Recent Advances in Nonfullerene Acceptor-Based Layer-by-Layer Organic Solar Cells Using a Solution Process

Abstract

Recently, sequential layer-by-layer (LbL) organic solar cells (OSCs) have attracted significant attention owing to their favorable p-i-n vertical phase separation, efficient charge transport/extraction, and potential for lab-to-fab large-scale production, achieving high power conversion efficiencies (PCEs) of over 18%. This review first summarizes recent studies on various approaches to obtain ideal vertical D/A phase separation in nonfullerene acceptor (NFAs)-based LbL OSCs by proper solvent selection, processing additives, protecting solvent treatment, ternary blends, etc. Additionally, the longer exciton diffusion length of NFAs compared with fullerene derivatives, which provides a new scope for further improvement in the performance of LbL OSCs, is been discussed. Large-area device/module production by LbL techniques and device stability issues, including thermal and mechanical stability, are also reviewed. Finally, the current challenges and prospects for further progress toward their eventual commercialization are discussed.

Keywords: layer-by-layer; nonfullerene acceptors; organic photovoltaics; pseudo-planar heterojunction.

Figure 1

a) Sequential LbL processes (spin casting, blade coating, and slot die coating) and evolution of p–i–n morphology. b) Recent developments in small‐ and large‐area solution‐processed LbL OSCs. c) Main discussion topics of this review.

Figure 2

Molecular structures of donor materials discussed in this review.

Figure 3

Molecular structures of acceptor materials discussed in this review.

Figure 3

Molecular structures of acceptor materials discussed in this review.

Figure 4

Schematic diagrams of film morphology in a) BHJ OSCs, b) LbL OSCs without o‐DCB, and c) LbL OSCs with 5% o‐DCB. d) Current density–voltage (J–V) curves of LbL OSCs with different contents of o‐DCB. Reproduced with permission.^{[ 18 ]} Copyright 2018, . e) Photographs for TDA (P3TEA) and non‐TDA (PBDB‐T‐2Cl) polymer‐based pristine films after spin‐coating several solvents on top of them. Reproduced with permission.^{[ 34 ]} Copyright 2019, Wiley‐VCH. f) Different solubility of PNTB‐Cl and PNTB6‐Cl in CF and g) schematic for their morphological evolution by deposition of N3 in CF. Reproduced with permission.^{[ 49 ]} Copyright 2021, Royal Society of Chemistry.

Figure 5

Fabrication of LbL OSCs using protective solvents. Reproduced with permission.^{[ 54 ]} Copyright 2021, Royal Society of Chemistry.

Figure 6

a) Schematic diagrams of film morphology in the LbL OSCs with or without DIO treatment. Reproduced with permission.^{[ 45 ]} Copyright 2021, Wiley‐VCH. b) Schematic illustration of vertical composition distribution in the LbL OSCs without and with DDO in PM6. Reproduced with permission.^{[ 52 ]} Copyright 2021, Wiley‐VCH. c) IHJ structure induced by introduction of wax additive. Reproduced with permission.^{[ 21 ]} Copyright 2021, Wiley‐VCH.

Figure 7

a) Schematic illustration of hybrid planar/bulk devices with ideal p–i–n heterojunction formation, b) the corresponding chemical and device structures, and c) cross‐sectional transmission electron microscope images of the devices. Reproduced with permission.^{[ 19 ]} Copyright 2021, Wiley‐VCH.

Figure 8

a) Enhanced exciton diffusion length of IDIC by self‐FRET and low σ. Reproduced with permission.^{[ 14 ]} Copyright 2019, American Chemical Society. b) Absorption spectra of NFAs and PL spectrum of polymer donor (PBDB‐T‐2F), c) exciton diffusion mechanism via FRET, and d) long‐range energy transfer from polymer donor to NFA layers in LbL OSCs. Reproduced with permission.^{[ 16 ]} Copyright 2021, Elsevier.

Figure 9

a) Schematic diagram of blade coating approach with different baseplate temperatures and b) evolution of morphological properties as a function of baseplate temperature. Reproduced with permission.^{[ 61 ]} Copyright 2021, Wiley‐VCH.

Figure 10

a) Schematic illustration of morphological evolution in PTB7‐Th:FOIC and PTB7‐Th/FOIC+N2200+DIO films, b) normalized PCEs as a function of storage time in nitrogen, and c) stress‐strain curves of PTB7‐Th:FOIC, PTB7‐Th/FOIC, PTB7‐Th/FOIC+DIO, and PTB7‐Th/FOIC+N2200+DIO devices. Reproduced with permission.^{[ 43 ]} Copyright 2020, Wiley‐VCH.

Figure 11

a) Schematic illustration of tensile test setup based on floated ultrathin films, b) stress‐strain curves of BHJ and LbL films, optical microscope images of c) BHJ and d) LbL processed films when the films were under different strains. Reproduced with permission.^{[ 55 ]} Copyright 2021, Wiley‐VCH.

Figure 12

a) Solar modules based on LbL PM6/Y6 films with an active area of 11.52 cm² and J–V curves of the best‐performing BHJ and LbL‐based devices, b) schematic illustration of section for the solar modules with the large area, c) the actual configuration of solar modules based on four series‐connected single cells of 8 mm × 36 mm (active area : 11.52 cm²), and d) LbL process flow diagram for fabricating large‐area solar modules. Reproduced with permission.^{[ 22 ]} Copyright 2020, Elsevier.