Synthesis, photophysical properties and two-photon absorption study of tetraazachrysene-based N-heteroacenes

Three novel N‐heteroacene molecules (SDNU‐1, SDNU‐2 and SDNU‐3) based on tetraazachrysene units as cores have been designed, synthesized and fully characterized. Their photophysical, electrochemical and fluorescence properties were investigated, and they exhibited blue to green emission in the solid state. Interestingly, SDNU‐2 exhibited high solid photoluminescence quantum efficiencies (75.3 %), which is the highest value of N‐heteroacenes derivatives to date. Two‐photon absorption studies have been conducted by using the open and close aperture Z‐san technique. SDNU‐3 showed a significant enhancement in the two‐photon absorption cross‐section with magnitudes as high as about 700 GM (1 GM=1×10−50 cm4 s/photon) when excited with 800 nm light, which is the largest value based on a heteroacene system measured by using a Z‐scan experiment so far. We attribute the outcome to sufficient electronic coupling between the strong charge transfer of quadrupolar substituents and the tetraazachrysene core. Our result would provide a new guideline to design novel efficient two‐photon materials based on N‐heteroacene cores

Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells.

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the Voc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells

Low Density of Conduction and Valence Band States Contribute to the High Open-Circuit Voltage in Perovskite Solar Cells

Hybrid perovskites are widely used for high-performance solar cells. Large diffusion lengths and long charge carrier lifetimes are considered two main factors for their high performance. Here, we argue that not only large diffusion lengths and long carrier lifetimes but also the low densities of the conduction and valence band states (<i>N</i>_c, <i>N</i>_v) contribute to high-performance perovskite solar cells. We estimated <i>N</i>_c and <i>N</i>_v of silicon, CdTe, and typical perovskites with two different methods. It was found that <i>N</i>_c and <i>N</i>_v of perovskites and CdTe are much lower than that of silicon. Using numerical models, we found that the solar cell of a material with same characteristics but lower <i>N</i>_c and <i>N</i>_v can realize a higher open-circuit voltage (<i>V</i>_oc) and higher power conversion efficiency (PCE). We put forward and proved that the low <i>N</i>_c and <i>N</i>_v in hybrid perovskite is one of the factors for its high performance. This provides a new guideline for finding and developing new photovoltaic candidate materials

High Performance Photovoltaic Applications Using Solution-Processed Small Molecules

Energy remains a critical issue for the survival and prosperity of humancivilization. Many experts believe that the eventual solution for sustainable energy is the use of direct solar energy as the main energy source. Among the options for renewable energy, photovoltaic technologies that harness solar energy offer a way to harness an unlimited resource and minimum environment impact in contrast with other alternatives such as water, nuclear, and wind energy.Currently, almost all commercial photovoltaic technologies use Si-based technology, which has a number of disadvantages including high cost, lack of flexibility, and the serious environmental impact of the Si industry. Other technologies, such as organic photovoltaic (OPV) cells, can overcome some of these issues. Today, polymer-based OPV (P-OPV) devices have achieved power conversion efficiencies (PCEs) that exceed 9%. Compared with P-OPV, small molecules based OPV (SM-OPV) offers further advantages, including a defined structure for more reproducible performance, higher mobility and open circuit voltage, and easier synthetic control that leads to more diversified structures. Therefore, while largely undeveloped, SM-OPV is an important emerging technology with performance comparable to P-OPV.In this Account, we summarize our recent results on solution-processed SM-OPV. We believe that solution processing is essential for taking full advantage of OPV technologies. Our work started with the synthesis of oligothiophene derivatives with an acceptor–donor–acceptor (A-D-A) structure. Both the backbone conjugation length and electron withdrawing terminal groups play an important role in the light absorption, energy levels and performance of the devices. Among those molecules, devices using a 7-thiophene-unit backbone and a 3-ethylrhodanine (RD) terminal unit produced a 6.1% PCE. With the optimized conjugation length and terminal unit, we borrowed from the results with P-OPV devices to optimize the backbone. Thus we selected BDT (benzo[1,2-b:4,5-b′]dithiophene) and DTS (dithienosilole) to replace the central thiophene unit, leading to a PCE of 8.12%. In addition to our molecules, Bazan and co-workers have developed another excellent system using DTS as the core unit that has also achieved a PCE greater than 8%

A perylene diimide (PDI)-based small molecule with tetrahedral configuration as a non-fullerene acceptor for organic solar cells

In this paper, a new perylene diimide (PDI)-based acceptor Me-PDI4 with tetrahedral configuration (or 3D) has been synthesized and characterized. Solution-processed organic solar cells (OSCs) based on Me-PDI4 have been investigated and our results show that the device performance can reach as high as 2.73%. Our new design with tetrahedral configuration (or 3D) could be an efficient approach to increase the PCE of OSCs with non-fullerene acceptors.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Influence of Hexagonal Boron Nitride on Electronic Structure of Graphene

By performing first-principles calculations, we studied hexagonal-boron-nitride (hBN)-supported graphene, in which moiré structures are formed due to lattice mismatch or interlayer rotation. A series of graphene/hBN systems has been studied to reveal the evolution of properties with respect to different twisting angles (21.78°, 13.1°, 9.43°, 7.34°, 5.1°, and 3.48°). Although AA- and AB-stacked graphene/hBN are gapped at the Dirac point by about 50 meV, the energy gap of the moiré graphene/hBN, which is much more asymmetric, is only about several meV. Although the Dirac cone of graphene residing in the wide gap of hBN is not much affected, the calculated Fermi velocity is found to decrease with the increase in the moiré super lattice constant due to charge transfer. The periodic potential imposed by hBN modulated charge distributions in graphene, leading to the shift of graphene bands. In agreement with experiments, there are dips in the calculated density of states, which get closer and closer to the Fermi energy as the moiré lattice grows larger

Fusing N-heteroacene analogues into one “kinked” molecule with slipped two-dimensional ladder-like packing

An unexpected “kinked” N-heteroacene with a slipped two-dimensional ladder-like packing feature is produced from the conventional condensation reaction. The as-obtained compound [2,2′]bi(5,12-bis(TIPS)piperazin-3-one[2,3-b]phenazine) (2BPP) consists of two identical backbones (5,12-bis(TIPS)piperazin-3-one[2,3-b]phenazine), which are fused together through a C[double bond, length as m-dash]C double bond and two intramolecular H-bonds. A study on the charge carrier transport indicates that a 2BPP single crystal has a hole mobility up to 0.3 cm2 V−1 s−1, while theoretical calculations suggest that this compound might possess potential well-balanced ambipolar charge-transport characteristics.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Cover picture : a concise method for synthesizing 1,4,8,11-tetraaza-6,13-dioxapentacene derivatives (Asian J. Org. Chem. 10/2013)

Five substituted tetraazadioxaacenes have been synthesized through the condensation of 2,3,7,8-tetraaminodibenzo-1,4-dioxin tetrahydrochloride and diketones. In their Full Paper on page 852 ff., Qichun Zhang and co-workers shown that these linearly extended molecules have relatively large bandgaps, and some of the compounds have less negative reduction potentials than is predicted by calculations

Singlet fission dynamics and optical spectra of pentacene and its derivatives

A multimode Brownian oscillator model is employed to investigate the absorption spectra of pentacene and its derivatives in solution and thin films. Excellent agreement has been obtained between simulated and measured absorption spectra. Furthermore, using parameters obtained from fitting the absorption spectra of these pentacene derivatives, the singlet fission dynamics and two-dimensional electronic spectra of an ab initio Hamiltonian are investigated by Dirac–Frenkel time-dependent variation with multiple Davydov trial states. It is found that the periodic wave packet motion induced in the displaced excited state, and the accompanying vibrational relaxation, can be visualized by two-dimensional electronic spectra at short times.Ministry of Education (MOE)Accepted versionSupport from the Singapore Ministry of Education Academic Research Fund (Grant No. 2018-T1-002-175, MOE2019-T2-1-085, and 2020-T1-002-075) is gratefully acknowledged. K. Sun also thanks the Natural Science Foundation of Zhejiang Province (Grant No. LY18A040005) for support

U-shaped helical azaarenes : synthesis, structures, and properties

The synthesis and properties of a series of U-shaped helical azaarenes are reported. Crystal structures of these helical azaarenes were obtained, and the solid-state structures unequivocally exhibited their helicity.Ministry of Education (MOE)Q.Z. acknowledges AcRF Tier 1 (Grants RG 111/17, RG 2/17, RG 114/16, RG 113/18) and Tier 2 (Grants MOE 2017-T2-1-021 and MOE 2018-T2-1-070), Singapore