  Recently, much effort has been devoted to improve the efficiency of organic
photovoltaic solar cells based on blends of donors and acceptors molecules in
bulk heterojunction architecture. One of the major losses in organic
photovoltaic devices has been recombination of polaron pairs at the
donor/acceptor domain interfaces. Here, we present a novel method to suppress
polaron pair recombination at the donor/acceptor domain interfaces and thus
improve the organic photovoltaic solar cell efficiency, by doping the device
active layer with spin 1/2 radical galvinoxyl. At an optimal doping level of 3
wt%, the efficiency of a standard
poly(3-hexylthiophene)/1-(3-(methoxycarbonyl)propyl)-1-1-phenyl)(6,6)C61 solar
cell improves by 18%. A spin-flip mechanism is proposed and supported by
magneto-photocurrent measurements, as well as by density functional theory
calculations in which polaron pair recombination rate is suppressed by resonant
exchange interaction between the spin 1/2 radicals and charged acceptors, which
convert the polaron pair spin state from singlet to triplet.
