ICMCTF2009 Session H4: Thin Films for Photovoltaics: Synthesis and Characterization

Wednesday, April 29, 2009 1:30 PM in Room Tiki Pavilion

Wednesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	H4-1 Manufacturing Photovoltaic Systems L. Kazmerski (NREL)
2:10 PM		H4-3 Steady State and Transient Photoconductivity of a-Se_90-xSb₁₀In_x (0 ≤ x ≤ 15) Thin Films M. Kamboj, F. Mohammadi (Ryerson University Toronto, Canada) Amorphous thin films of Se_90-xSb₁₀In_x (0 ≤ x ≤ 15) have been prepared by electron beam evaporation method. The steady state and transient photoconductivity measurements on the thin films of Se_90-xSb₁₀In_x (0≤x≤15) have been carried out at different level of light intensities (500 lx – 5000 lx) at room temperature (301 K). The plot of photocurrent (I_ph) versus light intensity (F) follows a power law I_ph= F^γ. The value of exponent lies between 0.5 and 1.0, which indicates there exists a continuous distribution of localized states in the mobility gap of Se_90-xSb₁₀In_x (0 ≤ x ≤ 15) thin films. For transient photoconductivity, when the samples were illuminated with light, the photocurrent quickly reaches the maximum value and thereafter, it start decreasing with the exposure time and becomes stable after 15 minutes of exposure. This kind of phenomenon is termed as photo-degradation of photocurrent. The results have been explained on the basis of charged defect model and the inter-cluster interaction model. High photocurrents are found for a-Se₇₅Sb₁₀In₁₅ system which is even higher than the parent system Se₉₀Sb₁₀. The photosensitivity shows a minimum value at 5 at% of Indium (In) concentration, which has been explained based on chemically ordered network and the topological models.
2:30 PM		H4-5 Glancing-Angle Deposited Titania Films for Dye-Sensitized Solar Cells H.Y. Yang, M.F. Lee, M.S. Wong (National Dong Hwa University, Taiwan) A series of sculptured porous nano-columnar titanium oxide films were prepared by glancing angle deposition (GLAD) method using an electron-beam evaporation system. The films were deposited on ITO glass and used as photo-anode in the dye-sensitized solar cell (DSSC). The as-deposited TiO₂ films have anatase phase and ordered porous nano-columnar structures. Both features of the photo-anode are advantageous for electron transfer and for high surface area, resulting in the enhanced dye-absorption, thus, promote the efficiency of the DSSC. In this study, we varied glancing angle and film thickness, and focused on the structure, crystallinity, dye-absorption, and light-absorption of the films, and their effect on the performance of DSSCs. The DSSCs exhibited a high fill-factor(FF) above 0.78. The best performing solar cell incorporating an 8 µm thick TiO₂ film fabricated at deposition angle of 73˚ had a photoelectric conversion efficiency of 3.23 %.
2:50 PM	Invited	H4-6 High Performance Polymer Soloar Cells and Optical Sensors Y. Yang (UCLA) In this presentation, we report the formation of polymer solar cells, with efficiency closed to 6% power conversation efficiency based on a new class of Si-containing conjugated polymer. The Si atom provides better pi-pi stacking on the polymer in the solid state format, hence improves the device performance significantly. The device with thin film with only 50% transparency reaches 5.6% efficiency. On the other hand, attempts to add quantum dots to further the efficiency has leaded to the discovery of high gain photoconductivity photo detector. By adding CdTe QDs into P3HT:PCBM blends, the polymer photo-device reaches 8000 times higher photo current.
3:30 PM		H4-8 Hybrid Solar Cells Based on Poly (3-Hexylthiophene):Fullerene Blend and TiO₂ Porous Film M.K. Fung, C.T. Yip, K.Y. Cheung, A.B. Djurišić, W.K. Chan (The University of Hong Kong) While inorganic solar cells have higher efficiency, faster electron transport and stability, organic solar cells can be fabricated by inexpensive methods on large area substrates, resulting in increasing interest in organic photovoltaics in spite of their inferior charge transport properties and efficiency compared to inorganic solar cells. One of the possible methods to combine the advantages of the two types of material is the use of hybrid organic/inorganic solar cells. However, the efficiencies of hybrid solar cells based on inorganic nanostructures and organic polymers are still relatively low. The reasons for low efficiency of hybrid cells are partially due to possible problems at the organic/inorganic interface¹, and also to the poor infiltration of polymer into the inorganic nanostructured network². In spite of this, it has been reported that the performance of pure polymer solar cells was improved by employing nanostructured semiconducting materials as charge collectors^3-5. We have previously reported solar cells based on TiO_x porous layer and poly(2-methoxy-5-(2’-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)⁴. Since the absorption spectrum of MEH-PPV is not as well matched with the solar spectrum as that of poly(3-hexylthiophene) (P3HT) or P3HT:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends, we investigate here the performance of solar cells based on P3HT:PCBM active layers as a function of the polymer/fullerene ratio and active layer thickness. The polymer infiltration and its relationship to the solar cell performance were examined by scanning electron microscopy, while the solar cell performance was characterized by I-V curve measurements in the dark and under simulated solar illumination. ¹H. Ishii, K.Sugiyama, E. Ito, and K. Seki, Adv. Mater. 1999, 11, 605-625. ²G. P. Bartholomew, A. J. Heeger, Adv. Funct. Mater. 2005, 15, 677-682. ³C. H. Chang, T. K. Hung, Y. T. Lin, Y. Y. Lin, C. W. Chen, T. H. Chu and W. F. Su, J. Mater. Chem. 2008, 18, 2201-2207 ⁴C. C. Oey, A. B. Djurišić, H. Wang, K. K. Y. Man, W. K. Chan, M. H. Xie, Y. H. Leung, A. Pandey, J-M Nunzi and P. C. Chui, Nanotechnology 2006, 17, 706-713. ⁵D. C. Olson, Yun-Ju Lee, M. S. White, N. Kopidakis, S. E. Shaheen, D. S. Ginley, J. A. Voigt, J. W. P. Hsu, J. Phys. Chem. C 2007, 111, 16640-16645.
3:50 PM		H4-10 Photoelectrochemical Properties of Nitrogen-Doped Indium Tin Oxide Thin Films Prepared by Reactive DC Magnetron Sputtering Technique K.R. Wu (National Kaohsiung Marine University, Taiwan); C.Y. Yeh (Kao Yuan University, Taiwan); C.H. Hung (National Kaohsiung First University of Science and Technology, Taiwan); C.Y. Chung (National kaohsiung Marine University, Taiwan) Nitrogen-doped indium tin oxide (N-ITO) thin films are deposited on unheated ITO glass substrates as visible-light enabling catalysts by direct current (DC) magnetron sputtering technique. Structural properties of the post annealed N-ITO thin films characterized by X-ray diffraction (XRD) show that some indium nitride (InN) nano-particles are embedded in the ITO matrix. A broad XRD peak around 2θ=33° is assigned to the (101) InN plane. This is also observed in a lattice image of cross-sectional high resolution transmission electron micrograph (HRTEM). The corresponding diffraction pattern confirms the formation of (101) and (112) InN planes. The InN nano-particles enable the light absorption of the N-ITO catalysts to longer wavelengths of about 500 nm, narrowing the band gap from 3.9 eV to approximately 2.5 eV. Under ultra-violet (365 nm) and visible-light (blue LED, 410＜λ＜550 nm) illumination, the N-ITO catalysts show the photocurrent densities of 80 and 2.5 μA/cm², respectively, with Na₂CO₃ as the sacrificial agent. These are attributed to the heterojunction arrays; the formation of InN phases embedded in the crystalline ITO matrix. The heterojunction arrays can facilitate the photoinduced electrons and holes separation for better photoelectron transfer. Although optimization will be needed to deliver high photocurrents, the present work shows that ITO can be suitably doped with N₂ gas to produce a promising catalyst with improved photoelectrochemical properties for solar water splitting for hydrogen evolution.