Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly suppressed charge trapping into electronic intra-bandgap tail states compared to other polymer/non-fullerene acceptor solar cells, attributed to lower energetic disorder. The presence of tail states is a key source of energetic loss in most organic solar cells, as charge carriers relax into these states, reducing the quasi-Fermi level splitting and therefore device VOC. DFT and Raman analyses indicate this suppression of tail state energetics disorder could be associated with a higher degree of conformational rigidity and uniformity for the Y6 acceptor. We attribute the origin of such conformational rigidity and uniformity of Y6 to the presence of the two alkyl side chains on the outer core that restricts end-group rotation by acting as a conformation locker. The resultant enhanced carrier dynamics and suppressed charge carrier trapping are proposed to be a key factor behind the high performance of this blend. Low energetic disorder is suggested to be a key factor enabling reasonably efficient charge generation in this low energy offset system. In the absence of either energetic disorder or a significant electronic energy offset, it is argued that charge separation in this system is primarily entropy driven. Nevertheless, photocurrent generation is still limited by slow hole transfer from Y6 to PM6, suggesting pathways for further efficiency improvement

Toward Improved Environmental Stability of Polymer:Fullerene and Polymer:Nonfullerene Organic Solar Cells: A Common Energetic Origin of Light- and Oxygen-Induced Degradation

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation

Correction to "Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution"

Tracking charge transfer to residual metal clusters in conjugated polymers for photocatalytic hydrogen evolution (Journal of the American Chemical Society (2020) 142:34 (14574-14587) DOI: 10.1021/jacs.0c06104) Page 14585. Appreciation for Dr. Yan-Gu Lin was inadvertently left out of the Acknowledgments. The scientific part of the paper remains unchanged. The complete correct Acknowledgments paragraph is as follows: ¦ ACKNOWLEDGMENTS M.S. is grateful to Imperial College for a President’s Ph.D. Scholarship and to the EPSRC for a Doctoral Prize Fellowship. J.R.D. and I.M. acknowledge support from KAUST (project numbers OSR-2015-CRG4-2572 and OSR-2018-CRG7- 3749.2). C.M.A., A.I.C., and R.S.S. acknowledge the Engineering and Physical Sciences Research Council (EPSRC, EP/ N004884/1). L.F. thanks the EU for a Marie Curie fellowship (658270). S.C. thanks Imperial College London for a Schro¨dinger Scholarship. R.G. is grateful to the FRQNT for a postdoctoral award and NSERC Discovery Grant funding. C.-L.C. appreciates his supervisor, Dr. Yan-Gu Lin, for his efforts on the beamtime support of XAS beamline and corresponding equipment/technical setup. All plotted data have been deposited on the open-access repository Zenodo and can be accessed via dx.doi.org/10.5281/zenodo.3932340

Correction to "Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution"

Tracking charge transfer to residual metal clusters in conjugated polymers for photocatalytic hydrogen evolution (Journal of the American Chemical Society (2020) 142:34 (14574-14587) DOI: 10.1021/jacs.0c06104) Page 14585. Appreciation for Dr. Yan-Gu Lin was inadvertently left out of the Acknowledgments. The scientific part of the paper remains unchanged. The complete correct Acknowledgments paragraph is as follows: ¦ ACKNOWLEDGMENTS M.S. is grateful to Imperial College for a President’s Ph.D. Scholarship and to the EPSRC for a Doctoral Prize Fellowship. J.R.D. and I.M. acknowledge support from KAUST (project numbers OSR-2015-CRG4-2572 and OSR-2018-CRG7- 3749.2). C.M.A., A.I.C., and R.S.S. acknowledge the Engineering and Physical Sciences Research Council (EPSRC, EP/ N004884/1). L.F. thanks the EU for a Marie Curie fellowship (658270). S.C. thanks Imperial College London for a Schro¨dinger Scholarship. R.G. is grateful to the FRQNT for a postdoctoral award and NSERC Discovery Grant funding. C.-L.C. appreciates his supervisor, Dr. Yan-Gu Lin, for his efforts on the beamtime support of XAS beamline and corresponding equipment/technical setup. All plotted data have been deposited on the open-access repository Zenodo and can be accessed via dx.doi.org/10.5281/zenodo.3932340

Efficient Hole Transfer from a Twisted Perylenediimide Acceptor to a Conjugated Polymer in Organic Bulk-Heterojunction Solar Cells

Non-fullerene acceptors have recently attracted tremendous interest due to their potential as alternatives to fullerene derivatives in bulk-heterojunction solar cells. Nevertheless, physical understanding of charge carrier generation and transfer mechanism that occurred at the interface between the non-fullerene molecule and donor polymer is still behind their enhanced photovoltaic performance. Here we report examples of a non-planar perylene dimer (TP) as an electron acceptor and achieve a power conversion efficiency of 6.29% in a fullerene-free solar cell. Photoluminescence (PL) measurements show high quenching efficiency driven by the excitons of both conjugated polymer and TP molecule, respectively, indicating efficient electron and hole transfer, which can support a highly intermixed phase of blends measured by atomic force microscopy (AFM) and grazing incident wide-angle X-ray diffraction (GIWAXS). Femtosecond transient absorption spectroscopy (fs-TAS) reveals that the fast exciton dissociation process from TP molecule to donor polymer contributes to additionally increasing current density, leading to stronger incident photon to current efficiency in the visible region

Efficient Hole Transfer from a Twisted Perylenediimide Acceptor to a Conjugated Polymer in Organic Bulk-Heterojunction Solar Cells

Non-fullerene acceptors have recently attracted tremendous interest due to their potential as alternatives to fullerene derivatives in bulk-heterojunction solar cells. Nevertheless, physical understanding of charge carrier generation and transfer mechanism that occurred at the interface between the non-fullerene molecule and donor polymer is still behind their enhanced photovoltaic performance. Here we report examples of a non-planar perylene dimer (TP) as an electron acceptor and achieve a power conversion efficiency of 6.29% in a fullerene-free solar cell. Photoluminescence (PL) measurements show high quenching efficiency driven by the excitons of both conjugated polymer and TP molecule, respectively, indicating efficient electron and hole transfer, which can support a highly intermixed phase of blends measured by atomic force microscopy (AFM) and grazing incident wide-angle X-ray diffraction (GIWAXS). Femtosecond transient absorption spectroscopy (fs-TAS) reveals that the fast exciton dissociation process from TP molecule to donor polymer contributes to additionally increasing current density, leading to stronger incident photon to current efficiency in the visible region

Charge Carrier Dynamics in Non-Fullerene Acceptor-Based Organic Solar Cells: Investigating the Influence of Processing Additives Using Transient Absorption Spectroscopy

In this study, we present a comprehensive investigation into the charge generation mechanism in bulk-heterojunction organic solar cells employing non-fullerene acceptors (NFAs) both with and without the presence of processing additives. While photovoltaic devices based on Y6 or BTP-eC9 have shown remarkable power conversion efficiencies, the underlying charge generation mechanism in polymer:NFA blends remains poorly understood. To shed light on this, we employ transient absorption (TA) spectroscopy to elucidate the charge transfer pathway within a blend of the donor polymer PM6 and NFAs. Interestingly, the charge carrier lifetimes of neat Y6 and BTP-eC9 are comparable, both reaching up to 20 ns. However, the PM6:BTP-eC9 blend exhibits substantially higher charge carrier generation and a longer carrier lifetime compared to PM6:Y6 blend films, leading to superior performance. By comparing TA data obtained from PM6:Y6 or PM6:BTP-eC9 blend films with and without processing additives, we observe significantly enhanced charge carrier generation and prolonged charge carrier lifetimes in the presence of these additives. These findings underscore the potential of manipulating excited species as a promising avenue for further enhancing the performance of organic solar cells. Moreover, this understanding contributes to the advancement of NFA-based systems and the optimization of charge transfer processes in polymer:NFA blends

Low Bandgap Quinoxaline-based D-A-type Copolymers: Synthesis, Characterization, and Photovoltaic Properties

Three classes of quinoxaline (Qx)-based donoracceptor (DA)-type copolymers, poly[thiophene-2,5-diyl-alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline-5,8-diyl] P(T-Qx), poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline-5,8-diy} P(BDT-Qx), and poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-(5',8'-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)-quinoxaline-5,5-diyl} P(BDT-DTQx), were synthesized via a Stille coupling reaction. The Qx unit was functionalized at the 2- and 3-positions with 4-(octyloxy)phenyl to provide good solubility and to reduce the steric hindrance. The absorption spectra of the Qx-containing copolymers could be tuned by incorporating three different electron-donating moieties. Among these, P(T-Qx) acted as an electron donor and yielded a high-performance solar cell by assuming a rigid planar structure, confirmed by differential scanning calorimetry, UVvis spectrophotometer, and density functional theory study. In contrast, the P(BDT-Qx)-based solar cell displayed a lower power conversion efficiency (PCE) with a large torsional angle (34.7 degrees) between the BDT and Qx units. The BDT unit in the P(BDT-DTQx) backbone acted as a linker and interfered with the formation of charge complexes or quinoidal electronic conformations in a polymer chain. The PCEs of the polymer solar cells based on these copolymers, in combination with [6,6]-phenyl C70 butyric acid methyl ester (PC71BM), were 3.3% [P(T-Qx)], 1.9% [P(BDT-Qx)], and 2.3% [P(BDT-DTQx)], respectively, under AM 1.5G illumination (100 mW cm-2). (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013X111517sciescopu

Covalent Networking of a Conjugated-Polymer Photocatalyst to Promote Exciton Diffusion in the Aqueous Phase for Efficient Hydrogen Production

A conjugated polymer particle in an aqueous phase is covalently networked in 3D by crosslinking with azide groups, leading to significantly enhanced activity—a high photocatalytic H2 evolution rate (11 024 µmol g−1 h−1 (λ > 420 nm)) and a high apparent quantum yield (up to 0.8%). The reaction between the photoactive azide and the alkyl chains of the conjugated polymer provides more intact intermolecular polymeric interactions in the colloidal state, thus preventing physical swelling and inhibiting the recombination of photoproduced carriers. The covalent network efficiently promotes exciton diffusion, which greatly facilitates charge separation and transfer. The azide photo-crosslinking also leads to more compact and better-packed nanoparticles in the aqueous phase and efficient transfer of excitons to the outer surface of the nanoparticles, where photocatalytic reactions occur. These results show that photo-crosslinking can suppress the adverse effects of alkyl chains which inhibit photocatalytic performance. Therefore, covalent crosslinking is a promising strategy for the development of solar and hydrogen energy. © 2022 Wiley-VCH GmbH1