Photoconductivity Dynamics Research Articles

Bandlike Transport and Charge-Carrier Dynamics in BiOI Films.

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant, and nontoxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping), which imposes a fundamental limit on long-range mobility. Here we report the terahertz (THz) photoconductivity dynamics in thin films of BiOI and demonstrate a lack of such self-trapping, with good charge-carrier mobility, reaching ∼3 cm2 V-1 s-1 at 295 K and increasing gradually to ∼13 cm2 V-1 s-1 at 5 K, indicative of prevailing bandlike transport. Using a combination of transient photoluminescence and THz- and microwave-conductivity spectroscopy, we further investigate charge-carrier recombination processes, revealing charge-specific trapping of electrons at defects in BiOI over nanoseconds and low bimolecular band-to-band recombination. Subject to the development of passivation protocols, BiOI thus emerges as a superior light-harvesting semiconductor among the family of bismuth-based semiconductors.

Electrical Relaxation and Transport Properties of ZnGeP2 and 4H-SiC Crystals Measured with Terahertz Spectroscopy

Terahertz photoconductivity and charge carrier recombination dynamics at two-photon (ZnGeP2) and three-photon (4H-SiC) excitation were studied. Thermally annealed, high-energy electron-irradiated and Sc-doped ZnGeP2 crystals were tested. The terahertz charge carrier mobilities were extracted from both the differential terahertz transmission at a specified photoexcitation condition and the Drude–Smith fitting of the photoconductivity spectra. The determined terahertz charge carrier mobility values are ~453 cm2/V·s for 4H-SiC and ~37 cm2/V·s for ZnGeP2 crystals. The charge carrier lifetimes and the contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of bulk and surface Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that ZnGeP2 possesses short charge carrier lifetimes (a~0.01 ps−1, b~6 × 10−19 cm3·ps−1 and c~7 × 10−40 cm6·ps−1) compared with 4H-SiC (a~0.001 ps−1, b~3 × 10−18 cm3·ps−1 and c~2 × 10−36 cm6·ps−1), i.e., τ~100 ps and τ~1 ns at the limit of relatively low injection, when the contribution from Auger and interband radiative recombination is small. The thermal annealing of as-grown ZnGeP2 crystals and the electron irradiation reduced the charge carrier lifetime, while their doping with 0.01 mass % of Sc increased the charger carrier lifetime and reduced mobility. It was found that the dark terahertz complex conductivity of the measured crystals is not fitted by the Drude–Smith model with reasonable parameters, while their terahertz photoconductivity can be fitted with acceptable accuracy.

Systematic analysis of semiconductor photoconductivity dynamics under different laser excitations: two- and three-level models

A numerical study of a model photoconductor under pulsed and continuous-wave laser excitation is presented. From the solution of a system of partial differential equations related to the populations of the conduction and valence bands, we analyze the time behavior of the photoexcited concentrations of electrons and holes where the diffusion phenomena and the presence of electric field are neglected. Additionally, for the three-level model, the population of trap levels is also introduced and analyzed as a third variable. The dynamics of electrons, holes and traps levels concentrations are studied in the presence of different types of pulse and harmonic laser photoexcitations. We show that an improved physical insight is obtained through a systematic analysis of the influence of the main physical parameters, such as generation and recombination rates, between the available energy levels.

Transient Photoconduction and Relaxation Photocurrent of ZnO Thin Films Produced by Pulsed Laser Deposition

This paper presents the results of a photoelectric study of cobalt-doped zinc oxide thin films. Layers were grown by pulsed laser deposition on Si, glass, and SiO2/Si substrates. The crystal structure of the layers was determined by X-ray diffraction methods. The time dependence of the photoconductivity was studied with zone-band excitation, excitation in the contaminant absorption region, and excitation in the dark. The analysis of the photoconductivity dynamics on the duration of the excitation pulse was carried out for the structural layers. The influence of the concentration of deep traps on the form of the photoconduction and long-term relaxation processes is evaluated. ZnO thin films produced by pulsed laser deposition are suitable for photosensors because of their photosensitivity in the UV spectral range.

Nanoscale Photoexcited Carrier Dynamics in Perovskites.

The optoelectronic properties of lead halide perovskite thin films can be tuned through compositional variations and strain, but the associated nanocrystalline structure makes it difficult to untangle the link between composition, processing conditions, and ultimately material properties and degradation. Here, we study the effect of processing conditions and degradation on the local photoconductivity dynamics in [(CsPbI3)0.05(FAPbI3)0.85(MAPbBr3)0.15] and (FA0.7Cs0.3PbI3) perovskite thin films using temporally and spectrally resolved microwave near-field microscopy with a temporal resolution as high as 5 ns and a spatial resolution better than 50 nm. For the latter FACs formulation, we find a clear effect of the process annealing temperature on film morphology, stability, and spatial photoconductivity distribution. After exposure of samples to ambient conditions and illumination, we find spectral evidence of halide segregation-induced degradation below the instrument resolution limit for the mixed halide formulation, while we find a clear spatially inhomogeneous increase in the carrier lifetime for the FACs formulation annealed at 180 °C.

Interrogating Light-initiated Dynamics in Metal-Organic Frameworks with Time-resolved Spectroscopy.

Time-resolved spectroscopy is an essential part of both fundamental and applied chemical research. Such techniques access light-initiated dynamics on time scales ranging from femtosecond to microsecond. Many techniques falling under this description have been applied to gain significant insight into metal-organic frameworks (MOFs), a diverse class of porous coordination polymers. MOFs are highly tunable, both compositionally and structurally, and unique challenges are encountered in applying time-resolved spectroscopy to interrogate their light-initiated properties. These properties involve various excited state mechanisms such as crystallographically defined energy transfer, charge transfer, and localization within the framework, photoconductivity, and structural dynamics. The field of time-resolved MOF spectroscopic studies is quite nascent; each original report cited in this review was published within the past decade. As such, this review is a timely and comprehensive summary of the most significant contributions in this emerging field, with focuses on the overarching spectroscopic concepts applied and on identifying key challenges and future outlooks moving forward.

Charge Transport in Networks of sp3-Functionalized Single-Walled Carbon Nanotubes

The controlled introduction of luminescent sp3 defects into semiconducting single-walled carbon nanotubes (SWCNTs) leads to narrowband, tunable emission in the near-infrared and increases their photoluminescence (PL) quantum yield for potential optoelectronic applications [ACS Nano 2019, 13, 9259]. While the spectroscopic properties of sp3-functionalized SWCNTs are already well-established, a detailed understanding of the impact of functionalization on their electrical performance especially in a network is still lacking. Here, we investigate charge transport in ambipolar, light-emitting field-effect transistors based on networks of pristine and sp3-functionalized (6,5) SWCNTs. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states. Charge-modulated PL spectroscopy, which exclusively probes the mobile charge carriers in SWCNT networks [ACS Nano 2020, 14, 2412], confirms that the defects are efficiently sampled by mobile carriers and consequently, sp3-functionalized SWCNTs actively participate in the charge transport within the network. Temperature-dependent transport measurements of transistors combined with ultrafast photoconductivity dynamics of individually dispersed SWCNTs measured by optical-pump THz-probe spectroscopy provide further insights into the impact of sp3-functionalization on charge transport within a single nanotube and within a network.

Single Copper Atoms Enhance Photoconductivity in g-C3N4.

Graphitic carbon nitride (g-C3N4) and its doped analogues have been studied over the past decade in part due to their promising applications in heterogeneous photocatalysis; however, the effect of doping on the photoconductivity is poorly understood. Herein, we investigate Cu doped g-C3N4 (Cu-g-C3N4) and demonstrate via extended X-ray absorption fine structure that Cu+ incorporates as an individual ion. Time-resolved optical pump terahertz probe spectroscopy was utilized to measure the ultrafast photoconductivity in response to a 400 nm pump pulse and showed that the Cu+ dopant significantly enhances photoconductivity of the as-prepared powdered sample, which decays within 10 ps. Furthermore, a film preparation technique was applied that further enhanced the photoconductivity and induced a longer-lived photoconductive state with a lifetime on the order of 100 ps. This study provides valuable insight into the ultrafast photoconductivity dynamics of g-C3N4 materials, which is essential toward developing efficient g-C3N4 photocatalysts.

Terahertz photoconductivity and photocarrier dynamics in graphene–mesoporous silicon nanocomposites

We investigate charge transport and photocarrier dynamics in graphene--mesoporous silicon nanocomposites using optical-pump terahertz-probe measurements. The nanocomposite material consists of a free-standing mesoporous silicon membrane whose specific surface is coated with a few-layer graphene shell. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial decay time of 5 ps and a longer decay time of about 25 ps. These decay times are significantly reduced compared to the values of ${\ensuremath{\tau}}_{1}\ensuremath{\sim}74$ ps and ${\ensuremath{\tau}}_{2}\ensuremath{\sim}730$ ps obtained for the uncoated mesoporous silicon membrane and this is attributed to the introduction of additional surface defects formed during the graphene deposition process. Based on the influence of the laser fluence on the time-resolved differential transmission curves, a capture/recombination model is proposed to describe the photocarrier dynamics in these nanocomposite materials. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz waveforms taken at different optical-pump THz-probe delays. These data curves are well reproduced using a modified Drude-Smith model taking into account diffusive-restoring currents. The $c$ parameter of this model, which describes the degree of carrier localization, is about $\ensuremath{-}0.73$ for the uncoated porous Si membrane and is approaching $\ensuremath{-}1$ for graphene--mesoporous Si nanocomposites formed at temperatures above $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. For all the nanocomposites, the characteristics of the photoconductive material, in terms of photocarrier capture/recombination time and effective mobility, are of interest for the fabrication of pulsed terahertz devices.

Carrier Recombination Processes in GaAs Wafers Passivated by Wet Nitridation.

As one of the successful approaches to GaAs surface passivation, wet-chemical nitridation is applied here to relate the effect of surface passivation to carrier recombination processes in bulk GaAs. By combining time-resolved photoluminescence and optical pump—THz probe measurements, we found that surface hole trapping dominates the decay of photoluminescence, while photoconductivity dynamics is limited by surface electron trapping. Compared to untreated sample dynamics, the optimized nitridation reduces hole- and electron-trapping rate by at least 2.6 and 3 times, respectively. Our results indicate that under ambient conditions, recovery of the fast hole trapping due to the oxide regrowth at the deoxidized GaAs surface takes tens of hours, while it is effectively inhibited by surface nitridation. Our study demonstrates that surface nitridation stabilizes the GaAs surface via reduction of both electron- and hole-trapping rates, which results in chemical and electronical passivation of the bulk GaAs surface.

Metal-Organic Framework Photoconductivity via Time-Resolved Terahertz Spectroscopy.

While metal-organic frameworks (MOFs) have been under thorough investigation over the pasttwo decades, photoconductive MOFs are an emerging class of materials with promising applications in light harvesting and photocatalysis. To date, there is not a general method to investigate the photoconductivity of polycrystalline MOF samples as-prepared. Herein, we utilize time-resolved terahertz spectroscopy along with a new sample preparation method to determine the photoconductivity of Zn2TTFTB, an archetypical conductive MOF, in a noncontact manner. Using this technique, we were able to gain insight into MOF photoconductivity dynamics with subpicosecond resolution, revealing two distinct carrier lifetimes of 0.6 and 31 ps and a long-lived component of several ns. Additionally, we determined the frequency dependent photoconductivity of Zn2TTFTB which was shown to follow Drude-Smith behavior. Such insights are crucially important with regard to developing the next generation of functional photoconductive MOF materials.

Transient photoconductivity and free carrier dynamics in a monolayer WS2 probed by time resolved Terahertz spectroscopy

The frequency and time resolved conductivity in a photoexcited large-area monolayer tungsten disulfide (WS2) have been simultaneously determined by using time-resolved terahertz spectroscopy. We use the Drude–Smith model to successfully reproduce the transient photoconductivity spectra, which demonstrate that localized free carriers, not bounded excitons, are responsible for the THz transport. Upon the optical excitation with 400 nm and 530 nm wavelength, the relaxation dynamics of the free carriers include fast and slow decay components with time constants approximately smaller than 1 ps and between 5–7 ps, respectively. The former sub-picosecond decay is attributed to the charge carrier loss induced by the exciton formation, while both the Auger recombination and the surface trapping can contribute to the slow relaxation.

The modulation of terahertz photoconductivity in CVD grown n-doped monolayer MoS2 with gas adsorption

The as-grown atomically thin transition metal dichalcogenides (TMDs) usually have extremely large surface-to-volume ratios. Hence, the electrical and optical properties are always affected by the adsorption of atmospheric gases. By using optical pump terahertz (THz) probe spectroscopy, THz photoconductivity dynamics of chemical vapor deposition grown monolayer MoS2 has been investigated in nitrogen, air and oxygen atmospheric environments. Our study reveals that the photoconductivity of MoS2 at THz frequencies is dramatically altered by the adsorption of oxygen. The relaxation dynamics of photoconductivity strongly depends on atmospheres, which is attributed to photo-excited different quasi-particles, such as free charge carriers, excitons and trions. This study highlights the role of oxygen passivation of the photoconductivity in monolayer MoS2, which may have important applications for the design of future MoS2-based electronic devices.

The determination of carrier lifetimes and associated mobility magnitudes using photoconductivity recovery dynamics in thin-film amorphous semiconductors

The extraction of trap-limited mobility information using photoconductivity relaxation dynamics in thin-film amorphous semiconductors is evaluated. Simulated photoconductivity dynamics suggest that free carrier decay is characterised by a spectrum of recombination lifetimes, where the spectra are sensitive to the underlying distribution of shallow trap densities and the experimental photoconductivity generation conditions employed. Weighted averaging of the spectral lifetimes is found to return trap-limited lifetime estimates which agree to within 5% of the lifetime values expected from free-to-trapped carrier ratios established under steady-state photoconductivity conditions. The averaged spectral lifetimes may consequently be used with mobility-lifetime magnitudes to determine the associated trap-limited mobility. The analysis procedure is evaluated using photoconductivity data obtained from a set of a-Si:H films deposited over a range of substrate temperatures.

Photoconductive Behavior of the PPV/RGO Composites: Insights of Charge Transfer Process

The paper deals with a study of composites based on poly(p‐phenylenevinylene) (PPV) and reduced graphene oxide (RGO) in terms of photoconductivity and photocurrent (PC) dynamics in charge‐discharge cyclic processes. The explanation for the photoconductive behavior is built with the support of DeVore and Onsager theories. Scanning samples in both directions involves charge transport, to and from, available energy states called defect centers. The existence of these centers is confirmed by a decrease in the composite bandgap caused by the RGO localized states which are situated slightly above the first HOMO level in the PPV bandgap. The contribution of RGO to the photoconductive properties of PPV is revealed through a photocurrent value with two orders of magnitude higher than for PPV.

Ultrafast Carrier Dynamics and Terahertz Photoconductivity of Mixed-Cation and Lead Mixed-Halide Hybrid Perovskites**Supported by the National Natural Science Foundation of China under Grant Nos 11604202, 11674213, 61735010 and 51603119, the Young Eastern Scholar under Grant Nos QD2015020 and QD2016027, the Shanghai Rising-Star Program under Grant No 18QA1401700, the ‘Chen Guang’ Project under Grant Nos 16CG45 and 16CG46, the Shanghai Municipal

Using time-dependent terahertz spectroscopy, we investigate the role of mixed-cation and mixed-halide on the ultrafast photoconductivity dynamics of two different methylammonium (MA) lead-iodide perovskite thin films. It is found that the dynamics of conductivity after photoexcitation reveals significant correlation on the microscopy crystalline features of the samples. Our results show that mixed-cation and lead mixed-halide affect the charge carrier dynamics of the lead-iodide perovskites. In the (5-AVA)0.05(MA)0.95PbI2.95Cl0.05/spiro thin film, we observe a much weaker saturation trend of the initial photoconductivity with high excitation fluence, which is attributed to the combined effect of sequential charge carrier generation, transfer, cooling and polaron formation.

Carrier Recombination Processes in Gallium Indium Phosphide Nanowires.

Understanding of recombination and photoconductivity dynamics of photogenerated charge carriers in GaxIn1-xP NWs is essential for their optoelectronic applications. In this letter, we have studied a series of GaxIn1-xP NWs with varied Ga composition. Time-resolved photoinduced luminescence, femtosecond transient absorption, and time-resolved THz transmission measurements were performed to assess radiative and nonradiative recombination and photoconductivity dynamics of photogenerated charges in the NWs. We conclude that radiative recombination dynamics is limited by hole trapping, whereas electrons are highly mobile until they recombine nonradiatively. We also resolve gradual decrease of mobility of photogenerated electrons assigned to electron trapping and detrapping in a distribution of trap states. We identify that the nonradiative recombination of charges is much slower than the decay of the photoluminescence signal. Further, we conclude that trapping of both electrons and holes as well as nonradiative recombination become faster with increasing Ga composition in GaxIn1-xP NWs. We have estimated early time electron mobility in GaxIn1-xP NWs and found it to be strongly dependent on Ga composition due to the contribution of electrons in the X-valley.

The influence of surfaces on the transient terahertz conductivity and electron mobility of GaAs nanowires

Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400–2100 cm2 V−1 s−1) and ultrashort charge carrier lifetimes (1–5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump–terahertz-probe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell–Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3 × 106 cm s−1. We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.

Light-induced Effects in Sillenite Crystals with Shallow and Deep Traps

This paper presents the light-induced effects in bismuth silicon and bismuth titanium oxide crystals associated both with the electron transitions into the conduction band and with the filling of shallow and deep traps, which determine the optical and electroconductive properties of these crystals. The dynamics of photoconductivity and light-induced absorption is analyzed under conditions of pulsed laser illumination at the wavelength of 532nm. The possibility to describe the relaxation processes of a population for trapping levels with the use of two-exponential function is demonstrated. The photoconductivity dynamics is characterized by two relaxation times on the order of 100ns and 10μs, whereas for light-induced absorption the lifetimes about 10μs and several days for short- and long-lived traps, respectively, have been obtained. Because of this, the relaxation transitions may be occurred both to the shallow trap centers with energy located close to the conduction band and to the deep-lying traps, which should be included into a diversified theoretical model adequately describing the light-induced phenomena in photorefractive sillenite-family crystals.

Giant negative photoconductivity of PbSnTe:In films with wavelength cutoff near 30 μm

Experimental results on the photoconductivity dynamics in PbSnTe:In films with a high SnTe content and corresponding fundamental absorption edge near 30 μm at liquid helium temperatures are reported. The possible causes for the giant (over two orders of magnitude) negative photoconductivity of the samples are discussed.