Exploring Polarization and Spin Effects in Organic-Inorganic Hybrid Perovskites towards Controlling the Key Optoelectronic Processes

Organic-inorganic hybrid perovskites, as emerging semiconductors for photovoltaic and light-emitting applications, have demonstrated unique polarization and spin properties, such as giant dielectric constants, ferroic behavior, and magneto-optical effects, etc. These bring out two important questions: (i) How do polarization and spin impact the optoelectronic properties? (ii) Is it possible to control the key optoelectronic processes by manipulating polarization and spin?To answer these questions, research in this dissertation was focused on exploring the effects of polarization and spin in hybrid perovskites, including polycrystalline thin film, single crystal, and their devices.

Photoinduced bulk polarization is revealed in perovskite solar cells (PSCs) by addressing an anomalous Capacitance-Voltage characteristic under photoexcitation in Chapter 2. The study suggests that photoinduced bulk polarization plays an important role in developing efficient photovoltaic actions. Inspired by this study, highly polar organic molecules are incorporated into PSCs in Chapter 3, which demonstrates an improvement in power conversion efficiency by 30% due to Coulombic screening effect from the local dipole field. In addition to bulk polarization, interfacial ionic polarization due to metal/ion interactions is demonstrated by interface dependent Seebeck effect study on perovskite single crystals and time-of-flight secondary ion mass spectrometry analysis on crystal/metal interface. This study demonstrates a robust method to control the semiconducting properties to realize a p- i-n junction for future optoelectronic design.

Spin effects on photovoltaic actions are investigated by optical operation of circularly polarized photoexcitation and magneto-optical measurements on both nanostructured PSCs and single crystal devices in Chapter 5. The spin-dependent photovoltaic actions suggest the importance of spin mixing in the high-efficiency PSCs. Finally, spin lifetime is investigated by combining time-resolved photoluminescence technique and circularly polarized excitation/detection in Chapter 6. Long-lived spin-polarized states with a lifetime in ~nanoseconds are demonstrated in perovskite single crystalline microcrystals. These studies open up a new opportunity to control the optoelectronic processes by manipulating polarization and spin in hybrid perovskites.

Portions of this document were previously published in journal.