Photovoltaic devices based on novel hybrid nanostructures

Energy supply has arguably become one of the most important problems facing humankind. The current global energy problem can be attributed to insufficient fossil fuel supplies and excessive greenhouse gas emissions resulting from increasing fossil fuel consumption. In order for 10 billion people to sustain the current lifestyle with the current energy consumption rate, we need a minimum of additional ten terawatts until the year 2050. The huge demand for clean energy can potentially be met by the solar-to-electricity conversion; howeve, the large-scale use of solar energy is not occurring due to the high cost and inadequate efficiencies of existing solar cells.;Nanostructured materials have offered new opportunities to design efficient solar cells. The objective of this study is to demonstrate photovoltaic devices (solar cells) using innovative hybird nanostructures that consist of semiconductor nanoparticles or quantum dots (QDs) supported on one-dimensional (1D) nanostructures (e.g., carbon nanotubes or CNTs) or two-dimensional (2D) nanostructures (e.g., graphene). The use of QDs provides the ability to tune the optical absorption in the solar cell (from infrared to ultraviolet) through the selection of semiconductor materials and nanoparticle sizes (quantum confinement effect). QDs can potentially display the recently-observed multiple electron-hole pair generation per photon to achieve higher solar cell efficiencies than that predicted by the classical Shockley and Queisser theory. The supporting 1D nanostructures offer additional opportunities to improve efficiencies of solar cells by facilitating photon absorption, electron transport, electron-hole separation at the hybrid nanostructure interface, and electron collection. Graphene as a 2D crystal of graphite monolayers exhibits impressive electronic properties and chemical stability. Graphene also has the lowest resistivity at room temperature. Use of graphene to support QDs provides tremendous opportunities to further boost the solar cell efficiency. This thesis entails the synthesis and characterization of vertically-aligned CNT arrays, vertically-oriented graphene sheets (VGs), CdSe nanoparticles (NPs), CdS QDs, and hybrids of these materials. Photo-generated charge transfer in CdSe-CNT and CdSe-graphene hybrids were investigated by using photo-sensitive field-effect transistors (FETs). Finally, these hybrid nanostructures were demonstrated for solar cell applications.;Vertically-aligned CNT arrays and CdSe NPs were synthesized using chemical vapor deposition (CVD). VG nanosheets were synthesized using an atmospheric pressure direct current plasma-enhanced CVD (PECVD) without catalysts. By controlling the electric field distribution on the substrate surface, patterned VG growth was accomplished. Graphene leaves were grafted onto the CNTs using the PECVD with CNTs as substrates. The CNT-graphene structure showed minimized tube-graphene junction resistance and exhibited improved gas sensing performance. The CdSe-CNT and CdSe-graphene hybrid structures were synthesized by direct deposition of aerosol CdSe NPs onto CNT and graphene surfaces. The feasibility of using CdSe-CNT and CdSe-graphene hybrid structures in photovoltaic devices was demonstrated through experimental observation of photo-electron transfer from CdSe NPs to CNTs or graphene in CdSe-CNT and CdSe-graphene FETs.;CNT and CNT-VG coated with CdS QDs and CdSe NPs were used as photoanodes in solar cells. The VG enhanced the photocurrent remarkably because it increased the QD/NP loading and enhanced photoelectron transport. The coexistence of CdS QDs and CdSe NPs in the solar cell anode led to more efficient light absorption and improved the power conversion efficiency. The VG was also studied as a catalytic counter electrode (CE) in dye-sensitized solar cells (DSSCs). The catalytic activity of VG and platinized fluorine-doped tin oxide (Pt/FTO) was rigorously studied and compared using electrochemical techniques. By tuning synthesis conditions, the catalytic activity of VG was optimized. The efficiency of a solar cell with most-oxidized VG CE exceeded that with Pt/FTO cathode by nearly 40%.