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Zhang, Kan,Ravishankar, Sandheep,Ma, Ming,Veerappan, Ganapathy,Bisquert, Juan,Fabregat-Santiago, Francisco,Park, Jong Hyeok Wiley 2017 ADVANCED ENERGY MATERIALS Vol.7 No.3
<P>Bulk and surface charge recombination of photoelectrode are two key processes that significantly hinder solar-to-fuel conversion of photoelectrochemical cell (PEC). In this study, the function of a crystal-deficient overlayer is unveiled, which outperforms a traditionally used amorphous or crystalline overlayer in PEC water splitting by exhibiting a high conductivity and large electron diffusion length to enable unlimited electron collection. The optimized approximate to 2.5 nm thickness of the crystal-deficient shell results in a depletion layer with a width of 3 nm, which overcomes the flat band limitation of the photovoltage and increases the light absorptivity in the wavelength range from 300 to 420 nm. In addition, a 50-fold increase in the conductivity yields a one-order-of-magnitude increase in the diffusion length of an electron (L-n)(approximate to 20 m), allowing for unlimited electron collection in the 1.9 m TiO2 nanowire array with the crystal-deficient shell. The controllable crystal-deficient overlayer in rutile TiO2 nanowires photoanode achieves a photocurrent density greater than 2.0 mA cm(-2) at 1.23 V versus reversible hydrogen electrode (RHE), a 1.18% applied bias photon-to-current efficiency at 0.49 V versus RHE, a faradaic efficiency greater than 93.5% at 0.6 V versus Pt under air mass 1.5G simulated solar light illumination (100 mW cm(-2)).</P>
Kim, Seul-Gi,Li, Cheng,Guerrero, Antonio,Yang, June-Mo,Zhong, Yu,Bisquert, Juan,Huettner, Sven,Park, Nam-Gyu The Royal Society of Chemistry 2019 Journal of materials chemistry. A, Materials for e Vol.7 No.32
<P>Since ion migration and interaction with external contacts has been regarded as one of the origins for photocurrent density (<I>J</I>)-voltage (<I>V</I>) hysteresis and phase segregation in perovskite solar cells (PSCs) under operational condition, control of ionic movement in organic-inorganic halide perovskites presents a big challenge for achieving hysteresis-free and stable PSCs. As a universal method, potassium doping into bulk perovskite films to minimize or eliminate the hysteresis was proposed. Here, we report direct observation of moderately retarded ion migration in K<SUP>+</SUP>-doped (FAPbI3)0.875(CsPbBr3)0.125 perovskite by <I>in situ</I> photoluminescence (PL) imaging. However, more impressive is the effect on the kinetics for generation of the ionic double layer in the vicinity of the contacts as it is reduced by two orders of magnitude on the time scale when devices are doped with K<SUP>+</SUP> as detected by impedance spectroscopy. A significantly reduced hysteresis in the K<SUP>+</SUP>-doped perovskite is responsible for more prolonged stability exhibiting ∼96% of initial power conversion efficiency (PCE) after 22 days than relatively short-lived perovskites undoped with K<SUP>+</SUP> ions. This work highlights the clear correlation of ion migration and a fast generation of the double layer close to the contacts with severe hysteresis and long-term instability in PSCs and the importance of K<SUP>+</SUP> ions in reducing the kinetics affecting the ionic attachment to the contact surface.</P>
Germanium coating boosts lithium uptake in Si nanotube battery anodes
Haro, Marta,Song, Taeseup,Guerrero, Antonio,Bertoluzzi, Luca,Bisquert, Juan,Paik, Ungyu,Garcia-Belmonte, Germà The Royal Society of Chemistry 2014 Physical chemistry chemical physics Vol.16 No.33
<P>Si nanotubes for reversible alloying reaction with lithium are able to accommodate large volume changes and offer improved cycle retention and reliable response when incorporated into battery anodes. However, Si nanotube electrodes exhibit poor rate capability because of their inherently low electron conductivity and Li ion diffusivity. Si/Ge double-layered nanotube electrodes show promise to improve structural stability and electrochemical kinetics, as compared to homogeneous Si nanotube arrays. The mechanism explaining the enhancement in the rate capabilities is revealed here by means of electrochemical impedance methods. The Ge shell efficiently provides electrons to the active materials, which increase the semiconductor conductivity thereby assisting Li<SUP>+</SUP> ion incorporation. The charge transfer resistance which accounts for the interfacial Li<SUP>+</SUP> ion intake from the electrolyte is reduced by two orders of magnitude, indicating the key role of the Ge layer as an electron supplier. Other resistive processes hindering the electrode charge–discharge process are observed to show comparable values for Si and Si/Ge array electrodes.</P> <P>Graphic Abstract</P><P>Interfacial charge transfer resistance accounting for Li intake extremely reduced by conductive germanium coating of Si nanotubes. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4cp02377c'> </P>
Shi, Xinjian,Herraiz-Cardona, Isaac,Bertoluzzi, Luca,Lopez-Varo, Pilar,Bisquert, Juan,Park, Jong Hyeok,Gimenez, Sixto The Royal Society of Chemistry 2016 Physical chemistry chemical physics Vol.18 No.13
<P>WO3-BiVO4 n-n heterostructures have demonstrated remarkable performance in photoelectrochemical water splitting due to the synergistic effect between the individual components. Although the enhanced functional capabilities of this system have been widely reported, in-depth mechanistic studies explaining the carrier dynamics of this heterostructure are limited. The main goal is to provide rational design strategies for further optimization as well as to extend these strategies to different candidate systems for solar fuel production. In the present study, we perform systematic optoelectronic and photoelectrochemical characterization to understand the carrier dynamics of the system and develop a simple physical model to highlight the importance of the selective contacts to minimize bulk recombination in this heterostructure. Our results collectively indicate that while BiVO4 is responsible for the enhanced optical properties, WO3 controls the transport properties of the heterostructured WO3-BiVO4 system, leading to reduced bulk recombination.</P>
Guerrero, Antonio,Garcia-Belmonte, Germà,Mora-Sero, Ivan,Bisquert, Juan,Kang, Yong Soo,Jacobsson, T. Jesper,Correa-Baena, Juan-Pablo,Hagfeldt, Anders American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.15
<P>Impedance spectroscopy offers access to all the different electronic and ionic processes taking place simultaneously in an operating solar cell. To date, its use on perovskite solar cells has been challenging because of the richness of the physical processes occurring within similar time domains. The aim of this work is to understand the general impedance response and propose a general equivalent circuit model that accounts for the different processes and gives access to quantitative analysis. When the electron-selective contacts and the thickness of the perovskite film are systematically modified, it is possible to distinguish between the characteristic impedance signals of the perovskite layer and those arising from the contacts. The study is carried out using mixed organic lead halogen perovskite (FA(0.85)MA(0.15)Pb(I0.85Br0.15)(3)) solar cells with three different electron-selective contacts: SnO2, TiO2, and Nb2O5. The contacts have been deposited by atomic layer deposition (ALD), which provides pinhole-free films and excellent thickness control in the absence of a mesoporous layer to simplify the impedance analysis. It was found that the interfacial impedance has a rich structure that reveals different capacitive processes, serial steps for electron extraction, and a prominent inductive loop related to negative capacitance at intermediate frequencies. Overall, the present report provides insights into the impedance response of perovskite solar cells which enable an understanding of the different electronic and ionic processes taking place during device operation.</P>
Cho, Woohyung,Lim, Jongchul,Kim, Tea-Yon,Kim, Young Rae,Song, Donghoon,Park, Taiho,Fabregat-Santiago, Francisco,Bisquert, Juan,Kang, Yong Soo American Chemical Society 2016 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.120 No.5
<P>The origin of the differences between the performance parameters found for dye-sensitized solar cells (DSCs) using liquid and poly(ethylene oxide)-based solid polymer electrolytes has been investigated. Limitations associated with poor polymer electrolyte penetration and ionic diffusion have been analyzed together with other effects such as the dye regeneration rate, the conduction band edge shift, and the electron recombination kinetics occurring in the solid polymer electrolyte. We have found that dye regeneration was faster for sensitized TiO2 films fully wetted with polymer electrolyte than that with liquid cells. This new result was attributed to a 0.2 eV decrease in the dye highest occupied molecular orbital energy yielding to an increase in the driving force for dye regeneration. These understandings may contribute to a further increase in the energy-conversion efficiency of DSCs employing solid polymer electrolyte.</P>
Boix, Pablo P.,Lee, Yong Hui,Fabregat-Santiago, Francisco,Im, Sang Hyuk,Mora-Sero, Ivan,Bisquert, Juan,Seok, Sang Il American Chemical Society 2012 ACS NANO Vol.6 No.1
<P>Nanoporous metal oxide electrodes provide a high internal area for dye anchoring in dye-sensitized solar cells, but the thickness required to extinguish the solar photons also enhances recombination at the TiO<SUB>2</SUB>/electrolyte interface. The high extinction coefficient of inorganic semiconductor absorber should allow the reduction of the film thickness, improving the photovoltage. Here we study all-solid semiconductor sensitized solar cells, in the promising TiO<SUB>2</SUB>/Sb<SUB>2</SUB>S<SUB>3</SUB>/P3HT configuration. Flat and nanostructured cells have been prepared and analyzed, developing a cell performance model, based on impedance spectroscopy results, that allows us to determine the impact of the reduction of metal oxide film thickness on the operation of the solar cell. Decreasing the effective surface area toward the limit of flat samples produces a reduction in the recombination rate, increasing the open circuit potential, <I>V</I><SUB>oc</SUB>, while providing a significant photocurrent. However, charge compensation problems as a consequence of inefficient charge screening in flat cells increase the hole transport resistance, lowering severely the cell fill factor. The use of novel structures balancing recombination and hole transport will enhance solid sensitized cell performance.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2012/ancac3.2012.6.issue-1/nn204382k/production/images/medium/nn-2011-04382k_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn204382k'>ACS Electronic Supporting Info</A></P>
Sudhagar, P.,Gonzá,lez-Pedro, Victoria,Mora-Seró,, Ivá,n,Fabregat-Santiago, Francisco,Bisquert, Juan,Kang, Yong Soo The Royal Society of Chemistry 2012 Journal of materials chemistry Vol.22 No.28
<P>Herein we report generic surface treatment approaches to improve the electronic interfaces of quantum dot-sensitized TiO<SUB>2</SUB> fiber electrodes, thereby promoting their photoanode performance. Highly dense, continuous and nanostructured TiO<SUB>2</SUB> fibrous membranes, without the inclusion of a scattering layer, unlike conventional TiO<SUB>2</SUB> particulate electrodes, showed feasible photoconversion performance under the proposed interfacial engineering modification. The proposed interfacial treatment concerns fibrous membranes both before and after calcination. The chemical vapor pre-treatment on an as-deposited fibrous membrane using tetrahydrofuran (THF) reinforces the physical contact between the fibrous membrane and the transparent conducting substrate and reduces significantly the recombination rate. In the case of post-treatment by F-ion on a fibrous surface, together with the interfacial engineering approach, the ZnS surface passivation layer markedly improves the photoanode performance of the TiO<SUB>2</SUB> fibrous membrane nearly to a factor of 3.2% with a remarkable open-circuit voltage <I>V</I><SUB>oc</SUB> = 0.69 V and <I>J</I><SUB>sc</SUB> = 13 mA cm<SUP>−2</SUP> under 1 sun illumination (100 mW cm<SUP>−2</SUP>). This report provides an excellent platform for studying and understanding the interfacial contacts and mechanisms related to the charge transfer at CdS/CdSe QD-sensitized TiO<SUB>2</SUB> fibrous assemblies. Such implications of this interfacial treatment strategy can be successfully extended to a wide range of photoanode candidates in energy conversion systems and confirm the effectiveness of some alternative nanostructured electrodes for the development of semiconductor-sensitized solar cells.</P> <P>Graphic Abstract</P><P>The generic surface treatment approaches were demonstrated to improve the electronic interfaces of quantum dot-sensitized TiO<SUB>2</SUB> fiber electrodes, thereby improving their photoanode performance in solar cells. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2jm31599h'> </P>