Billy

Red/near-infrared dyes are highly demanded for biological applications but most of them are far from satisfactory. In this work, a series of red/near-infrared fluorophores based on electron-withdrawing benzo[1,2-b:4,5-b′]dithiophene 1,1,5,5-tetraoxide (BDTO) are synthesized and characterized. They possess both aggregation-induced emission, and hybridized local and charge-transfer characteristics. Crystallographic, spectroscopic, electrochemical and computational results reveal that the oxidation of benzo[1,2-b:4,5-b′]dithiophene to BDTO can endow the fluorophores with greatly red-shifted emission, enhanced emission efficiency, reduced energy levels, enlarged two-photon absorption cross section, and increased reactive oxygen species generation efficiency. The nanoparticles fabricated with a near-infrared fluorophore TPA-BDTO show high photostability and biocompatibility with good performance in targeted photodynamic ablation of cancer cells and two-photon fluorescence imaging of intravital mouse brain vasculature.

Efficient near-infrared fluorophores based on benzo[1,2-b:4,5-b′]dithiophene 1,1,5,5-tetraoxide are developed to show promising applications in targeted photodynamic ablation of cancer cells and in vivo two-photon fluorescence visualization of brain blood vasculature.

All-polymer solar cells (all-PSCs) offer unique morphology stability for the application as flexible devices, but the lack of high-performance polymer acceptors limits their power conversion efficiency (PCE) to a value lower than those of the PSCs based on fullerene derivative or organic small molecule acceptors. We herein demonstrate a strategy to synthesize a high-performance polymer acceptor PZ1 by embedding an acceptor–donor–acceptor building block into the polymer main chain. PZ1 possesses broad absorption with a low band gap of 1.55 eV and high absorption coefficient (1.3×10⁵ cm⁻¹). The all-PSCs with the wide-band-gap polymer PBDB-T as donor and PZ1 as acceptor showed a record-high PCE of 9.19 % for the all-PSCs. The success of our polymerization strategy can provide a new way to develop efficient polymer acceptors for all-PSCs.

Energy conversion: Embedding an acceptor–donor–acceptor-structured organic semiconductor building block into a polymer main chain creates an excellent low-band-gap polymer acceptor with red-shifted absorption and high absorption coefficient. The polymer acceptor provides a record-high power conversion efficiency of 9.19 % for all-polymer solar cells.

Suppression of carrier recombination is critically important in realizing high-efficiency polymer solar cells. Herein, it is demonstrated difluoro-substitution of thiophene conjugated side chain on donor polymer can suppress triplet formation for reducing carrier recombination. A new medium bandgap 2D-conjugated D–A copolymer J91 is designed and synthesized with bi(alkyl-difluorothienyl)-benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit, for taking the advantages of the synergistic fluorination on the backbone and thiophene side chain. J91 demonstrates enhanced absorption, low-lying highest occupied molecular orbital energy level, and higher hole mobility, in comparison with its control polymer J52 without fluorination on the thiophene side chains. The transient absorption spectra indicate that J91 can suppress the triplet formation in its blend film with n-type organic semiconductor acceptor m-ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene). With these favorable properties, a higher power conversion efficiency of 11.63% with high V_OC of 0.984 V and high J_SC of 18.03 mA cm⁻² is obtained for the polymer solar cells based on J91/m-ITIC with thermal annealing. The improved photovoltaic performance by thermal annealing is explained from the morphology change upon thermal annealing as revealed by photoinduced force microscopy. The results indicate that side chain engineering can provide a new solution to suppress carrier recombination toward high efficiency, thus deserves further attention.

Suppression of carrier recombination is critically important for efficient polymer solar cells. Herein, it is demonstrated that difluoro-substitution of thiophene-conjugated side chains on the medium-bandgap polymer donor can suppress triplet formation for reducing carrier recombination and improving photovoltaic performance.

Two novel narrow bandgap π-conjugated polymers based on naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (NT) unit are developed, which contain the thiophene or benzodithiophene flanked with alkylthiophene as the electron-donating segment. Both copolymers exhibit strong aggregations both in solution and as thin films. The resulting copolymers with higher molecular weight show higher photovoltaic performance by virtue of the enhanced short-circuit current densities and fill factors, which can be attributed to their higher absorptivity and formation of more favorable film morphologies. Polymer solar cells (PSCs) fabricated with the copolymer PNTT achieve remarkable power conversion efficiencies (PCEs) > 11% based on both conventional and inverted structures at the photoactive layer thickness of 280 nm, which is the highest value so far observed from NT-based copolymers. Of particular interest is that the device performances are insensitive to the thickness of the photoactive layer, for which the PCEs > 10% can be achieved with film thickness ranging from 150 to 660 nm, and the PCE remains >9% at the thickness over 1 µm. These findings demonstrate that these NT-based copolymers can be promising candidates for the construction of thick film PSCs toward low-cost roll-to-roll processing technology.

Two novel conjugated polymers based on naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazol (NT) as the electron-deficient unit are developed for polymer solar cells (PSCs). The fabricated PSCs based on the high molecular weight copolymer and the fullerene acceptor ([6,6]-phenyl-C71-butyric acid methyl ester) present remarkable power conversion efficiencies over 10% with the bulk-heterojunction film thickness ranging from 150 to 660 nm.

Organic semiconductors are in general known to have an inherently lower charge carrier mobility compared to their inorganic counterparts. Bimolecular recombination of holes and electrons is an important loss mechanism and can often be described by the Langevin recombination model. Here, the device physics of bulk heterojunction solar cells based on a nonfullerene acceptor (IDTBR) in combination with poly(3-hexylthiophene) (P3HT) are elucidated, showing an unprecedentedly low bimolecular recombination rate. The high fill factor observed (above 65%) is attributed to non-Langevin behavior with a Langevin prefactor (β/β_L) of 1.9 × 10⁻⁴. The absence of parasitic recombination and high charge carrier lifetimes in P3HT:IDTBR solar cells inform an almost ideal bimolecular recombination behavior. This exceptional recombination behavior is explored to fabricate devices with layer thicknesses up to 450 nm without significant performance losses. The determination of the photoexcited carrier mobility by time-of-flight measurements reveals a long-lived and nonthermalized carrier transport as the origin for the exceptional transport physics. The crystalline microstructure arrangement of both components is suggested to be decisive for this slow recombination dynamics. Further, the thickness-independent power conversion efficiency is of utmost technological relevance for upscaling production and reiterates the importance of understanding material design in the context of low bimolecular recombination.

Nonfullerene-based organic solar cells with an unprecedentedly low bimolecular recombination rate are presented. The absence of parasitic recombination and high carrier lifetimes in the devices inform an almost ideal bimolecular recombination behavior with a Langevin prefactor (β/β_L) of 1.9 × 10⁻⁴. This exceptional recombination behavior allows the fabrication of solar cells with layer thicknesses up to 450 nm without significant performance losses.

Bulk heterojunction (BHJ) morphologies are vital to the device performance of organic solar cells (OSCs), including phase separation in lateral and vertical directions. However, the morphology developed from the blend solution is not easily predicted and controlled, especially in the vertical direction, because the BHJ morphology is kinetically frozen during the rapid solvent evaporation process. Here, a simple approach to control BHJ morphologies with optimized phase distribution for small molecule:[6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁ BM) blends by enhancing the substrate temperature during the spin-coating process. Three molecules with various fluorine atoms in the end acceptor units are selected. The relationship among molecular structures, substrate temperature effects on the morphology, and device performances are symmetrically investigated. Low temperature induces a multiple-sublayer-like architecture with significantly varied distributions of composition, morphology, and localized state energy, while high processing temperature induces more uniform film. The short-circuit current, open-circuit voltage, and fill factor of the devices are tuned with synergic improvement of efficiency toward over 10% and 11% for conventional and inverted devices. This work reveals the origination of vertical phase segregation, and provides a facile strategy to optimize the hierarchical phase separation for enhancing the performance of OSCs.

Vertical phase segregation in small molecule photovoltaic devices is manipulated via substrate temperature tuning. Low temperature induces multiple-sublayer-like architecture with significantly varied distributions of composition, morphology, and localized state energy, while high processing temperature induces more uniform film. The parameters of devices are largely tuned with synergic improvement of efficiency toward over 10% and 11% for conventional and inverted devices.

Tandem organic solar cells (TOSCs), which integrate multiple organic photovoltaic layers with complementary absorption in series, have been proved to be a strong contender in organic photovoltaic depending on their advantages in harvesting a greater part of the solar spectrum and more efficient photon utilization than traditional single-junction organic solar cells. However, simultaneously improving open circuit voltage (V_oc) and short current density (J_sc) is a still particularly tricky issue for highly efficient TOSCs. In this work, by employing the low-bandgap nonfullerene acceptor, IEICO, into the rear cell to extend absorption, and meanwhile introducing PBDD4T-2F into the front cell for improving V_oc, an impressive efficiency of 12.8% has been achieved in well-designed TOSC. This result is also one of the highest efficiencies reported in state-of-the-art organic solar cells.

Simultaneously improving the open-circuit voltage (V_oc) and short current density (J_sc) is a particularly tricky issue for tandem organic solar cells (TOSCs). By employing the low-bandgap nonfullerene acceptor, IEICO, in the rear cell to extend absorption, and meanwhile introducing PBDD4T-2F into the front cell for improving V_oc, an impressive efficiency of 12.8% is achieved in TOSCs. This result is also one of the highest efficiencies reported in state-of-the-art organic solar cells.

A new acceptor–donor–acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V _OC of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.

A new acceptor–donor–acceptor-structured nonfullerene acceptor, ITCC, is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V _OC of over 1 V is recorded in photovoltaic devices, suggesting great potential for applications in tandem organic solar cells.

The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.

The development of wide-bandgap (WBG) polymers is of significant importance in boosting the power conversion efficiency of organic solar cells. Recent advances in WBG polymers and their applications in different types of organic photovoltaic devices are reviewed. In addition, empirical regularities and invigorating perspectives are provided to help guide future designs of WBG photovoltaic materials.

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double-BHJ OSCs is constructed via stamp transferring of low bandgap BHJ atop of mediate bandgap active layers. Such devices allow a large gain in photocurrent to be obtained due to enhanced photoharvest, without suffering much from the fill factor drop usually seen in thick-layer-based devices. Overall, double-BHJ OSC with optimal ≈50 nm near-infrared PDPP3T:PC₇₁BM layer atop of ≈200 nm PTB7-Th:PC₇₁BM BHJ results in high power conversion efficiencies over 12%.

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double-BHJ OSCs is constructed via stamp-transferring of low bandgap BHJ layer atop of mediate bandgap active layers. Such devices obtain a large gain in photocurrent due to the enhanced photo harvest with little fill-factor drop. Overall, double-BHJ OSC results in high power conversion efficiencies over 12%.

Synthesis of fluorene-based conjugated polyelectrolytes was achieved via Suzuki polycondensation in water and completely open to air. The polyelectrolytes were conveniently purified by dialysis and analysis of the materials showed properties expected for fluorene-based conjugated polyelectrolytes. The materials were then employed in solar cell devices as an interlayer in conjunction with ZnO. The double interlayer led to enhanced power conversion efficiency of 10.75 % and 15.1 % for polymer and perovskite solar cells, respectively.

Fluorene-based conjugated polyelectrolytes were prepared by using a green synthetic route with polymerization in water and in air. High-performance polymer and perovskite solar cells were fabricated using the polyelectrolytes in the cathode interlayer giving power conversion efficiency of 10.75 % and 15.1 %.

A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flaked with a 2,1,3-benzothiadiazole (BT) and end-capped with highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile (RCN) units, has been successfully synthesized as a small molecular acceptor (SMA) for nonfullerene polymer solar cells (PSCs). This SMA exhibits a relatively wide optical bandgap of 2.03 eV, which provides a complementary absorption to commonly used low bandgap donor polymers, such as PTB7-Th. The strong electron-deficient BT and RCN units afford SFBRCN with a low-lying LUMO (lowest unoccupied molecular orbital) level, while the 3D structured spirobifluorene core can effectively suppress the self-aggregation tendency of the SMA, thus yielding a polymer:SMA blend with reasonably small domain size. As the results of such molecular design, SFBRCN enables nonfullerene PSCs with a high efficiency of 10.26%, which is the highest performance reported to date for a large bandgap nonfullerene SMA.

A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flanked with two 2,1,3-benzothiadiazole groups and end-capped with two highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile units, has been successfully synthesized as a small molecular acceptor for nonfullerene polymer solar cells with a high efficiency of 10.26%.

Nowadays, solvent additives are widely used in organic solar cells (OSCs) to tune the nano-morphology of the active blend film and enhance the device performance. With their help, power conversion efficiencies (PCEs) of OSCs have recently stepped over 10%. However, residual additive in the device can induce undesirable morphological change and also accelerate photo-oxidation degradation of the active blend film. Thereby, their involvements are actually unfavorable for practical applications. Here, a donor material PThBDTP is employed, and PThBDTP:PC71BM based OSCs are fabricated. A PCE of over 10% is achieved without using any additives and film post-treatments. The device displays a high open-circuit voltage of 0.977 V, a large short-circuit current density of 13.49 mA cm^-2, and a high fill factor of 76.3%. These results represent an important step towards developing high-efficiency additive-free OSCs.

Organic photovoltaic devices with PThBDTP as donor and PC₇₁BM as acceptor are fabricated, and a power conversion efficiency of over 10% is achieved without resorting to any additives and post treatments.

Aimed at achieving ideal morphology, illuminating morphology–performance relationship, and further improving the power conversion efficiency (PCE) of ternary polymer solar cells (TSCs), a ternary system is designed based on PTB7-Th:PffBT4T-2OD:PC₇₁BM in this work. The PffBT4T-2OD owns large absorption cross section, proper energy levels, and good crystallinity, which enhances exciton generation, charge dissociation and transport and suppresses charge recombination, thus remarkably increasing the short-circuit current density (J_sc) and fill factor (FF). Finally, a notable PCE of 10.72% is obtained for the TSCs with 15% weight ratio of PffBT4T-2OD. As for the working mechanism, it confirmed the energy transfer from PffBT4T-2OD to PTB7-Th, which contributes to the improved exciton generation. And morphology characterization indicates that the devices with 15% PffBT4T-2OD possess both appropriate domain size (25 nm) and enhanced domain purity. Under this condition, it affords numerous D/A interface for exciton dissociation and good bicontinuous nanostructure for charge transport simultaneously. As a result, the device with 15% PffBT4T-2OD exhibits improved exciton generation, enhanced charge dissociation possibility, elevated hole mobility and inhibited charge recombination, leading to elevated J_sc (19.02 mA cm⁻²) and FF (72.62%) simultaneously. This work indicates that morphology optimization as well as energy transfer plays a significant role in improving TSC performance.

Ternary polymer solar cells based on PTB7-Th:PffBT4T-2OD:PC₇₁BM are designed according to the complementary properties of PTB7-Th and PffBT4T-2OD. The highest efficiency of 10.72% for this ternary system is achieved with 15% PffBT4T-2OD. As for the working mechanism, this ternary system paves the way to investigate the morphology–performance relationship. Moreover, the energy transfer is involved in ternary blend.

A star-shaped electron acceptor based on porphyrin as a core and perylene bisimide as end groups was constructed for application in non-fullerene organic solar cells. The new conjugated molecule exhibits aligned energy levels, good electron mobility, and complementary absorption with a donor polymer. These advantages facilitate a high power conversion efficiency of 7.4 % in non-fullerene solar cells, which represents the highest photovoltaic performance based on porphyrin derivatives as the acceptor.

Within arm's reach: A star-shaped porphyrin-based molecule with four perylene bisimide arms (PBI-Por) was designed as a non-fullerene electron acceptor for application in solar cells. The combination of a donor polymer with PBI-Por in a solar cell resulted in a photoresponse from λ=300 to 850 nm, with a maximum external quantum efficiency (EQE) of almost 0.70, and a promising power conversion efficiency of 7.4 %.

The design of narrow band gap (NBG) donors or acceptors and their application in organic solar cells (OSCs) are of great importance in the conversion of solar photons to electrons. Limited by the inevitable energy loss from the optical band gap of the photovoltaic material to the open-circuit voltage of the OSC device, the improvement of the power conversion efficiency (PCE) of NBG-based OSCs faces great challenges. A novel acceptor–donor–acceptor structured non-fullerene acceptor is reported with an ultra-narrow band gap of 1.24 eV, which was achieved by an enhanced intramolecular charge transfer (ICT) effect. In the OSC device, despite a low energy loss of 0.509 eV, an impressive short-circuit current density of 25.3 mA cm⁻² is still recorded, which is the highest value for all OSC devices. The high 10.9 % PCE of the NBG-based OSC demonstrates that the design and application of ultra-narrow materials have the potential to further improve the PCE of OSC devices.

Mind the gap: A non-fullerene electron acceptor with an ultra-narrow optical band gap of 1.24 eV was prepared. Despite a low energy loss of 0.509 eV, the organic solar cell (OSC) device demonstrated a high power conversion efficiency of 10.9 % with an impressive short-circuit current density of 25.3 mA cm⁻².

Using a “multifluorination” strategy, ambipolar donor–acceptor conjugated polymer with hole and electron mobility (μ_h and μ_e) up to 3.94 and 3.50 cm² V⁻¹ s⁻¹, respectively, and unipolar n-type donor–acceptor conjugated polymers with μ_e up to 4.97 cm² V⁻¹ s⁻¹ is synthesized with isoindigo as acceptor units.

A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.

Benzene units are inserted into the backbone of a quaterthiophene-based polymer named PffBT4T, and the resulting polymer, PffBT4T-B, exhibits remarkably tight alkyl chain interdigitation, which can expel the ITIC-Th molecules from the polymer domains thus forming more pure and crystalline ITIC-Th domains. As a result, PffBT4T-B-based non-fullerene organic solar cells achieve a high power conversion efficiency of 9.4%.