Multi-component Copolymerized Donors enable Frozen Nano-morphology and Superior Ductility for Efficient Binary Organic Solar Cells.

Multi-component copolymerized donors (MCDs) have gained significant interest and have been rapidly developed in flexible organic solar cells (f-OSCs) in recent years. However, ensuring the power conversion efficiency (PCE) of f-OSCs while retaining ideal mechanical properties remains an enormous challenge. The fracture strain (FS) value of typical high-efficiency blend films is generally less than 8 %, which is far from the application standards of wearable photovoltaic devices. Therefore, we developed a series of novel MCDs after meticulous molecular design. Among them, the consistent MCD backbone and end-capped functional group formed a highly conjugated molecular plane, and the solubilization and mechanical properties were effectively optimized by modifying the proportion of solubilized alkyl chains. Consequently, due to the formation of entangled structures with a frozen blend film morphology considerably improved the high ductility of the active layer, P10.8/P20.2-TCl exhibited efficient PCE in rigid (18.53 %) and flexible (17.03 %) OSCs, along with excellent FS values (16.59 %) in pristine films, meanwhile, the outstanding FS values of 25.18 % and 12.3 % were achieved by P10.6/P20.4-TCl -based pristine and blend films, respectively, which were one of the highest records achieved by end-capped MCD-based binary OSCs, demonstrating promising application to synchronize the realization of high-efficiency and mechanically ductile flexible OSCs.

High Efficiency over 18.6% of Organic Solar Cells Enabled by PEDOT:PSS/Br-2PACz Dual-Anode Interface.

Organic solar cells (OSCs) with high performance were prepared using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and [2-(3,6-dibromo-9H-carbazol-9-yl)ethyl]phosphonic acid (Br-2PACz) double-layer films as the anode interface. By spin-coating a layer of Br-2PACz on PEDOT:PSS to form a PEDOT:PSS/Br-2PACz dual-anode interface, both the Jsc and FF of the device can be increased simultaneously, resulting in a high Jsc of 27.84 mA cm-2 and a high FF of 78.18%. The promising result indicates that the PEDOT:PSS/Br-2PACz dual-anode interface is an effective way to improve the performance of OSCs. The improvement of device performance is mainly attributed to (1) improved interface conductivity; (2) increased hole mobility and more balanced carrier transport efficiency; and (3) optimized morphology, which well explains the increase of Jsc and FF of the device. In addition, the OSC based on the PEDOT:PSS/Br-2PACz dual-anode interface exhibits exceptional stability, as it can maintain 94.7% of its initial efficiency even after 500 h of storage in a nitrogen environment. This work provides a promising strategy for improving the efficiency and stability of OSCs by dual-anode interface modulation.

Organic Photovoltaics Utilizing Small-Molecule Donors and Y-Series Nonfullerene Acceptors.

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure-property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

Ti3C2Tx/PEDOT:PSS Composite Interface Enables over 17% Efficiency Non-fullerene Organic Solar Cells.

Metal carbide Ti3C2Tx as a new two-dimensional material with excellent metallic conductivity, good water solubility, and superior transmittance in the visible light range shows great potential for applications in optoelectronic devices. Herein, Ti3C2Tx/PEDOT:PSS composite films were fabricated by a simple solution process and employed as an anode interfacial layer in organic solar cells. By introducing the Ti3C2Tx/PEDOT:PSS composite interface into the devices, the highest power conversion efficiency (PCE) of 17.26% was achieved while using PM6:Y6 as the active layer, with a high short-circuit current (Jsc) of 26.52 mA/cm2 and a fill factor of up to 0.76. The PCE is much higher than 15.89% for the pure PEDOT:PSS interfacial layer-based device without doping. The dramatically improved performance was attributed to the increased conductivity of the Ti3C2Tx/PEDOT:PSS composite interface and the increased charge extraction and collection efficiency of the devices. This work presents an effective method to prepare the Ti3C2Tx/PEDOT:PSS composite interface and high-performance organic solar cells.

Over 14% Efficiency Folding-Flexible ITO-free Organic Solar Cells Enabled by Eco-friendly Acid-Processed Electrodes.

Environment-friendly manufacturing and mechanical robustness are imperative for commercialization of flexible OSCs as green-energy source, especially in portable and wearable self-powered flexible electronics. Although, the commonly adopted PEDOT:PSS electrodes that are treated with severely corrosive and harmful acid lack foldability. Herein, efficient folding-flexible OSCs with highly conductive and foldable PEDOT:PSS electrodes processed with eco-friendly cost-effective acid and polyhydroxy compound are demonstrated. The acid treatment endows PEDOT:PSS electrodes with high conductivity. Meanwhile, polyhydroxy compound doping contributes to excellent bending flexibility and foldability due to the better film adhesion between PEDOT:PSS and PET substrate. Accordingly, folding-flexible OSCs with high efficiency of 14.17% were achieved. After 1,000 bending or folding cycles, the device retained over 90% or 80% of its initial efficiency, respectively. These results represent one of the best performances for ITO-free flexible OSC reported so far and demonstrate a novel approach toward commercialized efficient and foldable green-processed OSCs.

Synergistic Interface Energy Band Alignment Optimization and Defect Passivation toward Efficient and Simple-Structured Perovskite Solar Cell.

Efficient electron transport layer-free perovskite solar cells (ETL-free PSCs) with cost-effective and simplified design can greatly promote the large area flexible application of PSCs. However, the absence of ETL usually leads to the mismatched indium tin oxide (ITO)/perovskite interface energy levels, which limits charge transfer and collection, and results in severe energy loss and poor device performance. To address this, a polar nonconjugated small-molecule modifier is introduced to lower the work function of ITO and optimize interface energy level alignment by virtue of an inherent dipole, as verified by photoemission spectroscopy and Kelvin probe force microscopy measurements. The resultant barrier-free ITO/perovskite contact favors efficient charge transfer and suppresses nonradiative recombination, endowing the device with enhanced open circuit voltage, short circuit current density, and fill factor, simultaneously. Accordingly, power conversion efficiency increases greatly from 12.81% to a record breaking 20.55%, comparable to state-of-the-art PSCs with a sophisticated ETL. Also, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. This work facilitates the further development of highly efficient, flexible, and recyclable ETL-free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification.

13.34 % Efficiency Non-Fullerene All-Small-Molecule Organic Solar Cells Enabled by Modulating the Crystallinity of Donors via a Fluorination Strategy.

Non-fullerene all-small-molecule organic solar cells (NFSM-OSCs) have shown potential as OSCs, owing to their high purity, easy synthesis and good reproducibility. However, challenges in the modulation of phase separation morphology have limited their development. Herein, two novel small molecular donors, BTEC-1F and BTEC-2F, derived from the small molecule DCAO3TBDTT, are synthesized. Using Y6 as the acceptor, devices based on non-fluorinated DCAO3TBDTT showed an open circuit voltage (Voc ) of 0.804 V and a power conversion efficiency (PCE) of 10.64 %. Mono-fluorinated BTEC-1F showed an increased Voc of 0.870 V and a PCE of 11.33 %. The fill factor (FF) of di-fluorinated BTEC-2F-based NFSM-OSC was improved to 72.35 % resulting in a PCE of 13.34 %, which is higher than that of BTEC-1F (61.35 %) and DCAO3TBDTT (60.95 %). To our knowledge, this is the highest PCE for NFSM-OSCs. BTEC-2F had a more compact molecular stacking and a lower crystallinity which enhanced phase separation and carrier transport.

Improved Efficiency in All-Small-Molecule Organic Solar Cells with Ternary Blend of Nonfullerene Acceptor and Chlorinated and Nonchlorinated Donors.

Ternary nonfullerene all-small-molecule organic solar cells (NFSM-OSCs) were developed by incorporating a nonfullerene acceptor (IDIC) and two structurally similar small molecular donors (SM and SM-Cl), where SM-Cl is a novel small molecular donor derived from the reported molecular donor SM. When doping 10% SM-Cl in the SM:IDIC binary system, the power conversion efficiency (PCE) of the ternary solar cell was dramatically increased from 9.39 to 10.29%. Characterization studies indicated that the two donors tend to form an alloy state, which effectively down-shifted the highest occupied molecular orbital (HOMO) energy level of the donor, thus promoting a higher open-circuit voltage. Interestingly, incorporating a third component (SM-Cl) with a lower crystallinity was proven to facilitate the demixing between donors and acceptors, which was contrary to the traditional findings of enhanced phase separation through the incorporation of highly crystalline molecule. Although the morphological modulation has always been a bottleneck issue in NFSM-OSCs, the findings in this work indicated that the modulation on crystallinity deviation between donors and acceptors could be an effective method to further improve the performance of NFSM-OSCs, providing a new perspective on NFSM-OSCs.

Improving Performance of Nonfullerene Organic Solar Cells over 13% by Employing Silver Nanowires-Doped PEDOT:PSS Composite Interface.

Ag nanowires (NWs)/PEDOT:PSS composite was prepared by a facile solution-processing method and employed as anode interface in nonfullerene organic solar cells (OSCs). In the presence of a Ag NWs (5%, v/v%)/PEDOT:PSS interfacial layer, a high-power conversion efficiency up to 13.53% was achieved based on a PBDB-T-2Cl:IT-4F photoactive layer system, much higher than the efficiency of the controlled counterpart device with pristine PEDOT:PSS as anode modifier. Simultaneous enhancements in short-circuit current and fill factor were observed, in comparison to the case of the pristine PEDOT:PSS interface, due to the improved electrical conductivity of Ag NWs/PEDOT:PSS composites accompanied by the increased work function for a better matching with the indium tin oxide counter electrode, which facilitated increased charge transfer and reduced charge recombination at the anode/photoactive interface for improved device performance. The results clearly revealed that the Ag NWs/PEDOT:PSS composite interface is beneficial to improve the charge extraction and favor the realization of highly efficient nonfullerene OSCs.

Simultaneous Bottom-Up Interfacial and Bulk Defect Passivation in Highly Efficient Planar Perovskite Solar Cells using Nonconjugated Small-Molecule Electrolytes.

Recent perovskite solar cell (PSC) advances have pursued strategies for reducing interfacial energetic mismatches to mitigate energy losses, as well as to minimize interfacial and bulk defects and ion vacancies to maximize charge transfer. Here nonconjugated multi-zwitterionic small-molecule electrolytes (NSEs) are introduced, which act not only as charge-extracting layers for barrier-free charge collection at planar triple cation PSC cathodes but also passivate charged defects at the perovskite bulk/interface via a spontaneous bottom-up passivation effect. Implementing these synergistic properties affords NSE-based planar PSCs that deliver a remarkable power conversion efficiency of 21.18% with a maximum VOC = 1.19 V, in combination with suppressed hysteresis and enhanced environmental, thermal, and light-soaking stability. Thus, this work demonstrates that the bottom-up, simultaneous interfacial and bulk trap passivation using NSE modifiers is a promising strategy to overcome outstanding issues impeding further PSC advances.

16.67% Rigid and 14.06% Flexible Organic Solar Cells Enabled by Ternary Heterojunction Strategy.

Ternary heterojunction strategies appear to be an efficient approach to improve the efficiency of organic solar cells (OSCs) through harvesting more sunlight. Ternary OSCs are fabricated by employing wide bandgap polymer donor (PM6), narrow bandgap nonfullerene acceptor (Y6), and PC71 BM as the third component to tune the light absorption and morphologies of the blend films. A record power conversion efficiency (PCE) of 16.67% (certified as 16.0%) on rigid substrate is achieved in an optimized PM6:Y6:PC71 BM blend ratio of 1:1:0.2. The introduction of PC71 BM endows the blend with enhanced absorption in the range of 300-500 nm and optimises interpenetrating morphologies to promote photogenerated charge dissociation and extraction. More importantly, a PCE of 14.06% for flexible ITO-free ternary OSCs is obtained based on this ternary heterojunction system, which is the highest PCE reported for flexible state-of-the-art OSCs. A very promising ternary heterojunction strategy to develop highly efficient rigid and flexible OSCs is presented.

Efficient Enhancement of Electron Transport and Collection Capability in PTB7:PC71 BM-based Solar Cells Enabled by Sulforhodamine Cathode Interlayers.

Development of low-cost water-/alcohol-soluble interfacial materials is a crucial issue to promote the commercialization of polymer solar cells (PSCs). Herein, two derivatives of low-cost rhodamine, called sulforhodamine 101 (SR101) and sulforhodamine B (SRB), are applied as cathode interfacial layers (CILs) to effectively improve the charge-carrier transportation and collection, reduce the work function (WF) of Al counter electrode, and decrease the series resistance and charge recombination in the PSCs. As a result, SR101-based devices show excellent performance with the highest power conversion efficiency (PCE) of 9.10 %, superior to that of both the control devices with MeOH/Al and Ca/Al. Notably, sulforhodamine is commercially available with low cost and great solution-processability. This work demonstrates that sulforhodamine has a great potential as a CIL material,which is suitable for the large-area fabrication process and commercialization of highly efficient PSCs.

Efficient ternary organic solar cells based on a twin spiro-type non-fullerene acceptor.

A novel small-molecule (SM) acceptor DTF-IC is designed and synthesized in this work. The power conversion efficiency (PCE) of ternary OSCs increased up to 12.14% from 10.90% by incorporating 10 wt% of DTF-IC as second acceptors into the binary OSCs consisting of PBDB-T as donor and IT-M as acceptor. This was mainly due to the large increase in short-circuit current (Jsc) from 16.18 to 17.95 mA/cm2, without any drop in the open-circuit voltage (Voc) and fill factor (FF). The addition of DTF-IC enabled the donor and acceptor to form a distinct complementary absorption profile in the visible-light region, which boosted the photon harvesting in the range of 730-800 nm and consequently increased the Jsc of the ternary system by 11%. Moreover, there was an energy transfer between the two SM acceptors, favorable for enhancing charge separation and transfer as well as reducing charge recombination at PBDB-T:IT-M and PBDB-T:DTF-IC interface. Simultaneously, HOMO and LUMO energy levels of DTF-IC were lower than those of PBDB-T, but still higher than those of IT-M. Thus, DTF-IC is able to provide a cascading energy level with the host donor and acceptor which are beneficial for efficient charge transfer between the acceptors and facilitating exciton dissociation and carrier transport. Meanwhile, the highly crystalline DTF-IC as a third component can improve the crystallization process of the active layer while maintaining proper phase separation. This work proposes a novel idea for non-fullerene acceptors achieved via twin spiro-type structure modifying by indanone and provides a new direction for the selection of ternary solar cell materials.

All-Solution-Processed Metal-Oxide-Free Flexible Organic Solar Cells with Over 10% Efficiency.

All-solution-processing at low temperatures is important and desirable for making printed photovoltaic devices and also offers the possibility of a safe and cost-effective fabrication environment for the devices. Herein, an all-solution-processed flexible organic solar cell (OSC) using poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) electrodes is reported. The all-solution-processed flexible devices yield the highest power conversion efficiency of 10.12% with high fill factor of over 70%, which is the highest value for metal-oxide-free flexible OSCs reported so far. The enhanced performance is attributed to the newly developed gentle acid treatment at room temperature that enables a high-performance PEDOT:PSS/plastic underlying substrate with a matched work function (≈4.91 eV), and the interface engineering that endows the devices with better interface contacts and improved hole mobility. Furthermore, the flexible devices exhibit an excellent mechanical flexibility, as indicated by a high retention (≈94%) of the initial efficiency after 1000 bending cycles. This work provides a simple route to fabricate high-performance all-solution-processed flexible OSCs, which is important for the development of printing, blading, and roll-to-roll technologies.

A Methodological Study on Tuning the Thermally Activated Delayed Fluorescent Performance by Molecular Constitution in Acridine-Benzophenone Derivatives.

Thermally activated delayed fluorescent (TADF) emitters are usually designed as donor-acceptor structures with large dihedral angles, which tend to incur low fluorescent efficiency, and therefore, through molecular design various strategies have been proposed to increase the efficiency of emitters; however, few studies have compared these strategies in one TADF system. In this study, a novel TADF molecule, [4-(9,9-diphenylacridin-10-yl)phenyl](phenyl)methanone (BP-DPAC), was designed as a prototype, and two derivatives, BP-Ph-DPAC and DPAC-BP-DPAC, were also prepared to represent two common approaches to enhance TADF performance. Compared with the maximum external quantum efficiency (EQE) of 6.82 % for BP-DPAC, organic light-emitting diodes (OLED) devices based on DPAC-BP-DPAC exhibited enhanced TADF properties with the highest maximum EQE of 18.67 %, owing to an additional diphenylacridine donor, whereas BP-Ph-DPAC showed non-TADF properties and exhibited the lowest EQE of 4.25 %, owing to the insertion of a phenyl ring between donor and acceptor.

Ternary Nonfullerene Polymer Solar Cells with 12.16% Efficiency by Introducing One Acceptor with Cascading Energy Level and Complementary Absorption.

A novel small-molecule acceptor, (2,2'-((5E,5'E)-5,5'-((5,5'-(4,4,9,9-tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-(2-ethylhexyl)thiophene-5,2-diyl))bis(methanylylidene)) bis(3-hexyl-4-oxothiazolidine-5,2-diylidene))dimalononitrile (ITCN), end-capped with electron-deficient 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile groups, is designed, synthesized, and used as the third component in fullerene-free ternary polymer solar cells (PSCs). The cascaded energy-level structure enabled by the newly designed acceptor is beneficial to the carrier transport and separation. Meanwhile, the three materials show a complementary absorption in the visible region, resulting in efficient light harvesting. Hence, the PBDB-T:ITCN:IT-M ternary PSCs possess a high short-circuit current density (Jsc ) under an optimal weight ratio of donors and acceptors. Moreover, the open-circuit voltage (Voc ) of the ternary PSCs is enhanced with an increase of the third acceptor ITCN content, which is attributed to the higher lowest unoccupied molecular orbital energy level of ITCN than that of IT-M, thus exhibits a higher Voc in PBDB-T:ITCN binary system. Ultimately, the ternary PSCs achieve a power conversion efficiency of 12.16%, which is higher than the PBDB-T:ITM-based PSCs (10.89%) and PBDB-T:ITCN-based ones (2.21%). This work provides an effective strategy to improve the photovoltaic performance of PSCs.

High-Performance Polymer Solar Cells Employing Rhodamines as Cathode Interfacial Layers.

The development of simple and water-/alcohol-soluble interfacial materials is crucial for the cost-effective fabrication process of polymer solar cells (PSCs). Herein, highly efficient PSCs are reported employing water-/alcohol-soluble and low-cost rhodamines as cathode interfacial layers (CILs). The results reveal that rhodamine-based CILs can reduce the work function of the Al cathode and simultaneously increase the open-circuit voltage, current density, fill factor, and power conversion efficiency (PCE) of PSCs. The solution-processed rhodamine-based PSCs demonstrated a remarkable PCE of 10.39%, which is one of the best efficiencies reported for thieno[3,4-b]thiophene/benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester-based PSCs so far. The efficiency is also 42.3% higher than that of the vacuum-deposited Ca-based device (PCE of 7.30%) and 21.5% higher than that of the complicated solution-processable polymeric electrolyte poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]-based device (PCE of 8.55%). Notably, rhodamines are very economical and have been extensively used as dyes in industries. Our work indicates that rhodamines have shown a strong potential as CILs compared to their counterparts in the large-area fabrication process of PSCs.

Non-Doped Sky-Blue OLEDs Based on Simple Structured AIE Emitters with High Efficiencies at Low Driven Voltages.

Blue organic light-emitting diodes (OLEDs) are necessary for flat-panel display technologies and lighting applications. To make more energy-saving, low-cost and long-lasting OLEDs, efficient materials as well as simple structured devices are in high demand. However, a very limited number of blue OLEDs achieving high stability and color purity have been reported. Herein, three new sky-blue emitters, 1,4,5-triphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEI), 1-(4-methoxyphenyl)-4,5-diphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEMeOPhI) and 1-phenyl-2,4,5-tris(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (3TPEI), with a combination of imidazole and tetraphenylethene groups, have been developed. High photoluminescence quantum yields are obtained for these materials. All derivatives have demonstrated aggregation-induced emission (AIE) behavior, excellent thermal stability with high decomposition and glass transition temperatures. Non-doped sky-blue OLEDs with simple structure have been fabricated employing these materials as emitters and realized high efficiencies of 2.41 % (4.92 cd A-1 , 2.70 lm W-1 ), 2.16 (4.33 cd A-1 , 2.59 lm W-1 ) and 3.13 % (6.97 cd A-1 , 4.74 lm W-1 ) for TPEI, TPEMeOPhI and 3TPEI, with small efficiency roll-off. These are among excellent results for molecules constructed from the combination of imidazole and TPE reported so far. The high performance of a 3TPEI-based device shows the promising potential of the combination of imidazole and AIEgen for synthesizing efficient electroluminescent materials for OLED devices.

Chemically binding carboxylic acids onto TiO2 nanoparticles with adjustable coverage by solvothermal strategy.

This paper presents a solvothermal strategy for chemical modification of TiO(2) nanoparticles with carboxylic acids. Solvothermal reaction between the TiO(2) nanoparticles and carboxylic acid molecules in an autoclave at 100 degrees C provides carboxylic acid-modified TiO(2) particles with a modification efficiency much higher than the conventional immersion method. TiO(2) nanoparticles were prepared by hydrolysis of titanium isopropoxide in nitric acid solution; the modified nanoparticles were characterized by powder X-ray diffraction pattern, scanning electron microscopy, absorption and Fourier transform infrared spectra, and thermogravimetric analysis. Results show that the binding form of the modifier molecules on TiO(2) surface is in a bidentate chelating mode, the crystalline phase composition and morphological structure of the preformed TiO(2) nanoparticles are not affected by the solvothermal reaction, and the surface coverage of the modifier molecules can be adjusted by the weight ratio of modifier/TiO(2) during feeding. It is evident that the reaction processes in the solvothermal strategy involve the formation of double hydrogen bondings between carboxylic acid molecule and TiO(2) at the same Ti site and the coordination at solvothermal temperature by dehydration from the hydrogen bondings. The solvothermal strategy for modifying TiO(2) nanoparticles is expected to find potential applications in many fields; for example, our results demonstrate that the photovoltaic performance of the TiO(2) nanoparticles can be improved by the solvothermal modification even with an insulating modifier and controlled by the modifier coverage.