Y6 and its derivatives: molecular design and physical mechanism - Oxford Academic Journals
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National Science Review
PERSPECTIVE 8: nwab121, 2021
https://doi.org/10.1093/nsr/nwab121
Advance access publication 14 July 2021
MATERIALS SCIENCE
Special Topic: Advanced Materials for Solar Energy Conversion
Y6 and its derivatives: molecular design and physical mechanism
Qingya Wei1 , Jun Yuan1 , Yuanping Yi 2 , Chunfeng Zhang3 and Yingping Zou1,∗
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Non-fullerene-acceptor (NFA) materials Y-series acceptors is suppressed due to tical and electrochemical properties of
in the bulk heterojunction (BHJ) play the strengthened photoluminescence the Y-series acceptors. Halogenation was
an important role in high-performance quantum yield, which could result a simple but efficient method. For in-
organic solar cells (OSCs). Recently, from the electron-deficient A and the stance, Yan et al. altered the chlorine and
Y-series acceptors with an A-DA D-A molecular geometry. bromine positions on the terminal group
structure have been a research hotspot Subsequently, substantial efforts have to synthesize the BTP-ClBr. Enhanced
and advanced device efficiency to 15%– been paid to optimization of Y6 in the device performance, especially the VOC
18%. This perspective will focus on the side chains, backbone and end groups and FF, were achieved due to the shal-
discovery of Y6 and its derivatives as well (Fig. 1a). Zou and Yan et al. regulated lower LUMO energy level and the im-
as the intrinsic physical mechanism of the branched position of 2-ethylhexyl in proved blend morphology [7]. In 2017,
Y-series-based devices, which provides a the pyrrole of Y6 away from the nitro- Li et al. proposed a smart strategy of poly-
guideline for future molecular design. gen atoms to obtain N3 and N4 with merized small molecular acceptors (PS-
As the original molecule of the increasing solubility. Using PM6 as the MAs), which was very important for the
Y-series acceptors, BZIC (Y0) consists donor, the PM6 : N3 blend showed the development of all-polymer OSCs [8].
of a benzotriazole central core and a best molecular orientation and domain Using Y-series acceptors as key building
pentacyclic fused backbone with a simple size, resulting in the highest device effi- blocks combined with Li’s strategy, the
synthesis process of Stille coupling, dou- ciency [3]. Meanwhile, Sun et al. intro- PSMAs have boosted the efficiency of all-
ble intramolecular Cadogan reductive duced branched alkyl chains with differ- polymer OSCs by over 17% [9].
cyclization, N-alkylation, nucleophilic ent lengths to the thiophene β-position In addition to the above representa-
reaction and Knoevenagel condensation of Y6. As a result, the molecular stack- tive examples in the material design, the
[1]. After expanding the conjugated ing behavior was improved and an ex- intrinsic mechanism of Y-series acceptors
framework, attaching the terminal alkyl cellent efficiency of 18.32% with a high was also studied, revealing the reason
chains, replacing the electron-deficient fill factor (FF) of 81.5% was achieved for the outstanding device performance.
A core and fluorinating the end groups, [4]. These works revealed the impor- Zhang et al. investigated the charge sep-
a well-known Y6 was synthesized with an tance of branching positions and the aration mechanism in PM6 : Y6 blend
outstanding photovoltaic performance length of bulky side chains in NFAs. As using a method of broadband transient
exceeding 15% [2]. In terms of molecular for the backbone reformation, the asym- absorption (TA) spectroscopy. They
structure, the reason why Y6 performs metric strategy and introduction of het- demonstrated an intra-moiety excited
well can be summarized as the following. eroatoms are the most popular meth- (i-EX) state that formed in the neat Y6
Differently from the electron-donating ods. Selenium atoms were used to re- domain instead of the charge-transfer
fused backbone of ITIC, Y6 adopts a place the sulfur atoms of the benzothia- (CT) state at the PM6/Y6 interfaces,
fused donor-acceptor-donor (DA D) diazole (BT) or thieno[3,2-b]thiophene consequently enabling dissociation of
framework, fluorinated end groups and (TT) units in Y6 to synthesize Y6Se excitons into free holes and electrons.
a curved geometry, which enables Y6 to and CH1007. These two selenium het- Hence, manipulating the interplay
have more π –π stacking possibilities erocyclic electron acceptors possessed between intra-moiety and interfacial
and stronger near-infrared absorption, redshifted absorption of ∼950 nm and excited species in polymer/NFA blends
therefore, the intermolecular stacking, achieved a high JSC near 28 mA cm–2 could provide a promising strategy
optical absorption and charge transfer and power conversion efficiency (PCE) for future improvement of the charge
properties are enhanced. Meanwhile, over 17% [5,6]. Various terminal groups separation (Fig. 1b) [10]. Furthermore,
the non-radiative recombination of were also employed to modulate the op- Zhu et al. calculated the exciton binding
C The Author(s) 2021. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original
work is properly cited.
Natl Sci Rev, 2021, Vol. 8, nwab121
a b c d
Acceptor (Y6) Donor (PM6) E
Electron- Coulomb attraction
deficient core Hole
i-EX
~0.2 ps rr r
Backbone Pair 2
~15 ps Charge separation barrier
Side chain LE Pair 1 Pair 3
Electronic polarization
End group Electron
Figure 1. (a) The molecular structure of Y6 and molecular modification for its analogs. (b) Schematic diagram of the i-EX state formed in the neat
Y6 domain as an intermediate state to separate local excitation (LE) states into free polarons. Adapted with permission from ref. [10]. Copyright 2020
American Chemical Society. (c) Schematic description of polarization-assisted charge separation. Adapted with permission from ref. [12]. Copyright 2020
American Chemical Society. (d) Molecular pairs in the Y6 single crystal.
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energy (Eb ) in neat Y6 to be an extremely a specific ‘banana’ shape and could form preparation cost and long-term stabil-
small −0.11–0.15 eV, and further proved a twisted transport channel and slipped ity of OSCs should be dealt with. From
that the energy barrier for exciton disso- packing motif, therefore broadening the the perspective of electron-accepting ma-
ciation into free charge carriers was lower conjugated backbone and facilitating car- terials, non-fused acceptors with strong
than the thermal energy at room temper- rier transfer [15]. Furthermore, Yip et al. intramolecular non-covalent interactions
ature, therefore, direct charge generation demonstrated a unique π –π stacking be- will be one of the answers to reducing the
could be enabled without the help of tween the DA D framework (Pair 3) in synthesis cost while maintaining a high
D/A interfaces upon illumination [11]. the Y6 single crystal as well as a common efficiency. Regulating the molecular ag-
Yi et al. also demonstrated a barrierless π –π stacking between end groups (Pair gregation, avoiding light-sensitive groups
charge separation in Y6-based OSCs that 1 and Pair 2) resulting in an effective 3D (e.g. exocyclic double bond) and enhanc-
was due to the increase in polarization ambipolar transport network (Fig. 1d). ing the heat transfer property could im-
energies of hole and electron during The existence of the π –π stacking of Y6 prove the morphology, light and thermal
their separation that could overcome was observed not only in the single crys- stability of acceptors, and thus the OSCs.
the Coulomb attraction of the interfacial tal but also in the PM6 : Y6 blend film
CT state. The large polarization energies through grazing incidence wide-angle
could be attributed to the fluorination X-ray scattering (GIWAXS) and molec- FUNDING
of end groups and the incorporation ular dynamics simulations. This was the This work was supported by the National Key
of an electron-deficient A core in Y6 reason why the PM6 : Y6 blend simulta- Research & Development Program of China
(Fig. 1c) [12]. Neher et al. demonstrated neously showed a low voltage loss and (2017YFA0206600) and the National Natural
Science Foundation of China (21875286 and
an efficient photocurrent generation in high charge generation efficiency [16].
22005347).
PM6 : Y6 under a small driving energy As the most popular and successful
for dissociation of CT states. This was small molecular acceptors, Y-series
Conflict of interest statement. None declared.
because an electrostatic bias potential NFAs have been investigated system-
was provided by the large quadrupolar atically in recent years. However, there
Qingya Wei1 , Jun Yuan1 , Yuanping Yi 2,
moment of Y6 and its dimerization in are still some challenges in achieving
Chunfeng Zhang3 and Yingping Zou1,∗
a unit cell to make up for the Coulomb commercialization of OSCs. First of 1 College of Chemistry and Chemical Engineering,
binding of the CT states [13]. Mean- all, since the optimal JSC and FF have
Central South University, China; 2 Beijing National
while, Huang et al. demonstrated the reached ∼29 mA cm–2 [17] and 81.5%
Laboratory for Molecular Sciences, CAS Key
generation of triplet excitons in Y6 and [4], respectively, it is essential to further
Laboratory of Organic Solids, Institute of Chemistry,
its analogs, and the contribution of triplet enhance the photoluminescence quan-
Chinese Academy of Sciences, China and 3 National
states to the device performance was tum yield and reduce the non-radiative
Laboratory of Solid State Microstructures, School of
demonstrated by magneto-photocurrent recombination loss by introducing
Physics, and Collaborative Innovation Center for
and transient spectroscopy [14]. the groups with a photoluminescence
Advanced Microstructures, Nanjing University, China
More importantly, the single-crystal property, controlling the molecular ∗ Corresponding author.
structure of the Y6 molecule was charac- aggregation behavior for an improved
E-mail: yingpingzou@csu.edu.cn
terized to further understand the molecu- VOC and getting a balance of these
lar structure–properties relationship. Liu three parameters for a higher PCE.
et al. identified that the alkyl side chains Meanwhile, the intrinsic mechanism of REFERENCES
of Y6 onto the N-atoms could greatly af- Y-series acceptors should be further 1. Feng LL, Yuan J and Zhang ZZ et al. ACS Appl
fect the stacking of adjacent molecules, studied to predictably develop new Mater Interfaces 2017; 9: 31985–92.
which is totally different from that of typi- acceptors for special applications, such 2. Yuan J, Zhang YQ and Zhou LY et al. Joule 2019;
cal ITIC analogs; the molecule presented as indoor photovoltaics. More urgently, 3: 1140–51.
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