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\begin{document}

\title{Synergetic alkoxy side-chain and chlorine-contained end group strategy
toward high performance ultra-narrow bandgap small molecule acceptors}



\author[1]{Yingping Zou}%
\author[1]{Qingya Wei}%
\author[1]{Songting  Liang}%
\author[2]{Beibei  Qiu}%
\author[1]{Wei Liu}%
\author[1]{Xiang Xu}%
\author[3]{Yuang Fu}%
\author[3]{Xinhui Lu}%
\author[1]{Jun Yuan}%
\affil[1]{Central South University}%
\affil[2]{Zhejiang Normal University}%
\affil[3]{The Chinese University of Hong Kong}%


\vspace{-1em}



  
  \date{June 14, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Ultra-narrow bandgap (ultra-NBG) small molecule acceptors (SMAs) show
great potential in organic solar cells (OSCs) due to the extend-ed
near-infrared (NIR) absorption. In this work, a synergetic alkoxy
side-chain and chlorine-contained end group strategy is employed to
achieve A-DA'D-A type ultra-NBG SMAs by introducing alkoxy chains with
oxygen atom at the second position into the thiophene \selectlanguage{greek}β \selectlanguage{english}posi-tion as
well as replacing the F atoms with Cl atoms in the end group. As a
result, the heptacyclic BZO-4F shows a redshifted absorption onset (960
nm) than Y11 (932 nm) without oxygen atoms in the side chains. Then, the
fluorinated end groups are substituted with the chlorinated ones to
synthesize BZO-4Cl. The absorption onset of BZO-4Cl is further
redshifted to 990 nm, corresponding to an optical ultra-NBG of 1.25 eV.
When blending with the polymer donor PBDB-T, the binary devices based on
PBDB-T: BZO-4F and PBDB-T: BZO-4Cl delivers power conversion
efficiencies (PCEs) over 12\%. Furthermore, ternary devices with the
addition of BZ4F-O-1 into PBDB-T: BZO-4Cl system achieve the optimal PCE
of 15.51\%. This work proposes a synergetic alkoxy side-chain and
chlorine-contained end group strategy to achieve A-DA'D-A type ultra-NBG
SMAs, which is important for future molecular design.%
\end{abstract}%



\sloppy


\textbf{Cite this paper:}\emph{Chin. J. Chem.} \textbf{2022} , \emph{40}
, XXX---XXX. \textbf{DOI: 10.1002/cjoc.202200XXX}

Synergetic alkoxy side-chain and chlorine-contained end group strategy
toward high performance ultra-narrow bandgap small molecule acceptors

Qingya Wei, \textsuperscript{a}Songting Liang,\textsuperscript{a} Beibei
Qiu,\textsuperscript{b} Wei Liu,\textsuperscript{a} Xiang
Xu,\textsuperscript{a} Xinhui Lu,\textsuperscript{c} Yuang
Fu,\textsuperscript{c} Jun Yuan\textsuperscript{a} and Yingping
Zou*\textsuperscript{,\emph{a,d}}

\textsuperscript{a} State Key Laboratory of Powder Metallurgy, College
of Chemistry and Chemical Engineering, Central South University,
Changsha 410083, China \textsuperscript{b} Key Laboratory of Solid State
Optoelectronic Devices of Zhejiang Province, College of Physics and
Electronic Information Engineering, Zhejiang Normal University, Jinhua,
Zhejiang 321004, China\textsuperscript{c} Department of Physics, The
Chinese University of Hong Kong, New Territories, Hong Kong 999077,
China

\textsuperscript{d}School of Materials Science and Engineering,
Zhengzhou University, Zhengzhou 450001, China\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Comprehensive Summary}\tabularnewline
\midrule
\endhead
Ultra-narrow bandgap (ultra-NBG) small molecule acceptors (SMAs) show
great potential in organic solar cells (OSCs) due to the extended
near-infrared (NIR) absorption. In this work, a synergetic alkoxy
side-chain and chlorine-contained end group strategy is employed to
achieve A-DA'D-A type ultra-NBG SMAs by introducing alkoxy chains with
oxygen atom at the second position into the thiophene \selectlanguage{greek}β \selectlanguage{english}position as well
as replacing the F atoms with Cl atoms in the end group. As a result,
the heptacyclic BZO-4F shows a redshifted absorption onset (960 nm) than
Y11 (932 nm) without oxygen atoms in the side chains. Then, the
fluorinated end groups are substituted with the chlorinated ones to
synthesize BZO-4Cl. The absorption onset of BZO-4Cl is further
redshifted to 990 nm, corresponding to an optical ultra-NBG of 1.25 eV.
When blending with the polymer donor PBDB-T, the binary devices based on
PBDB-T: BZO-4F and PBDB-T: BZO-4Cl delivers power conversion
efficiencies (PCEs) over 12\%. Furthermore, ternary devices with the
addition of BZ4F-O-1 into PBDB-T: BZO-4Cl system achieve the optimal PCE
of 15.51\%. This work proposes a synergetic alkoxy side-chain and
chlorine-contained end group strategy to achieve A-DA'D-A type ultra-NBG
SMAs, which is important for future molecular design.\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
Organic solar cells \textbar{} A-DA'D-A type small molecule acceptors
\textbar{} Ultra-narrow bandgap \textbar{} Alkoxy side chains
\textbar{}\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Background and Originality Content

Organic solar cells (OSCs) have made continuous progress in recent years
due to the breakthrough in organic semiconducting
materials\textsuperscript{{[}1-11{]}}. Especially, the emerging A-DAꞌD-A
type small molecule acceptors (SMAs), Y6 and its
analogs\textsuperscript{{[}2, 12{]}}, promote the efficiency of OSCs to
15\%-20\% through the modification of the fused DAꞌD backbone,
electron-withdrawing end groups and side
chains\textsuperscript{{[}13-30{]}}. Meanwhile, some A-DAꞌD-A acceptors
with ultra-narrow bandgap (ultra-NBG) have been reported to realize
high\emph{J} \textsubscript{SC} for high-performance tandem or
semi-transparent OSCs\textsuperscript{{[}31-36{]}}. Extending the fused
backbone and modulating the push-pull effects of molecules are the
general methods to narrow the molecular optical bandgap (\emph{E}
\textsubscript{g}\textsuperscript{opt})\textsuperscript{{[}16,
37-40{]}}. And the molecular absorption onset could even be redshifted
to exceeding 1000 nm\textsuperscript{{[}31, 36{]}}. However, the
synthetic cost is increased due to the increased synthetic complexity
and reduced yield.

Side-chain engineering is one of the simplest methods to subtly tune the
molecular properties and device performance, which can effectively
reduce the synthetic difficulty and cost\textsuperscript{{[}21,
41-42{]}}. Particularly, the side chains with functional heteroatoms
could regulate the intra/intermolecular interactions due to the
electronegativity and conjugative/ inductive effects, therefore having
the potential in modulating the molecular packing and absorption
properties. For instance, alkoxy chains are widely used in
electron-donating materials for broadening the absorption
spectra\textsuperscript{{[}43-45{]}}. However, alkoxy substituents on
the SMAs usually cause abnormal blue-shifted
absorption\textsuperscript{{[}46-47{]}}.

In our most recent work, two efficient A-DA'D-A type pentacyclic
acceptors, BZ4F-O-2 and BZ4F-O-3, have been reported by moving the
oxygen atoms in the alkoxy side chains to the second or third position
for the first time\textsuperscript{{[}48{]}}. Through the simple
side-chain engineering strategy, the molecular absorption is obviously
redshifted. And the BZ4F-O-3 achieves one of the highest\emph{J}
\textsubscript{SC}s among the binary OSCs based on the pentacyclic
acceptors. Therefore, it is significant to extend the strategy into
heptacyclic acceptors for high-performance ultra-NBG SMAs with excellent
\emph{J} \textsubscript{SC}.

Accordingly, we insert oxygen atoms into the second position of the side
chains on our reported heptacyclic acceptor
(Y11\textsuperscript{{[}49{]}}) to synthesize BZO-4F. As predicted,
BZO-4F shows a redshifted absorption onset of 960 nm than that of Y11
(932 nm, Figure S1). By replacing the fluorine atoms on the end groups
with chlorine atoms, BZO-4Cl is obtained with a further redshifted
absorption onset of 990 nm. Choosing the low-cost polymer PBDB-T as the
donor, both the PBDB-T: BZO-4F and PBDB-T: BZO-4Cl-based devices exhibit
a moderate photovoltaic efficiency of \textasciitilde{}12.4\% with a
poor \emph{V} \textsubscript{OC}. However, the \emph{J}
\textsubscript{SC}of the PBDB-T: BZO-4Cl based device is increased due
to the redshifted absorption. To further optimize the energy level
arrangement, light absorption and blend morphology, BZ4F-O-1 with high
lowest unoccupied molecular orbital (LUMO) level and complementary
absorption is introduced as a third component into the PBDB-T: BZO-4Cl
blend. As a result, the PBDB-T: BZ4F-O-1: BZO-4Cl-based device achieves
a higher PCE of 15.51\% with the simultaneously boosted \emph{V}
\textsubscript{OC} of 0.77 V, \emph{J} \textsubscript{SC} of 27.85 mA
cm\textsuperscript{-2}and FF of 0.71. As far as we know, this is the
best efficiencies of single-junction OSCs based on PBDB-T and SMAs. This
work highlights the importance of a synergetic alkoxy side-chain and
chlorine-contained end group strategy to synthesize heptacyclic A-DA'D-A
type ultra-NBG SMAs for high-performance OSCs.

Results and Discussion

Figure 1a shows the chemical structures of PBDB-T, BZ4F-O-1, BZO-4F and
BZO-4Cl. PBDB-T is purchased from Solarmer Materials Inc., BZ4F-O-1 is
synthesized according to our previous work. The synthetic procedures of
two new acceptors, BZO-4F and BZO-4Cl are shown in Scheme S1 in the
supporting information (SI). Alkoxyl-substituted
thieno{[}3,2-b{]}thiophene unit (compound 4) is one of the key
intermediates that is synthesized through three-step reactions,
including Bouveault aldehyde synthesis, reduction reaction by sodium
borohydride and Williamson reaction. Subsequently, BZO-4F and BZO-4Cl
are obtained by the similar synthetic method of BZ4F-O-1. Chemical
structures of the intermediate molecules and target SMAs are determined
by nuclear magnetic resonance hydrogen and carbon spectra
(\textsuperscript{1}H and \textsuperscript{13}C NMR) and mass spectra
(MS), as shown in Figures S12-S24. BZO-4F and BZO-4Cl have good
solubility in common processing solvents, \emph{eg.} , chloroform,
chlorobenzene and toluene at room temperature. Thermogravimetric
analysis (TGA) is employed to evaluate the thermal stability of the two
acceptors, displayed in Figure S2. BZO-4F and BZO-4Cl exhibit high
decomposition temperatures of 305 \textsuperscript{o}C and
296\textsuperscript{o}C (\emph{T} \textsubscript{d}, 5\% weight loss),
respectively, revealing their good thermal stability.

The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra of
BZO-4F and BZO-4Cl in chloroform solution are displayed in Figure S3.
BZO-4F shows an optical absorption region in 670-830 nm, while both the
absorption peak and onset of BZO-4Cl are redshifted by
\textasciitilde{}20 nm. Similarly, in the film state, BZO-4Cl has a
bathochromic absorption spectrum with an ultra-NBG of 1.25 eV, 30 nm
redshift than that of BZO-4F (Figure 1b and Table 1). Electrochemical
energy levels of BZO-4F and BZO-4Cl were investigated by cyclic
voltammetry (CV) measurement. The CV curves are shown in Figure S4. The
highest occupied molecular orbital (HOMO)/ LUMO energy levels of BZO-4F
and BZO-4Cl are calculated as -5.61/-3.92 eV and -5.69/-3.93 eV,
respectively (Figure 1c and Table 1). The redshifted absorption from
BZO-4F to BZO-4Cl could be attributed to the enhanced ICT effect and the
\selectlanguage{greek}π-π \selectlanguage{english}stacking caused by the chlorine atoms. Different from the generally
lowered LUMO energy level of chlorinated SMA than that of the
fluorinated one, BZO-4Cl has a similar LUMO level with BZO-4F.
Therefore, the BZO-4Cl-based OSCs have the potential to achieve
high\emph{J} \textsubscript{SC} without the sacrifice of\emph{V}
\textsubscript{OC}.

Density functional theory (DFT) is employed to further explore the
molecular conformation and energy levels of BZO-4F and BZO-4Cl at the
B3LYP/6-31G* level. Undecyl and 2-ethylhexyl onto the thiophene/
benzotriazole units are simplified with methyl groups, while the
2-ethylhexyl side chains onto the pyrrole units are retained to reflect
the relatively true molecular conformation. The top/side view of BZO-4F
and BZO-4Cl in Figure S5 illustrates a planar molecular conformation
with a slightly twisted backbone that results from the steric hindrance
of the side chains in the pyrrole units. The twisted structure is
beneficial for limiting excessive aggregation. The HOMO/LUMO energy
levels of BZO-4F and BZO-4Cl are calculated as -5.37/ -3.45 eV and
-5.43/ -3.53 eV, respectively, which is consistent with the CV results.
Besides, electrochemical energy levels of BZO-4F and BZO-4Cl in dilute
chloroform solution further verify the theoretical calculation (Figure
S6).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 1.} a) Chemical structures of PBDB-T, BZ4F-O-1, BZO-4F
and BZO-4Cl; b) Normalized absorption spectra of PBDB-T, BZ4F-O-1,
BZO-4F and BZO-4Cl in neat films; c) The energy levels alignment of
PBDB-T, BZ4F-O-1, BZO-4F and BZO-4Cl.

\textbf{Table 1.} The optical absorption and electronic energy level
properties of PBDB-T, BZ4F-O-1, BZO-4F and BZO-4Cl.\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllll@{}}
\toprule
\begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
Acceptor\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{λ}\selectlanguage{english}\textsubscript{max}\textsuperscript{a)} (nm)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{λ}\selectlanguage{english}\textsubscript{onset}\textsuperscript{a)} (nm)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{λ}\selectlanguage{english}\textsubscript{max}\textsuperscript{b)} (nm)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{λ}\selectlanguage{english}\textsubscript{onset}\textsuperscript{b)} (nm)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
\emph{E}\textsubscript{g}\textsuperscript{opt c)} (eV)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
HOMO\textsuperscript{d)} (eV)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
HOMO\textsuperscript{d)} (eV)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
LUMO\textsuperscript{d)} (eV)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
LUMO\textsuperscript{d)} (eV)\strut
\end{minipage} & \begin{minipage}[b]{0.09\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
PBDB-T\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
620\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
666\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
631\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
679\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.83\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.83\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.39\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.39\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.36\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.36\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
BZ4F-O-1\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
702\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
739\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
769\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
815\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.52\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.52\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.73\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.73\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.82\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.82\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
BZO-4F\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
779\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
835\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
886\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
960\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.29\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.29\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.61\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.61\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.92\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.92\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
BZO-4Cl\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
795\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
852\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
909\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
990\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.25\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1.25\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.69\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.69\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.93\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.93\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textsuperscript{a)}In chloroform solution; \textsuperscript{b)}In neat
film; \textsuperscript{c)}Calculated from the formula:\emph{E}
\textsubscript{g}\textsuperscript{opt} = 1240/\selectlanguage{greek}\emph{λ}
\selectlanguage{english}\textsubscript{onset}\textsuperscript{b)};\textsuperscript{d)}By cyclic
voltammetry (CV) measurement.

To investigate the photovoltaic performance of BZO-4F and BZO-4Cl, we
fabricate two devices with a conventional structure of ITO/ PEDOT: PSS/
Donor: Acceptor /PDINN\textsuperscript{{[}50{]}}/ Ag. PBDB-T is selected
as the polymer donor to blend with the acceptors because of the matched
electrochemical energy levels and complementary optical absorption
(Figures 1b and 1c). The device blend films adopt a bulk heterojunction
(BHJ) structure with the optimized processing conditions such as
donor/acceptor (D/A) concentration and weight ratio, additive ratio
(chloronaphthalene, CN) and thermal annealing temperature/time (Table
S1). As shown in the current density-voltage (\emph{J} -\emph{V} )
graphs in Figure 2a, the BZO-4F-based binary device shows a moderate PCE
of 12.42\% with the \emph{V} \textsubscript{OC} of 0.73 V,\emph{J}
\textsubscript{SC} of 26.44 mA cm\textsuperscript{-2} and FF of 0.65.
And the further charge generation, transport, recombination and
morphology characterization are shown in Figures S8-S10. By comparison,
the BZO-4Cl-based binary device delivers a comparable efficiency of
12.48\% with an improved \emph{J} \textsubscript{SC} of 26.86 mA
cm\textsuperscript{-2} and a similar \emph{V} \textsubscript{OC}, which
agrees with the absorption and energy level analysis. The photovoltaic
parameters are summarized in Table 2. To further improve the
photovoltaic performance limited by the poor \emph{V}
\textsubscript{OC}and FF, a simple but efficient ternary strategy is
considered. BZ4F-O-1, an alkoxyl-substituted conjugated pentacyclic
acceptor reported by our group is chosen as the third component into the
PBDB-T: BZO-4Cl blend\textsuperscript{{[}48{]}}. BZ4F-O-1 has a strong
light capture ability in 600-800 nm, which can compensate for the
spectra gap between PBDB-T and BZO-4Cl (Figure 1b). Besides, the higher
LUMO level of BZ4F-O-1 is favorable for the VOC. Furthermore, the
introduction of BZ4F-O-1 might modulate the crystallinity and
miscibility properties of the blend, thus improving the blend
morphology.

First of all, the photovoltaic performance of PBDB-T: BZ4F-O-1 based
binary device is studied. As shown in Figure 2a and Table 2, the PBDB-T:
BZ4F-O-1 device shows a PCE of 10.48\% with a \emph{V}
\textsubscript{OC}of 0.774V, \emph{J} \textsubscript{SC} of 19.36 mA
cm\textsuperscript{-2}and FF of 0.70. The \emph{V} \textsubscript{OC}
and FF are much higher than that of the PBDB-T: BZO-4Cl binary devices.
When BZ4F-O-1 is introduced to the PBDB-T: BZO-4Cl binary system, the
PBDB-T: BZ4F-O-1: BZO-4Cl based ternary device achieved an optimal PCE
of 15.51\% with the\emph{V} \textsubscript{OC}, \emph{J}
\textsubscript{SC} and FF simultaneously improved (Figure S7 and Table
S2). The PCE-\emph{J} \textsubscript{SC} statistics of the PBDB-T-based
OSCs are shown in Figure 2d and Table S3. As far as we know, this is the
best value for single-junction OSCs based on the PBDB-T donor and SMAs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 2.} a) Current density-Voltage (\emph{J} -\emph{V} )
curves; b) Corresponding EQE spectra; c) \emph{J}
\textsubscript{ph}versus \emph{V} \textsubscript{eff} curves of the
optimized binary and ternary OSCs; d) Statistics of PCE and
corresponding\emph{J} \textsubscript{SC} values for single-junction OSCs
based on PBDB-T and SMAs.

The integrated \emph{J} \textsubscript{SC} values of the PBDB-T:
BZ4F-O-1, PBDB-T: BZO-4Cl and PBDB-T: BZ4F-O-1: BZO-4Cl based devices
calculated from the external quantum efficiency (EQE) curves (Figure 2b)
are 19.49, 25.80, and 26.50 mA cm\textsuperscript{-2}, respectively,
which agree well with the measured \emph{J} \textsubscript{SC}s
from\emph{J} -\emph{V} curves within a deviation of 5\%. The EQE spectra
valley at the 560-780 nm range of the PBDB-T: BZO-4Cl blend was
compensated by introducing BZ4F-O-1 as the third component, thus leading
to a significantly integrated \emph{J} \textsubscript{SC} increase of
1.29 mA cm\textsuperscript{-2} (560-780 nm). Besides, the maximum EQE
value is enhanced from 78.53\% to 82.39\%.

\textbf{Table 2.} The optimal photovoltaic parameters of the binary and
ternary OSCs under 1 sun illumination.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
\begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
Donor:Acceptor\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\emph{V}\textsubscript{OC} (V)\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\emph{J}\textsubscript{SC}\emph{\textsuperscript{a)}} (mA
cm\textsuperscript{-2})\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
FF (\%)\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
PCE (\%)\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{μ}\selectlanguage{english}\textsubscript{h}\emph{\textsuperscript{b)}}
(cm\textsuperscript{2} V\textsuperscript{-1}
s\textsuperscript{-1})\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{μ}\selectlanguage{english}\textsubscript{e}\emph{\textsuperscript{b)}}
(cm\textsuperscript{2} V\textsuperscript{-1}
s\textsuperscript{-1})\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
PBDB-T: BZ4F-O-1\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.77\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
19.36 (19.49)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.70\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
10.48\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
6.9\selectlanguage{ngerman}×10\textsuperscript{-5}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
6.6\selectlanguage{ngerman}×10\textsuperscript{-5}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
PBDB-T: BZO-4F\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.73\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
26.44 (25.80)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.65\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
12.42\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
8.3\selectlanguage{ngerman}×10\textsuperscript{-5}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.54\selectlanguage{ngerman}×10\textsuperscript{-4}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
PBDB-T: BZO-4Cl\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.72\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
26.86 (25.80)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.64\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
12.48\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
9.0\selectlanguage{ngerman}×10\textsuperscript{-5}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.51\selectlanguage{ngerman}×10\textsuperscript{-4}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
PBDB-T: BZ4F-O-1: BZO-4Cl\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.77\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
27.85 (26.50)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
0.71\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
15.51\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
3.17\selectlanguage{ngerman}×10\textsuperscript{-4}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
4.20\selectlanguage{ngerman}×10\textsuperscript{-4}\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

\textsuperscript{a)}The values in the bracket are integrated from EQE
spectrum; \textsuperscript{b)}Measured by space charge limited current
(SCLC) method.

\par\null

To further understand the exciton dissociation process of the binary and
ternary devices, the photocurrent density (\emph{J} \textsubscript{ph})
versus effective applied voltage (\emph{V} \textsubscript{eff}) was
measured (Figure 2c). \emph{J} \textsubscript{ph} is calculated by the
formula of \emph{J} \textsubscript{ph} =\emph{J}
\textsubscript{L}-\emph{J} \textsubscript{D}, where\emph{J}
\textsubscript{L} and \emph{J} \textsubscript{D} are the photocurrent
density in the light and in the dark.\emph{V} \textsubscript{eff} is
calculated by the formula of\emph{V} \textsubscript{eff} = \emph{V}
\textsubscript{o}-\emph{V} .\emph{V} \textsubscript{o} denotes the
voltage when the the\emph{J} \textsubscript{ph} =0, and V denotes the
applied voltage. All the devices obtain their saturated photocurrent
density (\emph{J} \textsubscript{sat}) when the \emph{V}
\textsubscript{eff} = 2V. The exciton dissociation probability (\emph{P}
\textsubscript{diss}) and charge collection efficiency (\emph{P}
\textsubscript{coll}) are calculated from \emph{J}
\textsubscript{ph}/\emph{J} \textsubscript{sat}and \emph{J}
\textsubscript{maxpower}/\emph{J} \textsubscript{sat} under the
short-circuit condition and maximal power output point, respectively.
The specific data are displayed at Table S4. All the devices showed
high\emph{P} \textsubscript{diss} over 97\% and \emph{P}
\textsubscript{coll}over 81\%. And the ternary device achieves the
highest\emph{P} \textsubscript{diss} of 98.2\% and \emph{P}
\textsubscript{coll}of 84.4\%, indicating the effective exciton
dissociation and charge collection processes.

The hole and electron mobilities (\selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{h} and\selectlanguage{greek}\emph{μ}
\selectlanguage{english}\textsubscript{e}) are measured by the space charge-limited current
(SCLC) method for investigating the charge transport of the blend films
(Figures 3a and 3b). As a result, the\selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{h} /
\selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{e} values of PBDB-T: BZ4F-O-1, PBDB-T: BZO-4Cl
and PBDB-T: BZ4F-O-1: BZO-4Cl based devices are
6.9\selectlanguage{ngerman}×10\textsuperscript{-5}/6.6×10\textsuperscript{-5},
9.0×10\textsuperscript{-5}/1.51×10\textsuperscript{-4} and
3.17×10\textsuperscript{-4}/4.20×10\textsuperscript{-4}cm\textsuperscript{2}V\textsuperscript{-1}s\textsuperscript{-1},
respectively (Table 2 and Table S5). The less balanced charge transport
of the PBDB-T: BZO-4Cl blend leads to a poor FF of 0.64. However, the
ternary blend achieves both the increased \selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{h}
/\selectlanguage{greek}\emph{μ} \selectlanguage{english}\textsubscript{e} values and a more balanced charge transport,
therefore, the \emph{J} \textsubscript{SC} and FF are simultaneously
improved.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 3.} a) Hole mobility; b) Electron mobility; c)\emph{J}
\textsubscript{SC} versus light intensity; d)\emph{V} \textsubscript{OC}
versus light intensity of the optimized binary and ternary OSCs.

The dependences of \emph{J} \textsubscript{SC} and\emph{V}
\textsubscript{OC} on light intensity (\emph{P} \textsubscript{light})
of the binary and ternary devices are shown in Figure 3c, 3d and Table
S6 to evaluate the charge recombination. The relationship between
\emph{J} \textsubscript{SC} and\emph{P} \textsubscript{light} can be
described by\emph{J} \textsubscript{SC} [?]\emph{P}
\textsubscript{light}\selectlanguage{greek}\textsuperscript{α}. \selectlanguage{english}All the \selectlanguage{greek}α \selectlanguage{english}values of the
devices are approaching 1, indicating negligible bimolecular
recombination in the devices. Meanwhile, \emph{V} \textsubscript{OC}
[?]\emph{nkT/e} ln(\emph{P} \textsubscript{light}) is used to define the
correlation between \emph{V} \textsubscript{OC} and Plight,
where\emph{k} is the Boltzmann constant, \emph{T} is the Kelvin
temperature, and \emph{e} is the elemental charge. Generally, if the
slope is close to \emph{kT/e} , the dominant recombination mechanism is
bimolecular recombination. If not, the mechanism is a complex
recombination process. The slopes of the \emph{V} \textsubscript{OC} -
ln(\emph{P} \textsubscript{light}) curves of the PBDB-T: BZ4F-O-1,
PBDB-T: BZO-4Cl and PBDB-T: BZ4F-O-1: BZO-4Cl devices are
1.48\emph{kT/q} , 1.30\emph{kT/q} and 1.26\emph{kT/q} , respectively.
The above results demonstrate that the ternary device possesses a more
suppressed charge recombination.

Active layer morphology has great effects on the exciton dissociation,
charge transport and recombination process, thus the photovoltaic
performance of the resulting OSCs. Differential scanning calorimetry
(DSC) is employed to understand the miscibility of the donor and
acceptor materials (Figure S11). In the heating process, BZ4F-O-1 and
BZO-4Cl exhibit their endothermic peaks at 240.4 \textsuperscript{o}C
and 215.9 \textsuperscript{o}C, respectively. The BZ4F-O-1: BZO-4Cl (1:
1, w/w) mixture shows a medium value of melting temperature at
223.4\textsuperscript{o}C, while the melting peaks of neat BZ4F-O-1 and
BZO-4Cl are disappeared, suggesting good miscibility between BZ4F-O-1
and BZO-4Cl\textsuperscript{{[}51{]}}.

Atomic force microscopy (AFM) is employed to investigate the surface
morphology of the devices based on PBDB-T: BZ4F-O-1, PBDB-T: BZO-4Cl and
PBDB-T: BZ4F-O-1: BZO-4Cl blends. The AFM images are shown in Figure 4.
The phase images present obvious fiber-like interpenetrating morphology.
According to the height images, all the blend films have uniform
surfaces with the root-mean-square (RMS) roughness of 1.510, 0.967 and
0.972 nm. PBDB-T: BZO-4Cl film possesses the lowest RMS value. When the
BZ4F-O-1 is introduced to the PBDB-T: BZO-4Cl binary system, the RMS
value of the ternary film is slightly increased. It is evident that the
crystallinity and miscibility of the blend might be finely improved,
favoring the charge dissociation and transport. In addition,
transmission electron microscopy (TEM) patterns (Figure 4) also prove
the uniform morphology of the binary and ternary blend films.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 4.} The AFM height images (2 \selectlanguage{greek}μ\selectlanguage{english}m \selectlanguage{ngerman}×2 \selectlanguage{greek}μ\selectlanguage{english}m) of a) PBDB-T:
BZ4F-O-1; b) PBDB-T: BZO-4Cl; c) PBDB-T: BZ4F-O-1: BZO-4Cl; The AFM
phase images (2 \selectlanguage{greek}μ\selectlanguage{english}m \selectlanguage{ngerman}×2 \selectlanguage{greek}μ\selectlanguage{english}m) of d) PBDB-T: BZ4F-O-1; e) PBDB-T: BZO-4Cl; f)
PBDB-T: BZ4F-O-1: BZO-4Cl; The TEM images of g) PBDB-T: BZ4F-O-1; h)
PBDB-T: BZO-4Cl; i) PBDB-T: BZ4F-O-1: BZO-4Cl

To further investigate the intermolecular stacking behavior of the neat
and blend films, 2D-grazing incidence wide-angle X-ray scattering
(GIWAXS) patterns and the corresponding 1D profiles are fully analyzed.
As shown in Figures 5, S10 and Table S7, all the neat films of the
acceptor materials show a strong (010) peak at \textasciitilde{}1.8
\selectlanguage{ngerman}Å\textsuperscript{-1} in the out-of-plane (OOP) direction and (100) peak
at \textasciitilde{}0.4 Å\textsuperscript{-1} in the in-plane (IP)
direction, which indicates a predominant face-on orientation. The (010)
peak is further studied to understand the \selectlanguage{greek}π-π \selectlanguage{english}stacking properties.
BZ4F-O-1 and BZO-4Cl show the \selectlanguage{greek}π-π \selectlanguage{english}stacking distance (d-spacing) of 3.50
and 3.42 \selectlanguage{ngerman}Å with the crystal coherence lengths (CCLs) of 22.5 and 27.0 Å,
respectively. BZO-4Cl presents a shorter \emph{d} -spacing and longer
CCL than that of BZ4F-O-1, thus a closer \selectlanguage{greek}π-π \selectlanguage{english}stacking, which could be
ascribed to the stronger intermolecular interaction. Compared with the
neat films, the binary blends retain the favorable face-on orientation.
PBDB-T: BZO-4Cl blend exhibits the closest \emph{d} -spacing among the
binary devices with the longest CCLs, which is favorable for charge
transport and consistent with the higher \emph{J} \textsubscript{SC}.
When BZ4F-O-1 is introduced into the PBDB-T: BZO-4Cl system, the\emph{d}
-spacing of the ternary blend is slightly increased and the CCL is
meanwhile reduced, indicating the slightly reduced crystallinity.
Combined with the suppressed recombination process and the improved
charge transport (Figure 3), we can conclude that a proper (not that
close) molecular \selectlanguage{greek}π-π \selectlanguage{english}stacking is achieved in the ternary blends due to
the introduction of BZ4F-O-1.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 5.} 2D-GIWAXS patterns of a) PBDB-T, b) BZO-4Cl, c)
PBDB-T: BZO-4Cl, d) PBDB-T: BZ4F-O-1 and e) PBDB-T: BZ4F-O-1: BZO-4Cl
films; f) In-plane (dashed) and out-of-plane (solid) GIWAXS curves of
the corresponding 2D patterns.

Conclusions

In conclusion, we employ a synergetic alkoxy side-chain and
chlorine-contained end group strategy for ultra-NBG heptacyclic SMAs.
And two A-DA'D-A type ultra-NBG acceptors, BZO-4F and BZO-4Cl are
designed and synthesized with the\emph{E}
\textsubscript{g}\textsuperscript{opt} of 1.29 eV and 1.25 eV,
respectively. By blending with the polymer donor PBDB-T, the
BZO-4Cl-based device shows a moderate PCE of 12.48\% with a\emph{J}
\textsubscript{SC} of 26.86 mA cm\textsuperscript{-2}, which is slightly
higher than that of the BZO-4F-based device. However, the photovoltaic
performance is limited by the low\emph{V} \textsubscript{OC} and FF.
BZ4F-O-1 with a high LUMO level and a complementary absorption is
considered to be introduced into the PBDB-T: BZO-4Cl system to construct
ternary OSCs. As well investigated, the ternary blend materials possess
good miscibility. Besides, the recombination process of the ternary
device is suppressed and the charge transport is significantly improved.
Furthermore, the molecular \selectlanguage{greek}π-π \selectlanguage{english}stacking is also subtly controlled,
leading to an optimal blend morphology. Consequently, the PBDB-T:
BZ4F-O-1: BZO-4Cl-based ternary device achieves a high PCE of 15.51\%
with the simultaneously increased\emph{V} \textsubscript{OC} (0.77 V),
\emph{J} \textsubscript{SC} (27.85 mA cm\textsuperscript{-2}) and FF
(0.71). This work highlights a molecular strategy of altering the oxygen
position along the side chains and chlorine-substitution for ultra-NBG
acceptor, which is a potential alternative in semi-transparent OSCs.

Experimental

\textbf{Synthesis of BZO-4F andBZO-4Cl:} Under argon, compound 9 (0.14
g,0.12 mmol) and
2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (0.11
g, 0.48 mmol) were dissolved in chloroform (30 mL). Pyridine (1.5 mL)
was added. After stirring at 65 \textsuperscript{o}C overnight, the
mixture was cooled to room temperature, the solution was concentrated
under reduced pressure, and poured into methanol and filtered. The
residue was purified in a silica gel column using petroleum
ether/dichloromethane (1:1, v/v) as the eluent. BZO-4F was obtained as a
dark blue solid (135 mg, 70\% yield). BZO-4Cl was synthesized by the
same method of BZO-4F.

\textbf{Device fabrication:} The optimized photovoltaic devices were
fabricated with a conventional structure of Glass/ITO/PEDOT: PSS/
PBDB-T: acceptor/PDINN/Ag. Pre-patterned ITO coated glass substrates
(Advanced Election Technology Co., Ltd) were washed with deionized water
and isopropyl alcohol in an ultrasonic bath for 15 minutes. After
blow-drying by high-purity nitrogen, all ITO substrates are cleaned in
the ultraviolet ozone cleaning system for 20 minutes. Subsequently, a
thin layer of PEDOT: PSS (Xi'an Polymer Light Technology Corp 4083) was
deposited through spin-coating on pre-cleaned ITO-coated glass at 4500
rpm for 30 s and dried subsequently at 150\selectlanguage{ngerman}°C for 15 minutes in
atmospheric air. Then the photovoltaic layers were spin-coated in a
glovebox from a solution of PBDB-T: acceptors (14.5mg/mL with 0.1vol\%
CN with the PBDB-T: acceptors weight ratios of 1:1.2, PBDB-T: BZ4F-O-1:
BZO-4Cl = 1: 0.6: 0.6) in CF. The optimal active layers were fabricated
by spin-coating at about 3000 rpm for the 30s. Then the blend films were
treated with thermal annealing at 100 \textsuperscript{o}C for 10 min.
After cooling to room temperature, a PDINN layer via a solution
concentration of 1 mg/mL was deposited at the top of the active layer at
a rate of 3000 rpm for 30 s. Finally, the top Ag electrode of 100 nm
thickness was thermally evaporated through a mask onto the cathode
buffer layer under a vacuum of 1.5×10\textsuperscript{-4} Pa.

Supporting Information

The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2021xxxxx.

Acknowledgement

This work was supported by National Natural Science Foundation of China
(52125306, 22005347).

Author contributions

Yingping Zou and Qingya Wei conceived the idea and designed the
acceptors. Qingya Wei synthesized the acceptor materials and tested the
molecular optoelectronic properties. Songting Liang fabricated the
organic solar cells and characterized the photovoltaic parameters.
Qingya Wei and Songting Liang tested the AFM and TEM. BeiBei Qiu helped
the theoretical calculations. Xinhui Lu and Yuang Fu supported the
GIWAXS. Qingya Wei wrote the original draft, and all the authors
including Jun Yuan, Wei Liu and Xiang Xu contributed to the data
analysis and manuscript revision. Yingping Zou directed this project.

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\begin{longtable}[]{@{}l@{}}
\toprule
(The following will be filled in by the editorial staff)\tabularnewline
Manuscript received: XXXX, 2022 Manuscript revised: XXXX, 2022
Manuscript accepted: XXXX, 2022 Accepted manuscript online: XXXX, 2022
Version of record online: XXXX, 2022\tabularnewline
\bottomrule
\end{longtable}

The Authors\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
& &\tabularnewline
& \textbf{Left to Right:} Qingya Wei, Songting Liang, Beibei Qiu, Wei
Liu, Xiang Xu, Xinhui Lu, Yuang Fu, Jun Yuan and Yingping Zou
&\tabularnewline
\bottomrule
\end{longtable}

\selectlanguage{english}
\FloatBarrier
\end{document}

