Modification on C217 by auxiliary acceptor toward efficient sensitiser for dye-sensitised solar cells: a theoretical study

In this work, to develop efficient organic dye sensitisers, a series of novel donor–acceptor–π–acceptor metal-free dyes were designed based on the C217 dye by means of modifying different auxiliary acceptors, and their photovoltaic performances were theoretically investigated with systematic density functional theory calculations coupled with the incoherent charge-hopping model. Results showed that the designed dyes possess lower highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels as well as narrower HOMO–LUMO gaps compared to C217, which indicate their higher light-harvesting efficiency. In addition, using the (TiO₂)₃₈ cluster and bidentate bridging model, we predicted that the photoelectric conversion efficiency (PCE) for the C217 dye is as high as 9.92% under air mass (AM) 1.5 illumination (100 mW·cm^?2), which is in good agreement with its experimental value (9.60%–9.90%). More interestingly, the cell sensitised by the dye 7 designed in this work exhibits a middle-sized open-circuit voltage of 0.737 V, large short-circuit photocurrent density of 21.16 mA?cm^?2 and a fill factor of 0.801, corresponding to a quite high PCE of 12.49%, denoting the dye 7 is a more promising sensitiser candidate than the C217, and is worth further experimental study.     

Computational study of new azo dyes with different anchoring groups for dye-sensitised solar cells

Some of new azo dyes with different anchoring groups, such as biscarbodithiolic acid, hydroxamic acid, phosphonic acid, carboxcylic acid and sulfonic acid have been investigated theoretically to evaluate the effects of various anchoring groups on the optical and electronic properties of the dyes in dye-sensitised solar cells. Optical and electronic properties, UV–Vis absorption spectra, light-harvesting efficiency, lifetime of the excited state, chemical hardness and lowest unoccupied molecular orbital (LUMO) orbital weight of the dyes on the anchoring groups, have been studied to shed light on how the various anchoring groups influence the properties of the dyes. The biscarbodithiolic acid-based dye shows the longest maximum absorption wavelength and the widest absorption spectra together with the highest light-harvesting efficiency, the longest lifetime of the excited state and the highest the LUMO orbital weight of the dye on the atoms of the anchoring group, suggesting the good ability in electron injection. Theoretical calculations have been also performed on the adsorption of these dyes on the TiO₂ anatase (101) surface. These results show that the biscarbodithiolic acid-based dye has the highest adsorption energy and the largest negative shift of the conduction band of TiO₂ due to the adsorption of the dye onto the TiO₂.     

Rational design and first-principles studies of phenothiazine-based dyes for dye-sensitised solar cells

We investigate a series of phenothiazine (PT)-based organic dyes by adopting different donors and different donor substitution positions as photosensitisers for application in dye-sensitised solar cells (DSSCs). First-principles calculations reveal systematic improvements of key parameters including light-harvesting efficiency, redox potential, electron injection and non-linear optical properties with donor substitutions. The non-planar structure also suppresses dye aggregation and reduces the rate of internal charge recombination. In particular, photosensitisers with combination of donor functional groups show outstanding performance on these key parameters. This study demonstrates that PT-based dyes with the studied donor groups could serve as excellent candidates of photosensitisers for future DSSC applications.     

Influence of bridging atom on the vertical phase separation of low band gap bulk heterojunction solar cells

Vertical phase separation of the polymer and fullerene molecules in bulk heterojunction organic solar cells influences the exciton dissociation, charge carrier transport and collection. This work compares the vertical phase separation of poly[2,1,3‐benzothiadiazole‐4,7‐diyl[4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta [2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl]] (C‐PCPDTBT):[6,6]‐phenyl C71 butyric acid methyl ester (PC₇₁BM) and poly[2,1,3‐benzothiadiazole‐4,7‐diyl[4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta [2,1‐b:3,4‐b′]dithiophene‐siloe2,6‐diyl]] (Si‐PCPDTBT):PC₇₁BM blend films, using X‐ray photoemission spectroscopy depth profiles. The difference between the two polymers is the bridging atom, which is carbon for C‐PCPDTBT and silicon for Si‐PCPDTBT. Si‐PCPDTBT exhibits enhanced polymer chain packing and crystallinity. We believe this enhanced chain packing provides a driving force during film drying which alters the vertical morphology. The different nature of vertical phase separation plays a role in determining the increased device performance observed for Si‐PCPDTBT:PC₇₁BM solar cells. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Theoretical study of electronic structure and excited states properties of two dyes for dye-sensitized solar cells

Two conjugated organic dyes comprising the benzo[b]furan moieties as the electron donor and cyanoacetic acid moieties as the electron acceptor/anchoring groups have been investigated using a quantum chemical method. The molecular equilibrium geometries and ground state character were studied using density functional theory. Absorption spectra were obtained using time-dependent density functional theory and semiempirical ZINDO. The nature of absorption spectra was further studied using 2D and 3D real-space analysis; here, 2D real-space analysis showed electron–hole coherence, and 3D real-space analysis showed intramolecular charge transfer during photo-excitation. As important parameters, excited state oxidation potential and driving force energy were obtained to reveal the relationship between molecular structure and performance of two compounds.     

DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells

The paper reports Time Dependent Density Functional Theory (TD DFT) calculations providing the structure, electronic properties and spectra of [Ru(II)(bpy)_{3? n} (dcbpy) _n ]²⁺ and [Rh(III)(bpy)_{3? n} (dcbpy) _n ]³⁺ complexes, where bpy?=?2,2′-bipyridyl, dcbpy?=?4,4′-dicarboxy-2,2′-bipyridyl, and n?=?0,?1,?2,?3, studied as possible pigments for dye-sensitized solar cells. The role of the metallic ion and of the COOH groups on the optical properties of these complexes are compared and contrasted and their relevance as dyes for hybrid organic–inorganic photovoltaic cells is discussed. It was found that the optical spectra are strongly influenced by the metallic ion, with visible absorption bands for the Ru(II) complexes and only ultraviolet bands for the Rh(III) complexes. Upon excitation, the extra positive charge of the Rh³⁺ centre tends to draw electrons towards the metal ion, facilitating some charge transfer from the ligand to the metal, whereas in the case of the Ru²⁺ ion the electron transfer is clearly from the metal to the ligand. The carboxyl groups play an important role in strengthening the absorption bands in solution in the visible region. Of the complexes studied, the most suited as pigments for dye-sensitized solar cells are the [Ru(II)(bpy)_{3? n} (dcbpy) _n ]²⁺ complexes with n?=?1 and 2. This is based on the following arguments: (i) their intense absorption band in the visible region, (ii) the presence of the anchoring groups allowing the bonding to the TiO₂ substrate and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte.     

Highly efficient perovskite solar cells by tuning electronic structures of thienothiophene-based as hole transport materials

The atomic substitutions were used to study the hole transport materials (HTM) properties of six thiophenothiophene molecules (HTM1-HTM6) to reveal the relationship between their core structures and photoelectric properties. To better investigate the difference between experimentally original and designed molecules, we calculated the hole mobility and some parameters (such as energy levels, stability, and optical properties, etc). The results showed that the molecular orbital levels of the original and designed molecules have well matched with perovskite and Ag electrode to ensure hole transport and inhibit the electron reflux. Among the designed HTMs, HTM5 has the smallest energy gap that results in the red-shifted absorption spectra. Furthermore, there is an obviously increased charge transfer integral V due to the introduction of the Si atom, which greatly improved the hole mobility. Therefore, atom substitution by introducing Si atoms (HTM5) will improve the energy levels and charge transport ability, and molecular design by means of atom substitution can be a potential way to tunable HTM performance in solar cells.     

Theory study on the properties of thiadiazole polymer donors for organic solar cells

In this work, the properties of [1,2,5] thiadiazolo [3,4‐c] pyridine ‐alt‐cyclopenta [2,1‐b:3,4‐b′] dithiophene (PT‐CDT) and [1,2,5] thiadiazolo [3,4‐c] pyridine‐6‐carbonitrile‐alt‐cyclopenta [2,1‐b:3,4‐b′] dithiophene (PCNT‐CDT) as donors were investigated by means of Density Functional Theory. The electronic properties and optical absorption properties were discussed, and hole‐transfer properties of donors were studied by Marcus electron transfer theory. The results indicate that the linear structure of PCNT‐CDT and PT‐CDT is more stable than the spiral structure of PCNT‐CDT and PT‐CDT; the absorption peak in visible region of PCNT‐CDT is stronger and wider, and the absorption spectrum is more matchable to solar spectrum than PT‐CDT, while the maximum absorption wavelength of PCNT‐CDT has an obvious red shift; the two designed materials show strong intramolecular and intermolecular charge transfer properties; PCNT‐CDT owns a large open‐circuit voltage and low reorganization energy, as well as high hole mobility. Therefore, the newly designed PCNT‐CDT can be a potential donor material of organic solar cell. Copyright © 2013 John Wiley & Sons, Ltd.     

Theoretical study of the optical,electronic, and charge-transfer properties of 1,2,4,5-tetrakis(phenylethynyl)benzene derivatives for solar cell applications

A series of 1,2,4,5-tetrakis(phenylethynyl)benzene derivatives has been investigated at the CAM-B3LYP/6-31G(d) and TD-CAM-B3LYP/6-31?+?G(d,p) levels to design materials with high performance with respect to suitable frontier molecular orbitals (FMOs), broad absorption spectra, and better and balanced charge-transfer properties. The calculated results reveal that the molecule possessing benzene has the largest torsion angle of these derivatives. Different branches have a slight influence on the distributions of the FMOs of the molecules. 2-vinyl-thieno[3,2-b]thiophene branches display a small HOMO–LUMO gap corresponding to red shifts of the absorption spectra. These molecules are potential ambipolar charge-transport materials under the appropriate operating conditions.     

Inverted structure perovskite solar cells: A theoretical study

We analysed perovskite CH₃NH₃PbI_3-xCl_x inverted planer structure solar cell with nickel oxide (NiO) and spiro-MeOTAD as hole conductors. This structure is free from electron transport layer. The thickness is optimized for NiO and spiro-MeOTAD hole conducting materials and the devices do not exhibit any significant variation for both hole transport materials. The back metal contact work function is varied for NiO hole conductor and observed that Ni and Co metals may be suitable back contacts for efficient carrier dynamics. The solar photovoltaic response showed a linear decrease in efficiency with increasing temperature. The electron affinity and band gap of transparent conducting oxide and NiO layers are varied to understand their impact on conduction and valence band offsets. A range of suitable band gap and electron affinity values are found essential for efficient device performance.     

Design of perylene-diimides-based small-molecules semiconductors for organic solar cells

A series of perylene-diimide-based small molecules have been designed to explore their optical, electronic and charge transport properties as organic solar cell materials. The frontier molecular orbitals analysis has turned out that the vertical electronic transitions of absorption are characterised as intramolecular charge transfer between perylene diimide moieties and substituent aromatic groups. Our results suggest that the optical and electronic properties and reorganisation energies are affected by the introduction of different aromatic groups to these molecules. The calculation results showed that the designed molecules own the large longest wavelength of absorption spectrum, the oscillator strength and absorption region values. On the basis of the investigated results, the designed molecules could be used as solar cell material with intense broad absorption spectra. Furthermore, they are expected to be the promising candidates for hole and/or electron transport materials.     

A silazane additive for CsPbI2Br perovskite solar cells

Adding additives into peroskite precursor solution has been proven as a simple and efficient strategy to improve the quality of peroskite films. In this work, we demonstrate an effective additive strategy to improve the quality of all-inorganic perovskite films by adding a novel silazane additive heptamethyldisilazane (HDMS). The power conversion efficiency (PCE) of the optimized devices is enhanced from 14.55% to 15.31% with an open-circuit voltage over 1.26 V due to the higher quality perovskite films with lower trap density after the incorporation of HDMS. More interestingly, the HDMS devices exhibit superior humidity and thermal stability compared with the control ones. This work provides a simple and efficient strategy to enhance the device performance and stability of all-inorganic perovskite solar cells, which could facilitate its commercialization.     

Design of new neutral organic super‐electron donors: a theoretical study

For the purpose of designing new functional super‐electron donors (SEDs), the structure and stability of analogous tetrathiafulvalene (TTF) compounds are investigated and compared with those of the parent TTF by quantum chemistry calculations. Density functional theory (DFT) method B3LYP in combination with the polarized continuum model (PCM) is employed to compute the standard redox potentials ( ) of about 80 neutral organic SEDs in acetonitrile. Theoretical models are evaluated through correlation with 40 available experimental data. Excellent agreement between the theoretical predictions and experimental data is found with a mean absolute deviation of 0.08 V. Furthermore, we designed new species related to tetraazafulvalene (TAF) and evaluated their redox potential in search of new SEDs. Copyright © 2009 John Wiley & Sons, Ltd.     

染料敏化太阳能电池光敏剂卟啉儿茶酚的密度泛函和含时密度泛函研究

用密度泛函理论的杂化密度泛函B3LYP方法研究了太阳能电池光敏荆5,10,15.三苯基-20-(3,4-二羟基苯)卟啉(卟啉儿荼酚,TPP-cat)的几何结构、电子结构、IR和Raman特性.用自然键轨道方法分析了电荷布居和成键性质.计算结果表明,最强的IR吸收峰位于1175.81 cm-1处,最强的Raman活性位于1587.18 cm-1处.采用含时密度泛函计算了TPP-cat在水溶液中的电子吸收谱,其Soret带和Q带均指认为π→π*跃迁,在大约354 cm-1处的跃迁与一个光诱导分子内电荷转移过程有关.     

Improved efficiency of small-molecular (S-M) tandem organic solar cells (TOSCs) by employing low work function alloy nanoparticle intermediate layer

We improved the power conversion efficiency (PCE) of the small molecular (S-M) tandem organic solar cells (TOSCs) by employing different low work function alloy nanoparticle intermediate layers. The enhancement of the PCE was mainly attributed to the gap states formed at the interface between the buffer layer and alloy nanoparticle intermediate layer. The gap states result in the disappearance of the electron injection barrier. Compared with the planar heterojunction (PHJ) TOSCs with single Ag nanoparticle intermediate layer, the PCE of the PHJ TOSC with the Mg-Ag alloy nanoparticle intermediate layer exhibits an enhancement of 7.5%. Moreover, the Mg-Ag alloy nanoparticle intermediate layer was also employed in the bulk-heterojunction (BHJ) TOSCs. Compared with the PHJ TOSCs, the PCE of the BHJ TOSCs with Mg-Ag alloy nanoparticle intermediate layer is doubled and achieves a value of 5.54%.     

First-principles study on Sb-doped SnS2 as a low cost and non-toxic absorber for intermediate band solar cell

Tin disulfide has attracted much attention on solar cell study due to its excellent optoelectronic properties in addition to just containing low-cost and non-toxic elements. Based on the HSE06-hybrid function calculations combined with Grimme's dispersion-correction method, a half-filled and delocalized intermediate band(IB) is presented in the main band gap of SnS₂ after partially Sb substituting on Sn site, which is made of the antibonding states of Sb-s and S-p states. Three-photon absorption can be realized in the doped sample and its corresponding absorption coefficient is enhanced at the visible light region thanks to the isolated and half-filled IB above the original valence band. Furthermore, Sb_Sn always has the lowest formation energy than other Sb-related defects (i.e. Sb_S and Sb_i) based on the defect formation energy calculations. Therefore, Sb-doped SnS₂ is suggested as a promising candidate for the absorber of intermediate band solar cell.     

Intermediate band insertion by group-IIIA elements alloying in a low cost solar cell absorber CuYSe2: A first-principles study

By using first-principles calculations based on HSE06 hybrid functional, the structural, electronic, and optical properties of CuYSe₂ as a low cost absorber material have been studied. Our results show that CuYSe₂ is a semiconductor with indirect band gap of 1.46 eV and optical band gap of 2.00 eV. Especially, an intermediate band has been found in Ga and In alloyed CuYSe₂, respectively, which can be served as a stepping stone to optical absorption on low energy photons. Therefore, Ga and In alloyed CuYSe₂ with an intermediate band as a new absorber material have been proposed.     

分子对飞秒激光场响应的含时密度泛函理论研究

采用含时密度泛函理论方法研究线性分子碳化锂(Li2C2)对飞秒激光场响应的电子-离子动力学行为.在典型的近共振和非共振的激光频率作用下,分别对比分析了分子的共振和非共振电离过程.研究发现:分子在共振频率激光场的作用下发生更强的电离过程,并倾向于发生库伦爆炸,键长的振荡断裂与电离相互促进影响,而分子在较弱的激光场作用下发生单光子电离过程;随着双脉冲时间间隔的增加,离化电子数在一定范围内呈振荡上升趋势,随后趋于常数.     

Secondary plasmon resonance in graphene nanostructures

The plasmon characteristics of two graphene nanostructures are studied using time-dependent density functional theory (TDDFT). The absorption spectrum has two main bands, which result from π and σ + π plasmon resonances. At low energies, the Fourier transform of the induced charge density maps exhibits anomalous behavior, with a π phase change in the charge density maps in the plane of the graphene and those in the plane 0.3 ? from the graphene. The charge density fluctuations close to the plane of the graphene are much smaller than those above and beneath the graphene plane. However, this phenomenon disappears at higher energies. By analyzing the electronic properties, we may conclude that the restoring force for the plasmon in the plane of the graphene does not result from fixed positive ions, but rather the Coulomb interactions with the plasmonic oscillations away from the plane of the graphene, which extend in the surface-normal direction. The collective oscillation in the graphene plane results in a forced vibration. Accordingly, the low-energy plasmon in the graphene can be split into two components: a normal component, which corresponds to direct feedback of the external perturbation, and a secondary component, which corresponds to feedback of the Coulombic interaction with the normal component.