Autodiffused Boron Emitter for N-Type Monocrystalline Si Thin-Film Solar Cells

The generation of an emitter in monocrystalline silicon thin-film solar cells by out-diffusion of dopant atoms from the growth substrate into the epitaxial layer is demonstrated. Starting with a boron-doped p ⁺-type substrate, a porous silicon surface layer is created to permit the layer transfer (PSI process). A p⁺-type emitter automatically forms by out-diffusion of boron atoms during the epitaxial growth of n-type silicon films. This "autodiffusion" process creates an emitter with a moderate surface concentration of 3times10¹⁸ cm^-3 and a junction depth of 1.1 mum. The sheet resistance is 330 Omega/square. n-type thin-film solar cells with an autodiffused boron emitter on the rear side of the cells are fabricated. An independently confirmed energy conversion efficiency of 14.5% with a short circuit current density of 33.3 mA/cm² as measured under standard testing conditions is achieved for a 4 cm² large cell with a thickness of 24 mum