Modification of D–A–π–A Configuration toward a High‐Performance Triphenylamine‐Based Sensitizer for Dye‐Sensitized Solar Cells: A Theoretical Investigation

In an attempt to shed light on how the addition of a benzothiadiazole (BTD) moiety influences the properties of dyes, a series of newly designed triphenylamine‐based sensitizers incorporating a BTD unit as an additional electron‐withdrawing group in a specific donor–acceptor–π‐acceptor architecture has been investigated. We found that different positions of the BTD unit provided significantly different responses for light absorption. Among these, it was established that the further the BTD unit is away from the donor part, the broader the absorption spectra, which is an observation that can be applied to improve light‐harvesting ability. However, when the BTD unit is connected to the anchoring group a faster, unfavorable charge recombination takes place; therefore, a thiophene unit was inserted between these two acceptors, providing redshifted absorption spectra as well as blocking unfavorable charge recombination. The results of our calculations provide valuable information and illustrate the potential benefits of using computation‐aided sensitizer design prior to further experimental synthesis.     

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.     

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.     

Influence of the Auxiliary Acceptor on the Absorption Response and Photovoltaic Performance of Dye‐Sensitized Solar Cells

Three new dyes with a 2‐(1,1‐dicyanomethylene)rhodanine (IDR‐ I , ‐ II , ‐ III ) electron acceptor as anchor were synthesized and applied to dye‐sensitized solar cells. We varied the bridging molecule to fine tune the electronic and optical properties of the dyes. It was demonstrated that incorporation of auxiliary acceptors effectively increased the molar extinction coefficient and extended the absorption spectra to the near‐infrared (NIR) region. Introduction of 2,1,3‐benzothiadiazole (BTD) improved the performance by nearly 50 %. The best performance of the dye‐sensitized solar cells (DSSCs) based on IDR‐ II reached 8.53 % (short‐circuit current density (J_sc)=16.73 mA cm^?2, open‐circuit voltage (V_oc)=0.71 V, fill factor (FF)=71.26 %) at AM 1.5 simulated sunlight. However, substitution of BTD with a group that featured the more strongly electron‐withdrawing thiadiazolo[3,4‐c]pyridine (PT) had a negative effect on the photovoltaic performance, in which IDR‐ III ‐based DSSCs showed the lowest efficiency of 4.02 %. We speculate that the stronger auxiliary acceptor acts as an electron trap, which might result in fast combination or hamper the electron transfer from donor to acceptor. This inference was confirmed by electrical impedance analysis and theoretical computations. Theoretical analysis indicates that the LUMO of IDR‐ III is mainly localized at the central acceptor group owing to its strong electron‐withdrawing character, which might in turn trap the electron or hamper the electron transfer from donor to acceptor, thereby finally decreasing the efficiency of electron injection into a TiO₂ semiconductor. This result inspired us to select moderated auxiliary acceptors to improve the performance in our further study.     

Synthesis and Characterization of Donor–π‐Acceptor‐Based Porphyrin Sensitizers: Potential Application of Dye‐Sensitized Solar Cells

New porphyrin sensitizers based on donor–π‐acceptor (D‐π‐A) approach have been designed, synthesized, characterized by various spectroscopic techniques and their photovoltaic properties explored. N,N′‐Diphenylamine acts as donor, the porphyrin is the π‐spacer, and either carboxylic acid or cyanoacryclic acid acts as acceptor. All compounds were characterized by using ¹H NMR spectroscopy, ESI‐MS, UV–visible emission spectroscopies as well as electrochemical methods. The presence of aromatic groups between porphyrin π‐plane and acceptor group push the absorption of both Soret and Q‐bands of porphyrin towards the red region. The electrochemical properties suggests that LUMO of these sensitizers above the TiO₂ conduction band. Finally, the device was fabricated using liquid redox electrolyte (I^?/I₃^?) and its efficiency was compared with that of a leading sensitizer.     

Dye‐Sensitized Solar Cells Based on a Donor–Acceptor System with a Pyridine Cation as an Electron‐Withdrawing Anchoring Group

New hemicyanine dyes ( CM101 , CM102 , CM103 , and CM104 ) in which tetrahydroquinoline derivatives are used as electron donors and N‐(carboxymethyl)‐pyridinium is used as an electron acceptor and anchoring group were designed and synthesized for dye‐sensitized solar cells (DSSCs). Compared with corresponding dyes that have cyanoacetic acid as the acceptor, N‐(carboxymethyl)‐pyridinium has a stronger electron‐withdrawing ability, which causes the absorption maximum of dyes to be redshifted. The photovoltaic performance of the DSSCs based on dyes CM101 – CM104 markedly depends on the molecular structures of the dyes in terms of the n‐hexyl chains and methoxyl. The device sensitized by dye CM104 achieved the best conversion efficiency of 7.0 % (J_sc=13.4 mA cm^?2, V_oc=704 mV, FF=74.8 %) under AM 1.5 irradiation (100 mW cm^?2). In contrast, the device sensitized by reference dye CMR104 with the same donor but the cyanoacetic acid as the acceptor gave an efficiency of 3.4 % (J_sc=6.2 mA cm^?2, V_oc=730 mV, FF=74.8 %). Under the same conditions, the cell fabricated with N719 sensitized porous TiO₂ exhibited an efficiency of 7.9 % (J_sc=15.4 mA cm^?2, V_oc=723 mV, FF=72.3 %). The dyes CM101 – CM104 show a broader spectral response compared with the reference dyes CMR101 – CMR104 and have high IPCE exceeding 90 % from 450 to 580 nm. Considering the reflection of sunlight, the photoelectric conversion efficiency could be almost 100 % during this region.     

1‐Alkyl‐1H‐imidazole‐Based Dipolar Organic Compounds for Dye‐Sensitized Solar Cells

A series of donor–π–acceptor‐type organic dyes based on 1‐alkyl‐1H‐imidazole spacers 1 , 2 , 3 , 4 , 5 have been developed and characterized. The two electron donors are at positions 4 and 5 of the imidazole, while the electron‐accepting cyanoacrylic acid is incorporated at position 2 by a spacer‐containing heteroaromatic rings, such as thiophene and thiazole. Detailed investigation on the relationship between the structure, spectral and electrochemical properties, and performance of DSSC is described here. Dye‐sensitized solar cells (DSSCs) using dyes as the sensitizers exhibit good efficiencies, ranging from 3.06 to 6.35 %, which reached 42–87 % with respect to that of N719‐based device (7.33 %) fabricated and measured under similar conditions. Time‐dependent density functional theory (TDDFT) calculations have been performed on the dyes, and the results show that both electron donors can contribute to electron injection upon photo‐excitation, either directly or indirectly by internal conversion to the lowest excited state.     

Metal‐Free Sensitizers for Dye‐Sensitized Solar Cells

This review focuses on our work on metal‐free sensitizers for dye‐sensitized solar cells (DSSCs). Sensitizers based on D?A′?π?A architecture (D is a donor, A is an acceptor, A′ is an electron‐deficient entity) exhibit better light harvesting than D?π?A‐type sensitizers. However, appropriate molecular design is needed to avoid excessive aggregation of negative charge at the electron‐deficient entity upon photoexcitation. Rigidified aromatics, including aromatic segments comprising fused electron‐excessive and ‐deficient units in the spacer, allow effective electronic communication, and good photoinduced charge transfer leads to excellent cell performance. Sensitizers with two anchors/acceptors, D(–π–A)₂, can more efficiently harvest light, inject electrons, and suppress dark current compared with congeners with a single anchor. Appropriate incorporation of heteroaromatic units in the spacer is beneficial to DSSC performance. High‐performance, aqueous‐based DSSCs can be achieved with a dual redox couple comprising imidazolium iodide and 2,2,6,6‐tetramethylpiperidin‐N‐oxyl, and/or using dyes of improved wettability through the incorporation of a triethylene oxide methyl ether chain.

     

Benzimidazole/Pyridoimidazole‐Based Organic Sensitizers for High‐Performance Dye‐Sensitized Solar Cells

A new series of benzimidazole ( BIm )‐based dyes ( SC32 and SC33 ) and pyridoimidazole‐( PIm ) based dyes ( SC35, SC36N and SC36 ) were synthesized as sensitizers for dye‐sensitized solar cells (DSSCs). The N‐substituent and C‐substituent at the BIm and PIm cores were found to be the dominating factor in determining the electronic properties of the dyes and their DSSCs performance. The efficiency of BIm ‐based dyes ( SC35 and SC36 ) was found to be higher than that of the PIm ‐based dyes ( SC32 and SC33 ) due to better light harvesting. The C‐substituents in SC36 , a 4‐hexylloxybenzene and a hexyl chain, are beneficial to dark current suppression, and hence SC36 achieves the best efficiency of 7.38 % (≈85 % of N719 ). The two BIm dyes have better cell efficiencies than their congeners with a bithiophene entity between the BIm and the anchor due to better light harvesting of the former.     

Benzoporphyrins: Selective Co‐sensitization in Dye‐Sensitized Solar Cells

A novel class of dyes, namely benzoporphyrins, was synthesized and implemented into dye‐sensitized solar cells. They feature complementary absorptions compared to N719 , which renders them promising candidates for co‐sensitization in DSSCs. Notably, metallated benzoporphyrins reveal a TiO₂–nanoparticle attachment that is size and aggregation dependent. Therefore, unproductive energy‐transfer events between the selectively attached dyes can be prevented. In light of the latter, an efficiency improvement of 39 % has been achieved upon selective adsorption of benzoporphyrins and N719 onto different layers of TiO₂ photoelectrode.     

Dipolar Compounds Containing Fluorene and a Heteroaromatic Ring as the Conjugating Bridge for High‐Performance Dye‐Sensitized Solar Cells

A novel series of dipolar organic dyes containing diarylamine as the electron donor, 2‐cyanoacrylic acid as the electron acceptor, and fluorene and a heteroaromatic ring as the conjugating bridge have been developed and characterized. These metal‐free dyes exhibited very high molar extinction coefficients in the electronic absorption spectra and have been successfully fabricated as efficient nanocrystalline TiO₂ dye‐sensitized solar cells (DSSCs). The solar‐energy‐to‐electricity conversion efficiencies of DSSCs ranged from 4.92 to 6.88 %, which reached 68–96 % of a standard device of N719 fabricated and measured under the same conditions. With a TiO₂ film thickness of 6 μm, DSSCs based on these dyes had photocurrents surpassing that of the N719‐based device. DFT computation results on these dyes also provide detailed structural information in connection with their high cell performance.     

Imidazole as a Donor/Acceptor Unit in Charge‐Transfer Chromophores with Extended π‐Linkers

Eleven new, stable, push–pull systems that feature 4,5‐bis[4‐(N,N‐dimethylamino)phenyl]imidazole and 4,5‐dicyanoimidazole as the donor and acceptor moieties and the systematically extended and varied π‐linker were prepared and investigated. Evaluation of the measured UV/Vis spectra, electrochemical data (cyclic voltammetry (CV), rotating‐disc voltammetry (RDV), and polarography) and calculated β and γ polarizabilities showed efficient charge transfer (CT) in biimidazole‐type chromophores. Push–pull system 27 , which features a planar thiophene‐derived π‐linker, was revealed to be the most efficient chromophore within the studied series. This chromophore possessed the most bathochromically shifted CT band, the lowest electrochemical gap, and highest β and γ polarizabilities. The CT transition was most significantly affected by structural features such as π‐linker length, planarity, conjugating arrangement, and the presence of olefinic/acetylenic or 1,4‐phenylene/thiophene subunits in the π‐linker.     

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.     

Ferrocene‐Donor and 4,5‐Dicyanoimidazole‐Acceptor Moieties in Charge‐Transfer Chromophores with π Linkers Tailored for Second‐Order Nonlinear Optics

A series of new nonlinear optical chromophores ( 1 – 15 ) that were comprised of ferrocene‐donor and 4,5‐dicyanoimidazole‐acceptor moieties and various π linkers of different length were synthesized. Support for the presence of significant D ? A interactions in these NLO‐phores was obtained from the evaluation of the quinoid character of the 1,4‐phenylene moieties and their electronic absorption spectra, which featured intense high‐energy (HE) bands that were accompanied by less‐intense low‐energy (LE) bands. The redox behavior of these compounds was investigated by cyclic voltammetry (CV) and by rotating‐disc voltammetry (RDV); their electrochemical gaps decreased steadily from 2.64 to 2.09 V. In addition to the experimentally obtained data, DFT calculations of their absorption spectra, HOMO/LUMO levels, and second‐order polarizabilities (β) (?2ω,ω,ω) were performed. A structure–property relationship study that was performed by systematically altering the π linker revealed that the intramolecular charge‐transfer and nonlinear optical properties of these inorganic–organic hybrid D? π? A systems ( 1 – 15 ) were primarily affected by: 1) The presence of olefinic/acetylenic subunits; 2) the length of the π linker; and 3) the spatial arrangement (planarity) of the π linker.     

Increasing Electron‐Transfer Rates with Increasing Donor–Acceptor Distance

Electron transfer can readily occur over long (≥15 Å) distances. Usually reaction rates decrease with increasing distance between donors and acceptors, but theory predicts a regime in which electron‐transfer rates increase with increasing donor–acceptor separation. This counter‐intuitive behavior can result from the interplay of reorganization energy and electronic coupling, but until now experimental studies have failed to provide unambiguous evidence for this effect. We report here on a homologous series of rigid rodlike donor‐bridge‐acceptor compounds in which the electron‐transfer rate increases by a factor of 8 when the donor–acceptor distance is extended from 22.0 to 30.6 Å, and then it decreases by a factor of 188 when the distance is increased further to 39.2 Å. This effect has important implications for solar energy conversion.     

Ultrafast Photoinduced Electron Transfer and Charge Stabilization in Donor–Acceptor Dyads Capable of Harvesting Near‐Infrared Light

To harvest energy from the near‐infrared (near‐IR) and infrared (IR) regions of the electromagnetic spectrum, which constitutes nearly 70 % of the solar radiation, there is a great demand for near‐IR and IR light‐absorbing sensitizers that are capable of undergoing ultrafast photoinduced electron transfer when connected to a suitable electron acceptor. Towards achieving this goal, in the present study, we report multistep syntheses of dyads derived from structurally modified BF₂‐chelated azadipyrromethene (ADP; to extend absorption and emission into the near‐IR region) and fullerene as electron‐donor and electron‐acceptor entities, respectively. The newly synthesized dyads were fully characterized based on optical absorbance, fluorescence, geometry optimization, and electrochemical studies. The established energy level diagram revealed the possibility of electron transfer either from the singlet excited near‐IR sensitizer or singlet excited fullerene. Femtosecond and nanosecond transient absorption studies were performed to gather evidence of excited state electron transfer and to evaluate the kinetics of charge separation and charge recombination processes. These studies revealed the occurrence of ultrafast photoinduced electron transfer leading to charge stabilization in the dyads, and populating the triplet states of ADP, benzanulated‐ADP and benzanulated thiophene‐ADP in the respective dyads, and triplet state of C₆₀ in the case of BF₂‐chelated dipyrromethene derived dyad during charge recombination. The present findings reveal that these sensitizers are suitable for harvesting light energy from the near‐IR region of the solar spectrum and for building fast‐responding optoelectronic devices operating under near‐IR radiation input.     

A p‐Type NiO‐Based Dye‐Sensitized Solar Cell with an Open‐Circuit Voltage of 0.35 V

In tandem : Employing a molecular dyad and a cobalt‐based electrolyte gives a threefold‐increase in open‐circuit voltage (V_OC) for a p‐type NiO device (V_OC=0.35 V), and a fourfold better energy conversion efficiency. Incorporating these improvements in a TiO₂/NiO tandem dye‐sensitized solar cell (TDSC), results in a TDSC with a V_OC=0.91 V (see figure; CB=conductance band, VB= valence band).