Xuan-Dung Dang

A modular molecular framework for utility in small-molecule solution-processed organic photovoltaic devices

We report on the design, synthesis and characterization of light harvesting small molecules for use in solution-processed small molecule bulk heterojunction (SM-BHJ) solar cell devices. These molecular materials are based upon an acceptor/donor/acceptor (A/D/A) core with donor endcapping units. Utilization of a dithieno(3,2-b;2′,3′-d)silole (DTS) donor and pyridal[2,1,3]thiadiazole (PT) acceptor leads to strong charge transfer characteristics, resulting in broad optical absorption spectra extending well beyond 700 nm. SM-BHJ solar cell devices fabricated with the specific example 5,5′-bis{7-(4-(5-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (6) as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor component showed short circuit currents above −10 mA cm−2 and power conversion efficiencies (PCEs) over 3%. Thermal processing is a critical factor in obtaining favorable active layer morphologies and high PCE values. A combination of UV-visible spectroscopy, conductive and photo-conductive atomic force microscopies, dynamic secondary mass ion spectrometry (DSIMS), and grazing incident wide angle X-ray scattering (GIWAXS) experiments were carried out to characterize how thermal treatment influences the active layer structure and organization.

Small molecule sensitizers for near-infrared absorption in polymer bulk heterojunction solar cells

A low band gap small molecule chromophore has been incorporated into a polymer/fullerene bulk heterojunction (BHJ) solar cell yielding increased carrier generation in the near infrared and increased overall short circuit current. The use of a small concentration of a soluble oligothiophene with a diketopyrrolopyrrole core can extend the absorption and photocurrent of poly(3-hexyl thiophene):[6,6]-phenyl C71 butyric acid methyl ester solar cells to 800 nm. Photocurrent from the dye embedded within the polymer BHJ is demonstrated, and the use of soluble small molecule sensitizers as a path toward high efficiency solar cells is discussed.

Conjugated Oligoelectrolyte Electron Transport/Injection Layers for Organic Optoelectronic Devices

The function of a conjugated oligoelectrolyte (COE) as the electron transport layer (ETL) in polymer light emitting diodes is demonstrated. Current density−luminance−voltage characteristics demonstrate that insertion of the COE adjacent to an Al electrode yields higher device efficiencies than those obtained using the low work function cathode Ba. Furthermore, the temporal response of the electroluminescence is faster than that observed when using conjugated polyelectrolytes as the ETL. That COEs have better defined structures than their polymeric counterparts may make them more attractive materials for the fabrication of efficient PLEDs prepared by spin coating methods.

Nanoscale characterization of tetrabenzoporphyrin and fullerene-based solar cells by photoconductive atomic force microscopy.

Organic photovoltaic devices (OPVs) have garnered signifi cant interest in the scientifi c community due to their potential applications as low-cost, light-weight and fl exible energy sources. [ 1–3 ] The power conversion effi ciency (PCE) of such devices is predominantly infl uenced by the optical and electronic properties of the donor and acceptor materials and the morphology of the donor and acceptor blend. [ 4–7 ] The sizes of donor and acceptor domains are often on the scale of nanometers; therefore, an understanding of how variations in local morphological features and charge generation at the nanoscale infl uence device performance is of the utmost importance. An array of microscopy techniques offering various types of local physical and chemical information has been utilized to characterize OPVs, including scanning electron microscopy, [ 8–9 ]

Morphology and conductivity modification of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) films induced by conductive atomic force microscopy measurements

Morphology and conductivity modifications in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films induced by conductive atomic force microscopy probe are investigated. At an applied bias of positive or negative 10 V, raised features of 12.8±1.8 nm in height are generated and the local film conductivity is reduced. The feature height formation is irreversible and dependent on both applied bias and tip velocity. The mechanism by which these features are generated is proposed to be mass transport of PSS− to the surface under the atomic force microscope tip. This finding may open up the possibility of patterning PEDOT:PSS films, and thereby organic optoelectronic devices.

Effects of Impurities on Operational Mechanism of Organic Bulk Heterojunction Solar Cells

Photoconductive atomic force microscopy in conjunction with transient absorption spectroscopy and charge transport study were used to investigate the effect of impurities (TCNQ and PC84 BM) on the performance of low band gap conjugated polymer and fullerene solar cells to gain insight into whether differing impurity shapes may lead to different loss mechanisms.

Reconstruction of Conjugated Oligoelectrolyte Electron Injection Layers

Surface reconstruction of electron injection layers based on conjugated oligoelectrolytes atop an electroluminescent layer occurs in the presence of air. The proposed mechanism involves hydration and concomitant increase of the interfacial energy with the underlying hydrophobic surface followed by dewetting via a nucleation process. No such changes are observed in the case of a conjugated polyelectrolyte, presumably because the lower mobility of the polymer chains leads to a kinetically locked bilayer.

High light intensity effects on nanoscale open-circuit voltage for three common donor materials in bulk heterojunction solar cells

White light photoconductive atomic force microscopy (pc-AFM) was employed to evaluate nanoscale open circuit (Voc) at high light intensities (up to 200 sun) of three donor:acceptor blends comprising two widely studied polymers and one small molecule donor material. By varying the work function of electron extraction contacts, the Voc observed in nanoscale measurements reveals a unified dependence on the electrode work functions regardless of the blend materials; in line with earlier macroscopic measurements at 1 sun. At high light intensities, an agreement between the nanoscale and bulk Voc is observed. Nonetheless, light intensity dependent Voc measurements suggest that under high light intensities, the Voc obtained by pc-AFM exhibits contact-limited behavior.