Large-area flexible electronics based on low-temperature solution-processed oxide semiconductors

Due to their high charge carrier mobility, optical transparency and mechanical flexibility, thin-film transistors (TFTs) based on metal oxide semiconductors represent an emerging technology that offers the potential to revolutionise the next-generations of large-area electronics. This thesis focuses on the development of high-performance TFTs based on low-temperature, solution-processed metal oxide semiconductors that are compatible with inexpensive flexible plastic substrates. The first part of the dissertation describes an ultraviolet light assisted processing method suitable for room-temperature activation of ZnO nanoparticles and their application as semiconducting channels in TFTs. The impact of the semiconductor/dielectric interface on electrical performance is studied using different device configurations and dielectric surface-passivation methods. Furthermore, a nanocomposite concept is proposed in order to assist electron transport between different crystalline domains. Using this approach, TFTs with electron mobilities of ~3 cm2/Vs are demonstrated. The second part of this work explores a carbon-free, aqueous-based Zn-ammine complex route for the synthesis of polycrystalline ZnO thin-films at low temperature and their subsequent use in TFTs. Concurrently, the development of a complementary high-κ oxide dielectric system enables the demonstration of high-performance ZnO TFTs with electron mobilities >10 cm2/Vs and operation voltage down to ~1.2 V. This low-temperature aqueous chemistry is further explored using a facile n-type doping approach. Enhancement in electrical performance is attributed to the different crystallographic properties of the Al-doped ZnO layers. The final part of the thesis introduces a novel TFT concept that exploits the enhanced electron transport properties of low-dimensional polycrystalline quasi-superlattices (QSLs), consisting of sequentially spin-cast layers of In2O3, Ga2O3 and ZnO deposited at temperatures 40 cm2/Vs - an order of magnitude higher than devices based on single binary oxide layers. Based on temperature dependent electron transport and capacitance-voltage measurements, it is reasoned that the enhanced electrical performance arises from the presence of quasi two-dimensional electron gas-like systems formed at the carefully engineered oxide heterointerfaces buried within the QSLs.Open Acces

Field-effect transistors based on Zinc oxide nanoparticles

This work reports the development of field-effect transistors (FETs), whose channel is based on zinc oxide (ZnO) nanoparticles (NPs). Using screen-printing as the primary deposition technique, different inks were developed, where the semiconducting ink is based on a ZnO NPs dispersion in ethyl cellulose (EC). These inks were used to print electrolyte-gated transistors (EGTs) in a staggered-top gate structure on glass substrates, using a lithium-based polymeric electrolyte. In another approach, FETs with a staggered-bottom gate structure on paper were developed using a sol-gel method to functionalize the paper’s surface with ZnO NPs, using zinc acetate dihydrate (ZnC4H6O4·2H2O) and sodium hydroxide (NaOH) as precursors. In this case, the paper itself was used as dielectric. The various layers of the two devices were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR), thermogravimetric and differential scanning calorimetric analyses (TG-DSC). Electrochemical impedance spectroscopy (EIS) was used in order to evaluate the electric double-layer (EDL) formation, in the case of the EGTs. The ZnO NPs EGTs present electrical modulation for annealing temperatures equal or superior to 300 ºC and in terms of electrical properties they showed On/Off ratios in the order of 103, saturation mobilities (μSat) of 1.49x10-1 cm2(Vs)-1 and transconductance (gm) of 10-5 S. On the other hand, the ZnO NPs FETs on paper exhibited On/Off ratios in the order of 102, μSat of 4.83x10- 3 cm2(Vs)-1and gm around 10-8 S

Solution processed metal oxide microelectronics: from materials to devices

Owing to their many interesting characteristics, the application of metal oxide based electronics has been growing at a considerable rate for the past ten years. High performance, optical transparency, chemical stability and suitability toward low cost deposition methods make them well suited to a number of new and interesting application areas which conventional materials such as silicon, or more recently organic materials, are unable to satisfy.The work presented in this thesis is focussed on the optimisation of high performance metal oxide based electronics combined with use of spray pyrolysis, as a low cost deposition method. The findings presented here are split into three main areas, starting with an initial discussion on the physical and electronic properties of films deposited by spray pyrolysis. The results demonstrate a number of deposition criteria that aid in the optimisation and fabrication of high performance zinc oxide (ZnO) based thin-film transistors (TFTs) with charge carrier mobilities as high a 20 cm2/Vs. Solution processed gallium oxide TFTs with charge carrier mobilities of ~0.5 cm2/Vs are also demonstrated, highlighting the flexibility of the deposition method. The second part of the work explores the use of facile chemical doping methods suitable for spray pyrolysed ZnO based TFTs. By blending different precursor materials in solution prior to deposition, it has been possible to adjust certain material characteristics, and in turn device performance. Through the addition of lithium it has been possible alter the films grain structure, leading to significantly improved charge carrier mobilities as high as ~54 cm2/Vs. Additionally the inclusion of beryllium during film deposition has been demonstrated to control TFT threshold voltages, leading to improved integrated circuit performance. The final segment of work demonstrates the flexibility of spray pyrolysis through the deposition of a number of high-k dielectric materials. These high performance dielectrics are integrated into the fabrication of TFTs already benefiting from the findings of the previously discussed work, leading to highly optimised low-voltage TFTs. The performance of these devices represent some of best currently available from solution processed ZnO TFTs with charge carrier mobilities as high as 85 cm2/Vs operating at 3.5 V.Open Acces

Influence of strain on the functionality of ink-jet printed thin films and devices on flexible substrates

Ink-jet printed devices on the flexible substrate are inexpensive and large area compatible as compared to rigid substrates. However, during fabrication and service they are subjected to complex strains, resulting in crack formation or delamination within the layers, affecting the device performance. Therefore, it is necessary to understand their failure mechanisms by correlating their electrical or structural properties with applied strain, supported by detailed microstructural investigations

Heteroepitaxial Growth Of Colloidal Nanocrystals Onto Substrate Films Via Hot-injection Routes

Hot-injection synthesis of colloidal nanocrystals (NCs) in a substrate-bound form is demonstrated. We show that polycrystalline films submerged into hot organic solvents can nucleate the heteroepitaxial growth of semiconductor NCs, for which the ensuing lattice quality and size distribution are on the par with those of isolated colloidal nanoparticles. This strategy is demonstrated by growing lead chalcogenide NCs directly onto solvent-submerged TiO(2) substrates. The resulting PbX/VTiO(2) (X = S, Se, Te) nanocomposites exhibit heteroepitaxial interfaces between lead chalcogenide and oxide domains and show an efficient separation of photoinduced charges, deployable for light-harvesting applications. The extendibility of the present method to other material systems was demonstrated through the synthesis of CdS/TiO(2) and Cu(2)S/TiO(2) heterostructures, fabricated from PbS/TiO(2) composites via cation exchange. The photovoltaic performance of nanocrystal/substrate composites comprising PbS NCs was evaluated by incorporating PbS/TiO2 films Into prototype solar cells

Electrodeposited semiconductor nanostructures & epitaxial thin films for flexible electronics

Single-crystal Si is the bedrock of semiconductor devices due to the high crystalline perfection which minimizes electron-hole recombination, and the dense native silicon oxide which minimizes surface states. To expand the palette of electronic materials beyond planar Si, an inexpensive source of highly ordered material is needed that can serve as an inert substrate for the epitaxial growth of grain boundary-free semiconductors, photonic materials, and superconductors. There is also a need for a simple, inexpensive, and scalable fabrication technique for the growth of semiconductor nanostructures and thin films. This dissertation focuses on the fabrication of semiconducting nanowires (polycrystalline Ge & epitaxial ZnO) and epitaxial thin films (Au & Cu₂O) using electrodeposition from an aqueous solution at ambient conditions as a simple benchtop process. Paper I describes a simple one-step electrodeposition of Ge nanowires on an indium-tin oxide substrate decorated with In nanoparticles. An In metal acts both as a catalyst for electrodeposition and as a solvent for recrystallization of the nanowires at ambient conditions. Ge nanowires are an attractive anode material for Li-ion batteries, due to their larger theoretical capacity compared to graphite. Paper II presents a scheme for epitaxial electrodeposition of ultrathin Au films on Si as an inexpensive proxy for single crystal Au for the electrodeposition of epitaxial Cu₂O thin films. A detailed study of the epitaxial growth, morphology, junction characteristics, and crystallinity is performed for both the Au and Cu₂O thin films. Paper III describes a technique for epitaxial lift-off of wafer-scale Au foils as transparent, single-crystal and flexible substrates for flexible electronics. The Au foils offer the order of traditional single-crystal semiconductors without the constraint of a rigid substrate. An organic light emitting diode is presented to evaluate the flexibility and transparency of Au foils. To study the single crystal nature of Au foil an epitaxial Cu₂O thin film inorganic diode with an improved diode quality factor is demonstrated --Abstract, page iv

Solution-Processed ZnO Nanoparticles for Optically Addressed Spatial Light Modulators and Other Applications

Solution-processable materials are becoming increasingly attractive due to their use in low cost, high throughput and relatively easy fabrications. In addition, the possibility of high-resolution patterning makes solution-based materials particularly suitable for integrated applications. The material that was investigated in this work is zinc oxide nanoparticles (ZnO NPs) dispersion, motivated by the highest resolution on record of optically addressed spatial light modulators (OASLMs) using solution-based ZnO NP as photoactive material. ZnO is a popular type of semiconductor compound from II-VI group and ZnO NPs are the nanocrystalline form of ZnO, which exhibit many unique and superior properties such as direct and wide bandgap, large surface-to-volume ratio, antibacterial and eco-friendly nature. Therefore, the investigation of ZnO NPs in terms of their physical properties, post processing effect, patterning techniques, and applications are of great significance. In this work, thin films made from ZnO NP dispersion in ethanol was characterized in detail including their structural, electrical, dielectric and optical properties. The post-processing effect such as thermal annealing and oxygen plasma treatment was also investigated. Then ZnO NP-based OASLM was researched by simulation and device characterization regarding electrical and optical properties. More importantly, the optimization of ZnO NP-based OASLMs was conducted in terms of diffraction efficiency and response speed, which are two key factors limiting the development of ZnO NP-based OASLMs. The diffraction efficiency was improved by pinpointing the optimum parameters of the driving signal such as waveform, amplitude and frequency. And the response time was reduced by several methods such as thermal annealing, introducing an interfacial layer and replacing the photoconductive ZnO NP layer with ZnO NP-based photodiode structure. The sensing of oxygen partial pressure in air by ZnO NP thin film was also observed and studied. Moreover, device miniaturization was achieved by the mould-guided drying technique, indicating a promising future for integrated applications. This patterning technique was also used for another type of solution-based material: PEDOT:PSS. And PEDOT:PSS-based organic electrochemical transistors (OECTs) with nanoscale channel length and channel width were realized by including a lift-off process, which demonstrated a great high-frequency response