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Photophysics of excitons and free charge polarons in organic photovoltaics
Thesis (MSc)--Stellenbosch University, 2025.
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Stellenbosch : Stellenbosch University
2025
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| _version_ | 1867614107550613504 |
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| access_status_str | Open Access |
| author | Everts, Kelsey |
| author2 | Bosman, Gurthwin W.
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| author_browse | Bosman, Gurthwin W.
Everts, Kelsey |
| author_facet | Bosman, Gurthwin W.
Everts, Kelsey |
| author_sort | Everts, Kelsey |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (MSc)--Stellenbosch University, 2025. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/132343 |
| institution | Stellenbosch University (South Africa) |
| last_indexed | 2026-06-10T12:46:46.943Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2025 |
| publishDateRange | 2025 |
| publishDateSort | 2025 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/132343 Photophysics of excitons and free charge polarons in organic photovoltaics Everts, Kelsey Bosman, Gurthwin W. Tegegne, N. Stellenbosch University. Faculty of Science. Dept. of Physics. Organic photovoltaic cells Polarons Electrons -- Properties Polythiophenes -- Optical properties Fullerenes -- Optical properties Photovoltaic power generation Light absorption -- Mathematical models Optical films Conjugated polymers UCTD Thesis (MSc)--Stellenbosch University, 2025. Everts, K. 2025. Photophysics of excitons and free charge polarons in organic photovoltaics. Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/e55a18a6-bc96-4612-a29b-8ef8b9f607c1 ENGLISH ABSTRACT: In the search for global clean energy solutions, organic photovoltaics (OPVs) offer a l ightweight, cost-effective and scalable alternative to silicon solar technology. To make OPVs more economically viable however, low OPV power conversion efficiencies must be improved by optimising the active light harvesting la yer. This work investigates polythiophene:fullerene (P3HT:PC₆₁BM) films which have already found widespread application in commercial OPV devices and are thus of interest to the OPV community. To gain a deeper insight into the ultrafast recombination losses which cannot be studied with power conversion efficiency tests, this work combines nanoscale imaging, steady state absorption, photoluminescence and femtosecond transient spectroscopy measurements. Several kinetic models describing the charge photo-generation process in P3HT:PCBM films are constructed. In P3HT films, photon absorption rapidly generates bound singlet electron-hole pairs (excitons) in less than 150 fs. These recombine geminately over around 300 ps. High incident intensities exceeding 160 µJ/cm² introduce an additional recombination channel: sub-picosecond exciton-exciton annihilation (EEA). This produces free charge carriers (polarons) in addition to excitons via hot exciton dissociation. By employing a new higher order spectroscopy procedure, we demonstrate to our knowledge the first successful extraction of exciton lifetimes from EEA contaminated data on these benchmark OPV P3HT films using this method. The results are also consistent with conventional spectral deconvolution analysis. Adding PC₆₁BM forms inter- spersed P3HT and PCBM domains (a bulk heterojunction) where excitons and polarons are also generated in less than 150 fs after photoexcitation. Unlike in P3HT, here polaron generation occurs via exciton dissociation and charge transfer at P3HT:PCBM interfaces. Long polaron lifetimes into the nanosecond regime, ideal for efficient charge collection in OPVs, were found. To further enhance photon harvesting, silver nanoparticles were also incorporated. These have demonstrated improved efficiencies in other polymer blends, but had not yet been tested in P3HT:PCBM or studied before with ultrafast spectroscopy. Results indicate the nanoparticles formed micron-sized clusters upon incorporation into P3HT:PCBM such that no light harvesting enhancements were observed. Instead the nanoparticles increased light scattering which did not affect the blend’s exciton and polaron generation or recombination rates. AFRIKAANSE OPSOMMING: In die soektog na globale oplossings vir skoon energie bied Organiese Fotovoltaïese Toestelle (OFVT) ’n ligge- wig, koste-effektiewe en skaalbare alternatief v ir s ilikon s onkrag t egnologie. Om OFVT’s e gter m eer ekonomies volhoubaar te maak, moet lae OFVT-kragomsettingsdoeltreffendheid verbeter word deur die aktiewe lig-oeslaag te optimaliseer. Hierdie werk ondersoek politiofeen:fullereen (P3HT:PC₆₁BM) films w at r eeds wydverspreide toepassing in kommersiële OFVT gevind het en dus van belang is vir die OFVT-gemeenskap. Om ’n dieper insig te verkry in die ultravinnige rekombinasieverliese wat nie met kragomskakelingsdoeltreffendheidstoetse bestudeer kan word nie, gebruik hierdie werk nanoskaalafbeelding, bestendige-toestand absorpsie, fotolumines- censie en femtosekonde-tydafhanklike spektroskopie metings. Verskeie kinetiese modelle wat die lading-foto- genererings proses in P3HT:PCBM-films b eskryf, w ord s aamgestel. I n P 3HT-films ge nereer fotonabsorpsie gebonde elektron-holte-pare (eksitone) binne 150 fs. Hierdie herkombineer oor ongeveer ∼300 ps. Indien hoë invalvalintensiteite van meer as 160 µJ/cm² gebruik word, kom daar ’n bykomende herkombinasiekanaal voor: sub-pikosekonde eksiton-eksiton vernietiging (EEV). Dit produseer ladingdraers (polarone) asook eksitone deur warm eksitondissosiasie. Deur gebruik te maak van ’n hoër-orde spektroskopie prosedure, demonstreer ons na ons kennis die eerste suksesvolle onttrekking van eksiton leeftye uit EEV besmette data van OFVT P3HT films. Die resultate stem ook ooreen met konvensionele spektrale dekonvolusie-analise. Deur PC₆₁BM by te voeg, vorm films m et a fgewisselde P 3HT- e n P CBM- d omeine ( groot h eterojunksie) w aar e ksitone e n p olarone ook binne 150 fs na foto-opwekking gegenereer word. Anders as in P3HT, vind hier polarongenerering plaas via eksiton-dissosiasie by die P3HT:PCBM-koppelvlakke. Lang polaron-leeftye tot in die nanosekonde-bereik, ideaal vir doeltreffende l adingversameling i n O FVT, i s g evind. O m f oton-oes v erder te verbeter, i s s ilwer nanoparti- kels ook ingewerk. Dit het verbeterde doeltreffendheid i n a nder p olimeermengsels g etoon, m aar i s n og n ie in P3HT:PCBM getoets of met ultravinnige spektroskopie bestudeer nie. Resultate dui aan dat die nanopartikels mikrongrootte trosse gevorm het by inkorporering in P3HT:PCBM sodat geen lig-oes verbeterings waargeneem is nie. In plaas daarvan het die nanopartikels ligverstrooiing verhoog wat nie die eksiton- en polarongenerering of rekombinasietempo’s beïnvloed het nie. Masters 2025-06-04T10:06:31Z 2025-06-04T10:06:31Z 2025-03 Thesis https://scholar.sun.ac.za/handle/10019.1/132343 Stellenbosch University vii, 80 pages : illustrations application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Organic photovoltaic cells Polarons Electrons -- Properties Polythiophenes -- Optical properties Fullerenes -- Optical properties Photovoltaic power generation Light absorption -- Mathematical models Optical films Conjugated polymers UCTD Everts, Kelsey Photophysics of excitons and free charge polarons in organic photovoltaics |
| title | Photophysics of excitons and free charge polarons in organic photovoltaics |
| title_full | Photophysics of excitons and free charge polarons in organic photovoltaics |
| title_fullStr | Photophysics of excitons and free charge polarons in organic photovoltaics |
| title_full_unstemmed | Photophysics of excitons and free charge polarons in organic photovoltaics |
| title_short | Photophysics of excitons and free charge polarons in organic photovoltaics |
| title_sort | photophysics of excitons and free charge polarons in organic photovoltaics |
| topic | Organic photovoltaic cells Polarons Electrons -- Properties Polythiophenes -- Optical properties Fullerenes -- Optical properties Photovoltaic power generation Light absorption -- Mathematical models Optical films Conjugated polymers UCTD |
| url | https://scholar.sun.ac.za/handle/10019.1/132343 |
| work_keys_str_mv | AT evertskelsey photophysicsofexcitonsandfreechargepolaronsinorganicphotovoltaics |