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published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy

Colloidal quantum dot photovoltaics (CQDPV) offer a big potential to be a renewable energy source due to low cost and tunable band-gap. Currently, the certified power conversion efficiency of CQDPV has reached 9.2%. Compared to the 31% theoretical efficiency limit of single junction solar cells, device performances have still have a large potential to be improved. For photovoltaic devices, a classical way to enhance absorption is to increase the thickness of the active layers. Although this approach can improve absorption, it reduces the charge carriers extraction efficiency. Photo-generated carriers, in fact, are prone to recombine within the defects inside CQD active layers. In an effort to solve this problem, we proposed to increase light absorption from a given thickness of colloidal quantum dot layers with the assistance of disorder. Our approach is to develop new types of electrodes with multi-scale disordered features, which localize energy into the active layer through plasmonic effects. We fabricated nanostructured gold substrates by electrochemical methods, which allow to control surface disorder as a function of deposition conditions. We demonstrated that the light absorption from 600 nm to 800 nm is impressively enhanced, when the disorder of the nanostructured surface increases. Compared to the planar case, the most disorder case increased 65% light absorption at the wavelength of λ = 700nm in the 100 nm PbS film. The average absorption enhancement across visible and infrared region in 100 nm PbS film is 49.94%. By developing a photovoltaic module, we measured a dramatic 34% improvement in the short-circuit current density of the device. The power conversion efficiency of the tested device in top-illumination configuration showed 25% enhancement.

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Colloidal quantum dots (CQDs) are an inexpensive and solution processable photovoltaic(PV) material reaching modest efficiencies of 6%. However, doping quantum dots still remains a challenge. This thesis explores the level of doping in lead sulfide (PbS)CQDs by surface ligands and bulk doping within the quantum dot lattice using metals. In light of the knowledge that oxygen creates traps on the surface of PbS CQDs, we have turned to the use of oxygen-free fabrication. We find that under a nitrogen environment, PbS CQD films are n-type and tunable in doping by use of halide ions. We show for the first time control over the doping density of n-type CQD films over a wide range. We also show the ability to fabricate p-type PbS films with high doping density that are compatible with n-type films. This compatibility enabled us to make the world’s first CQD homojunction PV device.

Colloidal quantum dots (CQD) are used in the fabrication of efficient, low-cost solar cells synthesized in and deposited from solution. Breakthroughs in CQD materials have led to a record efficiency of 7.0%. Looking forward, any path toward increasing efficiency must address the trade-off between short charge extraction lengths and long absorption lengths in the near-infrared spectral region. Here we exploit the localized surface plasmon resonance of metal nanoparticles to enhance absorption in CQD films. Finite-difference time-domain analysis directs our choice of plasmonic nanoparticles with minimal parasitic absorption and broadband response in the infrared. We find that gold nanoshells (NS) enhance absorption by up to 100% at λ = 820 nm by coupling of the plasmonic near-field to the surrounding CQD film. We engineer this enhancement for PbS CQD solar cells and observe a 13% improvement in short-circuit current and 11% enhancement in power conversion efficiency.

Colloidal quantum dots (CQDs) are an inexpensive and solution processable photovoltaic(PV) material reaching modest efficiencies of 6%. However, doping quantum dots still remains a challenge. This thesis explores the level of doping in lead sulfide (PbS)CQDs by surface ligands and bulk doping within the quantum dot lattice using metals. In light of the knowledge that oxygen creates traps on the surface of PbS CQDs, we have turned to the use of oxygen-free fabrication. We find that under a nitrogen environment, PbS CQD films are n-type and tunable in doping by use of halide ions. We show for the first time control over the doping density of n-type CQD films over a wide range. We also show the ability to fabricate p-type PbS films with high doping density that are compatible with n-type films. This compatibility enabled us to make the world’s first CQD homojunction PV device.