How Does Bridging Core Modification Alter the Photovoltaic Characteristics of Triphenylamine-Based Hole Transport Materials? Theoretical Understanding and Prediction

Perovskite solar cells have gained immense interest from researchers owing to their good photophysical properties, low-cost production, and high power conversion efficiencies. Hole transport materials (HTMs) play a dominant role in enhancing the power conversion efficiencies (PCEs) and long diffusion length of holes and electrons in perovskite solar cells. In hole transport materials, modification of π-linkers has proved to be an efficient approach for enhancing the overall PCE of perovskite solar cells. In this work, π-linker modification of a recently synthesized H−Bi molecule ( R ) is achieved with novel π-linkers. After structural modifications, ten novel HTMs ( HB1–HB10 ) with a D−π−D backbone are obtained. The structure–property relationship, and optoelectronic and photovoltaic characteristics of these newly designed hole transport materials are examined comprehensively and compared with reference molecules. In addition, different geometric parameters are also examined with the assistance of density functional theory (DFT) and time-dependent DFT. All the designed molecules exhibit narrow HOMO–LUMO energy gaps (E_g=2.82–2.99 eV) compared with the R molecule (E_g=3.05 eV). The designed molecules express redshifting in their absorption spectra with low values of excitation energy, which in return offer high power conversion efficiencies. Further, density of states and molecular electrostatic potential analysis is performed to locate the different charge sites in the molecules. The reorganizational energies of holes and electrons are found to have good values, suggesting that these novel designed molecules are efficient hole transport materials for perovskite solar cells. In addition, the low binding energy values of the designed molecules (compared with R ) offer high current charge density. Finally, complex study of HB9:PC₆₁BM is also undertaken to understand the charge transfer between the molecules of the complex. The results of all analyses advocate that these novel designed HTMs are promising candidates for the construction of future high-performance perovskite solar cells.     

Exploring the potential of tetraazaacene derivatives as photovoltaic materials with enhanced photovoltaic parameters

A series of D-π-A type molecules have been designed for their potential use in organic photovoltaic devices. Photovoltaic and optoelectronic properties of newly designed molecules have been explored by comparing with a reference molecule R comprising of the central core (2,3,8,9-tetrakis(thiophen-2-ylethynyl)-5,7,10,12-tetrakis((trimethylsilyl)ethynyl)pyrazino[2,3-b]phenazine) and π-bridge (thiophene). The end groups are (2-(2-ethylidene-3-oxo-2,3-dihydro-1H-inden-1 ylidene)malononitrile), (2-ethylidenemalonitrile), (methyl 2-cyanoacrylate) and (3-methyl-5-methylene-2-thioxothiazolidin-4-one) in the newly designed molecules. Among the investigated molecules M1 and M2 exhibit a broad absorption range of 627 and 626 nm with respect to the reference. All the designed molecules exhibited a lower bandgap as compared to R which indicates a better transfer of electron density from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). The reorganization energy values show that all designed molecules have efficient charge transport capability. This study proves that end-capped acceptor modification is an effective strategy for designing optimistic molecule for high performance future organic solar cells fabrication.     

Designing spirobifullerene core based three-dimensional cross shape acceptor materials with promising photovoltaic properties for high-efficiency organic solar cells

The development of organic electron acceptor materials is one of the key factors for realizing high-performance organic solar cells (OSCs). Nonfullerene electron acceptors, compared to traditional fullerene acceptor materials, have gained much impetus owing to their better optoelectronic tunabilities and lower cost, as well as higher stability. Therefore, 5 three-dimensional (3D) cross-shaped acceptor materials having a spirobifullerene core flanked with 2,1,3-benzothiadiazole are designed from a recently synthesized highly efficient acceptor molecule SF(BR) ₄ and are investigated in detail with regard to their use as acceptor molecules in OSCs. The density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed for the estimation of frontier molecular orbital (FMO) analysis, density of states analysis, reorganization energies of electron and hole, dipole moment, open-circuit voltage, photo-physical characteristics, and transition density matrix analysis. In addition, the structure-property relationship is studied, and the influence of end-capped acceptor modifications on photovoltaic, photo-physical, and electronic properties of newly selected molecules ( H1-H5 ) is calculated and compared with reference ( R ) acceptor molecule SF(BR) ₄ . The structural tailoring at terminals was found to effectively tune the FMO band gap, energy levels, absorption spectra, open-circuit voltage, reorganization energy, and binding energy value in selected molecules H1 to H5 . The 3D cross-shaped molecules H1 to H5 suppress the intermolecular aggregation in PTB7-Th blend, which leads to high efficiency of acceptor material H1 to H5 in OSCs. Consequently, better optoelectronic properties are achieved from designed molecules H1 to H5 . It is proposed that the conceptualized molecules are superior than highly efficient spirobifullerene core-based SF(BR) ₄ acceptor molecules and, thus, are recommended to experiments for future developments of highly efficient solar cells.     

Design and characterization of alkoxy-wrapped push-pull porphyrins for dye-sensitized solar cells

Three alkoxy-wrapped push-pull porphyrins were designed and synthesized for dye-sensitized solar cell (DSSC) applications. Spectral, electrochemical, photovoltaic and electrochemical impedance spectroscopy properties of these porphyrin sensitizers were well investigated to provide evidence for the molecular design.     

卟啉类光敏剂在染料敏化太阳能电池中的应用

染料敏化太阳能电池结合了染料光敏剂和无机半导体的优势，具有较宽的光谱响应范围，制造工艺简单、成本较低，对环境友好，应用前景广阔，因而备受人们的关注。本文以卟啉配合物为主线，介绍光敏太阳能电池的基本构造和光电原理，从改善电池性能的角度，综述了各种卟啉类光敏剂在染料太阳能电池中的应用，讨论了卟啉配合物及其超分子结构对光电转化率的影响机理。     

Atomistic mechanism of charge separation upon photoexcitation at the dye-semiconductor interface for photovoltaic applications

Charge separation in excited states upon visible light absorption is a central process in photovoltaic solar cell applications. Employing state-of-the-art first principles calculations based on time-dependent density functional theory (TDDFT), we simulate electron-hole dynamics in real time and illustrate the microscopic mechanism of charge separation at the interface between organic dye molecules and oxide semiconductor surfaces in dye-sensitized solar cells. We found that electron-hole separation proceeds non-adiabatically on an ultrafast timescale <100 fs at an anthocyanin/TiO(2) interface, and it is strongly mediated by the vibrations of interface Ti-O bonds, which anchor the dye onto the TiO(2) surface. The obtained absorption spectrum and electron injection timescale agree with experimental measurements.     

Molecular Acceptors Based on a Triarylborane Core Unit for Organic Solar Cells

Triarylboranes that exhibit p–π* conjugation serve as versatile building blocks to design n-type organic/polymer semiconductors. A series of new molecular acceptors based on triarylborane is reported here. These molecules are designed with a boron atom that bears a bulky 2,4,6-tri-tert-butylphenyl (Mes*) substituent at the core and strong electron-withdrawing 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) units as the end-capping groups that are linked to the core by bithiophene bridges. Butyl or butoxy groups are introduced to the bithiophene units to tune the optoelectronic properties. These molecules show nearly planar backbones with highly localized steric hindrance at the core, low LUMO/HOMO energy levels, and broad absorption bands spanning the visible region, which are all very desirable characteristics for use as electron acceptors in organic solar cell (OSC) applications. The attachment of butyl groups to the bithiophene bridges brings about a slightly twisted backbone, which in turn promotes good solubility and homogeneous donor/acceptor blend morphology, whereas the introduction of butoxy groups leads to improved planarity, favorable stacking in the film state, and a greatly reduced band gap. OSC devices based on these molecules exhibit encouraging photovoltaic performances with power conversion efficiencies reaching up to 4.07 %. These results further substantiate the strong potential of triarylboranes as the core unit of small molecule acceptors for OSC applications.     

Synthesis of tetracyanoethylene‐substituted ferrocene and its device properties

Small organic molecules are promising candidates for cheaper, flexible and good‐performance sources for organic solar cells (OSCs) due to their easy fabrication, low cost and slightly cheaper processing. However, the lower power conversion efficiency of OSCs is the main problem for their applications. Ferrocene structures could be the best candidates for the active layers of OSCs due to their unique properties such as thermal and chemical stability. The electrochemical, electro‐optical and solar cell performances of 2,5‐dicyano‐3‐ferrocenyl‐4‐ferrocenylethynylhexa‐2,4‐dienedinitrile (DiCN‐Fc) structures were investigated. First, the electrochemical and electro‐optical properties were examined for finding the highest occupied and lowest unoccupied molecular orbital values and bandgap of DiCN‐Fc. The best photovoltaic performance was obtained with 7 wt% of DiCN‐Fc loading, with a power conversion efficiency of about 4.27%. In the light of our investigations, ferrocenyl‐substituted small organic molecules could contribute to the development of organic photovoltaic devices.     

Tunable light‐harvesting polymers containing embedded dipolar chromophores for polymer solar cell applications

A series of light‐harvesting conjugated polymers were designed and synthesized for polymer solar cells. These newly designed polymers comprise an unusual two‐dimensional conjugated structure with an electron‐rich thiophene–triphenylamine backbone and stable planar indacenodithiophene π‐bridges terminated with tunable electron acceptors. It was found that the electron‐withdrawing strength of the acceptor could be used to manipulate the energy level of the lowest unoccupied molecular orbital and bandgap (as much as 0.3 eV), generating derivatives with complementary absorbance in the visible spectrum. This approach provides great flexibility in fine tuning the electronic and optical properties of the resultant polymers and facilitates the investigation of how these chemical modifications alter the subsequent photovoltaic properties of these materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Molecular adsorption-mediated control over the electrical characteristics of polycrystalline CdTe/CdS solar cells.

The effect of surface treatments on p-CdTe/n-CdS solar cell performance was examined. Adsorption of organic molecules with various magnitudes and directions of the dipole moment on p-CdTe resulted in controlled changes in electron affinity and surface bond bending. Similar adsorption on CdTe in state-of-the-art p-CdTe/n-CdS solar cells changes the cell performance, and we explain this by a combination of increased series resistance and changes in light absorption and in cell photovoltage. While at this stage no improvement in performance has been found with these cell structures, which are the result of years of empirical optimization, the molecular effect on the photovoltage shows that it is possible in this way to control the photovoltaic effect at this junction. Separate optimization may well lead to improvement by inserting a dipole layer near the photovoltaic interface. Our results also show that this is even possible when dipole adsorption is performed on the complete polycrystalline thin-film cell.     

Carbene-based ruthenium photosensitizers

A new series of N-heterocyclic carbene (NHC)-pyridine ruthenium complexes incorporating a carbene unit as an ancillary ligand were designed and successfully synthesized by using simple synthetic methods. The photophysical, electrochemical and photovoltaic properties of these NHC-pyridine based ruthenium complexes were investigated. These complexes showed photoelectric conversion efficiencies in the range of 6.43 ～ 7.24% under the illumination of AM 1.5 (100 mW cm(-2)). Interestingly, the modifications on the ancillary ligand of these sensitizers by removal of an alkoxyl group and replacement of the octyl chain with a 3,5-difluorobenzyl group showed a 13% increase in the conversion efficiency for the CifPR dye. These results demonstrated that structural modifications on the NHC-pyridine ancillary ligand of ruthenium complexes results in dye-sensitized solar cells exhibiting a comparable cell performance to that obtained using the standard N719 dye.     

Development of small-molecule materials for high-performance organic solar cells

With the rapid development in recent years, small-molecule organic solar cell is challenging the dominance of its counterpart, polymer solar cell. The top power conversion efficiencies of both single and tandem solar cells based on small molecules have surpassed 9%. In this mini review, achievements of small molecules with impressive photovoltaic performance especially reported in the last two years were highlighted. The relationship between molecular structure and device performance was analyzed, which draws some rules for rational molecular design. Five series of p- and n-type small molecules were selected based on the consideration of their competitiveness of power conversion efficiencies.     

Unsymmetric Platinum(II) Bis(aryleneethynylene) Complexes as Photosensitizers for Dye‐Sensitized Solar Cells

Four new unsymmetric platinum(II) bis(aryleneethynylene) derivatives have been designed and synthesized, which showed good light‐harvesting capabilities for application as photosensitizers in dye‐sensitized solar cells (DSSCs). The absorption, electrochemical, time‐dependent density functional theory (TD‐DFT), impedance spectroscopic, and photovoltaic properties of these platinum(II)‐based sensitizers have been fully characterized. The optical and TD‐DFT studies show that the incorporation of a strongly electron‐donating group significantly enhances the absorption abilities of the complexes. The maximum absorption wavelength of these four organometallic dyes can be tuned by various structural modifications of the triphenylamine and/or thiophene electron donor, improving the light absorption range up to 650 nm. The photovoltaic performance of these dyes as photosensitizers in mesoporous TiO₂ solar cells was investigated, and a power conversion efficiency as high as 1.57 % was achieved, with an open‐circuit voltage of 0.59 V, short‐circuit current density of 3.63 mA cm^?2, and fill factor of 0.73 under simulated AM 1.5G solar illumination.     

Effects of Metal Oxide Modifications on Photoelectrochemical Properties of Mesoporous TiO2 Nanoparticles Electrodes for Dye-Sensitized Solar Cells

Mesoporous TiO₂ (m-TiO₂) nanoparticles were used to prepare the porous film electrodes for dye-sensitized solar cells, and a second metal oxide (MgO, ZnO, Al₂O₃, or NiO) modifi-cation was carried out by dipping the m-TiO₂ electrode into their respective nitrate solution followed by annealing at 500 oC. Experimental results indicated that the above second metal oxide modifications on m-TiO₂ electrode are shown in all cases to act as barrier layer for the interfacial charge transfer processes, but film electron transport and interfacial charge recombination characteristics under applied bias voltage were dependent significantly on the existing states and kinds of these second metal oxides. Those changes based on sec-ond metal oxide modifications showed good correlation with the current-voltage analyses of dye-sensitized solar cell, and all modifications were found to increase the open-circuit photo-voltage in various degrees, while the MgO, ZnO, and NiO modifications result in 23%, 13%, and 6% improvement in cell conversion efficiency, respectively. The above observations indi-cate that controlling the charge transport and recombination is very important to improve the photovoltaic performance of TiO₂-based solar cell.     

Co-sensitization of organic dyes for efficient ionic liquid electrolyte-based dye-sensitized solar cells

The co-sensitization of two organic dyes (SQ1 and JK2), which are complementary in their spectral responses, shows enhanced photovoltaic performance compared with that of an individual organic dye-sensitized solar cell. The power conversion efficiency of the co-sensitized organic dye solar cell based on the newly developed binary ionic liquid (solvent-free) electrolyte gives 6.4% under AM 1.5 sunlight at 100 mW/cm2 irradiation, which is higher than that of individual dye-sensitized solar cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) of the co-sensitized solar cell shows typical absorption peaks at 530 and 650 nm corresponding to the two dyes and displays a broad spectral response over the entire visible spectrum with IPCE of >40% in the 400-700 nm wavelength domain.     

Computational prediction of the properties of bis(ethynylthienyl)dialcoxynaphthalene-based conjugated polymers for organic solar cell applications

A series of homo- ( P0 ) and copolymers ( P1-P5 ) based on the electron-donor building-block 2,2′-(2,3-bis(2-ethylhexyloxy)naphthalene-1,4-diyl)bis(ethyne-2,1-diyl)dithiophene (1,4-NET) including ethynyl linkers aiming to promote coplanarity were designed, and their properties predicted using theoretical methodologies to evaluate their potential in organic solar cell applications. The geometries, FMO levels, energy bandgaps, and absorption spectra of trimer models were determined using time-dependent density functional theory, while their photovoltaic and charge-transport properties were estimated by the Scharber's model and semiclassical Marcus theory, respectively. Compared to high-performance conjugated polymers (CPs), such as PTB7-Th or PM6, and similar systems based on the 1D-BDT unit, the HOMO and LUMO levels of P0-P5 tend to be higher. In addition, the new CPs have complementary absorptions with narrow-bandgap acceptors, such as ITIC and Y6, and adequate matches between their HOMO and LUMO levels. Although the simulated photovoltaic and charge-transport properties could be overestimated, the best candidate to be synthesized and tested in organic solar cells is P5 due to its suitable and well-balanced properties, demonstrating the positive effect of incorporating ethynyl bridges to improve the optoelectronic properties of CPs.     

Rapid development in two-dimensional layered perovskite materials and their application in solar cells

This review summarized recent research progresses of two-dimensional layered organic-inorganic hybrid perovskite materials and their photovoltaic performances in 2D perovskite solar cells.     

A Highly Conjugated Benzimidazole Carbene‐Based Ruthenium Sensitizer for Dye‐Sensitized Solar Cells

A new type of carbene‐based ruthenium sensitizer, CB104, with a highly conjugated ancillary ligand, diphenylvinylthiophene‐substituted benzimidazolepyridine, was designed and developed for dye‐sensitized solar cell applications. The influence of the thiophene antenna on the performance of the cell anchored with CB104 was investigated. Compared with the dye CBTR, the conjugated thiophene in the ancillary ligand of CB104 enhanced the molar extinction coefficient of the intraligand π–π* transition and the intensity of the lower energy metal‐to‐ligand charge‐transfer band. However, the incident photon‐to‐current conversion efficiency spectrum of the cell anchored with CB104 (0.15 mM ) showed a maximum of 63 % at 420 nm. The cell sensitized with the dye CB104 attained a power conversion efficiency of 7.30 %, which was lower than that of the cell with nonconjugated sensitizer CBTR (8.92 %) under the same fabrication conditions. The variation in the performance of these two dyes demonstrated that elongating the conjugated light‐harvesting antenna resulted in the reduction of short‐circuit photocurrent density, which might have been due to the aggregation of dye molecules. In the presence of a coabsorbate, chenodeoxycholic acid, the CB104‐sensitized cell exhibited an enhanced photocurrent density and achieved a photovoltaic efficiency of 8.36 %.     

Recent Progress and Emerging Applications of Rare Earth Doped Phosphor Materials for Dye‐Sensitized and Perovskite Solar Cells: A Review

Although the efficiency of Dye‐sensitized and Perovskite solar cell is still below the performance level of market dominance silicon solar cells, in last few years they have grabbed significant attention because of their fabrication ease using low‐cost materials, and henceforth these cells are considered as a promising alternative to commercial photovoltaic devices. However, third generation solar cells have significant absorption in the visible region of solar spectrum, which confines their power conversion efficiency. Subsequently, the performance of current photovoltaics is significantly hampered by the transmission loss of sub‐band‐gap photons. To overcome these issues, rare earth doped luminescent materials is the favorable route followed to convert these transmitted sub‐band‐gap photons into above‐band‐gap light, where solar cells typically have significant light‐scattering effects. Moreover, the rare earth based down/up conversion material facilitates the improvement in sensitization, light‐scattering and device stability of these devices. This review provides insight into the application of various down/up conversion materials for Dye‐sensitized and perovskite solar cell applications. Additionally, the paper discusses the techniques to improve the photovoltaic performance in terms of current density and photo voltage in detail.     

Temperature dependence of phonon modes, dielectric functions, and interband electronic transitions in Cu2ZnSnS4 semiconductor films

The quaternary semiconductor Cu(2)ZnSnS(4) (CZTS) has attracted a lot of attention as a possible absorber material for solar cells due to its direct bandgap and high absorption coefficient. In this study, photovoltaic CZTS nanocrystalline film with a grain size of about 10 nm has been grown on a c-plane sapphire substrate by radio-frequency magnetron sputtering. With increasing the temperature from 86 to 323 K, the A(1) phonon mode shows a red shift of about 9 cm(-1) due to the combined effects of thermal expansion and the anharmonic coupling to the other phonons. Optical and electronic properties of the CZTS film have been investigated by transmittance spectra in the temperature range of 8-300 K. Near-infrared-ultraviolet dielectric functions have been extracted with the Tauc-Lorentz dispersion model. The fundamental band gap E(0), and higher-energy critical points E(1) and E(2) are located at 1.5, 3.6, and 4.2 eV, respectively. Owing to the influences of electron-phonon interaction and the lattice expansion, the three interband transitions present a red shift trend with increasing temperature. It was found that the absorption coefficient in the visible region increases due to the modifications of electronic band structures. The detailed study of the optical properties of CZTS film can provide an experimental basis for CZTS-based solar cell applications.