Patterning of nanopillars-based CdS/CdTe thin films for photonic applications

In this study we propose a new design to place the contacts for both p-type and n-type semiconductors at the bottom of solar cell to increase photons entering the absorber layer. The fabricated device contains periodic nanopillars of similar to 243 nm radius and centre to centre pillar distance of similar to 550 nm. In addition to the fabrication, thin films of n-CdS and p-CdTe have been characterised for their material properties at different annealing temperatures in this study. The X-ray diffraction pattern for CdTe and CdS revealed that the growth is preferentially along the (111) plane with a cubic zinc blend and cubic structure, respectively. Optimum absorption of the solar spectrum for a single junction solar cell occurs with the optical band gap of 2.36 eV for CdS and 1.45 eV for CdTe. The significance of the work creates a new beginning for the fabrication of textured thin film photovoltaic cell

Novel patterning of CdS/CdTe thin film with back contacts for photovoltaic application

The heterostructure of patterned CdS/CdTe thin films with back contact have been devised with electron beam lithography and fabricated using sputter deposition technique. The metallic contacts for n-CdS and p-CdTe are patterned such that both are placed at the bottom of the cell. This avoids losses due to contact shading and increases absorption in the window layer. Patterning of the device surface helps in increasing the junction area which can modulate the absorption of more number of photons due to total internal reflection. Computing the surface area between a planar and a patterned device has revealed 133% increase in the junction area. The physical and optical properties of the sputter-deposited CdS/CdTe layers are also presented. J-V characteristics of the solar cell showed the fill factor to be 25.9%, open circuit voltage to be 17 mV and short-circuit current density to be 113.68 A/m(2). The increase in surface area is directly related to the increase in the short circuit current of the photovoltaic cell, which is observed from the results of simulated model in Atlas/Silvaco

Fabrication of back contacts using laser writer and photolithography for inscribing textured solar cells

Semiconductor fabrication process begins with photolithography. Preparing a photo mask is the key process step in photolithography. The photo mask was fabricated by inscribing patterns directly onto a soda lime glass with the help of a laser beam, as it is easily controllable. Laser writer LW405-A was used for preparing the mask in this study. Exposure wavelength of 405 nm was used, with which 1.2 mu m feature size can be written in direct write-mode over the soda lime glass plate. The advantage of using the fabricated mask is that it can be used to design back contacts for thin film Photovoltaic (PV) solar cells. To investigate the process capability of LW405-A, same pattern with different line widths was written on soda lime glass samples at different writing speeds. The pattern was inscribed without proximity effect and stitching errors, which was characterized using optical microscope and field emission scanning electron microscope (FE-SEM). It was proven that writing speed of a mask-writer is decided according to the intended feature size and line width. As the writing speed increases, the edges of the patterns become rougher due to uneven scattering of the laser beam. From the fabricated mask, the solar cell can be developed embedding both the contacts at the bottom layer, to increase the absorption of solar radiation on the top surface effectively by increasing light absorption area

Anodization of Aluminium using a fast two-step process

Ultra-fast two-step anodization method is developed for obtaining ordered nano-pores on aluminium (Al) foil. First anodization was carried out for 10 min, followed by 3 min of second anodization at high voltage (150 V) compared to previous reports of anodization times of 12 h (40-60 V). The pore dimensions on anodized alumina are 180 nm for pore diameter and 130 nm for inter-pore distance. It was evident that by increasing the anodization voltage to 150 V, the diameter of the pores formed was above 150 nm. The electrolyte and its temperature affect the shape and size of the pore formation. At lower anodization temperature, controlled pore formation was observed. The anodized samples were characterized using the field emission scanning electron microscope (FE-SEM) to determine the pore diameter and inter-pore distance. Using UV-Visible spectroscopy, the reflectance spectra of anodized samples were measured. The alumina (Al2O3) peaks were identified by x-ray diffraction (XRD) technique. The x-ray photo electron spectroscopy (XPS) analysis confirmed the Al 2p peak at 73.1 eV along with the oxygen O 1s at 530.9 eV and carbon traces C 1s at 283.6 eV

Effect of CuPc and PEDOT:PSS as hole transport layers in planar heterojunction CdS/CdTe solar cell

This paper presents the fabrication and photovoltaic performance of new architecture based planar heterojunction CdS/CdTe thin film solar cells which were employed with two hole transport layers (PEDOT:PSS as HTL1 and CuPc as HTL2). The reported solar cells were fabricated through various deposition techniques such as sputtering, thermal evaporation, spin coating and characterized by FESEM, AFM, XPS, UPS and AM 1.5 solar simulator. The interfacial layer growth and chemical state identification of the deposited thin films were studied by cross-sectional FESEM and XPS techniques. The band bending occurs between absorbing and transporting layer helps to inject the excited charge carriers effectively into electrode that was explained using UPS analysis. The present work intends to explain the role of additional window layer (TiO2), buffer layer (CdS) and hole transporting layers (PEDOT:PSS and CuPc) in the novel device architecture. Further, these findings will offer new research directions to address the double hole transport (back contact) layers selection concept in CdS/CdTe heterojunction based solar cells