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%.     

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%.     

0.4% absolute efficiency gain by novel back contact

Replacing the aluminum back contact of screen-printed multicrystalline silicon solar cells by a novel low-temperature layer sequence boosts the absolute power conversion efficiency η by Δη=0.4%. The optimized hydrogenated amorphous silicon (a-Si:H)-based back side junction provides efficient back side passivation and contacting at the same time. The improved passivation quality reduces the effective surface recombination velocity S_eff to S_eff<20 cm s^?1. Due to the optimized back side layer sequence, the open circuit voltage V_OC rises by ΔV_OC=15 mV up to V_OC=622 mV and the short circuit current increases by ΔJ_SC=0.8 mA cm^?2.     

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.     

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.     

Improvement of polymer/fullerene solar cells by controlling geometry of the ITO substrate surface

We have found that the short-circuit current, J_sc, of polymer/fullerene [RR-P3HT/C₆₀] solar cells has a clear dependence on the surface roughness of the ITO/glass substrate. We prepared an ITO surface with an average roughness, R_a, of 0.7–11 nm by chemical etching. At first J_sc increases with the increase in ITO surface roughness and then gradually decreases. The maximum performance was obtained at R_a≈4 nm. J_sc is also high with a very flat surface of R_a=0.7 nm. This feature can be attributed to the trade-off between the increase in absorption light path length and film-quality deterioration.     

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.     

Series-connected tandem dye-sensitized solar cell for improving efficiency to more than 10%

As one of the methods for improving the efficiency of a dye-sensitized solar cell (DSC), we investigated series-connected tandem DSCs. In this system, the top cell is made up of a transparent cell and the bottom cell utilizes only the light passing through the top cell. We investigated several combinations of dyes in tandem-type DSCs. The best efficiency obtained in our study is 10.4% (J_sc=10.8 mA/cm², V_oc=1.45 V, and FF=0.67) for a series-connected tandem DSC consisting of an N719 top cell and a black-dye bottom cell.     

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.     

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%.     

Synthesis of triarylamines with secondary electron-donating groups for dye-sensitized solar cells

Three organic dyes XS24–26 containing N,N-dimethylaniline and butoxybenzene have been designed, synthesized and applied in the dye-sensitized solar cells (DSSCs). The influence of secondary electron-donating groups on the performance of DSSCs is discussed. The dimethylaniline is beneficial to extend absorption spectrum, whereas butoxybenzene is useful to suppress electron recombination. XS26 containing butoxybenzene and thiophene unit gives the highest power efficiency η of 5.67%, with a J_SC of 12.36 mA cm^?2, V_OC of 680 mV, and ff of 0.67.     

A new organic dye bearing aldehyde electron-withdrawing group for dye-sensitized solar cell

A new metal-free dye (I) with a diketopyrrolopyrrole (DPP) core was synthesized, in which triphenylamine was used as electron donor, thiophene units as the π-conjugated bridge, aldehyde units as electron acceptor. The corresponding dye II containing carboxy group as the electron-withdrawing acceptor for the purpose of comparison was also synthesized. The absorption spectra, electrochemical and photovoltaic properties of I and II were extensively investigated. Electrochemical measurements data indicate that the tuning of HOMO and LUMO energy levels can be conveniently accomplished by alternating electron acceptor. The short-circuit photocurrent density and conversion efficiency of solar cell based on aldehyde-containing dye is more dominant than that bear a carboxy group as the electron withdrawing anchoring group. The new sensitizer I exhibited a photovoltaic performance: a short-circuit photocurrent density (J_sc) of 6.07 mA cm^?2, an open-circuit photovoltage (V_oc) of 568 mV, and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency of 2.27% under standard global AM 1.5 solar light condition. This work suggests that aldehyde units as new type of electron withdrawing anchoring group are promising candidates for improvement of the performance of DSSCs.     

A novel Al and Y codoped ZnO/n-Si heterojunction solar cells fabricated by pulsed laser deposition

Al and Y codoped ZnO (AZOY) transparent conducting oxide (TCO) thin films were first deposited on n-Si substrates by pulsed laser deposition (PLD) to form AZOY/n-Si heterojunction solar cells. However, the properties of the AZOY emitter layers are critical to the performance of AZOY/n-Si heterojunction solar cells. To estimate the properties of AZOY thin films, films deposited on glass substrates with various substrate temperatures (T_s) were analyzed. Based on the experimental results, optimal electrical properties (resistivity of 2.8 ± 0.14 × 10^?4 Ω cm, carrier mobility of 27.5 ± 0.55 cm²/Vs, and carrier concentration of 8.0 ± 0.24 × 10²⁰ cm^?3) of the AZOY thin films can be achieved at a T_s of 400 °C, and a high optical transmittance of AZOY is estimated to be >80% (with glass substrate) in the visible region under the same T_s. For the AZOY/n-Si heterojunction solar cells, the AZOY thin films acted not only as an emitter layer material, but also as an anti-reflected coating thin film. Thus, a notably high short-circuit current density (J_sc) of 31.51 ± 0.186 mA/cm² was achieved for the AZOY/n-Si heterojunction solar cells. Under an AM1.5 illumination condition, the conversion efficiency of the cells is estimated at only approximately 4% (a very low open-circuit voltage (V_oc) of 0.24 ± 0.001 V and a fill factor (FF) of 0.51 ± 0.011) without any optimization of the device structure.     

Investigation of bubble effect in microfluidic fuel cells by a simplified microfluidic reactor

This study experimentally examines the influence of two-phase flow on the fluid flow in membraneless microfluidic fuel cells. The gas production rate from such fuel cell is firstly estimated via corresponding electrochemical equations and stoichiometry from the published measured current–voltage curves in the literature to identify the existence of gas bubble. It is observed that O₂ bubble is likely to be generated in Hasegawa’s experiment when the current density exceeds 30 mA cm^?2 and 3 mA cm^?2 for volumetric flow rates of 100 μL min^?1 and 10 μL min^?1, respectively. Besides, CO₂ bubble is also likely to be presented in the Jayashree’s experiment at a current density above 110 mA cm^?2 at their operating volumetric liquid flow rate, 0.3 mL min^?1. Secondly, a 1000-μm-width and 50-μm-depth platinum-deposited microfluidic reactor is fabricated and tested to estimate the gas bubble effect on the mixing in the similar microchannel at different volumetric flow rates. Analysis of the mixing along with the flow visualization confirm that the membraneless fuel cell should be free from any bubble, since the mixing index of the two inlet streams with bubble generation is almost five times higher than that without any bubble at the downstream.     

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.     

Developing solar cells with recycled materials and household chemicals for drinking water chlorination by communities with limited resources

Ferric tannate-sensitized n-(ZnO, SnO₂)/Cu photoelectrochemical cells were constructed for drinking water chlorination using recycled waste materials and household chemicals and utilising Fe²⁺–Fe³⁺ and Cu²⁺–Cu redox couples for charge transfer. The solar cells, which were constructed in recycled clear plastic tubing and drinking straws in a home environment, produced an open-circuit voltages of 0.4–0.6 V and a short-circuit current densities of 1–2.5 mA cm^?2. Chlorine was produced at a rate of 4 mg h^?1 from a 1% salt solution using an array of cells with a combined voltage of 5 V and a current of 200 mA. This study has demonstrated that it is possible to construct viable solar cells for drinking water chlorination using waste materials and readily available chemicals. Further studies are needed to determine how practicable this would be in regions with drinking water quality and sanitation problems.     

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.     

LiMn2O4 cathode materials synthesized by the cellulose–citric acid method for lithium ion batteries

A new technique was employed to synthesize spinel LiMn₂O₄ cathode materials by adding cellulose and citric acid to an aqueous solution of lithium and manganese salts. Various synthesis conditions such as the calcination temperature and the citric acid-to-metal ion molar ratio (R) were investigated to determine the ideal conditions for preparing LiMn₂O₄ with the best electrochemical characteristics. The optimal synthesis conditions were found to be R = 1/3 and a calcination temperature of 800 °C. The initial discharge capacity of the material synthesized using the optimal conditions was 134 mAh g⁻¹, and the discharge capacity after 40 cycles was 125 mAh g⁻¹, at a current density of 0.15 mA cm⁻² between 3.0 and 4.35 V. Details of how the initial synthesis conditions affected the capacity and cycling performance of LiMn₂O₄ are discussed.     

Electrochemical lithium insertion in TiO2 with the ramsdellite structure

The electrochemical insertion of lithium in the ramsdellite polymorph of titanium dioxide, TiO₂ (R), is studied by electrochemical methods. At room temperature the maximal Li uptake under constant current densities of 0.1, 0.5 and 1.0 mA cm⁻² is 0.85, 0.8 and 0.7 Li/Ti, respectively. Between 2.3 and 1.3 V versus lithium, the specific capacity achieved is as high as 285 A h kg⁻¹ at 0.5 mA cm⁻². This corresponds to 85% of the maximum theoretical capacity (336 A h kg⁻¹), which may be reached by incorporation of one lithium per titanium under equilibrium conditions.     

Effect of titanium substitution in layered LiNiO2 cathode material prepared by molten-salt synthesis

LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) compounds have been synthesized by a direct molten-salt method that uses a eutectic mixture of LiNO₃ and LiOH salts. According to X-ray diffraction analysis, these materials have a well-developed layered structure (R3-m) and are an isostructure of LiNiO₂. The LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) compounds have average particle sizes of 1–5 μm depending on the amount of Ti salt. Charge–discharge tests show that a LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) cathode prepared at 700 °C has an initial discharge capacity as high as 171 mA h g⁻¹ and excellent capacity retention in the range 4.3–2.8 V at a current density of 0.2 mA cm⁻².