Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.

Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

[1]  Klaus Müllen,et al.  Exciton fission and fusion in bis(tetracene) molecules with different covalent linker structures. , 2007, Journal of the American Chemical Society.

[2]  R. Hesse,et al.  Absorption spectra of disordered solid tetracene and pentacene , 1980 .

[3]  Maurizio Prato,et al.  Excited-State Properties of C60 Fullerene Derivatives , 2000 .

[4]  Kelly P. Knutsen,et al.  Multiple exciton generation in colloidal silicon nanocrystals. , 2007, Nano letters.

[5]  Antoine Kahn,et al.  Energetics of metal–organic interfaces: New experiments and assessment of the field , 2009 .

[6]  Jean-Luc Brédas,et al.  Exciton-dissociation and charge-recombination processes in pentacene/C60 solar cells: theoretical insight into the impact of interface geometry. , 2009, Journal of the American Chemical Society.

[7]  A. S. Dhoot,et al.  Efficient exciton dissociation via two-step photoexcitation in polymeric semiconductors , 2001 .

[8]  M. G. Harrison,et al.  Field-effect transistor studies of precursor-pentacene thin films , 1997 .

[9]  David Beljonne,et al.  Electronic Structure and Geminate Pair Energetics at Organic–Organic Interfaces: The Case of Pentacene/C60 Heterojunctions , 2009 .

[10]  R. Friend,et al.  Ion-induced formation of charge-transfer states in conjugated polyelectrolytes. , 2009, Journal of the American Chemical Society.

[11]  A. A. Villaeys,et al.  Exciton dynamics in pentacene thin films studied by pump-probe spectroscopy , 1995 .

[12]  H. Bässler,et al.  Charge transfer transitions in solid tetracene and pentacene studied by electroabsorption , 1981 .

[13]  Igor Pugliesi,et al.  Ultrafast singlet and triplet dynamics in microcrystalline pentacene films , 2009 .

[14]  S. Arnold,et al.  Triplet exciton diffusion and double photon absorption in tetracene , 1977 .

[15]  M. Pope,et al.  Effect of Magnetic Field on the Fluorescence of Tetracene Crystals: Exciton Fission , 1969 .

[16]  T. Nishi,et al.  Determination of electron affinity of electron accepting molecules , 2009 .

[17]  Bernard Kippelen,et al.  Efficient thin-film organic solar cells based on pentacene/C60 heterojunctions , 2004 .

[18]  Paul M Zimmerman,et al.  Singlet fission in pentacene through multi-exciton quantum states. , 2010, Nature chemistry.

[19]  R. Friend,et al.  Subnanosecond geminate charge recombination in polymer-polymer photovoltaic devices. , 2010, Physical review letters.

[20]  R. Schaller,et al.  High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion. , 2004, Physical review letters.

[21]  B. Nickel,et al.  Ultrafast exciton relaxation in microcrystalline pentacene films. , 2007, Physical review letters.

[22]  P. Avakian,et al.  Spectroscopic Approach to Energetics of Exciton Fission and Fusion in Tetracene Crystals , 1971 .

[23]  Kurt V. Mikkelsen,et al.  Interpretation of the ultrafast photoinduced processes in pentacene thin films. , 2010, Journal of the American Chemical Society.

[24]  H. Queisser,et al.  Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells , 1961 .

[25]  Bernard Kippelen,et al.  Analysis of improved photovoltaic properties of pentacene/C60 organic solar cells: Effects of exciton blocking layer thickness and thermal annealing , 2007 .