Harvesting solar energy from sunlight to generate electricity is considered as one of the most important technologies to address the future sustainability of humans. Polymer solar cells (PSCs) have attracted tremendous interest and attention over the past two decades due to their potential advantage to be fabricated onto large area and light-weight flexible substrates by solution processing at a lower cost. PSCs based on the concept of bulk heterojunction (BHJ) configuration where an active layer comprises a composite of a p-type (donor) and an n-type (acceptor) material represents the most useful strategy to maximize the internal donor-acceptor interfacial area allowing for efficient charge separation. Fullerene derivatives such as [6,6]-phenyl-C61 or 71-butyric acid methyl ester (PCBM) are the ideal n-type materials ubiquitously used for BHJ solar cells. The major effort to develop photoactive materials is numerously focused on the p-type conjugated polymers which are generally synthesized by polymerization of electron-rich donor and electron-deficient acceptor monomers. Compared to the development of electron-deficient comonomers (acceptor segments), the development of electron-rich donor materials is considerably flourishing. Forced planarization by covalently fastening adjacent aromatic and heteroaromatic subunits leads to the formation of ladder-type conjugated structures which are capable of elongating effective conjugation, reducing the optical bandgap, promoting intermolecular π-π interactions and enhancing intrinsic charge mobility. In this review, we will summarize the recent progress on the development of various well-defined new ladder-type conjugated materials. These materials serve as the superb donor monomers to prepare a range of donor-acceptor semi-ladder copolymers with sufficient solution-processability for solar cell applications.

A new strategy to synthesize 4,9- and 5,10-dialkylated α-aNDTs as well as 4,9- and 5,10-dialkylated β-aNDTs is described. Four isomeric precursors with different dithienyl-ene-diyne arrangements undergo base-induced double 6π-cyclization to construct the central naphthalene cores, leading to the formation of the regiospecific products. These 2,7-distannylated dialkylated aNDT-based monomers can be used for Stille cross-coupling to produce promising conjugated materials for various optoelectronic applications.

A new tetracyclic 4,9-dialkyl angular-shaped naphthodiselenophene (4,9-α-aNDS) was designed and synthesized. The naphthalene core in 4,9-α-aNDS is formed by the DBU-induced 6π-cyclization of an (E)-1,2-bis(3-(tetradec-1-yn-1-yl)selenophen-2-yl)ethene intermediate followed by the second PtCl 2 -catalyzed benzannulation. This synthetic protocol allows for incorporating two dodecyl groups regiospecifically at 4,9-positions of the resulting α-aNDS. An ordered supramolecular self-assembly formed via noncovalent selenium−selenium interactions with a short contact of 3.5 Å was observed in the single-crystal structure of 4,9-α-aNDS. The distannylated α-aNDS building block was copolymerized with Br-DTFBT and Br-DPP acceptors by Stille cross coupling to form two new donor−acceptor polymers PαNDSDTFBT and PαNDSDPP, respectively. The bottom-gate/top-contact organic fieldeffect devices using the PαNDSDTFBT and PαNDSDPP semiconductors accomplished superior hole mobility of 3.77 and 2.17 cm 2 V −1 s −1 , respectively, which are among the highest mobilities reported to date.

A triarylamine-based crosslinked hole-transporting material is integrated with an ionic dopant to achieve high-performance PEDOT:PSS-free polymer solar cells.

Although the tetracyclic angular-shaped naphthodithiophenes (aNDTs) derivatives are an emergent building block for constructing promising semiconductor conjugated polymers, the absence of aliphatic side chains as solubilizing groups on the aNDTs greatly restricted their further application toward polymer synthesis. To create a new class of aNDT-based polymers for widespread applications in solution-processable OFETs and PSCs, side-chain engineering of the aNDT-based structures plays a pivotal role in improving solubility and optimizing electronic/steric properties associated with the resultant solar cell characteristics. In this research, we developed an efficient and straightforward methodology to construct the angular naphthodithophene core with regiospecific introduction of two aliphatic chains at its 4,9-positions via a base-induced double 6π-cyclization. For the first time, the corresponding 2,7-distannylated-4,9-dialkylated aNDT monomers were polymerized with FBT and DPP acceptors to make two new PaNDTDTFBT and PaNDTDPP donor−acceptor copolymers. The photovoltaic devices based on the PaNDTDTFBT:PC 71 BM blend not only showed a promising PCE of 6.52% with conventional configuration but also achieved a higher PCE of 6.86% with inverted configuration. Moreover, PaNDTDPP with strong intermolecular interaction achieved a high FET hole mobility of 0.202 cm 2 V −1 s −1 . ■ INTRODUCTIONDonor−acceptor (D−A) copolymers have been extensively utilized in organic photovoltaics (OPVs) and organic fieldeffect transistors (OFETs) due to their easy tunability of molecular properties and excellent solution processability. A variety of novel electron-rich conjugated building blocks have thus been developed for polymerization. 1 Compared to the tricyclic benzodithiophene derivatives representing the most successful component for constructing high-performance D−A copolymers of OPVs, 2 the tetracyclic naphthodithiophenes (NDTs) also attract growing attention considering their extended conjugation as well as diverse structures. 3 Depending on the molecular geometry, the NDT derivatives can have linear-shaped (lNDT) and angular-shaped (aNDT) isomeric structures (Figure 1). It was found that aNDT-based copolymers have higher ordered structures than the lNDTbased copolymers, thereby achieving higher OFET mobilities. 4 Furthermore, the aNDT molecules exhibit lower-lying HOMO energy levels than the lNDT counterparts, 5 which are beneficial for better air stability and greater open-circuit voltage (V oc ) for OPVs. To fully exploit the aNDT-containing polymers for the optoelectronic applications, side-chain engineering by introducing solubilizing groups onto an aNDT framework is highly essential to not only ensure the solution processability but also induce suitable molecular packing. 6 To address the issue, Takimiya and Osaka first introduced two alkyl groups at the outer positions (5,10-positions) of the central naphthalene moiety in the aNDT. 7 Compared to the polymer incorporating the unsubstituted aNDT units, 8 the 5,10-dialkyl aN...