Absorption Tails of Donor:C₆₀ Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation

Abstract

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D-A complexes occur at photon energies below the optical gaps of both the donors and the C acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

Molecular vibrations reduce the maximum achievable photovoltage in organic solar cells

“…In order to study the sub-gap lineshape of the CT EDOS in organic blends, we need to describe the effect of low-frequency vibrations in microscopic detail 29 . Here, a convenient simplification is to describe the skeletal intra-molecular modes (highfrequency vibrations with typical mode energies ħω hf > 125 meV) by their vibrational ground states because of the negligibly small population of these modes (k B T ≪ ħω hf ) at room temperature and below.…”

Absence of Charge Transfer State Enables Very Low VOC Losses in SWCNT:Fullerene Solar Cells

“…Toward lower energies, EQE(E) typically decreases faster than Φ BB,300K (E) rises. [15,33] The key parameter in our work is that we do not observe a CT state for (6,5) SWCNT:PC 70 BM. Accordingly, we calculate the thermodynamic voltage limit of V OC,rad = 0.851 V (see Table 2).…”

“…This relationship is explained via a simple energy gap law in which the nonradiative decay is mediated via high-frequency vibrational modes (160 meV for rubrene [33] ). Benduhn and coworkers relate the ΔV OC,nonrad losses to a strong vibrational coupling of the CT excited state to the ground state, causing a high rate of nonradiative recombination and therefore high ΔV OC,nonrad losses.…”

Full quantum treatment of charge dynamics in amorphous molecular semiconductors

“…In a recent study of low-energy tails of external quantum efficiency (EQE) spectra of OPV cells of C 60 mixed with different donor molecules, a failure of the application of Eq. (1) in describing the EQE tails was attributed to quantum-mechanical freeze-out of high-energy phonons [13]. These two examples point to the importance of considering the quantum character of phonons in studying charge dynamics.…”