Microstructural characterization of front-side Ag contact of crystalline Si solar cells with lightly doped emitter

Crystalline Si (c-Si) solar cell production has reached an annual scale of ~20 GW globally. Development of this leading technology has been boosted by continuous innovation in material science and reduced material and processing costs. An example of such innovation is the step-wise progression to more lightly doped emitters (LDE) that reduces recombination in the solar cell. Continuous improvement in front-side (FS) metallization pastes has enabled this progression to lower series resistance and higher cell efficiency. We report here homogeneous emitter LDE cells with efficiencies as high as 18.9%, printed with advanced FS Ag paste. A clear understanding of the microstructure of the interfacial region between Ag contact and Si emitter, and the associated electrical conduction mechanism of LDE cells can provide the guidance needed to drive overall efficiency higher and end-user cost lower. We report our latest investigation of the microstructure of the interface between FS Ag contact and lightly-doped emitter using scanning electron microscopy techniques. The microstructural features such as nano-Ag colloids, interfacial glass, and Ag crystallites are studied in detail. The relationship between microstructure, cell performance, and current conduction mechanism for LDE cells are discussed.