A near infrared femtosecond laser source for observation of charge transfer processes in semiconductors

Ahmed, Essraa Taha (2017-03)

Thesis (MSc)--Stellenbosch University, 2017

Thesis

ENGLISH ABSTRACT : Near infrared femtosecond laser pulses were generated in the spectral range 900 - 2400 nm using the idler of a noncollinear optical parametric amplifier (NOPA). The amplitude and phase of four exemplary pulses were retrieved using frequency resolved optical gating (XFROG). The durations of these pulses were found to be ≤ 151 fs. The generated pulses were used as a probe to detect the charge dynamics within two types of organic solar cells: indoline DN216/ZnO dye sensitized solar cell (DSSC) and fluorinated Zinc Phthalocyanine FnZnPc/C60 (n = 0, 4, 8, 16) bilayer photovoltaic cell. The two organic cells utilize organic materials as light absorbers. In the DN216/ZnO DSSC, electrons are injected from the dye, the electron donor, into the conduction band of the inorganic semiconductor ZnO upon photoexcitation. The injection into the conduction band of ZnO was found to occur directly in 200 fs, via a neutral charge transfer state (NCT) in 2 ps or via an ionic charge transfer state (ICT) in 10 ps [1]. In the FnZnPc/C60 (n = 0, 4, 8, 16) cells, electrons are injected into the C60 layer after dissociation of excitons at the donor/acceptor heterojunction [2]. A C− 60 anion is formed due to injection of electrons into the C60 layer as well as by direct photoexcitation of the molecule. The C− 60 formation was found to occur instantaneously after photoexcitation of either the donor or the acceptor material, independent of the energetics at the donor/acceptor heterojunction. Charge recombinations of the C− 60 anion within the FnZnPc/C60, n = 0, 4, 8, 16 cells were found to occur in the bulk of the C60 layer in 4 ps ± 1 ps as well as at the surface of the C60 layer with air in 43 ps ± 9 ps.

AFRIKAANSE OPSOMMING : Naby-infrarooi-femtosekonde laser pulse, in die spektrale gebied 900-2400 nm, is gegenereer met 'n nie-saamlynige parametriese optiese versterker deur gebruik te maak van die tweede uitset sein. Die amplitude en fase van vier stelle pulse is gemeet deur gebruik te maak van frekwensie analiese optiese monster metode (XFROG). Die durasie van die pulse was ≤ 151 fs. Die gegenereerde pulse is aangewend om die ladings dinamika te ondersoek van twee tipes organiese son selle: indolien DN216/ZnO kleursel-gesensiteerdeson-sel (DSSC) en gefloorineerde sink Phthalocyanine FnZnPc/C60 (n = 0, 4, 8, 16) dubbel-laag fotovoltaisesel. Die twee organiese selle maak gebruik van die organiese materiaale om lig te absorbeer. In die DN216/ZnO DSSC word elektrone gedoneer vanaf die kleursel, die elektron donor, en oorgedra aan die geleidende-band van die anorganiese semi-geleier ZnO waneer foto-opwekking plaas vind. Dit is bevind dat die elektron-oordrag na die geleidings-band van die anorganiese half-geleier ZnO plaas vind in 200fs, die oordrag vind plaas via die neutrale ladings oordrags toestand (NCT) in 2 ps of langs 'n ioniese ladings oordrags toestand (ICT) in 10 ps [1]. In die geval van FnZnPc/C60 (n = 0, 4, 8, 16) selle word elektrone oorgedra na die C60 vlak na dissosiasie van eksitone by die donor/akseptor intervlak [2]. 'n C− 60 anioon word gevorm weens die oordrag van elektrone na die C60 laag sowel as deur direkte foto-opwekking van die molekule. Dit is bevind dat die C60 formasie oombliklik plaas vind na foto-opwekking van enige van die donor of die akseptor materiaal, onafhanklik van die energie toestande by die donor/akseptor hetero-intervlak. Daar is bevind dat, ladings herkombinasie in die FnZnPc/C60, n = 0, 4, 8, 16 selle plaas vind in die bulk van die C60 laag in 4 ps ±1 ps sowel as by die intervlak van die oppervlak van die C60 laag en lug in 43 ps ± 9ps.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101048
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