Bulk Heterojunction Photovoltaic Cells with Low Donor Concentration

High-effi ciency organic photovoltaic (OPV) cells are mostly based on a bulk heterojunction [ 1 ] (BHJ) structure, which is essentially a thin fi lm of mixed electron donor and acceptor. With few exceptions, the acceptor component is a fullerene-based material such as C 60 [ 2 ] and PCBM, [ 3 ] whereas a large variety of materials has been found to be useful as the donor component. [ 4 ] In order to achieve high power conversion effi ciency, the donor–acceptor composition of the BHJ needs to be optimized with respect to light absorption and charge generation, including for instance the use of a low bandgap donor, [ 5 ] to complement the absorption of the acceptor and selecting a donor with proper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels to match that of the acceptor. In this work, we show that it is possible to achieve a large open-circuit voltage ( V oc > 1.0 V) in a fullerene-based OPV with almost any donor, provided that the donor is present in a small concentration and that MoO x is used as the Schottky barrier contact [ 6 ] to the BHJ. With fi ne tuning of the donor concentration to overcome the hole-transport limitation in the BHJ, high power conversion effi ciency ( η PCE > 5%) has been realized. In Figure 1 the schematic of the OPV cell layer sequence, indium tin oxide (ITO)/MoO x /BHJ/bathophenanthroline (Bphen)/Al, is shown along with the molecular structures of 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC), Bphen, C 60 , and C 70 . In this sequence, the BHJ layer is a mixed fi lm of TAPC [ 7 ] (donor) and C 60 (acceptor) with various ratios. The concentration of TAPC is indicated in volume fraction. As the major focus of this study was concerned with the composition of the BHJ layer, we kept the thickness and composition of all the other layers constant: ITO (90 nm), MoO x (2 nm), Bphen (8 nm), and Al (100 nm). First, we studied the effect of varying the TAPC concentration in the BHJ layer with a constant thickness of 40 nm. In Figure 2 a the current density–voltage ( J – V ) characteristics are

[1]  Yongli Gao,et al.  Strong interface p-doping and band bending in C60 on MoOx , 2011 .

[2]  Barry P Rand,et al.  4.2% efficient organic photovoltaic cells with low series resistances , 2004 .

[3]  C. Brabec,et al.  Origin of the Open Circuit Voltage of Plastic Solar Cells , 2001 .

[4]  R. Friend,et al.  Built-in field electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene dioxythiophene) hole injection layer , 1999 .

[5]  N. Armstrong,et al.  Titanyl phthalocyanine/C60 heterojunctions: Band-edge offsets and photovoltaic device performance , 2008 .

[6]  Wassim Hojeij,et al.  Optical modeling of organic solar cells based on CuPc and C60. , 2008, Applied optics.

[7]  Yang Yang,et al.  High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends , 2005 .

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

[9]  C. Tang,et al.  Effect of the highest occupied molecular orbital energy level offset on organic heterojunction photovoltaic cells , 2010 .

[10]  Kazuhiro Saito,et al.  Annealing effect in the sexithiophene:C70 small molecule bulk heterojunction organic photovoltaic cells , 2009 .

[11]  Kazuhiro Saito,et al.  Realization of Large Open-Circuit Photovoltage in Organic Thin-Film Solar Cells by Controlling Measurement Environment , 2006 .

[12]  Theo Siegrist,et al.  Physical vapor growth of organic semiconductors , 1998 .

[13]  Valentin D. Mihailetchi,et al.  Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells , 2007 .

[14]  P. Blom,et al.  Combined optical and electrical modeling of polymer: fullerene bulk heterojunction solar cells , 2008 .

[15]  Stephen R. Forrest,et al.  Solution-processed squaraine bulk heterojunction photovoltaic cells. , 2010, ACS nano.

[16]  Jingping Zhang,et al.  Naphthyl and Thionaphthyl End‐Capped Oligothiophenes as Organic Semiconductors: Effect of Chain Length and End‐Capping Groups , 2007 .

[17]  Ching Wan Tang,et al.  Organic Schottky barrier photovoltaic cells based on MoOx/C60 , 2010 .

[18]  L. Rothberg,et al.  Integer charge transfer states in organic light-emitting diodes: Optical detection of hole carriers at the anode∣organic interface , 2011 .

[19]  Gang Li,et al.  For the Bright Future—Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4% , 2010, Advanced materials.

[20]  M. Pederson,et al.  Static dielectric response of icosahedral fullerenes from C 60 to C 2160 characterized by an all-electron density functional theory , 2008, 0801.4433.

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

[22]  L. S. Roman,et al.  Modeling photocurrent action spectra of photovoltaic devices based on organic thin films , 1999 .

[23]  Yang Yang,et al.  Polymer solar cells with enhanced open-circuit voltage and efficiency , 2009 .

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

[25]  Martin Pfeiffer,et al.  Efficient Vacuum‐Deposited Organic Solar Cells Based on a New Low‐Bandgap Oligothiophene and Fullerene C60 , 2006 .

[26]  O. Inganäs,et al.  Optical optimization of polyfluorene-fullerene blend photodiodes , 2005 .

[27]  M. Hiramoto,et al.  Efficient Organic p-i-n Solar Cells Having Very Thick Codeposited i-Layer Consisting of Highly Purified Organic Semiconductors , 2008 .

[28]  Kazuhiro Saito,et al.  Efficient oligothiophene:fullerene bulk heterojunction organic photovoltaic cells , 2008 .

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

[30]  Xiong Gong,et al.  Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology , 2005 .

[31]  Valentin D. Mihailetchi,et al.  Device model for the operation of polymer/fullerene bulk heterojunction solar cells , 2005 .

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

[33]  L. Onsager.  Deviations from Ohm's Law in Weak Electrolytes , 1934 .

[34]  H. Looyenga.  Dielectric constants of heterogeneous mixtures , 1965 .