Organic polymer p-n heterostructures for optoelectronics

The photovoltaic effect due to the photogeneration of charge carriers at the interface in organic polymeric p-n-heterostructures based on the developed composites with hole (doped polyimides, conjugated polymers) and electron (doped polyetherimides and carbazolyl-containing polymers) conduction was observed and studied. The best performance was observed for heterostructures with diffuse and developed (bulk) interface. The maximum energy conversion efficiency was found to be 0.5% at λ = 400–650 nm. Components with enhanced photo and thermal stability were used. A weak photovoltaic effect was observed for heterostructures based on conjugated polymers (with a sharp interface). In this case, the rectification (diode) effect was observed for both dark current and photocurrent due to dark and photo injection of carriers from the electrodes at bias voltages across the cell above the injection threshold (V > V _i ). This conclusion is supported by the observation of recombination electroluminescence of the dopant in the n-layer, indicating a full or partial recombination of carriers at the interface. At V < V _i , photocurrent under reverse bias (+A1, ?ITO) is significantly higher than that under forward bias due to an efficient photogeneration of carriers at the interface and separate transport thereof. A sensitivity of about 100 mA/W at λ = 400–500 nm was reached.     

Analysis of electrical characteristics of Er/p-InP Schottky diode at high temperature range

We report on the temperature-dependent electrical characteristics of Er/p-InP Schottky barrier diodes. The current–voltage (I–V) and capacitance–voltage (C–V) measurements have been carried out in the temperature range of 300–400 K. Using thermionic emission (TE) theory, the zero-bias barrier height (Φ_bo) and ideality factor (n) are estimated from I–V characteristics. It is observed that there is a decrease in n and an increase in the Φ_bo with an increase in temperature. The barrier height inhomogenity at the metal/semiconductor (MS) interface resulted in Gaussian distribution of Φ_bo and n. The laterally homogeneous Schottky barrier height value of approximately 1.008 eV for the Er/p-InP Schottky barrier diodes is extracted from the linear relationship between the experimental zero-bias barrier heights and ideality factors. The series resistance (R_s) is calculated by Chenug's method and it is found that it increases with the decrease in temperature. The reverse-bias leakage current mechanism of Er/p-InP Schottky diode is investigated. Both Poole–Frenkel and Schottky emissions are described and discussed. Furthermore, capacitance–voltage (C–V) measurements of the Er/p-InP Schottky contacts are also carried out at room temperature in dark at different frequencies of 10, 100 and 1000 kHz. Using Terman's method, the interface state density is calculated for Er/p-InP Schottky diode at different temperatures.     

The influence of interband tunneling on leakage current in manganite/titanate heterojunction

The behavior of leakage current at reverse bias in p-La_0.9Sr_0.1MnO₃/n-SrNb_0.01Ti_0.99O₃ heterojunction has been theoretically studied by calculating interband tunneling current with various doping densities and temperatures. Our results reveal that the reduction of leakage current with decrease of doping density and increase of temperature originates from properties of interband tunneling.     

Temperature dependence of the current-voltage characteristics of the Al/Rhodamine-101/p-Si(1 0 0) contacts

The current-voltage (I-V) characteristics of Al/Rhodamine-101/p-Si/Al contacts have been measured at temperatures ranging from 280 to 400 K at 20 K intervals. A barrier height (BH) value of 0.817 eV for the Al/Rh101/p-Si/Al contact was obtained at the room temperature that is significantly larger than the value of 0.58 eV of the conventional Al/p-Si Schottky diode. While the barrier height Φ_b0 decreases the ideality factors (n) become larger with lowering temperature. The high values of n depending on the sample temperature may be ascribed to decrease of the exponentially increase rate in current due to space-charge injection into Rh101 thin film at higher voltage. Therefore, at all temperatures, it has been seen that the I-V characteristics show three different regions, the ohmic behavior at low voltages, and the space charge limited current with an exponential distribution of traps at high voltages.     

Investigation of the electrical and optical properties of InAs/InGaAs dot in a well solar cell

The electroreflectance (ER) and current–voltage (J–V) of InAs/InGaAs dots in a well (DWELL) solar cell (SC) were measured to examine the optical and electrical properties. To investigate the carrier capturing and escaping effects in the quantum dot (QD) states the above and below optical biases of the GaAs band gap were used. In the reverse bias region of the J–V curve, the tunneling effect in the QD states was observed at low temperature. The ideality factors (n) were calculated from the J–V curves taken from various optical bias intensities (I_ex). The changes in the ideality factor (n) and short circuit current (J_SC) were attributed mainly to carrier capture at low temperature, whereas the carrier escaping effect was dominant at room temperature. ER measurements revealed a decrease in the junction electric field (F_J) due to the photovoltaic effect, which was independent of the optical bias source at the same temperature. At low temperature, the reduction of photovoltaic effect could be explained by the enhancement carrier capturing effect due to the strong carrier confinement in QDs.     

Photovoltaic effect in the single-junction DBP/PTCBI organic system under low intensity of monochromatic light

Photoelectric properties of the planar ITO/MoO3/DBP/PTCBI/BCP/Ag system were characterized on the basis of short-circuit current, open-circuit voltage and absorption spectra, and current-voltage measurements in the dark and under monochromatic illumination of low intensity. Photovoltaic performance of the system was compared with the performance of ideal semiconductor and excitonic cells of chosen bandgaps. Such analysis shows, that the fabricated cell exhibits quite high value of the open-circuit voltage, in comparison to the SQ limits calculated for semiconductor devices of bandgaps close to the LUMO_PTCBI-HOMO_DBP offset or crystalline silicon cells of the same absorptivity. This confirms that the DBP/PTCBI junction exhibits good properties for conversion of exciton energy to chemical energy of electron-hole pair. Moreover, open-circuit voltage and short-circuit current of the investigated cell practically do not change within the 520 nm–620 nm range, for which they reach the maximum values, making the junction of DBP/PTCBI attractive for use in indoor photovoltaics.     

Temperature-dependent photovoltaic behavior of CdSe quantum dots/P3HT hybrid thinfilm

An organic–inorganic hybrid solar cell based on CdSe quantum dots (QDs) and poly(3-hexylthiophene) (P3HT) was fabricated. Its temperature-dependent photovoltaic behaviors, such as I–V characteristic curves and open circuit voltage (V_oc) transient response, were measured. The photovoltaic behavior of this hybrid thin film device was similar with that of organic thin film solar cells, according to analysis results based on the equivalent circuit method. The exact carrier lifetime was remarkably different between under low-temperature region and under temperature above 197 K.     

Current injected electron-hole plasma in GaP pin-device structures

We report first on an electrically generated electron-hole plasma (EHP) in GaP light emitting diodes having a pin-structure. For current densities above j ? 10³Acm^?2 and at T = 77 K a dense EHP is formed within the i-region whose carrier density can easily be changed by the injection level. The corresponding electron-hole pair densities n of the plasma and the resulting gap shrinkage ΔE_g have been determined using a least-squares fit of the EHP luminescence spectra. The lineshape analysis of the spectra yields a $\text{n}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ -dependence of the gap shrinkage in quantitative agreement with findings from optically excited EHP and corresponding theoretical work. From electrical investigations it can be concluded that the EHP causes a strong nonlinear behaviour of the current-voltage characteristics.     

Numerical study of overpopulation density for laser oscillation in recombining hydrogen plasmas

Overpopulation densities Δn_itij, which are defined as the differences between the population densities per unit statistical weight of the upper and lower excited levels i and j, are calculated as a function of the electron density n_e for various electron temperatures T_e in recombining hydrogen plasmas. Calculation have been made for line pairs with principal quantum numbers (2 ,3), (3, 4) and (4, 5). The effect of the ground-level population density n₁ on Δn_ij is calculated. In this calculation, the atom-atom collision and the self-absorption of the resonance lines are taken into account. The n₁-dependence of Δn_ij remains almost constant until the self-absorption becomes significant. The threshold condition for laser oscillation is discussed in relation to the calculated Δn_ij. Laser oscillation is possible for the line pair (2, 3) at an electron density and temperature higher than for the other pairs (3, 4) and (4, 5) when self-absorption is negligible.     

Transport and transient photovoltaic properties of La0.4Ca0.6MnO3/Nb:SrTiO3 heterojunction at high temperature

We have investigated the electronic transport and transient photovoltaic properties on the La_0.4Ca_0.6MnO₃/Nb(0.05 wt%):SrTiO₃ heterojunction at high temperatures up to 723 K. The temperature-dependent reversal of the rectifying polarity and the decrease of the photosensitivity and response time were observed in the junction with increasing temperature from 293 to 723 K. The mechanism of the energy-band structure evolvement with increasing temperature is presented to understand the experimental results.     

Analytical modelling of carrier transport mechanisms in long wavelength planar n+–p HgCdTe photovoltaic detectors

The dark electrical characteristic of n⁺ on p long wavelength Hg_1−xCd_xTe photovoltaic detectors has been studied as a function of applied bias voltages. The diodes under study were given different surface treatments prior to their passivation. The dark current components have been identified using the already known carrier transport mechanisms across the junction. It is reported that the diodes under investigation have an excess leakage current component that exhibits quadratic dependence on the applied reverse bias voltage across the diode. The origin of this excess current is found to be closely associated with the ohmic currents. The diodes with higher ohmic current exhibit higher excess leakage current.     

Mechanism of charge transfer in <Emphasis Type="Italic">n</Emphasis>-CdS/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions

The capacitance-voltage and current-voltage characteristics of the n-CdS/p-CdTe heterosystem are investigated. Analysis of these characteristics demonstrates that the CdTe_1?x S _x solid solution formed at the n-CdS/p-CdTe heterointerface is inhomogeneous in both the conductivity and composition. The thickness of solid solutions is estimated from the capacitance-voltage characteristics. It is shown that, for the n-CdS/p-CdTe heterosystem, the current-voltage characteristic in the current density range 10^?8-10^?5 A cm^?2 is governed by the thermal electron emission, whereas the current in the heterostructure at current densities in the range 10^?4-10^?2 A cm^?2 is limited by recombination of charge carriers in the electroneutral region of the CdTe_1?x S _x solid solution. The lifetime and the diffusion length of minority charge carriers in the CdTe_1?x S _x solid solution and the surface recombination rate at the interface between the CdS layer and the CdTe_1?x S _x solid solution are determined. It is demonstrated that the n-CdS/p-CdTe heterostructure operates as a p-i-n structure in which CdTe is a p layer, CdTe_1?x S _x is an i layer, and CdS is an n layer.     

Electrical conductivity,absorption and luminescence measurements in cubic boron nitride

Electrical and optical measurements were carried out on tiny crystals of cubic boron nitride. The dark current i_D was found to change exponentially with T, with activation energies in the range 0·2–0·4 eV. A red electroluminescence, of intensity i_EL. proportional to i_D was observed. Upon illumination at low temperatures a photocurrent i_p proportional to the square root of the excitation intensity appeared. It varied exponentially with T, with an activation energy of 0.05 ± 0.01 eV. The crystals exhibited a red thermoluminescence with several unresolved peaks covering the temperature range 100–400K, and having activation energies in the range 0·15–0·40 eV.     

Autosolitons in electron-hole plasma in p-Ge

The self-organization of an electron-hole plasma (EHP) heated by an electric field in pure p-Ge samples at T = 77 K has been studied experimentally. The derived current-voltage characteristics (CVCs) and the distributions of the electric field and IR emission of the hot carriers along the samples show that the segments of a steep rise or the S-shaped segments of the CVCs in samples with n-p junctions are related to the formation of longitudinal thermal-diffusion autosolitons (AS_‖); as a result, thin (d = 2–20 μm in diameter), melted-through current channels appear. Such AS_‖ are formed at high EHP densities (n ≥ 1 × 10¹⁶ cm⁻³), when the electron-hole scattering is dominant, and at electron temperatures T _e = (2–4.5)T ₀ (T ₀ is the lattice temperature). The saturation segments and the N-shaped segments in the CVCs are attributable to the generation of transverse thermal-diffusion high-field autosolitons (AS_⊥) in the form of narrow strata with electric field strengths = 1–20 kV cm⁻¹. High-field AS_⊥ are formed at EHP densities n = 5 × 10¹³−1 × 10¹⁶ cm⁻³, when the electron-phonon scattering is dominant, and at electron temperatures T _e ∼ Θ ≥ 5T ₀ (Θ is the Debye temperature). The generated longitudinal and transverse autosolitons have high temperatures (T _e ≥ 1000 K) and reduced carrier densities and can exist simultaneously in different parts of the sample. Original Russian Text ? M.N. Vinoslavskiĭ, P.A. Belevskii, A.V. Kravchenko, 2006, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2006, Vol. 129, No. 3, pp. 477–492.     

Boron-doped nanocrystalline silicon thin films for solar cells

This article reports on the structural, electronic, and optical properties of boron-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films. The films were deposited by plasma-enhanced chemical vapour deposition (PECVD) at a substrate temperature of 150 °C. Crystalline volume fraction and dark conductivity of the films were determined as a function of trimethylboron-to-silane flow ratio. Optical constants of doped and undoped nc-Si:H were obtained from transmission and reflection spectra. By employing p⁺ nc-Si:H as a window layer combined with a p′ a-SiC buffer layer, a-Si:H-based p-p′-i-n solar cells on ZnO:Al-coated glass substrates were fabricated. Device characteristics were obtained from current-voltage and spectral-response measurements.     

Current-voltage characteristics of a metal-semiconductor contact with a Mott barrier

An analytical solution for the current-voltage characteristics of a metal-semiconductor contact with a Mott barrier is derived with allowance made for the space charge of carriers in the n ⁺-i junction. The main assumption used in solving the Poisson equation is that the bulk doping of the i layer is ignored. The dependences of the electric current on the voltage are calculated for the characteristic cases of the thermionic emission and diffusion mechanisms of charge transfer. In contrast to the classical Mott result, the inclusion of the space charge of carriers in the calculation limits the increase in the electric current at a forward bias and decreases the nonlinearity of the current-voltage characteristics.     

Analytical Simulation of an InAsSb Photovoltaic Detector for Mid-Infrared Applications

A generic model of a mid-infrared photodetector based on a narrow bandgap semiconductor has been developed. The model has been applied for analysis and simulation of an InAs_0.89Sb_0.11 photovoltaic detector for operation at room temperature in 2–5 μm wavelength region. The model takes into account the effect of tunneling and other components of dark current on the detectivity of the device by considering all the three dominant recombination mechanisms e.g., radiative, Shockley-Read-Hall and Auger recombination. The study revealed that the dark current of the photodetector under reverse bias is dominated by trap-assisted tunneling component of current and this causes the detectivity of the device to decrease at high reverse bias. It is further concluded that by operating the device at a suitable low reverse bias it is possible to improve the room-temperature detectivity significantly as compared to its value at zero bias.     

Characteristics of a photovoltaic X-ray detector based on a GaAs epitaxial structure

The characteristics of a photovoltaic X-ray detector based on the GaAs p ⁺-n-n′-n ⁺ epitaxial structure grown using gas-phase epitaxy are studied. Typical current-voltage and capacitance-voltage characteristics of the epitaxial structures are analyzed together with the built-in electric field profile in the n-GaAs depleted region. The efficiency of charge accumulation in the photovoltaic detector is measured for zero bias and for a bias voltage of 17 V. It is shown that the GaAs-based photovoltaic X-ray detector can operate with zero bias voltage at room temperature. The sensitivity of the detector is measured as a function of the effective energy of X-rays and the angle of incidence of X-ray photons.     

CdTe/CdS太阳电池I-V，C-V特性研究

测量了CdTe太阳电池器件从50kHz至1MHz频率范围的电容-电压特性，计算了吸收层的载流子浓度和空间电荷区的位置，电容-电压特性测试结果出现两个峰，峰特征与测试频率有关，用多结模型进行模拟分析，解释了实验结果.测量了电池从220K至300K的变温暗电流-电压特性，得出电池的反向暗饱和电流密度J₀和二级管理想因子A，分析了J₀，A随测量温度的变化,并讨论了电池器件的电流特性. 关键词： CdTe太阳电池 电流-电压特性 电容-电压特性     

Temperature dependence of dark current-voltage characteristics of <Emphasis Type="Italic">n</Emphasis>InSb-<Emphasis Type="Italic">n</Emphasis>PbTe-<Emphasis Type="Italic">n</Emphasis>CdTe isotype structure

The temperature dependence of dark current-voltage characteristics of an nInSb-nPbTe-nCdTe structure is investigated. It is shown that in the temperature range from 115 K to 125 K an energy barrier exists for charge carriers through the InSb layer, which is strictly connected with different temperature dependences of electron concentrations in nInSb and nPbTe.