Maarten J. M. Wirix

Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu(3)N@C(80) (Lu(3)N@C(80)-PCBEH) show an open circuit voltage (V(OC)) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC(61)BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (J(SC)) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the J(SC) in blends containing the high voltage absorber Lu(3)N@C(80)-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu(3)N@C(80)-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC(61)BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu(3)N@C(80)-PCBEH are responsible for the reduced photocurrent.

Controlling Internal Pore Sizes in Bicontinuous Polymeric Nanospheres

Complex polymeric nanospheres were formed in water from comb-like amphiphilic block copolymers. Their internal morphology was determined by three-dimensional cryo-electron tomographic analysis. Varying the polymer molecular weight (MW) and the hydrophilic block weight content allowed for fine control over the internal structure. Construction of a partial phase diagram allowed us to determine the criteria for the formation of bicontinuous polymer nanosphere (BPN), namely for copolymers with MW of up to 17 kDa and hydrophilic weight fractions of ≤0.25; and varying the organic solvent to water ratio used in their preparation allowed for control over nanosphere diameters from 70 to 460 nm. Significantly, altering the block copolymer hydrophilic–hydrophobic balance enabled control of the internal pore diameter of the BPNs from 10 to 19 nm.

Visualizing order in dispersions and solid state morphology with CryoTEM and electron tomography: P3HT:PCBM organic solar cells

Building blocks for organic solar cells are made from P3HT in a P3HT:PCBM solution in toluene and used to tune the morphology of the photoactive layer. The approach presented here decouples the structure and morphology formation, providing precise control over both the structures in solution and the morphology of the photoactive layer. For the characterization of the nanostructures in the organic casting solutions, cryo-TEM was successfully employed, and reveals the P3HT crystals and even the 1.7 nm lamellar stacking, which in combination with cryogenic low dose electron diffraction clearly proves the high crystallinity of P3HT aggregates realized. The photoactive layers made from pre-crystallized solutions show a morphology that is closely related to the structures in solution. A clear trend of decreasing open circuit voltage and increasing short circuit current with increasing order in the casting solutions and the devices was observed, which correlates with the evolution of the morphology from very intermixed with small fibrillar structures to phase-separated with large polymer crystals, as evaluated from representative devices, characterized in 3D with electron tomography.

Suspended crystalline films of protein hydrophobin I (HFBI)

Protein interfaces play an essential role in both natural and man-made materials as stabilizers, sensors, catalysts, and selective channels for ions and small molecules. Probing the molecular arrangement within such interfaces is of prime importance to understand the relation between structure and functionality. Here we report on the preparation and characterization of large area suspended crystalline films of class II hydrophobin HFBI. This small, amphiphilic globular protein readily self-assembles at the air-water interface into a 2D hexagonal lattice which can be transferred onto a holey carbon electron microscopy grid yielding large areas of hundreds of square micrometers intact hydrophobin film spun across micron-sized holes. Fourier transform analysis of low-dose electron microscopy images and selected area electron diffraction profiles reveal a unit cell dimension a=5.6±0.1nm, in agreement with reported atomic force microscopy studies on solid substrates and grazing incidence X-ray scattering experiments at the air-water interface. These findings constitute the first step towards the utilization of large-area suspended crystalline hydrophobin films as membranes for ultrapurification and chiral separation or as biological substrates for ultrafast electron diffraction.