Tethered Helical Ladder-Type Aromatic Lactams.

Tethered nonplanar aromatics (TNAs) make up an important class of nonplanar aromatic compounds showing unique features. However, the knowledge on the synthesis, structures, and properties of TNAs remains insufficient. In this work, a new type of TNAs, the tethered aromatic lactams, is synthesized via Pd-catalyzed consecutive intramolecular direct arylations. These molecules possess a helical ladder-type conjugated system of up to 13 fused rings. The overall yields ranged from 3.4 to 4.3%. The largest of the tethered aromatic lactams, 6L-Bu-C14, demonstrates a guest-adaptive hosting capability of TNAs for the first time. When binding fullerene guests, the cavity of 6L-Bu-C14 became more circular to better accommodate spherical fullerene molecules. The host-guest interaction is thoroughly studied by X-ray crystallography, theoretical calculations, fluorescence titration, and nuclear magnetic resonance (NMR) titration experiments. 6L-Bu-C14 shows stronger binding with C70 than with C60 due to the better convex-concave π-π interaction. P and M enantiomers of all tethered aromatic lactams show distinct and persistent chiroptical properties and demonstrate the potential of chiral TNAs as circularly polarized luminescence (CPL) emitters.

Selenophene-fused Perylene Diimide-based Cathode Interlayer Enables 19% Efficiency Binary Organic Solar Cells via Stimulative Charge Extraction.

The cathode interlayer is crucial for the development of organic solar cells (OSCs), but the research on simple and efficient interlayer materials is lagging behind. Here, a donor-acceptor (D-A) typed selenophene-fused perylene diimide (PDI) derivative (SePDI3) is developed as cathode interlayer material (CIM) for OSCs, and a non-fused PDI derivative (PDI3) is used as the control CIM for comparison. Compared to PDI3, SePDI3 shows a stronger self-doping effect and better crystallinity, resulting in better charge transport ability. Furthermore, the interaction between SePDI3 and L8-BO can form an efficient extraction channel, leading to superior charge extraction behavior. Finally, benefitting from significantly enhanced charge transport and extraction capacity, the SePDI3-based device displays a champion PCE of 19.04% with an ultrahigh fill factor of 81.65% for binary OSCs based on PM6:L8-BO active layer, which is one of the top efficiencies reported to date in binary OSCs based novel CIMs. Our work prescribes a facile and effective fusion strategy to develop high-efficiency CIMs for OSCs.

Dithienoquinoxalineimide-Based Polymer Donor Enables All-Polymer Solar Cells Over 19 % Efficiency.

All-polymer solar cells (all-PSCs) have been regarded as one of the most promising candidates for commercial applications owing to their outstanding advantages such as mechanical flexibility, light weight and stable film morphology. However, compared to large amount of new-emerging excellent polymer acceptors, the development of high-performance polymer donor lags behind. Herein, a new D-π-A type polymer donor, namely QQ1, was developed based on dithienoquinoxalineimide (DTQI) as the A unit, benzodithiophene with thiophene-conjugated side chains (BDTT) as the D unit, and alkyl-thiophene as the π-bridge, respectively. QQ1 not only possesses a strong dipole moment, but also shows a wide band gap of 1.80 eV and a deep HOMO energy level of -5.47 eV, even without halogen substituents that are commonly indispensable for high-performance polymer donors. When blended with a classic polymer acceptor PY-IT, the QQ1-based all-PSC delivers an outstanding PCE of 18.81 %. After the introduction of F-BTA3 as the third component, a record PCE of 19.20 % was obtained, the highest value reported so far for all-PSCs. The impressive photovoltaic performance originates from broad absorption range, reduced energy loss, and compact π-π stacking. These results provide new insight in the rational design of novel nonhalogenated polymer donors for further development of all-PSCs.

Optimizing Molecular Crystallinity and Suppressing Electron-Phonon Coupling in Completely Non-Fused Ring Electron Acceptors for Organic Solar Cells.

High open-circuit voltage (Voc) organic solar cells (OSCs) have received increasing attention because of their promising application in tandem devices and indoor photovoltaics. However, the lack of a precise correlation between molecular structure and stacking behaviors of wide band gap electron acceptors has greatly limited its development. Here, we adopted an asymmetric halogenation strategy (AHS) and synthesized two completely non-fused ring electron acceptors (NFREAs), HF-BTA33 and HCl-BTA33. The results show that AHS significantly enhances the molecular dipoles and suppresses electron-phonon coupling, resulting in enhanced intramolecular/intermolecular interactions and decreased nonradiative decay. As a result, PTQ10 : HF-BTA33 realizes a power conversion efficiency (PCE) of 11.42 % with a Voc of 1.232 V, higher than that of symmetric analogue F-BTA33 (PCE=10.02 %, Voc=1.197 V). Notably, PTQ10 : HCl-BTA33 achieves the highest PCE of 12.54 % with a Voc of 1.201 V due to the long-range ordered π-π packing and enhanced surface electrostatic interactions thereby facilitating exciton dissociation and charge transport. This work not only proves that asymmetric halogenation of completely NFREAs is a simple and effective strategy for achieving both high PCE and Voc, but also provides deeper insights for the precise molecular design of low cost completely NFREAs.

Recent Advances in Organic Photovoltaic Materials Based on Thiazole-Containing Heterocycles.

Organic solar cells (OSCs) have achieved great progress, driven by the rapid development of wide bandgap electron donors and narrow bandgap non-fullerene acceptors (NFAs). Among a large number of electron-accepting (A) building blocks, thiazole (Tz) and its derived fused heterocycles have been widely used to construct photovoltaic materials, especially conjugated polymers. Benefiting from the electron deficiency, rigidity, high planarity, and enhanced intra/intermolecular interactions of Tz-containing heterocycles, some related photovoltaic materials exhibit proper energy levels, optimized molecular aggregation, and active layer morphology, leading to excellent photovoltaic performance. This review focuses on the progress of Tz-based photovoltaic materials in the field of OSCs. First, the Tz-based donor and acceptor photovoltaic materials are reviewed. Then, the materials based on promising Tz-containing heterocycles, mainly including thiazolo[5,4-d]thiazole (TzTz), benzo[1,2-d:4,5-d']bis(thiazole) (BBTz), and benzo[d]thiazole (BTz) are summarized and discussed. In addition, the new emerging Tz-fused structures and their application in OSCs are introduced. Finally, perspectives and outlooks for the further development of Tz-containing heterocycle-based photovoltaic materials are proposed.

Dithienobenzothiadiazole (DTBT)-Based Polymers Enable Organic Solar Cells with Ultrahigh VOC of ≈1.3 V.

Dithieno[3',2':3,4;2",3":5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT) is a newly emerging building block to construct effective photovoltaic polymers. Organic solar cells (OSCs) based on DTBT-based polymers have realized power conversion efficiency (PCEs) over 18%, despite their relatively low open-circuit voltage (VOC ) of 0.8-0.95 V. To extend the application of DTBT-based polymers in high-voltage OSCs, herein, D18-Cl and PE55 are used to combine with a wide-bandgap non-fullerene acceptor (NFA), BTA3, and achieve ultrahigh VOC of 1.30 and 1.28 V, respectively. Compared with D18-Cl based on tricyclic benzodithiophene (BDT) segment, PE55 containing the pentacyclic dithienobenzodithiophene (DTBDT) unit possesses better hole mobility, higher charge-transfer efficiency, and more desirable phase separation. Hence, PE55:BTA3 blend exhibits a higher efficiency of 9.36% than that of D18-Cl: BTA3 combination (6.30%), which is one of the highest values for OSCs at ≈1.3 V VOC . This work attests that DTBT-based p-type polymers are ideal for the application in high-voltage OSCs.

Benzotriazole-Based 3D Four-Arm Small Molecules Enable 19.1 % Efficiency for PM6 : Y6-Based Ternary Organic Solar Cells.

A third component featuring a planar backbone structure similar to the binary host molecule has been the preferred ingredient for improving the photovoltaic performance of ternary organic solar cells (OSCs). In this work, we explored a new avenue that introduces 3D-structured molecules as guest acceptors. Spirobifluorene (SF) is chosen as the core to combine with three different terminal-modified (rhodanine, thiazolidinedione, and dicyano-substituted rhodanine) benzotriazole (BTA) units, affording three four-arm molecules, SF-BTA1, SF-BTA2, and SF-BTA3, respectively. After adding these three materials to the classical system PM6 : Y6, the resulting ternary devices obtained ultra-high power-conversion efficiencies (PCEs) of 19.1 %, 18.7 %, and 18.8 %, respectively, compared with the binary OSCs (PCE=17.4 %). SF-BTA1-3 can work as energy donors to increase charge generation via energy transfer. In addition, the charge transfer between PM6 and SF-BTA1-3 also acts to enhance charge generation. Introducing SF-BTA1-3 could form acceptor alloys to modify the molecular energy level and inhibit the self-aggregation of Y6, thereby reducing energy loss and balancing charge transport. Our success in 3D multi-arm materials as the third component shows good universality and brings a new perspective. The further functional development of multi-arm materials could make OSCs more stable and efficient.

The Subtle Structure Modulation of A2 -A1 -D-A1 -A2 Type Nonfullerene Acceptors Extends the Photoelectric Response for High-Voltage Organic Photovoltaic Cells.

Molecular structural modifications are utilized to improve the short-circuit current (JSC ) of high-voltage organic photovoltaics (OPVs). Herein, the classic non-fullerene acceptor (NFA), BTA3, is chosen as a benchmark, with BTA3b containing the linear alkyl chains on the middle core and JC14 fusing thiophene on the benzotriazole (BTA) unit as a contrast. The photovoltaic devices based on J52-F: BTA3b and J52-F: JC14 achieve wider external quantum efficiency responses with band edges of 730 and 800 nm, respectively than that of the device based on J52-F: BTA3 (715 nm). The corresponding  JSC increases to 14.08 and 15.78 mA cm-2 , respectively, compared to BTA3 (11.56 mA cm-2 ). The smaller Urbach energy and higher electroluminescence efficiency guarantee J52-F: JC14 a decreased energy loss (0.528 eV) and a high open-circuit voltage (VOC ) of 1.07 V. Finally, J52-F: JC14 combination achieves an increased power conversion efficiency (PCE) of 10.33% than that of J52-F: BTA3b (PCE = 9.81%) and J52-F: BTA3 (PCE = 9.04%). Overall, the research results indicate that subtle structure modification of NFAs, especially introducing fused rings, is a simple and effective strategy to extend the photoelectric response, boosting the  JSC and ensuring a high VOC beyond 1.0 V.

Multiple-cation wide-bandgap perovskite solar cells grown using cesium formate as the Cs precursor with high efficiency under sunlight and indoor illumination.

Owing to the advantages of adjustable bandgap, low-cost fabrication and superior photovoltaic performance, wide-bandgap (WBG) perovskite solar cells (PSCs) are considered as the promising top-cell for multi-junction solar cells. At the same time, WBG PSCs have also shown great potential for indoor photovoltaic applications. To further improve the performance of WBG PSCs, in this work, we fabricated efficient WBG PSCs via introducing cesium formate (CsFa) as the Cs precursor. Due to the HCOO·Pb+ and HCOOH·Cs+ complex formation and HCOOH volatilization accompanying the crystallization process, the crystallization of the perovskite using the CsFa precursor (CsFa-perovskite) is promoted. Compared to the perovskite prepared using the CsBr precursor (CsBr-perovskite), the WBG CsFa-perovskite shows better perovskite crystallization, reduced trap-state density, and better phase stability under light illumination. Finally, the 1.63 eV WBG PSCs based on the CsFa-perovskite achieve a significant PCE of 20.01% under one sun illumination (AM 1.5G, 100 mW cm-2), which is higher than that of PSCs based on the CsBr-perovskite (18.27%). Moreover, the PCE of CsFa-perovskite PSCs also under indoor warm-white 2700 K LED light illumination (1000 lux) is as high as 38.52%. Our results demonstrate that CsFa as the Cs precursor is a promising candidate to promote the device performance of WBG PSCs.

Side-chain engineering of copolymers based on benzotriazole (BTA) and dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT) enables a high PCE of 14.6.

Dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT) is a kind of prospective candidate for constructing donor-π-acceptor (D-π-A) copolymer donors applied in organic solar cells but is restricted due to its relatively poor photovoltaic performance compared with benzo[1,2-b;4,5-b']dithiophenes (BDT)-based analog. Herein, three conjugated polymers (PE51,PE52andPE53)-based DTBDT and benzo[d][1,2,3]triazole (BTA) bearing different lengths of alkyl side chain were designed and synthesized. The change in alkyl chain length can obviously affect the energy level distribution, molecular stacking, miscibility and morphology with the non-fullerene acceptor ofY6. PolymerPE52with a moderate alkyl chain realized the highest short-current density (JSC) and fill factor (FF) of 25.36 mA cm-2and 71.94%, respectively. Compared with BDT-based analogJ52-Cl, the significantly enhanced crystallinity and intermolecular interaction ofPE52had effectively boosted the charge transport characteristic and optimized the surface morphology, thereby increasing the power conversion efficiency from 12.3% to an impressive 14.6%, which is the highest value among DTBDT-based and BTA-based polymers. Our results show that not only could high efficiency be achieved via using DTBDT as a D unit, but the length of the alkyl chain on BTA has a significant impact on the photovoltaic performance.

First-principles theoretical designing of planar non-fullerene small molecular acceptors for organic solar cells: manipulation of noncovalent interactions.

Non-fullerene small molecular acceptors (NFSMAs) exhibit promising photovoltaic performance; however, their electron mobilities are still relatively lower than those of fullerene derivatives. The construction of a highly planar conjugated system is an important strategy to achieve high charge mobility. In chemical parlance, it is tedious and costly to synthesize planar compounds by restricting the rotation at a specific bond. Recently, nonbonding intramolecular interactions, also termed "conformational locks," have been considered as an alternative way to achieve planar geometry. The successful implementation of this approach for designing polymers has been extensively reported. Recently, several examples of NFSMAs containing conformational locks have been presented in the literature. This situation encourages us to perform a detailed theoretical investigation in designing planar small molecular acceptors. Various nonbonding interactions were studied using accurate computational methods, and molecules with multiple nonbonding interactions showed high planarity. Planar acceptors showed red-shifted absorption with high oscillator strengths. In addition, backbone planarity plays a very important role in tuning the charge transport properties and decreasing reorganization energy. Our results could provide important information to guide the further design of promising NFSMA materials.

Design and Synthesis of a Novel n-Type Polymer Based on Asymmetric Rylene Diimide for the Application in All-Polymer Solar Cells.

A novel n-type polymer of PTDI-T based on asymmetric rylene diimide and thiophene is designed and synthesized. The highest power conversion efficiency of 4.70% is achieved for PTB7-Th:PTDI-T-based devices, which is obviously higher than those of the analogue polymers of PPDI-2T and PDTCDI. When using PBDB-T as a donor, an open-circuit voltage (VOC ) as high as 1.03 V is obtained. The results indicate asymmetric rylene diimide is a kind of promising building block to construct n-type photovoltaic polymers.

Control of Multi-Fluorination Number and Position in D-π-A Type Polymers and Their Impact on High-Voltage Organic Photovoltaics.

Exploring the structure-performance relationship of high-voltage organic solar cells (OSCs) is significant for pushing material design and promoting photovoltaic performance. Herein, we chose a D-π-A type polymer composed of 4,8-bis(thiophene-2-yl)-benzo[1,2-b:4,5-b']dithiophene (BDT-T) and benzotriazole (BTA) units as the benchmark to investigate the effect of the fluorination number and position of the polymers on the device performance of the high-voltage OSCs, with a benzotriazole-based small molecule (BTA3) as the acceptor. F00, F20, and F40 are the polymers with progressively increasing F atoms on the D units, while F02, F22, and F42 are the polymers with further attachment of F atoms to the BTA units based on the above three polymers. Fluorination positively affects the molecular planarity, dipole moment, and molecular aggregations. Our results show that VOC increases with the number of fluorine atoms, and fluorination on the D units has a greater effect on VOC than on the A unit. F42 with six fluorine atom substitutions achieves the highest VOC (1.23 V). When four F atoms are located on the D units, the short-circuit current (JSC) and fill factor (FF) plummet, and before that, they remain almost constant. The drop in JSC and FF in F40- and F42-based devices may be attributed to inefficient charge transfer and severe charge recombination. The F22:BTA3 system achieves the highest power conversion efficiency of 9.5% with a VOC of 1.20 V due to the excellent balance between the photovoltaic parameters. Our study provides insights for the future application of fluorination strategies in molecular design for high-voltage organic photovoltaics.

The Development of Quinoxaline-Based Electron Acceptors for High Performance Organic Solar Cells.

In the recent advances of organic solar cells (OSCs), quinoxaline (Qx)-based nonfullerene acceptors (QxNFAs) have attracted lots of attention and enabled the recorded power conversion efficiency approaching 20%. As an excellent electron-withdrawing unit, Qx possesses advantages of many modifiable sites, wide absorption range, low reorganization energy, and so on. To develop promising QxNFAs to further enhance the photovoltaic performance of OSCs, it is necessary to systematically summarize the QxNFAs reported so far. In this review, all the focused QxNFAs are classified into five categories as following: SM-Qx, YQx, fused-YQx, giant-YQx, and polymer-Qx according to the molecular skeletons. The molecular design concepts, relationships between the molecular structure and optoelectronic properties, intrinsic mechanisms of device performance are discussed in detail. At the end, the advantages of this kind of materials are summed up, the molecular develop direction is prospected, the challenges faced by QxNFAs are given, and constructive solutions to the existing problems are advised. Overall, this review presents unique viewpoints to conquer the challenge of QxNFAs and thus boost OSCs development further toward commercial applications.

Room Temperature Ionic Liquid Capping Layer for High Efficiency FAPbI3 Perovskite Solar Cells with Long-Term Stability.

Ionic liquid salts (ILs) are generally recognized as additives in perovskite precursor solutions to enhance the efficiency and stability of solar cells. However, the success of ILs incorporation as additives is highly dependent on the precursor formulation and perovskite crystallization process, posing challenges for industrial-scale implementation. In this study, a room-temperature spin-coated IL, n-butylamine acetate (BAAc), is identified as an ideal passivation agent for formamidinium lead iodide (FAPbI3) films. Compared with other passivation methods, the room-temperature BAAc capping layer (BAAc RT) demonstrates more uniform and thorough passivation of surface defects in the FAPbI3 perovskite. Additionally, it provides better energy level alignment for hole extraction. As a result, the champion n-i-p perovskite solar cell with a BAAc capping layer exhibits a power conversion efficiency (PCE) of 24.76%, with an open-circuit voltage (Voc) of 1.19 V, and a Voc loss of ≈330 mV. The PCE of the perovskite mini-module with BAAc RT reaches 20.47%, showcasing the effectiveness and viability of this method for manufacturing large-area perovskite solar cells. Moreover, the BAAc passivation layer also improves the long-term stability of unencapsulated FAPbI3 perovskite solar cells, enabling a T80 lifetime of  3500 h when stored at 35% relative humidity at room temperature in an air atmosphere.

Effect of Number and Position of Chlorine Atoms on the Photovoltaic Performance of Asymmetric Nonfullerene Acceptors.

It has been well proved that the introduction of halogen can effectively modify the optoelectronic properties of classic symmetric nonfullerene acceptors (NFAs). However, the relevant studies for asymmetric NFAs are limited, especially the effect of halogen substitution number and position on the photovoltaic performance is not clear. In this work, four asymmetric NFAs with A-D-A1-A2 structure are developed by tuning the number and position of chlorine atoms on the 1,1-dicyanomethylene-3-indanone end groups, namely, A303, A304, A305, and A306. The related NFAs show progressively deeper energy levels and red-shifted absorption spectra as the degree of chlorination increases. The PM6:A306-constructed organic solar cells (OSCs) give a champion power conversion efficiency (PCE) of 13.03%. This is mainly ascribed to the most efficient exciton dissociation and collection, suppressed charge recombination, and optimal morphology. Moreover, by alternating the substitution position, the PM6:A305-based device yielded a higher PCE of 12.53% than that of PM6:A304 (12.05%). This work offers fresh insights into establishing excellent asymmetric NFAs for OSCs.

Noncovalent Interaction Boosts Performance and Stability of Organic Solar Cells Based on Giant-Molecule Acceptors.

Designing giant-molecule acceptors is deemed as an up-and-coming strategy to construct stable organic solar cells (OSCs) with high performance. Herein, two giant dimeric acceptors, namely, DYV and DYFV, have been designed and synthesized by linking two Y-series derivatives with a vinyl unit. DYFV exhibits more red-shifted absorption, down-shifted energy levels, and enhanced intermolecular packing than DYV because the intramolecular noncovalent interaction (H···F) of DYFV leads to better coplanarity of the backbone. The D18:DYFV film owns a distinct nanofibrous nanophase separation structure, a more dominant face-on orientation, and more balanced carrier mobilities. Therefore, the D18:DYFV OSC achieves a higher photoelectron conversion efficiency of 17.88% and a longer-term stability with a t80 over 45,000 h compared with the D18:DYV device. The study demonstrates that the intramolecular noncovalent interaction is a superior strategy to design giant-molecule acceptors and boost the photovoltaic performance and stability of the OSCs.

Constructing D-π-A Type Polymers as Dopant-Free Hole Transport Materials for High-Performance CsPbI2Br Perovskite Solar Cells.

Hole-transporting materials (HTMs) play a major role in efficient and stable perovskite solar cells (PSCs), especially for CsPbI2Br inorganic PSC. Among them, dopant-free conjugated polymers attract more attention because of the advantages of high hole mobility and high stability. However, the relationship between the polymer structure and the photovoltaic performance is rarely investigated. In this work, we choose three similar D-π-A-type polymers, where the D unit and π-bridge are fixed into benzodithiophene and thiophene, respectively. By changing the A units from classic benzodithiophene-4,8-dione and benzotriazole to quinoxaline, three polymers PBDB-T, J52, and PE61 are utilized as dopant-free HTMs for CsPbI2Br PSCs. The energy levels, hole mobility, and molecular stacking of the three HTMs, as well as charge transfer between CsPbI2Br/HTMs, are fully investigated. Finally, the device based on PE61 HTM obtains the champion power conversion efficiency of 16.72%, obviously higher than PBDB-T (15.13%) and J52 (15.52%). In addition, the device based on PE61 HTM displays the best long-term stability. Those results demonstrate that quinoxaline is also an effective A unit to construct D-π-A-type polymers as HTMs and improve the photovoltaic performance of PSCs.

Fused Benzotriazole A-Unit Constructs a D-π-A Polymer Donor for Efficient Organic Photovoltaics.

Herein, we originally developed a fused ring building block as an acceptor unit, namely, 2,6,10-trihydro-carbazole[3,4-c:5,6-c]bis[1,2,5]-triazole (CTA), through fusing two benzotriazoles (BTA) with a pyrrole ring. A p-type polymer PE93 containing the CTA unit exhibits relatively high molecular energy levels and excellent luminescent properties. The PE93:BTA76-based solar cell obtained a device efficiency of 12.16%, with a VOC of 0.94 V and a low nonradiative recombination loss of 0.18 eV. The results suggest that the CTA unit is an efficient acceptor unit to achieve excellent photovoltaic performance.

A2 -A1 -D-A1 -A2 -Type Nonfullerene Acceptors.

The A2 -A1 -D-A1 -A2 -type molecules consist of one electron-donating (D) core flanked by two electron-accepting units (A1 and A2 ) and have emerged as an essential branch of nonfullerene acceptors (NFAs). These molecules generally possess higher molecular energy levels and wider optical bandgaps compared with those of the classic A-D-A- and A-DA'D-A-type NFAs, owing to the attenuated intramolecular charge transfer effect. These characteristics make them compelling choices for the fabrication of high-voltage organic photovoltaics (OPVs), ternary OPVs, and indoor OPVs. Herein, the recent progress in the A2 -A1 -D-A1 -A2 -type NFAs are reviewed, including the molecular engineering, structure-property relationships, voltage loss (Vloss ), device stability, and photovoltaic performance of binary, ternary, and indoor OPVs. Finally, the challenges and provided prospects are discussed for the further development of this type of NFAs.