A Narrow-Bandgap n-Type Polymer Semiconductor Enabling Efficient All-Polymer Solar Cells

The Promise and Challenges of Inverted Perovskite Solar Cells

Recently, there has been an extensive focus on inverted perovskite solar cells (PSCs) with a p-i-n architecture due to their attractive advantages, such as exceptional stability, high efficiency, low cost, low-temperature processing, and compatibility with tandem architectures, leading to a surge in their development. Single-junction and perovskite-silicon tandem solar cells (TSCs) with an inverted architecture have achieved certified PCEs of 26.15% and 33.9% respectively, showing great promise for commercial applications. To expedite real-world applications, it is crucial to investigate the key challenges for further performance enhancement. We first introduce representative methods, such as composition engineering, additive engineering, solvent engineering, processing engineering, innovation of charge transporting layers, and interface engineering, for fabricating high-efficiency and stable inverted PSCs. We then delve into the reasons behind the excellent stability of inverted PSCs. Subsequently, we review recent advances in TSCs with inverted PSCs, including perovskite-Si TSCs, all-perovskite TSCs, and perovskite-organic TSCs. To achieve final commercial deployment, we present efforts related to scaling up, harvesting indoor light, economic assessment, and reducing environmental impacts. Lastly, we discuss the potential and challenges of inverted PSCs in the future.

Functionalized Phenanthrene Imide-Based Polymers for n-Type Organic Thin-Film Transistors

High-performing n-type polymers are crucial for the advance of organic electronics field, however strong electron-deficient building blocks with optimized physicochemical properties for constructing them are still limited. The imide-functionalized polycyclic aromatic hydrocarbons (PAHs) with extended π-conjugated framework, high electron deficiency and good solubility serve as promising candidates for developing high-performance n-type polymers. Among the PAHs, phenanthrene (PhA) features a well-delocalized aromatic π-system with multiple modifiable active sites . However, the PhA-based imides are seldom studied, mainly attributed to the synthetic challenge. Herein, we report two functionalized PhAs, CPOI and CPCNI, by simultaneously incorporating imide with carbonyl or dicyanomethylene onto PhA. Notably, the dicyanomethylene-modified CPCNI exhibits a well stabilized LUMO energy level (-3.84 eV), attributed to the synergetic inductive effect from imide and cyano groups. Subsequently, based on CPOI and CPCNI, two polymers PCPOI-Tz and PCPCNI-Tz were developed. Applied to organic thin-film transistors, owing to the strong electron-deficiency of CPCNI, polymer PCPCNI-Tz shows an improved electron mobility and largely decreased threshold voltage compared with PCPOI-Tz. This work affords two structurally novel electron-deficient building blocks and highlights the effectiveness of dual functionalization of PhAs with strong electron-withdrawing groups for devising n-type polymers.

Intrinsically Stretchable and Healable Polymer Semiconductors

In recent decades, polymer semiconductors, extensively employed as charge transport layers in devices like organic field-effect transistors (OFETs), have undergone thorough investigation due to their capacity for large-area solution processing, making them promising for mass production. Research efforts have been twofold: enhancing the charge mobilities of polymer semiconductors and augmenting their mechanical properties to meet the demands of flexible devices. Significant progress has been made in both realms, propelling the practical application of polymer semiconductors in flexible electronics. However, integrating excellent semiconducting and mechanical properties into a single polymer still remains a significant challenge. This review intends to introduce the design strategies and discuss the properties of high-charge mobility stretchable conjugated polymers. In addition, another key challenge faced in this cutting-edge field is maintaining stable semiconducting performance during long-term mechanical deformations. Therefore, this review also discusses the development of healable polymer semiconductors as a promising avenue to improve the lifetime of stretchable device. In conclusion, challenges and outline future research perspectives in this interdisciplinary field are highlighted.

Buried-Metal-Grid Electrodes for Efficient Parallel-Connected Perovskite Solar Cells

The limited conductivity of existing transparent conducting oxide (TCO) greatly restricts the further performance improvement of perovskite solar cells (PSCs), especially for large-area devices. Herein, buried-metal-grid tin-doped indium oxide (BMG ITO) electrodes are developed to minimize the power loss caused by the undesirable high sheet resistance of TCOs. By burying 140-nm-thick metal grids into ITO using a photolithography technique, the sheet resistance of ITO is reduced from 15.0 to 2.7 Ω sq-1 . The metal step of BMG over ITO has a huge impact on the charge carrier transport in PSCs. The PSCs using BMG ITO with a low metal step deliver power conversion efficiencies (PCEs) significantly better than that of their counterparts with higher metal steps. Moreover, compared with the pristine ITO-based PSCs, the BMG ITO-based PSCs show a smaller PCE decrease when scaling up the active area of devices. The parallel-connected large-area PSCs with an active area of 102.8 mm2 reach a PCE of 22.5%. The BMG ITO electrodes are also compatible with the fabrication of inverted-structure PSCs and organic solar cells. The work demonstrates the great efficacy of improving the conductivity of TCO by BMG and opens up a promising avenue for constructing highly efficient large-area PSCs.

Biselenophene Imide: Enabling Polymer Acceptor with High Electron Mobility for High-Performance All-Polymer Solar Cells

The shortage of narrow band gap polymer acceptors with high electron mobility is the major bottleneck for developing efficient all-polymer solar cells (all-PSCs). Herein, we synthesize a distannylated electron-deficient biselenophene imide monomer (BSeI-Tin) with high purity/reactivity, affording an excellent chance to access acceptor-acceptor (A-A) type polymer acceptors. Copolymerizing BSeI-Tin with dibrominated monomer Y5-Br, the resulting A-A polymer PY5-BSeI shows a higher molecular weight, narrower band gap, deeper-lying frontier molecular orbital levels and larger electron mobility than the donor-acceptor type counterpart PY5-BSe. Consequently, the PY5-BSeI-based all-PSCs deliver a remarkable efficiency of 17.77 % with a high short-circuit current of 24.93 mA cm-2 and fill factor of 75.83 %. This efficiency is much higher than that (10.70 %) of the PY5-BSe-based devices. Our study demonstrates that BSeI is a promising building block for constructing high-performance polymer acceptors and stannylation of electron-deficient building blocks offers an excellent approach to developing A-A type polymers for all-PSCs and even beyond.

Manipulating Polymer Backbone Configuration via Halogenated Asymmetric End-Groups Enables Over 18% Efficiency All-Polymer Solar Cells

High-performance all-polymer solar cells (all-PSCs) deeply rely on the joint contributions of desirable optical absorption, adaptive energy levels, and appropriate morphology. Herein, two structural analogous polymerized small-molecule acceptors (PSMAs), PYFCl-T and PYF&PYCl-T, are synthesized, and then incorporated into the PM6:PY-IT binary blends to construct ternary all-PSCs. Due to the superior compatibility of PY-IT and PYFCl-T, the ternary all-PSC based on PM6:PY-IT:PYFCl-T with 10 wt% PYFCl-T, presents higher and more balanced charge mobility, suppressed charge recombination, and faster charge-transfer kinetics, resulting in an outstanding power conversion efficiency (PCE) of 18.12% with enhanced J_sc and FF, which is much higher than that (PCE of 16.09%) of the binary all-PSCs based on PM6:PY-IT. Besides, the ternary all-PSCs also exhibit improved photostability. The conspicuous performance enhancement principally should give the credit to the miscibility-driven phase optimization of the donor and acceptor. These findings highlight the significance of polymer-backbone configuration modulation of PSMAs in morphology optimization toward boosting the device properties of all-PSCs.

Efficient All-Polymer Solar Cells Enabled by Interface Engineering

All-polymer solar cells (all-PSCs) are organic solar cells in which both the electron donor and the acceptor are polymers and are considered more promising in large-scale production. Thanks to the polymerizing small molecule acceptor strategy, the power conversion efficiency of all-PSCs has ushered in a leap in recent years. However, due to the electrical properties of polymerized small-molecule acceptors (PSMAs), the FF of the devices is generally not high. The typical electron transport material widely used in these devices is PNDIT-F3N, and it is a common strategy to improve the device fill factor (FF) through interface engineering. This work improves the efficiency of all-polymer solar cells through interfacial layer engineering. Using PDINN as the electron transport layer, we boost the FF of the devices from 69.21% to 72.05% and the power conversion efficiency (PCE) from 15.47% to 16.41%. This is the highest efficiency for a PY-IT-based binary all-polymer solar cell. This improvement is demonstrated in different all-polymer material systems.

High-Performance All-Polymer Photodetectors Enabled by New Random Terpolymer Acceptor with Fine-Tuned Molecular Weight

Reducing the dark current density and enhancing the overall performance of the device is the focal point in research for organic photodetectors. Two novel random terpolymers (P3 and P4) with different molecular weights are synthesized and evaluated as acceptors in bulk heterojunction (BHJ) polymer photodetectors. Compared with known acceptor materials, such as N2200 (P1) and F-N2200 (P2), polymer P4 has a lower lowest unoccupied molecular orbital (LUMO) energy level, favorable morphology, and good miscibility with a donor material J71, which leads to proper phase separation of the blend film and better dissociation of excitons and transport of carriers. Therefore, a considerably low dark current density (Jd) of 1.9 × 10-10 A/cm2 and a high specific detectivity (D*) of 1.8 × 1013 cm Hz1/2/W (also "Jones") at 580 nm under a -0.1 V bias are realized for the P4-based photodetector. More importantly, the device also exhibits a fast response speed (τr/τf = 1.24/1.87 μs) and a wide linear dynamic range (LDR) of 109.2 dB. This work demonstrates that high-performance all-polymer photodetectors with ideal morphology can be realized by random polymer acceptors with a fine-tuned molecular weight.

Polymer Acceptors for High-Performance All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) have attracted considerable attention owing to their pronounced advantages of excellent mechanical flexibility/stretchability and greatly enhanced device stability as compared to other types of organic solar cells (OSCs). Thanks to the extensive research efforts dedicated to the development of polymer acceptors, all-PSCs have achieved remarkable improvement of photovoltaic performance, recently. This review summarizes the recent progress of polymer acceptors based on the key electron-deficient building blocks, which include bithiophene imide (BTI) derivatives, boron-nitrogen coordination bond (B←N)-incorporated (hetero)arenes, cyano-functionalized (hetero)arenes, and fused-ring electron acceptors (FREAs). In addition, single-component-based all-PSCs are also briefly discussed. The structure-property correlations of polymer acceptors are elaborated in detail. Finally, we offer our insights into the development of new electron-deficient building blocks with further optimized properties and the polymers built from them for efficient all-PSCs.

High-Performance, Flexible NO2 Chemiresistors Achieved by Design of Imine-Incorporated n-Type Conjugated Polymers

Flexible and mechanically robust gas sensors are the key technologies for wearable and implantable electronics. Herein, the authors demonstrate the high-performance, flexible nitrogen dioxide (NO2 ) chemiresistors using a series of n-type conjugated polymers (CPs: PNDIT2/IM-x) and a polymer dopant (poly(ethyleneimine), PEI). Imine double bonds (C = N) are incorporated into the backbones of the CPs with different imine contents (x) to facilitate strong and selective interactions with NO2 . The PEI provides doping stability, enhanced electrical conductivity, and flexibility. As a result, the NO2 sensors with PNDIT2/IM-0.1 and PEI (1:1 by weight ratio) exhibit outstanding sensing performances, such as excellent sensitivity (ΔR/Rb = 240% @ 1 ppm), ultralow detection limit (0.1 ppm), high selectivity (ΔR/Rb < 8% @ 1 ppm of interfering analytes), and high stability, thereby outperforming other state-of-the-art CP-based chemiresistors. Furthermore, the thin film of PNDIT2/IM-0.1 and PEI blend is stretchable and mechanically robust, providing excellent flexibility to the NO2 sensors. Our study contributes to the rational design of high-performance flexible gas sensors.

Polymerized Small Molecular Acceptor with Branched Side Chains for All Polymer Solar Cells with Efficiency over 16.7

The power conversion efficiencies (PCEs) of small molecule acceptor (SMA)-based organic solar cells have already exceeded 18%. However, the development of polymer acceptors still lags far behind their SMA counterparts mainly due to the lack of efficient polymer acceptors. Herein, a series of polymer acceptors named PY-X (with X being the branched alkyl chain) are designed and synthesized by employing the same central core with the SMA L8-BO but with different branched alkyl chains on the pyrrole motif. It is found that the molecular packing of SMA-HD featuring 2-hexyldecyl side chain used in the synthesis of PY-HD is similar to L8-BO, in which the branched alkyl chains lead to condensed and high-order molecular assembly in SMA-HD molecules. When combined with PM6, PY-HD-based all polymer solar cell (all-PSC) exhibits a high PCE of 16.41%, representing the highest efficiency for the binary all-PSCs. Moreover, the side-chain modification on the pyrrole site position further improves the performance of the all-PSCs, and the PY-DT-based device delivers a new record high efficiency of 16.76% (certified as 16.3%). The work provides new insights for understanding the structure-property relationship of polymer acceptors and paves a feasible avenue to develop efficient conjugated polymer acceptors.

Side-chain Substituents on Benzotriazole-based Polymer Acceptors Affecting the Performance of All-polymer Solar Cells

Recently, the strategy of polymerized small-molecule acceptors (PSMAs) has attracted extensive attention for applications in all-polymer solar cells (all-PSCs). Although side-chain engineering is considered as a simple and effective strategy for manipulating polymer properties, the corresponding effect on photovoltaic performance of PSMAs in all-PSCs has not been systemically investigated. Herein, we present a series of PSMAs based on the benzotriazole (BTz)-core fused SMAs with different N-alkyl chains including branched 2-butyloctyl, linear n-octyl, and methyl on the BTz unit, namely PZT-C12, PZT-C8, and PZT-C1, respectively. Comparative studies show that the size of alkyl chains has a significant impact on the solid-state behavior of PZT polymers, which in turn affects their light absorption and charge transporting capacities, and subsequently the all-PSC performances. When combining with the polymer donor PBDB-T, PZT-C1 affords a champion power conversion efficiency of 14.9%, compared to 13.1% of PZT-C12, and 13.8% of PZT-C8 in the resultant all-PSCs, mainly benefiting from its better crystallinity and the more favorable blend morphology. This work emphasizes the importance of optimizing side-chain substituents on PSMAs for improving the device efficiency of all-PSCs. This article is protected by copyright. All rights reserved.

Effects of the Selective Alkoxy Side Chain Position in Quinoxaline-Based Polymer Acceptors on the Performance of All-Polymer Solar Cells

The effects of the position of alkoxy side chains in quinoxaline (Qx)-based polymer acceptors (P_As) on the characteristics of materials and the device parameters of all-polymer solar cells (all-PSCs) are investigated. The alkoxy side chains are selectively located at the meta, para, and both positions in pendant benzenes of Qx units, constructing P_As denoted as P(QxCN-T2)-m, P(QxCN-T2)-p, and P(QxCN-T2), respectively. Among them, P(QxCN-T2)-m exhibits the deepest energy levels owing to the enhanced electron-withdrawing effect of meta-positioned alkoxy chains, which is in contrast to P(QxCN-T2)-p where para-positioned alkoxy chains have an electron-donating property. In addition, the meta-positioned alkoxy chains induce good electron-conducting pathways, while the para-positioned ones significantly interrupt crystallization and intermolecular interactions between the conjugated backbones. Thus, when the P_As are applied to all-PSCs, a power conversion efficiency (PCE) of 5.07% is attained in the device using P(QxCN-T2)-m with efficient exciton dissociation and good electron-transporting ability. On the contrary, the P(QxCN-T2)-p-based counterpart has a PCE of only 1.62%. These results demonstrate that introducing alkoxy side chains at a proper location in the Qx-based P_As is crucial for their application to all-PSCs.

N-Type Quinoidal Polymers Based on Dipyrrolopyrazinedione for Application in All-Polymer Solar Cells

Conjugated molecules and polymers with intrinsic quinoidal structure are promising n-type organic semiconductors, which have been reported for application in field-effect transistors and thermoelectric devices. In principle, the molecular and electronic characteristics of quinoidal polymers can also enable their application in organic solar cells. Herein, two quinoidal polymers, named PzDP-T and PzDP-ffT, based on dipyrrolopyrazinedione were synthesized and used as electron acceptors in all-polymer solar cells (all-PSCs). Both PzDP-T and PzDP-ffT showed suitable energy levels and wide light absorption range that extended to the near-infrared region. When combined with the polymer donor PBDB-T, the resulting all-PSCs based on PzDP-T and PzDP-ffT exhibited a power conversion efficiency (PCE) of 1.33 and 2.37 %, respectively. This is the first report on the application of intrinsic quinoidal conjugated polymers in all-PSCs. The photovoltaic performance of the all-PSCs was revealed to be mainly limited by the relatively poor and imbalanced charge transport, considerable charge recombination. Detailed investigations on the structure-performance relationship suggested that synergistic optimization of light absorption, energy levels, and charge transport properties is needed to achieve more successful application of intrinsic quinoidal conjugated polymers in all-PSCs.

Cyano-Functionalized Quinoxaline-Based Polymer Acceptors for All-Polymer Solar Cells and Organic Transistors

Quinoxaline (Qx) derivatives are promising building units for efficient photovoltaic polymers owing to their strong light absorption and high charge-transport abilities, but they have been used exclusively in the construction of polymer donors. Herein, for the first time, Qx-based polymer acceptors (P_A s) were developed by introducing electron-withdrawing cyano (CN) groups into the Qx moiety (QxCN). A series of QxCN-based P_A s, P(QxCN-T2), P(QxCN-TVT), and P(QxCN-T3), were synthesized by copolymerizing the QxCN unit with bithiophene, (E)-1,2-di(thiophene-2-yl)ethene, and terthiophene, respectively. All of the P_A s exhibited unipolar n-type characteristics with organic field-effect transistor (OFET) mobilities of around 10^-2  cm²  V^-1  s^-1 . In space-charge-limited current devices, P(QxCN-T2) and P(QxCN-TVT) exhibited electron mobilities greater than 1.0×10^-4  cm²  V^-1  s^-1 , due to the well-ordered structure with tight π-π stacking. When the P_A s were applied in all-polymer solar cells (all-PSCs), the highest performance of 5.32 % was achieved in the P(QxCN-T2)-based device. These results demonstrate the significant potential of Qx-based P_A s for high-performance all-PSCs and OFETs.

Highly Efficient All-Polymer Solar Cells Processed from Nonhalogenated Solvents

The remarkable advance of all-polymer solar cells (all-PSCs) achieved in the past decades is primarily powered by the innovation of polymer acceptors. However, most of high-performance all-PSCs are dominantly fabricated with halogenated solvents, which are detrimental to human bodies and the environment. Herein, eco-friendly solvent-processed all-PSCs were developed, based on wide-bandgap polymer poly[4,8-bis(5-(2-ethylhexylthio)thiophen-2-yl)-benzo-[1,2-b;4,5-b']dithiophene-alt-2,5-di(butyloctylthiophen-2-yl) -thiazolo[5,4-d]thiazole] (PSTZ) as donor and newly synthesized narrow-bandgap polymer 5,6-dicyano-2,1,3-benzothiadiazole indacenodithiophene (DCNBT-IDT) as acceptor. When processed with o-xylene and THF, PSTZ : DCNBT-IDT-based all-PSCs yielded remarkable power conversion efficiencies of 7.23 and 8.77 % with high short-circuit currents of 12.94 and 14.12 mA cm^-2 , respectively. The results indicated that the utilization of an all-polymer blend based on narrow polymer acceptor and compatible polymer donor is an effective strategy for advancing eco-friendly solvent-processed all-PSCs.

Achieving a Higher Energy Charge-Transfer State and Reduced Voltage Loss for Organic Solar Cells using Nonfullerene Acceptors with Norbornenyl-Functionalized Terminal Groups

Achieving a high-energy charge-transfer state (E_CT) and concurrently reduced energy loss is of vital importance in boosting the open-circuit voltage (V_oc) of organic solar cells (OSCs), but it is difficult to realize. We report herein a novel design tactic to achieve this goal by incorporating a three-dimensional (3D) shape-persistent norbornenyl group into the terminals of acceptor-donor-acceptor-type nonfullerene acceptors (NFAs). Compared with ITIC-based OSCs, norbornenyl-fused 1,1-dicyanomethylene-3-indanone (CBIC) terminals endow IDTT-CBIC-based OSCs with simultaneously higher E_CT and lower radiative and non-radiative voltage loss, hence enhancing V_oc by 90 mV. CBIC also improves the miscibility and modulates the molecular packing structures for efficient charge carrier transport and a better short-circuit current density in IDTT-CBIC-based OSCs. Consequently, the power conversion efficiency is improved by 22%, compared to that of the OSC based on ITIC. Furthermore, the effectiveness of the use of CBIC as the terminals is observed using different electron-donating cores. The utilization of the 3D shape-persistent building blocks represents a breakthrough in the design strategies for terminal groups toward efficient NFA-based OSCs with high V_oc.

One-Step Sixfold Cyanation of Benzothiadiazole Acceptor Units for Air-Stable High-Performance n-Type Organic Field-Effect Transistors

Reported here is a new high electron affinity acceptor end group for organic semiconductors, 2,1,3‐benzothiadiazole‐4,5,6‐tricarbonitrile (TCNBT). An n‐type organic semiconductor with an indacenodithiophene (IDT) core and TCNBT end groups was synthesized by a sixfold nucleophilic substitution with cyanide on a fluorinated precursor, itself prepared by a direct arylation approach. This one‐step chemical modification significantly impacted the molecular properties: the fluorinated precursor, TFBT IDT, a poor ambipolar semiconductor, was converted into TCNBT IDT, a good n‐type semiconductor. The electron‐deficient end group TCNBT dramatically decreased the energy of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO) compared to the fluorinated analogue and improved the molecular orientation when utilized in n‐type organic field‐effect transistors (OFETs). Solution‐processed OFETs based on TCNBT IDT exhibited a charge‐carrier mobility of up to μ_e≈0.15 cm² V⁻¹ s⁻¹ with excellent ambient stability for 100 hours, highlighting the benefits of the cyanated end group and the synthetic approach.

Cyano-Functionalized Bithiophene Imide-Based n-Type Polymer Semiconductors: Synthesis, Structure-Property Correlations, and Thermoelectric Performance

n-Type polymers with deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels are essential for enabling n-type organic thin-film transistors (OTFTs) with high stability and n-type organic thermoelectrics (OTEs) with high doping efficiency and promising thermoelectric performance. Bithiophene imide (BTI) and its derivatives have been demonstrated as promising acceptor units for constructing high-performance n-type polymers. However, the electron-rich thiophene moiety in BTI leads to elevated LUMOs for the resultant polymers and hence limits their n-type performance and intrinsic stability. Herein, we addressed this issue by introducing strong electron-withdrawing cyano functionality on BTI and its derivatives. We have successfully overcome the synthetic challenges and developed a series of novel acceptor building blocks, CNI, CNTI, and CNDTI, which show substantially higher electron deficiencies than does BTI. On the basis of these novel building blocks, acceptor-acceptor type homopolymers and copolymers were successfully synthesized and featured greatly suppressed LUMOs (-3.64 to -4.11 eV) versus that (-3.48 eV) of the control polymer PBTI. Their deep-positioned LUMOs resulted in improved stability in OTFTs and more efficient n-doping in OTEs for the corresponding polymers with a highest electrical conductivity of 23.3 S cm^-1 and a power factor of ∼10 μW m^-1 K^-2. The conductivity and power factor are among the highest values reported for solution-processed molecularly n-doped polymers. The new CNI, CNTI, and CNDTI offer a remarkable platform for constructing n-type polymers, and this study demonstrates that cyano-functionalization of BTI is a very effective strategy for developing polymers with deep-lying LUMOs for high-performance n-type organic electronic devices.

Impact of Chlorination Patterns of Naphthalenediimide-Based Polymers on Aggregated Structure, Crystallinity, and Device Performance of All-Polymer Solar Cells and Organic Transistors

The aggregation properties of conjugated polymers can play a crucial role in their thin film structures and performance of electronic devices. Control of these aggregated structures is particularly important in producing efficient all-polymer solar cells (all-PSCs), considering that strong demixing of the polymer donor and polymer acceptor typically occurs during film formation because of the low entropic contribution to the thermodynamics of the system. Here, three naphthalenediimide (NDI)-based polymer acceptors with different backbone chlorination patterns are developed to investigate the effect of the chlorination patterns on the aggregation tendencies of the polymer acceptors, which greatly influence their crystalline structures, electrical properties, and device performances of the resultant all-PSCs and organic field-effect transistors (OFETs). The counterparts of NDI units, dichlorinated bithiophene (Cl2T2), monochlorinated bithiophene (ClT2), and dichlorinated thienylene-vinylene-thienylene (Cl2TVT), are employed to synthesize a series of P(NDIOD-Cl2T2), P(NDIOD-ClT2), and P(NDIOD-Cl2TVT) polymers. The P(NDIOD-Cl2T2) polymer takes advantage of strong noncovalent bonding induced by its chlorine substituents, resulting in the formation of optimal face-on oriented crystalline structures which are suitable for efficient all-PSC devices. In comparison, the P(NDIOD-Cl2TVT) polymer forms bimodal crystalline structures in thin films to yield optimal performances in the resultant OFETs. When the three chlorinated polymers are applied to all-PSCs with the PBDTTTPD polymer donor, P(NDIOD-Cl2T2) achieves a maximum power conversion efficiency (PCE) of 7.22% with an appropriate blend morphology and high fill factor, outperforming P(NDIOD-ClT2) (PCE = 4.80%) and P(NDIOD-Cl2TVT) (PCE = 5.78%). Our observations highlight the effectiveness of the chlorination strategy for developing efficient polymer acceptors and demonstrate the important role of polymer aggregation in modulating the blend morphology and all-PSC performance.

Polymer Acceptors Containing B←N Units for Organic Photovoltaics

ConspectusOrganic photovoltaics (OPVs), in which blend films of organic or polymer electron donor and electron acceptor are used as the active layer, are a promising photovoltaic technology with the great advantages of solution processing, low cost, and flexibility. The development of small molecular or polymer electron acceptors has boosted power conversion efficiency (PCE) of OPVs from 10% to 18%. Among them, polymer acceptors have the merits of superior morphology stability and excellent mechanical properties. However, owing to the key requirement of very low-lying LUMO/HOMO energy levels for polymer acceptors, very few conjugated polymers can work as polymer acceptors in OPVs. The majority of polymer electron acceptors are based on strong electron-withdrawing imide units or cyano substituents. Since 2015, conjugated polymers containing the boron-nitrogen coordination bond (B←N) have emerged as a new kind of polymer electron acceptor with excellent photovoltaic performance in various kinds of organic photovoltaic devices. In this Account, we summarize our research progress on polymer acceptors containing B←N units.At first, we introduce the principle of B←N to greatly down shift LUMO/HOMO energy levels, which enables B←N to be used to design polymer acceptors. Then we describe the two molecular design strategies for polymer acceptors containing B←N units. For high-efficiency OPVs, polymer acceptors should have wide absorption spectra, proper LUMO/HOMO energy levels, high electron mobility, and good donor/acceptor blend morphology. We discuss how to use molecular design to finely tune the absorption spectra, energy levels, and electron mobility of the B←N-containing polymer acceptors. We also discuss how to improve the phase separation morphology of the blends of these polymer acceptors with small molecular donors or polymer donors. These improvements lead to excellent performance of the polymer acceptors containing B←N units in three kinds of organic photovoltaic devices. The small molecular donor/polymer acceptor type organic solar cells show excellent thermal stability and PCE of 8.0%, which is the highest value reported so far. The all-polymer solar cells exhibit PCE of 10.1%. The all-polymer indoor photovoltaics show PCE as high as 27.4% under fluorescent lamp illumination at 2000 lx. This PCE is fairly comparable to those of the best organic or inorganic indoor photovoltaics. These results provide a solid foundation for future advances. Finally, we propose that great attention should be paid to further PCE enhancement of OPVs and indoor photovoltaic applications of this new emerging kind of polymer acceptor.

High-Performance All-Polymer Solar Cells Enabled by n-Type Polymers with an Ultranarrow Bandgap Down to 1.28 eV

Compared to organic solar cells based on narrow-bandgap nonfullerene small-molecule acceptors, the performance of all-polymer solar cells (all-PSCs) lags much behind due to the lack of high-performance n-type polymers, which should have low-aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to the near-infrared region. Herein, two novel polymer acceptors, DCNBT-TPC and DCNBT-TPIC, are synthesized with ultranarrow bandgaps (ultra-NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm² V^-1 s^-1 obtained for DCNBT-TPC. Blended with a polymer donor, PBDTTT-E-T, the resultant all-PSCs based on DCNBT-TPC and DCNBT-TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short-circuit currents up to 19.44 and 22.52 mA cm^-2 , respectively. This is the first example that a PCE of over 10% can be achieved using ultra-NBG polymer acceptors with a photoresponse reaching 950 nm in all-PSCs. These results demonstrate that ultra-NBG polymer acceptors, in line with nonfullerene small-molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all-PSCs.

Low-Bandgap n-Type Polymer Based on a Fused-DAD-Type Heptacyclic Ring for All-Polymer Solar Cell Application with a Power Conversion Efficiency of 10.7

An n-type polymer (A701) is designed and synthesized with an alternative A'-DAD-A'-D' backbone, where 1,1-dicyanomethylene-3-indanone (IC), dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (TPBT), and benzodithiophene (BDT) are used as A', DAD, and D' units, respectively. A701 shows enhanced light absorption with a narrow bandgap of 1.42 eV and a high absorption coefficient of 6.85 × 10⁴ cm^-1 at 780 nm. It displays an uplifted LUMO (the lowest unoccupied molecular orbital) level of -3.80 eV. By introducing a high point solvent additive of 1,8-diiodooctane (DIO), all-polymer solar cells (all-PSCs) based on the PBDB-T:A701 blend exhibit efficient exciton dissociation, enhanced charge transport, and decreased bimolecular recombination. Thus, a high open-circuit voltage (V_OC) of 0.92 V, a short-circuit current (J_SC) of 18.27 mA cm^-2, and a fill factor (FF) of 0.64 are attained, affording an impressive power conversion efficiency (PCE) of 10.70%. The low voltage loss of 0.50 V and high efficiency of 10.7% are among the top values for all-PSCs. Our results indicate that the fused DAD-type heptacyclic ring can be utilized to construct not only nonfullerene small molecular acceptors but also promising polymer acceptors.