Scientific complications and controversies noted in the field of CdS/CdTe thin film solar cells and the way forward for further development

Cadmium telluride-based solar cell is the most successfully commercialised thin film solar cell today. The laboratory-scale small devices have achieved ~ 22%, and commercial solar panels have reached ~ 18% conversion efficiencies. However, there are various technical complications and some notable scientific contradictions that appear in the scientific literature published since the early 1970s. This review paper discusses some of these major complications and controversies in order to focus future research on issues of material growth and characterisation, post-growth processing, device architectures and interpretation of the results. Although CdTe can be grown using more than 14 different growth techniques, successful commercialisation has been taken place using close-space sublimation and electrodeposition techniques only. The experimental results presented in this review are mainly based on electrodeposition. Historical trends of research and commercial successes have also been discussed compared to the timeline of novel breakthroughs in this field. Deeper understanding of these issues may lead to further increase in conversion efficiencies of this solar cell. Some novel ideas for further development of thin film solar cells are also discussed towards the end of this paper

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

Morphology Controlled n-Type TiO2 and Stoichiometry Adjusted p-Type Cu2ZnSnS4 Thin Films for Photovoltaic Applications

This paper presents the fabrication and characterization of stoichiometry adjusted Cu2Zn1.5Sn1.2S4.4 thin film (FTO/TiO2/CdS/CZTS/Au) photovoltaic (PV) devices. The PV devices were developed using the window layer of rutile TiO2 nanoarchitecture arrays, i.e., one-dimensional (1D) nanorods and three-dimensional (3D) combined/hierarchical structures (nanorods with microspheres). One-dimensional (1D) nanorods and 3D combined structures of TiO2 window layers were synthesized by a hydrothermal method with different solvents without any assistance of surfactants and templates. We achieved two kinds of TiO2 nanostructures by tuning the precursor concentrations and volume by keeping a constant growth time and temperature. The detailed structural properties were studied using X-ray diffraction and high resolution transmission electron microscopy. Phase formation and chemical state of the prepared samples were examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The surface morphology and luminescence studies of TiO2 nanostructures were analyzed using field emission scanning electron microscopy and cathodoluminescence techniques. The current voltage performance of fabricated devices were measured under an AM 1.5 solar simulator. It is observed that combined structure PV device shows better efficiency (1.45%) than the nanorods alone structure (0.55%). Present work is a first attempt made to construct the inverted CZTS based solar cells. This study establishes the window layer of hierarchical TiO2 nanostructures based morphology that offers a great potential for the development of high-efficiency nonstoichiometric CZTS based solar cells

Effect of thickness variation of hole injection and hole blocking layers on the performance of fluorescent green organic light emitting diodes

In this paper we present the effect of thickness variation of hole injection and hole blocking layers on the performance of fluorescent green organic light emitting diodes (OLEDs). A number of OLED devices have been fabricated with combinations of hole injecting and hole blocking layers of varying thicknesses. Even though hole blocking and hole injection layers have opposite functions, yet there is a particular combination of their thicknesses when they function in conjunction and luminous efficiency and power efficiency are maximized. The optimum thickness of CuPc (Copper(II) phthalocyanine) layer, used as hole injection layer and BCP (2,9 dimethyl-4,7-diphenyl-1,10-phenanthroline) used as hole blocking layer were found to be 18 nm and 10 nm respectively. It is with this delicate adjustment of thicknesses, charge balancing is achieved and luminous efficiency and power efficiency were optimized. The maximum luminous efficiency of 3.82 cd/A at a current density of 24.45 mA/cm(2) and maximum power efficiency of 2.61 lm/W at a current density of 5.3 mA/cm(2) were achieved. We obtained luminance of 5993 cd/m(2) when current density was 140 mA/cm(2). The EL spectra was obtained for the LEDs and found that it has a peaking at 524 nm of wavelength. (C) 2012 Elsevier B.V. All rights reserved

Impact of spray flux density and vacuum annealing on the transparent conducting properties of doubly doped (Sn plus F) zinc oxide films deposited using a simplified spray technique

In this investigation transparent conducting properties of as-deposited and annealed ZnO:Sn:F films deposited using different spray flux density by changing the solvent volume (10 mL, 20 mL ... 50 mL) of the starting solutions have been studied and reported. The structural analyses of the films indicate that all the films have hexagonal wurtzite structure of ZnO with preferential orientation along (002) plane irrespective of the solvent volume and annealing treatment whereas, the overall crystalline quality of the films is found to be enhanced with the increase in solvent volume as well as with annealing. This observed enhancement is strongly supported by the optical and surface morphological results. From the measurements of electrical parameters, it is seen that, the annealed films exhibit better electrical properties compared to the as-deposited ones. Annealing has caused agglomeration of grains as confirmed by the surface morphological studies. Also, the annealing process has led to an improvement in the optical transparency as well as band gap. It is found from the analyses of the characteristics of the as- deposited and annealed films that the annealed film deposited from starting solution having solvent volume of 50 mL is optimal in all respects, as it possesses all the desirable characteristics including the quality factor (1.60 x 10(-4) (Omega/sq.)(-1)). (C) 2014 Elsevier Ltd. All rights reserved