Conspectus Organic 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.
Organometal halide perovskites (OHPs) are becoming a hot topic in the field of display and lighting. Unlike the strategy used for solar cells, that is, using several hundred nanometers thick OHP film ...for fully absorbing solar light to convert electricity, thin-film OHPs (<50 nm) are advantageous to restrain its self-absorption drawback and thus beneficial for preparing efficient light-emitting diodes (LEDs). Here we manipulate the excess molar ratio of MABr/PbBr2 precursors and the post-annealing temperature to obtain uniform thin-film OHPs and suppress the nonradiative defects. Using this simple process, high efficient green perovskite light-emitting diode (PeLED) was obtained, with a maximum luminance of 6124 cd m–2, current efficiency of 15.26 cd A–1, and external quantum efficiency of 3.38%, which is nearly three-fold enhancement with respect to the previous reported best PeLED based on thin perovskite films (<50 nm)
Piezoelectric materials convert between mechanical and electrical energy and are a basis for self-powered electronics. Current piezoelectrics exhibit either large charge (d
) or voltage (g
) ...coefficients but not both simultaneously, and yet the maximum energy density for energy harvesting is determined by the transduction coefficient: d
*g
. In prior piezoelectrics, an increase in polarization usually accompanies a dramatic rise in the dielectric constant, resulting in trade off between d
and g
. This recognition led us to a design concept: increase polarization through Jahn-Teller lattice distortion and reduce the dielectric constant using a highly confined 0D molecular architecture. With this in mind, we sought to insert a quasi-spherical cation into a Jahn-Teller distorted lattice, increasing the mechanical response for a large piezoelectric coefficient. We implemented this concept by developing EDABCO-CuCl
(EDABCO = N-ethyl-1,4-diazoniabicyclo2.2.2octonium), a molecular piezoelectric with a d
of 165 pm/V and g
of ~2110 × 10
V m N
, one that achieved thusly a combined transduction coefficient of 348 × 10
m
J
. This enables piezoelectric energy harvesting in EDABCO-CuCl
@PVDF (polyvinylidene fluoride) composite film with a peak power density of 43 µW/cm
(at 50 kPa), the highest value reported for mechanical energy harvesters based on heavy-metal-free molecular piezoelectric.
Optoelectronic polymers are always π-conjugated polymers consisting of C, H, N, S, and O elements. The main group element chemistry is a new tool to tune the optoelectronic properties of polymers. ...However, the resulting polymers generally show moderate optoelectronic device performance because they cannot meet the multiple requirements for solution-processed devices. Herein, we report an organoboron polymer, which can be used as a polymer acceptor in all-polymer solar cells (all-PSCs) to give power conversion efficiency of as high as 10.1%. By incorporating 2,1,3-benzothiadiazole unit to the organoboron polymer backbone, we tune its absorption spectrum, energy levels, electron mobility, and phase separation behavior. All of these improvements contribute to the excellent all-PSC device performance. These results prove that solution-processed polymers containing main group element atoms can give excellent optoelectronic device performance.
We demonstrate that polymer electron acceptors with excellent all‐polymer solar‐cell (all‐PSC) device performance can be developed from polymer electron donors by using B←N units. By alleviating the ...steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain B←N units, the π–π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all‐PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all‐PSCs with state‐of‐the‐art polymer acceptors.
From giver to taker: Incorporation of B←N units into polymer electron donors has resulted in a series of polymer electron acceptors. Extending the length of the repeating units of the conjugated polymers alleviates the effect of steric hindrance from the pendant groups and promotes the π–π stacking of the polymer backbones. The all‐polymer solar‐cell device shows a power conversion efficiency (PCE) exceeding 5.0 %.
Conspectus Since the discovery of conductive poly(acetylene), the study of conjugated polymers has remained an active and interdisciplinary frontier between polymer chemistry, polymer physics, ...computation, and device engineering. One of the ultimate goals of polymer science is to reliably synthesize structures, similar to small molecule synthesis. Kumada catalyst-transfer polymerization (KCTP) is a powerful tool for synthesizing conjugated polymers with predictable molecular weights, narrow dispersities, specific end groups, and complex backbone architectures. However, expanding the monomer scope beyond the well-studied 3-alkylthiophenes to include electron-deficient and complex heterocycles has been difficult. Revisiting the successful applications of KCTP can help us gain new insight into the CTP mechanisms and thus inspire breakthroughs in the controlled polymerization of challenging π-conjugated monomers. In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of homopolymers (p-type and n-type), copolymers (diblock and statistical), and monodisperse high oligomers. We first give a brief introduction of the mechanism and state-of-the-art of KCTP. Since the extent of polymerization control is determined by steric and electronic effects of both the catalyst and monomer, the polymerization can be optimized by modifying monomer and catalyst structures, as well as finding a well-matched monomer–catalyst system. We discuss the effects of side-chain steric hindrance and halogens in the context of heavy atom substituted monomers. By moving the side-chain branch point one carbon atom away from the heterocycle to alleviate steric crowding and stabilize the catalyst resting state, we were able to successfully control the polymerization of new tellurophene monomers. Inspired by innocent role of the sterically encumbered 2-transmetalated 3-alkylthiophene monomer, we introduce the treatment of hygroscopic monomers with a bulky Grignard compound as a water-scavenger for the improved synthesis of water-soluble conjugated polymers. For challenging electron-deficient monomers, we discuss the design of new Ni(II)diimine catalysts with electron-donating character which enhance the stability of the association complex between the catalyst and the growing polymer chain, resulting in the quasi-living synthesis of n-type polymers. Beyond n-type homopolymers, the Ni(II)diimine catalysts are also capable of producing electron-rich and electron-deficient diblock and statistical copolymers. We discuss how density functional theory (DFT) calculations elucidate the role of catalyst steric and electronic effects in controlling the synthesis of π-conjugated polymers. Moreover, we demonstrate the synthesis of monodisperse high oligomers by temperature cycling, which takes full advantage of the unique character of KCTP in that it proceeds through distinct intermediates that are not reactive. The insight we gained thus far leads to the first example of isolated living conjugated polymer chains prepared by a standard KCTP procedure, with general applicability to different monomers and catalytic systems. In summarizing a decade of innovation in KCTP, we hope this Account will inspire future development in the field to overcome key challenges including the controlled synthesis of electron-deficient heterocycles, complex and high-performance systems, and degradable and recyclable materials as well as cutting-edge catalyst design.
The morphology of a polymer donor/polymer acceptor active layer in all-polymer solar cells (all-PSCs) has a big influence on photovoltaic performance. It is difficult to tune the active layer ...morphology of all-PSCs by conventional methods, such as thermal annealing, solvent annealing, and utilization of solvent additive. In this article, we report that the active layer morphology of all-PSCs can be effectively optimized by the temperature of the solution for spin coating. We select poly4-(5-(4,8-bis(5-((2-butyloctyl)thio)thiophen-2-yl)-6- methylbenzo1,2-b:4,5-b′dithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-7-(5-methylthiophen-2-yl)-2H-benzod1,2,3triazole (CD1) as the polymer donor and poly4-(5-(5-(2-decyltetradecyl)-10-(2-dodecylhexadecyl)-4,4,9,9-tetrafluoro-2-methyl-4,5,9,10-tetrahydro-3a,5,8a,10-tetraaza-4,9-diborapyren-7-yl)thiophen-2-yl)-7-(5-methylthiophen-2-yl)benzoc1,2,5thiadiazole (PBN-12) as the polymer acceptor. Compared with the solution temperature of 30 °C, the CD1–PBN-12 active layer spin-coated at the solution temperature of 90 °C exhibits smaller phase-separation size and optimal crystallization degree of the polymers. The resulting all-PSCs device shows improved charge separation, balanced charge transporting, and suppressed bimolecular recombination. As a result, the power conversion efficiency is enhanced from 7.7% to 10.0%. The effect of solution temperature on phase-separation morphology of all-PSCs can be explained by the increased diffusion rate of polymer chains in the film-forming process and the disaggregation of polymer chains in solution at high temperature. These results indicate a new approach for improving active layer morphology of all-PSCs by solution temperature.
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 variety ...of polymer donors to produce efficient all-PSCs. The amorphous polymer acceptor (rr-PBN) is a regiorandom polymer consisting of alternating asymmetric B←N bridged thienylthiazole (BNTT) unit and pyridine-flanked diketopyrrolopyrrole (PyDPP) unit. It is amorphous in thin film because of its regiorandom structure and the large steric hindrance. rr-PBN shows deep LUMO/HOMO energy levels of −3.71/–5.81 eV, strong sunlight harvesting capability and high electron mobility of 2.20 × 10–4 cm2 V–1 s–1. As a polymer acceptor, rr-PBN matches well with three commercially available polymer donors, J71, PTB7-Th, and PffBT4T-2OD to give excellent percolating bicontinuous network morphology in all-PSCs. We propose that the crystallization of polymer donors governs the film-forming process and dominates the phase separation morphology, leading to good phase separation morphology. The all-PSC devices all show power conversion efficiencies (PCEs) of 5.2–6.6%. This study provides a new direction to design polymer acceptors and a novel approach to control phase separation morphology of all-PSCs.
Sequence‐defined synthetic oligomers and polymers provide unprecedented opportunities for polymer chemists to finely control properties such as chain folding, self‐assembly, and optoelectronic ...performance of materials. However, absolute control over both chain‐length and monomer sequence has been a long‐standing “grand challenge” for decades. Herein, we report a novel strategy to synthesize monodisperse sequence‐defined conjugated oligomers in a homogeneous manner by temperature cycling, thereby achieving single‐monomer precision in conjugated polyheterocycles. A series of sequence‐defined oligomers with up to twelve repeating units, four different monomers, and various sequences were successfully synthesized. Monomer sequence was also proved to affect optical properties. We believe this strategy not only exhibits general applicability to the synthesis of group 16 conjugated oligomers and polymers, but also has far‐reaching potential for other polymer systems.
Synthesis of structurally well‐defined macromolecules has been one of the ultimate goals and challenges for polymer chemists for decades. This work reports the first homogeneous synthesis of monodisperse sequence‐defined conjugated oligomers. The synthesis is supported by catalyst transfer polymerization and a custom Python‐based automated program.
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 efficiency (PCE) of 4.46% with a small photon energy loss (E loss) of 0.51 eV or a PCE of 3.77% with an extremely small E loss of 0.47 eV. This E loss is among the smallest values reported for organic solar cells. These results demonstrate that the “conjugated side chain” strategy can be used not only for high-efficiency polymer electron donors but also for high-performance polymer electron acceptors.
