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 high organic field-effect transistor mobility (0.29 cm(2) V(-1) s(-1) ) and bulk-heterojunction polymer solar cell performance (PCE of 6.82%) have been achieved in a low bandgap alternating copolymer consisting of axisymmetrical structural units, 5,6-difluorobenzo-2,1,3-thiadiazole. Introducing the fluorine substituents enhanced intermolecular interaction and improved the solid-state order, which consequently resulted in the highest device performances among the 2,1,3-thiadiazole-quarterthiophene based alternating copolymers.

A copolymerization strategy is developed to utilize porphyrin as a complementary light-harvesting unit (LHU) in D-A polymers. For polymer solar cells (PSCs), the presence of LHUs increases the short-circuit current density (Jsc ) without sacrificing the open-circuit voltage (Voc ) and fill factor (FF). Up to 8.0% power conversion efficiency (PCE) is delivered by PPor-2:PC71 BM single-junction PSCs. A PCE of 8.6% is achieved when a C-PCBSD cathodic interlayer is introduced.

Over the past few years, tremendous research effort has been made on all-solution processed bulk-heterojunction polymer solar cells (PSCs) in order to realize low-cost, lightweight, largearea and flexible photovoltaic devices. 1 To achieve high efficiency of PSCs, the most critical challenge at the molecular level is to develop the p-type conjugated polymers that possess (1) sufficient solubility to guarantee solution processability and miscibility with an n-type material, (2) low band gap (LBG) for strong and broad absorption spectrum to capture more solar photons and (3) high hole mobility for efficient charge transport. The general approach to produce a LBG polymer is to incorporate electron-rich donor and electrondeficient acceptor segments along the conjugated polymer backbone. Based on these polymers, researchers have made a breakthrough in fabricating PSC devices with PCEs over 5%. 2 Planarization of polyaromatic system facilitates π-electron delocalization and elongates effective conjugation length, providing another effective way to reduce the band gap. 3 Moreover, coplanar geometries and rigid structures can suppress the rotational disorder around interannular single bonds and lower the reorganization energy, which in turn enhances the intrinsic charge mobility. 4 Tricyclic 2, 7-carbazole 5 unit is an ideal electron-rich building block to construct donorÀacceptor polymers because its derivatives exhibit deeplying HOMO energy levels and good hole-transporting properties which are crucial prerequisites to achieve high open-circuit voltages (V oc ) and short circuit currents (J sc ), respectively. Poly-(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT) has been shown to act as a superior p-type photoactive material for the application in PSCs (Scheme 1). 5 Inspired by the skeletons of the PCDTBT polymers, for the first time, we have successfully utilized a facile Friedel-Craft cyclization to develop a novel carbazole-based coplanar π-conjugated system, carbazole-dicyclopentathiophene (CDCT), where the 3-positon of two outer thiophenes are covalently connected with the 3,6-position of central carbazole cores by a sp 3 -hybridized carbon bridge (Scheme 1). 6 By copolymerizing this heptacyclic structure with electron-deficient benzothiadizole unit, an alternating poly(carbazole-dicyclopentathiophene-alt-benzothiadiazole) (PCDCTBT) was synthesized. 6 The two cyclopentadiene rings embedded in the CDCT structure allows for introducing four 4-(2-ethylhexoxy)phenyl groups to guarantee solubility and

A new class of ladder‐type dithienosilolo‐carbazole (DTSC), dithienopyrrolo‐carbazole (DTPC), and dithienocyclopenta‐carbazole (DTCC) units is developed in which two outer thiophene subunits are covalently fastened to the central 2,7‐carbazole cores by silicon, nitrogen, and carbon bridges, respectively. The heptacyclic multifused monomers are polymerized with the benzothiadiazole (BT) acceptor by palladium‐catalyzed cross‐coupling to afford three alternating donor‐acceptor copolymers poly(dithienosilolo‐carbazole‐alt‐benzothiadiazole) (PDTSCBT), poly(dithienocyclopenta‐carbazole‐alt‐benzothiadiazole) (PDTCCBT), and poly(dithienopyrrolo‐carbazole‐alt‐benzothiadiazole) (PDTPCBT). The silole units in DTSC possess electron‐accepting ability that lowers the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PDTSCBT, whereas stronger electron‐donating ability of the pyrrole moiety in DTPC increases the HOMO and LUMO energy levels of PDTPCBT. The optical bandgaps (Egopt) deduced from the absorption edges of thin film spectra are in the following order: PDTSCBT (1.83 eV) > PDTCCBT (1.64 eV) > PDTPCBT (1.50 eV). This result indicated that the donor strength of the heptacyclic arenes is in the order: DTPC > DTCC > DTSC. The devices based on PDTSCBT and PDTCCBT exhibited high hole mobilities of 0.073 and 0.110 cm2 V−1 s−1, respectively, which are among the highest performance from the OFET devices based on the amorphous donor‐acceptor copolymers. The bulk heterojunction photovoltaic device using PDTSCBT as the p‐type material delivered a promising efficiency of 5.2% with an enhanced open circuit voltage, Voc, of 0.82 V.

A flexible solar device showing exceptional air and mechanical stability is produced by simultaneously optimizing molecular structure, active layer morphology, and interface characteristics. The PFDCTBT-C8-based devices with inverted architecture exhibited excellent power conversion efficiencies of 7.0% and 6.0% on glass and flexible substrates, respectively.

An isomerically pure anti-anthradithiophene (aADT) arranged in an angular shape is developed. Formation of the framework of aADT incorporating four lateral alkyl substituents was accomplished by a one-pot benzannulation via multiple Suzuki coupling. This newly designed 2,8-stannylated aADT monomer was copolymerized with a ditheniodiketopyrrolopyrrole (DPP) unit and a bithiophene unit, respectively, to furnish an alternating donor–acceptor copolymer poly­(anthradithiophene-alt-dithienyldiketopyrrolopyrrole) (PaADTDPP) and a thiophene-rich poly­(anthradithiophene-alt-bithiophene) (PaADTT). PaADTT with crystalline nature achieved a high FET mobility of 7.9 × 10–2 cm2 V–1 s–1 with an on–off ratio of 1.1 × 107. The photovoltaic device based on the PaADTDPP:PC71BM (1:2.5, w/w) blend exhibited a V oc of 0.66 V, a J sc of 9.49 mA/cm2, and a FF of 58.4%, delivering a power conversion efficiency (PCE) of 3.66%. By adding 1.5 vol % 1-chloronaphthalene (CN) as a processing additive, the PCE can be improved to 4.24%. We demonstrated that these angular-shaped and alkylated aADT-based polymers have better organic photovoltaic (OPVs) and field-effect transistor (FETs) characteristics than the linear-shaped ADT-containing polymers.

A new PC61BM‐based fullerene, [6,6]‐phenyl‐C61 butyric acid pentafluorophenyl ester (PC61BPF) is designed and synthesized. This new n‐type material can replace PC61BM to form a P3HT:PC61BPF binary blend or serve as an additive to form a P3HT:PC61BM:PC61BPF ternary blend. Supramolecular attraction between the pentafluorophenyl group of PC61BPF and the C60 cores of PC61BPF/PC61BM can effectively suppress the PC61BPF/PC61BM materials from severe aggregation. By doping only 8.3 wt% PC61BPF, device PC61BPF651 exhibits a PCE of 3.88% and decreases slightly to 3.68% after heating for 25 h, preserving 95% of its original value. When PC61BP with non‐fluorinated phenyl group is used to substitute PC61BPF, the stabilizing ability disappears completely. The efficiencies of PC61BP651 and PC61BP321 devices significantly decay to 0.44% and 0.11%, respectively, after 25 h isothermal heating. Most significantly, this strategy is demonstrated to be effective for a blend system incorporating a low band‐gap polymer. By adding only 10 wt% PC61BPF, the PDTBCDTB:PC71BM‐based device exhibits thermally stable morphology and device characteristics. These findings demonstrate that smart utilization of supramolecular interactions is an effective and practical strategy to control morphological evolution.