CdTe CDS Solar Cells Computer Science Essay


CdTe is one of the leading candidates for photovoltaic and optoelectronics applications due to its optimum band gap. Various techniques have been used to produce CdTe thin film such as Electro deposition, Vacuum evaporation, Close space sublimation method, Brush plating, Pulsed laser deposition, Chemical deposition method, and hot wall epitaxy (Kumar, et al. 2008; Bilevych et al., 2004). This chapter provides the necessary relative background knowledge of this research. Uncountable attempts have been made and are still in progress in this regard. The objective of this chapter is to review such attempts.

Kosyachenko et al. (2006) investigated the charge transport properties of the Schottky diodes fabricated on n-CdTe layers electrodeposited on stainless steel foil. The small area Au-CdTe contacts facilitated the investigation of the role of shunt and series resistances on the I-V characteristics of the thin-film CdTe device. The charge transport mechanism in these diodes was found to be generation-recombination in the depletion layer and over-barrier electron flow at low and higher bias voltages, respectively. By using this model a quantitative theoretical analysis of the current-voltage characteristics of the diodes was possible.

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Veeramania et al. (2008) investigated the influence of 100MeV Ag7+ ion irradiation on current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Au/CdTe and Au/Cd0.9Zn0.1Te Schottky barrier diodes as a function of fluence. The irradiation fluence was varied from 1 Ã- 1010 to 1 Ã- 1013 ions cm−2. The current transport across the metal-semiconductor junction for pure and irradiated Schottky barrier diodes was described by the thermionic-field emission process. Also, there were several mechanisms such as barrier tunneling, carrier compensation and generation-recombination mechanism, which may account for the I-V and C-V characteristics after irradiation. This variation in various diode parameters such as ideality factor (n), Schottky barrier height () and saturation current (Is) was studied as a function of irradiation fluence. The energy loss mechanism of swift heavy ions at the metal-semiconductor interface used to explain the change in Schottky barrier diode parameters. The observed modification at interface states of Schottky barrier height over a wide fluence range was mainly due to intense electronic energy loss mechanism and latent ion track formation created by swift heavy ions.

Horvath et al. (2001) studied the electrical and photoelectrical behaviour of Au/n-CdTe junctions prepared on CdTe monocrystalline substrates and CdTe epitaxial layers grown on n+ GaAs substrates. The electrical and photoresponse properties depended very strongly on the parameters of the compensated high-resistive layer at the CdTe surface formed by annealing during preparation.

Mathew et al. (2004) reviewed the development of CdTe/CdS solar cells on flexible substrates in this article. In this article, it was showed that Photovoltaic structures on lightweight and flexible substrates have several advantages over the heavy glass based structures in both terrestrial and space applications. It was also described that the cells mounted on flexible foil are not fragile, the requirements of the supporting structures are minimum and they can be wrapped onto any suitably oriented or curved structures. It was showed that the specific power of the solar cells is an important factor in space applications and hence development of photovoltaic devices on light weight substrates is interesting and CdTe is one of the leading candidates for photovoltaic applications due to its optimum band gap for the efficient photo-conversion and robustness for industrial production with a variety of film preparation methods. Flexible solar cells with conversion efficiencies exceeding 11% have been developed on polyimide foils. The development of CdTe devices on metallic substrates is impeded due to the lack of a proper ohmic contact between CdTe and the substrate. The polymer substrate has the advantage that the devices can be prepared in both ''superstrate'' and ''substrate''configurations.

Kosyachenko et al. (2007) investigated the Au/n-CdTe Schottky diodes with a high rectification factor (107-108 at |V| = 0.5 V), low reverse current (10−12-10−9 A at V = −1 V) and an open-circuit voltage of 0.75-0.85 V. Depending on the semiconductor resistivity (102-104 Ω cm at 300 K) and treatment of the surface of a single crystal before the vacuum deposition of Au, the observed current-voltage (I-V) characteristics of diodes exhibited a wide variety of shapes. The obtained experimental data on the charge transport mechanism in Schottky diodes based on both n- and p-type CdTe summarized taking into account the results reported earlier. The variety of the observed I-V characteristics explained in the framework of the Sah-Noyce-Shockley theory for generation recombination in the space-charge region of a p-n junction adapted to a Schottky diode.

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Pattabi et al. (2007) presented the results of the studies on the effect of temperature and 8 MeV electron irradiation on the current-voltage (I-V) characteristics of the Au/CdTe Schottky diodes. In this article, Schottky diodes were prepared by evaporating Au onto n-type CdTe films electrodeposited onto stainless steel substrates. The forward and reverse current-voltage characteristics of these diodes were studied as a function of temperature. The diodes were subjected to 8 MeV electron irradiation at various doses and their effect on the I-V characteristics was studied. Some intrinsic and contact properties such as barrier height, ideality factor, and series resistance were calculated from the I-V characteristics. Diode ideality factor of the junctions were greater than unity. The ideality factor and the series resistance Rs increase with decrease in temperature. The conduction was seemed to be predominantly due to thermionic emission-diffusion mechanism. The resistance was found to increase with increasing dose. The leakage current, ideality factor and barrier height were found to be unaffected by electron irradiation up to, a dose of about

40 kGy.

Mathew et al. (2000) investigated the charge transport mechanism in a typical Au/CdTe Schottky diode and observed the evidence for different types of charge transport at different temperature regions .The dominant transport mechanism in the 100-300K regions identified as the Poole-Frenkel type. The activation energy of the trap level detected in the 100-300K temperature range shows voltage dependence. The transport mechanism changed at a characteristic temperature of about 270 K.

Sarmah and Rahman (2001) prepared and studied the Schottky barriers of Ag, Al, Ni-(n)CdTe structures. The films were prepared by rf sputtering and doped with Cd metal. They showed that the diode ideality factor of these junctions is greater than unity and barrier height varies from 0.6-0.7 eV and are affected by room illumination. Photovoltaic effect of these junctions was very poor and fill factor below 0.4. Low doping concentration, high defect density, and presence of an interfacial layer and presence of high series resistance were perceived to affect the J-V characteristics.

Makhnij et al. (2009) studied current-voltage characteristics of surface-barrier diodes based on n-CdTe substrates treated in aqueous solutions of alkali metal salts. It was found that recombination processes in the space-charge region and the above-barrier carrier transport control the forward current.

Grushko et al. (2007) reported a study of the Au/CdTe Schottky diodes fabricated by vacuum evaporation of a semitransparent Au layer over an electrodeposited CdTe thin film. The theoretical model of the photocurrent spectra for an Au/CdTe Schottky diode based on the continuity equation and incorporating the surface recombination losses did not explain the measured spectra in the entire range of wavelengths, particularly the above-mentioned decay in the short-wavelength region. The satisfactory description of the measured spectra was achieved by proposing a model, in which the surface recombination along with the Schottky effect resulted in the presence of a dead layer in the space-charge region is taken into account. It was concluded that by varying the parameters such as uncompensated carrier concentration and carrier lifetime, the above model can explain the actual photoresponse spectra.

Mahesha et al. (2008) made a study on the behavior of Al/p-CdTe thin film junction grown by thermal evaporation method. It was reported that I-V characteristics which show that the Al makes Schottky contact with p-CdTe. The variation of junction capacitance with frequency and voltage was studied to evaluate the barrier height. The activation energy and band gap was estimated by studying variation of resistivity with temperature. Using all these data, band diagram of Al/p-CdTe was proposed.

Kleto et al. (2008) presented a comparative study of I-V characteristics of "metal-semiconductor-metal" structures without coatings and with protective coatings. Regimes for radio frequency deposition of dielectric coatings on the surface of pixeled Schottky junction based on Cl-doped CdTe single crystals with high resistivity (107-1010) Ω·cm were determined. In particular, the films with composition of (TeO2) x - (SiO2)1 − x have demonstrated high isolation properties. Possible applications of new CdTe pixeled detectors with the studied protective coatings for high-resolution spectrometry in X- and gamma ray sensor devices are discussed.

Al-Shibani (2002) grew single-crystal CdTe using the vapour phase technique. The dice were suitably etched and isothermally annealed in air for different times (0.5, 1, 2, 3, and 4 h) at 673 K. Schottky diodes were fabricated by vacuum evaporation of Au onto CdTe single-crystal substrates, which were isothermally annealed. The behaviour of Au-CdTe devices was studied as a function of time using capacitance-voltage and current-voltage analysis. The isothermal anneal technique was found to produce a significant change in the electrical properties of Au-CdTe Schottky barriers such as barrier height, carrier concentration, ideality factor and depletion width. The results were discussed in terms of various theories of Schottky barrier formation.

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Nollet et al. (2000) showed that the back contact on CdTe based solar cells is thought to be responsible for the non-idealities in the cell characteristics. Numerical modelling of the thin film cells was seemed to support this assumption: a simple model can explain all gross qualitative non-idealities such as rollover and fill-factor loss in I-V curves, the shape and frequency dependence of C-V curves. A more straightforward way to get convinced by the importance of the back contact was the use of monochromatic light, which penetrates to this contact. For CdTe, this comes down to light of about 840 nm. The influence of the light in this wavelength region on the I-V characteristics was studied in this paper. It was found that the I-V curves were only influenced by photons within this wavelength region. This can be explained by the generation of a new charge carrier distribution in the neighbourhood of the contact.

Kim et al. (2007) presented a semi-analytical model for Schottky diode with ideality factors () greater than 1.00 with an experimental verification from n-type GaAs Schottky diodes (=1.00-2.47 over T=83-323 K). Adopting a correcting ideality factor in the distribution function, an accurate modeling of temperature-sensitive of current-Voltage characteristics and accurate extraction of Schottky barriers were obtained with a modified Richardson constant. Temperature-dependent Schottky barriers (bn=0.928 V83 K -0.837 V 323 K), obtained from the semi-analytical model, were consistent with the variation of the energy bandgap with temperature which is known to be the main cause for the change of Schottky barriers.

Vorobets et al. [2007] investigated the influence of surface laser treatment on structural properties of metal-intrinsic CdTe contacts by metallographic method and scanning electronic microscopy techniques. At critical intensities of pulse laser irradiation on the metal-CdTe boundary re-structuring of structurally impurity defects in a solid state occured. A defect system of large grains of single crystalline CdTe was formed. Separate clusters were ordered over a long range and formed their own "crystalline sublattice", probably, of trimetric configuration. A diameter of the cluster in nodes of such a lattice equalled 1-3 μm, while distance between clusters was of about 10-15 μm. As radiation dose was increased the clusters acted as getters of dot defects. At optimal modes of laser irradiation their sizes increased in 2-3 times, consequently leading to decrease in concentration of deep levels in the semiconductor. As has shown investigation of the I-V and C-V characteristics such processes caused reduction of GR component of a current, promoting field charge transfer mechanisms in contacts with a Schottky barrier.

Chandramohan et al. (2008) reports in this research artical the effect of swift (80 MeV) oxygen (O+6) ion irradiation on the optical properties of CdTe thin films grown by conventional thermal evaporation on glass substrates. The films were found to be slightly Te-rich in composition and irradiation results no change in the elemental composition. The optical constants such as refractive index (n), absorption coefficient (α) and the optical band gap energy showed significant variation in their values with increase in ion fluence. Upon irradiation the band gap energy decreased from a value of 1.53 eV to 1.46 eV whereas the refractive index (n) increased from 2.38 to 3.12 at λ = 850 nm. The photoluminescence spectrum showed high density of native defects whose density strongly depends on the ion fluence. Both analyses indicate considerable defect production after swift ion beam irradiation.

Ghosh (2008) showed that CdTe and the associated materials are suffering from ohmic contacting problem due to their high electron affinity and consequently large function. Ni, Au, Pt and Pd have large work function and have possibility to match with CdTe. However, except Ni other materials have problems in large-scale applications. In the present paper, possibility of Ni was explored through work function engineering. Work function and bulk resistivity of Ni has been modulated with other materials like, Cu, Au, Mo, W and Co. A theoretical model was developed to calculate the effective work function and bulk resistivity after modulation. Modulated materials were deposited over thin film CdTe using electroless technique to evaluate the validity between the theoretical and experimental results. Results indicated about good matching between them.

Mahesha et al. (2009) grew nano-crystalline CdTe/CdS thin film hetero-junctions on glass substrate by thermal evaporation technique. The growth conditions to get stoichiometric compound films were optimized. The grown hetero-junctions were characterized for their I-V characteristics. Analysis of I-V characteristics was made to investigate the current conduction mechanism in p-CdTe/n-CdS hetero-junction. The band gap energy of cadmium telluride and cadmium sulfide films was computed from the study of variation of resistance with temperature. Based on the study, band diagram for p-CdTe/n-CdS hetero-junction was proposed.

Osvald (2009) showed that the ideality factor of inhomogeneous schottky diodes does not increase for decreasing temperature to such extent as is commonly observed for schottky diodes in experiment by numerical simulations of I-V-T curves. The main consequence of such a result was that in spite of the fact that the barrier height inhomogeneities fullfil the conditions for barrier height lowering for decreasing temperature they might not be a general or the only reason for occuring of this effect in experimental structures. It was found that much slower ideality factor temperature dependence than reported in the literature and the dependence was even not monotonous for simulation conditions used. It was conclude that some other reason as barrier inhomogeneity is responsible for ideality factor temperature dependence.

Wang et al. (2006) studied the effect of annealing time at 333 K in air on the ohmic property of Au/p-CdZnTe contact. Through I-V measurement, it was found that Au/p-CdZnTe had excellent ohmic property after 2 h annealing. SEM and XPS analyses showed that Au atoms diffused into CdZnTe during annealing. Diffused Au did not form any compound with any element in CdZnTe, but replaced Cd sites or occupied Cd vacancy as acceptors. Thus, the heavy p-type doping layer was formed and M-p+-p ohmic contact was obtained. At the same time, about 27.01% of Te in un-deposited CdZnTe surface layer was oxided into TeO2 during 2 h annealing.

Wagner and Sands (2000) formed the low resistance contacts to epitaxial n-CdTe on InSb by excimer laser induced diffusion of In with subsequent deposition of gold. Diodes was fabricated using a combination of laser processing and conventional photolithography, and the forward bias and reverse bias current-voltage characteristics measured. All the diodes were behaved effectively as Schottky diodes with high ideality factors (n≈3) and exhibited stable characteristics over several months. An estimate of the In-CdTe barrier height was made taking into account the effective current dependent series resistance which is known to arise from the rear InSb-CdTe junction. The barriers range from 0.7 eV (no diffusion) to 0.4 eV, the last having a specific contact resistance of 0.16 Ω/cm2. It was shown from the characteristics of the rear junction, together with an estimate of the maximum melt-depth, that In has not diffused to the rear InSb-CdTe junction.

Bozhkov and Zaitsev (2006) investigated a possibility of a more simple and exact representation of the current-voltage (I-V) characteristic of a tunnel metal-semiconductor contact. Analysis was made on the basis of the approach proposed earlier, according to which the behaviour of the (I-V) characteristics of the Schottky-barrier contacts at low temperatures depends on the nonlinear dependence of the true (or effective for a tunnel contact) barrier height on the bias voltage. A simple equation for the (I-V) characteristic of the tunnel contact sswas theoretically substantiated using measurable parameters - the ideality factor η and barrier height φbm where the quantity φbn ≡ ηφbm played the role of a barrier height. The calculation showed that this quantity is very close to the true barrier height of the tunnel contact (with allowance for the effect of image force) in wide current, temperature, and impurity-concentration ranges.

Gurumurthy et al. (1999) reported nitrogen plasma exposure (NPE) effects on indium doped bulk n-CdTe. Excellent rectifying characteristics of Au/n-CdTe Schottky diodes, with an increase in the barrier height, and large reverse breakdown voltages were observed after the plasma exposure. Surface damage was found to be absent in the plasma exposed samples. The breakdown mechanism of the heavily doped Schottky diodes was found to shift from the zener to avalanche after the nitrogen plasma exposure, pointing to a change in the doping close to the surface, which was also verified by C-V measurements. The thermal stability of the plasma exposure process was seen up to a temperature of 350 °C, thereby enabling the high temperature processing of the samples for device fabrication. The characteristics of the NPE diodes were stable over a year implying excellent diode quality. A plausible model based on Fermi level pinning by acceptor-like states created by plasma exposure was proposed to explain the observations.

Gulnahar and Efeoglu (2009) characterized the current-voltage (I-V) characteristics of Au/p-GaTe Schottky contact at 60-300 K temperature range and compared with those of Al/p-GaTe. The observed anomalies of Schottky barrier were interpreted on the basis of thermionic emission model. The barrier height and ideality factor values at 280 K for Au were 0.513 eV and 1.022, respectively. As temperature lowered down to 140 K, we observed that these parameters gradually decreased. The barrier height dependence on temperature demonstrated a double-Gaussian distribution. The weighting coefficients for each Gaussian distribution and their standard deviations were found to be 0.76-37.7 meV and 0.24-58.2 meV for Au, respectively. A linearization procedure based on the double barrier distribution was carried out, and the Richardson constant for Au/p-GaTe was found to be 148.03 A K−2 cm−2. This value is reasonably close to the theoretical value given in the literature as 119.4 A K−2 cm−2.

Diale and Auret (2009) studied Au/n-GaN Schottky barrier diodes. GaN surface was prepared by cleaning in HCl and (NH4)2S prior to metal deposition. The zero biased barrier heights and ideality factors were obtained from the current-voltage characteristics differ from diode to diode, although all the samples were prepared identically. The statistical analysis for the reverse bias C-V data yielded mean value of (1.35 ± 0.04) eV for Schottky barrier height of HCl treated sample and (1.20 ± 0.03) eV for (NH4)2S sample, where 9 dots were considered from each cleaning method. It was found that the barrier height values obtained from the C2 -V (1.43 eV) and I-V characteristics (0.89eV) are different from each other by 0.54 eV. The inhomogeneous barrier heights were found to be related to the effect of the high series resistance on diode parameters.

Borah et al. (2008) fabricated thin film (n)CdSe/(p)CdTe heterojunctions by depositing p-type CdTe thin films on n-type CdSe thin films using the thermal evaporation technique, and their electrical and optical properties at room temperature as well as elevated temperatures have been investigated by current-voltage measurements. The different junction parameters such as diode ideality factors, barrier heights, Richardson constant, short-circuit currents, open circuit voltage, etc. were determined from I-V characteristics and found changing on vacuum annealing. The ideality factors (>2 in dark at 303K) were found to decrease with increase in temperature. At room temperature (303K), the barrier height was found to be 0.7 eV in dark and showed no temperature dependence in the range from 303K to 333K. The photovoltaic performance of the junction was found to improve on vacuum annealing. The structure with concentrations Na=2.43x1016/cm3 for CdTe and Nd=9.03Ã-1015/cm3 for CdSe showed a photovoltaic effect with fill factor 0.46, open-circuit voltage 135 mV, short-circuit current 2.06Ã-10-4 A/cm2 for an annealed sample and corresponding quantities 0.37, 148 mV, 1.9Ã-10-4 A/cm2 for an untreated sample. The fill factor was found to decrease with increase in temperature. The junctions exhibited spectral response within 550-900 nm giving a peak at wave length 762 nm. Proper doping and annealing lead to reduction of series resistance for achieving an ideal solar cell.