Time-resolved Microwave Conductivity Technique Research Articles

Shallow traps-induced ultra-long lifetime of metal halide perovskites probed with light-biased time-resolved microwave conductivity

Metal halide perovskites have emerged as promising candidates for next-generation optoelectronics. However, the present understanding of their recombination processes and trap states within the devices are still limited, which is also inevitable in the state-of-the-art perovskite solar cells with multiple passivation strategies and various additives involved. Recent works have also demonstrated that metal cations incorporated perovskites could potentially reduce the non-radiative losses and improve the device performance to some extent. However, the underlying “doping” mechanism is not clear. In this work, we systematically investigated the trap-induced ultra-long carrier lifetime of the metal cation incorporated perovskites and found that some specific cations could extend the carrier lifetime up to ∼100 μs, which could be correlated with the formation of shallow trap states. In addition, such shallow trap-mediated charge dynamics could be effectively probed with light-biased time-resolved microwave conductivity technique, which provides additional information to conventional time-resolved photoluminescence.

Ultrathin Two Dimensional (2D) Supramolecular Assembly and Anisotropic Conductivity of an Amphiphilic Naphthalene-Diimide.

Two-dimensional (2D)-supramolecular assemblies of π-conjugated chromophores are relatively less common compared to a large number of recent examples on their low dimensional (0D or 1D) assemblies or 3D architectures. This article reports a rational design for the 2D supramolecular assembly of an amphiphilic core-substituted naphthalene-diimide derivative (cNDI-1). The building block contains a naphthalene-diimide (NDI) chromophore, symmetrically substituted with two dodecyl chains from the aromatic core while the imide positions are functionalized with two hydrophilic wedges containing oligo-oxyethylene chains. In water, it exhibits entropically favorable self-assembly with a critical aggregation concentration of 1.5 × 10-5 M and a lower critical solution temperature of 55 °C. The UV/vis absorption spectrum in water shows bathochromically shifted absorption bands compared to that of the monomeric dye in THF, indicating offset π-stacking among the NDI chromophores. C-H symmetric and asymmetric stretching frequencies in the FT-IR spectrum support the presence of organized hydrocarbon chains in trans conformation in the self-assembled state, similar to that in the crystalline n-alkanes, which is further supported by studying the general polarization (GP) values of a noncovalently entrapped Laurdan dye. The atomic force microscopy (AFM) image shows the formation of ultrathin (height < 2.0 nm) ribbons for the spontaneously assembled sample which eventually produces a large-area 2D nanosheet by the lateral organization. The powder X-ray diffraction pattern of the drop-casted film, prepared from the preformed aggregates, reveals sharp peaks that indicate a crystalline lamellar packing along the direction of the 2D growth. Differential scanning calorimetry trace shows the melting of the crystalline alkyl chain domain at T > 75 °C, which destroys the 2D assembly. Local-scale photoconductivity of the ordered 2D assembly, studied by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique, reveals an anisotropic conductivity with ∼3 times larger conductivity along the parallel direction compared to that along the perpendicular one.

The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Band-Like Charge Transport in Cs2AgBiBr6 and Mixed Antimony-Bismuth Cs2AgBi1-x Sb x Br6 Halide Double Perovskites.

Recently, halide double perovskites (HDPs), such as Cs2AgBiBr6, have been reported as promising nontoxic alternatives to lead halide perovskites. However, it remains unclear whether the charge-transport properties of these materials are as favorable as for lead-based perovskites. In this work, we study the mobilities of charges in Cs2AgBiBr6 and in mixed antimony–bismuth Cs2AgBi1–xSbxBr6, in which the band gap is tunable from 2.0 to 1.6 eV. Using temperature-dependent time-resolved microwave conductivity techniques, we find that the mobility is proportional to T–p (with p ≈ 1.5). Importantly, this indicates that phonon scattering is the dominant scattering mechanism determining the charge carrier mobility in these HDPs similar to the state-of-the-art lead-based perovskites. Finally, we show that wet chemical processing of Cs2AgBi1–xSbxBr6 powders is a successful route to prepare thin films of these materials, which paves the way toward photovoltaic devices based on nontoxic HDPs with tunable band gaps.

Charge Mobility and Dynamics in Spin-Crossover Nanoparticles Studied by Time-Resolved Microwave Conductivity.

We use the electrodeless time-resolved microwave conductivity (TRMC) technique to characterize spin-crossover (SCO) nanoparticles. We show that TRMC is a simple and accurate means for simultaneously assessing the magnetic state of SCO compounds and charge transport information on the nanometer length scale. In the low-spin state from liquid nitrogen temperature up to 360 K the TRMC measurements present two well-defined regimes in the mobility and in the half-life times, in which the former transition temperature TR occurs near 225 K. Below TR, we propose that an activationless regime taking place associated with short lifetimes of the charge carriers points at the presence of shallow-trap states. Above TR, these states are thermally released, yielding a thermally activated hopping regime where longer hops increase the mobility and, concomitantly, the barrier energy. The activation energy could originate not only from intricate contributions such as polaronic self-localizations but also from dynamic disorder due to phonons and/or thermal fluctuations of SCO moieties.

Minute-Scale Degradation and Shift of Valence-Band Maxima of (CH3NH3)SnI3 and HC(NH2)2SnI3 Perovskites upon Air Exposure

The development of lead-free, tin-based perovskite solar cells is becoming a pervasive topic; however, the inherent instabilities of such cells have prevented a boost of their power conversion efficiency and a deeper understanding of their fundamental properties. By using the photoelectron yield spectroscopy (PYS) and flash-photolysis time-resolved microwave conductivity (TRMC) techniques, we investigate the effects of air exposure on the valence-band maxima (VBMs) and photoconductivities of tin iodide perovskites (methylammonium cation, MASnI3; formamidinium cation, FASnI3). These perovskites exhibit a shift of the VBM (e.g., from −5.02 eV at 0 min to −5.17 eV at 18 min), deterioration of the PYS profiles, and progressive decrease of the TRMC transients on the minute scale of air exposure. The addition of SnF2 was found to suppress the initial defect-related density of the filled electronic states of MASnI3 and FASnI3, as revealed by PYS, and to partly mitigate the degradation of MASnI3, as revealed by T...

Vapour-Deposited Cesium Lead Iodide Perovskites: Microsecond Charge Carrier Lifetimes and Enhanced Photovoltaic Performance.

Metal halide perovskites such as methylammonium lead iodide (MAPbI3) are highly promising materials for photovoltaics. However, the relationship between the organic nature of the cation and the optoelectronic quality remains debated. In this work, we investigate the optoelectronic properties of fully inorganic vapour-deposited and spin-coated black-phase CsPbI3 thin films. Using the time-resolved microwave conductivity technique, we measure charge carrier mobilities up to 25 cm2/(V s) and impressively long charge carrier lifetimes exceeding 10 μs for vapour-deposited CsPbI3, while the carrier lifetime reaches less than 0.2 μs in the spin-coated samples. Finally, we show that these improved lifetimes result in enhanced device performance with power conversion efficiencies close to 9%. Altogether, these results suggest that the charge carrier mobility and recombination lifetime are mainly dictated by the inorganic framework rather than the organic nature of the cation.

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers.

Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm(-2), FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.

Long-lived photoinduced charge-carriers in Er3 + doped CaTiO3 for photocatalytic H2 production under UV irradiation

The incorporation of Er3+ ions in the CaTiO3 structure provides an enhanced H2 photoproduction due to the formation of long-lived charge-carriers in the semiconductor. Nitroblue tetrazolium reaction, photoluminescence and time resolved microwave conductivity techniques were used to investigate the generation of the superoxide species and the lifetime of the charge-carriers. The improved photoactivity has been explained in terms of the dopant agent, finding that phenomena such as a higher concentration of the superoxide radicals, a more efficient separation of the photogenerated charge-carriers, and a slower recombination process take place in the Er3+ doped CaTiO3 compared to the undoped CaTiO3.

Ferrocene-Substituted Naphthalenediimide with Broad Absorption and Electron-Transport Properties in the Segregated-Stack Structure.

A new naphthalenediimide (NDI) molecule, where two ferrocene (Fc) units were directly attached to both imide nitrogens (Fc-NDI-Fc), was synthesized. The Fc units provide high crystallinity to Fc-NDI-Fc with good solubility to conventional organic solvents. The Fc units also work as electron-donating substituents, in contrast to the electron-deficient NDI unit, resulting in broad charge-transfer absorption of Fc-NDI-Fc from the UV region to 1500 nm in the solid state. The crystal structure analysis revealed that Fc-NDI-Fc formed a segregated-stack structure. The DFT calculation based on the crystal structure showed that the NDI π-orbitals extended over two axes. The extended π-network of the NDI units led to the electron-transport properties of Fc-NDI-Fc, which was confirmed using a flash-photolysis time-resolved microwave conductivity technique.

Charge Carrier Lifetimes Exceeding 15 μs in Methylammonium Lead Iodide Single Crystals.

The charge carrier lifetime in organic-inorganic perovskites is one of the most important parameters for modeling and design of solar cells and other types of devices. In this work, we use CH3NH3PbI3 single crystal as a model system to study optical absorption, charge carrier generation, and recombination lifetimes. We show that commonly applied photoluminescence lifetime measurements may dramatically underestimate the intrinsic carrier lifetime in CH3NH3PbI3, which could be due to severe charge recombination at the crystal surface and/or fast electron-hole recombination close to the surface. By using the time-resolved microwave conductivity technique, we investigated the lifetime of free mobile charges inside the crystals. Most importantly, we find that for homogeneous excitation throughout the crystal, the charge carrier lifetime exceeds 15 μs. This means that the diffusion length in CH3NH3PbI3 can be as large as 50 μm if it is no longer limited by the dimensions of the crystallites.

Evaluation of the intrinsic charge carrier transporting properties of linear- and bent-shaped π-extended benzo-fused thieno[3,2-b]thiophenes.

The intrinsic charge carrier transporting properties of two isomeric linear- and bent-shaped 7-ring benzo-fused thieno[3,2-b] thiophenes and their octyl-substituted analogues were newly investigated using flash-photolysis (FP-) and field-induced (FI-) time-resolved microwave conductivity (TRMC) techniques. FP-TRMC study in the solid state revealed that octyl-substitution potentially improved the photoconductivity due to the enhanced crystalline lamellar packing. After this screening process, local-scale hole mobilities at the thienoacene-poly(methylmethacrylate) insulator interfaces were precisely recorded using FI-TRMC, reaching up to 4.5 cm(2) V(-1) s(-1) for the linear-shaped non-alkylated thienoacene. The combination of FP- and FI-TRMC measurements provides a rapid and quantitative evaluation scheme even for a variety of compounds with some issues in the processing conditions, leading to the optimized structure of the compounds used as active (interfacial) layers in practical electronic devices.

Time-resolved microwave conductivity study of charge carrier dynamics in commercially available TiO2 photocatalysts

Charge carrier dynamics in various commercially available TiO2 photocatalysts was studied by means of the time-resolved microwave conductivity (TRMC) technique.

Improved understanding of the electronic and energetic landscapes of perovskite solar cells: high local charge carrier mobility, reduced recombination, and extremely shallow traps.

The intriguing photoactive features of organic-inorganic hybrid perovskites have enabled the preparation of a new class of highly efficient solar cells. However, the fundamental properties, upon which the performance of these devices is based, are currently under-explored, making their elucidation a vital issue. Herein, we have investigated the local mobility, recombination, and energetic landscape of charge carriers in a prototype CH3NH3PbI3 perovskite (PVK) using a laser-flash time-resolved microwave conductivity (TRMC) technique. PVK was prepared on mesoporous TiO2 and Al2O3 by one or two-step sequential deposition. PVK on mesoporous TiO2 exhibited a charge carrier mobility of 20 cm(2) V(-1) s(-1), which was predominantly attributed to holes. PVK on mesoporous Al2O3, on the other hand, exhibited a 50% lower mobility, which was resolved into balanced contributions from both holes and electrons. A general correlation between crystal size and mobility was revealed irrespective of the fabrication process and underlying layer. Modulating the microwave frequency from 9 toward 23 GHz allowed us to determine the intrinsic mobilities of each PVK sample (60-75 cm(2) V(-1) s(-1)), which were mostly independent of the mesoporous scaffold. Kinetic and frequency analysis of the transient complex conductivity strongly support the superiority of the perovskite, based on a significant suppression of charge recombination, an extremely shallow trap depth (10 meV), and a low concentration of these trapped states (less than 10%). The transport mechanism was further investigated by examining the temperature dependence of the TRMC maxima. Our study provides a basis for understanding perovskite solar cell operation, while highlighting the importance of the mesoporous layer and the perovskite fabrication process.

Contactless photoconductance study on undoped and doped nanocrystalline diamond films.

Hydrogen and oxygen surface-terminated nanocrystalline diamond (NCD) films are studied by the contactless time-resolved microwave conductivity (TRMC) technique and X-ray photoelectron spectroscopy (XPS). The optoelectronic properties of undoped NCD films are strongly affected by the type of surface termination. Upon changing the surface termination from oxygen to hydrogen, the TRMC signal rises dramatically. For an estimated quantum yield of 1 for sub-bandgap optical excitation the hole mobility of the hydrogen-terminated undoped NCD was found to be ∼0.27 cm(2)/(V s) with a lifetime exceeding 1 μs. Assuming a similar mobility for the oxygen-terminated undoped NCD a lifetime of ∼100 ps was derived. Analysis of the valence band spectra obtained by XPS suggests that upon oxidation of undoped NCD the surface Fermi level shifts (toward an increased work function). This shift originates from the size and direction of the electronic dipole moment of the surface atoms, and leads to different types of band bending at the diamond/air interface in the presence of a water film. In the case of boron-doped NCD no shift of the work function is observed, which can be rationalized by pinning of the Fermi level. This is confirmed by TRMC results of boron-doped NCD, which show no dependency on the surface termination. We suggest that photoexcited electrons in boron-doped NCD occupy nonionized boron dopants, leaving relatively long-lived mobile holes in the valence band.

Photo-Induced Charge Transfer and Spectroscopy Across a Series of Pbttt Model Compounds

Organic-based excitonic photovoltaics are a promising low-cost renewable energy source. New solution-processable polymer and small molecule electron donor materials often utilize solubilizing alkyl tails. Herein we study the effects of alkyl tails on charge transfer between a fullerene and a model compound of the highly-conducting PBTTT polymer with varied alkyl tail lengths, ranging from one to thirteen carbons.First, we photophysically characterize the model compounds. Solution-phase spectroscopy and electrochemistry demonstrate similar energetics across the compounds series. Intermolecular aggregation in films leads to two distinct film absorption and photoluminescence features, which interestingly does not appear to follow tail length or odd-even trends. Second, we utilize the electrodeless time-resolved microwave conductivity technique to study photo-induced charge transfer and carrier dynamics. Specifically we create a donor-acceptor heterojunction with the soluble fullerene PCBM. Last, we create an electron acceptor “dilute regime” to study the tail-length effects on charge transfer. Our transient microwave absorption experiments show that the charge generation yield is intimately tied to tail length across the model compound series. Alkyl tails, which are often incorporated to enable solubility, can in fact act as a charge transfer barrier in bulk heterojunctions.

Characterization of a-Si:H/c-Si Heterojunctions by Time Resolved Microwave Conductivity Technique

In heterojunction solar cells, a-Si:H/c-Si heterointerface is of significant importance, since the heterointerface characteristics directly affect junction properties and thus solar cell efficiency. In this study, we have performed time resolved microwave conductivity (TRMC) measurements on n-type c-Si wafers passivated on both sides with intrinsic and doped a-Si:H layers in order to investigate electrical property and passivation quality of the a-Si:H/c-Si heterojunctions. It was found that the TRMC decay time and decay curve shape varied with the laser wavelength and power intensity and also depended on sample structures. By using 1064 nm laser pulse with high excitation, differences in the decay curve shape between samples with and without p-n junction were observed. The samples containing p-n junction(s) had unique slow decay mode, after the initial fast decay, which we ascribed to the release of carriers from the low-mobility amorphous layer into the high-mobility crystalline wafer as the built-in field of the junction was restored. Experimental results suggest that the TRMC is useful nondestructive technique which is suitable for primary check of the a-Si:H/c-Si heterojunctions during the solar cell fabrication process.

Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity

During the past decade, time-resolved microwave conductivity (TRMC) has evolved to an established, powerful technique to study photoactive layers. With this feature paper, we aim to fulfill two goals: (1) give a full description of the photoinduced TRMC technique, including experimental details and data analysis, and discuss to what extent the TRMC technique differs from more conventional DC techniques and (2) illustrate the potential of this technique for probing charge carrier dynamics in photoactive materials. For these reasons recent studies on conjugated polymer:fullerene blends will be presented and discussed. The findings from these studies have advanced the insight into the mechanism of charge carrier generation and decay in polymer:fullerene blends, which allows us to improve the efficiency of organic photovoltaic cells based on this active layer architecture. In short, it is shown how the TRMC technique can be used as a versatile method to screen the potential of new photovoltaic materials.

Understanding Local and Macroscopic Electron Mobilities in the Fullerene Network of Conjugated Polymer‐based Solar Cells: Time‐Resolved Microwave Conductivity and Theory

The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well‐connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash‐photolysis time‐resolved microwave conductivity (TRMC) experiments, and space‐charge‐limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so‐called ‘shuttlecock’ molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self‐assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl‐C60 butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM's superior local mobility comes from the near‐spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM.

Ion-Based Materials Derived from Positively and Negatively Charged Chloride Complexes of π-Conjugated Molecules

Oriented salts from planar charged species were prepared by combining positively and negatively charged receptor-anion complexes with similar geometries using dicationic and electronically neutral π-conjugated receptors. Phenylene- or pyrimidine-bridged bis(imidazolium) dicationic anion receptors formed monocationic receptor-Cl(-) complexes that were accompanied by a free Cl(-). This free Cl(-) was subsequently captured by pyrrole-based neutral anion receptors to form negatively charged receptor-Cl(-) complexes. The ion pair of the resulting positively and negatively charged planar receptor-Cl(-) complexes could produce a supramolecular octane gel, adopting a lamellar self-organized structure in its xerogel state. On the other hand, the solid-state ion pairs had hexagonal columnar mesophases, which formed via alternate stacking of the positively and negatively charged planar receptor-Cl(-) complexes. By use of the flash-photolysis time-resolved microwave conductivity technique, the one-dimensional charge-carrier transporting property, with a mobility of 0.05 cm(2) V(-1) s(-1), was determined for the newly prepared solid-state ion pairs.