Efficient Exciton Diffusion in Organic Bilayer Heterojunctions with Nonfullerene Small Molecular Acceptors

Abstract

Solution-processed bilayer organic solar cells (OSCs) with high performance are demonstrated for nonfullerene small molecular acceptors (NFAs). Unlike fullerene acceptors, NFAs show significant spectral overlap between their absorption and the photoluminescence (PL) of a polymer donor, which makes the design of an efficient exciton-harvesting bilayer heterojunction possible. Efficient exciton diffusion in the organic bilayer heterojunction is realized by long-range energy transfer between a polymer donor and NFAs. We observed efficient exciton diffusion from the polymer/ NFA bilayer heterojunctions via thickness-dependent PL quenching and timeresolved PL measurements. Despite the strongly reduced donor−acceptor interface area, a substantial density of charge-transfer states is observed for the polymer/NFA bilayer heterojunctions by electroluminescence measurements. Overall, polymer/NFA bilayer heterojunction OSCs demonstrate a power conversion efficiency of 9%−10%, which is comparable to the photovoltaic performance of bulk heterojunction OSCs, with the additional advantage of simplified microstructure formation.

Challenges to the Success of Commercial Organic Photovoltaic Products

“…Recently, with the development of NFAs, the bilayer architecture may be viable once more, for high efficiency OPVs. [ 116 ] The current trend in the literature is using NFAs with complementary absorption to the donor polymers for high photocurrent. This leads to a large spectral overlap between the NFA UV–vis and the photoluminescence (PL) spectra of the polymer donor and long‐range Förster resonant energy transfer (FRET), which allows excitons to efficiently diffuse and separate at the donor:acceptor interface of a bilayer structure.…”

“…It has been demonstrated that IDIC and ITIC when stacked on top of a PM6 donor polymer in the bilayer configuration, feature high L d and PCE of 10%, due to an almost complete exciton quenching in the donor polymer due to a long‐range energy transfer between the polymer donor and NFAs. [ 116 ] Further investigation of this configuration is crucial, not only for simple processing in device fabrication, without the necessity of tedious donor:acceptor blend morphology optimization, but also the lower diffusion of the donor and acceptor should prevent demixing, thereby improving the morphological stability in such devices.…”

Synergistic Effect of Dielectric Property and Energy Transfer on Charge Separation in Non‐Fullerene‐Based Solar Cells

“…Energy transfer from polymer donor to nonfullerene acceptor would occur in the bulk heterojunction non-fullerene-based devices due to their bandgap difference. [34] Optical absorption and photoluminescence (PL) properties of the pristine PM6 film, pristine non-fullerene acceptor films (ITIC,I T-4F,Y 6, and IEICO), and corresponding blend films are shown in Figure 4. Thee mission band of PM6 ranged from 650 to 1000 nm, partially overlapping with the absorption spectra of the selected ITIC acceptor.D ifferently,i tn early completely overlapped with the absorption spectra of IT-4F,Y6, and IEICO acceptors.As shown in PL of PM6 and corresponding blend films at the PM6 excitation wavelength of 560 nm, the PL of PM6 was quenched after blending with the non-fullerene acceptors, and the quenched wavelength range was consistent with the Angewandte Chemie overlapped range between the emission spectra of PM6 and the absorption spectra of selected non-fullerene acceptors.It is to say that the PL intensity of PM6 in ITIC based blend film was partly quenched while the PL intensity of PM6 was completely quenched upon blended with IT-4F,Y 6, and IEICO.I na ddition, the PL spectra of blend films at PM6s excitation wavelength exhibited different peak positions from those of PM6 films.S till, they were the same as those of the blend films at acceptors excitation wavelength.…”

Synergistic Effect of Dielectric Property and Energy Transfer on Charge Separation in Non‐Fullerene‐Based Solar Cells

“…Energy transfer from polymer donor to nonfullerene acceptor would occur in the bulk heterojunction non-fullerene-based devices due to their bandgap difference. [34] Optical absorption and photoluminescence (PL) properties of the pristine PM6 film, pristine non-fullerene acceptor films (ITIC,I T-4F,Y 6, and IEICO), and corresponding blend films are shown in Figure 4. Thee mission band of PM6 ranged from 650 to 1000 nm, partially overlapping with the absorption spectra of the selected ITIC acceptor.D ifferently,i tn early completely overlapped with the absorption spectra of IT-4F,Y6, and IEICO acceptors.As shown in PL of PM6 and corresponding blend films at the PM6 excitation wavelength of 560 nm, the PL of PM6 was quenched after blending with the non-fullerene acceptors, and the quenched wavelength range was consistent with the overlapped range between the emission spectra of PM6 and the absorption spectra of selected non-fullerene acceptors.It is to say that the PL intensity of PM6 in ITIC based blend film was partly quenched while the PL intensity of PM6 was completely quenched upon blended with IT-4F,Y 6, and IEICO.I na ddition, the PL spectra of blend films at PM6s excitation wavelength exhibited different peak positions from those of PM6 films.S till, they were the same as those of the blend films at acceptors excitation wavelength.…”