A Promising Candidate with D‐A‐A‐A Architecture as an Efficient Sensitizer for Dye‐Sensitized Solar Cells

A series of metal‐free organic dyes with electron‐rich (D) and electron‐deficient units (A) as π linkers have been studied theoretically by means of density functional theory (DFT) and time‐dependent DFT calculations to explore the effects of π spacers on the optical and electronic properties of triphenylamine dyes. The results show that Dye 1 with a structure of D‐A‐A‐A is superior to the typical C218 dye in various key aspects, including the maximum absorption (λ_max=511 nm), the charge‐transfer characteristics (D/Δq/t is 5.49 Å/0.818 e^?/4.41 Å), the driving force for charge‐carrier injection (ΔG_inject=1.35 eV)/dye regeneration (ΔG_regen=0.27 eV), and the lifetime of the first excited state (τ=3.1 ns). It is thus proposed to be a promising candidate in dye‐sensitized solar cell applications.     

Photophysical and Electrochemical Properties,and Molecular Structures of Organic Dyes for Dye‐Sensitized Solar Cells

Dye‐sensitized solar cells (DSSCs) based on organic dyes adsorbed on oxide semiconductor electrodes, such as TiO₂, ZnO, or NiO, which have emerged as a new generation of sustainable photovoltaic devices, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in not only their construction and operational principles, but also in their high incident‐solar‐light‐to‐electricity conversion efficiency and low cost of production. To develop high‐performance DSSCs, it is important to create efficient organic dye sensitizers, which should be optimized for the photophysical and electrochemical properties of the dyes themselves, with molecular structures that provide good light‐harvesting features, good electron communication between the dye and semiconductor electrode and between the dye and electrolyte, and to control the molecular orientation and arrangement of the dyes on a semiconductor surface. The aim of this Review is not to make a list of a number of organic dye sensitizers developed so far, but to provide a new direction in the epoch‐making molecular design of organic dyes for high photovoltaic performance and long‐term stability of DSSCs, based on the accumulated knowledge of their photophysical and electrochemical properties, and molecular structures of the organic dye sensitizers developed so far.     

High‐Performance Dye‐Sensitized Solar Cells Based on Phenothiazine Dyes Containing Double Anchors and Thiophene Spacers

A series of new push–pull phenothiazine‐based dyes ( HL1 , HL2 , HL3 , HL4 ) featuring various π spacers (thiophene, 3‐hexylthiophene, 4‐hexyl‐2,2′‐bithiophene) and double acceptors/anchors have been synthesized, characterized, and used as sensitizers for dye‐sensitized solar cells (DSSCs). Among them, the best conversion efficiency (7.31 %) reaches approximately 99 % of the N719‐based (7.38 %) DSSCs fabricated and measured under similar conditions. The dyes with two anchors have more efficient interfacial charge generation and transport compared with their congeners with only single anchor. Incorporation of hexyl chains into the π‐conjugated spacer of these double‐anchoring dyes can efficiently suppress dye aggregation and reduce charge recombination.     

New Dual Donor–Acceptor (2D‐π‐2A) Porphyrin Sensitizers for Stable and Cost‐Effective Dye‐Sensitized Solar Cells

A series of porphyrin sensitizers that featured two electron‐donating groups and dual anchoring groups that were connected through a porphine π‐bridging unit have been synthesized and successfully applied in dye‐sensitized solar cells (DSSCs). The presence of electron‐donating groups had a significant influence on their spectroscopic, electrochemical, and photovoltaic properties. Overall, the dual anchoring groups gave tunable electronic properties and stronger attachment to TiO₂. These new dyes were readily synthesized in a minimum number of steps in gram‐scale quantities. Optical and electrochemical data confirmed the advantages of these dyes for use as sensitizers in DSSCs. Porphyrins with electron‐donating amino moieties provided improved charge separation and better charge‐injection efficiencies for the studied dual‐push–pull dyes. Attenuated total reflectance–Fourier‐transform infrared (ATR‐FTIR) and X‐ray photoelectron spectroscopy of the porphyrin dyes on TiO₂ suggest that both p‐carboxyphenyl groups are attached onto TiO₂, thereby resulting in strong attachment. Among these dyes, cis-Zn2BC2A , with two electron‐donating 3,6‐ditertbutyl‐phenyl‐carbazole groups and dual‐anchoring p‐carboxyphenyl groups, showed the highest efficiency of 4.07 %, with J_SC=9.81 mA cm^?2, V_OC=0.63 V, and FF=66 %. Our results also indicated a better photostability of the studied dual‐anchored sensitizers compared to their mono‐anchored analogues under identical conditions. These results provide insight into the developments of a new generation of high‐efficiency and thermally stable porphyrin sensitizers.     

Molecular Design Principle of All‐organic Dyes for Dye‐Sensitized Solar Cells

All‐organic dyes have shown promising potential as an effective sensitizer in dye‐sensitized solar cells (DSSCs). The design concept of all‐organic dyes to improve light‐to‐electric‐energy conversion is discussed based on the absorption, electron injection, dye regeneration, and recombination. How the electron‐donor–acceptor‐type framework can provide better light harvesting through bandgap‐tuning and why proper arrangement of acceptor/anchoring groups within a conjugated dye frame is important in suppressing improper charge recombination in DSSCs are discussed. Separating the electron acceptor from the anchoring unit in the donor–acceptor‐type organic dye would be a promising strategy to reduce recombination and improve photocurrent generation.     

Structure–Performance Correlations of Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety for Dye‐Sensitized Solar Cells

The high performances of dye‐sensitized solar cells (DSSCs) based on seven new dyes are disclosed. Herein, the synthesis and electrochemical and photophysical properties of a series of intentionally designed dipolar organic dyes and their application in DSSCs are reported. The molecular structures of the seven organic dyes are composed of a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron‐deficient diphenylquinoxaline moiety integrated in the π‐conjugated spacer between the electron donor and acceptor moieties. The DSSCs based on the dye DJ104 gave the best overall cell performance of 8.06 %; the efficiency of the DSSC based on the standard N719 dye under the same experimental conditions was 8.82 %. The spectral coverage of incident photon‐to‐electron conversion efficiencies extends to the onset at the near‐infrared region due to strong internal charge‐transfer transition as well as the effect of electron‐deficient diphenylquinoxaline to lower the energy gap in these organic dyes. A combined tetraphenyl segment as a hydrophobic barrier in these organic dyes effectively slows down the charge recombination from TiO₂ to the electrolyte and boosts the photovoltage, comparable to their Ru^II counterparts. Detailed spectroscopic studies have revealed the dye structure–cell performance correlations, to allow future design of efficient light‐harvesting organic dyes.     

Metal‐Free Organic Dyes for Dye‐Sensitized Solar Cells: From Structure: Property Relationships to Design Rules

Works without ruthenium as well : Dye‐sensitized solar cells (DSSCs) incorporating metal‐free organic dyes have been considerably improved in recent years. Various design strategies have been established and are employed successfully in the synthesis of novel sensitizers. In this Review, structure–property–efficiency correlations are deduced from a vast number of dyes, which should help to design new and highly efficient sensitizers.

     

Triplication of the Photocurrent in Dye Solar Cells by Increasing the Elongation of the π‐conjugation in Zn‐Porphyrin Sensitizers

Porphyrins are promising sensitizers for dye solar cells (DSCs) but narrow absorption bands at 400–450 and 500–650 nm limit their light‐harvesting properties. Increasing elongation of the π‐conjugation and loss of symmetry causes broadening and a red‐shift of the absorption bands, which considerably improves the performance of the DSC. Herein we use an oligothienylenevinylene to bridge a Zn‐porphyrin system and the anchoring group of the sensitizer. We separately study the performance of the two basic units: oligothienylenevinylene and Zn‐porphyrin. The combined system provides a three‐fold enhancement of the photocurrent with respect to parent dyes. This is caused by an additional strong absorption in the region 400–650 nm that leads to flat IPCE of 60 %. Theoretical calculations support that the addition of the oligothienylenevinylene unit as a linking bridge creates a charge transfer band that transforms a Zn‐porphyrin dye into a push–pull type system with highly efficient charge injection properties.     

Opp‐Dibenzoporphyrins as a Light‐Harvester for Dye‐Sensitized Solar Cells

New opp‐dibenzoporphyrins were prepared in a concise method that was based on a Pd⁰‐catalyzed cascade reaction. These porphyrins, which contained carboxylic‐acid linker groups on benzene rings that were fused to the porphyrin at their β,β′‐positions, were examined as sensitizers for dye‐sensitized solar cells for the first time. Whereas all of the porphyrins showed solar‐energy‐to‐electricity conversion, an opp‐dibenzoporphyrin with conjugated carboxylic‐acid linkers displayed the highest conversion efficiency and an exceptionally high J_sc value. Cyclic voltammetry of these porphyrins suggested that the fusion of two aromatic benzene rings onto the periphery of the porphyrin lowered the HOMO–LUMO energy gap; the incorporation of a conjugated carboxylic‐acid linker group decreased the HOMO–LUMO gap even further. These CV data are consistent with DFT calculations for these porphyrins and agree well with the UV/Vis absorption‐ and fluorescence spectra of these porphyrins.     

Organic Dyes Containing Pyrenylamine‐Based Cascade Donor Systems with Different Aromatic π Linkers for Dye‐Sensitized Solar Cells: Optical,Electrochemical, and Device Characteristics

New organic dyes containing pyrenylamine donors in a cascade arrangement and cyanoacrylic acid acceptors have been synthesized and characterized by optical, electrochemical, and theoretical studies. The dyes inherit a D ‐π¹‐D ‐π²‐A (D=donor, A=acceptor) molecular architecture where the π linkers π¹ are changed from phenyl to biphenyl and fluorene, whereas the π linker π² that connects the donor fragment with the acceptor is a phenyl unit. The conjugation pathway linking the two donor segments has been found to play a major role in the optical and electrochemical properties. Shorter π linkers such as phenyl groups facilitate the donor–acceptor interaction while the nonplanar biphenyl spacer decreases the electronic communication between the donors and enhances the oxidation propensity of the corresponding dye. All the dyes display an intense longer wavelength electronic transition,which is attributable to the amine‐to‐cyanoacrylic acid charge transfer. The extinction coefficient of this peak grows dramatically on increasing the conjugation pathway length between the two donor segments. The dyes were used as sensitizers in nanocrystalline TiO₂‐based dye‐sensitized solar cells (DSSCs) and the cascade donor system contributed to the enhancement in the device efficiency due to favorable absorption and redox properties.     

Dye‐Sensitized Solar Cells Based on (Donor‐π‐Acceptor)2 Dyes With Dithiafulvalene as the Donor

Dipolar metal‐free sensitizers (D‐π‐A; D=donor, π=conjugated bridge, A=acceptor) consisting of a dithiafulvalene (DTF) unit as the electron donor, a benzene, thiophene, or fluorene moiety as the conjugated spacer, and 2‐cyanoacrylic acid as the electron acceptor have been synthesized. Dimeric congeners of these dyes, (D‐π‐A)₂, were also synthesized through iodine‐induced dimerization of an appropriate DTF‐containing segment. Dye‐sensitized solar cells (DSSCs) with the new dyes as the sensitizers have cell efficiencies that range from 2.11 to 5.24 %. In addition to better light harvesting, more effective suppression of the dark current than the D‐π‐A dyes is possible with the (D‐π‐A)₂ dyes.     

Benzotriazole‐Containing D–π–A Conjugated Organic Dyes for Dye‐Sensitized Solar Cells

A series of metal‐free benzotriazole‐based dipolar dyes have been developed as sensitizers for dye‐sensitized solar cells (DSSCs). Different heteroaromatic rings such as furan, thiophene, and selenophene, were used in combination with benzotriazole as the conjugated spacer group. Light harvesting, charge recombination, and electron injection of the cells fabricated are affected by the heteroaromatic ring used in the spacer. The DSSC with the thiophene‐containing dye has the highest conversion efficiency of 6.20 %, which reaches 85 % of the standard cell based on N719.     

Regioisomeric Effects on the Electronic Features of Indenothiophene‐Bridged D–π‐A′–A DSSC Sensitizers

Two D–π‐A′–A regioisomers (A‐IDT‐D and D‐IDT‐A) featuring 4,4′‐di‐p‐tolyl‐4 H‐indeno[1,2‐b]‐thiophene as a π linker (π) between the diarylamino donor (D) and the pyrimidine–cyanoacrylic acid acceptor (A′–A) have been successfully synthesized and characterized as efficient sensitizers for the dye‐sensitized solar cells (DSSCs). The different arrangements of the D and A′–A blocks on the unsymmetrical indenothiophene (IDT) core render the dipole of IDT being along (A‐IDT‐D) or opposite (D‐IDT‐A) to the direction of intramolecular (donor‐to‐acceptor) charge transfer, and thus induce variations in the physical properties. The experimental observations correlated well with the theoretical analyses, clearly revealing the trade‐off between the molar extinction coefficient (ε) and the S₀→S₁ transition energy. As a result, a superior ε value was observed for D‐IDT‐A, whereas a bathochromic shift in the absorption occurred in A‐IDT‐D. The larger ε value of D‐IDT‐A together with its more favorable energy level relative to TiO₂ led to a higher power conversion efficiency of 7.41 % for the D‐IDT‐A‐based DSSC, retaining approximately 95 % of the N719‐based DSSC efficiency. This work manifests the clear structure–property relationship for the case of donor and acceptor components being connected by an unsymmetrical π linker and provides insights for molecular engineering of organic sensitizers.     

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.     

New Synthetic Routes towards Soluble and Dissymmetric Triphenodioxazine Dyes Designed for Dye‐Sensitized Solar Cells

New π‐conjugated structures are constantly the subject of research in dyes and pigments industry and electronic organic field. In this context, the triphenodioxazine (TPDO) core has often been used as efficient photostable pigments and once integrated in air stable n‐type organic field‐effect transistor (OFET). However, little attention has been paid to the TPDO core as soluble materials for optoelectronic devices, possibly due to the harsh synthetic conditions and the insolubility of many compounds. To benefit from the photostability of TPDO in dye‐sensitized solar cells (DSCs), an original synthetic pathway has been established to provide soluble and dissymmetric molecules applied to a suitable design for the sensitizers of DSC. The study has been pursued by the theoretical modeling of opto‐electronic properties, the optical and electronic characterizations of dyes and elaboration of efficient devices. The discovery of new synthetic pathways opens the way to innovative designs of TPDO for materials used in organic electronics.     

Tailor‐Made Hole‐Conducting Coadsorbents for Highly Efficient Organic Dye‐Sensitized Solar Cells

The Y‐shaped, low molecular mass, hole‐conductor (HC), acidic coadsorbents 4‐{3,7‐bis[4‐(2‐ethylhexyloxy)phenyl]‐10H‐phenothiazin‐10‐yl}benzoic acid ( PTZ1 ) and 4‐{3,7‐bis[4‐(2‐ethylhexyloxy)phenyl]‐10H‐phenothiazin‐10‐yl}biphenyl‐4‐carboxylic acid ( PTZ2 ) were developed. Owing to their tuned and negative‐shifted HOMO levels (vs. NHE), they were used as HC coadsorbents in dye‐sensitized solar cells (DSSCs) to improve cell performance through desired cascade‐type hole‐transfer processes. Their detailed functions as HC coadsorbents in DSSCs were investigated to obtain evidence for the desired cascade‐type hole‐transfer processes. They have multiple functions, such as preventing π–π stacking of dye molecules, harvesting light of shorter wavelengths, and faster dye regeneration. By using PTZ2 as the tailor‐made HC coadsorbent on the TiO₂ surface with the organic dye NKX2677, an extremely high conversion efficiency of 8.95 % was achieved under 100 mW cm^?2 AM 1.5G simulated light (short‐circuit current J_SC=16.56 mA cm^?2, open‐circuit voltage V_OC=740 mV, and fill factor of 73 %). Moreover, J_SC was increased by 13 %, V_OC by 27 % and power‐conversion efficiency by 49 % in comparison to an NKX2677‐based DSSC without an HC coadsorbent. This is due to the HC coadsorbent having a HOMO energy level well matched to that of the NKX‐2677 dye to induce the desired cascade‐type hole‐transfer processes, which are associated with a slower charge recombination, fast dye regeneration, effective screening of liquid electrolytes, and an induced negative shift of the quasi‐Fermi level of the electrode. Thus, this new class of Y‐shaped, low molecular weight, organic, HC coadsorbents based on phenothiazine carboxylic acid derivatives hold promise for highly efficient organic DSSCs.     

Assessment of Luminescent Downshifting Layers for the Improvement of Light‐Harvesting Efficiency in Dye‐Sensitized Solar Cells

Luminescence downshifting (LDS) of light can be a practical photon management technique to compensate the narrow absorption band of high‐extinction‐coefficient dyes in dye‐sensitized solar cells (DSSCs). Herein, an optical analysis on the loss mechanisms in a reflective LDS (R‐LDS)/DSSC configuration is reported. For squaraine dye (550–700 nm absorption band) and CaAlSiN₃:Eu²⁺ LDS material (550–700 nm emission band), the major loss channels are found to be non‐unity luminescence quantum efficiency (QE) and electrolyte absorption. By using an ideal LDS layer (QE=100 %), a less absorbing electrolyte (Co‐based), and antireflection coatings, approximately 20 % better light harvesting is obtained. If the absorption/emission band of dye/LDS is shifted to 800 nm, a maximal short‐circuit current density (J_sc) of 22.1 mA cm^?2 can be achieved. By putting the LDS layer in front of the DSSC (transmissive mode), more significant loss channels are observed, and hence a lower overall efficiency than the R‐LDS configuration.