47 research outputs found
Opto-electronic Characterization of Perovskite Solar Cells
No full textSince the first report, in 2009, on solar cells based on organic-inorganic metal halide perovskites as the active light absorber, intensive research on performance improvement of these types of solar cells has led to a dramatic rise in their power conversion efficiencies from 3-4% (in 2009) to the recently certified efficiency of 22.1% for laboratory-scale devices. Researchers from various fields of physics, chemistry, material science and engineering have been attempting to understand the opto-electronic properties of these attractive crystalline semiconductors to (i) improve and develop new fabrication techniques of the perovskite layer so as to optimise their photovoltaic characteristics; (ii) develop stable and energetically appropriate charge carrier selective contacts; (iii) understand the underlying reasons of the current-voltage hysteresis in these solar cells; and (iv) overcome their long-term operational stability and to lessen the toxicity issues of the perovskite layer.
In two separate sections, this thesis is dedicated to the understanding of both the optical characteristics of most studied photovoltaic perovskite semiconductors, with the focus being on the exciton binding energy influencing light absorption, and also to develop fast, reliable and standard characterization technique for electrical examination of the corresponding solar cells which is based on the luminescence properties of perovskites. An attempt has been made to cover many of the reported binding energy values for polycrystalline perovskite thin films (methylammonium lead triiodide and tribromide) and further, the methods employed in the literature are discussed. The absorption spectrum of these materials with and without the inclusion of excitonic effects is discussed in detail. The experimental values for the binding energies extracted from temperature-dependent absorption spectra of high-quality thin films are compared with theory. In the theory employed, the interaction of electrons and holes with an effective optical phonon was considered for the first time in these semiconductors to better estimate the theoretical values for the binding energy. Good agreement between theory and experiments was shown to be achievable.
Performance characterization of perovskite solar cells was initiated in this thesis by exploration of the generalized Planck s radiation law for planar structured devices both under light- and electrical-bias conditions (photoluminescence and electroluminescence, respectively). In hysteresis affected devices, light-soaking was found to be an essential prerequisite in order to be able to correlate luminescence intensity and the device internal voltage. Long-term behaviour of planar solar cells with methylammonium lead triiodide active layer was investigated using full device electroluminescence imaging and it was realized that interfacial deterioration even when the device is stored in the dark under glovebox conditions (devices only used for light current-voltage measurements) is the critical factor suppressing the fill factor. The shortcoming of using titanium dioxide and Spiro-OMeTAD as electron and hole selective contacts was elucidated.
In the last section, using photoluminescence and electroluminescence imaging, the immediate response and long-term electrical evolution of the perovskite bulk and titanium dioxide/perovskite and perovskite/Spiro-OMeTAD interfaces after the initial short and prolonged selective illumination of the device is elaborated. Perovskite structure buckling over time resulting in the interfacial decoupling at the titanium dioxide/perovskite is demonstrated and the associated mechanism is discussed in detail. The latter is explained by the intrinsic ion migration feature of methylammonium lead triiodide perovskite which is influenced principally due to the change in the internal electric field of the device
Thermomechanical properties of poly(lactic acid) films reinforced with hydroxyapatite and regenerated cellulose microfibers
No full textNovel composite films constituted of poly(lactic acid) (PLA), hydroxyapatite (HAp), and two types of regenerated cellulose fillers—particulate and fibrous type—were produced by melt extrusion in a twin-screw micro-compounder. The effect of the film composition on the tensile and dynamic mechanical behavior and the HAp dispersion in the PLA matrix were investigated thoroughly. Appearance of crazed regions and prevention of HAp aggregation in the PLA matrix were elucidated in the composites with up to 15 wt % particulate cellulose content, which was the main reason for only slight reduction in the tensile properties, and consequently trivial degradation of their pre-failure energy absorption as compared to neat PLA films. Superior dynamical energy storage capacities were obtained for the particulate cellulose modified composites, while their fibrous counterparts had not as good properties. Additionally, the anisotropic mechanical behavior obtained for the extruded composites should be favorable for use as biomaterials aimed at bone tissue engineering applications.</p
Contactless Series Resistance Imaging of Perovskite Solar Cells via Inhomogeneous Illumination
No full textA contactless effective series resistance imaging method for large area
perovskite solar cells that is based on photoluminescence imaging with
non-uniform illumination is introduced and demonstrated experimentally. The
proposed technique is applicable to partially and fully processed perovskite
solar cells if laterally conductive layers are present. The capability of the
proposed contactless method to detect features with high effective series
resistance is validated by comparison with various contacted mode luminescence
imaging techniques. The method can reliably provide information regarding the
severeness of the detected series resistance through photo-excitation pattern
manipulation. Application of the method to sub-cells in monolithic tandem
devices, without the need for electrical contacting the terminals, appears
feasible.Comment: 17 pages, 5 figure
Luminescence Imaging Characterization of Perovskite Solar Cells: A Note on the Analysis and Reporting the Results
No full textPolaronic exciton binding energy in iodide and bromide organic-inorganic lead halide perovskites
No full textOptical Properties of Photovoltaic Organic-Inorganic Lead Halide Perovskites
No full textOver the last several years, organic-inorganic lead halide perovskites have rapidly emerged as a new photovoltaic contender. Although energy conversion efficiency above 20% has now been certified, improved understanding of the material properties contributing to these high performance levels may allow the progression to even higher efficiency, stable cells. The optical properties of these new materials are important not only to device design but also because of the insight they provide into less directly accessible properties, including energy-band structures, binding energies, and likely impact of excitons, as well as into absorption and inverse radiative recombination processes
Polaronic exciton binding energy in iodide and bromide organic-inorganic lead halide perovskites
No full textThe last 4 years have seen the rapid emergence of a new solar cell technology based on organic-inorganic lead halide perovskites, primarily CH3NH3PbI3 and related halides involving Cl and Br. Debate continues on the role of excitons and free carriers in these materials. Recent studies report values of exciton binding energy for the iodide ranging from 0.7 meV to 200 meV, with vastly different implications for device operation and design. In the present work, previously neglected polarons are shown likely to have a major impact in determining excitonic properties. Polaronic exciton binding energies calculated using effective longitudinal optical phonon energies, deduced from permittivity measurements, are shown consistent with experimental energies for good quality samples of CH3NH3PbI3 and CH3NH3PbBr3, as determined over a large temperature range from optical absorption data. Bandgaps determined simultaneously show a discontinuity at the orthorhombic to tetragonal phase transition for the iodide, but not for the bromide
Novel method for the extraction of the implied voltages of silicon wafers and solar cells from luminescence-based measurements
No full textElectro and photoluminescence imaging as fast screening technique of the layer uniformity and device degradation in planar perovskite solar cells
No full text© 2016 Author(s).In this study, we provide insights into planar structure methylammonium lead triiodide (MAPbI3) perovskite solar cells (PSCs) using electroluminescence and photoluminescence imaging techniques. We demonstrate the strength of these techniques in screening relatively large area PSCs, correlating the solar cell electrical parameters to the images and visualizing the features which contribute to the variation of the parameters extracted from current density-voltage characterizations. It is further used to investigate one of the major concerns about perovskite solar cells, their long term stability and aging. Upon storage under dark in dry glovebox condition for more than two months, the major parameter found to have deteriorated in electrical performance measurements was the fill factor; this was elucidated via electroluminescence image comparisons which revealed that the contacts' quality degrades. Interestingly, by deploying electroluminescence imaging, the significance of having a pin-hole free active layer is demonstrated. Pin-holes can grow over time and can cause degradation of the active layer surrounding them
