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.
This is a preview of subscription content, access via your institution
Access options
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 12 print issues and online access
$259.00 per year
only $21.58 per issue
Rent or buy this article
Prices vary by article type
from$1.95
to$39.95
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
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.
Change history
29 November 2021
A Correction to this paper has been published: https://doi.org/10.1038/s41563-021-01178-x
References
Yang, C. et al. Effects of energy-level offset between a donor and acceptor on the photovoltaic performance of non-fullerene organic solar cells. J. Mater. Chem. A 7, 18889–18897 (2019).
Cha, H. et al. Influence of blend morphology and energetics on charge separation and recombination dynamics in organic solar cells incorporating a nonfullerene acceptor. Adv. Funct. Mater. 28, 1704389 (2018).
Kaake, L. G. Towards the organic double heterojunction solar cell. Chem. Rec. 19, 1131–1141 (2019).
Menke, S. M., Ran, N. A., Bazan, G. C. & Friend, R. H. Understanding energy loss in organic solar cells: toward a new efficiency regime. Joule 2, 25–35 (2018).
Nakano, K. et al. Anatomy of the energetic driving force for charge generation in organic solar cells. Nat. Commun. 10, 2520 (2019).
Ward, A. J. et al. The impact of driving force on electron transfer rates in photovoltaic donor-acceptor blends. Adv. Mater. 27, 2496–2500 (2015).
Li, Y. et al. Non-fullerene acceptor with low energy loss and high external quantum efficiency: towards high performance polymer solar cells. J. Mater. Chem. A 4, 5890–5897 (2016).
Chen, S. et al. A wide-bandgap donor polymer for highly efficient non-fullerene organic solar cells with a small voltage loss. JACS 139, 6298–6301 (2017).
Zhang, J. et al. Accurate determination of the minimum HOMO offset for efficient charge generation using organic semiconducting alloys. Adv. Energy Mater. 10, 1903298 (2020).
Scharber, M. C. et al. Design rules for donors in bulk-heterojunction solar cells—towards 10% energy-conversion efficiency. Adv. Mater. 18, 789–794 (2006).
Ohkita, H. et al. Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy. JACS 130, 3030–3042 (2008).
Coffey, D. C. et al. An optimal driving force for converting excitons into free carriers in excitonic solar cells. J. Phys. Chem. C. 116, 8916–8923 (2012).
Hendriks, K. H., Wijpkema, A. S. G., van Franeker, J. J., Wienk, M. M. & Janssen, R. A. J. Dichotomous role of exciting the donor or the acceptor on charge generation in organic solar cells. JACS 138, 10026–10031 (2016).
Unger, T. et al. The impact of driving force and temperature on the electron transfer in donor–acceptor blend systems. J. Phys. Chem. C. 121, 22739–22752 (2017).
Zhou, Z. et al. Subtle molecular tailoring induces significant morphology optimization enabling over 16% efficiency organic solar cells with efficient charge generation. Adv. Mater. 32, 1906324 (2020).
Tereshchenko, I. V. et al. The role of semilabile oxygen atoms for intercalation chemistry of the metal-ion battery polyanion cathodes. JACS 140, 3994–4003 (2018).
Li, Y. et al. A fused-ring based electron acceptor for efficient non-fullerene polymer solar cells with small HOMO offset. Nano Energy 27, 430–438 (2016).
Li, S. et al. Highly efficient fullerene-free organic solar cells operate at near zero highest occupied molecular orbital offsets. JACS 141, 3073–3082 (2019).
Yan, C. et al. Non-fullerene acceptors for organic solar cells. Nat. Rev. Mater. 3, 18003(2018).
Chen, S. et al. Efficient nonfullerene organic solar cells with small driving forces for both hole and electron transfer. Adv. Mater. 30, 1804215 (2018).
Baran, D. et al. Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages. Energy Environ. Sci. 9, 3783–3793 (2016).
Wan, X., Li, C., Zhang, M. & Chen, Y. Acceptor–donor–acceptor type molecules for high performance organic photovoltaics – chemistry and mechanism. Chem. Soc. Rev. 49, 2828–2842 (2020).
Sworakowski, J. How accurate are energies of HOMO and LUMO levels in small-molecule organic semiconductors determined from cyclic voltammetry or optical spectroscopy? Synth. Met. 235, 125–130 (2018).
Janssen, R. A. J. & Nelson, J. Factors limiting device efficiency in organic photovoltaics. Adv. Mater. 25, 1847–1858 (2013).
Jin, F. et al. Improved charge generation via ultrafast effective hole-transfer in all-polymer photovoltaic blends with large highest occupied molecular orbital (HOMO) energy offset and proper crystal orientation. Adv. Funct. Mater. 28, 1801611 (2018).
Aplan, M. P. et al. Revealing the importance of energetic and entropic contributions to the driving force for charge photogeneration. ACS Appl. Mater. Interfaces 10, 39933–39941 (2018).
Perdigón‐Toro, L. et al. Barrierless free charge generation in the high‐performance PM6:Y6 bulk heterojunction non‐fullerene solar cell. Adv. Mater. 32, 1906763 (2020).
Ramirez, I., Causa’, M., Zhong, Y., Banerji, N. & Riede, M. Key tradeoffs limiting the performance of organic photovoltaics. Adv. Energy Mater. 8, 1703551 (2018).
Karki, A. et al. Unifying charge generation, recombination, and extraction in low‐offset non‐fullerene acceptor organic solar cells. Adv. Energy Mater. 10, 2001203 (2020).
Gautam, B. R., Younts, R., Carpenter, J., Ade, H. & Gundogdu, K. The role of FRET in non-fullerene organic solar cells: implications for molecular design. J. Phys. Chem. A 122, 3764–3771 (2018).
Mohapatra, A. A. et al. Förster resonance energy transfer drives higher efficiency in ternary blend organic solar cells. ACS Appl. Energy Mater. 1, 4874–4882 (2018).
Bi, P. et al. Dual Förster resonance energy transfer effects in non-fullerene ternary organic solar cells with the third component embedded in the donor and acceptor. J. Mater. Chem. A 5, 12120–12130 (2017).
Karuthedath, S. et al. Impact of fullerene on the photophysics of ternary small molecule organic solar cells. Adv. Energy Mater. 9, 1901443 (2019).
Zhong, Y. et al. Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers. Nat. Commun. 11, 833 (2020).
Liang, R.-Z. et al. Carrier transport and recombination in efficient “All-Small-Molecule” solar cells with the nonfullerene acceptor IDTBR. Adv. Energy Mater. 8, 1800264 (2018).
Alamoudi, M. A. et al. Impact of nonfullerene acceptor core structure on the photophysics and efficiency of polymer solar cells. ACS Energy Lett. 3, 802–811 (2018).
Yuan, J. et al. Single-junction organic solar cell with over 15% efficiency using fused-ring acceptor with electron-deficient core. Joule 3, 1140–1151 (2019).
Gao, F. A new acceptor for highly efficient organic solar cells. Joule 3, 908–909 (2019).
Yoshida, H. Note: low energy inverse photoemission spectroscopy apparatus. Rev. Sci. Instrum. 85, 16101 (2014).
Yao, H. et al. Design and synthesis of a low bandgap small molecule acceptor for efficient polymer solar cells. Adv. Mater. 28, 8283–8287 (2016).
Huang, J.-S. et al. Polymer bulk heterojunction solar cells employing Förster resonance energy transfer. Nat. Photon 7, 479–485 (2013).
Sapsford, K. E., Berti, L. & Medintz, I. L. Materials for fluorescence resonance energy transfer analysis: beyond traditional donor-acceptor combinations. Angew. Chem. (Int. ed. Engl.) 45, 4562–4589 (2006).
Poelking, C. & Andrienko, D. Design rules for organic donor-acceptor heterojunctions: pathway for charge splitting and detrapping. JACS 137, 6320–6326 (2015).
Poelking, C. et al. Impact of mesoscale order on open-circuit voltage in organic solar cells. Nat. Mater. 14, 434–439 (2015).
Castet, F., D’Avino, G., Muccioli, L., Cornil, J. & Beljonne, D. Charge separation energetics at organic heterojunctions: on the role of structural and electrostatic disorder. Phys. Chem. Chem. Phys. 16, 20279–20290 (2014).
Schwarze, M. et al. Band structure engineering in organic semiconductors. Science 352, 1446–1449 (2016).
D’Avino, G. et al. Electrostatic phenomena in organic semiconductors: fundamentals and implications for photovoltaics. J. Phys. Condens. Matter 28, 433002 (2016).
Schwarze, M. et al. Impact of molecular quadrupole moments on the energy levels at organic heterojunctions. Nat. Commun. 10, 2466 (2019).
Sousa, L. E., Coropceanu, V., da Silva Filho, D. A. & Sini, G. On the physical origins of charge separation at donor–acceptor interfaces in organic solar cells: energy bending versus energy disorder. Adv. Theory Simul. 3, 1900230 (2020).
Liang, R.-Z. et al. Additive-morphology interplay and loss channels in ‘All-Small-Molecule’ bulk-heterojunction (BHJ) solar cells with the nonfullerene acceptor IDTTBM. Adv. Funct. Mater. 28, 1705464 (2018).
Holliday, S. et al. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor. Nat. Commun. 7, 11585 (2016).
Yang, J.-P. et al. Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals. J. Phys. D. Appl. Phys. 50, 423002 (2017).
Kahn, A. Fermi level, work function and vacuum level. Mater. Horiz. 3, 7–10 (2016).
Yoshida, H. Near-ultraviolet inverse photoemission spectroscopy using ultra-low energy electrons. Chem. Phys. Lett. 539-540, 180–185 (2012).
Karuthedath, S. et al. Thermal annealing reduces geminate recombination in TQ1:N2200 all-polymer solar cells. J. Mater. Chem. A 6, 7428–7438 (2018).
Babics, M. et al. Negligible energyloss during charge generation in small-molecule/fullerene bulk-heterojunctionsolar cells leads to open-circuit voltage over 1.10 V. ACS Appl. Energy Mater. 2, 2717–2722 (2019). 4.
Frisch, M. J. et al. Gaussian 09 (Gaussian Inc., 2016).
Thole, B. T. Molecular polarizabilities calculated with a modified dipole interaction. Chem. Phys. 59, 341–350 (1981).
van Duijnen, P. T. & Swart, M. Molecular and atomic polarizabilities: Thole’s model revisited. J. Phys. Chem. A. 102, 2399–2407 (1998).
Stone, A. J. Distributed multipole analysis: stability for large basis sets. J. Chem. Theory Comput. 1, 1128–1132 (2005).
Rühle, V. et al. Microscopic simulations of charge transport in disordered organic semiconductors. J. Chem. Theory Comput. 7, 3335–3345 (2011).
Poelking, C. & Andrienko, D. Long-range embedding of molecular ions and excitations in a polarizable molecular environment. J. Chem. Theory Comput. 12, 4516–4523 (2016).
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.
Author information
Authors and Affiliations
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. and S.L. 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.
Corresponding authors
Ethics declarations
Competing interests
The authors declare no competing interests.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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).
Supplementary information
Supplementary Information
Supplementary Figs. 1–26, discussion and Tables 1–7.
Rights and permissions
About this article
Cite this article
Karuthedath, S., Gorenflot, J., Firdaus, Y. et al. Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells. Nat. Mater. 20, 378–384 (2021). https://doi.org/10.1038/s41563-020-00835-x
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/s41563-020-00835-x
This article is cited by
-
The role of interfacial donor–acceptor percolation in efficient and stable all-polymer solar cells
Nature Communications (2024)
-
Assessing intra- and inter-molecular charge transfer excitations in non-fullerene acceptors using electroabsorption spectroscopy
Nature Communications (2024)
-
Suppressing electron-phonon coupling in organic photovoltaics for high-efficiency power conversion
Nature Communications (2023)
-
Molecular orientation-dependent energetic shifts in solution-processed non-fullerene acceptors and their impact on organic photovoltaic performance
Nature Communications (2023)
-
Hexanary blends: a strategy towards thermally stable organic photovoltaics
Nature Communications (2023)