Photocurrent in bulk heterojunction solar cells

We investigated the photocurrent in poly(3-hexylthiophene-2,5-diyl)(P3HT):[6,6]-phenyl-${\text{C}}_{61}$ butyric acid methyl ester solar cells by applying a pulsed measurement technique. For annealed samples, a point of optimal symmetry with a corresponding voltage ${V}_{\text{POS}}$ of 0.52\char21{}0.64 V could be determined. Based on macroscopic simulations and results from capacitance-voltage measurements, we identify this voltage with flat band conditions in the bulk of the cell but not the built-in voltage as proposed by Ooi et al., J. Mater. Chem. 18, 1644 (2008). We calculated the field-dependent polaron pair dissociation after Onsager-Braun and the voltage-dependent extraction of charge carriers after Sokel and Hughes with respect to this point of symmetry. Our analysis allows to explain the experimental photocurrent in both forward and reverse directions. Also, we observed a voltage-independent offset of the photocurrent. As this offset is crucial for the device performance, we investigated its dependence on cathode material and thermal treatment. From our considerations we gain insight into the photocurrent's voltage dependence and the limitations of device efficiency.

[1]  C. Deibel.  Front Cover (Phys. Status Solidi A 12/2009) , 2009 .

[2]  L. Onsager.  Initial Recombination of Ions , 1938 .

[3]  Christoph J. Brabec,et al.  Simulation of light intensity dependent current characteristics of polymer solar cells , 2004 .

[4]  Temperature-dependent built-in potential in organic semiconductor devices , 2006 .

[5]  C. Deibel,et al.  Bipolar charge transport in poly(3-hexyl thiophene)/methanofullerene blends: A ratio dependent study , 2008, 0810.5460.

[6]  S. Selberherr.  Analysis and simulation of semiconductor devices , 1984 .

[7]  S. M. Sze.  Physics of semiconductor devices /2nd edition/ , 1981 .

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

[9]  F. Yakuphanoglu,et al.  Current–voltage and capacitance–voltage characteristics of the ITO/polyaniline doped boron trifloride/Al Schottky diode , 2008 .

[10]  A. J. Heeger,et al.  Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene , 1992, Science.

[11]  V. Mihailetchi,et al.  Photocurrent generation in polymer-fullerene bulk heterojunctions. , 2004, Physical review letters.

[12]  Ronald Österbacka,et al.  Time-dependent Langevin-type bimolecular charge carrier recombination in regiorandom poly(3-hexylthiophene) , 2005 .

[13]  C. Deibel,et al.  Origin of reduced polaron recombination in organic semiconductor devices , 2009, 0907.2428.

[14]  Gang Li,et al.  Accurate Measurement and Characterization of Organic Solar Cells , 2006 .

[15]  C. Deibel,et al.  Influence of charge carrier mobility on the performance of organic solar cells , 2008, 0806.2249.

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

[17]  John C. deMello,et al.  On the pseudo-symmetric current–voltage response of bulk heterojunction solar cells , 2008 .

[18]  Charles L. Braun,et al.  Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production , 1984 .

[19]  R. C. Hughes,et al.  Numerical analysis of transient photoconductivity in insulators , 1982 .

[20]  J. Bisquert,et al.  Band unpinning and photovoltaic model for P3HT:PCBM organic bulk heterojunctions under illumination , 2008 .

[21]  C. Deibel,et al.  Polaron recombination in pristine and annealed bulk heterojunction solar cells , 2008 .

[22]  Peter Blood,et al.  The Electrical Characterization of Semiconductors: Majority Carriers and Electron States , 1992 .