Non-fullerene System Research Articles

18.4% efficiency achieved by the cathode interface engineering in non-fullerene polymer solar cells

The suitable cathode interlayer (CIL) plays a vital role in improving the photovoltaic performances of polymer solar cells, and small-molecule CILs with simple structure and significant modification are believed to have a prospect for commercialization. Herein, we apply the molecular tailoring strategy and synthesize a conjugated small-molecule CIL named SME1, which shows the better reproducibility in different batches than that polymer counterpart and thus ensure the consistent modification effect. It is found that, except for optimizing the molecule/metal contact to decrease the traps, SME1 can also extract electrons from the polymer donor and establish the available n-doping with the non-fullerene acceptor, which are conducive to more efficient exciton dissociation and charge collection in varied non-fullerene systems. By applying SME1 as CIL, one of top power conversion efficiencies of 18.4% (17.9% by the third-party certification) is achieved in PM6:BTP-eC9-based device. In this sense, SME1 can indeed help the emerging photovoltaic materials realize their full potential.

Tackling voltage losses

Voltage losses limit the performance of organic solar cells, yet their origins are not fully understood. Now, a theoretical model encompassing electronic state hybridization and thermal population of vibrational states explains the reduced non-radiative voltage losses in efficient non-fullerene acceptor systems.

An antimonene modified hole extraction layer for high efficiency PEDOT:PSS-free nonfullerene organic solar cells

In this paper, a bilayer hole extraction layer (HEL) with solution-processed molybdenum trioxide (MoO 3 ) and two-dimensional (2D) material of antimonene was developed to achieve high performance nonfullerene organic solar cells (NF–OSCs). The application of antimonene facilitates effective charge extraction and lowered recombination loss, achieving improved photovoltaic performance. By inserting the antimonene layer, power conversion efficiency ( PCE ) of devices with MoO 3 HEL was increased from 8.92% to 11.30% in OSCs with non-fullerene systems of PBDB-T-2F:IT-4F, which was even much higher than that of the devices with PEDOT:PSS HEL (10.59%). Results make it clear that the solution-processed bilayer MoO 3 /antimonene HEL shows great potential for application in high performance PEDOT:PSS-free NF–OSCs. • A bilayer HEL with solution-processed MoO 3 and antimonene was developed to achieve high performance PEDOT:PSS-free NF-OSCs. • The MoO 3 /antimonene HEL favors the efficient operation of NF–OSCs, achieving suppressed trap-limited recombination and enhanced charge extraction efficiency.

Mitigating Dark Current for High-Performance Near-Infrared Organic Photodiodes via Charge Blocking and Defect Passivation.

Thin-film organic near-infrared (NIR) photodiodes can be essential building blocks in the rapidly emerging fields including the internet of things and wearable electronics. However, the demonstration of NIR organic photodiodes with not only high responsivity but also low dark current density that is comparable to that of inorganic photodiodes, for example, below 1 nA cm-2 for silicon photodiodes, remains a challenge. In this work, we have demonstrated non-fullerene acceptor-based NIR photodiodes with an ultralow dark current density of 0.2 nA cm-2 at -2 V by innovating on charge transport layers to mitigate the reverse charge injection and interfacial defect-induced current generation. The same device also shows a high external quantum efficiency approaching 70% at 850 nm and a specific detectivity of over 1013 Jones at wavelengths up to 940 nm. Furthermore, the versatility of our approach for mitigating dark current is demonstrated using a NIR photodetector utilizing different non-fullerene systems. Finally, the practical application of NIR organic photodiodes is demonstrated with an image sensor integrated on a silicon CMOS readout. This work provides new insight into the device stack design of low-dark current NIR organic photodiodes for weak light detection.

Phase Diagrams of Ternary π-Conjugated Polymer Solutions for Organic Photovoltaics.

Phase diagrams of ternary conjugated polymer solutions were constructed based on Flory-Huggins lattice theory with a constant interaction parameter. For this purpose, the poly(3-hexylthiophene-2,5-diyl) (P3HT) solution as a model system was investigated as a function of temperature, molecular weight (or chain length), solvent species, processing additives, and electron-accepting small molecules. Then, other high-performance conjugated polymers such as PTB7 and PffBT4T-2OD were also studied in the same vein of demixing processes. Herein, the liquid-liquid phase transition is processed through the nucleation and growth of the metastable phase or the spontaneous spinodal decomposition of the unstable phase. Resultantly, the versatile binodal, spinodal, tie line, and critical point were calculated depending on the Flory-Huggins interaction parameter as well as the relative molar volume of each component. These findings may pave the way to rationally understand the phase behavior of solvent-polymer-fullerene (or nonfullerene) systems at the interface of organic photovoltaics and molecular thermodynamics.

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Summary Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC).

High-Efficiency Thickness-Insensitive Organic Solar Cells with an Insulating Polymer.

Achieving high-efficiency thick-film bulk heterojunction (BHJ) organic solar cells (OSCs) with thickness-independent power conversion efficiencies (PCEs) in a wide thickness range is still a challenge for the roll-to-roll printing techniques. The concept of diluting the transport sites within BHJ films with insulating polymers can effectively eliminate charge trapping states and optimize the charge transport. Herein, we first adopted the concept with insulating polypropylene (PP) in the efficient non-fullerene system (PM6:Y6) and demonstrated its potential to fabricate thick-film OSCs. The PP can form an insulating matrix prior to PM6 and Y6 within the BHJ film, resulting in an enhanced molecular interaction and isolated charge transport by expelling Y6 molecules. We thus observed reduced trap state density and improved charge transport properties in the PP-blended device. At around 300 nm, the PM6:Y6:PP device enjoys a high PCE of 15.5% and achieves over 100% of the efficiency of the optimal thin-film device, which is significantly improved compared to the binary PM6:Y6 counterpart. This research promotes an effective strategy with insulating polymers and provides knowledge of commercial production with response to the roll-to-roll technique demands.

Calculation aided miscibility manipulation enables highly efficient polythiophene:nonfullerene photovoltaic cells

Polythiophenes, with merits of low cost and high scalability of synthesis, have received growing interest in organic solar cells. To date, the best-performing polythiophene:nonfullerene solar cells exhibit typical power conversion efficiencies (PCEs) of 10%–12%, which is much lower than those employing PM6- and D18-type polymers. This inferior performance is mostly limited by the improper miscibility between polythiophene and acceptors. Efforts on engineering the molecular structure to systematically tune the miscibility are required. With the aid of group contribution calculations, the miscibility of polythiophene:nonfullerene blend system was finely tuned by varying the ratios of siloxane-terminated chains and alkyl chains in ester-substituted polythiophenes through random copolymerization. Based on a series of the polythiophene and nonfullerene acceptors, the detailed analysis of blend miscibility and performance reveals a surprising anticorrelation between the Flory-Huggins interaction parameter ( χ aa) and the optimal time of solvent vapor annealing for device performance across these systems. Primarily due to the slightly higher χ aa, the blend of PDCBT-Cl-Si5 and ITIC-Th1 results in a record-high PCE of 12.85% in polythiophene:nonfullerene solar cells. The results not only provide a calculation-guided approach for molecular design but also prove that precise control of the miscibility is an effective way to design high-performance polythiophene:nonfullerene blends and beyond.

Molecular Engineering and Morphology Control of Polythiophene:Nonfullerene Acceptor Blends for High‐Performance Solar Cells

AbstractWith the advances in organic photovoltaics (OPVs), the development of low‐cost and easily accessible polymer donors is of vital importance for OPV commercialization. Polythiophene (PT) and its derivatives stand out as the most promising members of the photovoltaic material family for commercial applications, owing to their low cost and high scalability of synthesis. In recent years, PTs, paired with nonfullerene acceptors, have progressed rapidly in photovoltaic performance. This Review gives an overview of the strategies in designing PTs for nonfullerene OPVs from the perspective of energy level modulation. A survey of the typical classes of nonfullerene acceptors designed for pairing with the benchmark PT, i.e., poly(3‐hexylthiophene) (P3HT) is also presented. Furthermore, recent achievements in understanding and controlling the film morphology for PT:nonfullerene blends are discussed in depth. In addition to the effects of molecular weight and blend ratio on film morphology, the crucial roles of miscibility between PT and nonfullerene and processing solvent in determining film microstructure and morphology are highlighted, followed by a discussion on thermal annealing and ternary active layers. Finally, the remaining questions and the prospects of the low‐cost PT:nonfullerene systems are outlined. It is hoped that this review can guide the optimization of PT:nonfullerene blends and advance their commercial applications.

Addition of alkynes and osmium carbynes towards functionalized d\u03c0\u2013p\u03c0 conjugated systems

The metal-carbon triple bonds and carbon-carbon triple bonds are both highly unsaturated bonds. As a result, their reactions tend to afford cycloaddition intermediates or products. Herein, we report a reaction of M≡C and C≡C bonds that affords acyclic addition products. These newly discovered reactions are highly efficient, regio- and stereospecific, with good functional group tolerance, and are robust under air at room temperature. The isotope labeling NMR experiments and theoretical calculations reveal the reaction mechanism. Employing these reactions, functionalized dπ-pπ conjugated systems can be easily constructed and modified. The resulting dπ-pπ conjugated systems were found to be good electron transport layer materials in organic solar cells, with power conversion efficiency up to 16.28% based on the PM6: Y6 non-fullerene system. This work provides a facile, efficient methodology for the preparation of dπ-pπ conjugated systems for use in functional materials.

Ultrafast and Long-Range Exciton Migration through Anisotropic Coulombic Coupling in the Textured Films of Fused-Ring Electron Acceptors.

The exciton migration mechanism in organic photovoltaic devices is still an ambiguity owing to the insufficient understanding of molecular arrangement on a microscopic scale. Herein, we reveal the relationship between the molecular stacking modes and exciton migration for a representative fused-ring electron acceptor, namely, ITIC. The precise molecular stacking patterns are extracted, and directional Coulombic couplings are calculated based on the information of a single-crystal structure, which proves the anisotropic character for exciton motion. The theoretical analysis results indicate ultrafast exciton migration along the head-to-tail stacking directions with maximum migration length of 330 nm in the finite lifetime of 1 ns. Experimentally, the exciton diffusion length is determined to be 183 nm by exciton-exciton annihilation measurement. This work reveals head-to-tail type intermolecular stacking induces strong anisotropic Coulombic coupling, leading to the ultrafast and long-range exciton migration in nonfullerene systems.

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.

Polymer‐Modified ZnO Nanoparticles as Electron Transport Layer for Polymer‐Based Solar Cells

AbstractThe optimization of interfacial layer plays a critical role in the ultimate use of polymer‐based solar cells (PSCs). By introducing an insulating polymer, polystyrene (PS), into the ZnO nanoparticles (NPs) with large particle size, an electron transport layer (ETL) with a thickness of more than 130 nm is produced. The doping of PS not only improves the film quality of ZnO NPs to generate a denser, smoother, and more uniform ETL, but also increases the contact properties between the hydrophilic ZnO and hydrophobic active layer. In comparison to control devices, the power conversion efficiencies (PCEs), short circuit current densities, and fill factors of PSCs with the PS‐modified ETL for a typical fullerene system PTB7‐Th:PC71BM and, also, a nonfullerene system PBDB‐T:ITIC are increased, with PCEs from 8.49% to 9.54% and 10.03% to 11.05%, respectively. The reproducibility, mechanical endurance, and ambient stability of the PSCs with the PS‐modified ZnO NP ETL are significantly improved. The combination of the insulating polymer and ZnO NPs provides a simple, low‐cost way to realize the commercialization of high performance, flexible PSCs.

Developing low boiling point solvent additives directly based on non-fullerene based active layer: Higher efficiency and better thickness tolerance

Using solvent additive is an effective strategy to control the morphology of active layers. However, most additives with high boiling point (bp.) encounter an issue of long drying process of the active layer, which will increase the cost in commercial application. Herein, two different solvent additives 1,1-dimethoxyethylbenzene (DMB) and 6-mercapto-1-hexanol (MHA) with low bp. of ~200 and 225 °C, respectively, were employed to modify the PTB7-Th:EH-IDTBR non-fullerene system. The nonconjugated additive MHA has no solvation power on the PTB7-Th donor. Relative to the aromatic DMB, MHA can show sole affinity to the EH-IDTBR acceptor and the solidification of EH-IDTBR during film forming is more affected by the higher bp. MHA. It was found that phase separation, molecular aggregation, and balanced charge transport could be enhanced in an order of as-cast, DMB casted, and MHA casted blend films. At an active layer thickness of 100 nm, relative to 10.41% efficiency for as-cast device, solar cells casted with 0.5% DMB and MHA can show higher efficiencies of 10.81% and 11.36%, respectively. The morphological controls with DMB and MHA are also beneficial for thickness tolerance. DMB and MHA casted 300 nm thick active layers exhibit efficiencies of 9.07% and 8.11%, respectively, much higher than the 6.02% for the as-cast device. For the shelf-stability comparison up to 800 h in air, the DMB and MHA processed devices can remain 92.5% and 94% of the initial efficiency, respectively, better than 87.5% efficiency remaining for the as-cast device. Thereby, the DMB and MHA additives would be valuable toward highly efficient, more stable, and large scale NFA based solar cells.

Polyolefin Elastomer as the Anode Interfacial Layer for Improved Mechanical and Air Stabilities in Nonfullerene Solar Cells.

Despite the breakthroughs in power conversion efficiency (PCE) values of organic solar cells (OSCs), the other important issue concerns stability, which is urgently needed to be resolved for potential commercialization. A commercial and chemically stable polyolefin elastomer (POE) was incorporated into high-performance PBDB-T:ITIC, PM6:IT-4F, and PM6:Y6 nonfullerene systems to serve as the anode interfacial layer, affording remarkably improved mechanical and air stabilities when compared with those of the most studied MoO3 interfacial layer. The POE was found to selectively transport holes rather than electrons due to the upshifted surface contact potential of the active layer and the better ohmic contact between the active layer and the electrode. The POE serving as an encapsulating layer is supposed to suppress the penetration of water and oxygen in addition to the diffusion of Ag atoms into the active layer. After storing in an air environment with a humidity of approximately 70% for 150 days, the PCE of the device based on PM6:IT-4F with the POE anode interfacial layer decreased from 11.88 to 9.60%, retaining 80.8% of its original PCE value. The device using MoO3 as the anode interfacial layer showed a PCE value that was sharply reduced from 12.31 to 2.98% after storing for only 30 days. The POE could be potentially useful for flexible and large-scale device fabrication, accelerating the commercialization of OSCs.

2D Conjugated Polyelectrolytes Possessing Identical Backbone with Active-Layer Polymer as Cathode Interlayer for Organic Solar Cells.

A 2D conjugated polyelectrolyte (CPE), PBDTTh-TT-NBr, having the same backbone as the highly efficient donor polymer PTB7-Th and the quaternary ammonium pendant, is synthesized as a cathode interlayer (CIL) material for PTB7-Th-based fullerene and non-fullerene solar cells. The quaternary ammonium group is connected to the 2D conjugated backbone by a long, flexible alkyl chain, facilitating the modification of cathode via forming interface dipoles. Moreover, compared with another CPE analogue to PTB7, PBDT-TT-NBr, without the 2D conjugated backbone, the PBDTTh-TT-NBr presents a higher similarity in polymer structure to the donor polymer PTB7-Th. This feature makes it more compatible with the PTB7-Th-based active-layer film, improving the electron transport. With the PBDTTh-TT-NBr as the CIL, devices afford higher performances than those using the PBDT-TT-NBr in both fullerene and non-fullerene systems. This work offers guidance on choosing the CIL material that ought to possess a highly similar structure to the active-layer component.

Slot-die processing and encapsulation of non-fullerene based ITO-free organic solar cells and modules

Organic photovoltaic (OPV) devices have shown remarkable performance progress in recent years, reaching current record power conversion efficiency (PCE) values of 16.4% for single junction and 17.3% for multi junction devices, owing mostly to the impressive developments made within synthesis of new non-fullerene acceptors. This progress places organic solar cells at the forefront of thin-film photovoltaic technology. However, in order to meet industrial demands and reach high performance values in industrial settings, further research and development efforts within roll-to-roll (R2R) and sheet-to-sheet (S2S) processing of OPV devices under ambient conditions are required. Furthermore, OPV modules being manufactured through such up-scaled processing techniques should ideally be developed from low cost materials, and show good stability towards various different operational stress conditions. In this work, we demonstrate combined R2R and S2S development of ITO-free OPV devices, which are based on the non-fullerene material system PBDB-T:ITIC. The devices are processed from R2R vacuum sputtering and S2S slot-die coating at ambient conditions, and reach cell PCE values of 5.5%. In addition, we introduce a correlation between different barrier films, both commercial and sputtered inorganic coatings on ultra-clean PET, and the lifetime of the developed devices. The results therefore demonstrate an important step in the development of OPV devices from R2R and S2S processes in industrial settings.

Solution processed CuSCN/perylene hole extraction layer for highly efficient and stable organic solar cells

In this work, we report a solution processed hole extraction layer (HEL) for highly efficient organic solar cells (OSCs), which is formed by CuSCN and perylene. It shows that the introduction of perylene helps to polish the interface between HEL and active layers, leading to efficient charge transport and collection, diminished recombination loss, and thereby improving the photovoltaic performance. In poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]:[6,6]-phenyl C71-butyric acid methyl ester (PTB7-Th:PC71BM) based OSCs, over 9% enhancement of power conversion efficiency (PCE) is obtained in the cells using bilayer CuSCN/perylene as HEL compared with that of the reference cell using pure CuSCN as HEL. The advantage of bilayer CuSCN/perylene HEL is also confirmed in nonfullerene system. An improved PCE is also obtained after application of perylene in nonfullerene system based cell. Furthermore, superior air stability has been observed in CuSCN and CuSCN/perylene HEL based cells. The use of a bilayer CuSCN/perylene HEL proves a potential approach to obtain efficient and stable OSCs.

Regulating the phase separation of ternary organic solar cells via 3D architectured AIE molecules

An optimized bulk heterojunction (BHJ) interface, certifying enhanced exciton-splitting, charge separation and recombination inhibition, is vastly desired to obtain high power conversion efficiencies (PCEs). Herein, the ternary strategy has been employed to effectively modify the phase separation between the J71:ITIC blend by incorporating a 3D aggregation-induced emission (AIE) material, Tetraphenylethylene (TPE). Hence, as a consequence of improved charge mobility, lower bimolecular recombination and enhanced fill factor (FF), an excellent PCE of 12.16% has been achieved; a 21.23% increment over the PCE of binary devices. Likewise, Flory-Huggins parameter (χ) and surface free energy analysis reveals a high degree of miscibility between J71 and TPE, that leads to a rearrangement at the D-A interface such that TPE settles in between the D and A and thus forces the ITIC away from J71 and out of the mixed phase, indicating relatively higher average acceptor domain purity at the interface and ultimately better FF and PCE for the ternary devices. Likewise, the TPE inclusion in various other fullerene and nonfullerene systems also led to similar results, signifying this to be an effective methodology to boost the PCEs of the organic solar cells, especially for the systems with low FF.

Interface-enhanced organic solar cells with extrapolated T80 lifetimes of over 20 years

With recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) based on novel donor and non-fullerene acceptor (NFAs), improving the stability of these systems has become the most important issue for their practical applications. Herein, an efficient and highly stable OSC, containing a novel polymer donor and a non-fullerene acceptor system, is reported. The OSC is based on an inverted device structure that utilizes a self-assembled fullerene monolayer (C60-SAM) as the cathode modification layer, and an efficient and highly stable OSC composes of a polymer donor of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-alt-3-fluorothie-no[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and a non-fullerene acceptor of (2,2′-((2Z,2′Z)-(((4,4,9,9-Tetrakis(4-hexylphenyl)-4,9-dihydro-sindaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-((2ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene −2,1-diylidene))dimalononitrile) (IEICO-4F) is presented, showing a PCE of 10%. It further achieves an extrapolated T80 lifetime (the time required to reach 80% of initial performance) of 34,000 h, operating under one sun illumination equivalent. Based on an estimated solar irradiance of 1500 kWh/(m2 year) for China, a potential lifetime of 22 years is inferred for the OSC. Further investigation reveals that the reported C60-SAM modification stabilizes the OSC active layer morphology by lowering the surface energy of the underlying ZnO electron transport layer and suppressing trap-assisted recombination, thereby improving photostability. The results of this work establish important guidelines for the development of non-fullerene based OSCs with enhanced stability and pave the way for the commercialization of OSC technology.