Efficient Charge Generation via Hole Transfer in Dilute Organic Donor–Fullerene Blends

Abstract

Efficient organic photovoltaics (OPVs) require broadband charge photogeneration with near-unity quantum yield. This can only be achieved by exploiting all pathways that generate charge. Electron transfer from organic donors to acceptors has been well-studied and is considered the primary path to charge photogeneration in OPVs. In contrast, much less is known about the hole transfer pathway. Here we study charge photogeneration in an archetypical system comprising tetraphenyldibenzoperiflanthene:C70 blends using our recently developed multispectral twodimensional electronic spectroscopy (M-2DES), supported by time-dependent density functional theory and fully quantum-mechanical Fermi's golden rule rate calculations. Our approach identifies in real time two rapid charge transfer pathways that are confirmed through computational analysis. Surprisingly, we find that both electron and hole transfer occur with comparable rates and efficiencies, facilitated by donor-acceptor electronic interactions. Our results highlight the importance of the hole transfer pathway for optimizing the efficiency of OPV devices employing small-molecule heterojunctions.

Mechanistic Study of Charge Separation in a Nonfullerene Organic Donor–Acceptor Blend Using Multispectral Multidimensional Spectroscopy

“…We find that the broadband PA decays biexponentially with time constants of 2.2 ± 0.8 ps (52%) and 31 ± 8 ps (48%). The short lifetime of the PA is consistent with it arising from BPPs rather than long-lived polarons which typically have lifetimes of ns . Furthermore, we find that the broadband PA is not observed in the 2DEV of the ITIC solution as shown in Figure b, suggesting that the bound polaron pairs are intermolecular species.…”

“…The short lifetime of the PA is consistent with it arising from BPPs rather than longlived polarons which typically have lifetimes of ns. 52 Furthermore, we find that the broadband PA is not observed in the 2DEV of the ITIC solution as shown in Figure 2b, suggesting that the bound polaron pairs are intermolecular species. We note that the CN peak in solution appears at a lower frequency than the neat ITIC film.…”

Screened Range-Separated Hybrid Functional with Polarizable Continuum Model Overcomes Challenges in Describing Triplet Excitations in the Condensed Phase Using TDDFT

“…In fact, we confirm that the J TT measure is minimal at the optimal set of functional parameters determined by J OT , where only rubrene appears to present an insignificant deviation of this trend (see SI Table S5). As a result the success in calculating the triplet energies is achieved in addition to the high-quality performance in addressing singlet excitations including those with a CT character. ,− …”

Electric Field Facilitating Hole Transfer in Non-Fullerene Organic Solar Cells with a Negative HOMO Offset

“…In dilute donor devices, due to the fact that the incident photons are mainly absorbed in the acceptor phase, the photocurrent is dominated by the contribution via the hole transfer process from acceptor to donor, whereas the contribution from electron transfer will be decreased accordingly. This feature makes the dilute donor concept a better model for studying the hole transfer process …”

“…This feature makes the dilute donor concept a better model for studying the hole transfer process. 34 Building upon the above advantages, the OSCs are fabricated on the basis of D:A blends in the ratio of 1:9 with the same conventional device geometry. The corresponding J− V and EQE curves, as well as the detailed photovoltaic performance, are shown in Figure 4 and Table 2.…”