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Facilitating Exciton Dissociation with Organic-2D Hybrid Multilayer Structures
Wanigasekara, Shanika Shiwanthi
Wanigasekara, Shanika Shiwanthi
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Abstract
Organic Photovoltaic (OPV) devices are considered one of the promising technologies to convert solar energy into electricity due to low cost, excellent flexible feature, and light weight. In OPV cells, Frankel excitons are created upon absorption of light. At organic D-A interfaces, charge transfer (CT) occurs creating a coulombically bound electron and hole known as CT excitons. Separating the CT exciton at the interface limits the photo-to-electric conversion yield. In this thesis, different types of multilayer structures are design to enhance the charge separation (CS). For effective dissociation, the charges need to be extracted directly from the short-lived delocalized CT exciton before it relaxes into localized CT state. Using a prototype trilayer structure that has a cascade band structure, we show that the yield of charge separation can be doubled as compared to the bilayer counterpart when the thickness of the intermediate layer is in the electron delocalization size typically found in CT excitons. Electron delocalization, together with the band cascade, can effectively flatten the energetic pathway for charge separation. On the other hand avoiding the formation of bound localized CT state can promote the CT exciton dissociation. Our work demonstrates that it is possible to add nanometer-thick hexagonal boron nitride (h-BN) layer between the donor and the acceptor to avoid the formation of bound CT state by increasing the distance between electron and hole in CT state where wide band gap h-BN layer can spatially separate the electron and hole wave function in the CT exciton. We found that the h-BN increases the overall photon-to-free carrier conversion yield of the D-A heterostructure significantly compared to identical samples without the h-BN layer, even though the h-BN lowers the initial electron transfer rate from ZnPc to PTCDI. For the CT exciton dissociation, OPVs require a built-in E-field as the driving force. The built-in E-field creates an energy loss, resulting low open circuit voltage (VOC) in OPVs. Hence the efficiency of the OPV decrease compared to theoretical values. Therefore, it is necessary to experiment on the built-in E-field required to dissociate CT excitons in our previously studied samples. We utilize a capacitor structure where our sample is sandwiched between top and bottom electrodes to provide sufficient E-field for CS. Top graphene electrode acts as a 2D sensor to probe charge separation since graphene is charge sensitive. Finally, we use the same capacitor structure to study the ion migration in organometal halide perovskite material where we observed that ionic defect migration due to the applied E-field can inject both electrons and holes into perovskite. Graphene electrode solves the problem of metal diffusion into perovskite and acts as an ultrafast charge sensor in our experiment.
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2022-08-31
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University of Kansas
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Keywords
Condensed matter physics, Charge separation, Charge transfer, Exciton, Hexagonal Boron Nitride, Multilayer structures, Organic Photovoltaics