Formation of the Donor–Acceptor Charge-Transfer Exciton and Its Contribution to Charge Photogeneration and Recombination in Small-Molecule Bulk Heterojunctions

We report on the formation of charge-transfer (CT) excitons and their effect on photocurrent dynamics in composites with a fluorinated anthradithiophene (ADT-TES-F) donor (D) and acceptors (A) with (i) various LUMO energy offsets with respect to that of the donor (ΔLUMO) and (ii) several different side groups that modify spatial D/A separation at the D/A interface. Exciplexes and nonemissive CT excitons were formed in composites with ΔLUMO 0.6 eV, respectively. A competition between fast charge carrier photogeneration and CT exciton formation was observed, with outcomes depending on the applied electric field (E) and on the D/A separation. At low E fields, CT formation was dominant, and up to a factor of 2 increase in charge photogeneration efficiency due to CT exciton dissociation was observed in composites with large spatial D/A separation compared with that in pristine D films. At high E fields, fast charge carrier photogeneration was dominant in all composites, and no improvement in charge p...

[1]  Richard H. Friend,et al.  CHARGE- AND ENERGY-TRANSFER PROCESSES AT POLYMER/POLYMER INTERFACES : A JOINT EXPERIMENTAL AND THEORETICAL STUDY , 1999 .

[2]  Andrew J. Moad,et al.  Controlling the Microstructure of Solution-Processable Small Molecules in Thin-Film Transistors through Substrate Chemistry , 2011 .

[3]  L. Zakharov,et al.  Indeno[1,2-b]fluorenes: fully conjugated antiaromatic analogues of acenes. , 2011, Angewandte Chemie.

[4]  John E. Anthony,et al.  Photoconductivity in organic thin films: From picoseconds to seconds after excitation , 2008 .

[5]  Dirk Vanderzande,et al.  Influence of nanoscale phase separation on geminate versus bimolecular recombination in P3HT:fullerene blend films , 2010 .

[6]  Stephen R. Forrest,et al.  The path to ubiquitous and low-cost organic electronic appliances on plastic , 2004, Nature.

[7]  J. Brédas,et al.  Steric control of the donor/acceptor interface: implications in organic photovoltaic charge generation. , 2011, Journal of the American Chemical Society.

[8]  J. Brédas,et al.  Molecular understanding of organic solar cells: the challenges. , 2009, Accounts of chemical research.

[9]  Thomas Strobel,et al.  Role of the Charge Transfer State in Organic Donor–Acceptor Solar Cells , 2010, Advanced materials.

[10]  S. Meskers,et al.  Photoinduced charge and energy transfer in dye-doped conjugated polymers , 2006 .

[11]  L. Zakharov,et al.  Electron-accepting 6,12-diethynylindeno[1,2-b]fluorenes: synthesis, crystal structures, and photophysical properties. , 2011, Angewandte Chemie.

[12]  John E. Anthony,et al.  Soluble n-type pentacene derivatives as novel acceptors for organic solar cells , 2009 .

[13]  A. Heeger,et al.  Semiconducting polymers: the Third Generation. , 2010, Chemical Society reviews.

[14]  X. Zhu,et al.  Charge-transfer excitons at organic semiconductor surfaces and interfaces. , 2009, Accounts of chemical research.

[15]  John E. Anthony,et al.  Temperature dependence of exciton and charge carrier dynamics in organic thin films , 2011 .

[16]  M. J. Kendrick,et al.  Synthesis and charge transport studies of stable, soluble hexacenes. , 2012, Chemical communications.

[17]  Howard E. Katz,et al.  Thin-Film Organic Electronic Devices , 2009 .

[18]  H. Matsui,et al.  Competition between charge-transfer exciton dissociation and direct photocarrier generation in molecular donor-acceptor compounds. , 2010, Physical review letters.

[19]  John E. Anthony,et al.  High-mobility spin-cast organic thin film transistors , 2008 .

[20]  Weimin Zhang,et al.  Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy. , 2008, Journal of the American Chemical Society.

[21]  John E. Anthony,et al.  Energy Transfer and Exciplex Formation and Their Impact on Exciton and Charge Carrier Dynamics in Organic Films , 2011 .

[22]  F. Krebs,et al.  Direct observation of sub-100 fs mobile charge generation in a polymer-fullerene film. , 2012, Physical review letters.

[23]  Troy Van Voorhis,et al.  Charge transfer state versus hot exciton dissociation in polymer-fullerene blended solar cells. , 2010, Journal of the American Chemical Society.

[24]  Stefan C J Meskers,et al.  Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends. , 2008, Journal of the American Chemical Society.

[25]  D. Neher,et al.  Charge carrier generation and electron blocking at interlayers in polymer solar cells , 2007 .

[26]  John E. Anthony,et al.  Organic semiconductor composites: influence of additives on the transient photocurrent , 2009 .

[27]  Chi H. Lee,et al.  Interface effects on the external quantum efficiency of organic bulk heterojunction photodetectors , 2007 .

[28]  N. Stingelin,et al.  Charge separation in semicrystalline polymeric semiconductors by photoexcitation: is the mechanism intrinsic or extrinsic? , 2011, Physical review letters.

[29]  Carlos Silva,et al.  Exciton regeneration at polymeric semiconductor heterojunctions. , 2004, Physical review letters.

[30]  John E. Anthony,et al.  Photovoltaics from soluble small molecules , 2007 .

[31]  John E. Anthony,et al.  A survey of electron-deficient pentacenes as acceptors in polymer bulk heterojunction solar cells , 2011 .

[32]  Frédéric Laquai,et al.  Ultrafast exciton dissociation followed by nongeminate charge recombination in PCDTBT:PCBM photovoltaic blends. , 2011, Journal of the American Chemical Society.

[33]  Thuc-Quyen Nguyen,et al.  Small Molecule Solution-Processed Bulk Heterojunction Solar Cells† , 2011 .

[34]  Christopher R. Swartz,et al.  Synthesis and characterization of electron-deficient pentacenes. , 2005, Organic letters.

[35]  J. Anthony.  Small-Molecule, Nonfullerene Acceptors for Polymer Bulk Heterojunction Organic Photovoltaics† , 2011 .

[36]  David Beljonne,et al.  Electronic Processes at Organic−Organic Interfaces: Insight from Modeling and Implications for Opto-electronic Devices† , 2011 .

[37]  Thomas Kirchartz,et al.  Competition between the charge transfer state and the singlet states of donor or acceptor limiting the efficiency in polymer:fullerene solar cells. , 2012, Journal of the American Chemical Society.

[38]  Tracey M. Clarke,et al.  Charge photogeneration in organic solar cells. , 2010, Chemical reviews.

[39]  J S Brooks,et al.  Functionalized pentacene: improved electronic properties from control of solid-state order. , 2001, Journal of the American Chemical Society.

[40]  W E Moerner,et al.  Organic photorefractives: mechanisms, materials, and applications. , 2004, Chemical reviews.

[41]  J. Fréchet,et al.  Molecular design and ordering effects in π-functional materials for transistor and solar cell applications. , 2011, Journal of the American Chemical Society.

[42]  John E. Anthony,et al.  Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits. , 2008, Nature materials.

[43]  Wei Lin Leong,et al.  Solution-processed small-molecule solar cells with 6.7% efficiency. , 2011, Nature materials.

[44]  John E. Anthony,et al.  Bandlike transport in pentacene and functionalized pentacene thin films revealed by subpicosecond transient photoconductivity measurements , 2005 .

[45]  Jean Roncali,et al.  Molecular bulk heterojunctions: an emerging approach to organic solar cells. , 2009, Accounts of chemical research.

[46]  John E. Anthony,et al.  Influence of organic semiconductor-metal interfaces on the photoresponse of functionalized anthradithiophene thin films , 2009 .

[47]  David Hofer,et al.  Aggregate formation and its effect on (opto)electronic properties of guest-host organic semiconductors , 2010 .

[48]  Roberto Pacios,et al.  Charge transfer reactions in near IR absorbing small molecule solution processed organic bulk-heterojunction solar , 2011 .

[49]  John E. Anthony,et al.  Optical, Fluorescent, and (Photo)conductive Properties of High-Performance Functionalized Pentacene and Anthradithiophene Derivatives , 2009 .

[50]  Peter Bäuerle,et al.  Small molecule organic semiconductors on the move: promises for future solar energy technology. , 2012, Angewandte Chemie.

[51]  Raj René Janssen,et al.  The Energy of Charge‐Transfer States in Electron Donor–Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells , 2009 .

[52]  David L Eaton,et al.  A road map to stable, soluble, easily crystallized pentacene derivatives. , 2002, Organic letters.

[53]  David Beljonne,et al.  The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors , 2012, Science.

[54]  Ryan D. Pensack,et al.  Ultrafast probes of charge transfer states in organic photovoltaic materials , 2011 .

[55]  D. Hertel,et al.  Electric field assisted charge carrier photogeneration in poly(spirobifluorene-co-benzothiadiazole). , 2010, The Journal of chemical physics.

[56]  A. Zakhidov,et al.  Structural phase transition in pentacene caused by molecular doping and its effect on charge carrier mobility , 2012 .