Defect-Tolerance for Optoelectronic Semiconductors

The success of semiconductor technology is to a large extent driven by the industry’s ability to achieve perfection in manufacturing single crystals of remarkably low defect densities. Achieving the required purity in these materials is costly, hindering photovoltaics as a major and abundant source of renewable energy. However, lead halide perovskites (LHPs) are an emerging class of semiconductors that demonstrate competitive optoelectronic properties despite their fast and low-cost manufacturing processes. These locally ferroelectric semiconductors can be grown from solution synthesis at room temperature, which inevitably yields high defect densities and intrinsic disorder. Nonetheless, these imperfect materials show highly competitive optoelectronic properties. It was proposed that the high defect density could be compensated by efficient screening, building up on timescales relevant for carrier transport and electron-hole recombination.This proposal will not attempt to overcome current problems hindering the implementation of LHPs in today’s photovoltaics, but rather focuses on possible design principles derived from these materials. I propose to study how defect tolerance can arise from the interplay of charge carriers in a highly polarizable ferroelectric material. Carriers dressed by a cloud of lattice distortions are described in the framework of large polaron formation. The associated reduction in range of Coulomb interaction may be exceedingly beneficial for electrical transport as it reduced thecarrier scattering rate with charged defects and longitudinal optical phonons. The presence of fluctuating local polar nano-domains could further enhance screening of introduced charges, modifying the quasiparticle to become a ferroelectric large polaron.This mechanism may explain defect tolerance and low electron-hole recombination rates of charge carriers in LHPs. At the same time, it implies that conventional ferroelectrics are potential defect-tolerant semiconductors as well. While the Zhu group at Columbia is currently focusing efforts to verify the existence of ferroelectric large polarons in LHPs, I propose to test the general concept directly in conventional ferroelectrics. For this, I intend to study how large polarons form and carriers get trapped in the incipient ferroelectric n-SrTiO3 and the prototypical ferroelectric chalcohalides by carrying out measurements of the time-resolved photoluminescence, transient absorption, THz-conductivity and transientbirefringence as function of temperature. Our experiments will be able to identify and quantify the relevant actors and timescales of photoconduction and will clarify if in an anti-/ferroelectric semiconductor charge carriers can be protected from defect trapping by efficient Coulomb screening. The proposed research program will demonstrate that ferroelectric large polaron formation qualifies as the design rule for a new generation of optoelectronic semiconductors that promise to exceed current optoelectronic performances at much reduced costs.