Thanh Luan Nguyen

Semi-crystalline photovoltaic polymers with efficiency exceeding 9% in a ∼300 nm thick conventional single-cell device

We report a series of semi-crystalline, low band gap (LBG) polymers and demonstrate the fabrication of highly efficient polymer solar cells (PSCs) in a thick single-cell architecture. The devices achieve a power conversion efficiency (PCE) of over 7% without any post-treatment (annealing, solvent additive, etc.) and outstanding long-term thermal stability for 200 h at 130 °C. These excellent characteristics are closely related to the molecular structures where intra- and/or intermolecular noncovalent hydrogen bonds and dipole–dipole interactions assure strong interchain interactions without losing solution processability. The semi-crystalline polymers form a well-distributed nano-fibrillar networked morphology with PC70BM with balanced hole and electron mobilities (a h/e mobility ratio of 1–2) and tight interchain packing (a π–π stacking distance of 3.57–3.59 A) in the blend films. Furthermore, the device optimization with a processing additive and methanol treatment improves efficiencies up to 9.39% in a ∼300 nm thick conventional single-cell device structure. The thick active layer in the PPDT2FBT:PC70BM device attenuates incident light almost completely without damage in the fill factor (0.71–0.73), showing a high short-circuit current density of 15.7–16.3 mA cm−2. Notably, PPDT2FBT showed negligible changes in the carrier mobility even at ∼1 μm film thickness.

Enhanced Efficiency of Single and Tandem Organic Solar Cells Incorporating a Diketopyrrolopyrrole‐Based Low‐Bandgap Polymer by Utilizing Combined ZnO/Polyelectrolyte Electron‐Transport Layers

Power conversion efficiency up to 8.6% is achieved for a solution-processed tandem solar cell based on a diketopyrrolopyrrole-containing polymer as the low-bandgap material after using a thin polyelectrolyte layer to modify the electron-transport ZnO layers, indicating that interfacial engineering is a useful approach to further enhancing the efficiency of tandem organic solar cells.

A High Efficiency Nonfullerene Organic Solar Cell with Optimized Crystalline Organizations.

A well-organized donor-acceptor crystalline structure is examined for high -performance nonfullerene solar cells. By thermal annealing, nanoscale structures of both donor and acceptor domains are successfully modulated, followed by -significant changes in the resulting -photovoltaic characteristics. When annealed at 90 °C, a maximum power conversion efficiency of 7.64% with a -remarkable open-circuit voltage of 1.03 V is obtained.

Highly efficient red-emitting hybrid polymer light-emitting diodes via Förster resonance energy transfer based on homogeneous polymer blends with the same polyfluorene backbone.

Highly efficient inverted-type red-emitting hybrid polymeric light-emitting diodes (HyPLEDs) were successfully demonstrated via Förster resonance energy transfer (FRET) and interfacial engineering of metal oxide with a cationic conjugated polyelectrolyte (CPE). Similarly structured green- and red-emissive polyfluorene copolymers, F8BT and F8TBT, were homogeneously blended as a FRET donor (host) and acceptor (dopant). A cationic polyfluorene-based CPE was also used as an interfacial layer for optimizing the charge injection/transport and improving the contact problem between the hydrophilic ZnO and hydrophobic polymer layer. A long Förster radius (R0 = 5.32 nm) and high FRET efficiency (~80%) was calculated due to the almost-perfect spectral overlap between the emission of F8BT and the absorption of F8TBT. A HyPLED containing 2 wt % F8TBT showed a pure red emission (λmax = 640 nm) with a CIE coordinate of (0.62, 0.38), a maximum luminance of 26 400 cd/m(2) (at 12.8 V), a luminous efficiency of 7.14 cd/A (at 12.8 V), and a power efficiency of 1.75 lm/W (at 12.8 V). Our FRET-based HyPLED realized the one of the highest luminous efficiency values for pure red-emitting fluorescent polymeric light-emitting diodes reported so far.

Spontaneous and Selective Formation of HSNO, a Crucial Intermediate Linking H2S and Nitroso Chemistries

Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H2S biochemistries, but its existence and stability in vivo remain controversial. We establish that HSNO is spontaneously formed in high concentration when NO and H2S gases are mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed by the reaction H2S + N2O3 → HSNO + HNO2, where N2O3 is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H2S may form HNO and HSSH. The length of the S-N bond has been derived to high precision and is found to be unusually long: 1.84 Å, the longest S-N bond reported to date for an R-SNO compound. The present structural and, particularly, reactivity investigations of this elusive molecule provide a firm foundation to better understand its potential physiological chemistry and propensity to undergo S-N bond cleavage in vivo.

Ultrafast charge transfer in operating bulk heterojunction solar cells.

The ultrafast charge generation process in organic solar cell devices is investigated by transient reflection spectroscopy on five state-of-the-art bulk heterojunction systems. The charge generation process in operating devices is found to be a combination of an ultrafast generation mechanism over several hundred femto-seconds and a slow process from pico-seconds to nanoseconds, limited by exciton diffusion dynamics. In addition, the lack of electric field dependence in the charge dynamics rules out geminate recombination as an important loss mechanism.

Quinoxaline–thiophene based thick photovoltaic devices with an efficiency of ∼8%

A series of difluoroquinoxaline–thiophene based reduced band gap polymers was designed and synthesized by considering non-covalent coulombic interactions in a polymeric main chain. The insertion of different numbers of thiophene moieties allows for the adjustment of the absorption range, frontier energy levels, crystalline self-organization, film morphology and the resulting photovoltaic properties. A thick blend film of poly(thiophene-alt-(2,3-bis(3,4-bis(octyloxy)phenyl)-6,7-difluoroquinoxaline)) (PDFQx-T):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) showed a rough, inhomogeneous and largely phase-separated surface morphology compared to a typical film with ∼100 nm thickness. A similar trend was observed in the surface morphology of a poly(2,2′-bithiophene-alt-(2,3-bis(3,4-bis(octyloxy)phenyl)-6,7-difluoroquinoxaline)) (PDFQx-2T) blend film, showing deteriorated photovoltaic properties with increasing film thickness. In contrast, poly(2,2′:5′,2′′-terthiophene-alt-2,3-bis(3,4-bis(octyloxy)phenyl)-6,7-difluoroquinoxaline) (PDFQx-3T) had a similar blend film morphology for both thick and thin active layers, showing a homogeneous and smooth morphology with a face-on orientation and tight π–π stacking (d-spacing = 3.6 A). The optimized photovoltaic cell based on PDFQx-3T : PC71BM achieved a power conversion efficiency (PCE) of 8% with an open-circuit voltage of 0.74 V, a short-circuit current of 17.19 mA cm−2 and a fill factor of 0.63 at an active layer thickness of ∼270 nm. It is still a challenge to develop photovoltaic polymers which allow efficient charge transport and extraction at a device thickness of ∼300 nm. Fine-adjustment of intra- and interchain interactions must be considered carefully to achieve high device properties for thick devices without deterioration in the blend morphology and charge recombination. This high PCE at an active layer thickness of ∼300 nm may suggest great potential for the mass production of printed polymer solar cells via industrial solution processes.

Measurement of the Charge Carrier Mobility Distribution in Bulk Heterojunction Solar Cells

Charge carrier transport through organic solar cells is fundamentally dispersive due to the disordered structure and complex film morphology within the photoactive layer. A novel application of transient photocurrent and short-circuit variable time-delayed collection field measurements is used to reconstruct the complete charge carrier mobility distribution for the photogenerated carriers in optimized organic solar cells.

Thienothiophene-benzotriazole-based semicrystalline linear copolymers for organic field effect transistors

Abstract A series of thienothiophene-benzotriazole-based semicrystalline copolymers, PTTBTz, PTTBTz-F, and PTTBTz-OR, were synthesized by considering chain linearity, planarity and inter-chain packing by virtue of non-covalent attractive interaction. Fluorine and alkoxy substituents were introduced to modulate the intra- and inter-chain coulombic interactions and crystalline ordering. The fluorine and alkoxy-substituted PTTBTz-F and PTTBTz-OR showed pronounced inter-chain packing with edge-on orientation confirmed by UV-vis absorption and X-ray diffraction measurements. The well-resolved diffraction patterns were obtained for PTTBTz-F and PTTBTz-OR, showing (100)∼(500) inter-lamellar scattering peaks (d-spacing, 17∼18 Å) in the out-of-plane direction and a π-π stacking peak (d-spacing, 3.5∼4.1 Å) in the in-plane direction. Organic field effect transistor (OFET) devices were fabricated with a bottom gate and top contact geometry. PTTBTz-F (μh = 4.49 × 10–2 cm2 V–1 s–1, on/off ratio = 1.13 × 107) and PTTBTz-OR (μh = 8.39 × 10–3 cm2 V–1 s–1, on/off ratio = 2.98 × 104) showed nearly 3 and 2 orders of magnitude higher hole mobility upon annealing at 305 and 260 °C, with compared to the unsubstituted PTTBTz.

Synthesis and optical properties of pH-responsive conjugated polyampholytes

Conjugated polyampholytes (CPAs) containing both positive and negative ionic groups in their side-chains were designed and synthesized. Two types of random copolymers were prepared by the incorporation of fluorene, phenylene and 2,1,3-benzothiadiazole (BT) moieties in the main chain. Both quaternary ammonium bromide and carboxylic acid functionalities were introduced successfully in the side-chain through sequential protection, quaternization and deprotection reactions. The resulting ionic polymers were soluble in water and their optical characteristics were examined by changing pH. The fluorescence resonance energy transfer (FRET)-sensitized BT emission of the polymers increased with increasing solution pH. Under basic conditions, deprotonation of the carboxylic acid groups induced intra- and/or interchain aggregation via electrostatic complexation between the cationic ammonium and anionic carboxylate groups. The FRET ratio between the green and blue emissions showed a linear relationship with solution pH. This new type of water-soluble fluorescent bioassays and bioimaging applications through bioconjugation with a targeting moiety.