Cu implanted TiO2 based dye sensitized solar cells: Unraveling the effect of doping mechanism and type of metal ion on the photovoltaic properties

Abstract

Among noble metals, doping of copper (Cu) in TiO2 for dye sensitized solar cells (DSSCs) has always been controversial exhibiting both positive and negative results throughout the literature due to its relatively lesser known doping mechanism. We, hereby, have tried to resolve the controversy and provide detailed information on conditions under which Cu can increase performance of DSSCs. Firstly, for Cu doping, a very controlled and precise doping technique of ion implantation was employed in TiO2 photoanodes at variable fluence. From the X-ray diffraction spectra, Cu ions were found to be present at substitutional sites in TiO2 matrix. Interestingly X-ray photoelectron spectra reveal that lower Cu fluences result in production of Cu+ ion whereas at higher fluence Cu2+ ion starts dominating. Further, optical studies reveal that introduction of Cu2+ at higher fluence suffer from poor dye loading and higher recombinations due to distorted morphology and improper band alignment. Hence, DSSC prepared with maximum Cu+ content (fluence of 5 × 1014 ions.cm−2) exhibits efficiency of 7.35 % as compared to 6.22 % of pristine unimplanted DSSC, due to higher light harvesting, upward fermi level and lower recombinations. Further, DSSC with Cu2+ ion show poor performance (5.61 %) due to lower electron injection efficiency, despite of superior open circuit voltage values.