Preparation And Characterization Of Thin Film Photovoltaic Cells With Silver-plated Copper Powder Conductive Glue

Photovoltaic (PV) industry has become the fastest growing industries of renewable energy after wind power generation in China. PV technology is one focus of global research. PV products are divided into crystalline silicon solar cells and thin film solar cells. Thin film solar cells have a greater advantage in cost reduction. Firstly, it can greatly save the expensive semiconductor material; secondly, the thin film solar cells are forming with material preparation that saves a lot of processes; thirdly, thin film solar cells using low temperature technology, which is not only conducive to energy saving but also easy to use low-cost substrates. Low packaging temperature of solar module and non-solderability of substrate are the factors restricting the development of thin film solar cells.Conductive adhesive as a conventional solder alloy alternatives has aroused wide concern for the advantages of environmentally friendly, low operating temperature, fine pitch and less process steps. Heat-curable isotropic conductive adhesives (ICAs) can meet the needs of connectivity between the solar module and circuit in the thin film solar cell packaging process. ICAs are a promising connection material. Silver-plated copper powder filled ICAs is focused on more and more for whose price cheaper and conductivity better than silver powder filled ICAs. In order to improve the stability and reduce the cost of ICAs, this paper focuses on the preparation and property of ICAs for thin film solar cells use.1. In the present paper, ICAs was synthesized by using E-44 epoxy resin as the substrate of ICAs, triethylenetetramine (TETA) as curing agent, silver-plated copper powder and KH-550 silane coupling agent. The adhesion strength and conductive performance were studied by using pull and push dynamometer and digital ohmmeter respectively.2. The optimum formulations and process conditions of ICAs were that epoxy resin:curing agent equal to 100:13, silane coupling agent:25wt% silver-plated copper powder equal to 5:100, conductive filler accounts for 60% total mass of system. A wet film was cured for 40 minutes at 70℃in a drying oven. The conductive adhesive adhesion strength reached to 9.55MPa, and corresponding bulk resistance rate was 1.548×10-3Ω·cm.3. The resistance electrochemical migration (ECM) of different silver content copper was studied by applying constant voltage with an electrochemical workstation in epoxy resin and screen ink resin respectively. Distilled water was used as the test environment. The ability of resistance ECM was measured by Time to failure (TTF) and failure current. The copper powder of silver content equal or less than 25wt% showed the better resistance.4. Digital images showed that dendrite growth occurs as a result of dissolution of metal ions at the anode, which then plate out at cathode. SEM morphology of the ECM products showed that the primary and the secondary dendrite of the pure Ag is bolder and longer than those in silver-plated copper. EDS analysis of all ECM products, it was pointed out that the general copper/silver mass ratio in dendrites was reduced from cathode to anode. From the XRD pattern can be known that the ECM products of silver are metallic silver. The ECM products of silver-plated copper are mainly metallic cooper. The Cu2O has been found on the anode.5. The ECM inhibited mechanism was studied through electrochemical impedance spectroscopy (EIS) and Tafel polarization in 0.01mol/L NaOH with different contents of silver plating on copper powder. The results showed that the sacrifice anodic copper restrains cathodic silver dissolving in solution when in natural state so as to minimize the occurrence of silver migration. The anodic polarization effect has made the system of silver-plated copper powder differential dissolution when under DC bias.