Incorporation of Fluorine onto Different Positions of Phenyl Substituted Benzo[1,2-b:4,5-b′]dithiophene Unit: Influence on Photovoltaic Properties

Research progress and application of high efficiency organic solar cells based on benzodithiophene donor materials

In recent decades, the demand for clean and renewable energy has grown increasingly urgent due to the irreversible alteration of the global climate change. As a result, organic solar cells (OSCs) have emerged as a promising alternative to address this issue. In this review, we summarize the recent progress in the molecular design strategies of benzodithiophene (BDT)-based polymer and small molecule donor materials since their birth, focusing on the development of main-chain engineering, side-chain engineering and other unique molecular design paths. Up to now, the state-of-the-art power conversion efficiency (PCE) of binary OSCs prepared by BDT-based donor materials has approached 20%. This work discusses the potential relationship between the molecular changes of donor materials and photoelectric performance in corresponding OSC devices in detail, thereby presenting a rational molecular design guidance for stable and efficient donor materials in future.

Gradual Fluorination on the Phenyl Side Chains for Benzodithiophene-Based Linear Polymers to Improve the Photovoltaic Performance

To study the impact of introducing fluorine atoms onto the conjugated phenyl side chains of benzo[1,2-b:4,5-b']dithiophene (BDT)-based copolymers, three novel donor-π-acceptor (D-π-A) alternative polymers PE40, PE42, and PE44 were designed and synthesized. The phenyl-substituted-BDT, thieno[3,2-b]thiophene, and benzo[d][1,2,3]triazole (BTA) served as the donor, π-bridge, and acceptor units, respectively, to enable linear polymer backbones. When introducing two or four fluorine atoms into the phenyl side units of PE40, the polymers PE42 and PE44 demonstrate a gradual decrease of energy levels and an increase of crystallinity in the pristine and blend films. It was noted that the increase in fluorine atoms gradually improved the performance parameters of polymer solar cells (PSCs) with Y6 as the acceptor. The PE40:Y6 device yielded a power conversion efficiency (PCE) of up to 7.07% with a short-circuit (J_SC) of 21.36 mA cm^-2, an open-circuV_OC) of 0.65 V, and a fill factor (FF) of 0.51, and PE42:Y6 exhibited a better PCE of 10.11% (J_SC = 23.25 mA cm^-2, V_OC = 0.74 V, and FF = 0.59), while PE44:Y6 exhibited the best PCE of 13.62% (J_SC = 25.29 mA cm^-2, V_OC = 0.82 V, and FF = 0.66). The suitable energy offsets between the donor and the acceptor, high and balanced charge-carrier mobility, and the optimal morphology of the blend film contributed to the high performance of PE44:Y6 combination. Our results demonstrate that introducing more fluorine atoms onto the phenyl side units of BDT is a prospective approach to break the trade-offs between V_OC, J_SC, and FF, and finally improve the performance of PSCs.

Drastic Changes in Properties of Donor-Acceptor Polymers Induced by Asymmetric Structural Isomers for Application to Polymer Solar Cells

Appropriate design of donor-acceptor (D-A) conjugated polymers is important for enhancing their physical, optical, and electrochemical properties. The rapid development of D-A conjugated polymers based on fullerene and nonfullerene derivatives in the past decade has led to an improvement in the performance of polymer solar cells (PSCs). In this study, we designed and synthesized two donor polymers based on the DTffBT acceptor unit, with matching optical absorption range and energy levels with fullerene (PC₇₁BM) and nonfullerene acceptors (ITIC and IDIC), by introducing asymmetric structural isomers of donor units. We demonstrated that materials design by structural modification dramatically affects the physical, optical, and electrochemical properties as well as the crystallinity and photovoltaic performance of the polymers. The results provide valuable insights into materials design for efficient PSCs.

Nonhalogenated Solvent-Processed All-Polymer Solar Cells over 7.4% Efficiency from Quinoxaline-Based Polymers

Two conjugated polymers, with different side chains of alkoxy-substituted difluorobenzene and alkyl-substituted difluorobenzene based on quinoxaline (Qx) as the electron acceptor unit and benzodithiophene as the electron donor unit, named HFQx-T and HFAQx-T, were used as electron donor polymers to fabricate all-polymer solar cells (all-PSCs) with a naphthalenediimide-bithiophene n-type semiconducting polymer (N2200). Usually, halogenated solvents are harmful to natural environment and human beings, and solvent additives were disadvantageous in the process of roll-to-roll technology. The Qx-based polymers are successfully used to fabricate high-performance all-PSCs, which processed with the nonhalogenated solvent tetrahydrofuran (THF) at room temperature. With THF as the processing solvent, the active layer showed a higher absorption coefficient, better phase separation, exciton dissociation, and charge carrier mobilities than that processed with CHCl₃. Moreover, the photovoltaic properties have been dramatically improved with THF. The optimized device of HFAQx-T:N2200 processed with THF delivered an efficient power conversion efficiency (PCE) of 7.45%, which is the highest PCE for all-PSCs from Qx-based polymers processed by a nonhalogenated solvent.

Small Changes With Big Consequences: Swapping Two Atoms In Side Chains Changes Phenylene-Ethynylene Packing And Fluorescence

Engineering the properties of conjugated materials in the solid state is an unsolved, ongoing challenge important to fundamental understanding of how non-covalent interactions dictate packing and key properties, as well as the development of technologies based in organic optoelectronics. The most common design paradigm of such materials divide them into a "main chain" with extended conjugation, the chemical structure of which determines optoelectronic properties, and "side chains" not conjugated to the backbone, which provide solubility when they are long alkyl chains. This paper describes comparisons between phenylene-ethynylene molecules in which slight changes to the structure of "side chains"-swapping hydrogen and fluorine atomic position on an aromatic ring-results in unexpectedly large changes in the solid-state optical properties. In a pair of anisyl-terminated three-ring phenylene-ethynylenes, switching the side chain arenes of benzyl esters from 2,4,6-trifluoro to 2,3,6-trifluoro results in a shift in fluorescence emission spectra of over 100 nm, as well as the opposite direction of force-induced shifting of emission. Through a combination X-ray crystal structures, electronic structure calculations, and comparisons with other derivatives, we describe how the 2,4,6-trifluorinated side chains yield cofacial fluoroarene-arene stacking interactions that twist the PE backbone out of conjugation, while the 2,3,6-trifluoro side chains do not stack, instead yielding more coplanar PE backbones that form intermolecular aggregates. Overall, this work demonstrates how slight modifications to parts of conjugated materials normally considered ancillary to optoelectronic properties can determine their solid-state properties, epitomizing the challenge of rational design but at the same time offering opportunities for materials discovery and improved understanding of non-covalent interactions.

A new fluoropyrido[3,4-b]pyrazine based polymer for efficient photovoltaics

Using a fluoropyrido[3,4-b]pyrazine based 2D-conjugated polymer as an electron donor in polymer solar cells, a power conversion efficiency of 6.2% is obtained, which is the highest PCE among the PP-based polymers reported to date.

Naphthodifuran-based zigzag-type polycyclic arene with conjugated side chains for efficient photovoltaics

Two alkoxyphenyl-substituted naphthodifuran (zNDF)-based polymers (PzNDFP-BT and PzNDFP-ffQx) were synthesized. A promising PCE of 6.9% has been obtained, which is the highest PCE among zNDF-based polymers to date.

High-Performance Photovoltaic Polymers Employing Symmetry-Breaking Building Blocks

Two 1D-2D asymmetric benzodithiophenes (BDTs) as donor building blocks are designed and synthesized, combining the advantages of both 1D and 2D symmetric BDTs. The photovoltaic properties of the asymmetric BDT-based polymers are improved greatly in comparison with corresponding symmetric BDT-based polymers. This work provides a new approach to design prospective organic optoelectronic materials employing the symmetry-breaking strategy.

Novel wide band gap polymers based on dithienobenzoxadiazole for polymer solar cells with high open circuit voltages over 1 V

Just by replacing sulfur with oxygen atom, the V_oc of PBDT-fDTBO based PSCs is 0.2 V higher than PBDT-fDTBT based devices.

Two new fluorinated copolymers based on thieno[2,3-f]benzofuran for efficient polymer solar cells

Two new copolymers named TBFPF-BT and TBFPF-BO, composed of a fluorine substituted thieno[2,3-f]benzofuran donor unit and benzothiadiazole/benzooxadiazole acceptor unit, were synthesized for photovoltaic applications.

A simple strategy to the side chain functionalization on the quinoxaline unit for efficient polymer solar cells

A new tetrafluoridequinoxaline electron accepting block-ffQx from a quinoxaline core was designed. A new copolymer (PBDTT-ffQx) was synthesized from tetrafluoridequinoxaline and benzodithiophene and a high PCE of 8.6% was obtained.

A comparative study of the effect of fluorine substitution on the photovoltaic performance of benzothiadiazole-based copolymers

Fluorination on the acceptor unit is effective to lower both the HOMO and LUMO energy level of the copolymer.

Design and synthesis of the polymers based on alkylthiophenyl side chains and variant acceptor moieties for polymer solar cells

Three new BDT-based polymers with alkylthiophenyl side chains and the relationship between the acceptor structure and the photovoltaic performance.

Effect of Fluorine Substitution on Photovoltaic Properties of Alkoxyphenyl Substituted Benzo[1,2-b:4,5-b']dithiophene-Based Small Molecules

Two new small molecules, C3T-BDTP and C3T-BDTP-F with alkoxyphenyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT) and meta-fluorinated-alkoxyphenyl-substituted BDT as the central donor blocks, respectively, have been synthesized and used as donor materials in organic solar cells (OSCs). With the addition of 0.4% v/v 1,8-diiodooctane (DIO), the blend of C3T-BDTP-F/PC71BM showed a higher hole mobility of 8.67 × 10(-4) cm(2) V(-1) s(-1) compared to that of the blend of C3T-BDTP/PC71BM. Two types of interlayers, zirconium acetylacetonate (ZrAcac) and perylene diimide (PDI) derivatives (PDINO and PDIN), were used to further optimize the performance of OSCs. With a device structure of ITO/PEDOT:PSS/donor:PC71BM/PDIN/Al, the OSCs based on C3T-BDTP delivered a satisfying power conversion efficiency (PCE) of 5.27% with an open circuit voltage (V(oc)) of 0.91 V, whereas the devices based on C3T-BDTP-F showed an enhanced PCE of 5.42% with a higher V(oc) of 0.97 V.