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.     

Perylene imide dyes for solid-state dye-sensitized solar cells: Spectroscopy,energy levels and photovoltaic performance

We report the solid-state dye-sensitized solar cell performances of perylene imide using nanoporous TiO₂ electrodes. Solid-state dye-sensitized solar cells were fabricated using the organic hole-transporting medium (HTM) 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD). The experimental E_LUMO levels of perylene imide dyes are found to be 3.75 and 3.77 eV, respectively. Therefore, perylene imide dyes can inject electrons to the conduction band of titanium dioxide in organic dye-sensitized solar cells. TiO₂ thin films of about 2 μm in thicknesses were prepared. Both preparation and thickness of the compact TiO₂ layer were optimized using spray pyrolysis. The studies revealed that an optimum film thickness of 130–150 nm of compact TiO₂ yielded the best rectifying behavior and SDSC performance. In this work, our goal was to investigate the performance of perylene sensitizers in connection with spiro-OMeTAD hole transport material. Short-circuit current densities, open circuit voltages and overall conversion efficiencies of the solar cell with 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene(spiro-OMeTAD) as a hole conductor and perylene imide as sensitizer on mesoporous TiO₂ were investigated.     

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).     

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.     

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%.     

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.     

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.     

Twenty-two percent efficiency HIT solar cell

We have achieved the world's highest solar cell conversion efficiency of 22.3% (V_oc: 0.725 V, I_sc: 3.909 A, FF: 0.791, total area: 100.5 cm², confirmed by AIST) by using a heterojunction with intrinsic thin layer (HIT) structure. This is the world's first practical-size (>100 cm²) silicon solar cell that exceeds a conversion efficiency of 22% as a confirmed value. This high efficiency has been achieved mainly due to improvements in a-Si:H/c-Si hetero-interface properties and optical confinement.The excellent a-Si:H/c-Si hetero-interface of the HIT structure enables a high V_oc of over 0.720 V and results in better temperature properties. In order to reduce the power-generating cost, we are now investigating numerous technologies to further improve the conversion efficiency, especially the V_oc, of HIT solar cells, with the aim of achieving 23% efficiency in the laboratory by 2010.     

Design,preparation, and durability of TiO2/SiO2 and ZrO2/SiO2 double-layer antireflective coatings in crystalline silicon solar modules

This paper reports the use of a combination of numerical calculations and experimental work to establish the optimum photovoltaic transmittance (T_pv) and durability of the quarter wave, the quarter-half wave, and the non-quarter wave double-layer TiO₂–SiO₂ and ZrO₂–SiO₂ antireflective coatings (ARCs) on solar glass towards practical photovoltaic applications. Numerical calculations based on 4 × 4 propagation matrix method indicated that the non-quarter wave double-layer ARCs exhibited higher T_pv values than those of the quarter wave and the quarter-half wave ARCs. Such calculated values are in good agreement with the experimental T_pv values. For examples, the T_pv values for the non-quarter wave double-layer TiO₂–SiO₂ and ZrO₂–SiO₂ ARCs prepared by sol–gel reached 94.4 ± 0.1% and 94.3 ± 0.1%, respectively. In terms of the coating durability, the non-quarter wave double-layer coatings with a dense and thicker TiO₂ or ZrO₂ barrier layer on solar glass exhibited less than 1% reduction in T_pv after 96 h highly-accelerated temperature and humidity stress test (HAST), as compared with the standard single-layer porous SiO₂ used in industry which tested in the same HAST conditions to be greater than (15.4%) after 48 h. Single crystalline Si modules encapsulated by the non-quarter wave TiO₂–SiO₂ or ZrO₂–SiO₂ AR-coated glass are more durable, with only less than 10% degradation in efficiency after 48 h HAST, as compared with Si modules encapsulated by single-layer porous SiO₂ AR-coated glass which have signification loss in efficiency (circa. 21.8%).     

Study on dye-sensitized solar cell using novel infrared dye

The performances of infrared-dye-sensitized solar cells fabricated using two novel cyanine dyes (light absorption edge: 900 nm) were investigated. The performance of the cell using NK6037 dye was superior to that using NK4432; hence, an intimate interaction between the dye and the TiO₂ photoelectrode via the functional group is essential for efficient electron injection. The efficiency reached 1.9% by optimizing the light-confining effect of the TiO₂ photoelectrode and TiO₂ film thickness, and reached 2.3% by adjusting the concentration of deoxycholic acid (DCA) in the dye solution. The roles of the dye and DCA were clarified by photochemical and electrochemical characterization.     

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.     

Effects of addition of TiO2 nanoparticles on mechanical properties and ionic conductivity of solvent-free polymer electrolytes based on porous P(VdF-HFP)/P(EO-EC) membranes

To enhance the performance (i.e., mechanical properties and ionic conductivity) of pore-filling polymer electrolytes, titanium dioxide (TiO₂) nanoparticles are added to both a porous membrane and its included viscous electrolyte, poly(ethylene oxide-co-ethylene carbonate) copolymer (P(EO-EC)). A porous membrane with 10 wt.% TiO₂ shows better performance (e.g., homogeneous distribution, high uptake, and good mechanical properties) than the others studied and is therefore chosen as the matrix to prepare polymer electrolytes. A maximum conductivity of 5.1 × 10⁻⁵ S cm⁻¹ at 25 °C is obtained for a polymer electrolyte containing 1.5 wt.% TiO₂ in a viscous electrolyte, compared with 3.2 × 10⁻⁵ S cm⁻¹ for a polymer electrolyte without TiO₂. The glass transition temperature, T_g is lowered by the addition of TiO₂ (up to 1.5 wt.% in a viscous electrolyte) due to interaction between P(EO-EC) and TiO₂, which weakens the interaction between oxide groups of the P(EO-EC) and lithium cations. The overall results indicate that the sample prepared with 10 wt.% TiO₂ for a porous membrane and 1.5 wt.% TiO₂ for a viscous electrolyte is a promising polymer electrolyte for rechargeable lithium batteries.     

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.     

Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency

The p-type a-Si:H/n-type c-Si (P⁺ a-Si:H/N⁺ c-Si) heterojunction was simulated for developing solar cells with high conversion efficiency and low cost. The characteristic of such cells with different work function of transparent conductive oxide (TCO) were calculated. The energy band structure, quantum efficiency and electric field are analyzed in detail to understand the mechanism of the heterojunction cell. Our results show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work function, and the TCO work function should be large enough in order to achieve high conversion efficiency of P⁺ a-Si:H/N⁺ c-Si solar cells. With the optimized parameters set, the P⁺ a-Si:H/N⁺ c-Si solar cell reaches a high efficiency (η) up to 21.849% (FF: 0.866, V_OC: 0.861 V, J_SC: 29.32 mA/cm²).     

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.     

The effect of ZnO-coating on the performance of a dye-sensitized solar cell

This study investigates the effect of a ZnO-coated TiO₂ working electrode on the power conversion efficiency of a dye-sensitized solar cell (DSSC). This electrode was designed and fabricated by dipping the TiO₂ electrode with the TiCl₄ treatment in a solution of zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O] and ethanol. The effects of the concentration of Zn(CH₃COO)₂·2H₂O and the duration of dipping on the band gap of a working electrode and the power conversion efficiency of a DSSC were also examined. The band gap of the working electrode increases to 3.75 eV [TiO₂ electrode dipped in 0.05 M Zn(CH₃COO)₂·2H₂O) for 3 min] from 3.22 eV (TiO₂ electrode). Interestingly, the power conversion efficiency of the DSSC with a Zn-coated TiO₂ electrode (6.7%) substantially exceeds that of the conventional DSSC with a TiO₂ electrode (5.9%), and it may be originated from an increased energy barrier between ZnO and TiO₂ that reduces the electron recombination rate.     

Photovoltaic performance and long-term stability of quasi-solid-state fluoranthene dyes-sensitized solar cells

Two new fluoranthene-based organic dye sensitizers (I and II), in which 7, 12-diphenylbenzo[k]fluoranthene moiety is acted as electron donor, thiophene and phenylethynyl units as electron spacers and carboxylic acid as electron acceptor were successfully applied in quasi-solid-state dye-sensitized solar cells. The quasi-solid-state DSSCs based on the dye I showed the better photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 66%, a short-circuit photocurrent density (J_sc) of 3.53 mA cm^?2, an open-circuit photovoltage (V_oc) of 542 mV, and a fill factor (ff) of 0.70, corresponding to an overall conversion efficiency of 1.33% under standard global AM 1.5 solar condition. Moreover, the two sensitizers exhibited good stability during a long-term accelerated aging, in which the photovoltaic parameters retained more than 90% of its initial value even after 1000 h under light soaking at 60 °C.     

Cultivation of Spirulina platensis by continuous process using ammonium chloride as nitrogen source

This work is focused on the influence of dilution rate (0.08⩽D⩽0.32 d⁻¹) on the kinetics of continuous cultivation of Spirulina platensis at two different concentrations of ammonium chloride (N₀=1.0 and 10 mM) as nitrogen source. Cell productivity increased in both series of runs up to D≅0.12–0.16 d⁻¹, and then decreased. While at N₀=1.0 mM biomass washing was certainly the cause of progressive cell concentration decrease, a combination of this phenomenon with the toxic effect of excess ammonia was responsible, at N₀=10 mM and D⩾0.20 d⁻¹, for quick stop of cell growth just beyond the achievement of maximum cell productivity (92.4 mg l⁻¹ d⁻¹). Similar profile was observed for protein productivity, that achieved a maximum value of 67.0 mg l⁻¹ d⁻¹, because of the very high protein content (72.5%) of biomass produced under these conditions. The yield of nitrogen-to-biomass was much higher at the lower N₀, because of the low protein content, and reached a maximum value of 9.7 g g⁻¹ at D=0.08–0.12 d⁻¹. The yield of nitrogen-to-protein showed less marked difference, being most of the nitrogen present in the cell as proteins or free amino-acids.     

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.