Microstructures of front-side Ag contact of crystalline Si solar cells fired at temperatures from below to above optimal were systematically investigated using advanced electron microscopy. Ag pastes studied included commercial pastes and an experimental paste containing nano-sized metallic Zn additive. Microstructures of optimally fired cells determined from cross-sectional and top-view images were found to be consistent with a primary tunneling mechanism for current flow (a “nano-Ag colloids assisted tunneling” model) due to the lack of Ag crystallites connecting the silver conductor line to the silicon emitter. We mapped the evolution of the interfacial microstructures across the firing temperature window and correlated it with cell performance. Our result sheds light on the relative importance of the two current transport models as the peak firing temperature was swept from below to above optimal.

Four types of bis(imino)pyridine ligands and their corresponding Fe complexes with the same metaaryl pattern were synthesized for high-temperature R-olefin production (A: 9 and 13; B: 18, 19, 20 and 21, 22, 23, 24; C: 27 and 30; and D: 33 and 34). Pattern A (9 and 13) consists of four meta-aryl groups without ortho-methyls. The coordination of FeCl 2 with the ligand [3,5-bis(4-F-Ph) 2 -Ph-NdC(Me)-Py-C(Me)dN-Ph-3,5-bis(4-F-Ph) 2 ] (9) of type A resulted in the isolation of ion-paired complexes [L 2 Fe] 2+ FeCl 4 2-13. Coordination of ligands 18, 19, and 20 of pattern B with two meta-aryl groups on the same side with two ortho-methyls of the imino aryl group afforded the following 1:1 complexes:}FeCl 3 ] (24). Reduction of trivalent complex 24 in THF solution yields divalent complex 22. Ligand 27 of pattern C contains two meta-aryl groups on the side opposite two ortho-methyls of the imino aryl group and coordinates with FeCl 2 to form complex [{5-(3,5-bis-(CF 3 ) 2 -Ph)-2-Me-Ph-NdC(Me)-Py-C(Me)dN-Ph-2-Me-5-(3,5-bis-(CF 3 ) 2 -Ph)}FeCl 2 ] (30). Complex 34 of unsymmetrical pattern D has two meta-aryls in one imino aryl group and two ortho-methyls in the second imino aryl group: [{2,6-dimethyl-4-(3,5-bis-(CF 3 ) 2 -Ph)-2-Me-Ph-NdC(Me)-Py-C(Me)dN-Ph-2-Me-3,5-bis(3,5-bis-(CF 3 ) 2 -Ph)}FeCl 2 ]. The introduction of aryl groups into the meta positions of all four types of bis(imino)pyridine ligands was accomplished by palladium-catalyzed Suzuki cross-coupling reaction between aryl boronic acid 5, 16, and 17 and the appropriate meta-bromo-substituted bis(imino)pyridine ligands 4, 15, 25, and 32. Di-tert-butyl(2,2-dimethylpropyl)phosphane (7) and benzyl-di-tert-butylphosphane (28) were used as ligands for the Suzuki coupling. According to X-ray analysis, there are shortenings and therefore strengthening of the axial Fe-N bond lengths (up to 0.02 Å) in complex 22 of type B versus the Fe(II) complex without meta-aryl groups [{o-Me-Ph-NdC(Me)-Py-C(Me)dN-Ph-o-Me}FeCl 2 ](1). Complexes of patterns B and D (21, 22, 23, 24, and 34) afforded very active catalysts for the production of R-olefins with more ideal SchultzFlory distributions of R-olefins and with higher K values than the parent methyl-substituted Fe(II) complex 1 without meta-aryl groups.