High-Performance Green Thick-Film Ternary Organic Solar Cells Enabled by Crystallinity Regulation

The power conversion efficiency (PCE) of organic solar cells (OSCs) has reached high values of over 19%. However, most of the high-efficiency OSCs are fabricated by spin-coating with toxic solvents and the optimal photoactive layer thickness is limited to 100 nm, limiting practical development of OSCs. It is a great challenge to obtain ideal morphology for high-efficiency thick-film OSCs when using non-halogenated solvents due to the unfavorable film formation kinetics. Herein, high-efficiency ternary thick-film (300 nm) OSCs with PCE of 15.4% based on PM6:BTR-Cl:CH1007 are fabricated by hot slot-die coating using non-halogenated solvent (o-xylene) in the air. Compared to PM6:BTR-Cl:Y6 blends, the stronger pre-aggregation of CH1007 in solution induces the earlier aggregation of CH1007 molecules and longer aggregation time, and thus results in high and balanced crystallinity of donors and acceptor in CH1007-based ternary film, which led to high-carrier mobility and suppressed charge recombination. The ternary strategy is further used to fabricate high-efficiency, thick-film, large-area, and flexible devices processed from non-halogenated solvents, paving the way for industrial development of OSCs.     

Hot-Casting Boosts Efficiency of Halogen-Free Solvent Processed Non-Fullerene Organic Solar Cells

Despite the substantial climb of the power conversion efficiency (PCE) of organic solar cells (OSCs), the majority of processing solvent remains halogenated and stand as a critical issue for commercialization. Herein, a halogen-free solvent system consisting of toluene (Tol) and 1-phenylnaphthalene (PN) is used to replace the traditional halogenated chloroform (CF) and1-chloronaphthalene (CN) for the processing of the PM6:M36 OSC, reducing the maximum PCE from 15.0% to 13.3%. Hot-casting is demonstrated to boost the maximum PCE of halogen-free solvents processed OSCs back to 15.2%. The preheated substrate fastens the evaporation of Tol and enables similar film-forming kinetics to CF, resulting in the inhibition of immoderate molecular aggregation and excessive phase separation. Ternary OSCs, with either another donor or acceptor as the third component, can further improve device PCE to 15.8%, confirming the versatile photovoltaic systems that this hot-casting method can be applied to. Encouragingly, the hot-casting processed binary and ternary OSCs also exhibit retained storage stability. Therefore, hot-casting is demonstrated as a superior strategy to fabricate OSCs without efficiency and stability loss using halogen-free solvents.     

Tuning the viscosity of halogen free bulk heterojunction inks for inkjet printed organic solar cells

For the solution processing of organic photovoltaics on an industrial scale, the exclusion of halogenated solvents is a necessity. However, the limited solubility of most semiconducting polymer/fullerene blends in non-halogenated solvents results in ink formulations with low viscosities which poses limitations to the use of roll-to-roll compatible deposition processes, such as inkjet printing. We propose to add polystyrene as a rheological modifier to increase the viscosity of bulk heterojunction (BHJ) non-halogenated inks. The printing and performance of P3HT/PCBM photoactive layer inks are characterized as a function of polystyrene concentration and three different molecular weights. Addition of 1 wt% polystyrene provided a near two-fold gain in viscosity, with the largest viscosity gains coming from the polymer with the highest molecular weight. However, this coincided with greater viscoelastic behavior, which reduced the jetting performance of the inks. Differences in solvent compatibility of the polystyrene/P3HT/PCBM ternary blend resulted in phase separation upon layer drying, whereby polystyrene segregated to the layer-air interface to form an isolated domain or network like topology. Nevertheless, a 1.7-fold increase in dynamic viscosity was obtained for devices with printed BHJ layers containing polystyrene at the expense of a 20% reduction in OPV performance. The improved viscosity and good printing behavior achieved with small additions of polystyrene demonstrates its potential to overcome the limited viscosity resulting from typical non-halogenated ink formulations for semiconducting polymers. These results offer a step forward to the industrialization of inkjet printing as an effective deposition technique for functional layers of organic electronics.     

Ternary Blend Strategy for Achieving High‐Efficiency Organic Solar Cells with Nonfullerene Acceptors Involved

Nonfullerene acceptors have recently drawn considerable attention in bulk heterojunction organic solar cells (OSCs). The power conversion efficiency (PCE) over 14% is achieved in single‐junction fullerene‐free OSCs, which has surpassed that of fullerene‐based counterparts. For future commercial applications, however, a high and stable PCE > 15% is required, which entails rational material design and device optimization. In this context, three approaches are generally utilized—the synthesis of novel nonfullerene acceptors and the selection of suitable polymer donors to pair with them, the tandem or multijunction device architecture, and the ternary blend strategy. Compared to the former two methods, the ternary strategy allows to employ the existing photovoltaic materials and the single‐junction device. Therefore, an exploration of nonfullerene acceptor–based ternary blend OSCs (NFTSCs) has shown unprecedented progress since 2016. This review summarizes and classifies the photovoltaic materials utilized in NFTSCs, aiming to not only exhibit the recent development of NFTSCs but also elucidate the correlation among donor/acceptor materials, film morphology, transport dynamics, and device fabrication toward high‐efficiency OSCs. Lastly, the above key advances are highlighted along with the existing issues and insights into the viable path for the further research thrusts are offered.     

Asymmetric and Halogenated Fused-Ring Electron Acceptor for Efficient Organic Solar Cells

Fused-ring non-fullerene electron acceptors (NFAs) boost the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Asymmetric and halogenated NFAs have drawn increasing attention in recent years due to their unique optoelectronic properties. Starting from the symmetric NFA ITCC-M, this work systematically designs and synthesizes an asymmetric counterpart ITCC-M-2F, halogenated counterpart ITCC-Cl, and asymmetric and halogenated counterpart IDTT-Cl-2F. Among these NFAs, IDTT-Cl-2F shows the shallowest lowest unoccupied molecular orbital energy level, broader absorption range, and the tightest molecular packing. As a result, when blended with the donor PBDB-T-2Cl, IDTT-Cl-2F-based OSCs yield the highest PCE of 13.3% with an open-circuit voltage of 0.96 V, short-circuit current of 19.20 mA cm^–2, and fill factor of 71.1%, which is the highest PCE of OSCs employing 2-(2-chloro-6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene) malononitrile (ClIC) unit terminated NFA. The results demonstrate the synergistic effect of asymmetry and halogenation toward tuning of the optoelectronic properties of NFAs for high performance OSCs.     

Recent Progress in Flexible and Stretchable Organic Solar Cells

Flexible and stretchable organic solar cells (OSCs) have attracted enormous attention due to their potential applications in wearable and portable devices. To achieve flexibility and stretchability, many efforts have been made with regard to mechanically robust electrodes, interface layers, and photoactive semiconductors. This has greatly improved the performance of the devices. State‐of‐the‐art flexible and stretchable OSCs have achieved a power conversion efficiency of 15.21% (16.55% for tandem flexible devices) and 13%, respectively. Here, the recent progress of flexible and stretchable OSCs in terms of their components and processing methods are summarized and discussed. The future challenges and perspectives for flexible and stretchable OSCs are also presented.     

High-performance alloy-like ternary organic solar cells with two compatible non-fullerene acceptors

Ternary blending is one of the effective strategies to modulate the blend film morphology for achieving high efficiency organic solar cells (OSCs). In this work, high-performance ternary OSCs are fabricated by introducing a non-fullerene acceptor, namely IDTP-4F into the PM6:Y6 binary system to enhance the device performance. Detailed investigations indicate that IDTP-4F can form an alloy phase with Y6, resulting in the optimized morphology, which can facilitate the charge transport and reduce recombination, leading to enhanced open-circuit voltage (V_oc) and fill factor (FF) simultaneously. Consequently, the optimized ternary OSCs exhibit an excellent power conversion efficiency (PCE) of 17.1%, which is much higher than that of PM6:Y6 binary OSCs (15.9%). These results indicate that combining two compatible non-fullerene acceptors is an effective strategy to fabricate high efficiency ternary OSCs.     

Organic photovoltaic devices with enhanced efficiency processed from non-halogenated binary solvent blends

The development of processing routes to fabricate organic photovoltaic devices (OPVs) using non-halogenated solvents is a necessary step towards their eventual commercialisation. To address this issue, we have used Hansen solubility parameter analysis to identify a non-halogenated solvent blend based on a mixture of carbon disulphide and acetone. This solvent blend was then used to deposit a donor–acceptor polymer–fullerene thin-film that was then used as the active layer of bulk-heterojunction OPV. For the benchmark polymer:fullerene system PCDTBT:PC₇₀BM, a power conversion efficiency of 6.75% was achieved; a 20% relative improvement over reference cells processed using the chlorinated-solvent chlorobenzene. Improvements in device efficiency are attributed to an increase in electron and hole conductivity resulting from enhanced fullerene crystallisation; a property that leads to enhanced device efficiency through improved charge extraction.     

Conformation Locking on Fused‐Ring Electron Acceptor for High‐Performance Nonfullerene Organic Solar Cells

In this work, sidechain engineering on conjugated fused‐ring acceptors for conformation locking is demonstrated as an effective molecular design strategy for high‐performance nonfullerene organic solar cells (OSCs). A novel nonfullerene acceptor (ITC6‐IC) is designed and developed by introducing long alkyl chains into the terminal electron‐donating building blocks. ITC6‐IC has achieved definite conformation with a planar structure and better solubility in common organic solvents. The weak electron‐donating hexyl upshifts the lowest unoccupied molecular orbital level of ITC6‐IC, resulting in a higher V_OC in comparison to the widely used ITIC. The OSCs based on PBDB‐T:ITC6‐IC reveal a promising power conversion efficiency of 11.61% and an expected high V_OC of 0.97 V. The weaker π–π stacking induced by steric hindrance affords ITC6‐IC with enhanced compatibility with polymer donors. The blend film treated with suitable thermal annealing exhibits a fibril crystallization feature with a good bicontinuous network morphology. The results indicate that the molecular design approach of ITC6‐IC can be inspirational for future development of nonfullerene acceptors for high efficiency OSCs.     

A Versatile Self‐Organization Printing Method for Simplified Tandem Organic Photovoltaics

Despite recent dramatic enhancements in power conversion efficiencies (PCEs) resulting in values over 10%, the manufacturing of tandem organic solar cells (OSCs) via current printing technologies is subject to tremendous challenges. Existing complicated tandem structures consisting of six or more component layers have been a major obstacle that significantly increases the complexity of printing processes and substantially sacrifices the PCE for printed devices. Here, an innovative printing method is reported that simplifies the fabrication process of the tandem OSCs. By developing a new printing technique using a nanocomposites containing interfacial and photoactive materials, a simultaneously printed bilayer of consisting of interfacial and photoactive layers, achieved through vertical self‐organization, is successfully demonstrated, resulting in tandem OSCs with only four printed layers. Moreover, by rigorously controlling the molecular weight of the interfacial materials, the self‐assembly characteristics are improved and an efficient tandem OSC is yielded with a PCE of 9.1% achieved in printed layers.     

～1.2 V open-circuit voltage from organic solar cells

Organic solar cells (OSCs) have achieved rapid advance due to the continuous development of high-performance key materials.Recently,the power conversion efficiencies (PCEs)of OSCs under 1 Sun condition (AM 1.5 G,100 mW/cm2) are striving toward 19％[1-5].The PCE improvement benefits from the largely enhanced short-circuit current density (Jsc) and fill factor (FF).However,these cells show relatively low open-circuit voltage (Voc) around 0.8-0.9 V.The rise of Internet of Things (loT) industry has promoted the indoor application of solar cells.OSCs can afford higher PCEs under various indoor light as compared to 1 Sun condition[6,7],but they present lower Voc[8].Fabricating tandem devices is an effective strategy to boost the performance of OSCs.Sub-cells with syn-chronously high Voc,Jsc and FF are highly desired in tandem cells,while these sub-cells are still limited[9].Thus,improving Voc without sacrificing Jsc and FF is an urgent mission in OSCs.     

An efficient nonfullerene acceptor for all-small-molecule solar cells with versatile processability in environmentally benign solvents

A new structure of dicyanodistyrylbenzene-naphthalimide-based nonfullerene acceptor NIDCSN was synthesized and characterized to have a favorable electron accepting property and versatile processability in various organic solvents. The nonfullerene all-small-molecule solar cells comprising p-DTS(FBTTh₂)₂ as the donor and NIDCSN as the acceptor exhibited a maximum power conversion efficiency of 3.45% with a remarkable open-circuit voltage of 1.04 V, together with similar device performances when fabricated in five different solvents including environmentally benign non-halogenated ones.     

Vacuum deposited ternary mixture organic solar cells

Ternary mixtures of photo-active organic materials are an intuitive approach to achieve enhanced photocurrent in organic solar cells (OSCs). In this work, we study ternary mixtures of vacuum deposited small molecules, complementing the recent surge of interest in solution processed ternary OSCs. The mixed layer composition is systematically varied to study all possible film configurations, and the resulting OSCs are successful in harvesting photocurrent from all three components to grant broad spectral photoresponse. However, the performance of the ternary OSC is generally less than the binary OSC, largely due to reduced fill factors. By examining ternary OSC transient photocurrents and multi-donor planar heterojunction devices, we demonstrate that the ternary OSC is strongly affected by the energy levels of its constituent materials, with small differences in the two donor materials’ highest occupied molecular orbitals degrading hole transport. The results stress the importance of fine molecular engineering for ternary OSCs, and further hint that the enhancements commonly observed in solution processed ternary OSCs may in part be associated with morphological variations that are not present in vacuum deposited OSCs. The research verifies that, by designing small molecules with specific energy levels, ternary OSCs provide an alternative pathway to low cost, high efficiency photovoltaics in lieu of more complicated device architectures.     

Naphthalene tetracarboxylic diimide (NDI)-based polymer solar cells processed by non-halogenated solvents

Two non-fullerene acceptors, PNDIT-20 and PNDIT-16, based on naphthalenediimide (NDI) and thiophene comonomers, have been synthesized for all polymer solar cells (all-PSCs) application. The incorporation of long alkyl chains onto the NDI units endows the polymers with excellent solubility in both halogen and non-halogen solvents. Halogen-free solvents, e.g. 1,3,5-trimethylbenzene (1,3,5-TMB), 1,2,4-trimethylbenzene (1,2,4-TMB), and 1,2-dimethylbenzene (1,2-DMB), have been employed to fabricate all-PSCs based on PNDIT-20 or PNDIT-16 paired with a donor polymer PTB7-Th without use of additives or post-treatment. The devices using 1,2-DMB as the solvent demonstrated PCEs of 3.88% and 4.94% for PNDIT-20 and PNDIT-16, respectively, which are among the highest values reported for PNDIT-based all-PSCs produced using environmentally friendly solvent. The performance is superior than the control device fabricated from conventional but hazardous 1,2-dichlorobenzene (1,2-DCB). Space-charge-limited current (SCLC) measurements and active layer morphological investigation revealed that non-halogenated solvent processed devices show higher and more balanced hole and electron mobilities as well as favorable surface morphology for charge transfer. The results reported in this work suggest that non-halogenated solvents for all-PSCs processing are greatly promising for the development of high performance all-PSCs.     

Fibril Network Strategy Enables High‐Performance Semitransparent Organic Solar Cells

The development of semitransparent organic solar cells (ST‐OSCs) represents a significant step toward the commercialization of OSCs. However, the trade‐off between power conversion efficiency (PCE) and average visible transmittance (AVT) restricts further improvements of ST‐OSCs. Herein, it is demonstrated that a fibril network strategy can enable ST‐OSCs with a high PCE and AVT simultaneously. A wide‐bandgap polymer PBT1‐C‐2Cl that can self‐assemble into a fibril nanostructure is used as the donor and a near‐infrared small molecule Y6 is adopted as the acceptor. It is found that a tiny amount of PBT1‐C‐2Cl in the blend can form a high speed pathway for hole transport due to the well distributed fibril nanostructure, which increases the transmittance in the visible region. Meanwhile, the acceptor Y6 guarantees sufficient light absorption. Using this strategy, the optimized ST‐OSCs yield a high PCE of 9.1% with an AVT of over 40% and significant light utilization efficiency of 3.65% at donor/acceptor ratio of 0.25:1. This work demonstrates a simple and effective approach to realizing high PCE and AVT of ST‐OSCs simultaneously.     

Rational Design of Organic Semiconductors for Texture Control and Self‐Patterning on Halogenated Surfaces

Understanding the interactions at interfaces between the materials constituting consecutive layers within organic thin‐film transistors (OTFTs) is vital for optimizing charge injection and transport, tuning thin‐film microstructure, and designing new materials. Here, the influence of the interactions at the interface between a halogenated organic semiconductor (OSC) thin film and a halogenated self‐assembled monolayer on the formation of the crystalline texture directly affecting the performance of OTFTs is explored. By correlating the results from microbeam grazing incidence wide angle X‐ray scattering (μGIWAXS) measurements of structure and texture with OTFT characteristics, two or more interaction paths between the terminating atoms of the semiconductor and the halogenated surface are found to be vital to templating a highly ordered morphology in the first layer. These interactions are effective when the separating distance is lower than 2.5 d_w, where d_w represents the van der Waals distance. The ability to modulate charge carrier transport by several orders of magnitude by promoting “edge‐on” versus “face‐on” molecular orientation and crystallographic textures in OSCs is demonstrated. It is found that the “edge‐on” self‐assembly of molecules forms uniform, (001) lamellar‐textured crystallites which promote high charge carrier mobility, and that charge transport suffers as the fraction of the “face‐on” oriented crystallites increases.     

Ternary organic solar cells based on non-fullerene acceptors: A review

Organic solar cells (OSCs) have reached their second golden age in recent two years with a boosted number of publications. Non-fullerene acceptor (NFA) materials have become a rising star in the field which are widely applied in organic solar cells because of their excellent optoelectronic properties, such as strong light-harvesting ability and tunable energy level. Unlike the low synthetic flexibility and high production cost of fullerene materials, NFAs exhibit flexible structures, and relatively low fabrication costs. Recently, the ternary strategy has become another hot research topic in the field, which introduces a third component into the binary host system for OSCs. The application of a ternary strategy can break the limits of light absorption brought by the host system, improve the morphology and energy level alignment for the active layer and thus improved the efficiency of organic solar cell devices. Benefiting from the advancement in both NFA and ternary strategy, the power conversion efficiency (PCE) of organic solar cell has exceeded over 17.5% to date. A comprehensive review of the recent progress in NFA based ternary OSCs (TOSCs) is needed in the field. Herein, this review mainly focuses on recent research on ternary organic solar cells using NFA materials during the last two years. Firstly, device physics and frequently used active materials in NFA based TOSCs are summarized and discussed. Then, the recent reported high-performance NFA based TOSCs are reviewed. Finally, the outlook and future research direction in the field are proposed. This review aims to provide an insight into NFA based TOSCs and help researchers to explore the full potential of OSCs.     

Versatile Self-Assembled Hole Transport Monolayer Enables Facile Processing Organic Solar Cells over 18% Efficiency with Good Generality

Simplifying solution-processing of bulk-heterojunction (BHJ) organic solar cells (OSCs) via efficient interfacial layers with good generality is in great demand for pushing their large-scale applications. In this study, such a novel and cost-effective self-assembled monolayer (SAM) is reported herein as efficient hole transport layer (HTL) for high efficiency OSCs. The SAM-structured 4-(5,9-dibromo-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (DCB-BPA) enables not only enhanced photon harvesting in the active layer but also minimized nonradiative recombination losses to improve interface charge extraction/transport. As a consequence, high short-circuit current (≈28.07 mA cm⁻²) is achieved for PM6:BTP-eC9 based OSCs to deliver a champion power conversion efficiency of 18.16%, among the highest values for OSCs using small organic HTLs to date. Importantly, good generality of this SAMs is demonstrated for representative high-efficiency BHJ OSCs systems like PM6:Y6 and PM6:PC₆₁BM, outperforming conventional poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)-based counterparts. Excitingly, the SAM is applicable for large-area HTL processing via immersion method, affording 16.59% efficiency for PM6:BTP-BO-4Cl based OSCs. This study highlights the great potential of engineered SAMs for facile large-scale fabrication of high performance OSCs.     

Halogen-free solvent processing for sustainable development of high efficiency organic solar cells

Eliminating processing with halogenated solvents is desirable to achieve sustainable large-scale fabrication of organic solar cells. This work demonstrates a device processing approach completely free of halogenated solvents to yield high-performance (power conversion efficiency, η_P > 6%) polymer:fullerene bulk-heterojunction solar cells comprising a conjugated polymer PIDT-phanQ and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). Introducing 2% 1-methylnaphthalene (Me-naph) as a processing additive to toluene alleviates PC₇₁BM solubility problems, reduces phase domain size by two orders of magnitude, and boosts efficiency from η_P = 0.02% to 6.10%. Both AFM and TEM imaging show that the Me-naph additive promotes a more finely phase-separated morphology in spin-coated films, while photoluminescence quenching and photoinduced absorption spectroscopy confirm that this finer morphology results in both better exciton quenching and more efficient charge separation.     

Releasing Acceptor from Donor Matrix to Accelerate Crystallization Kinetics with a Second Donor toward High-Efficiency Green-Printable Organic Photovoltaics

The state-of-the-art power conversion efficiency (PCE) of organic solar cells (OSCs) is typically achieved in the devices fabricated by toxic halogen solvents with complex post-treatment processes in strictly inert atmosphere. Developing suitable processing method for printing in ambient air using eco-friendly solvents with continuous solution supply and fabricating efficient devices without any post-treatment are intensively desired. Controlling the crystallization kinetics to fine-tune the acceptor's assembly behavior with a second donor for favorable morphological evolution is an effective approach to achieve above requirements. Herein, a kinetics-controlling strategy is implemented by introducing a strong crystalline small molecule, BTR-Cl, to enhance the crystallinity of acceptors. The combined in situ spectra characterizations revealed that the earlier aggregation of acceptor and modulation in conformation of PM6 can be achieved. This unique aggregation behavior facilitated enhanced film crystallization with reduced paracrystallinity of π–π stacking, resulting in improved charge transport and inhibited charge recombination. An outstanding PCE of 17.50% is obtained for the device processed with o-xylene via ambient air printing without any post-treatment. More significantly, efficient all-printed inverted devices and large-area modules are prepared. The generalization of this strategy has been confirmed in other efficient systems, suggesting a great potential for universally fabricating high-efficiency and eco-friendly OSCs.