Over 19.2% Efficiency of Organic Solar Cells Enabled by Precisely Tuning the Charge Transfer State Via Donor Alloy Strategy

Precisely Regulating Intermolecular Interactions and Molecular Packing of Nonfused-Ring Electron Acceptors via Halogen Transposition for High-Performance Organic Solar Cells

The structure of molecular aggregates is crucial for charge transport and photovoltaic performance in organic solar cells (OSCs). Herein, the intermolecular interactions and aggregated structures of nonfused-ring electron acceptors (NFREAs) are precisely regulated through a halogen transposition strategy, resulting in a noteworthy transformation from a 2D-layered structure to a 3D-interconnected packing network. Based on the 3D electron transport pathway, the binary and ternary devices deliver outstanding power conversion efficiencies (PCEs) of 17.46 % and 18.24 %, respectively, marking the highest value for NFREA-based OSCs.

Molecular interaction induced dual fibrils towards organic solar cells with certified efficiency over 20

The nanoscale fibrillar morphology, featuring long-range structural order, provides abundant interfaces for efficient exciton dissociation and high-quality pathways for effective charge transport, is a promising morphology for high performance organic solar cells. Here, we synthesize a thiophene terminated non-fullerene acceptor, L8-ThCl, to induce the fibrillization of both polymer donor and host acceptor, that surpasses the 20% efficiency milestone of organic solar cells. After adding L8-ThCl, the original weak and less continuous nanofibrils of polymer donors, i.e. PM6 or D18, are well enlarged and refined, whilst the host acceptor L8-BO also assembles into nanofibrils with enhanced structural order. By adapting the layer-by-layer deposition method, the enhanced structural order can be retained to significantly boost the power conversion efficiency, with specific values of 19.4% and 20.1% for the PM6:L8-ThCl/L8-BO:L8-ThCl and D18:L8-ThCl/L8-BO:L8-ThCl devices, with the latter being certified 20.0%, which is the highest certified efficiency reported so far for single-junction organic solar cells.

High-Performance PM6:Y6-Based Ternary Solar Cells with Enhanced Open Circuit Voltage and Balanced Mobilities via Doping a Wide-Band-Gap Amorphous Acceptor

Great progress has been made in organic solar cells (OSCs) in recent years, especially after the report of the highly efficient small-molecule electron acceptor Y6. However, the relatively low open circuit voltage (VOC) and unbalanced charge mobilities remain two issues that need to be resolved for further improvement in the performance of OSCs. Herein, a wide-band-gap amorphous acceptor IO-4Cl, which possessed a shallower lowest unoccupied molecular orbital (LUMO) energy level than Y6, was introduced into the PM6:Y6 binary system to construct a ternary device. The mechanism study revealed that the introduced IO-4Cl was alloyed with Y6 to prevent the overaggregation of Y6 and offer dual channels for effective hole transportation, resulting in balanced hole and electron mobilities. Taking these advantages, an enhanced VOC of 0.894 V and an improved fill factor of 75.58% were achieved in the optimized PM6:Y6:IO-4Cl-based ternary device, yielding a promising power conversion efficiency (PCE) of 17.49%, which surpassed the 16.72% efficiency of the PM6:Y6 binary device. This work provides an alternative solution to balance the charge mobilities of PM6:Y6-based devices by incorporating an amorphous high-performance LUMO A-D-A small molecule as the third compound.

Derivatization of 2,1,3-Benzothiadiazole via Regioselective C-H Functionalization and Aryne Reactivity

Despite growing interest in 2,1,3-benzothiadiazole (BTD) as an integral component of many functional molecules, methods for the functionalization of its benzenoid ring have remained limited, and many even simply decorated BTDs have required de novo synthesis. We show that regioselective Ir-catalyzed C-H borylation allows access to versatile 5-boryl or 4,6-diboryl BTD building blocks, which undergo functionalization at the C4, C5, C6, and C7 positions. The optimization and regioselectivity of C-H borylation are discussed. A broad reaction scope is presented, encompassing ipso substitution at the C-B bond, the first examples of ortho-directed C-H functionalization of BTD, ring closing reactions to generate fused ring systems, as well as the generation and capture reactions of novel BTD-based heteroarynes. The regioselectivity of the latter is discussed with reference to the Aryne Distortion Model.

Efficient Dual Mechanisms Boost the Efficiency of Ternary Solar Cells with Two Compatible Polymer Donors to Exceed 19

Ternary strategyopens a simple avenue to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The introduction of wide bandgap polymer donors (PDs) as third component canbetter utilize sunlight and improve the mechanical and thermal stability of active layer. However, efficient ternary OSCs (TOSCs) with two PDs are rarely reported due to inferior compatibility and shortage of efficient PDs match with acceptors. Herein, two PDs-(PBB-F and PBB-Cl) are adopted in the dual-PDs ternary systems to explore the underlying mechanisms and improve their photovoltaic performance. The findings demonstrate that the third components exhibit excellent miscibility with PM6 and are embedded in the host donor to form alloy-like phase. A more profound mechanism for enhancing efficiency through dual mechanisms, that are the guest energy transfer to PM6 and charge transport at the donor/acceptor interface, has been proposed. Consequently, the PM6:PBB-Cl:BTP-eC9 TOSCs achieve PCE of over 19%. Furthermore, the TOSCs exhibit better thermal stability than that of binary OSCs due to the reduction in spatial site resistance resulting from a more tightly entangled long-chain structure. This work not only provides an effective approach to fabricate high-performance TOSCs, but also demonstrates the importance of developing dual compatible PD materials.

High-Efficiency Ternary Polymer Solar Cells with a Gradient-Blended Structure Fabricated by Sequential Deposition

Acquiring the ideal blend morphology of the active layer to optimize charge separation and collection is a constant goal of polymer solar cells (PSCs). In this paper, the ternary strategy and the sequential deposition process were combined to make sufficient use of the solar spectrum, optimize the energy-level structure, regulate the vertical phase separation morphology, and ultimately enhance the power conversion efficiency (PCE) and stability of the PSCs. Specifically, the donor and acceptor illustrated a gradient-blended distribution in the sequential deposition-processed films, thus resulting in facilitated carrier characteristics in the gradient-blended devices. Consequently, the PSCs based on D18-Cl/Y6:ZY-4Cl have achieved a device efficiency of over 18% with the synergetic improvement of open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). Therefore, this work reveals a facile approach to fabricating PSCs with improved performance and stability.

Precisely Manipulating Molecular Packing via Tuning Alkyl Side-Chain Topology Enabling High-Performance Nonfused-Ring Electron Acceptors

In the development of high-performance organic solar cells (OSCs), the self-organization of organic semiconductors plays a crucial role. This study focuses on the precisely manipulation of molecular assemble via tuning alkyl side-chain topology in a series of low-cost nonfused-ring electron acceptors (NFREAs). Among the three NFREAs investigated, DPA-4, which possesses an asymmetric alkyl side-chain length, exhibits a tight packing in the crystal and high crystallinity in the film, contributing to improved electron mobility and favorable film morphology for DPA-4. As a result, the OSC device based on DPA-4 achieves an excellent power conversion efficiency of 16.67 %, ranking among the highest efficiencies for NFREA-based OSCs.

NiOx Nanoparticles Hole-Transporting Layer Regulated by Ionic Radius-Controlled Doping and Reductive Agent for Organic Solar Cells with Efficiency of 19.18

Nickel oxide (NiOx ) has garnered considerable attention as a prospective hole-transporting layer (HTL) in organic solar cells (OSCs), offering a potential solution to the stability challenges posed by traditional HTL, PEDOT:PSS, arising from acidity and hygroscopicity. Nevertheless, the lower work function (WF) of NiOx relative to donor polymers reduces charge injection efficiency in OSCs. Herein, NiOx nanoparticles are tailored through rare earth doping to optimize WF and the impact of ionic radius on their electronic properties is explored. Lanthanum (La3+ ) and yttrium (Y3+ ) ions, with larger ionic radii, are effectively doped at 1 and 3%, respectively, while scandium (Sc3+ ), with a smaller ion radius, allows enhanced 5% doping. Higher doping ratios significantly enhance WF of NiOx . A 5% Sc3+ doping raises WF to 4.99 eV from 4.77 eV for neat NiOx while maintaining high conductivity. Consequently, using 5% Sc-doped NiOx as HTL improves the power conversion efficiency (PCE) of OSCs to 17.13%, surpassing the 15.64% with the neat NiOx . Further enhancement to 18.42% is achieved by introducing the reductant catechol, outperforming the PEDOT:PSS-based devices. Additionally, when employed in a ternary blend system (D18:N3:F-BTA3), an impressive PCE of 19.18 % is realized, top-performing among reported OSCs utilizing solution-processed inorganic nanoparticles.

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.

18.6% Efficiency All-Polymer Solar Cells Enabled by a Wide Bandgap Polymer Donor Based on Benzo[1,2-d:4,5-d']bisthiazole

The limited selection of wide bandgap polymer donors for all-polymer solar cells (all-PSCs) is a bottleneck problem restricting their further development and remains poorly studied. Herein, a new wide bandgap polymer, namely PBBTz-Cl, is designed and synthesized by bridging the benzobisthiazole acceptor block and chlorinated benzodithiophene donor block with thiophene units for application as an electron donor in all-PSCs. PBBTz-Cl not only possesses wide bandgap and deep energy levels but also displays strong absorption, high-planar structure, and good crystallinity, making it a promising candidate for application as a polymer donor in organic solar cells. When paired with the narrow bandgap polymer acceptor PY-IT, a fibril-like morphology forms, which facilitates exciton dissociation and charge transport, contributing to a power conversion efficiency (PCE) of 17.15% of the corresponding all-PSCs. Moreover, when introducing another crystalline polymer acceptor BTP-2T2F into the PBBTz-Cl:PY-IT host blend, the absorption ditch in the range of 600-750 nm is filled, and the blend morphology is further optimized with the trap density reducing. As a result, the ternary blend all-PSCs achieve a significantly improved PCE of 18.60%, which is among the highest values for all-PSCs to date.

Delayed Crystallization Kinetics Allowing High-Efficiency All-Polymer Photovoltaics with Superior Upscaled Manufacturing

Though encouraging performance is achieved in small-area organic photovoltaics (OPVs), reducing efficiency loss when evoluted to large-area modules is an important but unsolved issue. Considering that polymer materials show benefits in film-forming processability and mechanical robustness, a high-efficiency all-polymer OPV module is demonstrated in this work. First, a ternary blend consisting of two polymer donors, PM6 and PBQx-TCl, and one polymer acceptor, PY-IT, is developed, with which triplet state recombination is suppressed for a reduced energy loss, thus allowing a higher voltage; and donor-acceptor miscibility is compromised for enhanced charge transport, thus resulting in improved photocurrent and fill factor; all these contribute to a champion efficiency of 19% for all-polymer OPVs. Second, the delayed crystallization kinetics from solution to film solidification is achieved that gives a longer operation time window for optimized blend morphology in large-area module, thus relieving the loss of fill factor and allowing a record efficiency of 16.26% on an upscaled module with an area of 19.3 cm2 . Besides, this all-polymer system also shows excellent mechanical stability. This work demonstrates that all-polymer ternary systems are capable of solving the upscaled manufacturing issue, thereby enabling high-efficiency OPV modules.

Oligomerized Fused-Ring Electron Acceptors for Efficient and Stable Organic Solar Cells

Organic solar cells (OSCs) have attracted wide research attention in the past decades. Very recently, oligomerized fused-ring electron acceptors (OFREAs) have emerged as a promising alternative to small-molecular/polymeric acceptor-based OSCs due to their unique advantages such as well-defined structures, batch reproducibility, good film formation, low diffusion coefficient, and excellent stability. So far, rapid advances have been made in the development of OFREAs consisting of directly/rigidly/flexibly linked oligomers and fused ones. In this Minireview, we systematically summarized the recent research progress of OFREAs, including structural diversity, synthesis approach, molecular conformation and packing, and long-term stability. Finally, we conclude with future perspectives on the challenges to be addressed and potential research directions. We believe that this Minireview will encourage the development of novel OFREAs for OSC applications.

Achieved 18.9% Efficiency by Fine-Tuning Non-Fullerene Acceptor Content to Simultaneously Increase the Short-Circuit Current and Fill Factor of Organic Solar Cells

In this study, using PM6:L8-BO as the main system and non-fullerene acceptor IDIC as the third component, a series of ternary organic solar cells (TOSCs) are fabricated. The results reveal that IDIC plays a significant role in enhancing the performance of TOSCs by optimizing the morphology of blended films and forming interpenetrating nanostructure. The improved film morphology facilitates exciton dissociation and collection in TOSCs, which causes an increase in the short-circuit current density (JSC ) and fill factor (FF). Further, by optimizing the IDIC content, the power conversion efficiency (PCE) of TOSCs reaches 18.9%. Besides, the prepared TOSCs exhibit a JSC of 27.51 mA cm-2 and FF of 76.64%, which are much higher than those of PM6:L8-BO-based organic solar cells (OSCs). Furthermore, the addition of IDIC improves the long-term stability of the OSCs. Meanwhile, TOSCs with a large effective area of 1.00 cm2 have been prepared, which exhibit a PCE of 12.4%. These findings suggest that modifying the amount of the third component can be a useful strategy to construct hight-efficiency TOSCs with practical application potential.

Suppressing electron-phonon coupling in organic photovoltaics for high-efficiency power conversion

The nonradiative energy loss (∆Enr) is a critical factor to limit the efficiency of organic solar cells. Generally, strong electron-phonon coupling induced by molecular motion generates fast nonradiative decay and causes high ∆Enr. How to restrict molecular motion and achieve a low ∆Enr is a sticking point. Herein, the free volume ratio (FVR) is proposed as an indicator to evaluate molecular motion, providing new molecular design rationale to suppress nonradiative decay. Theoretical and experimental results indicate proper proliferation of alkyl side-chain can decrease FVR and restrict molecular motion, leading to reduced electron-phonon coupling while maintaining ideal nanomorphology. The reduced FVR and favorable morphology are simultaneously obtained in AQx-6 with pinpoint alkyl chain proliferation, achieving a high PCE of 18.6% with optimized VOC, JSC and FF. Our study discovered aggregation-state regulation is of great importance to the reduction of electron-phonon coupling, which paves the way to high-efficiency OSCs.

A narrow band gap non-fullerene electron acceptor based on a dithieno-3,2-b:2',3'-dlpyrrole unit for high performance organic solar cells with minimal highest occupied molecular orbital offset

Here, a new narrow band gap non-fullerene small molecular acceptor (NFSMA) based on a dithieno-3,2-b:2',3'-dlpyrrole(DTP) unit, namely SNIC-F, was designed and synthesized. Due to the strong electron-donating ability of the DTP-based fused-ring core, SNIC-F showed a strong intramolecular-charge transfer (ICT) effect and thus gave a narrow band gap of 1.32 eV. Benefiting from the low band gap and efficient charge separation, when pairing with a copolymer PBTIBDTT, the device optimized by 0.5% 1-CN gave a high short circuit current (J_sc) of 19.64 mA cm^-2. In addition, a high open-circuit voltage (V_oc) of 0.83 V was obtained due to the near 0 eV highest occupied molecular orbital (HOMO) offset between PBTIBDTT and SNIC-F. As a result, a high power conversion efficiency (PCE) of 11.25% was obtained, and the PCE was maintained above 9.2% as the active layer thickness increased from 100 nm to 250 nm. Our work indicated that designing a narrow band gap NFSMA-based DTP unit and blending it with a polymer donor with small HOMO offset is an efficient strategy for achieving high performance OSCs.

Large Steric Hindrance Enhanced Molecular Planarity for Low-Cost Non-Fused Electron Acceptors

Designing efficient non-fused ring electron acceptors is of great importance in decreasing the material cost of organic photovoltaic cells (OPVs). It is a challenge to construct a planar molecular skeleton in non-fused molecules as there are many torsions between adjacent units. Here, we design two non-fused electron acceptors based on bithieno[3,2-b]thiophene units as core structures and study the impact of steric hindrance of substituents on molecular planarity. We use 2,4,6-triisopropylphenyl and 4-hexylphenyl groups to prepare ATTP-1 and ATTP-2, respectively. Our results suggest that the enhanced steric hindrance is beneficial for obtaining a more planar molecular configuration, which significantly increases the optical absorption and charge transport properties. The power conversion efficiency (PCE) of PBDB-TF:ATTP-1 combination (11.3%) is superior to that of PBDB-TF:ATTP-2 combination (3.7%). In addition, an impressive PCE of 10.7% is recorded in ATTP-1-based devices when a low-cost polythiophene donor PDCBT is used, which is an outstanding value in OPVs fabricated by non-fused donor/acceptor combinations. Our work demonstrates that modulation of the steric hindrance effect is of great significance to control the molecular planarity and thus obtain excellent photovoltaic performance of low-cost non-fused electron acceptors.

Near-Infrared Acceptors with Imide-Containing End Groups for Organic Solar Cells

Near-infrared electron acceptors for organic solar cells (OSCs) mostly contain electron-withdrawing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) end groups, which can be modified by but limited to phenyl, thienyl, and naphthyl units with halogenated, methyl, and methyloxy substitution. In this work, we employed an imide-containing unit to construct a new IC end group, based on which a series of new electron acceptors were synthesized. The strong electron-deficient nature of imide units enables the new acceptors to show efficient intramolecular charge transfer and hence red-shifted absorption spectra compared to their IC counterparts. These new electron acceptors were applied to OSCs, providing efficiencies of over 17% with a low voltage loss of 0.52 eV. These results demonstrate that the new imide-containing end groups are promising fragments for the construction of near-infrared electron acceptors for high-performance OSCs.

Structural Fusion Yields Guest Acceptors that Enable Ternary Organic Solar Cells with 18.77 % Efficiency

The design and selection of a suitable guest acceptor are particularly important for improving the photovoltaic performance of ternary organic solar cells (OSCs). Herein, we designed and successfully synthesized two asymmetric silicon-oxygen bridged guest acceptors, which featured distinct blue-shifted absorption, upshifted lowest unoccupied molecular orbital energy levels, and larger dipole moments than symmetric silicon-oxygen-bridged acceptor. Ternary devices with the incorporation of 14.2 wt % these two asymmetric guest acceptors exhibited excellent performance with power conversion efficiencies (PCEs) of 18.22 % and 18.77 %, respectively. Our success in precise control of material properties via structural fusion of five-membered carbon linkages and six-membered silicon-oxygen connection at the central electron-donating core unit of fused-ring electron acceptors can attract considerable attention and bring new vigor and vitality for developing new materials toward more efficient OSCs.

A Multifluorination Strategy Toward Wide Bandgap Polymers for Highly Efficient Organic Solar Cells

Creating new electron-deficient unit is highly demanded to develop high-performance polymer donors for non-fullerene organic solar cells (OSCs). Herein, we reported a multifluorinated unit 4,5,6,7-tetrafluoronaphtho[2,1-b : 3,4-b']dithio-phene (FNT) and its polymers PFNT-F and PFNT-Cl. The advantages of multifluorination: (1) it enables the polymers to exhibit low-lying HOMO (≈-5.5 eV) and wide band gap (≈2.0 eV); (2) the short interactions (F⋅⋅⋅H, F⋅⋅⋅F) endow the polymers with properties of high film crystallinity and efficient hole transport; (3) well miscibility with NFAs that leads to a more well-defined nanofibrous morphology and face-on orientation in the blend films. Therefore, the PFNT-F/Cl : N3 based OSCs exhibit impressive FF values of 0.80, and remarkable PCEs of 17.53 % and 18.10 %, which make them ranked the best donor materials in OSCs. This work offers new insights into the rational design of high-performance polymers by multifluorination strategy.

Improved Molecular Ordering in a Ternary Blend Enables All-Polymer Solar Cells over 18% Efficiency

Although all-polymer solar cells (all-PSCs) show great commercialization prospects, their power conversion efficiencies (PCEs) still fall behind their small molecule acceptor-based counterparts. In all-polymer blends, the optimized morphology and high molecular ordering are difficult to achieve since there is troublesome competition between the crystallinity of the polymer donor and acceptor during the film-formation process. Therefore, it is challenging to improve the performance of all-PSCs. Herein, a ternary strategy is adopted to modulate the morphology and the molecular crystallinity of an all-polymer blend, in which PM6:PY-82 is selected as the host blend and PY-DT is employed as a guest component. Benefiting from the favorable miscibility of the two acceptors and the higher regularity of PY-DT, the ternary matrix features a well-defined fibrillar morphology and improved molecular ordering. Consequently, the champion PM6:PY-82:PY-DT device produces a record-high PCE of 18.03%, with simultaneously improved open-circuit voltage, short-circuit current and fill factor in comparison with the binary devices. High-performance large-area (1 cm² ) and thick-film (300 nm) all-PSCs are also successfully fabricated with PCEs of 16.35% and 15.70%, respectively.Moreover, 16.5 cm² organic solar module affords an encouraging PCE of 13.84% when using the non-halogenated solvent , showing the great potential of "Lab-to-Fab" transition of all-PSCs.

Unsymmetrically Chlorinated Non-Fused Electron Acceptor Leads to High-Efficiency and Stable Organic Solar Cells

Searching the cost-effective organic semiconductors is strongly needed in order to facilitate the practice of organic solar cells (OSCs), yet to be fulfilled. Herein, we have succeeded in developing two non-fused ring electron acceptors (NFREAs), leading to the highest efficiency of 16.2 % for the NFREA derived OSCs. These OSCs exhibit the superior operational stabilities under one sun equivalent illumination without ultraviolet (UV) filtration. It is revealed that the modulation of halogen substituents on aromatic side chains, as the new structural tool to tune the intermolecular interaction and optoelectronic properties of acceptors, not only promotes the interlocked tic-tac-toe frame of three-dimensional stacks in solid, but also improves charge dynamics of acceptors to enable high-performance and stable OSCs.