Investigation on the role of Sb doping on grain growth and properties of solution-processed CuIn(S,Se)2
Ho, John Chun Wan
Date of Issue2014
School of Materials Science and Engineering
Cu(In,Ga)(S,Se)2 (CIGSSe) has been one of the most promising materials used for photovoltaic application. Solution based synthesis of CuIn(S,Se)2 (CISSe) cells have achieved 12% efficiency, utilizing toxic hydrazine as the solvent. A shift towards an environmental friendly approach for CIS fabrication is thus necessary. In this thesis, solvothermal syntheses of wurtzite and chalcopyrite CIS nanoparticles are explored. Metastable wurtzite nanoparticles were successfully synthesized at much lower temperature, with a dual amine solvent under excess sulfur environment. Binary Cu9S5 and In2S3 nanoparticles were also synthesized and spray deposited to make a film. However, nanoparticle deposition resulted in rough and non-uniform film. Hence, aqueous spray pyrolysis of precursor solution was deployed to form CIS film directly on Mo substrate. Oxidation of Mo substrate is avoided by deposition of a precursor layer which acts as a barrier layer isolating Mo surface. The resultant film is dense and uniform but has very fine grains which are detrimental to carrier transport. A hybrid approach using nanoparticles incorporated into precursor solution for spray pyrolysis results in dense and uniform film, with significantly larger grains after selenization. However, the hybrid approach involves complex processing and precise experimental control. Hence, aqueous spray pyrolysis of precursor solution is a better option. High temperature annealing can enhance grain size of spray pyrolysed CIS film. Annealing under different vapors, namely S, Se and Ar, provides different extent of grain growth. Annealing in Se vapor, selenization, results in the most significant grain growth, due to the abundant formation of liquid CuxSe phases which enhances grain growth by liquid phase sintering. Annealing in S vapor is less efficient due to increased difficulty in liquid phase formation whereas annealing in inert Ar gas does not assist in grain growth. A high or saturated Se flux increases the probability of liquid phase existence, resulting in the most promising grain growth. Sb doping enhances grain growth during selenization by formation of low melting point Cu3SbSe4 compounds. Sb dopant most probably resided along grain boundaries after doping. Cu3SbSe4, which was formed along the grain boundaries during selenization, has a low melting point and exist as liquid phase facilitating atomic diffusion across grain boundaries. The formation of Cu3SbSe4 is further justified by selenization of CuSbS2 film formed by spray pyrolysis using Cu, Sb and S precursors. Cu3SbSe4 compounds provide additional liquid phase for effective sintering of the grains. Sufficient doping concentration enhances grain growth significantly by forming large grains of the same magnitude as the film thickness. The extent of grain growth is dependent on diffusion of dopants. Selective Sb doping at top layer of CIS creates a high dopant concentrate gradient, resulting in better grain growth due to increased dopant diffusion. The mechanism of grain growth is similar to liquid phase sintering of CISSe grains by formation of low melting point Cu3SbSe4 phases along the grain boundaries. Optical characterization on Sb doped CISSe film shows a negligible influence on the bandgap as formation of CuSbS2 is unlikely due to low doping level. Doped devices experienced superior J-V characteristics than undoped devices. An increase in doping level from 0.5 mol% to 1.0 mol% increased device performance further. In addition, selective doping of 1.0 mol% at the top layer proved to enhance device J-V characteristics relative to 0.333 mol% or 1.0mol% evenly doped sample. Cross-sectional TEM images show numerous pores within the film which are detrimental and a result of insufficient liquid phase for sintering. Selected area electron diffraction (SAED) pattern of a particular Sb doped CISSe grain gives an unknown superstructure, which indicates the presence of ordered defect compounds. Photoluminescence (PL) study conducted at low temperature indicated high potential fluctuation. Power law linear fit shows an excitonic recombination as the main recombination mechanism. High temperature PL intensity quenching fitting shows separation energy of 35 meV. Sb doping enhances carrier lifetimes, according to pump probe measurements. The enhanced lifetimes can be attributed to defect related recombination from less grain boundaries and the higher density of secondary (204)/(220) grain orientations which has lower density of non-radiative recombination centers. Conductive AFM mapping on Sb doped device with best device performance shows similar current map under positive bias and minimal current when the tip bias is negative, illustrating minimal leakage current in the device. Sb doping lowers carrier density, which might reduce probability of recombination, at the expense of lower carrier mobility.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films