Yingzhi Jin
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Thermal-Driven Phase Separation of Double-Cable Polymers Enables Efficient Single-Component Organic Solar Cells
Overcoming the energy loss in asymmetrical non-fullerene acceptor-based polymer solar cells by halogenation of polymer donors
DOI: 10.1039/C9TA02243K, Paper
PSCs based on polymer donors (PM6 and PM7) and an asymmetrical small molecule acceptor (IDT6CN-M) were developed and achieved a high PCE of 13.3% with a high Voc of over 1.0 V.
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Design of wide-bandgap polymers with deeper ionization potential enables efficient ternary non-fullerene polymer solar cells with 13% efficiency
DOI: 10.1039/C9TA04237G, Paper
Two regioisomeric wide-bandgap polymers with different nitrogen topologies along the conjugated backbone were developed and applied in non-fullerene polymer solar cells.
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Strategic end-halogenation of π-conjugated small molecules enabling fine morphological control and enhanced performance of organic solar cells
DOI: 10.1039/C9TA03869H, Paper
A novel family of photovoltaic small-molecule donors having the same conjugated backbone but different terminal halogen groups (F, Cl, Br, and I) are developed, and the impacts of end-halogenation on the photovoltaic and morphological properties are systematically investigated.
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Morphology of a thermally stable small molecule OPV blend comprising a liquid crystalline donor and fullerene acceptor
DOI: 10.1039/C9TA01989H, Paper
We combine thermodynamic modeling of molecular interactions in OPV blends with in situ measurements of morphology to link performance, structure and processing.
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All-printed solid-state supercapacitors with versatile shapes and superior flexibility for wearable energy storage
DOI: 10.1039/C9TA03513C, Paper
A diverse design pattern-based flexible supercapacitor is fabricated via a scalable screen printing method by using CoHCF as an electrode material.
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Thin Film Encapsulation of Organic Solar Cells by Direct Deposition of Polysilazanes from Solution
Perhydropolysilazane/organic polymer multilayer stacks directly coated on top of organic solar cells (OSCs) from solution act as flexible barriers against oxygen and moisture. They extend the lifetime of OCS from a few hours to several hundred hours under accelerated ageing conditions.
Abstract
Organic electronic devices (OEDs), e.g., organic solar cells, degrade quickly in the presence of ambient gases, such as water vapor and oxygen. Thus, in order to extend the lifetime of flexible OEDs, they have to be protected by encapsulation. A solution‐based encapsulation method is developed, which allows the direct deposition of the diffusion barrier on top of OEDs, thus avoiding lamination of barrier films. The method is based on the deposition of a perhydropolysilazane (PHPS) ink and its subsequent conversion into a silica layer by deep UV irradiation. The resulting barrier films show water vapor transmission rates (WVTRs) of <10−2 g m−2 d−1 (40 °C/85% relative humidity (RH)) and oxygen transmission rates (OTRs) of <10−2 cm3 m−2 d−1 bar−1 at ambient conditions. Flexibility of the resulting barrier films is improved by coating a barrier stack of several thin PHPS layers alternating with organic polymer interlayers. These stacks show an increase of WVTR values by less than 10% after 3000 bending cycles. Direct coating of the PHPS films on top of organic solar cells enhances the device lifetime in damp heat conditions from a few hours to beyond 300 h.
Insights from Machine Learning Techniques for Predicting the Efficiency of Fullerene Derivatives‐Based Ternary Organic Solar Cells at Ternary Blend Design
Machine‐learning approaches are utilized to build models for the prediction of efficiency using important frontier molecular orbital energy levels of organic materials as features. Furthermore, a versatile Random Forest model reveals that the lowest unoccupied molecular orbital energy of donor can be considered as a critical feature in design of ternary organic solar cells.
Abstract
Ternary organic solar cells (OSCs) have progressed significantly in recent years due to the sufficient photon harvesting of the blend photoactive layer including three absorption‐complementary materials. With the rapid development of highly efficient ternary OSCs in photovoltaics, the precise energy‐level alignment of the three active components within ternary OSC devices should be taken into account. The machine‐learning technique is a computational method that can effectively learn from previous historical data to build predictive models. In this study, a dataset of 124 fullerene derivatives‐based ternary OSCs is manually constructed from a diverse range of literature along with their frontier molecular orbital theory levels, and device structures. Different machine‐learning algorithms are trained based on these electronic parameters to predict photovoltaic efficiency. Thus, the best predictive capability is provided by using the Random Forest approach beyond other machine‐learning algorithms in the dataset. Furthermore, the Random Forest algorithm yields valuable insights into the crucial role of lowest unoccupied molecular orbital energy levels of organic donors in the performance of ternary OSCs. The outcome of this study demonstrates a smart strategy for extracting underlying complex correlations in fullerene derivatives‐based ternary OSCs, thereby accelerating the development of ternary OSCs and related research fields.
Charge Recombination Dynamics in Organic Photovoltaic Systems with Enhanced Dielectric Constant
An analysis of a fullerene derivative with an increased blend dielectric constant by the addition of a triethylene glycol appendage to the fullerene (TEG‐PCBM) is reported. The TEG‐PCBM is blended with donors P3HT and PTB7‐Th and the changes are examined in recombination dynamics in the enhanced dielectric constant systems by observing light intensity effects on open‐circuit voltage, short‐circuit current, fill factor, and exciton dissociation efficiency.
Abstract
Increasing the dielectric constant of organic photovoltaic materials to reduce recombination rates has long been pursued, however, material modification often results in the modification of multiple device characteristics, making system comparison difficult. In this study, a fullerene derivative with an increased blend dielectric constant is examined by the addition of a triethylene glycol appendage to the fullerene (TEG‐PCBM). Density functional theory calculations show a small change to the permanent dipole moment between TEG‐PCBM and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC60BM) resulting in similar solubility, morphology, and device performance. TEG‐PCBM is blended with donors P3HT and PTB7‐Th and a comparable performance to PC60BM is found. This model system shows the rarely reported characteristic of an increase in the dielectric constant while leaving its other properties unaltered. Looking at light intensity effects on open‐circuit voltage (Voc), short‐circuit current (Jsc), and fill factor (FF) along with exciton dissociation efficiency, it is observed that when switching to the TEG‐ modified fullerene derivative, geminate recombination is not reduced, and Shockley–Read–Hall recombination is increased. While triethlyene glycol appendages may prove to be ineffective in improving recombination through increased dielectric constant, an approach for studying recombination in future high dielectric systems is provided.
Hydrogen Bond Induced Green Solvent Processed High Performance Ternary Organic Solar Cells with Good Tolerance on Film Thickness and Blend Ratios
Intermolecular hydrogen bonding is a potential strategy for organic solar cells to realize low cost, high efficiency, good device and morphology stability, excellent composition, and film thickness tolerance.
Abstract
For comprehensive development of organic solar cells (OSCs), some factors such as environmental stability, low cost, insensitive film thickness, component contents tolerance, and green preparation processes are equally crucial to achieve high power conversion efficiencies (PCEs). In this work, a small molecule 3‐(diethylamino)‐7‐imino‐7H‐benzo[4,5]imidazo[1,2‐a]chromeno[3,2‐c]pyridine‐6‐carbonitrile (DIBC), which is commercially available at low cost, is utilized to realize high‐performance ternary OSCs. Demonstrated via Fourier transform infrared and 2D‐1HNMR, DIBC can form hydrogen bond interactions with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) in solid films. Further electrostatic potential (ESP) calculations indicate that the hydrogen bond interaction enhances the ESP of PC71BM and accelerates charge transport between donor and acceptor. As a result, poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b0]dithiophene‐2,6‐diylalt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl (PTB7‐Th):DIBC:PC71BM‐based ternary OSC achieves a maximum efficiency of 12.17%, which is the best result of green solvent processed fullerene OSCs at present. It is noteworthy that the ternary OSCs also show great tolerance to film thickness and blend ratios. These unique properties are attributed to the hydrogen‐bond‐linked DIBC and PC71BM, which modulates molecule distribution and improves film morphology with an interpenetrating network structure. Furthermore, the DIBC containing device also exhibits good thermal and light radiation stability. These results illustrate that intermolecular hydrogen bond interaction has great potential for realizing high‐performance OSCs.
Solution‐Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance
Flexible bandgap engineering is a crucial material science achievement that opens possibilities in the design of organic bulk heterojunctions for semitransparent photovoltaics. An additional feature of novel polymer:nonfullerene acceptor blends is efficient free‐charge‐carrier generation at small energetic offsets. These material properties are considered in conjunction with achievements in device physics and engineering of organic semitransparent solar cells.
Abstract
Recent research efforts on solution‐processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation–recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.
Single‐Junction Polymer Solar Cells with 16.35% Efficiency Enabled by a Platinum(II) Complexation Strategy
A platinum(II) complexation strategy is developed to regulate the crystallinity of a newly designed s‐tetrazine‐containing wide‐bandgap copolymer donor PSFTZ, and optimize the morphology of the PSFTZ:Y6 active blend film, which boosts successfully the power conversion efficiency of the resulting nonfullerene polymer solar cells (NF‐PSCs) from 13.03% to 16.35%. 16.35% is the new record for single‐junction NA‐PSCs at present.
Abstract
A new strategy of platinum(II) complexation is developed to regulate the crystallinity and molecular packing of polynitrogen heterocyclic polymers, optimize the morphology of the active blends, and improve the efficiency of the resulting nonfullerene polymer solar cells (NF‐PSCs). The newly designed s‐tetrazine (s‐TZ)‐containing copolymer of PSFTZ (4,8‐bis(5‐((2‐butyloctyl)thio)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3,6‐bis(4‐octylthiophen‐2‐yl)‐1,2,4,5‐tetrazine) has a strong aggregation property, which results in serious phase separation and large domains when blending with Y6 ((2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile)), and produces a power‐conversion efficiency (PCE) of 13.03%. By adding small amount of Pt(Ph)2(DMSO)2 (Ph, phenyl and DMSO, dimethyl sulfoxide), platinum(II) complexation would occur between Pt(Ph)2(DMSO)2 and PSFTZ. The bulky benzene ring on the platinum(II) complex increases the steric hindrance along the polymer main chain, inhibits the polymer aggregation strength, regulates the phase separation, optimizes the morphology, and thus improves the efficiency to 16.35% in the resulting devices. 16.35% is the highest efficiency for single‐junction PSCs reported so far.
Photoluminescence Quenching Probes Spin Conversion and Exciton Dynamics in Thermally Activated Delayed Fluorescence Materials
A new analytical model based on time‐resolved photoluminescence quenching is developed in order to measure the important excited‐state rate constants in materials that exhibit thermally activated delayed fluorescence (TADF). The method is applied to five different TADF materials, and structure–property relationships concerning intersystem crossing, reverse intersystem crossing, singlet exciton diffusion, and triplet exciton diffusion are highlighted.
Abstract
Fluorescent materials that efficiently convert triplet excitons into singlets through reverse intersystem crossing (RISC) rival the efficiencies of phosphorescent state‐of‐the‐art organic light‐emitting diodes. This upconversion process, a phenomenon known as thermally activated delayed fluorescence (TADF), is dictated by the rate of RISC, a material‐dependent property that is challenging to determine experimentally. In this work, a new analytical model is developed which unambiguously determines the magnitude of RISC, as well as several other important photophysical parameters such as exciton diffusion coefficients and lengths, all from straightforward time‐resolved photoluminescence measurements. From a detailed investigation of five TADF materials, important structure–property relationships are derived and a brominated derivative of 2,4,5,6‐tetrakis(carbazol‐9‐yl)isophthalonitrile that has an exciton diffusion length of over 40 nm and whose excitons interconvert between the singlet and triplet states ≈36 times during one lifetime is identified.
Ultranarrow Bandwidth Organic Photodiodes by Exchange Narrowing in Merocyanine H‐ and J‐Aggregate Excitonic Systems
Organic photodiodes composed of merocyanine dye H‐ and J‐aggregates are fabricated. The dye aggregates' absorption band can be tuned by the donor‐substituent‐dependent packing of the dyes to give ultranarrow bandwidths down to 30 nm and maximum external quantum efficiencies of up to 11% for the H‐band and 9% for the J‐band.
Abstract
Ultranarrowband organic photodiodes (OPDs) are demonstrated for thin film solid state materials composed of tightly packed dipolar merocyanine dyes. For these dyes the packing arrangement can be controlled by the bulkiness of the donor substituent, leading to either strong H‐ or strong J‐type exciton coupling in the interesting blue (H‐aggregate) and NIR (J‐aggregate) spectral ranges. Both bands are shown to arise from one single exciton band according to fluorescence measurements and are not just a mere consequence of different polymorphs within the same thin film. By fabrication of organic thin‐film transistors, these dyes are demonstrated to exhibit hole transport behavior in spin‐coated thin films. Moreover, when used as organic photodiodes in planar heterojunctions with C60 fullerene, they show wavelength‐selective photocurrents in the solid state with maximum external quantum efficiencies of up to 11% and ultranarrow bandwidths down to 30 nm. Thereby, narrowing the linewidths of optoelectronic functional materials by exciton coupling provides a powerful approach to produce ultranarrowband organic photodiodes.
Organic Photovoltaic Sensors for Photocapacitive Stimulation of Voltage‐Gated Ion Channels in Neuroblastoma Cells
The photocapacitive activation of voltage‐gated sodium channels in neuronal model cells by a small‐molecular organic photovoltaic sensor is studied with an electrophysiological patch‐clamp approach.
Abstract
Organic semiconductors are emerging as promising candidates for novel electrically self‐sufficient photovoltaic prosthetics for neurostimulation, especially for restoration of light sensitivity in degenerate retina. Considering future applications, it is essential to gain fundamental insight into the signaling mechanisms at the organic photosensor–electrolyte–neuron interface. Particularly, targeting voltage‐gated ion channels by a pure photocapacitive stimulation is a preferred therapeutic approach as it avoids redox reactions involved in Faradaic charge injection. The present study investigates whether single neuroblastoma (N2A) cells, grown on a photosensor based on a small molecular squaraine:fullerene photoactive layer blend, optionally covered with silicon dioxide, can be activated by photocapacitive stimulation. Indeed, upon pulsed illumination, a rapid transient photocurrent strongly depolarizes the membrane potential and subsequently activates fast‐responding voltage‐gated sodium channels. The dielectric top coating on the organic layer ensures sufficient capacitive charge injection efficiency while maintaining the rapid response of the device. Due to the high irradiance level required for photocapacitive stimulation, another slower, independent, and unintended, nonelectrical signaling pathway is identified. This activates voltage‐gated potassium channels, presumably by photothermal effects. The present study provides the basis for further improvements on standalone photovoltaic neurostimulating platforms based on organic photoactive layers.
Charge Recombination Dynamics in Organic Photovoltaic Systems with Enhanced Dielectric Constant
An analysis of a fullerene derivative with an increased blend dielectric constant by the addition of a triethylene glycol appendage to the fullerene (TEG‐PCBM) is reported. The TEG‐PCBM is blended with donors P3HT and PTB7‐Th and the changes are examined in recombination dynamics in the enhanced dielectric constant systems by observing light intensity effects on open‐circuit voltage, short‐circuit current, fill factor, and exciton dissociation efficiency.
Abstract
Increasing the dielectric constant of organic photovoltaic materials to reduce recombination rates has long been pursued, however, material modification often results in the modification of multiple device characteristics, making system comparison difficult. In this study, a fullerene derivative with an increased blend dielectric constant is examined by the addition of a triethylene glycol appendage to the fullerene (TEG‐PCBM). Density functional theory calculations show a small change to the permanent dipole moment between TEG‐PCBM and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC60BM) resulting in similar solubility, morphology, and device performance. TEG‐PCBM is blended with donors P3HT and PTB7‐Th and a comparable performance to PC60BM is found. This model system shows the rarely reported characteristic of an increase in the dielectric constant while leaving its other properties unaltered. Looking at light intensity effects on open‐circuit voltage (Voc), short‐circuit current (Jsc), and fill factor (FF) along with exciton dissociation efficiency, it is observed that when switching to the TEG‐ modified fullerene derivative, geminate recombination is not reduced, and Shockley–Read–Hall recombination is increased. While triethlyene glycol appendages may prove to be ineffective in improving recombination through increased dielectric constant, an approach for studying recombination in future high dielectric systems is provided.
Achieving a High Fill Factor and Stability in Perylene Diimide–Based Polymer Solar Cells Using the Molecular Lock Effect between 4,4′‐Bipyridine and a Tri(8‐hydroxyquinoline)aluminum(III) Core
Two perylene diimide (PDI)–based small molecular acceptors containing a 3D Alq3 core flanked by PDI or PDI2 end units are developed for efficient polymer solar cells. The coordination between Alq3 and the newly used 4,4′‐bipyridine additive allows a high fill factor, power conversion efficiency, and stability to be simultaneously achieved in PDI‐based polymer solar cells.
Abstract
Two novel perylene diimide (PDI)–based derivatives, Alq3‐PDI and Alq3‐PDI2, are synthesized by flanking a 3D tri(8‐hydroxyquinoline)aluminum(III) (Alq3) core with PDI and a helical PDI dimer (PDI2) to construct high‐performance small molecular nonfullerene acceptors (SMAs). The 3D Alq3 core significantly suppresses the molecular aggregation of the resulting SMAs, leading to a well‐mixed blend with a PTTEA donor polymer and weak phase separation. Compared with Alq3‐PDI, the extended π‐conjugation of Alq3‐PDI2 results in higher‐order molecular packing, which improves the absorption and phase separation behavior. Thus, the Alq3‐PDI2 devices have higher J sc and FF values and better device performance, which are further enhanced by a small amount of 4,4′‐bipyridine (Bipy) as an additive. The coordination between Bipy and the Alq3 core promotes molecular packing and phase separation, which lower charge recombination and enhanced charge collection in the resulting devices. Therefore, a largely improved J sc of 15.74 mA cm−2 and very high FF of 71.27% are obtained in the Alq3‐PDI2 devices, resulting in a power conversion efficiency of 9.54%, which is the best value reported for PDI‐based polymer solar cells. The coordination can also serve as a “molecular lock,” which prevents molecular motion and thus improves device stability.
Organic Photovoltaics with Multiple Donor–Acceptor Pairs
Utilizing multiple donor–acceptor pairs for organic solar cells (OSCs) is a very effective strategy for overcoming the limitations of conventional OSCs based on a single donor–acceptor pair. Recent cases of OSCs with multiple donor–acceptor pairs are not only summarized but their perspectives are also presented.
Abstract
Compared with conventional organic solar cells (OSCs) based on single donor–acceptor pairs, terpolymer‐ and ternary‐based OSCs featuring multiple donor–acceptor pairs are promising strategies for enhancing the performance while maintaining an easy and simple synthetic process. Using multiple donor–acceptor pairs in the active layer, the key photovoltaic parameters (i.e., short‐circuit current density, open‐circuit voltage, and fill factor) governing the OSC characteristics can be simultaneously or individually improved by positive changes in light‐harvesting ability, molecular energy levels, and blend morphology. Here, these three major contributions are discussed with the aim of offering in‐depth insights in combined terpolymers and ternary systems. Recent exemplary cases of OSCs with multiple donor–acceptor pairs are summarized and more advanced research and perspectives for further developments in this field are highlighted.
Recent progress in inkjet-printed solar cells
DOI: 10.1039/C9TA03155C, Review Article
In the past few decades, the fabrication of solar cells has been considered as one of the most promising ways to meet the increasing energy demands to support the development of modern society as well as to control the environmental pollution caused by the combustion of fossil fuels.
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A wide-bandgap D–A copolymer donor based on a chlorine substituted acceptor unit for high performance polymer solar cells
DOI: 10.1039/C9TA03272J, Paper
A new wide-bandgap chlorinated polymer, J101, was synthesized and successfully used as the donor polymer for application in non-fullerene PSCs, and the PSCs fabricated by combining the J101 donor with the electron acceptor ZITI demonstrated a remarkable PCE of 14.43%.
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What is the Binding Energy of a Charge Transfer State in an Organic Solar Cell?
The coulomb binding energy of an electron and a hole on adjacent chromophores is in the order of 0.5 eV, yet for efficient solar cells, very little activation energy is required for the photodissociation of excitations. It is shown here how the combined effects of interfacial electrostatics, wave function delocalization, and disorder can account for this.
Abstract
The high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge‐transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature‐dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron–hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield.
Asymmetric Nonfullerene Small Molecule Acceptors for Organic Solar Cells
Symmetry breaking provides a new material design strategy for nonfullerene small molecule acceptors (SMAs). The past 10 years have witnessed significant advances in asymmetric nonfullerene SMAs in organic solar cells (OSCs). In this review, the progress of asymmetric nonfullerene SMAs is reviewed. The structure–property relationships and the perspectives for future development of asymmetric non‐fullerene SMAs are also discussed.
Abstract
Symmetry breaking provides a new material design strategy for nonfullerene small molecule acceptors (SMAs). The past 10 years have witnessed significant advances in asymmetric nonfullerene SMAs in organic solar cells (OSCs) with power conversion efficiency (PCE) increasing from ≈1% to ≈14%. In this review, the progress of asymmetric nonfullerene SMAs, including early reports of asymmetric nonfullerene SMAs, asymmetric PDI‐based nonfullerene SMAs, and asymmetric acceptor–donor–acceptor (A–D–A)‐type nonfullerene SMAs, is summarized. The structure–property relationships and the perspectives for future development of asymmetric nonfullerene SMAs are also discussed.
An efficient binary cathode interlayer for large-bandgap non-fullerene organic solar cells
DOI: 10.1039/C9TA02844G, Communication
A binary cathode interlayer with both efficient charge extraction and transportation properties for large-bandgap non-fullerene OSCs.
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Efficiency enhancement of organic photovoltaics by introducing high-mobility curved small-molecule semiconductors as additives
DOI: 10.1039/C9TA02636C, Paper
Highly efficient OPVs are successfully fabricated by introducing high-mobility curved organic semiconductors. The significant enhancement of the device efficiency induced by the curved molecules can be attributed to increased hole mobility in the active layer and intimate interaction between the curved molecules and PC71BM.
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Electronic and optical properties of π-bridged perylenediimide derivatives: the role of π-bridges
DOI: 10.1039/C9TA03124C, Paper
For the π-bridged multi-PDI derivatives, intramolecular electron transfer is dictated by the super-exchange mechanism and can be greatly tuned by the π-bridge modes.
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High energy density and high efficiency all-organic polymers with enhanced dipolar polarization
DOI: 10.1039/C9TA03601F, Communication
The dielectric constant of polymers was increased by combining flexible segments and rigid polar segments in the polymer backbone.
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[ASAP] 14.7% Efficiency Organic Photovoltaic Cells Enabled by Active Materials with a Large Electrostatic Potential Difference
Facile Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)–Based Acceptors with Fine‐Tuned Optoelectronic Properties: Toward Efficient Additive‐Free Nonfullerene Organic Solar Cells
A series of polycyclic aromatic hydrocarbon (PAH) cores with distinct π‐conjugation size are incorporated to construct a new family of fused‐ring electron acceptors (FREAs) via a simple and low‐cost synthetic route. The optoelectronic properties can be fine‐tuned at a molecular level over a wide range, which enables pyrene‐based DTP‐IC‐4Ph achieving a promising power conversion efficiency (PCE) of 10.37% in additive‐free nonfullerene organic solar cells.
Abstract
A series of polycyclic aromatic hydrocarbons (PAHs) with extended π‐conjugated cores (from naphthalene, anthracene, pyrene, to perylene) are incorporated into nonfullerene acceptors for the first time. Four different fused‐ring electron acceptors (FREAs), i.e., DTN‐IC‐2Ph, DTA‐IC‐3Ph, DTP‐IC‐4Ph, and DTPy‐IC‐5Ph, are prepared via simple and facile synthetic procedures, yielding a remarkable platform to study the structure–property relationship for nonfullerene solar cells. With the PAH core being extended systematically, the gradually redshifted absorption with enhanced molar extinction coefficient (ε) is realized, the energy level of the highest occupied molecular orbital is up‐shifted, and the electron mobility is greatly enhanced. Meanwhile, the solubility decreases and the molecular packing becomes strengthened. As a result, with an optimized combination of these characteristics, DTP‐IC‐4Ph attains good solubility, high molar extinction coefficient, complementary absorption, suitable morphology, well‐matched energy levels, as well as efficient charge dissociation and transport in blend film. Consequently, the DTP‐IC‐4Ph‐based solar cells with a donor polymer, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T) exhibit a promising power conversion efficiency of 10.37% without any additives, which is close to the best performance achieved in additive‐free nonfullerene solar cells (NFSCs). The results demonstrate that the PAH building blocks have great potential for the construction of novel FREAs for efficient additive‐free NFSCs.
Unconjugated Side‐Chain Engineering Enables Small Molecular Acceptors for Highly Efficient Non‐Fullerene Organic Solar Cells: Insights into the Fine‐Tuning of Acceptor Properties and Micromorphology
Unconjugated side‐chain engineering is performed on non‐fullerene small molecule acceptors based on a fused‐benzodithiophene core. Thieno[3,2‐b]thiophene is superior to thiophene and benzene owing to its dual roles of promoting the molecular energy level (δ‐inductive effect) and optimizing the morphology. Thus, organic solar cells based on PBDB‐T:BTTIC‐TT achieve the highest power conversion efficiency of 13.44% among three devices.
Abstract
2D conjugated side‐chain engineering is an effective strategy that is widely utilized to construct benzodithiophene‐based polymers. Herein, an unconjugated side‐chain strategy to design fused‐benzodithiophene‐based non‐fullerene small molecule acceptors (SMAs) via vertical aromatic side‐chain engineering on the ladder‐type core is employed. Three SMAs named BTTIC‐Th, BTTIC‐TT, and BTTIC‐Ph with thiophene, thieno[3,2‐b]thiophene, and benzene, respectively, as side chains, are designed and synthesized. Three SMAs exhibit similar absorption ranges but different lowest unoccupied molecular orbital (LUMO) energy levels due to the different strength of the δ‐inductive effect between vertical aromatic side chains and their electron‐rich core. Organic solar cells based on PBDB‐T:BTTIC‐TT achieve a power conversion efficiency (PCE) of 13.44%, which is higher than the PCE of devices based on PBDB‐T:BTTIC‐Th (12.91%) and PBDB‐T:BTTIC‐Ph (9.14%). The difference in device performance is investigated by electrical and morphological characterizations. A large domain size and different types of π–π stacking are found in the bulk heterojunction layer of PBDB‐T:BTTIC‐Ph blend film, which are detrimental to exciton dissociation and charge transport. Overall, it is demonstrated that when designing unconjugated side chains, thieno[3,2‐b]thiophene is superior to thiophene and benzene through its dual roles of promoting the LUMO energy level and optimizing the morphology. These results shed light on the side‐chain engineering of high‐performance non‐fullerene SMAs.
[ASAP] High Exciton Diffusion Coefficients in Fused Ring Electron Acceptor Films
