable groups can be activated after the processing of the active layer by applying UV-light that initiates the crosslinking between the active specie and a neighboring group in the bulk heterojunction.DOI:10.1039/c2jm34284g The continuous interconnection of the polymer strands leads to an immobilization of the bulk and inhibits further growth of domains.

Figure 4. P3HT with incorporated alkyl-bromide side chains that up on activation with UV-light crosslink and creates a more stable morphology.DOI:10.1002/adfm.200900043

The above mentioned approaches all require additional synthetic steps in order to incorporate the active groups and quite often it deteriorate the optimum morphology obtained with the champion material which results in lower device performance. An elegant approach to circumvent this has been explored by Helgesen et al. who applied electron irradiation of both the active layer and as well as fully encapsulated PSC devices resulting in enhanced morphological stability and solvent resistance.DOI:10.1002/adfm.200900043 The electron irradiation generates active sites on the materials, without the need of active groups, and allows for crosslinking of not only the active layer but the entire device stack. The method is therefore generic for all polymer materials once the optimized processing conditions have been obtained it is a direct and non-invasive process.

Figure 5. Schematic drawing of the beam electron accelerator equipment.DOI:10.1002/app.40795

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