A low cost electro-deposition procedure for growth of n- type cds semiconductor material used in fabrication of cds/cdte thin film solar cells

Abstract

CdS/CdTe thin film solar cells have exceeded the efficiencies of nearly 20% and 16% in laboratory and industrial scale devices respectively. Owing to the large band gap of 2.42 eV of CdS semiconductor, it is used as the window material in these devices. A low cost electro-deposition experimental procedure which utilizes electro-purified analytical grade chemicals and a method which does not produce wastes need to be discarded into the environment is described here. CdS thin films with thickness <100 nm was potentiostasticaly electro-deposited utilizing EG & G Princeton Applied Research Model 366 A bipotentiostat on fluorine doped tin oxide(FTO) conducting glasses which function as working electrodes in three electrode cells comprising of silver/silver chloride reference electrodes and graphite counter electrodes. Prior to use, conducting glasses were mechanically cleaned in a dust free environment. Electro-purified CdCl2 and Na2S2O3 were used as the Cd and S sources for depositing thin films of CdS on FTO glasses. Ranges of working electrode potentials, relative proportions of the two salts and the pH in the electroplating baths which could yield CdS thin films that showed satisfactory photovoltaic activities were estimated. Bath temperature and deposition time were kept at previously established values of 470C and 1 hour for all depositions. Also all CdS thin films were annealed at 400 0C for 10 minutes in air prior to estimation of their photovoltaic properties. It was found that, deposition potentials in the range of -1300 to �1500 mV with respect to silver/silver chloride electrode, concentration ratios of [Cd]/[S] = 10:1 to 15:1 having [Cd] concentrations in the range of 10 to 15 mmol dm-3 in solution and pH in the range of (1.2�1.4) in the plating bath could produce CdS thin films with optical band gaps lying in the range of (2.3 � 0.1) eV. X-ray diffraction studies showed that all CdS deposits were to consist of cubic lattice structure. Current- Voltage measurements indicated that, the thin films grown were of n-type in electrical conductivity. Photo-electrochemical cell experiments produced open circuit voltages and short circuit current densities in the ranges of -150 to -250 mV and 3.75 to 20 ?A cm-2 respectively indicating the level of photovoltaic activity that the deposits of CdS could demonstrate. Studies are in progress for further improvements of the electro-deposition procedure.