Solar Cell Fabrication and Characterisation

Journal of Polytechnic, 2017

The polycrystalline p-type CdTe thin film was deposited to form a solar cell structure with n-type CdS thin film window layer. Material characterization of the deposited thin films were investigated by X-ray diffraction measurements and the preferred orientations were obtained along (111) direction at 2θ ≅ 24° and 2θ ≅ 26.5° for CdTe and CdS films, respectively. The optical behaviors were investigated according to the transmission spectrum and corresponding direct band gap values were found as 1.51 eV for CdTe and 2.22 eV for CdS thin films. The superstrate CdS/CdTe thin film heterostructure was investigated under the dark and illuminated current-voltage measurements. The heterostructure showed a photovoltaic behavior with the cell efficiency of %1.4. The effects of the etching process on the photovoltaic behavior of CdS/CdTe thin film heterostructure were also discussed and the cell efficiency was enhanced as about %1.6 eV.

1998

Solar Energy, 2004

Processing options for addressing critical issues associated with the fabrication of thin film CdTe solar cells are presented, including window and buffer layer processing, post-deposition treatment, and formation of stable low resistance contacts. The paper contains fundamental data, engineering relationships and device results. Chemical surface deposited CdS and Cd 1Àx Zn x S films are employed as the n-type heteropartner window layers. Maintaining junction quality with ultra-thin window layers is facilitated by use of a high resistance oxide buffer layer, such as SnO 2 , In 2 O 3 or Ga 2 O 3 , between the heteropartner and the transparent conductive oxide. Thermal annealing of the CdTe/CdS heterostructure in the presence of CdCl 2 and O 2 shifts the chemical equilibrium on the surface of the absorber layer, which influences the bulk electrical properties. Aspects of back contacting CdTe/CdS devices, including etching, Cu application, contact annealing, back contact chemistry and secondary contacts, are discussed. Two commonly employed etches used to produce a Te-rich layer, nitric acid/phosphoric acid mixtures and Br 2 /methanol are compared, including the nature and stability of the final treated CdTe surface. The diagnostic abilities of the surface sensitive VASE and GIXRD techniques are highlighted. Various methods of Cu delivery are discussed with consideration to; reaction with Te, processing simplicity, processing time and possible industrial scale-up. Some aspects of back contact stability are presented, including discussion of apparent robust back contacts, which contain a thick Te component.

Superficies y Vacío

CdTe semiconductor is an absorbent material used in “tandem” photovoltaic solar cells. This material is commonly deposited by thermal evaporation presenting electrical resistivity values about of 105 W·cm to 109 W·cm. CdTe is applied in thin solar cells as p-type layer which is in contact with metal back electrode in solar cells. In the CdTe/metal junction a Schottky barrier exits; and small number of charge carriers have enough energy to get over the barrier and cross to the metal back contact. To solve part of this problem, nanostructured Te thin films were used as intermediate layers between CdTe and metal contact. Te layers whit different physical properties were deposited on CdS/CdTe structure by thermal evaporation employing different growth parameters. The electrical parameters of CdTe solar cells were influenced by p+ Te regions. p+ Te regions used as intermediate layer with large deposition time increases the FF and VOC values from 30% to 60% and 560 mV to 730 mV respective...

The European Physical Journal Applied Physics, 2016

At present most of II-VI semiconductor based solar cells use the CdTe material as an absorber film. The simulation of its performance is realized by means of various numerical modelling programs. We have modelled a solar cell based on zinc telluride (ZnTe) thin film as absorber in substitution to the CdTe material, which contains the cadmium element known by its toxicity. The performance of such photovoltaic device has been numerically simulated and the thickness of the absorber layer has been optimized to give the optimal conversion efficiency. A photovoltaic device consisting of a ZnTe layer as absorber, CdS as the buffer layer and ZnO as a window layer was modelled through Solar Cell Capacitance Simulator Software. Dark and illuminated I-V characteristics and the results for different output parameters of ZnO/CdS/ZnTe solar cell were analyzed. The effect of ZnTe absorber thickness on different main working parameters such as: open-circuit voltage V oc, short-circuit current density Jsc, fill factor FF, photovoltaic conversion efficiency η was intensely studied in order to optimize ZnTe film thickness. This study reveals that increasing the thickness of ZnTe absorber layer results in higher efficiency until a maximum value and then decreases slightly. This maximum was found to be 10% at ZnTe optimum thickness close to 2 μm.

Asian Journal of Research and Reviews in Physics, 2022

Thin films of Cadmium Sulphide and Cadmium Telluride have gained a great deal of interest due to their potential applications in solar cells. Deposition of and thin films were performed on Soda Lime glass and FTO substrate at respectively using spray pyrolysis technique. The Hall Effect property was measured for the deposited and films. These results shows the resistivity and mobility of CdS films deposited at were and , respectively, The annealed thin film had a resistivity value of , while the annealed and etched thin film had a resistivity value of , The resultant films are observed to be good to make a solar cell with CdS as a window layer and CdTe as absorber layer.

Coatings, 2014

Thin film solar cells based on cadmium telluride (CdTe) are complex devices which have great potential for achieving high conversion efficiencies. Lack of understanding in materials issues and device physics slows down the rapid progress of these devices. This paper combines relevant results from the literature with new results from a research programme based on electro-plated CdS and CdTe. A wide range of analytical techniques was used to investigate the materials and device structures. It has been experimentally found that n-, i-and p-type CdTe can be grown easily by electroplating. These material layers consist of nano-and micro-rod type or columnar type grains, growing normal to the substrate. Stoichiometric materials exhibit the highest crystallinity and resistivity, and layers grown closer to these conditions show n → p or

Solar Energy Materials and Solar Cells, 2007

CdTe-based solar cells have long been of interest for terrestrial usage because of their high potential conversion efficiency (in the range of 18-24%) with low-cost manufacturability and concern over environmental effects. In order to conserve material and address environmental pollution concerns as well as to reduce carrier recombination loss throughout the absorber layer, efforts have been carried out to decrease the thickness of the CdTe absorption layer to 1 mm. As a result, to date, the experimental part of this study has realized cell efficiencies of 15.3% and 11.5% with 7 and 1.2-mm-thick CdTe layers, grown by close-spaced sublimation (195]. Since some problems remain with such thin 1 mm CdTe layers, possible methods to realize higher efficiency have been investigated using novel solar cell structures, with the help of numerical analyses tools. In the theory part of this study, numerical analysis with a 1-D simulation program named NSSP (Numerical Solar Cell Simulation Program) has been used to simulate these structures. We investigated the viability of CdTe thickness reduction to 1 mm together with the insertion of higher bandgap materials (i.e., ZnTe) at the back contacts to reduce carrier recombination loss there. The study shows potential results of the thickness reduction of CdTe absorption layer for a conventional CdS/CdTe/Cu-doped C structure with around 16% efficiency for cells below 3 mm CdTe. Decreases were found in spectral response that suggest from minority carrier recombination loss at the back contact interface. A higher band-gap material like ZnTe has been inserted to produce a back surface field (BSF) to inhibit the minority carrier loss at the back contact. An increase in the efficiency to about 20% has been found for a 1 mm-thin CdTe cell, which can be attributed to the increased BSF effect at the back contact of thinner CdTe-based cells.

Journal of vacuum science and technology, 2018

A study is reported comparing the electrical and optical properties of CdTe solar cells, prepared using CdS and CdSe buffer layers, to investigate defects in the bulk and interface, carrier transport, and recombination. Temperature dependent capacitance-voltage measurement and admittance spectroscopy were used to extract carrier concentration, resistivity, charge carrier mobility, and their temperature dependence. We identify the presence of two defect signatures corresponding to carrier freeze-out and the formation of a Schottky back-contact barrier. The back-contact barrier height (≈ 300 meV) extracted from the temperature dependent current density-voltage (JVT) experiment was confirmed by conventional admittance spectroscopy. The activation energies of mobility (resistivity) are 101.2 ± 2.5 meV (92.6 ± 2.3 meV) and 84.7 ± 2.7 meV (77.6 ± 4.5 meV) for CdS and CdSe buffer layers, respectively. Intensity dependent photoluminescence analysis demonstrates that the CdSe/CdTe device exhibits lower radiative efficiency than the CdS/CdTe device. This confirms the presence of higher defects in CdSe/CdTe device corroborated by temperature-dependent VOC analysis. The comparative electrical and optical analysis provides insight to improving the performance of CdTe solar cell device by selenization.

2020

This thesis presents a study on the optimisation of CdTe(1-X)SeX and MZO layers for CdTe PV applications. The first part of this work focused on the formation of the CdTe(1-X)SeX layers using CdSe layer and its impact on solar cell performance. Initially the incorporation of CdSe layer into conventional CdS/CdTe devices was investigated. This approach was found to be detrimental to all device parameters particularly JSC due to the formation of a CdS(1-X)SeX phase at the CdTe/CdSe/CdS interface. This phase increased the amount of parasitic absorption observed at short wavelength, reduced PV performance, and resulted in excessive void formation at the CdTe device interface. Replacement of CdS with SnO2 as the junction partner layer was found to increase the device photo response at both short and long wavelength due to removal of the CdS and efficient formation of the CdTe(1-X)SeX phase. This resulted in increased device performance of > 13% with notably high JSC values of > 29 ...

Thin Solid Films, 2015

Efficient thin film CdS/CdTe solar cell performance requires optimum parameters of each layer of this cell and of the barrier structure. Moreover, the effect of optical losses, recombination losses at front and back surface of CdTe and recombination losses in the space-charge region (SCR) must be considered in order to really analyze the role of these parameters on the performance of these cells. This work is focused on studying theoretically the effect of the thickness of the front contact (ITO), thickness of the window layer (CdS), thickness of the absorber layer (CdTe), width of the space-charge region and electron lifetime on the efficiency of CdS/CdTe solar cells. The reflection losses from interfaces and absorption losses in ITO and CdS, front and rear surface recombination losses of CdTe as well as recombination losses in SCR have been studied. It has been observed that the short-circuit current strongly depends on the thickness of ITO, thickness of CdS, thickness CdTe and electron lifetime. The concentration of uncompensated impurities (N a -N d ) in CdTe, which determines the width of SCR, plays a key role in the generation of photocurrent. The recombination losses in the SCR decrease rapidly with increasing the carrier lifetime in this region and can be ignored at lifetime of 10 -7 s. The reflectivity from the back contact introduces a small influence in increasing the short-current density particularly at thick absorber layer (5-8 μm). Under the conditions of N a -N d ~10 16 cm -3 , τ n = 10 -6 s, d CdTe = 8 μm, d ITO = 100 nm, and d CdS = 100 nm, the recombination and optical losses record their minimum ratio of 27%. Most of these losses (24%) are due to the optical losses. The efficiency of CdS/CdTe under these parameters is about 18.2% which is exactly matching with the recent experimental studies. Moreover, an ultrathin CdTe (= 1 μm) is sufficient to introduce high efficiency of 16.4%.

Current Applied Physics, 2003

Thin n-CdS and p-CdTe films were prepared by chemical spray pyrolysis and electrochemical deposition respectively. Excessive sulphur in the spray solution has promoted grain growth in CdS film. Microstructural features of CdS film with stoichiometric Cd:S concentration in the spray solution were more heterogeneous with grains, whereas film sprayed with excessive S show more uniformity, reduced grain boundary losses of current and improved shunt resistance through inhibition of leakage of current at narrow grain boundary or void site is expected and is indeed observed. Electrodeposition of CdTe films, beside the effect of the inherent process parameters, is also affected by crystalline and microstructural features of the underlying CdS. Nucleation of CdTe film is remarkably affected by CdS film spray deposited over glass substrate. Cell performance considerably depends upon the window layer CdS and the properties of sprayed CdS film depends considerably on the Cd:S ratio in the spray solution. A higher S content in the CdS film affects it optical transmission without changing the optical energy gap. This improves cell efficiency through reduction in CdS film resistivity. A typical increase in cell efficiency was found to increase from 8% to 10.5% using CdS film with Cd:S ratio as 1:1.1 and 1:1.3 respectively.

Progress in Photovoltaics: Research and Applications, 2012

The ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of nontransparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p-n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl 2-treatment procedure. For Cu-free cells, two activation treatments were applied, whereas Cu-containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe-CdS intermixing could be observed when the layers were treated consecutively. Remarkably high V OC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO 3 , Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu-free MoO 3 /Te buffer material, a V OC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained.

Polycrystalline thin film CdTe solar cell continues to be a leading candidate in the PV research and market because of its cost effectiveness and efficiency. In the conventional CdTe cell, polycrystalline cadmium sulfide (CdS) has been used as the best suited n-type heterojunction partner in the last few decades. This study demonstrates the use of novel CdS:O film as n-type heterojunction partner of CdTe cell, which has higher optical band gap (2.42-3.1eV), better lattice-match with CdTe and reduces the unwanted diffused layers than the poly-CdS layer. This novel CdS:O material is utilized in the baseline case of CdTe cell and a cell conversion efficiency as high as 18.5% (J sc =26.56 mA/cm 2 , Voc = 0.95 V and FF=0.8) has been found by numerical analysis utilizing AMPS-1D software. The cell normalized efficiency and Voc are found to decrease linearly at the operating temperature gradient of-0.2%/°C, indicating higher stability of the material at higher operating temperatures.

In this paper, various factors that affect the solar cell performances are investigated with emphasis given to the window layer (CdS:O) thickness, operating temperature and light intensity as well as the effect of front contact and BSF layers. Various parameters of solar cells such as quantum efficiency, series and shunt resistance associated with the front contact and BSF materials have been investigated also. Two materials ZnO and SnO 2 were used as front contact and two materials Sb 2 Te 3 and ZnTe were used as BSF layers to investigate their effects on ultra-thin cell performances. The results have shown that the BSF layers have no unfavourable effect on cell stability with temperature increase as the cell temperature coefficients with these BSF layers were 0.1% (ZnTe) to 0.3% (Sb 2 Te 3 ). However, the results of the comparisons demonstrate that the best option is Sb 2 Te 3 as BSF material, which exhibits lowest series resistance and behaves as an excellent minority carrier mirror whereas ZnTe showed unexpected series resistance problems because of the adverse heterojunction with CdTe.