Jayraj V. Vaghasiya

Improved molecular architecture of D–π–A carbazole dyes: 9% PCE with a cobalt redox shuttle in dye sensitized solar cells

Two new D–π–A dyes (SK2 & SK3) based on carbazole, and vinylene-phenylene (π-bridge) with rhodanine-3-acetic acid and cyanoacrylic acid as electron withdrawing–injecting as well as anchoring groups were designed and synthesised under conditions that were free from precious metal-catalysts and well characterized for dye-sensitized solar cell (DSSC) applications. A high power conversion efficiency (PCE) of 9% (AM 1.5 G, 100 mW cm−2) has been achieved using cyanoacrylic acid as an acceptor in D–π–A carbazole dye (SK3) with a cobalt based redox shuttle, while a PCE of 7.1% was exhibited in triiodide based redox mediators. A short-circuit current density, Jsc of ∼18.2 mA cm−2, an open-circuit voltage, Voc of ∼725 mV, and a fill factor, FF of ∼67% have been afforded by the SK3 based DSSC incorporating the Co2+/Co3+ electrolyte as the one-electron redox mediator. In contrast, SK2 dye based DSSCs with a cobalt based redox mediator have shown a Jsc ≈ 8.4 mA cm−2, a Voc ≈ 587 mV, and a FF ≈ 48%, yielding a PCE of 2.4%. The devices based on SK3 showed outstanding stability performance without significant degradation even after 1000 h of illumination under standard conditions in the Co2+/Co3+ electrolyte.

Role of a phenothiazine/phenoxazine donor in solid ionic conductors for efficient solid state dye sensitized solar cells

Efficient electron donors, phenothiazine (PTZ)/phenoxazine (POZ) substituted imidazolium (IMI) and benzimidazolium (BIMI) iodide solid organic ionic conductors (SOICs) possessing good thermal stability and high conductivity are synthesized. The high conductivity arises due to the presence of the effective electron donor moiety PTZ/POZ and hence, these SOICs were used as single component solid electrolytes in solid state dye sensitized solar cells (ss-DSSCs). The ss-DSSC devices operate proficiently without any post-treatment to dye loaded TiO2 and additives in the electrolyte matrix. The presence of unsaturation and hetero-atoms in PTZ/POZ is responsible for the hole mobility and enhancement in light harvesting properties. Hence, when SOICs were excited along with a metal-free sensitizer, SK 1,the higher LUMO levels of SOICs increased the total electron injection in the TiO2 interface with the electrons of SK 1. Under illumination of solar light (100 mW cm−2 with an AM1.5G filter), ss-DSSCs with POZ substituted IMI and BIMI as single component solid electrolytes showed power conversion efficiency (PCE) of about 5.7%, which is quite comparable with the conventional imidazolium/benzimidazolium salt based liquid electrolytes and SK 1 sensitizer. The ss-DSSC devices with all four SOICs exhibit good long-term stability (∼1000 h) under 1 sun illumination and ambient humidity conditions. The present report paves the way for the development of single component solid organic ionic conductors having high electronic conductivity and better light harvesting ability as well as thermal stability.

Humic Acid as a Sensitizer in Highly Stable Dye Solar Cells: Energy from an Abundant Natural Polymer Soil Component

Humic acid (HA), a natural polymer and soil component, was explored as a photosensitizer in dye-sensitized solar cells (DSSCs). Photophysical and electrochemical properties show that HA covers a broad visible range of the electromagnetic spectrum and exhibits a quasi-reversible nature in cyclic voltammetry (CV). Because of its abundant functionalities, HA was able to bind onto the nano-titania surface and possessed good thermal stability. HA was employed as a sensitizer in DSSCs and characterized by various photovoltaic techniques such as I–V, incident-photo-to-current conversion efficiency (IPCE), electrochemical impedance spectroscopy (EIS), and Tafel polarization. The HA-based device shows a power conversion efficiency (PCE) of 1.4% under 1 sun illumination. The device performance was enhanced when a coadsorbent, chenodeoxycholic acid (CDCA), along with HA was used and displayed 2.4% PCE under 0.5 sun illumination. The DSSCs employing HA with CDCA showed excellent stability up to 1000 h. The reported efficiency of devices with HA is better than that of devices with all natural sensitizers reported so far.

Sulphonate anchored hemicyanine dyes for dye solar cell: A study on dipole moment and polarity

The effect of the molecular architecture of dye on the performance of a hemicyanine based dye sensitized solar cell (DSSC) has been studied using theoretical and experimental tools. These dyes were utilized as sensitizers in DSSC and then later were characterized using various techniques like I-V, impedance, electron transfer kinetics, dielectrics, and Tafel polarization in order to study the photovoltaic performance. The dye having a p-OH group (INS 1) exhibited very good photon to electricity conversion efficiency (3%) along with good fill factor while the dye having a p-OMe group (INS 2) lagged behind in performance. The substituent effect was studied theoretically as well as experimentally emphasizing the dipole moment of the dye in various directions. Theoretical investigation reveals that the presence of a p–OH group in the donor unit enhances the dipole moment of the molecule in comparison to a p-OMe group, thereby imparting longer lifetime of excited state leading to better charge transfer at the ...

Structure-efficiency relationship of newly synthesized 4-substituted donor–π–acceptor coumarins for dye-sensitized solar cells

Four (MC1–MC4) 4-substituted coumarin dyes having N,N-diethyl aniline as donor and rhodanine-3-acetic acid as acceptor have been synthesized for use in nano-crystalline TiO2-based dye-sensitized solar cells (DSSCs). The alteration at the 4-position of coumarin was carried out by substituting H atom by chlorine (–Cl), piperidine (–NC5H10), and cyano group (–CN) to study the effect of substitution on the efficiency of the DSSCs. All the dyes were characterised by 1H-NMR and 13C-NMR spectroscopy and CHN analysis. The absorption and emission spectra of these coumarin dyes were recorded in eight different solvents and show positive solvatochromism. The TiO2-based DSSCs were fabricated using the four dyes and their photovoltaic properties were estimated. A maximum efficiency of 4.60% was obtained for the MC4 device, which has an electron-withdrawing cyano group at the 4-position of coumarin, while 2.64% efficiency was obtained for the MC1 device, which has only hydrogen at the 4-position. The efficiencies of the MC2 and MC3 devices lie between those of MC1 and MC4, suggesting the significance of substitution at the 4-position. The introduction of a cyano group, which acts as an electron trap, increases the electron-withdrawing capacity of the dye; thus, more electron density is withdrawn from the donor, resulting in enhanced efficiency. These dyes were studied by using density functional theory (DFT) and time dependent density functional theory (TD-DFT) to obtain the vertical excitation, HOMO–LUMO energy levels, band gap and electron distribution in the ground and excited state. The calculated values showed good correlation with the experimental values. Our results suggest that substitution at the 4-position is essential for enhancing the efficiency of coumarin-based DSSCs.

Dual functional hetero-anthracene based single component organic ionic conductors as redox mediator cum light harvester for solid state photoelectrochemical cells

We have synthesized a novel solid organic ionic conductor (SOIC) that acts as a redox mediator and a light absorbing material at the same time. Such dual function of SOICs has not been reported before. It was achieved by substituting N,N′-dimethyl benzimidazolium iodide (BIMI) with hetero-anthracene (phenoxazine (POZ)/phenothiazine (PTZ)) which are labeled as SOIC-1 and SOIC-2, respectively. These substitutions caused the absorption spectrum of BIMI to be extended over 100 nm into the red spectrum and at the same time exhibit excellent redox capability and ionic conductivity. In addition, these synthesized SOICs also enhance the total electron injection into the TiO2 matrix with the metal-free SK3 dye sensitizer. Density functional theory (DFT) was used to optimize the structure and geometrical arrangement of the SOICs. To evaluate the role of PTZ/POZ substitution on BIMI and how pore filling can affect the device performance, solid-state DSSC (ss-DSSC) devices were prepared with two different thickness of TiO2 (8 μm and 12 μm) photoanode. The 8 μm thick TiO2 photoanode with SOIC-1 has an overall PCE of 7.9% which is about 72% higher than the unmodified BIMI (PCE of 4.4%) under AM1.5 illumination. Last but not least, the synthesized SOICs have high thermal stability up to 120 °C which is way beyond the operating temperatures of solar cells which make them ideal for real-world applications.

Low toxicity environmentally friendly single component aqueous organic ionic conductors for high efficiency photoelectrochemical solar cells

Photoelectrochemical solar cells have gained impetus over the past two decades owing to several advantages over conventional Si solar cells. The redox capabilities of the electrolyte play a major role in determining the overall photo-conversion efficiency (PCE) of dye sensitized solar cells. In this work we report a novel low toxicity and environmentally friendly aqueous organic ionic conductor (AOIC) for high efficiency DSSCs. This study presents the effect of water on the photocurrent density (Jsc) and stability of DSSCs. Interestingly we have also found that the addition of water to the OICs strongly alters the flat band potential (Efb) of nano-crystalline TiO2, hinders the recombination at the TiO2–AOIC interface and positively shifts the redox potential of I−/I3−. Under 1 sun illumination, OICs with 3-methoxypropionitrile solvent and OICs in water show a PCE of 6.02% and 3.57% respectively. This reduced PCE and J–V characteristics are attributed to the negative influence of water on the electron lifetime, electron recombination kinetics and the dark current. Substituting the electron withdrawing group present in the para-position of pyridinium salts mitigates the toxicity and enhances the cationic effect in the electrolyte. The DSSCs fabricated using AOICs showed excellent stability for more than 20 days.

A Barbeque-Analog Route to Carbonize Moldy Bread for Efficient Steam Generation

Summary Sustainable reconversion of the large quantities of food waste generated every day is pivotal for a green urban development in future. Herein, we put forth a sustainable and cost-effective way to repurpose a commonly used food waste for solar steam generation, an important part of water desalination. Making use of moldy bread, a new route for steam generation is demonstrated. The moldy bread was converted into a solar absorber by a simple and cost-effective carbonization process mimicking outdoor barbeque cooking. Carbonizing food waste to facilitate better absorption of sunlight for effective evaporation of water is an unprecedented concept in this field. Interestingly, the carbonized bread repurposed from the food waste served as an effective solar steam generator with an efficiency as high as 71.4% under 1 sun illumination. The structural and thermal absorption properties of the carbonized bread facilitated efficient solar energy absorption, heat management, and water transpiration in the system.

A Smart Flexible Solid State Photovoltaic Device with Interfacial Cooling Recovery Feature through Thermoreversible Polymer Gel Electrolyte

Quasi-solid-state dye-sensitized solar cells (DSSCs) fabricated with lightweight flexible substrates have a great potential in wearable electronic devices for in situ powering. However, the poor lifespan of these DSSCs limits their practical application. Strong mechanical stresses involved in practical applications cause breakage of the electrode/electrolyte interface in the DSSCs greatly affecting their performance and lifetime. Here, a mechanically robust, low-cost, long-lasting, and environment-friendly quasi-solid-state DSSC using a smart thermoreversible water-based polymer gel electrolyte with self-healing characteristics at a low temperature (below 0 °C) is demonstrated. When the performance of the flexible DSSC is hindered by strong mechanical stresses (i.e., from multiple bending/twisting/shrinking actions), a simple cooling treatment can regenerate the electrode/electrolyte interface and recover the performance close to the initial level. A performance recovery as high as 94% is proven possible even after 300 cycles of 90° bending. To the best of our knowledge, this is the first aqueous DSSC device with self-healing behavior, using a smart thermoreversible polymer gel electrolyte, which provides a new perspective in flexible wearable solid-state photovoltaic devices.