Experimental aspects of hot electron distribution functions

A review is presented on the experimental efforts to obtain hot electron distribution functions in bulk materials in high d.c. electric fields. Three optical methods will be discussed in detail: (i) inter- and intraband absorption measurements, (ii) emission due to radiative transitions between band states, and band- and impurity states, (iii) inelastic light scattering experiments. A distinction is made between methods which yield the energy distribution functions and those which give information on the anisotropic distribution of hot carriers in the momentum space. The latter experiments involve a determination of the electric field induced dichroism in absorption or emission and the dependence of the scattering cross section on scattering geometry in inelastic light scattering experiments. The influence of the nonequilibrium phonon distribution on the interpretation of the experimental results is discussed, too. In addition, current work on energy distribution functions of hot carriers in quantizing magnetic fields as obtained from absorption or emission of infrared radiation due to transitions of carriers between Landau states or magnetic field split impurity states will be presented.     

Effect of the tin impurity on the energy spectrum and photoelectric properties of nanostructured In2O3 films

It is shown that the doping of colloidal indium-oxide nanocrystals with tin shifts the absorption edge to shorter wavelengths of the visible spectral region and induces considerable absorption in the infrared region. The shape of the absorption and transmittance spectra in the infrared region is characteristic of the local surface plasmon resonance. According to estimations, the concentration of free charge carriers in In₂O₃(Sn) reaches 10¹⁹ cm^?3. The temperature dependences of the photoconductivity in the range 77–300 K are indicative of hopping conduction in nanostructured films based on In₂O₃(Sn) nanocrystals. The mechanisms responsible for the effect are discussed.     

Near‐Infrared Photoresponse of One‐Sided Abrupt MAPbI3/TiO2 Heterojunction through a Tunneling Process

Trap states in semiconductors usually degrade charge separation and collection in photovoltaics due to trap‐mediated nonradiative recombination. Here, it is found that perovskite can be heavily doped in low concentration with non‐ignorable broadband infrared absorption in thick films and their trap states accumulate electrons through infrared excitation and hot carrier cooling. A hybrid one‐sided abrupt perovskite/TiO₂ p‐N heterojunction is demonstrated that enables partial collection of these trap‐filled charges through a tunneling process instead of detrimental recombination. The tunneling is from broadband trap states in the wide depleted p‐type perovskite, across the barrier of the narrow depleted TiO₂ region (<5 nm), to the N‐type TiO₂ electrode. The trap states inject carriers into TiO₂ through tunneling and produce around‐unity peak external quantum efficiency, giving rise to near‐infrared photovoltaics. The near‐infrared response allows photodetecting devices to work in both diode and conductor modes. This work opens a new avenue to explore the near‐infrared application of hybrid perovskites.     

Optical an photoelectric properties odf ZnSe:Ti crystals

The photoconductivity and photoluminescence spectra of ZnSe:Ti crystals in the visible and infrared spectral regions are studied. It is established that the high-temperature impurity-induced photoconductivity of ZnSe:Ti crystals is defined by the optical transitions of electrons from the ³ A ₂(F) ground state to highenergy excited states, with the subsequent thermal transitions of electrons to the conduction band. The efficient excitation of intracenter luminescence of ZnSe:Ti crystals is achieved by light from the region of the intrinsic absorption of Ti²⁺ ions.     

Optical properties of blue light-emitting diodes in the InGaN/GaN system at high current densities

The current-voltage and brightness-voltage characteristics and the electroluminescence spectra of blue InGaN/GaN-based light-emitting diodes are studied to clarify the cause of the decrease in the emission efficiency at high current densities and high temperatures. It is found that the linear increase in the emission intensity with increasing injection current changes into a sublinear increase, resulting in a decrease in efficiency as the observed photon energy shifts from the mobility edge. The emission intensity decreases with increasing temperature when the photon energy approaches the mobility edge; this results in the reduction in efficiency on overheating. With increasing temperature, the peak of the electroluminescence spectrum shifts to lower photon energies because of the narrowing of the band gap. The results are interpreted taking into account the fact that the density-of-states tails in InGaN are filled not only via trapping of free charge carriers, but also via tunneling transitions into the tail states. The decrease in the emission efficiency at high currents is attributed to the suppression of tunneling injection and the enhancement of losses via the nonradiative recombination channel “under” the quantum well.     

Electrical Conductivity, Thermoelectric Power, and Optical Properties of Organo-Soluble Polyaniline Organic Semiconductor

The electrical conductivity, thermoelectric power (TEP), and optical properties of organo-soluble polyaniline doped with HCl have been investigated. The electrical conductivity and TEP of the sample increase with increasing temperature. The electrical conductivity and TEP results of the polymer suggest that it is a p-type semiconductor. The fundamental absorption edge in the polymer is formed by the direct allowed transitions, and the optical band gap value was found to be 2.79 eV. The absorption spectra for an acidic solution of the polymer indicate two new absorption bands, which are due to polaron formation. The polaron bands are responsible for the conductivity of the polymer. The TEP results indicate that the conductivity mechanism of the polymer is controlled by the large polaron hopping model.     

Fine structure and Zeeman effect of the excited state of the green emitting copper centre in zinc oxide

In the excitation spectrum of the green emission band of ZnO:Cu crystals, zero phonon lines have been detected for the first time. In the liquid-helium temperature range, a line with Δ = 2 cm⁻¹ at λ = 433.5 nm coincides with the well-known zero-phonon line of the emission. Additional lines are located at 432.2 and 431.4 nm. In a distance of 525 cm⁻¹ from these zero-phonon lines, phonon satellites superpose on the low-energy part of the broad excitation band. On its high-energy slope pronounced structures appear in the excitonic region. With selected highly-doped crystals, the new zero-phonon lines could also be detected in absorption.Zeeman experiments produce anisotropic splittings of the common ground state of all three lines with g = 0.7 ± 0.1 and g = 1.5 ± 0.1 which are approximately the values known from emission. Different anisotropic g-factors around g = 2 are derived for the terminal states of the absorption lines.The experimental data are compatible with a model of the investigated centre in which the ground state is Cu²⁺ and the excited state is a Cu⁺ with a loosely bound hole: (Cu⁺)⁺ [3]. As to the position of the impurity levels within the forbidden gap, still two possibilities exist: the excited hole could either be localized near the oxygen neighbours so that a depth of some tenth of an eV results, or effective-mass-like states could be involved in a distance of only a few meV above the top of the valence band.     

Carrier Tunneling from Charge Transfer States in Organic Photovoltaic Cells

Charge transfer (CT) states play a key role in the functioning of organic solar cells; however, understanding the mechanism by which CT states dissociate efficiently into free charges remain a conceptual challenge. Here, the electric field dependent dynamics of charge generation in planar cyanine/fullerene photovoltaic cells is probed over a wide temperature range using time-resolved Stark effect experiments, transient absorption, and photocurrent measurements. Results indicate that dissociation of thermalized CT states is the rate-limiting step for all temperatures. The dissociation rate strongly depends on the field, but is temperature independent. The results also suggest that the yield of generated charges is temperature independent. Model electrostatic calculations illustrate that specific orientations of the cyanine crystal relative to C₆₀ create a repulsive potential for an electron near the interface that is largely due to the quadrupole moment of the unit cell. In combination with the electron-hole coulomb attraction and the electric field-induced barrier lowering, a high-energy potential barrier forms with a narrow width of a few nanometers. It is proposed that charge separation occurs via a field-dependent electron tunneling mechanism through that barrier, which is temperature independent. The results support a thus far overlooked pathway for CT state dissociation via carrier tunneling.     

The role of interface phonons in the formation of polaron states in quantum wells

A theory of a large-radius polaron in a quantum well is developed with consideration of the interaction of charged particles with different branches of the phonon spectrum. It is shown that, in narrow quantum wells, the major contribution to the polaron binding energy is made by interaction with symmetric interface phonons. As a result of such interaction, the polaron binding energy is defined by the effective mass of charge carriers in the quantum well and by the polarization properties of barriers. The possibilities of the formation of a polaron exciton in a quantum well in the case of strong interaction of charged particles with optical phonons are analyzed. The conditions in which the polarization fields produced by the electron and hole do not substantially compensate each other are established.     

纤锌矿氮化物半导体束缚极化子及压力效应

考虑到纤锌矿结构的氮化物半导体材料的单轴异性,采用变分法研究了两支异常光学声子LO-like和TO-like对杂质态结合能的影响,即极化子效应.计及电子有效质量,材料介电常数及晶格振动频率随流体静压力的变化,讨论了束缚极化子结合能的压力效应,数值结果表明,极化子效应使杂质态结合能明显降低,极化子效应的主要贡献来自杂质态与LO-like声子的相互作用.压力使得结合能增加,且增强了结合能的各向异性.     

Formation of Methanofullerene Cation in Bulk Heterojunction Polymer Solar Cells Studied by Transient Absorption Spectroscopy

Photogenerated charge carriers for blend films of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) have been investigated by transient absorption spectroscopy. The blend film with a low PCBM fraction (<10 wt %) exhibits a wide absorption that ranges from 900 to 1000 nm, which is characteristic of the MDMO‐PPV hole polaron and PCBM radical anion. On the other hand, the blend film with a higher PCBM fraction (> 30 wt %) exhibits a major absorption band at ∼900 nm, which is characteristic of the PCBM radical cation. For identification of charge carriers, the absorption spectrum and molar absorption coefficient of each charged species have been evaluated separately using various combinations of electron donor and acceptor materials. Consequently, the MDMO‐PPV hole polaron has been found to have a broad absorption at ∼950 nm and the PCBM radical anion and cation show a distinct absorption at 1020 and 890 nm, respectively. On the basis of these absorption spectra, the transient spectra observed for the blend films have been simulated. The spectrum for a low PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO‐PPV hole polaron and PCBM radical anion. On the other hand, the spectrum for a high PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO‐PPV hole polaron, PCBM radical anion, and PCBM radical cation, which indicates that the PCBM radical cation is formed in the blend films with PCBM at a high concentration. Possible mechanisms for the formation of the PCBM radical cation in the blend are also discussed.     

Carrier transport mechanism in aluminum nanoparticle embedded AlQ3 structures for organic bistable memory devices

Electrical bistability is demonstrated in organic memory devices based on tris-(8-hydroxyquinoline)aluminum (AlQ₃) and aluminum nanoparticles. The role of the thickness of middle aluminum layer and the size of the nanoparticles in device performance is investigated. Above a threshold voltage, the device suddenly switches from a low conductivity OFF state to a high conductivity ON state with a conductivity difference of several orders of magnitude. The OFF state of the device could be recovered by applying a relatively high voltage pulse. The electronic transition is attributed to an electric field induced transfer of charge between aluminum nanoparticles and AlQ_3. The type of charge carriers responsible for conductance switching is investigated. The charge carrier conduction mechanism through the device in ON and OFF states is studied by temperature dependent current–voltage characteristics and analyzed in the framework of existing theoretical models. The conduction mechanism in the OFF state is dominated by field-enhanced thermal excitation of charge carriers from localized centers, whereas it changes to Fowler–Nordheim tunneling of charge carriers in the ON state. The device exhibited excellent stability in either conductivity states. The results indicate the strong potential of the device towards its application as a nonvolatile electronic memory.     

Relations between IR induced photoconductivity and IR stimulated luminescence in ZnS

IR induced photoconductivity (INP) and IR stimulated luminescence (STL) have been simultaneously measured in self-activated ZnS crystals. The stimulation spectra of INP and STL for the λ_IR = 1.2 μm to 6.4 μm spectral range were identical including broad, structureless responses and many narrow characteristic lines. Transient characteristics of INP and STL were strongly temperature dependent and showed a large variation from sample to sample. In every case, STL began to decay while INP was still increasing during steady IR excitation. Many different features were found in the magnitudes of INP and STL as a function of temperature and in photoconductivity and luminescence decay processes after UV excitation. The photoelectronic effects observed in these IR sensitive ZnS crystals lead us to conclude the following: the same centers are responsible for INP and STL; charge transport through a band exists before recombination occurs; bound-bound transitions and bound-free transitions are possible; and a wide variety of shallow, intermediate, and deep traps are present.     

Influence of quantum capture and tunneling mechanisms on the dark current of bound-to-continuum quantum-well infrared photodetectors

We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques.     

The magnetophonon effect

This article is a review of the magnetophonon effect, covering its origins, uses and the information which derives from it. The magnetophonon effect arises due to resonant phonon emission or absorption by free charge carriers in a solid in a high magnetic field. The most usual example of this is when longitudinal optic (L.O.) phonons are absorbed by electrons or holes in a semiconductor, causing resonant transitions between Landau levels at magnetic fields given by the resonance condition B = (1/N)m¹ω_L.O./e, where ω_L.O. is the optic phonon frequency, and m¹ is the effective mass of the carriers in question. These resonant transitions cause resonances in a wide variety of different transport parameters, such as resistivity, Hall and Seebeck coefficients. The article reviews the observation of normal magnetophonon resonances in a wide variety of semiconducting and semi-metallic materials, including alloys, p-type materials and semiconductor heterostructures. A section is devoted to the hot-electron magnetophonon effect, which results from resonant electron cooling, and gives information on the electron energy loss mechanisms. This review was completed in December 1984.     

Refined model for the current-voltage characteristics of quantum-well infrared photodetectors

The temperature dependences of the dark current of quantum-well infrared photodetectors are investigated experimentally. It is established that the pre-exponential factor in the analytical expression for the photodetector current-voltage characteristics varies linearly with temperature. On the basis of the results obtained, it is suggested that the temperature dependence of the photodetector??s dark current is determined by the thermal excitation of charge carriers to a band characterized by a two-dimensional density of states. In the context of this suggestion, a refined model for the current-voltage characteristics is proposed. The model takes into account the thermal generation of charge carriers in a band with a two-dimensional density of states and the electric field dependence of the thermal activation energy for the quantum-well ground state and of the drift velocity of the carriers in the barrier conduction band.     

Scattering of conduction electrons at a spatially correlated system of charges in a heavily doped GaAs: Te semiconductor

The results of studying the absorption of infrared radiation by free charge carriers in the GaAs:Te single crystals, grown by the Czochralski method, had the electron concentration n ₀=5×10¹⁷?6×10¹⁸ cm^?3 are reported. An analysis of the spectral dependences of the absorption coefficient took into account the spatial correlation in the impurity-charge distribution. It is shown that the short-range correlation model makes it possible to account for the decrease in the absorption coefficient and a weakening of its spectral dependence, in the region of the impurity-mediated free-carrier absorption.     

Study of the impurity photoconductivity and luminescence in ZnSe:Ni crystals in the visible spectral region

The photoconductivity and photoluminescence spectra of ZnSe:Ni crystals in the visible spectral region are studied. It is established that the high-temperature impurity photoconductivity of ZnSe:Ni crystals is controlled by the optical transitions of electrons from the ground state ³ T ₁(F) to high-energy excited states, with subsequent thermally activated transitions of electrons to the conduction band. A photoconductivity band associated with the photoionization of Ni impurities is revealed. The intracenter luminescence of ZnSe:Ni crystals is efficiently excited with light corresponding to the intrinsic absorption region of Ni²⁺ ions.     

In Situ Monitoring of Polaron Localization and Cell Activity with Charge Accumulation Spectroscopy

Localization of charge carriers directly affects charge transport and local potential fluctuations and is a critical factor dictating the performance of organic bioelectronic devices. Understanding how the polaron localization at the solid−water interface is influenced by the microscopic structural ordering of the organic semiconducting films can provide complementary information to achieve better control on the electrochemical stability of bioelectronic devices. Here, charge accumulation spectroscopy is utilized to explore the structure-dependent polaron localization in an electrolyte-gated organic field-effect transistor structure. The results reveal that the ordered and closely packed systems exhibit much higher absorption cross-sections (σ_p) than disordered and isolated molecules. Therefore, the charge-induced absorption feature can be utilized to monitor in situ the microscopic morphology variations of organic semiconducting films under electrolyte and trace the entire cell detachment process noninvasively. This study suggests that the spectroscopic signature of polaronic charge carriers, which is highly sensitive to the local environment surrounding the probed chromophores, serves as an effective probe to detect the structure-dependent charge transport features in organic bioelectronic devices.