Spin-enhanced organic bulk heterojunction photovoltaic solar cells

Abstract

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)C 61 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.

Anomalous organic magnetoresistance from competing carrier-spin-dependent interactions with localized electronic and nuclear spins

“…Our results are consistent with the nonmonotonic behavior seen in the efficiency versus doping fraction in P3HT/PCBM solar cells [13,14]. Low and high doped solar cells displayed roughly the same efficiency marked by a substantial increase as intermediate fractions.…”

“…Lower right: HOMO, LUMO, and SOMO levels for MEH-PPV and galvinoxyl as well as the electron affinity and ionization potential for galvinxoyl [13,15].…”

“…Our conclusions also support the tentative interpretations of the mechanism in doped organic solar cells in Refs. [13,14].…”

“…Even though the exact mechanism behind OMAR is still debated [12], it is widely believed to be related to hyperfine interactions and radical pair effects as described above, at least in some materials. The effects of additional radical dopant spins on these phenomena were first studied in the context of organic solar cells [13,14]. The efficiency and short circuit current of doped regio-regular poly(3-hexylthiophene)/1-(3-(methoxycarbonyl)propyl)-1-phenyl)(6,6)C 61 (P3HT/PCBM) solar cells were maximum for a certain doping percentage (∼ 3%), which was suggested to originate from galvinoxyl spins near the PCBM side of the P3HT/PCBM boundary that interacted with electrons to decrease recombination losses by facilitating intersystem crossings to triplet states.…”

“…Our conclusions also support the tentative interpretations of the mechanism in doped organic solar cells in Refs. [13,14].Galvinoxyl (a commercially available free radical with excellent stability; see Fig. 1 for its chemical structure) was purchased from Aldrich and added at varying concentrations to MEH-PPV solutions in toluene.…”