Metal halide perovskites are semiconductor materials which have attracted considerable attention because of their high compositional freedom and unique electrical and optical properties, in combination with low-cost fabrication. They are being produced all around the world because they are extremely promising for multiple optoelectronic applications such as solar cells, light emitting diodes (LEDs), photodetectors, or as photocatalysts, but the fundamental understanding is lagging behind. We don’t know why they work so well. Our goal at Hofkens’ lab is to fill in the blanks. Establishing the structural changes and the charge carrier dynamics of metal halide perovskites under influence of external stimuli (light, temperature, humidity) at high spatial and temporal resolutions will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications.
The wide range of color tunability, high color purity and bright photoluminescence of lead halide perovskites has led to an impressive progress in the external quantum efficiency of perovskite-based LEDs. However, perovskites lose their emission efficiency owing to the materials’ poor stability due to the presence of unwanted surface defects. In Hofkens lab, we focus on the preparation and exploration of (low-dimensional) perovskite nanocrystals for application in LEDs by adjusting synthesis protocols which have been developed within our group. Surface engineering by post-synthetic treatment is applied in order to passivate nanocrystal surface defects and stabilize the material in environmental conditions. A combination of structural and micro-spectroscopic studies allows us to unravel the chemistry at the origin of efficient surface passivation.
Bhatia, H.; Steele, J.; Martin, C.; Keshavarz, M.; Solis-Fernandez, G.; Yuan, H.; Fleury, G.; Huang, H.; Dovgaliuk, I.; Chernyshov, D.; Hendrix, J.; Roeffaers, M.; Hofkens, J.; Debroye, E. Single-Step Synthesis of Dual Phase Bright Blue-Green Emitting Lead Halide Perovskite Nanocrystal Thin Films. Chemistry of Materials, 2019, 31, 6824-6832, DOI: 10.1021/acs.chemmater.9b01277.
Debroye, E.; Yuan, H.; Bladt, E.; Baekelant, W.; Van der Auweraer, M.; Hofkens, J.; Bals, S.; Roeffaers, M. Facile Morphology-Controlled Synthesis of Organolead Iodide Perovskite Nanocrystals Using Binary Capping Agents. ChemNanoMat, 2017, 3, 223, DOI: 10.1002/cnma.201700006.
The sensitive detection of X-rays embodies an important research area, being motivated by a common desire to minimize the radiation doses required for imaging. Among metal halide perovskites, the lead-free double perovskite Cs2AgBiBr6 system has emerged as a promising candidate for the detection of X-rays, capable of high X-ray stability and sensitivity. At Hofkens’ lab, the aim is to detail the important photophysical pathways in double perovskite single crystals, with emphasis made toward understanding the carrier dynamics that influence X-ray sensitivity. These studies rely upon several high-resolution structural and optical probes and a photophysical model is developed, being plagued by a large trap density and fast free carrier recombination pathways. This lab shows the importance of employing elemental substitution in the perovskite system to manipulate the microchemistry and intrinsic charge carrier dynamics, and enhance the X-ray response toward an improved material platform for next-generation imaging.
Keshavarz, M.; Debroye, E.; Ottesen, M.; Martin, C.; Zhang, H.; Fron, E.; Küchler, R.; Steele, J.; Bremholm, M.; Van de Vondel, J.; Wang, H.; Bonn, M.; Roeffaers, M.; Wiedmann, S.; Hofkens, J. Tuning the Structural and Optoelectronic Properties of Cs2AgBiBr6 Double Perovskite Single Crystals Through Alkali Metal Substitution. Advanced Materials, 2020, DOI: 10.1002/ adma202001878.
Steele, J.; Pan, W.; Martin, C.; Keshavarz, M.; Debroye, E.; Yuan,H.; Banerjee, S.; Fron, E.; Jonckheere, D.; Kim, C.W.; Baekelant, W.; Niu, G.; Tang, J.; Vanacken, J.; Van der Auweraer, M.; Hofkens, J.; Roeffaers, J. Photophysical Pathways in Highly Sensitive Cs2AgBiBr6 Double Perovskite Single-Crystal X-Ray Detectors. Advanced Materials, 2018, 30 (46), 1804450, DOI: 10.1002/adma.201804450.
Steele, J. A.; Puech, P.; Keshavarz, M.; Yang, R.; Banerjee, S.; Debroye, E.; Kim, C.W.; Yuan, H.; Heo, N.H.; Vanacken, J.; Walsh, A.; Hofkens, J.; Roeffaers, J. Giant Electron-Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite. ACS Nano, 2018, 12 (8), 8081-8090, DOI: 10.1021/acsnano.8b02936.
Perovskite phase polymorphism and stabilization
It is known that perovskites undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature. Our research approaches provide advanced insights into the evolution of the crystal structure that are essential to interpret future investigations of the electronic, optical, and photonic properties of lead-halide perovskite materials. Moreover, we introduce the concept of strain to render the high-temperature, metastable black-phase CsPbI3 thin films stable at room temperature. Synchrotron-based experiments and ab initio thermodynamic modeling indicated the introduction of crystal distortions within ‘strained’ black CsPbI3 thin films, of which the thermal stability is vastly improved.
Steele, J.; Lai, M; Zhang, Y.; Lin, Z.; Hofkens, J.; Roeffaers, M.; Yang, P. Phase Transitions and Anion Exchange in All-Inorganic Halide Perovskites. Acc. Mater. Res. 2020, 1, 1, 3–15. DOI: 10.1021/accountsmr.0c00009
Steele, J.; Jin, H.; Dovgaliuk, I.; Berger, R.; Braeckevelt, T.; Yuan, H.; Martin, C.; Solano, E.; Lejaeghere, K.; Rogge, S.; Notebaert, C.; Vandezande, W.; Janssen, K.; Goderis, B.; Debroye, E.; Wang, Y.-K.; Dong, Y.; Ma, D.; Saidaminov, M.; Tan, H.; Lu, Z.; Dyadkin, V.; Chernyshov, D.; Van Speybroeck, V.; Sargent, E.; Hofkens, J.; Roeffaers, M. Thermal unequilibrium of strained black CsPbI3 thin films. Science 2019, 365 (6454), 679-684, DOI: 10.1126/science.aax3878.
Keshavarz, M.; Ottesen, M.; Wiedmann, S.; Wharmby, M.; Küchler, R.; Yuan,H.; Debroye, E.; Steele, J.; Martens, J.; Hussey, N.; Bremholm, M.; Roeffaers, M.; Hofkens, J. Tracking Structural Phase Transitions in Lead-Halide Perovskites by Means of Thermal Expansion. Advanced Materials 2019, 31 (24), 1900521, DOI: 10.1002/adma.201900521
Steele, J.; Yuan, H.; Tan, C.; Keshavarz, M.; Steuwe, C.; Roeffaers, M.; Hofkens, J. Direct Laser Writing of δ- to α-Phase Transformation in Formamidinium Lead Iodide. ACS Nano 2017, 11, 8, 8072–8083, DOI: 10.1021/acsnano.7b02777
Study of defects at the single particle level
Applying high-resolution fluorescence microscopy, the underlying physical processes of photoluminescence intensity fluctuations (PL blinking) of perovskite single particles are studied. Hofkens’ lab detailed important mechanisms on the origin of blinking, which appeared to be strongly affected by defect states and environmental conditions. In fact, one defect state or trap can regulate radiative recombination dynamics in perovskite particles over distances of hundreds of nanometers to even micrometers. These concepts are very important to further understand and control charge carrier dynamics, which are key to great success in the future design of high-performance perovskite crystal-based optoelectronic devices.
Jin, H.; Debroye, E.; Keshavarz, M.; Scheblykin, I.; Roeffaers, M.; Hofkens, J.; Steele, J. It’s a Trap! On the Nature of Localised States and Charge Trapping in Lead Halide Perovskites. Materials Horizons, 2020, 7, 397-410, DOI: 10.1039/C9MH00500E.
Yuan, H.; Debroye, E.; Bladt, E.; Lu, G.; Keshavarz, M.; Janssen, K.; Roeffaers, M.; Bals, S.; Sargent, E.; Hofkens, J. Imaging Heterogeneously Distributed Photo‐Active Traps in Perovskite Single Crystals. Advanced Materials, 2018, 30, 1705494, DOI: 10.1002/adma.201705494.
Yuan, H.; Debroye, E.; Caliandro, G.; Janssen, K.; Van Loon, J.; Kirschhock, C.; Martens, J.; Hofkens, J.; Roeffaers, M. Photoluminescence Blinking of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods Induced by Surface Traps. ACS Omega, 2016, 1, 148-159, DOI: 10.1021/acsomega.6b00107.
Yuan, H.; Debroye, E.; Janssen, K. P. F.; Naiki, H.; Steuwe, C.; Lu, G.; Moris, M.; Orgiu, E.; Uji-i, H.; De Schryver, F.; Samori, P.; Hofkens, J.; Roeffaers, M. Degradation of Methylammonium Lead Iodide Perovskite Structures through Light and Electron Beam Driven Ion Migration. The Journal of Physical Chemistry Letters, 2016, 7 (3), 561-566, DOI: 10.1021/acs.jpclett.5b02828