Renee Kroon

Interlayer for Modified Cathode in Highly Efficient Inverted ITO-Free Organic Solar Cells

Inverted polymer solar cells with a bottom metal cathode modified by a conjugated polymer interlayer show considerable improvement of photocurrent and fill factor, which is due to hole blocking at the interlayer, and a modified surface energy which affects the nanostructure in the TQ1/[70]PCBM blend.

Thermoelectric plastics: from design to synthesis, processing and structure–property relationships

Thermoelectric plastics are a class of polymer-based materials that combine the ability to directly convert heat to electricity, and vice versa, with ease of processing.

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.

Conjugated polymers based on benzodithiophene and fluorinated quinoxaline for bulk heterojunction solar cells: thiophene versus thieno[3,2-b]thiophene as π-conjugated spacers

Two conjugated donor–acceptor copolymers based on a benzodithiophene donor unit and a fluorinated quinoxaline acceptor unit, spaced with either thiophene or thieno[3,2-b]thiophene π-bridges, were designed and synthesized. The effect of different π-bridges and of the processing conditions on optical, electrical, morphological and photovoltaic properties of the polymer:fullerene blend films were investigated. The polymer containing the thieno[3,2-b]thiophene π-bridge (PBDTFQ-TT) showed a red-shifted absorption and an enhanced charge carrier mobility, as compared to its analogue with the thiophene π-bridge (PBDTFQ-T), due to its narrower optical gap (by ∼0.1 eV) and stronger inter-chain interactions, favored by the structural planarity and increased linearity of the polymer backbone, as also supported by DFT calculations. The blend of PBDTFQ-TT and PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), compared to the PBDTFQ-T:PC61BM one processed under the same conditions (by blade-coating technique), showed greatly enhanced photovoltaic performance, with more than doubled power conversion efficiency (PCE up to 5.60% for the best device) due to the increased short-circuit current density and fill factor. However, similar PCEs were also achieved for PBDTFQ-T:PC61BM-based devices by optimizing the processing conditions through the addition of 1,8-diiodooctane (DIO) as the solvent additive. Through morphological and electrical analysis of the films, produced with and without additive, it was observed that the addition of DIO greatly enhances the self-organization, and consequently the charge mobility, of the thiophene π-bridge-based polymer, while it was detrimental for the nanoscale morphology and photovoltaic performances of the thieno[3,2-b]thiophene π-bridge-based polymer in the corresponding blend.

Inverted all-polymer solar cells based on a quinoxaline–thiophene/naphthalene-diimide polymer blend improved by annealing

We have investigated the effect of thermal annealing on the photovoltaic parameters of all-polymer solar cells based on a quinoxaline-thiophene donor polymer (TQ1) and a naphthalene diimide acceptor polymer (N2200). The annealed devices show a doubled power conversion efficiency compared to nonannealed devices, due to the higher short-circuit current (J(sc)) and fill factor (FF), but with a lower open circuit voltage (V-oc). On the basis of the morphology-mobility examination by several scanning force microscopy techniques, and by grazing-incidence wide-angle X-ray scattering, we conclude that better charge transport is achieved by higher order and better interconnected networks of the bulk heterojunction in the annealed active layers. The annealing improves charge transport and extends the conjugation length of the polymers, which do help in charge generation and meanwhile reduce recombination. Photoluminescence, electroluminescence, and light intensity dependence measurements reveal how this morphological change affects charge generation and recombination. As a result, the J(sc) and FF are significantly improved. However, the smaller band gap and the higher HOMO level of TQ1 upon annealing causes a lower V-oc. The blend of an amorphous polymer TQ1, and a semi-crystalline polymer N2200, can thus be modified by thermal annealing to double the power conversion efficiency.

A New Application Area for Fullerenes: Voltage Stabilizers for Power Cable Insulation

Fullerenes are shown to be efficient voltage-stabilizers for polyethylene, i.e., additives that increase the dielectric strength of the insulation material. Such compounds are highly sought-after because their use in power-cable insulation may considerably enhance the transmission efficiency of tomorrows power grids. On a molal basis, fullerenes are the most efficient voltage stabilizers reported to date.

Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene.

Molecular doping of organic semiconductors is critical for optimizing a range of optoelectronic devices such as field-effect transistors, solar cells, and thermoelectric generators. However, many dopant:polymer pairs suffer from poor solubility in common organic solvents, which leads to a suboptimal solid-state nanostructure and hence low electrical conductivity. A further drawback is the poor thermal stability through sublimation of the dopant. The use of oligo ethylene glycol side chains is demonstrated to significantly improve the processability of the conjugated polymer p(g4 2T-T)-a polythiophene-in polar aprotic solvents, which facilitates coprocessing of dopant:polymer pairs from the same solution at room temperature. The use of common molecular dopants such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is explored. Doping of p(g4 2T-T) with F4TCNQ results in an electrical conductivity of up to 100 S cm-1 . Moreover, the increased compatibility of the polar dopant F4TCNQ with the oligo ethylene glycol functionalized polythiophene results in a high degree of thermal stability at up to 150 °C.

Heterogeneity in the fluorescence of graphene and graphene oxide quantum dots

AbstractHeterogeneity is an inherent property of a wealth of real-world nanomaterials and yet rarely in the reporting of new properties is its effect sufficiently addressed. Graphene quantum dots (GQDs) – fluorescent, nanoscale fragments of graphene - are an extreme example of a heterogeneous nanomaterial. Here, top-down approaches – by far the most predominant – produce batches of particles with a distribution of sizes, shapes, extent of oxidation, chemical impurities and more. This makes characterization of these materials using bulk techniques particularly complex and comparisons of properties across different synthetic methods uninformative. In particular, it hinders the understanding of the structural origin of their fluorescence properties. We present a simple synthetic method, which produces graphene quantum dots with very low oxygen content that can be suspended in organic solvents, suggesting a very pristine material. We use this material to illustrate the limitations of interpreting complex data sets generated by heterogeneous materials and we highlight how misleading this “pristine” interpretation is by comparison with graphene oxide quantum dots synthesized using an established protocol. In addition, we report on the solvatochromic properties of these particles, discuss common characterization techniques and their limitations in attributing properties to heterogeneous materials. Graphical abstractGraphene quantum dots with very low oxygen content were synthesized using a simple method, suggesting a very pristine material. We highlight how misleading this “pristine” term is when applied to a heterogeneous material through comparison with graphene oxide quantum dots.

Enhanced Electrical Conductivity of Molecularly p-Doped Poly(3-hexylthiophene) through Understanding the Correlation with Solid-State Order

Molecular p-doping of the conjugated polymer poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is a widely studied model system. Underlying structure–property relationships are poorly understood because processing and doping are often carried out simultaneously. Here, we exploit doping from the vapor phase, which allows us to disentangle the influence of processing and doping. Through this approach, we are able to establish how the electrical conductivity varies with regard to a series of predefined structural parameters. We demonstrate that improving the degree of solid-state order, which we control through the choice of processing solvent and regioregularity, strongly increases the electrical conductivity. As a result, we achieve a value of up to 12.7 S cm–1 for P3HT:F4TCNQ. We determine the F4TCNQ anion concentration and find that the number of (bound + mobile) charge carriers of about 10–4 mol cm–3 is not influenced by the degree of solid-state order. Thus, the observed increase in electrical conductivity by almost 2 orders of magnitude can be attributed to an increase in charge-carrier mobility to more than 10–1 cm2 V–1 s–1. Surprisingly, in contrast to charge transport in undoped P3HT, we find that the molecular weight of the polymer does not strongly influence the electrical conductivity, which highlights the need for studies that elucidate structure–property relationships of strongly doped conjugated polymers.

Poly(4-vinylpyridine): A New Interface Layer for Organic Solar Cells

Poly(4-vinylpyridine) (P4VP) was used as a cathode interface layer in inverted organic solar cells (OSCs) fabricated using poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and PC71BM (phenyl C71 butyric acid methyl ester) as the donor and acceptor materials, respectively. We successfully demonstrate that the work function of underlying indium tin oxide (ITO) electrode can be significantly reduced by ∼0.7 eV, after modification of the surface with a thin film of P4VP. Photoconversion efficiency of 4.7% was achieved from OSCs incorporating P4VP interface layer between the ITO and bulk heterojunction (BHJ). Thin P4VP layer, when used to modify ZnO electron transport layer in inverted OSCs, reduced the ZnO work function from 3.7 to 3.4 eV, which resulted in a noteworthy increase in open-circuit voltage from 840 to 890 mV. On simultaneous modification of ZnO with P4VP and optimization of the BHJ morphology by using solvent additive chloronapthalene, photoconversion efficiency of OSCs was significantly increased from 4.6% to 6.3%. The enhanced device parameters are also attributed to an energetically favorable material stratification, as a result of an enrichment of PC71BM toward the P4VP interface.