As already addressed shortly in chapter 2, morphologies in organic solar cells tend to be thermo-dynamically unstable. This results in phase separation in bulk-heterojunction structures or the intermixing of adjacent layers in stacked architectures over time due to diffusion of low molecular weight compounds like fullerene acceptors or dopants. Crosslinking has proven to be a promising approach to tackle these issues and a lot of investigations on the potential use of crosslinkable materials in organic solar cells have emerged in the last decade.23,35,36 In general, it can be de-fined as covalently linking molecules applying various stimuli like temperature, pressure or light and often using additional compounds as agents. The linkage is usually mediated by specific functional groups attached to the molecules that are to be crosslinked.23,35,36 Transferring this concept to organic solar cells allows ”freezing” the initial morphology of a blend structure after optimization during processing and concomitantly suppressing the tendency of small molecules to diffuse within or into the active material. The latter has been nicely addressed for example in a work by Fischer et al.,315 where the diffusion coefficient has been shown to be reduced by three orders of magnitude as compared to the non-crosslinked reference in the case where every repeat unit in a model polyfluorene derivative carried a crosslinkable group. Furthermore, crosslinking renders the respective layers insoluble. In general, it has been shown that it is not absolutely necessary to have a crosslinkable group attached to every side chain, but smaller amounts are already enough to obtain a densely linked and insoluble network.329–333 The aspect of insolubility additionally makes crosslinking a promising alternative to the use of orthogonal solvents334 or inorganic blocking layers49,335,336 when fabricating multilayer structures via so-lution processing315,337–347 and is of special importance for the model bilayer systems that are frequently used throughout this thesis.
In terms of organic solar cells it is possible to crosslink either donor or acceptor to form a stable matrix for the respective other component or to directly link them.23,35,36 Here we will focus on the donor-to-donor linkage as this is the method utilized in this thesis. In the majority of devices, polymers are used as electron donating species. In this case, functional groups are usually attached to the solubilizing side chains of the donor in order to retain the electronic properties of the polymer and not severely disrupt the backbone conjugation. The general principle of this crosslinking approach is illustrated schematically in figure 7.1(a). Possible functional groups that may and have been applied in organic solar cells are bromine, acrylate, oxetane, azide and vinyl groups.23,35,36 In the framework of this thesis two of these concepts were used: acrylate and oxetane (figure 7.1(b)).
Crosslinking of acrylate groups takes place via a free radical mechanism.313,348,349 This process usually requires the addition of a (photo-)initiator, which is split into fragments with unpaired electrons upon exposure to UV-light and heat, thereby creating the primary (free) radicals
which then transfer to the polymer chain forming secondary propagating radicals. These are then the reactive species mediating the crosslinking between the acrylate containing side chains of individual polymers. A possible drawback in terms of application to optoelectronic devices is that the residues of the photoinitiator decomposition remain in the device and may act traps for charge carrier transport or as exciton quenching sites.23,350,351 This can be circumvented by mere thermally activated crosslinking with accompanying illumination. Both aspects, thermally activated crosslinking and the influence on charge transport when adding photoinitiators are addressed in my publication about the impact of crosslinking on charge carrier mobility (chapters 8.2.3 and 11). Nevertheless, crosslinking via acrylate groups is restricted to the use in layered, sequentially deposited architectures when intended to be used in the presence of compounds with strong electron scavanging ability such as fullerenes352–355 because these will efficiently terminate the radical mediated reaction thereby preventing the formation of a network.
PCDTBTOx PF2/6-A-n:m
(b)
Not crosslinked crosslinked
hν/
initiator/
heat
(a)
Figure 7.1.: (a) Schematic of the donor-to-donor crosslinking of a conjugated polymer. The process may be initiated via a (photo)initiator, heat and/or exposure to UV-lighthνresulting in a densely linked and insoluble network due to the formation of covalent bonds between the chains.
(b) Structure formulas of the two crosslinkable polymers used in this thesis. The respective functional groups used for crosslinking (left: acrylate, right: oxetane) are highlighted in red.
To allow for crosslinking in the presence of fullerenes, e.g. in a bulk heterojunction, another mechanism has to be applied. In this respect, the use of oxetane groups instead of acrylates is a promising approach, because oxetane mediated crosslinking proceeds via a cationic ring opening process (CROP).356 The protons needed for the initiation are typically provided from so called photoacid generators (PAGs).351,357 However, the interplay between a PAG and the oxetane-functionalized compound may lead to a number of unwanted side reactions. Among others, photoinduced electron transfer from the PAG to the semiconductor additionally re-sults in unintentional doping of the crosslinked layer, which in turn could deteriorate device
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performance.351 Also, residuals of the acid and the presence of counterions can cause problems in this respect.23,35,358 A very good alternative to the use of PAG is exposing the crosslinkable sample at elevated temperatures to trifluoroacetic acid (TFA) vapour, which is a very efficient initiator for the CROP process.359 TFA has the advantage of being very volatile with a boil-ing point of 72 ℃ and a high vapour pressure (97.5 mmHg @20 °C) so that residual acid may be easily removed after processing.360 In our case, we applied a combined thermal and vacuum treatment (90 ℃, 90 min, 10−4mbar) with a subsequent exposure to high vacuum (5·10−7mbar) for several hours prior to deposition of additional layers via evaporation. Again it was found, that even crosslinking via oxetane groups may be achieved by mere thermal activation,312 but this requires to expose the sample to heat for several tens of hours in order ot achieve a good crosslinking density.
A noteworthy example of crosslinking in an organic solar cell without addition of any further initiator has been shown by Peters et al. as well as Tournebize et al.274,313,314 They found that in a bulk heterjunction of PCDTBT with PCBM a special mechanism is at work that results in an impressive long-term stability of the respective devices with expected average lifetimes of up to seven years while still retaining 4 % PCE.274,314 The underlying process leads to the formation of actual covalent crosslinks between the polymer backbone and the fullerene, which is preceded by the photoinduced scission of the bond between the N-atom of the carbazole units of PCDTBT and the attached alkyl side chain.23,313
As a side remark, it shall be mentioned that the concept of crosslinking may also be used to stabilize nanostructures fabricated by nanoimprinting.23 This could be used to obtain a well-controlled morphology with defined phase intercalation and percolation pathways for organic solar cells.361–363 Unfortunately, Pfadler et al.364 have shown that the advantage of a larger interfacial area is finally outbalanced by increased recombination so that no net improvement of device performance could be achieved. For a more in-depth review about crosslinking in organic solar cells the reader is referred to chapter A in the appendix.