Effect of coadsorbent on the photovoltaic performance of zinc pthalocyanine-sensitized solar cells

The effect of chenodeoxycholic acid as a coadsorbent on TiO 2 nanocrystalline solar cells incorporating phthalocyanine sensitizers was studied under various conditions. Adding chenodeoxycholic acid onto TiO 2 nanoparticles not only reduces the adsorption of phthalocyanine sensitizers but also prevents sensitizer aggregation, leading to different photovoltaic performance. The inspection of IPCE and absorption spectra showed that the load of phthalocyanine sensitizers is strongly dependent on the molar concentration of chenodeoxycholic acid coadsorbent. The open circuit voltage of the solar cells with chenodeoxycholic acid coadsorbent increases due to the enhanced electron lifetime in TiO 2 nanoparticles coupled with the band edge shift of TiO 2 to negative potentials.     

Enhanced Charge Separation Efficiency in Pyridine‐Anchored Phthalocyanine‐Sensitized Solar Cells by Linker Elongation

A series of zinc phthalocyanine sensitizers ( PcS22 – 24 ) having a pyridine anchoring group are designed and synthesized to investigate the structural dependence on performance in dye‐sensitized solar cells. The pyridine‐anchor zinc phthalocyanine sensitizer PcS23 shows 79 % incident‐photon to current‐conversion efficiency (IPCE) and 6.1 % energy conversion efficiency, which are comparable with similar phthalocyanine dyes having a carboxylic acid anchoring group. Based on DFT calculations, the high IPCE is attributed with the mixture of an excited‐state molecular orbital of the sensitizer and the orbitals of TiO₂. Between pyridine and carboxylic acid anchor dyes, opposite trends are observed in the linker‐length dependence of the IPCE. The red‐absorbing PcS23 is applied for co‐sensitization with a carboxyl‐anchor organic dye D131 that has a complementary spectral response. The site‐selective adsorption of PcS23 and D131 on the TiO₂ surface results in a panchromatic photocurrent response for the whole visible‐light region of sun light.     

Quantification of the effect of 4-tert-butylpyridine addition to I-/I3- redox electrolytes in dye-sensitized nanostructured TiO2 solar cells

Addition of 4-tert-butylpyridine (4TBP) to redox electrolytes used in dye-sensitized TiO2 solar cells has a large effect on their performance. In an electrolyte containing 0.7 M LiI and 0.05 M I2 in 3-methoxypropionitrile, addition of 0.5 M 4TBP gave an increase of the open-circuit potential of 260 mV. Using charge extraction and electron lifetime measurements, this increases could be attributed to a shift of the TiO2 band edge toward negative potentials (responsible for 60% of the voltage increase) and to an increase of the electron lifetime (40%). At a lower 4TBP concentration the shift of the band edge was similar, but the effect on the electron lifetime was less pronounced. The working mechanism of 4TBP can be summarized as follows: (1) 4TBP affects the surface charge of TiO2 by decreasing the amount of adsorbed protons and/or Li+ ions. (2) It decreases the recombination of electrons in TiO2 with triiodide in the electrolyte by preventing triiodide access to the TiO2 surface and/or by complexation with iodine in the electrolyte.     

Oligothiophene-containing coumarin dyes for efficient dye-sensitized solar cells

We have developed oligothiophene-containing coumarin dyes fully functionalized for dye-sensitized nanocrystalline TiO(2) solar cells (DSSCs). DSSCs based on the dyes gave good performance in terms of incident photon-to-current conversion efficiency (IPCE) in the range of 400-800 nm. A solar energy-to-electricity conversion efficiency (eta) of 7.4% was obtained with a DSSC based on 2-cyano-3-[5'-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-[2,2']bithiophenyl-5-yl]acrylic acid (NKX-2677) under simulated AM 1.5G irradiation (100 mW cm(-2)) with a mask: short-circuit current density (J(sc)) = 13.5 mA cm(-2); open-circuit voltage (V(oc)) = 0.71 V; fill factor (FF) = 0.77. Transient absorption spectroscopy measurements indicated that electron injection from NKX-2677 to the conduction band of TiO(2) is very rapid (<100 fs), which is much faster than the emission lifetime of the dye (1.0 ns), giving a highly efficient electron injection yield of near unity.     

Preparation of a nanoporous CaCO3-coated TiO2 electrode and its application to a dye-sensitized solar cell

A nanoporous CaCO3 overlayer-coated TiO2 thick film was prepared by the topotactic thermal decomposition of Ca(OH)2, and its performance as an electrode of a dye-sensitized solar cell was investigated. As compared to bare TiO2, nanoporous CaCO3-coated TiO2 provided higher specific surface area and, subsequently, a larger amount of dye adsorption; this in turn increased short-circuit current (Jsc). Furthermore, the CaCO3 coating demonstrated increased impedance at the TiO2/dye/electrolyte interface and increased the lifetime of the photoelectrons, indicating the improved retardation of the back electron transfer, which increases Jsc, open-circuit voltage (Voc), and fill factor (ff). Thereby, the energy conversion efficiency (eta) of the solar cell improved from 7.8 to 9.7% (an improvement of 24.4%) as the nanoporous CaCO3 layer was coated onto TiO2 thick films.     

TiO(2) porous electrodes with hierarchical branched inner channels for charge transport in viscous electrolytes.

Titanium dioxide (TiO(2)) photoelectrodes with micro/nano hierarchical branched inner channels have been prepared by an electrohydrodynamic (EHD) technique and assembled to form dye-sensitized solar cells (DSSCs). Excellent penetration of ionic-liquid electrolytes and enhanced light harvesting in the longer wavelength region are realized within the composite-structure electrode, thus a better fill factor (ff) of 75.3 % and higher conversion efficiency (eta) of 7.1 % are obtained for viscous ionic-liquid electrolytes compared to pure nanostructured films. Hierarchical branched channels in the photoanodes can efficiently improve the transport properties of redox-active species in viscous electrolytes, which is demonstrated by electrical impedance spectroscopy (EIS). The incident monochromatic photon-to-electron conversion efficiency (IPCE) shows that enhanced light scattering in the composite film is of benefit for light harvesting and thus for solar energy conversion efficiency.     

Photophysical and (photo)electrochemical properties of a coumarin dye

A new coumarin dye, cyano-{5,5-dimethyl-3-[2-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)vinyl]cyclohex-2-enylidene}-acetic acid (NKX-2753), was prepared and characterized with respect to photophysical and electrochemical properties. It was employed as a dye sensitizer in dye-sensitized solar cells and showed efficient photon-to-electron conversion properties. The photocurrent action spectrum exhibited a broad feature with a maximum incident photon-to-electron conversion efficiency (IPCE) of 84% at 540 nm, which is comparable to that for the famous red dye RuL2(NCS)2 (known as N3), where L stands for 2,2'-bipyridyl-4,4'-dicarboxylic acid. The sandwich-type solar cell with NKX-2753, under illumination of full sun (AM1.5, 100 mW cm(-2)), produced 16.1 mA cm(-2) of short-circuit photocurrent, 0.60 V of open-circuit photovoltage, and 0.69 of fill factor, corresponding to 6.7% of overall energy conversion efficiency using 0.1 M LiI, 0.05 M I2, 0.1 M guanidinium thiocyanate, and 0.6 M 1,2-dimethyl-3-n-propyl-imidazolium iodide in dry acetonitrile as redox electrolyte. In comparison with its analogue NKX-2586 (Langmuir 2004, 20, 4205), NKX-2753 with an extra side ring on the alkene chain produced much higher IPCE values at the same conditions. The side ring acted as a spacer to efficiently prevent dye aggregation when adsorbed on the TiO2 surface, resulting in significant improvements of short-circuit photocurrent, open-circuit photovoltage, and fill factor compared with NKX-2586 that aggregated on the TiO2 surface.     

D-π-A dye system containing cyano-benzoic acid as anchoring group for dye-sensitized solar cells

A D-π-A dye (KM-1) incorporating cyano-benzoic acid as a new acceptor/anchoring group has been synthesized for dye-sensitized solar cells (DSCs) with a high molar extinction coefficient of 66,700 M(-1) cm(-1) at 437 nm. Theoretical calculations show that the hydrogen bond between -CN and surface hydroxyl leads to the most stable configuration on the surface of TiO(2). In addition, the adsorption of the dye on TiO(2) follows a Brunauer-Emmett-Teller (BET) isotherm. Multilayer adsorption of KM-1 on TiO(2) seems to take place particularly at higher dye concentrations. DSC device using KM-1 reached a maximum incident photon-to-current conversion efficiency (IPCE) of 84%, with a solar to electric power conversion efficiency (PCE) of 3.3% at AM1.5 G illumination (100 mW cm(-2)). This new type of anchoring group paves a way to design new dyes that combine good visible light harvesting with strong binding to the metal oxide surface.     

Effective enhancement of the performance of black dye based dye-sensitized solar cells by metal oxide surface modification of the TiO2 photoelectrode

Effective enhancement of the performance of black dye based dye-sensitized solar cells has been achieved by MgO or Al(2)O(3) surface modification of the TiO(2) photoelectrode. The conversion efficiency was improved from 10.4% to 10.8% due to the blocking effect of the thin overlayer at the TiO(2) surface.     

Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).     

Co-sensitization of organic dyes for efficient ionic liquid electrolyte-based dye-sensitized solar cells

The co-sensitization of two organic dyes (SQ1 and JK2), which are complementary in their spectral responses, shows enhanced photovoltaic performance compared with that of an individual organic dye-sensitized solar cell. The power conversion efficiency of the co-sensitized organic dye solar cell based on the newly developed binary ionic liquid (solvent-free) electrolyte gives 6.4% under AM 1.5 sunlight at 100 mW/cm2 irradiation, which is higher than that of individual dye-sensitized solar cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) of the co-sensitized solar cell shows typical absorption peaks at 530 and 650 nm corresponding to the two dyes and displays a broad spectral response over the entire visible spectrum with IPCE of >40% in the 400-700 nm wavelength domain.     

Interface modification effects of 4-tertbutylpyridine interacting with N3 molecules in quasi-solid dye-sensitized solar cells

In this paper, the interface modification effects of 4-tertbutylpyridine (TBP), especially the interaction with dye molecules, were discussed. The results of FTIR showed that TBP interacted with dye molecules, in addition to its interaction with the TiO(2) film. Reaction between N3 and TBP by the interaction force of the H atom in the -COOH group of N3 and the N atom of TBP could retard the aggregation of dye molecules, decreasing the electron quenching and charge recombination. Furthermore, the results of cyclic voltammograms and UV-vis absorption edge revealed the interaction between TBP and dye molecules could cause the energy level of the dye molecules to change, influencing the electron injection efficiency in DSCs. The IPCE results indicated that with TBP modification, the injection efficiency decreased, but the electron collection efficiency was enhanced.     

Effect of coadsorbent properties on the photovoltaic performance of dye-sensitized solar cells

Stearic acid as a coadsorbent, which has a low dipole moment and high solubility, retarded the rate of dye adsorption during the competitive anchoring process on the TiO(2) layer in dye-sensitized solar cells (DSCs), thereby increasing the content of strongly bound dye on the TiO(2) surface. This resulted in an approximately 25% improvement in both J(SC) and the power conversion efficiency of the DSCs, even for much lower dye coverage.     

TiO2纳米片/巢状分级结构纳米阵列薄膜的制备及其在染料敏化太阳能电池中的应用

采用水热合成法在氟掺杂二氧化锡(FTO)导电玻璃基底上得到TiO2纳米阵列薄膜,并进一步通过NaOH溶液水热处理制备了由巢状纳米阵列及纳米片覆盖层构成的TiO2纳米阵列分级结构一体化薄膜.采用场发射扫描电镜(FE-SEM),X射线衍射(XRD),紫外-可见(UV-Vis)漫反射光谱和吸收光谱技术对TiO2薄膜的结构和性质进行表征.FE-SEM结果表明:分级结构TiO2薄膜膜厚为1.5μm,薄膜由一层纳米片覆盖层(约0.2μm高)和一层巢状纳米阵列层(约1.3μm高)组成.XRD谱图表明TiO2薄膜为锐钛矿相.UV-Vis光谱显示分级结构TiO2薄膜具有较强的光捕获能力和染料吸附能力.TiO2纳米片/巢状分级结构纳米阵列薄膜作为光阳极,可有效地提高染料敏化太阳能电池的光电转换效率,其短路电流(Jsc)为7.79mA·cm-2,开路电压(Voc)为0.80V,填充因子(FF)为0.40,光电转换效率(η)为2.48%,其光电转换效率较TiO2纳米阵列薄膜提高了近10倍.     

Reconstruction of the (001) surface of TiO2 nanosheets induced by the fluorine-surfactant removal process under UV-irradiation for dye-sensitized solar cells

The champion dye-sensitized solar cells (DSSCs) based on TiO(2) nanoparticles nearly reach the limit of photo-current density using the black dye or zinc porphyrin dye as sensitizer. However, the way to make ordinary DSSCs more efficient as well as to understand the mechanism is still essential. Here we present an elegant UV irradiation treatment of TiO(2) nanosheets to enhance the performance of DSSCs based on the TiO(2) nanosheets via room temperature removal of inorganic surfactants and reconstruction of the (001) surface of TiO(2) nanosheets, killing two birds with one stone. UV irradiation was utilized to remove the fluorine-surfactant on the surface of anatase TiO(2) nanosheets with a high percentage of exposed {001} facets which were synthesized with the aid of hydrofluoric acid. The nanosheets treated with UV irradiation for 40 min had the advantage of improving the photoelectric conversion efficiency of DSSCs by 17.6%, compared to that without UV treatment when they were introduced into DSSCs as photoanode materials. The improved efficiency was ascribed to more dye adsorption. A theoretical calculation proposed that UV irradiation induced microfaceted steps on the TiO(2) surface by two domain (1 × 4) reconstruction after UV irradiating the (1 × 1) (001) surface. The microfaceted steps increase the active surface area of the TiO(2) nanosheets by increasing the exposure of titanium atoms and engendering active sites.     

Alkyl-functionalized organic dyes for efficient molecular photovoltaics

We designed and synthesized new alkyl-functionalized organic dyes, MK-1 and MK-2, for dye-sensitized solar cells (DSSCs). Based on the MK-2 dye, a high performance of efficiency (eta, 7.7%; short-circuit current density Jsc = 14.0 mA cm-2, open-circuit voltage Voc = 0.74 V, and fill factor FF = 0.74) was achieved under AM 1.5 G irradiation (100 mW cm-2). Remarkably, the relatively higher Voc for DSSCs based on MK-1 and MK-2 dyes, which have long alkyl chains, were observed among the organic dyes caused by the increasing of the electron lifetime in the conduction band of TiO2. Our molecular design of alkyl-functionalized dyes strongly suggests the promising performance of molecular photovoltaics based on organic dyes.     

Increasing the conversion efficiency of dye-sensitized TiO2 photoelectrochemical cells by coupling to photonic crystals

The mechanism of enhancing the light harvesting efficiency of dye-sensitized TiO(2) solar cells by coupling TiO(2) inverse opals or disordered scattering layers to conventional nanocrystalline TiO(2) films has been investigated. Monochromatic incident photon-to-current conversion efficiency (IPCE) at dye-sensitized TiO(2) inverse opals of varying stop band wavelengths and at disordered titania films was compared to the IPCE at bilayers of these structures coupled to nanocrystalline TiO(2) films and to the IPCE at nanocrystalline TiO(2) electrodes. The results showed that the bilayer architecture, rather than enhanced light harvesting within the inverse opal structures, is responsible for the bulk of the gain in IPCE. Several mechanisms of light interaction in these structures, including localization of heavy photons near the edges of a photonic gap, Bragg diffraction in the periodic lattice, and multiple scattering events at disordered regions in the photonic crystal or at disordered films, lead ultimately to enhanced backscattering. This largely accounts for the enhanced light conversion efficiency in the red spectral range (600-750 nm), where the sensitizer is a poor absorber.     

Synthesis and photophysical properties of ruthenium-based dendrimers and their use in dye sensitized solar cells

First- and second-generation dendrimers (Ru3 and Ru6) have been synthesized, and their photophysical properties were investigated in solution and when adsorbed on the nanocrystalline TiO2 surface. The performance of Ru3 and Ru6 as charge transfer photosensitizers in nanocrytalline TiO2 based solar cells was also investigated. The best photovoltaic performance was obtained by the Ru3 based solar cell yielding a short circuit current of J sc = 5.52 mA.cm (-2) and an open circuit voltage of V oc = 626 mV, corresponding to an overall conversion efficiency of eta = 1.80% that is approximately double the conversion efficiency of the reference compound Ru1 (eta = 0.91%) and of the second generation dendrimer Ru6 (eta = 0.95%). The particular efficiency of the first generation dendrimer, Ru3, is attributed to the better light-harvesting properties of the doped nanocrystalline TiO2 film when compared to Ru1, whereas the poor performance of the second generation dendrimer, Ru6, is attributed to the uneven adsorption of all of the ruthenium moieties to the nanocrystalline TiO2 surface at the same time.     

不对称菁染料敏化纳米TiO2的光生电流过程

用光电化学方法研究了不对称菁类染料敏化TiO2纳米结构电极的光电转换过程.结果表明,该染料的电子激发态能级位置与TiO2纳米粒子导带边位置匹配较好,光激发染料后,其激发态电子可以注入到TiO2纳米多孔膜的导带,从而使TiO2纳米结构电极的吸收光谱和光电流谱红移至可见光区,其 IPCE(Incident photon-to-electron conversion efficiency)值最高可达84.3%.并进一步结合现场紫外-可见吸收光谱研究了外加电势对激发态染料往TiO2纳米多孔膜注入电子过程的影响.     

Low-bandgap donor-acceptor conjugated polymer sensitizers for dye-sensitized solar cells

A set of two donor-acceptor type conjugated polymers with carboxylic acid side groups have been synthesized and utilized as active materials for dye-sensitized solar cells (DSSCs). The polymers feature a π-conjugated backbone consisting of an electron-poor 2,1,3-benzothiadiazole (BTD, acceptor) unit, alternating with either a thiophene-fluorene-thiophene triad (2a) or a terthiophene (3a) segment as the donor. The donor-acceptor polymers absorb broadly throughout the visible region, with terthiophene-BTD polymer 3a exhibiting an absorption onset at approximately 625 nm corresponding to a ～1.9 eV bandgap. The polymers adsorb onto the surface of nanostructured TiO(2) due to interaction of the polar carboxylic acid units with the metal oxide surface. The resulting films absorb visible light strongly, and their spectra approximately mirror the polymers' solution absorption. Interestingly, a series of samples of 3a with different molecular weight (M(n)) adsorb to TiO(2) to an extent that varies inversely with M(n). DSSCs that utilize the donor-acceptor polymers as sensitizers were tested using an I(-)/I(3)(-) electrolyte. Importantly, for the set of polymer sensitizers 3a with varying M(n), the DSSC efficiency varies inversely with M(n), a result that reflects the difference in adsorption efficiency observed in the film absorption experiments. The best DSSC cell tested is based on a sample of 3a with M(n) ～ 4000, and it exhibits a ～65% peak IPCE with J(sc) ～12.6 mA cm(-2) under AM1.5 illumination and an overall power conversion efficiency of ～3%.