Short and long-range electron transfer compete to determine free-charge yield in organic semiconductors

Abstract

Marcus theory explains photoinduced electron transfer from donor molecules to a fullerene host when all microstates are included, and formation of free charge competes with charge-transfer states.

Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems

“…We estimate the photoluminescence quenching by analyzing the difference between the overlapping spectra of the ZnPc:PS film with the ZnPc:PCBM film at 730 nm and find quenching efficiency of >90%. Applying similar methodology to the whole series of donors in this study shows the same result, except for SiPcBu, which also exhibits the lowest driving force for electron transfer (see SI Figure 1.5 and SI k FC and k CT values are calculated using the DRET model (eq 3) integrated over all possible microstates as is done in Carr et al 12 k FRET is determined through a series of experiments and calculations detailed in SI Section 5. k EET is held constant at a lower limit necessary to account for the discrepancy in ϕ FC from Figure 2. centered at 900 nm, that we attribute to CT state emission in this sample.…”

“…<2 nm for SiPcBu), and even though the absorption of neat PCBM is relatively weak in that region, the spectral overlap is sufficient to provide energy-transfer rate constants on the order of 1 × 10 10 to 1 × 10 11 s −1 (calculations for k FRET and simulated k EET provided in Consistent with our previous work, we use absorption and emission spectroscopy to provide evidence of isolated sensitization of the donor into the PCBM accepting host. 12 These data demonstrate spectral features that resemble molecular phthalocyanine and squaraine absorption, rather than typical aggregated features, such as dramatic red-shifting and/or broadening, which are absent here. 12,20−23 However, all of the sensitizer absorption peaks do broaden and red-shift to varying degrees in the PCBM host relative to polystyrene.…”

“…The E ex value for PCBM is estimated from absorption/PL spectra in SI Figure 1 contributes to the yield of free charges as we have observed in the past 15,35,36 and as described in our distributed range electron-transfer model. 12 Second, when the "blue sensitizers" are selectively excited, the yield of free carriers is dramatically suppressed relative to our prior work selectively exciting "red sensitizers" in the PCBM host. Third, exciting the "blue sensitizer" films at a wavelength that primarily excites the PCBM host recovers the peak yield, if not the exact shape of the curve obtained for the "red sensitizers".…”

“…3 nm) with small reorganization energy. In each case we use a Marcus rate expression and an integration over the available density of states to calculate the overall rate constants . Crucially, the CT states are posited to be too tightly bound to further dissociate and are not an intermediate to the FC states.…”

Electronic Mechanism of In Situ Inversion of Rectification Polarity in Supramolecular Engineered Monolayer

“…Indeed, it made it possible to achieve in situ inversion of polarity in molecular rectification. (iii) Our work adds to the repertoire of a handful of examples of Marcus inverted charge transport in molecular junctions, reported by Nijhuis, , and other solid-state devices. − (iv) The response of MO energy levels to an external electric field, as shown in this work, may provide insights into various research areas at the interface with electrochemistry, including electrosynthesis, electrocatalysis, electrodetection, and electrochemical spectroscopy …”