Session K50: Organic Electronics I: Organic Photovoltaics and Photophysics

Trapped Photons Induced Ultra-high External Quantum Efficiency and Photoresponsivity with Millisecond Response in Hybrid Graphene/Metal-Organic Framework Broadband Wearable Photodetectors

Metal-Organic frameworks (MOFs) are the promising new class of hybrid materials that has the tremendous attraction in sensing, gas storage, and drug delivery during past two decade due to its unique tunable properties and fascinating architectures. Optoelectronic application of MOF is challenging due to its high porosity and poor electrical conductivity. Combining the superior properties of the MOF along with ultra-high carrier mobility of graphene, we have fabricated and characterize the highly sensitive, broadband, and wearable photodetector on a polydimethylsiloxane substrate. The external quantum efficiency of the hybrid photodetector is found to be > 5×10⁸%, under the excitation of 325 nm laser of intensity ~ 0.5 µWcm^-2, which exceeds all the reported values of similar devices. The porosity of the MOF and ripple structure graphene assist the trapping of photons at the light-harvesting layer. The device photoresponsivity is found to be > 10⁶ AW^-1 with a response time of < 70 ms, which is ~100 times faster than the current standards of the graphene-organic hybrid photodetectors. In addition, utilizing the excellent flexibility of the graphene layer the wearability of the devices is demonstrated, which can be stretched up to 100% of its original dimension.




  

ULTRAFAST ENERGY DISSIPATIONS AND QUANTUM EFFICIENCIES OF CONJUGATED POLYMERS BY SEGMENTAL STRESSES

Quantum efficiencies of conjugated polymers (CPs) were found to increase dramatically by elevation of segmental stresses, therefore, we explore the emission behavior in the time scales comparable to segmental motions to unveil the relationship. We stretched CP (MEH-PPV) to ~300%, resulting photoluminescence (PL) enhancements of 20-60 folds depending on the stress level. By using up-conversion time-resolved confocal PL spectroscopy, we found the absorbed energy quickly dissipates via segmental rotations and vibrations, leading to PL red shifts. The damping via rotations was ~10 times faster than vibrations but stopped after ca. 2 ps, while the latter continued functioning, but abated slowly to diminishment. The segmental stresses can reduce or even switch off the rotation damping but yielded no influences on vibrations. The latter was found in fact heat dissipation, while the rotations, driven by local electrostatics from excitation, can trigger self-trapping and the 2 ps represents the time limit before escape from self-trapping for recombination. This model explains satisfactorily how segmental stresses interacting with CP backbones to result dramatic efficiency enhancements.   

Quantum-Entangled Triplet-Triplet Excitons in Acene Dimers and their role in Singlet Fission

Singlet fission (SF) in which a photoexcited spin singlet dissociates into a pair of low energy triplet excitons has been claimed in covalently linked acene dimers. Internal covnersion to a quantum entangled triplet-triplet state prior to dissociation makes the process spin-allowed. However, recent experiments based on electron spin resonance as well as transient absorption spectroscopy have both shown this intermediate biexciton to be stable against further decoherence thereby raising questions on SF itself. A disconnect between theory and experimental results have led us to calculate the excited state energies and their optical properties using a correlated electron model (Pariser-Parr-Pople Hamiltonian). We formulate a broad theory of quantum entanglement of triplet-triplet state in symmetric and asymmetric dimers. Based on our calculations, we conclude that i) spectroscopic differences in the ESA signals between free and bound triplets exist beyond the visible, ii) entanglement between the triplets is reduced as proximity between acene units is reduced or in the presence of significant steric hindrance, and iii) ESA from singlet exciton shows absorption in the short-wave infrared due to a transition to an optically forbidden even parity state.   

Comparative studies on fluorescent emission of organic semiconductors between optical and electrical pumping

Light-emitting devices of organic semiconductors (OSCs), such as organic light-emitting diodes, have widely been used in daily life. Especially, light-emitting organic field effect transistors (LE-OFETs) are considered as candidates for efficient light-emitting devices, such as OSC lasers, in the future. However, the detailed understanding of electroluminescence (EL) by electrical pumping has not fully been understood when it is compared to photoluminescence (PL) by optical excitations. For realizing such a highly performed light-emitting device, the fundamental understanding on the difference between EL and PL is necessary. Here, we report accurate comparative studies on the difference between PL and EL of 5,5’’-Bis(4-biphenylyl)-2,2’:5’,2’’-terthiophene (BP3T). The result shows that the spectrum shapes are different between PL- and EL- mode in the case of single-crystal thin films of BP3T and quenching of fluorescent emissions occurs in high current region in the EL-mode. In order to realize highly efficient light-emitting OFETs towards OSC lasers in the future, such annihilation would become a significant obstacle.   

Theoretical Study on Charge-Transfer Excitations of Buckycatcher-Fullerene Complexes

In particular, p/p stacking compounds have become of interest in new materials for photocatalysis, solar energy and phosphorescent organic light-emitting diodes. We investigate theoretically the photo-induced charge transfer in supramolecular chemistry. Buckycatcher (BC) molecules, bowl-shaped polycyclic hydrocarbons, have attracted attention as a new type of π conjugated compound. BC molecules with two corannulene or two sumanane subunits can form a complex (BC/C₆₀) with C₆₀ because of the π-π interaction between the convex surface of the C₆₀ and the concave faces of corannulene subunits of the BC. We have performed DFT and TD-DFT calculations on the structures and excitation spectrum of BC molecules and BC/C₆₀ complexes. The most stable structures are BC/C₆₀ in which the p–p stacking of the two corannulene subunits of BC and C₆₀ takes a concave–convex arrangement; therefore, the van der Waals contact is maximized. It is found that the charge-transfer-type excitations from BC to C₆₀ start at the ultraviolet light region. The BC/C₆₀ complexes and their derivatives are potential candidates as key elements for photovoltaic devices and other molecular electronic devices.   

The impact of atomic substitution on the photophysics of contorted hexabenzocoronene derivatives

The photophysical properties of organic semiconductors are strongly influenced by the spin states, lifetimes, populations, and energies of the lowest photoexcited species (S1 or T1). By favoring or limiting the rate of intersystem crossing (ISC), the relative population and lifetime of singlet and triplet excitons can be controlled through molecular design. The incorporation of heavy atoms, such as transition metals, have been reported to enhance spin-orbit coupling in organic systems. Here, we explore the role of peripheral moieties with heteroatoms on the photophysical properties of contorted hexabenzocoronene (cHBCs) derivatives. These chemical alterations may impose sterics or enhance spin-orbit coupling, significantly increasing the rate of ISC. With benzofuran moieties, cTBFDBC exhibits the highest degree of molecular contortedness in the series, which is correlated with a high rate of ISC. With much heavier benzothiophene moieties, cTBTDBC uniquely exhibits room temperature and higher phosphorescence yield compared to cTBFDBC, which we correlate to a high degree of spin-orbit coupling induced by the heavy sulfur atom. These findings offer valuable guidelines for enhancing the optoelectronic properties of cHBCs using simple molecular substitution approaches.   

Coherence and Electron Transport in Pi-Stacked Acceptor-Donor-Acceptor Molecular Triads

There has been a concerted effort to design non-fullerene acceptors (NFAs) for organic photovoltaic devices, in a large part, due to the processability and stability problems of fullerenes, but concomitantly there is a desire to further understand ultrafast charge generation in donor-acceptor organic molecules. Recently, covalently bonded molecular triads have shown substantial increases in device efficiency when blended with mid-gap polymers. There has been some advances in understanding the details of electron transport with NFA molecules, but there has yet to be quantitative investigation into the relationship of molecular organization in NFA device with its transport capabilities. Here, we present a detailed study of NFA aggregates that show slipped-stacked geometries and strong binding energies between the intermolecular layers. A consequence of strongly coupled LUMO orbitals is intermolecular delocalization leading to transport coherence. We find that for some slip-stacked arrangements favor coherent transport, while others do not, and we propose design strategies to get NFA packings with strong electronic couplings.   

Alloy or Blend: Analysis of a Ternary Organic Photovoltaic

The power conversion efficiency of organic solar cells can be enhanced by employing ternary blend bulk heterojunctions, which have been found to have an open-circuit voltage (V_OC) that depends on the blend ratio of acceptors/donors. It has been proposed that the variation in V_OC is due to the formation of molecular alloy between blended acceptors/donors. However, we assert that the formation of a “molecular alloy” should exhibit new electronic states distinguished from those of its constituents. Here, we study the electronic states and energy levels in the P3HT:(ICBA:PC₆₁BM) ternary blend system. We find that the ICBA:PC₆₁BM acceptors shows the same highest occupied molecular orbital levels and exciton energies as that of ICBA, indicating no new exciton state emerges from the blend. In the ternary blend, the charge transfer states are composed of a linear superposition of those in the corresponding binaries. From these results, no evidence of new emergent electronics state is found which would support the existence of a molecular alloy. Indeed, our results point to the conclusion that blends of these non-interacting molecules differ substantially from our understanding of a conventional alloy. In this talk, we will consider the general criteria in identifying a molecular alloy.   

Design of Ethanol/Water Soluble Polymers/Fullerenes for Aqueous Processed Organic Solar Cells and Importance of Water Contents for Enhancement of Processability and Device Performance

Most of the efficient organic electronic devices have been fabricated using toxic halogenated and/or aromatic organic solvents, which are not desirable in the industrial-scale solution process. Herein, we present a new series of ethanol/water soluble fullerene derivatives with different forms of oligoethylene glycol (OEG) side chains to be utilized in fabricating eco-friendly polymer solar cells (eco-PSCs). Intriguingly, the addition of a typical anti-solvent, water, to ethanol is found to markedly enhance the solubility of the non-ionic OEG side chain-based conjugated polymer (PPDT2FBT-A) and the newly designed fullerene mono-adducts (PC₆₁BO12, PC₆₁BO15, and PC₆₁BO27). A water-ethanol co-solvent with a 1:1 molar ratio provided an increased solubility of PPDT2FBT-A from 2.3 to 42.9 mg mL^-1 and that of PC₆₁BO12 from 0.3 to 40.5 mg mL^-1. Owing to the improved processability, efficient eco-PSCs with a power conversion efficiency of 2.05% were successfully fabricated. To date, this value is the highest among the devices based on the active layer of water/ethanol-soluble conjugated materials. Our results not only provide important guidelines for the design of electroactive materials, but also signifies the importance of water addition in fabricating environmentally benign eco-PSCs.   

Defective conjugated polymers for organic electronics

This contribution discusses recent progress in the area of the synthesis of conjugated polymers for organic electronics. A first focus is on the role of chemical defects that form during synthesis. Depending on monomer structure, synthetic method and reaction conditions, homocouplings form at varying extent in donor-acceptor copolymers. This deviation from the usually suggested, strictly alternating structure lowers performance and causes a low level of reproducibility. In PCDTBT, homocouplings lower the short circuit current of photovoltaic devices and may be considered as one source for the broad range of reported power conversion efficiencies. The second part includes n-type copolymers from the naphthalene diimide and diketopyrrolopyrrole families. Defect formation, molecular weight control and energy level tuning is elucidated and correlated with charge carrier mobility and photovoltaic performance. A central conclusion is that the general understanding of the occurrence and importance of chemical defects in conjugated polymers has improved, but their entire elimination remains to be solved rendering batch to batch reproducibility a challenge.   

Reduced mixed phase interface causes increased charge transfer state separation in polymer solar cells

Investigations into organic solar cells have shown that molecular mixing within domains and interfaces significantly impacts device performance. However, studies often use performance metrics that blur fundamental structure-function mechanisms – in particular, the role of the mixed phase in charge generation versus charge extraction. Here, we present a study based on time-delayed collection field to separately quantify each fundamental step in the charge generation process. Additionally, we utilize a novel resonant X-ray scattering analysis to quantify the state of the nanostructure as it exists within real devices. We find that in a model semicrystalline system, decreasing the mixed interface between donor and acceptor domains has minimal impact on charge transfer (CT) state generation while greatly enhances CT state separation. With both a high correlation (>99%) and a granular quantification of all dynamics, we eliminate competing processes in working devices to determine a causal relationship between the mixed phase volume and the efficiency of charge separation. Our results here support that a reduction in the interfacial mixed phase establishes a steep energy gradient between pure phases to aid charge generation in organic solar cells.   

Title: Understanding Quasi-random Nanostructures of the Bulkheterojunctions(BHJ) in Organic Solar Cell Active Layers with Spectral Density Function (SDF)

Bulkheterojunctions in organic photovoltaic solar cell (OPVC) active layers are believed to be determining the performance of the OPVCs. The domain features measured in the active layer exhibits quasi-random nanostructures. Though it is believed that the quasi-random nanostructures in the BHJ play a decisive role on determining the performance, it is unclear how to quantify this quasi-random nanostructures into a quantity that could be used to guide the synthesis. In this work, we will present mathematical description of SDF for two dimensional images followed by a simplified fitting equation for general SDF analysis. The simplified fitting equation will extract the information of domain sizes (average and deviation), and domain distances (average and deviation). Validation of this equation is tested by simulated model data. After testing with the simulated data, this equation is applied to two dimensional cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S) data and small angle X-ray diffraction data measured on the active layer made of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). It will be argued that this method could be applied to various types of data exhibit similar two-domain quasi-random features.   

Multiscale Modeling of Organic Photovoltaics

Obtaining a detailed understanding of how molecular configurations impact device performance in polymer-based solar cells is a key problem on the road to enabling improved rational design of these devices. Since direct observation of the relevant morphology is experimentally challenging, computer simulation has become an important tool in gaining insight into correlations between structure and performance in organic photovoltaics. Here, we report on the development and usage of a recent coarse-grained model of poly-3-hexylthiophene (P3HT), parameterized based on an information-theoretic Bayesian approach. On this basis, we discuss insights into not only how to build polymer models, but also which types of observables must be captured in the model in order to faithfully represent device morphology. Additionally, we consider spatial and temporal challenges present even in coarse-grained simulation and sampling and some approaches to alleviate them.