Academic literature on the topic 'Perovskites, photoluminescence spectroscopy'

Mn(II)-based perovskite materials are being intensively explored for lighting applications; understanding the role of ligands regarding their photobehavior is fundamental for their development. Herein, we report on two Mn (II) bromide perovskites using monovalent (perovskite 1, P1) and bivalent (perovskite 2, P2) alkyl interlayer spacers. The perovskites were characterized with powder X-ray diffraction (PXRD), electron spin paramagnetic resonance (EPR), steady-state, and time-resolved emission spectroscopy. The EPR experiments suggest octahedral coordination in P1 and tetrahedral coordination for P2, while the PXRD results demonstrate the presence of a hydrated phase in P2 when exposed to ambient conditions. P1 exhibits an orange-red emission, while P2 shows a green photoluminescence, as a result of the different types of coordination of Mn(II) ions. Furthermore, the P2 photoluminescence quantum yield (26%) is significantly higher than that of P1 (3.6 %), which we explain in terms of different electron-phonon couplings and Mn-Mn interactions. The encapsulation of both perovskites into a PMMA film largely increases their stability against moisture, being more than 1000 h for P2. Upon increasing the temperature, the emission intensity of both perovskites decreases without a significant shift in the emission spectrum, which is explained in terms of an increase in the electron-phonon interactions. The photoluminescence decays fit two components in the microsecond regime—the shortest lifetime for hydrated phases and the longest one for non-hydrated phases. Our findings provide insights into the effects of linear mono- and bivalent organic interlayer spacer cations on the photophysics of these kinds of Mn (II)-based perovskites. The results will help in better designs of Mn(II)-perovskites, to increase their lighting performance.

Emission characteristics of metal halide perovskites play a key role in the current widespread investigations into their potential uses in optoelectronics and photonics. However, a fundamental understanding of the molecular origin of the unusual blueshift of the bandgap and dual emission in perovskites is still lacking. In this direction, we investigated the extraordinary photoluminescence behavior of three representatives of this important class of photonic materials, that is, CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2)2PbBr3, which emerged from our thorough studies of the effects of temperature on their bandgap and emission decay dynamics using time-integrated and time-resolved photoluminescence spectroscopy. The low-temperature (<100 K) photoluminescence of CH3NH3PbI3and CH3NH3PbBr3reveals two distinct emission peaks, whereas that of CH(NH2)2PbBr3shows a single emission peak. Furthermore, irrespective of perovskite composition, the bandgap exhibits an unusual blueshift by raising the temperature from 15 to 300 K. Density functional theory and classical molecular dynamics simulations allow for assigning the additional photoluminescence peak to the presence of molecularly disordered orthorhombic domains and also rationalize that the unusual blueshift of the bandgap with increasing temperature is due to the stabilization of the valence band maximum. Our findings provide new insights into the salient emission properties of perovskite materials, which define their performance in solar cells and light-emitting devices.

The optical characteristics of lead-free nanocrystals with a crystal structure of the double perovskite type with the chemical composition Cs2AgInCl6, doped with bismuth and coated with silicon dioxide were studied, and the possibility of their further application was shown. The optical properties of the nanocrystals under study are analyzed by absorption and luminescence spectroscopy, including those with time resolution. Images were obtained using a scanning electron microscope. The influence of the amount of silicon dioxide precursor addition on the optical properties and morphology of lead-free nanocrystals was established. It is shown that the observed broad photoluminescence band is associated with the occurrence of self-trapped excitons in such nanocrystals. To demonstrate the possibility of practical application of these nanocrystals a light-emitting device based on them was developed and constructed. The light emitting device has a wide emission spectrum close to warm white light. Keywords: LEDs, lead-free perovskites, double perovskites, nanocrystals, photoluminescence.

We examine the role of surface passivation on carrier trapping and nonlinear recombination dynamics in hybrid metal-halide perovskites by means of excitation correlation photoluminescence (ECPL) spectroscopy.

Titanium based double perovskites have shown good optical properties along with remarkable stability, making them promising lead-free perovskites for optoelectronic applications. In this work, Potassium Titanium Halide (K2TiBr6, K2TiI6 and K2TiI2Br4) double perovskites are synthesized for the first time. Surface topology and chemical composition are studied. Photoluminescence characterization shows a peak in the UV region. The perovskites exhibit quasi bandgap with K2TiI6 having 1.62 eV direct bandgap, conducive for single junction solar cell fabrication. K2TiBr6 and K2TiI2Br4 have bandgaps 2.14 eV and 2.44 eV, respectively, which is favorable for tandem solar cell application. The decomposition temperature of K2TiI6 is notable at 678 °C, bestowing it with stability in ambient atmosphere. Inherent to its optical properties, Solar Cell Capacitance Simulator-1D (SCAPS-1D) is used to simulate perovskite solar cell (PSC) with K2TiI6 as the absorber. Utilizing the absorption data from UV-Vis spectroscopy, these PSCs are designed to give maximum efficiency. Upon numerical optimization of perovskite layer thickness, we propose an efficient, practically realizable PSC with a power conversion efficiency of 4.382%. Besides, various electron and hole transport layers are investigated and the effect of their thickness on the PSC performance are studied.

Two-dimensional (2D) layered hybrid organic–inorganic perovskites have potential applications in solar cells, electroluminescent devices and radiation detection because of their unique optoelectronic properties. In this paper, four 2D layered hybrid organic–inorganic halide perovskites of (C6H5CH2NH3)2PbCl4, (C6H5CH2NH3)2PbBr4, (C6H5CH2NH3)2PbI4 and (C4H9NH3)2PbBr4 were synthesized by solvent evaporation. Their crystal structure and surface morphology were studied. The effects of different halogens and organic amines on perovskites’ absorption spectra were investigated, and the photoluminescence (PL) properties were studied by femtosecond ultrafast spectroscopy. The experimental results show that the four perovskites are well crystallized and oriented. With the increase of halogen atom number (Cl, Br, I) in turn, the UV-Vis absorption spectra peaks of perovskites redshift due to the increasing of the layer spacing, but organic amines have little effect on the spectra of perovskites. The PL intensity increases with increasing laser power, but the lifetime decreases with increasing laser power, which is mainly due to the non-geminate recombination. This research is of great significance for realizing the spectral regulation of organic–inorganic hybrid perovskites and promoting their application in nano-photonics and optoelectronic devices.

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices.

All-inorganic halide perovskites, especially lead perovskite microcrystals, have attracted more and more attention because of their excellent photoelectric properties and chemical stability. Herein, high quality CsPbBr3 microcrystals with three different stable morphologies, namely microplate, frustum of a square pyramid and pyramid, were synthesized by the chemical vapor deposition (CVD) method through altering the flow rate of a carrier gas and were comparatively studied in structure and optical property. The photoluminescence (PL) results showed that the CsPbBr3 microplate has the best luminescence property. The structural characterization results by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray rocking curves (XRC) and Raman revealed that the flow rate of the carrier gas could manipulate the morphology evolution of CsPbBr3 microcrystals and further impact their luminescence properties.

Metal-halide perovskites show great promise as solution-processable semiconductors for efficient solar cells and LEDs. In particular, the diffusion range of photogenerated carriers is unexpectedly long and the luminescence yield is remarkably high. While much effort has been made to improve device performance, the barriers to improving charge transport and recombination properties remain unidentified. I first explore charge transport by investigating a back-contact architecture for measurement. In collaboration with the Snaith group at Oxford, we develop a new architecture to isolate charge carriers. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnOₓ and NiOₓ, respectively. Upon illumination, electrons (holes) generated over SnOₓ (NiOₓ) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. We characterise charge transport parameters of electrons and holes, separately, and demonstrate that grain boundaries do not prevent charge transport. Our results show that the low mobilities found in applied-field techniques do not reflect charge diffusivity in perovskite solar cells at operating conditions. We then use the back-contact architecture to investigate recombination under large excess of one charge carrier type. Recombination velocities under these conditions are found to be below 2 cm s⁻¹, approaching values of high quality silicon and an order of magnitude lower than under common bipolar conditions. Similarly, diffusion lengths of electrons and holes exceed 12 μm, an order of magnitude higher than reported in perovskite devices to date. We report back-contact solar cells with short-circuit currents as high as 18.4 mA cm⁻², giving 70% external charge-collection efficiency. We then explore the behaviour of charge carriers in continuously illuminated metal-halide perovskite devices. We show that continuous illumination of perovskite devices gives rise to a segregated charge carrier population, and we find that the distance photo-induced charges travel increases significantly under these conditions. Finally, we examine intermittancy in the photoluminescence intensity of metal-halide perovskite films.

Lead halide perovskites have shown remarkable device performance in both solar cells and LEDs. Whilst the research efforts so far have been mainly focussed on device optimisation, little is known about the photophysical properties. For example, the nature of the bandgap is still debated and an indirect bandgap due to a Rashba splitting has been suggested. In this thesis, we study the early-time carrier relaxation and its impact on photoluminescence emission. We first study ultrafast carrier thermalization processes using 2D electronic spectroscopy and extract characteristic carrier thermalization times from below 10 fs to 85 fs. We then investigate the early-time photoluminescence emission during carrier cooling. We observe that the luminescence signal shows a rise over 2 picoseconds in CH3NH3PbI3 while carriers cool to the band edge. This shows that luminescence of hot carriers is slower than that of cold carriers, as is found in direct gap semiconductors. We conclude that electrons and holes show strong overlap in momentum space, despite the potential presence of a small band offset arising from a Rashba effect. Recombination and device performance of perovskites are thus better described within a direct bandgap model. We finally study carrier recombination in perovskites and the impact of photon recycling. We show that, for an internal photoluminescence quantum yield of 70%, we measure external yields as low as 15% in planar films, where light out-coupling is inefficient, but observe values as high as 57% in films on textured substrates that enhance out-coupling. We study the photo-excited carrier dynamics and use a rate equation to relate radiative and non-radiative recombination events to measured photoluminescence efficiencies. We conclude that the use of textured active layers has the ability to improve power conversion efficiencies for both LEDs and solar cells.

Dye-sensitized solar cells (DSSCs) and perovskite solar cells are emerging as promising potential low-cost alternatives to established crystalline silicon photovoltaics. Of the employed functional materials, however, many fundamental optoelectronic properties governing photovoltaic device operation are not sufficiently well understood. This thesis reports on a series of studies using ultrafast THz and photoluminescence spectroscopy on two classes of such materials, providing insight into the dynamics of charge-transport and recombination processes following photoexcitation. For TiO₂-nanotubes, which have been proposed as easy-to-fabricate electron transporters for DSSCs, fast, shallow electron trapping is identified as a limiting factor for efficient charge collection. Trapping lifetimes are found to be about an order of magnitude shorter than in the prevalently employed sintered nanoparticles under similar excitation conditions and trap saturation effects are not observed, even at very high excitation densities. In organo-lead halide perovskites - specifically CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x, which have only recently emerged as highly efficient absorbers and charge transporters for thin-film solar cells, carrier mobilities and fundamental recombination dynamics are revealed. Extremely low bi-molecular recombination rates at least four orders of magnitude below the prediction of Langevin's model are found as well as relatively high charge-carrier mobilities in comparison to other solution-processable materials. Furthermore a very low influence of trap-mediated recombination channels was observed. Due to a combination of these factors, diffusion lengths reach hundreds of nanometres for CH₃NH₃PbI₃ and several microns for CH₃NH₃PbI_3-xCl_x. These results are shown to hold for both, solution processed and vapour-deposited CH₃NH₃PbI_3-xCl_x and underline the superb suitability of the materials as absorbers in solar cells, even in planar heterojunction architectures. The THz-frequency spectrum of the conductivity of the investigated perovskites is consistent with Drude-like charge transport additionally exhibiting weak signatures of phonon coupling. These coupling effects are also reflected in the luminescence of CH₃NH₃PbI_3-xCl_x, where they are believed to be the cause of the observed homogeneous spectral broadening. Further photoluminescence measurements were performed at temperatures between 4 K and room temperature to study the nature of recombination pathways in the material.

The purpose of applying a light management structure in a solar cell is to absorb the largest possible amount of incident radiation in the active layer. Independent of the material, thickness and structure of the solar cell, the fundamental approaches for light management are: A) reducing the proportion of the light being reflected out at the front surface and B) increasing the path length of the light within the absorbing substrate. Therefore, characterizing and optimizing the light management technique is essential for further improving solar cell efficiency. In this thesis, a method based on the generalized Planck’s law of radiation is developed to extract the band-to-band absorptance from the photoluminescence spectra of semiconductor materials. Unlike the traditional way of obtaining absorptance from the reflection and transmission measurement, this method is only sensitive to absorbed photons that can generate electron-hole pairs. Therefore, the parasitic losses such as free carrier absorption and the absorption in non-active layers can be automatically eliminated. With the extracted band-to-band absorptance, the implied photo-current of the sample is accurately estimated without the need of forming a p-n junction. By comparing the band-to-band absorptance of silicon wafers with and without light trapping structure, the optical enhancement is rapidly accessed. Using this method, a wide range of light trapping structures are studied on crystalline silicon wafers to improve the photo-current generation. Self-assembled plasmonic Ag particles (AgNP) together with a dielectric based diffuse reflector (DR) are applied on the rear side of silicon wafers to provide excellent optical enhancement without sacrificing the electrical performance of the device. AgNP have proven to scatter the light at wide angles, so that total internal reflection occurs and the light can be trapped in the solar cell. Dielectric based diffuse reflectors have the advantage of high reflectivity and low absorption loss compared to metal reflectors. The combination of DR and AgNP provides light trapping performance (62% of Lambertian enhancement) which is comparable with inverted pyramids (67% of Lambertian enhancement) on a 200 mm thick silicon wafer. By applying AgNP in the back of a silicon wafer with an interdigitated back contact (IBC) structure, a maximum of 53% of fraction of the Lambertian enhancement is achieved with an optimized capping layer. For a standard IBC cell with AgNP embedded in the rear side, the short circuit current density is estimated to enhance by 18% in the spectral range from 1000 nm to 1200 nm. Texturing is still the most widely applied light management technique in the c-Si solar cell industry. The textured surface produces broad-band anti-reflection properties as well as effective light trapping to the solar cell. We extend the application of the PL technique to evaluate the optical performance of textured samples. Several structures including reactive ion etched textures (RIE), metal-assisted textures (MET) and random pyramid textures (RAN) are experimentally evaluated with the photoluminescence technique. By fabricating a silicon wafer with RIE and RAN textures on the front and rear side respectively, we demonstrate a structure with near ideal absorption in the ultraviolet and visible spectrum and a light trapping efficiency of 55% in the near infrared region of the solar spectrum. Using an analytical model with independent angular distribution parameters on both surfaces, we carry out a quantitative analysis on the impact of front reflection, rear absorption and the angular distribution on the implied current generation of these silicon samples. With the origins of the optical loss of these light management structures revealed, the effective approach for reaching maximum photo-current for high efficiency silicon solar cells is discussed. We conclude that the non-perfect angular distribution is the main limitation for approaching Lambertian light trapping in high efficiency silicon solar cells. Dielectric based diffuse reflectors have excellent optical properties and can be applied on a wide range of solar cells. A diffuse reflector prepared by Snow Globe coating is applied on a-Si:H/mc-Si:H tandem solar cell. The reflector has close to 100% reflectance over a spectral range of 500-1300 nm which indicates much lower parasitic optical losses compared to the standard textured ZnO:Al/Ag reflector. The application of DR avoids the localized surface plasmon and propagating surface plasmon resonances existing on randomly textured ZnO:Al/Ag back contacts. Both of the resonances can couple with the incident light and introduce significant amount of parasitic absorption and thereby reducing the overall cell performance. By replacing ZnO:Al/Ag with SGC reflector on tandem thin film silicon solar cells as a rear reflector, the short circuit current of the bottom solar cell is enhanced and the overall cell efficiency is improved from 10.2% to 10.4%. Organic/inorganic hybrid perovskite material has the potential for being a lowcost and high efficiency photovoltaic technology. The knowledge of absorption coefficient of such novel absorber materials is essential in understanding the extent to which perovskite solar cells may suffer from parasitic absorption. The fundamental relationship between band-to-band absorptance and photoluminescence is used to measure the absorption coefficient absorption coefficient of organic-inorganic hybrid perovskite methylammonium lead iodide (CH3NH3PbI3) films from 675 nm to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10e-14 cm-1 are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of absorption coefficient is calculated from the photoluminescence spectra of CH3NH3PbI3 in the temperature range 80-360K with 10K intervals. Based on the temperature dependent absorption coefficient, a polynomial parameterization describing the product of the radiative recombination coefficient and square of the intrinsic carrier density is also presented. This thesis focuses on understanding and improving the light management of solar cells. The method of extracting band-to-band absorptance from photoluminescence spectra is used to compare a wide range of light trapping structures on silicon wafers and to extract the absorption coefficient of perovskite film at very low energy levels.

Metal halide perovskites have emerged as photoactive materials in solution-processed devices thanks to their unique properties such as high absorption coefficient, sharp absorption edge, long carrier diffusion lengths, and tunable bandgap, together with ease of fabrication. The single-junction perovskite solar cells have reached power conversion efficiencies of more than 25%. Although the efficiency of perovskite devices has increased tremendously in a very short time, the efficiency is still limited by carrier recombination at defects and interfaces. Thus, understanding these losses and how to reduce them is the way forward towards the Shockley-Queisser limit. This thesis aims to apply ultrafast optical spectroscopy techniques to investigate the recombination pathways in halide perovskites, and understand the charge extraction from perovskite to transport layers and nonradiative losses at the interface. The first part focuses on perovskite solar cells with planar n–i–p device architecture which offers significant advantages in terms of large scale processing, the potential use of flexible substrates, and applicability to tandems. In addition to the optimization of MAPbI3 solar cell fabrication using a modified sequential interdiffusion protocol, the photophysics of perovskites exposed to humid air and illumination are discussed. The MAPbI3 film processed with the addition of glycol ethers to the methylammonium iodide solution results in the control of PbI2 to perovskite conversion dynamics, thus enhanced morphology and crystallinity. For samples exposed to humid air and illumination, the formation of sub-bandgap states and increased trap-assisted recombination are observed, using highly-sensitive absorption and time-resolved photoluminescence measurements, respectively. It appears that such exposure primarily affects the perovskite surface. The second part discusses the hole extraction from Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45 to the polymeric hole transport layer and interfacial recombination using ultrafast transient absorption spectroscopy technique. To illustrate this, PDPP-3T was used as HTL, since its ground state absorption is red-shifted compared to the perovskite’s photobleach, thereby allowing direct probing of the interfacial hole extraction and recombination. Moreover, carrier diffusion is investigated by varying the perovskite film thickness, and carrier mobility is found to be 39 cm2V-1s-1. Finally, hole extraction is found to be one order of magnitude faster than the recombination at the interface.

碩士
中原大學
物理研究所
106
The organic-inorganic hybrid perovskite has very poor stability in the environment; it easily reacts with elements in the environment to degrade. To improve the stability of the methotrexate lead perovskite, we tried to using a cover layer to keep the perovskite sample from the harmful element in the environment. We use SiO2 and LiF as cover layer, because this two materials won’t affect the electrical measurement, and tens of nanometers thick of this two materials have good light transmittance without affecting the photoluminescence measurement. Then we used photoluminescence measurement and electrical measurement to verify the change of methyl iodide lead perovskite with time and with the number of measurements. From the experimental results, we found that both protective layer materials have the effect of insulating the gas in the atmosphere to improve the stability of the sample. However, in the production of the cover layer of SiO2, it’s easy to diffuse oxygen into the crystal lattice of the methyl iodide-lead perovskite, so that the sample after the measurement is rapidly degraded. The lithium fluoride protective layer sample can effectively play a role in greatly improving the stability of the MAPbI3 perovskite.