Showing papers by "Michael Grätzel published in 2011"
TL;DR: In this article, a Co(II/III)tris(bipyridyl)-based redox electrolyte was used in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8).
Abstract: The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, we report mesoscopic solar cells that incorporate a Co(II/III)tris(bipyridyl)–based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific molecular design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocrystalline titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 volt. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another organic dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight.
5,462 citations
TL;DR: The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are presented to define the obstacles that remain to be surmounted in order to fully exploit the potential of hematite for solar energy conversion.
Abstract: Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe(2)O(3)) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.
2,318 citations
TL;DR: A highly active photocathode for solar H(2) production is presented, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electroDeposited Pt nanoparticles.
Abstract: A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H(2) production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0 V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.
1,856 citations
TL;DR: In this article, the fate of photogenerated holes in photoelectrodes for water splitting was examined using H2O2 as an efficient hole scavenger, and all holes that arrived at the electrode/electrolyte interface were collected.
Abstract: We study hematite (α-Fe2O3) photoelectrodes for water splitting by examining the fate of photogenerated holes. Using H2O2 as an efficient hole scavenger, we collect all holes that arrive at the electrode/electrolyte interface. This provides the ability to distinguish between and quantify bulk and surface recombination processes involved in the photoelectrochemical oxidation of water. Below 1.0 VRHE, electrolyte oxidation kinetics limits the performance but above 1.2 VRHE bulk recombination becomes the limiting factor.
902 citations
TL;DR: In this paper, a short and simple overview of dye-sensitized solar cell technology from the working principles to the first commercial applications is given, emphasizing the role of the sensitizer and strategies to improve the performances of the dye as well as some recent development aiming to answer specific issues.
761 citations
TL;DR: In this article, an ultra-thin coating of Al2O3 was applied on the surface of high-performance hematite to reduce the overpotential of the material and increase the photocurrent.
Abstract: Hematite is a promising material for inexpensive solar energy conversion viawater splitting but has been limited by the large overpotential (0.5–0.6 V) that must be applied to afford high wateroxidation photocurrent. This has conventionally been addressed by coating it with a catalyst to increase the kinetics of the oxygen evolution reaction. However, surface recombination at trapping states is also thought to be an important factor for the overpotential, and herein we investigate a strategy to passivate trapping states using conformal overlayers applied by atomic layer deposition. While TiO2 overlayers show no beneficial effect, we find that an ultra-thin coating of Al2O3 reduces the overpotential required with state-of-the-art nano-structured photo-anodes by as much as 100 mV and increases the photocurrent by a factor of 3.5 (from 0.24 mA cm−2 to 0.85 mA cm−2) at +1.0 V vs. the reversible hydrogen electrode (RHE) under standard illumination conditions. The subsequent addition of Co2+ ions as a catalyst further decreases the overpotential and leads to a record photocurrent density at 0.9 V vs. RHE (0.42 mA cm−2). A detailed investigation into the effect of the Al2O3 overlayer by electrochemical impedance and photoluminescence spectroscopy reveals a significant change in the surface capacitance and radiative recombination, respectively, which distinguishes the observed overpotential reduction from a catalytic effect and confirms the passivation of surface states. Importantly, this work clearly demonstrates that two distinct loss processes are occurring on the surface of high-performance hematite and suggests a viable route to individually address them.
724 citations
TL;DR: A new class of Co(III) complexes as p-type dopants for triarylamine-based hole conductors such as spiro-MeOTAD and their application in solid-state dye-sensitized solar cells (ssDSCs) is reported on.
Abstract: Chemical doping is an important strategy to alter the charge-transport properties of both molecular and polymeric organic semiconductors that find widespread application in organic electronic devices We report on the use of a new class of Co(III) complexes as p-type dopants for triarylamine-based hole conductors such as spiro-MeOTAD and their application in solid-state dye-sensitized solar cells (ssDSCs) We show that the proposed compounds fulfill the requirements for this application and that the discussed strategy is promising for tuning the conductivity of spiro-MeOTAD in ssDSCs, without having to rely on the commonly employed photo-doping By using a recently developed high molar extinction coefficient organic D-π-A sensitizer and p-doped spiro-MeOTAD as hole conductor, we achieved a record power conversion efficiency of 72%, measured under standard solar conditions (AM15G, 100 mW cm–2) We expect these promising new dopants to find widespread applications in organic electronics in general and pho
701 citations
TL;DR: A photo-assisted electrodeposition approach was used to deposit a cobalt-phosphate water oxidation catalyst (Co-Pi) onto recently improved dendritic mesostructures of α-Fe2O3.
Abstract: A photo-assisted electrodeposition approach was used to deposit a cobalt–phosphate water oxidation catalyst (“Co–Pi”) onto recently improved dendritic mesostructures of α-Fe2O3. A comparison between this approach, electrodeposition of Co–Pi, and Co2+ wet impregnation showed that photo-assisted electrodeposition of Co–Pi yields superior α-Fe2O3 photoanodes for photoelectrochemical water oxidation. Stable photocurrent densities of 1.0 mA cm−2 at 1.0 V and 2.8 mA cm−2 at 1.23 V vs. RHE measured under standard illumination and basic conditions were achieved. By allowing deposition only where visible light generates oxidizing equivalents, photo-assisted electrodeposition provides a more uniform distribution of Co–Pi onto α-Fe2O3 than obtained by electrodeposition. This approach of fabricating catalyst-modified metal-oxide photoelectrodes may be attractive for optimization in conjunction with tandem or hybrid photoelectrochemical cells.
602 citations
TL;DR: It is proposed that the enhanced photoelectrochemical activity of the composite electrode for water photooxidation results, at least in part, from reduced recombination losses because of the formation of a Schottky-type heterojunction.
Abstract: Transient absorption spectroscopy was used to probe the dynamics of photogenerated charge carriers in α-Fe2O3/CoOx nanocomposite photoelectrodes for water splitting. The addition of cobalt-based electrocatalysts was observed to increase the lifetime of photogenerated holes in the photoelectrode by more than 3 orders of magnitude without the application of electrical bias. We therefore propose that the enhanced photoelectrochemical activity of the composite electrode for water photooxidation results, at least in part, from reduced recombination losses because of the formation of a Schottky-type heterojunction.
528 citations
TL;DR: It is predicted that the graphene composite is a strong candidate for replacing both Pt and FTO in cathodes for DSC and an order of magnitude decrease of R(CT) is still needed to improve the behavior of DSC near the open circuit potential and, consequently, the fill factor.
Abstract: Commercial graphene nanoplatelets in the form of optically transparent thin films on F-doped SnO2 (FTO) exhibited high electrocatalytic activity toward I3−/I− redox couple, particularly in electrolyte based on ionic liquid (Z952). The charge-transfer resistance, RCT, was smaller by a factor of 5−6 in ionic liquid, compared to values in traditional electrolyte based on methoxypropionitrile solution (Z946). Optical spectra and electrochemical impedance confirm that the film’s absorbance scales linearly with RCT−1. Electrocatalytic properties of graphene nanoplatelets for the I3−/I− redox reaction are proportional to the concentration of active sites (edge defects and oxidic groups), independent of the electrolyte medium. Dye-sensitized solar cell (DSC) was assembled with this material as a cathode. Semitransparent (>85%) film of graphene nanoplatelets presented no barrier to drain photocurrents at 1 Sun illumination and potentials between 0 and ca. 0.3 V, but an order of magnitude decrease of RCT is still n...
511 citations
TL;DR: An experimental study of the influence of gold nanoparticles on α-Fe(2)O(3) photoanodes for photoelectrochemical water splitting observed a relative enhancement in the water splitting efficiency at photon frequencies corresponding to the plasmon resonance in gold.
Abstract: An experimental study of the influence of gold nanoparticles on α-Fe2O3 photoanodes for photoelectrochemical water splitting is described. A relative enhancement in the water splitting efficiency at photon frequencies corresponding to the plasmon resonance in gold was observed. This relative enhancement was observed only for electrode geometries with metal particles that were localized at the semiconductor-electrolyte interface, consistent with the observation that minority carrier transport to the electrolyte is the most significant impediment to achieving high efficiencies in this system.
TL;DR: Graphene nanoplatelets in the form of thin semitransparent films on F-doped SnO2 (FTO) exhibit high electrocatalytic activity for the Co(bpy)3(3+/2+) redox couple in acetonitrile electrolyte solution.
Abstract: Graphene nanoplatelets (GNP) in the form of thin semitransparent films on F-doped SnO2 (FTO) exhibit high electrocatalytic activity for the Co(bpy)33+/2+ redox couple in acetonitrile electrolyte solution. The GNP film is superior to the traditional electrocatalyst, that is, platinum, both in charge-transfer resistance (exchange current) and in electrochemical stability under prolonged potential cycling. The good electrochemical performance of GNP is readily applicable for dye-sensitized solar cells with Y123-sensitized TiO2 photoanodes and Co(bpy)33+/2+ as the redox shuttle. The dye-sensitized solar cell with GNP cathode is superior to that with the Pt-FTO cathode particularly in fill factor and in power conversion efficiency at higher illumination intensity.
TL;DR: This work presents a new mesoporous composite material that can be used for dye-sensitized solar cells and its applications in photovoltaics and transport and performance research.
Abstract: Keywords: cobalt ; dye-sensitized solar cells ; electrochemistry ; photovoltaics ; sensitizers ; Photovoltaic Cells ; Transport ; Recombination ; Electrolyte ; Performance ; Mediators Reference EPFL-ARTICLE-166673doi:10.1002/cssc.201100120View record in Web of Science Record created on 2011-06-08, modified on 2017-05-12
TL;DR: Transient photovoltage and photocurrent decay measurements showed that the enhanced performance achieved with C220 partially stems from the high charge collection efficiency over a wide potential range.
Abstract: The high molar absorption coefficient organic D-pi-A dye C220 exhibits more than 6% certified electric power conversion efficiency at AM 1.5G solar irradiation (100 mW cm(-2)) in a solid-state dye sensitized solar cell using 2,2',7,7'-tetrakis(N,N-dimethoxyphenylamine)-9,9'-spirobi-fluorene (Spiro-MeOTAD) as the organic hole transporting material. This contributes to a new record (6.08% by NREL) for this type of sensitized heterojunction photovoltaic device. Efficient charge generation is proved by incident photon-to-current conversion efficiency spectra. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance achieved with C220 partially stems from the high charge collection efficiency over a wide potential range.
TL;DR: In this article, a dye-sensitized mesoscopic solar cell has been investigated as a promising photovoltaic cell and strategies for achieving panchromatic response in DSCs are discussed, where the primary step is performed by a sensitizer anchored to the surface of a wide band gap semiconductor.
Abstract: The dye-sensitized mesoscopic solar cell has been intensively investigated as a promising photovoltaic cell. Its ecological and economical fabrication processes make it attractive and credible alternative to conventional photovoltaic systems. In contrast to the latter design, the DSC approach separates tasks of light absorption and charge transport. The primary step of light absorption is performed by a sensitizer anchored to the surface of a wide band gap semiconductor. In order to reach a high conversion efficiency, the first requirement is that the sensitizer should absorb as much as possible of the incoming sunlight. Strategies for achieving panchromatic response in dye-sensitized mesoscopic solar cells are discussed.
TL;DR: The cell with GNP cathode is superior to that with Pt-FTO cathode particularly in fill factors and in the efficiency at higher illumination intensities.
Abstract: Graphene nanoplatelets (GNP) in the form of thin semitransparent film on F-doped SnO2 (FTO) exhibit high electrocatalytic activity for Co(L)2; where L is 6-(1H-pyrazol-1-yl)-2,2′-bipyridine. The exchange current densities for the Co2+/3+(L)2 redox reaction scaled linearly with the GNP film’s optical absorbance, and they were by 1–2 orders of magnitude larger than those for the I3–/I– couple on the same electrode. The electrocatalytic activity of GNP films with optical transmission below 88% is outperforming the activity of platinized FTO for the Co2+/3+(L)2 redox reaction. Dye-sensitized solar cells with Y123 dye adsorbed on TiO2 photoanode achieved energy conversion efficiencies between 8 and 10% for both GNP and Pt-based cathodes. However, the cell with GNP cathode is superior to that with Pt-FTO cathode particularly in fill factors and in the efficiency at higher illumination intensities.
TL;DR: In this paper, the onset potential of photoelectrochemical water oxidation on ultrathin hematite was improved by up to 200 mV by the chemical bath deposition of 13-group oxides as overlayers.
Abstract: The onset potential of photoelectrochemical water oxidation on ultrathin hematite was improved by up to 200 mV by the chemical bath deposition of 13-group oxides as overlayers. It is proposed that the corundum-type overlayers released lattice strain of the ultrathin hematite layer and decreased the density of surface states. Particularly, a Ga2O3 overlayer exhibited an enhanced photocurrent attributed to stoichiometric water splitting near the onset potential. The photocurrent was sustained over a day, attesting to its outstanding performance and durability for water splitting.
TL;DR: In this paper, a series of derivatives designed to address earlier problems in Bodipy-sensitized solar cells were reported, achieving an overall efficiency of a modest 2.46% but panchromatic nature of the dyes is quite impressive.
Abstract: Versatility of Bodipy (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes was further expanded in recent dye-sensitized solar cell applications. Here we report a series of derivatives designed to address earlier problems in Bodipy sensitized solar cells. In the best case example, an overall efficiency of a modest 2.46% was achieved, but panchromatic nature of the dyes is quite impressive. This is the best reported efficiency in liquid electrolyte solar cells with Bodipy dyes as photosensitizers.
TL;DR: Transient absorption spectroscopy on the μs-s time scale is used to monitor the yield and decay dynamics of photogenerated holes in nanocrystalline hematite photoanodes and results in the generation of long-lived photoholes.
TL;DR: It is demonstrated that one reason for the slow hole transfer on α-Fe(2)O(3) is the presence of a significant thermal barrier, the magnitude of which is found to be independent of the applied bias at the potentials examined.
Abstract: Competition between charge recombination and the forward reactions required for water splitting limits the efficiency of metal-oxide photocatalysts. A key requirement for the photochemical oxidation of water on both nanostructured α-Fe2O3 and TiO2 is the generation of photoholes with lifetimes on the order of milliseconds to seconds. Here we use transient absorption spectroscopy to directly probe the long-lived holes on both nc-TiO2 and α-Fe2O3 in complete PEC cells, and we investigate the factors controlling this slow hole decay, which can be described as the rate-limiting step in water oxidation. In both cases this rate-limiting step is tentatively assigned to the hole transfer from the metal oxide to a surface-bound water species. We demonstrate that one reason for the slow hole transfer on α-Fe2O3 is the presence of a significant thermal barrier, the magnitude of which is found to be independent of the applied bias at the potentials examined. This is in contrast to nanocrystalline nc-TiO2, where no di...
TL;DR: It is found that the cell performance was strongly correlated with the chemical interaction at the interface of Sb(2)S(3) as sensitizer and the HTMs through the thiophene moieties, which led to a higher fill factor (FF), open-circuit voltage (V(oc), and short-circuits current density (J(sc).
Abstract: Sb2S3-sensitized mesoporous-TiO2 solar cells using several conjugated polymers as hole-transporting materials (HTMs) are fabricated. We found that the cell performance was strongly cc rrelated with the chemical interaction at the interface of Sb2S3 as sensitizer and the HTMs through the thiophene moieties, which led to a higher fill factor (FF), open-circuit voltage (V-oc), and short-circuit current density (J(sc)). With the application of PCPDTBT (poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)) as a HTM in a Sb2S3-sensitized solar cell, overall power conversion efficiencies of 6.18, 6.57, and 6.53% at 100, 50, and 10% solar irradiation, respectively, were achieved with a metal mask.
TL;DR: In this article, the authors investigated the absorption spectrum and the alignment of ground and excited state energies for the prototypical N719 Ru(II) sensitizer adsorbed on an extended TiO2 model by means of high level DFT/TDDFT calculations.
Abstract: We have investigated the absorption spectrum and the alignment of ground and excited state energies for the prototypical N719 Ru(II) sensitizer adsorbed on an extended TiO2 model by means of high level DFT/TDDFT calculations. The calculated and experimental absorption spectra for the dye on TiO2 are in excellent agreement over the explored energy range, with an absorption maximum deviation below 0.1 eV, allowing us to assign the underlying electronic transitions. We find the lowest optically active excited state to lie ca. 0.3 eV above the lowest TiO2 state. This state has a sizable contribution from the dye π* orbitals, strongly mixed with unoccupied TiO2 states. A similarly strong coupling is calculated for the higher-lying transitions constituting the visible absorption band centered at ca. 530 nm in the combined system. An ultrafast, almost instantaneous, electron injection component can be predicted on the basis of the strong coupling and of the matching of the visible absorption spectrum and density...
TL;DR: The first solution-processed depleted bulk heterojunction colloidal quantum dot solar cells are presented, which allows high absorption with full depletion, thereby breaking the photon absorption/carrier extraction compromise inherent in planar devices.
Abstract: The first solution-processed depleted bulk heterojunction colloidal quantum dot solar cells are presented. The architecture allows high absorption with full depletion, thereby breaking the photon absorption/carrier extraction compromise inherent in planar devices. A record power conversion of 5.5% under simulated AM 1.5 illumination conditions is reported.
TL;DR: In this paper, energy relay dye was used for energy relay dyes and Spiro-Ometad was used to accelerate the process of energy relay relay dye processing and transport.
Abstract: Keywords: Energy Relay Dyes ; Spiro-Ometad ; Recombination ; Performance ; Transport ; Design ; Length Reference EPFL-ARTICLE-171308doi:10.1002/aenm.201000041View record in Web of Science Record created on 2011-12-16, modified on 2017-05-12
TL;DR: In this paper, the effect of the interfacial charge-transfer resistance at the counter electrode in dye-sensitized solar cells based on two cobalt redox shuttles was highlighted, with the former showing much lower charge transfer resistances for both cobalt complexes, leading to improved fill factors and linear response of the short circuit photo-current density to light intensity up to one sun.
Abstract: We highlight the effect of the interfacial charge-transfer resistance at the counter electrode in dye-sensitized solar cells based on two cobalt redox shuttles, namely cobalt(III/II) tris(2,2′-bipyridine) and cobalt(III/II) tris(1,10-phenanthroline). Highly porous counter electrodes based on poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by electro-oxidative polymerization are compared to the typically employed platinized FTO glass, with the former showing much lower charge transfer resistances for both cobalt complexes, leading to improved fill factors and to linear response of the short circuit photo-current density to light intensity up to one sun. Based on these findings, an excellent power conversion efficiency of 10.3% was achieved with a recently reported organic sensitizer and PEDOT as counter electrode.
TL;DR: Electrochemical data indicate that the oxidative behavior of the TPA and metal chelate units can be independently modulated except in cases where the anionic phenyl ring is in direct conjugation with the Tpa unit.
Abstract: The syntheses and the electrochemical spectroscopic properties of a suite of asymmetrical bistridentate cyclometalated Ru(II) complexes bearing terminal triphenylamine (TPA) substituents are reported. These complexes, which contain structural design elements common to both inorganic and organic dyes that exhibit superior power conversion efficiencies in the dye-sensitized solar cell (DSSC), are broadly formulated as [RuII(L-2,5′-thiophene-TPA-R1)(L-R2)]+ [L = tridentate chelating ligand (e.g., 2,2′:6′,2′′-terpyridine (tpy); deprotonated forms of 1,3-di(pyridin-2-yl)benzene (Hdpb) or 6-phenyl-2,2′-bipyridine (Hpbpy)); R1 = −H, −Me, −OMe; R2 = −H, −CO2Me, −CO2H]. The following structural attributes were systematically modified for the series: (i) electron-donating character of the terminal substituents (e.g., R1 = −H, −Me, −OMe) placed para to the amine of the “L-2,5′-thiophene-TPA-R1” ligand framework; (ii) electron-withdrawing character of the tridentate chelate distal to the TPA-substituted ligand (e.g.,...
TL;DR: This novel photoanode is designed to improve the charge extraction resulting in higher fill factor and photovoltage for DSCs and an increase in photvoltage of up to 110 mV over state-of-the-art DSC is demonstrated.
Abstract: Herein, we present a straightforward bottom-up synthesis of a high electron mobility and highly light scattering macroporous photoanode for dye-sensitized solar cells. The dense three-dimensional Al/ZnO, SnO2, or TiO2 host integrates a conformal passivation thin film to reduce recombination and a large surface-area mesoporous anatase guest for high dye loading. This novel photoanode is designed to improve the charge extraction resulting in higher fill factor and photovoltage for DSCs. An increase in photovoltage of up to 110 mV over state-of-the-art DSC is demonstrated.
TL;DR: In this paper, the effect of hole transport material (HTM) pore filling on the photovoltaic performance of solid-state dye-sensitized solar cells and the specifi c mechanisms involved are reported.
Abstract: A detailed investigation of the effect of hole transport material (HTM) pore fi lling on the photovoltaic performance of solid-state dye-sensitized solar cells (ss-DSCs) and the specifi c mechanisms involved is reported. It is demonstrated that the effi ciency and photovoltaic characteristics of ss-DSCs improve with the pore fi lling fraction (PFF) of the HTM, 2,2’,7,7’-tetrakis-( N , N -di- p -methoxyphenylamine)9,9’-spirobifl uorene(spiro-OMeTAD). The mechanisms through which the improvement of photovoltaic characteristics takes place were studied with transient absorption spectroscopy and transient photovoltage/photocurrent measurements. It is shown that as the spiroOMeTAD PFF is increased from 26% to 65%, there is a higher hole injection effi ciency from dye cations to spiro-OMeTAD because more dye molecules are covered with spiro-OMeTAD, an order-of-magnitude slower recombination rate because holes can diffuse further away from the dye/HTM interface, and a 50% higher ambipolar diffusion coeffi cient due to an improved percolation network. Device simulations predict that if 100% PFF could be achieved for thicker devices, the effi ciency of ss-DSCs using a conventional rutheniumdye would increase by 25% beyond its current value.
TL;DR: Investigation of the effect of temperature for dye adsorption on the photovoltaic performance of dye-sensitized solar cells (DSCs) found a significant efficiency enhancement upon lowering the temperature applied during the sensitizer uptake from solution.
Abstract: Employing a mesoscopic titania photoanode whose bilayer structure was judiciously selected to fit the optoelectronic characteristics of the Ru-based heteroleptic complex Na-cis-Ru(4,4'-(5-hexyltiophen-2-yl)-2,2'-bipyridine)(4-carboxylic-acid-4'-carboxylate-2,2'-bipyridine)(thiocyanate)(2), coded as C101, we investigated the effect of temperature for dye adsorption on the photovoltaic performance of dye-sensitized solar cells (DSCs). We found a significant efficiency enhancement upon lowering the temperature applied during the sensitizer uptake from solution. When the dye adsorption was performed at 4 °C, the photovoltaic performance parameters measured under standard reporting conditions (AM1.5 G sunlight at 1000 W/m(2) intensity and 25 °C), i.e., the open circuit voltage (V(oc)), the short circuit photocurrent density (J(sc)), the fill factor (FF), and consequently the power conversion efficiency (PCE), improved in comparison to cells stained at 20 and 60 °C. Results from electrochemical impedance spectroscopy (EIS) and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) show that the self-assembled layer of C101 formed at lower temperature impairs the back-electron transfer from the TiO(2) conduction band to the triiodide ions in the electrolyte more strongly than the film produced at 60 °C. Profiting from the favorable influence that the low-temperature dye uptake exerts on photovoltaic performance, we have realized DSCs showing a power conversion efficiency of 11.5%.
TL;DR: In this paper, the molecular structures and electronic and optical properties of 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl amine)-9,9′-spirobifluorene (spiro-MeOTAD) in different oxidation states have been investigated by means of DFT/TDDFT methods.
Abstract: The molecular structures and electronic and optical properties of 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl amine)-9,9′-spirobifluorene (spiro-MeOTAD) in different oxidation states have been investigated by means of DFT/TDDFT methods. Spiro-MeOTAD has been demonstrated to be an efficient hole-transport material (HTM) in organic light-emitting devices (OLEDs) and in solid-state dye-sensitized solar cells (ssDSCs), and to date spiro-MeOTAD, has yielded the highest ssDSC efficiency. The spiro-MeOTAD radical cation exhibits long-term stability, even though the 2+ and 4+ formal oxidation states are accessible. DFT and TDDFT allow the characterization of the excited states involved in the absorption processes of the spiro-MeOTAD-derived cations, an important aspect considering that the oxidized species absorb in the visible region. The excellent agreement between theory and experiment for both neutral spiro-MeOTAD and its oxidized forms opens the possibility for identifying the features that make it an efficien...