Karunakara Moorthy Boopathi

Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants

For inorganic thermoelectric materials, Seebeck coefficient and electrical conductivity are interdependent, and hence optimization of thermoelectric performance is challenging. In this work we show that thermoelectric performance of PEDOT:PSS can be enhanced by greatly improving its electrical conductivity in contrast to inorganic thermoelectric materials. Free-standing flexible and smooth PEDOT:PSS bulky papers were prepared using vacuum-assisted filtration. The electrical conductivity was enhanced to 640, 800, 1300, and 1900 S cm(-1) by treating PEDOT:PSS with ethylene glycol, polyethylene glycol, methanol, and formic acid, respectively. The Seebeck coefficient did not show significant variation with the tremendous conductivity enhancement being 21.4 and 20.6 μV K(-1) for ethylene glycol- and formic acid-treated papers, respectively. This is because secondary dopants, which increase electrical conductivity, do not change oxidation level of PEDOT. A maximum power factor of 80.6 μW m(-1) K(-2) was shown for formic acid-treated samples, while it was only 29.3 μW m(-1) K(-2) for ethylene glycol treatment. Coupled with intrinsically low thermal conductivity of PEDOT:PSS, ZT ≈ 0.32 was measured at room temperature using Harman method. We investigated the reasons behind the greatly enhanced thermoelectric performance.

Synergistic improvements in stability and performance of lead iodide perovskite solar cells incorporating salt additives

The main issues in planar perovskite solar cells are the coverage and crystallinity of the perovskite film on the PEDOT:PSS layer. To enhance these features, we introduced alkali metal halides (salts) as additives into the perovskite precursor solution used in a two-step preparation method. These alkali metal halides chelate with Pb2+ ions and enhance the crystal growth of PbI2 films, resulting in nanostructured morphologies. The nanostructured PbI2 films promote homogeneous nucleation and larger crystallite sizes, thereby enhancing the morphology and crystallinity of the perovskite films. The alkali metal halides recrystallize the small grains and passivate the grain boundaries and interface states, allowing effective charge generation and dissociation in perovskite films. Photoluminescence measurements indicated that perovskite films prepared with salt additives featured fewer charge traps and defects. The power conversion efficiency of the device incorporating a small amount of a salt additive increased by approximately 33%—from 11.4 to 15.08%. This device was more stable than a corresponding device prepared without the additive, with only 16.5% degradation occurring over a period of 50 days.

Using an Airbrush Pen for Layer-by-Layer Growth of Continuous Perovskite Thin Films for Hybrid Solar Cells

In this manuscript we describe hybrid heterojunction solar cells, having the device architecture glass/indium tin oxide/poly(3,4-ethylenedioxythiopene)/poly(styrenesulfonic acid)/perovskite/[6,6]-phenyl-C61-butyric acid methyl ester/C60/2,9-dimethyl- 4,7-diphenyl-1,10-phenanthroline/Al, fabricated using lead halide perovskite obtained through spray-coating at a low precursor concentration. To study the relationship between the morphology and device performance, we recorded scanning electron microscopy images of perovskite films prepared at various precursor ratios, spray volumes, substrate temperatures, and postspray annealing temperatures. Optimization of the spray conditions ensured uniform film growth and high surface area coverage at low substrate temperatures. Lead halide perovskite solar cells prepared under the optimal conditions displayed an average power conversion efficiency (PCE) of approximately 9.2%, with 85% of such devices having efficiencies of greater than 8.3%. The best-performing device exhibited a short-circuit current density of 17.3 mA cm(-2), a fill factor of 0.63, and an open-circuit voltage of 0.93 V, resulting in a PCE of 10.2%. Because spray-coating technology allows large-area deposition, we also fabricated devices having areas of 60 and 342 mm(2), achieving PCEs with these devices of 6.88 and 4.66%, respectively.

Planar Heterojunction Perovskite Solar Cells Incorporating Metal–Organic Framework Nanocrystals

Zr-based porphyrin metal-organic framework (MOF-525) nanocrystals with a crystal size of about 140 nm are synthesized and incorporated into perovskite solar cells. The morphology and crystallinity of the perovskite thin film are enhanced since the micropores of MOF-525 allow the crystallization of perovskite to occur inside; this observation results in a higher cell efficiency of the obtained MOF/perovskite solar cell.

A high performance electrochemical sensor for acetaminophen based on a rGO–PEDOT nanotube composite modified electrode

In this study, we perform an electrochemical sensing using a conductive composite film containing reduced graphene oxide (rGO) and poly(3,4-ethylenedioxythiophene) nanotubes (PEDOT NTs) as an electrode modifier on a glassy carbon electrode (GCE). Scanning electron microscopy suggests that the rGO covers the surface of GCE uniformly and the PEDOT NTs act as a conducting bridge to connect the isolated rGO sheets. By combining these two materials, the conductivity and the surface coverage of the film can be enhanced, which is beneficial for electrochemical sensing. The rGO–PEDOT NT composite modified electrode is applied for an effective sensor to analyze acetaminophen. The obtained electrochemical activity is much higher than those obtained by the rGO- and PEDOT NT-modified electrodes; the higher electrochemical activity may be attributed to the higher conductivity and greater coverage of the rGO–PEDOT NT composite film on the GCE. Furthermore, interference tests indicate that the rGO–PEDOT NT composite modified electrode exhibits high selectivity toward the analyte. This novel method for combining the rGO and PEDOT NTs establishes a new class of carbon material-based electrodes for electrochemical sensors.

Preparation of metal halide perovskite solar cells through a liquid droplet assisted method

Solution-processed organometal trihalide-based perovskites have attracted attention in the field of solar energy due to their high absorption and low temperature fabrication. We demonstrated the droplet-assisted two-step process of spin coating lead iodide (PbI2) followed by spray coating methylammonium iodide (CH3NH3I) to prepare a continuous lead iodide perovskite film. A simple airbrush gun was used to control the volume of CH3NH3I, in order to attain a uniform, stoichiometric and continuous perovskite film. An insufficient or excess volume of CH3NH3I gives poor crystallinity and morphology, which gradually reduces the device performance. A power conversion efficiency (PCE) of 11.66% was achieved for 100 nm of PbI2 followed by 300 μl of CH3NH3I and annealing at 100 °C for 120 min. To address the reproducibility of the device performance, 50 devices were fabricated for statistical analysis and 80% of the devices showed the average PCE of 10–11% with reproducible JSC, VOC and FF.

Bifunctional separator as a polysulfide mediator for highly stable Li–S batteries

The shuttling process involving lithium polysulfides is one of the major factors responsible for the degradation in capacity of lithium–sulfur batteries (LSBs). Herein, we demonstrate a novel and simple strategy—using a bifunctional separator, prepared by spraying poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on a pristine separator—to obtain long-cycle LSBs. The negatively charged SO3− groups present in PSS act as an electrostatic shield for soluble lithium polysulfides through mutual coulombic repulsion, whereas PEDOT provides chemical interactions with insoluble polysulfides (Li2S, Li2S2). The dual shielding effect can provide an efficient protection from the shuttling phenomenon by confining lithium polysulfides to the cathode side of the battery. Moreover, coating with PEDOT:PSS transforms the surface of the separator from hydrophobic to hydrophilic, thereby improving the electrochemical performance. We observed an ultralow decay of 0.0364% per cycle when we ran the battery for 1000 cycles at 0.25C—far superior to that of the pristine separator and one of the lowest recorded values reported at a low current density. We examined the versatility of our separator by preparing a flexible battery that functioned well under various stress conditions; it displayed flawless performance. Accordingly, this economical and simple strategy appears to be an ideal platform for commercialization of LSBs.

High electrocatalytic performance of platinum and manganese dioxide nanoparticle decorated reduced graphene oxide sheets for methanol electro-oxidation

In this study we report the synthesis of a novel Pt–MnO2–ERGO electrocatalyst by the deposition of MnO2 and Pt nanoparticles decorated on reduced graphene oxide sheets using a simple electrochemical method. The as prepared MnO2 and Pt nanoparticles decorated on the reduced graphene oxide sheets (Pt–MnO2–ERGO electrocatalysts) were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The cyclic voltammetric (CV), chronoamperometric and electrochemical impedance spectroscopic (EIS) measurements show high electrocatalytic activity and stability of the electrodes towards the methanol oxidation reaction in nitrogen saturated sulfuric acid aqueous solutions and in mixed sulfuric acid and methanol aqueous solutions. The voltammetric results show the electrocatalytic characteristics of the Pt–MnO2–ERGO electrocatalysts, which exhibit superior electrocatalytic activity (including good poison tolerance, and low onset potential) and stability toward electro-oxidation of methanol in a model reaction. The electrochemical impedance spectroscopic result shows good electrocatalytic activity in relation to methanol oxidation and improved tolerance of CO. In addition, the as designed Pt–MnO2–ERGO nanocomposite modified electrode with a novel structure can be directly employed for fuel cells.

Controlled mechanical cleavage of bulk niobium diselenide to nanoscaled sheet, rod, and particle structures for Pt-free dye-sensitized solar cells

In this study, we report a one-step process for the preparation of NbSe2 nanosheets, nanorods and nanoparticles from pristine materials under the effects of shear and friction forces. Nevertheless, simple and facile methods for the large-scale syntheses of well-defined NbSe2 nanostructures in high yield have yet to be realized and that will have a great impact in a wide range of applications. For example, developing platinum (Pt)-free and highly efficient counter electrodes is meaningful and necessary for the cost reduction of dye-sensitized solar cells (DSSCs). By integrating this approach with a simple method of thin film preparation (spray coating) allowed us to prepare large-area, conductive, semitransparent flexible thin films of NbSe2. We have used microscopic and macroscopic methods to examine the morphologies, compositions, crystallinity, and electrical and optical properties of the converted NbSe2 nanostructures. DSSCs with NbSe2 nanosheet counter electrodes (CEs) achieved a conversion efficiency of 7.73%, superior to an efficiency of 7.01% for Pt-based CEs. Our NbSe2 nanostructure provides a cost-effective CE alternative to the noble metal Pt in DSSCs.

Solution-processable antimony-based light-absorbing materials beyond lead halide perovskites

Organic–inorganic lead halide perovskites have recently emerged as highly competitive light absorbing materials for low cost solution-processable photovoltaic devices. With the high efficiency already achieved, removing the toxicity, i.e., lead-free and stability are the key obstacles for perovskite solar cells. Here, we report the synthesis of an antimony (Sb)-based hybrid material having the composition of A3Sb2I9 [A = CH3NH3 (MA), Cs] and an investigation of its potential photovoltaic applications. Sb-based perovskite-like materials exhibited attractive absorbance properties, with the band gaps of MA3Sb2I9 and Cs3Sb2I9 measured to be 1.95 and 2.0 eV, respectively. X-ray photoelectron spectroscopy confirmed the formation of stoichiometric perovskites from appropriate precursor molar ratios incorporated with hydroiodic acid (HI). Planar hybrid Sb-based solar cells exhibited negligible hysteresis and reproducible power output under working conditions. A power conversion efficiency of 2.04% was achieved by the MA3Sb2I9 perovskite-based device—the highest reported to date for a Sb-based perovskite solar cell.