Thomas Stergiopoulos

Ultrasmooth organic–inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells

To date, there have been a plethora of reports on different means to fabricate organic-inorganic metal halide perovskite thin films; however, the inorganic starting materials have been limited to halide-based anions. Here we study the role of the anions in the perovskite solution and their influence upon perovskite crystal growth, film formation and device performance. We find that by using a non-halide lead source (lead acetate) instead of lead chloride or iodide, the perovskite crystal growth is much faster, which allows us to obtain ultrasmooth and almost pinhole-free perovskite films by a simple one-step solution coating with only a few minutes annealing. This synthesis leads to improved device performance in planar heterojunction architectures and answers a critical question as to the role of the anion and excess organic component during crystallization. Our work paves the way to tune the crystal growth kinetics by simple chemistry.

Preparation, characterization and photocatalytic activity of nanocrystalline thin film TiO2 catalysts towards 3,5-dichlorophenol degradation

Both opaque and transparent TiO2 nanocrystalline thin films were developed on glass substrates by applying dip coating and doctor-blade deposition techniques, using titanium(IV) butoxide and Degussa P25 TiO2 powder as precursor and starting material, respectively. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) evaluated the surface characteristics of the films. Results on their structure and crystallinity were obtained by means of X-ray diffraction and Raman spectroscopy. The catalytic activity of the films towards photodegradation of 3,5-dichlorophenol (3,5-DCP) pollutant was examined and their efficiency was compared to that of the TiO2 powder (slurry) suspensions. Pseudo-first-order photodegradation kinetics were observed and the reaction constants were determined. It has been shown that the film photocatalysts can efficiently decompose the pollutant, although relatively higher decomposition rates were observed with the commercial starting powder. Differences in the film efficiencies can be attributed to differences in their grain size, surface roughness and fractal parameters. No altering on the doctor-blade films surface characteristics was observed for several hours of cyclic operation during which their photocatalytic efficiency remained remarkably stable.

Formation of thin films of organic-inorganic perovskites for high-efficiency solar cells.

Organic-inorganic perovskites are currently one of the hottest topics in photovoltaic (PV) research, with power conversion efficiencies (PCEs) of cells on a laboratory scale already competing with those of established thin-film PV technologies. Most enhancements have been achieved by improving the quality of the perovskite films, suggesting that the optimization of film formation and crystallization is of paramount importance for further advances. Here, we review the various techniques for film formation and the role of the solvents and precursors in the processes. We address the role chloride ions play in film formation of mixed-halide perovskites, which is an outstanding question in the field. We highlight the material properties that are essential for high-efficiency operation of solar cells, and identify how further improved morphologies might be achieved.

TiO2 Nanotubes in Dye‐Sensitized Solar Cells: Critical Factors for the Conversion Efficiency

Particle vs tube: The present paper systematically investigates a range of fundamental geometrical and structural features of TiO(2) nanotube layers and their effect on the dye-sensitized solar cell conversion efficiency, to deduce the most promising strategies for improvement. It is found that the performance of the cells strongly depends on the morphology and crystalline structure of the nanotubes.

Determination of the exciton binding energy and effective masses for methylammonium and formamidinium lead tri-halide perovskite semiconductors

The family of organic–inorganic halide perovskite materials has generated tremendous interest in the field of photovoltaics due to their high power conversion efficiencies. There has been intensive development of cells based on the archetypal methylammonium (MA) and recently introduced formamidinium (FA) materials, however, there is still considerable controversy over their fundamental electronic properties. Two of the most important parameters are the binding energy of the exciton (R*) and its reduced effective mass μ. Here we present extensive magneto optical studies of Cl assisted grown MAPbI3 as well as MAPbBr3 and the FA based materials FAPbI3 and FAPbBr3. We fit the excitonic states as a hydrogenic atom in magnetic field and the Landau levels for free carriers to give R* and μ. The values of the exciton binding energy are in the range 14–25 meV in the low temperature phase and fall considerably at higher temperatures for the tri-iodides, consistent with free carrier behaviour in all devices made from these materials. Both R* and μ increase approximately proportionally to the band gap, and the mass values, 0.09–0.117m0, are consistent with a simple k.p perturbation approach to the band structure which can be generalized to predict values for the effective mass and binding energy for other members of this perovskite family of materials.

Dye-sensitized solar cells based on thick highly ordered TiO2 nanotubes produced by controlled anodic oxidation in non-aqueous electrolytic media

Dye-sensitized solar cells (DSSCs) were prepared using TiO(2) nanotubes, grown by controlled Ti anodic oxidation in non-aqueous media. Smooth, vertically oriented TiO(2) nanotube arrays, presenting a high degree of self-organization and a length of 20 µm, have been grown using ethylene glycol electrolyte containing HF. As-grown nanotubes exhibit an amorphous structure, which transforms to the anatase TiO(2) crystalline phase upon post-annealing in air at 450 °C. Atomic force microscopy (AFM) revealed the porous morphology together with high roughness and fractality of the surface. The annealed tubes were sensitized by the standard N719 ruthenium dye and the adsorption was characterized using resonance micro-Raman spectroscopy and adsorption-desorption measurements. The sensitized tubes were further used as active photoelectrodes after incorporation in sandwich-type DSSCs using both liquid and solidified electrolytes. The efficiencies obtained under air mass (AM) 1.5 conditions, using a back-side illumination geometry, were very promising: 0.85% using a composite polymer redox electrolyte, while the efficiency was further increased up to 1.65% using a liquid electrolyte.

Sensitization of TiO2 by Polypyridine Dyes Role of the Electron Donor

Dye-sensitized photoelectrochemical cells (DSSC) are characterized by electrochemical impedance spectroscopy (EIS) and Raman spectroscopy during their polarization. Cells realized with a dye recently synthesized in one of our laboratories, containing two terpyridyl (terpy) ligands, are compared with cells using commercial dyes (Ru535 and Ru620) containing isothiocyanates and either bipyridyl (bpy) or terpy ligands. Here, two points are emphasized, first, the role of the functional group (carboxylate or phosphonate) which ensures the linkage to TiO 2 and, second, the role of the redox couple (I /I - 3 ) present in the electrolyte which can react with the dye D to give unwanted intermediate species. Two species, each of them giving a characteristic Raman band in the low wavenumber range, are characterized by Raman spectroscopy. The first of these species is triiodide; the nature of the second one, which directly implies the oxidized form of dye, D + , is discussed. During the DSSC functioning, EIS allows one to discriminate three potential ranges, the photocurrent plateau, the recombination range, and the direct current range when the voltage decreases from anodic to cathodic. The second intermediate exists only in the photocurrent plateau, while I - 3 exists also in the recombination range. These results do not depend on the nature (bpy or terpy) of the ligand.

Dye-sensitization of self-assembled titania nanotubes prepared by galvanostatic anodization of Ti sputtered on conductive glass

Self-organized porous TiO(2) nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titanium in an NH(4)F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 degrees C) was used to deposit 650 nm thick titanium films. After anodizing, NTs, 830 nm long, with an average external diameter of 92 nm, were grown; this gave a high conversion rate of oxide from titanium (1.9), with a 220 nm thick layer of titanium, which was not oxidized, located at the base of the tubes. The NTs revealed a mainly amorphous structure, which transformed mostly to anatase upon thermal treatment in air at 450 degrees C. The tubes were sensitized by the N719 complex and the resultant photoelectrodes were incorporated into liquid dye solar cells (DSCs) and further tested under back-side illumination. High values of V(oc) (714 mV) were obtained under 1 sun (AM 1.5), assigned to low dark current magnitude and large recombination resistance and electron lifetime. In addition, typical values of fill factors (of the order of 0.62) were attained, in agreement with the estimated ohmic resistance of the cells in combination with low electron transfer resistance at the platinum/electrolyte interface. The overall moderate power conversion efficiency (of the order of 0.3%) was mainly due to the low short-circuit photocurrents (J(sc) = 0.68 mA cm(-2)), which was confirmed further by the corresponding IPCE values (5.2% at 510 nm). The magnitude of J(sc) was attributed to absorbed light losses due to back-side illumination of the cells, the low dye loading (due to the limited thickness of anodic titania) and the high charge transfer resistance at the TiO(2)/conductive substrate due to the presence of barrier layer(s) underneath the tubes. These preliminary results encourage the DSC community to explore further the galvanostatic anodizing of titanium in order to produce highly efficient porous TiO(2) NTs directly on conductive glass. Current work is focusing on achieving complete anodizing of the metal substrate and full transparency for the photoelectrode in order to increase and optimize the resultant cell efficiencies.

A critical review on tin halide perovskite solar cells

APbI3−xBrx perovskite solar cells with the bandgap of ∼1.5–1.6 eV, where A represents caesium, methylammonium, formamidinium and mixtures thereof, currently present certified efficiencies very close to those of established thin film technologies (such as CIGS and CdTe) and are thus one of the most important optoelectronics. To restrict the use of lead, as well as to tune the band gap of the material close to the optimum according to the Shockley–Queisser limit (being 1.34 eV), substitution (total or partial) of Pb2+ by Sn2+ should take place. In this review, we present results on single junction solar cells, utilizing CsSnI3−xBrx, CH3NH3SnI3−xBrx or NH2CHNH2SnI3−xBrx perovskites as absorbers, as well as a mixture of Sn2+/Pb2+ being adopted as the metal binary cation, reducing the bandgap to 1.2–1.4 eV. We also highlight very recently recorded efficiencies of perovskite-on-perovskite tandem solar cells, produced by the combination of the above low band gap materials with typical highly performing semi-transparent APbI3−xBrx perovskites of a higher band gap (close to 1.6–1.8 eV). We discuss these fascinating results, focusing on some key points such as, among others, the role of the tin compensator/reducing agent (usually SnF2) during perovskite crystallization. In addition, we present the critical challenges that currently limit the efficiency/stability of these systems and propose prospects for future directions.

Effect of Structural Phase Transition on Charge-Carrier Lifetimes and Defects in CH3NH3SnI3 Perovskite

Methylammonium tin triiodide (MASnI3) has been successfully employed in lead-free perovskite solar cells, but overall power-conversion efficiencies are still significantly lower than for lead-based perovskites. Here we present photoluminescence (PL) spectra and time-resolved PL from 8 to 295 K and find a marked improvement in carrier lifetime and a substantial reduction in PL line width below ∼110 K, indicating that the cause of the hindered performance is activated at the orthorhombic to tetragonal phase transition. Our measurements therefore suggest that targeted structural change may be capable of tailoring the relative energy level alignment of defects (e.g., tin vacancies) to reduce the background dopant density and improve charge extraction. In addition, we observe for the first time an above-gap emission feature that may arise from higher-lying interband transitions, raising the prospect of excess energy harvesting.