Incident Photo-to-current Conversion Efficiency Research Articles

A dye-free photoelectrochemical solar cell based on BiVO4 with a long lifetime of photogenerated carriers

A dye-free photoelectrochemical (PEC) solar cell based on BiVO4 oxide film was developed. Without a bias, a high incident photo-to-current conversion efficiency (IPCE) of 43% was obtained and the solar energy conversion efficiency of the cell was 14 times higher than the record efficiency of Fe2O3 PEC solar cells. Ultra-fast transient absorption spectroscopy was utilized to investigate the mechanism of the different photoelectric performances. The results revealed that there was a longer lifetime of photogenerated carriers in BiVO4 than that in Fe2O3. It was helpful to develop other suitable photoelectrode materials for dye-free PEC solar cells.

A novel 2,7-diaminofluorene-based organic dye for a dye-sensitized solar cell

A novel 2,7-diaminofluorene-based organic dye (coded as JD2) with the diarylaminofluorene unit as the electron donor and the cyanoacrylic acid as the acceptor as well as anchoring group in a donor-π–donor-π–acceptor architecture is used in a dye-sensitized solar cell (DSSC). The TiO2 film thickness and the dye soaking time are optimized, and a coadsorbate of bis-(3,3-dimethyl-butyl)-phosphinic acid (DINHOP) is added to the dye solution to reform the cell performance. Effects of the additives, N-methylbenzimidazole (NMBI), guanidinium thiocyanate (GuSCN) or 4-tert-butylpyridine (TBP) in the electrolyte on the photovoltaic performance of the DSSC are studied. Effects of change of DINHOP concentration in JD2 dye solution, change of ratio of LiI to 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and change of concentration of I2 in the electrolyte on the photovoltaic performance of the DSSC are studied. A light-to-electricity conversion efficiency (η) of 6.31% is achieved for the JD2 dye-based DSSC, which is one of the best efficiencies for a cell with an organic dye. Explanations are made with electrochemical impedance spectra (EIS), laser-induced photo–voltage transients, and incident photo-to-current conversion efficiency (IPCE) curves.

High performance and reduced charge recombination of CdSe/CdS quantum dot-sensitized solar cells

CdSe and CdSe/CdS quantum dots (QDs) with narrow size distribution are attached onto TiO2 electrodes by linker-assisted chemical bath deposition (LACBD). Preparation is carried out at 60 °C or 160 °C, which results in the formation of CdSe or CdSe/CdS QDs onto the TiO2 nanoparticles connected by thioglycolic acid (TGA). The solar cell based on a CdSe/CdS QD-sensitized TiO2 photoelectrode shows higher JSC, VOC and power conversion efficiency comparing to single CdSe QD-sensitized TiO2, which can be ascribed to superior light absorption, faster electron transport and slower charge recombination for the former. Furthermore, the effects of a ZnS passivation layer and thermal annealing on the photovoltaic performance have been investigated by UV-vis spectra, incident photo-to-current conversion efficiency (IPCE) and electrochemical impedance spectra (EIS). As a result, a quantum dot-sensitized solar cell (QDSSC) based on a TiO2–CdSe/CdS–ZnS photoanode (400 °C, 300 s calcination), polysulfide electrolyte and Cu2S counter electrode achieves a power conversion efficiency of 4.21% under AM 1.5 G one sun illumination.

Structure–property relationship of naphthalene based donor–π–acceptor organic dyes for dye-sensitized solar cells: remarkable improvement of open-circuit photovoltage

Four new donor–π–acceptor organic dyes (YF01–04), containing naphthalene-substituted amines as an electron donor and cyanoacrylic acid as an electron acceptor, were designed and synthesized, and their photophysical properties and dye-sensitized solar cells (DSCs) performances were characterized. Dyes YF02 and YF04, with 2,6-disubstituted naphthalene frameworks, were superior than their analog dyes YF03 and YF01, having 1,2-disubstituted naphthalene moiety, in incident-photo-to-current conversion efficiency (IPCE) and total solar-to-electric conversion efficiency (η). The DSCs based on YF02, comprised of diphenylamine moiety as the donor, produced the highest η of 5.29% compared to 4.03% of the analog dye YF04, which has pyrrolidine as the donor. Remarkably, a high open-circuit photovoltage (Voc) of 0.799–0.807 V was achieved in the cases of YF02–03, which have diphenylamine-donors. To better understand the structure–property relationship for DSCs application, molecular modelling was performed on YF01–04 and vertical electronic excitations were calculated using long-range corrected energy functional WB97XD and CAM-B3LYP at the basis set level DGDZVP, which were in excellent agreement with the experimental results. Moreover, the equilibrium molecular geometries of dyes YF01–04 were calculated at the density function theory (DFT) level using the hybrid energy functional B3LYP and basis set DGDZVP. The torsion angles (θ) between the naphthalene moiety and diphenylamine donor in YF02 and YF03 were more twisted than that of the pyrrolidine-donor dyes YF01 and YF04, precluding efficient intermolecular π–π charge transfer, which translated into high Voc. Compared to the reference dye TA-St-CA, which is based on diphenylamine as an electron donor linked to a phenyl ring, YF02 achieved higher Voc, which indicated that naphthalene substituted with diphenylamine is more efficient in retarding charge recombinations.

Facile synthesis of ZnO nanocrystals via a solid state reaction for high performance plastic dye-sensitized solar cells

We report the facile synthesis of ZnO nanocrystals via a one-step solid state reaction at room temperature and their application as the photoanode in plastic dye-sensitized solar cells (DSCs). ZnO nanoparticles were prepared utilizing zinc acetate dihydrate and sodium hydroxide with a short grinding time and without a sintering process. The as-prepared samples with the polycrystalline hexagonal wurtzite structure were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The obtained ZnO nanoparticles exhibited high crystallinity even without a high temperature sintering treatment during the preparation process. The effects of compression post-treatment on the photovoltaic performance of DSCs were also investigated using intensity modulated photocurrent spectroscopy (IMPS), incident photo-to-current conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS). The results indicate that the improvement of power conversion efficiency after compression post-treatment of ZnO photoelectrode can be attributed to its high photoelectron collection efficiency and effective electron transport. Under the optimized conditions, a full plastic D149-sensitized ZnO solar cell measured under illumination of 100 mW·cm−2 (AM 1.5G) presents an energy conversion efficiency of 3.76% with open-circuit voltage of 0.688 V, short-circuit current density of 8.55 mA·cm−2, and fill factor of 0.64. These results demonstrate that the one-step solid state reaction is a convenient and effective method for the synthesis of ZnO nanocrystals for use in plastic DSCs. Open image in new window

Meta versus para substituent effect of organic dyes for sensitized solar cells

A series of new metal-free organic donor-bridge-acceptor dyes comprising a triphenylamine moiety as the electron donor, and a cyanoacrylic acid (dye series A) or a carboxylic acid moiety (dye series B) as the electron acceptor were synthesized and utilized for dye-sensitized solar cells (DSSCs). The triphenylamine moiety was linked to the main chromophore through either a para or a meta position across a phenyl group in order to examine the difference of structural effect. Both types of compounds exhibited apparent solvatochromic shift in their fluorescence spectra, showing the ready formation of charge separated states in both the para and meta-isomers. Quantum mechanical calculations were performed by using the density functional theory (DFT) at the B3LYP/6-31G(d,p) level to gain insight into the electron distributions surrounding the para and meta-substituted triphenylamine groups. The movement of an electron from the donor to the acceptor under the irradiation of light can be depicted by the time-dependent DFT (TDDFT) calculations, and the results were compared with experimental observations. These dyes were used effectively as sensitizers in DSSCs after being absorbed on surface of nanocrystalline TiO 2. The incident photo-to-current conversion efficiency (IPCE) spectra of the para isomers are broader than that of the corresponding meta isomers, and the short-circuit current density ( J sc) values of the former is also higher than the latter. Dyes of series A exhibited overall conversion efficiencies of 1.27–4.12%, depending on the relative values of J sc, under AM 1.5 G irradiation (100 mW cm −2). For dyes of series B, the para isomers showed overall better performance than the meta isomers, i.e., higher values for all three parameters J sc, V oc (open-circuit voltage), and ff (fill factor), thus led to a higher conversion efficiency.

Synthesis of hexagonal ZnO clubs with opposite faces of unequal dimensions for the photoanode of dye-sensitized solar cells

Novel sub-micro sized hexagonal clubs of ZnO (HC-ZnO), which are coated as a scattering layer (SL) for the photoanode of a DSSC, are synthesized. X-ray diffraction (XRD) patterns of the ZnO clubs show clear peaks corresponding to wurtzite crystal phase of ZnO. Scanning electron microscopic (SEM) images show that each club has two opposite hexagonal faces (parts) of unequal dimensions. High resolution transmission electron microscopic (HR-TEM) image of a single ZnO club reveals that the ZnO is single crystalline and has wurtzite crystal structure; the image indicates a lattice spacing (d) of 0.26 nm; this is ascribed to the (002) planar spacing of the hexagonal ZnO. A solar-to-electricity conversion efficiency (η) of 3.36% is achieved for the cell with the double layer (DL) film, which is 16% higher than that of the cell with only transparent layer (TL) of commercial ZnO (2.89%) and far higher than that of the cell with SL (0.05%). The η of the cell with the DL (3.36%) could further be improved to 4.28% through the modification of the DL surface with TiO(x). Incident photo-to-current conversion efficiency (IPCE) curves, UV-vis absorption spectra, energy dispersive X-ray (EDX) spectra, and electrochemical impedance spectra (EIS) are also used to substantiate the results.

Morphology Dependence of Silicon Nanowire/Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Heterojunction Solar Cells

The characteristics of well-aligned silicon nanowire (SiNW)/poly(3,4-ethylenedioxy- thiophene):poly(styrenesulfonate) (PEDOT) heterojunction solar cells with varying nanowire lengths are investigated. PEDOT adheres on the hydrophilic n-type silicon nanowire surface to form a core−sheath heterojunction structure through the solution process. Such a structure increases the area of heterojunctions and shortens the carrier diffusion distance, and therefore, it greatly increases carrier collection efficiency. The cells exhibit stable rectifying diode behavior. Compared to the cells without a nanowire structure, the series resistance of the SiNW/PEDOT cell decreases from 60.42 Ω cm2 to 1.47 Ω cm2, and the power conversion efficiency improves from 0.08% to 5.09%. The SiNW/PEDOT solar cell harvests photons from 400 nm to 1100 nm and a maximum incident phototo-current conversion efficiency of ∼32% at 700 nm.

Novel ruthenium bipyridyl dyes with S-donor ligands and their application in dye-sensitized solar cells

Two series of novel ruthenium bipyridyl dyes incorporating sulfur-donor bidentate ligands with general formula [Ru(R-bpy) 2C 2N 2S 2] and [Ru(R-bpy) 2(S 2COEt)][NO 3] (where R = H, CO 2Et, CO 2H; C 2N 2S 2 = cyanodithioimidocarbonate and S 2COEt = ethyl xanthogenate) have been synthesized and characterized spectroscopically, electrochemically and computationally. The acid derivatives in both series (C 2N 2S 2 3 and S 2COEt 6) were used as a photosensitizer in a dye-sensitized solar cell (DSSC) and the incident photo-to-current conversion efficiency (IPCE), overall efficiency ( η) and kinetics of the dye/TiO 2 system were investigated. It was found that 6 gave a higher efficiency cell than 3 despite the latter dye's more favorable electronic properties, such as greater absorption range, higher molar extinction coefficient and large degree of delocalization of the HOMO. The transient absorption spectroscopy studies revealed that the recombination kinetics of 3 were unexpectedly fast, which was attributed to the terminal CN on the ligand binding to the TiO 2, as evidenced by an absorption study of R = H and CO 2Et dyes sensitized on TiO 2, and hence leading to a lower efficiency DSSC.

Photoelectrocatalytic treatment of pentachlorophenol in aqueous solution using a rutile nanotube-like TiO2/Ti electrode

Taking pentachlorophenol (PCP) as a reference, we investigated the photoelectrocatalytic degradation of organic pollutants using a rutile nanotube-like TiO(2)/Ti film electrode. The nanotube-like TiO2 electrode was prepared by first oxidizing the surface of a titanium sheet to form rutile TiO2 and then treating it to form the tubular structure in NaOH aqueous solution. The occurrence of PCP degradation was indicated by the decrease in pH, concentration of PCP and TOC, and by the formation of chloride ions. The photoelectrochemical (PEC) efficiency of the nanotube-like TiO2/Ti electrode has been determined in terms of degradation of PCP and the incident photo-to-current conversion efficiency (IPCE). The experimental results showed that PCP could be degraded more efficiently by a photoelectrocatalytic process than by a photocatalytic or electrochemical oxidation alone. A significant photoelectrochemical synergetic effect was observed. The kinetic constant of PEC degradation of PCP using a nanotube-like TiO2 electrode was over 60% higher than that using a TiO2 film electrode. It is noted that chloride ion and hydrogen ion concentration increased with irradiation time in the PEC degradation of PCP; PCP was gradually mineralized and the complete minimization of PCP took more time than its degradation.

Novel Carboxylated Oligothiophenes as Sensitizers in Photoelectric Conversion Systems

Novel carboxylated oligothiophenes with different thiophene units were designed and synthesized as photosensitizers in dye-sensitized solar cells (DSSCs) for efficient opto-electric materials. The introduction of -COOH into thiophene molecules can lead to a red shift of UV-visible absorption, increase light-harvesting efficiency, and enhance photoinduced charge transport by forming efficient covalent bonds to the substrate surface. A red shift of the absorption spectrum of oligothiophene is also achieved by the increase in the number of thiophene units. The DSSCs based on the oligomers have excellent photovoltaic performances. Under 100 mW cm(-2) irradiation a short-circuit current of 10.57 mA cm(-2) and an overall energy conversion efficiency of 3.36 % is achieved when pentathiophene dicarboxylated acid was used as a sensitizer. The incident photo-to-current conversion efficiency (IPCE) has a maximum as high as 80 %. In addition, photovoltage and photocurrent transients show that slow charge recombination in DSSCs is important for efficient charge separation and excellent photoelectric conversion properties of the oligomers. These initial and promising results suggest that carboxylated oligothiophenes are efficient photosensitizers.

Synthesis of phthalocyanines with two carboxylic acid groups and their utilization in solar cells based on nano-structured TiO2

A way of anchoring unsymmetrical phthalocyaninato-metal complexes (metal ion: zinc and ruthenium) is described. The synthesis and characterization of these complexes are presented. In case of the zinc complex, the obtained product is an aggregate, while only the monomer is obtained in the case of the ruthenium derivative. Both complexes could be attached onto the TiO 2 surface by using the reported method. Both dyes are expected to form monolayers with dye molecules standing on the surface of nano-structured TiO 2, forming higher-order aggregates with the zinc but not with the ruthenium complex. A highest monochromatic incident photo-to-current conversion efficiency (IPCE) of 1.6% at 690 nm was obtained for a solar cell based on the Pc-Zn sensitized nano-structured TiO 2 electrode, while an IPCE of 23% at 630 nm was obtained for the Pc-Ru sensitized electrode. Overall conversion efficiencies (η) at a simulated AM 1.5 (100 W.m-2) of 0.03% and 0.40% for the zinc and ruthenium complexes were achieved, respectively. The difference in efficiencies could be due to the formation of face-to-face aggregation in the former case. This work shows that the ruthenium complex, with two axial methylpyridine ligands, does not form aggregates in solution nor on the surface of TiO 2, making it possible for further construction of supramolecular systems with such types of phthalocyanine.

Modified phthalocyanines for efficient near-IR sensitization of nanostructured TiO(2) electrode.

A zinc phthalocyanine with tyrosine substituents (ZnPcTyr), modified for efficient far-red/near-IR performance in dye-sensitized nanostructured TiO(2) solar cells, and its reference, glycine-substituted zinc phthalocyanine (ZnPcGly), were synthesized and characterized. The compounds were studied spectroscopically, electrochemically, and photoelectrochemically. Incorporating tyrosine groups into phthalocyanine makes the dye ethanol-soluble and reduces surface aggregation as a result of steric effects. The performance of a solar cell based on ZnPcTyr is much better than that based on ZnPcGly. Addition of 3alpha,7alpha-dihydroxy-5beta-cholic acid (cheno) and 4-tert-butylpyridine (TBP) to the dye solution when preparing a dye-sensitized TiO(2) electrode diminishes significantly the surface aggregation and, therefore, improves the performance of solar cells based on these phthalocyanines. The highest monochromatic incident photo-to-current conversion efficiency (IPCE) of approximately 24% at 690 nm and an overall conversion efficiency (eta) of 0.54% were achieved for a cell based on a ZnPcTyr-sensitized TiO(2) electrode. Addition of TBP in the electrolyte decreases the IPCE and eta considerably, although it increases the open-circuit photovoltage. Time-resolved transient absorption measurements of interfacial electron-transfer kinetics in a ZnPcTyr-sensitized nanostructured TiO(2) thin film show that electron injection from the excited state of the dye into the conduction band of TiO(2) is completed in approximately 500 fs and that more than half of the injected electrons recombines with the oxidized dye molecules in approximately 300 ps. In addition to surface aggregation, the very fast electron recombination is most likely responsible for the low performance of the solar cell based on ZnPcTyr.

Electrochemically promoted photocatalytic oxidation of nitrite ion by using rutile form of TiO 2/Ti electrode

The photocatalytic oxidation of nitrite ion in a NaCl aqueous solution using the rutile form of TiO 2/Ti as the working electrode was studied. Experimental results indicate that the rutile form of TiO 2/Ti film electrode has excellent photoactivity by applying a bias potential and irradiation simultaneously. The incident photo-to-current conversion efficiency (IPCE) of this working electrode is a function of the applying bias potential. The photocurrent efficiency of nitrite ion oxidation was 33–40% at a pH of about 7. The oxidation rate of the nitrite ion in brine wastewater using the rutile form of TiO 2/Ti electrode can be estimated by photocurrent measurements. The applying bias potential, light power and pH value were the major factors affecting the oxidation rate and the photocurrent efficiency of nitrite ion oxidation, while the concentrations of nitrite ion was minor.