New organic dye-sensitized solar cells based on the D-A-π-A structure for efficient DSSCs: DFT/TD-DFT investigations

Global energy consumption has increased due to population growth and economic development. Solar energy is one of the most important renewable energy sources for human consumption. In this research, four novel organic dyes (D2-D5) of the D-A-π-A structure based on triphenylamine (TPA) were studied theoretically using DFT and TD-DFT techniques for future usage as dye-sensitized solar cells (DSSCs). The effects of modifying the π-spacer of the reference molecule D1 on the structural, electronic, photovoltaic, and optical characteristics of the D2-D5 dyes were studied in detail. D2-D5 exhibited band gaps (E _gap) in the range from 1.89 to 2.10 eV with λ _abs in the range of 508 to 563 nm. The results obtained show that modifying the π-spacer of the dye D1 increased its hole injection and reinforced the intramolecular charge-transfer (ICT) impact, which resulted in a red-shifted ICT absorption with a greater molar extinction coefficient. The theoretically calculated open-circuit voltage (V _oc) values ranged from 0.69 to 1.06 eV, while the light-harvesting efficiency (LHE) values varied from 0.95 to 0.99. Indeed, this theoretical research could guide chemists to synthesize effective dyes for DSSCs.

Novel dual acceptor (D–D′–A′–π–A) dye-sensitized solar cells based on the triarylamine structure and benzothiadiazole double electron withdrawing unit

Three D–D′–A′–π–A dyes BT4–6 using triarylamine derivatives with varied D′ units as donors were synthesized. The result suggests that the difference of D′ units have significant effects on their physicochemical property and photovoltaic performance.

DFT/TD-DFT study of novel T shaped phenothiazine-based organic dyes for dye-sensitized solar cells applications

Five novel T shaped phenothiazine-based organic dyes DTTP1~5 with different spacers at N (10) position were designed. The geometries, electronic structures, absorption spectra, electron transfer and injection properties of these isolated dyes and dye/(TiO₂)₉ systems were investigated via density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculation. The optimized geometries indicate that these T shaped dyes show non-planar conformations, which are helpful in suppressing the close intermolecular π-π aggregation in device and enhancing thermal stability. The calculated results indicate that type of π-conjugated spacers can affect the molecular absorption spectra. Introduction of thiophene-benzothiadizole-thiophene unit as π-conjugated spacer can most effectively shift the light absorption to near infrared region and enhance the light harvesting efficiency (LHE). Moreover, it is found that these dyes show a good performance of electron injection and dye regeneration owing to the proper electron injection driving force (ΔG_inject) and dye regeneration driving force (ΔG_reg). The theoretical results reveal that these dyes could be used as potential sensitizers for DSSCs, and DTTP4 would be the most plausible sensitizer for high-efficiency DSSCs due to the narrow HOMO-LUMO energy gap (Δ_H-L), broad absorption spectrum, high LHE value, and large dipole moment (μ_normal).

Molecular Engineering of Quinoxaline-Based D-A-π-A Organic Sensitizers: Taking the Merits of a Large and Rigid Auxiliary Acceptor

The continuing efforts of creating novel D-A-π-A structured organic sensitizers with excellent optoelectronic properties have resulted in substantial improvement of power conversion efficiency (PCE) as well as stability of dye-sensitized solar cells (DSSCs). Here, we report a new molecular engineering strategy for enhancing optical gain and improving excited-state features in D-A-π-A structured organic sensitizers by improving the conjugation size and rigidity of the auxiliary acceptor functional group. A series of phenanthrene-fused-quinoxaline (PFQ)-based D-A-π-A organic sensitizers (WS-82, WS-83, and WS-84) are designed and synthesized for applications in DSSCs. Compared to 2,3-diphenylquinoxaline (DPQ)-based dye IQ-4, PFQ dyes show extended absorption spectra and improved open-circuit voltage performance. Upon a systematical engineering of alkyl chains and π-spacer structure, the unfavorable issues of PFQ dyes including low solubility and high energy barrier in intramolecular charge transition are successfully eliminated. When applied in iodine electrolyte-based DSSCs, the best performing PFQ dye WS-84 shows a PCE of 10.11%, which is much higher than that of our previous champion DPQ dye IQ-4 under the same conditions.

Phenothiazine-Based D-A-π-A Dyes for Highly Efficient Dye-Sensitized Solar Cells: Effect of Internal Acceptor and Non-Conjugated π-Spacer on Device Performance

Three new D-A-π-A metal-free organic dyes based on phenothiazine as a donor (D) and non-conjugated π-spacer were designed and synthesized. The incorporation of different 'internal acceptors' (electron traps) such as benzothiadiazole (BTD), benzotriazole (BTA), and pyridine were shown to allow systematic tuning of the energy levels and the photophysical properties. The AI-1 dye showed lower electronic disorder compared with the other two dyes. The efficiencies achieved with AI-1, AI-2, and AI-3 dyes were 8.5 % (J_sc =15.42 mA cm^-2 , V_oc =0.78 V, FF=68 %), 7 % (J_sc =12.8 mA cm^-2 , V_oc =0.78 V, FF=68 %) and 6.7 % (J_sc =11.57 mA cm^-2 , V_oc =0.82 V, FF=68.26 %), respectively. The incorporation of non-conjugated phenothiazine as a π-spacer in D-A-π-A dyes showed remarkable enhancement in the photovoltaic performance of dye-sensitized solar cell (DSSC) devices. The sealed DSSC devices with iodide/tri-iodide (I^- /I₃ ^- )-based liquid electrolyte showed promising stability under ambient conditions.

Bi-anchoring Organic Dyes that Contain Benzimidazole Branches for Dye-Sensitized Solar Cells: Effects of π Spacer and Peripheral Donor Groups

Benzimidazole-branched bi-anchoring organic dyes that contained triphenylamine/phenothiazine donors, 2-cyanoacrylic acid acceptors, and various π linkers were synthesized and examined as sensitizers for dye-sensitized solar cells. The structure-activity relationships in these dyes were systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The wavelength of the absorption peak was more-heavily influenced by the nature of the π linker than by the nature of the donor. For a given donor, the absorption maximum (λmax ) was red-shifted on changing the π linker from phenyl to 2,2'-bithiophene, whilst the dyes that contained triphenylamine units displayed higher molar extinction coefficients (ϵ) than their analogous phenothiazine-based triphenylamine dyes, which led to good light-harvesting properties in the triphenylamine-based dyes. Electrochemical data for the dyes indicated that the triphenylamine-based dyes possessed relatively low-lying HOMOs, which could be beneficial for suppressing back electron transfer from the conduction band of TiO2 to the oxidized dyes, owing to facile regeneration of the oxidized dye by the electrolyte. The best performance in the DSSCs was observed for a dye that possessed a triphenylamine donor and 2,2'-bithiophene π linkers. Electron impedance spectroscopy (EIS) studies revealed that the use of triphenylamine as the donor and phenyl or 2,2'-bithiophene as the π linkers was beneficial for disrupting the dark current and charge-recombination kinetics, which led to a long electron lifetime of the injected electrons in the conduction band of TiO2 .

Molecular engineering of new phenothiazine-based D–A–π–A dyes for dye-sensitized solar cells

We report new D–A–π–A dyes gained by molecular engineering for DSSCs, achieving a highest photoelectric conversion efficiency of 6.35%.

Insight into D-A-π-A Structured Sensitizers: A Promising Route to Highly Efficient and Stable Dye-Sensitized Solar Cells

The dye-sensitized solar cell (DSSC) is one of the most promising photovoltaic technologies with potential of low cost, light weight, and good flexibility. The practical application of DSSCs requires further improvement in power conversion efficiency and long-term stability. Recently, significant progress has been witnessed in DSSC research owing to the novel concept of the D-A-π-A motif for the molecular engineering of organic photosensitizers. New organic and porphyrin dyes based on the D-A-π-A motif can not only enhance photovoltaic performance, but also improve durability in DSSC applications. This Spotlight on Applications highlights recent advances in the D-A-π-A-based photosensitizers, specifically focusing on the mechanism of efficiency and stability enhancements. Also, we find insight into the additional acceptor as well as the trade-off of long wavelength response. The basic principles are involved in molecular engineering of efficient D-A-π-A sensitizers, providing a clear road map showing how to modulate the energy bands, rationally extending the response wavelength, and optimizing photovoltaic efficiency step by step.

Computational design of small phenothiazine dyes for dye-sensitized solar cells by functionalizations affecting the thiophene unit

We present a computational density functional theory study of the potential to improve the solar absorbance of small organic dyes featuring a phenothiazine donor and an acceptor moiety that combines a thiophene unit and a cyanoacrylic group. We consider different conjugation orders and functional groups on and around the thiophene unit, including electron-donating and electron-withdrawing moieties (H, F, CH3, CF3, and CN). We predict that by combining change of conjugation order and functionalization with electron withdrawing CN groups, it must be possible to decrease the excitation energy by up to 60 % vs. the parent dye (which would correspond to a redshift of the absorption peak maximum from 450 nm to 726 nm), effectively enabling red light absorption with small dyes. The contraction of the band gap is mostly due to the stabilization of the LUMO (by up to 1.8 eV), so that-in spite of the kinetic redundancy of the parent dye with respect to the conduction-band minimum of TiO2-care must be taken to ensure efficient injection when using the dyes in dye-sensitized solar cells. By studying 50 dyes, of which 44 are new dyes that are studied for the first time in this work, we identify parameters (such as charges, dihedral angles between donor and acceptor groups, bond length alternation) which can serve as predictors of the band gap. We find that bond length alternation or dihedral angles are not good predictors, while the charge on the thiophene unit is.

Tri(N-carbazolyl)triphenylamine-based starburst organic dyes: effects of different acceptors on the optical, electrochemical and photovoltaic properties

Novel tri(N-carbazolyl)triphenylamine-based organic dyes were designed, synthesized and applied as photosensitizers for DSSCs.

Biindole-based double D–π–A branched organic dyes for efficient dye-sensitized solar cells

Novel biindole-based double D–π–A branched organic dyes have been designed and synthesized for dye-sensitized solar cells.

Effect of a long alkyl group on cyclopentadithiophene as a conjugated bridge for D-A-π-A organic sensitizers: IPCE, electron diffusion length, and charge recombination

The option of using conjugated π-linkers is critical for rational molecular design toward an energy-level strategy for organic sensitizers. To further optimize photovoltaic performance, methyl- and octyl-substituted 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) are introduced into D-A-π-A featured sensitizers. Along with CPDT, instead of thiophene as conjugated bridge, WS-39 and WS-43 exhibit an extended spectral response due to the excellent conjugation and coplanarity of CPDT. Specifically, we focused on the critical effect of length of the alkyl group linked to the bridging carbon atoms of CPDT on the photovoltaic performances. Octyl-substituted WS-39 shows a broader IPCE onset with an enhanced photovoltage relative to the analogue WS-5. In contrast, WS-43, with methyl substituted on the CPDT moiety, presents a relatively low quantum conversion efficiency within the whole spectral response region, along with low photocurrent density. WS-43 displays a distinctly low IPCE platform, predominately arising from the short electron diffusion length with significant electron loss during the electron transport. The relative movement of the conduction band edge (E(CB)) and charge transfer resistance as well as lifetime of injected electrons are studied in detail. Under standard AM 1.5 conditions, WS-39-based solar cells show a promising photovoltaic efficiency of 9.07% (J(SC) = 16.61 mA cm(-2), V(OC) = 770 mV, FF = 0.71). The octyl chains attached on CPDT can provide dual protection and exhibit a high propensity to prevent binding of the iodide-triiodide redox couple, producing an efficient shielding effect to retard the charge recombination and resulting in improvement of V(OC). Our research paves the way to explore more efficient sensitizers through ingenious molecular engineering.