Efficient Non‐Fullerene Organic Photovoltaic Modules Incorporating As‐Cast and Thickness‐Insensitive Photoactive Layers

In this work, a new combination of a wide bandgap polymer poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene‐alt‐N‐(2‐hexyldecyl)‐5′5‐bis[3‐(decylthio)thiophene‐2‐yl]‐2′2‐bithiophene‐3′3‐dicarboximide] (PBTIBDTT) and a non‐fullerene small molecule acceptor based on a bulky seven‐ring fused core (indacenodithieno[3,2‐b]thiophene) end‐capped with 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile groups with one fluorine substituent (ITIC‐F) is proposed, and as‐cast non‐fullerene organic solar cells (NFOSCs) with 11.2% efficiency are achieved. The device efficiencies are also insensitive to the variation of photoactive layer (PAL) thickness and can maintain over 9% efficiency as PAL thickness increases to 350 nm, which is one of the best results for as‐cast organic solar cells. More importantly, non‐fullerene organic photovoltaic (OPV) modules are demonstrated via laser ablation technique for the first time, which delivers a record efficiency of 8.6% (with active area of 3.48 cm²) among large‐area OPV modules. Furthermore, the morphology and performance evolutions of the as‐cast NFOSCs and the ones processed with solvent additive are systematically investigated. The results demonstrate the great advantage of as‐cast solar cells in achieving constant morphology and high performance with thick PALs. The NFOSCs fabricated with simple procedure, insensitive to film thickness and compatible with large‐area OPV modules, show significant potential for application the future.     

Conductive,Solution‐Processed Dioxythiophene Copolymers for Thermoelectric and Transparent Electrode Applications

Conjugated polymers with high electrical conductivities are attractive for applications in capacitors, biosensors, organic thermoelectrics, and transparent electrodes. Here, a series of solution processable dioxythiophene copolymers based on 3,4‐propylenedioxythiophene (ProDOT) and 3,4‐ethylenedioxythiophene (EDOT) is investigated as thermoelectric and transparent electrode materials. Through structural manipulation of the polymer repeat unit, the conductivity of the polymers upon oxidative solution doping is tuned from 1 × 10^?3 to 3 S cm^?1, with a polymer consisting of a solubilizing alkylated ProDOT unit and an electron‐rich biEDOT unit (referred to as PE₂) showing the highest electrical conductivity. Optimization of the film casting method and screening of dopants result in AgPF₆‐doped PE₂ achieving a high electrical conductivity of over 250 S cm^?1 and a thermoelectric power factor of 7 μW m^?1 K^?2. Oxidized spray cast films of PE₂ are also assessed as a transparent electrode material for use with another electrochromic polymer. This bilayer shows reversible electrochemical switching from a colored charge‐neutral state to a highly transmissive color‐neutral, oxidized state. These results demonstrate that dioxythiophene‐based copolymers are a promising class of materials, with ProDOT–biEDOT serving as a soluble analog to the well‐studied PEDOT as a p‐type thermoelectric and electrode material.     

Carrier‐Selectivity‐Dependent Charge Recombination Dynamics in Organic Photovoltaic Cells with a Ferroelectric Blend Interlayer

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the “carrier selectivity” at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution‐processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non‐Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier‐selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier‐selectivity‐controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions.     

Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells

Charge transport in organic photovoltaic (OPV) devices is often characterized by space‐charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi‐equilibrium energies in the disorder‐broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time‐resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time‐resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum‐power point (short‐circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.     

Quantitative Morphology–Performance Correlations in Organic Solar Cells: Insights from Soft X‐Ray Scattering

Organic/polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. One of the key factors contributing to the rapid and continuous progress of organic photovoltaics (OPVs) is the control and optimization of photoactive‐layer morphology. The impact of morphology on photovoltaic parameters has been widely observed. However, the highly complex and multilength‐scale morphology often formed in efficient OPV devices consisting of compositionally similar components impose obstacles to conventional morphological characterizations. In contrast, due to the high compositional and orientational sensitivity, resonant soft X‐ray scattering (R‐SoXS), and related techniques lead to tremendous progress of characterization and comprehension regarding the complex mesoscale morphology in OPVs. R‐SoXS is capable of quantifying the domain characteristics, and polarized soft X‐ray scattering (P‐SoXS) provides quantitative information on orientational ordering. These morphological parameters strongly correlate the fill factor (FF), open‐circuit voltage (V_oc), as well as short‐circuit current (J_sc) in a wider range of OPV devices, including recent record‐efficiency polymer:fullerene solar cells and 12%‐efficiency fullerene‐free OPVs. This progress report will delineate the soft X‐ray scattering methodology and its future challenges to characterize and understand functional organic materials and provide a non‐exhaustive overview of R‐SoXS characterization and its implication to date.     

Nanofiber‐Based Bulk‐Heterojunction Organic Solar Cells Using Coaxial Electrospinning

Nanofibers consisting of the bulk heterojunction organic photovoltaic (BHJ–OPV) electron donor–electron acceptor pair poly(3‐hexylthiophene):phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) are produced through a coaxial electrospinning process. While P3HT:PCBM blends are not directly electrospinnable, P3HT:PCBM‐containing fibers are produced in a coaxial fashion by utilizing polycaprolactone (PCL) as an electrospinnable sheath material. Pure P3HT:PCBM fibers are easily obtained after electrospinning by selectively removing the PCL sheath with cyclopentanone (average diameter 120 ± 30 nm). These fibers are then incorporated into the active layer of a BHJ–OPV device, which results in improved short‐circuit current densities, fill factors, and power‐conversion efficiencies (PCE) as compared to thin‐film devices of identical chemical composition. The best‐performing fiber‐based devices exhibit a PCE of 4.0%, while the best thin‐film devices have a PCE of 3.2%. This increase in device performance is attributed to the increased in‐plane alignment of P3HT polymer chains on the nanoscale, caused by the electrospun fibers, which leads to increased optical absorption and subsequent exciton generation. This methodology for improving device performance of BHJ–OPVs could also be implemented for other electron donor–electron acceptor systems, as nanofiber formation is largely independent of the PV material.     

ITO‐Free and Fully Solution‐Processed Semitransparent Organic Solar Cells with High Fill Factors

Organic photovoltaic (OPV) solar cells that can be simply processed from solution are in the focus of the academic and industrial community because of their enormous potential to reduce cost. One big challenge in developing a fully solution‐processed OPV technology is the design of a well‐performing electrode system, allowing the replacement of ITO. Several solution‐processed electrode systems were already discussed, but none of them could match the performance of ITO. Here, we report efficient ITO‐free and fully solution‐processed semitransparent inverted organic solar cells based on silver nanowire (AgNW) electrodes. To demonstrate the potential of these AgNW electrodes, they were employed as both the bottom and top electrodes. Record devices achieved fill factors as high as 63.0%, which is comparable to ITO based reference devices. These results provide important progress for fully printed organic solar cells and indicate that ITO‐free, transparent as well as non‐transparent organic solar cells can indeed be fully solution‐processed without losses.     

Plasmon‐Active Nano‐Aperture Window Electrodes for Organic Photovoltaics

A lithography free approach to fabricating optically thin (～10 nm) noble metal electrodes with a dense array of sub‐wavelength apertures is reported. These nano‐structured electrodes support surface plasmon resonances which couple strongly with visible light concentrating it near to the electrode surface. They are also remarkably robust and can be fabricated on glass and plastic substrates with a sheet resistance of <15 Ω sq^?1. As the window electrode in solution processed and vacuum deposited organic photovoltaics (OPV) the photocurrent is increased by as much as 28% as compared to identical devices without apertures, demonstrating that the apertures do not need to have a tight size and/or shape distribution to be effective. As a drop‐in replacement for the indium‐tin oxide electrode in flexible OPV these plasmon‐active electrodes offer superior performance; 5.1% vs. 4.6%, demonstrating that this class of electrode is a truly viable alternative to conducting oxide window electrodes for OPV.     

Porphyrin‐Based Bulk Heterojunction Organic Photovoltaics: The Rise of the Colors of Life

Organic photovoltaics (OPV) represent a thin‐film PV technology that offers attractive prospects for low‐cost and aesthetically appealing (colored, flexible, uniform, semitransparent) solar cells that are printable on large surfaces. In bulk heterojunction (BHJ) OPV devices, organic electron donor and acceptor molecules are intimately mixed within the photoactive layer. Since 2005, the power conversion efficiency of said devices has increased substantially due to insights in the underlying physical processes, device optimization, and chemical engineering of a vast number of novel light‐harvesting organic materials, either small molecules or conjugated polymers. As Nature itself has developed porphyrin chromophores for solar light to energy conversion, it seems reasonable to pursue artificial systems based on the same types of molecules. Porphyrins and their analogues have already been successfully implemented in certain device types, notably in dye‐sensitized solar cells, but they have remained largely unexplored in BHJ organic solar cells. Very recent successes do show, however, the strong (latent) prospects of porphyrinoid semiconductors as light‐harvesting and charge transporting materials in such devices. Here, an overview on the state‐of‐the‐art of porphyrin‐based solution‐processed BHJ OPV is provided and insights are given into the pathways to follow and hurdles to overcome toward further improvements of porphyrinic materials and devices.     

Study of Burn‐In Loss in Green Solvent‐Processed Ternary Blended Organic Photovoltaics Derived from UV‐Crosslinkable Semiconducting Polymers and Nonfullerene Acceptors

This work deals with the investigation of burn‐in loss in ternary blended organic photovoltaics (OPVs) prepared from a UV‐crosslinkable semiconducting polymer (P2FBTT‐Br) and a nonfullerene acceptor (IEICO‐4F) via a green solvent process. The synthesized P2FBTT‐Br can be crosslinked by UV irradiation for 150 s and dissolved in 2‐methylanisole due to its asymmetric structure. In OPV performance and burn‐in loss tests performed at 75 °C or AM 1.5G Sun illumination for 90 h, UV‐crosslinked devices with PC₇₁BM show 9.2% power conversion efficiency (PCE) and better stability against burn‐in loss than pristine devices. The frozen morphology resulting from the crosslinking prevents lateral crystallization and aggregation related to morphological degradation. When IEICO‐4F is introduced in place of a fullerene‐based acceptor, the burn‐in loss due to thermal aging and light soaking is dramatically suppressed because of the frozen morphology and high miscibility of the nonfullerene acceptor (18.7% → 90.8% after 90 h at 75 °C and 37.9% → 77.5% after 90 h at AM 1.5G). The resulting crosslinked device shows 9.4% PCE (9.8% in chlorobenzene), which is the highest value reported to date for crosslinked active materials, in the first green processing approach.     

Influence of Aggregation on the Performance of All‐Polymer Solar Cells Containing Low‐Bandgap Naphthalenediimide Copolymers

The authors present efficient all‐polymer solar cells comprising two different low‐bandgap naphthalenediimide (NDI)‐based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near‐field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large‐scale phase‐separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donor–acceptor heterojunction.     

Allosteric HIV‐1 integrase inhibitors promote aberrant protein multimerization by directly mediating inter‐subunit interactions: Structural and thermodynamic modeling studies

Allosteric HIV‐1 integrase (IN) inhibitors (ALLINIs) bind at the dimer interface of the IN catalytic core domain (CCD), and potently inhibit HIV‐1 by promoting aberrant, higher‐order IN multimerization. Little is known about the structural organization of the inhibitor‐induced IN multimers and important questions regarding how ALLINIs promote aberrant IN multimerization remain to be answered. On the basis of physical chemistry principles and from our analysis of experimental information, we propose that inhibitor‐induced multimerization is mediated by ALLINIs directly promoting inter‐subunit interactions between the CCD dimer and a C‐terminal domain (CTD) of another IN dimer. Guided by this hypothesis, we have built atomic models of inter‐subunit interfaces in IN multimers by incorporating information from hydrogen‐deuterium exchange (HDX) measurements to drive protein‐protein docking. We have also developed a novel free energy simulation method to estimate the effects of ALLINI binding on the association of the CCD and CTD. Using this structural and thermodynamic modeling approach, we show that multimer inter‐subunit interface models can account for several experimental observations about ALLINI‐induced multimerization, including large differences in the potencies of various ALLINIs, the mechanisms of resistance mutations, and the crucial role of solvent exposed R‐groups in the high potency of certain ALLINIs. Our study predicts that CTD residues Tyr226, Trp235 and Lys266 are involved in the aberrant multimer interfaces. The key finding of the study is that it suggests the possibility of ALLINIs facilitating inter‐subunit interactions between an external CTD and the CCD‐CCD dimer interface.     

ITO‐Free,Small‐Molecule Organic Solar Cells on Spray‐Coated Copper‐Nanowire‐Based Transparent Electrodes

The great potential of solution‐processed metal nanowire networks utilized as a transparent electrode has attracted much attention in the last years. Typically, silver nanowires are applied, although their replacement by more abundant and cheaper materials is of interest. Here, a hydrazine‐free synthesis route for high aspect ratio copper nanowires is used to prepare conductive networks showing an enhanced electrode performance. The network deposition is done with a scalable spray‐coating process on glass and on polymer foils. By a pressing or an annealing step, highly conductive transparent electrodes are obtained, and they reveal transmittance‐resistance values similar to indium tin oxide (ITO) and networks made of silver nanowires. The application potential of the copper nanowire electrodes is demonstrated by integrating them into an evaporated small‐molecule organic solar cell with 3% efficiency.     

Multi‐armed poly(L‐glutamic acid)‐graft‐oligoethylenimine copolymers as efficient nonviral gene delivery vectors

Background

The application of polyethylenimine (PEI) in gene delivery has been severely limited by significant cytotoxicity that results from a nondegradable methylene backbone and high cationic charge density. It is therefore necessary to develop novel biodegradable PEI derivates for low‐toxic, highly efficient gene delivery.

Methods

A series of novel cationic copolymers with various charge density were designed and synthesized by grafting different kinds of oligoethylenimine (OEI) onto a determinate multi‐armed poly(L ‐glutamic acid) backbone. The molecular structures of multi‐armed poly(L ‐glutamic acid)‐graft‐OEI (MP‐g‐OEI) copolymers were characterized using nuclear magnetic resonance, viscosimetry and gel permeation chromatography. Moreover, the MP‐g‐OEI/DNA complexes were measured by a gel retardation assay, dynamic light scattering and atomic force microscopy to determine DNA binding ability, particle size, zeta potential, complex formation and shape, respectively. MP‐g‐OEI copolymers were also evaluated in Chinese hamster ovary and human embryonic kidney‐293 cells for their cytotoxicity and transfection efficiency.

Results

The particle sizes of MP‐g‐OEI/DNA complexes were in a range of 109.6–182.6 nm and the zeta potentials were in a range of 29.2–44.5 mV above the N/P ratio of 5. All the MP‐g‐OEI copolymers exhibited lower cytotoxicity and higher gene transfection efficiency than PEI25k in the absence and presence of serum with different cell lines. Importantly, the 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay revealed that the cytotoxicity of MP‐g‐OEI copolymers varied with their molecular weight and charge density, and two of MP‐g‐OEI copolymers (OEI600‐MP and OEI1800‐MP) could achieve optimal transfection efficiency at a similar low N/P ratio as that for PEI25k.

Conclusions

MP‐g‐OEI copolymers demonstrated considerable potential as nonviral vectors for gene therapy. Copyright © 2009 John Wiley & Sons, Ltd.     

Mole Fraction Control of Poly([R]‐3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHB/HV) Synthesized by Paracoccus denitrificans

Different substrate mixtures of acetic acid and valeric acid were used to synthesize copolymers of poly([R]‐3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHB/HV) with Paracoccus denitrificans under N‐limited conditions. A correlation between the substrate ratio and HV content was found in batch experiments, which seemed to be suitable to produce a number of defined copolymers. In fed‐batch fermentation, such correlation could only be found with carbon substrate mixtures of very restricted composition. Due to the individual substrate consumption rates with this technique, a polymer with 16.5 mol.‐% HV content [w/w] could be reproducibly synthesized. However, under N‐limited chemostatic cultivation conditions it was possible to produce a spectrum of definitely composed copolymers (3.0 %–46.3 mol.‐% HV) from different mixtures of acetic acid and valeric acid.     

Biomimetic potential of some methacrylate‐based copolymers: A comparative study

Preparation of new biocompatible materials for bone recovery has consistently gained interest in the last few decades. Special attention was given to polymers that contain negatively charged groups, such as phosphate, carboxyl, and sulfonic groups toward calcification. This present paper work demonstrates that other functional groups present also potential application in bone pathology. New copolymers of 2‐hydroxyethyl methacrylate with diallyldimethylammonium chloride (DADMAC), glycidyl methacrylate (GlyMA), methacrylic acid (MAA), 2‐methacryloyloxymethyl acetoacetate (MOEAA), 2‐methacryloyloxyethyltriethylammonium chloride (MOETAC), and tetrahydrofurfuryl methacrylate (THFMA) were obtained. The copolymers were characterized by FTIR, swelling potential, and they were submitted to in vitro tests for calcification and cytotoxicity evaluation. GlyMA and MOETAC‐containing copolymers show promising results for further in vivo mineralization tests, as a potential alternative to the classical bone grafts, in bone tissue engineering. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 966–973, 2009. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com     

Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well‐Known PBDTTT Polymers

With the advances in organic photovoltaics (OPVs), the invention of model polymers with superior properties and wide applicability is of vital importance to both the academic and industrial communities. The recent inspiring advances in OPV research have included the emergence of poly(benzodithiophene‐co‐thieno[3,4‐b]thiophene) (PBDTTT)‐based materials. Through the combined efforts on PBDTTT polymers, over 10% efficiencies have been realized recently in various types of OPV devices. This review attempts to critically summarize the recent advances with respect to five well‐known PBDTTT polymers and their design considerations, basic properties, photovoltaic performance, as well as device application in conventional, inverted, tandem solar cells. These PBDTTT polymers also make great contributions to the rapid advances in the field of emerging ternary blends and fullerene‐free OPVs with top performances. Addtionally, new challenges in developing novel photovoltaic polymers with more superior properties are prospected. More importantly, the research of highly efficient PBDTTT‐based polymers provides useful insights and builds fundamentals for new types of OPV applications with various architectures.     

A High‐Performance D–A Copolymer Based on Dithieno[3,2‐b:2′,3′‐d]Pyridin‐5(4H)‐One Unit Compatible with Fullerene and Nonfullerene Acceptors in Solar Cells

Development of high‐performance donor–acceptor (D–A) copolymers is vital in the research of polymer solar cells (PSCs). In this work, a low‐bandgap D–A copolymer based on dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one unit (DTP), PDTP4TFBT, is developed and used as the donor material for PSCs with PC₇₁BM or ITIC as the acceptor. PDTP4TFBT:PC₇₁BM and PDTP4TFBT:ITIC solar cells give power conversion efficiencies (PCEs) up to 8.75% and 7.58%, respectively. 1,8‐Diiodooctane affects film morphology and device performance for fullerene and nonfullerene solar cells. It inhibits the active materials from forming large domains and improves PCE for PDTP4TFBT:PC₇₁BM cells, while it promotes the aggregation and deteriorates performance for PDTP4TFBT:ITIC cells. The ternary‐blend cells based on PDTP4TFBT:PC₇₁BM:ITIC (1:1.2:0.3) give a decent PCE of 9.20%.