Time-resolved Microwave Conductivity Measurements Research Articles

Low‐Temperature Synthesis of Stable CaZn2P2 Zintl Phosphide Thin Films as Candidate Top Absorbers

AbstractThe development of tandem photovoltaics and photoelectrochemical solar cells requires new absorber materials with bandgaps in the range of ≈1.5–2.3 eV, for use in the top cell paired with a narrower‐gap bottom cell. An outstanding challenge is finding materials with suitable optoelectronic and defect properties, good operational stability, and synthesis conditions that preserve underlying device layers. This study demonstrates the Zintl phosphide compound CaZn2P2 as a compelling candidate semiconductor for these applications. Phase‐pure, ≈500 nm‐thick CaZn2P2 thin films are prepared using a scalable reactive sputter deposition process at growth temperatures as low as 100 °C, which is desirable for device integration. Ultraviolet‐visible spectroscopy shows that CaZn2P2 films exhibit an optical absorptivity of ≈104 cm−1 at ≈1.95 eV direct bandgap. Room‐temperature photoluminescence (PL) measurements show near‐band‐edge optical emission, and time‐resolved microwave conductivity (TRMC) measurements indicate a photoexcited carrier lifetime of ≈30 ns. CaZn2P2 is highly stable in both ambient conditions and moisture, as evidenced by PL and TRMC measurements. Experimental data are supported by first‐principles calculations, which indicate the absence of low‐formation‐energy, deep intrinsic defects. Overall, this study shall motivate future work integrating this potential top cell absorber material into tandem solar cells.

Electron Transport over 2D Molecular Materials and Assemblies.

ConspectusTwo-dimensional (2D) molecular materials, in which the major interactions are confined in 2D planes with contrasted force fields acting in between the planes, have been key electronic functional materials since the past decade. Even without referring to the functionals of graphene-based systems, 2D electronic conjugated systems are expected to show extrawide dynamic ranges in electronic density of states (DOS) tuning, effective electron mass, electron mobility, and conductivity. A major advantage of 2D electronic systems is their compatibility with the ubiquitous electronic devices designed using planar structures, such as transistors and memories, which is associated with the utility of 2D active materials. The mobility of electrons in 2D systems is the key to their utility, and various conjugated molecular and 2D materials have been designed to optimize the mobility. This Account begins with an introduction for mobility assessment: using noncontact time-resolved microwave conductivity (TRMC) measurements as a technique to probe differential conductivity upon transient charge carrier injection into the materials. Electronic transport over 2D electronic materials such as graphenes, covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) is discussed with a special emphasis on molecular building blocks, fine-tuning conducting species and linkages, topology of the framework, and controlling molecular doping. The superiority of β-ketoenamine-linked COF over imine-linked COF films in charge transport and dominant in-plane charge carrier mobility over out-of-plane mobility is also illustrated. Systematic molecular engineering of the building blocks of β-ketoenamine-linked COFs with varying degrees of donor-acceptor (D-A) conjugation, torsional angles, and reaction conditions resulted in the modulation of the efficiency of charge carrier generation/transport as well as exciton migration. The advantages of 2D systems are finally discussed in terms of the mobility interplaying with spatial arrangements of molecules as well as the substantial role of intermolecular interactions in stabilizing their condensed phases. The strong correlation between the dispersion of mobility and hierarchical intermolecular interactions sheds light on the way to overcome structural fluctuation on the optimization of charge transport in molecular electronic materials. The point of singularity in the dispersion at an intermolecular distance of d ∼ 0.3 nm is deduced from the overall mobility assessment in condensed phases of conjugated molecules, suggesting key roles of intermolecular electronic coupling: the new concept of electronic conjugation. Exceptional electronic coupling with relatively high charge carrier mobility was also observed, particularly in 2D spatial arrangements of chiral molecules in contrast to 3D analogues, where the reduction of gravitational density of the molecular condensates was impacting DOS: the Wallach's rule. 2D electronic systems are strong candidates for the violation of the long-lasting Wallach's rule in terms of DOS.

(Invited) Dynamic Relaxation of Charge Carrier Mobilities in Organic Photovoltaics

A non-fullerene small molecular acceptor (NFA) is a prominent molecule that shows moderate electron mobility and a narrow bandgap complementary to middle-bandgap p-type conjugated polymers, which leads to great improvement in the performance of organic photovoltaic (OPV) cells. However, little is known about the relaxation of charge carriers, which is key to efficient charge transport. Herein, we report simultaneous time-of-flight (TOF) and time-resolved microwave conductivity (TRMC) measurements[1] employing benzodithiophene-based polymer (PBDB-T):soluble C70-fullerere (PCBM) and PBDB-T:NFA (ITIC or Y6) blends, as benchmark systems[2, 3]. In addition to the conventional TOF mobilities, relaxation of the hole and electron mobility was evaluated by TRMC under an external electric field. Although PBDB-T:ITIC exhibited much faster relaxation than PBDB-T:PCBM, the relaxation in PBDB-T:Y6 was considerably moderate. This is consistent with the energetic disorder estimated from the photoabsorption onset. Interestingly, the slower relaxation of the electrons compared to the holes in PBDB-T:Y6 is in line with the preferred normal device structure. Our work deepens the understanding of the energetics of polymer:NFA blends and offers a basis for achieving efficient NFA properties.Reference[1] Y. Shimata, A. Saeki, J. Phys. Chem. C 121 (2017) 18351.[2] F. Hamada, A. Saeki, ChemSusChem 14 (2021) 3528.[3] S. Li, F. Hamada, R. Nishikubo, A. Saeki, Sustainable Energy Fuels 6 (2022) 756.

Combined Charge Extraction by Linearly Increasing Voltage and Time-Resolved Microwave Conductivity to Reveal the Dynamic Charge Carrier Mobilities in Thin-Film Organic Solar Cells.

This article reports a purely experiment-based method to evaluate the time-dependent charge carrier mobilities in thin-film organic solar cells (OSCs) using simultaneous charge extraction by linearly increasing the voltage (CELIV) and time-resolved microwave conductivity (TRMC) measurements. This method enables the separate measurement of electron mobility (μe) and hole mobility (μh) in a metal-insulator-semiconductor (MIS) device. A slope-injection-restoration voltage profile for MIS-CELIV is also proposed to accurately determine the charge densities. The dynamic behavior of μe and μh is examined in five bulk heterojunction (BHJ) OSCs of polymer:fullerene (P3HT:PCBM and PffBT4T:PCBM) and polymer:nonfullerene acceptor (PM6:ITIC, PM6:IT4F, and PM6:Y6). While the former exhibits fast decays of μh and μe, the latter, in particular, PM6:IT4F and PM6:Y6, exhibits slow decays. Notably, the high-performing PM6:Y6 demonstrates both a balanced mobility (μe/μh) of 1.0-1.1 within 30 μs and relatively large CELIV-TRMC mobility values among the five BHJs. The results exhibit reasonable consistency with a high fill factor. The proposed new CELIV-TRMC technique offers a path toward a comprehensive understanding of dynamic mobility and its correlation with the OSC performance.

Electronic relaxation in PLD TiO2 films processed with femtosecond UV-laser

Modifications of shallow electron trap states near conduction band was analyzed after surface structuring of TiO2 thin films prepared via pulsed laser deposition method. Laser-induced periodic surface structures (LIPSS) were produced on the large-area films after irradiation with femtosecond KrF laser (λ = 248 nm). While spatial period of LIPSS Λ ≈ 190 ± 10 nm was not affected by laser fluence, coherence was increased with an increase of laser dose. Time-resolved microwave conductivity measurements accompanied by theoretical modelling revealed energetic positions of shallow electron trap states, connected to LIPSS. A possibility to alter the trap states energy via surface structuring of the material was shown.

Modification of Visible-Light-Responsive Pb2Ti2O5.4F1.2 with Metal Oxide Cocatalysts to Improve Photocatalytic O2 Evolution toward Z-Scheme Overall Water Splitting.

The development of a highly active photocatalyst for visible-light water splitting requires a high-quality semiconductor material and a cocatalyst, which promote both the migration of photogenerated charge carriers and surface redox reactions. In this work, a cocatalyst was loaded onto an oxyfluoride photocatalyst, Pb2Ti2O5.4F1.2, to improve the water oxidation activity. Among the metal oxides examined as cocatalysts, RuO2 was found to be the most suitable, and the O2 evolution activity depended on the preparation conditions for Ru/Pb2Ti2O5.4F1.2. The highest activity was obtained with RuCl3-impregnated Pb2Ti2O5.4F1.2 heated under a flow of H2 at 523 K. The H2-treated Ru/Pb2Ti2O5.4F1.2 showed an O2 evolution rate an order of magnitude higher than those for the analogues without the H2 treatment (e. g., RuO2/Pb2Ti2O5.4F1.2). Physicochemical analyses by X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and time-resolved microwave conductivity measurements indicated that the optimized photocatalyst contained partially reduced RuO2 species with a particle size of ~5 nm. These partially reduced species effectively trapped the photogenerated charge carriers and promoted the oxidation of water into O2. The optimized Ru/Pb2Ti2O5.4F1.2 could function as an O2-evolving photocatalyst in Z-scheme overall water splitting, in combination with an Ru-loaded, Rh-doped SrTiO3 photocatalyst.

Tail states suppression via surface-modification of wide-bandgap perovskites for high-efficiency all-perovskite photovoltaic tandems

As an essential light-absorbing material in perovskite-based tandem solar cells, wide-bandgap (WBG) perovskite has garnered extensive attention. Nevertheless, due to severe non-radiative recombination from tail states, as well as photoinduced phase segregation and unoptimized charge transfer, WBG perovskite solar cells (PSCs) typically display significant open-circuit voltage (VOC) loss and low fill factor (FF). In this work, we report to modify the 1.68 eV WBG perovskite film upon optimized surface passivation with 1,3-propane-diammonium iodide (PDAI2). Time-resolved microwave conductivity measurement is employed to analyze the tail states, mobilities of charge carriers and the rate constants of recombination in perovskite films. The results indicate that the optimized surface modification strategy not only enhances film quality, boosts carrier transport, but also suppresses tail states and therefore diminishes non-radiative recombination, consequently elevating the VOC and FF of the device. Hence, the PDAI2-assisted WBG PSCs deliver an optimal efficiency of 21.48 %, accompanied by a VOC of 1.243 V. Furthermore, the integration of a narrow-bandgap tin–lead PSC with a semi-transparent WBG PSC results in a four-terminal perovskite/perovskite tandem solar cell, showcasing an efficiency surpassing 28 %. Our research offers a practical approach for producing efficient WBG PSCs and tandem solar cells.

Perovskite Solar Cell Using Isonicotinic Acid as a Gap-Filling Self-Assembled Monolayer with High Photovoltaic Performance and Light Stability.

High photovoltaic performance and light stability are required for the practical outdoor use of lead-halide perovskite solar cells. To improve the light stability of perovskite solar cells, it is effective to introduce a self-assembled monolayer (SAM) between the carrier transport layer and the perovskite layer. Several alternative approaches in their molecular design and combination with multiple SAMs support high photovoltaic conversion efficiency (PCE). Herein, we report a new structure for improving both PCE and light stability, in which the surface of an electron transport layer (ETL) was modified by combining a fullerene-functionalized self-assembled monolayer (C60SAM) and a suitable gap-filling self-assembled monolayer (GFSAM). Small-sized GFSAMs can enter the gap space of the C60SAM and terminate the unterminated sites on the ETL surface. The best GFSAM in this study was formed using an isonicotinic acid solution. After a light stability test for 68 h at 50 °C under 1 sun illumination, the best cell with C60SAM and GFSAM showed a PCE of 18.68% with a retention rate of over 99%. Moreover, following outdoor exposure for six months, the cells with C60SAM and GFSAM exhibited almost unchanged PCE. From the valence band spectra of the ETLs obtained using hard X-ray photoelectron spectroscopy, we confirmed a decrease in the offset at the ETL/perovskite interface owing to the additional GFSAM treatment on the C60SAM-modified ETL surface. Time-resolved microwave conductivity measurements demonstrated that the additional GFSAM improved electron extraction at the C60SAM-modified ETL/perovskite interface.

Modeling Phase Changing Property of VO2 for Reconfigurable Microwave Frequency Selection Applications via Time-Resolved Microwave Conductivity

Vanadium dioxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is characterized using the time-resolved microwave conductivity (TRMC) method for applications requiring reconfigurable microwave frequency selection. We demonstrate that TRMC is a flexible method for determining the microwave conductivity of thin film compounds of varied thicknesses that are both phase-changing and non-phase changing in nature. The method makes use of Ka-band microwave resonant waveguides to examine how thermally excited samples’ microwave conductivity changes in response to microwave-transmitted power through the system’s sensitivity factor. The sensitivity factor is illustrated by a study of incident waves (electromagnetic fields) on medium and waveguide perturbation analysis in VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films. The TRMC measurements show a low regime in the microwave conductivity below the transition temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_c</i> ) of VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> where <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_c</i> occurs near 65°C; this is associated with the dielectric characteristics of the charge carriers and points to the presence of shallow trap electron states. Longer electron hops enhance charge mobility and, consequently, the barrier energy above <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_c</i> , where these states are thermally discharged, resulting in a high thermally triggered microwave conductivity regime. The activation energy may come from a variety of complex contributions, like polaronic self-localizations, as well as a dynamic disorder brought on by the thermal oscillations of VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> molecules. The calculated conductance values are simulated in CST Studio as plane wave incidence on medium to ascertain the fields’ microwave transmission behavior and contrast it with the experimental results to validate the model. Additionally, using the derived conductivities, the characteristics of well-known materials like copper (Cu) were confirmed.

Preparation and Physicochemical Properties of Nanomaterial/Fullerene Composites

Bismuthene, an exfoliated two-dimensional material obtained from bulk bismuth, has drawn significant attention because of its unique electronic properties. Few-layered bismuthene (FLBi) with an average thickness of 1.0 nm was synthesized by the ball mill and sonication method. The FLBi film was fabricated onto a semiconducting SnO2 electrode by electrophoretic deposition (SnO2/FLBi). In the flash-photolysis time-resolved microwave conductivity measurement, the SnO2/FLBi film showed a rise of conductivity upon photoexcitation, supporting the occurrence of the electron injection from the photoexcited FLBi to the conduction band of SnO2. The SnO2/FLBi on a transparent fluorine-doped tin oxide (FTO) electrode was used for photoelectrochemical devices. Photocurrents produced by the FTO/SnO2/FLBi electrode were larger than those produced by the FTO/SnO2 and FTO/FLBi electrodes because of the efficient electron injection. Moreover, FLBi was noncovalently functionalized with fullerene C60 in a mixed solvent of toluene and acetonitrile. Upon photoexcitation, the composite of FLBi and C60 exclusively led to the occurrence of photoinduced energy transfer from C60 to FLBi without generating the charge-separated state. These results give fundamental insights into the feasibility toward the construction of FLBi-based optoelectronic devices.Supramolecular composites consisting of fullerene C60 and carbon nanodiamond (ND) were prepared through spontaneous complexation of C60 aggregates onto the surface of ND aggregates in N-methylpyrrolidone. The resultant C60-ND composite was fabricated onto a nanostructured SnO2 electrode by an electrophoretic deposition method. Formation of the C60-ND composite was supported by dynamic light scattering and field-emission scanning electron microscopy. The C60-ND composite on the SnO2 electrode revealed high incident photon-to-current efficiencies in the visible region in comparison with the single component system of C60 or ND. The enhanced photocurrent generation of the C60-ND composite may originate from the photoinduced charge separation at the interface between C60 and ND. These results will give important insight into the design of all-nanocarbon optoelectronic devices.[1] T. Umeyama, Y. Okawada, T. Ohara, and H. Imahori, Chem. Asian J., 14, 4042-4047 (2019).[2] T. Umeyama, H. Xu, T. Ohara, Y. Tsutsui, S. Seki, H. Imahori, J. Phys. Chem. C, 125, 13954-13962 (2021).

Long-range charge carrier mobility in metal halide perovskite thin-films and single crystals via transient photo-conductivity

Charge carrier mobility is a fundamental property of semiconductor materials that governs many electronic device characteristics. For metal halide perovskites, a wide range of charge carrier mobilities have been reported using different techniques. Mobilities are often estimated via transient methods assuming an initial charge carrier population after pulsed photoexcitation and measurement of photoconductivity via non-contact or contact techniques. For nanosecond to millisecond transient methods, early-time recombination and exciton-to-free-carrier ratio hinder accurate determination of free-carrier population after photoexcitation. By considering both effects, we estimate long-range charge carrier mobilities over a wide range of photoexcitation densities via transient photoconductivity measurements. We determine long-range mobilities for FA0.83Cs0.17Pb(I0.9Br0.1)3, (FA0.83MA0.17)0.95Cs0.05Pb(I0.9Br0.1)3 and CH3NH3PbI3-xClx polycrystalline films in the range of 0.3 to 6.7 cm2 V−1 s−1. We demonstrate how our data-processing technique can also reveal more precise mobility estimates from non-contact time-resolved microwave conductivity measurements. Importantly, our results indicate that the processing of polycrystalline films significantly affects their long-range mobility.

(Invited, Digital Presentation) Composites of 2D Materials and Organic Molecules

Atomically thin two-dimensional (2D) materials have aroused an explosive growth of scientific interest for their outstanding physicochemical properties. In addition, decorations of 2D materials with organic semiconductors have also begun to receive scientific attention owing to the tunable optoelectronic properties and the excellent solution-based processability of the organic molecules. In this study, composite films that consisted of C60 with well-exfoliated nanosheets of transition metal dichalcogenides (TMDs) [1], antimony [2], or bismuth [3] were successfully fabricated onto a semiconducting SnO2 electrode via a two-step methodology: self-assembly into their composite aggregates by injection of a poor solvent into a good solvent with the dispersion, and subsequent electrophoretic deposition. In addition, photophysical and photoelectrochemical properties of the composite films were investigated in detail. For example, in a flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurement of a composite material consisting of C60 and few-layered antimonene (FLSb) nanosheets (denoted as (FLSb+C60) m ), a rapid rise in transient conductivity was observed, whereas no conductivity signal was observed in the single components, FLSb and C60. This demonstrated the occurrence of photoinduced charge separation between FLSb and C60 in (FLSb+C60) m . Furthermore, a photoelectrochemical device with an electrophoretically deposited (FLSb+C60) m film exhibited an enhanced efficiency of photocurrent generation, compared to those of the single-components, FLSb and C60, due to the photoinduced charge separation between FLSb and C60. This work provides a promising approach for fabrication of 2D material–organic molecule composites and paves the way for their application in optoelectronics.[1] J. Baek, T. Umeyama, W. Choi, Y. Tsutsui, H. Yamada, S. Seki, and H. Imahori, Chem. Eur. J., 24, 1561-1572 (2018).[2] T. Umeyama, T. Ohara, Y. Tsutsui, S. Nakano, S. Seki, and H. Imahori, Chem. Eur. J., 26, 6726-6735 (2020).[3] T. Umeyama, H. Xu, T. Ohara, Y. Tsutsui, S. Seki, and H. Imahori, J. Phys. Chem. C, 125, 13954-13962 (2021).

Solid-solid transition of microcrystalline oligo(phenylene ethynylene)s: Impact of crystalline structure on optoelectronic properties

Conjugated oligomers with distinct chain lengths, the trimers and pentamers of phenyleneethynylene units, have been investigated in this work with an aim to obtain ordered crystalline phases of the linear chain molecules. As such, we have observed a unique phase transition only in the pentamer. Unlike the case of the crystalline-liquid crystalline phase transition, the pentamer exhibited a unique solid-solid transition with uniaxial expansion in the condensed phase. Time-resolved microwave conductivity measurements suggested an interplay of the thermal activation of charge carriers, which enhanced the hopping probability, and uniaxial expansion reduced the electronic coupling in the π-stacking of adjacent molecules.

Synthesis and Strong π-π Interaction of Hexaazatriphenylene Derivatives with Alternating Electron-Withdrawing and -Donating Groups.

Hexaazatriphenylene (HAT) derivatives have attracted wide attention because of their electron-deficient nature and unique self-assembly properties. In this work, a facile synthesis method for obtaining HAT derivatives with alternating electron-withdrawing nitrile and electron-donating alkoxy groups (HATCNOCn) is proposed. Crystal structure analysis indicated that HATCNOCn forms a one-dimensional columnar structure via strong π-π interactions. Density functional theory calculations revealed that the edge of HATCNOCn is divided into positively and negatively charged sites owing to the presence of alternating nitrile and alkoxy groups, which would induce strong π-π interactions. Thermal analysis and polarizing optical microscopy revealed that HATCNOCn exhibits columnar liquid-crystal phases. Time-resolved microwave conductivity measurements further demonstrated the photoconductive nature of HATCNOCn. The proposed strategy could provide a new strategy for the design of novel organic semiconductive materials.

Quantifying the optimal thickness in polymer:fullerene solar cells from the analysis of charge transport dynamics and photoabsorption

We provide a semi-empirical equation that quantifies the optimal layer thickness in bulk heterojunction organic solar cells, which is based on time-of-flight and time-resolved microwave conductivity measurements and photoabsorption of a film.

Controlling the charge carrier dynamics by modulating the orientation diversity of perovskites

Photo-induced charge carrier dynamics over a wide time domain from sub-nanoseconds to microseconds, via time-correlated single photon counting and flash photolysis time-resolved microwave conductivity measurements.

Recombination and localization: Unfolding the pathways behind conductivity losses in Cs2AgBiBr6 thin films

Cs2AgBiBr6 (CABB) has been proposed as a promising nontoxic alternative to lead halide perovskites. However, low charge carrier collection efficiencies remain an obstacle for the incorporation of this material in optoelectronic applications. In this work, we study the optoelectronic properties of CABB thin films using steady state and transient absorption and reflectance spectroscopy. We find that optical measurements on such thin films are distorted as a consequence of multiple reflections within the film. Moreover, we discuss the pathways behind conductivity loss in these thin films, using a combination of microsecond transient absorption spectroscopy and time-resolved microwave conductivity measurements. We demonstrate that a combined effect of carrier loss and localization results in the conductivity loss in CABB thin films. Moreover, we find that the charge carrier diffusion length and grain size are of the same order of magnitude. This suggests that the material's surface is an important contributor to charge-carrier loss.

Naphthalenediimide/Formamidinium-Based Low-Dimensional Perovskites

Low-dimensional hybrid perovskites have emerged as promising materials for optoelectronic applications. Although these materials have already demonstrated enhanced stability as compared to their three-dimensional perovskite analogues, their functionality has been limited by the insulating character of the organic moieties that primarily play a structure-directing role. This is particularly the case for the layered (2D) perovskite materials based on formamidinium lead iodide (FAPbI3) that remain scarce. We demonstrate a low-dimensional hybrid perovskite material based on a SPbI4 composition incorporating an electroactive naphthalenediimide (NDI) moiety as an organic spacer (S) between the perovskite slabs and evidence the propensity of the spacer to stabilize the α-FAPbI3 perovskite phase in hybrid low-dimensional SFAn–1PbnI3n+1 perovskite compositions. This has been investigated by means of solid-state nuclear magnetic resonance spectroscopy in conjunction with molecular dynamics simulations and density functional theory calculations. Theoretical calculations suggest an electronic contribution of the organic spacer to the resulting optoelectronic properties, which was confirmed by transient absorption spectroscopy. We have further analyzed these materials by time-resolved microwave conductivity measurements, revealing challenges for their application in photovoltaics.

(Invited) Optoelectronic and Dielectric Properties of Organic Cation Tin-Based Perovskite Solar Cells

Tin iodide perovskite (ASnI3) is one of the most feasible, less toxic alternatives to lead halide perovskite (APbX3), owing to its readily formed three-dimensional (D) structure and more suited bandgap (1.2–1.4 eV) of the solar cell application than APbX3 (1.5–2.3 eV). Regardless of the skyrocketing boost in Pb perovskite solar cells throughout this decade, Sn-based counterparts have remained constrained because of the intrinsically unstable Sn2+ that is easily oxidized to Sn4+. The resultant p-doping causes electron trapping and a shorting of the diode circuit, which is partly mitigated by SnF2 addition. Nevertheless, the power conversion efficiency (PCE) values of MASnI3 (MA: methylammonium cation) based on a normal cell structure (mesoporous TiO2/active layer/hole transport layer: HTL) have been suppressed due to the p-doping and low film quality. Notably, progress in an inverted cell structure (HTL/active layer/electron transport layer: ETL) has been reported for FASnI3 (FA: formamidinium cation) mixed with either MA, PEA (phenylethylamine cation), EDA (ethylenediammonium dication), or GA (guanidium cation). In addition to this A-site cation mixing strategy associated with 2D/3D perovskites, the use of {en} (ethylenediammonium), SnX2-(pyrazine, trimethylamine, or solvent) complexes, hot anti-solvent treatment (HAT), poly(vinyl alcohol) (PVA) addition, and GeI2 dopingreportedly showed improvement in PCEs. Thus, the updated PCE in the forefront of the Sn perovskite solar cell is ~9–10%, which is enabled by MA/FA mixing, FA/PEA mixing, GA/FA mixing, as well as the addition of 4-fuorobenzohydrazide (FBH), 2-cyano-3-[5-[4-(diphenylamino)phenyl]-2-thienyl]-propenoic acid (CDTA), or passivation with ethylenediamine (edamine). Although the mixing of A-site cations, along with additives, has been found to smooth the film surface, decrease the Sn4+ impurities, and tune the valence band maximum (VBM), their effects on charge carrier dynamics and morphological changes are the subject of ongoing debate.We characterized the solar cells composed of ternary A-site cation-mixed Sn perovskite: ASnI3 [A = (GAxFA1–x)0.9PEA0.1, the ratios are the precursor stoichiometry] by changing the GA content (x = 0–1) and investigated their charge carrier dynamics, surface morphologies, and energetics.[1] The inclusion of 10 mol% PEA was found necessary for performing these evaluations over a wide range of GA:FA ratios. Electrode-less, time-resolved microwave conductivity (TRMC) measurements[2] were applied to a pristine ASnI3 film, double layers of HTL/ASnI3 and ASnI3/ETL, as well as a triple layer of HTL/ASnI3/ETL, where PEDOT:PSS and thermally evaporated fullerene (C60) were used as the HTL and ETL, respectively. These comparative samples with different GA contents discerned the contributions of hole and electron mobilities in the TRMC transients. Notably, the change in electron mobilities corresponded to the PCE dependence on the GA content, highlighting the pivotal role of electron mobility in solar cell performance. We further examined the atomic force microscopy (AFM) surface images, VBM measured by photoelectron spectroscopy (PYS), and crystalline features measured by X-ray diffraction (XRD), which supports the dramatic change of charge carrier mobility and lifetime.On one hand, the dielectric properties of organic-inorganic hybrid perovskite materials have remained poorly understood, despite probably influencing delayed charge recombination and device capacitance. We characterize the unprecedented dielectric behavior of MHPs comprising methylammonium cations, Pb/Sn as metals, and Br/I as halides using TRMC measurements.[3] At specific compositions, the above MHPs exhibit negative real and positive imaginary photoconductivities, the polarities of which are opposite to those observed for conventional photogenerated charge carriers. Comparing the observed TRMC kinetics with that of inorganic perovskites (SrTiO3 and BaTiO3) and characterizing its dependence on temperature, frequency and near-infrared second push pulse, we conclude that the above behavior is due to the trapping of polaronic holes/electrons by oriented dipoles of organic cations, which opens a hitherto unexplored route to the dynamical control of dielectric permittivity by photoirradiation.Reference Nakanishi, R. Nishikubo, A. Wakamiya, A. Saeki, J. Phys. Chem. Lett. 11 (2020) 4043.Saeki, A. Polym. J. 52 (2020) 1307.Yamada, R. Nishikubo, H. Oga, Y. Ogomi, S. Hayase, S. Kanno, Y. Imamura, M. Hada, A. Saeki, ACS Photonics 5 (2018) 3189.

(Invited) Controlling Monolayer and Few-Layer MoS2 and WS2 Optoelectronic and Catalytic Properties

By functionalizing, doping, and changing phases, we can control the optoelectronic and catalytic properties of 2D transition metal dichalcogenides (TMDCs), specifically MoS2 and WS2. These 2D layered nanomaterials can be made via a top-down (exfoliation) or bottom-up (chemical vapor deposition) approach. The optoelectronic properties of monolayer TMDCs are strongly influenced by their surroundings and can be used as a humidity sensor, which we demonstrate with photoluminescence and time-resolved microwave conductivity measurements. Furthermore, we quantify the stability of metallic, monolayer TMDCs when it is converted from the semiconducting to the metallic phase and functionalized, where comparisons are made to few-layer TMDCs. By controlling these optoelectronic properties, we can better understand the catalytic performance for hydrogen evolution and other other catalytic reactions.