Modelling of polymer-carbon nanotube heterojunctions for photovoltaic applications

A thesis presented to the University of the Basque Country UPV/EHU in fulfilment of the thesis requirement for the degree of Doctor of Philosophy in Physics of Nanostructures and Advanced Materials.Semiconducting single-walled carbon nanotubes (s-SWNT) are promising materialsfor efficient organic photovoltaics (OPVs). Unfortunately, the implementation of s-SWNTs has so far not lead to the expected increase in power conversion efficienciesof OPVs. For this reason, we want to study the electronic processes within polymer-SWNT heterojunctions. Transient spectroscopy provides direct information aboutphotoexcitation processes in blends. We modelled the transient spectrum of apolymer:fullerene:s-SWNT blend using Linear Response. Based on our results, weare able to explain the structure transient spectra of s-SWNT systems and theelectronic dynamics linked to it. Further, we studied the internal quantum efficiencyof different donor/acceptor blends by carrying out calculations of the Landauer-Bütticker conductance of prototypical donor/acceptor heterojunctions. We find adependence of the conductivity on the level alignment. By improving the levelalignment of the polymer and SWNT through the use of larger band gap SWNTs,one may obtain a dramatic improvement in OPV efficiency. In summary, our resultsprovide a deeper insight into the photoexcitation and electronic processes ofpolymer-carbon nanotube heterojunctions and thus support the development of moreefficient polymer-SWNT OPVs.I acknowledge funding from from the European Projects POCAONTAS (FP7-PEOPLE-2012- ITN- 316633), DYNamo (ERC-2010-AdG-267374), MOSTOPHOS (SEP-210187476), and EUSpec (COST Action MP-1306), and CRONOS (280879-2 CRONOS CP-FP7); Spanish Grants (FIS2012-37549-C05-02, FIS2010-21282-C02-01, PIB2010US-00652, JCI-2010-08156); and Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-578-13); and the computational time from the BSC Red Espagnola de Supercomputacion.Peer reviewe