Efficient Charge Generation Research Articles

Excited-state structural relaxation and exciton delocalization dynamics in linear and cyclic π-conjugated oligothiophenes.

π-Conjugated oligothiophene is considered a chain segment of its polymeric counterpart, whose size and shape can be precisely controlled. Because of its simplified structure, it is possible to understand complex excited-state dynamics of the π-conjugated polymers by employing a bottom-up approach. We review theoretical and experimental aspects of π-conjugated oligothiophenes by summarizing recent works employing time-resolved spectroscopy. The extent of exciton delocalization, which is a prerequisite to efficient charge generation at organic heterojunctions, is described sequentially in model linear and cyclic oligothiophenes, and their analogues. The heterogeneous nature of these systems is highlighted by illustrating the results at both ensemble and single-molecule levels. Exciton dynamics that arise in the polymers are also covered and the signifcance of exciton and charge delocalization in photovoltaic materials is highlighted.

Influence of conjugation length on ultrafast electronic tunneling in organic semiconductor thin films.

Electron delocalization in conjugated organic molecules is a rate-limiting step in maximizing the charge generation efficiency of next generation photovoltaics and molecular electronics. In particular, ultrafast (<50 fs) delocalization is an important aspect that has been beyond the scope of traditional optical experiments. In this work, we use resonant photoemission spectroscopy to probe electron delocalization timescales as a function of conjugation length by examining an oligothiophene chemical series containing 4-, 5- and 6-mers. We find that above a certain photon energy threshold, the 5-mer, quinquenthiophene, displays the largest ultrafast tunneling rates, roughly three times faster than the 6-mer, sexithiophene. We argue that differences in thin-film molecular packing cannot satisfactorily explain our results, and we speculate that the differences in ultrafast electron dynamics may be a manifestation of the odd/even effect.

Simultaneously Achieved High Open‐Circuit Voltage and Efficient Charge Generation by Fine‐Tuning Charge‐Transfer Driving Force in Nonfullerene Polymer Solar Cells

AbstractTo maximize the short‐circuit current density (JSC) and the open circuit voltage (VOC) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole‐based p‐type polymer (J61) and three benzotriazole‐based nonfullerene small molecule acceptors (BTA1‐3) are chosen to investigate the energetic driving force for the efficient charge transfer. The lowest unoccupied molecular orbital (LUMO) energy levels of small molecule acceptors can be fine‐tuned by modifying the end‐capping units, leading to high VOC (1.15–1.30 V) of OSCs. Particularly, the LUMO energy level of BTA3 satisfies the criteria for efficient charge generation, which results in a high VOC of 1.15 V, nearly 65% external quantum efficiency, and a high power conversion efficiency (PCE) of 8.25%. This is one of the highest VOC in the high‐performance OSCs reported to date. The results imply that it is promising to achieve both high JSC and VOC to realize high PCE with the carefully designed nonfullerene acceptors.

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.

Microwave exposure as a fast and cost-effective alternative of oxygen plasma treatment of indium-tin oxide electrode for application in organic solar cells

Pre-treatment methods are commonly employed to clean as well as to modify electrode surfaces. Many previous reports suggest that modifying the surface properties of indium tin oxide (ITO) by oxygen plasma treatment is a crucial step for the fabrication of high performance organic solar cells. In this work, we propose a fast and cost-effective microwave exposure step for the modification of the surface properties of ITO anode electrodes used in organic solar cells. It is demonstrated that a short microwave exposure improves the hydrophilicity and reduces the roughness of the ITO surface, as revealed by contact angle and atomic force microscopy (AFM) measurements, respectively, leading to a better quality of the PEDOT:PSS film coated on top of it. Similar results were obtained with the commonly used oxygen plasma treatment of ITO suggesting that microwave exposure is an effective process for modifying the surface properties of ITO with the benefits of low-cost, easy and fast processing. In addition, the influence of the microwave exposure of ITO anode electrode on the performance of an organic solar cell based on the poly(3-hexylthiophene):[6,6]-phenyl C70 butyric acid methyl ester (P3HT:PC70BM) blend is investigated. The 71% efficiency enhancement obtained in the microwave annealed-ITO based device as compared to the device with the as-received ITO was mainly attributed to the improvement in the short circuit current (Jsc) and decreased leakage current caused by the reduced series and the increased shunt resistances and also by the higher charge generation efficiency, and the reduced recombination losses.

Solution-Processed Metal-Oxide p–n Charge Generation Junction for High-Performance Inverted Quantum-Dot Light-Emitting Diodes

We report solution-processed metal-oxide p-n junction, Li-doped CuO (Li:CuO) and Li-doped ZnO (Li:ZnO), as a charge generation junction (CGJ) in quantum-dot light-emitting diode (QLED) at reverse bias. Efficient charge generation is demonstrated in a stack of air-annealed Li:CuO and Li:ZnO layers in QLEDs. Air annealing of Li:ZnO on Li:CuO turns out to be a key process to decrease oxygen vacancy (Vo) and increase the copper (II) oxide (CuO) fraction at the Li:CuO/Li:ZnO interface for efficient charge generation. Green QLEDs incorporating Li:CuO/Li:ZnO CGJ show the maximum current and power efficiencies of 35.4 cd/A and 33.5 lm/W, respectively.

High-Performance Photoelectronic Sensor Using Mesostructured ZnO Nanowires.

Semiconductor photoelectrodes that simultaneously possess rapid charge transport and high surface area are highly desirable for efficient charge generation and collection in photoelectrochemical devices. Herein, we report mesostructured ZnO nanowires (NWs) that not only demonstrate a surface area as high as 50.7 m2/g, comparable to that of conventional nanoparticles (NPs), but also exhibit a 100 times faster electron transport rate than that in NP films. Moreover, using the comparison between NWs and NPs as an exploratory platform, we show that the synergistic effect between rapid charge transport and high surface area leads to a high performance photoelectronic formaldehyde sensor that exhibits a detection limit of as low as 5 ppb and a response of 1223% (at 10 ppm), which are, respectively, over 100 times lower and 20 times higher than those of conventional NPs-based device. Our work establishes a foundational pathway toward a better photoelectronic system by materials design.

Quantum dynamical studies of ultrafast charge separation in nanostructured organic polymer materials: Effects of vibronic interactions and molecular packing

AbstractWe review recent work employing high‐dimensional quantum dynamical techniques to study ultrafast charge separation in functional organic materials, in view of understanding the key microscopic factors that lead to efficient charge generation in photovoltaics applications. As highlighted by recent experiments, these processes can be guided by quantum coherence, despite the presence of static and dynamic disorder. The present approach combines first‐principles parametrized lattice Hamiltonians, based on Time‐Dependent Density Functional Theory (TDDFT) and/or high‐level electronic structure calculations, with accurate quantum dynamics simulations using the Multi‐Configuration Time‐Dependent Hartree (MCTDH) method. This contribution specifically addresses the mechanism of charge generation in (i) regioregular oligothiophene‐fullerene aggregates, and (ii) highly ordered oligothiophene‐perylene diimide co‐oligomer assemblies. These studies highlight that chemical design of donor–acceptor combinations needs to account for the effects of electronic delocalization and the modified energetics due to molecular packing, as well as multiple transfer pathways and internal conversion channels induced by vibronic interactions.

Highly efficient charge generation and electron injection of m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction on ITO cathode for high efficiency inverted white organic light-emitting diodes

The charge generation and electron injection characteristics of m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction made of 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) p-type organic semiconductor and 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN) n-type semiconductor were well studied. It was found that m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction showed better charge generation ability than m-MTDATA/HAT-CN organic heterojuntion, and realized highly efficient electron injection when using it as charge generator on indium tin oxide (ITO) cathode. The investigations of capacitance-frequency and current density-voltage characteristics of the electron-only devices based on m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction demonstrated that the amounts of the injected electrons were dependent on the properties of the used n-doping electron transporting layer (n-ETL). Therefore, by optimization, high efficiency inverted white organic light-emitting diodes (OLEDs) with ITO as cathode were successfully fabricated using m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction as charge injector and lithium carbonate (Li2CO3) doped bathophenanthroline (BPhen) as n-ETL. The resulting inverted white OLEDs achieved the maximum external quantum efficiency of 12.9%, current efficiency of 42.6 cd/A and power efficiency of 50.5 lm/W, and still remained 12.5%, 41.0 cd/A and 45.0 lm/W at the luminance of 1000 cd/m2, respectively, exhibiting extremely low efficiency roll-off.

Design of Cyanovinylene‐Containing Polymer Acceptors with Large Dipole Moment Change for Efficient Charge Generation in High‐Performance All‐Polymer Solar Cells

AbstractDesigning polymers that facilitate exciton dissociation and charge transport is critical for the production of highly efficient all‐polymer solar cells (all‐PSCs). Here, the development of a new class of high‐performance naphthalenediimide (NDI)‐based polymers with large dipole moment change (Δµge) and delocalized lowest unoccupied molecular orbital (LUMO) as electron acceptors for all‐PSCs is reported. A series of NDI‐based copolymers incorporating electron‐withdrawing cyanovinylene groups into the backbone (PNDITCVT‐R) is designed and synthesized with 2‐hexyldecyl (R = HD) and 2‐octyldodecyl (R = OD) side chains. Density functional theory calculations reveal an enhancement in Δµge and delocalization of the LUMO upon the incorporation of cyanovinylene groups. All‐PSCs fabricated from these new NDI‐based polymer acceptors exhibit outstanding power conversion efficiencies (7.4%) and high fill factors (65%), which is attributed to efficient exciton dissociation, well‐balanced charge transport, and suppressed monomolecular recombination. Morphological studies by grazing X‐ray scattering and resonant soft X‐ray scattering measurements show the blend films containing polymer donor and PNDITCVT‐R acceptors to exhibit favorable face‐on orientation and well‐mixed morphology with small domain spacing (30–40 nm).

Origin of Charge Separation at Organic Photovoltaic Heterojunctions: A Mesoscale Quantum Mechanical View

The high efficiency of charge generation within organic photovoltaic blends apparently contrasts with the strong "classical" attraction between newly formed electron-hole pairs. Several factors have been identified as possible facilitators of charge dissociation, such as quantum mechanical coherence and delocalization, structural and energetic disorder, built-in electric fields, nanoscale intermixing of the donor and acceptor components of the blends. Our mesoscale quantum-chemical model allows an unbiased assessment of their relative importance, through excited-state calculations on systems containing thousands of donor and acceptor sites. The results on several model heterojunctions confirm that the classical model severely overestimates the binding energy of the electron-hole pairs, produced by vertical excitation from the electronic ground state. Using physically sensible parameters for the individual materials, we find that the quantum mechanical energy difference between the lowest interfacial charge transfer states and the fully separated electron and hole is of the order of the thermal energy.

Polymer Main‐Chain Substitution Effects on the Efficiency of Nonfullerene BHJ Solar Cells

Abstract“Nonfullerene” acceptors are proving effective in bulk heterojunction (BHJ) solar cells when paired with selected polymer donors. However, the principles that guide the selection of adequate polymer donors for high‐efficiency BHJ solar cells with nonfullerene acceptors remain a matter of some debate and, while polymer main‐chain substitutions may have a direct influence on the donor–acceptor interplay, those effects should be examined and correlated with BHJ device performance patterns. This report examines a set of wide‐bandgap polymer donor analogues composed of benzo[1,2‐b:4,5‐b′]dithiophene (BDT), and thienyl ([2H]T) or 3,4‐difluorothiophene ([2F]T) motifs, and their BHJ device performance pattern with the nonfullerene acceptor “ITIC”. Studies show that the fluorine‐ and ring‐substituted derivative PBDT(T)[2F]T largely outperforms its other two polymer donor counterparts, reaching power conversion efficiencies as high as 9.8%. Combining several characterization techniques, the gradual device performance improvements observed on swapping PBDT[2H]T for PBDT[2F]T, and then for PBDT(T)[2F]T, are found to result from (i) notably improved charge generation and collection efficiencies (estimated as ≈60%, 80%, and 90%, respectively), and (ii) reduced geminate recombination (being suppressed from ≈30%, 25% to 10%) and bimolecular recombination (inferred from recombination rate constant comparisons). These examinations will have broader implications for further studies on the optimization of BHJ solar cell efficiencies with polymer donors and a wider range of nonfullerene acceptors.

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics—however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.

Potential of Nonfullerene Small Molecules with High Photovoltaic Performance

Over the past decades, fullerene derivatives have become the most successful electron acceptors in organic solar cells (OSCs) and have achieved great progress, with power conversion efficiencies (PCEs) of over 11 %. However, fullerenes have some drawbacks, such as weak absorption, limited energy-level tunability, and morphological instability. In addition, fullerene-based OSCs usually suffer from large energy losses of over 0.7 eV, which limits further improvements in the PCE. Recently, nonfullerene small molecules have emerged as promising electron acceptors in OSCs. Their highly tunable absorption spectra and molecular energy levels have enabled fine optimization of the resulting devices, and the highest PCE has surpassed 12 %. Furthermore, several studies have shown that OSCs based on small-molecule acceptors (SMA) have very efficient charge generation and transport efficiency at relatively low energy losses of below 0.6 eV, which suggests great potential for the further improvement of OSCs. In this focus review, we analyze the challenges and potential of SMA-based OSCs and discuss molecular design strategies for highly efficient SMAs.

Syntheses of Novel Lanthanide Metal–Organic Frameworks for Highly Efficient Visible-Light-Driven Dye Degradation

Series of lanthanide metal–organic frameworks (Ln-MOFs) have been solvothermally synthesized using an anthracene-derived dicarboxylate ligand. The synthesized Ln-MOFs (Ce, Tb, Dy) show characteristics of broad-band visible-light absorption and efficient photoinduced charge generation. For the first time, Ln-MOFs have been employed as visible-light photocatalysts for rhodamine B (RhB) degradation in the presence of oxidant of hydrogen peroxide (H2O2). Results show that these Ln-MOFs are highly efficient for visible-light-driven RhB degradation in aqueous solution. The degradation reactions show notable reaction rate constants. It is interesting to observe that the premixing between Ln-MOFs and H2O2 is vital to improve photocatalytic performance, indicating the advantage of porous MOF catalysts. This study not only presents rare examples of visible-light-driven MOF photocatalysts constructed from lanthanide ions, but also reveals that the H2O2-involved advanced oxidation process is feasible for enhancing ph...

Ultrafast Spectroscopy Reveals Electron-Transfer Cascade That Improves Hydrogen Evolution with Carbon Nitride Photocatalysts.

Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-C3N4) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-C3N4 electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and near-infrared femtosecond transient absorption (TA) spectroscopy to reveal an electron-transfer cascade that correlates with a near-doubling in photocatalytic activity from 2050 to 3810 μmol h-1 g-1 when we infuse a suspension of bulk g-C3N4 with 10% mass loading of chemically exfoliated carbon nitride. TA spectroscopy indicates that exfoliated carbon nitride quenches photogenerated electrons on g-C3N4 at rates approaching the molecular diffusion limit. The TA signal for photogenerated electrons on g-C3N4 decays with a time constant of 1/ke' = 660 ps in the mixture versus 1/ke = 4.1 ns in g-C3N4 alone. Our TA measurements suggest that the charge generation efficiency in g-C3N4 is greater than 65%. Exfoliated carbon nitride, which liberates only trace hydrogen levels when photoexcited directly, does not appear to independently sustain appreciable long-lived charge generation. Thus, the activity enhancement in the two-component infusion evidently results from a cooperative effect in which charge is generated on g-C3N4, followed by electron transfer to exfoliated carbon nitride containing photocatalytic chain terminations. This correlation between electron transfer and photocatalytic activity provides new insight into structural modifications for controlling charge separation dynamics and activity of carbon-based photocatalysts.

P‐186: Realization of Low‐Voltage Drop across Charge Generation Layer using Bi‐layer Oxide Semiconductors for Tandem OLEDs

The efficient charge generation ability of the MoOx/transparent amorphous oxide semiconductor (TAOS) bi‐layer is reported. The MoOx/TAOS bi‐layer charge generation layer (CGL) exhibited high optical transparency, low carrier injection barrier and low driving voltage for tandem OLEDs. It was also discovered that TAOS has ohmic contact with MoOx, which resulted in very small voltage drop across the charge generation layer. These results suggest that novel tandem white OLEDs with low driving voltage would be realized using the MoOx/ TAOS CGL.

Electrostatic charging of single granules by repeated sliding along inclined metal plates

In this work, repeated sliding tests for single granules were investigated for their electrostatics generation. Several factors were considered including granule length-ratio, sliding face shape, sliding times, sliding area, sliding velocity, front-facing edge, plate inclined angle and humidity. Generally, it is found that electrostatics increases with granule length-ratio. Two kinds of granular sliding face shapes were used in this work, half circle and rectangle. Under the same working conditions, a granule with the sliding face shape of half-circle tends to produce more electrostatics than that of rectangle. In addition, the efficiency of granule charge generation increases with sliding times although the amount of impact charge is decreased by the initial charge. Electrostatics increases with sliding area, which is independent of granule sliding-face shape and sliding times. Electrostatics also increases with granule sliding velocity. Front-facing sliding with a short edge tends to generate more electrostatics than that with a long edge. In this work, three sliding-plate angles were chosen as 30°,54°,70°, where granules sliding along the inclined plate at 54° acquired the highest electrostatics in comparison with other two angles. Humidity has significant effect on electrostatics as that electrostatics decreases with humidity. At lower relative humidity, the granule length-ratio is found to have more effect on electrostatics.

Comparing the device physics, dynamics and morphology of polymer solar cells employing conventional PCBM and non-fullerene polymer acceptor N2200

Current all polymer solar cells still suffer from low fill factors (FF) and short-circuit current density (Jsc) compared with the conventional polymer/fullerene system. Herein in this work, devices using PTP8 as the electron donor and [70]PCBM as well as widely used polymer N2200 as the electron acceptor were systematically studied and compared. The major loss mechanisms in the all polymer solar cells were investigated to understand their relatively lower performance than the PTP8/fullerene system. By performing in-depth analysis on ultrafast transient transmission spectroscopy results, we estimated that in PTP8/N2200 device nearly half of the charges recombine geminately, which is confirmed as the major factor hindering the device performance of all polymer solar cells compared with polymer/fullerene system. Through thorough morphology analysis, the low charge generation efficiency is attributed to the reduced crystallinity of N2200 in the blend film and the unfavorable face-to-edge orientation at the donor/acceptor heterojunction. Coupling these results with knowledge from efficient polymer/fullerene systems, the future design of new polymers can devote to increase the attraction between the π face of donor and acceptor, leading to enhanced face-to-face orientation at the heterojunction, while maintaining a high π-π stacking order for each polymer.

C70/C70:pentacene/pentacene organic heterojunction as the connecting layer for high performance tandem organic light-emitting diodes: Mechanism investigation of electron injection and transport

A high performance tandem organic light-emitting diode (OLED) is realized by employing a C70/C70:pentacene/pentacene organic heterojunction as the efficient charge generation layer (CGL). Not only more than two time enhancement of external quantum efficiency but also significant improvement in both power efficiency and lifetime are well achieved. The mechanism investigations find that the electron injection from the CGL to the adjacent electron transport layer (ETL) in tandem devices is injection rate-limited due to the high interface energy barrier between the CGL and the ETL. By the capacitance-frequency (C-F) and low temperature current density-voltage (J-V) characteristic analysis, we confirm that the electron transport is a space-charge-limited current process with exponential trap distribution. These traps are localized states below the lowest unoccupied molecular orbital edge inside the gap and would be filled with the upward shift of the Fermi level during the n-doping process. Furthermore, both the trap density (Ht) and the activation energy (Ea) could be carefully worked out through low temperature J-V measurements, which is very important for developing high performance tandem OLEDs.