N-type Dye-sensitized Solar Cells Research Articles

First-Principles Study of Cu-Based Inorganic Hole Transport Materials for Solar Cell Applications.

Perovskite solar cells (PSCs) and dye-sensitized solar cells (DSCs) both represent promising strategies for the sustainable conversion of sunlight into electricity and fuels. However, a few flaws of current devices hinder the large-scale establishment of such technologies. On one hand, PSCs suffer from instabilities and undesired phenomena mostly linked to the perovskite/hole transport layer (HTL) interface. Most of the currently employed organic HTL (e.g., Spiro-OMeTAD) are supposed to contribute to the perovskite decomposition and to be responsible for charge recombination processes and polarization barriers. On the other hand, power conversion efficiencies (PCEs) of DSCs are still too low to compete with other conversion technologies. Tandem cells are built by assembling p-type and n-type DSCs in a cascade architecture and, since each dye absorbs on a different portion of the solar spectrum, the harvesting window is increased and the theoretical efficiency limit for a single chromophore (i.e., the Shockley–Queisser limit) is overcome. However, such a strategy is hindered by the lack of a p-type semiconductor with optimal photocathode features. Nickel oxide has been, by far, the first-choice inorganic p-type semiconductor for both PV technologies, but its toxicity and non-optimal features (e.g., too low open circuit voltage and the presence of trap states) call for alternatives. Herein, we study of three p-type semiconductors as possible alternative to NiO, namely CuI, CuSCN and Cu2O. To this aim, we compare the structural and electronic features of the three materials by means of a unified theoretical approach based on the state-of-the art density functional theory (DFT). We focus on the calculation of their valence band edge energies and compare such values with those of two widely employed photo-absorbers, i.e., methylammonium lead iodide (MAPI) and the triple cation MAFACsPbBrI in PSCs and P1 and Y123 dyes in DSCs, given that the band alignment and the energy offset are crucial for the charge transport at the interfaces and have direct implications on the final efficiency. We dissect the effect a copper vacancy (i.e., intrinsic p-type doping) on the alignment pattern and rationalize it from both a structural and an electronic perspective. Our data show how defects can represent a crucial degree of freedom to control the driving force for hole injection in these devices.

Photophysics and Electrochemistry of Biomimetic Pyranoflavyliums: What Can Bioinspiration from Red Wines Offer?

Natural dyes and pigments offer incomparable diversity of structures and functionalities, making them an excellent source of inspiration for the design and development of synthetic chromophores with a myriad of emerging properties. Formed during maturation of red wines, pyranoanthocyanins are electron-deficient cationic pyranoflavylium dyes with broad absorption in the visible spectral region and pronounced chemical and photostability. Herein, we survey the optical and electrochemical properties of synthetic pyranoflavylium dyes functionalized with different electron-donating and electron-withdrawing groups, which vary their reduction potentials over a range of about 400 mV. Despite their highly electron-deficient cores, the exploration of pyranoflavyliums as photosensitizers has been limited to the “classical” n-type dye-sensitized solar cells (DSSCs) where they act as electron donors. In light of their electrochemical and spectroscopic properties, however, these biomimetic synthetic dyes should prove to be immensely beneficial as chromophores in p-type DSSCs, where their ability to act as photooxidants, along with their pronounced photostability, can benefit key advances in solar-energy science and engineering.

Effect of Electron-Withdrawing/-Donating Groups on the Sensitizing Action of the Novel Organic Dye “3-(5-(4-(Diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic Acid” for N-Type Dye-Sensitized Solar Cells: A Theoretical Study

Computational studies using density functional theory and time-dependent DFT approaches have been performed to study a series of metal free donor−π-linker–acceptor (D−π–A) dyes. Taking the novel or...

Schiff Base Ancillary Ligands in Bis(diimine) Copper(I) Dye-Sensitized Solar Cells.

Five 6,6′-dimethyl-2,2′-bipyridine ligands bearing N-arylmethaniminyl substituents in the 4- and 4′-positions were prepared by Schiff base condensation in which the aryl group is Ph (1), 4-tolyl (2), 4-tBuC6H4 (3), 4-MeOC6H4 (4), and 4-Me2NC6H4 (5). The homoleptic copper(I) complexes [CuL2][PF6] (L = 1–5) were synthesized and characterized, and the single crystal structure of [Cu(1)2][PF6]·Et2O was determined. By using the “surfaces-as-ligands, surfaces-as-complexes” (SALSAC) approach, the heteroleptic complexes [Cu(6)(Lancillary)]+ in which 6 is the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid)) and Lancillary = 1–5 were assembled on FTO-TiO2 electrodes and incorporated as dyes into n-type dye-sensitized solar cells (DSCs). Data from triplicate, fully-masked DSCs for each dye revealed that the best-performing sensitizer is [Cu(6)(1)]+, which exhibits photoconversion efficiencies (η) of up to 1.51% compared to 5.74% for the standard reference dye N719. The introduction of the electron-donating MeO and Me2N groups (Lancillary = 4 and 5) is detrimental, leading to a decrease in the short-circuit current densities and external quantum efficiencies of the solar cells. In addition, a significant loss in open-circuit voltage is observed for DSCs sensitized with [Cu(6)(5)]+, which contributes to low values of η for this dye. Comparisons between performances of DSCs containing [Cu(6)(1)]+ and [Cu(6)(4)]+ with those sensitized by analogous dyes lacking the imine bond indicate that the latter prevents efficient electron transfer across the dye.

Noncovalent interactions based self-assembled bichromophoric sensitizer for dye-sensitized solar cells

A noncovalent interaction based self-assembled ruthenium (II) phthalocyanine (RuPc) and N-pyridyl-peryleneimide (PyPMI) dyad has been exploited to fabricate n-type dye-sensitized solar cells (DSSCs). This supramolecular dyad design is an alternative method to replace the most challenging synthesis of covalent-linked dyads. Metal-coordinated-based dyad complex improved the light-harvesting properties of the photoanodes as opposed to when individual dye anchored on TiO2 surface alone. DSSCs comprise of RuPc⋅PyPMI dyad convert light-to-electrical energy more efficiently (η = 2.29%) than those made of single dye under one sun irradiation (100 mW cm−2) condition. The enhanced photovoltaic performance of the dyad-based devices is due to the broader light absorption of the dyad in the longer wavelengths, enabling better electron injection into the conduction band of TiO2. The combined effect of efficient electron-hole charge separation and the long-lived charge-separated states facilitated the higher short-circuit current density (Jsc) and open-circuit voltage (Voc) of the devices. The enhancement of Voc and Jsc of the devices is confirmed by measuring current–voltage (I–V) curve and incident photon to current conversion efficiency (IPCE) spectrum of each device.

Can a temporary bond between dye and redox mediator increase the efficiency of p-type dye-sensitized solar cells?

Efficient n-type dye-sensitized solar cells are known since the seminal work of O'Reagan and Grätzel in 1991. However, highly efficient p-type dye-sensitized solar cells have not been developed so far. This hinders the construction of tandem dye-sensitized solar cells, which can surpass the performance of n-type devices. Within this work, we investigate if a temporary coordination of transition metal-based redox mediators at a sensitizer can increase the efficiency of p-type dye-sensitized solar cells. Based on a computational screening, diverse Cu, Ni, and Co redox mediators were selected to construct p-type dye-sensitized solar cells. Unfortunately, the efficiency of the investigated devices does not surpass analogous cells with iodide-triiodide as redox mediator. While Ni and Cu complexes might be reduced to Ni(0) and Cu(0), respectively, the investigated Co-complex quenches the excited state efficiently. As a result, the necessary electron injection from the semiconductor is too slow, which hinders the construction of a highly efficient p-type dye-sensitized solar cell. Graphical Abstract Comparison of the mode of action of p-type dye-sensitized solar cells. While top shows the traditional one, bottom shows the investigated devices where a temporary link between dye and redox mediator plays a crucial role.

Exploration on Charge Transfer and Absorption Spectra of Spiro[fluorene-9,90-xanthene]-based Polyoxometalate Hybrids Toward High Performance Dye-sensitized Solar Cell

Based on spiro[fluorene-9,90-xanthene](SFX, dye 1), the Lindqvist-type polyoxometalate(POM) functio-nalized with SFX and its derivatives(dyes 2―4) used in dye-sensitized solar cells(DSSCs) were designed and inves-tigated with the density functional theory(DFT) and time-dependent DFT(TD-DFT) calculations. The results indicate that Lindqvist-type POM is the main contribution to the lowest unoccupied molecular orbital(LUMO) and affects the LUMO energies of dyes 2―4. The maximum absorptions of the designed dyes containing POM(dyes 2―4) are red shifted comparing with that of dye 1. The introduction of electron-donating group onto SFX segment is helpful to red shift the absorption spectra. The major factors affecting the performance of DSSCs, including light harvesting and electron injection were evaluated. Considering the absorption spectra and photovoltaic parameters, dyes 3 and 4 are promising high performance dye sensitizers in n-type DSSCs.

Singlet Fission for Photovoltaics with 130 % Injection Efficiency.

A novel pentacene dimer (P2) and a structurally analogous monomer (P1) were synthesized for use in n-type dye-sensitized solar cells. In P2, the triplet excited states formed by the rapid, spin-allowed process singlet fission were expected to enable carrier multiplication in comparison to the slow, spin-forbidden intersystem crossing seen in P1. A meta-positioning of the two pentacenes and the carboxylate anchor were chosen in P2 to balance the intramolecular dynamics of singlet fission and electron injection. Electron injection from energetically low-lying triplet excited states of pentacene units necessitated the intrinsic and extrinsic lowering of the Fermi level of the semiconductor. Indium-zinc oxide in the presence of Li+ was found to be the optimum choice for the photoelectrodes. Efficient electron injection from the triplet excited states of P1 and P2 was found, with a carrier multiplication of nearly 130 %.

Effects of Introducing Methoxy Groups into the Ancillary Ligands in Bis(diimine) Copper(I) Dyes for Dye-Sensitized Solar Cells

A systematic investigation of four heteroleptic bis(diimine) copper(I) dyes in n-type Dye-Sensitized Solar Cells (DSSCs) is presented. The dyes are assembled using a stepwise, on-surface assembly. The dyes contain a phosphonic acid-functionalized 2,2′-bipyridine (bpy) anchoring domain (5) and ancillary bpy ligands that bear peripheral phenyl (1), 4-methoxyphenyl (2), 3,5-dimethoxyphenyl (3), or 3,4,5-trimethoxyphenyl (4) substituents. In masked DSSCs, the best overall photoconversion efficiency was obtained with the dye [Cu(5)(4)]+ (1.96% versus 5.79% for N719). Values of JSC for both [Cu(5)(2)]+ (in which the 4-MeO group is electron releasing) and [Cu(5)(4)]+ (which combines electron-releasing and electron-withdrawing effects of the 4- and 3,5-substituents) and are enhanced with respect to [Cu(5)(1)]+. DSSCs with [Cu(5)(3)]+ show the lowest JSC. Solid-state absorption spectra and external quantum efficiency spectra reveal that [Cu(5)(4)]+ benefits from an extended spectral range at higher energies. Values of VOC are in the order [Cu(5)(4)]+ > [Cu(5)(1)]+ > [Cu(5)(2)]+ > [Cu(5)(3)]+. Density functional theory calculations suggest that methoxyphenyl character in MOs within the HOMO manifold in [Cu(5)(2)]+ and [Cu(5)(4)]+ may contribute to the enhanced performances of these dyes with respect to [Cu(5)(1)]+.

The influence of phosphonic acid protonation state on the efficiency of bis(diimine)copper(i) dye-sensitized solar cells

The effects of a change in the protonation state of the phosphonic acid anchoring ligand in a heteroleptic copper(i) sensitizer on the performance of n-type dye-sensitized solar cells are presented.

Pristine and Al-doped hematite printed films as photoanodes of p-type dye-sensitized solar cells

We hereby propose a non-expensive method for the deposition of pure and Al-doped hematite photoanodes in the configuration of thin films for the application of dye-sensitized solar cells (DSSC). The electrodes have been prepared from hematite nanoparticles that were obtained by thermal degradation of a chemical precursor. The particles have been used in the preparation of a paste, suitable for both screen printing and doctor blade deposition. The paste was then spread on fluorine-doped tin oxide (FTO) to obtain porous hematite electrodes. The electrodes have been sensitized using N3 and D5 dyes and were characterized through current/voltage curves under simulated sun light (1 sun, AM 1.5) with a Pt counter electrode. Al-doping of hematite showed interesting changes in the physical and electrochemical characteristics of sensitized photoanodes since we could notice the growth of AlFe2O4 (hercynite) as a secondary crystal phase into the oxides obtained by firing the mixtures of two chemical precursors at different molar ratios. Pure and Al-doped hematite electrodes have been used in a complete n-type DSSCs. The kinetics of charge transfer through the interface dye/electrolyte was studied and compared to that of a typical p-type DSSC based on NiO photocathodes sensitized with erythrosine B. The results suggest a potential application of both Fe2O3 and Fe2O3/AlFe2O4 as photoanodes of a tandem DSSC.

Influence of an Al-blended TiO2 photoanode on the photovoltaic properties of n-DSSCs

ABSTRACTThe performance of n-type dye-sensitized solar cells (n-DSSCs) based on a Co complex redox electrolyte were investigated with a photoanode composed with mixer consists of both TiO2 and AlCl3. The surface of TiO2 films coated on FTO glass substrates was amended by soaking them in a TiCl4:AlCl3 blended aqueous solution at different molar ratios, and calcining them to yield the surface-modified photoelectrodes (Ti:Al/TiO2). An enhancement in the power conversion efficiency 2.02% of the resulting n-DSSCs with Ti:Al (9:1)/TiO2 compared to the reference cell by the plain TiO2. The dark current measurements, incident photon-to-current conversion, and efficiency electrochemical impedance spectroscopy (EIS) revealed that the combination of a mixer onto the TiO2 film led to the delay of the charge recombination among the photo-injected electrons and Co complex redox electrolyte, resulting in an enhanced short-circuit current and open-circuit current, compared to device without surface modification.

Donor design and modification strategies of metal-free sensitizers for highly-efficient n-type dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) cannot be developed without the research on sensitizers. As the key of light harvesting and electron generation, thousands of sensitizers have been designed for the application in DSSC devices. Among them, organic sensitizers have drawn a lot of attention because of the flexible molecular design, easy synthesis and good photovoltaic performance. Recently, new record photovoltaic conversion efficiencies of 11.5% for DSSCs with iodide electrolyte and 14.3% for DSSCs with cobalt electrolyte and co-sensitization have been achieved with organic sensitizers. Here we focus on the donor design and modification of organic sensitizers. Several useful strategies and corresponding typical examples are presented.

The Effect of the Scattering Layer in Dye-Sensitized Solar Cells Employing a Cobalt-Based Aqueous Gel Electrolyte.

We developed an aqueous gel electrolyte based on the [Co(bpy)3](2+/3+) (bpy=2,2'-bipyridine) redox mediator for use in n-type dye-sensitized solar cells. Application of this electrolyte in combination with titania nanoparticle-based photoanodes and mesoporous titania bead scattering layers resulted in devices with efficiencies of 4.1% under simulated AM 1.5 sunlight, compared with 3.5% for devices using scattering layers made from a commercial paste. This difference was largely a result of a 25% improvement in the short-circuit current density and was ascribed to lower diffusion resistance for the gel electrolyte within mesoporous TiO2 beads, as confirmed by electrochemical impedance spectroscopy. Transient photocurrent measurements identified mass-diffusion problems for DSCs employing the commercial TiO2 paste. Moreover, under continuous 1 sun illumination, the devices based on an aqueous gel electrolyte displayed higher stability relative to those assembled with the corresponding liquid electrolyte.

Structure and energy level alignment at the dye-electrode interface in p-type DSSCs: new hints on the role of anchoring modes from ab initio calculations.

p-type dye-sensitized solar cells (DSSCs) represent the complementary photocathodes to the well-studied n-type DSSCs (Grätzel cells), but their low performances have hindered the development of convenient tandem solar cells based on cost-effective n- and p-type DSSCs. Because of their low efficiencies, experimental investigations highlighted the role of hole-electron transport processes at the dye-electrode interface. However, the effects of the dye anchoring groups on interfacial electronic features are still unclear. We report here a first principles study of a benchmark p-type DSSC model, namely the widely used Coumarin-based dye C343 adsorbed on the p-NiO surface. Together with the original carboxylic acid, we test the alternative phosphonic acid as the anchoring group. We investigate binding energies, structural features and electronic energy level alignments: our results highlight that these properties are highly sensitive to the binding modes. In particular, both the chemical nature of the anchoring group and the binding mode strongly affect the thermodynamic driving force for the dye-electrode hole injection process. From analysis of the electronic densities, we find that favorable driving forces are correlated with small values of the interfacial electrostatic dipole that is formed upon dye adsorption. From our results, we derive new hints for improving open circuit potential and the hole injection process in p-type DSSCs based on NiO electrodes.

Recent progress on tandem structured dye-sensitized solar cells

Tandem structured dye-sensitized solar cells (DSSCs) can take full advantage of sunlight, effectively broadening the absorption spectrum of the cell, resulting in a higher open circuit voltage or short circuit current than that of the conventional DSSC with single light absorber. The theoretical maximum efficiency is therefore suggested to be over the Schottky-Queisser limit of 33%. Accordingly, tandem design of DSSC is thought to be a promising way to break the performance bottleneck of DSSC. Besides, the tandem designs also broaden the application diversity of DSSC technology, which will accelerate its scale-up industrial application. In this paper, we have reviewed the recent progress on photo-electrochemical applications associated with kinds of tandem designs of DSSCs, in general, which are divided into three kinds: “n-type DSSC + n-type DSSC,” “n-type DSSC + p-type DSSC” and “n-type DSSC + other solar conversion devices.” The working principles, advantages and challenges of these tandem structured DSSCs have been discussed. Some possible solutions for further studies have been also pointed out together.

Thieno[3,4‐b]thiophene‐Based Organic Dyes for Dye‐Sensitized Solar Cells

New dipolar sensitizers containing an ethyl thieno[3,4-b]thiophene-2-carboxylate (ETTC) entity in the conjugated spacer have been synthesized in two isomeric forms. These compounds were used as the sensitizers of n-type dye-sensitized solar cells (DSSCs). The best conversion efficiency (5.31%) reaches approximately 70% of the N719-based (7.41%) DSSC fabricated and measured under similar conditions. The ETTC-containing compounds exhibit a bathochromic shift of the absorption compared to their thiophene congeners due to the quinoid effect, however, charge-trapping at the ester group of ETTC was found to jeopardize the electron injection and lower the cell efficiency. Charge trapping is alleviated as the ester group of ETTC is replaced with a hydrogen atom, as evidenced from the theoretical computation.