Ni dopants typically exist as Ni2+ oxidation state in metal oxides. We report that as Ni is co-doped with P in Ni(P):SnO2, P promotes Ni2+ into Ni3+, which acts as acceptor and coverts n-type SnO2 to p-type conduction. Significant hole concentration (4.5 x 10(18) cm(-3)) of p-type Ni(P):SnO2 can be obtained. The chemical state of Ni3+ in Ni(P):SnO2 is verified by ultra-violet and x-ray photoelectron spectroscopy. With the developed p-Ni(P):SnO2, transparent p-Ni(P):SnO2/i-SnO2/n-ITO p-i-n structure was fabricated and characterized: low leakage-current (8.41 x 10(-5) A at -5 V), turn-on voltage (4.68 eV), good transmittance (85%), and small ideality-factor (1.73). (C) 2015 Elsevier B.V. All rights reserved.