Charge dynamics in hybrid and organic-inorganic light harvesting thin films followed with femtosecond transient absorption spectroscopy

Minda, Iulia (2017-12)

Thesis (PhD)--Stellenbosch University, 2017

Thesis

ENGLISH ABSTRACT : In order to bridge the gap between traditional sources of electricity and the increasing global demand for it, as a society we must move towards renewable sources of energy such as solar radiation. Photovoltaic devices (PVs) harness solar power and convert it to electrical power. In order to be commercially viable, they need to be efficient, cost effective, simple to fabricate and environmentally friendly. To address these requirements, the class of emerging PVs arose, which includes dye sensitised solar cells (DSSCs) and perovskite solar cells. Femtosecond transient absorption spectroscopy (TAS) is an experimental technique which allows us to follow the ultrafast photoinduced charge dynamics in real time in light harvesting thin films and PVs. By assigning time and rate constants to various processes governing the charge generation and extraction in solar cells, we construct charge dynamics models, and therefore learn the fundamental photophysics reasons behind what makes the power conversion efficiencies (PCEs) of some solar cells superior. In particular, this study focused on the charge transfer processes in indoline dye (DN216) sensitised electrodeposited ZnO solar cells, and the charge recombination dynamics in FA0.85MA0.15PbI2.55Br0.45 perovskite thin films. To construct the simplest fully consistent charge dynamics models, we match the visible and near-infrared spectroscopic signals of our samples to the allowed electronic transitions, and follow their temporal evolutions on the femtosecond and picosecond time scales. From our measured time and rate constants we observed that ZnO based DSSCs are less efficient than their TiO2 counterparts because the electron injection from the photoexcited indoline dye into the ZnO CB doesn’t just occur directly (< 200 fs), but also stepwise via neutral (∼ 2 ps) and ionic (∼ 10 ps) intermediate charge transfer states, resulting from surface trap states characteristic of electrodeposited ZnO. Moreover, FA0.85MA0.15PbI2.55Br0.45 is an excellent hybrid photoabsorber in record efficiency perovskite solar cells because even at high charge carrier densities of 1019 cm−3 , the third order non-radiative Auger recombination mechanism is not dominant. Furthermore we determined the associated geminate, non-geminate and Auger recombination rate constants as A = 5 × 109 s −1 , B = 10−10 s −1 cm3 and C = 50 × 10−32 s −1 cm6 .

AFRIKAANSE OPSOMMING : Die gaping tussen tradisionele bronne van elektrisiteit en die aanvraag daarvoor moet oorbrug word, as ’n wereld gemeenskap moet ons beweeg in die rigting van herwin- ˆ bare energie bronne soos son krag. Fotovolta¨ıse toestelle (PVs) omskep son krag na elektriese krag. Om ekonomies vatbaar te wees, moet hulle effektief, koste effektief en eenvoudig wees om te vervaardig in ’n omgewings vriendelike manier. Om hierdie vereistes aan te spreek het ’n klas PVs ontstaan waaronder kleursel-gesensiteerdesonselle (DSSCs) en perovskiet son-selle val. Femtosekonde leeftyd absorpsie spektroskopie (TAS) is ’n eksperimentele tegniek wat ons toelaat om die super vinnige fotogeinduseerde dinamika soos dit plaas vind in lig opnemende dun film materiaal waar te neem. Deur leeftyds konstante waardes toe te ken vir die verskillende prosesse wat by dra tot ladings generasie en ekstraksie in son-selle kan ons dinamiese modelle saam te stel om sodoende iets te leer oor die fundamentele fotofisika agter die sonsel kragopwekking effektiwiteit (PCEs). In hierdie studie het ons meer spesifiek gefokus op die ladings-oordrag prosesse van indolien kleursel (DN216) gesensiteerde sink elektrolities gedeponeerde son-selle sowel as die ladings herkombinasie dinamika in FA0.85- MA0.15PbI2.55Br0.45 perofskiet dun films. Om sodanig die eenvoudigste heeltemal konsistente ladings dinamika modelle op te stel vind ons die ooreenkomstige sigbare en naby-infrarooi spektroskopiese seine van ons monsters wat korrek vir die toelaatbare elektroniese transisies is en volg hulle evolusie in tyd op ’n femtosekonde tydskaal. Vanaf ons gemete tyd konstantes neem ons waar dat ZnO gebaseerde DSSCs minder effektief is as TiO2 omdat die elektron inspuiting vanaf die foto-opgewekte indolien kleursel na die ZnO CB nie net direk plaas vind (< 200 fs), maar ook stapsgewys via natuurlike (∼ 2 ps) en ioniese (∼ 10 ps) interim ladings oordrag toestande as gevolg van die oppervlak vasvang toestand eienskappe van elektronies gedeponeerde ZnO. Verder ook, FA0.85MA0.15PbI2.55Br0.45 is ’n uitstekende hibriede fotoabsorbeerder in rekord effektiwiteit perovskiet son-selle omdat selfs by hoer ladingsdraer digthede van ¨ 1019 cm−3 is die nie-stralende Auger herkombinasie meganisme dominant. Verder het ons ook vasgestel dat die geassosieerde geminatus, nie-geminatus en Auger herkombinasie tyd konstantes A = 5 × 109 s −1 , B = 10−10 s −1 cm3 en C = 50 × 10−32 s −1 cm6 is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102765
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