Jasper J. Michels

Predicting morphologies of solution processed polymer : fullerene blends

The performance of solution processed polymer:fullerene thin film photovoltaic cells is largely determined by the nanoscopic and mesoscopic morphology of these blends that is formed during the drying of the layer. Although blend morphologies have been studied in detail using a variety of microscopic, spectroscopic, and scattering techniques and a large degree of control has been obtained, the current understanding of the processes involved is limited. Hence, predicting the optimized processing conditions and the corresponding device performance remains a challenge. We present an experimental and modeling study on blends of a small band gap diketopyrrolopyrrole-quinquethiophene alternating copolymer (PDPP5T) and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) cast from chloroform solution. The model uses the homogeneous Flory-Huggins free energy of the multicomponent blend and accounts for interfacial interactions between (locally) separated phases, based on physical properties of the polymer, fullerene, and solvent. We show that the spinodal liquid-liquid demixing that occurs during drying is responsible for the observed morphologies. The model predicts an increasing feature size and decreasing fullerene concentration in the polymer matrix with increasing drying time in accordance with experimental observations and device performance. The results represent a first step toward a predictive model for morphology formation.

Elimination of charge carrier trapping in diluted semiconductors

In 1962, Mark and Helfrich demonstrated that the current in a semiconductor containing traps is reduced by N/Nt(r), with N the amount of transport sites, Nt the amount of traps and r a number that depends on the trap energy distribution. For r > 1, the possibility opens that trapping effects can be nearly eliminated when N and Nt are simultaneously reduced. Solution-processed conjugated polymers are an excellent model system to test this hypothesis, because they can be easily diluted by blending them with a high-bandgap semiconductor. We demonstrate that in conjugated polymer blends with 10% active semiconductor and 90% high-bandgap host, the typical strong electron trapping can be effectively eliminated. As a result we were able to fabricate polymer light-emitting diodes with balanced electron and hole transport and reduced non-radiative trap-assisted recombination, leading to a doubling of their efficiency at nearly ten times lower material costs.

Analysis of Nanoporosity in Moisture Permeation Barrier Layers by Electrochemical Impedance Spectroscopy

Water permeation in inorganic moisture permeation barriers occurs through macroscale defects/pinholes and nanopores, the latter with size approaching the water kinetic diameter (0.27 nm). Both permeation paths can be identified by the calcium test, i.e., a time-consuming and expensive optical method for determining the water vapor transmission rate (WVTR) through barrier layers. Recently, we have shown that ellipsometric porosimetry (i.e., a combination of spectroscopic ellipsometry and isothermal adsorption studies) is a valid method to classify and quantify the nanoporosity and correlate it with the WVTR values. Nevertheless, no information is obtained about the macroscale defects or the kinetics of water permeation through the barrier, both essential in assessing the quality of the barrier layer. In this study, electrochemical impedance spectroscopy (EIS) is shown as a sensitive and versatile method to obtain information on nanoporosity and macroscale defects, water permeation, and diffusivity of moisture barrier layers, complementing the barrier property characterization obtained by means of EP and calcium test. EIS is performed on thin SiO2 barrier layers deposited by plasma enhanced-CVD. It allows the determination of the relative water uptake in the SiO2 layers, found to be in agreement with the nanoporosity content inferred by EP. Furthermore, the kinetics of water permeation is followed by EIS, and the diffusivity (D) is determined and found to be in accordance with literature values. Moreover, differently from EP, EIS data are shown to be sensitive to the presence of local macrodefects, correlated with the barrier failure during the calcium test.

Complexation of porphyrin-appended guests by calix[4]arene-appended cyclodextrins

A calix[4]arene based β-cyclodextrin dimerand tetramer (1 and2, Figure 1) were synthesized by covalentattachment of amono(2-O-xylylamino)-β-cyclodextrinderivative to calix[4]areneplatforms, bi- or tetrafunctionalized withcarboxylic acid groups at their upperrims. The complexation of porphyrin-basedguest molecules by these hosts inwater was studied using microcalorimetry.Tetrakis(4-phenylsulfonato)porphyrin(TsPP) binds to 2 in a 1 : 2 (host : guest)fashion with enhanced bindingstrength (K1 = 6.6 × 106 M -1) ascompared to the monomeric TsPP–CDinteraction (K = 8.8 × 105 M -1). Thisenhancement is attributed tothe involvement of two cyclodextrin units in theaccommodation of one TsPP guest.Increase of the number of 4-sulfonatophenyl siteson the guest by generating theμ-oxo-dimer of the iron(III) complex of TsPPled to further increase of thebinding strength owing to participation of threeβ-cyclodextrin cavities of2 (K = 1.5 × 107 M -1). The geometricincompatibility betweenhost and guest, stemming from the fact that both TsPPand its μ-oxo-dimer arefairly small compared to the multi-cyclodextrin hosts,probably explains why theenhancement is still moderate. A much more pronouncedincrease in complexationstrength was achieved with p- and m-pyridylporphyrinextended withp-tert-butylbenzyl guest sites. Theseguests are large enough to accommodatethree to four β-cyclodextrin units. The bettermatch in size between host and guestgave association constants up 108 and 109 M -1for the β-cyclodextrindimer and tetramer, respectively. In fact, the 1 : 1complex betweentetrakis(p-tert-butylbenzyl)-p-pyridylporphyrinand 2(K = 5 × 109 M -1) is the strongestreported for cyclodextrin–porphyrininteractions.

Origin of fullerene-induced vitrification of fullerene:donor polymer photovoltaic blends and its impact on solar cell performance

Organic solar cell blends comprised of an electron donating polymer and electron accepting fullerene typically form upon solution casting a thin-film structure made up of a complex mixture of phases. These phases can vary greatly in: composition, order and thermodynamic stability; and they are dramatically influenced by the processing history. Understanding the processes that govern the formation of these phases and their subsequent effect on the efficiency of photo-generating and extracting charge carriers is of utmost importance to enable rational design and processing of these blends. Here we show that the vitrifying effect of three fullerene derivatives ([60]PCBM, bis[60]PCBM, and [60]ICBA) on the prototypical donor polymer (rr-P3HT) can dominate microstructure formation of fullerene/donor polymer blends cast from solution. Using a dynamic crystallization model based on an amalgamation of Flory–Huggins and Lauritzen–Hoffman theory coupled to solvent evaporation we demonstrate that this vitrification, which can result in a large fraction of highly intermixed amorphous solid solution of the fullerene and the polymer, is due to kinetic and thermodynamic reasons. The former is partly determined by the glass transition temperature of the individual components while donor polymer:fullerene miscibility, strongly influenced by the chemical nature of the donor and the fullerene and leading to thermodynamic mixing, dictates the second phenomena. We show that our approximate dynamic crystallization model assists understanding the different solid-state structure formation of rr-P3HT:fullerene blends. Due to the generality of the assumptions used, our model should be widely applicable and assist to capture the influence of the different vitrification mechanisms also of other photovoltaic blends, including the high-efficiency systems based on the strongly aggregating PCE11 (PffBT4T-2OD), which also feature clear signs of vitirfication upon blending with, e.g., [60]PCBM. Hence, our model will provide essential materials design criteria and enable identification of suitable processing guidelines for existing and new high-performing blends from the outset.

Disruption of the Electrical Conductivity of Highly Conductive Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) by Hypochlorite

The effect of hypochlorite treatment on the layer thickness and conductivity of a state-of-the-art high conducting poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is investigated as a function of exposure time and hypochlorite concentration. Because of overoxidation by the hypochlorite the PEDOT:PSS conductivity is decreased by 10 orders of magnitude. Comparison of thickness and conductivity as a function of time shows that a residual insulating layer remains on the substrate upon treatment. Going from a low (<0.01%) to a high (>0.1%) hypochlorite concentration the interaction between PEDOT:PSS and hypochlorite changes from reaction- to diffusion limited. The decrease in conductivity can be interpreted in terms of the interruption of percolating conductive pathways by the reaction between PEDOT and hypochlorite.

Surface‐Directed Spinodal Decomposition of Solvent‐Quenched Organic Transistor Blends

This paper describes the first example of the application of a combination of the Flory-Huggins and Cahn-Hilliard theories to model and simulate microstructure evolution in solution-processed functional blend layers of organic semiconductors, as used in organic electronics devices. Specifically, the work considers phase separation of the active blend components of organic transistors based on triisopropylsilylpentacene (TIPS-pentacene) and poly(α-methylstyrene) (PαMS). By calculation and estimation of relevant physical parameters, it is shown that the vertically phase-separated structure observed in as-cast blend layers containing PαMS of a sufficiently high molecular weight (of the order of 10(2) kDa) evolves via surface-directed spinodal decomposition. The surface-directed effect can already be triggered by small differences in substrate- and/or air-interface interaction energies of the separating phases. During phase separation, which commences at the interfaces, bulk features of the TIPS-enriched phase formed by thermal noise collapse to give the experimentally observed trilayer structure of TIPS-PαMS-TIPS. The reported near absence of solution-state phase separation of as-cast blend layers containing a low molecular weight PαMS (of the order of 1 kDa) is also reproduced.

Synthesis of poly(para-phenylenevinylene) rotaxanes by aqueous Suzuki coupling

PPV-based polyrotaxanes have been prepared by coupling vinyl boronic acids to aryl iodides in the presence of cyclodextrins, and the crystal structure of a [2]rotaxane of this type has been determined.

Dendrimer–cyclodextrin assemblies as stabilizers for gold and platinum nanoparticles

Aqueous assemblies of adamantyl-derivatized poly(propylene imine) (PPI) dendrimers and B-cyclodextrin (B-CD) have been used as nanoreactors in the preparation of gold and platinum nanoparticles in water. These particles have been formed by the reduction of aurate or platinate anions in the presence of the generation 4 (4·(B-CD)32) and 5 ( 5·(B-CD)40) assemblies. Lower generation assemblies did not provide stable nanoparticles. A kinetic model is proposed in which the particles form inside the dendrimer assemblies owing to preferred nucleation as a result of the electrostatic attraction between the polycationic core and the metallate anions. The persistent shape of the adamantyl-derivatized dendrimers and the dense shell of adamantyl-B-CD complexes provide a kinetic barrier for nanoparticle escape thus prolonging their lifetime. Exchange of the dendrimers for a cationic disulfide provided stable, water-soluble metal nanoparticles without change of their size distribution.

On the short circuit resilience of organic solar cells: prediction and validation

UNLABELLED The operational characteristics of organic solar cells manufactured with large area processing methods suffers from the occurrence of short-circuits due to defects in the photoactive thin film stack. In this work we study the effect of a shunt resistance on an organic solar cell and demonstrate that device performance is not affected negatively as long as the shunt resistance is higher than approximately 1000 Ohm. By studying charge transport across PEDOT PSS-lithium fluoride/aluminum (LiF/Al) shunting junctions we show that this prerequisite is already met by applying a sufficiently thick (>1.5 nm) LiF layer. We demonstrate that this remarkable shunt-resilience stems from the formation of a significant charge transport barrier at the PEDOT PSS-LiF/Al interface. We validate our predictions by fabricating devices with deliberately severed photoactive layers and find an excellent agreement between the calculated and experimental current-voltage characteristics.