Thin crystalline silicon solar cells based on epitaxial films grown at 165 °C by RF-PECVD

We report on heterojunction solar cells whose thin intrinsic crystalline absorber layer has been obtained by plasma enhanced chemical vapor deposition at 165 °C on highly doped p-type (1 0 0) crystalline silicon substrates. We have studied the effect of the epitaxial intrinsic layer thickness in the range from 1 to 2.5 μm. This absorber is responsible for photo-generated current whereas highly doped wafer behave like electric contact, as confirmed by external quantum efficiency measurements and simulations. A best conversion efficiency of 7% is obtained for a 2.4 μm thick cell with an area of 4 cm², without any light trapping features. Moreover, the achievement of a fill factor as high as 78.6% is a proof that excellent quality of the epitaxial layers can be produced at such low temperatures.     

Microstructural and electrical properties of different-sized aluminum-alloyed contacts and their layer system on silicon surfaces

The firing of screen-printed aluminum pastes is well established for the formation of a back surface field (BSF) and back contacts since many years in silicon solar cell fabrication. In this paper we investigate the electrical and microstructural properties of Al-alloyed contacts and their layer system, consisting of (i) the Al-doped p⁺-layer (2-14 μm), the eutectic layer (1-15 μm) and the layer of paste residuals (20-100 μm). We show the influence of process parameters like the amount of printed paste, the alloying time and the peak temperature. Special emphasis is devoted to the properties of small alloyed screen-printed aluminum structures for the formation of local aluminum back contacts. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), electrochemical capacitance voltage (ECV) and conductivity measurements have been applied to characterize the samples. A simple model is qualitatively augmented to describe all effects occurring in a technical alloying process. For example, for increasing aluminum amounts, a saturation of the p⁺-layer thickness was found in the range of 10 mg/cm². For small screen-printed structures, the p⁺-layer is formed very homogenously and with a greater thickness compared to samples with a full-area Al metallization, which have been processed with similar alloying conditions. For the eutectic layer a high electrical conductivity of about 16×10⁶ S/m, only 2-3 times below that of pure aluminum has been determined. This is advantageous for the lateral conductivity especially for high paste amounts and small structures, which feature a relatively thick eutectic layer.     

Improved fuel use efficiency in microchannel direct methanol fuel cells using a hydrophilic macroporous layer

We demonstrate state-of-the-art room temperature operation of silicon microchannel-based micro-direct methanol fuel cells (μDMFC) having a very high fuel use efficiency of 75.4% operating at an output power density of 9.25 mW cm⁻² for an input fuel (3 M aqueous methanol solution) flow rate as low as 0.55 μL min⁻¹. In addition, an output power density of 12.7 mW cm⁻² has been observed for a fuel flow rate of 2.76 μL min⁻¹. These results were obtained via the insertion of novel hydrophilic macroporous layer between the standard hydrophobic carbon gas diffusion layer (GDL) and the anode catalyst layer of a μDMFC; the hydrophilic macroporous layer acts to improve mass transport, as a wicking layer for the fuel, enhancing fuel supply to the anode at low flow rates. The results were obtained with the fuel being supplied to the anode catalyst layer via a network of microscopic microchannels etched in a silicon wafer.     

Integration of silicon micro-hole arrays as a gas diffusion layer in a micro-fuel cell

This work examines silicon micro-hole arrays (Si-MHA) as a gas diffusion layer (GDL) in a micro-fuel cell that was fabricated using micro-electro-mechanical systems (MEMS) fabrication technique. Pt was deposited on the surface of the Si-MHA, to increase the conductivity of the micro-fuel cell. The Si-MHA with three micro-holes, replaces the traditional GDL, and the performance of the micro-proton exchange membrane fuel cell was discussed. Wet etching was performed on a 500 μm-thick layer of silicon to yield fuel channels with a depth of 450 μm and a width of 200 μm. The Si-MHA formed by deep reactive ion etching (DRIE) in the fabricated structure had diameters of 10 μm, 30 μm and 50 μm; the thickness of the structure was 50 μm.     

Fabrication of Ni nanowires for hydrogen evolution reaction in a neutral electrolyte

Hydrogen evolution reaction in 1 M Na₂SO₄ was investigated using Ni nanowires in diameter of 250 nm with exposed lengths of 20, 35, and 45 μm, respectively. The Ni nanowires were fabricated by a direct-current pulse electrodeposition technique using an anodic aluminum oxide template, followed by selective removal of the supporting pore walls. Scanning Electron Microscope images revealed structural stabilities and X-ray diffraction pattern indicated a polycrystalline fcc phase. In current–potential (i–V) polarizations, the Ni nanowires with longer exposed lengths demonstrated larger current responses. Analysis from impedance spectroscopy confirmed increasing double-layer capacitances with longer Ni nanowires. In galvanostatic lifetime experiments, the free-standing Ni nanowires exhibited a reduced overpotential over that of supported ones. Similar procedures were performed for the oxygen evolution reaction in both i–V and lifetime measurements. For the Ni nanowires of 45 μm length, we estimated the energy cost for hydrogen production was 5.24 × 10⁵ J/mole.     

Formation of highly aluminum-doped p-type silicon regions by in-line high-rate evaporation

Highly aluminum-doped p-type silicon regions are formed by in-line high-rate evaporation of aluminum. We deposit aluminum layers of 28 μm thickness at dynamic deposition rates of 20 μm×m/min on p-type silicon substrates. Due to the high substrate temperature of up to 770 °C during deposition an Al-doped p⁺ region is formed. Using the camera-based dynamic infrared lifetime mapping technique we measure emitter saturation current densities of 695±65 fA/cm² for the fully metalized Al-p⁺ regions, which corresponds to an implied solar cell open-circuit voltage of 635±2 mV.     

Electric double layer capacitance on hierarchical porous carbons in an organic electrolyte

Nanoporous carbons were prepared by using colloidal crystal as a template. Nitrogen adsorption/desorption isotherms and transmission electron microscope images revealed that the porous carbons exhibit hierarchical porous structures with meso/macropores and micropores. Electric double layer capacitor performance of the porous carbons was investigated in an organic electrolyte of 1 M LiClO₄ in propylene carbonate and dimethoxy ethane. The hierarchical porous carbons exhibited large specific double layer capacitance of ca. 120 F g⁻¹ due to their large surface areas. In addition, the large capacitance was still obtained at a large current density up to 10 A g⁻¹, which satisfies demands from the high power application such as hybrid electric vehicles. Capacitance analysis of the hierarchical porous structures revealed the contribution of meso/macropores and micropore to the electric double layer capacitance to be 8.4 and 8.1 μF cm⁻², respectively. The results indicated electric double layer is formed even when solvated ions are larger than pore diameters.     

Local aluminum-silicon contacts by layer selective laser ablation

We demonstrate damage free selective laser ablation of silicon nitride from a silicon nitride/amorphous silicon double layer. This approach allows local contact formation to passivated silicon. Thereby the remaining amorphous silicon dissolves in evaporated aluminum by annealing. This technique is especially useful for contacting thin emitters since it avoids any damage to the silicon substrate. We demonstrate a local contact resistivity of 0.8±0.3 mΩ cm² on a phosphorous diffused emitter with a peak doping density of 2×10²⁰ cm⁻³. Laser treated as well as non-treated areas show the same carrier lifetime of 2000 μs on 100 Ω cm mono-crystalline silicon, proving the selective ablation.     

Surface passivation of crystalline silicon wafer via hydrogen plasma pre-treatment for solar cells

The carrier lifetime of crystalline silicon wafers that were passivated with hydrogenated silicon nitride (SiN_x:H) films using plasma enhanced chemical vapor deposition was investigated in order to study the effects of hydrogen plasma pre-treatment on passivation. The decrease in the native oxide, the dangling bonds and the contamination on the silicon wafer led to an increase in the minority carrier lifetime. The silicon wafer was treated using a wet process, and the SiN_x:H film was deposited on the back surface. Hydrogen plasma was applied to the front surface of the wafer, and the SiN_x:H film was deposited on the hydrogen plasma treated surface using an in-situ process. The SiN_x:H film deposition was carried out at a low temperature (<350 °C) in a direct plasma reactor operated at 13.6 MHz. The surface recombination velocity measurement after the hydrogen plasma pre-treatment and the comparison with the ammonia plasma pre-treatment were made using Fourier transform infrared spectroscopy and secondary ion mass spectrometry measurements. The passivation qualities were measured using quasi-steady-state photoconductance. The hydrogen atom concentration increased at the SiN_x:H/Si interface, and the minority carrier lifetime increased from 36.6 to 75.2 μs. The carbon concentration decreased at the SiN_x:H/Si interfacial region after the hydrogen plasma pre-treatment.     

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current-voltage (I-V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450-950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm²). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.     

Preparation and characterization of graded cathode La0.6Sr0.4Co0.2Fe0.8O3−δ

Perovskite oxide La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF6428), a wonderful electronic–ionic conductor could be used as cathode of solid oxide fuel cell (SOFC). Graded cathode with coarse layer and fine layer, could improve the diffusion rate and electrochemical reaction activity of oxidant. The fabrication and properties of graded LSCF6428 cathode were discussed in this paper. First, pure perovskite LSCF6428 powders were prepared by citrate–EDTA method (CEM), citrate method (CM) and solid phase synthesis (SPS). The powders with higher specific surface area and smaller grain size are easier to be sintered and densified. Single LSCF6428 cathode with thickness of 30 μm was prepared by SPS powders, the porosity of cathode was high about 30% and pore size was about 5 μm. Graded LSCF6428 cathode including 30 μm outer layer and 10 μm inner layer was prepared by SPS and CM powders, respectively. Clear double-layer cathode was observed by SEM, which combined tightly and transited gradually. Porosity of outer layer is high about 30% and pore size is about 1–5 μm; inner layer is finer and pore size is about 0.2–1 μm. Based on the above research, 300 μm yttria stabilized zirconia (YSZ) electrolyte supported cell with single LSCF6428 cathode and double-layer LSCF6428 cathode were prepared, and the properties of two type cells were tested in H₂. Power density of graded cell is 197 mW cm⁻² at 950 °C, and improved about 46% comparing that of single layer LSCF6428 cell (135 mW cm⁻²).     

Theoretical investigation on the absorption enhancement of the crystalline silicon solar cells by pyramid texture coated with SiNx:H layer

The absorption enhancement of the crystalline silicon (c-Si) solar cells by pyramid texture coated with SiN_x:H layer was investigated by theoretical simulation via rigorous coupled-wave analysis (RCWA). It was found that in order to maximize the spectrally weighted absorptance of the solar cells for the Air Mass 1.5 (AM1.5) solar spectrum (A_AM1.5), the required pyramid size (d) was dependent on the thickness of the c-Si substrate. The thinner the c-Si substrate is, the larger the pyramids should be. Pyramids with d > 0.5 μm can make A_AM1.5 maximal if the c-Si substrate thickness is larger than 50 μm. But d > 1.0 μm is needed when the c-Si substrate thickness is less than 25 μm. If the c-Si substrate is thinner than 5 μm, even d > 4.0 μm is required. The underlying mechanism was analyzed according to the diffraction theory. The pyramid texture acts as not only an antireflective (AR) component, but also a light trapping element. Then, the optimized refractive index and the thickness of SiN_x:H layer to further enhance the absorption were given out. The potential solar cell efficiency was also estimated.     

The circuit point of view of the temperature dependent open circuit voltage decay of the solar cell

The open circuit voltage decay (OCVD) technique has been used to determine the minority carrier lifetime. In this study, an experimental and analytical method is described for determination of minority carrier lifetime at porous Si based solar cell by photo induced OCVD technique. The cell is illuminated by a monochromatic light source (λ = 658 nm) in the open circuit configuration, and the decay of voltage is measured after abruptly terminating the excitation. For the analysis of the OCVD characteristic of solar cell device, equivalent electrical circuit has been proposed in which the diffusion capacitance is connected in series with the contribution of the solar cell interface. Exact minority carrier lifetimes at low (50-170 K) and high (190-330 K) temperature regions have been obtained as 28.9 and 2.65 μs from the temperature dependent OCVD measurements by using an alternative extraction technique.     

Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Although phosphorus (P) diffusion gettering process has been wildly used to improve the performance of Si solar cells in photovoltaic technology, it is a new attempt to apply P diffusion gettering process to upgraded metallurgical grade silicon (UMG-Si) wafers with the purity of 99.999%. In this paper, improvements on the electrical properties of UMG-Si wafers and solar cells were investigated with the application of P diffusion gettering process. To enhance the improvements, the gettering parameters were optimized on the aspects of gettering temperature, gettering duration and POCl₃ flow rate, respectively. As we expected, the electrical properties of both multicrystalline Si (multi-Si) and monocrystalline Si (mono-Si) wafers were significantly improved. The average minority carrier lifetime increased from 0.35 μs to nearly about 2.7 μs for multi-Si wafers and from 4.21 μs to 5.75 μs for mono-Si wafers, respectively. Accordingly, the average conversion efficiency of the UMG-Si solar cells increased from 5.69% to 7.03% for multi-Si solar cells (without surface texturization) and from 13.55% to 14.55% for mono-Si solar cells, respectively. The impurity concentrations of as-grown and P-gettered UMG-Si wafers were determined quantitively so that the mechanism of P diffusion gettering process on UMG-Si wafers and solar cells could be further understood. The results show that application of P diffusion gettering process has a great potential to improve the electrical properties of UMG-Si wafers and thus the conversion efficiencies of UMG-Si solar cells.     

Charge/discharge behavior of plasma-fluorinated natural graphites in propylene carbonate-containing solvent

Plasma-fluorination of natural graphite samples with average particle sizes of 5 μm, 10 μm and 15 μm (NG5μm, NG10μm and NG15μm) was performed using CF₄ and charge/discharge characteristics of surface-fluorinated samples were investigated in 1 mol dm⁻³ LiClO₄–ethylene carbonate (EC)/diethyl carbonate (DEC)/propylene carbonate (PC) (1:1:1, v/v/v). Fluorine contents obtained by elemental analysis were in the range of 0.3–0.6 at.% and surface fluorine concentrations determined by X-ray photoelectron microscopy (XPS) were 14.8–17.3 at.%. Plasma-fluorination increased surface disorder of natural graphite samples though reduced surface areas due to its surface etching effect. Electrochemical decomposition of PC was highly reduced on surface-fluorinated NG10μm and NG15μm with high disorder. First coulombic efficiencies of plasma-fluorinated NG10μm and NG15μm increased by 9.7 and 19.3% at 150 mA g⁻¹, respectively.     

Three-dimensionally ordered macroporous polyimide composite membrane with controlled pore size for direct methanol fuel cells

Three-dimensionally ordered macroporous (3DOM) polyimide matrix with different pore size (1.3 μm–200 nm) was fabricated, and its structural effect on some properties of composite membrane was investigated. The composite membrane prepared by impregnation of 2-acrylamido-2-methylpropanesulfonic acid polymer (PAMPS) exhibited swelling ratios as low as 2–3% in water or methanol solutions, compared with about 400% of PAMPS itself. The swelling ratio of composite membrane was constant regardless of the 3DOM pore size. However, methanol permeability strongly depended on the pore size. In particular, it was drastically reduced when connecting windows among macropores became less than 100 nm. On the other hand, proton conductivity changed with 3DOM matrix porosity according to Archie's law. The porosity of 3DOM matrix is basically constant even if the pore size changes. Therefore, we suppressed the methanol crossover without lowering of proton conductivity due to reducing the matrix pore size, and the selectivity (proton conductivity/methanol permeability) of 1.2 × 10⁵ S cm⁻³ s, which was one order of magnitude greater than that of Nafion^®, was achieved.     

Low-temperature solid oxide fuel cells with novel La0.6Sr0.4Co0.8Cu0.2O3−δ perovskite cathode and functional graded anode

The perovskite La_0.6Sr_0.4Co_0.8Cu_0.2O_3−δ (LSCCu) oxide is synthesized by a modified Pechini method and examined as a novel cathode material for low-temperature solid oxide fuel cells (LT-SOFCs) based upon functional graded anode. The perovskite LSCCu exhibits excellent ionic and electronic conductivities in the intermediate-to-low-temperature range (400-800 °C). Thin Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte and NiO-SDC anode functional layer are prepared over macroporous anode substrates composed of NiO-SDC by a one-step dry-pressing/co-firing process. A single cell with 20 μm thick SDC electrolyte on a porous anode support and LSCCu-SDC cathode shows peak power densities of only 583.2 mW cm⁻² at 650 °C and 309.4 mW cm⁻² for 550 °C. While a cell with 20 μm thick SDC electrolyte and an anode functional layer on the macroporous anode substrate shows peak power densities of 867.3 and 490.3 mW cm⁻² at 650 and 550 °C, respectively. The dramatic improvement of cell performance is attributed to the much improved anode microstructure that is confirmed by both SEM observation and impedance spectroscopy. The results indicate that LSCCu is a very promising cathode material for LT-SOFCs and the one-step dry-pressing/co-firing process is a suitable technique to fabricate high performance SOFCs.     

Effect of air ambient on surface recombination and determination of diffusion length in silicon wafer using photocurrent generation method

An n⁺-p-p⁺ structure was formed by depositing a thin Al layer on one side and a semitransparent Pd layer on other side of a p-Si wafer, and the diffusion length L was determined from the slope of short circuit current density J_sc vs. illumination intensity P_in characteristics by illuminating the p⁺ side with a laser light of wavelength λ = 789 nm, following the photocurrent generation method. The slope was found to decrease after the specimen was exposed to air for a few hours before the measurement. The present investigation revealed that the degradation of slope was owing to increase in surface recombination velocity S significantly, e.g. to 2800 cm/s for air exposure of 8 h, above its initial value 10 cm/s. This resulted from adsorption of moisture on Pd surface which decreases work function of Pd and lowers the accumulation potential barrier. The adsorbed water molecules could be removed from Pd surface by vacuum pumping and then S again became small and the slope regained its original value. The true value of L, smaller or larger than the wafer thickness, can be determined using single monochromatic illumination if S is known or has a negligibly small value, as in our case where the specimen was stored in vacuum. In our specimen we determined L = 56 μm when S was negligibly small. However, using the experimental slopes of J_sc-P_in curves at two monochromatic illuminations obtained by Sharma et al. we determined both S and L and found their values 1390 cm/s and 94 μm, respectively.     

Amorphous Si film anode coupled with LiCoO2 cathode in Li-ion cell

An amorphous silicon film with an average thickness of up to 2 μm was deposited on copper foil by direct-circuit (dc) magnetron sputtering and coupled with commercial LiCoO₂ cathode to fabricate cells. Their cycle performance and high rate capability at room temperature have been investigated. In the voltage range 2.5–3.9 V at the current density of 0.2C (0.11 mA cm⁻²), the lithiation and delithiation capacity of this cell was first increased to 0.55 mAh cm⁻² within 80 cycles and maintained stable during the following cycles. After 300 cycles its capacity still retained 0.54 mAh cm⁻². High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) image indicated that the sputtered film could keep an amorphous structure although the volume expansion ratio during the lithiation and delithiation was still up to 300% after 300 cycles observed from scanning electron microscopy (SEM) image. This recovered amorphous structure was believed to be beneficial for the improvement of the cycle life of the cell. Rate performance showed that the cells had a promising high rate capability. At 30C, its lithiation/delithiation capacity remained 25% of that at 0.2C.     

Factors limiting minority carrier lifetime in solar grade silicon produced by the metallurgical route

Solar grade, p-type multicrystalline silicon wafers with large grains from different parts of silicon ingots produced by the metallurgical route (SoG-Si) at ELKEM Solar were studied using a number of complementary methods such as microwave photoconductivity decay, deep level transient spectroscopy, transmission and scanning electron microscopy, X-ray fluorescence, and secondary ion mass spectroscopy. Wafers from the top of the ingots have uniform spatial distributions of both minority carrier lifetime (average lifetime τ=3.2 μs) and concentrations of illumination-sensitive recombination centers (N_rc=3×10¹⁰−2×10¹¹ cm⁻³) over the whole wafers. Wafers from the bottom of the ingots have regions of very low lifetimes (τ=0.3 μs) and high concentrations of illumination-sensitive recombination centers (N_rc=2×10¹² cm⁻³). In the top part of the ingots the observed DLTS peaks can be attributed to copper-related extended defects, and the DLTS results from grains and grain boundaries are not significantly different. The main factors limiting the lifetime in the high lifetime regions are concluded to be illumination-sensitive recombination centers such as Fe-B pairs, B-O complexes, and Cu-related extended defects. The low lifetimes in the bottom part of the ingots are explained by a combination of several factors including high concentrations of illumination-sensitive recombination centers and of some deleterious elements (S, Na and Al), and a large amount of structural defects.