Abstract Coulomb interactions govern most optoelectronic properties in semiconductors including bandgap, excitons, polaron formation, light absorption, interaction of carriers with defects, charge and energy transport. A clear signature of the strength of many-body Coulomb interactions is the exciton binding energy (Eb). As such, the importance of Eb is perhaps second only to the fundamental band gap (Eg) in understanding the physics and efficiency of low-dimensional semiconductors. However, despite their importance, Eg and Eb are difficult to measure. Here, by choosing 2D halide perovskites as the material and electroabsorption as the experimental probe, we measure Eg and Eb with a precision that is an order of magnitude better than typical methods. The Eb values are both lower than previous literature reports and lower than expected from standard theory. Using dielectric spectroscopy, density functional theory, and quantum mechanical modeling, we demonstrate these low Eb values are a consequence of unique screening effects, such as superlattice screening and phonon screening. We demonstrate a clear correlation between Eb and Eg and provide design principles in order to a priori tune Eg and Eb to their optimal values. As such, this work lays the blueprint for Eg-Eb engineering of low-dimensional semiconductors, as an even more useful replacement to simply band gap engineering.

As the field of 2D halide perovskites (HPs) matures, state-of-the-art techniques to measure important properties, such as the band gap (Eg) and exciton binding energy (Eb), continue to produce inconsistent values. Here, we tackle this long-standing problem by obtaining direct measurements of Eg and Eb for 31 unique HP structures. The Eb values are lower than in previous literature reports and lower than expected from standard theory that assumes excitons are screened by optical-frequency dielectric constants. These low Eb values are shown to be a consequence of unique screening effects, such as superlattice screening and phonon screening. We find a strikingly strong correlation between Eb and Eg and provide design principles to a priori tune Eg and Eb to their optimal values. As such, this work offers a blueprint for Eg-Eb engineering of low-dimensional semiconductors as an even more useful replacement for simply band-gap engineering.

Coulomb interactions govern most optoelectronic properties in semiconductors including bandgap, excitons, polaron formation, light absorption, interaction of carriers with defects, charge and energy transport. A clear signature of the strength of many-body Coulomb interactions is the exciton binding energy (Eb) and the importance of Eb is perhaps second only to the fundamental band gap (Eg) in determining the optoelectronic properties of low-dimensional semiconductors. However, despite their importance, Eg and Eb have still evaded consensus in the literature for even the most commonly studied 2D halide perovskites (HPs). Here, we use electroabsorption to obtain direct and precise measurements Eg and Eb for 31 unique HP structures. The Eb values are much lower than previous literature reports and also much lower than expected from standard theory. Using dielectric spectroscopy, density functional theory, and quantum mechanical modeling, we demonstrate these low Eb values are a consequence of unique screening effects, such as superlattice screening and phonon screening. We demonstrate a clear correlation between Eb and Eg and provide design principles to a priori tune Eg and Eb to their optimal values. As such, this work lays the blueprint for Eg-Eb engineering of low-dimensional semiconductors, as an even more useful replacement to simply band gap engineering.

Aside from band gap reduction, little is understood about the effect of the tin‐for‐lead substitution on the fundamental optical and optoelectronic properties of metal halide perovskites (MHPs), especially when transitioning from 3D to lower dimensional structures. Herein, we take advantage of the spectroscopic isolation of excitons in 2D MHPs to study the intrinsic differences between lead and tin MHPs. The exciton's spectral fine structure indicates a larger polaron binding energy in tin MHPs. Additionally, the electroabsorption responses of the 2D MHPs demonstrates that tin MHPs have exciton binding energies 1.5–2× lower than that of their lead counterparts. Despite the lower binding energy, the excitons in tin MHPs are more Frenkel‐like with small radii, small polarizabilities, and large dipole moments. These results are interpreted as consequences of small polaron formation and disorder‐induced dipole moments. This work highlights the wide range of intrinsic differences between lead and tin MHPs as well as the complexity of excited states in these systems.

Self-doping is a particular doping method that has been applied to a wide range of organic semiconductors. However, there is a lack of understanding regarding the relationship between dopant structure and function. A structurally diverse series of self-n-doped perylene diimides (PDIs) is investigated to study the impact of steric encumbrance, counterion selection, and dopant/PDI tether distance on functional parameters such as doping, stability, morphology, and charge-carrier mobility. The studies show that self-n-doping is best enabled by the use of sterically encumbered ammoniums with short tethers and Lewis basic counterions. Additionally, water is found to inhibit doping, which concludes that thermal degradation is merely a phenomenological feature of certain dopants, and that residual solvent evaporation is the primary driver of thermally activated doping. In situ grazing-incidence wide-angle X-ray scattering studies show that sample annealing increases the π-π stacking distance and shrinks grain boundaries for improved long-range ordering. These features are then correlated to contactless carrier-mobility measurements with time-resolved microwave conductivity before and after thermal annealing. The collective relationships between structural features and functionality are finally used to establish explicit self-n-dopant design principles for the future design of materials with improved functionality.

We use single-crystal-to-single-crystal transformations to directly image with sub-angstrom resolution distortions of the host lattice, Li site occupancies, and diffusion pathways in a 2D metastable intercalation material. We capture atomistic descriptions of Li-ion diffusion pathways and reconstruct a Li-eye view of available interstitial sites through a 2D intercalation host to address a long-standing challenge in the design of cathode materials and pave the way to atom-precise structural modifications.

The development of Sn-based metal halide perovskites (MHP) for thermoelectric applications along with many other device applications has thus far been limited by self-doping degradation and ion migration. Each of these effects decreases the electronic stability and prevents Sn MHPs from performing at their full potential. Motivated by the hypothesis that a pi-conjugated crosslinking agent may fill or block halide vacancies that would otherwise be oxygen-doped, we sought to perform a detailed study of the morphological and electronic degradation of Sn MHPs when 3-aminophenyl boronic acid (3APBA) is introduced. The level of p-type self-doping in Sn MHPs was modified by controlling the ratio of pi-conjugated crosslinking agent to Sn MHP in solution, which significantly impacts the electronic structure and thermoelectric properties of Sn MHPs. Additionally, these findings suggest that the incorporation of crosslinking agents can limit voltage bias stress effects caused by ion migration, which could be applied to a broad range of MHP materials that experience internal instabilities due to ionic effects.

Self-doping is an essential method of increasing carrier concentrations in organic electronics that eliminates the need to tailor host-dopant miscibility, a necessary step when employing molecular dopants. Self-n-doping can be accomplished using amines or ammonium counterions as an electron source, which are being incorporated into an ever-increasingly diverse range of organic materials spanning many applications. Self-n-doped materials have demonstrated exemplary and, in many cases, benchmark performances in a variety of applications. However, an in-depth review of the method is lacking. Perylene diimide (PDI) chromophores are an important mainstay in the semiconductor literature with well-known structure-function characteristics and are also one of the most widely utilized scaffolds for self-n-doping. In this review, we describe the unique properties of self-n-doped PDIs, delineate structure-function relationships, and discuss self-n-doped PDI performance in a range of applications. In particular, the impact of amine/ammonium incorporation into the PDI scaffold on doping efficiency is reviewed with regard to attachment mode, tether distance, counterion selection, and steric encumbrance. Self-n-doped PDIs are a unique set of PDI structural derivatives whose properties are amenable to a broad range of applications such as biochemistry, solar energy conversion, thermoelectric modules, batteries, and photocatalysis. Finally, we discuss challenges and the future outlook of self-n-doping principles.

Poor electrochemical communication between biocatalysts and electrodes is a ubiquitous limitation to bioelectrocatalysis efficiency. An extensive library of polymers has been developed to modify biocatalyst-electrode interfaces to alleviate this limitation. As such, conducting redox polymers (CRPs) are a versatile tool with high structural and functional tunability. While charge transport in CRPs is well characterized, the understanding of charge transport mechanisms facilitated by CRPs within decisively complex photobioelectrocatalytic systems remains very limited. This study is a comprehensive analysis that dissects the complex kinetics of photobioelectrodes into fundamental blocks based on rational assumptions, providing a mechanistic overview of charge transfer during photobioelectrocatalysis. We quantitatively compare two biohybrids of metal-free unbranched CRP (polydihydroxy aniline) and photobiocatalyst (intact chloroplasts), formed utilizing two deposition strategies ( “mixed” and “layered” depositions). The superior photobioelectrocatalytic performance of the “ layered” biohybrid compared to the “ mixed” counterpart is justified in terms of rate ( D app ), thermodynamic and kinetic barriers (H ≠ , E a ), frequency of molecular collisions ( D 0 ) during electron transport across depositions, and rate and resistance to heterogeneous electron transfer ( k 0 , R CT ). Our results indicate that the primary electron transfer mechanism across the biohybrids, constituting the unbranched CRP, is thermally activated intra- and inter-molecular electron hopping, as opposed to a non-thermally activated polaron transfer model typical for branched CRP- or conducting polymer (CP)-containing biohybrids in literature. This work underscores the significance of subtle interplay between CRP structure and deposition strategy in tuning the polymer-catalyst interfaces, and the branched/unbranched structural classification of CRPs in the bioelectrocatalysis context.

Perylene diimides (PDIs) have garnered attention as organic photocatalysis in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and olefin iodoperfluoroalkylation. Previous work in this area employs spectator pendant groups attached to the imide nitrogen positions of PDIs that are only added to impart solubility. In this work, we employ electron-rich ammonium iodide or ammonium hydroxide pendant groups capable of self-n-doping the PDI core to form radical anions (R●―) and dianions (D●●―). We observe R●― formation is favored at low concentrations where aliphatic linkers are able to freely rotate, while D●●― formation is favored at elevated concentrations likely due to Coulombic stabilization between adjacent chromophores in a similar manner to that of Kasha exciton stabilization. Cyclic voltammetric measurements are consistent with steric encumbrance increasing the Lewis basicity of anions through Coulombic destabilization. However, sterics also inhibit dianion formation by disrupting aggregation. Finally, femtosecond transient absorption measurements reveal that low wavelength excitation (400 nm) preferentially favors the excitation of R●― to the strongly reducing doublet excited state 2[R●―]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state 1[N]*. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on R●― and D●●― formation, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient systems based on self-n-doped PDIs.

The demand for multifunctional devices continues to drive the evolution and parametrization of technology. Simultaneously, the breadth of structural, morphological, and electronic information that can be used to study materials widens and grows in complexity. To keep up with the demand for superior technologies in disparate fields, such as energy, electronics, and biotechnology, an unparalleled amount of manpower and financial resources have been devoted to the development of materials that can integrate multiple functionalities, particularly functions that ostensibly exclude one another. To address different functionalities at once, materials have become highly complex and often exhibit multiple structural and morphological phases, hierarchical dependencies, and far-from-equilibrium dynamic structures with little to no long-range atomic ordering. A major area of study that is quickly evolving deals with the quantitative characterization of structure-property-function relationships in complex materials based on pi-conjugated organic and organometal systems-particularly in thin film formulations. In this review, we discuss various research avenues where cleverly engineered self-assembly protocols, as well as characterization methods for probing morphology and electronic structure, are implemented to enable the fabrication of well-defined emergent materials.

To close the technological gap between laboratory-based conducting polymers and commercially available inorganic semiconductors, it is imperative to develop synthesis approaches that allow for the fabrication of morphology-controlled polymers where charge carriers can be readily fine-tuned. Herein, our studies provide a detailed investigation of the polymerization protocols needed to afford the fabrication of well-defined and highly ordered polymer thin films. Using poly(3,4-ethylenedioxythiophene (PEDOT) as a case study, we demonstrate strong correlations between the polymerization protocols and counterion dopants in driving the observed changes in crystallinity and pi-stacking ordering of fabricated thin films. Upon the fabrication of thin films with controlled morphologies, we can provide an in-depth characterization of the thermoelectric properties (i.e., electrical conductivity, Seebeck coefficient, and power factor) of doped PEDOT chains as a function of counterion dopants and oxidation levels. Given the high power factors, we fit the thermoelectric characteristics of our samples to determine the charge transport mechanism in PEDOT thin films as a function of polymerization protocols and counterion dopants. On the basis of the fits, we report record-breaking transport function values which are indicative of bandlike transport in PEDOT thin films. As such, these high transport function values may point to the fabrication of well-defined and highly conducting polymers that can achieve thermoelectric figure of merit values above 0.6 at room temperature.

At the very heart of the global semiconductor industry lies the omnipresent push for new materials discovery. New materials constantly rise and fall out of fashion in the scientific literature, with those passing an initial phase of research scrutiny becoming hotbeds of characterization and optimization efforts. Yet, innumerable hours of painstaking research have been devoted to materials that have ultimately fallen by the wayside after crossing over an indefinable threshold, whereupon historical optimism is met with newfound skepticism. Materials have to perform well, and they have to do it quickly. In the past decade, metal-halide perovskites (MHPs) have garnered widespread attention. The hegemonic view in both academic and industrial circles is that these materials could be engineered to meet the demands of the semiconductor industry. Their promise as inexpensive solar cell devices is highly attractive, and it has been nothing short of remarkable that efficiencies have risen from 3.8% in 2009 to more than 25.5% in 2021. Moreover, MHPs are poised to be revolutionary materials in more ways than one. The highest MHP LED efficiency was recently reported (23.4%), and MHPs have demonstrated promise in photodetectors, memristors, and transistors. However, the many excellent properties of MHPs are contrasted by longstanding stability and reproducibility limitations that have hindered their commercialization. Overcoming the limitations of MHPs is ultimately a materials engineering problem, which should be solved by mapping more precise relationships between structure, composition, and device performance. In 1958, Francis Crick famously developed the central dogma of molecular biology which describes the unidirectional flow of information in biological systems. In the words of Crick, "nature has devised a unique instrument in which an underlying simplicity is used to express great subtlety and versatility." In this Account, taking inspiration from the hierarchical organization of nature, we describe a hierarchical approach to materials engineering of organic metal-halide semiconductors. We demonstrate that organo-metal halide semiconductors' dimensionality, composition, and morphology dictate their optoelectronic properties and can be exploited in defining more explicit relationships between structure and function. Here, we traverse three-dimensional (3D), two-dimensional (2D), and one-dimensional (1D) organo-metal halide semiconductors, detailing the morphological and compositional differences in each and the implications that can be drawn within each domain on the engineering process. Control over ion migration pathways via morphology engineering as well as control over charge formation in organic-inorganic semiconductors is demonstrated. Fundamental insights into the amount of static and dynamic disorder in the MHP lattice are provided, which can be continuously tuned as a function of composition and morphology. Using electroabsorption spectroscopy on 2D MHPs, a disorder-induced dipole moment in the exciton proportional to the summed value of static and dynamic disorder is measured. Spectroscopic isolation of exciton features in 2D MHP electroabsorption spectra allows us to obtain precise, model-independent measurements of exciton binding energies to study the effect of chemical substitutions, such as Sn2+ → Pb2+, on the value of the exciton binding energy. Finally, we conclude that this multidimensional platform, with the aid of machine learning and robotics, will be foundational in accurately predicting structure-property-device relationships in organo-metal halide semiconductors in the future.

We determined how morphology, electronic and interfacial interactions affect perovskite PVs under voltage bias stress. Our findings provide insights into the discrepancies in the solar cell efficiencies observed across many different research groups.

We provide fundamental design principles on the effect of dopant structure (steric hindrance) on the doping efficiency in highly oriented self-doped organic semiconducting thin films.

Thin film materials have become increasingly complex in morphological and structural design. When characterizing the structure of these films, a crucial field of study is the role that crystallite orientation plays in giving rise to unique electronic properties. It is therefore important to have a comparative tool for understanding differences in crystallite orientation within a thin film, and also the ability to compare the structural orientation between different thin films. Herein, we designed a new method dubbed the mosaicity factor (MF) to quantify crystallite orientation in thin films using grazing incidence wide-angle X-ray scattering (GIWAXS) patterns. This method for quantifying the orientation of thin films overcomes many limitations inherent in previous approaches such as noise sensitivity, the ability to compare orientation distributions along different axes, and the ability to quantify multiple crystallite orientations observed within the same Miller index. Following the presentation of MF, we proceed to discussing case studies to show the efficacy and range of application available for the use of MF. These studies show how using the MF approach yields quantitative orientation information for various materials assembled on a substrate.