Bifacial transparent solid-state dye-sensitized solar cell with sputtered indium-tin-oxide counter electrode

In this study, we report a solid state dye-sensitized solar cell (SSDSC) made with a transparent ITO film as a counter electrode using the sputtering technology. For the first time, a bifacial transparent SSDSC is realized and irradiated from FTO and ITO side. The SSDSCs give short circuit photocurrent density (J_sc) of 4.16 mA cm^?2, open circuit voltage (V_oc) of 0.74 V, and fill factor (FF) of 0.64, corresponding to the photoelectric conversion efficiency of 1.96% from FTO side illumination (AM 1.5G, 100 mW cm^?2). Moreover, it is found that J_sc of SSDSCs (2.85 mA cm^?2) when irradiated from ITO side is less than that from FTO side. This result is because of the cut-off of incident photons in the blue region by the ITO film and the light screening effect by the hole transport material (HTM) absorption. Our results demonstrate the possibility of production scalable sputtering process for SSDSCs electrodes fabrication and pave the avenue for tandem design application which requires a transparent intermediate layer for interconnection.     

Optoelectronic properties of chemically deposited Bi2S3 thin films and the photovoltaic performance of Bi2S3/P3OT solar cells

Thin films of bismuth sulfide (Bi₂S₃), prepared on conductive tin-doped indium oxide (ITO)-glass substrates by chemical deposition showed a variation of optical band gap with thickness: 1.8 eV for a 50 nm film (deposited at 40 °C for 30 min) to 1.5 eV for a 200 nm film deposited for 2 h. The electronegativity for Bi₂S₃ compound is 5.3 eV, as estimated from the ionization energy and electron affinity of elemental Bi and S, and thus the electron affinity of chemically deposited Bi₂S₃ film was deduced to be 4.5 eV. In the energy level analysis of ITO/Bi₂S₃/P3OT/Au structure, Bi₂S₃ was established as an electron acceptor. To produce ITO/Bi₂S₃/P3OT/Au solar cell structures, poly3-octylthiophene (P3OT), prepared in the laboratory was dissolved in toluene and was drop-cast on the Bi₂S₃ film and a gold film was thermally evaporated. Under 100 mW/cm² tungsten-halogen irradiation incident from the ITO-side, the cell using a Bi₂S₃ film with thickness of 50 nm has the highest open circuit voltage (V_oc) of 440 mV and short-circuit current density (J_sc) of 0.022 mA/cm². The addition of a CdS thin film (90 nm) between ITO and B₂S₃ would increase V_oc to 480 mV, and J_sc to 0.035 mA/cm². The role of surface morphology and optoelectronic properties of the Bi₂S₃ film in the photovoltaic performance of the junction is discussed.     

Tuning photocurrent response through size control of CdTe quantum dots sensitized solar cells

The photovoltaic performance of CdTe quantum dots (QDs) sensitized solar cells (QDSSCs) as a function of tuning the band gap of CdTe QDs size is studied. The tuning of band gap was carried out through controlling the size of QDs. Presynthesized CdTe QDs of radii from 2.1 nm to 2.5 nm) were deposited by direct adsorption (DA) technique onto a layer of TiO₂ nanoparticles (NPs) to serve as sensitizers for the solar cells. The characteristic parameters of the assembled QDSSCs were measured under AM 1.5 sun illuminations. The values of current density (J_sc) and overall efficiency (η) increase with decreasing CdTe QDs size, since the lowest unoccupied molecular orbital (LUMO) levels shifts closer to vacuum level, which causes an increase in the driving force. Consequently the electrons’ injections to the conduction band (CB) of TiO₂ NPs become faster. The maximum values of J_sc (1105 μA/cm²) and η (0.190%) were obtained for the smallest CdTe QDs size (2.10 nm). The open circuit voltages (V_oc) varies slightly with the size of the CdTe QDs, however it is only dictated by the CB level of TiO₂ NPs and the VB of the electrolyte. Furthermore, the photocurrent response of the assembled cells to ON–OFF cycles of the illumination indicates the prompt generation of anodic current.     

Effect of ultra-thin polymer membrane electrolytes on dye-sensitized solar cells

In-situ ultra-thin porous poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF–HFP) membranes were prepared by a phase inversion method on TiO₂ electrodes coated with Ru N-719 dye. These membranes were then soaked in the organic liquid electrolyte to form the in-situ ultra-thin porous P(VDF–HFP) membrane electrolytes. Dye-sensitized solar cell (DSC) using the membrane electrolyte exhibited an open-circuit voltage (V_oc) of 0.751 V, a short-circuit current (J_sc) of 16.260 mA cm^?2 and a fill factor (FF) of 0.684 under an incident light intensity of 1000 W m^?2 yielding an energy conversion efficiency (η) of 8.35%. The V_oc, FF and η of the solar cell using the membrane electrolyte increased by about 5.8%, 2.2% and 5.7%, respectively, when compared with the corresponding values of a cell using liquid electrolyte. However, the J_sc decreased by about 2.1%.     

TiO2–Au plasmonic nanocomposite for enhanced dye-sensitized solar cell (DSSC) performance

Anatase TiO₂ nanoparticles dressed with gold nanoparticles were synthesized by hydrothermal process by using mixed precursor and controlled conditions. Diffused Reflectance Spectra (DRS) reveal that in addition to the expected TiO₂ interband absorption below 360 nm gold surface plasmon feature occurs near 564 nm. It is shown that the dye sensitized solar cells made using TiO₂–Au plasmonic nanocomposite yield superior performance with conversion efficiency (CE) of ～6% (no light harvesting), current density (J_SC) of ～13.2 mA/cm², open circuit voltage (V_oc) of ～0.74 V and fill factor (FF) 0.61; considerably better than that with only TiO₂ nanoparticles (CE ～ 5%, J_SC ～ 12.6 mA/cm², V_oc ～ 0.70 V, FF ～ 0.56).     

Organic thin-film solar cell employing a novel electron-donor material

A highly efficient organic thin-film solar cell based on a heterojunction structure employing a novel electron-donor (ED) material, tetraphenyldibenzoperiflanthene (DBP), has been demonstrated for the first time. An organic photovoltaic (OPV) cell with 0.033-cm² active area, comprising DBP as an ED layer, fullerene C₆₀ as an electron-acceptor (EA) layer, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as an exciton-blocking (EB) layer, has exhibited an open-circuit voltage (V_oc) of 0.92 V, a short-circuit current density (J_sc) of 6.3 mA/cm² and a conversion efficiency of 3.6% at 100-mW/cm² simulated AM1.5G sunlight. Meanwhile, those of a conventional cell employing copper phthalocyanine (CuPc) for an ED layer are 0.51 V, 4.3 mA/cm², and 1.4%, respectively. The high V_oc and J_sc of the DBP-based cell is attributed to the DBP's highest occupied molecular orbital (HOMO) level 5.5 eV and the effective light absorption, respectively.     

Analytical derivation of explicit J–V model of a solar cell from physics based implicit model

Recently a simple explicit model was introduced to represent the J–V characteristics of an illuminated solar cell with parasitic resistances and bias dependent photocurrent as v^m + jⁿ = 1. Here the normalized voltage, v and normalized current density j can be represented as v = V/V_oc and j = J/J_sc respectively, where V_oc is the open circuit voltage and J_sc is the short circuit current density. This model is useful for design, characterization and simple fill factor calculation and its applicability was demonstrated with the measured data of a wide variety of solar cells. This explicit form is intuitive and hence the model lacks the analytical support. In this paper an analytical derivation of this closed form explicit model is presented, which is derived from the physics based implicit J–V equation. The derivation expands the scope of model applicability and provides a new insight of analytical modeling of the solar cell.     

New organic photosensitizers incorporating carbazole and dimethylarylamine moieties for dye-sensitized solar cells

Four organic dyes (XS10–13) employing carbazole unit as electron donor and N,N-dimethylarylamine moieties as electron-donating groups were designed and synthesized for nanocrystalline TiO₂ dye-sensitized solar cells. The electron-donating groups of dimethylarylamine increase the electron density of donor moiety and enhance the molar extinction coefficient of dyes. For a typical device the maximum IPCE value could reach 86%, with a short-circuit photocurrent density (J_sc) of 9.8 mA cm^?2, an open-circuit photovoltage (V_oc) of 642 mV, and fill factor (FF) of 0.63, which corresponds to an overall conversion efficiency (η) of 4.0%. For a comparison, the N719-sensitized TiO₂ solar cell showed an efficiency of 6.4%.     

Effect of composition gradient in Cu(In,Al)Se2 solar cells

Cu(In,Al)Se₂ (CIAS) thin films were prepared by a three-stage evaporation process. In this experiment, the composition ratio of Cu/(In+Al) at the end of the second stage (Cu/III_2nd) was changed from 1.1 to 1.7. The CIAS films showed an Al distribution with a V-shape profile. The valley depth of the V-shape from the surface increased with increasing the Cu/III_2nd ratio. The valleys of the V-shape for the films with the Cu/III_2nd ratio of 1.1–1.7 were located at approximately 0.3–1.0 μm from the film surface, respectively. The rms surface roughness increased from 40 nm for Cu/III_2nd=1.1 to 90 nm at Cu/III_2nd=1.3 and then saturated for greater Cu/III_2nd ratios. Solar cells with the Al/ITO/ZnO/CdS/CIAS/Mo/soda-lime glass structure were fabricated. The fill factor was seen to decrease while the product of short-circuit current and open-circuit voltage remained constant. The reverse saturation current increased when the Cu/III_2nd ratio is greater than 1.3 which is a behavior of the surface roughness. Cu/III_2nd ratios greater than 1.3 lead to the distant position of V-shape from the surface and the increase in surface roughness.     

An analytical expression for the instantaneous efficiency of a flat plate solar water heater and the influence of absorber plate absorptance and emittance

Standard test results to quantify the instantaneous efficiency, η, of a glazed flat plate solar water heater are normally expressed in terms of a reduced temperature parameter, x, and global insolation, G, as η = η₀ ? a₁x ? a₂Gx². We show that the Hottel–Whillier–Bliss relation for the efficiency can be expressed in the same form with each of the coefficients η₀, a₁, and a₂ in terms of algebraic expressions of standard mechanical, fluid and thermal parameters of a single glazed, finned heater, including the absorber plate absorptance, α, and thermal emittance, ε. The advantage of the derived expression is that the effect on the efficiency of changes in various heater parameters can be readily evaluated. Furthermore, it is shown that for selectivity α/ε > 2, each coefficient η₀, a₁, and a₂ can be expressed as η₀ = η_0C ? εη_0R, etc., in order to separate out the role of absorber radiation from other losses. This allows one to easily compare selective solar absorbers with different α and ε and, for example, to suggest an optimum coating thickness for thickness sensitive selective solar absorbers. In particular it can be seen that care should be taken in reducing ε at the expense of also reducing α in order to increase the selectivity, α/ε, since this will often be detrimental to the efficiency. The analytical expressions for η₀, a₁, and a₂ can be easily programmed on a spreadsheet and, for convenience, are summarised in an appendix.     

Anthocyanins and betalains as light-harvesting pigments for dye-sensitized solar cells

We present the photoelectrochemical properties of dye-sensitized solar cells using natural pigments containing betalains and anthocyanins as sensitizers. The dyes extracted from grape, mulberry, blackberry, red Sicilian orange, Sicilian prickly pear, eggplant and radicchio have shown a monochromatic incident photon to current efficiency (IPCE) ranging from 40% to 69%. Short circuit photocurrent densities (J_sc) up to 8.8 mA/cm², and open circuit voltage (V_oc) ranging from 316 to 419 mV, were obtained from these natural dyes under 100 mW/cm² (AM 1.5) simulated sunlight. The best solar conversion efficiency of 2.06% was achieved with Sicilian prickly pear fruits extract. The influence of pH and co-absorbers on natural sensitizers, were investigated and discussed.     

Investigation of coated tubes in cross-flow boiling

The boiling in cross-flow is investigated for coated tubes (low-porosity, flame-sprayed) in this paper. The effect of surface roughness on flow boiling heat transfer for a horizontal tube surface in cross-flow is studied for saturated boiling of water at atmospheric pressure. The parameters varied were for flow velocity up to 3.24 kg/s (G = 258.49 kg/m² s), heat flux from 12 to 45 kW/m², surface roughness (Ra) from 0.3296 to 4.731 μm. Nominal enhancement in heat transfer coefficient at higher mass flux may be attributed to the continued nucleation at the uppermost surfaces (in the wake region of the flow) of the rougher tubes thereby increasing the overall heat transfer rate. The flow boiling data was found to best fit the Kutateladze asymptotic equation h = h_l[1 + (h_npb/h_l)ⁿ]^1/n with the value of n = 2.258 (which is close to the value of n = 2 suggested by Kutateladze).     

A chart for predicting the possible advantage of adopting a suction/liquid heat exchanger in refrigerating system

This paper presents the effects produced by a suction/liquid heat exchanger installed in a refrigerating cycle, evidencing that, its use can improve or decrease the system performance depending on the operating conditions. Attention is focused on developing an easy operating method in order to predict the behaviour of the system introducing the heat exchanger, changing the operating conditions and/or the refrigerant fluids. To this aim, 19 different ozone friendly fluids (R-22, R-32, R-152a, R-125, R-134a, R-236a, R-227a, RC-318, R-410A, R-413A, R-407C, R-417, R-502, R-507A, R-717, R-290, R-600, R-600a and R-1270) have been considered, varying evaporating and condensation temperatures, respectively in the range −40 °C/10 °C and 25 °C/50 °C. The advisability of the installation of the heat exchanger can be evaluated as a function of thermodynamic properties. Furthermore, a simple chart allowing to verify the effectiveness of installation of heat exchanger has been developed for each refrigerating fluids and for the specified operating conditions.     

Measuring temperature of the ice surface during its formation by using infrared instrumentation

A non-destructive remote sensing technique was used to measure the surface temperature of a thin macroscopic water film flowing on a growing asymmetric ice accretion during its formation inside an icing research wind tunnel. Given the underlying thermodynamic conditions of this experimental series, the recorded surface temperature was always below the temperature of water fusion, T_m = 273.15 K, even when water shedding from growing ice accretions was observed visually. The surface temperature of ice accretions, T_s, ranged from −1 °C, for angular positions near the stagnation line, down to a certain minimum above the ambient temperature, T_a, for the greater angular positions, i.e. T_m > T_s > T_a.     

Fabrication and characterisations of n-CdS/p-PbS heterojunction solar cells using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cells were prepared via microwave-assisted chemical bath deposition method. A cadmium sulfide (CdS) window layer (340 nm thickness) was deposited on an indium tin oxide (ITO) glass. A lead sulfide (PbS) absorber layer (985–1380 nm thickness) with different molar concentrations (0.02, 0.05, 0.075, and 0.1 M) was then grown on ITO/CdS to fabricate a p–n junction. The effects of changing molar concentration of the absorber layer on structural and optical properties of the corresponding PbS thin films and solar cells were investigated. The optical band gap of the films decreased as the molarity increased. The photovoltaic properties (J–V characteristics, short circuit current, open circuit voltage, fill factor, and efficiency) of the CdS/PbS heterostructure cells were examined under 30 mW/cm² solar radiation. Interestingly, changing molar concentration improved the photovoltaic cells performances, the solar cell exhibited its highest efficiency (1.68%) at 0.1 M molar concentration.     

Optimization of lead- and cadmium-free front contact silver paste formulation to achieve high fill factors for industrial screen-printed Si solar cells

Screen-printed n⁺–p–p⁺ solar cells were fabricated on Cz single crystalline Si material, with a 45 Ω/sq emitter and PECVD SiNx antireflective coating with a thickness of 700 Å, using different Ag pastes and commercial leaded reference paste (CN33-462, Ferro Corp.). Ag and Al contacts were co-fired using a mass-production line equipped with mesh belt conveyer furnace systems (Centrotherm thermal solution GmbH & Co. KG). The average results for single crystalline Si solar cells (156 cm²) are: I_sc=5.043 A, V_oc=0.621 V, R_s=0.0087 Ω, R_sh=15.3 Ω, FF=0.773, and E_ff=16.45%. R_sh and fill factor values of fabricated cells were slightly higher when compared with the commercial leaded Ag paste, although cells were fabricated by metallizing the lead-free silver pastes. For the lead-free Ag paste used in this study, the line pattern continuity is retained with improved edge definition in sharp contrast to that of reference Ag paste. Average value of R_s was also equivalent approximately to that of the leaded Ag paste.     

Novel quasi-solid electrolyte for dye-sensitized solar cells

A novel alkyloxy-imidazole polymer was prepared by in situ co-polymerization of alkyloxy-imidazole and diiodide to develop an ionic polymer gel electrolyte for quasi-solid dye-sensitized solar cells (DSCs). The DSCs with the polymer gel electrolyte of 1-methyl-3-propylimidazolium iodide (MPII) showed good photovoltaic performance including the short-circuit photocurrent density (J_sc) of 3.6 mA cm⁻², the open-circuit voltage (V_oc) of 714.8 mV, the fill factor (FF) of 0.60 and the light-to-electricity conversion efficiency (η) of 1.56% under AM 1.5 (100 mW cm⁻²). As a comparison, the DSCs with the polymer gel electrolyte of 1,2-dimethyl-3-propylimidazolium iodide (DMPII) yielded a light-to-electricity conversion efficiency of 1.33%. The results indicated that the as-prepared polymers were suitable for the solidification of liquid electrolytes in DSCs.     

Homogeneous p+ emitter diffused using boron tribromide for record 16.4% screen-printed large area n-type mc-Si solar cell

A record efficiency of 16.4% (156.25 cm²) has been achieved for an n-type wafer-based (hereafter, “n-based”) mc-Si solar cell. A horizontal quartz tube furnace with an industry-compatible scale is employed for forming a p⁺-emitter using boron tribromide (BBr₃) as the boron source, in which system less contamination is confirmed than in other options of boron diffusion. A significantly homogeneous emitter is achieved with the standard deviation of 1.5 Ω/sq. n-Based mc-Si solar cells are fabricated with phosphorus-diffused BSF, SiN deposition, and fire-through screen-printed contacts. The properties of the best cell are; η: 16.4%, V_oc: 607 mV, J_sc: 35.2 mA/cm², and FF: 76.7%.     

Optimal design of geometric parameters of double-layered microchannel heat sinks

This work uses an optimization procedure consisting of a simplified conjugate-gradient method and a three-dimensional fluid flow and heat transfer model to investigate the optimal geometric parameters of a double-layered microchannel heat sink (DL-MCHS). The overall thermal resistance R_T is the objective function to be minimized, and the number of channels N, channel width ratio β, lower channel aspect ratio α_l, and upper channel aspect ratio α_u are the search variables. For a given bottom area (10 × 10 mm) and heat flux (100 W/cm²), the optimal (minimum) thermal resistance of the double-layered microchannel heat sink is about R_T = 0.12 °C/m²W. The corresponding optimal geometric parameters are N = 73, β = 0.50, α_l = 3.52, and, α_u = 7.21 under a total pumping power of 0.1 W. These parameters reduce the overall thermal resistance by 52.8% compared to that yielded by an initial guess (N = 112, β = 0.37, α_l = 10.32, and α_u = 10.93). Furthermore, the optimal thermal resistance decreases rapidly with the pumping power and then tends to approach an constant value. As the pumping power increases, the optimal values of N, α_l, and α_u increase, whereas the optimal β value decreases. However, increasing the pumping power further is not always cost-effective for practical heat sink designs.     

High voltage stability of nanostructured thin film catalysts for PEM fuel cells

This paper provides a comparative evaluation of electrocatalyst surface area stability in PEM fuel cells under accelerated durability testing. The two basic electrocatalyst types are conventional carbon-supported dispersed Pt catalysts (Pt/C), and nanostructured thin film (NSTF) catalysts. Both types of fuel cell electrocatalysts were exposed to continuous cycling between 0.6 and 1.2 V, at various temperatures between 65 and 95 °C, with H₂/N₂ on the anode and cathode, while periodic measurements of electrochemical surface area were recorded as a function of the number of cycles. The NSTF electrocatalyst surface areas were observed to be significantly more stable than the Pt/C electrocatalysts. A first order rate kinetic model was applied to the normalized surface area changes as a function of number of cycles and temperature, and two parameters extracted, viz. the minimum stable surface area, S_min, and the activation energy, E_a, for surface area loss in this voltage range. S_min was found to be 10% versus 66%, and E_a 23 kJ mole⁻¹ versus 52 kJ mole⁻¹, for Pt/C versus NSTF-Pt, respectively. The loss of surface area in both cases is primarily the result of Pt grain size increases, but the Pt/C XRD grain sizes increase significantly more than the NSTF grain sizes. In addition, substantial peak shifts occur in the Pt/C CVs, which ultimately end up aligning with the NSTF peak positions, which do not change substantially due to the voltage cycling. NSTF catalysts should be more robust against shut down/start-up, operation near OCV and local H₂ starvation effects.