Current solar cell contacts are made by screen-printing silver pastes and then sintering the silver particles together at high temperatures to form a conductive electrode. However, the high-temperature sintering step is costly and is not compatible with thin-film solar cell technologies. One solution is to use low-temperature reactive inks. Reactive inks print chemicals (instead of printing particles) that react at low temperatures, and can potentially produce dense materials with good electrical conductivity. Unfortunately, non-optimized reactive inks tend to produce high porosity in printed materials. This award supports fundamental research to enable development of new silver and copper reactive inks that can print conductive materials with very low porosity. These new inks can have applications in photovoltaics and low-cost flexible electronics.
The first research objective is to understand the effect of ink composition (including ligand type, reactant concentration, and solvent) on the chemical and fluid properties of reactive inks. Relevant ink properties under investigation include reaction activation energy, Arrhenius pre-exponential factor, heat-of-vaporization, diffusion coefficient, viscosity, and surface tension. This objective will be achieved by experimentally measuring relevant properties as ligand type (examples: [(NH3)2CH3CO2]-, [(NH3)2NO3]-, and 2-amino-2-methyl-1-propanol), reactant concentration (10 µmol/L to 1 mol/L), and solvent type (ethanol, water, glycerol, and 2,3-butandiol) are varied. Activation energy, Arrhenius pre-exponential factor, and heat of vaporization will be measured using a differential scanning calorimeter and a thermogravimetric analyzer; diffusion coefficient will be measured using chronoamperometry and cyclic voltammetry; viscosity will be measured using a microVISC rheometer; and surface tension will be measured using a custom goniometer. The second research objective is to understand the effect of ink composition and processing parameters on chemical potential distribution in evaporating reactive ink droplets. To achieve this objective, simulations will be conducted using Comsol Multiphysics equipped with heat transfer, computational fluid dynamics, chemical reaction engineering, and particle tracing modules. These simulations will predict the chemical potential distribution in evaporating reactive ink droplets as ink composition and processing parameters are varied. The third research objective is to understand the effect of reaction kinetics on particle nucleation and growth rate distributions in evaporating reactive ink droplets. To achieve this objective, particle nucleation and growth rate distributions will be modeled using the classical nucleation theory with activation energies taken from the literature. The reaction kinetics will be calculated using the measured reaction activation energy, the measured Arrhenius pre-exponential factor, and the simulated chemical potential distribution. Some predicted results will be compared with experimental observations.
