Exciton Formation Dynamics and Band‐Like Free Charge‐Carrier Transport in 2D Metal Halide Perovskite Semiconductors

• Published in: Advanced functional materials 2023-08, Vol.33 (32), p.n/a
• Main Authors: Motti, Silvia G., Kober‐Czerny, Manuel, Righetto, Marcello, Holzhey, Philippe, Smith, Joel, Kraus, Hans, Snaith, Henry J., Johnston, Michael B., Herz, Laura M.
• Format: Article
• Language: English
• Subjects: 2D perovskites; Carrier transport; Current carriers; Excitons; Light emission; Light sources; Low temperature; Materials science; Metal halides; Optimization; Optoelectronics; perovskite semiconductors; Perovskites; Phonons; Photoconductivity; Photovoltaic cells; Saha equations; Semiconductors; Solar cells; Temperature dependence
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202300363

Metal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high‐efficiency photovoltaic applications. Low‐dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge‐carrier mobilities, and exciton formation in 2D (PEA)2PbI4 and (BA)2PbI4 (where PEA is phenylethylammonium and BA is butylammonium). Temperature‐dependent measurements of free charge‐carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in‐plane mobilities. Enhanced charge‐phonon coupling is shown to reduce charge‐carrier mobilities in (BA)2PbI4 with respect to (PEA)2PbI4. Exciton and free charge‐carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge‐carrier population is observed, surpassing 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. Temperature‐dependent charge‐carrier mobilities and exciton formation dynamics in 2D Ruddlesden‐Popper perovskite semiconductors are recorded by femtosecond optical and THz spectroscopy. The results reveal efficient band transport of free charge carriers, greatly surpassing the theoretical expectations for these materials in spite of strong quantum confinement.