Stereoisomeric Non-Fullerene Acceptors-Based Organic Solar Cells.

Chiral alkyl chains are ubiquitously observed in organic semiconductor materials and can regulate solution processability and active layer morphology, but the effect of stereoisomers on photovoltaic performance has rarely been investigated. For the racemic Y-type acceptors widely used in organic solar cells, it remains unknown if the individual chiral molecules separate into the conglomerate phase or if racemic phase prevails. Here, the photovoltaic performance of enantiomerically pure Y6 derivatives, (S,S)/(R,R)-BTP-4F, and their chiral mixtures are compared. It is found that (S,S) and (R,R)-BTP-4F molecule in the racemic mixtures tends to interact with its enantiomer. The racemic mixtures enable efficient light harvesting, fast hole transfer, and long polaron lifetime, which is conducive to charge generation and suppresses the recombination losses. Moreover, abundant charge diffusion pathways provided by the racemate contribute to efficient charge transport. As a result, the racemate system maximizes the power output and minimizes losses, leading to a higher efficiency of 18.16% and a reduced energy loss of 0.549 eV, as compared to the enantiomerically pure molecules. This study demonstrates that the chirality of non-fullerene acceptors should receive more attention and be designed rationally to enhance the efficiency of organic solar cells.

Molecular-Level Insight into Impact of Additives on Film Formation and Molecular Packing in Y6-based Organic Solar Cells.

Additive engineering is widely utilized to optimize film morphology in active layers of organic solar cells (OSCs). However, the role of additive in film formation and adjustment of film morphology remains unclear at the molecular level. Here, taking high-efficiency Y6-based OSC films as an example, this work thus employs all-atom molecular-dynamics simulations to investigate how introduction of additives with different π-conjugation degree thermodynamically and dynamically impacts nanoscale molecular packings. These results demonstrate that the van der Waals (vdW) interactions of the Y6 end groups with the studied additives are strongest. The larger the π-conjugation degree of the additive molecules, the stronger the vdW interactions between additive and Y6 molecules. Due to such vdW interactions, the π-conjugated additive molecules insert into the neighboring Y6 molecules, thus opening more space for relaxation of Y6 molecules to trigger more ordered packing. Increasing the interactions between the Y6 end groups and the additive molecules not only accelerates formation of the Y6 ordered packing, but also induces shorter Y6-intermolecular distances. This work reveals the fundamental molecular-level mechanism behind film formation and adjustment of film morphology via additive engineering, providing an insight into molecular design of additives toward optimizing morphologies of organic semiconductor films.

Halogenated 9H-Indeno[1,2-b]Pyrazine-2,3-Dicarbonitrile End Groups Based Asymmetric Non-Fullerene Acceptors for Green Solvent-Processable, Additive-Free, and Stable Organic Solar Cells.

Non-fullerene acceptors (NFAs) significantly enhance photovoltaic performance in organic solar cells (OSCs) using halogenated solvents and additives. However, these solvents are environmentally detrimental and unsuitable for industrial-scale production, and the issue of OSCs' poor long-term stability persists. This report introduces eight asymmetric NFAs (IPCnF-BBO-IC2F, IPCnF-BBO-IC2Cl, IPCnCl-BBO-IC2F, and IPCnCl-BBO-IC2Cl, where n = 1 and 2). These NFAs comprise a 12,13-bis(2-butyloctyl)-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 (BBO) core. One end of the core attaches to a mono- or di-halogenated 9H-indeno[1,2-b]pyrazine-2,3-dicarbonitrile (IPC) end group (IPC1F, IPC1Cl, IPC2F, or IPC2Cl), while the other end connects to a 2-(5,6-dihalo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) end group (IC2F or IC2Cl). The optical and electronic properties of these NFAs can be finely tuned by controlling the number of halogen atoms. Crucially, these NFAs demonstrate excellent compatibility with PM6 even in o-xylene, facilitating the production of additive-free OSCs. The di-halogenated IPC-based NFAs outperform their mono-halogenated counterparts in photovoltaic performance within OSCs. Remarkably, the di-halogenated IPC-based NFAs maintain 94‒98% of their initial PCEs over 2000 h in air without encapsulation, indicating superior long-term device stability. These findings imply that the integration of di-halogenated IPCs in asymmetric NFA design offers a promising route to efficient, stable OSCs manufactured through environmentally friendly processes.

The Influence of Donor/Acceptor Interfaces on Organic Solar Cells Efficiency and Stability Revealed through Theoretical Calculations and Morphology Characterizations.

End-groups halogenation strategies, generally refers to fluorination and chlorination, have been confirmed as simple and efficient methods to regulate the photoelectric performance of non-fullerene acceptors (NFAs), but a controversy over which one is better has existed for a long time. Here, two novel NFAs, C9N3-4F and C9N3-4Cl, featured with different end-groups were successfully synthesized and blended with two renowned donors, D18 and PM6, featured with different electron-withdrawing units. Detailed theoretical calculations and morphology characterizations of the interface structures indicate NFAs based on different end-groups possess different binding energy and miscibility with donors, which shows an obvious influence on phase-separation morphology, charge transport behavior and device performance. After verified by other three pairs of reported NFAs, a universal conclusion obtained as the devices based on D18 with fluorination-end-groups-based NFAs and PM6 with chlorination-end-groups-based NFAs generally show excellent efficiencies, high fill factors and stability. Finally, the devices based on D18: C9N3-4F and PM6: C9N3-4Cl yield outstanding efficiency of 18.53 % and 18.00 %, respectively. Suitably selecting donor and regulating donor/acceptor interface can accurately present the photoelectric conversion ability of a novel NFAs, which points out the way for further molecular design and selection for high-performance and stable organic solar cells.

Over 19 % Efficiency Organic Solar Cells Enabled by Manipulating the Intermolecular Interactions through Side Chain Fluorine Functionalization.

Fluorine side chain functionalization of non-fullerene acceptors (NFAs) represents an effective strategy for enhancing the performance of organic solar cells (OSCs). However, a knowledge gap persists regarding the relationship between structural changes induced by fluorine functionalization and the resultant impact on device performance. In this work, varying amounts of fluorine atoms were introduced into the outer side chains of Y-series NFAs to construct two acceptors named BTP-F0 and BTP-F5. Theoretical and experimental investigations reveal that side-chain fluorination significantly increase the overall average electrostatic potential (ESP) and charge balance factor, thereby effectively improving the ESP-induced intermolecular electrostatic interaction, and thus precisely tuning the molecular packing and bulk-heterojunction morphology. Therefore, the BTP-F5-based OSC exhibited enhanced crystallinity, domain purity, reduced domain spacing, and optimized phase distribution in the vertical direction. This facilitates exciton diffusion, suppresses charge recombination, and improves charge extraction. Consequently, the promising power conversion efficiency (PCE) of 17.3 % and 19.2 % were achieved in BTP-F5-based binary and ternary devices, respectively, surpassing the PCE of 16.1 % for BTP-F0-based OSCs. This work establishes a structure-performance relationship and demonstrates that fluorine functionalization of the outer side chains of Y-series NFAs is a compelling strategy for achieving ideal phase separation for highly efficient OSCs.

Unraveling the Role of Non-Fullerene Acceptor with High Dielectric Constant in Organic Solar Cells.

Increasing the relative dielectric constant is a constant pursuit of organic semiconductors, but it often leads to multiple changes in device characteristics, hindering the establishment of a reliable relationship between dielectric constant and photovoltaic performance. Herein, a new non-fullerene acceptor named BTP-OE is reported by replacing the branched alkyl chains on Y6-BO with branched oligoethylene oxide chains. This replacement successfully increases the relative dielectric constant from 3.28 to 4.62. To surprise, BTP-OE offers consistently lower device performance relative to Y6-BO in organic solar cells (16.27% vs 17.44%) due to the losses in open-circuit voltage and fill factor. Further investigations unravel that BTP-OE has resulted in reduced electron mobility, increased trap density, enhanced first order recombination, and enlarged energetic disorder. These results demonstrate the complex relationship between dielectric constant and device performance, which provide valuable implications for the development of organic semiconductors with high dielectric constant for photovoltaic application.

Recent Progress in All-Small-Molecule Organic Solar Cells.

Active layer material plays a critical role in promoting the performance of an organic solar cell (OSC). Small-molecule (SM) materials have the merits of well-defined chemical structures, few batch-to-batch variations, facile synthesis and purification procedures, and easily tuned properties. SM-donor and non-fullerene acceptor (NFA) innovations have recently produced all-small-molecule (ASM) devices with power conversion efficiencies that exceed 17% and approach those of their polymer-based counterparts, thereby demonstrating their great future commercialization potential. In this review, recent progress in both SM donors and NFAs to illustrate structure-property relationships and various morphology-regulation strategies are summarized. Finally, ASM-OSC challenges and outlook are discussed.

Directly Cross-Linked Conjugated Polymer Donor Enables Efficient Polymer Solar Cells with Extraordinary Mechanical Robustness.

A cross-linking strategy can result in a three-dimensional network of interconnected chains for the copolymers, thereby improving their mechanical performance. In this work, a series of cross-linked conjugated copolymers, named PC2, PC5, and PC8, constructed with different ratios of monomers are designed and synthesized. For comparison, a random linear copolymer, PR2 is also synthesized based on the similar monomers. When blended with Y6 acceptor, the cross-linked polymers PC2, PC5, and PC8-based polymer solar cells (PSCs) achieve superior power conversion efficiencies (PCEs) of 17.58%, 17.02%, and 16.12%, respectively, which are higher than that (15.84%) of the random copolymer PR2-based devices. Moreover, the PCE of PC2:Y6-based flexible PSC retains ≈88% of the initial efficiency value after 2000 bending cycles, overwhelming the PR2:Y6-based device with the remaining 12.8% of the initial PCE. These results demonstrate that the cross-linking strategy is a feasible and facile approach to developing high-performance polymer donors for the fabrication of flexible PSCs.

Molecular Design of A-D-A Electron Acceptors Towards Low Energy Loss for Organic Solar Cells.

Low energy loss is a prerequisite for organic solar cells to achieve high photovoltaic efficiency. Electron-vibration coupling (i. e., intramolecular reorganization energy) plays a crucial role in the photoelectrical conversion and energy loss processes. In this Concept article, we summarize our recent theoretical advances on revealing the energy loss mechanisms at the molecular level of A-D-A electron acceptors. We underline the importance of electron-vibration couplings on reducing the energy loss and describe the effective molecular design strategies towards low energy loss through decreasing the electron-vibration couplings.

Spatial Balance of Photogenerated Charge Carriers in Active Layers of Polymer Solar Cells.

Bulk heterojunction polymer solar cells (PSCs) blended with non-fullerene-type acceptors (NFAs) possess good solar power conversion efficiency and compatibility with flexible electronics, rendering them good candidates for mobile photovoltaic applications. However, their internal absorption performance and mechanism are yet to be fully elucidated because of their complicated interference effect caused by their multilayer device structure. The transfer matrix method (TMM) is ideal for analyzing complex optical electric fields by considering multilayer interference effects. In this study, an active layer (AL) thickness-dependent TMM is used to obtain accurate information on the photon-capturing mechanisms of NFA-based PSCs for comparison with experimental results. Devices with AL thicknesses of 40-350 nm were prepared, and the AL-thickness-dependent device parameters with incident photon-to-current efficiency spectra were compared with the calculated internal absorption spectra of the TMM. The spectrally and spatially resolved spectra as a function of the AL thickness and excitation wavelength revealed that the power conversion efficiency of the NFA-blended PSC decreased with the increasing AL thickness after reaching a maximum of ~100 nm; by contrast, the internal absorption efficiency showed the opposite trend. Furthermore, the TMM spectra indicated that the spatial distribution of the photogenerated charge carriers became significantly imbalanced as the AL thickness increased, implying that the AL-dependent loss stemmed from the discrepancy between the absorption and the extracted charge carriers.

Design of Furan-Based Acceptors for Organic Photovoltaics.

We explore a series of furan-based non-fullerene acceptors and report their optoelectronic properties, solid-state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced band gap and red-shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels-Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to most non-fullerene acceptors, and the photostability depends strongly on the molecular structure. Placement of furans next to the acceptor termini leads to better photostability, well-balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker offers the best photostability and power conversion efficiency (>14 %), outperforming all furan-based acceptors reported to date and all indacenodithiophene-based acceptors. Our findings show the possibility of photostable furan-based alternatives to the currently omnipresent thiophene-based photovoltaic materials.

Quinoxaline derivatives as attractive electron-transporting materials.

This review article provides a comprehensive overview of recent advancements in electron transport materials derived from quinoxaline, along with their applications in various electronic devices. We focus on their utilization in organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), organic field-effect transistors (OFETs), organic-light emitting diodes (OLEDs) and other organic electronic technologies. Notably, the potential of quinoxaline derivatives as non-fullerene acceptors in OSCs, auxiliary acceptors and bridging materials in DSSCs, and n-type semiconductors in transistor devices is discussed in detail. Additionally, their significance as thermally activated delayed fluorescence emitters and chromophores for OLEDs, sensors and electrochromic devices is explored. The review emphasizes the remarkable characteristics and versatility of quinoxaline derivatives in electron transport applications. Furthermore, ongoing research efforts aimed at enhancing their performance and addressing key challenges in various applications are presented.

The Importance of Nonequilibrium to Equilibrium Transition Pathways for the Efficiency and Stability of Organic Solar Cells.

Controlled morphology of solution-processed thin films have realized impressive achievements for non-fullerene acceptor (NFA)-based organic solar cells (OSCs). Given the large set of donor-acceptor pairs, employing various processing conditions to realize optimal morphology for high efficiency and stable OSCs is a strenuous task. Therefore, comprehensive correlations between processing conditions and morphology evolution pathways have to be developed for efficient performance and stability of devices. Within the framework of the blend system, crystallization transitions of NFA molecules are tracked utilizing the first heating scan of differential scanning calorimeter (DSC) measurement correlating with respective morphology evolution of blend films. Real-time dynamics measurements and morphology characterizations are combined to provide optimal morphology transition pathways as NFA molecules are shown to be released from the mixed-phase to form balanced ordered packing with variant processing conditions. Polymer:NFA films are fabricated using blade coating incorporating solvent additive or thermal annealing as processing conditions as a correlation is formulated between performance and stability of solar cells with morphology transition pathways. This work demonstrates the significance of processing condition-controlled transition pathways for the realization of optimal morphology leading to superior OSC devices.

Origin of the Additive-Induced VOC Change in Non-Fullerene Organic Solar Cells.

Additives are often used to adjust the morphology of the active layer to improve the performance of organic solar cells (OSCs). Here, taking typical high-efficiency non-fullerene systems as examples, the effect of the additive on the device performance in non-fullerene OSCs is systematically investigated. Surprisingly, an unpresented VOC change is observed in the opposite direction of the two typical systems (PM6:Y6 and PTB7-Th: ITIC) appearing after the incorporation of the additive DIO, which can be affected by the morphological differences as indicated by the several morphological studies. The bewildering VOC change caused by the additive in different material systems is supposed to originate from the different energy level variations as verified by the energy level studies. Molecular dynamic (MD) and density functional theory (DFT) calculations are also included to get an insight into the dynamic of the additive-induced morphological differences that are supposed to contribute to the energy level changes. Combining a series of morphological and energic studies as well as the theoretical calculations, the origin of unforeseeable VOC changes caused by additives in non-fullerene OSCs is clarified, and provides in-depth insights into the effects of additives on device performance.

All-Small-Molecule Organic Solar Cells with Efficiency Approaching 16% and FF over 80.

Molecule engineering has been demonstrated as a valid strategy to adjust the active layer morphology in all-small-molecule organic solar cells (ASM-OSCs). In this work, two non-fullerene acceptors (NFAs), FO-2Cl and FO-EH-2Cl, with different alkyl side chains are reported and applied in ASC-OSCs. Compared with FO-2Cl, FO-EH-2Cl is designed by replacing the octyl alkyl chains with branched iso-octyl alkyl chains, leading to an enhanced molecular packing, crystallinity, and redshifted absorption. With a small molecule BSFTR as donor, the device of BSFTR:FO-EH-2Cl obtains a better morphology and achieves a higher power conversion efficiency (PCE) of 15.78% with a notable fill factor (FF) of 80.44% than that of the FO-2Cl-based device with a PCE of 15.27% and FF of 78.41%. To the authors' knowledge, the FF of 80.44% is the highest value in ASM-OSCs. These results demonstrate a good example of fine-tuning the molecular structure to achieve suitable active layer morphology with promising performance for ASM-OSCs, which can provide valuable insight into material design for high-efficiency ASM-OSCs.

Molecular Insights of Non-fused Ring Acceptors for High-Performance Non-fullerene Organic Solar Cells.

Non-fullerene acceptors with fused-ring structures have rapidly improved the performance of organic solar cells over the past five years, but they still suffer from synthetic complexity and thus high material costs, one of the major obstacles of hindering their commercialization process. The construction of non-fused ring acceptors (NFRAs) has recently been regarded as a feasible solution due to their facile synthesis and satisfactory device performances. Thus in this concept, we highlight the important progress of NFRAs in recent years, and discuss the key relationship between molecular design strategies and device performance. Finally, we provide some potential molecular insights for the future design of high-performance NFRAs.

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.

New Bithiophene Extended IDIC-Based Non-Fullerene Acceptors and Organic Photovoltaics Thereof.

We developed new bithiophene extended electron acceptors based on m-alkoxythenyl-substituted IDIC with three different end groups, named as IDT-BT-IC, IDT-BT-IC4F, and IDT-BT-IC4Cl, respectively. The ultraviolet absorption maximum was redshifted and the bandgap was decreased as the strong electron accepting ability of the end group increased. A differential scanning calorimetry thermogram analysis revealed that all the new acceptors have a crystalline character. Using these acceptors and a bulk heterojunction structure using PBDB-T, inverted organic photovoltaic (OPV) devices were fabricated, and their performance was analyzed. Due to the red shift of the electron acceptors, the OPV active layer particularly, which was derived from IDT-BT-IC4F, exhibited increased absorption at long wavelengths over 800 nm. The OPV prepared using IDT-BT-IC exhibited a short-circuit current density (Jsc) of 2.30 mA/cm2, an open-circuit voltage (Voc) of 0.95 V, a fill factor (FF) of 45%, and a photocurrent efficiency (PCE) of 1.00%. Using IDT-BT-IC4F, the corresponding OPV device showed Jsc = 8.31 mA/cm2, Voc = 0.86 V, FF = 47%, and PCE = 3.37%. The IDT-BT-IC4Cl-derived OPV had Jsc = 3.00 mA/cm2, Voc = 0.89 V, FF = 29%, and PCE = 0.76%. When IDT-BT-IC4F was used as the electron acceptor, the highest Jsc and PCE values were achieved. The results show that the low average roughness (0.263 nm) of the active layer improves the extraction of electrons.

Simultaneous Improvement of Efficiency and Stability of Organic Photovoltaic Cells by using a Cross-Linkable Fullerene Derivative.

Improving power conversion efficiencies (PCEs) and stability are two main tasks for organic photovoltaic (OPV) cells. In the past few years, although the PCE of the OPV cells has been considerably improved, the research on device stability is limited. Herein, a cross-linkable material, cross-linked [6,6]-phenyl-C61-butyric styryl dendron ester (c-PCBSD), is applied as an interfacial modification layer on the surface of zinc oxide and as the third component into the PBDB-TF:Y6-based OPV cells to enhance photovoltaic performance and long-term stability. The PCE of the OPV cells that underwent the two-step modification increased from 15.1 to 16.1%. In particular, such OPV cells exhibited much better stability under both thermal and air conditions because of the decreased number of interfacial defects and stable interfacial and active layer morphologies. The results demonstrated that the introduction of a cross-linkable fullerene derivative into the interfacial and active layers is a feasible method to improve the PCE and stability of OPV cells.

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.