Open-circuit voltage limit caused by recombination through tail states in bulk heterojunction polymer-fullerene solar cells

The output open-circuit voltage of bulk heterojuntion polymer-fullerene solar cells exhibts an offset of Δ≈0.3 V with the difference between the donor highest-occupied molecular orbital (HOMO) and the acceptor lowest-unoccupied molecular orbital (LUMO), as discussed by Scharber et al. [Adv. Mater. (Weinheim, Ger.) 18, 789 (2006)]. We show that the energetic disorder of the electronic density-of-states (DOS) of acceptor and donor materials reduces the splitting of the electron and hole Fermi levels in bulk heterojunction solar cells, by restricting the accumulation of carriers into the tails of the DOS. A simple model based on the kinetic balance between photogeneration and recombination fluxes provides a lower limit to the energy offset Δ>(σn2+σp2)/kBT in terms of the disorder parameters.

[1]  D. Bradley,et al.  Formation of a Ground‐State Charge‐Transfer Complex in Polyfluorene//[6,6]‐Phenyl‐C61 Butyric Acid Methyl Ester (PCBM) Blend Films and Its Role in the Function of Polymer/PCBM Solar Cells , 2007 .

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

[3]  Juan Bisquert,et al.  Simultaneous determination of carrier lifetime and electron density-of-states in P3HT:PCBM organic solar cells under illumination by impedance spectroscopy , 2010 .

[4]  Christoph J. Brabec,et al.  Design Rules for Donors in Bulk‐Heterojunction Solar Cells—Towards 10 % Energy‐Conversion Efficiency , 2006 .

[5]  Nelson E. Coates,et al.  Bulk heterojunction solar cells with internal quantum efficiency approaching 100 , 2009 .

[6]  Germà Garcia-Belmonte,et al.  Determination of gap defect states in organic bulk heterojunction solar cells from capacitance measurements , 2009 .

[7]  D. Vanmaekelbergh,et al.  Wide energy-window view on the density of states and hole mobility in poly(p-phenylene vinylene). , 2004, Physical review letters.

[8]  P. Heremans,et al.  Analytic model of hopping mobility at large charge carrier concentrations in disordered organic semiconductors: Polarons versus bare charge carriers , 2007 .

[9]  D. Rauh,et al.  Charge Carrier Concentration and Temperature Dependent Recombination in Polymer-Fullerene Solar Cells , 2009, 0907.1401.

[10]  M. Green.  Limits on the open-circuit voltage and efficiency of silicon solar cells imposed by intrinsic Auger processes , 1984, IEEE Transactions on Electron Devices.

[11]  Jean Manca,et al.  The Relation Between Open‐Circuit Voltage and the Onset of Photocurrent Generation by Charge‐Transfer Absorption in Polymer : Fullerene Bulk Heterojunction Solar Cells , 2008 .

[12]  Jiří Stuchlík,et al.  Non-Langevin bimolecular recombination in low-mobility materials , 2006 .

[13]  H. Bässler.  Charge Transport in Disordered Organic Photoconductors a Monte Carlo Simulation Study , 1993 .

[14]  F. Krebs,et al.  Low band gap polymers for organic photovoltaics , 2007 .

[15]  Olle Inganäs,et al.  On the origin of the open-circuit voltage of polymer-fullerene solar cells. , 2009, Nature materials.

[16]  J. Bisquert.  Interpretation of electron diffusion coefficient in organic and inorganic semiconductors with broad distributions of states. , 2008, Physical chemistry chemical physics : PCCP.

[17]  Donal D. C. Bradley,et al.  Bimolecular recombination losses in polythiophene: Fullerene solar cells , 2008 .

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

[19]  Thomas Kirchartz,et al.  Efficiency Limits of Organic Bulk Heterojunction Solar Cells , 2009 .

[20]  T. Tiedje,et al.  Band tail recombination limit to the output voltage of amorphous silicon solar cells , 1982 .

[21]  Jean Manca,et al.  Electroluminescence from charge transfer states in polymer solar cells. , 2009, Journal of the American Chemical Society.

[22]  Juan Bisquert,et al.  Physical Chemical Principles of Photovoltaic Conversion with Nanoparticulate, Mesoporous Dye-Sensitized Solar Cells , 2004 .

[23]  J. Manca,et al.  Varying polymer crystallinity in nanofiber poly(3-alkylthiophene): PCBM solar cells: Influence on charge-transfer state energy and open-circuit voltage , 2009 .

[24]  E. Yablonovitch,et al.  Limiting efficiency of silicon solar cells , 1984, IEEE Transactions on Electron Devices.

[25]  Stephen R. Forrest,et al.  Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells , 2007 .

[26]  O. Inganäs,et al.  Correlation between oxidation potential and open-circuit voltage of composite solar cells based on blends of polythiophenes/ fullerene derivative , 2004 .

[27]  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 .

[28]  A. Cravino.  Origin of the open circuit voltage of donor-acceptor solar cells: Do polaronic energy levels play a role? , 2007 .