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E1.01 - Material Design for Novel-Concept-Based Solar Cells --Sulfurization or Oxidization of "Cheap'' Metals 
April 22, 2014   8:30am - 9:00am

Even though extraordinary progress has been made in the field of Si and Cu(In,Ga)(S,Se)2 (CIGS) solar cells/modules over the years and their performance significantly improved, fabrication/material costs and safety still remain major concerns. In terms of material design for compound semiconductors, the most cost-effective, safe, and easy-handling anions are sulfur and oxygen. Nowadays, to obtain compound semiconductors by reacting inexpensive metal cations (precursors) with gas-phase anions has become a significant issue. For realizing this, sulfurization and oxidation are appropriate physical processes because they are based on simple thermal diffusion of anions into a metal precursor. In fact, sulfurization/selenization techniques are adopted for commercially manufacturing CIGS solar modules because of the cost effectiveness, scalability, and uniformity of such techniques.Tin monosulfide (SnS) has a direct energy bandgap of 1.3 eV and a high optical absorption coefficient of 104 cm−1. Therefore, SnS is perceived to be a promising candidate as a cost-effective and earth-abundant inorganic material for use in the fabrication of next-generation solar cells. Although the theoretical conversion efficiency of SnS-based solar cells is high, the demonstrated efficiencies of such cells are still low. Some of the reasons for low efficiencies are considered to be the poor crystal quality of a SnS layer, and the mismatch in the band diagram at a pn-heterojunction. In fact, most researchers simply refer to the CIGS solar cells structure. The growth mechanism of SnS or the appropriate band offset of SnS-based solar cells has not yet been clarified because it shows metastable phase.Wide-bandgap transparent conducting oxide (TCO) films exhibit only n-type conductivity. On the other hand, NiO shows only p-type conductivity, and is composed of inexpensive and less toxic elements. Recently, NiO has also been used in visibly transparent solar cells, which are attractive because their optical transparency permits greater flexibility in terms of installation locations. In addition, NiO films can be easily obtained just by the oxidation of Ni. We have proposed a “NiO-based invisible sensor” that receives electrical power from a “NiO-based invisible solar cell”. This sensor can be fixed on a wall or ceiling without a wired connection. Moreover, the user will not be aware of these invisible sensors and will not experience the stress associated with being monitored. Therefore, there is a great market opportunity for various environments, and the application possibilities for invisible devices based on NiO-based solar cells are limited only by imagination.In this presentation, we will introduce some earth-abundant solar absorbers (SnS, Cu2SnS3, Cu2O, and related compounds) and TCOs (NiO, CuAlO2, and related compounds) for use in photovoltaics, with focus on materials research and device development, in terms of the most recent advances and experimental results.

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Keynote Address
Panel Discussion - Different Approaches to Commercializing Materials Research
Business Challenges to Starting a Materials-Based Company
Fred Kavli Distinguished Lectureship in Nanoscience
Application of In-situ X-ray Absorption, Emission and Powder Diffraction Studies in Nanomaterials Research - From the Design of an In-situ Experiment to Data Analysis