Lili Du

Efficiency Enhancement of Perovskite Solar Cells through Fast Electron Extraction: The Role of Graphene Quantum Dots

We report on a significant power conversion efficiency improvement of perovskite solar cells from 8.81% to 10.15% due to insertion of an ultrathin graphene quantum dots (GQDs) layer between perovskite and TiO2. A strong quenching of perovskite photoluminescence was observed at ∼760 nm upon the addition of the GQDs, which is pronouncedly correlated with the increase of the IPCE and the APCE of the respective cells. From the transient absorption measurements, the improved cell efficiency can be attributed to the much faster electron extraction with the presence of GQDs (90-106 ps) than without their presence (260-307 ps). This work highlights that GQDs can act as a superfast electron tunnel for optoelectronic devices.

Effects of hydrogen bond and solvent polarity on the C=O stretching of bis(2-thienyl)ketone in solution.

The optimized structural parameters, the absorption and the resonance Raman spectra have been investigated for the bis(2-thienyl)ketone in gas phase, in cyclohexane, methanol, and acetonitrile solvents by means of time dependent density functional theory calculations, the solvent electronic polarization effect on the solvation shift is examined and in well accordance with the calculation. The effect of increasing the polarity of the solvent is well represented by the polarizable continuum model, both for the absorption spectra and resonance Raman intensities. The Raman spectra of the C=O stretching mode, which is sensitive to the intermolecular interaction for bis(2-thienyl)ketone dissolved in solvents, were systematically studied. It was found that the hydrogen bond effect plays an important role in reducing the carbonyl stretching wavenumbers. The results of Raman shifts were interpreted through the dilution effect, solvation effects, and hydrogen bond-forming effects. Furthermore, the excitation profiles of several important Raman bands of bis(2-thienyl)ketone molecule in different solvents have been critically analyzed. The solvent effects on structural and symmetry properties of the molecule in S2 electronic state as well as the short-time photo relaxation dynamics have been discussed.

Investigation of high performance TiO2 nanorod array perovskite solar cells

In this paper, systematic investigations on the fabrication and characterization of high performance TiO2 nanorod array perovskite solar cells (NAPSCs) are reported. The TiO2 nanorods, of length around 350–400 nm, were grown by solvothermal technique directly on glass/FTO substrates. From the scanning transmission electron microscopy (STEM) we demonstrate that excellent crystallinity for the TiO2 nanorods can be produced using the solvothermal technique. Precursor consisting of a mixture of PbI2, CH3NH3I (MAI) and CH3NH3Cl (MACl) was used for the growth of perovskite thin films on the glass/FTO/TiO2 nanorod array (TiO2-NA) substrates. It is found that the morphology and quality of the perovskite layer depend strongly on the concentration of MACl in the precursor. Experimental studies on femtosecond transient absorption (fs-TA) indicate that the incorporation of TiO2-NA greatly enhances the collection efficiency of the photo-generated carriers due to substantial increase of interfacial area between the perovskite and TiO2-NA, leading to a reduction in carrier diffusion distance. It is shown to be the key factor that the proposed technique facilitates the use of a thicker perovskite absorber layer (∼500 nm) without compromising on the series resistance. Detailed J–V characterization shows that the NAPSCs exhibit negligible hysteresis with a power conversion efficiency (PCE) >19% for the champion device.

Photoconversion of β-Lapachone to α-Lapachone via a Protonation-Assisted Singlet Excited State Pathway in Aqueous Solution: A Time-Resolved Spectroscopic Study

The photophysical and photochemical reactions of β-lapachone were studied using femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopy techniques and density functional theory calculations. In acetonitrile, β-lapachone underwent an efficient intersystem crossing to form the triplet state of β-lapachone. However, in water-rich solutions, the singlet state of β-lapachone was predominantly quenched by the photoinduced protonation of the carbonyl group at the β position (O9). After protonation, a series of fast reaction steps occurred to eventually generate the triplet state α-lapachone intermediate. This triplet state of α-lapachone then underwent intersystem crossing to produce the ground singlet state of α-lapachone as the final product. 1,2-Naphthoquinone is examined in acetonitrile and water solutions in order to elucidate the important roles that water and the pyran ring play during the photoconversion from β-lapachone to α-lapachone. β-Lapachone can also be converted to α-lapachone in the ground state when a strong acid is added to an aqueous solution. Our investigation indicates that β-lapachone can be converted to α-lapachone by photoconversion in aqueous solutions by a protonation-assisted singlet excited state reaction or by an acid-assisted ground state reaction.

Direct time-resolved spectroscopic observation of arylnitrenium ion reactions with guanine-containing DNA oligomers.

The metabolic activation of a number of aromatic amine compounds to arylnitrenium ions that can react with DNA to form covalent adducts has been linked to carcinogenesis. Guanine in DNA has been shown to be the main target of N-containing carcinogens, and many monomeric guanine derivatives have been utilized as models for product analysis and spectroscopic investigations to attempt to better understand the reaction mechanisms of DNA with arylnitrenium ions. However, there are still important unresolved issues regarding how arylnitrenium ions attack guanine residues in DNA oligomers. In this article, we employed ns-TA and ns-TR(3) spectroscopies to directly observe the reaction of the 2-fluorenylnitrenium ion with selected DNA oligomers, and we detected an intermediate possessing a similar C8 structure as the intermediates produced from the reaction of monomeric guanosine derivatives with arylnitrenium ions. Our results suggest that the oligomeric structure can lead to a faster reaction rate of arylnitrenium ions with guanine residues in DNA oligomers and the reaction of arylnitrenium ions take place in a manner similar to reactions with monomeric guanosine derivatives.

Photochemical Activation of Tertiary Amines for Applications in Studying Cell Physiology

Representative tertiary amines were linked to the 8-cyano-7-hydroxyquinolinyl (CyHQ) photoremovable protecting group (PPG) to create photoactivatable forms suitable for use in studying cell physiology. The photoactivation of tamoxifen and 4-hydroxytamoxifen, which can be used to activate Cre recombinase and CRISPR-Cas9 gene editing, demonstrated that highly efficient release of bioactive molecules could be achieved through one- and two-photon excitation (1PE and 2PE). CyHQ-protected anilines underwent a photoaza-Claisen rearrangement instead of releasing amines. Time-resolved spectroscopic studies revealed that photorelease of the tertiary amines was extremely fast, occurring from a singlet excited state of CyHQ on the 70 ps time scale.

Direct Observation of an Efficient Triplet Exciton Diffusion Process in a Platinum-Containing Conjugated Polymer

We report the synthesis and characterization of a conjugated polymer incorporated with cyclometalated platinum complexes on the main chain. The polymer may serve as an efficient triplet sensitizer in light-harvesting systems. The photophysical properties of the polymer were studied by nanosecond and femtosecond time-resolved transient absorption spectroscopies. After excitation, an energy-transfer process from the thiophene units on the conjugated main chain to the singlet excited state of the Pt complex moieties occurred in less than 150 fs. The subsequent intersystem crossing process resulted in the formation of a triplet excited state at the Pt complex moieties in ∼3.2 ps, which was then followed by an efficient triplet diffusion process that led to the formation of triplet excitons on the polymer main chain in ∼283 ps. This proposed efficient triplet sensitized polymer system not only enhances the exciton diffusion length but also reduces energy loss in the process, which displays remarkable implications in the design of novel materials for triplet sensitized solar cells.

Direct Detection of the Open-Shell Singlet Phenyloxenium ion: An Atom-Centered Diradical Reacts as an Electrophile

A new photoprecursor to the phenyloxenium ion, 4-methoxyphenoxypyridinium tetrafluoroborate, was investigated using trapping studies, product analysis, computational investigations, and laser flash photolysis experiments ranging from the femtosecond to the millisecond time scale. These experiments allowed us to trace the complete arc of the photophysics and photochemistry of this photoprecursor beginning with the initially populated excited states to its sequential formation of transient intermediates and ultimate formation of stable photoproducts. We find that the excited state of the photoprecursor undergoes heterolysis to generate the phenyloxenium ion in ∼2 ps but surprisingly generates the ion in its open-shell singlet diradical configuration (1A2), permitting an unexpected look at the reactivity of an atom-centered open-shell singlet diradical. The open-shell phenyloxenium ion (1A2) has a much shorter lifetime (τ ∼ 0.2 ns) in acetonitrile than the previously observed closed-shell singlet (1A1) phenyloxenium ion (τ ∼ 5 ns). Remarkably, despite possessing no empty valence orbitals, this open-shell singlet oxenium ion behaves as an even more powerful electrophile than the closed-shell singlet oxenium ion, undergoing solvent trapping by weakly nucleophilic solvents such as water and acetonitrile or externally added nucleophiles (e.g., azide) rather than engaging in typical diradical chemistry, such as H atom abstraction, which we have previously observed for a triplet oxenium ion. In acetonitrile, the open-shell singlet oxenium ion is trapped to generate ortho and para Ritter intermediates, one of which (para) is directly observed as a longer-lived species (τ ∼ 0.1 ms) in time-resolved resonance Raman experiments. The Ritter intermediates are ultimately trapped by either the 4-methoxypyridine leaving group (in the case of para addition) or trapped internally via an essentially barrierless rearrangement (in the case of ortho addition) to generate a cyclized product. The expectation that singlet diradicals react similarly to triplet or uncoupled diradicals needs to be reconsidered, as a recent study by Perrin and Reyes-Rodríguez (J. Am. Chem. Soc. 2014, 136, 15263) suggested the unsettling possibility that singlet p-benzyne could suffer nucleophilic attack to generate a naked phenyl anion. Now, this study provides direct spectroscopic observation of this phenomenon, with an atom-centered open-shell singlet diradical reacting as a powerful electrophile. To the question of whether a nucleophile can attack a singly occupied molecular orbital, the answer is apparently yes, at least if another partially occupied orbital is available to avoid violation of the rules of valence.

Study of electronic interactions and photo-induced electron transfer dynamics in a metalloconjugated polymer–single-walled carbon nanotube hybrid by ultrafast transient absorption spectroscopy

Regioregular poly(thiophene) functionalized with pendant photosensitizing ruthenium complexes was synthesized. The conjugated polymer serves dual functions as a dispersant for single-walled carbon nanotubes (SWCNTs) and also as a photosensitizer. SWCNTs can be dispersed effectively by the polymer, and the polymer/SWCNT hybrid dispersion obtained was stable for months. Raman spectroscopic results showed that there was no structural damage or defects present in SWCNTs after the functionalization process and SWCNTs with both small (0.9 nm) and large (1.6 nm) diameter can be dispersed. Transmission electron microscopy revealed the presence of polymers on the SWCNT surface. The dynamics of the photo-induced electron transfer process between the ruthenium sensitizers and SWCNTs was probed by femtosecond transient absorption spectroscopy. Rapid electron injection from the ruthenium sensitizers to SWCNTs with a time constant of 167 ps was observed. By using this synthesis approach, it is possible to incorporate different types of sensitizers in one polymer chain so as to fine-tune the absorption spectra, and a new light harvesting system can be developed by coupling the polymer with SWCNTs.

Direct Observation of 4-Phenoxyphenylnitrenium Ion: A Transient Absorption and Transient Resonance Raman Study.

Femtosecond (fs) and nanosecond (ns) transient absorption (TA) and single pulse transient resonance Raman spectroscopic investigation of the intermediates after laser photolysis of 4-phenoxyphenyl azide in acetonitrile and mixed aqueous solution is reported. fs-TA results show that the singlet 4-phenoxyphenylnitrene was produced immediately after photolysis of the azide. Then, the singlet nitrene underwent intersystem crossing (ISC) and ring expansion to generate triplet nitrene and ketenimine in acetonitrile with t = 346 ps or protonation in mixed aqueous solution with t = 37 ps, respectively, a little slower than the counterparts of the methoxy one (108 and 5.4 ps for ISC and protonation processes, respectively). The transient Raman spectrum combined density functional theory (DFT) calculation predicting the structure and vibrational frequencies suggested that phenoxyphenylnitrenium ion has a comparable quinoidal character to that of methoxy- and ethoxy-phenylnitrenium ions. All of these results indicated that the phenoxy substitution has some impact on the reactivity of phenylnitrene but a slight influence on the structure of phenylnitrenium ion.