Graded NixAlyO hole transport layer in silicon solar cells

NiO alloyed with aluminum, Ni1-xAlxO, is analyzed in terms of its stoichiomery, electronic and transport properties, as well as interfacial band alignment with Si considering its potential use as a hole transport layer (HTL) in p-i-n type solar cells. The analysis is based on the component material and slab structural simulations, as well as simulated and measured angle-resolved valence-band photoemission spectroscopy (UV PES) data, in order to reveal the best suitable stoichiometry. It is concluded that the ionization energy from the highest occupied states tends to increase with Al content as the simulated work function grows from 4.1 eV in pure NiO to 4.7 eV in heavily alloyed Al0.50Ni0.50O. The electronic structure as a function of the interface design between crystalline silicon and the transport layer is used to assess the band lineup and its correlation with the discontinuity of the affinities. The self-energy of the hole carriers is estimated by contrasting simulated and measured UPS data, which in case of non-annealed Al-rich samples is shown to be by an order of magnitude higher due to the disorder effects. The work functions derived from the measured UPS data for the ALD grown oxide films with nearly same alloy stoichiometry correlate well with the simulated values. The findings suggest Ni coating of the back of silicon first, followed by a graded doping AlxNi1-xO, with x running from 1 at contact/oxide interface to 0 at oxide/semiconductor, as the best design for HTL.