Visualizing charge separation in bulk heterojunction organic solar cells

Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (<1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.

[1]  M. Doi.  Theory-Experiment and Simulation , 2004 .

[2]  H. Bässler,et al.  Dynamic Stark effect as a probe of the evolution of geminate electron-hole pairs in a conjugated polymer , 2002 .

[3]  Richard H. Friend,et al.  The origin of the open-circuit voltage in polyfluorene-based photovoltaic devices , 2002 .

[4]  Martin Weis,et al.  Probing and modeling of interfacial carrier motion in organic devices by optical second harmonic generation , 2010 .

[5]  M. Stutzmann,et al.  Role of structural order and excess energy on ultrafast free charge generation in hybrid polythiophene/Si photovoltaics probed in real time by near-infrared broadband transient absorption. , 2011, Journal of the American Chemical Society.

[6]  Valentin D. Mihailetchi,et al.  Charge Transport and Photocurrent Generation in Poly(3‐hexylthiophene): Methanofullerene Bulk‐Heterojunction Solar Cells , 2006 .

[7]  N. S. Sariciftci,et al.  Temperature dependence of the charge carrier mobility in disordered organic semiconductors at large carrier concentrations , 2010 .

[8]  B. Gregg.  Entropy of Charge Separation in Organic Photovoltaic Cells: The Benefit of Higher Dimensionality , 2011 .

[9]  G. Lanzani,et al.  Photoinduced transient stark spectroscopy in organic semiconductors: a method for charge mobility determination in the picosecond regime. , 2006, Physical review letters.

[10]  C. Deibel,et al.  Binding energy of singlet excitons and charge transfer complexes in MDMO‐PPV:PCBM solar cells , 2011, 1110.2671.

[11]  V. Sundström,et al.  Electron and Hole Contributions to the Terahertz Photoconductivity of a Conjugated Polymer:Fullerene Blend Identified. , 2012, The journal of physical chemistry letters.

[12]  G. Cerullo,et al.  Hot exciton dissociation in polymer solar cells. , 2013, Nature materials.

[13]  Vidmantas Gulbinas,et al.  Excited state and charge photogeneration dynamics in conjugated polymers. , 2007, The journal of physical chemistry. B.

[14]  H. Ohkita,et al.  Near-IR femtosecond transient absorption spectroscopy of ultrafast polaron and triplet exciton formation in polythiophene films with different regioregularities. , 2009, Journal of the American Chemical Society.

[15]  Stephen R. Forrest,et al.  Separation of geminate charge-pairs at donor–acceptor interfaces in disordered solids , 2004 .

[16]  M. Kemerink,et al.  Mechanism for Efficient Photoinduced Charge Separation at Disordered Organic Heterointerfaces , 2012 .

[17]  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.

[18]  Amy M. Ballantyne,et al.  Free Energy Control of Charge Photogeneration in Polythiophene/Fullerene Solar Cells: The Influence of Thermal Annealing on P3HT/PCBM Blends , 2008 .

[19]  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.

[20]  Jenny Nelson.  Organic photovoltaic films , 2002 .

[21]  P. E. Keivanidis,et al.  Dependence of charge separation efficiency on film microstructure in poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61Butyric acid methyl ester blend films , 2010 .

[22]  Christoph J. Brabec,et al.  Organic photovoltaics: technology and market , 2004 .

[23]  P. Blom,et al.  Validity of the Einstein relation in disordered organic semiconductors. , 2011, Physical review letters.

[24]  Olle Inganäs,et al.  Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends. , 2007, Journal of the American Chemical Society.

[25]  Laura M. Herz,et al.  Efficient generation of charges via below-gap photoexcitation of polymer-fullerene blend films investigated by terahertz spectroscopy , 2008 .

[26]  Technology and the market , 1998 .

[27]  D. Hertel,et al.  Ultrafast dynamics of carrier mobility in a conjugated polymer probed at molecular and microscopic length scales. , 2009, Physical review letters.

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

[29]  Miao Xu,et al.  Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure , 2012, Nature Photonics.

[30]  Richard H. Friend,et al.  Direct Measurement of Electric Field‐Assisted Charge Separation in Polymer:Fullerene Photovoltaic Diodes , 2010, Advanced materials.

[31]  Guglielmo Lanzani,et al.  Pump‐Probe Spectroscopy in Organic Semiconductors: Monitoring Fundamental Processes of Relevance in Optoelectronics , 2011, Advanced materials.

[32]  J. Hummelen,et al.  Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions , 1995, Science.

[33]  Frédéric Laquai,et al.  Effect of morphology on ultrafast free carrier generation in polythiophene:fullerene organic solar cells. , 2010, Journal of the American Chemical Society.

[34]  Thomas Strobel,et al.  Origin of the efficient polaron-pair dissociation in polymer-Fullerene blends. , 2009, Physical review letters.

[35]  G. Lanzani,et al.  Transient Absorption Imaging of P3HT:PCBM Photovoltaic Blend: Evidence For Interfacial Charge Transfer State , 2011 .

[36]  H. Bässler,et al.  Does Conjugation Help Exciton Dissociation? A Study on Poly(p‐phenylene)s in Planar Heterojunctions with C60 or TNF , 2012, Advanced materials.

[37]  Richard H Friend,et al.  Effect of annealing on P3HT:PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy. , 2010, Nano letters.

[38]  V. Sundström,et al.  Charge formation and transport in bulk-heterojunction solar cells based on alternating polyfluorene copolymers blended with fullerenes , 2006 .

[39]  E. Abrahams,et al.  Impurity Conduction at Low Concentrations , 1960 .

[40]  V. Sundström,et al.  Ultrafast terahertz photoconductivity of bulk heterojunction materials reveals high carrier mobility up to nanosecond time scale. , 2012, Journal of the American Chemical Society.

[41]  C. A. Walsh,et al.  Efficient photodiodes from interpenetrating polymer networks , 1995, Nature.

[42]  Brian A. Gregg,et al.  Comparing organic to inorganic photovoltaic cells: Theory, experiment, and simulation , 2003 .