Tenghooi Goh

Improved efficiency of smooth and aligned single walled carbon nanotube/silicon hybrid solar cells

Smooth and aligned single walled carbon nanotube (SWNT) thin films with improved optoelectronic performance are fabricated using a superacid slide casting method. Deposition of as made SWNT thin film on silicon (Si) together with post treatments result in SWNT/Si hybrid solar cells with unprecedented high fill factor of 73.8%, low ideality factor of 1.08 as well as overall dry cell power conversion efficiency of 11.5%.

A Mesoporous Catalytic Membrane Architecture for Lithium–Oxygen Battery Systems

Controlling the mesoscale geometric configuration of catalysts on the oxygen electrode is an effective strategy to achieve high reversibility and efficiency in Li-O2 batteries. Here we introduce a new Li-O2 cell architecture that employs a catalytic polymer-based membrane between the oxygen electrode and the separator. The catalytic membrane was prepared by immobilization of Pd nanoparticles on a polyacrylonitrile (PAN) nanofiber membrane and is adjacent to a carbon nanotube electrode loaded with Ru nanoparticles. During oxide product formation, the insulating PAN polymer scaffold restricts direct electron transfer to the Pd catalyst particles and prevents the direct blockage of Pd catalytic sites. The modified Li-O2 battery with a catalytic membrane showed a stable cyclability for 60 cycles with a capacity of 1000 mAh/g and a reduced degree of polarization (∼ 0.3 V) compared to cells without a catalytic membrane. We demonstrate the effects of a catalytic membrane on the reaction characteristics associated with morphological and structural features of the discharge products via detailed ex situ characterization.

Toward Efficient Thick Active PTB7 Photovoltaic Layers Using Diphenyl Ether as a Solvent Additive

The development of thick organic photovoltaics (OPV) could increase absorption in the active layer and ease manufacturing constraints in large-scale solar panel production. However, the efficiencies of most low-bandgap OPVs decrease substantially when the active layers exceed ∼100 nm in thickness (because of low crystallinity and a short exciton diffusion length). Herein, we report the use of solvent additive diphenyl ether (DPE) that facilitates the fabrication of thick (180 nm) active layers and triples the power conversion efficiency (PCE) of conventional thienothiophene-co-benzodithiophene polymer (PTB7)-based OPVs from 1.75 to 6.19%. These results demonstrate a PCE 20% higher than those of conventional (PTB7)-based OPV devices using 1,8-diiodooctane. Morphology studies reveal that DPE promotes the formation of nanofibrillar networks and ordered packing of PTB7 in the active layer that facilitate charge transport over longer distances. We further demonstrate that DPE improves the fill factor and photocurrent collection by enhancing the overall optical absorption, reducing the series resistance, and suppressing bimolecular recombination.

Panchromatic polymer–polymer ternary solar cells enhanced by Förster resonance energy transfer and solvent vapor annealing

Thanks to the bulk-heterojunction (BHJ) feature of polymer solar cells (PSC), additional light active components can be added with ease to form ternary solar cells. This strategy has achieved great success largely due to expanded spectral response range and improved power conversion efficiency (PCE) without incurring excessive processing costs. Here, we report ternary blend polymer–polymer solar cells comprised of PTB7, P3HT, and PC71BM with PCE as high as 8.2%. Analyses of femtosecond time resolved photoluminescence and transient absorption spectroscopy data confirm that P3HT is effective in transferring energy non-radiatively by inducing excitons and prolonging their overall lifetime in PTB7. Furthermore, solvent vapor annealing (SVA) treatment was employed to rectify the overly-coarse morphology, thus enhancing the fill factor, reducing interfacial recombination, and boosting the PCE to 8.7%.

Ultrathin Nanotube/Nanowire Electrodes by Spin–Spray Layer-by-Layer Assembly: A Concept for Transparent Energy Storage

Fully integrated transparent devices require versatile architectures for energy storage, yet typical battery electrodes are thick (20-100 μm) and composed of optically absorbent materials. Reducing the length scale of active materials, assembling them with a controllable method and minimizing electrode thickness should bring transparent batteries closer to reality. In this work, the rapid and controllable spin-spray layer-by-layer (SSLbL) method is used to generate high quality networks of 1D nanomaterials: single-walled carbon nanotubes (SWNT) and vanadium pentoxide (V2O5) nanowires for anode and cathode electrodes, respectively. These ultrathin films, deposited with ∼2 nm/bilayer precision are transparent when deposited on a transparent substrate (>87% transmittance) and electrochemically active in Li-ion cells. SSLbL-assembled ultrathin SWNT anodes and V2O5 cathodes exhibit reversible lithiation capacities of 23 and 7 μAh/cm(2), respectively at a current density of 5 μA/cm(2). When these electrodes are combined in a full cell, they retain ∼5 μAh/cm(2) capacity over 100 cycles, equivalent to the prelithiation capacity of the limiting V2O5 cathode. The SSLbL technique employed here to generate functional thin films is uniquely suited to the generation of transparent electrodes and offers a compelling path to realize the potential of fully integrated transparent devices.

Improving the Performance of P3HT/PCBM Solar Cells with Squaraine Dye

Expanding the spectral absorption breadth and efficiently harvesting excitons are crucial towards creating highly efficient polymer solar cells. Here we describe a strategy to realize broad-band light harvesting in poly(3-hexylthiophene) (P3HT)-based solar cells. We introduce the use of squaraine dye molecules that play a dual role towards improving P3HT-based solar cells. The first benefit is an increase in the spectral absorption in the near infrared region. The second advantage is the collection of excitons close to the interfacial heterojunctions via Förster resonance energy transfer (FRET). Unlike traditional multi-blend systems, where each donor works independently in separate spectral responses, FRET-based systems enable the effective use of multiple donors with significant improvements in light absorption and conversion. Ultrafast transient absorption experiments show that the excitation energy from P3HT can be transferred rapidly (within a few picoseconds) and efficiently (up to 96%) to the squaraine via FRET. As a result, the overall power conversion efficiency is improved. This architecture opens up a new paradigm towards transformative improvements of polymer solar cells.

An additive dripping technique using diphenyl ether for tuning perovskite crystallization for high-efficiency solar cells

Controlling the morphology of the MAPbI3−xClx active layer has remained a challenge towards advancing perovskite solar cells (PvSCs). Here, we demonstrate that a low temperature additive dripping (AD) treatment step, using diphenyl ether (DPE), can significantly improve the power conversion efficiency (PCE), compared to the control device using chlorobenzene (CB), by 15% up to 16.64%, with a high current density (JSC) of 22.67 mA/cm2. We chose DPE for its small and appropriate dipole moment to adjust the solubility of the MAPbI3−xClx precursor during the formation of the intermediate phase and the MAPbI3−xClx phase. The low DPE vapor pressure provides a longer processing window for the removal of residual dimethylformamide (DMF), during the annealing process, for improved perovskite formation. Imaging and X-ray analysis both reveal that the MAPbI3−xClx film exhibits enlarged grains with increased crystallinity. Together, these improvements result in reduced carrier recombination and hole trap-state density in the MAPbI3−xClx film, while minimizing the hysteresis problem typical of PvSCs. These results show thatthe AD approach is a promising technique for improving PvSCs.

PEOz-PEDOT:PSS Composite Layer: A Route to Suppressed Hysteresis and Enhanced Open-Circuit Voltage in a Planar Perovskite Solar Cell

The successful commercialization of perovskite solar cells (Pvs-SCs) calls for the need to find low-temperature processable interlayers with outstanding charge-transport features. In this work, we strategically blend poly(2-ethyl-2-oxazoline) (PEOz) with PEDOT:PSS as the modified hole transport layer (HTL) to achieve high-efficiency P-I-N CH3NH3PbI3 Pvs-SCs. The PEOz-PEDOT:PSS HTL exhibits enhanced features over the conventional layer including the following: (1) promoting perovskite with enlarged grain sizes to decrease the perovskite layers recombination, (2) increasing the work function of the HTL, and (3) decreasing the noncapacitive current in Pvs-SCs. Remarkably, we demonstrate a 17.39% power conversion efficiency with very low hysteresis and high Voc values of 1.075 V for Pvs-SCs with PEOz-PEDOT:PSS.