Design of organic ternary blends and small-molecule bulk heterojunctions: photophysical considerations

Abstract. We explored relationships between photophysical processes and solar cell characteristics in solution-processable bulk heterojunctions (BHJs), in particular: (1) polymer donor:fullerene acceptor:small-molecule (SM) nonfullerene acceptor, (2) polymer donor:SM donor:SM nonfullerene acceptor, and (3) SM donor:SM nonfullerene or fullerene acceptor. Addition of a nonfullerene SM acceptor to “efficient” polymer:fullerene BHJs led to a reduction in power conversion efficiency (PCE), mostly due to decreased charge photogeneration efficiency and increased disorder. By contrast, addition of an SM donor to “inefficient” polymer:SM nonfullerene acceptor BHJs led to a factor of two to three improvement in the PCE, due to improved charge photogeneration efficiency and transport. In most blends, exciplex formation was observed and correlated with a reduced short-circuit current (Jsc) without negatively impacting the open-circuit voltage (Voc). A factor of ∼5 higher PCE was observed in SM donor:fullerene acceptor BHJs as compared to SMBHJs with the same SM donor but nonfullerene acceptor, due to enhanced charge carrier photogeneration in the blend with fullerene. Our study revealed that the HOMO and LUMO energies of molecules comprising a blend are not reliable parameters for predicting Voc of the blend, and an understanding of the photophysics is necessary for interpreting solar cell characteristics and improving the molecular design of BHJs.

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

[2]  Tao Wang,et al.  Rationalizing Phase Transitions with Thermal Annealing Temperatures for P3HT:PCBM Organic Photovoltaic Devices , 2012 .

[3]  Mark A. Ratner,et al.  Organic solar cells: A new look at traditional models , 2011 .

[4]  Yang Yang,et al.  Solution-processed small-molecule solar cells: breaking the 10% power conversion efficiency , 2013, Scientific Reports.

[5]  John E. Anthony,et al.  Enhanced charge photogeneration promoted by crystallinity in small-molecule donor-acceptor bulk heterojunctions , 2014, Applied Physics Letters.

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

[7]  Jizheng Wang,et al.  Fill factor in organic solar cells. , 2013, Physical chemistry chemical physics : PCCP.

[8]  Germà Garcia-Belmonte,et al.  Polymer defect states modulate open-circuit voltage in bulk-heterojunction solar cells , 2013 .

[9]  Christoph J. Brabec,et al.  Organic Ternary Solar Cells: A Review , 2013, Advanced materials.

[10]  Sankar Subramanian,et al.  Chromophore fluorination enhances crystallization and stability of soluble anthradithiophene semiconductors. , 2008, Journal of the American Chemical Society.

[11]  John E. Anthony,et al.  Effects of polymorphism on charge transport in organic semiconductors , 2009 .

[12]  N. S. Sariciftci,et al.  Efficiency of bulk-heterojunction organic solar cells , 2013, Progress in polymer science.

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

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

[15]  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 .

[16]  John E. Anthony,et al.  Small-Molecule Bulk Heterojunctions: Distinguishing Between Effects of Energy Offsets and Molecular Packing on Optoelectronic Properties , 2013 .

[17]  Christoph J. Brabec,et al.  Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells , 2005 .

[18]  Brian F. G. Johnson,et al.  Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: Numerical modeling of time-resolved photocurrent , 2013 .

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

[20]  James R. Durrant,et al.  Materials Design Considerations for Charge Generation in Organic Solar Cells , 2014 .

[21]  Carlos Silva,et al.  Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions , 2013, Nature Communications.

[22]  Akihiro Furube,et al.  Analysis of the excited states of regioregular polythiophene P3HT , 2008 .

[23]  Jenny Nelson,et al.  Non‐Geminate Recombination as the Primary Determinant of Open‐Circuit Voltage in Polythiophene:Fullerene Blend Solar Cells: an Analysis of the Influence of Device Processing Conditions , 2011 .

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

[25]  Robert A. Street,et al.  Origin of the tunable open-circuit voltage in ternary blend bulk heterojunction organic solar cells. , 2013, Journal of the American Chemical Society.

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

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

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

[29]  A. Troisi,et al.  What Makes Fullerene Acceptors Special as Electron Acceptors in Organic Solar Cells and How to Replace Them , 2013, Advanced materials.

[30]  John E. Anthony,et al.  Organic Single-Crystal Field-Effect Transistors of a Soluble Anthradithiophene , 2008 .

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

[32]  Alberto Salleo,et al.  Morphology‐Dependent Trap Formation in High Performance Polymer Bulk Heterojunction Solar Cells , 2011 .

[33]  Ben Minnaert,et al.  Empirical study of the characteristics of current-state organic bulk heterojunction solar cells , 2007 .

[34]  John E. Anthony,et al.  Formation of the Donor–Acceptor Charge-Transfer Exciton and Its Contribution to Charge Photogeneration and Recombination in Small-Molecule Bulk Heterojunctions , 2012 .

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

[36]  Yongfang Li,et al.  Small Molecule Semiconductors for High-Efficiency Organic Photovoltaics , 2012 .

[37]  Christopher M. Proctor,et al.  Charge carrier recombination in organic solar cells , 2013 .

[38]  X. Zhu,et al.  How to Draw Energy Level Diagrams in Excitonic Solar Cells. , 2014, The journal of physical chemistry letters.

[39]  Oksana Ostroverkhova,et al.  Handbook of organic materials for optical and (opto)electronic devices , 2013 .

[40]  Alan J. Heeger,et al.  Barium: An Efficient Cathode Layer for Bulk-heterojunction Solar Cells , 2013, Scientific Reports.

[41]  Jenny Clark,et al.  Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes , 2014, Science.

[42]  John E. Anthony,et al.  Photophysical and photoconductive properties of organic semiconductor composites , 2009, Organic Photonics + Electronics.

[43]  Franky So,et al.  Dielectric Effect on the Photovoltage Loss in Organic Photovoltaic Cells , 2014, Advanced materials.

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