Enhancement of optical absorption for below 5 μm thin-film poly-Si solar cell on glass substrate

Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 μm. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H/(0.45 μm)/poly-Si (5 μm) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (_sc above 27 mA/cm² at the 4.7 μm thin-film poly-Si solar cell.     

Monolithic series-interconnection for a thin film silicon solar cell

To raise the output voltage of silicon solar cells several solar cells on one wafer can be monolithically interconnected. A solar cell system consisting of 20 solar cells on a 2×2 cm² area has been produced on a 4” SOI-wafer with a 15 μm thick monocrystalline active layer. Under irradiation with an AM1.5G spectrum an open-circuit voltage of 7.5 V and current densities up to 17 mA/cm² for the system have been measured. An increase in performance is expected, when the doping and contact processing is better suited and a light trapping structure is realized for the solar cell system.     

Direct methanol fuel cell operation with pre-stretched recast Nafion

Direct methanol fuel cell operation with uniaxially pre-stretched recast Nafion^® membranes (draw ratio of 4) was investigated and compared to that with commercial (un-stretched) Nafion^®. The effects of membrane thickness (60–250 μm) and methanol feed concentration (0.5–10.0 M) on fuel cell power output were quantified for a cell temperature of 60 °C, ambient pressure air, and anode/cathode catalyst loadings of 4.0 mg cm⁻². Pre-stretched recast Nafion^® in the 130–180 μm thickness range produced the highest power at 0.4 V (84 mW cm⁻²), as compared to 58 mW cm⁻² for Nafion^® 117. MEAs with pre-stretched recast Nafion^® consistently out-performed Nafion^® 117 at all methanol feed concentrations, with 33–48% higher power densities at 0.4 V, due to a combination of low area-specific resistance (the use of a thinner pre-stretched membrane, where the conductivity was the same as that for commercial Nafion^®) and low methanol crossover (due to low methanol solubility in the membrane). Very high power was generated with a 180-μm thick pre-stretched recast Nafion^® membrane by increasing the cell temperature to 80 °C, increasing the anode/cathode catalyst loading to 8.0 mg cm⁻², and increasing the cathode air pressure to 25 psig. Under these conditions the power density at 0.4 V for a 1.0-M methanol feed solution was 240 mW cm⁻² and the maximum power density was 252 mW cm⁻².     

Highly efficient 1 μm thick CdTe solar cells with textured TCOs

Thickness reduction of CdTe absorption layer down to 1 μm has been achieved by controlling the temperature profile used during the close-spaced sublimation (CSS) growth. Transparent conducting oxides, such as indium tin oxide (ITO) and textured fluorine doped tin oxide (SnO₂:F) films have been investigated as transparent electrodes for such 1-μm-thick CdTe absorption layers to increase the incident light confinement and thus to achieve higher conversion efficiency. The contribution in solar cell performance has been found in the case of textured TCOs with optimum haze ratio (roughness). Conversion efficiencies of 10.6% (V_oc: 0.75 V, J_sc: 22.02 mA/cm², FF: 0.64, area: 1 cm²) and 11.2% (V_oc: 0.78 V, J_sc: 22.6 mA/cm², FF: 0.63) have been achieved for only 0.6-μm-thick CdTe absorption layers with SnO₂:F-TCO of 11% and 3% of haze ratios, respectively.     

Electrooxidation study of pure ethanol/methanol and their mixture for the application in direct alcohol alkaline fuel cells (DAAFCs)

Aliphatic alcohol mainly, ethanol, methanol and their mixture were subjected to electrooxidation study using cyclic voltammetry (CV) technique in a three electrodes half cell assembly (PGSTAT204, Autolab Netherlands). A single cell set up of direct alcohol alkaline fuel cell (DAAFC) was fabricated using laboratory synthesized alkaline membrane to validate the CV results. The DAAFC conditions were kept similar as that of CV experiments. The anode and cathode electrocatalysts were Pt-Ru (30%:15% by wt.)/Carbon black (C) (Alfa Aesar, USA) and Pt (40% by wt.)/High Surface Area Carbon (C_HSA) (Alfa Aesar, USA) respectively. The CV and single cell experiments were performed at a temperature of 30 °C. The anode electrocatalyst was in the range of 0.5 mg/cm² to 1.5 mg/cm² for half cell CV analysis. The cell voltage and current density data were recorded for different concentrations of fuel (ethanol or methanol) and their mixture mixed with different concentration of KOH as electrolyte. The optimum electrocatalyst loading in half cell study was found to be 1 mg/cm² of Pt-Ru/C irrespective of fuel used. The single cell was tested using optimum anode loading of 1 mg/cm² of Pt-Ru/C which was found in CV experiment. Cathode loading was kept similar, in the order of 1 mg/cm² Pt/C_HSA. In single cell experiment, the maximum open circuit voltage (OCV) of 0.75 V and power density of 3.57 mW/cm² at a current density of 17.76 mA/cm² were obtained for the fuel of 2 M ethanol mixed with 1 M KOH. Whereas, maximum OCV of 0.62 V and power density of 7.10 mW/cm² at a current density of 23.53 mA/cm² were obtained for the fuel of 3 M methanol mixed with 6 M KOH. The mixture of methanol and ethanol (1:3) mixed with 0.5 M KOH produced the maximum OCV of 0.66 V and power density of 1.98 mW/cm² at a current density of 11.54 mA/cm².     

Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

Influence of sputter oxygen partial pressure on photoelectrochemical performance of tungsten oxide films

Polycrystalline tungsten oxide films of 1–1.2μm thickness were prepared by reactive sputtering at elevated substrate temperature (270 °C) and under different oxygen partial pressures in the range from 0.8 to 2.1 mTorr. At the lowest partial pressure the films were substoichiometric, showed increased disorder, and exhibited photocurrents of 0.6 mA/cm² at 1.8 V vs SCE in 0.33 M H₃PO₄. At partial pressures of 1.4 mTorr and greater, stoichiometric WO₃ films were produced which exhibited photocurrents of 2.4 mA/cm² at 1.8 V vs SCE. It has been determined that the photoelectrochemical performance of slightly substoichiometric films is adversely affected by changes in optical properties, while the photocurrents of severely substoichiometric films suffer additionally from poor carrier collection.     

A thin-film solar cell of high-quality -FeSi2/Si heterojunction prepared by sputtering

High-quality (1 1 0)/(1 0 1)-oriented epitaxial β-FeSi₂ films were fabricated on Si (1 1 1) substrate by the sputtering method. The critical feature was the formation of a high-quality thin β-FeSi₂ template buffer layer on Si (1 1 1) substrate at low temperature. It was demonstrated that the template is very important for the epitaxial growth of thick β-FeSi₂ films and for the blocking of Fe diffusion into the Si at the β-FeSi₂/Si interface. Hall effect measurements for β-FeSi₂ films showed n-type conductivity, with residual electron concentration around 2.0 × 10¹⁷ cm⁻³ and mobility of 50–400 cm²/V s. A prototype thin-film solar cell was fabricated by depositing n-β-FeSi₂ on p-Si (1 1 1). Under 100 mW/cm² sunlight, an energy conversion efficiency of 3.7%, with an open-circuit voltage of 0.45 V, a short-circuit current density of 14.8 mA/cm² and a fill factor of 0.55, was obtained.     

High current, thin silicon-on-ceramic solar cell

Solar cells utilizing thin-film polycrystalline silicon can achieve photovoltaic power conversion efficiencies greater than 19%. These high efficiencies are a result of light trapping and back surface passivation. Optimum silicon thickness, for devices employing such technology, has been determined to be 20 μm (Blakers et al., Appl. Phys. Lett. 60 (1992) 2572) to 35 μm (Rand et al., Proceedings of the IEEE Photovoltaic Specialist Conference, Orlando, FL, May 1990, p. 263). Low cost is achieved by minimizing the required amount of silicon feedstock per watt of power output. The use of an electrically insulating supporting substrate allows for monolithic, series connected sub-modules. A solar cell with a 20 μm thick polycrystalline Si-layer on a ceramic substrate, utilizing both light trapping and back-surface passivation, was fabricated and characterized. A short-circuit current of 25.8 mA/cm² was measured and verified by the National Renewable Energy Laboratory (NREL).     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.     

Heterogeneous growth of microcrystalline silicon germanium

Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10⁻⁵/Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E_a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm² has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 Å/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V_oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).     

Boron-doped diamond-like amorphous carbon as photovoltaic films in solar cell

In this paper, the photovoltaic feature of metal-boron carbide-silicon (MCS) solar cell was reported. The boron-doped diamond-like carbon thin film on n-silicon substrate has been prepared using arc-discharge plasma chemical vapor deposition (PCVD) technique. The conductivity and the resistivity of the film were measured by Bio-Rad Hall5500PC system to be p-type semiconductor and 3–12 Ω cm/□, respectively. The boron content in the films was about 0.8–1.2%, obtained from Auger electron spectroscopy (AES), and some microcrystalline diamond grains (0.5–1.0 μm) embedded in the mainly amorphous network were revealed through scanning electron microscope (SEM) and Raman spectrum. The performance of Au/C(B)/n-Si heterojunction solar cells has been given under dark I–V rectifying curve and I–V working curve (with 100 mW cm⁻² illumination). A measurement of open-circuit voltage V_oc=580 mV and short-circuit current density J_sc=32.5 mA cm⁻² was obtained. Accordingly, the energy conversion efficiency of the device was tentatively determined to be about 7.9% in AM 1.5, 100 mW/cm² illuminated.     

Large-area high efficiency single crystalline silicon solar cells

This paper reports on a 100 cm² single crystalline silicon solar cell with a conversion efficiency of 19.44% (J_sc = 37.65 mA/cm², V_oc = 638 mV, FF = 0.809). The cell structure is as simple as only applying the textured surface, oxide passivation, and back surface field by the screen printing method. The comparison between cell performances of the CZ (Czochralski) and FZ (Floating zone) silicon substrates was investigated. The higher efficiency cells were obtained for the FZ substrate rather than the CZ substrate. The influence of the phosphorus concentration of the emitter on the cell efficiency has also been investigated. A good result was obtained when the surface concentration of phosphorus was 3 × 10²⁰ cm⁻³ and the junction depth was about 0.6 μm.     

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.     

Design of high-efficiency solid-state dye-sensitized solar cells using coupled dye mixtures

A solid-state dye-sensitized solar cell comprising dye mixtures of [Ru(2,2-bpy-4,4′-dicarboxylic acid)(NCS)₂] and [Ru(4,4′,4″-tricarboxy-2,2;6,2″-terpy)(NCS)₃] on TiO₂ thin film was fabricated. The different optical properties of dyes results in increased photocurrent and incident photon to photocurrent efficiency (IPCE). The multiple dye system showed the short circuit current (I_sc) of 10.2 mA/cm² and a cell efficiency (η) of 2.8 while broadening the spectral sensitivity of the cell. When a single dye is used, I_sc of 6 and 5 mA/cm² and cell efficiency of 1.7 and 1.2 were observed for [Ru(4,4-bis(carboxy)-bpy)₂(NCS)₂] (dye 1) and [Ru(2,2′,2″-(COOH)₃-terpy)(NCS)₃] (dye 2), respectively. Additionally, the resulting IPCE for the solar cell consisting of dye mixture was 50% at wide wavelength range from 530 to 650 nm while for the dye 1, 32% IPCE was observed at 535 nm while for the dye 2, highest IPCE value observed was 20% at 620 nm.     

Hydrogen PEMFC stack performance analysis through experimental study of operating parameters by using response surface methodology (RSM)

Although many studies have been done on finding operating conditions of hydrogen-fed fuel cells before, it remains one of the most critical points in determining its parameters in the process. So this paper aims to investigate experimentally the reactant gases flow rate and cell voltage which have a significant impact on the current density of a 3-cell Proton Exchange Membrane fuel cell stack having a 150 cm² active layer. In this case, to determine the optimum values, Design of Experiment and Response Surface Methodology was applied to the experimental system at low 1.5 V, medium 1.8 V, and high 2.1 V. Then, they were compared with each other. In this context, keeping the hydrogen flow rate low and obtaining high current density is one of the main targets; at low voltage values, it was concluded that the flow rate should be increased due to the reaction rate increases with temperature. In general, the effect of humidification and cell temperature on performance was seen more prominently at 1.8 V. The highest current density values that were 313.66 mA/cm², 336.75 mA/cm², and 323.48 mA/cm², respectively, were reached at flow rates of 1 L/min,1.3 L/min,1.6 L/min.     

Preparation and characterization of Ni-plated polytetrafluoroethylene plate as an electrode for alkaline fuel cell

At about 50 wt% Ni content, Ni-plated polytetrafluoroethylene (Ni-PTFE) particles show conductivity of 300 S m⁻¹ when plated on 25 μm PTFE particles. For this study, Ni-PTFE particles were formed into the Ni-PTFE plate using heat treatment at 350 °C after 300 kg cm⁻² pressing. The Ni-PTFE plate displayed electrical conductivity and gas permeability. The plate was used as an electrode in an alkaline fuel cell (AFC). In terms of the current density, AFC using the Ni-PTFE electrode plated with Pt or Pd by immersion plating showed improved performance.     

High-Ts amorphous top cells for increased top cell currents in micromorph tandem cells

In the present paper, the authors discuss the application of amorphous p–i–n solar cells containing i-layers which are deposited at high substrate temperatures as top cells in amorphous silicon/microcrystalline silicon tandem (“micromorph”) solar cells. Increasing the substrate temperature for the deposition of intrinsic a-Si : H results in a reduced optical gap. The optical absorption is enhanced and thereby the current generation. A high-current generation within a relatively thin amorphous top cell is very interesting in the context of micromorph tandem cells, where the amorphous top cell should contribute a current of at least 13 mA/cm² for a total cell current density of 26 mA/cm². A detailed study of the intrinsic material deposited by VHF-GD at 70 MHz at substrate temperatures between 220°C and 360°C is presented, including samples deposited from hydrogen-diluted silane plasmas. The stability of the films against light soaking is investigated employing the μ₀τ₀ parameter, which has been shown to be directly correlated to the cell performance. The paper discusses in detail the technological problems arising from the insertion of i-layers deposited at high substrate temperatures into solar cells. These problems are specially pronounced in the case of cells in the p–i–n (superstrate) structure. The authors demonstrate that an appropriate interface layer at the p/i-interface can largely reduce the detrimental effects of i-layer deposition at high temperatures. Finally, the application of such optimized high-temperature amorphous cells as top cells in micromorph tandem cells is discussed. Current densities of 13 mA/cm² in the top cell are available with a top cell i-layer thickness of only 250 nm.     

Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.     

Self-adaptive amorphous Co2P@Co2P/Co-polyoxometalate/nickel foam as an effective electrode for electrocatalytic water splitting in alkaline electrolyte

Noble-metal-free transition metal based phosphides (TMPs) display great potential as candidates to replace the state-of-the-art noble metal-based catalysts for electronic water splitting. In this study, amorphous Co₂P was decorated on Co-polyoxometalate (POM) and conductive cobalt phosphide forming integrated Co₂P@ Co₂P/Co-POM/NF electrode, through in suit growth, low-temperature phosphating and electrocatalytic self-adaption pathway by the stripping of superficial Co-POM when subjected to persistent bubbles. The fantastic design simultaneously offers excellent electrical conductivity for fast electron transfer, a large surface area with numerous active edge sites and a conductive current collector facilitating mass transfer and gas release. The electrode showed high catalytic activity, requiring overpotential of 130 mV for HER to achieve a current density of 50 mA/cm², 336 mV for OER to achieve a current density of 50 mA/cm², affording a water-splitting current density of 10 mA/cm² at a low cell voltage of 1.6 V. The results and facile synthesis method also offer an exciting avenue for the design of amorphous phase TMPs on a current collector with high specific area and excellent electrical conductivity for energy storage and conversion devices.