Barrier-Free Charge Separation Enabled by Electronic Polarization in High-Efficiency Non-fullerene Organic Solar Cells

Abstract

The separation of charge-transfer states into free charges at the donor/acceptor (D/A) interfaces plays a central role in organic solar cells (OSCs). Because of strong Coulomb attraction, the separation mechanisms are elusive, particularly for the high-efficiency nonfullerene (NF) OSCs with low exciton-dissociation driving forces. Here, we demonstrate that the Coulomb barriers can be substantially overcome by electronic polarization for OSCs based on a series of A-D-A acceptors (ITIC, IT-4F, and Y6). In contrast to fullerene-based D/A heterojunctions, the polarization energies for both donor holes and acceptor electrons are remarkably increased from the interfaces to pure regions in the NF heterojunctions because of strong stabilization on electrons but destabilization on holes by electrostatic interactions in the A-D-A acceptors. In particular, upon incorporation of fluorine substituents and electron-poor cores into ITIC, the increased polarization energies can completely compensate for the Coulomb attraction in the IT-4F-and Y6-based heterojunctions, leading to barrierless charge separation.

The Path to 20% Power Conversion Efficiencies in Nonfullerene Acceptor Organic Solar Cells

“…Thus, much effort has been dedicated to studying the low-offset hole transfer process from the NFA to the D, which appears to be significantly slower than the charge transfer in FA OSCs; in FA OSCs, charge transfer and separation into FCs is typically completed on sub-ps timescales, [87,93,94,103,104] while hole transfer alone can take tens of ps in low-offset NFA systems. [29,109,111,114,[116][117][118][119] In order to explain how hole transfer and CT state dissociation can occur in NFA systems with apparently minimal driving energy, the involvement of intermolecular NFA excited states with a reduced binding energy, [118][119][120] as well as interfacial electrostatics that can drive the charge transfer, [121][122][123][124][125] have both been invoked in addition to entropic factors. [89,90,[95][96][97] In our recent study, we investigated the effect of the driving energy on the hole transfer process for a series of NFAs blended with the polymer PTB7-Th.…”

Y6 and its derivatives: molecular design and physical mechanism

“…The large polarization energies could be attributed to the fluorination of end groups and the incorporation of an electron-deficient A′ core in Y6 (Fig. 1c ) [ 12 ]. Neher et al.…”

A History and Perspective of Non‐Fullerene Electron Acceptors for Organic Solar Cells

“…Alternatively, Tu et al proposed that barrier-free charge separation in Y6 and related compounds is enabled by the electronic polarization which stabilizes the separated electron in the neat Y6 phase. [220] This is because the polarization energy is larger for the charge separated state where the electron and hole reside in their respective pure domains, compared to the CT state at the DA interface. For Y6 in combination with a prototypical thiophene-based oligomer donor, they calculated the difference between the polarization energy in the bulk and at the interface to be as high as 490 meV, overcompensating the calculated CT binding energy of 249 meV.…”