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The fusion of multiple B/O-heterocycles onto hexa-peri-hexabenzocoronene has led to the generation of two B/O-containing nanographenes that exhibit distorted geometries, red-shifted absorptions and fluorescence.

Two chiral tetracoordinate organoboranes based on double B←N bridged bipyridine were synthesized via an Et2AlCl-mediated reaction, and they possess excellent circularly polarized luminescence performance.

Heterocyclic diradicaloids with atom-precise control over open-shell nature are promising materials for organic electronics and spintronics. Herein, we disclose quinoidal π-extension of a B/N-heterocycle for generating B/N-type organic diradicaloids. Two quinoidal π-extended B/N-doped polycyclic hydrocarbons that feature fusion of the B/N-heterocycle motif with the antiaromatic s-indacene or dicyclopenta[b,g]naphthalene core were synthesized. This quinoidal π-extension and B/N-heterocycle leads to their open-shell electronic nature, which stands in contrast to the multiple-resonance effect of conventional B/N-type emitters. These B/N-type diradicaloids have modulated (anti)aromaticity and enhanced diradical characters comparing with the all-carbon analogues, as well as intriguing properties, such as magnetic activities, narrow energy gaps and highly red-shifted absorptions. This study thus opens the new space for both of B/N-doped polycyclic π-systems and heterocyclic diradicaloids.

Incorporation of heteroatoms into polyarenes has been developed as an effective approach to alter their intrinsic structures and properties. Herein, we designed and synthesized two PO-containing dibenzopentaarene isomers (5a and 5b) and studied their structures and properties, along with those of dibenzopentaarenes containing six-membered Si- and B-heterocycles (3 and 4). These heterocyclic polyarenes have similar frameworks to well-known heptazethrene, and thus can be regarded as members of the heteroatom-doped zethrene system. The heterocycles greatly affect not only the molecular and packing structures but also the electronic structures and properties. Notably, while compounds 3 and 4 adopt almost planar geometries, 5a possesses a clearly curved conformation, leading to its brick-type slipped and dense π-π stacking mode. Moreover, the electron-withdrawing PO groups endow 5a and 5b with simultaneously lowered lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) levels, whereas the p-π conjugation of the B atoms in 4 leads to its smaller energy gap and thus remarkably red-shifted absorption and fluorescence bands by over 80 nm, though all of these molecules possess similar closed-shell structures. This study thus deepens the understanding of heteroatom-doping effects, which may be expanded to develop other heteroatom-doped zethrene materials.

Molecular ribbons (MRs), one-dimentional topological polycyclic aromatic hydrocarbons (PAHs), are of importance for synthetic chemistry and material sciences. Herein, we disclose an effective strategy to develop boron-doped MRs, i.e. photochemical cyclization on conjugated organoboranes for rapid π-extension. A series of ribbon-type boron-doped PAHs that own multiple cove edges were synthesized using Mallory photoreaction in solution. Two of them feature isomeric C

For organnoboron compounds, the substituents on boron atoms are very important because they not only impact on the molecular stability but also significantly modulate the electronic structures and properties. In this manuscript, we synthesized two new B⟵N-containing azaacenes with propynyl groups on boron atoms through one-step Grignard reaction. Replacing fluorine atoms by propynyl groups greatly impacts on the electronic energy levels, especially enhancing the HOMO levels, thus leading to the narrowed HOMO-LUMO bandgaps. These B⟵N-containing azaacenes exhibit the NIR light-absorption (λabs = 706 nm for 2a and 762 nm for 2b) and fluorescence properties (λem = 740 nm for 2a and 802 nm for 2b), as well as multiple reversible redox behaviors, which are significantly different from the analogs with fluorine atoms. This study thus provides a functional substituent of boron atom, which may lead to new organoboron materials with fascinating properties.

Tuning diradical character is an important topic for organic diradicaloids. Herein, we report the precise borylation enabling structural isomerism as an effective strategy to modulate diradical character and thereby properties of organic diradicaloids. We synthesized a new B-containing polycyclic hydrocarbon that has the indeno[1,2-b]fluorene π-skeleton with the β-carbons bonding to two boron atoms. Detailed theoretical and experimental results show that this bonding pattern leads to its distinctive electronic structures and properties in comparison to that of its isomeric molecule. This molecule has the efficient conjugation between boron atoms and π-skeleton, resulting in downshifted LUMO and HOMO levels. Moreover, it exhibits smaller diradical character and thereby inhibited diradical properties, such as significantly blue-shifted light absorption, larger energy bandgap and weak para-magnetic resonance. Notably, this B-containing polycyclic hydrocarbon possesses much stronger Lewis acidity and its Lewis acid-base adducts display enhanced diradical character, demonstrating the positive effects of Lewis coordination on modulating diradical performance.

Organic diradicaloids have unique open-shell structures and properties and promising applications in organic electronics and spintronics. Incorporation of heteroatoms is an effective strategy to alter the electronic structures of organic diradicaloids. However, B-containing organic diradicaloids are very challenging due to their high reactivities, which are caused by not only diradical nature but also the B atom. In this article, we report a new kind of organic diradicaloids containing boron atoms. Our strategy is to incorporate planarized triarylboranes to antiaromatic polycyclic hydrocarbons (PHs). We synthesized two isomeric B-containing PHs composed of indenofluorene π-skeletons and two dioxa-bridged triphenylborane moieties. As proved by theoretical and experimental results, both of them have excellent ambient stability and open-shell singlet diradical structures, as well as intriguing magnetic and optoelectronic properties, such as thermally accessible triplet species, reversible multiredox ability, and narrow energy gaps. Notably, they possess sufficient Lewis acidity, which has never been observed for organic diradicaloids. In addition, they can coordinate with Lewis bases to form Lewis adducts, achieving unprecedented dynamic modulations of (anti)aromaticity and thus diradical character of organic diradicaloids.

To effectively harvest sunlight, polymer donors with near infrared (NIR) absorption properties are important for high-performance polymer solar cells (PSCs), especially semitransparent PSCs. However, building blocks to design this kind of polymer donor are rare. In this manuscript, we report 3,11-dibromo-5,9-bis(5-(2-decyltetradecyl)thiophen-2-yl)-7,7-difluoro-7H-6λ4,7λ4-[1,3,2]diazaborinino[4,3-a:6,1-a′]diisoindole (BOPT) as a new building block to design polymer donors with NIR absorption. In BOPT, the 4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) core unit is fused with two benzene rings for a reduced bandgap and is attached with two alkylthienyl groups as conjugated side chains. We synthesized two polymers consisting of alternating BOPT units and thiophene or bithiophene units, respectively. The two polymers displayed near-infrared (NIR) light absorption with the maximum absorption peaks at wavelengths exceeding 800 nm and small bandgaps of ca. 1.3 eV. They showed high highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of about −5.0 eV/−3.5 eV and high hole mobilities of ca. 3 × 10−3 cm2 V−1 s−1. PSC devices using the two polymers as electron donors exhibited power conversion efficiencies of 2.6%. These results indicate that BOPT is a promising building block in designing polymer donors with near-infrared absorption for organic photovoltaics.

Two donor–acceptor (D–A) conjugated polymers designed on same 8,10-bis(2-octyldodecyl)-8,10-dihydro-9H-bisthieno[2′,3′:7,8;3″,2″:5,6]naphtho[2,3- d]imidazole-9-one donor and dissimilar acceptor uni...

Organic solar cells (OSCs) based on small molecular donor/polymer acceptor (MD/PA) blends have the advantage of excellent thermal stability, but they suffer from low photovoltaic efficiency because...

Polymer acceptors for high-efficiency all-polymer solar cells (all-PSCs) are generally semicrystalline. In this manuscript, we report an amorphous polymer acceptor, which matches well with a variet...

The general strategy to construct D-A type conjugated polymers is alternating copolymerization of electron-donating (D) monomer and electron-accepting (A) monomer. In this article, we report a new strategy to develop D-A type conjugated polymers, i.e. first fuse the D and A units into a polycyclic structure to produce a building block and then polymerize the building block with another unit. We develop a new building block with ladder structure based on B←N unit, B←N bridged dipyridylbenzene (BNDPB). In the skeleton of BNDPB, one diamine-substituted phenylene ring (D unit) and two B←N-linked pyridyl rings (A unit) are fused together to produce the polycyclic structure. Owning to the presence of intramolecular D-A character, the building block itself exhibits narrow bandgap of 1.74 eV. The conjugated polymers based on BNDPB show unique electronic structures, i.e. localized HOMOs and delocalized LUMOs, which are rarely observed for conventional D-A conjugated polymers. The polymers exhibit smaller bandgap than that of the building block BNDPB and display near-infrared (NIR) light absorption ( λ abs=ca. 700 nm). This study thus provides not only a new strategy to design D-A conjugated polymers but also a new kind of building block with narrow bandgap.

All-polymer solar cells (all-PSCs) use a blend of polymer donor and polymer acceptor as the active layer. The active layer morphology in all-PSCs cannot be effectively tuned by conventional methods...

The B←N unit has a large dipole and it is isoelectronic to C-C moiety with no dipole. Incorporating B←N units into π-conjugated system is a powerful strategy to design organic small molecules and polymers with intriguing opto-electronic properties and excellent opto-electronic device performance. However, it is unclear how the B←N unit affects electronic structures and opto-electronic properties of large π-conjugated molecules. In this work, to address this question, we developed three dibenzo-azaacene molecules in which two B←N units were introduced at different positions. Although the dibenzo-azaacene skeleton is fully π-conjugated, the effect of B←N unit on the electronic structures of the adjacent rings is much stronger than that of the distant rings. As a result, the three molecules with isomerized B←N incorporation patterns possess different electronic structures and exhibit tunable opto-electronic properties. Among the three molecules, the centrosymmetrical molecule exhibits higher LUMO/HOMO energy levels than those of the two axisymmetrical molecules. When used as the active layer in organic field-effect transistors (OFETs), while the two axisymmetrical molecules show unipolar electron transporting property, the centrosymmetrical molecule exhibits ambipolar hole and electron transporting behavior. This work not only deepens our understanding on organoboron π-conjugated molecules, but also indicates a new strategy to tune opto-electronic properties of organic semiconductors for excellent device performance.

With the aim of developing polymer electron acceptors with high electron mobilities (μe) for all-polymer solar cells (all-PSCs), we synthesized two novel polymer acceptors (P-BNBP-2f and P-BNBP-4f) containing alternate double B←N bridged bipyridine (BNBP) and 2,2′-(2,5-difluoro-1,4-phenylene)dithiophene/2,2′-(perfluoro-1,4-phenylene)dithiophene with extended conjugated structures. In comparison to P-BNBP-4f, P-BNBP-2f exhibits a small π–π stacking distance of 3.60 A, leading to a sufficient electron mobility of 5.40 × 10−4 cm2 V−1 s−1 (measured by the space-charge-limited current method). This μe value is among the highest values of the conventional polymer acceptors, and is close to the hole mobilities (μh) of high-efficiency polymer donors. Moreover, P-BNBP-2f possesses high-lying LUMO/HOMO energy levels of −3.42/−5.81 eV, which match well with that of the reported excellent polymer donors of 2D-conjugated bithienyl-benzodithiophene-alt-fluorobenzotriazole (J61) and thienyl-substituted BDT with alkoxycarbonyl-substituted thieno[3,4-b]thiophene (PBDTTT-E-T). In addition, the blend films based on P-BNBP-2f exhibit intermixed morphologies, which are beneficial for efficient excition dissociation. As a result, all-PSCs with P-BNBP-2f as an electron acceptor work very well and exhibit a power conversion efficiency of 5.46%. These results demonstrate that high electron mobility of a polymer electron acceptor is very important to produce efficient all-PSCs.

Organic solar cells (OSCs) based on small molecular donor and polymer acceptor (MD/PA) blends display inferior device performance due to poor active layer morphology. However, the phase separation behaviour and morphology evolution process of MD/PA-blends are scarcely investigated and less understood. Here, the morphology of MD/PA-blends based on two small molecular donors with different π–π stacking capability was systemically investigated. The small molecular donor with strong π–π stacking crystallizes into discontinuous large domains, which disturbs the ordered aggregation of the polymer acceptor. In contrast, the small molecular donor with weak π–π stacking remains amorphous and the polymer acceptor forms ordered aggregates in the as-cast film, while post-annealing promotes the small molecular donor to crystallize into nano-sized domains, which results in interpenetrating donor/acceptor networks. Due to the improved exciton dissociation and charge transport, the MD/PA-type OSC from the small molecular donor with weak π–π stacking shows superior photovoltaic performance. These results suggest that the π–π stacking capability of small molecular donors is critical to the active layer morphology of MD/PA-type OSCs.

The development of molecular donor/polymer acceptor blend (MD/PA)-type organic solar cells (OSCs) lags far behind other type OSCs. It is due to the large-size phase separation morphology of MD/PA blend, which results from the high crystallinity of molecular donors. In this article, to suppress the crystallinity of molecular donors, we use ternary blends to develop OSCs based on one polymer acceptor (P-BNBP-fBT) and two molecular donors (DR3TBDTT and BTR) with similar chemical structures. The ternary OSC exhibits a power conversion efficiency (PCE) of 4.85%, which is higher than those of the binary OSCs (PCE=3.60% or 3.86%). To our best knowledge, it is the first report of ternary MD/PA-type OSCs and this PCE is among the highest for MD/PA-type OSCs reported so far. Compared with the binary blends, the ternary blend exhibits decreased crystalline size and improved face-on orientation of the donors. As a result, the ternary blend exhibits improved and balanced charge mobilities, suppressed charge recombination and increased donor/acceptor interfacial areas, which leads to the higher short-circuit current density. These results suggest that using ternary blend is an effective strategy to manipulate active layer morphology and enhance photovoltaic performance of MD/PA-type OSCs.

We disclose a novel strategy to design n-type acenes through the introduction of boron-nitrogen coordination bonds (B←N). We synthesized two azaacenes composed of two B←N units and six/eight linearly annelated rings. The B←N unit significantly perturbed the electronic structures of the azaacenes: Unique LUMOs delocalized over the entire acene skeletons and decreased aromaticity of the B←N-adjacent rings. Most importantly, these B←N-containing azaacenes exhibited low-lying LUMO energy levels and high electron affinities, thus leading to n-type character. The solution-processed organic field-effect transistor based on one such azaacene exhibited unipolar n-type characteristics with an electron mobility of 0.21 cm2 V-1 s-1 .

The LUMO and HOMO energy levels (ELUMO/EHOMO) are key parameters for conjugated polymers, which can greatly affect their applications in organic opto-electronic devices. In this manuscript, with donor–acceptor (D–A) type conjugated polymers based on double B←N bridged bipyridine (BNBP) unit, we report fine-tuning of ELUMO of conjugated polymers in a wide range via substitutions on both D unit and A unit. We synthesize eight D–A type conjugated polymers with alternating electron-deficient BNBP unit and electron-rich bithiophene (BT) unit in the main chain. By changing the substitutes on BNBP or BT, the ELUMO of these polymers can be finely tuned in a wide range from −3.3 eV to −3.7 eV. We comprehensively investigate the electronic structures, photophysical properties, charge-transporting properties and polymer solar cell (PSC) device applications of these polymers. In PSC devices, these BNBP-based polymers can be used either as electron donors (with high-lying ELUMO/EHOMO) or as electron acceptors (with lo...

We report 4,4-difluoro-4H-cyclopenta[2,1-b:3,4-b']dithiophene (fCDT) as a new electron-rich unit to design polymer electron acceptors. Owing to the fluoro substitutes, fCDT unit exhibits downshifted LUMO energy level, diminished steric hindrance effect and strong intermolecular interaction. The resulting polymer electron acceptor exhibits low-lying LUMO energy level and high electron mobility, as well as good all-polymer solar cell device performance.

The development of polymer electron acceptors lags far behind that of polymer electron donors. A general approach to improve photovoltaic performance of polymer electron donors is to incorporate conjugated side chains to the electron-rich unit. In this article, we introduce the “conjugated side chain” strategy to molecular design of polymer electron acceptors by incorporating conjugated side chains to the electron-deficient unit. The polymer backbones consist of alternating electron-deficient double B←N bridged bipyridine (BNBP) unit and electron-rich thiophene or selenophene unit. Polymer electron acceptors are developed by incorporating conjugated alkoxyphenyl side chains to the BNBP unit. Compared with conventional alkyl side chains, the conjugated alkoxyphenyl side chains endow the polymer electron acceptors with low-lying LUMO energy levels, enhanced π–π stacking, and high electron mobilities, which are very desirable for electron acceptors. The resulting all-PSCs exhibit an enhanced power conversion...

Polymer electron acceptors are the key materials in all-polymer solar cells (all-PSCs). In this review, we focused on introducing the principle of boron-nitrogen coordination bond (B←N), and summarizing our recent research on polymer electron acceptors containing B←N unit for efficient all-PSC devices. Two approaches have been reported to design polymer electron acceptors using B←N unit. One is to replace a C–C unit by a B ← N unit in conjugated polymers to transform a polymer electron donor to a polymer electron acceptor. The other approach is to construct novel electron-deficient building block based on B←N unit for polymer electron acceptors. The polymer electron acceptors containing B←N unit showed tunable lowest unoccupied molecular orbital (LUMO) energy levels and exhibited excellent all-PSC device performance with power conversion efficiency of exceeding 6%. These results indicate that organic boron chemistry is a new toolbox to develop functional polymer materials for optoelectronic device applications.

For many years, organoboron compounds have been expected to show excellent electron-injecting and -transporting properties. However, lowest unoccupied molecular orbital (LUMO) energy levels (ELUMO) of B-containing π-conjugated molecules are mostly higher than −4.0 eV and their electron mobilities are usually less than 10–2 cm2 V–1 s–1. In this work, we experimentally prove the remarkably high electron affinity and high electron mobility of organoboron compounds. Our strategy is to incorporate multiple boron–nitrogen coordination bonds (B←N) into azaacenes. We synthesized quadruply B←N-fused dibenzo-azaacene (QBNA) through one-pot multifold borylation cyclization reaction. The incorporation of four B←N units greatly changes the electronic structures and properties and significantly downshifts the electronic energy levels of QBNA. QBNA shows a ELUMO of as low as −4.58 eV, which is among the lowest for n-type organic semiconductors. Single-crystal organic field-effect transistors of QBNA display unipolar n-t...

In this manuscript, we report a disk-type polyarene consisting of an all-hexagon π-framework and four B←N units. It exhibits intriguing electronic structures and opto-electronic properties, such as locally modulated (anti)aromaticity, strong red fluorescence and n-type semiconductor characteristics, thus representing a new kind of π-molecule containing B←N units.

Open-circuit voltage (VOC) of polymer solar cells (PSCs) is always in the range of 0.6–0.9 V and rarely exceeds 1.0 V. The common strategy to enhance VOC of PSCs is to use polymer donors with low-lying highest occupied molecular orbital energy level (EHOMO). In this manuscript, we report high VOC of 1.3 V for PSCs by further using a polymer acceptor with high-lying lowest unoccupied molecular orbital energy level (ELUMO). With poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole] (PCDTBT) as the donor and poly[(N,N′-bis(2-hexyldecyl)-diamine-bis(difluoro-borane)−2,2′-bipyridine)-alt-(2,5-thiophene)] (P-BNBP-T) as the acceptor, the PSC devices show the VOC of 1.30 V and power conversion efficiency (PCE) of 3.20%. This VOC is higher than that of the conventional acceptor materials by 0.4 V owing to the high-lying ELUMO of P-BNBP-T. We have also investigated the effect of processing solvents on the blend morphology and the device performance. Compared with high boiling-point solvent o-dichlorobenzene, low boiling-point solvent chloroform gives finer phase separation size, improved exciton dissociation and consequently better photovoltaic device performance. The demonstration of high VOC in this work indicates that there is large room for PCE enhancement of PSCs.

A new polymer electron acceptor (P-BNBP-CDT) composed of an alternating double B←N bridged bipyridine (BNBP) unit and a cyclopenta-[2,1-b:3,4-b′]-dithiophene (CDT) unit has been developed. P-BNBP-CDT exhibits strong light absorption in the visible range of 500–650 nm and suitable LUMO/HOMO energy levels (ELUMO/HOMO) of −3.45 eV/−5.64 eV, which are very complementary to that (ELUMO/HOMO = −3.2 eV/−5.2 eV) of the widely-used polymer donor, poly(3-hexylthiophene) (P3HT). All-polymer solar cells (all-PSCs) with P3HT as an electron donor and P-BNBP-CDT as an electron acceptor exhibit power conversion efficiencies (PCEs) exceeding 1.0% with high donor : acceptor blend ratios (w : w, from 0.5 : 1 to 9 : 1). The highest PCE of these devices is 1.76% with a high donor : acceptor blend ratio of 5 : 1. These results not only indicate that BNBP-based polymers are promising for P3HT : polymer acceptor devices, but also suggest the potential for low cost and facile device processing of all-PSCs.

Organic solar cells (OSCs) based on a small molecule donor (SD) and a polymer acceptor (PA) exhibit low power conversion efficiency (PCE) due to the limited number of small molecule donor–polymer acceptor combinations. In this work, we employ a polymer acceptor based on the double B ← N bridged bipyridyl (BNBP) unit to develop SD/PA-type OSCs. With poly[(N,N′-bis(2-hexyldecyl)-diamine-bis(difluoro-borane)-2,2-bipyridine)-alt-(2,5-thiophene)] (P-BNBP-T) as the acceptor and 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) as the donor, the OSC device shows a high open-circuit voltage (VOC) of 1.08 V and a PCE of 3.50%. The VOC is ca. 0.3 V greater than that of other OSCs based on p-DTS(FBTTh2)2 due to the larger offset between the HOMO energy level of p-DTS(FBTTh2)2 and the higher-lying LUMO energy level of P-BNBP-T. The PCE of p-DTS(FBTTh2)2/P-BNBP-T is higher than that of any other OSCs based on the p-DTS(FBTTh2)2/polymer acceptor blend reported so far. These results indicate that the BNBP-based polymer acceptors are promising for high-performance SD/PA-type OSCs. While the as-cast p-DTS(FBTTh2)2/P-BNBP-T blend film exhibits low molecular packing order and large-size phase separation, processing with solvent additive 1,8-diiodoctane (DIO) leads to continuous networks with small crystalline grains of p-DTS(FBTTh2)2 in the blend film. The resulting OSC device exhibits the best photovoltaic performance because of the improved exciton dissociation efficiency and charge transport ability.

In this communication, we report a series of polymer semiconductors based on a novel electron-deficient building block, double B←N bridged bipyridine (BNBP). These polymers show ambipolar or unipolar n-channel charge-transporting characteristics with electron mobilities in the range of 0.02–0.32 cm2 V−1 s−1 in organic thin film transistors.

B←N coordination bond can be used to develop polymer electron acceptors for efficient all-polymer solar cells (all-PSCs). Here, we report a new alternating conjugated polymer containing two building blocks based on B←N unit. The polymer exhibits strong light absorption in the visible range, low-lying LUMO/HOMO energy levels and moderate electron mobility. The resulting all-PSC devices exhibit power conversion efficiencies of 1.50%–2.47%.

Strongly red luminescent and water-soluble probes are very important for studying biological events and processes. Fluorescent nanoparticles (NPs) built from the aggregation-induced emission luminogen (AIEgen) and amphipathic polymeric matrixes have been considered as promising candidates for bioimaging. However, AIE NPs with long-wavelength absorption suitable for in vivo application are still scarce. In this work, three AIE-active red-emissive BODIPY derivatives with long-wavelength absorption were rationally designed and synthesized. Then three NPs based on these AIEgens exhibit bright red photoluminescence with high fluorescence quantum yield in aqueous media. These NPs uniformly dispersed in water and showed excellent stability and good biocompatibility. They can be readily internalized by HeLa cells, and the staining process is performed by simply shaking the culture with cells for just a few seconds at room temperature, which indicates an ultrafast and easy-to-operate staining protocol. More importantly, long-term tracing in living cells and mouse over 15 days is successfully achieved. The strong fluorescence signals, ultrafast staining procedure, and long-term tracing abilities indicate that these AIE NPs hold great potential for monitoring biological processes.

Efficient organic solar cells (OSCs) often use combination of polymer donor and small molecule acceptor. Herein we demonstrate efficient and thermally stable OSCs with combination of small molecule donor and polymer acceptor, which is expected to expand the research field of OSCs. Typical small molecule donors show strong intermolecular interactions and high crystallinity, and consequently do not match polymer acceptors because of large-size phase separation. We develop a small molecule donor with suppressed π-π stacking between molecular backbones by introducing large steric hindrance. As the result, the OSC exhibits small-size phase separation in the active layer and shows a power conversion efficiency of 8.0%. Moreover, this OSC exhibits much improved thermal stability, i.e. maintaining 89% of its initial efficiency after thermal annealing the active layer at 180 °C for 7 days. These results indicate a different kind of efficient and stable OSCs., The organic solar cells are typically based on a binary combination of polymeric donor and molecular acceptor. Here Zhang et al. develop alternative combination based on molecular donor and polymeric acceptor featuring 8% efficiency and high stability up to 7 days at 180 °C.

Organoboron chemistry is a new tool to tune the electronic structures and properties of conjugated polymers, which are important for applications in organic optoelectronic devices. To investigate the effects of substituents of boron atoms on conjugated polymers, we synthesized three conjugated polymers based on double B←N-bridged bipyridine (BNBP) with various substituents on the boron atoms. By changing the substituents from four phenyl groups and two phenyl groups/two fluorine atoms to four fluorine atoms, the BNBP-based polymers show blue-shifted absorption spectra, decreased LUMO/HOMO energy levels, and enhanced electron affinities, as well as increased electron mobilities. Moreover, these BNBP-based polymers can be used as electron acceptors for all-polymer solar cells. These results demonstrate that substituents of boron atoms can effectively modulate the electronic properties and applications of conjugated polymers.

A new polymer acceptor based on the BNBP unit with an optimal LUMO energy level has been developed. The resulting all-polymer solar cells show high PCEs, remarkably high Voc values and small photon energy losses., A key parameter for polymer electron acceptors is the lowest unoccupied molecular orbital (LUMO) energy level (ELUMO). For state-of-the-art polymer electron acceptors based on the naphthalene diimide (NDI) unit, their ELUMO are low-lying and cannot be tuned, leading to a low open-circuit voltage (Voc) of the resulting all-polymer solar cells (all-PSCs). We report that polymer electron acceptors based on the double B←N bridged bipyridine (BNBP) unit exhibit tunable ELUMO because of their delocalized LUMOs over polymer backbones. The ELUMO of the copolymer of the BNBP unit and selenophene unit (P-BNBP-Se) is lower by 0.16 eV than that of the copolymer of the BNBP unit and thiophene unit (P-BNBP-T). As a result, the energy levels of P-BNBP-Se match well with the widely-used polymer donor, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th). The electron mobility of P-BNBP-Se (μe = 2.07 × 10–4 cm2 V–1 s–1) is also higher than that of P-BNBP-T (μe = 7.16 × 10–5 cm2 V–1 s–1). While the all-PSC device based on the PTB7-Th:P-BNBP-T blend shows a moderate power conversion efficiency (PCE) of 2.27%, the corresponding device with P-BNBP-Se as the acceptor exhibits a PCE as high as 4.26%. Moreover, owing to the suitable ELUMO of P-BNBP-Se, the all-PSC device of P-BNBP-Se shows a Voc up to 1.03 V, which is higher by 0.22 V than that with the conventional NDI-based polymer acceptor. These results indicate that BNBP-based polymers can give all-PSCs with high PCEs, remarkably high Voc values and small photon energy losses.

Broad absorption spectra and small optical bandgaps of polymer electron acceptors are very important for the sunlight harvesting of all-polymer solar cells (all-PSCs). Conjugated polymers based on the double B ← N bridged bipyridine (BNBP) unit are a new class of polymer electron acceptors, which suffer from narrow absorption spectra and large bandgaps. In this manuscript, we report a new polymer electron acceptor (P-BNBP-DPP) based on the BNBP unit and the dithienyl-diketopyrrolopyrrole (DPP) unit with a small bandgap and improved sunlight-harvesting capability. P-BNBP-DPP exhibits a broad absorption band with the onset absorbance at 796 nm and a small optical bandgap of 1.56 eV. Moreover, P-BNBP-DPP shows the low LUMO/HOMO energy levels of −3.87 eV/−5.45 eV and a high electron mobility of 2.1 × 10−4 cm2 V−1 s−1. An all-PSC device with P-BNBP-DPP as the acceptor and poly[(ethylhexyl-oxy)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7) as the donor produces a power conversion efficiency of 2.69% with a broad external quantum efficiency response in the range of 300–800 nm. These results suggest an effective approach to tune the absorption spectra of BNBP-based polymer electron acceptors.

A double B←N bridged bipyridyl (BNBP) is a novel electron-deficient building block for polymer electron acceptors in all-polymer solar cells. The B←N bridging units endow BNBP with fixed planar configuration and low-lying LUMO/HOMO energy levels. As a result, the polymer based on BNBP units (P-BNBP-T) exhibits high electron mobility, low-lying LUMO/HOMO energy levels, and strong absorbance in the visible region, which is desirable for polymer electron acceptors. Preliminary all-polymer solar cell (all-PSC) devices with P-BNBP-T as the electron acceptor and PTB7 as the electron donor exhibit a power conversion efficiency (PCE) of 3.38 %, which is among the highest values of all-PSCs with PTB7 as the electron donor.

The LUMO/HOMO energy levels of conjugated polymers are key parameters for their applications as polymer electron donors for polymer solar cells (PSCs). The widely-used strategy to tune the LUMO/HOMO levels of polymer donors is to develop D–A type polymers based on an alternating electron-donating unit (D) and an electron-accepting unit (A). In this paper, we report a novel approach to tune the LUMO/HOMO levels of polymer donors via replacing a C–C unit by a B ← N unit for enhanced PSC device performance. The control polymer PCPDT shows the LUMO/HOMO levels of −2.71 eV/−4.98 eV, which are both much higher than those required for an ideal polymer donor. By replacing a C–C unit with a B ← N unit, the resulting polymer PBNCPDT exhibits much lower LUMO/HOMO levels of −3.23 eV/−5.20 eV. PBNCPDT also shows a narrower optical bandgap (Eg = 1.73 eV) than that (Eg = 1.85 eV) of PCPDT, which is helpful for harvesting of sunlight. Moreover, PBNCPDT with the B ← N unit is not a typical D–A type conjugated polymer because its LUMO and HOMO are both delocalized over the whole conjugated framework. As the control PSC device based on PCPDT exhibits an open-circuit voltage (Voc) of 0.45 V and power conversion efficiency (PCE) of 0.63%, the device of PBNCPDT shows much improved Voc of 0.82 V and PCE of 3.74%. These results indicate that a B ← N unit can be used to develop polymer donors for high-performance PSC devices.