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PLASMONICALLY ENHANCED COLLOIDAL QUANTUM DOT PHOTODETECTORS AND PHOTOVOLTAICS

Final Report Summary - PECQDPV (PLASMONICALLY ENHANCED COLLOIDAL QUANTUM DOT PHOTODETECTORS AND PHOTOVOLTAICS)

Colloidal quantum dots (CQDs) have recently attracted significant attention as a candidate material for optoelectronic devices, and in particular photo-detectors and solar cells. These materials can be manufactured in solution and spin-cast onto a variety of substrates, significantly reducing the cost of device fabrication. Additionally, the band-gap of CQD films can be tuned to allow absorption of specific wavelength regions by varying the diameter of the CQDs, due to the quantum confinement size effect. To maintain efficient charge extraction in these devices, the thickness of the CQD layer is restricted, resulting in devices that are limited by incomplete absorption. To improve efficiencies it is necessary to decouple the optical thickness from the electrical thickness by employing novel light-trapping schemes.

Plasmonic light trapping employs metal nanostructures to manipulate incident light and trap it inside absorbing layers, increasing the probability of it being absorbed. Plasmonic enhancement is particularly suited to ultra-thin absorber layers that are usually necessary for solution processed materials like CQD, and can be engineered to address a given spectral window for solar harnessing or photodetection applications. Additionally, nanostructures can be fabricated on a substrate before the CQD are spin cast allowing plasmonic scattering structures to be incorporated into the devices without significantly increasing the complexity or cost of cell fabrication. The challenge is then to exploit the potential of plasmonic light trapping by designing the most effective structures.

There were two main aims for this project; firstly to provide effective light trapping for optoelectronic devices based on colloidal quantum dot (CQD) semiconducting films and to study the electrical effects of embedding metal nanostructures in these devices. The second was to explore the physical mechanisms behind plasmonic enhancement and to develop simple design rules for optimal light trapping schemes. These aims were achieved by using PbS CQD optoelectronic devices as test structures, by employing full-field optical simulations and developing an intuitive conceptual model.

To address the first challenge, we studied simple photoconductor devices, fabricated from PbS CQD films with embedded arrays of random, self-assembled metal nanoparticles that are known to scatter strongly. With this architecture, we were able to demonstrate a 2.4 fold enhancement in photocurrent at wavelengths near the exciton peak associated with PbS quantum dots (QDs) of a given size. We then employed the same device structure to study periodic arrays of nanoparticles fabricated from different metals, incorporated into PbS CQD films. The periodic arrangement allowed us to accurately model the absorption in these devices and thus isolate the electrical effect of integrating metal nanostructures inside semiconducting CQD films. We found that direct contact with the nanoparticles lead to suppression or enhancement of the photocurrent depending on the work function of the metal. This was attributed to the alignment of the Fermi level of the metals with that of the p-type PbS QDs. These findings are important for designing plasmonic CQD optoelectronic devices as they indicate that incorporating metal nanostructures can change the nature of existing p-n junctions and/or the type of electrical contact formed. With this insight, we were able to propose a novel approach to exploit plasmonic near-field absorption enhancement by tailoring the charge generation profile across the absorbing layer of a device. Schottky nano-junctions were formed between Ag nanoparticles and PbS CQD films. The Ag nanoparticles support strong plasmonic resonances in spectral regions where the active layer is weakly absorbing, leading to enhanced absorption at these wavelengths. By enhancing photo generation very close to the junction where charges are separated, the carrier generation profile inside the cell was optimized for charge extraction.

To address the second challenge, a number of studies of the light trapping potential of plasmonic structures in CQD PbS devices have been completed, employing full-field optical simulations and simple analytical models. Simulations of single nanoparticles were performed to extract scattering properties, demonstrating that large scattering and coupling efficiencies can be achieved with Ag nanoparticles embedded in PbS CQD films. However, the angular distribution of the scattered light is relatively narrow due to the low refractive index contrast between the glass substrate and the PbS CQD film, lowering the over-all light trapping potential from random scattering nanoparticles. We experimentally studied the dependence of photocurrent enhancement due to light trapping with random plasmonic nanoparticles on the CQD PbS layer thickness. In doing so, we showed that the mode structure of the thin semiconductor film is important in determining how much light is trapped: as fewer modes are available in thinner films the efficiency of isotropic scattering layers in providing light trapping is reduced.

To order to increase the efficiency of light trapping beyond that provided by random structures, periodically arranged nanostructures were investigated. Periodic arrangements of scattering nanoparticles form diffraction gratings with parameters that can be chosen to target particular guided modes of a thin film in a given spectral window. We developed a conceptual model to provide simple design rules for light trapping in thin films with grating couplers, allowing quick and easy optimisation of plasmonic light trapping and providing targeted photocurrent enhancement for solar harnessing or photodetection applications. Using these insights we designed optimal 2D nanostructured gratings to couple incident light at the exciton peak to the surface plasmon polariton modes (SPP) mode propagating on the metal-semiconductor interface of PbS CQD photodiodes. The SPP couplers were integrated into homo-junction photodiodes as the back contact, and achieved absorption enhancements at the exciton peak of a factor of 3 for thin diodes and 1.5 for thick diodes, relative to planar reference devices of similar thickness.

Taken together these results provide a good understanding of the challenges and opportunities of plasmonic light trapping for solution processed optoelectronics. They enable an informed choice of plasmonic material to avoid or exploit electronic effects due to embedding these nanostructures, and offer a simple and effective method for tailoring light trapping to a particular device geometry and spectral window.

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