Aram Amassian, King Abdullah University of Science and Technology
Joseph J. Berry, National Renewable Energy Laboratory
Martyn A. McLachlan, Imperial College London
Erin L. Ratcliff, University of Arizona
Symposium Support Angstrom Engineering Inc.
Journal of Materials Chemistry
P2: Materials Processing
Monday PM, December 01, 2014
Hynes, Level 3, Room 308
2:30 AM - P2.01
Printing Organic Semiconductors for Organic Circuits with Low Patterning Errors and Electrical Variability
Gaurav Giri 1 Steve Park 2 Zhenan Bao 2
1Massachusetts Inst Tech Cambridge USA2Stanford University Stanford USAShow Abstract
Logic circuits are necessary to fulfill the vision of low cost, large area organic electronics, made with organic semiconductors (OSC) as the charge transfer layer. However, these circuits have stringent requirements. Primarily, all the thin film transistors (TFTs) participating in the circuit need to have a low variation in charge transfer characteristics (charge carrier mobility, threshold voltage, current, etc.). Additionally, organic circuits should be operated with low power consumption. To this end, research is being performed to pattern OSCs on the organic circuit to reduce parasitic current leakage. These twin requirements of low variability and OSC patterning set up conflicting goals, as the variability increases if each TFT is patterned individually. Moreover, patterning TFTs such that every TFT works, for the numerous TFTs required for logic circuits, is difficult with conventional methods such as ink jet printing due to patterning errors over large areas.
Here, we show a self-patterning method that does not require an extra patterning step to deposit the OSC layer onto the organic circuit. We have developed a surface functionalization procedure for a variety of oxide and metal surfaces, which, when paired with controlled solvent flow, can pattern OSCs in the TFT channel region only. Using this method, we show 100% viability of the patterned TFTs with low variability of charge carrier mobility and current. This method has been used to form logic gates and other organic circuits.
2:45 AM - P2.02
Low Temperature, Solution-Processed Dye Sensitized In2O3 Phototransistors
Alexander David Mottram 1 Yen-Hung Lin 1 Pichaya Pattanasattayavong 1 Thomas D Anthopoulos 1
1Imperial College London London United KingdomShow Abstract
Research into photodetectors has been intensified by the desire to produce complex products such as touch screens with integrated photodetectors , stacked colour pixel sensors  and photosensitive screens . As a result, there is an ongoing search for photodetectors with a specific set of properties including: colour selectivity, transparency, high responsivity, low-temperature and low-cost manufacturing, and the ability to be integrated with driving devices/circuitry. The combination of research from the field of metal-oxide semiconductors, as applied to transistors, and certain organic light absorbing dyes from the field of dye sensitized solar cells (DSSC), has led to a possible novel solution. Dye-sensitized transistors combine the ultra-high gain mechanisms of a transistor with the colour selectivity of the chosen dye allowing for the modular design of the photoresponse via the selection of an appropriate light-absorbing dye. Previous work in this field has involved the functionalization of SnO2 nanowire field effect transistors (FET) using chlorophyll and fluorescein  and ZnO thin film transistors (TFT) functionalized using the well-known organic light absorbing dye D102 .
Here we report the demonstration of a dye-sensitized thin film transistor (DSTFT) consisting of indium oxide (In2O3) and the dye D102. The devices have been grown using solution-based processes at temperatures le;200 °C. Control and active devices show the dye producing a colour selective photoresponse around the wavelength of green light (~510 nm) with a maximum photosensitivity of 105 and a responsivity value of 3×102 A/W. It is hypothesised that holes originating from the photo-generated electron-hole pairs in the excited dye assist in the desorption of oxygen from the semiconductor surface . This process leads to the production of excess free electrons that dope the metal oxide channel, increasing conductivity and explaining the very high photoresponse. Furthermore, the active device layers, i.e., the semiconductor/light absorbing dye, are highly transparent with an average transmission level of over 93 % across all visible wavelengths (300-650 nm). This unique characteristic makes this dye-sensitized device a promising candidate for applications such as a highly transparent photodetector.
 You, B. H., et al., SID Symposium Digest of Technical Papers, 40, 439, (2009)
 Gidon, P., et al., Physica Status Solidi (a), 207, 704-707, (2010)
 Jeon, S., et al., Nature Materials, 11, 301, (2012)
 Wu, H.-C., et al., Advanced Functional Materials, 21, 474-479, (2011)
 Pattanasattayavong, P., et al., Journal of Applied Physics, 112, 074507, (2012)
 Verbakel, F., et al., Journal of Applied Physics, 102, 083701, (2007)
3:00 AM - P2.03
Universal Method for the Synthesis and Solution Processing of Ultrasmall Metal Oxide Nanoparticles as Interfacial Contact Layers in Organic Photovoltaics
Yun-Ju Lee 1 Jian Wang 1 Diego Barrera 1 Julia W.P. Hsu 1
1University of Texas at Dallas Richardson USAShow Abstract
Interfacial contact layers (ICLs) improve the performance of organic photovoltaic (OPV) devices, by establishing a drift field within the bulk heterojunction (BHJ) and by blocking exciton quenching and carrier recombination at the electrodes. Metal oxide nanoparticle suspensions that can be solution processed at low temperature into pinhole free thin films (~ 10 nm) represent a promising route for ICLs because of the inherent compatibility with scalable device fabrication processes. However, most synthesis methods of nanoparticle suspensions require separate optimization for each metal oxide to achieve the desired combination of suspension stability, ease of film formation, and superior optical and electronic properties. Here, we present a universal method to fabricate stable suspensions of metal oxide nanoparticles where their stoichiometry and electronic properties were optimized prior to solution deposition, and utilize these nanoparticles as hole contact layers (HCLs) in OPV devices. Using hydrolysis of micron sized metal powders in the presence of excess H2O2, we synthesized charge stabilized suspensions of MoO3, NiO, Co3O4, Cr2O3 and other metal oxide nanoparticles with average diameter < 5 nm and high yield. Spin coating of the nanoparticle suspensions without any post-processing resulted in uniform thin films with work function and stoichiometry comparable to evaporated films, as determined by Kelvin probe and X-ray photoelectron spectroscopy measurements. We also determined the electronic band diagrams of the metal oxide nanoparticle films using a combination of UV-visible spectroscopy and UV photoelectron spectroscopy in air and in vacuum. When the metal oxide nanoparticles films were used as HCLs in P3HT:PC60BM and PCDTBT:PC70BM devices, we found that the performance corresponded most strongly to work function of the ICL, although conductivity and bandgap of the ICLs also have effects on carrier extraction. Finally, we will discuss the synthesis of doped metal oxides and ternary metal oxides using the same general synthesis method, and examine their effect on electronic properties and OPV device performance. This project is sponsored by National Science Foundation DMR-1305893.
3:15 AM - *P2.04
Harnessing Small Molecule Diffusion to Direct Organic/Inorganic Interfaces in Photovoltaic Devices
Gitti Frey 1
1Technion Haifa IsraelShow Abstract
The key processes of charge generation and charge extraction in organic and hybrid solar cells occur at the organic-inorganic interfaces, either in the active layer and/or at the active layer-metal interface. These processes are predominantly influenced by interfacial properties such as chemical composition, physical interactions and dipoles. Efficient photovoltaic performance, therefore, requires control over the non-homogeneous interfacial structure and composition. Here we show that diffusion of small molecules through the polymer film can be used to promote the desired organic/inorganic interfacial structure and composition. This approach is demonstrated in two systems: the active layer/metal interface in organic devises, and polymer/metal oxide donor/acceptor interface in hybrid devices. In the former, a polyethylene oxide (PEG) interlayer is spontaneously formed at the active layer/metal interface by its migration from the active layer to the interface. Using a plethora of techniques we unambiguously show that PEG migration occurs during the metal evaporation process and is driven by the metal-PEG interactions. In the second case, atomic layer deposition is used to process a hybrid bulk-heterojunction with exceptional control over film composition and morphology. The BHJ is prepared by diffusion of a ZnO precursor, diethylzinc (DEZ), into pre-formed P3HT films, followed by its in-situ conversion into crystalline ZnO. Finally, the morphology and interfacial interactions directed by the small molecule diffusion are correlated with photo-excitation dynamics and the macroscopic photovoltaic device performance.
4:15 AM - *P2.05
Solution Processable Inorganic-Organic Hybrids for Solar Cells and Water Splitting
Maria Antonietta Loi 1
1University of Groningen Groningen NetherlandsShow Abstract
Colloidal semiconductor nanocrystals (NCs) are solution processable semiconductors, which have great potential for optoelectronic device fabrication. Their remarkably broad absorption, their large tunability, their high dielectric constant, and their high stability under ambient conditions, are all qualities that makes of NCs solids ideal for solar light harvesting.
The use of small bi-dentate ligands to substitute the long insulating ligands with which NCs are synthetized, allows charge transport between them, so that high performances field effect transistors  and efficient solar cells have been reported [2,3].
In this presentation I will report about the open circuit voltage enhancement, respect the one of devices made with core only PbS NCs, obtained using PbS/CdS NCs. The mechanism determining this increased performances is investigated by mean of photoluminescence time resolved spectroscopy, impedance spectroscopy and transport measurements . Finally, I will show as colloidal NCs together with conjugated polymers can give new possibilities towards broad absorption of the solar spectrum not only in solar cells but also for photocatalytic water splitting.
 S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss and M. A. Loi, Adv. Mater. Adv. Mater. 25, 4309 (2013); D. M. Balazs, M. I. Nugraha, S. Z. Bisri, M. Sytnyk, W. Heiss, M. A. Loi, Appl. Phys. Lett. 104, art# 112104 (2014).
 Z. Ning , D. Zhitomirsky , V. Adinolfi, B. Sutherland, J. Xu , O. Voznyy , P. Maraghechi , X. Lan , S. Hoogland , Y. Ren, and E. H. Sargent, Adv. Mater. 25, 1719 (2013).
 C. Piliego, L. Protesescu, S. Z. Bisri, M. V. Kovalenko, M. A. Loi, Energy Environ. Sci., 6, 3054 (2013).
 M. J. Speirs, D. M. Balazs, H-H. Fang, L-H. Lai, L. Protesescu, M. Kovalenko, M. A. Loi (submitted).
 L-H. Lai, W. Gomulya, L. Protesescu, M. Kovalenko, M. A. Loi, (submitted).
4:45 AM - P2.06
Spray Pyrolysis of MoOx Layers for Incorporation in the Anode Contact of Organic Electronic Devices
Edward Lofts 1 Donal D C Bradley 1
1Imperial College London London United KingdomShow Abstract
One of the driving interests into the research of organic electroluminescent devices, and in particular polymer light emitting diodes (PLEDs), is the fabrication of large area, low cost devices for use in applications such as large area lighting. PLED devices are particular suited to this application due to their solution based nature, leading to ease of processing. However, deep highest occupied molecular orbital levels (HOMOs) of the polymer materials require interlayers to assist hole injection into the material in devices. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the standard interlayer used.
However, PEDOT:PSS has several disadvantages, one of the most limiting being its work function of 5.2eV, which is not deep enough to achieve ideal ohmic contact with common light emitting polymers. Metal oxides, in particular Molybdenum Trioxide (MoO3), have been used as an alternative interlayer to achieve ohmic injection into organic layers. However, MoO3 is usually evaporated for inclusion in small molecule organic devices, and to retain the advantage of PLED materials a solution processed route is required.
Here, we present an investigation of spray pyrolysis of a MoOx interlayer for use in PLED devices. Spray pyrolysis involves spraying of a precursor material dissolved in solvent onto a heated substrate. The decomposition of the precursor allows deposition of the target material in a manner with similarities to chemical vapour deposition, and control of the deposition temperature allows investigation of the effect of oxygen vacancies in the MoOx layer on device performance.
We present a comprehensive study of the various stages of the spray pyrolysis process. Decomposition of the precursor material is discussed. Morphological, optical and electrical measurements of the resulting MoOx layers are then considered. The injection properties of the layer are then investigated through the fabrication of single carrier devices. Finally, luminescent devices are fabricated using poly(9,9'-dioctylfluorene-alt-benzothiadiazole) (F8BT), which show a more than threefold increase in device efficiency compared to PEDOT:PSS.
5:00 AM - P2.07
Root-Cause Failure Analysis Reveals Harmful Interaction between Non-Adjacent MoO3 and Al Layers
Eszter Voroshazi 1 Griet Uytterhoeven 1 Paola Favia 1 Hugo Bende 1 Thierry Conard 1 Kjell Cnops 1 2 David Cheyns 1
1imec Leuven Belgium2University of Leuven Leuven BelgiumShow Abstract
Device stability of organic photovoltaic (OPV) devices has proven to be largely determined by the transport layers and electrodes. Integration of transition metal oxides as interface layers has enhanced both device stability and performance compared to their organic and/or inorganic counterparts. This, combined with their compatibility with large-area depositing techniques, motivates their integration in OPVs. Here, we propose the stability assessment of MoO3 in inverted and packaged polythiophene:fullerene cells with Ag/Al composite electrode in operational conditions combining solar radiation and 65°C. The Ag/Al electrode balances the enhanced air stability, owing to the combined metal thickness of 300 nm, with low-cost material selection. In the over 1000 h ageing study the performance loss of over 50% in the first 200 h is dominated by a drop in the short-circuit current (Jsc). We reveal a concurrent loss in reflectance from 85% to 40% below 1.9 eV, which is below the optical gap of the used photo-active materials, hence excluding any major degradation in the bulk of this layer. Correlating the responses of aged devices to a series of test structures comprised of the bottom ITO/ZnO cathode, the top MoO3/Ag or MoO3/Ag/Al anode and their combinations with the active layer allowed us to identify that the combination of Ag and Al reproduces the low reflectance of aged devices. This suggests that Al plays a critical role in the failure independent of the photo-active layer. Systematic single-stress ageing further indicates that elevated heat solely triggers this mechanism. Cross-section transmission electron microscope coupled with elemental analysis revealed the unsuspected role of Al, notably it diffuses through the entire 150 nm thick Ag layer and accumulates at the MoO3/Ag interface. Depth profiling with X-ray photoelectron spectroscopy advanced our understanding by indicating the oxidation of Al only at the MoO3/Ag interface hinting that MoO3 is the source of oxygen and is most probably reduced. Control X-ray diffraction measurement, not relying on sputtering, allowed us to confirm the diffusion of Al and exclude any contribution from metal induced crystallization in MoO3. Finally, the input from the material characterization to optical simulations allowed to reproduce the reflectance response of aged devices and estimated that 20% of the loss in Jsc is ascribed to optical changes in MoO3. Concurrent shift in its energy levels could account for the remaining performance loss as well as other failure mechanisms might also contribute. In conclusion, our root-cause failure analysis provides insight into the harmful interaction of two non-adjacent layers and their consequently changed optical properties resulting in overall electrical performance loss. This insight provides guidelines for heat-stable MoO3/metal contacts, independent of the active layer and illustrates the much needed but still rare root-cause failure analysis in OPVs.
5:15 AM - P2.08
Metal Oxide Films and Organic Nanowires as Interlayers for Opto-Electronic Devices
Antonio Facchetti 1 2
1Northwestern University Evanston USA2Polyera Corporation skokie USAShow Abstract
In this presentation new approaches to metal oxide and hybrid materials for unconventional electronic applications are presented. Particularly, we will discuss our latest results in developing amorphous and polycrystalline In-Y-X-O (Y, X = none, Ga, Sn, Zn, Y, La) formulations for combustion synthesis in which the corresponding films can be annealed at temperatures < 300 0C. Depending on composition, ITO workfunction can be changed by several eV enabling their use with various photoactive layers for solar cell. A simila strategy has been employed for tuning the conductivity of ZnO using nanowires of perfluorocupperphthalocyanine. Finally, these interlayes can be use to tune injection ability into organic transistors.
5:30 AM - *P2.09
Next Generation Metal Oxide Transistors Based on Solution-Processed Superlattices: Overcoming Single Material Limitations by Exploring Low-Dimensional Electron Transport Phenomena
Thomas Anthopoulos 1
1Imperial College London London United KingdomShow Abstract
Metal oxide semiconductors that can be processed via solution-based methods at low temperatures in air represent an emerging class of electronic materials for application in a range of existing as well as fast emerging opto/electronics. In this talk I will discuss the development of low temperature solution-processable metal oxide semiconductors and their application in thin-film transistors and integrated circuits. I will describe how low-dimensional metal oxide superlattices can be grown from solution in a highly controlled manner and the unique electronic properties of these multilayer systems can be explored to overcome the intrinsic electron transport limitations encountered in conventional single semiconductor channel transistors. These novel prototype transistors exhibit band-like transport with electron mobility values approximately a tenfold higher than control devices based on single metal oxide semiconductors. The development of other exotic devices, such as double-barrier resonant tunnelling diodes, that exploit the low-dimensional nature of these solution grown oxide superlattices will also be discussed.
P3: Poster Session
Monday PM, December 01, 2014
Hynes, Level 1, Hall B
9:00 AM - P3.01
Electrical Behavior of Zinc Oxide/Organic Layer Junctions with Modified Hybrid Interface
Grzegorz Luka 1 Krzysztof Goscinski 1 Marek Godlewski 1 2
1Institute of Physics PAS Warsaw Poland2Cardinal S. Wyszynski University Warsaw PolandShow Abstract
Ideally, combining organic and inorganic materials could result in a synergy of the unique properties of both classes of materials. In practice, however, this can be limited by several factors. In case of zinc oxide/organic interfaces, ZnO surface properties highly affect electrical or photovoltaic performance of hybrid organic-inorganic devices. This motivates us to investigate the influence of ZnO surface modification on the electrical and impedance behavior of the ZnO/organic layer hybrid structures. We studied two kinds of ZnO films, i.e. conducting Al-doped ZnO (ZnO:Al) films serving as transparent electrodes, and semiconducting ZnO films with lower carrier concentrations as n-type partner to organic semiconductors. In the first case, we deposited additional very thin layer of ZnMgO:Al to decrease the work function of ZnO:Al electrodes. In the latter case, we modified ZnO surface by depositing a very thin layer of high dielectric constant oxide (Al2O3, HfO2 or TiO2) to, on the one hand, passivate ZnO and prevent formation of ZnO surface states, and, on the other hand, to form as low potential barrier for the charge carrier flow as possible. All the oxide films were grown by atomic layer deposition. As organic layers, we chose small-molecule materials having different HOMO and LUMO levels (tetracene and two squaraine derivatives). We will show electrical and impedance characterization results of the unmodified as well as modified ZnO/organic layer junction devices. This work was partially supported by The National Science Centre (NCN, Poland) under decision No. DEC-2012/07/D/ST3/02145.
9:00 AM - P3.02
Ultrathin Ag-Oxide for Conjugated Polymer Optoelectronics
Peter D Spatocco 1 Benjamin Agyei-Tuffour 2 3 4 Christopher Petoukhoff 1 4 Deirdre M O'Carroll 1 4
1Rutgers, The State University of New Jersey Piscataway USA2African University of Science and Technology Abuja Nigeria3University of Ghana Legon-Accra Ghana4Institute of Advanced Materials, Devices and Nanotechnology Piscataway USAShow Abstract
Ultrathin p-type conductive native metal oxide layers are typically categorized as thin films that are less than 10 nm in thickness and find application in a host of optoelectronic devices. In photovoltaics, ultrathin p-type silver oxides can be used to increase the surface work function of the silver back electrodes while maintaining the electrode&’s high reflectivity. P-type metal oxide films can also be used for lighting devices such as top-emitting LEDs to improve hole injection of transparent electrodes without significantly inhibiting light emission. However, the optical transparency and electrical conductivity of p-type native metal oxide films depend heavily on the thickness of these films. If the films are too thick, they would no longer be transparent, but if they are too thin a significant change in work function may not be observed. Therefore, it is very important to be able to control and accurately measure the thickness of these native oxide films. In this project we have investigated two methods to grow ultrathin native silver oxide films on 100-nm-thick thermally evaporated silver films in a controlled manner: electrochemical oxidation and ultra violet ozone oxidation. Electrochemical oxidation was accomplished by exposing the silver film to a brief, reduction electrochemical potential, and, subsequently, applying an oxidization potential for 400 seconds. Ultra violet ozone oxidation was accomplished by exposing the silver film to an ozone environment for either 1 or 2 minutes to produce an optically-transparent oxide films. The oxide films were characterized with x-ray photoelectron spectroscopy (XPS) and grazing incidence wide angle x-ray scattering (GIWAXS). To measure the film thickness, grazing incidence small angle x-ray scattering (GISAXS), spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) techniques are being adopted. The AFM can measure the change in height of steps in the oxide film created by using a selective chemical etchant that attacks the oxide layer but not the metal layer. Some of the etchants that are being investigated are dilute oxalic acid and dilute ammonium solutions. In the XPS data, the binding energies (Eb) for O1s and Ag3d5/2 photoelectrons were shifted from Eb,O1s = 531.6 eV and Eb,Ag3d = 368.2 eV for un-oxidized Ag to Eb,O1s = 529.4 eV and Eb,Ag3d = 367.6 eV for electrochemically oxidized Ag and Eb,O1s=530.6 eV and Eb,Ag3d=367.7 eV for UV-ozone oxidized Ag indicating formation of Ag2O. The shift in GIWAXS scattering vector from 2.5 Å-1 to 2.24 Å-1 following ozone exposure of Ag for at least 2 minutes further confirmed ultrathin Ag2O formation. Future work will determine the electronic workfunction and bandgaps of the oxides using ultraviolet photoelectron spectroscopy (UPS). The implications of the results will be used to inform the optimum p-type native oxide thickness required for organic optoelectronic device applications.
9:00 AM - P3.03
Dependence of Processing and Characterization Conditions in the Performance of Hybrid Organic/Inorganic Amorphous Metal-Oxide Thin-Film Transistors
Joao Paulo Braga 1 Cleber Alexandre Amorin 2 Giovani Gozzi 2 Dante Luis Chinaglia 2 Lucas Fugikawa Santos 1
1UNESP - Univ Estadual Paulista Sao Jose do Rio Preto Brazil2Univ Estadual Paulista - UNESP Rio Claro BrazilShow Abstract
In the present work, we propose the study of the atmosphere conditions on device performance of hybrid organic/inorganic thin-film transistors (TFTs) comprising sol-gel processed indium-zinc oxide (IZO) and poly(3-hexyl thiophene) (P3HT). The electrical properties of amorphous transparent metal oxides films produced by sol-gel process have been found to depend strongly on the ambient conditions during film processing and device characterization. The performance of the TFTs was evaluated from parameters obtained from the characteristic transfer transistor curves: majority charge-carrier mobility, threshold voltage and on/off current modulation ratio. A very rigorous control of the ambient oxygen content (from 10 ppm up to completely oxygen saturated atmosphere) during the metal-oxide film processing and also during the device characterization was carried out in order to find the most appropriate conditions for device preparation and the dynamics of oxygen release by the device during characterization. The results show that device performance is almost atmosphere independent for an oxygen content higher than 5% during the metal-oxide preparation. However, device performance changes dramatically on time for characterization at completely inert atmosphere conditions (below 10 ppm), within a time span of several weeks. A extensive analysis of the device performance parameters is carried out to determine the most appropriate preparation and characterization conditions for the device achieve a stable response after a minimal time interval. These information are valuable to continuosly improve device output and performance. (Acknowledgements to Fapesp Proc. 2013/24461-7)
 T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv. Mater. 11, 044305 (2010).
 G.H. Kim, H.S. Kim, H.S. Shin, B. Du Ahn, K.H. Kim, and H.J. Kim, Thin Solid Films517, 4007 (2009).
 K. K. Banger, Y. Yamashita, K. Mori, R. L. Peterson, T. Leedham, J. Rickard, H. Sirringhaus, Nature Materials DOI:10.1038/NMAT2914 (2011).
9:00 AM - P3.04
Developing Standards of CuPc, C60, P3HT, and PCBM Using Electron Energy-Loss Spectroscopy and Spectroscopic Ellipsometry for Studies of Interfaces in Organic-Based Photovoltaics
Jessica A. Alexander 1 Michael F. Durstock 2 Christopher E. Tabor 2 Benjamin J. Leever 2 Lawrence F. Drummy 2 Michael D. Clark 2 3 Dennis P. Butcher 2 3 Frank J. Scheltens 1 David W. McComb 1
1The Ohio State University Columbus USA2Air Force Research Laboratory WPAFB USA3UES, Inc. Dayton USAShow Abstract
The processes that generate current in organic photovoltaics (OPVs) are dependent on the micro- and nano-structure of the devices, especially at the donor-acceptor interface. Light trapping strategies, including the incorporation of plasmonic nanostructures, have been proposed to tailor absorption of incident sunlight and generate more photocurrent at the donor-acceptor interface to improve the power conversion efficiency. These plasmonic nanostructures are known to enhance the optical absorption and current density in an OPV without increasing the thickness of the active layers, but little is known about the detailed structure, chemistry, and bonding between the active layer and the plasmonic nanostructures. The understanding of this interface is vital to understanding why these nanoparticles improve the efficiency of such devices. Electron energy-loss spectroscopy (EELS) may be used for the study of such interfaces in OPV structures. From the collected valence loss spectra, it is possible to extract the energy-loss function and calculate the complex dielectric function from which the frequency-dependent refractive index (eta;), and the extinction coefficient (κ) can be obtained. This knowledge allows peaks associated with single electron transitions to be distinguished from collective excitations. This analysis would result in a signal that is related to the interactions between the acceptor/donor and nanoparticle interfaces, which could be studied and analyzed to try and understand why the insertion of nanoparticles in the active layer of the OPV device improves the efficiency of the devices. However, organic materials are extremely susceptible to electron beam damage, so it is first necessary to determine the allowable electron beam doses that will not change the chemistry of specific organic samples. Using spectroscopic ellipsometry (SE), the optical properties of a material may be determined. Kramers-Kronig analysis can be applied to determine the dielectric function from which the energy-loss function may be calculated. This energy-loss function can then be compared to the energy-loss function measured using EELS at different electron beam doses to determine the maximum beam dose that will not change the chemistry of the sample. Four organic materials have been studied in this manner, including copper phthalocyanine (CuPc), fullerene-C60, [6,6] phenyl C61 butyric acid methyl ester (PCBM), and poly(3-hexylthiophene) (P3HT), in order to determine the acceptable electron beam doses for EELS measurements of these organics and develop standards for EELS measurements.
9:00 AM - P3.05
Fabrication and Characterization of a 3D Photonic TiO2/P3HT Nanocomposite for Enhancing Charge Generation in a Bulk Heterojunction Solar Cell
Nicholas Tulsiram 1
1York University Mississauga CanadaShow Abstract
Photonic crystals are highly ordered structures that exhibit unique light controlling abilities, such as being able to theoretically reduce the group velocity of light to zero. They have been shown to increase light absorption and reduce charge-carrier recombination, which would be desirable for solar conversion processes. We present an organic/inorganic nanocomposite for bulk heterojunction cell applications using a titanium dioxide (TiO2) 3D photonic crystal coated with poly (3-hexylthiophene) (P3HT). TiO2 inverse opals are used as the photonic crystal in this application. Inverse opals were fabricated by infiltration of an opal template of polystyrene spheres that were deposited onto a glass substrate via colloidal self-assembly. We fabricated TiO2 inverse opals with different periodicities, number of layers and wall thicknesses to systematically study the effects of different factors on charge generation. Charge generation is probed by photoinduced absorption spectroscopy (PIA) under different excitation wavelengths and modulation frequencies. The TiO2/P3HT nanocomposites show a significant increase in charge generation in comparison to conventional P3HT/PCBM films. We elucidate the structural and photonic effects from the nanocomposites and address the potential of charge enhancement by photonic crystals for organic photovoltaics. The proof of concept may serve as blueprint that can be applied to different photovoltaic devices in the future.
9:00 AM - P3.06
Polyhexylthiophene with Boron Nitride Hybrid Blend Photoelectrochemical Cell
Nunzio Giambrone 2 Manoj K Ram 1 Ashok Kumar 3
1University of South Florida Tampa USA2University of South Florida Tampa USA3University of South Florida Tampa USAShow Abstract
Recently, we have studied the photoelectrochemical properties of nanodiamond (ND)-ragioregular polyhexylthiophene (RRPHTh), ND- poly (3-dodecylthiophene), another derivative of polythiophene [1-4]. The photocurrent of 8 to 20 times has been observed in the nanodiamond ND (RRPHTh) nanostructured blend film than most of nanoparticles containing blend films of RRPHTh conducting polymer. Under this investigation, the effect of various sizes of (ND) from micro to nano in the blend structure of ND wirh RRPHTh conducting polymer has been studied. The nano-diamonds (ND)-RRPHTh showed increased photovoltaic efficiency than micro-diamonds (MD)-RRPHTH based blend films.
Further, the photoelectrochemical properties of boron nitride (BN) blend with RRPHTh conducting polymer has been studied. The BN posseses the similar characteristics of diamond and, recently has gained interest in the mechanical and optical fields. The UV-vis spectroscopy, FTIR spectroscopy, and SEM properties of blend films were studied and compared. The electrochemical properties such as cyclic voltammetry, chronoamperometric, impedance spectroscopy with and without light were studied. The ND-RRPHTh and BN-RRPHTh hybrid based photoelectrochemical cells were studied and compared for short circuit current, power conversion efficiency (eta;) and fill factor. Our work shows for the first time the use of boron nitride with RRPHTh conducting polymer based photoelectrochemical photovoltaic devices.
MK Ram, H Gomez, F Alvi, E Stefanakos, Y Goswami, A Kumar, Novel Nanohybrid Structured Regioregular Polyhexylthiophene Blend Films for Photoelectrochemical Energy Applications The Journal of Physical Chemistry C 115 (44), 21987-21995
PA Basnayaka, P Villalba, MK Ram, L Stefanakos, A Kumar, Photovoltaic properties of multi walled carbon nanotubes-poly (3-octathiophene) conducting polymer blends, structures, MRS Proceedings 1493, 139-144
MK Ram, P Basnayaka, A Kumar Materials Science & Technology 2013 Photoelectrochemical Properties of Conjugated Polymer and Nanomaterial, Hybrid Organic: Inorganic Materials for Alternative Energy, October 27-31, 2013: Montreal, QC
F. M Abdelmola, M K Ram, A Takshi, E Stefanakos, A Kumar, Y.Goswami Photoelectrochemical cell of hybrid regioregular poly(3-hexylthiophen,2,5,diyl) and molybdenum disulfide film, Electro Chimica Acta, Communicated. (2014).
9:00 AM - P3.07
The Synthesis and Properties of Solution Processable a Red Phosphorescent Iridium(III) Complex
Bona Yang 1 Dong-Myung Shin 1
1Hongik University Seoul Korea (the Republic of)Show Abstract
A new phosphorescent (TMP-AM)2Ir(acac) with 2-bromo-4-(trifluoromethyl)pyridine and 3-Aminophenylboronic acid was synthesized for organic light-emitting diodes (OLEDs). This material was designed by result of Gaussian modeling program. The ligands have sites of both the electron donor and acceptor in structure. There are pyridyl group which decrease electron density and thiophen group which increase electron. So, it showed Intramolecular Charge Transfer (ICT) property. The dopant was synthesized by Suzuki coupling reaction and Nonoyama reaction. Red dopant was observed with an emission peak at approximately 600nm. The device structures were ITO / NPB / CBP: Ir complex / Bphen / Liq / Al. Electroluminescent properties were observed with devices doped with different doping concentrations.
9:00 AM - P3.08
Structured Layer of Rhenium Dye on SiO2 and TiO2 Surfaces by Langmuirminus;Blodgett Technique
Yongho Joo 1 Josef W. Spalenka 1 Kyle M. McElhinny 1 Samantha K. Schmitt 1 Paul G. Evans 1 Padma Gopalan 1
1University of Wisconsin-Madison Madison USAShow Abstract
Bottom-up self-assembly processes for the uniform deposition of monolayers or controlled multilayers have been investigated to develop a basic understanding of the relationship between the molecular structure and ordering at interfaces. At the same time, these studies have led to advances in optical devices, photonics, and electronic devices where structure and ordering at interfaces play a key role in device properties. Among the different strategies to fabricating ultrathin films, such as spin-coating, dip-coating, layer-by-layer deposition, and Langmuirminus;Blodgett (LB) assembly. Of these methods, the LB method offers assembly of the molecules at an interface using compressive forces to drive close packing. LB is traditionally used to prepare ultrathin films of amphiphilic molecules at the air/water interface, which are then transferred on to a variety of substrates such as TiO2, glass, mica, or SiO2. Here we outline the use of LB to create ordered monolayers of two metal-bipyridine complexes on SiO2 and TiO2. We show through a combination of x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and x-ray reflectivity (XRR). The dye is a rhenium-bipyridine complex where the molecular bridge to the carboxylic acid anchoring group to the oxide is either an aliphatic chain (ReEC) or a bithiophene (Re2TC). Both of these molecules have the potential to form stable LB films, and the change in the bridge characteristics from aliphatic to aromatic alters the driving forces for packing such as cohesive and repulsive forces between the various components of the dye, stiffness of the molecule, and hence the characteristics of the isotherm.
The two rhenium-bipyridine complexes ReEC and Re2TC studied here both form stable LB films. The hydrophilic carboxylic acid group anchors the molecule into the water subphase with the organometallic head group facing the air. The presence of a flexible aliphatic linker in ReEC aids in faster packing of the molecules hence compressing the liquid phase in the isotherm, compared to the more rigid aromatic link in Re2TC. Given that the cross sectional dimensions of the head group (CO-Re-Cl distance is approximately 4.8 Å) is larger than the effective distance for pi-pi interactions, the interactions between the aromatic bridges is minimal in Re2TC even when close-packed. Hence, the observed differences between the two molecules, is consistent with the greater rigidity of the 2TC link in comparison with EC. AFM and XRR characterization shows that the molecules are extended close to their calculated maximum length and are oriented with the long axis of the molecule close to normal to the substrate. These molecules can be assembled in ordered monolayers at the air/water interface and transferred to oxide surfaces, including TiO2 substrates, hence opening up a path to study the effect of specific molecular orientation or aggregation states on the charge injection dynamics at donor/semiconductor interfaces.
9:00 AM - P3.09
Synthesis and Characterization of Pulse-Laser Deposited ZnO/Polymer Heterojunction
Leandro Gutierrez 1 William A. Manners 1 Arya A. Nabizadeh 1 Mehmet A. Sahiner 1 Weining Wang 1
1Seton Hall University South Orange USAShow Abstract
ZnO/polymer heterojunction has been studied by many groups for its potential application in solar cell, LED, UV photodetection and other applications. However, there are few studies on ZnO/polymer heterojunction by synthesizing ZnO using pulsed laser deposition. Comparing with other methods, PLD has the advantage of congruent evaporation, and being able to grow high quality thin films at relatively low temperature. In our previous work in CdTe/CdS based thin films we have seen correlations between the pulsed laser deposition parameters and the electrical performance of the thin film solar cells.
In this work, we report our studies on pulsed-laser-deposited (PLD) ZnO/polymer heterojunctions based on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). We studied how the barrier height and built-in-potential of (PLD) ZnO/PEDOT:PSS heterojunctions depend on the conductivitiy of PEDOT:PSS and the pulse-laser deposition conditions of ZnO, such as deposition temperature, background pressure of oxygen, and ZnO film thickness. X-ray diffraction (XRD) and scanning electron microscopy were used to characterize the pulsed-laser-deposited (PLD) ZnO film. Current-Voltage and Capacitance-Voltage measurements were used to characterize the (PLD) ZnO/PEDOT:PSS heterojunctions.
9:00 AM - P3.10
Transferable Graphene Oxide Electron-Transport Layer by Stamping Nanotechnology for Efficient Organic Photovoltaic Cells
Dong Hwan Wang 1 Jung Kyu Kim 2 Jong Hyeok Park 2
1Chung-Ang University Seoul Korea (the Republic of)2Sungkyunkwan Univ. Suwon Korea (the Republic of)Show Abstract
In this research, transferable graphene oxide (GO) serves as an electron transport layer (ETL) in bulk heterojuction solar cells. The GO is inserted by stamping nanotechnology, with the help of a transfer film, on poly[N-9"-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C71 butyric acid methyl-ester (PC71BM). The BHJ solar cells with the GO ETL exhibits improved short circuit current, Jsc, and improved Power Conversion Efficiency, PCE, because of efficient electron transport from the BHJ layer to the Al cathode resulting in reduced series resistance and better charge extraction compared to similar devices fabricated without the GO interlayer. Moreover, solar cells with sequentially coated GO/titanium oxide (TiOx) ETL show a synergistic effect of increased electric field amplitude as inferred from optical simulation. Addition of the GO/titanium oxide (TiOx) ETL leads to the highest PCE of 7.5 %. BHJ solar cells fabricated with the GO layer by stamping transfer are promising as candidates for high performance devices because of the new ETL architecture which serves to reduce the electronic charge barrier from the UPS analysis.
9:00 AM - P3.11
Hybrid, Solution Processable and Photocrosslinkable High-K Dielectric Based on Titanium Oxide Nanorods for Organic Electronic Applications
Emanuele Verrelli 1 Fei Cheng 2 Fahad Alharthi 2 Neil Kemp 1 Mary O'neill 1 Steve Kelly 2
1University of Hull Hull United Kingdom2University of Hull Hull United KingdomShow Abstract
Nanocomposite hybrid materials based on inorganic dielectric nanoparticles or nanorods are rapidly attracting increasing attention due to their potential as solution processable high-k dielectrics for organic electronics. Although there are several reports claiming the successful implementation of such dielectrics in organic-field-effect-transistors (OFETs), very little has been done on the in-depth characterization of the dielectric (impedance spectroscopy) and electric (leakage current) response of such hybrid systems. Here we present one of the first works addressing this matter by investigating and discussing the dielectric and electric behaviour of several functionalized titanium oxide nanorods films. Both anatase and rutile nanorods were prepared modifying an existing technique found in the literature. The anatase titanium oxide nanorods have an average diameter of 5 nm and length of 20 nm while the rutile nanorods have a mean diameter of 5 nm and length of 40 nm. Ligands of different types, including oleic-acid, phosphonic-acid and photocrosslinkable phosphonate terminated coumarin, were successfully used to functionalize the nanorods. The use of photocrosslinkable ligands is particularly attractive because it enables the fabrication of 1) devices (OFETs) without using orthogonal solvents as well as 2) devices based on stacks of hybrid thin films allowing thus to further tune the properties of the final dielectric stack. The functionalized nanorods can be solution processed in several common solvents and spin coated producing uniform thin films with RMS surface roughness of the order of 1-2 nm. Metal-insulator-metal (MIM) and metal-oxide-semiconductor (MOS) devices were fabricated to assess the dielectric and electric properties of the hybrid materials. In those samples incorporating the crosslinkable material, ultraviolet light irradiation was used to make insoluble films prior to depositing the top electrode. The highest value of the relative dielectric constant is of the order of 30 at 1 MHz and the dielectric loss is of the order of 0.01. The nature of the ligand shell around the titanium oxide core is very important in determining the final dielectric and electric properties of the hybrid film. For this reason, particular attention is devoted to the analysis of the data in order to extract information that allows us to design functionalized nanorods with desired dielectric properties (in terms of total dielectric constant and leakage current of the film). We will discuss the differences in the dielectric and electric behaviour of devices based on 1) nanorods functionalized with different ligands, 2) nanorods with anatase or rutile crystalline phase and 3) stacks of hybrid titanium oxide films (bilayer approach). It should be stressed that the material preparation, device fabrication and measurements are all carried out in air showing the huge potential of this approach in the organic electronic field.
9:00 AM - P3.12
Large Thermopower of delta;-Doped LaTiO3/SrTiO3 Interfaces and Its Field Dependence
Shubhankar Das 1 P. C. Joshi 1 A. Rastogi 1 Z. Hossain 1 R. C. Budhani 1 2
1Indian Institute of Technology, Kanpur Kanpur India2National Physical Laboratory New Delhi IndiaShow Abstract
Measurements of magneto-thermopower (S(H,T)) of interfacial delta doped LaTiO3/SrTiO3 (LTO/STO) heterostructure by an iso-structural antiferromagnetic perovskite LaCrO3 are reported. The thermoelectricpower of the pure LTO/STO interface at 300 K is asymp; 118 mu;V/K, but increases dramatically on δ-doping. The observed linear temperature dependence of S(T) over the range 100 K to 300 K is in agreement with the theory of diffusion thermopower of a two-dimensional electron gas. The S(T) displays a distinct enhancement in the temperature range (T < 100 K) where the sheet resistance shows a Kondo-type minimum. We attributed this maximum in S(T) to Kondo scattering of conduction electron by localized impurity spins at the interface. The suppression of S by a magnetic field, and the isotropic nature of the suppression in out-of-plane and in-plane field geometries further strengthen the Kondo model based on interpretation of S(H, T).
9:00 AM - P3.13
Monochromated Electron Energy-Loss Spectroscopy of Organic Photovoltaic Devices
Frank J. Scheltens 1 Michael F. Durstock 2 Christopher E. Tabor 2 Benjamin J. Leever 2 Lawrence F. Drummy 2 Michael D. Clark 3 1 Dennis P. Butcher 3 2 Jessica A. Alexander 1 David W. McComb 1
1The Ohio State University Columbus USA2Air Force Research Laboratory Wright-Patterson AFB USA3UES, Inc. Dayton USAShow Abstract
Bulk heterojunction (BHJ) organic photovoltaic (OPV) based solar cells hold the promise of achieving the highly desirable objective of low cost, clean renewable electrical power via solar energy collection in a flexible device architecture. Recent developments in polymeric OPV device technology have increased power conversion efficiencies (PCE) into the 10% range, pushing OPV based devices closer to being viable low-cost, environmentally friendly alternatives to contemporary inorganic based solar cells. Extending OPV performance significantly beyond the 10% PCE barrier is a critical challenge. Factors such as the amount of light absorption, efficiency of photo-generation of electrons and holes, and their collection efficiency at the respective electrodes must be optimized in order to improve the device PCE. The development of light trapping strategies such as the utilization of surface plasmon polaritons (SPPs) in metal nanostructures can be an effective method of improving the amount of light absorption inside the active layer of a thin-film solar cell . While incorporation of plasmonic nanostructures into thin-film PV cells is an attractive solution for enhancement of the optical absorption and current density in an OPV, little is known about the detailed structure, chemistry and bonding between the active layer and the plasmonic structure.
High resolution electron energy loss spectroscopy (EELS) can be used to probe the interfaces in the blended film active layer containing plasmonic nanostructures. In particular, monochromated scanning transmission electron microscopy (STEM) valence loss spectra can probe the complex dielectric function of a material with high spatial resolution yielding information about the effect of the interface between the organic phase and metal nano-particle on the observed resonances. Knowledge about the changes in the chemical bonding at this interface can be gained from studying the single electron transitions and collective excitations in the organic phase resulting from Kramers-Kronig analysis of these spectra. This analysis would then result in a signal that is related to the interactions between the acceptor/donor and gold nanoparticle interfaces. A successful Kramers-Kronig analysis requires as input the energy-loss function (ELF) and the extraction of this function from the collected valence loss data is not a trivial process. In this presentation we will explore the application of monochromated STEM and high resolution EELS to the topic of OPV characterization and, in particular, issues related to the removal of the monochromated zero-loss peak (ZLP) from the low loss EELS spectra prior to Kramers-Kronig analysis will be addressed.
The authors acknowledge the Air Force Office of Scientific Research and the Air Force Research Laboratory Materials and Manufacturing Directorate for funding as well as The Ohio State University Center for Electron Microscopy and Analysis for technical support.
9:00 AM - P3.14
Interfacial Structure of Substituted Poly(phenyleneethynylene)s in Contact with Ligand-Stabilized CdS Nanoparticles
Hua Liu 1 Matthew P Espe 2 David A Modarelli 2 Eduardo Arias 3 Ivana Moggio 3 Ronald F Ziolo 3 Hendrik Heinz 1
1University of Akron Akron USA2University of Akron Akron USA3Centro de Investigaciones en Quimica Aplicada Saltillo MexicoShow Abstract
The interfacial region between surface-modified semiconducting nanoparticles and polymers remains difficult to characterize experimentally in atomic resolution and contributes to the limited efficiency of hybrid photovoltaic cells and luminescent devices. Therefore, molecular dynamics simulation was employed to investigate the structure of cadmium sulfide nanoparticles capped with 3-mercaptopropyltrimethoxysilane (MPS) in contact with four substituted poly(phenyleneethynylene)s using a new force field for CdS and the polymer consistent force field. The results show that polymers with long alkyl side chains tend to wrap around the nanoparticles, and reduce backbone bending as well as polymer diffusion. The absence of alkyl side chains decreases the distance of conjugated backbones from the surface. Differences in the preferred location of functional groups of the polymers on the nanoparticle surface and of covalent versus non-covalent bonding were also monitored. Polymers containing terminal hydroxyl groups on alkyl side chains approach the surfactant corona and the core of the CdS-MPS nanoparticles. Close contact supports the formation of silyl ether cross-links although the interfacial structure upon bond formation remains similar to that of the non-covalently attached polymers. Ester groups bound to aromatic rings in the poly(phenyleneethynylene) backbone did not closely approach the nanoparticle surface. The results are the first step to understand nanoparticle-
polymer interfaces at length scales of 10 nm and explore correlations with photovoltaic performance.
9:00 AM - P3.15
Structure Prediction at Oxide Surfaces: The Case of ZnO
Navid Abedi Khaledi 1 Philipp Herrmann 1 Georg Heimel 1
1Humboldt-Universitamp;#228;t zu Berlin Berlin GermanyShow Abstract
Despite their great potential as transparent electrodes in organic electronics and as integral, active components in hybrid (opto-)electronic devices, full control over the surface properties of transition-metal oxides has remained elusive. Atomistic details of their surface structure have proven hard to assess, rendering application-relevant surface properties, such as electronic defect states or work function, highly dependent on environment and preparation conditions.
Here, on the example of zinc oxide (ZnO), we present a novel approach to theoretical ab-initio methods for the prediction of the atomistic surface structure as a function of environmental conditions, specifically temperature and atmospheric composition. Covering both, steady-state structures and kinetic barriers in reaching them, predicted ZnO structures will also be discussed in terms of their experimental signatures in core-level spectroscopy (XPS). The possibility of tuning the properties of ZnO surfaces via self-assembled monolayers (SAMs) of small organic molecules will subsequently be explored and, further extending our methodology, the environmental stability of such SAMs will be elucidated.
9:00 AM - P3.16
Fabrication and Lifetime Testing of Polymer Light-Emitting Diodes
Sivarampragadeesh Siva 1 Catrice Carter 1 Christopher Petoukhoff 1 Deirdre O'Carroll 1
1Rutgers University Dayton USAShow Abstract
The fabrication of organic polymer-based light-emitting diodes (LEDs) is investigated as an option for more environmentally-friendly lighting and displays. These polymer light-emitting diodes (PLEDs) can be a sustainable alternative to incandescent and fluorescent light sources, inorganic light-emitting diodes and liquid-crystal displays. The main problem hindering the use of PLEDs in lighting and display applications is the electroluminescence efficiency of blue light-emitting PLEDs, which is due to the electronic properties of the materials used. A conventional PLED device features the following layers: an ITO on glass anode, a hole transport layer composed of (poly(3,4-ethylenedioxythiophene);poly(styrenesulfonate)) (PEDOT:PSS), an active layer of (poly(9,9-di-n-dodecylfluorenyl-2,7-diyl)) (PFO), an electron transport layer of lithium fluoride, and an aluminum cathode. In this device, the work function of PEDOT:PSS does not provide a good match to the energy of the blue light-emitting PLEDs due to the low HOMO energy level of the conjugated polymer . Furthermore, PEDOT:PSS exhibits poor stability over time resulting in a low operational lifetime. These factors traditionally limit the use of blue light-emitting PLEDs.
In this work, we aim to investigate alternative device architectures such as top-emitting and inverted PLED devices that eliminate the PEDOT:PSS hole transport layer and compare their photoluminescence and electroluminescence lifetime to those of conventional blue PLED devices. Alternative hole transport layers that are under investigation are ultra-thin (< 10 nm) MoO3 and p-type metal oxides (CuOx, NiOx and AgOx). For device construction, polymer spin coating, annealing, and metal thermal evaporation processes are employed. Bright- and dark-field optical microscopy as well as atomic force microscopy is employed to characterize and optimize the homogeneity and thickness of the individual layers present in the device. Device operational lifetime testing will occur through time dependent electroluminescence testing to determine the operational lifetime of the devices (where its lifetime is considered to end when its luminance reaches 50% of its initial value). Furthermore, photoluminescence and electroluminescence testing of the devices will be carried out at both room temperature (25 oC) and elevated temperatures (70 oC) in both inert and ambient environments to compare the relative stabilities of the various device architectures.
 Deng, Xian-Yu. "Light-Emitting Devices with Conjugated Polymers." International Journal of Molecular Sciences 12 (2011): 1575-594. Web. 28 Dec. 2013.
P1: Interfacial Charge Transfer and Energy Storage
Monday AM, December 01, 2014
Hynes, Level 3, Room 308
9:30 AM - *P1.01
Inorganic Nanostructure-Organic Hybrid Materials for Next Generation of Energy Storage
Yi Cui 1
1Stanford University Stanford USAShow Abstract
Storing electricity for portable electronics, electrical vehicles and grid-scale storage calls for next generation of batteries. Inorganic nanostructures functionalized with organic molecules are a powerful materials platform to enable high energy, long cycle life, high power and good safety. Here I present several exciting examples on inorganic-organic hybrids: 1) In-situ polymerization of conducting hydrogel on Si nanoparticles as high capacity cathodes; 2) Self-healing batteries; 3) Polymer-encapsulated hollow S nanoparticle as high-capacity cathodes. 4) Amphiphilic polymer modification for trapping polysulfides.
10:00 AM - P1.02
Well-Defined All-Conducting Block Copolymer Bilayer Hybrid Nanostructure: Selective Positioning of Lithium Ions and Efficient Charge Collection
Taiho Park 1 In Young Song 1
1Pohang University of Science and Technology (POSTECH) Pohang Korea (the Republic of)Show Abstract
The properties of interfaces between organic and inorganic materials in heterojunctions are relevant to a variety of electronic applications. The performances of such devices strongly depend on the kinetics of the charge transfer reactions and the thermodynamic properties (e.g., energetic difference at the junctions) at the heterogeneous interfaces.1-2 Heterogeneous interfaces, especially in iodine-free dye-sensitized solar cells (DSCs),3-5 can be controlled by modifying the interfacial properties to improve the photovoltaic performance. Herein, we present a novel synthesis of well-defined conducting block copolymers,6 which are formed on the surfaces of nanocrystalline TiO2 particles. This approach permitted the selective positioning of lithium ions on the PEDOT-TB blocks7 only and the realization of an ideal device, thereby affording an improvement in the photovoltaic performances in iodine-free DSCs.
1) T. Leijtens, J. Lim, J. Teuscher, T. Park, H. Snaith, Adv. Mater. 2013, 25, 3227.
2) S.-H. Park, J. Lim, I. Y. Song, T. Park, Adv. Energy Mater. 2014, 4, 1300489.
3) S.-H. Park, I. Y. Song, J. Lim, Y. S. Kwon, J. Choi, S. Song, J.-R. Lee, T Park, Energy Environ. Sci. 2013, 6, 1559.
4) S.-H. Park, J. Lim, Y. S. Kwon, I. Y. Song, J. M. Choi, S. Song, T. Park, Adv. Energy Mater. 2013, 3, 184.
5) Y. S. Kwon, J. Lim, H.-J. Yun, Y.-H. Kim, T. Park, Energy Environ. Sci. 2014, 7, 1454.
6) I. Y. Song, Y. S. Kwon, J. Lim, T. Park, ACS Nano2014, DOI: 10.1021/nn5016083 (ASAP).
7) I. Y. Song, S.-H. Park, J. Lim, Y. S. Kown, T. Park. Chem. Commun.2011, 47, 10395.
10:15 AM - P1.03
Rectifying Electrical Noise with an Ionic-Organic Ratchet
Oleksandr V. Mikhnenko 1 Samuel D Collins 1 Thuc-Quyen Nguyen 1
1UCSB Santa Barbara USAShow Abstract
Electronic ratchets can rectify AC signals that are extracted from unpredictable energy fluctuations. Such rectification is needed in radio frequency identification tags (RFIDs) and in other energy harvesting modules. However, currently available electronic ratchets suffer from poor output currents and voltages, and are generally not able to rectify efficiently random noise signals. We present an ionic-organic ratchet device that delivers record high electrical currents of 2.6 and 1.7 mu;A when driven with an AC signal of square wave and random amplitude, respectively. The device is based on a poly(3-hexylthiophene-2,5-diyl):salt blend that acquires rectification properties after a voltage stress in a transistor configuration. The frequency response of the ratchet can be tuned by the geometry of the contacts to obtain peak response above 5 MHz, making it an attractive candidate for use in RFIDs. The working mechanism of the ratchet is a charge pump with current generation efficiency of 62%. We show that the device performance can be increased even further when ZnO is implemented to the active layer due to formation of the rectifying junctions at ZnO-metal interface. The experimental results are supported with a Monte Carlo simulation of charge transport.
10:30 AM - P1.04
Investigations into Cathode-Electrolyte Interfaces in Model Li-Ion Batteries Systems
Joseph Franklin 1 Madhavi Srinivasan 1
1Nanyang Technological University Singapore SingaporeShow Abstract
The cathode/electrolyte interface is the location of vital exchange reactions during charging and discharging cycles and the location of degradation processes such as cation dissolution and cathode surface layer (CSL) formation; however very little is known about the chemical nature of this crucial interface or the modification of its properties during use. Investigations into its properties are made difficult by complicated cathode chemistries and exacerbated further by intricate surface geometries. In this work, a model system is proposed with the deposition of epitaxial spinel cathode films (LiMn2O4 and LiNi0.5Mn1.5O4) on conducting lattice matched substrates, Nb-doped-STO. The low roughness (<1 nm) of these films and the choice of material makes them an ideal system for several in-situ surface characterisation methodologies. The creation of these model systems will be discussed and results presented on their application for investigating the role of the interface in performance limitation and lifetime reduction in lithium-ion battery systems.
11:15 AM - *P1.05
Fundamental Charge Transfer Processes in Stable Free-Radical Organic Polymer Systems
Wade Braunecker 2 Barbara Hughes 2 Madison Martinez 2 1 Travis Kemper 2 Ross Larsen 2 Andrew Ferguson 2 Steven George 1 Thomas Gennett 2
1University of Colorado Boulder USA2National Renewable Energy Laboratory Golden USAShow Abstract
Polymers with stable pendant radical groups are a unique class of redox-active materials emerging as potentially the next generation energy storage breakthrough. These polymers facilitate apparently remarkably rapid, efficient and reversible multi-step charge-transfer processes. The focus of our work is to advance the fundamental understanding of the structure-property relationships associated with the mechanism(s) of electron transfer and ion transport, along with associated interfacial mass-transfer processes that impact the charge-transfer processes of a unique class of organic free-radical polymeric redox active materials. A greater understanding of the inherent charge transfer limitations in such systems will ultimately be paramount to further advancements in the understanding of both inter-film and interfacial ion and electron transfer reactions.
The project involves an integrated approach of chemical synthesis, electrochemistry, spectroscopy and theoretical modeling of a series of stable organic radical materials. Initially, we have focused on the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) organic radical moiety incorporated into a complex materials set of poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (PTMA). Our initial synthetic efforts focused on developing strategies to alter the fundamental charge transport phenomena in the nitroxide radical polymers through structure control. For example we systematically changed the radical density within the polymers by introducing “blanks”, or monomers not bearing any nitroxide radical. Several series of copolymers and oligomers bearing 20, 40, 60, 80, and 100 mol % of the radical moiety were synthesized. The electrochemical, spectroelectrochemical, EPR analysis and theoretical models then coalesced on a unique property of the 20 mol % PTMA derivative where the change in electron-transfer rates, ionic mobility and observed overpotentials correlated to some of the changes in predicted molecular packing.
Controlled radical polymerization was used to also grow brushes of PTMA on an indium tin oxide (ITO) surface. Subsequently we evaluated the 1-dimensional, intra-molecular charge transport properties along the polymer chain within these brushes, especially across the hard-soft inorganic-organic interfacial barrier.
In our presentation we will discuss in detail the synthesis, electrochemical and spectro-electrochemical properties of these unique PTMA materials, and discuss how variation in oxidation states of nitroxide radical in the films affects electrochemical properties of electrodes.
11:45 AM - P1.06
Charge Generation at Polymer/Metal Oxide Interface: From Molecular Scale Dynamics to Mesoscopic Effects
Ajay Ram Srimath Kandada 1 Simone Guarnera 1 2 Francesco Tassone 1 Guglielmo Lanzani 1 2 Annamaria Petrozza 1
1Istituto Italiano di Tecnologia Milano Italy2Politecnico di Milano Milano ItalyShow Abstract
The correlation between molecular scale morphology and charge generation across hybrid photovoltaic interfaces made of metal oxides (ZnO and TiO2) and a prototypical electron donor polymer, P3HT, is investigated. Device characterization and UV-NIR transient absorption spectroscopy are used to demonstrate that the local disorder of the polymer chains on the surface of the metal-oxide film provides better electron injection efficiencies than the crystalline phases1, though the latter are essential for energy and charge transport. Charge injection dynamics from the polymer to the metal-oxide have been largely investigated based on the excitation dynamics in the polymer that include both bulk and interface phenomenon 2,3. Here, an unambiguous spectroscopic tool is demonstrated to probe the occupation of the conduction band of ZnO following the electron injection from the polymer through the ultrafast tracking of the Burstein-Moss effect4.
 Kandada et al, Adv. Func. Mater. 24, 3094-3099 (2014).
 Meister et al, J. Phys, Chem. Lett. 3, 265-2670 (2012).
 Bansal et al, Sci. Rep. 3, 1-8 (2013).
 Burstein, Phys. Rev. 93, 632 (1954).
12:00 PM - P1.07
Polymer Photocurrent Contribution in Hybrid Metal Oxide-Polymer Solar Cells
Jonas Weickert 1 Philipp Ehrenreich 1 Eugen Zimmermann 1 Lukas Schmidt-Mende 1
1University of Konstanz Konstanz GermanyShow Abstract
Hybrid solar cells (HSCs) have the potential to combine the advantages of organic solar cells and solid-state dye-sensitized solar cells (SS-DSCs) by implementation of an organic hole transporter instead of the transparent Spiro-OMeTAD in SS-DSCs. High light absorption is possible due to the use of monolayers of dye molecules or thin coating with highly absorbing semiconductors as sensitizers on the high surface area TiO2 electrodes. Additional light absorption becomes possible if high performance OPV materials with high extinction coefficients are implemented as hole transporters. In many instances throughout literature, however, this polymer light absorption is not efficiently utilized in order to generate additional photocurrent.
This project focusses on the polymer photocurrent contribution in different hybrid solar cell systems. Our results show how the choice of the dye directly influences how efficiently photocurrent is generated upon photon absorption in the polymer and how charge carrier recombination can be suppressed. Furthermore, the interfacial modifier 4-mercaptopyridine is found to enhance the polymer contribution in systems where electron or energy transfer from the polymer to the dye is energetically allowed. We also show a combined electronic and optical study outlining the higher efficiency of hybrid solar cells with an energy transfer mechanism at the polymer-metal oxide interface. This is inferred from photoinduced absorption experiments that are in accordance with external quantum efficiency spectra. Our investigations are based on a model system of three different dye molecules in combination with the conjugated polymer poly(3-hexylthiophene) (P3HT). A squaraine dye is employed in order to allow energy transfer from the polymer to the dye, while a fullerene derivative serves as a charge separation model system. An indoline dye with a high lowest unoccupied molecular orbital further is used in devices where the polymer only serves as hole transporter and cannot contribute to the photocurrent.
 J. Weickert, L. Schmidt-Mende, APL Materials 2013, 1, 020901.
 J. Weickert, F. Auras, T. Bein, L. Schmidt-Mende, Journal of Physical Chemistry C 2011, 115, 15081.
 J. Weickert, E. Zimmermann, J. B. Reindl, T. Pfadler, J. A. Dorman, A. Petrozza, L. Schmidt-Mende, APL Materials 2013, 1.
12:15 PM - P1.08
Interfacial Charge Transfer and Charge Transport in Dye-Sensitized Solar Cells Based on Porous Zinc Oxide and Combinations of Organic Dyes
Melanie Rudolph 1 Tsukasa Yoshida 2 Derck Schlettwein 1
1Justus Liebig University Gieamp;#223;en Giessen Germany2Yamagata University Yonezawa JapanShow Abstract
Electrodeposited porous ZnO shows excellent electron transport properties without high-temperature treatment, and therefore presents a promising alternative to TiO2 as photoelectrode material in dye-sensitized solar cells (DSCs). Combined with the organic dye D149, a conversion efficiency of around 6 % has been attained. While this result comes close to the top efficiency of 7.5% of ZnO-based DSCs, it remains below the record of 12-13 % among TiO2-based cells. In order to fully exploit the potential of electrodeposited porous ZnO as a low-temperature alternative to TiO2, new strategies to improve the photovoltaic performance by modifying the oxide-dye-electrolyte interface need to be developed. Presently, the best cells based on electrodeposited ZnO rely on co-adsorption of cholic acid to prevent aggregation of adsorbed D149 molecules, as photoelectrochemical studies have found that the presence of D149 aggregates accelerates recombination and thereby lowers the fill factor. While cholic acid is effective in reducing aggregation, it does not absorb visible light, and therefore does not actively contribute to the photovoltaic operation of the cell.
The approach of the present study was to combine D149 with light-absorbing co-adsorbates, i.e. co-sensitizers, in order to investigate whether they are capable of preventing D149 from aggregating on the ZnO surface, while simultaneously participating in the conversion of sunlight. Three organic co-sensitizers with different spectral light absorption were employed: the indoline dye D131, a sulfonated zinc phthalocyanine as well as the squaraine dye Sq2. Moreover, the influence of additional adsorption of the co-adsorbates cholic acid and octanoic acid was investigated. Sandwich-type solar cells with an iodide/triiodide redox electrolyte were built from the sensitized ZnO films, and were analyzed in their optical properties, global device performance and microscopic charge transport and transfer processes by means of absorption spectroscopy, current-voltage characterization, quantum efficiency measurements, time-resolved photocurrent and photovoltage measurements, intensity-modulated photovoltage and photocurrent spectroscopy (IMVS and IMPS) and electrochemical impedance spectroscopy (EIS).
Combination of D149 with both D131 and Sq2 led to efficient sensitization of electrodeposited ZnO films and improved performance compared to reference cells with D149 only. For all dye combinations, an additional improvement of the fill factor and the open-circuit photovoltage was found upon co-adsorption of cholic acid or octanoic acid. The observed dye-dye, dye-co-adsorbate and dye-ZnO interactions will be discussed in detail in conjunction with the resulting effects on interfacial charge transfer processes and charge transport. Particular emphasis will be put on correlations between the properties of the dye/co-adsorbate layer and electron recombination across the ZnO-dye-electrolyte interface.
12:30 PM - *P1.09
Charge Transfer Dynamics at Buried Interfaces
Xiaoyang Zhu 1
1Columbia University New York USAShow Abstract
Charge and energy transfer across interfaces are fundamental processes in semiconductor electronics and optoelectronics. Using model organic/inorganic semiconductor interfaces and femtosecond nonlinear laser spectroscopies, we provide real time views on how exciton disscociation across the interface sets up transient electric fields on femtosecond time scales, how Coulomb attraction leads to the formation of charge transfer excitons, and how energy relaxation competes with charge separation. These measurements reveal fundamental mechanisms responsible for electronic and optoelectronic devices.
Aram Amassian, King Abdullah University of Science and Technology
Joseph J. Berry, National Renewable Energy Laboratory
Martyn A. McLachlan, Imperial College London
Erin L. Ratcliff, University of Arizona
Symposium Support Angstrom Engineering Inc.
Journal of Materials Chemistry
P5: Interface and Electrode Modification
Tuesday PM, December 02, 2014
Hynes, Level 3, Room 308
2:30 AM - P5.00
Modification of ITO Electrodes Using a TiO2 Interlayer for Electron Selective Contacts in OPVs
Hyungchul Kim 1 Kai-Lin Ou 2 Xin Wu 2 Neal R. Armstrong 2 Samuel Graham 1 3
1Georgia Institute of Technology Atlanta USA2University of Arizona Tucson USA3Georgia Institute of Technology Atlanta USAShow Abstract
We demonstrate the use of ultra-thin (0.5-3 nm) titanium dioxide (TiO2) films for the application of hole-blocking interlayers in inverted organic photovoltaics (OPVs) using plasma-enhanced atomic layer deposition (PE-ALD). In order to evaluate the hole blocking capability of the ultra-thin TiO2 films, we fabricated diodes from TiO2/p-doped Si heterojunction devices with aluminum and silver contacts on front and backsides to isolate the rectification effect of the TiO2 layer. Subsequently, we integrated these films as a modifier of ITO electrodes in organic photovoltaics to create inverted solar cells. This was done using poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC61BM) bulk-heterojuction OPV devices with TiO2 interlayers on ITO to evaluate OPV performance. The TiO2/Si diode device showed remarkable current flow density being more than 6000 mA/cm2 at 0.5 V under forward bias while less than 50 mA/cm2 under -0.5 V backward bias, which suggests excellent rectification performance without losing electron transport rate. The fabricated OPVs showed 2.7-3.0% power conversion efficiency, 9.2-9.7 mA/cm2 of short-circuit current density, and 0.61-0.64 V of open-circuit photopotential under light illumination. X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) of TiO2 films suggested that fabricated TiO2 films are titanium dioxide with low density of defects, and has a deep valence band energy that can block holes effectively. Based on these results and the environmental stability of TiO2, these results show the thinnest ALD films used to create rectifying contacts in organic solar cells. In addition, the aqueous stability of these films also suggest that these interfaces may present an effective way of creating robust, environmentally stable low workfunction contacts for OPV as compared to more traditional methods using ZnO, thus producing enhanced reliability.
2:45 AM - *P5.01
c-Si/PEDOT:PSS Interface Modification as Model System for Studying Hybrid Organic Inorganic Systems
David Cahen 1 Ann Erickson 1 Nir Kedem 1 Arava Zohar 1
1Weizmann institute of Science Rehovot IsraelShow Abstract
Coupling the properties of electronically active organic materials with those of inorganic semiconductors has long been recognized as an advantageous device design, combining low processing temperatures, tunable structure and properties, and low surface state density given by the first with high carrier mobility, low exciton binding energy and long photo-excited charge carrier lifetime of the other. In many cases, though, this approach results in a system where the materials&’ limitations, rather than their advantages, dominate device performance. By systematically studying and modifying the interfaces of a well-controlled Si/PEDOT:PSS model system we explore ways to mitigate the effect of these limitations on photovoltaic device performance. We find that the Si surface passivation is the most crucial parameter in determining the devices electronic behavior. Well passivated surface by H termination, molecular monolayer or an oxide film results in high performance devices.
Inversion layer photovoltaic devices rely on majority carrier inversion at a semiconductor surface to form a p-n homojunction. The junction formation occurs by charge transfer upon interface formation between the semiconductor and a specifically selected metal (or a metal-like polymer). Using single crystal (sc) n-Si/PEDOT:PSS as we study the effects of interfacial energy level alignment, bulk doping level and mobility, and surface and interface passivation on device performance. With H termination, known to passivate Si for a limited time only, a close to maximal Vbi can be achieved. To obtain more enduring stability we modified the Si surface with several dipole bearing molecular monolayers. The molecular layers serve to provide more lasting passivation and to allow work function tuning in order to vary the Vbi of the junction with PEDOT-PSS. The Vbi can be derived from capacitance-voltage (CV) and current-voltage (IV) measurements. Although the CV results show that the molecular surface modification is preserved at the interface, if the modification implies an increase in Vbi, then only very small effects in the IV characteristics are seen. This can be understood by assuming inhomogeneity of the monolayer. Higher capacity comes from densely covered areas (high Vbi), dominating the CV measurement. Higher current passes through loosely covered area (low Vbi) areas, dominating the IV curve. Alternatively, we used ultra-thin film of highly charged Al2O3 to induce inversion electrostatically and stably passivate the Si surfaces.In this approach charges collection is done via small areas from which Al2O3 was removed, and Si/PEDOT:PSS interface is formed.
Even though the (sc) n-Si/PEDOT:PSS system has been well-studied, we find it to be quite useful as a model to investigate hybrid systems, not only with PEDOT:PSS. In addition, with PCE up to ~12% and room temperature processing of the wafer, the system is interesting in itself for real applications.
3:15 AM - P5.02
Surface Modification of TiO2 Nanoparticle as the Low Temperature Processed Electron Transport Layer for Perovskite Solar Cells
Meng-Huan Jao 1 Wei-Fang Su 1
1National Taiwan University Taipei TaiwanShow Abstract
Recently, perovskite photovoltaic technique has attracted an intense interest and great progress has been made for this kind of organolead trihalide based solar cells. Nowadays, efficiency higher than 17% has been realized for this hybrid solid state devices. Different from other high performance thin film solar cells, perovskite soalr cells are solution processable, and processing temperature higher than 150 oC is not needed. All those special properties make perovskite solar cells one of the most promising techniques as future energy sources. However, most of the high efficiency perovskite devices are fabricated on substrate with crystalline TiO2 as electron transport layer, which needs to be sintered at temperature higher than 450 oC. This extreme sintering condition limits the choices of substrate and increases the production cost and embedded energy.
Here in our presentation we aim for replacing high-temperature sintered TiO2 electron transport layer by anatase TiO2 nanoparticle, which is synthesized by hydrolysis of titanium tetraisopropoxide in oleic acid. The room-temperature processed TiO2 nanoparticle based perovskite solar cells shows promising efficiency of 11.5%. Further improvement of the device performance can be made by treating this oleic acid-capped TiO2 nanoparticle with adequate ligand exchange methods to eliminate the surface traps of the nanoparticles. Our study demonstrate sintering-free TiO2 nanoparticle electron transport layer for perovskite solar cells by solid state ligand exchange method. This work relax the substrate choices and enables the fabrication of perovskite devices on flexible polymer substrate, thus opens the door for roll-to-roll solution-precessed perovskite devices.
3:30 AM - P5.03
Simultaneous Protection of Organic p- and n-Channel Layers against Aging and Bias-Stress by Guanine/Al2O3 Double Layer on Complementary Inverter
Junyeong Lee 1 Hyuncheol Hwang 1 Sung-Wook Min 1 Hee Sung Lee 1 Jae Min Shin 1 Seongil Im 1
1Younsei University, Korea Seoul Korea (the Republic of)Show Abstract
Organic field effect transistors (OFETs) have been developed for over a decade with various benefits of light weight, low cost, flexibility. The development of electrical property has been of particular concern and nowadays the mobility of several organic semiconductors becomes to exceed that of amorphous Si based FET which is used in display industry. What makes practical application of OFETs still problematic, however, is operational-bias and environmental stability. The factors to determine the stability are generally categorized into four parts; semiconductor channel, dielectric, channel/dielectric interface, and semiconductor channel surface. For the cases of dielectric and interface, the most detrimental factor is thought to be charge trapping phenomena originating from hydroxyl functional group or other ionic impurity. Those trapping effect can be dramatically reduced using polymer dielectric such as fluoropolymer or self-assembled monolayers treatment on conventional inorganic oxide. For the case of organic semiconductor and its surface, on the other hand, the vulnerability of charge carriers to penetrated ambient oxygen and water molecules induces detrimental effects on device stability. To protect OFET from such molecule-induced instabilities, several strategies are also introduced: formation a kinetic barrier by more compact packing, lowering the level of molecule orbitals below such as oxidation energy level, and increasing the band-gap for defending even from energetic photons. However, although finding proper passivation layer can be the most promising strategy to protect the organic semiconductor from penetrated atmospheric species, passivation related works have not been reported as much as other methods, particularly for n-type organic semiconductors.
Here we propose DNA-base small molecule guanine and Al2O3 bilayer as a unique passivation layer to solve above issues at once, simultaneously covering the aging and bias stress stabilities of both n- and p-channel OFETs in an air ambient. Although the Al2O3 layer shows very low oxygen and water permeability, direct deposition of such an inorganic insulator on organic material usually cause serious damage because the deposition process contains several chemical impurities from precursor and water. So we adopted an organic layer to buffer/or minimize the damaging effects; 20 nm-thin layer by guanine, one of DNA-base small molecules as extracted from DNA polymer, was deposited on n- and p-channel OFETs before atomic layer deposition of thin Al2O3 layer, since unintentional doping of hydrogen is efficiently prohibited by guanine. Using guanine/Al2O3 passivation layer, both p- and n-type OFETs demonstrated dramatic endurance under gate-bias stress and 30 days-long aging, showing little threshold and mobility change. As a result, a highly stable complementary-type logic inverter was even accomplished by combining p- and n-type FETs, successfully operating at 5 V as a minimum low voltage.
4:15 AM - *P5.04
Polymer Zwitterions: Strong Work Function Reducers for All Electrodes
Alejandro L. Briseno 1 Hyunbok Lee 1 Todd Emrick 1
1University of Massachusetts Amherst Amherst USAShow Abstract
We demonstrate the use of polymeric zwitterions, namely, poly(sulfobetaine methacrylate) (PSBMA), as solution-processable work function reducers for inverted organic electronic devices. A notable feature of PSBMA is orthogonal solubility relative to solvents typically employed in the processing of organic semiconductors. A strong permanent dipole moment on the sulfobetaine moiety was calculated by density functional theory. PSBMA interlayers reduced the work function of a broad range of electrodes [indium tin oxide (ITO), Au, Ag, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), Cu, Al, and even graphene] by over 1 eV. By employing an ultrathin interlayer of PSBMA, one can reduce the electron injection barrier between ITO and C70 by 0.67 eV. As a result, the device performance of OPVs with PSBMA interlayers are significantly improved, and enhanced electron injection is demonstrated in electron-only devices with ITO, PEDOT:PSS and graphene electrodes. This work makes available a new class of dipole-rich, counterion-free, pH insensitive interlayers for use as strong work function reducers for any electrode.
4:45 AM - P5.05
Characterizing the Selective Contacts to Methyl-Ammonium Pb Halide Perovskites: A Surface Photovoltage Spectroscopy Study
Lee Barnea-Nehoshtan 1 Saar Kirmayer 1 Eran Edri 1 Gary Hodes 1 David Cahen 1
1Weizmann Institute of Science Rehovot IsraelShow Abstract
The field of organo-lead perovskite absorbers for solar cells is developing rapidly, with open circuit voltages of some of the reported devices approaching the maximal achievable theoretical voltage. Obtaining such high voltages on spun-cast or evaporated thin films is intriguing, and calls for detailed investigation of the source of photovoltage in those devices. We present a study of the roles of the selective contacts to methyl ammonium lead iodide (MAPbI3) and methyl ammonium lead iodide chloride (MAPbI3-xClx) using surface photovoltage spectroscopy. By depositing and characterizing each layer at a time, we find that the electron-extracting interface is more than twice as effective as the hole-extracting interface in generating photovoltage, for several combinations of electrode materials. We find indications for the existence of strong internal fields, which we attribute to some band bending in the MAPbI3-xClxat both the electron-extracting and hole-extracting interfaces. We further observe the existence of an electron-injection related spectral feature at 1.1 eV, which might be significant for the cell&’s operation. By varying the granularity of the MAPbI3 we demonstrate that this IR feature is related to grain boundaries. Our results show how SPV spectroscopy helps pinpoint processes that limit the important charge injection processes in perovskite-based solar cells, thus providing an additional tool to improve cell performance based on understanding.
5:00 AM - P5.06
Molecular Monolayer Based Inorganic-Organic Interfaces: A Key to Improve the Performance of Perovskite Solar Cells
Pabitra K. Nayak 1 Sandeep K. Pathak 1 Nakita K. Noel 1 Henry J. Snaith 1
1University of Oxford Oxford United KingdomShow Abstract
Photovoltaic (PV) solar cells promise to be a major contributor to our future energy supply. The ramping up in production and affordable global uptake of solar energy requires a significant reduction in materials and manufacture costs. Furthermore, a solar cell industry on the TW scale must be based on sustainable materials and manufacturing techniques.
Hybrid organic -inorganic based perovskite cells prepared by low cost material and technique have shown impressive progress in solar to electric power conversion efficiency in last two years. These types of cells have potential to show even better power conversion efficiency in near future if the growth of perovskite films is further optimized for solar cell application. Though many techniques are used to prepare trihalide-organo-lead perovskite thin films, spin coating method appears to be an attractive option due to the low fabrication cost. Planar heterojunction cells (i.e. without mesoscopic TiO2/Al2O3 layer) prepared in spincoating method shows power conversion efficiency ~12%. These solution processed perovskite thin films suffer from non-uniformity, partial coverage and band gap states. Moreover, the defects at the TiO2 and perovskite interface can also play a major role in cell performance. In order to improve the performance of these types of cells, it is required to have appropriate passivation of defects, interface engineering for better coverage and higher crystallinity. To this end, we have used silane based organic molecular monolayers to modify compact TiO2 surfaces using a simple, yet powerful wet chemical method. Perovskite thin films prepared by spincoating on these modified TiO2 surfaces show better crystallinity than the unmodified surface, observed by Xray diffraction and scanning electron microscope (SEM) measurements. Photothermal deflection spectroscopy (PDS) suggests that perovskite films prepared on modified surfaces has lower density of bandgap (tail) states, which often causes non-radiative recombination that lowers the power conversion efficiency. Time resolved photo-luminesces measurement and other photo-physical measurements also show that higher crystallinity translates into higher carrier life time which is essential for planar heterojunction cells. Solar cells prepared with modified interfaces show best power conversion efficiency over 15% compared to 12% for cells with unmodified interfaces. We attribute the enhancement in efficiency to the better crystallinity in perovskite thinfilm and passivation of defect states at the interfaces which in turn are achieved by interface engineering with small organic molecules.
5:15 AM - *P5.07
The Critical Impact of Film Formation and Crystallisation upon the Operation of Organic-Inorganic Perovskite Solar Cells
Henry Snaith 1
1University of Oxford Oxford United KingdomShow Abstract
Organic-inorganic metal halide perovskite absorbers have rocketed to the forefront of PV research as efficient solar cell materials, which seem to be both simple to process and promise to reach the highest efficiencies. Great advances have been made in performance and this appears to be largely driven by improved control of thin film formation and crystalisation. Thin films from the archetypical perovskite, CH3NH3PbI3, can be fabricated in many ways, ranging from one-pot solution casting to sequential deposition to vacuum based sublimation. There are many open questions concerning the mechanism for film formation and crystallisation and the ensuing impact on electronic quality of the perovskite absorber. Here I will present a detailed study understanding how crystallisation proceeds when the film is cast from a common solution of the precursor salts and correlate materials properties to optoelectronic and device characteristics. I will elucidate the role of halide within the precursor solution and the role of excess organic component, finally shedding light in the critical role Cl plays in the mixed halide perovskite CH3NH3PbI3-xClx.
P4: Characterization of Buried Interfaces
Tuesday AM, December 02, 2014
Hynes, Level 3, Room 308
9:30 AM - *P4.01
Mapping the Effects of Heterogeneity in the Buried Oxide/Organic Electronic Interface in OPVs
David S Ginger 1
1University of Washington Seattle USAShow Abstract
Recently, a number of interlayers and chemical interface modifications - ranging from NiOx to the use of self-assembled monolayers to the use of chemical dopants- have been used to tailor the properties of the transparent conductive oxide (TCO) interface with the active layer in organic semiconductor devices. Here, we show how chemistry at the buried interface can affect carrier recombination lifetimes in organic bulk heterojunction solar cells. Furthermore, we show how the heterogeneity inherent in many chemical interfaces can manifest as heterogeneity in local carrier recombination lifetimes, as probed via intensity-modulated scanning kelvin probe microscopy (IM-SKPM).
10:00 AM - P4.02
Deformation Dynamics in Azobenznene Liquid Crystal Polymer Films Measured by Time-Resolved Techniques and Microscopic Observation
Kenji Katayama 1 Tomomi Fujii 1 Shota Kuwahara 1 Kiyohide Takado 1 Tomiki Ikeda 2
1Chuo University Tokyo Japan2Research and Development Initiative, Chuo University Tokyo JapanShow Abstract
Crosslinked azobenzene liquid crystal (LC) polymer films are bent by irradiation of UV light. Utilizing the feature, a plastic motor , a robotic arm  etc. have been developed. The overall mechanism was understood as the surface region of the film was contracted. Although the mechanism has not been fully understood yet on what kinds of interactions between molecules, molecular chains, domains are involved and which time scale they happen. On the other hand, we have developed a new type of the time-resolved technique, called the heterodyne transient grating (HD-TG) method [3,4], which features a highly sensitive detection of the refractive index change and wide temporal response from nanoseconds to seconds. In this study, we studied the molecular dynamics in an azobenzene LC polymer film by using the HD-TG technique, combined with the transient absorption (TA) technique. By observation of the refractive index change induced by a pulse laser, contraction of the film was observed on the order of several hundreds of nanoseconds, and the subsequent reorientation and molecular rotation dynamics was observed from a few microseconds to a hundred milliseconds. Finally, the cis isomer of azobenzene was thermally returned back to the trans isomer about ten seconds because the film could not be bent in the liquid crystal cell. Since the contraction, reorientation and molecular rotation took place before the cis to trans back-transformation, these processes correspond to the preliminary molecular motion preceding the macroscopic bending of the film. . Furthermore, we studied the structure and ordering change in the domains in the LC polymer film by using a phase microscopy with a depth slicing function, which helped understanding how the LC molecules behave for the bending.
1. M. Yamada, M. Kondo, J. I. Mamiya, Y. L. Yu, M. Kinoshita, C. J. Barrett and T. Ikeda, Angew. Chem. Int. Ed., 2008, 47, 4986-4988.
2. M. Yamada, M. Kondo, R. Miyasato, Y. Naka, J.-i. Mamiya, M. Kinoshita, A. Shishido, Y. Yu, C. J. Barrett and T. Ikeda, J. Mater. Chem., 2009, 19, 60-62.
3. K. Katayama, M. Yamaguchi and T. Sawada, Appl. Phys .Lett. 2003, 82, 2775-2777.
4. M. Okuda and K. Katayama, Chem. Phys. Lett., 2007, 443, 158-162.
5. T. Fujii, S. Kuwahara, K. Katayama, K. Takado, T. Ube and T. Ikeda, Phys. Chem. Chem. Phys., 2014, 16, 10485-10490.
10:15 AM - P4.03
Infrared Spectroscopic Investigation of Charge Transfer at Interfaces of Organic Semiconductors
Sebastian Beck 1 2 David Gerbert 1 2 3 Tobias Glaser 1 2 Annemarie Pucci 1 2 4
1Heidelberg University Heidelberg Germany2InnovationLab GmbH Heidelberg Germany3Heidelberg University Heidelberg Germany4Heidelberg University Heidelberg GermanyShow Abstract
The mechanisms of charge transfer (CT) in organic electronics are of fundamental importance for device functionality. Especially a more detailed understanding of interactions at organic/organic and organic/inorganic interfaces is needed to improve device performance and to develop new device architectures. Overall, its basic principles are still under strong investigation and subject of a severe discussion.
Here we show that in-situ infrared (IR) spectroscopy is a powerful tool to investigate CT effects at such interfaces. The prototypical interfaces between hole transport materials, such as 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) and N,N'-Di-[(1-naphthyl)-N,N'-diphenyl]-1,1'-biphenyl)-4,4'-diamine (alpha-NPD), and the hole injection layer material MoO3 were investigated. When the organic molecules are deposited on MoO3 charged and non-charged molecular species are formed which can be spectroscopically identified and quantitatively analyzed with in-situ IR-spectroscopy. The different species can be distinguished by their specific mid IR absorption features. For the inverted layer structure, MoO3 deposited on a thin organic layer, a significantly different behavior was observed. The organic layer is highly doped, due to a strong diffusion of the deposited MoO3 into the organic layer.
Financial support by BMBF via MESOMERIE Project (FKZ 13N10724) is gratefully acknowledged.
10:30 AM - P4.04
Electron Transport and Inelastic Electron Tunneling Spectroscopy of Porphyrin in a Molecular Junction
Teresa A Esposito 1 Alexandra Krawicz 2 Peter H Dinolfo 2 Kim Lewis 1
1Rensselaer Polytechnic Institute Troy USA2Rensselaer Polytechnic Institute Troy USAShow Abstract
Organometallic molecules, such as porphyrin, are being investigated as circuit elements for organic electronics. Porphyrins are highly conjugated aromatic molecules that have shown electronic properties that exhibit high and low conductance states. These conductance states may be related to conformational changes in the molecule. To investigate these conductance states we plan to perform inelastic electron tunneling (IET) spectroscopy of porphyrin molecules in an electromigrated nanogap. IET spectroscopy is an important tool in determining the vibrational modes of a molecule adsorbed to a metal oxide and is found by identifying peaks in the second derivative of the junction characteristics (d2I/dV2).
We form nanogaps via electromigration of a 20x100 nm gold wire, producing gaps of ~2-3 nm, which is approximately the length of a porphyrin molecule. Each end of the porphyrin molecule is functionalized with a thiol group (-SH) which will covalently bond to the gold, forming a molecular junction. We simultaneously measure I-V, dI/dV, d2I/dV2 of the junction at 4.2 K and 300 K. Electron transport will be compared between an empty nanogap and a molecular junction. The size of the junction will be calculated using Landauer theory, and then compared to scanning electron microscopy (SEM) images of the gap.
11:15 AM - *P4.05
Charge Dynamics at Hybrid Organic - Metal Oxide Interfaces
P. Tiwana 1 C. T. Weisspfennig 1 P. Docampo 1 M. B. Johnston 1 H. J. Snaith 1 Laura M Herz 1
1University of Oxford Oxford United KingdomShow Abstract
We have explored hybrid interfaces comprising organic dyes as sensitizer monolayers on metal-oxide mesostructures films, which have been highly successful when implemented in dye-sensitized solar cells (DSSCs). High-performance dye-sensitized solar cells are usually fabricated using nanostructured TiO2 as a thin-film electron-collecting material. However, alternative metal-oxides are currently being explored that may offer advantages through ease of processing, higher electron mobility, or interface band energetics. We have conducted a comparative study of electron mobility and injection dynamics in thin films of TiO2, ZnO, and SnO2 nanoparticles sensitized with Z907 ruthenium dye. Using time-resolved terahertz photoconductivity measurements, we show that, for ZnO and SnO2 nanoporous films, electron injection from the sensitizer has substantial slow components lasting over tens to hundreds of picoseconds, while for TiO2, the process is predominantly concluded within a few picoseconds . These results correlate well with the overall electron injection efficiencies we determine from photovoltaic cells fabricated from identical nanoporous films, suggesting that such slow components limit the overall photocurrent generated by the solar cell. We conclude that these injection dynamics are not substantially influenced by bulk energy level offsets but rather by the local environment of the dye nanoparticle interface that is governed by dye binding modes and densities of states available for injection, both of which may vary from site to site. For SnO2 in particular, such effects appear to be influenced by a “light-soaking” effects, i.e. we observe a monotonic speeding-up of electron transfer from the photoexcited dye into the semiconductor following exposure by a pulse train at 550 nm wavelength . We postulate that these effects are caused by photoinduced charging of the SnO2 inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. In addition, we explore how surface coating and percolation pathway formation occurs when a solid-state hole transporting material (spiro-OMeTAD) is infiltrated into the pores of a nanoporous TiO2 network used in solid-state dye-sensitized solar cells . We find that as the hole-transporter coats the surface of the dye-sensitized TiO2, the yield of hole transfer from dye sensitizer to hole transporter increases with pore-filling fraction and saturates around ~30%. Using a simple model of random infiltration of spiro-OMeTAD into the TiO2 porous network, we hence show that that charge diffusion through the dye monolayer network must precede transfer to the hole-transporting material. We further observe sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory predicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls.
 P. Tiwana, P. Docampo, M. B. Johnston, H. J. Snaith, and L. M. Herz, ACS Nano 5, 5158 (2011).
 P. Tiwana, P. Docampo, M. B. Johnston, L. Herz, and H. Snaith, Energy Environ. Sci. 5, 9566 (2012).
 C. T. Weisspfennig, D. J. Hollman, C. Menelaou, S. D. Stranks, H. J. Joyce, M. B. Johnston, H. J. Snaith, and L. M. Herz, #8232;Adv. Func. Mater. 24, 668 (2014).
11:45 AM - P4.06
Charge Separation and Energy Transfer at Metal Oxide/Organic Interfaces: A Case Study with ZnO
Sylke Blumstengel 1 Francesco Bianchi 1 Stefan Hecht 2 Fritz Henneberger 1 Bjoern Kobin 2 Norbert Koch 1 Seth R. Marder 3 Karttikay Moudgil 3 Raphael Schlesinger 1 Mino Sparenberg 1
1Humboldt University of Berlin Berlin Germany2Humboldt University of Berlin Berlin Germany3Georgia Institute of Technology Atlanta USAShow Abstract
ZnO is currently attracting significant interest as a candidate for hybrid photovoltaic and light-emitting devices. We studied - in an all-ultrahigh vacuum approach - the interfacing of ZnO with various conjugated organic molecules, including perylene derivatives, oligo-phenylenes as well as ladder-type oligo-phenylenes whose fundamental optical excitation is resonant to the ZnO band gap. The morphology and electronic structure of the hybrid interfaces were determined by in-situ electron diffraction, scanning probe microscopies, photoemission and reflection spectroscopy, complemented by ex-situ transmission electron microscopy and X-ray diffraction analyses. By appropriate interfacial design, we are able to tune electron-hole separation at the ZnO/organic interface and to achieve excitonic energy transfer with efficiencies of up to 80 %.
12:00 PM - P4.07
Probing the Relative Sensitization Efficiencies of Dye Monomers and Aggregates by Simultaneous Photocurrent and Attenuated Reflection Spectroscopies on ZnO Single Crystal
Laurie Ann King 1 Bruce A Parkinson 1
1University of Wyoming Laramie USAShow Abstract
Cyanine dyes, often utilized in dye-sensitized solar cells (DSSC), are prone to aggregation forming a range of molecular species from monomer to H- and J- aggregates in both solution, and when adsorbed on an oxide semiconductor photoelectrode. The relative efficiency of the different adsorbed dye species to inject photo-excited electrons into single crystal zinc oxide conduction band and produce photocurrent was determined by simultaneous attenuated reflection (ATR) UV-vis absorption and incident photon current efficiency (IPCE) spectroscopy measurements. ATR measurements enable identification of the dye species populating the surface, while IPCE spectroscopy pinpoints the species contributing to the photocurrent.
We report studies of the sensitization of the dicarboxylate cyanine dye, R8 (2,2&’ carboxymethylthiadicarbocyanine bromide), adsorbed on a zinc oxide (ZnO) single crystal surface. Sequential sensitization photocurrent measurements over a range of R8 dye concentrations from 10-6 M to 10-5 M on the single crystal ZnO produced a mixture of adsorbed species from monomer to aggregate. Previous work by the Parkinson group where ZnO was sensitized with G15 (2,2&’ carboxymethylthiacarbocyanine bromide) dye revealed strong agreement between ATR UV-vis absorption spectra and IPCE spectrum indicating all absorbing species were equally contributing to the photocurrent.1 On the contrary, for R8 we observe a disproportionately greater contribution to photocurrent from the R8 aggregates compared to the monomeric species as measured by ATR dye absorption spectra.
(1) Rowley, J. G.; Parkinson, B. A. Simultaneous Measurement of Absorbance and Quantum Yields for Photocurrent Generation at Dye-Sensitized Single-Crystal ZnO Electrodes. Langmuir2013, 29, 13790-13796.
12:15 PM - P4.08
Controlling Molecular Orientation: Exploiting the Dipole-Dipole Interaction between Molecules and Ferroelectric Substrates
Alexandra Ramadan 1 Luke Rochford 2 Tim S. Jones 2 Mary P Ryan 1 Sandrine Heutz 1
1Imperial College London London United Kingdom2University of Warwick Coventry United KingdomShow Abstract
The orientation of organic semiconducting molecules within electronic and optoelectronic devices has been shown to have significant effects on device performance . It is known that the molecular orientation of organic films on a surface can be influenced by the complex interplay of the intermolecular (molecule-molecule) interactions and interactions between the molecules and the surface . The use of substrates to independently control molecular orientation and charge injection has previously received little attention.
Ferroelectric materials possess a permanent surface dipole moment and as such can be expected to behave as a type of interacting substrate. The effect of ferroelectric materials on the molecular orientation of organic films has not been previously investigated.
Electrodes modified to have a surface dipole moment have previously been shown to improve device efficiency in OLEDs by decreasing the energy barrier to charge injection . The influence of this surface dipole on device efficiency and the potential for influence over molecular orientation makes the use of ferroelectrics as an interacting substrate very attractive.
In this work the interaction between ferroelectric substrates and organic films has been studied for the first time. Two different molecular systems similar in chemical structure but with different dipolar characteristics, vanadyl phthalocyanine (VOPc) and metal free phthalocyanine (H2Pc), have been investigated. Diffraction and microscopy studies have been conducted to elucidate the effects of the permanent dipole moment of the substrates on the morphology, texture and molecular orientation of the deposited organic layers. Ultra-thin films (1-5ML) of VOPc and H2Pc have been grown on single crystal ferroelectrics and the effect of the surface on the molecular structure has been studied using STM and LEED. To gather a more cohesive picture of our system thicker organic films (up to 50 nm) have also been examined using AFM and XRD. The potential for incorporation of ferroelectric thin films into devices will also be discussed.
 B. Rand et al, Adv. Fun. Mat.22, 2987 (2012)
 A. Cruickshank et al, J. Am. Chem. Soc. 35, 14302 (2012)
 S. Khodabaksh et al, Adv. Fun. Mat. 14, 1205 (2004)
 W. Chen et al, Adv. Fun. Mat. 21, 410 (2011)
12:30 PM - *P4.09
Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Dependence on Molecular Orientation
Michael F Toney 1
1Stanford Synchrotron Radiation Lightsource Menlo Park USAShow Abstract
The nature of the donor/acceptor interface in an organic heterojunction solar cell plays a critical role in determining functional properties,