Browsing by Author "Chen, Lei"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Development and characterization of (Ag,Cu)(In,Ga)Se2 thin films deposited by three-stage co-evaporation(University of Delaware, 2016) Chen, LeiA novel thin-film semiconductor material (Ag,Cu)(In,Ga)Se2 (ACIGS) has kindled interest on new chalcopyrite and even kesterite system alloyed with silver (Ag) with its achievement in photovoltaic application. Solar cells based on ACIGS absorbers have reached several efficiency milestones at a variety of bandgaps (Eg), including a champion device built on a soda-lime glass substrate at Eg=1.2 eV achieving 19.9%, a device on glass substrate at Eg=1.5 eV with 13.0% efficiency, and a device on polyimide substrate at Eg=1.35 eV with 17.9% efficiency. Compared with the more mature thin-film photovoltaic absorbers Cu(In,Ga)Se2 (CIGS), ACIGS features lower melting temperature (Tm), wider Eg and even presumably different defect formations. The lowered melting temperature could lead to larger grain size and possibly reduced defects, if films are deposited at the same growth temperature (Tss). The adjustability of the Eg via Ag incorporation offers a new pathway to wide-bandgap CIGS absorber which historically has been obstructed by the defects introduced from widening Eg via Ga alloying only. Different synthesis methods have been adopted to ACIGS deposition from CIGS growth including element co-evaporation and sputtered metal precursor reaction. A particular co-evaporation method call three-stage co-evaporation is the focus of the study, and this dissertation investigates and discusses how the material evolves throughout this deposition process and explores unique properties arising from the incorporation of Ag. ☐ The three-stage co-evaporation process has been widely used and successfully produced high performance CIGS absorbers. The suitability of depositing ACIGS films using this process and growth at higher substrate temperature were evaluated with the use of specialty high temperature glass as substrates. Thin-film ACIGS was deposited by the three-stage process at both Tss=580°C and 650°C. It was determined by secondary ion mass spectroscopy (SIMS) that composition ratios Ag/(Ag+Cu) and Ga/(In+Ga) formed a hump and notch shape gradient profiles, respectively, and the gradients were flattened by the 650°C growth temperature. Using an intercept counting method, the grain size of ACIGS films was found almost twice as large as that of corresponding CIGS films grown under the same conditions, and also twice as large at higher growth temperature. These observations are consistent with a zone growth model with grain growth affected by Tm/Tss. ACIGS films exhibited the structure of a single chalcopyrite phase and their film texture wasn't affected by different growth temperature. ☐ Secondary phases, besides the primary chalcopyrite phase, have been shown in literature to be one of the key features in CIGS three-stage deposition influencing films' surface morphology, Ga gradient, distribution of sodium diffused from glass substrates and eventually device performance. To understand and optimize the ACIGS three-stage growth, a systematic study was conducted on possible secondary phases formed during the group-I rich [(Ag+Cu)/(In+Ga)>1] ACIGS growth, by depositing films with Ag/(Ag+Cu)=0, 0.1 and 0.5 until the end of the second stage of the three-stage process. SIMS depth profiling located secondary phases containing Ag-Cu-Se compounds near the film surfaces with composition ratio (Ag+Cu)/(In+Ga)>1 and higher Ag/(Ag+Cu) than in the bulk films. Scrutiny of this surface region by transmission electron microscopy discovered an intermixture of the secondary and chalcopyrite phases when Ag was added, distinct from a separate layer of secondary phases when there was no Ag. X-ray diffraction identified the secondary phases as HT--Cu2-xSe, RT--Cu2Se, HT--(Ag,Cu)2-xSe, RT--AgCuSe and RT--Ag2Se structures. Their existence in the films depended on Ag/(Ag+Cu), which is consistent with the phases along the Cu2Se -- Ag2Se pseudo-binary tie-line. Through changes in the secondary phases, the increase in Ag/(Ag+Cu) smoothed the film surfaces, reduced facets and voids, suppressed sodium accumulation, and decreased the Ga gradient. ☐ Possible preferential reaction involving Ag and other constituent elements urge an examination of the composition variation in ACIGS films at microscopic scales. Such variation has been suspected to be a source of non-uniformity in CIGS films that were detected by electron beam-induced current, surface electric potential and photoluminescence (PL) etc. Using energy dispersive x-ray spectroscopy in a scanning transmission electron microscope, the composition gradients along ACIGS films' growth direction were found independent of grain distribution and exist even within a single grain. The gradients matched those measured by SIMS at sub-millimeter scales, indicating small composition variation along films' planar direction. The planar composition variation was not correlated with Ga/(In+Ga) or presence of Ag. However, even small, this variation can generate Eg fluctuation that is comparable with the non-uniformity derived from absorptance, PL and electroluminescence measurements. As extracted from microscopy images, grain size clearly increased along the film growth direction, and the grain growth seemed affected by both Ag and Ga composition. ☐ The findings in this dissertation can aid the optimization of the ACIGS deposition process, and provide reference for studies on junction formation with buffer layers and device non-uniformity etc.Item Problematizing the critical thinking concept: perspectives of Chinese undergraduate students and their U.S. university faculty(University of Delaware, 2017) Chen, LeiThis qualitative study examined reflections of 12 Chinese students who studied in a U.S. college, and 10 of their U.S. faculty in terms of their conceptualization of critical thinking. Throughout the study, a situated cognitive framework was applied to analyze the interview data and explore the concept of critical thinking. ☐ The participants of this study were asked to explicitly define critical thinking, and to assess the importance of critical thinking to students’ learning and students’ lives. The study also explored whether and how critical thinking was related to some of the challenges that the Chinese students faced as students studying abroad in the U.S. ☐ The results showed that the definitions of critical thinking provided by the U.S. faculty and Chinese students were embedded in different contexts. The professors were more likely to define critical thinking as universal thinking skills embedded in certain disciplines. The Chinese students were more likely to contextualize critical thinking in their study abroad experiences, as well as in the purpose of activities, and in their future jobs. The U.S. faculty and Chinese students also assessed the importance of critical thinking in this study. Generally speaking, U.S. faculty attached more importance to critical thinking than the Chinese students. Some of the Chinese students regarded universal critical thinking skill as not transferable to their work and their life, and hence, not that important. Finally, U.S. faculty and Chinese students reflected that attributing some of the challenges that Chinese students faced while studying abroad to critical thinking might be problematic. Chinese students’ descriptions of the challenges that they faced in Western university were not necessarily related to critical thinking. Those descriptions tend to focus on whether it was safe to express themselves in class, whether their voices and experiences were valued in the class community, whether they understood the interactional patterns present in a U.S. university and their English language proficiency. ☐ Examining the concept of critical thinking through Chinese students’ perspectives and experiences is significant to the field of international education for several reasons. To start, it problematizes critical thinking concept as a standard that is used to measures students’ skills and abilities. The study also demonstrates conceptualizations of critical thinking in ways that consider Chinese students’ social, cultural, and educational backgrounds instead of in ways that focus on deficits of Chinese students while they study abroad in Western countries. Further, this study is informative because the challenges that Chinese students faced in this study might be faced by other Chinese students who are studying abroad. The U.S. faculty and Chinese students’ reflections on what helped Chinese student conquer their difficulties might be helpful for teachers and researchers of Chinese students. Finally, by interviewing Chinese students, this study incorporated Chinese students’ voices in studies that consider the role of critical thinking.