Wannier-Like Delocalized Exciton Generation in C₆₀ Fullerene Clusters: A Density Functional Theory Study

Abstract

C 60 fullerene is widely used in organic devices; however, there are still unaccountable phenomena, such as the strong light absorption around 2.8 eV of C 60 films and favorable device performance of organic photovoltaics (OPVs) using C 60rich bulk heterojunctions. We studied the excited states of C 60 fullerene clusters using large-scale time-dependent density functional theory taking thermal vibrations into account. A strong absorption peak around 2.8 eV appeared because of aggregation and we found that Wannier-like delocalized excitons spread over multiple molecules were generated in this energy region. This is contrary to the accepted theory that only Frenkel or charge transfer (CT) excitons are generated in organic materials. It is considered that the delocalized excitons with energies greater than electrical gap (E g ) become CT excitons after thermalization whereas those with energies lower than E g become either CT excitons by obtaining thermal energy or Frenkel excitons after thermalization. In the exciton dissociation process, there is an essential channel in which the delocalized excitons are generated first and then subsequently become CT excitons. The delocalized exciton generation can explain both the strong absorption around 2.8 eV and enhanced OPV performance using C 60 -rich bulk heterojunctions.

Electronic Spectra of C60 Films Using Screened Range Separated Hybrid Functionals

“…Using RSH-PCM (Figure a), we found the first high absorption peak at 4.19 eV with an OS of 1.16 and in good agreement with earlier calculated results that are based on a similar functional . This result, however, presents a significant overestimation by 0.6 eV over the measured low-lying spectral peak of single C 60 at 3.6 eV.…”

“…To better understand the excitonic nature of the low-energy states, we consider the calculated excitonic binding energy, E bind , of the different absorbing states following the usual definition, ,, where E bind = E gap – E opt . Here, E opt is the excitation energy reported above in Tables and ; E gap is the energy difference between IP and EA.…”

Impact of packing arrangement on the optical properties of C60 cluster aggregates

“…Apparently, the significant presence of induced charge density distributions in the gap regions indicates that TCMs corresponding to these peaks represent intermolecular charge transfer excitons. 62–64 This is clearly visible from the TCM plots corresponding to the different photoabsorption peaks since one finds mixed red and blue spots on the TCM plane. Notably, there is a systematic reduction in the number of red and blue spots on the TCM plane by moving from the low energy peak centered at around 1.74 eV to the high energy peak centered at around 4.47 eV.…”

Direct observation and evolution of electronic coupling between organic semiconductors

“…A consequence of this coupling is the delocalization of the corresponding electronic wave functions under excitation (or excited state wave functions) in C60, -a well-known phenomenon in C60 [30][31][32][33], -and analogous to band formation in inorganic semiconductors or J-type interaction [34,35] causing a gradual red-shift (along with polarization effects [36,37]) of the primary transitions with deposition. If there were no electronic coupling between the molecules, and consequently no delocalization, the response δρ should be the same for every deposition resulting in the same δρ for every δd.…”

“…Such variation of δρ with thickness shows that the optoelectronic properties of C60 or any such interacting systems is not uniform along the z direction. Moreover, because the wave functions of C60 delocalize, the optical response δρ of a layer δd deposited at time t cannot be solely assigned to that layer δd [30][31][32][33]. The C60 molecules at the top and bottom of the film will be experiencing different environments, and thus coupling, compared to the molecule in the middle.…”