Improvement of Photovoltaic Response Using Triplet Excitons
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0974-CC04-05
Improvement of Photovoltaic Response Using Triplet Excitons Zhihua Xu and Bin Hu University of Tennessee, knoxville, TN, 37996
ABSTRACT We report an enhancement of photovoltaic response by dispersing phosphorescent dye fac tris (2-phenylpyridine) iridium (Ir(ppy)3) in organic solar cells of poly[2-methoxy-5-(2’ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6] C61 (PCBM). It is known that photoexcitation generates both singlet and triplet states through intersystem crossing caused by hyperfine or spinorbital coupling. Due to long diffusion length the triplet excitons can migrate from their generation sites to the interfaces of donor-acceptor interaction and directly dissociate into charge carriers. We found, based on the studies of magnetic field-dependent photocurrent, that the dispersed Ir(ppy)3 molecules increase the spin-orbital coupling strength and triplet density in the MEH-PPV matrix due to the penetration of MEH-PPV π electrons into the large field of orbital dipoles of the Ir(ppy)3. Especially, the triplet excitons facilitate the direct dissociation into charge carriers at the donor-acceptor interacting interfaces in the composite of MEH-PPV and PCBM, and consequently improve the photovoltaic response in organic solar cells.
INTRODUCTION Since last decade organic photovoltaic (PV) cells have attracted much interest in developing low cost, large scale, flexible solar energy-electricity conversion devices [1-6]. In organic semiconducting materials, singlet excitons are first generated upon light absorption. However, the hyperfine interaction or spin-orbital coupling can flip spin polarization of electrons and thus induces intersystem system crossing between singlet and triplet excitons. As a consequence, singlet and triplet excitons coexist in organic semiconducting materials under photon absorption. In bulk or layered heterojuction PV cells, the donor-acceptor interaction can overcome the binding energies of excitons and dissociates them into charge carriers for the generation of photocurrent. Due to the long lifetime and diffusion length, the triplet exciton can more effectively diffuse to the locations of donor-acceptor interaction for dissociation before they vanish through nonradiation. Therefore, control of triplet states may form a mechanism to enhance the PV response in organic solar cells [7-10]. We know that increasing spin-orbital coupling can boost the triplet density through the singlet-triplet intersystem crossing in organic materials. There are two ways to increase the spin-orbital coupling: chemically attaching heavy metal atoms to organic molecules [11], namely internal heavy atom effect or physically dispersing heavy metal particles into organic materials [12,13], so called “external heavy atom
effect”. The internal heavy-atom effect requires delicate organometallic reactions to systematically change the spin-orbital coupling strength. The external heavy-atom effect can be readily obtained
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