Low bandgap semiconducting polymers for polymeric photovoltaics.

In order to develop high performance polymer solar cells (PSCs), full exploitation of the sun-irradiation from ultraviolet (UV) to near infrared (NIR) is one of the key factors to ensure high photocurrents and thus high efficiency. In this review, five of the effective design rules for approaching LBG semiconducting polymers with high molar absorptivity, suitable energy levels, high charge carrier mobility and high solubility in organic solvents are overviewed. These design stratagems include fused heterocycles for facilitating π-electron flowing along the polymer backbone, groups/atoms bridging adjacent rings for maintaining a high planarity, introduction of electron-withdrawing units for lowering the bandgap (Eg), donor-acceptor (D-A) copolymerization for narrowing Eg and 2-dimensional conjugation for broadened absorption and enhanced hole mobility. It has been demonstrated that LBG semiconducting polymers based on electron-donor units combined with strong electron-withdrawing units possess excellent electronic and optic properties, emerging as excellent candidates for efficient PSCs. While for ultrasensitive photodetectors (PDs), which have intensive applications in both scientific and industrial sectors, sensing from the UV to the NIR region is of critical importance. For polymer PDs, Eg as low as 0.8 eV has been obtained through a rational design stratagem, covering a broad wavelength range from the UV to the NIR region (1450 nm). However, the response time of the polymer PDs are severely limited by the hole mobility of LBG semiconducting polymers, which is significantly lower than those of the inorganic materials. Thus, further advancing the hole mobility of LBG semiconducting polymers is of equal importance as broadening the spectral response for approaching uncooled ultrasensitive broadband polymer PDs in the future study.

Correction: Low bandgap semiconducting polymers for polymeric photovoltaics.

Correction for 'Low bandgap semiconducting polymers for polymeric photovoltaics' by Chang Liu et al., Chem. Soc. Rev., 2016, DOI: .

Further correction: Low bandgap semiconducting polymers for polymeric photovoltaics.

Further correction for 'Low bandgap semiconducting polymers for polymeric photovoltaics' by Chang Liu et al., Chem. Soc. Rev., 2016, DOI: 10.1039/c5cs00650c.

Flexible Organic Transistors with Controlled Nanomorphology.

We report the controlled nanomorphology of semiconducting polymers on chemically and mechanically stable nanogrooved polymer substrates. By employing silicon dioxide thin films with finely adjusted thicknesses on nanogrooved polymer substrates, semiconducting polymer thin films oriented and aligned along the nanogrooves were obtained. Organic field-effect transistors (OFETs) fabricated from the oriented semiconducting polymer, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT), yielded saturation hole mobilities as high as 19.3 cm(2) V(-1 )s(-1), and the flexible "plastic" transistors demonstrated excellent mechanical stability under various bending conditions. These results represent important progress for solution-processed flexible OFETs and demonstrate that directed self-assembly of semiconducting polymers can be achieved by soft nanostructures.

Molecularly Smooth Self-Assembled Monolayer for High-Mobility Organic Field-Effect Transistors.

Despite the need for molecularly smooth self-assembled monolayers (SAMs) on silicon dioxide surfaces (the most common dielectric surface), current techniques are limited to nonideal silane grafting. Here, we show unique bioinspired zwitterionic molecules forming a molecularly smooth and uniformly thin SAM in "water" in <1 min on various dielectric surfaces, which enables a dip-coating process that is essential for organic electronics to become reality. This monomolecular layer leads to high mobility of organic field-effect transistors (OFETs) based on various organic semiconductors and source/drain electrodes. A combination of experimental and computational techniques confirms strong adsorption (Wad > 20 mJ m-2), uniform thickness (∼0.5 or ∼1 nm) and orientation (all catechol head groups facing the oxide surface) of the "monomolecular" layers. This robust (strong adsorption), rapid, and green SAM represents a promising advancement toward the next generation of nanofabrication compared to the current nonuniform and inconsistent polysiloxane-based SAM involving toxic chemicals, long processing time (>10 h), or heat (>80 °C).

High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics.

We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = -5.66 eV, LUMO = -3.93 eV; ITIC: HOMO = -5.48 eV, LUMO = -3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-band-gap polymer donors and wide-band-gap polymer donors. ITIC-Th has higher electron mobility (6.1 × 10(-4) cm(2) V(-1) s(-1)) than ITIC (2.6 × 10(-4) cm(2) V(-1) s(-1)) due to enhanced intermolecular interaction induced by sulfur-sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-band-gap and wide-band-gap polymer donors. In one case, a power conversion efficiency of 9.6% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.

High-Performance Solution-Processed Non-Fullerene Organic Solar Cells Based on Selenophene-Containing Perylene Bisimide Acceptor.

Non-fullerene acceptors have recently attracted tremendous interest because of their potential as alternatives to fullerene derivatives in bulk heterojunction organic solar cells. However, the power conversion efficiencies (PCEs) have lagged far behind those of the polymer/fullerene system, mainly because of the low fill factor (FF) and photocurrent. Here we report a novel perylene bisimide (PBI) acceptor, SdiPBI-Se, in which selenium atoms were introduced into the perylene core. With a well-established wide-band-gap polymer (PDBT-T1) as the donor, a high efficiency of 8.4% with an unprecedented high FF of 70.2% is achieved for solution-processed non-fullerene organic solar cells. Efficient photon absorption, high and balanced charge carrier mobility, and ultrafast charge generation processes in PDBT-T1:SdiPBI-Se films account for the high photovoltaic performance. Our results suggest that non-fullerene acceptors have enormous potential to rival or even surpass the performance of their fullerene counterparts.

General strategy for self-assembly of highly oriented nanocrystalline semiconducting polymers with high mobility.

Solution processable semiconducting polymers with excellent film forming capacity and mechanical flexibility are considered among the most progressive alternatives to conventional inorganic semiconductors. However, the random packing of polymer chains and the disorder of the polymer matrix typically result in low charge transport mobilities (10(-5)-10(-2) cm(2) V(-1) s(-1)). These low mobilities compromise their performance and development. Here, we present a strategy, by utilizing capillary action, to mediate polymer chain self-assembly and unidirectional alignment on nanogrooved substrates. We designed a sandwich tunnel system separated by functionalized glass spacers to induce capillary action for controlling the polymer nanostructure, crystallinity, and charge transport. Using capillary action, we demonstrate saturation mobilities with average values of 21.3 and 18.5 cm(2) V(-1 )s(-1) on two different semiconducting polymers at a transistor channel length of 80 µm. These values are limited by the source-drain contact resistance, Rc. Using a longer channel length of 140 µm where the contact resistance is less important, we measured µh = 36.3 cm(2) v(-1) s(-1). Extrapolating to infinite channel length where Rc is unimportant, the intrinsic mobility for poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (Mn = 140 kDa) at this degree of chain alignment and structural order is µh ≈ 47 cm(2 )v(-1) s(-1). Our results create a promising pathway toward high performance, solution processable, and low-cost organic electronics.

High thermal stability solution-processable narrow-band gap molecular semiconductors.

A series of narrow-band gap conjugated molecules with specific fluorine substitution patterns has been synthesized in order to study the effect of fluorination on bulk thermal stability. As the number of fluorine substituents on the backbone increase, one finds more thermally robust bulk structures both under inert and ambient conditions as well as an increase in phase transition temperatures in the solid state. When integrated into field-effect transistor devices, the molecule with the highest degree of fluorination shows a hole mobility of 0.15 cm(2)/V·s and a device thermal stability of >300 °C. Generally, the enhancement in thermal robustness of bulk organization and device performance correlates with the level of C-H for C-F substitution. These findings are relevant for the design of molecular semiconductors that can be introduced into optoelectronic devices to be operated under a wide range of conditions.

Design and properties of intermediate-sized narrow band-gap conjugated molecules relevant to solution-processed organic solar cells.

Increases in the molecular length of narrow band gap conjugated chromophores reveal potentially beneficial optical and electronic properties, thermal stabilities, and high power conversion efficiencies when integrated into optoelectronic devices, such as bulk heterojunction organic solar cells. With the objective of providing useful information for understanding the transition from small-sized molecules to polymers, as well as providing a general chemical design platform for extracting relationships between molecular structure and bulk properties, we set out to vary the electron affinity of the molecular backbone. Therefore, a series of donor (D)-acceptor (A) alternating narrow band gap conjugated chromophores were synthesized based on the general molecular frameworks: D(1)-A(1)-D(2)-A(2)-D(2)-A(1)-D(1) and D(1)-A(1)-D(2)-A(2)-D(2)-A(2)-D(2)-A(1)-D(1). When the central electron-accepting moiety (A(2)) was varied or modified, two classes of molecules could be compared. First, we showed that the alteration of one single electron-accepting group, while maintaining the shape of the molecular framework, can effectively impact the optical properties and energy levels of the molecules. DFT ground state structure optimizations show similar "U" shape conformations among these molecules. Second, we examined how the site-specific introduction of fluorine atom(s) modifies the thermal properties in the solid state, while maintaining relatively similar optical and electrochemical features of interest. Structure-property relationship of such molecular systems could be rationally evaluated in the aspects of thermal-responsive molecular organizations in the solid state and dipole moments both in the ground and excited states. The impact of molecular structure on charge carrier mobilities in field effect transistors and the performance of photovoltaic devices were also studied.

Silaindacenodithiophene-based molecular donor: morphological features and use in the fabrication of compositionally tolerant, high-efficiency bulk heterojunction solar cells.

A novel solution-processable small molecule, namely, benzo[1,2-b:4,5-b]bis(4,4'-dihexyl-4H-silolo[3,2-b]thiophene-2,2'-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole (p-SIDT(FBTTh2)2), was designed and synthesized by utilizing the silaindacenodithiophene (SIDT) framework as the central D(2) donor unit within the D(1)AD(2)AD(1) chromophore configuration. Relative to the widely studied 7,7'-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl]bis[6-fluoro-4-(5'-hexyl-[2,2'-bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole] (p-DTS(FBTTh2)2), which contains the stronger donor fragment dithienosilole (DTS) as D(2), one finds that p-SIDT(FBTTh2)2 exhibits a wider band gap and can be used to fabricate bulk heterojunction solar cells with higher open circuit voltage (0.91 V). Most remarkably, thin films comprising p-SIDT(FBTTh2)2 can achieve exceptional levels of self-organization directly via solution deposition. For example, high-resolution transmission electron microscopy analysis shows that p-SIDT(FBTTh2)2 spin-cast from chlorobenzene organizes into crystalline domains with lattice planes that extend over length scales on the order of hundreds of nanometers. Such features suggest liquid crystalline properties during the evolution of the film. Moreover, grazing incidence wide-angle X-ray scattering analysis shows a strong tendency for the molecules to exist with a strong "face-on" orientation relative to the substrate plane. Similar structural features, albeit of more restricted dimensions, can be observed within p-SIDT(FBTTh2)2:PC71BM bulk heterojunction thin films when the films are processed with 0.4% diiodooctane (DIO) solvent additive. DIO use also increases the solar cell power conversion efficiencies (PCEs) from 1.7% to 6.4%. Of significance from a practical device fabrication perspective is that, for p-SIDT(FBTTh2)2:PC71BM blends, there is a wide range of compositions (from 20:80 to 70:30 p-SIDT(FBTTh2)2:PC71BM) that provide good photovoltaic response, i.e., PCE = 4-6%, indicating a robust tendency to form the necessary continuous phases for charge carrier collection. Light intensity photocurrent measurements, charge selective diode fabrication, and internal quantum efficiency determinations were carried out to obtain insight into the mechanism of device operation. Inclusion of DIO in the casting solution results in films that exhibit much lower photocurrent dependence on voltage and a concomitant increase in fill factor. At the optimum blend ratio, devices show high charge carrier mobilities, while mismatched hole and electron mobilities in blends with high or low donor content result in reduced fill factors and device performance.

Improved light harvesting and improved efficiency by insertion of an optical spacer (ZnO) in solution-processed small-molecule solar cells.

We demonstrate that the power conversion efficiency can be significantly improved in solution-processed small-molecule solar cells by tuning the thickness of the active layer and inserting an optical spacer (ZnO) between the active layer and the Al electrode. The enhancement in light absorption in the cell was measured with UV-vis absorption spectroscopy and by measurements of the photoinduced carriers generation rate. The ZnO layer used to improve the light-harvesting increases the charge collection efficiency, serves as a blocking layer for holes, and reduces the recombination rate. The combined optical and electrical improvements raise the power conversion efficiency of solution-processed small-molecule solar cells to 8.9%, that is, comparable to that of polymer counterparts.

Remarkable order of a high-performance polymer.

We directly image the rich nanoscale organization of the high performance, n-type polymer poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) using a combination of high-resolution transmission electron microscopy and scanning transmission electron microscopy. We demonstrate that it is possible to spatially resolve "face-on" lamella through the 2.4 nm alkyl stacking distance corresponding to the (100) reflection. The lamella locally transition between ordered and disordered states over a length scale on the order of 10 nm; however, the polymer backbones retain long-range correlations over length-scales approaching a micrometer. Moreover, we frequently observe overlapping structure implying a number of layers may exist throughout the thickness of the film (~20 nm). The results provide a simple picture, a highly ordered lamella nanostructure over nearly the entire film and ordered domains with overlapping layers providing additional interconnectivity, which unifies prior seemingly contradictory conclusions surrounding this remarkable, high-mobility material.

Colorimetric detection of DNA, small molecules, proteins, and ions using unmodified gold nanoparticles and conjugated polyelectrolytes.

We have demonstrated a novel sensing strategy employing single-stranded probe DNA, unmodified gold nanoparticles, and a positively charged, water-soluble conjugated polyelectrolyte to detect a broad range of targets including nucleic acid (DNA) sequences, proteins, small molecules, and inorganic ions. This nearly "universal" biosensor approach is based on the observation that, while the conjugated polyelectrolyte specifically inhibits the ability of single-stranded DNA to prevent the aggregation of gold-nanoparticles, no such inhibition is observed with double-stranded or otherwise "folded" DNA structures. Colorimetric assays employing this mechanism for the detection of hybridization are sensitive and convenient--picomolar concentrations of target DNA are readily detected with the naked eye, and the sensor works even when challenged with complex sample matrices such as blood serum. Likewise, by employing the binding-induced folding or association of aptamers we have generalized the approach to the specific and convenient detection of proteins, small molecules, and inorganic ions. Finally, this new biosensor approach is quite straightforward and can be completed in minutes without significant equipment or training overhead.

High mobility field effect transistors based on macroscopically oriented regioregular copolymers.

Field-effect transistors fabricated from semiconducting conjugated polymers are candidates for flexible and low-cost electronic applications. Here, we demonstrate that the mobility of high molecular weight (300 kDa) regioregular, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] can be significantly improved by introducing long-range orientation of the polymer chains. By annealing for short periods, hole mobilities of 6.7 cm(2)/(V s) have been demonstrated. The transport is anisotropic, with a higher mobility (approximately 6:1) parallel to the polymer backbone than that perpendicular to the polymer backbone.

Facile doping of anionic narrow-band-gap conjugated polyelectrolytes during dialysis.

PCPDTBTSO3 K, an anionic, narrow-band-gap conjugated polyelectrolyte, was found to be doped after dialysis. The proposed doping mechanism involves protonation of the polymer backbone, followed by electron transfer from a neutral chain, to generate radical cations, which are stabilized by the pendant sulfonate anions. Formation of polarons is supported by spectroscopy and electrical-conductivity measurements.

Photoinduced charge generation in a molecular bulk heterojunction material.

Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providing the necessary guidelines to improve overall device efficiency. Despite more than 15 years of experimental and theoretical studies, a universal picture describing the generation and recombination processes operating in organic photovoltaic devices is still being forged. We report here the results of ultrafast transient absorption spectroscopy measurements of charge photogeneration and recombination processes in a high-performing solution-processed molecular BHJ. For comparison, we also studied a high-performing polymer-based BHJ material. We find that the majority of charge carriers in both systems are generated on <100 fs time scales and posit that excited state delocalization is responsible for the ultrafast charge transfer. This initial delocalization is consistent with the fundamental uncertainty associated with the photon absorption process (in the visible, λ/4π > 30 nm) and is comparable with the phase-separated domain size. In addition, exciton diffusion to charge-separating heterojunctions is observed at longer times (1-500 ps). Finally, charge generation in pure films of the solution processed molecule was studied. Polarization anisotropy measurements clearly demonstrate that the optical properties are dominated by molecular (Frenkel) exictons and delocalized charges are promptly produced (t < 100 fs).

Narrow-band-gap conjugated chromophores with extended molecular lengths.

The influence of extending the molecular length of donor-acceptor chromophores on properties relevant to organic optoelectronic devices has been studied by using two new narrow-band-gap systems. Most significantly, we find that the higher molecular weight systems exhibit higher thermal stabilities (beyond 200 °C) when introduced into field effect transistor devices. It is also possible to fabricate bulk heterojunction solar cells using PC(61)BM with power conversion efficiencies >6%. These high values are not heavily influenced by the blend composition and are achieved without the influence of solvent additives or postdeposition thermal annealing.

Solar cell efficiency, self-assembly, and dipole-dipole interactions of isomorphic narrow-band-gap molecules.

We examine the correlations of the dipole moment and conformational stability to the self-assembly and solar cell performance within a series of isomorphic, solution-processable molecules. These charge-transfer chromophores are described by a D(1)-A-D-A-D(1) structure comprising electron-rich 2-hexylbithiophene and 3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene moieties as the donor units D(1) and D, respectively. The building blocks 2,1,3-benzothiadiazole (BT) and [1,2,5]thiadiazolo[3,4-c]pyridine (PT) were used as the electron-deficient acceptor units A. Using a combination of UV-visible spectroscopy, field-effect transistors, solar cell devices, grazing incident wide-angle X-ray scattering, and transmission electron microscopy, three PT-containing compounds (1-3) with varying regiochemistry and symmetry, together with the BT-based compound 5,5'-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolobenzene}-3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene (4), are compared and contrasted in solution, in thin films, and as blends with the electron acceptor [6,6]-phenyl-C(70)-butyric acid methyl ester. The molecules with symmetric orientations of the PT acceptor, 1 and 2, yield highly ordered blended thin films. The best films, processed with the solvent additive 1,8-diiodooctane, show donor "crystallite" length scales on the order of 15-35 nm and photovoltaic power conversion efficiencies (PCEs) of 7.0 and 5.6%, respectively. Compound 3, with an unsymmetrical orientation of PT heterocycles, shows subtle differences in the crystallization behavior and a best PCE of 3.2%. In contrast, blends of the BT-containing donor 4 are highly disordered and give PCEs below 0.2%. We speculate that the differences in self-assembly arise from the strong influence of the BT acceptor and its orientation on the net dipole moment and geometric description of the chromophore.

Polymer-fullerene miscibility: a metric for screening new materials for high-performance organic solar cells.

The improvement of the power conversion efficiency (PCE) of polymer bulk heterojunction (BHJ) solar cells has generally been achieved through synthetic design to control frontier molecular orbital energies and molecular ordering of the electron-donating polymer. An alternate approach to control the PCE of a BHJ is to tune the miscibility of the fullerene and a semiconducting polymer by varying the structure of the fullerene. The miscibility of a series of 1,4-fullerene adducts in the semiconducting polymer, poly(3-hexylselenophene), P3HS, was measured by dynamic secondary ion mass spectrometry using a model bilayer structure. The microstructure of the bilayer was investigated using high-angle annular dark-field scanning transmission microscopy and linked to the polymer-fullerene miscibility. Finally, P3HS:fullerene BHJ solar cells were fabricated from each fullerene derivative, enabling the correlation of the active layer microstructure to the charge collection efficiency and resulting PCE of each system. The volume fraction of polymer-rich, fullerene-rich, and polymer-fullerene mixed domains can be tuned using the miscibility leading to improvement in the charge collection efficiency and PCE in P3HS:fullerene BHJ solar cells. These results suggest a rational approach to the design of fullerenes for improved BHJ solar cells.