Tailored Donor–Acceptor Polymers with an A–D1–A–D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices

The Importance of End Groups for Solution-Processed Small-Molecule Bulk-Heterojunction Photovoltaic Cells

End groups in small-molecule photovoltaic materials are important owing to their strong influence on molecular stability, solubility, energy levels, and aggregation behaviors. In this work, a series of donor-acceptor pentads (D2 -A-D1 -A-D2 ) were designed and synthesized, aiming to investigate the effect of the end groups on the materials properties and photovoltaic device performance. These molecules share identical central A-D1 -A triads (with benzodithiophene as D1 and 6-carbonyl-thieno[3,4-b]thiophene as A), but with various D2 end groups composed of alkyl-substituted thiophene (T), thieno[3,2-b]thiophene (TT), and 2,2'-bithiophene (BT). The results indicate a relationship between conjugated segment/alkyl chain length of the end groups and the photovoltaic performance, which contributes to the evolving molecular design principles for high efficiency organic solar cells.

Photon-absorbing charge-bridging states in organic bulk heterojunctions consisting of diketopyrrolopyrrole derivatives and PCBM

We have investigated the photo- and electrochemical properties of five diketopyrrolopyrrole (DPP) derivatives both experimentally and theoretically. In the experimental study, we found that a blend of a DPP derivative named D2 and phenyl-C61-butyric acid methyl ester (PCBM) exhibits the highest internal quantum efficiency (IQE) and power convergence efficiency (PCE) among the five derivatives investigated. In the theoretical study, we found that the open-circuit voltage can be estimated from the difference between the energy gap of frontier orbitals and the voltage loss and that the latter is suppressed when the IQE is large. Then, to investigate the factors that influence the IQE, investigations on charge recombination, hole transfer, and charge transfer induced by photoabsorption were conducted for the complexes of each DPP derivative and PCBM. It was found that D2/PCBM exhibits the largest charge-bridging upon photoabsorption, which leads to the highest IQE and PCE among the five DPP derivatives.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

Solution-processed bulk heterojunction solar cells have experienced a remarkable acceleration in performances in the last two decades, reaching power conversion efficiencies above 10%. This impressive progress is the outcome of a simultaneous development of more advanced device architectures and of optimized semiconducting polymers. Several chemical approaches have been developed to fine-tune the optoelectronics and structural polymer parameters required to reach high efficiencies. Fluorination of the conjugated polymer backbone has appeared recently to be an especially promising approach for the development of efficient semiconducting polymers. As a matter of fact, most currently best-performing semiconducting polymers are using fluorine atoms in their conjugated backbone. In this review, we attempt to give an up-to-date overview of the latest results achieved on fluorinated polymers for solar cells and to highlight general polymer properties’ evolution trends related to the fluorination of their conjugated backbone.

Regioisomeric donor–acceptor–donor triads based on benzodithiophene and BODIPY with distinct optical properties and mobilities

Two regioisomeric donor–acceptor–donor triads composed of benzodithiophene and BODIPY exhibit distinct optical and charge transfer properties, and mobilities of ∼10⁻⁴ cm² V⁻¹ s⁻¹.

Synthesis and photophysical properties of regioregular low bandgap copolymers with controlled 5-fluorobenzotriazole orientation for photovoltaic application

Two regioregular P1 and random P2 copolymers were synthesized and examined as electron donors in BHJ solar cells. The high PCE achieved of 7.66% for P1 is attributed to increased hole mobility.

Enhancing the photovoltaic properties of low bandgap terpolymers based on benzodithiophene and phenanthrophenazine by introducing different second acceptor units

Two D–A type random terpolymers PBDTT-PPzIID and PBDTT-PPzDPP were synthesized by copolymerizing a donor and two acceptor units. The PBDTT-PPzDPP-based device shown a PCE of 5.91%.

Side-Chain Engineering for Enhancing the Thermal Stability of Polymer Solar Cells

An effective strategy of engineering side chains is proposed for enhancing solar-cell-device thermal stability. As the conjugated length of the side chains increases, the morphological stability of the blend film is enhanced. The thermal stability of corresponding devices is consequently improved.

One-Nanometer Thin Monolayers Remove the Deleterious Effect of Substrate Defects in Molecular Tunnel Junctions

Defects in self-assembled monolayer (SAMs) based junctions cause the largest deviation between predicted and measured values of the tunnelling current. We report the remarkable, seemingly counterintuitive finding that shorter, less-ordered SAMs provide, unlike taller crystalline-like SAMs, higher quality tunnelling barriers on defective substrates, which points to self-repair of liquid-like SAMs on defects. The molecular dynamics show that short-chain molecules can more easily rotate into low-density boundary regions and smoothen out defects than thick solid-like SAMs. Our findings point to an attractive means of removing their deleterious effects simply by using flexible molecules.

Side Chain Influence on the Morphology and Photovoltaic Performance of 5-Fluoro-6-alkyloxybenzothiadiazole and Benzodithiophene Based Conjugated Polymers

Three conjugated polymers (P1-P3) with benzodithiophene derivatives as the donor unit, 5-fluoro-6-(2-hexyldecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole as the acceptor unit and thiophene as the spacer were designed, synthesized, and used as donor materials for polymer solar cells (PSCs). The influence of side chains at the benzodithiophene unit on the performance of PSCs was investigated. PSCs with the blend of P2:PC71BM (1:2, by weight) as the active layer show the highest power conversion efficiency (PCE) of 6.88%, with an open circuit voltage (Voc) of 0.76 V, a short circuit current (Jsc) of 14.67 mA/cm(2), and a fill factor (FF) of 0.62. Our research revealed that the variation of side chains had a great influence on the morphology of blend films, which is crucial to the performance of PSCs. As indicated by transmission electron microscopy, the blends of P1:PC71BM (1:2) and P2:PC71BM (1:2) formed nanofibers, whereas the blends of P3:PC71BM (1:2) formed spherical domains. Therefore, we concluded that formation of a more interpenetrating phase-separated donor-acceptor network with a larger interfacial area and proper percolation in the blends from P1 to P2 is mainly responsible for better performance in the corresponding devices.

Red-emitting DPSB-based conjugated polymer nanoparticles with high two-photon brightness for cell membrane imaging

New red-emitting conjugated polymers have been successfully synthesized by incorporating classical two-photon absorption (TPA) units, electron-rich units, and a small amount of electron-deficient units along the polymer backbones. Water-dispersible nanoparticles (NPs) based on these polymers were also fabricated for applications in two-photon excitation fluorescence imaging of cell membrane. Through optimization of the polymer/matrix mass ratio and the initial feed concentration of the polymer solution, a high quantum yield (QY) of 24% was achieved for the red-emitting NPs in water. TPA cross section and two-photon action cross section values of these polymers at 750 nm reached up to 1000 GM and 190 GM per repeat unit in aqueous media, 2.5 × 10(5) GM and 4.7 × 10(4) GM per NP, respectively. Furthermore, these NPs displayed excellent photostability and biocompatibility. Their applications as two-photon excitation fluorescence probes for cell membrane imaging have been demonstrated in three different cell lines with excellent imaging contrast. These results demonstrated that these polymer NPs hold great potentials as excellent two-photon excitation fluorescence probes in various biological applications.

2,1,3-Benzothiadiazole-5,6-dicarboxylic imide--a versatile building block for additive- and annealing-free processing of organic solar cells with efficiencies exceeding 8%

A new photoactive polymer comprising benzo[1,2-b:3,4-b':5,6-d']trithiophene and 2,1,3-benzothiadiazole-5,6-dicarboxylic imide is reported. The synthetic design allows for alkyl chains to be introduced on both electron-rich and electron-deficient components, which in turn allows for rapid optimization of the alkyl chain substitution pattern. Consequently, the optimized polymer shows a maximum efficiency of 8.3% in organic photovoltaic devices processed in a commercially viable fashion without solvent additives, annealing, or device engineering.

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

Effect of fluorine substitution on the photovoltaic performance of poly(thiophene-quinoxaline) copolymers

Three new conjugated polymers with or without fluorine substituents on quinoxaline ring were investigated in detail to assess the role of F in OPV performance.

Highly efficient photovoltaics and field-effect transistors based on copolymers of mono-fluorinated benzothiadiazole and quaterthiophene: synthesis and effect of the molecular weight on device performance

A D–A conjugated polymer based on mono-fluorinated benzothiadiazole (FBT) was designed and synthesized, and high performance photovoltaics and FETs were achieved.

Solution-processed new porphyrin-based small molecules as electron donors for highly efficient organic photovoltaics

A series of new A–D–A structural 5,15-dialkylated porphyrin-cored small molecules have been developed as donors in bulk heterojunction organic solar cells, and the highest power conversion efficiency of 6.49% has been achieved.

Improved photovoltaic performance of a 2D-conjugated benzodithiophene-based polymer by the side chain engineering of quinoxaline

To investigate the influence of side chains of quinoxaline on the photovoltaic performances, a novel D–A-type polymer of PBDTDT(Qx-3)-T was synthesized and characterized, in which BDT-T, T and Qx-3 were used as the donor (D) unit, π-bridge and acceptor (A) unit, respectively.

Dithienosilole-benzothiadiazole-based ternary copolymers with a D1–A–D2–A structure for polymer solar cells

A series of regularly alternating D₁–A–D₂–A type of ternary copolymers were synthesized for the application in polymer solar cells.

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.

Water/alcohol soluble conjugated polymers for the interface engineering of highly efficient polymer light-emitting diodes and polymer solar cells

This review provides a summary of the recent developments and applications of water/alcohol soluble conjugated polymers in highly efficient polymer light-emitting diodes and polymer solar cells.

Effect of molecular weight on the properties and organic solar cell device performance of a donor–acceptor conjugated polymer

The effect of molecular weight of a conjugated polymer on its photophysical properties and solar cell device performance was investigated.

The enhanced performance of fluorinated quinoxaline-containing polymers by replacing carbon with silicon bridging atoms on the dithiophene donor skeleton

The bridging atom effect shows a great influence on the thermal stability, absorption, energy levels, carrier mobility and photovoltaic performance of fluorinated quinoxaline-dithiophene based low band gap copolymers.

Photocurrent enhancement of BODIPY-based solution-processed small-molecule solar cells by dimerization via the meso position

Three 4,4-difluoro-4-bora-3a,4a-diaza-s-indancene (BODIPY)-based small molecule donors H-T-BO, Br-T-BO, and DIMER were synthesized and fully characterized. Although modification at the meso position has a subtle influence on the light-harvesting ability, energy levels, and phase sizes, it has a striking effect on the packing behavior in solid film as two-dimension grazing incidence X-ray diffraction (2D GIXRD) and X-ray diffraction (XRD) confirm. Br-T-BO exhibits better packing ordering than H-T-BO in pristine film, which is beneficial from reinforced intermolecular interaction from halogen atoms. However, when [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is blended, no diffraction patterns corresponding to the monomeric donor can be seen from the XRD data and both H-T-BO- and Br-T-BO-based blend films give a slightly blue-shifting absorption peak with respect to their neat ones, both of which imply destruction of the crystalline structure. As for DIMER, the enhancement of the intermolecular interaction arises not only from the expansion of the backbone but the "steric pairing effect" brought on by its twisted structure. When blended with PC71BM, the diffraction patterns of DIMER are, however, kept well and the absorption peak position remains unchanged, which indicates the ordered packing of DIMER is held well in blend film. In coincidence with the fact that packing ordering improves from H-T-BO to Br-T-BO and DIMER in pristine films and the ordered packing of DIMER even in blend film, DIMER-based devices show the highest and most balanced hole/electron mobility of 1.16 × 10(-3)/0.90 × 10(-3) cm(2) V(-1) s(-1)with respect to Br-T-BO (4.71 × 10(-4)/2.09 × 10(-4) cm(2) V(-1) s(-1)) and H-T-BO (4.27 × 10(-5)/1.00 × 10(-5) cm(2) V(-1) s(-1)) based ones. The short-circuit current density of the three molecule-based cells follows the same trend from H-T-BO (6.80) to Br-T-BO (7.62) and then to DIMER (11.28 mA cm(-2)). Finally, the H-T-BO-, Br-T-BO-, and DIMER-based optimal device exhibits a power conversion efficiency of 1.56%, 1.96%, and 3.13%, respectively.