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

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.

Concepts and principles of self-n-doping in perylene diimide chromophores for applications in biochemistry, energy harvesting, energy storage, and catalysis

Self-doping is an essential method of increasing carrier concentrations in organic electronics that eliminates the need to tailor host-dopant miscibility, a necessary step when employing molecular dopants. Self-n-doping can be accomplished using amines or ammonium counterions as an electron source, which are being incorporated into an ever-increasingly diverse range of organic materials spanning many applications. Self-n-doped materials have demonstrated exemplary and, in many cases, benchmark performances in a variety of applications. However, an in-depth review of the method is lacking. Perylene diimide (PDI) chromophores are an important mainstay in the semiconductor literature with well-known structure-function characteristics and are also one of the most widely utilized scaffolds for self-n-doping. In this review, we describe the unique properties of self-n-doped PDIs, delineate structure-function relationships, and discuss self-n-doped PDI performance in a range of applications. In particular, the impact of amine/ammonium incorporation into the PDI scaffold on doping efficiency is reviewed with regard to attachment mode, tether distance, counterion selection, and steric encumbrance. Self-n-doped PDIs are a unique set of PDI structural derivatives whose properties are amenable to a broad range of applications such as biochemistry, solar energy conversion, thermoelectric modules, batteries, and photocatalysis. Finally, we discuss challenges and the future outlook of self-n-doping principles.

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.

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.

Photovoltaic Small Molecules of TPA(FxBT-T-Cz)3: Tuning Open-Circuit Voltage over 1.0 V for Their Organic Solar Cells by Increasing Fluorine Substitution

To simultaneously improve both open-circuit voltage (V_oc) and short-circuit current density (J_sc) for organic solar cells, a novel D(A-π-Ar)₃ type of photovoltaic small molecules of TPA(F_xBT-T-3Cz)₃ was designed and synthesized, which contain central triphenylamine (TPA), terminal carbazole (Cz), armed fluorine-substituted benzothiadiazole (F_xBT, where x = 1 or 2), and bridged thiophene (T) units. A narrowed ultraviolet-visible absorption and a decreasing highest occupied molecular orbital energy level were observed from TPA(F₁BT-T-3Cz)₃ to TPA(F₂BT-T-3Cz)₃ with increasing fluorine substitution. However, the TPA(F₂BT-T-3Cz)₃/PC₇₁BM-based solar devices showed a rising V_oc of 1.01 V and an enhanced J_sc of 10.84 mA cm^-2 as well as a comparable power conversion efficiency of 4.81% in comparison to the TPA(F₁BT-T-3Cz)₃/PC₇₁BM-based devices. Furthermore, in comparison to the parent TPA(BT-T-3Cz)₃ molecule without fluorine substitution, the fluorine-substituted TPA(F_xBT-T-3Cz)₃ molecules exhibited significantly incremental V_oc and J_sc values in their bulk heterojunction organic solar cells, owing to fluorine incorporation in the electron-deficient benzothiadiazole unit.

Effect of the π-conjugation length on the properties and photovoltaic performance of A-π-D-π-A type oligothiophenes with a 4,8-bis(thienyl)benzo[1,2-b:4,5-b']dithiophene core

Benzo[1,2-b:4,5-b′]dithiophene (BDT) is an excellent building block for constructing π-conjugated molecules for the use in organic solar cells. In this paper, four 4,8-bis(5-alkyl-2-thienyl)benzo[1,2-b:4,5-b′]dithiophene (TBDT)-containing A–π–D–π–A-type small molecules (COOP-nHT-TBDT, n = 1, 2, 3, 4), having 2-cyano-3-octyloxy-3-oxo-1-propenyl (COOP) as terminal group and regioregular oligo(3-hexylthiophene) (nHT) as the π-conjugated bridge unit were synthesized. The optical and electrochemical properties of these compounds were systematically investigated. All these four compounds displayed broad absorption bands over 350–600 nm. The optical band gap becomes narrower (from 1.94 to 1.82 eV) and the HOMO energy levels increased (from −5.68 to −5.34 eV) with the increase of the length of the π-conjugated bridge. Organic solar cells using the synthesized compounds as the electron donor and PC₆₁BM as the electron acceptor were fabricated and tested. Results showed that compounds with longer oligothiophene π-bridges have better power conversion efficiency and higher device stability. The device based on the quaterthiophene-bridged compound 4 gave a highest power conversion efficiency of 5.62% with a V_OC of 0.93 V, J_SC of 9.60 mA·cm⁻², and a FF of 0.63.

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.