Research

The most exciting innovations in solar energy technology over the past few years have been driven by hybrid metal halide perovskite semiconductors. They offer high performance and low-cost fabrication. Crucially, they are also exceptionally versatile and tunable, which makes them ideal for multi-junction devices that can surpass the fundamental efficiency limits of traditional devices. Furthermoe, their bandgap tunability, high radiative efficiency, and defect tolerance also make them excellent light emitters. In addition to their exciting technological potential, these materials present intriguing and unusual optoelectronic properties that are regulated by vibration, fluctuations, and heterogeneities of the lattice. 

Understanding the interaction between light and matter is key for developing new and more efficient technologies for solar cells, light emitters, communication networks, imaging, and sensing. Optical spectroscopy offers a wide variety of tools to study and optimise charge generation, transport, and recombination dynamics in semiconductor materials and devices. Of special interest is the use of short laser pulses for monitoring charge-carrier dynamics in semiconductor over the femtosecond to microsecond scale. Among the main time-resolved tools for studying semiconductor photophysics are photoluminescence lifetimes, transient absorption, and terahertz photoconductivity.

Developed in the group of Prof Laura Herz at the University of Oxford.

Low-dimensional perovskite semiconductors have promising prospects in light emission and quantum technologies. Here we investigate phonon energies, charge-carrier mobilities, and exciton formation in 2D perovskites formed with two different large organic cations. Temperature-dependent charge-carrier mobilities reveal band transport with surprisingly high in-plane mobilities. We disentangle exciton and free charge-carrier dynamics by simultaneous monitoring transient absorption and THz photoconductivity, and observe a sustained free charge-carrier population that surpasses the Saha equation predictions even at low temperature. These findings provide new insights into the temperature-dependent interplay of exciton and free-carrier populations in 2D MHPs. Furthermore, such sustained free charge-carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.

Developed in the group of Prof Laura Herz at the University of Oxford.

Mixed halide perovskites provide ideal bandgaps for multijunction solar cells, but their soft lattice is subject to instabilities that result in phase segregation. In this work we use optical photoconductivity spectroscopy to probe the impact of ionic migration in mixed-halide perovskites. We show how the charge funneling dynamics in the heterogeneous material results in enhanced recombination and reduced diffusion lengths, despite high charge-carrier mobilities being preserved. We also show how the ionic migration is associated with the lattice anharmonicity, evidenced by photoinduced phonon shifts. These findings provide insight into strategies to minimise losses in mixed-halide perovskite photovoltaics.

Developed in the group of Prof Laura Herz at the University of Oxford.

We investigated charge-carrier recombination and transport in perovskite thin films comprising a quasi-2D and a 3D layer. Using time-resolved photoluminescence, photoconductivity spectroscopy and modelling, we demonstrate that charge diffusion and photon recycling effectively compensate for the confinement effects within the quasi-2D domains. These results show how the enhanced environmental stability of quasi-2D perovskites can be combined with efficient charge transport of 3D perovskite for efficient and stable photovoltaics. 

Developed in the group of Annamaria Petrozza at IIT, Milano, Italy, in collaboration with Prof Filippo de Angelis at CNR and University of Perugia.

The discovery of emissive trap states at room temperature (ACS Energy Letters 1, 726–730, 2016) and the identification of long-lived trap states in lead halide perovskites (Energy & Environmental Science 11, 702–713, 2018), in combination with ab-initio calculations, lead to a comprehensive understanding of the defect activity in these semiconductors (Advanced Materials 31, 1901183, 2019). This understanding provided an explanation to the remarkable defect tolerance in these materials. 

Furthermore, these advances supported a systematic study that resulted in the identification of the mechanisms behind defect formation and self-healing in perovskite thin films (Nature Photonics 13, 532–539, 2019). These results reveal important fundamental properties of perovskite thin films and unravel the contradictions between previous reports in the literature. 

Such advances allowed for the rational engineering of processing methods. For instance, the application of polyethylene oxide as an interlayer for moisture protection and passivation of undercoordinated surface lead (Energy & Environmental Science 11, 2609–2619, 2018) resulted in enhanced performance and stability of photovoltaic devices.