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Preparation and investigation of heterojunction solar cells  p-Si/n-Cd 1-x Zn x S 1-y Te y

Preparation and investigation of heterojunction solar cells  p-Si/n-Cd 1-x Zn x S 1-y Te y H.M.MAMEDOV Baku State University.

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Preparation and investigation of heterojunction solar cells  p-Si/n-Cd 1-x Zn x S 1-y Te y

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  1. Preparation and investigationof heterojunction solar cells p-Si/n-Cd1-xZnxS1-yTey H.M.MAMEDOV Baku State University

  2. It is well known that, films with nano-sized grains embedded inside are called nano-granular thin films. As a branch of the family of nanostructured materials, the structures and properties of nano-granular films differ from those of the micron, submicron polycrystalline thin films and amorphous films with the same composition. This should be attributed to their ultrafine grains and up to 50% atoms located on the boundary [1], which gives rise to some special effects exclusive to nanostructured materials. Therefore, study of the nano-granular thin films is gaining increasing attention. Many nano-structured materials are now being investigated for their potential applications in photovoltaic, electro-optical, micromechanical and sensor devices [2–3]. Our interest lies in taking advantage of the benefits offered by nanotechnology to make inexpensive and efficient solar cells on a large scale. Nanostructured layers in thin film solar cells offer three important advantages. • light generated electrons and holes need to travel over a much shorter path and thus recombination losses are greatly reduced. As a result, the absorber layer thickness in nano-structured solar cells can be as thin as 150 nm instead of several micrometers in the traditional thin film solar cells [4]. • the energy band gap of various layers can be tailored to the desired design value by varying the size of nano-particles. This allows for more design flexibility in the absorber and window layers in the solar cell. In particular nano-structured CdS, CdTe and TiO2 are of interest as window and absorber layers in thin film solar cells [5–7]. • due to multiple reflections, the effective optical path for absorption is much larger than the actual film thickness. 1. H. Gleiter, NanoStructured Mater. 6 (1995) 3-7. 2. Xiangfeng Duan, Chunming Niu, Vijendra Sahi, Jian Chen, J.W. Parce, S. Empedocles, J. Goldman, Nature 425 (2003) 274–278. 3. M.C. McAlpine, R.S. Friedman, Song Jin, Keng-hui Lin, Wayne U. Wang, C.M. Lieber, Nano Lett. 3 (2003) 1531–1535.

  3. According to results in scientific literature and to our results, it is possible to change the energy gap width, i.e. photosensitivity region (0.3-1.5 m) of solar cells by fixing lattice parameter of films Cd1-xZnxS1-yTey (0x1; 0y1) (in accordance with lattice parameters of absorber layer Si). The replacement of binary films with the higher energy band gap Cd1-xZnxS1-yTey allows has led to a decrease in window absorption losses and has resulted in an increase in the short-circuit current in the solarcell. Our primary investigations shows that it is possible to prepare thin films with different size (0.6-1 cm2) and surface morphology by controlling the electrochemical depositing regime and composition of reaction solution. Also it is possible to change the size (from nano up to micro) of grainsat film surface by controlling the cathodic voltage and heat treatment regime (temperature and duration). Thin film heterojunctions Si/A2B6currently is manufacture by different methods (magnetron sputtering, vapor-phase epitaxy, ion implantation ets.). However, complexity of technological cycle, difficulties arising at manufacturing of structures with great active area results in high cost price of the solar cells. Method of electrochemical deposition allows eliminating these difficulties. It is easily to control the method of electrochemical deposition because the process carried out at room temperature. Also, choice of an optimum thickness of films for decrease of series resistance of solar cells, is easily by the method of electrochemical deposition. This method does not demand moreover expensive technological cycles and equipments, allows preparing thin films on the surface of substrates with any geometrical form, structure and size. 4.K. Ernst, A. Belaidi, R. Konenkamp, Semicond. Sci. Technol. 18 (2003) 475. 5. V.P. Singh, R.S. Singh, G.W. Thompson, V. Jayaraman, S. Sanagapalli, V.K. Rangari, Sol. Energy Mater. Sol. Cells 81 (2004) 293–303. 6. V.P. Singh, J.C. McClure, Sol. Energy Mater. Sol. Cells 76 (2003) 369–385. 7. D.L. Linam, V.P. Singh, J.C. McClure, G.B. Lush, X. Mathew, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 70 (2001) 335–344.

  4. The process of electrodeposition of films CdZnSTe in the composition range 0 <x< 1 and 0 <y < 1 form aqueous solution of : CdCl2+ZnCl2+Na2S2O3+TeO2+H2O onto Si substrates, with resistivity p = 0.8 Ohmcm, was carried out at room temperature and 80 C. A carbon rod and a saturated calomel electrode were used as an anode and a reference electrode, respectively. The thickness of the substrates was varying within 0.6 – 0.8 mm. The surface of the substrates were etched in an aqueous solution of KOH+KNO3 (1:3) mixture and further washed in distilled water, which it was maintained at various temperatures in the range of 250 – 500С. The deposition potential was controlled at a value between – 0.4 to 2.5 V for the various values of x and y. The current density during the deposition was increased from 8 to 45 mA/cm2, with the increase of negative deposition potential. Depending on the deposition duration the thickness of the films was 0.1 – 9 m. The composition of the mixed films altered with the concentration of Zn and Te. Concentration of charge carriers at 300 K was 21016 – 81016 cm-3 depending on the x and y. Evaporated Al and In was used as a low resistance front contact to the Si and to the CdZnSTefilms, respectively. The active area of heterojunctions was 0.82 cm2.

  5. Cyclic voltammogram for (a) 0.01M CdCl2, (b) 0.09M ZnCl2, (c1) 0.02M Na2S2O3, (c2) 0.08M TeO2and (d) mixture of (a), (b) and (c) solutions at room temperature

  6. Dependence of film thickness on the deposition time

  7. TGA and DSC curves of Cd0.25Zn0.75S0.8Te0.2 thin films TGA: 1- air; 2- oxygen; 3- argone DSC: 4- air

  8. d) e) m) AFM images of Cd0.25Zn0.75S0.8Te0.2 thin films at various cathodic potentials (a, b, c) and at various regimes of heat treatment (d, e, m) (350 C for 8 min) –0.5 V; b) –0.7 V; c) –0.88 V d) air HT; e) oxygen HT; m) argon HT

  9. X-ray diffraction patterns of Cd0.25Zn0.75S0.8Te0.2films at various deposition potentials 1) – 0.5 V ; 2) –0.7 V; 3) –0.88 V

  10. X-ray diffraction patterns of Cd0.25Zn0.75S0.8Te0.2films at various heat tratment regimes 1) before HT; 2) air HT; 3) oxygen HT; 4) argon HT

  11. Optical transmittance spectrums of Cd0.25Zn0.75S0.8Te0.2 films before and after HT in air, oxygen and argone at 3500C for 8 min

  12. Optical transmittance spectrums of Cd1-xZnxS1-yTey films for various films compositions, after HT in argone at 3500C for 8 min 

  13. Temperature dependences of dark and photoconductivity of the Cd0.25Zn0.75S0.8Te0.2 thin films

  14. CdZn ST e Al In 10 mm h 8,2 mm Si

  15. DarkJ-V curves forp- Si/n- Cd0.25Zn0.75S0.8Te0.2 heterojunctions

  16. Semilogarithmic plot of forward bias of J-V curves for p- Si/n- Cd0.25Zn0.75S0.8Te0.2heterojunctions at different temperatures.

  17. Capacitance-voltagedependences for heterostructuresfor p- Si/n- Cd0.25Zn0.75S0.8Te0.2heterojunctions

  18. Spectral distribution of photocurrent in p-Si/Cd0.25Zn0.75S0.8Te0.2 thin films heterojunctions after HT in argon for 3500C, 8 min

  19. J-V curves under illumination AM 1.5 of the p- Si/n- Cd0.25Zn0.75S0.8Te0.2heterojunctions

  20. Thank You for your attention! E-mail: mhhuseyng@yahoo.co.uk

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