Electron Injection Mechanism Research Articles

Plasmonic Pd Nanoparticle- and Plasmonic Pd Nanorod-Decorated BiVO4 Electrodes with Enhanced Photoelectrochemical Water Splitting Efficiency Across Visible-NIR Region.

The photoelectrochemical (PEC) water splitting performance of BiVO4 is partially hindered by insufficient photoresponse in the spectral region with energy below the band gap. Here, we demonstrate that the PEC water splitting efficiency of BiVO4 electrodes can be effectively enhanced by decorating Pd nanoparticles (NPs) and nanorods (NRs). The results indicate that the Pd NPs and NRs with different surface plasmon resonance (SPR) features delivered an enhanced PEC water splitting performance in the visible and near-infrared (NIR) regions, respectively. Considering that there is barely no absorption overlap between Pd nanostructures and BiVO4 and the finite-difference time domain (FDTD) simulation indicating there are substantial energetic hot electrons in the vicinity of Pd nanostructures, the enhanced PEC performance of Pd NP-decorated BiVO4 and Pd NR-decorated BiVO4 could both benefit from the hot electron injection mechanism instead of the plasmon resonance energy transfer process. Moreover, a combination of Pd NPs and NRs decorated on the BiVO4 electrodes leads to a broad-band enhancement across visible-NIR region.

Study on geomagnetic storms driving motion of 0.1–2 MeV radiation belt electrons

Using more than five years’ worth of data observed by the Instrument for the Detection of Particles (IDP) spectrometer onboard the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite, we studied the motion characteristics of energetic electrons in different regions, i.e., the inner radiation belt, the outer radiation belt, and the slot region in geomagnetic storms. We investigated the flux change of 0.1–2.4MeV electrons and the energy change of 0.1–1.0MeV electrons in these different regions. By cross correlation analysis, we came to the following conclusions. First, when Dst<−50, the correlation coefficient (c.c.) of the electron flux and Dst index ranges from −0.63 to −0.86, and the enhancement of the electron flux generally occurs during the storm’s main and recovery phases. Second, the storms greatly influence the lower energy region of the electron energy spectrum in the inner radiation belt, while the enhancement in the higher energy region is more significant in the outer radiation belt and the slot region. Third, the effects of geomagnetic storms on electrons are not distinguished significantly between in the day and night, and independent of the timing of the events. For storms with −50<Dst<−30, there is a negative correlation of −0.51 to −0.57 between the Dst index and the electron flux in the outer radiation belt. Our analysis suggests that strong storms cause energetic electron ejections across a wide range, and the ejection level is affected by the storm intensity. Furthermore, the electron energy region influenced by the strong geomagnetic storms is opposite in the inner and outer radiation belts. The proportion of electrons accelerated to relativistic energies is greater in the outer radiation and slot regions, while the ejection energetic electrons are more concentrated in the low energy region of the inner radiation belt. This phenomenon reflects the different electron injection mechanisms and accelerating processes responsible for spectral index variations in different L regions during geomagnetic storms.

Hot Electron‐Based Solid State TiO2|Ag Solar Cells

The present work reports a simple and direct sputtering deposition to form solid state TiO2|Ag independent plasmonic solar cells. The independent plasmonic solar cells are based on a Schottky barrier between two materials, TiO2 and Ag. The Ag functions as the absorber generating “hot” electrons, as well as the contact for the solar cell. The Ag sputtering is performed for different durations, to form Ag nanoparticles with a wide size distribution on the surface of rough spray pyrolysis deposited TiO2. Incident photon to current efficiency (IPCE) measurements show photovoltaic activity below the TiO2 bandgap, which is caused by the silver nanoparticles that have a wide plasmonic band, leading to the generation of “hot” electrons. X‐ray photoelectron spectroscopy analysis supports the “hot” electron injection mechanism by following the Ag plasmon band and detecting local photovoltages. The measurements show that electrons are formed in the Ag upon illumination and are injected into the TiO2, producing photovoltaic activity. J–V measurements show photocurrents up to 1.18 mA cm−2 and photovoltages up to 430 mV are achieved, with overall efficiencies of 0.2%. This is, to our knowledge, the highest performance reported for such independent plasmonic solar cells.

Surface Plasmon Catalytic Aerobic Oxidation of Aromatic Amines in Metal/Molecule/Metal Junctions

The surface plasmon catalytic selective aerobic oxidation of aromatic amines to aromatic azo compounds in metal/molecule/metal junctions was explored by density functional theory. The overall reaction could be divided into the initial plasmon-induced oxygen activation and the subsequent photothermal-driven dehydrogenation process. The activation of oxygen on silver and gold surfaces is proposed through a surface plasmon-mediated hot electron injection mechanism at solid/gas interface. Resonance absorption of incident light by metal nanostructures generates energetic electron–hole pairs. Time-dependent density functional theory calculations illustrate that the excited hot electrons created on metal surfaces can transfer to the antibonding 2π* orbital of adsorbed oxygen, which facilitates the dissociation of O2 on metal surfaces. Silver shows better catalytic performance for the oxygen activation due to its stronger surface plasmon resonance absorption intensity and higher hot electron energy level. Aromati...

Large enhancement in photocurrent by Mn doping in CdSe/ZTO quantum dot sensitized solar cells.

We find a large enhancement in the efficiency of CdSe quantum dot sensitized solar cells by doping with manganese. In the presence of Mn impurities in relatively small concentrations (2.3%) the photoelectric current increases by up to 190%. The average photocurrent enhancement is about 160%. This effect cannot be explained by a light absorption mechanism because the experimental and theoretical absorption spectra demonstrate that there is no change in the absorption coefficient in the presence of the Mn impurities. To explain such a large increase in the injection current we propose a tunneling mechanism of electron injection from the quantum dot LUMO state to the Zn2SnO4 (ZTO) semiconductor photoanode. The calculated enhancement is approximately equal to 150% which is very close to the experimental average value of 160%. The relative discrepancy between the calculated and experimentally measured ratios of the IPCE currents is only 6.25%. For other mechanisms (such as electron trapping, etc.) the remaining 6.25% cannot explain the large change in the experimental IPCE. Thus we have indirectly proved that electron tunneling is the major mechanism of photocurrent enhancement. This work proposes a new approach for a significant improvement in the efficiency of quantum dot sensitized solar cells.

Coumarin-based donor–π–acceptor organic dyes for a dye-sensitized solar cell: photophysical properties and electron injection mechanism

The electronic structure and photophysical properties of five coumarin-based donor–π–acceptor (D–π–A)-type organic dyes for a dye-sensitized solar cell (DSSC) which were recently developed have been investigated using the time-dependent density functional theory and the symmetry-adapted cluster configuration interaction method. Theoretical calculations including the solvent effect in state-specific and linear-response scheme reproduced the experimental UV–Vis absorption spectra of these dyes satisfactorily. The π-spacers, thiophene and thiophene–phenylene mixed units, affect the planarity of the molecular structures which is relevant to the photophysical properties and charge polarization. Energy levels of the frontier orbitals and charge separation were analyzed, and the thiophene linker was found to be effective for the electron injection in DSSC. The adsorption of these dyes on the TiO2 anatase (101) surface and the electron injection mechanism were also investigated using a dye–(TiO2)38 cluster model employing PBE and TD-CAM-B3LYP calculations, respectively. The adsorption energies of these dyes were estimated to be ~14 kcal/mol, indicating strong adsorption of dye to a TiO2 surface by carboxylate group. The possible direct electron injection mechanism was suggested in the present coumarin-based D–π–A dyes in a dye–TiO2 interacting system.

Mechanism of Charge Transfer and Recombination Dynamics in Organo Metal Halide Perovskites and Organic Electrodes, PCBM, and Spiro-OMeTAD: Role of Dark Carriers.

Despite the unprecedented interest in organic-inorganic metal halide perovskite solar cells, quantitative information on the charge transfer dynamics into selective electrodes is still lacking. In this paper, we report the time scales and mechanisms of electron and hole injection and recombination dynamics at organic PCBM and Spiro-OMeTAD electrode interfaces. On the one hand, hole transfer is complete on the subpicosecond time scale in MAPbI3/Spiro-OMeTAD, and its recombination rate is similar to that in neat MAPbI3. This was found to be due to a high concentration of dark charges, i.e., holes brought about by unintentional p-type doping of MAPbI3. Hence, the total concentration of holes in the perovskite is hardly affected by optical excitation, which manifested as similar decay kinetics. On the other hand, the decay of the photoinduced conductivity in MAPbI3/PCBM is on the time scale of hundreds of picoseconds to several nanoseconds, due to electron injection into PCBM and electron-hole recombination at the interface occurring at similar rates. These results highlight the importance of understanding the role of dark carriers in deconvoluting the complex photophysical processes in these materials. Moreover, optimizing the preparation processes wherein undesired doping is minimized could prompt the use of organic molecules as a more viable electrode substitute for perovskite solar cell devices.

The Role of Porphyrin-Free-Base in the Electronic Structures and Related Properties of N-Fused Carbazole-Zinc Porphyrin Dye Sensitizers.

Dye sensitizers can significantly affect power conversion efficiency of dye-sensitized solar cells (DSSCs). Porphyrin-based dyes are promising sensitizers due to their performances in DSSCs. Here, based upon a N-fused carbazole-zinc porphyrin-free-base porphyrin triad containing an ethynyl-linkage (coded as DTBC), the novel porphyrin dyes named DTBC-MP and DTBC-TP were designed by varying the porphyrin-free-base units in the π conjugation of DTBC in order to study the effect of porphyrin-free-base in the modification of electronic structures and related properties. The calculated results indicate that, the extension of the conjugate bridge with the porphyrin-free-base unit results in elevation of the highest occupied molecular orbital (HOMO) energies, decrease of the lowest unoccupied molecular orbital (LUMO) energies, reduction of the HOMO-LUMO gap, red-shift of the absorption bands, and enhancement of the absorbance. The free energy changes demonstrate that introducing more porphyrin-free-base units in the conjugate bridge induces a faster rate of electron injection. The transition properties and molecular orbital characters suggest that the different transition properties might lead to a different electron injection mechanism. In terms of electronic structure, absorption spectra, light harvesting capability, and free energy changes, the designed DTBC-TP is a promising candidate dye sensitizer for DSSCs.

A striking performance improvement of fullerene n-channel field-effect transistors via synergistic interfacial modifications

For fullerene based n-channel transistors, remarkably improved device characteristics were achieved via charge injection and transport interfacial synergistic modifications using low-cost aluminium source/drain electrodes. Compared with the reference device without any modifications (device A), the as-fabricated transistor (device H) showed a dramatic improvement of saturation mobility from 0.0026 to 0.3078 cm2 V−1 s−1 with a maximum on–off current ratio of 106 and a minimum subthreshold slope of 1.52 V decade−1. AFM and XRD analysis manifested that the deposited C60 films on PVA/OTS successive-modified SiO2 substrate were highly dense polycrystalline and uniform with larger crystalline grain and less grain boundary. A gap state assisted electron injection mechanism was proposed to explicate the enhanced electrical conductivity considering BCP modification for charge injection interface, which has been well corroborated by a diode-based injection experiment and a theoretical calculation of contact resistances. We further demonstrated the application of the concept modification method to enable comparative time-stable operation of fullerene n-channel transistors. Given many key merits, we believed that this general method using multi-interface modifications could be extended to fabricate other n-channel OFETs with superior electrical performance and stability.

Multilevel Resistive Switching with Oxygen Vacancy Filaments in Pt/TaOx/Cu and Pt/TaOx/Pt Devices

Paper demonstrates that multilevel switching of oxygen vacancy (VO) conductive filaments (CF) in Cu/TaOx/Pt devices is characterized by the relation Ron=A/Icc n between ON-state resistance (Ron) and compliance current (Icc). However, in contrast to metallic (Cu, Ag) CFs, where the exponent n for various devices is found universally to be n≈1, we find for VO CFs to be n=1.36 with A=0.022. This constitutes a significant departure from n=1 found for all metallic CFs. As a consequence of n>1, the constant A can no longer be interpreted as the universal minimum switching voltage. In fact, our experiments confirmed that no minimum Vset for Vo could be ascertained. Locally, for a small Icc intervals, one can still fit the data with n=1 and extract following values for the constant A: for Icc ≈ 10nA A=1.7V, for Icc ≈ 0.1mA A=0.52 V, and for Icc=10mA A=0.06V. One concludes that at low Icc currents the minimum set voltage is high and at high Icc currents very low. We have measured the Vset of Vo CF as a function of voltage sweep rate v. We find that, as in the case of metallic CFs, the SET voltage decreases linearly with the log(v). However, in contrast to metallic CFs, the Vset for Vo CF does not saturate at a minimum set voltage, even at such slow ramp rates as v=1x10-5 V/s. We argue that an exponent n>1 implies two distinct mechanisms responsible for the formation of VO CFs. 1st mechanism is similar to the formation mechanism for metallic CFs and accounts for the unity part of the exponent, while the 2nd mechanism is responsible for the remainder (n-1). There is a consensus that at the interface between the active electrode (Cu) and the metal oxide dielectric a redox reaction is taking place: Cu ←→ Cu++e-. When negative bias is applied to Cu electrode, the redox reaction provides an efficient surface injection mechanism of electrons into the oxide while Cu+ ions are returned to the Cu electrode. The electrons are injected and may charge and dislodge a negative oxygen ion from the metal oxide matrix leaving behind a neutral oxygen vacancy Vo according to the reaction: TaOx+2e-→TaOx-1+O2-=TaOxVo+O2-. Since the redox reaction takes place at the Cu/TaOx interface and produces a large number of electrons, the creation of Vo will take place predominantly at the interface. Since the oxygen vacancy is known to provide a local Fermi level close to the conduction band, the vacancy will extend the active electrode into the oxide and act as a conduit for more electrons. Thus it is likely that another Vo will be formed below the first one extending thus the partial Vo filament as an extension of the electrode deeper into the oxide. This leads to an increased electric field between the end of the partial Vo CF and the inert electrode Pt. The self-accelerating mechanism is similar to the formation of Cu CF at positive bias. The 2nd mechanism for creation of oxygen vacancies consists in electron migration into the oxide, at random, and at some point creating a (Vo,O2-) pair but now in the bulk rather than at the surface. Over the stress time, more and more of vacancies will be generated in the same way. The increased density of vacancies will reach a critical concentration of vacancies allowing for a conductive percolation path between the two electrodes. The 2nd mechanism is verified on Pt/TaOx/Pt devices where the 1st surface electron injection mechanism is eliminated. We measure Vset for Vo CF for Pt/TaOx/Pt devices to be significantly higher, Vset≈-8V than for Cu/TaOx/Pt with Vset≈-2V. Also for Pt/TaOx/Pt devices the relation Ron=A/Icc n cannot be fully constructed: Only for Icc>1mA stable non-volatile Vo CFs could be formed. At higher set currents the cell could be set and reset but less than 10-20 times before the device is permanently damaged. To investigate the two Vo mechanisms further, we fabricated and characterized Cu/SiO2/Pt and Pt/SiO2/Pt devices. The thickness of SiO2 layer is dSiO2=32,16, and 8nm. SiO2 is known to be very stable and not susceptible to oxide damage induced by Vo. For both kind of devices with dSiO2=32nm no Vo CF could be formed even at voltages as high as 15V. For dSiO2=16nm devices Vo CF could be formed but not reliably set and reset. For dSiO2=8nm the Vo CF could be readily formed, set and reset. This behavior is explained in terms of critical size of Vo cluster and compared for ALDTa2O5, TaOx, and SiO2 dielectrics, and complements the two mechanisms discussed above.

Theoretical studies of fluorine substituent effect on organic photo-sensitizers in dye sensitized solar cells

Based on titanium dioxide model and LJBs sensitizers (triphenylamine as the donor, 3,4-ethylene dioxy thiophene plus various functionalized phenylenes as the π-spacer, and cyanoacrylic acid as the anchoring group) with different connection types on TiO2 substrate, the reasonable dye-TiO2 connection has been located. The results show that the dissociative adsorption of LJBs is more thermodynamically favorable than the neutral molecule bound configurations. LJBs adsorb onto TiO2 via interaction between 3d orbital of surface Ti atom and 2p orbital of N/O atoms in the acceptors. This result is confirmed in both neutral and dissociation forms of LJBs molecules. According to our calculations, adding a meta-fluorine substituent to the phenyl group of cyanoacrylic acid (LJB-Fm) may damage the planarity and conjugation. Consequently, the light harvesting efficiency decreased, and that is particularly unfavorable for the DSSCs application. The Ortho F-substituted dye (LJB-Fo), however, exhibited enhanced light absorption and more efficient intra-molecular charge transport. The bigger Jsc, Voc values of LJB-Fo system predicts its superior DSSC performance. Additionally, LJBs anchored on TiO2 surface via group –COO (LJB-H and LJB-Fo) lead to an indirect mechanism for electron injection. While LJB-Fm preferred to direct electron injection mechanism due to the strong orbital-coupling between sensitizer and the TiO2 substrate.

Mechanism of performance enhancement via fluorine doped titanium dioxide nanoparticles in dye sensitized solar cells

Abstract Fluorine doped anatase TiO2 nanoparticles are synthesized by hydrothermal method and applied in dye-sensitized solar cells (DSCs) as excellent photoelectrode material. The result shows that fluorine-doping can positively shift the conduction band of TiO2, thus enlarge the driving force for efficient electron injection, reduce the interfacial losses from the charge recombination and consequently facilitate the charge transfer. The conversion efficiency of DSCs with fluorine-doped TiO2 electrodes is improved by 16.2% and results in a 23.1% photocurrent gain. This paper provides a comprehensive understanding on the behaviors of fluorine-doping TiO2 in DSCs from the conduction band shift and the mechanism of electron injection, transfer and recombination and a promising strategy to explore high efficiency DSCs.

Mechanism and Dynamics of Electron Injection and Charge Recombination in DNA. Dependence on Neighboring Pyrimidines.

The mechanism and dynamics of photoinduced electron injection and charge recombination have been investigated for several series of DNA hairpins. The hairpins possess a stilbenediether linker, which serves as an electron donor and base pair stems that possess different pyrimidine bases adjacent to the linker. Hairpins with adjacent thymine-adenine (T-A) base pairs undergo fast electron injection and relatively slow charge recombination with rate constants that are not strongly dependent upon the following base pair. Hairpins with adjacent cytosine-guanine (C-G) base pairs undergo reversible electron injection and much faster charge recombination than those with adjacent T-A base pairs. Hairpins with 5-fluorouracil or other halogenated pyrimidines in their first and second base pair undergo fast electron injection and multiexponential charge recombination. The difference in kinetic behavior for the different series of hairpins and its implications for the formation of long-lived charge-separated states are discussed and compared to results reported previously for other electron-donor chromophores.

Electron Transfer of Squaraine-Derived Dyes Adsorbed on TiO2 Clusters in Dye-Sensitized Solar Cells: A Density Functional Theory Investigation

Because the intense absorptions of squaraine (SQ) dyes in the red/near-infrared spectral regions closely match the spectral response of sunlight, SQ dyes have great potential for use in dye-sensitized solar cells (DSCs). In this study, we employed density functional theory (DFT) and time-dependent DFT to investigate the structural, optical, and electron transfer properties of seven recently reported SQ-derived dyes adsorbed on a (TiO2)38 cluster having an anatase (101) surface, as a model for corresponding DSCs. In particular, we calculated the proportions of the electron densities in the dye–(TiO2)38 systems that were transferred to the TiO2 moieties upon excitation, allowing us to investigate their electron injection mechanisms. We found that the dye–(TiO2)38 systems followed different direct and indirect mechanisms of electron injection to TiO2 depending on the localization of the excited state electron density and the driving force for excited-state electron injection. JD10 and YR6 owning two intense absorption bands, which have significant proportions of electron density delocalized into the TiO2 moiety upon excitation and have driving forces for excited-state electron injection, followed direct electron injection mechanisms to TiO2. These results are compatible with their higher experimentally observed short-circuit currents (Jsc) than those of the SQ dyes. In contrast, SQ12, SQ2, and SQ4 followed the indirect electron injection mechanism due to their negligible proportion of electron density injected into TiO2; in addition, SQ2 and SQ4 do not provide driving force for electron injection. SQ12, SQ2, and SQ4 dyes had lower experimentally observed Jsc values than those of the other SQ dyes. The calculated probabilities of electron density being delocalized into TiO2 and driving force for excited-state electron injection from these studied SQ dyes are compatible with their experimentally observed Jsc values. This study provides insight into the electron injection mechanisms of SQ-derived dyes adsorbed on TiO2 upon photoexcitation. Furthermore, our calculations and findings give clues for designing new SQ-derived sensitizers for DSC applications.

Hole‐Trapping Effect of the Aliphatic‐Amine Based Electron Injection Materials in the Operation of OLEDs to Facilitate the Electron Injection

The organic materials containing aliphatic amine groups (AAGs) are widely used as the electron injection layer (EIL) in the application of organic light‐emitting diodes (OLED) to significantly improve the device performance. The electron injection mechanism is commonly explained by the interface dipole model, which suggests that the dipole layer induced by the electron injection layer at the organic/metal interface elevates the vacuum level on the metal side, thereby reducing the electron injection barrier. The experiments supporting the interface dipole model are generally carried out under the nonoperating condition of the OLEDs in which few holes are present. Our experiments on the OLED devices with neutralized AAG‐EIL dipole reveal that the interface dipole model cannot fully explain the AAG‐EIL's electron injection ability. During the operation of the OLEDs, the aliphatic amine function groups of the AAG‐EIL trap a huge amount of holes, bending the energy level and decreasing the electron tunneling distance. As a result, the reduced electron tunneling distance facilitates the electron injection.

The Impact of the Electrical Nature of the Metal Oxide on the Performance in Dye-Sensitized Solar Cells: New Look at Old Paradigms

The process of primary charge separation in Ru-based dye-sensitized solar cells (DSSCs) based on both TiO2 and ZnO photoanodes has been studied in fully working devices by time-resolved measurements, including emission and transient absorption (from femtoseconds to microseconds), and electrochemical techniques (current–voltage characteristics and impedance spectroscopy). By studying the effect of different electrolyte compositions using potential-determining additives (Li+ ions, 4-tert-butylpiridine), we have been able to provide novel insights into the mechanisms of electron injection and dye regeneration across the oxide/dye/electrolyte system. In this respect, the shift of the conduction band that is commonly reported for TiO2 in the presence of additives is not observed for ZnO. In addition, both injection and regeneration for ZnO-based cells are shown to be much slower than those for TiO2 and independent of the electrolyte composition. The slower injection and regeneration for ZnO and its lower sensitivity with respect to the addition of potential-determining additives strongly suggest that the electrical nature of the oxide is crucial to facilitate charge separation across the oxide/dye/electrolyte interface. In addition, electron injection from singlet and triplet states has been identified for both metal oxides. The former process occurs on the ultrafast time scale (<100 fs), while the latter extends over many time scales from single ps to single ns. The present work provides novel insights about the processes of electron injection and dye regeneration in DSSCs as well as the reasons for the lower performance of ZnO-based dye solar cells.

Effect of capping ligands and TiO2 supporting on the optical properties of a (CdSe)13 cluster.

The influence of different aliphatic and aromatic ligand molecules on the electronic properties of CdSe quantum dots (QDs) has been examined by employing density functional theory (DFT). Optical spectra were simulated with the real-time time-dependent DFT (RT-TDDFT) methodology. The assignment of the first absorption peak features that occur in these spectra was done by taking into account the composition of the frontier molecular orbitals (MOs) of the different systems. While the aliphatic ligands considered-amine, thiol, and phospine oxides-did not show any major influence on the electronic absorption spectra, some of the aromatic ligands do have a noticeable impact on the optoelectronic properties of the QD. Aromatic ligands are mainly aniline-type molecules; additionally, a thiophenol and uracil were employed to saturate the dangling bonds on the Cd atoms. Finally, a more realistic model of a QD-sensitized solar cell consisting of methylamine-capped (CdSe)13 cluster linked to a TiO2 nanoparticle through a mercaptopropionate bridge was considered. The simulations again show that the lowest electronic excitation takes place within the QD subunit, demonstrating the indirect nature of the electron injection mechanism operating in these solar cells.

Transient states and the role of excited state self-quenching of indoline dyes in complete dye-sensitized solar cells

The photo behaviour of indoline dye D149 on different metal oxide nanoparticles in functioning solar cells is investigated by time-resolved studies in the time range from 100 fs to several ns. The cells are also characterized by standard photovoltaic measurements. The electron injection is found to occur on the time scales from <200 fs to several tens of ps. Locally excited (LE) and charge transfer (CT) excited states of the dye are identified to participate in the electron injection and dye deactivation mechanisms. The dependence of the ultrafast dynamics on the coadsorbent concentration and energy density of the pump pulse indicates the important role of excited state self-quenching. A decrease in the photocurrent of the cells upon aging (very fast for ZnO and slower for TiO2 nanoparticles) is found to be correlated with the transient absorption kinetics, with a probable explanation suggested as electrolyte-induced dye rearrangement and aggregate formation.

Organic sensitizers with modified di(thiophen-2-yl)phenylamine donor units for dye-sensitized solar cells: a computational study

A new series of organic donor–π–acceptor (D–π–A) dyes namely B1–6 with modification of donor groups by introducing thiophene (as D) and fluorene-connected carbazole on top of thiophene (as 2D–D) in phenylamine moieties, and with elongation of π-spacer unit of thiophene (1–3 units) in the π-spacer, were molecularly designed by using density functional theory (DFT) and time-dependent DFT. The nature of intramolecular charge transfer of all dyes was elucidated by means of frontier molecular orbital analysis, electronic structures, and absorption spectra to provide their potential use for dye-sensitized solar cells (DSSCs). The structural results show that the 2D–D–π–A dyes have a nonplanar structure on the D–D moiety that may suppress the aggregation of dye and yet maintain the conjugation in the whole D–π–A moiety. The systematically elongating π-spacer of B4–6 dyes with increasing number of thiophene group and the introducing 2D into D–π–A dyes give the redshift on absorption peak and broaden the absorption range, which are in excellent agreement with available experiment. Thus, this redshift improves their overall light-harvesting efficiency (LHE) better than the B1–3 dyes. Among the six dyes, B6 would have the best performance because it has the highest predicted LHE at the maximum absorption wavelength (λ max) and the suitable driving force ΔG inject of the electron injection from the excited state of dyes to the conduction band of TiO2. The prototype of DSSCs performance of selected dyes was further simulated using the chemisorption of dyes onto the (TiO2)38 cluster to reveal the nature of the electron injection mechanism. This current work is expected to assist in the molecular design of new metal-free organic dyes for use in DSSCs yielding highly efficient performance.

Unsymmetrical squaraine dye containing dithieno[3,2-b:2′,3′-d]pyrrole as a π-spacer: A potential photosensitizer for dye-sensitized solar cells

Designed with electron-rich and electron-deficient groups as π-linkers, a series of novel unsymmetrical squaraine dyes (SQ) for dye-sensitized solar cells (DSSCs) have been simulated using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Dye/TiO2 complexes were also calculated to explore various properties on the interface. The results show that, compared with the prototype dye of WCH-SQ11, the modified WCH-SQ11-1, obtained by replacing the 3,4-ethylenedioxythiophene (EDOT) unit in WCH-SQ11 with dithieno[3,2-b:2′,3′-d]pyrrole (DTP), keeps a good balance in various key properties including light harvesting efficiency (LHE), driving force for electron injection (ΔGinject), charge transfer (CT) character, and vertical dipole moment (μnormal). Further investigation of the adsorbed WCH-SQ11-1 (i.e. the WCH-SQ11-1/(TiO2)38 complex) reveals a red-shifted absorption spectrum compared to the free dye. The analysis of the molecular orbitals responsible for the maximum absorption indicate a direct electron injection mechanism for the WCH-SQ11-1/(TiO2)38 system.