Free Carrier Density Research Articles

Determination of Free Charge Carrier Luminescence and Quasi‐Fermi Level Separation in Organic Solar Cells via Transient Photoluminescence Measurements

AbstractThe detection of photoluminescence (PL) is extremely valuable for photovoltaic technologies as it provides direct information about the free charge carrier densities and therefore about the separation of the quasi‐Fermi levels (ΔEF). However, for organic solar cells the PL is strongly dominated by photogenerated excitons that in part decay radiatively before they form free charge carriers via dissociation at a donor/acceptor interface. For this reason, it is impossible to deduce the free charge carrier densities and ΔEF directly from the PL signal. To overcome this severe limitation, a new approach is developed that allows disentangling the luminescence of photogenerated excitons from the one of free charge carriers. Due to the large difference in respective lifetimes—for photogenerated excitons it is ≤ ns whereas for free charge carriers it is in the µs‐range—this is achieved by time‐resolved PL measurements. For highly efficient organic solar cells, it is found that ΔEF determined from these PL transients matches perfectly with the electrical voltage. Moreover, the new method also allows for the determination of ΔEF from mere absorber films without electrodes and thus paves the way for a better understanding of organic solar cells.

Spectral Response of Solar Blind M-S-M Photodetector With InGaZnO Film Sputter Deposited in Diluted Oxygen Ambience

Indium-gallium-zinc-oxide (InGaZnO) metal-semiconductor-metal (M-S-M) photodetector has been demonstrated here, which shows enhanced spectral response in the solar blind window. It had been found that, while IGZO film (100 nm) for the MSM photodetector was sputtered in argon diluted oxygen ambience (Ar:O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> = 20:2 sccm) deposited film contained higher atomic percentage of gallium as compared to film sputtered in pure argon ambience. Higher Ga content (26.92 – 28.14 %) present in diluted O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ambience sputtered film was found advantageous for stretching spectral response (SR) of detector till the solar blind window. Fabricated detector showed peak responsivity of 1.12 A/W at 255 nm and at 8V when as-deposited diluted O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sputtered film in it received two consecutive anneals beginning with ozone (O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) at 90°C for 30 minutes followed by a 300°C thermal anneal for 1 hour in nitrogen (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). Substantial improvement in responsivity as found in solar blind window has been attributed to enhanced absorption by IGZO in deep UV band separately characterized using absorption spectra of diluted O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sputtered and post deposition annealed IGZO films. Enhanced UV-absorption was resulted from structural relaxation of lattice prompted by higher Ga content present in as-deposited films, which reduces ionization energy of deep lying neutral oxygen vacancy states within the IGZO bandgap thereby increases free carrier density impacting extinction coefficient.

Theoretical study on time response of semiconductor photorefractive effects under subpicosecond ultra-fast X-rays.

A theoretical model that can efficiently calculate the refractive index response of semiconductors under ultrafast X-ray radiation is established based on the photorefractive effect of semiconductors. The proposed model is used to interpret X-ray diagnostics experiments, and the results are in good agreement with experiments. In the proposed model, a rate equation model of free carrier density calculation is adopted with the X-ray absorption cross-sections calculated by atomic codes. The two-temperature model is used to describe the electron-lattice equilibration and the extended Drude model is applied to calculate the transient refractive index change. It is found that faster time response can be achieved for semiconductors with shorter carrier lifetime and sub-picosecond resolution can be obtained for InP and [Formula: see text]. The material response time is not sensitive to X-ray energy and the diagnostics can be used in the 1-10 keV energy range. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'.

Photocatalytic degradation performance and mechanism of tetracycline by Pd-loaded titanium dioxide

The development of effective and environmentally safe photocatalysts for the removal of antibiotics from aquatic environments remains an important issue. In this work, materials with different metal Pd mass loading ratios were prepared by loading TiO2 with noble metal Pd after the introduction of oxygen vacancies. After the photocatalytic degradation of TC, it was found that the 0.5% Pd/TiO2-Vo photocatalyst showed good photocatalytic activity in the degradation of tetracycline, with a degradation efficiency of 95.04% in 2 h and a reaction rate constant 2.18 times higher than that of TiO2. It was found that the introduction of oxygen vacancies and Pd loading could enhance the visible light response to overcome the drawback that TiO2 could only be irradiated by UV while increasing the free carrier density and inhibiting electron-hole recombination to improve its photocatalytic degradation efficiency. Finally, the possible decomposition pathways and degradation products of TC were inferred by combining liquid chromatographymass spectrometry (LCMS). The findings of this work may provide a new approach to improving the photocatalytic activity of TiO2.

Understanding and Mitigating the Degradation of Perovskite Solar Cells Based on a Nickel Oxide Hole Transport Material during Damp Heat Testing.

The development of stable materials, processable on a large area, is a prerequisite for perovskite industrialization. Beyond the perovskite absorber itself, this should also guide the development of all other layers in the solar cell. In this regard, the use of NiOx as a hole transport material (HTM) offers several advantages, as it can be deposited with high throughput on large areas and on flat or textured surfaces via sputtering, a well-established industrial method. However, NiOx may trigger the degradation of perovskite solar cells (PSCs) when exposed to environmental stressors. Already after 100 h of damp heat stressing, a strong fill factor (FF) loss appears in conjunction with a characteristic S-shaped J-V curve. By performing a wide range of analysis on cells and materials, completed by device simulation, the cause of the degradation is pinpointed and mitigation strategies are proposed. When NiOx is heated in an air-tight environment, its free charge carrier density drops, resulting in a band misalignment at the NiOx/perovskite interface and in the formation of a barrier impeding hole extraction. Adding an organic layer between the NiOx and the perovskite enables higher performances but not long-term thermal stability, for which reducing the NiOx thickness is necessary.

The effect of dopant concentration and annealing treatments on N-type Iodine doped CdTe

We report properties of highly conducting n-type cadmium telluride single crystals doped with iodine (CdTe:I). These crystals were grown with dopant concentrations in the range of 1017 cm−3 to 1019 cm−3 by Modified Vertical Bridgman (MVB) melt growth. Post-growth dopant activation, including Cd annealing, Te annealing, and rapid thermal annealing (RTA), was applied to improve free carrier density. The structural, optical, and electrical properties were analyzed by Glow Discharge Mass Spectroscopy (GDMS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Photoluminescence Spectroscopy (PL), optical absorption, Hall measurements, Capacitance-Voltage (CV) measurements, and time-resolved photoluminescence (TRPL). The results indicate that Cd annealing is the most effective activation method to get 100 % donor activation (n ≈ 2 ×1018 cm−3), which is close to the room temperature solubility limit of iodine. This leads to the lowest resistivity and the highest mobility. Moreover, this data suggests a potential role of Cd vacancy-related defects on electrical self-compensation.

Dynamics of Free Carrier Absorption and Refractive Index Dispersion in Si and Si/PolySi Microrings

We report pump-probe measurements of time resolved optical transmission spectra of Si and Si/poly-Si microrings after high free carrier densities have been generated by two-photon absorption of the pump pulse. From measurements, we can extract the recovery dynamics of free carrier absorption, refractive index dispersion, generated free carriers and finally the effective initial free carrier lifetimes. The method is validated by comparing modelling and simulations with measurements; the obtained results are in very good agreement with what predicted by the Shockley-Read-Hall recombination model for trap assisted recombination. We also propose a method for determining the empirical relations for free carrier absorption and refractive index dispersion in poly-Si waveguides.

High-field 1/f noise in hBN-encapsulated graphene transistors

$1/f$ electronic noise is a conductance fluctuation, expressed in terms of a mobility ``$\ensuremath{\alpha}$-noise'' by Hooge and Kleinpenning. Understanding this noise in graphene is key for high-performance electronics. Early investigations pointed out a deviation from the standard Hooge formula, with the free-carrier density substituted by a constant density ${n}_{\mathrm{\ensuremath{\Delta}}}\ensuremath{\sim}{10}^{12}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Here we investigate hBN-encapsulated graphene transistors where high mobility gives access to the velocity-saturation regime. We show that $\ensuremath{\alpha}$-noise is still accounted for by the Hooge formula on substituting conductance by differential conductance $G$, resulting in a bell-shaped dependence of flicker noise with bias voltage. The same analysis holds in the Zener regime at even larger bias, with two main differences. The first one is a strong enhancement of the Hooge parameter reflecting the hundred-times larger coupling of interband excitations to the hyperbolic phonon-polariton (HPhP) modes of the midinfrared Reststrahlen (RS) bands of hBN, which is supported by microwave noise thermometry measurements. The second is an exponential suppression of this coupling at large fields, which we attribute to decoherence effects. The phenomenology of $1/f$ noise in graphene supports a quantum-coherent bremsstrahlung interpretation of $\ensuremath{\alpha}$-noise.

Chemical Defects and Energetic Disorder Impact the Energy‐Level Alignment of Functionalized Hexaazatriphenylene Thin Films

The energy‐level alignment (ELA) at interfaces ubiquitous in organic optoelectronic applications is decisive for the device performance. Due to the notoriously low density of free charge carriers in organic thin films, the ELA at organic––inorganic interfaces is determined by gap states and/or tailing states of the frontier molecular orbitals, that is, the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO). Informed by modeling defect energies on the density‐functional theory level, it is deduced from ultraviolet and X‐ray photoelectron spectroscopy data that chemical‐defect induced gap states lead to the substrate‐independent pinning of the Fermi level (EF) to the LUMO for 1,4,5,8,9,12‐hexaazatriphenylene‐2,3,6,7,10,11‐hexacarbonitrile thin films. For 5,6,11,12,17,18‐hexaazatrinaphthylene thin films, the ELA is instead governed by tailing states due to energetic disorder, which put the EF closer to midgap position. It is highlighted in the study that the susceptibility of conjugated organic material to forming chemical and structural defects is key for the ELA at interfaces and, therefore, must be considered in the synthesis of novel materials and their processing into functional structures.

Controlled coherent-coupling and dynamics of exciton complexes in a MoSe2 monolayer

Quantifying and controlling the coherent dynamics and couplings of optically active excitations in solids is of paramount importance in fundamental research in condensed matter optics and for their prospective optoelectronic applications in quantum technologies. Here, we perform ultrafast coherent nonlinear spectroscopy of a charge-tunable MoSe2 monolayer. The experiments show that the homogeneous and inhomogeneous line width and the population decay of exciton complexes hosted by this material can be directly tuned by an applied gate bias, which governs the Fermi level and therefore the free carrier density. By performing two-dimensional spectroscopy, we also show that the same bias-tuning approach permits us to control the coherent coupling strength between charged and neutral exciton complexes.

Invertible optical nonlinearity in epsilon-near-zero materials

Epsilon-near-zero (ENZ) materials such as indium tin oxide (ITO) have recently emerged as a new platform to enhance optical nonlinearities. Here we report a theoretical and experimental study on the origin of nonlinearities in ITO films that are dominated by intraband and interband transitions. We show that there are two competing factors that jointly contribute to a spectrally invertible nonlinearity of ITO near its ENZ region, i.e., the nonparabolicity of the bands that results in a larger effective mass, and the Fermi energy shift that determines the free carrier density. Our work reveals the relationship between the large nonlinearity and the intrinsic material properties of the ITO films, which will enable design and development of photonic materials and nonlinear devices made of transparent conductive oxides.

Heavily Doped Si Nanocrystals Formed in P-(SiO/SiO2) Multilayers: A Promising Route for Si-Based Infrared Plasmonics

As building blocks of multifunctional materials involving coupling at the nanoscale, highly doped semiconductor nanocrystals are of great interest for potential applications in nanophotonics. In this work, we investigate the plasmonic properties of highly doped Si nanocrystals embedded in a silica matrix. These materials are obtained by evaporation of heavily phosphorus-doped SiO/SiO2 multilayers in an ultrahigh vacuum chamber followed by rapid thermal annealing. For P contents between 0.7 and 1.9 atom %, structural investigations at the nanoscale give clear evidence that P atoms are mainly located in the core of Si nanocrystals with concentrations reaching up to 10 atom %, i.e., well beyond the solid solubility limit of P in bulk Si. Alloying and formation of SiP nanoparticles are observed for P contents exceeding 4 atom % in the multilayer. Infrared absorption measurements give evidence of a localized surface plasmon resonance located in the 3–6 μm range. A core–shell structure was used to model Si nanocrystals embedded in a silica matrix. Based on the Mie theory and the Drude model, both the mobility and the free charge carrier density were extracted from the simulation, with values reaching 27 cm2 V–1 s–1 and 2.3 × 1020 cm–3, respectively. This results in a dopant activation rate of about 8%.

Non-linear propagation effects of intense femtosecond pulses on below bandgap harmonics in solids

The non-linear propagation of the intense near-infrared (NIR) driving field in crystals poses a challenge and can offer an opportunity to control the spectral properties of harmonics in solids. Here, we have investigated the non-linear propagation effects in wide bandgap dielectrics such as Magnesium Oxide (MgO), Chromium (Cr) doped MgO (Cr: MgO), Sapphire (Sa) crystals, and fused silica (FS). Furthermore, we have generated second and third harmonics (TH) and measured the linear polarization dependence of harmonics in these thin solids to explore the non-linear response at a strong field. We observe spectral shifts and broadening of the driving field spectrum which is imprinted on the harmonics. We attribute these effects to the strong photoionization, generation of free-carrier density, and self-phase modulation effects. This work shows the sensitivity to control the spectral profile of harmonics by manipulating the driving field, showing the possibility of new tailored solid-state ultraviolet sources for optical diagnostics.

Characterizing Density and Spatial Distribution of Trap States in Ta3N5 Thin Films for Rational Defect Passivation.

Tantalum nitride (Ta3N5) has gained significant attention as a potential photoanode material, yet it has been challenged by material quality issues. Defect-induced trap states are detrimental to the performance of any semiconductor material. Beyond influencing the performance of Ta3N5 films, defects can also accelerate the degradation in water during desired electrochemical applications. Defect passivation has provided an enormous boost to the development of many semiconductor materials but is currently in its infancy for Ta3N5. This is in part due to a lack of experimental understanding regarding the spatial and energetic distribution of trap states throughout Ta3N5 thin films. Here, we employ drive-level capacitance profiling (DLCP) to experimentally resolve the spatial and energetic distribution of trap states throughout Ta3N5 thin films. The density of deeper energetic traps is found to reach ∼2.5 to 6 × 1022 cm-3 at the interfaces of neat Ta3N5 thin films, over an order of magnitude greater than the bulk. In addition to the spatial profile of deep trap states, we report neat Ta3N5 thin films to be highly n-type in nature, owning a free carrier density of ∼9.74 × 1017 cm-3. This information, coupled with the present understanding of native oxide layers on Ta3N5, has facilitated the rational design of a targeted passivation strategy that simultaneously provides a means for catalyst immobilization. Loading catalyst via silatrane moieties suppresses the density of defects at the surface of Ta3N5 thin films by two orders of magnitude, while also reducing the free carrier density of films by over one order of magnitude, effectively dedoping the films to ∼2.40 × 1016 cm-3. The surface passivation of Ta3N5 films translates to suppressed defect-induced trapping and recombination of photoexcited carriers, as determined through absorption, photoluminescence, and transient photovoltage. This illustrates how developing a deeper understanding of the distribution and influence of defects in Ta3N5 thin films has the potential to guide future works and ultimately accelerate the integration and development of high-performance Ta3N5 thin film devices.

Resistive-like Behavior of Ferroelectric p–n Bilayer Structures Based on Epitaxial Pb(Zr0.2Ti0.8)O3 Thin Films

The p–n junctions are the building blocks of nowadays electronic devices. The n- or p-type conductivity is obtained in classic semiconductors, like Si, by doping with atoms acting as donors or acceptors, respectively. Doping was used in ferroelectrics to influence the transition temperature, magnitude of some physical properties, but not necessarily conduction type. Therefore, comprehensive studies to obtain true ferroelectric p–n junctions by controlled doping are missing. Recently, it has been shown that Pb(Zr0.2Ti0.8)O3 films doped with ≈1% atomic Nb (n-type doping) or Fe (p-type doping) have different orientations of polarization in the as-grown state. Knowing that polarization orientation depends on doping type, the next step is to build ferroelectric p–n homojunctions and to study their properties in relation to ferroelectric polarization. p–n and n–p structures were grown for this purpose by successive deposition of Nb-doped and Fe-doped Pb(Zr,Ti)O3 layers with different thicknesses. We find that these p–n homojunctions are ferroelectric, but the magnitude of the polarization and coercive field, as well as the dominant polarization orientation in the as-grown state, depend on the conduction type of the first grown layer. The I–V characteristics are quasi-linear, although the interfaces with the electrodes behaves as Schottky contacts. The resistance extracted from the I–V characteristics displays an exponential dependence on temperature, with an activation energy in the range of 0.14–0.17 eV. These results are explained assuming that the total current in the junction is the total of electron and hole injections at the electrode interfaces. It is shown that for relatively low doping concentrations, the current density contains a dominant term with a linear voltage dependence and an exponential temperature dependence, as observed experimentally, and a secondary (correction) term that is dependent on the free carrier density and can induce non-linear voltage dependence when this density is significant.

Incomplete activation and ionization of dopants in Si at room temperature

A new model for incomplete ionization of dopants in Si is presented, where the Fermi level of free carriers may displace with respect to the case of full activation of dopants. The curves of the ratio of free-carrier density and active-dopants density vs doping, which are calculated at partial activation of dopants with the new model, overlap exactly with the curves of the same quantity calculated at full activation of dopants with a reported model. Calculations are performed with and without reported parameterizations of the density of states and occupancy probability of the dopant band simulating incomplete ionization around the Mott concentration. With parameterizations, comparisons with Hall-mobility data show that the curves of free-carrier density calculated at partial dopant activation with the new model are more accurate than the curves of the same quantity calculated at full dopant activation with the reported model. Without parameterizations, the new model allows calculating for the same carrier species curves of majority-carrier mobility that fit measured data of minority-carrier mobility at high dopings and agree with the Klaassen mobility model for minority carriers. The consistency with the band theory of the new and reported models is discussed, and the new model is found to be the most appropriate in this respect. The free-carrier density calculated with the new model without parameterizations overlaps at high dopings with free-carrier density calculated with reported models for band-gap narrowing and allows calculating curves of Auger lifetime of majority carriers that fit measured lifetime data of minority carriers.

Carrier density tuning in CuS nanoparticles and thin films by Zn doping via ion exchange.

Copper sulphide (covellite) nanoplatelets have recently emerged as a plasmonic platform in the near-infrared with ultrafast nonlinear optical properties. Here we demonstrate that the free-carrier density in CuS, which is an order of magnitude lower than in traditional plasmonic metals, can be further tuned by chemical doping. Using ion exchange to replace Cu with an increasing content of Zn in the nanoparticles, the free-hole density can be lowered, resulting in a long-wavelength shift of the localised plasmon resonances from 1250 nm to 1750 nm. The proposed approach provides new opportunities for tuning the plasmonic response of covellite nanocrystals as well as the carrier relaxation time which decreases for lower free-carrier densities.

ZnO, Cu-doped ZnO, Al-doped ZnO and Cu-Al doped ZnO thin films: Advanced micro-morphology, crystalline structures and optical properties

The thin film coatings composed of: undoped ZnO film, ZnO doped with Al, ZnO doped with Cu, and ZnO simultaneously doped with Al and Cu (co-doping) were separately deposited on quartz substrates using RF sputtering method with different targets. The advanced fractal features, crystalline structure and optical properties of sputtered samples were investigated by atomic force microscopy (AFM), X‐ray diffraction (XRD) and UV–vis spectroscopy. Microstructural studies revealed homogeneously granular structure of ZnO layer and axially oriented granular structure of AZO thin film.The transmission spectra of undoped, mono-doped and co-doped ZnO thin films were measured revealing relatively large transmittance of more than 80 % for un-doped and co-doped samples and less than that value for mono-doped thin films in both visible and infrared regions. CAZO thin film was found the most transparent thin film in the visible area being a prerequisite for good TCO. Analysis of absorption coefficients demonstrated that excitonic effects are invisible in mono-doped and co-doped samples. Also, PL spectra show that in these samples there are very high densities of free carriers and presence of impurities, which is important for conductivity of thin films as well as their optical applications. The optical band gap of ZnO thin films decreases by Cu doping from 3.12 eV to 3.09 eV and increases by Al doping to 4.30 eV, but remains exactly between those values in terms of co-doped sample (3.75 eV).

Broadband nonlinear optical response of titanium nitride in the visible spectral range

Epsilon-near-zero (ENZ) materials, a new class of materials with vanishing permittivity, which can significantly enhance the interaction between incident light and matters. Here, we investigated the third-order nonlinear optical response of titanium nitride (TiN), an ENZ material, in the visible range by using Z-scan method, and obtained a maximum nonlinear absorption coefficient β of −3.78 × 103 cm/GW at its ENZ wavelength. Furthermore, the carriers dynamic characteristics of TiN are revealed by time-resolved pump-probe measurement, including the intraband and interband excitation, the former is mainly derived from laser-induced to form a non-equilibrium distribution of hot electrons, resulting in the electron energy redistribution process in the conduction band, and the latter is the electrons located in the valence band being promoted to the conduction band, increasing the total free carrier density and thus causing a change in the optical properties. Our results indicate the great potential applications of TiN in nonlinear optical and all-optical modulation devices.

Ultrafast Spectroscopy of Plasmons and Free Carriers in 2D MXenes.

2D MXenes have diverse and chemically tunable optical properties that arise from an interplay between free carriers, interband transitions, and plasmon resonances. The nature of photoexcitations and their dynamics in three different members of the MXene family, Ti3 C2 , Mo2 Ti2 C3 , and Nb2 C, are investigated using two complementary pump-probe techniques, transient optical absorption, and time-resolved terahertz (THz) spectroscopy. Measurements reveal pronounced plasmonic effects in the visible and near-IR in all three. Optical excitation, with either 400 or 800nm pulses, results in a rapid increase in lattice temperature, evidenced by a pronounced broadening of the plasmon mode that presents as a plasmon bleach in transient absorption measurements. Observed kinetics of plasmon bleach recovery provide a means to monitor lattice cooling. Remarkably slow cooling, proceeding over hundreds of picoseconds to nanoseconds time scales, implies MXenes have low thermal conductivities. The slowest recovery kinetics are observed in the MXene with the highest free carrier density, viz. Ti3 C2 , that supports phonon scattering by free carriers as a possible mechanism limiting thermal conductivity. These new insights into photoexcitation dynamics can facilitate their applications in photothermal solar energy conversion, plasmonic devices, and even photothermal therapy and drug delivery.