Energy transfer between materials in organic photovoltaics often assists energy transport to the site of free charge generation. Here we present a case where the opposite is true: dilute donor molecules sensitizing a fullerene matrix. We show via a combination of time-resolved microwave conductivity (TRMC), femtosecond transient absorption (fsTA), and photoluminescence excitation (PLE) spectroscopy that fast energy transfer from the donor to the acceptor ultimately results in charge transfer, but not photoconductivity. Instead, the excited states are lost as tightly bound charge-transfer states that do not subsequently dissociate to from free charge in this system. This behavior is caused by an asymmetry in the entropy associated with charge transfer in each direction and is well described by a model in which free charge generation is governed by a combination of entropic gain and competition between multiple Marcus-like charge transfer events to a distribution of distances.
We present evidence that electron-transfer in model organic photovoltaic blends can be modeled as a competition between short and long-range electron transfer events, each described by a Marcus parabola having different reorganization energies for the most localized charge-transfer (CT) state and the mobile free charge (CT) state. Time-resolved Microwave Conductivity (TRMC) combined with photoluminescence excitation (PLE), photoinduced-absorption detected magnetic resonance (PADMR), and femtosecond transient absorption (fsTA) spectroscopy show that when electron transfer is confined to the immediate interfacial region between the donor and the acceptor very little free charge is produced. Instead, excitons split into a highly localized charge transfer state that does not produce photoconductivity. These results provide an alternative way of thinking about charge separation in organic photovoltaic materials, unify solid-state and solution phase models of charge separation, and provide unique design rules for functional donor/acceptor interfaces.
In this work, charge pairs of varying separation distance are interpreted from photoinduced absorption detected magnetic resonance (PADMR) spectroscopy of organic, small molecule dilute donor/acceptor thin films. We report that a donor/acceptor film that generates few free charges at room temperature yet has a relatively large, calculated driving force for electron transfer generates a large concentration of tightly bound CT states when measured with PADMR. These states are markedly absent in films with smaller driving forces yet higher free charge yields which instead only show charge-separated state signals with weaker spin coupling. We interpret this result to be in support of a hypothesis where a larger reorganization energy associated with charge transfer to tightly bound CT states means that they are primarily generated in systems far from the Marcus optimum for free charge yield. And the highest free charge yielding systems instead predominantly undergo long-range charge separation into the acceptor host.
We introduce the concept that free-charge generation in organic photovoltaic (OPV) materials may best be described by competition between long- and short-range electron transfer events, and that the distribution of rates as a function of distance follows the predictions of Marcus theory. Our results reveal the fundamental connection between solution-phase electron transfer research that has been conducted in the chemistry community over many decades, and the younger materials science effort to develop efficient OPV materials. The model that emerges provides insight into how the microstructure of OPV materials influences the electron transfer process via both entropic and quantum-mechanical mechanisms.
The helical nanofilament (HNF) liquid crystal phase is a member of an unusual class of thermotropic phases with lamellar structures dominated by a tendency towards developing negative Gaussian curvature of the layers. Members of this family are sometimes termed “dark conglomerates,” due to their behavior in polarized light microscopy. These include a fluid phases - the high temperature dark conglomerate phase, which is a kind of sponge phase, and the low temperature dark conglomerate phase, also seemingly a sponge phase with structural details currently under investigation. The HNF phase, also a “dark conglomerate,” seems to be unique in the family, since slow conformational dynamics indicate a quasi-crystalline structure within layers, but no long range positional correlations across layers. We have been exploring possible applications of the HNF phase, which is highly porous, as a host for the formation of alignable composites for photovoltaics and other organic semiconductor applications. Recent results regarding the structure of these composites, including data suggesting a remarkably elegant nanostructure for HNF-chiral nematic composites, will be discussed.
Solid-state energy upconversion has many potential applications, from nonlinear photonics and biophotonics to expanding the spectrum available for solar energy harvest. In organic molecular systems, upconversion is frequently done in solution to mitigate aggregation-induced photoluminescence quenching or to facilitate the diffusion of triplet donors in Triplet-Triplet Annihilation (TTA) systems. Here we demonstrate an organic thin film upconversion system utilizing two-photon absorption (TPA) properties to improve upconversion efficiency. In blend films of Stilbene-420 and Rhodamine 6G we observe a tenfold increase in up-converted fluorescence compared to the fluorescence yield of TPA in pristine stilbene films. While TPA normally has quadratic dependence on excitation intensity, these blend films exhibit sub-quadratic intensity dependence, indicating a combination of linear and quadratic upconversion processes and dramatically improving upconversion efficiency at lower excitation intensities. This improvement in intensity dependence allows for relatively efficient upconversion upon excitation by a nanosecond laser pulse, in contrast to the more expensive femtosecond lasers generally required for excitation in TPA microscopy and similar systems. Time-resolved photoluminescence decay measurements reveal that all excited states involved in this upconversion process are singlets, which indicates the potential for reduced energy losses when compared to TTA upconversion systems and their inherent intersystem-crossing energy losses. We observe emission from both the Rhodamine 6G donor molecules and Stilbene-420 acceptor molecules, indicating the presence of prompt fluorescence from the donor as well as upconversion to the acceptor, and FRET losses from acceptor back to donor. By fitting to a kinetic model we extract rates for these competing processes.
With increasing knowledge of the role of the different phases in the bulk heterojunction organic solar cell, the primary site for charge generation is now considered to be the mixed phase, and not the clean interface between neat polymer and neat fullerene. To gain a better understanding of the primary charge generating and recombination steps in this region of the system, we focus our studies on the role of the solid-state microstructure of neat polymers and light-doping of these polymers with a variety of electron-accepting dopants at low concentration.
This presentation will describe some recent work on the doping of polythiophene and polyfluorene derivatives with fullerenes, phthalocyanines and perylenes, which provide a range of reduction potentials that serve to control the driving force for electron transfer processes. Results from flash photolysis, time-resolved microwave conductivity (fp-TRMC), femtosecond transient absorption spectroscopy (fTA) and photoluminescence spectroscopy will be presented.
We examined photoinduced charge-generation dynamics of the poly(3-hexylthiophene) (P3HT)/titanyl phthalocyanine (TiOPc) bilayer and the P3HT/TiOPc/C60 trilayer using the combination of flash-photolysis time-resolved microwave conductivity experiments (fp-TRMC) and classic pump-probe transient absorption (TA) spectroscopy following dominant excitation of the P3HT layer. The superlinear increase of φΣμ for the P3HT/TiOPc bilayer, compared to the φΣμ sum of each P3HT and TiOPc layer suggest photoinduced carrier-generation. Furthermore, the superlinear increase of φΣμ of the P3HT/TiOPc/C60 trilayer with respect to the each P3HT/TiOPc and TiOPc/C60 bilayers evinces charge migration from one interface to the other interface. In addition, with selective photoexcitation on the P3HT layer, both amorphous and H-aggregated P3HT domains participate in electron transfer ([P3HT*/TiOPc]→[P3HT•+/TiOPc•-]), contrasting to the previous observation where with selective excitation of the TiOPc layer, only the H-aggregated P3HT domain involves in hole transfer ([P3HT/TiOPc→[P3HT•+/TiOPc•-]) to produce P3HT•+/TiOPc•- in J. Phys. Chem. B 119(24), 7729—7739 (2015). These results under different excitation conditions are consistent with calculated energetic driving force (ΔECS) for charge generation which is -0.58 eV and -0.73 eV for amorphous and H-aggregated P3HT domains under the P3HT layer excitation, while 0.04 eV and -0.11 eV for amorphous and H-aggregated P3HT domains under the TiOPc layer excitation.
Matthew Bird, Gina Mauro, Lori Zaikowski, Xiang Li, Obadiah Reid, Brianne Karten, Sadayuki Asaoka, Hung-Cheng Chen, Andrew Cook, Garry Rumbles, John Miller
The diffusion of singlet and triplet excitons along single polyfluorene chains in solution has been studied by monitoring their transport to end traps. Time-resolved transient absorption and steady state fluorescence were used to determine fractions of excitons that reach the end caps. In order to accurately determine the singlet diffusion coefficient, the fraction of polymer ends that have end traps was determined through a combination of NMR and triplet quenching experiments. The distributions of polymer lengths were also taken into account and the resulting analysis points to a surprisingly long singlet diffusion length of 34 nm. Experiments on triplet transport also suggest that the entire 100nm+ chain is accessible to the triplet during its lifetime suggesting a lack of hindrance by defects or traps on this timescale. Time Resolved Microwave Conductivity measurements were also performed on a series of different length oligo- and polyfluorenes in solution allowing a global fit to be performed to extract an accurate intrachain mobility of 1.1 cm2/Vs.
KEYWORDS: Polymers, Solar cells, Heterojunctions, Composites, Electron transport, Energy efficiency, Photovoltaics, Nanoparticles, Absorption, Solar energy
We report photovoltaic devices based on composites of a branched nanoheterostructure containing a CdTe core and CdSe arms, CdTe(c)-CdSe(a), combined with either poly(3-hexylthiophene), P3HT, or poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, with solar power conversion efficiencies of 1.2% and 1.8%, respectively. A comparison with previously reported composite devices of a related branched nanoheterostructure: CdSe(c)-CdTe(a) reveals an improved device performance that is attributed to a better electron percolation pathway provided by the dominant, higher electron affinity CdSe arms of the nanoheterostructures.
We present time-resolved photoluminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ~4-fold and ~15-fold improvements in short-circuit currents, respectively, over neat-film P3HT on TiO2 solgel. Despite efficient charge-transfer in P3HT:PCBM films, as evidenced by enhanced device performance and quenched steady-state luminescence, we observe only moderate reduction of the excited state lifetime, due to the already efficient non-radiative pathways in P3HT. We observe evidence for a new state that we assign to an exciplex in blends of P3HT with the electron-transporting polymer CN-ether-PPV. The exciplex state, which confirms the existence of charge-transfer between the two polymers, may account for the enhanced device performance of these blends by acting as a scavenger for excitons that would otherwise decay rapidly via non-radiative pathways. The long-range order of P3HT is disrupted when spin-cast on rough TiO2 nanoparticles, and this results in a blueshift of the PL spectrum and a new long-lived decay component that we attribute to long-lived intrachain polarons. P3HT on smooth TiO2 solgel films shows little or no quenching of the excited state, despite known charge transfer from P3HT to TiO2.
We measure the concentration of single-walled nanotubes (SWNTs) present in aqueous suspensions by a technique that involves surfactant removal followed by high-temperature oxidation and mass spectroscopy of the resulting products. We also analyze the shift in SWNT emission energy evident from photoluminescence excitation spectroscopy as the surfactant molecule is changed. Next we study spectroscopic changes as surfactant is gently removed by dialysis.
KEYWORDS: Near field scanning optical microscopy, Proteins, Quantum dots, Spectroscopy, Near field, Sensors, Polymers, Near field optics, Luminescence, Avalanche photodetectors
We investigated the engineered bioconjugate of cadmium selenide core/zinc sulfide shell, (CdSe)ZnS, quantum dots (QDs) with genetically modified proteins using near-field scanning optical microscopy (NSOM). A genetically engineered protein polymer was expressed and purified from E. coli. The protein polymer was allowed to self-assemble to the bacterial microcrystalline cellulose surface through the cellulosic binding domain. QDs were then conjugated to the protein/cellulose assembly through interaction with the 6x-histidine tag on the protein. The transmitted near-field optical signals are collected and detected by both a PMT (near-field scanning optical microscopy, NSOM) and a spectrometer (near-field scanning optical spectroscopy, NSOS). Results from the sample containing the QDs/protein/cellulose assemblies suggest that QDs were arrayed along the cellulose surface. The near-field spectroscopic study also showed that the slight change of spectroscopic properties of the QDs upon bioconjugation, indicating the strong interaction between the constructed protein and QDs.
The work reported here explores the impact of polymer morphology on the physics and performance of perylene benzimidazole/poly(3-hexylthiophene) bilayer photovoltaic devices. By varying both the annealing temperature and the solvent used for polymer deposition, we demonstrate control of the polymer chain morphology. An increase in the relative ordering of the polymer chain conformation is observed through a shift in the absorption onset and absorption spectral shape, and results in improved photovoltaic performance.
The photophysics of a soluble, cynao-substituted poly (p- phenylene vinylene) and some related model compounds are compared using steady-state and time-resolved photoluminescence spectroscopy. The nature of the excited states in the model compounds are shown to differ markedly from the polymer as steric hindrance between the cyano group and an alkoxy group disrupts electron delocalization resulting in a twist in the molecular structure. In solution the polymer exhibits emission from both isolated chains, and from aggregates formed when the solution is allowed to equilibrate over a long period of time. The aggregate has its own distinct absorption and emission features and is assigned to a dimer- like species. When spun from the same solvent, thin films of the polymer exhibit the same emission features as the solution aggregates.
We report a study of the photophysics of solutions and films of the conjugated polymer poly(3-hexylthiophene) (P3HT). We have performed measurements of absorption, photoluminescence (PL), PL quantum yield, and time-resolved PL. For P3HT in a good solvent the PL decays with a time constant of 510 +/- 20 ps, and the PL quantum yield is 0.42 +/- 0.04, we assign the PL to the intra-chain singlet exciton, and deduce a natural radiative lifetime for this species of 1.2 +/- 0.1 ns. We explore the use of solvent mixture as a way of controlling the degree of intermolecular interaction. We find that there is a red-shift of absorption and PL in poor solvent mixtures, and the spectra and PL quantum yield become similar to the film. The PL decay becomes faster and non-exponential in poor solvent mixtures and the thin film.
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