Search

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...

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.

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...

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.

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%.

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.

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.

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.

A novel polymer acceptor based on the double B←N bridged bipyridine building block is reported. All-polymer solar cells based on the new polymer acceptor show a power conversion efficiency of as high as 6.26% at a photon energy loss of only 0.51 eV.

A series of cholesterol-appended quinacridone (QA) derivatives 1a–1d have been synthesized, in which 1b and 1c could form stable organogels in a wide range of organic solvents upon ultrasound irradiation. Field emission scanning electronic microscope (FESEM) and transmission electron microscopy (TEM) of xerogels or precipitates indicated that 1b and 1c formed 1D fibrous nanostructure, while 1a assembled into 3D flower-like microstructures. The ultrasound-induced organogel process was characterized by kinetic UV-vis and photoluminescence spectroscopic methods suggesting the formation of π-π aggregates in the gel state. Experimental results demonstrated that the ultrasound could promote molecules to contact frequently in the solution and induce semistable initial aggregates, which propagate to form nano/micro superstructures. The aggregation model was optimized by semiempirical AM1 calculation suggesting the hierarchical self-assembly process. In addition, the formed xerogel film exhibited mechanochromic property, and the phase transition process was accompanied by the fluorescence changes between yellowish green and orange.

An electron donor−acceptor structured π-conjugated organic compound 1 composed of trifluoromethyl-biphenyl and cyano-stilbene-amine was designed and exhibited multi-stimuli-responsive fluorescence switching behaviors. The synthesized solid exhibited piezochromism in that grinding and heating could change the emission colors between orange-red and yellow. The amorphous 1 also showed interesting vapochromic behavior in that organic vapor could convert the yellow color into orange. The solution of 1 exhibited nearly no fluorescence at room temperature and intensive yellowish green emission at 77 K, while adding CF3COOH (TFA) resulted in green emissive state at room temperature and blue fluorescent state at 77 K.

The impact of boron doping on the nature of nanographene was investigated at the molecular level in terms of chemical adsorption with various Lewis bases, spin multiplicity of the two electron-reduced species, and performance as a battery electrode., The preparation of boron-doped nanocarbon scaffolds with well-defined structures is important for the understanding of the impact of boron doping on their properties and behavior at the molecular level. We recently succeeded in the synthesis of a structurally well-defined nanographene molecule, bearing two boron atoms at the central positions. In this study, the characteristic properties and functions of this boron-doped nanographene were investigated in terms of (1) Lewis acidity, (2) redox properties, and (3) electrode performance in a battery. This boron-doped nanographene was susceptible to chemical adsorption with various Lewis bases, resulting in significant changes in the absorption and fluorescence properties, as well as in the conformation of the honeycomb framework. The two-electron reduction of this boron-doped nanographene produced a dianionic species that showed a substantial biradical character with a triplet ground state. A Li battery electrode, composed of a boron-doped nanographene with small peripheral substituents, displayed a stable performance in the 1.5–4.0 V range with a first discharge capacity of 160 mA h g–1. These results provide important insights into the effect of boron doping on nanocarbon compounds.