Application In Organic Solar Cells Research Articles

Enhancing performance and scalability of HTL-free organic solar cells with keggin polyoxometalates-type additive

Interfacial layers play a crucial role in enhancing the performances and stabilities of organic solar cells (OSCs). Despite achieving high efficiencies from OSCs, the challenges associated with large-area manufacturing and the need for nanoscale uniformity during coating have hindered the large-scale production of these devices. To address these issues, we propose the concept of hole transport layer (HTL)-free OSCs to simplify the fabrication processes and improve the efficiency of production. In this study, we explore the potential of phosphotungstic acid (PTA), a Keggin polyoxometalate, both as a promising HTL and as an additive in HTL-free OSCs to enhance device performances and scalabilities in cell-to-module OSCs. Remarkably, incorporating PTA as an annealing-free HTL and as an additive in the photoactive layer (PM6:Y6:PC71BM) significantly increases power conversion efficiency (PCE) up to 17.28 % and 16.33 %, respectively, with respect to that of the reference device with PEDOT:PSS (15.88 %). We successfully fabricate a large-area OSC module, exhibiting an impressive PCE of 15.18 % and an active area of 54 cm2, achieving approximately 85 % of the effectiveness of small-area OSCs. The improved performance is attributed to the enhanced transmittance, excellent carrier-dynamic capacity, suppressed carrier recombination, and reduced trap-states in the ternary bulk-heterojunction OSCs. This study demonstrates the promising versatility of PTA for application in OSCs, offering potential enhancements in device performance and scalability for future applications.

(Invited) Performance Improvement and Potential Applications of Ultrathin Organic Solar Cells

Flexible and thin photovoltaics can supply a variety of applications such as power sources for wearable electronics and soft robots. We reduce the total thickness of such organic photovoltaics down to several μm, which enables extreme bendability and lightweight properties. The important parameters include high efficiency and stability in ambient air conditions. Here we show our recent progress with ultra-thin organic solar cells and potential applications for wearable electronics and soft robots.The current best power conversion efficiency (PCE) for ultrathin organic solar cells is 15.8% reported by ourselves1). We have solved the specific issues behind the usage of ultrathin substrate films to achieve almost comparable PCE with those using normal flexible organic solar cells. One specific technology is the usage of polymer films having good thermal stability and smooth surface roughness2). Another important technology is the development of stable charge carrier injection layers and interfaces at the active layer. The state-of-the-art active layer materials such as PM6:Y6 is known not to be stable in ambient air and other environmental stresses. We verified the degradation occurred by the diffusion of Zn atoms from the conventional electron-transport layer (ETL) of ZnO obtained from the sol-gel process to the active layer, and solved it by replacing the ETL with more stable organic/inorganic hybrid material1,3). Such improved ETL and interface achieved highly balanced performance and stability, enabling good PCE on ultrathin substrates.We improved both PCE and environmental stabilities simultaneously. The stabilities include mechanical robustness3), water-proof property4), thermal stability2), and storage stability in ambient air5). Material development and process engineering enable such improved stabilities. In addition, such environmental stability can offer another practical usage. Our ultrathin solar cells have excellent thermal stability up to 120 ℃. This is capable of the hot-melt process which is a mature technology for the apparel industry. We can adhere our ultrathin organic solar cells directly onto textiles with such hot-melt films without any degraded performances. Apart from the improvement of the performance of ultrathin organic solar cells, we are focusing on how to implement such flexible devices onto three-dimensional surfaces5).We demonstrated some potential applications using our ultrathin organic solar cells used as a power source for wearable sensor applications7,8). We fabricated ultrathin organic solar cell modules and sensors. The sensor was powered by the solar cells under light illumination and monitored electrocardiogram signals with a high signal-to-noise ratio. In addition to this, we also demonstrated rechargeable cyborg insects9). In this research, we adhered our ultrathin organic solar cells onto a living insect abdomen and demonstrated a recharging wireless locomotion control system.We are focusing on both performance improvements of ultrathin organic solar cells and the integration of these cells to supply power to wearable electronics. By accelerating system-level integration research in the next decade to meet the required specifications of appropriate target applications, ultrathin organic solar cells will solve the problems associated with the supply of power for various electronics and will contribute to the overall development of society.1) S. Xiong et al., Adv. Sci. 2022, 9, 2105288.2) X. Xu et al., Proc. Natl. Acad. Sci. 2018, 115, 4589.3) F. Qin et al., Nat. Commun. 2020, 11, 4508 .4) H. Jinno et al., Nat. Energy 2017, 2, 780.5) Z. Jiang et al., Proc. Natl. Acad. Sci. 2020, 117, 6391.6) S. I. Rich, S. Lee, K. Fukuda, and T. Someya, 2022, 34, 2106683.7) S. Park et al., Nature 2018, 561, 516.8) H. Jinno et al., Nat. Commun. 2021, 12, 2234.9) Y. Kakei et al., npj Flex. Electron. 2022, 6, 78. Figure 1

Effect of Benzothiadiazole-Based π-Spacers on Fine-Tuning of Optoelectronic Properties of Oligothiophene-Core Donor Materials for Efficient Organic Solar Cells: A DFT Study.

In this work, five novel A-π-D-π-A type molecules D1-D5 were designed by adding unusual benzothiadiazole derivatives as π-spacer blocks to the efficient reference molecule DRCN5T for application as donor materials in organic solar cells (OSCs). Based on a density functional theory approach, a comprehensive theoretical study was performed with different functionals (B3LYP, B3LYP-GD3, B3LYP-GD3BJ, CAM-B3LYP, M06, M062X, and wB97XD) and with different solvent types (PCM and SMD) at the extended basis set 6-311+g(d,p) to evaluate the structural, optoelectronic, and intramolecular charge transfer properties of these molecules. The B3LYP-GD3BJ hybrid functional was used to optimize the studied molecules in CHCl3 solution with the SMD model solvent as it provided the best results compared to experimental data. Transition density matrix maps were simulated to examine the hole-electron localization and the electronic excitation processes in the excited state, and photovoltaic parameters including open-circuit photovoltage and fill factor were investigated to predict the efficiency of these materials. All the designed materials showed promising optoelectronic and photovoltaic characteristics, and for most of them, a red shift. Out of the proposed molecules, [1,2,5]thiadiazolo[3,4-d]pyridazine was selected as a promising π-spacer block to evaluate its interaction with PC61BM in a composite to understand the charge transfer between the donor and acceptor subparts. Overall, this study showed that adding π-spacer building blocks to the molecular structure is undoubtedly a potential strategy to further enhance the performance of donor materials for OSC applications.

Synthesis of novel star-shaped truxene cored small molecules and applications in organic solar cells

Synthesis of novel star-shaped truxene cored small molecules and applications in organic solar cells

Recent Advance in Solution‐Processed Hole Transporting Materials for Organic Solar Cells

AbstractSolution‐processed hole transporting layers (HTLs) not only play a crucial role in realizing high performance of organic solar cells (OSCs), but also possess excellent compatibility with low‐cost and large‐area processing methods of industrialized productions. However, the number and species of HTL materials are obviously fewer than that of electron‐transporting materials, which limits the development and application of OSCs. In particular, the large energy level difference between anode and organic active layer leads to the serious energy barrier for hole collection, bringing much difficulty in developing efficient HTL materials. In this review, it is focused on the recent advances in solution‐processed HTLs in OSCs. Initially, the working mechanism, property requirement, and existing issues of solution‐processed HTLs are systematically analyzed. Afterward, the main classes of solution‐processed HTL materials are discussed, including PEDOT:PSS, conjugated polyelectrolytes (CPEs), TMOs, and others. The structure‐property relationships of solution‐processed HTL materials are analyzed, and some important design rules for such materials toward efficient and stable OSCs are presented. Finally, a brief summary is presented along with some perspectives to help researchers understanding the challenges and opportunities in this field.

Precise synthesis and photovoltaic properties of giant molecule acceptors

Series of giant molecule acceptors DY, TY and QY with two, three and four small molecule acceptor subunits are synthesized by a stepwise synthetic method and used for systematically investigating the influence of subunit numbers on the structure-property relationship from small molecule acceptor YDT to giant molecule acceptors and to polymerized small molecule acceptor PY-IT. Among these acceptors-based devices, the TY-based film shows proper donor/acceptor phase separation, higher charge transfer state yield and longer charge transfer state lifetime. Combining with the highest electron mobility, more efficient exciton dissociation and lower charge carrier recombination properties, the TY-based device exhibits the highest power conversion efficiency of 16.32%. These results indicate that the subunit number in these acceptors has significant influence on their photovoltaic properties. This stepwise synthetic method of giant molecule acceptors will be beneficial to diversify their structures and promote their applications in high-efficiency and stable organic solar cells.

Synthesis of Long‐Chain Oligomeric Donor and Acceptors via Direct Arylation for Organic Solar Cells†

Comprehensive SummaryThe rapid synthesis of structurally complicated electron donors & acceptors still remains a major challenge in organic solar cells (OSC). In this work, we developed a highly efficient strategy to access long‐chain oligomeric donor and acceptors for OSC applications. A series of cyclopentadithiophene (CPDT) and benzothiadiazole (BT)‐based π‐conjugated oligomers, i.e., three oligomeric acceptors (BTDT)n‐IC (n = 1—3) and one long‐chain oligomeric donor (BTDT)4‐RD, are facilely synthesized by an atom‐ and step‐economical, and labor‐saving direct C—H arylation (DACH) reaction (i.e., C—H/C—Br cross coupling). Note that (BTDT)4‐RD involving five CPDT, four BT and two rhodamine (RD) building blocks is the longest oligomeric donor in the fullerene‐free OSC devices ever reported. The dependence of the structure‐property‐performance correlation of (BTDT)n‐IC (n = 1—3) and (BTDT)4‐RD on the π‐conjugation lengths is thoroughly investigated by opto‐electrochemical measurements, bulk heterojunction (BHJ) OSC devices and microscopies. The (BTDT)1‐IC:PBDB‐T and (BTDT)4‐RD:Y6 BHJs achieve power conversion efficiencies of 9.14% and 4.51%, respectively. Our findings demonstrate that DACH reaction is a powerful tool to tune the opto‐electronic properties and device performances by regulating the lengths of π‐conjugated oligomers with varied numbers of repeating units.

Computational prediction of the properties of bis(ethynylthienyl)dialcoxynaphthalene‐based conjugated polymers for organic solar cell applications

AbstractA series of homo‐ (P0) and copolymers (P1‐P5) based on the electron‐donor building‐block 2,2′‐(2,3‐bis(2‐ethylhexyloxy)naphthalene‐1,4‐diyl)bis(ethyne‐2,1‐diyl)dithiophene (1,4‐NET) including ethynyl linkers aiming to promote coplanarity were designed, and their properties predicted using theoretical methodologies to evaluate their potential in organic solar cell applications. The geometries, FMO levels, energy bandgaps, and absorption spectra of trimer models were determined using time‐dependent density functional theory, while their photovoltaic and charge‐transport properties were estimated by the Scharber's model and semiclassical Marcus theory, respectively. Compared to high‐performance conjugated polymers (CPs), such as PTB7‐Th or PM6, and similar systems based on the 1D‐BDT unit, the HOMO and LUMO levels of P0‐P5 tend to be higher. In addition, the new CPs have complementary absorptions with narrow‐bandgap acceptors, such as ITIC and Y6, and adequate matches between their HOMO and LUMO levels. Although the simulated photovoltaic and charge‐transport properties could be overestimated, the best candidate to be synthesized and tested in organic solar cells is P5 due to its suitable and well‐balanced properties, demonstrating the positive effect of incorporating ethynyl bridges to improve the optoelectronic properties of CPs.

Computational design of efficient corannulene-based Non-Fullerene acceptors for organic solar cells applications

In our study, we aimed to enhance the optoelectronic properties of corannulene through functionalization for use as a non-fullerene acceptor (NFA) in organic solar cells. To achieve this, we designed new corannulene derivatives by introducing various electron-donating and electron-withdrawing groups onto the corannulene core. Geometric, electronic, optical, and charge transport properties of newly designed molecules are analyzed using long-range corrected density functional theory methods. The potential NFA is identified by conducting extensive analysis, such as light absorption properties, ground-state dipole moments, and the energy difference between LUMO and LUMO+1 orbitals. Also, a model donor–acceptor interface is constructed by considering PTQ10 oligomer as the donor and corannulene derivatives as acceptors. Charge-transfer states and exciton dissociation processes are analyzed at the polymer chain and NFA interface. Overall, the results obtained from this study can provide useful guidance in designing high-performing NFAs.

Strategies Toward End-group Engineering of Chrysene Core-based Non-fullerene Acceptors for High Performance Organic Solar Cells: A DFT Study

In this study, seven new molecules (OA1–OA7) were introduced with the goal of enhancing the photovoltaic efficiency of solar cells. All these molecules were evaluated by using density functional theory (DFT). All the designed molecules have shorter band gap and higher values of [Formula: see text] than the reference. For confirming the charge mobility, the optoelectronic characterization has been carried out by densities of states (DOS), transition density matrix (TDM), molecular electrostatic potential (MEP) and reorganization energy. The [Formula: see text] values in designed molecules were also improved. The stability of designed molecules was also confirmed by the analysis of non-covalent interactions (NCIs). These results elaborate on the superiority of these novel-designed molecules over pre-existing (R) molecule as potential blocks for better organic solar cell applications.

Recent Advances of Benzodithiophene-Based Donor Materials for Organic Solar Cells.

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has increased dramatically, making a big step toward the industrial application of OSCs. Among numerous OSCs, benzodithiophene (BDT)-based OSCs stand out in achieving efficient PCE. Notably, single-junction OSCs using BDT-based polymers as donor materials have completed a PCE of over 19%, indicating a dramatic potential for preparing high-performance large-scale OSCs. This paper reviews the recent progress of OSCs based on BDT polymer donor materials (PDMs). The development of BDT-based OSCs is concisely summarized. Meanwhile, the relationship between the structure of PDMs and the performance of OSCs is further described in this review. Besides, the development and prospect of single junction OSCs are also discussed.

Efficient calculation of electronic coupling integrals with the dimer projection method via a density matrix tight-binding potential.

Designing organic semiconductors for practical applications in organic solar cells, organic field-effect transistors, and organic light-emitting diodes requires understanding charge transfer mechanisms across different length and time scales. The underlying electron transfer mechanisms can be efficiently explored using semiempirical quantum mechanical (SQM) methods. The dimer projection (DIPRO) method combined with the recently introduced non-self-consistent density matrix tight-binding potential (PTB) [Grimme et al., J. Chem. Phys. 158, 124111 (2023)] is used in this study to evaluate charge transfer integrals important for understanding charge transport mechanisms. PTB, parameterized for the entire Periodic Table up to Z = 86, incorporates approximate non-local exchange, allowing for efficient and accurate calculations for large hetero-organic compounds. Benchmarking against established databases, such as Blumberger's HAB sets, or our newly introduced JAB69 set and comparing with high-level reference data from ωB97X-D4 calculations confirm that DIPRO@PTB consistently performs well among the tested SQM approaches for calculating coupling integrals. DIPRO@PTB yields reasonably accurate results at low computational cost, making it suitable for screening purposes and applications to large systems, such as metal-organic frameworks and cyanine-based molecular aggregates further discussed in this work.

Synthesis and characterization of P3HT:ZnO-FWCNTs for possible application in organic solar cells

Zinc oxide (ZnO) and few-walled carbon nanotubes (FWCNTs) mixed in poly(3-hexylthiophene-2,5-diyl) (P3HT) were prepared for application in organic solar cells (OSCs) as photoactive materials. ZnO was synthesized by microwave-assisted sol-gel method. Improved properties were found for the nanocomposite with 3 wt% of FWCNTs concentration. XRD revealed the hexagonal wurtzite structure of ZnO and the nanocomposites (ZnO-FWCNTs) showed both hexagonal wurtzite and carbon structure. UV/Vis/NIR and PL studies revealed a relatively higher photon absorption efficiency and emission intensity from the ZnO-3% FWCNTs. PL intensity of P3HT was quenched from the sample P3HT:ZnO-3% FWCNTs with the ratio of 1:1, indicating charge transfer phenomena between the electron donor (P3HT) and an acceptor (ZnO-FWCNTs) materials. FESEM showed well-mixed ZnO nanorods with P3HT. Current-voltage measurements indicated an improved electrical conductivity for P3HT:ZnO-3% FWCNTs (ratio 1:1). The obtained results suggest that this sample can potentially improve the photoelectrical conversion efficiency of the OSCs.

Asymmetric Molecule and Thermal Condition Enable Green Solvent‐Processed Organic Solar Cells Achieve 17.89% Efficiency

Using green solvents to fabricate high‐efficiency organic solar cells (OSCs) is the ultimate choice for realizing commercial application of OSCs. However, the low solubility of conventional materials in green solvents and unfavorable phase separation limit the development of green solvent‐processed organic solar cells (GSP‐OSCs). To solve the above problems, a new strategy in which asymmetric acceptor and thermal processing jointly optimize the device performance is proposed. PM6 with temperature‐dependent aggregation property is chosen as the donor, and thermal treatment can improve its solubility. The asymmetric small molecule acceptor BTP‐2F2Cl is chosen as the third material due to its strong crystallinity and good miscibility with the main system materials. Meanwhile, BTP‐2F2Cl can induce the formation of well‐ordered crystallization and molecular stacking in the active layer material, which promotes charge transport and inhibits recombination. Therefore, the PM6: L8‐BO: BTP‐2F2Cl‐based ternary GSP‐OSCs achieve a champion power conversion efficiency of 17.89% compared with the binary devices (16.99%). The results indicate that the introduction of asymmetric acceptor and thermal treatment are an effective strategy to regulate the active layer morphology and enhance the performance of GSP‐OSCs.

Design of benzotriazole-based A2-A1-D-A1-A2-type fully non-fused non-fullerene acceptors for P3HT

Designing fitted acceptors for the homopolymer poly(3-hexylthiophene) (P3HT) has been a hot topic in research field of organic solar cells (OSCs) because of its unique advantages of low-cost, stability and simplicity of synthesis. In resent years, we have been committed to the development of fused non-fullerene acceptors (NFA) containing benzotriazole (BTA) unit in order to achieve high power conversion efficiency (PCE). The modulation of the crystallinity of P3HT in the active layer affecting exciton dissociation, charge transfer and recombination has always been a challenge. Here, two simple non-fused NFAs containing BTA with different terminal units were easily prepared. P3HT:BTA31 device delivered an initial PCE of 3.95 % due to the unbalanced charge mobility and adverse trap-assisted recombination. Due to the efficient charge separation and suppressed charge recombination, P3HT:BTA33 device showed an increased PCE of 5.68 %. Our results suggest that these non-fused NFAs are promising candidates for application of P3HT-based OSCs.

Facile access to C-N bonds via unexpected side reactions of Knoevenagel condensation and their application in high-efficiency organic solar cells

Facile access to C-N bonds via unexpected side reactions of Knoevenagel condensation and their application in high-efficiency organic solar cells

Low‐Cost Pentacyclic Nonfullerene Acceptors to Exceed 13% Efficiency by Thermal Annealing‐Free Treatment

Nonfullerene electron acceptors (NFAs) improve the photoelectrical conversion efficiency of organic solar cells (OSCs), but they suffer from high synthetic cost and time‐consuming thermal annealing treatment. Nevertheless, low‐cost NFAs and simple device fabrication are still crucial issues to boosting the commercial application of OSCs. Herein, one kind of low‐cost pentacyclic NFAs based on benzothiadiazole and thiophene derivatives named L1‐EH and L1‐BO with different substitutions is successfully synthesized and characterized. The optimized OSCs based on PM6:L1‐BO (2‐butyloctyl side chain) display an outstanding power conversion efficiency of 13.15%. Moreover, OSC devices based on these acceptors operate in thermal annealing‐free conditions. Further studies manifest that the proper side chains in NFAs can induce more favorable face‐on molecule orientation, enhance carrier mobility, balance charge transport, and reduce recombination losses. This research demonstrates that these types of low‐cost NFAs are promising active layer candidates of high‐efficiency organic solar cells featuring simple device manufacture.

Ductile Oligomeric Acceptor-Modified Flexible Organic Solar Cells Show Excellent Mechanical Robustness and Near 18% Efficiency.

High power conversion efficiency (PCE) and mechanical robustness are key requirements for wearable applications of organic solar cells (OSCs). However, almost all highly efficient photoactive films comprising polymer donors (PD ) and small molecule acceptors (SMAs) are mechanically brittle. In this study, highly efficient (PCE = 17.91%) and mechanically robust (crack-onset strain [COS] = 11.7%) flexible OSCs are fabricated by incorporating a ductile oligomeric acceptor (DOA) into the PD :SMA system, representing the most flexible OSCs to date. The photophysical, mechanical, and photovoltaic properties of D18:N3 with different DOAs are characterized. By introducing DOA DOY-C4 with a longer flexible alkyl linker and lower polymerization, the D18:N3:DOY-C4-based flexible OSCs exhibit a significantly higher PCE (17.91%) and 50% higher COS (11.7%) than the D18:N3-based device (PCE = 17.06%, COS = 7.8%). The flexible OSCs based on D18:N3:DOY-C4 retain 98% of the initial PCE after 2000 consecutive bending cycles, showing greater mechanical stability than the reference device (maintaining 89% of initial PCE). After careful investigation, it is hypothesized that the enhancement in mechanical properties is mainly due to the formation of tie chains or entanglement in the ternary blend films. These results demonstrate that DOAs have great potential for achieving high-performance flexible OSCs.

Exploring novel naphthalene-fused octacyclic core-based non-fullerene acceptor materials with augmented optoelectronic attributes for stable and efficient solar cells

To meet the growing need for high-efficiency photovoltaic materials in today's high-tech applications, ten acceptor molecules (SH01-SH10) with naphthalene-fused octacyclic electron-donating central core NITT are proposed. End-capped structural-alterations on reference R (NITT-2F) with strong electron-drawing groups (A01-A10) are performed for attaining improved optoelectronic characteristics and possible applications in organic solar cells (OSCs). In comparison to the reference R (NITT-2F), key parameters such as FMO analysis, energy gap (Egap), open-circuit voltage (VOC), absorption spectra (λmax), excitation energy (Ex), transition density matrix (TDM) with binding energy (Eb), the density of states (DOS), reorganization energy of the electron (λe) and hole (λh), and charge transfer with PM6 donor substance are perceived. It is found that all proposed molecules (SH01-SH10) have a reduced energy gap (2.104–1.901 eV), a broader absorption spectrum (707–820 nm), and smaller excitation energy (1.75–1.51 eV) as compared to reference molecule NITT-2F values 2.132 eV, 675 nm and 1.79 eV respectively. The significant charge transfers and 0.23 eV decrease in the Egap is noted for the designed molecule SH05. Results confirmed that all proposed molecules, especially SH05 could be an excellent choice for OSCs candidate due to the combination of strong electron-withdrawing effect in end-capped acceptor unit and extended conjugation, as well as its promising photovoltaic properties, which include the lowest Egap (1.901 eV), highest λmax (820 nm (in chloroform) and 729 nm (in gas phase)) with Ex values of 1.51 and 1.70 eV respectively, slightest electron mobility (λe = 0.0090 eV) and hole mobility (λh = 0.0073 eV), highest ionization potential (6.826 eV) and electron affinity (3.384 eV), and fine value of VOC=1.02 V with respect to (ǀEPM6HOMOǀ - ǀESH05LUMOǀ). End-capped acceptor alterations are an acceptable way to attain the desired optoelectronic characteristics demonstrated in this research work. This study can be essential in achieving efficient novel non-fullerene OSCs with higher performance, low cost, and higher optical absorption coefficient. Thus, proposed molecules (SH01-SH10) with proficient electron and hole transfer mobilities and optoelectronic features are recommended to develop higher-efficiency solar cells.

Stability enhancement of silver nanowire-based flexible transparent electrodes for organic solar cells

Flexible organic solar cells (OSCs) are one of the most promising power sources for wearable electronics. A high-quality transparent electrode is one of the indispensable components for OSCs because it guarantees sufficient light transparency and electrical conductivity. Silver nanowires (AgNWs) are considered to be an ideal candidate over commercial rigid indium tin oxide as flexible transparent electrodes (FTEs) because of their superior optoelectronic properties, excellent mechanical flexibility, solution treatability, and high compatibility with semiconductors. However, the stability of AgNWs-based FTEs (AgNWs-FTEs) hinders their further application. In this perspective, we provide state-of-the-art solution-processed AgNWs with enhanced stability and their applications in flexible OSCs. We start with a systematic analysis of the failure mechanisms and evaluation methods of AgNWs-FTEs. Then the key reinforced materials are summarized. Next, the main stability enhancement strategies are briefly discussed. Applications of AgNWs-FTEs in flexible and stretchable OSCs are further surveyed. Finally, an outlook on the current status, future challenges, and emerging strategies in this field is presented.