Hole‐transporting materials (HTMs) play a critical role in realizing efficient and stable perovskite solar cells (PVSCs). Considering their capability of enabling PVSCs with good device reproducibility and long‐term stability, high‐performance dopant‐free small‐molecule HTMs (SM‐HTMs) are greatly desired. However, such dopant‐free SM‐HTMs are highly elusive, limiting the current record efficiencies of inverted PVSCs to around 19%. Here, two novel donor–acceptor‐type SM‐HTMs (MPA‐BTI and MPA‐BTTI) are devised, which synergistically integrate several design principles for high‐performance HTMs, and exhibit comparable optoelectronic properties but distinct molecular configuration and film properties. Consequently, the dopant‐free MPA‐BTTI‐based inverted PVSCs achieve a remarkable efficiency of 21.17% with negligible hysteresis and superior thermal stability and long‐term stability under illumination, which breaks the long‐time standing bottleneck in the development of dopant‐free SM‐HTMs for highly efficient inverted PVSCs. Such a breakthrough is attributed to the well‐aligned energy levels, appropriate hole mobility, and most importantly, the excellent film morphology of the MPA‐BTTI. The results underscore the effectiveness of the design tactics, providing a new avenue for developing high‐performance dopant‐free SM‐HTMs in PVSCs.

Chemical doping is a key process for investigating charge transport in organic semiconductors and improving certain (opto)electronic devices 1-9 . N-(electron)doping is fundamentally more challenging than p-(hole)doping and typically achieves very low doping efficiency (η) <10% 1,10 . An efficient molecular n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability 1,5,6,9,11 , which is very challenging. Here we show a general concept of catalysed n-doping of organic semiconductors using air-stable precursor-type molecular dopants. Incorporation of a transition metal as vapor-deposited nanoparticles (e.g. Pt, Au, Pd) or solution-processable 2 organometallic complexes (e.g. Pd 2 (dba) 3 ) catalyses the reaction, as assessed by experimental and theoretical evidence, enabling drastically increased η in a much shorter doping time and high electrical conductivities >100 S cm −1 12 . This methodology has technological implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, molecular dopants, and semiconductors, thus opening new opportunities in n-doping research and applications.N-doping of organic semiconductors is important for developing light-emitting diodes 1,6-9 , solar cells 7,8 , thin-film transistors 10 , and thermoelectric devices 12,13 . Although solution-based ndoping is widely investigated, only few air-stable n-dopants have been developed (Fig. S1), with the most prominent being organic hydrides 5,9,14-18 such as benzoimidazole derivatives, dimers of organic radicals 11,19,20 such as nineteen-electron organometallic sandwich compounds, and mono-/multi-valent anions 8,21,22 such as OH − , F − and Ox 2− . These air-stable dopants have a deep ionization potential (IP) in their initial forms, thus, cannot directly transfer electrons to n-dope organic semiconductors with a low electron affinity (EA). For anions, it was shown that dispersion into small anhydrous clusters enables sufficiently high donor levels for n-doping organic semiconductors with EAs up to 2.4 eV 8 . Hydride and dimer dopant precursors (or referred as precursor-type dopants) most undergo a C-H and C-C bond cleavage reaction, respectively, to generate active-doping-species in situ before electron transfer can occur [23][24][25][26] . Thus, their reducing strength and reaction kinetics are strongly affected by the thermodynamics and the activation energies of the doping reaction [23][24][25][26] . If the activation energy to the product is reduced, it is expected that the reaction rate, and extent of doping, will greatly increase (Fig. 1a). 3Transition metal (TM) catalysed C-H and C-C bond cleavage reactions are widely used in organic synthesis, with the most common TMs belonging to group 8-11 elements and the catalysts in the form of nanoparticles (NPs) and organometallic complexes 27,28 . Nanoparticle size, supporting material, and chemical structure of the complex can greatly affect catalytic activities. Thus, an i...

As a key component in perovskite solar cells (PVSCs), hole-transporting materials (HTMs) have been extensively explored and studied. Aiming to meet the requirements for future commercialization of PVSCs, HTMs which can enable excellent device performance with low cost and eco-friendly processability are urgently needed but rarely reported. In this work, a traditional anchoring group (2-cyanoacrylic acid) widely used in molecules for dye-sensitized solar cells is incorporated into donor–acceptor-type HTMs to afford MPA-BT-CA, which enables effective regulation of the frontier molecular orbital energy levels, interfacial modification of an ITO electrode, efficient defect passivation toward the perovskite layer, and more importantly alcohol solubility. Consequently, inverted PVSCs with this low-cost HTM exhibit excellent device performance with a remarkable power conversion efficiency (PCE) of 21.24% and good long-term stability in ambient conditions. More encouragingly, when processing MPA-BT-CA films with the green solvent ethanol, the corresponding PVSCs also deliver a substantial PCE as high as 20.52% with negligible hysteresis. Such molecular design of anchoring group-based materials represents great progress for developing efficient HTMs which combine the advantages of low cost, eco-friendly processability, and high performance. We believe that such design strategy will pave a new path for the exploration of highly efficient HTMs applicable to commercialization of PVSCs.

of these FREAs can be fine-tuned by optimizing the intramolecular charge transfer character while maintaining their key characteristics as efficient electron acceptor materials. [4-6] The FREAs with fine-tailored properties enable the state-of-the-art power conversion efficiencies (PCEs) surpassing 16% in OSCs. [7-12] Among various OSCs, all-polymer solar cells (all-PSCs) employing conjugated polymers as both the electron donors and acceptors, have recently attracted great attention because of some unique advantages including superior stability and mechanical robustness. [13-20] However, only a few polymer acceptors can yield PCEs over 8% till now (see Figure S1 and Table S1, Supporting Information) due to the scarcity of highly electron-deficient building blocks. For instance, classical naphthalene diimide and perylene diimidebased donor-acceptor (D-A)-type polymer acceptors (see Figure S2a, Supporting Information) suffer from poor absorption coefficient in the long-wavelength region and (or) the localized lowest unoccupied molecular orbital (LUMO), which limits the short-circuit currents density (J sc) and V oc , together with Narrow-bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow-bandgap polymer acceptor L14, featuring an acceptor-acceptor (A-A) type backbone, is synthesized by copolymerizing a dibrominated fused-ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A-A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low-lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open-circuit voltage (V oc), which is attributed to a small nonradiative recombination loss (E loss,nr) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and V oc , an excellent efficiency of 14.3% is achieved, which is among the highest values for all-polymer solar cells (all-PSCs). The results demonstrate the superiority of narrow-bandgap A-A type polymers for improving all-PSC performance and pave a way toward developing high-performance polymer acceptors for all-PSCs.

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.

Unpaired electrons of organic radicals can offer high electrical conductivity without doping, but they typically suffer from low stability. Herein, we report two organic diradicaloids based on quinoidal oligothiophene derivative (QOT), that is, BTICN and QTICN, with high stability and conductivity by employing imide-bridged fused molecular frameworks. The attachment of a strong electron-withdrawing imide group to the tetracyano-capped QOT backbones enables extremely deeply aligned LUMO levels (from −4.58 to −4.69 eV), cross-conjugated diradical characters, and remarkable ambient stabilities of the diradicaloids with half-lives > 60 days, which are among the highest for QOT diradicals and also the widely explored polyaromatic hydrocarbon (PAH)-based diradicals. Specifically, QTICN based on a tetrathiophene imide exhibits a cross-conjugation assisted self-doping in the film state as revealed by XPS and Raman studies. This property in combination with its ordered packing yields a high electrical conductivity of 0.34 S cm −1 for the QTICN films with substantial ambient stability, which is also among the highest values in organic radical-based undoped conductive materials reported to date. When used as an n-type thermoelectric material, QTICN shows a promising power factor of 1.52 uW m −1 K −2 . Our results not only provide new insights into the electron conduction mechanism of the self-doped QOT diradicaloids but also demonstrate the great potential of fused quinoidal oligothiophene imides in developing stable diradicals and high-performance doping-free n-type conductive materials.

Compared to organic solar cells based on narrow bandgap non-fullerene small molecule acceptors, the performance of all-polymer solar cells (all-PSCs) lags much behind due to the lack of highperformance n-type polymers, which should have low-aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to near-infrared region. Herein, two novel polymer acceptors, DCNBT-TPC and DCNBT-TPIC, are synthesized with an ultra-narrow bandgap (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 cm 2 V -1 s -1 obtained for DCNBT-TPC. Blended with 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 acceptor with a photo-response reaching 950 nm in all-PSCs. These results demonstrate that ultra-NBG polymer acceptors, in line with non-fullerene small molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all-PSCs.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

A distannylated electron‐deficient bithiophene imide (BTI‐Tin) monomer was synthesized and polymerized with imide‐functionalized co‐units to afford homopolymer PBTI and copolymer P(BTI‐BTI2), both featuring an acceptor–acceptor backbone with high molecular weight. Both polymers exhibited excellent unipolar n‐type character in transistors with electron mobility up to 2.60 cm2 V−1 s−1. When applied as acceptor materials in all‐polymer solar cells, PBTI and P(BTI‐BTI2) achieved high power‐conversion efficiency (PCE) of 6.67 % and 8.61 %, respectively. The PCE (6.67 %) of polymer PBTI, synthesized from the distannylated monomer, is much higher than that (0.14 %) of the same polymer PBTI*, synthesized from typical dibrominated monomer. The 8.61 % PCE of copolymer P(BTI‐BTI2) is also higher than those (<1 %) of homopolymers synthesized from dibrominated monomers. The results demonstrate the success of BTI‐Tin for accessing n‐type polymers with greatly improved device performance.