Liquid silicon and its application in electronics

Abstract

We have been attempting to use liquid silicon (Si) in a solution process to develop semiconductor materials. We chose cyclopentasilane (CPS) as the raw material, which can be converted to poly(dihydrosilane) by photoinduced polymerization. Poly(dihydrosilane) is mixed with an organic solvent to form Si ink. We fabricated not only intrinsic Si ink but also both n- and p-type doped-Si inks. In the solution process, coating and pyrolysis are essential for device development. The parameters of these processes and the quality of the resultant solid film strongly depend on the properties and behavior of liquid Si, including those of CPS, poly(dihydrosilane), and Si ink. Here, we clarified the structure and properties of CPS, the photopolymerization of CPS, the structure of the polymer [poly(dihydrosilane)] in solution, the criteria for forming a uniform polymer film on a substrate, and the pyrolysis of a polymer film to an amorphous Si film. We also evaluated the properties of the resultant amorphous films. The quality of a solution-processed film was inferior to that of a vacuum-processed film just after the pyrolysis; however, it can be improved to a devicegrade film by hydrogen radical treatment. So far, the devices that we have developed with liquid Si include polycrystalline Si thin-film transistors (TFTs), single-grained Si-TFTs, and thin-film solar cells. For TFTs, their excellent properties have been demonstrated. In this review article, we introduce the development of solar cells using hydrogenated amorphous Si (a-Si:H) films for the p-i-n structure. We also show that the solutionprocessed a-Si:H solar cells exhibit 0.31-0.51% efficiency under AM-1.5G (100mW/cm 2) illumination. © 2014 The Japan Society of Applied Physics.