{"id":{"repo_id":"uiuc","oai_identifier":"oai:www.ideals.illinois.edu:2142/95315"},"canonical_url":"https://search.dev.ndltd.org/etd/uiuc/oai:www.ideals.illinois.edu:2142/95315","repository":{"repo_id":"uiuc","name":"University of Illinois - Urbana-Champaign","base_url":"https://www.ideals.illinois.edu/oai-pmh"},"display":{"title":"Carrier transport, recombination, and the effects of grain boundaries in polycrystalline cadmium telluride thin films for photovoltaics","abstract":"Cadmium Telluride (CdTe), a chalcogenide semiconductor, is currently used as the absorber layer in one of the highest efficiency thin film solar cell technologies. Current efficiency records are over 22%. In 2011, CdTe solar cells accounted for ~8% of all solar cells installed. This is because, in part, CdTe has a low degradation rate, high optical absorption coefficient, and high tolerance to intrinsic defects. Solar cells based on polycrystalline CdTe exhibit a higher short-circuit current, fill factor, and power conversion efficiency than their single crystal counterparts. This is despite the fact that polycrystalline CdTe devices exhibit lower open-circuit voltages. This is contrary to the observation for silicon and III-V semiconductors, where material defects cause a dramatic drop in device performance. For example, grain boundaries in covalently-bonded semiconductors (a) act as carrier recombination centers, and (b) lead to localized energy states, causing carrier trapping. Despite significant research to date, the mechanism responsible for the superior current collection properties of polycrystalline CdTe solar cells has not been conclusively answered. This dissertation focuses on the macro-scale electronic band structure, and micro scale electronic properties of grains and grain boundaries in device-grade CdTe thin films to answer this open question. My research utilized a variety of experimental techniques. Samples were obtained from leading groups fabricating the material and devices. A CdCl2 anneal is commonly performed as part of this fabrication and its effects were also investigated. Photoluminescence (PL) spectroscopy was employed to study the band structure and defect states in CdTe polycrystals. Cadmium vacancy- and chlorine-related states lead to carrier recombination, as in CdTe films grown by other methods. Comparing polycrystalline and single crystal CdTe, showed that the key to explaining the improved performance of polycrystalline CdTe does not lie in macroscopic analysis. The nanoscale majority carrier concentration was studied using scanning microwave impedance microscopy, which revealed an existence of majority carrier depletion along the grain boundaries, independent of the growth process used, which was absent in films that were not subjected to CdCl2 annealing. This effect promotes carrier separation and collection. Conductive atomic force microscopy showed enhanced conduction of electrons along the grain boundaries in samples subjected to the CdCl2 anneal treatment while holes were shown to move through the grain bulk. The separation of conduction channels minimizes recombination while simultaneously reducing series resistance and hence enhancing fill factor. Several technical capabilities demonstrated in this work can be easily extended to other semiconductor materials.","abstract_html":"Cadmium Telluride (CdTe), a chalcogenide semiconductor, is currently used as the absorber layer in one of the highest efficiency thin film solar cell technologies. Current efficiency records are over 22%. In 2011, CdTe solar cells accounted for ~8% of all solar cells installed. This is because, in part, CdTe has a low degradation rate, high optical absorption coefficient, and high tolerance to intrinsic defects. Solar cells based on polycrystalline CdTe exhibit a higher short-circuit current, fill factor, and power conversion efficiency than their single crystal counterparts. This is despite the fact that polycrystalline CdTe devices exhibit lower open-circuit voltages. This is contrary to the observation for silicon and III-V semiconductors, where material defects cause a dramatic drop in device performance. For example, grain boundaries in covalently-bonded semiconductors (a) act as carrier recombination centers, and (b) lead to localized energy states, causing carrier trapping. Despite significant research to date, the mechanism responsible for the superior current collection properties of polycrystalline CdTe solar cells has not been conclusively answered. This dissertation focuses on the macro-scale electronic band structure, and micro scale electronic properties of grains and grain boundaries in device-grade CdTe thin films to answer this open question. My research utilized a variety of experimental techniques. Samples were obtained from leading groups fabricating the material and devices. A CdCl2 anneal is commonly performed as part of this fabrication and its effects were also investigated. Photoluminescence (PL) spectroscopy was employed to study the band structure and defect states in CdTe polycrystals. Cadmium vacancy- and chlorine-related states lead to carrier recombination, as in CdTe films grown by other methods. Comparing polycrystalline and single crystal CdTe, showed that the key to explaining the improved performance of polycrystalline CdTe does not lie in macroscopic analysis. The nanoscale majority carrier concentration was studied using scanning microwave impedance microscopy, which revealed an existence of majority carrier depletion along the grain boundaries, independent of the growth process used, which was absent in films that were not subjected to CdCl2 annealing. This effect promotes carrier separation and collection. Conductive atomic force microscopy showed enhanced conduction of electrons along the grain boundaries in samples subjected to the CdCl2 anneal treatment while holes were shown to move through the grain bulk. The separation of conduction channels minimizes recombination while simultaneously reducing series resistance and hence enhancing fill factor. Several technical capabilities demonstrated in this work can be easily extended to other semiconductor materials.","abstract_has_math":false,"creators":["Tuteja, Mohit"],"institution":"University of Illinois at Urbana-Champaign","degree_name":"Ph.D.","degree_level":"Dissertation","degree_discipline":"Materials Science & Engr","degree_department":null,"school":null,"contributors":["Rockett, Angus A.","Abelson, John R.","Ertekin, Elif","Schleife, André"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2017,"date_issued":"2017-03-01T15:48:41Z","date_published":"2017-03-01T15:48:41Z","updated_at":"2026-07-22T22:26:37Z","subjects":["Cadmium Telluride","Grain Boundaries","Carrier Concentrations","Conductivity","Cadmium Chloride Anneal Treatment"],"languages":["en"],"rights":["All rights reserved by Mohit Tuteja."],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"http://hdl.handle.net/2142/95315","outbound_label":"Handle","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Rockett, Angus A.","Abelson, John R.","Ertekin, Elif","Schleife, André"]},{"key":"dc:creator","label":"Author","values":["Tuteja, Mohit"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2017-03-01T15:48:41Z","2016-11-16","2016-12"]},{"key":"dc:type","label":"Dc Type","values":["text"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Materials Science & Engr"]},{"key":"thesis:degree_level","label":"Degree Level","values":["Dissertation"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Ph.D."]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["University of Illinois at Urbana-Champaign"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Cadmium Telluride","Grain Boundaries","Carrier Concentrations","Conductivity","Cadmium Chloride Anneal Treatment"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["All rights reserved by Mohit Tuteja."]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["http://hdl.handle.net/2142/95315"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["Cadmium Telluride (CdTe), a chalcogenide semiconductor, is currently used as the absorber layer in one of the highest efficiency thin film solar cell technologies. Current efficiency records are over 22%. In 2011, CdTe solar cells accounted for ~8% of all solar cells installed. This is because, in part, CdTe has a low degradation rate, high optical absorption coefficient, and high tolerance to intrinsic defects. Solar cells based on polycrystalline CdTe exhibit a higher short-circuit current, fill factor, and power conversion efficiency than their single crystal counterparts. This is despite the fact that polycrystalline CdTe devices exhibit lower open-circuit voltages. This is contrary to the observation for silicon and III-V semiconductors, where material defects cause a dramatic drop in device performance. For example, grain boundaries in covalently-bonded semiconductors (a) act as carrier recombination centers, and (b) lead to localized energy states, causing carrier trapping. Despite significant research to date, the mechanism responsible for the superior current collection properties of polycrystalline CdTe solar cells has not been conclusively answered. This dissertation focuses on the macro-scale electronic band structure, and micro scale electronic properties of grains and grain boundaries in device-grade CdTe thin films to answer this open question. My research utilized a variety of experimental techniques. Samples were obtained from leading groups fabricating the material and devices. A CdCl2 anneal is commonly performed as part of this fabrication and its effects were also investigated. Photoluminescence (PL) spectroscopy was employed to study the band structure and defect states in CdTe polycrystals. Cadmium vacancy- and chlorine-related states lead to carrier recombination, as in CdTe films grown by other methods. Comparing polycrystalline and single crystal CdTe, showed that the key to explaining the improved performance of polycrystalline CdTe does not lie in macroscopic analysis. The nanoscale majority carrier concentration was studied using scanning microwave impedance microscopy, which revealed an existence of majority carrier depletion along the grain boundaries, independent of the growth process used, which was absent in films that were not subjected to CdCl2 annealing. This effect promotes carrier separation and collection. Conductive atomic force microscopy showed enhanced conduction of electrons along the grain boundaries in samples subjected to the CdCl2 anneal treatment while holes were shown to move through the grain bulk. The separation of conduction channels minimizes recombination while simultaneously reducing series resistance and hence enhancing fill factor. Several technical capabilities demonstrated in this work can be easily extended to other semiconductor materials.","Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2017-02-28 without embargo terms","The student, Mohit Tuteja, accepted the attached license on 2016-11-14 at 12:22.","The student, Mohit Tuteja, submitted this Dissertation for approval on 2016-11-14 at 12:55.","This Dissertation was approved for publication on 2016-11-16 at 11:41.","DSpace SAF Submission Ingestion Package generated from Vireo submission #10240 on 2017-02-28 at 14:47:32","Made available in DSpace on 2017-03-01T15:48:41Z (GMT). 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Current efficiency records are over 22%. In 2011, CdTe solar cells accounted for ~8% of all solar cells installed. This is because, in part, CdTe has a low degradation rate, high optical absorption coefficient, and high tolerance to intrinsic defects. Solar cells based on polycrystalline CdTe exhibit a higher short-circuit current, fill factor, and power conversion efficiency than their single crystal counterparts. This is despite the fact that polycrystalline CdTe devices exhibit lower open-circuit voltages. This is contrary to the observation for silicon and III-V semiconductors, where material defects cause a dramatic drop in device performance. For example, grain boundaries in covalently-bonded semiconductors (a) act as carrier recombination centers, and (b) lead to localized energy states, causing carrier trapping. Despite significant research to date, the mechanism responsible for the superior current collection properties of polycrystalline CdTe solar cells has not been conclusively answered. This dissertation focuses on the macro-scale electronic band structure, and micro scale electronic properties of grains and grain boundaries in device-grade CdTe thin films to answer this open question. My research utilized a variety of experimental techniques. Samples were obtained from leading groups fabricating the material and devices. A CdCl2 anneal is commonly performed as part of this fabrication and its effects were also investigated. Photoluminescence (PL) spectroscopy was employed to study the band structure and defect states in CdTe polycrystals. Cadmium vacancy- and chlorine-related states lead to carrier recombination, as in CdTe films grown by other methods. Comparing polycrystalline and single crystal CdTe, showed that the key to explaining the improved performance of polycrystalline CdTe does not lie in macroscopic analysis. The nanoscale majority carrier concentration was studied using scanning microwave impedance microscopy, which revealed an existence of majority carrier depletion along the grain boundaries, independent of the growth process used, which was absent in films that were not subjected to CdCl2 annealing. This effect promotes carrier separation and collection. Conductive atomic force microscopy showed enhanced conduction of electrons along the grain boundaries in samples subjected to the CdCl2 anneal treatment while holes were shown to move through the grain bulk. The separation of conduction channels minimizes recombination while simultaneously reducing series resistance and hence enhancing fill factor. Several technical capabilities demonstrated in this work can be easily extended to other semiconductor materials.","Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2017-02-28 without embargo terms","The student, Mohit Tuteja, accepted the attached license on 2016-11-14 at 12:22.","The student, Mohit Tuteja, submitted this Dissertation for approval on 2016-11-14 at 12:55.","This Dissertation was approved for publication on 2016-11-16 at 11:41.","DSpace SAF Submission Ingestion Package generated from Vireo submission #10240 on 2017-02-28 at 14:47:32","Made available in DSpace on 2017-03-01T15:48:41Z (GMT). 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