Identifying high performance and durable methylammonium-free lead halide perovskites through high throughput synthesis and characterization

One of the organic component in the perovskite photo-absorber, the methylammonium cation, has been suggested to be a roadblock to long-term operation of organic-inorganic hybrid perovskite-based solar cells. Methylammonium-free perovskites thus represent a possible direction for more stable photo-absorbers that are also compatible with multijunction solar cells. However, most work on methylammonium-free perovskites involves cesium and formamidinium as the A-site cations, which are thermodynamically less stable than the methylammonium-based materials. In this work we systematically explore the crystallographic and optical properties of the compositional space of mixed cation and mixed halide lead perovskites, where formamidinium (FA+) is gradually replaced by cesium (Cs+), and iodide (I-) is substituted by bromide (Br-), i.e., CsyFA1–yPb(BrxI1–x)3. The crystal phases, which could be tuned by changing the tolerance factor for mixed perovskite alloys, are qualitatively determined and the composition–structure relationship is established in the CsyFA1–yPb(BrxI1–x)3 compositional space. We find that higher tolerance factors lead to more cubic structures, whereas lower tolerance factors lead to more orthorhombic. We also find that while some correlation exists between tolerance factor and structure, tolerance factor does not provide a holistic understanding of whether a perovskite structure will fully form. Given the wide range of bandgaps produced by this compositional space, an empirical expression is devised to predict the optical bandgap of CsyFA1–yPb(BrxI1–x)3 perovskites – which changes as a function of composition –, conducive to the design of absorbers with bandgaps tailor-made for specific tandem and single-junction applications. By screening 26 solar cells with different compositions, we find that Cs1/6FA5/6PbI3 delivers the highest efficiency and long-term stability among I-rich compositions. This work sheds light on the fundamental structure-property relationships in the CsyFA1–yPb(BrxI1–x)3 compositional space, providing vital insight to the design of durable perovskite materials. Our approach provides a library of structural and optoelectronic information of this compositional space.

A Channel Based Fair Scheduling Scheme For Downlink High Speed Data In CDMA Networks

High speed data transmission in wireless networks demands better radio resource management schemes. The research work for this thesis considers packet scheduling in downlinks of a cellular CDMA system for delay-tolerant applications. In this thesis, a packet scheduling scheme is proposed that attempts to provide fair allocation of individuals throughout and obtain relatively high system throughput. It is based on combined consideration of channel conditions, required throughput and achieved average throughput. A priority factor and system tolerance factor are introduced. We confirmed the trade-off between system throughout (i.e., efficiency) and individual throughput (i.e., fairness) by both analysis and simulation. Relative performance between the proposed scheme and traditional schemes is evaluated through simulation to confirm the analytical observations. The sensitivity of system tolerance factor towards efficiency and fairness was also investigated. Overall, the proposed scheme performs between absolute unfairness scheme and absolute fairness scheme in term s of system throughput and fair allocation of individual throughput.

A Channel Based Fair Scheduling Scheme For Downlink High Speed Data In CDMA Networks

High speed data transmission in wireless networks demands better radio resource management schemes. The research work for this thesis considers packet scheduling in downlinks of a cellular CDMA system for delay-tolerant applications. In this thesis, a packet scheduling scheme is proposed that attempts to provide fair allocation of individuals throughout and obtain relatively high system throughput. It is based on combined consideration of channel conditions, required throughput and achieved average throughput. A priority factor and system tolerance factor are introduced. We confirmed the trade-off between system throughout (i.e., efficiency) and individual throughput (i.e., fairness) by both analysis and simulation. Relative performance between the proposed scheme and traditional schemes is evaluated through simulation to confirm the analytical observations. The sensitivity of system tolerance factor towards efficiency and fairness was also investigated. Overall, the proposed scheme performs between absolute unfairness scheme and absolute fairness scheme in term s of system throughput and fair allocation of individual throughput.

G. Czédli’s tolerance factor lattice construction and weak ordered relations

G. Czédli proved that the blocks of any compatible tolerance T of a lattice L can be ordered in such a way that they form a lattice L/T called the factor lattice of L modulo T. Here we show that the Dedekind–MacNeille completion of the lattice L/T is isomorphic to the concept lattice of the context (L, L, R), where R stands for the reflexive weak ordered relation $$ \mathord {\le } \circ T$$ ≤ ∘ T . Weak ordered relations constitute the generalization of the ordered relations introduced by S. Valentini. Reflexive weak ordered relations can be characterized as compatible reflexive relations $$R\subseteq L^{2}$$ R ⊆ L 2 satisfying $$R=\ \mathord {\le } \circ R\circ \mathord {\le } $$ R = ≤ ∘ R ∘ ≤ .