Recent improvements and arising challenges in dye-sensitized solar cells

Work demand for energy is expected high and finding sufficient route to produce clean energy is an ever more pressing problem. Science had identified the key research challenges if solar energy is to provide a significant fraction of our energy needs. The huge gap between our present use of solar energy and its enormous undeveloped potential defines a grand challenge in energy research. One of the most attractive methods currently being developed is “dye sensitization” in solar cells in order to increase the efficiency of conversion of solar radiation into electricity. Although large improvements in present technology will be required, the review points to progress in nanoscience, in particular as a reason to be optimistic.     

Amine functionalized homoleptic ruthenium(II) sensitizer for dye-sensitized solar cells: A combined effect of ancillary ligands and co-sensitization

In this work, we propose a homoleptic ruthenium(II) complex, [Ru(dabpy)₃]Cl₂ (RDAB3) containing amine functionalized electron donating anchoring units for DSSCs. The DSSCs were co-sensitized by coumarin-based thiophene (CT) and indole (CI) units. The presence of four ancillary amine groups in RDAB3 influences its photovoltaic performance and the introduction of coumarin-based co-sensitizers significantly enhances the efficiency. The fabricated DSSCs were explored by UV-Visible absorption, photocurrent-voltage assessments, and impedance spectral studies. Co-sensitized DSSCs showed an improved photovoltaic efficiency than the device sensitized by RDAB3 alone. Under optimized condition, the device made up of RDAB3+CI exhibited a high Jsc = 9.9 mA/cm² with Voc of 0.7 V, fill factor of 0.773, and solar to power conversion efficiency of 5.35% in standard global AM 1.5 solar irradiation. This performance is found to be higher than the DSSC sensitized with RDAB3 (η = 3.84%) fabricated under same circumstances.     

Near-IR dye-sensitized solar cells using a new type of ruthenium complexes having 2,6-bis(quinolin-2-yl)pyridine derivatives

New ruthenium(II)-polypyridyl complexes 1a (X=H) and 1b (X=Cl) having 2,6-bis(4-carboxyquinolin-2-yl)pyridine derivatives were synthesized as a sensitizer for dye-sensitized solar cells (DSCs), and their photophysical and photochemical properties were characterized. Both of the complexes showed broad electronic absorption bands in the near-IR region, which were assigned to the metal-to-ligand charge transfer (MLCT) transitions. On the other hand, the photovoltaic performance of the DSCs sensitized with them were different from each other. The DSC sensitized with 1a exhibited higher IPCE value than that of the one sensitized with 1b. The substituent effects on the ligand on photovoltaic performance of the DSCs were examined.     

On the structural variations of Ru(II) complexes for dye-sensitized solar cells

Ruthenium(II) complexes with new phenanthrenyl ligand (TAPNB) have been synthesized and examined. The spectroscopic and electrochemical measurements showed that the excited states of those complexes matched the conduction band of titanium dioxide. The overall power conversion efficiencies of the solar cells utilized these new complexes as sensitizers for TiO₂ films were less than that of N3-sensitized cell. Although the open-circuit voltage was similar to that of N3-sensitized cell, the short-circuit current was one order lower. Such outcome may be attributed to the less amount of dyes adsorbed due to the steric congestion of the complex. When NCS ligand was replaced by pyridyl ligand, the energy of metal-to-ligand charge transfer (Ru(II)→TAPNB) increased and resulted in blue shift of the absorption band. Anchoring of carboxylic acid at the surface of TiO₂ slightly lowered the energy of Ru(II)→TAPNB charge transfer band. As carboxylic acid anchor was replaced by acetyl ester, the weaker interaction between the semiconductor and the ligand led to diminishing amount of the complex adsorbed and less photocurrent was detected.     

Recent advances of high-efficiency single crystalline silicon solar cells in processing technologies and substrate materials

Single crystalline silicon solar cells have demonstrated high-energy conversion efficiencies up to 24.7% in a laboratory environment. One of the recent trends in high-efficiency silicon solar cells is to fabricate these cells on different silicon substrates. Some silicon wafer suppliers are also involved in such development. Another recent trend is the increased production of high-efficiency silicon cells, some of them with low-cost structures. This paper will discuss the progress at the University of New South Wales, and these trends in other organisations.     

Fabrication of n–p junction electrodes made of n-type SnO2 and p-type NiO for control of charge recombination in dye sensitized solar cells

The n–p junction electrode fabricated coating nanocrystalline SnO₂ thin film with a thin layer of p-type NiO was found to increase the sensitized photocurrent and photovoltage. In addition, increase in the fill factor was noticed due to inhibition of electron back transfer from SnO₂ to the redox electrolyte (I₃⁻) by both junction effect and presence of NiO barrier, resulting in much better energy conversion efficiencies. The highest cell efficiency was obtained for the cell fabricated by immersing SnO₂ thin films in soluble Ni salts, and converting them to NiO by firing. The optimum NiO coating thickness was found to be only a few angstroms and the energy levels of the excited dye and the conduction band position of NiO suggest that the electron transfer from the excited dye to the underlying SnO₂ layer occurs by tunneling through the p-type NiO layer.     

Photoelectrochemically polymerized polythiophene layers on ruthenium photosensitizers in dye-sensitized solar cells and their beneficial effects

Beneficial effects of polythiophen layers on ruthenium (Ru) photosensitizer in dye-sensitized solar cells (DSSCs) are observed. The thiophene is photoelectrochemically polymerized by the photoexcitation of Ru photosensitizers on TiO₂. In situ polymerization enables the formation of uniform polythiophene layers on Ru-complexes/TiO₂. The polythiophene layers have hydrophobic nature and bridge the energy level between Ru-complexes and iodine/iodide, which suppress the recombination reaction and improve the concentration of photoinjected electron in TiO₂ layers. The DSSCs with polythiophene protecting layers shows a power conversion efficiency of 6.01%, which is a 30% increase compared to one without polythiophene layers.     

Numerical and experimental analysis of convection heat transfer in passive solar heating room with greenhouse and heat storage

In this paper, heat transfer and air flow in passive solar heating room with greenhouse and heat storage are studied. Thermal insulation of solar heating room has significant effects on temperature distribution and airflow in the heating chamber of this solar system. Heat transfer and air flow in a rock bed, which is used as solar absorber and storage layer, are also studied. If porosity is kept within certain range, increasing the rock size causes an increase of the capability of thermal storage and heating effects; increasing the porosity of thermal storage materials results in an increase of the bed temperature but a decrease of the rock mass. The specific heat capacity and thermal conductivity have a remarkable effect on the average temperature of rock bed. All these factors should be taken into account when designing a solar heating system.     

Life cycle environmental impact of a thermosyphonic domestic solar hot water system in comparison with electrical and gas water heating

A methodology for the environmental impact evaluation over the life span of a Domestic Solar Hot Water System (DSHWS) is presented. The results are compared to the environmental consequences of the conventional energy form substituted and the total environmental gain is calculated. For the purposes of this analysis, the “Eco-indicator ’99” Life Cycle Impact Assessment methodology was adopted and the materials and procedures of the DSHWS production and utilization are evaluated.     

Solvent-free synthesis of alkylbenzimidazolium iodides and their applications in dye-sensitized solar cells

In this paper, the synthesis of 1-hexyl-3-methylbenzimidazolium iodide (HMBI) and 1-hexyl-3-propylbenzimidazolium iodide (HPBI) was developed by quaternization reaction of 1-hexylbenzimidazole and alkyl iodide under solvent-free condition using Teflon-lined, stainless autoclaves. Their thermal properties were measured on the thermo gravimetric analysis and differential scanning calorimeter. The influence of HMBI, HPBI and 1-methyl-3-propylimidazolium iodide (MPII) on redox behavior of and I⁻ was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. It was found that the resulting HMBI and HPBI had high purity and the reaction time was shortened to 3 h. The thermal stability of HMBI and HPBI was better than that of alkylimidazolium iodides, and HMBI and HPBI were prone to exhibit the supercooling phenomena. The DSCs with HMBI, HPBI and MPII gave photoelectric conversion efficiency of 5.49%, 5.34% and 5.54%, respectively.     

Measurement and comparison of AC parameters of silicon (BSR and BSFR) and gallium arsenide (GaAs/Ge) solar cells used in space applications

The AC parameters of silicon (BSR and BSFR) solar cells and GaAs/Ge solar cell have been measured using impedance spectroscopy. Each cell capacitance, dynamic resistance and series resistance were measured and compared. GaAs/Ge solar cell has shown only the transition capacitance throughout its operating range while silicon (BSR and BSFR) solar cells exhibited both transition and diffusion capacitance. The theoretical and experimental values of dynamic resistance were compared and found in good agreement while the diode factor in silicon solar cells varies from 2 to 1, where as in GaAs/Ge solar cell it varies from 4 to 2 to 1.     

Synthesis and characterization of ZnO nanowires grown on different seed layers: The application for dye-sensitized solar cells

Zinc oxide (ZnO) nanowire electrodes which were grown on different seed layers and examination of their significant effects on the performance of dye sensitized solar cells were studied. Through chemical bath deposition process, the ZnO nanowires were grown on an indium tin oxide (ITO) coated glass using sputter-deposited aluminum doped zinc oxide (AZO) and ZnO seed layers. Afterward, main parameters such as solution concentration, growth temperature, and time were systematically investigated based on morphology of nanowires. The X-ray diffraction (XRD), field emission scanning microscopy (FESEM), and photoluminescence (PL) were applied to investigate the characteristics of the samples. The results showed ZnO nanowires, which were grown by AZO seed layer, had a high density array with hexagonal wurtzite structure distributed vertically and uniformly on ITO coated glass. The mentioned zinc-oxide nanowires grown under an optimum condition on different seed layer were used to fabricate dye solar cells afterward. The seed layer was effective on morphologic, optical, and structural features. The overall light-conversion efficiency of dye sensitized solar cell with ZnO nanowires grown on AZO seed layer was almost 2 times higher than that of those grown on ZnO seed layer. Electrochemical impedance spectroscopy analysis was measured under standard light to investigate the electron transport properties in the both ZnO-NW DSSCs. As the results showed, photoanode electron recombination rate with electrolyte was 6.02 Hz for dye solar cells of zinc oxide (ZnO-NWDSSC) produced by ZnO seed layer, which is 2.5 times faster than cells with AZO seed layer.     

The synthesis and characterization of carbon dots and their application in dye sensitized solar cell

Carbon dots (CDOTs) are increasingly becoming popular in the areas ranging from sensing and bioimaging to electronics. The interesting optical properties of CDOTs make it vital to explore its potential in the development of sustainable energy. In this work, one-step hydrothermally synthesized CDOTs were used as sensitizing agent in the fabrication of dye sensitized solar cell. The fabrication of the CDOT-based dye sensitized solar cell and its performance characteristics are explored in depth. The fabricated dye sensitized solar cell performance in terms of efficiency, voltage, and current was evaluated using a standard illumination of air-mass 1.5 global (AM 1.5 G) having an irradiance of 100 mW/cm [2]. The photon-to-current conversion efficiency (η) of only the carbon dot sensitized solar cell was 0.10% whereas the efficiency of the solar cell fabricated with a sensitizing dye made up of CDOT and N719 was 0.19%. As compared with the performance DSSCs fabricated with only 719 dye, it was observed that when CDOT was used in combination with N719 as sensitizing dye, the open circuit voltage increases yet the overall efficiency of the resulting solar cells decreases. It is clear from the result that CDOT could be used as a sensitizing dye in DSSCs. However, it is not very useful when used in combination with other sensitizing dyes due to energy transfer.     

Comparison of charge accumulation and transport in nanostructured dye-sensitized solar cells with electrolyte or CuSCN as hole conductor

The charge transport properties of the dye-sensitized solar cells consisting of Ru(dcbpyH₂)₂(NCS)₂-sensitized nanostructured TiO₂ with either redox electrolyte or CuSCN as hole conductor have been compared. The electron transport time and the electron charge in the TiO₂ varies in a similar way with the incident light intensity for both hole conductors: electron transport becomes faster and electron accumulation increases with increasing light intensity. Electron transport in the CuSCN-based cells is significantly faster than in electrolyte cells under conditions where the accumulated charge is equal. An ultra-thin aluminum oxide layer on the nanocrystalline titanium oxide has a beneficial effect as it reduces the recombination and increases the open-circuit potential.     

Synthesis and characterization of trivalent metal porphyrin with NCS ligand for application in dye-sensitized solar cells

Two novel trivalent metal porphyrin dyes, P_Mn-HT-SCN and P_Ga-HT-SCN, were designed, synthesized and firstly applied in dye-sensitized solar cells (DSSCs). These two dyes possess porphyrin donor modified with manganese (III) and gallium (III) as coordination metal and NCS as the second ligand, cyanoacrylic acid as electron-accepting moiety and 4-hexylthiophene as π-spacers. Each of the porphyrin showed different adsorption behavior and saturated coverage on the TiO₂ surface. Between the two dyes, the P_Mn-HT-SCN-based DSSCs afforded the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 4.32 mA/cm², an open-circuit photovoltage (V_oc) of 0.61 V and a fill factor (FF) of 0.58, corresponding to a solar-to-electricity conversion efficiency of 1.53% under 100 mW/cm² irradiation.     

Synthesis, characterization and application of sol–gel derived mesoporous TiO2 nanoparticles for dye-sensitized solar cells

Nanocrystalline mesoporous titania of anatase crystal phase were prepared by sol–gel route by varying calcination (400 °C and 600 °C) conditions, and the photo-electrochemical properties were investigated for dye-sensitized solar cell applications. The TTIP precursor in n-heptane solvent with ratio of water to TTIP (5:1) was found to be effective substrate for the working electrodes. The overall conversion efficiency of 7.59% was achieved under 1 sun irradiation with open circuit voltage of 0.77 V, current density of 17.00 mA/cm² and FF of 51.12. The high efficiency of the 400 °C calcined sample were attributed to its mesopores, high BET surface area (80.1 m²/g) and large pore volume of prepared titania substrate which provide better surface for the absorption of dye, improves light harvesting efficiency and better charge injection. The prepared samples were characterized by XRD, small angle XRD, FE-SEM, TEM, IPCE, I–V curve, BET surface area and BJH plot techniques.     

Synthesis of TiO2 submicro-rings and their application in dye-sensitized solar cell

In this paper, novel TiO₂ submicro-rings were synthesized via potentiostatic anodization of titanium powder coated on transparent conducting oxide glass. The TiO₂ submicro-rings film was characterized by SEM, XPS and 3D optical profiling. Accordingly, a possible growth mechanism of submicro-rings was discussed. The TiO₂ submicro-rings based dye-sensitized solar cell (DSSC) with the film thickness of ca. 3.1 μm was assembled and a conversion efficiency of 1.36% was achieved under AM 1.5 illumination. The photoelectron transport properties of TiO₂ submicro-rings based DSSC were also discussed according to the electron impedance spectroscopy.     

A polymer gel electrolyte composed of a poly(ethylene oxide) copolymer and the influence of its composition on the dynamics and performance of dye-sensitized solar cells

A polymer gel electrolyte composed of a poly(ethylene oxide) derivative, poly(ethylene oxide-co-2-(2-methoxyethoxy) ethyl glycidyl ether), mixed with gamma-butyrolactone (GBL), LiI and I₂ is employed in dye sensitized solar cells (DSSC). The electrolyte is characterized by conductivity experiments, Raman spectroscopy and thermal analysis. The influence of the electrolyte composition on the kinetics of DSSC is also investigated by transient absorption spectroscopy (TAS). The electrolyte containing 70 wt.% of GBL and 20 wt.% of LiI presents the highest conductivity (1.9 × 10⁻³ S cm⁻¹). An efficiency of 4.4% is achieved using this composition. The increase in I_SC as a function of GBL can be attributed an increase in the mobility of the iodide (polyiodide) species. The increase in the yield of the intermediate species, I₂⁻, originating in the regeneration reaction, is confirmed by TAS. However, the charge recombination process is faster at this composition and a decrease in the V_oc is observed. Photovoltage decay experiments confirm an acceleration in charge recombination for the DSSC assembled with the electrolyte containing more GBL. Raman investigations show that in this electrolyte the I₅⁻/I₃⁻ ratio is higher. Theoretical calculations also indicate that the I₅⁻ species is a better electron acceptor.     

Synthesis of new low band gap dyes with BF2-azopyrrole complex and their use for dye-sensitized solar cells

The diazonium salt derived from 4-aminobenzoic acid, 4-aminophenol or 2-aminophenol reacted with half equivalent of pyrrole to afford symmetrical 2,5-bisazopyrroles. They reacted subsequently with boron trifluoride in the presence of triethylamine to afford the corresponding BF₂-azopyrrole complexes D1, D2 and D3 respectively. They were soluble and stable in nonprotic solvents such as chloroform, dichloromethane and tetrahydrofuran but unstable in protic solvents such as ethanol. Their absorption spectra were broad with optical band gap of 1.49-1.70 eV. Among these dyes D2 displayed the broader absorption spectrum with low band gap of 1.49 eV. We have utilized these complexes as photosensitizers for quasi solid state dye-sensitized solar cells (DSSCs) and achieved power conversion efficiency in the range of 4.0-6.0%. We have also found that the co-adsorption of citric acid hindered the formation of dye aggregates and might improve the electron injection efficiency leading to an enhancement in short circuit photocurrent. This work suggests that metal-free dyes based on BF₂-azopyrrole complex are promising candidates for improvement of the DSSC performance.     

Efficiency enhancement of dye‐sensitized TiO2 solar cell based on ruthenium(II) terpyridyl complex photosensitizer

We theoretically investigated the influence of the ligands on the enhancement of the efficiency toward absorption in the solar spectrum of ruthenium(II) terpyridyl complex. In the present work, the NCS ligands of the parent black dye 4,4′,4″‐tricarboxy‐2,2′:6′,2″‐terpyridine)ruthenium(II) complex (BD0) were kept unchanged, while π‐conjugating spacers were introduced between the terpyridine ligands and the anchoring groups ―COOH. Molecular structures, electronic, and spectroscopic properties of four designed black dyes, in addition to the parent black dye (BD0), were examined. Compared with the parent black dye BD0, broad and intense absorption bands in the infrared and near‐infrared region (red‐shifted) were found, featuring the enhancement of the absorption efficiency resulting from the insertion of the proposed π‐conjugating spacers. Copyright © 2015 John Wiley & Sons, Ltd.