Thiolate/Disulfide Based Electrolytes for p-type and Tandem Dye-Sensitized Solar Cells

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.

Naphthalene imides as novel p-type sensitizers for NiO-based p-type dye-sensitized solar cells

We report two efficient materials for p-type dye-sensitized solar cells, namely S64 and S85. Novel p-type photosensitizers for NiO-based p-type dye-sensitized solar cells are presented in this study.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

Size dependence of efficiency of PbS quantum dots in NiO-based dye sensitised solar cells and mechanistic charge transfer investigation

Quantum dots (QDs) are very attractive materials for solar cells due to their high absorption coefficients, size dependence and easy tunability of their optical and electronic properties due to quantum confinement. Particularly interesting are PbS QDs owing to their broad spectral absorption until long wavelengths, their easy processability and low cost. Here, we used control of the PbS QD size to understand charge transfer processes at the interfaces of a NiO semiconductor and explain the optimal QD size in photovoltaic devices. Towards this goal, we have synthesized a series of PbS QDs with different diameters (2.8 nm to 4 nm) and investigated charge transfer dynamics by time resolved spectroscopy and their ability to act as sensitizers in nanocrystalline NiO based solar cells using the cobalt tris(4,4'-ditert-butyl-2,2'-bipyridine) complex as a redox mediator. We found that PbS QDs with an average diameter of 3.0 nm show the highest performance in terms of efficient charge transfer and light harvesting efficiency. Our study showed that hole injection from the PbS QDs to the NiO valence band (VB) is an efficient process even with low injection driving force (-0.3 eV) and occurs in 6-10 ns. Furthermore we found that direct electrolyte reduction (photoinduced electron transfer to the cobalt redox mediator) also occurs in parallel to the hole injection with a rate constant of similar magnitude (10-20 ns). In spite of its large driving force, the rate constant of the oxidative quenching of PbS by Co(iii) diminishes more steeply than hole injection on NiO when the diameter of PbS increases. This is understood as the consequence of increasing the trap states that limit electron shift. We believe that our detailed findings will advance the future design of QD sensitized photocathodes.

A Blue Diketopyrrolopyrrole Sensitizer with High Efficiency in Nickel-Oxide-based Dye-Sensitized Solar Cells

We prepared a series of four new diketopyrrolopyrroles (DPPs)-based sensitizers that exhibit high-molar extinction coefficients, extended absorption into the long wavelengths, and well-suited photoredox properties to act as sensitizers in p-type dye-sensitized solar cells (p-DSSCs). These new DPP dyes, composed of a thienyl DPP core, are substituted on one end either by a thiophene carboxylic (Th) or a 4,4'-[(phenyl)aza]dibenzoic acid as anchoring group and, on the other extremity, either by a proton or a naphthalene diimide (NDI) moiety. These new dyes were completely characterized by absorption and emission spectroscopy along with electrochemistry and they were modeled by time-dependent DFT (TD-DFT) quantum chemical calculations. The photovoltaic study in p-DSSC with iodine-based electrolyte reveals that the Th-DPP-NDI dye is particularly efficient (J_sc =7.38 mA cm^-2 ; V_oc =147 mV; FF=0.32; η=0.35 %) and quite active in the low-energy region of the solar spectrum (above 700 nm), where only a few NiO dyes are effective. To illustrate the potential of DPP dyes in photocathodes, we designed a highly efficient tandem DSSC composed of a TiO₂ photoanode sensitized by the dye D35 and a NiO photocathode sensitized by Th-DPP-NDI. This tandem DSSC gives the highest performances ever reported (J_sc =6.73 mA cm^-2 ; V_oc =910 mV; η=4.1 %) and, importantly, the tandem cell outcompetes with the sub-cells.

Designing Squaraines to Control Charge Injection and Recombination Processes in NiO-based Dye-Sensitized Solar Cells

Herein, the synthesis of a new family of squaraines (SQs) and their application in p-type dye-sensitized solar cells (DSSCs) is presented. In particular, two sets of SQs were designed featuring either two or four anchoring carboxylic groups combined with either oxygen or dicyanovinyl central groups. The SQs were characterized by using a joint theoretical, photophysical, and electrochemical approach. Importantly, the presence of different central groups forces a frozen cis (dicyanovinyl group) or a trans (oxygen group) SQ conformation. Based on the latter, the current work enables a direct comparison between cis and trans isomers as well as the impact of a different number of anchors. Considering their electron-accepting and light-harvesting character, they were tested in NiO-based DSSCs. Photocurrent-voltage, incident photon-to-current conversion efficiency (IPCE), and electrochemical impedance spectroscopy measurements were performed. By virtue of their different symmetry, stereochemistry, and number of carboxylic groups, altered adsorption behavior onto NiO electrodes as well as diverse charge injection and charge recombination dynamics were noted under operation conditions. SQs with four linkers in a frozen cis isomerism show the best charge collection properties among the investigated SQs, providing a valuable guideline for the molecular design of future SQs for p-type DSSCs. In addition, we assembled tandem DSSCs featuring SQ/NiO photocathodes and N719/TiO₂ photoanodes. The IPCE of the resulting tandem DSSCs implies light harvesting throughout most of the visible part of the solar spectrum owing to the complementary absorption features of SQ and N719.

Discrete photoelectrodes with dyes having different absorption wavelengths for efficient cobalt-based tandem dye-sensitised solar cells

A pn-tandem dye-sensitised solar cell (pn-DSSC) employing a set of sensitisers with complementary absorption spectra and a less-corrosive cobalt-based electrolyte is presented. We applied three organic sensitisers (denoted C343, DCBZ, and SQ) featuring different absorption wavelengths for the p-DSSC, while keeping the n-DSSC sensitiser (denoted DCA10CN2) constant. Characterisation of the Co+2/+3-based DSSC devices revealed that SQ dye, with a longer absorption wavelength, showed broader spectra and increased photocurrent activity in the visible and near-infrared region compared to the other two devices with C343 and DCBZ in the pn-DSSCs. As a result, the short-circuit current density increased significantly to 4.00 mA cm-2, and the devices displayed overall power conversion efficiencies of as high as 1.41%, which is comparable to that of the best pn-DSSCs in the literature. Our results demonstrate that complementary absorption between the two photoelectrodes is important for enhancing the photovoltaic performance of pn-DSSCs.

KuQuinones as sensitizers for NiO based p-type dye-sensitized solar cells

KuQuinones have been used for the first time as dyes in NiO-based p-type DSSCs.

Highly efficient dye-sensitized solar cells based on low concentration organic thiolate/disulfide redox couples

An organic thiolate/disulfide (BMIT/BMIDT) redox couple was synthesized and applied in DSSCs with very low concentration.