The long carrier lifetimes in perovskite single crystals have drawn significant attention recently on account of their irreplaceable contribution to high-performance photovoltaic (PV) devices. Herein, the optical and optoelectronic properties of CH3NH3PbI3 and CH3NH3PbI3–xBrx (with five different contents of Br doped) single crystals were investigated. Notably, a superior carrier lifetime of up to 262 μs was observed in the CH3NH3PbI3–xBrx (I/Br = 10:1 in the precursor) single-crystal PV device under 1 sun illumination, which is two times longer than that in the CH3NH3PbI3 single crystal. Further study confirmed that the ultralong carrier lifetime was ascribed to the integrated superiority derived from both the low trap-state density and high charge-injection efficiency of the device interface. On this basis, appropriate incorporation of Br is useful in the design of better PV devices.Keywords: carrier lifetime; charge-injection efficiency; photovoltaic device; single crystal; trap-state density;

The radiative and nonradiative decay processes of four platinum(II) complexes chelated with triarylboron (TAB)-functionalized N-heterocyclic carbenes (NHC) are investigated by using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculation, for probing into the influence of different numbers of TAB on the phosphorescent emission properties. For the radiative decay processes, zero-field splitting energies, radiative rates, and lifetimes are explored, and corresponding factors including transition dipole moments, singlet–triplet splitting energies as well as spin–orbit coupling matrix elements are also analyzed in detail. Additionally, energy-gap law is considered in the temperature-independent nonradiative decay processes; meanwhile, potential energy profiles are obtained to elaborate the temperature-dependent nonradiative decay processes. As a result, radiative rates declined slightly with the increased numbers of TAB. The minimum temperature-independent nonradiative decay may occur in BC-3 due to its smallest structural distortion between S0 and T1 states. According to the potential energy profiles of the deactivation pathways, four investigated phosphors have the similar temperature-dependent nonradiative decay processes because of the incredibly analogous energy barriers. We speculate that it does not mean greater phosphorescent emission and higher phosphorescent quantum yield with more TAB units, which would provide extraordinary assistance for further research in potential phosphors of organic light-emitting diodes.

To screen efficient sensitizers for dye-sensitized solar cells (DSSCs), two series of porphyrin sensitizers have been reengineered based on one of the best sensitizers YD2-o-C8 by introducing different heterocycles into acceptor part to form stronger acceptors. The electronic structures and optical properties of these sensitizers have been investigated using density functional theory and its time-dependent density functional theory version. The computational results suggest that the stronger acceptor can result in a narrower HOMO–LUMO energy gap, an obvious red-shift and stronger absorption in long-wavelength region compared with YD2-o-C8. Meanwhile, the analyses of electron density difference plots suggest that all designed sensitizers possess longer electron transfer distance, larger fraction of electron exchange, and smaller overlap between the zones of density depletion and increment than these of YD2-o-C8, indicating enhanced electron transfer ability from donor to acceptor groups. Moreover, the designed dyes exhibit good performance in terms of the electron injection ability, the excited state lifetime, and the strength of the interaction between dye and the TiO2 surface. As a whole, all the designed dyes, especially P4 and P6 may act as excellent sensitizers for high-efficiency DSSCs.

•The five dyes containing heteroatom with different atomic sizes and electronegativity have been investigated.•The sizes and electronegativity of the heteroatoms can affect on Voc and Jsc.•The reason for the changes of the light harvesting efficiency was explored.•Vibrationally resolved electronic spectra were simulated to explain the vibronic Contributions.The vibrationally resolved electronic spectra of five metal-free NKX-2587 derivatives containing heteroatom with different atomic sizes and electronegativity, were simulated within the Franck-Condon approximation including the Herzberg-Teller and Duschinsky effects, aimed at exploring the correlation of vibronic structure associated with the spectrum and efficiency of dye sensitized solar cells (DSSCs). The parameters of short-circuit current density (Jsc) and open circuit voltage (Voc) involving efficiency of DSSCs, such as total dipole moments (μnormal), the light harvesting efficiency (LHE), injection driving force (∆ Ginject), and the number of electrons in the conduction band (nc), were calculated and discussed in detail. Results showed that the heteroatoms in the same period with large size and weak electronegativity and the ones in the same main group with large size and weak electronegativity are beneficial to Voc. The sizes and electronegativity of the heteroatoms have a weak effect on Jsc. The low-frequency modes play important roles in enhancing the intensities of the electronic spectra and structures can affect light harvesting efficiency (LHE). In this sense, our results provided guidance for understanding the sources of spectral intensities of dye molecules, and a valuable help for rational design of new molecules to improve the energy conversion efficiency (η) of DSSCs.Download high-res image (82KB)Download full-size image

Hole transporting materials (HTMs) play an important role in most efficient perovskite solar cells (PSCs). In particular, donor-π-bridge-donor type oligomers (D-π-D) have been explored extensively as alternative and economical HTMs. In the present work, a series of triphenylamine-based derivatives as alternatives to the expensive Spiro-OMeTAD were explored by using first-principles calculations combined with the Marcus theory. The electronic structures, optical properties and hole mobilities of all the molecules were investigated to reveal the relationship between their charge-transport properties and the π-bridge conjugation. The HOMO levels decrease with the extension of the π-bridge conjugation length, which may lead to higher open-circuit voltages. Moreover, we employed a quantum mechanical (QM) methodology to estimate the carrier mobility for organic crystals. Specifically, an orientation function μΦ (V, λ, r, θ, γ; Φ) is first applied to quantitatively evaluate the overall carrier mobility of HTMs in PSCs. The theoretically calculated results validate that this model predicts the hole mobility of HTMs correctly. More importantly, it is revealed that enhancing the π-bridge conjugation in HTMs can improve the hole mobility, which will definitely improve the performance of PSCs. We hope that our theoretical investigation will offer a reliable calculation method to estimate the charge-transport properties of novel HTMs applied in perovskite solar cells.

Long electron–hole diffusion lengths in organolead trihalide compounds play a key role in achieving the remarkable performance of perovskite photovoltaics. Diffusion lengths in solution-grown CH3NH3PbI3 single crystals have been found to be greater than 175 micrometer (μm). Herein, we report the diffusion lengths in CH3NH3PbI3−xClx single crystals exceeding 380 μm under 1 Sun illumination, which is twice that in CH3NH3PbI3 single crystals. Incorporation of chlorine is found to increase the density of trap-states and reduce the valence band level; these two factors, which dominate the carrier recombination and the charge transfer, respectively, are in a competing relation. As a result, the electron–hole diffusion lengths in a CH3NH3PbI3−xClx single crystal with an optimum Cl proportion (x = 0.005) reach the maximum values. This study provides a strategy for the design of perovskite optoelectronics.

•Two regeneration mechanisms of the metal-free sensitizer are discussed.•Dye/(TiO2)6 system is applied to explore the regeneration process for the first time.•Considering a variety of possible reactions between the dye and electrolyte.•Desolvation effect is taken into account in the study of SIP and TIP.The regeneration mechanism of the oxidized metal-free sensitizer reacting with electrolyte containing iodide/triiodide(I−/I3-) as a redox mediator in dye-sensitized solar cells (DSSC) has been investigated by the density functional theory (DFT). Reported mechanisms which are usually discussed in ruthenium sensitizer-based DSSC, single iodide process (SIP) and two iodide process (TIP), are theoretically evaluated in the metal-free sensitizer containing carbon chains with heteroatoms (S atoms). Calculation indicates that TIP plays a critical role when regeneration reactions occur in different reaction sites of the cationic dye from the view of charges and energetics. While, reduction reactions at two special sites, where iodide ion reacts with S atom of the carbon chain in the direction of being perpendicular to the nearby benzene plane, are more likely to proceed along with the SIP. The occurrence of the SIP lies in a fast inner-sphere electron transfer (ISET) process, which is usually caused by the partially desolvated in acetonitrile when solvent effects are considered. In addition, calculated Mulliken spin densities also imply that partial charges are transferred from the iodide ion to dye+. Once the second iodide participates, oxidized dye is reduced completely. Besides I−, reactions involving I3− and I2− are also taken into consideration due to the complexity of the regeneration process. This exploration on the regeneration mechanism of the metal-free sensitizer confirms the reported regeneration process and provides assistance for further research in the regeneration process of the metal-free sensitizer.The regeneration mechanisms of the metal-free sensitizer, single iodide process (SIP) and two iodide process (TIP), are investigated theoretically.

•Represented π-spacers with different induced effects of electron are investigated.•Insight into some performance parameters of p-type photosensitizers theoretically.•Isolated dyes and dyes linking with (NiO)9 are considered.•Comparing regenerations of sensitizers reacting with I3− as well as I2−.Two classical p-type photosensitizers with modified conjugation spacers by the electron-rich unit (thiophene) and electron-deficient unit (pyrimidine) are calculated and investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Creatively, regeneration reactions between reduced sensitizers and I3− as well as I2− are explored theoretically, reappearing the regeneration process that electrons are transferred from reduced dyes to I3− or I2− in p-type dye-sensitized solar cells (p-type DSSCs) intuitively, and uncovering the difference of π-spacers with different electron-induced effects in the process of dye regeneration. Besides, calculation also indicates that thiophene-based dye (PS) has better light capture efficiency but unfavorable performance of charge transfer when compared with the pyrimidine-based dye (PN). From another perspective, both sensitizers have efficient driving forces of injection. In addition, dyes absorbing on the (NiO)9 semiconductor are also considered. Our comparison discussion of two sensitizers with different π-spacers provides great assistance for deeply investigating p-type photosensitizes with higher efficiencies and their regeneration processes, contributing to the potential tandem DSSCs.P-type “push-pull” sensitizers with modified conjugation bridges (thiophene units and pyrimidine units) as well as their regeneration processes are investigated theoretically.

•Spectra of group IV dichlorides and their anions were systematically investigated.•Herzberg–Teller effect and anharmonic correction were taken into account.•The simulated vibrational modes were confirmed and tentatively assigned.•Resolved vibrational electronic spectrum of PbCl2 was reported for the first time.We systematically studied the vibrational-resolved electronic spectra of group IV dichlorides using the Franck–Condon approximation combined with the Duschinsky and Herzberg–Teller effects in harmonic and anharmonic frameworks (only the simulation of absorption spectra includes the anharmonicity). Calculated results showed that the band shapes of simulated spectra are in accordance with those of the corresponding experimental or theoretical ones. We found that the symmetric bend mode in progression of absorption is the most active one, whereas the main contributor in photoelectron spectra is the symmetric stretching mode. Moreover, the Duschinsky and anharmonic effects exert weak influence on the absorption spectra, except for PbCl2 molecule. The theoretical insights presented in this work are significant in understanding the photophysical properties of MCl2 (M = C, Si, Ge, Sn, Pb) and studying the Herzberg–Teller and the anharmonic effects on the absorption spectra of new dichlorides of this main group.

A series of “push–pull” sensitizers with modified conjugation bridges are designed and investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT), with the purpose of revealing the effect of different linker moieties on the performance of p-type dye-sensitized solar cells (DSSCs). Creatively, the electron-rich unit (thiophene) and the electron-deficient unit (pyrimidine) are studied as the linking groups in p-type sensitizers from a comparative perspective, two special bridge-sites and the lengths of conjugation bridges are also taken into account. Calculations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicate that there is efficient hole injection and dye regeneration for all the sensitizers. Importantly, the influence of the number of thiophene and pyrimidine moieties is seen mainly on the long-wavelength region and the short-wavelength region, respectively. According to the charge transfer properties and the driving forces of hole injection, dye regeneration and charge recombination (ΔGinj, ΔGreg and ΔGCR, respectively), the increased length of the thiophene-based bridge close to the carboxyl group has a positive impact on the device performance. Likewise, for the pyrimidine-based bridges, it is probably the increased conjugation length between the donor and acceptor that significantly improves the device efficiency. Our intensive analysis on the π-bridges provides assistance for designing more efficient p-type photosensitizers, which contributes to the rational design for tandem DSSCs.

•Simulated absorption and emission spectra matched the experimental ones well.•The HT and Duschinsky effects were essential to weak vibrational transitions.•Anharmonic correction is important.•The possible reasons for mirror symmetry breakdown were discussed.•The simulated vibrational modes were confirmed and tentatively assigned.Based on the density functional theory and its time-dependent extension, the properties of the ground and the first excited states of phenanthrene were calculated. In harmonic and anharmonic approximations, the well-resolved absorption and emission spectra of phenanthrene were simulated using the Franck–Condon approximation combined with the Herzberg–Teller and Duschinsky effects, and the results reproduced the experimental spectra very well. The mirror symmetry breakdown between absorption and emission spectra is induced mainly from the Herzberg–Teller effect and Duschinsky mode mixing. Moreover, most of the vibrational modes were tentatively assigned and compared with the experiment.

•Simulated absorption and emission spectra match well with the experimental ones.•The HT and Duschinsky effects are essential to weak dipole-allowed transitions.•The HT and Duschinsky contributions are discussed intuitively.•Some uncertain modes in experiment are tentatively assigned in this work.Geometrical optimizations of the ground and first excited states of benzimidazole and indene were performed using the density functional theory (DFT) and its time-dependent extension methods (TD-DFT), respectively. Their vibrationally resolved 1Lb (1A′) ↔ S0 (1A′) absorption and fluorescence spectra were simulated within the Franck–Condon approximation including the Herzberg–Teller (HT) and Duschinsky effects. Calculated results revealed that, with the HT and Duschinsky effects getting involved, the simulated weak 1Lb (1A′) ↔ S0 (1A′) electronic spectra of the two molecules excellently reproduce the experimental findings. Based on the experimental data and other theoretical results, we tentatively assigned most of the vibrational normal modes which emerged in the experimental spectra of the two molecules. The present theoretical insights are expected to help us understand the nature of electronic transitions in the vibrationally resolved absorption and fluorescence spectra of benzimidazole and its analogues.

Several novel indoline dyes configured with donor–acceptor–bridge–acceptor (D–A–π–A) structures were designed and applied to organic dye-sensitized solar cells. These D–A–π–A dye molecules are composed of indoline (electron donating group), benzothiadiazole (BDT) (auxiliary acceptor), two furan rings (π-conjugated group), and 2-cyanoacrylic acid (electron accepting group). The influence of position of auxiliary acceptor in D–A–π–A organic sensitizer on the performance of photosensitize is investigated in detail. Calculated results show that the sensitizer could achieve a red-shifted absorption in long-wavelength region and a stronger absorption in short-wavelength region when the position of auxiliary acceptor changes from the donor to the acceptor. Moreover, among these dyes, WS-12, whose auxiliary acceptor nearing the 2-cyanoacrylic acid, possesses the better performance in terms of the charge transfer characteristics, lifetime of excited state as well as the vertical dipole moment when compared with WS-1 and WS-11. We hope that the present results could provide theoretical guidance for designing photosensitizes with higher efficiencies.

An efficient strategy was provided by adopting different numbers of electron-deficient units (pyrimidyl and quinolyl) into parent coumarin sensitizers to obtain excellent absorption in the short-wavelength region (B2 band), which eventually improves the performance of DSSCs. Density functional theory calculations were performed on both free dyes and dye–TiO2 complexes. As expected, introducing a single electron-deficient unit results in a positive influence on the power conversion efficiency (η) of DSSCs because of the larger short-circuit current density (Jsc is proportional to optical absorption (φLHE), charge separation, dye regeneration (φreg) and electron injection (φinject)) and the higher open circuit voltage (Voc). The introduction of more pyrimidine facilitates charge separation and favors effective electron injection, whereas the second quinoline displays the opposite effect. The results give guidance to design promising candidates for future DSSCs applications.

A series of dyes based on a porphyrin donor and a cyanoacrylic acid anchor/acceptor group for solar cell application are investigated with regards to varied-length π-spacers affecting the photo-to-electric conversion efficiency (PCE). Investigations are firstly performed on three porphyrin sensitizers with 1–3 conjugated phenylethynyl (PE) units, which have experimentally proved that the efficiency of power conversion decreases systematically with increasing spacer length. The distances and amounts of charge transfer after photoexcitation are calculated. In the PE bridged porphyrin dyes, the calculated electron injection driving forces and the regeneration driving forces gradually decrease as the distance of the π-spacer increases. Our theoretical calculations can reproduce well the experimental conclusion, showing that the photo-to-electric efficiency has a strong distance dependence for the electron-rich phenyl spacer. Then we replace the phenyl group with a pyrimidyl (PM) group to uncover how the characteristics of the π-spacer affect the performance of optical absorption, charge separation, and the regeneration process, to further improve the power conversion. We find that the adoption of electron-deficient pyrimidyl can break and even remove the distance dependence of the π-spacer. Some integral factors affecting the dye performance, such as short-circuit photocurrent, open-circuit voltage and charge collection efficiency are analyzed. It would help to interpret what role the electron deficient π-spacers with varied lengths will play and how they are expected to behave in the performance of sensitizers. In this regard, this study presents us with a promising way to design novel functional dyes and to utilize the potential advantages of the lengthy spacer dyes.

In the present letter we investigate the noncovalent interactions in the mixed coaggregates of 1,3,5-triphenyl-2-pyrazoline (TPP) and 1,4-dicyanonaphthalene (DCN) and their influence on the excited-state properties of the TPP–DCN. The theoretical results show that the π–π stacking interactions play an important role in the noncovalent interactions of the TPP–DCN coaggregates. The effect of the π–π interactions on the excited-state properties of the TPP–DCN is also fully investigated, and the results indicate that the TPP and DCN do not form an intermolecular charge-transfer complex in the ground state, whereas they form an exciplex in the excited state.Graphical abstractHighlights► The ground and excited state geometries of the mixed aggregates are obtained. ► The effect of the π–π stacking interaction on the excited-state properties of the mixed. ► TPP–DCN coaggregates is also fully investigated through analysing the absorption and fluorescence spectra and the frontier molecular orbitals. ► The nature of the noncovalent interactions is described.

Although zinc porphyrin is a suitable specimen for studying the electronic structures and fine optical spectra of large molecules, few theoretical investigations have been performed to simulate and assign the vibrational bands in highly resolved spectra. On the basis of density functional theory and its TD extension, the S0 → S1 absorption spectrum (that is, the Q band) of zinc porphyrin was simulated using the Franck–Condon (FC) approximation and incorporating the Herzberg–Teller (HT) and Duschinsky contributions to electronic transition dipole moments. Our theoretical results fit well with the optical spectrum obtained by experimental observations. The spectral profile of the Q band of zinc porphyrin is primarily described by the transition of the vibrational normal modes v8, v18, v49, v56, v57, v63, v64 and v87. Comparison of the FC and FCHT spectra with experimental results indicate that the S0 → S1 absorption spectrum of zinc porphyrin is primarily due to the HT mechanism. In this regard, for the vibronically allowed or very weak transitions like those of the Q band in zinc porphyrin, the results indicate that the HT effect is more dominant than the FC contribution. In addition, we have tentatively assigned the vibrational lines in the simulated absorption spectrum. When compared with the available experimental spectra, our simulated results accurately reproduce most of the dominant transitions.Graphical abstractHighlights► The vibronic fine structure of S0 → S1 transition (the Q band) of zinc porphyrin was simulated theoretically. ► The FC and FCHT approximations were used to simulate the highly resolved spectra. ► The assignment and origin of the observed vibrational transitions in the Q band of zinc porphyrin were provided.

A mass of porphyrin sensitizers have been designed and synthesized for dye-sensitized solar cells in previous works, and almost all of them incorporated an electron-rich system as the π-spacer. We here adopted the electron-deficient pyrimidine as an effective π-spacer and combined a cyanoacrylic acid anchoring group, as such a design yields a more bathochromic shift of the spectral absorption of the dye and results in an improved spectral overlap with the solar spectrum and an enhanced light-harvesting efficiency. The result does tally with the performance of sensitizer adsorbing on a semiconductor. From the electron density difference plots of electron transitions, we found that not all electron transitions could make for the effective electron transfer from donor to acceptor groups, which means the sensitizer performance in dye-sensitized solar cells not only relies on the extrinsic spectral absorption intensity but also depends on the intrinsic character of electron movement related to electron excitation. Moreover, the introduction of electron-deficient pyrimidine could affect the energy levels of excited molecules in solution, further affecting the kinds of electron transfer processes. We presented several novel porphyrin sensitizers for comparison on how the π-spacer and anchoring group influence the optical absorption, electron transfer processes, and regeneration of the oxidized dyes, thereby gaining potential dye-sensitized solar cells with highly efficient photo-to-electric conversion performances.

Although difluorobenzenes (DFBs) are well-known organic molecules to understand the electronic structure and spectroscopy of benzene and its derivatives, few theoretical investigations have been performed to simulate their fine spectra and assign their vibrational bands. In this work, the fluorescence excitation (FEX) spectra of the first excited singlet states for three DFBs molecules (para-, meta- and ortho-difluorobenzene) were simulated by the Franck–Condon calculations with the displaced harmonic oscillator approximation plus the distorted correction. The calculated results indicated that the spectral profiles of three DFBs are primarily described by the Franck–Condon progression of their totally symmetric vibrational modes. Specifically, it is found that modes v3 and v5 of para-DFB, v8 and v9 of meta-DFB, and ortho-DFB play the most important roles in the fluorescence spectra. By taking into account the contributions of the distorted effect, we could assign most of the dominant overtones from the nontotally symmetric vibrational modes, and the results agree well with the experimental assignments. Some inferred and unassigned vibrational transitions in experiment were confirmed according to the present calculated results. In addition, in the simulated fluorescence spectra, we tentatively assigned several combination bands with relative moderate intensity and weak vibrational lines which appeared in the experimental observations but the corresponding assignments were not given. The present work reproduced satisfactorily the experimental FEX spectra of p-, m-, and o-DFBs derivatives and provided a useful method to simulate the FEX spectra of dihalogenated benzene molecules.