Hytham Elbohy

TiO2 coated urchin-like SnO2 microspheres for efficient dye-sensitized solar cells

AbstractUrchin-like SnO2 microspheres have been grown for use as photoanodes in dye-sensitized solar cells (DSSCs). We observed that a thin layer coating of TiO2 on urchin-like SnO2 microsphere photoanodes greatly enhanced dye loading capability and light scattering ability, and achieved comparable solar cell performance even at half the thickness of a typical nanocrystalline TiO2 photoanode. In addition, this photoanode only required attaching ∼55% of the amount of dye for efficient light harvesting compared to one based on nanocrystalline TiO2. Longer decay of transient photovoltage and higher charge recombination resistance evidenced from electrochemical impedance spectroscopy of the devices based on TiO2 coated urchin-like SnO2 revealed slower recombination rates of electrons as a result of the thin blocking layer of TiO2 coated on urchinlike SnO2. TiO2 coated urchin-like SnO2 showed the highest value (76.1 ms) of electron lifetime (τ) compared to 2.4 ms for bare urchin-like SnO2 and 14.9 ms for nanocrystalline TiO2. TiO2 coated SnO2 showed greatly enhanced open circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) leading to a four-fold increase in efficiency increase compared to bare SnO2. Although TiO2 coated urchin-like SnO2 showed slightly lower cell efficiency than nanocrystalline TiO2, it only used a half thickness of photoanode and saved ∼45% of the amount of dye for efficient light harvesting compared to normal nanocrystalline TiO2.

Dye-sensitized solar cells based on spray-coated carbon nanofiber/TiO2 nanoparticle composite counter electrodes

Electrospun carbon nanofiber (ECN)/TiO2 nanoparticle composite counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) were successfully prepared by spray-coating an ECN/TiO2 (1 : 1 by weight) mixture on a fluorine doped tin oxide (FTO)-glass substrate. TiO2 particles (Degussa P25) were used to bind carbon nanofibers and adhere them to the FTO-glass substrate. Electrochemical impedance spectroscopy (EIS) measurements revealed that the spray-coated ECN/TiO2 composite CEs have lower charge transfer resistance (Rct) and higher interfacial capacitance (Q) than those of Pt CEs. Cyclic voltammograms (CV) further indicated that ECN/TiO2 composite CEs have a faster tri-iodide reduction rate than those of Pt CEs. DSSCs fabricated using ECN/TiO2 CEs showed a power conversion efficiency (η) of 7.25% under 100 mW cm−2 light intensity, which is comparable to that of thermally deposited Pt based DSSCs (η = 7.57%). Moreover, ECN/TiO2 composite CE based DSSCs demonstrated almost equal power conversion efficiency to that of Pt based cells by adding only 8 wt% Pt, which unveiled a cost-effective alternative of costly Pt CEs in DSSCs.

Graphene-embedded carbon nanofibers decorated with Pt nanoneedles for high efficiency dye-sensitized solar cells

Graphene-embedded carbon nanofibers (GCNFs) were developed as a new counter electrode nanomaterial for high efficiency dye-sensitized solar cells (DSCs). GCNFs were produced by electrospinning polyacrylonitrile (PAN) with graphene nanoplatelets followed by stabilization and carbonization. GCNFs decorated with surface-attached platinum nanoneedles (GCNFs-PtNNs) were subsequently prepared by a redox reaction and then deposited onto fluorine doped tin oxide (FTO) glass to make a counter electrode for DSCs. Graphene inside the carbon nanofibers and Pt nanoneedles on the surface demonstrated a synergistic effect to improve the DSC performance. Compared to DSCs with conventional planar Pt counter electrodes, the DSCs with GCNFs-PtNNs significantly improved the energy conversion efficiency from ∼8.63% to ∼9.70% using a mask under AM1.5 illumination. This is the highest conversion efficiency so far with a carbon nanofiber based counter electrode.

Incorporation of plasmonic Au nanostars into photoanodes for high efficiency dye-sensitized solar cells

We have significantly improved the performance of dye-sensitized solar cells (DSSCs) by incorporating plasmonic Au nanostars into the TiO2 photoanode. Gold nanostars were synthesized and used as localized surface plasmons (LSPs) to enhance the light absorption of DSSCs. The Au nanostars exhibit near infrared (NIR) light absorption at ∼785 nm. The N719 and N749 dye based DSSC devices were fabricated with and without the incorporation of Au nanostars. The power conversion efficiency (PCE) of DSSCs using Au nanostars was increased by ∼20% from 7.1% to 8.4% for N719 cells and by ∼30% from 3.9% to 5.0% for N749 devices. The incident photon-to-current conversion efficiency (IPCE) was improved and the spectral response was broadened over the wavelength range of 380–1000 nm. Electrochemical impedance spectroscopy (EIS) showed that by incorporating Au nanostars, the charge recombination resistance Rct value decreased under both open-circuit and illumination conditions, which indicated high electron density generated in the device photoanode.

Electrospun carbon nano-felt derived from alkali lignin for cost-effective counter electrodes of dye-sensitized solar cells

In this study, freestanding and mechanically flexible nano-felt consisting of electrospun carbon nanofibers (ECNFs) derived from alkali lignin with a BET specific surface area of ∼583 cm2 g−1 and average pore size of ∼3.5 nm was prepared and then surface-deposited with Pt nanoparticles (Pt NPs). Both nano-felts of ECNFs and ECNFs–Pt were studied as cost-effective counter electrodes of dye-sensitized solar cells (DSSCs). The energy-dispersive X-ray spectroscopy (EDS) results showed that the amount of Pt NPs in ECNFs–Pt nano-felt was ∼9.9 wt%, and the scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) results indicated that the Pt NPs with small sizes in the range of 2–20 nm were randomly distributed on the surface of ECNFs. The electrochemical impedance spectroscopy (EIS) tests revealed that the ECNFs-based counter electrode had low charge transfer resistance (Rct = 1.94 Ω cm2), and the Rct value was reduced to 1.2 Ω cm2 upon surface-deposition of Pt NPs. The prototype DSSCs based on ECNFs and ECNFs–Pt counter electrodes exhibited comparable performances to the DSSC based on a conventional Pt counter electrode in terms of short circuit density (Jsc), open circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (η).

Characteristics of SnO2 nanofiber/TiO2 nanoparticle composite for dye-sensitized solar cells

SnO2 nanofibers and their composites based photoanodes were fabricated and investigated in the application of dye-sensitized solar cells. The photoanode made of SnO2/TiO2 composites yielded an over 2-fold improvement in overall conversion efficiency. The microstructure of SnO2 nanofibers was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). A compact morphology of composites was observed using scanning electron microscopy (SEM). A long charge diffusion length (62.42 μm) in the composites was derived from time constant in transient photovoltage and photocurrent analysis. These experimental results demonstrate that one-dimensional nanostructured SnO2/TiO2 composites have a great potential for application in solar cells.

Incorporation of

Incorporation of TiO2 nanoparticles into SnO2 nanofibers as photoanode in dye-sensitized solar cells improved the cell efficiency from 4.63% to 6.17%. The addition of TiO2 nanoparticles increased the surface area of the photoanode that led to the enhancement in the dye attachment. In addition, the incorporation of TiO2 nanoparticles helped in the reduction of the recombination of electrons in the photoanode with the electrolyte resulting in the increase in the open circuit voltage (Voc) and fill factor.

{\rm TiO}_{2}

Porous hollow tin oxide (SnO2) nanofibers and their composite with titanium dioxide (TiO2) particles (Degussa P25) were investigated as a photoanode for dye-sensitized solar cells. Incorporation of TiO2 particles in porous hollow SnO2 fibers enhanced the power conversion efficiency (η) from 4.06% to 5.72% under 100-mW/cm2 light intensity. The enhancement of efficiency was mainly attributed to increase in current density (Jsc) and improvement in fill factor (FF). Increase in Jsc was caused by higher dye loading as indicated by UV-Vis absorption spectra and the improvement in FF was attributed to faster charge transport time as obtained from transient analysis. The microstructure of SnO2 fibers was studied using transmission electron microscope, scanning electron microscope, and X-ray diffraction. The electron transfer and recombination life times were studied using transient analysis, whereas interfacial charge transfer was studied using electrochemical impedance spectroscopy.

Nanoparticles Into

A hierarchical mesoporous carbon foam (ECF) with an interconnected micro-/mesoporous architecture was prepared and used as a binder-free, low-cost, high-performance anode for lithium ion batteries. Due to its high specific surface area (980.6 m2/g), high porosity (99.6%), light weight (5 mg/cm3) and narrow pore size distribution (~2 to 5 nm), the ECF anode exhibited a high reversible specific capacity of 455 mAh/g. Experimental results also demonstrated that the anode thickness significantly influence the specific capacity of the battery. Meanwhile, the ECF anode retained a high rate performance and an excellent cycling performance approaching 100% of its initial capacity over 300 cycles at 0.1 A/g. In addition, no binders, carbon additives or current collectors are added to the ECF based cells that will increase the total weight of devices. The high electrochemical performance was mainly attributed to the combined favorable hierarchical structures which can facilitate the Li+ accessibility and also enable the fast diffusion of electron into the electrode during the charge and discharge process. The synthesis process used to make this elastic carbon foam is readily scalable to industrial applications in energy storage devices such as li-ion battery and supercapacitor.

{\rm SnO}_{2}

A series of counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) was fabricated using different weight ratios of electrospun carbon nanofibers (ECNs) and carbon nanoparticles (CNPs). The conductivity of neat ECN was 838 S/m, which is more than twice than that of neat CNP, and the bulk resistance of CEs decreased as the ECN ratios increased in the composite, leading to lower transport resistance in the CEs. However, as the concentration of CNPs increased, the surface area of CEs also improved because CNPs have a much smaller dimension than ECNs, leading to higher electrocatalytic property. The CEs with higher ratio of CNPs possessed several superiorities compared with those with higher ratio ECNs, such as larger surface area for triiodide reduction, faster reaction rate, and less charge transfer resistance at the interface of CE and electrolyte. Evidenced from cyclic voltammograms and electrochemical impedance spectroscopy, the devices with higher ratio CNPs exhibited lower Nernst diffusion impedance and higher efficiency electrocatalytic performance than those with higher ratio ECNs. When the materials of CE switched from neat ECN to those with a higher concentration of CNPs, the DSSC fill factor, current density, and efficiency were improved.