Green Solvent Processed Conjugated Polymers for Organic Solar Cells: The Impact of Oligoethylene Glycol Side Chains

Organic photovoltaics (OPVs) possess the advantageous trait of solution processability, yet OPV blends typically use hazardous chlorinated solvents for processing. In order to realize the full advantages of OPVs, as well as growing to an industrial scale, the use of environmentally friendly solvents for processing OPVs needs to be pursued. In this study, we utilized the well-studied polymer PBnDT-FTAZ system as the model conjugated polymer, and synthesized a series of structurally similar conjugated polymers with oligo(ethylene glycol) (OEG) side-chains, aiming to understand the structural requirements to convert conventional conjugated polymers into green-processable alternatives. We elucidated the impact of these OEG chains on the properties of modified polymers when compared with the original PBnDT-FTAZ, including solubility and optoelectronic properties. Finally, aiming to understand the impact of changing side chains to the device performance, we fabricated solar cells with a nonfullerene acceptor (IT-M), achieving decent device efficiencies (over 7%). Additionally, using renewable and green solvent, 2-methyltetrahydrofuran (2-MeTHF), we were able to achieve device efficiencies of over 2%

Revealing the Impact of F4-TCNQ as Additive on Morphology and Performance of High-Efficiency Nonfullerene Organic Solar Cells

Fluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells

The Importance of Entanglements in Optimizing the Mechanical and Electrical Performance of All-Polymer Solar Cells

Organic solar cells that have all-polymer active layers may have several advantages compared with polymer-small molecule systems including improved mechanical and thermodynamic stability; however, an all-polymer active layer does not guarantee robust mechanical behavior. Here, we consider key parameters that may influence the mechanical behavior and power conversion efficiency of all-polymer solar cells (all-PSCs). Considerations include the thermal transition temperature of the polymers, the molecular weight (MW) of the polymers, and film morphology. The impact these features have on mechanical behavior is probed by measuring the cohesive fracture energy (Gc), crack onset strain, and elastic modulus. We find that the selection of ductile polymers with high MW enhances interchain interactions that improve the mechanical resilience of the films. High-MW polymers are also found to maximize the power conversion efficiency (PCE). Using this strategy, BHJ films with the best reported combination of Gc (7.96 J m-2) and PCE (6.94%) are demonstrated. Finally, it is found that increasing the film thickness increases the fracture energy of the films but at the cost of PCE. These findings provide a fundamental perspective on the design strategy to achieve high performance and mechanically robust organic solar cells

Utilizing Difluorinated Thiophene Units to Improve the Performance of Polymer Solar Cells

While there are numerous approaches to functionalize conjugated polymers for organic solar cells (OSCs), one widely adopted approach is fluorination. Of the many different locations for fluorination, one of the least studied is the conjugated linker which connects the donor and acceptor moieties; further, all existing reports primarily explore monofluorinated thiophene units. Herein, we synthesize and compare two conjugated polymers, HTAZ and DFT-HTAZ, which have different thiophene linkers. In HTAZ, a bare thiophene unit connects the donor and acceptor moieties, while DFT-HTAZ utilizes difluorinated thiophene (DFT) linkers. These polymers serve as the model system to explore the impact of DFT units in OSCs; additionally, this is the first publication to investigate polymers containing DFT units paired with non-fullerene acceptors. Compared to HTAZ, the incorporation of the DFT units maintained the optical properties while lowering the energy levels by â0.4 eV, which allowed for a much improved Voc value of â1 V. Importantly, when compared with the appropriate non-fullerene acceptor, DFT-HTAZ:ITIC-Th1 blends reached an efficiency of â10%, which is nearly 3× that of the nonfluorinated HTAZ. As most OSC polymers have thiophene linkers, using DFT units could serve as a proficient method to increase OSC performance in many polymer systems, especially those that do not have locations for functionalization on the acceptor moiety

A carbon-oxygen-bridged hexacyclic ladder-type building block for low-bandgap nonfullerene acceptors

A hexacyclic carbon-oxygen-bridged ladder-type unit, COi6, was developed. Three nonfullerene acceptors (COi6IC, COi6FIC and COi6DFIC) based on COi6 were prepared. They present low optical bandgaps of 1.31-1.37 eV and strong absorbance in the near-infrared region. A 9.12% power conversion efficiency was achieved from the solar cells based on COi6FIC and a wide-bandgap copolymer donor (FTAZ)

Highly Efficient, Stable, and Ductile Ternary Nonfullerene Organic Solar Cells from a Two-Donor Polymer Blend

Organic solar cells (OSCs) are one of the most promising cost‐effective options for utilizing solar energy, and, while the field of OSCs has progressed rapidly in device performance in the past few years, the stability of nonfullerene OSCs has received less attention. Developing devices with both high performance and long‐term stability remains challenging, particularly if the material choice is restricted by roll‐to‐roll and benign solvent processing requirements and desirable mechanical durability. Building upon the ink (toluene:FTAZ:IT‐M) that broke the 10% benchmark when blade‐coated in air, a second donor material (PBDB‐T) is introduced to stabilize and enhance performance with power conversion efficiency over 13% while keeping toluene as the solvent. More importantly, the ternary OSCs exhibit excellent thermal stability and storage stability while retaining high ductility. The excellent performance and stability are mainly attributed to the inhibition of the crystallization of nonfullerene small‐molecular acceptors (SMAs) by introducing a stiff donor that also shows low miscibility with the nonfullerene SMA and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer. The study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of nonfullerene OSCs

Fluorinated Thiophene Units Improve Photovoltaic Device Performance of Donor-Acceptor Copolymers

Fluorinated conjugated polymers leading to enhanced photovoltaic device performance has been widely observed in a variety of donor-acceptor copolymers; however, almost all these polymers have fluorine substituents on the acceptor unit. Building upon our previously reported PBnDT-FTAZ, a fluorinated donor-acceptor conjugated polymer with impressive device performance, we set this study to explore the effect of adding the fluorine substituents onto the flanking thiophene units between the donor unit (BnDT) and the acceptor unit (TAZ). We developed new synthetic approaches to control the position of the fluorination (3′ or 4′) on the thiophene unit, and synthesized four additional PBnDT-TAZ polymers incorporating the fluorine-substituted-thiophene (FT) units, 3′-FT-HTAZ, 4′-FT-HTAZ, 3′-FT-FTAZ, and 4′-FT-FTAZ. We discover that relocating the fluorine substituents from the acceptor to the flanking thiophene units have a negligible impact on the device characteristics (short circuit current, open circuit voltage, and fill factor) when comparing 4′-FT-HTAZ with the original FTAZ. Combining these two fluorination approaches together, 4′-FT-FTAZ shows even higher device performance than FTAZ (7.7% vs 6.6%) with active layers over 200 nm in thickness. Furthermore, high values of fill factor∼70% are all achieved for photovoltaic devices based on 3′-FT-HTAZ, 4′-FT-HTAZ, or 4′-FT-FTAZ, ascribed to the observed high hole mobilities (over 1 × 10-3 cm2/(V s)) in these devices. Our study offers a new approach to utilize the fluorinated thiophene units in developing new conjugated polymers to further improve the device performance of polymer solar cells

Fluorination of Donor-Acceptor Copolymer Active Layers Enhances Charge Mobilities in Thin-Film Transistors

Several recent reports have demonstrated that fluorinated analogues of donor/acceptor copolymers surpass nonfluorinated counterparts in terms of performance in electronic devices. Using a copolymer series consisting of fluorinated, partially fluorinated, and nonfluorinated benzotriazole, we confirm that the addition of fluorine substituents beneficially impacts charge transport in polymer semiconductors. Transistor measurements demonstrated a factor of 5 increase in carrier mobilities with the degree of fluorination of the backbone. Furthermore, grazing-incidence X-ray diffraction data indicates progressively closer packing between the conjugated cores and an overall greater amount of π-stacking in the fluorinated materials. It is likely that attractive interactions between the electron-rich donor and fluorinated electron-deficient acceptor units induce very tightly stacking crystallites, which reduce the energetic barrier for charge hopping. In addition, a change in crystallite orientation was observed from primarily edge-on without fluorine substituents to mostly face-on with fluorinated benzotriazole

A Fused Ring Electron Acceptor with Decacyclic Core Enables over 13.5% Efficiency for Organic Solar Cells

Extending π-conjugation of donor units in fused ring electron acceptors (FREAs) promises to reinforce intramolecular charge transfer for smaller bandgaps, and enhances intermolecular interactions for higher charge mobility. Despite these advantages, power conversion efficiencies (PCEs) of FREAs with a large planar donor core are still below 12%. Herein, a fused decacyclic donor unit, IDC, is developed, and the FREA, IDCIC, whose optical bandgap is 1.45 eV, is synthesized. The FTAZ:IDCIC-based organic solar cell (OSC) with a binary additive, chloronaphthalene and 1,8-diiodooctane (CN&DIO), affords a remarkable PCE of 13.58%, which is among the highest efficiencies of OSCs. The binary additive plays a crucial role in the morphology of FTAZ:IDCIC-based OSCs. The addition of DIO promotes IDCIC to aggregate and enhances domain size and domain purity in FTAZ:IDCIC blend film for its poor solubility in DIO, while the addition of chloronaphthalene (CN) inhibits the aggregation, facilitates IDCIC to diffuse into the donor, and reduces domain size and domain purity for its excellent solubility in CN. The binary additive, CN&DIO, compromises the effect of the two; thus FTAZ:IDCIC-based OSCs with 0.25% CN&DIO obtain moderate domain size and high domain purity simultaneously, achieving the least charge recombination for the highest JSC (21.98 mA cm−2) and fill factor (FF) (71.03%)

The crucial role of end group planarity for fused-ring electron acceptors in organic solar cells

Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. Of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ∼1 Å, but have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. Our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance