A contribution to the optical design of dye-sensitized nanocrystalline solar cells

The absorption of a large fraction of the incident solar radiation by the photoactive layer of dye-sensitized, liquid-junction photovoltaic cells is an important factor for achieving useful photoelectric conversion efficiencies. A model is presented that estimates the enhancement of optical absorption that can be obtained from light scattering in the porous nanocrystalline films used in these cells and from reflection at the back electrode. The model is applied to the optical characterization of two films, a transparent, and a strongly scattering porous titania sample.     

2-Ethyl-1-hexanol based screen-printed titania thin films for dye-sensitized solar cells

Nanocrystalline titania thin films were prepared by screen printing in order to efficiently control and optimize the main step of the dye-sensitized solar cells (DSSCs) fabrication process. Different compositions of nanocrystalline titanium dioxide screen-printing pastes are described, based on 2-ethyl-1-hexanol solvent and commercial Degussa P25 TiO₂. The rheological properties of the prepared pastes are presented as the crucial parameter of the deposition procedure. The produced titania thin films are extensively characterized by means of spectroscopy (Raman, XRD) and microscopy (SEM, AFM). The performance (induced photon-to-current efficiency—IPCE% and overall energy conversion efficiency—η%) of the corresponding DSSCs is also reported.     

Synthesis of nanocrystalline ZnO powder via sol-gel route for dye-sensitized solar cells

This paper reports the growth mechanism of sol-gel-derived nanocrystalline ZnO powder. The influence of pH value of the sol on the crystallite size, morphology and structure of ZnO powder was investigated by using X-ray diffraction, transmission and scanning electron microscopy. Maximum size nanocrystallite (∼14 nm) of ZnO powder was obtained for pH value of 9. An increase in the band gap (blue shift) was observed with decrease in the size of the ZnO nanocrystallites. The variation in band gap was found to be in agreement with theoretical calculations using effective mass model. The growth mechanism of ZnO particles from zinc acetate dihydrate precursor by the sol-gel process has been discussed in terms of solvation, hydrolysis and polymerization. The synthesized ZnO powders were successfully used as the electrode material for dye-sensitized solar cells.     

Performance of a new polymer electrolyte incorporated with diphenylamine in nanocrystalline dye-sensitized solar cell

A new solvent-free composite polymer electrolyte consisting of poly(ethylene oxide) (PEO) incorporated into diphenyl amine (DPA) along with KI and I₂ has been developed. The current-voltage characteristics of this nanocrystalline dye-sensitized solar cell measured under simulated sunlight with 1.5 AM at 60 mW/cm² have indicated that this cell generates a photocurrent of 10.2 mA/cm², together with a photovoltage of 810 mV and fill factor of 0.47 yielding an overall energy conversion efficiency of 6.5%. This result suggests that the electron donicity of DPA influences the interaction of nanocrystalline TiO₂ electrode and I⁻/I₃⁻ electrolyte, leading to a high performance of the fabricated solar cell.     

Regenerative effects by temperature variations in dye-sensitized solar cells

The effect of repeated temperature variations on the performance of both fresh and aged dye-sensitized solar cells with liquid and semi-solid electrolytes has been studied. The cell performance was characterized with IV-curves obtained at different cell operating temperatures and electrochemical impedance spectroscopy measurements before and after the temperature treatments. Consecutive temperature rampings of the aged cells did regenerate the cell function, so that the total efficiency drop over the observation period was on average 18%/unit less for the temperature-treated cells than for reference cells aged at constant temperature.     

Porous TiO2 thin films synthesized by a spray pyrolysis deposition (SPD) technique and their application to dye-sensitized solar cells

Titanium dioxide (TiO₂) thin films were synthesized on glass substrates from titanium(IV)oxy acetylacetonate 2-butanol solution by a spray pyrolysis deposition (SPD) technique. The films consisted of TiO₂ leaflets and showed the oriented growth along the (2 0 0) direction. The surface area of the film was successfully increased by adding a small amount of aluminum(III) acetylacetonate (AA) in the source solution. This is because AA sublimates easily during the film formation to leave many pores within the film. A dye-sensitized solar cell was constructed with the TiO₂ film which was deposited on the fluorine-doped tin(IV) oxide layer by the SPD technique. The conversion efficiency of the cell was effectively enhanced as high as 3.2% at AA content of 0.6 at% in the source solution, attributing to the fact that the amount of a dye anchored on the surface of TiO₂ layer was the highest at this AA content. Although the conversion efficiency is relatively low, this finding leads to the possibility of an industrial production of a dye-sensitized solar cell in the near future.     

Pyridinium molten salts as co-adsorbents in dye-sensitized solar cells

The influence of using pyridinium molten salts as co-adsorbents to modify the monolayer of a TiO₂ semiconductor on the performance of a dye-sensitized solar cell is studied. The current-voltage characteristics are measured under AM 1.5 (100 mW cm⁻²). The pyridinium molten salts significantly enhance the open-circuit photovoltage (V_oc), the short circuit photocurrent density (J_sc) as well as the solar energy conversion efficiency (η). 1-Ethyl-3-carboxypyridinium iodide ([ECP][I]) is applied successfully to prepare an insulating molecular layer with N719, and achieve high energy conversion efficiency as high as 4.49% at 100 mW cm⁻² and AM 1.5. The resulting efficiency is 20% higher than that of a non-additive device. This enhancement of conversion efficiency is attributed to the negative shift of the conduction band (CB) edge and the abundant concentration of I⁻ on the surface of the electrode when using [ECP][I] as the co-adsorbent.     

On the modeling of the dye-sensitized solar cell

A simplified electric model of the dye-sensitized electrochemical solar cell (DSC) is presented. It permits the calculation of internal steady-state cell characteristics like particle density distributions or the electric field as a function of the (measured) external current I_ext. The cell is modeled as an one-dimensional pseudo-homogeneous medium of thickness L, where all the electroactive particles involved in the current supporting process move according to different effective transport coefficients (i.e. effective diffusivities D and effective mobilities μ). The electroactive particles are the electrons e⁻ injected into the nanoporous TiO₂ layer after light absorption by the dye, the reduced and the oxidized counterpart of the redox electrolyte El_Red and El_Ox, and the positively charged cation Kat⁺ being brought into the cell together with the electrolyte. By applying the continuity equation, the transport-equation and Poisson's equation to all the electroactive species involved (e⁻, El_Red, El_Ox and Kat⁺) and by assuming a linear Boltzmann relaxation approximation for the back reaction, a system of differential equations is derived, describing particle densities, particle currents and the electric field within the cell. The underlying simplifying assumptions as well as the resulting limits of the model are stated, and some possible extensions are given. This paper aims to outline the general ideas and limitations of the proposed electric modeling, numerical calculations have been successfully implemented, but will be presented in a future paper.     

High-performance carbon counter electrode for dye-sensitized solar cells

Here, we reported that a new carbon electrode prepared with an activated carbon was superior to a Pt sputtered electrode as the counter electrode of dye-sensitized solar cells. The photovoltaic performance was largely influenced by the roughness factor of carbon electrode. The open-circuit voltage increased by about 60 mV using the carbon counter electrode compared to the Pt counter electrode because of positive shift of the formal potential for I₃⁻/I⁻ couple.     

Molten and solid metal-iodide-doped trialkylsulphonium iodides and polyiodides as electrolytes in dye-sensitized nanocrystalline solar cells

The conductivity and solar cell performance of metal-iodide-doped trialkylsulphonium iodides and polyiodides have been investigated as electrolytes in dye-sensitized nanocrystalline solar cells (DNSCs). Nine different metal-iodide-containing (R₂R′S)I with additional iodine provided overall solar-to-electric energy conversion efficiencies of over 2%, while used as electrolytes in DNSCs in simulated AM 1.5 solar light at the light intensity of 100 W m⁻². The highest overall conversion efficiency, 3.1%, was achieved by using the electrolyte (Bu₂MeS)I:AgI:I₂ in the proportions (1:0.03:0.05). The effects from 4-tert-butylpyridine treatment of the electrodes were studied. The effects of metal-iodide doping were also investigated with respect to speciation in the electrolytes and potential influence on electrochemical conductivity.     

High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells

Polypyrrole (PPy) nanoparticle was synthesized and coated on a conducting FTO glass to construct PPy counter electrode used in dye-sensitized solar cell (DSSC). Scanning electron microscope images show that PPy with porous and particle diameter in 40–60 nm is covered on the FTO glass uniformly and tightly. Cyclic voltammograms of I₂/I⁻ system measurement reveals that the PPy electrode has smaller charge-transfer resistance and higher electrocatalytic activity for the I₂/I⁻ redox reaction than that Pt electrode does. Overall energy conversion efficiency of the DSSC with the PPy counter electrode reaches 7.66%, which is higher (11%) than that of the DSSC with Pt counter electrode. The excellent photoelectric properties, simple preparation procedure and inexpensive cost allow the PPy electrode to be a credible alternative used in DSSCs.     

Effects of silver particles on the photovoltaic properties of dye-sensitized TiO2 thin films

To evaluate the possibility of using the plasmon resonance effect to enhance the efficiency of photochemical cells, cis-(SCN)₂Bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) dye-sensitized cells were used to measure the photoresponse of TiO₂ film electrodes before and after deposition of Ag particles. The deposited Ag particles created a film with Ag islands. We found that the photoresponse in the visible region increased as the mass-equivalent Ag-island film thickness, t_Ag, increased to 3.3 nm, but decreased when t_Ag was further increased to 6 nm. On the other hand, compared with bare TiO₂ films, the photoresponse in the UV region decreased for any level of Ag islands. These results suggest that under proper conditions, enhancement of the optical absorption of the dye by the Ag plasmon resonance effect contributes to the photocurrent, and indicates the possibility of improving the energy conversion efficiency of photoelectrochemical cells with Ag-island films.     

Investigations on ZnxCd1−xO thin films obtained by spray pyrolysis

Zn_xCd_1−xO thin films were prepared on glass substrates by spray pyrolysis technique. The precursor solutions were obtained by varying the concentration of Zn(NO₃)₂·6H₂O and Cd(NO₃)₂·4H₂O in bi-distilled water. The structural properties have been studied using X-ray diffraction spectra. All the structures include the basic compounds, i.e. ZnO and CdO. The orientation and the crystalline phases of the deposited films were specified. With the addition of Zn to the precursor solution, we can observe the preferential orientation of the CdO in the [2 0 0] direction. The electrical measurements were performed using method of four contacts. Thin films transmittances, in the 1.5–4.3 eV range, for different compositions have been measured and the optical gaps have been determined. The variations are explained considering the gaps of the two pure films. The influence of increased Cd concentration in the films on the structural, electrical and optical properties is investigated in this study.     

A phenylcarbazole functionalized ruthenium dye for efficient dye-sensitized solar cells

A new heteroleptic Ru(II) complex of [Ru(Hcpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hcpip = 2-(4-(9H-carbazol-9-yl)phenyl)-1H-imidazo[4,5-f] [1,10]phenanthroline, Hdcbpy = 4-carboxylic acid-4′-carboxylate-2,2′-bipyridine} has been synthesized and demonstrated to function as an efficient sensitizer for nanocrystalline TiO₂-based dye-sensitized solar cell (DSSC). The DSSC based on this Ru(II) complex showed a short-circuit photocurrent density of 19.2 mA cm⁻², an open-circuit photovoltage of 630 mV, a fill factor of 57.7%, corresponding to an overall light to electricity conversion efficiency of 6.98% under simulated solar light irradiation at 100 mW cm⁻². This efficiency value is 2.81- and 1.08-fold efficiency values of 2.48% and 6.47% observed for carbazole-free parent complex [Ru(Hpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline}- and cis-bis(isothiocyanato)bis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) N3-based solar cells respectively, under identical experimental conditions. The molecular structures and electronic properties of the Ru(II) complexes were also investigated by means of density functional theory calculations in an effort to understand the device performance observed.     

Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature

Quasi-solid-state dye-sensitized solar cells with enhanced performance were made by using nanocrystalline TiO₂ films without any template deposited on plastic or glass substrates at low temperature. A simple and benign procedure was developed to synthesize the low-temperature TiO₂ nanostructured films. According to this method, a small quantity of titanium isopropoxide (TTIP) was added in an ethanolic dispersion of TiO₂ powder consisting of nanoparticles at room temperature, which after alkoxide's hydrolysis helps to the connection between TiO₂ particles and to the formation of mechanically stable thick films on plastic or glass substrates. Pure TiO₂ films without any organic residuals consisting of nanoparticles were formed with surface area of 56 m²/g and pore volume of 0.383 cm³/g similar to that obtained for Degussa-P25 powder. The structural properties of the films were characterized by microscopy techniques, X-ray diffractometry, and porosimetry. Overall solar to electric energy conversion efficiencies of 5.3% and 3.2% (under 1sun) were achieved for quasi-solid-state dye-sensitized solar cells employing such TiO₂ films on F:SnO₂ glass and ITO plastic substrates, respectively. Thus, the quasi-solid-state device based on low-temperature TiO₂ attains a conversion efficiency which is very close to that obtained for cells consisting of TiO₂ nanoparticles sintered at high temperature.     

Faradaic impedance of dye-sensitized solar cells

Theoretical equations of the Faradaic impedance of the photoelectrode and the counter electrode of dye-sensitized solar cell (DSC) were derived. The Faradaic impedance is the frequency dependent resistance related to the time constants of elementary electrode processes like photoexcitation, electron transfer, charge transfer reaction and diffusion. The typical cell impedance spectrum describes the locus of three semicircles on the Nyquist plane. The locus of three semicircles is generally analyzed by using the equivalent circuit composed of charge transfer resistance (R_ct,1) and capacitance (C_dl,1) of counter electrode, charge transfer resistance (R_ct,2) and capacitance (C_dl,2) of photoelectrode, the finite diffusion impedance due to the diffusion of I₃⁻ on the counter electrode (Z_w), and total resistance of the substrate and solution (R_s). The physical meanings of R_ct,1 and R_ct,2 can be elucidated by the interpretations of Faradaic impedance derived in the present paper. The R_ct,1 is represented as the function of the potential-dependent rate constants of I₃⁻ reduction and I⁻ oxidation. On the other hand, the R_ct,2 is the function of the photoelectrode potential, the surface concentration of I₃⁻ and the potential-independent rate constant of the back electron transfer reaction. The theoretical expressions of the current-voltage (I-V) curve of the DSC can be also derived. In the present paper, the relations between the impedance and I-V curve of the DSC are discussed.     

Single-step fabrication of phase-controllable nanocrystalline TiO2 films for enhanced photoelectrochemical water splitting and dye-sensitized solar cells

We introduce a single-step procedure for growing a phase-controllable bilayer-structured TiO₂ film directly onto transparent conductive oxide glass by precipitation from hydrolysis of TiCl₄ in acid solution containing sulfate ions. The obtained bilayer-structured film with anatase nanoparticles in the inner layer which provide high surface area, and an outer layer of larger rutile particles for incident light scattering. In both the water splitting and the dye-sensitized solar cells under AM 1.5 simulated solar light, the bilayer-structured film outperformed the single layer-structured films with either anatase or rutile TiO₂ alone by at least 50%.     

Optical properties of nano-structured dye-sensitized solar cells

Radiative transfer computations are carried out to describe the intrinsic and effective optical properties of light diffusing and absorbing materials consisting of anatase titania pigments hosted in an electrolyte medium. The intrinsic visible absorption of some of the pigments has been increased by coating them with an absorbing dye monolayer. A multiple scattering approach is applied to compute average path-length parameters and forward-scattering ratios used in four-flux radiative transfer calculations. It is shown that the effective absorption coefficient of the inhomogeneous medium is maximized when the size of the pigments is around 12 nm in diameter, and the effective scattering coefficient is optimized for diameters of the pigments around 250 nm. The intrinsic solar absorptance of the medium is optimized when the diameter of the pigments is around 60 nm.     

Improvement of the performance of dye-sensitized solar cells using Sn-doped ZnO nanoparticles

Sn-doped and undoped ZnO nanoparticles were synthesized by hydrothermal method and their performance as the photoanode of dye-sensitized solar cells (DSSCs) was investigated. Energy dispersive X-ray spectroscopy and X-ray diffraction showed that the Sn had been doped into the ZnO lattice. A red shift of photoluminescence spectra which was induced by Sn doping was observed. The photocurrent density-voltage curves of DSSCs indicated that the efficiency was increased by as high as 140% on bare-FTO substrate and 105% on ZnO compact layer/FTO substrate via Sn doping. Also the effect of the ZnO compact layer was discussed by both of Sn-doped or undoped DSSCs.     

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.