Song Jin, University of Wisconsin-Madison
Kevin Sivula, Ecole Polytechnique Federale de Lausanne
Gengfeng Zheng, Fudan University
James Stevens, The Dow Chemical Company
Symposium Support Royal Society of Chemistry
Z3: Artificial Photosynthesis
Tuesday PM, December 03, 2013
Hynes, Level 3, Room 304
2:30 AM - *Z3.01
Semiconductor Nanowires for Artificial Photosynthesis
Peidong Yang 1 N. P. Desgupta 1
1UC, Berkeley Berkeley USAShow Abstract
Nanowires, with their unique capability to bridge the nanoscopic and macroscopic worlds, have already been demonstrated as important materials for different energy conversion. One emerging and exciting direction is their application for solar to fuel conversion. The generation of fuels by the direct conversion of solar energy in a fully integrated system is an attractive goal, but no such system has been demonstrated that shows the required efficiency, is sufficiently durable, or can be manufactured at reasonable cost. One of the most critical issues in solar water splitting is the development of suitable photoelectrodes with high efficiency and long-term durability in an aqueous environment. Semiconductor nanowires represent an important class of nanostructure building block for direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. Nanowires can be readily designed and synthesized to deterministically incorporate heterojunctions with improved light absorption, charge separation and vectorial transport. Meanwhile, it is also possible to selectively decorate different oxidation or reduction catalysts onto specific segments of the nanowires to mimic the compartmentalized reactions in natural photosynthesis.
Recently, We have developed a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance.
3:00 AM - Z3.02
Resistive and pH Gradient Losses in Membrane-Buffered Electrolyte Based Water-Splitting Photoelectrochemical Cells
Nella Marie Vargas-Barbosa 1 Emil A. Hernandez-Pagan 1 Thomas E. Mallouk 1 Eugene S. Smotkin 2
1The Pennsylvania State University State College USA2Northeastern University Boston USAShow Abstract
The push for the development of inexpensive, earth-abundant catalysts for photoelectrochemical water-splitting has demonstrated significant advancements. For example, Nocera&’s cobalt-phosphate and nickel borate catalysts have been successfully used for the oxidation of water with moderate overpotentials. However, these catalysts require near-neutral and weakly basic pH electrolytes in order to maintain their performance and usability. In order to successfully collect the generated fuels in photoelectrochemical cells (PECs), the use of electrolytic membranes and porous separators is necessity. In this study, we focus on identifying and measuring the series resistance losses that could arise in aqueous buffered-membrane electrolytes in PECs. Potentiometric and pH measurements were used to study the contribution of solution resistance, membrane resistance and pH gradient formation at 25 mA cm-2. A comparison between different combinations of membranes and buffered electrolytes was done, in which we identified membrane pH gradient formation as the greatest contributor to voltage losses on the PECs.
3:15 AM - Z3.03
An Efficient Approach to Solar Water Splitting Based on CuInxGa1-xSe2 Reaching beyond 8% Overall Solar-to-Hydrogen Efficiency
Jesper Jacobsson 1 Viktor Fjamp;#228;llstroem 1 Marika Edoff 1 Tomas Edvinsson 1
1Uppsala University Uppsala SwedenShow Abstract
CIGS (CuInxGa1-xSe2) is a well-known solar cell material, and should thus be an interesting material for solar water splitting applications. Despite this almost no work has been directed towards utilizing CIGS for renewable hydrogen production.
Here we demonstrate that CISG is a highly interesting material for solar hydrogen applications, with the potential of deliver photocurrents of technological importance. CIGS in itself has a suitable conduction band position for the hydrogen producing half-reaction, and photocurrents of 6 mA/cm2 for the photo reduction are demonstrated. The stability in water under illumination is however a problem as for most other efficient materials. We demonstrate how the problem can be solved by spatially separating the charge carrier generation process from the catalysis step by increasing the distance of charge transport.
Finally we report results from a monolithic CIGS-device with over 8% solar-to-hydrogen efficiency for the total reaction.
3:30 AM - *Z3.04
Inorganic Nanoscale Assemblies for Artificial Photosynthesis
Heinz M Frei 1
1Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
The long term goal of our research is the direct conversion of carbon dioxide and water with visible light to a liquid fuel in a nanoscale assembly made of robust, Earth abundant materials. Focusing on inorganic heterobinuclear light absorbers and metal oxide catalysts, nanoscale assemblies are explored that afford the coupling of light absorbers and catalysts across a nanometer thin proton transmitting silica layer under separation of the water oxidation catalysis from all other photosynthetic processes. Effort ranges from establishing efficient visible light driven water oxidation catalysts and units for carbon dioxide photoreduction, to methods for the efficient charge transport across product separating silica membranes.
A photosynthetic unit consisting of a heterobinuclear ZrOCo(II) site anchored on a silica surface has been shown to reduce CO2 to CO, with the electrons provided by an Ir oxide nanocluster catalyst for water oxidation. Transient optical spectroscopy of such all-inorganic heterobinuclear groups excited to the metal-to-metal charge-transfer states revealed long lifetimes consistent with the photocatalytic activity of these light absorber-catalyst units. Very active Co3O4 nanoparticles for visible light sensitized water oxidation allowed us to detect surface reaction intermediates of oxygen evolution on a metal oxide catalyst in a time resolved manner for the first time. The technique used was rapid-scan FT-IR spectroscopy of an aqueous suspension in the attenuated total reflection mode. The temporal behavior and isotopic composition of intermediates allowed us to establish their kinetic relevancy and mechanistic role. In parallel, we have developed Co3O4(4 nm)/SiO2(2 nm) core/shell constructs with embedded molecular wires (oligo paraphenylenevinylene) for controlled charge transport from the visible light chromophore on the outside across the silica layer to the Co oxide catalyst core on the inside. Efficient hole transfer was demonstrated by transient absorption spectroscopy using spherical core-shell particles. Implementation in the form of Co3O4/SiO2 core shell nanotubes and nanotube arrays for closing of the photosynthetic cycle under product separation is in progress.
 H.S. Soo, A. Agiral, A. Bachmeier, and H. Frei, J. Am. Chem. Soc. 134, 17104 (2012).
 A. Agiral, H.S. Soo, and H. Frei, Chem. Mater. 25, 2264 (2013).
 M. Zhang, M. De Respinis, and H. Frei, submitted.
4:30 AM - *Z3.05
La-Ti Oxysulfides as Photocatalysts and Photoelectrodes
Guijun Ma 1 Aki Iwanaga 1 Jingyuan Liu 1 Yosuke Moriya 1 Takashi Hisatomi 1 Tsutomu Minegishi 1 Masao Katayama 1 Jun Kubota 1 Kazunari Domen 1
1the University of Tokyo Tokyo JapanShow Abstract
Photoelectrochemical (PEC) water splitting is one of the ideal alternatives for hydrogen production from clean and abundant solar energy. La5Ti2CuS5O7 (LTC) was reported as a visible light responded oxysulfide semiconductor whose wavelength of absorption edge was at 650 nm equal with a band gap value of 1.9 eV [1-3]. Furthermore, the abundance of Ti, Cu and La element in the earth&’s crustal rocks are 6320, 68 and 35 ppm respectively, which is much higher than the widely studied PEC materials such as Ga (19 ppm) and In (0.24 ppm) . We had reported that LTC exhibit photocatalytic activity for both water reduction and oxidation under visible light irradiation in the presence of sacrificial reagents [2,3]. These results indicate that LTC has the potential for overall water splitting to utilize a large portion of sunlight (lambda;< 650 nm) as a photocatalyst. Then, the LTC thin-film photoelectrode was fabricated by a novel particle transfer (PT) technology . The PEC water splitting reaction was carried in a commonly used three-electrode system under visible light irradiation. The current-voltage curve for LTC thin film electrode under intermittent visible-light irradiation showed a clear photocathodic current, which implied a p-type semiconductor character. The correlation between the amount of H2 evolution and photocatodic current showed a nearly 100% faradic efficiency on the LTC photoelectrode. Then, the zero-bias PEC water splitting was carried out by combining the LTC photocathode with other photoanode prepared by the PT method, which finally accomplished stoichiometric H2 and O2 production from water under visible light irradiation.
1. V. Meignen, L. Cario, A. Lafond, Y. Moelo, C. Guillot-Deudon, and A. Meerschaut, J. Solid State Chem., 177 (2004) 2810.
2. M. Katayama, D. Yokoyama, Y. Maeda, Y. Ozaki, M. Tabata, Y. Matsumoto, A. Ishikawa, J. Kubota, K. Domen, Mater. Sci. Eng. B, 173 (2010) 275.
3. T. Suzuki, T. Hisatomi, K. Teramura, Y. Shimodaira, H. Kobayashi and Kazunari Domen, Phys. Chem. Chem. Phys., 14 (2012) 15475.
4. N. N. Greenwood and A. earnshaw, Chemistry of the Elements, 2nd Edition, 1997.
5. T. Minegishi, N. Nishimura, J.kubota and Kazunari Domen, Chem. Sci., 4 (2013) 1120.
5:00 AM - Z3.06
Computational Screening of Materials for Water Splitting Applications
Ivano Eligio Castelli 1 Kristian Sommer Thygesen 1 Karsten Wedel Jacobsen 1
1Technical University of Denmark Kgs. Lyngby DenmarkShow Abstract
The development of sustainable energy forms is one of the most important problems of our time because of the ever increasing energy consumption together with the CO2 related climate problems. The conversion of solar light into electrons and holes which are used to split water into hydrogen and oxygen is one of the possible ways to address the world's pressing energy supply and storage problem. The properties determining the usefulness of a material to be used as light harvester in a photoelectrochemical cell include (i) a band gap in the visible range with band edges well positioned with respect to the redox levels of water, (ii) good mobility allowing electrons and holes to reach the surface before recombining, (iii) chemical/structural stability under irradiation, and (iv) low cost and nontoxicity. In previous works, we performed a computational screening for new materials with focus on one- and two-photon water splitting [1,2]. We found 20 and 12 promising materials for visible light harvesting in the one- and two-photon schemes in the space of 20000 cubic perovskites. Now, we move from the perovskite crystal structure and extend the screening to a more general collection of materials that are all known to exist in nature (as present in the Materials Project database ). The descriptors are the heat of formation, the bandgap, and the band edge positions. The heat of formation is calculated with respect to solid and dissolved phases using Pourbaix
diagrams . The bandgaps are evaluated using the GLLB-SC functional recently implemented in the GPAW electronic structure code. The band edges are calculated using an empirical formula based on the electronegativities of the constituent atoms. Based on the screening, we suggest a handful of materials for the one- and two-photon water splitting devices for further experimental investigation.
 I.E. Castelli, T. Olsen, S. Datta, D.D. Landis, S. Dahl, K.S. Thygesen, and K.W. Jacobsen, Energy Environ. Sci., 5, 5814 (2012).
 I.E. Castelli, D.D. Landis, K.S. Thygesen, S. Dahl, I. Chorkendorff, T.F. Jaramillo, and K.W. Jacobsen, Energy Environ. Sci., 5, 9034 (2012).
 I.E. Castelli, K.S. Thygesen, and K.W. Jacobsen, Accepted in Topics in Catalysis (2013).
5:15 AM - Z3.07
Graphene Catalyst on Silicon Photocathode for Hydrogen Production
Uk Sim 1 Tae-Youl Yang 1 Joonhee Moon 2 Junghyun An 1 Jinyeon Hwang 1 Byung Hee Hong 2 Ki Tae Nam 1
1Seoul National University Seoul Republic of Korea2Seoul National University Seoul Republic of KoreaShow Abstract
Carbon-based catalysts have been attracting attention in renewable energy technologies due to the low cost and high stability, but their insufficient activity is still a challenging issue. Here, we demonstrate that graphene (Gr) can catalyze the hydrogen evolution reaction (HER); thus, Gr can enhance the performance of silicon (Si) photocathodes through a significant decrease in the overpotential.
To evaluate the photocathodic behavior of Gr loaded on a p-type Si (Gr-Si) electrode, a current density was measured as the potential was swept from 0.4 V to -1.0 V vs. Reversible Hydrogen Electrode (RHE). The potential of Gr-Si at -1 mA/cm2 (the onset potential, VOS) is 0.01 V vs. RHE, and this VOS is a positive shift by 0.18 V compared to that of bare Si (-0.17 V vs. RHE). Dark current density was also measured using heavily arsenic doped n+ type Si electrode. In the dark condition, the positive shift in 0.14 V of VOS (-0.49 V vs. RHE for Gr-Si) also shows higher activity for HER compared to that of the bare Si (-0.63 V vs. RHE), which results in the photovoltage of 0.50 V by depositing Gr on bare Si. This result indicates that Gr acts as an effective catalyst for HERs on the Si photocathode.
To investigate the electrocatalytic activity of Gr, cyclic voltammetry was measured with a rotating disk electrode using Gr-loaded glassy carbon (Gr-GC) electrode. VOS for Gr-GC was -0.18 V vs. RHE; this VOS is shifted positive by 60 mV compared to that for the bare GC. This result means that the Gr has electrocatalytic activity for HER. To quantitatively gain more insight into the catalytic activity of Gr, the potential-current density curves were converted into a plot of the potential as a function of the logarithm of current density; this plot is called a Tafel plot. If the electrochemical desorption step (Hads + H3O+ + e- = H2 + H2O, the Heyrovsky reaction) is the rate-determining step for HER, a Tafel slope of 40 ~ 118 mV/decade is measured and is dependent of the value of the adsorbed hydrogen coverage (theta;H = 0 ~ 1). The observed Tafel slope of 74 mV/decade in the current work indicates that the kinetics of the HER on Gr-GC electrodes is determined by the Heyrovsky reaction because theta;H has an intermediate value (0 ~ 1). Moreover, the Gr-GC electrode also showed an enhanced exchange current density (J0) of 2.73e-6 A/cm2, which is higher than the J0 value for bare GC (1.63e-6 A/cm2). The higher J0 indicates that electron transfer or the adsorption/desorption of protons at the electrode/electrolyte can occur more easily with a lower kinetic barrier. From the Tafel analysis, the HER catalytic activity of the Gr catalyst is identified from the increase in J0.
In summary, we have presented Gr catalyst that enhanced the photoelectrochemical performance of the Si-photocathode. Our approach in this study exploits a strategy to develop metal-free carbon-based catalysts with high efficiency for solar-driven hydrogen fuel production.
5:30 AM - Z3.08
Highly Efficient Visible Light Photocatalytic Hydrogen Evolution over Graphitic Carbon Nitride Coupled with NiS2 as a Noble Metal Free Co-Catalyst
Lisha Yin 1 Yupeng Yuan 1 Can Xue 1
1Nanyang Technological University Singapore SingaporeShow Abstract
Recently, transition metal sulfides, such as MoS2 and NiS, have attracted considerable attentions as noble-metal-free co-catalysts for efficient photocatalytic H2 production. Herein, we developed a transition metal sulfide, NiS2, as a new noble-metal-free co-catalyst for photocatalytic H2 production. NiS2 co-catalyst was deposited onto the surface of graphitic carbon nitride (g-C3N4) via hydrothermal method. Results show that NiS2 co-catalyst exhibits an excellent H2 evolution performance which is even better than that of the commonly used noble metal cocatalyst, Pt. The 2wt% NiS2 deposited g-C3N4 photocatalyst displays a highest photocatalytic H2 production rate of 406.3 umol/h-1/g-1 from triethanolamine (TEOA) aqueous solution under visible light irradiation., which is 3 times higher than that of 1 wt% Pt loaded g-C3N4. Further studies reveal that the enhancement in photocatalytic H2 production by NiS2 deposition is due to efficient suppression of photoexcited electrons and holes recombination. This research also demonstrates the great potential of transition metal sulfides as noble-metal free co-catalysts for photocatalytic H2 production and provides a new insight on developing active metal sulfide co-catalysts.
5:45 AM - Z3.09
Enhanced Schottky Barrier Height by a Thin ALD- Al2O3 Interlayer Improves a Photoelectrochemical Performance of p-Si/Electrolyte
Min-Joon Park 1 Jin-Young Jung 1 Jae-Won Song 1 Yoon-Ho Nam 1 Sun-Mi Shin 1 Jung-Ho Lee 1
1Hanyang University Ansan Republic of KoreaShow Abstract
Silicon is cheap, earth abundant material which is suitable for mass producible, solar energy conversion for water splitting. Although a p-type Si (p-Si) photocathode shows a negative potential appropriate for the hydrogen evolution reaction (HER) at the conduction band-edge, a large amount of applied overpotential is required to drive water splitting because the position of a conduction band is not negative enough to result in a kinetically reasonable HER. Adding metal-catalyzed nanoparticles,  metal-oxide thin film,  or an interface passivation layer  has recently been reported to overcome the kinetic limitation of Si photocathodes.
Here, we focus on how the schottky barrier height (SBH) has been increased by inserting an atomic-layer-deposited Al2O3 interlayer (ALD-Al2O3) into an interface between electrolyte and a Si substrate. This attempt provides a solution to mitigate the requirement of overpotential without additional optical losses. 5~20cycles of ALD-Al2O3 were applied on a lightly boron-doped (1~10Omega;cm) p-Si wafer. HER currents were measured in 0.5M sulfuric acid used for electrolyte. A significant amount of positive fixed charges observed in ALD-Al2O3 films electrically induced electrons to segregate at the Si surface such that an accumulation layer has been formed at the Si band-edge to enhance the SBH. As a result, a charge transfer resistance (Rct) of the photocathode was greatly reduced, and the overpotential of ~150mV was also decreased at a current density of 20mA/cm2. The remarkable increase in photocurrent was observed by a factor of 5.84 (from 1.80 to 10.52mA/cm2) at -0.4 V versus reversible hydrogen electrode (RHE). Tafel-plots and Mott-Schottky analyses reveal that a significant improvement in charge carrier transfer with a SBH enhancement is attributed to a thin interlayer of ALD-Al2O3 formed at the Si/electrolyte interface.
 I. Oh, J. Kye, and S. Hwang, Nano. Lett. 12(1), 292 (2012)
 K. Sen, N. Park, Z. Sun, J. Zhou, J. Wang, X. Pang, S. Shen, S. Y. Noh, Y. Jing, S. Jin, P. K. L. Yu, and D. Wang, Energy Environ. Sci. 5, 7872 (2012)
 F. L. Formal, N. Tétreault, M. Corniz, T. Moehl, M. Grätzel, and K. Sivula, Chem. Sci. 2, 737 (2011)
Z1: New Materials I
Tuesday AM, December 03, 2013
Hynes, Level 3, Room 304
9:15 AM - *Z1.01
Towards Commercialization of Earth Abundant Photovoltaic (PV) Materials
Michael E Mills 1 Jim Stevens 1 Rebekah Feist 1
1Dow Chemical Midland USAShow Abstract
Earth abundant photovoltaic materials attracted significant attention about 10 years ago as the world focused on alternate energy generation to supplement and eventually replace fossil fuels as sources. Future projections at that time, of substantial long term market penetration by solar to achieve its full potential, drove the realization that the envisioned Terawatts capacity needed to satisfy the world demand could only be addressed by abundant photovoltaic materials for the anticipated capacity. At the beginning of this earth abundant PV development activity, the benchmark incumbent technology consisted of primarily both multi-crystalline and single crystal silicon, providing efficiencies of 11 - 18%, panel prices of $3 to $5/watt and global manufacturing capacity of single digit GW. Estimations of efficiency, cost and capacity projected 10 years forward to current (2013) at that time for both types of silicon seemed to be achievable by earth abundant PV solutions. Significant technological progress has been made since then by both the incumbent technology, silicon, as well as several earth abundant PV solutions. The economic viability of commercializing an earth abundant PV technological solution includes not only the requirement for abundance but the commercial solar solution also must integrate the overall material cost of the active device structure and supporting materials, the commercial scale manufacturing costs of the PV solution, the product requirements and system performance value to the customer relative to the incumbent offering. We will share our insights as to whether the earth abundant PV community is now closer to a viable commercial offering then 10 years ago.
9:45 AM - *Z1.02
Interfaces Rule: Assessment of Heterostructures in New Oxide, Phosphide and Nitride Materials for Photovoltaics and Solar Fuels
Harry Atwater 1
1California Institute of Technology Pasadena USAShow Abstract
Terawatt scale deployment of solar energy will demands the exploration of new high performance earth abundant optoelectronic materials options for photovoltaics and solar fuels. That implies a need for materials that are both abundant and sustainably harvestable from the earth&’s crust, and which also can be designed as high efficiency heterostructures. In this context, we outline the critical role of heterostructure interfaces in optoelectronic performance with case examples in cuprous oxide, zinc phosphide photovoltaic, and Zn-IV nitride semiconductors. We examine the dramatic dependence of open circuit voltage on interface stoichiometry at zinc oxide/cuprous oxide interfaces, and the effect of heterostructure design on current transport interfaces between zinc phosphide and ZnS, ZnSe, ZnO and CdS. Design of electrochemically stable photoanodes and photocathodes for solar fuels applications poses an additional constraint of anode-electolyte robustness against corrosion under widely varying pH conditions. We compare solar fuel photoanode designs based on stable oxides such as bismuth vanadium oxide with approaches based on interface passivation and protection of less-stable III-V compound semiconductor interfaces for solar fuels devices.
10:15 AM - Z1.03
Optoelectronic Properties and Defect Physics in ZnSnxGe1-xN2 Semiconductors
Prineha Narang 1 Aashrita Mangu 1 Shiyou Chen 2 Naomi Coronel 1 Sheraz Gul 4 Junko Yano 4 Lin-Wang Wang 3 Nathan S. Lewis 2 Harry Atwater 1
1California Institute of Technology (Caltech) Pasadena USA2California Institute of Technology (Caltech) Pasadena USA3Lawrence Berkeley National Laboratory Berkeley USA4Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
The II-IV-N2 compounds are closely related to the wurtzite-structure III-N semiconductors, but have a mixed A-site composition. The choice of different group II and group IV elements provides chemical diversity that can be exploited to tune the structural and electronic properties of the II-IV- N2 compounds. Specifically, ZnSnxGe1-xN2 alloys with optical band gaps ranging from 2-3.1eV can be tuned to span a large portion of the solar spectrum, and can therefore be a viable earth-abundant light absorber and replacement for InGaN in nitride optoelectronic devices. They exhibit local order as demonstrated via X-ray absorption fine structure spectroscopy (EXAFS) and a linear relationship between the (002) peak position and composition in X-ray diffraction studies, indicating continuous access to the entire range of band gap values without phase separation or the need for more complicated growth strategies. The bowing parameter is 0.29 eV for the measured band gaps of ZnSnxGe1-xN2, and 0.67 eV for the calculated band gaps. Although they are different, both values are significantly smaller than that of In1-xGaxN, indicating that the ZnSnxGe1-xN2 alloy band gaps can be tuned almost linearly by controlling the Sn/Ge composition.
In this presentation we will describe theoretical studies of the optoelectronic behavior and defect physics of the ZnSnN2 alloy series, as well as experimental investigations via X-ray absorption and emission spectroscopy of the alloys that probe the conduction and valence band partial density of states, which are in excellent agreement with first principles theory. Resonant inelastic scattering observations to provide understanding of the role of Ge when incorporated into the ZnSnN2 lattice will be presented along with carrier dynamics of ZnSnxGe1-xN2 alloys as elucidated from photoluminescence (room and low temperature) and pump-probe spectroscopy.
10:30 AM - Z1.04
Band Alignment and Device Properties of II-VI/Zn3P2 Heterojunction Photovoltaics
Jeffrey Paul Bosco 1 Steven Rozeveld 2 Seokmin Jeon 1 David Scanlon 3 Harry Atwater 1 Samantha Wilson 1
1Caltech Pasadena USA2The DOW Chemical Company Midland USA3University College London London United KingdomShow Abstract
Zinc phosphide (Zn3P2) is a promising candidate for scalable photovoltaics, with a reported direct band gap of 1.5 eV and a long minority-carrier diffusion length (>5µm). However understanding the interface electronic properties of Zn3P2 is important to the performance of heterojunction devices incorporating Zn3P2 as a thin-film absorber. In this work, we have determined the energy-band alignment of epitaxial heterojunctions between Zn3P2 and n-type II-VI materials: ZnS, ZnSe, ZnO, and CdS as well as III-V materials: GaAs and GaP. The valence-band discontinuities were determined using high-resolution X-ray photoelectron spectroscopy measurements via the Kraut method. Amongst the band alignments measured, the ZnSe/Zn3P2 heterojunction demonstrated a large valence-band offset of -1.21 ± 0.11 eV and a negligible conduction-band offset of -0.03 ± 0.11 eV, indicating a nearly ideal alignment for a photovoltaic device. High-resolution transmission electron micrographs of the ZnSe/Zn3P2 interface showed that the morphological properties of the ZnSe epilayer were dominated by a thin (~1.5 nm) amorphous layer at the crystalline Zn3P2 surface. Various growth conditions were investigated in an attempt to remove this amorphous interfacial layer and improve the ZnSe crystallinity and the extent to which the films could be doped. Finally, the device properties of the ZnSe/Zn3P2 heterojunctions, including substrate and superstrate configurations grown epitaxially on GaAs, were characterized using current-voltage measurements performed under dark and simulated Air Mass (AM) 1.5, 1-Sun illumination. These results represent significant progress towards the realization of efficient, earth abundant Zn3P2 solar cells.
10:45 AM - Z1.05
Characterization of Defects in Photovoltaic Materials by Three-Dimensional Atom Probe Tomography: Transition Metal Impurities
Austin Akey 1 Amanda Youssef 1 Daniel Recht 2 Jim Williams 3 Michael J. Aziz 2 Tonio Buonassisi 1
1Massachusetts Institute of Technology Cambridge USA2Harvard School of Engineering and Applied Sciences Cambridge USA3Australian National University Canberra AustraliaShow Abstract
Semiconductors intended for use in photovoltaics (PV) are often limited by the unintentional incorporation of chemical impurities. Transition metals have long been viewed as one of the most problematic contaminants in PV absorbers; much effort has been invested into developing techniques to avoid or remove contamination that adversely affects device performance. One of the most important unanswered questions in this field involves the actual spatial distribution of contaminant atoms in processed material, which requires compositional characterization on the sub-100 nm scale. Recent advances have enabled Atom Probe Tomography to measure the atomic-scale spatial distribution of impurities in semiconductors with parts-per-million chemical sensitivity and sub-nanometer resolution, representing an entirely new, and very powerful, form of metrology. Atom Probe is not limited to crystalline samples, and has been shown to yield complete, detailed three-dimensional atomic reconstructions of compound semiconductors as well as oxides. Using silicon as a well-understood model system, we examine by Atom Probe Tomography the incorporation and segregation of transition metal impurities. Challenges involving the evaporation, ionization, and detection of these impurities are addressed, and prospects for rapidly localizing and identifying contaminants in other, novel Earth-abundant materials are discussed.
Tuesday AM, December 03, 2013
Hynes, Level 3, Room 304
11:15 AM - Z2.01
Iron Sulfide Ink and Their Conversion to Pyrite Thin-Films for Use in Solar Devices
Alec Kirkeminde 1 Maogang Gong 1 Shenqiang Ren 1
1University of Kansas Lawrence USAShow Abstract
Pyrite (FeS2) is an earth abundant semiconductor that has experienced increased attention in the past few years. Much time has been invested on studying how to creating thin films of pyrite material for optoelectric devices utilizing spincoating/dipcoating solution-based nanocrystals and also chemical vapor deposition growth. Here we present a novel Iron Sulfide (FeS) nanowire ink precursor which can be used to create very uniform films, which can then be sulfurized to convert to the final pyrite material. Solar devices will be presented utilizing the pyrite thin-film which features a unique ZnO nanowire electron blocking layer which is shown to help improve device performance. This study opens up new, easily scalable strategies for pyrite thin film creation coupled with distinctive device fabrication.
11:30 AM - Z2.02
Doping and Transport Mechanisms in Ex Situ Sulfidized FeS2 Thin Films
Xin Zhang 1 Mike Manno 1 Melissa Johnson 1 Yuqi Yan 1 Tyler Socha 1 Eray S Aydil 1 Chris Leighton 1
1University of Minnesota, Twin Cities Minneapolis USAShow Abstract
Pyrite FeS2 is undergoing a tremendous resurgence of interest as a candidate thin-film solar absorber based on abundant, low-cost, and non-toxic elements. However, FeS2-based solar cells have suffered from low open circuit voltages (~0.1 V), and low efficiency, although the origins are not clear. Conduction mechanisms and doping are similarly poorly understood in FeS2, a simple example being the commonly observed p-doping in thin films, in contrast with the n-doping in typical bulk crystals. Understanding these issues could contribute significantly to improvements in FeS2-based solar cells. It is in this context that we have performed a comprehensive study of conduction mechanisms in FeS2 films synthesized via ex situ sulfidizing Al2O3(0001)/Fe films at temperatures in the range 100 C le; TS le; 700 C. In our initial work, we detected a crossover in transport mechanism around 450 C, from intergranular hopping to conventional charge transport . Detailed analysis identified the origin as residual nanoscopic Fe clusters embedded in an FeS2 matrix, eventually extinguished at high TS. Significantly, the crossover was also accompanied by a three order of magnitude increase in Hall coefficient and a reversal of its sign. The apparent p-type conduction in the hopping regime was argued to be an artifact of hopping , challenging the viewpoint of predominantly p-type behavior in FeS2 films.
More recently, we have performed a detailed study of the response of these high TS transport properties to vacuum and S annealing, to probe possible Fe:S stoichiometry effects. As a function of vacuum annealing (up to 550 C) we observe a rapid crossover from close to diffusive to intergranular hopping transport. Structural characterization identifies formation of pyrrhotite (a FeS1±x phase), which we confirm to be highly conductive, again implying hopping due to nanoscopic secondary phase formation. Complementary annealing experiments in S were plagued by porosity problems. As a whole the annealing results show little promise for precise manipulation of the Fe:S ratio to control doping, at least when starting from these relatively heavily unintentionally doped (1020-1021 cm-3) low mobility (0.1-0.01 cm2/Vs) films. To address this high doping/low mobility, and to determine whether the n-type behavior we observe is general, we also synthesized FeS2 on multiple substrates. This is important given the possibility of potential dopant out-diffusion from the substrate. A variety of substrates were employed, including glasses, and other single crystal and amorphous substrates. Hall measurements revealed n-type behavior in all cases, but interestingly, with quite different electron density and mobility. Trends with respect to chemical constituents in the substrate will be discussed in detail, and have important implications for dopants in FeS2 films.
Work supported by UMN-IREE grant (RL-0004-11).
 Zhang, Manno, Baruth, Johnson, Aydil and Leighton, ACS Nano 7, 2781 (2013).
11:45 AM - *Z2.03
A Hole Inversion Layer at the Surface of Iron Pyrite
Matt Law 1 2
1University of California, Irvine Irvine USA2University of California, Irvine Irvine USAShow Abstract
Numerical modeling of Hall effect data is used to demonstrate the existence of a strong hole inversion layer at the surface of high-quality n-type single crystals of iron pyrite (cubic FeS2) grown by a flux method. The presence of the hole inversion layer is corroborated by photoemission spectroscopy and electrochemical impedance measurements. The inversion layer can explain both the low photovoltage of pyrite solar cells and the universal heavy p-type conductivity of polycrystalline pyrite thin films that have together perplexed researchers for the past thirty years. We find that the thickness and hole concentration of the inversion layer can be modified by mechanical and chemical treatments of the pyrite surface, suggesting that it may be possible to eliminate this hole-rich layer by passivating surface and near-surface defects. Furthermore, an analysis of high-temperature electrical conductivity and optical transmission data firmly establishes that the electronic and optical band gap is in fact ~0.90 eV at room temperature, confirming that photovoltages in excess of 500 mV should be attainable from pyrite under solar illumination.
12:15 PM - Z2.04
Investigation of Surface Inversion and Improvement of Pyrite Single Crystals and Nanostructures for Solar Energy Conversion Application
Miguel Caban-Acevedo 1 Dong Liang 1 Nicholas S. Kaiser 1 Song Jin 1
1University of Wisconsin-Madison Madsion USAShow Abstract
Iron pyrite (FeS2), is an earth-abundant semiconductor that has generated recent interest due to its promising properties for solar energy conversion (band gap of 0.95 eV and high absorption coefficient α ~ 105 cm-1). Despite intensive efforts in synthesizing pyrite thin films there has not been any report of solar conversion efficiency. The only reported conversion efficiency has been for photoelectrochemical cells of pyrite single crystals which displays low photovoltage and low efficiency (< 3%). We hypothesize that the lack of photovoltage and the low performance in pyrite materials originates from intrinsic bulk and surface defects. In order to demonstrate our hypothesis, we report electrochemical and photoelectrochemical studies on pyrite single crystals grown by CVT, in contrast with device transport studies on single crystal pyrite nanorods and nanoribbons synthesized via sulfidation. Herein, we report the J-V characteristics, electrochemical impedance spectroscopy, and Mott-Schottky analysis of pyrite single crystals and the gating of field-effect transistors based on pyrite nanostructures, and discuss how the observed behaviors are a result of bulk defects and a Fermi level pinning which causes a surface inversion. We provide a comprehensive explanation into how a degenerately doped p-type surface inversion layer of pyrite that originates from a high density of intrinsic surface states is the main reason for the low solar performance in pyrite single crystals and the observed heavy p-type like transport in pyrite nanostructures. We will further discuss our efforts in passivating the surface defects in both pyrite single crystals and nanostructures.
12:30 PM - Z2.05
Gated Hall Effect of Pyrite Nanoplates with Surface Inversion Layer
Dong Liang 1 Miguel Caban-Acevedo 1 John P DeGrave 1 Song Jin 1
1University of Wisconsin-Madison Madison USAShow Abstract
Iron pyrite (cubic β-FeS2), an earth abundant and nontoxic semiconductor, has attracted resurgent attention as a promising candidate for solar energy conversion thanks to its suitable band gap (0.95 eV indirect, 1.03 eV direct), high absorption coefficient (~6×10^5 cm^-1), excellent resistance to photocorrosion for photoelectrochemical applications. However, the application of pyrite for solar cells has been hindered by its low open circuit voltage (< 200 mV) and thus low efficiency (~3%), which is likely the results of phase impurities, and rich bulk and surface defects. Therefore, it is pivotal to understand and reveal the mystery of pyrite electrical transport mechanism arising from bulk and surface defects. Single crystalline phase-pure pyrite nanoplates offer a versatile platform to study their physical properties. Here, we employ electrolyte field-effect gating and Hall effect measurements of pyrite nanoplates to investigate and confirm the n-type bulk pyrite with heavily p-doped surface inversion layer. Furthermore, by combining the multilayer Hall effect model and Poisson equation, we obtain key physical parameters including the bulk electron and surface hole carrier concentrations, mobilities, the thicknesses of surface inversion layer and depletion layer. This fundamental study confirms that the surface Fermi level pinning is one of the main obstacles that prevent pyrite to become a high performance solar material and will allow us to improve the solar performance of pyrite nanostructures and thin films.
12:45 PM - Z2.06
In situ X-Ray Studies of Surface and Electronic Structures in Pyrite Thin Films
Yu Liu 1 2 Nicholas Berry 1 Yanning N. Zhang 1 Cheng-Chien Chen 3 Hendrik Bluhm 4 Zhi Liu 4 Ruqian Wu 1 Matt Law 1 5 John C. Hemminger 1 5
1UC Irvine Irvine USA2UC Irvine Irvine USA3Argonne National Laboratory Lemont USA4Lawrence Berkeley National Laboratory Berkeley USA5UC Irvine Irvine USAShow Abstract
Iron pyrite (cubic FeS2) with its exceptional optical absorption and suitable band gap is a promising candidate for earth-abundant thin-film solar cells. Using ambient pressure synchrotron x-ray spectroscopies, we report the electronic properties and phase transformation of pyrite thin films under in situ heating in ultrahigh-vacuum environment. The low-temperature Fe and S L-edge absorption spectra indicate an increasing density of states (DOS) above the valence band, and the nondestructive photoemission depth profiles also suggest an increasing DOS below the conduction band. Together they reveal a band gap narrowing related to surface states of defects and sulfur vacancies created by heating. Above 430 °C, x-ray photoelectron spectroscopy and in situ x-ray diffraction study indicate a phase transformation from pyrite to pyrrhotite, where a significant change of the S/Fe ratio from 2:1 to 1:1 occurs. The results provide crucial information on surface states and phase transition of pyrite based thin-film solar cells.
Song Jin, University of Wisconsin-Madison
Kevin Sivula, Ecole Polytechnique Federale de Lausanne
Gengfeng Zheng, Fudan University
James Stevens, The Dow Chemical Company
Symposium Support Royal Society of Chemistry
Z5/W11: Joint Session: CZTS II
Wednesday PM, December 04, 2013
Hynes, Level 3, Room 304
2:30 AM - Z5.01/W11.01
Highly Efficient CZTSSe Thin Film Solar Cells Prepared via Electrodeposition
Jong Ok Jun 1 Kee Doo Lee 1 Lee Seul Oh 1 Jin Young Kim 1
1Korea Institute of Science and Technology (KIST) Seoul Republic of KoreaShow Abstract
Kesterite Cu2ZnSn(S,Se)4 (CZTS) thin films are attracting a lot of interest as an alternative system to Cu(In,Ga)Se2 (CIGS) thin films, owing to their majority carrier type (p-type), proper band gap energy (1.0-1.5 eV), and high optical absorption coefficient (> 10^4 cm-1). More promisingly, the CZTSSe is composed of earth-abundant (cf. In in CIGS), environmentally-friendly (cf. Cd in CdTe), and relatively cheap elements. Here, we fabricated metallic Cu-Zn-Sn (CZT) precursor thin films via electrochemical deposition from aqueous metal salt solution on Mo-coated soda-lime glass substrates, and the influence of the subsequent sulfurization/selenization condition on the structural, electrical, and photovoltaic properties of the CZTSSe thin films was investigated. The as-deposited films are composed of binary metallic alloys, which can be converted to the highly crystalline CZTS phase after sulfurization at temperatures above 500 oC. The composition of the CZT film barely changes during the sulfurization, and small amount of CuS-based secondary phases exists even at 550 oC. However, a quick post-annealing KCN treatment effectively and selectively removes the secondary phase, evidenced by the Raman spectroscopy. The formation of the CuS-based secondary phase can be suppressed by slowing down the hearting rate during the sulfurization process, leading to an increased conversion efficiency of ~ 4%. The selenization process has been found to accelerate the crystallization process to CZTSe and the grain growth compared to the sulfurization process, and thus, to enhance the photovoltaic properties, exhibiting a high conversion efficiency of ~ 8%.
2:45 AM - *Z5.02/W11.02
Kesterite Solar Cells from Molecular-Inks and Nanocrystal-Inks: Mapping the Effects of Composition to Material Quality and Device Performance
Hugh W. Hillhouse 1
1University of Washington Seattle USAShow Abstract
Given the terawatt-scale of future energy needs, the most promising future photovoltaic materials should be Earth abundant with their primary mineral resources distributed across several geographic regions and their supply chains robust to reduce concerns of price volatility. In addition, the process of forming the solar cell should be scalable, low-cost, and not utilize dangerous or toxic materials. The strongest initial candidate appears to be kesterite structures of Cu2ZnSn(S,Se)4 (CZTSSe) and similar alloy materials.
Conventionally, thin film chalcopyrite and kesterite solar cells have been synthesized by evaporating or sputtering metals followed by sulfurization or selenization. More recently, two potentially low-cost high-throughput approaches have been demonstrated that form the quaternary or pentenary chalcogenide directly from solution-phase processes. One is based on first synthesizing multinary sulfide nanocrystals and then sintering them to form a dense layer. The other approach utilizes molecular precursors dissolved in hydrazine. Both approaches reach their highest device efficiencies by incorporating Se to form Cu2ZnSn(Sx,Se1-x)4 devices, and each has yielded higher efficiency devices than the best vacuum deposited absorbers. The hydrazine route has yielded the most efficient CZTS-based devices thus far.
The presentation will focus on our development of the nanocrystal-ink based routes to materials and devices and a new molecular-ink route that utilizes benign solvents (avoiding the use of hydrazine). For both systems we will show the results of composition spread experiments coupled with spatially resolved photoluminescence and Raman scattering to reveal the effects of native point defects and doping on material quality and device performance. Finally, the current state-of-the art and performance limitations for the material will be review.
3:15 AM - Z5.03/W11.03
Photoluminescence Study and Observation of Unusual Optical Transitions in Cu2ZnSnSe4/CdS/ZnO Solar Cells
Souhaib Oueslati 1 2 4 Marc A. Meuris 2 3 Jef Poortmans 6 8 Marie Buffiere 6 8 Guy Brammertz 2 3 Oualid Touyar 5 Christine Koble 7
1KACST-Intel Consortium Center of Excellence in Nano-manufacturing Applications (CENA) Riyadh Saudi Arabia2imec Division IMOMEC - Partner in Solliance Leuven Belgium3Institute for Material Research (IMO) Hasselt University Leuven Belgium4Faculty of Sciences of Tunis, El Maner Tunis Tunisia5National Institute of Applied Sciences and Technology, INSAT Tunis Tunisia6Catholic University of Leuven Leuven Belgium7Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH Berlin Germany8imec Leuven BelgiumShow Abstract
We examine photoluminescence spectra (PL) of Cu2ZnSnSe4/CdS/ZnO solar cells via temperature-dependent and illumination power-dependent measurements. Our cells are fabricated by H2Se selenization of sputtered Cu, Zn, Sn multilayers and show a total area efficiency of 9.2 % with an open circuit voltage of 416 mV and a short circuit current density of 38 mA/cm2. PL measurements offer an opportunity to study the defect states in the absorber layer.
The experimental results lead us to propose a recombination model for our cells that is able to explain both temperature dependent PL as well as power dependent PL results. At low temperatures and moderate excitation power, the quasi-donor-acceptor recombination (Q-DAP) between electrons localized at distorted donor states and holes at distorted acceptor states dominates. At higher temperatures and/or higher excitation power, the recombination involves electrons in the conduction band and distorted acceptor levels and the transition changes from Q-DAP to a quasi-free to bound transition (Q-FB). We can link the low Voc values generally observed in CZTSe solar cells to the presence of strong potential fluctuations in the absorber layer, as these potential fluctuations will give rise to tunneling enhanced recombination, thereby increasing the recombination currents as compared to the case without potential fluctuations.
3:30 AM - Z5.04/W11.04
Effects of Alkali Metal Impurities on the Microstructure and Electronic Properties of Cu2ZnSnS4 Thin Films
Melissa Johnson 1 Sergey V. Baryshev 2 Elijah Thimsen 1 Michael Manno 1 Xin Zhang 1 Chris Leighton 1 Eray S. Aydil 1
1University of Minnesota Minneapolis USA2Argonne National Laboratory Argonne USAShow Abstract
Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) solar cells with the highest power conversion efficiencies are fabricated on Mo-coated soda lime glass (SLG), a carryover from Cu(InxGa1-x)Se2 (CIGS) solar cells. In CIGS solar cells, Na diffusion from the SLG into the CIGS film has been shown to enhance the power conversion efficiency. Na diffusion is also expected when CZTS is deposited on Mo-coated SLG. In fact, SLG hosts a variety of other impurities such as K, Ca, Mg, and Al that may also diffuse into CZTS. However, a systematic investigation of whether these impurities diffuse into CZTS and how they affect the film properties has not yet been conducted. To this end, we have investigated the effects of the substrate and the intentional addition of individual impurities on the microstructure and electronic properties of CZTS films. Thin CZTS films were synthesized via ex situ sulfidation of Cu-Zn-Sn films co-sputtered on a variety of substrates, including, crystalline quartz, amorphous quartz, sapphire, SLG and Pyrex. These Cu-Zn-Sn precursor films were then loaded into quartz ampoules with 1 mg of S, evacuated to 10-6 Torr, sealed and sulfidized at 600 oC for 8 hours. The sulfidized films were then characterized using a suite of techniques including X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Concentration depth profiles were examined using time-of-flight secondary ion mass spectrometry (TOF-SIMS). CZTS films synthesized on SLG were found to have significantly larger grains than films grown on any of the other substrates. Furthermore, we found that by simply including a bare additional piece of SLG in the sulfidation vessel, the grain size of films grown on impurity-free quartz increases from 100's of nm to greater than 1 mu;m. This conclusively demonstrates that the impurity atoms found in SLG are volatilized in a sulfur atmosphere and transported via the vapor phase to neighboring films. TOF-SIMS experiments implicated Na, K and Ca as the impurities responsible for this enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of either Na, K, or Ca into the sulfidation ampoule. By including impurities at levels as low as 10-6 moles of Na, or 10-7 moles of K, in the 8 cm3 sulfidation ampoule, the grain size of CZTS on quartz substrates was increased dramatically from 100's of nm to greater than 1 mu;m, while Ca loading had little effect. The electronic properties of CZTS films synthesized with different amounts of impurities in the sulfidation tube were also studied using temperature dependent conductivity and Hall effect. The effects of alkali metal impurities on the microstructure and electric