Accelerating Stochastic Composition Optimization

Consider the stochastic composition optimization problem where the objective is a composition of two expected-value functions. We propose a new stochastic first-order method, namely the accelerated stochastic compositional proximal gradient (ASC-PG) method, which updates based on queries to the sampling oracle using two different timescales. The ASC-PG is the first proximal gradient method for the stochastic composition problem that can deal with nonsmooth regularization penalty. We show that the ASC-PG exhibits faster convergence than the best known algorithms, and that it achieves the optimal sample-error complexity in several important special cases. We further demonstrate the application of ASC-PG to reinforcement learning and conduct numerical experiments

Diffusion Approximations for Online Principal Component Estimation and Global Convergence

In this paper, we propose to adopt the diffusion approximation tools to study the dynamics of Oja's iteration which is an online stochastic gradient descent method for the principal component analysis. Oja's iteration maintains a running estimate of the true principal component from streaming data and enjoys less temporal and spatial complexities. We show that the Oja's iteration for the top eigenvector generates a continuous-state discrete-time Markov chain over the unit sphere. We characterize the Oja's iteration in three phases using diffusion approximation and weak convergence tools. Our three-phase analysis further provides a finite-sample error bound for the running estimate, which matches the minimax information lower bound for principal component analysis under the additional assumption of bounded samples.Comment: Appeared in NIPS 201

Ta₃N₅/Polymeric g-C₃N₄ as Hybrid Photoanode for Solar Water Splitting:

Thesis advisor: Dunwei WangWater splitting has been recognized as a promising solution to challenges associated with the intermittent nature of solar energy for over four decades. A great deal of research has been done to develop high efficient and cost-effective catalysts for this process. Among which tantalum nitride (Ta₃N₅) has been considered as a promising candidate to serve as a good catalyst for solar water splitting based on its suitable band structure, chemical stability and high theoretical efficiency. However, this semiconductor is suffered from its special self-oxidation problem under photoelectrochemical water splitting conditions. Several key unique properties of graphitic carbon nitride (g-C₃N₄) render it an ideal choice for the protection of Ta₃N₅. In this work, Ta₃N₅/g-C₃N₄ hybrid photoanode was successfully synthesized. After addition of co-catalyst, the solar water splitting performance of this hybrid photoanode was enhanced. And this protection method could also act as a potential general protection strategy for other unstable semiconductors.Thesis (MS) — Boston College, 2018.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Chemistry

Cost-effective Copper-plated Metallisation for III-V Solar Cells: A Development Path

III-V solar cells can achieve the highest solar energy conversion efficiency of any photovoltaic device. However, since the technology has been developed for use on spacecraft, the manufacturing techniques developed to date are specialised and costly and inhibit its commercial development for terrestrial applications. To meet terrestrial industrial needs and make III-V solar cells cost competitive, cost reduction of front metallisation is of interest. In the conventional III-V solar cell fabrication processes, precious and costly metal gold (Au) and time-consuming vacuum evaporation are typically used. Copper (Cu) plating can provide a solution to reduce the consumption of Au and to decrease the costs for III-V photovoltaic manufacturing. While Cu plating technology has been extensively researched and explored in the silicon (Si) photovoltaic industry for many years, there has been a lack of attention given to studying its application on III-V solar cells. This work developed and fabricated a Cu-plated front metallisation process for GaAs-based III-V solar cells, focusing on the metallurgy associated with the different interfaces. Then, the characterisation results of GaAs-based solar cells with Cu-based front electrodes are presented for performance comparison with conventionally fabricated GaAs-based solar cells. The diffusion barrier systems designed in this work is proven to efficiently exhibit Cu penetration to GaAs. Moreover, III-V solar cells with Cu-based front contacts developed in this work showed similar output performance to the cells with traditional Au-based electrodes, suggesting the possibility of achieving comparable efficiencies with lower cost in III-V solar cell manufacturing. This work also compared the optical performance of five light management finger designs in concentrated photovoltaics systems with two different refractive homogenizers for further reduction of optical losses. In summary, a development path of copper-plated metallisation for III-V solar cell is presented in this work, providing potentials to largely decrease manufacturing costs via replacing the current Au metallisation, without sacrifices of high efficiencies of III-V solar cells