M.Sc Thesis


M.Sc StudentOded Nahor
SubjectCharge Transfer in Hybrid Photovoltaic Devices
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Full Professors Frey Gitti


Abstract

Hybrid organic/inorganic solar cells composed of conjugated polymers and metal-oxides have been suggested as a promising alternative to conventional Si-based solar cells. This combination offers low cost solution processing, high optical absorption and chemical versatility, endowed by the organic component; along with the good charge transport capability, and the mechanical and environmental stability of the inorganic component. To allow efficient charge separation, the hybrid film must fulfill some critical requirements such as sub 20nm organic/inorganic phase separation and continuous pathways of each phase for charge transport. Owing to the different nature of the materials involved, surfactants are often used to compatibilize between the organic and inorganic phases. As a result, the non-optoelectronically active surfactant is present in the hybrid film and often located at the interface between the organic and inorganic phases and hinders the donor/acceptor electron-coupling thus reducing the device efficiency.

 In this study we replace the conventional surfactants with an optoelectronically active surfactant that both compatibilizes the organic and inorganic phases, and plays an active role in the device. Specifically, an amphiphilic polythiophene polymer (UPT) with hydrophilic urethane side-groups “surfactant” is used as a photo-active mediator between poly(3-hexylthiophene-2,5-diyl) (P3HT) and ZnO. Using a combination of FTIR, optical absorption and electron microscopy we find that the UPT associates with the ZnO and directs the formation of a sub-20-nm organic/inorganic phase separation with continuity of both phases through the film. However, increasing UPT concentration in the film hinders charge generation and charge transports, as evident from photoluminescence life-time spectroscopy and photocurrent measurements, respectively. Therefore, conditions are found to optimize the UPT content that will direct the preferred morphology while maintaining charge transport in corresponding photovoltaic devices.  This study is the first demonstration that an amphiphilic photo-active polymer can replace conventional non-active surfactants to compatibilize organic and inorganic components in hybrid PV films, to control the morphology of the film and generate continuous conductive pathways of both phases, for charge generation and charge transfer.