Engineering of a novel ruthenium sensitizer and its application in dye-sensitized solar cells for conversion of sunlight into electricity

A novel ligand 4,4'-bis(carboxyvinyl)-2,2'-bipyridine (L) and its ruthenium(II) complex [Ru(II)L(2)(NCS)(2)] (K8) were synthesized and characterized by analytical, spectroscopic, and electrochemical techniques. The performance of the K8 complex as a charge transfer photosensitizer in nanocrystalline TiO(2) based solar cells was studied. When the K8 complex anchored onto a nanocrystalline TiO(2) film, we achieved very efficient sensitization yielding 77 +/-5% incident photon-to-current efficiencies (IPCE) in the visible region using an electrolyte consisting of 0.6 M methyl-N-butyl imidiazolium iodide, 0.05 M iodine, 0.05 M LiI, and 0.5 M 4-tert-butylpyridine in a 50/50 (v/v) mixture of valeronitrile and acetonitrile. Under standard AM 1.5 sunlight, the complex K8 gave a short circuit photocurrent density of 18 +/- 0.5 mA/cm(2), and the open circuit voltage was 640 +/- 50 mV with fill factor of 0.75 +/- 0.05, corresponding to an overall conversion efficiency of 8.64 +/- 0.5%.     

Enhanced performance of ruthenium dye-sensitized solar cell by employing an organic co-adsorbent of N,N'-bis((pyridin-2-yl)(methyl) methylene)-o-phenylenediamine

A pyridine-anchor co-adsorbent of N,N'-bis((pyridin-2-yl)(methyl) methylene)-o-phenylenediamine(named BPPI) is prepared and employed as co-adsorbent in dye-sensitized solar cells(DSSCs). The prepared co-adsorbent could overcome the deficiency of N719 absorption in the low wavelength region of visible spectrum, offset competitive visible light absorption of I₃^-, enhance the spectral responses of the co-adsorbed TiO₂ film in region from 300 nm to 750 nm, suppress charge recombination, prolong electron lifetime, and decrease the total resistance of DSSCs. The optimized cell device co-sensitized by BPPI/N719 dye gives a short circuit current density of 12.98 mA cm^-2, an open circuit voltage of 0.73 V, and a fill factor of 0.66 corresponding to an overall conversion efficiency of 6.22% under standard global AM 1.5 solar irradiation, which is much higher than that of device solely sensitized by N719(5.29%) under the same conditions. Mechanistic investigations are carried out by various spectral and electrochemical characterizations.     

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.     

A highly efficient organic sensitizer for dye-sensitized solar cells

We have synthesized a highly efficient organic dye for a dye-sensitized solar cell; the overall solar-to-energy conversion efficiency was 9.1% at AM 1.5 illumination (100 mW cm(-2)): short-circuit current density (J(sc)) = 18.1 mA cm(-2), open circuit photovoltage (V(oc)) = 743 mV and fill factor (ff) = 0.675.     

Near infrared thieno[3,4-b]pyrazine sensitizers for efficient quasi-solid-state dye-sensitized solar cells

Three near infrared (NIR) metal-free organic sensitizers (FNE32, FNE34, FNE36) based on the thieno[3,4-b]pyrazine derivative have been designed and synthesized for application in quasi-solid-state dye-sensitized solar cells (DSSCs). These organic dyes demonstrate maximum absorption bands at 596-625 nm due to the presence of the thieno[3,4-b]pyrazine derivative, which facilitates the intramolecular electron transfer from the donor to the acceptor. Quasi-solid-state DSSCs based on FNE34 display efficient photoelectric conversion over the whole visible range extending into the NIR region up to 900 nm with maximum incident monochromatic photon-to-electron conversion efficiency (IPCE) of 77%, yielding a short-circuit photocurrent density of 16.24 mA cm(-2) and a power conversion efficiency of 5.30%. To the best of our knowledge, this is the highest efficiency for quasi-solid-state DSSCs based on an organic NIR dye. When exposed to one-sun illumination for 1000 h, the quasi-solid-state DSSC based on FNE34 exhibits good long-term stability with almost constant power conversion efficiency.     

MEH-PPV和PCBM共混体系光电池中电荷传输及性能与光强的关系

以MEH-PPV{poly[2-methoxy-5-(2'-ethylhexoxy)]-1,4-phenylene vinylene}作为电子给体材料, PCBM[1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61]作为电子受体材料, 制成了共混体系的高性能太阳电池. 光电池在100 mW/cm2强度光照下, 其开路电压Voc为0.8 V, 短路电流密度Jsc为5.06 mA/cm2, 填充因子FF为48.1%, 能量转换效率η为1.93%. UV-Vis及PL图表明, MEH-PPV与PCBM之间没有发生化学变化, 但有明显的荧光猝灭, 说明光生激子能有效地快速分离, 并在各自的传输网络中传递. 分析了光照及暗导I-V曲线的物理意义, 探讨了MEH-PPV与PCBM之间的电荷传输, 研究了在不同强度的光照下器件性能的变化. 随着光强的增加, 器件的短路电流密度线性增大, 开路电压也略有升高, 并联电阻和填充因子下降, 串联电阻变化不明显. 分析了其物理机理, 并进行了合理的解释.     

A swift dye uptake procedure for dye sensitized solar cells

Solar cells based on swift self-assembled sensitizer bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N'-bis(4-carboxylato-4'-carboxylic acid-2,2'-bipyridine)ruthenium(II) (N719) on double layers of 12 + 4 microm thick nanocrystalline TiO2 films exhibit the incident monochromatic photon-to-current conversion efficiency (IPCE) 90% and show a short circuit current density of 17 mA cm(-2), 750 mV open circuit potential and 0.72 fill factor yielding power conversion efficiencies over 9.18% under AM 1.5 sun. For the first time highest power conversion efficiencies are obtained for dye sensitized solar cells using a swift self-assembled procedure.     

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.     

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.     

Platinum-free tungsten carbides as an efficient counter electrode for dye sensitized solar cells

Mesoporous tungsten carbides displayed an excellent solar conversion efficiency (7.01%) as a counter electrode for dye sensitized solar cells under 100 mW cm(-2), AM 1.5G illumination, which corresponded to ca. 85% of the efficiency of the conventional platinum electrode.     

Novel polyene dyes for highly efficient dye-sensitized solar cells

We have developed an efficient and novel polyene-dye-sensitized nanocrystalline TiO2 solar cells producing a 6.8% solar energy-to-electricity conversion efficiency (eta) under AM 1.5 irradiation (100 mW cm(-2)): short-circuit current density (Jsc), 12.9 mA cm(-2), open-circuit photovoltage (Voc), 0.71 V, fill factor (ff), 0.74.     

New type of organic sensitizers with a planar amine unit for efficient dye-sensitized solar cells

A new type of organic sensitizers incorporating a planar amine unit have been synthesized and demonstrated to be a highly efficient sensitizers, showing evidence of lateral interactions on the TiO(2) surface. Under standard global air mass 1.5 solar conditions, the JK-98 sensitized cell gave a short circuit photocurrent density (J(sc)) of 16.78 mA cm(-2), an open-circuit voltage (V(oc)) of 0.745 V, and a fill factor (ff) of 0.70, corresponding to an overall conversion efficiency (η) of 8.71%.     

Constructing organic D-A-π-A-featured sensitizers with a quinoxaline unit for high-efficiency solar cells: the effect of an auxiliary acceptor on the absorption and the energy level alignment

Four organic D-A-π-A-featured sensitizers (TQ1, TQ2, IQ1, and IQ2) have been studied for high-efficiency dye-sensitized solar cells (DSSCs). We employed an indoline or a triphenylamine unit as the donor, cyanoacetic acid as the acceptor/anchor, and a thiophene moiety as the conjugation bridge. Additionally, an electron-withdrawing quinoxaline unit was incorporated between the donor and the π-conjugation unit. These sensitizers show an additional absorption band covering the broad visible range in solution. The contribution from the incorporated quinoxaline was investigated theoretically by using DFT and time-dependent DFT. The incorporated low-band-gap quinoxaline unit as an auxiliary acceptor has several merits, such as decreasing the band gap, optimizing the energy levels, and realizing a facile structural modification on several positions in the quinoxaline unit. As demonstrated, the observed additional absorption band is favorable to the photon-to-electron conversion because it corresponds to the efficient electron transitions to the LUMO orbital. Electrochemical impedance spectroscopy (EIS) Bode plots reveal that the replacement of a methoxy group with an octyloxy group can increase the injection electron lifetime by a factor of 2.4. IQ2 and TQ2 can perform well without any co-adsorbent, successfully suppress the charge recombination from TiO(2) conduction band to I(3)(-) in the electrolyte, and enhance the electron lifetime, resulting in a decreased dark current and enhanced open circuit voltage (V(oc)) values. By using a liquid electrolyte, DSSCs based on dye IQ2 exhibited a broad incident photon-to-current conversion efficiency (IPCE) action spectrum and high efficiency (η=8.50?%) with a short circuit current density (J(sc)) of 15.65?mA?cm(-2), a V(oc) value of 776?mV, a fill factor (FF) of 0.70 under AM 1.5 illumination (100?mW?cm(-2)). Moreover, the overall efficiency remained at 97% of the initial value after 1000?h of visible-light soaking.     

An iodine-free electrolyte based on ionic liquid polymers for all-solid-state dye-sensitized solar cells

An ionic liquid polymer, poly (1-alkyl-3-(acryloyloxy)hexylimidazolium iodide), was employed as an iodine-free electrolyte in all-solid-state dye-sensitized solar cells with an overall conversion efficiency of 5.29% under AM 1.5 simulated solar light (100 mW cm(-2)) illumination.     

Photovoltaic cells based on sequentially adsorbed multilayers of conjugated poly(p-phenylene ethynylene)s and a water-soluble fullerene derivative

We describe the layer-by-layer (LBL) fabrication of multilayer films and photovoltaic cells using poly(phenylene ethynylene)-based anionic conjugated polyelectrolytes as electron donors and water-soluble cationic fullerene C60 derivatives as acceptors. LBL film deposition was found to be linearly related to the number of bilayers as monitored by UV-vis absorption. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) of the multilayer films revealed an aggregated but relatively uniform morphology devoid of any long-range phase separation. The maximum incident monochromatic photon to current conversion efficiency (IPCE) of the photovoltaic cells was 5.5%, the highest efficiency reported to date for cells fabricated by using the LBL fabrication technique, and since the thin film cells do not provide complete absorption of the incident light, the current generation per photon absorbed may be as much as 10%. The cells exhibited open circuit voltages of 200-250 mV with highest measured short circuit currents up to 0.5 mA/cm2 and fill factors around 30%. The power conversion efficiencies measured at AM 1.5 solar conditions (100 mW/cm2) varied between 0.01 and 0.04%, and similar to the IPCE results, the efficiency is a function of the thickness of the PV active layer.     

Synthesis, characterization, and DFT-TDDFT computational study of a ruthenium complex containing a functionalized tetradentate ligand

A ruthenium complex trans-[Ru(L)(NCS)2], L = 4,4' '-di-tert-butyl-4',4' '-bis(carboxylic acid)-2,2':6',2' ':6' ',2' '-quaterpyridine (N886), was synthesized and characterized by spectroscopic and electrochemical methods. The absorption spectrum of the N886 complex shows metal-to-ligand charge-transfer transitions in the entire visible region and quasi-reversible oxidation and reduction potentials at E(1/2) = +0.38 and -1.92 V vs ferrocene, respectively. The electronic spectra of the N886 complex were calculated by density functional theory (DFT)-time-dependent DFT, which qualitatively reproduces the experimental absorption spectra for both the protonated and deprotonated species. From the analysis of the computed optical transitions of N886, we assign its absorption bands as mixed Ru/SCN-to-quaterpyridine charge-transfer transitions, which extend from the near-IR to the UV regions. The panchromatic response of the N886 complex renders it as a suitable sensitizer for solar energy conversion applications based on titanium dioxide mesoporous electrodes. The preliminary results using the N886 complex as a sensitizer in a dye-sensitized solar cell, with an electrolyte containing 0.60 M butylmethylimidazolium iodide, 0.03 M I2, and 0.50 M tert-butylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio 1:1), show 40% incident photon-to-current efficiencies, yielding under standard AM 1.5 sunlight a short-circuit photocurrent density of 11.8 +/- 0.2 mA/cm(2), an open-circuit voltage of 680 +/- 30 mV, and a fill factor of 0.73 +/- 0.03, corresponding to an overall conversion efficiency of 5.85%.     

Enhancing the Photocurrent in Diketopyrrolopyrrole-Based Polymer Solar Cells via Energy Level Control

A series of diketopyrrolopyrrole (DPP)-based small band gap polymers has been designed and synthesized by Suzuki or Stille polymerization for use in polymer solar cells. The new polymers contain extended aromatic π-conjugated segments alternating with the DPP units and are designed to increase the free energy for charge generation to overcome current limitations in photocurrent generation of DPP-based polymers. In optimized solar cells with [6,6]phenyl-C(71)-butyric acid methyl ester ([70]PCBM) as acceptor, the new DPP-polymers provide significantly enhanced external and internal quantum efficiencies for conversion of photons into collected electrons. This provides short-circuit current densities in excess of 16 mA cm(-2), higher than obtained so far, with power conversion efficiencies of 5.8% in simulated solar light. We analyze external and internal photon to collected electron quantum efficiencies for the new polymers as a function of the photon energy loss, defined as the offset between optical band gap and open circuit voltage, and compare the results to those of some of the best DPP-based polymers solar cells reported in the literature. We find that for the best solar cells there is an empirical relation between quantum efficiency and photon energy loss that presently limits the power conversion efficiency in these devices.     

Triphenylamine-thienylenevinylene hybrid systems with internal charge transfer as donor materials for heterojunction solar cells

Star-shaped molecules based on a triphenylamine core derivatized with various combinations of thienylenevinylene conjugated branches and electron-withdrawing indanedione or dicyanovinyl groups have been synthesized. UV-vis absorption and fluorescence emission data show that the introduction of the electron-acceptor groups induces an intramolecular charge transfer that results in a shift of the absorption onset toward longer wavelengths and a quenching of photoluminescence. Cyclic voltammetry shows that all compounds present a reversible first oxidation process whose potential increases with the number of electron-withdrawing groups in the structure. Prototype bulk and bilayer heterojunction solar cells have been realized using fullerene C60 derivatives as acceptor material. The results obtained with both kinds of devices show that the introduction of electron-acceptor groups in the donor structure induces an extension of the photoresponse in the visible spectral region, an increase of the maximum external quantum efficiency, and an increase of the open-circuit voltage under white light illumination. These synergistic effects allow reaching power conversion efficiencies of approximately 1.20% under simulated AM 1.5 solar irradiation at 100 mW cm(-2).     

Novel D-π-A structured Zn(ii)-porphyrin dyes containing a bis(3,3-dimethylfluorenyl)amine moiety for dye-sensitised solar cells

A high solar-to-electricity conversion efficiency of 7.22% was achieved with a short circuit current (J(sc)) of 15.30 mA cm(-2), an open circuit voltage (V(oc)) of 669 mV and a fill factor (FF) of 0.71 for the 2Flu-ZnP-CN-COOH dye with a multi-functional co-adsorbent, under 100 mW cm(-2) AM 1.5 G simulated light.     

Synthesis, characterization, and photovoltaic properties of a low band gap polymer based on silole-containing polythiophenes and 2,1,3-benzothiadiazole

A new low band gap silole-containing conjugated polymer, PSBTBT, was designed and synthesized. Photovoltaic properties of PSBTBT were initially investigated, and an average power conversion efficiency (PCE) of 4.7% with a best PCE of 5.1% was recorded under illumination (AM 1.5G, 100 mW/cm(2)). The response range of the device covers the whole visible range from 380 to 800 nm. These results indicate that PSBTBT is a promising polymer material for applications in polymer solar cells.