Infoscience

Thesis

Nanoscale Organic / Inorganic Hybrid Photovoltaic Devices

There is an increasing trend of using various types of nanostructures as components of solar cells. This highlights the need to gain a deeper understanding of the nanoscale interface in such devices. The present thesis aims to study individual photoactive junctions as the elementary functional unit of bulk photovoltaic devices, and exploit the gained knowledge toward further improving the solar cells' efficiency. Along this direction, Schottky-contacts to cadmium sulphide nanowires were thoroughly studied using scanning photocurrent microscopy (SPCM). In conjunction with theoretical simulations of the measured photocurrent profiles, it was found that charge carriers can be very efficiently extracted out of a few micrometer long nanowire segments below the metal contact. Moreover, the carrier combination rate in this section could be determined as a function of an applied bias or backgate voltage. These findings provide valuable clues about how electric fields are distributed within semiconductor nanowire-based devices. Moreover, SPCM was employed to explore the local photoresponse along graphene/CdS heterojunctions, representing a first step toward implementing graphene as an active acceptor material into solar cells. The short circuit current of such devices could be enhanced by two orders of magnitudes through chemical tailoring of the graphene-CdS interface. In addition, evidence was obtained that surface plasmon excitation in the metal contacts can make significant contributions to the generated photocurrent. This effect was exploited in the fabrication of novel surface plasmon detectors, which support different plasmon modes depending on the polarization direction of the incoming light. Finally, it was attempted to realize photovoltaic devices comprising a pn-junction implemented into a π-conjugated organic polymer. While the electric-field assisted exciton dissociation in this system could be successfully modelled in the framework of the Braun-Onsager model, photoinduced charge separation at the interface between differently gated regions of the polymer could not be achieved due to insufficient electron conduction of the material.

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