p-doped μc-Si:H window layers prepared by hot-wire CVD for amorphous solar cell application

We report on boron-doped μc-Si:H films prepared by hot-wire chemical vapor deposition (HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in the range of 1–10 (Ω cm)⁻¹. The open circuit voltage V_oc of the solar cells was found to decrease from 840 mV at low hydrogen dilution H-dil=91% to 770 mV at high H-dil =97% during p-layer deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent band edge discontinuity between μc-Si:H p- and amorphous i-layer. The short circuit current density J_sc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for amorphous solar cells with and without the seed layer are presented. The I–V parameters for the best p–i–n solar cell obtained so far are J_sc=13.95 mA/cm², V_oc=834 mV, FF=65% and η=7.6%, where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847 mV could be achieved at higher TMB concentrations.     

Photovoltaic activity in a ZnTe/PEO–chitosan blend electrolyte junction

A ZnTe/polymer junction has been fabricated and the photovoltaic properties studied. The polymer is a blend of 50 wt% chitosan and 50 wt% polyethylene oxide (PEO). The polymer blend was complexed with ammonium iodide (NH₄I) and some iodine crystals were added to the polymer–NH₄I solution to provide the I⁻/I³⁻ redox couple. The ionic conductivity of the polymer electrolyte is 4.32×10⁻⁶ S cm⁻¹ at room temperature. ZnTe was electrodeposited on ITO conducting glass. The polymer film was sandwiched between the ZnTe semiconductor and an ITO glass to form a ZnTe/polymer electrolyte/ITO photovoltaic cell. The open circuit voltage (V_oc) of the fabricated cells ranges between 300 and 400 mV and the short circuit current between 2 and 5 μA.     

An alkaline direct NaBH4–H2O2 fuel cell with high power density

A high performance alkaline direct borohydride–hydrogen peroxide fuel cell with Pt–Ru catalyzed nickel foam as anode and Pd–Ir catalyzed nickel foam as cathode is reported. The electrodes were prepared by electrodeposition of the catalyst components on nickel foam. Their morphology and composition were analyzed by SEM–EDX. The effects of concentrations of NaBH₄ and H₂O₂ as well as operation temperature on the cell performance were investigated. The cell exhibited an open circuit voltage of about 1.0 V and a peak power density of 198 mW cm⁻² at a current density of 397 mA cm⁻² and a cell voltage of 0.5 V using 0.2 mol dm⁻³ NaBH₄ as fuel and 0.4 mol dm⁻³ H₂O₂ as oxidant operating at room temperature. Electrooxidation of NaBH₄ on Pt–Ru nanoparticles was studied using a rotating disk electrode and complete 8e⁻ oxidation was observed in 2 mol dm⁻³ NaOH solution containing 0.01 mol dm⁻³ NaBH₄.     

CaBr2 hydrolysis for HBr production using a direct sparging contactor

The calcium–bromine cycle being investigated is a novel continuous hybrid cycle for hydrogen production employing both heat and electricity. Calcium bromide (CaBr₂) hydrolysis generates hydrogen bromide (HBr) which is electrolyzed to produce hydrogen. The CaBr₂ hydrolysis at 1050 K (777 °C) is endothermic with the heat of reaction δG_T = 181.5 KJ/mol (43.38 kcal/mol) and the Gibbs free energy change is positive at 99.6 kJ/mol (23.81 kcal/mol). What makes this hydrolysis reaction attractive is both its rate and that well over half the thermodynamic requirements for water-splitting heat of reaction of δG_T = 285.8 KJ/mol (68.32 kcal/mol) are supplied at this stage using heat rather than electricity. Molten-phase calcium bromide reactors may overcome the technical barriers associated with earlier hydrolysis approaches using supported solid-phase calcium bromide studied in the Japanese UT-3 cycle. Before constructing the experiment two design concepts were evaluated using COMSOL™ multi-physics models; 1) the first involved sparging steam into a calcium-bromide melt, while 2) the second considered a “spray-dryer” contactor spraying molten calcium bromide counter-currently to upward-flowing steam. A recent paper describes this work [6]. These studies indicated that sparging steam into a calcium-bromide melt is more feasible than spraying molten calcium bromide droplets into steam. Hence, an experimental sparging hydrolysis reactor using a mullite tube (ID 70 mm) was constructed capable of holding 0.3–0.5 kg (1.5–2.5 × 10⁻³ kg mol) CaBr₂ forming a melt with a maximum 0.08 m (8 cm) depth. Sparging steam at a steam rate of 0.02 mol/mol of CaBr₂ per minute (1.2–2.3 × 10⁻⁵ kg/s), into this molten bath promptly yielded HBr in a stable operation that converted up to 25% of the calcium bromide. The kinetic constant derived from the experimental data was 2.17 × 10⁻¹² kmol s⁻¹ m⁻² MPa⁻¹ for the hydrolysis reaction. The conversion rate is highly dependent on melt depth and the design for steam sparging. This experimental data provides a basis for designing a larger-scale sparging hydrolysis reactor for the calcium bromide thermochemical cycle where the endothermic heat of reaction can be effectively supplied by heat transfer coils embedded in the melt.     

Bulk-heterojunction photovoltaic devices based on donor–acceptor organic small molecule blends

We present a systematic study on photovoltaic devices that combine an organic small molecule photoactive donor–acceptor bulk heterojunction system with controlled doping of the charge transport layers. The doped transport layers are formed using high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on zinc-phthalocyanine (ZnPc) as donor (D) and fullerene (C₆₀) as electron acceptor (A) with doped charge transport layers. The cells show a short circuit current, I_sc=1.5 mA/cm², an open circuit voltage, V_oc=450 mV, a fill factor, FF=0.5, and a power conversion efficiency, η_e=3.37% under sun (10 mW/cm²) white light illumination. In addition, these bulk-heterojunction photovoltaic devices were characterized under 1 sun (100 mW/cm²) white light illumination showing I_sc=6.3 mA/cm², V_oc=500 mV, and η_e=1.04%. We have observed that the performance of such ‘bulk-heterojunction’ photovoltaic devices is critically dependent on the transport properties of the interpenetrating network D/A system and doped charge transport layers.     

Ribbon Si solar cells with efficiencies over 18% by hydrogenation of defects

We have fabricated 4 cm² solar cells on String Ribbon Si wafers and edge-defined film-fed grown (EFG) Si wafers with using a combination of laboratory and industrial processes. The highest efficiency on String Ribbon Si wafer is 17.8% with an open circuit voltage (V_oc) of 620 mV, a short circuit current density (J_sc) of 36.8 mA/cm² and a fill factor (FF) of 0.78. The maximum efficiency on EFG Si is 18.2% with a V_oc of 620 mV, a J_sc of 37.5 mA/cm² and a FF of 0.78. These are the most efficient ribbon Si devices made to date, demonstrating the high quality of the processed Si ribbon and its potential for industrial cells. Co-firing of SiN_x and Al by rapid thermal processing was used to boost the minority carrier lifetime of bulk Si from 3–5 μs to 70–100 μs. Photolithography-defined front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The effects of firing temperature and time were studied to understand the trade-off between hydrogen retention and Al-doped back surface field (Al-BSF) formation. Excellent bulk defect hydrogenation and high-quality thick Al-BSF formation was achieved in a very short time (1 s) at firing temperatures of 740–750 °C. It was found that the bulk lifetime decreases at annealing temperatures above 750 °C or annealing time above 1 s due to dissociation of hydrogenated defects.     

Blue sensitizers for solar cells: Natural dyes from Calafate and Jaboticaba

Blue-violet anthocyanins from Jaboticaba (Myrtus cauliflora Mart) and Calafate (Berberies buxifolia Lam) were employed as TiO₂ dye-sensitizers. Solar cells sensitized by Jaboticaba extracts achieved up to J_sc=9.0 mA cm⁻², V_oc=0.59 V, P_max=1.9 mW cm⁻² and ff=0.54, while for Calafate sensitized cells the values determined were up to J_sc=6.2 mA cm⁻², V_oc=0.47 V, P_max=1.1 mW cm⁻² and ff=0.36. Other natural dyes were evaluated without significant photocurrent, demonstrating that only selected extracts are capable of converting sunlight in electricity. The results obtained with extracts of Jaboticaba and Calafate show a successful conversion of visible light into electricity by using natural dyes as wide band-gap semiconductor sensitizers in dye-sensitized solar cells. It also represents an environmentally friendly alternative for dye-sensitized solar cells with low cost production and an excellent system for educational purposes.     

Hybrid solar cells based on MEH-PPV and thin film semiconductor oxides (TiO2, Nb2O5, ZnO, CeO2 and CeO2–TiO2): Performance improvement during long-time irradiation

Performance improvement of hybrid solar cells (HSC) applying five different thin film semiconductor oxides has been observed during long-time irradiation in ambient atmosphere. This behavior shows a direct relation between HSC and oxygen content from the environment. Photovoltaic devices were prepared as bi-layers of thin film semiconducting oxides (TiO₂, Nb₂O₅, ZnO, CeO₂–TiO₂ and CeO₂) and the polymer MEH-PPV, with a final device configuration of ITO/Oxide_{thin film}/MEH-PPV/Ag. The oxides were prepared as thin transparent films from sol–gel solutions. The photovoltaic cells were studied in ambient atmosphere by recording the initial values of open circuit voltage (V_oc) and current density (I_sc). Solar decay curves presented as the measurement of the short circuit current as a function of time, IV curves and photophysical analyses were also carried out for each type of device. Solar cells with TiO₂ thin films showed the best performance with maximum V_oc as high as −0.74 V and I_sc of 0.4 mA/cm². Solar decay analyses showed that the devices require a stabilization period of several hours in order to reach maximum performance. In the case of TiO₂, Nb₂O₅ and CeO₂–TiO₂, the maximum current density was observed after 15 h; for CeO₂, the maximum performance was observed after 30 h. The only exception was observed with devices applying ZnO in which the current density decreased drastically and degraded the polymer in just a couple of hours.     

Solar cells using iodine-doped polythiophene–porphyrin polymer films

Wet-type organic solar cells containing 5,10,15,20-3-tetrathienylporphyrin (TThP) and polythiophene (PTh) films were fabricated. The TThP/PTh film was prepared on indium-tin-oxide (ITO) glass using an electrochemical polymerization method in an n-Bu₄NPF₆/CH₂Cl₂ solution. It was found that a small amount of iodine doping of the film improved the incident photon-to-electron conversion efficiency (IPCE) of a solar cell consisting of a TThP/PTh film and an aqueous electrolyte. A HOMO level measurement suggested that a modified HOMO level of the low iodine-doped TThP/PTh film allowed a fast electron transfer from PTh to a porphyrin moiety. To obtain further improvement, a sandwich-type solar cell using a 5% (w/w) aqueous solution of acetonitrile containing 0.05 M iodine and 0.5 M lithium iodide as an electrolyte was then fabricated. The solar cell absorbed light in the 300–800 nm wavelength range, converting this to a cathodic photocurrent with a maximum IPCE of 32% at 430 nm under irradiation of 5.0×10¹⁴ photon cm⁻² s⁻¹. This value is about 10 times higher than that of the solar cells using an aqueous electrolyte. The total energy conversion efficiency (η) of the solar cell under simulated sunlight reached 0.12% for 2.59 mW cm⁻² at AM1.5 and 0.05% for 100 mW cm⁻² at air mass 1.5.     

Precursor route poly(thienylene vinylene) for organic solar cells: Photophysics and photovoltaic performance

Photophysical studies and photovoltaic devices on a low bandgap, high-charge carrier mobility poly(thienylene vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the “dithiocarbamate route”, are reported. In composites with an electron acceptor ([6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), a soluble fullerene derivative), photoinduced absorption characteristic for charged excitations together with photoluminescence quenching are observed indicating photoinduced electron transfer. The “bulk heterojunction” photovoltaic devices using PTV and PCBM composites show short-circuit currents up to 4 mA/cm² under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset about 1.65 eV (750 nm), which corresponds to the absorption spectrum of the polymer.     

Improvement of a-Si solar cell properties by using SnO2:F TCO films coated with an ultra-thin TiO2 layer prepared by APCVD

TiO₂-overcoated SnO₂:F transparent conductive oxide films were prepared by atmospheric pressure chemical vapor deposition (APCVD) and an effect of TiO₂ layer thickness on a-Si solar cell properties was investigated. The optical properties and the structure of the TiO₂ films were evaluated by spectroscopic ellipsometry and X-ray difractometry. a-Si thin film solar cells were fabricated on the SnO₂:F films over-coated with TiO₂ films of various thicknesses (1.0, 1.5 and 2.0 nm) and I–V characteristics of these cells were measured under 1 sun (100 mW/cm² AM-1.5) illumination. It was found that the TiO₂ film deposited by APCVD has a refractive index of 2.4 at 550 nm and anatase crystal structure. The conversion efficiency of the a-Si solar cell fabricated on the 2.0 nm TiO₂-overcoated SnO₂:F film increased by 3%, which is mainly attributed to an increase in open circuit voltage (V_oc) of 30 mV.     

Photoresponse of n-TiO2 thin film and nanowire electrodes

Thin film and nanowire electrodes of n-type titanium oxide (n-TiO₂) were fabricated and their photoresponses towards water-splitting reaction were studied. A more than twofold increase in maximum photoconversion efficiency was observed when a single-layer thin film of n-TiO₂ was replaced by nanowires. Highest photoconversion efficiency was observed at an applied potential of 0.61 V vs. E_aoc where the electrode potential at open circuit, E_aoc was found to be −1.0 V SCE⁻¹ at illumination intensity of 40 mW cm⁻² in 5.0 M KOH solution. The maximum photoconversion efficiency was approximately of another twofold increase for water splitting at nanowire electrodes when methanol was used as a sacrifacial hole scavenger (depolarizing agent) in the electrolyte. Also, this maximum photoconversion efficiency was found at a lower applied potential of 0.41 V vs. E_aoc in presence of methanol. The band-gap energy of the n-TiO₂ films and nanowires annealed at 750°C was found to be 2.98 eV, which indicates their rutile structure.     

Nanostructured solar cell based on spray pyrolysis deposited ZnO nanorod array

In this paper we present a realization of an extremely thin absorber (ETA) layer solar cell by the chemical spray pyrolysis method. CuInS₂ absorber was deposited onto a blocking layer coated ZnO nanorods grown on a transparent conductive oxide. Layers and cells were characterized by optical and Raman spectroscopy, and scanning electron microscopy. Current–voltage, spectral response and electron beam induced current measurements were applied to solar cells. ZnO nanorod cell showed twice higher short circuit current density than the flat reference. ETA cells with efficiency of 2.2% (j=12 mA/cm², V_oc=425 mV, FF=43%) and of 2.5% were prepared using TiO₂-anatase and an indium sulfide blocking layer, respectively.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Stability of thin film solar cells having less-hydrogenated amorphous silicon i-layers

In this study, highly stabilized hydrogenated amorphous silicon films and their solar cells were developed. The films were fabricated using the triode deposition system, where a mesh was installed between the cathode and the anode (substrate) in a plasma-enhanced chemical vapor deposition system. At a substrate temperature of 250 °C, the hydrogen concentration of the resulting film (Si–H=4.0 at%, Si–H₂<1×10²⁰ cm⁻³) was significantly less than that of conventionally prepared films. The films were used to develop the i-layers of solar cells that exhibited a significantly low degradation ratio of 7.96%.     

Concentrator silicon solar cells from silicon industry rejects

Two types of silicon (Si) substrates (40 n-type with uniform base doping and 40 n/n⁺ epitaxial wafers) from the silicon industry rejects were chosen as the starting material for low-cost concentrator solar cells. They were divided into four groups, each consisting of 20 substrates: 10 are n/n⁺ and 10 are n substrates, and the solar cells were prepared for different diffusion times (45, 60, 75 and 90 min). The fabricated solar cells on n/n⁺ substrates (prepared with a diffusion time of 75 min) showed better parameters. In order to improve their performances, particularly the fill factor, 20 new solar cells on n/n⁺ substrates were fabricated using the same procedure (the diffusion time was 75 min)—but with four new front contact patterns. Investigation of current–voltage (I–V) characteristics under AM 1.5 showed that the parameters of these 20 new solar cells have improved in comparison to previous solar cells' parameters, and were as follows: open-circuit voltage (V_OC=0.57 V); short circuit current (I_SC=910 mA), and efficiency (η=9.1%). Their fill factor has increased about 33%. The I–V characteristics of these solar cells were also investigated under different concentration ratios (X), and they exhibited the following parameters (under X=100 suns): V_OC=0.62 V and I_SC=36 A.     

Electronic and photovoltaic properties of Al/p-Si/copper phthalocyanine photodiode junction barrier

Al/p-Si/copper phthalocyanine photovoltaic device has been fabricated and characterised by current–voltage and capacitance–voltage measurements. Electrical properties of the device were determined by current–voltage characterizations under dark and illumination conditions. The density distribution of the interface states of the photodiode was found to vary from 8.88×10¹² eV⁻¹ cm⁻² in E_ss-0.54 eV to 4.51×10¹² eV⁻¹ cm⁻² in E_ss-0.61 eV. The device shows a photovoltaic behaviour with a maximum open circuit voltage Voc of 0.16 V and short-circuits current I_sc of 0.45 μA under 3500 lux light intensity.     

Advanced mathematical model for the passive direct borohydride/peroxide fuel cell

In the literature a mathematical model has been developed for the direct borohydride fuel cells by Verma et al. [1]. This model simply simulates the fuel cell system via kinetic mechanisms of the borohydride and oxygen. Their mathematical expression contains the activation losses caused by the oxidation of the borohydride and the concentration overpotential increased by the reduction of oxygen. In this study a direct borohydride/peroxide fuel cell has been constructed using hydrogen peroxide (H₂O₂) as oxidant instead of the oxygen. Therefore we created an advanced model for peroxide fuel cells, including the activation overpotential of the peroxide. The goal of our model is to provide the information about the peroxide reduction effect on the cell performance. Our comprehensive mathematical model has been developed by taking Verma’s model into account. KH₂O₂ used in the advanced model was calculated as 6.72 × 10⁻⁴ mol cm⁻² s⁻¹ by the cyclic voltammogram of Pt electrode in the acidic peroxide solution.     

Co-free, iron perovskites as cathode materials for intermediate-temperature solid oxide fuel cells

We have developed a Co-free solid oxide fuel cell (SOFC) based upon Fe mixed oxides that gives an extraordinary performance in test-cells with H₂ as fuel. As cathode material, the perovskite Sr_0.9K_0.1FeO_3−δ (SKFO) has been selected since it has an excellent ionic and electronic conductivity and long-term stability under oxidizing conditions; the characterization of this material included X-ray diffraction (XRD), thermal analysis, scanning microscopy and conductivity measurements. The electrodes were supported on a 300-μm thick pellet of the electrolyte La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM) with Sr₂MgMoO₆ as the anode and SKFO as the cathode. The test cells gave a maximum power density of 680 mW cm⁻² at 800°C and 850 mW cm⁻² at 850 °C, with pure H₂ as fuel. The electronic conductivity shows a change of regime at T ≈ 350 °C that could correspond to the phase transition from tetragonal to cubic symmetry. The high-temperature regime is characterized by a metallic-like behavior. At 800 °C the crystal structure contains 0.20(1) oxygen vacancies per formula unit randomly distributed over the oxygen sites (if a cubic symmetry is assumed). The presence of disordered vacancies could account, by itself, for the oxide-ion conductivity that is required for the mass transport across the cathode. The result is a competitive cathode material containing no cobalt that meets the target for the intermediate-temperature SOFC.     

Photovoltaic effect in single-layer organic solar cell devices fabricated with two new imidazolin-5-one molecules

Organic solar cells were fabricated with two new imidazolin-5-one molecules as active layers. The use of imidazolin-5-ones, derivatives of a biomolecule chromophore, for photovoltaic applications is particularly attractive due to its biodegradable nature and tunable properties. Single-layer devices with two analogues of imidazolin-5-ones were prepared and characterized. Devices fabricated with one of the molecules as the active layer showed a maximum J_sc of 0.52 μA cm⁻² and V_oc of 0.68 V at an incident power of 20.32 mW cm⁻², while the other set of devices showed a maximum J_sc of 0.63 μA cm⁻² and V_oc of 0.57 V at the same incident power.