Alloy-like ternary polymer solar cells with over 17.2% efficiency

Over 18.2% efficiency of layer-by-layer all-polymer solar cells enabled by homoleptic iridium(III) carbene complex as solid additive

The vertical phase distribution of active layers plays a vital role in balancing exciton dissociation and charge transport for achieving efficient polymer solar cells (PSCs). The layer-by-layer (LbL) PSCs are commonly prepared by using sequential spin-coating method from donor and acceptor solutions with distinct solvents and solvent additives. The enhanced exciton dissociation is expected in the LbL PSCs with efficient charge transport in the relatively neat donor or acceptor layers. In this work, a series of LbL all-polymer solar cells (APSCs) were fabricated with PM6 as donor and PY-DT as acceptor, and triplet material m-Ir(CPmPB)3 is deliberately incorporated into PY-DT layer to prolong exciton lifetimes of active layers. The power conversion efficiency (PCE) of LbL APSCs is improved to 18.24% from 17.32% by incorporating 0.3 wt% m-Ir(CPmPB)3 in PY-DT layer, benefiting from the simultaneously enhanced short-circuit current density (JSC) of 25.17 mA cm-2 and fill factor (FF) of 74.70%. The enhancement of PCE is attributed to the efficient energy transfer of m-Ir(CPmPB)3 to PM6 and PY-DT, resulting in the prolonged exciton lifetime in the active layer and the increased exciton diffusion distance. The efficient energy transfer from m-Ir(CPmPB)3 to PM6 and PY-DT layer can be confirmed by the increased photoluminescence (PL) intensity and the prolonged PL lifetime of PM6 and PY-DT in PM6 + m-Ir(CPmPB)3 and PY-DT + m-Ir(CPmPB)3 films. This study indicates that the triplet material as solid additive has great potential in fabricating efficient LbL APSCs by prolonging exciton lifetimes in active layers.

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.

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.

Printing and Coating Techniques for Scalable Organic Photovoltaic Fabrication

Within recent years, there has been an increased interest towards organic photovoltaics (OPVs), especially with their significant device performance reaching beyond 19% since 2022. With these advances in the device performance of laboratory-scaled OPVs, there has also been more attention directed towards using printing and coating methods that are compatible with large-scale fabrication. Though large-area (>100 cm2) OPVs have reached an efficiency of 15%, this is still behind that of laboratory-scale OPVs. There also needs to be more focus on determining strategies for improving the lifetime of OPVs that are suitable for scalable manufacturing, as well as methods for reducing material and manufacturing costs. In this paper, we compare several printing and coating methods that are employed to fabricate OPVs, with the main focus towards the deposition of the active layer. This includes a comparison of performances at laboratory (<1 cm2), small (1-10 cm2), medium (10-100 cm2), and large (>100 cm2) active area fabrications, encompassing devices that use scalable printing and coating methods for only the active layer, as well as "fully printed/coated" devices. The article also compares the research focus of each of the printing and coating techniques and predicts the general direction that scalable and large-scale OPVs will head towards.

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

Hot-Casting Strategy Empowers High-Boiling Solvent-Processed Organic Solar Cells with Over 18.5% Efficiency

Most top-rank organic solar cells (OSCs) are manufactured by the halogenated solvent chloroform, which possesses a narrow processing window due to its low-boiling point. Herein, based on two high-boiling solvents, halogenated solvent chlorobenzene (CB) and non-halogenated green solvent ortho-xylene (OX), preparing active layers with the hot solution is put forward to enhance the performance of the OSCs. In situ test and morphological characterization clarify that the hot-casting strategy assists in the fast and synchronous molecular assembly of both donor and acceptor in the active layer, contributing to preferable donor/acceptor ratio, vertical phase separation, and molecular stacking, which is beneficial to charge generation and extraction. Based on the PM6:BO-4Cl, the hot-casting OSCs with a wide processing window achieve efficiencies of 18.03% in CB and 18.12% in OX, which are much higher than the devices processed with room temperature solution. Moreover, the hot-casting devices with PM6:BTP-eC9 deliver a remarkable fill factor of 80.31% and efficiency of 18.52% in OX, representing the record value among binary devices with green solvent. This work demonstrates a facile strategy to manipulate the molecular distribution and arrangement for boosting the efficiency of OSCs with high-boiling solvents.

Ternary Organic Solar Cells with Power Conversion Efficiency Approaching 15% by Fine-Selecting the Third Component

Nonfullerene acceptors with mediate bandgap play a crucial role in ternary devices as the third component, further boosting the performance of organic solar cells (OSCs). Herein, three F-series acceptors (F-H, F-Cl, and F-2Cl) with mediate bandgap are selected and introduced into the PM6:BDT-Br binary system as third component to find the detailed influence of end groups with chlorine (Cl) atom substitution on the performance of ternary organic solar cells. Due to the increased substitution of Cl atoms on the end groups, F-Cl and F-2Cl as guest acceptors reveal a superior ability to regulate the morphology of blend films, contributing to the ordered packing properties and high crystallinity. As a result, F-Cl and F-2Cl based ternary OSCs achieve significantly improved PCEs of 13.89% and 14.67%, respectively, compared with the binary devices (12.70%). On the contrary, F-H without Cl atom displays a poor compatibility with the host system, resulting in an inferior ternary device with a low PCE of 10.79%. This work indicates that F-series acceptors with mediate bandgap are a promising class of third component for high-performance ternary OSCs. And introducing more Cl atoms substitution on the end groups, especially F-2Cl, will own a broad applicability for other binary devices.

Study of Tunable Dielectric Permittivity of PBDB-T-2CL Polymer in Ternary Organic Blend Thin Films Using Spectroscopic Ellipsometry

The ellipsometric analyses reported in this paper present a novelty by bringing an in-depth optical investigation of some ternary organic blends. This study focuses on the tunability and control of the relative permittivity of active layers by varying the weight ratio of blended materials spin-coated as thin films. To investigate this, an extensive approach based on spectroscopic ellipsometry was conducted on ternary blend (D:A1:A2) thin films, involving a donor [D = chlorinated conjugated polymer (PBDB-T-2Cl)] and two acceptor materials [A1 = a non-fullerene (ITIC-F) and A2 = a fullerene (PCBM)]. The refractive index constitutes a key parameter that exposes insights into the feasibility of photovoltaic cells by predicting the trajectory of light as it transits the device. In this term, higher obtained refractive indexes support higher absorption coefficients. Notably, the dielectric constant can be rigorously tuned and finely calibrated by modest variations in the quantity of the third element, resulting in considerable modifications. Moreover, the inclusion of fullerene in blends, as the third element, results in a smooth topographical profile, further refining the surface of the film. From an electrical point of view, the ternary blends outperform the polymer thin films. The synergistic interaction of constituents emphasizes their potential to enhance solar conversion devices.

New Medium-Bandgap Nonfused Ring Guest Acceptor with a Higher-Lying LUMO Level Enables High-Performance Ternary Organic Solar Cells

Adding another constituent into a binary system, known as a ternary strategy, represents a simple and effective approach to boosting the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, we have prepared a new nonfused ring small-molecule acceptor with a medium bandgap, named DFTQA-2FIC, which possesses a high-lying lowest unoccupied molecular orbital energy level and a strong intramolecular charge-transfer effect. We elaborately utilized it as a third component in a typical PM6:Y6 blend to obtain high-performance ternary OSCs. The resulting ternary blend film exhibited superior and balanced hole/electron mobility, enhanced favorable aggregation morphology, and reduced charge carrier recombination. Consequently, an optimized ternary OSC presented a distinctly increased PCE of 17.29%, accompanied by synchronous enhancements in crucial parameters, representing a 7.46% improvement over the binary OSC based on PM6:Y6 with a PCE of 16.09%. This study highlights that incorporating DFTQA-2FIC as a third component in a binary system is suitable for optimizing photovoltaic performance.

Simultaneously enhancing the photovoltaic parameters of ternary organic solar cells by incorporating a fused ring electron acceptor

The ternary strategy has been recognized as an effective method to improve the photovoltaic performance of organic solar cells (OSCs). In ternary OSCs, the complementary or broadened absorption spectrum, optimized morphology, and enhanced photovoltaic performance could be obtained by selecting a third rational component for the host system. In this work, a fused ring electron acceptor named BTMe-C8-2F, which possesses a high-lying lowest unoccupied molecular orbital (LUMO) energy level and a complementary absorption spectrum to PM6:Y6, was introduced to a PM6:Y6 binary system. The ternary blend film PM6:Y6:BTMe-C8-2F showed high and more balanced charge mobilities, and low charge recombination. Therefore, the OSC based on the PM6:Y6:BTMe-C8-2F (1 : 1.2 : 0.3, w/w/w) blend film achieved the highest power conversion efficiency (PCE) of 17.68%, with an open-circuit voltage (VOC) of 0.87 V, a short-circuit current (JSC) of 27.32 mA cm-2, and a fill factor (FF) of 74.05%, which are much higher than the binary devices of PM6:Y6 (PCE = 15.86%) and PM6:BTMe-C8-2F (PCE = 11.98%). This work provides more insight into the role of introducing a fused ring electron acceptor with a high-lying LUMO energy level and complementary spectrum for simultaneously enhancing the VOC and JSC to promote the performance of ternary OSCs.

In-depth analysis on PTB7 based semi-transparent solar cell employing MoO3/Ag/WO3 contact for advanced optical performance and light utilization

Novel semi-transparent organic solar cells (ST-OSC) can be designed with high average visible transmittance (AVT) while at the same time exhibiting superior photovoltaic performance. This reach requires their design to be based not only on conventional window applications but also on functional industrial applications that require exceptional optical performance. In ST-OSC, high AVT can be achieved by photonic-based dielectric/metal/dielectric (DMD) transparent contact engineering. Functional optical modification can also be made with a fine-tuned design of DMD that includes a light management engineering-based approach. Thus, ST-OSCs can be suitable for aesthetic, colourful and decorative industrial windows that provide natural lighting. In this study, we determined optimal ST-OSCs based on a novel PTB7:PC₇₁BM polymer blend with MoO₃/Ag/WO₃ asymmetric DMD top contact by examining extraordinary optical properties such as AVT, colour rendering index, correlated colour temperature and colour perception over 10 thousand designs. In addition to determining the optimality and extraordinary optical limits for PTB7, we also evaluated the photon-harvesting and photovoltaic performance of ST-OSCs from external quantum efficiency and quantum utilization efficiency. In optimal situations, ST-OSCs offering 48.75% AVT, 99.08 CRI, and sky-blue colours were designed and determined to generate short-circuit current densities of 9.88 mA·cm^-2, 13.64 mA·cm^-2, and 13.06 mA·cm^-2, respectively.

Atomic-level chemical reaction promoting external quantum efficiency of organic photomultiplication photodetector exceeding 108% for weak-light detection

Low-cost, solution-processed photomultiplication organic photodetectors (PM-OPDs) with external quantum efficiency (EQE) above unity have attracted enormous attention. However, their weak-light detection is unpleasant because the anode Ohmic contact causes exacerbation in dark current. Here, we introduce atomic-level chemical reaction in PM-OPDs which can simultaneously suppress dark current and increase EQE via depositing a 0.8 nm thick Al₂O₃ by the atomic layer deposition. Suppression in dark current mainly originates from the built-in anode Schottky junction as a result of work function decrease of hole-transporting layer of which the chemical groups can react chemically with the bottom surface of Al₂O₃ layer at the atomic-level. Such strategy of suppressing dark current is not adverse to charge injection under illumination; instead, responsivity enhancement is realized because charge injection can shift from cathode to anode, of which the neighborhood possesses increased photogenerated carriers. Consequently, weak-light detection limit of the forwardly-biased PM-OPD with Al₂O₃ treatment reaches a remarkable level of 2.5 nW cm^-2, while that of the reversely-biased control is 25 times inferior. Meanwhile, the PM-OPD yields a record high EQE and responsivity of 4.31 × 10⁸% and 1.85 × 10⁶ A W^-1, respectively, outperforming all other polymer-based PM-OPDs.

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

Research on organic solar cells (OSCs) has progressed through material innovation and device engineering. However, well-known and ubiquitous intermolecular interactions, and particularly their synergistic effects, have received little attention. Herein, the complicated relationship between photovoltaic conversion and multidimensional intermolecular interactions in the active layers is investigated. These interactions are dually regulated by side-chain isomerization and end-cap engineering of the acceptors. The phenylalkyl featured acceptors (LA-series) exhibit stronger crystallinity with preferential face-on interactions relative to the alkylphenyl attached isomers (ITIC-series). In addition, the PM6 and LA-series acceptors exhibit moderate donor/acceptor interactions compared to those of the strongly interacting PM6/ITIC-series pairs, which helps to enhance phase separation and charge transport. Consequently, the output efficiencies of all LA series acceptors are over 14%. Moreover, LA-series acceptors show appropriate compatibility, host/guest interactions, and crystallinity relationships with BTP-eC9, thereby leading to uniform and well-organized "alloy-like" mixed phases. In particular, the highly crystalline LA23 further optimizes multiple interactions and ternary microstructures, which results in a high efficiency of 19.12%. Thus, these results highlight the importance of multidimensional intermolecular interactions in the photovoltaic performance of OSCs.

Medium Bandgap Nonfullerene Acceptor for Efficient Ternary Polymer Solar Cells with High Open-Circuit Voltage

We have designed a new medium bandgap non-fullerene small-molecule acceptor consisting of an IDT donor core flanked with 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]-thiophene-4-ylidene) malononitrile (TC) acceptor terminal groups (IDT-TC) and compared its optical and electrochemical properties with the IDT-IC acceptor. IDT-TC showed an absorption profile from 300 to 760 nm, and it has an optical bandgap of 1.65 eV and HOMO and LUMO energy levels of -5.55 and -3.83 eV, respectively. In contrast to IDT-IC, IDT-TC has an upshifted LUMO energy level, which is advantageous for achieving high open-circuit voltage. Moreover, IDT-TC showed higher crystallinity and high electron mobility than IDT-IC. Using a wide bandgap D-A copolymer P as the donor, we compared the photovoltaic performance of IDT-TC, IDT-IC, and IDT-IC-Cl nonfullerene acceptors (NFAs). Polymer solar cells (PSCs) using P: IDT-TC, P: IDT-IC, and P:IDT-IC-Cl active layers achieved a power conversion efficiency (PCE) of 14.26, 11.56, and 13.34%, respectively. As the absorption profiles of IDT-IC-Cl and IDT-TC are complementary to each other, we have incorporated IDT-TC as the guest acceptor in the P: IDT-IC-Cl active layer to fabricate the ternary (P:IDT-TC: IDT-IC-Cl) PSC, demonstrating a PCE of 16.44%, which is significantly higher than that of the binary BHJ devices. The improvement in PCE for ternary PSCs is attributed to the efficient exploitation of excitons via energy transfer from IDT-TC to IDT-IC-Cl, suitable nanoscale phase separation, compact stacking distance, and more evenly distributed charge transport.

Optimized Morphology Enables High-Efficiency Nonfullerene Ternary Organic Solar Cells

Tuning the three-dimensional morphology in the active layer is an effective method to improve the performance of bulk heterojunction organic solar cells (OSCs). In this work, an acceptor-donor-acceptor structured small molecule ST10-CN-1 was synthesized and employed as the guest donor to fabricate ternary OSCs based on a PBDB-T:IT-M host binary system. The incorporation of ST10-CN-1 could broaden the active layer's absorption range of solar light thereby leading to a promotional short-circuit current. Moreover, adding an appropriate amount of ST10-CN-1 could effectively regulate the morphology of the active layer in both the lateral direction and vertical stratification. All of these morphological alterations helped to speed up the exciton dissociation, charge transit, and charge collecting processes, which in turn increased the power conversion efficiency. As a result, an excellent PCE of 11.5% for the ternary device based on PBDB-T:IT-M:ST10-CN-1 was obtained. The enhanced PCE was also linked to the formation of an alloylike state between PBDB-T and ST10-CN-1, as evidenced by the fact that the open circuit voltage of ternary OSCs lay between those for PBDB-T:IT-M (0.925 V) and ST10-CN-1:IT-M (1.064 V). This work illustrates that refining the morphology of the active layer by incorporating an appropriate third component is an effective way to further enhance the device's performance.

High-Performance Ternary Organic Solar Cells Enabled by Introducing a New A-DA'D-A Guest Acceptor with Higher-Lying LUMO Level

A ternary strategy is viable to minimize the trade-off between short-circuit current density (J_sc) and open-circuit voltage (V_oc) in organic solar cells. Generally, the ternary OSCs can achieve a higher PCE than the binary counterparts by subtly utilizing the particular photoelectric properties of the third material. In this regard, we choose BTP-CC with a higher-lying LUMO level based on a fused TPBT (dithienothiophen[3.2-b]-pyrrolobenzothiadiazole) central framework and CC (2-(6-oxo-5,6-dihydro-4H-cyclopenta [b]thiophen-4-ylidene) malononitrile) flanking groups as the third component to broaden the light-absorption spectrum, regulate the bulk heterojunction (BHJ) morphology, improve the V_oc, and reduce the charge recombination in OSCs. In addition, BTP-CC demonstrates intense intermolecular energy transfer to Y6 by fluorescence resonance energy transfer (FRET) pathway, which is due to the photoluminescence (PL) spectrum of BTP-CC covering the absorption region of Y6. The PM6:Y6:BTP-CC based ternary OSC achieves a champion PCE of 17.55%. Further investigation indicates that introduction of BTP-CC could reduce the trap states in OSCs, leading to an increased charge carrier density. Moreover, the incorporation of BTP-CC could improve the device stability. These results demonstrated that BTP-CC is important in improving the photovoltaic performance of ternary OSCs, and this work also provides a guideline for constructing ideal ternary OSCs in the future.

Tunable Color-Variable Solar Absorber Based on Phase Change Material Sb2Se3

In this paper, a dynamic color-variable solar absorber is designed based on the phase change material Sb2Se3. High absorption is maintained under both amorphous Sb2Se3 (aSb2Se3) and crystalline Sb2Se3 (cSb2Se3). Before and after the phase transition leading to the peak change, the structure shows a clear color contrast. Due to peak displacement, the color change is also evident for different crystalline fractions during the phase transition. Different incident angles irradiate the structure, which can also cause the structure to show rich color variations. The structure is insensitive to the polarization angle because of the high symmetry. At the same time, different geometric parameters enable different color displays, so the structure can have good application prospects.

Novel Third Components with (Thio)barbituric Acid as the End Groups Improving the Efficiency of Ternary Solar Cells

Developing novel third component is critical for the ternary organic solar cells (TOSCs). Herein, we design and synthesize two novel third components, MAZ-1 and MAZ-2, with 1,3-diethyl-2-thiobarbituric acid and 1,3-dimethylbarbituric acid as the weak electron withdrawing end groups, respectively. Both MAZ-1 and MAZ-2 could improve the photovoltaic performance of the binary OSCs based on D18:Y6 which exhibit the power conversion efficiency (PCE) of 17%, because the third components can optimize the phase separation, suppress the bimolecular recombination, and decrease the nonradiative energy loss in ternary blends. The PCE of the optimized TOSCs approaches 18% along with the simultaneous increase in open circuit voltage, short circuit current density, and fill factor by incorporating 10 wt % MAZ-1 and MAZ-2 in acceptors. This work enriches the building blocks for novel third components for achieving highly efficient TOSCs.

Polymers in High-Efficiency Solar Cells: The Latest Reports

Third-generation solar cells, including dye-sensitized solar cells, bulk-heterojunction solar cells, and perovskite solar cells, are being intensively researched to obtain high efficiencies in converting solar energy into electricity. However, it is also important to note their stability over time and the devices' thermal or operating temperature range. Today's widely used polymeric materials are also used at various stages of the preparation of the complete device-it is worth mentioning that in dye-sensitized solar cells, suitable polymers can be used as flexible substrates counter-electrodes, gel electrolytes, and even dyes. In the case of bulk-heterojunction solar cells, they are used primarily as donor materials; however, there are reports in the literature of their use as acceptors. In perovskite devices, they are used as additives to improve the morphology of the perovskite, mainly as hole transport materials and also as additives to electron transport layers. Polymers, thanks to their numerous advantages, such as the possibility of practically any modification of their chemical structure and thus their physical and chemical properties, are increasingly used in devices that convert solar radiation into electrical energy, which is presented in this paper.

Revealing the Sole Impact of Acceptor's Molecular Conformation to Energy Loss and Device Performance of Organic Solar Cells through Positional Isomers

Two new fused-ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m-BAIDIC and p-BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p-BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p-BAIDIC-based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open-circuit voltage (VOC ) and short-current current density (JSC ) of the devices. Therefore, the linear FREA, p-BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m-BAIDIC. When p-BAIDIC is incorporated into the binary PM6/BO-4Cl system to form a ternary system, synergistic enhancements in VOC , JSC , fill factor (FF), and ultimately a high efficiency of 17.6% are achieved.

Boosted efficiency over 18.1% of polymer solar cells by employing large extinction coefficients material as the third component

Series of binary and ternary polymer solar cells (PSCs) were successfully fabricated with PM6 as donor, m-BTP-PhC6 and Y6 as acceptor. The optimal ternary PSCs achieve a power conversion efficiency (PCE) of 18.14% by incorporating 20 wt% Y6 in acceptors, benefiting from the increased short circuit current density (J_SC ) of 26.53 mA cm^-2 and fill factor (FF) of 78.51% in comparison with the J_SC s (25.05 mA cm^-2 and 25.65 mA cm^-2 ) and the FFs (77.13% and 76.55%) of binary PSCs with m-BTP-PhC6 or Y6 as acceptor. The photon harvesting ability of ternary active layers can be enhanced by incorporating appropriate Y6, which can be confirmed from the EQE spectral difference of the optimized ternary and binary PSCs, especially in the wavelength range from 680 nm to 800 nm. The refractive index and extinction coefficients of binary and ternary blend films were measured, which can well support the effect of Y6 incorporation on photon harvesting ability in different wavelength range. Meanwhile, the appropriate Y6 incorporation with large extinction coefficients can be considered as morphology regulator, which can be confirmed from the enhanced FF and the more balanced charge transport in the optimal ternary PSCs. Photogenerated exciton distribution in active layers was simulated by transmission matrix method based on the Beer-Lambert law. The photogenerated exciton density can be enhanced in the middle of active layers by incorporating 20 wt% Y6 in acceptors, which is conducive to charge collection by individual electrode, resulting in the simultaneously enhanced J_SC and FF of the optimal ternary PSCs. This work indicates that an appropriate third component will play versatile roles in improving the performance of PSCs via enhancing photon harvesting and optimizing photogeneration exciton distribution for better charge collection. This article is protected by copyright. All rights reserved.

Realizing Broadband NIR Photodetection and Ultrahigh Responsivity with Ternary Blend Organic Photodetector

With the advancement of portable optoelectronics, organic semiconductors have been attracting attention for their use in the sensing of white and near-infrared light. Ideally, an organic photodiode (OPD) should simultaneously display high responsivity and a high response frequency. In this study we used a ternary blend strategy to prepare PM6: BTP-eC9: PCBM-based OPDs with a broad bandwidth (350-950 nm), ultrahigh responsivity, and a high response frequency. We monitored the dark currents of the OPDs prepared at various PC71BM blend ratios and evaluated their blend film morphologies using optical microscopy, atomic force microscopy, and grazing-incidence wide-angle X-ray scattering. Optimization of the morphology and energy level alignment of the blend films resulted in the OPD prepared with a PM6:BTP-eC9:PC71BM ternary blend weight ratio of 1:1.2:0.5 displaying an extremely low dark current (3.27 × 10-9 A cm-2) under reverse bias at -1 V, with an ultrahigh cut-off frequency (610 kHz, at 530 nm), high responsivity (0.59 A W-1, at -1.5 V), and high detectivity (1.10 × 1013 Jones, under a reverse bias of -1 V at 860 nm). Furthermore, the rise and fall times of this OPD were rapid (114 and 110 ns), respectively.

The Excited State and Charge Transfer of Two Nonfullerene Acceptors from First-Principles Many-Body Green’s Function Theory

Nonfullerene acceptors (NFAs) have shown an outstanding performance upon producing highly efficient and sustainable organic solar cells (OSC). Recently, a growing group of researchers denoted to modify the structures of acceptor−donor−acceptor-type NFAs to raise the power conversion efficiencies (PCEs) when they are blended with a variety of polymer donors in OSC. In 2020, the ketone on the ending groups of BTP-IC were substituted for sulfonyl; the new NFA named BTP-IS was synthetized. The PCE of BTP-IS based OSC is 5.25% higher than that of the BTP-IC device. Based on this, the many-body Green’s function theory, combined with other quantum chemical methods, is conducted to study their ground electronic structures, excited states, and absorption spectra. The ground-state geometries, ionization energies, and the excited state energies are deeply sensitive to exchange-correlation functionals used in calculations. The lowest excited state energies calculated by full-BSE method using DFT-PBE as the starting point is 0.07~0.14 eV smaller than that by TDDFT-PBE method, which is more consistent with experimental data. This provided a methodology for future research on similar NFA systems. The first charge-transfer states and transfer mechanism of two molecules are proposed in this paper. Furthermore, we found that the reason for more efficient charge transport in BTP-IS-based OSC is the larger ionization energies and much weaker electron-hole interaction in BTP-IS. This finding is conducive to the better application of BTP-IS in OSC field.

Nonalloy Model-Based Ternary Organic Solar Cells

Ternary blending based on an alloy-like model has been proved as an efficient strategy for high-efficiency organic solar cells (OSCs). However, the third component that possesses excellent miscibility with host materials in the alloy-like model may trigger adverse effects for the active layer, especially at a high doping ratio. In this work, we propose a new concept of nonalloy model for the ternary OSCs in which the third component presents moderate miscibility with the acceptor and distributes at the interspace between donor and acceptor domains. The nonalloy model is constructed based on the PM6:Y6 system, and a Y6 analogue (BTP-MCA) is synthesized as the third component. The BTP-MCA can maintain initial excellent morphology of the active layer and enhance the morphological stability by acting as a frame around the host materials. As a result, ternary OSCs based on the PM6:Y6:BTP-MCA blend exhibit an impressive efficiency of 17.0% with a high open-circuit voltage of 0.87 V. Moreover, the devices present a high doping tolerance (keeping high efficiency with a doping ratio of 50%) and improved stability. This work indicates that the nonalloy model can be a promising method to fabricate efficient and stable ternary OSCs apart from the conventional alloy-like model.

Accelerated Explore of Efficient Ternary Solar Cell with PTB7:PC71BM:SMPV1 Using Machine-Learning Methods

Machine learning(ML) provides an efficient tool for predicting the photoelectric conversion efficiency(PCE) of organic solar cells(OSCs). In this paper, random forest (RF), K-nearest neighbors , and support vector machine in...

Smart Ternary Strategy in Promoting the Performance of Polymer Solar Cells Based on Bulk-Heterojunction or Layer-By-Layer Structure

Although the rapid development of polymer solar cells (PSCs) has been achieved, it is still a great challenge to explore efficient ways for improving power conversion efficiency (PCE) of PSCs from materials and device engineering. Ternary strategy has been confirmed as an efficient way to improve PCE of PSCs by employing three kinds of materials. In this work, one polymer donor PM6, and two non-fullerene materials N3 and MF1 are selected to prepare ternary PSCs with layer-by-layer (LbL) or bulk-heterojunction (BHJ) structure. The LbL and BHJ-PSCs exhibit PCEs of 16.75% and 16.76% with 15 wt% MF1 content in acceptors, corresponding to over 5% or 4% PCE improvement compared with N3-based binary PSCs with LbL or BHJ structure. The PCE improvement is mainly attributed to the fill factor enhancement from 73.29% to 76.95% for LbL-PSCs or from 74.13% to 77.51% for BHJ-PSCs by employing the ternary strategy. This work indicates that ternary strategy has great potential in preparing highly efficient LbL-PSCs via simultaneously optimizing molecular arrangement and the thickness of each layer.

High-performance ternary organic photovoltaics with NC70BA as the third component material enabling thickness-insensitive photoactive performance

In this work, a thinner (100 nm) and thicker (150 and 200 nm) ternary organic photovoltaic (OPV) are fabricate by D18 as donor, Y6 as acceptor and NC₇₀BA as third component materials. The addition of the hollow 3D spherical structure of NC₇₀BA into D18:Y6 binary films is helpful for improving phase separation and smooth surface of ternary photoactive layer, and form more continuous electron transport channels in ternary photoactive layers. It is enhance photovoltaic performance under not only thinner photoactive layer thickness but also thicker photoactive layer thickness. Our results demonstrate the feasibility of employing D18:Y6 as a binary photovoltaic layer and fullerene derivative NC₇₀BA as a third component material and has construct high-efficiency thickness-insensitive ternary OPVs; this approach would promote the development of thicker photoactive layer ternary OPVs to fulfill the requirements of solution coating processes.

Designs and understanding of small molecule-based non-fullerene acceptors for realizing commercially viable organic photovoltaics

Organic photovoltaics (OPVs) have emerged as a promising next-generation technology with great potential for portable, wearable, and transparent photovoltaic applications. Over the past few decades, remarkable advances have been made in non-fullerene acceptor (NFA)-based OPVs, with their power conversion efficiency exceeding 18%, which is close to the requirements for commercial realization. Novel molecular NFA designs have emerged and evolved in the progress of understanding the physical features of NFA-based OPVs in relation to their high performance, while there is room for further improvement. In this review, the molecular design of representative NFAs is described, and their blend characteristics are assessed via statistical comparisons. Meanwhile, the current understanding of photocurrent generation is reviewed along with the significant physical features observed in high-performance NFA-based OPVs, while the challenging issues and the strategic perspectives for the commercialization of OPV technology are also discussed.

Charge Transfer Mechanisms Regulated by the Third Component in Ternary Organic Solar Cells

For ternary organic solar cells (T-OSCs), introducing the third component (D₂) can significantly enhance the efficiency of cell while still maintaining easy fabrication. However, it brings difficulty in physical understanding of the fundamental mechanism because of the more complicated photophysical processes in T-OSCs. Accordingly, how the guest donor D₂ regulates the charge transfer mechanism was explored in theory using three T-OSCs containing two donors and an acceptor. The results point out that larger differences in molecular weight and/or backbone between D₂ and the host donor D₁ cause different charge transfer mechanisms, which hardly provide a coexisting charge transfer path. Besides, strong absorption capacity of D₂ with a high oscillator strength would produce favorable regulation of the charge transfer mechanism. Therefore, this work clarifies the influence of D₂ on the charge transfer mechanism in T-OSCs, which suggests that the method of improving the power conversion efficiency cannot be generalized but rather must be tailored to specific conditions.

Concurrently Improved Jsc, Fill Factor, and Stability in a Ternary Organic Solar Cell Enabled by a C-Shaped Non-fullerene Acceptor and Its Structurally Similar Third Component

A ternary strategy is recognized as a promising approach that enjoys both the simplicity of fabrication conditions and potential to improve performance in organic solar cells. Herein, a C-shaped narrow band gap non-fullerene acceptor GL1 with a C_2v symmetry based on a new core was designed and synthesized. A power conversion efficiency (PCE) of 11.43% was achieved by employing PBDB-T:GL1 as an active layer to fabricate photovoltaic devices. To further promote photovoltaic performance, following a similar-structure prescreen principle, a middle band gap acceptor F-2Cl with the same backbone shape, side-chain distribution, and dipole moment orientation as GL1 was introduced as the guest acceptor into the active layer. Thus, benefiting from the collaboration of complementary absorption, cascade energy levels, and well-modified microstructure of the active layer, a 13.17% PCE was obtained with concurrently elevated J_sc, fill factor, and stability for the optimized ternary device. This work presents a successful example of prescreening the third component to simplify the workload for a high-performance ternary device.

Recent Progress of Organic Photovoltaics with Efficiency over 17%

The power conversion efficiency (PCE) of organic photovoltaics (OPVs) has exceeded 18% with narrow bandgap, non-fullerene materials Y6 or its derivatives when used as an electron acceptor. The PCE improvement of OPVs is due to strong photon harvesting in near-infrared light range and low energy loss. Meanwhile, ternary strategy is commonly recognized as a convenient and efficient means to improve the PCE of OPVs. In this review article, typical donor and acceptor materials in prepared efficient OPVs are summarized. From the device engineering perspective, the typical research work on ternary strategy and tandem structure is introduced for understanding the device design and materials selection for preparing efficient OPVs.

Hybrid Hole Extraction Layer Enabled High Efficiency in Polymer Solar Cells

Charge extraction layers with excellent charge extraction capability are essential for achieving high photovoltaic performance in cells. In this work, a hole extraction layer (HEL) is developed by doping conductive polymer TFB into CuSCN (CuSCN:TFB(X)), which exhibits good light transparency and high affinity for the light absorber. Compared to the reference cell, the CuSCN:TFB(X) HEL-based cells show impressive enhancement owing to the increased exciton dissociation and charge extraction processes and weak recombination losses. Furthermore, matched work function, better interface contact, and appropriate domain size also contribute to the enhanced power conversion efficiency. As a consequence, the highest conversion efficiency of 15.28% is observed in a cell based on the PM6:Y6 blend film and CuSCN:TFB(1.0%) HEL, which is >16% higher than the efficiency of 13.13% in a cell with CuSCN HEL.

Optimized Active Layer Morphologies via Ternary Copolymerization of Polymer Donors for 17.6 % Efficiency Organic Solar Cells with Enhanced Fill Factor

Regulating molecular structure to optimize the active layer morphology is of considerable significance for improving the power conversion efficiencies (PCEs) in organic solar cells (OSCs). Herein, we demonstrated a simple ternary copolymerization approach to develop a terpolymer donor PM6-Tz20 by incorporating the 5,5'-dithienyl-2,2'-bithiazole (DTBTz, 20 mol%) unit into the backbone of PM6 (PM6-Tz00). This method can effectively tailor the molecular orientation and aggregation of the polymer, and then optimize the active layer morphology and the corresponding physical processes of devices, ultimately boosting FF and then PCE. Hence, the PM6-Tz20: Y6-based OSCs achieved a PCE of up to 17.1% with a significantly enhanced FF of 0.77. Using Ag (220 nm) instead of Al (100 nm) as cathode, the champion PCE was further improved to 17.6%. This work provides a simple and effective molecular design strategy to optimize the active layer morphology of OSCs for improving photovoltaic performance.

A Narrow-Bandgap n-Type Polymer with an Acceptor-Acceptor Backbone Enabling Efficient All-Polymer Solar Cells

Narrow-bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow-bandgap polymer acceptor L14, featuring an acceptor-acceptor (A-A) type backbone, is synthesized by copolymerizing a dibrominated fused-ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A-A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low-lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open-circuit voltage (V_oc ), which is attributed to a small nonradiative recombination loss (E_loss,nr ) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and V_oc , an excellent efficiency of 14.3% is achieved, which is among the highest values for all-polymer solar cells (all-PSCs). The results demonstrate the superiority of narrow-bandgap A-A type polymers for improving all-PSC performance and pave a way toward developing high-performance polymer acceptors for all-PSCs.

Effect of Nitro-Substituted Ending Groups on the Photovoltaic Properties of Nonfullerene Acceptors

Chemical modification of end groups has proved to be an effective way to design new acceptor-donor-acceptor (A-D-A)-structured nonfullerene acceptors (NFAs) for high-performance organic solar cells (OSCs). Herein, we designed and synthesized two nitro-substituted end groups, N1 and N2. Using the two end groups as A units, two A-D-A acceptors, F-N1 and F-N2, were obtained. It also has been found that the nitro substitution position on end groups affects not only the absorptions and energy levels of the resultant acceptor materials but also their molecular packing behavior and active layer morphologies. In addition, the devices based on the two acceptors showed different energy losses. Power-conversion efficiencies (PCEs) of 10.66 and 11.86% were achieved for F-N1- and F-N2-based devices, respectively. This work reveals that the nitration of end groups is one of the potential strategies for designing high-performance photovoltaic active layer materials.

Efficient Fullerene-Free Organic Solar Cells Using a Coumarin-Based Wide-Band-Gap Donor Material

In recent years, tremendous growth has been seen for solution-processed bulk heterojunction solar cells (BHJSCs) using fullerene-free molecular acceptors. Herein, we report the synthesis, characterization of a coumarin-based organic semiconducting molecule C1, and its use in BHJSCs as an electron donor. The compound exhibited an absorption band at 472 nm in chloroform solution with an optical energy gap of 2.33 eV. The HOMO/LUMO energy levels of C1 were found to be ideal for use in BHJSCs. Using PC₇₁BM and a fullerene-free acceptor IT-4F, the device generated power conversion efficiencies (PCEs) of 6.17 and 8.31%, respectively. The success of the device based on a fullerene-free acceptor is a result of complementary absorption and well-matched energy levels, resulting in an improved photocurrent and photovoltage in the device. Moreover, ternary solar cells fabricated by employing C1 (20 wt%) as a secondary donor, i.e., an active layer of C1:PM6:IT-4F (0.2:0.8:1.5), generated an enhanced PCE of 11.56% with a high short-circuit current density (J_SC) of 16.42 mA cm^-2, an open-circuit voltage (V_OC) of 1.02 V, and a fill factor of 0.69 under 1 sun spectral illumination, which is ∼8% higher than that for the PM6:IT-4F-based binary device (PCE = 10.70%). The increased PCE for the ternary organic solar cell may be related to the efficient exciton generation and its dissociation via Forster resonance energy transfer, which guarantees enough time for an exciton to diffuse toward the D/A interfaces.

Enhanced and Balanced Charge Transport Boosting Ternary Solar Cells Over 17% Efficiency

Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, an OSC with a ternary architecture featuring a highly crystalline molecular donor DRTB-T-C4 as a third component to the host binary system consisting of a polymer donor PM6 and a nonfullerene acceptor Y6 is reported. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor (FF) of 0.813 and yielding an impressive PCE of 17.13%. The heterojunctions are designed using so-called pinning energies to promote exciton separation and reduce recombination loss. In addition, the preferential location of DRTB-T-C4 at the interface between PM6 and Y6 plays an important role in optimizing the morphology of the active layer.

Comparing Benzodithiophene Unit with Alkylthionaphthyl and Alkylthiobiphenyl Side-Chains in Constructing High-Performance Nonfullerene Solar Cells

Using single-bonded and fused aromatic rings are two methods for extending the π-conjugation in the vertical direction of benzo [1,2-b:4,5-b'] dithiophene (BDT) unit. To investigate which method is more efficient in nonfullerene systems, two novel polymers based on alkylthionaphthyl and alkylthiobiphenyl substituted BDT named PBDTNS-FTAZ and PBDTBPS-FTAZ are designed and synthesized. Two polymers only exhibit small differences in structure, but huge differences in photovoltaic properties. They are studied by blended with 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno [2,3-d':2,3'-d']-s-indaceno [1,2-b:5,6-b'] dithiophene (ITIC). The device based on PBDTNS-FTAZ:ITIC showed the best power conversion efficiency (PCE) of 9.63% with the Voc of 0.87 V, a Jsc of 18.06 mA/cm2 and a fill factor of 61.21%, while the PBDTBPS-FTAZ:ITIC only exhibit a maximum PCE of 7.79% with a Voc of 0.86 V, a Jsc of 16.24 mA/cm2 and a relatively low fill factor of 55.92%. Therefore, extending π-conjugation with alkylthionaphthyl is more effective against constructing nonfullerene solar cells.

Conformation-Tuning Effect of Asymmetric Small Molecule Acceptors on Molecular Packing, Interaction, and Photovoltaic Performance

Understanding the conformation effect on molecular packing, miscibility, and photovoltaic performance is important to open a new avenue for small-molecule acceptor (SMA) design. Herein, two novel acceptor-(donor-acceptor1-donor)-acceptor (A-DA1D-A)-type asymmetric SMAs are developed, namely C-shaped BDTP-4F and S-shaped BTDTP-4F. The BDTP-4F-based polymer solar cells (PSCs) with PM6 as donor, yields a power conversion efficiency (PCE) of 15.24%, significantly higher than that of the BTDTP-4F-based device (13.12%). The better PCE for BDTP-4F-based device is mainly attributed to more balanced charge transport, weaker bimolecular recombination, and more favorable morphology. Additionally, two traditional A-D-A-type SMAs (IDTP-4F and IDTTP-4F) are also synthesized to investigate the conformation effect on morphology and device performance. Different from the device result above, here, IDTP-4F with S-shape conformation outperforms than IDTTP-4F with C-shape conformation. Importantly, it is found that for these two different types of SMA, the better performing binary blend has similar morphological characteristics. Specifically, both PM6:BDTP-4F and PM6:IDTP-4F blend exhibit perfect nanofibril network structure with proper domain size, obvious face-on orientation and enhance donor-acceptor interactions, thereby better device performance. This work indicates tuning molecular conformation plays pivotal role in morphology and device effciciency, shining a light on the molecular design of the SMAs.

Over 15% Efficiency in Ternary Organic Solar Cells by Enhanced Charge Transport and Reduced Energy Loss

In this study, an efficient ternary bulk-heterojunction (BHJ) organic solar cell (OSC) is demonstrated by incorporating two acceptors, PC₆₁BM and ITC6-4F, with a polymer donor (PM6). This reveals that the addition of PC₆₁BM not only enhances the electron mobility of the derived BHJ blend but also facilitates exciton dissociation, resulting in a more balanced charge transport alongside with reduced trap-assisted charge recombination. Consequently, as compared to the pristine PM6/ITC6-4F device, the optimal ternary OSC is revealed to deliver an improved power conversion efficiency (PCE) of 15.11% with a boosted J_SC, V_OC, and fill factor (FF) simultaneously. The resultant V_OC and FF are among the highest values recorded in the literature for the ternary OSCs with a PCE exceeding 15%. This result thus suggests that besides improving the charge transport characteristics in devices, incorporating a fullerene derivative as part of the acceptor can also improve the resultant V_OC, which can reduce the energy loss to realize efficient organic photovoltaics.