Conformal P-N junctions in macroporous silicon for photovoltaic energy conversion

We report on the fabrication of a macroporous silicon diode that successfully operates in a photovoltaic mode of energy conversion. Typical device structures are fabricated on 4″, Cz grown, , p-type, 5 – 30 Ω-cm, Si substrates and make use of random or pre-patterned macroporous silicon films up to 100 μm thick with pores ∼1 μm in diameter and a center-to-center pore spacing of 2.5 μm. Phosphorous dopants are introduced throughout the porous silicon region with a newly adapted technique based on proximity rapid thermal diffusion. This development effort has enhanced the uniformity of dopant diffusion into the pores and subsequently improved device performance. The resulting p-n diode within the porous silicon is of a three-dimensional nature and exhibits an extremely high internal surface area up to ∼6690 cm2/cm3. To our knowledge, this is the first use of such a technique to fabricate conformal, three-dimensional p-n junctions for photovoltaic applications. The large internal surface area of this diode makes it applicable to next generation photovoltaics. Moreover, the porous silicon within this device is not simply an antireflection coating; rather it is a light trapping medium that has the ability to become a large surface area diode and can serve as a host matrix for photoactive compounds. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)