Symposium Organizers
Vivian Ferry, University of Minnesota
Ali Javey, University of California Berkeley
Evelyn Wang, Massachusetts Institute of Technology
Jia Zhu, Nanjing University
OO3: Excitonic Solar Cells
Session Chairs
Monday PM, November 30, 2015
Hynes, Level 3, Room 302
2:30 AM - *OO3.01
Engineering Nanostructures and Interfaces in Excitonic Solar Cells
Guozhong Cao 1
1Univ of Washington Seattle United States
Show AbstractExcitonic solar cells including dye-sensitized solar cells, quantum dot-sensitized solar cells, bulk heterojunction organic photovoltaics, are built upon nanostructures of various functional materials. Nanostructures are essential for the high power conversion efficiency, for example, in dye-sensitized solar cells and quantum dot-sensitized solar cells, mesoporous photoanodes made of nanoparticles offer large specific surface area for loading a large amount of dyes or quantum dots so as to capture a sufficient fraction of photons. However, the large surface area of such nanostructures also provide easy pathways for charge recombination, and surface defects and connections between adjacent nanoparticles may retard effective charge injection and charge transport, leading to a loss of power conversion efficiency. Surface facets and chemistry may also affect the conformal coating and adhesion of dye molecules and polymer layers. In this presentation, I will present and discuss our recent work on the design and control of (1) nanostructures and surface chemistry of photoanodes for quantum dot - sensitized solar cells, (2) nonstoichiometric composition, doping, and allignment of quantum dots in quantum dot-sensitized solar cells, and (3) the incorporation of plasmonic nanocrystals. Our research has demonstrated that the power conversion efficiency can be significantly enhanced with excellent device stability when both nanostructures and interface chemistry are properly enginnered.
3:00 AM - OO3.02
Controlling Charge Carrier Recombination in Nanocrystal-Based Solar Cells
Deniz Bozyigit 1 Nuri Abraham Yazdani 1 Weyde Matteo Mario Lin 1 Maksym Yarema 1 Olesya Yarema 1 Vanessa C. Wood 1
1ETH Zurich Zurich Switzerland
Show AbstractA high efficiency device requires that the useful process, such as charge extraction in the case of a solar cell, be faster than the loss of energy as heat, which occurs as electrons release their energy in the form of mechanical vibration quanta (‘phonons&’). While electron-phonon coupling is well understood in bulk crystalline semiconductors, where electron and phonon density of states have been measured and calculated and coupling rates measured, electron-phonon coupling in nanomaterials is not understood.
Here, we perform inelastic neutron scattering (INS) and density-functional-theory calculations to study the electron-phonon interactions in nanocrystal-solids. We show that mechanical weakness of the nanocrystal surface enables the strong coupling of low-energy surface-phonon modes to electronic transitions. Using thermal admittance spectroscopy to quantify the electronic transition rates in nanocrystal-based diodes, we provide evidence that these low-energy phonon modes can drive large energy transitions (~1 eV) at unusually high rates, due to large entropy change in these transitions.
Our findings explain the role of surface defects and passivation on nanocrystal-based device performance and guide the bottom-up engineering of next generation semiconductors, where optical, electronic, and phononic properties can be tailored.
3:15 AM - OO3.03
Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots
Freddy Rabouw 1 Marko Kamp 2 Relinde van Dijk-Moes 1 Daniel Gamelin 3 Femius Koenderink 2 Andries Meijerink 1 Daniel Vanmaekelbergh 1
1Utrecht University Utrecht Netherlands2FOM Institute AMOLF Amsterdam Netherlands3University of Washington Seattle United States
Show AbstractThe efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is negatively affected by ‘blinking&’ on the single-nanocrystal level, i.e. random alternation of bright and dark periods. The timescales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. In this work we investigate photoluminescence from QDs ‘delayed&’ over microseconds to milliseconds, thus connecting the timescales of exciton decay to those of blinking. With single-QD as well as ensemble measurements, we examine the dynamics and spectral characteristics of this delayed emission, and quantify radiative and nonradiative decay constants by systematically varying the photonic environment of the QDs. Our results indicate that delayed emission and blinking have the same physical origin, namely charge carrier separation, storage for microseconds to seconds, and eventual recovery of the exciton state. A new microscopic model is developed that connects our new results on delayed emission to exciton recombination and blinking. While existing models for blinking have in common that they seek an explanation for uninterrupted optical cycling during long ON ‘periods&’ of up to many seconds, our results on delayed emission show that such long periods of uninterrupted cycling do not occur. The new blinking model, which assumes a significant (of order 10%) probability of charge carrier separation at all times during optical cycling, reproduces the photoluminescence decay dynamics of QDs, including the delayed component, as well as the blinking characteristics.
3:30 AM - OO3.04
Thiol-Capping of Type-II Heterojunction Nanorods and Their Improved Photovoltaic Performance
Joseph Flanagan 1 Moonsub Shim 1
1University of Illinois Urbana United States
Show AbstractCdSe/CdTe heterojunction nanorods (HNRs) with type-II staggered band offset can allow directional and efficient separation of photogenerated charge carriers. However, CdTe nanocrystals can be easily oxidized with just post-synthesis processing in air, which can then lead to charge traps that negate the benefits of the type-II band offset. Here, we introduce a simple ligand exchange method immediately post-synthesis to replace the native ligands on CdSe/CdTe or CdSe/CdSexCdTe1-x HNRs with 1-octanethiol, resulting in improved photoluminescence and good stability in air. Transient absorption measurements reveal that electron transfer from CdTe to CdSe remains fast and efficient (~400 fs) despite the hole trapping nature of thiol ligands for CdSe. Absorption bleach arising from CdTe-to-CdSe electron transfer can be observed out to 1 mu;s even after days of storage in air, an order of magnitude longer than HNRs with native ligands kept rigorously air-free and three orders of magnitude longer than HNRs with native ligands exposed to air. This improved environmental stability that preserves efficient charge separation and minimizes trapping translates to enhanced photocurrent generation, especially with respect to contribution from photoexcitation of CdTe transitions. When used as light harvesters in sensitized solar cells, CdSe/CdSe0.4CdTe0.6 HNRs capped with thiol ligands show an enhanced ability to extract photogenerated charges, resulting in a 3.02% power conversion efficiency.
3:45 AM - OO3.05
Electron Transport Regimes and the Hall Effect in Networks of Heavily Doped Semiconductor Nanocrystals Embedded in Insulating Matrix
Deanna Lanigan 1 Elijah Thimsen 1
1Washington Univ in Saint Louis Saint Louis United States
Show AbstractThin films comprised of nanocrystals continue to attract interest as functional layers in photovoltaic devices. The focus of this paper will be on films comprised of heavily doped ZnO nanocrystals for use as transparent conducting oxide (TCO) layer. Majority carrier transport, electrons in the case of ZnO, is imperative to performance of TCO layers; yet it remains relatively poorly understood in films comprised of nanocrystals. The interpretation of results from routine electrical characterization techniques, such as Hall effect measurements, has historically been difficult. In this experimental work, we present results from assemblies of ZnO nanocrystals embedded in Al2O3. The insulating matrix is required to remove acceptor defects from the particle surfaces and render them conductive. It is demonstrated that the key parameter controlling the electron transport mechanism is the contact resistance between nanocrystals in the assembly, which can be controlled by systematically adjusting the contact area between particles using atomic layer deposition. The magnitude of the contact resistance between particles compared to the quantum resistance determines the transport mechanism. If contact resistance is greater than the quantum resistance, transport is in the dielectric regime wherein a variable range hopping mechanism is observed. If contact resistance is less than the quantum resistance, disordered metallic conduction is observed with relatively high electron mobility of 5 cm2 V-1 s-1 if nanocrystals are well-connected. In the dielectric regime, an anomalous hall effect is observed at moderately low temperature (100 ~ 250 K). This anomalous Hall effect has several unexpected features, which to our knowledge have not been reported before. Alternatively, if the nanocrystals are well-connected and the electron transport mechanism is metallic, the Hall coefficient displays explicable features of a disordered, heavily doped, n-type semiconductor. In the metallic regime, a negative hall coefficient is observed that is independent of temperature and corresponds to a reasonable value for the carrier concentration.
OO4: Nanostructured I
Session Chairs
Monday PM, November 30, 2015
Hynes, Level 3, Room 302
4:30 AM - *OO4.01
Nanostructures for Light Management in Chalcopyrite Solar Cells
Martina Schmid 1 2 Phillip Manley 1 Guanchao Yin 1
1Helmholtz-Zentrum Berlin Berlin Germany2Freie Universitauml;t Berlin Berlin Germany
Show AbstractChalcopyrite (Cu(In,Ga)Se2) solar cells have achieved the highest stable efficiency amongst polycrystalline thin-film devices with a latest record of 21.7%. For increased competitiveness on the market and addressing energy production on the TW scale, a reduced consumption of Indium - their element with the highest supply risk - is desired. A reduction of absorber layer thickness to below 500 nm with efficiencies above 10% has proven possible, but still leaves significant room for increasing short circuit current density. For light absorption enhancement in the ultra-thin Chalcopyrite layers plasmonic and photonic nanostructures are highly promising due to their light concentrating and scattering behavior.
Plasmonic nanoparticles made from e.g. Ag or Au can be fabricated with a well-defined size and density distribution leading to a controllable spectral response. Integrating these nanoparticle distributions into thin-film solar cells allows the absorption to be significantly enhanced based on 1) high near-field enhancement around the nanoparticles and 2) scattering towards the absorber layer and leading to light trapping inside. These mechanisms however require a short distance of the optically active nanostructures from the absorber layer which has turned out to be challenging for Chalcopyrite solar cells. High temperature fabrication processes and the formation of e.g. Ag-Chalcopyrite compounds makes the direct integration of metallic nanoparticles into Cu(In,Ga)Se2 absorbers impossible. Core-shell nanoparticles are a promising option for chemical and also electrical passivation; we found that a refractive index of the shell higher than the one of the surrounding medium is crucial for effective use of enhanced near-field and high scattering from the metal core.
Photonic nanostructures, made from insulating dielectric material only, are found to have high potential for replacing their metallic counterparts due to absence of parasitic absorption. They show comparable scattering cross sections of more than five times their geometrical cross section and can also be brought into direct contact with the Chalcopyrite absorber. We have proven an increase in short circuit current density of more than 2 mA/cm2 in ultra-thin Cu(In,Ga)Se2 solar cells by integrating SiO2 nanopsheres and -patterns at the front or rear side of the structure. Going beyond the pure optical properties of nanostructures integrated into ultra-thin-film devices, electrical properties also need to be considered and optimized effectively. Point contact structures from e.g. Al2O3 at the absorber&’s back or also front interface are investigated with respect to reduced recombination leading to a combined electrical and optical benefit for the solar cell performance.
5:00 AM - OO4.02
MoSe2: Poly(Thiophene) Bulk Heterojunctions for Hybrid Photovoltaic Applications : A Nanoscale Analysis of the (Photo)Electrical Properties
Laurie Letertre 1 Olivier Douheret 2 Moussa Bougouma 3 Thomas Doneux 4 Roberto Lazzaroni 1 Claudine Buess-Herman 4 Philippe E. Leclere 1
1Univ Mons Mons Belgium2Materia Nova Mons Belgium3Ouagadougou University Ouagadougou Burkina Faso4Universite Libre de Bruxelles Bruxelles Belgium
Show AbstractHybrid solar cells are in sustained development among the photovoltaic (PV) technologies. Molybdenum diselenide (MoSe2) is seen as a promising alternative semiconductor for hybrid PV due to its absorption in the near-infrared spectrum, its good carrier transport and its ability to form 2D structures. Scanning Spreading Resistance Microscopy (SSRM) is used for the electrical characterization of crystalline Nb-doped MoSe2. A n-type nature of doping is demonstrated, with local accumulation of dopants in the crystal when substituting Mo atoms by Nb in a 0.01 % stoichiometric ratio. The study of the local electrical resistance of the Nb-doped MoSe2 surface also shows more conductive spots with a nanometric size, most likely due to a local accumulation of dopants and therefore carrier density within the crystal. A mechanical exfoliation of the single crystals led to micrometric and nanometric fragments suitable for hybrid bulk heterojunction (HBH) fabrication. An intrinsic photo-electrical effect has been observed by PhotoConductive Atomic Force Microscopy (pc-AFM) in Nb-doped MoSe2 dispersed in a poly(methyl methacrylate) matrix. A negative photo-current has been recorded, showing a spontaneous charge separation of the photo-generated excitons in the material, along with an efficient collection of the charges. On the basis of the SSRM measurements, a band structure for the probe-sample system is proposed.
Finally, a photoactive effect has been shown by blending the Nb-doped MoSe2 “flakes” with a typical p-type semiconducting polymer: the poly(3-hexylthiophene) (P3HT). The photo-electrical properties of those Nb-doped MoSe2:P3HT systems are studied at the nanoscale by pc-AFM. This study hence demonstrates the capability of MoSe2 to be considered as a promising acceptor material with a very good stability in future hybrid photovoltaic devices. Having described this system, one could expect to get HBH with a more controlled nanomorphology by monitoring 2D MoSe2 nanostructures. As the bandgap of the MoSe2 crystal widens when the number of unit lamellae decreases, operating with 2D nanostructrures should improve the charge separation at the Nb-doped MoSe2:P3HT interfaces, since a lowering of the valence band of the 2D MoSe2 relative to the bulk can be expected.
5:15 AM - OO4.03
Highly Efficient Large-Area Colourless Luminescent Solar Concentrators with Reduced Re-Absorption Losses Using Heavy Metal Free Colloidal Quantum Dots
Francesco Meinardi 1 Hunter McDaniel 2 Francesco Carulli 1 Roberto Simonutti 1 Nikolay Makarov 3 Kirill A Velizhanin 3 Victor I. Klimov 3 Sergio Brovelli 1
1Universitagrave; degli Studi di Milano Bicocca Milano Italy2UBIQD Los Alamos United States3Los Alamos National Laboratory Los Alamos United States
Show AbstractIn the near future, building integrated photovoltaics (PV) could revolutionize urban architecture by allowing one to reach the ambitious goal of net zero energy consumption buildings. Luminescent solar concentrators (LSCs) can play an important role in this transition as they provide a means to realize semitransparent PV windows that are able to convert the energy passive facades of urban buildings into distributed energy generation units[1]. Wide use of LSCs has, however, so far been hindered by the lack of suitable emitters especially with regard to the limited coverage of the solar spectrum and significant optical losses associated with re-absorption of guided luminescence that prevent the realization of large area devices and lead to strong coloring of devices, limiting their usage in architecture. Colloidal quantum dots (QDs) can help overcome these limitations thanks to material engineering strategies for increasing Stokes shift, including heterostructuring[2] and doping[3]. Despite their great potential, LSCs fabricated using CdSe/CdS heterostructures or doped QDs still suffer from incomplete coverage of the solar spectrum due to a large energy gap of the absorber material. Furthermore, core/shell structures contain hazardous heavy metal ions and thus require expensive disposal/recycling protocols.
An interesting class of nontoxic materials that provide a large, hundreds of meV Stokes shift without the need for heterostructuring are QDs of ternary I-III-VI2 semiconductors such as CuInS2 (CIS), CuInSe2 (CISe), and their alloys (CuInSexS2minus;x or CISeS). An attractive feature of these QDs is that they do not contain heavy metals and can be fabricated in large quantities via high-throughput, non-injection techniques using inexpensive precursors. Furthermore, their large absorption cross-sections and a spectrally tunable, near-IR absorption onset are well suited for harvesting solar radiation[4]. Here we use CISeS QDs coated with ZnS to realize the first large-area IR QD-LSCs with reduced re-absorption losses and extended coverage of the solar spectrum. We focus on CISeS QDs with emission at 960 nm (1.3 eV), which is near optimal for LSCs coupled to Si PVs and also allows for the realization of colourless QD-LSCs. By incorporating QDs into a photopolymerized PLMA matrix, we obtain freestanding, colorless polymer slabs of excellent optical quality that introduce no distortion to transmitted colors and are thus well suited for integration into buildings in the form of PV windows. Using this approach, we achieve an optical power conversion efficiency of 3.2% of the incident solar radiation, which is the highest value ever reported for large area LSCs with any types of chromophores.[5]
References
[1]M. G. Debije, et al., Adv En Mater 2012, 2, 12.
[2]F. Meinardi, et al., Nature Photonics 2014, 8, 392.
[3]C. S. Erickson, et al., Acs Nano 2014, 8, 3461.
[4]D. Aldakov, et al., J Mat Chem C 2013, 1, 3756.
[5] F. Meinardi, et al., Nature Nano 2015, under review
5:30 AM - OO4.04
Quantum Dot Luminescent Concentrator Cavity Exhibiting 30-Fold Concentration
Noah Bronstein 1 Yuan Yao 2 Lu Xu 2 Erin O'Brien 1 Alexander Powers 1 Vivian Ferry 3 Ralph G. Nuzzo 2 A. Paul Alivisatos 1
1Univ of California-Berkeley Berkeley United States2University of Illinois Urbana United States3University of Minnesota Minneapolis United States
Show AbstractLuminescent solar concentrators doped with CdSe/CdS quantum dots provide a potentially low-cost and high-performance alternative to costly high band-gap III-V semiconductor materials to serve as a top junction in multi-junction photovoltaic devices for efficient utilization of blue photons. In this study, a photonic mirror was coupled with such a luminescent waveguide to form an optical cavity where emitted luminescence was trapped omnidirectionally. By mitigating escape cone and scattering losses, 82% of luminesced photons travel the length of the waveguide. Using quantum dots with a 60% luminescent quantum yield, this allows a concentration ratio of 30.3 for blue photons in a waveguide with a geometric gain of 61. Further, we study the photon transport inside the luminescent waveguide, showing unimpeded photon collection across the entire length of the waveguide.
5:45 AM - OO4.05
Electron-Injection and Charge-Recombination at Chalcogenorhodamine-TiO2 Interfaces
Kacie R Liwosz 1 Michael Detty 2 David Watson 2
1D'Youville College Buffalo United States2University at Buffalo Buffalo United States
Show AbstractAlthough organic dyes may be attractive alternatives to metal-containing dyes for dye-sensitized solar cells (DSCs) they are often less efficient and remain relatively less explored. We have examined two essential processes for the success of an organic DSC, electron injection from the sensitizer into TiO2 and minimization of non-favorable charge recombination. The influences of surface-attachment functionality on electron-injection yield (Phi;inj), inertness of dye-TiO2 linkages, and photoelectrochemical performance of DSCs are reported. In addition, the electron lifetime (tau;e) in a phosphonate-attached chalcogenorhodamine dye-DSC is compared to the tau;e in a high-performing cis-bis(4,4'-dicarboxy-2,2'bipyridine)dithiocyanatoruthenium(II)-DSC (N3-DSC). The chalcogenorhodamine dyes were constructed around 3,6-bis(dimethylamino)selenoxanthylium and varied in the 9-substituent: 5-carboxythien-2-yl (1-Se) and 5-phosphonothien-2-yl (3-Se). Transient absorption spectroscopy (TA) revealed the Phi;inj via carboxylates was 2-fold greater than via phosphonates [1]. The favorable Phi;inj of 1-Se was offset by rapid desorption from TiO2 into acidified CH3CN. Incident photon-to-current efficiencies (IPCEs) of DSCs ranged from 66% to 91% and were initially higher for 1-Se. The performance of 1-Se degraded rapidly, whereas the performance of 3-Se remained stable [2]. Thus, utilizing phosphonic acids increased the inertness of chalcogenorhodamine-TiO2 interfaces without greatly impacting the photoelectrochemical performance. Despite the high IPCEs achieved with these DSCs, photocurrent-photovoltage (J-V) experiments reveal overall power conversion efficiencies (eta;) of <1% for each chalcogenorhodamine-sensitized solar cell, compared to 5.9% for N3-based DSCs. The low eta; from J-V experiments under simulated solar illumination stands in sharp contrast to the high IPCEs under low-intensity monochromatic illumination and the high relative Phi;inj from TA experiments. These differences led to the design of ongoing transient photovoltage (TPV) experiments, which aim to characterize charge recombination and dye regeneration at TiO2-chalcogenorhodamine-electolyte interfaces, and will be emphasized in this talk. We compared tau;e for 3-Se-DSCs and N3-DSCs with and without supporting electrolyte. Our results indicate the presence of an alternative non-favorable electron-recombination pathway in the 3-Se-DSCs compared to the N3-DSCs.
[1] Mulhern, K. R.*; Detty, M. R.; Watson, D. F., Effects of surface-anchoring mode and aggregation state on electron injection from chalcogenorhodamine dyes to titanium dioxide. Journal of Photochemistry and Photobiology A: Chemistry 2013,264, 18-25.
[2] Mulhern, K. R.*Dagger;; Orchard, A.Dagger;; Watson, D. F.; Detty, M. R., Influence of Surface-Attachment Functionality on the Aggregation, Persistence, and Electron-Transfer Reactivity of Chalcogenorhodamine Dyes on TiO2. Langmuir 2012,28, 7071-7082.
*Now Kacie R. Liwosz Dagger;Indicates equal contribution.
OO1: Photon Management
Session Chairs
Monday AM, November 30, 2015
Hynes, Level 3, Room 302
9:00 AM - *OO1.01
Energy Conversion Using Nanophotonics in Macro-Scopic Systems
Marin Soljacic 1
1MIT Cambridge United States
Show AbstractNanophotonics provides superb opportunities for tailoring the flow of light. This way, many novel physical phenomena can be enabled, as well as many important functionalities for novel energy applications (e.g. energy conversion, lighting). In order to make these phenomena useful for large systems, large-area nano-fabrication techniques have to be successfully implemented. In this talk, I will present some of our recent theoretical and experimental progress in exploring these opportunities.
9:30 AM - *OO1.02
Engineering the Photovolatic Voltage with Nanostructures
Zongfu Yu 1
1University of Wisconsin Madison Madison United States
Show AbstractNanostrutures will modify the spectral and angular emissvity of solar materials. Consequently, the photovolatic voltage will be enhanced or suppressed. We will discuss the way to model such changes and describe a few examples. In particular, based on voltage enhancement, we will show how to go beyond one-sun Shockley-Quiesser limit using a single semiconductor material in a tandem configuration.
10:00 AM - OO1.03
Novel Architectures Combining a Luminescent Down-Shifting Layer and a 2D Photonic Structure for Enhanced Light Management
Ngoc-Vu Hoang 1 2 Matias Calderini 1 Thierry Deschamps 1 Emmanuel Drouard 2 Bernard Moine 1 Hai Son Nguyen 2 Regis Orobtchouk 3 Anne Pillonnet 1 Christian Seassal 2 Antonio Pereira 1
1Institut Lumiegrave;re Matiegrave;re Villeurbanne France2Institut des Nanotechnologies de Lyon Ecully France3Institut des Nanotechnologies de Lyon Villeurbanne France
Show AbstractIn crystalline silicon-based solar cells, a substantial part of the energy losses is related to the carriers&’ thermalization in the UV-blue range. This issue can be partially circumvented by a down-shifting process inside a luminescent rare-earth (RE) doped thin layer. However, due to the low absorption cross-section of the RE ions, the efficiency of the converting layer needs to be increased. In this study, we introduce a new concept which combines a rare-earth doped thin layer (Y2O3: Eu3+) with a 2D photonic crystal (SiNx), in order to take control of the frequency conversion process. A Y2O3:Eu3+ 100 nm thick layer was first deposited by Pulsed Laser Deposition. Above this converter layer, a SiNx thin film was then deposited and periodically patterned using laser interference lithography and reactive ion etching, to ensure an exaltation of the guided electromagnetic field inside the luminescent layer. This nanophotonic structure was designed and optimized using RCWA simulation in order to enhance separately absorption and emission of the rare-earth layer or both of them at the same time.
In this communication, we will discuss on the design, fabrication and measurements performed on these new architecture enabling the control of light-matter interaction. We will show that a substantial enhancement of the conversion yield was achieved, which could pave the way to the development of more efficient photovoltaic devices.
10:15 AM - OO1.04
Controlling Emission Using One Dimensional Integrated Photonic Fluorescent Solar Concentrators
Thomas Stephen Parel 1 Tomas Markvart 1
1University of Southampton Southampton United Kingdom
Show AbstractOne dimensional photonic crystals (Bragg stacks) can be used to control the directionality of fluorescence emission. In this paper we report on how this property can be used to improve the operation of fluorescent solar concentrators. Fluorescent solar concentrators aim to concentrate light onto solar cells by trapping fluorescence through total internal reflection. In a good fluorescent solar concentrator the major obstacle to efficient photon transport is the loss of photons through the top and bottom escape cones. One possible method to decrease this loss and improve the efficiency of these devices is to fabricate one-dimensional photonic crystals doped with fluorescent molecules. If these photonic crystals are tuned to exhibit a photonic band gap in the escape cone directions and at the emission frequencies of the fluorescent molecules, a suppression of the escape cone emission and an enhancement of the edge emission is expected. In this paper, we detail the fabrication of the first one dimensional integrated photonic solar concentrator and show the suppression of the escape cone emission. This suppression and also enhancements in the edge emission will also be shown to correspond well with the calculated band structure of the device. The control of emission inside fluorescent solar concentrators opens up a number of additional possibilities for efficiency enhancements that will also be discussed.
10:30 AM - OO1.05
Optical Concentration in Nanostructures for Photovoltaic Voltage Enhancements
Sander A. Mann 1 Richard Ryan Grote 2 Erik Garnett 1
1FOM Inst AMOLF Amsterdam Netherlands2University of Pennsylvania Philadelphia United States
Show AbstractIn recent years the interest in solar energy conversion based on nanoparticles has surged due to their promising material and optical properties. One of the promising optical properties is that nanostructures are known to exhibit absorption cross sections that can be considerably larger than their own geometrical cross section, thereby effectively collecting current from a larger area. This phenomenon appears to be very similar to macroscopic concentrator photovoltaics, where the intensity of light incident on a solar cell is increased by means of a lens. As a result the carrier concentration increases, which results in higher voltages and larger conversion efficiencies. Due to the large optical absorption cross sections of nanostructures, this enhanced efficiency might thus be available without the use of external optics.
In this work we discuss how macroscopic and nanophotonic concentration are actually quite different, and that care needs to be taken when designing a nanophotonic solar cell. We show that large optical cross sections for nanoparticles help to reduce bulk recombination losses, but that this by itself never leads to efficiencies above the one sun Shockley-Queisser limit. To exceed this limit angular anisotropy in the absorption of the nanoparticle is required, but we show that in general this is actually very hard to achieve. As a numerical demonstration we discuss the voltage and current characteristics of single finite GaAs nanowire solar cells, a very promising building block of highly efficient photovoltaic devices. The angularly dependent absorption cross section of such a finite wire is complicated and involves Mie modes as well as the excitation of guided modes, but we derive simple yet accurate analytical approximations. We show that indeed defect recombination can be reduced with respect to bulk values, but that the angular anisotropy is low and therefore efficiencies do not exceed regular Shockley-Queisser values. Finally, we propose several approaches to actually achieve optical concentration with nanomaterials, for instance by utilizing array effects in large-scale systems.
OO2: Perovskite Solar Cells I
Session Chairs
Monday AM, November 30, 2015
Hynes, Level 3, Room 302
11:15 AM - *OO2.01
Highly Reproducible Hysteresis-Free Perovskite Solar Cell with Efficiency Exceeding 20% via Acid-Base Adduct Chemistry
Dae-Yong Son 1 Namyoung Ahn 2 In-Hyuk Jang 1 Nam-Gyu Park 1
1Sungkyunkwan Univ Suwon Korea (the Republic of)2Seoul National University Seoul Korea (the Republic of)
Show AbstractSince the report on the long-term durable solid-state perovskite solar cell employing MAPbI3 (MA = CH3NH3) perovskite in 2012, following two important pioneer works on perovskite-sensitized solar cells in reported 2009 and 2011, there has been a surge of interest in perovskite solar cell. We report here highly reproducible hysteresis-free perovskite solar cell with mean power conversion efficiency (PCE) exceeding 19% and best PCE exceeding 20% via well-defined acid-base adduct approach. The device includes basically a MAPbI3 layer with small fraction of mesoprous TiO2 layer and a spiro-MeOTAD hole transport layer. The Lewis base adducts of PbI2 was found to yield high quality pinhole free MAPbI3 film at ambient condition, leading to high photocurrent and voltage. Compared to conventional methods, Lewis base adduct approach showed higher charge extraction ability as evidenced by photo-CELIV measurement. Furthermore, molecular-level interfacial nanoengineering improved fill factor significantly. Hysteresis-free perovskite solar cell in a normal structure was realized by structural and electronic modifications.
11:45 AM - OO2.02
Reduced Non-Radiative Recombination via Surface Passivation in CH3NH3PbI3 Perovskite Films
Dane W. Dequilettes 1 Liam Bradshaw 1 Mark Ziffer 1 Susanne Koch 2 Samuel David Stranks 3 David Ginger 1
1Univ of Washington Shoreline United States2University of Konstanz Konstanz Germany3Massachusetts Institute of Technology Boston United States
Show AbstractConfocal photoluminescence (PL) microscopy and fluorescence lifetime imaging microscopy (FLIM) reveal significant local heterogeneity in non-radiative recombination rates in perovskite thin films, and point to the role of surface and grain boundary recombination in these films. We use PL spectroscopy, including FLIM, to study the effects of different passivating chemical treatments on carrier recombination in CH3NH3PbI3 films. We both identify compounds with the potential to further reduce non-radiative recombination in perovskite films, and quantify the effects on local trap state density using kinetic models. Using picosecond PL spectroscopy, we further elucidate the various kinetic pathways that carriers traverse before repopulation of the ground state. Our study highlights the nature of non-radiative loss at the surface of perovskite films and points the way for further improvement in these materials for light emitting and solar cell applications.
12:00 PM - OO2.03
20% Efficiency Perovskite Solar Cell via Molecular-Level Interfacial Nanoengineering
Dae-Yong Son 1 Nam-Gyu Park 1
1SungKyunKwan Univ Kyunkido Korea (the Republic of)
Show AbstractWe report here an effective methodology for MAPbI3 (MA = CH3NH3) perovskite solar cell with power conversion efficiency (PCE) exceeding 20% via molecular-level interfacial nanoengineering. The basic cell configuration is FTO/thin film TiO2/mesoporous TiO2/MAPbI3/spiro-MeOTAD/Ag, where 40-50 nm-sized TiO2 particles are used for the mesoporous TiO2 layer. MAPbI3 layer are prepared via adduct approach, where Lewis acid PbI2 is reacted with Lewis base polar aprotic solvent such as N,N-dimethylsulfoxie (DMSO) to form MAI-PbI2-DMSO adduct film. The adduct approach yields average PCE of 18-19%. However, the open-circuit voltage is lower than 1.1 V despite the theoretical output voltage expecting 1.3 V. Moreover, fill factor is hard to approach 80%. Those are related to non-radiative recombination in bulk perovskite and/or interfacial charge recombination. We have introduced thin interfacial layer on the perovskite layer contacting either p-type or n-type layer. Molecular-level engineering by varying interfacial gap between perovskite layer and n- or p-type layer results in open-circuit voltage as high as 1.13 V and fill factor of 78%, which leads to a PCE of 20.7%. The molecular orientation and opto-electronic role of interfacial layer is investigated using AFM and spectroscopic tools, along with impedance at interfaces.
12:15 PM - OO2.04
A New Strategy to Get High Efficiency Perovskite Solar Cell by Sequential Deposition
Chenyi Yi 1 Xiong Li 1 Jingshan Luo 1 Michael Graetzel 1
1EPFL Ecublens Switzerland
Show AbstractHybrid organic inorganic lead halide perovskites emerged as attractive solar cell materials. Two-step sequential deposition is an important method to fabricate perovskite film. It usually involves depositing a uniform condense PbI2 film, and subsequent conversion of the PbI2 into perovskite. A homogenous perovskite upper layer on top of the TiO2 is important to prevent the charge recombination between hole transporter and electron transporter, thus to get a high power conversion efficiency. However, the condense PbI2 atop the TiO2 inhibits its conversion into perovskite. In this presentation, we will show a new strategy to assist the conversion of the PbI2. With the new method, a homogeneous condense perovskite upper layer on mesoporous TiO2 with well-infiltrated perovskite can be produced. High power conversion efficiency up to 20% can be achieved.
12:30 PM - OO2.05
Optoelectronic Properties of Lead Trihalide Perovskites for Photovoltaics
Rebecca Milot 1 Christian Wehrenfennig 1 Mingzhen Liu 1 Giles Eperon 1 Henry Snaith 1 Michael Johnston 1 Laura Herz 1
1University of Oxford Oxford United Kingdom
Show AbstractThe organic-inorganic metal halide perovskite CH3NH3PbI3 has emerged as a new material for photovoltaic cells with power conversion efficiencies near 20%. CH3NH3PbI3 and the related CH3NH3PbI3minus;xClx exhibit long charge-carrier diffusion lengths [1], high charge-carrier mobilities and unusually low bi-molecular charge recombination rates [2]. Yet relatively little is known about how nano-morphology and structural phase transitions affect charge transport in photovoltaic cells.
We show that at room temperature, CH3NH3PbI3minus;xClx films exhibit extraordinarily low energetic disorder, with an emission spectrum that is predominantly homogenously broadened by ~100meV, through coupling of charge-carriers to phonons [3]. However, the charge-carrier dynamics are likely to be affected by structural phase transitions that occur at ~160K and ~315K. An understanding of changes in optoelectronic properties occurring across tetragonal-to-cubic transition at 315K is particularly important since the actual working temperature of a high-efficiency perovskite solar cell in field conditions can exceed 70°C.
We have investigated the transient photoluminescence and photoconductivity in CH3NH3PbI3 and CH3NH3PbI3minus;xClx thin films from 8 K to 370 K across three structural phases. Analysis of the charge-carrier recombination dynamics reveals a variety of starkly differing recombination mechanisms. Evidence of charge-carrier localization is observed only at low temperature. Below the orthorhombic-to-tetragonal phase transition at 160K complex PL decay channels are observed [4] that may derive from nanoscopic inclusions of domains adopting the high-temperature phase. However, high mobility and diffusion length are maintained at high temperature beyond the tetragonal-to-cubic phase transition at ~310K. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH3NH3PbI3 under typical field conditions.
[1] Stranks, Eperon, Grancini, Menelaou, Alcocer, Leijtens, Herz, Petrozza, Snaith, Science 342 (2013) 341.
[2] Wehrenfennig, Eperon, Johnston, Snaith, Herz, Adv. Mater. 26 (2014), 1584.
[3] Wehrenfennig, Liu, Snaith, Johnston, Herz, J. Phys. Chem. Lett. 5 (2014), 1300.
[4] Wehrenfennig, Liu, Snaith, Johnston, Herz, APL Materials 2 (2014), 081513.
Symposium Organizers
Vivian Ferry, University of Minnesota
Ali Javey, University of California Berkeley
Evelyn Wang, Massachusetts Institute of Technology
Jia Zhu, Nanjing University
OO7: Nanostructured II
Session Chairs
Tuesday PM, December 01, 2015
Hynes, Level 3, Room 302
2:30 AM - *OO7.01
Enhanced Radiative Cooling in GaAs Nanowire Solar Cells
Shao-Hua Wu 1 Michelle L. Povinelli 1
1University of Southern California Los Angeles United States
Show AbstractA great deal of recent work has examined the optical properties of nanowire solar cells. It is now well known that properly designed nanowires concentrate incoming light, achieving high absorption with minimal material volume. Volume reduction can relax the requirements on material quality, as the distance electrical carriers must travel to reach the contacts is minimized. However, little attention has been paid to the thermal consequences of the nanowire design. In particular, the thermal conductivity of nanowires is known to be several orders of magnitude lower than for bulk. Moreover, the concentration of light absorption inside the nanowire implies more concentrated heat generation, as a portion of absorbed energy is converted to heat inside the nanowire. It is thus of crucial importance to consider how the operating temperature of a nanowire solar cell compares to that of a traditional, planar one.
In this work, we carry out a detailed simulation of the temperature rise of a GaAs nanowire solar cell under solar illumination. We compare the temperature to that of an equivalent height planar structure. Our model takes into account coupled thermal and optical effects. We simulate the detailed spatial absorption profile in the nanowires using the finite-difference time domain method. To obtain the spatial heat generation profile, we assume that some fraction of the absorbed energy (equal to one minus the cell efficiency) is converted to heat. We then calculate the IR emissivity of the structure using the rigorous coupled-wave equations. The results of the electromagnetic simulations are then incorporated into a full 3D simulation of the heat equation including both convective and radiative boundary conditions.
We show that in fact, the nanowire cell heats up slightly less than the planar structure. Physically, this is due to the higher emissivity of the nanowire structure at thermal wavelengths, which enhances the radiative cooling rate. We find that the thermal conductivity of the nanowires has only minimal effect on the operating temperature. In steady state, the temperature is determined by a power balance equation between input and output flux. Despite the concentrated heat source within the nanowire, the temperature variation over the unit cell of the nanowire is extremely small compared to the overall temperature rise. As a result, the temperature is nearly constant over the whole structure, and so to first approximation, the power balance equation is independent of thermal conductivity.
Further, we find that a much higher cooling rate can be obtained by infiltrating the nanowires with BCB (benzocyclobutene), a polymer commonly used for electrical insulation and structural support. For a heat input of 900 W/m^2, this results in a reduction of the operating temperature by approximately 7K relative to a structure without BCB.
We conclude that overall, from a thermal standpoint, GaAs nanowire solar cells have similar performance to their planar counterparts, and that BCB-infiltrated nanowire cells significantly outperform bare structures.
3:00 AM - OO7.02
A Semi-classical Treatment for Understanding Carrier Transport Process at Semiconductor-Liquid Junction Using Energy Band Diagram Representation.
Asif Iqbal 1 2 Kirk H. Bevan 1 2
1McGill Univ Montreal Canada2McGill University Montreal Canada
Show AbstractPhotoelectrochemical (PEC) devices capable of converting sunlight into solar fuels hold out the promise of a renewable energy economic paradigm. However, in order to systematically engineer PEC devices, several key scientific questions need to be addressed: in particular, the carrier transport process from oxide or semiconductor materials, where electron-hole pairs are generated by incident sunlight, to the liquid environment. In this work a semi-classical method for understanding charge transfer processes in PEC devices is presented, building upon the energy band diagram approach common in the solid-state device community. Our approach is based upon integrating band theory theory with Gerischer&’s picture of heterogeneous electron transfer, coupled through Boltzmann statistics and variable dielectric electrostatics. We show that this approach is cable of integrating key concepts, such as solid-liquid Fermi level alignment and band-edge pinning/unpinning. In general the method elucidates an approach towards the systematic design of PEC devices.
3:15 AM - OO7.03
Fouml;rster Resonance Energy Transfer to Overcome Deficiencies in Luminescent Solar Concentrators
Clemens Tummeltshammer 1 Alaric Taylor 1 Anthony J Kenyon 1 Ioannis Papakonstantinou 1
1University College London London United Kingdom
Show AbstractLuminescent Solar Concentrators (LSCs) offer a means to reduce the cost of solar energy by concentrating incoming light. A LSC consists of a flat, transparent host matrix that is doped with fluorophores and has solar cells attached to its sides. Incident light is absorbed by the fluorophores and concentrated towards the sides of the host matrix via total internal reflection. LSCs are capable of absorbing diffuse sunlight which makes them very suitable for urban environments and building-integrated photovoltaics. Most commonly organic dye molecules are used as fluorophores; however, the absorption of dye molecules is spectrally quite narrow and therefore only a small part of the AM1.5 will be harvested. Quantum dots absorb over a wider spectrum but often suffer from lower quantum yields than organic dye molecules.
We have shown theoretically that linking quantum dots to dye molecules can enhance the performance of a LSC. The linkage induces Förster resonance energy transfer (FRET) from the quantum dot to the dye molecule. The quantum dot serves as an absorption center due to its spectrally wide absorption. Due to the proximity, the energy transfer will be efficient despite a potentially lower quantum yield of the quantum dot. The dye molecule will then emit the photon with a double Stokes shift which will reduce the probability of re-absorption by the quantum dot. This system exploits the advantages of both, the quantum dots and the organic dye molecules: the spectrally wide absorption of the quantum dots and the high quantum yield of dye molecules.
At the conference we will present our latest experimental results. CdSe/ZnS quantum dots functionalized with amine-derivatized PEG are used as the donor fluorophore. An Alexa Fluor dye molecule with a NHS ester reactive group serves as acceptor. The absorption profile of the dye is overlapping with the emission profile of the quantum dot to induce efficient FRET. The quantum dots and dye molecules are conjugated in a buffer solution and unconjugated dye molecules are removed using a centrifuge. The FRET efficiency is determined by comparing the decay times of quantum dots by themselves with conjugated samples. Quantum dot/dye conjugates in solution are then injected into a glass cell to determine their performance as LSC fluorophores. An experimental method developed by our group determines the optical efficiency of the quantum dot/dye conjugate LSC using an integrating sphere. Measurements are compared to unconjugated quantum dots and dye molecules to verify that FRET can boost the performance of LSCs.
3:30 AM - OO7.04
Thermoelectric-Photoelectric Composite Nanocables Largely Enhancing Efficiency of Dye-Sensitized Solar Cells
Hongcai He 1 2 Chuanbo Zhang 1 2 Wei Hu 1 2 Tao Liu 1 2 Ning Wang 1 2
1University of Electronic Science and Technology of China Chengdu China2University of Electronic Science and Technology of China Chengdu China
Show AbstractComposite nanocables (CNCs) with thermoelectric NaCo2O4 cores and TiO2 shells have successfully been prepared by electrospinning technique with a coaxial syringe needle. The CNCs were added into the TiO2 nanocrystals to obtain a new thermoelectric-photoelectric composite photoanode, and TiO2 shells of the CNCs achieve a good match with dyes while thermoelectric NaCo2O4 cores with high conductivity play a role of fast conduction channels of electrons. The different additive amount of TiO2-NaCo2O4 CNCs caused different device performances. The DSSC with 10wt% CNCs in TiO2 photoanode exhibits best photovoltaic performance with high Jsc of 21.16 mA cm-2, Voc of 0.754 V, FF of 68.09% and PCE of 10.86%, due to the reduced series resistance and improved electron transport. When the content of CNCs further increases to 20wt% and 40wt%, the DSSC performance decreases due to the reduced dye loading capacity of CNCs. In addition, the temperature gradient effect on photovoltaic performances of the DSSCs with 10wt% TiO2-NaCo2O4 CNCs and without any CNCs were investigated. The negative temperature gradient of -5 K increases PCE of the DSSC with 10wt% CNCs from 10.86% to 12.07%, while the positive temperature gradient decreases its PCE to 8.58%, due to the built-in electric field inside thermoelectric materials according to thermoelectric Seebeck effect. The temperature gradient effect implies that a further performance improvement is possible for the DSSCs with the thermoelectric-photoelectric CNCs because a temperature gradient between two sides of solar cells is inevitably caused by irradiation differences on two sides of solar cells and heating effect of sunlight irradiation. These results demonstrate the positive effect of the new thermoelectric-photoelectric composite nanocables for high efficiency dye-sensitized solar cells. This work not only is significant for developing novel photoanode of DSSCs with high performance, but also provides a new idea for integrated utilization of different new energy technologies including photovoltaic devices and thermoelectric devices.
3:45 AM - OO7.05
Light-Trapping Assessment in Silicon Thin Film Solar Cells Fabricated on Nano-Wrinkle Textured Surfaces Using Raman Spectroscopy
Derese Gugsa 1 Bjarke Rolighed 3 Michele Bellettato 4 Sanjay K. Ram 3 Rita Rizzoli 4 Caterina Summonte 4 Pia Bomholt Jensen 3 Rui N. Pereira 1 2 Peter Balling 3 Arne Nylandsted Larsen 3
1University of Aveiro Aveiro Portugal2Technische Universitauml;t Muuml;nchen Garching Germany3Aarhus University Aarhus Denmark4IMM- Consiglio Nazionale delle Ricerche Bologna Italy
Show AbstractInternal light scattering in textured solar cells is a currently used strategy to increase the optical path length of incident photons within the absorber layer (light-trapping), which leads to increased charge carrier generation. The characterization of the light-trapping capability of surface-textured solar cells has not been an easy task, since the most commonly used methods based on electrical measurements, do not probe exclusively the optical properties of the films but are also affected by the films&’ electronic performance. However, it has been recently proposed that Raman spectroscopy is a particularly valuable tool for probing the light-trapping in the case of textured microcrystalline silicon solar cells deposited on front textured ZnO superstrates [1]. This exploits the direct correlation between intensity of the Raman spectrum from a textured film and its optical thickness. In this study, we apply a similar approach based on Raman spectroscopy to quantify light-trapping in superstrate type p-i-n amorphous silicon (a-Si) thin film solar cells fabricated on nanotextured surfaces. Our sub-wavelength scale nanotextures are quasi-periodic nano-wrinkled surfaces, with varying periodicity and size range of the features. The absolute Raman signal intensities of the amorphous Si absorber layer of the solar cells fabricated on flat and different nano-wrinkle surfaces were measured using three laser wavelengths of 455 nm, 514 nm and 785 nm, with which we are capable of probing different depths in the a-Si thin films. We successfully correlate our findings in terms of light-trapping performance of the nanotextured surfaces to the efficiency of the corresponding solar cells. Our experimental results show a significant photocurrent enhancement in the solar cells fabricated on these nano-wrinkle surfaces, consistent with an improved photon harvesting. The similarity in the variation of the measured absolute Raman signal intensities and the photocurrent values with respect to different nano-wrinkle features shows that Raman spectroscopy can indeed be used to predict the light-trapping efficacy of irregular nanotextures applied in solar cells.
[1] M. Ledinsky, E. Moulin, G. Bugnon, K. Ganzerová, A. Vetushka, F. Meillaud, A. Fejfar, C. Ballif, Appl. Phys. Lett. 105, 111106 (2014)
OO8: Solar Materials
Session Chairs
Tuesday PM, December 01, 2015
Hynes, Level 3, Room 302
4:30 AM - *OO8.01
Selective Area Growth of GaAs on Si Using Nanoimprinted Templates
Adele Tamboli 1 Emily Warren 1 Emily Makoutz 2 Jeramy D. Zimmerman 2 Pauls Stradins 1 Andrew Gordon Norman 1 Bill McMahon 1 Daniel J. Friedman 1
1NREL Golden United States2Colorado School of Mines Golden United States
Show AbstractSilicon-based multijunction photovoltaics are desirable due to their high efficiency potential and leveraging of existing crystalline Si market dominance. III-V materials have the highest efficiency potential as top cell materials on Si, but numerous challenges- including lattice mismatch and antiphase domains- have hindered the development of high quality III-V/Si hybrid structures. Our approach to growing high-quality III-Vs on Si relies on selective area growth using nanopatterned templates. GaAs, which has a 4% lattice mismatch with Si, can be selectively nucleated in nanoscale holes in a SiO2 layer. Below a critical feature width, defect propagation is blocked, enabling heteroepitaxial growth of high quality GaAs for Si-based photovoltaics. An additional benefit is the excellent surface passivation provided by SiO2 on Si, compared to a typically defect-rich interface with a III-V layer. In this work, we will discuss the fabrication of Si templates patterned using substrate-conformal imprint lithography, an inexpensive and scalable technique for achieving nanoscale features. These patterned SiO2 on Si structures are then used for growth of GaAs films. Ultimately, this platform enables the fabrication of GaInP/GaAs/Si triple junction solar cells with an efficiency potential of > 45% and costs compatible with one-sun photovoltaic installations
5:00 AM - OO8.02
Coupled Organic-Inorganic Nanostructures? Charge Transport Properties of Thiadiazole-Functionalized CuInSe2 Nanocrystal Thin Films
Friederieke E. S. Gorris 1 Horst Weller 1
1University of Hamburg Hamburg Germany
Show AbstractSolution-processable semiconductor nanocrystals as thin films are a promising low-cost alternative to vapor-deposited materials in electronic devices such as solar cells. The exchange of the synthesis-induced ligand shell of these nanocrystals for suitable semiconducting organic ligands can lead to coupled organic-inorganic nanostructures (COIN). These organic ligands are supposed to couple the inorganic nanocrystals first by stronger binding interactions and second by near-resonant alignment of energy levels.[1] By this coupling the electronic properties such as conductivity or charge carrier mobility in nanocrystal thin films can be significantly optimized.[2]
The goal of our work is to improve charge transport of CuInSe2 nanocrystal thin films[3] in regard to facilitate solar cell assembly. This should be accomplished by exchange of insulating ligands for semiconducting organic ligands. We have determined energy level positions of two thiadiazoles (2,5-dimercapto-1,3,4-thiadiazole and 5-amino-2-mercapto-1,3,4-thiadiazole) by cyclic voltammetry to confirm the required alignment of suitable energy levels between these organic ligands and the nanocrystals. In addition we performed the ligand exchange in a thin film and analyzed charge transport properties.
In our contribution we show the results of the cyclic voltammetry measurements in comparison to the energy level positions of CuInSe2 nanocrystals to evaluate electronic coupling of this combination. Furthermore we present the effect of the ligand exchange on the nanocrystal thin film and charge transport properties.
[1] M. Scheele, W. Brütting and F. Schreiber, Phys. Chem. Chem. Phys.2015, 17, 97-111.
[2] M. Scheele, D. Hanifi, D. Zherebetskyy, S. T. Chourou, S. Axnanda, B. J. Rancatore, K. Thorkelsson, T. Xu, Z. Liu, L.-W. Wang, Y. Liu and A. P. Alivisatos, ACS Nano.2014, 8 (3), 2532-2540.
[3] J. Lauth, J. Marbach, A. Meyer, S. Dogan, C. Klinke, A. Kornowski and H. Weller, Adv. Funct. Mater.2014, 24, 1081-1088.
5:15 AM - OO8.03
Nontoxic, Abundant AgBiS2 Nanocrystal Solar Cells with Efficiencies Approaching 6%
Nichole Cates Miller 1 Maria Bernechea 1 Guillem Xercavins 1 Gerasimos Konstantatos 1
1ICFO - The Institute of Photonic Sciences Castelldefels Spain
Show AbstractSolar cells made from colloidal nanocrystals have shown great potential because of their solution processability, tunable absorption spectra, and promising efficiencies. However, the most efficient nanocrystal solar cells use toxic elements such as lead, cadmium, or antimony.(1-3) We will present colloidal AgBiS2 nanocrystals based on nontoxic, abundant elements and their use in solar cells with efficiencies approaching 6%. To our knowledge, this is the highest efficiency of any nontoxic, abundant nanocrystal solar cell to date. Furthermore, AgBiS2 nanocrystals can be solution processed close to room temperature, unlike many other ternary and quaternary chalcogenide systems such as CIGS and CZTS.
We have developed a synthesis for AgBiS2 nanocrystals with high, tunable absorption spectra (bandgap = 1.0 - 1.4 eV) that are stable in colloidal solutions for many months. These nanocrystals can be incorporated into planar or nanostructured solar cells with record efficiencies. As no selenization is required, both the material synthesis and the solar-cell fabrication are performed at low temperatures (le;100°C). Furthermore, the highest-efficiency cells yield short-circuit currents of up to 20 mA/cm2 with a very thin (~40 nm) active layer. These cells therefore offer the additional advantage of having an ultrathin active layer, which lowers material costs and paves the way for further improvement through incorporation of ultrathin absorbing layers in new device architectures such as tandem and sensitized solar cells.
References
1. C. M. Chuang, P. R. Brown, V. Bulovicacute;, M. G. Bawendi, Improved performance and stability in quantum dot solar cells through band alignment engineering, Nat. Mater. , 1-6 (2014).
2. I. J. Kramer, E. H. Sargent, The architecture of colloidal quantum dot solar cells: Materials to devices, Chem. Rev.114, 863-882 (2014).
3. Z. Pan et al., High-efficiency “green” quantum dot solar cells., J. Am. Chem. Soc.136, 9203-10 (2014).
5:30 AM - OO8.04
Optical Quantum Confinement and Photocatalytic Properties in Low Dimensional Hematite
Tomas Edvinsson 1
1Uppsala University Uppsala Sweden
Show AbstractBy being one of the common forms of iron oxide and one of the main components of iron ore, hematite is of immense importance to our industrialized society. In the presence of rust it is also one of few semiconductors visible, although not appreciated, in everyday life. Despite this it turns out that behind the familiar rusty faccedil;ade our understanding of the materials optical and catalytic properties, it is still insufficient for an efficient technological application. Hematite has been suggested in application in everything from cancer treatment and water cleaning, but also as a non-toxic, cheap and abundant material for photocatalytic water splitting. Here, the optical absorption is analyzed in detail as a function of film thickness for 35 thin films of hematite ranging between 2 and 70 nm. The hematite was deposited by atomic layer deposition on FTO-substrates using Fe(CO)5 and O2 as precursors. For film thicknesses below 20 nm the optical properties are severely affected as a consequence of quantum confinement. One of the more marked effects is a blue shift of up to 0.3 eV for thinner films of both the indirect and direct transitions, as well as a 0.2 eV shift of the absorption maximum.[1] Raman measurements revealed no peak shift or change in relative intensity for vibrations for the thinnest films indicating that the decrease in indirect transition could not directly be assigned to depression of any specific phonon but instead seems to be a consequence of isotropic phonon confinement. Elemental diffusion of Si, Sn and F [2] from the Fluorine doped SnO2 substrate upon thermal treatment of the hematite electrodes and the implications for the catalytic activity in water splitting and the resulting photocurrents are also discussed.
[1] Fondell, M., Jacobsson, T. J., Boman, M., Edvinsson, T. Optical quantum confinement in low dimensional hematite, J. of Materials Chem. A, 2 , 2014, 3352-3363
[2] Diffusion of Sn, Si and F in Hematite: Implications for Photocatalytic Water Splitting Applications, Fondell, M., Jacobsson, T. J., Boman, M., Edvinsson, T., Gorgoi, M. , von Fiendt, L. , Boman, M., Edvinsson, T. and Lindblad. T., submitted
5:45 AM - OO8.05
Analysis of Charge Collection in Highly Photoactive Ultrathin Hematite Photoanodes Produced by Low-Temperature Atomic Layer Deposition
Ludmilla Steier 1 Jingshan Luo 1 Marcel Schreier 1 Matthew T. Mayer 1 Michael Graetzel 1
1EPFL Lausanne Switzerland
Show AbstractHematite is one of the most stable and sustainable photoanode materials for solar water splitting that do not require any protection layer engineering. It has a suitable band gap of 2 eV and a well-aligned energetic position of its valence band for the oxidation of water. However, its breakthrough for application in a water splitting device is hampered by its poor charge collection efficiency. Charge transport limitations urge the photoanode design towards thin films deposited on high surface area nanostructured conductive scaffolds,1, 2 where recombination losses within the thin absorber material are minimized while light absorption can be adapted by varying the nanostructure thickness. Though thin hematite films deposited by, for example, ultrasonic spray pyrolysis have reached high photoactivities,3-5 this method is not very suitable to homogenously coat within a porous nanostructure. Thus, the ideal method is atomic layer deposition (ALD) that produces highly ordered thin films in a layer-by-layer deposition.6 Here, we present a study on the nature of the charge transport processes in hematite on high quality conformal thin films deposited via a novel low-temperature ALD route. We show that these hematite films are phase-pure, crystalline as-deposited and, therefore, fully photoactive without the need for post-annealing treatments. Implementing a functional underlayer, hematite thicknesses of less than 10 nm will be shown to yield the best charge collection efficiency and, in addition, remarkable photocurrents for films this thin. We will discuss the dependence of the optimal film thickness on the space charge layer width and hole diffusion length and will present a general understanding for the prediction of the optimal hematite film thickness.
References
1. Sivula, K.; Le Formal, F.; Grätzel, M. Chem. Mater. 2009.
2. Stefik, M.; Cornuz, M.; Mathews, N.; Hisatomi, T.; Mhaisalkar, S.; Gratzel, M. Nano Lett. 2012.
3. Steier, L.; Herraiz-Cardona, I.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Tilley, S. D.; Gratzel, M. Adv. Funct. Mater. 2014.
4. Hisatomi, T.; Dotan, H.; Stefik, M.; Sivula, K.; Rothschild, A.; Gratzel, M.; Mathews, N. Adv. Mater. 2012.
5. Hisatomi, T.; Le Formal, F.; Cornuz, M.; Brillet, J.; Tétreault, N.; Sivula, K.; Grätzel, M. Energy Environ. Sci. 2011.
6. Martinson, A. B. F.; DeVries, M. J.; Libera, J. A.; Christensen, S. T.; Hupp, J. T.; Pellin, M. J.; Elam, J. W. J. Phys. Chem. C 2011.
OO/NN Rump Session: Perovskite Based Photovoltaic and Optoelectronic Devices
Session Chairs
Tuesday PM, December 01, 2015
Hynes, Level 3, Ballroom B
OO5: Si and Group IV
Session Chairs
Tuesday AM, December 01, 2015
Hynes, Level 3, Room 302
9:30 AM - *OO5.01
Thin Silicon Solar Cells with Nanoscale Photon Management
Yi Cui 1
1Stanford Univ Stanford United States
Show AbstractSilicon solar cells have been dominating the PV industry. Thin silicon represents an exciting direction for future solar cells. Here I will show our recent progress on thin single-crystal Si down to 2 micron: fabrication of thin Si, nanoscale photon management, device physics and remarkable mechanical flexibility and robustness. We demonstrate 10 micron thick Si solar cells with 13.7% power efficiency.
10:00 AM - OO5.02
Down-Conversion Tb3+-Yb3+ Co-Doped SiNx Layers for Si Solar Cell Efficiency Improvement
Lucile Dumont 1 Julien Cardin 1 Florian Ehre 1 Christophe Labbe 1 Marzia Carrada 2 Andrea L. Richard 3 David C. Ingram 3 Jadwisienczak M Wojciech 4 Fabrice Gourbilleau 1
1CIMAP CNRS/CEA/ENSICAEN/UCBN Caen France2CEMES-CNRS Toulouse France3Ohio University Athens United States4Ohio University Athens United States
Show AbstractIncreasing solar cell (SC) efficiency while keeping low cost process is one of the big challenges that the silicon solar industry is facing. One of the solutions consists in tuning the light received by the SC for absorbing more IR photons thanks to a frequency conversion layer. This work focus on down-conversion layers (DCLs) which was deposited on top of SC for increasing the number of IR photons that energetically match the Si SC band gap. In general, the single DCL absorbs UV photons and converts them in two IR ones. This new distribution of photons maximizes the number of IR photons at the expense of UV one and allows to maximize photogenerated carriers with energy slightly above the SC band gap at the expense of hot carriers. This results in a decrease of the energy losses due to the thermalization of hot carriers in the SC. Previous study of our group on SiOxNy: Tb3+-Yb3+ [1] had shown promising results with achieving a quantum internal efficiency up to 183%. However, a silicon SC industry compatible process is required to be integrated in any SC system improvement. From this stand point, we have fabricated a DCL constituted of a SiNx matrix doped with Tb3+ and Yb3+ ions by reactive magnetron co-sputtering. Indeed, SiNx is already used in the Si solar industry as antireflective material which ensures its compatibility with Si SC and offers low cost production. In addition, removing the oxygen from the matrix allows the incorporation of rare earths without clusters formation that would limit the rare earth ions luminescence efficiency.
The optical properties of the undoped SiNx host matrix were investigated prior doping with Tb3+ ions. We have optimized the system obtained (via the deposition parameters) with the aim of achieving an intense photoluminescence (PL) emission from Tb3+ ions. Subsequently, Yb3+ ions were incorporated to generate an intense PL emission at 980 nm through a wide UV domain excitation. Quantum efficiency as high as ~200% has been obtained demonstrating the successful optimization of the excitation and energy transfer processes in developed DCL system. The cooperative energy transfer taking place between Tb3+-Yb3+ ions involved in the down-conversion is highly dependent on the distance between the two ions. Thus, in order to achieve an even higher quantum efficiency, a multilayer approach based on alternating stack of Tb- and Yb-doped sublayers has been developed to increase the number of excited Yb3+ ions. The effect, how this approach influences the properties of DCL frequency conversion process will be discussed in details during the presentation.
References:
[1] Y.-T. An, C. Labbé, J. Cardin, M. Morales, F. Gourbilleau, Highly Efficient Infrared Quantum Cutting in Tb3+- Yb3+ Codoped Silicon Oxynitride for Solar Cell Applications, Adv. Opt. Mater. 1 (2013) 855-862.
10:15 AM - OO5.03
Full Range SnxGe1-x Alloy Nanocrystals: Toward Direct Gap Group IV Semiconductors
Karthik Ramasamy 1 Jeffrey M. Pietryga 1 Sergei Ivanov 1
1Los Alamos Nat'l Laboratory Los Alamos United States
Show AbstractSuch elements of the group IV as silicon and germanium are the most common building blocks of modern electronic devices due to their desirable band gap values, abundance and the relative ease of processing. Relatively small band gaps of these materials have ensured their applications in IR detectors and solar cells, despite the indirect nature of their band gaps, which gives rise to the reduced ability to absorb light. It has been shown from theoretical analysis that Si or Ge alloying with tin would produce better absorbing direct band gap materials. However, miniscule solubility of Si and Ge in Sn due to large lattice mismatch and much higher cohesion energies of Si and Ge compared to that of Sn make alloying difficult to achieve in bulk. Nevertheless, the high surface-to-volume ratios and degree of curvature, available in small NCs, open up a unique possibility for a structural relaxation that reduces the strain from lattice inhomogeneity, making a wider range of alloys stable in the nanocrystalline form. Towards the goal of creating better absorbing Group IV materials, we have already achieved1 the synthesis of SnxGe1-x nanocrystals with x up to 0.4 and now we report on the synthesis of alloy nanocrystals with the full compositional range where tin content reaches 95%. X-ray diffraction, TEM and EDX mapping analysis have confirmed the presence of homogenous alloys over the completely compositional range. Absorption spectroscopy of NCs of varying composition demonstrates a pronounced red-shift and increase in molar absorptivity as a true indicator of solar relevance with increasing Sn-content. Although definitive evidence of a transition to direct-gap behavior remains elusive, weak photoluminescence is observed in a substantial fraction of NC samples in which Sn-content exceeds 35%. These results suggest that solution-processible NCs based on alloys of non-toxic and abundant Group IV elements are a potentially promising material system for solar energy capture applications. The details of the synthetic approach together with results of the structural and optical characterizations will be presented and discussed. The mechanistic insight about the reaction pathways leading to the formation of SnxGe1-x alloy was also used to attempt the synthesis of nanocrystalline SnxSi1-x analogue. Preliminary results of this study will also be discussed.
References:
1. Ramasamy, K.; Kotula, P. G.; Fidler, A. F.; Brumbach, M. T.; Pietryga, J. M.; Ivanov, S. A. Chem Mater., DOI: 10.1021/acs.chemmater.5b01041.
10:30 AM - OO5.04
Hot Carrier Relaxation and Transport Rates in Amorphous and Nanocrystalline Silicon
Tae-Ho Park 1 Reuben T. Collins 1 Craig Taylor 1 Susan L. Dexheimer 2 Pauls Stradins 3 Mark Thomas Lusk 1
1Colorado School of Mines Golden United States2Washington State University Pullman United States3National Renewable Energy Laboratory (NREL) Golden United States
Show AbstractSolar energy and optoelectronics applications of quantum confined silicon dots generally require an efficient means of collecting and distributing charge carriers. This has led us to consider quantum dots encapsulated within a hydrogenated amorphous silicon matrix that plays a critical role in photon collection, hot carrier cooling and transport. Cooling rates and transport mobilities are governed by phonon-assisted hopping events that we can now quantify in terms of nonadiabatic coupling (NAC). The approach provides significant advantages over more conventional methods of characterizing electron-phonon coupling for systems in which the electronic states are spatially localized. Dynamical effects associated with atomic motion and accurate excited electronic state energies are both accounted for. NAC is calculated using a new, parallelized Time-Dependent Density Functional Theory (TD-DFT) routine based on Density Functional Perturbation Theory (DFPT). This computational methodology is applied to predict cooling rates and transport mobilities for amorphous silicon, crystalline silicon and layered structures in which the morphologies are alternated. Tailoring the size and spacing of the nanocrystals modifies the interplay of electronic and vibrational states so as to optimize carrier transport.
10:45 AM - OO5.05
Optical Analysis of AgNW-TCO Hybrid Window Electrode for Silicon Heterojunction Solar Cells via Angular Matrix Framework
Zi Ouyang 1 Yang Li 1 Alison Lennon 1
1The University of New South Wales Sydney Australia
Show AbstractSilicon heterojunction solar cells with open-circuit voltages (Voc) above 700 mV and efficiencies above 25% have been reported and shown great potentials. The high voltages are due to the unique hydrogenated amorphous silicon (a-Si:H) layers that are capable of both passivating (intrinsic a-Si:H) and extracting (doped a-Si:H) the photogenerated carriers. Unlike the diffused emitters, the thin doped a-Si:H layer is not laterally-conductive enough for the charges to travel millimeters and be collected. As a result, a conductive window layer, usually transparent conductive oxide (TCO), is needed for on-site charge collection. Unfortunately, any conductive layer can absorb light parasitically and consequently, the high Voc is partially offset by the loss of short-circuit current density (Jsc). Therefore, it is of significance to seek for better window electrode materials that are highly transparent and conductive. The authors have demonstrated that silver nanowire (AgNW) network can be such a candidate with superior conductance and transmittance to TCOs. However, a very thin spacer layer of TCO underneath the AgNW network is still necessary. It is for the same reason that the sub-micron gaps between randomly-arranged NWs make the charges difficult to travel laterally.
Thickness of the TCO spacer layer, diameter of the AgNWs and density of the AgNW network are the key parameters affecting the optical and electrical performance of the proposed AgNW-TCO hybrid electrode. For better understanding and optimizing, an effective optical model is necessary. However, the challenge of modeling is that multiple optical structures are involved in the device, e.g., NWs (~ 100 nm), TCO (1 - 100 nm), a-Si:H (1 - 50 nm), textured surface (1 - 5 mu;m), wafer substrate (180 mu;m), etc. These structures are of massive scale difference from a few nanometers to hundreds of microns, making individual modeling methods incapable, i.e., the device is too computationally intensive for electrodynamic modeling (e.g., Finite-Difference Time-Domain method, FDTD) and too coarse for ray-based simulators (e.g., ray-tracer).
In this paper, we use a novel angular matrix framework (AMF) to resolve the challenge, where matrices are used to describe the transition of the angular distribution of the light when it is reflected or transmitted at the interface, or absorbed in the bulk. Three steps are involved in the AMF: (i) ray-tracing is used for textured surface and light propagation in the wafer substrate. The FDTD is implemented on the NWs and the thin-film coatings. (ii) The inputs to the models are decomposed from the angular matrices, and the outputs are composed to the matrices. (iii) The interactions of the adjacent optical structures are realized by matrix operations. The framework is capable of effectively integrating ray-tracing and electrodynamic simulation for the hererojunction solar cells with AgNW-TCO hybrid electrode and quickly providing optimizing guidance.
OO6: Photochemistry I
Session Chairs
Tuesday AM, December 01, 2015
Hynes, Level 3, Room 302
11:30 AM - *OO6.01
Artificial Photosynthesis: Progress, Science Outlook and Technology Prospects
Harry A. Atwater 1
1California Inst of Technology Pasadena United States
Show AbstractThe design of highly efficient, non#8208;biological, molecular#8208;level energy conversion “machines” that generate fuels directly from sunlight, water, and carbon dioxide is both a formidable challenge and an opportunity that, if realized, could have a revolutionary impact on our energy system. In the past five years, considerable progress has been made in scientific discovery of key materials and mechanisms needed to realize artificial solar fuels generators, and advances modeling and design have generated a now quite widely embraced conceptual paradigm for a solar fuels generator. While we still lack sufficient knowledge to design solar-fuel generation systems with the ultimate efficiency, scalability, and sustainability to be economically viable, considerable advances have been made, particularly for water-splitting solar fuels devices. I will also survey outstanding scientific challenges to realization of artificial photosynthesis for generation of fuels and chemicals by reduction of carbon dioxide.
12:00 PM - OO6.02
Thermally Activating Minority Carrier Hopping in Monolithic Si/SnO2/Mo:BiVO4 Photoanode for Solar Water Splitting
Xiaofei Ye 1 Liming Zhang 1 Madhur Boloor 1 Andrey D Poletayev 1 Nicolas A Melosh 1 William C. Chueh 1
1Stanford Univ Stanford United States
Show AbstractTransition-metal-oxide semiconductors are promising photoanodes for solar water splitting due to their excellent chemical stability and appropriate band gaps. However, in absorbers such as BiVO4, TiO2, α-Fe2O3, and WO3, charge carriers localize as small polarons, leading to low minority carrier mobilities. This limits the minority carrier collection efficiency in the quasi-neutral region of the light absorber, and lowers the overall photoactivity. In this work, we demonstrate that modestly elevating the temperature activates small-polaron hopping in monoclinic BiVO4, significantly enhancing the saturation photocurrent without a substantial anodic shift of the onset potential. Specifically, using a Si/SnO2/Mo:BiVO4 tandem photoanode/photovoltaic, increasing the absolute temperature by 11% from 10 to 42 °C elevates the saturation photocurrent from 1.8 to 4.0 mA cm-2. Concurrently, the onset potential shifts slightly from 0.02 V to 0.08 V versus the reversible hydrogen electrode (or equivalently, from -1.22 V to -1.13 V versus the equilibrium potential of oxygen evolution). Our observation contrasts with the prevailing understanding that decreasing photovoltage with temperature dominates the energy conversion efficiency. Thermally-activating minority carrier transport represents a general pathway towards enhancing the photoactivity of light absorbers exhibiting small-polaron conduction.
12:15 PM - OO6.03
Spontaneous Solar Water Splitting by Re-Grown Hematite and Amorphous Silicon
Chun Du 1 Ji-Wook Jang 1 Yumin He 1 Dunwei Wang 1
1Boston College Chestnut Hill United States
Show AbstractAs an economically competitive pathway to clean chemical fuel (hydrogen gas), photoelectrochemical (PEC) water splitting is a kinetically and thermodynamically difficult process. Spontaneous reaction needs a large photovoltage of 1.23 eV plus the overpotentials and energy losses due to the multi-electron transfer kinetics and non-ideal interfacial thermodynamics. Hematite (α-Fe2O3), with a suitable bandgap ca. 2.1 eV, represents a promising photoanode semiconductor material which is stable, inexpensive and nontoxic. Previously, performance of hematite in photoelectrochemical water oxidation was mainly impeded by its low photovoltage production. It was reported that structural defects and surface trap states are the primary reasons for the unnecessary potential loss at photoelectrode and electrolyte surface. Herein, a facile re-growth method is developed to reduce structural disorders and modify hematite surface. More specifically, nanostructured hematite is synthesized by post annealing FeOOH nanowire at 800 °C for 5 min. Afterward, when growth and calcination are repeated under the same conditions, surface disorder and defects are gradually reduced. When coupled with a layer of robust amorphous NiFeOx electrocatalyst, a greater photovoltage of 0.80 V and correspondingly low turn on voltage of 0.45 V (vs. reversible hydrogen electrode) are measured. X-ray diffraction, Raman spectrum and soft X-ray absorption analysis clearly show the structural difference of hematite with and without re-growth treatment in bulk as well as on the surface. This excellent performance allows us to construct unassisted PEC devices with hematite/NiFeOx as photoanode and amorphous-Si as photocathode to split water at an overall efficiency of 0.91%. This work successfully pushes the performance limit of hematite and points to a research direction for the study of semiconductor electrode and electrolyte interface.
12:30 PM - *OO6.04
Quantum Dot Luminescent Concentrators
A. Paul Alivisatos 1
1University of California, Berkeley Berkeley United States
Show AbstractBright and photo-stable quantum dots with narrowband emission can be used as downconverters in a new generation of luminescent concentrator. Such concentrators can be integrated with existing solar cell technologies to provide improvements in performance. This talk will describe recent programs in implementing such a design.
Symposium Organizers
Vivian Ferry, University of Minnesota
Ali Javey, University of California Berkeley
Evelyn Wang, Massachusetts Institute of Technology
Jia Zhu, Nanjing University
OO11: Solar Thermal
Session Chairs
Wednesday PM, December 02, 2015
Hynes, Level 3, Room 302
2:30 AM - *OO11.01
Large-Scale Nanophotonic Solar Selective Absorbers for High-Efficiency Solar Thermal Energy Conversion
Sheng Shen 1
1Carnegie Mellon Univ Pittsburgh United States
Show AbstractSolar thermal energy conversion is superior to traditional solar photovoltaics because it can utilize nearly the entire solar spectrum, thus enabling higher energy conversion efficiency. Solar absorbers are generally required for any solar thermal technologies in order to convert solar flux into heat. To maximize the conversion efficiency, ideal solar absorbers should exhibit near-blackbody absorption in the solar spectrum while suppressing infrared emission at elevated temperatures. However, the development of such solar selective absorbers has been hurdled by the challenges associated with their spectral performance, thermal stability for high temperature applications, and manufacturing cost. Here, we demonstrate large-scale nanophotonic solar selective absorbers with excellent solar selective absorptivity and thermal stability. The measured absorptivity/emissivity of the nanophotonic absorbers has spectrum selection with ~ 95 % solar absorptivity and ~10 % emissivity in the infrared range. The fabricated structures demonstrate stable spectral performance at 800 oC. Due to the three-dimensional nanophotonic design, the excellent selective absorption maintains for a wide range of incident angles +/- 50 degree , which indicates the omnidirectional absorption of the solar absorbers.
3:00 AM - OO11.02
Hybrid PV and Thermal Solar Receiver Using Silica Aerogel and Thin-Film Multi-Layer Spectral Splitting
Lee Weinstein 1 James Loomis 1 Xiaopeng Huang 1 Sungwoo Yang 1 Lin Zhao 1 Yi Huang 1 Feng Cao 2 Tianyi Sun 2 Bikram Bhatia 1 David Bierman 1 Elise Strobach 1 Wei-Chun Hsu 1 George Ni 1 Lu Tang 2 Svetlana Boriskina 1 Zhifeng Ren 2 Evelyn Wang 1 Gang Chen 1
1MIT Cambridge United States2University of Houston Houston United States
Show AbstractTwo promising methods to convert sunlight to electricity are through the use of photovoltaics (PV) and concentrating solar power (CSP), however each method has distinct drawbacks. The large reduction in manufacturing costs for PV cells over the past few years is well-known; however, single-gap PV cells cannot utilize solar photons with energy below the band gap and convert photons far above the band gap very inefficiently. Additionally, storage of PV-generated electricity for use at off-peak times remains expensive using current technologies. On the other hand, CSP utilizes the entire solar spectrum and can be paired with inexpensive thermal energy storage; however, the overall cost and efficiency of CSP plants are not competitive with PV. We propose a novel, high efficiency, hybrid (combined PV and thermal) solar receiver to optimally utilize the full solar spectrum, which is enabled due to high performance silica aerogels and spectral splitting using a thin-film multi-layer coating. Silica aerogels are highly porous matrices made from agglomerated nanometer scale particles. Aerogels have exceptionally low thermal conductivity due to the low solid volume fraction and the small pore sizes suppressing convection and limiting gas molecule mean free path length. With proper synthesis, silica aerogels can also be made very transparent in the solar spectrum. The low thermal conductivity and high transparency allows sunlight in, but minimizes radiative and convective heat losses out, enabling efficient conversion of sunlight to high temperature thermal energy. For spectral splitting, a thin-film multi-layer stack of alternating high and low index of refraction materials allows targeted reflection of a narrow spectral band where PV conversion is most efficient. Our investigations into the proposed hybrid solar receiver suggest that it could produce electricity at a lower levelized cost of energy than current state of the art CSP technologies and advance the adoption of dispatchable solar energy.
This work was funded by Advanced Research Projects Agency - Energy (ARPA-E) under award number DE-AR0000471 for development of a Full Spectrum Stacked Solar-Thermal and PV Receiver.
3:15 AM - OO11.03
Integrated Photonic Crystal Selective Structures for Solar Thermophotovoltaics
Zhiguang Zhou 1 Enas Sakr 1 Peter Bermel 1
1Purdue Univ West Lafayette United States
Show AbstractSolar thermophotovoltaic (TPV) systems convert sunlight into electricity via thermal radiation. The efficiency of this process depends critically on both the selective absorber and the selective emitter, which are controlled by both the materials and the photonic design. For high concentration solar TPV applications, 2D photonic crystals made of refractory metals such as tungsten have demonstrated promising results. On the other hand, it is difficult to exceed world record efficiencies for photovoltaics with such an approach. Thus, we propose an integrated photonic crystal selective structure (IPSS), which combines 2D photonic crystals and filters into a single device to both collect solar heat and reradiate above-gap photons. Finite difference time domain (FDTD) and current transport simulations show that an IPSS can significantly suppress sub-bandgap photons. This increases sunlight-to-electricity conversion for photonic crystal-based emitters above 40% at 1000 suns concentration using a Ga0.47In0.53As PV diode with a bandgap of 0.74 eV (lattice-matched to InP). The physical basis of this enhancement is a shift from a perturbative to a non-perturbative regime that maximizes photon recycling. Furthermore, combining an IPSS with non-conductive optical waveguides eliminates a key difficulty associated with solar TPV: the need for precise alignment between the solar-heated selective emitter and the cool PV diode.
4:30 AM - OO11.04
Strong Photon Absorption in 2-D Material-Based Spiral Photovoltaic Cells
Mohammad Tahersima 1 Volker J. Sorger 1
1George Washington Univ Washington United States
Show AbstractRecent investigations of semiconducting two-dimensional (2D) transition metal dichalcogenides have provided evidence for strong light absorption relative to its thickness. This can be attributed to high density of states in these atom-thing material systems. Stacking a combination of metallic, insulating, and semiconducting 2D materials enables functional devices while keeping the overall volume small. While photovoltaic cells based on 2D materials have been demonstrated, the reported absorption is still just a few percent of the incident light due to their sub-wavelength thickness leading to low cell efficiencies. Here we show that taking advantage of the mechanical flexibility of 2D materials by rolling a molybdenum disulfide (MoS2)/graphene (Gr)/hexagonal boron nitride (hBN) stack to a spiral solar cell allows for optical absorption up to 90%. That is, we investigate the optical absorption of a 1 um-long hetero-material spiral cell consisting of the aforementioned hetero stack forming an optical nanocavity. Our results show that the absorption is about 50% stronger compared to a planar MoS2 cell of the same thickness despite the volumetric absorbing material ratio being only 6%. We find that a core-shell structure exhibits enhanced absorption and pronounced absorption peaks with respect to a spiral structure without metallic contacts. A study of the eigenmodes and group-index of the nanocavity confirms the enhancement of the light-matter interactions by the optical cavity. However, a spectral analysis shows that the core-shell nanocavity design provides the aforementioned high absorption over a several hundreds of nanometer spectral bandwidth suitable for solar cell applications. In conclusion, we anticipate these results to provide guidance for photonic structures that take advantage of the unique properties of 2D materials in solar energy conversion applications.
4:45 AM - *OO11.05
Nanomaterials-Based Solar-Thermal Technologies
Gang Chen 1 Daniel Kraemer 1 Lee Weinstein 1 James Loomis 1 George Ni 1 Jonathan Tong 1 Yi Huang 1 Svetlana Boriskina 1
1MIT Cambridge United States
Show AbstractSolar energy holds a great deal of promise as the best clean energy source for human beings, but its increasing utilization depends on the development of technologies that are cost-competitive with other energy sources. Cost reduction can be achieved by either incrementally improving existing technologies or by inventing disruptive new approaches. This talk will give a few examples from our research of solar technology approaches which aim at reducing the cost of solar energy systems using nanostructured materials. For example, efficient solar steam generation is possible with the addition of a porous absorber that localizes solar energy at the surface of water. This technology has potential applications in both clean water and power generation. The second technology is an aerogel-based solar receiver for concentrated solar power plants, with the aim of high-temperature operation under atmospheric conditions. The third example is solar thermoelectric generation devices, which are a solid-state technology that also have the advantage of utilizing the full solar spectrum. The final approach I will discuss is thermophotovoltaic energy conversion in both the near and far fields. By extending the concept of quantum-wells from electrons to photons, we show that thin-film based thermophotovoltaic cells can achieve high efficiency. The fundamental science guiding and stimulating the development of these technologies will be elaborated.
5:15 AM - OO11.06
Self-Assembled Nanostructured Plasmonic Absorbers for Solar Steam Generation
Lin Zhou 1 Yingling Tan 1 Jia Zhu 1
1Nanjing University Nanjing China
Show AbstractPlasmonic absorbers, which can absorb light efficiently through metallic nanostructures, have attracted a lot of attention due to various solar harvesting applications. However, up to now, most of the reported metallic absorbers are predominantly fabricated by the top down approach, the performance and scalability of which need to be further improved to enable wide spread applications. In this work, we report a plasmonic absorber fabricated by a bottom up fabrication process, which can enable an average measured absorbance of ~ 99% across the wavelengths from 400 nm to 20 µm, the most efficient and broadband plasmonic absorber reported so far. The bandwidth and absorption efficiency of the proposed absorber can be structurally tuned from visible to infrared region. An optimized absorber is fabricated through self-assembly of gold nanoparticles onto a nanoporous template by one step deposition process. Because of its efficient light absorption and strong field enhancement, it can enable solar steam generation with over 90% efficiency under solar irradiation of 4 kWm-2. The proposed design of ideal plasmonic absorbers can be applied to other plasmonic metals, such as aluminum. The pronounced light absorption effect coupled with the high throughput self-assembly process can lead towards large scale manufacturing of other novel nanophotonic structures and devices. Our absorbers may be good candidate for many applications, such as solar steam generation, thermophotovoltaics, as well as light/thermal detectors.
5:30 AM - OO11.07
Solar Filtering Applications of Indium Tin Oxide Nanoparticles in Heat Transfer Fluids
Ebrima Tunkara 1 Mit Muni 1 Drew DeJarnette 1 Todd Otanicar 1 Kenneth Roberts 1
1University of Tulsa Tulsa United States
Show AbstractWe investigated the use of indium tin oxide (ITO), a semiconductor nanoparticle, as a spectral filter for use in concentrated GaAs photovoltaic (PV) cells. The synthesized nanoparticles filter high energy UV and infrared photons that usually degrade PV performances. The energy of the absorbed solar radiation will be transferred to a heat transfer fluid for photothermal energy conversion. Synthesis of ITO nanoparticles tailored to achieve optimal optical performance while remaining stable in a heat transfer fluid is ncessary. Particles were synthesized using a one-pot synthesis with indium acetylacetonate (In(acac)3, tin bis (acetylacetonate) dichloride (Sn(acac)2Cl2) , and myristic acid (CH#8323;(CH#8322;)#8321;#8322;COOH) as the starting materials and oleylamine as a reducing agent and surface passivation ligand. In order to enhance the solubility and stability of the as-synthesized ITO particles in heat transfer fluids, different strategies such as ligand exchange, silica encapsulation, and synthesis with alternate passivating ligands were employed. Characterization of the nanoparticles was accomplished by Fourier transform infrared spectroscopy, UV-Visible spectrometry, dynamic light scattering, scanning electron microscopy, and solution stability tests at various temperatures.
5:45 AM - OO11.08
Using Photonic Nanostructures to Modify the Optoelectronic Response of Photovoltaic Materials
Yulu Xu 1 Taqiyyah Safi 1 Jeremy N. Munday 1
1Univ of Maryland College Park United States
Show AbstractOptical nanostructures are well known to for their ability to absorb and scatter light. However, by modifying the absorption in a semiconductor, one can also modify the electronic properties of the device, include the voltage response. Here we will discuss ways in which we can surpass the efficiency limit of traditional photovoltaic devices using novel new methods involving nanoscale optical concentration [1], photonic crystals to effectively modify the semiconductor bandgap [2], and modified radiative thermal emission [3]. We will show our recent experimental results involving the use of photonic materials to modify the effective semiconductor bandgap of a GaAs solar cell and how nanoscale optical concentration has the potential to increase cell efficiencies to >40%. Finally, we will show that by combining specifically designed radiative cooling structures with solar cells, higher power conversion efficiencies can be obtained for solar cells in both terrestrial and extraterrestrial environments. Specifically, we show that a solar cell can be improved by up to 4% absolute efficiency in an extraterrestrial environment in near-earth orbit using radiative emission. By combining these strategies, we will layout a pathway to achieving ultra-high efficiency solar cells based purely of photonic design considerations.
[1] Yunlu Xu and Jeremy N. Munday, IEEE Journal of Photovoltaics, 4, 233 - 236 (2014)
[2] Yunlu Xu, Tao Gong, and Jeremy N. Munday, Scientific Reports, in press (2015)
[3] Taqiyyah Safi and Jeremy Munday (in review)
OO9: Perovskite Solar Cells II
Session Chairs
Wednesday AM, December 02, 2015
Hynes, Level 3, Room 302
9:00 AM - *OO9.01
Organic-Inorganic Perovskites: Opportunities and Challenges for Photovoltaic Materials Design
David B. Mitzi 1 Bayrammurad Saparov 1 Hsin-Sheng Duan 1 Feng Hong 2 Weiwei Meng 2 Yanfa Yan 2
1Duke Univ Durham United States2University of Toledo Toledo United States
Show AbstractOrganic-inorganic perovskites have recently attracted great interest for use in solar cell technology, because of the high carrier mobilities, long minority carrier lifetimes, tunable band gaps and relatively benign defects and grain boundaries for systems based on Group 14 metals (e.g., Ge, Sn and Pb). Indeed, these materials have enabled unprecedented rapid improvement in device performance to levels above 20% power conversion efficiency and with open circuit voltages above 1V for a single junction photovoltaic (PV) device. This talk will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight and review the outstanding chemical flexibility and potential of the broader class of 3-D and lower-dimensional organic-based perovskite family. Challenges for this materials class include replacing lead with more environmentally benign metals, improving PV device stability (moisture, UV and air) and controlling hysteresis. The concept of a multifunctional organic-inorganic hybrid, in which the organic and inorganic structural components are well ordered on the nanoscale and provide unique and hopefully synergistic features to the compound, represents another important contemporary target. Further exploration within this structural and chemical space is expected to yield important opportunities for broader energy materials design and fundamental understanding.
OO12: Poster Session I: Solar Energy Conversion I
Session Chairs
Wednesday PM, December 02, 2015
Hynes, Level 1, Hall B
9:00 AM - OO12.01
Ultrafast Charge Dynamics in Trap-Free and Surface-Trapping Colloidal Quantum Dots
Charles T Smith 1 2 Marina A Leontiadou 1 2 Robert Page 1 Paul O'Brien 1 David J Binks 1 2
1University of Manchester Manchester United Kingdom2Photon Science Institute Manchester United Kingdom
Show AbstractColloidal quantum dots (CQD) are promising as the light absorbing species in next generation photovoltaic solar cells and as the luminophores in light emitting devices, and bio-labelling. However, in CQDs the photoluminescence quantum yield, charge extraction efficiency and lifetime of photo-generated charges are often limited by surface-mediated non-radiative recombination, which can occur on a sub-nanosecond time-scale. Treatment with halide ions has emerged recently as a particularly effective method of surface passivation resulting in significant improvements in device performance, such as the demonstration of record efficiency for a CQD-sensitised solar cell.
In this study ultrafast transient absorption spectroscopy is used to study sub-nanosecond charge dynamics in CdTe colloidal quantum dots. After treatment with chloride ions, these can become free of surface traps that produce non-radiative recombination. This enables direct observation of the effects of surface trapping by comparing CQD charge dynamics before and after treatment. The surface-traps typically increase the rate of electron cooling to the band edge by 70% and introduce a recombination pathway that depopulates the conduction band minimum of single excitons on a sub-nanosecond timescale, regardless of whether the sample is stirred or flowed. It is also shown that surface-trapping significantly reduces the peak bleach obtained for a particular pump fluence and introduces a rapid sub-nanosecond decay from the peak bleach, which has important implications for the interpretation of transient absorption data, including the estimation of absorption cross-sections and multiple exciton generation yields.
9:00 AM - OO12.02
Solution-Based Synthesis and Optical Property Evaluation of Cu2ZnSn(S1-xSex)4 Nanoparticles
Jhon Lehman Cuya Huaman 1 Keisuke Kitagishi 1 Tsuyoshi Akiyama 1 Hiroshi Miyamura 1 Maurice Nuys 2 Stefan Muthmann 2 Balachandran Jeyadevan 1
1The University of Shiga Prefecture Hikone Japan2Forschungszentrum Juelich Juelich Germany
Show AbstractInorganic semiconductors such as Cu2ZnSnS4(CZTS) have gained a lot of attention as low-cost solar cell materials and for optimal use of the visible spectrum, Cu2ZnSn(S1-xSex)4(CZTSSe) has been proposed. Although the main advantage of this kind of solar cell materials is the use of earth abundant elements, the synthesis of highly pure CZTSSe through of chemical routes has been a challenge and the optimal Se/S ratio for suitable band gap has not been experimentally verified. Here, we report the results of the study undertaken to evaluate the composition, structure and optical properties of CZTSSe nanoparticles with different Se/S ratio using a solution-based technique. For synthesizing the semiconducting nanoparticles, metal precursors such as copper(II) acetate, zinc acetate dihydrate, tin(II) chloride dihydrate, and the chalcogenide sources, sulfur and selenide mixed at different molar ratios, were introduced in oleylamine and heated at 260#730;C for 300 min under an N2 atmosphere. The nanoparticle diameters ranged between 10-15 nm and the Se/S ratio between 0 and 1. The as-synthesized CZTSSe particles with varying S/Se ratios exhibited band gap values lower, 0.7-1.1 eV, than the theoretical values. However, the discrepancy was reduced when the particles were heat treated at temperatures higher than 300 oC in sulfur rich-atmospheres. Thus, the band gap value shifted to high energy region, 1.14-1.27 eV, whereas the composition remained invariable. On the other hand, XRD pattern shows that as-prepared CZTSSe matched very well with stannite/kesterite type structure; although the (002) and (101) peaks were not observed, simulated measurements for perfect kesterite crystals indicate the presence of both peaks at around 17° and 18°, respectively. However, these peaks appeared when annealed and considered to have enhanced the crystallinity of the powders. Consequently, we found that the crystallinity of CZTSSe has a strong effect on the optical properties, and this could be achieved by subjecting these particles to annealing at higher temperatures.
9:00 AM - OO12.03
Stable Fiber-Shaped Dye-Sensitized Solar Cells Based on Eutectic Melts
Houpu Li 1 Longbin Qiu 1 Hao Sun 1 Huisheng Peng 1
1Fudan University Shanghai China
Show AbstractAs energy issue draw more and more attention, Dye sensitized solar cells (DSSC) had been reported to be a promising candidate for industrial application for its high efficiency as well as low cost. Yet the electrolyte using organic solvent suffer from problems like leakage and vaporization. Since working condition of solar cells would be under strong light intensity and temperature up to 60 °C, and encapsulation material would easily fail due to the aging, it would be a great challenge for stability of electrolytes in DSSCs. For fiber-shaped solar cell, which is most likely to industrialize thanks to the tide of wearable electronics, this problem is even more severe. Solid-state electrolytes are resistive to these problems in nature, and have been shown to have high stability. Herein, we reported a kind of solid state electrolyte made from eutectic melts of ionic liquid crystal, combining different ionic liquid, which is solid-state and non-volatile, providing good ionic conductivity without any volatile solvent. Besides acceptable efficiency, it provides extraordinary stability, solar cells using these electrolytes can recover full efficiency even after heated to 110 °C, and have excellent stability for different storage and working condition. Long time performance of the fiber-shaped solar cell was greatly improved.
9:00 AM - OO12.04
Deciphering the Interface and Structure of Graphene-TiO2 Composite Materials
Brandon Bukowski 2 Nathaniel Aaron Deskins 1
1Worcester Polytechnic Institute Worcester United States2Purdue University West Lafayette United States
Show AbstractTiO2-graphene photoactive composites show promise for a wide variety of environmental applications1, including pollutant degradation, water treatment, and energy/fuel production. When combined with graphene, TiO2 shows increased photocatalytic activity, which has been attributed to a variety of reasons1. Narrowing of the band gap of TiO2 into the visible region may occur due to formation of TiO2-C moieties. The large surface area of graphene may facilitate surface reactions between adsorbates and catalyst. Finally, increased charge separation (which lowers charge recombination rates, the opposite of photoexcitation) may occur as electrons are shuttled away by graphene. Still many questions exist for these materials. For instance, experimental evidence suggests that graphene with low number of defects makes for an ideal TiO2-graphene photocatalyst2. However, many popular production methods involve reduction of graphene oxide, which may produce graphene with many defects. It is crucial to identify the role of such defects in these photocatalysts. Other work also suggests that intimate contact between TiO2 and graphene will increase catalytic activity3.
We present density functional theory simulations of TiO2-graphene systems in order to elucidate the nature of the interface between the two materials. We have specifically focused on TiO2 clusters up to 1 nm in size and rutile (110) surfaces. We considered graphene as defect-free or with common defects such as hydroxyls, vacancies, or epoxides. Our results indicate that significant binding occurs at defect sites, much more so than over pristine graphene. Most notably defects on the graphene surface lower the band gap of TiO2, indicating that new composite electronic states may arise and increase photoexcitation yield in such materials. We also analyzed how surface-surface distance (indicative of degree of contact between graphene and TiO2) affects the binding and TiO2 band gaps. Our results highlight the potential role of the graphene surface state in TiO2-graphene photocatalyst materials, and suggest design strategies for graphene-based composite materials based on the TiO2-graphene interface structure.
(1) An, X. Q.; Yu, J. C. Rsc Advances2011, 1, 1426.
(2) Liang, Y. T.; Vijayan, B. K.; Gray, K. A.; Hersam, M. C. Nano Letters2011, 11, 2865.
(3) Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Physical Chemistry Chemical Physics2012, 14, 9167.
9:00 AM - OO12.05
SiNx Dielectric Layer Coating for Light Trapping and Crucial Carrier Collection in Nanostructured Si Solar Cells
Yunae Cho 1 Dong-Wook Kim 1 Minji Gwon 1 Eunah Kim 1 Joondong Kim 2 Hyeong Ho Park 3
1Ewha Womans University Seoul Korea (the Republic of)2Incheon National University Incheon Korea (the Republic of)3Korea Advanced Nano Fab Center (KANC) Suwon Korea (the Republic of)
Show AbstractThe nanostructured Si solar cells are expected to overcome the limit of conventional light trapping strategies such as anti-reflection coating and surface texturing. In spite of superior optical absorption gain, the nanostructured solar cells often exhibit poor energy conversion efficiency, mostly due to the surface recombination-induced electrical loss. In this work, we have investigated influence of SiNx dielectric layer coating on photovoltaic (PV) performance of Si absorbers with surface hexagonal nanoconical frustum (NCF) arrays. We fabricated the NCF array solar cells using nanoimprint lithography and achieved a very high photocurrent of 36.94 mA/cm2. The SiNx layer significantly reduced the surface recombination rate and improved the quantum efficiency QE of the nanostructured cells. The Mie resonance of the NCF array enhanced the optical absorption in broadband and also increased near surface optical intensity. Such high density of charge carriers near the surface may lead to large recombination losses, which often can lead to poor PV performance of the nanostructured cells, in spite of the notable optical gain. Therefore, proper surface passivation via the deposition of a dielectric layer is crucial to exploit the enhanced optical absorption of the nanostructured solar cells.
9:00 AM - OO12.06
Dual-Functional Photoelectrochemical Power Sources for Energy Efficient Glass Facades
Michele Manca 1 Mara Serrapede 1 Roberto Giannuzzi 1 Praveen Pattathil 1 Riccardo Scarfiello 1
1Istituto Italiano di Tecnologia Lecce Italy
Show AbstractDespite the great market expectations for buiding-integrated photovoltaics sector, glass-on-glass mass production of dye-sensitized solar cells (DSSCs) is still under development and companies involved are trying to find compatible solutions of technical (device optimization in terms of production cost, long term stability and performances) and marketing issues (best product for smart windows in terms of aestethic and perfomances). The availability of affordable, efficient and easily up-scalable photovoltaic (PV) technologies is a pivotal challenge in view of an upcoming diffusion of a next generation of building-integrated energy conversion systems.In this respect, during the last two decades, DSSCs have attracted widespread academic and industrial interest because, in comparison to silicon based technologies, they can be manufactured using low cost materials and relatively easy fabrication processes, often borrowed from the printing industry. These issues alone, however, do not constitute a sufficient and effective driving force toward the full scale industrialization of DSSC technology. Competition with silicon-based PV technologies in terms of cost-effectiveness has become hardly sustainable. The natural technological evolution of this class of devices should be represented by an integrated photovoltaic powered electrochromic (PV-EC) window, also considering that the operational characteristics of both PV and EC technologies are highly compatible. A small area of PV cells could in fact provide sufficient electric power to operate a large-area EC window.
A photovoltachromic cell (PVCC) may potentially act as a complex artificial skin, by generating electric energy as a photovoltaic system but even ‘‘perceiving&’&’ small variations in external radiation and controlling the energy fluxes by means of a smart variation of their optical transmittance. Incoming solar light is partially harvested by a dye-sensitized photoelectrode made on the front glass of the cell which fully overlaps a bi-functional counter. When the cell is illuminated, the photovoltage drives electrons into the electrochromic stripes through the photoelectrochromic circuit and promotes the Li+ diffusion towards the electrochromic region, which thus turns in its colored state. At the same time an efficient photovoltaic functionality is executed by the catalytic region. A photocoloration efficiency of about 20 cm2*min-1*W-1 has been demonstrated at low illumination intensities along with fast response (coloration time < 30 sec and bleaching time < 60 sec). Regarding the PV functionality, a power conversion efficiency ranging from 2% to 5% is achievable depending on the light absorbing prerogatives of the dye-sensitized electrode as well as on the specific electrolyte&’s chemical composition.
9:00 AM - OO12.07
A Facile Strategy to Fabricate TiO2 Nanostructures with Controllable Crystalline Polymorphs and Morphologies and Their Applications: Photoelectrochemical Cell and Field Emission
Mingi Choi 1 Zhuo Zhang 1 Kijung Yong 1
1POSTECH Pohang Korea (the Republic of)
Show AbstractTiO2 has been considered as promising materials for optoelectrical applications including photovoltaic cells, and dye-sensitized solar cells, because it is low cost materials with appropriate bandgap, high photocatalytic activity, and chemical durability. TiO2 has three major crystalline polymorphs : rutile(tetragonal, space group: P42/mnm), anatse(tetragonal, space group: I41/amd), and brookite(orthorhombic, space group: Pbca). Rutile is a stable phase, whereas anatase and brookite are metastable phases which can be transformed to rutile when annealed. Research on Brookite has been less focused on compared to anatase and rutile, because brookite rarely exists in nature, and it is difficult to synthesize.
In this research, various TiO2 nanostructures were synthesized on titanium foil, by facile one-step hydrothermal reaction. Cleaned titanium foil was placed on Teflon-lined stainless steel autoclave filled with 0.5M NaOH aqueous solution. After the reaction at 220&’C, the titanium foil was immersed in 0.1M HCl solution to replace Na+ ion to H+ ion. Then, the titanium foil was put in a muffle furnace for heat treatment. Synthesized TiO2 can have four morphologies which are sheet, tube, wire, and bullet shape, depending on the experimental parameter such as reaction temperature, reaction time and concentration of the additive. Nanowire structure was synthesized at 220&’C with 0.5M NaOH. Nanosheet structure was synthesized at lower temperature, and nanotube and bullet structure was synthesized at lower concentration of additive.
The morphology was observed using SEM and TEM. Nanosheet and nanotube structure were transformed to nanowire after appropriate reaction time. Average length of nanowires could be modified by changing the reaction time, while the bullet structure didn&’t change much depending on the reaction time. The crystalline structure was confirmed by SAED pattern of TEM and XRD analysis. As a result of TEM and XRD analysis, we found wire, sheet, and tube structures were anatase, and bullet structure was brookite.
The photoelectrochemical properties of various TiO2 nanostructured electrodes were studied in a three electrode cell with Ag/AgCl and Pt as the reference and counter electrodes. Linear sweep was conducted under chopped illumination (AM 1.5G, 100mW/cm2). Photocurrents had different values depending on their morphologies. The brookite had the highest photoelectrochemical properties compared to other morphologies of anatase with similar length.
The field emission properties which can be affected by the morphologies of emitter of various TiO2 nanoarrays were measured in vacuum chamber. Diverse length of the TiO2 nanowires and nanotubes showed different field emission properties, and they were analyzed and optimized by theoretical calculation with “Zero Thickness Charge Disc (ZTCD)” model.
9:00 AM - OO12.08
Dynamics of Charge Carrier Trapping and De-Trapping in Electronically Coupled PbS Nanocrystal Solids
Rachel Hoffman Gilmore 1 William Tisdale 1
1MIT Cambridge United States
Show AbstractLead sulfide (PbS) colloidal nanocrystals (NCs) are a promising material for infrared optoelectronic devices. Understanding charge transport in solids made from these materials is essential to designing more efficient devices. Current devices are believed to be limited by surface defects that act as recombination centers and trap charge carriers before they can be extracted. This work uses transient absorption and photoluminescence spectroscopies to monitor the trap state and band edge charge carrier populations in films made from highly monodisperse (<5%) PbS NCs. Charges in strongly coupled solids are found to escape trap states that are located below the band gap by several times the thermal energy. The effects of NC size, sample temperature, and ligand length on the TA and PL spectra are studied to understand the activation mechanism and role of charge transfer in allowing charge carriers to escape trap states.
9:00 AM - OO12.09
Fine-Tuning Optical and Electronic Properties of Graphene Oxide for Highly Efficient Perovskite Solar Cells
Abd Rashid Mohd Yusoff 1 Jin Jang 1
1Kyung Hee University Seoul Korea (the Republic of)
Show AbstractSimplifying the process of fine-tuning the electronic and optical properties of graphene oxide (GO) is of importance in order to fully utilize it as the hole interfacial layer (HIL). We introduced silver trifluoromethanesulfonate (AgOTf) inorganic chemical dopant that tune and control the properties of single-layer GO films synthesized by chemical vapor deposition. The morphology, work function, mobility, sheet resistance, and transmittance of the GO film were systematically tuned by various doping concentrations. We further developed solution-processable low-temperature hole interfacial layer (HIL) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS):AgOTf-doped GO HIL in highly efficient perovskite solar cells. The PEDOT:PSS:AgOTf-doped GO HIL grants desirable charge-collection in the HIL allowing the entire device to be prepared at temperatures less than 120 #730;C. The fabricated perovskite solar cells utilize rigid substrate demonstrate compelling photovoltaic performance with a power conversion efficiency (PCE) of 11.90%. Moreover, flexible devices prepared using a polyethylene terephthalate (PET)/ITO demonstrate a PCE of 9.67%, while ITO-free flexible devices adopting PET/aluminum doped zinc oxide (AZO)/silver (Ag)/AZO demonstrate a PCE of 7.97%. This study shows that PEDOT:PSS:AgOTf-doped GO HIL has significant potential to contribute to the development of low-cost solar cells.
9:00 AM - OO12.10
Effects of Surface Passivation on the Optical Properties and Charge Dynamics of Colloidal Quantum Dots
Marina Leontiadou 1 Charles T Smith 1 Robert Page 2 Daniel Espinobarro-Velazquez 1 Edward Lewis 3 Sarah Haigh 3 Hanna Radtke 1 Atip Pengpad 1 Federica Bondino 5 Elena Magnano 5 Igor Pis 5 Marco Califano 4 Wendy Flavell 1 Paul O'Brien 2 David J Binks 1
1University of Manchester Manchester United Kingdom2University of Manchester Manchester United Kingdom3University of Manchester Manchester United Kingdom4University of Leeds Leeds United Kingdom5Laboratorio Nazionale TASC Basovizza Italy
Show AbstractColloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion [1-3]. The efficacy of CQDs in applications such is light emitting diodes and solar cells is reduced by the non-radiative recombination associated with surface traps [4,5]. Thus understanding and suppression of “unwanted” surface-related recombination is essential in order to maximize the performance of devices incorporating CQDs. With a post-synthetic ion chloride treatment (without formation of any thick inorganic shell) of CdTe CQDs we succeed a near-complete suppression of surface trapping that resulted in almost completely suppression of non-radiative decay pathways and near-unity photoluminescence (PL) quantum yield [6]. We performed a systematic investigation where the effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The chloride was shown to reside largely on the surface of the quantum dots, and does not affect crystallinity.
The effect on the recombination dynamics was also investigated with or without chloride passivation and after air exposure [7]. Radiative recombination was found to be dominant in the passivated CQDs and the radiative lifetime scales linearly with the CQD volume. Non-radiative recombination contributions appear for non passivated sample cases and after air exposure. Sample-dependent lifetimes ranged from less than 1 ns to 10s of ns, while the non-radiative dynamics can be explained by an atomistic model for Auger-mediated trapping of holes in surface oxidized CQDs. The chloride treatment also significantly increased the stability of the CdTe quantum dots when exposed to the air, which will facilitate their inclusion in many optoelectronic devices.
References
[1] A. H. Ip , S. M. Thon , S. Hoogland , O. Voznyy , D. Zhitomirsky , R. Debnath , L. Levina , L. R. Rollny , G. H. Carey , A. Fischer , K. W. Kemp , I. J. Kramer , Z. Ning , A. J. Labelle , K. W. Chou , A. Amassian , E. H. Sargent , Nat Nano 2012 , 7 , 577.
[2] E. H. Sargent, Nat Photon 2012 , 6 , 133.
[3] P. V. Kamat, J. Phys. Chem. Lett. 2013 , 4 , 908.
[4] K. Katsiev, A. H. Ip, A. Fischer, I. Tanabe, X. Zhang, A. R. Kirmani, O. Voznyy, L. R. Rollny, K. W. Chou, S. M. Thon, G. H. Carey, X. Y. Cui, A. Amassian, P. Dowben, E. H. Sargent, O. M. Bakr, Adv. Mater. 2014, 26, 937- 942.
[5] D. Zhitomirsky, O. Voznyy, S. Hoogland, E. H. Sargent, ACS Nano 2013, 7, 5282 -5290.
[6] Robert C. Page , Daniel Espinobarro-Velazquez , Marina A. Leontiadou , Charles Smith , Edward A. Lewis , Sarah J. Haigh , Chen Li , Hanna Radtke , Atip Pengpad , Federica Bondino , Elena Magnano , Igor Pis , Wendy. R. Flavell , Paul O&’Brien , and David J. Binks, Small 2015, 11, No. 13, 1548-1554.
[7] Daniel Espinobarro-Velazquez, Marina A. Leontiadou, Robert C. Page, Marco Califano, Prof. Paul O'Brien and David J. Binks, ChemPhysChem 2015, 16,1239 -1244.
9:00 AM - OO12.11
Hybrid Carbon Nanomaterials for Highly Efficient Dye Sensitized Solar Cells (DSSCs)
Aabhash Shrestha 1 Munkhbayar Batmunkh 1 Joe Shapter 2 Shizhang Qiao 1 Sheng Dai 1
1Univ of Adelaide Adelaide Australia2Flinders University Adelaide Australia
Show AbstractDye sensitized solar cells (DSSC) have received growing interest due to their potential for low cost, easy fabrication and high efficiency photovoltaic (PV) device. Usually, a conventional DSSC utilize Platinum (Pt) as a counter electrode (CE) to catalyse the electrolyte (e.g. iodine redox couple). Nevertheless, the development of low-cost DSSCs is still hindered due to the use of expensive and easily corroded Pt CE. With this in mind, alternatives for Pt as CEs in DSSCs are highly desirable. Ideally, a CE material should have a combination of excellent catalytic activities, high electrical conductivities, and superior electrochemical stabilities. Recently, carbon nanomaterials (such as Carbon black, carbon nanotubes (CNTs) and graphene) are seen to meet these requirements and be an ideal alternative to Pt CEs. Although the carbon nanomaterials have been successfully applied as CEs in DSSCs, these materials still suffer from high charge transfer resistance (Rct), and less catalytic surface area leading to inferior photo-conversion efficiency. The high catalytic activities of carbon nanomaterials are closely related to their defective sites and the electronic structures within the carbon matrix. An attempt to increase these defective sites however leads to a decrease in electrical conductivity and thus lower performance. As such, it still remains a challenge to seek for new carbon nanomaterials with abundant catalytic surface areas whilst also maintaining the high electrical conductivity. Here, we find that the heteroatom (sulphur and nitrogen) doped hybrid carbon materials with CNT and graphene fulfils these requirements and presents a viable alternative to Pt CE. We also selectively control the defective sites (doping levels) in these hybrid carbon nanomaterials while still maintaining the high conductivity of pristine graphene and CNTs. Consequently, we find that higher PV efficiencies is achieved using these hybrid carbon nanomaterials.
9:00 AM - OO12.12
Tungsten Passivation of the Defects in Ta2O5 Nanotubes for Efficient Solar Energy Conversion
Nageh K. Allam 1
1American Univ in Cairo New Cairo Egypt
Show AbstractVertically oriented Ta-W-O nanotube array films were fabricated via the anodization of Ta-W alloy foils in HF-containing electrolytes. HF concentration is a key parameter in achieving well-adhered nanotube array structure. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements confirm the staggered band-alignment between Ta2O5 and WO3, which facilitates the separation of charge carriers. The nanotubes made of Ta-W films containing 10% W showed 100-fold improvement in the measured photocurrent compared to pristine Ta2O5 upon their use to split water photoelectrochemically. This enhancement was related to the efficient charge transport and the red shift in absorption spectrum with increase of the W content, which was asserted by ultrafast transient absorption (TA) spectroscopy measurements. The TA measurements showed the elimination of trap states upon annealing Ta-W-O nanotubes and, hence, minimizing the charge carrier trapping, whereas the trap states remain in pristine Ta2O5 nanotubes even after annealing.
9:00 AM - OO12.13
Photoinduced Bimolecular Electron Transfer with Surprisingly Long-Lived Radical Ions
Amani Abdu Alsam 1 Shawkat M. Aly 1 Dilshad Masih 1 Erkki Alarousu 1 Omar F. Mohammed 1
1King Abdullah University of Science and Technology (KAUST) Jeddah Saudi Arabia
Show AbstractThe excited-state interactions of bimolecular non-covalent systems consisting of cationic poly[(9,9-di(3,3&’-N,N&’-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and 1,4-dicyanobenzene (DCB) have been explored by steady-state and time-resolved techniques including femto- and nanosecond transient absorption (fs-ns-TA) and fs-Infrared (IR) spectroscopies with broadband capabilities. The experimental results demonstrate that the photo-induced electron transfer (PET) from PFN to DCB occurs in picosecond time scale, leading to the formation of PFN+bull; and DCB-bull; radical ions. Interestingly, following vibrational marker modes on the acceptor side in real time provides direct evidence and insight into the electron transfer process concluded indirectly from UV-Vis experiments. A band narrowing on a time scale of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion provide a clear experimental evidence for a substantial part of the excess energy is channeled into vibrational modes of the electron transfer product, and the dissociation of the geminate ion pairs. More importantly, our nanosecond time-resolved data indicate long-lived charge separated state (~ 30 ns) due to the dissociation of the contact radical ion pair into free ions which is very promising and unique feature for organic solar applications.
9:00 AM - OO12.14
The Effect of Charge Localization on Ultrafast Electron Injection at the Cationic Porphyrin-Graphene Interface
Manas Ranjan Parida 1 Shawkat M. Aly 1 Erkki Alarousu 1 Omar F. Mohammed 1
1KAUST Jeddah Saudi Arabia
Show AbstractUltrafast charge transfer, separation and recombination at the donor-acceptor interface, has been demonstrated to be key in solar cell performance. How to control these ultrafast processes remains a topic for debate. Here, we use porphyrin (TMPyP)-graphene carboxylate (GC) as a unique model system to understand these processes at the molecular level.1,2 The steady state interaction between GC and porphyrin results in a red-shifted absorption spectrum providing a clear indication for the binding affinity between porphyrin and GC via electrostatic and π-π interaction. Our steady-state and femtosecond (fs) time-resolved data clearly demonstrate that the charge transfer process at GC interfaces can be tuned by changing the electronic structure of the meso unit and the redox properties of the porphyrin cavity. More specifically, only positively-charged porphyrins can be strongly attracted on the surface of GC, which allows ultrafast interfacial electron injection upon laser pulse excitation. In addition, our results indicate that with careful control of the charge localization of the TMPyP cavity using Cyclodextrin (β-CD) as an external cage, we successfully improved the interfacial-electron injection efficiency from cationic TMPyP to GC by 120% compared to TMPyP alone. In this case, β-CD not only restricts free pyridinium rotation, but also blocks ICT between the macrocycle and the meso substituents of TMPyP; subsequently, electron density is localized on the cavity of TMPyP and its oxidation potential is reduced, increasing the ability to donate electrons to GC. Interestingly, unlike the interaction between TMPyP and GC, which is static and dynamic in nature, the interaction of TMPyP- β-CD with GC takes places via an efficient static mechanism. Finally, the novel new insights in this study holds promise for a variety of potential application that principally rely on interfacial charge transfer, such as photocatalysis.
Reference :
1.Ultrafast Electron Injection at the Cationic Porphyrin- Graphene Interface Assisted by Molecular Flattening shawkat Aly, Manas Parida, Erkki Alarousu and Omar AbdelsaboorChem. Commun., 50, 10452-10455. (2014)
2. To what extent can charge localization influence electron injection efficiency at graphene-porphyrin interfaces? Manas R. Parida, Shawkat M. Aly, Erkki Alarousu, A. Sridharan, D.H. Nagaraju, Husam N. Alshareef, and Omar F. MohammedPhys. Chem. Chem. Phys.,17, 14513-14517 (2015)
9:00 AM - OO12.15
WO3 Nanoflakes for Enhanced Photoelectrochemical Conversion
Peimei Da 1 Gengfeng Zheng 1
1Fudan University Shanghai China
Show AbstractWe developed a postgrowth modification method of two-dimensional WO3 nanoflakes by a simultaneous solution etching and reducing process in a weakly acidic condition. The obtained dual etched and reduced WO3 nanoflakes have a much rougher surface, in which oxygen vacancies are created during the simultaneous etching/reducing process for optimized photoelectrochemical performance. The obtained photoanodes show an enhanced photocurrent density ofsim;1.10 mA/cm2 at 1.0 V vs Ag/AgCl (sim;1.23 V vs reversible hydrogen electrode), compared to 0.62 mA/cm2 of pristine WO3 nanoflakes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that this improved performance of dual etched and reduced WO3 nanoflakes is attributed to the increase of charge carrier density as a result of the synergetic effect of etching and reducing. This simultaneous etching and reducing method is convenient and facile and requires only mild solution conditions, which may provide a general approach to tune the surface morphology of other transition metal oxide materials.
9:00 AM - OO12.16
Transparent Electrode Optimization for Organic and Hybrid Solar Cells
Kwang-Dae Kim 1 Thomas Pfadler 1 Eugen Zimmermann 1 Yuyi Feng 1 James A. Dorman 1 Jonas Weickert 1 Lukas Schmidt-Mende 1
1Univ of Konstanz Constance Germany
Show AbstractHighly effective light-trapping and optimal in-coupling of light is crucial to enhance the overall device performance in thin-film photovoltaics, such as organic and hybrid solar cells. Here we present two ways how the transparent electrode architecture influences the device performance.
We prepared an electrode structured with a TiO2/Ag/TiO2 (TAT) multilayer as ITO replacement. This electrode allows to directly tune the optical cavity mode towards maximized photocurrent generation by varying the thickness of the layers in the sandwich structure. This enables tailored optimization of the transparent electrode for different organic thin film photovoltaics without alteration of their electro-optical properties. OPV featuring our TAT multilayer show an average improvement of ~15% over the ITO reference and allow power conversion efficiencies up to 8.7% in PTB7:PC71BM devices.
We also investigated wavelength-scale structured ITO/TiO2 electrode prepared via direct laser interference patterning (DLIP) the TiO2 layer. Two representative thin-film solar cell architectures are deposited on top: an organic solar cell featuring blended P3HT:PCBM as active material, and a hybrid solar cell with Sb2S3 as inorganic active material. A direct correlation in the asymmetry in total absorption enhancement and in structure-induced light in-coupling is spectroscopically observed for the two systems.
9:00 AM - OO12.17
Carbon Passivated Porous Silicon Counter Electrodes for Dye Sensitized Solar Cells Enable Integrated Energy Conversion and Energy Storage
William R Erwin 1 Adam Paul Cohn 3 Landon Oakes 2 Shahana Chatterjee 3 Holly Zarick 1 Keith Share 2 Rachel Carter 3 Cary Pint 3 Rizia Bardhan 1
1Vanderbilt University Nashville United States2Vanderbilt University Nashville United States3Vanderbilt University Nashville United States
Show AbstractDye-sensitized solar cells (DSSCs) have emerged as a promising solar energy harvesting technology cell due to efficiencies so far measured up to 11.9% and straightforward processes for solar cell manufacturing. Platinum is traditionally used in DSSCs as the counter electrode due to its corrosion resistance to iodide species and its excellent electrocatalytic activity toward the iodide/triiodide redox couple. However the use of this precious metal ($50/gram) remains a barrier to scalability and commercialization of DSSCs providing an incentive to develop DSSC counter electrodes using cheaper, sustainable materials. In this work we demonstrate the passivation of highly reactive porous silicon with graphenic carbon (C-Psi) and their use as a counter electrode in DSSCs exhibiting an efficiency of 5.38%, which is comparable to the 5.80% efficiency of a DSSC made with the conventional platinized counter electrode. We have studied the effect of chemical vapor deposition carbonization parameters on coverage and quality of graphene coating, and shown how this affects overall solar cell performance. An atomically thin coating of few layer graphene was found to be optimal for passivation of porous silicon as well as activation of the surface for triiodide reduction.
The design of next generation of energy systems requires the co-existence of both energy conversion and energy storage modalities within a single component. The versatility of materials such as silicon to be utilized simultaneously for energy storage and energy harvesting opens unique opportunities for the integration of these systems on a single platform. By using the C-PSi counter electrode as a multifunctional electrochemical platform, we designed a single device that captures solar energy and converts it to chemical energy via charging of an electrochemical supercapacitor. This architecture enables constant power supply on the timescale of tens of seconds, even when the light source is blocked, achieving an overall efficiency of 2.1%.
9:00 AM - OO12.18
Hematite Decorated with Mono- and Multi-Layers Iridium Oxygen-Evolution Catalysts for Solar Water Splitting in Acidic Solutions
Wei Li 1 Stafford Wheeler Sheehan 2 Da He 1 Yumin He 1 Xiahui Yao 1 Ronald Grimm 3 Gary Brudvig 2 Dunwei Wang 1
1Boston College Chestnut Hill United States2Yale University New Haven United States3Worcester Polytechnic Institute Worcester United States
Show AbstractTo meet rapidly rising energy demand without devastating the environment, we need a new energy scheme where renewable sources will play a key role. Of many chemical candidates for this new energy infrastructure, H2 produced by H2O splitting holds great promise owing to its high energy density per kilogram and potential ease of transportation and utilization. More importantly, the energetics required match those of the solar spectrum well. To date, the most efficient approaches reported in the literature either involve or are entirely powered by photovoltaic modules. The comparatively simpler, lower cost approach of direct water photolysis (also known as photoelectrochemistry, PEC) suffers from low efficiency, poor durability or both. Poor integration of catalysts with photoelectrodes is one important reason for this slow development. On a system level, its operation needs to be matched with a photocathode. Because the only commercially successful membranes to separate the photoanode and photocathode are proton exchange ones, PEC H2O oxidation in acidic solutions deserves particular attention, but has not been demonstrated to date in a dual absorber photoelectrochemical cell. The lack of functionally stable water-oxidation catalysts (WOCs) in acids is a key reason for this slow development. The only WOCs that are stable at low pH are Ir-based ones, which are typically too expensive to be implemented broadly. Herein, we show that this deficiency may be corrected by applying an ultra-thin monolayer of a molecular Ir WOC to hematite for solar water splitting in acidic solutions. Hematite in this case is prepared by solution synthesized method, the unique nano-structure developed by re-growth strategy helps heal the surface deficiencies to provide good PEC performance. The turn-on voltage is observed to shift cathodically by 250 mV upon the application of a monolayer of the molecular Ir WOC. X-ray photoelectron spectroscopy (XPS) proved the observed performance enhancement was indeed a result of Ir het-WOC decoration, and evolved O2 detection (the Faradaic efficiency is almost 100 %) experimentally verified the measured photocurrents were indeed water oxidation. When the molecular WOC is replaced by a heterogeneous multi-layer derivative, which yielded comparable results on the surface of hematite as the Ir het-WOC, stable solar water splitting for over 5 h is achieved with near unity Faradaic efficiency which promises the integration with amorphous silicon photocathode as un-assisted dual absorber PEC cell for water splitting in acidic solutions.
9:00 AM - OO12.19
Carrier Dynamics in Chlorine-Passivated CdSe Nanocrystal Quantum Dots: Surface-Property Relationship in Photocatalytic Hydrogen Evolution from Water
Whi Dong Kim 1 Sooho Lee 1 Seokwon Lee 1 Ju Young Woo 1 Doh C Lee 1
1Korea Advanced Institute of Science and Technology (KAIST) Daejeon Korea (the Republic of)
Show AbstractSurface passivation of semiconductor nanocrystal quantum dots (NQDs) relates to the properties of NQDs as the dangling bonds of the surface atoms dictate the dynamics of photo-generated carriers. Long-chain alkyl ligands, typical capping ligands for colloidal NQDs, fall short of covering nanocrystal surface due to steric hindrance. The incomplete passivation plays a major role in nanocrystal stability, or lack ther of. In addition, optical properties of nanocrystals alter significantly due to dangling bonds and trap states.
In this presentation, we will discuss the effect of chlorine (Cl) ion on the surface of CdSe NQDs (Cl-CdSe NQDs). Transient absorption spectroscopy reveals that passivated Cl diminishes electron trap states in CdSe NCs, resulting in an increase of photoluminescence quantum yield, e.g., from 4.6 % to 7.6 % after Cl treatment. In order to elucidate the effect of Cl treatment on electron trap states, we compared the photocatalytic hydrogen evolution rate by bare CdSe and Cl-CdSe NQDs. In the case of large NQDs, Cl-CdSe NQDs show high hydrogen evolution rate, as the passivation of trap states with energy levels below the reduction potential necessary for H2O reduction, while bare CdSe NQDs exhibit slower reduction. By contrast, in the case of small NQDs with higher degree of quantum confinement effect, trap states do not come into play as the conduction band-edge and trap states are above the water reduction potential. In this case, the lack of trap states via Cl passivation reduces electron-hole separation, resulting in reduced photocatalytic activity.
9:00 AM - OO12.20
Highly Efficient Inverted Perovskite Solar Cells with Graphene Electrodes
Hyangki Sung 1 2 Namyoung Ahn 1 2 Min Seok Jang 2 Jong-Kwon Lee 2 Heetae Yoon 1 2 Nam-Gyu Park 3 Mansoo Choi 1 2
1Seoul National University Seoul Korea (the Republic of)2Seoul National University Seoul Korea (the Republic of)3Sungkyunkwan University Suwon Korea (the Republic of)
Show AbstractWe have demonstrated over 18% power conversion efficiency (PCE) of inverted perovskite solar cells consisting of graphene/MoO3/PEDOT:PSS/CH3NH3PbI3/C60/BCP/LiF/Al, which is comparable to that of perovskite solar cells (~19%) with indium tin oxide (ITO) electrodes. A single-layer graphene was synthesized using chemical vapor deposition on thin copper layer and transferred to a glass substrate. The PEDOT:PSS and CH3NH3PbI3 layers were prepared by spin-coating and the other layers were deposited by vacuum thermal evaporation. A MoO3/PEDOT:PSS bilayer was used as a hole transporting layer on the graphene electrodes because i) a MoO3 layer can provide hydrophilicity to the graphene surface, allowing for PEDOT:PSS to wet and form a continuous layer on graphene, and ii) a MoO3 layer can dope holes into graphene and increase its low work function (-4.3 eV) to the higher level, resulting in nearly ohmic contacts at graphene and PEDOT:PSS interfaces. As the thickness of the thermally evaporated MoO3 layer was varied from 0.5 to 2 nm, sheet resistance of the pristine single-layer graphene (2.2 kOmega;/cm2) was decreased up to ~500Omega;/cm2. Although the resistance of the doped graphene is still much higher than that of ITO (9.2 Omega;/cm2), graphene is optically more transparent (~98% transmittance) than ITO (~90%) and its energy level is more favorably aligned with the hole-transporting layer. As a result, the overall PCEs of the cells with graphene and ITO electrodes were observed to be similar. Both types of cells were nearly hysteresis-free, likely due to well-formed interfaces between the smooth perovskite film and the organic layers. Given that graphene is mechanically robust and flexible than common transparent electrodes such as ITO and FTO (Fluorine-doped thin oxide), this work demonstrates great potential of graphene for developing flexible solar cells with high efficiency and stable performance.
9:00 AM - OO12.21
Enhancing Stability of Quantum-Confined Colloidal Nanocrystals for Efficient Photocatalytic Hydrogen Generation
Frank Jaeckel 1 Wei Li 1 Jonathan Lee 1
1Univ of Liverpool Liverpool United Kingdom
Show AbstractColloidal, catalyst decorated semiconductor nanocrystal are receiving increasing attention as an possibly cheap and scalable means for the photocatalytic generation of hydrogen from water.1,2 A major shortcoming in many colloidal nanocrystal systems, however, is the short lifetime due to ligand or nanocrystal oxidation leading to aggregation and precipitation. Here we show, that the colloidal stability as well as hydrogen generation efficiency of cysteine coated, Pt-decorated CdS nanocrystals (using Na2SO3 as hole scavengers) can be significantly enhanced by replacing the original ligands in situ with triethanolamine (TEOA).3 The presence of TEOA extends the highly efficient and steady photocatalytic H2 generation period by an order of magnitude. External quantum efficiencies up to 31.5% and turnover frequencies up to 0.11 H2/Pt/s are achieved. The short, dendritic structure and rather weak adsorption of TEOA to the nanocrystal as well as sufficient free TEOA in solution are the keys to obtaining enhanced stability and efficient H2 evolution simultaneously.
1. M. Berr, A. Vaneski, A. S. Susha, J. Rodri#769;guez-Ferna#769;ndez, M. Do#776;blinger, F. Ja#776;ckel, A. L. Rogach, and J. Feldmann, Appl. Phys. Lett., 2010, 97, 093108.
2. T. Simon, N. Bouchonville, M. J. Berr, A. Vaneski, A. Adrovic, D. Volbers, R. Wyrwich, M. Döblinger, A. S. Susha, A. L. Rogach, F. Jäckel, J. K. Stolarczyk, J. Feldmann, Nature Materials 2014, 13, 1013.
3. W. Li et al. submitted
9:00 AM - OO12.22
Investigation of Hole Transfer Rate in Efficient p-Type Quantum Dot Sensitized Solar Cells
Muhammad Tariq Sajjad 1 Jinhyung Park 2 3 4 Dmitry Aldakov 2 3 4 Pierre-Henri Jouneau 5 Peter Reiss 2 3 4 Ifor Samuel 1
1University of ST Andrews St Andrews United Kingdom2INAC-SPRAM Grenoble France3INAC-SPRAM Grenoble France4Univ. Grenoble Alpes Grenoble France5INAC/SP2M (UMR-E CEA-UJF)/LEMMA Grenoble France
Show AbstractQuantum dots (QDs) are very attractive materials for photovoltaic devices due to their high absorption coefficients, size dependence and easy tunability of their optical and electronic properties due to quantum confinement. Furthermore, semiconductor QDs offer the possibility of multiple exciton generation,1 which could allow them to overcome the Shockleyminus;Queisser limit in solar cells.2 QD based solar cells are widely considered as a viable alternative to silicon and inorganic thin film based cells.3 However, typically colloidal quantum dots used for solar cells contain toxic heavy metals (Cd or Pb), which limits their industrial applications. So it is very important to explore alternative non-toxic materials, such as CuInS2 derivatives. In our study we used “eco-friendly” quantum dots (CuInS2 and CuInSxSe2-x) along with p-type electrodes (NiO or CuScN nanowires) for solar cell fabrication.
In the classical QD sensitized solar cells the light is absorbed by the QDs, which then inject electrons into an n-type material (TiO2), while the hole is regenerated by the liquid electrolyte. The main limiting step in such cells is hole transfer, which occurs slower and less efficiently than electron transfer. One way to overcome this is to invert the configuration of the cell to benefit from photoinduced hole injection from the QDs into a p-type material. We investigated the hole transfer rate from QDs to p-type semiconductors using steady state and time-resolved photoluminescence. We found hole transfer rate of 2- 5 x 107/s from the CuInS2 to CuScN nanowires and 3-4 x 107/s to NiO. However, this transfer rate becomes an order of magnitude higher (i.e. 4-6 x 108/s) for the case of CuInSxSe2-x when we used them with NiO. Moreover, the resulting hole transfer rates also follow solar efficiencies.
1 A. J. Nozik, M. C. Beard, J. M. Luther, M. Law, R. J. Ellingson and J. C. Johnson, Chem. Rev., 2010, 110, 6873-90.
2 J. B. Sambur, T. Novet and B. A. Parkinson, Science, 2010, 330, 63-6.
3 P. V Kamat, J. Phys. Chem. Lett., 2013, 4, 908-918.
9:00 AM - OO12.23
Band Gap Narrowing of Iron Oxide Nanotubes upon Doping with Zinc and Their Spectral Sensitivity Used as Photoelectrode
Yuta Kosugi 2 Takuya Tomiyasu 2 Shunji Bandow 1
1Meijo Univ. Nagoya Japan2Meijo Univ. Nagoya Japan
Show AbstractZinc doped iron oxide nanotubes (ZnFe-ox-NTs) were prepared by the polycondensation of a mixture of zinc(II)-nitrate-hexahydrate and iron(III)-nitrate-nonahydrate taken on the surface of self-organized block copolymer (Pluronic F-127) in a 1-propanol. This reaction was conducted at 318 K for 5 days to make gel slowly. The gel prepared was dried at 393 K in open air oven, and then the temperature was increased to 543 K in order to complete the polycondensation reaction and to burn out Pluronic F-127. The product obtained was well ground by using an agate mortar and rinsed several time by EtOH. Next the structural annealing of crude nanotubes was carried at 543 K for 6 hours. Nanotube structure was confirmed by TEM. Compositional formulae for the products were determined by the combination study of XRD and XPS, formulated by ZnxFe3-xO4 where x can be controlled in the range between 0 and 0.66 as function of the mixing ratio of zinc-nitrate.
Band gap was determined by a Tauc plot method using optical absorption spectrum in the range between 200 and 1000 nm. For Zn undoped iron oxide nanotube sample (Zn0Fe3O4-NTs), band gap was estimated to be ~2.3 eV, and its value decreased by ~2.0 eV for Zn0.27Fe2.73O4-NTs. Photoelectrode was made by dropping viscous aqueous solution of ZnFe-ox-NTs/acetylacetone onto FTO glass spun at 3000 rpm, then heat-treated at 270 °C in open air. Pt was used as counter electrode. Spectral sensitivity was measured by irradiating band-pass filtered light using the electrolyte of LiClO4/LiI/I2/acetonitrile. Resulting that maximum sensitivity was located at the wavelength of ~510-540 nm, which is longer than that of Fe2O3 film (~390 nm). In addition, the sensitivity in the long wavelength region (< 540 nm) was improved by Zn doping. This can be explained by narrowing the band gap for nanotube samples and also by widening of the LUMO level band. Detailed thickness dependence of nanotube layer on IPCE will be reported at the meeting.
9:00 AM - OO12.24
Chiral Molecule-Enhanced Hydrogen Photo-Production
Wilbert Mtangi 1 Kiran Vankayala 1 Claudio Fontanesi 2 Ron Naaman 1
1Weizmann Institute of Science Rehovot Israel2Modena University Modena Italy
Show AbstractKey biochemical reactions in nature like photosynthesis and respiration, are multiple electron reactions. In nature these reactions are highly efficient. Artificial hydrogen production by photochemical cells is an example of performing a similar reaction in an artificial setting. However, the practical production of hydrogen is hampered by the need to apply additional voltage to initiate the reaction; the over-potential, which results in low efficiency and lack of selectivity in the oxidation process, as other molecules besides water tend to be oxidized. Hence, although hydrogen is considered to be the ultimate fuel of the future its efficient production remains an important challenge.
We found that in a cell in which the anode is coated with chiral molecules, the over-potential required for hydrogen production drops remarkably, as compared with cells containing achiral molecules. We propose that the spin specificity of electron transfer through chiral molecules is the origin of a more efficient oxidation process in which a molecule (either oxygen or disulfide) is formed in its triplet ground state. The lower over-potential is therefore due to a correlation that exists in the spins&’ alignment in the atoms composing these molecules. This finding opens the way for efficient production of hydrogen.
In the present study, we investigated a cell based on a TiO2 electrode to which CdSe nanoparticles (NPs) are attached by molecules. The NPs serve as photocatalysts for hydrogen evolution from an aqueous Na2S/Na2SO3 (sacrificial electrolyte) solution when irradiated with visible light. We investigated the evolution of hydrogen when the nanoparticles are tethered to TiO2 via achiral and chiral molecules. Different chiral molecules were investigated, i.e. two oligopeptides having different lengths and double-stranded DNA. Correlation was found between the efficiency of these molecules as spin filters and the reduction in the over-potential.
9:00 AM - OO12.25
Morphology-Controlled Photonic Silicon Nanowires
David John Hill 1 Seokhyoung Kim 1 Christopher Pinion 1 Hong-Gyu Park 2 James Cahoon 1
1University of North Carolina-Chapel Hill Chapel Hill United States2Korea University Seoul Korea (the Republic of)
Show AbstractBottom-up control of the structure of nanoscale materials provides the opportunity to tune the absorption and scattering through the introduction of photonic crystals and photonic modes. The structural parameters of Si nanowires (NWs) grown by the vapor-liquid-solid mechanism can be manipulated axially through a selective doping and etching process, or axially through shell deposition, affording a controllable platform for developing structurally determined light interactions in nanoscale materials. The axial doping and etching process is used to construct photonic crystals along the axis of the NW, and the photonic properties of these structures are observed experimentally through spectroscopy along with a novel optical waveguide microscopy technique. The waveguide-based microscopy we have developed enhances control of the illumination polarization and wavevector in these experiments while simultaneously reducing background scattering. Improved photovoltaic properties from the shell deposition process are experimentally realized through the construction of radial P-I-N devices with dielectric shells deposited by plasma-enhanced chemical vapor deposition, which exhibit radial photonic modes based on the size and composition of the shell. Through theoretical studies and controlled synthesis, we are able to harness the unique interactions of nanoscale materials with light to produce enhanced photonic and photovoltaic properties in silicon nanowires.
9:00 AM - OO12.26
Quantum Cutting Effect Based on Ce3+-Yb3+ Doped Silicon Oxynitride Thin Films for Solar Cell Applications
Florian Ehre 1 Christian Dufour 1 Lucile Dumont 1 Fabrice Gourbilleau 1 Cedric Frilay 1 Xavier Portier 1 Julien Cardin 1 Philippe Marie 1 Christophe Labbe 1
1CIMAP CNRS/CEA/Ensicaen/UCBN Caen France
Show AbstractSolar panels have become one of the most promising answers to green and renewable energy issues. Commercial mono c-Si solar cells have actually an efficiency of 22-25% which is mainly limited by the thermalization of high energy photogenerated carriers due to the mismatch between their energy and the c-Si band gap energy.
The quantum cutting (QC) is one of the interesting ways to improve the solar cells efficiency by overcoming the thermalization of those hot carriers. A conventional solar cell is then topped by a QC layer where a down converting (DC) effect takes place which consists in converting the incident UV-Visible photon energy into two IR photons which are more efficiently absorbed by Si solar cell. To create such a QC effect, many systems were studied such as Pr3+-Yb3+(1) or Tb3+-Yb3+ (2) co-doped matrices. Unfortunately, up to now, the proposed host matrices were hardly compatible with the silicon technology. Thus, the last five years researches have been focusing on silicon integration: Zn2SiO4 :Tb3+ minus;Yb3+(3), Er3+ in SiO2(4), or recently Ce3+-Yb3+ in SiOx(5) and in SiOxNy with Tb3+-Yb3+ (6) co-doped systems by our group, revealing a semi-empirical additional external quantum efficiency of about 2%.
In this study, we propose to investigate the Ce3+-Yb3+ co-doped system in a SiOxNy matrix. Indeed, one of the advantages of this matrix is the reduction of the rare earth clustering observed in other matrices such as SiOx. The UV energy is absorbed by Ce3+ ions between the 4f level and the 5d band. This energy is transferred by a cooperative energy transfer mechanism to two Yb3+ ions which emit at a 980 nm wavelength (1.2 eV), slightly above the Si gap of the solar cell.
Our layers are deposited by using a radio frequency co-sputtering technique, with help of CeO2, Yb and Si targets under a nitrogen flow. The results reveal an intense photoluminescence of Ce3+ ions. Such intensity is optimized by means of several deposition parameters: target power, the nitrogen flow, deposition and annealing temperature. Structural investigations are performed with help of RBS and HRTEM. The ellipsometry experiment shows a refractive index between 1.9 and 3.8 at 1.95 eV with FTIR measurements revealing low oxygen content.
For the sample presenting the most intense Ce3+ emission, a co-doping with Yb3+ ions is carried out to test the cooperative energy transfer mechanism with the Ce3+ ions. Quantum efficiency of such mechanism is also investigated by using decay time measurements to estimate the quantum efficiency.
1. Li, D. H. et al.Phys. B Condens. Matter446, 12-16 (2014).
2. Terra, I. A. A. et al.J. Appl. Phys.113, 073105 (2013).
3. Huang, X. Y. & Zhang, Q. Y. J. Appl. Phys.105, 053521 (2009).
4. Ha, N. N. et al.Phys. Rev. B84, (2011).
5. Heng, C. L., Li, J. T., Su, W. Y., Yin, P. G. & Finstad, J. Appl. Phys.117, 043101 (2015).
6. An, Y.-T., Labbé, C., Cardin, J., Morales, M. & Gourbilleau, F. Adv. Opt. Mater.1, 855-862 (2013).
9:00 AM - OO12.27
Photoelectrochemical Hydrogen Production through Hybrid Organic/Nanostructured Inorganic Photocathodes
Hansel Comas 1 Sebastiano Bellani 1 Francesco Fumagalli 1 Gabriele Tullii 1 Silvia Leonardi 1 Matthew T. Mayer 2 Ludmilla Steier 2 Michael Graetzel 2 Guglielmo Lanzani 1 Fabio Di Fonzo 1 Maria Rosa Antognazza 1
1Center of Nanoscience and Technology, Istituto Italiano di Tecnologia Milan Italy2Ecole Polytechnique Federale de Lausanne Laussanne Switzerland
Show AbstractThe generation of renewable hydrogen through the photoelectrochemical water splitting route requires transfer of photogenerated electrons from the light harvester semiconductor to an efficient electrocatalyst [1]. Durability, stability and economic viability are also critical requirements those photocathodes need to face [2]. State-of-the-art inorganic semiconductors-based photocathodes are very limited and prone to photocorrosion [3] whereas organic ones are many more, offer greater flexibility but have been much less studied [4]. Recently, some organic semiconductors have been proved to be active towards hydrogen evolution [5] as well as stable in aqueous environment [6]. The Photogenerated hydrogen by organic catalytic systems (PHOCS) European project is devoted to such state of the art problem, based on the role of organic electronics for energy applications [7]. In this work we report our latest findings in producing and characterizing hydrogen evolving photocathodes based on hybrid organic/inorganic interfaces. The active layer concerns a conventional donor-acceptor bulk heterojunction blend while the charge selective layers were introduced to maximize charge collection. In this regard, nanostructured titanium oxide grown from the gas phase was used as an electron selective layer being critical for greater performance. Photocurrents densities as high as 8 mA/cm2 at 0 V have been achieved exhibiting 100% faradic efficiency for hydrogen generation as well as positive and stable onset potentials, hence desirable towards a further tandem configuration. However, device performance is compromised by the porous nature of the inorganic nanostructured top layer, therefore some protective strategies and alternatives were assessed. These results pave the way towards the forthcoming use of organic semiconductors interfaced with inorganic nanostructures for hydrogen generation, thus strengthening a wider and more flexible exploitation of organic electronics for energy applications.
References
1. M. A. Gross, A. Reynal, J. R. Durrant et al., J. Am. Chem. Soc. 2014, 136, 356-366.
2. P. M. Laurence, W. K. G. Upul, Chem. Phys. Chem. 2014, 15, 1983 - 1995
3. O. Khaselev and J. A. Turner, Science 1998, 280, 17, 425-427; T. Hisatomi, J. Kubota, K. Domen, Chem. Soc. Rev. 2014, 43, 7520-7535; S. D. Tilley, M. Schreier, J. Azevedo et al. Adv. Funct. Mater. 2014, 24, 303-311.
4. M. Haro, C. Solis, G. Molina et al. J. Phys. Chem. C 2015, 119, 6488-6494.
5. E. Lanzarini, M.R. Antognazza, M. Biso et al., J. Phys. Chem. C 2012, 116, 10944-10949.
6. S. Bellani, D. Fazzi, P. Bruno et al. J. Phys. Chem. C 2014, 118, 6291-6299; M. R. Antognazza, D. Ghezzi, D. Musitelli et al. Appl Phys Lett. 2009, 94, 243501, 0003-6951; R. Porrazzo, S. Bellani, A. Luzio et al. Organic Electronics 2014, 15, 2126- 2134.
7. www.phocs.eu
9:00 AM - OO12.28
Electrophoretic Deposition of TiO2 with In-Situ Rutile/Anatase Phase Separation for Flexible Dye-Sensitized Solar Cells
Bahadir Can Kocaoglu 1 Ahmet Macit Ozenbas 1
1Middle East Technical Univ Ankara Turkey
Show AbstractOne of the most remarkable aspects of dye-sensitized solar cells is the possibility of manufacturing on flexible substrates. Therefore, it is vital to optimize a production technique (sequence) in order to manufacture large area cells and modules for mobile applications. Therefore, in this study we aimed to develop a highly versatile, cost and time effective fabrication process for flexible photoanodes for dye sensitized solar cells (DSSC). Considering the impossibility of heat treatment process in order to obtain a nanoporous TiO2 layer for flexible photoanodes as in the commercial ones, electrophoretic deposition (EPD) and subsequent cold isostatic pressing (CIP) were optimized. However, upon large-scale production of TiO2 nanoparticles, the batches inevitably contain both anatase and rutile phases. Therefore, using isoelectric point and zeta potential concepts of EPD processes, the anatase and rutile phases are separated during deposition of nanoparticles, eliminating the rutile phase. In order to achieve this property, several binder-free, low additive alcohol based colloidal solutions were prepared and their coating uniformity, morphology and phase separation performances are investigated. Comparing the EPD solutions using SEM, XRD, Zeta potential analysis methods, a low additive ethanol based solution has exhibited excellent coating uniformity performances eliminating almost all rutile phase from prepared photoanode. The cells were fabricated having 0.5x0.5 cm2 , 1.0x1.0 cm2 and 1.0x1.0 in2 were investigated under AM1.5 conditions and efficiencies were obtained as 4.22%, 4.13% and 4.01% respectively.
9:00 AM - OO12.29
All Silicon Electrode Photo-Capacitor for Integrated Energy Storage and Conversion
Adam Paul Cohn 1 William R Erwin 1 Cary Pint 1
1Vanderbilt University Nashville United States
Show AbstractWe demonstrate a simple wafer-scale process by which an individual silicon wafer can be processed into a multifunctional platform where one side is adapted to replace platinum and enable triiodide reduction in a dye-sensitized solar cell (DSSC), and the other side provides on-board charge storage as an electrochemical supercapacitor. This builds upon electrochemical fabrication of dual-sided porous silicon, and subsequent carbon surface passivation for silicon electrochemical stability. The utilization of this silicon multifunctional platform as a combined energy storage and conversion system yields a total device efficiency of 2.1%, where the high frequency discharge capability of the integrated supercapacitor gives promise for dynamic load-leveling operations to overcome current and voltage fluctuations during solar energy harvesting. On a large scale, our advances in integrated energy storage and conversion technology may prove to be key to developing decentralized energy grids, which currently have low tolerance to overcome the variability introduced by intermittent generation, and allow for greater penetration of solar resources.
9:00 AM - OO12.30
Probing the Energetic Distribution of Electrons Transferred from Photoexcited CdSe Quantum Dots to Molecularly-Tethered TiO2 Nanoparticles
Saurabh Chauhan 1 David Watson 1
1UB Buffalo United States
Show AbstractSemiconductor quantum dots (QDs) are intriguing light-harvesters and excited-state charge donors for solar energy conversion, due to their size-dependent optical properties and band-edge potentials, high oscillator strengths, and the possibilities of hot-carrier extraction and multiexciton generation. However, excited-state relaxation mechanisms of QDs are complex and multiexponential, which in turn complicates the mechanisms of photoinduced charge-transfer processes at QD-containing interfaces. In this regard, excited-state charge-transfer process of QDs differ fundamentally from than those of molecular chromophores.
This presentation will focus on the mechanism and dynamics of excited-state electron transfer from CdSe QDs to molecularly-linked TiO2 nanoparticles with particular emphasis on the relative rate constants and quantum yields of electron transfer from band-edge vs. trap states of CdSe QDs. CdSe QDs were covalently linked to TiO2 by using 3-mercaptopropionic acid (MPA) linker. Steady-state and time-resolved emission spectroscopy were used to study dynamics of electron transfer from CdSe QDs to TiO2. The time-resolved photoluminescence decays were acquired using time-correlated single photon counting (TCSPC). Electrons were injected from both band-edge and trap states of CdSe with rate constants of (1.6±0.8)×108 sec-1 and (8±6)×107 sec-1, respectively. Rate constants of electron injection from trap states were measurably slower than from the band edge. Control experiments were done by using QD-MPA-ZrO2 assemblies for which, due to the interfacial energetic offsets, electron transfer was assumed to be absent.
Ongoing experiments on charge-transfer dynamics of CdSe/ZnS core/shell QDs tethered to TiO2 via MPA will be discussed as well. Our goals are to study the charge transfer in absence of trap states and to establish conditions in which electrons can be extracted efficiently from band-edge states without the deleterious decrease of the electron&’s potential energy upon trapping. This research provides new insight into the contributions of both bulk-like conduction band states and surface-localized trap states of QDs to charge-transfer processes relevant to solar energy conversion.
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9:00 AM - OO12.31
Optical Properties and Charge Transfer in Porphyrin-Fullerene Dyads
Oscar Granas 1 2 Grigory Kolesov 1 Efthimios Kaxiras 1 3 4
1Harvard University Cambridge United States2Uppsala University Uppsala Sweden3Harvard University Cambridge United States4Harvard University Cambridge United States
Show Abstract
Porphyrin molecules are common chromophores in both natural photosynthesis and in synthetic solar harvesting, such as the dye-sensitized solar cell. In order to harvest the energy of the photon the system has to i) absorb the photon, ii) facilitate charge separation and iii) inhibit charge recombination. Using non-adiabatic excited states molecular dynamics through real-time propagation TD-DFT we investigate how light interacts with the electronic structure of the porphyrin chromophore. The calculated absorption spectra reproduces the Soret band and Q band of the porphyrin molecule very well, allowing us to investigate the influence of the central metal-ion on the absorption. Further, we investigate electron transport properties of porphyrin-fullerene dyads bridged with pi-conjugated molecular wires. The photo-excited state in the chromophore is described using delta-SCF. The state is further propagated in time to investigate how the electron is transported from the chromophore to the C60 electron acceptor. This provides a microscopic insight to vibrational and steric contributions to charge transport properties in liked porphyrin-fullerene dyads.
Support from the Swedish Research Council (VR) is thankfully acknowledged. Simulations are performed on allocations from XSEDE, NERSC and Harvard RC.
9:00 AM - OO12.32
Synthesis of ZnO via CVD for Fluorescence-Assisted DSSC Application by Controlling Partial Pressure Ratio of Reactants
Taehoon Lim 1 2 Alfredo A. Martinez-Morales 2
1University of California, Riverside Riverside United States2University of California, Riverside Riverside United States
Show AbstractSolar cell devices have attracted much interest from the research community because they are a sustainable and cost-effective technology. Dye-sensitized solar cells (DSSCs), in particular, have gained tremendous attention due to its ease of fabrication and low production cost. DSSCs also have a wide range of applications, such as building integrated photovoltaic (BIPV) technologies and flexible/wearable solar cells.
In an effort to improve DSSC photovoltaic characteristics, a fluorescence-assisted approach was considered, by using a fluorescent photoelectrode material made out of zinc oxide (ZnO) nanostructures. The fluorescence emissions from the photoelectrode layer can be used as a secondary light source to enhance photovoltaic characteristics. Furthermore, the transparency of ZnO allows it to be used as a photoelectrode for DSSCs.
Since both fluorescence and transparency are essential characteristics for developing a fluorescence-assisted DSSC photoelectrodes, transparent ZnO with fluorescence emission have been synthesized by adjusting synthesis conditions. By adjusting the length and diameter of 1-dimensional ZnO nanostructures, optical and electrical characteristics were optimized. Here we demonstrate the successful synthesis of transparent and fluorescent 1-dimensional semiconducting ZnO nanostructures for DSSC photoelectrode application.
9:00 AM - OO12.33
A Challenge beyond Bottom Cells: Nanostructured Dielectric/Metal/Polymer Films for Top-Illuminated Flexible Organic Solar Cells on Opaque Substrates
Juyoung Ham 1 Jong-Lam Lee 1
1POSTECH Pohang Korea (the Republic of)
Show AbstractRecently, Dielectric/Metal/Dielectric (DMD) electrodes as ITO alternatives have attracted much interest because of the unique optical and electronic properties as well as mechanical flexibility. However, there were still many problems in application of DMD electrodes to highly efficient top-illuminated organic solar cells (OSCs). First, there is Fresnel reflection loss at the air-DMD interface due to the highly complex refractive index (N = n - ik) of the top dielectrics, such as MoO3, WO3, V2O5, and ZnS. Second, DMD electrodes have peak transparency at a specific dielectric thickness for a given wavelength. Therefore, if the thickness of the dielectric layer deviates from the optimal thickness in the DMD structure, the electrode suffers from drastically reduced transmission. Third, nano-structural patterns have not yet been implemented on inorganic dielectric materials in the DMD electrodes, which are only produced by physical deposition. Even though the inductively coupled plasma (ICP) etching process, laser interference lithography, electron beam, and ion beam lithography can be used to produce nanostructures on inorganic dielectric materials, many of these techniques involve harsh processing conditions, which readily cause damages to organic active materials. This is why conventional DMDs have a planar geometry structure.
In this work, we report the novel design of nanostructured Dielectric/Metal/Polymer (DMP) with high transmittance and excellent haze in top-illuminated OSCs, resulting in increase of efficiency by 28% compared to bottom cells. This improvement was achieved by overcoming critical problems in planar Dielectric/Metal/Dielectric (DMD) based solar cells. To design advanced structure, PDMS, known as a hybrid polymer with dielectric property, was employed instead of an inorganic dielectric layer. With the low refractive index (n = 1.45, k = 0.0001) of PDMS, a transparency insensitive to the polymer thickness up to 300 mm could be achieved even in on nanostructured electrode. Consequently, incident light could pass through the PDMS layer, without reflection loss, resulting in the enhanced transmittance. Additionally, we integrated inverted hexagonal pyramid patterned PDMS with on front side of MoO3/Ag by a simple lamination technique without damages to organic active layer. The nanostructured MAP allows incident lights to be diffracted to enhance light path length in the active layer, resulting in enhanced omnidirectional optical characteristics and power conversion efficiency of 6.75 %. From the device simulation using rigorous coupled wave analysis software, we inferred that the lateral distribution of light propagation due to nanostructures could increase light path length in the active PTB7:PC70BM layer. Finally, the results offer new opportunities in development of top-illuminated OSCs based on flexible metal foil or stainless steel substrates, which assist industrialization of very cost-efficient solar modules.
OO9: Perovskite Solar Cells II
Session Chairs
Wednesday AM, December 02, 2015
Hynes, Level 3, Room 302
9:30 AM - OO9.02
High-Performance Perovskite Photoanode Enabled by Ni Passivation and Catalysis
Peimei Da 1 Gengfeng Zheng 1
1Fudan Univ Shanghai China
Show AbstractLead halide perovskites have achieved phenomenal successes in photovoltaics due to their suitable bandgaps, long diffusion lengths, and balanced charge transport. However, the extreme susceptibility of perovskites to water or air has imposed a seemingly insurmountable barrier for leveraging these unique materials into solar-to-fuel applications such as photoelectrochemical conversion. Here we developed a CH3NH3PbI3-based photoanode with an ultrathin Ni (8 nm) surface layer, which functions as both a physical passivation barrier and a hole-transferring catalyst. Remarkably, a much enhanced photocurrent density, an unassisted photoelectrochemical conversion capability, and a substantially better stability against water have been achieved, which are exceeding most of the previously reported photoanodes as well as a similar CH3NH3PbI3-based device structure but without the Ni surface layer. The photocurrent density is measured over 10 mA/cm2 in 0.1 M Na2S at 0 V vs Ag/AgCl, this PEC photocurrent density can be maintained over 2 mA/cm2 after 15minus;20 min of continuous PEC tests, which is strikingly phenomenal for the CH3NH3PbI3 electrode and also substantially higher than the similar device structure but without Ni coating. Ni 2p spectra for the same sample taken after three different Ar ion milling times into the surface reveal the gradual change from oxidized Ni to metallic nickel. This leads to a fast transfer of the photogenerated holes from the inner CH3NH3PbI3 layer toward the external Au and Ni surface and subsequently to the electrolyte for oxidation, as confirmed by the significantly increased photocurrent, which in the meantime suppresses the photocorrosion of the electrode. In comparison, additional Au layers with equal or even much larger thickness only yield a negligible effect on both the PEC activity and stability enhancement, indicating the imperative role of the Ni surface layer. Our study indicates that the recent research breakthroughs in perovskite can be well inherited into the PEC field for hydrogen production and artificial photosynthesis, and the development on water-resistive perovskite devices can also be leveraged to improve long-term stability of perovskite solar cells under ambient conditions. The perovskite solar cell with 8 nm Ni coating maintains much higher power conversion efficiency than bare perovskite solar cell after being stored in ambient environment for 12 days. Further improvement on the Au and Ni layer fabrication quality such as using molecular beam epitaxy (MBE) may lead to higher stability of the perovskite photoelectrodes.
9:45 AM - OO9.03
The Assignment of Optical Transitions in CH3NH3PbX3 (X = I, Br, Cl) Perovskites for the Interpretation of Steady State and Transient Absorption Measurements
Aurelien M. A. Leguy 1 Pooya Azarhoosh 2 Mariano Campoy-Quiles 3 M. Isabel Alonso 3 Oliver J. Weber 4 Jizhong Yao 1 Daniel Bryant 1 Mark T. Weller 4 Jenny Nelson 1 Aron Walsh 4 Mark van Schilfgaarde 2 Piers R.F. Barnes 1
1Imperial College London London United Kingdom2King's College London London United Kingdom3Institut de Ciegrave;ncia de Materials Barcelona Spain4University of Bath Bath United Kingdom
Show AbstractWe present the optical constants determined from spectroscopic ellipsometry of single crystals of CH3NH3PbX3 (where X = I, Br, Cl). These results are interpreted by comparison with optical constants and band structure derived from the highest level of ab initio calculations using the relativistic the quasiparticle self-consistent GW approximation (QS-GW). The calculations reproduce the majority of observed absorption features. However the increasingly strong absorption peak observed at the band edge moving from X = I to Br to Cl attributed to excitonic absorption is not predicted by QS-GW since electron-hole interactions are not accounted for by the theory.
Strikingly, the analysis shows that transitions from the highest valence band (VB1) to the lowest conduction band (CB1) at different momenta are responsible for all three of the main absorption peaks at (1.6, 2.5 and 3.1 eV) in CH3NH3PbI3 (only minor contributions from the second highest VB and to the second lowest CB are involved).
This observation of is particular relevance to the interpretation of transient absorption spectroscopy (TAS) measurements, the nature of which are the subject of considerable debate. We use our optical constants and energy band diagram of CH3NH3PbI3 to simulate its TAS and find good agreement with measurement. The results suggest that the bleaching states observed at 480 and 760 nm can be assigned to a reduction in the VB1 → CB1 transition at two different regions of momentum space. It is not necessary to invoke additional transitions from VB2 to explain the spectra.
Our calculations also indicate that the orientation of CH3NH3 cations can have a significant influence (up to ~0.14 eV) on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties.
10:00 AM - OO9.04
Zero I-V Hysteresis in Normal Planar Structured Perovskite Solar Cell Realized by Interfacial Engineering
In-Hyuk Jang 1 Namyoung Ahn 2 Dae-Yong Son 1 Nam-Gyu Park 1
1Sungkyunkwan University Suwon Korea (the Republic of)2Seoul National University Seoul Korea (the Republic of)
Show AbstractWe report here normal planar structured perovskite solar cell without I-V hysteresis via interfacial engineering. In normal planar structure with cell configuration of FTO/thin film TiO2/perovskite/hole transport material/Ag, considerable I-V hysteresis has been unavoidably observed. To solve this problem, we analyzed potential factors affecting I-V hysteresis, such as ferroelectricity, ion migration and interface using a simple equivalent circuit. Since little hysteresis has been reported from the cell configuration being analogue to organic photovoltaics, bulk property of perovskite layer may not be mainly involved in I-V hysteresis. Interface between perovskite and thin film TiO2 or FTO was modified to reduce interfacial capacitance and by doing so we realized no I-V hysteresis in the normal planar structure. Charge mobility and extraction was analyzed by photo induced charge extraction by linearly increasing voltage method (photo-CELIV). A capacitance component of perovskite-substrate interface was analyzed by impedance spectroscopy. Interfacial modification led to changes in charge extraction property and capacitance, which was found to be responsible for no I-V hysteresis. Theoretical simulation further supported the observed results. Among the interfaces contacting perovskite layer, we extracted the interface contributing to mainly I-V hysteresis by investigating different selective contacts and hole transport materials.
10:15 AM - OO9.05
Hysteresis and Stability Measurements on MaPbBr3-Based Perovskite Solar Cells under Operation Conditions
Katsuya Ono 1 Atsushi Gabe 1 Zafer Hawash 1 Sonia Ruiz Raga 1 Mikas Remeika 1 Shenghao Wang 1 Yuichi Kato 1 Yabing Qi 1
1Okinawa Inst of Samp;T Okinawa Japan
Show AbstractOrgano-lead-halide perovskite based solar cells are the new solar cells that hold promises for large-scale solar-to-electricity conversion at low-cost. Since the first few reports, optimization in perovskite synthesis and device architecture has led to record power conversion efficiencies (PCEs) as high as 20.1% in just a few years. On the other hand, there are few studies on the lifetime and degradation mechanisms of fabricated OHP-based solar cells under operation conditions. In this work, we have studied the stability of FTO/compact-TiO2/mesoporous-Al2O3/MAPbBr3/spiro-MeOTAD/Au cells under continuous light illumination (AM1.5G) and applied bias tracking the maximum power point (MPP) for 100 hours. The cells were kept in controlled environment of dry-N2 and air with controlled relative humidity (~30%). We have taken the caution to determine the steady-state solar cell parameters (PCE, Jsc, Voc, and FF) because of large hysteresis effects observed on MAPbBr3 based solar cells. Time-evolution of stability profile operated in N2 and air showed the complexity in behavior. Post-mortem analysis of cells and systematic studies of individual layers by multiple techniques (X-ray photoelectron spectroscopy X-ray diffraction, X-ray fluorescence, UV-vis) reveals the mechanisms for the solar cell behaviors.
10:30 AM - OO9.06
Spectrum-Dependent Mechanism on the Generation of Oxidized Spiro-MeOTAD in Perovskite Solar Cell
Shen Wang 1 Shirley Meng 1
1University of California, San Diego La Jolla United States
Show AbstractAmmonium metal halide perovskite solar cells (PSCs) have attracted great attention recently as a new research hotspot in the fields of photovoltaic devices. Solar cells with over 20% power conversion efficiency (PCE) have been reported using ammonium lead iodide perovskite as an absorbing layer1. 2,2prime;,7,7prime;-tetrakis(N,N-di-p-methoxyphenylamine)-9,9prime;-spirobifluorene (Spiro-MeOTAD) is one of the most commonly used hole transport materials in PSCs. However, the intrinsic conductivity of Spiro-MeOTAD is relatively low unless it is doped with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) to oxidize the Spiro-MeOTAD. The mechanism for the generation of oxidized Spiro-MeOTAD in PSCs with LiTFSI is still under debate. Here we report a spectrum-dependent mechanism for the formation of oxidized Spiro-MeOTAD with LiTFSI. In our research, optical long pass filters are integrated with a solar simulator to control the spectral range and subsequently illuminate our samples. According to ultraviolet-visible spectroscopy (UV-Vis) and four-point probe conductivity measurements, two different Spiro-MeOTAD oxidization processes are observed depending on the spectral range of illumination and the participation of ammonium metal halide perovskite. The influence of oxidized Spiro-MeOTAD on device performance is investigated with J-V test and electrochemical impedance spectroscopy (EIS) as well. This is further confirmation of the spectrum-dependent mechanism.
1 W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Science, 348, 1234 (2015)
10:45 AM - OO9.07
Lewis Base Adduct Approach for Highly Reproducible 19.7% Efficient Perovskite Solar Cell
Namyoung Ahn 1 Mansoo Choi 1 Nam-Gyu Park 2
1Seoul National University Seoul Korea (the Republic of)2Sungkyunkwan University Suwon Korea (the Republic of)
Show AbstractWe report highly reproducible perovskite solar cell with power conversion efficiency (PCE) of 19.7% via acid-base adduct approach. Adduct of PbI2 formed with Lewis base N,N-dimethylsulfoxide (DMSO) and iodide (I-) in methylammonium iodide (CH3NH3I). Contrary to conventional perovskite precursors including PbI2 and equimolar CH3NH3I in N,N-dimethylformamide (DMF) solvent, we prepared a perovskite solutions with additionally equimolar DMSO (1:1:1 mol%) in order to form a well-defined CH3NH3Ibull;PbI2bull;DMSO adduct film, where the solvent DMF was selectively dissolved. The transparent adduct film was converted to a dark brown perovskite film by removing the DMSO in the adduct film at mild temperature by taking advantage of volatile characteristics of DMSO in adduct. Infrared (IR) spectroscopic studies showed that the S=O stretching frequency of DMSO at 1045 cm-1 shifted to lower frequency of 1015 cm-1 upon reacting DMSO with PbI2 and CH3NH3I, which is indicative of adduct formation. Charge extraction and mobility characteristics of the perovskite fabricated by the adduct approach were improved, compared with the conventional perovskite fabricated without DMSO-induced adduct. In addition, pinhole free large grains were obtained by adduct approach. As a result, a best PCE as high as 19.7% was achieved along with average PCE of 18.3%.
OO10: Perovskite Solar Cells III
Session Chairs
Wednesday AM, December 02, 2015
Hynes, Level 3, Room 302
11:30 AM - *OO10.01
Light Absorption in Perovskites and Silicon for Solar Cells
Kylie Catchpole 1
1Australian National Univ Canberra Australia
Show AbstractThe rapid advancement of thin-#64257;lm photovoltaic (PV) technology increases the real possibility of large-area Si-based tandems reaching 30% ef#64257;ciency, although light in these devices must be managed carefully. We identify the optical requirements to reach high ef#64257;ciencies. Strict conditions are placed on material parasitic absorption and transmission of contacts: Absorption of 20% of sub-bandgap light leads to the required top-cell ef#64257;ciencies of 18% at a bandgap of 1.5 eV to break even and 23% to reach tandem ef#64257;ciencies of 30%. Perovskite-silicon tandem cells present the #64257;rst low-cost devices capable of improving standalone 25% ef#64257;ciencies and we quantify the ef#64257;ciency gains and reduced thickness afforded by wavelength-selective light trapping. Applying these principles to a perovskite-based top cell characterized by strong absorption and high luminescence ef#64257;ciency, our modeling shows that tandem ef#64257;ciencies greater than 30% are possible with a bandgap of E g = 1.55 eV and carrier diffusion lengths less than 100 nm. At an optimal top-cell bandgap of 1.7 eV, with diffusion lengths of current vapor-deposited CH3NH3PbIxCl1 minus; x perovskites, we show numerically that tandem ef#64257;ciencies beyond 35% are achievable with careful light management.
We also use spectrally-resolved photoluminescence to experimentally determine the absolute value of the band-to-band absorption coefficient of perovskite methylammonium lead iodide CH3NH3PbI3 film from 675nm to 1400nm. Unlike other methods that are used to extract the absorption coefficient, photoluminescence is only affected by the band-to-band absorption and is capable of detecting absorption events at very low energy level. Absorption coefficient values as low as 10-14cm-1 are detected at room temperature, which extends the range of absorption data for this material by 14 orders of magnitude.
Finally, we study the band-to-band absorption enhancement due to various types of light trapping structures with photoluminescence (PL) on monocrystalline silicon wafers. Four basic light trapping structures are examined: reactive ion etched texture (RIE), metal-assisted etched texture (MET), random pyramid texture (RAN) and plasmonic Ag nanoparticles with a diffusive reflector (Ag/DR). We also compare two novel combined structures of front side RIE/rear side RAN and front side RIE/rear side Ag/DR. The results show that by combining RIE with RAN and Ag/DR, we can fabricate two structures with excellent light trapping efficiencies of 55% and 52% respectively, which is well above previously reported values for similar wafer thicknesses. A comparison of the measured band-band absorption and the EQE of back-contact silicon solar cells demonstrates that PL extracted absorption provides a very good indication of long wavelength performance for high efficiency silicon solar cells.
12:00 PM - OO10.02
Tuning the Optical and Electrical Properties of Organometal Halide Perovskite Nanoparticles
Alexander S. Urban 1 Jasmina Sichert 1 Yu Tong 1 Lakshminarayana Polavarapu 1 Verena Hintermayr 1 Niklas Mutz 1 Mathias Vollmer 1 Karolina Milowska 1 Carlos Cardenas-Daw 1 Stefan Fischer 2 Bert Nickel 2 Jacek Stolarczyk 1 Jochen Feldmann 1
1Ludwig-Maximilians-Universitauml;t Munich Germany2Ludwig-Maximilians-Universitauml;t Munich Germany
Show AbstractOrganometal halide perovskites have raced to the forefront of photovoltaic and light emission research due to their unique optical and electrical properties. While most of the current research has focused on thin films, the use of nanoparticles could further enhance the potential of this new material. To this end we synthesize colloidal perovskite nanocrystals with different solution-based fabrication strategies. By specifically selecting ligands and the precursor materials used we are able to not only control the morphology and dimensionality of the resulting nanocrystals, but can tune their optical and electronic properties as well. We look into quantum confinement in perovskite nanoplatelets, especially focusing on the properties of the exciton in these highly confined systems. Additionally, we vary the precursors to further tune the bandgap of such structures and we investigate energy transfer between such nanostructures as well as exciton diffusion.
12:15 PM - OO10.03
Ultra-Smooth MeNH3PbI3 Films with High Charge Carrier Mobilities Made by Physical Vapour Deposition
Eline Hutter 1 Carlito Ponseca 2 Michiel Moes 1 Ruben Abellon 1 Arkady Yartsev 2 Villy Sundstrom 2 Tom J. Savenije 1
1TU Delft Delft Netherlands2Lund University Lund Sweden
Show AbstractThe reported efficiencies of devices based on organometal halide perovskites (OMHPs) are rapidly increasing, however a solid understanding of the photophysical properties is still lacking. In this work, we study the kinetics of charge carriers in 200 nm thin films of CH3NH3PbI3 prepared by sequential physical vapour deposition (PVD) of the precursors CH3NH3I and PbI2. SEM and AFM microscopy show that this preparation method yields smooth films with a thickness variation less than 2 nm. The X-ray diffraction pattern shows only the (110) and (220) directions. Terahertz spectroscopy and time-resolved microwave conductivity (TRMC) are used to examine the opto-electronic properties of the perovskite layers. Mobilities are > 50 cm2/Vs, which are related to large crystalline domains within the film. In addition, for low light intensities (<1012 photons/cm2) we find no relaxation of the charge carrier mobility on time scales up to 1 ns. We apply a kinetic model to describe the TRMC traces recorded using laser intensities varying over 3 orders of magnitude with one set of kinetic parameters, yielding a trap density of less than 2 × 1015 cm-3 and a dark carrier concentration of 4 × 1015 cm-3. Altogether, we show that stoichiometric PVD results in perfectly flat nearly intrinsic films in which the charge carrier mobilities are substantially higher than in solution-processed OMHPs.
12:30 PM - OO10.04
Striking Improvement in Performance and Stability of Perovskite Solar Cells by Crystal Crosslinking with Aminoalkylphosphonates
Xiong Li 1
1Ecole Polytechnique Federale de Lausanne Lausanne Switzerland
Show AbstractDeveloping a strategy that enables perovskite solar cells to show high performance and simultaneously exhibit long term stability is one of the main challenges to exploit the true potential of perovskites for photovoltaic applications. Herein, we demonstrate that using judiciously designed aminoalkylphosphonates to modify the surface of methylammonium lead triiodide (MAPbI3) perovskite allows controlling the crystal nucleation and growth leading to dramatic enhancement in device performance and durability. We apply a simple one-step method, which involves spin-coating MAPbI3 precursor solution containing 4-aminobutylphosphonic acid chloride (4-ABPACl). Using STEM-EDS, XRD and 1H NMR analyses, we establish that 4-ABPACl surround the MAPbI3 structures via strong hydrogen bonding of the -PO(OH)2 and -NH3+ terminal groups to the perovskite surface affording crosslinking of MAPbI3 structures. Such architecture not only achieves a dramatic increase of the photovoltaic performance from 8.8% to 16.7% but also enhances greatly the cell stability in ambient condition or at 85 oC.
Symposium Organizers
Vivian Ferry, University of Minnesota
Ali Javey, University of California Berkeley
Evelyn Wang, Massachusetts Institute of Technology
Jia Zhu, Nanjing University
OO15: Nanostructured III
Session Chairs
Thursday PM, December 03, 2015
Hynes, Level 3, Room 302
2:45 AM - *OO15.01
Advanced Synthesis and Characterization for Nanowire Solar Cells
Erik C. Garnett 1
1FOM Inst AMOLF Amsterdam Netherlands
Show AbstractSemiconductor nanowires are among the most promising candidates for next generation high-efficiency photovoltaics. This is primarily due to their outstanding optical and electrical properties which provide large optical cross sections while simultaneously decoupling the photon absorption and charge carrier extraction length scales. These effects relax the requirements for both the minority carrier diffusion length and the amount of semiconductor necessary for the optimal utilization of the incident light power. This talk will detail recent efforts in our group focused on synthesis and characterization of nanowires for solar energy conversion. Synthetic descriptions will focus on novel metal-semiconductor epitaxial core-shell nanowires grown in solution as well as novel nanowire materials (e.g. Lenaite AgFeS2) and traditional growth mechanisms applied to new materials (VLS growth of CH3NH3PbI3 hybrid perovskite). Next a novel method for 3D mapping of the internal structure of core-shell nanomaterials, using only a standard SEM operated at a variety of voltages will be presented. Finally, a unique laser spectroscopy setup that allows for diffraction limited mapping of single nanowire solar cell absorption, photocurrent, external and internal quantum efficiency, and photoluminescence will be described and applied to Si, Cu2O, InP and CH3NH3PbBr3. By combining all these different characterization methods on the same nanowire solar cell, we hope to gain new insights into the working mechanisms and limiting factors in conversion efficiency.
3:15 AM - *OO15.02
Flexible Thin Film Solar Cells on Nanostructured Substrates with Performance Enhancement
Zhiyong Fan 1
1The Hong Kong University of Science and Technology Hong Kong Hong Kong
Show AbstractThin film solar cells have advantages of being low-cost and flexible. However, materials such as amorphous silicon (a-Si), Cadmium telluride (CdTe), etc., have relatively lower power conversion efficiency as compared with the conventional crystalline silicon solar cells. Meanwhile, bendability and reliability of the devices also depend on adhesion of the thin film materials on the flexible substrate, especially when plastic substrates are used. Introducing substrate nano/micro-texture is one of the promising methods to improve the performance of the thin film solar devices by enhanced optical absorption. Recently, we have developed a set of low cost and scalable approaches to fabricate various three-dimensional (3-D) nanostructures on metallic and plastic flexible substrates. By controlling the periodicity and the roughness of the nano-textures, it is discovered that optical absorption and the energy conversion efficiency of amorphous Si and organometallic perovskite materials based thin film solar cells can be largely improved over the planar counterparts. In addition, experiments suggest that thin film solar cells fabricated on nanostructured substrates also have significantly improved reliability. These results can shed light on design optimization of high performance flexible thin film solar cells.
3:45 AM - OO15.03
Broadband Absorption Enhancement in Lead Sulfide Nanowire Resonantors
Yiming Yang 1 Xingyue Peng 1 Steven Hyatt 1 Dong Yu 1
1Univ of California-Davis Davis United States
Show AbstractLight trapping in sub-wavelength semiconductor nanowires offers a promising approach to simultaneously reducing material consumption and enhancing photovoltaic performance. Nevertheless, the absorption efficiency of a nanowire, defined by the ratio of optical absorption cross section to the nanowire diameter, lingers around 1 in existing nanowire photonic devices, and the absorption enhancement suffers from a narrow spectral width. Here, we show that the absorption efficiency can be significantly improved in nanowires with higher refractive indices, by an experimental observation of up to 350% apparent external quantum efficiency (EQE) in lead sulfide (PbS) nanowire resonators, a 3-fold increase compared to Si nanowires. Furthermore, broadband absorption enhancement is achieved in single tapered NWs, where light of various wavelengths is absorbed at segments with different diameters. Overall, we fabricate and demonstrate single PbS nanowire Schottky solar cells with a power conversion efficiency (PCE) of 3.9%, which is comparable to single Si nanowire coaxial p-n junction cells, but with much simpler fabrication processes. The high efficiency is enabled by a combination of optical resonance, near bandgap open circuit voltage, and long minority carrier diffusion length. This work provides valuable insights for optimizing light absorption and solar conversion efficiency by harnessing the resonance absorption particularly of high refractive index materials and engineering the doping concentration of semiconductor nanowires.
OO16: Nanowires and PEC
Session Chairs
Thursday PM, December 03, 2015
Hynes, Level 3, Room 302
4:30 AM - *OO16.01
III-V Nanowires for Photovoltaics
Magnus Borgstrom 1
1Lund Univ Lund Sweden
Show AbstractSemiconducting nanowires have been recognized as promising materials for high-performance electronics and optics where optical and electrical properties can be tuned individually. The feasibility of III-V nanowire integration with existing silicon processing technology due to the small footprint between the silicon substrate and the nanowire material has further sparked that interest. For NWs to provide the new architecture for next generation photovoltaics there is a strong need to take complete control over synthesis. By optimizing growth conditions with respect to tapering we created nanowire-InP nanowire based solar cells using Au seed particles for growth.
We will report on the growth, processing and characterization of nanowire array-based solar cells with 13.8 % efficiency [1]. First, gold particles were patterned on InP substrates with a 500 nm pitch, using nanoimprint lithography. Then, about 1.5 mu;m long InP nanowires were grown using DEZn and TESn as doping precursors, to create an axially defined p-i-n junction. HCl was used to prevent radial overgrowth. The nanowires were processed as-grown with a transparent conductive oxide as top contact to create 1x1 mm2 solar cells, with 4 million nanowires per cell.
The solar cells were investigated using a sun simulator at Fraunhofer ISE CalLab reference setup. Although the 180 nm-diameter NWs only covered 12 % of the surface, the photocurrents were 71 % of the theoretical maximum for an InP solar cell. This is six times the limit in a simple ray optics description, and comparable to the record planar InP cell. To understand the absorption, we used three-dimensional electromagnetic optical modeling [2, 3]. We find excellent agreement between the spectra of modeled absorption and the experimentally measured external quantum efficiency.
1. J. Wallentin et al. Science, 339, 1057 (2013)
2. N. Anttu et al., Phys. Rev. B 83, 165431 (2011)
3. J. Kupec et al., Opt. Express 18, 27589 (2010)
5:00 AM - OO16.02
Single InP Nanowire Solar Cells
Sebastian Oener 2 Sander Mann 2 Alessandro Cavalli 1 Erik Bakkers 1 Erik Garnett 2
1Eindhoven University of Technology Eindhoven Netherlands2FOM Institute AMOLF Amsterdam Netherlands
Show AbstractInP nanowire solar cells have been proven to be among the most promising candidates for next generation photovoltaics by surpassing the energy conversion efficiency of their traditional counterpart, InP thin film solar cells. Their low surface recombination velocity and close to ideal band gap of ~ 1.4 eV make them an ideal platform to study the limits of semiconductor nanowire based solar cells. Here we present for the first time spatially resolved quantitative absorption and internal quantum efficiency (IQE) measurements of single InP p-i-n nanowire solar cells. We show how those measurements can be used to gain more insight into the light absorption, charge carrier separation and extraction mechanisms present in a device. Finally we use those insights to study the effects of surface functionalization on the nanowire solar cell performance.
5:15 AM - OO16.03
Integrating beta;-PbxV2O5 Nanowires with CdSe Quantum Dots: Toward Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer
Christopher C Milleville 1 Kate E Pelcher 2 Sarbajit Banerjee 2 David Watson 1
1University at Buffalo, State University of New York Buffalo United States2Texas Aamp;M University College Station United States
Show AbstractAchieving directional charge transfer across semiconductor interfaces requires careful consideration of relative band alignments. Here, we demonstrate a promising tunable platform for light harvesting and excited-state charge transfer based on interfacing β-PbxV2O5 nanowires with CdSe quantum dots. Two distinct routes are developed for assembling the heterostructures: linker-assisted assembly mediated by a bifunctional ligand and successive ionic layer adsorption and reaction (SILAR). In the former case, the thiol end of a molecular linker is found to bind to the quantum dot surfaces, whereas a protonated amine moiety interacts electrostatically with the negatively charged nanowire surfaces. In the alternative SILAR route, the surface coverage of CdSe nanostructures on the β-PbxV2O5 nanowires is tuned by varying the number of successive precipitation cycles. High-energy valence band X-ray photoelectron spectroscopy measurements indicate that “mid-gap” states of the β-PbxV2O5 nanowires derived from the stereoactive lone pairs on the intercalated lead cations are closely overlapped in energy with the valence band edges of CdSe quantum dots that are primarily Se 4p in origin. Both the midgap states and the valence-band levels are in principle tunable by variation of cation stoichiometry and particle size, respectively, providing a means to modulate the thermodynamic driving force for charge transfer. Steady-state and time-resolved photoluminescence measurements reveal dynamic quenching of the trapstate emission of CdSe quantum dots upon exposure to β PbxV2O5 nanowires. This result is consistent with a mechanism involving the transfer of photogenerated holes from CdSe quantum dots to the midgap states of β-PbxV2O5 nanowires.
5:30 AM - OO16.04
Self-Assembled Bulk Hetero-Junction Organic Solar Cell Based on Functionalized Rosette Nanotube
Ehsan Keyvani-Someh 1 Hicham Fenniri 1 Liang Shuai 1
1Northeastern University Boston United States
Show AbstractDue to their cost effective and flexible nature, organic photovoltaics are considered as a new and viable alternative for the silicon-based solar cells. However, power conversion efficiency (PCE), which depends on different factors (such as the morphology of the photoactive materials used, structure of the cell, and fabrication technique), is low due to the low conductivity and narrow absorption spectra. Since highly ordered nanoscale morphologies of the photoactive materials are ideal for the development of high efficient organic photovoltaics, self-assembly is an attractive pathway to introduce order from the nanoscale to the macroscale in order to enhance transport properties and, as a result, efficiency. In this study, self-assembled rosette nanotubes (RNTs) functionalized with porphyrin and/or oligothiophene with thermodynamically stable structures have been used as photoactive layer. Our results have shown that organic solar cells made of RNTs have significantly higher PCE than the corresponding non-assembled (disordered) devices.
5:45 AM - OO16.05
Vertically Aligned Highly Ordered Copper Indium Selenide(CISe) Nanotube Arrays for High Efficient Solar Energy Conversion
Wipula Priya Rasika Liyanage 1 Manashi Nath 1
1Missouri University of Science and Technology Rolla United States
Show AbstractNano-structuring the photoactive semiconducting materials have become an attractive alternative path for increasing the efficiency of photovoltaic devices. However, maintaining the monodispersity over the entire nanostructured pattern still presents a significant challenge to overcome since the overall functionality of nanodevice is highly dependent on the morphology of the individual nanofeatures. In this report, we describe a method for fabrication of vertically aligned and highly ordered copper indium selenide (CISe) nanotube and nanorod arrays for solar energy conversion. These ordered nanostructures were grown by confined electrodeposition on nanoelectrode patterns defined by electron beam lithography. The simplicity and the ability to manipulate morphological parameters such as nanotube/nanorod diameter, length, tube wall thickness, and the gap between nanotubes/nanorods with a high degree of precision and reproducibility throughout the entire pattern make this protocol especially attractive for the study of effect of physical dimensions of the photoabsorber on the solar energy conversion efficiency. Photoelectrochemical characterization of the as prepared nanotube/nanorod arrays show that a photocurrent comparable to that from a thin film device fabricated under similar conditions could be obtained from the nanodevice with the use of only a fraction (~10%) of the photoactive material compared to the thin film device. These nanotube/nanorod arrays have been characterized through powder X-ray diffraction, SEM, EDS, and UV-Vis studies for elucidating their structure, morphology, elemental composition and electronic nature. Photoconversion efficiency has been studied through photoelectrochemical studies in a liquid junction cell.
OO17: Poster Session II: Solar Energy Conversion II
Session Chairs
Thursday PM, December 03, 2015
Hynes, Level 1, Hall B
9:00 AM - OO17.01
Probing Electron Dynamics in Plasmon-Enhanced Mesoscopic Solar Cells Using Transient Absorption Spectroscopy
Holly F. Zarick 1 Abdelaziz Boulesbaa 2 Alexander Puretzky 2 Naiya Soetan 1 Rizia Bardhan 1
1Vanderbilt University Nashville United States2Oak Ridge National Laboratory Oak Ridge United States
Show AbstractPlasmonic enhancement of mesoscopic solar cells, including dye-sensitized solar cells (DSSCs) and perovskite-sensitized solar cells (PSSCs), has gained tremendous attention due to enhanced light harvesting characteristics generating 30-60% improvement in power conversion efficiencies as well as straightforward incorporation into standard processing techniques. Despite the progress, however, the fundamental mechanism that contributes to enhanced light harvesting and improved charge injection and charge separation remains poorly understood. Here we have investigated the impact of incorporating Au/Ag core/shell bimetallic metal nanostructures in three different mesoscopic solar cell systems: N719 sensitized DSSCs, methylammonium lead triiodide (MAPbI3) PSSCs, and methylammonium lead tribromide (MAPbBr3) PSSCs. We have utilized transient absorption spectroscopy to examine exciton generation, electron injection, and charge recombination within the various systems over a range of nanoparticle loadings and compared these parameters to non-enhanced devices. Our results demonstrate plasmon-enhanced light trapping and corresponding charge-carrier generation occurs at time scales of several hundred picoseconds, suggesting metal nanostructures increase light absorption in the active layer of solar cells, enhancing the number of excitons generated. Therefore, a larger number of electrons is available to rapidly transfer to the TiO2 conduction band when plasmonic nanostructures are present, which results in faster electron injection into the TiO2 conduction band before recombination can occur in the bulk. We have provided a detailed mechanistic understanding of our results. This fundamental insight will ultimately result in implementation of plasmonic enhancement to other mesoscopic and planar solar devices to achieve unparalleled light harvesting characteristics.
9:00 AM - OO17.02
Engineering the Photonic Density of States in Woodpile and Inverse Woodpile Photonic Nanocrystalline Silicon Solar Cells for Light Trapping
Baomin Wang 1 Paul Leu 1
1University of Pittsburgh Pittsburgh United States
Show AbstractIn this work, we demonstrate that woodpile (WP) and inverse woodpile (IWP) photonic crystal nanocrystalline silicon (nano-Si) structures may be engineered for light trapping in solar cells. We use finite difference time domain simulations to show that the geometry of these photonic crystals may be varied such that there is an increased photonic density of states above the band gap. We additionally show that the utilization of a silver back contact as opposed to a perfectly electric conductor only decreases the solar absorption by a small amount. Finally, we show that these photonic crystals exhibit enhanced absorption over a wide range of incidence angles.
For a fixed equivalent thickness of 200 nm, we found a 104% and 77% absorption enhancement in the optimized WP and IWP structures respectively compared to nano-Si thin film. The WP and IWP structures have significantly superior absorption than the nano-Si thin film in almost the whole range of the solar spectrum. These structures have several additional resonance peaks in the infrared and visible parts of the spectrum. There is an increased photonic density of states in these parts of the spectrum, such that the absorption is enhanced. We also investigate the dispersive band structures for the IWP and WP structures. This study demonstrates a new structure for light trapping over a broad range of photon energies and incidence angles. Other equivalent thicknesses are additionally studied.
9:00 AM - OO17.03
Band Gap Narrowing and Increased Lifetimes in N-Doped La2Ti2O7 for Hydrogen Generation
Brandon Tyler Yost 2 Scott Kevin Cushing 2 1 Nianqiang Wu 1 Alan D Bristow 2
1West Virginia University Morgantown United States2West Virginia University Morgantown United States
Show AbstractNitrogen doping was found to extend lanthanum dititanate&’s (LTO), La2Ti2O7, absorption from 380 nm to 550 nm without decreasing ultraviolet photoactivity, giving a promising 2.3 eV bandgap for solar water splitting. In this presentation, transient absorption spectroscopy with a supercontinuum and THz probe confirm N-doping creates a continuum of states above the intrinsic LTO&’s valence band (VB), shrinking the band gap without diminishing excited carrier lifetimes. The supercontinuum probe reveals excited state absorption and sub-band gap state densities match density functional theory (DFT) predictions. The lifetime is measured for excitation at the VB-CB and dopant continuum-CB transitions, showing that the added states increase initial thermalization of excited carriers but maintain lifetimes comparable to intrinsic LTO. The THz probe confirms the visible light excited carriers are mobile and not trapped, allowing a large increase in photoactivity. Further, by adding reduced graphene oxide (RGO) and plasmonic gold nanoparticles to the N-doped LTO, charge extraction and light absorption is improved, tripling the hydrogen generation rate.
9:00 AM - OO17.04
Ni Nanochain/SiOx (x
Xiaobai Yu 1 Xiaoxin Wang 1 Qinglin Zhang 2 Jifeng Liu 1
1Dartmouth College Hanover United States2University of Kentucky Lexington United States
Show AbstractThe oxidation of cermet solar absorbers has become one of the major concerns in high-temperature concentrated solar power (CSP) systems. The prevention of metal nanostructure oxidation in cermet absorbers and the development of atmospherically stable CSP systems at high working temperatures are thus very important. Recently, we have demonstrated a high-optical-performance, solution processed Ni nanochain/SiOx with promising anti-oxidizing behavior at temperatures up to 600 °C for several hours in air, while the mechanism and interfacial transformation are not completely understood [1]. In this paper, we focus on the interfacial analysis of these anti-oxidation high-performance Ni nanochain-SiOx (x<2) selective solar thermal absorbers, studying interfacial formation of nickel silicides at the Ni/SiOx interface and the corresponding mechanisms of the anti-oxidation behavior. To further enhance the anti-oxidation effect, we investigate high-temperature (>900 oC), pre-operation annealing in N2 ambient to form desirable nickel silicide phases at the Ni/SiOx interface, as confirmed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). A remarkable saturation of the oxidation process shows up after annealing at 550 °C in air for 12 h from X-ray diffraction (XRD) and optical reflectance spectra analyses. The oxidation vs. time curve derived from XRD shows clear saturation behavior deviating from the kinetic Deal-Grove oxidation model. In addition, air annealing at > 550 °C for up to 32 h doesn&’t induce any further impact on the optical or structural properties of the solar thermal coatings. Note that the surface plasma polariton (SPP) response in metal nanostructures can be used as a very sensitive technique to detect any interfacial changes. Therefore, the optical reflectance spectra evolution of these plasmonic structures can be associated with the interfacial formation of Ni oxide and silicide, as well as self-terminated oxidation upon 12h annealing at 550 °C. These results are unusual considering that the average diameter of the Ni nanoparticles is only 80 nm. They strongly suggest the possibility of anti-oxidizing Ni/SiOx cermet solar selective absorbers towards a thermodynamically stable state via interfacial engineering. More promisingly, the principles of self-terminated oxidation could apply to many other metal nanomaterials to achieve desirable performance.
Reference
[1] X. B. Yu, X. X. Wang, Q. L. Zhang, J. C. Li, and J. F. Liu, J. Appl. Phys. 116, 073508 (2014).
9:00 AM - OO17.05
Hierarchically Self-Assembled Super Structural TiO2 Microspheres: Enhanced Excitonic Efficiency as Photocatalyst and Photoanode Material
Arunkumar Shanmugasundaram 1 Pratyay Basak 1 Manorama V Sunkara 1
1CSIR-Indian Institute of Chemical Technology Hyderabad India
Show AbstractComplex three-dimensional (3-D) super structural titanium dioxide (TiO2) architectures with controlled shape and size were synthesised by facile hydrothermal synthesis route using titanium tetrachloride (TiCl4) as the metal precursor and hydrochloric acid (HCl)/ammonium chloride (NH4Cl) as the mineraliser/structure directing agent. Interesting hierarchical architectures like nanoparticles assembled spheres, nanowires assembled spheres, nanorods assembled spheres, and nanosheets assembled spheres were achieved by controlled variation of crystal structure modifier. The synthesised materials were characterised in detail by different analytical techniques and the findings are consistent with each other. A detailed mechanism of formation for these hierarchical architectures is postulated based on detailed analysis. The as-prepared TiO2 architectures exhibit superior performance as visible light photocatalyst for the degradation of Rhodamine-B (RhB) and photoanode for dye sensitized solar cells.
Owing to its unique physico-chemical and opto-electronic properties TiO2 is considered as, one of the most important metal oxide semiconductor and its potential importance have been demonstrated in many areas of science and technology.1 Over the years several forms of TiO2 hierarchical architectures have been reported.2 However, the development of facile and effective methods for creating novel super structural architectures which can perform better than the existing materials is a topic of research and remains a great challenge for materials society.3 Herein, we demonstrate in detail the preparation of 3-D hierarchical rutile TiO2 architectures with controlled morphologies through facile hydrothermal synthesis method notably without use of any surfactant or template. The synthesized materials were characterized extensively by XRD, TG-DTA, micro-Raman, and UV-DRS. Surface area and pore size distribution were determined from N2 adsorption and desorption isotherms. Morphological evaluations of the materials were carried out by electron microscopy in scanning and transmission mode. The TiO2 architectures were used as photocatalyst for the degradation of Rhodamine-B (RhB) and photo-anode for DSSCs. The results demonstrate that the synthesised material exhibits ~10 times better enhanced photocatalytic activity towards the degradation of RhB compared to commercial TiO2. The DSSCs thus fabricated exhibited ~40% much higher energy conversion efficiency compared to commercial P25. This could be attributed to the unique hierarchical structure, high surface area, higher light scattering ability, and lower electron recombination rates.
References
Zeai et al. ACS Appl. Mater. Interfaces. 2013, 5, 8663minus;8669.
Kim et.al. Adv. Mater. 2009, 21, 3668-3673.
Pan et.al Science, 2001, 291, 1947-1949.
9:00 AM - OO17.06
Block Copolymer Self Assembly Based Surface Nanotextures for Broadband Antireflection in Solar Cells
Charles T. Black 1 Atikur Rahman 1 Andreas Liapis 1 Antonio Checco 2
1Brookhaven National Laboratory Upton United States2Brookhaven National Laboratory Upton United States
Show AbstractWe present a new approach to designing highly-antireflecting and broadband surface nanotextures with densely packed feature sizes smaller than 50 nanometers using block copolymer self assembly. Block copolymer thin films provide a robust method for generating regular, uniform patterns at length scales in the range of ten nanometers, over arbitrarily large areas. A significant advantage of this approach is its ease of integration with all other aspects of traditional thin-film processing, including plasma-based etching and metallization.
Here, we precisely tailor the dimensions of the nanotexture design in the range of ~10 to 70 nanometers, reducing the average reflectance from a polished silicon wafer to less than 1 percent across a broad wavelength range from 350 nm to 1 micron using textures less than 200 nm tall. The surface nanotextures maintain less than 5 percent broadband reflectance even for incident light angles as high as 60 degrees. Implementing this efficient antireflection approach in crystalline silicon solar cells significantly betters the performance gain compared to an optimized, planar antireflection coating.
We will further highlight the richness of this self-assembly based fabrication approach through examples of its implementation in silicon, silicon nitride, glass, and plastic thin films, and its ability to render solar cell surfaces highly water repellent.
9:00 AM - OO17.07
Charge Transport and Interfacial Recombination in Extremely Thin Absorber Solar Cells
Michael E Edley 1 Treavor Z. Jones 1 Jason Baxter 1
1Drexel University Philadelphia United States
Show AbstractSolar cells can employ semiconductor-sensitized nanostructured architectures to decouple the functions of light harvesting and charge transport into different materials. This allows use of inexpensive materials and processing routes that would not be possible in planar solar cells. We report on extremely thin absorber (ETA) solar cells that use ZnO nanowire arrays coated with an ultrathin (<5nm) CdS buffer layer and a thin (10-40 nm) CdSe absorber layer. CdSe absorbs visible light, and photoexcited electrons are injected into ZnO while photoexcited holes oxidize the redox species in an electrolyte. After charge separation, transport of electrons through ZnO and holes through liquid electrolyte must occur faster than interfacial recombination. Understanding the fundamental carrier dynamics of charge transport and interfacial recombination in ETA cell is vital to cell design.
Our approach correlates I-V characterization with transient photocurrent (TPC), transient photovoltage (TPV), and electrochemical impedance spectroscopy (EIS) to study electron transport and interfacial recombination in ETA cells. I-V characteristics of our ETA cells reveal that Jsc is maximized with a CdSe thickness of ~30 nm for a given nanowire array, which balances light harvesting and charge collection effects. TPC at short circuit showed electron transport times of 70 mu;s for ETA cells made from 500 nm long nanowires coated with 10 nm of CdSe. Transport times increased with nanowire length but were independent of CdSe thickness for thin absorbers, indicating that transport through the nanowires dominates the collection time. Recombination times were on the millisecond scale, as measured by TPV and EIS at open circuit. Transport times are much faster than recombination times, indicating efficient charge collection. Interestingly, transport times decreased 10-fold for CdSe coatings above a critical thickness of 40 nm, which is estimated by Mott-Schottky analysis to be the depletion width. Additionally, cells with thin CdSe coatings showed reasonably good fill factors, but thicker CdSe coatings exhibited significant bias-dependent collection. Bias-dependent EIS revealed that cells with the thinnest CdSe shells had the highest recombination resistance. Though counterintuitive in the context of pinholes, this result indicates that the depletion region formed at the electrolyte-CdSe interface extends into the ZnO for thin CdSe, thus suppressing interfacial recombination but also slowing transport within the nanowires by confining electrons to a very narrow core. In contrast, when the CdSe thickness exceeds the depletion width, photocurrent is limited by charge separation across the CdSe coating; recombination resistance is low but independent of bias. These results reveal the importance of managing relative length scales of absorber coating thickness and depletion width to control charge separation and transport in semiconductor-sensitized nanostructured solar cells.
9:00 AM - OO17.08
Ferroic Domain Pattern in micro;m-Sized Methylammonium Lead Halide Grains
Ilka Hermes 1 Simon Anselm Bretschneider 1 Victor Wolfgang Bergmann 1 Dan Li 1 Alexander Klasen 2 Julian Mars 1 Wolfgang Tremel 2 Markus Mezger 2 Ruediger Berger 1 Brian Rodriguez 3 Stefan A L Weber 1
1Max Planck Institute for Polymer Research Mainz Mainz Germany2Johannes Gutenberg University Mainz Germany3University College Dublin Dublin Ireland
Show AbstractMethylammonium lead halides (MAPbX3) are promising materials for photovoltaic applications, but the physical processes behind their performance are not yet fully understood. Unusual phenomena such as the photocurrent-voltage hysteresis continue to puzzle the scientific community. This necessitates the ongoing research on the material properties of the perovskite compound.
In our piezoresponse force microscopy (PFM) study on µm-sized MAPbI3-xClx grains we observed a periodically alternating structure reminiscent of ferroelastic domain patterns. We propose a domain orientation and suggest that the ferroelastic behavior is induced by internal strain during the crystallization. Although ferroelasticity is often connected to ferrelectricity we did not find any evidence of ferroelectricty in PFM switching experiments. However, this result could be explained by the domain orientation in respect to the sample surface.
This first observation of a ferroelastic domain pattern will provide a better understanding of the ferroic behavior of MAPbX3.
9:00 AM - OO17.09
Electronic Energy Landscape Engineering in Colloidal Quantum Dot Solids through Connectivity Motifs for Solar Energy Harvesting
David Zhitomirsky 1 Huashan Li 1 Shreya Dave 1 Jeffrey C. Grossman 1
1Massachusetts Institute of Technology Cambridge United States
Show AbstractColloidal Quantum Dots (CQDs) are nanoengineered solution-processable materials that offer size-bandgap tunability making them attractive materials for tunable optoelectronic devices. In order to make solid state films, these materials must undergo surface manipulation often involving replacing long insulating ligand molecules with shorter ones, thus enabling electrical connectivity. Recently, there have been wide reports of ligands influencing several other properties, such as Stokes shift, optical extinction coefficients, band alignment, and electronic doping. Hence, CQDs offer the unique opportunity of simultaneously engineering materials properties through a combinatorial ligand approach.
We took the view that understanding the impact of different types of ligands would be best realized through a combined experimental and theoretical approach. We combined high-resolution transmission electron microscopy (HR-TEM) imaging with density functional theory (DFT) in order to reveal and compare the impact of different types of connectivity between PbS CQDs, spanning organic, atomic, and fused linking motifs. We carried out the studies on CQDs with different bandgaps, in order to understand the size-dependence of making connected films. Our findings suggest that there while large 8 nm nanoparticles may preserve order and packing within the film, and offer a more controlled way of connecting CQDs, small 3 nm photovoltaics-relevant nanocrystals are highly sensitive to the ligand treatment conditions, often resulting in completely random packing and clustering.
We then focus on the specific application of photovoltaic energy conversion using these materials. We discuss the tradeoffs between attaining high mobility and maintaining a trap-free energy gap, which may be realized by combining different types of ligands that offer the best control for connectivity and passivation, respectively. Our findings suggest that optimal CQD films may be realized through a highly selective, facet-dependent, ligand-binding strategy that will involve several different types of ligands to serve as levers for critical CQD materials properties. Such advancement in surface engineering would result in unprecedented control over materials properties and pave the way for novel applications for CQD solid-state materials.
9:00 AM - OO17.10
Au NPs Decorated WO3 NWs-Branched TiO2/WO3 Heterostructure Photoanode by Direct Imprinting-Assisted Hydrothermal Growth for Photoelectrochemical Water Splitting
Hak-Jong Choi 1 Yang Doo Kim 1 Ju-Hyeon Shin 1 Sung-Jin Moon 1 Chaehyun Kim 1 Jun-Ho Jun 1 Heon Lee 1
1Korea Univ Seoul Korea (the Republic of)
Show AbstractRecently, solar energy has a lot of attention in the fields of renewable energy due to cleanness, safety, and unlimitness. Photoelectrochemical water splitting is one of the most promising solar conversion technology, which solar energy can be converted as chemical energy in form of H2. In order to efficient solar-to-hydrogen conversion, controlled semiconductors are needed to absorb sunlight and make some photoelectrochemical reactions. Several semiconductor metal oxide, including TiO2, Fe2O3, WO3, and BiVO4, have suitable electronic properties. Especially, TiO2 has been considered one of the most promising materials for photoelectrochemical water splitting due to its photostability, relatively low cost, and non-toxicity. However, the photoelectrochemical water splitting of TiO2 has some limitation for efficiency because TiO2 can only absorb ultra-violet light according to their electronic band gap, which accounts for only 4% of the incoming solar energy. In order to overcome this low efficiency, heterostructure of two different metal oxides has been recently considered as a new photoanode materials.
In this study, we fabricated hierarchical TiO2/WO3 heterostructure photoanode using nanoimprinting of TiO2/WO3 nanoaprticle (NP) dispersed resin and subsequent hydrothermal growth. First, WO3/TiO2 NP dispersed resin is synthesized using TiO2 NP solution, WO3 NP solution, monomer, and photo-initiator. Then, WO3/TiO3 microstructure is formed on transparent conducting layer using nanoimprinting of WO3/TiO2 NP dispersed resin. Subsequently, WO3/TiO2 microstructure is sintered at high temperature over 500 °C. Finally, WiO3 nanowires (NWs) are grown on WO3/TiO2 microstructure in order to fabricate hierarchical WO3/TiO2 heterostructure and then, Au nanoparticles(NPs) are decorated for enhance the light absorption. Optical properties of Au NPs decorated hierarchical WO3/TiO2 heterostructure are characterized using UV-Visible spectroscopy. In addition, photocurrent and external quantum efficiency of Au NPs decorated hierarchical WO3/TiO2 heterostructure photoanode are measured using solar simulator and incident photon-to-current efficiency measurement. Compared with hierarchical TiO2 structure, Au NPs decorated hierarchical WO3/TiO2 heterostructure exhibit the high efficiency over 2 times and photocurrent in the range of visible light.
9:00 AM - OO17.11
Revealing the Effect of Electromagnetic Coupling Modes on the Spectral Response in a Nanoparticle-Mediated Polymer/Metal Thin Film System
Binxing Yu 1 Deirdre O'Carroll 2 1 3
1Rutgers Univ Piscataway United States2Rutgers Univ. Piscataway United States3Rutgers Univ. Piscataway United States
Show Abstract
We identify the optical modes existing in a sphere-on-plane system consisting of an ultra-thin (16 - 50 nm) conjugated polymer spacer layer sandwiched between a plasmonic Au nanoparticle and a thin metal film, and successfully manipulate these modes to enhance the absorptive and emissive function of the polymer[1]. The scattering, absorption and photoluminescence spectral responses in this complex system are mediated by one or more of the following modes: a horizontal image dipole coupling mode at a wavelength of ~550 nm; a vertical image dipole coupling mode at ~620 nm; and horizontal and vertical coupling modes between localized surface plasmon resonances (LSPRs) supported by the nanoparticle and surface plasmon polaritons (SPPs) supported by the metal film at wavelengths above 630 nm. Additionally, single-particle dark-field spectroscopy indicates the existence of a sub-set of particles for which the signal of the horizontal image dipole coupling mode is quenched. This is attributed to the partial-embedding of Au nanoparticles into the polymer spacer and leads to higher scattered light intensities at longer wavelengths. With the aid of electromagnetic simulations, we succeeded in developing an interpretation of the experimentally observed dark-field spectral variations with changes in the spacer thickness and degree of nanoparticle embedding. Furthermore, absorption enhancement in the polymer spacer increases with decreasing thickness, demonstrating the increased light trapping ability of the Au nanoparticles for ultra-thin optically absorbing layers. In addition, since this sphere-on-plane system provides large electric field enhancement in the conjugated polymer spacer at controlled resonance wavelengths[2], whereas the underlying metal film typically results in rapid quenching of the fluorescence of the conjugated polymer, photoluminescence of different polymer spacers in such sphere-on-plane systems is studied to fully characterize the competing plasmonic mechanisms. This work shows how electromagnetic coupling modes can be manipulated in a sphere-on-plane system with an ultra-thin conjugated polymer spacer and is a foundation for studies and applications of plasmonic structures for light management in optoelectronic devices with active layer thickness well below that used in more conventional devices.
B. Yu, J. Woo, M. Kong, D. M. O&’Carroll, Nanoscale, in-press.
B. Yu, S. Goodman, A. Abdelaziz, D. M. O&’Carroll, Appl. Phys. Lett., 2012, 101, 151106.
9:00 AM - OO17.12
Interfacial Modification of Heterojunction Metal Oxide Photo Anodes for Efficient Solar Water Splitting
Yakup Goenuellue 1 Jennifer Leduc 1 Thomas Fischer 1 Sanjay Mathur 1
1Univ of Cologne Cologne Germany
Show AbstractPhotoelectrochemical splitting of water to produce hydrogen is a viable approach to transform sunlight into chemical energy, which has triggered a quest for suitable photocatalysts. Transition metal oxides such as α-Fe2O3 and TiO2 are favorable candidates and provide intrinsic advantages in terms of high photo-stability and sufficient mobility of charge carriers, besides their earth-abundance. Despite these advantages, no commercially viable material exists that is able to maintain the proposed minimum 10 % requirement for solar energy to hydrogen fuel efficiency (STH). In the search for strategies enabling an enhancement of the photo electrochemical properties of metal oxide, various methods such as doping of metal oxide, co-catalyst or mutlilayering were conducted.
In this study, we have focused on Interfacial modification of α-metal oxide multilayer photoanodes deposited by plasma enhanced chemical vapor deposition (PE-CVD). Different mechanisms such as patterning of multilayering with different structure (bar structure or line structure) or graphene supporting were examined in this study. The α- bilayer electrode exhibited enhanced PEC responses in terms of a lower onset potential and a higher photocurrent density when compared to the single layer α-Fe2O3 electrode. This enhancement was observed to be due to synergistic light absorption with the bilayer electrode, although charge carrier recombination occurred faster due to interfacial defect states. The incorporation of a graphene layer between the α-Fe2O3/TiO2 double layer and the FTO substrate resulted in a doubling of the photocurrent, but lead to a loss of the synergistic effect between the two active metal oxide layers. However, depositing the graphene between the two metal oxide layers, α-Fe2O3/graphene/TiO2 resulted in an even higher photocurrent, while retaining the enhanced onset potential of the double layer electrode. This enhancement was observed to be due to either the passivation of the oxide defect states or enhancement of the charge transfer between the two oxide layers.
9:00 AM - OO17.13
Environmental Effects on Perovskite Degradation
Eric Grulke 1 Alexander G. Agrios 1
1Univ of Connecticut Storrs United States
Show AbstractPerovskite solar cells (PSC) have emerged as a breakthrough photovoltaic technology, with solar power conversion efficiencies already exceeding 20%. Yet the records carry a heavy asterisk, as the stability of the devices remains poor. Shaping the PSC into a commercially viable technology for low-cost, high-efficiency solar energy requires ensuring that the devices can provide a reasonable service life. This requires greatly enhancing the stability of the materials over present performance, and the problem appears to lie mainly with the perovskite material itself, which undergoes decomposition and/or phase changes, especially upon exposure to moisture. Improving device stability, therefore, requires an understanding of the mechanisms and circumstances that result in degradation of the perovskite layer.
In this work, organolead-halide perovskites, fabricated using different techniques and compositions, are coated on glass substrates and subjected to a controlled atmosphere where temperature, moisture, and oxygen content are varied. Gaseous products evolved from the perovskite material are captured and analyzed by an online GC-MS, indicative of the chemical reactions responsible for degradation. XRD spectra of the films over time report on changes in crystal structure resulting from these and other reactions. Whole PSC devices subjected to the same treatments, or made from perovskite films subjected to such treatments and then finished with a hole-transport layer and contacts, are measured to correlate the chemical and materials shifts with device performance. Together, the characterizations give insight as to the mechanisms and effects of degradation as an effect of key parameters of the environment.
9:00 AM - OO17.14
Impact of the Size Distribution Controls in Colloidal Quantum Dot Solids on the Charge Carrier Mobility and Photovoltaic Performance
Sangjin Lee 1 David Zhitomirsky 1 Jeffrey C. Grossman 1
1MIT Cambridge United States
Show AbstractThe effect of size-dispersion in colloidal quantum dot (CQDs) solids on the charge carrier mobility was computed using charge hopping transport models. The experimental film deposition processes were replicated using a molecular dynamics simulation where the equilibrium positions of CQDs with a certain radii distribution were determined under the granular potential. Those radii and positions of CQDs were then used in the charge hopping transport simulator where the carrier mobility was estimated. We observed large decreases (up to 70%) in electron mobility for typical experimental polydispersity (about 10%) in CQD films. These large degradations in hopping charge transport were investigated using transport vector analysis with which suggested that the site energy differences raised the portion of the off-axis rate of charge transport to the electric field direction. Furthermore, we have shown that controlling the size distribution remarkably impacted the charge carrier mobility and suggested that tailored and potentially experimentally achievable re-arrangement of the CQD size ensemble can mediate the mobility drops even in highly dispersive cases, and presents an avenue towards improved charge transport. In addition, we studied the degradation in CQD solar cells with respect to the polydispersity and how these enhanced charge transport can boost the photovoltaic performances.
9:00 AM - OO17.16
Ligand-assisted co-assembly approach toward mesoporous hybrid catalysts of transition-metal oxides and noble metals for photochemical water splitting
Ben Liu 1 Steven Suib 1 Jie He 1
1University of Connecticut Storrs United States
Show AbstractThe utilization of noble metal nanoparticles (NPs) (e.g. Au and Ag) as photo antennas in metal oxides having a large band gap is of particular interest for surface plasmon enhanced photochemical water splitting. Current synthetic approaches of metal oxide/noble metal hybrids, however, lack controllability in engineering their nanostructures. We herein present a new bottom-up synthetic approach to preparing mesoporous transition metal oxide/noble metal hybrid catalysts via a ligand-assisted co-assembly of amphiphilic block polymer micelles and polymer-tethered noble metal NPs. The use of polymer ligands that can improve the stability of metal NPs under the extreme conditions used for the sol-gel chemistry is a key to the controllable synthesis of hybrid catalysts. It is found that the hybrid catalyst possesses long-range, periodically ordered mesoporous nanostructures with an average pore size of 25 nm and a wall thickness of 18 nm. Noble metal NPs are homogenously embed into the mesostructural channels of the hybrid catalysts without NP fusion or migration. Our synthetic approach offers a general and straightforward method to precisely tune the sizes and loading amounts of noble metal NPs in metal oxides; and this system, thus, provides a solid platform to clearly understand the role of noble metal NPs in photochemical water splitting. We further demonstrate that the presence of trace amounts of metal NPs (~0.1 wt%) can largely enhance the photocatalytic activity for water oxidation up to 4 fold. Our findings can conceivably be applied to other semiconductors/noble metal catalysts which may stand out as a new methodology to build up highly efficient solar conversion systems.
9:00 AM - OO17.17
Surfactant-Assisted Synthesis of Monodisperse Fe-Doped TiO2 Nanocrystals for Applications in Photocatalysis
Swati Naik 1 2 Gabriel Caruntu 1 2
1Central Michigan University Mount Pleasant United States2Central Michigan University Mount Pleasant United States
Show AbstractTiO2 is a well-studied photocatalyst due to its several advantages such as a high chemical stability, low cost, non-toxicity and chemical inertness. Among the various polymorphs of TiO2, anatase is known to be photocatalytically active and technologically important. An efficient photocatalyst should have a high surface area and a narrow bandgap for efficient solar energy conversion. Surfactant-assisted synthesis regulates the shape and size of the nanocrystals, whereas the doping with either anions or cations allows for the engineering of the band gap. In this work, we have used hydrothermal method to synthesize uniform monodisperse nanocrystals of both pristine and Fe-doped TiO2 and tested their photocatalytic performance in dye degradation reactions. X-Ray diffraction studies performed were in good agreement with Raman spectroscopy experiments showing the formation of phase pure anatase TiO2 under solvothermal conditions. Electron microscopy showed that the nanocrystals retain diamond-type morphology, they are well-separated from each other as a result of the retention of oleic acid on their surfaces and have the tendency to self-assemble onto planar surfaces. Diffuse Reflectance Spectroscopy (DRS) was employed to understand the effect of dopant ion on the band structure of TiO2 and showed a progressive decrease of the band gap value with increasing the content of transition metal. BET studies will be used for surface area analysis, being therefore suitable as catalysts in various types of photocatalytic processes.
9:00 AM - OO17.18
Photoluminescence Quenching of Single-Walled Carbon Nanotubes through a Flavin-Helical Encase via a Proximal C60 Moiety
Fotios Papadimitrakopoulos 2 Mehdi Mollahosseini 2 Erandika Karunaratne 2 Jose A Gascon 1 George Gibson 1
1Univ of Connecticut Storrs United States2Univ. of Connecticut Storrs United States
Show AbstractThe strong light absorption of single-walled carbon nanotubes (SWNTs) in the visible and near infrared (NIR) regions, alongside their outstanding transport characteristics, have generated considerable interest in using them as active components in photovoltaic (PV) applications. Harvesting such potential typically utilizes heterojunctions of semiconducting (sem)-SWNTs with fullerenes, π-conjugated or combination of both. These heterojunctions however, exhibit considerable inhomogeneity in terms of the localized environment around SWNTs. In order to improve on such inhomogeneity, our group has relied on the highly organized flavin mononucleotide (FMN) assembly around various SWNTs.1 By appropriate substitution of the flexible side group, C60 can be linked to an organic soluble flavin derivative (FC12)1a to afford an intramolecular linked structure, termed FC60, which can then be organized around SWNTs. In this contribution, we describe the synthesis and characterization of FC60 and discuss methods on how to successfully incorporate it within the FC12 helix. Moreover, we show that the incorporation of as much as 1% FC60, causes effective photo-induced charge transfer quenching of the NIR emission of SWNTs, as indicated by steady state and transient spectroscopy. The orderly insertion of FC60 within the FC12 helix induces minimal dispersion (in terms of line broadening of the ESii transitions of SWNTs), which can ultimately enable a new class of highly organized supramolecular architectures with spectrally sharp response in the visible and NIR regions.
1. (a) S.-Y. Ju, W. Kopcha, F. Papadimitrakopoulos, Science, 2009, 323, 1319-1323; (b) R. Sharifi, M. Samaraweera, J. A. Gascoacute;n, F. Papadimitrakopoulos, J. Am. Chem. Soc., 2014, 136(20), 7452-7463.
9:00 AM - OO17.19
Spectroelectrochemical Characterization of Charge Dynamics at the Interface of Molecular Functionalized Nanostructured Thin Films
Davide Moia 1 Anna Szumska 1 Valerie Vaissier 1 Tomas Leijtens 2 Henry James Snaith 2 Brian Orsquo;Regan 3 Jenny Nelson 1 Piers R.F. Barnes 1
1Imperial College London London United Kingdom2University of Oxford Oxford United Kingdom3Imperial College London London United Kingdom
Show AbstractUnderstanding interfacial charge transfer dynamics in thin films of semiconducting materials is a critical factor to achieve high performance solar energy conversion and storage devices. In this work we propose a multiscale characterization of interfacial processes via transient spectroelectrochemistry and polarization spectroscopy techniques. This approach enables the investigation of charge transfer between different phases but also lateral charge transfer phenomena at functionalized interfaces.
Intermolecular charge transport between molecules chemically anchored to the surface of oxide nanocrystals has been applied to high performance lithium ion batteries,1 electrochromic devices2 and solar fuel production.3 The role of this process in solar energy photo-electrochemical devices has been proposed4 but not demonstrated before now. We show evidence that charge diffusion on the surface of sensitized nanostructured is of fundamental importance to the functioning of different optoelectronic systems.
We address the optimization of the dye regeneration process occurring during photoconversion in solid state dye sensitized solar cells. We find that the fraction of dye regeneration attributable to inter-dye hole percolation can be >50% of the overall regenerated dyes under low pore filling conditions by the hole transporting material spiro OMeTAD. This fraction is reduced to about 5% when the infiltration of the HTM in the pores is optimized. We have therefore clarified the process of charge collection in this class of devices and demonstrated that hole percolation in the dye monolayer is necessary to achieve 100% regeneration yield.
Secondly, we show transient absorption and transient absorption anisotropy spectroscopy data suggesting correlation between the electron-hole recombination kinetics in dye sensitized mesoporous TiO2 on the mobility of holes on the surface of the nanocrystals. Strikingly, our study shows that control on “surface charges” can delay photo-generated carriers recombination by one order of magnitude. The result is supported by quantum chemical calculations and Monte Carlo simulations of charge transport and recombination which show agreement with our multiscale experimental characterization. Our finding has deep relevance for the community investigating interfacial processes for efficient solar fuel and photovoltaic devices using functionalized nanostructures.
Finally, we present a transient spectroelectrochemical approach to investigate the optoelectronic properties, charge transport and electron transfer dynamics in thin film semiconductors and molecular functionalized nanostructured materials for solar energy conversion and storage.
References:
(1) Wang, Q.; Evans, N.; Zakeeruddin, S. M.; Exnar, I.; Grätzel, M. Molecular Wiring of Insulators: Charging and Discharging Electrode Materials for High-Energy Lithium-Ion Batteries by Molecular Charge Transport Layers. J. Am. Chem. Soc. 2007, 129, 3163-3167.
(2) Wang, Q.; Zakeeruddin, S. M.; Cremer, J.; Bäuerle, P.; Humphry-Baker, R.; Grätzel, M. Cross Surface Ambipolar Charge Percolation in Molecular Triads on Mesoscopic Oxide Films. J. Am. Chem. Soc. 2005, 127, 5706-5713.
(3) Ardo, S.; Meyer, G. J. Characterization of Photoinduced Self-Exchange Reactions at Molecule-Semiconductor Interfaces by Transient Polarization Spectroscopy: Lateral Intermolecular Energy and Hole Transfer across Sensitized TiO2 Thin Films. J. Am. Chem. Soc. 2011, 133, 15384-15396.
(4) Weisspfennig, C. T.; Hollman, D. J.; Menelaou, C.; Stranks, S. D.; Joyce, H. J.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Dependence of Dye Regeneration and Charge Collection on the Pore-Filling Fraction in Solid-State Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2013, 24, 668-677.
9:00 AM - OO17.20
Light-Induced Morphological and Photophysical Changes of Polycrystalline Methylammonium Lead-Halide Perovskite Films
Simon Anselm Bretschneider 1 Dominik Werner Gehrig 1 Alexander Klasen 2 Ilka Hermes 1 Stefan A L Weber 1 3 Wolfgang Tremel 2 Frederic Laquai 1 4
1Max Planck Institute for Polymer Research Mainz Germany2University Mainz Mainz Germany3University Mainz Mainz Germany4King Abdullah University of Science and Technology (KAUST) Thuwal Saudi Arabia
Show AbstractLead-halide perovskites have emerged as a new class of materials for electro-optical, especially photovoltaic applications with power conversion efficiencies up to 24 %. [1] Beyond solar cell efficiencies, methylammonium lead iodide perovskites are investigated extensively to understand the unique properties of the material. In addition to 3-dimensional perovskites, 2-dimensional perovskites are also investigated, especially since they allow better tuning of the optoelectronic properties.
Time-resolved transient absorption and photoluminescence spectroscopy in the visible spectral range allow precise characterization of the charge generation and recombination mechanisms in lead-halide perovskite films. In this work, we use these techniques to investigate the effects of light-induced changes of solvent-annealed methylammonium lead iodide films under nitrogen atmosphere. To gain further insight, we use complementary characterization techniques, namely scanning-probe and electron microscopy, to study the morphology of the film. Transient absorption measurements of solvent-annealed perovskite films under vacuum resemble a large share of the published photophysical properties. [2,3] Here we show light-induced morphology changes - formation of quantum-well like nano flakes - and corresponding transient absorption and photoluminescence measurements. Our findings indicate that the formation of nano flakes on top of the perovskite film dramatically changes the photophysics. The photophysical changes are visible in the quenching of the spectral signature of a depopulated ground state and the appearance of a derivative-like spectral feature with decay time similar to the photoluminescence of the film. Similar derivative-like features have been observed for solution-grown perovskite quantum wells. [4]
[1] WCPEC-6 Conference Highlights (18.06.2015).
[2] Christians et al.: J. Phys. Chem. Lett., 2015, 6 (11), pp 2086-2095
[3] Chen et al.: J. Phys. Chem. Lett., 2015, 6 (1), pp 153-158
[4] Wu et al.: J. Phys. Chem. C, 10.1021/acs.jpcc.5b00148
9:00 AM - OO17.21
Nanoparticle-Based Coatings for Enhanced Solar Cell Performance in Desert Conditions
Harry Apostoleris 1 Chia-Yun Lai 1 Llorenc Domingo Capdevila 2 Sergio Santos 1 Matteo Chiesa 1
1Masdar Inst of Samp;T Abu Dhabi United Arab Emirates2Nano 4 Smart Barcelona Spain
Show AbstractDesert environments are characterized simultaneously by high solar irradiance and by windy and sandy conditions. In order to fully exploit the solar resource, fouling of solar panel surfaces by dust accumulation must be avoided. Furthermore, in such environments, where water is a scarce resource, cleaning requirements should also be minimized. This is a problem that applies not only to solar panels but also glass building facades and vehicle windshields in desert societies such as the Gulf states. We present a study of the fouling behavior of glass treated with commercial nanoparticle-based coatings, and an analysis on the long-term impact of such coating on solar cell performance under real conditions in Abu Dhabi, UAE. Studies of surface wettability will also be included as part of an investigation of the self-cleaning properties of the coatings.
9:00 AM - OO17.22
Intra-Bandgap Defect States in Ternary/Quaternary Semiconductor Nanocrystals: From Photophysics to Device Fabrication
Riya Bose 1 Osman M Bakr 1 Omar Mohammed 1
1King Abdullah University of Science and Technology Thuwal Saudi Arabia
Show AbstractManipulating the charge carrier dynamics of semiconductor nanocrystals is the key element to utilize them for photoactive applications. The presence of intra bandgap defect states can trap the excited carriers shortening their lifetime significantly, and consequently limit their applicability. To remove these undesirable defect states, a profound knowledge about their origin and nature is required. In this report, we monitor the ultrafast charge carrier dynamics of ternary CuInS2, CuInSSe, and CuInSe2 nanocrystals before and after ZnS shelling using state-of-the-art time-resolved laser techniques including femtosecond broadband transient absorption spectroscopy.1 The experimental results demonstrate that ZnS shelling efficiently removes a significant fraction of carrier trapping sites and the carrier lifetime can be elongated upto ~1.8 times. Further, we extended this study for quaternary copper indium gallium selenide (CIGSe) nanocrystals which have many desirable attributes to be used as a photovoltaic material, but relative abundance of intra bandgap trap states and lack of their knowledge limit their applicability. We synthesized sub-10 nm CIGSe nanocrystals for the first time by proper manipulation of reaction parameters and monitored their carrier dynamics precisely in real time as a function of trap states. Here also, ZnS shelling is found to be efficient to remove carrier trapping sites and can elongate the carrier lifetime significantly upto ~120 ns. As a result, the photon conversion efficiency of solution processed photovoltaic device incorporating the nanocrystals can be increased to a factor of ~2.2 and the clear signature of trap state removal is evident by the transient photocurrent experiments.2 We report a clear quantitative insight of the charge carrier dynamics and effect of shelling in ternary/quaternary nanocrystals, and also their immediate effect to enhance the photoresponse of the nanocrystals, providing a better understanding to unlock their full potential for photovoltaics and light emitting applications.
References:
Bose, R.; Ahmed, G. H.; Alarousu, E.; Parida, M. R.; Abdelhady, A. L.; Bakr, O. M.; Mohammed, O. F., Direct Femtosecond Observation of Charge Carrier Recombination in Ternary Semiconductor Nanocrystals: The Effect of Composition and Shelling. J. Phys. Chem. C,2015, 119, 3439-3446.
Bose, R.; Bera, A.; Parida, M.; Abdelhady, A. L.; Alarousu, E.; Bakr, O. M.; Mohammed, O. F., Enhanced Photoresponse of CIGSe nanocrystals by Trap States Engineering- An Ultrafast Study. Chem. Mater., To be submitted.
9:00 AM - OO17.23
Nanostructured Bilayered Uranium Oxide- Metal Oxide Films for Solar Hydrogen Production
Jennifer Leduc 1 Thomas Fischer 1 Yakup Goenuellue 1 Sanjay Mathur 1
1Univ of Cologne Cologne Germany
Show AbstractAlthough being considered a scarce element, huge amounts of depleted uranium sources (e.g. alone 700,000 tons UF6 in the USA), originally produced as waste streams in the enrichment process of nuclear fuels, are currently stored without any prospect for further applications which poses a perpetual environmental hazard, due to accidental release of volatile, corrosive and toxic compounds. In order to reduce this amount of uranium waste, its reuse is discussed in the context of semiconductor materials as well as catalysts for thermochemical water splitting, destruction of air pollutants and CO2 activation.
Herein we present the use of air-stable and volatile uranium metal-organic precursors for the gas phase deposition (thermal and plasma-assisted chemical vapor deposition) of uranium oxide thin-films and their use as photoelectrodes in photoelectrochemical water splitting devices. Uranium oxides with band-gap energies in the range of 2.0 - 2.6 eV coupled with their good electrical and catalytic properties mostly driven by facile valence dynamics of the uranium cations are promising electrode materials in photoelectrochemical water splitting reactions. However, when tested in a standard PEC setup using different acidic and basic electrolytes, they were found to decompose, when electric potential was applied. In order to prevent dissolution of the uranium oxide layers, we prepared nanostructured bilayered thin films with a corrosion-resistant metal-oxide layer for protection against dissolution at the semiconductor/electrolyte interface.
9:00 AM - OO17.24
Mechanochemical Synthesis of ZnxCo3-xO4 as a Mesoscale Solar Photocathode
Shannon Marie McCullough 1 Cory Flynn 1 Candy Mercado 2 Arthur Nozik 2 3 James Cahoon 1
1University of North Carolina at Chapel Hill Chapel Hill United States2University of Colorado, Boulder Boulder United States3National Renewable Energy Laboratory Golden United States
Show AbstractThe search for the next generation of low-cost high-performance solar cells is underway. Several classes of potential generation III solar energy devices, including dye-sensitized solar cells (DSSCs) and organo-halide perovskite devices, make use of high surface area, mesoporous p-type cathodes for charge separation and transport of charge carriers. However, the performance of these p-type cathode materials typically lags behind n-type anode materials such as TiO2. In this work, the spinel, ZnxCo3-xO4, is presented as a candidate p-type solar photocathode. A solid-state mechanochemical synthesis was developed, producing nanospheres with an average diameter of 24 ± 5 nm. The solid state preparation allows for highly tunable Zn concentration in the spinel lattice. The mesoscale photocathodes&’ performance was evaluated in a p-DSSC configuration with the molecular chromophore P1 and an I-/I3- electrolyte. DSSCs were tested with variable Zn concentration in the particles, and showed that increased Zn substitution resulted in increased performance. Poor surface dye-loading was identified as a preliminary performance limiter, and various surface modifications on the material were investigated to maximize dye-loading and increase DSSC performance. Surface modifications included controlling both the temperature and atmosphere while annealing the thin film photocathode, yielding increases in dye-loading, photocurrent, and overall DSSC performance. The results suggest that ZnxCo3-xO4 could be developed into a high performance material for a variety of solar energy applications.
9:00 AM - OO17.25
Nanocomposite Architecture for Rapid, Spectrally-Selective Electrochromic Modulation of Solar Transmittance
Jongwook Kim 1 Gary K. Ong 1 2 Yang Wang 1 Gabriel LeBlanc 1 Teresa E Williams 3 4 Tracy Mattox 4 Brett Anthony Helms 4 Delia Milliron 1
1University of Texas at Austin Austin United States2University of California Berkeley Berkeley United States3University of California Berkeley Berkeley United States4Lawrence Berkeley National Laboratory Berkeley United States
Show AbstractHeating, cooling, and lighting in buildings account for 24% of the total energy consumption in the US. If near infrared (NIR, heat) and visible (VIS, light) solar radiation through windows can be actively and independently controlled in response to exterior conditions, this enormous energy demand can be significantly reduced. We present the design and fabrication of a nanocomposite electrochromic material that performs near-ideal solar modulation.
Two active electrochromic materials, NIR-plasmonic tungsten oxide nanocrystals (WO3-x) and VIS-polaronic niobia glass (NbOx), are organized into a mesostructured architecture. The inherently different switching potentials of WO3-x and NbOx is a prerequisite to NIR-VIS dual-band switching[1]. However, a judicious arrangement of the two components was essential to avoid deleterious interactions such as blocking of charge transport pathways. Our nanocomposite architecture comprises a mesoscale WO3-x framework, interpenetrating NbOx domains, and small mesopore channels introduced at the WO3-x-NbOx heterointerface. The high density of the heterointerface and the open pore channels allow efficient electrolyte percolation promoting exceptionally rapid and independent charging and discharging of each component. Moreover, enhancements in charge capacity, optical contrast, and cycling durability (>2000 cycles) were also observed. In result, we achieved unprecedented modulation of thermal loads at the ‘Cool mode&’ (36%TNIR : 73%TVis), while the neutral color ‘Dark mode&’ (7%TNIR : 22%TVis) rivals the best tinting performance of VIS-switching windows commercialized today[2].
[1] Llordes, A., Garcia, G., Gazquez, J. & Milliron, D. J., Nature500, 323-326
[2] Kim, J., Ong, G. K., Wang, Y., LeBlanc, G., Williams, T. E., Mattox. T. M., Helms, B. A., Milliron, D. J., Submitted
9:00 AM - OO17.26
Scanning Force Microscopy on Perovskites
Ilka Hermes 1 Victor Wolfgang Bergmann 1 Dan Li 1 Simon Anselm Bretschneider 1 Brian J. Rodriguez 3 Ruediger Berger 1 Stefan A L Weber 1 2
1Max-Planck-Inst Mainz Germany2Johannes Gutenberg University Mainz Germany3University College Dublin Dublin Ireland
Show AbstractScanning force microscopy (SFM) is a versatile tool for studying nano- and sub-nanoscale surface structures. Using advanced SFM methods, morphological features can be correlated with a vast number of material properties, such as mechanical or electrical properties. This information is of particular interest in the field of novel photovoltaic materials such as perovskites, where the nanoscale structure of the materials has a huge impact on the device performance. Additional to topographic information, electrical SFM methods can map various surface properties on a nanometer length scale. The accessible surface properties include electrical conductivity (conductive SFM), surface potential in Kelvin probe force microscopy, (KPFM) or electromechanical coupling in Piezoresponse force microscopy (PFM).
In order to study light induced electrical effects on the level of single nanostructures, a new SFM setup in inert atmosphere with a sample illumination was built. With this photoelectric SFM, we are studying the electrical properties of perovskite films and solar cells [1]. In order to investigate operating devices across the internal interfaces, we developed a procedure to fine polish a device cross section in order to get a smooth topography. Subsequently, we used high-resolution frequency modulation KPFM to map the potential distribution in dark and under illumination with white light. Our measurements showed that SFM can reveal potential distributions inside a working device with nanometer resolution [2]. With piezoresponse force microscopy (PFM) on micron-sized perovskite grains we observed a periodically alternating structure reminiscent of ferroelastic domain patterns. From vertical and lateral PFM, a domain orientation with respect to the sample surface is proposed. Structural changes due to degradation by water and air exposure in perovskite films were investigated in-situ. Finally, conductive SFM experiments provide information about conducting pathways and photocurrent generation in the perovskite layers.
Such experiments provide valuable information for the optimization of the light harvesting abilities and lifetime in perovskite materials.
[1] R. Berger, A.L. Domanski and S.A.L. Weber Electrical Characterization of Organic Solar Cell Materials based on Scanning Force Microscope Methods, Eur. Polym. J., 49, 1907 (2013).
[2] Bergmann, V.W.; Weber, S.A.L.; Ramos, J.A.; Nazeeruddin, M.K.; Grätzel, M.; Li, D; Domanski, A.L.; Lieberwirth, I.; Ahmad, S. and Berger, R. Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell, Nature Communications, 5, 5001 (2014).
9:00 AM - OO17.27
Molecular Dynamics Calculations of C60-Functionalized Flavin Helices around Single-Walled Carbon Nanotubes
Erandika Karunaratne 1 2 Mehdi Mollahosseini 1 2 Jose A Gascon 2 Fotios Papadimitrakopoulos 1 2
1University of Connecticut Storrs United States2University of Connecticut Storrs United States
Show AbstractRecently, there has been considerable interest in using single-walled carbon nanotubes (SWNTs) as active components in photovoltaic (PV) applications. To harvest such potential, heterojunctions were made from: (i) semiconducting (sem)-SWNTs and fullerenes, (ii)π-conjugated polymers and sem-SWNTs, and (iii) hybrid approaches, where both C60 and SWNTs were used in combination with conjugated polymers. These heterojunctions however display considerable dispersion in terms of the localized environment around SWNTs. In order to improve on such dispersion, our group has relied on the highly organized flavin mononucleotide (FMN) assembly around various SWNTs.1 By appropriate substitution of the flexible side group, C60 can be linked to an organic soluble flavin (FC12)1a to afford an intramolecular linked structure termed FC60, which can then be organized around SWNTs. In this contribution, we present the molecular dynamics (MD) results of FC12-wrapped SWNTs, where one FC12 molecule of the helix has been exchanged with an FC60. Our results indicate that the flexible side chain of FC60 linking the C60 with the isoalloxazine ring bends in such fashion to form a ground state complex that affords direct π-π stacking overlap with the flavin helix. Different N10 nitrogen hybridizations (i.e.sp2 and sp3) of the isoalloxazine ring lead to different bended conformations in both free (in solution) and helix-bound FC60.
1. (a) S.-Y. Ju, W. Kopcha, F. Papadimitrakopoulos, Science, 2009, 323, 1319-1323; (b) R. Sharifi, M. Samaraweera, J. A. Gascoacute;n, F. Papadimitrakopoulos, J. Am. Chem. Soc., 2014, 136(20), 7452-7463.
9:00 AM - OO17.28
Homogeneous Architecture of Fullerenes around Individualized SWNTs
Mehdi Mollahosseini 1 Fotios Papadimitrakopoulos 1
1University of Connecticut Tolland United States
Show AbstractThe potential roll-to-roll, low-cost fabrication of organic photovoltaics has become increasingly attractive for power generation to even the most remote locations. P3HT (poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C-61-butyric acid methyl ester) are the most widely used donor/acceptor materials in organic photovoltaics (OPVs). One of the biggest challenges of organic semiconductors is their low charge carrier mobility. The four to five orders of magnitude higher charge carrier mobility of semiconducting (sem-) single walled carbon nanotubes (SWNTs) can potentially address this issue, if one manages to appropriately configure them in heterojunction structure with the proper carrier extraction architecture. To this effect, the supramolecular organizations of flavin mononucleotide (FMN) around single walled carbon nanotubes (SWNTs) was shown to provide effective nanotube dispersions and the ability to impart selective enrichment of sem-SWNTs by recognizing the underlying nanotube helical pattern. Utilizing this approach, we herein present our recent work in outfitting the organic analogue of FMN (FC12) with a terminal PCBM functionality. The presence of PCBM moiety provides an effective photoluminescence (PL) quenching for both FC60 and FC12-dispersed SWNTs with ca. 1% incorporated FC60. Spectroscopic characterization enables to verify the incorporation of FC60 within the FC12 helix. The suggested system here, helps to improve homogeneous environment around the SWNTs and SWNT individualization which can promote exciton dissociation and then single layer processability of organic photovoltaics.
9:00 AM - OO17.29
Energy-Down-Shift Having Energy-Tuning Effect of Mn Doped Cd0.5Zn0.5S-ZnS Core-Shell Quantum Dots on Power-Conversion Efficiency Enhancement in Silicon Solar Cells
Yun-hyuk Ko 1 Seung-Jae Lee 1 Mohammed Jalalah 1 Jae-hyoung Shim 1 Tae-Hun Shim 1 Jea-Gun Park 1
1Hanyang University Seoul Korea (the Republic of)
Show AbstractWe investigated the energy-down-shift having energy-tuning effect of Mn doped Cd0.5Zn0.5S-ZnS core-shell quantum dots on power-conversion efficiency enhancement in silicon solar cells for the first time. We found that Mn doped Cd0.5Zn0.5S-ZnS core (8.0 nm in diameter)-shell (4.8 nm in thickness) quantum dots (QDs) demonstrated a typical energy-down-shift (2.76-4.96 → 2.05 eV), which absorb ultra-violet(UV) light (250-450 nm in wavelength) and emit orange visible light (~602 nm in wavelength). They showed the quantum yield of ~59 % and their coating on the SiNX film textured p-type silicon solar-cells enhanced the external-quantum-efficiency(EQE) of ~20 % at 300-450 nm in wavelength, thereby enhancing the short-circuit-current-density(JSC) of ~1.42 mA/cm2 and the power-conversion-efficiency(PCE) of ~0.55 % (relatively ~3.07 % increase compared with the reference without QDs for p-type silicon solar cells). In particular, the PCE peaked at a specific coating thickness of the Cd0.5Zn0.5S-ZnS core-shell QD layer; i.e., the PCE enhancement of 0.55 % at an 8.8 nm thick QD layer.
*This work was supported by Basic Science Research Program through the Ministry of Higher Education, Kingdom of Saudi Arabia for supporting this research through a grant (PCSED-008-14) under the Promising Center for Sensors and Electronic Devices (PCSED) at Najran University, Kingdom of Saudi Arabia and Brain Korea 21 PLUS Program in 2014.
9:00 AM - OO17.30
Efficient Photocatalytic Hydrogen Generation from Ni Nanoparticle Decorated CdS Nanosheets
Maksym Zhukovskyi 1 Pornthip Tongying 1 Halyna Yashan 1 Yuanxing Wang 1 Masaru Kuno 1
1University of Notre Dame Notre Dame United States
Show AbstractHigh quality, thickness-controlled, CdS nanosheets (NSs) have been obtained through the thermal decomposition of cadmium diethyldithiocarbamate in octadecene. Ensembles with discrete thicknesses of 1.50 nm, 1.80 nm, and 2.16 nm have been made with corresponding lateral dimensions on the order of 90 nm x 20 nm. In tandem, associated Ni nanoparticle (NP) decorated counterparts have been made through the photodeposition of Ni onto NS basal planes.
Subsequent photocatalytic hydrogen generation measurements have compared the performance of CdS NSs with their Ni NP decorated counterparts in water/ethanol mixtures. Of interest is how the large absorption efficiencies, room temperature excitonic resonances, long lifetimes and corresponding suppressed Auger responses of 2D NSs impact their photocatalytic response. Apparent quantum yields as large as 25% have been seen for Ni NP decorated NSs with transient yields as large as 64% within the first 2 hours of irradiation. Ensemble femtosecond transient differential absorption spectroscopy show that the high efficiency of CdS/Ni NS photocatalysts ultimately stems from efficient electron transfer from CdS to Ni. In this regard, the CdS/Ni metal/semiconductor heterojunction acts to dissociate strongly bound excitons in CdS NSs, creating free carriers needed to carry out relevant reduction chemistries. This, in turn, suggests that the combination of several intrinsic features of 2D CdS NSs namely, their large absorption efficiencies, their strong exciton binding energies, and, more importantly, their long exciton lifetimes -all coupled with the presence of a metal/semiconductor heterojunction- results in efficient hydrogen generation.
9:00 AM - OO17.31
Optimizing Wide Band Gap ZrO2 Blocking Layer for TiO2 Film-Based Dye Sensitized Solar Cells
Halil Yavuz 1 Ahmet Macit Ozenbas 1
1Middle East Technical Univ Ankara Turkey
Show AbstractA ZrO2 thin film was deposited on a fluorine-doped tin oxide (FTO) electrode by hydrothermal treatment and its application as a new blocking layer material for dye-sensitized solar cells (DSSCs) was investigated. According to current-voltage (I-V) characteristics and electrochemical impedance spectra (EIS), it was found that the ZrO2 layer functioned as both a blocking layer and a heat treatment protector for transparent conducting oxide (TCO) layer. The use of ZrO2 layer as blocking layer increases the electron lifetime and decreases the recombination from TCO to the electrolyte. In addition to photovoltaic performance, ZrO2 keeps resistivity of TCO stable after heat treatment compared to TiO2 blocking layer. As a result, the overall energy conversion efficiency of the DSSC with ZrO2 blocking layer was enhanced by 47 % for front side illumination compared to that of bare FTO substrate and 30 % compared to that of commercial TiO2 blocking layer for backside illumination. This study demonstrated that ZrO2 could be a promising alternative to the conventional TiO2 blocking layer for high efficiency DSSCs.
9:00 AM - OO17.32
Combinatorial Synthesis and High-Throughput Photoelectrochemical Assessment of Thin Film Materials Libraries for the Identification of Photoelectrodes for Solar Water Splitting
Chinmay Khare 1 Kirill Sliozberg 2 Robert Meyer 1 Helge Stein 1 Bruce A. Parkinson 3 Wolfgang Schuhmann 2 4 Alfred Ludwig 1 4
1Ruhr-University Bochum Bochum Germany2Ruhr-University Bochum Bochum Germany3University of Wyoming Laramie United States4Ruhr University Bochum Bochum Germany
Show AbstractMetal oxides semiconductors are promising for photoelectrodes in a photoelectrochemical (PEC) solar water-splitting device due to their abundance, light absorption properties and stability in aqueous media. Identification of optimized photoelectrode materials can be achieved by combinatorial fabrication and high-throughput characterization methods. By systematically mixing materials of interest, in a reactive sputter deposition process, thin film materials libraries, exhibiting combined thickness and compositional gradients as well as structural variations can be synthesized. Additionally, glancing angle deposition, varying sputter deposition conditions and selective dissolution methods present facile routes to synthesis of highly porous nanostructured thin films. Nanostructuring results in effectively increasing the surface area to allow larger solid/electrolyte interface for efficient charge transfer process. With this approach, Ti-W-O, Fe-W-O and Fe-Cr-Al-O materials libraries were thoroughly screened for potential photoelectrode materials. High-throughput investigation of compositional, structural and functional data on the materials libraries were performed by automated EDX, XRD, thickness and PEC measurements. The analysis of the data enabled to establish correlations between composition, morphology, thickness, crystallinity and PEC properties. For the Ti-W-O system exhibiting a compositional range between (Ti80W20)Ox and (Ti97W3)Ox, the PEC measurements showed the maximum quantum efficiency of 37.5 % (at lambda; = 350 nm) and a maximum peak photocurrent density of 70.3 µA/cm2 at 94.4 at.% Ti. Extensive phase analysis in the case of Fe-W-O indicated the presence of a ternary phase Fe2O6W at the local photocurrent maxima, while the sub-stoichiometric W5O14 phase (with 15 at.% Fe) demonstratedthe maximum quantum efficiency of 45 % at lambda; = 300 nm and a highest photocurrent density values of 65 µA/cmsup2;. In the Fe-Cr-Al-O system, two distinct semiconductor compositions p-type Fe36.5Cr55.5Al8Ox and n-type Fe51Cr47Al2Ox were identified. The p-type material exhibited a bandgap of 1.55 eV and an open circuit potential of 0.85 V vs RHE. On the other hand, the n-type material demonstrated a bandgap of 1.97 eV and an open circuit potential of 0.6 V. The detailed structural analysis suggested presence of spinel-type structures.
9:00 AM - OO17
Abstract OO17.15 Transferred to OO12.33
Show AbstractOO13: Quantum Dots
Session Chairs
Thursday AM, December 03, 2015
Hynes, Level 3, Room 302
9:30 AM - OO13.01
Improved Efficiency and Stability of Cadmium Chalcogenide Nanoparticles by Photodeposition of Co-Catalysts
Philip Kalisman 1 Lilac Amirav 1
1Technion - Israel Institute of Technology Technion City Israel
Show AbstractThe production of hydrogen by photocatalytic water splitting is a potentially clean and renewable source for hydrogen fuel. Cadmium chalcogenides are attractive photocatalysts because they have the potential to convert water into hydrogen and oxygen using photons in the visible spectrum. Cadmium sulfide rods with embedded cadmium selenide quantum dots (CdSe@CdS) are particularly attractive because their energy bands can be ideally tuned for this task; however, two crucial drawbacks hinder the implementation of these materials in wide spread use: poor charge transfer and photochemical instability. Utilizing photochemical deposition of co-catalysts onto CdSe@CdS substrates we can address each of these weaknesses. We report conditions under which metal capped CdSe@CdS can achieve record efficiency for the water reduction half-reaction. We also report long term photostability for CdSe@CdS under high intensity 455nm light (a viable wavelength for photocatalytic water reduction by CdSe@CdS) by growing metal oxide co-catalysts on the surface of our rods.
9:45 AM - OO13.02
Solar-Driven Photoelectrochemical Probing of Nanodot/Nanowire/Cell Interface
Jing Tang 1
1Fudan Univ Shanghai China
Show AbstractWe report a nitrogen-doped carbon nanodot (N-Cdot)/TiO2 nanowire photoanode for solar-driven, real-time, and sensitive photoelectrochemical probing of the cellular generation of H2S, an important endogenous gasotransmitter based on a tunable interfacial charge carrier transfer mechanism. Synthesized by a microwave-assisted solvothermal method and subsequent surface chemical conjugation, the obtained N-Cdot/TiO2 nanowire photoanode shows much enhanced photoelectrochemical photocurrent compared with pristine TiO2 nanowires. This photocurrent increase is attributed to the injection of photogenerated electrons from N-Cdots to TiO2 nanowires, confirmed by density functional theory simulation. In addition, the charge transfer efficiency is quenched by Cu2+, whereas the introduction of H2S or S2- ions resets the charge transfer and subsequently the photocurrent, thus leading to sensitive photoelectrochemical recording of the H2S level in buffer and cellular environments. Moreover, this N-Cdot-TiO2 nanowire photoanode has been demonstrated for direct growth and interfacing of H9c2 cardiac myoblasts, with the capability of interrogating H2S cellular generation pathways by vascular endothelial growth factor stimulation as well as inhibition.
10:00 AM - OO13.03
Solid-State Infrared-to-Visible Upconversion via Colloidal-Nanocrystal-Sensitized Triplet-Triplet Annihilation
Mengfei Wu 1 Daniel Congreve 1 Mark Wilson 1 Vladimir Bulovic 1 Moungi Bawendi 1 Marc Baldo 1
1Massachusetts Institute of Technology Cambridge United States
Show AbstractOptical upconversion converts two or more low-energy photons into a higher-energy photon. Upconversion that converts infrared light into the visible is particularly useful for photovoltaic applications, and can help surpass the Shockley-Queisser limit by capturing the sub-bandgap photons. To date, two main processes have been studied for upconversion under incoherent excitation: luminescence from lanthanide ions, and sensitized triplet-triplet-annihilation (TTA). While the latter typically works efficiently at much lower excitation intensities than the lanthanides, demonstrations have been from pump wavelength lambda; < 790 nm, and mostly in solution. Here, we report TTA-based upconversion in solid-state thin film from excitation beyond lambda; = 1 mu;m to emission at lambda; = 612 nm, utilizing lead sulfide colloidal nanocrystals as the triplet sensitizer. We show that upconversion becomes efficient at an absorbed power density of 12 mW cm-2 under lambda; = 808 nm monochromatic excitation, which is equivalent to less than one sun of AM 1.5 broad-spectrum excitation in photon flux. Given the wide bandgap tunability (up to lambda; = 2 mu;m) of the nanocrystals and their broadband absorption, upconversion via nanocrystal-sensitized TTA promises to harness the infrared for conventional solar cell technologies.
10:15 AM - OO13.04
Linear and Nonlinear Optical Properties of Perovskite Quantum Dots Evaluated at the Single-Particle and Ensemble Level
Nikolay Makarov 1 Shaojun Guo 1 Young-Shin Park 1 Oleksandr Isaienko 1 Jeffrey M. Pietryga 1 Istvan Robel 1 Victor I. Klimov 1
1Los Alamos National Laboratory Los Alamos United States
Show AbstractPerovskites have emerged as a highly flexible materials platform for the realization of solution processable solar cells with efficiencies rapidly approaching those of established commercial photovoltaic technologies. Recently, advances in colloidal synthesis have permitted the preparation of quantum dots of perovskite materials with good size distributions and high photoluminescence (PL) quantum yields. Here we explore the size and composition dependent optical properties of perovskite quantum dots at both the single-particle and ensemble levels. Transient absorption and time resolved PL experiments allow us to quantify the degeneracy of the band edge exciton states, characteristic time constants of radiative and nonradiative Auger recombination, the rates of intraband relaxation, and the strength of exciton-exciton coupling. Single-dot experiments reveal fluorescence intermittency (“blinking”) in both PL intensity and time domains, with statistical properties similar to those of conventional quantum dots. Further, the analysis of pump-flux dependent PL intensity and lifetime trajectories allows us to identify the mechanism for intermittency. We will also present the results of ongoing studies of carrier multiplication in composition- and shaped-controlled perovskite quantum dots and discuss the issues of their photodegradation. Our studies indicate that while some of the optical properties of perovskite quantum dots are similar to those of well studied II-VI quantum dots there are also certain distinctions especially with regards to the properties of multicarrier states.
10:30 AM - OO13.05
Upconversion between Nanocrystals and Molecules across the Entire Visible and Infrared Spectrum
Ming Lee Tang 1
1Univ of California-Riverside Riverside United States
Show AbstractPhoton upconversion has many optoelectronic applications (for example, in solar cells, photodetectors or data storage optical devices). However, the conversion of infrared light to visible light has been limited by the low absorption cross-sections and tunability of lanthanide-doped materials. We demonstrate efficient upconversion with semiconductor nanocrystals and molecular emitters using continuous wave light. The absorption of low energy photons by the nanocrystal results in the sensitization of the triplet state of the molecule, which then undergoes triplet-triplet annihilation to emit upconverted light. The ligands bound to the nanoparticles during the upconversion process mediate this energy transfer. The use of different combinations of nanocrystals and emitters shows that this is a general platform with great flexibility, both in the choice of excitation and emission wavelengths for upconversion.
10:45 AM - OO13.06
Energy Transfer from CdSe/ZnS Quantum Dots to Single and Few Layers of SnS2 Nano-Sheets: From Single Quantum Dots to Device
Huidong Zang 1 Yuan Huang 1 Peter Sutter 1 Mircea Cotlet 1
1Brookhaven National Laboratory Upton United States
Show AbstractLayered metal dichalcogenides (LMDs) have emerged as exciting 2D semiconductors for energy device applications, partly due to the strong dependence of their bandgaps on the number of LMD layers. MoS2, WS2 and WSe2 show a transition from indirect bandgap in multilayer to direct bandgap in monolayer, and this bandgap inversion provides enhanced photoluminescence (PL) for monolayer LMDs, making them appealing light harvesting materials for photovoltaics (PVs). The large surface area of these light emitting LMDs is also suitable for catalysis when doped with appropriate nanoparticles or it can provide support for the development of heterostructures with enhanced light absorbing capabilities such as colloidal quantum dots. Recently we introduced SnS2 as a 2D LMD with great potential for the development of photovoltaic devices and photodetectors [1] Here we combined SnS2 with colloidal (0-D) nanomaterials in the form of CdSe/ZnS quantum dots to obtain heterostructured nanomaterials with improved light harvesting properties. Specifically, we demonstrate efficient energy transfer from isolated Qdots to single and few layers of SnS2 with time-resolved single nanoparticle microscopy and find that the rate of energy transfer between such Qdots and SnS2 nanosheets increases with the number of SnS2 layers. We then move to device to demonstrate incorporation of such heterostructures in a field effect transistor architecture and to find that such devices exhibit improved photocurrent generation due to the addition of colloidal Qdots.
[1]. Yuan Huang , Eli Sutter , Jerzy T. Sadowski , Mircea Cotlet , Oliver L.A. Monti , David A Racke , Mahesh R Neupane , Darshana Wickramaratne , Roger K. Lake , Bruce A Parkinson , and Peter SutterTin Disulfide - An Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics. ACS Nano, 2014, 8 (10), pp 10743-10755.
OO14: Photochemistry II
Session Chairs
Thursday AM, December 03, 2015
Hynes, Level 3, Room 302
11:30 AM - *OO14.01
Photochemistry via Plasmonic Metal Nanoparticles from First Principles
Emily A. Carter 1
1Princeton University Princeton United States
Show AbstractPhotoelectrochemical energy conversion involves light-induced charge transfer excitations prior to chemical bond breaking and making. Quantum mechanical simulations usually employ density functional theory (DFT), but conventional DFT fails to treat these types of excitations correctly due to exchange-correlation functional limitations. By contrast, embedded correlated wavefunction (ECW) theory treats charge transfer accurately by properly including exact electron exchange and correlation in a region of interest while the extended background is described via periodic DFT, encapsulated in a so-called embedding potential. We will describe how an unusual form of photoelectrocatalysis can also be captured by this theory, namely plasmon-induced hot electron dissociation of molecules on gold and aluminum nanoparticles.
12:00 PM - OO14.02
Sculpting Photocatalysts on the Nano Scale
Lilac Amirav 1
1Technion - Israel Institute of Technology Haifa Israel
Show Abstract
The solar-driven photocatalytic splitting of water into hydrogen and oxygen is a potential source of clean and renewable fuels. However, four decades of global research have proven this multi-step reaction to be highly challenging. The design of effective artificial photo-catalytic systems will depend on our ability to correlate the photocatalyst structure, composition, and morphology with its activity.
I will present our strategies, and most recent results, in taking photocatalyst production to new and unexplored frontiers. I will focus on unique design of innovative nano scale particles, which harness nano phenomena for improved activity, and methodologies for the construction of sophisticated heterostructures. I will demonstrate how vital is the ability to characterize our hybrid nanostructures on the atomic level, and how we can benefit from information on the structure-properties relationship for the future design of an efficient photocatalyst for solar-to-fuel energy conversion.
12:15 PM - OO14.03
Photovoltage and Fill Factor Design for ALD Metal Oxide Protected Silicon Anodes for Tandem Water Splitting Cells
Andrew Scheuermann 1 John Lawrence 1 Kyle Kemp 1 Olivia Hendricks 3 Adrian Walsh 2 Ian Povey 2 Martyn Pemble 2 4 Christopher Chidsey 3 Paul K. Hurley 2 Paul C. McIntyre 1
1Stanford University Stanford United States2Tyndall National Institute Cork Ireland3Stanford University Stanford United States4University College Cork Cork Ireland
Show AbstractMetal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis. ALD-TiO2 remains the most widely used material because of its excellent stability under water oxidation conditions and potential for high electrical conductivity both as an ultrathin film and with thicknesses exceeding 100 nm [1-3]. However, the devices fabricated with the most conductive ALD-TiO2 films exhibit poor photovoltages of ~ 400 mV and less [3] while photoanodes reported to have higher voltages required ultrathin protection layers for efficient operation [1]. To achieve overall water splitting efficiencies over 20%, photovoltages over 600 mV with sufficiently thick TiO2 for long-term protection are required for silicon to be a viable bottom cell in a tandem architecture.
Here we investigate the trade-off between conductivity of the ALD-grown TiO2 protection layer on silicon anodes and the photovoltage. We report a novel observation of photovoltage loss associated with charge transfer in these metal oxide protected devices. By eliminating the loss, we are able to achieve photovoltages as high as 630 mV with thicker (> 5 nm) protection layers, the maximum reported to date for single-junction water-splitting silicon cells. The loss mechanism is systematically probed in MOS Schottky junction cells compared to buried junction p+n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A capacitor model that predicts this loss is developed, and is related to the dielectric properties of the protective oxide, achieving excellent agreement with the data. From these findings, we extract design principles for simultaneous optimization of charge transfer resistance and interface quality to maximize the photovoltage of insulator-protected water splitting devices.
Building on this work, we investigate strategies of controlling conductivity in ALD-TiO2 to optimize fill factor without sacrificing voltage. Photovoltages exceeding 600 mV for both nSi MIS and p+n Si buried junction anodes in water splitting cells are demonstrated, uncovering new and reliable pathways to high conductivity and overall high efficiency.
[1] Y.W. Chen, et al. Nature Mat. 2011, 10, 539-544.
[2] A. G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487-2496.
[3] S. Hu, et al. Science 2014, 344, 1005minus;1009.
12:30 PM - OO14.04
Development of a Fully Integrated, Stable and Efficient Solar-Driven Water-Splitting Prototype with Earth Abundant Catalysts
Rui Liu 1 Erik Verlage 1 Karl Walczak 2 Ryan Jones 1 Chris Kapr 1 Jian Jin 2 Harry A. Atwater 1 Nathan S. Lewis 1 3 Chengxiang Xiang 1
1California Institution of Technology Pasadena United States2Lawrence Berkeley National Laboratory Berkeley United States3California Institute of Technology Pasadena United States
Show AbstractA fully integrated, efficient, stable and scalable solar-driven water-splitting cell requires synergistic incorporation of multiple material assemblies, including light absorber, protection layer, catalysts, gas separation membrane and electrolye, into a single device. Significant advancements in integrating and prototyping development of solar-hydrogen devices have been made at Joint Center for Artificial Photosynthesis (JCAP).
Herein, two strategies that achieved stable and efficient prototype cell are presented. One strategy leverages the uniform coating of “leaky” TiO2 for stable operation in alkaline conditions. A monolithically integrated device consisting of a tandem-junction GaAs/InGaP photoanode exhibited a solar-to-hydrogen conversion efficiency of 10.5% under 1 sun illumination, with stable performance for > 40 h of continuous operation. The monolithically integrated, membrane-base, wireless solar-hydrogen prototype system (~1 cm2) comprised of NiMo/GaAs/InGaP/TiO2/Ni was constructed and a hydrogen production rate of 0.81 mu;L s-1 and a solar-to-hydrogen conversion efficiency of 8.6% was observed at 1 sun illumination in 1.0 M KOH in a full cell configuration with minimal product crossover.
Alternatively, photoelectrodes patterned with transparent dielectrics and dark/metallic catalyst islands with a low filling fraction have the potential to achieve stable and efficient prototype with minimal parasitic absorption of electrocatalysts. A redox inactive underlying metal in conjunction with Ni-based catalyst for oxygen evolution reaction was employed to prevent (photo)electrochemical corrosion under anodic conditions. This general strategy was applied to a range of semiconductor substrates and achieved long-term stability in large-scale prototypes. Specifically, in a triple junction III-V photoabsober, in which excess photovoltage was readily available, prototypes with patterned catalysts exhibited higher energy conversion efficiency in relative to a device incorporating uniformly coated catalysts. In addition, preliminary results on photoelectrochemical generation and compression of hydrogen in a solar water-splitting prototype will also be presented.
12:45 PM - OO14.05
Copper-Based Photoactive Nanoparticles: Preparation, Characterization and Performance
Gang Wang 1 Roy van den Berg 1 Krijn de Jong 1 Celso de Mello Donega 2 Petra E. de Jongh 1
1Utrecht University Utrecht Netherlands2Utrecht University Utrecht Netherlands
Show AbstractCu2O is a p-type semiconductor with a band gap of ~2.2 eV which attracts much attention for application in photovoltaics, photocatalysis and solar water splitting, but is not intrinsically stable under illumination in aqueous solutions. CuxS compounds have a much higher intrinsic stability, and fascinating, though not completely understood, optoelectronic properties. Building on methods from the field of catalysis, we explore new strategies to assemble Cu-based semiconductor nanoparticles with controlled size, amonst others using mesoporous silica supports. For instance the size of pure-phase Cu2O nanoparticles was tuned from 2 nm to 16 nm by varying either the concentration of the Cu-precursor or the pore diameter of the supporting silica, followed by subsequent conversion to Cu2O by gas-phase reduction with carbon monoxide. The activity, stability and selectivity of the as-obtained nanoparticles, for different sizes and compositions, were examined for solar-driven H2 evolution and in photocatalytic oxidation. A few examples will be highlighted, such as the relatively high stability of silica-supported Cu2O nanoparticles, and the possibility to influence the selectivity of the photocatalyzed oxidation reactions.