Thermally Evaporated MAPbI₃ Perovskite Multiple-Quantum Wells for Enhanced Optoelectronic Properties

Metal halide perovskites have delivered rapid advances in emissive and absorbing functionalities in a short period of time, demonstrating enhanced properties such as increased carrier mobilities, and improved luminescent device performances.¹<br/>Two-dimensional confinement has demonstrated advantageous optoelectronic properties and facilitated fundamental studies in a variety of materials. Sequential stacking of a semiconducting material with a layer thickness below its Bohr diameter with another material of a different bandgap produces a meta-structure known as a multi-quantum well (MQW), providing advantageous optoelectronic properties such as bandgap tunability and increased exciton binding energy.^2,3 Quantum wells have previously shown wide and prosperous use in III-V semiconductor materials, both for photovoltaics and light emission.^4,5<br/>Thermal evaporation provides a method to produce highly uniform, large area depositions with a high degree of thickness accuracy.⁶ This method has allowed fully inorganic perovskite MQWs to be produced, demonstrating the viability and optoelectronic advantages of the structure over bulk counterparts.^{2,7,8,9,10,11}<br/>Here we present our work on the first type-I co-evaporated hybrid organic-inorganic perovskite MQWs, demonstrating enhanced luminescent properties and increased hot carrier temperatures. Study of sequentially decreasing thicknesses of MAPbI₃from bulk to ultrathin layers shows a persistent composition, with morphological analysis displaying continuous films at ultrathin thicknesses. Using bathocuproine (BCP) as the barrier material to produce a type-I bandgap alignment, single quantum wells (SQWs) exhibited a 50x increase in integrated PL intensity. Ultrafast spectroscopy was used to uncover the mechanisms behind this enhancement, finding a significant increase in radiative recombination. In addition, through Maxwell-Boltzmann distribution fitting, this examination exposed a significant increase in hot carrier temperature as the MAPbI₃ well thickness is reduced, opening the possibility for further study and utilisation of hot carriers in thermally evaporated organic-inorganic perovskite MQWs.<br/>A secondary material, lead phthalocyanine (PbPC), was used to generate a type-II band alignment with the MAPbI₃ so as to further study the charge dynamics between the well and barrier materials. Both type-I and type-II MQWs were integrated into lateral photodetector devices, with minimal increase from the type-I, and a significant increase from the type-II structures, when comparing both to MAPbI₃ only layers of the same thickness. This exhibits the advantage of using type-II aligned MQWs for absorbing devices, where the charges separate into the well and barrier individually, reducing recombination.<br/>[1] Advanced Materials 34, 21, https://doi.org/10.1002/adma.202108132<br/>[2] Nano Letters, 19, 6, https://doi.org/10.1021/acs.nanolett.9b00384<br/>[3] Journal of Applied Physics, 91, 3, https://doi.org/10.1063/1.1445280<br/>[4] Optics Express, 26, 25, https://doi.org/10.1364/oe.26.033108<br/>[5] Joule, 6, 5, https://doi.org/10.1016/j.joule.2022.04.024<br/>[6] Nanoscale, 11, 30, https://doi.org/10.1039/c9nr04104d<br/>[7] Advanced Materials, 33, 17, https://doi.org/10.1002/adma.202005166<br/>[8] ACS Energy Letters, 6, https://doi.org/10.1021/acsenergylett.1c01142<br/>[9] Nano Letters, 22, 19, https://doi.org/10.1021/acs.nanolett.2c02953<br/>[10] Advanced Science, 9, 24, https://doi.org/10.1002/advs.202200379<br/>[11] Advanced Optical Materials, 12, 13, https://doi.org/10.1002/adom.202302701