Over 19% Efficiency Organic Solar Cells by Regulating Multidimensional Intermolecular Interactions

Indacenodithiophene-Based Medium-Bandgap Guest Acceptor Enables High-Efficiency Ternary Organic Solar Cells

The ternary organic solar cells (OSCs) have been proven to be an effective strategy for achieving high power conversion efficiency (PCE), exhibiting substantial potential for continuous enhancement of device performance. In this work, a novel nonfullerene acceptor, IDT-FN, is developed utilizing a renowned indacenodithiophene (IDT) core and moderately intense electron-withdrawing terminal groups, serving as the third component in ternary OSCs. IDT-FN demonstrates excellent complementary light absorption and cascaded energy levels with the host materials D18 and CH-6F, resulting in enhanced photon harvesting and charge transport within the ternary blend. Therefore, even the as-cast ternary device manages to surpass the optimal binary host device, achieving a superior PCE of 17.34% compared to the latter's 17.08%. Through optimization, the optimal ternary devices attain an impressive PCE of 18.32%, accompanied by a high open-circuit voltage (Voc) of 0.897 V, a fill factor of 0.745, and a short-circuit current density (Jsc) of 27.41 mA cm-2. This demonstrates a significant success in utilizing IDT-based medium-bandgap guests to achieve state-of-the-art ternary OSCs.

Dominant Face-On Oriented Perylene-Diimide Interlayers for High-Performance Organic Solar Cells

Electron transport properties of cathode interlayers are crucial to high-performance organic solar cells (OSCs). We propose a novel approach to enhance electron transport of cathode interlayers through controlling a preferential face-on molecular orientation of non-ionic perylene-diimide- (PDI) based cathode interlayers with restricted n-doping effects. 1-(2,5,8-trioxadec-10-yl)-1,2,3-triazole (TOT) units as bulky and extended side chains were incorporated into brominated-PDIs via click chemistry to yield PDIBr-TOT. TOT side chains impart PDI-based interlayers with a dominant face-on orientation, meanwhile leading to a negligible doping effect due to their weak electron-donating properties. Impressively, at a slight doping level, higher electron mobility is gained through efficient vertical charge transport channels built by preferred face-on molecular orientations of PDIBr-TOT, beating the results acquired through strong doping effects of traditional PDIBr-N with an edge-on orientation. Thus, PDIBr-TOT can suppress exciton recombination and lower the surface energies for good contact with active layers, consequently leading to increases in fill factor and short-circuit current. Integrating PDIBr-TOT with various active layers, a remarkable efficiency of 19.52 % is obtained. Moreover, device stability is enhanced by restrained doping effects. Modulating face-on orientations of cathode interlayers prescribed here will encourage further innovative designs of high-performance cathode interlayers towards OSC advances.

Balanced Crystallization Enhances Morphology and Efficiency in Binary Organic Solar Cells

Achieving high-performance organic solar cells (OSCs) relies heavily on precise morphology optimization, a challenging task due to the intrinsic differences in crystallization kinetics and interfacial compatibility between polymer donors and small-molecule acceptors. In this work, 2,7-dibromonaphthalene (DBN) is introduced as an innovative solid additive that uniquely regulates crystallization in both donor and acceptor phases within the PM6:Y6 system. Unlike conventional liquid additives, which often induce excessive Y6 crystallization, DBN achieves a balanced crystallization, enhancing molecular order in PM6 while mitigating over-aggregation in Y6. This dual-phase effect improves light absorption, exciton generation and dissociation, charge transport, and reduces recombination losses. As a result, OSCs treated with DBN achieved a remarkable power conversion efficiency (PCE) of 18.5%, with an open-circuit voltage (VOC) of 0.848 V, a high short-circuit current density (JSC) of 28.15 mA cm-2, and an enhanced fill factor (FF) of 77.7%. Adding an anti-reflection MgF2 layer further boosts efficiency to 19.0%, setting a new benchmark for PM6:Y6 binary devices. This study establishes DBN as a promising crystallization regulator and presents a robust strategy for morphology control, advancing the development of high-performance photovoltaic applications.

Tuning Active Layer Morphology via Ternary Copolymerization with an Asymmetric Benzothiazole Unit as the Third Component to Enhance Photovoltaic Performance

Controlling the morphology and phase separation of the active layer is of great significance for the development of efficient organic solar cells. This study employs ternary copolymerization to optimize the phase separation and surface morphology of the active layer. Three terpolymers (PMz-5, PMz-10, and PMz-20) are synthesized by incorporating an asymmetric benzo[d]thiazole (BTz) unit as the third component to suppress the strong aggregation of the PM6:L8-BO blend film. Compared to the PM6 blend film, terpolymer blend films exhibit improved surface morphology, an appropriate phase separation scale, and, thus, an enhanced device performance. Furthermore, the incorporation of a BTz unit can lower the highest occupied molecular orbital levels of three terpolymers, which is conducive to charge transport. The champion device PMz-5:L8-BO system achieves an efficiency of 16.57%, featuring a high open-circuit voltage of 0.886 V, a high short-circuit current density of 25.49 mA cm-2, and a remarkable fill factor of 73.35%. This work provides an efficient and straightforward ternary copolymerization strategy that can be used to enhance the device performance.

Effects of Self-Assembled Polymer-Based Hole Transport Monolayer on Organic Photovoltaics

In this study, the first attempt is made to implement conjugated polymer-based self-assembled monolayer (SAM), poly[3-(6-carboxyhexyl) thiophene-2,5-diyl] (P3HT-COOH), is implemented as the hole transport layer (HTL) in fabricatiing organic photovoltaics (OPVs). The scanning tunneling microscopy (STM) results show that those P3HT-COOH molecules with periodic carboxylic acid anchoring groups pack periodically on the indium tin oxide (ITO) surface and form a monolayer. Further, this monolayer is smooth and dense with a polar feature that minimizes defects, forms an excellent interface with the photoactive layer, and tunes the work function of ITO beneficial for hole extraction. OPVs with this P3HT-COOH polar monolayer as HTL exhibit an improved exciton dissociation rate, enhanced polymer crystallinity of the photoactive layer with increased hole mobility for more balanced charge transport, reduced trap state density, and weaker bimolecular recombination with larger recombination resistance. The improved charge transport properties lead to a ≈9% increment in power conversion efficiency (PCE) of OPVs relative to those using well-known PEDOT:PSS as HTLs. Additionally, the hydrophobic feature of P3HT-COOH SAM stabilizes the OPVs with residual PCE of ≈80% even after 5.3 months. The proposed approach is very useful and timely and provides a key step for developing optoelectronic devices with high-efficiency, high stability, and cost-effective production.

Multilength-Scale Morphological Engineering for Stable Organic Solar Cells

Organic solar cells (OSCs) have garnered significant attention owing to the light weight, flexibility, and low cost. Continuous improvement in molecular design, morphology control, and device fabrication has propelled the power conversion efficiency of OSCs beyond 20%. While obtaining long-term device stability is still a critical obstacle for the commercialization of OSCs. The nano- and microstructural characteristics of the active layer morphology-including molecular stacking, phase separation, and domain sizes-play a pivotal role in determining device performance. Consequently, a comprehensive understanding of how film structure impacting device stability and the methods to control film morphology are vital for improving device lifetime. This review seeks to elucidate the structure-performance relationship between active layer morphology from the nanoscale to microscale and device stability. It can provide rational guidance to enhance device stability from morphology control, accelerating the commercialization of OSCs.

Understanding the Effect of Regioselective Fluorination in Tuning Miscibility and Crystallinity of Polymer Donors for Efficient Organic Solar Cells

Fluorination is a widely adopted strategy for modifying polymer donors (PDs) in organic solar cells (OSCs). The incorporation of fluorine atoms is known to enhance crystallinity and facilitate charge transport of PDs, thereby boosting the short-circuit current density  of related OSCs. However, improperly executed fluorination can impair miscibility with acceptors, leading to excessive self-aggregation, unfavorable phase separation, serious charge reorganization, and increased energy loss. It is crucial to balance the crystallinity and miscibility of PDs for efficient OSCs. In this study, four PDs, SL1 to SL4, are designed with varying degrees of fluorination by precisely controlling the fluorination sites within D-A conjugate polymers. Notably, SL2, fluorinated on the A unit, exhibits optimal aggregation behavior and crystallinity while maintaining good miscibility with acceptors. Consequently, SL2 achieved reduced energy loss and delivered an impressive power conversion efficiency of 15.35% in binary devices. Furthermore, D18:Y6:SL2 based ternary device achieved a remarkable PCE of over 19%. This work offers new insights into the fluorination of PDs and a strategy for synergistic enhancement in crystallinity and miscibility for OSCs.

Morphology Regulation Is Achieved by Volatile Solid Additives in Halogen-Free Solvents to Fabricate Efficient Polymer Solar Cells

The meticulous control of micromorphology in high power conversion efficiency (PCE) of polymer solar cells (PSCs) typically relies on halogenated solvents, which pose serious threats to both environmental sustainability and human health. In this work, a green and efficient method for fabricating high PCE PSCs with halogen-free solvents is developed. By introducing volatile solid additives 1-bromo-2,6-dichlorobenzene (DIB) and 1-bromo-2,3,5-trichlorobenzene (TIB) into toluene solvents, the aggregation behaviors of PM6:L8-BO were meticulously regulated, forming distinct fibrous morphology; in detail, the micromorphology of vertical direction exhibited a distinct pattern of acceptor enrichment at the top and donor enrichment at the bottom, which leads to enhanced exciton dissociation efficiency, improved charge transport performance, significantly reducing charge recombination, and finally improved PCEs, as the maximum PCEs were 18.56 and 17.67%, respectively, which are notably higher than those of devices without additives. Furthermore, since the solid additives can be completely removed from the active layer, the additive-treated devices exhibit superior morphology and photovoltaic stability. This work, therefore, unveils a straightforward and environmentally friendly method for preparing efficient PSCs, which is instrumental in facilitating the large-scale commercialization of PSC technology.

Rational Design Dithieno[3,2-f:2',3'-h]Quinoxaline-Based Polymers with Optimized Molecular Configuration and Charge Transfer for High-Performance All-Polymer Solar Cells

Due to poor planarity and weak intermolecular interactions of acceptor units which have two-dimensional configuration, the development of related polymers is slow. In this work, we synthesized a dithieno[3,2-f:2',3'-h]quinoxaline-based unit substituted with fluorinated thiophene, which can effectively expand the area of the acceptor unit. Then, it paired with three different benzodithiophene-based donor units, and three new polymers of PQTF-BT, PQTF-BTCl and PQTF-DTCl were obtained. PQTF-BT with thiophene side chain exhibits appropriate aggregation and crystallization performance, and the PQTF-BT:PY-IT all-polymer solar cells exhibit high device efficiency of 15.54 %. Meanwhile, PQTF-BTCl with chlorination strategy on thiophene side chain further enhances the open-circuit voltage of related devices. However, it has negative impact on the miscibility and charge transfer of the material, leading to a poor efficiency of 8.71 % in the PQTF-BTCl:PY-IT device. To reinforce miscibility, PQTF-DTCl with expanded backbone planarity was further studied. However, the big dihedral angles of the donor side chains disrupted the effective stacking of the molecules, resulting in great recovery of device performance to 13.42 %. This work provides research ideas for the design of polymers with two-dimensional configuration side chain and match selection of donor units.

Non-fullerene acceptors with high crystallinity and photoluminescence quantum yield enable >20% efficiency organic solar cells

The rational design of non-fullerene acceptors (NFAs) with both high crystallinity and photoluminescence quantum yield (PLQY) is of crucial importance for achieving high-efficiency and low-energy-loss organic solar cells (OSCs). However, increasing the crystallinity of an NFA tends to decrease its PLQY, which results in a high non-radiative energy loss in OSCs. Here we demonstrate that the crystallinity and PLQY of NFAs can be fine-tuned by asymmetrically adapting the branching position of alkyl chains on the thiophene unit of the L8-BO acceptor. It was found that L8-BO-C4, with 2-butyloctyl on one side and 4-butyldecyl on the other side, can simultaneously achieve high crystallinity and PLQY. A high efficiency of 20.42% (certified as 20.1%) with an open-circuit voltage of 0.894 V and a fill factor of 81.6% is achieved for the single-junction OSC. This work reveals how important the strategy of shifting the alkyl chain branching position is in developing high-performance NFAs for efficient OSCs.

Influence of alkyl chain length on solar cell performance of molecular organic semiconductors: a review

Structural fine-tuning of organic semiconductors by side-chain modifications is indeed an important factor in enhancing the efficiency of organic solar cells (OSC). In this review, we recapitulate the recent efforts on side chain engineering of organic small molecules and their effect on molecular packing, crystallinity, charge transport, and solar cell properties. The influence of the length and branching position of side-chains on device performance are comprehensively discussed. The challenges and prospects of structural manipulations at the molecular level are discussed, with a focus on their relevance to the future technology of OSCs.

Polymer-like tetramer acceptor enables stable and 19.75% efficiency binary organic solar cells

Limited by large batch differences and inferior polymerization degree of current polymer acceptors, the potential high efficiency and stability advantages of all-polymer solar cells (all-PSCs) cannot be fully utilized. Alternatively, largely π-extended and structurally definite oligomer acceptors are effective strategies to realize the overall performance of polymer acceptors. Herein, we report a linear tetramer acceptor namely 4Y-BO with identical molecular skeleton and comparable molecular-weight relative to the control polymer acceptor PY-BO. The structurally definite tetramer shows refined film-forming kinetics and improved molecular ordering, offering uniform crystallinity with polymer donor and hence well-defined fibrous heterojunction textures. Encouragingly, the PM6:4Y-BO devices achieve an efficiency up to 19.75% (certified efficiency:19.58%), largely surpassing that of the control PM6:PY-BO device (15.66%) and ranks the highest among solar cells based on oligomer and polymer acceptors. More noticeably, thermal stability, photostability and mechanical flexibility are collectively enhanced for PM6:4Y-BO devices. Our study provides an important approach for fabricating high performance and stable organic photovoltaics.

Side Chain Phenyl Isomerization-Induced Spatial Conjugation for Achieving Efficient Near-Infrared II Phototheranostic Agents

The contradiction of near-infrared II (NIR-II) emission and photothermal effects limits the development of phototheranostic agents (PTAs) in many emerging cutting-edge applications. Organic aggregates present a promising opportunity for the balance of competitive relaxation processes through the manipulation of molecular structure and packing. Herein, side chain phenyl isomerization-induced spatial conjugation was proposed for constructing A-D-A type NIR-II PTAs with simultaneous enhancement of fluorescence brightness and photothermal properties. Three pairs of mutually isomeric fluorophores, whose phenyls respectively located at the outside (o-series) and inside (i-series) of the side chain, were designed and synthesized. The positional isomerization of the phenyl endows the o-series crystals with strong spatial conjugation between the phenyl group on the side chain and the backbone, as well as interlocked planar network, which is different to that observed in the i-series. Thus, all o-series nanoparticles (NPs) exhibit red-shifted absorption, enhanced NIR-II emission, and superior photothermal properties than their i-series counterparts. A prominent member of the o-series, o-ITNP NPs, demonstrated efficacy in facilitating NIR-II angiography, tumor localization, and NIR-II imaging-guided tumor photothermal therapy. The success of this side chain phenyl isomerization strategy paves the way for precise control of the aggregation behavior and for further development of efficient NIR-II PTAs.

Donor-Interacting Arylated Carbazole Self-Assembled Monolayer Enables Highly Efficient and Stable Organic Photovoltaics

Carbazole-derived self-assembled monolayers (SAMs) are promising materials for hole-extraction layer (HEL) in conventional organic photovoltaics (OPVs). Here, a SAM Cbz-2Ph derived from 3,6-diphenylcarbazole is demonstrated. The large molecular dipole moment of Cbz-2Ph allows the modulation of electrode work function to facilitate hole extraction and maximize photovoltage, thus improving the OPV performance. Additionally, the flanking aryls of Cbz-2Ph help establish CH-π interactions for forming a dense and well-organized SAM HEL and exhibit stronger van der Waals interactions with the donor PM6 than acceptor BTP-eC9. The stronger SAM-donor interactions modulate the PM6 distribution in PM6:BTP-eC9 bulk-heterojunction film, leading to PM6 enrichment near HEL to facilitate efficient hole extraction to the ITO anode in conventional p-i-n OPVs. Consequently, binary PM6:BTP-eC9-based devices incorporating the Cbz-2Ph HEL demonstrate an impressive efficiency of 19.18%. These cells also showcase excellent operational stability, with a T80 lifetime of ≈1260 h at the maximum power point, over 10 times longer than those using the traditional PEDOT:PSS HEL (T80 ≈96 h). Furthermore, the universal applicability of Cbz-2Ph as a HEL is evident through its successful implementation in PM6:BTP-eC9:L8-BO-F-based ternary devices and PM6:BTP-eC9-based printed OPV devices, achieving a PCE of 19.30% and 16.96%, respectively.

New Polymeric Acceptors Based on Benzo[1,2-b:4,5-b'] Difuran Moiety for Efficient All-Polymer Solar Cells

In order to realize high-performance bulk-heterojunction (BHJ) all-polymer solar cells, achieving appropriate aggregation and moderate miscibility of the polymer blends is one critical factor. Herein, this study designs and synthesizes two new polymer acceptors (PAs), namely PYF and PYF-Cl, containing benzo[1,2-b:4,5-b'] difuran (BDF) moiety with/without chlorine atoms on the thiophene side groups. Thanks to the preferred planar structure and high electronegativity of the BDF units, the resultant PAs generate strong intermolecular interactions and π-π stacking in both the neat and blend films. At the same time, the BDF moieties flanked with bulky 2D side groups increase intermolecular space and restrain the excessive entanglement of polymer chains for developed heterojunction miscibility attributes. Consequently, appropriate polymer crystallinity and moderate miscibility between PAs and polymer donor (PD) contribute to harmonious blending morphologies. Eventually, the PM6:PYF-based solar cells achieve an optimal efficiency of 11.82%. More encouragingly, the PYF serves as an efficient guest acceptor and realizes an improved efficiency of 17.05% in the PM6:PYIT:PYF ternary systems, which is much higher than that of the host system (15.87%). This study highlights the importance of BDF moiety in designing new PAs for high-performance all-polymer solar cells.

Integrated Omnidirectional Design of Non-Volatile Solid Additive Enables Binary Organic Solar Cells with Efficiency Exceeding 19.5

Solid additives have drawn great attention due to their numerous appealing benefits in enhancing the power conversion efficiencies (PCEs) of organic solar cells (OSCs). To date, various strategies have been reported for the selection or design of non-volatile solid additives. However, the lack of a general design/evaluation principles for developing non-volatile solid additives often results in individual solid additives offering only one or two efficiency-boosting attributes. In this work, we propose an integrated omnidirectional strategy for designing non-volatile solid additives. By validating the method on the 4,5,9,10-pyrene diimide (PyDI) system, a novel non-volatile solid additive named PyMC5 was designed. PyMC5 is capable of enhancing device performance by establishing synergistic dual charge transfer channels, forming appropriate interactions with active layer materials, reducing non-radiative voltage loss and optimizing film morphology. Notably, the binary device (PM6 : L8-BO) treated by PyMC5 achieved a PCE over 19.5 %, ranking among the highest reported to date. In addition, the integration of PyMC5 mitigated the degradation process of the devices under photo- and thermal-stress conditions. This work demonstrates an efficient integrated omnidirectional approach for designing non-volatile solid additives, offering a promising avenue for further advancements in OSC development.

Organic Solar Cells Based on Non-Fullerene Low Molecular Weight Organic Semiconductor Molecules

The development of narrow bandgap A-D-A- and ADA'DA-type non-fullerene small molecule acceptors (NFSMAs) along with small molecule donors (SMDs) have led to significant progress in all-small molecule organic solar cells. Remarkable power conversion efficiencies, nearing the range of 17-18 %, have been realized. These efficiency values are on par with those achieved in OSCs based on polymeric donors. The commercial application of organic photovoltaic technology requires the design of more efficient organic conjugated small molecule donors and acceptors. In recent years the precise tuning of optoelectronic properties in small molecule donors and acceptors has attracted considerable attention and has contributed greatly to the advancement of all-SM-OSCs. Several reviews have been published in this field, but the focus of this review concerns the advances in research on OSCs using SMDs and NFSMAs from 2018 to the present. The review covers the progress made in binary and ternary OSCs, the effects of solid additives on the performance of all-SM-OSCs, and the recently developed layer-by-layer deposition method for these OSCs. Finally, we present our perspectives and a concise outlook on further advances in all-SM-OSCs for their commercial application.

Non-fused Nonfullerene Acceptors with an Asymmetric Benzo[1,2-b:3,4-b', 6,5-b"]trithiophene (BTT) Donor Core and Different AcceptorTerminal Units for Organic Solar Cells

Here in, we have designed two new unfused non-fullerene small molecules using asymmetric benzo[1,2-b:3.4-b', 6,5-b"]trithiophene (BTT) as the central donor core and different terminal units i. e., 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (NFA-4) and 1,3-diethyl-2-thioxodi hydropyrimidine-4,6 (1H,5H)-dione (NFA-5) and examined their optical and electrochemical properties. Using a wide band-gap copolymer D18, organic solar cells (OSCs) based on bulk heterojunction of D18:NFA-4 and D18:NFA-5 showed overall power conversion efficiency (PCE) of about 17.07 % and 11.27 %, respectively. The increased PCE for the NFA-4-based OSC, compared to NFA-5 counterpart, is due higher value of short circuit current (JSC), open circuit voltage (VOC), and fill factor (FF). Following the addition of small amount of NFA-5 to the binary bulk heterojunction D18:NFA-4, the ternary organic solar cells attained a PCE of 18.05 %, surpassing that of the binary counterparts due to the higher values of which is higher than that for the binary counterparts and attributed to the increased values of JSC, FF, and VOC. The higher value of JSC is linked to the efficient use to excitons transferred from NFA-5 to NFA-4 with a greated dipole moment than NFA-5 and subsequently dissociated into a free charge carrier efficiently.

Regiospecific Incorporation of Fluorine Atoms in Polythiophene Derivatives for Efficient Organic Solar Cells

Derivatives of polythiophene (PT) have garnered considerable attention in organic solar cells (OSCs) because of their relatively uncomplicated molecular structures and cost-effective synthesis. Herein, we have developed two regioisomeric fluorinated PT donors, PEI3T-FITVT and PEI3T-FOTVT, to realize efficient OSCs. PEI3T-FITVT and PEI3T-FOTVT are strategically designed with different fluorine atom arrangements on thiophene-vinyl-thiophene (TVT) units. Notably, PEI3T-FOTVT possesses enhanced backbone planarity induced by F···S noncovalent interactions between two constituent building blocks. Consequently, PEI3T-FOTVT with the higher aggregation and crystalline properties leads to a 2.5-fold increase in hole mobility over PEI3T-FITVT (from 1.4 × 10-4 to 3.6 × 10-4 cm2 V-1 s-1). Furthermore, PEI3T-FOTVT exhibits higher domain purity than PEI3T-FITVT, leading to faster charge transport and reduced charge recombination in OSC devices. These characteristics lead to a higher power conversion efficiency of 14.4% for PEI3T-FOTVT-based OSCs, compared to 12.9% for PEI3T-FITVT-based OSCs.

Non-Fused π-Extension of Endcaps of Small Molecular Acceptors Enabling High-Performance Organic Solar Cells

The modular structure of small molecular acceptors (SMAs) allows for versatile modifications of the materials and boosts the photovoltaic efficiencies of organic solar cells (OSCs) in recent years. As a critical component, the endcaps of SMAs have been intensively investigated and modified to control the molecular aggregation and photo-electronic conversion. However, most of the studies focus on halogenation or π-fusion extension of the endcap moieties, but overlook the non-fused π-extension approach, which could be a promising strategy to balance the self-aggregation and compatibility behaviors. Herein, we reported two new acceptors namely BTP-Th and BTP-FTh based on non-fused π-extension of the endcap by chlorinated-thiophene, of which the latter molecule has better co-planarity and crystallinity because of the intramolecular noncovalent interactions. Paired with donor PBDB-T, the optimal device of BTP-FTh reveals a greater efficiency of 14.81 % that that of BTP-Th (13.91 %). Nevertheless, the BTP-Th based device realizes a lower energy loss, enabling BTP-Th as a good candidate to serve as guest acceptor. As a result, the ternary solar cells of PM6 : BTP-eC9 : BTP-Th output a champion efficiency up to 18.71 % with enhanced open-circuit voltage. This study highlights the significance of rational decoration of endcaps for the design of high-performance SMAs and photovoltaic cells.

Gradual Optimization of Molecular Aggregation and Stacking Enables Over 19% Efficiency in Binary Organic Solar Cells

Volatile solid additive is an effective and simple strategy for morphology control in organic solar cells (OSCs). The development of environmentally friendly new additives which can also be easily removed without high-temperature thermal annealing treatment is currently a trend, and the working mechanism needs to be further studied. Herein, a highly volatile and non-halogenated solid additive 1-benzothiophene (BBT) is reported to regulate molecular aggregation and stacking of active layer components. According to the film-forming kinetics process, a momentary intermediate phase is formed during spin-coating, which slows down the film-forming process and leads to more ordered molecular stacking in the solid film after introducing solid additive BBT. Subsequently, after solvent vapor annealing (SVA) further treatment, the resultant blend films exhibit a tighter and more ordered molecular stacking. Consequently, the synergistic effect of solid additive BBT and SVA treatment can effectively control morphology of active layer and improve carrier transport characteristics, thereby enhancing the performance of OSCs. Finally, in D18-Cl:N3 system, an impressive power conversion efficiency of 19.53% is achieved. The work demonstrates that the combination of highly volatile solid additives and SVA treatment is an effective morphology control strategy, guiding the development of efficient OSCs.

Loosely Bounded Exciton with Enhanced Delocalization Capability Boosting Efficiency of Organic Solar Cells

In organic solar cells (OSCs), electron acceptors have undergone multiple updates, from the initial fullerene derivatives, to the later acceptor-donor-acceptor type non-fullerene acceptors (NFAs), and now to Y-series NFAs, based on which efficiencies have reached over 19%. However, the key property responsible for further improved efficiency from molecular structure design is remained unclear. Herein, the material properties are comprehensively scanned by selecting PC71BM, IT-4F, and L8-BO as the representatives for different development stages of acceptors. For comparison, asymmetric acceptor of BTP-H5 with desired loosely bounded excitons is designed and synthesized. It's identified that the reduction of intrinsically exciton binding energy (Eb) and the enhancement of exciton delocalization capability act as the key roles in boosting the performance. Notably, 100 meV reduction in Eb has been observed from PC71BM to BTP-H5, correspondingly, electron-hole pair distance of BTP-H5 is almost two times over PC71BM. As a result, efficiency is improved from 40% of S-Q limit for PC71BM-based OSC to 60% for BTP-H5-based one, which achieves an efficiency of 19.07%, among the highest values for binary OSCs. This work reveals the confirmed function of exciton delocalization capability quantitatively in pushing the efficiency of OSCs, thus providing an enlightenment for future molecular design.

Approaching 20% Efficiency in Ortho-Xylene Processed Organic Solar Cells by a Benzo[a]phenazine-Core-Based 3D Network Acceptor with Large Electronic Coupling and Long Exciton Diffusion Length

High-performance organic solar cells often rely on halogen-containing solvents, which restrict the photovoltaic industry. Therefore, it is imperative to develop efficient organic photovoltaic materials compatible with halogen-free solvents. Herein, a series of benzo[a]phenazine (BP)-core-based small-molecule acceptors (SMAs) achieved through an isomerization chlorination strategy is presented, comprising unchlorinated NA1, 10-chlorine substituted NA2, 8-chlorine substituted NA3, and 7-chlorine substituted NA4. Theoretical simulations highlight NA3's superior orbit overlap length and tight molecular packing, attributed to interactions between the end group and BP unit. Furthermore, NA3 demonstrates dense 3D network structures and a record electronic coupling of 104.5 meV. These characteristics empower the ortho-xylene (o-XY) processed PM6:NA3 device with superior power conversion efficiency (PCE) of 18.94%, surpassing PM6:NA1 (15.34%), PM6:NA2 (7.18%), and PM6:NA4 (16.02%). Notably, the significantly lower PCE in the PM6:NA2 device is attributed to excessive self-aggregation characteristics of NA2 in o-XY. Importantly, the incorporation of D18-Cl into the PM6:NA3 binary blend enhances crystallographic ordering and increases the exciton diffusion length of the donor phase, resulting in a ternary device efficiency of 19.75% (certified as 19.39%). These findings underscore the significance of incorporating new electron-deficient units in the design of efficient SMAs tailored for environmentally benign solvent processing of OSCs.

Fully Fused Indacenodithiophene-Centered Small-Molecule n-Type Semiconductors for High-Performance Organic Electronics

Developing novel n-type organic semiconductors is an on-going research endeavour, given their pivotal roles in organic electronics and their relative scarcity compared to p-type counterparts. In this study, a new strategy was employed to synthesize n-type organic semiconductors featuring a fully fused conjugated backbone. By attaching two sets of adjacent amino and formyl groups to the indacenodithiophene-based central cores and triggering a tandem reaction sequence of a Knoevenagel condensation-intramolecular cyclization, DFA1 and DFA2 were realized. The solution-processed organic field effect transistors based on DFA1 exhibited unipolar n-type transport character with a decent electron mobility of ca. 0.10 cm2 V-1 s-1 (ca. 0.038 cm2 V-1 s-1 for DFA2 based devices). When employing DFA1 as a third component in organic solar cells, a high power conversion efficiency of 19.2 % can be achieved in ternary devices fabricated with PM6 : L8-BO : DFA1. This work provides a new pathway in the molecular engineering of n-type organic semiconductors, propelling relevant research forward.

A theoretical investigation for improving the performance of non-fullerene organic solar cells through side-chain engineering of BTR non-fused-ring electron acceptors

In the current quantum chemical study, indacenodithiophene donor core-based the end-capped alterations of the reference chromophore BTR drafted eight A2-A1-D-A1-A2 type small non-fullerene acceptors. All the computational simulations were executed under MPW1PW91/6-31G (d, p) level of DFT. The UV-Vis absorption, open circuit voltage, electron affinity, ionization potential, the density of states, reorganization energy, orbital analysis, and non-covalent interactions were studied and compared with BTR. Several molecules of our modeled series BT1-BT8 have shown distinctive features that are better than those of the BTR. The open circuit voltage (VOC) of BT5 has a favorable impact, allowing it to replace BTR in the field of organic solar cells. The charge carrier motilities for proposed molecules generated extraordinary findings when matched to the reference one (BTR). Further charge transmission was confirmed by creating the complex with a PM6 donor molecule. The remarkable dipole moment contributes to the formation of non-covalent bond interactions with chloroform, resulting in superior charge mobility. Based on these findings, it can be said that every tailored molecule has the potential to surpass chromophore molecule (BTR) in OSCs. So, all tailored molecules may enhance the efficiency of photovoltaic cells due to the involvement of potent terminal electron-capturing acceptor2 moieties. Considering these obtained results, these newly presented molecules can be regarded for developing efficient solar devices in the future.

Manipulating Alkyl Inner Side Chain of Acceptor for Efficient As-Cast Organic Solar Cells

As-cast organic solar cells (OSCs) possess tremendous potential for low-cost commercial applications. Herein, five small-molecule acceptors (A1-A5) are designed and synthesized by selectively and elaborately extending the alkyl inner side chain flanking on the pyrrole motif to prepare efficient as-cast devices. As the extension of the alkyl chain, the absorption spectra of the films are gradually blue-shifted from A1 to A5 along with slightly uplifted lowest unoccupied molecular orbital energy levels, which is conducive for optimizing the trade-off between short-circuit current density and open-circuit voltage of the devices. Moreover, a longer alkyl chain improves compatibility between the acceptor and donor. The in situ technique clarifies that good compatibility will prolong molecular assembly time and assist in the preferential formation of the donor phase, where the acceptor precipitates in the framework formed by the donor. The corresponding film-formation dynamics facilitate the realization of favorable film morphology with a suitable fibrillar structure, molecular stacking, and vertical phase separation, resulting in an incremental fill factor from A1 to A5-based devices. Consequently, the A3-based as-cast OSCs achieve a top-ranked efficiency of 18.29%. This work proposes an ingenious strategy to manipulate intermolecular interactions and control the film-formation process for constructing high-performance as-cast devices.

Inner/Outer Side Chain Engineering of Non-Fullerene Acceptors for Efficient Large-Area Organic Solar Modules Based on Non-Halogenated Solution Processing in Air

Achieving efficient and large-area organic solar modules via non-halogenated solution processing is vital for the commercialization yet challenging. The primary hurdle is the conservation of the ideal film-formation kinetics and bulk-heterojunction (BHJ) morphology of large-area organic solar cells (OSCs). A cutting-edge non-fullerene acceptor (NFA), Y6, shows efficient power conversion efficiencies (PCEs) when processed with toxic halogenated solvents, but exhibits poor solubility in non-halogenated solvents, resulting in suboptimal morphology. Therefore, in this study, the impact of modifying the inner and outer side-chains of Y6 on OSC performance is investigated. The study reveals that blending a polymer donor, PM6, with one of the modified NFAs, namely N-HD, achieved an impressive PCE of 18.3% on a small-area OSC. This modified NFA displays improved solubility in o-xylene at room temperature, which facilitated the formation of a favorable BHJ morphology. A large-area (55 cm2) sub-module delivered an impressive PCE of 12.2% based on N-HD using o-xylene under ambient conditions. These findings underscore the significant impact of the modified Y6 derivatives on structural arrangements and film processing over a large-area module at room temperature. Consequently, these results are poised to deepen the comprehension of the scaling challenges encountered in OSCs and may contribute to their commercialization.

MolPackL: Quantification and Interpretation of Intermolecular Interactions Driven by Molecular Packing

In organic optoelectronic devices, the properties of the aggregated organic materials depend not only on individual molecules or monomers but also significantly on their packing modes. Different from their inorganic counterparts linked by explicit covalent bonds, organic solids exhibit intricate and numerous intermolecular interactions (IMIs). Due to the intrinsic complexity and disorder of IMIs, identifying and understanding them is a formidable challenge in experimental, theoretical, and data-driven approaches. In this work, we constructed an innovative algorithm framework, Molecular Packing Learning (MolPackL), which can accurately quantify elusive IMIs using contact density histograms (CDHs) and efficiently extract intermolecular features for further property prediction of organic solids. It performs satisfactorily in training predictive models of IMI-related properties in molecular crystals. Particularly, the band gap predictive model based on MolPackL achieved the best-reported performance, with an MAE of 0.20 eV and an impressive R2 of 0.92. Class activation mapping (CAM) visually demonstrates MolPackL's accurate identification of effective interaction sites as the molecular packing changes. What is more, the elemental importance analysis verified that the superior score benefits from MolPackL's ability to comprehensively consider multiple influencing factors of IMIs. In summary, MolPackL provides a new framework for quantitative assessment and understanding of the effect of IMIs. The development of MolPackL marks a significant advancement in establishing predictive models of molecular aggregates, deepening the comprehension of IMIs on the material properties. Given the superior performance, we believe that MolPackL will also become a powerful tool in the design of high-performance organic optoelectronic materials.

Efficient Ternary Organic Solar Cells with Suppressed Nonradiative Recombination and Fine-Tuned Morphology via IT-4F as Guest Acceptor

The large open circuit voltage (VOC) loss is currently one of the main obstacles to achieving efficient organic solar cells (OSCs). In this study, the ternary OSCs comprising PM6:BTP-eC9:IT-4F demonstrate a superior efficiency of 18.2 %. Notably, the utilization of the medium bandgap acceptor IT-4F as the third component results in an exceptionally low nonradiative recombination energy loss of 0.28 V. The desirable energy level cascade is formed among PM6, BTP-eC9, and IT-4F due to the low-lying HOMO and LUMO energy levels of IT-4F. More importantly, the VOC of PM6:BTP-eC9:IT-4F OSCs can reach as high as 0.86 V, which is higher than both binary OSCs without sacrificing JSC and FF. Besides, this strategy proved that IT-4F can not only broaden the absorption range but also work as a morphology modifier in PM6:BTP-eC9:IT-4F OSCs, and there also exists efficient energy transfer between BTP-eC9 and IT-4F. This result provides a promising way to suppress the nonradiative recombination energy loss and realize higher VOC than the two binary OSCs in ternary OSCs to obtain high power conversion efficiencies.

Electrostatic Potential Design of Solid Additives for Enhanced Molecular Order of Polymer Donor in Efficient Organic Solar Cells

Polymeric semiconducting materials struggle to achieve fast charge mobility due to low structural order. In this work, five 1H-indene-1,3(2H)dione-benzene structured halogenated solid additives namely INB-5F, INB-3F, INB-1F, INB-1Cl, and INB-1Br with gradually varied electrostatic potential are designed and utilized to regulate the structural order of polymer donor PM6. Molecular dynamics simulations demonstrate that although the dione unit of these additives tends to adsorb on the backbone of PM6, the reduced electrostatic potential of the halogen-substituted benzene can shift the benzene interacting site from alkyl side chains to the conjugated backbone of PM6, not only leading to enhanced π-π stacking in out-of-plane but also arising new π-π stacking in in-plane together with the appearance of multiple backbone stacking in out-of-plane, consequent to the co-existence of face-on and edge-on molecular orientations. This molecular packing transformation further translates to enhanced charge transport and suppressed carrier recombination in their photovoltaics, with a maximum power conversion efficiency of 19.4% received in PM6/L8-BO layer-by-layer deposited organic solar cells.

An Indacenodithienothiophene-Based Wide Bandgap Small Molecule Guest for Efficient and Stable Ternary Organic Solar Cells

The strategic and logical development of the third component (guest materials) plays a pivotal and intricate role in improving the efficiency and stability of ternary organic solar cells (OSCs). In this study, a novel guest material with a wide bandgap, named IDTR, is designed, synthesized, and incorporated as the third component. IDTR exhibits complementary absorption characteristics and cascade band alignment with the PM6:Y6 binary system. Morphological analysis reveals that the introduction of IDTR results in strong crystallinity, good miscibility, and proper vertical phase distribution, thereby realizing heightened and balanced charge transport behavior. Remarkably, the novel ternary OSCs have exhibited a significant enhancement in photovoltaic performance. Consequently, open-circuit voltage (VOC), short-circuit current (JSC), and fill factor (FF) have all witnessed substantial improvements with a remarkable power conversion efficiency (PCE) of 18.94% when L8-BO replaced Y6. Beyond the pronounced improvement in photovoltaic performance, superior device stability with a T80 approaching 400 h is successfully achieved. This achievement is attributed to the synergistic interplay of IDTR, providing robust support for the overall enhancement of performance. These findings offer crucial guidance and reference for the design and development of efficient and stable OSCs.

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.

Small molecular donor materials based on β-β-bridged BODIPY dimers with a triphenylamine or carbazole unit for efficient organic solar cells

Herein, we have designed and synthesized two novel BODIPY dimer-based small molecules, denoted as ZMH-1 and ZMH-2, covalently linked and functionalized with triphenylamine (TPA) (ZMH-1) and carbazole (CZ) (ZMH-2) units as the electron donor at the 3- and 5-positions of the BODIPY core, respectively. Their optical and electrochemical properties were investigated. We have fabricated all small molecule bulk heterojunction organic solar cells using these BODIPY-based small molecules as electron donors along with fullerene derivative (PC71BM) and medium bandgap non-fullerene acceptor IDT-TC as electron acceptors. The optimized OSCs based on ZMH-1:PC71BM, ZMH-2:PC71BM, ZMH-1:IDT-IC, and ZMH-2:IDT-IC attain overall PCEs of 8.91%, 6.61%, 11.28%, and 5.48%, respectively. Moreover, when a small amount of PC71BM as guest acceptor is added to the binary host ZMH-1:IDT-TC and ZMH-2:IDT-TC, the ternary OSCs based on ZMH-1 and ZMH-2 reach PCEs of 13.70% and 12.71%, respectively.

Insights Into Preaggregation Control of Y-Series Nonfullerene Acceptors in Liquid State for Highly Efficient Binary Organic Solar Cells

Leveraging breakthroughs in Y-series nonfullerene acceptors (NFAs), organic solar cells (OSCs) have achieved impressive power conversion efficiencies (PCEs) exceeding 19%. However, progress in advancing OSCs has decelerated due to constraints in realizing the full potential of the Y-series NFAs. Herein, a simple yet effective solid additive-induced preaggregation control method employing 2-chloro-5-iodopyridine (PDCI) is reported to unlock the full potential of the Y-series NFAs. Specifically, PDCI interacts predominantly with Y-series NFAs enabling enhanced and ordered phase-aggregation in solution. This method leads to a notable improvement and a redshifted absorption of the acceptor phase during film formation, along with improved crystallinity. Moreover, the PDCI-induced preaggregation of NFAs in the solution enables ordered molecule packing during the film-formation process through delicate intermediate states transition. Consequently, the PDCI-induced preaggregated significantly improves the PCE of PM6:Y6 OSCs from 16.12% to 18.12%, among the best values reported for PM6:Y6 OSCs. Importantly, this approach is universally applicable to other Y-series NFA-based OSCs, achieving a champion PCE of 19.02% for the PM6:BTP-eC9 system. Thus, the preaggregation control strategy further unlocks the potential of Y-series NFAs, offering a promising avenue for enhancing the photovoltaic performance of Y-series NFA-based OSCs.

Efficient and Stable All-Polymer Solar Cells Enabled by Dual Working Mechanism

Ternary strategy with integration characteristics and adaptability is a simple and effective method for blooming of the performance of photovoltaic devices. Herein, a novel wideband gap polymer donor PBB2-Hs is synthesized as the guest component to optimize all-polymer solar cells (all-PSCs). High-energy photon absorption and long exciton lifetime of PBB2-Hs constitute efficient energy transfer. Good miscibility and cascade energy levels promote the formation of alloy-like structure between PBB2-Hs and host system. The dual working mechanisms greatly improve photon capture and charge transfer in active layers. Additionally, the introduction of PBB2-Hs also optimizes the ordered molecular stacking of acceptors and suppresses molecular peristalsis. Upon adding 15 wt% PBB2-Hs, the ternary all-PSC achieved a champion efficiency of 17.66%, and can still maintain 82% photostability (24 h) and 91% storage stability (1000 h) of the original PCE. Moreover, the strong molecular stacking and entanglement between PBB2-Hs and the host material increased the elongation at break of ternary blend film by 1.6 times (16.2%), allowing the flexible device to maintain 83% of the original efficiency after 800 bends (R = 5 mm). This work highlights the effectiveness of guest polymer on simultaneously improving photovoltaic performance, photostability and mechanical stability in all-PSCs.

Combined Charge Extraction by Linearly Increasing Voltage and Time-Resolved Microwave Conductivity to Reveal the Dynamic Charge Carrier Mobilities in Thin-Film Organic Solar Cells

This article reports a purely experiment-based method to evaluate the time-dependent charge carrier mobilities in thin-film organic solar cells (OSCs) using simultaneous charge extraction by linearly increasing the voltage (CELIV) and time-resolved microwave conductivity (TRMC) measurements. This method enables the separate measurement of electron mobility (μe) and hole mobility (μh) in a metal-insulator-semiconductor (MIS) device. A slope-injection-restoration voltage profile for MIS-CELIV is also proposed to accurately determine the charge densities. The dynamic behavior of μe and μh is examined in five bulk heterojunction (BHJ) OSCs of polymer:fullerene (P3HT:PCBM and PffBT4T:PCBM) and polymer:nonfullerene acceptor (PM6:ITIC, PM6:IT4F, and PM6:Y6). While the former exhibits fast decays of μh and μe, the latter, in particular, PM6:IT4F and PM6:Y6, exhibits slow decays. Notably, the high-performing PM6:Y6 demonstrates both a balanced mobility (μe/μh) of 1.0-1.1 within 30 μs and relatively large CELIV-TRMC mobility values among the five BHJs. The results exhibit reasonable consistency with a high fill factor. The proposed new CELIV-TRMC technique offers a path toward a comprehensive understanding of dynamic mobility and its correlation with the OSC performance.

Layer-by-Layer-Processed All-Polymer Solar Cells with Enhanced Performance Enabled by Regulating the Microstructure of Upper Layer

The layer-by-layer (LBL) fabrication method allows for controlled microstructure morphology and vertical component distribution, and also offers a reproducible and efficient technique for fabricating large-scale organic solar cells (OSCs). In this study, the polymers D18 and PYIT-OD are employed to fabricate all-polymer solar cells (all-PSCs) using the LBL method. Morphological studies reveal that the use of additives optimizes the microstructure of the active layer, enhancing the cells' crystallinity and charge transport capability. The optimized device with 2% CN additive significantly reduces bimolecular recombination and trap-assisted recombination. All-PSCs fabricated by the LBL method based on D18/PYIT-OD deliver a power conversion efficiency (PCE) of 15.07%. Our study demonstrates the great potential of additive engineering via the LBL fabrication method in regulating the microstructure of active layers, suppressing charge recombination, and enhancing the photovoltaic performance of devices.

High Open-Circuit Voltage Organic Solar Cells with 19.2% Efficiency Enabled by Synergistic Side-Chain Engineering

Restricted by the energy-gap law, state-of-the-art organic solar cells (OSCs) exhibit relatively low open-circuit voltage (VOC) because of large nonradiative energy losses (ΔEnonrad). Moreover, the trade-off between VOC and external quantum efficiency (EQE) of OSCs is more distinctive; the power conversion efficiencies (PCEs) of OSCs are still <15% with VOCs of >1.0 V. Herein, the electronic properties and aggregation behaviors of non-fullerene acceptors (NFAs) are carefully considered and then a new NFA (Z19) is delicately designed by simultaneously introducing alkoxy and phenyl-substituted alkyl chains to the conjugated backbone. Z19 exhibits a hypochromatic-shifted absorption spectrum, high-lying lowest unoccupied molecular orbital energy level and ordered 2D packing mode. The D18:Z19-based blend film exhibits favorable phase separation with face-on dominated molecular orientation, facilitating charge transport properties. Consequently, D18:Z19 binary devices afford an exciting PCE of 19.2% with a high VOC of 1.002 V, surpassing Y6-2O-based devices. The former is the highest PCE reported to date for OSCs with VOCs of >1.0 V. Moreover, the ΔEnonrad of Z19- (0.200 eV) and Y6-2O-based (0.155 eV) devices are lower than that of Y6-based (0.239 eV) devices. Indications are that the design of such NFA, considering the energy-gap law, could promote a new breakthrough in OSCs.

Guest Acceptors with Lower Electrostatic Potential in Ternary Organic Solar Cells for Minimizing Voltage Losses

The ternary strategy, in which one guest component is introduced into one host binary system, is considered to be one of the most effective ways to realize high-efficiency organic solar cells (OSCs). To date, there is no efficient method to predict the effectiveness of guest components in ternary OSCs. Herein, three guest compositions (i.e., ANF-1, ANF-2 and ANF-3) with different electrostatic potential (ESP) are designed and synthesized by modulating the electron-withdrawing ability of the terminal groups through density functional theory simulations. The effects of the introduction of guest component into the host system (D18:N3) on the photovoltaic properties are investigated. The theoretical and experimental studies provide a key rule for guest acceptor in ternary OSCs to improve the open-circuit voltage, that is, the larger ESP difference between the guest and host acceptor, the stronger the intermolecular interactions and the higher the miscibility, which improves the luminescent efficiency of the blend film and the electroluminescence quantum yield (EQEEL) of the device by reducing the aggregation-caused-quenching, thereby effectively decreasing the non-radiative voltage loss of ternary OSCs. This work will greatly contribute to the development of highly efficient guest components, thereby promoting the rapid breakthrough of the 20% efficiency bottleneck for single-junction OSCs.

Dual Function of the Third Component in Ternary Organic Solar Cells: Broaden the Spectrum and Optimize the Morphology

Ternary organic solar cells (T-OSCs) have attracted significant attention as high-performance devices. In recent years, T-OSCs have achieved remarkable progress with power conversion efficiency (PCE) exceeding 19%. However, the introduction of the third component complicates the intermolecular interaction compared to the binary blend, resulting in poor controllability of active layer and limiting performance improvement. To address these issues, dual-functional third components have been developed that not only broaden the spectral range but also optimize morphology. In this review, the effect of the third component on expanding the absorption range of T-OSCs is first discussed. Second, the extra functions of the third component are introduced, including adjusting the crystallinity and molecular stack in active layer, regulating phase separation and purity, altering molecular orientation of the donor or acceptor. Finally, a summary of the current research progress is provided, followed by a discussion of future research directions.

Dimeric Giant Molecule Acceptors Featuring N-type Linker: Enhancing Intramolecular Coupling for High-Performance Polymer Solar Cells

Giant molecular acceptors (GMAs) are typically designed through the conjugated linking of individual small molecule acceptors (SMAs). This design imparts an extended molecular size, elevating the glass transition temperature (Tg) relative to their SMA counterparts. Consequently, it effectively suppresses the thermodynamic relaxation of the acceptor component when blended with polymer donors to construct stable polymer solar cells (PSCs). Despite their merits, the optimization of their chemical structure for further enhancing of device performance remains challenge. Different from previous reports utilizing p-type linkers, here, we explore an n-type linker, specifically the benzothiadiazole unit, to dimerize the SMA units via a click-like Knoevenagel condensation, affording BT-DL. In comparison with B-DL with a benzene linkage, BT-DL exhibits significantly stronger intramolecular super-exchange coupling, a desirable property for the acceptor component. Furthermore, BT-DL demonstrates a higher film absorption coefficient, redshifted absorption, larger crystalline coherence, and higher electron mobility. These inherent advantages of BT-DL translate into a higher power conversion efficiency of 18.49 % in PSCs, a substantial improvement over the 9.17 % efficiency observed in corresponding devices with B-DL as the acceptor. Notably, the BT-DL based device exhibits exceptional stability, retaining over 90 % of its initial efficiency even after enduring 1000 hours of thermal stress at 90 °C. This work provides a cost-effective approach to the synthesis of n-type linker-dimerized GMAs, and highlight their potential advantage in enhancing intramolecular coupling for more efficient and durable photovoltaic technologies.

Boosting the Performances of Semitransparent Organic Photovoltaics via Synergetic Near-Infrared Light Management

Semitransparent organic photovoltaics (ST-OPVs) offer promising prospects for application in building-integrated photovoltaic systems and greenhouses, but further improvement of their performance faces a delicate trade-off between the two competing indexes of power conversion efficiency (PCE) and average visible transmittance (AVT). Herein, the authors take advantage of coupling plasmonics with the optical design of ST-OPVs to enhance near-infrared absorption and hence simultaneously improve efficiency and visible transparency to the maximum extent. By integrating core-bishell PdCu@Au@SiO2 nanotripods that act as optically isotropic Lambertian sources with near-infrared-customized localized surface plasmon resonance in an optimal ternary PM6:BTP-eC9:L8-BO-based ST-OPV, it is shown that their interplay with a multilayer optical coupling layer, consisting of ZnS(130 nm)/Na3AlF6(60 nm)/WO3(100 nm)/LaF3(50 nm) identified from high-throughput optical screening, leads to a record-high PCE of 16.14% (certified as 15.90%) along with an excellent AVT of 33.02%. The strong enhancement of the light utilization efficiency by ≈50% as compared to the counterpart device without optical engineering provides an encouraging and universal pathway for promoting breakthroughs in ST-OPVs from meticulous optical design.

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.

Green Printing for Scalable Organic Photovoltaic Modules by Controlling the Gradient Marangoni Flow

Despite the rapid development in the performances of organic solar cells (OSCs), high-performance OSC modules based on green printing are still limited. The severe Coffee-ring effect (CRE) is considered to be the primary reason for the nonuniform distribution of active layer films. To solve this key printing problem, the cosolvent strategy is presented to deposit the active layer films. The guest solvent Mesitylene with a higher boiling point and a lower surface tension is incorporated into the host solvent o-XY to optimize the rheological properties, such as surface tension and viscosity of the active layer solutions. And the synergistic effect of inward Marangoni flow generation and solution thickening caused by the cosolvent strategy can effectively restrain CRE, resulting in highly homogeneous large-area active layer films. In addition, the optimized crystallization and phase separation of active layer films effectively accelerate the charge transport and exciton dissociation of devices. Consequently, based on PM6:BTP-eC9 system, the device prepared with the co-solvent strategy shows the a power conversion efficiency of 17.80%. Moreover, as the effective area scales to 1 and 16.94 cm2, the recorded performances are altered to 16.71% and 14.58%. This study provides a universal pathway for the development of green-printed high-efficiency organic photovoltaics.

A New Framework for Understanding Recombination-Limited Charge Extraction in Disordered Semiconductors

Recombination of free charges is a key loss mechanism limiting the performance of organic semiconductor-based photovoltaics such as solar cells and photodetectors. The carrier density-dependence of the rate of recombination and the associated rate coefficients are often estimated using transient charge extraction (CE) experiments. These experiments, however, often neglect the effect of recombination during the transient extraction process. In this work, the validity of the CE experiment for low-mobility devices, such as organic semiconductor-based photovoltaics, is investigated using transient drift-diffusion simulations. We find that recombination leads to incomplete CE, resulting in carrier density-dependent recombination rate constants and overestimated recombination orders; an effect that depends on both the charge carrier mobilities and the resistance-capacitance time constant. To overcome this intrinsic limitation of the CE experiment, we present an analytical model that accounts for charge carrier recombination, validate it using numerical simulations, and employ it to correct the carrier density-dependence observed in experimentally determined bimolecular recombination rate constants.

Eliminating the Burn-in Loss of Efficiency in Organic Solar Cells by Applying Dimer Acceptors as Supramolecular Stabilizers

The meta-stable active layer morphology of organic solar cells (OSCs) is identified as the main cause of the rapid burn-in loss of power conversion efficiency (PCE) during long-term device operation. However, effective strategies to eliminate the associated loss mechanisms from the initial stage of device operation are still lacking, especially for high-efficiency material systems. Herein, the introduction of molecularly engineered dimer acceptors with adjustable thermal transition properties into the active layer of OSCs to serve as supramolecular stabilizers for regulating the thermal transitions and optimizing the crystallization of the absorber composites is reported. By establishing intimate π-π interactions with small-molecule acceptors, these stabilizers can effectively reduce the trap-state density (Nt) in the devices to achieve excellent PCEs over 19%. More importantly, the low Nt associated with an initially optimized morphology can be maintained under external stresses to significantly reduce the PCE burn-in loss in devices. This research reveals a judicious approach to improving OPV stability by establishing a comprehensive correlation between material properties, active-layer morphology, and device performance, for developing burn-in-free OSCs.

CPE-Na-Based Hole Transport Layers for Improving the Stability in Nonfullerene Organic Solar Cells: A Comprehensive Study

Organic photovoltaic (OPV) cells have experienced significant development in the last decades after the introduction of nonfullerene acceptor molecules with top power conversion efficiencies reported over 19% and considerable versatility, for example, with application in transparent/semitransparent and flexible photovoltaics. Yet, the optimization of the operational stability continues to be a challenge. This study presents a comprehensive investigation of the use of a conjugated polyelectrolyte polymer (CPE-Na) as a hole layer (HTL) to improve the performance and longevity of OPV cells. Two different fabrication approaches were adopted: integrating CPE-Na with PEDOT:PSS to create a composite HTL and using CPE-Na as a stand-alone bilayer deposited beneath PEDOT:PSS on the ITO substrate. These configurations were compared against a reference device employing PEDOT:PSS alone, as the HTL increased efficiency and fill factor. The instruments with CPE-Na also demonstrated increased stability in the dark and under simulated operational conditions. Device-based PEDOT:PSS as an HTL reached T80 after 2500 h while involving CPE-Na in the device kept at T90 in the same period, evidenced by a reduced degradation rate. Furthermore, the impedance spectroscopy and photoinduced transient methods suggest optimized charge transfer and reduced charge carrier recombination. These findings collectively highlight the potential of CPE-Na as a HTL optimizer material for nonfluorine OPV cells.

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.

Research Advances of Nonfused Ring Acceptors for Organic Solar Cells

The last few decades have witnessed the rapid development of organic solar cells (OSCs). High power conversion efficiencies (PCEs) of over 19% have been successfully achieved due to the emergence of fused-ring acceptors (FRAs). However, the high complexity and low yield for the material synthesis result in high production costs of FRAs, limiting the further commercial application of OSCs. In contrast, nonfused ring acceptors (NFRAs) with the merits of facile synthesis, high yield, and preferable stability can promote the development of low-cost OSCs. Currently, the PCEs of NFRAs-based OSCs have exceeded 17%, which is expected to reach efficiency comparable to that of the FRAs-based OSCs. This review describes the advantages of the recent advances in NFRAs, which emphasizes exploring how the chemical structures of NFRAs influence molecular conformation, aggregation, and packing modes. In addition, the further development of NFRA materials is prospected from molecular design, morphological control, and stability perspectives.

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.