PhD Thesis Colloquium: Mr. Manvendra Singh (07/11/25)
Thesis title:
Fully inkjet-printed inorganic-organic heterojunction diodes for advanced electronic and optoelectronic applications
Faculty advisor(s):
Prof. Subho Dasgupta
When?
7th November, 2025 (Friday), 4:00 PM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
Printed electronics has rapidly advanced as a promising technology to fabricate lightweight, flexible, and low-cost electronic systems. Unlike conventional vacuum-based methods, these techniques enable direct patterning of semiconducting and conducting layers onto wide range of substrates, including plastics, papers, and textiles, opening the way for applications in wearables, wireless communication, energy storage, and optoelectronics. Among the various candidate materials, oxide semiconductors have gained prominence because of their wide band gap, high carrier mobility, optical transparency, and environmental stability. In particular, indium gallium zinc oxide (IGZO) and other indium rich crystalline and amorphous oxides have grown into prominence due to their high electron mobility, environmental stability and reliability.
While a large part of research activity in printed electronics, thus far, have focused only on field-effect transistors and relevant circuit elements, the other crucial building block of printed and flexible circuits, namely diodes have attracted much less attention, in comparison. Diodes are in fact the basis of rectifiers, energy-harvesting, energy transfer modules, and various optoelectronic systems. Hybrid oxide–polymer heterojunctions, combining printed oxides with conducting polymers such as PEDOT:PSS, provide a versatile route to achieve both electrical and optical functionalities in flexible form factors. This thesis investigates printed oxide-based heterojunction diodes across multiple application domains, spanning from wireless power transfer, integrated energy storage, neuromorphic computing, and ultraviolet light detection.
The first part of the work demonstrates the potential of printed oxide-based heterojunction diodes for wireless power transfer applications. Using indium–gallium oxide (IGO) precursor inks, heterojunction diodes have been fabricated on both rigid glass and flexible polyimide substrates, achieving rectification ratio exceeding 104 and operation frequency up to 25 MHz on glass and 15 MHz on flexible substrates. These printed diodes are further used to build full-wave and double half-wave rectifier circuits, enabling efficient AC to DC conversion. Wireless power transfer has been demonstrated from a distance of 3 cm at 125 kHz, highlighting their potential as power receivers in wearable or implantable devices. At the next step, dynamic mechanical tests have been carried out; bending fatigue tests have shown that the printed diodes on flexible substrates retained functionality under tensile strains up to 2.5%, confirming their robustness for mechanically demanding applications.
Building on this, the next part of the study focuses on extending printed diodes to eco-friendly, disposable substrates such as paper. To overcome the limitations of precursor inks that require higher processing temperatures, a method has been developed to fabricate diodes using surfactant-free polycrystalline IGO nanoparticle inks, processed at low thermal budgets compatible with paper substrates. These diodes exhibited unprecedented current density of 6 A cm-2 at 1 V, along with rectification performance up to 10 MHz and a 3 dB cutoff frequency of 1 MHz. They also have shown excellent durability, maintaining stability over 600 days and withstanding 1000 bending cycles at 1.25% strain. To demonstrate practical functionality, these diodes have been integrated with printed supercapacitors into an integrated wireless power storage system (IWPSS). In this architecture, the printed diodes wirelessly charge supercapacitors, enabling reliable energy storage and delivery. For instance, a paper diode is shown to charge a bulk supercapacitor with an energy density of 8.1 Wh kg-1 and an aerial capacitance of 28.9 mF cm-2 within 30 minutes to 0.4 V as well as a screen-printed micro-supercapacitor with an energy density of 5.1 μWh cm-2 and a capacitance of 14.3 mF cm-2 within 500 ms to 0.72 V. This work addresses one of the major challenges of printed power modules, low power output, by demonstrating robust diode–supercapacitor integration on a sustainable substrate.
In parallel to these wireless power studies, the thesis also explores the optical functionalities of oxide-based heterojunctions for neuromorphic and sensing applications. A major focus of the study has been on exploiting the persistent photoconductivity (PPC) in IGZO to realize optoelectronic synapses. By combining printed IGZO with PEDOT:PSS in two-terminal heterojunction structures, the role of oxide composition and electrode choice have been systematically investigated. Devices have displayed a range of synaptic behaviours under optical stimulation, including excitatory post-synaptic currents (EPSC), paired-pulse facilitation (PPF), and transformations from short-term to long-term plasticity. These behaviours are strongly linked to oxygen-vacancy states and interfacial energetics. Notably, electrode choice, oxygen-rich versus oxygen-deficient, significantly influenced PPC dynamics. A software-based artificial neural network is implemented using these devices, achieving handwritten digit recognition (MNIST dataset) with up to 65% accuracy at 1 V operation with 512 discrete conductance states. This highlights the potential of printed optoelectronic synapses for future neuromorphic vision systems that merge sensing, memory, and computation.
The final part of the thesis demonstrates electrolyte-assisted tuning of oxide–polymer heterojunctions for ultraviolet (UV) detection and energy harvesting. Inkjet-printed IGO/PEDOT:PSS heterojunctions have been encapsulated with a composite solid polymer electrolyte (CSPE), where dimethyl sulfoxide enabled ionic transport. Mobile Li⁺ ions penetrated through PEDOT:PSS to modulate IGO surface charges, enhancing band bending and raising the built-in potential from 0.37 eV to 0.7 eV. This band structure tuning suppresses the off-currents and significantly improves optoelectronic performance, with photosensitivity enhanced from 103 to beyond 106. A self-biased UV photodetector has also been demonstrated, operating without external bias while delivering responsivity of ~1 mA/W and photosensitivity of ~1.5×103. Furthermore, applying a gate bias across the electrolyte has enabled dynamic control of the junction, with rectifying behaviour transitioning to Ohmic conduction in real time. This work highlights how electrolyte-assisted ionic modulation provides a reconfigurable platform for energy-efficient sensing and multifunctional device applications.
In summary, this thesis establishes a comprehensive framework for printed oxide heterojunction diodes, spanning high-frequency rectifiers for wireless power transfer, integrated diode–supercapacitor systems on eco-friendly substrates, optoelectronic synapses for neuromorphic vision, and reconfigurable UV photodetectors. Together, these studies demonstrate how material design, interfacial engineering, and printing strategies can overcome the limitations of printed electronics in terms of power, stability, and multifunctionality. By bridging the domains of energy, sensing, and computation, the work positions printed oxide–polymer heterojunction diodes as versatile building blocks for the next generation of sustainable and intelligent electronic systems.