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.     

Cost-efficient thermophotovoltaic cells based on germanium substrates

This paper describes the realisation of cost-efficient thermophotovoltaic (TPV) cells based on germanium substrates. Because the majority of the photons incident on the TPV cell in a typical TPV system will have a long wavelength it is important to apply optical confinement in the TPV cell. In this paper this has been done by using a highly reflective rear contact. Electrical contact at the rear has been created with a laser (laser fired contact, LFC) such that the metal is locally heated and contact is formed.The optimal TPV cell is based on a low doped p-type germanium substrate having a 500 nm thick n-type emitter , where front and rear are both passivated with hydrogen rich PECVD amorphous silicon. An AM1.5G record efficiency has been measured for a germanium TPV cell with an LFC rear contact. The application of optical confinement leads to a clear improvement of the spectral response in the wavelength region which is dominant in TPV systems and results in a potential 20% increase of current density compared to the use of a classical germanium photovoltaic cell.     

Thin-film solar cells on commercial ceramic tiles

Amorphous silicon solar cells were deposited on porcelain stoneware tiles in order to develop a fully integrated PV building element. In a previous work we demonstrated the feasibility of adopting porcelain stoneware tiles as thin-film solar cell substrates and we fabricated solar cells on “industrial-level” ceramic substrates showing more than 4% efficiency. In this study we focus our attention on larger area devices deposited on tiles. As the active area increases additional problems arise from the surface of the substrate. In particular we find that short-circuit paths originate from the unfavorable properties of the substrate tiles (roughness, porosity, etc.) having dramatic consequences on the performance of larger area devices. With the assumption that spot-like regions, in which the back layer and the front layer are in electrical contact (local short-circuits), are present all over the device, we propose a new device structure to overcome the substrate drawbacks. A new contacts arrangement was designed in order to minimize the shunting effect of short-circuit paths. As a result an initial efficiency of 2.5% has been obtained using the new device structure for devices deposited on a PV mini-module tile.     

Transparent anodes for polymer photovoltaics: Oxygen permeability of PEDOT

The oxygen permeability of the transparent organic anode poly(3,4,-ethylene dioxythiophene) with paratoluenesulphonate as the anion (PEDOT:pTS) was determined to be (STP) , and is thus comparable in magnitude to the oxygen permeability of polyethyleneterephthalate (PET). The oxygen diffusion through bilayers of polyethylene (PE) and PEDOT:pTS and bilayers of PET and PEDOT:pTS was established. The bilayer structures were applied as the carrier substrate and the transparent anode in polymer-based photovoltaic devices employing a mixture of poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) and [6,6]-phenyl-C₆₁-butanoicacidmethylester (PCBM) as the active layer and aluminium as the cathode. The oxygen permeability of the layers and the aluminium cathode was correlated with the lifetime of the solar cell devices. It was found that the performance of the devices with PET as the carrier substrate degraded more slowly due to the lower oxygen and water permeability, whereas devices using PE as the carrier substrate gave devices with a very short lifetime. It was found that PEDOT:pTS on its own is a not a significant barrier for oxygen in the context of photovoltaic devices where long lifetimes are anticipated. It is concluded that the large oxygen permeability of the barrier layers contribute to the short device lifetimes while other permeates such as water also contribute to device degradation.     

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.     

Microcrystalline silicon deposited at high rate on large areas from pure silane with efficient gas utilization

Microcrystalline silicon thin film deposited by RF-PECVD and integrated in a tandem structure is a promising material for low cost photovoltaic solar cells compared to solar cells based on crystalline silicon. However, in order to allow a cost-effective mass production of solar cells based on this material, deposition processes should fulfill several conditions such as high deposition rate, good uniformity over large area and efficient gas utilization. In this work, it is shown that the atomic hydrogen density can be high enough to form microcrystalline thin films even from a pure silane RF discharge and that the pure silane regime is more efficient in terms of gas utilization. In situ Fourier transform infrared absorption and ex situ Raman spectroscopy measurements have been used to determine the fraction of dissociated silane in the discharge and the crystallinity of the deposited layers. Results have shown that microcrystalline silicon can be deposited uniformly on a large area substrate with a deposition rate of more than with a low powder formation and an input power density of from a pure silane discharge.     

Organic photovoltaic devices based on polythiophene films electrodeposited on FTO substrates

We investigated photovoltaic devices based on electrochemically deposited monolayer of neat polythiophene (PT) films onto fluorine-doped tin oxide (FTO)/glass substrates. The photo-electrical behavior of these devices, using FTO and aluminum as electrodes, presented symbatic and antibatic response. These devices presented V_oc700 mV, under monochromatic irradiation (λ=610 nm; 1 W/m²) and the Incident Photon Converted to Electron Efficiency (IPCE) around 5%, with illumination through the FTO electrode (λ=610 nm; 1 W/m²). Cyclic voltammogramms and optical measurements were used to estimate the PT HOMO and LUMO energy levels, as well to demonstrate that the potential synthesis did not produce any polymer degradation. Using the Schottky model expression in the dark current voltage characteristics it was possible to obtain the barrier height value (_b), for the interface PT/Al. The _b was quite near to the difference between the aluminum work function and the PT electronic affinity and coherent with the V_oc values.     

Pad printing as a film forming technique for polymer solar cells

Pad printing as a technique for preparing the active layer in polymer solar cells is presented. The technique employs a silicone rubber stamp to pick up the motif from a gravure plate and transfer it to the substrate. The strengths and limitations of pad printing are discussed and polymer solar cells prepared by pad printing are presented. Devices were prepared on indium tin oxide substrates but in principle the entire photovoltaic device comprising front and back electrodes, barrier layers and active layer could be printed with no need for vacuum steps. The device geometry comprises a spin coated transparent zinc oxide front electrode, a pad printed active layer based on a bulk heterojunction of the thermocleavable polymer poly(3-(2-methylhexyloxycarbonyl)thiophene-co-thiopene) (P3MHOCT) and zinc oxide nanoparticles, spin coated PEDOT:PSS and finally a manually cast thermally cured silver paste back electrode. The P3MHOCT was converted to poly(3-carboxy-dithiophene) (P3CT) in situ by heating the film to for a brief period. The entire printing and device preparation was carried out in the ambient atmosphere and the devices obtained had a good stability in air during storage and operation.     

AB based solar cells and concentrating optical elements for space PV modules

This paper presents briefly the results of the development of , and GaSb cells manufactured for tandem solar cells, as well as tandems designed for point and line-focus concentrator modules. The maximum efficiency 23–23.8% (25°C, AM0) under 20–100 suns has been reached in the infrared transparent cells with prismatic cover. The efficiency 27.5% under AM1.5, 140 suns conditions has been reached as well. The bottom cells are based on lattice-matched or GaSb homo-junction Zn-diffused structures. The summation of the highest efficiencies measured in the top and bottom cells gave the values 28.8%–29.4% (AM0, 20–70 suns, 25°C) and 33.2% (AM1.5, 100 suns, 30°C). Two types of concentrator photovoltaic modules employing the reflective optical elements have been developed. The first type is based on compound parabolic concentrators, the second one on line-focus parabolic troughs. The estimated specific parameters of these modules with single-junction solar cells are the following: 230–240 W · m⁻² (AM0) and 3 kg · m⁻². The usage of tandem cells will allow to increase specific power of these modules on the value of 20–25%.     

Solar collector systems to provide hot air in rural applications

A simple solar system is designed and studied, its thermal performance and economics are evaluated. A mathematical model and a code are developed based on monthly average meteorological data. The collector field is built using indigenous and locally available materials. Two kinds of solar collector field are considered: (i) collector field for which the ground is used as absorber and with glazing; (ii) collector field for which roofing sheet is used as absorber without glazing (bare absorber plate collector). In a case study, the system is used to provide thermal energy for drying tobacco in an existing propane burning heating plant at Bokito, Cameroon. The results showed that in the first case, the thermal efficiency is about 38%, the useful annual solar energy collected is and the cost of thermal energy is 2.03 $/GJ, and in the second case, the corresponding values are for collector with galvanized iron roofing sheet absorber, 22%, , 1.46 $/GJ, and for collector with aluminum roofing sheet absorber, 24.5%, , 1.28 $/GJ. The estimated solar energy cost compares favorably with the thermal energy cost of 36.5 $/GJ from the existing propane fired system.     

Phenomenological modeling of dye-sensitized solar cells under transient conditions

A phenomenological model is proposed for a better understanding of the basic working mechanisms of dye-sensitized solar cells (DSCs). A steady-state approach allows the construction of the I-V characteristics, giving important informations about the main factors that influence DSCs’ performance. On the other hand, the transient approach model is an important tool to relate the phenomenological behavior with certain dynamic techniques, such as Electrochemical Impedance Spectroscopy (EIS). Bearing in mind the uncertainty arising from fitting the experimental Nyquist diagrams to general electrical analogues, this transient model contributes for a deeper understanding of the DSCs and for obtaining the relevant kinetic parameters with higher accuracy. The one-dimensional transient phenomenological model presented here assumes that the injected conduction-band electrons may recombine only with the electrolyte redox species. Due to the small dimension of the titania particles, no significant electrical potential gradient is considered, resulting only in a diffusive electron transport across the semiconductor. For modeling purposes, the mesoscopic porous structure, consisting of TiO₂ nanoparticles covered with light-absorbing dye molecules and interpenetrated by the redox mediator (electrolyte), is considered as a homogeneous nanocrystalline structure of thickness L. The continuity and transport governing equations are defined for the mobile species involved: electrons in the TiO₂ conduction band and ions in the electrolyte. The simulated results are in straight agreement with the experimental data.     

Increase in photovoltaic performances of dye-sensitized solar cells—Modification of interface between TiO2 nano-porous layers and F-doped SnO2 layers

In order to improve the physical and chemical contacts between a porous TiO₂ layer and an F-doped SnO₂ transparent conductive layer (FTO), the surface of the FTO layer is polished. After polishing, the surface roughness decreased. However, light transmittance and sheet resistance did not vary largely. The short circuit current (J_sc) and efficiencies increased after the FTO was polished. It was found that the interfacial charge transfer between a TiO₂ layer and an FTO layer decreased by impedance measurement, which suggests that contacts between an FTO and a TiO₂ layer are improved because of the flatted surfaces or removal of electrical impurities. We propose one of the industrially important phenomena that surface polishing of FTO is one of the ways to increase photovoltaic performances for DSCs.     

Characterization of sky conditions by the use of solar radiation data

A method is proposed to characterize the sky and cloud conditions (SCC) by using ground based global/diffuse irradiance data. The term SCC is defined for the purpose of this paper as the bulk effect of clouds, and other atmospheric constituents on the values of the diffuse fraction of solar radiation (), the ratio between, the global to extra terrestrial radiation (K_T) and the transmissivity T_r measured at the earth's surface. However, there is a difficulty in using the measured values of K_T and () for describing the SCC since these values depend on the solar zenith angle and not only on meteorological factors. This difficulty is overcome by the use of a simple semi-empirical procedure by which one can eliminate the seasonal and latitudinal variations (due to variations in the solar zenith angle) in and K_T, or similar relations, leaving only variations due to SCC. The procedure is based on a paper by Zangvil and Aviv on the effect of latitude and season on the relation between the diffuse fraction of solar radiation and the ratio of global to extraterrestrial radiation, Solar Energy39, 321–327 (1987) where a numerical simulation of the seasonal and latitudinal (zenith angle dependent) changes of positions of points representing different simulated SCC on a diagram was performed. The method is demonstrated on monthly mean relations calculated from three years of global and diffuse irradiance measured at Sede Boker, Israel. Results show that the large scatter of the original data points on a diagram is, to a considerable extent due to a seasonal effect (variations of the solar zenith angle). After these effects are removed by the use of the proposed technique the transformed monthly data points have much less scatter and they describe the observed seasonal variations of sky and cloud conditions correctly.     

Novel hydrophobic ionic liquids electrolyte based on cyclic sulfonium used in dye-sensitized solar cells

A novel series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations were first time synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte system. The chronoamperograms result showed that the length of substituent on sulfonium cations could inhibit the diffusion and the five-ring structure of sulfonium was benefit for fast triiodide ion diffusion. The electrochemical impendence spectra measurement of dye-sensitized solar cells with these ionic liquid electrolytes was carried out and the result indicated that the cations’ structure had indeed influence on the cells’ performance especially for the fill factor, which was further proved by the measurement result of I-V curves of these dye-sensitized solar cells. The conclusion was obtained that the electron exchange reaction on Pt counter electrode/electrolyte interface dominated the cells’ performance for these ionic liquid electrolyte-based DSCs.