All Roads Lead to Rome: Isomers with Divergent Cathode Modification Mechanisms toward Ohmic Contact.

Cathode interfacial layers (CILs) hold utmost importance for achieving ohmic contact at the organic semiconductor-cathode interface of organic photovoltaic devices. Delving deep into diverse design principles and working mechanisms is of great significance for designing novel CILs with high performance. Herein, two novel nonamine-based CILs are designed: one featuring a cyclopentadiene unit, designated as CIL-cp; while the other, lacking cyclopentadiene, is referred to as CIL-ph, which is an isomer of CIL-cp. The subtle changes in chemical structures result in distinct modification mechanisms toward ohmic contact. On one hand, the robust electron-withdrawing characteristic of cyclopentadiene endows CIL-cp with lower energy levels, resulting in an interfacial dipole at the active layer-CIL-cp interface due to electron transfer from D18 to CIL-cp. On the other hand, CIL-ph exhibits a strong interfacial dipole at the CIL-Ag interface, which significantly reduces the work function (WF) of the silver electrode. Both CIL-cp and CIL-ph demonstrate excellent interfacial modification capability, whereas CIL-cp possesses a stronger electron extraction ability, thus leading to a high power conversion efficiency of 19.31% in the D18:L8-BO system. Our results reveal the distinctive operational mechanism of cyclopentadiene-based CILs, thus offering innovative design ideas for CIL materials.

A 3,3'-Difluoro-2,2'-Bithiophene Based Donor Polymer Realizing High Efficiency (>17%) Single Junction Binary Organic Solar Cells.

In this study, two novel donor-acceptor (D-A) copolymers are designed and synthesized, DTBT-2T and DTBT-2T2F with 2,2'-bithiophene or 3,3'-difluoro-2,2'-bithiophene as the donor unit and dithienobenzothiadiazole as the acceptor unit, and used them as donor materials in non-fullerene organic solar cells (OSCs). Due to enhanced planarity of polymer chains resulted by the intramolecular F···S noncovalent interactions, the incorporation of 3,3'-difluoro-2,2'-bithiophene unit instead of 2,2'-bithiophene into the polymers can enhance their molecular packing, crystallinity and hole mobility. The DTBT-2T:L8-BO based binary OSCs deliver a power conversion efficiency (PCE) of only 9.71% with a Voc of 0.78 V, a Jsc of 20.69 mA cm-2 , and an FF of 59.67%. Moreover, the introduction of fluoro atoms can lower the highest occupied molecular orbital levels. As a result, DTBT-2T2F:L8-BO based single-junction binary OSCs exhibited less recombination loss, more balanced charge mobility, and more favorable morphology, resulting in an impressive PCE of 17.03% with a higher Voc of 0.89 V, a Jsc of 25.40 mA cm-2, and an FF of 75.74%. These results indicate that 3,3'-difluoro-2,2'-bithiophene unit can be used as an effective building block to synthesize high performance polymer donor materials. This work greatly expands the selection range of donor units for constructing high-performance polymers.

The efficient triplet states formation of Se-modified PDI dimers and tetramers in solvents.

The triplet excited states of molecules play an important role in photophysical processes, which has attracted great research interest. Perylene diimide (PDI) is a widely studied material closely associated with the generation of triplet states, and it is highly anticipated to become an electron acceptor material for improving photovoltaic conversion efficiency. In this work, we prepared dimers and tetramers composed of selenium-modified PDI-C5 (N,N'-bis(6-undecyl) perylene-3,4,9,10-bis(dicarboximide)) units. We investigated the photophysical processes of these dimers and tetramers in chloroform and toluene using UV-visible absorption spectroscopy, fluorescence spectroscopy, and femtosecond transient absorption spectroscopy. Both the dimers and tetramers undergo efficient triplet state formation processes in the solvents. Solvents with higher polarity facilitate charge transfer thereby promote the triplet states formation. The differences in the configurations of the dimer and tetramer molecules lead to variations in triplet states generation. The twisted angles in the tetramer restricted the intramolecular electronic coupling, posing certain hindrances to exciton coupling and lowering the intramolecular CT characteristics. The emission of excimer in tetramers also competes with the triplet states formation. The research demonstrates the influence of various factors on the generation of triplet states of PDI oligomers.

Designing A-D-A Type Fused-Ring Electron Acceptors with a Bulky 3D Substituent at the Central Donor Core to Minimize Non-Radiative Losses and Enhance Organic Solar Cell Efficiency.

Designing and synthesizing narrow band gap acceptors that exhibit high photoluminescence quantum yield (PLQY) and strong crystallinity is a highly effective, yet challenging, approach to reducing non-radiative energy losses (▵Enr) and boosting the performance of organic solar cells (OSCs). We have successfully designed and synthesized an A-D-A type fused-ring electron acceptor, named DM-F, which features a planar molecular backbone adorned with bulky three-dimensional camphane side groups at its central core. These bulky substituents effectively hinder the formation of H-aggregates of the acceptors, promoting the formation of more J-aggregates and notably elevating the PLQY of the acceptor in the film. As anticipated, DM-F showcases pronounced near-infrared absorption coupled with impressive crystallinity. Organic solar cells (OSCs) leveraging DM-F exhibit a high EQEEL value and remarkably low ▵Enr of 0.14 eV-currently the most minimal reported value for OSCs. Moreover, the power conversion efficiency (PCE) of binary and ternary OSCs utilizing DM-F has reached 16.16 % and 20.09 %, respectively, marking a new apex in reported efficiency within the OSCs field. In conclusion, our study reveals that designing narrow band gap acceptors with high PLQY is an effective way to reduce ▵Enr and improve the PCE of OSCs.

Hierarchical Solid-Additive Strategy for Achieving Layer-by-Layer Organic Solar Cells with Over 19 % Efficiency.

Layer-by-layer (LbL) deposition of active layers in organic solar cells (OSCs) offers immense potential for optimizing performance through precise tailoring of each layer. However, achieving high-performance LbL OSCs with distinct solid additives in each layer remains challenging. In this study, we explore a novel approach that strategically incorporates different solid additives into specific layers of LbL devices. To this end, we introduce FeCl3 into the lower donor (D18) layer as a p-type dopant to enhance hole concentration and mobility. Concurrently, we incorporate the wide-band gap conjugated polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) into the upper acceptor (L8-BO) layer to improve the morphology and prolong exciton lifetime. Unlike previous studies, our approach combines these two strategies to achieve higher and more balanced electron and hole mobility without affecting device open-circuit voltage, while also suppressing charge recombination. Consequently, the power conversion efficiency (PCE) of the D18+FeCl3/L8-BO device increases to 18.12 %, while the D18/L8-BO+PFO device attains a PCE of 18.79 %. These values represent substantial improvements over the control device's PCE of 17.59 %. Notably, when both FeCl3 and PFO are incorporated, the D18+FeCl3/L8-BO+PFO device achieves a remarkable PCE of 19.17 %. In summary, our research results demonstrate the effectiveness of the layered solid additive strategy in improving OSC performance.

Precise Methylation Yields Acceptor with Hydrogen-Bonding Network for High-Efficiency and Thermally Stable Polymer Solar Cells.

Utilizing intermolecular hydrogen-bonding interactions stands for an effective approach in advancing the efficiency and stability of small-molecule acceptors (SMAs) for polymer solar cells. Herein, we synthesized three SMAs (Qo1, Qo2, and Qo3) using indeno[1,2-b]quinoxalin-11-one (Qox) as the electron-deficient group, with the incorporation of a methylation strategy. Through crystallographic analysis, it is observed that two Qox-based methylated acceptors (Qo2 and Qo3) exhibit multiple hydrogen bond-assisted 3D network transport structures, in contrast to the 2D transport structure observed in gem-dichlorinated counterpart (Qo4). Notably, Qo2 exhibits multiple and stronger hydrogen-bonding interactions compared with Qo3. Consequently, PM6 : Qo2 device realizes the highest power conversion efficiency (PCE) of 18.4 %, surpassing the efficiencies of devices based on Qo1 (15.8 %), Qo3 (16.7 %), and Qo4 (2.4 %). This remarkable PCE in PM6 : Qo2 device can be primarily ascribed to the enhanced donor-acceptor miscibility, more favorable medium structure, and more efficient charge transfer and collection behavior. Moreover, the PM6 : Qo2 device demonstrates exceptional thermal stability, retaining 82.8 % of its initial PCE after undergoing annealing at 65 °C for 250 hours. Our research showcases that precise methylation, particularly targeting the formation of intermolecular hydrogen-bonding interactions to tune crystal packing patterns, represents a promising strategy in the molecular design of efficient and stable SMAs.

Boosting Organic Solar Cells to Over 18% Efficiency through Dipole-Dipole Interactions in Fluorinated Nonfused Ring Electron Acceptors.

This study successfully designed and synthesized two nonfused ring electron acceptors, 412-6F and 412-6Cl, modified with fluorine and chlorine substituents, respectively. Single-crystal analysis revealed that 412-6F possesses a planar molecular backbone and exhibits pronounced dipole-dipole interactions between the fluorine atoms on the lateral phenyl groups and the carbonyl oxygen atoms on the end groups. This specific interaction promotes dense end-group stacking, leading to a reduced interlayer spacing. Improved crystallinity and coherence length are observed in the D18:412-6F blend film. Conversely, 412-6Cl adopts a more distorted configuration and lacks these interactions. As a result, the organic solar cell (OSC) based on D18:412-6F achieved a remarkable power conversion efficiency of 18.03%, surpassing the performance of the D18:412-6Cl OSC. This underscores the importance of designing novel acceptors with beneficial intermolecular interactions to enhance OSC efficiency, thus providing a new direction for organic photovoltaic advancement.

Formation of Core-Shell AuCu@Ag Nanocrystals through the Nanoscale Kirkendall Effect.

Polymetallic nanocrystals (NCs) consist of multiple metal elements. A powerful platform to achieve the flexible construction of polymetallic NCs is highly desired but challenging. Herein, we devise a model system that realizes metal atom diffusion between different NCs, resulting in the formation of polymetallic NCs. The differential bond strength between different metal atoms is proposed to initiate such metal atom diffusion, and the specific high surface-to-volume ratio of the NCs can expedite the diffusion process. Taking the Au-Cu-Ag trimetallic system as an example, core-shell AuCu@Ag NCs were successfully formed by combining AgCu NCs with Au NCs. The evolution process was explored, and the gradual fusion of simple NCs into AuCu@Ag NCs was unambiguously observed, which could be attributed to the larger bond strength of Au-Cu than that of Ag-Cu. This work offers an opportunity/platform in theory and experiment to expand the synthesis framework as well as the polymetallic NC list.

Intramolecular and Intermolecular Interaction Switching in the Aggregates of Perylene Diimide Trimer: Effect of Hydrophobicity.

The research on perylene diimide (PDI) aggregates effectively promotes their applications in organic photovoltaic solar cells and fluorescent sensors. In this paper, a PDI fabricated with three peripheral PDI units (N, N'-bis(6-undecyl) perylene-3,4,9,10-bis(dicarboximide)) is investigated. The trimer shows different absorption and fluorescence properties due to hydrophobicity when dissolved in the mixed solvent of tetrahydrofuran (THF) and water. Through comprehensive analysis of the fluorescence lifetime and transient absorption spectroscopic results, we concluded that the trimer underwent different excited state kinetic pathways with different concentrations of water in THF. When dissolved in pure THF solvent, both the intramolecular charge-transfer and excimer states are formed. When the water concentration increases from 0 to 50% (v/v), the formation time of the excimer state and its structural relaxation time are prolonged, illustrating the arising of the intermolecular excimer state. It is interesting to determine that the probability of the intramolecular charge-transfer pathway will first decrease and then increase as the speed of intermolecular excimer formation slows down. The two inflection points appear when the water concentration is above 10% and 40%. The results not only highlight the importance of hydrophobicity on the aggregate properties of PDI multimers but also guide the further design of PDI-based organic photovoltaic solar cells.

High-Pressure Fabrication of Binary Organic Solar Cells with High Molecular Weight D18 Yields Record 19.65 % Efficiency.

In this work, inspired by the principles of a pressure cooker, we utilized a high-pressure method to address the processing challenges associated with high molecular weight polymers. Through this approach, we successfully dissolved high molecular weight D18 in chloroform at 100 °C within a pressure-tight vial. The increased steam pressure raised the boiling point and dissolving capacity of chloroform, enabling the creation of a hybrid film with superior properties, including more ordered molecular arrangement, increased crystallinity, extended exciton diffusion length, and improved phase morphology. Organic solar cells (OSCs) based on D18 : L8-BO prepared using this high-pressure method achieved an outstanding power conversion efficiency of 19.65 %, setting a new record for binary devices to date. Furthermore, this high-pressure method was successfully applied to fabricate OSCs based on other common systems, leading to significant enhancements in device performance. In summary, this research introduces a universal method for processing high molecular weight D18 materials, ultimately resulting in the highest performance reported for binary organic solar cells.

Macrocyclic Encapsulation in a Non-fused Tetrathiophene Acceptor for Efficient Organic Solar Cells with High Short-Circuit Current Density.

Non-fullerene acceptors have shown great promise for organic solar cells (OSCs). However, challenges in achieving high efficiency molecular system with conformational unicity and effective molecular stacking remain. In this study, we present a new design of non-fused tetrathiophene acceptor R4T-1 via employing the encapsulation of tetrathiophene with macrocyclic ring. The single crystal structure analysis reveals that cyclic alkyl side chains can perfectly encapsulate the central part of molecule and generate a conformational stable and planar molecular backbone. Whereas, the control 4T-5 without the encapsulation restriction displays cis- and twisted conformation. As a result, R4T-1 based OSCs achieved an outstanding power conversion efficiency (PCE) exceeding 15.10 % with a high short-circuit current density (Jsc ) of 25.48 mA/cm2 , which is significantly improved by ≈30 % in relative to that of the control. Our findings demonstrate that the macrocyclic encapsulation strategy could assist fully non-fused electron acceptors (FNEAs) to achieve a high photovoltaic performance and pave a new way for FNEAs design.

High-Performance Nonfused Ring Electron Acceptors with V-Shaped Side Chains.

Motivated by simplifying the synthesis of nonfullerene acceptor and establishing the relation between molecular structure and photovoltaic performance, two isomeric nonfused ring electron acceptors (o-TT-Cl and m-TT-Cl), whose properties can be adjusted by changing the side chains, are designed and synthesized with several high-yield steps. o-TT-Cl with V-shaped side chain induces a dominated J-aggregation and displays much better solubility and more ordered packing than m-TT-Cl with linear side chain. Thus, the o-TT-Cl-based blend film generates better phase morphology and charge transport than m-TT-Cl-based one. Finally, the power conversion efficiency of o-TT-Cl-based devices is 12.84%, which is much higher than that of m-TT-Cl-based ones (6.54%). This work highlights the importance of side chains engineering on improving photovoltaic performance of nonfused ring electron acceptors.

Room-Temperature Phosphorescence Invoked Through Norbornyl-Driven Intermolecular Interaction Intensification with Anomalous Reversible Solid-State Photochromism.

Herein, norbornyl (NB), a bulky annular nonconjugated spacer, is melded into π systems to construct two groups of ladder-type room-temperature phosphorescence (RTP) luminogens. The effect of the NB on π-π interactions, packing modes and RTP performance is explored systematically. The experimental and computational results demonstrate the versatility of NB in reducing π-π distances and synergistically intensifying the intermolecular interactions, which not only induces intersystem crossing from S1 to Tn but also diminishes the nonradiative decay of triplet excitons. Impressively, 1800-fold phosphorescence lifetime enhancement is achieved in comparison with the reference compounds without NB. The molecular packing and RTP performance can be further modulated by the length of the backbones and terminal end-groups. It is quite peculiar that NB-annulated phthalic acid exhibits reversible photochromism in the solid state, likely due to the formation of persistent radical pairs. Our study paves an ingenious avenue towards enhancing intermolecular interactions and provides significant implications for a better comprehensive understanding of the origin of their RTP and the inherent photophysical mechanism.

A Fully Non-fused Ring Acceptor with Planar Backbone and Near-IR Absorption for High Performance Polymer Solar Cells.

Fused-ring electron acceptors have made significant progress in recent years, while the development of fully non-fused ring acceptors has been unsatisfactory. Here, two fully non-fused ring acceptors, o-4TBC-2F and m-4TBC-2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o-4TBC-2F formed planar backbones, while m-4TBC-2F displayed a twisted backbone. Additionally, the o-4TBC-2F film showed a markedly red-shifted absorption after thermal annealing, which indicated the formation of J-aggregates. For fabrication of organic solar cells (OSCs), PBDB-T was used as a donor and blended with the two acceptors. The o-4TBC-2F-based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o-4TBC-2F gave a PCE of 10.26 %, which was much higher than those based on m-4TBC-2F at 2.63 %, and it is one of the highest reported PCE values for fully non-fused ring electron acceptors.

Exploiting Noncovalently Conformational Locking as a Design Strategy for High Performance Fused-Ring Electron Acceptor Used in Polymer Solar Cells.

We have developed a kind of novel fused-ring small molecular acceptor, whose planar conformation can be locked by intramolecular noncovalent interaction. The formation of planar supramolecular fused-ring structure by conformation locking can effectively broaden its absorption spectrum, enhance the electron mobility, and reduce the nonradiative energy loss. Polymer solar cells (PSCs) based on this acceptor afforded a power conversion efficiency (PCE) of 9.6%. In contrast, PSCs based on similar acceptor, which cannot form a flat conformation, only gave a PCE of 2.3%. Such design strategy, which can make the synthesis of small molecular acceptor much easier, will be promising in developing a new acceptor for high efficiency polymer solar cells.

Highly Efficient Planar Perovskite Solar Cells Via Interfacial Modification with Fullerene Derivatives.

Planar heterojunction perovskite solar cells with a high efficiency up to 17.76% are fabricated by modifying the compact TiO2 (c-TiO2) with a [6,6]-phenyl-C61-butyric acid (PCBA) monolayer. High quality CH3NH3PbI3 films can be easily fabricated on PCBA-modified c-TiO2 substrates by a one-step solution processing method. Significant improvements of the device parameters are observed after PCBA modification. A high open-circuit voltage (Voc) of 1.16 V has been achieved, indicating that the PCBA monolayer can act as a hole blocking layer to reduce the trap site density atop the c-TiO2 and the hole recombination at the c-TiO2 /perovskite interface. The enhancement of the fill factor, as well as the partial quenching of the fluorescence of perovskite after modification with PCBA, reveals that the charge extraction is improved.

Investigation into the Sensing Process of High-Performance H2S Sensors Based on Polymer Transistors.

Herein a H2S sensor based on a polymer field-effect transistor is reported and the sensor shows high sensitivity, excellent selectivity, fast response, and good operational stability. A concentration as low as 1 ppb H2 is detectable, which is to date the most sensitive H2S sensor based on organic semiconducting film. Thinning the thickness of active layer does not necessarily improve the sensitivity, but rather leads to the reduction of performance if the thickness is too low. Further analysis proposes a mechanism that the changing rate of absorption and desorption of H2S molecules is different when the thickness of active layer varies, indicating the necessity for thickness optimization.

Engineered Ionic Gates for Ion Conduction Based on Sodium and Potassium Activated Nanochannels.

In living systems, ion conduction plays a major role in numerous cellular processes and can be controlled by biological ion channels in response to specific environmental stimuli. This article describes biomimetic ionic gates for ion conduction based on sodium and potassium activated nanochannels. The Na(+) activated ionic gate and K(+) activated ionic gate were developed by immobilizing the alkali metal cation-responsive functional molecules, 4'-aminobenzo-15-crown-5 and 4'-aminobenzo-18-crown-6, respectively, onto the conical polyimide nanochannels. When the ionic gate was in the presence of the specific alkali metal cation, positively charged complexes formed between the crown ether and the alkali metal cation. On the basis of the resulting changes in surface charge, wettability and effective pore size, the nanochannel can achieve reversible switching. The switching behaviors of the two complexes differed due to the differences in binding strength between the two complexes. The Na(+) activated ionic gate is able to open and close to control the ion conduction through the nanochannel, and the K(+) activated ionic gate enables selective cation and anion conduction through the nanochannel. The Na(+) and K(+) activated ionic gates show great promise for use in clinical medicine, biosensors and drug delivery based on their high sensitivity and selectivity of being activated, and good stability.

Performance Enhancement of Polymer Solar Cells by Using Two Polymer Donors with Complementary Absorption Spectra.

Performance enhancement of polymer solar cells (PSCs) is achieved by expanding the absorption of the active layer of devices. To better match the spectrum of solar radiation, two polymers with different band gaps are used as the donor material to fabricate ternary polymer cells. Ternary blend PSCs exhibit an enhanced short-circuit current density and open-circuit voltage in comparison with the corresponding HD-PDFC-DTBT (HD)- and DT-PDPPTPT (DPP)-based binary polymer solar cells, respectively. Ternary PSCs show a power conversion efficiency (PCE) of 6.71%, surpassing the corresponding binary PSCs. This work demonstrates that the fabrication of ternary PSCs by using two polymers with complementary absorption is an effective way to improve the device performance.

High-efficiency large-bandgap material for polymer solar cells.

High-molecular-weight conjugated polymer HD-PDFC-DTBT with N-(2-hexyldecyl)-3,6-difluorocarbazole as the donor unit, 5,6-bis(octyloxy)benzothiadiazole as the acceptor unit, and thiophene as the spacer is synthesized by Suzuki polycondensation. HD-PDFC-DTBT shows a large bandgap of 1.96 eV and a high hole mobility of 0.16 cm(2) V(-1) s(-1) . HD-PDFC-DTBT:PC71 BM-based inverted polymer solar cells (PSCs) give a power conversion efficiency (PCE) of 7.39% with a Voc of 0.93 V, a Jsc of 14.11 mA cm(-2) , and an FF of 0.56.