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\begin{document}

\title{Enhancing Photovoltaic Performance of Nonfused-Ring Electron Acceptors
via Asymmetric End-group Engineering and Noncova-lently Conformational
Locks}



\author[1]{Bo Liu}%
\author[2]{Congqi Li}%
\author[2]{Xiaobin Gu}%
\author[2]{Yinghui Han}%
\author[3]{Zhixiang Wei}%
\author[2]{Yunhao Cai}%
\author[2]{Xin Zhang}%
\author[1]{Yanping Huo}%
\affil[1]{Guangdong University of Technology}%
\affil[2]{University of the Chinese Academy of Sciences}%
\affil[3]{National Center for Nanoscience and Technology}%


\vspace{-1em}



  
  \date{May 27, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
By employing the asymmetric end-group engineering, an asymmetric
nonfused-ring electron acceptors (NFREAs) was designed and synthesized.
Compared with the symmetric analogs (NoCA-17 and NoCA-18), NoCA-19
possesses broader light absorption range, more coplanar \selectlanguage{greek}π-\selectlanguage{english}conjugated
backbone, and appropriate crystallinity according to the experimental
and theoretical results. The organic solar cells based on J52:NoCA-19
exhibited a power conversion efficiency as high as 12.26\%, which is
much higher than those of J52:NoCA-17 (9.50\%) and J52:NoCA-18
(11.77\%), mainly due to more efficient exciton dissociation, better and
balanced charge mobility, suppressed recombination loss, shorter charge
extraction time, longer charge carrier lifetimes, and more favorable
blend film morphology. These findings demonstrate the great potential of
asymmetric end-group engineering in exploring low-cost and
high-performance NFREAs.%
\end{abstract}%



\sloppy


\textbf{Cite this paper:}\emph{Chin. J. Chem.} \textbf{2023} , \emph{41}
, XXX---XXX. \textbf{DOI: 10.1002/cjoc.202300XXX}

Enhancing Photovoltaic Performance of Nonfused-Ring Electron Acceptors
via Asymmetric End-group Engineering and Noncovalently Conformational
Locks\textsuperscript{+}

Bo Liu,\textsuperscript{\emph{a} ,\emph{b}} Congqi
Li,\emph{\textsuperscript{b}} Xiaobin
Gu,\emph{\textsuperscript{b}}Yinghui Han,*\textsuperscript{,\emph{c}}
Zhixiang Wei,\emph{\textsuperscript{d}} Yunhao
Cai,\emph{\textsuperscript{b}} Xin Zhang*\textsuperscript{,\emph{b}} and
Yanping Huo*\textsuperscript{,\emph{a}}

\textsuperscript{a} School of Chemical Engineering and Light Industry,
Guangdong University of Technology, Guangzhou 510006,
China\textsuperscript{b} College of Materials Science and
Opto-Electronic Technology \& Center of Materials Science and
Optoelectronics Engineering \& CAS Center for Excellence in Topological
Quantum Computation \& CAS Key Laboratory of Vacuum Physics, University
of Chinese Academy of Sciences, Beijing 100049, China\textsuperscript{c}
College Resources and Environment, University of Chinese Academy of
Sciences, Beijing 100049, China\textsuperscript{d} Center for Excellence
in Nanoscience (CAS) \& Key Laboratory of Nanosystem and Hierarchical
Fabrication (CAS), National Center for Nanoscience and Technology,
Beijing 100190, China\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
Organic solar cells \textbar{} Nonfused-ring electron acceptors
\textbar{} Asymmetric end-group engineering \textbar{} Noncovalently
conformational locks \textbar{} Low-cost acceptors\tabularnewline
\textbf{Comprehensive Summary}\tabularnewline
By employing the asymmetric end-group engineering, an asymmetric
nonfused-ring electron acceptors (NFREAs) was designed and synthesized.
Compared with the symmetric analogs (NoCA-17 and NoCA-18), NoCA-19
possesses broader light absorption range, more coplanar \selectlanguage{greek}π-\selectlanguage{english}conjugated
backbone, and appropriate crystallinity according to the experimental
and theoretical results. The organic solar cells based on J52:NoCA-19
exhibited a power conversion efficiency as high as 12.26\%, which is
much higher than those of J52:NoCA-17 (9.50\%) and J52:NoCA-18
(11.77\%), mainly due to more efficient exciton dissociation, better and
balanced charge mobility, suppressed recombination loss, shorter charge
extraction time, longer charge carrier lifetimes, and more favorable
blend film morphology. These findings demonstrate the great potential of
asymmetric end-group engineering in exploring low-cost and
high-performance NFREAs.\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Background and Originality Content

Organic solar cells (OSCs) have gained attention for their light weight,
adjustable absorption range, flexibility, and solution
processability.\textsuperscript{{[}1-3{]}}Recent advancements in this
field have been primarily driven by polymeric and small-molecule
donors/acceptors, with the power conversion efficiencies (PCEs)
surpassing 19\% for single-junction devices.\textsuperscript{{[}4-14{]}}
Among the high-performance acceptor materials, the
acceptor-donor-acceptor (A-D-A) and A-DA'D-A-structured fused-ring
electron acceptors (FREAs), exemplified by Y6 and ITIC respectively,
stand out.\textsuperscript{{[}15-20{]}} With the aim of exploring
superior acceptors, considerable research has focused on molecular
engineering, such as \selectlanguage{greek}π-\selectlanguage{english}conjugated extension, side-chain engineering,
end-group halogenation, and isomerization, to regulate the light
harvesting abilities, energy levels, and packing behaviors of
FREAs.\textsuperscript{{[}21-24{]}} In addition, asymmetric strategy
serves as a key molecular design approach in enhancing the dipole moment
and dielectric constant while reducing the exciton binding energy,
thereby promoting exciton dissociation and charge
transport.\textsuperscript{{[}25, 26{]}}

With the rapid development of FREAs, another type of nonfused-ring
electron acceptors (NFREAs), aimed at reducing product costs, has
emerged.\textsuperscript{{[}27-32{]}} This type of acceptors typically
possess a partially or completely non-fused ring cores, greatly
simplifying molecular design and opening the opportunity for
constructing cost-effective acceptor materials.\textsuperscript{{[}33,
34{]}} However, the additionally introduced rotatable \selectlanguage{greek}\emph{σ} -\selectlanguage{english}single
bonds can result in conformational isomers and twisted backbones. To
address this issue, large steric hindrance of side-chains and weak
intramolecular interactions (also known as noncovalently conformational
locks, NoCLs) are introduced in molecular design to lock-in the
conformation, thereby eliminating conformational isomers and achieving a
highly coplanar \selectlanguage{greek}\emph{π} -\selectlanguage{english}conjugated
backbones.\textsuperscript{{[}35-42{]}} Recently, we reported a
PhO4T-series of symmetric NFREAs, demonstrating higher figure-of-merit
values than those of FREAs, due to their simple structures and concise
synthesis routes.\textsuperscript{{[}43{]}} However, the PCEs of NFREAs
have still lagged far behind those of FREAs. Therefore, how to
rationally design NFREAs for low-cost and high-performance OSCs remains
challenging.

Herein, asymmetric engineering was employed to construct a new NFREA,
namely NoCA-19, with two distinct end-groups
(2-(5,6-dichloro-3-oxo-2,3-dihydro-1\emph{H}
-inden-1-ylidene)malononitrile, IC-2Cl; and
2-(6,7-difluoro-3-oxo-2,3-dihydro-1\emph{H} -cyclopenta{[}\emph{b}
{]}naphthalen-1-ylidene)malononitrile, NC-2F) on the basis of two
symmetric NFREAs (one is previously reported acceptor NoCA-17, and
another one is newly synthesized acceptor NoCA-18) (Figure 1a).
Experimental and theoretical results show that asymmetric acceptor
NoCA-19 possesses broader light absorption range, more coplanar\selectlanguage{greek}\emph{π}
-\selectlanguage{english}conjugated backbone, and appropriate crystallinity, compared to the two
symmetric acceptors. When blending with the polymer donor J52, the
NoCA-19-based blend film afforded more balanced charge mobility,
suppressed recombination loss, shorter charge extraction time, and more
favorable morphology. Therefore, the asymmetric NFREA NoCA-19-based OSCs
achieved the highest PCE of 12.26\% among the three blend systems. This
work highlights the importance of asymmetric end-group engineering in
exploring low-cost and high-performance NFREAs.

Results and Discussion

Synthesis and characterization

As depicted in Scheme S1, the three NFREAs were synthesized in one-pot
Knoevenagel condensation reaction, and then separated individually by
column chromatography. The synthesis route is shown in Scheme S1, and
the detailed synthesis procedures and the
\textsuperscript{1}H-/\textsuperscript{13}C-NMR spectra were given in
Supporting Information. All the three acceptors show good solubility in
common solvents, such as chloroform, toluene, and chlorobenzene.

The UV--vis absorption of these three acceptors in both dilute
chloroform solution and thin-film state are shown in Figures 1b and 1c,
and the corresponding optical parameters are summarized in Table 1. In
dilute chloroform solutions, the shapes of the normalized absorption
curves for three NFREAs are almost identical in the range of 550--850 nm
(Figure 1b). The maximum absorption peaks at 724 nm for NoCA-17 and 734
nm for NoCA-18 indicate a stronger electron-withdrawing ability of NC-2F\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 1} (a) Chemical structures, normalized dilute solution
(b) and thin film (c) optical absorption spectra of the three NRFEAs.
(d) Energy level diagram of donor and acceptor materials in this work.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 2} (a) Optimized geometries (top: top view; bottom: side
view) of the three NFREAs. (b) 2D-GIWAXS patterns of neat NoCA-17,
NoCA-18, and NoCA-19 films.

compared to IC-2Cl, leading to a more efficient intramolecular charge
transfer effect. Accordingly, NoCA-19 displays a blue-shifted absorption
peak (728 nm) in comparison NoCA-18. Both symmetric acceptors, NoCA-17
and NoCA-18, in the thin film exhibit comparable absorption spectra. In
contrast, the asymmetric acceptor NoCA-19 demonstrates a broadening of
the absorption spectrum in the high-energy region, indicating a unique
stacking arrangement in the solid state.

\textbf{Table 1} Basic properties of NoCA-17, NoCA-18, and NoCA-19\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
NFREAs\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{λ}\selectlanguage{english}\textsubscript{abs.max}\textsuperscript{sol.} {[}nm{]}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{λ}\selectlanguage{english}\textsubscript{abs.max}\textsuperscript{film} {[}nm{]}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\emph{E}\textsubscript{HOMO}/\emph{E}\textsubscript{LUMO}\emph{\textsuperscript{a}}
{[}eV{]}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\emph{E}\textsubscript{HOMO}/\emph{E}\textsubscript{LUMO}\emph{\textsuperscript{b}}
{[}eV{]}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{NoCA-17}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
724\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
784\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-5.62/-4.00\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-5.25/-3.30\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{NoCA-18}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
734\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
781\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-5.49/-3.90\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-5.17/-3.22\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{NoCA-19}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
728\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
770\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-5.57/-3.97\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-5.21/-3.27\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\emph{\textsuperscript{a}} Calculated by CV
measurements.\emph{\textsuperscript{b}} Derived from DFT calculations.

Furthermore, cyclic voltammetry (CV) was employed to estimate the energy
levels of the NFREAs in solid state. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy
levels derived from the oxidation/reduction onsets of the CV curves
(Figure S1) are -5.62/-4.00, -5.49/-3.90, and -5.57/-3.97 eV for
NoCA-17, NoCA-18, and NoCA-19, respectively (Figure 1d and Table 1).
Moreover, the trend of DFT calculated \emph{E}
\textsubscript{HOMO/LUMO}values for three NFREAs is well consistent with
CV results (Figure S2).

The optimized molecular geometries of the three NFREAs were investigated
by performing density function theory (DFT) calculations at the
B3LYP/6-31G(d,p) level. The intramolecular distances between the sulfur
atom of the thiophene ring and oxygen atom of the phenylalkoxy motif are
found to be \textasciitilde{}2.71 \selectlanguage{ngerman}Å for all these three acceptors
(Figure 2a), much shorter than the sum of the van der Waals radius
(\emph{r} \textsubscript{w, S···O} = 3.25 Å), indicating the existence
of S···O NoCLs. The dihedral angles between the central phenyl core and
the adjacent thiophene ring are 7.1° and 7.4° for NoCA-17 and NoCA-18,
respectively. In the case of NoCA-19, the dihedral angles decrease
slightly to 7.1/6.6°, indicating a more coplanar \selectlanguage{greek}\emph{π} -\selectlanguage{english}conjugated
backbone due to asymmetric substitution of end-groups.

The packing behaviors in neat films were explored by measuring the
two-dimensional grazing incidence wide-angle X-ray scattering
(2D-GIWAXS). In the in-plane (IP) direction, the neat NoCA-17 film
exhibits a lamellar packing diffraction peak at\emph{q}
\textsubscript{xy} = 0.30 \selectlanguage{ngerman}Å\selectlanguage{english}\textsuperscript{-1}
(\emph{d}\textasciitilde{} 22.67 A), as shown in Figures 1b and S3.
Additionally, the \selectlanguage{greek}\emph{π} -\selectlanguage{english}\selectlanguage{greek}\emph{π} \selectlanguage{english}stacking diffraction signal is
observed in the out-of-plane (OOP) direction at \emph{q}
\textsubscript{z} = 1.61 \selectlanguage{ngerman}Å\textsuperscript{--1} (\emph{d}
\textasciitilde{} 3.90 Å), suggesting the predominant face-on
orientation of NoCA-17. In the neat NoCA-18 film, the lamellar packing
diffraction peak is also observed at\emph{q} \textsubscript{xy} = 0.30
Å\selectlanguage{english}\textsuperscript{-1} in the IP direction, while the \selectlanguage{greek}\emph{π} -\selectlanguage{english}\selectlanguage{greek}\emph{π}
\selectlanguage{english}diffraction peak is located at\emph{q} \textsubscript{z} = 1.78
\selectlanguage{ngerman}Å\textsuperscript{--1} (\emph{d}\textasciitilde{} 3.53 Å), indicating
that the end-group NC-2F can enhance intermolecular interactions and
achieve tighter packing. Surprisingly, the neat NoCA-19 film exhibits a
balanced\selectlanguage{greek}\emph{π} -\selectlanguage{english}\selectlanguage{greek}\emph{π} \selectlanguage{english}stacking distance (\emph{q} \textsubscript{z}
= 1.74 \selectlanguage{ngerman}Å\textsuperscript{--1}, \emph{d} \textasciitilde{} 3.62 Å), which
is closer than NoCA-17 and looser than NoCA-18. The results demonstrate
that the asymmetric end-group engineering can effectively regulate the
packing behaviors in the neat films, enabling NoCA-19 to possess
moderate \selectlanguage{greek}\emph{π} -\selectlanguage{english}\selectlanguage{greek}\emph{π} \selectlanguage{english}stacking distance and appropriate
crystallinity.

Photovoltaic properties

To investigate the photovoltaic performance of the three NFREAs, OSC
devices were fabricated with a conventional structure of
ITO/PEDOT:PSS/active layer/PDINO/Al based on J52:NoCA-series acceptors
blends. The current density-voltage (\emph{J} --\emph{V} )
characteristics are shown in Figure 3a. The detailed device parameters
and the optimization process can be found in Tables 2 and S1-S3. The
optimized device based on J52:NoCA-17 exhibits an inferior PCE of
9.50\%, with an open circuit voltage (\emph{V} \textsubscript{oc}) of
0.804 V, a short circuit current density (\emph{J} \textsubscript{sc})
of 21.55 mA cm\textsuperscript{-2}, and a fill factor (FF) of 54.81\%.
Furthermore, the device based on J52:NoCA-18 shows a decrease in\emph{V}
\textsubscript{oc} (0.792 V) but an improvement in\emph{J}
\textsubscript{sc} (22.72 mA cm\textsuperscript{-2}) and FF (65.51\%),
resulting in a higher PCE of 11.77\%. Impressively, the NoCA-19 based
device exhibits the champion PCE of 12.26\% with an much higher \emph{J}
\textsubscript{s} (23.11 mA cm\textsuperscript{-2}) and FF (67.11\%).
External quantum efficiency (EQE) measurements were carried out to
verify the above \emph{J} \textsubscript{sc} values (Figure 3b). The
integrated \emph{J} \textsubscript{sc} values of the devices based on
J52:NoCA-17, J52:NoCA-18, and J52:NoCA-19 are 20.80 mA
cm\textsuperscript{-2}, 21.90 mA cm\textsuperscript{-2}, and 22.16 mA
cm\textsuperscript{-2}, respectively, The corresponding mismatches are
less than 5\%, as compared to the values from \emph{J}
--\emph{V}characteristics.

Space charge limited current method was used to evaluate charge
transport properties in the three blend films, as shown in Figure S4.
The hole mobility (\selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{h}) and electron mobility
(\selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{e}) were calculated to be 8.74 \selectlanguage{ngerman}×
10\selectlanguage{english}\textsuperscript{-4} and 6.69 x
10\selectlanguage{english}\textsuperscript{-4}cm\textsuperscript{2} V\selectlanguage{english}\textsuperscript{-1}
s\selectlanguage{english}\textsuperscript{-1} for the NoCA-19 blend film, respectively. These
values were found to be much higher than those of the NoCA-17-based
blend film (6.42 x 10\selectlanguage{english}\textsuperscript{-4} and 4.10 x
10\selectlanguage{english}\textsuperscript{-4}cm\textsuperscript{2} V\selectlanguage{english}\textsuperscript{-1}
s\selectlanguage{english}\textsuperscript{-1}) and NoCA-18-based blend film (7.81 x
10\selectlanguage{english}\textsuperscript{-4} and 5.85 x 10\selectlanguage{english}\textsuperscript{-4}
cm\textsuperscript{2} V\selectlanguage{english}\textsuperscript{-1}s\selectlanguage{english}\textsuperscript{-1}).
Furthermore, the J52:NoCA-19 blend film exhibited more balanced charge
carrier mobilities (\selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{h}/\selectlanguage{greek}\emph{μ}
\selectlanguage{english}\textsubscript{e} = 1.31) compared to the other two blends, which may
contribute to the observed improvement in FF.

\textbf{Table 2} Photovoltaic parameters of OSCs based on PM6:NFREAs
under AM1.5G illumination, 100 mW cm\textsuperscript{-2}.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
Active layers\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\emph{V}\textsubscript{oc}\emph{\textsuperscript{a}} {[}V{]}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\emph{J}\textsubscript{sc} {[}mA cm\textsuperscript{-2}{]}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
FF {[}\%{]}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
PCE {[}\%{]}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
J52:NoCA-17\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
0.804 (0.803\selectlanguage{ngerman}±0.001)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
21.55 (21.48\selectlanguage{ngerman}±0.17)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
54.81 (54.52\selectlanguage{ngerman}±0.44)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
9.50 (9.40\selectlanguage{ngerman}±0.07)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
J52:NoCA-18\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
0.792 (0.792\selectlanguage{ngerman}±0.001)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
22.72 (22.60\selectlanguage{ngerman}±0.15)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
65.51 (65.22\selectlanguage{ngerman}±0.57)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
11.77 (11.67\selectlanguage{ngerman}±0.10)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
J52:NoCA-19\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
0.841 (0.790\selectlanguage{ngerman}±0.001)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
23.11 (23.07\selectlanguage{ngerman}±0.10)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
67.11 (67.08\selectlanguage{ngerman}±0.26)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
12.26 (12.22\selectlanguage{ngerman}±0.02)\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

\emph{\textsuperscript{a}} The average values shown in parentheses were
obtained from 15 devices.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 3} (a) \emph{J} -\emph{V} characteristics, (b) EQE
responses, (c) \emph{J} \textsubscript{ph}-\emph{V} \textsubscript{eff},
(d) \emph{J} \textsubscript{sc}-\emph{P} \textsubscript{light}, (e) TPC,
and (f) TPV measurements of the optimized OSCs based on J52:NoCA-17,
J52:NoCA-18, and J52:NoCA-19.

To deep understand the variation in photovoltaic performance among the
three OSC devices, charge generation and recombination were
systematically studied. The photocurrent density (\emph{J}
\textsubscript{ph}) versus effective voltage (\emph{V}
\textsubscript{eff}) curves were measured to investigate the exciton
dissociation and charge extraction mechanism. The exciton dissociation
probability \emph{P} (\emph{E} , \emph{T} ) can be reflected by the
ratio of \emph{J} \textsubscript{ph} to \emph{J} \textsubscript{sat},
where\emph{J} \textsubscript{sat}. is defined as the saturation\emph{J}
\textsubscript{ph} at a sufficiently high\emph{V} \textsubscript{eff},
indicating that all photogenerated excitons can be dissociated into free
carriers and almost totally collected by the individual electrodes. As
shown in Figure 3c, the\emph{P} \textsubscript{diss} values were
calculated as 95.1\%, 96.0\%, and 97.3\% for the NoCA-17, NoCA-18, and
NoCA-19-based devices, respectively. The higher \emph{P} (E, T) suggests
that the J52:NoCA-19 blend film exhibits more efficient exciton
dissociation and charge collection, which is consistent with the
higher\emph{J} \textsubscript{sc} value. In addition, the \emph{J}
-\emph{V}curves were measured under various light intensities (\emph{P}
\textsubscript{light}) to investigate the charge recombination
properties. The relationship between \emph{J} \textsubscript{sc}
and\emph{P} \textsubscript{light} relationship can be described
by\emph{J} \textsubscript{sc} [?]\emph{P}
\textsubscript{light}\selectlanguage{greek}\emph{\textsuperscript{α}} , \selectlanguage{english}where the bimolecular
recombination can be completely suppressed when the ? value approaches
unity. As shown in Figure 3d, the \selectlanguage{greek}\emph{α} \selectlanguage{english}values were calculated as
0.98, 0.96, and 0.99 for the NoCA-17, NoCA-18, and NoCA-19-based
devices, respectively. The higher \selectlanguage{greek}\emph{α} \selectlanguage{english}for the NoCA-19 blend system
indicates suppressed bimolecular recombination, which may contribute to
the higher \emph{J} \textsubscript{sc} and FF relative to the other two
devices.

The charge extraction and recombination processes were further
investigated by measuring the transient photocurrent (TPC) and transient
photovoltage (TPV) decay kinetics of three devices. The charge sweep-out
times at short-circuit condition were evaluated to be 0.58, 0.48, and
0.30 \selectlanguage{greek}μ\selectlanguage{english}s for NoCA-17, NoCA-18, and NoCA-19 based blends, respectively,
indicating higher charge extraction efficiency and higher electron
mobility (Figure 3e). The carrier lifetimes obtained from the decay
traces of TPV measurements (Figure 3f) were 2.35, 3.30, and 8.89 \selectlanguage{greek}μ\selectlanguage{english}s for
NoCA-17, NoCA-18, and NoCA-19 based devices, respectively. At
open-circuit condition, the relatively longer lifetime of free carriers
may imply less recombination. These above results are consistent to the
improved \emph{J} \textsubscript{sc} and FF in J52:NoCA-19 based device.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 4} (a) Tapping-mode AFM height images (2 \selectlanguage{greek}μ\selectlanguage{english}m \selectlanguage{ngerman}× 2 \selectlanguage{greek}μ\selectlanguage{english}m)
(insets are phase images of 1 \selectlanguage{greek}μ\selectlanguage{english}m \selectlanguage{ngerman}× 1 \selectlanguage{greek}μ\selectlanguage{english}m size), (b) TEM images, and (c)
2D-GIWAXS patterns of J52:NoCA-17, J52:NoCA-18, and J52:NoCA-19 blend
films.

Morphology investigation

The surface and bulk morphologies of blend films were investigated by
atomic force microscopy (AFM) and transmission electron microscopy
(TEM). As shown in Figure 4a, the J52:NoCA-19 blend film exhibits a
uniform surface with a higher root-mean-square (\emph{R}
\textsubscript{q}) surface roughness of 1.57 nm compared to the
J52:NoCA-17 (1.09 nm) and J52:NoCA-18 (1.43 nm) blend films.
Furthermore, Figure 4b shows that the J52:NoCA-19 blend film exhibits
smaller domain sizes and nano-scale phase separation. To provide further
insight into the molecular packing in the blend films, 2D-GIWAXS was
employed (Figures 4c and S5). The NoCA-17-based blend film demonstrates
overlapping lamellar packing diffraction peaks with the polymer J52.
Interestingly, the lamellar packing diffraction peaks of the acceptors
in the NoCA-18 and NoCA-19-based blend films remain clear, indicating
that the order packings of NoCA-18 and NoCA-19 were not vulnerable to be
disturbed when blending with J52. This features in film morphology are
beneficial for charge transport and thus device performance.

Conclusions

In conclusion, the asymmetric end-group engineering was employed to
construct a new NFREA, namely NoCA-19 containing two distinct
end-groups, IC-2Cl and NC-2F. Together with two symmetrical NFREAs
(NoCA-17 and NoCA-18), experimental and theoretical studies were
systematically carried out, showing that asymmetric acceptor NoCA-19
possesses broad light absorption range, more coplanar\selectlanguage{greek}\emph{π}
-\selectlanguage{english}conjugated backbone, and appropriate crystallinity. The NoCA-19-based
device delivered a champion PCE of 12.26\%, mainly due to the best and
most balanced carrier mobility, least charge recombination, shortest
charge extraction time, and most favorable morphology among the three
blend systems. This work sheds light on the potential of asymmetric
end-group engineering in designing low-cost and high-performance NFREAs.

Supporting Information

The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2023xxxxx.

Acknowledgement

B. Liu and C. Li contributed equally to this work. The authors
acknowledge the financial support from the NSFC (2197505, U2001222,
52103352, 52120105006, and 51925306), National Key R\&D Program of China
(2018FYA 0305800), Key Research Program of Chinese Academy of Sciences
(XDPB08-2), the Youth Innovation Promotion Association of Chinese
Academy of Sciences (2022165), and the Fundamental Research Funds for
the Central Universities. DFT results described in this article were
obtained from the National Supercomputing Centre in Shenzhen (Shenzhen
Cloud Computing Centre).

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\begin{longtable}[]{@{}l@{}}
\toprule
Manuscript received: XXXX, 2023 Manuscript revised: XXXX, 2023
Manuscript accepted: XXXX, 2023 Accepted manuscript online: XXXX, 2023
Version of record online: XXXX, 2023\tabularnewline
\bottomrule
\end{longtable}

\textbf{Entry for the Table of Contents}\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Enhancing Photovoltaic Performance of Nonfused-Ring Electron
Acceptors via Asymmetric End-group Engineering and Noncova-lently
Conformational Locks} Bo Liu, Congqi Li, Xiaobin Gu, Yinghui Han,*
Zhixiang Wei, Yunhao Cai, Xin Zhang* and Yanping Huo*\textsuperscript{,}
\emph{Chin. J. Chem.} \textbf{2023}, \emph{41}, XXX---XXX. \textbf{DOI:
10.1002/cjoc.202300XXX}\tabularnewline
The asymmetric end-group engineering was employed to construct a new
nonfused-ring electron acceptors (NFREAs), namely NoCA-19 containing two
distinct end-groups. Compared with symmetric acceptors, asymmetric
acceptor NoCA-19 possesses broad light absorption range, more coplanar
\selectlanguage{greek}\emph{π}-\selectlanguage{english}conjugated backbone, and appropriate crystallinity. Thus,
NoCA-19-based organic solar cell shows a considerable power conversion
efficiency of 12.26\%, demonstrating the potential of asymmetric
end-group engineering in achieving low-cost and high-performance
NFREAs.\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
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