Four donor (D)–acceptor (A) conjugated polymers with isoindigo[7,6-g]isoindigo ([3E,8E]-3,8-bis(2-oxoindolin-3-ylidene)-6,8-dihydroindolo[7,6-g]indole-2,7(1H,3H)-dione, DIID) as A-unit and thiophene derivatives as D-units were synthesized by Stille polycondensation. Optical and electrochemical properties of the polymers were studied by UV–vis–NIR absorption spectrometer and cyclic voltammetry. Compared with isoindigo-based analogues, the polymers display much broader absorption spectra (covering 400–950 nm) and remarkably lower bandgaps (ca. 1.3 eV). All polymers showed ambipolar transport properties as evaluated by bottom-gate/top-contact (BGTC) and top-gate/bottom-contact (TGBC) organic thin film transistors (OTFTs) in air. Gate-voltage-dependent hole mobility (μh) was observed for BGTC devices, while the mobility of TGBC devices exhibited weak gate-voltage dependence. P3 delivered the best device performance. At the optimized thermal annealing temperature (200 °C), a μh of 1.79 cm2/(V s) and an electron mobility (μe) of 0.087 cm2/(V s) were demonstrated with BGTC devices, and μh calculated from the higher VGS region is decreased to 0.35 cm2/(V s). The relatively balanced hole and electron mobilities were observed for TGBC devices based on P3, which were 0.45 and 0.16 cm2/(V s), respectively.

A polycyclic aromatic unit comprising six rings, i.e., 5,11-bis(2-octyldodecyl)dithieno[3,2-b:3′,2′-b′]naphtho[1,2-b:5,6-b′]dithiophene (DTNDT), was developed. Four donor–acceptor (D–A) conjugated polymers, which are named P-BT, P-2FBT, P-IID, and P-DPP, were synthesized with DTNDT as the D-unit and 2,1,3-benzothiadiazole (BT), 5,6-difluorobenzo[c][1,2,5]thiadiazole (2FBT), isoindigo (IID), and diketopyrrolopyrrole (DPP) as the A-unit, respectively. All four polymers are thermally stable with decomposition temperature above 390 °C and show pseudo-straight-shaped backbones. Their ordered thin films were prepared via solution spin-casting, in which conjugated backbones mainly adopted edge-on alignment on the substrate. The semiconducting properties of the polymers were characterized with bottom gate and top contact (BGTC) organic thin film transistors (OTFTs). All four polymers showed p-type transport behavior, and the hole mobilities were 0.023, 0.078, 0.50, and 1.80 cm2/(V s) for P-BT, P-2FBT, P-IID, and P-DPP, respectively. P-DPP exhibited the highest film order, a short π–π stacking distance (3.52 Å), and therefore the highest device mobility.

Three low bandgap conjugated polymers based on 7-fluorinated isoindigo (IID1F) and dithieno[3,2-b;6,7-b]carbazole (DTC), i.e., poly[N-dodecyldithieno[3,2-b;6,7-b]carbazole-alt-7-fluoro-N,N-di(2-octyldodecyl)isoindigo] (P1), poly[N-dodecyldithieno[3,2-b;6,7-b]carbazole-alt-7-fluoro-N,N-di(3-octyltridecyl)isoindigo] (P2) and poly[N-dodecyldithieno[3,2-b;6,7-b]carbazole-alt-7-fluoro-N,N-di(4-octyltetradecyl)isoindigo] (P3), were synthesized. All three polymers are soluble in non-chlorinated solvent o-xylene owing to regio-random distribution of F-atoms along the conjugated backbone. The position of the alkyl-branching point has a negligible influence on energy levels and absorption spectra of the polymers, but has a small effect on their charge transport properties. Bulk heterojunction (BHJ) polymer solar cells (PSCs) of the polymers were fabricated with phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor. When o-xylene was used as a solvent, all three polymers delivered power conversion efficiencies (PCEs) above 7%. P2 exhibited the best device performance with a PCE of 7.5%. The devices processed with o-xylene showed higher device efficiency than those fabricated with o-dichlorobenzene (o-DCB) since the films of polymer:PC61BM blends prepared with o-xylene exhibited better morphology and higher and more balanced charge-carrier mobilities, leading to less recombination loss and higher fill factor (FF).

Four indeno[1,2-b]thiophene- or benzo[5,6]indeno[1,2-b]thiophene-flanked diketopyrrolopyrrole (DPP) derivatives, i.e., DPP-PhCO, DPP-PhCN, DPP-NaCO and DPP-NaCN in which phenyl or naphthyl and thiophene units were bridged with a carbonyl (DPP-PhCO and DPP-NaCO) group or a malononitrile-substituted carbon atom (DPP-PhCN and DPP-NaCN), were synthesized via intramolecular Friedel–Crafts acylation and Knoevenagel condensation. Compared to 5-phenylthiophen-2-yl substituted DPP (DPP-Ph) which has no bridge between the phenyl and thiophene rings, DPP-PhCO, DPP-PhCN, DPP-NaCO and DPP-NaCN have more planar structures and exhibit deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The semiconducting properties of the compounds were studied using organic thin film transistors (OTFTs). DPP-Ph, DPP-PhCO, DPP-NaCO and DPP-NaCN exhibited p-type transport behavior and DPP-PhCO showed the highest mobility (up to 0.052 cm2 V−1 s−1), while DPP-PhCN displayed ambipolar transport properties with hole and electron mobilities of 0.017 and 8.6 × 10−4 cm2 V−1 s−1, respectively.

Three donor–acceptor–donor (D–A–D) conjugated oligomers, i.e., 2ATT-IID-C8C10, 2ATT-IID-C6C8 and 2ATT-IID-C4C6, have been synthesized using anthra[1,2-b]thieno[2,3-d]thiophene (ATT) as an electron-donor unit and isoindigo (IID) as an electron-acceptor unit by combining the planar and rigid structure of fused aromatics and the strong intramolecular interaction of D–A conjugated molecules, and their semiconducting properties were studied via organic thin-film transistors (OTFTs). The alkyl chains, which are 2-octyldodecyl (C8C10), 2-hexyldecyl (C6C8) and 2-butyloctyl (C4C6), respectively, were employed in the IID unit in order to study the effect of alkyl bulkiness on the properties of the oligomers. All three oligomers adopted an edge-on alignment in thin films. Decreasing the bulkiness or length of the alkyls caused a noticeable improvement of the packing order of the oligomers, leading to a remarkably enhanced charge carrier mobility. 2ATT-IID-C8C10 could only form the film featured with one-dimensional order. Consequently, it exhibited the lowest OTFT mobility (<0.1 cm2 V−1 s−1). In contrast, both 2ATT-IID-C6C8 and 2ATT-IID-C4C6 adopted two-dimensional-ordered packing structures after thermal annealing with a π–π stacking distance of ∼3.6 Å, thereby exhibiting promising semiconducting properties. 2ATT-IID-C4C6 showed the best OTFT performance with a mobility of 0.72 cm2 V−1 s−1. This mobility is among the highest for the solution processible D–A conjugated oligomers to date.

A low bandgap conjugated polymer, i.e., poly[2,2′-bithiophene-alt-7-fluoro-N,N′-bis(2-octyldodecyl)isoindigo] (P(1FIID-BT)), was synthesized. The polymer was soluble in non-chlorinated solvents such as o-xylene and mesitylene, and exhibited a film optical bandgap of 1.61 eV as calculated from the absorption onset and a deep highest occupied molecular orbital (HOMO) energy level of −5.46 eV as measured by an electrochemical method. Remarkably high mobilities of 0.33 and 0.42 cm2 V−1 s−1 were obtained for organic field-effect transistors (OFETs) based on pristine and annealed films, respectively. Inverted polymer solar cells (PSCs) were fabricated with phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor material and o-xylene as the processing solvent. The device efficiency was insensitive to the thickness of the active layer, and all devices with film thickness in the range of 105–340 nm showed a power conversion efficiency (PCE) higher than 6.5%. The devices based on 270 nm thick films displayed the highest performance with a VOC of 0.89 V, a JSC of 14.50 mA cm−2 and a FF of 0.58, leading to a PCE of 7.46%. This is the first low bandgap conjugated polymer for PSCs which combine three features: non-chlorinated solvent processability, high efficiency and insensitivity of efficiency to the active layer thickness.

The controlled synthesis of poly(3-hexylthiophene)s (P3HTs) with number-average molecular weights (Mns) up to 350 kg mol−1 has been realized with Ni(IPr)(acac)2 as the catalyst.

Three new bis(diphenylphosphino)propane (dppp) derivatives, i.e., 1,3-bis(di(4-methylphenyl)phosphino)propane (L1), 1,3-bis(di(3-methylphenyl)phosphino)propane (L2) and 1,3-bis(di(2-methylphenyl)phosphino)propane (L3), were synthesized for studying the effect of the ligand on the Kumada catalyst transfer polycondensation (KCTP) of 7-bromo-9,9-dialkylfluorenylmagnesium chloride (M1) using Ni(acac)2/L (L = dppp, L1, L2 or L3) as the catalyst. Ni(acac)2/L1 exhibited a polymerization performance comparable to Ni(acac)2/dppp, and the polymerization could be well-controlled with ≥1 mol% catalyst. When the most hindered ligand L3 was used, the polymerization was out-of-control and only poly(9,9-dioctylfluorene)s (PF8s) with relatively low number-average molecular weights (Mn) and large polydispersity indices (PDIs) could be obtained. In contrast, the moderately hindered catalyst Ni(acac)2/L2 outperformed Ni(acac)2/dppp. The polymerization was well-controlled with a catalyst of ≥0.5 mol%, as confirmed by the linear correlation of Mnversus [monomer]/[Ni], and PF8s with Mn up to 91.1 kDa and fluorene-thiophene block copolymers (PF8-b-P3HTs) with Mn up to 78.4 kDa could be prepared in a controlled manner. Density functional theory (DFT) calculations, 31P NMR spectroscopy studies and block copolymerizations revealed that the great performance of L2 can be attributed to the stronger affinity of the L2Ni(0)–polymer π-complex and the higher stability of active chain ends. The active chain ends of the high molecular weight polymers were unstable, probably owing to unknown side reactions assisted by chain aggregation or entanglement, which may be responsible for the out-of-control nature of the polymerization at low catalyst loadings.

•A blend of P(IID-DTC):di-PBI is used to fabricate polymer solar cells.•The absorption of blends exhibits full coverage of 400–750 nm to solar spectrum.•Dominated bimolecular recombination is suppressed by using CH2Cl2 vapor annealing.•The electron mobility is enhanced by more than two orders of magnitude.•The P(IID-DTC):di-PBI polymer solar cell shows a PCE of 2.95%.A singly-linked perylene bisimide dimer (di-PBI) and a low band gap polymer based on dithienocarbazole and isoindigo moieties (P(IID-DTC)) are selected as electron acceptor and donor respectively to fabricate non-fullerene based polymer solar cells. The combination of di-PBI acceptor and P(IID-DTC) donor provides well overlapped absorption profile with solar light in the visible region. The P(IID-DTC):di-PBI blend morphology and its effect on charge transport and recombination have been investigated in detail. The dominant bimolecular recombination in commonly processed P(IID-DTC):di-PBI blend results in a low power conversion efficiency of merely 1%. We find that CH2Cl2 vapor annealing can effectively improve bi-continuous phase separation and boost the electron transport by more than two orders of magnitude due to the detrap of di-PBI molecules from amorphous P(IID-DTC) matrix. More importantly, the dominant bimolecular recombination in the P(IID-DTC):di-PBI blend films is strongly suppressed. Finally, the power conversion efficiency of non-fullerene P(IID-DTC):di-PBI blend solar cell is dramatically improved to 2.95%.A blend comprising singly-linked perylene bisimide dimer (di-PBI) acceptor and low band gap polymer P(IID-DTC) donor is utilized to fabricate non-fullerene polymer solar cells. The absorption of P(IID-DTC):di-PBI blend fully covers 400–750 nm of solar spectrum. Dominated bimolecular charge recombination is effectively suppressed by using CH2Cl2 vapor to treat the blend films. The power conversion efficiency of the P(IID-DTC):di-PBI polymer solar cell is finally enhanced to 2.95%.

Donor-acceptor (D-A)-conjugated polymers P(BT-C1) and P(BT-C2), with dithieno[2,3-b;7,6-b]carbazole (C1) or dithieno[ 3,2-b;6,7-b]carbazole (C2) as D-unit and benzothiadiazole (BT) as A-unit, were synthesized. The optical bandgaps of the polymers are similar (1.84 and 1.88 eV, respectively). The structures of donor units noticeably influence the energy levels and backbone curvature of the polymers. P(BT-C1) shows a large backbone curvature; its highest occupied molecular orbital (HOMO) energy level is −5.18 eV, whereas P(BT-C2) displays a pseudo-straight backbone and has a HOMO energy level of −5.37 eV. The hole mobilities of the polymers without thermal annealing are 1.9×10−3 and 2.7×10−3 cm2V−1s−1 for P(BT-C1) and P(BT-C2), respectively, as measured by organic thin-film transistors (OTFTs). Polymer solar cells using P(BT-C1) and P(BT-C2) as the donor and phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor were fabricated. Power conversion efficiencies (PCEs) of 4.9% and 5.0% were achieved for P(BT-C1) and P(BT-C2), respectively. The devices based on P(BT-C2) exhibited a higher Voc due to the deeper HOMO level of the polymer, which led to a slightly higher PCE.

The current challenges for efficient bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on organic/polymeric (non-fullerene) acceptors involve difficult control of neat phase separation in the nanoscale, severe geminate charge recombination, etc. Herein, a new molecular design concept, that is to construct donor–spacer–acceptor (D–S–A) co-oligomers with self-assembly properties, is proposed in order to realize ideal film morphology and manipulate the exciton dissociation and geminate charge recombination processes simultaneously. Three D–S–A co-oligomers, i.e.F5T8P-C2, F5T8P-C4 and F5T8P-C6 with oligo(fluorene-alt-bithiophene), perylene diimide (PDI) and alkyl as D-, A- and S-segments, respectively, were synthesized. All three D–S–A co-oligomers can form D–A alternating lamellar nanostructures with periods of ∼15 nm, an ideal nanostructure for BHJ OPVs. Compared to D–A co-oligomer F5T8-epP in which the D- and A-segments are directly connected without the alkyl spacer, the D–S–A co-oligomers not only show higher electron mobilities due to closer packing of PDI moieties, but also exhibit longer lifetimes of the charge-transfer states that can potentially restrain the geminate charge recombination and improve the charge generation efficiency. Accordingly, the single-molecule photovoltaic cells based on the D–S–A co-oligomers exhibit an improved fill factor of 0.47 and a high open-circuit voltage of 1.04 V. In particular, an external quantum efficiency of ∼65%, which is the highest for BHJ OPVs based on non-fullerene acceptor materials, has been demonstrated. By further extending the absorption onset of D–S–A co-oligomers to ∼600 nm, a single-molecule photovoltaic device with a power conversion efficiency of 2.70% has been fabricated. These results prove that high-efficiency BHJ OPVs based on non-fullerene acceptors are achievable if both the film morphology of the D–A blend and D–A interfaces are suitably manipulated.

Oligo(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene)s Tn and Tn-2CN (n = 4, 8, 12, 20, n represents the number of thiophene rings), which have H and CN end-groups, respectively, were synthesized, and the effect of chain-length and end-groups on their properties was studied. For both Tn and Tn-2CN, a red-shift of absorption spectra was observed as the chain length increased. Introduction of electron-withdrawing CN end-groups resulted in a remarkable red-shift of absorption spectra in both solution and film states for shorter oligomers (n = 4 and 8). However, for oligomers with n = 12 and 20, obvious red-shift of absorption spectra was only observed in the film state. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were both lowered by introducing CN end-groups, and the level of reduction decreased with an increase of the chain length. Both the chain length and end-groups influenced the thin film microstructures of the oligomers as revealed by two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and atomic force microscopy (AFM) studies. The oligomers with n = 4 and 8 stood on the substrates with a tilt angle, and the films of T4-2CN and T8-2CN were characterized with 3D crystal structures. In contrast, the oligomers with n = 12 and 20 adopted an edge-on arrangement with the long axes of the molecules parallel to the substrates, like polymeric counterparts. Compared to the films of T12 and T20, those of T12-2CN and T20-2CN comprised larger and more oriented crystal-domains to the substrates along with shorter lamellar distances. Charge-transport properties of the oligomers were influenced by both the chain length and end-groups due to their remarkable effect on the film morphology and packing structures. The film of T8-2CN combined three features: large domain sizes, low surface-roughness and diminished grain boundaries, and therefore exhibited the highest field-effect mobility.

Two benzothienobenzothiophene (BTBT)-based conjugated oligomers, i.e., 2,2′-bi[1]benzothieno[3,2-b][1]benzothiophene (1) and 5,5′-bis([1]benzothieno[3,2-b][1]benzothiophen-2-yl)-2,2′-bithiophene (2), were prepared and characterized. Both oligomers exhibit excellent thermal stability, with 5% weight-loss temperatures (TL) above 370 °C; no phase transition was observed before decomposition. The highest occupied molecular orbital (HOMO) levels of 1 and 2 are −5.3 and −4.9 eV, respectively, as measured by ultraviolet photoelectron spectroscopy. Thin-film X-ray diffraction and atomic force microscopy characterizations indicate that both oligomers form highly crystalline films with large domain sizes on octadecyltrimethoxysilane-modified substrates. Organic thin-film transistors with top-contact and bottom-gate geometry based on 1 and 2 exhibited mobilities up to 2.12 cm2/V·s for 1 and 1.39 cm2/V·s for 2 in an ambient atmosphere. 1-based devices exhibited great air and thermal stabilities, as evidenced by the slight performance degradation after 2 months of storage under ambient conditions and after thermal annealing at temperatures below 250 °C.Keywords: benzothienobenzothiophene; conjugated oligomers; field-effect mobility; organic semiconductors; organic thin-film transistors;

Controlled Suzuki–Miyaura catalyst-transfer polycondensations (SCTPs) of fluorene- and thiophene-based AB-type monomers have been demonstrated with a N-heterocyclic carbene (NHC)-based Pd complex, Pd(IPr)(OAc)2, as the catalyst. The number average molecular weights (Mns) of the resulting poly(9,9-dioctyl-9H-fluorene)s (PF8s) were linear to the conversions of the monomer. PF8s with Mns in the range of 10.5–69.2 kDa and polydispersity indices (PDIs) of ∼1.60 were successfully synthesized by tuning the feed ratios ([monomer]0/[Pd]). The protocol is also applicable to the controlled synthesis of poly(3-hexylthiophene) (P3HT). Polymers with Mns of 9.5–63.8 kDa, which were linearly correlated to feed ratios, were obtained when the catalyst loading was tuned to 5–0.5 mol%. The “living” characteristics of the polymerization were also confirmed by monomer-addition and block copolymerization experiments. In addition, PF8 with a moderate molecular weight (MW) was mainly end-capped with Br/H end groups as evidenced by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass and 1H NMR spectra, indicating that this polymerization involves a catalyst-transfer mechanism. The mechanism was further confirmed by the fact that the cross-coupling of dibromofluorene (1 equiv.) and biphenylboronic acid ester (1 equiv.) with Pd(IPr)(OAc)2 as the catalyst preferentially gave a di-substituted product.

•Oligo(2,6-naphthalene)s with 3–5 repeating units have been synthesized.•Highly ordered films can be prepared by vacuum deposition.•Organic thin-film transistors (OTFTs) of the oligomers exhibited mobilities up to 0.50 cm2/V s.Oligo(2,6-naphthalene)s (nNs) with the number of repeating unit (n) of 3–5 and dioctyl substituted trimer (DO-3N) and tetramer (DO-4N) were synthesized and characterized. Except of DO-4N that decomposed at 331 °C, all other oligomers display good thermal stability with 5% weight loss temperatures beyond 350 °C. For 4N and DO-4N, only one phase transition corresponding to melting can be found. However, DO-3N and 5N exhibit multiple phase transitions. The film absorption maximum of the oligomers at the long wavelength exhibits a red-shift with increasing the number of repeating unit. Meanwhile, the highest occupied molecular orbital (HOMO) levels increase from −5.96 eV of 3N to −5.91 eV of 4N and then −5.85 eV of 5N. The introduction of octyl group has negligible effect on the photophysical properties. Highly ordered films of 4N, 5N, DO-3N and DO-4N in which the molecules orient with their long axis standing on the substrates can be prepared by vacuum deposition. Organic thin-film transistors (OTFTs) with top-contact and bottom-gate geometry have been fabricated based on these films. The introduction of octyl groups results in the films with higher order and therefore higher field-effect mobilities (μ) of OTFT devices. Among these oligomers, DO-3N exhibits the highest mobility of 0.50 cm2/V s when the film deposited on the octyltrichlorosilane (OTS) modified substrate.Graphical abstract

π-Extended thienoacenes that comprise alternatively arranged anthracene and thieno[3,2-b]thiophene moieties and have 8 or 13 aromatic rings were synthesized. The delocalization of their HOMO and LUMO orbitals is over the entire molecules, and low-lying HOMO levels endow them good stability.

Four new low-band-gap alternating copolymers (P-1, P-2, P-3 and P-4) based on electron-rich benzodithiophene and newly developed electron-deficient units, thienopyrazine or dithiadiazatrindene derivatives, were synthesized by Stille polycondensation. All polymers exhibit good solubility in common organic solvents and a broad absorption band in the visible to near-infrared regions. The film optical band gaps of the polymers are in the range of 1.28–2.07 eV and the highest occupied molecular orbital (HOMO) energy levels are in the range of −4.99 eV to −5.28 eV. Bulk heterojunction polymer solar cells (PSCs) of the polymers were fabricated with phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor material, and a power conversion efficiency of 0.80% was realized with P-1 as donor material.

A cyano-terminated dimer of dithienyldiketopyrrolopyrrole (TDPP), DPP2-CN, is a solution processable ambipolar semiconductor with field-effect hole and electron mobilities of 0.066 and 0.033 cm2 V−1 s−1, respectively, under ambient conditions.

The controlled synthesis of poly(3-hexylthiophene)s (P3HTs) with number-average molecular weights (Mns) up to 350 kg mol−1 has been realized with Ni(IPr)(acac)2 as the catalyst.

Oligo(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene)s Tn and Tn-2CN (n = 4, 8, 12, 20, n represents the number of thiophene rings), which have H and CN end-groups, respectively, were synthesized, and the effect of chain-length and end-groups on their properties was studied. For both Tn and Tn-2CN, a red-shift of absorption spectra was observed as the chain length increased. Introduction of electron-withdrawing CN end-groups resulted in a remarkable red-shift of absorption spectra in both solution and film states for shorter oligomers (n = 4 and 8). However, for oligomers with n = 12 and 20, obvious red-shift of absorption spectra was only observed in the film state. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were both lowered by introducing CN end-groups, and the level of reduction decreased with an increase of the chain length. Both the chain length and end-groups influenced the thin film microstructures of the oligomers as revealed by two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and atomic force microscopy (AFM) studies. The oligomers with n = 4 and 8 stood on the substrates with a tilt angle, and the films of T4-2CN and T8-2CN were characterized with 3D crystal structures. In contrast, the oligomers with n = 12 and 20 adopted an edge-on arrangement with the long axes of the molecules parallel to the substrates, like polymeric counterparts. Compared to the films of T12 and T20, those of T12-2CN and T20-2CN comprised larger and more oriented crystal-domains to the substrates along with shorter lamellar distances. Charge-transport properties of the oligomers were influenced by both the chain length and end-groups due to their remarkable effect on the film morphology and packing structures. The film of T8-2CN combined three features: large domain sizes, low surface-roughness and diminished grain boundaries, and therefore exhibited the highest field-effect mobility.

Four indeno[1,2-b]thiophene- or benzo[5,6]indeno[1,2-b]thiophene-flanked diketopyrrolopyrrole (DPP) derivatives, i.e., DPP-PhCO, DPP-PhCN, DPP-NaCO and DPP-NaCN in which phenyl or naphthyl and thiophene units were bridged with a carbonyl (DPP-PhCO and DPP-NaCO) group or a malononitrile-substituted carbon atom (DPP-PhCN and DPP-NaCN), were synthesized via intramolecular Friedel–Crafts acylation and Knoevenagel condensation. Compared to 5-phenylthiophen-2-yl substituted DPP (DPP-Ph) which has no bridge between the phenyl and thiophene rings, DPP-PhCO, DPP-PhCN, DPP-NaCO and DPP-NaCN have more planar structures and exhibit deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The semiconducting properties of the compounds were studied using organic thin film transistors (OTFTs). DPP-Ph, DPP-PhCO, DPP-NaCO and DPP-NaCN exhibited p-type transport behavior and DPP-PhCO showed the highest mobility (up to 0.052 cm2 V−1 s−1), while DPP-PhCN displayed ambipolar transport properties with hole and electron mobilities of 0.017 and 8.6 × 10−4 cm2 V−1 s−1, respectively.

Three donor–acceptor–donor (D–A–D) conjugated oligomers, i.e., 2ATT-IID-C8C10, 2ATT-IID-C6C8 and 2ATT-IID-C4C6, have been synthesized using anthra[1,2-b]thieno[2,3-d]thiophene (ATT) as an electron-donor unit and isoindigo (IID) as an electron-acceptor unit by combining the planar and rigid structure of fused aromatics and the strong intramolecular interaction of D–A conjugated molecules, and their semiconducting properties were studied via organic thin-film transistors (OTFTs). The alkyl chains, which are 2-octyldodecyl (C8C10), 2-hexyldecyl (C6C8) and 2-butyloctyl (C4C6), respectively, were employed in the IID unit in order to study the effect of alkyl bulkiness on the properties of the oligomers. All three oligomers adopted an edge-on alignment in thin films. Decreasing the bulkiness or length of the alkyls caused a noticeable improvement of the packing order of the oligomers, leading to a remarkably enhanced charge carrier mobility. 2ATT-IID-C8C10 could only form the film featured with one-dimensional order. Consequently, it exhibited the lowest OTFT mobility (<0.1 cm2 V−1 s−1). In contrast, both 2ATT-IID-C6C8 and 2ATT-IID-C4C6 adopted two-dimensional-ordered packing structures after thermal annealing with a π–π stacking distance of ∼3.6 Å, thereby exhibiting promising semiconducting properties. 2ATT-IID-C4C6 showed the best OTFT performance with a mobility of 0.72 cm2 V−1 s−1. This mobility is among the highest for the solution processible D–A conjugated oligomers to date.

A low bandgap conjugated polymer, i.e., poly[2,2′-bithiophene-alt-7-fluoro-N,N′-bis(2-octyldodecyl)isoindigo] (P(1FIID-BT)), was synthesized. The polymer was soluble in non-chlorinated solvents such as o-xylene and mesitylene, and exhibited a film optical bandgap of 1.61 eV as calculated from the absorption onset and a deep highest occupied molecular orbital (HOMO) energy level of −5.46 eV as measured by an electrochemical method. Remarkably high mobilities of 0.33 and 0.42 cm2 V−1 s−1 were obtained for organic field-effect transistors (OFETs) based on pristine and annealed films, respectively. Inverted polymer solar cells (PSCs) were fabricated with phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor material and o-xylene as the processing solvent. The device efficiency was insensitive to the thickness of the active layer, and all devices with film thickness in the range of 105–340 nm showed a power conversion efficiency (PCE) higher than 6.5%. The devices based on 270 nm thick films displayed the highest performance with a VOC of 0.89 V, a JSC of 14.50 mA cm−2 and a FF of 0.58, leading to a PCE of 7.46%. This is the first low bandgap conjugated polymer for PSCs which combine three features: non-chlorinated solvent processability, high efficiency and insensitivity of efficiency to the active layer thickness.

The current challenges for efficient bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on organic/polymeric (non-fullerene) acceptors involve difficult control of neat phase separation in the nanoscale, severe geminate charge recombination, etc. Herein, a new molecular design concept, that is to construct donor–spacer–acceptor (D–S–A) co-oligomers with self-assembly properties, is proposed in order to realize ideal film morphology and manipulate the exciton dissociation and geminate charge recombination processes simultaneously. Three D–S–A co-oligomers, i.e.F5T8P-C2, F5T8P-C4 and F5T8P-C6 with oligo(fluorene-alt-bithiophene), perylene diimide (PDI) and alkyl as D-, A- and S-segments, respectively, were synthesized. All three D–S–A co-oligomers can form D–A alternating lamellar nanostructures with periods of ∼15 nm, an ideal nanostructure for BHJ OPVs. Compared to D–A co-oligomer F5T8-epP in which the D- and A-segments are directly connected without the alkyl spacer, the D–S–A co-oligomers not only show higher electron mobilities due to closer packing of PDI moieties, but also exhibit longer lifetimes of the charge-transfer states that can potentially restrain the geminate charge recombination and improve the charge generation efficiency. Accordingly, the single-molecule photovoltaic cells based on the D–S–A co-oligomers exhibit an improved fill factor of 0.47 and a high open-circuit voltage of 1.04 V. In particular, an external quantum efficiency of ∼65%, which is the highest for BHJ OPVs based on non-fullerene acceptor materials, has been demonstrated. By further extending the absorption onset of D–S–A co-oligomers to ∼600 nm, a single-molecule photovoltaic device with a power conversion efficiency of 2.70% has been fabricated. These results prove that high-efficiency BHJ OPVs based on non-fullerene acceptors are achievable if both the film morphology of the D–A blend and D–A interfaces are suitably manipulated.

A cyano-terminated dimer of dithienyldiketopyrrolopyrrole (TDPP), DPP2-CN, is a solution processable ambipolar semiconductor with field-effect hole and electron mobilities of 0.066 and 0.033 cm2 V−1 s−1, respectively, under ambient conditions.