Siva Krishna Karuturi

Quantum‐Dot‐Sensitized TiO2 Inverse Opals for Photoelectrochemical Hydrogen Generation

A new nanoarchitecture photoelectrode design comprising CdS quantum-dot-sensitized, optically and electrically active TiO(2) inverse opals is developed for photoelectrochemical water splitting. The photoelectrochemical performance shows high photocurrent density (4.84 mA cm(-2) at 0 V vs. Ag/AgCl) under simulated solar-light illumination.

Photon Upconversion in Hetero‐nanostructured Photoanodes for Enhanced Near‐Infrared Light Harvesting

A hetero-nanostructured photoanode with enhanced near-infrared light harvesting is developed for photo-electrochemical cells. By spatially coating upconversion nanoparticles and quantum dot photosensitizers onto TiO2 inverse opal, this architecture allows direct irradiation of upconversion nanoparticles to emit visible light that excites quantum dots for charge separation. Electrons are injected into TiO2 with minimal carrier losses due to continuous electron conducting interface.

TiO2 inverse-opal electrode fabricated by atomic layer deposition for dye-sensitized solar cell applications

TiO2 inverse opals (TIO) fabricated by the atomic layer deposition (ALD) technique showed a superior infiltration result when compared to those fabricated by the conventional nanoparticles-infiltration method reported in previous studies. The ALD can achieve high filling fractions of more than ca. 96% of the maximum possible infiltration by conformal filling of 288, 390 and 510 nm opals, giving rise to high quality TIO. The photoelectrochemical performances of the ALD-fabricated TIO photoanodes of different sizes are investigated systematically for the first time in dye-sensitized solar cells (DSCs). When the TIO with a size of 288 nm was used as photoanode and indoline dye as a sensitizer in DSCs, the power conversion efficiency of the cell could attain 2.22% (Air Mass 1.5). It is found that the efficiency increases with decreasing lattice size of TIO electrode due to the larger surface area for dye loading. Owing to the selective reflectivity of the inverse opal, IPCE spectra of TIO electrodes revealed a strong wavelength dependence. Strategies relating to the characteristics of selective reflection and the design of composite photoanodes to enhance the efficiency of DSCs are discussed.

Homogeneous Photosensitization of Complex TiO2 Nanostructures for Efficient Solar Energy Conversion

TiO2 nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO2 in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO2 such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO2 substrates. PEC tests made to CdSe-sensitized TiO2 inverse opal photoanodes result in a drastically improved photocurrent level, up to ~15.7 mA/cm2 at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction.

A Novel Photoanode with Three‐Dimensionally, Hierarchically Ordered Nanobushes for Highly Efficient Photoelectrochemical Cells

In recent years, photoelectrochemical (PEC) cells have attracted worldwide attention as cheap alternatives to conventional devices for solar energy conversion. Crucial to the light harvesting and conversion effi ciency of a PEC cell is a nanostructured photoanode, in which the incident photons are captured, electron–hole pairs are generated, and the subsequent electron transfer takes place. [ 1 , 2 ] To realize highly effi cient PEC cells, a nanostructured photoanode should possess several favorable intrinsic characteristics, such as adequate specifi c surface area to permit high photosensitizer loading (in the case of TiO 2 ), direct electron transport pathways for long electron diffusion length, and strong light scattering to promote the light harvesting ability by confi ning the light within the cell. [ 3–6 ] It is thus highly desirable to develop a photoanode that meets all the above requirements. Towards this goal, immense efforts have been concentrated on tailoring the nanometer-scale features of photoanode materials. [ 7 ] Nanoparticle fi lms provide very high surface areas to increase the amount of sensitizer loading, but they lack direct electrical contacts and light-scattering ability. [ 8 , 9 ]

Applications of atomic layer deposition in solar cells

Atomic layer deposition (ALD) provides a unique tool for the growth of thin films with excellent conformity and thickness control down to atomic levels. The application of ALD in energy research has received increasing attention in recent years. In this review, the versatility of ALD in solar cells will be discussed. This is specifically focused on the fabrication of nanostructured photoelectrodes, surface passivation, surface sensitization, and band-structure engineering of solar cell materials. Challenges and future directions of ALD in the applications of solar cells are also discussed.

Inverted Hysteresis in CH3NH3PbI3 Solar Cells: Role of Stoichiometry and Band Alignment

J-V hysteresis in perovskite solar cells is known to be strongly dependent on many factors ranging from the cell structure to the preparation methods. Here we uncover one likely reason for such sensitivity by linking the stoichiometry in pure CH3NH3PbI3 (MAPbI3) perovskite cells with the character of their hysteresis behavior through the influence of internal band offsets. We present evidence indicating that in some cells the ion accumulation occurring at large forward biases causes a temporary and localized increase in recombination at the MAPbI3/TiO2 interface, leading to inverted hysteresis at fast scan rates. Numerical semiconductor models including ion accumulation are used to propose and analyze two possible origins for these localized recombination losses: one based on band bending and the other on an accumulation of ionic charge in the perovskite bulk.

Gradient inverse opal photonic crystals via spatially controlled template replication of self-assembled opals

A method of spatially controlled template filling and replication is reported herein using Knudsen diffusion-limited atomic layer deposition (ALD). Experimental and theoretical investigations based on the porous framework of self-assembled polystyrene opals further confirm the well controlled z-directional TiO2 spatial gradients paving the way for the fabrication of gradient index inverse opal photonic crystals for the first time.

Mechanically-stacked perovskite/CIGS tandem solar cells with efficiency of 23.9% and reduced oxygen sensitivity

A perovskite/CIGS tandem configuration is an attractive and viable approach to achieve an ultra-high efficiency and cost-effective all-thin-film solar cell. In this work, we developed a semi-transparent perovskite solar cell (PSC) with a maximum efficiency of 18.1% at a bandgap of ∼1.62 eV. Combining this cell in a mechanically stacked tandem configuration with a 16.5% CIGS cell results in a tandem efficiency of 23.9%. We also present a semi-transparent high bandgap (∼1.75 eV) PSC with a champion efficiency of 16.0% that enables a tandem efficiency of 23.4%. Optical simulation predicts that a perovskite/CIGS tandem efficiency of over 30% is feasible with a high bandgap perovskite top cell. The multiple-cation perovskite absorbers enabling high tandem efficiencies in this work are found to be remarkably less sensitive towards oxygen exposure compared to the widely used CH3NH3PbI3 (MAPbI3). By combining systematic compositional tuning of perovskite materials and the simultaneous probe of terminal open-circuit voltage (Voc) and Photoluminence (PL) of PSCs, it is deduced that an interaction between methylamonnium (MA) cations and oxygen molecules results in an increased surface recombination rate, and this is the main driver for oxygen-induced degradation. The extraordinary device performance and stability reported in this work pave the way for ultimately realizing the commercialization of all-thin-film photovoltaic technology.

Improved photoelectrochemical performance of GaN nanopillar photoanodes

In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.