27 research outputs found
Metal-halide perovskite for photoconversion: fabrication at ambient conditions and photoelectrochemical characterization
Programa de Doctorado en Medio Ambiente y SociedadLĂnea de InvestigaciĂłn: SimulaciĂłn Molecular de Sistemas Complejos con Aplicaciones MedioambientalesClave Programa: DAMCĂłdigo LĂnea: 89As a result of the increasing worldwide energy demand and the environmental
impacts resulting from the use of fossil fuels, the search for alternative energy
resources is gaining more and more importance. In this context, among the
different renewable energies, the field of photovoltaics has undergone rapid
progress in the last few years due to the development of solar cells based on
hybrid organic-inorganic halide perovskite materials. The natural abundance of
the precursors employed to synthesise these materials and their excellent
optoelectronic properties make them potential competitors of well-established
thin-film photovoltaic technologies such as those based on silicon. However,
perovskite solar cells have several fundamental issues such as the lack of longterm
stability under environmental conditions (moisture, oxygen, heat and
light), the requirement of expensive materials as contacts and technical
limitations to their industrial scaling that restrict their widespread
commercialisation.
The main aim of this thesis is to provide a fundamental knowledge aimed at
understanding the physicochemical processes that determine the stability and
photovoltaic performance of perovskite solar devices. In particular, small
perturbation optoelectronic techniques have been used to look at electronicionic
processes that cause hysteresis phenomena. They have also been used to
identify the main routes of charge recombination for different perovskite
devices and under different moisture conditions. Regarding this, simple models
for the interpretation of the different signals obtained from small perturbation
techniques are also provided in this thesis. Another important contribution of
this work refers to the preparation of perovskites under ambient conditions.
Here we have found that the traditional use of relative humidity as control
parameter to fabricate cells should be replaced by the absolute water amount in
the atmosphere in the form of partial water vapour pressure. This last point
opens a new window to facilitate the industrial implementation of perovskite
solar cells because no glove box would limit the area of the devices.Universidad Pablo de Olavide de Sevilla. Departamento de Sistemas FĂsicos, QuĂmicos y NaturalesPostprin
Impedance analysis of perovskite solar cells: a case study
Metal halide perovskites are mixed electronic-ionic semiconductors with an extraordinary rich optoelectronic behavior and
the capability to function very efficiently as active layers in solar cells, with a record efficiency surpassing 23% nowadays. In
this work, we carry out an impedance spectroscopy analysis of two perovskite solar cells with quite distinct optical and
electrical characteristics, i.e. MAPbI3 and CsPbBr3-based devices. The main aim of the analysis is to establish how, regardless
the inherent complexity of the impedance spectrum due to ionic effects, information like ideality factors, recombination
losses and the collection efficiency can be qualitative and quantitatively assessed from impedance experiments at operating
conditions
Homeopathic Perovskite Solar Cells: Effect of Humidity during Fabrication on the Performance and Stability of the Device
Rapid degradation in humid environments is a major drawback of methylammonium lead iodide (CH3NH3PbI3), which is the archetypical component of perovskite solar cells. In this work, we have investigated the aging and degradation kinetics of CH3NH3PbI3 films and devices fabricated under controlled conditions as a function of relative humidity (RH) and compared their performance with those that were prepared under dry conditions. The aging and degradation kinetics is monitored by optical absorption and impedance spectroscopy measurements under monochromatic illumination at two different wavelengths. Aged devices show a substantial difference between the recombination rate under red and blue light illumination, which is attributed to the enhancement of local recombination routes upon aging. Interestingly, we observe that this feature is less pronounced in devices prepared under conditions of the highest RH of 50%. In general, we found that these devices keep their original electric properties and withstand a humid environment better. Chemical analysis by X-ray photoelectron spectroscopy reveals the presence of coordinating water in the CH3NH3PbI3 crystalline structure. This indicates that the presence of a small amount of water has a beneficial effect against degradation in a humid environment
Ultrathin Plasma Polymer Passivation of Perovskite Solar Cells for Improved Stability and Reproducibility
Despite the youthfulness of hybrid halide perovskite solar cells, their efficiencies are currently comparable to commercial silicon and have surpassed quantum-dots solar cells. Yet, the scalability of these devices is a challenge due to their low reproducibility and stability under environmental conditions. However, the techniques reported to date to tackle such issues recurrently involve the use of solvent methods that would further complicate their transfer to industry. Herein a reliable alternative relaying in the implementation of an ultrathin plasma polymer as a passivation interface between the electron transport layer and the hybrid perovskite layer is presented. Such a nanoengineered interface provides solar devices with increased long-term stability under ambient conditions. Thus, without involving any additional encapsulation step, the cells retain more than 80% of their efficiency after being exposed to the ambient atmosphere for more than 1000 h. Moreover, this plasma polymer passivation strategy significantly improves the coverage of the mesoporous scaffold by the perovskite layer, providing the solar cells with enhanced performance, with a champion efficiency of 19.2%, a remarkable value for Li-free standard mesoporous n-i-p architectures, as well as significantly improved reproducibility
Conformal TiO aerogel-like films by plasma deposition: from omniphobic antireflective coatings to perovskite solar cells photoelectrodes
The ability to control porosity in oxide thin films is one of the key factors
that determine their properties. Despite the abundance of dry processes for the
synthesis of oxide porous layers, the high porosity range is typically achieved
by spin-coating-based wet chemical methods. Besides, special techniques such as
supercritical drying are required to replace the pore liquid with air while
maintaining the porous network. In this study, we propose a new method for the
fabrication of ultra-porous titanium dioxide thin films at room or mild
temperatures (T lower or equal to 120 degrees Celsius) by the sequential
process involving plasma deposition and etching. These films are conformal to
the substrate topography even for high-aspect-ratio substrates and show
percolated porosity values above 85 percent that are comparable to advanced
aerogels. The films deposited at room temperature are amorphous. However, they
become partly crystalline at slightly higher temperatures presenting a
distribution of anatase clusters embedded in the sponge-like structure.
Surprisingly, the porous structure remains after annealing the films at 450
degrees Celsius in air, which increases the fraction of the embedded anatase
nanocrystals. The films are antireflective, omniphobic, and photoactive
becoming super-hydrophilic subjected to UV light irradiation The supported
percolated nanoporous structure can be used as an electron-conducting electrode
in perovskite solar cells. The properties of the cells depend on the aerogel
film thickness reaching efficiencies close to those of commercial mesoporous
anatase electrodes. This generic solvent-free synthesis is scalable and is
applicable to ultra-high porous conformal oxides of different compositions with
potential applications in photonics, optoelectronics, energy storage, and
controlled wetting.Comment: 31 pages, 10 Figs. plus Supporting Information 7 pags, 6 figs. Full
Pape
Enhanced stability of perovskite solar cells incorporating dopant-free Crystalline spiro-OMeTAD layers by vacuum sublimation
The main handicap still hindering the eventual exploitation of organometal halide perovskite-based solar cells is their poor stability under prolonged illumination, ambient conditions, and increased temperatures. This article shows for the first time the vacuum processing of the most widely used solid-state hole conductor (SSHC), i.e., the Spiro-OMeTAD [2,2âČ,7,7âČ-tetrakis (N,N-di-p-methoxyphenyl-amine) 9,9âČ-spirobifluorene], and how its dopant-free crystalline formation unprecedently improves perovskite solar cell (PSC) stability under continuous illumination by about two orders of magnitude with respect to the solution-processed reference and after annealing in air up to 200 °C. It is demonstrated that the control over the temperature of the samples during the vacuum deposition enhances the crystallinity of the SSHC, obtaining a preferential orientation along the ÏâÏ stacking direction. These results may represent a milestone toward the full vacuum processing of hybrid organic halide PSCs as well as light-emitting diodes, with promising impacts on the development of durable devices. The microstructure, purity, and crystallinity of the vacuum sublimated Spiro-OMeTAD layers are fully elucidated by applying an unparalleled set of complementary characterization techniques, including scanning electron microscopy, X-ray diffraction, grazing-incidence small-angle X-ray scattering and grazing-incidence wide-angle X-ray scattering, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy.The authors thank the âAgencia Estatal de InvestigaciĂłnâ, âConsejerĂa de EconomĂa y Conocimiento de la Junta de AndalucĂaâ (USâ1263142), âMinisterio de EconomĂa y Competitividadâ (MAT2016â79866âR, MAT2013â42900âP, FPA2016â77689âC2â1âR, and MAT2016â76892âC3â2âR) and the European Union (EU) through cohesion fund and FEDER 2014â2020 programs for financial support. J.R.S.âV. and A.B. acknowledge the EU project PlasmaPerovSol and funding from the European Union's Horizon 2020 research and innovation programme under the Marie SkĆodowskaâCurie grant agreement ID 661480. J.R.S.âVâ and M.C.L.âS. thank the University of Seville through the VI âPlan Propio de InvestigaciĂłn y Transferencia de la USâ (VI PPITâUS). This research has received funding from the EUâH2020 research and innovation programme under Grant Agreement No. 654360 having benefitted from the access provided by Technische UniversitĂ€t Graz at ElettraâTUG in Trieste (IT) within the framework on the NFFA (Nanoscience Foundries & Fine Analysis) Europe Transnational Access Activity. F.J.A. and J.R.S.âV. acknowledge the âJuan de la Ciervaâ and âRamon y Cajalâ national programs, respectively
Highly Anisotropic Organometal Halide Perovskite Nanowalls Grown by Glancing-Angle Deposition
Polarizers are ubiquitous components in current optoelectronic devices as displays or photographic cameras. Yet, control over light polarization is an unsolved challenge, since the main drawback of the existing display technologies is the significant optical losses. In such a context, organometal halide perovskites (OMHP) can play a decisive role given their flexible synthesis with tunable optical properties such as bandgap and photoluminescence, and excellent light emission with a low non-radiative recombination rate. Therefore, along with their outstanding electrical properties have elevated hybrid perovskites as the material of choice in photovoltaics and optoelectronics. Among the different OMHP nanostructures, nanowires and nanorods have lately arisen as key players in the control of light polarization for lighting or detector applications. Herein, the fabrication of highly aligned and anisotropic methylammonium lead iodide perovskite nanowalls by glancing-angle deposition, which is compatible with most substrates, is presented. Their high alignment degree provides the samples with anisotropic optical properties such as light absorption and photoluminescence. Furthermore, their implementation in photovoltaic devices provides them with a polarization-sensitive response. This facile vacuum-based approach embodies a milestone in the development of last-generation polarization-sensitive perovskite-based optoelectronic devices such as lighting appliances or self-powered photodetectors
Inverted Hysteresis in n-i-p and p-i-n Perovskite Solar Cells
A combination of experimental studies and drift-diffusion modeling has been used to investigate the appearance of inverted hysteresis, where the area under the J-V curve for the reverse scan is lower than in the forward scan, in perovskite solar cells. It is found that solar cells in the p-i-n configuration show inverted hysteresis at a sufficiently high scan rate, whereas n-i-p solar cells tend to have normal hysteresis. By examining the influence of the composition of charge transport layers, the perovskite film crystallinity and the preconditioning treatment, the possible causes of the presence of normal and inverted hysteresis are identified. Simulated current-voltage measurements from a coupled electron-hole-ion driftdiffusion model that replicate the experimental hysteresis trends are presented. It is shown that during current-voltage scans, the accumulation and depletion of ionic charge at the interfaces modifies carrier transport within the perovskite layer and alters the injection and recombination of carriers at the interfaces. Additionally, it is shown that the scan rate dependence of the degree of hysteresis has a universal shape, where the crossover scan rate between normal and inverted hysteresis depends on the ion diffusion coefficient and the nature of the transport layers.Universidad Pablo Olavide. Departamento de Sistemas FĂsicos, QuĂmicos y Naturale
Synthesis of hyperpolarizable biomaterials at molecular level based on pyridinium-chitosan complexes
Potential NLO-phore materials based on chitosan are described for the
first time in this study. A series of fluorescent and quaternized pyridiniumâ
chitosan derivatives have been synthesized by reaction of this polymer with easily available tunable pyrylium tetra fluoroborate salts. Among other spectroscopic techniques, 19 F NMR, 13 C CPMAS NMR and 2D diffusion experiments, were used to con firm the structures of the new pyridiniumâ
chitosan complexes that show high fluorescence intensity. Degrees of N -substitution were achieved lower than 4.3%, allowing the original
physicochemical properties of the biopolymer to be preserved. DFT calculations have been performed to investigate the molecular features related to the NLO properties in these compounds. NLO behavior was found to be clearly dependent on the nature and location of the substituent into the pyridinium core. Theoretical data reveal a large permanent dipolar moment, polarizabilities and hyperpolarizabilities making these molecules promising candidates as supramolecular devices exhibiting NLO properties with potentially enhanced solvatochromic properties.Agencia Española de CooperaciĂłn Internacional para el Desarrollo A/023577/09, A/030422/10Junta de AndalucĂa FQM 142, P09-AGR-459