Influence of Redox Couple on the Performance of ZnO Dye Solar Cells and Minimodules with Benzothiadiazole-Based Photosensitizers

ZnO-based dye-sensitized solar cells exhibit lower efficiencies than TiO₂-based systems despite advantageous charge transport dynamics and versatility in terms of synthesis methods, which can be primarily ascribed to compatibility issues of ZnO with the dyes and the redox couples originally optimized for TiO₂. We evaluate the performance of solar cells based on ZnO nanomaterial prepared by microwave-assisted solvothermal synthesis, using three fully organic benzothiadiazole-based dyes YKP-88, YKP-137, and MG-207, and alternative electrolyte solutions with the I^-/I₃ ^-, Co(bpy)₃ ^2+/3+, and Cu(dmp)₂ ^1+/2+ redox couples. The best cell performance is achieved for the dye-redox couple combination YKP-88 and Co(bpy)₃ ^2+/3+, reaching an average efficiency of 4.7% and 5.0% for the best cell, compared to 3.7% and 3.9% for the I^-/I₃ ^- couple with the same dye. Electrical impedance spectroscopy highlights the influence of dye and redox couple chemistry on the balance of recombination and regeneration kinetics. Combined with the effects of the interaction of the redox couple with the ZnO surface, these aspects are shown to determine the solar cell performance. Minimodules based on the best systems in both parallel and series configurations reach 1.5% efficiency for an area of 23.8 cm².

Stable Cobalt-Mediated Monolithic Dye-Sensitized Solar Cells by Full Glass Encapsulation

Dye-sensitized solar cells (DSSCs) emerged in the market as one of the most promising indoor photovoltaic technologies to address the need for wireless powering of low-consuming electronics and sensor nodes of the internet of things (IoT). The monolithic design structure of the cell (M-DSSCs) makes the devices simpler and cheaper, and it is straightforward for constructing in-series modules. The most efficient DSSCs reported so far are Co(III/II)-mediated liquid junction cells with acetonitrile electrolytes; however, they are mostly unstable. This study reports on highly stable cobalt-mediated M-DSSCs, passing thermal cycling tests up to 85 °C according to ISOS standard protocols. Under 1000 h of aging in the dark and under simulated solar and artificial light soaking, all tested cells improved or retained their initial power conversion efficiency. Advanced long-term stability was achieved by eliminating the extrinsic factors of degradation, such as the interaction of the cell components with the environment and electrolyte leakage. This was obtained by encapsulation of the devices using a glass-frit sealant, including the holes for filling up the liquid components of the cells. The hermeticity of the encapsulation complies with the MIL-STD-883 standard fine helium gas leakage test, and its hermeticity remained unchanged after humidity-freeze cycles according to IEC 61646. The elimination of extrinsic degradation factors allowed reliable assessment of inner factors accountable for aging. The impact of the ISOS-protocol test conditions on the intrinsic device stability and long-term photovoltaic history of the M-DSSCs is discussed.

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.

Effect of Donor Strength and Bulk on Thieno[3,4-b]-pyrazine-Based Panchromatic Dyes in Dye-Sensitized Solar Cells

Near-infrared-absorbing organic dyes are critically needed in dye-sensitized solar cells (DSCs). Thieno[3,4-b]pyrazine (TPz) based dyes can access the NIR spectral region and show power conversion efficiencies (PCEs) of up to 8.1 % with sunlight being converted at wavelengths up to 800 nm for 17.6 mA cm^-2 of photocurrent in a co-sensitized DSC device. Precisely controlling dye excited-state energies is critical for good performances in NIR DSCs. Strategies to control TPz dye energetics with stronger donor groups and TPz substituent choice are evaluated here. Additionally, donor size influence versus dye loading on TPz dyes is analyzed with respect to the TiO₂ surface protection designed to prevent recombination of electrons in TiO₂ with the redox shuttle. Importantly, the dyes evaluated were demonstrated to work well with low Li⁺ concentration electrolytes, with iodine and cobalt redox shuttle systems, and efficiently as part of co-sensitized devices.

Supramolecular Hemicage Cobalt Mediators for Dye-Sensitized Solar Cells

A new class of dye-sensitized solar cells (DSSCs) using the hemicage cobalt-based mediator [Co(ttb)]2+/3+ with the highly preorganized hexadentate ligand 5,5'',5''''-((2,4,6-triethyl benzene-1,3,5-triyl)tris(ethane-2,1-diyl))tri-2,2'-bipyridine (ttb) has been fully investigated. The performances of DSSCs sensitized with organic D-π-A dyes utilizing either [Co(ttb)]2+/3+ or the conventional [Co(bpy)3 ]2+/3+ (bpy=2,2'-bipyridine) redox mediator are comparable under 1000 W m-2 AM 1.5 G illumination. However, the hemicage complexes exhibit exceptional stability under thermal and light stress. In particular, a 120-hour continuous light illumination stability test for DSSCs using [Co(ttb)]2+/3+ resulted in a 10 % increase in the performance, whereas a 40 % decrease in performance was found for [Co(bpy)3 ]2+/3+ electrolyte-based DSSCs under the same conditions. These results demonstrate the great promise of [Co(ttb)]2+/3+ complexes as redox mediators for efficient, cost-effective, large-scale DSSC devices.

Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO₂ and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu^(II/I)(dmby)₂TFSI_2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu^(II/I)(tmby)₂TFSI_2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu^(II/I)(dmp)₂TFSI_2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu^(II/I)(tmby)₂TFSI_2/1 as evidenced by longer lifetimes. The solar-to-electrical power conversion efficiencies for [Cu(tmby)₂]^2+/1+, [Cu(dmby)₂]^2+/1+, and [Cu(dmp)₂]^2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m^-2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.

Highly stable tandem solar cell monolithically integrating dye-sensitized and CIGS solar cells

A highly stable monolithic tandem solar cell was developed by combining the heterogeneous photovoltaic technologies of dye-sensitized solar cell (DSSC) and solution-processed CuIn_xGa_1-xSe_yS_1-y (CIGS) thin film solar cells. The durability of the tandem cell was dramatically enhanced by replacing the redox couple from to [Co(bpy)₃]²⁺ /[Co(bpy)₃]³⁺), accompanied by a well-matched counter electrode (PEDOT:PSS) and sensitizer (Y123). A 1000 h durability test of the DSSC/CIGS tandem solar cell in ambient conditions resulted in only a 5% decrease in solar cell efficiency. Based on electrochemical impedance spectroscopy and photoelectrochemical cell measurement, the enhanced stability of the tandem cell is attributed to minimal corrosion by the cobalt-based polypyridine complex redox couple.

Direct light-induced polymerization of cobalt-based redox shuttles: an ultrafast way towards stable dye-sensitized solar cells

The photopolymerization of Co(II)/Co(III) complexes for dye-sensitized solar cells (DSSCs) by means of a fast, inexpensive, in situ and inhibition-free process has been examined. We have succeeded in fabricating high-performance DSSCs able to retain a light-to-electricity power conversion efficiency exceeding 6.5% (8.5% at low intensity) after 1800 h of mixed (light on/off, temperature high/low) accelerated aging tests, thus revealing a possible way for the stabilization of these record-holding redox pairs.

Long-term stability of novel double rhodanine indoline dyes having one and two anchor carboxyl group(s) in dye-sensitized solar cells

An indoline dye having both α-cyanoacrylic and rhodanine acetic acid anchor groups at the acceptor moiety exhibited improved stability upon fluorescence lamp irradiation.

Recent Advances of Cobalt(II/III) Redox Couples for Dye-Sensitized Solar Cell Applications

In recent years dye-sensitized solar cells (DSSCs) have emerged as one of the alternatives for the global energy crisis. DSSCs have achieved a certified efficiency of >11% by using the I(-) /I3 (-) redox couple. In order to commercialize the technology almost all components of the device have to be improved. Among the various components of DSSCs, the redox couple that regenerates the oxidized sensitizer plays a crucial role in achieving high efficiency and durability of the cell. However, the I(-) /I3 (-) redox couple has certain limitations such as the absorption of triiodide up to 430 nm and the volatile nature of iodine, which also corrodes the silver-based current collectors. These limitations are obstructing the commercialization of this technology. For this reason, one has to identify alternative redox couples. In this regard, the Co(II/III) redox couple is found to be the best alternative to the existing I(-) /I3 (-) redox couple. Recently, DSSC test cell efficiency has risen up to 13% by using the cobalt redox couple. This review emphasizes the recent development of Co(II/III) redox couples for DSSC applications.