J. Toušková

Preparation and characterization of CdS/CdTe thin film solar cells

Abstract Thin film n-CdS/p-CdTe heterojunctions for solar cells were prepared by chemical deposition of CdS and electrodeposition of CdTe layers. No chlorine was intentionally put into the deposition solution for CdTe preparation (the A samples), whereas chloride ions were added in the case of the B samples. No post-depobition treatment with CdCl 2 followed. The structure and morphology of the cells were analyzed using X-ray diffraction and scanning electron microscopy, The photovoltaic properties were tested using illuminated current-voltage characteristics. The relationship between the structure and morphology of the CdTe layers and the efficiency of the cells was found, and it was confirmed that the presence of chloride ions was essential to achieve an efficiency of 9.4%. The contactless pholovoltage (CPV) method was used to measure the minority carrier diffusion length in CdTe. Values of the diffusion length and some other parameters of the cells were deduced by fitting the theoretical curve of the CPV spectral response to the experiment.

Diffusion length in CdTe by measurement of photovoltage spectra in CdS/CdTe solar cells

Modified method of surface photovoltage (SPV), spectral response measurement, and constant photocurrent method (CPM) were applied to thin film CdS/CdTe solar cells with the aim of finding diffusion length of minority carriers (L) in the CdTe material. The SPV signal was theoretically calculated without constraints of absorption coefficients for the incident radiation and thickness of the sample assuming one space charge region (SCR) located on the CdS/CdTe interface. In addition to the diffusion length, the SPV is a function of the surface recombination velocity and the parameters of the SCR, which complicates the evaluation. Illuminating the back side of the solar cell (without ohmic contact) we obtain a photovoltage spectrum predominantly influenced by the diffusion length. On the other hand, the standard measurement using light penetrating from the CdS side strongly depends on the thickness of the SCR. The small signal approximation model presented here successfully explains both measured spectra and p...

Influence of growth rate on charge transport in GaSb homojunctions prepared by metalorganic vapor phase epitaxy

Dark current–voltage (I–V) characteristic measurement in the temperature range from −190 °C to 65 °C was carried out on GaSb p-n homojunctions prepared by low-pressure metalorganic vapor phase epitaxy. It was shown that the charge transport mechanism in these homojunctions is strongly affected by the growth rate of GaSb epitaxial layers. Samples prepared at higher growth rate (40 nm/min.) exhibit an anomalous low-temperature peak of tunneling current which can be explained by the presence of a narrow band of energies due to high concentration of native defects, probably GaSb antisites. The same defect levels are responsible for the generation–recombination current which dominates in these samples at higher temperatures. On the other hand, quite different behavior was found in the case of slowly grown (20 nm/min) samples. At sufficiently low temperatures, a current maximum near 50 mV of forward voltage points out a band-to-band tunneling as a prevailing transport mechanism. With increasing temperature, how...

Photovoltage spectroscopy of InAs/GaAs quantum dot structures

In addition to widely used photoluminescence spectroscopy photovoltaic measurement of quantum dot structures can give complementary information about electron and hole transitions. Structures with self-organized InAs quantum dots in GaAs matrix were grown by the Stranski–Krastanov mechanism using the low pressure metalorganic vapor phase epitaxy technique. Two types of samples were studied, with single and multiple quantum dot layers. We have shown that surface photovoltage spectroscopy can be used for the study of single, as well as multiple quantum dot layer structures.

Contactless photovoltage method for measurement of diffusion length of minority carriers in solar cells

Contactless photovoltage method is presented as a generalization of conventional surface photovoltage investigation of semiconductor parameters. Existence of two space charge regions causing two signals of opposite sign was taken into account and calculated photovoltage was compared with experimental data. The method has been used for evaluation of parameters of silicon solar cells.

Improved photovoltaic method for measurement of minority carrier diffusion length applied to silicon solar cells

Abstract The photovoltage spectrum measured on back illuminated silicon solar cells of the (passivated emitor solar cell) (PESC) type without original bottom ohmic electrode is evaluated with the aim to find the diffusion length of minority carriers in bulk of the absorber ( L ). Two junctions, namely pn + junction of the cell and that spontaneously created on the free surface generally exist in such samples. They give rise to two signals of opposite signs with one point of exact compensation. Six parameters (including L ) are needed to characterize the spectrum. Special simple arrangement removes influence of spontaneously created junction on the free surface, which, in this way, reduces the number of parameters needed for fitting to three and enhances reliability of the measurement.

Interpretation of minority carrier diffusion length measurements in thin silicon wafers

Diffusion length of minority carriers measured in thin wafers reflects not only properties of the material but also the state of its surface and, consequently, becomes an effective quantity Leff depending on the thickness of the sample. The function describing this dependence is presented here and compared with the data from surface photovoltage experiments. It allows one to find the correct diffusion length from measurement of two samples with different thicknesses.

Some properties of electrodeposited CdTe thin layers

Polycrystalline CdTe layers have been deposited cathodically on nickel substrates from an aqueous solution of CdSO4 and TeO2. The structure of the deposits was studied by means of X-ray diffraction. Measurements of current-voltage and capitance-voltage characteristics and spectral dependence of the short-circuit current of a Au-CdTe Schottky diode prepared on the basis of these layers, were performed. Parameters as height of both Schottky and intergranular barrier, impurity concentration in the bulk of the grains and minority carriers effective diffusion length were determined.

Transport-controlling deep defects in MOVPE grown GaSb

Deep level transient spectroscopy (DLTS) measurements were used to directly observe growth-rate induced defects in MOVPE grown GaSb layers. The DLTS spectra of p–n homojunctions prepared at a growth rate of 40 nm min−1 show a broad minority carrier peak in the temperature range of 190–230 K. The shape of the peak does not correspond to the exponential transient. Assuming a band of localized states with Gaussian distribution, a mean energy of about 0.3 eV above the valence band edge and a characteristic broadening of 75 meV match the measurement. On the other hand, the DLTS spectra of the samples grown at a growth rate of 20 nm min−1 exhibit only a slowly varying background. We conclude that the observed defects are due to the high growth rate of the GaSb layers and control defect-assisted tunnelling previously observed from current–voltage characteristics.

Current–voltage characteristics of GaSb homojunctions prepared by MOVPE

Mechanism of charge transport through GaSb p–n homojunctions prepared by low pressure metal organic vapour phase epitaxy technology was studied.Calculation of current–voltage characteristics based on Shockley–Read–Hall recombination model was performed accounting for specific features of the band structure of GaSb with the lowest minimum of the conduction band at C point and higher minima at L points.It is shown that this type of transport prevails if the samples were prepared at higher growth rate.Analysis of temperature dependencies of some parameters of the experimental characteristics gave, besides the expected value of band gap energy, also the activation energy of the recombination level as (Ec � 0:22) eV or (Ev þ 0:29) eV.The latter value agrees reasonably well with published data on deep levels in GaSb. On the other hand, the charge transport in more slowly grown samples was nearly temperature independent, especially in the reverse direction, probably due to prevailing tunnelling mechanism. 2003 Elsevier Science Ltd.All rights reserved.