New low-cost, transparent electrodes for optoelectronics
Indium-based materials form the essential element in virtually all current-generation flat-panel displays, high-efficiency solar cells, and are part of organic and inorganic LEDs. However, indium is increasingly rare and expensive. The main objective of the EU-funded project INREP was to create strong but economically viable candidates to replace indium in optoelectronic components. The multi-disciplinary consortium took a holistic approach with renowned companies in the transparent conducting oxide (TCO) value chain as well as research institutes investigating options for indium-free TCOs and deposition technologies. “We used a combination of fundamental physics and material studies, conventional thin-film deposition, as well as growth and characterisation techniques to develop TCO films that match the specific application requirements in novel optoelectronic devices,” points out Dr Duncan Allsopp. The newly developed TCO thin films are currently being used as electrodes for solar cells, touch screen monitors, organic and inorganic LEDs. High-efficiency photovoltaic cells Front and back TCOs play a crucial role in making highly efficient solar cells. “Rather than indium tin oxide (ITO), which has become a standard material in the design of TCOs in solar cells, we proved that aluminium zinc oxide (AZO) can make very efficient back reflectors, both for front- and rear-emitter solar cells. As a material, AZO is a cheap, more readily available, equally transparent and electrically conducting alternative to indium,” notes Dr Allsopp. For rear-emitter solar cells, AZO offers a viable alternative to front surface electrodes based on ITO, with similar conductivity, fill factor and open-circuit voltage. For p-type amorphous silicon heterojunction solar cells, AZO-based TCO thin films prepared by atomic layer deposition (ALD) increased the fill factor at the back reflector of rear-emitter solar cells. Gallium nitride (GaN) on silicon GaN is the main semiconductor used for the fabrication of visible light LEDs. In standard growth conditions, GaN crystallises in the wurtzite structure which is not centro-symetric: the growths along the Ga-polarity and N-polarity axes are not equivalent. This polarity has a profound impact on the material characteristics and device performance. The preparation of LEDs with AZO films as electrodes involved the challenging task of synthesising TCOs on nitrogen polar n-type GaN and exploring new LED architectures. The team successfully developed low-resistance contacts between the TCO and the nitrogen polar n-type GaN through careful interface engineering via plasma pre-treatment. “The newly developed TCO materials have opened the door to novel visible LED device designs that are more efficient, reliable and less expensive,” notes Dr Allsopp. Indium-free anodes for OLEDs In contrast to LEDs, organic LEDs (OLEDs) can emit soft diffused light and also allow easier heat dissipation. Furthermore, as thin-film encapsulation techniques improve, they can provide panels of light as flexible as a piece of paper. The need to improve transparency and conductivity at elevated temperatures makes use of ITO in combination with a flexible substrate and encapsulation materials more challenging. Researchers have overcome this challenge, showing that AZO anodes in OLED devices prepared either by sputtering, chemical vapour deposition or ALD yield high luminous efficacy, similar to that of reference devices with ITO anodes. Touch screen sensors INREP researchers investigated alternatives to ITO electrode materials produced by screen printing for transparent touch sensors. The challenge was to synthesise printing pastes based on silver nanowire ink formulations with rheological characteristics compatible with transparent touch sensors. “Silver nanowire touch screen displays have proved to be greatly superior to equivalent structures fabricated with ITO in terms of transparency,” says Dr Allsopp. A major achievement, the consortium has developed the technologies and processes capable of exploiting silver nanowire inks for the commercial manufacture of a wide range of touch screen displays. They have also established firmware platforms needed for these applications.
Keywords
INREP, transparent conducting oxide (TCO), solar cell, aluminium zinc oxide (AZO), touch screen, silver nanowire, transparent electrode, organic LED (OLED), indium-based electrode
