Keval K. Sonigara

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.

A Smart Flexible Zinc Battery with Cooling Recovery Ability

Flexible batteries are essential for wearable electronic devices. To meet practical applications, they need to be mechanically robust and stable. However, strong or multiple bending may sever the interfacial contact between electrode and electrolyte, causing capacity fading or even battery failure. Herein we present a new cooling-recovery concept for flexible batteries, which involves a temperature-sensitive sol-gel transition behavior of the thermoreversible polymer hydrogel electrolyte. Once a battery has suffered from strong mechanical stresses, a simple cooling process can refresh the electrode-electrolyte interface. The energy-storage capability can be recovered with a healing efficiency higher than 98 %. It is believed that this study not only offers new valuable insights, but also opens up new perspectives to develop functional wearable devices.

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.

Effect of structural manipulation in hetero-tri-aryl amine donor-based D–A′–π–A sensitizers in dye-sensitized solar cells

The role of hetero-atom manipulation/hetero-aryl group insertion in the triarylamine to obtain hetero triarylamine as a donor in highly efficient photosensitizers was investigated to study the structure–efficiency relationship in dye-sensitized solar cells (DSSCs). A newly synthesized sensitizer was explored containing N-phenyl-N-(pyridin-2-yl) pyridine-2-amine (DPPA) and N-(pyridin-2-yl)-N-(thiophen-2-yl) pyridine-2-amine (DPTA) as the donor along with a strong electron-withdrawing cyano group (–CN) as the auxiliary acceptor group and cyanoacetic acid and rhodamine-3-acetic acid as anchoring groups. The triphenylamine donor was manipulated for the first time with the insertion of a nitrogen atom in the aryl ring for DSSCs. These hetero-aryl-based sensitizers showed a significant improvement in the photophysical as well as photovoltaic performance. The replacement of cyanoacetic acid by rhodanine-3-acetic acid as an anchoring unit resulted in a significant red-shift in absorption as well as emission maxima. The methylene group in rhodanine-3-acetic acid interrupted the LUMO delocalization on the anchoring group in sensitizers DP3 and DP4, as shown by DFT calculations. The presence of cyanoacetic acid in sensitizers DP1 and DP2 showed effective charge transfer from HOMO to LUMO and efficient electron injection from LUMO to the conduction band of the TiO2 semiconductor. The sensitizer DP2 showed a maximum efficiency of 4.7%, a short-circuit current Jsc = 11.78 mA cm−2, an open-circuit voltage Voc = 0.608 V and a fill factor FF = 0.62. The enhanced efficiency of sensitizer DP2 was attributed to the presence of the strong electron-withdrawing cyanoacetic acid anchoring group and the presence of the thiophene linker at the N-aryl core.

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.

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.