title: Integrating high efficiency energy conversion nanostructures on flexible substrates
Creators: name: Zumeit, Ayoub
Publisher: name: University of Glasgow
Year: 2023

description: Flexible electronics that can bend, fold, stretch, or conform to curvy surfaces continue to have a huge impact on numerous aspects of our daily lives, such as in healthcare, prosthetics, etc. Recent developments have shown that high-performance flexible electronics are required to advance several applications, particularly where faster communications and computation are required. In this regard, the use of high-quality inorganic semiconductors such as monocrystalline silicon and compound semiconductors can open new opportunities for flexible electronics. These materials formed the basis of the semiconductor industry over last several decades and transformed almost every sector of the economy worldwide. For more than 60 years the electronic devices have been manufactured using a sequence of lithographic processes and chemical processing steps, creating in the end electronic circuits on planar silicon (Si) and/or other rigid semiconductor substrates. These process sequences do not work well for flexible electronics due to thermal budget-related issues. For instance, the implementation of certain fabrication process steps, such as doping by thermal diffusion and dielectric deposition via chemical vapor deposition (CVD), becomes challenging on flexible substrates due to their incompatibility with harsh temperatures required by these processes. To this end, transfer printing of silicon and compound semiconductor micro/nanostructures on flexible substrates such as nanoribbons (NRs) provides an attractive solution. This thesis explores this direction to address the longstanding issue of attaining electronic on flexible substrates and at the same time exhibit the performance similar to conventions micro/nanofabricated Si based electronics. In particular, the thesis explores the transfer printing approach, and its advanced version i.e., direct roll transfer printing, to integrate high mobility inorganic nanostructures on flexible substrate. First, the conventional transfer printing method is modified to achieve high transfer yield of silicon nanoribbons on flexible polyimide substrate. Here, the new part with respect to conventional transfer printing -is that the high-quality dielectric layer is deposited after the transfer printing step. The developed method led to high mobility Si nanoribbon based flexible transistors (&gt; 600 cm2/V. s). However, the developed devices suffered from shortcomings such as poor uniformity over large areas, impurities, lower transfer yield, presence of elastomeric residues on Si nanoribbons and poor registration. Such issues would limit the scalability of presented approach and to mitigate the potential adverse consequences, I developed a novel direct roll transfer printing method to print micro/nanostructures arrays of high mobility inorganic semiconducting materials such as Si, GaAs in a single step. These micro/nanostructures arrays were printed on a variety of flexible substrates including polyimide, polyethylene terephthalate, and metal foils, etc. The developed technique has the following distinct advantages: (i) unlike conventional transfer printing, the presented method does not require an elastomeric (e.g., Polydimethylsiloxane (PDMS)) transfer stamp (hence, named as direct transfer printing). Besides solving the challenges issues related to conventional transfer printing, this method also led to a reduced number of printing steps and hence saving in terms of cost and time. Further, it reduces the chance of breakage and/or wrinkling of printed nanostructures and hence helps to preserve their morphology and structure. This also offers an excellent opportunity to enhance the transfer yield and registration of printing nanostructures; (ii) The process helps to achieve high device-to-device uniformity by avoiding contamination from PDMS stamps, and (iii) the process is compatible with R2R fabrication which is advantageous for future large area electronics (LAE) manufacturing. The versatility of direct roll printing is demonstrated by obtaining the high-performance flexible field-effect transistors based on n- and p-channel silicon nanoribbons, high speed broadband photodetectors based on GaAs microstructures and multifunctional Si NR based micro solar cells. The silicon NR based n-channel transistors consistently show high performance i.e., high on-state current (Ion) &gt;1 mA, high mobility (μeff) &gt;600 cm2/V.s, high on/off ratio (Ion/off) of around 106, and low hysteresis (&lt;0.4 V). The direct roll transfer printed GaAs microstructures-based photodetectors exhibit excellent performance under ultraviolet and near-infrared illumination, including ultrafast response (2.5 ms) and recovery (8 ms) times, high responsivity (&gt;104 A/W), detectivity (&gt;1014 Jones), external quantum efficiency (&gt;106), and photoconductive gain (&gt;104) at low operating voltage of 1 V. Furthermore, the developed direct roll transfer printing has been demonstrated as an effective method to realize miniaturized solar cells with dual functionality: energy harvesting and self-powered photodetection. These micro solar cells have an area of approximately 315 µm2 and exhibit a maximum power density of around 11 µW/cm2. The developed photodiodes or micro solar cells can also function as a self-powered photo sensors with distinctive photo response under visible-UV-NIR light illumination. This photo sensor module shows high-speed response, peak responsivity of 2.48 A/W at 365 nm, external quantum efficiency of 8.30×102 %, and detectivity of 2.74×1013 jones. Additionally, the device demonstrates an exceptionally fast response speed (rise time τRise = 205μs and fall time τFall = 200μs) and stable detection performance. These flexible photodiodes present a potential pathway to develop self-powered broadband photodetectors for future smart optoelectronic applications, such as light imaging, light wave communication, integrated wireless sensor networks (WSNs), and wire-free routes for artificial e-skin. Heterogeneous integration of high mobility inorganic nanostructure such as silicon and compound semiconductors on flexible substrates, shown in this thesis work, also provides a potential route towards implementation of the high-performance energy autonomous flexible electronic systems.