Efficient Non-fullerene Organic Solar Cells Research Articles

Regioisomeric thieno[3,4-d]thiazole-based A-Q-D-Q-A-type NIR acceptors for efficient non-fullerene organic solar cells.

This study explores the potential of regioisomeric quinoidal-resonance π-spacers in designing near-infrared (NIR) non-fullerene acceptors (NFAs) for high-performance organic solar cell devices. Adopting thienothiazole as the π-spacer, two new isomeric A-Q-D-Q-A NFAs, TzN-S and TzS-S, are designed and synthesized. Both NFAs demonstrate a broad spectral response extended to the NIR region. However, they exhibit different photovoltaic properties when they were mixed with the PCE10 donor to fabricate respective solar cells. The optimal device of TzS-S achieves a PCE of 10.75%, much higher than that of TzN-S based ones (6.13%). The more favorable energetic offset and better molecular packing contribute to the better charge generation and transport, which explains the relative superiority of TzS-S NFA. This work sheds new light on the regioisomeric effect of component materials for optoelectronic applications.

Designing thickness-insensitive cathode interlayers via constructing noncovalently conformational locks for highly efficient non-fullerene organic solar cells

A notable PCE of 15.31% was achieved for a thick cathode interlayer (53 nm) based on PF-BDT with “Noncovalent conformational lock”, which stands out as one of the highest reported PCEs for organic cathode interlayer with a thickness exceeding 50 nm.

Highly Efficient Nonfullerene Organic Solar Cells: Morphology Control and Characterizations

Nonfullerene acceptors (NFAs) are currently a major research focus in the development of organic solar cells (OSCs) because of their readily tunable optical and electronic properties, enabling bulk heterojunction (BHJ) NFA‐based OSCs to achieve photovoltaic efficiencies exceeding 19%. Significant efforts have been made to yield the optimal nanoscale morphology, enabling the achievement of highly efficient NFA‐based OSCs. This review discusses the structural characteristics of NFAs and their relationship with morphology. Subsequently, the correlation between the morphology and photovoltaic parameters is introduced, which provides a fundamental basis for morphology modulation. This review then points out the major challenges of morphological characterization of NFA‐based blend films while using some conventional real‐space techniques due to low phase contrast and summarizes recently emerging characterization techniques capable of characterizing high‐contrast phase morphologies at multiple length scales. Finally, strategies for targeted morphological optimization through materials design, processing solvents, posttreatment, and ternary strategies are presented, although it is challenging to obtain an ideal morphology (e.g., appropriate phase separation and ideal donor/acceptor molecular interactions) due to the anisotropic structural characteristics of NFAs in the as‐cast films. This review is expected to provide guidance to continuously advancing the success of NFA‐based OSCs from the morphology perspective in the future.

Introducing thiazole units into small-molecule acceptors for efficient non-fullerene organic solar cells

The end-group modification engineering has been widely applied in non-fullerene acceptors (NFAs) to achieve high-performance organic solar cells (OSCs). Herein, we designed and synthesized a series of small-molecule acceptors (SMAs), namely ODTz-γ, ODTz-m, DTTz-γ and DTTz-m, which containing the electron-withdrawing thiazole units on the end groups at different position. These SMAs not only exhibited strong and broad absorption region from 500 nm to 900 nm, identical band gaps, but also the similar energy levels, indicating the position of thiazole units have trivial effect on their absorbance and energy levels. OSCs were fabricated by integrating an electron-donating polymer PBDB-T with these resultant electron-accepting materials. Devices based on ODTz-γexhibited an obviously higher power conversion efficiency over 12.0%, which outperformed those of obtained from devices based on other SMAs. Further characterization of the devices revealed that introducing the thiazole units into the γ position of end groups can enhance the charge transport mobility, suppress recombination, and improve the blend film morphology.

Impact of Intermolecular Interactions between Halogenated Volatile Solid Additives and the Nonfullerene Acceptor in Organic Solar Cells

AbstractThe halogenated volatile solid additives can delicately optimize the active layer morphology of organic solar cells, improving the devices' performance, stability, and reproducibility. However, what type of intermolecular interaction occurs between the solid additives and the active layer and whether the interaction truly impacts the donor or acceptor remains debatable. Herein, the focus is on halogenated volatile solid additives with conjugated benzene rings and their influence on the morphology of the active layer composed of PM6:Y6 as they evaporated. The absorbance spectra exhibit apparent red‐shift features in Y6 absorption regions, while the donor part is unaffected. The theoretical calculation results reveal that the additives can stay between two Y6 molecules and form halogen bonds, affecting the π–π aggregation properties of Y6. As a result, the crystalline features of the active layer are altered, leading to increased charge carrier mobilities, extended charge carrier diffusion lengths, reduced bimolecular charge recombination, and thus the device performance. Especially when 1,3,5‐tri bromobenzene is used, a champion power conversion efficiency of 17.9% is attained, among the best‐performed organic solar cells comprising PM6:Y6. The findings shed light on theoretical and experimental guidelines for designing and developing volatile solid additives for highly efficient nonfullerene organic solar cells.

Double-Dipole Induced by Incorporating Nitrogen-Bromine Hybrid Cathode Interlayers Leads to Suppressed Current Leakage and Enhanced Charge Extraction in Non-Fullerene Organic Solar Cells.

The cathode interlayer plays a vital role in organic solar cells, which can modify the work function of electrodes, lower the electron extraction barriers, smooth the surface of the active layer, and remove solvent residuals. However, the development of organic cathode interlayer lags behind the rapidly improved organic solar cells because their intrinsic high surface tension can lead to poor contact with the active layers. Herein, a double-dipole strategy is proposed to enhance the properties of organic cathode interlayers, which is induced by incorporating nitrogen- and bromine-containing interlayer materials. To verify this approach, the state-of-the-art active layer composed of PM6:Y6 and two prototypical cathode interlayer materials, PDIN and PFN-Br is selected. Using the cathode interlayer PDIN: PFN-Br (0.9:0.1, in wt.%) in the devices can reduce the electrode work function, suppress the dark current leakage, and improve charge extractions, leading to enhanced short circuit current density and fill factor. The bromine ions tend to break from PFN-Br and form a new chemical bond with the silver electrode, which can adsorb extra dipoles directed from the interlayer to silver. These findings on the double-dipole strategy provide insights into the hybrid cathode interlayers for efficient non-fullerene organic solar cells.

Efficient non-fullerene organic solar cells employing aqueous solution-processed MoO3 as a hole-transporting layer

Organic solar cell (OSC) has drawn considerable interest in recent decades owing to their advantages of light weight, flexible, large area and potentially low-cost. Employing an appropriate hole-transporting layer (HTL) into an OSC device has been proved as an efficient method to obtain high efficiency OSC due to the enhancement of the hole transporting and extraction of the device. In this work, aqueous solution-processed MoO3 (s-MoO3) thin films were employed as HTLs to construct non-fullerene PM6:Y6 OSCs. The s-MoO3 thin film was prepared by using an aqueous solution process from an isopolymolybdate [NH4]6Mo7O24.4H2O precursor followed by thermal annealing treatment to convert the precursor to MoO3. The s-MoO3 HTL based PM6:Y6 device demonstrates a power conversion efficiency of 15.75%, which is 38% improved than that of the device with thermally evaporated-MoO3 as HTL and 8% improved than that of the device with PEDOT:PSS as HTL. The enhancement of the device performance could be attributed to the enhanced hole mobility and better band matching of the s-MoO3 HTL. Moreover, the s-MoO3 HTL based PM6:Y6 device exhibited higher device stability than those of the reference devices. Our finding indicates that this s-MoO3 film has great potential as efficient HTL for high performance non fullerene OSCs.

All‐Green Solvent and Additive Combination Enables Efficient Nonfullerene Organic Solar Cells via Sequential Deposition Strategy

The performance of organic solar cells mainly depends on the morphology of the active layer. Traditional solution‐processed active layers have poor performance due to the random distribution of donor and acceptor materials during solution processing. In addition, halogenated solvents and additives widely used in traditional fabrication processes will have a huge impact on human health and the environment. Herein, an all‐green solvent and additive combination strategy is proposed to assist the morphology control of the active layer with sequential deposition (SD) technique. Ultimately, with adopting benzyl viologen (BV) additive as well as ethyl alcohol treatment, PM6:Y6‐2BrO‐based devices yield a power conversion efficiency of 14.3%, higher than those of the control device (13.24%) and the device solely with BV additives (13.62%). The result demonstrates the feasibility of SD technique to finely control the morphology of active layer in future all‐green industrial production.

Halogen-Free -Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility.

In -conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a -conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (VOC s) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and VOC s of more than 0.84V are observed, both of which are among the top values reported so far for "halogen-free" polymers. This study will serve as an important reference for designing -conjugated polymers to achieve highly efficient and solution-processable OPVs.

Highly Efficient Nonfullerene Organic Solar Cells with Nickel Oxide Hole‐Transporting Layer: Using Dipole‐Induced Energy‐Level Modification

To date, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most commonly used hole‐transporting material (HTM) for nonfullerene acceptor (NFA) organic solar cell (OSCs) due to its high hole conductivity and reproducibility. However, considering the significant breakthroughs made by novel donor and NFA combinations such as PM6 and Y6, the development of HTMs which can outperform PEDOT:PSS is very limited. Here, for the first time, high‐performance NFA OSCs based on simple and solution‐processed nickel oxide HTMs with high power conversion efficiency (PCE) and stability are reported. Two different kinds of hydroquinone sulfonic acid (HSA) salts, which have a permanent dipole moment, are added to NiO solution. Due to the interfacial dipole moment generated by these salts and effective work function modulation of NiO, efficient hole transportation at the interface between hole‐transporting layer and bulk heterojunction (BHJ) layer by minimizing the energetic barrier is promoted. As a result, using PM6 and the Y6‐BO‐4F‐based BHJ layer, the PCE of 14.19% with very high open‐circuit voltage of 0.86 V is achieved, which is comparable with that of PEDOT:PSS‐based OSCs. For 1200 h under ambient atmosphere, OSCs based on NiO containing HSA salts demonstrate enhanced ambient stability compared to that of PEDOT:PSS.

Passivating the Defects and Modulating the Surface Energy of ZnO Cathode Interlayer for Efficient Nonfullerene Organic Solar Cells

Charge transport is a meaningful process in nonfullerene organic solar cells (NOSCs). However, owing to the inherent defects of its metal oxide, the classic ZnO electron transport layer will severely hinder charge transport, and its high surface energy can cause a mismatch contact with the low surface energy active layer. Therefore, polyethylenimine ethoxylated (PEIE) is treated with quaternization and esterification to obtain polyethylenimine ethoxylated‐phenyl acetyl chloride (PEIE‐PAC). ZnO:PEIE‐PAC (ZnO‐P) with fewer defects and lower surface energy film is prepared by blending PEIE‐PAC with the precursor solution of ZnO. Compared with ZnO‐based control device with a power conversion efficiency (PCE) of 15.30%, the PCE of ZnO‐P‐based device is increased to 16.47% in PM6:Y6 system due to the optimization of oxygen defects and surface roughness. Meanwhile, owing to the decrease of surface energy in ZnO‐P, the contact between ZnO‐P and hydrophobic nonfullerene acceptors is closer, and it can isolate water and oxygen in the external environment, thus enhancing the stability in air of NOSCs. The ZnO‐P‐based PM6:Y6 device maintains long‐term storage stability in both N2 and ambient temperature, and after 1000 h aging, the PCE of ZnO‐P‐based device is still more than 85% of the initial PCE.

Enhancing Efficiency of Nonfullerene Organic Solar Cells via Using Polyelectrolyte-Coated Plasmonic Gold Nanorods as Rear Interfacial Modifiers.

Sufficient sunlight absorption and exciton generation are critical for developing efficient nonfullerene organic solar cells (OSCs). In this work, polyelectrolyte polystyrenesulfonate (PSS)-coated plasmonic gold nanorods (GNRs@PSS) were incorporated, for the first time, into the inverted nonfullerene OSCs as rear interfacial modifiers to improve sunlight absorption and charge generation via the near-field plasmonic and backscattering effects. The plasmonic GNRs effectively improved the sunlight absorption and enhanced the charge generation. Meanwhile, the negatively charged PSS shell ensured the uniform dispersion of the GNRs on the surface of the photoactive layer, optimized the interfacial contact, and further promoted the hole transport to the electrode. These concerted synergistic effects augmented the efficiency (10.11%) by nearly 20% relative to the control device (8.47%). Remarkably, the ultrathin (∼2.2 nm) organic layer on the surface of GNRs was closely examined by acquiring the carbon contrast image through energy-filtered transmission electron microscopy (EF-TEM), which clearly confirmed the coating uniformity from the side to end-cap of GNRs. The surface plasmon resonance (SPR) effect of the GNRs@PSS on the surface of the photoactive layer was unprecedentedly mapped by photoinduced force microscopy (PiFM) under the illumination of a tunable wavelength supercontinuum laser mimicking sunlight. Furthermore, investigations into the effect of size, surface coverage, and incorporation location of GNRs@PSS on the performance of OSCs revealed that the appropriate design and incorporation of the plasmonic nanostructures are crucial, otherwise the performance can be decreased, as evidenced in the case of front interface integration.

Twisted A‐D‐A Type Acceptors with Thermally‐Activated Delayed Crystallization Behavior for Efficient Nonfullerene Organic Solar Cells

AbstractMolecular aggregation and crystallization during film coating play a crucial role in the realization of high‐performing organic photovoltaics. Strong intermolecular interactions and high solid‐state crystallinity are beneficial for charge transport. However, fast crystallization during thin‐film drying often limits the formation of the finely phase‐separated morphology required for efficient charge generation. Herein, the authors show that twisted acceptor‐donor‐acceptor (A‐D‐A) type compounds, containing an indacenodithiophene (IDT) electron‐rich core and two naphthalenediimide (NDI) electron‐poor units, leads to formation of mostly amorphous phases in the as‐cast film, which can be readily converted into more crystalline domains by means of thermal annealing. This design strategy solves the aforementioned conundrum, leading to an optimal morphology in terms of reduced donor/acceptor domain‐separation sizes (ca. 13 nm) and increased packing order. Solar cells based on these acceptors with a PBDB‐T polymer donor show a power conversion efficiency over 10% and stable morphology, which results from the combined properties of desirable excited‐state dynamics, high charge mobility, and optimal aggregation/crystallization characteristics. These results demonstrate that the twisted A‐D‐A motif featuring thermally‐induced crystallization behavior is indeed a promising alternative design approach toward more morphologically robust materials for efficient organic photovoltaics.

Block copolymer compatibilizer for efficient and stable nonfullerene organic solar cells

High-efficiency nonfullerene organic solar cells with long-term stability is crucial for future commercialization in the field of organic photovoltaics. Herein, we designed and synthesized a block copolymer, namely PM6-b-PTY6, containing both donor PM6 and Y6 segments in the polymer backbone. We then first demonstrated that the use of such block copolymer as a compatibilizer in high-efficiency PM6:Y6 based benchmark nonfullerene solar cells, simultaneously enhancing their power conversion efficiency and long-term stability. Detailed experimental results reveal that the addition of PM6-b-PTY6 into PM6:Y6 blend can effectively optimize carrier dynamic process, enhance donor–acceptor miscibility, and stabilize the blend morphology. Additionally, this strategy shows similar function when applied to other system like D18-Cl:Y6. As a result, enhanced efficiencies of 16.48% and 17.55% for PM6:Y6 and D18-Cl:Y6 solar cell are achieved, respectively. More importantly, the inverted PM6:Y6 devices exhibit improved stability, retaining 0.81 and 0.86 of the initial efficiency after 550 h aging under ambient and thermal stress, respectively. These results indicate that the compatible block copolymer could provide a fresh strategy to design more efficient nonfullerene organic solar cells.

Quasi-three-dimensional self-doped conjugated polyelectrolytes based on a triphenylamine skeleton for non-fullerene organic solar cells

Two quasi-three-dimensional self-doped conjugated polyelectrolytes based on a triphenylamine skeleton are designed and employed as ETLs to modulate the cathode interface for efficient non-fullerene organic solar cells.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties.

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018–2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Boosting the Efficiency of Non-fullerene Organic Solar Cells via a Simple Cathode Modification Method.

This work demonstrates a simple yet effective method to significantly improve the power conversion efficiency (PCE) of highly efficient non-fullerene organic solar cells by mixing two electron transport materials. The new electron transport layer shows an energy level better aligned with the active layer and an improved morphology that could reduce the active layer-electrode contact. These improvements lead to enhanced charge extraction, better charge selectivity, suppressed exciton recombination, and finally a boosted PCE in the PM6:Y6-based solar cells. When applied in conjunction with the non-halogenated solvent-processed PM6:PY-IT-based active layer, the mixed ETL also gives rise to a leading result for binary all-polymer solar cells (PCE of >16%) with a concurrent increase in VOC, JSC, and FF.

N-Type Self-Doped Hyperbranched Conjugated Polyelectrolyte as Electron Transport Layer for Efficient Nonfullerene Organic Solar Cells.

The electron transport layer (ETL) exerts a dramatic influence on the power conversion efficiency (PCE) of the nonfullerene organic solar cells (NOSCs). Currently, the majority of the organic ETLs possess a relatively poor conductivity, which is not conducive to carrier transport and collection. Herein, we design and develop a novel hyperbranched conjugated polyelectrolyte (CPE) based on n-type perylene diimide (PDI) as the center core and quaternary ammonium salt as the side polar groups. The lone pair electrons of the nitrogen atoms can transfer to the electron deficient PDI core and endow the molecule with an efficient n-type self-doping effect. Moreover, the hyperbranched structure makes the molecule functionalized with more side polar groups, favoring forming more dipoles and stronger dipole moments. Therefore, the CPE PTPAPDINO possesses a high conductivity and can notably decrease the work function (WF) of the electrode, contributing to the carrier transport and collection of the device. The NOSC with PTPAPDINO as ETL delivers an excellent PCE of 15.62%, which is even superior to the device using the classical PDINO ETL. Moreover, the PCE can retain 82.6% of the optimal device when the thickness has been increased to 28 nm. These results manifest that it is a feasible strategy to design an n-type self-doping hyperbranched CPE as efficient ETL, and PTPAPDINO is a promising alternative ETL for high performance NOSCs.

Effects of electron-withdrawing group and π-conjugation length in donor-acceptor oligothiophenes on their properties and performance in non-fullerene organic solar cells

The development of small-molecule donor materials for highly efficient non-fullerene (NF) organic solar cells (OSC) remains an urgent research task. In this work, the synthesis of donor-acceptor molecules containing either five (5T) or seven (7T) thiophene units end-capped with methyl dicyanovinyl (DCV) or ethyl cyanoacetate (CNA) electron-withdrawing groups was reported. Their thermal stability, phase behaviour, optical and electrochemical properties as well as photovoltaic performance in blends with IDIC or Y6 as acceptor materials were comprehensively investigated and compared. All of the obtained donor molecules are characterized by high thermal stability, efficient sunlight absorption and excellent solubility in combination with a good crystallinity. The results obtained allowed us to reveal the impact of the oligothiophene conjugation length and type of electron-withdrawing group on the properties and performance of the D-A oligothiophenes in NF OSC. The results clearly demonstrate that the devices based on donor molecules with 7T or CNA fragments significantly outperform those based on 5T or DCV groups.

Developing Halogen‐Free Polymer Donors for Efficient Nonfullerene Organic Solar Cells by Addition of Highly Electron‐Deficient Diketopyrrolopyrrole Unit

High‐performance polymer donors when paired with nonfullerene acceptors are mainly limited to flanking halogenated benzodithiophene (BDT)‐based π‐conjugated copolymers, which however involve complex synthetic procedures. Herein, a series of halogen‐free polymer donors that link BDT moiety with two highly electron‐deficient benzodithiophene‐dione (BDD) and diketopyrrolopyrrole (DPP) units with various molar ratios is developed. Compared with the benchmark PBDB‐T donor containing BDD unit, additional incorporation of a stronger electron‐negative DPP unit markedly lowers frontier molecular orbital levels and extends optical absorption, potentially leading to simultaneously enhanced VOC and JSC in organic solar cells. A remarkable power conversion efficiency (PCE) of 10.28% is thus obtained in the optimal P75 (BDD : DPP = 3:1 mol%) and Y6 blend cells in comparison with the reference PBDB‐T:Y6 (9.20%). A slight addition of PC71BM into the blend is found to further generate finer phase‐separated domains and thus increase the best efficiency up to 12.20%. The subtly critical roles of PC71BM are determined by transient absorption measurements on both thin‐film and in situ devices to be the prolonged free charge carrier lifetime and the shallow charge transfer states, which enhance JSC and fill factor in the device, respectively.