DEFECTS BY DESIGN: DEFECT-PROPERTY ENGINEERING IN EMERGING SEMICONDUCTORS

Abstract

Conventional semiconductor photovoltaics have approached their practical efficiency limits, motivating the search for solution-processable materials that can match or surpass their performance at reduced fabrication cost. Hybrid organic-inorganic perovskites are promising next-generation materials due to their exceptional optoelectronic performance, solution processability, and unique defect-tolerant electronic structures. Unlike conventional semiconductors such as silicon (Si) or gallium arsenide (GaAs), hydrbid perovskites retain favorable electronic properties despite high intrinsic equilibrium concentrations of crystallographic defects (e.g. vacancies, interstitials). While this defect tolerance can enable inexpensive fabrication and long carrier diffusion lengths, it also presents a fundamental challenge: the low defect formation energies and mobile defect reservoirs can inhibit intentional doping and Fermi level control. This limitation is particularly acute in tin(II) based perovskites, which can exhibit metallic-like intrinsic p-type carrier concentrations arising in part from the facile oxidation of Sn(II) to Sn(IV) and the formation of compensating tin vacancies. This dissertation investigates how crystal chemistry controlled via the organic cation identity and defect equilibria is leveraged to intentionally manipulate intrinsic defect populations in tin-based hybrid halide perovskites. Rather than treating defects as unavoidable, unwanted byproducts of synthesis, this work seeks to re-frame defect formation as a thermodynamically tunable equilibrium that can be redirected between ionic and electronic compensation pathways. This outlines the manipulation of intrinsic defect chemistry as an indirect chemical handle on carrier populations, and thus electronic properties of traditionally“undopable” materials. The thesis begins with the three-dimensional (3D) “hollow” perovskite family, (CH3NH3)1 − x(NH3(CH2)2NH3)xSn1 − 0.7xI3 − 0.4x, where the incorporation of the divalent ethylenediammonium cation generates ionicallycoupled tin and iodine vacancies to size and charge compensate while preserving 3D connectivity that is adventitious for electronic applications. With a new solvent-free mechanochemical synthesis approach, compositional control is decoupled from the high iodine chemical potentials typically inherent to hybrid-halide solution processing. Structural (powder X-ray diffraction, proton nuclear magnetic resonance spectroscopy), optical (UVvisible diffuse reflectance spectroscopy), and bulk density measurements combined with dark microwave conductivity (DMC) and time-resolved microwave conductivity (TRMC) experiments reveal two distinct regimes of conductivity suppression. At low ethylenediammonium incorporation, intrinsic carrier density decreases by order of magnitude, consistent with ionic compensation of tin vacancies in lieu of mobile holes thereby reducing the overall carrier concentration. At higher substitution levels (x ≥ 0.15), carrier mobility is reduced as the result of increased defect concentrations and enhanced polaronic behavior. These results demonstrate that intentional vacancy generation can redirect compensation mechanisms and effectively suppress self-doping in tin-based perovskites. Building upon this framework, the organic cation size limits of defect accommodation are examined through systematic size increases of the organic cations in the series, (CH3NH3)1 − xAxSn1 − 0.7xI3 − 0.4x,where A = 2C (NH3(CH2)2NH 2+3 ), 3C (NH3(CH2)3NH 2+3 ), 4C (NH3(CH2)4NH2+3), and 5C (NH3(CH2)5NH 2+3 ). Solvent-free mechanochemical synthesis yields nominally cubic perovskites at low substitution levels (e.g. x ≤ 0.05), with quantitative 1H NMR confirming the nominal targeted substitution amount. The drastic decreases in bulk powder density with increasing large-cation content are consistent with coupled tin and iodine vacancy generation. While the smallest diammonium cation (2C) exhibits extended solid solution behavior across a broad compositional range with minimal microstrain, larger cations (3C-5C) display limited solubility and increasing microstructural strain. This reflects their (expected) steric incompatibility with the 3D framework, but shows that even at the very low incorporation limits of the large cation, they can be extremely effective in reducing hing intrinsic carrier densities. Microwave conductivity measurements indicate that even as the organic backbone length is increased, the suppression of intrinsic conductivity occurs through reductions in carrier density, analogous to the behavior observed in the 2C system. However, this is paired with a faster carrier mobility loss and rate of recombination. Together, these findings demonstrate that steric design of the large organic “dopant” can modulate both the carrier density as well as the carrier dynamics, but in an undesired way. These results further reveal the limits of ‘defect tolerance’ in these materials. Finally, we demonstrate that the protonation state of incorporated organic cations constitutes a previously unrecognized thermodynamic handle on defect stoichiometry in hollow tin halide perovskites. The substitution of ethylenediammonium for methylammonium has been assumed to occur in an aliovalent manner, such that each substituted cation carries a +2 charge that drives vacancy-mediated ionic compensation. Counterintuitive to the use of HI during synthesis, this work provides evidence that solution-grown materials instead incorporate a mixture of singly protonated (en+, isovalent with methylammonium) and doubly protonated (en2+, aliovalent) ethylenediammonium. This mixed protonation is thermodynamically reasoned by the nonstoichiometric incorporation of ethylenediamine into the solid, demonstrating that the product coexists in a solubility equilibrium with the mother liquor (HI). A Boltzmann analysis of the experimentally hypothesized protonation distributions yields a free energy difference between the two incorporation modes of only |ΔΔGf | ≈ 18–24 meV which is well within kBT and indicates near degeneracy in the solid-state host despite a ∼0.6 eV barrier in solution. We then systematically controlled this protonation state via stoichiometric solid-state ball milling, which reproduces the microstrain, bulk density, and optical band gap signatures of solution-grown materials as a function of the singly protonated fraction, and three correlated 1H NMR observables evolve systematically with w and are quantitatively eliminated upon re-exposure to HI vapor. These findings reveal that proton chemical potential during crystallization is a critical and underappreciated dimension of synthetic control over defect compensation in this materials family. Together, this work demonstrates that intrinsic carrier concentrations in tin-based hybrid perovskites – materials historically regarded as “undopable” – can be systematically manipulated through defect engineering enabled by crystal chemistry and chemical equilibria. By elucidating the interplay between structure, defect formation, and electronic compensation, this dissertation provides guiding principles for the rational design of defect-tolerant, electronically dopable hybrid semiconductors for photovoltaic and optoelectronic applications. In the concluding chapter, we examine a different area inorganic materials synthesis: the data-driven identification of low-temperature solid-state routes to phase-pure functional oxides. Here, computational thermodynamic screening of reaction networks is applied to the narrow band gap semiconductor bismuth vanadate (BiVO4, monoclinic), an emerging photoanode and commercial pigment material that conventionally requires high-temperature, time-consuming syntheses. Three new reaction classes are identified computationally and experimentally validated at 500 °C including 1) a ternary metathesis via alkali metavanadates and bismuth oxychloride, 2) a one-pot assisted metathesis of bismuth chloride with alkali carbonates and vanadium oxide, and 3) salt-mediated binary oxide reactions. Potassium-based variants consistently outperform their sodium counterparts, yielding phase-pure monoclinic BiVO4 with no detectable impurity phases and optical band gaps of 2.4 eV. These results demonstrate that thermodynamic screening of metathesis reaction networks provides a reliable, predictive methodology for designing efficient solid-state syntheses of complex semiconductor oxides.