Measuring small field profiles and output factors with a stemless plastic scintillator array.

Purpose: To fabricate a 1D stemless plastic scintillation detector (SPSD) array using organic photodiodes and to use the detector to measure small field profiles and output factors.

Methods: An organic photodiode array was fabricated by spin coating a mixture of P3HT and PCBM organic semiconductors onto an ITO-coated glass substrate and depositing aluminum top contacts. Four bulk scintillators of various dimensions were placed on top of the photodiode array.

Method for the differentiation of radiation-induced photocurrent from total measured current in P3HT/PCBM BHJ photodiodes.

Thin film radiation-detecting diodes fabricated in the laboratory, such as an organic bulk heterojunction, often contain conductive leads, indium tin oxide traces and metallic interconnects which are exposed to the high-energy photon beam during operation. These components generate extraneous radiation-induced currents, that, if not accounted for, will erroneously be attributed to the detector. In commercial devices, these contributions are mitigated by minimizing the size of these components, an approach that is often not feasible in a research lab.

Fabrication and characterization of a stemless plastic scintillation detector.

Purpose: To fabricate a stemless plastic scintillation detector (SPSD) and characterize its linearity and reproducibility, and its dependence on energy and dose per pulse; and to apply it to clinical PDD and output factor measurements.

Methods: An organic bulk heterojunction photodiode was fabricated by spin coating a blend of P3HT and PCBM onto an ITO-coated glass substrate and depositing aluminum top contacts. Eljen scintillators (~5 × 5 × 5 mm ; EJ-204, EJ-208, and EJ-260) or Saint-Gobain scintillators (~3 × 3 × 2 mm ; BC-400 and BC-412) were placed on the opposite side of the glass using a silicone grease (optical coupling agent) creating the SPSD.

Ultrafast acoustic phonon scattering in CHNHPbI revealed by femtosecond four-wave mixing.

Carrier scattering processes are studied in CHNHPbI using temperature-dependent four-wave mixing experiments. Our results indicate that scattering by ionized impurities limits the interband dephasing time (T) below 30 K, with strong electron-phonon scattering dominating at higher temperatures (with a time scale of 125 fs at 100 K). Our theoretical simulations provide quantitative agreement with the measured carrier scattering rate and show that the rate of acoustic phonon scattering is enhanced by strong spin-orbit coupling, which modifies the band-edge density of states.

Simultaneous observation of free and defect-bound excitons in CHNHPbI using four-wave mixing spectroscopy.

Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CHNHPbI films using four-wave mixing (FWM) spectroscopy.

Understanding the morphology of solution processed fullerene-free small molecule bulk heterojunction blends.

Bulk-heterojunction (BHJ) molecular blends prepared from small molecules based on diketopyrrolopyrrole (DPP) and perylene-diimide (PDI) chromophores have been studied using optical absorption, cyclic voltammetry, photoluminescence quenching, X-ray diffraction, atomic force microscopy, and current-voltage measurements. The results provided useful insights into the use of DPP and PDI based molecules as donor-acceptor composites for organic photovoltaic (OPV) applications. Beside optoelectronic compatibility, the choice of active layer processing conditions is of key importance to improve the performance of BHJ solar cells.

The Silicon:Colloidal Quantum Dot Heterojunction.

A heterojunction between crystalline silicon and colloidal quantum dots (CQDs) is realized. A special interface modification is developed to overcome an inherent energetic band mismatch between the two semiconductors, and realize the efficient collection of infrared photocarriers generated in the CQD film. This junction is used to produce a sensitive near infrared photodetector.

Perovskite-fullerene hybrid materials suppress hysteresis in planar diodes.

Solution-processed planar perovskite devices are highly desirable in a wide variety of optoelectronic applications; however, they are prone to hysteresis and current instabilities. Here we report the first perovskite-PCBM hybrid solid with significantly reduced hysteresis and recombination loss achieved in a single step. This new material displays an efficient electrically coupled microstructure: PCBM is homogeneously distributed throughout the film at perovskite grain boundaries.

An electron-deficient small molecule accessible from sustainable synthesis and building blocks for use as a fullerene alternative in organic photovoltaics.

An electron-deficient small molecule accessible from sustainable isoindigo and phthalimide building blocks was synthesized via optimized synthetic procedures that incorporate microwave-assisted synthesis and a heterogeneous catalyst for Suzuki coupling, and direct heteroarylation carbon-carbon bond forming reactions. The material was designed as a non-fullerene acceptor with the help of DFT calculations and characterized by optical, electronic, and thermal analysis. Further investigation of the material revealed a differing solid-state morphology with the use of three well-known processing conditions: thermal annealing, solvent vapor annealing and small volume fractions of 1,8-diiodooctane (DIO) additive.

Patterning of nanocrystalline cellulose gel phase by electrodissolution of a metallic electrode.

At high concentration or in the presence of electrolytes and organic solvents, solutions of cellulose nanocrystals (CNCs) can form gels exhibiting optical properties similar to the ones of liquid crystal phases. In an attempt to pattern such a gel phase, we have studied the electrodissolution of a metallic electrode in a water suspension of carboxylated CNCs (cCNCs). Depending on the metal used, the electrodissolution process was observed at a different positive potential.

Rapid thermal conductivity measurements for combinatorial thin films.

A simple and inexpensive automated method for determining the thermal conductivity of a combinatorial library of thin films is demonstrated by measuring the thermal conductivity of a sputtered silicon dioxide film of varying thickness deposited on single crystal silicon. Using 3ω measurements, two methods for calculating the substrate thermal conductivity and two methods for determining the film thermal conductivity are demonstrated and compared. The substrate thermal conductivity was found to be 139 ± 3 W/m·K.

Comparative interface metrics for metal-free monolayer-based dye-sensitized solar cells.

The first quantitative comparison between self-assembled monolayers of homologous carboxylate- and phosphonate-terminated organic dyes that are of use in dye-sensitized solar cells (DSSCs) is reported. (Cyanovinyl)phosphonate-terminated oligothiophenes and (cyanovinyl)carboxylate-terminated oligothiophenes were synthesized on TiO(2) thin film electrodes. Structurally analogous organics were compared for the effect of the anchoring groups on photochemical properties in solution as measured by UV/vis spectroscopy and for reactivity with the electrode surface.

Electric field induced assembly of vimentin microscaffolds around metallic electrodes.

The self-assembly properties of fibrous proteins such as collagen are frequently used to form three-dimensional scaffolds. In this study we investigated the effect of nonuniform alternating and static electric-fields on the self-assembly properties of a dilute solution of vimentin. In the presence of both types of fields at the same time, vimentin was observed to accumulate at the positive electrode and to form microscaffolds bridging the two electrodes in 20-30 min.

Organophosphonate self-assembled monolayers for gate dielectric surface modification of pentacene-based organic thin-film transistors: a comparative study.

Organic thin-film transistors using pentacene as the semiconductor were fabricated on silicon. A series of phosphonate-based self-assembled monolayers (SAMs) was used as a buffer between the silicon dioxide gate dielectric and the active pentacene channel region. Octadecylphosphonate, (quarterthiophene)phosphonate, and (9-anthracene)phosphonate SAMs were examined.