Improved hybrid solar cells via in situ UV polymerization.

One approach for making inexpensive inorganic-organic hybrid photovoltaic (PV) cells is to fill highly ordered TiO(2) nanotube (NT) arrays with solid organic hole conductors such as conjugated polymers. Here, a new in situ UV polymerization method for growing polythiophene (UV-PT) inside TiO(2) NTs is presented and compared to the conventional approach of infiltrating NTs with pre-synthesized polymer. A nanotubular TiO(2) substrate is immersed in a 2,5-diiodothiophene (DIT) monomer precursor solution and then irradiated with UV light. The selective UV photodissociation of the C--I bond produces monomer radicals with intact pi-ring structure that further produce longer oligothiophene/PT molecules. Complete photoluminescence quenching upon UV irradiation suggests coupling between radicals created from DIT and at the TiO(2) surface via a charge transfer complex. Coupling with the TiO(2) surface improves UV-PT crystallinity and pi-pi stacking; flat photocurrent values show that charge recombination during hole transport through the polymer is negligible. A non-ideal, backside-illuminated setup under illumination of 620-nm light yields a photocurrent density of approximately 5 microA cm(2)-surprisingly much stronger than with comparable devices fabricated with polymer synthesized ex situ. Since in this backside architecture setup we illuminate the cell through the Ag top electrode, there is a possibility for Ag plasmon-enhanced solar energy conversion. By using this simple in situ UV polymerization method that couples the conjugated polymer to the TiO(2) surface, the absorption of sunlight can be improved and the charge carrier mobility of the photoactive layer can be enhanced.

[1]  Tom J. Savenije,et al.  Visible light sensitisation of titanium dioxide using a phenylene vinylene polymer , 1998 .

[2]  D. Grebner,et al.  Theoretical studies and spectroscopic investigations of ground and excited electronic states of thiophene oligomers , 1995 .

[3]  Seth B Darling,et al.  Isolating the effect of torsional defects on mobility and band gap in conjugated polymers. , 2008, The journal of physical chemistry. B.

[4]  Chengwei Wang,et al.  Surface-initiated growth of conjugated polymers for functionalization of electronically active nanoporous networks: synthesis, structure and optical properties , 2006 .

[5]  Sue A. Carter,et al.  Charge transfer in photovoltaics consisting of interpenetrating networks of conjugated polymer and TiO2 nanoparticles , 1999 .

[6]  Yunzhi Liu,et al.  Infiltrating Semiconducting Polymers into Self‐Assembled Mesoporous Titania Films for Photovoltaic Applications , 2003 .

[7]  Craig A. Grimes,et al.  A new benchmark for TiO2 nanotube array growth by anodization , 2007 .

[8]  Michael Grätzel,et al.  Solar energy conversion by dye-sensitized photovoltaic cells. , 2005, Inorganic chemistry.

[9]  Craig A. Grimes,et al.  Anodic Growth of Highly Ordered TiO2 Nanotube Arrays to 134 μm in Length , 2006 .

[10]  Paul A. van Hal,et al.  Photoinduced electron transfer from conjugated polymers to TiO2 , 1999 .

[11]  P. Hones,et al.  Optical dispersion analysis of TiO2 thin films based on variable-angle spectroscopic ellipsometry measurements , 1999 .

[12]  S. Babu,et al.  Two-step regression procedure for the optical characterization of thin films. , 1991, Applied optics.

[13]  C. Grimes,et al.  Application of highly-ordered TiO2 nanotube-arrays in heterojunction dye-sensitized solar cells , 2006 .

[14]  Craig A Grimes,et al.  Self-assembled hybrid polymer-TiO2 nanotube array heterojunction solar cells. , 2007, Langmuir : the ACS journal of surfaces and colloids.

[15]  Xiaoniu Yang,et al.  Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells. , 2005, The journal of physical chemistry. B.

[16]  K. Yoshino,et al.  Crystal structural change in poly(3-alkyl thiophene)s induced by iodine doping as studied by an organized combination of X-ray diffraction, infrared/Raman spectroscopy and computer simulation techniques , 1997 .

[17]  Michael D. McGehee,et al.  Photovoltaic cells made from conjugated polymers infiltrated into mesoporous titania , 2003 .

[18]  Seong H. Kim,et al.  Photochemical synthesis of oligothiophene thin films and nano-patterns in condensed multilayer films of 2,5-diiodothiophene—Effects of surface chemistry of substrates , 2007 .

[19]  W. Su,et al.  Effect of chemical structure of interface modifier of TiO2 on photovoltaic properties of poly(3-hexylthiophene)/TiO2 layered solar cells. , 2009, Journal of colloid and interface science.

[20]  Kai Zhu,et al.  Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. , 2007, Nano letters.

[21]  M. McGehee,et al.  Nanostructuring titania by embossing with polymer molds made from anodic alumina templates. , 2005, Nano letters.

[22]  C. Grimes,et al.  Effect of device geometry on the performance of TiO2 nanotube array-organic semiconductor double heterojunction solar cells , 2008 .

[23]  P. M. Kumar,et al.  Nanocrystalline TiO2 studied by optical, FTIR and X-ray photoelectron spectroscopy: correlation to presence of surface states , 2000 .

[24]  Hans-Heinrich Hörhold,et al.  Efficient Titanium Oxide/Conjugated Polymer Photovoltaics for Solar Energy Conversion , 2000 .

[25]  Thomas H. Reilly,et al.  Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics , 2008 .

[26]  J. Schoonman,et al.  UV Polymerization of Oligothiophenes and Their Application in Nanostructured Heterojunction Solar Cells , 2004 .

[27]  Seong H. Kim,et al.  Photochemical conversion of 2,5-diiodothiophene condensed on substrates to oligothiophene and polythiophene thin films and micro-patterns , 2006 .

[28]  C. Grimes,et al.  Cation Effect on the Electrochemical Formation of Very High Aspect Ratio TiO2 Nanotube Arrays in Formamide−Water Mixtures , 2007 .

[29]  Jow-Lay Huang,et al.  High quality transparent conductive ZnO/Ag/ZnO multilayer films deposited at room temperature , 2006 .

[30]  Seong H. Kim,et al.  Photochemical production of oligothiophene and polythiophene micropatterns from 2,5-diiodothiophene on Au in UHV , 2005 .

[31]  Anusorn Kongkanand,et al.  Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe-TiO2 architecture. , 2008, Journal of the American Chemical Society.

[32]  Ligui Li,et al.  Improving performance of polymer photovoltaic devices using an annealing-free approach via construction of ordered aggregates in solution , 2008 .

[33]  Sean E. Shaheen,et al.  Effect of Polymer Processing on the Performance of Poly(3-hexylthiophene)/ZnO Nanorod Photovoltaic Devices , 2007 .