Conjugated Side-Chain Isolated Polythiophene: Synthesis and Photovoltaic Application

Biaxially Conjugated Materials for Organic Solar Cells

Organic solar cells (OSCs) represent one of the most important emerging photovoltaic technologies that can implement solar energy conversion efficiently. The chemical structure of organic semiconductors deployed in the active layer of OSCs plays a critical role in the photovoltaic performance and chemical/physical stability of relevant devices. With the structure innovation of organic semiconductors, especially nonfullerene acceptors (NFAs), the performance of OSCs have been promoted rapidly in recent years, with state-of-the-art power conversion efficiencies (PCEs) exceeding 19.5%. Compared with other photovoltaics like perovskite, the shortcoming of OSCs mainly lies in the high nonradiative recombination loss. However, the photocurrent density is superior in OSCs owing to the easy modulation of the NFA band gap toward the near-infrared region. In these regards, the effort to further boost the PCE of OSCs to achieve a milestone >21% should be devoted to reducing the nonradiative loss while further broadening the absorption band. Developing organic semiconductors with biaxially extended conjugated structures has provided a potential solution to achieve these goals. Herein, we summarize the design rules and performance progress of biaxially extended conjugated materials for OSCs. The descriptions are divided into two major categories, i.e., polymers and NFAs. For p-type polymers, we focus on the biaxial conjugation on some representative building blocks, e.g., polythiophene, triphenylamine, and quinoxaline. Whereas for n-type polymers, some structures with large conjugated planes in the normal direction are presented. We also elaborate on the biaxial conjugation strategies in NFAs with modification site at either the π-core or side-group. The general structure-property relationships are further retrieved within these materials, with focus on the short-wavelength absorption and nonradiative energy loss. Finally, we provide an outlook for the further structure modification strategies of biaxially conjugated materials toward highly efficient, stable, and industry-compatible OSCs.

Cu(0)-RDRP of acrylates based on p-type organic semiconductors

A series of four acrylic monomers were synthesized based on p-type organic semiconductor motifs found commonly in organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs) and organic photovoltaics (OPVs).

Synthesis of di(ethylene glycol)-functionalized diketopyrrolopyrrole derivative-based side chain-conjugated polymers for bulk heterojunction solar cells

Synthesis of diketopyrrolopyrrole derivatives based side-chain conjugated polymers for solar cells.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Influence of the terminal donor on the performance of 4,8-dialkoxybenzo[1,2-b:4,5′]dithiophene based small molecules for efficient solution-processed organic solar cells

New dialkoxybenzo[1,2-b:4,5′]dithiophene based small molecules, BDT(PTBT)₂ and BDT(TTBT)₂, are designed and synthesized. BDT(TTBT)₂ shows higher PCE than BDT(PTBT)₂.

Tunable light harvesting properties of a highly crystalline alternating terpolymer for high-performing solar cells

To investigate the influence of a polymer structure on photovoltaic performances, an A–D₁–A–D₂ type terpolymer system of PDPPPyT was synthesized and characterized.

Impact of constitution of the terthiophene-vinylene conjugated side chain on the optical and photovoltaic properties of two-dimensional polythiophenes

The effects of the spatial arrangement of the conjugated side chains of two-dimensional polymers on their optical, electrochemical, molecular-packing, and photovoltaic characteristics were investigated. Accordingly, novel polythiophenes with horizontally (PBTTTV-h) and vertically (PBTTTV-v) grafted terthiophene–vinylene (TTV) conjugated side chains were synthesized that display two and one UV-vis peaks, respectively; the difference is due to the different constitutions of the conjugated side-chains. Because the spatial arrangement affects the molecular self-assembly, PBTTTV-h shows stronger crystallinity than PBTTTV-v, which enhances the charge mobility in devices. Moreover, PBTTTV-h has a lower HOMO energy level (−5.49 eV) than PBTTTV-v (−5.40 eV). Bulk heterojunction solar cells fabricated from PBTTTV-h/PC71BM and PBTTTV-v/PC71BM exhibit power conversion efficiencies of 4.75% and 4.00%, respectively, and Voc values of 800 and 730 mV, respectively, under AM1.5G illumination (100 mW cm(−2)). Thus, the architecture of the TTV conjugated side chains affects the optical, electrochemical, and photovoltaic properties; this study provides more ideas for improving 2-D conjugated polymers for semiconductor devices.

Polythiophenes Comprising Conjugated Pendants for Polymer Solar Cells: A Review

Polythiophene (PT) is one of the widely used donor materials for solution-processable polymer solar cells (PSCs). Much progress in PT-based PSCs can be attributed to the design of novel PTs exhibiting intense and broad visible absorption with high charge carrier mobility to increase short-circuit current density (J_sc), along with low-lying highest occupied molecular orbital (HOMO) levels to achieve large open circuit voltage (V_oc) values. A promising strategy to tailor the photophysical properties and energy levels via covalently attaching electron donor and acceptor pendants on PTs backbone has attracted much attention recently. The geometry, electron-donating capacity, and composition of conjugated pendants are supposed to be the crucial factors in adjusting the conformation, energy levels, and photovoltaic performance of PTs. This review will go over the most recent approaches that enable researchers to obtain in-depth information in the development of PTs comprising conjugated pendants for PSCs.

Fullerene-bisadduct acceptors for polymer solar cells

Polymer solar cells (PSCs) have drawn great attention in recent years for their simple device structure, light weight, and low-cost fabrication in comparison with inorganic semiconductor solar cells. However, the power-conversion efficiency (PCE) of PSCs needs to be increased for their future application. The key issue for improving the PCE of PSCs is the design and synthesis of high-efficiency conjugated polymer donors and fullerene acceptors for the photovoltaic materials. For the acceptor materials, several fullerene-bisadduct acceptors with high LUMO energy levels have demonstrated excellent photovoltaic performance in PSCs with P3HT as a donor. In this Focus Review, recent progress in high-efficiency fullerene-bisadduct acceptors is discussed, including the bisadduct of PCBM, indene-C60 bisadduct (ICBA), indene-C70 bisadduct (IC70BA), DMPCBA, NCBA, and bisTOQC. The LUMO levels and photovoltaic performance of these bisadduct acceptors with P3HT as a donor are summarized and compared. In addition, the applications of an ICBA acceptor in new device structures and with other conjugated polymer donors than P3HT are also introduced and discussed.

Synthesis and photovoltaic properties of acceptor materials based on the dimerization of fullerene C60 for use in efficient polymer solar cells

We demonstrate that dimeric fullerene derivatives work as an excellent n-type material for polymer solar cells in combination with P3HT or PTB7, showing the best PCE of 3.3% or 6.1%, respectively. The device performance was influenced by the position of the two C60 moieties linked together.

Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption

Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs. In this Account, I discuss the basic requirements and scientific issues in the molecular design of high efficiency photovoltaic molecules. I also summarize recent progress in electronic energy level engineering and absorption spectral broadening of the donor and acceptor photovoltaic materials by my research group and others. For high-efficiency conjugated polymer donors, key requirements are a narrower energy bandgap (E(g)) and broad absorption, relatively lower-lying HOMO (the highest occupied molecular orbital) level, and higher hole mobility. There are three strategies to meet these requirements: D-A copolymerization for narrower E(g) and lower-lying HOMO, substitution with electron-withdrawing groups for lower-lying HOMO, and two-dimensional conjugation for broad absorption and higher hole mobility. Moreover, better main chain planarity and less side chain steric hindrance could strengthen π-π stacking and increase hole mobility. Furthermore, the molecular weight of the polymers also influences their photovoltaic performance. To produce high efficiency photovoltaic polymers, researchers should attempt to increase molecular weight while maintaining solubility. High-efficiency D-A copolymers have been obtained by using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) donor unit and benzothiadiazole (BT), thienopyrrole-dione (TPD), or thiazolothiazole (TTz) acceptor units. The BDT unit with two thienyl conjugated side chains is a highly promising unit in constructing high-efficiency copolymer donor materials. The electron-withdrawing groups of ester, ketone, fluorine, or sulfonyl can effectively tune the HOMO energy levels downward. To improve the performance of fullerene derivative acceptors, researchers will need to strengthen absorption in the visible spectrum, upshift the LUMO (the lowest unoccupied molecular orbital) energy level, and increase the electron mobility. [6,6]-Phenyl-C(71)-butyric acid methyl ester (PC(70)BM) is superior to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) because C(70) absorbs visible light more efficiently. Indene-C(60) bisadduct (ICBA) and Indene-C(70) bisadduct (IC(70)BA) show 0.17 and 0.19 eV higher LUMO energy levels, respectively, than PCBM, due to the electron-rich character of indene and the effect of bisadduct. ICBA and IC(70)BA are excellent acceptors for the P3HT-based PSCs.