Novel Blue Organic Dye for Dye-Sensitized Solar Cells Achieving High Efficiency in Cobalt-Based Electrolytes and by Co-Sensitization

Enhanced Efficiency of Dye-Sensitized Solar Cells by Controlled Co-Adsorption Self-Assembly of Dyes

Single dyes typically exhibit limited light absorption in dye-sensitized solar cells (DSSCs). Thus, cosensitization using two or more dyes to enhance light-harvesting efficiency has been explored; however, the aggregation of dyes can adversely affect electron injection capabilities. This study focused on the design and synthesis of three dyes with a common carbazole donor for DSSCs: DZ102, TZ101, and JM102. JM102 broadens the absorption spectrum by replacing the benzoic acid electron acceptor of TZ101 with acetylenic benzoic acid. A cosensitized DSSC device based on CO-1 [DZ102:TZ101 = 1:1 (50 μM:50 μM)] achieved a short-circuit current density of 19.4 mA/cm2 and a power conversion efficiency of 10.9%. For the first time, the molecular interactions between the dyes in the photoanode were demonstrated using cyclic voltammetry, which revealed the presence of intermolecular forces. Adsorption kinetics further indicated that these forces promoted the self-assembly of dyes during adsorption, which resulted in a cosensitization adsorption amount greater than the sum of the individual dye adsorptions. This study provides novel insights into the selection of cosensitizing dyes for DSSCs.

Visible-Light-Active Unsymmetrical Squaraine Dyes with Pyridyl Anchoring Groups for Dye-Sensitized Solar Cells

Visible-light-active alkyl group-wrapped unsymmetrical squaraine dyes SD1-SD3 were synthesized, featuring an indoline donor and pyridine and carboxylic acid anchoring groups. Their photophysical, electrochemical, and photovoltaic characteristics were examined by fabricating a dye-sensitized solar cell (DSSC) device. Both carboxylic acid and pyridine anchoring groups containing squaraine dyes SD3 and SD2 possess similar photophysical and electrochemical characteristics. However, their photovoltaic performances were completely different. The SD3 dye with the carboxylic acid anchoring group displayed a DSSC device efficiency of 7.20% (VOC 0.81 V; JSC 12.29 mA/cm2) using iodolyte (I-/I3-) electrolyte, compared to SD1 (VOC 0.659 V; JSC 4.97 mA/cm2; and η - 2.34%) and SD2 (VOC 0.629 V; JSC 1.68 mA/cm2; and η - 0.84%), which were featured with pyridyl anchoring groups. These results were attributed to dye loading on the Lewis and Brønsted acidic sites of TiO2 and the importance of aggregated structures for photocurrent generation. In the incident photon-to-current efficiency (IPCE) analysis, SD1 dye-sensitized devices exhibited photocurrent generation from both monomeric and aggregated dyes on the TiO2 surface. In contrast, SD2 showed photocurrent generation solely from aggregated states. Despite the introduction of long alkyl chains to reduce dye aggregation and charge recombination, the results indicated preferential charge injection from only the aggregated SD2 dye on TiO2. Fluorescence-quenching experiments indicated an efficient charge transfer from the aggregated SD2 dye to TiO2 compared to that of the monomeric dye. Cosensitization, a method to enhance the light-harvesting efficiency and photocurrent generation in DSSCs, was explored by simultaneously cosensitizing pyridyl-based dyes (SD1 and SD2) with a blue-colored carboxylic acid-based squaraine dye SD4. IPCE analysis demonstrated that both SD1 and SD4 contributed to generating a photocurrent of 9.11 mA/cm2. The sequential cosensitization of SD1 and SD4 with the coadsorbent CDCA showed the highest performance, with a VOC of 0.663 V, a JSC of 11.43 mA/cm2, and an efficiency (η) of 5.20%.

Recent progress in one dimensional TiO2 nanomaterials as photoanodes in dye-sensitized solar cells

Exploiting the vast possibilities of crystal and electronic structural modifications in TiO2 based nanomaterials creatively attracted the scientific community to various energy applications. A dye sensitised solar cell, which converts photons into electricity, is considered a viable solution for the generation of electricity. TiO2 nanomaterials were well accepted as photoanode materials in dye-sensitized solar cells, and possess non-toxicity, high surface area, high electron transport rates, fine tuneable band gap, high resistance to photo corrosion and optimum pore size for better diffusion of dye and electrolyte. This review focuses on various aspects of TiO2 nanomaterials as photoanodes in dye-sensitized solar cells. TiO2 photoanode modification via doping and morphological variations were discussed in detail. The impact of various morphologies on the design of TiO2 photoanodes was particularly stressed.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

A soft x-ray probe of a titania photoelectrode sensitized with a triphenylamine dye

We present a thorough soft x-ray photoelectron spectroscopy (XPS) study of a mesoporous titanium dioxide electrode sensitized with the dye 4-(diphenylamino)phenylcyanoacrylic acid, referred to as "L0." Supported by calculations, the suite of XPS, x-ray absorption spectroscopy, and resonant photoelectron spectroscopy allows us to examine bonding interactions between the dye and the surface and the frontier electronic structure at the molecule-oxide interface. While placing these measurements in the context of existing literature, this paper is intended as a useful reference for further studies of more complex triphenylamine based sensitizers.

New Benzo[h]quinolin-10-ol Derivatives as Co-sensitizers for DSSCs

New benzo[h]quinolin-10-ol derivatives with one or two 2-cyanoacrylic acid units were synthesized with a good yield in a one-step condensation reaction. Chemical structure and purity were confirmed using NMR spectroscopy and elemental analysis, respectively. The investigation of their thermal, electrochemical and optical properties was carried out based on differential scanning calorimetry, cyclic voltammetry, electronic absorption and photoluminescence measurements. The analysis of the optical, electrochemical and properties was supported by density functional theory studies. The synthesized molecules were applied in dye-sensitized solar cells as sensitizers and co-sensitizers with commercial N719. The thickness and surface morphology of prepared photoanodes was studied using optical, scanning electron and atomic force microscopes. Due to the utilization of benzo[h]quinolin-10-ol derivatives as co-sensitizers, the better photovoltaic performance of fabricated devices compared to a reference cell based on a neat N719 was demonstrated. Additionally, the effect of co-adsorbent chemical structure (cholic acid, deoxycholic acid and chenodeoxycholic acid) on DSSC efficiency was explained based on the density functional theory.

Origin of the electrocatalytic activity in carbon nanotube fiber counter-electrodes for solar-energy conversion

Carbon nanotubes are a versatile platform to develop sustainable and stable electrodes for energy-related applications. However, their electrocatalytic activity is still poorly understood. This work deciphers the origin of the catalytic activity of counter-electrodes (CEs)/current collectors made of self-standing carbon nanotube fibers (CNTfs) using Co2+/Co3+ redox couple electrolytes. This is based on comprehensive electrochemical and spectroscopic characterization of fresh and used electrodes applied to symmetric electrochemical cells using platinum-based CEs as a reference. As the most relevant findings, two straight relationships were established: (i) the limiting current and stability increase rapidly with the surface concentration of oxygen-containing functional groups, and (ii) the catalytic potential is inversely related to the amount of residual metallic Fe catalyst nanoparticles interspersed in the CNTf network. Finally, the fine tuning of the metal nanoparticle content and the degree of functionalization enabled fabrication of efficient and stable dye-sensitized solar cells with cobalt electrolytes and CNTf-CEs outperforming those with reference Pt-CEs.

Synthesis of Novel 3,6-Dithienyl Diketopyrrolopyrrole Dyes by Direct C‒H Arylation

Direct C-H arylation coupling is potentially a more economical and sustainable process than conventional cross-coupling. However, this method has found limited application in the synthesis of organic dyes for dye-sensitized solar cells. Although direct C-H arylation is not an universal solution to any cross-coupling reactions, it efficiently complements conventional sp²−sp² bond formation and can provide shorter and more efficient routes to diketopyrrolopyrrole dyes. Here, we have applied palladium catalyzed direct C-H arylation in the synthesis of five new 3,6-dithienyl diketopyrrolopyrrole dyes. All prepared sensitizers display broad absorption from 350 nm up to 800 nm with high molar extinction coefficients. The dye-sensitized solar cells based on these dyes exhibit a power conversion efficiency in the range of 2.9 to 3.4%.

New Blue Donor-Acceptor Pechmann Dyes: Synthesis, Spectroscopic, Electrochemical, and Computational Studies

The design, synthesis, and characterization of a new class of blue-colored thiophene-substituted Pechmann dyes are reported. Due to a distinguishing blue coloration and the capability to absorb light in one of the most photon-dense regions of the solar spectrum, such compounds are of great interest for application as photoactive materials in organic optoelectronics, in particular, in dye-sensitized solar cells. To achieve fine tuning of the optical and electrochemical properties, the electron-poor thiophene-bis-lactone moiety has been decorated with donor (D) and acceptor groups (A), targeting fully conjugated D-A-π-A structures. The designed structures have been investigated by means of DFT and time-dependent DFT calculations, and the most promising dyes have been synthesized. These molecules represent the very first preparation of unsymmetrical Pechmann derivatives. Optical and electrochemical properties of the new dyes have been studied by cyclic voltammetry and UV-vis and fluorescence spectroscopy. In two cases, test cells were built proving that a photocurrent can indeed be generated when using electrolytes especially formulated for narrow-band-gap dyes, although with a very low efficiency.

Role of hexyloxy groups in zinc phthalocyanines bearing sulfonic acid anchoring groups for dye-sensitized solar cells

Zinc phthalocyanine dyes bearing four sulfonic acid anchoring groups with (A-ZnPc) and without (H-ZnPc) four chloro and eight hexyloxy groups were used as sensitizers for dye-sensitized solar cells (DSSCs). The dyes were investigated in terms of their optical, electrochemical and photovoltaic properties. The presence of these groups in dye A-ZnPc resulted in both red-shifted absorption and decreased dye aggregation, which are beneficial for the improvement of device performance. In the presence of chenodeoxycholic acid (CDCA) as a coadsorbent, the DSSC based on H-ZnPc shows a power conversion efficiency (PCE) of 0.96%, which is improved by [Formula: see text]40% as compared to the device without CDCA. However, the PCE of an A-ZnPc-based device with CDCA slightly enhances from 1.15% (without CDCA) to 1.22%, indicating that the bulky hexyloxy groups with large steric hindrance can effectively suppress aggregation of the adsorbed dye. The results showed that the zinc phthalocyanine dye bearing bulky hexyloxy groups is a promising candidate to construct efficient coadsorbent-free DSSCs.

Improving the Performance of Dye-Sensitized Solar Cells

Dye-sensitized solar cells have been investigated intensively during the last three decades. Nevertheless, there are still many aspects to be explored to further improve their performance. Dye molecules can be modified endlessly for better performance. For instance, steric groups can be introduced to slow down recombination reactions and avoid unfavorable aggregation. There is a need for more optimal dye packing on the mesoporous TiO₂ surface to increase light absorption and promote a better blocking effect. Novel redox mediators and HTMs are key elements to reach higher performing DSC as they can offer much higher output voltage than the traditional triiodide/iodide redox couple.

Molecular Engineering of D-π-A Type of Blue-Colored Dyes for Highly Efficient Solid-State Dye-Sensitized Solar Cells through Co-Sensitization

A novel blue-colored organic donor-π-acceptor sensitizer, the so-called MKA16 dye, has been employed to construct solid-state dye-sensitized solar cells (ssDSSCs). Using 2,2',7-,7'-tetrakis( N, N-di- p-methoxyphenyl-amine) 9,9'-spirobifuorene (Spiro-OMeTAD) as hole-transport material, a good conversion efficiency of 5.8% was recorded for cells based on the MKA16 dye and a high photovoltage of 840 mV in comparison with 5.6% efficiency using the known (Dyenamo Blue) dye. By co-sensitization using the orange-colored D35 dye and MKA16 together, the solid-state solar cells showed an excellent efficiency of 7.5%, with a high photocurrent of 12.41 mA cm^-2 and open-circuit voltage of 850 mV. The results show that the photocurrent of ssDSSCs can be significantly improved by co-sensitization mainly attributed to the wider light absorption range contributing to the photocurrent. In addition, results from photo-induced absorption spectroscopy show that the dye regeneration is efficient in co-sensitized solar cells. The current results possible routes of improving the design of aesthetic and highly efficient ssDSSCs.

Physical Insight on Mechanism of Photoinduced Charge Transfer in Multipolar Photoactive Molecules

Two series of novel dyes were designed based on the multipolar structures of the red dye D35 and blue dye DB, by introducing the furan (F), benzene ring (B) and benzo[c]thiophene (BT) groups into the conjugated bridge of D35 in proper order and adjusting the position of diketopyrrolopyrrole(DPP) unit and the incorporation of fluorine in the conjugated bridge of DB, respectively. We performed the quantum chemistry calculation to investigate the ground state and excited properties in a direct correlation with the spectra properties and abilities of losing or accepting electron for the original and designed molecules. Furthermore, the absorption spectra characteristics in consideration of the aggregation of dyes on the TiO₂ layer and intermolecular charge transfer rate of the dimers were calculated. The obtained results indicate that the larger intermolecular charge transfer rate leads to the poor photoelectrical properties of the dyes, and the designed dyes D35-3 and DB-2 would exhibit the best photoelectrical properties among the investigated dyes due to their lower energy gaps, widened absorption spectra and prominent charge transfer properties.

A Stable Blue Photosensitizer for Color Palette of Dye-Sensitized Solar Cells Reaching 12.6% Efficiency

We report a blue dye, coded as R6, which features a polycyclic aromatic hydrocarbon, 9,19-dihydrobenzo[1',10']phenanthro[3',4':4,5]thieno[3,2-b]benzo[1,10]phenanthro[3,4-d]thiophene, coupled with a diarylamine electron donor and 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid acceptor. Dye R6 displays a brilliant sapphire color in a sensitized TiO₂ mesoporous film with a Co(II/III) tris(bipyridyl)-based redox electrolyte. The R6 based dye-sensitized solar cell achieves an impressive power conversion efficiency of 12.6% under standard air mass 1.5 global, 100 mW cm^-2, and shows a remarkable photostability.

1,1′-Bis(diphenylphosphino)ferrocene-appended nickel(ii) dithiolates as sensitizers in dye-sensitized solar cells

Herein, two dppf-appended Ni(ii) dithiolates with 2,2-diacetyl and 2-nitro anchors were prepared, and their light harvesting properties were explored in dye-sensitized solar cells (DSSCs).

Low-Recombination Thieno[3,4-b]thiophene-Based Photosensitizers for Dye-Sensitized Solar Cells with Panchromatic Photoresponses

We report four NIR photosensitizers employing a low-recombination donor and a thieno[3,4-b]thiophene (3,4-TT) π bridge for use in dye-sensitized solar cells. The inclusion of electron rich π spacers red-shifts the dye absorbance with solution absorption onsets reaching 700 nm. Dyes were found to have suitable energy levels for rapid electron transfers using cyclic voltammetry and UV/Vis-NIR absorption spectroscopy. Computationally optimized ground-state geometries show an increased torsional angle between π spacer and π bridge brought about by an added alkyl chain. This results in a widened optical band gap and increased oxidation potentials owing to a weakening of the electron-accepting ability of 3,4-TT for solution-state measurements. Interestingly in terms of device parameters, the alkylated π spacer had a nearly identical incident photon-to-current conversion efficiency (IPCE) curve onset when compared to a non-alkylated analogue, suggesting more similar dye geometries on the surface of TiO₂ . Elevated short-circuit current density (J_SC ) values and comparable open-circuit voltage (V_OC ) values were observed in the alkylated-π-spacer-dye-based devices with power conversion efficiencies up to 6.8 % observed with IPCE onsets exceeding 800 nm.

Au/TiO2 Hollow Spheres with Synergistic Effect of Plasmonic Enhancement and Light Scattering for Improved Dye-Sensitized Solar Cells

Au-decorated TiO₂ hollow spheres (Au-THS) have been successfully synthesized via a facile one-pot solvothermal method. The Au-THS hybrid features unique hollow structure with a large specific surface area of 120 m² g^-1 and homogeneous decoration of Au nanoparticles, giving rise to enhanced light harvesting and charge generation/separation efficiency. When incorporated into the active layer of dye-sensitized solar cells (DSSCs), an improved power conversion efficiency of 7.3% is obtained, which is increased by 37.7% compared with the controlled P25 DSSC. The underlying mechanism to rationalize the efficiency enhancement can be mainly attributed to the strong synergistic effect of superior light scattering ability of the THS and the plasmonic-enhanced effect rendered by the Au nanoparticles.

2-(4-Butoxyphenyl)-N-hydroxyacetamide: An Efficient Preadsorber for Dye-Sensitized Solar Cells

The effect of chemical modification of mesoporous TiO₂ electrodes by 2-(4-butoxyphenyl)-N-hydroxyacetamide (BPHA) before dye adsorption is investigated in dye-sensitized solar cells (DSCs). Two organic dyes, LEG4 and Dyenamo blue, were used in combination with the cobalt (II/III) tris(bipyridine) redox couple. The photovoltaic performance of the DSCs is clearly enhanced by BPHA. Preadsorption of mesoporous TiO₂ electrodes with BPHA lowered the amount of adsorbed dye but improved the short-circuit current densities and the power conversion efficiencies by 10-20%, while keeping the open-circuit potential essentially unaffected. Notably, BPHA improved the LEG4 performance, whereas it has been reported for this dye that chenodeoxycholic acid as a coadsorbent lowers solar cell efficiency. Faster dye regeneration was found to be one reason for improved performance, but improved electron injection efficiency may also contribute to the favorable effect of BPHA.

New D–π–A dyes incorporating dithieno[3,2-b:2′,3′-d]pyrrole (DTP)-based π-spacers for efficient dye-sensitized solar cells

Bulky substituents on the N-position of the DTP spacer optimize the photovoltaic performances of DSSCs.

A small electron donor in cobalt complex electrolyte significantly improves efficiency in dye-sensitized solar cells

Photoelectrochemical approach to solar energy conversion demands a kinetic optimization of various light-induced electron transfer processes. Of great importance are the redox mediator systems accomplishing the electron transfer processes at the semiconductor/electrolyte interface, therefore affecting profoundly the performance of various photoelectrochemical cells. Here, we develop a strategy—by addition of a small organic electron donor, tris(4-methoxyphenyl)amine, into state-of-art cobalt tris(bipyridine) redox electrolyte—to significantly improve the efficiency of dye-sensitized solar cells. The developed solar cells exhibit efficiency of 11.7 and 10.5%, at 0.46 and one-sun illumination, respectively, corresponding to a 26% efficiency improvement compared with the standard electrolyte. Preliminary stability tests showed the solar cell retained 90% of its initial efficiency after 250 h continuous one-sun light soaking. Detailed mechanistic studies reveal the crucial role of the electron transfer cascade processes within the new redox system.

The electrolyte is an important component of dye-sensitized solar cells. Here, Hao et al. use an electron donor additive in the cobalt-based electrolyte, which speeds up the dye regeneration and slows down recombinations. The resulting devices are stable and more efficient than those without additive.