Maikel F. A. M. van Hest National Renewable Energy Laboratory
Patrick J. Smith The University of Sheffield
David B. Mitzi IBM T. J. Watson Research Center
Aoife Morrin Dublin City University
S3: Poster Session: Solution-Processed PV, TCOs, TFTs
Tuesday PM, November 29, 2011
Exhibition Hall C (Hynes)
1:00 AM -
S3.1 TRANSFERRED TO S1.9
S1: Solution-Processed Chalcogenides for Thin Film Photovoltaic Devices
Maikel van Hest
Tuesday AM, November 29, 2011
Room 310 (Hynes)
9:30 AM - **S1.1
Nanocrystal-Ink Routes and High-Yield Molecular Precursor Routes to Chalcopyrite and Kesterite Thin Film Solar Cells.
Hugh Hillhouse 1 Show Abstract
1 Chemical Engineering, University of Washington, Seattle, Washington, United States
The presentation will briefly summarize recent progress on nanocrystal-ink based routes to CIGS. However, given the terawatt scale of future energy needs, the most promising future photovoltaic materials should be Earth abundant with their primary mineral resources distributed across several geographic regions and their supply chains robust to reduce concerns of price volatility. In addition, the process of forming the solar cell should be scalable, low-cost, and not utilize dangerous or toxic materials. The strongest initial candidate appears to be Cu2ZnSnS4 (CZTS). Up until 2009, CZTS thin film solar cells were synthesized primarily by evaporating or sputtering metals (Cu, Zn, & Sn) followed by sulfurization. More recently, two potentially low-cost high-throughput approaches have been demonstrated that form the quaternary or pentenary chalcogenide directly from solution-phase processes. One is based on first synthesizing multinary sulfide nanocrystals and then sintering them to form a dense layer. The other approach utilizes molecular precursors dissolved in hydrazine. Both new approaches reach their highest device efficiencies by incorporating Se to form Cu2ZnSn(Sx,Se1-x)4 devices, and each has yielded substantially higher efficiency devices than the best vacuum deposited absorbers. The hydrazine route has yielded the most efficient CZTS-based devices thus far. The presentation will focus on our recent progress in CZTS-based nanocrystal-ink devices. In particular, we have shown that germanium may be alloyed with CTZS (at least up to Ge/(Sn+Ge) ratios of 0.7) to form Cu2Zn(Sn,Ge)S4 nanocrystals that have an increased bandgap . The defect chemistry is serendipitous, and allows for at least 6.8% efficient devices at high germanium content. This exciting prospect may be used to create a back surface field and direct carriers in a similar manner to how gallium is used in high efficiency CIGS devices. In addition, we will report recent results on a scalable chemical route to Earth abundant element thin film solar cells by coating a solution of highly soluble, inexpensive, and commercially available precursors in an environmentally friendly non-toxic solvent to form device quality films (without using nanocrystals or hydrazine).  Ford, G.M., Guo, Q., Agrawal, R., and Hillhouse, H.W., “Earth Abundant Element Cu2Zn(Sn1-xGex)S4 Nanocrystals for Tunable Band Gap Solar Cells: 6.8% Efficient Device Fabrication,” Chemistry of Materials 23 (10), 2626–2629 (2011).
10:00 AM - **S1.2
Advances in Kesterite Solar Cells by Solution-Nanoparticle Ink Processing.
Teodor Todorov 1 , David Aaron Barkhouse 1 , Santanu Bag 1 , Oki Gunawan 1 , Tayfun Gokmen 1 , Goislard de Monsabert Thomas 1 , S. Jay Chey 1 , David Mitzi 1 Show Abstract
1 Photovoltaic Science and Technology, IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Future terawatt-scale solar electricity requires lower photovoltaic (PV) module fabrication costs, reliance on more abundant materials and efficiency equal to or greater than current commercial technologies. Thin-film indium-based chalcopyrite devices have one of the fastest growth potentials and also highest efficiencies among thin-film PV technologies. Replacement of the scarce indium element is viewed as a strategic step towards unlimited growth of chalcogenide PV production beyond 100GWp/year. A strong candidate in this category is the kesterite family, including Cu2ZnSnS4 and Cu2ZnSn(S,Se)4 (CZTS and CZTSSe). We have developed kesterite solar cells operating in the 10% efficiency range. This performance is achieved by a simple low-cost liquid-based deposition approach based on chalcogenide solutions in combination with particle-based precursors that yield homogeneous CZTS and CZTSSe phases after thermal treatment. Here we will present recent advances in this field.
10:30 AM - S1.3
Air-Stable Hybrid Inks for Low-Cost CZTS Solar Cells.
Kyoohee Woo 1 , Youngwoo Kim 1 , Joosun Kim 2 , Jooho Moon 1 Show Abstract
1 Department of Materials Science and Engineering, Yonsei University, Seoul Korea (the Republic of), 2 Principal Research Scientist Center for Energy Materials Research, Korea Institute of Science & Technology (KIST), Seoul Korea (the Republic of)
Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is a very promising material as a low cost absorber alternative to other chalcopyrite-type semiconductors based on Ga or In because it is only composed of abundant and economical elements. In addition, CZTS has a direct band-gap energy of 1.0~1.5 eV and large absorption coefficient over ~104 cm-1, which are similar to those of Cu(In,Ga)Se2(CIGS) that is regarded as one of the most successful absorber materials. Typically, metal chalcogenide films such as CIGS and CZTS are deposited by vacuum process such as evaporation or sputtering. However, this vacuum deposition suffers from relatively low throughput, low material utilization, and difficulties associated with large-scale production. In this regard, solution-based deposition methods are being developed because they have advantages including suitability for large-area substrates, higher throughput, and more efficient materials usage. Various solution based approaches for producing absorber layer have been reported including sol-gel and nanocrystal dispersion, but they are still facing some limitations; sol-gel method is vulnerable to the contamination of carbon, oxygen, and other impurities from the precursor solution and inevitably leads to the formation of porous structure. Nanocrystal dispersion method needs the complex synthesis of nanocrystals. Recently, Todorov et al. reported the fabrication of CZTS thin film solar cells with 9.6% power conversion efficiency using a hydrazine based approach. However, hydrazine is a highly toxic and very unstable compound that requires extreme caution during handling and strorage. Furthermore, due to the reactive nature of this solvent, all processings for slurry and film preparations must be performed under inert atmospheric conditions and thus it would not be easily adapted for large-scale solar cell fabrication. With these considerations, it is highly desirable to develop a robust, easily scalable and relatively safe solution-based process for the fabrication of high quality CZTS absorber layer. Here, we devise the air-stable hybrid solution-particle approach for the fabrication of dense CZTS absorber layer. Our air-stable ink comprises of commercially available powder mixture of Zn, Sn, and S dispersed in an environmentally benign solvent in which Cu and Zn precursors are dissolved. The metal precursor solution is vulcanized by sulphur element to create a rubberlike polymer, providing the viscosity and wetting that allows us to prepare highly stable hybrid ink containing heavy metallic powders. Our readily achievable hybrid ink, without the involvements of complex particle synthesis and high toxic solvent, enables a convenient access to fabricate uniform, dense, contaminant-free, large-grained CZTS absorber layer. Our simple approach reported here will be the first step in realizing the low-cost and large-area solar cells with high efficiency.
10:45 AM - S1.4
Sonication Assisted Phase and Microstructure Evolution of CuInSe2 by Elemental Cu,In,Se Precursors.
Emre Yassitepe 1 , William Shafarman 2 , S. Ismat Shah 1 3 Show Abstract
1 Material Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States, 3 Physics and Astronomy, University of Delaware, Newark, Delaware, United States
Non toxic chemical routes that enable formation of high quality CuInSe2 thin films with high materials utilization are desired for low production cost of solar cells. We will present results on the effects of ultrasound on Cu,Se and In,Se elemental precursors with different solvents. Depending on the solvent, the reaction between these elemental precursors facilitates binary selenide phases. These crystalline phases are compared with the effective heat of formation model of Cu-Se and In-Se compounds. The formed binary selenide phases gave an exact match with the predicted effective heat of formation model. However, sonication of Cu,In and Se elemental particles for short times did not yield any single phase CuInSe2 due to the unfavored reaction pathways between CuSe2 and In4Se3 phases. Further annealing these binary phases led to the single phase formation of CuInSe2 at 350C. SEM studies revealed upon sonication above one micron sized Cu,In,Se elemental particles are broken to smaller particles during sonication and moreover upon completion of the reaction CuInSe2 nanocrystals are obtained.
11:30 AM - **S1.5
Understanding Hydrazine Solution Processed CuIn(Se,S)2 Solar Cells.
Yang Yang 1 , Choong-Heui Chung 1 , Bao Lei 1 Show Abstract
1 Materials Science and Engreeing, Univ. of California Los Angeles, Los Angeles, California, United States
CuIn(Se,S)2 and other chalcopyrite-based solar cells have received substantial research interest owing to their high optical absorption coefficient, a tunable band gap and demonstrated the highest power conversion efficiency among thin film solar cells. However, the use of vacuum technique in the preparation of absorber layers imposes hurdles in the the production of low cost and large area photovoltaic modules. Solution-based absorber deposition approaches offer an alternative to many of these issues, and have thus been extensively pursued for more than a decade. Recently, a hydrazine solution based processing method has demonstrated copper chalcopyrite based thin film solar cells with efficiencies of up to 13.6% despite the early developmental stage of this technique. In order to further improve this approached solar cells performance, deeper understanding on dissolving mechanism, the structure of molecular precursors, phase formation, defect physics and recombination mechanisms in such devices would be necessary. In this talk, we present the molecular structures present in solutions containing Cu2S, In2Se3, or mixtures of the two, and the CuIn(Se,S)2 phase formation after annealing the solution-processed precursor films. We also examine the effects of introducing an additional Cd ion solution soaking step into our device fabrication process on the defect energy levels and carrier concentration which in turn consistently improves the open circuit voltage of solar cell devices.
12:00 PM - S1.6
Studies of Carrier Recombination in CuIn(S,Se)2 through Photoluminescence Spectroscopy.
Hsin-Sheng Duan 1 , Wan-Ching Hsu 1 , Kuo-Chun Tang 2 , Bao Lei 1 , Pi-Tai Chou 2 , Yang Yang 1 Show Abstract
1 , University of California, Los Angeles, Los Angeles, California, United States, 2 , National Taiwan University, Taipei Taiwan
Solution-processed CuIn(S,Se)2 has shown considerable promise as an absorber material in the fabrication of high performance solar cells. However, it is crucial to indentify the primary factors limiting photovoltaic performance. In this study, photoluminescence spectroscopy is employed to investigate the carrier recombination in CuIn(S,Se)2 with emphasis on the effect of cadmium sulfide (CdS) deposition. In steady state photoluminescence, it has been found that CdS deposition (1) leads to an altered photoluminescence profile due to cadmium diffusion into the CuIn(S,Se)2 layer as well as defect passivation at the absorber surface. (2) CdS deposition helps to protect the CuIn(S,Se)2 film from chemical degradation, and also (3) the shallow defects in CuIn(S,Se)2 film at roughly 31 meV above the valence band and with density about 1015cm-3 are estimated through temperature-dependent photoluminescence measurements. These effects have been studied quantitatively, and the results are presented along with suggestions for how future devices can be optimized with respect to material parameters and fabrication strategies.
12:15 PM - S1.7
Solution-Processed Sintered Nanocrystal Solar Cells and Semiconductor Quantum Dot Energy Levels.
Scott Watkins 1 , Jacek Jasieniak 1 , Brandon MacDonald 1 2 , Paul Mulvaney 2 Show Abstract
1 Materials Science and Engineering, CSIRO, Melbourne, Victoria, Australia, 2 Chemistry, University of Melbourne, Melbourne, Victoria, Australia
The use and characterisation of solution processable inorganic semiconductors is highly relevant to the development of low-cost electronic devices such as solar cells. In this paper we will report on two aspects of our work in this area. Firstly, we will describe our layer-by-layer process that enables the fabrication of totally solution processable solar cells from inorganic nanocrystal inks in air at temperatures as low as 300 degrees Celsius. Focusing on a CdTe/ZnO thin-film system we report solar cells that achieve power conversion efficiencies of over 7% with greater than 90% internal quantum efficiency (IQE). We will describe the characterisation of the devices and, through capacitance-voltage measurements, demonstrate that the CdTe layer is fully depleted which enables charge carrier collection to be maximized. Finally, we also discuss the use of photoelectron spectroscopy in air (PESA) to investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS and PbSe semiconductor quantum dots (QDs). We will compare the results to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k-p theory and state-of-the-art atomistic semi-empirical pseudopotential modelling.
12:30 PM - S1.8
Improved Efficiency in a PbS Nanocrystal Heterojunction Solar Cell by Employing a Thin TiO2 Film and a Hole Transporting Polymer Layer.
Jangwon Seo 1 , Dongho Lee 1 , Alexander Cartwright 1 , Paras Prasad 1 Show Abstract
1 , University at Buffalo, Buffalo, New York, United States
We report a highly efficient hybrid nanostructured solar cell consisting of a densely-packed TiO2 film; a PbS nanocrystal film; a hole transporting polymer layer (HTL); and metal contacts. The TiO2 film was spray deposited on an ITO coated glass slide. Subsequently, a PbS nanocrystal film was deposited, layer-by-layer, on the TiO2 film to establish a planar donor-acceptor heterojunction. Two device geometries were fabricated on this structure by subsequently depositing either: i) a single Au contact layer or ii) a HTL followed by the Au contact layer. Drastic increases in the photocurrent, and stability in ambient air, was observed in the device with the HTL, relative to the device without the HTL. The device with the HTL layer had a power conversion efficiency of more than 5% under AM1.5G (100 mW/cm2). Moreover, the external quantum efficiencies of this device was very high (~90% in the visible range [400~600 nm] and ~30% in the near infrared range). The dependence of photovoltaic performance of the device on the thickness of the PbS nanocrystal film and the TiO2 film will be presented.
12:45 PM - S1.9
Adhesion Study of CZTS Layer Deposited from Ink Type Precursor onto Back Contact for Photovoltaic Applications.
Prashant Sarswat 1 , Michael Free 1 Show Abstract
1 Metallurgical Engineering, University of Utah, Salt Lake city, Utah, United States
CZTS (Cu2ZnSnS4) based ink has been emerged as a potential candidate for high throughput production of p-type layer on a back contact. An ink based absorber material can simplify device production process because of fast printing technology. The pre-synthesized liquid can also be used as a spray to get faster output. Although this technique is very fast, some issues such as adhesion of ink on back contact need to be addressed. A systematic study of adhesion of ink on different substrates using various solvents, binders and annealing temperatures is discussed. Some techniques for improved adhesion were proposed and demonstrated.
S2: Solution-Processed Oxides
Maikel van Hest
Tuesday PM, November 29, 2011
Room 310 (Hynes)
2:30 PM - **S2.1
Electrodeposition and Chemical Bath Deposition of Zinc Oxide Semiconducting Films and Nanostructures: A Case Example of a Potential Alternative to Classical Vacuum Deposition Routes.
Daniel Lincot 1 Show Abstract
1 , Institute of Research and Development of Photovoltaic Energy (IRDEP), Joint Laboratory CNRS-EDF-Chimie Paristech, Chatou France
Zinc oxide has become in recent years one of the key materials in the search for advanced applications in optoelectronics, photovoltaics, sensing and magnetic applications, as a low cost alternative to existing state of the art materials. ZnO, in contrast with many other key wide gap semiconductors has the unique advantage to be easily prepared in high quality from solution routes. This allows low temperature cost effective deposition processes (mostly <100°C) and also to take the advantage of the solution environment to promote highly-selective self-assembling processes to generate a wide range of structures from classical thin films to nanostructures . This explains why tremendous attention is now paid, mostly since 2000, to these new solution synthetic approaches. Three main approaches are Chemical bath deposition (CBD), also named chemical solution deposition (CSD), Hydrothermal deposition and electrodeposition (ED). All these methods, even if they are often presented separately, possess common roots based on solution chemistry, interfacial reactions and general concepts of heterogeneous nucleation and growth. The aim of this presentation is first to highlight these aspects. Then recent advances in the electrodeposition of ZnO thin films for TCO applications for thin film CIGS solar cells will be presented which may represent a breakthrough for replacing sputtering deposition. The deposition of nanostructures consisting in nanocolumns and nanoporous will be presented with possible applications in nanostructured solar cells. Recent results on epitaxial growth will be highlighted. Next avenues for research will be discussed. D. Lincot, MRS Bulletin, 35(2010)778-788 and references therein.
3:00 PM - S2.2
A Unifying Modular Approach to Solution Processed Dielectric Semiconducting and Conducting Mixed Metal Oxides: Synthesis, Characterization and Electronic Performance.
Joerg Schneider 1 Show Abstract
1 , TU Darmstadt, Darmstadt Germany
Transparent metal oxides are promising materials for applications in thin film electronic transisistors (TFTs). Due to their variable dopant content, their tunable properties their electronic performance can sometimes be tailored from insulating dielectric, to semiconducting to metallic behaviour. Especially, amorphous metal oxides of Zn, Al, Ga, In, and Sn for semiconducting and conducting oxides, and Zr and Hf for dielectric transparent oxides are interesting and thus have been studied extensively over the last couple of years worldwide. However, solution processing of such metal oxides under ambient conditions is still a major challenge since processing conditions under ambient conditions are necessary e.g. to use soft substrates, e.g. polymers for the thin film deposition. We report on a solely molecular based, modular approach which allows to tailor the elemental composition of resulting metal oxide films with respect to composition and performance obtained from such precursors as single, binary or ternary phases. This offers a synthetic route to dielectric, semiconducting and electrically conducting thin films under very similar processing conditions. T. Minami, New n-type transparent conducting oxides, MRS Bull. 25, 2000, 38-44
3:15 PM - S2.3
Solution Processing of Nb-Doped TiO2 and Ti-Doped In2O3 Transparent Conductors.
Robert Pasquarelli 1 , Maikel van Hest 2 , Alexander Miedaner 2 , Calvin Curtis 2 , Joseph Berry 2 , John Perkins 2 , Ryan O'Hayre 1 , David Ginley 2 Show Abstract
1 Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Transparent conducting oxide (TCO) thin-films play a critical role in many current and emerging opto-electronic devices due to their combination of high transparency in the visible region of the spectrum and tunable electronic conductivity. Nb-doped anatase TiO2 deposited by physical vapor deposition methods and on lattice-matched substrates has demonstrated potential as a replacement for indium-based TCOs such as indium tin oxide. Atmospheric-pressure solution deposition is an attractive alternative to these conventional vacuum-based TCO deposition techniques due to its ease and potential to lower device manufacturing costs. Here we report on 5 at% Nb-doped TiO2 films deposited from solutions of Ti(OEt)2(acac)2 and Nb(OEt)5. The desired anatase phase formed from an amorphous state on glass substrates at 500°C without the need of lattice-matching or a seed layer. Films demonstrated conductivities of 20-60 S/cm with mobilities ranging 0.1-0.6 cm2/Vs and a carrier concentration of 5x1020 /cm3. Processing conditions had a significant effect on the final electronic properties and will be presented. The Ti precursor was also used in combination with the novel low-temperature In precursor, In5O(OPr)13, to prepare 3 at% Ti-doped In2O3 films. Undoped films have demonstrated conductivities of 60-100 S/cm with mobilities as high as 10 cm2/Vs.
3:30 PM - S2.4
Electrochemically Triggered Deposition of Porous Zinc Oxide from Aqueous Precursor Solutions on Textile- Compatible Microelectrodes on the Way towards Textile-Based Photovoltaics.
Derck Schlettwein 1 Show Abstract
1 Institute of Applied Physics, Justus-Liebig-University Giessen, Giessen Germany
Over the last years electronics have been more and more integrated into textiles, mainly for sensing purposes. Interest rose to also integrate photovoltaics as an independent energy supply. Preparation of semiconductor films from precursor solutions represents the most promising approach because of good compatibility of the processes to a low thermal stability of textiles and to the need of a three- dimensional coating process. Water as solvent is optimum because of perfect environmental and textile chemical compatibility. Dye-sensitized solar cells represent a suitable photovoltaic cell technology since electrolytes are capable to bridge an insulating gap between the active electrodes of typically 10 - 100 µm in most finely woven fabrics.Thin porous films of ZnO were electrochemically deposited on silver- coated polyamide (ELITEX®) threads, filaments and knitted fabrics and subsequently sensitized by appropriate dyes. Such textiles had been established earlier as quite well conducting textiles, stable under environmental conditions. They were therefore chosen in a first step towards textile- integrated photovoltaics. Electrodeposition of zinc oxide from oxygen-saturated aqueous solutions was presented as a successful strategy to realize semiconductor structures on textiles and detailed control and influence of the deposition conditions was discussed . The role of microelectrode effects at such threads, options of forced convection, deposition under pulsed potential, alternative deposition baths and different substrate metals were investigated . However, corrosion problems in the typically used iodine- containing electrolyte hampered the use of ELITEX® in dye-sensitized cells. Thin tungsten wires served as model for a passivated surface. ZnO could still be deposited electrochemically but the active electrode structure was protected against corrosion. Recent work therefore concentrated on utilization of corrosion- resistant micro-fibers of stainless steel which are compatible to textile technologies. Structures with conversion efficiency close to 1 % under AM 1.5 conditions could be realized and thereby open the door towards devices of technical interest .1. T. Loewenstein, A. Hastall, M. Mingebach, Y. Zimmermann, A. Neudeck and D. Schlettwein, "Textile electrodes as substrates for the electrodeposition of porous ZnO," Phys. Chem. Chem. Phys. 10(14), 1844-1847 (2008).2. M. Rudolph, T. Loewenstein, E. Arndt, Y. Zimmermann, A. Neudeck and D. Schlettwein, "Pulsed electrodeposition of porous ZnO on Ag-coated polyamide filaments," Phys. Chem. Chem. Phys. 11(17), 3313-3319 (2009).3. T. Loewenstein, M. Rudolph, M. Mingebach, K. Strauch, Y. Zimmermann, A. Neudeck, S. Sensfuss and D. Schlettwein, "Textile-Compatible Substrate Electrodes with Electrodeposited ZnO-A New Pathway to Textile-Based Photovoltaics," Chemphyschem 11(4), 783-788 (2010).
3:45 PM - S2.5
Thin Film Formation of Transparent Conductive Oxides and Organic Materials by Solution-Based Mist Deposition Method toward Hybrid Device Applications.
Takumi Ikenoue 1 2 , Shizuo Fujita 2 Show Abstract
1 Department of Electronic Science and Engineering, Kyoto University, Kyoto Japan, 2 Photonics and Electronics Science and Engineering Center, Kyoto University, Kyoto Japan
Solution processable organic devices such as solar cells and light emitting diodes have attracted increasing interest due to its rapid efficiency-improvement, ease of processing, and cost-effective large-area processability. Many of organic devices have generally been fabricated by spin-coating technique, but material efficiency of this method is low and it is not suitable for large area fabrication. In addition, indium-tin oxide (ITO), which is used as a transparent electrode, is fabricated by vacuum processes, therefore it cannot take full advantages of the solution based processes. We report, in this presentation, a novel vapor-like-deposition technique based on a solution process, namely ultrasonic spray assisted mist deposition technique. Transparent conductive oxides such as ITO and zinc oxide can be fabricated by this technique without any vacuum systems. Additionally, this technique offers many advantages, for example, (i) it is possible to grow high quality thin organic films, (ii) this technique is suitable for large area substrates and roll-to-roll processes, (iii) source materials can be formed into films without many wastes, and (iv) owing to a vapor-deposition technique this deposition method allows hard-mask patterning without photolithography, which can simplify device fabrication processes, and easy formation of multilayer structures. In the mist deposition technique, the source solution is ultrasonically atomized and the aerosol particles formed are supplied onto a substrate using nitrogen carrier gas to form thin films. To fabricate ITO thin films, we used indium acetylacetonate and tin acetate as a starting source, which were diluted in methanol and deionized water with concentrations of 0.020 and 0.002 mol/L, respectively. A flow rate of nitrogen carrier gas and the substrate temperature were set at 5 L/min and 400 degrees centigrade, respectively. The ITO films were indicated the high transmittance and sheet resistance of less than 10 Ω/square, which was reasonable to be applied as a transparent electrode. Similarly, by optimizing the deposition conditions of organic thin film formation, thicknesses of organic thin films have been well controlled and the properties such as conductivity values and roughness of those were comparable to those deposited by spin-coating. The detailed properties of the films and some examples of device applications such as solar cells will be demonstrated at the presentation.
4:30 PM - **S2.6
Solution Based Processing for Cost Effective Smart Windows.
Robert Tenent 1 , Chi-Ping Li 2 , Feng Lin 4 , David Alie 3 , Colin Wolden 2 , Ryan Richards 4 , Sean Shaheen 3 , Anne Dillon 1 Show Abstract
1 Chemical and Materials Sciences, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Materials Science, Colorado School of Mines, Golden, Colorado, United States, 4 Chemistry, Colorado School of Mines, Golden, Colorado, United States, 3 Physics, University of Denver, Denver, Colorado, United States
Internal environmental control in buildings is the largest source of electrical energy use in the United States. “Smart” windows which dynamically control the transmission of both heat and visible light could potentially save roughly 30% of this energy amounting to almost $40 billion in total energy savings annually. At present, the smart windows market is dominated by vacuum processed metal oxide-based electrochromic devices. These devices typically consist of a five layer structure that includes two electrode materials separated by an ion conductor with these three layers sandwiched between transparent, but electrically conducting contacts. These devices change tint upon applying a small voltage (~1V) and act via an ion intercalation mechanism similar to battery materials. A consensus based on energy efficiency modeling as well as windows industry input has determined that prices must drop to between $20-$25/ft2 in order for this technology to be market viable. Present market prices vary from $50 to $100/ft2.Here we will present what we believe to be fruitful routes toward low-cost production of multilayer electrochromic devices. Our work is focused on solution processing of electrochromic metal oxide layers as well as ion conductors using the ultrasonic spray deposition method. Two possible manufacturing options will be presented. The first will be deposition on flat glass substrates and possible integration with the well known float glass process. We will also present work aimed at the development of roll-to-roll (R2R) processing methods using inexpensive flexible plastic substrates. To date we have demonstrated a high temperature deposition of lithium-doped nickel oxide1 material that is likely compatible with float glass processing as well as a low temperature deposition of tungsten oxide2 that may be suitable for R2R methods. We will touch briefly on these results and primarily discuss further work toward deposition of ion conductor materials and initial demonstrations of integrated device structures. Ion conductor work will primarily focus on development of gel electrolyte materials that may be compatible with atmospheric pressure deposition onto flexible substrates.1.Tenent, R. C.; Gillaspie, D. T.; Miedaner, A.; Parilla, P. A.; Curtis, C. J.; Dillon, A. C., Fast-Switching Electrochromic Li+Doped NiO Films by Ultrasonic Spray Deposition. Journal of the Electrochemical Society 2010, 157 (3), H318-H322.2.Li, C.-P.; Tenent, R. C.; Wolden, C.; Dillon, A. C., Electrochromic Films Produced by Ultrasonic Spray Deposition of Tungsten Oxide Nanoparticles. Solar Energy Materials and Solar Cells, 2011, in press, doi:10.1016/j.solmat.2011.03.034
5:00 PM - S2.7
Structure, Sodium Ion Role, and Practical Issues for β-Alumina as a High-K Solution-Processed Gate Layer for Transparent and Low Voltage Electronics.
Bo Zhang 1 , Yu Liu 1 , Howard Katz 1 Show Abstract
1 , Johns Hopkins University, Baltimore, Maryland, United States
β-alumina (SBA) has been discovered as a promising high dielectric constant gate material for solution processed, transparent and low voltage transistor devices. Some experimental evidence indicates that the mobile Na ion within two spinel blocks made by Al and O is responsible for the high dielectric constant. Transistors (W/L ratio 10) using SBA as gate layer and sol-gel-processed zinc tin oxide (ZTO) as active layer only need 2V to obtain 0.7mA output current. The calculated field driven mobility for the devices is 22cm2/Vs. SBA material is compatible with organic semiconductors such as NTCDI and pentacene as well. Some key issues regarding using SBA for real applications, such as device stability in the ambient atmosphere, response under high frequency, and threshold voltage shift under gate bias have also been studied. It is found that encapsulating the device with CYTOP fluorinated polymer is an effective way to increase the operational stability of the devices in the ambient environment.
5:15 PM - S2.8
Solution-Processed Oxide Thin-Film Transistors Using La-Ta-O/Bi-Nb-O Stacked Gate Insulator.
Takaaki Miyasako 1 3 , Masatoshi Onoue 1 , Eisuke Tokumitsu 1 3 , Tatsuya Shimoda 1 2 Show Abstract
1 ERATO SHIMODA Nano-Liquid Process Project, JST, Nomi, Ishikawa, Japan, 3 Precision and Intelligence Laboratory, Tokyo institute of Tecnology, Yokohama, Kanagawa, Japan, 2 School of Material Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan
More than a decade, a solution-based fabrication process has attracted much attention as an alternative to the conventional vacuum deposition process for fabrication of various electronic devices, because this process could facilitate dramatic low cost and low energy processing due to its low equipment costs, process simplicity and direct patternability by using various printing techniques. In recent years, oxide-based thin-film transistors (TFTs) with solution-processed conductive oxide channels such as ZnO, In-Zn-O and In-Ga-Zn-O have been reported. However, most of those TFTs employed solution process only for a channel layer while the rest parts of transistors were fabricated by conventional vacuum deposition processes. In order to achieve ultra low cost and low-energy fabrication, it is very important to realize a total solution-based process, which means solution derived materials are applied not only to a channel layer but also to other layers in the transistor including a gate insulator layer and electrodes. Meanwhile it is needless to say that the totally solution processed transistors ought to have a high device performance for putting them into practical use. With those motivations, we already developed a ferroelectric-gate thin-film transistor in which all parts of the device were fabricated from solution-derived materials only and confirmed their good device performance  In this study, we focused on solution-processed oxide TFTs using stacked paraelectric gate insulator to achieve good electrical performance. We fabricated oxide TFTs on a glass substrate in which a gate electrode, a gate insulator and a channel were fabricated by chemical solution deposition (CSD) process. We adopted a LaNiO3 (LNO) film as a gate electrode, La-Ta-O (LTO)/Bi-Nb-O (BNO) stacked film as a gate insulator, In-Zn-O film as a channel and ITO film as source-drain electrodes. In particular, the LTO thin film was an amorphous insulator with a relative permittivity of ~22 and the leakage current of ~10-6A/cm2 at 1MV/cm applied field while the BNO thin film was a polycrystalline insulator with the relative permittivity of ~170 and the leakage current of ~10-4A/cm2 at 1MV/cm applied field. By stacking LTO and BNO films, a good gate insulator was developed, in which the relative permittivity, the leakage current and the surface flatness were simultaneously satisfied. The fabricated TFTs exhibited typical n-channel transfer characteristics with no hysteresis and good saturation in out-put characteristics. The obtained on/off current ratio, subthreshold voltage swing and field-effect mobility were about 108, and 220mV/decade and 8.5cm2/Vs, respectively.  T. Miyasako, B. N. Q Trinh, M. Onoue, T. Kaneda, P. T. Tue E. Tokumitsu, T. Shimoda Appl. Phys. Lett., 97, 173509 (2010).
5:30 PM - S2.9
All-Solution-Processed Fully Transparent ITO/GaZnO/ITO Resistive Random Access Memory.
Areum Kim 1 , Keunkyu Song 1 , Youngwoo Kim 1 , Jooho Moon 1 Show Abstract
1 Department of Materials Science and Engineering, Yonsei University, Seoul Korea (the Republic of)
Transparent electronics has attracted great interests in recent years due to their fascinating potential to make significant impacts in the applications like transparent displays, transparent supercapacitors, ultraviolet (UV) detectors, and solar cells. Transparent non-volatile memory device based on the oxide semiconductor TFT has also been demonstrated, but they suffer from high voltage operation, complicate structure, and short retention time. Resistive random access memory (RRAM) is a promising device due to its great characteristics such as nonvolatility, low power consumption, fast operation speed, and simple structure, which can replace flash memory that faces scaling-down limitation. Resistive switching phenomenon has been observed in various oxides such as TiO2, NiO, ZnO, HfO2, Ta2O3, WO3, MnO2 and ZrO2. These oxides are mostly transparent having wide-band gap energies, so that RRAM can be a good candidate enabling transparent memory.After the first demonstration of fully transparent RRAM (TRRAM), various configurations such as ITO/TiO2/ITO, ITO/ZnO/ITO, ITO/ZnO:Mg/FTO, ITO/Gd2O3/ITO, ITO/InGaZnO/ITO, and Ga:ZnO/ZnO/Ga:ZnO have been reported. The active layers are either vacuum-deposited or solution processed, whereas the transparent electrodes are usually deposited by radio frequency (rf) magnetron sputter. This vacuum processing raises manufacturing costs, posing an obstacle for modern, mass-produced, large electronics. Scalable large-area fabrications based on solution processing are the object of growing interest with the ultimate goal of fabricating invisible electronics. Here, we demonstrate all-solution-processed fully transparent RRAM (sol-TRRAM) using ITO/GaZnO(GZO)/ITO configuration. All layers including an active layer and top and bottom electrodes were fabricated by solution-processing, and the transmittance of ITO/GZO/ITO/glass stacks was 87 % at 400 nm. The microstructure of ITO/GZO/ITO device was analyzed by high-resolution transmission electron microscopy and X-ray diffraction. X-ray photoelectron spectroscopy was performed on GZO thin film to characterize its structural information and defect state. To clarify the role of ITO electrode during switching, top electrode was varied to Pt, Al, and Ti, and resistive switching behavior were compared as a function of the electrode types. Our sol-TRRAM shows bipolar current-voltage (I-V) switching characteristics with a low operation current (<10-4A) and good endurance property under room temperature with few current fluctuations in the ON and OFF states.
S3: Poster Session: Solution-Processed PV, TCOs, TFTs
Tuesday PM, November 29, 2011
Exhibition Hall C (Hynes)
9:00 PM - S3.11
Sol-Gel Processing of Silver-Incorporated Hybrid Indium Tin Oxide (ITO) Thin Films.
Mehmet Kesim 1 , Caner Durucan 1 Show Abstract
1 Metallurgical and Materials Engineering, Middle East Technical University, Ankara Turkey
The performance of ITO films in optoelectronic applications as transparent electrode is mainly defined by their intrinsic electrical conductivity and optical properties. The conductivity of ITO films can be further improved by incorporation of noble metal nanoparticles as they provide additional charge carriers. In this study, development of such silver-incorporated hybrid ITO film (Ag-ITO) on soda-lime-silica glass has been realized by aqueous processing routes, by sol-gel. As a wet processing method which is also advantageous due to its feasibility in coating large area substrates and also due to simple processing needs compared to vacuum techniques. The ITO sols were prepared from metal salts (In(NO3)3.xH2O, SnCl4.5H2O), acetylacetone and ethanol in which silver nitrate (AgNO3) was used as silver source. The effect of silver amount and number of Ag-ITO coating layers on microstructural, electrical and optical properties of the resultant spin-coated films have been reported. It has been shown that stable Ag-ITO coating sol can be prepared by diluting a plain ITO sol in the presence of polyvinylpyrrolidone (PVP) serving as stabilizing agent. Addition of silver at all proportions (0.01-0.05 M) enhances the film conductivity, however the improvement in electrical conductivity was more pronounced at high silver contents. On the other hand, high silver containing ITO films exhibit lower transparency (around 86%) in the visible range due to surface plasmon absorption of silver nanoparticles. It was observed that increasing the number of Ag-ITO coating layers do not only lowers the transparency of the ITO films, but also leads to stronger particle-particle and particle-matrix interactions and enhanced growth of silver nanoparticles after each deposition-calcination cycle. The results illustrate the potential of processing Ag-ITO films of improved conductivity by sol-gel routes, where the performance properties can be easily controlled and optimized by the choice of a couple of processing parameters, by the amount of silver in the coating sol and by the number of coating layers.
9:00 PM - S3.12
Low Temperature Microwave-Assisted Solvothermal Synthesis of TiO2 Thin Films.
Katharine Harrison 1 , B. Reeja Jayan 2 , Arumugam Manthiram 1 2 Show Abstract
1 Mechanical Engineering, University of Texas at Austin, Austin, Texas, United States, 2 Materials Science and Engineering, University of Texas at Austin, Austin, Texas, United States
Titanium dioxide thin films with the anatase structure have widespread applications including solar cells, batteries, and electronics such as detectors, sensors, and transistors. Anatase films typically must be sintered at temperatures in the range of 450-600 oC, which is too high for some applications. Therefore, it is critical to develop low-temperature thin film synthesis methods for applications such as organic solar cells since the TiO2 must be deposited on plastic substrates which cannot withstand high temperatures. We have developed a novel sol-gel based microwave-assisted solvothermal method to synthesize thin anatase films on indium tin oxide (ITO) coated glass slides. We have confirmed through glancing angle X-ray diffraction (XRD) analysis that anatase films form at a solution temperature as low as 150 oC and the films form only on the indium tin oxide. This is likely because the microwaves interact strongly with the ITO leading to local heating of the ITO film, which acts as a nucleation site for TiO2. The films can be synthesized directly by placing an ITO coated glass slide in the microwave vessels or the sol-gel can be spin coated onto the ITO and then microwaved. Additionally, the film can be patterned by etching the ITO, and the TiO2 only forms on the ITO. The effects of temperature, reaction time, solvent, and slide orientation on film thickness, uniformity, and film cracking have been examined in this work.
9:00 PM - S3.13
Deposition of Metal Oxide Thin Films in Supercritical Carbon Dioxide.
Akiteru Kojima 1 , Yuichiro Hirota 1 , Koki Nakata 1 , Takeyasu Saito 1 , Naoki Okamoto 1 , Kazuo Kondo 1 , Seiichi Takami 2 Show Abstract
1 , Osaka Prefecture University, Sakai-shi Japan, 2 , Institute of Multidisciplinary Research for Advanced Materials Tohoku University , Sendai-shi Japan
Deposition of Metal Oxide Thin Films in Supercritical Carbon DioxideMetal oxide thin films are of great interest for various applications depending on their chemical, electrical and magnetic properties. A chemical vapor deposition (CVD) method can deposit uniform metal oxide thin films, like SiO2, Al2O3 and ZnO etc, however, it is difficult to deposit the compositionally uniform multi-component oxide thin films. Therefore, a novel film-forming method should be developed. A method using supercritical carbon dioxide (scCO2) is one of the promising alternatives. Supercritical carbon dioxide possesses unique characteristics, such as high diffusivity, high solvent capability, low viscosity and zero surface tension, which allows possible candidate for multi-component thin films deposition with conformal step coverage and superior compositional uniformity. In this work, deposition characteristics of the Al2O3 thin films and ferroelectric Bi4Ti3O12 (BIT) thin films in scCO2 were investigated. Al(acac)3 was used as a precursor for Al2O3 deposition and Bi(C6H5)3 and Ti(O-i-C3H7)2(DPM)2 were used as precursors for BIT deposition, in which the ratio of Bi(C6H5)3/Ti(O-i-C3H7)2(DPM)2 was varied from 0.75 to 1.5. A precleaned Si substrate (1 cm×4 cm) was set in reactor, then prescribed amount of precursors and additive agents were introduced in the reactor. After sealing, CO2 was pressurized to desired pressure and the temperature of the reactor was increased to growth temperature set point (260-400°C) and the pressure in reactor was then approximately 16 MPa. After reaction was completed, surface morphology and thickness of deposited films were evaluated by field emission scanning electron microscopy (FE-SEM) and ellipsometry, respectively. The crystallographic structures of BIT films were determined by X-ray diffraction before and after 1 hour annealing at 800°C in oxygen.For Al2O3 deposition, the growth rate was enhanced about 5 times by isopropyl alcohol addition and the growth rate ranged from 9.8 to 4.5 nm/min at the temperature from 320 to 260°C. The activation energy calculated from the Al2O3 deposition rate in scCO2 was 59.2 kJ/mol without additives and 32.5 kJ/mol with additives, respectively. This activation energy was lower than that of a vacuum-based process (80-100 kJ/mol). For BIT deposition, only Bi2O3 peaks were observed before annealing, however, BIT peaks appeared after were annealing when the ratio, Bi(C6H5)3/Ti(O-i-C3H7)2(DPM)2, was 1.5, which suggests that this films consists of Bi2O3 and BIT. This trend was almost the same with other precursor ratios. Further compositional evaluation and optimization is under study. Q. Peng et al., Thin Solid Films 516 (2008) 4997-5003.
9:00 PM - S3.15
High Performance Bi-Nb-Ox Thin-Film Capacitors Fabricated by Chemical Solution Deposition Process.
Masatoshi Onoue 1 , Takaaki Miyasako 1 2 , Eisuke Tokumitsu 1 2 , Tatsuya Shimoda 1 3 Show Abstract
1 , Japan Science and Technology Agency, Nomi Japan, 2 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama Japan, 3 , Japan Advanced Institute of Science and Tecnology, Nomi Japan
Various metal-oxide paraelectric insulators have been studied for long time, and those have been used not only in passive components such as voltage-tunable devices, high density storage condensers for telecommunication system but also in active components such as gate insulators for thin-film transistors and metal-oxide-semiconductor field effect transistors. The required performances for those insulators are dependent on device applications. In particular, we focused on paraelectric thin-film capacitors which have a high relative permittivity (>100) and a low dielectric loss. Among them, (Ba, Sr)TiO3 (BST) paraelectric thin films have been widely investigated because of their high relative permittivity (~400). However, they have inherent problem of large dielectric loss. Meanwhile, bismuth based paraelectric insulators such as Bi-Nb-Ox (BNO) and Bi-Zn-Nb-Ox (BZN) have been considered as an alternative to BST due to their low dielectric loss as well as high relative permittivity. But there have been not many reports about BNO (Bi/Nb=1/1) thin-films and the obtained relative permittivity was only around 50 by using mixed oxide method . In this study, we report a new type of the BNO (Bi/Nb=1/1) film having very high relative permittivity via chemical solution deposition (CSD) process. We fabricated a BiNbOx (BNO) thin film (~170nm) on Pt substrate (Pt/TiOx/SiO2/Si) by using CSD process. The precursor solution of the BNO film was prepared by dissolving metal organic based bismuth and niobium in alcohol solvent. The BNO precursor solution was spin-coated on a Pt substrate, dried at 250°C and then annealed at 550°C in O2 for 20min. Then, top electrodes of Pt (~150nm) were deposited on the BNO film by sputtering to make a capacitor. As a reference, a sputtered BNO capacitor was also fabricated on a Pt substrate. The obtained relative permittivity and dielectric loss at room temperature of the solution processed BNO capacitor were about 170 and 1.6×10-3 at 1kHz applied frequency, respectively, while those of the sputtered BNO capacitor were about 65 and 4.8×10-3, respectively. These high properties of the solution processed BNO films were repeatedly confirmed, resulting in the values between 150 and 200. We observed cross sectional TEM images and found that there existed a unique phase structure, which could be considered as a cause of the high relative permittivity. It is concluded that during the decomposition process from solution to a solid film through a gel condition BNO precursor is put in a proper condition to decompose to such a special phase as would cause a high properties. It is clearly said the solution process created a new type of material. Details will be reported on the conference. D. Zhou et al., Appl. Phys. Lett. 90, 172910 (2007)
9:00 PM - S3.16
Donor and Defect States in Solution-Processed Zinc Tin Oxide.
W. Hu 1 , I. Bonachera 1 , R. Peterson 1 Show Abstract
1 Electrical and Computer Engineering, University of Michigan, Ann Arbor, Michigan, United States
Solution processes such as dip-coating, spin-coating and ink-jet printing are now commonly used techniques for the deposition of zinc oxide and zinc tin oxide (ZTO), which are promising materials for the active layer of transparent thin film transistors (TFTs) used in large area displays. Others have made solution-processed ZTO transistors with mobilities from 0.03 cm2/(V●s) to 16.2 cm2/(V●s) and vacuum-processed ZTO from 5 cm2/(V●s) to 50 cm2/(V●s), and ZTO TFTs have shown good electrical stress stability. Here we investigate the temperature dependence of solution-processed TFT performance to explore the donor and defect states of these materials.Back gate, top contact TFTs are fabricated. Heavily doped Si is used as the gate and thermally grown SiO2 as the gate dielectric. The active layer is deposited by spin-coating precursor solutions and subsequent annealing. The solutions are prepared using zinc acetate dihydrate and/or tin (II) acetate in 2-methoxyethanol. Contact electrodes are sputtered Aluminum. ZTO TFTs exhibit room temperature mobilities of 0.5 cm2/(V●s), turn-on voltages of 12 V, sub-threshold slopes of 1.6 V/dec, and ON/OFF current ratios of > 105 (limited by gate current).Cryogenic properties of the fabricated ZnO and ZTO TFTs are studied. The field-effect mobility of ZTO TFTs annealed at 500°C obeys an Arrhenius function over temperatures ranging from 300K to 77K. The activation energy is 14.7meV. This shows that ZTO has a large concentration of shallow donors, presumably oxygen-vacancy related, so that mobile electrons can easily populate the conduction band at room temperature. The ZnO films, which are typically resistive with sheet resistance of about 600 kΩ per square, show similarly shallow dopants with only a 3.4x reduction in conductivity from 300K to 77K.Moreover, as the temperature is reduced from 300K to 77K the turn-on voltage shifts from +12 V to +42 V, while the sub-threshold slope increases only slightly. The positive shift in transistor turn-on as temperature is reduced reveals the existence of positively charged trap states deep in the band, with an areal density of 2.9x1012 q+/cm2 which compares favorably with typical interface state densities for SiO2. In ZTO TFTs, these trap states might be associated with dangling bonds, grain boundary defects, insulator-semiconductor interface states or states at the exposed semiconductor surface. Work is ongoing to characterize more precisely the nature of these states.
9:00 PM - S3.17
P-Type Amorphous Oxide Semiconductors Ln-Ru-O from Solution Processing.
Jinwang Li 1 , Eisuke Tokumitsu 1 2 , Mikio Koyano 3 , Tatsuya Shimoda 1 3 Show Abstract
1 ERATO, Shimoda Nano-Liquid Process Project, Japan Science and Technology Agency (JST), Nomi, Ishikawa, Japan, 2 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama Japan, 3 School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan
We have synthesized a class of amorphous semiconductors Ln-Ru-O (Ln = lanthanide elements except Ce) using a simple solution deposition process. The films show low resistivity (down to 10-2 – 10-3 Ω cm) around room temperature. La-Ru-O was analyzed as a representative, with comparison to sputtered samples. Resistivity-temperature relation analysis revealed semiconducting behavior with carrier transport by variable range hopping. Seebeck measurement indicated the carriers were p-type. The valence band structure was analyzed using ultraviolet and X-ray photoelectron spectroscopy, which showed a valence band hybridized with O 2p and Ru 4d orbitals, explaining the origin of p-type conduction. The optical band gap is around 1 eV. The film surfaces have excellent smoothness (root mean square roughness <0.3 nm and peak-valley < 3 nm by AFM on 1 μm × 1 μm area). The solution process is not only simple and of low cost, but also resulted in films of lower resistivity. Compared to other oxides and processes, these materials have several advantages. First, they may be the lowest in resistivity (around 2 orders lower) among amorphous oxides deposited by solution techniques. Second, they are novel p-type amorphous oxides originated by a solution process. Before these oxides, only sputtered Zn-Rh-O with a resistivity of ~0.5 Ω cm was known as a p-type amorphous oxide. Third, they have excellent thermal stability, remaining amorphous state up to 800°C. Fourth, the solution processing temperature can be 400°C or lower. Further, an advantage to stress for amorphous materials over crystals is that amorphous materials are well compatible with scaling down. By using these, it would be very easy to make a nano-pattern, because amorphous materials are neither constrained by the crystal size, nor distorted/damaged during crystallization as found in the process of crystalline materials. Therefore, these materials have a high potential in electronic applications. The solution process offers further advantages for low cost and direct patterning (e.g., by imprinting).
9:00 PM - S3.18
Influence of Dye Adsorption Conditions on the Properties of Electrodeposited, Dye-Sensitized Zinc Oxide Films.
Melanie Rudolph 1 , Silvia Schmandt 1 , Derck Schlettwein 1 Show Abstract
1 Institute of Applied Physics, Justus-Liebig-University Giessen, Giessen Germany
Electrodeposition of nanoporous zinc oxide from aqueous solutions is a viable low-cost and low-temperature way to obtain high surface area films suitable as photoelectrodes in dye-sensitized solar cells (DSCs). Nanostructuration of the films is achieved directly during growth by the presence of organic template molecules like eosinY or coumarin 343 in the plating bath. Such molecules selectively bind to the surface of zinc oxide and thereby act as structure-directing agents. This kind of electrochemical self-assembly leads to formation of inorganic/organic hybrid materials in which the inorganic and organic phases are intimately mixed on the nm scale. The template molecules can be completely removed afterwards by dipping the hybrid films into a slightly alkaline solution at room temperature, yielding a sponge-like, pure ZnO matrix of well-defined crystallinity and electrical connectivity. In order to obtain dye-sensitized photoelectrodes, the nanostructured semiconductor films are processed in a solution of a photosensitizer like, e.g., the indoline dye D149, which binds to the oxide surface via its carboxylic acid function. Optimization of the processing conditions is crucial to obtain high-efficiency DSCs, because the semiconductor-dye bond and the molecular arrangement on the surface are critical factors determining the rate of electron injection, their recombination and hence the quantum efficiency of overall photon-to-electron conversion. The present study comprised variation of the conditions of dye adsorption on nanostructured zinc oxide films electrodeposited in the presence of eosinY. For this purpose, ZnO samples of different film thickness and porosity were prepared. The films were characterized by profilometry, color confocal scanning laser microscopy, atomic absorption spectrometry and scanning electron microscopy. Functionalization was reached by immersion into an organic solution of D149 at room temperature, where the composition of the dye bath and the residence time in the solution were varied. The amount of sensitizer molecules on the films was determined by UV/Vis absorption spectroscopy in solution and in solid state. The influence of structural features on the uptake of dye was studied. Photoelectrochemical characterization (I-V curves, time and frequency resolved photocurrent and photovoltage measurements) of the dye-sensitized ZnO films in contact with an organic iodide/triiodide electrolyte was performed. The influence of overall dye content and of the dye distribution on the inner electrode surface on individual steps of photoelectrochemical energy conversion will be discussed.
9:00 PM - S3.2
Aqueous Solution Deposition of CIS/CIGS Solar Absorber.
Wei Wang 1 , Benjamin Clark 2 , Robert Kykyneshi 1 , Douglas Keszler 1 Show Abstract
1 Chemistry, Oregon State University, Corvallis, Oregon, United States, 2 , Inpria Corp., Corvallis, Oregon, United States
CuInS2 and CuIn1-xGaxS2 (0 ≤ x ≤ 0.5) thin films have been deposited from a one-pot aqueous precursor via spin coating in air followed by rapid thermal annealing in an inert atmosphere. Single crystals of the Cu and In precursors have been isolated and characterized by single crystal X-ray diffraction. Thin-film X-ray diffraction results are consistent with the formation of single phase sulfide films. The change in unit-cell dimensions are consistent with the replacement of In by Ga in the CuIn1-xGaxS2 materials. Complete results on the thin films with respect to composition (TOF-SIMS, EPMA); film morphology (SEM); optical absorption coefficients (transmission, reflection, spectroscopic ellipsometry); and electrical properties (majority carrier type, carrier concentration, and mobility) will be described. Progress towards production of micron-thick films on Mo-coated glass will be presented.
9:00 PM - S3.20
Solution Transformation of Metal Thin Films to II-VI Semiconductor Films.
Adam Gross 1 Show Abstract
1 , HRL Laboratories, LLC, Malibu, California, United States
Direct deposition of semiconductor layers over large areas is challenging and the films frequently require high temperature sintering to attain low sheet resistances. Another route to forming II-VI semiconductor materials is through the low temperature solution based sulfurization and ion exchange of silver nanoparticles. Because silver films may be scalably deposited on many substrates through evaporation, electro- and electroless plating, we investigate the low temperature solution based transformation of these layers to II-VI thin films. This chemical transformation is problematic in thin films because diffusion occurs directionally and the ~50% increase in lattice expansion from silver to semiconductor layers may debond the film. Silver films were deposited on glass with adhesion layers and successfully transformed to well adhered CdS, MnS, and CuS films at temperatures ≤ 60 °C. Sheet resistances as low as 56 ohms per square for CuS and 97 ohms per square for Cu doped MnS were obtained. Characterization of the II-VI films using optical absorption spectroscopy, SEM, and XRD will be presented. These results demonstrate that solution based sulfurization and ion-exchange is a viable mechanism for forming II-VI semiconductor thin films with good conductivities from metal films at low temperatures.
9:00 PM - S3.21
Preliminary Work on Electrodeposition of a Photovoltaic Using E-ALD.
Brian Perdue 1 , John Stickney 1 Show Abstract
1 , University of Georgia, Athens, Georgia, United States
Current progress in the formation of a CdTe/CdS photovoltaic by Electrochemical Atomic Layer Deposition (E-ALD) will be presented. E-ALD is a condensed phase variant of atomic layer deposition (ALD). In ALD the deposition of a compound is achieved by means of sequential surface limiting reactions in which the components are deposited one atomic layer at a time. In E-ALD the individual atomic layers are formed using underpotential deposition (UPD), an electrochemical surface limited reaction. XRD results show that the CdTe layer has a strong preferred <111> orientation as deposited. EPMA results show stoichiometric Cd, with a Cd/Te ratio of 1.02. Photoelectrochemical results indicated a band gap of 1.5 eV, p-type as deposited, no annealing. Different CdS deposition methods were also investigated. CdS was deposited by chemical bath deposition (CBD), successive ionic layer absorption and reaction (SILAR) and E-ALD. Results showed that the average Cd/S ratio, determined using EPMA, was 1.07, 1.10, and 1.05 for CBD, SILAR and ALD respectively. Photoelectrochemical results show that the photoresponse for E-ALD grown CdS was the highest and CBD was the least photoactive. As exciton creation in the CdS layer is parasitic to the photovoltaic, CBD CdS was used in first attempts at PV construction. Solar cells were fabricated in the substrate configuration, using 200 nm of E-ALD grown CdTe and 80 nm of CBD grown CdS. 100 nm ITO was magnetron sputtered on top to complete the cell. Cells achieve a maximum current density of 103 μA/cm2 when exposed to light; however appear to suffer from shunts, work on these defects is underway.
9:00 PM - S3.22
Solution Processing of Ag Nanowire Hybrid Electrode for Organic Photovoltaic Devices.
Ju Hwan Choi 1 , Sujin Baek 1 , Dang-Mo Yoon 1 , Hak-jun Chung 1 , Young Kyu Hong 1 , BumJoo Lee 1 , Jin-Koog Shin 1 Show Abstract
1 Korea Printed Electronics Center, Korea Electronics Technology Institute, Jeonju, Jeonrabook-do, Korea (the Republic of)
Printed devices especially organic photovoltaic (OPV) devices have gained strong attention due to the possibilities of low-cost fabrication as well as mechanical flexibility. Manufacturing printed device is thought to be inexpensive but the materials related it is remained persistently expensive. Many have addressed this issue by replacing expensive transparent conductive indium tin oxide (ITO). Furthermore, market dominant ITO films on flexible substrates readily crack leading to device failure. Achieving highly conductive and transparent electrode materials are one of the challenging tasks towards inexpensive, high yield printed OPV. Various materials including Ag nanowire have been investigated as candidate materials for printed electrodes, wiring, and conductive film substituting the oxide-based electrode materials. Solution processing of Ag nanowires which forms random network  or patterned grid  for transparent conductive electrodes showed promising performance compared to ITO. The combination of Ag nanowire and conjugated polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as mixture and/or multilayer were investigated and optimized with various processing conditions as well as additives. We have explored the effects of these silver nanowires and PEDOT:PSS combinations on the performance for the printed OPV. In this work, different types of conductive PEDOT:PSS mixtures with additives for Organic Photovoltaic were printed forming Ag NWs random network or multilayer and followed by layers of Poly-(3-hexylthiophene-2,5-diyl) and C60-butyric-acidmethyl-ester (P3HT:PCBM). Introducing these electrodes in our OPV cells, the short-circuit current and the fill factor increased with optimized printing process conditions. Utilizing these results, equal or even better efficiencies for large area printed OPV devices can be reached. These combinations of Ag nanowire and PEDOT:PSS have proven to be an effective way as a low-cost and easy process for printed conductive transparent substrates which can be applied to other printed devices.  J. Y. Lee, S. T. Connor, Y. Cui, and P. Peumans, Nano Lett. vol. 8, 689-692 (2008).  J. Zou, H.-L. Yip, S. K. Hau, and A. K.-Y. Jen, Appl. Phys. Lett. 96 203301 (2010).
9:00 PM - S3.23
Large-Area Solution-Processed Superhydrophobic and Broadband Antireflective Organically Modified Silica Coatings for Solar Cells.
Adem Yildirim 1 2 , Hulya Budunoglu 1 2 , Bihter Daglar 1 2 , Mehmet Bayindir 1 2 3 Show Abstract
1 , UNAM-National Nanotechnology Research Center, Ankara Turkey, 2 Material Science and Nanotechnology Institute, Engineering and Science , Ankara Turkey, 3 Department of Physics, Bilkent University, Ankara Turkey
Dust accumulation on the surface of solar cells degrades solar cell performance. To prevent this transparent superhydrophobic surfaces are desired because of their self-cleaning property. However, preparation of transparent superhydrophobic surfaces is still challenging because high surface roughness needed for superhydrophobicity. Highly rough surfaces decrease the transmission by scattering especially at visible wavelengths and result in opaque or translucent appearance. In order to achieve a transparent superhydrophobic surface, surface roughness must be carefully engineered. Also superhydrophobic surfaces must be mechanically and thermally stable to resist the outdoor conditions in order to use them in photovoltaic applications. In this context, we synthesized nanoporous organically modified silica (ormosil) coatings from MTMS and TMOS monomers by a template-free sol-gel method. We observed that when the TMOS ratio (v/v) is between 5-9% transparent and superhydrophobic surfaces can be obtained. Superhydrophobic films can be coated to surfaces directly from the ormosil colloidal dispersions by spin, dip or spray coating methods which enable the low-cost and large-area fabrication. Also, coatings can be prepared on almost any substrate including, glass, silicon, plastics and paper. Furthermore, we investigated the mechanical properties of the coatings by adhesive tape and water dripping tests and thermal properties by heating the coatings to the elevated temperatures. The coatings exhibited mechanically and thermally stable (up to 500 °C) superhydrophobic property. In addition, we prepared a three layered coating on an organic solar by using ormosil thin films with different refractive indices. The coating demonstrates broadband antireflective property together with superhydrophobicity. Because of the broadband antireflective property the solar cell performance found to be improved as ~%3. Such mechanically and thermally stable large-area broadband anti-reflective and self-cleaning surfaces can be used both to improve of the solar cell performance and to protect it from environmental harms.
9:00 PM - S3.3
Solution-Processed Photovoltaic and Photodetector Based on Iron Disulfide (FeS2) Nanocrystal.
Diyan Wang 1 , You-Ting Jiang 2 , Chia-Chun Chen 2 3 , Chun-Wei Chen 1 Show Abstract
1 Department of Materials Science and Engineering, National Taiwan University, Taipei Taiwan, 2 Department of Chemistry, National Taiwan Normal University, Taipei Taiwan, 3 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Iron disulfide (FeS2) is a promising earth abundant and environmentally friendly semiconductor with a large optical absorption coefficient (6.0 × 105 cm-1) and a narrow bandgap of 0.95 eV. In this work, we would like to present the fabrication of the solution-processable photovoltaic and photodetector using FeS2 nanocrystal ink. The device performances depending on various nanocrystal shapes and sizes will be addressed and the mechanisms of carrier generation and transport will be also investigated by various transient techniques. The extended near infrared (NIR) light harvesting of the photoactive layer consisting of solution processed FeS2 nanocrystals indicates the potential applications in large-area and low-cost NIR photodetecting and photovoltaic devices.
9:00 PM - S3.4
Solution Processable CdS, CdS-AgInS2 Semiconductor Nanocrystals-Benzothiaziazole Polymer Nanocomposites.
Pradipta Maiti 1 , Venkatesh Mamidala 2 , Venkartam Nalla 2 , Ji Wei 2 , Suresh Valiyaveettil 1 Show Abstract
1 Chemistry, National University of Singapore, Singapore Singapore, 2 Physics, National University of Singapore, Singapore Singapore
Hybrid inorganic-organic nanomaterials are interesting owing to the integrated functionalities of the components present in the system. The combined properties endow the nanocomposites with promising applications in the field of FET, OPV, LED and LASER. Because interfaces between polymer and nanocrystals have substantial effect on the electronic interaction, and aggregation of nanocrystals causes problems in preparations of composite materials. On the other hand physical blending of the components is not suitable for producing new materials with the desired properties. Here we discuss a one pot synthesis and characterization of CdS, CdS-AgInS2 nanocrystals in benzothiadiazole based semiconducting polymer solution. The CdS nanorods synthesized were 20 nm in length and 5 nm in diameter whereas the CdS-AgInS2 nanocrystals were anisotropic in nature. Photophysical studies of these nanocomposites showed considerable amount of charge transfer from the nanocrystals to the polymer. Those synthesized nanocomposites show strong non-linear absorption in solution. We estimated effective scattering coefficient for the nanocomposites, which is absent in case of pure polymers. These differences indicate that the non-linear optical property arises from the surface chemistry of nanocrystals and Benzothiadiazole copolymers. The presentation will focus on synthesis, characterization and photophysical studies of the hybrid materials.
9:00 PM - S3.5
Effect of Nanosized CdS in Photovoltaic Properties of CdS- Poly3-Hexylthiophene Heterojunctions.
Claudia Martinez-Alonso 1 , Hugo Cortina 1 , Ma.Elena Nicho 2 , Hailin Hu 1 Show Abstract
1 , Centro de Investigacion en Energia, UNAM, Temixco, Morelos, Mexico, 2 , Centro de Investigacion en Ingenieria y Ciencias Aplicadas, UAEM, Cuernavaca, Morelos, Mexico
Hybrid photovoltaic (PV) solar cells studied in this work were formed by n-type inorganic cadmium sulfide (CdS) and p-type conjugated polymer poly3-hexylthiophene (P3HT). CdS nanoparticles of about 10 nm diameter were obtained by a simple and fast solution precipitation method, whereas P3HT was polymerized from 3-hexylthiophene by an oxidative method. Three geometric structures of CdS/P3HT solar cells have been chosen: (1) the bulk one whose active layer consisted of a blend of CdS powder with the P3HT solution, (2) the quasi-bilayer one formed by collocating the CdS powder on conductive substrate and then drop casting the P3HT solution, and (3) the planar one consisting in thin films of CdS and P3HT for comparison purpose. First of all, it is observed that the type of solvent used for P3HT product and the drying rate of the active layer influence significantly on the PV performance of all the cells. A solvent with an intermediate vapor pressure was adequate for hybrid bulk solar cells to balance the phase separation effect and macromolecular rearrangement during the polymer solution drying process. PV performance of each type of the cells has been analyzed, and the most interesting observation is that the presence of CdS nanoparticles in the first two geometric structure cells enhances both open circuit voltage (Voc) and short circuit current (Jsc) values compared to planar one. The similarity and difference in PV performance between bulk and quasi-bilayer cells were studied from materials as well as device point of view: the morphology (SEM and AFM) and optical properties of active layers as functions of type of solvents, the drying process, the annealing process, and analysis of current- voltage (I-V) curves and quantum efficiency of the cells prepared under different procedures. A hypothesis has been given to explain the origin of the improvement in PV performance of nanostructured hybrid CdS-P3HT solar cells.
9:00 PM - S3.6
Optimization of Solution Processed Nanocrystal Quantum Dot Tandem Solar Cells Guided by Optical Modeling Based on Scattering Matrix Formalism.
Joshua Choi 1 2 , Apurva Sisodia 1 , Whitney Wenger 1 , Tobias Hanrath 1 Show Abstract
1 Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 2 Applied and Engineering Physics, Cornell Universiy, Ithaca, New York, United States
Semiconductor nanocrystal quantum dots (NQDs) are a promising material system for low-cost multijunction solar cells. Their high absorption cross section, size-tunable energy gaps and solution processability offer a unique opportunity to realize high-conversion efficiency in low-cost device structures by allowing fabrication of solution processed multijunction solar cells composed of absorber layers with cascaded energy gaps. We recently demonstrated proof-of-principle tandem solar cells produced from colloidal suspensions of size-tuned semiconductor NQDs. Using size-tuned PbS NQDs with energy gaps in the range of 0.7 eV and 2.0 eV, we demonstrated NQD tandem cells with an open circuit voltage (Voc) approaching 1V. We reported a study of the structural, optical and electrical properties of the interlayer comprised of ZnO, thin metal layer, and hole conducting polymer and showed that the device performance depends critically on the optical and electrical properties of the interlayer. Optimization of the device performance is contingent upon better management of light absorption and charge collection in the multilayered device stack. Towards that goal, we modeled the optical electric field in the tandem device stack using scattering matrix formalism. The model used complex index of refraction of each layer obtained from spectroscopic ellipsometry measurements. From the model, we show that thin film interference effects and the ZnO layer as ‘optical spacer’ (allowing NQD film to be away from optical electric field quenching metal electrode) can be exploited to place peaks of optical electric field distribution inside the NQD films. For initial experiments, we focus on thin and fixed thickness (50 nm) NQD layers to decouple effect of electrical transport properties from the device performance while varying the thicknesses of interlayer and electron and hole transporting layers to maximize short circuit current density and experimentally demonstrate corresponding tandem devices. Choi, J. J., Wenger, W. N., Hoffman, R. S., Lim, Y. F., Luria, J. Jasieniak, J., Marohn, J. A., Hanrath, T. Advanced Materials DOI: 10.1002/adma.201100723
9:00 PM - S3.7
Surfactant-Free CuInSe2 Nanocrystals Transformed from In2Se3 Nanoparticles and Their Application for a Flexible Ultraviolet Detector.
Yuho Min 1 , Minwoo Park 1 , Minkwan Shin 1 , Unyong Jeong 1 Show Abstract
1 , Yonsei University, Seoul Korea (the Republic of)
Synthesis of semicondutor nanoparticles has been extensively studied because of their outstanding physical and chemical properties. The chalcogenide compounds which belong to III-VI and I-III-VI are one of the most attractive materials for various electronic applications. Although a large number of solution-based methods to synthesize semicondutor nanoparticles have been estalished, many approaches require high processing temperature, long reaction duration or specific and toxic agents. In this talk, In2Se3 nanoparticles were synthesized in an aqueous solution without using any surfactant and then they were chemically transformed into CuInSe2 nanocrystals. The transformation was thermodynamically favorable and fast in mild reaction conditions, which allowed massive production of CuInSe2 nanocrystals. By the virtue of the surface charges, the CuInSe2 nanocrystals were well-dispersed in polar solvents. The surfactant-free nanocrystals enabled the formation of a semiconducting CuInSe2 film on a flexible polymer substrate without any thermal treatment. We took the advantage to fabricate a flexible UV photodetector. The detection sensitivity remained the same until the bending radius was reduced down to 4 mm. The dynamic response of the film device was stable and reproducible during the light irradiation (350 nm).
9:00 PM - S3.8
Nanocrystal Quantum Dot Tandem Solar Cells – Controlling Charge Recombination at the Interlayer.
Whitney Wenger 1 , Joshua Choi 1 2 , Tobias Hanrath 1 Show Abstract
1 Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 2 Applied and Engineering Physics, Cornell University, Ithaca, New York, United States
Advances in the creation of nanomaterials with optical and electrical properties by design have created a fertile opportunity space for renewable energy technologies. A particularly intriguing case in point is the prospect of exploiting size-tuned energy levels in semiconductor colloidal nanocrystal quantum dots to create multi-junction solar cells with cascaded energy gaps. We have recently demonstrated a proof-of-principle tandem photovoltaic device based on colloidal nanocrystal quantum dots. One of the most daunting challenges in the fabrication of tandem solar cells is designing an interlayer connecting the front and back cell. The interlayer must meet two critical optical and electrical demands: first, it should be optically transparent to pass light to the underlying cell; second, it must provide appropriate energy level alignment to accept electrons and holes from the subcells and efficiently recombine them without degrading the overall photovoltage of the cell. Many multijunction solar cells utilize an interlayer created between an n-type metal oxide and p-type PEDOT:PSS. In some cases, an ohmic junction is obtained with the addition of an ultrathin metal film possessing an energy level between those of the n- and p-type materials. Our first demonstration of a multijunction cell using solution-processed nanocrystal active layers featured an ultra-thin film of vacuum-deposited gold between n- and p-type materials. Fundamentally, the understanding of the effects governing the creation of the ohmic junction within the interlayer remains not well understood. To address this knowledge gap we probed interfacial charge transfer processes in the interlayer. We replaced the vacuum-deposited ultra-thin metal film with solution-processed gold and silver metal nanoparticles. Colloidal metal nanoparticle films offer several promising advantages including: (i) better control over the metal particle size, (ii) solution-processing, (iii) improved control over the chemical interface between the metal and adjacent layers through adjustment of the metal nanoparticle ligand chemistry, and (iv) opportunities to manage light absorption in the device stack through plasmonic effects. We compare the interlayer performance in tandem photovoltaic devices based on ultra-thin vacuum-deposited metal films with metal-nanoparticle based interlayers processed entirely from solution. We systematically studied charge recombination in the metal interlayer as a function of ligand chemistry, metal nanoparticle composition and size.  Choi, J. J., Wenger, W. N., Hoffman, R. S., Lim, Y. F., Luria, J. Jasieniak, J., Marohn, J. A., Hanrath, T. Advanced Materials DOI: 10.1002/adma.201100723
9:00 PM - S3.9
CIS Thin Film Synthesis Route with Non-Vacuum Precursor Deposition and Thermal Treatment.
Dongwook Lee 1 Show Abstract
1 Chemical Engineering, POSTECH, Pohang Korea (the Republic of)
In polycrystalline semiconductor thin films, CuInSe2(CIS) is widely studied for photovoltaic applications. CIS thin film solar cell has great potential to substitute for a light-absorber layer of solar cell because of high absorption coefficient, and tuneable, direct band gap structure compared to Si-based photovoltaics. However, high quality CIS thin films which recorded much than 15% of efficiency were synthesized through high vacuum deposition process such as thermal evaporation and sputtering technique. Molecular level precursor solution approaches have a lot of advantages for continuous deposition, material usage and process cost reduction. In this work, we successfully synthesized stoichiometric CIS thin films by combination of non-toxic, stable precursor paste and post-heat treatment. And film growth mechanism (precursor decomposition, inter-mediate phase formation, CIS film formation) was suggested with varied annealing temperature using SEM, XRD and TGA analysis. Precursor layer containing metal and chalcogen ion gradually decomposed and formed metal-chalcogenide (CuxSe, In2Se3) phases. At higher temperature, CIS chalcopyrite phases such as (112), (204), and (312) planes were appeared and showed 500 nm thickness of continuous morphology. And we observed that CIS films showed high optical absorbance behaviour at visible and near-IR light region, so it presents great potential to reduce fabrication cost for thin film solar cells.