Sneha A. Kulkarni

High Efficiency Solid-State Sensitized Solar Cell-Based on Submicrometer Rutile TiO2 Nanorod and CH3NH3PbI3 Perovskite Sensitizer

We report a highly efficient solar cell based on a submicrometer (~0.6 μm) rutile TiO2 nanorod sensitized with CH3NH3PbI3 perovskite nanodots. Rutile nanorods were grown hydrothermally and their lengths were varied through the control of the reaction time. Infiltration of spiro-MeOTAD hole transport material into the perovskite-sensitized nanorod films demonstrated photocurrent density of 15.6 mA/cm(2), voltage of 955 mV, and fill factor of 0.63, leading to a power conversion efficiency (PCE) of 9.4% under the simulated AM 1.5G one sun illumination. Photovoltaic performance was significantly dependent on the length of the nanorods, where both photocurrent and voltage decreased with increasing nanorod lengths. A continuous drop of voltage with increasing nanorod length correlated with charge generation efficiency rather than recombination kinetics with impedance spectroscopic characterization displaying similar recombination regardless of the nanorod length.

Laminated Carbon Nanotube Networks for Metal Electrode-Free Efficient Perovskite Solar Cells

Organic-inorganic metal halide perovskite solar cells were fabricated by laminating films of a carbon nanotube (CNT) network onto a CH3NH3PbI3 substrate as a hole collector, bypassing the energy-consuming vacuum process of metal deposition. In the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 6.87%. The CH3NH3PbI3/CNTs solar cells were semitransparent and showed photovoltaic output with dual side illuminations due to the transparency of the CNT electrode. Adding spiro-OMeTAD to the CNT network forms a composite electrode that improved the efficiency to 9.90% due to the enhanced hole extraction and reduced recombination in solar cells. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. The flexible and transparent CNT network film shows great potential for realizing flexible and semitransparent perovskite solar cells.

Band-gap tuning of lead halide perovskites using a sequential deposition process

Band-gap tuning of mixed anion lead halide perovskites (MAPb(I1−xBrx)2 (0 ≤ x ≤ 1)) has been demonstrated by means of a sequential deposition process. The optical properties of perovskite hybrids can be flexibly modified by changing (mixing) the concentration of halogen precursors. The concentrations of precursor solution as well as the conversion time play an important role in determining the band-gap of perovskites. A systematic shift of the absorption band edge to shorter wavelengths is observed with increasing Br content in the perovskite films, which results in the decrement of the photocurrent. Nanorod like morphological features are also observed for perovskite films with an iodide to bromide molar ratio of <0.7.

A large area (70 cm2) monolithic perovskite solar module with a high efficiency and stability

Monolithic perovskite modules with active areas of 31 cm2 and 70 cm2 and with power conversion efficiencies (PCE) of 10.46% and 10.74%, respectively, were fabricated using scalable printing processes. An ambient stability of more than 2000 h with less than a 5% reduction in efficiency is demonstrated. The electrical quality of the mesoscopic hole transporter and its facilitation of efficient infiltration is paramount.

A selective co-sensitization approach to increase photon conversion efficiency and electron lifetime in dye-sensitized solar cells

Ruthenium-based C106 and organic D131 sensitizers have been judicially chosen for co-sensitization due to their complementary absorption properties and different molecular sizes. Co-sensitization yields a higher light-harvesting efficiency as well as better dye coverage to passivate the surface of TiO(2). The co-sensitized devices C106 + D131 showed significant enhancement in the performance (η = 11.1%), which is a marked improvement over baseline devices sensitized with either D131 (η = 5.6%) or C106 (η = 9.5%). The improved performance of the co-sensitized cell is attributed to the combined enhancement in the short circuit current, open circuit voltage, and the fill-factor of the solar cells. J(sc) is improved because of the complementary absorption spectra and favorable energy level alignments of both dyes; whereas, V(oc) is improved because of the better surface coverage helping to reduce the recombination and increase the electron life time. The origins of these enhancements have been systematically studied through dye desorption, absorption spectroscopy, and intensity modulated photovoltage spectroscopy investigations.

Nanoclay Gelation Approach toward Improved Dye-Sensitized Solar Cell Efficiencies: An Investigation of Charge Transport and Shift in the TiO2 Conduction Band

Nanoclay minerals play a promising role as additives in the liquid electrolyte to form a gel electrolyte for quasi-solid-state dye-sensitized solar cells, because of the high chemical stability, unique swelling capability, ion exchange capacity, and rheological properties of nanoclays. Here, we report the improved performance of a quasi-solid-state gel electrolyte that is made from a liquid electrolyte and synthetic nitrate-hydrotalcite nanoclay. Charge transport mechanisms in the gel electrolyte and nanoclay interactions with TiO(2)/electrolyte interface are discussed in detail. The electrochemical analysis reveals that the charge transport is solely based on physical diffusion at the ratio of [PMII]:[I(2)] = 10:1 (where PMII is 1-propyl-3-methylimidazolium iodide). The calculated physical diffusion coefficient shows that the diffusion of redox ions is not affected much by the viscosity of nanoclay gel. The addition of nitrate-hydrotalcite clay in the electrolyte has the effect of buffering the protonation process at the TiO(2)/electrolyte interface, resulting in an upward shift in the conduction band and a boost in open-circuit voltage (V(OC)). Higher V(OC) values with undiminished photocurrent is achieved with nitrate-hydrotalcite nanoclay gel electrolyte for organic as well as for inorganic dye (D35 and N719) systems. The efficiency for hydrotalcite clay gel electrolyte solar cells is increased by 10%, compared to that of the liquid electrolyte. The power conversion efficiency can reach 10.1% under 0.25 sun and 9.6% under full sun. This study demonstrates that nitrate-hydrotalcite nanoclay in the electrolyte not only solidifies the liquid electrolyte to prevent solvent leakage, but also facilitates the improvement in cell efficiency.

Investigation of the role of anions in hydrotalcite for quasi-solid state dye-sensitized solar cells application

In recent research in clean energy applications, clay has gained significant interest, especially as a gelator in dye-sensitized solar cells (DSSCs), for its capability to resolve the leakage issue of liquid electrolyte. In this paper, anionic hydrotalcite is utilized as a gelator to assist the formation of gel electrolyte in DSSCs. Three types of hydrotalcite with exchangeable anions, viz. NO3− (CL-N), CO32− (CL-C) and SO42− (CL-S), were synthesized with similar morphologies via the co-precipitation method. It is observed that the gel formation of hydrotalcite strongly depends on the exchangeable anions present in the hydrotalcite. The objective of this work is to understand the effect of hydrotalcite anions on the photovoltage and the photocurrent in the gel electrolyte through electrochemical analysis. With increasing ion affinity, the Voc increases. This is attributed to Li+ intercalation with hydrotalcite compound resulting in the elevation of the conduction band of TiO2. With increasing ion affinity, the Jsc decreases. This is attributed to the decreasing diffusion coefficient of triiodide and the increasing difficulty in the injection process. For anions with low ion affinity in hydrotalcite, the diffusion of the redox couple is not significantly affected by the high viscosity of the gel. Furthermore, the study indicates that proper selection of hydrotalcite compounds not only produces a quasi-solid gel electrolyte, but also increases the efficiency of the solar cells: the device performance was improved from 7.8% (liquid electrolyte) to 8.4% (hydrotalcite gel electrolyte).

Wire-shaped perovskite solar cell based on TiO2 nanotubes

In this work, a wire-shaped perovskite solar cell based on TiO2 nanotube (TNT) arrays is demonstrated for the first time by integrating a perovskite absorber on TNT-coated Ti wire. Anodization was adopted for the conformal growth of TNTs on Ti wire, together with the simultaneous formation of a compact TiO2 layer. A sequential step dipping process is employed to produce a uniform and compact perovskite layer on top of TNTs with conformal coverage as the efficient light absorber. Transparent carbon nanotube film is wrapped around Ti wire as the hole collector and counter electrode. The integrated perovskite solar cell wire by facile fabrication approaches shows a promising future in portable and wearable textile electronics.

Evolution of hydrogen by few-layered black phosphorus under visible illumination

Recently, a new class of two-dimensional black phosphorus (BP) with a visible direct band gap is predicted as a potential candidate for photo-catalysis applications. Here, we present the first experimental evidence of hydrogen (H2) evolution from aqueous solution by using BP (nanosheets and nanoparticles) under visible light illumination. Our experimental results describe that liquid phase exfoliated BP nanosheets and BP nanoparticles exhibit suitable energy level alignments for electron transfer and further proton reduction reactions in the solution under visible light illumination. Density functional theory (DFT) calculations predict that the H2 evolution activity of bilayer BP is independent of edge or center positions, which is unique in BP as compared to those of other 2D materials.

Modulating Excitonic Recombination Effects through One-Step Synthesis of Perovskite Nanoparticles for Light-Emitting Diodes

The primary advantages of halide perovskites for light-emitting diodes (LEDs) are solution processability, direct band gap, good charge-carrier diffusion lengths, low trap density, and reasonable carrier mobility. The luminescence in 3 D halide perovskite thin films originates from free electron-hole bimolecular recombination. However, the slow bimolecular recombination rate is a fundamental performance limitation. Perovskite nanoparticles could result in improved performance but processability and cumbersome synthetic procedures remain challenges. Herein, these constraints are overcome by tailoring the 3 D perovskite as a near monodisperse nanoparticle film prepared through a one-step in situ deposition method. Replacing methyl ammonium bromide (CH3 NH3 Br, MABr) partially by octyl ammonium bromide [CH3 (CH2 )7 NH3 Br, OABr] in defined mole ratios in the perovskite precursor proved crucial for the nanoparticle formation. Films consisting of the in situ formed nanoparticles displayed signatures associated with excitonic recombination, rather than that of bimolecular recombination associated with 3 D perovskites. This transition was accompanied by enhanced photoluminescence quantum yield (PLQY≈20.5 % vs. 3.40 %). Perovskite LEDs fabricated from the nanoparticle films exhibit a one order of magnitude improvement in current efficiency and doubling in luminance efficiency. The material processing systematics derived from this study provides the means to control perovskite morphologies through the selection and mixing of appropriate additives.