Fullerenes have been (and still are) very successfully used as electron accepting materials in or-ganic photovoltaics. This is mainly attributed to the spherical shape of these molecules and their fully conjugated structure,18,19 as these properties result in a strong electron accepting ability as well as an effective three-dimensional system. This is entropically favourable for CT dissoci-ation (cf. chapter 3.3) and gives rise to isotropic electron transport ability with high mobility in the order of 10−2 cmVs2.84,110,204,255 The spherical aromaticity is also regarded to facilitate elec-tron delocalization at D/A interfaces,131,171,174 yet there is still an ongoing discussion whether electron delocalization actually occurs in a fullerene aggregate and if so whether this delocaliza-tion helps in the CT dissociadelocaliza-tion process. This is discussed in detail in chapters 3.2, 8.2.1 and 9. Due to their isotropic interaction capability, fullerenes in addition have the advantage that there is no special need for a specific orientation with respect to the donor, so that they are com-patible and efficient with a wide range of donor materials (be they polymers or small molecules).
A common drawback ofC60 and its derivatives in solar cell applications is their limited absorp-tion in the VIS-range. The singlet energy ofC60is located at 1.85 eV, but significant absorption only sets in around 2.25 eV.46,256–258Unfortunately, the absorption range is only hardly tunable, as the core structure of the molecule is always the same, i.e. C60 fullerene, and side-chains have only small influence on the conjugation of theC60core and thus on its electronic structure that in turn determines the optical properties.3 This is why a lot of effort was put into the develop-ment of low bandgap polymers to cover a larger portion of the red part of the electromagnetic spectrum and complement the absorption ofC60 and its derivatives.20–24,49 To some extend the issue of weak absorption in the VIS range could be also addressed by usingC70or its derivatives instead of C60 based compounds, as C70 features a stronger absorption around 2.5 eV (figure 5.1(a)).257,259–261 Yet, C70 is more difficult to obtain and thus much more expensive making it less relevant to a possible commercial application.44,262 Thus, some groups suggested to use mixtures of C60 and C70 because these are the natural result of fullerene synthesis and would render additional separation steps unnecessary.44,262
Apart from this drawback in absorption coefficient,C60tends to crystallize and form aggregates.
23,25,30,31,35,36,263 This is on the one hand a desired property, as a certain domain size of the acceptor phase is favourable in terms of CT dissociation and charge transport and too strong intermixing would increase charge carrier recombination during the extraction process. On the other hand, a morphology that is optimized concerning domain sizes and percolation is usually not thermodynamically stable and phase separation takes place under thermal stress like it is the case under operating conditions.23,35,36 As already noted in chapter 2, the driving force for this demixing is either crystallization of the polymeric donor, thereby expelling fullerenes from
the mixed phase, or Oswald ripening1.23,35,36,60 Over the last years a lot of effort has been and still is put into understanding the formation and temporal evolution of specific morphologies as well as the underlying thermodynamics.25–31,57–59 One possible approach to stabilize blend morphologies in fullerene based devices was found to be the deliberate formation of a densely linked network via chemical cross-linking. This mechanism is addressed in chapter 7.
a)
b)
c)
Figure 5.1.: (a) Absorption spectra of C60 (– –) and C70-films (—), as well as dipole moment as function of energy forC60 (–•–) andC70 (–∆–) measured at 77K. Reprinted with permission from Kazaoui et al., Phys. Rev. B58, 7689 (1998). Copyright ©(1998) by the American Physical Society. (b) Absorption spectrum of a C60 film measured at 77K (—) and Electroabsorption spectra of C60 films measured at 300K (–∆–) and 77K (–•–). Reprinted with permission from Kazaoui et al., Phys. Rev. B 58, 7689 (1998). Copyright ©(1998) by the American Physical Society. (c) Illustration of the autoionization mechansim of CT states in neat C60 films. IE and EA are the ionization potential and the electron affinity ofC60, respectively. r0 denotes the intrapair distance after thermalization. The latter process only occurs for excitation energies higher than the electrical gap. Adapted with permission from Hahn et al., J. Phys. Chem. C, 2016, 120 (43), pp 25083–25091. Copyright ©(2016) American Chemical Society.
Another possibility to suppress the aggregation tendency ofC60is to add bulky side chains to the molecule that prevent efficient clustering and mutual alignment. Beside this effect, additional side chains are usually designed to increase the solubility of the fullerene, as pure C60 only fea-tures a very low solubility in common organic solvents used in device processing264,265. Several
1 Ostwald ripening is a thermodynamically-driven process that results in the growth of larger particles at the cost of smaller ones as a result of the tendency to minimize the surface area and achieve an energetically more favourable state.60
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derivatives with solubilizing side-chains were developed,32,33,179,266with the most prominent one being PCBM.266,267 Common examples of higher adduct fullerenes are ICBA or ICTA.32,179
33,34 These fullerene derivatives enabled organic photovoltaic devices with VOC exceeding 1 V, yet at the cost ofjsc and thus charge generation efficiency.33,34,179 The increase in Voc may be attributed to the lower electron affinity (EA) as compared to C60 or monoadduct derivatives.
A concomitant decrease in the EA- as well as ionization energy (IE)-difference between donor and acceptor results in increased exciton recycling to donor or acceptor and concomitantly in-efficient CT dissociation as shown by Hoke et al. and Faist et al.33,34 Furthermore, the addition of side chains decreases electron mobility which in addition impedes efficient charge extraction and increases recombination.32,33 According to Faist et al., for these systems the donor has to be chosen carefully to compensate the drawbacks associated with higher adduct fullerenes.33 Apart from aggregation, fullerenes may under certain conditions, especially photoexcitation and electrical bias, also form covalently bound dimers.268–273 This mechanism was thoroughly inves-tigated by Heumüller et al.268and found to represent a major contribution to the often observed burn-in loss as a result of a gradual reduction in short-circuit current Jsc in organic solar cell systems based on fullerene acceptors.274–277 According to their study, the light-induced dimer-ization process of PCBM depends on both film morphology and electrical bias of the respective organic solar cell. Concerning film morphology they found that a higher degree in fullerene crystallinity in neat fullerene phases as well as strong intermixing of fullerenes with amorphous polymeric donors inhibits or even prevents the dimerization reaction. This was attributed to geometric constraints in the first case and a lower probability of two fullerenes being in close proximity and aligned in the right way in the second case.278 Due to the observed clear correla-tion of PL quenching as well as changes in absorpcorrela-tion with fullerene dimerizacorrela-tion, they suggest the reaction mechanism to be based on the light induced presence of (triplet-) excitons.279 Meanwhile, it has been shown that fullerenes may not only be efficient acceptors in combination with another donor component, but that neat phases of fullerenes are able to contribute to the photogeneration process via autoionization of CT states in the bulk of C60. The principal existence of exited states with CT character at energies above 2.2 eV has been already investi-gated before by Könenkamp et al. and Kazaoui et al. by spectroscopic means.256,258 Kazaoui et al. also performed electroabsorption measurements in order to assess polarizability ∆p and dipole momentµof the involved states and found that neatC60 already features a pronounced, extended CT-state at 2.43 eV that is characterized by ∆p = 880·10−24cm3,µ = 22.9 D and a fraction of transferred charge of 0.48 (figure 5.1(a) and (b)). An in-depth study about the role of such CT states in the charge generation process in organic solar cells as well as the under-lying mechanism of their autoionization was finally conducted by Hahn et al.46. In their work, they performed measurements on single layer devices of C60 and PCBM and investigated the photogeneration as a function of the internal electric field. They found that significant e-h-pair dissociation via autoionization of charge transfer states starts at about 2.25 eV, i.e. about 0.4 eV above the S1 state of C60. This process can be described and analysed in the framework of Onsager’s 1938 theory, yielding an initial intrapair distance of about 2 nm at the onset energy of 2.25 eV. At higher excitation energy, the resulting e-h-pairs are slightly more extended and gen-erated incoherently via thermalization from higher energy states (figure 5.1(c)). Furthermore, Hahn et al. found that the coupling between charge transfer and charge transporting states is by
a factor of three lower in PCBM than in C60. This was inferred from the fraction of generated charges in relation to the number of photoexcitations at a certain electric field. Apart from all this, their works also indirectly shows thatC60 may also fairly well transport holes, albeit with a somewhat lower mobility in the order of 10−4 cmVs2 as shown by Könenkamp et al.256.
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