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Colloidal quantum dot solar cells have drawn great attention in the past decade due to their easy synthesis, tunable bandgaps and low cost. However, the quantum dots’ low absorbance together with limits placed on the thickness of the absorbing layer due to poor transport properties is hindering the development of quantum dot solar cells. In this research, for further developing quantum dot solar cells, a tandem solar cell strategy was utilised to address the above problem. A tandem solar cell stacks materials with different bandgaps to more efficiently convert the energy of a wider range of photons to electricity for higher solar cell efficiency. This project developed high performance monolithic tandem solar cells based on PbS colloidal quantum dots. Firstly, single-junction colloidal quantum dot cells were optimised as the building blocks of the tandem cells. MgCl2 passivated ZnO-NP electron transporting layers were applied to improve single-junction colloidal quantum dot cells by increasing both fill factor FF and short circuit current Jsc. Significant efficiency improvement has been observed based on single-junction PbS solar cells. Secondly, an optical modelling tool was created for the first time to simulate tandem colloidal quantum dot solar cells. This model could offer exact simulation on photon absorbance and current generation in tandem cells’ functional layers. The model provided very instructive and quite essential guidance for designing the tandem cell structure and experimental works of fabricating quantum dot tandem cells. Thirdly, based on improved single-junction PbS solar cell and an established directive model, high performance tandem colloidal quantum dot solar cells were successfully fabricated with different interlayers of both Au and ITO. A graded band alignment employing both ZnO-doping and quantum dot size tuning has been applied to further improve tandem cell performance. Due to improved carriers extraction, this strategy successfully pushed the efficiency of a tandem quantum dot solar cell to be around 7%.

Photovoltaic devices that directly convert sunlight to electrical power have the potential to generate terawatts of usable power if they can be produced economically at scale. Solution-processed bulk heterojunction organic photovoltaics (BHJ OPVs) are a particularly interesting photovoltaic technology primarily because they have demonstrated power conversion efficiencies (PCEs) exceeding 10% and can be solution-processed over large areas, indicating their potential as a cost-effective, scalable source of renewable energy. Whereas BHJ OPVs utilizing polymer donors and functionalized fullerene acceptors dominate the organic solar cell literature, BHJ OPVs have recently been fabricated using small molecule donors with PCEs on par with their polymeric counterparts. The work comprising this dissertation therefore consists of two separate but related goals: 1) understanding and controlling the crystallization of small molecule donor materials in BHJ blends with functionalized fullerene acceptors and 2) identifying the efficiency limiting processes of organic solar cells utilizing perylene diimide (a commercial dye) acceptor molecules and small molecule donor materials. It is well established that the specifics of the bulk heterojunction morphology (phase separation, crystallinity, etc.) profoundly affect BHJ OPV device PCE. Controlling the BHJ morphology is thus of great importance. Using in-situ thermal annealing grazing incidence wide and small angle x-ray scattering it was discovered that the driving force for phase separation in BHJ OPV systems utilizing small molecule donor materials and functionalized fullerene acceptors is the crystallization of the small molecule donor material. Additionally, it was shown that this crystallization process, and therefore the development of the bulk heterojunction morphology and device performance, can be controlled by using commercially available nucleating agents designed for the clarifying of isotactic polypropylene. Functionalized fullerene acceptors, ubiquitous in high efficiency BHJ OPVs, are produced via particularly solvent and energy intensive techniques and exhibit small extinction coefficients across the terrestrial solar spectrum. There then exists obvious benefits for using inexpensive, easily mass-produced fullerene alternatives that strongly absorb solar photons. Perylene diimides (PDIs) are a class of organic dye molecules with high electron affinity similar to fullerenes, large extinction coefficients across the terrestrial solar spectrum and relatively high electron mobilities, thus making them attractive for use as electron acceptors in organic solar cells. A BHJ OPV with a PCE of 3.1% was fabricated using a small molecule donor material and a PDI acceptor, making this one of the most efficient BHJ OPV devices utilizing a non-fullerene acceptor. Use of the solvent additive 1,8-diiodooctane (DIO) was shown to greatly improve the PCE of these devices. Using UV-Vis and transient absorption pump-probe experiments it was shown that use of DIO increases the structural order of both the donor and acceptor molecule, corresponding to a drastic increase in the efficiency with which excited states separate into free charge carriers and therefore largely explaining the drastic increase in solar cell figures of merit when using the solvent additive.

Organic field-effect transistors (OFETs) have attracted tremendous attention due to their flexibility, transparency, easy processiblity and low cost of fabrication. High-performance OFETs are required for their potential applications in the organic electronic devices such as flexible display, integrated circuit, and radiofrequency identification tags. One of the major limiting factors in fabricating high-performance OFET is the large interfacial barrier between metal electrodes and OSC which results in low charge injection from the metal electrodes to OSC. In order to overcome the challenge of low charge injection, carbon nanotubes (CNTs) have been suggested as a promising electrode material for organic electronic devices. In this dissertation, we study the effect of carbon nanotube (CNT) density in CNT electrodes on the performance of organic field effect transistor (OFETs). The devices were fabricated by thermal evaporation of pentacene on the Pd/single walled CNT (SWCNT) electrodes where SWCNTs of different density (0-30/um) were aligned on Pd using dielectrophoresis (DEP) and cut via oxygen plasma etching to keep the length of nanotube short compared to the channel length. From the electronic transport measurements of 40 devices, we show that the average saturation mobility of the devices increased from 0.02 for zero SWCNT to 0.06, 0.13 and 0.19 cm2/Vs for low (1-5 /µm), medium (10-15 /µm) and high (25-30 /µm) SWCNT density in the electrodes, respectively. The increase is three, six and nine times for low, medium and high density SWCNTs in the electrode compared to the devices that did not contain any SWCNT. In addition, the current on-off ratio and on-current of the devices are increased up v to 40 times and 20 times with increasing SWCNT density in the electrodes. Our study shows that although a few nanotubes in the electrode can improve the OFET device performance, significant improvement can be achieved by maximizing SWCNT/OSC interfacial area. The improved OFET performance can be explained due to a reduced barrier height of SWCNT/pentacene interface compared to metal/pentacene interface which provides more efficient charge injection pathways with increased SWCNT/pentacene interfacial area.

Nanomanufacturing is a term used to describe either the production of nanoscale materials, or to describe the manufacturing of parts `bottom up' from nanoscale material or `top down' in smallest steps for high precision. In recent years, nanomanufacturing is facing a new and broadened definition the essential themes lie on low cost, scalability, reliability and sustainability, besides the conventional requirements. The emphasis on this technology is towards realizing the low cost and large area manufacturing with control and benefits from nanoscale structures and nano enabled properties. The newly defined nanomanufacturing is expected to find extensive new applications in the large area electronics and renewable energy industries.Nanomanufacturing based on solution processed nanoparticles is a promising approach towards this goal and is currently under intense research. This process significantly simplifies the manufacturing process and enables potential applications that are low temperature and extremely low cost. The solution deposition eliminates the need of multiple steps and vacuum deposition. It is capable of being integrated into the simple process, i.e. roll-to-roll manufacturing, to increase yield and process area. The suppressed melting temperature of nanoparticles also eliminates the need of high temperature processing. In solution processing, in general, nanoparticles prepared chemically with controlled stability and compositions are deposited in solution phase (e.g. printing) followed by annealing and/or nanopatterning steps for achieving the required spatial resolution and reasonable electrical and optical properties.Laser processing, including annealing/sintering and ablation, of solution deposited nanoparticles is a new and interesting process that has barely been explored. Not only this process can take advantage of laser processing for low cost, low temperature device fabrication, but also a large amount of physics can be explored including laser-nanomaterial interaction, nanoscale heat/mass transport, defects / grain boundaries evolution, semiconductor device physics and photonics, etc. The present study examined various components in the low cost nanomanufacturing process, including laser annealing of semiconductor and metallic nanoparticles, optical characterization of sintering process and scalable nanopatterning techniques. The emphasis is on both demonstration of novel processes and working devices as well as fundamental experimental and numerical studies of transport phenomena involved in laser induced solid state and melt-mediated nanoparticle coalescence.Firstly, to provide insight into the transport phenomena involved in laser induced non-melt and melt-mediated nanoparticle coalescence, experimental and numerical studies were performed. The Molecular Dynamics simulation was adopted to study the solid-state nanoparticle sintering by laser heating. The comparison between MD and macroscopic model reveals the unique mechanisms in sub-10nm nanoparticles sintering suggesting enhanced neck growth rate can be expected for nanoparticle with size below 10nm. The melt-mediated nanoparticle coalescence induced by ns laser is probed in-situ by a pump-and-probe technique. The probing results reveal several important characteristic times during laser annealing that will determine the morphology and crystalline structures of the laser processed film: characteristic coalescence time, cooling and resolidification times. The coalescence time compares favorably with MD simulation. Secondly, the laser annealing of metal oxide nanoparticles for electronics and photovoltaics are demonstrated. Solution deposited ZnO nanoparticles can be transformed into active channel layer of thin film transistors (TFTs) by Excimer laser annealing (ELA). The device shows mobility ~0.1 cm2/V-s and current on/off ratios of more than 104. The properties of the laser annealed films were characterized by scanning electron microscopy, high-resolution TEM, DC conductance, and photoluminescence measurements. Furthermore, ELA is combined with spray deposition of TiO2 nanoparticle to fabricate dye sensitized solar cells (DSSCs) on glass and plastic substrates. All-laser-annealed TiO2 photoelectrodes achieve efficiency at ~3.8% with Ruthenium dyes.Optical characterization of optical properties and film thickness by ellipsometry is also demonstrated for silver nanoparticle film sintering. The ellipsometric measurement is found to be robust in capturing the percolation transition, dielectric constant and film thickness evolutions. The non-contact optical measurement is also found to correlate well with DC measurement. Finally, large-area optical near-field nanoprocessing, i.e. nanosintering and nanoablation, of solution deposited nanoparticle is demonstrated. The feature size down to ~150nm is obtained and annealing in furnace further reduces the feature size down to ~50nm. Nanoscale organic transistors are fabricated by nanosintered electrodes and solution deposited air-stable semiconductor polymer. Molecular Dynamics simulation is also employed to calculate the effective thermal conductivity of the nanoparticle film to facilitate thermal modeling of the nanoscale laser processing.