Sharp Optical Edge Research Articles

Titanium Disulphide (TiS2) Dichalcogenide Thin Films as Inorganic Hole Transport Layer for Perovskite Solar Cells Synthesized from Ionic Liquid Electrodeposition

AbstractIn high efficiency organic-inorganic perovskite solar cells formed as a multilayer structure, the hole transporting layer (HTL) at the perovskite absorber layer interface has a critical role. Organic HTLs based on Spiro-OMeTAD and PTAA have led to high efficiencies but displayed poor long-term stability and involves expensive purification processes that hinders universal low-cost commercialization goals for perovskite solar cells. Though as an inorganic alternative, transition metal chalcogenides have been investigated for HTL recently, the hot-injection method often used in synthesis has shown poor reproducibility and difficulty in scaling-up. In this work we demonstrate an ab initio facile inexpensive scalable synthesis of transition metal dichalcogenide (TiS2) by electrodeposition from ionic liquids as a low-cost inorganic HTL for perovskite solar cells. The TiS2 thin films were electrodeposited from choline chloride–urea eutectic based ionic liquid electrolytes at 80°C with Na2S2O3 as sulphur and TiCl4 as titanium source. From cyclic voltammetry studies the deposition potential of TiS2 was optimized at -0.8V vs Pt. The as-deposited TiS2 HTL exhibited polycrystalline structure with preferential growth along (001), (100), (002), (102), (110), (111) planes. The Raman spectroscopy of the films showed peaks around 225 cm−1 and 332 cm−1 attributed to the Eg and A1 g Raman modes respectively. The synthesized thin films demonstrated sharp optical bandgap edge along with bandgap tunability as the bandgap (direct) decreased from 1.53 eV to 1.49 eV, 1.40 eV, and 1.34 eV with gradual change in deposition potential from −0.8 V to −0.9 V, −1.0 V, and −1.1 V vs Pt, respectively. This aspect has potential for alignment of valance band edge in facilitating the hole transport at the perovskite-TiS2 interface. The absorption coefficient in visible-light range of the as-deposited TiS2 thin films likewise has shown a dependence on the synthesis potential which is highly conducive for application as an HTL in multilayer solar cell structure. The TiS2 thin films were observed to be p-type as shown from the Hall effect studies with a carrier mobility up to 14.4 cm2V−1s−1. A detailed study of the effect of the synthesis parameters on the structural, optical, band-edge, and electronic properties of TiS2 thin films suitable for application as HTL in perovskite solar cells is presented.

In situ NMR Investigation of the Photoresponse of Perovskite Crystal

Summary Hybrid perovskites are outstanding candidates for use in optoelectronic devices. Nonetheless, hybrid perovskites present numerous fundamental issues that are not yet fully understood. This study seeks to address this problem by reporting on the occurrence of ultraslow dynamic orientation changes in the methylammonium (MA) cations and the corresponding changes in the inorganic lattice of MAPbI3 perovskite crystals occurring over a time scale of 100 s under continuous illumination based on in situ single-crystal 2H nuclear magnetic resonance and in situ X-ray diffraction results. The demonstrated ultraslow dynamic changes in MAPbI3 are far slower than elementary molecular motions typically having time scales of femtoseconds and picoseconds and have never been reported previously. The ultraslow dynamic changes have been correlated with the reduction of band gap and the increase of absorption coefficient of MAPbI3, providing the molecular origins of the stabilization process in the photoresponse of the MAPbI3 device.

Water-Stable 1D Hybrid Tin(II) Iodide Emits Broad Light with 36% Photoluminescence Quantum Efficiency.

The optical and light emission properties of tin and lead halide perovskites are remarkable because of the robust room-temperature (RT) performance, broad wavelength tunability, high efficiency, and good quenching resistance to defects. These highly desirable attributes promise to transform current light-emitting devices, phosphors, and lasers. One disadvantage in most of these materials is the sensitivity to moisture. Here, we report a new air-stable one-dimensional (1D) hybrid lead-free halide material (DAO)Sn2I6 (DAO, 1,8-octyldiammonium) that is resistant to water for more than 15 h. The material exhibits a sharp optical absorption edge at 2.70 eV and a strong broad orange light emission centered at 634 nm, with a full width at half-maximum (fwhm) of 142 nm (0.44 eV). The emission has a long photoluminescence (PL) lifetime of 582 ns, while the intensity is constant over a very broad temperature range (145-415 K) with a photoluminescence quantum yield (PLQY) of at least 20.3% at RT. Above 415 K the material undergoes a structural phase transition from monoclinic (C2/c) to orthorhombic (Ibam) accompanied by a red shift in the band gap and a quench in the photoluminescence emission. Density functional theory calculations support the trend in the optical properties and the 1D electronic nature of the structure, where the calculated carrier effective masses along the inorganic chain are significantly lower than those perpendicular to the chain. Thin films of the compound readily fabricated from solutions exhibit the same optical properties, but with improved PLQY of 36%, for a 60 nm thick film, among the highest reported for lead-free low-dimensional 2D and 1D perovskites and metal halides.

Dynamic shortening of disorder potentials in anharmonic halide perovskites

Halide perovskites are semiconductors that exhibit sharp optical absorption edges and small Urbach energies allowing for efficient collection of sunlight in thin-film photovoltaic devices. However, halide perovskites also exhibit large nuclear anharmonic effects and disorder, which is unusual for efficient optoelectronic materials and difficult to rationalize in view of the small Urbach energies that indicate a low amount of disorder. To address this important issue, the disorder potential induced for electronic states by the nuclear dynamics in various paradigmatic halide perovskites is studied with molecular dynamics and density functional theory. We find that the disorder potential is dynamically shortened due to the nuclear motions in the perovskite, such that it is short-range correlated, which is shown to lead to favorable distributions of band edge energies. This dynamic mechanism allows for sharp optical absorption edges and small Urbach energies, which are highly desired properties of any solar absorber material.

Tuning Bandgap of Mixed‐Halide Perovskite for Improved Photovoltaic Performance Under Monochromatic‐Light Illumination

Organic–inorganic halide perovskites have emerged as promising materials for optoelectronic devices. This paper focuses on a new application field for perovskite materials as monochromatic‐light conversion devices. First the optical properties of organic–inorganic perovskite semiconductors with bandgaps varying from near‐infrared to visible at room temperature are presented. Two types of hybrid organic–inorganic mixed‐halide perovskites, (FAPbI3)x(MAPbBr3)1‐x and FA0.85MA0.15Pb(IxBr1‐x)3, are adopted for bandgap tuning, an approximate linear variation of bandgaps with the x value is obtained. The relationship between thin film composition and device performance are investigated. Based on the results of the above bandgap tuning, two kinds of devices with bandgap near the wavelength of 683 nm are characterized under monochromatic‐light illumination. A conversion efficiency of up to 40% under 60 mW cm−2 monochromatic‐light illumination is achieved. The results confirm that the perovskite films exhibit sharp optical absorption edge, enabling highly efficient monochromatic‐light conversion device.

Perovskite Solar Cells: From the Atomic Level to Film Quality and Device Performance.

Organic-inorganic perovskites have made tremendous progress in recent years due to exceptional material properties such as high panchromatic absorption, charge carrier diffusion lengths, and a sharp optical band edge. The combination of high-quality semiconductor performance with low-cost deposition techniques seems to be a match made in heaven, creating great excitement far beyond academic ivory towers. This is particularly true for perovskite solar cells (PSCs) that have shown unprecedented gains in efficiency and stability over a time span of just five years. Now there are serious efforts for commercialization with the hope that PSCs can make a major impact in generating inexpensive, sustainable solar electricity. In this Review, we will focus on perovskite material properties as well as on devices from the atomic to the thin film level to highlight the remaining challenges and to anticipate the future developments of PSCs.

Perowskit‐Solarzellen: atomare Ebene, Schichtqualität und Leistungsfähigkeit der Zellen

AbstractOrganisch‐anorganische Perowskite fanden in den letzten Jahren enormes Interesse aufgrund ihrer außergewöhnlichen Materialeigenschaften wie hohe panchromatische Absorption, große Ladungsträgerdiffusionslängen und eine scharfe optische Bandlücke. Die Kombination qualitativ hochwertiger Halbleiter mit günstigen Herstellungsmethoden scheint eine ideale Paarung zu sein, die auch außerhalb der akademischen Welt für große Begeisterung sorgt. Das gilt insbesondere für Perowskit‐Solarzellen (PSZs), die in nur 5 Jahren beispiellose Steigerungen ihrer Effizienz und Stabilität gezeigt haben. Derzeit gibt es ernsthafte Kommerzialisierungsbestrebungen mit der Hoffnung, dass PSZs eine große Rolle bei der Erzeugung von kostengünstigem, nachhaltigem Solarstrom spielen werden. In diesem Aufsatz betrachten wir die Materialeigenschaften und Solarzellstrukturen von PSZs vom atomaren Niveau bis zu Dünnfilmen, um die verbleibenden Herausforderungen bei PSZs zu identifizieren und zukünftige Richtungen aufzuzeigen.

Low temperature perovskite solar cells with an evaporated TiO2 compact layer for perovskite silicon tandem solar cells

Silicon-based tandem solar cells can overcome the efficiency limit of single junction silicon solar cells. Perovskite solar cells are particularly promising as a top cell in monolithic tandem devices due to their rapid development towards high efficiencies, a tunable band gap with a sharp optical absorption edge and a simple production process. In monolithic tandem devices, the perovskite solar cell is deposited directly on the silicon cell, requiring low-temperature processes (< 200°C) to maintain functionality of under-lying layers of the silicon cell in case of highly efficient silicon hetero-junction (SHJ) bottom solar cell. In this work, we present a complete low-temperature process for perovskite solar cells including a mesoporous titanium oxide (TiO2) scaffold - a structure yielding the highest efficiencies for single-junction perovskite solar cells. We show that evaporation of the compact TiO2 hole blocking layer and ultra-violet (UV) curing for the mesoporous TiO2 layer allows for good performance, comparable to high-temperature (> 500°C) processes. With both manufacturing routes, we obtain short-circuit current densities (JSC) of about 20 mA/cm², open-circuit voltages (VOC) over 1 V, fill factors (FF) between 0.7 and 0.8 and efficiencies (η) of more than 15%. We further show that the evaporated TiO2 layer is suitable for the application in tandem devices. The series resistance of the layer itself and the contact resistance to an indium doped tin oxide (ITO) interconnection layer between the two sub-cells are low. In addition, the low parasitic absorption for wavelengths above the perovskite band gap allow a higher absorption in the silicon bottom solar cell, which is essential to achieve high tandem efficiencies.

Structural and opto-electronic features of pulsed laser ablation grown Cu2ZnSnS4 films for photovoltaic applications

We report synthesis of Cu2ZnSnS4 thin films of kesterite structure by sulfurization of pulsed laser deposited (PLD) CuZnSn composite films at optimized temperature. These films show sharp optical absorption edge in the range of 1.4–1.6 eV, when prepared under optimized conditions of deposition and sulfurization. In general, the processed films are found to be non-stoichiometric with deficiency of S and excess of Zn. Electrical conductivity measurements in the temperature range 50–300 K reveal hopping transport with activation energies ≤20 meV at T < 80 K and a room temperature resistivity varying from 0.1 to 2.7 Ω-cm.

Preparation of Single-Phase Films of CH3NH3Pb(I1-xBrx)3 with Sharp Optical Band Edges.

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide-iodide lead perovskite films CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) by spin-coating from solution and obtain films with monotonically varying bandgaps across the full composition range. Photothermal deflection spectroscopy, photoluminescence, and X-ray diffraction show that following suitable fabrication protocols these mixed lead-halide perovskite films form a single phase. The optical absorption edge of the pure triiodide and tribromide perovskites is sharp with Urbach energies of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for CH3NH3PbI1.2Br1.8. We demonstrate a bromide-iodide lead perovskite film (CH3NH3PbI1.2Br1.8) with an optical bandgap of 1.94 eV, which is optimal for tandem cells of these materials with crystalline silicon devices.

Microstructure and optical properties of zinc nitride films deposited by magnetron sputtering method

Zinc nitride films were coated on a glass substrate by reactive magnetron sputtering method using a zinc target and a mixture of N2 and Ar gasses. The deposited films were characterized for microstructure, surface morphology and optical properties. These films possess a polycrystalline structure and a high electrical conductivity. The optical properties indicate that the films have high transmittance in the infra-red (IR) region with a very sharp optical absorption edge in the IR range corresponding to the band gap of zinc nitride. The band gap energy of deposited films was estimated to be 1.2eV, which slightly decreased when the films were annealed at 300°C. This effect is interpreted as due to release of extra nitrogen from the films during annealing. The electrical conductivity of the as-deposited films is 5×102Ω·cm, and it decreases by annealing of these films in the air or O2. The zinc nitride films annealed in the air or O2 exhibited some photoconductivity; however, no photoconductivity was observed for the as-deposited films.

A novel fiber-optic temperature sensor based on high temperature-dependent optical properties of ZnO film on sapphire fiber-ending

We report the growth of high-quality thin films of ZnO via an electron-beam evaporation technique. Studies of the transmittance spectra have revealed a sharp optical absorption edge and a significant redshift. After annealing at 673 K, the ZnO films again demonstrated a sharp absorption edge in a manner similar to the as-deposited samples. This illustrates the excellent thermal stability of the thin films and, as such, demonstrates their potential as fiber-optic temperature sensors. Utilizing the influence of optical absorption spectra at different temperatures, a novel fiber-optic temperature sensor based on this material has been designed and tested. This technique could offer a simple, robust and cost-effective method to be used in high temperature sensing applications.

The temperature-dependent optical properties of ZnO film deposited on sapphire fiber-ending

In this paper, ZnO thin film was first grown on the sapphire fiber-ending by electron-beam evaporation, which has good morphology and high crystallization quality. The transmittance spectra showed that the sharp optical absorption edge has a red shift with increasing temperature, and the obtained temperature-dependent band gap energy for ZnO follows to the linear relation Eg（T）=340-491×１０-4T. This formula presents the key to fabricate new broad\|measurement range fiber-optic temperature sensors based on semiconductor ZnO by utilizing the optical absorption spectra at different temperatures.

Photoluminescence enhancement in nanocomposite thin films of CdS–ZnO

We show that the photoluminescence emitted from a dense, two-component quantum dot ensemble on a thin film is significantly higher and decays much faster than that from quantum dots of either of the two pure systems (CdS and ZnO). The semiconductor nanocomposite, in which the characteristic grain size of each species was 2–3nm, was deposited directly on Si wafers by high-pressure magnetron sputtering, and exhibits a single, relatively sharp optical absorption edge.

Effect of compensation of electron and hole scattering potentials on the optical band edge of heavily dopedGaAs∕AlxGa1−xAssuperlattices

The optical broadenings studied by the photoluminescence in the intentionally disordered $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ superlattices were compared with the broadenings of the individual electron states measured by the Shubnikov-de Haas oscillations. It was shown that the combined effect of the electron and hole energy blurrings is to decrease the optical broadening with respect to the individual state broadenings resulting in very sharp optical edges even in highly disordered superlattices. It was also found that the impurities almost equally influence the electron and hole scattering potentials, contrary to what happens due to the structural superlattice disorder.

Structural and optical characterizations of nanostructured diamond films grown on Si(0 0 1) substrates

Nanostructured diamond films (NDFs) were grown on fine-polished Si(0 0 1) substrates by radio-frequency plasma-enhanced hot-filament chemical vapor deposition at low gas pressure of approximately 2 Torr. High resolution field-emission scanning electron microscopy (FESEM), X-ray diffraction and Raman scattering spectroscopy were employed to characterize the NDFs. FESEM measurement indicated that diamond nuclei density exceeded 10 10 cm −2 after growth for 20 min, which should render potential applications, such as in the fabrication of diamond-related field-emission devices. Photoluminescence excitation characterizations revealed the sharp optical absorption band edge of NDFs, and the band gap is 5.34 eV.

Orientation-controlled epitaxy of A2CuO3 (A: Sr, Ca) films with large optical nonlinearity

The one-dimensional Cu–O chains responsible for the large optical nonlinearity in Sr2CuO3 (SCO) and Ca2CuO3 (CCO) are aligned in one direction in films grown on LaSrAlO4 (LSAO) (1 0 0) substrates by using pulsed laser deposition. The CCO films deposited directly on lattice matched LSAO substrates have a single orientation with high crystallinity, while the SCO films include small portions of misoriented domains. Using a simple model of lattice mismatch, we developed a Sr2TiO4 buffer layer in order to grow orientation-controlled SCO films with both high crystallinity and a sharp optical absorption edge. This technique will be a key to fabricating ultrafast all-optical switches with a waveguide structure.

Thin film II–VI photovoltaics

With the exception of HgSe and HgTe, II–VI compounds are direct gap semiconductors with sharp optical absorption edge and large absorption coefficients at above bandgap wavelengths. Device quality polycrystalline films of II–VI compounds can be prepared from inexpensive raw materials by a number of low-cost methods. They are well-suited for thin film solar cells and provide an economically viable approach to the terrestrial utilization of solar energy. Thin film II–VI solar cells are usually of the heterojunction type consisting of a high bandgap window (or collector) and a lower bandgap absorber. The grain boundary effects in polycrystalline II–VI films are considerably less pronounced than those in III–V films and can be passivated, at least partially, by chemical treatment. The use of CdS, ZnO, ZnSe and Cd 1 − x Zn x S as the window and the use of CdTe and Cd 1 − x Zn x Te as the absorber are reviewed in this paper. The fabrication and characteristics of a number of the thin film solar cell structures are discussed with emphasis on the thin film CdS CdTe solar cell.

Properties of Hg-based films and superlattices grown by molecular-beam epitaxy

We report the electrical, optical, and structural properties of thin films of HgTe, Hg1−xCdxTe, and Hg1−xMnxTe and superlattices of HgTe–CdTe and Hg1−xMnxTe–HgTe. All of these epitaxial films were deposited by molecular-beam epitaxy. The Hg1−xCdxTe films exhibit large electron Hall mobilities, specular reflectances, and sharp optical absorption edges. The Hg1−xMnxTe films, grown with x values of 0.03 and 0.10, also exhibit excellent optical and electrical properties. The p-type HgTe–CdTe superlattices exhibit large hole mobilities. Superlattices containing alternating layers of Hg1−xMnxTe and HgTe have also been successfully prepared. The Hg1−xMnxTe–HgTe multilayers are the first superlattices containing alternating layers of a Hg-based dilute magnetic semiconductor to be grown by any thin-film technique.

Comments on the evidence for sharp and gradual optical absorption edges in amorphous germanium

AbstractSeveral investigators have analyzed optical data on amorphous Ge films prepared by evaporation on room‐temperature substrates to derive a sharp absorption edge near 0.5 eV. A re‐examination of their work shows that the sharp edges, found hitherto, may be artifacts resulting from either experimental or calculational errors and that the preponderance of the current evidence favors a gradual absorption edge.