Pavao Roko Andricevic
In this dissertation, carbon is proposed as an alternative electrode material to interface with semiconducting organic-inorganic metal halide perovskites in various optoelectronic applications. Carbon electrodes may offer, good device performance, while providing improved stability due to their inert nature. The focus will be on a special architecture of carbon nanotubes, called the vertically aligned carbon nanotube forest (VACNTs), in which the CNTs are self-assembled into a vertically oriented array during growth on a substrate.
I start discussing, the unexpected discovery, namely, that single crystals of methylammonium lead perovskites (MAPbBr3 and MAPbCl3) grow directly on these VACNT forests. The peculiarity is that fast-growing single crystals engulfed the individual CNTs, as protogenetic inclusions, resulting in a three-dimensionally enlarged photosensitive interface. Sensitive photodetector devices were fabricated utilizing this ¿exotic¿ junction, detecting low light intensities (< 20 nW) from the UV range to 550 nm. Moreover, a photocurrent was recorded at zero external bias voltage, which points to a plausible formation of a p-n junction of the MAPbBr3 single crystal and VACNT forest interface.
While, attempting to push the I-V characteristics of these photodetectors to the limits, we were granted with another unexpected discovery, the bright green electroluminescence of these devices, observed at room temperature. Under an applied electric field, charged ions in the crystal drifted and accumulated near the electrodes, resulting in an in operando formed p¿i¿n heterojunction. The decreased interface energy barrier and the strong charge injection due to the CNT tip enhanced electric field, resulted in a bright green light emission up to 1800 cd m-2 at room temperature.
Furthermore, the possibility of designing perovskite-based ultrasensitive, low cost detectors for high-energy radiation was demonstrated. MAPbBr3 single crystal ¿-ray detectors, equipped with carbon electrodes, were fabricated allowing radiation detection by photocurrent measurements at room temperature with record sensitivities (333.8 µC Gy-1 cm-2). Importantly, the devices operated at low bias voltages (< 1.0 V), which may enable, future low-power operation in energy-sparse environments, including space. The detector prototypes were exposed to radiation from a 60Co source at dose rates up to 2.3 Gy h-1 under ambient and operational conditions for over 100 hours, without any sign of degradation. We postulate that the excellent radiation tolerance stems from the intrinsic structural plasticity of the organic-inorganic halide perovskites, which can be attributed to a defect-healing process by fast ion migration at the nanoscale level.
Since, the sensitivity of the ¿-ray detectors is proportional to the volume of the employed MAPbBr3 crystals, a unique crystal growth technique was introduced, baptized as the `oriented crystal-crystal intergrowth¿ or OC2G method, yielding crystal specimens with a volume and mass of over 1¿000 cm3 and 3.8 kg, respectively. Large volume specimens have a clear advantage for radiation detection, however, the demonstrated kilogram scale crystallogenesis coupled with future cutting and slicing technologies may have additional benefits, for instance, enable the development for the first time crystalline perovskite wafers, which may challenge the status quo of present and future performance limitations in all optoelectronic applications.
EPFL2019
