Higher Charge Recombination Resistance Research Articles

A facile strategy for highly efficient perovskite crystals fabricated in a high moisture environment via solvent engineering process

BackgroundHybrid organic perovskite solar cells (PSCs) have recently gained recognition for their excellent photovoltaic properties. However, most PSCs were fabricated in a nitrogen-filled glovebox to prevent damage from moisture. Overcoming humidity damage is crucial for PSC fabrication in an ambient atmosphere. Fabricating perovskite solar cells (PSCs) under ambient atmosphere conditions needs the aid of substrate heating or air-knife flow to rapidly evaporate solvents. MethodsHerein, we propose a facile solvent engineering method without extra equipment or processes for fabricating highly efficient PSCs in a high moisture condition by a one-step spin-coating process. Acetone incorporation into the perovskite precursor solution not only prevents the ingress of moisture, but also accelerates the volatilization of solvents during the crystallization process. Significant findingsThe as-prepared PSCs feature low trap-state density, high charge recombination resistance, low hysteresis, and long charge lifetime. Little MABr and PbBr2 are separated out to passivate interface/grain-boundary defects. At a 10% acetone incorporation, the highest power conversion efficiency can reach 17.75% for the PSC fabricated in 70% relative humidity. The work provides a facile, promising method to prepare highly efficient PSCs in an ambient atmosphere.

Dye‐Sensitized Solar Cells with Efficiency over 36% under Ambient Light Achieved by Cosensitized Tandem Structure

The cosensitization process is an effective way to improve the performance of dye‐sensitized solar cells (DSSCs), especially for tandem cells that employ the complementary light harvest ranges of different dyes. Herein, various cosensitized DSSCs are prepared using D35, XY1b, and Y123 dyes and employed to construct cobalt tandem DSSCs. First, the performance of single cells with and without scattering layers is studied. The results show that the cosensitized cells using D35 + XY1b and XY1b + Y123 dyes have higher power conversion efficiency (PCE) than the others, attributed to their longer light absorption wavelength and higher charge recombination resistance. Furthermore, the XY1b + Y123 cell has significantly higher open circuit voltage (Voc) but slightly lower current density (Jsc) than those of the D35 + XY1b cell. The cosensitized systems are further utilized to prepare tandem cells. Photoelectrodes without and with scattering layers are used as top and bottom cells, respectively. It shows that by using D35 + XY1b (higher Jsc) and XY1b + Y123 (higher Voc) as sensitizers of top and bottom cells, respectively, a PCE as high as 36.27% can be achieved under fluorescent lighting. The cells using these dyes have high long‐term stability at room temperature.

Passivation and energy-level change of the SnO2 electron transport layer by reactive titania for perovskite solar cells

Reactive anatase titania (TAc) are incorporated into SnO2 to form a single TAc-SnO2 electron transport layer (ETL) for mixed-cation perovskite solar cells (PSCs). The TAc-SnO2 layer is prepared in a low-temperature process by the advantage of high crystallinity and surface reactivity of TAc. The influence and mechanism of the TAc incorporation on the photovoltaic performance of PSCs are investigated. The TAc-SnO2 ETL presents rapid electron transfer, low trap density, high charge recombination resistance, low hysteresis and good stability. The 2.5-wt% TAc incorporation reveals the highest power conversion efficiency of 20.12 %, a 19 %-fold enhancement compared to the pristine cell with the SnO2 ETL. The enhancement arises from the fact that the TAc can fill up breaching in the pristine SnO2 layer and adjust the ETL energy level. The carboxyl groups of TAc facilitate the formation of solid bonding among SnO2/TAc colloids and prevent ions from accumulation at the interface of ETL/perovskite. The work provides a facile way to fabricate a low-temperature, single-layer and effective ETL for highly efficient PSCs.

The effect of graphene quantum dots/ ZnS co‐passivation on enhancing the photovoltaic performance of CdS quantum dot sensitized solar cells

The effect of graphene quantum dots (GQDs) as a passivating layer for cadmium sulfide (CdS) quantum dot-sensitized solar cell (QDSSC) has been investigated. GQDs were prepared by the liquid exfoliation method. The optical properties of GQDs were studied by UV-Vis and photoluminescence spectroscopies. The morphology of GQDs was studied by transmission emission morphology (TEM) analysis. Further, the photoexcitation behavior of GQDs has been studied in detail using emission spectroscopy and its effect on the photovoltaic performance of CdS QDSSC has been explored using J-V analysis. The efficacy of GQDs as passivation layer over the CdS loaded titanium dioxide (TiO2) photoanode in QDSSC is high and thereby improved the open-circuit voltage (Voc) (555 mV) compared to zinc sulfide (ZnS) passivated QDSSC (509 mV). As a result, the photoconversion efficiency (PCE) of GQD-passivated QDSSC is relatively enhanced up to 25% higher than that of ZnS-passivated QDSSC under similar conditions. A high PCE of 4.26% has been achieved for GQD/ZnS co-passivated QDSSC, which is 31% higher than ZnS-passivated QDSSC. Besides, the impedance analysis revealed low internal resistance and high charge recombination resistance in the GQD-passivated QDSSC when compared to other cells. The presence of GQD layer suppressed the recombination with high electron transport and thereby improved the PCE of QDSSC. The present study shows the possibility of using GQDs as an interfacial passivation layer between the sensitizer and electrolytes for enhancing the photovoltaic performance of CdS-based QDSSCs.

Layered Ruddlesden–Popper Perovskites with Various Thicknesses for Stable Solid-State Solar Cells

The present research comes up with optimizing the layers thickness of a Ruddlesden–Popper perovskite with the general formula of (S0.97$${\text{S}}_{{0.03}}^{'}$$)2[Cs0.05(FA0.097MA0.03)0.95]n – 1Pbn(I0.97Br0.03)3n + 1 for efficient, stable solar cell applications. Such a triple-cation quasi-two-dimensional (2D) structure simultaneously contains two spacers, namely 5-ammonium valeric acid iodide (S) and tetra-n-octylammonium bromide (S'). Systematic studies showed that morphology, crystal structure, optical properties, photovoltaic performance, and internal resistances of this compound depended upon the value of the n integer. Field emission scanning electron microscopy set forth that the deposited films were composed of various morphologies depending on the n value. An increase in the n value resulted in improving the light absorption, reducing the band gap energy, and blue-shifting the photoluminescence peak. So as to fabricate solar cells, CuInS2 nanoparticles were employed as a novel hole-transporting material. The device based on the film having n = 4 value showed the highest power conversion efficiency of 10.2%. Electrochemical impedance spectroscopy demonstrated that the improved performance of this cell was mainly thanks to its low series resistance (11.68 Ω), high charge recombination resistance (922.35 Ω), and long electron lifetime (8.05 μs) as compared to all the fabricated cells. Moreover, this cell displayed a maximum external quantum efficiency of 82% among all the devices. The un-encapsulated solar cells showed that the output reduction directly depended on the n value so that the cell based on the n = 4 reached 82% of its initial power over 2500 h in ambient conditions.

Development of a triple-cation Ruddlesden–Popper perovskite structure with various morphologies for solar cell applications

The present research sheds new light on the development of a triple-cation quasi-two-dimensional (2D) perovskite family with the general formula of (S1−xS′x)2[Cs0.05(FA1−xMAx)0.95]3Pb4(I1−xBrx)13, in which two spacers, namely 5-ammonium valeric acid iodide (S) and tetra-n-octylammonium bromide (S′) were simultaneously incorporated. Morphology, crystal structure, optical properties, photovoltaic performance, and internal resistances of such compound were systemically studied in comparison with an analogous single-cation 2D counterpart (i.e. (S)2(FA)3Pb4I13) as a reference. X-ray diffraction set forth that the films deposited based upon these compounds had a 2D perovskite crystal structure owing to the pronounced (0 k 0) peak series at low angles. Field emission scanning electron microscopy propounded that the layered ultrathin 2D structures were stacked up to produce various 3D solids such as particles, flowers, needles, sheets, blades, and cubes, depending on the spacers atomic ratios (i.e. x = S′/S + S′) in the range of 0–0.05. Moreover, the photoluminescence spectra of the films exhibited that the triple-cation perovskites had lower bandgap of around 1.68 eV compared to the reference film (ca., 1.74 eV), confirming the formation of 2D perovskites. The device fabricated based on 97 at% S and 3 at% S′ (i.e. x = 0.03) showed the highest power conversion efficiency of 10.2% due mainly to its low series resistance (11.7 Ω), high charge recombination resistance (922.4 Ω), and long electron lifetime (8.0 µs) among the fabricated cells. The un-encapsulated x = 0.03 cell displayed a maximum external quantum efficiency of 82% and lost just 18% of its initial efficiency after 2500 h in ambient conditions.

Synthesis, characterization of ((CH3)3S)2SnI6-nCln and ((CH3)3S)2SnI6-nBrn (n=1, 2) perovskites and use in dye-sensitized solar cells

New air-stable ((CH3)3S)2SnI6-nCln and ((CH3)3S)2SnI6-nBrn (n = 1, 2) defect perovskites were synthesized and their physicochemical properties were established. Rietveld analysis on the recorded XRD patterns revealed the presence of cubic structural modification with a 0D network of [SnI6-nCln] and [SnI6-nBrn] octahedra. The vibrational and electronic properties of the mixed-anion based trimethylsulfoinum Tin (IV) perovskites investigated using Raman and UV–vis spectroscopy showed that upon substitution of Cl/Br for I, the electronic band gap slightly increased, while the lattice vibrations were also largely affected. Density functional theory (DFT) calculations permitted to determine the density of states (DOS) distribution and corresponding band energy structures, confirming the obtained increase of direct band gap values with halogen substitution. The lead-free Sn(IV)-based compounds were successfully incorporated as hole transporting materials (HTMs) in sensitized nanocrystalline solar cells (DSCs). For these devices, power conversion efficiencies as high as 5% were obtained, under 1 sun (A.M. 1.5G) illumination. Electrochemical impedance spectroscopic analysis indicated that the maximum device performance is associated with high charge recombination resistance and low electron transfer resistance at dye/perovskite and perovskite/Pt interfaces, respectively.

New pyridyl-based dyes for co-sensitization in dye sensitized solar cells

Co-sensitization is very advantageous in increasing a dye sensitized solar cell’s (DSSC) efficiency. It relies on sensitizing the same titania film with two dyes that have complementary absorption spectra. In this report we describe co-sensitization of dyes with pyridine anchoring groups with carboxylic-acid based ones having complementary absorption spectra. Six new pyridine based dyes with colors ranging from yellow to turquoise have been synthesized and characterized. T220, a yellow dye, was co-sensitized with Dyenamo Blue, a commercially available blue dye. This approach showed good performance enhancement compared to individual dye sensitized cells with a Co(II/III) redox mediator. An increase in the photocurrent (JSC), photo-voltage (Voc) and thus total efficiency (PCE%) were seen. The close packing of the two dyes in the co-sensitized devise caused a decrease in the electron recombination processes at the TiO2/electrolyte interface, thus resulting in higher Voc values. Such an increase in dye coverage results in a blocking behavior at the TiO2/electrolyte interface, as mirrored in the performed electrochemical impedance spectroscopy (EIS) experiments, where higher charge recombination resistance (RCT) and longer electron lifetime were measured in the co-sensitized DSSCs.

Structure-property relationships: “Double-tail versus double-flap” ruthenium complex structures for high efficiency dye-sensitized solar cells

Six novel heteroleptic amphiphilic polypyridyl Ruthenium (II) complexes, coded FS01-FS06, with hetero-aromatic electron-donor ancillary ligands containing julolidine-derived moieties were synthesized to investigate the relationship between structure modulations of electron donors of Ru(II) dyes and their photophysical, electrochemical and photovoltaic properties for dye-sensitized solar cells (DSSCs). These modulations include: Ru(II) complexes with double “tails” (i.e. tetramethyl groups attached to the end of julolidine-based antennas, FS04) compared to the ones without double “tails” (FS01); complexes with double small “flaps” (i.e. small acyclic electron donor auxochromes ortho to the CH = CH bridge of stilbazole, FS02, FS05) compared to the ones with double large “flaps” (FS03, FS06). Their low energy metal-to-ligand charge transfers (MLCT) band and molar absorptivities were all better than those of the benchmark, N719. It was also shown that the incorporation of double “flaps” into the ancillary ligands caused a slight red shift of light absorption. The photovoltaic properties were evaluated under 1.5 AM standard illumination condition and compared to N719. The highest photocurrent density (JSC) was observed for the complex with double “tails” and double small “flaps” (FS05). The overall conversion efficiency for devices employing julolidine-derived Ru (II) complexes was in the following order FS05 > FS02 > FS04 > FS01 > FS06 > FS03. FS05 (8.16%) outperformed the benchmark N719 (7.75%) in the photovoltaic performance, which is due to its best light-harvesting ability, highest molar extinction coefficient and smallest energy band gap among all the six dyes. To probe the interrelationship among julolidine-based electron donors of ancillary ligands, photocurrent and photovoltage of these dyes, the equilibrium molecular geometries of the ancillary ligands were calculated using DFT. The equilibrium molecular geometries of these dyes the photocurrent and photovoltage are dependent on the donating effect of alkyloxy auxochromes, the steric effect generated from the auxochromes and julolidine moieties, and the orientation of longer alkyloxy group. The introduction of double “tails” resulted in less dye aggregation and higher charge recombination resistance, leading to higher photocurrents and photovoltages in the solar cell performances. Despite of the donating effect of alkyloxy groups, the bulky double “flaps” mainly jammed the hole transportation between the redox couple of the electrolyte and the HOMO of thiocyanate groups (−NCS), translating into the decrease of photocurrent.

Spray-Coated CsPbBr3 Quantum Dot Films for Perovskite Photodiodes.

Large-area film deposition and high material utilization ratio are the crucial factors for large-scale application of perovskite optoelectronics. Recently, all-inorganic halide perovskite CsPbBr3 has attracted great attention because of its high phase stability, thermal stability, and photostability. However, most reported perovskite devices were fabricated by spin-coating, suffering from a low material utilization ratio of 1% and a small coverage area. Here, we developed a spray-coating technique to fabricate a CsPbBr3 quantum dot (QD) film photodiode which had a high material utilization ratio of 32% and a deposition rate of 9 nm/s. The film growth process was studied, and substrate temperature and spray time were two key factors for the deposition of uniform and crack-free QD films. The spray-coated photodiode was demonstrated to be more suitable for working in the photodetector mode because a low dark current density of 4 × 10-4 mA cm-2 resulting from an extremely low recombination current contributed to a high detectivity of 1 × 1014 Jones. A high responsivity of 3 A W-1 was obtained at -0.7 V under 365 nm illumination, resulting from a low charge-transfer resistance and a high charge recombination resistance. We believe that the spray deposition technique will benefit the fabrication of perovskite QD film optoelectronics on a large scale.

Electrochemical synthesis of one-dimensional ZnO nanostructures on ZnO seed layer for DSSC applications

Abstract Electrochemical deposition of vertically aligned zinc oxide (ZnO) nanorods were prepared on ZnO seeded fluorine doped tin oxide (FTO) substrate in the solutions consisting of different concentrations of hexamethylenetetramine (HMTA). The electrochemical, structural, morphological, vibrational and optical properties were characterized by cyclic voltammetry (CV), X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectroscopy and photoluminescence (PL) spectroscopy, respectively. CV curves confirm that metallic zinc phase is not deposited as the HMTA concentration is about 9 mM in a deposition solution. XRD patterns of the as-prepared films show that the increasing HMTA concentrations from 0 mM to 9 mM not only increase the formation of zinc hydrate chloride (Zn 5 (OH) 8 Cl 2 ·H 2 O) but also decrease and finally disappear the metallic Zn deposition. After the as-prepared films annealed at 450 ° C, the crystalline phases of Zn and Zn 5 (OH) 8 Cl 2 ·H 2 O are completely converted to ZnO hexagonal wurtzite phase with high intense growth (002) plane orientation. SEM images support that the vertical growth of ZnO nanostructures (nanorods and petals) with a few flowers is found to be in the cordillera structure as the films are deposited in the solutions consisting of 3 mM, 6 mM and 9 mM HMTA respectively. Raman and PL spectra confirm that the ZnO film deposited in the solution consisting of 9 mM HMTA has a higher crystalline nature with lesser atomic defects and is also higher c-axis growth than that of other films deposited in the solutions consisting of 0 mM, 3 mM and 6 mM, respectively. UV–vis absorbance spectra corroborate that the ZnO film deposited in the solution consisting of 9 mM HMTA shows a high dye absorbance as compared with other films. The efficiency of DSSCs based on ZnO photoanodes deposited in the solutions consisting of 0 mM and 9 mM HMTA was 1.79 and 3.75%, respectively. Electrochemical impedance spectra revealed that DSSC based on ZnO photoanode deposited in the solution consisting of 9 mM HMTA has a higher charge recombination resistance (Rrec) than that of another DSSC.

Conjugated Small Molecule for Efficient Hole Transport in High‐Performance p‐i‐n Type Perovskite Solar Cells

The π‐conjugated organic small molecule 4,4′‐cyclohexylidenebis[N,N‐bis(4‐methylphenyl) benzenamine] (TAPC) has been explored as an efficient hole transport material to replace poly(3,4‐ethylenedio‐xythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the preparation of p‐i‐n type CH3NH3PbI3 perovskite solar cells. Smooth, uniform, and hydrophobic TAPC hole transport layers can be facilely deposited through solution casting without the need for any dopants. The power conversion efficiency of perovskite solar cells shows very weak TAPC layer thickness dependence across the range from 5 to 90 nm. Thermal annealing enables improved hole conductivity and efficient charge transport through an increase in TAPC crystallinity. The perovskite photoactive layer cast onto thermally annealed TAPC displays large grains and low residual PbI2, leading to a high charge recombination resistance. After optimization, a stabilized power conversion efficiency of 18.80% is achieved with marginal hysteresis, much higher than the value of 12.90% achieved using PEDOT:PSS. The TAPC‐based devices also demonstrate superior stability compared with the PEDOT:PSS‐based devices when stored in ambient circumstances, with a relatively high humidity ranging from 50 to 85%.

Inorganic Ligand Thiosulfate-Capped Quantum Dots for Efficient Quantum Dot Sensitized Solar Cells.

The insulating nature of organic ligands containing long hydrocarbon tails brings forward serious limitations for presynthesized quantum dots (QDs) in photovoltaic applications. Replacing the initial organic hydrocarbon chain ligands with simple, cheap, and small inorganic ligands is regarded as an efficient strategy for improving the performance of the resulting photovoltaic devices. Herein, thiosulfate (S2O32-), and sulfide (S2-) were employed as ligand-exchange reagents to get access to the inorganic ligand S2O32-- and S2--capped CdSe QDs. The obtained inorganic ligand-capped QDs, together with the initial oleylamine-capped QDs, were used as light-absorbing materials in the construction of quantum dot sensitized solar cells (QDSCs). Photovoltaic results indicate that thiosulfate-capped QDs give excellent power conversion efficiency (PCE) of 6.11% under the illumination of full one sun, which is remarkably higher than those of sulfide- (3.36%) and OAm-capped QDs (0.84%) and is comparable to the state-of-the-art value based on mercaptocarboxylic acid capped QDs. Photoluminescence (PL) decay characterization demonstrates that thiosulfate-based QDSCs have a much-faster electron injection rate from QD to TiO2 substrate in comparison with those of sulfide- and OAm-based QDSCs. Electrochemical impedance spectroscopy (EIS) results indicate that higher charge-recombination resistance between potoanode and eletrolyte interfaces were observed in the thiosulfate-based cells. To the best of our knowledge, this is the first application of thiosulfate-capped QDs in the fabrication of efficient QDSCs. This will lend a new perspective to boosting the performance of QDSCs furthermore.

Positional isomers of pyridine linked triphenylamine-based donor-acceptor organic dyes for efficient dye-sensitized solar cells

Two donor-acceptor (D-A) metal free dyes 3-{4-[Bis-(4-pyridin-2-ylethynyl-phenyl)-amino]-phenyl}-2-cyano-acrylic acid (D1) and 3-{4-[Bis-(4-pyridin-3-ylethynyl-phenyl)-amino]-phenyl}-2-cyano-acrylic acid (D2) have been designed, synthesized and used as sensitizers for the dye sensitized solar cells (DSSCs). The DSSCs based on D1 and D2 exhibit the power conversion efficiency (PCE) of 5.16% and 4.27%, respectively. The higher PCE value of D1 based DSSC is attributed to its enhanced short circuit current (Jsc) and open-circuit voltage (Voc) and fill factor (FF) values. The electrochemical impedance spectra demonstrated shorter electron transport time, longer electron lifetime and high charge recombination resistance for DSSC based on D1, as well as larger dye loading onto TiO2 are attributed to the higher PCE.

Preparation of nano-flower-like SnO2 particles and their applications in efficient CdS quantum dots sensitized solar cells

Two kinds of hierarchical SnO2 particles as nano-flower-like SnO2 particles (NF-SnO2) and mesoporous-sphere-like SnO2 particles (MS-SnO2) have been synthesized and used in CdS quantum dots (QDs) sensitized solar cells (QDSSC). TEM images show that NF-SnO2 particles compose well interconnected nanoplates and MS-SnO2 spheres are consisted with large numbers of SnO2 nanoparticles with size about 7 nm. Owing to the larger Brunauer–Emmett–Teller surface area of NF-SnO2 particles, NF-SnO2 photoelectrode can load higher amounts of CdS QDs and shows enhanced light absorbance, resulting in higher short-circuit current density compared with that of MS-SnO2 photoelectrode. The electrochemical impedance spectroscopy analysis results show that the QDSSC with NF-SnO2 photoelectrode has a higher charge recombination resistance and a longer electron lifetime, so its photovoltaic performance is better than that of the QDSSC with MS-SnO2 photoelectrode. The QDSSC with NF-SnO2 photoelectrode can attain 3 % power conversion efficiency, which is higher than that of the cell with MS-SnO2 photoelectrode (1.97 %).

"Spider"-shaped porphyrins with conjugated pyridyl anchoring groups as efficient sensitizers for dye-sensitized solar cells.

Two novel "spider-shaped" porphyrins, meso-tetraaryl-substituted 1PV-Por and zinc-metalated 1PV-Zn-Por, bearing four oligo(p-phenylenevinylene) (oPPV) pyridyl groups with long dodecyloxy chains on the phenyl groups, have been synthesized. The presence of four pyridyl groups in both porphyrins, which allow them to act as anchoring groups upon coordination to various Lewis acid sites, the conjugated oPPV bridges, which offer the possibility of electronic communication between the porphyrin core and the pyridyl groups, and the dodecyloxy groups, which offer the advantage of high solubility in a variety of organic solvents of different polarities and could prevent porphyrin aggregation, renders porphyrins 1PV-Por and 1PV-Zn-Por very promising sensitizers for dye-sensitized solar cells (DSSCs). Photophysical measurements, together with electrochemistry experiments and density functional theory calculations, suggest that both porphyrins have frontier molecular orbital energy levels that favor electron injection and dye regeneration in DSSCs. Solar cells sensitized by 1PV-Por and 1PV-Zn-Por were fabricated, and it was found that they show power conversion efficiencies (PCEs) of 3.28 and 5.12%, respectively. Photovoltaic measurements (J-V curves) together with incident photon-to-electron conversion efficiency spectra of the two cells reveal that the higher PCE value of the DSSC based on 1PV-Zn-Por is ascribed to higher short-circuit current (Jsc), open-circuit voltage (Voc), and dye loading values. Emission spectra and electrochemistry experiments suggest a greater driving force for injection of the photogenerated electrons into the TiO2 conduction band for 1PV-Zn-Por rather than its free-base analogue. Furthermore, electrochemical impedance spectroscopy measurements prove that the utilization of 1PV-Zn-Por as a sensitizer offers a high charge recombination resistance and, therefore, leads to a longer electron lifetime.

Mass scale sugar-mediated green synthesis and DSSCs application of tin oxide nanostructured photoanode: Effect of zinc sulphide layering on charge collection efficiency

Table-top (market quality) sugar-mediated green synthesis of tin oxide (SnO2) spherical nanocrystallites (NCs) of 40-60nm is reported, passivated with 2, 4 and 6 layers (L) of zinc sulphide (ZnS) and envisaged in dye sensitized solar cells (DSSCs). All photoanodes i.e. without and ZnS-passivated SnO2 (SnO2-ZnS NCs) are characterized for their structure, morphology, optoelectrochemical properties. SnO2-ZnS NC-electrodes show appreciable DSSCs performance due to an enhanced dye loading capability, light scattering ability, and suppressed recombination rate. The optimized SnO2-(4L) ZnS NC-photoanode demonstrates 200% enhancement in the power conversion efficiency compared to SnO2 NC-based photoanode. This is due to prolonged transient photovoltage decay and higher charge recombination resistance with a minimized recombination rate of photoelectrons. Mott-Schottky measurements confirm a negative shift in the conduction band position of SnO2-(4L) ZnS NCs photoanode (0.41V) compared to SnO2 NC-photoanode.

Efficient Sensitization of Dye-Sensitized Solar Cells by Novel Triazine-Bridged Porphyrin–Porphyrin Dyads

Two novel porphyrin-porphyrin dyads, the symmetrical Zn[Porph]-Zn[Porph] (2) and unsymmetrical Zn[Porph]-H2[Porph] (4), where Zn[Porph] and H2[Porph] are the metalated and free-base forms of 5-(4-aminophenyl)-10,15,20-triphenylporphyrin, respectively, in which two porphyrin units are covalently bridged by 1,3,5-triazine, have been synthesized via the stepwise amination of cyanuric chloride. The dyads are also functionalized by a terminal carboxylic acid group of a glycine moiety attached to the triazine group. Photophysical measurements of 2 and 4 showed broaden and strengthened absorptions in their visible spectra, while electrochemistry experiments and density functional theory calculations revealed negligible interaction between the two porphyrin units in their ground states but appropriate frontier orbital energy levels for use in dye-sensitized solar cells (DSSCs). The 2- and 4-based solar cells have been fabricated and found to exhibit power conversion efficiencies (PCEs) of 3.61% and 4.46%, respectively (under an illumination intensity of 100 mW/cm(2) with TiO2 films of 10 μm thickness). The higher PCE value of the 4-based DSSC, as revealed by photovoltaic measurements (J-V curves) and incident photon-to-current conversion efficiency (IPCE) spectra of the two cells, is attributed to its enhanced short-circuit current (J(sc)) under illumination, high open-circuit voltage (V(oc)), and fill factor (FF) values. Electrochemical impedance spectra demonstrated shorter electron-transport time (τd), longer electron lifetime (τe), and high charge recombination resistance for the 4-based cell, as well as larger dye loading onto TiO2.

Electrophoretically self-assembled mixed metal oxide-TiO2 nano-composite film structures for photoelectrochemical energy conversion: Probing of charge recombination and electron transport resistances

Nanotitania/metal oxide composite films are prepared via the coupling of low DC voltage Electrophoretic Deposition (EPD) of P25 TiO2 nanoparticles with simultaneous electrolytic deposition of hydrous metal oxides, namely ZnO, MgO and Al2O3. The nanocomposite films are built into dye-sensitized solar cell photoanodes and their interfacial charge recombination and electronic resistances are investigated. The nano hydrous oxides were found to co-deposit (1–3 wt%) uniformly within the TiO2 film (forming island-like nanodeposits) significantly increasing film adhesion. Analysis via Electrochemical Impedance Spectroscopy and Open Circuit Voltage Decay techniques found, among the three composite films, the TiO2–Al2O3 electrode to exhibit the highest charge recombination resistance at the TiO2/electrolyte interface (Rrec) and as consequence an increase in Voc. However, its conversion efficiency (4.14%) was the lowest because it suffered from very high electron transport resistance (Rt) in the TiO2 network. By comparison, the TiO2–MgO film resulted in 5.40% efficiency and the TiO2–ZnO film in 5.85% efficiency-both exhibiting significantly lower Rt resistance. The obtained results point to the need for simultaneous optimization of the nanocomposite TiO2/metal oxide film structure in terms of high interfacial charge recombination resistance and low overall electron transport resistance.

Dye-sensitized solar cells containing plasma jet deposited hierarchically nanostructured TiO2 thin photoanodes

The investigation of dye-sensitized solar cells (DSCs) using a very thin (0.8 to 4.7 μm) transparent photoanode based on a hierarchically nanostructured TiO2 film prepared via Plasma Assisted Supersonic Jet Deposition (PA-SJD), in combination with the DSC benchmark photosensitizer N719, is presented. The cell photovoltages (nearly 0.8 V) and the amount of adsorbed dye and photocurrent densities, normalized over the film thickness, are higher than those of DSCs prepared with thicker (9 μm) conventional screen-printed TiO2 films. A record efficiency value of 5.7% (standard AM 1.5G solar irradiation) for very thin nanostructured titania photoanodes is obtained with a 2 μm PA-SJD TiO2 film. Electrochemical impedance spectroscopy shows that PA-SJD devices are endowed with higher charge recombination resistance (lower recombination losses) and electron lifetimes compared to conventional nanocrystalline films.