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SCIENCE CHINA Chemistry, Volume 62, Issue 9: 1221-1229(2019) https://doi.org/10.1007/s11426-019-9484-8

Regulating the morphology of fluorinated non-fullerene acceptor and polymer donor via binary solvent mixture for high efficiency polymer solar cells

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  • ReceivedMar 20, 2019
  • AcceptedApr 23, 2019
  • PublishedMay 30, 2019

Abstract

Fluorinated non-fullerene acceptors (NFAs) usually have planar backbone and a higher tendency to crystallize compared to their non-fluorinated counterparts, which leads to enhanced charge mobility in organic solar cells (OSCs). However, this self-organization behavior may result in excessive phase separation with electron donors and thereby deteriorate device efficiency. Herein, we demonstrate an effective approach to tune the molecular organization of a fluorinated NFA (INPIC-4F), and its phase separation with the donor PBDB-T, by varying the casting solvent. A prolonged film drying time encourages the crystallization of INPIC-4F into spherulites and consequently results in excessive phase separation, leading to a low device power conversion efficiency (PCE) of 8.1%. Contrarily, a drying time leads to fine mixed domains with inefficient charge transport properties, resulting in a moderate device PCE of 11.4%. An intermediate film drying time results in the formation of face-on π-π stacked PBDB-T and INPIC-4F domains with continuous phase-separated networks, which facilitates light absorption, exciton dissociation as well as balanced charge transport towards the electrode, and achieves a remarkable PCE of 13.1%. This work provides a rational guide for optimizing the molecular ordering of NFAs and electron donors for high device efficiency.


Funded by

the Natural Science Foundation of Hubei Province(Grant,No.,2018CFA055)

the National Natural Science Foundation of China(Grants,No.,21774097,21504065,51573077,21875111)


Acknowledgment

This work was supported by the Natural Science Foundation of Hubei Province (2018CFA055), the National Natural Science Foundation of China (21774097, 21504065, 51573077, 21875111), and the Fundamental Research Funds For the Central Universities (WUT: 195201017, 2019IVB081). We thank beamline I07 at Diamond Light Source (UK) for providing beam time to perform GIWAXS measurements. D.G. Lidzey thanks the U.K. EPSRC for funding High Resolution Mapping of Performance and Degradation Mechanisms in Printable Photovoltaic Devices (EP/M025020/1), and EPSRC for funding a studentship for E.L.K. Spooner via the Centre for Doctoral Training in New and Sustainable PV (EP/L01551X/1) and for a DTA studentship award for R.C. Kilbride.


Contributions statement

These authors contributed equally to this work.


Supplement

The supporting information is available online at http://chem.scichina.com and http://link.springer.com/journal/11426. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.


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  • Figure 1

    (a) Chemical structures of INPIC-4F and PBDB-T; (b) energy level diagrams of ITO/ZnO/PBDB-T:INPIC-4F/MoO3/Ag inverted devices (color online).

  • Figure 2

    (a) J-V characteristics, (b) EQE of PBDB-T:INPIC-4F solar cells cast using different solvents; (c) absorbance and (d) PL spectra of PBDB-T:INPIC-4F films cast using different solvents (color online).

  • Figure 3

    Optical microscope images of INPIC-4F films cast from (a) CB, (b) CB:CF (1.5:1, v/v) and (c) CF; AFM images of PBDB-T:INPIC-4F blend films cast from (d) CB, (e) CB:CF (1.5:1, v/v) and (f) CF (color online).

  • Figure 4

    2D GIWAXS patterns of PBDB-T:INPIC-4F blend films prepared from (a) CB, (b) CB:CF (1.5:1, v/v) and (c) CF. (d) Out-of-plane 1D profiles of GIWAXS patterns along the qz axis of PBDB-T:INPIC-4F films under different conditions. Multi-peak fitting results from 1.4 to 2.0 Å−1 of PBDB-T:INPIC-4F films (e) CB, (f) CB:CF (1.5:1, v/v) and (g) CF (color online).

  • Figure 5

    (a) Photocurrent density versus effective voltage curves of PBDB-T:INPIC-4F films cast from different solvents; (b) Nyquist plots of impedance spectra of various devices under 1 sun irradiation with an applied bias at Voc (color online).

  • Table 1   Photovoltaic parameters of PBDB-T:INPIC-4F OSCs measured at an illumination of AM 1.5 G, The statistical data were obtained from over 25 individual devices

    Solvent

    FF (%)

    Jsc(mA/cm2)

    Calculated Jsc (mA/cm2)

    Voc (V)

    PCEmax (PCEavg) (%)

    CB

    66.5

    14.6

    14.0

    0.84

    8.1 (7.4±0.9)

    CB:CF (1.5:1, v/v)

    72.7

    22.0

    21.4

    0.82

    13.1 (12.9±0.3)

    CF

    72.2

    19.1

    18.5

    0.83

    11.4 (11.0±0.5)

  • Table 2   , , , hole and electron mobilities of PBDB-T:INPIC-4F OSCs prepared under different conditions

    Solvent

    Jsat(mA/cm2)

    Pdiss (%)

    Pcoll (%)

    Hole mobility (μh)(cm2/(V s))

    Electron mobility (μe)(cm2/(V s))

    μh/μe

    CB

    14.9

    97.9

    86.3

    5.1×10−4

    6.8×10−4

    0.75

    CB:CF (1.5:1, v/v)

    22.6

    98.6

    87.2

    7.2×10−4

    7.8×10−4

    0.92

    CF

    19.4

    98.4

    85.6

    5.2×10−4

    6.8×10−4

    0.76

  • Table 3   Summary of the electrical parameters of OSCs obtained by fitting the Nyquist plots

    Solvent

    Rs (Ω)

    Rp (Ω)

    CPE1-T (F)

    R1 (Ω)

    CPE2-T (F)

    CB

    64.2

    108.5

    1.28×10−8

    10.8

    2.64×10−8

    CB:CF (1.5:1, v/v)

    71.3

    75.4

    2.87×10−8

    19.4

    3.98×10−8

    CF

    74.3

    83.4

    1.83×10−8

    17.3

    1.59×10−8

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