High open-circuit voltage solid-state dye-sensitized solar cells with organic dye

A theoretical study on the role of the π-spacer in the thoughtful design of good light-absorbing dyes with phenothiazine for efficient dye-sensitized solar cells (DSSCs)

CONTEXT

In this work, we designed ten new organic phenothiazine dyes bridged by different πi-spacers (PTZ1-PTZ10) of D-π-A type based on the synthesized dye CC202-III for their efficacy in dye-sensitized solar cells (DSSC) applications. To learn how various π-spacers affect their performance in DSSCs, these isolated dyes and dye-cluster systems have had their geometries, electronic structures, absorption spectra, dipole moments, and molecular electrostatic potential examined and talked about. Additionally, a number of quantization parameters that affect power conversion efficiency (PCE), including light collection efficiency (LHE), reorganization energy (λtotal), vertical dipole moment (μnormal), strength electron injection driving force (ΔGinject), regeneration driving force (ΔGreg), excited state lifetime (τ), and open circuit voltage (VOC), were calculated in order to identify the organic dyes that would be best suited for DSSC applications. Calculated results revealed that the designed dyes PTZ3, PTZ4, PTZ5, and PTZ10 exhibit a lower energy gap among all dyes compared to the corresponding CC202-III. Additionally, PTZ3, PTZ4, PTZ5, PTZ7, PTZ8, PTZ9, and PTZ10 exhibit significant red-shifted absorption spectra compared to the other dyes with a larger oscillator strength, which improves the photocurrent density of the devices. The findings thus imply that bridge modification is a workable tactic to raise DSSC effectiveness.

METHOD

We used density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to study the electronic and photovoltaic properties of the dyes designed (PTZ1-PTZ10) to assess their effectiveness in DSSCs. DFT and TD-DFT simulations are theoretically used to deeply analyze key characteristics of all organic dyes that affect open-circuit voltage (VOC) and short-circuit current (JSC) to identify structure-property relationships.

Optical and electronic properties enhancement via chalcogenides: promising materials for DSSC applications

CONTEXT

Comparatively, metal-free sensitizers featuring the chalcogen family receive less attention despite known electronic properties for metal-chalcogenide materials. This work reports an array of optoelectronic properties using quantum chemical methods. Observed red-shifted bands within the UV/Vis to NIR regions with absorption maxima > 500 nm were consistent with increasing chalcogenide size. There is a monotonic down-shift in the LUMO and ESOP energy consistent with O 2p, S 3p, Se 4p, to Te 5p atomic orbital energies. Excited-state lifetime and charge injection free energies follow the decreasing order of chalcogenide electronegativity. Adsorption energies of dyes on TiO₂ anatase (101) range between - 0.08 and - 0.77 eV. Based on evaluated properties, selenium- and tellurium-based materials show potential use in DSSCs and futuristic device applications. Therefore, this work motivates continued investigation of the chalcogenide sensitizers and their application.

METHODS

The geometry optimization was performed at B3LYP/6-31 + G(d,p) and B3LYP/LANL2DZ level of theory for lighter and heavier atoms, respectively, using Gaussian 09. The equilibrium geometries were confirmed by the absence of imaginary frequencies. Electronic spectra were obtained at CAM-B3LYP/6-31G + (d,p)/LANL2DZ level of theory. Adsorption energies for dyes on a 4 × 5 supercell TiO₂ anatase (101) were obtained using VASP. The dye-TiO₂ optimizations were employed using GGA and PBE with the PAW pseudo-potentials. The energy cutoff was set at 400 eV and convergence threshold for self-consistent iteration was set to 10^-4, and van der Waals were accounted using DFT-D3 model and on-site Coulomb repulsion potential set at 8.5 eV for Ti.

Single- and co-sensitization of triphenylamine-based and asymmetrical squaraine dyes on the anatase (001) surface for DSSC applications: Periodic DFT calculations

Dye aggregation causes poor performance of dye-sensitized solar cell (DSSC) applications through faster charge recombination of the photosensitizer with electrolyte. Triphenylamine (TBA)-based dyes feature a higher molar absorption coefficient and broadened wavelength but cannot absorb sunlight in the near-infrared (NIR) region. In contrast, the squaraine (SQ) photosensitizer, which is also called an NIR photosensitizer, has a maximum wavelength in the NIR region with high intensity. However, SQ dye suffers from dye aggregation due to its planar structure. The use of a co-sensitizer is one well-tested way to increase the power conversion efficiency (η) of solar cells by reducing dye aggregation and charge recombination. Using density functional theory (DFT) and time-dependent DFT (TDDFT), this work explains from a theoretical perspective the higher η values of the TZC1 and TZC2 dyes compared to that of asymmetric the SQ sensitizer (YR6) as free dyes. The electronic properties, reorganization energies, absorption and emission spectra, ICT parameters, and photovoltage parameters of the TZC1, TZC2, and YR6 dyes were computed using the M06/6-31G(d,p) level of theory in the gas phase and CH₂Cl₂ solvent (CPCM method). Additionally, the mono- and co-adsorption processes of TZC-based sensitizers with YR6 on the anatase (001) surface were investigated using periodic DFT calculations with the PBE + U/PAW method and the dispersion correction of the Grimme method D3. The results reveal that the use of the co-sensitized led to significant stabilization of the formed complexes by at least 1.21 eV, the panchromatic effect on the absorption spectra, and an increase in the light-harvesting ability in the NIR region, which improves the performance of DSSCs.

Theoretical study on the interaction of iodide electrolyte/organic dye with the TiO2 surface in dye-sensitized solar cells

The iodide/triiodide interaction with the dye on a semiconductor surface plays a significant role in understanding the dye-sensitized solar cells (DSSCs) mechanism and improving its efficiency. In the present study, density functional theory (DFT) calculations were used to determine the interaction between the complexed iodide redox couple with dye/TiO2 for the relevance of DSSCs. Three new metal-free organic dyes noted as D1Y, D2Y and D3Y, featured with D-π-A configuration were designed by varying functional groups on the donor moiety. We analyzed the structural and electronic properties of these dyes when standing alone and being adsorbed on the oxide surface with the iodide electrolyte. Of the designed dyes, the incorporation of a strong donor unit in D1Y and D2Y sensitizers in conjunction with iodide electrolytes on the TiO2 surface provides better adsorption and electronic properties in comparison to those from the dye alone on the TiO2 surface. Analysis of density of states (DOS) indicates that the introduction of a strong electron-donating group into the organic dye, mainly D1Y and D2Y with an iodide electrolyte on the surface remarkably upshifts the Fermi energy, thereby improving the efficiency of the DSSCs by an increase of the open-circuit voltage (Voc). The present finding constitutes the basis for achieving a deeper understanding of the intrinsic interaction taking place at the electrolyte/dye/TiO2 interface and provides us with directions for the design of efficient dyes and redox electrolytes for improving DSSCs.

Photochromic dye-sensitized solar cells with light-driven adjustable optical transmission and power conversion efficiency

Semi-transparent photovoltaics only allows for the fabrication of solar cells with an optical transmission that is fixed during their manufacturing resulting in a trade-off between transparency and efficiency. For the integration of semi-transparent devices in building, ideally solar cells should generate electricity while offering the comfort for users to self-adjust their light transmission with the intensity of the daylight. Here we report a photochromic dye-sensitized solar cell (DSSC) based on donor-π-conjugated bridge-acceptor structures where the π-conjugated bridge is substituted for a diphenyl-naphthopyran photochromic unit. DSSCs show change in colour and self-adjustable light transmittance when irradiated with visible light and a power conversion efficiency up to 4.17%. The colouration-decolouration process is reversible and these DSSCs are stable over 50 days. We also report semi-transparent photo-chromo-voltaic mini-modules (23 cm²) exhibiting a maximum power output of 32.5 mW after colouration.

A DFT study to probe homo-conjugated norbornylogous bridged spacers in dye-sensitized solar cells: an approach to suppressing agglomeration of dye molecules

This work reports a sigma-bridged framework as spacers to design new dye-sensitized solar cells. The norbornylogous bridged spacer can avoid π–π aggregation of dye molecules on the semiconductor surface in DSSCs. These sesquinorbornatrienes are known to exhibit electron propagation through the interaction of sigma and π orbitals via through bond (OITB) and through space (OITS) mechanisms. Density functional theory (DFT) calculations performed with these spacers and a modelled simple donor unit like N,N-dimethylamine and cyanoacrylic acid as the anchoring group showed significant results with the requisite optical parameters for DSSCs. The newly designed dyes have shown comparable or better optical properties compared to the reference dye molecule with π-conjugated thiophene spacer units. The ΔG_injection, V_OC and μ_normal values calculated for the designed dyes were found to be higher than those of the reference system. The trans-sesquinorbornatriene system spacer (6-D) showed a V_OC of 3.3 eV, ΔG_injection of 2.4 eV and oscillatory strength (f) of 0.96. The total and partial density of states indicates a good communication between the valence and conduction band for the designed dyes. Transition density matrix results suggest that the exciton dissociation in the excited state is sufficiently high to overcome the coulombic attraction of the hole. These results are promising for the design of dye molecules with such scaffolds, to achieve better efficiency and to eliminate one of the major issues with π-spacer units in DSSCs.

This work reports homoconjugated norbornylogous spacers to supress agglomeration of dye molecules with improved efficiency of dye sensitized solar cells.

Helical Peptides Design for Molecular Dipoles Functionalization of Wide Band Gap Oxides

The use of helical hexapeptides to establish a surface dipole layer on a TiO₂ substrate, with the goal of influencing the energy levels of a coadsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric acid (Aib) residues, were protected at the N-terminus with a carboxybenzyl group (Z) and at the C-terminus carried either a carboxylic acid or an isophthalic acid (Ipa) anchor group to form Z-(Aib)₆-COOH or Z-(Aib)₆-Ipa, respectively. Using a combination of vibrational and photoemission spectroscopies, bonding of the two peptides to TiO₂ surfaces (either nanostructured or single-crystal TiO₂(110)) was found to be highly dependent on the anchor group, with Ipa establishing a monolayer much more efficiently than COOH. Furthermore, a monolayer of Z-(Aib)₆-Ipa on TiO₂(110) was exposed for different binding times to a solution of a zinc tetraphenylporphyrin (ZnTPP) derivative terminated with an Ipa anchor group (ZnTPP-P-Ipa). Photoemission spectroscopy revealed that ZnTPP-P-Ipa partly displaced Z-(Aib)₆-Ipa, forming a coadsorbed monolayer on the oxide surface. The presence of the peptide molecular dipole shifted the HOMO levels of the ZnTPP group to lower energy by ∼300 meV, in accordance with a simple parallel plate capacitor model. These results suggest that a mixed-layer approach, involving coadsorption of a strong molecular dipole compound with a chromophore, is a versatile method to shift the energy levels of such chromophores with respect to the band edges of the substrate.

A perspective on using experiment and theory to identify design principles in dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) have been the subject of wide-ranging studies for many years because of their potential for large-scale manufacturing using roll-to-roll processing allied to their use of earth abundant raw materials. Two main challenges exist for DSC devices to achieve this goal; uplifting device efficiency from the 12 to 14% currently achieved for laboratory-scale 'hero' cells and replacement of the widely-used liquid electrolytes which can limit device lifetimes. To increase device efficiency requires optimized dye injection and regeneration, most likely from multiple dyes while replacement of liquid electrolytes requires solid charge transporters (most likely hole transport materials - HTMs). While theoretical and experimental work have both been widely applied to different aspects of DSC research, these approaches are most effective when working in tandem. In this context, this perspective paper considers the key parameters which influence electron transfer processes in DSC devices using one or more dye molecules and how modelling and experimental approaches can work together to optimize electron injection and dye regeneration.

A comparative study of the influence of N,N'-dialkyl vs. N,N'-diaryl-based electron donor ancillary ligands on photocurrent and photovoltage in dye-sensitized solar cells (DSSCs)

In this study, we report the synthesis of a novel heteroleptic Ru(ii)-sensitizer, (Ru(2,2'-bipyridine-4,4'-dicarboxylic acid)-4,4'-bis(4-piperidin-1-yl)phenyl ethenyl)-(2,2'-bipyridine) (NCS)₂, denoted as SD-1; moreover, its photophysical, electrochemical, and photovoltaic performances were compared with those of N719 and K77-7 (N,N'-diaryl Ru-sensitizer, namely Ru(2,2'-bipyridine-4,4'-dicarboxylic-acid)-4,4'-bis(2-(4-N,N'-diphenylaminophenyl)ethenyl)-2,2'-bipyridine (NCS)₂). The photovoltaic performance of SD-1 outperformed those of N-719 and K77-7, particularly in the red region, and the overall efficiency of SD-1 was 8.5% as compared to 8.0% of K77-7 and 7.7% of N719 under the same experimental device conditions. The superior light harvesting efficiency of SD-1 can be attributed to the strong electron donor sp³-nitrogen, which is attached to two sp³-carbons (dialkyl), whereas in the case of K77-7, all carbon atoms attached to the sp³-nitrogen are sp², which decrease the electron density on the latter and minimize the electron-donating power of the ancillary ligand in K77-7. To gain a quantitative understanding of the electron density on nitrogen in SD-1 and K77-7, first-principle calculations using molecular and thermodynamic descriptors, such as frontier molecular orbitals, ground-state oxidation potential (GSOP), excited-state oxidation potential (ESOP), optical gap (E_0-0), and charge distributions, were conducted in solution. In addition, for understanding the anchored structures of dyes on Ti₂₄O₄₈, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were utilized. Results of computational studies are in excellent agreement with the experimental results, which can be used as a screening tool for the design of more efficient molecular motifs for DSSCs.

Efficiency difference between furan- and thiophene-based D–π–A dyes in DSSCs explained by theoretical calculations

The performance of two donor–π-bridge–acceptor type phenothiazine dyes bearing different π-bridges (furan and thiophene) was investigated by density functional theory and time-dependent density functional theory to explore the reasons for the differences in DSSC efficiency. It was revealed that dye1 with furan showed higher short-circuit photocurrent density due to its larger driving force and better light harvesting efficiency compared with dye2. Moreover, a larger number of photo-injected electrons into TiO₂ for dye1 leads to higher open-circuit photovoltage. Our results indicate that furan could be used as a promising π-bridge to improve the efficiency of PTZ dyes. We hope that our work can provide a theoretical basis and view for designing efficient dyes in dye-sensitized solar cells (DSSCs).

The performance of two D–π–A type phenothiazine dyes bearing different π-bridges (furan and thiophene) was investigated by DFT and TDDFT to explore the reasons for the differences in DSSC efficiency.

Orthogonally Functionalized Donor/Acceptor Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: An Approach to Introduce Panchromaticity and Control the Charge Recombination

Organic dyes possessing conjugated π-framework forms closely packed monolayers on photoanode in dye-sensitized solar cell (DSSC), because of the limitation to control the orientation and the extend of intermolecular π-π interaction, self-aggregation of dyes leads to reduced cell performance. In this report, a series of homodimeric (D₁-D₁ and D₂-D₂) and heterodimeric (D₁-D₂ and D₂-D₄) donor/acceptor (D/A) dyes containing spiroBiProDOT π-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and correlates the device performance with monomeric dyes (D₁ and D₂). Both the thiophenes (π-spacer) of spiroBiProDOT were functionalized with same or different donor groups which led to homodimeric and heterodimeric chromophores in a single sensitizer. The homodimeric spiro-dye D₁-D₁ showed higher power conversion efficiency (PCE), of 7.6% with a V_oc and J_sc of 0.672 V and 16.16 mA/cm², respectively. On the other hand, the monomeric D₁ exhibited a PCE of 3.2% (V_oc of 0.64 V and J_sc of 7.2 mA/cm²), which is lower by 2.4 fold compared to dimeric analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO₂ due to four sp³-centers, which arrest the molecular motions after chemisorption. This study shows a new molecular approach to incorporate two chromophores in the dimeric dye possessing complementary absorption characteristics toward panchromatic absorption. The attenuated charge recombination at TiO₂/Dye/redox couple interface in case of D₁-D₁, owing to better passivation of TiO₂ surface, was elucidated through impedance analysis. The FT-IR spectrum of D₁-D₁ adsorbed on TiO₂ film indicated both the carboxylic units were involved in chemisorption which makes strong coupling between dye and TiO₂.

Charge Transfer Enhancement in the D-π-A Type Porphyrin Dyes: A Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) Study

The electronic geometries and optical properties of two D-π-A type zinc porphyrin dyes (NCH₃-YD2 and TPhe-YD) were systematically investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to reveal the origin of significantly altered charge transfer enhancement by changing the electron donor of the famous porphyrin-based sensitizer YD2-o-C8. The molecular geometries and photophysical properties of dyes before and after binding to the TiO₂ cluster were fully investigated. From the analyses of natural bond orbital (NBO), extended charge decomposition analysis (ECDA), and electron density variations (Δρ) between the excited state and ground state, it was found that the introduction of N(CH₃)₂ and 1,1,2-triphenylethene groups enhanced the intramolecular charge-transfer (ICT) character compared to YD2-o-C8. The absorption wavelength and transition possess character were significantly influenced by N(CH₃)₂ and 1,1,2-triphenylethene groups. NCH₃-YD2 with N(CH₃)₂ groups in the donor part is an effective way to improve the interactions between the dyes and TiO₂ surface, light having efficiency (LHE), and free energy change (ΔG_inject), which is expected to be an efficient dye for use in dye-sensitized solar cells (DSSCs).

Ultrafast Charge Transfer and Enhanced Absorption in MoS2-Organic van der Waals Heterojunctions Using Plasmonic Metasurfaces

Hybrid organic-inorganic heterostructures are attracting tremendous attention for optoelectronic applications due to their low-cost processing and high performance in devices. In particular, van der Waals p-n heterojunctions formed between inorganic two-dimensional (2D) materials and organic semiconductors are of interest due to the quantum confinement effects of 2D materials and the synthetic control of the physical properties of organic semiconductors, enabling a high degree of tunable optoelectronic properties for the heterostructure. However, for photovoltaic applications, hybrid 2D-organic heterojunctions have demonstrated low power conversion efficiencies due to the limited absorption from constraints on the physical thickness of each layer. Here, we investigate the ultrafast charge transfer dynamics between an organic polymer:fullerene blend and 2D n-type MoS₂ using transient pump-probe reflectometry. We employ plasmonic metasurfaces to enhance the absorption and charge photogeneration within the physically thin hybrid MoS₂-organic heterojunction. For the hybrid MoS₂-organic heterojunction in the presence of the plasmonic metasurface, the charge generation within the polymer is enhanced 6-fold, and the total active layer absorption bandwidth is increased by 90 nm relative to the polymer:fullerene blend alone. We demonstrate that MoS₂-organic heterojunctions can serve as hybrid solar cells, and their efficiencies can be improved using plasmonic metasurfaces.

Achievement of over 1.4 V photovoltage in a dye-sensitized solar cell by the application of a silyl-anchor coumarin dye

A dye-sensitized solar cell (DSSC) fabricated by using a novel silyl-anchor coumarin dye with alkyl-chain substitutes, a Br₃⁻/Br⁻ redox electrolyte solution containing water, and a Mg²⁺-doped anatase-TiO₂ electrode with twofold surface modification by MgO and Al₂O₃ exhibited an open-circuit photovoltage over 1.4 V, demonstrating the possibility of DSSCs as practical photovoltaic devices.

Does the position of the electron-donating nitrogen atom in the ring system influence the efficiency of a dye-sensitized solar cell? A computational study

We have reported a number of new metal-free organic dyes (2-6) that have cyclic asymmetric benzotripyrrole derivatives as donor groups with peripheral nitrogen atoms in the ring, fluorine and thiophene groups as π-spacers, and a cyanoacrylic acid acceptor group. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were employed to examine the influence of the position of the donor nitrogen atom and π-conjugation on solar cell performance. The calculated electron-injection driving force (ΔG inject), electron-regeneration driving force (ΔG regen), light-harvesting efficiency (LHE), dipole moment (μ normal), and number of electrons transferred (∆q) indicate that dyes 3, 4, and 6 have significantly higher efficiencies than reference dye 1, which exhibits high efficiency. We also extended our comparison to some other reported dyes, 7-9, which have a donor nitrogen atom in the middle of the ring system. The computed results suggest that dye 6 possesses a higher incident photon to current conversion efficiency (IPCE) than reported dyes 7-9. Thus, the use of donor groups with peripheral nitrogen atoms appears to lead to more efficient dyes than those in which the nitrogen atom is present in the middle of the donor ring system. Graphical Abstract The locations of the nitrogen atoms in the donor groups in the designed dye molecules have an important influence on DSSC efficiency.

Synthesis and spectral characterization of bis(4-amino-5-mercapto-1,2,4-triazol-3-yl)propane

Bis(4-amino-5-mercapto-1,2,4-triazol-3-yl)propane (BAMTP) was synthesized and characterized by FT-IR and FT-Raman spectra. Gas phase structure of BAMTP was examined under density functional theory B3LYP/6-311++G(d, p) level of basis set, wherein the molecule was subjected to conformational analysis. Thus the identified stable structure utilized for the calculations such as geometry optimization, vibrational behavior, hyperpolarizability analysis, natural bond orbital analysis, band gap, chemical hard/softness and stability. Geometry of BAMTP has been discussed elaborately with related crystal data. The results found from experimental and theoretical methods were reported herewith.

Tuning Thiophene-Based Phenothiazines for Stable Photocatalytic Hydrogen Production

Dibranched donor-(π-acceptor)2 dyes, where phenothiazine is the donor core, cyanoacrylic acid is the acceptor/anchoring group, and π is represented by mono- and poly-cyclic simple and fused thiophene derivatives, were tested as photosensitizers in the photocatalytic production of H2 , in combination with a Pt/TiO2 catalyst. The optical and electrochemical properties of the dyes were investigated, showing that careful design of the thiophene-based π spacer afforded enhanced optical properties. In the H2 production over 20 h, the new thiophene-based sensitizers revealed improved stability after longer irradiation times and enhanced performances, in terms of H2 production rates and light-to-fuel efficiencies, after an initial activation period, which were for the first time associated with enhanced stability under photocatalytic production of H2 and the absence of critical dye degradation.

Triphenylamine-based indoline derivatives for dye-sensitized solar cells: a density functional theory investigation

A new series of triphenylamine-based indoline dye sensitizers were molecularly designed and investigated for their potential use in dye-sensitized solar cells (DSSCs). Theoretical calculations revealed that modifying donor part of D149 by triphenylamine significantly altered the electronic structures, MO energies, and intramolecular charge transfer (ICT) absorption band. Key parameters associated with the light-harvesting efficiency at a given wavelength LHE(λ), the driving force ΔG inject, and the open-circuit photovoltage V oc were characterized. More importantly, these designed (dimeric) dye sensitizers were found to have similar broad absorption spectra to their corresponding monomers, indicating that modifying the donor part with triphenylamine may stop unfavorable dye aggregation. Further analyses of the dye-(TiO2)9 cluster interaction confirmed that there was strong electronic coupling at the interface. These results are expected to provide useful guidance in the molecular design of new highly efficient metal-free organic dyes.

How to Optimize the Interface between Photosensitizers and TiO2 Nanocrystals with Molecular Engineering to Enhance Performances of Dye-Sensitized Solar Cells?

In this work, the interfacial properties of a series of metal-free organic naphthodithienothiophene (NDTT)-based photosensitizers adsorbed on TiO2 surfaces were investigated by a combination of ab initio calculations and experimental measurements. The calculations and experiments reveal that because of the efficient charge transfer from the adsorbed dyes to TiO2 nanocrystal surface there is an upward shift for the energy levels of dyes and a downward shift for the conduction band of surface TiO2 and that the band gaps for both of them are also reduced. Such electronic level alignments at the interface would lead to increased light absorption range by adsorbed dyes and increased driving force for charge injection but reduced open-circuit potential (V(oc)). More interestingly, we found that molecule engineering of the donor group and introducing additional electron-withdrawing unit have little effect on the electronic level alignments at the interface (because band gaps of the dyes adsorbed on TiO2 surfaces become approximately identical when compared with those of the dyes measured in solution) but that they can affect the steric effect and the charge separation at the interface to tune V(oc) and the short-circuit current density (J(sc)) effectively. All these findings suggest that optimizing the interfacial properties of dyes adsorbed on TiO2 surfaces by synchronously modifying steric effects of dye molecules anchored on TiO2 and charge-transfer and separation properties at the interfaces is important to construct efficient dye-sensitized solar cells.

Molecular engineering of organic dyes with a hole-extending donor tail for efficient all-solid-state dye-sensitized solar cells

We report a new concept for the design of metal-free organic dyes (OD5-OD9) with an extended donor-π-acceptor (D-π-A) molecular framework, in which the donor terminal unit is attached by a hole-extending side chain to retard back electron transfer and charge recombination; the π-bridge component contains varied thiophene-based substituents to enhance the light-harvesting ability of the device. The best dye (OD9) has a D-A-π-A configuration with the hexyloxyphenylthiophene (HPT) side chain as a hole-extension component and a benzothiadiazole (BTD) internal acceptor as a π-extension component. The co-sensitization of OD9 with the new porphyrin dye LW24 enhanced the light-harvesting ability to 800 nm; thus, a power conversion efficiency 5.5 % was achieved. Photoinduced absorption (PIA) and transient absorption spectral (TAS) techniques were applied to account for the observed trend of the open-circuit voltage (VOC ) of the devices. This work provides insights into the molecular design, photovoltaic performance, and kinetics of charge recombination.

New generation solar cells: concepts, trends and perspectives

Organic, dye-sensitized and perovskite solar cell technologies have triggered widespread interest in recent years due to their very promising potential towards a high solar electricity future. A number of important milestones have marked the roadmap of each sector on the way to today's outstanding performances, but there still remains plenty of scope for further improvement. The most influential landmarks, together with basic concepts and future perspectives, are unraveled in this review.

Metal-free tetrathienoacene sensitizers for high-performance dye-sensitized solar cells

A new series of metal-free organic chromophores (TPA-TTAR-A (1), TPA-T-TTAR-A (2), TPA-TTAR-T-A (3), and TPA-T-TTAR-T-A (4)) are synthesized for application in dye-sensitized solar cells (DSSC) based on a donor-π-bridge-acceptor (D-π-A) design. Here a simple triphenylamine (TPA) moiety serves as the electron donor, a cyanoacrylic acid as the electron acceptor and anchoring group, and a novel tetrathienoacene (TTA) as the π-bridge unit. Because of the extensively conjugated TTA π-bridge, these dyes exhibit high extinction coefficients (4.5-5.2 × 10(4) M(-1) cm(-1)). By strategically inserting a thiophene spacer on the donor or acceptor side of the molecules, the electronic structures of these TTA-based dyes can be readily tuned. Furthermore, addition of a thiophene spacer has a significant influence on the dye orientation and self-assembly modality on TiO2 surfaces. The insertion of a thiophene between the π-bridge and the cyanoacrylic acid anchoring group in TPA-TTAR-T-A (dye 3) promotes more vertical dye orientation and denser packing on TiO2 (molecular footprint = 79 Å(2)), thus enabling optimal dye loading. Using dye 3, a DSSC power conversion efficiency (PCE) of 10.1% with Voc = 0.833 V, Jsc = 16.5 mA/cm(2), and FF = 70.0% is achieved, among the highest reported to date for metal-free organic DSSC sensitizers using an I(-)/I3(-) redox shuttle. Photophysical measurements on dye-grafted TiO2 films reveal that the additional thiophene unit in dye 3 enhances the electron injection efficiency, in agreement with the high quantum efficiency.

Molecular interfaces for plasmonic hot electron photovoltaics

The use of self-assembled monolayers (SAMs) to improve and tailor the photovoltaic performance of plasmonic hot-electron Schottky solar cells is presented. SAMs allow the simultaneous control of open-circuit voltage, hot-electron injection and short-circuit current. To that end, a plurality of molecule structural parameters can be adjusted: SAM molecule's length can be adjusted to control plasmonic hot electron injection. Modifying SAMs dipole moment allows for a precise tuning of the open-circuit voltage. The functionalization of the SAM can also be selected to modify short-circuit current. This allows the simultaneous achievement of high open-circuit voltages (0.56 V) and fill-factors (0.58), IPCE above 5% at the plasmon resonance and maximum power-conversion efficiencies of 0.11%, record for this class of devices.