Olof Bäcke

A new tetracyclic lactam building block for thick, broad-bandgap photovoltaics.

A new tetracyclic lactam building block for polymer semiconductors is reported that was designed to combine the many favorable properties that larger fused and/or amide-containing building blocks can induce, including improved solid-state packing, high charge carrier mobility, and improved charge separation. Copolymerization with thiophene resulted in a semicrystalline conjugated polymer, PTNT, with a broad bandgap of 2.2 eV. Grazing incidence wide-angle X-ray scattering of PTNT thin films revealed a strong tendency for face-on π-stacking of the polymer backbone, which was retained in PTNT:fullerene blends. Corresponding solar cells featured a high open-circuit voltage of 0.9 V, a fill factor around 0.6, and a power conversion efficiency as high as 5% for >200 nm thick active layers, regardless of variations in blend stoichiometry and nanostructure. Moreover, efficiencies of >4% could be retained when thick active layers of ∼400 nm were employed. Overall, these values are the highest reported for a conjugated polymer with such a broad bandgap and are unprecedented in materials for tandem and particularly ternary blend photovoltaics. Hence, the newly developed tetracyclic lactam unit has significant potential as a conjugated building block in future organic electronic materials.

Fullerene mixtures enhance the thermal stability of a non-crystalline polymer solar cell blend

Printing of polymer:fullerene solar cells at high speed requires annealing at temperatures up to 140 C. However, bulk-heterojunction blends that comprise a non-crystalline donor polymer often suffer from insufficient thermal stability and hence rapidly coarsen upon annealing above the glass transition temperature of the blend. In addition, micrometer-sized fullerene crystals grow, which are detrimental for the solar cell performance. In this manuscript, we present a strategy to limit fullerene crystallization, which is based on the use of fullerene mixtures of the two most common derivatives, PC61BM and PC71BM, as the acceptor material. Blends of this fullerene mixture and a non-crystalline thiophene-quinoxaline copolymer display considerably enhanced thermal stability and largely retain their photovoltaic performance upon annealing at elevated temperatures as high as 170 C.

Sub-glass transition annealing enhances polymer solar cell performance

Thermal annealing of non-crystalline polymer:fullerene blends typically results in a drastic decrease in solar cell performance. In particular aggressive annealing above the glass transition temperature results in a detrimental coarsening of the blend nanostructure. We demonstrate that mild annealing below the glass transition temperature is a viable avenue to control the nanostructure of a non-crystalline thiophene–quinoxaline copolymer:fullerene blend. Direct imaging methods indicate that coarsening of the blend nanostructure can be avoided. However, a combination of absorption and luminescence spectroscopy reveals that local changes in the polymer conformation as well as limited fullerene aggregation are permitted to occur. As a result, we are able to optimise the solar cell performance evenly across different positions of the coated area, which is a necessary criterion for large-scale, high throughput production.

Effects of side chain isomerism on the physical and photovoltaic properties of indacenodithieno[3,2-b]thiophene–quinoxaline copolymers: toward a side chain design for enhanced photovoltaic performance

Four new D–A polymers PIDTT-Q-p, PIDTT-Q-m, PIDTT-QF-p and PIDTT-QF-m, using indacenodithieno[3,2-b]thiophene (IDTT) as an electron-rich unit and quinoxaline (Q) as an electron-deficient unit, were synthesized via a Pd-catalyzed Stille polymerization. The side chains on the pendant phenyl rings of IDTT were varied from the para- to the meta-position, and the effect of the inclusion of fluorine on the quinoxaline unit was simultaneously investigated. The influence on the optical and electrochemical properties, film topography and photovoltaic properties of the four copolymers were thoroughly examined via a range of techniques. The inductively electron-withdrawing properties of the fluorine atoms result in a decrease of the highest occupied molecular orbital (HOMO) energy levels. The effect of meta-substitution on the PIDTT-Q-m polymer leads to good solubility and in turn higher molecular weight. More importantly, it exhibits optimal morphological properties in the PIDTT-Q-m/PC71BM blends. As a result, the corresponding solar cells (ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al) attain the best power conversion efficiency (PCE) of 6.8%. The structure–property correlations demonstrate that the meta-alkyl-phenyl substituted IDTT unit is a promising building block for efficient organic photovoltaic materials. This result also extends our strategy with regards to side chain isomerism of IDTT-based copolymers for enhanced photovoltaic performance.

Direct Chemical Vapor Deposition of Large-Area Carbon Thin Films on Gallium Nitride for Transparent Electrodes: A First Attempt

Direct formation of large-area carbon thin films on gallium nitride by chemical vapor deposition without metallic catalysts is demonstrated. A high flow of ammonia is used to stabilize the surface of the GaN (0001)/sapphire substrate during the deposition at 950°C. Various characterization methods verify that the synthesized thin films are largely sp2 bonded, macroscopically uniform, and electrically conducting. The carbon thin films possess optical transparencies comparable to that of exfoliated graphene. This paper offers a viable route toward the use of carbon-based materials for future transparent electrodes in III-nitride optoelectronics, such as GaN-based light emitting diodes and laser diodes.

Mapping fullerene crystallization in a photovoltaic blend: an electron tomography study

The formation of fullerene crystals represents a major degradation pathway of polymer/fullerene bulk-heterojunction thin films that inexorably deteriorates their photovoltaic performance. Currently no tools exist that reveal the origin of fullerene crystal formation vertically through the film. Here, we show that electron tomography can be used to study nucleation and growth of fullerene crystals. A model bulk-heterojunction blend based on a thiophene-quinoxaline copolymer and a fullerene derivative is examined after controlled annealing above the glass transition temperature. We image a number of fullerene nanocrystals, ranging in size from 70 to 400 nanometers, and observe that their center is located close to the free-surface of spin-coated films. The results show that the nucleation of fullerene crystals predominately occurs in the upper part of the films. Moreover, electron tomography reveals that the nucleation is preceded by more pronounced phase separation of the blend components.

Correction to "a new tetracyclic lactam building block for thick, broad-bandgap photovoltaics".

Page 11579. Further analysis of the spectroscopic data of the NT monomer suggested that the majority product is the Oalkylated instead of N-alkylated product. The amide functionality displays ambident reactivity, and the ratio of Nor Oalkylation is governed by factors such as the employed halide on the alkyl reactant and the thermodynamic stability of the final product. For amide-containing structures employed in conjugated polymers, the N-alkylated product is usually the majority product. However, recently He et al. reported a new conjugated building block that favored O-alkylation over Nalkylation under reaction conditions similar to those we employed. To unambiguously determine which isomer was formed, we synthesized a crystalline C8-NT unit under conditions similar to those used for the initial 2-hexyldecyl-substituted NT unit. Analysis by single-crystal X-ray diffraction and comparison of the other spectroscopic data confirmed that the majority product after alkylation is O-alkylated. The correct structures of the NT unit and PTNT are depicted in Figure 1.

Plasmon-enhanced four-wave mixing by nanoholes in thin gold films

Nonlinear plasmonics opens up for wavelength conversion, reduced interaction/emission volumes, and nonlinear enhancement effects at the nanoscale with many compelling nanophotonic applications foreseen. We investigate nonlinear plasmonic responses of nanoholes in thin gold films by exciting the holes individually with tightly focused laser beams, employing a degenerated pump/probe and Stokes excitation scheme. Excitation of the holes results in efficient generation of both narrowband four-wave mixing (FWM) and broadband multiphoton excited luminescence, blueshifted relative to the excitation beams. Clear enhancements were observed when matching the pump/probe wavelength with the hole plasmon resonance. These observations show that the FWM generation is locally excited by nanoholes and has a resonant behavior primarily governed by the dimensions of the individual holes.

A Fullerene Alloy Based Photovoltaic Blend with a Glass Transition Temperature above 200 °C

Organic solar cells with a high degree of thermal stability require bulk-heterojunction blends that feature a high glass transition, which must occur considerably above the temperatures encountered during device fabrication and operation. Here, we demonstrate for the first time a polymer : fullerene blend with a glass transition temperature above 200 °C, which we determine by plasmonic nanospectroscopy. We achieve this strong tendency for glass formation through the use of an alloy of neat, unsubstituted C60 and C70, which we combine with the fluorothieno-benzodithiophene copolymer PTB7. A stable photovoltaic performance of PTB7 : C60 : C70 ternary blends is preserved despite annealing the active layer at up to 180 °C, which coincides with the onset of the glass transition. Rapid deterioration of the power conversion efficiency from initially above 5% only occurs upon exceeding the glass transition temperature of 224 °C of the ternary blend.

Enhanced thermal stability of a polymer solar cell blend induced by electron beam irradiation in the transmission electron microscope

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.