TiO2-based Solar Cells Research Articles

Grape pigment (malvidin-3-fructoside) as natural sensitizer for dye-sensitized solar cells

TiO2 nanocrystalline thin films have various applications, among them dye-sensitized solar cells are more promising and low-cost alternative to conventional inorganic photovoltaic devices. TiO2 nanocrystalline thin films have been sensitized with the natural dye (malvidin-3-fructoside) extracted from grape fruits. The interaction between the semiconductor and the dye has been studied. The maximum absorption band of grape extract at 525 nm has been shifted to 545 nm after incorporation of the dye indicating interaction of the dye molecules with TiO2. The grape extract has polyphenolic anthocyanin pigment (malvidin-3-fructoside), which has carboxylic and hydroxyl groups that can attach effectively to the surface of TiO2 film. The dye-sensitized TiO2-based solar cell sensitized using grape dye exhibited a Jsc of 4.06 mA/cm2, Voc of 0.43 V, FF of 0.33 and efficiency of 0.55 %. Natural dye-sensitized TiO2 photo electrodes present the prospect to be used as an environment-friendly, low-cost alternative system.

Basella alba rubra spinach pigment-sensitized TiO2 thin film-based solar cells

Nanocrystalline TiO2 thin films have been prepared by sol–gel dip coating method. The X-ray diffraction results showed that TiO2 thin films annealed at 400, 450 and 500 °C are of anatase phase and the peak corresponding to the (101) plane is present in all the samples. The grain size of TiO2 thin films was found to increase with increasing annealing temperature. The grain size is found to be 20, 25 and 33 nm for the films annealed at 400, 450 and 500 °C. The structure of the TiO2 nanocrystalline thin films have been examined by high-resolution transmission electron microscope, Raman spectroscopy and FTIR spectroscopy. TiO2 thin films were sensitized by natural dyes extracted from basella alba rubra spinach. It was found that the absorption peak of basella alba rubra extract is at about 665 nm. The dye-sensitized TiO2-based solar cell sensitized using basella alba rubra exhibited a Jsc of 4.35 mA cm−2, Voc of 0.48 V, FF of 0.35 and efficiency of 0.70 %. Natural dyes as sensitizers for dye-sensitized solar cells are promising because of their environmental friendliness, low-cost production and fully biodegradable.

The Importance of Perovskite Pore Filling in Organometal Mixed Halide Sensitized TiO2-Based Solar Cells.

Emerging from the field of dye-sensitized solar cells, organometal halide perovskite-based solar cells have recently attracted considerable attention. In these devices, the perovskite light absorbers can also be used as charge transporting materials, changing the requirements for efficient device architectures. The perovskite deposition can vary from merely sensitizing the TiO2 electron transporting scaffold as an endowment of small nanoparticles, to completely filling the pores where it acts as both light absorber and hole transporting material in one. By decreasing the TiO2 scaffold layer thickness, we change the solar cell architecture from perovskite-sensitized to completely perovskite-filled. We find that the latter case leads to improvements in device performance because higher electron densities can be sustained in the TiO2, improving electron transport rates and photovoltage. Importantly, the primary recombination pathway between the TiO2 and the hole transporting material is blocked by the perovskite itself. This understanding helps to rationalize the high voltages attainable on mesoporous TiO2-based perovskite solar cells.

Rutile TiO2-based perovskite solar cells

A perovskite solar cell based on rutile TiO2 film was prepared and its photovoltaic performance was compared to an anatase TiO2-based perovskite solar cell. Rutile TiO2 nanoparticles with aspect ratio of 0.2 (20 nm wide and 100 nm long) were prepared by hydrolysis of TiCl4 at ambient temperature. Anatase TiO2 nanoparticles with diameter of about 50 nm were hydrothermally synthesized. The annealed rutile film showed porosity of 60.6%, while lower porosity of 49.1% was detected in the anatase TiO2 film. CH3NH3PbI3 perovskite was deposited on TiO2 film using either a one-step spin coating or two-step dipping method. 2,2′,7,7′-Tetrakis(N,N-p-dimethoxy-phenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) was used as a hole transporting material. One-step deposition led to average power conversion efficiency (PCE) of 8.19% from the rutile-perovskite solar cells and 7.23% from the anatase-perovskite solar cells, while two-step deposition resulted in higher average PCE of 13.75% for the former device and 13.99% for the latter one. Regardless of the deposition methodologies, the rutile–perovskite solar cell showed generally higher Jsc and lower Voc. Slower electron transport and longer electron lifetime were observed for the rutile-based perovskite solar cell than for the anatase-based one. Although the same perovskite material was used for both rutile and anatase TiO2, the difference in electronic behavior indicates that photo-excited electrons are in part injected to TiO2 and the extent of electron injection can be influenced by the crystal phase of TiO2. Despite longer electron lifetime, the slightly lower voltage of the rutile-based device might be due to the fact that the amount of injected electrons was relatively larger for rutile than anatase, leading to a lower Fermi energy level at equilibrium between TiO2 and perovskite. Using a 260 nm-thick rutile TiO2 film, the highest PCE of 14.46% was achieved by depositing CH3NH3PbI3 using a two-step method, in which photocurrent density of 20.02 mA cm−2, open-circuit voltage of 1.022 V and fill factor of 0.71 were demonstrated.

Dye-sensitized solar cells based on ZnO nanoflowers and TiO2 nanoparticles composite photoanodes

ZnO nanoflowers were synthesized by low temperature solution-phase method. ZnO nanoflowers/TiO2 nanoparticles composite photoanodes with various mass ratios were prepared on transparent conductive fluorine-doped tin oxide substrates by doctor-blade technique. The dye-sensitized solar cells (DSSCs) were assembled. The morphology characteristics of ZnO nanoflowers and ZnO/TiO2 composite photoanodes have been analyzed by scanning electron microscopy. The effect of the ZnO nanoflowers/TiO2 nanoparticles mass ratio on the performance of DSSCs was systematically investigated by I–V characteristics and electrochemical impedance spectroscopy. Results show that the conversion efficiency of the dye-sensitized solar cell with a ZnO/TiO2 mass ratio of 25:75 was increased by about 35 % compared to that of pure TiO2-based solar cell. Addition of ZnO nanoflowers can enhance the light harvesting and improve electron transport.

Cu-Based Metal–Organic Frameworks for Photovoltaic Application

In the present study, a thin layer of Cu-based metal–organic frameworks (MOFs, copper(II) benzene-1,3,5-tricarboxylate) is fabricated using a layer-by-layer technique, and the layer is investigated as a light-absorbing layer in TiO2-based solar cells. Iodine doping of the MOFs is performed to improve the conductivity and charge-transfer reaction across the TiO2/MOF/electrolyte interface. The HOMO and LUMO energy states of the MOF films are estimated to be −5.37 and −3.82 eV (vs vacuum), respectively, which show a well-matched energy cascade with TiO2. For the first time, a TiO2-based solar cell is fabricated successfully using iodine-doped Cu-MOFs as an active layer, demonstrating a cell performance with Jsc = 1.25 mA cm–2 and Eff = 0.26% under illumination of 1 sun radiation. In contrast, the cell with an undoped MOF layer exhibited Jsc = 0.05 mA cm–2 and Eff = 0.008%. Electrochemical impedance spectroscopy of the cells suggests that iodine doping significantly reduces the charge-transfer resistance.

N-doped carbon quantum dots for TiO2-based photocatalysts and dye-sensitized solar cells

N-doped nanostructured carbon materials have attracted much attention because the doping can induce new unique physical and chemical properties of carbon. In this study, we report, for the first time, that N-doped carbon quantum dots (NCQDs) can combine with rutile TiO2 and form NCQDs/TiO2 hierarchical microspheres assembled by nanorods with enhanced photocatalytic activities under visible light irradiation. As a result, the pseudo-first-order reaction constant for the degradation of rhodamine B (RhB) on NCQDs/TiO2 (0.11min−1) is 215.7, 7.3 and 1.3 times much larger than that of NCQDs (0.00051min−1), TiO2 (0.015min−1), and CQDs/TiO2 (0.086min−1), respectively. The functions of NCQDs were discussed and a new synergistic photosensitized mechanism was proposed for the degradation of RhB on the as-synthesized NCQDs/TiO2 nanocomposites based on the experimental results. NCQDs were also applied as metal-free sensitizers in dye-sensitized solar cells (DSSCs) for the first time. A preliminary study shows that under one sun illumination (AM 1.5), the open circuit voltage and fill factor values reach 0.46V and 43%, respectively, achieving a power conversion efficiency of 0.13% in the proof-of-concept TiO2-based DSSCs device. The performance of NCQDs sensitized solar cells surpasses that of N-free counterpart as well. N-doping lowered the work function of CQDs, which was responsible for enhanced photocatalytic activity of NCQDs/TiO2 and better NCQDs sensitized TiO2-based solar cell performance.

Spectroelectrochemical Photoluminescence of Trap States of Nanocrystalline TiO2 in Aqueous Media

Trap state photoluminescence of nanocrystalline TiO2 electrodes is investigated as a function of applied bias and pH in aqueous electrolyte. Films composed of the anatase polymorph reveal an increase in a broad red emission at increasingly negative potentials, with an onset about 200 mV positive of the pH-dependent literature value of the conduction band potential, followed by conversion to the green emission characteristic of hole traps at more negative bias. Green photoluminescence is the only emission seen from mixed-phase (P25, anatase/rutile) films at any applied potential, while red-emitting electron traps in P25 appear to be quenched by electron transfer to rutile hole traps. The influence of surface treatment by TiCl4 is investigated for both anatase and P25 in order to shed light on the mechanism by which this treatment improves the performance of TiO2-based solar cells. Our results reveal the difference between trap state distributions of P25 and anatase nanoparticles and address the molecular b...

Effect of the acceptor on the performance of dye-sensitized solar cells

Three new phenothiazine dyes were designed and synthesized, utilizing different acceptor groups. Upon application to TiO2-based solar cells, the effects of different acceptors on the photophysical and electrochemical properties of the dyes and the solar cell performance are detailed. The introduction of a pyridinium unit or 5-carboxy-1-hexyl-2,3,3-trimethyl-indolium unit into the molecular frame as the acceptor instead of cyano acrylic acid can effectively cause a red shift in the absorption spectra. Applied to DSSCs, the devices sensitized by CM502 with the pyridinium unit as the acceptor show the highest efficiency of 7.3%. The devices fabricated with dye CM501 with cyano acrylic acid as the acceptor exhibited the highest Voc while for the devices sensitized by the dye CM503 with 5-carboxy-1-hexyl-2,3,3-trimethyl-3H-indolium unit as the acceptor, the Voc value was the lowest, at 494 mV. The addition of TBP in the electrolyte can improve the performance of DSSCs fabricated using CM501 and CM502, with the Voc value greatly improved but the Jsc value slightly decreased. However, with the addition of TBP in the electrolyte, the efficiency of the cells sensitized by CM503 dropped significantly (from 4.9% to 1.0% when 0.1 M TBP was added).

Effects of Metal Oxide Modifications on Photoelectrochemical Properties of Mesoporous TiO2 Nanoparticles Electrodes for Dye-Sensitized Solar Cells

Mesoporous TiO2 (m-TiO2) nanoparticles were used to prepare the porous film electrodes for dye-sensitized solar cells, and a second metal oxide (MgO, ZnO, Al2O3, or NiO) modification was carried out by dipping the m-TiO2 electrode into their respective nitrate solution followed by annealing at 500 °C. Experimental results indicated that the above second metal oxide modifications on m-TiO2 electrode are shown in all cases to act as barrier layer for the interfacial charge transfer processes, but film electron transport and interfacial charge recombination characteristics under applied bias voltage were dependent significantly on the existing states and kinds of these second metal oxides. Those changes based on second metal oxide modifications showed good correlation with the current-voltage analyses of dye-sensitized solar cell, and all modifications were found to increase the open-circuit photo-voltage in various degrees, while the MgO, ZnO, and NiO modifications result in 23%, 13%, and 6% improvement in cell conversion efficiency, respectively. The above observations indicate that controlling the charge transport and recombination is very important to improve the photovoltaic performance of TiO2-based solar cell.

Determination of electron and hole lifetimes of rutile and anatase TiO2 single crystals

The dynamical behavior of photoexcited states of TiO2 governs the activities of TiO2-based solar cells and photocatalysts. We determined the lifetimes of photoexcited electrons and holes in rutile and anatase TiO2 single crystals by combining advantages of time-resolved photoluminescence, photoconductance, and transient absorption spectroscopy. Electrons and holes in rutile show exponential decays with the lifetime of a few tens of nanoseconds, while non-exponential decays are observed in anatase, indicating the presence of multiple carrier trapping processes. We revealed the generic features of the carrier recombination processes in rutile and anatase TiO2.

Fabrication of highly efficient dye-sensitized solar cell and CO2 reduction photocatalyst using TiO2 nanoparticles prepared by spin coating-assisted sol–gel method

A sol–gel method was applied for fabrication of nanocrystalline anatase TiO2 thin films on ITO glass substrates and followed by rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). TiO2 nanoparticles were characterized by X-ray diffraction (XRD) pattern and scanning electron microscopy (SEM) and the absorption of dye on the TiO2 electrode was shown by UV–vis spectroscopy. By controlling different parameters including numbers of coated layers, the gap between two electrodes, sensitization time, and light source power, TiO2-based solar cells with high efficiency was achieved. The results show that a five time spin-coated TiO2 electrode with applying sealant and sensitization time of 24 h in N3 dye under illumination of 100 W cm−2 tungsten lamp give the optimum power conversion efficiency (η) of 6.61%. The increases in thickness of TiO2 films by increasing the numbers of coated layers can improve adsorption of the N3 dye through TiO2 layers to increase the open-circuit voltage (V oc). However, short-circuit photocurrents (J sc) of DSSCs with a one-coated layer of TiO2 films are smaller than those of DSSCs with five-coated layer of TiO2 films. It could be due to the fact that the increased thickness of TiO2 thin films also resulted in a decrease in the transmittance of TiO2 thin films. Also, this electrode was employed to photoreduce CO2 with H2O under tungsten lamp as light source.

Energy transfer versus charge separation in hybrid systems of semiconductor quantum dots and Ru-dyes as potential co-sensitizers of TiO2-based solar cells

Hybrid structures of colloidal quantum dots (QDs) with Ru-dyes have been studied as candidates for panchromatic sensitizers for TiO2-based solar cells. Steady-state and time resolved photoluminescence spectroscopy and photocurrent measurements have been employed to identify the prevailing transfer mechanisms for photogenerated excitons between CdSe QDs capped with a traditional bulky organic ligand trioctylphosphine and Ru-dyes (N3 or Ru505) deposited onto inert glass or mesoporous TiO2 substrates. The type II energy level alignment between the QDs and both N3 and Ru505 offers a possibility for the directional charge separation, with electrons transferred to the QDs and holes to the dye. This scenario is indeed valid for the QD/Ru505 and TiO2/QD/Ru505 hybrid systems, with the negligible spectral overlap between the emission of the QDs and the absorption of the Ru505 dye. For the QD/N3 and TiO2/QD/N3 hybrid systems, the spectral overlap favors the longer range energy transfer from the QDs to N3, independently of the presence of the electron acceptor TiO2.

Understanding the Improved Stability of Hybrid Polymer Solar Cells Fabricated with Copper Electrodes

It is known that atmospheric oxygen is essential for realizing the photovoltaic properties of P3HT-TiO₂-based hybrid polymer solar cells because oxygen vacancies created in TiO₂ can become recombination sites for charge carriers, causing photovoltaic properties like open-circuit voltage (V(oc)) to decline quickly in an inert atmosphere. We demonstrate here that using an annealed Cu layer as hole collecting electrode results in a remarkably stable hybrid solar cell that maintains its photovoltaic parameters during 1 h of continuous testing in an inert atmosphere. An analysis of the data from photovoltaic device performance tests and X-ray photoelectron spectroscopy (XPS) attributes this improvement to the tendency of Cu to form sulfide-like complexes with the S atoms on P3HT, thereby inducing a chemically driven vertical segregation of P3HT toward the hole-collecting metal electrode. Additionally, XPS depth profiling analysis shows that Cu atoms can diffuse up to the TiO₂ layer and assist in filling up oxygen vacancies on the TiO₂ surface, thus eliminating defects that can act as donors of free electrons and degrade photovoltaic performance in an inert atmosphere. We analyze these improvements by examining in situ the effect of Cu on the P3HT and TiO₂ layers and on the organic-inorganic interface formed between them inside a hybrid solar cell.

Enhanced Energy Conversion Efficiency of Mg2+-Modified Mesoporous TiO2 Nanoparticles Electrodes for Dye-Sensitized Solar Cells

Mesoporous TiO2 (m-TiO2) nanoparticles were used to prepare the porous electrodes for dye-sensitized solar cells (DSSCs). Experimental results indicate that the MgO covered on TiO2 electrode can act as barrier layer for the interfacial charge recombination processes, whereas some MgO immingled among TiO2 networks due to the part reconstruction of the electrode during the second annealing process can retard the electron transport within electrode. The optimal conversion efficiency is obtained from solar cell fabricated with TiO2 electrode modified with 0.2 M Mg2+ solution, with a 39% improvement in the efficiency as compared to the unmodified one. The suppression of interfacial charge recombination and the enhanced dye adsorption capacity contribute to the improved photovoltaic performance for the Mg2+-modified electrode, and the retardation of electron transport and injection by excessive Mg2+ modification is responsible for the efficiency losses of the corresponding TiO2-based solar cell. This indicates ...

A Triphenylamine-Grafted Imidazo[4,5-f][1,10]phenanthroline Ruthenium(II) Complex: Acid−Base and Photoelectric Properties

A new heteroleptic ruthenium(II) complex of [Ru(Hipdpa)(Hdcbpy)(NCS)(2)](-).0.5H(+).0.5[N(C(4)H(9))(4)](+) Ru(Hipdpa) {where Hdcbpy = monodeprotonated 4,4'-dicarboxy-2,2'-bipyridine and Hipdpa = 4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-N,N-diphenylaniline} was synthesized and characterized by elementary analysis, standard spectroscopy techniques, and cyclic voltammetry. The ground- and excited-state acid-base properties of Ru(Hipdpa) were studied by means of UV-vis absorption spectrophotometric and spectrofluorimetric titrations in 4:1(v/v) Britton-Robinson/dimethylformamide buffer solution. The four-step separate protonation/deprotonation processes were found in the ground states, and one of which taking place near the physiological pH range. The two observable excited-state protonation/deprotonation processes were found for the Ru(Hipdpa), constituting pH-induced "off-on-off" emission switches. The performance of the complexes as photosensitizers in nanocrystalline TiO(2)-based liquid solar cells containing an electrolyte solution (0.05 M I(2), 0.5 M LiI, and 0.5 M 4-tert-butylpyridine in 50% acetonitrile and 50% propylene carbonate) was investigated and found to achieve a much improved device performance (a short-circuit photocurrent density of 18.7 mA cm(-2), an open-circuit voltage of 630 mV, and an overall conversion efficiency of 6.85%) compared to a triphenylamine-free parent complex [Ru(Hpip)(Hdcbpy)(NCS)(2)](-).[N(C(4)H(9))(4)](+)-based device {Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline} and a comparable performance to that of cis-bis(isothiocyanato)bis(2,2'-bipyridine-4,4'-dicarboxylic acid)ruthenium(II) (N3) under identical experimental conditions. A density functional theory calculation of the molecular structures and electronic properties of the complexes was also carried out in an effort to understand their effectiveness in TiO(2)-based solar cells.

Numerical Simulation of the Current−Voltage Curve in Dye-Sensitized Solar Cells

A theoretical model based on the numerical integration of the continuity equation for electrons with trap-limited density-dependent diffusion and recombination constants is implemented to describe the functioning of dye-sensitized solar cells (DSSC). The application of the model combines recent theory on charge transport in nanocrystalline materials with parameters extracted from five simple measurements: the UV/vis spectrum of the dye in solution, the steady-state current−voltage curve, the open circuit photovoltage versus light intensity curve, photocurrent transient upon switching on an illumination source, and open-circuit voltage decay upon switching off the illumination source. As a novel feature not previously included in this kind of calculations, the model includes an additional term that accounts for the charge transfer from the transparent conducting oxide (TCO) substrate to the electrolyte solution. The general applicability of the model is illustrated by applying it to two different types of cell: a TiO2-based solar cell with an organic solvent electrolyte and a ZnO-based solar cell with a solvent-free electrolyte. It is found that the numerical model is capable of adequately fitting all data for both systems, resulting in quantitative estimates for the main parameters controlling solar cell functioning and efficiency. The results show that it is possible to provide a global description of DSSCs based on fundamental theories for trap-limited transport and recombination using simple experimental techniques available to every solar cell laboratory. The present paper tries to help fill the gap between pure theoreticians and experimentalists working on this kind of system.

High Molar Extinction Coefficient Ruthenium Sensitizers for Thin Film Dye-Sensitized Solar Cells

We present two sensitizers, Ru(4,4′-dicarboxyvinyl)-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2 (K9) and Ru(4,4′-dicarboxy(phenylethenyl)-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2 (K23), which were synthesized and characterized by analytical, spectroscopic, and electrochemical impedance measurements. The two sensitizers that contain hydrophobic alkyl chains and an extended π-conjugation bridge between carboxylic acid groups and the ruthenium chromophore center exhibit enhanced red response and higher molar extinction coefficients when compared to the Ru(4,4′-dicarboxy-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2 (Z907) sensitizer. The performances of the K9, K23, and Z907 sensitizers in nanocrystalline TiO2-based thin film solar cells were studied using transparent 2.6-μm-thick mesoporous layers, which show short-circuit photocurrent densities (Jsc) of 7.80, 8.31, and 7.16 mA/cm2, corresponding to overall conversion efficiencies of 4.14, 4.41, and 4.06%, respectively. The increase in the Jsc of the cells with K9 and K23 with respect to that with Z907 is due to the enhanced molar extinction coefficient of the new sensitizers. The incident monochromatic photon-to-current conversion efficiency for the K23 is plotted as a function of excitation wavelength show in the plateau region 79% and even at 700 nm 69%.

Efficient Panchromatic Sensitization of Nanocrystalline TiO2-based Solar Cells Using 2-Pyridinecarboxylate-substituted Ruthenium(II) Complexes

Abstract Ruthenium(II) complexes containing a 2-pyridinecarboxylate ligand (1 and 2) were synthesized as sensitizers for dye-sensitized solar cells (DSCs). The solar cells containing 2 exhibited efficient panchromatic sensitization over the entire visible wavelength range, extending into the near-IR region. An overall solar light-to-electrical conversion efficiency of 9.66% was achieved under standard air mass (AM) 1.5 irradiation (100 mW cm−2). This efficiency was superior to that of black dye (9.58%) under the same cell fabrication and measuring procedure.

A novel ruthenium sensitizer with a hydrophobic 2-thiophen-2-yl-vinyl-conjugated bipyridyl ligand for effective dye sensitized TiO2 solar cells

A hydrophobic and 2-thiophen-2-yl-vinyl-conjugated ruthenium complex, cis-Ru(dhtbpy)(dcbpy)(NCS)2 [dhtbpy = 4,4'-di(hexylthienylvinyl)-2,2'-bipyridyl; dcbpy = 4,4'-dicarboxy-2,2'-bipyridyl], was newly designed, synthesized and applied successfully to sensitization of nanocrystalline TiO2-based solar cells, giving a conversion efficiency of 9.5% under irradiation with AM 1.5 solar light.