Abstract
It is common knowledge that molecular energy level offsets of a type II heterojunction formed at the donor–acceptor interface are considered to be the driving force for photoinduced charge transfer in organic solar cells. Usually, these offsets – present between molecular energy levels of the donor and acceptor – are obtained via cyclic voltammetry (CV) measurements of organic semiconductors cast in a film or dissolved in solution. Simply transferring such determined energy levels from solution or film of single materials to blend films may be obviously limited and not be possible in full generality. Herein, we report various cases of material combinations in which novel non-fullerene acceptors did not yield successful charge transfer, although energy levels obtained by CV on constituting single materials indicate a type II heterojunction. Whilst the integer charge transfer (ICT) model provides one explanation for a relative rise of molecular energy levels of acceptors, further details and other cases have not been studied so far in great detail. By applying energy-resolved electrochemical impedance spectroscopy (ER-EIS) on several donor–acceptor combinations, a Fano-like resonance feature associated with a distinctive molecular energy level of the acceptor as well as various relative molecular energy level shifts of different kinds could be observed. By analyzing ER-EIS and absorption spectra, not only the exciton binding energy within single materials could be determined, but also the commonly unknown binding energy of the CT state with regard to the joint density of states (jDOS) of the effective semiconductor. The latter is defined by transitions between the highest occupied molecular orbitals (HOMO) of the donor and the lowest unoccupied molecular orbitals (LUMO) of the acceptor. Using this technique among others, we identified cases in which charge generation may occur either via uphill or by downhill processes between the charge transfer exciton and the electronic gap of the effective semiconductor. Exceptionally high CT-exciton binding energies and thus low charge generation yields were obtained for a case in which the donor and acceptor yielded a too intimate blend morphology, indicating π–π stacking as a potential cause for unfavorable molecular energy level alignment.
| Original language | English (US) |
|---|---|
| Journal | Journal of Materials Chemistry C |
| DOIs | |
| State | Published - Oct 6 2021 |
Bibliographical note
KAUST Repository Item: Exported on 2021-10-12Acknowledged KAUST grant number(s): OSR-2018-CARF/CCF-3079, OSR-CRG2018-3746
Acknowledgements: SA, DAME, FL, WJDB, and HH are grateful for financial support via DFG/FWF in the frame of the “PhotoGenOrder” D-A-CH project. USS is grateful to the Thüringer Ministerium für Wirtschaft, Wissenschaft und Digitale Gesellschaft (TMWWDG) for funding the CEEC Jena. The research of VN was funded by VEGA Project No. 2/0081/18 and was performed during the implementation of the project Building-up Centre for advanced materials application of the Slovak Academy of Sciences, ITMS project code 313021T081 supported by the Research & Innovation Operational Programme funded by the ERDF. AMA and FK acknowledge financial support from the DFG in the framework of the collaborative research center (Sonderforschungsbereich) SFB/TRR 102 project B08. CG and CD thank the Deutsche Forschungsgemeinschaft (DFG) for funding this work within project Photogen (no. 362992821). AMA is thankful for founding the fellowship AN 1523/1-1. SD, AS, and VD acknowledge EU H2020 for funding through the Grant SEPOMO (Marie Skłodowska-Curie Grant Agreement 722651). Some advanced characterization techniques were supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. OSR-2018-CARF/CCF-3079 and Award No. OSR-CRG2018-3746. Finally, SA and HH are grateful to Moidul Islam, for some support in data analysis.