β-Functionalized Push-Pull Porphyrin Sensitizers in Dye-Sensitized Solar Cells: Effect of π-Conjugated Spacers

Excited state electron transfer in A2 and A2B2 functionalized zinc porphyrins carrying rigid and flexible β-pyrrole π-extended substituents

Optical absorption and emission, electrochemical, and photochemical properties of peripherally functionalized with flexible and rigid [Formula: see text]-extended substituents on A2 and A2B2 type zinc porphyrins is investigated. The significance of rigid [Formula: see text]-substituents over flexible ones in governing the spectral properties is unraveled. Flexible [Formula: see text]-substituents on the porphyrin ring caused appreciable spectral broadening compared to porphyrin carrying rigid [Formula: see text]-substituents. Further, supramolecular dyads are formed by coordinating phenyl imidazole functionalized fullerene, C[Formula: see text]Im. The calculated binding constants for the 1:1 complexes is in the order of 2–7 × 105 M[Formula: see text] suggesting stable complex formation. Free-energy calculations performed according to the Rehm–Weller approach suggested possibility of excited state electron transfer in these dyads. Femtosecond transient absorption studies of the dyads performed in [Formula: see text]-dichlorobenzene showed evidence of occurrence of electron transfer from the singlet-excited state that was in competition with the intersystem crossing process to populate the triplet-excited state of porphyrins.

Synthesis and Characterization of Novel β-Bis( N, N-diarylamino)-Substituted Porphyrin for Dye-Sensitized Solar Cells under 1 sun and Dim Light Conditions

In this work, we have synthesized a novel porphyrin dye named SK7, which contains two N, N-diarylamino moieties at two β-positions as electron-donating units and one carboxy phenylethynyl moiety at the meso-position as an electron-withdrawing, anchoring group. This novel dye was tested for the application in dye-sensitized solar cells. The light-harvesting behavior of SK7 and YD2 was investigated using UV-vis absorption and density functional calculation. The electron transport properties at the TiO₂/dye/electrolyte interface for SK7- and YD2-based devices were evaluated by electrochemical impedance spectroscopy. X-ray crystallographic characterization was conducted to understand the influence of two N, N-diarylamino units at two β-positions. The power conversion efficiencies of ca. 6.54% under 1 sun illumination (AM 1.5G) and ca. 19.72% under a T5 light source were noted for the SK7 dye. The performance of SK7 is comparable to that of dye YD2, which contains only one N, N-diarylamino moiety at the meso-position (ca. 7.78 and 20.00% under 1 sun and T5 light, respectively).

Investigation of the push–pull effects on β-functionalized zinc porphyrin coordinated to C60 donor–acceptor conjugates

Two novel β-functionalized push–pull zinc porphyrins with amine or phenyl push-groups and cyclic imide or carboxylic esters pull-groups have been newly synthesized for light energy harvesting applications. The ethynylphenyl spacers extended the conjugation of the porphyrin π-systems, as reflected by their red-shifted absorbance and fluorescence spectra. Computational studies performed at the B3LYP/6-31G* level indicated no steric hindrance between the porphyrin π-system and the substituents. The calculated HOMO and LUMO orbitals displayed significant delocalization in 1, where the electron density in the HOMO was localized over the push groups, whereas the LUMO was extended over the porphyrin π- and pull-groups. On the other hand, 2 did not display significant orbital segregation, which is attributed to its weaker push–pull character. Electrochemical studies revealed smaller HOMO–LUMO gaps for the push–pull zinc porphyrins. As a consequence of the push–pull effects, reduction in fluorescence intensity and lifetime was observed. Femtosecond transient absorption spectral studies revealed successful formation of singlet excited state in both of the push–pull porphyrins. Donor–acceptor conjugates were subsequently built and characterized by coordinating an electron acceptor, C60Im, via metal–ligand axial coordination. Efficient photo-induced charge separation in both donor–acceptor conjugates was witnessed wherein the charge-separated states persisted tens of nanoseconds prior returning to the ground state.

Designing Push-Pull Porphyrins for Efficient Dye-Sensitized Solar Cells

Over the past decade, tremendous effort has been made to improve the light-harvesting ability of push-pull porphyrin dyes. Despite notable success achieved in this direction, push-pull porphyrin dyes still suffer from a poor light-harvesting efficiency owing to the lack of absorption between the Soret and Q-bands. To tackle this issue, here we design a series of push-pull porphyrin dyes with anchoring groups either at meso- or β-position using calculations based on first-principles time-dependent density functional theory. In contrast to the common perception, we find that porphyrin dyes bearing an electron-donor at the meso-position and an electron-acceptor at the β-position produce an additional extended band between the Soret and Q-bands appearing at around 500 nm due to S₀ → S₃ excitation, leading to a much higher light-harvesting performances compared to meso- and β-disubstituted ones. In addition, changing the π-conjugated linker at the acceptor site from ethylene linker (C═C) to acetylene linker (C≡C) further improves the light-harvesting ability of meso-β-porphyrin dyes, making them promising candidates for dye-sensitized solar cell application.

Porphyrin-sensitized solar cells: systematic molecular optimization, coadsorption and cosensitization

As a promising low-cost solar energy conversion technique, dye-sensitized solar cells have undergone spectacular development since 1991. For practical applications, improvement of power conversion efficiency has always been one of the major research topics. Porphyrins are outstanding sensitizers endowed with strong sunlight harvesting ability in the visible region and multiple reaction sites available for functionalization. However, judicious molecular design in consideration of light-harvest, energy levels, operational dynamics, adsorption geometry and suppression of back reactions is specifically required for achieving excellent photovoltaic performance. This feature article highlights some of the recently developed porphyrin sensitizers, especially focusing on the systematic dye structure optimization approach in combination with coadsorption and cosensitization methods in pursuing higher efficiencies. Herein, we expect to provide more insights into the structure-performance correlation and molecular engineering strategies in a stepwise manner.

Theoretical investigations on the unsymmetrical effect of β-link Zn-porphyrin sensitizers on the performance for dye-sensitized solar cells

Dye sensitizers play an important role in dye-sensitized solar cells (DSSCs). As a promising strategy for the design of novel porphyrin sensitizers, the asymmetric modification of the porphyrin ring to meso-link porphyrin sensitizer has emerged in recent years, which can improve the light-harvesting properties and enhance the electron distribution. In this work, in order to reveal the essence of the effect of unsymmetrical substitution on the performance of β-link porphyrin dyes in DSSCs, four kinds of common β-link porphyrin dyes with different structures are calculated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The electronic structures and optical properties of these studied dyes in dimethylformamide (DMF) are also investigated. The key parameters of the short-circuit current density (J_sc), including light harvesting efficiency (LHE), electron injection driving force (ΔG_inject), and intra-molecular charge transfer (ICT) are discussed in detail. In addition, the periodic DFT calculations in the dye-TiO₂ systems are also employed to investigate the geometrical and electronic injection process of the different connection types of these studied dyes adsorbed on the periodic TiO₂ model with an exposed anatase (101) surface. We expect the present study would deepen the understanding of the alternative function of unsymmetrical substitution and may contribute to future DSSC design.

4D-π-1A type β-substituted ZnII-porphyrins: ideal green sensitizers for building-integrated photovoltaics

Two novel green β-substituted Zn^II-porphyrins, G1 and G2, based on a 4D-π-1A type substitution pattern have been synthesized. Their enhanced push-pull character, by reduction of H-L energy gaps, promotes broadening and red-shifting of absorption bands. The effective synthetic pathway and the remarkable spectroscopic properties make G2 ideal for BIPV application.

Efficient Solar Cells Based on Porphyrin Dyes with Flexible Chains Attached to the Auxiliary Benzothiadiazole Acceptor: Suppression of Dye Aggregation and the Effect of Distortion

Donor-π-acceptor-type porphyrin dyes have been widely used for the fabrication of efficient dye-sensitized solar cells (DSSCs) owing to their strong absorption in the visible region and the ease of modifying their chemical structures and photovoltaic behavior. On the basis of our previously reported efficient porphyrin dye XW11, which contains a phenothiazine-based electron donor, a π-extending ethynylene unit, and an auxiliary benzothiadiazole acceptor, we herein report the syntheses of novel porphyrin dyes XW26-XW28 by introducing one or two alkyl/alkoxy chains into the auxiliary acceptor. The introduced chains can effectively suppress dye aggregation. As a result, XW26-XW28 show excellent photovoltages of 700, 701, and 711 mV, respectively, obviously higher than 645 mV obtained for XW11. Nevertheless, the optimized structures of XW26 and XW27 exhibit severe distortion, showing large dihedral angles of 57.2° and 44.0°, respectively, between the benzothiadiazole and benzoic acid units, resulting from the steric hindrance between the benzoic acid unit and the neighboring alkyl/alkoxy chain on the benzothiadiazole unit, and thus blue-shifted absorption, decreased photocurrents. and low efficiencies of 5.19% and 6.42% were observed for XW26 and XW27, respectively. Interestingly, XW26 exhibits a more blue-shifted absorption spectrum relative to XW27, indicating that the steric hindrance of the alkyl/alkoxy chains has a more pronounced effect than the electronic effect. Different from XW26 and XW27, XW28 contains only one alkyl chain neighboring the ethynylene unit, which does not induce obvious steric hindrance with the benzoic acid unit, and thus distortion of the molecule is not seriously aggravated compared with XW11. Hence, its absorption spectrum and photocurrent are similar to those of XW11. As a result, a higher efficiency of 9.12% was achieved for XW28 because of its suppressed dye aggregation and higher photovoltage. It is worth noting that a high efficiency of 10.14% was successfully achieved for XW28 upon coadsorption with CDCA, which is also higher than the corresponding efficiency obtained for XW11. These results provide a novel approach for developing efficient porphyrin dyes by introducing chains into the suitable position of the auxiliary benzothiadiazolyl moiety to suppress dye aggregation, without seriously aggravating distortion of the dye molecules.

Diphenyl-2-pyridylamine-Substituted Porphyrins as Hole-Transporting Materials for Perovskite Solar Cells

The susceptibility of porphyrin derivatives to light-harvesting and charge-transport operations have enabled these materials to be employed in solar cell applications. The potential of porphyrin derivatives as hole-transporting materials (HTMs) for perovskite solar cells (PSCs) has recently been demonstrated, but knowledge of the relationships between the porphyrin structure and device performance remains insufficient. In this work, a series of novel zinc porphyrin (PZn) derivatives has been developed and employed as HTMs for low-temperature processed PSCs. Key to the design strategy is the incorporation of an electron-deficient pyridine moiety to down-shift the HOMO levels of porphyrin HTMs. The porphyrin HTMs incorporating diphenyl-2-pyridylamine (DPPA) have HOMO levels that are in good agreement with the perovskite active layers, thus facilitating hole transfers from the perovskite to the HTMs. The DPPA-containing zinc porphyrin-based PSCs gave the best performance, with efficiency levels comparable to those of PSCs using spiro-OMeTAD, a current state-of-the-art HTM. In particular, PZn-DPPA-based PSCs show superior air stability, in both doped and undoped forms, to spiro-OMeTAD based devices.

Unsymmetrically Substituted Donor-π-Acceptor-Type 5,15-Diazaporphyrin Sensitizers: Synthesis, Optical and Photovoltaic Properties

The first examples of unsymmetrical β-substituted donor-π-acceptor (D-π-A)-type 5,15-diazaporphyrin (DAP) sensitizers with both p-aminophenyl and p-carboxyphenyl groups at their peripheral 3-, 7-, 13-, and/or 17-positions have been synthesized for use in dye-sensitized solar cells (DSSCs). UV/Vis absorption and emission spectroscopy, electrochemical measurements, and DFT calculations revealed that these D-π-A dyes exhibit high light-harvesting properties over the whole visible range because of the intrinsic charge-transfer character of their electronic transitions. The cell performances of TiO₂ -based DSSCs fabricated with the newly prepared DAP derivatives were evaluated under standard AM1.5 conditions. Among the four dyes examined, 13,17-bis(p-carboxyphenyl)-3,7-bis[p-(N,N-dimethylamino)phenyl]-DAP showed the highest power conversion efficiency (2.0 %), which was 20 times larger than that obtained with 3-(p-carboxyphenyl)-DAP. These results show that the DAP chromophore could be used as the electron-accepting π unit in various types of functional dyes.

Monobenzoporphyrins as Sensitizers for Dye-Sensitized Solar Cells: Observation of Significant Spacer-Group Effect

A series of monobenzoporphyrins (WH1-WH4) bearing different conjugated spacer groups were designed and synthesized as sensitizers for dye-sensitized solar cells. Although a phenyl spacer only has a minimal impact on the absorption bands of the monobenzoporphyrin, an ethynylphenyl (WH3) or a vinyl (WH4) spacer redshifts and broadens the absorption bands of the dyes to result in much enhanced light-harvesting ability. Dye-sensitized solar cells based on these monobenzoporphyrin dyes displayed remarkable differences in power conversion efficiencies (PCEs). The monobenzoporphyrin bearing no spacer (WH1) resulted in a PCE of only 0.5 %; in contrast, the monobenzoporphyrin bearing vinyl spacers (WH4) achieved a PCE of 5.2 %. The high efficiency of the WH4 cell is attributed to the higher light-harvesting ability, the lesser extent of aggregation on the TiO2 surface, and the more favorable electron-density distributions of the HOMO and LUMO for electron injection and collection. This work demonstrates the exceptional tunability of benzoporphyrins as sensitizers for dye-sensitized solar cells.

Benzoporphyrins bearing pyridine or pyridine-N-oxide anchoring groups as sensitizers for dye-sensitized solar cell

Novel benzoporphyrins bearing pyridine or pyridine-[Formula: see text]-oxide groups were prepared through a concise method based on a Pd0 catalyzed cascade reaction. These benzoporphyrins were examined as sensitizers for dye-sensitized solar cells. Vicinal pyridine and vicinal pyridine-[Formula: see text]-oxide groups were introduced as new types of anchoring/acceptor groups for dye-sensitized solar cells for the first time. While all the porphyrins showed solar to electricity conversion, benzoporphyrins bearing pyridine-[Formula: see text]-oxide anchoring groups displayed higher conversion efficiency than benzoporphyrins bearing pyridine-anchoring groups.Opp-dibenzoporphyrins display broadened and red-shifted UV-vis absorption and emission bands as compared with those of the monobenzoporphyrins, which arises from the fusion of one more benzene ring and the attachment of two more electron-withdrawing groups to the porphyrin [Formula: see text]-positions. Cyclic voltammetry (CV) data and DFT calculation data obtained for these porphyrins agree well with their UV-vis absorption and fluorescence spectroscopic data. The HOMO energy level derived from the first oxidation potentials indicate that regeneration of the resulting porphyrin radical cation by the redox mediator (I[Formula: see text]/I[Formula: see text] is thermodynamically feasible for all these benzoporphyrin sensitizers (3, 5, 8 and 10). On the other hand, excited state energy levels of these benzoporphyrins calculated from the CV data, the UV-vis and fluorescence spectroscopic data are all slightly lower than the energy level of the conduction band of TiO2, suggesting insufficient driving force for efficient electron injection from the porphyrin excited singlet state to the conduction band of TiO2.

Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Accelerated inner charge transfer in porphyrins promotes a broad light-harvesting ability up to 840 nm and a conversion efficiency of 9.2%.