Due to their low cost and high efficiency, polymer/nanocrystal hybrid solar cells (HSCs) have attracted much attention in recent years. In this work, water-soluble hybrid materials consisting of amphiphilic block copolymers (ABCPs) and cadmium telluride nanocrystals (CdTe NCs) were used as the active layer to fabricate the HSCs via aqueous processing. The ABCPs composed of poly(3-hexylthiophene) (P3HT) and poly(acrylic acid) (PAA) self-assembled into ordered nanostructured micelles which then transformed to nanowires by comicellization with P3HT additives. Furthermore, after annealing, the hybrid materials formed an interpenetrating network which resulted in a maximum power conversion efficiency of 4.8% in the HSCs. The properties of the hybrid materials and the film morphology were studied and correlated to the device performance. The results illustrate how the inclusion of ABCPs for directed assembly and homo-P3HT for charge transport and light absorption improves device performance. The aqueous-processed HSCs based on the ABCPs and NCs offer an effective method for the fabrication of efficient solar cells.Keywords: amphiphilic block copolymer; aqueous-processed; hybrid; self-assembled; solar cells;

Organic photovoltaic cells made with semiconducting polymers remain one of the most promising technologies for low-cost solar energy due to their compatibility with roll-to-roll printing techniques. The development of new light-absorbing polymers has driven tremendous advances in the power conversion efficiency of these devices. In particular, the use of alternating electron rich (donor) and electron poor (acceptor) segments along the polymer backbone can produce low optical bandgap materials that capture more of the solar spectrum. As a result, power conversion efficiencies over 10% are increasingly common for this technology. This review summarizes the recent advances in donor-acceptor polymer design and synthesis, highlighting the structural features that are key to providing high efficiency, scalable and stable devices.

An alkyl-substituted indacenodithiophene-based donor–acceptor π-conjugated polymer (PIDTBPD) with low stiffness and high ductility is reported. The polymer was synthesized after DFT calculations predicted that it would have a kinked backbone conformation while showing strong intramolecular charge transfer (ICT), suggestive of the fact that it would be beneficial to the polymer's elasticity and charge mobility. Atom-efficient direct arylation polymerization (DArP) was exploited to synthesize the polymer. Mechanical studies indicate that PIDTBPD has relatively rapid stress-relaxation properties, which lead to a low elastic modulus of 200 MPa and high crack-onset strain of ca. 40% (lower limit). A moderate charge carrier mobility of 2 × 10−3 cm2 V−1 s−1 with a current on/off ratio of 2.5 × 106 was obtained from the fabricated OFETs. Further experiments were performed to elucidate the structural aspects of this polymer: UV-Vis and PL spectra suggest that minimal conformational change occurs in the polymer between its diluted solution and thin film states; DSC measurements indicate that the polymer's Tg is below −20 °C, allowing it to be in a rubbery state at room temperature; and XRD studies support this observation suggesting that the polymer is mostly amorphous at room temperature.

The nanoscale structure and macroscopic morphology of π-conjugated polymers are very important for their electronic application. While ordered single crystals of small molecules have been obtained via solution deposition, macroscopically aligned films of π-conjugated polymers deposited directly from solution have always required surface modification or complex pre-deposition processing of the solution. Here, ordered nanowires were obtained via shear-enhanced crystallization of π-conjugated polymers at the air–liquid–solid interface using simple deposition of the polymer solution onto an inclined substrate. The formation of macroscopically aligned nanowire arrays was found to be due to the synergy between intrinsic (π-conjugated backbone) and external (crystallization conditions) effects. The oriented nanowires showed remarkable improvement in the charge carrier mobility compared to spin-coated films as characterized in organic field-effect transistors (OFETs). Considering the simplicity and large-scale applicability, shear-enhanced crystallization of π-conjugated polymers provides a promising strategy to achieve high-performance polymer semiconductor films for electronics applications.

Thin-film photovoltaic (PV) devices can be fabricated using a solution-based synthesis procedure in which metal-chalcogenide nanocrystals with aliphatic coordinating ligands are suspended in a solvent to produce a printable ink (NC-ink). However, the aliphatic ligands that are used to solubilize and stabilize the nanocrystals operate as a significant source of carbon impurities that are incorporated into the final device absorber layer. Despite the ubiquity of this technique and the fact that carbon defects have been reported to be found across a spectrum of devices, the structure, properties, and influence of the carbon on PV device performance remain relatively unexplored. Our findings indicate that these organic ligands undergo a pyrolysis reaction during annealing, producing an electrically conductive, graphitic carbon that also reacts with chalcogens (S or Se) to produce heterocyclic moieties. In this work, we used oleylamine (OLA) and dodecylamine (DDA) to fabricate Cu2ZnSn(SxSe1–x)4 (CZTSSe) photovoltaic devices from a NC-ink. DDA, which has fewer carbon atoms and contains no double bond, produces CZTSSe devices with less carbon in the absorber layer; but this reduced carbon content does not translate to improved device performance. OLA, which is a larger molecule and contains one double bond, produces CZTSSe devices with more carbon in the absorber layer. However, OLA also produces more crystalline graphitic carbon and allows for the CZTS nanocrystals to grow to a larger size during annealing, which improves device performance significantly.

C–H activation reactions have allowed us to react traditionally chemically inert bonds in molecules to develop new methods for cross-coupling reactions. This type of reactivity can be applied to conjugated polymer materials in an effort to improve existing synthetic difficulties including harsh reaction conditions, multiple monomer functionalization steps, and organometallic reagent waste. In this Viewpoint, we highlight some of the encouraging advances in direct arylation polymerization (DArP) as well as ongoing challenges for future improvement and utility.

In the search for new synthetic routes toward greener and more facile syntheses of conjugated polymers, C–H functionalization provides a promising solution by minimizing the production and processing of aryl halide monomer precursors used in traditional organometallic coupling reactions. In this paper, we investigate the use of Au(I) and its ability to directly C–H activate 2-bromo-3-hexylthiophene to form a reactive monomer species, bypassing the typical Grignard monomer formation from a dihalogenated thiophene. Addition of Pd-PEPPSI-iPr as a palladium catalyst source in the presence of the resultant aurylated thiophene monomer yielded poly(3-hexylthiophene) as observed by both NMR and GPC. Studies on the growth of these polymers show linear dependence between Mn and monomer conversion, low dispersities, as well as Mn predicted by catalyst loading, which is supportive of a living-type chain growth mechanism. This Au–Pd system represents a novel methodology for incorporating C–H activation into the synthesis of P3HT with control over Mn.

Correction for ‘The future of organic photovoltaics’ by Katherine A. Mazzio et al., Chem. Soc. Rev., 2015, 44, 78–90.

Increasing global demand for energy, along with dwindling fossil fuel resources and a better understanding of the hidden costs associated with these energy sources, have spurred substantial political, academic, and industrial interest in alternative energy resources. Photovoltaics based on organic semiconductors have emerged as promising low-cost alternatives for electricity generation that relies on sunlight. In this tutorial review we discuss the relevance of these organic photovoltaics beginning with some of the economic drivers for these technologies. We then examine the basic properties of these devices, including operation and materials requirements, in addition to presenting the development of the field from a historical perspective. Potential future directions are also briefly discussed. This tutorial review is intended to be an essential overview of the progress of the field, in addition to aiding in the discussion of the future of OPV technologies.

Organic coordinating ligands are ubiquitously used to solubilize, stabilize and functionalize colloidal nanoparticles. Aliphatic organic ligands are typically used to control size during the nanoparticle growth period and are used as a high boiling point solvent for solution-based synthesis procedures. However, these aliphatic ligands are typically not well suited for the end use of the nanoparticles, so additional ligand exchange or ligand stripping procedures must be implemented after the nanoparticle synthesis. Herein we present a ligand-free CdS nanoparticle synthesis procedure using a unique sulfur copolymer. The sulfur copolymer is derived from elemental sulfur, which is a cheap and abundant material. This copolymer is used as a sulfur source and high boiling point solvent, which produces stabilized metal-sulfide nanoparticles that are suspended within a sulfur copolymer matrix. The copolymer can then be removed, thereby yielding ligand-free metal-sulfide nanoparticles.

A series of n-type hyperbranched polymers exhibiting variable porosity and excellent electrochemical stability are presented for use as the cathodes of asymmetric supercapacitors. The polymers are designed with triphenylamine (TPA) cores and naphthalene diimide (NDI) terminal units, with NDI being chosen for its electrochemical stability under reduction. A different number of thiophene rings between the TPA and NDI units are used to alter the porosity. These devices show very good stability over 500 cycles as a result of the stable NDI unit, and nitrogen adsorption experiments confirm that the addition of thiophene spacers results in a concomitant increase in the polymer matrix pore size. Electrochemical impedance spectroscopy (EIS) characterization reveals that as the porosity of the polymer increases, the diffusion resistance decreases. However, as the pore size increases, the charge transfer resistance and equivalent series resistance increases. Finally, one of the polymers is used to fabricate proof-of-concept symmetric supercapacitors.

This study explores the role of very small changes in poly(3-hexylthiophene-2,5-diyl) (P3HT) regioregularity on the physical and electronic properties of P3HT nanowires. Due to a high level of synthetic control, we are able to isolate the effects of regioregularity from those of polymer molecular weight and dispersity for the first time. A series of P3HTs with regioregularities from 96 to 99%, similar molecular weights, and low dispersities are synthesized. The charge transport properties of these polymers, along with a Soxhlet extracted 93% regioregular P3HT purchased from Rieke metals, are investigated in both thin film and nanowire transistors. The resulting structural characteristics are examined by atomic force microscopy and X-ray diffraction, and the optical characteristics are explored by UV–vis absorption. It is found that increasing the P3HT regioregularity results in improved charge transport characteristics, with an increase in mobility by a factor of 4 for the regioregularities examined. The increased mobility is shown to reflect increasing structural coherence lengths in the (010) direction, as well as improved J-aggregate characteristics due to greater planarity and reduced numbers of defect sites along the polymer nanowires. Overall, this study serves to emphasize the importance of determining and reporting even small changes in polymer regioregularity.

The charge carrier dynamics of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) organic bulk-heterojunction photovoltaics with and without TiO2 nanowires are studied. Using inorganic nanowires as electron transport pathways improves the charge transit time and electron diffusion coefficient, which is the origin of fill factor and power conversion efficiency improvement observed in these devices. Through further comparison of devices with surface-modified nanowires (PCB–TiO2-NW), it is found that under AM 1.5 light illumination, charge recombination is dominant in the organic layer rather than at the TiO2 nanowire surface.

Conjugated polymers with fluorine substituents on their backbone have exhibited improved performance over their un-fluorinated analogues by lowering the polymer HOMO level, thereby increasing the open-circuit voltage (VOC). To further investigate how fluorine substituents improve device performance, three polymers with the same donor and acceptor co-monomers, but differing by the number of fluorine atoms on the acceptor unit, were synthesized. Although the HOMO levels of the mono-(P1F) and di-fluorinated (P2F) polymers are essentially the same, an increase in VOC was still observed in the OPV device incorporating P2F. This implies that correlating the VOC to the donor polymer HOMO level is inadequate to fully explain the improvement in VOC. By calculating the charge transfer exciton binding energy from the measured film dielectric constant, it was found that the increase in VOC in going from P1F to P2F matches the decrease in charge transfer exciton binding energy.

The efficient synthesis of regioregular poly(3-hexylthiophene-2,5-diyl)s (rr-P3HTs) capped with chalcogens using a simple quenching method is reported. Thiol (SH) end groups are selectively installed at the terminating end (ω-end) or at both the initiating (α-) and ω-ends using sulphur powder or triisopropylsilanethiol (TIPS–SH), respectively.

A one pot method for organic/colloidal CdSe nanoparticle hybrid material synthesis is presented. Relative to traditional ligand exchange processes, these materials require smaller amounts of the desired capping ligand, shorter syntheses and fewer processing steps, while maintaining nanoparticle morphology.

Charge transport is one of the five main steps in the operation of organic photovoltaics, but achieving balanced hole and electron transport with high mobility has been challenging in devices. Here, we report improved charge transport in organic photovoltaics via incorporation of nanostructured inorganic electron transport materials into the active layers of devices. Co-depositing TiO2 nanowires with the organic active layer solution embeds the nanowires directly within active layers of the solar cell. The ability of these nanowires to transport electrons is compared with neat P3HT:PCBM active layers and also devices containing TiO2 nanotube aggregates. Incorporation of TiO2 nanowires yields a six-fold increase in the electron mobility relative to unmodified devices, leading to a 19% improvement in the power conversion efficiency. Lower energetic disorder of the film and more balanced charge transport are also observed upon incorporating TiO2 nanowires. These advantageous effects correlate with the TiO2 nanowire length.

The nanoscale structure and macroscopic morphology of π-conjugated polymers are very important for their electronic application. While ordered single crystals of small molecules have been obtained via solution deposition, macroscopically aligned films of π-conjugated polymers deposited directly from solution have always required surface modification or complex pre-deposition processing of the solution. Here, ordered nanowires were obtained via shear-enhanced crystallization of π-conjugated polymers at the air–liquid–solid interface using simple deposition of the polymer solution onto an inclined substrate. The formation of macroscopically aligned nanowire arrays was found to be due to the synergy between intrinsic (π-conjugated backbone) and external (crystallization conditions) effects. The oriented nanowires showed remarkable improvement in the charge carrier mobility compared to spin-coated films as characterized in organic field-effect transistors (OFETs). Considering the simplicity and large-scale applicability, shear-enhanced crystallization of π-conjugated polymers provides a promising strategy to achieve high-performance polymer semiconductor films for electronics applications.

Organic coordinating ligands are ubiquitously used to solubilize, stabilize and functionalize colloidal nanoparticles. Aliphatic organic ligands are typically used to control size during the nanoparticle growth period and are used as a high boiling point solvent for solution-based synthesis procedures. However, these aliphatic ligands are typically not well suited for the end use of the nanoparticles, so additional ligand exchange or ligand stripping procedures must be implemented after the nanoparticle synthesis. Herein we present a ligand-free CdS nanoparticle synthesis procedure using a unique sulfur copolymer. The sulfur copolymer is derived from elemental sulfur, which is a cheap and abundant material. This copolymer is used as a sulfur source and high boiling point solvent, which produces stabilized metal-sulfide nanoparticles that are suspended within a sulfur copolymer matrix. The copolymer can then be removed, thereby yielding ligand-free metal-sulfide nanoparticles.

The efficient synthesis of regioregular poly(3-hexylthiophene-2,5-diyl)s (rr-P3HTs) capped with chalcogens using a simple quenching method is reported. Thiol (SH) end groups are selectively installed at the terminating end (ω-end) or at both the initiating (α-) and ω-ends using sulphur powder or triisopropylsilanethiol (TIPS–SH), respectively.

A one pot method for organic/colloidal CdSe nanoparticle hybrid material synthesis is presented. Relative to traditional ligand exchange processes, these materials require smaller amounts of the desired capping ligand, shorter syntheses and fewer processing steps, while maintaining nanoparticle morphology.

Correction for ‘The future of organic photovoltaics’ by Katherine A. Mazzio et al., Chem. Soc. Rev., 2015, 44, 78–90.

Conjugated polymers with fluorine substituents on their backbone have exhibited improved performance over their un-fluorinated analogues by lowering the polymer HOMO level, thereby increasing the open-circuit voltage (VOC). To further investigate how fluorine substituents improve device performance, three polymers with the same donor and acceptor co-monomers, but differing by the number of fluorine atoms on the acceptor unit, were synthesized. Although the HOMO levels of the mono-(P1F) and di-fluorinated (P2F) polymers are essentially the same, an increase in VOC was still observed in the OPV device incorporating P2F. This implies that correlating the VOC to the donor polymer HOMO level is inadequate to fully explain the improvement in VOC. By calculating the charge transfer exciton binding energy from the measured film dielectric constant, it was found that the increase in VOC in going from P1F to P2F matches the decrease in charge transfer exciton binding energy.

Charge transport is one of the five main steps in the operation of organic photovoltaics, but achieving balanced hole and electron transport with high mobility has been challenging in devices. Here, we report improved charge transport in organic photovoltaics via incorporation of nanostructured inorganic electron transport materials into the active layers of devices. Co-depositing TiO2 nanowires with the organic active layer solution embeds the nanowires directly within active layers of the solar cell. The ability of these nanowires to transport electrons is compared with neat P3HT:PCBM active layers and also devices containing TiO2 nanotube aggregates. Incorporation of TiO2 nanowires yields a six-fold increase in the electron mobility relative to unmodified devices, leading to a 19% improvement in the power conversion efficiency. Lower energetic disorder of the film and more balanced charge transport are also observed upon incorporating TiO2 nanowires. These advantageous effects correlate with the TiO2 nanowire length.

The charge carrier dynamics of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) organic bulk-heterojunction photovoltaics with and without TiO2 nanowires are studied. Using inorganic nanowires as electron transport pathways improves the charge transit time and electron diffusion coefficient, which is the origin of fill factor and power conversion efficiency improvement observed in these devices. Through further comparison of devices with surface-modified nanowires (PCB–TiO2-NW), it is found that under AM 1.5 light illumination, charge recombination is dominant in the organic layer rather than at the TiO2 nanowire surface.

Increasing global demand for energy, along with dwindling fossil fuel resources and a better understanding of the hidden costs associated with these energy sources, have spurred substantial political, academic, and industrial interest in alternative energy resources. Photovoltaics based on organic semiconductors have emerged as promising low-cost alternatives for electricity generation that relies on sunlight. In this tutorial review we discuss the relevance of these organic photovoltaics beginning with some of the economic drivers for these technologies. We then examine the basic properties of these devices, including operation and materials requirements, in addition to presenting the development of the field from a historical perspective. Potential future directions are also briefly discussed. This tutorial review is intended to be an essential overview of the progress of the field, in addition to aiding in the discussion of the future of OPV technologies.