Solar cell materials and their basic parameters

Parameters such as the energy gapE _g , open circuit voltageV _oc , short circuit current densityj _sc , fill factor F.F., efficiency η, antireflection coating condition A.R. and illumination conditions of currently known photovoltaic diodes are reviewed and tabulated, followed by some critical comments.     

Characterization of limited-diffusion-volume HgCdTe/CdTe photodiodes

Thin epitaxial HgCdTe/CdTe photodiodes are shown to have lower dark current noise than similar thick devices. This is explained by the limited-diffusion-volume model which requires a thin diode and a low surface recombination rate. Electron beam induced current is used for characterizing minority electrons distribution; the characteristic interface recombinations(L/D) has been found to be equal to 0.05 at 80 and 210 K.R ₀ A measurements give values consistent with this model.     

Acceptor Concentration Effects on Photovoltaic Response in the La1-xSrxMnO3/SrNbyTi1-yO3 Heterojunction

Photovoltaic response in the heterojunction of La1-x SrxMnO3/SrNby Ti1-yO3 （LSMO/SNTO） is analyzed theoretically based on the drift-diffusion model. It is found that the decrease of acceptor concentration in the La1-xSrxMnO3 layer of hereto junction can increase the peak value of photovoltaic signal and the speed of photovoltaic response, whereas the changing of donor concentration in the SrNby Ti1-yO3 layer has no such evident effect. Furthermore, the result also indicates that the modulation of Sr doping in La1-xSrxMnO3 is an effective method to accommodate the sensitivity and the speed of photovoltaic response for LSMO/SNTO photoelectric devices.     

Effect of annealing and Mn doping on the photoluminescence of strontium tartrate crystals

Strontium tartrate crystals (STC) were grown in gel using the single tube diffusion method. Powder XRD and FTIR spectroscopy were used for the characterization of the crystal. The optical band-gap (Eg) of STC is found to be 5.46 eV. Photoluminescence (PL) spectra of STC are recorded at different annealing temperature and concentration of dopant Mn. The spectral peaks (λ_em) of strontium tartrate photoluminor lie around 417, 440, 513 and 620 nm with excitation wavelength (λ_exc)=379 nm. The peaks at 417, 513 and 620 nm correspond to transitions ²P_1/2⁰→²S_1/2, ¹D⁰→³P⁰ and ¹S→³P⁰, respectively of Sr. The PL peak observed around 440 nm corresponds to the a⁴D_7/2→a⁶S_5/2 transition of Mn.     

Dependence of Photovoltaic Property of ZnO/Si Heterojunction Solar Cell on Thickness of ZnO Films

N-ZnO/p-Si heterojunctions are prepared by sputtering deposition of intrinsic ZnO films on p-Si substrates. Thicknesses of ZnO films are altered by varying the deposition time from I h to 3h. The electrical properties of these structures are analysed from capacitance-voltage （C V） and current-voltage （I-V） characteristics performed in a dark room. The results demonstrated that all the samples show strong rectifying behaviour. Photovoltaie property for the samples with different thicknesses of ZnO films are investigated by measuring open circuit voltage and short circuit current. It is found that photovoltages are kept to be almost constant of 320 m V along with the thickness while photoeurrents changing a lot. The variation mechanism of the photovoltade effect as a function of thickness of ZnO films is investigated.     

Light–induced charge transport processes in photorefractive barium titanate crystals doped with iron

3 ) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe²⁺/Fe³⁺. The photoconductivity exceeds the dark conductivity for intensities above 1 kWm^-2. A relation between the absorption constant and the Fe²⁺ concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO₃:Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm^-2 are estimated. Received: 22 January 1997/Accepted: 23 January 1997     

The time recovery of the spectroscopic properties of heavily irradiated CdZnTe and CdTe detectors

The time recovery of the spectroscopic capabilities of CdZnTe and CdTe detectors, irradiated with increasing doses of high- and low-energy neutrons, as well as electrons, has been investigated by studying their spectroscopic behavior at different photon energies using leakage current measurements and PICTS (photo-induced current transient spectroscopy) analysis. The detectors were stored at room temperature for up to one year to study the time evolution of their spectroscopic performance and to correlate it with the presence of defective states in the material. We have observed a clear improvement in the material’s detection properties with time, though only in those detectors which have not been severely degraded by the irradiation. The recovery can be associated with a decrease in the concentration of some defective states, thus allowing the assessment of the crucial role these play in determining the charge collection processes in the material and its spectroscopic capabilities. Received: 1 August 2001 / Accepted: 3 August 2001 / Published online: 20 December 2001     

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.     

Light-induced charge transport processes in photorefractive barium titanate doped with rhodium and iron

Absorption, light-induced absorption changes, photo and dark conductivities, bulk photovoltaic currents and two-beam coupling gain coefficients are measured for oxidized, as grown and reduced BaTiO₃:Rh,Fe crystals. Theoxidized sample shows hole conductivity and three photorefractive levels are present: Rh^4+/5+ Fe^4+/5+ and Rh^3+/4+. The measurements indicate that Rh^3+/4+ is responsible for the photo conductivity and that its energy level is located (0.9 ± 0.1) eV above the valence band edge. Theas grown sample also shows hole conductivity and two photorefractive levels are involved: Rh^3+/4+ and Fe 3+/4+. The results yield that the Fe^3+/4+ level is located (0.95 ± 0.05) eV above the valence band edge. In thereduced sample electrons are the dominant charge carriers and two levels are of importance. The shallow one is located (0.39 ± 0.05) eV below the conduction band edge. Here the performance of this crystal is strongly impaired by a pronounced sublinear photo conductivity.Dedicated to O. F. Schirmer on the occasion of his 60th birthday     

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well nanoparticles

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well (QDQW) nanoparticles synthesized by the colloidal method are investigated in this study. Strong exciton bands were observed in absorption and photoluminescence (PL) spectra taken for the CdTe/HgTe/CdTe QDQW nanoparticles. The energy difference between the exciton absorption and PL bands is larger than those obtained with CdTe and HgTe nanoparticles. Photocurrent-voltage curves and time-dependent photocurrent curves were obtained for the CdTe/HgTe/CdTe QDQW nanoparticles. With regard to the photocurrent mechanism of these QDQW nanoparticles, those charge carriers participating in the formation of excitons may not contribute to the photocurrent, because of the large binding energy of the excitons. Moreover, it is suggested in this paper that free holes in the HgTe quantum-well in the valance band, rather than free electrons, are the main contributors to the photocurrent.     

Impurity distribution and electrical characteristics of boron-doped Si1−x Ge x /Si p +/N heterojunction diodes produced using pulsed UV-laser-induced epitaxy and Gas-immersion laser doping

A two-step pulsed UV-laser process which independently controls the metallurgical and electrical junction depth of a Si_1–x Ge _x /Si heterojunction diode has been implemented. Pulsed Laser-Induced Epitaxy (PLIE) combined with Gas-immersion Laser Doping (GILD) are used to fabricate boron-doped heteroepitaxial p ⁺/N Si_1–x Ge _x /Si layers and diodes. Borontrifluoride is used as the gaseous dopant source in the GILD process step. Boron incorporation and activation are investigated as a function of laser energy fluence and the number of laser pulses using SIMS and Halleffect measurements. The dose of incorporated dopant is on the order of 10¹³ cm^–2 per pulse. The B profiles obtained are flat except for a peak at the interface resulting from segregation effects. The B and Ge distributions are compared with shifts in the turn-on voltage of p ⁺/N Si_1–x /Si heterojunction diodes produced by the process. The GILD/PLIE process is spatially selective with the resulting diodes fabricated being quasiplanar. Hole mobilities in the heavily doped Si_1–x Ge _x films are found to be slightly lower than in comparable Si films.Presently at the Oregon Graduate Institute, Beaverton, OR 97006, USA     

Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering

ZnO and Al-doped ZnO(ZAO) thin films have been prepared on glass substrates by direct current (dc) magnetron sputtering from 99.99% pure Zn metallic and ZnO:3 wt%Al₂O₃ ceramic targets, the effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. It shows that the surface morphologies of ZAO films exhibit difference from that of ZnO films, while their preferential crystalline growth orientation revealed by X-ray diffraction remains always the (0 0 2). The optical transmittance and photoluminescence (PL) spectra of both ZnO and ZAO films are obviously influenced by the substrate temperature. All films exhibit a transmittance higher than 86% in the visible region, while the optical transmittance of ZAO films is slightly smaller than that of ZnO films. More significantly, Al-doping leads to a larger optical band gap (E_g) of the films. It is found from the PL measurement that near-band-edge (NBE) emission and deep-level (DL) emission are observed in pure ZnO thin films. However, when Al was doped into thin films, the DL emission of the thin films is depressed. As the substrate temperature increases, the peak of NBE emission has a blueshift to region of higher photon energy, which shows a trend similar to the E_g in optical transmittance measurement.     

Effect of high energy ion irradiation on silicon substrate in a pulsed plasma device

We have performed an experimental analysis on the investigation of high energy ion beam irradiation on Si(1 0 0) substrates at room temperature using a low energy plasma focus (PF) device operating in methane gas. The surface modifications induced by the ion beams are characterized using standard surface science diagnostic tools, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), photothermal beam deflection, energy-dispersive X-ray (EDX) analysis and atomic force microscope (AFM) and the results are reported. In particular, it has been found that with silicon targets, the application of PF carbon ion beams results in the formation of a surface layer of hexagonal (6H) silicon carbide, with embedded self-organized step/terrace structures.     

Fabrication of doped nano‐electromechanical systems

We present a new generation of nano‐electromechanical systems (NEMS), which are realized by doping the semiconductor base material. In contrast to the traditional approach these doped NEMS (D‐NEMS) do not require a metallization layer. This makes them far lighter and hence increases resonance frequency and quality factor. Additionally, D‐NEMS can be tuned from the conductive state into an insulating one. This will enable a host of new device designs, like mechanically tunable pin‐junctions and nanomechanical single electron switches. We demonstrate D‐NEMS fabrication and operation from the intrinsic, to the light, and to the heavy regime of doping. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurement of the thorium-228 activity in solutions cavitated by ultrasonic sound

We show that cavitation of a solution of thorium-228 in water does not induce its transformation at a faster rate than the natural radioactive decay. We measured the activity of a thorium-228 solution in water before, and after, it was subjected to a cavitation at 44 kHz and 250 W for 90 min in order to observe any change in the thorium half-life. The results were compared to the original activity of the sample and we observed no change. Our results and conclusions conflict with those in a recent paper by F. Cardone et al. [1].     

Time-dependent photocurrent of a CdTe nanoparticle film under the above-gap illumination

Time-dependent photocurrent of a solid film of 1-thioglycerol-capped CdTe nanoparticles under the illumination of the 325-nm wavelength light is characterized to investigate the transport mechanism of photo-generated charge carriers in this nanoparticle film. Under the illumination of the above-gap light, photocurrent rises rapidly, and subsequently it decays or rises slowly, depending on the magnitude of bias voltage. A careful investigation into the variation in the magnitude of the current measured as a function of time while the light was switched on and off periodically reveals that rapidly and slowly respondent photocurrents overlap in the time-dependent photocurrent. Charge carriers contributing to the rapidly and slowly respondent photocurrents are electrons and holes separated from a fraction of excitons excited by the above-gap light. The transport behaviors of these charge carriers may explain the monotonously decay and slow rising of the photocurrent after its rapidly rising under the illumination for the solid film at unbiased and biased voltage, respectively.     

Determination of bulk and surface transport properties by photocurrent spectral measurements

Reliable minority carrier diffusion length and surface recombination velocity values have been obtained from stationary photocurrent measurements. A modified surface photovoltage method has been used to determine diffusion lengths longer than the wafer thickness in high-purity Si, whereas the spectral variation of the photocurrent has been employed to measure the surface recombination velocity. The novelty presented in this paper is that a Schottky diode has been employed in both the methods to collect generated charged carriers. Moreover the same Schottky diode has been employed in both the methods in order to avoid any a priori assumptions on the material transport parameters. This combined application of the two methods at the same device enables the determination of highly reliable results. Received: 17 February 2000 / Accepted: 28 March 2000 / Published online: 30 June 2000     

Waveguides induced by steady-state gray solitons in biased photorefractive-photovoltaic crystals

We carry out a theoretical investigation of the properties of waveguides induced by photorefractive one-dimensional steady-state gray spatial solitons (i.e., screening solitons, photovoltaic solitons, and screening-photovoltaic solitons). We demonstrate that waveguides induced by photorefractive steady-state gray spatial solitons are only a single guided mode for both all soliton graynesses and all values of ρ, where ρ is the ratio between the soliton peak intensity and the dark irradiance, and moreover, waveguides induced by gray photovoltaic solitons for closed-circuit condition are also only a single guided mode for all electric current densities. We find that the confined energy near the center of a photorefractive steady-state gray spatial soliton increases with ρ and decreases with an increase in the soliton grayness. We also find that the confined energy near the center of a gray photovoltaic soliton for closed-circuit condition increases with the electric current density. On the other hand, waveguides induced by gray screening-photovoltaic solitons are gray screening soliton-induced waveguides when the bulk photovoltaic effect is neglectable and are gray photovoltaic soliton-induced waveguides when the external bias field is absent.     

Electrical Characteristics of Co/n-Si Schottky Barrier Diodes Using I-V and C-V Measurements

Electrical characteristics of Co/n-Si Schottky barrier diodes are analysed by current- voltage （I- V） and capacitancevoltage （C- V） techniques at room temperature. The electronic parameters such as ideality factor, barrier height and average series resistance are determined. The barrier height 0. 76 eV obtained from the C - V measurements is higher than that of the value 0. 70 eV obtained from the I - V measurements. The series resistance Rs and the ideality factor n are determined from the d ln（ I） /dV plot and are found to be 193.62 Ω and 1.34, respectively. The barrier height and the Rs value are calculated from the H（I） - I plot and are found to be 0.71 eV and 205.95Ω. Furthermore, the energy distribution of the interface state density is determined from the forward bias I - V characteristics by taking into account the bias dependence of the effective barrier height. The interface state density Nss ranges from 6.484×10^11 cm^-2eV^-1 in （Ec - 0.446） eV to 2.801×10^10 cm^-2eV^-1 in （Ec - 0.631） eV, of the Co/n-Si Schottky barrier diode. The results show the presence of a thin interracial layer between the metal and the semiconductor.     

Fast photovoltaic characteristic of silver nano-cluster doped ZnO thin films induced by 1.064 μm pulsed laser

Fast photovoltaic characteristic was found from silver nano-cluster doped ZnO thin films when it was irradiated by 1.064 μm infrared laser. Its arising time is about 1 ns with an open-circuit photovoltage of ∼2 ns full width at half-maximum. A transient photovoltaic signal of ∼80 mV occurred with a full width at half-maximum of about 2 ns was observed under the illumination of a 4 mJ pulsed laser in duration of 25 ps. This fast photovoltaic response perhaps relate to the non-uniform distribution of the silver clusters along direction perpendicular to ZnO surface.