Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells

Abstract

In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.

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Fig. 1: Materials’ structures, energy levels and IQEs in photovoltaic devices.
Fig. 2: The charge carrier generation yield is independent of donor or acceptor excitation, but it increases with the IE offset.
Fig. 3: Spectral overlap of donor PL and acceptor absorption facilitates ultrafast energy transfer.
Fig. 4: The IQE follows the exciton quenching yield controlled by the IE offset and energy level bending at the donor–acceptor interface.

Data availability

The datasets generated and analysed during this study and presented in this article are available online at https://doi.org/10.6084/m9.figshare.c.5110076.v1.

Code availability

The codes or algorithms used to analyse the data reported in this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This publication is based on work supported by the KAUST Office of Sponsored Research (OSR) under award nos. OSR-2018-CARF/CCF-3079 and OSR-CRG2018-3746. D.A. acknowledges funding from the BMBF grant InterPhase and MESOMERIE (grant nos. FKZ 13N13661, FKZ 13N13656) and the European Union Horizon 2020 research and innovation program ‘Widening materials models’ under grant agreement no. 646259 (MOSTOPHOS). D.A. also acknowledges the KAUST PSE Division for hosting his sabbatical in the framework of the Division’s Visiting Faculty program. A.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 844655 (SMOLAC). We thank L. Sinatra of KAUST and Quantum Solutions LLC for assisting with the PLQY measurements. G.T.H acknowledges K. Graham and A. Amassian (and previous group members including M. Tietze and G.O.N. Ndjawa) for having designed and installed and worked on the IPES setup. In particular, G.T.H. acknowledges K. Graham’s kind assistance during the reconfiguration and optimization of the IPES setup, as well as U. Sharif for technical assistance.

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Contributions

S.K. performed the transient spectroscopy experiments, data analysis and prepared the first draft of the manuscript. J.G. conceived the study, analysed the transient and steady-state spectroscopic experiments, carried out the FRET calculations and contributed to the manuscript. Y.F. performed the DR3, PTB7-Th and PBDB-T-2F-based solar cell device preparation and characterization, and the ellipsometry measurements and analysis. N.C. prepared and characterized PBDB-T-SF-based devices and prepared thin-film blends for TRPL experiments. C.S.P.D.C. prepared samples and characterized the steady-state absorption of thin films, PLQY and PL quenching in DR3, PTB7-Th and PBDB-T-2F-based blends via TRPL. G.T.H. performed the UPS and LE–IPES experiments and data analysis. J.I.K. provided steady-state absorption, PL and TRPL data of PBDB-T-SF-based blends. A.M. calculated the solid-state IEs, EAs and bias potentials. W.L. performed quantum-chemical calculations of donor–acceptor dimers. A.H.B. and T.A.D.P. determined the CT state energies from electroluminescence spectra. R.-Z.L. prepared DR3-based devices and characterized their performance. W.Z. synthesized the O-IDTBR acceptor. Y.L. prepared thin films for steady-state characterization. D.H.A. performed thin-film imaging and EELS analysis. E.A. contributed to the development and maintenance of the ultrafast laser spectroscopy setups. S.H.K.P determined the IQE of PTB7-Th-based devices. A.S. determined the IE and EA of IEICO-4F and IT-4F by UPS and LE–IPES, respectively. P.M.B. supervised the device preparation of DR3-based solar cells and synthesis of IDTTBM used in this work. S.D.W. oversaw the UPS/LE–IPES experiments and data analysis. I.M. supervised the synthesis of O-IDTBR used in this work. T.D.A. supervised the PBDB-T-2F, and some of the R3 and PTB7-Th donor device preparation and characterization. D.B. supervised some of the PTB7-Th donor device preparation and characterization. D.A. supervised the computational part of the work, conceived the model describing the energy level bending and contributed to the manuscript. F.L. conceived the study and supervised the steady-state and transient optical spectroscopy experiments and data analysis, as well as the PBDB-T-SF donor device preparation and characterization and contributed to the manuscript. All authors contributed to the revision of the final version of the manuscript.

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Correspondence to Denis Andrienko or Frédéric Laquai.

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Extended data

Extended Data Fig. 1 UPS and LE-IPES spectra of IT-4F, IT-4Cl, ITIC, ICC6 and PCBM.

UPS and LE-IPES spectra of IT-4F), IT-4Cl, ITIC, ICC6, and PC71BM thin films on evaporated Au100nm/Cr10nm/n-type Si(100). a) SECO region normalized to the SEC peak height and offset with work function (WF) values labelled. b) UPS (left) and LE-IPES (right) spectra displayed with respect to the Fermi level at 0. The intersection between the flat baseline and tangent of the respective frontier orbitals band defines here the HOMO and LUMO level and IE and EA with respect to vacuum. These are identified by black vertical lines and value labeled. The respective vacuum levels are displayed by the vertical lines.

Extended Data Fig. 2 UPS and LE-IPES spectra of PBDB-T-SF, IDTTBM, O-IDTBR, Y6 and BT-CIC.

UPS and LE-IPES spectra of PBDB-T-SF, IDTTBM, O-IDTBR, Y6, and BT-CIC thin films on evaporated Au100nm/Cr10nm/n-type Si(100). a) SECO region normalized to the SEC peak height and offset with work function (WF) values labeled. b) UPS (left) and LE-IPES (right) spectra displayed with respect to the Fermi level at 0. The intersection between the flat baseline and tangent of the respective frontier orbitals band defines here the HOMO and LUMO level and IE and EA with respect to vacuum. These are identified by black vertical lines and value labeled. The respective vacuum levels are displayed by the vertical lines.

Extended Data Fig. 3 UPS and LE-IPES spectra of PBDB-T-2F, PTB7-Th, DR3, IEICO and IEICO-4F.

UPS and LE-IPES spectra of PBDB-T-2F, PTB7-Th, DR3, IEICO, and IEICO-4F thin films on evaporated Au100nm/Cr10nm/n-type Si(100). a) SECO region normalized to the SEC peak height and offset with work function (WF) values labeled. b) UPS (left) and LE-IPES (right) spectra displayed with respect to the Fermi level (0 eV). Vertical marks identify the estimated HOMO and LUMO energetic values (IE and EA with respect to vacuum) with lines used in the linear extrapolation to baseline shown. Vertical lines identify the vacuum energy (Evac). (UPS photoelectron intensity (a.u), IPES PMT photon intensity (a.u)).

Extended Data Fig. 4 J-V curves and EQE spectra of DR3-, PBDB-T-2F-, PTB7-Th- and PBDB-T-SF-based devices.

J-V curves measured under 1 sun illumination for a) DR3-, c) PBDB-T-2F-, e) PTB7-Th-, and h) PDBDT-SF-based BHJ devices as indicated in the legend. b, d), f) and i) EQE spectra of the respective devices (see legend for BHJ system). g) and j) Layout of the device structure of the studied solar cells (inverted was used for PDBD-T-SF- based devices, normal for all other donors).

Extended Data Fig. 5 Internal quantum efficiency of DR3-, PBDB-T-2F-, PTB7-Th- and PBDB-T-SF-based devices.

Internal quantum efficiency (IQE) spectra of DR3- (a), PBDB-T-2F- (b) PTB7-Th- (c), and PBDB-T-SF-based devices. The IQE of each solar cell was calculated using: IQE(%) = 100*EQE(%)/(100% – Reflectance(%) – Parasitic Absorption (%)). The reflectance spectra were collected with an integrating sphere using the same EQE measurement system, while the parasitic absorption was calculated using transfer matrix modelling.

Extended Data Fig. 6 Fill factor vs IE offset.

Qualitatively, the solar cells’ fill factors (FF) increase with the IE offset in all donor-NFA systems. Quantitatively, the increase depends on the specific donor used.

Extended Data Fig. 7 Fluence-dependent nanosecond to microsecond TA kinetics of DR3 blends.

Fluence-dependent nanosecond to microsecond TA kinetics of DR3 blends (photo-induced absorption region) as indicated in the legend after excitation at 532 nm. The percentage of free charge generation as obtained from the two-pool recombination model fit to the experimental data is shown in each panel. Details of this calculation can be found in the SI (two-pool charge recombination model).

Extended Data Fig. 8 Time-resolved photoluminescence spectra and kinetics of PBDB-T-2F blends and respective neat acceptors.

Time-resolved photoluminescence a-b, d-e, g-h, j-k, m-n) spectra and c, f, i, l o) kinetics of IEICO and PBDB-T-2F:IEICO, IEICO-4F and PBDB-T-2F:IEICO-4F, BT-CIC and PBDB-T-2F:BT-CIC, Y6 and PBDB-T-2F:Y6, IT-4F and PBDB-T-2F:IT-4F, respectively, under inert nitrogen atmosphere, λexc= 690 nm.

Extended Data Fig. 9 Thin-film morphology analyses of optimized IT-4F, BT-CIC, IEICO-4F and IEICO blends with PBDB-T-2F.

Thin-film morphology analyses of optimized BHJ active layers. (a, c, e, g) dark-field STEM image and (b, d, f, h) EELS map of (a-b) IT-4F, (c-d) BT-CIC, (e-f) IEICO-4F and (g-h) IEICO blends with PBDB-T-2F. The EELS maps depict the component-separation as donor-rich (red) and NFA-rich domains (green). Note that 60 - 70 nm thick films were used for these analyses to improve spectral resolution and reduce domain overlap when projected through the film thickness.

Extended Data Fig. 10 Electroluminescence spectra of PBDB-T-2F:NFA-based devices.

Reduced electroluminescence spectra of PBDB-T-2F:NFA-based devices (black symbols), their decomposition into neat NFA reduced electroluminescence (colored symbols), and a Gaussian-shape CT state emission (blue dashed lines), and the sum of the two contributions (solid green line). The gap between 1.3 and 1.36 eV corresponds to the spectral region, where both the infrared and visible detector used to measure the EL spectra are less sensitive (about 80% of the maximum sensitivity).

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Karuthedath, S., Gorenflot, J., Firdaus, Y. et al. Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-00835-x

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