Mohammad Shaad Ansari

Graphitic carbon nitride as a photovoltaic booster in quantum dot sensitized solar cells: a synergistic approach for enhanced charge separation and injection

A ∼70% improvement in power conversion efficiency (PCE, η) is observed for the devices fabricated with a binary hybrid composite of graphitic carbon nitride and zinc oxide nanorods, i.e., (g-C3N4–ZnO NR) [η ≈ 2.43%, for an optimized weight ratio (0.5 : 1)] as compared to the pristine ZnO NR device (η ≈ 0.65%). Systematic investigations reveal that g-C3N4 boosts the light harvesting ability of the photovoltaic devices primarily by impeding photo-induced electron interception to the redox couple and injecting electrons into the conduction band of the semiconductor. Electrochemical impedance spectroscopy (EIS) analysis shows a reduced tunneling of photo-induced electrons to the sulfide–polysulfide (S2−/Sn2−) redox shuttle in the case of (g-C3N4–ZnO NR) composite devices. Higher recombination resistance (Rk) indicates that the g-C3N4 sheet acts as a barrier for photo-induced electron interception at the working electrode/electrolyte interface. Preliminary investigation using steady state and dynamic photoluminescence analyses suggest a similar fact about the photo-induced electron injection from g-C3N4 sheets to ZnO, contributing to the enhanced light harvesting ability of (g-C3N4–ZnO NR) composite devices.

Ethyl Cellulose and Cetrimonium Bromide Assisted Synthesis of Mesoporous, Hexagon Shaped ZnO Nanodisks with Exposed ±{0001} Polar Facets for Enhanced Photovoltaic Performance in Quantum Dot Sensitized Solar Cells

Hexagon shaped mesoporous zinc oxide nanodisks (ZnO NDs) with exposed ±{0001} polar facets have been synthesized by using ethyl cellulose (EC) and cetrimonium bromide (CTAB) as the capping and structure directing agents. We have characterized ZnO NDs using analytical techniques, such as powder X-ray diffraction (PXRD), diffuse reflectance UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) surface area analysis and proposed a plausible mechanism for the formation of ZnO NDs. EC molecules form a colloidal solution in a 1-butanol:water (3:1) solvent system having a negative zeta potential (ζ ≈ -32 mV) value which can inhibit CTAB assisted c-axis growth of ZnO crystal and encourage the formation of ZnO NDs. In the control reactions carried out in presence of only CTAB and only EC, formation of hexagonal ZnO nanorods (NRs) and ZnO nanosheets (NSs) composed of numerous ZnO nanoparticles are observed, respectively. Photovoltaic properties of ZnO NDs as compared to ZnO NRs, ZnO NSs, and conventional ZnO nanoparticles (NPs) are investigated by co-sensitizing with CdS/CdSe quantum dots (QDs). An ∼35% increase in power conversion efficiency (PCE, η) is observed in ZnO NDs (η ≈ 4.86%) as compared to ZnO NPs (η ≈ 3.14%) while the values of PCE for ZnO NR and ZnO NS based devices are found to be ∼2.52% and ∼1.64%, respectively. Enhanced photovoltaic performance of the ZnO NDs based solar cell is attributed to an efficient charge separation and collection, boosted by the exposed ±(0001) facets apart from the single crystalline nature, better light-scattering effects, and high BET surface area for sensitizer particle adsorption. Electrochemical impedance spectroscopy (EIS) analysis further reveals that the charge recombination resistance and photoinduced electron lifetime are substantially higher in the ZnO ND based device than in ZnO NR, ZnO NP, and ZnO NS based devices, which demonstrates a slower electron-hole (e(-)-h(+)) recombination rate and faster charge migration through the single crystalline ZnO NDs.

Enhanced photovoltaic performance of meso-porous SnO2 based solar cells utilizing 2D MgO nanosheets sensitized by a metal-free carbazole derivative

Herein, we report the power conversion efficiency (PCE) of ∼3.71%, achieved in a mesoporous SnO2 based solar cell by introducing 15 wt% of 3D porous hierarchical MgO composed of 2D nanosheets by a simple sonochemical route followed by a mixing process, sensitized with a metal-free carbazole dye, namely, 2-cyano-3-(4-(2-(9-p-tolyl-9H-fluoren-6-yl)vinyl)phenyl)acrylic acid (i.e. SK1 dye). We have optimized the performance of solar cell devices with the addition of MgO and performed a comparative study on photovoltaic performances of the fabricated devices, such as SnO2–MgO with pristine SnO2, by employing two different redox mediators, namely, (I−/I3−) and [Co(bpy)3]2+/3+. We observed a significant improvement in the open circuit voltage (Voc) and fill factor (FF) for the SnO2–MgO based dye sensitized solar cell (DSSC) over the pristine SnO2 device, i.e. from 357 mV to 550 mV and from ∼38% to ∼50% respectively, wherein an ∼60% enhancement in PCE for the SnO2–MgO device as compared to bare SnO2 device using the redox mediator (I−/I3−) is achieved. Interestingly, in the case of cobalt tris(2,2′-bipyridyl) redox shuttle, we observed further improvement in the PCE value of the photovoltaic device by 74%, i.e. from ∼1% (pristine SnO2 device, redox mediator I−/I3−) to ∼3.71% (SnO2–MgO device). From electrochemical impedance spectroscopy (EIS) of the devices, we conclude that the charge transfer resistance at the SnO2–MgO/SK1/electrolyte interfaces is lower as compared to SnO2/SK1/electrolyte interfaces, which demonstrates faster charge migration across the interfaces and a slower electron–hole (e−–h+) recombination rate. It was observed that in the presence of MgO, the life time of photoinduced electrons in the conduction band (CB) of SnO2 microspheres increases to τe = 15.9 ms from 7.1 ms (for pristine SnO2 device), by employing [Co(bpy)3]2+/3+ as a redox shuttle, and thus resulting in higher values of open circuit voltage (Voc) and short circuit current density (Jsc). Therefore, hierarchical MgO not only improves the PCE of the photovoltaic devices by minimizing the leakage of trapped electrons in the conduction band level of SnO2/electrolyte interface but also provides more surface area for the adsorption of dye molecules.

Understanding the role of silica nanospheres with their light scattering and energy barrier properties in enhancing the photovoltaic performance of ZnO based solar cells

The present study discusses the design and development of a dye sensitized solar cell (DSSC) using a hybrid composite of ZnO nanoparticles (ZnO NP) and silica nanospheres (SiO2 NS). A ≈22% enhancement in the overall power conversion efficiency (PCE, η) was observed for the device fabricated with a binary hybrid composite of 1 wt% SiO2 NS and ZnO NP compared to the pristine ZnO NP device. A systematic investigation revealed the dual function of the silica nanospheres in enhancing the device efficacy compared to the bare ZnO NP based device. Sub-micron sized SiO2 NS can boost the light harvesting efficiency of the photoanode by optical confinement, resulting in increased propagation length of the incident light by multiple internal reflections, which was confirmed by UV-Vis diffused reflectance spectroscopy. Electrochemical impedance spectroscopic (EIS) analysis showed a reduced recombination of photo-generated electrons to the I-/I3- redox shuttle in the case of the composite photoanode. The higher recombination resistance (Rct) in the case of a 1 wt% composite indicates that the SiO2 NS serves as a partial energy barrier layer to retard the interfacial recombination (back transfer) of photo-generated electrons at the working electrode/electrolyte interface, increasing the device efficiency.

Cs-Symmetric Triphenylamine-Linked Bisthiazole-Based Metal-Free Donor–Acceptor Organic Dye for Efficient ZnO Nanoparticles-Based Dye-Sensitized Solar Cells: Synthesis, Theoretical Studies, and Photovoltaic Properties

Herein, we have designed a metal-free donor–acceptor dye by incorporating an electron deficient bisthiazole moiety as a linker in between the electron donor triphenylamine and cyanoacetic acid acceptor. The bisthiazole-based organic dye D1 was synthesized using the Pd-catalyzed Suzuki cross-coupling and Knoevenagel condensation reactions. On the basis of the optical, electrochemical, and computational studies, dye D1 showed a better electronic interaction between the donor and acceptor moieties. As-synthesized C2 symmetric triphenylamine-linked bisthiazole-based organic dye D1 has four anchoring groups, which play a significant role for better adsorption on the ZnO surface along with the enhanced kinetics of photoexcited electron injection. Consequently, photovoltaic properties of the organic dye D1 has been carried out by fabricating the ZnO nanoparticles (ZnO NPs)-based solar device. We obtained the maximum incident photon-to-current conversion efficiency of about 56.20%, with a short-circuit photocurrent density (Jsc) of 13.60 mA cm–2, which results in a power conversion efficiency (PCE) of 4.94% under AM 1.5 irradiation (100 mW cm–2). The high PCE value is the result of proficient electron injection from ELUMO of dye D1 to the conduction band of ZnO NPs, as suggested by the computational calculations. Electrochemical impedance spectroscopy measurement is carried out to calculate the electron lifetime and also reveals the insight to the reduced charge recombinations at the various active interfaces of the photovoltaic device.

Multifunctional hierarchical 3-D ZnO superstructures directly grown over FTO glass substrates: enhanced photovoltaic and selective sensing applications

Ammonia has been extensively utilized in many applications such as agrochemicals, pharmaceuticals, organic dyes, synthetic fibres, and it can diffuse into the atmosphere and cause severe effects on human health as well as the environment. Ammonium nitrate is usually found in many explosives and they release trace amounts of ammonia upon decomposition, the monitoring of which is very crucial in order to prevent lethal accidents. It is therefore necessary to develop a highly-sensitive room-temperature-efficient NH3 gas sensor. Sensing devices that rely on electron transport often suffer from the drawback of higher ohmic contact between the active materials and the collecting electrodes, which are transparent conducting oxides in most cases. Designing such systems is important, especially when the vapor pressure of the compounds yield very low concentrations of sensing elements. Herein, we report the in situ growth of hierarchical three-dimensional zinc oxide superstructures over a conductive glass substrate, i.e., fluorine-doped tin oxide, under a controlled hydrothermal route for low ohmic contact, allowing efficient charge injection. An anionic polysaccharide “k-carrageenan” was employed for assisting the heteroepitaxial aggregated growth of the 1-D nanocrystals. We have successfully demonstrated the applications of the as-characterized multifunctional 3-D ZnO hierarchical structures in photovoltaic and selective chemical vapor sensing. A significant enrichment (∼33%) in power conversion efficiency (η) for the hierarchical 3-D ZnO superstructure-based photovoltaic device, as compared to the 1-D ZnO nanowires, was observed, mainly due to the larger surface to volume ratio for sensitizer loading, better light-scattering effect, better charge separation and collection. Two terminal sensor devices displayed high sensitivity and selectivity for NH3 vapors with the limit of detection value of ∼5 (±3%) parts per billion (ppb) for three dimensional ZnO hierarchical superstructures and ∼17 (±3%) ppb for 1-D ZnO NWs, which is very small as compared to the maximum permissible limit, i.e., 25 parts per million (ppm). Selectivity, recyclability, response/recovery time and sensitivity toward primary, secondary and tertiary amines have been studied to understand the probable mechanism for the high sensing ability of the hierarchical superstructures.