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\begin{document}

\title{PY-IT, an excellent polymer acceptor}



\author[1]{Qingqing Bai}%
\author[1]{Qiming Liang}%
\author[1]{Qian Liu}%
\author[2]{Bin Liu}%
\author[2]{Xugang Guo}%
\author[1]{Li Niu}%
\author[1]{Huiliang Sun}%
\affil[1]{Guangzhou University}%
\affil[2]{Southern University of Science and Technology}%


\vspace{-1em}



  
  \date{June 15, 2024}


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\selectlanguage{english}
\begin{abstract}
All-Polymer solar cells (all-PSCs) have attracted considerable attention
due to their inherent advantages over other types of organic solar
cells, including superior optical and thermal stability, as well as
exceptional mechanical durability. Recently, all-PSCs have experi-enced
remarkable advancements in device performance thanks to the invention of
polymerized small-molecule acceptors (PSMAs) since 2017. Among these
PSMAs, PY-IT has garnered immense interest from the scientific community
due to its exceptional perfor-mance in all-PSCs. In this review, we
presented the design principles of PY-IT and discussed the various
strategies employed in device engineering for PY-IT-based all-PSCs.
These strategies include additive and interface engineering,
layer-by-layer processing methods, meniscus-assisted coating methods,
and ternary strategy. Furthermore, this review highlighted several novel
polymeric donor materials that are paired with PY-IT to achieve
efficient all-PSCs. Lastly, we summarized the inspiring strategies for
further advancing all-PSCs based on PY-IT. These strategies aim to
enhance the overall performance and stability of all-PSCs by exploring
new materials, optimizing device architectures, and improving
fabrication techniques. By leveraging these approaches, we anticipate
significant progress in the development of all-PSCs and their potential
as a viable renewable energy source.%
\end{abstract}%



\sloppy


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

PY-IT, an excellent polymer acceptor

Qingqing Bai,\emph{\textsuperscript{a}} Qiming
Liang,\emph{\textsuperscript{a}} Qian Liu\textsuperscript{a}, Bin
Liu,\textsuperscript{b} Xugang Guo,\textsuperscript{b} Li
Niu,\textsuperscript{a} and Huiliang Sun*\textsuperscript{,\emph{a}}

\textsuperscript{a} Center for Advanced Analytical Science, Guangzhou
Key Laboratory of Sensing Materials and Devices, Guangdong Engineering
Technology Research Center for Photoelectric Sensing Materials and
Devices, School of Chemistry and Chemical Engineering, Guangzhou
University Guangzhou, Guangdong 510006, China

\textsuperscript{b} Department of Materials Science and Engineering,
Southern University of Science and Technology (SUSTech), Shenzhen,
Guangdong 518055, China\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
PY-IT \textbar{} all-polymer solar cells \textbar{} polymerized small
molecule acceptors \textbar{} device engineering \textbar{} ternary
strategy\tabularnewline
\textbf{Comprehensive Summary}\tabularnewline
All-Polymer solar cells (all-PSCs) have attracted considerable attention
due to their inherent advantages over other types of organic solar
cells, including superior optical and thermal stability, as well as
exceptional mechanical durability. Recently, all-PSCs have experienced
remarkable advancements in device performance thanks to the invention of
polymerized small-molecule acceptors (PSMAs) since 2017. Among these
PSMAs, PY-IT has garnered immense interest from the scientific community
due to its exceptional performance in all-PSCs. In this review, we
presented the design principles of PY-IT and discussed the various
strategies employed in device engineering for PY-IT-based all-PSCs.
These strategies include additive and interface engineering,
layer-by-layer processing methods, meniscus-assisted coating methods,
and ternary strategy. Furthermore, this review highlighted several novel
polymeric donor materials that are paired with PY-IT to achieve
efficient all-PSCs. Lastly, we summarized the inspiring strategies for
further advancing all-PSCs based on PY-IT. These strategies aim to
enhance the overall performance and stability of all-PSCs by exploring
new materials, optimizing device architectures, and improving
fabrication techniques. By leveraging these approaches, we anticipate
significant progress in the development of all-PSCs and their potential
as a viable renewable energy source.\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Contents

1. Introduction Page No.

2. Material Design and Device Engineering Page No.

2.1. Design principles of PY-IT Page No.

2.2. Additive and interface engineering Page No.

2.3. Solution processing method Page No.

2.4. Ternary strategy Page No.

3. High-Performance Polymer Donors Matched with PY-IT Page No.

4. Conclusions and Perspectives Page No.

1. Introduction

Photovoltaic energy has experienced remarkable growth in the past three
decades and is considered a promising solution to future energy
challenges. Polymer solar cells (PSCs) have garnered significant
interest due to their potential in eco-friendly roll-to-roll
manufacturing processes, lightweight, and semi-transparent properties.
PSCs based on polymer donors\textsuperscript{{[}1, 2{]}} and
non-fullerene small molecule acceptors (NF-SMAs) have recently achieved
power conversion efficiencies (PCEs) exceeding
19\%\textsuperscript{{[}3, 4{]}}. Among various PSCs, all-polymer solar
cells (all-PSCs)\textsuperscript{{[}5-16{]}} using polymers as both the
electron donor and the electron acceptor materials exhibit good
morphological stability, light stability, and mechanical flexibility,
making them more suitable for flexible device
applications\textsuperscript{{[}7-11, 17, 18{]}}. However, the PCEs of
all-PSCs still need to catch up with those achieved by NF-SMA-based
PSCs. In the initial stages of all-PSCs research, the focus of polymer
acceptor design and development revolved around utilizing building
blocks such as perylene diimide (PDI)\textsuperscript{{[}19, 20{]}} and
naphthalene diimide (NDI)\textsuperscript{{[}21-24{]}}. However, the
device performance of NDI/PDI-based polymer acceptors were constrained
by their comparatively low extinction coefficient and strong
crystallization tendency. Besides, B-N-bridged bipyridine and
bithiophene imide-based polymer
acceptors\textsuperscript{{[}25-27{]}}exhibit narrow absorption bands.
Consequently, all-PSCs utilizing these polymer acceptors exhibit PCEs of
below 12\%\textsuperscript{{[}28{]}}.

In 2017, Zhang et al. introduced the groundbreaking concept of
``polymerized small molecule acceptor (PSMA)'', effectively addressing
the primary performance bottleneck of all-PSCs in terms of the poor
absorptivity at long wavelengths\textsuperscript{{[}29{]}}. In their
pioneering work, they developed a narrow bandgap acceptor called PZ1,
incorporating NF-SMA IDIC as the main building block and a monothiophene
as the linker. The resulting polymer PZ1 yielded a remarkable PCE of
9.19\% in all-PSCs thanks to its significantly enhanced light absorption
(absorption coefficient = 1.33 \selectlanguage{ngerman}×
10\textsuperscript{5}cm\selectlanguage{english}\textsuperscript{-1}) compared with most other
polymer acceptors. The success of PZ1 further encouraged researchers to
pursue innovative PSMAs\textsuperscript{{[}30-34{]}}. Among them, Luo et
al. designed and synthesized the first well-regular PSMA, PY-IT, which
achieved an impressive PCE of 15.05\% in
all-PSCs\textsuperscript{{[}35{]}}. This is the first time that the PCE
of all-PSCs has exceeded 15\%. The success of PY-IT has triggered
several research groups to develop well-regular PMSAs. Since 2020, the
PCEs of all-PSCs have risen sharply from \selectlanguage{english}~15\% to \selectlanguage{english}~18.5\% because of
PSMAs (mainly PY-IT) and several newly invented polymer donors together
with device engineering (\textbf{Figure 1}
)\textsuperscript{{[}36-39{]}}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 1} Progress in the photovoltaic performance of
PY-IT-based all-PSCs (The content of PY-IT in the active layer is more
than 50\%).

In this review, we aim to provide valuable insights into the underlying
mechanisms that contribute to the enhanced device performance of the
cutting-edge PY-IT-based all-polymer system. To achieve this, we will
primarily focus on the PY-IT's design principles and explore the various
device engineering strategies employed in PY-IT-based all-PSCs,
including additive and interface engineering, layer-by-layer (LbL)
processing methods, and meniscus-assisted coating methods. Furthermore,
we will investigate the utilization of ternary strategies to further
enhance the efficiency and stability of PY-IT-based all-PSCs. We will
examine the synergistic effects and optimization strategies employed in
ternary systems to understand the underlying mechanisms better. In
addition, several new polymeric donor materials for high-efficiency
all-PSCs will also be highlighted in our review. Finally, we will
thoroughly discuss the opportunities and challenges associated with the
practical implementation of PY-IT in PSC applications. By addressing
these issues, we aim to pave the way for future research and development
efforts in overcoming these obstacles, ultimately facilitating the
widespread adoption of PY-IT-based all-PSCs in real-world applications.
The parameters of the PY-IT-based all-PSCs are listed in Table 1
and\textbf{Figure 1} .

\textbf{Table 1} The summary of performance parameters for PY-IT-based
all-PSCs.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
Active layer & \emph{V}\textsubscript{OC} (V) &
\emph{J}\textsubscript{SC} (mA cm\textsuperscript{-2}) & FF (\%) & PCE
(\%) & Ref.\tabularnewline
\midrule
\endhead
PM6:PY-IT & 0.933 & 22.3 & 72.3 & 15.05 &
\textsuperscript{{[}35{]}}\tabularnewline
PM6:PY-IT:BN-T & 0.955 & 22.65 & 74.3 & 16.09 &
\textsuperscript{{[}40{]}}\tabularnewline
PM6:PY-IT:N2200 & 0.947 & 22.6 & 74.9 & 16.04 &
\textsuperscript{{[}41{]}}\tabularnewline
PM6:PY-IT & 0.936 & 22.77 & 73.2 & 15.62 &
\textsuperscript{{[}42{]}}\tabularnewline
PM6:PY-IT & 0.933 & 22.52 & 73.8 & 15.51 &
\textsuperscript{{[}43{]}}\tabularnewline
PM6:PY-IT & 0.947 & 22.78 & 74.6 & 16.1 &
\textsuperscript{{[}44{]}}\tabularnewline
PM6:PY-IT & 0.945 & 23.27 & 70.77 & 15.53 &
\textsuperscript{{[}45{]}}\tabularnewline
PBDF-NS:PY-IT & 0.859 & 25.24 & 74.6 & 16.17 &
\textsuperscript{{[}46{]}}\tabularnewline
PM6:J71:PY-IT & 0.942 & 23.29 & 75.2 & 16.52 &
\textsuperscript{{[}47{]}}\tabularnewline
PM6:PY-IT:PDI-2T & 0.949 & 23.97 & 70.48 & 16.03 &
\textsuperscript{{[}48{]}}\tabularnewline
PM6:PTQ10:PY-IT & 0.94 & 23.79 & 73.87 & 16.52 &
\textsuperscript{{[}49{]}}\tabularnewline
PM6:PY-IT:PYCl-T & 0.92 & 24.64 & 73.31 & 16.62 &
\textsuperscript{{[}50{]}}\tabularnewline
PM6:PY-IT & 0.95 & 22.61 & 73.62 & 15.81 &
\textsuperscript{{[}51{]}}\tabularnewline
PM6:PY-IT & 0.938 & 23.15 & 74.7 & 16.22 &
\textsuperscript{{[}52{]}}\tabularnewline
PQM-Cl:PY-IT & 0.92 & 24.3 & 80.7 & 18 &
\textsuperscript{{[}53{]}}\tabularnewline
PM6:PM6TPO:PY-IT & 0.945 & 24.8 & 72.52 & 17 &
\textsuperscript{{[}54{]}}\tabularnewline
PM6:PY-IT:PYF-IT & 0.92 & 24.1 & 75 & 16.6 &
\textsuperscript{{[}55{]}}\tabularnewline
PBQ8:PY-IT & 0.91 & 25.14 & 74.5 & 17.04 &
\textsuperscript{{[}56{]}}\tabularnewline
PM6:PY-IT & 0.945 & 23.57 & 76.9 & 17.14 &
\textsuperscript{{[}57{]}}\tabularnewline
PM6:PY-IT:PPCBMB & 0.956 & 24.16 & 78.1 & 18.04 &
\textsuperscript{{[}58{]}}\tabularnewline
PBQx-TF:PM6:PY-IT & 0.931 & 24.5 & 79.6 & 18.2 &
\textsuperscript{{[}8{]}}\tabularnewline
PM6:PY-IT: PYFCl-T & 0.94 & 25.76 & 74.84 & 18.12 &
\textsuperscript{{[}9{]}}\tabularnewline
PQB-2:PY-IT & 0.942 & 24.2 & 79.5 & 18.1 &
\textsuperscript{{[}59{]}}\tabularnewline
PQM-Cl:PTQ10:PY-IT & 0.943 & 24.8 & 78.9 & 18.45 &
\textsuperscript{{[}7{]}}\tabularnewline
PM6:PY-IT & 0.95 & 24.74 & 74.25 & 17.45 &
\textsuperscript{{[}60{]}}\tabularnewline
PM6:PBPD:PY-IT & 0.938 & 24.77 & 73.84 & 17.16 &
\textsuperscript{{[}61{]}}\tabularnewline
PM6:PBB2-H:PY-IT & 0.936 & 24.69 & 76.34 & 17.64 &
\textsuperscript{{[}62{]}}\tabularnewline
PDCBT:PY-IT & 0.929 & 22.8 & 72.2 & 15.3 &
\textsuperscript{{[}63{]}}\tabularnewline
PM6:D18-Cl:PY-IT & 0.941 & 23.85 & 73.9 & 16.6 &
\textsuperscript{{[}64{]}}\tabularnewline
\bottomrule
\end{longtable}

2. Material Design and Device Engineering

2.1. Design principles of PY-IT

Although tremendous progress has been made in PSMA strategies, the
regioisomeric issue associated with brominated
1,1-dicyanomethylene-3-indanone (IC-Br) still needs to be addressed.
IC-Br is, in fact, a mixture of two isomers with similar polarities:
2-(5-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-Br-\selectlanguage{greek}γ\selectlanguage{english})
and 2-(6-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile
(IC-Br-\selectlanguage{greek}δ\selectlanguage{english}), which poses a considerable challenge in terms of its
separation\textsuperscript{{[}65{]}}. Therefore, NF-SMAs capped with
IC-Br become a mixture comprising three isomers, which greatly affects
the reproducibility of the corresponding PSMAs from batch-to-batch and
subsequently impacts the device performances. To this end, Luo et al.
successfully addressed this challenge by separating IC-Br (in) and IC-Br
(out) from IC-Br through recrystallization in different solvents (where
''in'' and ''out'' denote the bromine group and carbonyl group on the
same side and opposite side, respectively) and developed two polymer
acceptors PY-IT and PY-OT (\textbf{Figure 2} ). Besides, a polymer
acceptor PY-IOT was developed by randomly copolymerizing two acceptors
in identical ratios, aiming to further investigate the influence of
mixed isomers on photovoltaic performance. It was observed that there is
a gradual redshift of the absorption edge, a deepening in the lowest
unoccupied molecular orbitals (LUMOs) energy level, and an increase in
electron mobility from PY-OT, PY-IOT to PY-IT. Density functional theory
(DFT) calculations presented that the LUMOs of PY-IT were distributed
throughout its molecular backbone. Compared with PY-OT, the more
delocalized LUMO in PY-IT enhances the charge transfer, resulting in a
redshift of the absorption spectrum. Consequently, by eliminating the
structural disorganization caused by the regiorandom dibrominated
monomers, all-PSCs based on PY-IT delivered an excellent PCE of 15.05\%,
much higher than those for PY-OT (10.04\%) and PY-IOT
(12.12\%)\textsuperscript{{[}35{]}}. Soon after, inspired by PY-IT,
various well-regular PMSAs have been designed and
synthesized\textsuperscript{{[}66-72{]}}. In this review article, we
only discuss the widely used PY-IT.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 2} (a) The synthetic routes of IC-Br (in) and IC-Br
(out). (b) Chemical structures of PY-IT, PY-OT and PY-IOT.

2.2. Additive and interface engineering

In the realm of all-PSCs, the morphology of the active layer plays a
crucial role in determining the dynamics of excitons and charge
carriers. However, achieving the desired morphology becomes increasingly
challenging due to the complex blend of two intimately entangled
materials. Fine-tuning the morphology of polymer/polymer blend films is
crucial for further enhancing the performance of conventional bulk
heterojunction (BHJ) all-PSCs. Appropriate additives play a key role in
optimizing phase separation for effective exciton dissociation and
improving the crystallization and orientation of molecules for efficient
hole and electron transport in the active layer. Various additives can
be employed in this regard, including solvent, solid volatile, and solid
non-volatile\textsuperscript{{[}73{]}}. For instance, the volatile solid
additive 2-methoxynaphthalene (2-MN) has been found to be effective in
fine-regulating the aggregation, molecular packing and phase separation
morphology of PM6:PY-DT system during film deposition and thermal
annealing, fueling the PM6:PY-DT-based all-PSCs to achieve an impressive
PCE of 17.32\%\textsuperscript{{[}74{]}}.

Ma et al.\textsuperscript{{[}41{]}} finely tuned the morphology of the
all-polymer active layer by combining solvent additive (1-CN) and the
solid non-volatile additive (N2200\textsuperscript{{[}75{]}}) in the
film preparation process to improve device performance. The 2D-grazing
incidence wide angle X-ray scattering (2D-GIWAXS) measurements showed
that the improved photovoltaic performances were associated with the
change in the crystal molecular packing from a ``wide and short'' shape
to a ``narrow and tall'' form. This structural transformation enhanced
charge transport, reduced recombination, and suppressed energy loss,
resulting in an increased PCE from 14.93\% to 16.04\% for
PM6:PY-IT-based all-PSCs with the combination of 0.7 vol\% CN and 3 wt\%
N2200 additives.

Recently, solvent additives have been introduced as a simple and
convenient strategy to further improve the performance of LbL all-PSCs
by optimizing the molecular arrangement and microcrystalline structure
of the layers in the donor or acceptor solutions. For example, adding 1
vol\% CN to the PY-IT solution or the PM6:PY-IT blend resulted in a more
than 15\% improvement in PCE for LbL or BHJ
all-PSCs\textsuperscript{{[}51{]}}. The introduction of CN led to a
synergistic optimization of the charge transport, recombination, and
extraction processes, which improved the short-circuit current (\emph{J}
\textsubscript{SC}) and the fill factor (FF) of the all-PSCs. Notably,
the LbL all-PSC obtained a PCE of 15.81\% greater than the corresponding
BHJ all-PSC (15.29\%), mainly due to the formation of more efficient
charge transport channels by the LbL processing method, indicating that
the LbL processing method combined with additives should be a
prospective strategy for the fabrication of efficient all-PSCs.

Modifying the interface is crucial for improving the device performance
of PSCs. Ma et al.\textsuperscript{{[}42{]}} developed three modified
hole transport layers (HTLs), namely PEDOT:PSS-PA, PEDOT:PSS-TA, and
PEDOT:PSS-DA, by simply doping a phenylethylamine derivative into
commercially available Al 4083. The presence of hydroxyl groups on the
phenyl ring allowed for the formation of more hydrogen bonds, leading to
higher work functions and improved electronic conductivity
(\textbf{Figure 5} (a)). Among these modified HTLs, PEDOT:PSS-PA
processed devices achieved the highest PCE of 15.62\% based on the
PM6:PY-IT system. In addition, Shi et al. reported a simple dopant-free
approach to modulate PEDOT:PSS using orthogonal solvents (chloroform
(CF)), where CF can move the insoluble PEDOT and PSS phases during the
rotation process. Subsequently, the composition distribution can be
adjusted to obtain a smoother surface after thermal annealing, which
leads to significantly improved conductivity and device mobility. This
advancement resulted in an impressive efficiency increase of 17.14\% for
PM6:PY-IT\textsuperscript{{[}57{]}}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 3} Chemical structures of the polymer donors for
PY-IT-based all-PSCs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 4} Chemical structures of PY-IT and other related polymer
acceptors.

Ding et al.\textsuperscript{{[}44{]}} focused on enhancing the
performance of electron transport layers (ETLs) by blending two
high-performance ETL materials, PNDIT-F3N and PDINN. This blending
strategy led to more efficient charge extraction, better selectivity,
and reduced charge recombination, primarily due to improved energy
alignment and surface morphology. When applied to a PM6:PY-IT-based
all-PSC processed with a non-halogen solvent (toluene), the
PM6:PY-IT-based all-PSC resulted in a PCE of 16.10\%. This work
illustrates the power of a simple yet effective charge transport layer
design strategy.

During the formation of film morphology, the manipulation of aggregated
structures of polymer donor/acceptor (D/A) and the dynamic drying
process are crucially influenced by the selection of processing
solvents\textsuperscript{{[}76{]}}. In recent years, the exploration of
green solvents\textsuperscript{{[}77-80{]}} to process active layers has
received significant attention due to their eco-friendly nature and
potential for large-scale manufacturing. Jin et
al.\textsuperscript{{[}43{]}} conducted a comparison between CF and
toluene-processed all-PSCs based on PM6:PY-IT. By employing identical
blend solution recipes, film deposition speeds, and post-casting
treatments, toluene-processed all-PSCs obtained a higher PCE of 15.51\%
than that of CF-processed all-PSCs (15.00\%). This improvement was
attributed to the fine-tuned morphology of the active layers facilitated
by toluene solvent (\textbf{Figure 5} (b-e)), which led to better charge
transport and extraction, as well as the inhibition of charge
recombination. Another study by Yang et
al.\textsuperscript{{[}52{]}}demonstrated a PCE of 16.22\% using o-XY as
the solvent in the PM6:PY-IT system, surpassing the PCE values achieved
with toluene or TMB-based all-PSCs.

2.3. Solution processing method

Currently, the majority of highly efficient all-PSCs are fabricated
using the BHJ structure, which offers a bi-continuous interpenetrating
network that facilitates effective charge separation within the active
layer by providing ample D/A interfaces. However, traditional one-step
spin-coating methods suffer from challenges such as closely mixed
molecular crystallinity and relatively low phase purity, preventing
suitable vertical phase separation, efficient charge transfer, and
collection in the BHJ morphology. The LbL processing method has emerged
as a promising alternative for constructing all-PSCs by sequentially
depositing donor and acceptor solutions, allowing independent
optimization of the microstructure in each layer\textsuperscript{{[}81,
82{]}}. This processing method conquers the thermodynamic
incompatibility arising from the blend of polymer donors and acceptors
and achieves a desirable vertical component distribution.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Figure 5} (a) AFM height images of top surfaces and proposed
mechanism of doping's effect on surface morphology of PEDOT: PSS, PEDOT:
PSS-PA, and PEDOT: PSS-DA. Reproduced with
permission,\textsuperscript{{[}42{]}} Copyright 2021, Wiley. (b)
2D-GIWAXS patterns of active layers. (c) AFM phase images of active
layers. (d) \emph{J--V} characteristics. (e) Performance comparison.
Reproduced with permission,\textsuperscript{{[}43{]}} Copyright 2021,
American Chemical Society.

In the quest for further advancements, exciton dissociation in the
acceptor layer near the electrode poses a challenge based on energy
transfer theory because energy cannot be transferred from the narrow
bandgap acceptor to the wide bandgap donor. To address this, a strategy
proposed by Xu et al. involves incorporating fewer donors in the
acceptor layer to achieve exciton dissociation and generate free charges
near the electrode\textsuperscript{{[}60{]}}. By incorporating 10 wt\%
of PM6 in the PY-IT layer, the all-PSC fabricated by the LbL process
achieved an optimum PCE of 17.45\%, which is one of the highest levels
among the LbL all-PSCs prepared using PM6 and PY-IT. Additionally, the
molecular arrangement in the PY-IT layer can be optimized by introducing
the appropriate PM6 to improve the \emph{J} \textsubscript{SC} and FF of
LbL all-PSCs for enhanced charge transfer and collection efficiency
(\textbf{Figure 6} (a-c)). This work also shows that incorporating a
donor in the acceptor layer is an effective strategy to fabricate
efficient LbL all-PSCs by improving the exciton utilization efficiency
in the acceptor layer prepared from narrow bandgap materials.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure 6} (a) Schematic image of LbL active layer with
sequentially spin-coating method. (b) The 2D-GIWAXS patterns of
PM6/PY-IT:PM6 films (c) the out-of-plane and in-plane line-cut profiles
abstracted from the corresponding 2D-GIWAXS patterns. Reproduced with
permission,\textsuperscript{{[}60{]}} Copyright 2023, Wiley. (d)
Schematic illustration of the mechanism for the film-deposition process
via the meniscus, showing the asymmetry in the forward and backward
capillary liquid bridge. (e) In situ observation of the TCL in
active-layer solution by MAC at different advancing speeds of 1, 2, and
3 mm s\textsuperscript{-1}. (f) In situ UV--vis absorption spectra
during MAC preparation at different advancing speed of 1, 2, and 3 mm
s\textsuperscript{-1}. (g) Schematic illustration of evolution process
from transition state to solid film for different\emph{V}
\textsubscript{a}. Reproduced with permission,\textsuperscript{{[}45{]}}
Copyright 2022, Wiley.

Furthermore, solution printing emerges as a cost-effective and suitable
film deposition technique for large-area fabrication. By applying
directional movement of the solution, shear forces accelerate the
crystallization process of the film. This printing strategy has been
successfully applied to prepare highly ordered conjugated polymer films
in high-mobility organic field effect transistors. Given their inherent
film ductility and stability, all-PSCs also hold great potential for
large-area fabrication of cells. Yue et al. introduced a
meniscus-assisted-coating (MAC) strategy that precisely controls the
liquid advancement velocity, maintaining a dynamic balance state between
the solution and the solid film (\textbf{Figure 6} (d-g)). The MAC
strategy focuses on maintaining a linear state of the three-phase
contact line and promoting the formation of a binary nanofiber network
morphology with enhanced crystallization in a uniformly advancing way.
The MAC-based all-PSCs with PM6:PY-IT as active layer showed superior
PCEs of 15.53\%, higher than that of spin coating method
(14.58\%)\textsuperscript{{[}45{]}}.

2.4. Ternary strategy

The ternary device engineering contributes to complementary absorption
behavior, coordinated electronic energy levels, positively tuned blend
film morphology, and balanced electron-hole mobility. Yan's
group\textsuperscript{{[}40{]}} incorporated a B-N-type polymer acceptor
BN-T (\textbf{Figure 4} ) into the PM6:PY-IT system to construct a
ternary blend. The study revealed that BN-T plays a crucial role in
enhancing the crystallization of the active layer in the PM6:PY-IT:BN-T
ternary blend. Simultaneously, it induces a slight reduction in phase
separation. The appropriate size of the crystalline domains and
decreased mixed domains afford higher charge generation and extraction
efficiency and more balanced charge transport in the resulting ternary
all-PSCs. Thermodynamically, BN-T prefers to be located between PM6 and
PY-IT, which fine-tunes the morphology in that region. Moreover, the
ternary blend experiences a significant decrease in non-radiative energy
losses with the coexistence of energy and charge transfer between the
two acceptors. Consequently, all-PSCs based on PM6:PY-IT:BN-T achieved a
PCE of 16.09\%\textsuperscript{{[}40{]}}. This is the first time to
achieve an all-PSCs efficiency greater than 16\%. Later on, the same
group employed two well-miscible polymer donors, PM6 and J71
(\textbf{Figure 3} ), along with PY-IT as the polymer acceptor, to
fabricate a ternary all-PSC with a PCE of
16.52\%\textsuperscript{{[}47{]}}. The improvement can be attributed to
effective energy transfer between the two donors, faster charge transfer
within the ternary blend, more balanced charge transport, suppressed
exciton recombination, and enhanced charge extraction. Morphological
studies have further demonstrated that the favorable miscibility of the
two donors plays a crucial role in maintaining the overall structure of
the nanoscale network within the active layer, leading to more desirable
phase separation. Similarly, another study of ternary all-PSCs based on
PM6:D18-Cl:PY-IT-based showed a PCE of 16.6\%\textsuperscript{{[}64{]}}.

Achieving an ideal vertical-phase distribution of polymer/polymer blends
remains a major challenge, primarily attributed to non-ideal molecular
conformations and mixing behaviors. Cui et al. adopted a
ternary-assisted sequential solution deposition (SSD) strategy to
regulate the vertical compositional profiles of all-PSCs. A favorable
acceptor (donor)-enriched phase can be obtained near the cathode (anode)
by a ternary-assisted SSD strategy, allowing the exciton yield and
carrier density to be improved by a vertical composition gradient. The
improved exciton diffusion length (15.36 nm) in PY-IT:PDI-2T phase
suppressed the non-geminate recombination. Transient absorption
spectroscopy showed that the third component, PDI-2T, markedly enhanced
the hole transfer kinetics in the PM6:PY-IT-based active layer. As a
result, the ternary PM6:PY-IT:PDI-2T (\textbf{Figure 4} ) devices using
SSD strategy obtained a PCE exceeding 16\%. This work demonstrates the
effective contribution of PDI-2T as a third component in assisting
exciton diffusion and charge transfer within PM6:PY-IT systems, and the
success of ternary-assisted SSD strategy in reconfiguring the
vertical-phase distribution, which offers a viable approach to potential
ternary device construction for efficient
all-PSCs.\textsuperscript{{[}48{]}}

To address the relative difficulty in controlling the morphology of
all-polymer blends, Zhang et al. introduced low-cost PTQ10
(\textbf{Figure 3} ) as a second polymer donor (third component) into
the PM6:PY-IT blends to finely tune the energy level matching and
micromorphology of the all-polymer blend. The addition of PTQ10 leads to
reduced \selectlanguage{greek}π-π \selectlanguage{english}stacking distances, increased \selectlanguage{greek}π-π \selectlanguage{english}stacking coherence length
(CL), and a more ordered face-on molecular stacking orientation in the
vertical direction of the substrate, resulting in improved exciton
dissociation, reduced charge-carrier recombination, and better charge
transport capability of the device. Moreover, PTQ10 has a deeper highest
occupied molecular orbital (HOMO) energy level compared with PM6,
leading to an increase in open-circuit voltage (\emph{V}
\textsubscript{OC}) of the ternary device\textsuperscript{{[}49{]}}. As
a result, the PM6:PTQ10:PY-IT-based ternary all-PSC exhibited an
excellent PCE of 16.52\%, benefiting from the simultaneously enhanced
\emph{V} \textsubscript{OC},\emph{J} \textsubscript{SC}, and FF. In
addition, these devices still showed high PCEs of 15.27\% and 13.91\% at
active layer thicknesses of [?]205 and [?]306 nm, respectively.

Terminal group engineering is one of the common strategies to modulate
absorption, energy level, crystallinity, and molecular polarity with
different substituents. Hu et al. developed a novel A-DA'D-A PSMA (named
PYCl-T (\textbf{Figure 4} )) that features a polymer backbone similar to
PY-IT, with the distinction of chlorine substitution on the A-terminal
groups of the A-DA'D-A structure. PYCl-T was incorporated as the third
component in the PM6:PY-IT system, which facilitated the formation of a
homogeneous mixture (alloy) phase and benign molecular ordering in the
ternary blend films, thus promoting the exciton separation and
inhibiting the carrier recombination. Ultimately, the optimized ternary
all-PSCs based on PM6:PY-IT:PYCl-T showed a PCE of 16.62\% (certified
value of 16.3\%). Furthermore, when the effective area was scaled up to
1 cm\textsuperscript{2}, the enlarged ternary all-PSCs achieved a
notable PCE of 15.52\%, showcasing their great potential for large-scale
applications\textsuperscript{{[}50{]}}. Particularly, the ternary device
based on PM6:PY-IT:PYCl-T retained 97.2\% of its initial PCE after
undergoing a 150 \selectlanguage{ngerman}°C thermal stress for 600 minutes, higher than that of
PM6:PY-IT-based binary device (94.6\%). The ternary all-PSC demonstrated
a slight enhancement in its ability under 150 °C thermal stress,
primarily due to the stability of FF.

Designing asymmetric polymer acceptors can enable efficient PSCs.
Precise control of the polarity of polymers is a strategy for regulating
molecular interactions based on the ''like dissolves like'' principle.
Molecular polarity is closely related to substituents and molecular
backbone structure. Considering all the factors mentioned above, Guo et
al.\textsuperscript{{[}9{]}} synthesized two structural analogous
polymer acceptors, namely PYFCl-T and PYF\&PYCl-T (\textbf{Figure 4} ),
incorporating distinct asymmetric backbone configurations. Due to the
comparable molecular polarity indexes of the polymer acceptors PYFCl-T
and PY-IT, indicating their good compatibility, it is easy to form
alloyed structures of the acceptors in the ternary blend of
PM6:PY-IT:PYFCl-T, enabling the modulation of the morphology and energy
levels of the active layer photovoltaic materials. Ternary devices based
on PM6:PY-IT:PYFCl-T obtained proper phase separation, higher and more
balanced charge mobility, reduced charge carrier recombination, and
faster charge transfer kinetics at 10 wt\% PYFCl-T content, resulting in
a remarkable PCE of 18.12\%. This work highlights the feasibility and
effectiveness of advancing all-PSC development by tuning the backbone
configuration of polymer acceptors. Yue et
al.\textsuperscript{{[}54{]}}also proposed an ''end-capped'' ternary
strategy to construct efficient all-PSCs by introducing PM6TPO
(\textbf{Figure 3} ) as a third component, which was synthesized by
Stille coupling reaction of the stannylated end-capped group with the
bromine-terminated side of PM6. Specifically, the ''end-capped'' ternary
strategy avoids the complex selection and additional synthesis of the
third component. PM6TPO exhibited moderate aggregation and excellent
miscibility with end-capped groups with steric effects, indicating
excellent potential for achieving more suitable phase separation and
optimal morphology (\textbf{Figure 7} (a)). The optimized ternary
all-PSC based on PM6:PM6TPO:PY-IT achieved an impressive PCE of 17.0\%
(weight ratio of 0.5:0.5:1), exceeding the PCE of PM6:PY-IT (15.49\%)
and PM6TPO:PY-IT 15.26\% based binary all-PSCs, respectively.
Furthermore, the devices based on PM6:PM6TPO:PY-IT demonstrated
outstanding storage stability and decent tolerance to film thickness.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Figure 7} (a) Schematic illustration of the morphology evolution
mechanism. Both the binary films exhibit larger phase separation, but
the ternary blend show significant more suitable phase separation.
Reproduced with permission,\textsuperscript{{[}54{]}} Copyright 2022,
Wiley. (b) Photovoltaic parameters of PM6:PY-IT:PYF-IT-based devices
with different PYF-IT contents. The evolution plots of normalized PCE
with storage time for the three blend films and the extrapolated T80
lifetime. The stress-strain curves of PM6:PY-IT, PM6:PYF-IT, and
PM6:PY-IT:PYF-IT (50\%) blend films tested by the FOW meth. Reproduced
with permission,\textsuperscript{{[}55{]}} Copyright 2022, SIOC, CAS,
Shanghai, \& Wiley. (c) Schematic diagrams of PBB2-H as the third
component regulating the microstructures of BHJ and PPHJ active layers.
Reproduced with permission,\textsuperscript{{[}62{]}} Copyright 2023,
Royal Society of Chemistry. (d) The evolutions of molecular stacking in
neat films and blend films. Reproduced with
permission,\textsuperscript{{[}61{]}} Copyright 2023, Wiley.

In-depth studies on the relationship between microstructural
photovoltaic properties and the mechanical robustness of the active
layer in all-PSCs are still insufficient. To investigate the
relationship between the microstructural photovoltaic properties and the
mechanical robustness of the active layer in all-PSCs, Xian et
al.\textsuperscript{{[}55{]}} chose the binary blend system of PM6 and
PY-IT as the model system. Considering that the poor solubility and
strong aggregation of PY-IT inhibit the photovoltaic properties and
mechanical robustness of the binary devices, Xian et al. introduced a
fluorinated version named PYF-IT (\textbf{Figure 4} ) as the third
component to regulate the aggregation and crystallization behavior of
the acceptor precisely. The ternary all-PSC based on PM6:PY-IT:PYF-IT
obtained a PCE of 16.6\%, which is higher than that of PM6:PY-IT
(15.0\%) and PM6:PYF-IT (15.1\%) based binary devices. Besides, the
interentanglement between the acceptor polymer chains provided better
film flexibility and stretchability (\textbf{Figure 7} (b)). The
optimized ternary polymer blend demonstrates large-area devices with an
area of 1.0 cm\textsuperscript{2}, showing an ideal photovoltaic PCE of
15.0\% after 1000 consecutive bending cycles and improved mechanical
stability on flexible substrates.

In addition, Wang et al.\textsuperscript{{[}62{]}} presented an
effective approach to induce independent D/A stacking by introducing a
novel polymer donor, PBB2-H (\textbf{Figure 3} ), into PM6:PY-IT systems
for the construction of ternary all-PSCs with BHJ and pseudo-planar
heterojunction (PPHJ) structures, respectively. PBB2-H was found could
penetrate D/A aggregation domains, facilitating optimal molecular
alignment and phase separation in the blends. Upon the addition of 20
wt\% PBB2-H, the PCE of PM6:PBB2-H:PY-IT-BHJ based all-PSCs increased to
17.64\% (thin film) and 16.56\% (thick film), with corresponding
enhancements observed in flexible devices to 15.46\% (thin film) and
14.98\% (thick film). Specifically, the well-miscible PBB2-H component
enables independent optimization of D/A stacking (\textbf{Figure 7}(c)).
Therefore, when the ratio of PM6:PBB2-H at the bottom and PY-IT:PBB2-H
at the top was adjusted to 10:1.5/7:0.5, the PPHJ-based ternary all-PSCs
achieved a champion PCE of 17.94\% and a high FF of 77.45\%, while also
demonstrating remarkable shelf stability and light stability. These
findings demonstrate the significant potential of incorporating
well-miscible third components to induce D/A stacking and construct BHJ
and PPHJ-based all-PSCs. The ability to independently optimize D/A
stacking contributes to improved performance and stability in all-PSCs.

Combining fullerene and non-fullerene acceptors in these studies to form
a ternary matrix is a classical strategy. Analogous to polymerized
Y-series acceptors, polymerized fullerene materials effectively achieve
efficient and stable all-polymer blends, but only small molecules of
PCBM have been attempted as guests of the host system. In a pioneering
study by Liu et al,\textsuperscript{{[}58{]}} a novel polymeric
fullerene material called PPCBMB (\textbf{Figure 4} ) was introduced and
incorporated into PM6:PY-IT to form a ternary device.
PM6:PY-IT:PPCBMB-based ternary delivered the highest PCE of 18.04\% at a
mass ratio of 1:0.8:0.2, superior to the binary control and other
ternary blends with different proportions. The addition of PPCBMB in the
blend resulted in a monotonous decrease in crystallinity and a decrease
followed by an increase in phase separation in the film. Therefore, the
optimal blend exhibits the most suitable film morphology, enabling
efficient charge generation and maintaining excellent charge transport.
Furthermore, Chen and Yan's group developed a poly(fullerene-alt-xylene)
acceptor (PFBO-C12) as a guest into PM6:PY-V-\selectlanguage{greek}γ \selectlanguage{english}host system. The ternary
devices based on PM6:PY-V-\selectlanguage{greek}γ:\selectlanguage{english}PFBO-C12 showed a significant improvement
PCE of 18.0\% compared with the binary devices (16.9\%). These work
further strengthens the effectiveness of developing polyfullerene guests
into ternary systems for achieving highly efficient, stable, and
mechanical robustness ternary all-PSCs\textsuperscript{{[}83{]}}. These
studies highlight the significance of ternary strategies in achieving
efficient all-PSCs by improving phase separation, optimizing morphology,
facilitating charge transfer, and enhancing overall device performance.

3. High-Performance Polymer Donors Matched with PY-IT

The molecular design of polymer donors plays a critical role in
achieving high-performance all-PSCs. In addition to device engineering,
numerous high-performance donors have been developed, and binary devices
fabricated by blending them with PY-IT have exhibited impressive
efficiencies in all-PSCs. Sun's group\textsuperscript{{[}46{]}} designed
and synthesized a polymer donor named PBDF-NS (\textbf{Figure 3} ) with
naphthalene-substituted benzo{[}1,2-b:4,5-b'{]}difuran as the
electron-sufficient units and fluorinated benzotriazole (BTz) as the
electron-deficient units. PBDF-NS demonstrates a significantly low HOMO
energy level of -5.44 eV and a wide bandgap of 1.87 eV. When paired with
PY-IT, the PBDF-NS-based all-PSC exhibited a remarkable PCE of 16.17\%.
Li's group\textsuperscript{{[}56{]}} synthesized a novel
quinoxaline-based polymer donor, PBQ8 (\textbf{Figure 3} ), with an
n-octyl substituent on the difluoro-quinoxaline (DFQ) unit. The impact
of alkyl side chains was investigated by comparing it with a similar D-A
copolymer, PBQ5, based on a DFQ unit with an isooctyl substituent, as
previously reported by the group. PBQ8 exhibited similar energy levels
compared to PBQ5, but it demonstrated enhanced intermolecular
interactions, tighter \selectlanguage{greek}π-π \selectlanguage{english}stacking, and stronger molecular packing
compared to PBQ5. All-PSCs based on PBQ8:PY-IT exhibited a significantly
high PCE of 17.04\%. Furthermore, PBQ8:PY-IT-based devices displayed
excellent tolerance to active layer thickness, maintaining a high PCE of
15.80\% even at an active layer thickness of 250 nm.

When it comes to enhancing stability, reducing the burn-in loss of
morphology in PSC devices stands out as one of the most effective
approaches. In addition to the widely recognized trap-assisted
recombination mechanism, the morphological degradation, characterized by
the mixing or separation of donor and acceptor materials, has also been
identified as an important factor. Given that both of these evolutionary
behaviors can result in significant burn-in loss, it needs to find a
balance point that minimizes undesirable morphological degradation.
Consequently, combining two efficient binary D-A systems which exhibit
distinct phase evolution directions holds great potential to achieve
efficient and stable PSCs. Recently, Ma et
al.\textsuperscript{{[}7{]}}developed efficient all-PSCs based on the
PQM-Cl:PTQ10:PY-IT active layer, achieving an impressive PCE of 18.45\%
and improved device operating stability at maximum power point tracking.
Device lifetime elongation was achieved by mitigating burn-in loss, with
PQM-Cl:PTQ10:PY-IT (0.8:0.2:1.2) exhibiting an alleviated burn-in loss
in morphology and efficiency under light soaking. The combined
characterization of the fresh and light-soaked active layers illustrates
the opposite morphological evolutionary trends and physical degradation
directions of PQM-Cl and PTQ10, resulting in a delicate balance in the
optimal ternary system. In terms of phase separation, the expanding
tendency of PQM-Cl and the shrinking tendency of PTQ10 counteracted each
other leading to the morphological stabilization of its mixed phase;
Moreover, the hole transfer kinetics of the PQM-Cl:PY-IT host was
stabilized by introducing PTQ10.

Moreover, Zhang et al.\textsuperscript{{[}61{]}} successfully reported a
novel ultrawide bandgap polymer donor, PBPD (\textbf{Figure 3} ), with
the \emph{E} \textsubscript{opt} value of up to 2.24 eV by rationally
inhibiting the intramolecular charge transfer (ICT) and quinoid
resonance effects in the D-A conjugated system. Due to its large
transition dipole moments and strong oscillator strengths, PBPD features
high absorption and light harvesting efficiency. The hypsochromic and
hyperchromic absorption spectra of PBPD are well-matched to the most
intense radiation region of the solar spectrum, thus significantly
improving the light utilization in all-PSCs. Furthermore, the face-on
oriented PBPD optimizes the crystallization kinetics of the ternary
matrix, resulting in favorable fibrous microstructures and reduced traps
in the ternary active layer (\textbf{Figure 7} (d)). As a result,
all-PSCs based on PM6:PBPD:PY-IT demonstrated an ideal photon
utilization and achieved a high PCE of up to 17.16\% with an
impressive\emph{J} \textsubscript{SC} of 24.50 mA
cm\textsuperscript{-2}.

Remarkably, Hou's group developed a novel polymer donor PQM-Cl
(\textbf{Figure 3} ) with chlorinated benzodithiophene (BDT-Cl) as the
donor unit and methyl groups substituted
dithieno-{[}3,2-f:2',3'-h{]}quinoxaline (DTQx) as the acceptor unit.
Compared with the commonly used PM6, PQM-Cl surface displayed a negative
electrostatic potential (ESP) value and a low average local ionization
energy distribution, facilitating the efficient generation and transfer
of charges. PQM-Cl:PY-IT-based all-PSCs obtained a record-breaking PCE
of 18.0\% with a rare high FF of 80.7\% and a \emph{J}
\textsubscript{SC}of 24.3 mA cm\textsuperscript{-2{[}53{]}}. A
surprisingly good 91.5\% of the original PCE was retained by the
PQM-Cl:PY-IT-based devices after 2000 consecutive bending cycles at a
bending radius of 5 mm, which exhibited better mechanical resilience
than the PM6:PY-IT-based devices ([?]85\%)\textsuperscript{{[}61{]}}. In
another work from the same group, PM6 with high weight-average molecular
weight (\emph{M} \textsubscript{w}= 151630 kg mol\textsuperscript{-1})
was introduced into the PBQx-TF (\textbf{Figure 3} ) (\emph{M}
\textsubscript{w} = 93036 kg mol\textsuperscript{-1}) and PY-IT
(\emph{M} \textsubscript{w} = 15516 kg mol\textsuperscript{-1})
system\textsuperscript{{[}8{]}}. The incorporation of PBDB-TF imparts
favorable morphological characteristics, including a delicate and smooth
fibrillar network and closer \selectlanguage{greek}π-π \selectlanguage{english}stacking, which facilitates the
generation and transport of charge carriers. As a result,
PBQx-TF:PM6:PY-IT (0.8:0.2:1.2)-based all-PSCs yielded a PCE of 18.2\%r
with an FF of 79.6\% and a\emph{J} \textsubscript{SC} of 24.5 mA
cm\textsuperscript{-2{[}8{]}}. Moreover, the high \emph{M}
\textsubscript{w}of PM6 plays a significant role in stress dissipation
and enhancing the stretchability of the ternary blend film, thereby
resulting in a PCE of 16.5\% in flexible all-PSCs. Simultaneously, the
ternary devices with the addition of PM6 exhibited better flexural
endurance compared with the PBQxTF:PY-IT-based devices ([?]85\%),
maintaining 91\% of initial PCE after 2000 consecutive bending cycles.
To better utilization of PBDB-TF and PBQx-TF, Hou's group then developed
three random terpolymers (PQB-x, x = 1, 2, and 3) (\textbf{Figure 3} )
composed of fluorinated benzodithiophene (BDT-F) and
benzo{[}1,2-c:4,5-c'{]}dithiophene-4,8-dione (BDD) donor units and DTQx
acceptor units\textsuperscript{{[}59{]}}. The terpolymers shifted the
molecular orientation from both face-on and edge-on to exclusively
face-on, effectively suppressing energy disorder (44, 47, and 53 meV for
PQB-2, PBQxTF, and PBDB-TF, respectively). All-PSCs based on the polymer
acceptor PY-IT, PQB-2 exhibited reduced energy loss, leading to an
increased \emph{V} \textsubscript{OC} of 0.942 V. The maximum PCE
achieved by PQB-2:PY-IT based devices reached the PCE of 18.1\%
(certified as 17.6\%), which is one of the highest values among
all-PSCs. This study demonstrates that ternary copolymerization is a
molecular design strategy for tailoring optical and electronic
properties, and it is a viable approach to suppress energy disorder in
all-PSCs, playing a crucial role in enhancing photovoltaic performance.

In the material design of all-PSCs, the interaction between the donor
and acceptor is crucial for avoiding excessive interactions and
miscibility issues. Hou's group\textsuperscript{{[}63{]}} introduced
strong electron-withdrawing functional groups into P3HT, which
suppressed the interaction energy between the donor (PDCBT
(\textbf{Figure 3} )) and acceptor (PY-IT), thereby facilitating the
desired phase separation and high FF in the PDCBT:PY-IT blend. The
research findings revealed the key role of electrostatic forces in
regulating the intermolecular interactions between the donor and
acceptor. A larger difference in electrostatic forces enhanced the
miscibility of the donor and acceptor, suppressing phase separation in
the BHJ blend and reducing the blend's domain purity. P3HT exhibited
relatively low molecular ESPs, resulting in strong interactions with
PY-IT and excessive miscibility. The introduction of ester groups
significantly increased the ESP of PDCBT, reducing its interaction
energy with PY-IT and suppressing the blend's miscibility. Therefore,
maintaining an appropriate difference in electrostatic forces between
the donor and acceptor is crucial for avoiding excessive interactions
and excessive miscibility issues.

4. Conclusions and Perspectives

In this review, we present the development of PSMA PY-IT-based all-PSCs
and summarize the chemical structures of polymer donors and acceptors
associated with PY-IT-based all-PSCs in \textbf{Figure 3}
and\textbf{Figure 4} . The polymer acceptor PY-IT displays broad
absorption, deep LUMO energy levels, high electron mobility, and good
mechanical properties and thus has great potential for photovoltaic
applications. To date, both binary and ternary all-PSCs based on PY-IT
have achieved impressive PCEs of exceeding 18\%. Inspired by the design
principles of PY-IT and the matching high-performance polymer donors and
device engineering to boost device performance, we summarize the
following strategies for improving the overall performance and stability
of all-PSCs.

Interface engineering plays a crucial role in optimizing the device
architecture and improving the performance of all-PSC. For example,
introducing specific dopants at the interface can modify the work
functions and enhance charge transport and collection, or using
orthogonal solvents to drag the composition and thereby control the
interfacial morphology and surface roughness. By carefully selecting the
solvents, we can achieve smoother and suppressed phase separation,
facilitating efficient charge transport. Additionally, interface
modifications can enhance the stability and durability of the devices by
reducing degradation mechanisms and improving long-term performance.

The ability to produce uniform and controlled heterojunction structures
over large areas is essential for the commercialization of all-PSCs. The
LbL deposition method is expected to facilitate the formation of
quasi-planar or cross-linked heterojunctions, enabling vertical phase
separation. Moreover, the LbL and MAC techniques are highly adaptable to
large-scale production, which can be easily scaled up to accommodate
high-throughput manufacturing, making it attractive for industrial
applications.

Incorporating an appropriate third component has been demonstrated to
effectively improve the mechanical durability, stability, and thickness
insensitivity of all-PSCs. Ternary all-PSCs based on PY-IT have achieved
an impressive PCE of 18.45\%, the highest reported for ternary all-PSCs.
Specifically, polymerized asymmetric FREAs show advantages as a third
component for constructing ternary all-PSCs. Polymerized asymmetric
FREAs can form an alloyed phase with the host acceptors, facilitating
charge transfer and transport processes and enhancing\emph{V}
\textsubscript{OC} and FF. Developing polyfullerenes as guests into
all-PSCs is another effective strategy to achieve efficient devices by
synergistically exploit the unique properties of fullerenes and
non-fullerenes. Furthermore, adding high-\emph{M}
\textsubscript{w}material not only allows for precise adjustment of the
nanoscale bicontinuous network structure of the BHJ to enhance its
efficiency but also enhances the mechanical performance of all-PSCs.

Developing high-performance polymer donors that are compatible with
PSMAs is crucial. For instance, quinoxaline building blocks have been
demonstrated to construct high-performance donors, which can match with
PY-IT to fabricate flexible devices with excellent efficiency and
flexural durability. Additionally, thiophene derivatives functionalized
with different electron-withdrawing groups\textsuperscript{{[}36{]}}have
regained attention as they enable control over the polymer's morphology
by adjusting the ESPs and other factors.

Understanding and mitigating the degradation mechanisms in all-PSCs is
crucial for achieving long-term stability and improving the device
lifetime. Factors such as light-induced degradation, thermal
instability, and environmental factors can significantly impact the
performance and durability of the devices. Developing stable materials,
device engineering methods can help mitigate degradation issues and
enhance the long-term stability of all-PSCs.

Beyond their application in PSCs, PY-IT can also be used as an interface
regulator to optimize the interface between the perovskite layer and the
transport layer, enhancing the photovoltaic performance of inverted
perovskite solar cells. PY-IT has shown remarkable effectiveness in
passivating defects and promoting electron transfer performance, with
the advantages of strong planarity and rotatable linkers, significantly
optimizing perovskite grain growth orientation and increasing charge
transfer channels\textsuperscript{{[}84{]}}.

In general, we have put forth a series of strategies to explore novel
materials, optimize device structures, and improve manufacturing
techniques to enhance the overall performance and stability of all-PSCs
and promote their application in markets such as flexible and portable
electronics.

Acknowledgement

Q.B. and Q.L. contributed equally to this work. We are especially
grateful to the inventors of PY-IT, including Prof. Zhenghui Luo and
Prof. Chuluo Yang from Shenzhen University, Prof. Tao Liu from Guangxi
University, Prof. He Yan from Hong Kong University of Science and
Technology, Dr. Ruijie Ma from Hong Kong Polytechnic University and all
collaborators in the article (\emph{Adv. Mater.} \textbf{2020}
,\emph{32} , 2005942). H.S. thanks the support from the National Natural
Science Foundation of China (52173172), the Natural Science Foundation
for Distinguished Young Scholars of Guangdong Province
(2021B1515020027), the Shenzhen Science and Technology Innovation
Commission (JCYJ202103243104813035), the Open Fund of the State Key
Laboratory of Luminescent Materials and Devices (South China University
of Technology, 2022-skllmd-17), the Research \& Development Projects in
Key Areas of Guangdong Province, China (2019B010933001), Science and
Technology Projects in Guangzhou (202201000002), Department of Science
\& Technology of Guangdong Province (2022A156), and University Students'
Innovation Training Program (No. XJ202311078037). This work is also
supported by the Shenzhen Science and Technology Program
(KCXST20221021111413031) and Key Discipline of Materials Science and
Engineering, Bureau of Education of Guangzhou (No. 202255464).

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\begin{longtable}[]{@{}l@{}}
\toprule
(The following will be filled in by the editorial staff)\tabularnewline
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{Photo-} \emph{Chin. J. Chem.} \textbf{2023}, \emph{41},
XXX---XXX. \textbf{DOI: 10.1002/cjoc.202200XXX}\tabularnewline
\textbf{PY-IT, boosting efficiency breakthrough in all-polymer solar
cells.}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
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